ALBERT

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: 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: Textural and chemical characterization of pyrite was used to reconstruct the hydrothermal evolution of the Bracemac-McLeod Archean volcanogenic massive sulfide (VMS) deposits (~6 Mt) in the Matagami district, Abitibi, Canada. The mineralization, hosted in a bimodal volcanic sequence, is divided into a (1) Zn-rich zone, concordant to the Key Tuffite— a marker horizon at the district-scale, 2) a Cu-rich zone which locally crosscuts the stratigraphic pile, and 3) some localized magnetite-rich zones replacing sphalerite and pyrite. Detailed petrographic and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) studies of whole pyrite grains (including inclusions and zonation) from the Key Tuffite and from the different ore zones of the deposits, has revealed five pyrite types. Nodular pyrites (I) in the Key Tuffite and subidiomorphic pyrites from the low temperature (250°C) Zn-rich zone (II) have the same chemical signature, suggesting that both precipitated under similar physicochemical conditions. A wide range of trace elements (Cu, Zn, Ag, Sn, Sb, Te, Au, Pb, Bi, Tl) are present in both, but compared to the other types of pyrite, they are significantly enriched in Sb and Tl. Subidiomorphic pyrites from the copper-rich zone (III) have a different signature, enriched only in Se ± Co, In, Pb, and Bi. These pyrites commonly overgrow the nodular pyrites and are related to later higher-temperature fluids (300°C). Pyrite preserved during a later magnetite-replacement (〉350°C) stage (IV) also are enriched in Se, but are depleted in all other elements. A similar signature is found in idiomorphic metamorphic pyrite (V), which occurs at the district scale. During high temperature recrystallization of pyrite IV and V, only the contents of Ni, Co, As, and Se are mostly preserved, whereas other base metals are expelled from the pyrite structure. Consequently, only Ni, Co, As, Se, Sb, and Tl were useful to reconstruct the hydrothermal evolution of the VMS system and used to build a suite of discrimination diagrams. Specifically, the ratio Se/Tl can discriminate pyrites from mineralized Zn-rich zone (Se/Tl 〈10), from pyrites associated with Cu-rich zone (10〈 Se/Tl 〈10,000) and pyrites from noneconomic parts of the deposit (replaced by magnetite or metamorphogenic; Se/Tl 〉10,000). The innovative approach of this study, based on LA-ICP-MS acquisition of the chemistry of entire pyrite grains, makes our results directly applicable for exploration. Consequently, pyrite concentrate samples, analyzed for As, Tl, and Se by more classical mass-spectrometric techniques, have the potential to be used for vectoring to ore by using the proposed discrimination diagrams.

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: 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: 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: 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.