ALBERT

Description: Genesis of porphyry Cu deposits remains controversial. A common view suggests that the efficient partitioning of Cu into a fluid exsolved from a calc-alkaline magma with normal Cu but elevated S concentrations is critical to ore formation. However, this view was recently challenged by two new ideas, which emphasize that the formation of porphyry Cu deposits requires anomalously Cu rich sources of either a mantle domain or intracrustal magmatic cumulate. Here we report data on an intracrustal magmatic hornblendite, genetically associated with the large Mujicun porphyry Cu deposit in North China craton. Petrographic observations, geochemical data, and P-T calculations indicate that the hornblendite formed at a depth of ~28 km by amphibole accumulation from a basaltic magma. These cumulates, despite containing traces of Cu-bearing sulfides, have Cu contents of only 17 to 60 ppm, close to primitive arc magmas. Their parental magma, initially not Cu enriched, evolved to ≥60 ppm Cu in resultant gabbronorite emplaced at ca. 8 km. This was followed by strong Cu depletion (〈10 ppm) in the quartz-diorite porphyries associated with Cu mineralization. Most of the gabbronorites contain magmatic sulfide phases that have been partially dissolved by hydrothermal fluids. One sample contains sulfide droplets and coexisting silicate glass and fluid inclusions that were locally trapped in interstitial magmatic quartz. We conclude that the exsolution of a Cu-rich sulfide melt and its subsequent redissolution into aqueous fluids was a significant factor in the formation of the Mujicun porphyry Cu deposit, and that anomalous Cu enrichments in the mantle source regime or intracrustal cumulates were not required to form the ore deposit.

Description: The Southern Feeder Dike Complex is part of the Franklin Large Igneous Province (LIP), exposed in the Minto Inlier of Victoria Island in the Canadian Arctic. Previous field and geochemical studies on the Franklin LIP considered its igneous rocks to be prospective for Fe-Ni-Cu mineralization. The Southern Feeder Dike Complex comprises a series of NW-SE-trending gabbroic intrusions and sedimentary hosts. Field and textural relationships show that the Complex intrusions were emplaced contemporaneously with Neoproterozoic normal faulting. Faulted contact zones correspond to prominent first derivative magnetic lineaments. Gabbroic dikes have intrusive contacts against brecciated country rock, and diabasic microxenoliths in basaltic matrices indicate multiple intrusive/brecciation events. Intrusive breccias are commonly overprinted by hydrothermal greenschist facies assemblages, with calcite + pyrite veins filling open spaces between breccia fragments. Late dikes emplaced into these heterogeneous breccias contain disseminated globular and net-textured sulfides suggesting that sulfide immiscibility was triggered on a local scale by assimilation of local wall rock. This inference is supported by elevated 34 S values of sulfides in these dikes, consistent with assimilation of country rocks. Wall-rock assimilation would have been facilitated by fault-related brecciation and cataclasis, which would expose extensive xenolith surface areas to fresh magma. Gossanous and meter-scale semimassive sulfide showings associated with dikes and sills located upsection from the Southern Feeder Dike Complex suggest that immiscible sulfide liquids may have been flushed downstream (or upsection) during replenishment of composite dike systems. Fault-mediated melt ascent along northwest-southeast faults has been documented elsewhere in the Minto Inlier, providing equivalent opportunities for wall-rock assimilation and consequent triggering of sulfide immiscibility and sulfide melt redistribution. The evidence preserved in the Complex confirms the Fe-Ni-Cu potential of the Franklin LIP and informs current models of ore deposit formation in conduit-type magmatic plumbing systems.

Description: Subseafloor replacement-style volcanogenic massive sulfide (VMS) deposits are a subset of VMS deposits where sulfides have replaced unconsolidated volcanic, volcano-sedimentary, and sedimentary material. These deposits are anomalously large and are important global sources of metals. They have distinct textures at the sulfide-ore interface, including bed-by-bed replacement of sedimentary layers, and typically fill void space between unconsolidated volcaniclastic detritus or fractures in flows or intrusions. At the microscale, metalbearing sulfides have partially to fully replaced framboidal (bacteriogenic) sulfides, or the framboidal sulfides have acted as nuclei upon which additional metalliferous massive sulfide is deposited. The textures presented are reconciled within a semipermeable interface model for replacement. In this model unconsolidated sediment, volcaniclastic rocks, or fractured coherent volcanic rocks provide a permeable to semipermeable interface that allowed ingress of cold seawater into the pore spaces of the stratigraphic sequence prior to and during lulls in hydrothermal activity. Seawater sulfate in the pore water is partially reduced by bacteria to provide reduced sulfur (H 2 S) as well as framboidal pyrite in the host sequence(s). The reduced sulfur and framboidal pyrite, as well as the cool pore water, provided a thermal, redox, and chemical gradient in which upwelling hydrothermal fluids interact. In such an environment rising hydrothermal fluids mix with cold water, not only at the seawater interface leading to exhalative sulfide deposition, but also in the subseafloor leading to sulfide precipitation via replacement. The upwelling hydrothermal fluids can also interact with bacterial H 2 S in the pore spaces of the unconsolidated material, resulting in additional subseafloor precipitation of metal sulfides. The fluids also result in replacement of framboidal pyrite nuclei pseudomorphous after the original framboidal masses. This semipermeable interface also favors enhanced zone refining, assuming the hydrothermal system is sufficiently long lived, leading to upgrading of the tenor of the sulfides with well-developed metal zoning, as observed in many ancient replacement-type deposits. Furthermore, the precipitation of a significant subseafloor sulfide mineralization results in greater trapping of metals from upwelling fluids and larger tonnage deposits with greater contained metal. This model may also be applicable to other replacement-type deposits in broadly similar geologic and hydrothermal environments (e.g., sediment-hosted and Irish-type Zn-Pb deposits). Additional, critical tests are required to validate and refute the model and potential tests are presented herein.

Description: The Mount Keith region of the Agnew-Wiluna belt in Western Australia is perhaps best known for hosting the giant komatiite-associated disseminated Ni sulfide deposit known as MKD5. It also hosts the sizeable Cliffs massive Ni sulfide deposit, which is the eighth largest in the Archean Yilgarn block. A concerted program of regional-scale three-dimensional model construction has integrated a plethora of geological, geochemical, and geophysical data and led to a much better understanding of the tectonic and volcanic architecture of this ancient greenstone belt. The disseminated MKD5 orebody occupies a curvilinear thickening in the olivine adcumulate core of the host Mount Keith ultramafic unit. The orebody plunges shallowly to the south from surface, over a distance of 3.5 km, before bending sharply to plunge steeply to the north-northwest at depth. The thickest part of the orebody and the best Ni grades are associated with the bend in the overall ore trend. This also marks the approximate intersection point of two growth faults that appear to control the thickness variations in the adcumulate core and the plunge variations in the ore trend. The olivine adcumulate core is mantled by olivine orthocumulate rocks, which are best developed above and below the flanks of the orebody, and a fractionated sequence of pyroxenitic and gabbroic rocks occurs at the top of the Mount Keith ultramafic unit above the inside of the bend in the ore trend. These observations suggest a strong volcanological control on the position of the MKD5 orebody, and the local curvilinear thickening in the adcumulate core is interpreted as a fluid pathway, the locus of which was in turn controlled by the intersection of two broadly orthogonal growth faults. The Cliffs ultramafic unit consists of a lower olivine ortho- to mesocumulate unit, which varies in thickness along strike, overlain by a sequence of thin, spinifex-textured komatiite flow units intercalated with variably sulfidic tuffaceous shales. The Cliffs massive Ni sulfide deposit occurs at the base of a relative thickening (to ~100 m) in the lower olivine ortho- to mesocumulate unit. The orebody is typically less than 6 m thick, 500 m wide and plunges gently south from surface over a distance of 1.5 km, terminating in a structurally complex zone across which there are marked differences in the internal stratigraphy and thickness of the Cliffs ultramafic unit. The orebody has relatively low tenor (maximum ~8% Ni, typically ~5% Ni) and the tenor is strongly zoned with extremely low tenors (1–3% Ni) developed on the flanks. A barren exhalative massive Fe sulfide unit is typically present at the base of the Cliffs ultramafic unit on the flanks of the orebody and probably acted as a source of sulfur. The tenor zonation pattern suggests that the melting and entrainment of the underlying exhalative sulfide horizon occurred more vigorously in the center of the lava pathway. The structurally complex zone at the southern end of the Cliffs orebody is interpreted as a possible vent location; this would explain the relatively low tenor of the orebody and the tenor zonation (through limited mixing and entrainment of melted exhalative iron sulfide melt with the komatiite lava), the poorly incised nature of the lava pathway and the abrupt change in internal komatiite stratigraphy across this zone. There is good evidence that the southern terminations of both the MKD5 and the Cliffs orebodies are marked by growth faults. At a belt scale these points correspond roughly with the intersection of NNE- and NW-trending lineaments. These lineaments are on the order of 5 to 10 km in strike length and are visible in regional geophysical datasets (both gravity and magnetics). They are likely to mark the position of accretionary structures that were active at the time of volcanism and that probably controlled the volcanic architecture of the komatiite flow field and the location of Ni sulfide accumulations within komatiite lava pathways. Repeated reactivation of these early crustal-scale structures during subsequent orogenesis resulted in their upward propagation as sets of late brittle structures. Of broader significance to the understanding of the generation of komatiite-associated nickel sulfide deposits is the conclusion that many such deposits may be developed proximal to vent rather than in a distal setting, as has been proposed for the Kambalda camp.

Description: Red Chris is a Late Triassic porphyry Cu-Au-(Mo) deposit in the Stikinia island-arc terrane in northern B.C. Late Triassic sedimentary and volcanic rocks of the Stuhini Group host a series of Late Triassic to Early Jurassic (204–198 Ma) diorite to quartz monzonite stocks and dikes. The Red Chris deposit is hosted in the 204 Ma Red stock, the largest (6.5 x 1.5 km) and most altered intrusion in the area. Volcanic rocks approximately time equivalent with the stock are exposed 2 km to the southwest and west. Postmineral Early and Middle Jurassic sedimentary rocks unconformably overlie the Stuhini Group and Red stock to the south and southeast. The composite Red stock consists of multiple phases of crowded porphyries (30–65% plagioclase + hornblende ± biotite phenocrysts) including abundant premineral "P1" leucodiorite; multiple subphases of synmineral "P2" quartz monzonite porphyries; and volumetrically minor late mineral monzonite ("P3") and monzodiorite ("P4") porphyries. The stock evolved from subalkaline to silica-saturated alkalic composition, with the mineral-related P2 porphyries being high K calcalkalic and high Sr/Y. Common septa of Stuhini biotitic hornfels are entrapped within the stock. Minor postmineral, weakly altered mafic dikes cut the stock. The zone of 〉0.25% Cu equiv (Cu + Au) is 〉2 km long (WSW-ENE) and up to 650 m wide. Early A-type quartz veins with disseminated bornite accompanied biotitic and K-feldspar-magnetite alteration, which was coeval with P2 porphyries. Higher Cu and Au grades correlate in a general way with zones of more abundant A veins. Chlorite-carbonate-epidote-actinolite replaced hornblende peripheral to biotitic alteration, and chlorite overprinted much of the secondary biotite. Late alteration affected all porphyry phases and is subdivided into earlier sericite-clay-pyrite, and later low sulfide carbonate-clay-hematite. Late sericite-clay-pyrite alteration sulfidized much of the primary bornite to chalcopyrite with or without pyrite, and carbonate-clay-hematite alteration converted most magnetite to hematite. Highest Cu-Au grades (〉2% Cu equiv) occur in and around the early and intermediate stage P2 porphyries. Zoning of Cu, Au, Au/Cu ratios, sulfides, and quartz veins is centered and mostly symmetrical around a core of high-grade Cu-Au within and surrounding apices of P2 porphyry. Contact relationships confirm the close association of P2 porphyry phases with Cu-Au. Zones of 〉50 ppm Mo surround the highest grade Cu-Au zone at depth. West-northwest and east-northeast strike-slip to oblique-slip faults offset sulfide, Cu-Au, and Mo patterns. Several lines of evidence indicate 15° to 20° of southward tilt since the system formed. Due to glaciation, oxidation is very thin and supergene enrichment negligible. Red Chris was previously considered by most workers as an alkalic or "hybrid" alkalic deposit. New chemical data on synmineral porphyry phases instead suggest high K calc-alkalic composition, similar to the monzonitic Cu-(Mo-Au) class of porphyry deposits, which includes Bingham and Bajo de la Alumbrera. Red Chris is also classified as an A vein type deposit due to strong control of higher grade Cu-Au by A vein stockworks with abundant disseminated Cu sulfides.

Description: The middle to late Miocene Altar porphyry Cu-(Au-Mo) deposit, located in the Andean Main Cordillera of San Juan Province (Argentina), is characterized by the superposition of multiple vein generations consisting of both porphyry-type and high sulfidation epithermal-style alteration and mineralization. We constrain the physical and chemical evolution of the hydrothermal fluids that formed this deposit based on description and distribution of vein types, scanning electron microscopy, cathodoluminescence (CL) imaging, trace elements in quartz veins, and fluid inclusion microthermometry. Quartz CL textures and trace elements (chiefly Li, Al, Ti, and Ge) differentiate among quartz generations precipitated during different mineralization and alteration events. Early quartz ± chalcopyrite ± pyrite veins and quartz ± molybdenite veins (A and B veins) show considerable complexity and were commonly reopened, and some underwent quartz dissolution. Early quartz ± chalcopyrite ± pyrite veins (A veins) are dominated by equigranular bright CL quartz with homogeneous texture. Most of these veins contain higher Ti concentrations than any other vein type (average: 100 ppm) and have low to intermediate Al concentrations (65–448 ppm). Quartz ± molybdenite (B veins) and chlorite + rutile ± hematite (C veins) veins contain quartz of intermediate CL intensity that commonly shows growth zones with oscillatory CL intensity. Quartz from these veins has intermediate Ti concentrations (~20 ppm) and Al concentrations similar to those of A veins. Quartz from later quartz + pyrite veins with quartz + muscovite ± tourmaline halos (D veins) has significantly lower CL intensity, low Ti (〈15 ppm) and elevated Al concentrations (up to 1,000 ppm), and typically contains euhedral growth zones. Late veins rich in sulfides and sulfosalts show CL textures typical of epithermal deposits (dark CL quartz, crustiform banding, and euhedral growth zones). Quartz from these veins typically contains less than 5 ppm Ti, and Al, Li, and Ge concentrations are elevated relative to other vein types. Based on experimentally established relationships between Ti concentration in quartz and temperature, the decrease in Ti content in successively later quartz generations indicates that the temperature of the hydrothermal fluids decreased through time during the evolution of the system. Vein formation at Altar occurred at progressively lower pressure, shallower paleodepth, and lower temperature. Under lithostatic pressures, the magma supplied low-salinity aqueous fluids at depths of ~6 to 6.8 km (pressures of 1.6–1.8 kbar) and temperatures of 670° to 730°C (first quartz generation of early quartz ± chalcopyrite ± pyrite veins). This parental fluid episodically depressurized and cooled at temperatures and pressures below the brine-vapor solvus. Quartz ± molybdenite veins precipitated from fluids at temperatures of 510° to 540°C and pressures of 800 to 1,000 bars, corresponding to depths of 3 to 3.7 km under lithostatic pressures. Further cooling of hydrothermal fluids to temperatures between 425° and 370°C under hydrostatic pressures of 200 to 350 bars produced pyrite-quartz veins and pervasive quartz + muscovite ± tourmaline and illite alteration that overprinted the early hydrothermal assemblages. Late veins rich in sulfides and sulfosalts that overlapped the deep and intermediate high-temperature veins formed from fluids at temperatures of 250° to 280°C and pressures of 20 to 150 bars. The epithermal siliceous ledges formed from low-temperature fluids (〈230°C) at hydrostatic pressures of 〈100 bars corresponding to depths of 〈〈1 km.

Description: Gold mineralization at the Damang deposit is unique among currently known orogenic gold deposits in Ghana, comprising gold hosted within metasediments of the Tarkwaian System and contained in a subhorizontal, extensional quartz vein array that formed during regional compression. The Damang region has an extended paragenesis involving numerous structural, metamorphic, igneous, and metasomatic events. Orogenic gold mineralization occurred late in the geologic paragenesis at Damang, postdating regional metamorphism and an earlier episode of hydrothermal alteration, locally termed "pink hematite" alteration, associated with the intrusion of mafic sills and dikes. This earlier pink hematite alteration event involves extensive silicification that changed the rheology of the altered rocks and promoted later fracturing. Following peak regional metamorphism at around 2005 Ma, the Damang region underwent a short period of rapid exhumation, as constrained through numerical thermal modeling of existing pressure-temperature-time data. This exhumation triggered the generation of a subhorizontal fracture array that was fed by fluids released through decompression-driven metamorphic devolatilization. The interaction between these fluids and the host rock resulted in precipitation of gold in association with sulfide-carbonate-potassic alteration halos around quartz veins. Such postpeak metamorphic, exhumation-driven, devolatilization is unlikely to be a singular occurrence and represents a potentially important source of fluid for orogenic gold deposits elsewhere in Ghana, West Africa, and globally.

Description: Various ultramafic Ni-Cu-platinum group element (PGE) deposits associated with the North American Midcontinent rift have been attributed to formation in a magma conduit setting, whereby PGE concentration is controlled by various fluid dynamic processes. The Marathon Cu-PGE sulfide deposit located within the Midcontinent rift-related Coldwell Alkaline Complex has been classified as a gabbro-associated contact-type deposit; however, both magmatic and hydrothermal processes have been proposed to account for the significant concentration of PGE. In light of the growing field of evidence for magma conduit-type settings, this study comprised a comprehensive geochemical investigation of the complicated crosscutting gabbroic to ultramafic units in the immediate vicinity of the Marathon deposit; and a thorough three-dimensional investigation of the distribution of Cu and Pd within the Main mineralized zone. The main objectives of this study were to test the applicability of the magma conduit deposit model to the Marathon deposit and to identify key exploration criteria for use elsewhere in the Coldwell Alkaline Complex. Mineralization is hosted by the Two Duck Lake gabbro, a 4-km-long and 250-m-thick unit of the Marathon Series. The Marathon Series is the latest of three magmatic series that make up the 1- to 2-km-thick Eastern Gabbro Suite, which wraps around the eastern and northern margin of the Coldwell Alkaline Complex. The three magmatic series are shown here to have distinct trace element signatures that enable reliable discrimination of potentially sulfide and PGE-bearing units of the Marathon Series from the barren rocks of either the Fine-Grained or Layered Series. At the Marathon deposit, sulfides consist of disseminated chalcopyrite, pyrrhotite, and minor bornite and occur within the Main, Footwall, and Hanging-wall zones and in the PGE-enriched W Horizon. This paper focused on sulfides located within the Main zone, including the keel-shaped feeder channel that continues downdip to over 550-m depth. The spatial distribution of Cu, Pd, and Cu/Pd were examined in relation to a three-dimensional surface model for the footwall contact; in a vertical profile through the Main zone; and in a longitudinal section that cuts the feeder channel. There are three important observations: (1) trends for elevated Cu and Pd are parallel to numerous troughs and ridges in the footwall, (2) Cu, Pd, and Cu/Pd varies up section in a saw-toothed pattern from high to low values, and (3) the proportion of high Cu/Pd sulfides is greatest within the thickest accumulations of sulfides within the feeder channel. Evaluation of interelement relationships between Cu and Pd and between Pd and Ir, Rh, Pt, and Au for mineralization within the Main zone indicate positive associative, but nonlinear behavior for all elements. Briefly, the data show nonlinear correlations between Cu and Pd in which Cu/Pd decreases with increasing Pd; and coherent but nonlinear behavior for Ir, Rh, Pt, and Pd in which Pd/Ir, Pd/Rh, Pd/Pt, and Pd/Au all increase with increasing Pd. The observed variation in Cu/Pd is consistent with a magmatic model calculated by others for deposits in the Duluth Complex, in which sulfides accumulated in a closed system from a melt with mantlelike Cu/Pd and an elevated silicate to sulfide ratio. The observed variations in Pd/Ir, Pd/Rh, and Pd/Pt are consistent with R factor fractionation related to differences in the relative partition coefficients between sulfide and silicate melts, and rule out the possibility that processes such as fractionation of sulfide melt by monosulfide solid solution (mss) or redistribution of metals during hydrothermal alteration played a significant role in the mineralizing event. The Two Duck Lake gabbro and associated sulfides of the Marathon deposit are proposed to have formed by multiple injections of plagioclase crystal mush that carried droplets of sulfide liquid along a conduit system that was controlled by radial and ring fault structures in the Coldwell Alkaline Complex. The accumulation of sulfides was controlled by flow dynamic processes within the magma channels, but Cu/Pd was controlled by local proportions of silicate melt to sulfide liquid. Key characteristics of the deposit that are critical to exploration elsewhere in the Coldwell Alkaline Complex include the following: (1) the recognition that gabbroic to ultramafic intrusions of the Marathon Series are the host for Cu and PGE mineralization, (2) the distribution of Cu/Pd data within sulfide occurrences are useful as vectors toward the feeder channel, (3) topographic lineaments are indicators of potential mineralized feeder zones, and (4) oxide- and apatite-rich, irregularly shaped gabbroic to ultramafic pods are potential indicators of an underlying feeder channel.

Description: We have investigated the application of ground, laboratory, and airborne optical remote sensing methods for the detection of hydrothermal alteration zones associated with the Izok Lake volcanogenic massive sulfide (VMS) deposit in Nunavut, Canada. This bimodal-felsic Zn-Cu-Pb-Ag deposit is located above the tree line in a subarctic environment where lichens are the dominant cryptogamic species coating the rocks. The immediate host rhyolitic rocks have been hydrothermally altered and contain biotite, chlorite, and white micas as dominant alteration minerals. These minerals have spectral Al-OH and Fe-OH absorption features in the short-wave infrared wavelength region that display wavelength shifts, which are documented to be due to chemical compositional changes. Our ground spectrometer measurements indicate that there is a systematic trend in the Fe-OH absorption feature wavelength position of biotite/chlorite with increasing distance from the VMS deposit: the average Fe-OH absorption feature wavelength position of the proximal areas (398–3,146 m from mineralization) is observed at 2,254 nm, and that of the distal areas (5,782–6,812 m) at 2,251 nm. Moreover, the proximal areas have an average Al-OH absorption feature wavelength position at 2,203 nm, in contrast with the average wavelength position at 2,201 nm in the distal areas, implying a spectral shift of 2 nm. These findings indicate that hydrothermal alteration zones can be detected by hyperspectral remote sensing, despite the presence of abundant lichen cover. However, the airborne results discussed in this study required the screening out of more than 99% of the pixels in the area.

Description: Diamond-bearing kimberlite dikes (fissures) are present in the deeply eroded Man Shield of West Africa. Small kimberlite pipes, generally less than 1 hectare in area, are known in Guinea, Sierra Leone, and Liberia. Exploration for larger bodies has been severely hampered by thick tropical vegetation, and the lack of distinct geophysical contrasts between weathered kimberlite and the nonresponsive nature of country-rock granites and granodioritic gneisses. Following several years of intense exploration in the highly active artisanal diamond district of northwestern Liberia (which was a major source of alluvial "blood diamonds") by several large companies and the present study, we report that an elusive diamond-bearing kimberlite pipe has finally been located. A bonus to the pipe location is that an unusual botanical indicator, Pandanus candelabrum , is now recognized exclusively on the pipe and not in eluvium covering the adjacent kimberlite dikes. Plants ( Lychnis alpina ) have been widely used since medieval times for copper in Sweden, and with Haumaniastrum katangese, more recently in Africa. Other plants have evolved to physiologically stabilize heavy metals (U, Pb, Zn, Ni, Cr, Ba, Pb, Zn) in leaves and bark. Termite hills have been used in diamond exploration for kimberlitic indicator minerals (ilmenite, chromite, garnet, pyroxene) in Botswana, the United States, and Australia, but the identification of Pandanus candelabrum, with stilt-like aerial roots, is the first plant to be described that has a marked affinity for kimberlite pipes. This could dramatically change the exploration dynamics for diamonds in West Africa, as geobotanical mapping and sampling is cost-effective in tough terrain.

Description: The 2.67 Ga Hackett River volcanogenic massive sulfide (VMS) deposits located in the northeastern Slave province, Nunavut, Canada, are among the largest undeveloped massive sulfide resources in Canada and are silver rich compared to other such deposits of similar age, with Ag grades up to 3,000 g/t. The deposits are hosted by the Ignerit Formation of the felsic to intermediate calc-alkaline Hackett River Group metavolcanic rocks that are part of the province-wide supracrustal Yellowknife Supergroup. One of the most economically significant of the Hackett River deposits is the Hackett River Main zone (Main zone), which consists of two parts: a stratigraphically lower chalcopyrite-rich stringer zone and an upper massive to semimassive polymetallic sulfide lens. The mineralization is subdivided into five types based on mineralogy, textures, and approximate stratigraphic position: (1) disseminated footwall sulfides, (2) copper-rich stringer sulfides, (3) pyrite-poor sphalerite-pyrrhotite-chalcopyrite mineralization at the top of the stringer zone, (4) mineralization in calc-silicate–altered calcareous tuff units, and (5) sphalerite-pyrite massive sulfide. In type 1 mineralization, disseminated pyrite, pyrrhotite, and sphalerite contain negligible Ag and in type 2, Bi-Ag-(Pb) sulfides, Ag-Bi-Se–enriched galena and chalcopyrite are the dominant Ag hosts. Within type 3, Ag-rich tetrahedrite (freibergite) and galena are the main Ag hosts. In type 4, Ag is hosted in disseminated electrum and freibergite, and within type 5 mineralization, freibergite hosts 99% of the Ag. Overall within the Main zone, Ag-rich freibergite contains 79.4% of the Ag, whereas chalcopyrite hosts 6.3% and galena contains 1.8%. Trace minerals such as electrum host the remainder of the Ag, and these have a limited spatial distribution. Zone refining is the most important control on the distribution of Ag within the Main zone and the principal controls on Ag residence are mineralizing fluid temperature, deposit-scale relative redox conditions, sulfidation state, location of the mineralization relative to the hydrothermal conduit, and the ratio of Bi to Sb in the mineralizing fluid available for coupled substitution. Within the freibergite and chalcopyrite, Ag directly substitutes for Cu and replaces Pb in galena by coupled substitution with Bi and, to a lesser extent, Sb. Lower temperatures 〈ca. 250°C and more oxidizing conditions favored partitioning of Ag into freibergite and less oxidizing conditions favored galena as a host. At higher temperatures, 〉ca. 250°C, the most reducing conditions favored incorporation in Ag-Bi-rich galena (plus Se) and Bi-bearing sulfides or Ag-rich chalcopyrite under lesser reducing conditions.

Description: The Bayinwula roll front-type uranium deposit is hosted in sandstones of the Early Cretaceous Saihan Formation deposited during the postrift stage of the Erlian basin, in northeast China. The present study aims to characterize: (1) the roles of sulfate-reducing bacteria and (2) the roles of Fe-Ti oxides in the genesis of the Bayinwula deposit. Drill cores of the host sandstone have been investigated with emphasis on petrographic observations, whole-rock geochemistry, and geochemical and/or mineralogical study of Fe-Ti oxides, iron disulfide, uranium minerals (EMP, LA-ICPMS), and organic matter (rock-eval pyrolysis). The 34 S value has been measured in situ by SIMS on the different generations of iron disulfide. Within regional sandstones, preore uranium enrichment (U mean = 30.4 ppm in whole rock) was identified on altered Fe-Ti oxides (38.5% of the whole-rock U content) and constitutes a major source of uranium for the mineralization. Petrographic observations and rock-eval data indicate that organic matter occurring in the host sandstone is mainly inherited from land plants and corresponds to type III or type IV kerogens. Organic matter fragments disseminated in sandstones may also contain significant preore uranium concentration. Framboidal and collomorph ore-stage iron disulfides have moderate to high concentrations of As, Ni, and Co and have a light sulfur isotope signature characterized by 34 S values from –30.5 to –7.5, suggesting that sulfur originated from bacterial sulfate reduction, which was mainly responsible for (1) the liberation of U from Fe-Ti oxides and organic matter, (2) the generation of ore-stage iron disulfides, and (3) the production of a secondary H 2 S-rich reducing barrier involved in the reduction of U(VI) and the precipitation of U(IV). Uranyl and sulfate ions were transported through the host sandstone by low-temperature oxygenated groundwater and U(IV) was precipitated at the redox front as P-rich coffinite and ningyoite, dominantly as replacement of ore-stage iron disulfides which have partly to totally replaced organic matter and Fe-Ti oxides. Therefore, the combined mineralogical, geochemical, and isotopic characteristics of the Bayinwula roll-front uranium deposit support the theory that biogenic processes have widely contributed to the genesis of the uranium mineralization.

Description: Supergene nonsulfide Zn-Pb deposits consist mainly of Zn-Pb carbonates, Zn silicates, Fe hydroxides, and minor Zn-Pb phosphates, commonly associated with remnants of primary sulfides (sphalerite and galena). The relative abundances of these mineral phases are strongly dependent on the type of host rock. Their variable mineralogy is complex to characterize, and can lead to problems during the processing and recovery of the minerals from the ores. In this case study, mineralogical characterization and quantitative evaluation of the Jabali (Yemen) nonsulfide Zn-Pb deposit was carried out using automated scanning electron microscopy (QEMSCAN ® ). Both primary and secondary ores at Jabali are hosted in Jurassic dolostones. Smithsonite is the most abundant economic ore mineral in the supergene deposit, and it is locally intergrown with Fe hydroxides and remnants of primary sulfides (sphalerite and galena). The host dolomite is locally replaced by broad bands of Zn-rich dolomite, where Zn has substituted for Mg. The ZnO content in this dolomite can reach 22 wt %. Other Zn minerals such as hemimorphite and hydrozincite occur in limited amounts, as well as cerussite and anglesite. Small particles of Ag sulfide are associated with the secondary minerals. Gypsum, Fe (hydr)oxides (goethite 〉 hematite), Zn-Mn (hydr)oxides, and Pb-Mn (hydr)oxides have been detected locally. The QEMSCAN technique, combined with data previously obtained from other analytical techniques (X-ray diffraction [XRD]), scanning electron microscope-energy dispersive spectrometry [SEM-EDS], optical petrography), has provided detailed mineralogical and textural information on the Jabali mineralization. A key outcome from this QEMSCAN study is the textural data and quantification of the Zn dolomite, which is fairly abundant in this deposit. The combination of techniques used to examine the Jabali supergene ore provides high-quality information that not only characterizes the deposit in detail, but also offers a better understanding for the design of ore processing options and a more realistic predicted recovery of economic minerals.

Description: Gahnite-bearing rocks are common throughout the Proterozoic Broken Hill domain, New South Wales, Australia, where they are spatially associated with Broken Hill-type Pb-Zn-Ag mineralization, including the supergiant Broken Hill deposit. In the past, such rocks have been utilized as exploration guides to ores of this type, but their presence has had mixed success in discovering new occurrences of sulfide mineralization. Major element chemistry of gahnite has previously been used to define a compositional range associated with metamorphosed massive sulfides deposits, including Broken Hill-type deposits, but it fails to distinguish sulfide-rich from sulfide-poor occurrences. Major and trace element data from LA-ICP-MS and electron microprobe analyses were obtained for gahnite from twelve Broken Hill-type deposits to determine whether or not gahnite chemistry may be used to distinguish prospective exploration targets from nonprospective occurrences. Major and trace element data were discriminated using a principal component analysis, and in a bivariate plot of Zn/Fe versus Ni + Cr + V to distinguish gahnite associated with the Broken Hill deposit from that associated with sulfide-poor lode pegmatite, and sillimanite gneiss. Bivariate plots of Zn/Fe versus trace element contents (e.g., Ga, Co, Mn, Co, Ni, V, Cd) suggest gahnite from the Broken Hill deposit has a relatively restricted compositional range that overlaps with some minor Broken Hill-type occurrences. Based on the ore grade (wt % Pb + Zn) of rocks hosting gahnite at each locality, gahnite in the highest grade mineralization from minor Broken Hill-type deposits possess compositions that plot within the field for gahnite from the Broken Hill deposit, which suggests that major and trace element chemistry (e.g., Zn/Fe = 2–4 vs. Co = 10–110 ppm, Ga = 110–400 ppm, Mn = 500–2,250 ppm; and Co = 25–100 ppm vs. Ga = 125–375 ppm) may be used as an exploration guide to high-grade ore.

Description: A key step in the formation of many magmatic Ni-Cu-PGE sulfide deposits is the addition of crustal sulfur to mafic or ultramafic magmas. Sulfur addition has been proposed to take place via two different types of processes: (1) production of sulfurous fluids within the thermal aureole around an intrusion accompanying breakdown of sulfide- or sulfate-bearing minerals during devolatilization, followed by diffusive or advective transport of these fluids into the magma, and (2) by direct melting and assimilation of wall rock and xenoliths. We consider physical and chemical controls on the timescales of these processes and show that wall-rock and xenolith melting is by far the most efficient and quickest process for adding crustal sulfur, with melting processes taking place on a scale of minutes to years. In contrast, liberation of sulfur from a thermal aureole via diffusion is much slower and requires timescales of millions of years—two orders of magnitude longer than the time required for an intrusion to solidify by diffusion. We conclude that sulfur, which may be liberated in thermal aureoles (produced either via devolatilization reactions or dissolution involving hydrothermal fluids) and which must be transported via diffusional processes, has a negligible effect on the formation of magmatic Ni-Cu-PGE deposits.

Description: Sulfur isotope ratios of sulfide ores and their host rocks, and the overlying main mass sequence of norites, quartz gabbros, and granophyres from the Sudbury Igneous Complex have been analyzed to establish the extent of isotopic heterogeneity of the Complex and the impact melt sheet from which it was produced. Noritic rocks located above the sublayer in the lower portion of the main mass of the Sudbury Igneous Complex are characterized by uniform sulfur isotope ratios with 34 S values of 2.59 ± 0.48. Rocks from the granophyric portion of the main mass are characterized by elevated and variable 34 S values from 4.4 to 14.1. The more 34 S-enriched S in the granophyre was produced by secondary sulfide precipitation related to hydrothermal alteration systems developed in the upper altered granophyric part of the main mass as it cooled. Footwall and contact-style mineralization from the sublayer show variable 34 S values from 0.2 to 4.3. South Range mineralization is characterized by low ratios from 0.2 to 2.5. North and Northeast Range mineralization exhibits locally elevated but variable 34 S values; for example mineralization from Whistle mine varies from 3.1 to 4.3, whereas mineralization from Victor varies between 2.4 and 2.9. There is typically only a small difference in S isotope ratio of contact and proximal footwall mineralization. Country rocks in the area of the Sudbury Igneous Complex are also characterized by a wide range in 34 S values and the sulfur isotope homogeneity of the noritic rocks can be explained by mixing of sulfur from several potential sources. However, the variability present between deposits at the lower contact of the Complex requires that country rocks of locally variable sulfur isotope composition interacted with magma at the base of the cooling melt sheet. There is no evidence that deposit size correlates with 34 S value; variations between deposits may involve local S isotope exchange reactions between the country rock and the melt sheet as the melt digested the footwall. This study illustrates the application of S isotope data in constraining the scale of convective mixing and contamination in the largest known terrestrial melt sheet.

Description: Re-Os dating of three samples of hydrothermal molybdenite collected from the Pueblo Viejo Au-Ag-Cu district (5.5 Moz gold, past production, 16.2 Moz proven plus probable gold ore reserves) yields ages of 112.1 ± 0.4, 112.0 ± 0.4, and 111.5 ± 0.4 Ma. These Re-Os dates confirm an Early Cretaceous age for mineralization, coeval with tholeiitic, intraoceanic island arc volcanism and, in particular, with an episode of felsic magmatism and extension across the Early Cretaceous arc (110–118 Ma). Re-Os dating of a sample of hydrothermal molybdenite collected from the Douvray porphyry Cu-Mo deposit (with inferred resources of 1,257 Mlb Cu, 0.276 Moz Au, 6.84 Moz Ag, and 4,367 tons Mo) yields an age of 93.3 ± 0.3 Ma, coeval with calc-alkaline magmatism along the Greater Antilles island arc.

Description: The Chahnaly low-sulfidation epithermal Au deposit and nearby Au prospects are located northwest of the intermittently active Bazman stratovolcano on the western end of the Makran volcanic arc, which formed as the result of subduction of the remnant Neo-Tethyan oceanic crust beneath the Lut block. The arc hosts the Siah Jangal epithermal and Kharestan porphyry prospects, near Taftan volcano, as well as the Saindak Cu-Au porphyry deposit and world-class Reko Diq Cu-Au porphyry deposit, near Koh-i-Sultan volcano to the east-northeast in Pakistan. The host rocks for the Chahnaly deposit include early Miocene andesite and andesitic volcaniclastic rocks that are intruded by younger dacitic domes. Unaltered late Miocene dacitic ignimbrites overlie these rocks. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology data yield ages between 21.8 and 9.9 Ma for the acidic-intermediate regional volcanism. The most recent volcanic activity of the Bazman stratovolcano involved extrusion of an olivine basalt during Pliocene to Quaternary times. Interpretation of geochemical data indicate that the volcanic rocks are synsubduction and calc-alkaline to subalkaline. The lack of a significant negative Eu anomaly, a listric-shaped rare earth element pattern, and moderate La/Yb ratios of host suites indicate a high water content of the source magma. Gold and electrum are temporally and spatially related to a series of structurally controlled, 030°-trending, subvertical hydrothermal breccias with chalcedony-adularia that cut porphyritic andesite and andesitic volcaniclastic rocks. Gold is associated with pyrite, a siliceous matrix of hydrothermal breccia, and previously formed vein clasts, as well as with iron oxides and hydroxides in oxidized zones. Rare silver minerals include Ag-bearing electrum and naumannite, iodargyrite, an unnamed silver diiodide, and hessite. Hydrothermal alteration is generally well developed surrounding the ore-bearing hydrothermal breccia. The main types of alteration in the area include an inner ~0.5- to 20-m-thick gold-bearing hydrothermal breccia composed of quartz-chalcedonyadularia-illite-pyrite, a ~5- to 50-m-thick zone of quartz, chalcedony, pyrite, illitic phengite, phengite, illitic muscovite, illite, illitic paragonite, paragonite, muscovite, montmorillonite and, rarely, siderite, and a 30- to 70-m outer propylitic zone of Fe-Mg chlorite, calcite, ankerite, dolomite, epidote, palygorskite, and pyrite. The Chahnaly Au deposit formed during the early stages of magmatism. LA-ICP-MS zircon U-Pb geochronology of host andesite and 40Ar/39Ar dating of two samples of gold-associated adularia show that the ore-stage adularia (19.83 ± 0.10 and 19.2 ± 0.5 Ma) is younger, by as much as 1.5 million years, than the volcanic host rock (20.32 ± 0.4 Ma). Therefore, either hydrothermal activity continued well after volcanism or a second magmatic event rejuvenated hydrothermal activity. This second magmatic event may be related to eruption of porphyritic andesite at ~20.32 ± 0.40 Ma, which is within error of ~19.83 ± 0.10 Ma adularia. The new LA-ICP-MS zircon U-Pb host rock and vein adularia 40Ar/39Ar ages suggest that early Miocene magmatism and mineralization in the Bazman area is of a similar age to that of the Saindak porphyry and Tanjeel porphyry center of the giant Reko Diq deposit. This confirms the existence of early Miocene arc magmatism and mineralization along the Iranian part of the Makran volcanic arc. Ore, alteration mineralogy, and alteration patterns indicate that the Chahnaly deposit is a typical low-sulfidation epithermal Au deposit, located in a poorly explored part of the Makran volcanic arc in Iran.

Description: Early Cambrian black shale and chert in the Qinling-Daba region along the northern margin of the Yangtze (South China) craton host important high-grade strata-bound barite and witherite deposits. Barite-dominated deposits occur in a northern thrust domain, whereas witherite-dominated deposits occur in a thrust domain a few tens of km to the southwest. The authors studied the strontium isotope ( 87 Sr/ 86 Sr), and stable isotope composition ( 34 S, 13 C, and 18 O) of barite and witherite from the Shenxiantai and Chiyan deposits, two representative Ba deposits in the region, and compared these data to previously reported Sr, S, C, and O isotope data on a variety of Ba deposits in a similar stratigraphic position along the Early Cambrian rifted margin of the Yangtze craton. Most barite and witherite deposits in the Qinling-Daba region have consistent 87 Sr/ 86 Sr ratios with an average value of 0.70845 ± 0.00017 (1 , n = 53), in the range of Early Cambrian seawater from 0.7081 to 0.7090. Barite 34 S and 18 O VSMOW values vary from 8.0 to 76.1 (avg 40.6 ± 13.2, 1 , n = 35) and from 9.9 to 24.7 (avg 19.0 ± 3.0, 1 , n = 103), respectively, generally higher than those of seawater sulfate in the Early Cambrian. We propose that seawater-derived barium was precipitated in the anoxic water column and during early diagenesis. Bacterial sulfate reduction (BSR) and reoxidation under partial sulfate limitation produced higher 34 S and 18 O values of the diagenetic barite compared to contemporaneous seawater sulfate. The 13 C values of Ba carbonate minerals (witherite and barytocalcite) vary from –10.8 to –23.6 (avg –14.1 ± 3.2, 1 , n = 84) and indicate the involvement of carbon from oxidized organic matter. Although the 87 Sr/ 86 Sr ratios, 34 S, and 13 C values of barite and witherite are very close, barite has slightly higher 18 O VSMOW values (avg 22.0 ± 1.6, 1 , n = 19) than witherite and barytocalcite (avg 18.0 ± 2.5, 1 , n = 84), possibly revealing the role of CO 2 -rich cold seeps under sulfate-limited sedimentary-diagenetic conditions during the formation of the witherite-dominated deposits. The Sr isotope composition of barite in the Shenxiantai deposit is different from the other barite deposits and has lower 87 Sr/ 86 Sr ratios, which suggests the involvement of less radiogenic Sr in diagenetic fluids which interacted with underlying mafic volcanic rocks.

Description: Carbonate-hosted Pb and Zn mineralization in the Quesnel Lake district (Flipper Creek, Dolomite Flats, Main, Gunn, and Que deposits) forms an ~8-km-long belt in metasedimentary and metavolcanic rocks of the Cariboo terrane, a displaced piece of the ancestral North American margin. The mineralization is strata-bound, dolostone hosted, and consists of Pb- and Zn-bearing sulfide and nonsulfide minerals. The supergene nonsulfides mimic the morphology of preexisting sulfides occurring as disseminations, veins, and fracture fillings, and irregular replacement zones in the Neoproterozoic to Upper Cambrian dolostones of the Cunningham Formation. The main sulfides are galena, sphalerite, and minor amounts of pyrite, which have been partially or completely transformed into nonsulfide minerals by supergene processes. The main nonsulfide minerals are smithsonite [ZnCO 3 ] and hemimorphite [Zn 4 Si 2 O 7 (OH) 2 ⋅(H 2 O)], with variable amounts of cerussite [PbCO 3 ], anglesite [PbSO 4 ], hydrozincite [Zn 5 (CO 3 ) 2 (OH) 6 ], and iron oxides. The 18 O VSMOW , 13 C VPDB , and 87 Sr/ 86 Sr values of the nonmineralized and mineralized dolostones are indistinguishable and consistent with the composition of carbonates that underwent postdepositional recrystallization and isotopic reequilibration during deep burial fluid-rock interaction. The 18 O VSMOW and 13 C VPDB values range from 17.4 to 20 and –0.8 to +1.4, respectively, and 87 Sr/ 86 Sr values are above 0.71048. Carbon and oxygen ratios of smithsonite show a limited range of values, which are slightly lower than those of the host dolostones with 18 O VSMOW values of 16.9 to 17.8 and 13 C VPDB values of –1.6 to –0.9. The carbon isotope values suggest that the host dolostones are the carbon source for smithsonite. The 18 O smithsonite values are lower than typical values for nonsulfides formed under warm, humid, and arid-to-hyperarid climates elsewhere in the world. The precipitation temperatures derived from the isotope data for the Quesnel Lake smithsonite vary between 9° and 33°C. This temperature range is favorable for supergene smithsonite formation. For most deposits of the Quesnel Lake district examined in this study, close spatial relationships (including textures and morphological similarities) between sulfide and nonsulfide zones, coexisting sulfide-nonsulfide assemblages, and stable oxygen and carbon isotope data suggest that the oxidation of sulfides occurred after the Laramide orogeny (〈55 Ma), during intervals when the climate was relatively warm. Several periods of warm temperatures during the Eocene, early Oligocene, early to middle Miocene, and Holocene may have been favorable for the supergene oxidation of sulfides and the formation of nonsulfides. This process may be ongoing today. The occurrence of nonsulfides in northern latitudes has been poorly documented in the past. This paper raises awareness about the presence of carbonate-hosted nonsulfide Pb-Zn deposits in the northern Cordillera and their potential as a source of zinc.

Description: The Kianna deposit is an unconformity-related uranium deposit in the western Athabasca Basin, hosting mineralization in three zones: (1) perched in sandstone above the unconformity, (2) unconformity and shallow basement hosted, and (3) deep basement hosted. In situ secondary ion mass spectrometry was used to obtain radiogenic and stable isotope data to update the genetic model for the Kianna deposit. Primary basement-hosted ingress-style mineralization, intergrown with hematite and muscovite, has a minimum U-Pb age of ~1500 Ma. Recrystallization of basement uraninite occurred at ~1100 Ma with the precipitation of coarse-grained illite. Late basement uraninite precipitated with fine-grained illite at ~850 Ma. A separate, deeper basement pod formed at ~1280 Ma. Egress-style mineralization at the unconformity and perched uraninite in the sandstone, intergrown with aluminophosphate sulfate minerals and chalcopyrite, formed at ~750 Ma. Later unconformity and perched uraninite precipitated with hematite, pyrite, and chalcopyrite at ~500 Ma. Sulfides coeval with unconformity and perched uraninite have 34 S values ranging from –1.9 to +8.1 and 15.1 to 25.4, indicating two sources of sulfur: sulfides in the metamorphosed basement and aluminophosphate sulfate minerals in the sandstone. Average 18 O and D mineral values for muscovite are 0.7 ± 4.3 and –33 ± 12, respectively, suggesting formation from a marine brine. Average 18 O and D mineral values for coarse-grained illite are 0.4 ± 4.1 and –79 ± 16, respectively, indicating formation from hydrothermal fluids, whereas fine-grained illite 18 O and D mineral values are 6.5 ± 1.6 and –144 ± 21, respectively, suggesting formation from meteoric fluids.

Description: The Xincheng gold deposit, hosted by the Early Cretaceous 132 to 123 Ma Guojialing-type granitoids in northwest Jiaodong Peninsula, southeast North China craton, formed about 2 billion years later than regional metamorphism of the Archean Jiaodong basement rocks. The Xincheng deposit comprises mineralized zones with three types of hydrothermal pyrite associated with gold, tellurides, and a variety of sulfides: py 1 as disseminated euhedral to subhedral grains in altered granitoids around quartz veins; py 2 as subhedral grains with brittle cataclastic textures and fractures in quartz-pyrite veins; and py 3 as subhedral, partially corroded crystals in sulfide-rich veins or veinlets. All three generations of pyrite are unzoned and have low trace element contents, including very low lattice-bound gold contents: (py 1 : 0.180 ppm; py 2 : 0.053 ppm; py 3 : 0.060 ppm). Given that there is 10 to 15% pyrite in the ore zone at Xincheng, its very low gold content indicates that it contributes 〈0.2% of gold to the 7.75 g/t gold in the orebody. Instead, over 99% of the gold is present as discrete electrum and/or gold (total range 0.02–59% silver) grains, which are largely sited in fractures at all scales in pyrite, other ore minerals, and quartz. Importantly, visible gold in py 3 is also sited on solution-corroded pyrite grains. The pyrite textural and geochemical data indicate that it is impossible to derive the high gold-grade orebodies through local remobilization of originally lattice-bound gold in pyrite. Instead, the gold is interpreted to have been deposited through sulfidation reactions and phase separation of a H 2 O-CO 2 ore fluid during progressive brittle cataclastic deformation associated with seismic activity and regional sinistral transtensional shear movement. This concomitant fluid infiltration and deformation caused episodic deposition and fracturing and corrosion of earlier formed pyrite and deposited visible gold in dilational cracks. The coupled development of the transtensional, rather than normal transpressional setting, and precipitation of gold within dilational veins and wall-rock alteration facilitated the deposition of visible gold and an exceptionally high gold tenor. All deposit characteristics indicate that the Xincheng gold deposit is a member of the epizonal orogenic deposit class.

Description: The Glacier Creek volcanogenic massive sulfide (VMS) deposit, Alaska, is hosted within Late Triassic, oceanic back-arc or intraarc, rift-related bimodal volcanic rocks of the allochthonous Alexander terrane, known as the Alexander Triassic metallogenic belt. The Alexander Triassic metallogenic belt is host to the world-class Greens Creek Zn-Pb-Ag VMS deposit near Juneau in the south and the giant Windy Craggy Cu-Co VMS deposit in British Columbia, about 250 km to the north. The Glacier Creek deposit, located ~80 km southeast of Windy Craggy, consists of four tabular massive sulfide lenses within a bimodal mafic volcaniclastic and rhyolitic sequence. The mineralization-hosting stratigraphy is folded by a deposit-scale anticline and offset by a thrust fault near the axial surface of the fold. A resource of 8.13 Mt has been inferred from drilling, with grades of 1.41% Cu, 5.25% Zn, 0.15% Pb, 0.32 g/t Au, and 31.7 g/t Ag. Six main mineralization types are recognized, dominated by massive barite-sphalerite-pyrite, which is replaced at the base and center of the main lenses by massive and semimassive chalcopyrite-pyrite-quartz. The flanks and tops of the lenses are carbonate rich and consist of interbedded calcite-dolomite, barite and sulfide, resedimented massive barite-sulfide, and mineralized massive carbonate rocks. Tuffaceous hydrothermal sediment, with a distinct positive Eu anomaly, overlies the massive sulfide. Pyrrhotite and chalcopyrite in stringers constitute the main "feeder zone." Stringer-style sphalerite-pyrite mineralization occurs above and below the lenses. Fe-poor sphalerite is dominant throughout the lenses, whereas Fe-rich sphalerite occurs at the stratigraphic top and bottom of the lenses in pyrrhotite-rich zones. Galena, tennantite-tetrahedrite, and arsenopyrite are the most important trace minerals within massive barite-sphalerite-pyrite mineralization, which is generally enriched in Sb, Hg, and Tl. Mineralization-related gangue minerals include barite, quartz, barian muscovite, calcite, dolomite, albite, chlorite, hyalophane, and celsian. Four types of alteration are recognized in the dominantly basaltic host rocks: pervasive muscovite-rich alteration, quartz-pyrite alteration associated with sulfide stringers, stratabound carbonate-bearing alteration, and background epidote-bearing alteration. Mass balance calculations indicate gains of S, Fe, Si, and K with coincident losses of Ca, Na, and Mg in all of the alteration types. Trace elements, Tl, Sb, Hg, Ba, Zn, Cu, and As were added to the rocks, whereas Sr was lost. Short wavelength infrared (SWIR) spectroscopy shows an increase in the wavelength of the AlOH absorption feature toward mineralization at a scale of 30 to 50 m, coincident with a general decrease in the Na, K, and Al and increase in the Fe, Mg, and Ba content of muscovite. The Glacier Creek deposit is transitional in character between Greens Creek, which is more Zn, Pb, and precious metal rich, and the Windy Craggy deposit, which is more Cu and Co rich, reflecting differences in the basement rocks and depositional settings within the Alexander Triassic metallogenic belt. Mineral-chemical studies and sulfur isotope data suggest that the Glacier Creek deposit formed under initially oxidized and sulfate-rich conditions that evolved to more reduced conditions in the latest stages of mineralization. The abundant argillite and presence of hyalophane rather than barite in the immediate hanging wall of the deposit may be an indication of a deepening basin and development of local anoxia, similar to Greens Creek.

Description: The Caledonian orogenic belt of northern Britain hosts some significant quartz vein-hosted gold deposits. However, as in orogenic belts worldwide, the relationship between gold mineralization and regional tectonics, magmatism, and metamorphism is a matter of debate. This is primarily due to the absence of precise temporal constraints for the mineralization. Here we report high-precision 40 Ar/ 39 Ar and Re-Os ages for the largest known gold deposit at Curraghinalt (2.7 Moz) in Northern Ireland and use these ages to constrain the regional geologic setting of the gold mineralization and establish a genetic model. The gold resource is contained in a suite of quartz sulfide veins hosted by Neoproterozoic (Dalradian) metasediments, which have been thrust over an Ordovician island arc (Tyrone Igneous Complex). Previous studies recognized two generations of gold sulfide mineralization and we have identified a third in microshears that cut the veins. In the absence of precise geochronological data, mineralization ages from Ordovician to Carboniferous have been proposed. We have dated muscovite ( 40 Ar/ 39 Ar) in quartz vein-hosted clasts of Dalradian wall rock to 459.3 ± 3.4 Ma (all 40 Ar/ 39 Ar and Re-Os ages herein are reported at the 2 confidence level including all sources of uncertainty), an age that we interpret as representing the regional cooling path and which provides a maximum age constraint for all gold mineralization. This is consistent with the quartz veins postdating the end of main-stage deformation in the Grampian event of the Caledonian orogeny (ca. 465 Ma). Molybdenite (Re-Os) and sericite ( 40 Ar/ 39 Ar) from the newly identified gold-bearing microshears (third generation of gold mineralization) yield indistinguishable Re-Os models and 40 Ar/ 39 Ar ages, with a combined age of 455.8 ± 3.0 Ma. The radioisotope ages and field evidence temporally constrain gold mineralization at Curraghinalt to the lower Late Ordovician. Data show that the gold mineralization was emplaced during the Grampian event of the Caledonian orogeny. The ca. 10 Ma maximum possible mineralization interval (462.7–452.8 Ma) for all three episodes of gold emplacement is postpeak metamorphism and main deformation, coinciding with a period of rapid uplift and extensional tectonics following orogenic collapse. While previous studies have suggested the involvement of magmatic fluids in the deposition of the primary gold resource, the absence of magmatism throughout most of the mineralization interval and the nature of the geologic setting suggest that crustal orogenic fluids should also be considered. Overall Curraghinalt displays most of the characteristics of orogenic gold deposits but also some important differences, which may be explained by the geologic setting. The timing of mineralization at Curraghinalt broadly coincides with the shift from compressional to extensional tectonics. The extensional regime, rapid uplift, and a crustal profile comprising metasediments overlying a still hot island arc were ideal for creating large and long-lasting hydrothermal systems deriving heat, metals, and some of the fluids from the underlying arc.

Description: Zircon LA-ICP-MS and CA-TIMS and molybdenite Re-Os geochronology for the El Abra porphyry copper deposit (Chile) document a ~8.6 Ma protracted magmatic-hydrothermal history. Initial magmatic events occurred at ~45 Ma, and continued to ~36 Ma. An ~1.8 m.y. magmatic lull is recorded in the early stages of magmatism, which is characterized by equigranular rocks from the Pajonal suite. Magmatism from ~41 to ~36 Ma, corresponding to the El Abra Granodiorite Complex, is more recurrent, ending in a ~1.4 m.y. period of porphyritic intrusions, which are coeval with Cu-Mo mineralization. Late porphyritic units reveal subtle zircon inheritance likely associated with early magmatic pulses. The young stages of magmatism indicate system rejuvenation based on plagioclase phenocryst compositions, suggesting that the porphyry system is ultimately linked to a less evolved magma. Although there are multiple porphyry and veining events, molybdenite Re-Os ages define a focused mineralization episode from 36.34 to 36.18 Ma. Molybdenite Re-Os ages from nearby related prospects belonging to the El Abra cluster expand the mineralization period in the district up to ~1.0 m.y., suggesting a multistage, long-lived hydrothermal system. Integration of the molybdenite Re-Os dates and those from previous works at the El Abra and Toki clusters, accompanied by available tectonic reconstruction along the Domeyko fault system suggest the development, by late Eocene times, of a single, large (~30 km across) porphyry copper cluster, which was subsequently separated by an offset of ~35 km along the West fault.

Description: Mississippi Valley-type (MVT) deposits of the Viburnum Trend are typically lead dominant (Pb/Zn 〉5) and occur mainly in the reef-grainstone facies of the upper Bonneterre Dolomite (Cambrian). Recent drilling has encountered economic mineralization within the lower Bonneterre Dolomite of the Brushy Creek mine, more than 30 m below the main ore-bearing horizon of the district. These ores are more zinc and copper rich than typical deposits (Zn/Pb commonly 〉2 and Cu up to 8 wt %), with notable enrichments in Ni, Co, and Ag. There is pronounced zoning of Ni-Co, Cu, Zn, and Pb with increasing distance above the Lamotte Sandstone. Observations of drill cores and samples from active mine faces, and mineral textural relationships in thin section indicate that the lower ore mineralization contains dominantly early Zn- and Cu-bearing sulfides, which are overprinted by main-stage Pb-Zn mineralization. The lower ores contain multiple generations of chalcopyrite and sphalerite that display frequent breccia textures at the meso- and microscale. This style of breccia is uncommon for the Viburnum Trend and is composed dominantly of sulfides supported by clay (insoluble residue), which are cemented by subsequent generations of sulfides and carbonates, resulting in massive, high-grade ore. Cathodoluminescence microscopy reveals that ore is associated with two generations of dolomite cement that appear to predate the regional dolomite cement associated with main-stage Pb-Zn mineralization in the Viburnum Trend. The stratigraphic position, massive character, metal contents, mineral zoning and textures, and gangue dolomite cements of the orebody are unusual for the Viburnum Trend. Mineralogically and stratigraphically, these ores are more similar to the historically mined ores of the Indian Creek and Fredericktown-Mine La Motte districts to the north and east of the Viburnum Trend. In the Brushy Creek mine, the lower ores are not related to obvious stratigraphic controls, such as pinch-outs of the Lamotte Sandstone against Precambrian knobs, but appear to be localized along early zones of fracture-enhanced porosity and permeability within the Lamotte Sandstone, which promoted extreme dissolution of host rocks at the base of the Bonneterre Dolomite and development of high-grade Cu-(Ni-Co)-Zn-rich ore. The presence of distinct Ni- and Ni-Co-rich areas and the spatial variability of sphalerite types across the orebody indicate that there were likely multiple sites of ore fluid introduction whose influence varied with time during development of the ores.

Description: 〈span〉〈div〉Abstract〈/div〉Ore assemblages in uranium roll-front deposits are highly variable and heavily dependent on Eh/pH conditions. Sulfur isotopes in pyrite traditionally have been employed to distinguish between biogenic or abiogenic redox pathways as drivers of roll-front propagation. However, the extent of and constraints on bacterial productivity have never been quantified, nor have the chemical conditions imposed by either primary formation mechanism. Moreover, this approach implicitly assumes that deposits form via one process or the other and disregards the possibility that both processes participate simultaneously in generating some orebodies. In this study, we analyzed sulfur isotopes from pyrite coprecipitated with uranium in two Wyoming roll-front deposits: Lost Creek and Willow Creek Mine Unit 10. The results document contrasting isotopic fractionation that correlates with pyrite morphology. Both deposits evolved with both abiogenic and biogenic redox mechanisms as active contributors to ore formation. In the past, bimodal fractionation behavior with pyrite morphology has been attributed to distinct temporal episodes of pyrite formation, driven by either a change in redox mechanism or multiple independent fluid events with unique isotopic signatures. However, neither explanation is appropriate for the isotopic trends identified in this study, where the two pyrite morphologies appear coeval in both deposits. Moreover, the contemporaneous formation of both pyrite morphologies cannot occur under the same conditions by the same precipitation mechanism because of the difference in their free energies of formation. The data suggest a third alternative in which pyrite morphology correlates to its biogenic or abiogenic mode of formation. Given the isotopic composition of pre-ore pyrite, sulfur isotope fractionation trends within the ore zone can be applied to establish prolificacy of bacteria and chemical conditions of the ore-forming solution.In both study sites, framboidal pyrite occurred as the primary by-product of sulfur-reducing bacteria, and the corresponding fractionation pattern constrains the sulfur availability and bacterial productivity. Euhedral to anhedral pyrite precipitated from abiogenic redox, the sulfur fractionation recording Eh/pH gradients during ore evolution. At Lost Creek, framboidal pyrite produced 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values from –50.8 ± 0.5‰ to +142.8 ± 0.3‰, while subhedral pyrite ranged from –68.1 ± 0.4‰ to +33.8 ± 0.3‰ with 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values increasing toward the barren, unaltered contact. Pre-ore pyrite at Lost Creek ranged from –0.8 ± 0.5‰ to +70.6 ± 0.3‰. 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values from biogenically derived pyrite at Lost Creek indicate a closed system with limited sulfate availability and a slow rate of bacterial reduction, implying restricted bacterial activity. Abiogenic fractionation behavior indicates a system driven by an Eh drop under neutral or basic pH conditions, and pyrite distribution across the roll identifies abiogenic pyrite recycling as the dominant redox mechanism at Lost Creek. At Willow Creek Mine Unit 10, framboidal pyrite ranged from –32.5 ± 0.4‰ to +68.2 ± 0.4‰, and subhedral pyrite ranged from –45.1 ± 0.4‰ to +5.4 ± 0.4‰. The subhedral pyrite 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values initially increased into the center of the roll and subsequently decreased again approaching the barren, unaltered contact. Pre-ore pyrite ranged from –48.1 ± 0.4‰ to +15.6 ± 0.5‰. Willow Creek Mine Unit 10 biogenically produced 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values show minimal fractionation from pre-ore pyrite, indicating an open system with abundant sulfate and rapid reduction from prolific bacterial activity. The abiogenic trends indicate an Eh drop and low pH at the barren, altered contact progressively neutralized across the orebody. This correlates to the anticipated Eh/pH gradients in a system dominated by biogenic redox.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Karangahake deposit is the third largest gold producer in the Hauraki goldfield, Coromandel, New Zealand. Production was mainly from the Maria and Welcome/Crown veins that were mined over a strike length of 〉1,300 m and vertical interval of 700 m. The veins are hosted in andesite that is strongly altered over an area of approximately 4.2 × 2.7 km. By using petrography, qualitative and quantitative X-ray diffraction (XRD), and automated mineralogy from a scanning electron microscope (SEM), hydrothermal alteration has been characterized in detail for surface rocks exposed along the Ohinemuri and Waitawheta rivers and along the underground Keillors crosscut, which links the Maria and Welcome/Crown veins. The chemistry of hydrothermally altered rocks was analyzed using portable X-ray fluorescence (pXRF). The automated mineralogy reveals alteration mineral occurrences, abundances, and some textural relationships that are not as apparent from conventional studies.Hydrothermal alteration of the andesite is typically strong to intense, with 100% conversion of primary to secondary minerals, but decreases to moderate to weak, comprising 〈25% secondary minerals toward the west and east, where the margins of alteration are exposed. The andesite is altered by combinations of quartz, chlorite, adularia, albite, illite, pyrite, calcite, mixed-layered illite-smectite, smectite, and rare epidote. Quartz and chlorite are common and present in both strongly and weakly altered rocks. Adularia is widespread and extends laterally up to 500 m from the Maria and Welcome/Crown veins. Illite is broadly coextensive with and locally occurs beyond the adularia halo. The zone of illite alteration is bordered to the east by patchy mixed-layered illite-smectite and outlying smectite alteration. Along the Keillors crosscut, the rocks between the Maria and Welcome/Crown veins are altered to adularia, albite, illite, chlorite, and rare calcite and epidote.Automated mineralogy and quantitative XRD mineral estimates indicate many rocks have been altered by 〉30% adularia (max. 74%) and that the amount of adularia can exceed that of quartz. Adularia and albite occur throughout the Keillors crosscut, with adularia most abundant within 20 to 50 m of the Maria, Mystery, and Welcome/Crown veins, whereas albite is more abundant between these veins.Hydrothermal alteration has resulted in the variable gains in K and Rb forming anomalous halos that surround veins, which are attributed to adularia and illite. By contrast, Ca, Sr, and to a lesser extent Fe are variably lost around veins in broadly overlapping zones of depletion.The key hydrothermal alteration minerals are illite, illite-smectite, smectite, adularia, and albite. The systemscale zonation of illite, illite-smectite, and smectite outlines the broad thermal structure, with illite stability indicating 〉230°C, illite-smectite indicating 130° to 230°C, and smectite indicating 〈130°C. Adularia indicates zones of inferred high permeability and the central upflow of boiling hydrothermal fluids, whereas albite indicates lower inferred permeability and peripheral-style alteration. Adularia represents a broad-scale vector toward veins, and its occurrence and abundance can be determined using field portable XRD or chemically by proxy from the K/Al value.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Chengchao Fe skarn deposit (280 Mt @ 45% Fe; nearly half of the total Fe reserve has an ore grade higher than 53 wt %) is the largest high-grade magnetite skarn deposit in the Middle-Lower Yangtze River metallogenic belt. The magnetite orebodies and skarns occur mainly along the contact zones between granitic (129 ± 2 Ma) rocks and diorite (133 ± 1 Ma) and Triassic strata. The paragenesis of the Chengchao deposit includes the following five stages: Na-K alteration stage (albite + K-feldspar), prograde skarn stage (garnet + pyroxene), retrograde skarn stage (magnetite + phlogopite + amphibole), sulfate-sulfide stage (anhydrite + pyrite ± garnet ± magnetite), and carbonate stage (calcite).Four generations of garnet and magnetite with distinct textures are recognized. The elemental compositions and fluid inclusion data of the three generations of garnet from the prograde skarn stage indicate fluctuating changes in the oxygen fugacity during their precipitation. Some magnetite grains from the retrograde skarn stage are characterized by well-developed zonation with respect to Al, K, Ca, and Si, with concentrations of up to hundreds of parts per million. The higher contents of (V + Ti) and (Al + Mn) in Mt2 relative to Mt1 (0.8 and 2,321 ppm for Mt1, and 182 and 10,315 ppm for Mt2, respectively) may be indicative of increasing fluid temperatures from episodic magmatic events. Fluid inclusion data show that the fluids responsible for the prograde skarn stage have high temperature (〉750°C), high salinity (〉50 wt % NaCl equiv) and high Fe concentration (e.g., magnetite and pyrite daughter crystals within garnet and pyroxene). Oxygen and hydrogen isotope data suggest that magmatic water was dominant during the prograde skarn stage and that increasing percentages of meteoric water were involved during later stages. Multiple episodic incursions of Fe-rich fluids, which were probably related to the emplacement of granitic rocks and diorite, were crucial for controlling the formation of the high-grade Chengchao Fe skarn deposit.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉High- and low-temperature platinum nuggets from two historical localities, Chocó in Colombia and Córrego Bom Sucesso in Brazil, are compared with respect to trace elements. Supergene platinum nuggets from Córrego Bom Sucesso are enriched in Se and Hg, but depleted in siderophile and chalcophile elements, and have fractionated platinum group element patterns, in comparison with magmatic platinum nuggets from Chocó. In particular, Se concentrations over ~100 〈span〉μ〈/span〉g/g Se and S/Se ratios above unity indicate Se recycling in a supergene environment with abundant organic matter.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The transport and deposition of gold from colloidal suspensions in hydrothermal fluids has been a persistent theme in ore deposits research. Studies of active geothermal systems show that a complete model of gold transport must include both dissolved and particulate forms. However, samples of the hydrothermal fluids are commonly spiked with aqua regia after collection in order to put any solids back into solution, thus preventing a quantitative assessment of the particle load. Although attempts have been made to filter the solids, gold nanoparticles (Au NPs) will mostly pass the 0.2-〈span〉μ〈/span〉m filters that are in common use, and a simple technique for analyzing suspended particles in the liquids has been lacking. In this study, we demonstrate how time-resolved acquisition of mass 197 in a conventional inductively coupled plasma-mass spectrometer (ICP-MS) can be used to detect and measure Au NPs in the filtered liquids, with an example of well-characterized fluids from the Reykjanes geothermal field on Iceland. The technique allows for precise monitoring of the solution as it is introduced into the plasma with the capability of identifying individual particles carried in suspension. Results show that Au particles passing the 0.2-〈span〉μ〈/span〉m filters are abundant in the studied samples, and measurements of the individual particles can be used to determine their size. The experiment highlights the potential of emerging ICP-MS techniques, including very fast data acquisition and multielement analysis of single particles in time-of-flight mode, for characterization of NPs in hydrothermal fluids.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉We report the first Ag isotope data for the Paleozoic orogenic Au deposits in the Victorian Goldfields, southeast Australia, a world-class province with a historic production of 2,400 tonnes of Au. Despite their relatively uniform geology—similar host-rock types, age, mineralization style—deposits in Victoria show a wide range in 〈sup〉107〈/sup〉Ag/〈sup〉109〈/sup〉Ag ratios in native Au (〈span〉ε〈/span〉〈sup〉107〈/sup〉Ag −6.6 to +8.3, relative to the NIST SRM 978a Ag standard), comparable to the entire previously known terrestrial range (−9.4 to +5.3). The data show no correlation with mineralization age or host-rock composition, and there is no obvious isotopic link to established “mantle” or “crustal” Ag isotope values, implying that source rock signatures are unlikely to be the main control on Ag isotope variations. Instead, it is suggested that the Ag isotope variation is primarily related to physicochemical processes, particularly Ag isotope fractionation during redox reactions such as conversion of Ag〈sup〉0〈/sup〉 in native Au to Ag〈sup〉+〈/sup〉 in dissolved Ag(HS)〈sub〉2〈/sub〉〈sup〉−〈/sup〉 or sulfide-borne Ag. Repeated Ag〈sup〉0〈/sup〉〈strong〉←→〈/strong〉 Ag〈sup〉+〈/sup〉 reactions along transport pathways and at sites of ore accumulation could generate a wide range in 〈span〉ε〈/span〉〈sup〉107〈/sup〉Ag, and evidence of this range is presented in the data here. Silver isotope fractionation via numerous deposition-dissolution cycles provides a different perspective into large-scale ore genesis that has not previously been recognized for orogenic gold systems; multistaged metal remobilization along fluid transport pathways is standard during their formation. Detailed Ag isotope studies have considerable potential for understanding the history of episodic metal addition and within-deposit redistribution.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉Four porphyry Cu-Mo systems were investigated by Re-Os molybdenite geochronology to constrain their timing with respect to the geodynamic and magmatic evolution of the eastern Pontides, Turkey. Molybdenite from the Ispir-Ulutaş deposit yielded an Re-Os age of 131.0 ± 0.7 Ma, which is consistent with Early Cretaceous U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon ages of local calc-alkaline intrusions. It demonstrates that porphyry deposits were already formed during Early Cretaceous subduction of the Neotethys along the eastern Pontides, and that they can be correlated with porphyry Cu events in the adjacent Lesser Caucasus. Molybdenite Re-Os ages of 76.0 ± 0.4 and 75.7 ± 0.4 Ma at the Elbeyli prospect and 77.2 ± 1.0 Ma at the Emeksen prospect overlap with U-Pb LA-ICP-MS zircon ages of shoshonitic to high-K calc-alkaline intrusions in the region, which were emplaced during Late Cretaceous Neotethys subduction. A 50.7 ± 0.3 Ma molybdenite Re-Os age at the Güzelyayla deposit confirms porphyry Cu-Mo emplacement coeval with Eocene postcollisional, calc-alkaline adakitic magmatism of the eastern Pontides.An electron microprobe study of molybdenite samples, supplemented by data obtained during Re-Os dating, shows that the Eocene Güzelyayla deposit and the Late Cretaceous Emeksen prospect have the highest Re enrichment. Postcollisional melting of a thickened mafic lower continental crust and melting of a metasomatized lithospheric mantle with little to no interaction with upper crustal rocks may explain the Re enrichment at Güzelyayla and Emeksen, respectively.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The trace element composition of zircon, especially in tandem with U-Pb geochronology, has become a powerful tool for tracing magmatic processes associated with the formation of porphyry copper deposits. However, the use of the redox-sensitive Eu and Ce anomalies as a potential mineral exploration proxy is controversial. This study presents a comprehensive, temporally constrained data set of zircon trace element compositions (n = 645) for three compositionally distinct magmatic series identified in the Meghri-Ordubad pluton, southernmost Lesser Caucasus. The 30 million years of Cenozoic magmatism in the Meghri-Ordubad pluton are associated with several ore-forming pulses leading to the formation of porphyry copper deposits and epithermal-style mineralization. Our zircon geochemical data constrain the thermal and chemical evolution of this complex intrusive suite and allow an evaluation of the usefulness of zircon as a mineral exploration proxy for porphyry copper deposits. Our results combined with Rayleigh fractionation modeling indicate that the trace element composition of zircon (Th/U, Hf, Ti, Yb〈sub〉N〈/sub〉/Dy〈sub〉N〈/sub〉, Eu anomalies) is influenced by the composition and the water concentration of the parental magma, as well as by co-crystallizing titanite and apatite. In contrast, the variations of Ce anomalies remain difficult to explain by magmatic processes and could rather be ascribed to relative fluctuations of the redox conditions. In the Meghri-Ordubad pluton, we do not observe any systematic patterns between the trace element composition in zircons and the different ore-forming pulses. This questions the reliability of using the trace element composition in zircon as an exploration mineral proxy, and it rather emphasizes that a good knowledge of the entire magmatic evolution of a metallogenic province is required.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Muratdere Cu-Mo (Au) porphyry deposit in western Turkey contains elevated levels of rhenium and is hosted within granodioritic intrusions into an ophiolitic mélange sequence in the Anatolian belt. The deposit contains several stages of mineralization: early microfracture-hosted molybdenite and chalcopyrite, followed by a quartz-pyrite-chalcopyrite vein set associated with Cu-Au grade, a quartz-chalcopyrite-pyrite-molybdenite vein set associated with Cu-Mo-Re grade, and a later polymetallic quartz-barite-sphalerite-galena-pyrite vein set. The rhenium in Muratdere is hosted within two generations of molybdenite: early microfracture-hosted molybdenite and later vein-hosted molybdenite. In situ laser ablation-inductively coupled plasma-mass spectrometry analysis of sulfides shows that the later molybdenite has significantly higher concentrations of Re (average 1,124 ppm, 〈span〉σ〈/span〉 = 730 ppm, n = 43) than the early microfracture-hosted molybdenite (average 566 ppm, 〈span〉σ〈/span〉 = 423 ppm, n = 28). Pyrite crystals associated with the Re-rich molybdenite have higher Co and As concentrations than those in other vein sets, with Au associated with As. The microfracture-hosted sulfides have 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values between −2.2‰ and +4.6‰, consistent with a magmatic source. The vein-hosted sulfides associated with the high-Re molybdenite have a 〈span〉δ〈/span〉〈sup〉34〈/sup〉S signature of 5.6‰ to 8.8‰, similar to values found in peridotite lenses in the Anatolian belt. The later enrichment in Re and 〈span〉δ〈/span〉〈sup〉34〈/sup〉S-enriched S may be sourced from the surrounding ophiolitic country rock or may be the result of changing redox conditions during deposit formation.〈/span〉

Description: 〈span〉〈div〉Abstract〈/div〉The Kapan mining district in the southernmost Lesser Caucasus is one of the few locations along the central Tethyan metallogenic belt where ore-forming processes were associated with magmatic arc growth during Jurassic Tethys subduction along the Eurasian margin. Three ore deposits of the Kapan district were investigated in this study: Centralni West, Centralni East, and Shahumyan. The ore deposits are hosted by Middle Jurassic andesitic to dacitic volcanic and volcaniclastic rocks of tholeiitic to transitional affinities below a late Oxfordian unconformity, which is covered by calc-alkaline to transitional Late Jurassic-Early Cretaceous volcanic rocks interlayered with sedimentary rocks.The mineralization consists of veins, subsidiary stockwork, and partial matrix replacement of breccia host rocks, with chalcopyrite, pyrite, tennantite-tetrahedrite, sphalerite, and galena as the main ore minerals. Centralni West is a dominantly Cu deposit, and its host rocks are altered to chlorite, carbonate, epidote, and sericite. At Centralni East, Au is associated with Cu, and the Shahumyan deposit is enriched in Pb and Zn as well as precious metals. Both deposits contain high-sulfidation mineral assemblages with enargite and luzonite. Dickite, sericite, and diaspore prevail in altered host rocks in the Centralni East deposit. At the Shahumyan deposit, phyllic to argillic alteration with sericite, quartz, pyrite, and dickite is dominant with polymetallic veins, and advanced argillic alteration with quartz-alunite ± kaolinite and dickite is locally developed.The lead isotope composition of sulfides and alunite (〈sup〉206〈/sup〉Pb/〈sup〉204〈/sup〉Pb = 18.17–18.32, 〈sup〉207〈/sup〉Pb/〈sup〉204〈/sup〉Pb = 15.57–15.61, 〈sup〉208〈/sup〉Pb/〈sup〉204〈/sup〉Pb = 38.17–38.41) indicates a common metal source for the three deposits and suggests that metals were derived from magmatic fluids that were exsolved upon crystallization of Middle Jurassic intrusive rocks or leached from Middle Jurassic country rocks. The 〈span〉δ〈/span〉〈sup〉18〈/sup〉O values of hydrothermal quartz (8.3–16.4‰) and the 〈span〉δ〈/span〉〈sup〉34〈/sup〉S values of sulfides (2.0–6.5‰) reveal a dominantly magmatic source at all three deposits. Combined oxygen, carbon, and strontium isotope compositions of hydrothermal calcite (〈span〉δ〈/span〉〈sup〉18〈/sup〉O = 7.7–15.4‰, 〈span〉δ〈/span〉〈sup〉13〈/sup〉C = −3.4−+0.7‰, 〈sup〉87〈/sup〉Sr/〈sup〉86〈/sup〉Sr = 0.70537–0.70586) support mixing of magmatic-derived fluids with seawater during the last stages of ore formation at Shahumyan and Centralni West.〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar dating of hydrothermal muscovite at Centralni West and of magmatic-hydrothermal alunite at Shahumyan yield, respectively, a robust plateau age of 161.78 ± 0.79 Ma and a disturbed plateau age of 156.14 ± 0.79 Ma. Re-Os dating of pyrite from the Centralni East deposit yields an isochron age of 144.7 ± 4.2 Ma and a weighted average age of the model dates of 146.2 ± 3.4 Ma, which are younger than the age of the immediate host rocks. Two different models are offered, depending on the reliability attributed to the disturbed 〈sup〉40〈/sup〉Ar/〈sup〉39〈/sup〉Ar alunite age and the young Re-Os age. The preferred interpretation is that the Centralni West Cu deposit is a volcanogenic massive sulfide deposit and the Shahumyan and Centralni East deposits are parts of porphyryepithermal systems, with the three deposits being broadly coeval or formed within a short time interval in a nascent magmatic arc setting, before the late Oxfordian. Alternatively, but less likely, the three deposits could represent different mineralization styles successively emplaced during evolution and growth of a magmatic arc during a longer time frame between the Middle and Late Jurassic.〈/span〉

Description: The Hongniu-Hongshan Cu skarn deposit (77.8 Mt at 1.8% Cu) is located in the central part of the Zhongdian porphyry and skarn Cu belt in southwestern China. Skarn and orebodies occur mainly between the different units of the Upper Triassic Qugasi Formation or within altered limestone adjacent to Late Cretaceous intrusions (78–76 Ma). Three main paragenetic stages of skarn formation and ore deposition have been recognized on the basis of petrographic observations: (1) pre-ore-stage hornfels with diopside (Di 87–72 Hd 12–7 ), small-scale endoskarn with reddish grossular (Adr 22–57 Gr 78–43 ), diopside (Di 83–92 Hd 7–15 ), vesuvianite, and abundant exoskarn with red-brown andradite (Adr 75–98 Gr 2–22 ), sahlite (Di 28–41 Hd 58–71 ), and wollastonite; (2) syn-ore-stage retrograde minerals, sulfides (pyrite, chalcopyrite, pyrrhotite, molybdenite, galena, and sphalerite), quartz, and calcite; and (3) post-ore-stage calcite veins. Sulfur isotope values of sulfides are relatively high, with an average 34 S = 4.9 (n = 40), suggesting that the ore-forming fluid was magmatic and that the sulfides precipitated from a relatively reducing ore fluid. The coexistence of silicate melt and primary fluid inclusions in quartz phenocrysts of the mineralization-related quartz monzonite porphyry indicates the simultaneous entrapment of fluid and melt, and records the process of the aqueous fluid exsolving from the crystallizing melt. The initial single-phase fluid has a salinity of 8.8 to 12.7 wt % NaCl equiv and homogenization temperatures of 566° to 650°C, corresponding to pressures of 680 to 940 bar and lithostatic depth of 2.5 to 3.5 km. The primary fluid inclusions in the pre-ore-stage garnet and pyroxene composed of coeval vapor-rich (V type) and halite-bearing (S-I and S-II types containing sylvite) inclusions (32–〉79 total wt % salts) share similar homogenization temperatures (450°–550°C), indicative of the occurrence of fluid unmixing under lithostatic pressures of ~550 to 780 bar (〉2.0-km depth). Primary fluid inclusions trapped in syn-ore quartz, calcite, and epidote show the common development of S-type inclusions (~37.3 wt % NaCl equiv) with coexisting V-type, liquid-rich (L type), and CO 2 -bearing (C-I type) inclusions, all of which have homogenization temperatures of 300° to 400°C and trapping pressures of 100 to 400 bar (~1.5-km depth). Brine inclusions homogenized by halite dissolution after vapor disappearance in both the pre- and syn-ore stages are interpreted to have been trapped under overpressured conditions (〉1,520 bar). Oxygen isotope analyses were conducted on garnet, wollastonite, epidote, quartz, and calcite. The pre-ore-stage garnet and wollastonite have 18 O fluid values of 5.6 to 8.1, whereas the syn-ore-stage epidote, quartz, and calcite have more variable 18 O fluid values in the range of 3.9 to 17.5. The 18 O fluid values of the post-ore-stage vein calcite (15.2–21.3) are much higher than both the pre- and syn-ore stages. The vapor phase of inclusions contains H 2 S, CH 4 , and C 2 H 6 in the syn-ore stages. All these observations reveal that (1) the formation of the Cu skarn deposit was dominated by a magmatic hydrothermal system, (2) multiple fluid pulses contributed to the formation of the pre- and syn-ore-stage skarn minerals and sulfides, and (3) the increase in pH due to the neutralization of the acidic fluid could be the main factor controlling the large-scale ore deposition in Hongniu-Hongshan.

Description: Interspinifex ores are developed where pools of sulfide liquid overlie thermally eroded komatiite flows, such that sulfide occupies the space between spinifex olivine plates. Microbeam X-ray fluorescence mapping and 3-D X-ray computed tomography have been used to investigate microstructures and chemical zonation within interspinifex ores from Coronet shoot, Kambalda. Sulfide compositions in the interspinifex space match the composition of the overlying sulfide pool. Aluminous silicate, interpreted as displaced silicate melt, forms a film at the silicate-sulfide interface, locally developing dome-like plumes. The film and plumes are characterized by fine skeletal chromite that diminishes in abundance over about a decimeter downward into the interspinifex zone. These relationships are strong evidence for a primary magmatic origin for this ore type, driven by the strong tendency of dense, inviscid sulfide liquid to infiltrate and melt underlying rocks. Such infiltration-melting interfaces may be a common feature at the base of massive sulfide ores, taking different forms depending on the lithological and fracturing characteristics of the footwall rocks.

Description: The metamorphosed Cambro-Ordovician Ming volcanogenic massive sulfide deposit in northern Newfoundland, Canada, is locally overlain by a unit consisting of mafic to intermediate medium- to coarse-grained volcaniclastic breccia with up to 10 vol % sulfide clasts. Analysis via mineral liberation analyzer of two sulfide clasts, completed using scanning electron microscope observations, allowed the identification of a number of microscopic and submicroscopic electrum grains. These electrum grains occur in three types of textural settings: (1) free electrum grains with tellurides within gangue minerals, (2) inclusions of electrum with tellurides in pyrrhotite grains, and (3) free electrum grains with base metals and tellurides interstitial between base metals and along cataclastic fractures in pyrite. These three textural settings are similar to those in the underlying massive sulfide orebodies that represent very likely sources to the sulfide clasts. The polylithic nature and angularity of the volcaniclastic fragments in the breccia suggest a postmineralization gravity-controlled debris flow proximal to its source. The mineral assemblage and textures of electrum in the sulfide clasts implies evidence in support of syngenetic and predeformation Au introduction in the volcanogenic massive sulfide deposit, and argues against an orogenic overprint as the cause for Au enrichment.

Description: The Poyi magmatic Ni-Cu sulfide deposit is situated in the Beishan fold belt in the northeastern rim of the Tarim craton. Many Permian magmatic Ni-Cu sulfide deposits, such as those in East Tianshan, are present in the adjacent Central Asian orogenic belt to the north. The Poyi deposit is hosted in a small dike-like ultramafic-troctolitic body that was emplaced into a much larger gabbroic intrusion. Our new zircon U-Pb isotope data reveal that these two intrusions formed ~6 Ma apart. The ultramafic-troctolitic intrusion was emplaced at 269.9 ± 1.7 Ma, whereas the gabbroic intrusion was emplaced at 276.1 ± 1.9 Ma. The results show that the Poyi deposit is the youngest among the major magmatic Ni-Cu sulfide deposits (≥0.2 Mt Ni) of Permian ages in the Beishan-Tianshan region. Sulfide mineralization in the Poyi deposit occurs as steeply dipping disseminated sulfide lenses mostly associated with wehrlites in the center of the dike. Olivine from the Poyi ultramafic rocks has Fo content up to 91 mol %, which is similar to the lower limit of mantle olivine and the most primitive within the Permian Beishan-Tianshan nickel belt. Like other magmatic sulfide deposits in this belt, olivine from the Poyi deposit is depleted in Ca (〈1,000 ppm). The estimated parental magma for the Poyi most primitive ultramafic rocks contains 15 ± 2 wt % MgO. Cotectic olivine-sulfide segregation from the Poyi magma is inferred from systematic variation of Fo-Ni contents in olivine from some sulfide-barren ultramafic rock samples, and supported by the occurrence of sulfide droplets as small inclusions in olivine (Fo 90 mol %) in these rocks. The involvement of multiple pulses of sulfide-charged magma with different compositions is indicated by the abrupt change of olivine Fo content with depth and the presence of olivine with similar Fo contents but dramatically different Ni abundances in the different parts of the deposit. The ( 87 Sr/ 86 Sr) i ratios and Nd(total) of the Poyi ultramafic rocks and troctolites range from 0.7042 to 0.7052 and from 4.9 to 6.0, respectively, which are close to the isotope compositions of depleted mantle and within the ranges of major magmatic Ni-Cu sulfide deposits of Permian ages in East Tianshan. No more than 5 wt % of bulk crustal contamination is required to explain the variations of Sr-Nd isotopes in the Poyi ultramafic-troctolitic intrusion. The abundances of incompatible trace elements in whole rocks and clinopyroxene crystals indicate very weak light REE enrichments coupled by significant negative Nb-Ta anomalies in the parental melts. Bulk sulfides in the Poyi deposit are characterized by positive correlations between any pair of platinum-group elements (PGE), indicating that PGE tenor variations in the deposit are mainly controlled by variable R-factors (magma/sulfide mass ratios). The estimated initial concentrations of PGE in the parental magma for the Poyi deposit are almost two orders of magnitude lower than the abundances of PGE in some continental picrites. Given that the parental magma for the Poyi deposit is as primitive as a primary mantle-derived magma, the depletion of PGE in the Poyi deposit is most likely due to previous sulfide segregation at depth. Based on these observations, we conclude that sulfide saturation in the Poyi PGE-depleted, Mg-rich magma was triggered by addition of crustal sulfur during magma ascent and that the Poyi deposit was a dynamic conduit used by multiple pulses of olivine- and sulfide-charged magma.

Description: There is a strong association between regions containing orogenic gold mineralization and exploitation of placer gold, although, in many cases, the nature of the source mineralization for these placer deposits remains unclear. This study describes a novel approach to evaluating the economic potential of in situ orogenic gold mineralization through characterization of both lode and placer gold mineralogy, followed by synthesis of this information with records of both mineral occurrences and historical placer mining. The northern Cariboo Gold District in east-central British Columbia, Canada, was chosen as a location for the study because of the gold endowment of the area (1.2 million ounces [Moz] of lode gold and between 0.5 and 3 Moz. of placer gold) and the information available from both placer and lode gold mining. Compositional analysis of 533 gold grains from 21 lode localities and 1,914 gold grains from 30 placer localities from throughout the Cariboo Gold District has identified four main compositional types in terms of their alloy compositions and associated suite of mineral inclusions revealed in polished section. A distinctive low (4–7%)-Ag gold that exhibits a strong Bi association in the mineral inclusion suite is geographically limited to the Wells area, where it is recorded in both lode mineralization and its placer expression. Regionally pervasive mineralization yields gold of binary Au-Ag alloy and a simple inclusion suite of sulfides and sulfarsenides. Gold in most large placers in trunk drainages was derived from multiple (mostly small) occurrences of this type. The nature of compositional variation between gold grains liberated from hypogene ore has informed the history of episodic mineralization and suggests multiple stages of gold emplacement at some localities, while others are dominated by gold deposited in a single stage. The new information from gold grain analysis has been considered in the context of other information. Classification of a placer as either allocthonous or autocthonous both informs interpretation of compositional characteristics of the detrital gold grains and provides information on distance to the hypogene source. Mineral inclusion assemblages observed in sample populations of placer gold grains have been correlated with reports of hypogene vein mineralogy described in mineral occurrence records to clarify the geographical extent of specific mineralization types. The compositional range of alloys of different gold types has been compared to historical records of gold production and gold fineness (Au-Ag ratio) to reconstruct the size and distribution of hypogene sources prior to erosion. Synthesis of gold compositional studies with other publicly accessible data sets provides a new generic approach capable of evaluating the most attractive targets for future exploration and highlighting compositional signatures of placer gold which relate to undiscovered in situ sources.

Description: The Kapai Slate is a continuous, pyrite-rich carbonaceous shale horizon within the St. Ives Au district that is spatially related to high-grade Au mineralization. In situ laser ablation-inductively coupled mass spectrometry (LA-ICPMS) trace element analyses, in situ sensitive high resolution ion microprobe, stable isotope (SHRIMP-SI) S isotope analyses, and optical microscopy pyrite texture analyses were used to examine the different pyrite types in the Kapai Slate and Au deposits. These data were also used to confirm that the trace element signature of sedimentary pyrite can be preserved in rocks that underwent upper to mid-greenschist facies metamorphism and significant hydrothermal overprint. The data were further utilized to gain a more detailed understanding of the ocean conditions during deposition of the Kapai Slate and determine whether some of the Au and S in the St. Ives district could have been sourced from the Kapai Slate. Seven different types of pyrite were identified: fine-grained sedimentary pyrite (Py 1 ), nodular sedimentary pyrite (Py 2 ), remobilized sedimentary pyrite (Py 3 ), coarse-grained, inclusions poor late pyrite (Py 4 ), inclusion-rich magnetite series pyrite (Py 5 ), ore stage pyrite (Py 6 ), and pyrite associated with the mafic units (Py 7 ). Each type of pyrite was found to have distinctive trace element compositions and S isotope signatures. The results of the LA-ICPMS analyses provide evidence for early trace element enrichment in the Kapai Slate sedimentary pyrite (median values of 158 ppm Ni, 387 ppm Co, 82 ppm Cu, 727 ppm As, 1.91 ppm Mo, 13 ppm Se, 0.25 ppm Au, 7.72 ppm Te and 3.36 ppm Ag for Py 1 and 223 ppm Ni, 158 ppm Co, 99 ppm Cu, 856 ppm As, 1.27 ppm Mo, 10.2 ppm Se, 0.57 ppm Au, 10.09 ppm Te, and 6.62 ppm Ag for Py 2 ). Concentrations of Ni and Co are low, relative to other late Archean sedimentary pyrite (median of 813 and 465 ppm, respectively) and Mo levels are near that of the euxinic shales of the similar-aged Jeerinah Formation in the Hamersley Basin, Western Australia. These data suggest that the Kapai Slate was deposited in an anoxic to euxinic basin with relatively low biological productivity. The 33 S and 34 S signatures of the sedimentary pyrite suggest two different sources of S. Positive 34 S and negative 33 S signatures indicate bacterial reduction of SO 4 2– from seawater, whereas positive 34 S and positive 33 S signatures indicate an elemental S 8 source, indicating the pyrite formed later during diagenesis. This S isotope signature is consistent with a transition between a near-sediment environment to a more distal environment source. Analyses of the ore-phase pyrite yield weakly positive 33 S values. This suggests there was a minor contribution of sedimentary S to the more significant oxidized ore-forming fluids, which is consistent with a small contribution of Au from a sedimentary source. Approximations of the degree of sedimentary pyrite destruction in the pyrrhotite/pyrite dominated zones and pyrrhotite/magnetite/pyrite zones of the northern part of the St. Ives district were used to calculate the amount of Au released from the early sedimentary pyrite. The calculation suggests that a minor, though possibly locally significant, amount of Au could have been sourced from the Kapai Slate.

Description: The large Cerro de Pasco Cordilleran base metal deposit in central Peru is the result of three successive mineralizing stages comprising both low- and high-sulfidation mineral associations: (A) several pyrrhotite pipes grading outward to sphalerite and galena replacement bodies, (B) a massive, funnel-shaped pyrite-quartz replacement orebody, and (C) E-W–trending Cu-Ag-(Au-Zn-Pb) enargite-pyrite veins and well-zoned Zn-Pb-(Bi-Ag-Cu) carbonate-replacement orebodies. This superposition of hydrothermal events leads to complex replacement textures and crosscutting relationships. A detailed study of the textures and mineral composition of the up to 15-m-wide replacement front existing between the pyrrhotite pipes and the pyrite-quartz body allows for clarification of the relative chronology of the hydrothermal events. The results show that, in contrast to previous interpretations, the emplacement of the pyrrhotite pipes and their Zn-Pb mineralized rims precedes that of the pyrite-quartz body. The replacement textures affecting pyrrhotite and arsenopyrite and the nature of the newly formed minerals have been used as a qualitative way to track the evolution of f S 2 , f O 2 , and pH of the mineralizing fluids. Two steps of pyrrhotite replacement have been recorded. The first one takes place under moderate acidity and relatively reduced to moderately oxidized conditions and is marked by replacement of pyrrhotite by euhedral nonporous pyrite. The second step occurs under more acidic and oxidized conditions and is characterized by replacement of pyrrhotite by porous marcasite and replacement of arsenopyrite by pyrite. Subsequently, marcasite is partly replaced by fine-grained euhedral nonporous pyrite. LA-ICP-MS trace element analyses of the replaced pyrrhotite and arsenopyrite and of the newly formed marcasite and pyrite support dissolution-reprecipitation as the main mechanism for replacement. Positive correlations between some of the elements (e.g., Pb-Sb, Pb-Ag) are indicative of the possible presence of nanoscale solid inclusions as main carriers for those elements; however, coupled substitutions and incorporation of some of the elements at a ppm level into the pyrite and marcasite structures cannot be excluded. The obtained As, Sb, Pb, and Bi values in pyrite are systematically higher than published data of pyrite in epithermal and porphyry systems. Nature and trace element content of the newly formed minerals yield information on the physicochemical conditions during their precipitation, the initial trace element content of replaced minerals, and the subsequently dissolved neighboring phases. The results show that the metal concentration of the fluid is locally influenced by the composition of the dissolved minerals. This study leads to a simpler interpretation of the fluid evolution than previously proposed, with a progressive increase of f S 2 , f O 2 , and pH as a result of decreasing wall-rock buffering during the three successive mineralizing stages at Cerro de Pasco.

Description: A gold-bearing ore from the San Salvador vein, Capillitas mine, Argentina, was exposed to an enriched, iron- and sulfur-oxidizing bacterial consortium for two months in an experimental system that represented an oxidized, acid-leached weathering environment. Within this laboratory model, the dissolution of metal sulfide minerals by the bacterial consortium liberated gold grains that floated on water. Surficial crevices on grains contained detrital material associated with μ m-scale, gold-rich bacteriomorphic structures interpreted to be relics of gold dissolution. The presence of nanophase gold particles, i.e., colloids and octahedral platelets, was attributed to gold reprecipitation. These secondary gold structures suggest that gold dissolution/reprecipitation, i.e., cycling, was occurring concurrently with the bacterially catalyzed dissolution of metal sulfides. The flake-like morphology and small size of gold grains, i.e., high surface area to volume ratio increased by μ m-scale surface dissolution textures, would have enhanced their propensity to float. The liberation of buoyant gold grains and secondary gold particles could contribute to rapid gold mobility and dispersion in natural environments.

Description: Apatite is a common resistate mineral occurring in a range of host rocks and ore-related hydrothermal alteration assemblages. Apatite in several porphyry copper deposits in British Columbia has a unique set of physical and compositional characteristics that can be used to evaluate the chemical conditions of magmas that formed the causative intrusions or associated hydrothermal alteration. Apatite under visible light and SEM shows no notable variations between unaltered and altered varieties but cathodoluminescence reveals significant differences. Apatite in unaltered rocks displays yellow, yellow-brown, and brown luminescence, whereas in K silicate-altered rocks apatite displays a characteristic green luminescence. The green-luminescent apatite replaces yellow- or brown-luminescent apatite and locally overgrows it. Apatite occurring with muscovite (i.e., phyllic)-altered rocks displays characteristic gray luminescence. The chemistry of apatite, as determined by electron microprobe and laser ICP-MS analyses, directly reflects its alteration and luminescence. The unaltered yellow-luminescent apatite has high concentrations of Mn (0.3–0.5 wt % MnO) and a high Mn/Fe ratio (〉1), whereas the brown-luminescent apatite has low Mn, but higher concentrations of S and REE + Y. The green K silicate alteration-related luminescence is caused by lower Mn/Fe ratios (ca. 1) along with depletions of other trace elements such as Cl, S, and Na. Gray-luminescent apatite occurring with muscovite-altered rocks results from significant Mn loss (〈0.15% MnO) contemporaneous with depletion in Na, S, Cl, and REE during low pH phyllic alteration in calc-alkalic porphyry deposits. The correlation between apatite texture, luminescence, and chemical composition with the type and intensity of porphyry alteration offers a potentially fast and effective method to utilize it as an indicator for porphyry mineralization in a range of exploration materials including soils, regoliths, and heavy mineral concentrates from glacial and fluvial materials.

Description: Volcanic-hosted massive sulfide (VMS) occurrences in the Quinns district are hosted predominantly by ca. 2814–2800 Ma banded iron formation (BIF) within a sequence of rhyolite, basalt, and minor siltstone. The VMS occurrences and their surrounding rocks are folded, metamorphosed, and deeply weathered and variably covered by transported regolith. This study uses an integrated petrological and geochemical approach to map gradients in synvolcanic mineral and element abundances, with the aim of understanding the effects of postvolcanic processes on Archean VMS ore. Rhyolite-dominant footwall rocks and the BIF record kilometer-scale gradients in alteration mineral patterns and geochemistry. Rhyolite exposed throughout the district and intersected by drill holes records distal alteration assemblages of quartz-white mica ± chlorite. At the Austin deposit in the western part of the district, abundant talc and anthophyllite with minor cummingtonite and hornblende are associated with mineralization in the BIF, indicating intense magnesium metasomatism. Further away from mineralization, chlorite is the dominant alteration mineral. Adjacent rhyolite is altered to chlorite in proximal zones and white micas in more distal areas. Approaching known VMS prospects in the eastern half of the district, the rhyolite grades into a 2 x 1 km zone of schistose rhyolite with generally dispersed, but locally abundant, coarse-grained andalusite ± kyanite ± garnet rhyolitic schist. This broad Al-rich silicate alteration zone envelops two discrete 1 km x 500 m proximal alteration zones in rhyolite, defined by chlorite ± talc, with minor disseminated magnetite, pyrite, and chalcopyrite. The northernmost proximal alteration zone lies stratigraphically below the BIF-hosted Cu-Zn-rich gossan exposed near the Tasman prospect. The proximal zones are interpreted to be the result of the interaction between synvolcanic, Mg-rich fluids and rhyolite footwall at the time of VMS mineralization and BIF deposition. In contrast, outer zones of andalusite ± kyanite ± garnet suggest the removal of silica and alkali elements and residual concentration of aluminium in rhyolite footwall by acidic synvolcanic hydrothermal fluids prior to peak regional metamorphism. The mapped patterns in alteration minerals mirror bulk rock geochemical gradients in rhyolite; proximal alteration zones are enriched in Cu, Zn, Ag, Au, Bi, Fe 2 O 3 , In, MgO, Mo, S, Se, and Te, with local enrichments in As, Cd, MnO, Pb, and V. These rocks are depleted in Ba, K 2 O, Li, Na 2 O, and Rb relative to least-altered rhyolite. Rhyolite located up to 100 m from proximal alteration zones is enriched in Ag, Fe 2 O 3 , In, K 2 O, MgO, S, and V. Banded iron formation hosting massive sulfides is altered to talc, anthophyllite, chlorite, and magnetite; these rocks are enriched in Cu, Zn, Pb, Ag, and Sn, with local enrichments in Bi, In, and MgO. Positive Eu anomalies are associated with mineralized BIF in the Austin VMS deposit and contrast with the mostly flat REE slopes for the BIF sampled from less mineralized areas in the district. The kilometer-scale gradients in hydrothermal alteration minerals and bulk rock geochemistry in rhyolite and the BIF are interpreted to be mostly controlled by synvolcanic hydrothermal alteration related to the development of VMS-related hydrothermal fluid pathways. Metamorphism of the synvolcanic alteration was isochemical, apart from localized metasomatism related to later shear zones or fault zones. The defined alteration mineral and geochemical gradients serve as useful tools for detecting fluid alteration pathways related to VMS systems in complexly deformed and metamorphosed districts globally.

Description: The Wolverine volcanogenic massive sulfide (VMS) deposit is a polymetallic, felsic-siliciclastic deposit hosted by ~352 to 347 Ma volcanic and sedimentary rocks of the Yukon-Tanana terrane, Yukon, Canada. Shales are located at various stratigraphic levels and in various mineralized zones within the Wolverine deposit area (e.g., Fisher, Puck, and Sable zones). Shales present along the mineralized horizon near the immediate hanging wall and footwall were deposited under anoxic conditions (e.g., low Mn, anoxic V-Cr-Mo-U systematics), whereas in the stratigraphically deeper footwall and uppermost hanging wall, the redox signatures imply deposition under suboxic to oxic conditions. The Mo-U systematics of the shales suggest that during the time of sulfide mineralization, the ambient basin was periodically euxinic with aqueous H 2 S present in the water column that this H 2 S contributed to the sulfur budget of the deposit. Furthermore, the Mo-U and C org -Ni systematics favor deposition in a restricted basin (i.e., nutrient trap) where restriction of the water column led to H 2 S formation via sulfate reduction associated with excess organic carbon preservation. There is also evidence of a progressive shift upward in the stratigraphy to more oxygenated conditions in the uppermost hanging wall. The shift from euxinic to oxic-suboxic conditions is consistent with regional tectonic models that indicate a change from rifting during Wolverine deposit formation, where the basin was partially restricted with minimal circulation (i.e., restricted depocenter), to an incipient back-arc basin accompanied by extension and likely ingress of oxygenated seawater. The rare earth element and Y (REY) systematics in Wolverine shales illustrate that proximal to the mineralized horizon, shales have higher Y/Ho (〉27), and Ce/Ce*〈〈1 and negative Ce anomalies indicative of oxygenated seawater. The Ce/Ce* values in the Wolverine shales have an inverse correlation with P 2 O 5 content and suggest partial control by detrital apatite. It is envisioned that apatite formed in the upper, oxygenated portion of the water column, inherited the REE signature of oxygenated seawater (i.e., Ce/Ce*〈〈1), and was subsequently deposited into deeper waters as detrital grains. The Ce/Ce* and Y/Ho also correlate with CO 2 and carbonate content of the shales. Moreover, the shales that have the strongest REY signature of oxygenated seawater also coincide with the strongest euxinic signatures. This paradox can be reconciled by enhanced deposition of apatite coincident with deposit formation, coupled with a late hydrothermal overprint on the shales from low-temperature, CO 2 -rich (oxygenated?) hydrothermal fluids (i.e., high Y/Ho and Ce/Ce*〈〈1) in a vent-proximal environment. This model is consistent with the geology, stratigraphy, and hydrothermal alteration in the immediate footwall and hanging wall of the deposit. Prospective shales in the Wolverine basin, and similar sediment-rich hydrothermal basins globally, should exhibit evidence for deposition under anoxic conditions (i.e., Mn 〈1,000 ppm; high V-Mo-U, U EF , Mo EF ), hydrothermal Fe-Al-Mn systematics (i.e., high Fe/Al), and evidence for hydrothermal alteration (i.e., high CIW values, high molar sericite and chlorite). Identification of samples having these features is useful in targeting prospective VMS environments in shale-dominated successions and potential targets within shale-rich basins.

Description: Water column redox conditions, degree of restriction of the depositional basin, and other paleoenvironmental parameters have been determined for the Mississippian Kuna Formation of northwestern Alaska from stratigraphic profiles of Mo, Fe/Al, and S isotopes in pyrite, C isotopes in organic matter, and N isotopes in bulk rock. This unit is important because it hosts the Red Dog and Anarraaq Zn-Pb-Ag ± barite deposits, which together constitute one of the largest zinc resources in the world. The isotopic and chemical proxies record a deep basin environment that became isolated from the open ocean, became increasingly reducing, and ultimately became euxinic. The basin was ventilated briefly and then became isolated again just prior to its demise as a discrete depocenter with the transition to the overlying Siksikpuk Formation. Ventilation corresponded approximately to the initiation of bedded barite deposition in the district, whereas the demise of the basin corresponded approximately to the formation of the massive sulfide deposits. The changes in basin circulation during deposition of the upper Kuna Formation may have had multiple immediate causes, but the underlying driver was probably extensional tectonic activity that also facilitated fluid flow beneath the basin floor. Although the formation of sediment-hosted sulfide deposits is generally favored by highly reducing conditions, the Zn-Pb deposits of the Red Dog district are not found in the major euxinic facies of the Kuna basin, nor did they form during the main period of euxinia. Rather, the deposits occur where strata were permeable to migrating fluids and where excess H 2 S was available beyond what was produced in situ by decomposition of local sedimentary organic matter. The known deposits formed mainly by replacement of calcareous strata that gained H 2 S from nearby highly carbonaceous beds (Anarraaq deposit) or by fracturing and vein formation in strata that produced excess H 2 S by reductive dissolution of preexisting barite (Red Dog deposits).

Description: Metamorphosed Zn-Pb occurrences (Fable Lake, George, Sito South, Sito Southwest, Sito West, Sito East, Robyn Lake, and Mackie Lake) with high Zn:Pb ratios occur in the Foster River area, northern Saskatchewan, on the southeast margin of the Paleoproterozoic Wollaston Domain. The Sito East prospect, the largest of these occurrences, contains ~50,000 tonnes of 4.5% Zn, with one drill intercept containing 11 m of 4.2% Zn and 0.6% Pb. Most prospects are hosted in a thin, areally extensive metaquartzite within a thick sequence of metasiltstones and metaquartzites in the middle of the Paleoproterozoic Wollaston Group. Occurrences in metaquartzites and metaarkoses, which locally contain gahnite, spessartine, and/or graphite, are spatially associated with metaexhalites (iron formation and quartz garnetite) and strata-bound metamorphosed hydrothermal alteration zones (nodular sillimanite rock). Post-Archean Australian shale-normalized rare earth element (REE) patterns of all three rock types are characterized by light REE enrichment, positive Ce anomalies, neutral to strongly negative Eu anomalies, and heavy REE depletion, consistent with an environment of deposition distinguished by a high (〉30 wt %) continental detrital component and formation from or interaction with low-temperature (≤250°C), near-neutral pH and high f O 2 fluids. Sulfur isotope compositions of sulfides at Foster River are highly enriched in 34 S ( 34 S = 26.2–38.1, n = 20) and resulted from microbial reduction of partially reduced seawater sulfate in a restricted basin. The spatial association of spessartine- and gahnite-bearing lithologies and silicate facies iron formation with Paleoproterozoic base metal sulfides in a psammopelitic host sequence implies similarities between sulfide mineralization in the Foster River area and Broken Hill-type Pb-Zn-Ag deposits. However, the markedly enriched 34 S sulfide compositions, low Pb/(Pb + Zn) and Ag/(Pb + Zn + Ag) ratios, low Ag contents, and apparent absence of metavolcanic and metavolcaniclastic rocks in the dominantly clastic rock sequence hosting the sulfides in the Foster River area suggest that these occurrences are part of a metamorphosed sedimentary exhalative system with Broken Hill-type affinities, similar to that associated with the formation of the Gamsberg Zn (South Africa), Kanmantoo Group Pb-Zn (South Australia), and Sullivan Pb-Zn (Canada) deposits.

Description: The Whalesback Cu-rich volcanogenic massive sulfide deposit in the Newfoundland Appalachians is a highly deformed deposit found on a steep limb of a closed and boudinaged overturned fold. The deposit was intensely deformed at low temperature but medium pressure (〉175 MPa) during the accretion of the composite Lushs Bight oceanic tract-Dashwoods terrane onto the Humber margin at ca. 480 Ma. The ore mineralogy consists of chalcopyrite, pyrrhotite, and pyrite with lesser sphalerite and trace Ag, Bi, and Hg tellurides. Four styles of sulfide mineralization are present: (1) disseminated (5%); (2) vein (50%); (3) breccia (25%); and (4) semimassive to massive (20%). Independent of mineralization style, massive pyrite and pyrrhotite (and some chalcopyrite) are commonly parallel to main S 2 schistosity in the deposit, whereas late chalcopyrite piercement veins occur at a high angle to S 2 . The progressive increase in pressure and temperature produced a remobilization sequence wherein sphalerite was the first sulfide phase to cross the brittle-ductile boundary, followed by pyrrhotite and, finally, chalcopyrite. Maximum temperature was not high enough for the pyrite to cross the brittle-ductile boundary. Instead, pyrite grains were incorporated and transported by pyrrhotite and chalcopyrite during the ductile remobilization events, rounding and fracturing them. Remobilization of the sulfides occurred mainly by plastic flow, but some solution transport and reprecipitation is locally observed. In situ secondary ion mass spectrometry sulfur isotope geochemistry of sulfides yielded values of 34 S ranging from 2.7 to 4.7 for pyrite, 2.1 to 4.0 for pyrrhotite, and 1.3 to 4.7 for chalcopyrite. Sulfur isotope modeling suggests that at least 60% of the sulfur was derived from leaching of igneous rocks (i.e., basalts), with the remainder derived from thermochemical sulfate reduction of seawater sulfate during alteration of the basalts by seawater. At the deposit scale, sulfur isotopes retained their original signature and did not reequilibrate during the secondary deformation and remobilization events.

Description: A strong relationship was documented in the charnockitic footwall at the Spruce Road Cu-Ni sulfide deposit (South Kawishiwi intrusion, Duluth Complex) between the contact metamorphic processes, mineralogy, texture, and ore mineral assemblages. Textural evidences of partial melting, including partial melt films, granophyric intergrowth of feldspar and quartz, as well as in situ partial melt pockets and veins, indicate that partial melting occurred in the granitic footwall even up to 125 m from the footwall-intrusion contact. Temperature in the proximal 10-m segment of the footwall was around 850° to 920°C based on the observed mineral assemblage and two-pyroxene thermometry and decreases to around 700°C 100 m from the intrusion footwall contact. Partial melting of the footwall granite in this segment gave rise to sulfide liquid to sink and infiltrate into the partially molten rock. Fe- and Cu-rich sulfide mineral assemblages have been distinguished in the partially molten footwall. In the proximal 10-m part of the footwall, the sulfide assemblage is pyrrhotite and pentlandite rich, whereas in the distal part, 10 to 100 m from the contact the dominant minerals are chalcopyrite, bornite, millerite, platinum group minerals (PGMs), native gold, and other base and semimetal minerals. Systematic variation of Pd/Rh and Au/Pd ratios indicate that the formation of the sulfide mineral zonation can be interpreted with fractional crystallization of a parent sulfide liquid. Biotite, associated with the Cu-rich assemblage, indicates that fluids may have expelled from the fractionated sulfide liquid. Variation of the Cl/F ratio in the biotite and apatite that crystallized during the peak metamorphism indicates that fluids may have also played a role in the metal and sulfur transport.

Description: The nickeliferous Madziwa Igneous Complex consists of several lenticular, mafic-ultramafic enclaves within late Archean gneissic terrain. The lenses of igneous rock are locally deformed and metamorphosed to lower greenschist grade. They are here interpreted to be remnants of a large, composite, magmatic structure with two main components: (1) a set of narrow (10s–100s of meters wide) dikes intruding the gneissic foliation, and (2) a large lopolith made up of leuconorite. Some dikes contain a differentiated and vertically oriented layered suite comprising a central pyroxenite layer, plus norite as both a continuous marginal layer and intermittent layers within the pyroxenite. The pyroxenite-norite suite has a preliminary U-Pb zircon age of 2684 Ma. Other dikes are made up of diorite and/or ferrodiorite; in places, the dioritic rocks also intrude the pyroxenite-norite suite. The leuconorite lopolith transects both the dikes and the gneissic country rocks, its basal contacts with the pyroxenite-norite suite varying locally from intrusive to gradational to (magmatic) erosional. Although modified in places by secondary mobilization, disseminated Ni-Cu sulfides are primarily hosted within the central (ortho- to mesocumulate) pyroxenite adjacent to internal norite layers. Whole-rock geochemical data establish the comagmatic origin of the principal rock types and indicate an Archean, D-type, basaltic source magma with ca. 8% MgO, the two dioritic rock types representing late, immiscible, silica- and Fe-Ti-P-rich derivatives of pyroxenite-norite(-leuconorite) crystallization. In addition to the major rock types, peridotites occur in several outlying dikes and may represent fractionates of a komatiitic precursor to the basalt. Geologic and geochemical evidence points to bulk assimilation of country rock gneiss by the Madziwa magma and sulfide segregation triggered by felsic contamination. These processes did not occur locally within the dikes but rather in the conduit system prior to the emplacement of the magma charged with sulfide droplets. The unusual vertical layering of the pyroxenite-norite sequence and the localization of the sulfide ores are attributed to the strong outward, cooling gradient across such narrow dikes and to large-scale, lateral movement of sulfide droplets through the solidifying, pyroxene crystal framework ahead of an advancing postcumulus plagioclase crystallization front.

Description: The Grass Valley orogenic gold district in the Sierra Nevada foothills province, central California, the largest historic gold producer of the North American Cordillera, comprises both steeply dipping E-W veins located along lithologic contacts in accreted ca. 300 and 200 Ma oceanic rocks and shallowly dipping N-S veins hosted by the Grass Valley granodiorite; the latter have yielded about 70% of the 13 million ounces of historic lode gold production in the district. The oceanic host rocks were accreted to the western margin of North America between 200 and 170 Ma, metamorphosed to greenschist and amphibolite facies, and uplifted between 175 and 160 Ma. Large-scale magmatism in the Sierra Nevada occurred between 170 to 140 Ma and 120 to 80 Ma, with the Grass Valley granodiorite being emplaced during the older episode of magmatism. Uranium-lead isotope dating of hydrothermal xenotime yielded the first absolute age of 162 ± 5 Ma for the economically more significant N-S veins. The vein-hosted xenotime, as well as associated monazite, are unequivocally of hydrothermal origin as indicated by textural and chemical characteristics, including grain shape, lack of truncated growth banding, lack of an Eu anomaly, and low U and Th concentrations. Furthermore, the crack-seal texture of the veins, with abundant wall-rock slivers, suggests their formation as a result of episodic fluid flow possibly related to reoccurring seismic events, rather than a period of fluid exsolution from an evolving magma. The N-S veins are temporally distinct from a younger 153 to 151 Ma gold event that was previously reported for the E-W veins. Overlapping U-Pb zircon (159.9 ± 2.2 Ma) and 40 Ar/ 39 Ar biotite and hornblende (159.7 ± 0.6–161.9 ± 1.4 Ma) ages and geothermobarometric calculations indicate that the Grass Valley granodiorite was emplaced at ca. 160 Ma at elevated temperatures (~800°C) within approximately 3 km of the paleosurface and rapidly cooled to the ambient temperature of the surrounding country rocks (〈300°C). The age of the granodiorite is indistinguishable from that of the N-S veins, as recorded by the U-Pb age of xenotime in those veins. Consequently, the N-S veins must have formed between 162 and 157 Ma, the maximum permissive age of magma emplacement and the youngest permissive xenotime U-Pb age, respectively, during an E- to ENE-directed compressional regime. The geochemistry of the Grass Valley granodiorite is consistent with it being the product of arc magmatism. It served as a receptive host for mineralization, but it is has no direct genetic relationship to gold mineralization. Initial uplift of the intrusive mass correlates with the initial voluminous fluid flow event and vein formation at depths of no greater than 3 km. The E-W gold-bearing veins hosted within greenschist-facies country rocks adjacent to the intrusion formed during a second hydrothermal event 5 to 10 million years later than the magmatism and were contemporaneous with a shift to transtensional deformation denoted by sinistral strike-slip faulting.

Description: The Kalgoorlie goldfield (~50 Moz Au produced), famous for its long mining history and diversity of precious metal telluride minerals, is a world-class Neoarchean Au-Ag-Te district, which includes the Golden Mile Super Pit, the largest single gold deposit in the Eastern Goldfields of Western Australia, and the smaller but nonetheless significant Mount Charlotte deposit, 3 km to the north. The gold ore at Kalgoorlie is of two types—Au- and Te-rich first stage (Golden Mile, or Fimiston, ore), which constitutes the bulk of the Au endowment, followed by a relatively Te-poor second stage (Mount Charlotte ore). Fimiston-stage ore is characterized by deformed quartz-carbonate structures termed "lodes:" thin (1–10 cm) zones of quartz/ankerite/gold/telluride-rich vein breccias with halos of fine-grained pyrite, muscovite, ankerite, and tourmaline. Charlotte-stage ore is primarily hosted by ankerite-pyrite-rich alteration selvages around flat-sided, undeformed bucky quartz veins and is the only ore style present in the Mount Charlotte mine itself. The primary host unit for both mineralization styles is the Golden Mile Dolerite, one of several dolerite intrusions in the mafic-ultramafic volcanic succession of the Kalgoorlie terrane. Along with the large amount of mafic metavolcanics, consistent with typical greenstone belt stratigraphy, the Kalgoorlie goldfield contains at least three fine-grained carbonaceous (meta)black shale units (from oldest to youngest: the Kapai Slate; an unnamed interflow shale near the top of the Paringa Basalt; and black shale forming the base of the Black Flag Group). Each of these units contains varying amounts of synsedimentary, diagenetic, and hydrothermal-metamorphic pyrite and pyrrhotite, including well-preserved pyrite nodules. Nodules at the Golden Mile Super Pit vary in diameter from a few millimeters to several centimeters, can have several concentric zones of pyrite with internally variant textures, and are commonly deformed into ovoid shapes. There are also horizons of pyrrhotite nodules within certain sections of these units; like their pyrite counterparts, these are commonly concentrically zoned and show evidence of later deformation. Rare examples of thin massive sulfide beds are also present in the interflow shale near the top of the Paringa Basalt. LA-ICP-MS imaging of pyrite nodules from each of the three black shale units reveals complex (and sometimes spectacular) concentric compositional zonation that parallels the growth zones. Trace element concentrations vary within different nodule bands in a coherent pattern, with Au, Ag, Te, and As typically enriched together in certain zones. Gold content is particularly high in the Paringa Basalt interflow shale nodules, which average 3 to 4 ppm Au as well as 30 to 40 ppm Ag, 30 to 40 ppm Te, and 1,000 ppm As. Samples taken several kilometers to the south (along strike) and west of the Golden Mile of the Kapai Slate and Black Flag Group shale also contain disseminated and nodular pyrite enriched in Au, Ag, Te, and As at levels comparable to samples of those formations within the deposit. However, in distal samples of the Paringa interflow shale, there is only laminated and nodular pyrrhotite, marked by enrichments in Au, Ag, Sb, Te, Tl, Pb, and Bi relative to a later (and presumably metamorphic) pyrrhotite which crosscuts and partially replaces the earlier pyrrhotite. Lead isotope studies of nodules from the three shale units, as well as pyritic ore samples from two separate Fimiston-stage lodes and one Mount Charlotte-stage sample, have been undertaken to help resolve relative timing issues. Nodular pyrite from each shale formation has a distinct isotopic composition, with the Kapai Slate samples being the least radiogenic, followed by those from the Paringa interflow shale and, lastly, the Black Flag shale. These data result in progressively younger Pb-Pb model ages, in keeping with the established stratigraphic order. In contrast, ore pyrites contain a wide spread of relatively unradiogenic to radiogenic isotope compositions, partially overlapping with the nodular pyrites. Sulfur isotope studies ( 32 S, 33 S, and 34 S) have provided evidence on S source(s) for the nodules and ore-stage pyrites. Whereas the cores of most nodules contain pyrite with negative 33 S, a signal thought to be derived from seawater sulfate, the rims of the same have positive 33 S, which may result from metabolization of atmospheric elemental S. By contrast, ore-related pyrites (both Fimiston- and Mount Charlotte-stage) have no or little 33 S anomalies. The shape, internal textures, and distinct trace element enrichment and zonation, evidently little affected by ore-forming processes, suggest the nodules are synsedimentary to early diagenetic. There is virtually no evidence that gold or other elements have been added to the nodules during hydrothermal ore events; gold, along with many other elements, remains a coherent part of the primary nodule structure. Lead and S isotope studies on the pyrite nodules provide strong supportive evidence of an early marine sedimentary age for the nodules: the Pb isotopes give an age roughly equivalent to progressive sedimentation of the black shale host rocks, and the S isotopes are best explained by marine sulfate being the original S source for the nodules. The evidence is compelling that there was enrichment of Au-Ag-Te-Hg-As during intervolcanic sedimentation and diagenesis in the Kapai Slate, the interflow shale near the top of the Paringa Basalt, and Black Flag shale, before the formation of the Fimiston-stage gold-telluride lodes. While this work does not permit us to comment on the gold source issue in the Kalgoorlie deposits, the fact remains that syngenetic/diagenetic gold preconcentration in fine-grained, sulfidic, moderate- to deep-water sediments likely occurred across the Eastern Goldfields between ~2700 to 2680 Ma.

Description: The Yinachang deposit in the Kangdian Fe-Cu metallogenic province, Southwest China, contains approximately 20 Mt ore @ 41.9 to 44.5 wt % Fe and 15 Mt ore @ 0.85 to 0.97 wt % Cu, with potentially significant REEs. Orebodies are hosted in the late Paleoproterozoic Dongchuan Group, and consist mainly of massive and banded replacement ores. The paragenetic sequence of this deposit includes pre-ore Na-(Fe) alteration (stage I), Fe-(REE) mineralization dominated by magnetite and siderite with subsidiary apatite and fluorite (stage II), and Cu-(REE) mineralization with chalcopyrite, ankerite, biotite, and subordinate apatite, fluorite, allanite, and synchysite (stage III). This sequence is similar to those of many iron oxide copper-gold (IOCG) deposits elsewhere. There are two distinct types of fluid inclusions within apatite of both stages II and III. Type 1 fluid inclusions with liquid and vapor phases have relatively low homogenization temperatures (98°–345°C) and salinities (4.7–16.2 wt % NaCl equiv), whereas type 2 fluid inclusions with one or more solid phases, a liquid phase, and a vapor phase have higher homogenization temperatures (273°–〉500°C) and salinities (36.2–〉59.8 wt % NaCl equiv). Fluids in sulfur isotope equilibrium with chalcopyrite of stages III have 34 S CDT values ranging from –2.8 to +2.7, consistent with a magmatic origin for the sulfur. Thus, sulfur isotopie compositions imply that magmatic-hydrothermal fluids, probably represented by the hot and saline type 2 fluid inclusions, were involved in the ore-forming process, although 34 S close to 0 might also mean that sulfur was leached from igneous rocks by nonmagmatic fluids. Siderite of stage II and ankerite of stage III have 13 C PDB values ranging from –12.4 to –7.5 and from –8.5 to –4.4, respectively. The large variation of 13 C PDB values reveals a strong interaction between the magmatic-hydrothermal fluids and the dolostone in the ore-hosting sequence. Calculated 18 O SMOW values of the fluids in stages II and III are broadly similar, and vary from 11.1 to 17.1 (avg = 13.5). Such values are higher than that of magmatic-hydrothermal fluids and thus, may indicate extensive fluid–wall-rock interaction and/or mixing of magmatic-hydrothermal fluids with shallow-level nonmagmatic fluids. This interpretation is consistent with the radiogenic Sr isotope compositions of bulk ores (( 87 Sr/ 86 Sr) i = 0.710404–0.734034) and fluorite separated from ores (( 87 Sr/ 86 Sr) i = 0.709851–0.723658). These nonmagmatic fluids are probably representative of the type 1 fluid inclusions. Bulk ores and mineral separates have Nd(t) values ranging from –8.1 to +0.4 (mostly –0.9 to +0.4), similar to the coeval mafic intrusions in the region (+2.0 to +2.8). This similarity suggests that REEs in the Yinachang deposit might have been derived dominantly from mantle-derived magmas. On the other hand, relatively low Nd(t) values (–5.5 and –8.1) of some samples indicate insignificant REE contributions from wall rocks. The Yinachang deposit is temporally associated with regional mantle-derived mafic magmatism, and may have a genetic relationship with it. Fluids derived from deep-seated magmas may have supplied abundant sulfur and carbon as well as some of the ore metals in this deposit. The mafic magmatism could also have provided heat to induce the shallow level nonmagmatic fluids to interact with the sedimentary-volcanic sequence, thus leaching additional ore metals into the ore-forming system. Fluid mixing can effectively trigger saturation and deposition of ore minerals. Our work highlights that nonmagmatic fluids are not a prerequisite for Cu-(Au-REE) mineralization, but may facilitate metal deposition in magmatic-hydrothermal IOCG systems.

Description: The development of the North American Midcontinent rift can be understood in the context of a tectonic and magmatic event in which the far-field effects of continental drift influenced magmatism produced by a mantle plume. We report and interpret new data for samples collected from stratigraphically controlled sequences of basaltic rocks in the Osler Volcanic Group and the Mamainse Point Volcanic Group in Ontario, Canada. We confirm that the earliest phase of rift development involved primitive melts that produced basalts in the Lower Formation of the Osler Volcanic Group with Fo 74–80 olivine (1,490–2,280 ppm Ni). The earliest basalts are primitive in both the Mamainse Point Volcanic Group and Osler Volcanic Group sections, with elevated MgO, Cr, and Ni; these rocks also have high TiO 2 and high Gd/Yb, which are features that indicate low degrees of melting of a garnet-bearing lherzolite mantle. The Central Formation A and B basalts of the Osler Volcanic Group that overlie the Lower Formation lavas record an upward increase and then decrease in the degree of contamination of basaltic rocks with normal Ni and platinum group element (PGE) abundance levels that require no sulfide saturation event to accompany contamination. The Upper Formation of the Osler Volcanic Group is characterized by a limited number of PGE-depleted flows; although these basalts are highly fractionated and typically develop textures with stellate to laminated plagioclase feldspar, they are usually not significantly contaminated. The Lower and Upper formations of the Mamainse Point Volcanic Group show a diversity in geochemistry consistent with contributions from different mantle sources, but contamination is restricted to a small number of flows with normal Ni and PGE abundance levels. There is no indication of a regional metal depletion signature on the scale of the Nadezhdinsky Formation at Noril’sk, but there is evidence that local S saturation events were triggered in the unexposed conduits feeding some of the upper sequence flows in response to fractionation of the magma. Massive to heavy disseminated Ni-Cu-PGE sulfide mineralization in the Midcontinent rift is hosted by small primitive intrusions that were formed early in the evolution of the rift and are typically associated with transtensional structures in the cratonic blocks adjacent to the main rift. Examples include the Eagle deposit in Michigan, the Tamarack mineralization in Minnesota, and the Current Lake Complex in Ontario. The American examples are associated with splays of the Great Lakes tectonic zone and the Canadian example is associated with the Quetico fault zone. The intrusions share the geochemical fingerprints of the more primitive lowermost volcanic rocks of the Midcontinent rift and they are broadly contemporaneous with the development of the lower stratigraphy of the Osler Volcanic Group and Mamainse Point Volcanic Group. Although the low-grade but large-tonnage Cu-Ni-PGE deposits of the Duluth were formed significantly later than the small primitive intrusions, their host rocks share the same lithophile trace element signature as these early intrusions. The parental magmas that produced all of these intrusions were derived from deeper mantle depths, where garnet was a stable phase, than the magmas that produced the Central Formation basalts and some of the Upper Formation basalts investigated in this study.

Description: The NICO Au-Co-Bi(±Cu±W) deposit is located in the Great Bear magmatic zone, NWT, Canada, where numerous polymetallic, iron oxide-dominated mineralized systems have been recognized. Petrographic, electron microprobe analysis (EMPA), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) studies of host-rock alteration and ore mineralogy, together with sulfarsenide geothermometry, have been carried out to constrain the nature of alteration and/or mineralization assemblages in this deposit. Metasedimentary rocks of the Treasure Lake Group host NICO and are pervasively altered to an assemblage of ferrohornblende I + actinolite I + biotite I + magnetite I ± orthoclase, which is cut by barren veins composed of quartz ± ferrohornblende-orthoclase-calcite (Set 1). These alteration events are overprinted by metasomatic prograde and retrograde mineralized assemblages and both brittle and ductile deformation accompanied the metasomatism. The prograde assemblage (〉400°C) consists of cobaltite, Co-rich loellingite, and Co-rich arsenopyrite (stage I), magnetite II, ferrohornblende II, actinolite II, biotite II, pyrite, and minor scheelite and orthoclase. The earliest retrograde mineralization consists of arsenopyrite (stages II and III), which contains variable amounts of Co, together with native Bi (±bismuthinite) and Au, with lesser magnetite, marcasite, pyrite, hastingsite, and minor quartz. The preservation of solidified native Bi droplets suggests a temperature range of 270° to 〈400°C for precipitation of this assemblage. The final stage of retrograde mineralization consists of a chalcopyrite-bismuthinite-hematite-chlorite assemblage, together with hastingsite ± emplectite, which formed at temperatures of less than 270°C. Textural and trace element evidence indicates that the Au and Bi present within arsenides and sulfarsenides in the NICO system resulted from the initial partitioning of structurally bound Au and/or "invisible" (nanometer-sized particles) of Au and Bi into the prograde sulfarsenide and arsenide phases, which contain up to 81 ppm Au. The Au and Bi were remobilized following retrograde alteration of those minerals to arsenopyrite II. Molten Bi droplets are interpreted to have scavenged Au insitu, resulting in the formation of the Bi-Au inclusions observed in arsenopyrite II. The second mechanism of gold refining is explained by the occurrence of contemporaneous Bi (±Te) melt and hydrothermal fluids that also could have fractionated gold during transport in solution and deposited it in fractures, interstitially to earlier mineral grains, and as disseminations within Ca-Fe-amphibole-magnetite-biotite-altered rocks. Overall, the gold upgrading at NICO is consistent with the liquid bismuth collector model, suggesting that this process was an important control on gold concentration in this and potentially other Au-Bi-Te-Fe-As-S-rich iron oxide-copper-gold (IOCG) deposits.

Description: Gold mineralization at the Tropicana mine occurs within the Plumridge terrane along the eastern margin of the Archean Yilgarn craton in the Albany-Fraser orogen, Western Australia. Mineralization is hosted in a favorable syenitic lithofacies of the Tropicana Gneiss with a minimum igneous age of 2638 ± 4 Ma (2 ) and which was metamorphosed to mid-amphibolite to lower granulite facies in the period ca. 2638 to 2520 Ma. The Tropicana Gneiss was exhumed to crustal levels equivalent to greenschist-facies conditions by the time of economic gold mineralization. The major gold-bearing pyrite-biotite-sericite mineralization formed in association with shear zones during northeast-southwest compression (D 3 ) that postdated W- to NW-verging thrusting (D 2 ). The late fluid-induced event (Tropicana event; Doyle et al., 2013 ; Blenkinsop and Doyle, 2014 ) produced a mineral assemblage indicative of greenschist-facies conditions. The paucity of water in granulite-facies gneisses under retrograde conditions suggests that fluids were introduced from an external source for both mineralization and the younger metamorphic event. This occurred at ca. 2520 Ma as determined from biotite (ca. 2515 Ma, 40 Ar/ 39 Ar age), pyrite (ca. 2505 Ma, Re-Os, Pb/Pb ages), and tungsten-rich rutile (2521 ± 5 Ma, U-Pb age): the latter is considered to provide the best direct measurement age of gold mineralization. The Tropicana Gneiss was derived from the upthrusted easternmost margin of the underlying Yilgan craton, or represents a relatively small (160 km long x 50 km wide) remnant crustal block accreted to the Yilgarn craton sometime between ca. 2.6 and 2.5 Ga. The timing of the Tropicana mineralization is distinctly younger than greenstone-associated gold deposits elsewhere in the Yilgarn craton. As in the Dharwar craton, India, and the Limpopo belt, Zimbabwe, economic gold mineralization at Tropicana postdates the major peak of Late Archean world-class gold mineralization (ca. 2.65 Ga) by more than 100 m.y. The Tropicana Gneiss was relatively unaffected during the younger Albany-Fraser orogeny, with only minor remobilization of gold. Deformation may have been localized at the margins of the domain, between constituent ridged structural blocks, and/or within discrete high-strain shears.

Description: The Late Archean Hutti-Maski greenstone belt hosts the largest operational gold mine in India at Hutti, as well as two satellite mines at Hira-Buddini and Uti. Tourmaline is a common accessory mineral in the auriferous altered wall rocks, proximal to the gold-bearing veins of these orogenic gold deposits. We report here major, rare earth element (REE), and selected trace element compositions of tourmalines from these alteration zones at Hutti and Hira-Buddini, and from spatially associated volcaniclastic rocks at Uti, in order to compare and evaluate possible fluid sources related to tourmaline precipitation and gold mineralization. Tourmalines at Hutti occur as tourmaline-calcite microveins within proximal alteration zones, and as disseminated grains along the biotite-chlorite–defined mylonitic foliation. These tourmalines belong to the dravite/oxydravite group, with an evolutionary trend towards magnesio-foitite, and commonly are Mg rich (up to 2.06 apfu) with low Na (〈0.61 apfu) and up to 53% X-site vacancies. Characteristics are low abundances of total REE (0.25–0.67 x chondrite) and a flat trend, with minor HREE enrichment accompanied by strong positive Eu anomalies. At Hira-Buddini, tourmalines of the oxydravite-povondraite series replace amphiboles and hydrothermal biotite. Based on textural and chemical characteristics, three generations of tourmalines are present at Hira-Buddini. Earliest type I tourmaline has moderate Fe content (up to 1.46 apfu), high Na (average 0.80 apfu), and moderate X-site vacancies up to 22%. Type II tends to be relatively enriched in Fe (up to 2.08 apfu), have similar Na contents (average 0.79 apfu), and low X-site vacancies (up to 8%). These tourmalines contain higher total REE (0.93–2.23 x chondrite) and display a flat trend with slight middle rare earth element (MREE) depletion accompanied by a small positive Eu anomaly. The latest tourmalines (type III) are Fe-poor (up to 1.52 apfu) and Na-poor (average 0.67 apfu), with relatively high X-site vacancies (up to 21%). This tourmaline type has relatively low total REEs (0.21–1.05 x chondrite) and similar abundance patterns to those of Hutti tourmalines, accompanied by large positive Eu anomalies. Similarity in tourmaline compositions with evolutionary trends in compositional space from different textural associations suggests that a single low-salinity and reduced metamorphogenic fluid, with inherently low REE, was responsible for proximal alteration and gold mineralization at Hutti. The type I and type II tourmalines from Hira-Buddini display oxydravite-povondraite trends, with compositional overlap that suggests fluid mixing at a paragenetically early stage. The early hydrothermal fluid was more saline, oxidizing, and likely had a granite-derived component with high REE contents. A later evolved, relatively low-salinity and reduced fluid is recorded by the last generation (type III) tourmalines at Hira-Buddini. The tourmalines from Uti metapelites are characterized by dravitic composition, low Na contents, high X-site vacancies (31–47%), and an evolutionary trend toward magnesio-foitite. These tourmalines have higher REE contents (1.16–2.08 x chondrite), LREE-enriched patterns together with large positive Eu anomalies, and low concentrations of most trace elements, all features that suggest a metamorphic origin.

Description: The remobilization of metals during postdeposition hydrothermal alteration of magmatic sulfide ores has the potential to result in large haloes, the recognition of which could potentially enlarge the detectable footprint of this ore type. The Miitel komatiite-hosted nickel sulfide deposit in Western Australia was used as a case study to investigate the nature and 3-D geometry of the geochemical halo created by the remobilization of base metals, gold, and platinum group elements (PGE) into the rocks surrounding the mineralization. At Miitel, anomalous metal enrichment is found in the country rocks surrounding the massive sulfides, up to 250 m away from the ore. This enrichment, detected using portable X-ray fluorescence (pXRF) and backed up by laboratory analyses, occurs in the Mount Edwards footwall basalt within decimeters of the contact with the overlying Widgiemooltha komatiites. It is associated with the presence of nickel arsenides. Gersdorffite and minor nickeline are concentrated within small quartz and carbonate veinlets, and are interpreted to form during the circulation of arsenic-rich hydrothermal fluids. Results of lead fire assay analyses and in situ laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) analyses indicate high PGE concentrations (Pd and Pt) and minor gold associated with the observed nickel and arsenic enrichment. Results from a larger, regional-scale study, combined with this PGE enrichment, suggest that the massive nickel sulfides from the Miitel ore are the source of the remobilized nickel in the country rocks. The presence of Pd- and Pt-enriched trace arsenide phases in country rocks and shear zones may be a generally applicable proximity indicator for nickel sulfides in hydrothermally altered terranes.

Description: The Middle Cambrian Mount Read Volcanics of western Tasmania, Australia, host several world-class volcanic-hosted massive sulfide (VHMS) deposits, representing a wide range of deposit styles. Although the deposits and their host sequences are variably deformed and locally preserve spectacular examples of primary textures and structures, rapid lateral and vertical facies changes, and faults with uncertain sense and magnitude of displacement, have made it impossible to correlate stratigraphy across the belt. Previous dating studies in the area have yielded relatively imprecise crystallization ages. We have employed the chemical abrasion isotope dilution thermal ionization mass spectroscopy (ID-TIMS) U-Pb zircon method to obtain highly precise crystallization ages for a total of 18 samples of volcanic and intrusive rock units from throughout the Mount Read Volcanics and underlying mafic-ultramafic complexes. The new data permit detailed resolution of age relationships within the belt. The study establishes an age of 516.0 ± 0.9 Ma for the McIvor Hill gabbro, which is part of a mafic-ultramafic complex interpreted to underlie the Mount Read Volcanics. Magmatism in the central Mount Read Volcanics lasted at least 12.7 m.y., from 506.8 ± 1.0 Ma for a massive dacite unit in the lower part of the Central Volcanic Complex to 496.0 ± 0.9 Ma for a welded ignimbrite in the lower Tyndall Group. Together with previous age constraints, results of the study provide a precise chronostratigraphic framework for magmatism and VHMS deposit formation within the Mount Read Volcanics. The precise age data indicate that, north of the Henty fault, magmatism occurred in three discrete pulses, at least two of which were separated by periods of sedimentation. We demonstrate that VHMS deposits in the Mount Lyell, Roseberry-Hercules, and Que-Hellyer districts, comprising the majority of the known, significant VHMS deposits in the belt, formed within a narrow time interval at ~500 ± 1 Ma, at a relatively late stage in the evolution of the belt. Some of the larger intrusions in the belt (e.g., the Bonds Range porphyry; 500.4 ± 0.8 Ma) were emplaced contemporaneously with VHMS deposit formation; however, other bodies such as the Murchison granite (497.3 ± 0.9 Ma) are younger than the deposits and are unlikely to have been involved in their genesis as has previously been suggested. These conclusions will aid explorers in targeting VHMS mineralization in the Mount Read Volcanics.

Description: Deposits of Cu-Ni-platinum group elements (PGE)–rich sulfides occur along the North Range footwall of the Sudbury Igneous Complex, Ontario, Canada. The deposits have been modified chemically, mineralogically, and texturally by multiple syn- to postmagmatic hydrothermal events and are hosted mainly within Sudbury breccia, a contact metamorphosed and hydrothermally altered impact breccia. The composition of C1 to C6 unsaturated and saturated hydrocarbons in bulk fluid inclusion volatiles in the matrix of the Sudbury breccia was investigated using in-line rock-crushing gas chromatography (GC) in order to compare the hydrocarbon composition of this rock type in breccia zones containing economic footwall sulfide deposits at the Morrison deposit (Levack mine, North Range, Sudbury, Ontario) with barren breccia zones. Trace hydrocarbons, possibly abiogenic in origin, occur throughout the Sudbury Igneous Complex footwall and country rocks investigated. However, there are differences in the composition and abundance of bulk hydrocarbons released from mineralized and barren Sudbury breccia when fluid inclusions are opened. Volatiles released from samples of Sudbury breccia from sulfide-mineralized and sulfide-barren environments showed the following: (1) higher average abundances of light saturated hydrocarbons (C1–C4) and an approximately 2 x higher total abundance of hydrocarbons (mol/g of rock basis) associated with samples from the mineralized study area, and (2) comparable average unsaturated hydrocarbon abundances in both settings, but a larger range in unsaturated hydrocarbon abundances in the mineralized study area. Differences in bulk hydrocarbon abundance and composition reflect differences in the abundance and hydrocarbon content of fluid inclusions trapped in the barren and mineralized Sudbury breccia zones. In addition, the compositional modification of circulating fluids by processes and conditions unique to the mineralized Sudbury breccia environment may have occurred prior to inclusion entrapment (e.g., modification by mineral catalysts, differences in temperature and oxygen fugacity). Preliminary spatial analysis of the hydrocarbon distribution within and surrounding the Morrison deposit shows that the total hydrocarbon abundance in the Sudbury breccia does not change with orebody proximity within a single host breccia zone. However, volatiles trapped in the Sudbury breccia matrix near the top of the footwall deposit, where it is physically connected to contact-style (Ni-Cu-PGE) ores, have elevated unsaturated hydrocarbon abundances (e.g., propene) compared to other parts of the same breccia zone distal to this zone of physical connection. Samples of the PGE-enriched Sudbury breccia and mineralized veins containing alteration assemblages characteristic of the footwall ore styles have higher unsaturated hydrocarbon abundances and unsaturated/saturated ratios compared to Sudbury breccia and vein samples that are PGE poor. Localized enrichments in unsaturated hydrocarbons in Sudbury breccia zones may be the result of specific ore minerals and metal complexes (involving the PGE and group VIII metals) that acted as catalytic agents, modifying the composition of initially light saturated hydrocarbon species prior to entrapment in fluid inclusions. To apply the analysis of hydrocarbon compositions in trapped fluid inclusions to discriminating mineralized from barren zones of Sudbury breccia, samples from targeted areas were compared to samples of barren Sudbury breccia ("background"). Although a large data set is presented here to establish these compositional fields, analysis of smaller sample subsets from each environment yields a statistically significant separation between the two environments. Total hydrocarbon abundance, C3/C1 ratios, and unsaturated/saturated hydrocarbon ratios are useful indicators of sulfide ore proximity and may help delineate regions enriched in PGE.

Description: The syenite-associated Beattie deposit (measured and indicated resources of 60.9 Mt grading 1.59 g/t Au, and inferred resources of 29.7 Mt grading 1.51 g/t Au), located in the Abitibi greenstone belt, consists of two styles of gold mineralization: lithology controlled and structure controlled. Lithology-controlled mineralization is hosted by the syenite intrusion and associated with iron carbonate and sericite alteration. Lithology-controlled mineralization is low grade (1–2 g/t Au) and associated with arsenian pyrite and arsenopyrite, with gold being in solid solution within the spongy As-rich cores of pyrite. Structure-controlled mineralization is present within fault zones that are within the syenite intrusion and adjacent to its margins. This type of ore consists of high-grade mineralization (5 g/t Au) in silicified breccia with both hydraulic and tectonic features, cherty veins, and polymetallic veins. In this facies gold is visible as electrum filling microfractures of brecciated pyrite. The deposit has a distinct paragenesis. The initial stage of alteration involved hematite alteration due to deuteric oxidation. This was followed by a shift toward more reducing conditions, triggered by the introduction of CO 2 -rich hydrothermal fluids that led to sulfide precipitation and gold deposition in As-rich pyrite and arsenopyrite. A later-stage alteration event implies the input of silica-rich fluids that produced sulfide brecciation and their redistribution in the fault zones and gold remobilization in microfractures of brecciated pyrite. Calculated composition for 18 O and measured composition for D on quartz veins associated with this late event are respectively 6.9 to 10.8 and –53 to –83, indicating a potential magmatic-metamorphic fluid mixing. This event is associated with enrichment in Te, Hg, Mo, As, Au, Se, Ag, and Sb. In the Beattie deposit, interaction between magmatic and hydrothermal activities, coupled with external fluid ingress, led to a multistage process of sulfide and gold deposition.

Description: The Leinster area is situated toward the southern end of the Agnew-Wiluna Belt in the Yilgarn craton of Western Australia and hosts the giant Perseverance komatiite-associated nickel sulfide deposit. At in excess of 1.2 Mt contained Ni, this is the biggest type 1 (basal sulfide-rich accumulation) deposit in the world, and it is associated with a number of smaller deposits that extend over a strike length of 20 km adjacent to the Perseverance fault on the eastern margin of the belt. In broad terms, the greenstone sequence comprises an eastern, rift-proximal komatiite-dacite sequence and a western and more marginal basalt-dominated sequence with minor komatiite. The komatiites display a wide range of compositions and textures, from highly magnesian, olivine meso- to adcumulate rocks composed of olivine with forsterite contents of up to 94.5% to sequences of thin, spinifex-textured komatiite flow units. This compositional range indicates considerable variation in volcanic facies, from rift-proximal to distal. Loci of focused flow (termed pathways), indicated by thickened cord-like bodies of more olivine rich rock, occur at all scales and are intimately associated with the Ni sulfide mineralization. The pathways are linked by thinner sequences of lower-MgO rocks, either layered olivine orthocumulate rocks or sequences of thin spinifex-textured flow units. Some of the lower-MgO komatiite sequences appear to emanate from the tops of pathways, in the manner of breakout flows in tube-fed basaltic terrains. Variably sulfidic black shales and cherts and barren exhalative iron sulfide horizons are intimately interlayered with the komatiitic rocks. The overprinting effects of regional deformation and metamorphism, spanning at least 80 m.y., are considerable. At least eight local deformation events have been recognized by previous workers. These have been used to constrain the 3-D model and can be linked to craton-wide structural schemes. Some of the major faults are clearly accretionary and reflect reactivation of underlying basin-forming structures. Early north-over-south thrusting was followed by E-W–directed basin closure and the development of NNW-trending open regional folds. Extension accompanied batholithic granitoid intrusion and reactivated rift-parallel NNW-trending faults, producing the major strike-slip fault systems that divide the greenstone succession into a series of structural and stratigraphic domains. The ensuing orogenic collapse resulted in overturning of the stratigraphy to the east. Subsequent brittle deformation events have produced fault sets in a variety of orientations that can be related to ongoing shortening with a variable principal stress orientation. There is a complex interplay of structural and volcanological controls on the formation and subsequent deformation of the Ni sulfide deposits in this ancient terrane. Three-dimensional modeling has furthered a detailed understanding of the geologic and structural evolution of the belt, allowed the identification of komatiite pathway positions and their plunges, and provided quantitative data on the magnitude and direction of fault offsets of different generations. The modeling exercise has shown that it is possible to strip away the effects of overprinting metamorphism and deformation to reveal some of the primary controls on komatiite volcanism and the location of Ni sulfide deposits, even in those portions of greenstone belts that are highly deformed and have undergone amphibolite facies metamorphism. This is illustrated with case studies of the Perseverance, Venus, Rocky’s Reward, Harmony, and Sir Lancelot deposits. Three-dimensional modeling has the capacity to integrate geologic, geochemical, and geophysical datasets in a way that fully utilizes the results of past exploration programs and produces a premium geologic product that can be used to guide further exploration. The recent discovery of the Venus deposit is, in part, a tribute to the power of this approach.

Description: The Diyanqinamu porphyry Mo deposit in the southern Greater Khingan Range of the Central Asian orogenic belt contains 800 million metric tons (Mt) of ore with an average grade of 0.097% molybdenum. The deposit is hosted in Late Jurassic volcanic rocks of tuff, andesite, and volcanic breccia. Multiple-stage hydrothermal activities have resulted in propylitic, phyllic, and argillic alteration in this deposit. Five stages (I–V) of hydrothermal activity are identified. Stage I is represented by a mineral assemblage of epidote, chlorite, and magnetite, with some discontinuous barren veinlets of quartz + K-feldspar ± fluorite ± magnetite ± epidote ± chlorite. Stage II is marked by occurrence of quartz + fluorite + molybdenite + magnetite ± pyrite ± sericite ± siderite veinlets/veins with phyllic halos. Stage III consists of fluorite + siderite + quartz + molybdenite + pyrite ± ankerite ± calcite ± chalcopyrite veins that are commonly related to phyllic alteration and dissemination of fluorite in the altered rocks. Stage IV has an assemblage of fluorite + quartz + pyrite ± ankerite ± calcite ± molybdenite ± chalcopyrite ± sphalerite ± galena in coarse veins (10–20 mm wide). Stage V consists of narrow (≤5-mm wide) veinlets of calcite + fluorite + pyrite ± quartz. Molybdenite mainly occurs in Stages II and III. Re-Os dating results for molybdenite samples from these two stages yielded an isochron age of 156.2 ± 4.2 Ma (2 , MSWD = 0.96, n = 10). Most molybdenite samples have high 34 S values (≥8.4) relative to other sulfide minerals (i.e., galena, sphalerite, pyrite, and chalcopyrite) of Stages II to V ( 34 S = 2.5–8.3, n = 22). Molybdenite also has low 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios relative to other sulfide minerals although there are minor overlaps. In a diagram of 206 Pb/ 204 Pb versus 207 Pb/ 204 Pb, these Pb isotope data display a positive trend transecting the growth curves of crustal lead, which could be invoked by mixing of crustal and mantle sources with distinct Pb isotopes. In combination with the S isotope data and mineral paragenesis, we suggest that magmas were the main source of molybdenum, whereas other metals (i.e., Pb, Zn, and Cu) were possibly sourced from the country rocks.

Description: The El Teniente Cu-Mo porphyry deposit, Chile, is one of the world’s largest and most complex porphyry ore systems, containing an estimated premining resource of approximately 95 Mt Cu and 2.5 Mt Mo. Although Cu mineralization at the deposit is quite well studied, little work has focused specifically on the distribution and timing of Mo mineralization. Combined grade, vein, and breccia distribution analysis reveals that deposit-wide Mo grades of 0.01 to 0.06 wt % are strongly controlled by the abundance of main mineralization (type 6a) quartz ± molybdenite veins. These show a clear spatial relationship with several felsic-intermediate intrusions and appear to develop outward and upward into Cu-rich (type 6b–7b) quartz-chalcopyrite veins and (type 8) chalcopyrite-anhydrite ± bornite veins with sericitic alteration halos. High-precision Re-Os molybdenite dating reveals that these linked vein types did not develop in a single, deposit-wide evolution, but are diachronous, related to distinct episodes of hydrothermal activity associated with the emplacement of diorite finger porphyries and the composite Teniente Dacite Porphyry. These units acted as effective, short-lived (〈100,000 years) conduits for pulses of Mo- and Cu-bearing hydrothermal fluids between 6.3 and 4.6 Ma. The rapid thermal contraction of each system during mineralization led to extensive overprinting of Mo-rich veins by their lower-temperature, Cu-rich equivalents. Separate pulses in magmatic-hydrothermal activity are separated by distinct gaps of up to 300,000 years, during which Mo-mineralizing activity appears to have gone into quiescence. Mo grades exceeding 0.06 wt % correspond to the presence of molybdenite-bearing, late mineralization-stage, tourmaline-cemented (type 9), and anhydrite-carbonate ± gypsum (type 10) veins and breccias. These are abundant at shallow mine levels and show a close spatial relationship with a series of concentric faults associated with the Braden Breccia Pipe. Mineralization in this paragenetic stage is relatively short-lived and occurs in all parts of the deposit between 4.80 and 4.58 Ma. The generally Cu poor nature of the late mineralization stage is attributed to the prior preferential extraction of Cu from the underlying magma chamber in earlier mineralizing events. This led to the late exsolution of oxidized, Mo-rich fluids that may have undergone further enrichment by remobilizing Mo from main mineralization-type veins associated with the Teniente Dacite Porphyry. The formation of the Braden Breccia Pipe is likely to have occurred in a single cataclysmic event at approximately 4.58 Ma, which cut the Mo-rich tourmaline breccias and created a distinct Mo-rich grade halo at shallow mine levels. With the exception of minor mineralization associated with small dacitic dikes at approximately 4.42 Ma, the Braden event marked the termination of Mo deposition.

Description: Precise and accurate determination of the timing and duration of ore-forming processes in porphyry systems is a fundamental step in understanding their genesis and placing them in a regional context. Here, we take advantage of the considerable improvements in the field of geochronology over the last decade to provide a robust geochronologic framework for hydrothermal and magmatic events in the Eocene Coroccohuayco porphyry-skarn Cu deposit, and the first robust dating of an ore system in the emerging Andahuaylas-Yauri batholith and metallogenic belt, southern Peru. This batholith and associated porphyry systems were emplaced during the Incaic orogeny, in a context of slab flattening, compression, exhumation, uplift, and the initiation of the bending of the Bolivian orocline. High-precision ages from early skarn (U-Pb, hydrothermal titanite) and later-stage mineralization (Re-Os, molybdenite) in the Coroccohuayco deposit are indistinguishable from each other and from available high-precision U-Pb zircon ages of the porphyries. All together, they indicate that the deposit was formed in less than 100 k.y. between 35.7 and 35.6 Ma. We also highlight a previously unrecognized pre-ore high-temperature hydrothermal event (U-Pb, hydrothermal titanite) that corresponds to the emplacement of a precursor gabbrodiorite complex at ca. 40.2 Ma. A new 40 Ar/ 39 Ar age at 26.6 Ma of a post-ore alkali basalt is interpreted as recording the initiation of slab roll-back following the flat slab episode and is therefore not related to the magmatic-hydrothermal system at Coroccohuayco. These data, together with structural measurements at the Coroccohuayco deposit and available regional data, suggest that the Coroccohuayco deposit was formed toward the end of Eocene arc magmatism, in a context of transpressional stress, intense erosion, and exhumation associated with Incaic orogeny. At the scale of the Tintaya ore district (which hosts the Coroccohuayco, Tintaya, and Antapaccay deposits), available data and a new molybdenite Re-Os age obtained for the Tintaya deposit suggest that mineralizing events were spatially focused and episodic over several millions of years, while a single economic deposit may have been formed within less than 100 k.y.

Description: Magmatic-hydrothermal ore deposits in collisional orogens are new targets for modern mineral exploration, yet it is unclear why they preferentially occur in some specific tectonic environments within these orogenic belts. We integrate geologic and geochemical data (especially zircon U-Pb dating and Lu-Hf isotope data) for Mesozoic-Cenozoic magmatic rocks and associated ore deposits in the Lhasa terrane, a highly endowed tectonic unit within the Himalayan-Tibetan orogen, and provide the first example in a continental collision terrane of the application of zircon Hf isotope data to image the lithospheric architecture and its relationship with ore deposits. Three crustal blocks are identified within the Lhasa terrane by the Hf isotope mapping method. They include a central long-lived Precambrian microcontinent with local reworking and two surrounding juvenile Phanerozoic crustal blocks with significant mantle contributions to constituent magmatic rocks. The three crustal blocks are bounded by two E-W–trending terrane-boundary faults, and each block is cut by two N-S–striking concealed faults. Isotopic signatures of zircons from the juvenile crustal blocks indicate that the Phanerozoic continental crust grew from several Mesozoic volcanic-plutonic arcs and by underplating of mantle-derived magmas generated during Mesozoic accretion and Cenozoic collision. Mesozoic subduction-related porphyry Cu-Au deposits and Cenozoic collision-related Cu-Mo deposits are exclusively located in regions with high Hf (〉5) juvenile crust. Cu enrichment during differentiation of high f o 2 arc magmas is the key for the formation of Mesozoic subduction-related porphyry Cu-Au. By contrast, remelting of the lower crustal Cu sulfide-rich magmatic cumulates within the juvenile crust is interpreted to have played a key role in the formation of Cenozoic collision-related Cu-Mo deposits. Granite-related Pb-Zn deposits cluster in the oldest crustal regions or developed along the margin of the old crustal block bounded by lithospheric faults. The porphyry Mo deposits are localized along the reworked margins of the old crustal block. It is suggested that crustal reworking released Mo from the old crust to form porphyry Mo deposits, whereas leaching of Pb and Zn from the Paleozoic carbonate cover strata by felsic intrusion-driven fluids is critical to the formation of Pb-Zn ore deposits. Skarn Fe-Cu ore deposits are typically localized along a terrane boundary fault, i.e., lithospheric discontinuity, through which crust-derived felsic melt mixed with Cu-rich mantle-derived mafic magmas ascending upward. Associated granitoid rocks usually bear microgranular mafic enclaves and show a zircon Hf isotope array from negative to positive Hf values (–7.3 to +6.7), supporting mixing of juvenile mantle and evolved crustal sources. The Hf isotope maps show temporal-spatial relationships between crustal structure and the location of ore deposits, demonstrating that the structure, nature, and composition of the crust controlled the localization of ore deposits and the migration of ore-forming metals in the terrane. This study shows that the lithospheric architecture of an orogenic terrane can be imaged by Hf isotope mapping to provide mappable units which can be used to explore for ore deposits at the terrane scale.

Description: The gold endowment of the western Churchill Province is conventionally attributed to reworking and attendant metamorphism during the Trans-Hudson Orogeny (1.9–1.8 Ga). The Meliadine gold district is regarded as a type example of this inferred Paleoproterozoic gold metallotect and also represents one of Canada’s largest emerging orogenic greenstone- and banded iron formation (BIF)-hosted gold districts (2.8 Moz contained Au in reserves and total resource of 5.8 Moz Au). The largest gold deposits are cospatial with the Pyke fault and associated quartz (± ankerite) veining, which cuts Neoarchean (ca. 2.66 Ga) plutonic and supracrustal rocks comprising the Rankin Inlet greenstone belt. Meliadine gold occurs as inclusions within idioblastic arsenopyrite crystals, at sulfide crystal boundaries, and/or as sulfide fracture fills in hydrothermally altered, sulfidized, and veined BIF. Clusters of gold, Bi-Mo telluride, chalcopyrite, and galena inclusions characterize high-grade ore zones and are spatially associated with sieve-textured arsenopyrite domains. These microtextural relationships suggest that incipient sulfide recrystallization liberated gold, which, along with other precious and base metals, was redistributed during late, fluid-assisted and deformation/metamorphic-driven remobilization. Late precious and base metal enrichment is also demonstrated by in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) arsenopyrite mapping. Hydrothermal xenotime crystals occur with gold in low-strain microtextural sites and yield U-Pb ages at ca. 1.86 Ga, suggesting that remobilization was concomitant with Trans Hudson orogenesis. New Re-Os arsenopyrite model ages range from 2.3 to 1.8 Ga and document a hitherto unrecognized, pre-1.86 Ga hydrothermal activity. The range of Re-Os model ages tends to support partial open-system behavior and/or mixing of disparate arsenopyrite generations that are evident from microtextural observations and in situ LA-ICP-MS element mapping. Replicate analyses of the two most rhenium rich arsenopyrite samples yield reproducible Re-Os model ages at ca. 2.27 and 1.90 Ga. These rhenium-rich samples are also gold poor and likely yield ages that predate gold remobilization and subsequent enrichment along arsenopyrite crystal boundaries and fractures. We suggest that pre-1.86 Ga hydrothermal activity was likely critical to the gold endowment of the district, but is obscured by reworking during the later stages of the Trans-Hudson Orogeny at 1.86 Ga. If correct, these new ages imply that the Paleoproterozoic gold metallotect, which is recorded across the western Churchill Province, in fact comprises multiple, temporally distinct gold events. Differentiating between disparate Paleoproterozoic gold events represents a critical step for effective mineral exploration in the western Churchill Province and at other reworked Archean terranes.

Description: The Beiya skarn gold deposit is located in the eastern Tethyan orogenic belt in western Yunnan province, China. It is one of the largest gold deposits in China, with significant amounts of silver and base metals. To the end of 2014, the estimated resources are 125 million tonnes (Mt) of ore, grading 2.42 g/t Au, 0.48 wt % Cu, 25.5 wt % Fe, 38.85 g/t Ag, 1.24 wt % Pb, and 0.53 wt % Zn. Skarn alteration and mineralization are related to shoshonitic quartz monzonite porphyries that were emplaced in Triassic carbonates (Beiya Formation). Re-Os dating on molybdenite from a skarn orebody indicates an ore-forming age of 36.82 ± 0.48 Ma, which is consistent with previous dating results of the quartz monzonite porphyries. At least two paragenetic stages of skarn minerals and associated sulfides were recognized, with the early stage typified by garnet ± pyroxene, magnetite, and calcite, and the late stage characterized by epidote, amphibole, chlorite, quartz, and calcite, containing up to 70% sulfides (pyrite, chalcopyrite, and minor pyrrhotite). The early skarn is dominated by anhydrous minerals, which were replaced by hydrous minerals formed during the late stage. The garnet in the Beiya deposit is andradite rich (Ad 36–97 Gr 3–61 ), and pyroxene is relatively diopside rich (Di 8–91 Hd 7–89 ). This mineral assemblage indicates an oxidized skarn system, similar to other Au-Cu, Fe-bearing skarn deposits around the world. Fluid inclusions from pyroxene indicate precipitation from high-temperature and high- to moderate-salinity fluids (420°–530°C, 11.1–43.3 wt % NaCl equiv), which probably results from boiling of a moderately saline magmatic fluid. Cooler (180°–365°C) and moderate- to low-salinity fluids (1.6–16.5 wt % NaCl equiv) were trapped in garnet and quartz and are interpreted to be responsible for gold deposition. Chlorite chemistry indicates ore-forming temperatures between 300° and 340°C, in agreement with fluid inclusion data. It appears that gold was transported as chloride complexes under oxidized conditions and was deposited at temperatures of about 300°C, when transport of chloride complexes as gold carriers was less efficient.

Description: Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of 1,407 sedimentary (diagenetic and syngenetic) pyrites from 45 carbonaceous shale and unconsolidated sulfidic sediment samples, ranging in age from Paleoarchean to present day, show a considerable range of trace element compositions. Arsenic, Ni, Pb, Cu, and Co are among the most abundant trace elements, with medians ranging from 100 to 1,000 ppm. Less abundant elements Mo, Sb, Zn, and Se have median ranges of 10 to 100 ppm, and Ag, Bi, Te, Cd, and Au have median ranges of 0.01 to 10 ppm. Our dataset reveals three main groups of trace elements that are incorporated into pyrite in different ways. Group 1 elements (As, Ni, Co, Sb, Se, and Mo) are contained uniformly throughout the pyrite and may be held within the pyrite crystal structure or as nanoinclusions evenly distributed within pyrite. Group 2 elements (Bi, Pb, Ag, Au, Te, and Cu) generally occur uniformly at low concentrations and may be incorporated into the pyrite structure but are highly variable at high concentrations, where they may also occur as microinclusions. Group 3 elements (Zn and Cd) tend to have highly variable abundances and generally occur in pyrite as microinclusions of sphalerite. Factor analyses of the dataset identified five factors that account for 65.4% of the variance in pyrite trace element concentrations. Factor 1 includes Pb, Bi, Au, and Te, and explains 18.1% of the variance, possibly due to As(II) ( Qian et al., 2013 ) or As(III) substituting for Fe in pyrite, which induces the uptake of these elements. Factor 2 includes Co, Ni, and As and accounts for 13.6% of the variance, possibly due to the presence of As(–I) substituting for S(–II) in pyrite, which, in turn, promotes the uptake of Ni and Co. Factor 3 includes Zn and Cd and explains 12.3% of the variance and is due to the presence of sphalerite inclusions. Factor 4 includes Se, Ag, and Sb and explains 11.0% of the variance, which is believed to reflect coeval input of these elements into the oceans during periods of increased oxygenation. Factor 5 includes Mn, Cu, and Mo and explains 10.4% of the variance. It is likely that this behavior is due to these elements being delivered together to the sediments by adsorbing to Mn (hydro)oxides, which are released when the Mn (hydro)oxides dissolve in reducing bottom waters or pore waters. Variations in pyrite texture do not show consistent compositional patterns between different samples, though within the same sample later formed pyrite tends to have lower trace element abundance. Many trace elements associated with mafic extrusions/circulation of fluids through mafic rocks (Ni, Co) are more enriched in Archean sedimentary pyrite at times when mafic volcanism/circulation of fluids through mafic rocks was more active. Similarly, some trace elements tend to be more enriched in Phanerozoic pyrite due to increasing levels of atmospheric oxidation.

Description: Short-wave infrared (SWIR, 1.90–2.36 μ m) and long-wave infrared (LWIR, 8.1–11.1 μ m) hyperspectral images collected using the SisuROCK system were used to develop an automated methodology for generating kimberlite dilution maps. Smoothed and denoised images from two Snap Lake (Northwest Territories, Canada) kimberlite drill cores were processed, and SWIR and LWIR spectral endmembers were extracted from the images with each mineralogical endmember assigned to one of four compositional groups: undiluted kimberlite, microdiluted kimberlite, micro- and macrodiluted kimberlite, and crustal rocks. These endmembers were used to classify the SWIR and LWIR images, and the results were validated using linescan data, drill core logs, petrology reports, and the results of X-ray diffraction, Raman spectroscopy, and Micro-Fourier Transform Infrared (FTIR) spectroscopy. This study demonstrates that hyperspectral imagery can be used to generate dilution maps for hypabyssal kimberlites that far supersede other current techniques in terms of spatial resolution.

Description: We report sensitive high mass resolution ion microprobe, stable isotopes (SHRIMP SI) multiple sulfur isotope analyses ( 32 S, 33 S, 34 S) to constrain the sources of sulfur in three Archean VMS deposits—Teutonic Bore, Bentley, and Jaguar—from the Teutonic Bore volcanic complex of the Yilgarn Craton, Western Australia, together with sedimentary pyrites from associated black shales and interpillow pyrites. The pyrites from VMS mineralization are dominated by mantle sulfur but include a small amount of slightly negative mass-independent fractionation (MIF) anomalies, whereas sulfur from the pyrites in the sedimentary rocks has pronounced positive MIF, with 33 S values that lie between 0.19 and 6.20 (with one outlier at –1.62). The wall rocks to the mineralization include sedimentary rocks that have contributed no detectable positive MIF sulfur to the VMS deposits, which is difficult to reconcile with the leaching model for the formation of these deposits. The sulfur isotope data are best explained by mixing between sulfur derived from a magmatic-hydrothermal fluid and seawater sulfur as represented by the interpillow pyrites. The massive sulfide lens pyrites have a weighted mean 33 S value of –0.27 ± 0.05 (MSWD = 1.6) nearly identical with –0.31 ± 0.08 (MSWD = 2.4) for pyrites from the stringer zone, which requires mixing to have occurred below the sea floor. We employed a two-component mixing model to estimate the contribution of seawater sulfur to the total sulfur budget of the two Teutonic Bore volcanic complex VMS deposits. The results are 15 to 18% for both Teutonic Bore and Bentley, much higher than the 3% obtained by Jamieson et al. (2013) for the giant Kidd Creek deposit. Similar calculations, carried out for other Neoarchean VMS deposits give value between 2% and 30%, which are similar to modern hydrothermal VMS deposits. We suggest that multiple sulfur isotope analyses may be used to predict the size of Archean VMS deposits and to provide a vector to ore deposit but further studies are needed to test these suggestions.

Description: Exploration for heavy minerals along the coastal strip of southwest Africa led to the discovery and subsequent delineation of the Namakwa Sands deposit near Brand-se-baai in 1987. Currently, Tronox Limited controls the Namakwa Sands operation, which has developed into a significant, global producer of titanium slag, pig iron, ceramics quality zircon and rutile concentrates. The Namakwa Sands deposit is classified as a megaresource with 1.17 billion tons (Bt) of in situ ore, containing an estimated 92.6 million metric tons (Mt) of total heavy minerals and the equivalent of 23.9 Mt TiO 2 units and 9.4 Mt zircon. The siliciclastic, arenaceous deposit is divided into two adjacent orebodies, referred to as Graauwduinen West and Graauwduinen East. The Graauwduinen West orebody consists of three strandline-dune couplets set in a transitional shallow marine-aeolian environment, whereas the Graauwduinen East orebody comprises a largely unrelated dune deposit. Superimposed onto the ore-bearing sequence is a pseudostratigraphy of duricrust that effectively cemented the bulk of the mineralized sands to various degrees of hardness. The two orebodies are profoundly different in many aspects. Spatially, the planar mineralization bears northeasterly in the Graauwduinen West orebody, strikingly perpendicular to the southeasterly trend observed in the Graauwduinen East orebody. Contrasts in their bulk chemistry and mineralogy are statistically significant. The Graauwduinen West orebody features noticeably better total heavy mineral grades, accompanied by conspicuously high proportions of the gangue minerals garnet and pyroxene. The valuable mineral fraction is predominated by ilmenite (FeTiO 3 ), followed in abundance by zircon (ZrSiO 4 ), leucoxene (an alteration product of ilmenite), and rutile (TiO 2 ). Their geochemistry, mineralogy, and petrography are extremely variable, but the bulk of the current valuable mineral fraction is recoverable to premium-grade products. The proximal, Mesoproterozoic Namaqualand Metamorphic Province is considered the main contributor to the current heavy mineral population. The Graauwduinen West orebody received source sediment via established fluvial-marine courses, whereas the source of the Graauwduinen East orebody is considered to originate from a distinct fluvial-aeolian corridor. The genesis of the Namakwa Sands deposit is strongly entrenched in the geomorphic evolution of the west coast of southern Africa since the Cenozoic, which was driven by regional tectonism, fluctuating sea levels, and alternating climatic patterns.

Description: The Cenozoic Ambohimirahavavy alkaline complex in Madagascar consists of several syenitic to granitic intrusions (24.2 ± 0.6 Ma) the largest of which, the Ampasibitika intrusion, is characterized by the presence in its outer flanks of late peralkaline granitic dikes intruding mudstone and limestone of the Isalo Group. A network of dikelets and veinlets propagates from these dikes, intruding along bedding or obliquely to bedding. At the contact between the dikes and dikelets and a limestone, a reaction zone enriched in rare metals, dominated by calc-silicate minerals such diopside and andradite-grossular, forms a rare example of skarn resulting from peralkaline igneous activity. Much of the rare-metal mineralization (REE, Zr, Nb, Th, Sn, and Ti) occurs as secondary phases in the dikelets and skarn. In the dikelets and endoskarn, high field strength element (HFSE)-rich phases consist mainly of zircon, bastnäsite-(Ce), and Ca-REE-, and Ca-HFSE-rich phases in pseudomorphs after aegirine-augite. In the exoskarn, the main HFSE-rich phases are bastnäsite-(Ce), zircon, pyrochlore, Nb-rich titanite, and an unidentified F-rich Ca-zirconosilicate finely disseminated in a matrix composed of calcite, diopside, andradite, phlogopite, quartz, fluorite, and fluorapatite. The secondary zircon is characterized by a low Zr content and by the presence of REE, Ca, Al, and Fe. Three types of primary fluid inclusions were observed in dikelets and skarn in quartz, calcite, and diopside: liquid-rich inclusions (L-V) (20 to 40 vol % vapor) occur in all three minerals and homogenize to liquid; vapor-rich inclusions (V) (〉90 vol % vapor) occur in diopside and quartz, and homogenize to vapor; and halite-bearing L-V inclusions (L-V-H) occur in diopside and quartz, and homogenize either by disappearance of the vapor bubble or by halite dissolution. The L-V inclusions have low to intermediate salinity and homogenize at temperatures ranging from 200° to 380°C. The V inclusions have low salinity and homogenize at higher temperature (350°–395°C). The L-V-H inclusions mainly contain NaCl (35–45 wt % NaCl equiv), and homogenize by three modes, namely, bubble disappearance (mode A), halite dissolution (mode B), and simultaneous bubble and halite disappearance (mode C); the homogenization temperatures range from 260° to 380°C. We propose a model in which rare metals were transported by a Cl – , F – and HFSE-rich orthomagmatic fluid exsolved at 400° to 450°C and about 20 MPa. At these conditions, the fluid was in the two-phase region and vapor dominated. The rare metals were deposited as a result of the interaction of this fluid with limestone and mixing with an external fluid. This interaction/mixing buffered the orthomagmatic fluid to higher pH and lower temperature, resulting in the destabilization of REE-chloride complexes and deposition of fluorocarbonate minerals in the limestone; Zr and Nb, which were likely transported as hydroxyl-fluoride complexes, precipitated as zircon and pyrochlore due to deposition of fluorite and a consequent decrease in fluoride activity.

Description: The black shale in the Lower Cambrian Niutitang Formation, southwest China, which has been dated to 532.3 ± 0.7 Ma by U-Pb radiometric measurements, hosts a sedimentary layer with abnormally high Ni-Mo-PGE-Au contents. This layer is enriched in Ni (up to 3.8 wt %), Mo (up to 7.7 wt %), and U (up to 595 ppm), but its genesis is still controversial. Here we report the first direct observation of uranium-bearing minerals and their radioactive effects on the surrounding matter in the polymetallic sulfide ore. X-ray absorption fine structure analyses confirmed the reduced valence state of uranium. In combination with high-resolution electron microscopy and electron probe microanalysis, the mineral was identified as coffinite (USiO 4 ). A strong positive correlation between the sizes of the coffinite crystals and their surrounding carbonized rings reveals that the coffinite is authigenic, and its crystallization-produced radiation resulted in the radiolysis of surrounding organic matter. The association of various biogenic metal sulfides, phosphates, and abundant organic substances within the Ni-Mo sulfide-enriched ore suggests that biological adsorption may have participated in the enrichment of soluble U(VI), and that microbial sulfate reduction might have facilitated the uranium mineralization.

Description: The Touissit-Bou Beker Mississippi Valley-type (MVT) district of northeastern Morocco includes five major strata-bound Pb-Zn-Ag deposits that produced, over a century of mining, more than 70 Mt of ore at an average grade of 4.0 wt % Pb, 3.5 wt % Zn, ~1 wt % Cu, and 120 g/t Ag. Economic orebodies are hosted by a 25-m-thick sequence of unmetamorphosed, flat-lying, diagenetically and hydrothermally dolomitized carbonate platform rocks of Aalenian-Bajocian (174–168 Ma) age. The sulfide mineralization consists principally of galena and sphalerite, and occurs as open-space fillings and partial to massive replacements of medium- to coarse-grained dolostones. Throughout the district and surrounding prospects, sulfur isotope compositions of sulfide minerals range from –8.6 to 12.9 CDT ( n = 194) and display spatial and temporal variations characterized by an overall progressive decrease of 34 S values: (1) with advancing paragenetic sequence, and upward stratigraphic position; (2) toward distal prospects; and (3) within single grains from core to rim. Similarly, the distribution of 3 He/ 4 He ratios shows an overall lowering from 0.5 R A to 〈0.01 R A with advancing paragenetic stage. Conversely, lead isotope compositions of galena display fairly uniform 206 Pb/ 204 Pb (18.31–18.37), 207 Pb/ 204 Pb (15.61–15.66), and 208 Pb/ 204 Pb (38.45–38.62) ratios, which contrast significantly with the highly heterogeneous isotopic compositions ( 206 Pb/ 204 Pb = 17.76–18.49, 207 Pb/ 204 Pb = 15.61–15.70, 208 Pb/ 204 Pb = 37.76–39.55) shown by potential metal source rocks. The large variations in 34 S values are explained in terms of inorganic and/or bacteriogenic reduction, at different rates, of seawater sulfate in seawater or pore fluids, in reservoirs both open and closed with respect to sulfate. The highly variable 3 He/ 4 He values reflect different degrees of mixing between mantle and crustal He components along the fluid flow path. Estimated contributions of the mantle-derived ( 4 He) component involved in the mineralizing fluids of the paragenetically early Zn-rich ore-stage I, and in the later Pb-rich main ore-stage II, are in the range of ~3 to 8%. With time, the mantle-derived input to the mineralizing fluids decreased, and contemporaneously the involvement of surface waters increased, consistent with the lowering of 34 S with advancing paragenetic stage. This geochemical evolution toward lower isotopic values indicates that the Touissit-Bou Beker hydrothermal system became exclusively dominated by crustally derived fluids, and probably incorporated at shallower levels the influx of progressively larger volumes of cooler, dilute, and oxidized meteoric waters. Whole-rock and galena lead isotope compositions are consistent with a model in which metals were extracted from the local country rocks, particularly the basement Visean rhyodacites and associated siliciclastic rocks, and from the overlying Triassic to Upper Bajocian sediments. The extraction of lead likely occurred during the late Neogene, coincident with the Messinian salinity crisis and anorogenic alkaline magmatism. Conventional MVT genetic models cannot satisfactorily account for Pb-Zn-Ag ore deposition in the Touissit-Bou Beker district. Accordingly, we propose an alternative model involving a genetic link among extensional tectonics, Neogene-Quaternary mafic magmatism, the Messinian salinity crisis, and formation of the Touissit-Bou Beker deposits. We suggest that this mineralization resulted from the regional circulation of warm saline brines, sourced within the Rif corridor, during Messinian-Tortonian time (11.6–5.3 Ma). Messinian paleogeography would have favored the evaporation of seawater and development of a reflux-circulation system, synchronous with the emplacement of Neogene-Quaternary alkaline magmas. The associated elevated heat flow and subsequent increased geothermal gradient initiated buoyancy-driven fluid convection of downward-flowing Messinian seawater, which ultimately promoted the mobilization of older, high-temperature, rock-buffered basement dense brines stored within the Paleozoic basement, and the formation of base-metal–bearing chloride complexes. Mixing of Messinian seawater and basement-derived hydrothermal brines triggered deposition of Pb- and Zn-rich stage I and II mineralization. The resulting mixed brines were mainly centered on the Touissit Shelf and its flanks, and then flowed laterally away from the basement high, giving rise to the lower grade mineralization of the distal prospects. The predominance of a high heat flow regime centered on a positive paleogeographic structure (i.e., basement high) and more importantly the availability of appropriate sulfur nutrients (i.e., "fertile" vs. biogenic sulfur), may have constituted the critical ore controls that governed the size of the ore deposits. Finally, during late Pb-rich cuboctahedral stage III mineralization, interpreted to have been emplaced during a magmatic interlude contemporaneous with the closure of the Rif corridor and its final exhumation, the hydrothermal system became open to the incursion of meteoric waters; subsequent mixing with deep-seated, reduced, H 2 S-rich, saline waters led to precipitation of the sulfides of stage III mineralization.

Description: The Paleoproterozoic (~2.38–2.21 Ga) Yuanjiacun banded iron formation (BIF), located in Shanxi Province, is a Superior-type BIF in the North China craton. This BIF is within a metasedimentary rock succession of the Yuanjiacun Formation, in the lower Lüliang Group, which has undergone lower greenschist-facies metamorphism. Iron oxide (magnetite and hematite), carbonate, and silicate facies are all present within the iron-rich layers. The eastward transition from carbonate- into oxide-facies iron formations is accompanied by a change in mineralogical composition from siderite in the west through magnetite-ankerite and magnetite-stilpnomelane assemblages in the transition zone to magnetite and then hematite in the east. These distinct lateral facies are also observed vertically within the BIF, i.e., the iron mineral assemblage changes upsection from siderite through magnetite into hematite-rich iron formation. The oxide-facies BIF formed near shore, whereas carbonate (siderite)- and silicate-facies assemblages formed in deeper waters. Based on detailed analyses of these variations on a basinal scale, the BIF precipitated during a transgressive event within an environment that ranged from deep waters below storm wave base to relatively shallow waters. The BIF samples display distinctively seawater-like REEs + Y profiles that are characterized by positive La and Y anomalies and HREEs enrichment relative to LREEs in Post-Archean Australian shale-normalized diagrams. Consistently positive Eu anomalies are also observed, which are typical of reduced, high-temperature hydrothermal fluids. In addition, slightly negative to positive Ce anomalies, and a large range in ratios of light to heavy REEs, are present in the oxide-facies BIF. These characteristics, in combination with consistently positive 56 Fe values, suggest that deposition of the BIF took place along the chemocline where upwelling of deep, anoxic, iron- and silica-rich hydrothermal fluids mixed with shallower and slightly oxygenated seawater. The ankerite displays highly depleted 13 C values and the carbonate-rich BIF has a high content of organic carbon, suggesting dissimilatory Fe(III) reduction of a ferric oxyhydroxide precursor during burial of biomass deposited from the water column; that same biomass was likely tied to the original oxidation of dissolved Fe(II). The fact that the more ferric BIF facies formed in shallower waters suggests that river-sourced nutrients would have been minimal, thus limiting primary productivity in the shallow waters and minimizing the organic carbon source necessary for reducing the hematite via dissimilatory Fe(III) reduction. By contrast, in deeper waters more proximal to the hydrothermal vents, nutrients were abundant, and high biomass productivity was coupled to increased carbon burial, leading to the deposition of iron-rich carbonates. The deposition of the Yuanjiacun BIF during the onset of the Great Oxidation Event (GOE; ca. 2.4–2.2 Ga) confirms that deep marine waters during this time period were still episodically ferruginous, but that shallow waters were sufficiently oxygenated that Fe(II) oxidation no longer needed to be tied directly to proximal cyanobacterial activity.

Description: The Thelon basin, Nunavut, shares many similarities with the uranium-producing Athabasca basin, Saskatchewan; however, the uranium deposits associated with the Thelon basin are still poorly understood. The Kiggavik project area (AREVA Resources Canada) is located near the northeastern terminus of the Thelon basin and comprises multiple uranium deposits hosted exclusively in basement rocks. The Bong deposit is hosted dominantly by Neoarchean metagraywacke of the Woodburn Lake group. A five-phase metallogenetic model is proposed for the Bong deposit, with three stages of uraninite identified. The premineralization phase is characterized by host-rock silicification. Mineralization is separated into three main stages. Stage 1 uraninite (U1; ca. 1500 Ma) is preserved in highly fractured and altered disseminated grains that are overgrown by later stages of uraninite. Stage 2 uraninite (U2; ca. 1100 Ma) forms veinlets parallel to D 1 foliation and coats and fills fractures in organic matter nodules and blebs. Stage 2 uraninite is associated with pervasive illite that formed from ~190°C fluids ( 18 O: –6.4, 2 H: –97), which remobilized much of Stage 1 uraninite and completely overprinted Stage 1 alteration. At ~1000 Ma an oxidizing fluid deposited uraninite along redox fronts (U3) while altering and remobilizing Stage 1 and 2 uraninite. Post-uranium-oxide minerals include drusy quartz, calcite, and illite accompanied by uranyl phases (e.g., uranophane, Ca-U).

Description: The behavior of niobium and tantalum is poorly understood in rocks that have undergone significant hydrothermal alteration, and niobium-tantalum minerals of hydrothermal origin are rarely mentioned in the literature. Consequently, the mobility of these critical metals, although widely considered to be negligible, has not been evaluated. In this paper, we present the results of a study of the genesis of niobium and tantalum mineralization in the Nechalacho rare metal deposit, Northwest Territories, Canada, which contains one of the largest known resources of these metals in rocks that have undergone intense hydrothermal alteration. Analyses and examination of samples using the electron microprobe has led to the identification of a variety of niobium- and tantalum-bearing minerals in the Nechalacho deposit. Niobium-bearing zircon, columbite-(Fe), fergusonite-(Y), and samarskite-(Y) were identified in the ore zones of the deposit, uranopyrochlore, and columbite-(Fe) were found outside the ore zones, and magmatic fluornatropyrochlore was shown to be the sole niobium-tantalum mineral in relatively unaltered syenites below the Basal ore zone. Based on the paragenetic relationships among the above minerals, variations in the composition of the columbite group minerals as a function of location in the Nechalacho Layered Suite and the distribution of niobium, tantalum, zirconium, and uranium in the bulk rocks, we have developed a model to explain the occurrence of niobium and tantalum in the Nechalacho deposit. The first step in the concentration of these elements was the crystallization of niobium- and tantalum-bearing zircon and eudialyte in the subhorizontal Upper and Basal ore zones, respectively. This was accompanied by the crystallization of magmatic columbite-(Fe) in the Upper ore zone. Fergusonite-(Y) crystallized in the Basal ore zone and also formed due to the breakdown of eudialyte. Outside the ore zones, there was crystallization of pyrochlore and to a lesser extent magmatic columbite-(Fe). This step led to the development of strong spatial associations among niobium, zirconium, and uranium that are evident as strong positive correlations in the bulk-rock concentrations of these elements at the meter scale. During the ensuing intense and widespread hydrothermal alteration, niobium was locally remobilized. Hydrothermal columbite-(Fe) and fergusonite-(Y) formed at the cores of altered zircon grains. Wholesale replacement of magmatic columbite-(Fe) and fergusonite-(Y) by hydrothermal anhedral crystals occurred in the two ore zones. The estimated relative proportions of the sources of these minerals in the ore zones, although varying to some extent because of a dependence on the amount of niobium mobilized from zircon, is ~40/60. Outside the ore zones, columbite-(Fe) and uranopyrochlore are the present manifestations of the former pyrochlore. With the exception of magmatic fluornatropyrochlore in the fresher syenites below the Basal ore zone and a single example of magmatic columbite-(Fe) in an Upper ore zone sample, all niobium and tantalum minerals have a hydrothermal origin as a result of this pervasive alteration.

Description: Trace element concentrations in quartz from 188 granitic pegmatites in the Froland and Evje-Iveland pegmatite fields, southern Norway, have been determined to establish exploration targets for high-purity quartz and to gain a better understanding of the genesis of pegmatites hosting these deposits. Both pegmatite fields were formed during the Sveconorwegian (Grenvillian) orogeny (1145–900 Ma) at the western margin of the Fennoscandian Shield. In situ raster analyses within single quartz crystals were undertaken by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS); spot size 75 μ m) to assess levels of lattice-bound impurities, rather than mineral and fluid inclusions that are relatively easily removed during high-purity quartz processing. Quartz in the Froland pegmatites has relatively pure and homogeneous compositions containing 46 ± 24 μ gg –1 Al, 8 ± 3 μ gg –1 Ti, 1.4 ± 0.8 μ gg –1 Ge, and 11 ± 7 μ gg –1 Li. The Ti-in-quartz geothermobarometer gives an average pegmatite crystallization temperature of 537° ± 39°C. Temperature estimates are highest along the northwestern margin of the pegmatite field (〉550°C), whereas the most differentiated pegmatites occur toward the northeast. The area of greatest economic potential for high-purity quartz lies just north of the central part of the field where individual pegmatites contain 〉1 million metric tons (Mt) quartz with low average trace element contents of 67 ± 11 μ gg –1 . From mineral-chemical criteria, and a range of other geologic factors, we propose that pegmatite melts in the Froland field were generated by fluid-present crustal melting at about 1060 Ma, in zones of localized high-strain deformation during progressive thrusting along the Porsgrunn-Kristiansand fault zone. Quartz in the Evje-Iveland pegmatites has more variable compositions with 69 ± 57 μ gg –1 Al, 19 ± 11 μ gg –1 Ti, 2.3 ± 1.8 μ gg –1 Ge, and 7 ± 5 μ gg –1 Li. From its Ti content, it crystallized at temperatures of 613° ± 70°C. The regional spatial distribution of Ti-in-quartz temperatures appears irregular mainly due to the scattered distributions of chemical evolved pegmatites with "amazonite"-"cleavelandite" replacement zones, which show crystallization temperatures down to 442°C. Quartz from the Evje-Iveland pegmatites is unlikely to be of current economic interest due to its moderate to high trace element contents, heterogeneous chemistry, and low volume. The Evje-Iveland pegmatites show no apparent genetic link to a granite intrusion; instead they probably formed as a result of partial melting at the depth of their amphibolite country rocks at around 910 Ma. This is related to a regional low-pressure/high-temperature metamorphic event at about 930 to 920 Ma.