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Hydrothermally altered country rocks surrounding hydrothermal ore deposits are commonly characterized by altered δ 18 O and δ 13 C values relative to distal, unaltered rocks. Stable isotopes have been recognized as a powerful tool to complement traditional geochemical and geophysical approaches during mineral exploration, particularly in carbonate replacement and Carlin-type deposits. However, wall-rock alteration and carbonate veining during ore formation is often accompanied by the precipitation of sulfide minerals, complicating stable isotope analysis. The classic phosphoric acid extraction technique for carbonate minerals results in the release of additional H 2 S gas, interfering with isotope ratio mass spectrometry (IRMS), and laser absorption spectrometry (LAS) analysis of CO 2 . Here we present the first results from a novel off-axis integrated cavity output spectrometer (OA-ICOS) instrument, capable of determining the stable isotope composition ( δ 13 C, δ 18 O) of mixed CO 2 -H 2 S gas evolved from carbonate-sulfide mixtures. Instrument calibration was performed by analyzing in-house and commercially available CaCO 3 stable isotope reference materials. Analyses of mixed CO 2 -H 2 S tank gas and sulfide-spiked stable isotope reference materials containing up to 40 wt % chalcopyrite and sphalerite demonstrate that the method outlined here is capable of effectively analyzing previously problematic high sulfide samples. The analytical capabilities, together with the robust nature of the OA-ICOS instrument, may be of direct practical use for mineral exploration in carbonate-hosted hydrothermal systems including ore deposits.

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

The Guelb Moghrein Cu-Au deposit occurs within metacarbonate bodies derived from sedimentary carbonate beds of probable late Archean age. The metacarbonate lenses are enclosed within mafic meta-igneous rocks that display a coarse-grained, amphibolite-facies (hornblende-albite) assemblage that is mineralogically anomalous relative to the greenschist-facies metamorphic rocks present regionally within the district and may represent a zone of synmetamorphic hydrothermal metasomatism. Crosscutting relationships and textural evidence for progressive replacement of different mineral assemblages demonstrate that the deposit had a complex, multiphase alteration-mineralization history. Early sodic alteration at Guelb Moghrein resulted in the formation of albitized zones along moderately to shallowly dipping structures. Sodic alteration was followed by syndeformational ferroan potassic alteration that resulted in the growth of biotite and grunerite-cummingtonite in meta-igneous rocks and conversion of calcite-dolomite in metacarbonate bodies to ankerite- and siderite-rich assemblages. Progressive alteration resulted in breakdown of siderite to magnetite and graphite at approximately 400° to 450°C. Sulfide mineralization took place at ca. 2.5 Ga, concurrently with or shortly after ferroan potassic alteration and magnetite-graphite formation. An early assemblage of Co-, Ni-, and As-rich sulfide minerals was followed by precipitation of pyrrhotite and chalcopyrite. Sulfide mineralization was focused within brittle deformation zones along the margins of the metacarbonate bodies, which represent high-permeability zones adjacent to more ductile structures. The nucleation of sulfides on graphite during siderite breakdown suggests redox reactions may have largely controlled sulfide precipitation. A late magnesian alteration event at ca. 1.74 Ga resulted in the formation of chlorite-rich shear zones and precipitation and/or recrystallization of sulfides. The Guelb Moghrein deposit is a member of the iron oxide Cu-Au class of deposits but is unusual in being hosted in carbonate strata.

The Tootoo and Mequillon Ni-Cu-Pt-Pd deposits in the South Raglan Trend of the New Quebec orogen host significant but as yet unexploited nickel-copper-platinum group element (PGE) mineralization within the mafic-ultramafic rocks of the Expo Intrusive Suite. The olivine melagabbronorite host rocks have been pervasively altered to hydrous mineral assemblages dominated by chlorite, tremolite-actinolite, and relict late-magmatic hornblende, characteristic of greenschist facies metamorphism. In greenschist-altered rocks, the PGE are spatially associated with intercumulus blebs and networks of pyrrhotite, chalcopyrite, and pentlandite after primary magmatic sulfides. Palladium is significantly enriched in the vicinity of veins associated with local replacement of the early hydrous silicate mineral assemblage by talc, calcite, dolomite, and minor serpentine and disseminated magnetite. The varieties of platinum group minerals (PGM) are similar in samples both with and without carbonate alteration, with platinum hosted exclusively in discrete sperrylite grains, and palladium dominantly hosted by the Pd-Sb intermetallic phases of sudburyite, ungavaite, and naldrettite, as well as merenskyite, kotulskite, and michenerite. Palladium minerals commonly form composite grains with hessite, altaite, and other PGM. Most of the PGM are hosted by silicate minerals, but some also occur attached to sulfide minerals, and a few PGM grains are completely enclosed within sulfides. In samples disturbed by carbonate alteration, the PGM are present in greater abundance and larger size. We suggest that palladium was transferred from massive ores to the net-textured ores via an oxidizing, CO2-bearing, neutral to mildly alkaline fluid capable of mobilizing the semimetals such as antimony as hydroxide complexes. Upon reaching zones where oxygen fugacity was buffered to lower values by an assemblage containing pyrrhotite but not magnetite, antimony in the fluid was reduced to its metallic state, causing coprecipitation of palladium in intermetallic phases. This process appears to have been localized at a reaction front near the outer edges of moderately oxidized carbonate-altered domains surrounding carbonate veins. This reaction front has high Pd/Pt whereas lower Pd/Pt ratios are observed in magnetite-bearing, carbonate-altered samples closer to the carbonate veins. PGE grades in the vicinity of carbonate alteration attain levels of potential economic significance, possibly meriting attention as PGE resources distinct from the Ni-Cu-PGE sulfide assemblage extracted by current beneficiation practices in nearby deposits. If the magnetite-free, carbonate-altered lithofacies can be confirmed by further study to show consistently elevated Pd grades, then it may be possible to improve Pd recoveries.

Approximately 10% of copper resources in the Central African Copperbelt, the world’s largest, sediment-hosted stratiform copper province, occur in the Domes region of northwestern Zambia. The copper deposits and prospects are commonly hosted by amphibolite-facies metamorphic rocks and lie within or adjacent to several basement inliers. Minor molybdenite accompanies the copper-iron sulfide minerals in two deposits and four prospects within the Mwombezhi dome as well as in deposits elsewhere in the Domes region. The results of Re-Os dating of 11 molybdenite samples reveal the existence of two discrete copper-mineralizing epochs, separated by >500 m.y. Most of the mineralization, including that in the Chimiwungu orebody at Lumwana, the largest deposit in the Mwombezhi dome, formed within a maximum interval of ~42 m.y. (538.9–497.1 Ma), which spans the peak of the Lufilian collisional orogeny. In contrast, three samples (four determinations) from the Nyungu prospect returned Mesoproterozoic ages of 1084 to 1059 ± 5 Ma, coincident with the Irumide collisional orogeny. Although several investigators have speculated that the copper in the Central African Copperbelt deposits was extracted from the basement, this is the first unambiguous evidence for an appreciable copper concentration of pre-Katangan (pre-Neoproterozoic) age. Based on this evidence, additional Mesoproterozoic and possibly even older copper concentrations seem likely to exist elsewhere beneath and/or alongside the Copperbelt in both Zambia and the Democratic Republic of Congo. Such preenrichment of the basement in copper may have contributed to the enormous metal endowment of the Central African Copperbelt.

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.

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.

The Bovine Igneous Complex (referred to as the BIC intrusion) occurs within the Marquette-Baraga dike swarm in the Upper Peninsula of Michigan. The intrusion is likely related to the ~1.1 Ga Midcontinent Rift system based on chemical similarities with the nearby Eagle mine and other Midcontinent Rift-related picrites, the lack of deformation fabric, and well-developed hornfels that overprints Penokean deformation fabrics. However, there is currently no age date for the BIC intrusion to confirm that it is a rift-related intrusion. The BIC is a relatively small layered intrusion with dimensions of ~1,100 × 400 × 732 m thick. Units include feldspathic wehrlite/olivine melagabbros, feldspathic olivine clinopyroxenite, and gabbro. The Little BIC, a satellite intrusion located ~100 m from the main intrusion, is primarily feldspathic wehrlite. Ni-Cu sulfide mineralization occurs in both intrusions, with a higher relative proportion in the Little BIC intrusion. Sulfide assemblages are comprised predominantly of pyrrhotite, chalcopyrite, and pentlandite; textural types vary from disseminated through globular to semimassive net-textured and locally massive. The δ 34S values of sulfide minerals in the BIC intrusion range from −6.0 to +1.5‰, with ~ 80% of the values between 0 and 1.5‰. The Proterozoic Michigamme Formation country rocks show δ 34S values between 3.0 and 9.8‰. The δ 34S values of the semimassive sulfides in the Little BIC intrusion are similar to those in country rock (3.8−6.7‰), whereas the massive sulfide values are similar to those of the sulfide minerals in the BIC intrusion. Although many values are near those of mantle-derived sulfur, the presence of strongly negative values in both intrusions indicates that crustally derived sulfur was involved in the mineralization process. In addition the distinctly positive values of disseminated sulfides in the Little BIC intrusion indicate that a large fraction of the sulfur involved in the mineralization was of country-rock origin and that at least two pulses of sulfide-saturated liquids were involved in its genesis. The Δ33S values for sulfide minerals in the BIC and the Little BIC intrusions range from −0.33 to +0.35‰, indicating that a component of Archean sulfur was also involved in the mineralization. Olivine Fo and Ni contents are consistent with rock types being related via fractional crystallization, but this process cannot account for the wide range of sulfide zones found throughout both intrusions. Multiple pulses of magma, several sulfide saturated and with distinct sulfur isotope compositions, were involved in the formation of the BIC and the Little BIC intrusions. Both sulfide concentration and isotopic composition suggest that sulfide saturation was promoted via the addition of country rock-derived sulfur.

The El Callao mining district is the most important gold-producing region in Venezuela. It is hosted in the Paleoproterozoic Guasipati-El Callao greenstone belt, which forms part of the Guayana craton, the Venezuelan extension of the Guiana Shield of South America. It consists of volcanic and volcanosedimentary sequences that were affected by several deformation events, particularly localized shear zones. The Colombia mine, the largest active mine in the district, produces 4 tonnes (t) (128,600 oz) of gold annually with reserves estimated at 740 t (24 Moz) and grades of up to 60 g/t. Gold mineralization is concentrated within a vein network in the Colombia corridor, a shear fracture-hosted mesh of interconnected quartz-ankerite-albite veins enclosing fragments of altered metabasaltic host rocks. Gold occurs mostly in the metabasaltic fragments and is spatially associated with pyrite, in which it occurs as invisible gold, micron-sized native gold inclusions, and filling fractures. Based on optical and scanning electron microscopy-backscattered electron observations, two types of pyrite are recognized: a simple-zoned pyrite and a less common oscillatory-zoned pyrite. Both types consist of a mineral inclusion-rich core and a clearer rim; however, in oscillatory-zoned pyrite, the latter is composed of complex rhythmic overgrowths of alternating As-rich and As-poor bands. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis and elemental mapping reveal the presence of invisible gold in all generations of pyrite. The highest concentrations (5–23 ppm Au) are found in oscillatory-zoned pyrite rims, which correlate with the highest As concentrations (16,000–23,000 ppm). In As-poor bands, Au (up to 1.5 ppm) and As (300–6,000 ppm) concentrations decrease by about an order of magnitude. Copper, Bi, Te, Sb, Pb, and Ag always occur with invisible gold, particularly in pyrite cores, suggesting that at least part of the gold occurs in sulfosalt nanoparticles of these metals and metalloids. Visible native gold grains occur as small inclusions throughout core and rim of both pyrite types, as well as in fractures within it. In both occurrences, chalcopyrite, sphalerite, tellurobismuthite, ankerite, albite, and chlorite accompany native gold, and gold fineness ranges between 900 and 930. At an early stage of vein mesh formation, pyrite formed in the metabasaltic fragments at the expense of ankerite, which, in turn, resulted from alteration of Fe-Ti oxides. Gold, together with other chalcophile elements, was incorporated within the structure of pyrite, most likely by destabilization of metal sulfide complexes during ankerite replacement. Subsequent cyclic reactivations of the shear zone caused development of pressure shadows around pyrite, generating local and repeated decreases in pressure, which triggered local boiling of the hydrothermal fluid, as evidenced by the presence of primary fluid inclusions containing immiscible liquid-rich and vapor-rich aqueous-carbonic fluids. This process was responsible for a number of physical-chemical changes in the liquid, all of which contributed to the formation of the As- and Au-rich overgrowths in pyrite: (1) removal of H 2 O into the vapor phase, inducing saturation of dissolved metals in the remaining liquid; (2) an increase in pH due to partition of H 2 S and CO 2 into the vapor, thus decreasing the solubility of sulfide minerals; and (3) an adiabatic decrease in temperature, lowering the solubility of As and Au in the liquid. Waning of this process restored precipitation of As-poor pyrite, until the onset of a new cycle. Because pressure drops are more significant adjacent to open spaces, oscillatory-zoned pyrite probably crystallized near newly formed veins whereas simple-zoned pyrite formed away from them. Previously formed pyrite underwent fracturing during reactivation of the deformation, especially through the brittle deformation events that postdated shearing, resulting in local pulverization of pyrite. This newly created porosity facilitated fluid circulation and remobilization of structurally bound gold, as well as of other chalcophile elements (Ag, Cu, Bi, Te, Pb, and Sb), which reprecipitated together with pyrite in the form of native gold, sulfides, and tellurides, either as small inclusions or as larger grains within fractures. This remobilization process facilitates the exceptionally high gold tenor found in the deposit, where the Colombia corridor is intersected by the Santa Maria fault.

The Kamoa copper deposit is located in the Katanga Province of the southern Democratic Republic of Congo outside of the area previously considered prospective for large copper deposits. With a 1% Cu cutoff, the hypogene ore zone of Kamoa spans over 81 km2, varying in thickness from 2 to 15 m, and is currently laterally unconstrained. The deposit is hosted at the stratigraphic contact between hematitic and locally pyritic Mwashya Subgroup sandstones and overlying fine-grained pyritic diamictites of the Grand Conglomerat unit of the Nguba Group. The Mwashya sandstones appear to have been deposited in a marginal marine to fluvial environment. The Grand Conglomerat diamictite contains glacially derived mass transport and sediment gravity flow deposits. The unit appears to have been deposited in a tectonically active, locally anoxic marine environment. The contact between the Mwashya and Grand Conglomerat units represents a major change in depositional style during what was probably a period of rapid subsidence or sea level rise. The majority of the Kamoa orebody occurs within the lowermost portion of the Grand Conglomerat unit that contains siltstones with high concentrations of diagenetic framboidal and later cubic pyrite that may be indicative of early hydrothermal activity. Later hydrothermal alteration mineral assemblages within the Grand Conglomerat are stratigraphically zoned, changing from a potassic alteration and silicification assemblage in the lowermost stratigraphic units to a dominantly magnesian alteration assemblage higher in the stratigraphy. Ore stage sulfide minerals are zoned vertically away from the Mwashya-Grand Conglomerat contact, changing from chalcocite to bornite to chalcopyrite to pyrite with increased stratigraphic height. Copper sulfide minerals occur as fine-grained disseminations within the diamictite matrix that probably represent replaced diagenetic pyrite as well as in coarse-grained mineral rims on diamictite clasts. The rims are best developed in the lowermost stratigraphic units and gradually lessen in size, vertical elongation, and abundance up stratigraphic section. Sulfur isotope studies indicate that most of the sulfur in the copper sulfides was derived from earlier formed diagenetic iron sulfide. Fluid inclusion analyses indicate that the ore-forming fluid was saline, ~23 to 26 wt % NaCl wt equiv, and had homogenization temperatures (T h ) ranging from 210° to 240°C.

Strata-bound carbonate-replacement Pb-Zn-Ag deposits in the Lavrion district, Greece, are spatially related to a late Miocene granodiorite intrusion (7–10 Ma) and various sills and dikes of mafic to felsic composition. The Plaka granodiorite contains porphyry molybdenum mineralization and is locally associated with a Ca-Fe skarn. Carbonate-replacement deposits occur predominantly in marbles (Upper and Lower Marble of the Basal unit), Kaesariani schists, and along a major detachment fault that separates the Basal unit from the Upper unit. Orebodies are mainly strata bound carbonate-replacement, although sulfides also occur in veins. The mineralogy of carbonate-replacement deposits is dominated by base metal sulfides and sulfosalts of Ag, Bi, Sn, Sb, As, and Pb, particularly at Plaka and Kamariza. Carbonates are intergrown with earlier formed sulfides and sulfosalts but are more abundant late in the paragenetic sequence with fluorite and barite. Fluid inclusion studies of sphalerite, fluorite, calcite, and quartz in carbonate-replacement deposits suggest that they were deposited from 132° to 365°C from CO2-poor, low- to high-salinity fluids (1–20 wt % NaCl equiv). Carbon and oxygen isotope compositions of calcite ( δ 13C = −15.6 to −1.5‰ and δ 18O = −9.2 to +17.3‰) intergrown with sulfides reflect variable exchange of the ore-bearing fluid with the Upper and Lower Marbles and proximity to the Plaka granodiorite. Post-Archean Australian Shale (PAAS)-normalized rare earth and yttrium patterns of the Upper and Lower Marbles, and calcite intergrown with sulfides show positive Eu and negative Ce anomalies as well as Y/Ho ratios between 40 and 80. Normalized rare earth and yttrium patterns of fluorite also have positive Eu and negative Ce anomalies. Such anomalies for both the carbonates and fluorite reflect the high pH or high f O2 conditions of the late-stage hydrothermal fluids and the likely derivation of calcium from marine carbonates (precursors of the Upper and Lower Marbles). The range of sulfur isotope compositions for sulfides ( δ 34S = −4.9 to +5.3‰, with one outlier of 9.4‰) in carbonate-replacement and vein deposits is due likely to a magmatic sulfur source with a contribution of reduced seawater sulfate. Sulfur isotope compositions of barite from carbonate-replacement range from δ 34S = 17.2 to 23.7 per mil and reflect Miocene seawater sulfate values. If a magmatic source of sulfur is assumed along with an average temperature of 250°C for the ore-forming fluids, as based on fluid inclusion studies, sulfides in carbonate-replacement deposits were deposited at values of log f O2 = −41 to −36 and a pH = 5.8 to 9.1. However, the range of sulfur isotope values does not rule out the possibility that sulfur in sulfides could have been produced by the reduction of seawater sulfate with no contribution from a magmatic source. The carbonate-replacement deposits resemble manto-type sulfide deposits in Mexico, central Colorado, South Korea, Nevada, and northern Greece.

The South Filson Creek mineralization occurs above the basal units of the South Kawishiwi intrusion and represents an uncommon geologic setting of Cu-Ni-platinum-group element (PGE) mineralization within the Duluth Complex. Unlike other mineralization of economic interest in the basal heterogeneous troctolite units along the lower contact, it is located at an approximate stratigraphic height of 1,000 m above the contact with the footwall granitoids in the usually unmineralized homogeneous troctolite unit. Magmatic sulfide mineralization forms disseminated fine-grained patches and pockets in an area of approximately ½ km2 of homogeneous troctolitic rocks in the northeastern part of the South Kawishiwi intrusion. Pyrrhotite, chalcopyrite, pentlandite, and cubanite are the main sulfide minerals with subordinate amounts of other copper-bearing phases (bornite, covellite, talnakhite). Cu/Pd ratios for unaltered mineralization indicate that sulfides were formed from multiple injections of magma that segregated sulfides with different R factors or having different initial compositions. Variation in the halogen contents of apatite from pegmatoids within troctolitic rocks revealed that a fluid has segregated from the crystallizing melt into which fluid Cl and rare earth elements partitioned. Amphibole, chlorite, sericite, prehnite, pumpellyite, and carbonate are associated with alteration along brittle structures and isolated patches in the area. The hydrothermal fluid-rock interaction has affected the composition of primary magmatic sulfide mineralization and is also responsible for mineralization in the anorthositic hanging wall of the South Kawishiwi intrusion. Hydrothermal alteration resulted in a slight Pd depletion in magmatic sulfide mineralization, which might indicate remobilization of precious metals to an unknown location. The hanging-wall mineralization is characterized by relative Cu enrichment in comparison to the magmatic sulfide mineralization with no PGE enrichment. Partial remobilization of platinum group elements (PGE) has also occurred as a result of a separate alteration event, on the evidence that platinum group minerals (PGM) are also associated with serpentinization of olivine and Ca enrichment of plagioclase along grain boundaries with sulfides. This alteration was probably effective on a very local scale.

Microorganisms alter minerals for a variety of reasons, including generation of more hospitable surroundings, extraction of nutrients, and sequestration of toxic substances. Minerals also may be altered passively by the products of microbial metabolism, such as production of acid. Microbes use minerals as sources and sinks of electrons for coupled oxidation-reduction reactions, and many of these reactions enable the release and capture of energy from unstable or metastable minerals. These processes can have dramatic results, such as the production of intensely acidic solutions during oxidation of sulfide minerals, or they can slowly and discretely transform minerals during weathering and diagenesis over geologic time. In either case, mineral sources of energy are transformed irreversibly by microbial metabolism. The purpose of this paper is to explore several central questions about microbial-mineral interactions, including the following: Under what conditions does a mineral represent an energy source for microbial metabolism? How can we evaluate the amount of energy available, and upon which factors does that energy supply depend most critically? How do we get from field and analytical data to an energy analysis for a natural system? This framework is illustrated with examples of known and postulated microbially mediated energy-yielding reactions involving minerals.

Over a century of mining and smelting of the world-class porphyry lode ore deposit at Butte, Montana, has resulted in extensive environmental damage. In addition to its being the location of one of the world’s largest and most acidic mining pit lakes, Butte is host to over 16,000 km of flooded underground mine workings. Of the more than 60 mine shafts that have historically operated in Butte, approximately one dozen are presently accessible for groundwater sampling. The geochemistry of the mine shaft waters is zoned and roughly coincides with a district-wide zonation in hydrothermal alteration and mineralization. Mine waters in the so-called “Central zone” of intense phyllic and advanced argillic alteration have lower pH and very high concentrations of As, Fe, Mn, and Zn, but very low concentrations of dissolved Cu. The scarcity of Cu is attributed to cementation onto scrap iron left in the mines, and/or to replacement of preexisting sulfide minerals below the water table in a manner analogous to supergene enrichment processes. At the other extreme, mine waters in the “Peripheral zone” of weakest alteration have near-neutral pH, low metal concentrations, and contain dissolved sulfide (H2S, HS–). These waters are close to equilibrium with calcite, siderite, crystalline or amorphous MnCO3, and mackinawite (poorly crystalline FeS). A suite of deep groundwater monitoring wells completed in fractured and mineralized Butte Quartz Monzonite, unassociated with the mining complex, shows a similar range in groundwater chemistries to the mine shaft waters, suggesting a fundamental control of bedrock geology on water quality. Based on its isotopic composition, aqueous sulfate in the Butte mine waters was sourced from a combination of pyrite oxidation and leaching of hydrothermal anhydrite associated with early, porphyry-style Cu-Mo mineralization. Aqueous sulfide in the Peripheral zone mine workings is 28 to 50 per mil depleted in 34S relative to coexisting aqueous sulfate, consistent with microbial sulfate reduction. Dissolved inorganic carbon in the majority of the waters sampled appears to have isotopically equilibrated at low temperature with hydrothermal rhodochrosite, an abundant mineral at Butte. Waters with the highest H2S concentrations also have unusually high dissolved inorganic carbon concentrations that are depleted in 13C, consistent with an influx of CO2 from microbial oxidation of organic carbon. The source of organic carbon is not known, but may include timbers used to reinforce the tunnels and stopes. In contrast to the large horizontal gradients in mine water chemistry on a district scale, vertical gradients in chemistry and temperature within each individual shaft at Butte are negligible, possibly due to vertical water circulation. The circulation model is consistent with data on the local geothermal gradient in Butte, and explains why the deepest mine shafts tend to have the warmest water. The Kelley mine has an anomalously warm temperature (~35°C), and some of the excess heat in this mine shaft may have come from pyrite oxidation, a highly exothermic reaction. The flooded underground mine complex of Butte has potential for heat recovery using modern heat pump technology.

The Co-Ni arsenide ore deposits at Bou Azzer, Morocco, represent an uncommon type of Co mineralization. Several Co-Ni-As-Au-Ag mines are active in Bou Azzer and adjacent areas. The Co-Ni arsenide ores are structurally controlled and concentrated mainly within quartz-carbonate veins along the boundaries of Neoproterozoic serpentinized mantle peridotites, quartz diorite, and Precambrian volcanic rocks. They have varying shapes and sizes; however, all deposits have the same general mineralogy in decreasing order of abundance: arsenides, sulfarsenides, and sulfides. Arsenide minerals include skutterudite (CoAs3), safflorite (CoAs2), loellingite (FeAs2), nickeline (NiAs), rammelsbergite (NiAs2), and S-rich Ni-Co diarsenide. Co-rich ores are predominant over Ni-rich ones in almost all deposits. Arsenopyrite and, to a lesser extent, gersdorffite are the main sulfarsenide minerals in almost all samples studied. Bornite, an unidentified Cu-Zn sulfide, chalcocite, chalcopyrite, pyrite, pyrrhotite, sphalerite, and tetrahedrite, in decreasing order of abundance, are the main sulfide minerals present in Bou Azzer ores. Gold-silver alloys, with about 15 wt percent Ag on average, are commonly associated with Ni-Co arsenides and/or disseminated in quartz veins. In the Ni-rich assemblage pure nickeline with high Ni content (~48 wt %) and very low Co content (≤ 0.1 wt %) grades outward to compositions with less Ni (~20 wt %) and very high Co (up to 18 wt %). This suggests a depositional sequence from Ni monoarsenide (nickeline) to Ni-Co diarsenide to cobalt triarsenides (i.e., a progressive increase in Co and As). The ore-forming elements (Co, Ni, Fe, and As) were originally leached from serpentinites by acidic magmatic fluids under moderately reducing conditions at high fluid/rock ratios. Deposition of ore minerals occurred in response to increasing pH and decreasing oxygen fugacity of the mineralizing fluids caused by mixing between the magmatic brines and meteoric water. The predominance of Co over Ni arsenide minerals in the Bou Azzer mineralization despite the high Ni/Co ratio of the serpentinites may be attributable to the different solubilities of Ni and Co in the hydrothermal system.

Mineralized hydrothermal breccias and veins formed during and after the waning stages of maar-diatreme-related volcanic activity at Kelian, Kalimantan, Indonesia. Subsurface phreatic explosions occurred around the margins of the diatreme breccia complex, focusing high-temperature fluid flow and generating several large, mineralized hydrothermal breccia bodies. Tectonic, phreatomagmatic, and hydraulic processes also contributed to hydrothermal brecciation. Explosive phreatic brecciation was followed by in situ hydraulic brecciation, and then by minor veining as the system returned to steady-state geothermal conditions. Brecciation and mineralization mostly occurred 400 to 700 m below the paleowater table. The Kelian deposit contained more than 240 t Au prior to mining. Precious metals occur with sulfide minerals as disseminations, in sheeted and conjugate veins and as breccia cement. There is a progression from pyrite-dominated (stage 1) to base metal sulfide-dominated (stages 2 and 3) to sulfosalt-dominated hydrothermal breccias and veins (stage 4). In terms of gangue minerals, the system evolved from illite-quartz to adularia- and/or quartz- and/or illite-dominated and then carbonate-dominated. Boiling produced abundant bladed carbonate during stages 3 and 4. Overall, carbonate and base metal sulfide minerals are abundant, whereas quartz is comparatively minor. Free gold is most abundant in stages 3 and 4 but also occurs in stages 1 and 2. Native gold occurs principally as inclusions in pyrite, sphalerite, galena, arsenopyrite, quartz, bladed carbonate, and sulfosalts. Hydrothermal alteration assemblages are zoned about contacts, faults, breccias and veins, and consist of secondary quartz, illite, pyrite, chlorite, and various carbonate minerals. Kelian preserves evidence of a magmatic component to the mineralizing fluids. Fluid inclusion analyses revealed the presence of saline fluid inclusions, in particular from stage 3. Isothermal mixing of low-salinity (∼0 to 2 wt % NaCl equiv) with moderate-salinity fluids (10 to 25 wt % NaCl equiv), rather than boiling, resulted in the range of salinity and homogenization temperatures of fluid inclusions in sphalerite, carbonate and quartz deposited during stage 3. Salinities of ∼4 to 6 wt percent NaCl equiv for inclusions in adularia and quartz from stages 2 and 4 and in rhodochrosite and proustite-pyrargyrite from stage 4 may also reflect a component of mixing of low-salinity water with a moderately saline fluid. Sulfur isotope values from Kelian sulfides (δ 34S = -1.4 to 5.5 per mil) are consistent with a magmatic sulfur source. Both boiling and fluid mixing contributed to high-grade Au mineralization at Kelian. Rapid formation (hundreds of years or less) is inferred for the individual hydrothermal breccia bodies. © 2008 Society of Economic Geologists, Inc.

The Wolverine deposit is a high-grade (~6.2 Mt at 12.7% Zn, 1.3% Cu, 1.6% Pb, 370.9 g/t Ag, and 1.8 g/t Au) volcanic and sedimentary rock-hosted massive sulfide deposit in the Finlayson Lake area, Yukon Territory. The area has the potential to become an important mining camp and currently contains more than 30 million tons (Mt) of massive sulfide ore in five deposits. A succession of metamorphosed early Mississippian felsic volcanic and sedimentary rocks that are interpreted to have formed on the margin of an ensialic back-arc ocean basin between the Yukon-Tanana terrane and the ancestral North American craton hosts the Wolverine deposit. The massive sulfide ore occurs at a single horizon that marks a significant change in the character of the volcanism from quartz- and feldspar-phyric rhyolite of the footwall to aphyric rhyolite and chemical sedimentary exhalative rock of the hanging wall. The deposit consists of two discrete Zn-Pb-Ag massive sulfide lenses situated at the same stratigraphic level connected by strata-bound semimassive, replacement-style Zn-Pb-Ag mineralization. Copper-rich massive sulfide commonly replaces the Zn-Pb-Ag massive sulfide at the base of the lenses and in the footwall replacement zones beneath the massive mineralization. Several zones of sulfide stringer veins and chlorite-sericite-carbonate alteration occur within permeable footwall volcaniclastic rocks. Alteration minerals preferentially associated with massive sulfide mineralization include Ba-rich phengitic mica, biotite, Mg-rich chlorite and siderite. Microthermometric measurements of fluid inclusions in the massive sulfide bodies and accompanying stringer zones indicate formation at temperatures of 235° to 353°C. Primary, aqueous, liquid-rich fluid inclusions in hydrothermal quartz from quartz-sulfide stringer veins contain low-salinity fluids (2.1–8.5 wt % NaCl equiv; mean = 6.0). There is no evidence of fluid boiling in the inclusions, and based on the average fluid salinity and temperature of homogenization, mineralization took place under a minimum water depth of ~1,000 m. In situ analyses of δ34SV-CDT in sulfide minerals from massive sulfide lenses (means 0.8‰) and sulfide stringer veins (mean 12.0‰) display a bimodal distribution. Lower δ34S values characterize sulfides near the top of the lenses versus higher (more positive) values in underlying veins may reflect different processes of reduction of seawater sulfate. Sulfur in sulfides from the tops of the lenses was likely derived mainly from bacterial reduction of seawater sulfate, whereas sulfur in the stringer veins and at the base of the lenses was mainly from inorganic reduction of seawater sulfate. Sulfide minerals precipitated both on the sea floor and in the subsea floor from hydrothermal fluids that vented from multiple mounds. The presence of black shales and the strongly positive sulfur isotope compositions of the sulfides provide evidence that the ambient bottom water in the basin in which the Wolverine deposit formed was likely anoxic. This environment may have contributed to the preservation of the sulfide mounds. Based on its tectonic setting, host-rock types, local geologic setting, metal grades, age, and source(s) of sulfur, the Wolverine deposit best exemplifies the volcanic sediment-hosted massive sulfide (VSHMS) class of syngenetic massive sulfide deposits in the district.

The Feitais deposit is one of six polymetallic, volcanic-hosted massive sulfide deposits located adjacent to the town of Aljustrel in the Iberian Pyrite Belt of southern Portugal. The deposit occurs on the upright limb of the Feitais anticline, within the upper part of the regional Volcano-Sedimentary Complex of Devonian to Mississippian age and near the contact with the overlying Culm Flysch Group. It is stratigraphically underlain by felsic tuffs and flows and overlain by a unit of feldspar-bearing felsic tuff that passes upward into chert, then argillite, and finally graywacke turbidites. The massive sulfide lens is about 1,000 m in length, 500 m wide, and up to 100 m thick. Although mainly fine-grained pyrite, it commonly has a Cu-rich base that passes upward through a low-grade pyritic zone to a Zn-Pb–enriched upper portion. Sulfide minerals include sphalerite, galena, chalcopyrite, tetrahedrite, and minor arsenopyrite. Copper stockwork veins in rhyolite underlie the thicker sections of the deposit and appear to be spatially associated with growth faults that controlled basin geometry. These faults also influenced the distribution of tuffaceous and cherty (exhalative) units that overlie the sulfides. In terms of mined plus reserve ores, the Feitais deposit was estimated in 1998 to contain 54.5 Mt grading 3.72 percent Zn, 0.42 percent Cu, 1.2 percent Pb, and 44 g/t Ag (Leistel et al., 1998). Recent drilling has identified a new Zn resource in the upper part of the massive sulfide lens consisting of a measured and indicated resource of 15.22 Mt grading 6.00 percent Zn, 0.21 percent Cu, 1.85 percent Pb, and 67.4 g/t Ag; and a deeper Cu resource straddling the lower part of the massive sulfide lens and the underlying stockwork mineralization and consisting of an indicated resource of 4.87 Mt grading 0.88 percent Zn, 2.12 percent Cu, 0.24 percent Pb, and 13.5 g/t Ag. The deposit is open downplunge to the northwest for 200 m. It is truncated by the northeast-striking, steeply dipping Represa fault. The deposit is offset right laterally about 500 m along this fault, north of which it is called the Estacao deposit. Application of lithogeochemical methods to 170 new and 80 published analyses of the Feitais host rocks led to the chemical identification of five main felsic rock types that have distinctive immobile element signatures. These signatures are maintained even where the rocks are strongly altered. The deposit is underlain mainly by a thick (>150 m) unit of rhyolite A, the most fractionated of the felsic rocks. Rhyolite X occurs structurally below rhyolite A in two deep footwall holes. Rhyolite B occurs mainly downdip of the massive sulfide lens but also is present as meter-scale intervals near the base of the sulfide lens. The deposit is overlain stratigraphically by 5 to 30 m of tuffaceous rhyolite C, which is followed by 2 to 20 m of Fe-bearing chert. A centimeter-scale crystal tuff bed with a distinctive felsic composition overlies the cherts and forms a marker horizon that can be traced at least 700 m along the strike of the deposit. The massive sulfide lens becomes thicker downdip to the northeast, then terminates laterally against an uplifted block of footwall rhyolite A capped by rhyolite B. Field relationships and isopach data suggest that an asymmetrical sea-floor graben bounded by a synvolcanic fault was filled by massive sulfides, followed by rhyolite C, then chert, with the chert also extending laterally beyond the graben margin. Footwall and hanging-wall felsic volcanic rocks are altered to chloritesericite-pyrite-quartz-carbonate assemblages and contain stockwork chalcopyrite-pyrite-quartz-carbonate veins. Hydrothermal alteration is strongest in the upper 50 m of rhyolite A below the orebody but extends at least 200 m into the footwall along the main synvolcanic fault zone. Rhyolite C above the orebody is also strongly altered. These areas have experienced significant additions of Fe and Mg, with variable gains or losses of Si. Barium is enriched for up to a few tens of meters above and laterally from the orebody. Dominating the central part of the Aljustrel area is a quartz-feldspar (QF) porphyry that shows primary enrichment in Fe, Ti, and V relative to the rhyolites. At Feitais, this lithology was only encountered deep in the footwall, structurally below rhyolite X. No andesitic or mafic extrusive rocks have been identified. The QF porphyry is chemically unrelated to the four rhyolite types. The rare earth element (REE) patterns of the rhyolites show moderate enrichment in the light REE, with near-flat middle to heavy REE. The rhyolites are of tholeiitic to transitional affinity, with moderately high Zr (180–380 ppm) but low Nb (7–15 ppm) contents. These collective features are consistent with a rift-related, continental-margin setting, behind an arc, for the rhyolite volcanism.

Three significant base metal deposits were recently discovered in the Finlayson Lake district, Yukon Territory, Canada; however, widely varying selenium contents (e.g., 1,100 ppm avg Se at Wolverine, 200 ppm avg Se at Kudz Ze Kayah, and 7 ppm avg Se at GP4F) have hindered their rapid exploitation. Observations and data from the deposits in the Finlayson Lake district show a strong positive correlation between high-temperature (>300°C), copper-rich sulfide assemblages at the base of volcanic-hosted massive sulfide (VHMS) and the selenium content of all sulfide minerals. Thermodynamic calculations for selenide and sulfide minerals indicate significantly higher temperatures of formation and lower solubilities of selenide versus sulfide minerals, which is consistent with petrographic observations and can explain the distribution of selenium in zone-refined VHMS and volcanic sediment-hosted massive sulfide deposits. Volumetric mass-balance calculations indicate that some selenium could be sourced from contemporaneous volatile degassing of SeO 4 , its rapid reduction to H 2 Se, and subsequent entrainment into hydrothermal ore fluid. However, estimates for the Kudz Ze Kayah and Wolverine deposits suggest an additional large Se reservoir. Volumetric mass-balance estimates indicate that contemporaneous volcanic rocks and black shales are likely selenium reservoirs for the Kudz Ze Kayah and Wolverine deposits, respectively.

Recent developments in the measurement of δ34S and δ33S in sulfide minerals provide a new framework to evaluate isotopic equilibrium among sulfide minerals and a new tool to investigate mineralizing processes or sources of sulfur in ore-forming systems. Isotopic equilibrium requires that values of δ34S and δ33S are highly correlated between mineral pairs and that deviations (quantified as Δ33S) from a reference mass-dependent fractionation are the same for both minerals within experimental error. In this contribution, we apply this framework for the first time to natural samples, and evaluate isotopic equilibrium among sulfide minerals from the Archean Kidd Creek volcanogenic massive sulfide deposit in Ontario, Canada. Two of eight sulfide mineral pairs showed clear isotopic disequilibrium, in agreement with interpreted paragenetic relationships; six other pairs have isotopic compositions that are consistent with equilibrium. We discuss the implications of these results for the interpretation of mineral paragenesis and sulfur isotope geothermometry at Kidd Creek. This technique offers new possibilities for evaluating sulfur isotope systematics in Archean (>2.45 Ga) ore deposits, in which the range of δ34S values is small but non-zero values of Δ33S are common.

The Jinchuan intrusion hosts one of the largest magmatic Ni-Cu sulfide deposits in the world. Net-textured and disseminated pyrrhotite, pentlandite, and chalcopyrite are found in olivine-rich rocks (60–80 vol %) that occur as a northwest-southeast–striking dike. Olivine in the intrusion has been variably converted to mixtures of serpentine and fine-grained magnetite. Interstitial pyroxene and minor plagioclase have been altered to mixtures of chlorite, amphibole, epidote, clinozoisite, Na-rich feldspar, and lesser amounts of calcite. Although the interstitial net texture of sulfide minerals surrounding olivine grains is preserved, up to 30 vol percent of sulfide minerals are replaced by magnetite, serpentine, and chlorite. An external origin for Mg and Si needed for the isovolumetric replacement of sulfides cannot be discounted, but a local origin related to olivine and pyroxene alteration is favored. Serpentine and chlorite may be enriched in Cu and Ni, but the enrichment can account for less than 1 percent of the metals lost during the replacement of sulfide minerals by silicates and oxides. Some of the metals and sulfur that were removed from the net-textured sulfide assemblages have been redeposited on a centimeter scale in fine ribbons and veins within serpentine, chlorite, and amphibole. Sulfur isotope values of the pyrrhotite-pentlandite-chalcopyrite assemblage at Jinchuan fall in the range of −2 to 8 per mil, with over 80 percent of the values between −2 and 2 per mil. Immediate country rocks in the Jinchuan area are Precambrian granitoids, schists, and marbles with sulfur concentrations less than 100 ppm. Although the δ 34 S values of the main sulfide assemblage at Jinchuan are in the range that is characteristic of mantle-derived sulfur, assimilation of sulfur from unexposed country rocks cannot yet be discounted. Secondary pyrite occurs in veins and stringers, and rarely as a replacement of pyrrhotite, and is characterized by δ 34 S values between −7 and −27 per mil. The strongly negative δ 34 S values of secondary pyrite are thought to be a result of oxidation of primary magmatic sulfide assemblages and partial reduction of aqueous sulfate. Oxygen and hydrogen isotope values of silicate minerals suggest that at least two fluids have been involved in the hydrothermal alteration at Jinchuan. The δ 18 O values of plagioclase, amphibole, and serpentine all suggest the involvement of a fluid with a δ 18 O value between ~2 and 5 per mil at 300° to 400°C. The δD values of amphibole and one population of serpentine suggest a δD value for the fluid in the range of −45 to −63 per mil. These values are consistent with the involvement of evolved seawater or a metamorphic fluid in the early stages of alteration. The possible involvement of evolved seawater in hydrothermal alteration is consistent with a rift setting for the emplacement of the Jinchuan intrusion. However, the wide range in serpentine δD values (computed δD of the fluid = −45 to −103‰) suggest either that serpentinization involved a mixture of evolved seawater and/or metamorphic water and meteoric water or, more likely, that serpentine continued to exchange hydrogen isotopes with late-stage meteoric waters after it formed.

Bacterial metabolism, involving redox reactions with carbon, sulfur, and metals, appears to have been important since the dawn of life on Earth. In the Archean, anaerobic bacteria thrived before the Proterozoic oxidation of the atmosphere and the oceans, and these organisms continue to prosper in niches removed from molecular oxygen. Both aerobes and anaerobes have profound effects on the geochemistry of dissolved metals and metal-bearing minerals. Aerobes can oxidize dissolved metals and reduced sulfur, as well as sulfur and metals in sulfide minerals can contribute to the supergene enrichment of sulfide ores, and can catalyze the formation of acid mine drainage. Heterotrophic anaerobes, which require organic carbon for their metabolism, catalyze a number of thermodynamically favorable reactions such as Fe-Mn oxyhydroxide reductive dissolution (and the release of sorbed metals to solution) and sulfate reduction. Bacterial sulfate reduction to H S can be very rapid if reactive organic carbon is present and can lead to precipitation of metal sulfides and perhaps increase the solubility of elements such as silver, gold, and arsenic that form stable Me-H S aqueous complexes. Similarly, the bacterial degradation of complex organic compounds such as cellulose and hemicellulose to simpler molecules, such as acetate, oxalate, and citrate, can enhance metal solubility by forming Me organic complexes and cause dissolution of silicate minerals. Bacterially induced mineralization is being used for the bioremediation of metal-contaminated environments. Through similar processes, bacteria may have been important contributors in some sedimentary ore-forming environments and could be important along the low-temperature edges of high-temperature systems such as those that form volcanogenic massive sulfides.

Gold and silver from a selection of springs at Waiotapu, New Zealand, was preconcentrated onto activated charcoal and analyzed by ICP-MS. The measured gold concentrations were combined with field and laboratory analyses of other components, thermodynamic data and geochemical modeling software to calculate the gold saturation index in each spring, assuming that reduced sulfur ligands largely control gold solubility. The springs that were selected for analysis have elevated concentrations of components that are residually enriched by boiling, so their composition is strongly related to that of the geothermal reservoir. Therefore, these analyses provide information about geochemical processes that operate beneath the springs as well as those at work within the springs. Previous geochemical investigations at Waiotapu indicated that dissolved gold concentrations are elevated within Champagne Pool and in precipitates surrounding this spring. Therefore, it is likely that the reservoir fluid that feeds springs at Waiotapu also contains dissolved gold. Champagne Pool has the highest gold concentration measured in this study, 109 ngL–1 dissolved and 362 ngL–1 total. This spring is slightly undersaturated with gold in solution, but the total concentration is higher than the solubility of Au(s). Undersaturation with respect to Au(s) is consistent with deposition of gold by adsorption and concentration within an As- and Sb-sulfide precipitate that forms around Champagne Pool and is similar in magnitude to that expected from modeling of gold adsorption onto As- and Sb-sulfide precipitates. Elevated Au concentrations of gold (40–90 ngL–1) at two sites downstream from Champagne Pool indicate that deposition of gold within Champagne Pool is inefficient. The two downstream sites are substantially oversaturated with respect to reduced S-Au species; however, assumptions used in solubility calculations are unlikely to be valid at these sites because the oxidation state of the fluids after discharge was not determined. Despite uncertainty in the saturation index calculated at these sites, the elevated concentrations of gold and apparent oversaturation indicate that gold is transported by other ligands, such as polysulfides and thiosulfates or as colloidal particles downstream from Champagne Pool. Several springs at Waiotapu with moderate concentrations of dissolved reduced sulfur (1–6 mgL–1) have very low dissolved concentrations of gold and are substantially undersaturated with respect to Au(s). Deposition of gold by precipitation requires loss of reduced sulfur ligands through processes such as boiling, oxidation, acidification or sulfide precipitation. Therefore, the low concentrations of gold in springs that contain substantial reduced sulfur indicates that gold is deposited by processes that can cause substantial undersaturation of Au(s), such as adsorption, coprecipitation, or both. Deposition of gold by adsorption or coprecipitation is consistent with the occurrence of gold as impurities in epithermal sulfide minerals. These results do not rule out direct precipitation of Au(s) at Waiotapu but indicate that other depositional mechanisms could also be important in the geothermal system beneath the springs.

The Anarraaq deposit in northern Alaska consists of a barite body, estimated to be as much as 1 billion metric tons, and a Zn-Pb-Ag massive sulfide zone with an estimated resource of about 18 Mt at 18 percent Zn, 5.4 percent Pb, and 85 g/t Ag. The barite and sulfide minerals are hosted by the uppermost part of the Mississippian Kuna Formation (Ikalukrok unit) that consists of carbonaceous and siliceous mudstone or shale interbedded with carbonate. The amount of interbedded carbonate in the Anarraaq deposit is atypical of the district as a whole, comprising as much as one third of the section. The total thickness of the Ikalukrok unit is considerably greater in the area of the deposit (210 to almost 350 m) than to the north and south (maximum of 164 m). The mineralized zone at Anarraaq is lens shaped and has a relatively flat top and a convex base. It also ranges greatly in thickness, from a few meters to more than 100 m. Textures of some of the carbonate layers are distinctive, consisting of nodules within siliceous mudstone or layers interbedded with shale. Many of the layers contain calcitized sponge spicules or radiolarians in a carbonate matrix. Textures of barite and sulfide minerals mimic those of carbonate and provide unequivocal evidence that replacement of precursor carbonate was an important process. Barite and sulfide textures include either nodular, bladed grains of various sizes that resemble spicules (observed only with iron sulfides) or well-rounded forms that are replaced radiolarians. Mineralization at Anarraaq probably occurred in a fault-bounded Carboniferous basin during early diagenesis in the shallow subsurface. The shape and size of the mineralized body suggest that barite and sulfides replaced calcareous mass flow deposits in a submarine channel. The distribution of biogenic and/or early diagenetic silica may have served as impermeable barriers to the fluids, thereby focusing and controlling fluid flow through unreplaced carbonate layers.

The Red Dog Zn-Pb deposits are hosted in organic-rich mudstone and shale of the Mississippian Kuna Formation. A complex mineralization history is defined by four sphalerite types or stages: (1) early brown sphalerite, (2) yellow-brown sphalerite, (3) red-brown sphalerite, and (4) late tan sphalerite. Stages 2 and 3 constitute the main ore-forming event and are volumetrically the most important. Sulfides in stages 1 and 2 were deposited with barite, whereas stage 3 largely replaces barite. Distinct chemical differences exist among the different stages of sphalerite. From early brown sphalerite to later yellow-brown sphalerite and red-brown sphalerite, Fe and Co content generally increase and Mn and Tl content generally decrease. Early brown sphalerite contains no more than 1.9 wt percent Fe and 63 ppm Co, with high Mn (up to 37 ppm) and Tl (126 ppm), whereas yellow-brown sphalerite and red-brown sphalerite contain high Fe (up to 7.3 wt %) and Co (up to 382 ppm), and low Mn (

The El Mochito mine, located in west-central Honduras, exploits a Zn-Pb-Ag skarn deposit. Sulfide minerals including sphalerite, galena, pyrrhotite, and pyrite postdate a gangue consisting mainly of pyroxene in the hedenbergite-diopside series and garnet in the andradite-grossular series. A strong structural control is apparent, with most mineralization located along or near fault zones. No igneous body, which could have contributed to the formation of the skarn, appears spatially associated with the El Mochito deposit, and a source of fluid and metals has not been identified previously. δ 34S values of sphalerite, galena, and pyrrhotite cluster between –1 and +2 per mil, suggesting that the sulfur at El Mochito was primarily igneous in origin. Lead isotope ratios for galena have mean values of 18.766, 15.616, and 38.572 for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, respectively, and are similar to Pb isotope ratios for the Miocene-Pliocene Padre Miguel ignimbrite. A progressive decrease in the δ 34S values of sphalerite and a corresponding increase in the whole-rock Pb isotoperatios of the host limestone correlate with decreasing alteration from northeast to southwest along the trend of the Salva Vida and Nacional orebodies. These observations are interpreted to indicate that the fluid and lead source was located to the northeast of the deposit, and that magma genetically related to the Padre Miguel ignimbrite produced the hydrothermal system, which formed the deposit.

The hydrothermal fluids that deposited the ores of the Viburnum Trend of the Southeast Missouri district have long been recognized to be sedimentary basinal brines. The major element composition of these brines has been well characterized; however, there have been no attempts to measure their metal content, which is only loosely constrained by theoretical solubility relations. The present study is an effort to assess the metal content of these mineralizing brines by analyzing fluid inclusions from gangue minerals in the Viburnum Trend using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and microthermometry. By focusing on gangue minerals instead of metal sulfide minerals, interferences from the mineral matrix are avoided. A high priority was to determine whether brines of anomalously high metal content were circulating through southeast Missouri during the time of Mississippi Valley-type mineralization. Fluid inclusions in dolomite, quartz, and calcite gangue from throughout the Viburnum Trend and representative of the entire paragenesis were analyzed. None of the inclusions contained measurable quantities of Pb, Zn, or Cu and only a few contained measurable quantities of Ba, which ranged from about 100 to 500 ppm. Where detection limits were not exceeded, they indicated maximum possible concentrations of the element in the fluid inclusion. Maximum possible concentrations varied from inclusion to inclusion and were controlled by the mass of analyte within the fluid volume—the larger or more saline the inclusion, the lower the detection limit. These results showed that the fluid that deposited quartz, calcite, and most of the dolomite gangue in the Viburnum Trend is unlikely to have had order-of-magnitude concentrations of Pb and Zn higher than 102 ppm or order-of-magnitude concentrations of Cu higher than 101 ppm. Concentrations of Pb and Zn during deposition of stage 3 dolomite could not have been higher than 103 ppm, and concentrations of Cu could not have been higher than 102 ppm. These upper limits on possible metal concentrations, with the principal exception of those in stage 3 dolomite fluid inclusions, are of magnitude similar to the highest measured concentrations of Pb, Zn, Cu, and Ba found in typical sedimentary brines. Thus, the brines responsible for depositing gangue minerals in the Viburnum Trend are unlikely to have had higher ore metal concentrations than typical basinal brines. The lack of temporal overlap between gangue and metal sulfide deposition, and the higher K/Na and lower Ca/Mg ratios in fluid inclusions in the gangue minerals compared with fluid inclusions in the metal sulfides, leaves open the possibility that the metal content of the fluid that deposited the sulfides was different from that responsible for depositing gangue. However, the data show that fluid entering the Southeast Missouri district was not pervasively metal rich throughout time, and that if metal-enriched fluid ever entered the district it did so at discrete intervals.

Lead isotope compositions of bulk mineral samples (fluorite, orpiment, and realgar) determined using conventional techniques and of ore-stage arsenian pyrite using the Sensitive High Resolution Ion-Microprobe (SHRIMP) in the Getchell and Turquoise Ridge Carlin-type gold deposits (Osgood Mountains) require contribution from two different Pb sources. One Pb source dominates the ore stage. It has a limited Pb isotope range characterized by 208Pb/206Pb values of 2.000 to 2.005 and 207Pb/206Pb valuesof 0.8031 to 0.8075, as recorded by 10-mm-diameter spot SHRIMP analyses of ore-stage arsenian pyrite. These values approximately correspond to 206Pb/204Pb of 19.3 to 19.6, 207Pb/204Pb of 15.65 to 15.75, and 208Pb/204Pb of 39.2 to 39.5. This Pb source is isotopically similar to that in average Neoproterozoic and Cambrian clastic rocks but not to any potential magmatic sources. Whether those clastic rocks provided Pb to the ore fluid cannot be unequivocally proven because their Pb isotope compositions over the same range as in ore-stage arsenian pyrite are similar to those of Ordovician to Devonian siliciclastic and calcareous rocks. The Pb source in the calcareous rocks most likely is largely detrital minerals, since that detritus was derived from the same sources as the detritus in the Neoproterozoic and Cambrian clastic rocks. The second Pb source is characterized by a large range of 206Pb/204Pb values (18–34) with a limited range of 208Pb/204Pb values (38.1–39.5), indicating low but variable Th/U and high and variable U/Pb values. The second Pb source dominates late and postore-stage minerals but is also found in preore sulfide minerals. These Pb isotope characteristics typify Ordovician to Devonian siliciclastic and calcareous rocks around the Carlin trend in northeast Nevada. Petrologically similar rocks host the Getchell and Turquoise Ridge deposits. Lead from the second source was either contributed from the host sedimentary rock sequences or brought into the hydrothermal system by oxidized ground water as the system collapsed. Late ore- and postore-stage sulfide minerals (pyrite, orpiment, and stibnite) from the Betze-Post and Meikle deposits in the Carlin trend and from the Jerritt Canyon mining district have Pb isotope characteristics similar to those determined in Getchell and Turquoise Ridge. This observation suggests that the Pb isotope compositions of their ore fluids may be similar to those at Getchell and Turquoise Ridge. Two models can explain the Pb isotope compositions of the ore-stage arsenian pyrite versus the late ore or postore sulfide minerals. In either model, Pb from the Ordovician to Devonian siliciclastic and calcareous rock source enters the hydrothermal system late in the ore stage but not to any extent during the main stage of ore deposition. In one model, ore-stage Pb was derived from a source with Pb isotope compositions similar to those of the Neoproterozoic and Cambrian clastic sequence, transported as part of the ore fluid and then deposited in the ore-stage arsenian pyrite and fluorite. The second model is based on the observation that the Pb isotope characteristics of the ore-stage minerals also are found in some Ordovician to Devonian calcareous and siliciclastic rocks. Hence, ore-stage Pb could have been derived locally and simply concentrated during the ore stage. Critical to the second model is the removal of all high 206Pb/204Pb (>20) material during alteration. It also requires the retention of only the low 206Pb/204Pb component of the Ordovician to Devonian sedimentary rocks. This critical step is possible only if the high 206Pb/204Pb values are contained in readily dissolvable mineral phases, whereas the low 206Pb/204Pb values are found only in refractory minerals that released Pb during a final alteration stage just prior deposition of auriferous arsenian pyrite. Distinguishing between Pb transported with the ore fluid or inherited from the site of mineral deposition is not straightforward; however, it is simpler to explain the Pb isotope compositions of ore-stage arsenian pyrite and fluorite in two different but spatially related Carlin-type deposits (Getchell and Turquoise Ridge) with different host rocks by input of Pb with the ore fluid. Once the limited Pb in the hydrothermal fluid was exhausted by incorporation in ore-stage arsenian pyrite or other ore-stage minerals, Pb from the second source, the Ordovician to Devonian sedimentary rock sequences, became available for incorporation in some but not all of the late-stage sulfide minerals.

A number of occurrences and deposits of nonsulfide zinc ores in Europe were the historical basis for the development of the zinc mining and smelting industry. The principal occurrences in Silesia (Poland), Sardinia (Italy), and northern Spain are described. These deposits are products of the supergene oxidation of primary carbonate-hosted sulfide minerals during the complex interplay of tectonic uplift, karst development, changes in the level of the water table, and weathering. The nonsulfide zinc mineral deposits in the Irish Midlands may represent an example of surface oxidation of primary sulfide mineral deposits, redeposition, and preservation under glacial till. The willemite-dominated mineralization at La Calamine, Belgium, may be related to paleoweathering or be of possible hydrothermal origin, similar to other willemite deposits in the world.

The Skorpion zinc deposit in southwestern Namibia comprises a significant nonsulfide orebody (24.6 Mt at 10.6% Zn) and subordinate amounts of primary base metal sulfide mineralization, which structurally underlies the nonsulfide ores at depth. The host rocks of the blind, nonsulfide orebody and relict sulfide mineralization consist of a mixed volcano-sedimentary sequence of Neoproterozoic age that has formed in an initial continental rift system between the Kalahari cratonic province and the Rio de la Plata cratonic province. The metavolcanic and subvolcanic rocks display a distinct bimodal geochemical composition of within-plate affinity and have been emplaced under an extensional crustal regime. Zircons from a rhyolitic vesicular flow, extruded approximately contemporaneously with the volcanic-hosted sulfide mineralization, have been dated by SHRIMP U-Pb analysis and yield an age of 751.9 ± 5.5 Ma. The basin that hosts the Skorpion deposit has been shaped by horst and (half-)graben tectonics, which also controlled the sedimentary deposition of siliciclastic and carbonate sediments within both shallow and deeper water environments. The rock sequence was affected by a major tectono-metamorphic event the Pan African-Brasiliano orogeny at approximately 550 to 545 Ma. Metamorphism reached uppermost greenschist to lowermost amphibolite facies, and the complex deformational style of the region is the result of intense faulting, folding, and thrusting. The Skorpion orebody is hosted by arkosic metaarenites and subordinately by volcaniclastic metasedimentary rocks. It is irregularly shaped, transgressive to sedimentary layering, and has a relatively flat top, which is covered by a blanket of unmineralized overburden consisting of calcrete, boulder beds and recent sand dunes. At depth, the orebody has a relatively sharp contact with a barren marble unit on the east and south and a more gradational boundary within the siliciclastic host rocks on the west and north. The nonsulfide ore minerals comprise predominantly sauconite (Zn smectite), substantial amounts of hemimorphite and smithsonite, and subordinate amounts of scholzite, tarbuttite, atacamite, hydrozincite, and hetearolite. The ore minerals are mainly euhedral and subhedral crystals and occur as open space fillings in intergranular voids, fractures, and breccias and as replacements of feldspar and mica. The metasedimentary host rocks of the nonsulfide ore contain abundant iron hydroxide pseudomorphs after disseminated sulfides and stringers and after rare semimassive stratiform layers of sulfides. Gossanous, manganese and iron hydroxide-rich, siliciclastic, metasedimentary rocks are common in the western and northwestern vicinity of the nonsulfide orebody. Alteration of the supergene mineralized host rocks includes removal of calcite cement and the breakdown of detrital albite-rich plagioclase and of minor orthoclase. Local alteration phenomena also include silicification and rare secondary barite, both a result of the alteration and breakdown of feldspar. Minor base metal sulfide mineral deposits occur in metarhyolitic flows and felsic hyaloclastic rocks, structurally below the nonsulfide orebody. The primary sulfide minerals comprise pyrite, pyrrhotite, sphalerite, minor chalcopyrite, and galena as well as secondary chalcocite and colloidal sphalerite (brunckite). The volcanic-hosted sulfide ore minerals are deformed and partly recrystallized and thus of premetamorphic and predeformation origin. In contrast, the Skorpion nonsulfide zinc deposit is of postmetamorphic origin, as shown by two hydrous, low-P, low-T ore assemblages. The secondary assemblages occur, at least partly, as delicate euhedral and subhedral crystals in secondary pore space, open fractures, veins, and breccias. The nonsulfide ore contains pseudomorphs from in situ oxidation and replacement of preexisting sulfides and ore and also from the remobilization or precipitation of base metals. The Skorpion nonsulfide zinc deposit is of supergene origin and formed from the oxidation of sulfide protore by circulating meteoric water, as suggested by textural evidence and by initial stable isotope data. The timing of ore formation is still undefined but lies within the large age bracket between 545 Ma (peak metamorphism) and subsequent partial exhumation and the deposition of the nonmineralized overburden of Namib Desert gravels (

The Voisey’s Bay Ni-Cu-Co deposit is located in ca 1.33 Ga troctolitic rocks of the Nain Plutonic Suite, northern Labrador. It is widely regarded as a prime example of sulfide mineralization associated with a magmatic conduit system. Previous geological and geochemical studies have indicated that the Voisey’s Bay intrusion was contaminated by interaction with both sulfide-poor Archean gneisses and with Paleoproterozoic sulfide-bearing Tasiuyak metasedimentary gneiss. Limited sulfur isotope data previously indicated that only one of the four ore zones at Voisey’s Bay is characterized by δ 34S values outside the range considered normal for uncontaminated, mantle-derived mafic magmas (0 ± 2‰). New S isotope data presented here extends the previously reported range in δ 34S values for the Tasiuyak gneiss from –17.0 to +18.3 per mil, with an average of –1.4 per mil. Assimilation of this sulfur, followed by isotope homogenization in the magmatic system, is consistent with the isotope data from the ore zones, and in line with the wealth of evidence that suggests that interaction with Tasiuyak gneiss was an essential ingredient for ore formation in all the zones. Se/S values of sulfide minerals in the ore zones at Voisey’s Bay are also in the same range as those of whole-rock samples from the Tasiuyak gneiss. δ 13C values of both mineralized and nonmineralized parts of the Voisey’s Bay intrusion are anomalous (–18.3 to –24.8‰), and similar to values of graphite in the Tasiuyak gneiss. Both S and C contamination of the Voisey’s Bay intrusion via diffusive transfer in a fluid phase derived from inclusions and wall rocks are considered unlikely because the area had been subjected to granulite-grade metamorphism prior to emplacement of the Nain Plutonic Suite. Either fractional melting or total fusion of xenoliths is considered a more likely mechanism for S addition to the mafic magma via pyrrhotite melting and for C addition in the form of CO2 and CH4 produced by reaction of graphite with O- and H-bearing melt species.

Economic sulfide deposits have been identified in only a handful of Proterozoic massif-type anorthosite complexes. Yet exploration interest has expanded greatly in the last decade since discovery of the Voisey’s Bay deposits. The paucity of new discoveries may reflect the unique confluence of geologic conditions required to concentrate sulfides in this environment. Sulfide minerals in anorthosite complexes are generated when the silicate magma reaches sulfide saturation and exsolves an immiscible sulfide melt. If this occurs early in magma emplacement, the dense sulfide melt will tend to stall in feeders or near the base of the magma body. Thus, sulfide concentrations will be spatially confined in large anorthosite complexes, with sulfide exposure dependent entirely on erosional level. Generation of economic sulfide concentrations may require assimilation of crustal sulfide, either from sulfidic metasedimentary rocks or from volcanogenic or sediment-hosted massive sulfides. These sulfide sources are prevalent in rift settings and hence in the suture zones that commonly characterize the country rocks of major anorthosite-associated intrusive complexes. Nevertheless, development of sulfide concentrations may depend on the fortuitous intersection of magma with sulfidic country rocks. Rhenium-osmium (Re-Os) isotope data are available for parts of three Proterozoic anorthosite complexes; in each case, high initial 187 Os/ 188 Os requires a predominantly crustal source for the Os in the sulfide minerals. Modeling of Re-Os isotope data from the 1559 Ma Suwalki anorthosite massif leads to the hypothesis that sulfide saturation results from direct assimilation of crustal sulfide by a mantle-derived magma. Subsequent interaction of the sulfide phase with additional, uncontaminated silicate melt increases the concentrations of Re, Os, and other chalcophile elements in the sulfides and shifts the 187 Os/ 188 Os of the crustally derived sulfide toward that of the mantle-derived silicate melt. Whereas we cannot rule out a crustal source for the parental magma at Suwalki, Re-Os data also permit a mantle-derived melt modified by crustal contamination. We model two end-member processes that can form sulfide concentrations in mantle-derived melts by assimilation of crustal material. The first process assumes that upwelling magmas are contaminated by bulk silicate crust, bringing the magma to sulfide saturation and producing an immiscible sulfide melt. The magma continuously assimilates crustal material so that the silicate melt maintains high 187 Os/ 188 Os. With increasing R (the mass ratio of silicate to sulfide liquid), concentrations of chalcophile elements, including Re and Os, increase in the sulfide melt but 187 Os/ 188 Os remains roughly constant. The second process assumes that upwelling magmas are contaminated by Re-Os-rich crustal sulfide, generating copious immiscible sulfide melt with Re and Os concentrations and 187 Os/ 188 Os not far removed from those of the contaminant. Assimilation of crustal material declines as the magma establishes armored conduits and/or chemical and thermal gradients between the magma and pristine country rock. With increasing R , continued flux of relatively uncontaminated magma increases Re and Os concentrations in the sulfide but draws 187 Os/ 188 Os downward toward lower mantle values. If this second process dominates at Suwalki, the high γ Os limits R to very low values, probably 187 Os/ 188 Os values at Suwalki.

The Voisey’s Bay Ni-Cu deposit is one of the most significant discoveries made in Canada in the last 30 yr. Known reserves, plus indicated and inferred resources, amount to 136.7 106 metric tons (t), grading 1.59 wt percent Ni and 0.85 wt percent Cu. The deposit can be described in terms of four geologic settings: the Eastern Deeps, consisting of massive and disseminated sulfide along the line of intersection of a feeder sheet with the base of a troctolite intrusion; the Ovoid, a 600-m-long by 350-m-wide 350 by 110-m-deep lens of massive sulfide underlain by a feeder sheet; the Discovery Hill zone, which comprises a series of swellings containing disseminated and massive sulfides in the troctolite feeder; and the Reid Brook zone, which is where the feeder sheet opens out into a deeper level intrusion. The principal sulfide minerals are hexagonal pyrrhotite, troilite, pentlandite, chalcopyrite, cubanite, and magnetite. Troilite is restricted to the Reid Brook, Discovery Hill, and Ovoid zones, where it occurs as exsolution lamellae in hexagonal pyrrhotite. The texture of pentlandite varies with the presence of troilite in coexisting pyrrhotite; where troilite is present, pentlandite occurs as equant masses, up to several centimeters across; where troilite is absent; it occurs around pyrrhotite grain margins and as lamellae in a pyrrhotite host. The Ni content of hexagonal pyrrhotite averages 0.28 wt percent (14 analyses) in troilite-bearing zones, and 0.51 wt percent (8 analyses) in troilite-absent zones. Cubanite is restricted to troilite-bearing zones. The presence of troilite indicates a metal-rich high-temperature hexagonal monosulfide solid solution from which pentlandite will exsolve at a higher temperature than from a more sulfur-rich monosulfide solid solution, thus developing as larger masses; cubanite can exist with metal-rich pyrrhotite but not with sulfur-rich pyrrhotite. Metal-rich pyrrhotite indicates an original metal-rich sulfide melt, which, in turn, indicates a low oxygen fugacity in the silicate magma with which the sulfides equilibrated; this is thought to be due to the presence of graphite in the region where the magma extracted sulfide from surrounding gneisses.

The Rosebery deposit in western Tasmania is a polymetallic massive sulfide deposit hosted in felsic volcanics of the Cambrian Mount Read Volcanic belt. The deposit underwent upper greenschist facies regional metamorphism and related deformation during the Devonian Tabberabberan orogeny, resulting in folding, shearing, and faulting (thrusting) of the ore lenses. The south end of the Rosebery deposit has undergone metasomatic replacement related to a postorogenic Devonian granitoid pluton, whose existence is inferred from geophysical (gravity) data. Metasomatic mineral assemblages related to this pluton have been recognized transgressing Cambrian volcanic-hosted massive sulfide minerals at the south end of the Rosebery orebody. Cambrian volcanic-hosted massive sulfide mineralization at Rosebery consists of three primary zones: a lowermost pyrite-chalcopyrite zone, overlain by a sphalerite-galena ± pyrite zone, and further overlain by a massive barite ± carbonate zone. Three major Devonian replacement zones are also present (1) a magnetite-biotite ± chalcopyrite zone, (2) a pyrrhotite-pyrite zone, and (3) a tourmaline-quartz ± magnetite zone. Minerals such as fluorite, gamet, and helvite are also present in the replacement zones. Metal zonation studies indicate that zinc occurs dominantly in a blanketlike enrichment zone in the primary sulfide lenses of the Rosebery deposit. Gold is largely concentrated at the top of the ore lenses. In the transgressive pyrrhotite-pyrite replacement zone at the south end of the Rosebery deposit, zinc is conspicuously depleted but gold concentrations of more than 20 glt are observed. In contrast, the biotite-, magnetite-, and tourmalinebearing replacement zones contain low gold grades (generally less than 5.0 glt Au). Copper concentrations as high as 2 percent are observed in the biotite-magnetite zone, as well as in the pyrrhotite-pyrite zone. Microthermometry and laser Raman spectroscopy studies of fluid inclusions from tlle Devonian replacement zone indicate that the early biotite-magnetite and pyrrhotite-pyrite assemblages formed from the interaction of moderate- to high temperature (>330°C), saline (10-25 wt % NaCl equiv), CO2-bearing fluid with the original stratiform lead-zinc mineralization. The later stage tourmaline veining and associated replacement assemblages resulted from lower temperature (

In the Pioche district, lead, zinc, silver, gold, copper, and manganese were mined from intrusion-related bedding replacement, fissure replacement, and fissure vein deposits in Late Proterozoic and Cambrian siliciclastic and carbonate rocks. Radioisotopic ages of intrusions, lead and sulfur isotope compositions of sulfide minerals, and metal abundances define two periods of metal mineralization: Late Cretaceous (ca. 100 to 90 Ma) and late Oligocene (ca. 28 to 27 Ma). Cretaceous deposits are characterized by spatial association with Cretaceous dikes and small stocks, high sulfur isotope compositions of sulfide minerals (delta 34 S > 10ppm), mostly radiogenic lead isotope compositions ( 207 Pb/ 204 Pb > 15.65), stratigraphic distribution restricted to Late Proterozoic-Early Cambrian rocks, and in many cases, low abundances of copper. Unifying features of Oligocene deposits are proximity to Oligocene intrusions and buried, magnetic apophyses of Oligocene intrusions, low sulfur isotope compositions (delta 34 S

Apatite- and oxide (ilmenite + magnetite)-rich dunite, pyroxenite, peridotite, melatroctolite, and gabbro occur as transgressive bodies in the Boulder Lake area of the southern Duluth Complex. Nelsonite (apatite + ilmenite) is locally found as vein or dikelike bodies whose geometric form resembles veinlike massive Cu-Ni sulfide mineralization in the Babbitt deposit. Massive sulfide mineralization at Babbitt has been attributed to the separation of an immiscible sulfide magma from a tholeiitic parent. Nelsonite contains assessory minerals such as Zn-rich hercynitic spinel, biotite, zircon, baddeleyite, and sulfide minerals. Massive sulfide mineralization at Babbitt is characterized by the presence of minor (1-10 vol %) amounts of euhedral apatite, as well as Fe-Ti oxides and the same assemblages of accessory minerals that is found in the nelsonite. Textures developed in the nelsonite, together with the unusual composition, are supportive of an origin involving Fe-Ti-P and S liquid immiscibility. Apatite from the nelsonites and related rocks contain very low REE and Cl contents relative to that in the massive sulfide. These data are thought to reflect the evolution of a Cl- and REE-bearing fluid from the magma that gave rise to the nelsonite and a later separation of an Fe-Ti-P-rich immiscible liquid in comparison to the early separation of an immiscible sulfide-rich liquid from a basaltic magma in the Babbitt area.Sulfur, oxygen, and hydrogen isotope data are not supportive of an origin for the apatite-oxide-rich rocks that is linked to assimilation of metasedimentary country rock. However, elevated delta 18 O values (up to 13.2 in plagioclase) in many of the apatite-oxide-rich rocks, together with anomalously high An content of plagioclase, suggest that subsolidus interaction with a Ca-rich fluid derived from dehydration reactions in the pelitic rocks of the Virginia Formation has been important.Although an origin involving immiscibility of an Fe-Ti-P-rich liquid is strongly indicated for the nelsonites, the origin of spatially associated apatite-oxide-rich rock types is problematic. The SiO 2 content of most of the crosscutting bodies is less than 30 wt percent, and derivation by extreme closed-system fractional crystallization of a tholeiitic parent is considered unlikely due to predicted early saturation in Fe-Ti oxides. The oxideapatite-rich rocks may themselves be products of liquid immiscibility, reflecting a continuum in compositions with nelsonite as an end member.

Many aspects of the timing and genesis of the Archean Hemlo gold deposit are unresolved; pre-, syn- and postmetamorphic models have been proposed. The presence of abundant low-temperature sulfide minerals within highly deformed, sillimanite-grade host rocks has been used to support a postmetamorphic origin for the deposit. However, the present Sb-As-Hg ore assemblage, including cinnabar-realgar-orpiment assemblages, developed through a sequence of exsolution events during postpeak metamorphic cooling. At peak metamorphism, most of the As and Hg, along with Tl, Cu, Zn, and minor Fe, was incorporated into a high-temperature antimonian sulfosalt. In the lower amphibolite facies or upper greenschist facies, impure (Hg, Zn)S exsolved from the high-temperature phase. An impure chalcopyrite and native antimony exsolved from the host sulfosalt soon after, leaving behind an Sb 2 S 3 -As 2 S 3 solid solution. Further cooling resulted in the exsolution of cinnabar and mercurian sphalerite from the (Hg,Zn)S. Impurities in the cinnabar and chalcopyrite exsolved to form aktashite, arsenopyrite and native antimony. Eventually, orpiment exsolved from the arsenic-bearing stibnite. Late, retrograde events converted the free orpiment to realgar and remobilized the realgar over short distances, along with cinnabar and native antimony. The history of sequential sulfide exsolution, along with the deformed nature of the Sb-As-Hg-bearing veins, and their metamorphosed alteration selvages, suggests that the metals were introduced prior to, or during, peak metamorphism.

The Crofoot-Lewis deposit is an adularia-sericite-type (low-sulfidation) epithermal Au-Ag deposit, whose well-preserved paleosurface includes abundant opaline sinters, widespread and intense silicification, bedded hydrothermal eruption breccias, and a large zone of acid sulfate alteration. Radiogenic isotope ages indicate that the system was relatively long-lived, with hydrothermal activity starting around 4 Ma and extending, at least intermittently, for the next 3 m.y.Field evidence indicates that the surficial zone of acid sulfate alteration formed in a steam-heated environment within an active geothermal system. A drop in the water table enabled descending acid sulfate waters to leach Au and Ag from zones of low-grade disseminated mineralization, resulting in the redistribution and concentration of Au and Ag into ore-grade concentrations. These zones of secondary Au-Ag enrichment are associated with opal + alunite + kaolinite + montmorillonite + or - hematite and were deposited in open space fractures at, and within a few tens of meters below, the paleowater table.The stable isotope systematics of alunite and kaolinite in the steam-heated environment are relatively complex, due to variations in the residence time of aqueous SO 4 that formed from the oxidation of H 2 S prior to precipitation of alunite, and the susceptibility of fine-grained kaolinites to hydrogen isotope exchange with later waters. Most of the alunites are enriched in 34 S relative to early sulfide minerals, reflecting partial S isotope exchange between aqueous SO 4 and H 2 S. About half of the alunites give reasonable calculated delta 18 O (sub SO 4 -OH) temperatures for a steam-heated environment indicating O isotope equilibrium between aqueous SO 4 and water. The delta D (sub H 2 O) values of the hydrothermal fluids varied by almost 60 per rail over the life of the meteoric water-dominated system, suggesting significant climate changes.Mineralization is believed to have resulted from large-scale convection of meteoric water controlled largely by basin and range fractures and a high geothermal gradient with H 2 S for Au complexing derived from organic matter in basin sediments. A wet climate resulted in the formation of a large inland lake which provided abundant recharge water for the hydrothermal system. A fluctuating water table controlled by changing climatic conditions enabled steam-heated acid sulfate fluids to overprint lower grade mineralization resulting in ore-grade precious metal enrichment.

Jurassic (166 + or - 2 Ma) gold vein deposits of the Samhwanghak mine, Youngdong area, are examples of mesothermal gold deposits in the Republic of Korea. The deposits are composed of gold-bearing but relatively sulfide-poor massive quartz veins which occupy shear zones in graphite-bearing paragneiss of Precambrian age.Vein quartz was deposited mostly prior to deposition of sulfides and gold, and contains three main types of fluid inclusions: (I) CO 2 -CH 4 fluids, (II) low-salinity ( 2 kbars. Initial hydrothermal fluids were near their critical states at temperatures of 410 degrees to 480 degrees C and were homogeneous compositionally with X (sub CO 2 ) +CH 4 of 0.45 (X (sub CH 4 ) = 0.14) and salinity of [asymp] 9 wt percent NaCl. Type III inclusions appear to be secondary and represent later fluids which formed through extensive fluid unmixing of type II fluids with progressive decreases in temperature (below 370 degrees C) and pressure (down to [asymp] 1 kbar), but may also include deep circulating meteoric waters.Thermochemical calculations indicate the following evolution of hydrothermal fluids: fugacity of sulfur decreased progressively with decreasing temperature near the pyrite-pyrrhotite sulfidation curve; fugacity of oxygen was controlled largely through graphite-fluid interaction and evolved near the QFM buffer; pH was maintained near neutrality. Temperature versus f (sub O 2 ) relationships indicate that hydrothermal fluids were derived originally from a nearby S-type magma, the Middle Jurassic Kimcheon Granite. Gold occurs as gold-rich (avg 72 at. % Au) electrum intimately associated with sulfide minerals, and precipitated from later aqueous (type III) fluids at temperatures of 340 degrees to 240 degrees C, as a combined result of cooling and decrease of sulfur activity (by sulfide precipitation and/or H 2 S loss accompanying fluid unmixing).Calculated sulfur isotope compositions of hydrothermal fluids (delta 34 S (sub Sigma S) ) near 2 per mil indicate that ore sulfur was derived from an ilmenite series (S-type) granitic melt. Measured and calculated oxygen and hydrogen isotope compositions of ore fluids (delta 18 O water = 6.1-8.4ppm; delta D water = -62 to -72ppm) are consistent with derivation from and interaction with magmatic rocks. Mesothermal gold vein deposits of the Youngdong area were formed by extensive fracturing and veining during the Jurassic Daebo orogeny and developed from fluids that may have included an important component of magmatic fluids. Mineralogy and ore fluid chemistry of the deposits of the Youngdong area are similar to those of Archcan mesothermal gold vein deposits, possibly validating a more general application of a magmatic hydrothermal model for the formation of mesothermal gold vein deposits.

The Silver City stock, one of several resurgent intrusions along the margin of the Tintic caldera, has been historically accepted as the source for ore fluids at Tintic, or at least responsible for the heat to drive the hydrothermal system. Fluid inclusions in quartz from fissure veins, however, reveal a thermal gradient that traverses exposures of the Silver City stock, reaching a thermal maximum 3 km south of the exposed stock. Sulfur isotope values from sulfide minerals in the vein and replacement deposits are typical of igneous values (about 1ppm) near the thermal high, but are distinctly lower (as low as -6.8ppm) to the north, though still located in the Silver City stock.Main-stage mineralization at Tintic may have been the product of fluids emanating from a very late resurgent granitoid intrusion in the Tintic district, one that is still largely unexposed. An apophysis of this intrusion may approach the surface near the thermal high delineated by fluid inclusion homogenization temperatures. Additional evidence for the existence of such an intrusion includes widespread propylitic alteration of the Silver City stock and surrounding volcanic rocks, a ladder-style pattern of fissure veins traversing the district, and rare exposures of granitic aplite dikes crosscutting the Silver City stock.

Published thermodynamic properties of selenide minerals were used to construct fugacity-temperature diagrams to estimate relative stabilities of the minerals as a function of temperature. The phase relations among selenides, sulfides, tellurides, and oxides were investigated by constructing fugacity-fugacity diagrams at temperatures between 100 degrees and 300 degrees C. These diagrams are used to predict equilibrium mineral assemblages and their variation with temperature, the relative tendency of various metals to form selenides, and to estimate the prevailing f (sub Se (sub 2(g)) ) , f (sub S (sub 2(g)) ) , f (sub Te (sub (g)) ) and f (sub O (sub 2(g)) ) , and their evolution during mineral deposition and/or reequilibration from mineral assemblages recognized in the deposit. These diagrams are also used to explain the occurrence of selenide minerals in ore deposits and to predict the mineral assemblages of some economically important chemical elements including Au, Ag, and Cu. The stability fields of selenide minerals are generally more restricted than those of sulfide minerals but are larger than those of corresponding telluride minerals for similar values of f (sub S (sub 2(g)) ) , f (sub Te (sub (g)) ) , and f (sub Se (sub 2(g)) ) . Exceptions include hessire (Ag 2 Te) and calaverite (AuTe 2 ), whose stability fields are larger than those of naumannite (Ag 2 Se) and AuSe, respectively. Most oxide minerals, except cassiterite, magnetite, hematite, and uraninite are unstable relative to the corresponding selenide minerals in typical hydrothermal fluids. The larger the stability field of the selenide, the more common it is in ore deposits (e.g., clausthalite, PbSe). Some transition metal selenide minerals that have not been found in nature (e.g., Ni 3 Se 2 are stable only at very low f (sub S (sub 2(g)) ) and/or f (sub O (sub 2(g)) ) and are unlikely to occur in the earth's crust. Some compounds such as AuSe and those in the Tl-Se binary system are likely to be found in natural assemblages. They have not been observed because of our failure to detect them or because ternary gold and thallium minerals, respectively, are formed instead. The electrum-naumannite assemblage may act as a sliding-scale indicator of f (sub Se (sub 2(g)) ) , similar to the electrum-Ag 2 S system for f (sub S (sub 2(g)) ) .

Laboratory measurements of the density (rho ) and compressional wave velocity (v P ) of massive sulfides and host rocks at elevated pressures show that ores occupy a V P - rho field which is distinct from that of common silicate rocks and governed by simple mixing rules between the properties of host rocks and pure sulfide minerals. The velocities of the most common sulfides are extremely variable, ranging from 3.7 km/s for galena, through 4.7 km/s for pyrrhotite, 5.6 km/s for sphalerite and chalcopyrite, to 8.0 km/s for pyrite, but their densities are always high (7.6, 4.6, 4.1 - 4.2, and 5.0 g/cc, respectively), causing ores of even intermediate grade to have high acoustic impedances. This suggests that large massive sulfide bodies should make strong to spectacular seismic reflectors, depending on their grade, mineralogy, size, and setting. In principle, it should thus be possible to use high-resolution seismic reflection techniques for base metal exploration and delineation.

Sulfur isotope analyses were carried out on ore minerals from Mississippi Valley-type deposits throughout the southern Appalachians, as well as on evaporite sulfate minerals that might have supplied this sulfur, in an effort to determine the origin(s) of sulfur in the deposits. Mississippi Valley-type deposits are found largely in two major paleoaquifers in the region, the Lower Cambrian Shady Dolomite and the Lower Ordovician Knox Group karst zone. In general, Mississippi Valley-type sulfide delta 34 S values are different in these two paleoaquifers, but appear to be relatively homogeneous within each paleoaquifer over areas of thousands of square kilometers. In the Knox paleoaquifer, the most common sulfide sulfur has delta 34 S values of 31 to 36 per mil and appears to have been derived by complete reduction of seawater sulfate of Lower Ordovician age, with little or no fractionation. A second population of sulfide sulfur in the Knox paleoaquifer has delta 34 S values of 26 to 29 per mil and appears to have been derived by similar reduction of Lower Cambrian seawater sulfate. Barite deposits in the Knox paleoaquifer have delta 34 S values that also fall in the range of Lower Ordovician seawater but extend to values that could reflect mixing with sulfate from underlying Upper Cambrian evaporites. The isotopic composition of sulfate sulfur in the Lower Cambrian paleoaquifer is similar to that of local evaporites, but sulfide minerals in the northern part of the paleoaquifer may have been derived from Mississippian evaporites.These results indicate that sulfur isotope compositions are not specific to individual Mississippi Valley-type deposits but, rather, have similar compositions over large regions. Compositions are most similar within specific stratigraphic units, with local evidence for cross-formational flow. The close similarity of sulfide and sulfate delta 34 S values suggests that large volumes of sulfate were reduced quantitatively with little or no fractionation. The lack of isotopic zonation within individual crystals indicates that sulfate reduction took place largely before sulfide deposition. Although thermochemical sulfate reduction appears to be the most likely mechanism to produce this reduced sulfur, some might have been produced by bacterial reduction in bottom waters of the overlying anoxic Sevier Shale basin, which recharged the Knox paleoaquifer.

Proterozoic metal-rich black shales (metasiltstones) in the Bohemian Massif are always closely associated with submarine mafic volcanic rocks and occur in (semi)isolated basins that formed most likely in connection with the evolution of an Upper Proterozoic intracontinental rift. The black shales are characterized by an average of 3.5 wt percent C organic , and 5.2 wt percent S and carry up to 0.42 wt percent Zn, 0.21 wt percent V, 0.12 wt percent Ni, 0.12 wt percent Cr, 0.12 wt percent Cu, 0.06 wt percent As, 0.03 wt percent Mo, 132 ppb Au, 102 ppb Pd, 25 ppb Pt, 1.7 ppb Rh, and 1.3 ppb Ru. Their thickness varies from about 1 to 45 m. Sulfide minerals often include abundant framboidal pyrite, which occurs widely in C organic -rich layers. Grainy pyrite, Zn, Ni, Cu, and Mo sulfides appear to be restricted to late quartz and carbonate veinlets. Mo selenide, Mo telluride, galena, berthierite, and gold are very rare. Sulfur isotope results suggest the dominant source of sulfur to have been seawater sulfate, reduced by bacteria under different conditions. The delta 13 C values of the organic matter (-24.2 to -37.5ppm) reflect the combination of the carbon isotope composition of dissolved HCO 3 , its high concentration, and the type of organisms. The results of Rock-Eval pyrolysis as well as laser Raman spectroscopy show a high maturity, close to that of well-crystallized graphite. Fluid inclusion and stable isotope data from quartz and carbonate veinlets of very low grade regionally metamorphosed metalrich black shales at Kamenec and highly metamorphosed facies at Hromnice indicate that they resulted from the interaction between 18 O-rich fluids, volcanics, and metasediments. The interaction could have occurred during subsea-floor or later regional metamorphism.

Geochemical and mineralogical data have been obtained from black carbonaceous shales interbedded with dolomite in the Chuniespoort Group of the Late Archean-Early Proterozoic Transvaal Sequence. The black shales are fine grained and generally argillaceous and contain on average 1.2 percent total C organic , 0.7 percent S, 7.5 ppb Au, 60 ppm Cu, 330 ppm Cr, and 125 ppm Ni. These metal concentrations are similar to, or slightly higher than, the values given for the U.S. Geological Survey black shale standard SDO-1. All other heavy and transition metals, however, are depleted in the Chuniespoort shales relative to SDO-1. It is well known that high concentrations of Cr and Ni occur in Archcan shales. The enhanced Cr and Ni values found in the Chuniespoort shales are a reflection of their antiquity and provenance and are not linked to preferential uptake of these metals during sedimentation and/or diagenesis. The Chuniespoort shales are characterized by excess Fe with respect to that required to form pyrite and have extremely variable C/S ratios as well. This, together with the wide-ranging S isotope ratios, is taken as an indication that biogenically produced sulfide was augmented by sulfide introduced from an extraneous source. Whole-rock S isotopes further indicate that S could have been derived from biogenic and inorganic reduction of sulfate as well as from hydrothermal fluids. Interelement correlations suggest that organic carbon does not play a role in hosting significant concentrations of base and transition metals. Rather, element associations indicate that V and Cr are preferentially contained in illite-hydromuscovite and that Ni, Co, Zn, Cu, Fe, Pb, and Mo probably reside in pyrite and/or chalcopyrite. Gold is only weakly correlated with other elements, which may indicate that its distribution is not solely determined by the presence of sulfide minerals. The fact that Au, Cu, and S/C ratios show an increase in samples with delta 34 S values around 0 per mil suggests that these elements are likely to have an epigenetic hydrothermal origin. The limited syngenetic metal accumulation can be explained by the stable cratonic environment in which the sediments were deposited and the absence of submarine hydrothermal springs venting metalliferous fluids into the basin. Other factors, such as the low metal potential of the provenance, the rate of sedimentation, and the degree of oxygenation of the bottom waters of the basin, may have been additional factors limiting the concentration in, and extraction of, metals from seawater. The fact that the Chuniespoort shales host significant epigenetic gold occurrences, particularly in the Sabie-Pilgrim9s Rest gold field, is largely related to their chemical characteristics, which promoted permeability and high absorption capacities for metals in hydrothermal fluids. Because of their low metal concentrations elsewhere in the basin, the shales cannot, however, be considered the source rocks for gold mineralization.

Part I. Pyrite, chalcopyrite, and sphalerite from six volcanic-hosted massive sulfide (Mount Chalmers, Rosebery, Waterloo, Agincourt, Dry River South, and Balcooma) deposits in eastern Australia were analyzed using a proton microprobe to determine trace element abundances. In pyrite, trace elements can be divided into three groups according to the most likely occurrence of the element: (1) elements that occur mainly as inclusions (Cu, Zn, Pb, Ba, Bi, Ag, and Sb), (2) elements that occur as nonstoichiometric substitutions in the lattice (As, Tl, Au, and possibly Mo), and (3) elements that occur as stoichiometric substitutions for Fe (Co and Ni) or S (Se and Te). Hydrothermal and metamorphic recrystallization cleans pyrite of group 1 and group 2 elements, but does not appear to affect the concentrations of group 3 elements. Colloform pyrite grains have the highest levels of As and Au (up to 200 ppm), suggesting that rapid precipitation is important in incorporating Au into auriferous pyrite. Elements that occur as inclusions in chalcopyrite include Pb, Bi, Zn (P), and Ba. The occurrence of As and Sb is unresolved, although consistently high values of As in some samples suggest that As may substitute into the lattice of chalcopyrite. Elements that substitute into the lattice include Ag (for Cu), In, Sn and Zn (?) (for Fe), and Se (for S). Lead, Ba, Sb, possibly, and in some cases, Cu, occur commonly as inclusions in sphalerite. Lattice substitutions in sphalerite include Fe, Cd, Cu (to 4,500 ppm), Ni, In, Ag, Te, Ga and possibly Mo. In addition, consistently high (2,000-4,000 ppm) levels of As in the Rosebery barite zone may indicate As lattice substitution. Part II. The Se content of pyrite in volcanic-hosted massive sulfide deposits varies as follows: in Cu-poor, Zn-rich deposits, Se levels are low (mainly

Mississippi Valley-type mineralization from two major host formations in the central Appalachians was studied in order to gain a better understanding of depositional conditions, sources of ore constituents, and the distribution of mineralizing fluids in the region. Sulfur isotope data obtained in this study and from the literature (Ault and Kulp, 1960; Howe, 1981; Crawford and Beales, 1983; LeHuray, 1984; Wilbur et al., 1990) show that mineralization in each formation acquired sulfur from sources of similar isotopic composition. Barite deposits in the Beckmantown Group have (delta 34 S values ranging from 25 to 36 per mil; the values probably closely reflect the isotopic composition of the sulfur-bearing fluid involved in the barite formation. Sulfide deposits in this formation have more variable compositions and more complex isotopic systematics. In both the Timberville district and Nittany Arch area, main-stage sulfide minerals have delta 34 S values ranging from about 15 to 30 per mil. Each district also has a smaller, isotopically lighter substage of sphalerite mineralization that has (delta 34 S values between 1 and 19 per mil. Sulfate minerals in the Nittany Arch area are isotopically heavier than the sulfides, with delta 34 S values between 30 and 40 per mil. The lightest sulfur in the central Appalachian Mississippi Valley-type districts is found in the Friedensville district, where delta 34 S values of sulfide minerals range between -9 and +1 per mil. Sulfide minerals from the two Silurian Tuscarora-Shawangunk-hosted deposits have similar (delta 34 S values of 21 to 35 per mil. Minor amounts of sulfate minerals in this formation are isotopically heavy with delta 34 S values between 34 and 43 per mil.New fluid inclusion data were also obtained from the Timberville district and reveal the presence of at least two distinct fluids involved in mineralization-a more saline or calcium-rich fluid with final melting temperatures between -42 degrees and -29 degrees C, and a less saline or more calcium-poor fluid with final melting temperatures of -24 degrees to -12 degrees C. Homogenization temperatures of both inclusion types ranged for the most part between 100 degrees and 170 degrees C.Geologic evidence suggests that the Beckmantown Group and Tuscarora-Shawangunk Formations were part of distinct palcoaquifers, and together with the sulfur isotope data, also suggests that the source of sulfur for Mississippi Valley-type deposits in each palcoaquifer was cormate (formational) fluids derived from seawater, or alternatively in the case of the Beckmantown Group, intraformational evaporites. Furthermore, evidence of mixing and a temporal increase in the oxidation state of mineralizing conditions in at least two of the districts suggests that much of the Mississippi Valley-type mineralization in the central Appalachian region may be the result of mixing of oxidizing tectonically driven Zn-Pb or Ba-rich fluids with sulfur-rich fluids of varying oxidation state present in the Beckmantown Group and Tuscarora-Shawangunk Formations.

The Beregovo vein-type gold-base metal deposit is hosted by volcanic sedimentary sequences of the Pannonian basin in the Carpathian Neogene volcanic belt. Four stages of mineralization were distinguished: (1) sulfide, (2) quartz-barite, (3) carbonate-quartz, and (4) carbonate-goethite. Homogenization temperature, salinity, and chemical and isotopic composition of fluid inclusions in sphalerite, galena, barite, quartz, and carbonates, combined with the delta 18 O values of quartz, carbonates, and barite, suggest the presence of three major fluid types: (1) deep, relatively high temperature (250 degrees C) and moderate salinity (about 8 wt % NaCl equiv) Na-K-Cl fluid with relatively high delta 18 O (5ppm), delta D (-60ppm), and delta 13 C (-5ppm) values; (2) high-salinity (up to 14 wt % NaCl equiv) Ca-Mg-Na-Cl formation water with higher delta D values (-40ppm); and (3) near-surface steam-heated (about 200 degrees C) sulfate-bicarbonate-dominated fluid delta 18 O from -5 to -1ppm, and delta D from -110 to -56ppm). Low delta 13 C values (-25ppm) for CO 2 in type 2 and 3 fluids reflect a significant contribution of carbon from organic-rich sedimentary wall rocks.The delta 34 S values of sulfide minerals range from -2 to +2.5 for pyrite, -1 to +6 for sphalerite, and -2 to +8 per mil for galena. Two sources of sulfur are identified for the Beregovo sulfide ores. Ores with delta 34 S values close to 0 per mil may have been magmatic and those with higher delta 34 S values (up to 8ppm) perhaps resulted from organic matter (5ppm) in the wall rocks.Two main isotopically and genetically different barite generations were found: hypogene barites with delta 34 S values from 13 to 26 per mil, and supergene barites with values from 4.0 to 5.5 per mil. None of the barites had isotopically equilibrated with the sulfide minerals. If equilibrium in the isotopic exchange between sulfate and water were realized, the delta 18 O values of the fluid would range from -5.0 to -2.0 and -10.5 to -5.9 per mil for the two generations, respectively. For comparison, oxygen isotope composition for present-day precipitation is about -10 per mil.

Middle Valley is a sediment-covered rift near the northern end of Juan de Fuca Ridge. Hydrothermal fluids are presently being discharged at two vent fields about 3 km apart, Bent Hill and the area of active venting. The hydrothermally active chimneys at both Bent Hill and the area of active venting consist of anhydrite and Mg-rich silicates with minor pyrite, Cu-Fe sulfide, sphalerite, and galena. Hydrothermal discharge in these areas appears to be focused along extensional faults. At the Bent Hill massive sulfide deposit, clastic sulfide layers are interbedded with hydrothermally altered and unaltered hemipelagic and turbiditic sediment along the flanks of the sulfide mound. Sulfide textures and mineralogy suggest that the Bent Hill sulfide mound formed by the build-up and collapse of sulfide chimneys, the resedimentation of sulfide debris and the formation of clastic sulfide layers, and the infilling and replacement of clastic sulfides by hydrothermal fluids near vents. Sulfur isotope values that are consistently more positive than basaltic sulfur support the addition of seawater sulfur. Pb isotope values for the Bent Hill deposit that are transitional between midocean ridge basalt (MORB) and Middle Valley sediments indicate that the sulfides probably formed from fluids which originated in the oceanicmore » crust but which have been modified by reaction with lower temperature (

Geologic investigations with the submersible Nautile in the Lau basin represent one of the first detailed studies of hydrothermal activity on a modern back-arc volcanic ridge, the Valu Fa Ridge. Three major hydrothermal fields (Hine Hina, Vai Lili, and White Church) were discovered in the areas of the greatest differention of volcanic rocks. The type of hydrothermal deposit is controlled by the type of volcanism and by tectonic activity which increases from south to north. Three stages of sulfide formation are proposed and described. Vent fluids at Valu Fa have much higher metal contents than those at midocean ridges. Cl enrichment is best explained by mixing with deep brine rather than by subcritical phase separation and there is no evidence for a magmatic fluid contribution. Some characteristics of the fluids, such as low pH (2) and low concentration of H[sub 2]S, can be explained by subsea-floor sulfide formation. Vertical mineralogical zonation within the mound differs from midocean ridge deposits but is similar to that of a kuroko deposit. At the surface of the deposit, the virtual absence of pyrite and the high amount of sphalerite, barite, tennantite, galena, and locally, native gold are remarkable. Compared to midocean ridges, Lau basinmore » deposits are enriched in Ba, Zn, As, Pb, Ag, Au, and Hg and depleted in Mo, Se, and Co. Their mineralogy, chemical composition, and geologic setting show that the southern Lau basin deposits are intermediate between oceanic and continental back-arc deposits.« less