ALBERT

Description: Near-bottom magnetic field data were collected using a towed magnetometer over selected parts of Palinuro and Marsili submarine volcanoes in the southern Tyrrhenian Sea, Italy. We obtained equivalent magnetizations maps at these sites by inverting the corresponding magnetic anomalies, highlighting the seafloor expression of hydrothermal alteration. Zones of reduced magnetization are interpreted as evidence for alteration related to hydrothermal processes; they are associated with water-column and seafloor observations of hydrothermal activity and altered host rocks. At Marsili volcano, a large elliptical area of reduced magnetization is located south of the summit cone and perpendicular to the trend of Marsili volcano itself. This confirms the presence of a large hydrothermal system centered over the more recent eastern volcanic ridge and its associated magma chamber. Similarly, areas of reduced magnetization over Palinuro caldera are consistent with hydrothermal venting occurring along the caldera walls, consistent with permeability structures related to caldera ring faults providing preferred pathways for the upflow of hydrothermal fluids.

Description: SuSu Knolls comprises three steep-sided conical volcanic peaks, standing on a N-NW-trending ridge in the eastern Manus basin, a complex zone of convergence between the major Indo-Australian and Pacific plates. The knolls consist of three porphyritic andesite-to-dacite domes, each 1.0 to 1.5 km in diameter, with crests ranging from 1,150 to 1,520 m below sea level. An intense hydrothermal plume, with peak transmission anomalies in excess of 40%, originates from SuSu Knolls, and associated hydrothermal venting, with fluid temperatures exceeding 300°C and sulfide mineralization, has been detected on the crests of all three edifices. The most important of these is the 2.47 million metric ton (Mt) Solwara 1 copper-gold deposit on Suzette Knoll, slated for mining by Nautilus Minerals. The porphyritic dacite is commonly strongly altered by reaction with acidic fluids. Fragments of sulfide mineralization, collected as part of the seafloor talus surrounding the crest of North Su and South Su, are characterized by an assemblage of pyrite-(fukuchilite)-enargite ± covellite-chalcopyrite. By contrast, there are actively venting chimneys on the crest of Suzette that are typically zoned, with chalcopyrite-pyrite-tennantite inner zones and barite-dominated outer zones. The knolls are covered by black sulfidic sediments that contain up to 2.4 wt % Cu and 3.2 ppm Au. Primary feldspars have been obliterated by the hydrothermal activity that ranges from incipient to intense and which is characterized by natroalunite, alunite (North Su only), cristobalite, tridymite, and rare quartz and kaolinite. Most volcanic rocks exhibit a patchy surficial coating of native sulfur, which may also fill vesicles. No altered rocks were dredged from Suzette because of the thick sulfidic sediment cover. Mass-balance calculations for altered and unaltered rock from North and South Su show that major and trace lithophile elements are depleted in the altered rocks, except for Ti, Al, Si, and Zr (all near-immobile), in concert with the destruction of the primary minerals and removal of primary components by acidic fluids. Sulfur and chalcophile trace elements (i.e., As, Au, Ba, Cd, Cu, Mo, Pb, and Se), typically associated with magmatic Cu-Au mineralization, are enriched by orders-of-magnitude in both leached and mineralized rock. Sulfide and native sulfur 34 S values from all three domes range from –7.4 to +0.4, indicating a magmatic component for the sulfur. A small group of ore elements (i.e., Ag, Bi, In, Sb, and Zn) are strongly enriched in mineralized breccias and depleted in the altered dacites, suggesting redistribution during alteration. The presence of advanced argillic alteration, the paragenesis of the Fe-Cu-As sulfide assemblage and presence of native sulfur, together with the sulfur isotope evidence for magmatic input into the hydrothermal fluid at SuSu Knolls, are consistent with a submarine high sulfidation magmatic Cu-Au hydrothermal system. Furthermore, the sulfide assemblage pyrite-enargite (±covellite-chalcopyrite) observed at North and South Su, and orders-of-magnitude enrichment of chalcophile elements in both altered and mineralized volcanic rocks, relative to unaltered volcanic rock, are also characteristic of subaerial high sulfidation epithermal mineralization. The SuSu Knolls hydrothermal field is a good example of a modern, high sulfidation, Cu-Au submarine hydrothermal system.

Description: East Diamante is a submarine volcano in the southern Mariana arc that is host to a complex caldera ~5 x 10 km (elongated ENE-WSW) that is breached along its northern and southwestern sectors. A large field of barite-sulfide mounds was discovered in June 2009 and revisited in July 2010 with the R/V Natsushima , using the ROV Hyper-Dolphin . The mound field occurs on the northeast flank of a cluster of resurgent dacite domes in the central caldera, near an active black smoker vent field. A 40 Ar/ 39 Ar age of 20,000 ± 4000 years was obtained from a dacite sample. The mound field is aligned along a series of fractures and extends for more than 180 m east-west and 〉120 m north-south. Individual mounds are typically 1 to 3 m tall and 0.5 to 2 m wide, with lengths from about 3 to 8 m. The mounds are dominated by barite + sphalerite layers with the margins of each layer composed of barite with disseminated sulfides. Rare, inactive spires and chimneys sit atop some mounds and also occur as clusters away from the mounds. Iron and Mn oxides are currently forming small (〈1-m diam, ~0.5-m tall) knolls on the top surface of some of the barite-sulfide mounds and may also drape their flanks. Both diffusely and focused fluids emanate from the small oxide knolls. Radiometric ages of the layered barite-sulfide mounds and chimneys vary from ~3,920 to 3,350 years. One layer, from an outcrop of 10- to 100-cm-thick Cu-rich layers, is notably younger with an age of 2,180 years. The Fe-Mn oxides were 〈5 years old at the time of collection in 2009. Most mound, chimney, and layered outcrop samples are dominated by barite, silica, and sphalerite; other sulfides, in decreasing order of abundance, are galena, chalcopyrite, and rare pyrite. Anglesite, cerussite, and unidentified Pb oxychloride and Pb phosphate minerals occur as late-stage interstitial phases. The samples contain high Zn (up to 23 wt %), Pb (to 16 wt %), Ag (to 487 ppm), and Au (to 19 ppm) contents. Some layered outcrop samples are dominated by chalcopyrite resulting in ≤4.78 wt % Cu in a bulk sample (28 wt % for a single lens), with a mean of 0.28 wt % for other samples. Other significant metal enrichments are Sb (to 1,320 ppm), Cd (to 1,150 ppm), and Hg (to 55 ppm). The East Diamante mound field has a unique set of characteristics compared to other hydrothermal sites in the Mariana arc and elsewhere. The geochemical differences may predominantly reflect the distribution of fractures and faults and consequently the rock/water ratio, temperature of the fluid in the upper parts of the circulation system, and extensive and prolonged mixing with seawater. The location of mineralization is controlled by fractures. Following resurgent doming within the caldera, mineralization resulted from focused flow along small segments of linear fractures rather than from a point source, typical of hydrothermal chimney fields. Based on the mineral assemblage, the maximum fluid temperatures were ~260°C, near the boiling point for the water depths of the mound field (367–406 m). Lateral fluid flow within the mounds precipitated interstitial sphalerite, silica, and Pb minerals within a network of barite with disseminated sulfides; silica was the final phase to precipitate. The current low-temperature precipitation of Fe and Mn oxides and silica may represent rejuvenation of the system.

Description: Forearcs within intraoceanic arcs preserve a record of earliest arc volcanism that took place at the initiation of subduction. In this study we have investigated the submarine section of the Bonin Ridge, which forms part of the Izu-Bonin intraoceanic arc and/or forearc system. The crustal stratigraphy of this forearc is found to have many similarities with suprasubduction zone ophiolite sequences, including the occurrence of sheeted dikes overlain by depleted tholeiites and/or boninitic magmatism. We propose that the Izu-Bonin forearc is potentially analogous to suprasubduction ophiolites. Hydrothermal alteration, together with sulfide disseminations, is exposed on some of the subaerial Bonin Islands. Based on geologic observations, the hydrothermal activity appears, in general, to have been contemporaneous with the boninite and boninitic andesite host volcanics. The sulfide mineral assemblage associated with the hydrothermal alteration is dominated by pyrite accompanied by minor chalcopyrite, sphalerite, and pyrrhotite. These sulfides are mainly found as disseminations in pillow breccias and hyaloclastite, or along the margins of dikes, but are also found as sulfide nodules in hydrothermal clay-rich zones. This forearc mineral assemblage contrasts with the Pb-rich mineralization associated with the Quaternary submarine hydrothermal deposits found along the main Izu-Bonin arc. Hydrothermal alteration in the Eocene Izu-Bonin forearc represents the style of hydrothermal activity generated by magmatism immediately after the initiation of intraoceanic subduction. Similarities in the style of hydrothermal activity and in the stratigraphy of the hosting volcanic sequence imply that the Bonin Ridge can be regarded as an in situ (i.e., not obducted or dismembered) analogue for a suprasubduction ophiolite including Cyprus-type massive sulfide deposits.

Description: A subseafloor replacement-style barite and sulfide occurrence was drilled in shallow waters at the Palinuro volcanic complex, the northernmost Aeolian arc volcano in the Tyrrhenian Sea, Italy. Using a lander-type drilling device, 11 successful drill holes yielded a total of 13.5 m of core from a sediment-filled depression located at a water depth of 630 to 650 m. The longest continuous drill core recovered consists of 4.84 m of massive to semimassive barite and sulfides with abundant late, native sulfur overprint. Seafloor observations suggest that the hydrothermal system associated with the formation of the subseafloor barite and sulfide ore zone is still active, although black smoker activity does not occur on the seafloor. The recovered drill core shows that the subseafloor deposit is zoned with depth. The top of the mineralized zone is comprised of a variably silicified vuggy barite-sulfide facies that shows notable polymetallic metal enrichment, while the deeper portion of the mineralized zone is dominated by massive pyrite having distinctly lower base and precious metal grades. Metal zonation of the barite and sulfide deposit is related to the evolution of the hydrothermal fluids in space and time. The barite cap and the massive pyrite present in the deeper portion of the mineralized zone appear to have formed early in the paragenesis. During the main stage of the mineralization, the barite cap was brecciated and cemented by a polymetallic assemblage of barite and pyrite with minor chalcopyrite and tetrahedrite, trace famatinite, and rare cinnabar. Lower temperature precipitates formed during the main stage of mineralization include sphalerite, galena, pyrite, opal-A, and barite, which are associated with traces of Pb-Sb-As sulfosalts such as bournonite-seligmannite, or semseyite. A distinct mineral assemblage of fine-grained anhedral enargite, hypogene covellite, chalcopyrite, and galena is commonly associated with colloform sphalerite, galena, and pyrite as a late phase of this main stage. Colloform pyrite and marcasite are the last sulfides formed in the paragenetic sequence. The deposit is interpreted to have formed from fluids having an intermediate-sulfidation state, although excursions to high- and very high sulfidation states are indicated by the presence of abundant enargite and hypogene covellite. Laser ablation and conventional sulfur isotope analyses show that pyrite formed close to the seafloor within the zone of polymetallic metal enrichment has a variable sulfur isotope composition ( 34 S = –39 to +3), whereas a more narrow range is observed in the massive pyrite at depth ( 34 S = –10 to 0). Similar variations were also documented for the late native sulfur overprint. Overall, the negative sulfur isotope ratios at depth, the intermediate- to very high sulfidation conditions during mineralization, and the abundance of native sulfur suggest contributions of magmatic volatiles to the mineralizing fluids from a degassing magma chamber at depth. Biological processes are interpreted to have played a major role during late stages of ore formation. The combination of a subseafloor replacement deposit with a massive to semimassive barite cap rock overlying massive pyrite, the intermediate- to high-sulfidation characteristics, and the strong biological influence on the late stages of mineralization are distinct from other modern seafloor massive sulfide deposits and represents a style of submarine mineralization not previously recognized in a modern volcanic arc environment. The barite and sulfide occurrence at Palinuro shares many characteristics with porphyry-related base metal veins and intermediate-sulfidation epithermal deposits, suggesting that metallogenic processes associated with arc-related magmatic-hydrothermal systems are not restricted to the subaerial environment.

Description: Hydrothermal alteration processes involve mineralogical, chemical, and textural changes as a result of hot aqueous fluid-rock interaction under evolving boundary conditions. These changes affect the physico-chemical properties of the rocks, enabling high-resolution geophysical prospecting to be an important tool in the detection of seafloor hydrothermal alteration. Here we present the results of recent geophysical investigations of the Marsili and Palinuro volcanic complexes, southern Tyrrhenian Sea, during the 2010 TIR10 and 2011 MAVA2011 cruises by the R/V Urania . The new dataset includes a dense grid of multibeam bathymetry; seafloor reflectivity, magnetic and gravity lines; and high-resolution single (CHIRP) and multichannel seismic profiles. The surveys were focused on areas known to host intense hydrothermal alteration in order to provide a more detailed description of the Marsili and Palinuro hydrothermal systems. Ground-truthing was based on earlier discoveries of hydrothermal vents and their associated deposits, and on direct observations made by ROV dives. High-resolution morpho-bathymetry, sonar reflectivity, rock magnetization, and density distribution together enabled us to assess the extent of seafloor hydrothermal alteration and its relationship to local volcanic and tectonic structures. Hydrothermal alteration associated with the Marsili seamount is largely distributed along primary volcano-tectonic structures at the ridge crest. By contrast, at Palinuro hydrothermal alteration is mostly associated with secondary volcanic structures such as collapsed calderas and volcanism reactivation along ring faults. In particular, evidence for intense hydrothermal activity occurs at Palinuro where volcanotectonic features interact with regional tectonic structures.

Description: The geochronology of seafloor hydrothermal activity from several mineralized, active and inactive vent fields at Suzette and Pual Ridge, eastern Manus back-arc basin, Bismarck Sea, Papua New Guinea, has been established using the 228 Th/ 228 Ra, 228 Ra/ 226 Ra, and 226 Ra/Ba uranium-series disequilibrium methods for barite-rich samples. These hydrothermal systems underwent episodes of hydrothermal fluid circulation that produced barite over the last ~6,000 years. Precipitation of hydrothermal barite, disseminated within the host rock and lining fractures, vugs, and conduits, was controlled by decreasing temperatures and mixing of hydrothermal fluids with oxidized seawater. Repeated fluctuations in hydrothermal fluid temperature and chemistry resulted in complex sequences of barite and sulfide minerals lining vugs and conduits, whereas rapid cooling following fracture events led to precipitation of only barite and some Fe oxide coatings on barite crystals. A wide range in ages of barite within drill cores (i.e., 380–5,990 yrs before present (BP)) reflect repeated pulses of hydrothermal fluids through the altered volcanic and mineralized rocks that make up the hydrothermal mound structures, with the younger ages (i.e., 34–220 yrs BP), representing more recent barite-precipitating fluid circulation accessing fractures, veins, and vugs. Barite from chimney samples is mostly young (i.e., 23–1,680 yrs BP), with the majority precipitated between 23 to 41 yrs BP; reflecting the structural stability (standing life) of the chimneys and possible influences from varying hydrothermal fluid chemistry or reactivation of vent fields. Three generalized age groupings, representing major episodes of barite-precipitating hydrothermal fluid circulation, are resolved from the Suzette area (19–45, 810–1,900, and 3,740–4,710 yrs BP, including 1 errors); with the most recent ages recorded from both the Suzette and Pual Ridge areas. Fast-spreading rates within the Manus basin, an abundance of faults and variable magmatic inputs are major factors controlling the frequency and longevity of these episodes of hydrothermal activity. Mineral resource models and comparisons indicate that the Cu-Zn-Ag-Au–rich Suzette deposit is small; however the measured age range is comparable to ages given for modeled (typical) seafloor massive sulfide deposits from back-arc and arc environments. We suggest that the scale of the Suzette deposit may have been limited during formation by a low hydrothermal fluid flux and/or inappropriate physical and chemical conditions for sufficient metal precipitation from hydrothermal fluids.

Description: The 2.7 m Bikpela chimney differs from sulfide-rich chimneys at PACMANUS, eastern Manus basin, by virtue of high silica content, a stromatolite-like layered structure, and the presence of a large internal collapse cavity. Its interior is dominated by 5- to 10-mm-thick primary laminae of opal-A-dominated fronds clouded by submicroscopic sphalerite inclusions, set between thin subhorizontal growth borders. Larger sphalerites with sulfosalt-galena intergrowths at frond ends project into discoid cavities under the growth borders. A steeper, second-order lamination reflects arrested expansion of the 50-cm-high internal collapse cavity. Underlying the cavity floor an axial breccia pipe contains distorted fragments of laminated interior modified by dissolution and then redeposition of opal and further growth of pyrite and sphalerite. Beneath a thin Mn oxide crust, a hard outer sphalerite-barite carapace ~5 cm thick and devoid of opal is banded parallel to the chimney exterior. At least three stages of chimney growth are evident, beginning with repetitive deposition of opaline frond layers probably developed from microbial mat precursors by overgrowth of hydrothermal opal and fine sphalerite. Next, with rising fluid temperatures, hydrothermal Fe-poor sphalerite with Pb sulfosalts and galena deposited at frond ends and under growth borders and, via lateral diffusion and mixing with seawater, formed abundantly in the outer carapace of the chimney. Passage of hot, more acid fluids into the chimney axial zone then caused dissolution of primary opal and consequent formation of the collapse cavity and underlying breccia pipe, together with deposition of Fe-rich sphalerite and pyrite in second-order fractures and cavities related to the collapse. Finally, conductive cooling during or after this latter stage produced films and spherules of clear opal coating pores and grain margins throughout the structure, apart from in the carapace. Bikpela contains significant gold and silver (avg: 8.7 ppm Au, 250 ppm Ag; n = 9), but it is low overall in Cu (up to 0.5 wt % in the pipe) and Pb (up to 0.7 wt % in the carapace). Its carapace is relatively rich in Zn (up to 54 wt %), Ag, As, and Sb, with one sample containing 0.12 wt % silver.

Description: Clark volcano of the Kermadec arc, northeast of New Zealand, is a large stratovolcano comprised of two coalescing volcanic cones; an apparently younger, more coherent, twin-peaked edifice to the northwest and a relatively older, more degraded and tectonized cone to the southeast. High-resolution water column surveys show an active hydrothermal system at the summit of the NW cone largely along a ridge spur connecting the two peaks, with activity also noted at the head of scarps related to sector collapse. Clark is the only known cone volcano along the Kermadec arc to host sulfide mineralization. Volcano-scale gravity and magnetic surveys over Clark show that it is highly magnetized, and that a strong gravity gradient exists between the two edifices. Modeling suggests that a crustal-scale fault lies between these two edifices, with thinner crust beneath the NW cone. Locations of regional earthquake epicenters show a southwest-northeast trend bisecting the two Clark cones, striking northeastward into Tangaroa volcano. Detailed mapping of magnetics above the NW cone summit shows a highly magnetized "ring structure" ~350 m below the summit that is not apparent in the bathymetry; we believe this structure represents the top of a caldera. Oblate zones of low (weak) magnetization caused by hydrothermal fluid upflow, here termed "burn holes," form a pattern in the regional magnetization resembling Swiss cheese. Presumably older burn holes occupy the inner margin of the ring structure and show no signs of hydrothermal activity, while younger burn holes are coincident with active venting on the summit. A combination of mineralogy, geochemistry, and seafloor mapping of the NW cone shows that hydrothermal activity today is largely manifest by widespread diffuse venting, with temperatures ranging between 56° and 106°C. Numerous, small (≤30 cm high) chimneys populate the summit area, with one site host to the ~7-m-tall "Twin Towers" chimneys with maximum vent fluid temperatures of 221°C (pH 4.9), consistent with 34 S anhydrite-pyrite values indicating formation temperatures of ~228° to 249°C. Mineralization is dominated by pyrite-marcasite-barite-anhydrite. Radiometric dating using the 228 Ra/ 226 Ra and 226 Ra/Ba methods shows active chimneys to be 〈20 with most 〈2 years old. However, the chimneys at Clark show evidence for mixing with, and remobilizing of, barite as old as 19,000 years. This is consistent with Nd and Sr isotope compositions of Clark chimney and sulfate crust samples that indicate mixing of ~40% seawater with a vent fluid derived from low K lavas. Similarly, REE data show the hydrothermal fluids have interacted with a plagioclase-rich source rock. A holistic approach to the study of the Clark hydrothermal system has revealed a two-stage process whereby a caldera-forming volcanic event preceded a later cone-building event. This ensured a protracted (at least 20 ka yrs) history of hydrothermal activity and associated mineral deposition. If we assume at least 200-m-high walls for the postulated (buried) caldera, then hydrothermal fluids would have exited the seafloor 20 ka years ago at least 550 m deeper than they do today, with fluid discharge temperatures potentially much hotter (~350°C). Subsequent to caldera infilling, relatively porous volcaniclastic and other units making up the cone acted as large-scale filters, enabling ascending hydrothermal fluids to boil and mix with seawater subseafloor, effectively removing the metals (including remobilized Cu) in solution before they reached the seafloor. This has implications for estimates for the metal inventory of seafloor hydrothermal systems pertaining to arc hydrothermal systems.

Description: The results of three decades of seafloor research provide the most reliable information on the importance of water depth in massive sulfide formation. Available data from over 130 occurrences show that water depths of seafloor vent sites vary with plate tectonic setting and the regional magmatic and volcanic environment. The shallowest hydrothermal systems in subduction-related settings are hosted by arc volcanoes. These shallow vent sites have a number of features in common with subaerial epithermal systems. Massive sulfide occurrences in arc-related rifts, the most likely setting for many ancient analogs, are generally restricted to water depths from ~700 to 2,000 m, with rifts developing within old arc crust at the deeper end of this range. Back-arc spreading centers proximal to arcs host massive sulfide deposits at depths of ~1,500 to 2,000 m. The deepest hydrothermal systems occur along mature back-arc spreading centers distal to volcanic arcs where water depths range from ~2,000 to 3,700 m. These deeper vent sites probably represent the best modern analogues of ophiolite-hosted massive sulfide deposits. Boiling of the hydrothermal fluids is common at volcanic arcs and in arc-related rifts. In these environments, elevated magmatic gas contents of the hydrothermal fluids can contribute to the widespread occurrence of phase separation and associated gas loss. By contrast, the high ambient pressures in deep marine hydrothermal systems along mature back-arc spreading centers prevent fluids from boiling during their ascent to the seafloor. Boiling controls the maximum temperature at which hydrothermal fluids discharge at the seafloor and, therefore, influences the metal content of seafloor sulfide deposits. Copper-rich massive sulfides typically occur at water depths exceeding ~1,000 m, whereas Zn- and Pb-rich occurrences may form at any water depth. Boiling can be an important control on Ag and Au grades but is not the only factor controlling precious metal enrichment in massive sulfides. Shallow marine hot spring deposits can be highly enriched in trace metals such as As, Hg, and Sb. Submarine volcanic arc and back-arc settings are geologically complex and significant variations in water depth can occur over short distances. Paleoenvironmental reconstruction of these environments in ancient volcanic terranes is hampered by the lack of unequivocal volcanological or sedimentological criteria that indicate water depth. The relationships established here using modern seafloor observations provide important constraints on the paleoenvironmental setting of ancient volcanic-hosted massive sulfide deposits.

Description: Nickel is a metal that reflects the technological advances of the twentieth and twenty-first centuries, emerging as critically important for stainless steel and a variety of specialty metal alloys as well as currency, chemicals, and batteries. Although mineral resources are commonly considered to be limited or finite, global Ni production has grown steadily throughout the twentieth century and has been matched by substantial growth in estimated Ni reserves and resources. While there is growing concern about "peak oil," there is very little research about "peak minerals." In this paper, we present a detailed compilation and assessment of globally reported Ni resources by project and split into standard mineral deposit types for the year 2011. The minimum amount of Ni reported globally as mineral resources is 296.2 million metric tons (Mt) Ni, split over a total of 253 sulfide projects containing 118.0 Mt Ni and 224 laterite projects containing 178.1 Mt Ni, with a further 3.38 Mt Ni in China (excluding Jinchuan, which is included in our sulfide compilation)—i.e., a global total of some 299.6 Mt Ni. Our data compilation indicates that the majority of global Ni resources are hosted by laterite deposits (59.5%), especially Ni laterite deposits located around the tropics. In addition, our compiled data indicate that global Ni resources continue to increase, despite a coincident increase in Ni production over time, along with declining cut-off and ore grades, increasing awareness of environmental issues and other related aspects. Overall, there are abundant Ni resources already identified which can meet growing global demands for some decades to come—the primary factors which govern whether a given project is developed (or not) will be social, economic, and environmental in nature.

Description: The Shuteen area is located in the Gurvansaikhan island arc terrane of South Mongolia. It contains a large domain of intensely developed silicic and advanced argillic alteration that has strongly affected the adakitic volcanoplutonic Shuteen Complex. The local occurrence of low-grade copper mineralization within porphyritic intrusive rocks is indicative of porphyry-style mineralization. Hydrothermal quartz veins host gold mineralization locally. The results of scanning electron microscope-cathodoluminescence imaging and fluid inclusion microthermometry indicate that the veins and altered rocks at Shuteen were produced by multiple hydrothermal events. The occurrence of CO 2 inclusions in quartz from granitoids and the 34 S values obtained for sulfide and sulfate within the hydrothermal breccias provide evidence of the involvement of magmatically derived fluids in the early stages of the Shuteen magmatic-hydrothermal system, inferred to be sourced from a porphyry-style intrusive center. Late-stage fluids produced abundant barren quartz veins and clay alteration assemblages. The weak mineralization and widespread alteration in the Shuteen Complex are typical of lithocaps found in high-level porphyry copper-(gold-molybdenum) and high sulfidation epithermal districts. The presentday erosion level at Shuteen is relatively shallow, implying that economic porphyry-style mineralization may exist at depth within or beneath the Shuteen lithocap.

Description: The Paleoproterozoic Great Bear magmatic zone is the focus of ongoing exploration for iron oxide copper-gold (IOCG) deposits and also hosts iron oxide-apatite occurrences. Examples of IOCG deposits in the Great Bear magmatic zone include Sue-Dianne and NICO, and other smaller prospects, including Damp, Fab, and Nori/Ra. The past-producing Terra mine property hosts significant IOCG-like alteration that contains dome-shaped, iron oxide-apatite bodies. Petrographic study has identified multiple generations of magnetite at NICO, Fab, and Nori/Ra and, for the most part, a single generation of magnetite at Sue-Dianne, Damp, and Terra. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) documents important geochemical differences in V, Ni, Cr, and Co concentrations within the magnetite. Variations of trace elements in magnetite from the Great Bear magmatic zone could be a result of (1) host rock-fluid equilibration during regional metamorphism, (2) postmetamorphic hydrothermal metasomatism of Treasure Lake Group metasedimentary rocks, (3) preferential solubility of Co over Ni within the Fe-rich fluids, (4) changes in oxygen fugacity ( f O 2 ), and (5) partitioning of elements into coprecipitating sulfides. Regionally, the Cr/Co ratio is higher in barren and pre-ore magnetite compared to magnetite coprecipitated with ore minerals and/or present in ore-rich veins and breccias. Locally, at the Nori/Ra prospect, the V/Ni ratio in magnetite differentiates between barren and ore-related magnetite, and at Damp and Sue-Dianne the Co/Ni ratio is extremely high and clearly different from that of other Great Bear magmatic zone magnetite samples. These results provide the first database for geochemically characterized magnetite from different stages of IOCG alteration and illustrate the potential use of magnetite as an indicator mineral in the exploration for IOCG deposits.

Description: New constraints on the geologic history and timing of mineralization at the Haile gold deposit in South Carolina are provided by radiometric dating of igneous and metamorphic samples using Re-Os, U-Pb, and 40 Ar/ 39 Ar methods. Seven Re-Os measurements were made on impure mineral separates of fine-grained molybdenite containing variable quantities of pyrite, pyrrhotite, muscovite, and illite. Four measurements produced an isochron at 548.7 ± 2.0 Ma (MSWD = 1.04). An additional age of 541 ± 2 Ma suggests that molybdenite may have been produced over a ~8 Ma timespan (541–549 Ma), but the preferred interpretation is that molybdenite formed primarily at ~549 Ma. Broadly correlative Mo and Au abundances in drill samples, and the common presence of molybdenite in Au-bearing zones, indicate that Au mineralization at Haile probably also occurred at ~549 Ma. Contrasting Re-Os ages produced at 564 ± 2 and 529 ± 2 Ma are interpreted to be inaccurate, due to the effects of impure mineral separates, and possibly resulting from loss of radiogenic Os from fine-grained molybdenite to other sulfide minerals. Some mobilization and loss of Re, possibly under oxidizing conditions during late Paleozoic metamorphism, may explain two relatively old Re-Os ages determined here and by Stein et al. (1997) . Laser ablation dating of zircons from a metavolcanic sample at Haile yielded a weighted mean age of 560 ± 8 Ma. This age is similar to the published time window of 551 to 555 Ma for magmatism at Haile, based on U-Pb zircon dating by ion probe ( Ayuso et al., 2005 ). Similar timing of magmatism and Au mineralization indicates that the hydrothermal systems at Haile were likely driven by magmatism and were therefore not the product of collision, orogeny, and metamorphism. These results confirm that magmatism and mineralization at Haile were active at a time when the Carolina terrane was still in a peri-Gondwana location, well prior to its accretion to eastern Laurasia. Ages of some detrital zircons imply an anomalously young depositional age for the metasedimentary unit that hosts the Haile orebodies (〈540 Ma). These ages conflict with an older age of 560 ± 8 Ma determined for the overlying metavolcanic unit. This conflict is interpreted to indicate that folding has overturned the geologic units at Haile. A possible Devonian-age metamorphic event at ~390 Ma is documented by 40 Ar- 39 Ar dating of porphyroblastic biotite collected from mineralized rock at Haile. Zircon ages from the Dutchman Creek Gabbro (weighted mean of 314 ± 2 Ma) and 40 Ar- 39 Ar ages on biotite separates from dikes that crosscut the Haile property (311 ± 3, 311 ± 2 Ma) document a pulse of mafic and alkaline magmatism in the late Carboniferous. These late-stage metamorphic and magmatic events appear to be unrelated to mineralization at Haile.

Description: The northern Shoshone and Toiyabe Ranges in north-central Nevada expose numerous areas of mineralized Paleozoic rock, including major Carlin-type gold deposits at Pipeline and Cortez. Paleozoic rocks in these areas were previously interpreted to have undergone negligible postmineralization extension and tilting, but here we present new data that suggest major post-Eocene extension along west-dipping normal faults. Tertiary rocks in the northern Shoshone Range crop out in two W-NW–trending belts that locally overlie and intrude highly deformed Lower Paleozoic rocks of the Roberts Mountains allochthon. Tertiary exposures in the more extensive, northern belt were interpreted as subvertical breccia pipes (intrusions), but new field data indicate that these "pipes" consist of a 35.8 Ma densely welded dacitic ash flow tuff (informally named the tuff of Mount Lewis) interbedded with sandstones and coarse volcaniclastic deposits. Both tuff and sedimentary rocks strike N-S and dip 30° to 70° E; the steeply dipping compaction foliation in the tuffs was interpreted as subvertical flow foliation in breccia pipes. The southern belt along Mill Creek, previously mapped as undivided welded tuff, includes the tuff of Cove mine (34.4 Ma) and unit B of the Bates Mountain Tuff (30.6 Ma). These tuffs dip 30° to 50° east, suggesting that their west-dipping contacts with underlying Paleozoic rocks (previously mapped as depositional) are normal faults. Tertiary rocks in both belts were deposited on Paleozoic basement and none appear to be breccia pipes. We infer that their present east tilt is due to extension on west-dipping normal faults. Some of these faults may be the northern strands of middle Miocene (ca. 16 Ma) faults that cut and tilted the 34.0 Ma Caetano caldera ~40° east in the central Shoshone Range (〈5 km south of Mill Creek), but further mapping is necessary to trace the faults through the highly deformed Paleozoic rocks that surround the isolated Tertiary outcrops. Significant post-Eocene extensional faulting in the northern Shoshone Range may have important implications for both the structure of the Roberts Mountains allochthon and the exposure of potentially mineralized rocks in its lower plate, both of which were likely east-tilted and repeated by west-dipping faults together with overlying Tertiary rocks.

Description: The Serra Pelada Au-Pd-Pt deposit is located within the Carajás mineral province, which also hosts the world-class iron ore and iron oxide-hosted copper-gold deposits of the Amazon craton in Brazil. The unusual low-temperature hydrothermal mineralization at Serra Pelada is epigenetic, hosted by metasedimentary rocks of the Águas Claras Formation and structurally localized in the Serra Pelada overturned syncline. The orebodies are controlled by the intersection of subvertical NE-trending fault zones with metasiltstones, mainly at the syncline’s hinge, with minor ore occurrences at the upper and lower limb. Intense tropical weathering over the last 70 m.y. has completely overprinted the shallow ore in and near the flooded open pit, but primary hydrothermal features are preserved in deeper drill core delineating the remaining resource. Gold, platinum, and palladium mineralization is associated with intense argillic alteration, hematite breccias, and silicification, with the highest grade ore hosted by brecciated metasiltstones that are highly enriched in amorphous carbon. Distal alteration zones comprise a reducing and oxidizing alteration front. The hydrothermal mineral paragenesis comprises kaolinite, quartz, sericite, amesite (Mg-rich Al-silicate), amorphous carbon, hematite, monazite, rutile, pyrite, and a complex assemblage of Bi-, Ag-, Pb-, Cu-, Co-, Ni-, Pt-, Pd-, and Au-bearing, chalcogenide (S, Se), and arsenide (As, Sb) minerals. Major element changes during hydrothermal alteration include C and Mg addition, K depletion, localized silica loss, and silicification with notable introduction of trace elements including light rare earth elements (LREE), Bi, Pb, U, V, Cu, Co, Ni, and As. The hydrothermal alteration and element association of the Serra Pelada deposit show geochemical similarities with unconformity-related uranium deposits, which may also be enriched in Au, Pd, and Pt and were formed by mixing of fluids that interacted with a highly oxidized cover sequence and highly reduced rock packages in structures of brittle-ductile strain.

Description: SHRIMP-RG U-Pb zircon geochronologic data together with published U-Pb ages from the succession of quartz diorite porphyry intrusions in the western porphyry Cu-Au centers at Reko Diq, western Balochistan, Pakistan, indicate the emplacement of four spatially distinct porphyry Cu centers over ~1 m.y. in the Miocene between ~12.9 and 11.9 Ma. Intrusive activity in the western porphyry cluster began with emplacement of hornblende-rich quartz diorite porphyry associated with the northern and uneconomic H79 porphyry Cu center with U-Pb ages of 12.87 ± 0.27 and 12.72 ± 0.3 Ma. To the south of the H79 porphyry Cu center, successively younger porphyry intrusive centers formed along a south-southwest trend. The economic H15 center has U-Pb zircon ages for four quartz diorite porphyry intrusions between 12.58 ± 0.29 and 12.18 ± 0.19 Ma with three of the porphyry intrusions that span the intrusive sequence in the H15 center having essentially identical ages between 12.58 ± 0.29 and 12.49 ± 0.20 Ma. The adjacent economic H14 center to the south has U-Pb ages on zircons for five quartz diorite porphyry intrusions between 12.22 ± 0.15 and 12.00 ± 0.16 Ma with four of the porphyry intrusions spanning the intrusive sequence having ages between 12.14 ± 0.14 and 12.00 ± 0.16 Ma. The southern end of the trend is the subeconomic H13 complex where mineralized quartz diorite porphyry has a U-Pb age on zircon of 11.93 ± 0.14 Ma. Two new Re-Os ages on molybdenite reported herein agree with the magmatic ages for the H15 (12.43 ± 0.05 Ma) and H14 complexes (11.95 ± 0.05 Ma), whereas previously published Re-Os ages on molybdenite are younger (H15: 11.73 ± 0.04 Ma; H14: 11.82 ± 0.04 Ma) than the youngest dated intrusion in those centers. Published K-Ar ages on hydrothermal biotite from the H79 center (13.0 ± 0.2 Ma), H14 (12.0 ± 0.2 Ma), and H13 (12.0 ± 0.2 Ma) indicate the individual system cooled rapidly. Overall the combined U-Pb geochronology dataset for the four porphyry intrusive centers suggests short-lived magmatic systems characterized each center. However, the available Re-Os ages on molybdenite suggest the potential for complexity in associated hydrothermal systems at the H15 and H14 porphyry Cu-Au centers.

Description: The Hitachi volcanogenic massive sulfide (VMS) deposit in the Hitachi belt is one of the largest Cu-Zn sulfide deposits in Japan. However, its primary formation age has not been precisely determined because regional and contact metamorphism have disturbed some radiometric isotope systems, e.g., K-Ar ages on wall rocks. Further metamorphism has greatly reduced the fossil record, which has limited the use of biostratigraphy. To establish the ore deposit age, we applied Re-Os isotope dating to samples from two representative sulfide deposits of the greater Hitachi VMS deposit: the Fudotaki and Fujimi deposits. Re-Os isotope data from the Fudotaki deposit yields a model 3 age of 533 ± 13 Ma, with an initial 187 Os/ 188 Os ratio of 0.35 ± 0.20. In contrast, Re-Os isotope data from the Fujimi sulfide ore did not exhibit a linear correlation, suggesting that the Re-Os isotope system there has been disturbed by regional metamorphism of higher grade than the Fudotaki deposit as well as contact metamorphism during intrusion of Cretaceous granitic rocks. Although the Fudotaki sulfide ores have been metamorphosed to lower epidote-amphibolite facies, the well-defined Re-Os isochron allowed us to interpret the timing of primary sulfide deposition on the paleoseafloor. Some orebodies of the greater Hitachi VMS deposit have been estimated to be Early Carboniferous based on the scarce fossil records and considered to be distributed in the Daioin Formation, but our results revealed that the Fudotaki deposit were formed during the Cambrian and occurs in the Upper Akazawa Formation, which strongly supports the idea that the Cambrian beds are widely distributed in the Hitachi belt and that a significant unconformity exists between the Cambrian and Early Carboniferous above the Fudotaki deposit. The Cambrian Re-Os date classifies the Hitachi VMS deposit as the oldest dated ore deposit in Japan, and in turn provides an insight to understand the geologic framework and genesis of the proto-Japanese islands and the East Asian region.

Description: Economic stratiform copper mineralization in the fine-grained, carbonaceous basal Nonesuch Formation of the ~50-km-wide White Pine-Presque Isle mining district, northern Michigan, has long been explained by the essentially vertical infiltration of cupriferous brine across the full width of basal Nonesuch strata from the underlying Copper Harbor Conglomerate aquifer, followed by copper precipitation mainly by reactions with in situ iron sulfide. Based on that model, why do lesser amounts of total copper, measured in vertical sections, occur in coarser grained (presumably more permeable) basal Nonesuch strata situated laterally to the east and west of the mining district? The answer may lie in increased infiltration rates for Copper Harbor-hosted ore-forming brine warmed locally by latent volcanic heat from a resurgent caldera (or calderas) located within the large Porcupine Volcanics dome underlying the mine district. Not only would caldera heat have warmed the brine and lowered its density, but more importantly it would have produced a remarkable decrease in the brine viscosity. An increase in temperature from 15° to 100°C could have increased the brine infiltration rate by more than four times in very early diagenetic time when the sediments were poorly compacted and highly porous. Because progressive compaction would have opposed the temperature effect, a very early diagenetic timing is suggested for main-stage copper mineralization over the Porcupine Volcanics caldera. Textural evidence has also long supported an early mineralization event. At the rift-basin scale, the exceptionally rapid upward flux of warmed brine into fine-grained Nonesuch over the caldera would have required equivalent volumes of replacement brine, which would in turn have required a lateral convergence of brine within the footwall Copper Harbor Conglomerate aquifer toward the White Pine-Presque Isle district. This lateral focus of brine toward the warm caldera area would have created a more efficient basin-scale ore-forming system than that resulting from a linear hydrogeologic flow. Most concepts described here deserve further evaluation through quantitative hydrogeologic modeling. Thermal effects on ore brine viscosities and densities, developed here for the sediment-hosted stratiform copper mineralization of the White Pine-Presque Isle district, could be considered in genetic models for other deposits and deposit types. The effects are especially strong for solutions warmed from low temperatures (e.g., 20°C or lower) to 100°C, and could also be important to ore fluid flow for ores involving much higher hydrothermal temperatures if solutions with initially low temperatures played a role in their genesis.

Description: In the Gangdese belt of southern Tibet, Paleocene-Eocene magmas record the final stage of Neo-Tethyan subduction and are associated with few economic porphyry deposits. In contrast, magmas formed during later stages of the India-Asia collision in the Oligo-Miocene are associated with several large porphyry Cu-Mo ± Au deposits, especially in the eastern part of the belt (~89°E–93°E). In a previous study, we showed that these Oligocene-Miocene magmas were more hydrous than the earlier Paleocene-Eocene magmas. In this study, we show that the later magmas were also more oxidized. Paleocene-Eocene rocks from the eastern Gangdese belt are characterized by low zircon Ce 4+ /Ce 3+ ratios (mostly 〈50; 6.0–66.8; average = 25.7 ± 18.4, n = 26) and low to moderate FMQ values (–1.2 to +0.8, average = –0.2 ± 0.8, n = 5; estimated from ilmenite-magnetite mineral pairs). In comparison, Miocene igneous rocks from the eastern Gangdese belt show higher zircon Ce 4+ /Ce 3+ ratios (mostly 〉50; 32.3–141.9, average = 74.3 ± 30.1, n = 33) and higher f O 2 values (FMQ 0.8–2.9, average = 1.8 ± 0.8, n = 6; estimated from magnetite-ilmenite pairs). Estimates of magmatic oxidation state from amphibole compositions also show an increase from FMQ 1.2 to 2.1 (average = 1.6 ± 0.2, n = 40) in the Paleocene-Eocene to 2.0 to 2.8 (average = 2.5 ± 0.2, n = 58) in the Miocene. Sparse whole-rock Fe 3+ /Fe 2+ ratios show the same general trend. The amphibole results are systematically shifted to higher FMQ compared to data from magnetite-ilmenite pairs, but their trend is internally consistent. The higher oxidation states (and water contents) of Miocene igneous rocks from the eastern Gangdese belt may explain their unique association with large porphyry deposits in Tibet, because it has been shown from other studies that the potential for hydrous calc-alkaline magmas to transport Au is maximized near FMQ 1.0, and Cu at higher f O 2 (FMQ 〉 1.0). In comparison, the somewhat less oxidized and less hydrous Paleocene- Eocene magmas would have been less fertile for the formation of such deposits. We suggest that these differences reflect the tectonomagmatic evolution of the Cenozoic Gangdese collisional orogen, from early collision-related magmatism in the Paleocene-Eocene to late collisional magmatism in the Miocene. Asthenospheric upwelling following slab breakoff in the Miocene caused interaction between mantle-derived magmas and previously subduction modified Tibetan lithosphere. The resultant evolved magmas had high oxidation states and water contents, which are favorable properties for the subsequent formation of magmatic-hydrothermal porphyry Cu-Mo ± Au deposits.

Description: The Xietongmen district is located 260 km west-southwest of Lhasa in the Tibet Autonomous Region, China. The district occurs within the Gangdese belt, which forms the eastern part of the Trans-Himalayan magmatic belt and is the product of complex magmatic activity that began during the Late Triassic or Early Jurassic and ended in the Eocene. The Xietongmen Cu-Au and Newtongmen Cu-Au-Mo deposits contain a total measured and indicated resource of approximately 610 million metric tons, with additional mineralization in the Langtongmen and Olitongmen Cu-Au prospects. Porphyry mineralization in the Xietongmen district formed during Middle Jurassic volcanic arc activity in the Lhasa terrane, prior to its accretion to the southern margin of Eurasia, and establishes that an economically important, but only recently recognized, metallogenic event is present in the region. Rock types in the Xietongmen district range from Early Jurassic to Eocene in age. Early Jurassic (~188-177 Ma) volcanic, volcaniclastic, and coeval intrusive rock types are crosscut by Middle Jurassic (176-171 Ma) hornblende diorite and quartz diorite porphyry dikes and stocks, including intrusions related to porphyry Cu-Au ± Mo mineralization. The Jurassic igneous assemblage was intruded by mafic dikes between the Late Jurassic and the Cretaceous, then by an Eocene (50-47 Ma) biotite granodiorite batholith and related dikes, and finally by, volumetrically minor lamprophyre dikes. The most important structures in the Xietongmen district are four E-striking, moderately N-dipping, sinistral-oblique thrust faults. Crosscutting and suturing relationships between the TSF-2 thrust fault, located in the south part of the district, and intrusions dated to between 174 and 180 Ma constrain the main stage of thrust fault activity to the Middle Jurassic. The Contact and Adit-1 thrust faults truncate the Xietongmen deposit and form the footwall and hanging wall to mineralization, respectively. Numerous zones of cataclasis deform the Xietongmen deposit between these bounding thrust surfaces. The strongly deformed Langtongmen Cu-Au prospect is located ~1.3 km west of the Xietongmen deposit and occurs in the immediate hanging wall of the Adit-1 thrust fault. The Newtongmen deposit and the Olitongmen Cu-Au prospect occur to the north in the hanging wall to the SBF thrust fault and are not strongly deformed. Mineralization and hydrothermal features in the Xietongmen district are fully compatible with porphyry Cu-Au ± Mo deposits. Alteration, vein types, and mineralization are zoned around quartz diorite porphyry intrusions. Early K silicate alteration and related veins occur within and proximal to the intrusions and contain the highest grade mineralization. In the Xietongmen deposit, the grade of mineralization decreases outward from a core of early biotite-rich K silicate alteration, through a transitional zone in which early K silicate alteration is partially overprinted by quartz-sericite-pyrite alteration, to a peripheral zone of poorly mineralized quartz-sericite-pyrite ± pyrrhotite alteration. Incipient sodic alteration occurs as albite alteration envelopes to quartz-sulfide veinlets in the deepest part of the deposit. Late polymetallic veins and veinlets contain sphalerite, galena, and other base metal sulfides and sulfosalts, occur throughout the Xietongmen deposit, and reflect telescoping during late-stage collapse of the hydrothermal system. Partially developed supergene mineralization forms less than 10% of the Xietongmen deposit. Underlying hypogene mineralization comprises ubiquitous pyrite, chalcopyrite, lesser and more erratically distributed pyrrhotite, and rare molybdenite. The characteristics of the Langtongmen prospect are identical to those found in the deeper parts of the Xietongmen deposit. Characteristics of the Newtongmen deposit are generally similar to those in the Xietongmen deposit but Newtongmen contains only minor supergene mineralization, is cut by very few late polymetallic veinlets, and contains zones of strong, weakly mineralized sodic alteration related to a relatively later stage of quartz diorite porphyry intrusion. The Olitongmen prospect has characteristics similar to the Newtongmen deposit. The Xietongmen deposit and the Olitongmen prospect were thermally recrystallized in a hornfels aureole to the Eocene biotite granodiorite batholith, whereas thermal effects are minor to absent at Langtongmen and Newtongmen. Copper and gold (Au/Cu; ppm / % ) are closely correlated within each of the two main deposits and ratios range from 1.5 to 1.2 in the Xietongmen deposit to between 0.8 and 0.6 in the Newtongmen deposit. Mineralization in the Xietongmen district formed in several coeval mineralized centers and the vein types, alteration and metal assemblages among these centers span a continuum in hydrothermal characteristics. The differences between the mineralized zones are interpreted to reflect exposure at different relative paleodepths as a result of displacement and deformation by posthydrothermal, sinistral-oblique movement on thrust faults. The Xietongmen deposit was transposed to the south by deformation on and between the bounding Adit-1 and Contact thrust faults. The Langtongmen deposit was separated from the deep hanging wall of the Xietongmen deposit and displaced approximately 600 m vertically and 1,300 m to the west by the Adit-1 thrust fault. The Newtongmen deposit and the Olitongmen prospect were uplifted relative to Xietongmen and Langtongmen in the hanging wall to the SBF thrust fault and were not significantly deformed. The genesis and relationship between porphyry deposits in the Xietongmen district can be reconciled by the combined effects of vertical and lateral displacement by thrust faults, preservation of the deposits at different relative paleodepths, and varying degrees of posthydrothermal mechanical and thermal recrystallization.

Description: The Enterprise nickel deposit (40 Mt of 1.07% Ni) is located on the eastern edge of the Kabompo dome in the North-Western Province of Zambia. The deposit area contains basement schists overlain by Neoproterozoic metasedimentary rocks. Nickel sulfides are hosted within a sequence of quartz-, carbonate-, and carbon-rich metasedimentary rocks that interfinger with and overlie siliciclastic metasedimentary rocks. The host rocks contain significant kyanite, talc, and magnesian chlorite. Silicification and magnesian metasomatism occurred prior to or concurrent with a regional metamorphic event (590–500 Ma). Mineralization resulted in the precipitation of nickel and iron-nickel sulfides in veins and as semimassive replacements of the host rocks. Nickel sulfides precipitated in two main stages: a millerite-vaesite-pyrite assemblage that formed disseminations and semimassive replacements in vuggy textured rocks, and a later millerite-bravoite-(molybdenite) assemblage in quartz-kyanite veins and local semimassive replacements. The deposit contains minor copper and trace amounts of cobalt and platinum group elements (PGEs). A discrete zone of copper sulfides underlies a portion of the nickel sulfide zone. Re-Os geochronology on molybdenite yields a 540.6 ± 1.8 Ma age for mineralization at Enterprise, the approximate age of metamorphism. Sulfur isotope results indicate that the sulfur at Enterprise was derived largely from Neoproterozoic marine sulfate by thermochemical sulfate reduction. Significant volumes of mafic igneous rocks are not present in the immediate area of the Enterprise deposit. No evidence of prealteration concentrations of nickel exists within the sedimentary rock sequence at the deposit. The alteration and mineralization style of the Enterprise deposit is similar to the much less metamorphosed nickel-bearing Shinkolobwe uranium deposit in the Democratic Republic of Congo (DRC), though the Enterprise deposit does not contain significant uranium. The sediment-hosted nickel-rich deposits of Central African Copperbelt exemplified by Enterprise appear to represent a new style of hydrothermal nickel mineralization.

Description: Textural and compositional data for magnetite from nine iron skarn deposits in Canada, Romania, and China show that most samples have reequilibrated by dissolution and reprecipitation, oxy-exsolution, and/or recrystallization. The dissolution and reprecipitation processes are most extensive and are present in most magnetite samples examined, whereas the oxy-exsolution occurs only in high-Ti magnetite, forming exsolution lamellae of Fe-Ti-Al oxides. Electron microprobe analysis indicates that the reequilibration processes have significantly modified the minor and trace element compositions of magnetite, notably Si, Mg, Ca, Al, Mn, and Ti, whereas oxy-exsolution is effective in decreasing the Ti content of high-Ti magnetite. Many analyses of magnetite grains from the skarn deposits plot variably in the banded iron formations (BIF), iron oxide–copper-gold (IOCG), or porphyry Cu fields using the Ti + V versus Ca + Al + Mn discrimination diagram. This pattern suggests that trace element data for magnetite that has unusual composition and/or reequilibrated cannot be reliably used as a petrogenetic indicator. Mixing of externally derived saline fluids with Fe-rich magmatic-hydrothermal solutions, an increase in temperature, and local decreasing pressure and f O 2 are considered the most important causes for the dissolution and reprecipitation, or recrystallization, of the magnetite; increasing f O 2 and decreasing temperature may facilitate oxy-exsolution of Fe-Ti-Al oxides in high-Ti magnetite. Results presented here highlight the importance of detailed textural characterization prior to in situ chemical analysis of magnetite grains so that mineral compositions can be properly evaluated in terms of the genesis and evolution of iron skarn deposits.

Description: Strata-bound, nonstratiform, epigenetic galena-sphalerite-cement mineralization in Ediacaran-Cambrian sandstone, including the previously mined deposits at Laisvall and Vassbo, occurs along the eastern erosional front of the Caledonian orogen in Sweden. The sandstone is part of an autochthonous siliciclastic sedimentary sequence that rests unconformably on top of Proterozoic crystalline basement beneath the Caledonian thrust nappes. Linear anomalies have been identified in high-resolution airborne magnetic data that correspond to geologic features in the Proterozoic basement. Furthermore, the Laisvall and Vassbo strata-bound Pb-Zn deposits are both spatially associated with areas of change in the trend of the magnetic lineaments. Magnetic anomalies, trending either N-S to NE-SW and WNW-ESE to NW-SE in the Laisvall area, and NNE-SSW to NNW-SSE and NW-SE to W-E in the Vassbo area, were identified. In the Laisvall area, some magnetic minima and edges along magnetic gradients can be correlated with faults in the Proterozoic basement. The reactivation of these basement structures is expressed in the Ediacaran-Cambrian sedimentary cover rocks as newly formed faults with Phanerozoic displacement. Along individual faults belonging to two sets (NE-SW to N-S and WNW-ESE to NW-SE), synsedimentary block movement has been recognized. The highest Pb and Zn grades in Laisvall delineate orebodies and orebody trends that follow these faults. Areas where the faults change strike contain some of the largest and richest orebodies. In the Vassbo area, the orebody footprint reflects a folded dolerite dike in the underlying Proterozoic basement. The dike, modeled on the basis of borehole data, is recognized by a magnetic maximum and an edge along a magnetic gradient. No faults have been mapped at the ground surface as being related to the location of dolerite dikes in the basement. However, it is considered that the basement dikes illustrate a structural control, emplacement either producing a local fracture network or being driven by preexisting basement structures. The main orebodies in both deposits display funnel-shape geometry, fault-rooted in Laisvall and located close to the hinges of the folded dolerite dike in the basement at Vassbo. Metal distribution patterns are similar in both deposits and are characterized by Pb-rich cores proximal to the basement-steered structures while Zn-rich shells are distal from these structures. The funnel-shaped ore geometry is interpreted to reflect a fault-rooted migration path and the metal precipitation mechanism. In both deposits, the highest Pb and Zn grades occur at the top of sandstone paleoaquifers. Similar mineralization footprints, variation in grades, and paleoaquifer settings were recognized in several carbonate-hosted Mississippi Valley-type (MVT) Zn-Pb deposits (e.g., San Vicente deposit, Peru; Topla-Mežica deposits, Slovenia). This geometry is suggestive of a sour gas trap that accumulated by density at the top of paleoaquifers. This gas could have provided H 2 S by thermogenic sulfate reduction to the metal-bearing fluids and triggered precipitation of Pb-Zn sulfides. The combined evidence from the airborne magnetic data, the structural analysis and the geometry of the orebodies, and metal distribution suggests that the basement faults reactivated during the Ediacaran-Cambrian sedimentation, acted at a later time as feeders for the metal-bearing fluids to fertile horizons for mineralization, and localized deformation during postsedimentary and postmineralization tectonics.

Description: Despite the numerous industrial and scientific applications of gallium, its behavior in nature and the processes that concentrate it to potentially economic levels are poorly understood. Although the main supply of this metal is as a by-product of the mining of bauxite, it is also concentrated by magmatic-hydrothermal processes in peralkaline igneous systems. Here we report the results of a study of the distribution of gallium and the controls on this distribution in the Nechalacho rare metal deposit, Northwest Territories, Canada, which has been shown to contain significant reserves of this critical metal. Electron microprobe analyses and X-ray element maps of gallium-bearing minerals were used to determine the mineralogical distribution of gallium in the Nechalacho intrusive suite. Elevated gallium concentrations were identified in albite, biotite, orthoclase, chlorite, and allanite. Of these aluminum-bearing minerals, the most important hosts of gallium are albite, biotite, and orthoclase. Ferric iron-bearing minerals, including magnetite and aegirine, which were considered potential candidates for gallium sequestration, contain relatively low concentrations of the metal. This behavior of gallium, at least from a magmatic perspective, is consistent with its predicted partitioning between phenocrysts and melt. However, there is also evidence that gallium was redistributed by hydrothermal fluids. Chloritization of biotite resulted in the enrichment of gallium in the secondary mineral (chlorite), and the development of secondary albite (albitization) led to a depletion of gallium in primary albite. On the basis of these results, we argue that the overall distribution of gallium within the Nechalacho deposit was controlled by magmatic crystal fractionation, whereas hydrothermal processes led to local remobilization of the metal. During fractional crystallization, gallium was moderately compatible in minerals such as albite and biotite, whereas it bordered between compatibility and incompatibility in minerals such as magnetite and orthoclase, and was incompatible in aegirine. This resulted in a relatively constant bulk gallium concentration in the Nechalacho deposit, although locally, gallium was remobilized hydrothermally, particularly within the most altered parts of the intrusion, notably, the albitite.

Description: The Shetland ophiolite, located to the northeast of the Scottish mainland, contains podiform chromitites. Two of these are known to host extremely anomalous PGE concentrations. The chromitite at Cliff is enriched in Pt, Pd, and Rh PGE (PPGE) and the Harold’s Grave chromitite is enriched in Ir, Ru, and Os PGE (IPGE). Analysis of multiple samples of chromitite from chromite quarry spoil tips and surrounding chromite-bearing dunite and chromite-bearing harzburgite has revealed that there are links between the chromite geochemistry and the PGE. Both Cliff and Harold’s Grave have a characteristic chromite geochemistry, which distinguishes them from the other PGE-poor chromitites. The IPGE-rich Harold’s Grave chromite contains elevated trace elements (TiO 2 , V 2 O 5 , and Zn), lower Mg#, and a lower Fe 3+ /Fe 2+ ratio than in the other chromitite deposits. This suggests formation from a more reduced magma that favors the concentration of IPGE in chromitite. It is proposed that the IPGE have been concentrated to such enriched and consistent concentrations due to large throughput of magma, leaving an unusually large dunite envelope that surrounds a rather small concentration of chromitite. The geochemistry of the Harold’s Grave deposit suggests formation from a mid-ocean ridge basalt (MORB)/island-arc tholeiite-type magma. There is a range of chromite compositions at Cliff that suggests formation of the chromitite from a variety of mantle melts from mantle of variable composition. PPGE-rich Cliff chromite has a lower Cr# combined with a higher TiO 2 than the other chromite in chromitites at Cliff. It is suggested that a more island-arc tholeiite-type magma formed these PPGE-rich chromitites compared to a more boninitic magma that formed the other chromitites at Cliff. This island-arc tholeiite-type magma was closer to sulfur saturation than the boninitic magma and reached sulfur saturation, producing the very PPGE-rich chromite at Cliff.

Description: The Dahu Au-Mo deposit is located in the Xiaoqinling region and belongs to the Qinling-Dabie orogen of central China. The quartz vein-style Dahu Au-Mo deposit (31 t Au, average grade: 4.7 g/t; 30,000 t Mo, average grade: 0.13 %) formed in four stages, i.e., an early quartz-K-feldspar stage (I), a pyrite-molybdenite stage (II), a sulfide-telluride-sulfosalt-gold stage (III), and a late carbonate-barite stage (IV). Abundant tellurides and sulfosalts occur in stage III: altaite, tellurobismuthite, buckhornite, hessite, petzite, calaverite, aikinite, kupcikite, lindstromite, salzburgite, wittichenite, and tetradymite. The system is characterized by carbono-aqueous fluids of low to moderate salinity and high oxygen fugacity. Total homogenization temperatures of the H 2 O-CO 2 fluid inclusions in stage I and stage III quartz range from 230° to 440°C and 198° to 320°C, respectively. The variation in CO 2 /H 2 O ratios and microthermometric data of the H 2 O-CO 2 fluid inclusions results from a combination of fluid unmixing and sequential trapping of fluid inclusions, but also from postentrapment reequilibration. Abundant minerals are present as solid phases in H 2 O-CO 2 fluid inclusions in stage I quartz: Cu 1.65 S, covellite, chalcopyrite, bornite, molybdenite, pyrite, colusite, anhydrite, and celestine. 18 O quartz values of stage I quartz range from 10.2 to 12.0, with calculated 18 O fluid values ranging from 5.6 to 7.4. The D values of fluid inclusion waters (thermal decrepitation) in stage I quartz range from –117 to –54. The low and scattered D values reflect a mixture of ore-forming fluids trapped by the H 2 O-CO 2 fluid inclusions at deep crustal levels and postmineralization fluids, possibly meteoric waters, trapped by secondary aqueous fluid inclusions at higher crustal levels during uplift of the Xiaoqinling core complex. Four molybdenite samples yield an Re-Os isochron age of 206.4 ± 3.9 Ma (MSWD = 0.88), suggesting ore formation in the Late Triassic. Metal signature, mineral association, fluid characteristics, and isotope data of the Dahu deposit, together with the tectonomagmatic setting, imply that the ore-forming fluids were derived from an unexposed and oxidized magmatic system, which likely stems from partial melting of enriched lithospheric mantle beneath the North China craton in a postcollisional extensional setting.

Description: Diamond grade distributions within individual kimberlite bodies are controlled by physical emplacement processes, which are generally determined from volcaniclastic facies characteristics. However, the effects and distribution of secondary alteration styles, which usually mask primary kimberlite facies, have been given little attention. At the Fort à la Corne kimberlite field, Saskatchewan, complex overprinting alteration textures and mineral phases are recognized. This study documents the characteristics, overprinting relationships, and spatial distribution of alteration styles within a crater-filling volcaniclastic deposit, Body 219 of the Orion Central volcanic complex. Data were acquired from macroscopic drill core observations, microscopic petrography, SEM imaging, EDS element mapping, and XRD analyses. Olivine crystals display a mesh fracture pattern and have been replaced by multiple generations of serpentine and magnetite. Late-stage serpentine has migrated into the interstitial medium. Bulk-rock alteration textures include (1) fine-grained crystal-rich domains, which are alteration pseudoclasts and/or relict juvenile pyroclasts; (2) a heterogeneous interstitial medium, varying from early-stage disseminated calcite-rich serpentine to amorphous serpentine and crystalline calcite; and (3) overprinting serpentine, carbonate and magnetite veins, and serpentine nodules. Alteration styles defined by unique alteration characteristics are spatially zoned through the Body 219 volcaniclastic deposit, and this is a result of multiple reaction fronts associated with the influx of fluids of varying composition over time and permeating different parts of the deposit. Recognition of alteration processes, their paragenesis and spatial controls can assist in defining and constraining the distribution of primary kimberlite facies, and by consequence the distribution of diamond grades.

Description: Southern Peru contains several small- and medium-sized epithermal Au-Ag (± base metals) deposits related to Miocene-Pliocene metallogenetic belts. Specifically, the characterization of the geology and mapping of hydrothermal alteration zones of the Chapi Chiara prospect, Canahuire deposit, and Cerro Millo and San Antonio de Esquilache targets were done with data acquired by Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) spaceborne sensor. ASTER mapping techniques included (1) band ratio and principal component analysis (Crósta technique) applied to the visible and near-infrared and short-wave infrared bands to produce a regional hydrothermal alteration map (alunite and/or kaolinite, illite-muscovite and/or smectite, iron-bearing minerals) and (2) spectral indices and selective principal component analysis applied to the thermal infrared bands to detect quartz- and carbonates-bearing targets, respectively. These methods were used to establish a hydrothermal zoning pattern in paleostratovolcanoes, where the Chapi Chiara, Cerro Millo, and San Antonio de Esquilache targets are located. This zoning pattern was used to infer erosion conditions and the potential for metal deposits based on the mineralogy, which was also analyzed using reflectance spectroscopy and petrography. In addition, ASTER data were used to characterize the carbonate host rocks, the quartz-bearing units of the Yura Group, and the quartz-poor unit associated with the phreatic and phreatomagmatic breccias in the Canahuire deposit region. This characterization led to the development of a favorability model for the occurrence of "Canahuire-type" deposits based on spatial analysis using the fuzzy logic technique.

Description: Porphyry Cu (±Mo ±Au) and epithermal Au-Ag deposits are major sources of mined metals and are commonly formed by magmatic-hydrothermal fluids derived from hydrous magmas in Phanerozoic convergent margin settings. The igneous rock assemblages associated with porphyry mineral deposits are common in modern convergent margin settings, but while many have produced acidic magmatic fluids, very few, past or present, have produced sufficient metal, chlorine, and sulfur enrichments necessary to engender an ore deposit. The reasons for this remain uncertain. We report SHRIMP-RG ion microprobe analyses of hafnium, titanium and rare earth element (REE) abundances in zircon, a nearly ubiquitous and robust trace mineral in crustal magmas. Comparison of the compositions of zircons in ore-forming and barren granitic plutons indicate that ore-forming granites crystallized at relatively low temperature and have relatively small negative europium anomalies (mostly Eu N /Eu N * ≥0.4). We interpret these small zircon europium anomalies to indicate oxidizing magmatic conditions and hypothesize that in many cases this reflects oxidation due to SO 2 degassing from magmas with a relatively low Fe/S ratio. Oxidation of europium and iron in the melt is produced by reduction of magmatic sulfate (S 6+ ) to SO 2 (S 4+ ) upon degassing. This interpretation reinforces the important role of oxidized sulfur-rich fluids in porphyry and epithermal mineral deposit formation. Zircon compositions thus may be used to identify ancient magmas that released significant amounts of SO 2 -rich gases, and regional surveys of zircon composition are potentially a valuable tool for mineral exploration.

Description: The Beaverlodge area in Saskatchewan, Canada, hosts numerous fault-controlled uranium deposits that are geochemically and structurally complex because of multiple deformation events. Field, petrographic, and geochemical data indicate there are six distinct styles of uraninite mineralization, with five temporally distinct mineralization events. The earliest two styles of mineralization are hosted in cataclasite and veins that formed at ca. 2.29 Ga from 300°C fluids derived from retrograde metamorphic processes during late stages of the early Paleoproterozoic Arrowsmith orogen. The second style of mineralization is restricted to the albitized granite of the Gunnar deposit and formed between ca. 2.3 and 1.9 Ga from reducing magmatic-hydrothermal fluids near 300°C. However, uranium associated with this metasomatic style of mineralization is overprinted by the more significant breccia-vein and minor volcanic-type mineralization, thus the Gunnar deposit contains three styles of mineralization. Reactivation of major fault zones resulted in breccia-type mineralization, the dominant mineralizing event, at ca. 1.85 Ga, from Ca-Na-F-rich fluids at about 330°C during the postpeak Thelon-Taltson orogen and early stages of the Trans-Hudson orogen. Inversion followed by subsidence resulted in formation of the Martin Lake basin, a successor basin filled with ca. 5 km of arkose, conglomerates, siltstone, and alkaline mafic flows and sills that formed during back-arc extension following peak Trans-Hudson orogeny. Volcanic-style mineralization at 1.82 Ga. resulted from late exsolution of magmatic fluids from the alkaline mafic dikes. Overlying the Martin Lake basin are outliers of the Athabasca Formation associated with the last stage of uranium mineralization, at ca. 1.62 Ga, from oxidizing basinal brines near 230°C. Subsequent erosion of the Athabasca and Martin Lake basin rocks and weathering of the deposits resulted in secondary uranium minerals and late veins. Multiple stages of deformation and fluid overprinting suggest that uranium mineralization record progressively higher levels in the crust. Each successive event overprinted to varying degrees the previous events. Although the Beaverlodge area represents several distinct mineralizing events, the major event is the breccia-type that occurred at 1.85 Ga. The other styles of mineralization are minor, and their presence complicates both exploration and our understanding of ore deposit genesis in the area.

Description: Porphyry-related base metal vein and replacement mineralization (i.e., Cordilleran polymetallic mineralization) in the Morococha district, central Peru, is part of a large magmatic-hydrothermal system associated with the emplacement of several late Miocene porphyry intrusions and formation of important Cu-Mo mineralization. Zn-Pb-Ag-Cu veins overprint the giant Toromocho porphyry Cu-Mo deposit in the center of the district and display a typical concentric base metal zonation (Cu -〉 Zn, Pb -〉 Ag) covering an area of approximately 50km 2 . A detailed fluid inclusion study supports the hypothesis that base metal mineralization precipitated from cooled and evolved metal-rich, intermediate-density porphyry-type fluids. In early stages of Cordilleran base metal vein formation, fluid inclusions have low salinities of ~2 to 5 wt % NaCl equiv, CO 2 contents of 3 to 10mol %, and homogenization temperatures (T h ) of 380° to 340°C. They are similar to intermediate-density fluid inclusions trapped in a milky quartz vein predating Cordilleran polymetallic mineralization, with similar low salinities (3.0–3.8 wt % NaCl equiv) and low CO 2 contents (6.5–8 mol %), but higher T h of ~420° to 410°C. During cooling of the intermediate-density fluids from 400° to 300°C, the lithostatic pressure regime changed to a hydrostatic one. The fluids underwent pressure drop as well as phase separation (i.e., unmixing) and lost most of their CO 2 . They acquired moderate salinities, in some cases intermediate (~up to 16 wt % NaCl) to brine compositions. However, the bulk of the magmatic fluid retained low salinity while it continued to cool under open-system conditions and precipitated tennantite-tetrahedrite, chalcopyrite, enargite, sphalerite, and galena. Upon cooling below 270°C, the fluids deposited abundant rhodochrosite and quartz, while following the boiling curve toward lower P-T conditions. These data record an evolution of mineral precipitation from deep (minimum depth of 2–1.5 km) to shallow environments (300–800 m). Oxygen, hydrogen, and carbon stable isotope data indicate that the hydrothermal fluids have a dominantly magmatic signature and were diluted by meteoric waters during the carbonate stage. Copper (5,000–18,000 μ g/g), sulfur (up to 12,000 μ g/g), and iron (2,100–6,000 μ g/g) concentrations in the intermediate-density fluid inclusions in the milky quartz veins are approximately 5 to 10 times higher than in intermediate-density inclusions of the early Cordilleran base metal veins. The base metals Zn, Pb, and Mn have comparable concentrations between 100 and 1,000 μ g/g for both types of fluid. These findings suggest that the fluids identified in Cordilleran polymetallic veins are compositionally similar to the porphyry-type fluids and could have derived from the latter after precipitation of Cu- and Fe-bearing sulfides in a deeper porphyry environment. The new data explain the commonly observed base and precious metal zonation patterns encountered in porphyry-centered districts (e.g., Bingham, Butte) and show that both porphyry and polymetallic mineralization can precipitate from similar magmatic-hydrothermal fluid pulses.

Description: Bonanza-style mineralization of palladiferous gold in specular hematite-rich lodes engendered the most important gold rush in Brazilian history, which took place at the end of the seventeenth century in the province that became known as Minas Gerais. The timing of this itabirite-hosted vein mineralization, locally referred to as jacutinga, is controversial; different views have ascribed jacutinga to one of the two orogenic events recorded in the Quadrilátero Ferrífero of Minas Gerais, the ~2.0 Ga Transamazonian event and the ~0.6 Ga Brasiliano event. This controversy is due mainly to the absence of reliable age data for hydrothermal minerals. Here we report the first U-Pb age for a jacutinga lode, obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry (LA-SF-ICP-MS) on monazite grains recovered from the Conceição Fe-ore deposit within the Itabira district. The monazite analyses yield a precise Late Cambrian age of 495.6 ± 2.2 Ma, which is well within the time period between ca. 524 and 444 Ma, suggested by recently published geochronologic data on regionally distributed similar hydrothermal systems. The age of 496 Ma for the jacutinga monazite corroborates an orogen-scale hydrothermal overprint at later stages of the Brasiliano event. This hydrothermal overprint also affected surrounding Neoarchean rocks of the Minas Supergroup, as well as younger rocks of the southern Serra do Espinhaço, along the platiniferous Au-Pd belt of Minas Gerais.

Description: Previous studies attributed a supergene origin to sphalerite rimming copper minerals occurring in Chuquicamata in a mineral assemblage including typically chalcocite-digenite (Cu 1.85 S 1.12 and Cu 1.99 S 1.01 ), covellite (CuS and Cu 1.08 S 0.92 ), and sphalerite (up to 1.2 wt % Fe). Microscopic observations on samples from a central and a southern section, completed by scanning electronic microscope (SEM) backscattering electron imaging and electron microprobe analyses, suggest that all sphalerite in Chuquicamata is hypogene. A scenario, backed by observations in each step, that explains the formation of the peculiar "sphalerite rims" is the following: (1) precipitation of chalcopyrite, typical of the early and main hydrothermal stages; (2) precipitation of sphalerite rimming chalcopyrite and in voids and in weakness sites during the late hydrothermal stage; (3) partial or total replacement of chalcopyrite by chalcocite-digenite during the late hydrothermal stage; and (4) formation of lamellar covellite, principally at the expense of chalcopyrite, suggesting increasingly oxidizing and/or acidic conditions. This covellite may be linked to supergene processes and/or to the late hydrothermal stage. The findings of the present work have implications for the position of the lower limit of the supergene enrichment in the eastern-southern part of the deposit, as the sphalerite rims, now interpreted as hypogene, were the only potential supergene sulfide in the eastern vertical section of the southern section.

Description: Conventional diamond exploration seldom involves searching for diamonds in rock and soil samples; rather, it focuses on the search for "indicator minerals." Kimberlite indicator minerals include garnet, olivine, chromite, pyroxene, and ilmenite, and these can be used to infer the presence of kimberlites and lamproites in the vicinity of where the samples were collected. Ilmenite has served as an effective indicator mineral for more than 40 years due to its resistance to chemical and physical weathering. As a result of its relatively high density compared to other indicator minerals, ilmenite grains often accumulate in placer deposits downstream from a kimberlite source. Although the ideal formula for ilmenite is FeTiO 3 , the crystal structure is also favorable to cation substitution owing to similarities in ionic radii and charge between Ti and Fe and other trace elements associated with its formation. We have investigated ilmenite trace element chemical signatures that can be related to the presence of diamond-bearing or diamond-free kimberlites. Our results suggest that the diamond potential of kimberlites is best reflected in the Zr/Nb ratio of ilmenite—these elements substitute for Ti in the ilmenite structure. An extensive compilation of compositions of ilmenite collected from heavy-mineral placers and from 14 kimberlites in northern Siberia (Yakutia) indicates that diamond pipes that have economically favorable diamond grades and abundances are associated with ilmenites having a Zr/Nb ratio of 〉0.37. Because of this, we suggest that ilmenite trace-element chemistry can be a useful tool to identify high-priority targets for diamond potential on the Siberian craton, and reconnaissance studies of other areas suggest that this relationship may be universally applicable.

Description: The UG-2 chromitite of the Bushveld Complex in South Africa contains the world’s largest resources of platinum group elements (PGEs). However, only limited work has been conducted on the fine-scale chemical variation of chromite, and the distribution of PGEs and platinum group minerals (PGMs) within the UG-2. In the present study, one drill core (155 cm long) covering the sequence of the UG-2 chromitite at the Karee mine in the western Bushveld Complex was studied in detail. Geochemically, Pt correlates well with Pd, but not with Ni, Cu, and S, and Pt and Pd show high concentrations at the bottom and in the center of the main UG-2 layer. The mineral chemistry of chromite is characterized by distinct cryptic variation, leading to the division of the UG-2 sequence into 10 distinct sublayers. From bottom to top (with exception of the basal sublayer), each sublayer is defined by an upward decrease of Mg# combined with increases of Cr# and TiO 2 . It is proposed that these trends reflect magmatic differentiation of individual chromitite entities, which coalesced to form a massive chromitite seam. Sulfides (pentlandite, chalcopyrite) interstitial to chromite grains are rare, and associated PGM grains are mainly Pt-Fe alloy, laurite, and cooperite-braggite. The PGEs are bimodally distributed in the UG-2. Platinum and the IPGE (Os, Ir, Ru) are dominantly present as discrete PGMs, whereas large proportions of Pd and Rh are hosted in the crystal lattice of pentlandite. Palladium and rhodium contents of pentlandite reach a maxima of 2.2 wt % Pd and 3.0 wt % Rh. The present data suggest that a succession of chromitite sublayers formed due to magmatic differentiation and segregated sequentially on top of each other, finally forming a massive chromitite seam. Contemporaneously with chromite segregation, PGE-rich magmatic sulfides segregated with chromite grains, but large proportions of these sulfides were subsequently removed mainly by reaction of sulfide with chromite, thereby upgrading the PGE contents of the remaining sulfides. Upon cooling, the PGE-bearing sulfide droplets recrystallized, leading to a bimodal distribution of the PGEs in the UG-2. The bulk of the Pd and Rh is hosted in pentlandite, whereas Pt and the IPGEs, apparently incompatible in the remaining sulfides, formed a distinct suite of discrete PGMs.

Description: Lorraine is a copper-gold porphyry deposit (6.4 Mt indicated resource at 0.61% copper and 0.23 g/t gold, and 28.8 Mt inferred resource at 0.45% copper and 0.19 g/t gold) in north-central British Columbia—one of the youngest known alkalic porphyry deposits within the Triassic-Jurassic Quesnel arc. The deposit is hosted by the Early Jurassic Duckling Creek Syenite Complex, an alkalic multiphase intrusive dike complex of pyroxenite, syenite, and monzonite composition. The southern part of the complex hosts numerous copper-gold porphyry prospects in addition to Lorraine; together, they constitute the Duckling Creek district. Main-stage Cu-Au porphyry mineralization at Lorraine is hosted by phase 1 of the Duckling Creek Syenite Complex, an SW-dipping, early intrusive suite of silica-undersaturated pyroxenites and syenites. Mineralized zones are up to 2.5 km long by 500 m wide, in the form of northwesterly elongate, SW-dipping panels that generally parallel the phase 1 intrusive contacts. Mineralization is attributed to "blind" porphyries within the phase 1 sequence, which was tilted after main-stage mineralization. Copper sulfides define deposit-scale zonation patterns of bornite-chalcocite cores rimmed by chalcopyrite-rich and outer, weakly developed pyrite-dominated domains. Alteration patterns and intensity are consistent across mineralized zones. Strong, pervasive K-feldspar-biotite (potassic) core zones are rimmed by albite-diopside-magnetite (calc-potassic) alteration that is pervasive near the center of the zones, but varies laterally outward to vein-style alteration. Pervasive albite alteration (sodic) is locally present and is interpreted to be an early alteration event. Mineralization at Lorraine has been cut by later phases of the Duckling Creek Syenite Complex. A second cycle of pyroxenite-syenite-monzonite intrusion (phase 2) is mineralogically similar to phase 1, but is unmineralized and has vertical intrusive contacts that strike northwest. A change to late, silica-saturated intrusions is marked by subvertical phase 3 silica-saturated leucosyenite and pegmatite dikes. U-Pb zircon and Ar-Ar geochronology constrain the timing of main-stage mineralization at Lorraine to an approximately 1.5 m.y. interval between 180 Ma and 178.5 Ma. Phase 1 intrusions are as old as 181.68 ± 0.95 Ma based on an Ar-Ar age on biotite from pyroxenite, but may range to as young as 180.2 ± 0.28 Ma based on a U-Pb zircon age from a mineralized phase 1 syenite. The upper age limit for mineralization is 178.75 ± 0.21 Ma, based on U-Pb data for a postmineral phase 2 syenite. An alternate interpretation considers the presence of zircon antecrysts in an analyzed phase 1 rock, which leads to a more narrow time window of 0.31 m.y. that includes phase 1 intrusion, main-stage porphyry mineralization, and structural tilting, which was followed by phase 2 intrusion. Magmatic and hydrothermal activity in the Lorraine area persisted until at least 176 Ma, based on the U-Pb zircon age of 176.77 ± 0.36 Ma from a phase 3 leucosyenite dike, and a late biotite-albite-apatite vein with an Ar-Ar biotite age of 175.21 ± 0.91 Ma. Several intrusive-hosted porphyry prospects within the Duckling Creek district share mineralogical and alteration characteristics, as well as relative igneous paragenesis, with Lorraine. The model for magmatic and hydrothermal development at Lorraine may broadly be applied to the district as a whole, providing a geologic framework upon which to base future exploration in the district.

Description: Porphyry Cu-Au-Mo deposits are characterized by numerous fracturing events. Brittle fracture of mineralized intrusions and the surrounding host rocks is a consequence of hydraulic breakage caused by the interplay of the changing physiochemical properties of magmatic fluids and/or the local tectonic regime. We have mapped more than 1,400 quartz-sulfide vein sets at the Ordovician Endeavour 26 North (E26) deposit, central New South Wales, in order to constrain the processes influencing fracture development around small (~200 m) porphyritic intrusions. Veins from intermediate levels in the E26 deposit define complicated patterns within 10 m of the porphyritic intrusions, whereas more systematic patterns occur up to 50 m from the intrusive contact. The outer parts of the ore deposit are characterized by quartz veins with orientations of ~340° and subordinate orthogonal sets of bisecting veins at ~240° and ~290°. The data show a broad range of vein orientations that define a conical distribution around a vertical axis. There is a rare subset of near-horizontal veins that formed early relative to the main stage of ore-bearing vein emplacement. Many quartz veins exhibit fault displacements (at both micro- and macroscopic scales) and fabrics (including serrate fractures, slickenlines, and asymmetric fiber trails). The distribution of veins suggests that the dominant fracture array developed perpendicular to the direction of bulk finite extension, but significant vein dilation also occurred in orientations with a component of shear strain. We propose a physical model that combines intrusion-driven hydraulic fracturing into a preexisting fracture mesh to help explain the observed vein patterns and to better link fracture development and vein emplacement. Hydraulically driven deformation was important locally at E26, contributing to the formation of the irregular fracture stockwork immediately adjacent to the small intrusive bodies. The more predictable vein patterns that occur away from the porphyritic intrusions require part of the hydraulically driven extension to have reactivated the existing fracture mesh. Parts of this more systematic geometry, including the occurrence of flat veins, are probably related to the emplacement of larger (10-km diameter) magma bodies adjacent to E26, with doming causing the subordinate fracture arrays, including the gross conical distribution. Fluctuating fluid pressures, combined with hydraulically driven extension, caused dilation of preexisting and synmineralization fractures and superposition of veins with similar orientations. Vein patterns suggest that the regional stress state that existed prior to or synchronously with porphyry ore deposit formation strongly influenced vein orientation to within tens of meters away from the mineralizing intrusions. Fractures that localized ore in the E26 porphyry deposit developed in response to the infiltration of fluid batches, including magma and magmatic-hydrothermal fluids, into a regional deformation environment.

Description: The Lower Main zone of the Lorraine alkalic porphyry Cu-Au district, BC, Canada, is hosted in an intrusive complex that comprises pre- and late mineral biotite pyroxenites, monzonites, and syenites. Some of the biotite pyroxenites have interstitial sulfides, which appear similar to net-textured sulfides in magmatic ore deposits; however, the pyroxenites have undergone several stages of hydrothermal alteration. Main-stage mineralization produced sulfide zonation patterns consisting of a bornite-chalcocite or bornite-chalcopyrite core grading outward to domains of chalcopyrite, chalcopyrite 〉 pyrite, and a peripheral domain of pyrite with minor chalcopyrite. Syenite in the inner bornite-chalcopyrite zone typically contains abundant turbid K-feldspar (〉70%), whereas syenite marginal to the bornite-chalcopyrite core contains less K-feldspar (50–70%), indicating an increase in K metasomatism of syenites toward the core of the deposit. Main-stage mineralization predominantly occurs as fine-grained disseminated sulfides in syenite, biotite pyroxenite, and fine-grained K-feldspar biotite rock. Textural analyses have shown that (1) primary magmatic diopside in contact with sulfides has corroded and actinolite-altered margins, suggesting alteration of primary minerals, (2) scalloped relicts of biotite and diopside occur as inclusions in sulfides, implying that sulfides have replaced primary minerals, and (3) the deposit-scale sulfide zonation patterns overprinted numerous rock types, including the biotite pyroxenites that contain interstitial sulfides. These results provide evidence for replacement-style mineralization at Lorraine, where primary magmatic biotite and diopside were totally or partially replaced by sulfide minerals during main-stage alteration. U-Pb zircon dates from pre- and late mineral syenite dikes show that the timing of main-stage mineralization occurred between 178.8 and 176.0 Ma. Lorraine is hosted within the Quesnel island arc terrane, and mineralization and magmatism at Lorraine postdate accretion of the Quesnel terrane to ancestral North America by approximately 7 to 10 m.y. The Lorraine deposit represents the youngest known alkalic Cu-Au porphyry deposit within the Quesnel terrane, and appears to coincide with the last gasp of alkalic magmatism within the Quesnel terrane during the Mesozoic.

Description: The Late Triassic Southwest zone alkalic porphyry Cu-Au deposit in the Galore Creek district, northwestern British Columbia, is hosted in hydrothermally cemented breccias that overprinted older matrix-rich breccia and megacrystic orthoclase-phyric syenite and monzonite dikes. Synmineralization cemented breccias and biotite-phyric monzodiorite dikes are localized at the contact of the matrix-rich breccia and host syenite and monzonite dikes, as well as in crosscutting faults. Regional shortening tilted the Southwest zone deposit so that the cemented breccias now dip 50° to 60° southwest. Four paragenetic stages formed the deposit. A premineralization stage (1) consists of the emplacement of coherent syenite and monzonite host rocks that are cut by a matrix-rich breccia. The synmineralization stage (2) produced potassic-altered rocks and phlogopite-magnetite-K-feldspar–cemented breccia and veins, and most of the Cu-Fe sulfides. A halo of stage 2 diopside-magnetite-grandite-phlogopite calcic-potassic veins and altered rocks formed around the potassic domains and Cu ore zones. Alteration during stage 2 probably formed at temperatures of about 500° to 425°C from oxidized saline magmatic-hydrothermal fluids. Contemporaneous high (a Ca 2+ )/(a H+ ) 2 conditions peripheral to the potassic domains are required to form the marginal calcic-potassic and calcic assemblages. The potassic and calcic-potassic alteration domains were overprinted by propylitic and calcic alteration assemblages due to the retrograde influx of an admixture of cooler magmatic and nonmagmatic fluids. Sphene from a stage 2 calcic-potassic vein yielded a 202.8 ± 2.1 Ma age (U-Pb SHRIMP-RG) that is interpreted to reflect the approximate time of hydrothermal alteration. Late-mineralization stage (3) alteration overprinted the cemented breccias and comprises sericite-anhydrite (sericitic), grandite (calcic), and chlorite-epidote-carbonate-pyrite (propylitic) assemblages. Localized late-mineralization and Cu-poor stage (4) K-feldspar-Fe carbonate-specularite (carbonate-potassic) and quartz-pyrite veins (quartz veins) cut stage 2 and 3 assemblages. Chalcopyrite and bornite dominate the core of the deposit, and grade outward to chalcopyrite 〉 pyrite and to pyrite 〉 chalcopyrite, flanked by a pyrite-only halo. The core of the system is Cu-Au rich, grading outward to an Au-enriched, Cu-poor halo coincident with pyrite-bearing propylitic alteration. Gold-copper ratios increase upward within stage 2 potassic zones and outward from the core. Variations in pH, redox, and temperature along and across fluid flow paths best explain the change to higher Au/Cu values upward and away from the Cu-rich potassic domains.

Description: Paired belts of alkalic and calc-alkalic porphyry deposits are genetically linked to early Mesozoic volcanic arcs preserved as the Quesnel and Stikine terranes in western Canada. These parallel arc terranes extend for 2,000 km along the axis of the Canadian Cordillera. They are joined at their northern ends, but are otherwise separated by relics of Tethyan ocean basin and oceanic arc rocks collectively referred to as the Cache Creek terrane. Porphyry Cu ± Au-Mo-Ag deposits are concentrated within the Stikine-Quesnel arc terranes, with most of their economic metal endowment a consequence of a 15-m.y. mineralizing epoch. A particularly prolific period within the mineralizing epoch is a 6-m.y. pulse centered on 205 Ma when more than 90% of the known copper endowment was acquired. Distinct trends of Cu-Au ± Ag-Mo mineralization within both arc terranes coincide in time and space with events that are attributed to effects of slab subduction. Stratigraphy within both Stikine and Quesnel arc terranes is equivalent, with greater variation along the arcs than between them. Magmatism within both arcs began in the Late Devonian, following initiation of subduction near eastern Panthalassa and formation of the back-arc Slide Mountain ocean basin margin adjacent to Laurentia. Both terranes record further arc development in the lower Carboniferous and cessation in the Permian. Between the Late Permian and Middle Triassic, both arcs foundered, and then recorded a cryptic Early Triassic deformational event, coincident with closing of the Slide Mountain ocean. Permo-Triassic subduction and arc magmatism are recorded in the Sitlika-Kutcho-Venables arc, but this occurred far to the west, within the Tethyan realm. Early in the Late Triassic (early Carnian), both the Stikine and Quesnel arcs reignited. Later, near the end of the Triassic (late Norian), the arcs were uplifted, picrites were erupted locally, and magmatism was dominantly strongly alkaline. Monzonitic plutons hosting alkalic Cu porphyry deposits were emplaced along the arcs, followed by a widespread erosional hiatus that extends throughout much of the latest Triassic (Rhaetian). Late Triassic events culminating in generation of Cu porphyry deposits are attributed to a model wherein delivery of the Permo-Triassic Sitlika-Kutcho-Venables arc to the Stikine-Quesnel arc trench led to stalled subduction and arc-parallel tearing of the slab. Ingress of hot subslab mantle into the tear resulted in high-degree partial melts, as recorded by picrite within the arc circa 210 Ma. Widening of the tear and decay of the initial thermal spike raised temperatures across broad regions of the metasomatized mantle wedge, with low-degree partial melts generated from the most hydrated, metal-enriched portions circa 203 Ma. These melts produced the well-defined belt of alkalic Cu-Au-Ag porphyries. Parts of the slab tear may have nucleated near the forearc to generate Mo-rich Cu porphyries, such as the Gibraltar deposit, which partly cuts the accreted Cache Creek terrane. Metal-rich hydrous melts may have traveled along preexisting intrusive pathways to invade batholithic bodies along the arc axis, such as at Highland Valley, where 204 Ma mineralization is hosted within a larger ~210 Ma plutonic complex. Migration of the locus of mineralized porphyry intrusions toward the backarc may track the leading edge of the slab gap, with the formation of Cu-Au and younger Au-rich porphyries until ~195 Ma. Late alkalic porphyry formation, following emplacement of the Quesnel arc onto the ancestral North American margin at ~186 Ma, may have been produced through cannibalism of mineralized intrusions emplaced at deeper crustal levels during the ~203 Ma Cu porphyry event. Two calc-alkalic porphyry deposits that predate Sitlika-Kutcho-Venables arc collision—Fin and Schaft Creek (~220 Ma)—are temporally related to arc uplift and erosion across the northern Stikine arc, but little is known about the extent and cause of this event and how it relates to porphyry Cu genesis. Slab tears may play a dominant role in the formation of porphyry deposits globally. Previous workers have linked episodes of porphyry formation with subduction of ocean plateaus, changes in subduction polarity, or termination of subduction. In each case, slab tears are the likely underlying cause of porphyry formation. We believe them to be fundamental to mineralization in the early Mesozoic arc terranes of Stikine and Quesnel.

Description: The moderately tilted and faulted Early Jurassic Mt. Milligan Au-Cu deposit provides a cross-section view of the hydrothermal alteration, sulfide mineralogy, and geochemical zonation of a silica-saturated alkalic porphyry system over a vertical distance approaching 700 m. Magnetite-bearing potassic alteration and associated Au-Cu form a core to the system in the central monzonitic stock and adjacent basaltic trachyandesite host rock. Lateral to the high-temperature core are sodic-calcic and inner and outer propylitic alteration assemblages. Chalcopyrite dominates the high-temperature potassic core whereas pyrite is the predominant sulfide within and outboard from the sodic-calcic assemblage. A funnel-shaped remnant of carbonate-rich phyllic alteration of the host supracrustal rocks in the fault-bounded 66 zone represents the upper auriferous alteration in the alkalic porphyry Au-Cu system. Alteration mineral assemblage, S isotope (ranging from 34 S –5 relative to Canon Diablo Troilite in the core to 34 S +0.5 in the periphery), and limited fluid inclusion data suggest mineralization at the Au-Cu alkalic porphyry system was derived from an oxidized, CO 2 -bearing magmatic fluid that rose upward along the margins and through the Magnetite Breccia stock. Laterally from the Au-Cu mineralized potassic core, magmatic fluid evolved through water-rock interaction, mixing with an external fluid as shown by a shift in calculated 87 Sr/ 86 Sr 0 for alteration minerals to values higher than magmatic values, declining temperature, or some combination of all. Variations in trace element concentrations of epidote (V, Mn, Sb, Zr, As, and Bi) and pyrite (Mn, As, Zr, Pb, and Bi) across the deposit show a local high degree of variability, but general increases or decreases in overall trends in their median values are inferred to reflect the hydrothermal evolution of the system. Pistachite ratios of epidote show an outward decrease in ferric iron as recorded in the pistachite ratio changing from P S36 to P S25 , suggesting less oxidizing conditions on the system periphery. Additionally, light rare-earth elements in epidote fractionate toward the core of the deposit, and the height of positive Eu anomalies also appears to have a similar spatial trend. In pyrite, there is a general increase in trace element concentration toward the epidote-pyrite–rich outer propylitic assemblage forming the system periphery. Alteration mineralogy and trace element signatures indicate the southeastern portion of the deposit, the 66 zone, is a down-dropped segment from higher in the paleohydrothermal system. S isotope signatures of sulfides within and surrounding the stratiform Upper Trachyte unit in the 66 zone indicate structural channeling of oxidizing fluids that likely were the distal and cooler expression of more oxidizing magmatic-derived hydrothermal fluids responsible for potassic alteration and Au-Cu in the subjacent Magnetite Breccia zone, the main orebody.

Description: Polyphase veins at the low-grade, structurally controlled epithermal gold deposit Endeavour 42 in the Junee-Narromine volcanic belt of the Ordovician to Silurian Macquarie Arc, central New South Wales (Australia), record multiple tectonically driven fluid-flow events during the mid-Paleozoic. Ordovician host rocks consist of a volcano-sedimentary succession intruded by trachyandesite and andesitic lavas and intrusions, as well as a volumetrically significant Ordovician diorite sill. The entire succession was tilted and faulted prior to the emplacement of multiple mafic to intermediate dike phases, commonly localized along faults. Gold-bearing veins cut all lithologic units at the Endeavour 42 deposit, with the exception of rare late-stage intrusions. Economic gold is restricted to faults, fault-hosted breccias, and veins with two distinct populations: (1) an older, steeply dipping conjugate vein set, cut by (2) two younger, moderately dipping vein sets. Both steep and inclined, moderately dipping vein sets contain a similar mineral assemblage, implying formation during a single epithermal system. Veins are generally 1 to 10 mm thick, commonly straight walled, but locally form overlapping veins with hard linkages. The steep veins reactivated two sets of older faults, fault-parallel fractures, and dike contacts, and accommodated subhorizontal extension. High gold grade, hydrothermally cemented breccias are associated with early gold-bearing veins and localized along deposit-scale faults. Crosscutting the early steep gold-bearing vein sets are inclined gold-bearing veins with moderate dips to the southwest and west. These opened as a new vein set and as veins localized along and at a slight angle to westward-dipping bedding planes, respectively. The geometry of the inclined, moderately dipping veins either represents a sudden change in the far-field stress state to near-vertical minimum compressive stress, thereby suggesting a change from an extensional to a contractional environment, or it may reflect local changes in the differential stress state with subhorizontal northeast-southwest tension. Late veins and thrust faults in the deposit indicate a postmineral contractional environment. The Endeavour 42 deposit underwent tectonic burial until it was exhumed in the early Carboniferous, as indicated by 40 Ar/ 39 Ar plateau ages of 362.7 ± 2.1, 356.3 ± 1.8, and 344.5 ± 2.1 Ma, toward the end of the Kanimblan Cycle.

Description: Mount Polley is a Late Triassic (~205 Ma) alkalic porphyry Cu-Au-Ag deposit (226.3 thousand tonnes (t) Cu, 21.5 t Au, and 65.1 t Ag), hosted by silica-undersaturated to silica-saturated monzonitic intrusions of the Mount Polley Complex, located in British Columbia, Canada. The Northeast ore zone at Mount Polley is hosted by magmatic-hydrothermal breccia. Copper and precious metals occur in sulfide minerals primarily as coarse- to fine-grained breccia cement. Local wall rocks include equigranular to porphyritic diorite, monzodiorite, and monzonite. Alteration, breccia cement, and veins of the Northeast ore zone formed in five paragenetic stages: prebreccia (stage 1), brecciation and main-stage mineralization (stage 2), late-stage mineralization (stage 3), unmineralized postbreccia dikes and veins (stage 4), and epithermal-style veins (stage 5). Intense pervasive K and Fe metasomatism ± calcite and calc-silicate alteration occurred prior to brecciation caused by the intrusion of megacrystic K-feldspar-phyric monzonite. Stage 2 fluids were silica undersaturated, high temperature (〉350°C), CO 2 enriched, and of near-neutral to alkaline pH. Potassic, sodic, and calc-potassic assemblages precipitated with mineralization during stage 2 with moderate temperatures at the deposit periphery and in stages 3 and 4. Evidence for more acidic and lower-temperature conditions is preserved in stage 5 veins. The 34 S sulfide isotope compositions of stages 2 and 3 chalcopyrite, pyrite, and bornite range from –7.1 to +1.4. Sulfur isotope compositions of anhydrite and gypsum are mostly between 6.2 and 9.8. These values, together with the presence of hematite, are consistent with deposition from an oxidized, sulfate-dominant, high-temperature magmatic-hydrothermal fluid. Limited sulfur isotope geothermometry indicates that Cu sulfides precipitated at temperatures from ~480° to ~250°C. Hydrothermal calcite occurs in all paragenetic stages at Mount Polley. Calcite 13 C values range from –0.2 to –10.5, and 18 O values from 4.0 to 20.9. The enriched C-O isotope values are not consistent with simple precipitation from an entirely magmatic source of hydrothermal fluid. Interaction of the fluid and/or magma with limestone is considered a likely process to explain the C and O isotope signature. Lead isotope data suggest mixing of mantle and crustal sources during mineralization. Main-stage chalcopyrite and pyrite as well as late-stage galena have 206/204 Pb values of 18.77 to 18.92, 207/204 Pb of 15.56 to 15.59, and 208/204 Pb of 38.22 to 38.32. Strontium isotope data (0.70331–0.70371) provide evidence of a strongly depleted mantle source of Sr with minor crustal input. Epsilon Nd values for main-stage apatite range between 5.9 and 6.5, also indicating a depleted mantle source. Stage 5 carbonate 206/204 Pb values of 18.96 to 19.04, 207/204 Pb of 15.57 to 15.59, and 208/204 Pb of 38.26 to 38.36 suggest superposition of an epithermal system onto the Northeast ore zone, potentially as late as ~100 m.y. after breccia formation. The data presented are consistent with the hypothesis that the silica-undersaturated alkalic Mount Polley Complex formed due to carbonate assimilation prior to mineralization. This process can explain the 13 C- 18 O isotope data, calcite precipitation concurrent with Cu-Au mineralization, and silica undersaturation of the magma. The CO 2 released during assimilation of carbonate also could have promoted magmatic-hydrothermal brecciation. Silica-undersaturated alkalic porphyry systems may preferentially form in arc terranes built on a carbonate-bearing substrate.

Description: The Late Triassic Galore Creek porphyry district is the largest accumulation of Cu-Au prospects in British Columbia, Canada, in terms of contained metal. The principal economic resource is the Central zone deposit. It comprises three high-grade centers: the Au-rich North and South gold lenses and the Cu-dominated Central replacement zone. Alteration footprints, ore shells, and porphyry intrusions in these centers dip 45° to 60° to the west in the North gold lens and southwest in the South gold lens and Central replacement zone, indicative of significant postmineral tilting. This inclination created an oblique vertical expression of the Central zone at surface and at depth, providing a +700-m depth profile through the deposit. Galore Creek is considered to be the end-member of the silica-undersaturated class of alkalic porphyry Cu-Au deposits. It displays a unique hydrothermal alteration footprint, generally devoid of quartz veining. Despite the lack of crosscutting veins, a paragenetic sequence of several discrete potassic and calcic alteration events has been established. During main-stage alteration, two mineralization events occurred that formed the economic resource in the Central zone. Highly oxidizing fluids derived from porphyritic syenite to monzonite intrusions produced initial potassic alteration and sulfide mineralization in the North gold lens and South gold lens. Cu-Au ore shells are characterized by a gold-rich core dominated by bornite associated with hematite-dusted orthoclase, specular hematite, and anhydrite. The second period of hydrothermal activity caused brecciation and calcic alteration in the Central replacement zone. The calcic fluids formed hydrothermal cement grading from Ti-rich andraditic to grossularitic garnet in the core of the breccia to a diopside and magnetite-dominated assemblage at the margins. Calcic alteration also formed in the surrounding host rock. This alteration event was followed by potassic alteration and mineralization, characterized by biotite, anhydrite, and chalcopyrite in the Cu-rich ore shell of the Central replacement zone. Most of the Au is hosted in bornite that is spatially associated with oxidized (reddened) rock units dominated by an alteration assemblage of orthoclase ± hematite ± specular hematite ± garnet ± anhydrite. High-grade Cu in the Central zone is related to abundant chalcopyrite that has commonly replaced an assemblage of biotite ± magnetite ± diopside. This distinct spatial separation strongly suggests that the local redox environment within the wall rocks governed sulfide and gold distributions. The switch from early Au-rich mineralization to late-stage Cu-dominated mineralization appears to have been controlled by varying redox conditions within the magma. The change from more oxidized alteration assemblages (Fe 3+ 〉 Fe 2+ ) to more reduced assemblages (Fe 3+ ≤ Fe 2+ ) is indicative of a change in oxidation potential that may have propelled sulfide saturation in the magma and, thus, the subsequent depletion of most of the Au.

Description: The volcanosedimentary rocks that host the Au-rich porphyry Cu deposits of the Cadia Valley preserve the products of episodic volcanism that erupted into a large sedimentary basin. Volcanogenic sedimentation, including the Forest Reefs Volcanics, overwhelmed the fine-grained sedimentary component that characterized much of the Weemalla Formation. The Forest Reefs Volcanics evolved as a relatively low relief, multiple-vent submarine volcanic complex. The vents comprised mafic to intermediate lava flows, cryptodomes, and subvolcanic intrusions (dikes and sills). Stacked lava sequences, including hyaloclastites, massive lavas, and their reworked equivalents, are up to 1 km thick, forming significant intrabasinal topography. Explosive volcanism occurred during the late stages of Forest Reefs Volcanics deposition. These air-fall deposits, combined with coexisting shallow-water faunal assemblages, imply that volcanism became locally emergent. Continuity of sedimentation between underlying deep marine basin deposits of the Weemalla Formation and Forest Reefs Volcanics, coupled with the predominance of sheet-like, laterally continuous debris flow and other coarse-grained sedimentary deposits, implies that volcanism and related sedimentation persisted in an active sedimentary basin marginal to an oceanic island arc. Deposition of the Forest Reefs Volcanics spanned the Late Ordovician to Early Silurian. Monzonite fragments (identical to the ore-related intrusions) are abundant in sedimentary breccias found at the top of the preserved volcanic stratigraphy. This finding, combined with available absolute ages of crosscutting intrusions and associated hydrothermal alteration and mineralization, suggests that some volcanosedimentary units were deposited synchronously with or immediately after the last known porphyry-related hydrothermal event in the Cadia Valley. The Au-rich porphyry deposits were therefore emplaced into an evolving sedimentary basin with episodic intrabasinal magmatism. Permeable horizons and volcanic lithofacies can preferentially host alteration and mineralization that can extend over several kilometers in lateral extent. This finding suggests that hydrological models of fluid flow in porphyry systems need to take basin architecture into account.

Description: The 46-million-ounce Ladolam gold deposit, the largest alkalic epithermal gold deposit in the world in terms of contained gold, is composed of four ore zones: Minifie, Lienetz, Kapit, and Coastal. This detailed lithofacies study of the Minifie ore zone recognized three evolutionary stages: (1) volcanosedimentary strata (part of an alkalic composite volcanic island), (2) a porphyry-style breccia dike, and (3) epithermal-style breccias. The Plio-Pleistocene volcanosedimentary stratigraphy reflects the transition from subaerial deposition of pyroclastic surge deposits close to vent to a subaqueous, quiet depositional environment into which a cryptodome was emplaced. The stratigraphy is predominantly composed of polymictic, matrix-supported breccia and sandstone interbedded with lavas and shallow intrusions. The Minifie strata have a shallow southward dip as a result of Quaternary uplift. Overprinting the volcanosedimentary stratigraphy are three hydrothermally cemented breccia facies—one deposited in the porphyry environment and two deposited in the epithermal environment. Porphyry-style alteration assemblages and a 3- to 5-m-wide biotite-K-feldspar-calcite-anhydrite–cemented breccia dike are focused around the central "Minifie shear" fault. The porphyry alteration assemblages are overprinted by shallow-level and deeper-level epithermal-style features. The deeper-level (below the present mining surface; 〉140 m below sea level) epithermal vein stockwork is composed of quartz-calcite-adularia-anhydrite–cemented breccias and shallowly northward to near-horizontal dipping veins that have gold grades of 1 to 〉60 g/t Au. Shallow-level epithermal facies (adularia-quartz-pyrite–cemented breccias, veins, and associated alteration) host the bulk mineable gold ore, and typically yield 〉4 g/t Au over 12-m blast hole samples.

Description: Endeavour 41 is a deep-level, structurally controlled epithermal gold deposit hosted by Early Ordovician subaqueous volcanosedimentary rocks. Calc-alkalic to shoshonitic, mafic to intermediate sills, dikes, and stocks intruded the volcanosedimentary units in the Middle Ordovician. The most felsic intrusions, together with pyroxene-bearing dikes, are temporally related to gold mineralization. Postmineralization intrusions are exclusively of mafic character. Endeavour 41 evolved from early, high-temperature porphyry-style veins and alteration to lower-temperature epithermal-style gold mineralization. Early magnetite and garnet-bearing veins (stage 1 and 2, respectively) associated with actinolite, magnetite, and biotite-bearing alteration assemblages have been cut by gold-bearing veins and associated alteration assemblages. There were two main epithermal-style gold mineralizing events: (1) quartz-pyrite ± calcite ± adularia ± chlorite veins (stage 3) and (2) carbonate-base metal sulfide veins (calcite, ankerite, quartz, pyrite, sphalerite, galena, chalcopyrite, Ag tellurides, arsenopyrite, apatite, hematite, illite-muscovite, and chlorite [stage 4]). Gold occurs principally as a refractory phase in pyrite. It also occurs as grains of Au-Ag tellurides and as inclusions of free gold in pyrite, sphalerite, and chalcopyrite. Hydrothermal alteration associated with gold-mineralized veins produced early epidote and K-feldspar-epidote–bearing alteration halos and later-stage illite-muscovite-K-feldspar and calcite-rich alteration halos. The highest gold grades are associated with muscovite and illite alteration. Stable isotope analyses and fluid inclusion data provide evidence of a magmatic-hydrothermal component to the mineralizing fluids. Fluid inclusion data suggest that gold precipitated from boiling saline waters (~9.0 wt % NaCl) at temperatures of about 310°C. Stage 3 veins are estimated to have formed approximately 1 km below the paleosurface at hydrostatic pressure (~90 bars). Stage 4 illite formed at temperatures below ~280°C. Stage 3 calcite has 13 C calcite and 18 O calcite values that range from –5.2 to –4.6 and from 11.6 to 12.1, respectively. Calculated fluids for these mineral values at 300°C ( 13 C fluid = –3; 18 O fluid = 6) are consistent with a magmatic-hydrothermal source of carbon and oxygen during stage 3. A component of meteoric waters is inferred for stage 4, because 13 C carbonate and 18 O carbonate values range from –6.9 to –0.5 and from 10.9 to 30.1, respectively, corresponding to 13 C fluid and 18 O fluid values of –5 and –2 at 200° to 250°C. The 34 S sulfide values for early vein stages range between –4.9 and –0.5. Stage 3 has 34 S sulfide values ranging from –5.2 to +0.8 with the most 34 S enriched values deposited away from the mineralized center. Stage 4 sulfides have isotopic compositions from –7.5 to +2.5. The negative isotopic values are consistent with oxidized (sulfate-predominant) magmatic-hydrothermal fluids. Sulfur isotopic zonation patterns show that the most negative 34 S values correlate with gold-enriched domains and also with areas that contain high-temperature, porphyry-style alteration facies. The negative sulfur isotope values define zones of upflow for the mineralizing magmatic-hydrothermal fluids. The paragenetic history of Endeavour 41 records a transition from deep-level to shallow-level magmatic-hydrothermal activity. This transition implies erosion and unroofing of the system synchronous with mineralization. High-temperature assemblages (e.g., actinolite-magnetite, biotite, and K-feldspar-epidote) indicate that epithermal mineralization occurred proximal to a magmatic-hydrothermal center and that there is potential for the discovery of porphyry copper-gold mineralization below the current level of diamond drilling.

Description: Spatial and temporal variations in quartz textures and the types of quartz-hosted fluid inclusions in the no. 3 vein of the Pleistocene Koryu epithermal Au-Ag deposit were studied. Bonanza-grade ore zones contain multiple generations of quartz with six distinct textures—crustiform, comb, microcrystalline, colloform, cockade, and platy. Early-formed parts of the vein are mainly composed of comb quartz that is overgrown by later quartz with a variety of textures, including comb, microcrystalline, and colloform; the latter two are the most common textures of later parts of the vein. Comb, microcrystalline, and colloform textures can be traced along each mineral band at the scale of the ore deposit, suggesting the existence of uniform silica-supersaturated conditions during the deposition of the band. Interpretation of petrographic features of fluid inclusions in quartz at the same mine level reveals three types of fluid conditions—intense boiling (flashing), gentle boiling, and nonboiling conditions. The presence of comb quartz texture corresponds to fluid inclusion evidence for gentle boiling or nonboiling conditions. The presence of microcrystalline and colloform quartz textures indicates intense boiling, as suggested in previous studies, although the secondary fluid inclusions in earlier-formed quartz only correspond to fluid inclusion evidence for intense boiling. A fluid inclusion microthermometric study using fluid inclusion assemblages in comb quartz trapped under both gentle boiling and nonboiling conditions shows hydrothermal temperature fluctuates mostly between 243° and 268°C. The relationship between boiling conditions and quartz textures suggests that the intensity of boiling of silica-supersaturated hydrothermal fluid fluctuates at a given depth over time. The close association between precious metal content and microcrystalline and colloform quartz supports the idea that the metals precipitated due to intense boiling. Intense boiling at the base of the boiling zone likely was the main mechanism for bonanza precious metal precipitation, whereas physical transportation of the metals by gently boiling fluids was only a minor mechanism for local precipitation of precious metals at Koryu. On the basis of the observations of this study and previous paleodepth data, quartz textures are overprinted; a deep assemblage of the early comb quartz, formed at 〉500-m depth below the paleowater table, was overprinted by a late microcrystalline and colloform quartz assemblage formed at 〈500-m depth. Such textural variations may be attributed to a decrease in the depth of the water table caused by nearby volcanic eruptions.

Description: Re-Os dating of two molybdenite samples from the Squaw Peak porphyry copper-molybdenum prospect in central Arizona returned essentially identical ages of 1,729 ± 7 and 1,738 ± 7 Ma. Therefore the prospect is not a component of the Laramide (Late Cretaceous-early Tertiary) porphyry copper province of southwestern North America as previously presumed. These Paleoproterozoic ages are similar to that of 1,740 ± 15 Ma for the I-type, magnetite-series Cherry batholith, within which the Squaw Peak porphyry stock and associated mineralization are located. Squaw Peak cannot be more than a few million years younger than volcanogenic massive sulfide (VMS) copper deposits in the nearby Jerome district, which are part of the Yavapai Supergroup, host to the Cherry batholith. These volcanic and intrusive rocks and their associated copper mineralization were formed in a juvenile island-arc setting and are now part of the Yavapai province, which was assembled and accreted to the Archean nucleus of North America by ~1.68 Ga. The Paleoproterozoic age for Squaw Peak in conjunction with the existence of the slightly older VMS deposits shows that the Laramide province of southwestern North America first developed its copper metallogenic signature 〉1,700 m.y. ago. The presence of this Paleoproterozoic copper mineralization may be taken as further support for recently proposed metasomatism of the mantle lithosphere during Paleoproterozoic subduction as a precursor to formation of at least part of the Laramide porphyry copper province. Similar spatial associations between inferred metal sources in Proterozoic mantle lithosphere and lowermost crust, relatively minor copper and/or molybdenum mineralization in Proterozoic magmatic arcs, and important post-Paleozoic porphyry copper and/or molybdenum provinces have recently been documented elsewhere, particularly in eastern China, and could prove to be of more general exploration significance.

Description: Petrographic and geochemical profiles collected through a ~250-m-thick Nipigon diabase sill near Black Sturgeon Lake, Ontario, constrain the complex emplacement and differentiation processes that controlled the magmatic evolution of these rocks: several nested reinjections of phenocryst-bearing magma, compactiondriven redistribution of mildly evolved interstitial liquids in the central portion of the sill, and segregation of highly-evolved interstitial liquids into micropegmatitic veins near the upper contact. These processes impact not only the silicate mineralogy of the rocks, but also distribution of sulfide phases. Both petrographic and geochemical data demonstrate that the sill was gradually inflated via several discrete injections of olivine-bearing magma. Plagioclase crystal size trends exhibit several reversals that are correlated with increases in olivine abundance and bulk-rock MgO content. These textural and compositional reversals are attributed to reinjections of olivine-bearing magma into partially solidified magma from earlier injections. Mixing between evolved resident magma and reinjections of less-evolved magma is commonly proposed as a mechanism for triggering sulfide mineralization. In this sill, which contains no significant mineralization, localized minor enrichments and depletions in copper content are associated with the margins of a well-constrained reinjection horizon. Chemical differentiation of the magma was controlled by three distinct processes: flow differentiation, crystal-mush compaction, and solidification front instability. Although the mineralogy of the sill is dominated by plagioclase and clinopyroxene, major-oxide compositional variations are largely controlled by the mechanical concentration of olivine via flow differentiation or phenocryst redistribution within the individual reinjection pulses. This redistribution of phenocryst phases is the primary factor controlling the major-element composition of the rocks in most of the sill. In the central portion of the sill, the abundances of incompatible trace elements are influenced by the compaction of partially crystalline mush and the expulsion of incompatible-rich interstitial liquid. This is recognized by a negative correlation between the alignment of plagioclase crystals and concentrations of incompatible trace elements such as Zr. Finally, there is a ~2.5-m-thick silicic segregation approximately 32 m below the upper contact, whose composition is consistent with residual liquid after ~65 to 85% fractional crystallization of magma with a composition equivalent to that of the marginal rocks. Similar veins and patches are common throughout the upper 50 m of the sill, indicating that this segregation process was common and widespread during the cooling and crystallization of the sill. The complexity of intrusion and differentiation in this sill, despite its generally unremarkable texture and composition, suggests that many diabase sills may have experienced a range of processes that have previously been recognized to impact the petrologic evolution of larger igneous systems, and that these processes can be identified in minor intrusions such as sills and dikes using simple and robust analytical techniques.

Description: For the first time in the Rudny Altai, we propose to distinguish Besshi- and Kuroko-type deposits, previously considered as members of a single massive sulfide-polymetallic group. The ores of the two deposit types differ in mineral composition, with pyrite-pyrrhotite-chalcopyrite ± sphalerite mineralization in the Besshi-type deposits versus pyrite-sphalerite-galena-chalcopyrite-gold mineralization in the Kuroko-type deposits. The Besshi-type deposits are restricted to the Irtysh terrane and associated with deformed and metamorphosed (greenschist to mid-amphibolite facies) mafic-siliciclastic rock sequences. This contrasts with the well-preserved Kuroko-type deposits, hosted in bimodal volcanic sequences in the Devonian-Carboniferous Rudny Altai magmatic arc. Our U-Pb, 40 Ar/ 39 Ar, and Re-Os isotope dating of sulfides and host rocks of the Besshi-type deposits suggests that they were probably formed during the early Paleozoic in a back-arc rift setting. They were then metamorphosed during Devonian times within the accretionary wedge in front of the Rudny Altai arc, synchronously with formation of the Kuroko-type deposits.

Description: Mineral maps based on Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data were used to study late Miocene advanced argillic alteration at Cuprite, Nevada. Distributions of Fe-bearing minerals, clays, micas, sulfates, and carbonates were mapped using the Tetracorder spectral-shape matching system. The Al content of white micas increases toward altered areas and near intrusive rocks. Alunite composition varies from pure K to intimate mixtures of Na-K endmembers with subpixel occurrences of huangite, the Ca analogue of alunite. Intimately mixed Na-K alunite marks areas of relatively lower alteration temperature, whereas co-occurring Na-alunite and dickite may delineate relict hydrothermal conduits. The presence of dickite, halloysite, and well-ordered kaolinite, but absence of disordered kaolinite, is consistent with acidic conditions during hydrothermal alteration. Partial lichen cover on opal spectrally mimics chalcedony, limiting its detection to lichen-free areas. Pods of buddingtonite are remnants of initial quartz-adularia-smectite alteration. Thus, spectral maps provide a synoptic view of the surface mineralogy, and define a previously unrecognized early steam-heated hydrothermal event. Faulting and episodes of hydrothermal alteration at Cuprite were intimately linked to upper plate movements above the Silver Peak-Lone Mountain detachment and growth, collapse, and resurgence of the nearby Stonewall Mountain volcanic complex between 8 and 5 Ma. Isotopic dating indicates that hydrothermal activity started at least by 7.61 Ma and ended by about 6.2 Ma. Spectral and stable isotope data suggest that Cuprite is a late Miocene low-sulfidation adularia-sericite type hot spring deposit overprinted by late-stage, steam-heated advanced argillic alteration formed along the margin of the Stonewall Mountain caldera.

Description: The Jbel Tirremi fluorite-barite ± sulfide deposit in northeastern Morocco is hosted in a Jurassic-aged structurally high carbonate platform known as the Jbel Tirremi dome. The host rocks consist of unmetamorphosed, flat-lying early Jurassic dolomitized limestones, locally intruded by Eocene lamprophyre dikes. The orebodies consist mostly of fluorite and barite, and occur as open-space fillings and partial to massive replacement of the enclosing medium- to coarse-grained dolomitized limestones. The ore mineralogy is dominated by fluorite of different colors and habits, barite, and, to a lesser extent, sulfides. Rare earth element compositions along with fluid inclusion, halogen and isotopic data suggest that the fluorite barite mineralization and the spatially associated Eocene alkaline magmatism are petrogenetically unrelated, pointing instead to the regional circulation of hydrothermal basinal brines mixed to various degrees with meteoric water in a dominantly closed rock-buffered system at progressively higher temperatures and fluid/rock ratios. In this respect, fluid inclusion microthermometric measurements show that the ore-bearing hydrothermal system developed in two separate stages of fluorite-barite mineralization, as also revealed by isotopic data. Both stages precipitated from saline fluids at shallow crustal levels (i.e., 〈5 km), and were related, in varying degrees, to different stages of basin evolution and salt dome growth (salt mobilization and mineralization). During the first stage, the ore fluid was a highly saline aqueous brine with a total salinity up to 44.2 wt % NaCl + KCl equiv, at temperatures ≥82°C and possibly up to 218°C, whereas in the second stage the mineralizing fluid had a similar temperature range, but lower salinities (~20–10 wt % NaCl equiv). The recorded high salinities are interpreted to represent the involvement of a mixture of halite dissolution water and evaporated seawater component. Oxygen ( 18 O = 21.7 to 29.6 V-SMOW) and carbon ( 13 C = –7.9 to 0.2 V-PDB) isotope data along with strontium ( 87 Sr/ 86 Sr = 0.70300–070789) and lead ( 206 Pb/ 204 Pb = 17.961–20.96, 207 Pb/ 204 Pb = 15.511–15.697, 208 Pb/ 204 Pb = 37.784–39.993) isotope ratios suggest the involvement of a mixture of oil-bearing fluids, basinal brines, and meteoric fluids that interacted extensively with the early Jurassic host carbonates, the underlying Triassic salt-bearing diapir, associated siliciclastic rocks, and the highly fractionated and greisenized Hercynian granitic crystalline basement, resulting in the release of fluoride, metals, and other constituents to form the Jbel Tirremi deposit. Petroleum-bearing fluid, released from overpressured portions of the Guercif Basin at lithostatic pressures, and bittern brines dominated the first stage of mineralization. Mixing of saline, oxidized, CaCl 2 - and sulfate-rich bittern brine with oil-bearing fluid resulted in fluorite precipitation of stage I. Conversely, during the second stage of mineralization, the hydrothermal system was open to the influx of oxidized meteoric water as a consequence of the upward migration of the Triassic salt-bearing diapir and associated pressure decrease. The shift from stage I to stage II is associated with the evolution of the system from lithostatic to mostly hydrostatic pressure conditions. Stage I mineralization is thought to have occurred during the Late Miocene in response to rapid sedimentation and high subsidence rates and subsequent hydrocarbon migration associated with the outward migration of the Rif thrust front. Conversely, stage II mineralization occurred coevally with the uplift phase during Tortonian time.

Description: The ca. 2720 Ma Winston Lake greenstone belt of the Wawa-Abitibi terrane, Superior province, is located on the northern margin of the Wawa subprovince and has important regional tectonostratigraphic implications for the development of the Neoarchean Superior province. The metallogenic importance of the belt is evident from the presence of the Winston Lake, Pick Lake, and Zenith volcanogenic massive sulfide (VMS) deposits. However, the chemostratigraphic evolution has not been fully examined at the greenstone belt scale, nor have the VMS deposits been examined from the perspective of the belt-scale petrogenesis and volcanic reconstruction. This study (1) reviews the geology and VMS mineralization in the Winston Lake greenstone belt, and (2) reconstructs its geodynamic setting, and its magmatic and metallogenetic history using new geological, trace element, and Nd isotope data constrained by new U-Pb geochronology. The belt is subdivided into two main lithotectonic assemblages: the Winston Lake Assemblage composed of tholeiitic to calc-alkalic bimodal volcanic lithofacies, and the Big Duck Lake Assemblage composed of alkalic to tholeiitic mafic flows and sills. The contact between the Winston Lake and Big Duck Lake Assemblages is intruded by the differentiated Zenith gabbro, which hosts the 0.16 Mt Zenith VMS orebody as a series of xenoliths. The variably altered felsic lithofacies of the Winston Lake Assemblage host the 3.1 Mt Winston Lake Main and 1.3 Mt Pick Lake Zn-rich VMS deposits. The felsic volcanic lithofacies of the Winston Lake Assemblage are FIII-type rhyolites and have a bimodal distribution of Zr/Ti indicating two distinct magmas formed by different temperatures of partial melting. Mafic lithofacies of the Winston Lake Assemblage are typically calc-alkalic to transitional between calc-alkalic and tholeiitic in composition. They have arc to back-arc trace element contents with distinctive negative Nb and Ti anomalies, enrichment in the light rare earth elements (LREE), and higher Th/Nb ratios than the mafic rocks from the Big Duck Lake Assemblage. Mafic strata from the Big Duck Lake Assemblage have trace element characteristics similar to mature back-arc tholeiitic basalts, and OIB-like or oceanic plateau alkalic basalts with high Nb/Th and Ti/V ratios. The chemostratigraphy of the Winston Lake greenstone belt is consistent with a geodynamic model where arc rifting is followed by back-arc development. Nd isotope data indicate that strata from the Winston Lake and Big Duck Lake Assemblages are juvenile and show no evidence of contamination by significantly older crust. Therefore, crustal contamination signatures in the mafic flows from the Winston Lake Assemblage must have formed by contamination from crust that is of similar age and supports a comagmatic relationship between the mafic and felsic strata. The mafic flows of the Big Duck Lake Assemblage represent a mature back-arc setting with evidence for enriched plume-generated basalts in the upper mafic unit. The Winston VMS deposit formed during the early rifting of the arc and the timing is tightly constrained by the hydrothermally altered 2721.2 ± 0.9 Ma VMS-hosting felsic strata and younger, postmineralization, 2719 ± 4 Ma Zenith gabbro. The Zn-dominated VMS mineralization suggests that temperature of the hydrothermal fluid during discharge was relatively cool (〈300°C) and did not maintain sufficiently high temperatures to precipitate significant amounts of copper relative to zinc. This is likely due to adiabatic cooling of the fluid because of a low-pressure (〈0.5 kb) sea-floor environment and/or permeable volcaniclastic strata. A U-Pb age of emplacement of 2692.1 ± 0.7 Ma from the syndeformation quartz-feldspar porphyry in the Big Duck Lake Assemblage indicate it is comagmatic with syndeformation magmatic events typical of other greenstone belts along the northern margin of the Wawa-Abitibi terrane. This event is interpreted to represent the timing of terrane accretion of the Wawa-Abitibi terrane to the Superior province.

Description: The Santa Cruz porphyry Cu-(Mo) system near Casa Grande, Arizona, includes the Sacaton mine deposits and at least five other concealed, mineralized fault blocks with an estimated minimum resource of 1.5 Gt @ 0.6% Cu. The Late Cretaceous-Paleocene system has been dismembered and rotated by Tertiary extension, partially eroded, and covered by Tertiary-Quaternary basin-fill deposits. The mine and mineralized fault blocks, which form an 11 km (~7 miles) by 1.6 km (~1 mile) NE-SW–trending alignment, represent either pieces of one large deposit, several deposits, or pieces of several deposits. The southwestern part of the known system is penetrated by three or more diatremes that consist of heterolithic breccia pipes with basalt and clastic matrices, and subannular tuff ring and maar-fill sedimentary deposits associated with vents. The tephra and maar-fill deposits, which are covered by ~485 to 910 m (~1,600–3,000 ft) of basin fill, lie on a mid-Tertiary erosion surface of Middle Proterozoic granite and Late Cretaceous porphyry, which compose most xenoliths in pipes and are the host rocks of the system. Some igneous xenoliths in the pipes contain bornite-chalcopyrite-covellite assemblages with hypogene grades 〉1 wt % Cu, 0.01 ounces per ton (oz/t) Au, 0.5 oz/t Ag, and small amounts of Mo (〈0.01 wt %). These xenoliths were derived from mineralized rocks that have not been encountered in drill holes, and attest to additional, possibly higher-grade deposits within or subjacent to the known system. The geometry, stratigraphy, and temporal relationships of pipes and tephras, interpreted from drill hole spacing and intercepts, multigenerational breccias and matrices, reequilibrated and partially decomposed sulfide-oxide mineral assemblages, melted xenoliths, and breccia matrix compositions show that the diatremes formed in repeated stages. Initial pulses of basalt magma fractured granite, porphyry, and other crustal rocks during intrusion, transported multi-sized fragments of these rocks upward, and partially melted small fragments. Rapid decompression of magma induced catastrophic devolatilization that ruptured overlying rocks to the surface, and generated fragment-volatile suspensions that abraded conduits into near-vertical cylindrical structures. Fragments entrained in suspensions were milled and sorted, and ejected as basal surge, pyroclastic deposits, and airfall tephra that built tuff rings around vents and filled vent depressions. Comminuted m- to mm-sized fragments of wall rocks in magma and suspensions that remained in conduits solidified as heterolithic breccias. Subsequent pulses of basalt magma ascended through the same conduits, brecciated older heterolithic breccias, devolatilized, and quenched, leaving two or more generations of nested and mingled heterolithic breccias and internal zones of fluidized fragments. Tephra and maar-fill deposits from later eruptions are composed of more hydrous and oxidized minerals than earlier tephras, reflecting a higher proportion of water in transport fluid which, based on fluid inclusion populations in mineralized xenoliths, was saline water and CO 2 . The large vertical extent (~600 m; ~2,000 ft) of basalt matrix in pipes, near-paleosurface matrix vesiculation, and plastically deformed basalt lapilli indicates that diatreme eruptions were predominantly phreatic. Diatreme xenoliths represent crustal stratigraphy and, as in the Santa Cruz system, provide evidence of concealed mineral resources that can guide exploration drilling through cover. Vectors to the source of bornite-dominant xenoliths containing 〉1% Cu and significant Au and Ag could be determined by refinement of breccia pipe geometries, by reassembly of mineralized fault blocks using modal, chemical, and temporal characteristics of hydrothermal mineral assemblages and fluid inclusions, and by paleodrainage analysis.

Description: Thin horizons of sulfide minerals enriched in platinum group elements (PGEs), known as PGE "reefs," are a feature of many layered mafic intrusions, including the Sonju Lake intrusion (Midcontinent rift, Minnesota). In traditional petrologic models, PGE reefs are thought to form during upward accumulation of mineral deposits, either by downward settling of immiscible sulfides through a silicate melt (the orthomagmatic model) or by concentration during upward flow and deposition in a fluid (the hydromagmatic model). While each model explains some of the observations from reefs, neither provides a consistent explanation for both the overall PGE distribution and evidence for offsets in peak concentrations of PGEs and base metals. If, alternatively, layered intrusions form by a top-down process of sill injection and reaction (thermal migration zone refining) as postulated in Lundstrom et al. (2011a) , then PGE reefs could form as a moving sulfide band passes downward through a mineral-melt mush at the particular temperature of sulfide saturation. A simple temperature gradient experiment in a sealed quartz tube illustrates how sulfide moves and aligns parallel to a temperature gradient. Thus, while silicates form in a mush zone and remain immobile, the immiscible sulfide liquid position is controlled by the temperature of sulfide saturation and thus moves down relative to the silicates as new sills underplate previous ones; effectively, all PGE derived from the upper portion of the intrusion (above the reef) partition into this layer resulting in the enriched reef layer, consistent with observed distribution and mass balance in the Sonju Lake intrusion. The higher sulfide-silicate mineral partition coefficients for the PGE relative to Cu as sulfides move relative to the silicates results in Cu being chromatographically separated from PGEs during the process. Thus, the mechanism can account for the two major observations, the overall distribution of PGEs in layered intrusions and the chromatographic separation of PGEs from Cu, perhaps better than existing models. The model provides a sharp alternative to current models, with its predictive aspects potentially improving future exploration for PGE deposits.

Description: Many Carlin-type gold deposits in northern Nevada are found adjacent to steeply dipping faults that channeled hydrothermal fluids upward into mineralized regions. However, some Carlin deposits comprise gently dipping tabular ore zones lacking any obvious large, steeply dipping, feeder structure beneath them. The goal of this paper is to test whether fluid flow into such ore zones also involved largely vertical upwelling, or whether lateral fluid flow, particularly along low-angle fault structures, may have played a significant role in their formation. In this study we use carbon and oxygen isotopes in conjunction with trace element geochemistry to investigate pathways taken by hydrothermal ore-forming fluids into the tabular subhorizontal ore zones of the Pipeline deposit, a giant carbonate rock-hosted Carlin-type deposit in Nevada. We sampled deep drill holes within, below, and to the side of the main ore zone to assess the extent of lateral versus vertical fluid flow up to 600 m below mineralization. The main ore zone at the Pipeline deposit is dominated by carbonate host rocks having 18 O and 13 C values that are depleted in the heavier 18 O and 13 C isotopes compared to global background values for rocks of the same age. Depletion results from the progressive buffering of the rock by an infiltrating auriferous hydrothermal fluid. There is significant heterogeneity in the spatial pattern and relative degree of 18 O and 13 C depletion through the ore zone, most likely reflecting multiple flow paths with different total integrated fluid fluxes, path lengths, rates of fluid cooling, and possibly fluid mixing. A lack of a pervasive reduction in 18 O and 13 C values, and low concentrations of trace elements in the rocks immediately beneath the main ore zone indicate that the deposit was not a product of large-scale vertical upwelling of auriferous hydrothermal fluid directly into the mineralized region. Rather, flow was focused along preexisting low-angle thrusts, particularly the Abyss fault, with fluids flowing laterally underneath and then up into the area of the main ore zone. Enhanced permeability along these low-angle structures most likely derived from fault reactivation and the generation of fracture networks in damage zones peripheral to a relatively impermeable fault core. Such reactivation would have required suprahydrostatic fluid pressures, relatively shallow crustal depths, and a lack of more optimally oriented, high angle faults (〉50°) that would otherwise have preferentially failed and focused fluid flow steeply upward.

Description: The duration of hydrothermal activity required to form ore deposits is poorly constrained. We demonstrate that thermochronology data, coupled with thermal modeling, can be used to constrain the duration of hydrothermal fluid flow. Apatite fission-track (AFT) thermochronology data define a conductive halo around an Eocene hydrothermal system that formed the Betze-Post gold deposit on the northern Carlin trend in Nevada. The premineralization Goldstrike stock acted as an essentially impermeable side to the auriferous Carlin hydrothermal system. The hydrothermal fluid conductively heated the intrusion over the time that it flowed past it. To derive first-order estimates for the maximum duration of this flow we numerically modeled one-dimensional conductive heat flow into the intrusion and used the results to forward model ensuing AFT annealing. Modeled levels of annealing were compared to AFT dates and track length data measured across the intrusion. Our results indicate that the episode of main ore-stage hydrothermal fluid flow (mean temperature of 200°C) that formed the ~1,150 metric ton (t) Betze-Post gold deposit had a maximum duration of 〈15 to 45 ka. The average gold flux over this period was ~80 to 30 kg yr –1 , comparable to that measured in the deep reservoirs of several modern geothermal fields. Conservative estimates of gold concentration in the main ore-stage fluids imply that fluid upflow rates and total advective heat flow were also comparable to modern geothermal systems. This suggests that the most important factors for generating the large gold deposits of the northern Carlin trend were a large and/or continuous source of gold, and a very efficient means of removing it from the fluid, rather than the hydrologic system itself. The short duration of main ore-stage fluid flow is unlikely to represent a steady-state convective system. Instead, it most likely reflects a transient period of flow following slip and permeability generation along the steeply dipping Post-Genesis fault system that hosts many of the deposits along the northern Carlin trend. A sudden increase in the permeability of a fault may have led to a transitory period of peak fluid temperature as the fault initially tapped meteoric fluid that had resided at depth and had thermally equilibrated with the host rocks. With continued convection the flow drew cooler, less rock-buffered meteoric water down from higher in the system.

Description: The Altar porphyry Cu-(Mo-Au) deposit occurs in the Miocene to early Pliocene copper belt of the high Andes of western Argentina, close to the Chilean border. A cluster of late Miocene porphyries intruded a more extensive complex of early Miocene rhyolitic to andesitic volcanic and subvolcanic rocks. Potassic alteration in the Central Porphyry was partially overprinted by feldspar-destructive alteration and finally by argillic to advanced argillic alteration. Early stockwork veins have been cut successively by quartz-pyrite veins, tourmaline veins, and enargite veins. Based on correlation analysis of assay results and mapped abundances of these four vein types, we estimate that approximately 11 to 26% of the copper in the Altar orebody is associated with enargite veins, whereas the remainder is associated with the early stockwork veining and potassic alteration. Gold concentration is low compared with typical Cu-Au porphyries of the Andean back-arc region, but higher than in the giant Miocene deposits of Chile, at an average Au/Cu ratio of 0.14 x 10 –4 by weight across the Central Porphyry orebody at Altar. Gold is dominantly associated with chalcopyrite in the domain of stockwork veining and potassic alteration. Small-scale assays in combination with petrographic observations and laser ablation-inductively coupled plasma-mass spectrometry microanalyses show that a significant fraction of the gold in the stockwork veins is enclosed in pyrite as submicroscopic particles attached to inclusions of a Bi-Te-Pb-Ag–enriched Cu-Fe sulfide phase. Microscopic grains of native gold also occur along grain boundaries between pyrite and chalcopyrite, and rarely as larger particles in enargite veins.

Description: The eastern Gangdese magmatic belt (between ~88°E and 94°E) in southern Tibet preserves a series of Cenozoic collision-related igneous rocks that have formed since the start of the India-Asia collision at ~55 to 50 Ma. The Paleocene-Eocene intrusive rocks have intermediate [La/Yb] N ratios (average = 12.0 ± 8.6, n = 150, range = 1.15–64.0), intermediate to low Sr/Y ratios (mostly 〈 40), and negative Eu anomalies (average Eu n /Eu* = 0.76 ± 0.19, n = 150, range = 0.23–1.33). They are mainly composed of equigranular pyroxene and plagioclase in mafic intrusions or porphyritic plagioclase and quartz in intermediate-felsic intrusions, with minor interstitial amphibole and biotite. In contrast, Oligo-Miocene intrusive igneous rocks have high [La/Yb] N ratios (Oligocene: average [La/Yb] N = 42.8 ± 17.0, n = 35, range = 23.2–93.2; Miocene: average [La/Yb] N = 28.5 ± 8.8, n = 93, range = 6.7–49.1), high Sr/Y ratios (〉40), and weak or absent Eu anomalies (Oligocene: average Eu n /Eu* = 0.99 ± 0.27, n = 35, range = 0.67–1.58; Miocene: average Eu n /Eu* = 0.95 ± 0.13, n = 93, range = 0.44–1.46), and their mineralogy consists mainly of plagioclase, quartz, and amphibole as phenocrysts. These geochemical and mineralogical characteristics suggest that the Paleocene-Eocene magmas were relatively dry and evolved primarily by fractionation of pyroxene and plagioclase, whereas the Oligo-Miocene magmas were more hydrous and fractionated significant amounts of hornblende and lesser plagioclase prior to upper crustal emplacement. Oligocene and Miocene magmatic rocks crop out as small-volume intrusions and are associated with several large- to giant-sized porphyry Cu-Mo ± Au deposits, as well as numerous smaller porphyry and skarn deposits. In contrast, earlier, more voluminous Paleocene-Eocene magmatism is only known to be associated with three small porphyry deposits. Erosional loss of subvolcanic porphyry systems from the older Paleocene-Eocene sequence is not thought to be a controlling factor in this temporal difference of porphyry deposit distribution, because exhumation rates after 55 Ma were relatively low and coeval Linzizong volcanic rocks are extensively preserved in the Gangdese belt. Instead, we suggest that differences in magmatic history and petrogenesis led to this restricted temporal distribution of porphyry deposits. The Paleocene-Eocene magmas were generated during the onset of collision between India and Asia and were triggered by rollback of the Neo-Tethyan oceanic slab. This magmatism records the final stage of subduction of oceanic lithosphere beneath southern Tibet, and the relatively low water contents of these magmas may reflect final dehydration of the remnant Neo-Tethyan slab. After oceanic slab breakoff at ~40 to 38 Ma, the Greater India slab began to subduct beneath the Gangdese belt until hard collision with the Indian continent at ~35 Ma and breakoff of the Greater India slab at ~25 to 10 Ma. This last event is suggested to have caused asthenospheric mantle upwelling and partial melting of subduction-modified, hydrated Tibetan lithosphere. The resultant partial melts may have remobilized metals residual in the deep lithosphere from previous arc magmatism, giving rise to a suite of postsubduction porphyry-type ore deposits in the Oligo-Miocene.

Description: The magmatic to hydrothermal transition in the Late Cretaceous Elatsite porphyry Cu-Au-(Mo-platinum group element) deposit has been studied in a suite of samples with clear timing relations between porphyry dikes, magmatic-hydrothermal veins, silicate melt inclusions in quartz veins, fluid inclusion generations, and ore minerals. Ore mineralization occurs late, at temperatures ~200° to 300°C below those of the early, multistage interplay between magmatic and hydrothermal processes at near-magmatic temperature-pressure conditions. Crosscutting relations and petrography indicate that shortly after the intrusion of the earliest, monzodioritic dikes, fluids precipitated a first generation of granular quartz veins accompanied by potassic alteration. The second vein generation with crystalline quartz textures and K-feldspar alteration halos formed at the same time as the second, granodioritic pulse of porphyry intrusions. Cathodoluminescence imaging of quartz growth textures reveals that the earliest fluid inclusions in the crystalline quartz veins are of intermediate density and ~8 wt % NaCl equiv salinity, probably trapped at near-lithostatic pressures of ~1,200 to 1,300 bars at near-magmatic temperatures (≤730°C), and implies a depth of ~4 to 5 km. Depressurization led to phase separation, indicated by a first generation of coexisting brine and vapor inclusions trapped at temperatures of ≥640°C and suprahydrostatic pressures of ≥920 bars. A second quartz generation in crystalline quartz veins precipitated during progressive depressurization and hosts assemblages of coexisting brine, vapor, and silicate melt inclusions, trapped at temperatures in excess of 600°C and suprahydrostatic pressures of 630 to 880 bars. Some open-spaced quartz veins were filled with aplite during this stage. Field relations and geochemical evidence suggest that the aplites as well as the silicate melt inclusions in hydrothermal quartz veins represent volumetrically minor residual melts that evolved directly from granodiorite porphyries at the level of deposit formation, and do not represent aliquots of metal-supplying magma at depth. Fluid inclusions coexisting with the silicate melt inclusions are metal rich, but these fluids predate sulfide precipitation and are, therefore, not the dominant fluids responsible for the Cu-Au mineralization at Elatsite. Melt-fluid-metal separation processes recorded in these co-trapped silicate melt and fluid inclusions in vein quartz are small-scale, local phenomena and do not appear to be suited for understanding and quantifying metal segregation in the deeper source. Microthermometry data of fluid inclusions, trapped during the later stages of the formation of the second quartz generation, show further depressurization to ~260 to 325 bars and cooling to ~460°C, representing a hot hydrostatic regime. Bornite, chalcopyrite, and magnetite seem to be slightly later precipitated at temperatures ≤460°C in separate veins or opened spaces of preexisting veins. Dissolution textures of the second quartz generation indicate a subsequent local redissolution of vein quartz, most likely as a result of passing through a window of retrograde quartz solubility upon further cooling below 460°C. The next, economically most important chalcopyrite-pyrite stage is largely devoid of quartz precipitation and is mostly expressed as "paint veins." Due to the lack of fluid inclusions, the temperature-pressure conditions of formation of these veins could not be constrained. The waning phase of hydrothermal activity is represented by a quartz-carbonate-zeolite stage, formed at low temperature (~145°C), as indicated by microthermometry of fluid inclusions trapped in quartz from this stage. Laser ablation-inductively coupled plasma-mass spectrometry analyses of fluid inclusions show very high Cu contents in early intermediate-density fluid inclusions and in the first vapor inclusion generation, followed by a drastic decrease in the second generation of vapor inclusions. The decrease correlates with the appearance of anhydrite inclusions in the second quartz generation and may indicate that a lack of sulfur in these later vapor inclusions led to Cu partitioning into the brine. Alternatively, it is also possible that, unlike early vapor inclusions, the later vapor inclusions were not susceptible to postentrapment copper enrichment, in accordance with recent experiments. A progressive Cu enrichment in the brine phase correlates well with depressurization prior to mineralization. Mass balance considerations based on analyzed fluid components and fluid phase relations indicate that brine was volumetrically minor and therefore likely stagnant, but this may have prepared ore precipitation by accumulating Cu stripped from ascending vapor.

Description: The Biga Peninsula in northwestern Turkey hosts a large number of high and low sulfidation epithermal gold-silver-(copper) and associated copper-gold porphyry deposits and prospects, associated with voluminous, Eocene-Miocene calc-alkaline volcanism and plutonism. Approximately 15 million gold-equiv oz have been discovered within the last 10 years in this area, with a number of new discoveries now in the drill-testing stage and two deposits in the development stage. At the TV Tower property in the central Biga Peninsula, Eocene(?) to Oligocene intermediate to felsic volcanic and volcaniclastic strata and associated intrusive rocks overlie metamorphosed basement rocks of the Çamlıca Group. The volcanic sequence is broadly divisible into a lower sequence of andesitic dikes and flows intruded by monzonite porphyry stocks and dikes, a middle sequence of andesitic breccias and flows, and an upper sequence consisting largely of dacitic tuffs, flows, and epiclastic strata. The middle and upper sequences are strongly altered over an area covering several tens of km2, including argillic, advanced argillic, and massive to vuggy quartz alteration, the latter largely manifested as ledges hosted in dacitic volcaniclastic rocks. Moderate- and high-angle normal and oblique faults are common in the area and may have influenced the distribution of mineralization. The Küçükdağ zone was drill-tested in 2010 on the basis of strong surface soil and rock sample data. The second hole returned a near-surface intercept of 136.2 m averaging 4.30 g/t Au, 15.0 g/t Ag, and 0.68% Cu. As of the date of this communication, 169 holes have been drilled to test the zone, which measures approximately 600 x 400 m and consists of a lower gold-rich zone and an upper silver-rich zone. Total mineral endowment is estimated at 966,000 indicated and 351,000 inferred gold-equiv oz. Mineralization is strongly controlled by stratigraphy, with gold hosted primarily within a massive lithic lapilli-bearing tuff unit and silver largely constrained to an overlying fluvial-lacustrine sequence capped by andesite breccias. Copper is distributed throughout the zone in association with both gold and silver. The zone is located primarily in the hanging wall of a moderate-angle fault that separates the dacitic volcaniclastic sequence from andesite breccias in the footwall. Gold is associated with tectonic-hydrothermal breccias, zones of vuggy quartz and veins, and alteration minerals consisting primarily of alunite and dickite with sulfides including pyrite, enargite, and covellite. Spatially, silver is largely separate from gold, and is associated with silicification, very fine grained pyrite and/or marcasite and enargite. Paragenetic evidence suggests that the silver event may predate the gold event, but that both may have shared common fluid pathways. A number of other targets are currently being explored on the property, including silicified ledge or rib-style, vuggy quartz-hosted oxide gold, Cu-Au porphyry and low sulfidation epithermal gold-silver veins.

Description: The Devonian subduction-related Heishan mafic-ultramafic intrusion hosting a magmatic Ni-Cu-(PGE) deposit is situated at the southern margin of the Central Asian orogenic belt. The outcrop of the intrusion is ~800 m long and ~470 m wide and dips to the southwest to depths up to 1,300 m. The Heishan intrusion is emplaced in Neoproterozoic dolomitic marble and siliceous slate and dominantly comprises of harzburgite and lherzolite. The stratigraphic reversals of Fo and Ni contents of olivine within the lerzolite and harzburgite indicate injections of several pulses of magmas. The two orebodies (Nos. 1 and 4) within the harzburgite and lherzolite at the lower part of the intrusion host ~35 million metric tons (Mt) of disseminated sulfide mineralization with average grades of 0.6 wt % Ni and 0.3 wt % Cu. The sulfides of the No. 4 orebody are lower in PGE contents on the basis of 100% sulfide (580–1,860 ppb Pt, 720–1,450 ppb Pd, 50–120 ppb Ir) and higher in Cu/Pd ratios (48,000–75,000) relative to the sulfides of the No. 1 orebody and mineralized harzburgites, which have 2,350 to 4,110 ppb Pt, 3,460 to 5,840 ppb Pd, 130 to 160 ppb Ir and Cu/Pd ratios of 16,000 to 26,000. Additionally, the olivines in the No. 1 orebody and mineralized harzburgites have higher forsterite (Fo) and Ni contents than those in the No. 4 orebody. We propose that the sulfide segregation of the No. 4 orebody was associated with extensive introduction of crustal sulfur, which is indicated by high 34 S values (1.9–6.1) and low Se/S ratios (230–390 x 10 –6 ). Reworking of early fractionated massive or net-textured sulfides by the new wave of magma containing unfractionated sulfide droplets resulted in the positive correlation between Pd/Ir and Ni/Ir for the disseminated sulfides of the No. 4 orebody. In contrast, low 34 S values (0.43–1.01) and restricted range of Pd/Ir ratios indicate that the high PGE contents of the sulfides in the No. 1 orebody and mineralized harzburgites resulted from reaction between the sulfides and new pulses of S-undersaturated magmas. The low PGE grades of the Heishan sulfides indicate that the sulfides were segregated from PGE-depleted parental magmas.

Description: The Misery syenitic intrusion in northern Quebec is host to a potentially important, recently discovered rare earth element (REE)-Zr-Nb prospect, containing significant concentrations of both light and heavy REEs, and is conspicuous on aeromagnetic maps as a well-defined, ring-shaped anomaly. Felsic syenite composed of idiomorphic perthite with interstitial mafic minerals, comprising fayalite, hedenbergite, ferropargasite and annite dominates the exposed outer part of the intrusion (the core is covered by Misery Lake). This unit is accompanied by ferrosyenite containing up to 50 vol % mafic minerals, including cumulate fayalite. The ferrosyenite, which occurs as amoeboid-like inclusions in the felsic syenite, is interpreted to have formed by fractional crystallization of ferromagnesian minerals, leaving behind a residual magma which produced the felsic syenites. This latter magma was progressively enriched in alkalis and silica, and only became saturated in ferromagnesian minerals at a very late stage of crystallization. The bulk of the REE mineralization was initially concentrated in pods and layers of fluorapatite that accumulated as a result of gravitational settling. The fluorapatite crystals were partly or completely replaced by britholite-(Ce), which further enriched the rocks in LREEs. Locally, thin quartz-fayalite dikes cut the felsic syenite. These dikes contain up to 10 vol % fergusonite-(Y) and 8 vol % zircon, and are interpreted to be products of an immiscible FeO-SiO 2 -rich magma into which Zr, Nb, and HREEs were preferentially fractionated from the conjugate felsic syenite magma.

Description: High-grade iron ore of the 4EE orebody of the 4E deposit (〉200 Mt at 63.5 wt % Fe) occurs as a southerly dipping sheet within banded iron formation (BIF) of the Paleoproterozoic Dales Gorge and Joffre members of the Brockman Iron Formation. Structural reconstruction of the 4E deposit shows that reactivation of the 18E fault and development of the NW-striking, steeply SW dipping 4E and 4EE normal faults resulted in preservation of the 4EE orebody below the 4E deposit, and 400 m below the modern topographic surface. Three hypogene alteration zones between low-grade BIF and high-grade iron ore are observed: (1) distal magnetite-quartz-dolomite-stilpnomelane-hematite ± pyrite, (2) intermediate magnetite-dolomite-hematite-chlorite-quartz-stilpnomelane, and (3) proximal hematite-dolomite-chlorite ± pyrite ± magnetite. Hydrothermal alteration is temporally and spatially constrained by NW-trending dolerite dikes that intruded the 4E and 4EE faults prior to hypogene alteration. Six vein types (V 1 –V 6 ) are recognized at the 4E deposit. The veins both cut and parallel the primary BIF layers and were emplaced contemporaneously with the hydrothermal alteration zones that record the transformation of low-grade BIF to high-grade iron ore. Our integrated structural-hydrothermal alteration and fluid flow model proposes that during early stage 1a, hypogene fluid flow in the 4E orebody occurred during a period of continental extension and enhanced heat flow within sedimentary basins to the south of the Paraburdoo Range. Heated basinal brines were focused by the NW-striking, steeply SW dipping 4E and 4EE normal faults and reacted with BIF of the Dales Gorge and Joffre members. The warm to hot (160°–255°C), Ca-rich (26.6–31.9 equiv wt % CaCl 2 ) basinal brine interacted with magnetite-chert layers, transforming them into magnetite-quartz-dolomite-stilpnomelane-hematite-pyrite BIF. The iron-rich brine (up to 2.8 wt % Fe) likely originated from evaporated seawater that had lost Mg and Na and gained Li and Ca through fluid-rock reactions with volcaniclastic rocks and carbonate successions within the Wittenoom Formation. The first incursion of deeply circulating, low-salinity (5.8–9.5 wt % NaCl equiv), heated (106°–201°C) modified meteoric water is recorded in late stage 1a minerals. This modified meteoric water had lost some of its Na through wall rock interaction with plagioclase, possibly by interaction with dolerite of the Weeli Wooli Formation that directly overlies the Joffre and Dales Gorge members. Stage 1b involved continuing reactions between the hydrothermal fluids and the magnetite-quartz-dolomite-stilpnomelane-hematite-pyrite BIF, and produced both the intermediate magnetite-dolomite-hematite-chlorite-pyrite and the proximal hematite-dolomite-magnetite-stilpnomelane alteration assemblages. Microplaty (10–80 μ m), platy (100–250 μ m), and anhedral hematite increasingly replace magnetite in the intermediate alteration zone, forming the proximal alteration zones that consist of microplaty, platy, anhedral hematite and magnetite. The intermediate and proximal alteration zones represent the mixing of a hot (250°–400°C), high-salinity, Ca-rich (30–40 wt % CaCl 2 equiv), Sr-rich basinal brine with low-temperature and low-salinity (~5 wt % NaCl equiv) modified meteoric water that was heated (~100°–200°C) during its descent into the upper crust. Heterogeneous mixing of the two end-member fluids resulted in the trapping of primary fluid inclusion assemblages containing a wide range of trapping temperatures (up to 200°C) and salinities (up to 25 wt % NaCl equiv). Stage 1c of the hypogene hydrothermal fluid is characterized by low-temperature (〈110°C), low-salinity (~5 wt % NaCl) meteoric water that interacted with the proximal hematite-dolomite-magnetite-stilpnomelane–altered BIF, leaving a porous, hematite-apatite high-grade ore. Supergene alteration affected the orebody since the Cretaceous and produced a hematite-goethite alteration assemblage, resulting in destruction of the hypogene alteration zones that are only preserved below the depth of modern weathering. Discovery of the concealed 4EE orebody of the 4E deposit demonstrates that structural geology plays a critical role in the exploration for high-grade iron orebodies. Structural reconstruction should be considered a critical exploration activity in structurally complex terranes where concealed orebodies may exist.

Description: The Yanque nonsulfide Zn-Pb deposit (26,491 kilotonnes of indicated resources at 2.37% Zn and 2.18% Pb [1.67% ZnEq cutoff]; 8,701 kilotonnes of indicated resources at 4.05% Zn and 3.32% Pb [4.00% ZnEq cutoff]) is located in the Andahuaylas-Yauri metallogenic province, southern Peru. The deposit occurs in a base metal district, with the Dolores porphyry copper deposit at its center. Sedimentary and igneous rocks predominate in the Yanque-Dolores area. The sedientary rocks belong to the Soraya, Mara, and Ferrobamba formations (Upper Jurassic-Upper Cretaceous). Igneous rocks are mainly intrusive products associated with the Andahuaylas-Yauri batholith (diorite/tonalite, tonalite porphyry, and quartz diorite/monzonite porphyry). The Yanque deposit is hosted by two different sedimentary facies consisting ofa siliciclastic conglomerate and a sedimentary dolomite breccia at the transition between the Mara and Ferrobamba formations. Hypogene mineralization consisted of galena, pyrite, and sphalerite, associated with a strong muscovite/illite hydrothermal alteration of the host rock (similar to alteration observed in porphyry copper deposits) and minor kaolinite, dolomite, and quartz. Minor element geochemistry, characterized by Sb, As, Mn, Ag, and Cu, suggests a magmatic-hydrothermal source for primary mineralization at Yanque. The Pb isotope compositions of Yanque and Dolores sulfides (pyrite and galena) are similar: 206 Pb/ 204 Pb ratios are between 18.545 ± 0.003 and 18.656 ± 0.002, 207 Pb/ 204 Pb ratios between 15.632 ± 0.003 and 15.653 ± 0.006, and 208 Pb/ 204 Pb ratios between 38.531 ± 0.009 and 38.649 ± 0.007. This indicates that the Yanque primary Zn-Pb mineralization likely formed from the same hydrothermal system that produced the Dolores porphyry copper deposit. The Pb isotope values of sulfides in both deposits are typical of the Tertiary magmatically derived ores in this part of Peru. Economic mineralization at Yanque consists of oxidized minerals resulting fromweathering and alteration of hypogene sulfide mineralization. Sauconite (Zn smectite clay) and cerussite (Pb carbonate) with minor hemimorphite (Zn hydrosilicate) and smithsonite (Zn carbonate) predominate. Sauconite formed as replacements of detrital feldspar and hydrothermal muscovite/illite and kaolinite. In contrast to carbonate-hosted nonsulfide Zn-Pb deposits, sauconite at Yanque occurs in comparable amounts as in the world-class Skorpion deposit (Namibia). Cerussite replaced galena and precipitated as cement in the host rock. Hemimorphite and smithsonite mainly crystallized in late-forming veins. Unlike most supergene Zn-Pb deposits, the Yanque orebody shows comparable ore rades for both Pb and Zn with an inverse supergene chemical zoning. Zinc is most abundant at the top of the deposit, whereas lead increases with depth. These two elements have different mobility in solution, which implies that the primary source of zinc in the Yanque nonsulfide mineralization was originally located far from the primary galena mineralization, the remnants of which are present in situ. Supergene zinc mineralization was derived from the dissolution of primary sphalerite hosted in the now-eroded rocks above the Yanque mine site. Zinc-rich solution migrated through the sedimentary host rocks and precipitated Zn minerals, mainly sauconite, in pore spaces and replaced detrital and hydrothermal alteration minerals.

Description: This study tests the utility of the minor to trace element composition of sphalerite to discriminate between possible sources of ore-forming fluids, and to constrain processes involved in ore genesis of the world-class Irish-type Navan Zn-Pb orebody, Ireland. Detailed petrography and electron microprobe microanalyses were performed on layered sphalerite previously analyzed for Zn, Fe, and S isotope compositions. Layered sphalerite displays a wide range of chemical composition at both sample and crystal scales. The color, style, and scale of layering show variations with chemical composition, but none of these correlations are consistent between samples. However, there are strong intersample correlations between chemical and S-Fe-Zn isotope compositions at the millimeter scale. Sphalerite precipitated from deep, hydrothermal fluids with 34 S-enriched sulfide is enriched in Cd, Sb, Cu, and Ag, whereas Fe and As are enriched in sphalerite precipitated from shallow, bacteriogenic brines with isotopically light S. Significant chemical variations also occur at the micrometer scale in sphalerite regardless of its genetic affinity. These variations are interpreted as being due to variations in temperature, pH, and sulfur activity following the arrival at the site of deposition of pulses of hydrothermal fluids at specific stages of sphalerite growth. Therefore the chemistry of sphalerite, coupled with its texture, appears to be a powerful tool to elucidate fluid typing and ore genesis. The results support the hypothesis that layered sphalerite forms by rapid crystallization due to mixing of two fluids of contrasting physicochemical properties, a process required for the formation of Irish-type and some other hydrothermal deposits including, most notably, Mississippi Valley-type (MVT) deposits. The existence of layered sphalerite, coupled with its texture and chemical composition, may therefore provide useful insight for the mineral exploration industry in Ireland and elsewhere.

Description: Although gold in high-sulfidation epithermal deposits generally occurs as the native metal or electrum, in some deposits, a significant proportion of the gold is hosted in pyrite. Here we use a combination of petrography, whole-rock geochemistry, pyrite chemistry, crystallography, and phase stability relationships to determine how gold was transported and incorporated into pyrite in two relatively young high-sulfidation epithermal deposits, where the gold occurs almost exclusively in solid solution or as nanoparticles in pyrite. The genetically related Bawone and Binebase Au (Cu-Ag) deposits, located 1 km apart on the volcanic island of Sangihe, northeastern Indonesia, are hosted by andesitic volcaniclastic rocks that were altered to a proximal advanced argillic association of quartz + pyrite (py I) + pyrophyllite + natroalunite + alunite + dickite + kaolinite and a more distal intermediate argillic association of quartz + pyrite (py I) + kaolinite + dickite + illite. The economic mineralization takes the form of multiple generations of auriferous pyrite, the first of which, pyrite I (py I), developed during advanced argillic alteration. Mass balance calculations show that all elements were mobile with the exception of Nb, Ti, some rare earth elements, and possibly Al. The highest gold concentration is in pyrite II (py II), which occurs in veins that cut pyrite I. This drusy variety of pyrite is characterized by complex growth and sector zoning, and contains as much as 6.0 wt % Cu. The elevated Cu concentrations correlate positively with Au and As concentrations, whereas the Ag concentration correlates strongly with Au but not Cu. Later barite-enargite mineralization exploited py II veins and vugs, and significant concentrations of Ag and Au are hosted by enargite, although the Au concentration in enargite is lower than in py II or py I. A model is presented in which the fluid responsible for advanced argillic and intermediate argillic alteration and associated stage 1 gold mineralization was a condensed magmatic vapor derived from an oxidized magma. The gold and other metals were transported as hydrated species that ascended through the volcanic pile via fractures and zones of enhanced permeability to a depth between 900 and 1300 m, where the vapor condensed at a temperature between 250 and 340°C to form an acidic liquid with a pH of ~2.5; f o 2 ranged up to four log units above the hematite-magnetite buffer. Interaction of this liquid with the host andesites caused advanced argillic and intermediate argillic alteration, including sulfidation of mafic minerals to form py I. During crystallization of py I, Au, Cu and Ag were adsorbed onto the surface of the pyrite and deposited as nanoparticles, or were incorporated in the pyrite structure. Adsorption of Au, Cu, and Ag from the condensed vapor reached a peak during the crystallization of vein-hosted py II, and the uptake of Ag and minor Au continued during later crystallization of enargite. From the distribution of metals among growth and sector zones in py II, incorporation of gold and other metals appears to have been maximized when physicochemical conditions were relatively stable. This is in contrast to the requirement for native gold precipitation, namely that physicochemical gradients be steep to ensure supersaturation of gold in the ore fluid.

Description: The Ming deposit, Baie Verte, Newfoundland, Canada, is an Early Ordovician bimodal-mafic, Cu-Au-(Zn-Ag) volcanogenic massive sulfide (VMS) deposit. The deposit consists of a number of ore lenses that are stacked in the uppermost part of the Rambler rhyolite. One of the uppermost lenses, the 1806 zone, is enriched in Au and Ag. The deposit has been affected by Silurian-Devonian greenschist to amphibolite grade metamorphism and polyphase deformation and this has led to debates as to whether the Au-Ag enrichment in the deposit is syngenetic or a product of a later metamorphic and structural overprint. The 1806 zone consists predominantly of discordant sulfide stringer, stratabound semimassive to massive sulfides and a weakly mineralized silicified cap zone. The ore is largely hosted within the footwall Rambler rhyolite that is strongly altered to quartz-sericite ± green mica with sporadic chlorite-biotite. Base and precious metal zoning is developed from the down-plunge portion (Cu-Au) to the up-plunge portion (Cu-Zn-Au-Ag-Pb) of the 1806 zone. The ore mineralogy and mineral chemistry are complex and are interpreted to be of an intermediate sulfidation type. Pyrite and chalcopyrite are the dominant sulfide species with minor to accessory sphalerite, galena, pyrrhotite, and arsenopyrite. Sulfosalts including Ag-bearing tennantite-tetrahedrite, stannite, boulangerite, and loellingite, and precious metal-rich phases are common throughout the deposit. Precious metals occur as (1) mercurian electrum (7.93–20.57 wt % Hg) and (2) various Ag phases (miargyrite, pyrargyrite, mercurian stephanite, unnamed AgCuFeS phase, Ag-Hg ± Au alloys) that are more abundant in the up-plunge portion of the 1806 zone. Tellurides and bismuthides are present in trace amounts. Oxides (cassiterite and magnetite) are present exclusively in the down-plunge portion of the 1806 zone. The deposit geometry, metals distribution, complex ore assemblages, abundant sulfosalts, and the significant concentration of the epithermal suite of elements (e.g., Au, Ag, As, Hg, Sb, Bi) in the ore strongly contrast with those of orogenic Au deposits and support a syngenetic or synvolcanic origin for precious metals in the Ming VMS deposit, including a possible magmatic input into the 1806 zone ore-forming system. Evidence for a late, syndeformation precious metal emplacement via orogenic overprinting is lacking at the 1806 zone and all data point to a syngenetic origin for precious metal enrichment, indicating that Ordovician Appalachian VMS deposits are favorable targets for Au. Nevertheless, Silurian-Devonian metamorphism and deformation have resulted in textural recrystallization and local remobilization of precious metals and the main fabrics largely control the current geometry of the host succession and of the Ming orebody.

Description: This investigation of the space-time progression of magmatism and hydrothermal activity in the Patagonia Mountains of southern Arizona is based on field and paragenetic relationships, and on U-Pb and 40 Ar/ 39 Ar geochronology of igneous and hydrothermal minerals. The Patagonia Mountains consist of Precambrian, Paleozoic, and Mesozoic sedimentary, granitic, and volcanic rocks, Laramide volcanic rocks, and a core of Laramide intrusions that comprise the Patagonia Mountains batholith. Laramide igneous rocks and adjacent Paleozoic and Mesozoic rocks contain significant porphyry Cu-Mo deposits, Mo-Cu breccia pipes, Ag replacement deposits, and numerous other Cu-Pb-Zn-Ag replacement and vein deposits. Ages of igneous and hydrothermal minerals from 20 U-Pb and 52 40 Ar/ 39 Ar determinations define four magmatic and magmatic-hydrothermal events that formed the batholith and altered parts of it and adjacent rocks; cumulatively the events span at least 16 m.y., from ~74 to 58 Ma. The oldest event of this succession includes the 74 Ma Washington Camp stock and spatially associated Cu-Pb-Zn-Ag replacement deposits in Paleozoic carbonate rocks of the Washington Camp-Duquesne district. Eruption of 73 to 68 Ma volcanic rocks in the northern part of the range was the next youngest event, which coincides temporally with replacement and vein deposits in Paleozoic carbonate rocks at the Flux mine (~71 Ma). An event at 65 to 62 Ma is marked by emplacement of small-volume quartz monzonite, granodiorite, and diorite intrusions, formation of the Ventura breccia deposit in Jurassic granite at 65 to 64 Ma, and formation of other Pb-Zn-Ag-Cu replacement and vein deposits (~62 Ma; Blue Nose and Morning Glory). The Red Mountain porphyry Cu-Mo system is hosted by ~62 Ma granodiorite and Laramide volcanic rocks (73–68 Ma) at the northern end of the batholith. It includes a deep, chalcopyrite-bornite resource (~60 Ma) that is associated with potassic and sericitic alteration and a near-surface chalcocite-enargite resource (60 Ma) that is associated with advanced, supergene-enriched argillic alteration. The youngest event includes the Sunnyside porphyry Cu-Mo system and a Cu-Mo breccia deposit at Red Hill (Four Metals mine), both of which formed in large-volume quartz monzonite, granodiorite, quartz monzonite porphyry, and quartz feldspar porphyry (~61–59 Ma). Similar to the Red Mountain system, the Sunnyside system consists of a deep chalcopyrite resource that occurs in ~60 to 59 Ma quartz feldspar porphyry, and a near-surface, slightly younger (~59–58 Ma) enargite-chalcocite-tennantite resource that occurs in quartz feldspar porphyry, quartz monzonite porphyry, and Mesozoic rocks. The Red Hill Cu-Mo breccia deposit is hosted by large-volume quartz monzonite, granodiorite, and quartz monzonite porphyry (~61–59 Ma). Discrepancies between field and paragenetic relationships and some analytic ages at Sunnyside and Red Hill preclude precise dating of mineralization stages, and may reflect disturbance of isotope systems by multiple, co-spatial to juxtaposed intrusive and hydrothermal events, and/or by unrecognized intrusions. Numerous vein and replacement deposits at the northern end of the batholith, including the Hardshell Ag resource and the Three R supergene chalcocite resource, are distal deposits of the Sunnyside and Red Mountain systems. Small, ~61 to 59 Ma Cu-Mo deposits in large-volume intrusions in the southern part of the batholith consist of hydrothermal quartz, biotite, K-feldspar, muscovite, chalcopyrite, and molybdenite. The age span of magmatic and magmatic-hydrothermal events in the Patagonia Mountains, minimally 16 m.y., is comparable to that of certain other magmatic-hydrothermal successions that contain porphyry Cu-Mo systems. Magmatic-hydrothermal events of the Wasatch-Oquirrh igneous trend, Utah, and the Boulder batholith, Montana, both span ~17 m.y. and include the Bingham and Butte porphyry Cu-Mo, vein and replacement deposits, respectively. Plutons and mineral deposits in the Pima district, Arizona, which includes the porphyry Cu-Mo deposits at Sierrita-Esperanza, Mission-Pima-San Xavier North, and Twin Buttes, formed over an interval of ~14 m.y. The diversity of igneous and hydrothermal products likely reflects evolutionary processes occurring at multiple sites in the lithosphere and at different time scales from 〉10 m.y. to less than the geochronologic precision currently achievable.

Description: The Semail ophiolite in Oman is capped by up to 2 km of basaltic-andesitic lavas that host copper-dominant, Cyprus-type, volcanogenic massive sulfide (VMS) deposits. This study identifies multiple volcanostratigraphic horizons on which the deposits are situated, based on characterization of footwall and hanging-wall lavas from 16 deposits or deposit clusters. Comparison of field and petrographic features, compositions of igneous clinopyroxenes, and whole-rock geochemical signatures permits classification of the lavas within a modified version of the established regional volcanostratigraphy. Four extrusive units host deposits: Geotimes (earliest), Lasail, Alley, and Boninitic Alley (latest). The latter was previously known only at few localities, but this study reveals its regional extent and significance as a host for VMS deposits. The Geotimes and Lasail units represent Late Cretaceous, ocean spreading ridge and related off-axis volcanic environments, respectively. The Alley and Boninitic Alley units represent younger, subduction-related volcanism prior to Coniacian-Santonian obduction of the ophiolite. Our results show that VMS deposits occur on or near the Geotimes/Lasail and Geotimes/Alley contacts as well as entirely within the Geotimes, Lasail, Alley, and Boninitic Alley units. Highest Cu grades tend to occur in deposits lying on or within the Geotimes, whereas highest Au grades occur in deposits within the Boninitic Alley. In contrast to earlier studies, we conclude that essentially every horizon marking a hiatus in lava deposition in the Semail ophiolite, i.e., contacts between the four major eruptive units, and umbers and sedimentary chert layers within the units, has exploration potential for Cu-Au VMS deposits.

Description: Many ~260 Ma mafic-ultramafic layered intrusions, including Hongge and Panzhihua, in the Emeishan large igneous province, southwestern China host world-class Fe-Ti-V oxide ore deposits. These two most important ore-bearing intrusions show differences in lithology and mineral chemistry. The most important orebodies in the Hongge intrusion occur as concordant layers in the middle part of the intrusion, closely associated with clinopyroxenites. Titanomagnetite and Mg-rich ilmenite are the major ore minerals of the Hongge deposit. Coexisting clinopyroxene contains 〉1.7 wt % TiO 2 . These data indicate high Ti parental magma for the Hongge ore-bearing clinopyroxenites. In the Panzhihua intrusion, the most important orebodies also occur as concordant layers but in its lower part instead of middle part. In contrast with the Hongge deposit, the most important host rocks of the Panzhihua deposit are gabbros, not clinopyroxenites. In addition, ilmenite is rare and titanomagnetite is predominant in the Panzhihua deposit. Coexisting clinopyroxene in the Panzhihua deposit contains 〈1.6 wt % TiO 2 . The contrasting lithologic and mineral compositions indicate that the parental magma for the Hongge deposit has higher TiO 2 than that for the Panzhihua deposit. The compositions of clinopyroxene from the Panzhihua and Hongge deposits indicate that their parental magmas are also different in MgO/FeO and trace element ratios as well. Calculations using average experimental Mg-Fe exchange coefficient and trace element partition coefficients between clinopyroxene and magma show that the Hongge parental magma has higher MgO/FeO ratios and more fractionated mantle-normalized trace element patterns than the Panzhihua parental magma. The estimated compositions of the parental magmas for the Hongge and Panzhihua ore-bearing lithologies resemble the average compositions of the Longzhoushan-type high Ti basalts and Ertan-type intermediate Ti basalts in the Emeishan large igneous province, respectively. The new trace element data from this study, together with available Sr-Nd isotope data for the ore-bearing intrusions from literature, support a new petrogenetic model involving selective assimilation of newly subducted, stagnant oceanic gabbroic slab above the deep-seated Emeishan mantle plume. This process and subsequent contamination with the upper crust played an important role in the variation of parental magma compositions between the Hongge and Panzhihua magmatic oxide ore deposits. Abundant Fe-Ti oxide ore deposits associated with less evolved basaltic magma in the Emeishan large igneous province than elsewhere in the world are attributed to selective assimilation of newly subducted, stagnant oceanic lithospheric slab by the ascending mantle plume-derived picritic magma that was originally undersaturated with Fe-Ti oxides.

Description: Mineralized occurrences in the Baiganhu district were discovered in 2002 after extensive exploration in the eastern Kunlun domain in the Kunlun terrane. Tungsten-Sn deposits in the area contain total resources of 174, 913 metric tons (t) of WO 3 and 79, 091 t of Sn, which makes the Baiganhu field a new large W-Sn metallogenic province after the Nanling region of southern China. The W-Sn mineralization in the Baiganhu field is spatially associated with monzogranite that yielded a 238 U- 206 Pb zircon age of 430.5 ± 1.2 Ma ( n = 25). Cassiterite yielded a 206 Pb/ 207 Pb- 238 U/ 207 Pb isochron age of 427 ± 13 Ma ( n = 32), which confirms a close relationship of the early Silurian intrusion and the W-Sn mineralization. The timing of the mineralization indicates another important W-Sn metallogenic event in China in addition to the period of Late Jurassic (from 160–150 Ma) in the Nanling region. The mineralization is divided into three stages: (I) scheelite-bearing skarn stage, (II) wolframite- and scheelite-bearing greisenization stage, and (III) wolframite- and cassiterite-bearing quartz-veining stage. Quartz in the wolframite- and cassiterite-bearing quartz veins shows two types of fluid inclusions: liquid-rich two-phase aqueous inclusions and CO 2 -rich and CH 4 -bearing three-phase inclusions. Inclusions have medium salinity (10–14 wt % NaCl equiv), low density (0.60–1.06 g/cm3), and moderate homogenization temperatures (240°–270°C). The CO 2 phase in three-phase inclusions shows a large variation from 10 to 70 vol %, which is attributed to immiscible separation of a CO 2 -rich phase from saline aqueous fluids. The immiscible separation likely contributed to the mineralization in quartz veins. The $${\delta }^{18}{\mathrm{O}}_{{\mathrm{H}}_{2}\mathrm{O}}$$ values of the mineralizing fluids calculated from quartz and D of inclusion fluids in quartz vary from +4.5 to +6.4 and –65 to –43, respectively, supporting that the mineralizing fluids originated from the parental magmas.

Description: Altar (31° 29' S, 70° 28' W) is a large porphyry Cu-(Au-Mo) deposit with associated epithermal Au-(Ag-Cu) veins located in the Cordillera Principal of southwest San Juan province (Argentina). Altar is a complex magmatic-hydrothermal system formed from several magmatic and hydrothermal pulses during the middle-late Miocene. New LA-ICPMS U-Pb ages in zircons from the Altar porphyries indicate four discrete events of intrusions over an extended magmatic life time of ca. 3 m.y. It comprises a premineralization porphyry (11.75 ± 0.24 Ma), three mineralized porphyries (11.62 ± 0.21 and 11.68 ± 0.27 Ma, 11.13 ± 0.26 Ma, 10.35 ± 0.32 Ma) related to hydrothermal breccias, two postmineralization intrusions, and a postmineralization breccia (8.9 ± 0.4 Ma). The three mineralized porphyries (porphyries 2, 3, and 4) were emplaced within ~0.7 to 1.3 m.y. Amphibole phenocrysts from the porphyries crystallized from oxidized magmas ( f O2 = NNO +1 to +2) at temperatures of 780° to 850°C and pressures between 0.9 and 1.8 kbars corresponding to depths of ~4 to 7 km. Anorthite- and Fe-rich rims in the plagioclase phenocrysts suggest that the magmatic chambers were episodically recharged by a less evolved magma. The middle-late Miocene intrusions are interpreted to have been derived from a deeper and relatively large magmatic reservoir that supplied magmas to smaller chambers located in the upper crust. The focused magmatic output to shallow levels during a period of a few million years in the Altar area has been a main requirement in the formation of this large porphyry copper deposit.