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The Prominent Hill deposit is a large iron oxide copper-gold (IOCG) resource located in the Olympic IOCG province of South Australia. The deposit is hosted by brecciated sedimentary rocks and structurally underlying lavas of the ca. 1.6 Ga Gawler Range Volcanics. Both rock units are altered and mineralized, forming characteristic hematite breccias. They are located in the footwall of the Southern overthrust separating the host-rock package in the footwall from Paleoproterozoic metasedimentary rocks in the hanging wall. The metasedimentary rocks were intruded by the Hiltaba Suite granites, which are comagmatie with the Gawler Range Volcanics and show widespread magnetite-rich alteration. Economic mineralization was formed dirough a two-stage process. Early pyrite and minor chalcopyrite were deposited from moderately reduced fluids during sulfide stage I and are hosted in subeconomic magnetite skarns and in the brecciated sedimentary host rocks. This preore stage was overprinted by the economically important stage II sulfides, deposited from hypogene, oxidized fluids ultimately sourced from the paleosurface. The high-grade Cu ores contain dominantly chalcocite, bomite, chalcopyrite, and gangue minerals including fluorite, barite, and minor quartz hosting mineralization-related fluid inclusion assemblages. Petrography, microthermometry, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) microanalysis were used to characterize pre-, syn- and postmineralization fluid inclusion assemblages. The results permit discrimination of four fluid end members (A, B, C, and D). Fluid A is the main ore fluid and hosted in fluorite and barite intergrown with Cu sulfides in the breccia matrix. It is weakly saline (< 10 wt % NaCl equiv) and contains low concentrations of K, Pb, Cs, and Fe (600 ppm) but is rich in Cu (1,000 ppm) and U (0.5-40 ppm). A magmatic origin of the salinity is supported by the low molar Br/Cl ratio of 0.003. We suggest that the solute inventory was derived from shallow fluid exsolution and degassing of late Gawler Range Volcanics and subsequent complete oxidation of the fluid via contact with atmospheric oxygen. Fluid A migrated through oxidized aquifers to the site of the Prominent Hill deposit, where it became the main driver of stage II copper mineralization. Fluid B occurs in fluid inclusions in siderite + quartz-bearing veins crosscutting the hematite breccia. It is die most saline fluid, with a total NaCl + CaCb concentration of 36 to 45 wt % and a low Ca/Na mass ratio of 0.3. Fluid B is rich in K, Fe, Pb, and Cs and contains modest Cu (-70 ppm). Its composition is typical of a moderately reduced magmatic-hydrothermal brine modified by fluid-rock interaction. Fluid C is hosted by fluid inclusions in fluorite and barite within bomite + chalcocite-bearing ores. It is a calcic-sodic brine witii 16 to 28 wt % NaCl + CaCl2 and has an elevated Ca/Na (0.6) and high Br/Cl ratios characteristic of basin brines of residual bittern origin. It is quite rich in Cu (-200 ppm) and likely contributed metals to economic mineralization. Fluid D is hosted by inclusions in fluorite in late veins crosscutting the hematite breccia. The total NaCl + CaCl2 salinity ranges between 19 and 30 wt %, and the concentrations of K, Mn, Cs, and Pb are similar to those of fluid C. Fluid D is a basement brine with characteristically high Ca/Na ratios of about 2 and contains modest Cu (-100 ppm). The high-grade copper mineralization in the Prominent Hill deposit originated dominantly from magmatic components, including both die metal inventory (Cu, minor Au, and U) and sulfur. The essential factor controlling the high-grade IOCG mineralization was wholesale oxidation of magmatic Cu and S from Gawler Range volcanic eruptions; volcanic gases reacted with atmospheric oxygen at or near the Earth's surface-for example, in an acidic volcanic lake environment. Oxidized surface fluids charged with Cu, U, and sulfate and high acidity then infiltrated the host rocks at Prominent Hill and became reduced by ferrous iron in residual magmatic brine, in nonmagmatic basin brine, and by ferrous iron-bearing minerals. Copper sulfides and hematite precipitated in a zone of fluid mixing and by interaction of the fluids with acid-neutralizing host-rock strata. Limited sulfide for the low-sulfidation ore mineral assemblage was supplied, to presently unknown proportions, from magmatic-hydrothermal pyrite derived in a preore stage of mineralization and from magmatic sulfate reduced by aqueous Fe2+ to precipitate hematite. Exploration-relevant factors for Prominent Hill-style high-grade copper ores are the combination of (1) active volcanism during the late stages of the Gawler Range Volcanic Province, (2) a fluid-mixing interface between deep brines and an acidic, surface-oxidized but magmatically charged water as the main ore fluid, and (3) a mappable gradient of acid neutralization in brecciated and chemically reactive host rocks, ideally containing carbonates and some earlier pyrite. [ABSTRACT FROM AUTHOR]

Geologic field relationships combined with spatially resolved geochemistry and high-precision U-Pb geochronology of zircons allow quantification of magmatic processes leading to porphyry emplacement and Au-Cu ore formation at Ok Tedi, the Earths youngest giant porphyry-skarn deposit. Trace element contents in zircons were obtained by laser ablation-inductively coupled plasma-mass spectrometry before selected grains were removed from the sample mount, chemically abraded, spiked, and dissolved for high-precision U-Pb geochronology by chemical abrasion-isotope dilution-thermal ionization mass spectrometry. U-Pb geochronology by laser ablation-sector field-inductively coupled plasma-mass spectrometry on abundant inherited zircon cores that occur in all intrusions together with Hf isotope analyses by solution multicollector-inductively coupled plasma-mass spectrometry permitted further assessment of the degree of crustal assimilation of the upper crustal magma reservoir. The combined chemical data and precise crystallization ages of zircons indicate closed system fractional crystallization and concurrent cooling in a common magma reservoir, from which the intrusions at Ok Tedi were successively extracted. Inherited zircon populations and Hf isotope analyses of Pleistocene zircons record lower crustal assimilation with Proterozoic basement. Based on cathodoluminescence (CL) imaging a chemically distinct zircon population with low Th/U was identified in the youngest synore intrusive rocks. These zircons indicate injection of a distinct magma into the common magma reservoir, which triggered the emplacement of the youngest porphyry, intrusion of a polymictic breccia with partly igneous matrix, and intimately associated hydrothermal Au-Cu mineralization. Crystallization ages of chemically characterized zircon crystals extend over a total time span of 212 ± 44 k.y., recording the minimum duration of fractional crystallization in the large magma reservoir from the time it reached zircon saturation. The youngest zircons in each intrusion resolve the emplacement of early thorite intrusions, low-grade monzodiorite porphyries, including the Sydney monzodiorite and Fubilan porphyry, into three time-resolved pulses separated by a total of ~160 k.y. The Sydney monzodiorite contains barren quartz veins and produced endoskarn and exoskarn; however, the exact timing of skarn-hosted Au-Cu mineralization is not yet clear. The emplacement of the ore-bearing Fubilan porphyry and the intrusive breccia overlap at 1.187 ± 0.022 Ma, constraining the time and duration of porphyry emplacement with associated Au-Cu mineralization. The injection of a distinct, probably more mafic magma batch into the magma reservoir preceded the emplacement of the Fubilan porphyry and the intrusive breccia by less than the resolution of 30 k.y., indicating that this event was the immediate trigger for Au-Cu mineralization at Ok Tedi. [ABSTRACT FROM AUTHOR]

The Prominent Hill iron oxide-copper-gold (IOCG) deposit, located in the Gawler craton of South Australia, contains ca. 278 million metric tons (Mt) of ore at 0.98% Cu, 0.75 g/t An, and 2.5 g/t Ag. In contrast to the predominantly granite-hosted Olympic Dam IOCG deposit, Prominent Hill is mainly within unmetamorphosed sedimentary rocks comprising coarse clastic to laminated argillaceous lithologies with some volcaniclastic components and variable carbonate, including local massive dolomite. Essentially unmetamorphosed sedimentary rocks and structurally underlying mafic to intermediate-composition lavas, inferred to be members of the lower Gawler Range Voleanics, host the economically mineralized hematite breccias. The volcanic-sedimentary package was downfaulted and tilted along a major east-west fault, north of which similar but regionally low-grade metamorphosed rocks were affected by subeconomic skarn mineralization, and (on a more regional scale of the Mount Woods domain) intruded by granitic and gabbroic bodies. Hydrothermal alteration and mineralization at Prominent Hill involved pervasive and texturally destructive replacement of formerly calcareous, dolomitic, and siliciclastic breccia components. Hydrothermal alteration minerals comprise hematite, magnetite, siderite, ankerite, quartz, sericite, chlorite, kaolinite, fluorapatite, fluorite, barite, REE-U minerals (including monazite), uraninite, and coffinite, together with Cu sulfides including chalcopyrite, bornite, and chalcocite in the highest grade ore. Brecciation and replacement caused mechanical mixing as well as chemical alteration of primary lithologies, such that sedimentary contacts became obscured. Mass-balance calculations identify Al, Ti, Si, and Zr as least mobile components during hematite-chlorite-sericite to weak hematite-quartz alteration. Because Zr was not regularly assayed in drill cores, we use concentration ratios of Ti, Al, and Si from the deposit-scale assay database to delineate the distribution of lithochemical units prior to hydrothermal alteration and Cu mineralization. The resulting lithochemical model, based on one horizontal and five vertical cross sections, is used as a basis for mapping alteration patterns calculated from molar (Fe + Si)/(Fe + Si + Al), K/Na, and K/Al ratios. These chemical patterns, in conjunction with mineral stoichiometry, indicate that the spatial distribution of hematite, chlorite, variably phengitic sericite (and/or illite) ± kaolinite ± quartz-bearing alteration is superimposed on the pattern of interpreted lithologic contacts. The alteration patterns confirm visual logging results, showing that hematite enrichment correlates only partially with the distribution of Cu grades of >0.25 wt %. A subvertical body of complete replacement by hematite and quartz with consistent but subeconomic gold enrichment forms a Cu-barren core in the central and eastern parts of the deposit. Zones of increasing K/Al and K/Na ratios extend upward and westward from this Cu-barren core, transgressively overprinting lithologic contacts. The degree of hematite-quartz replacement can be measured by a hematite-quartz alteration index, here termed the HMSI value [(Fe + Si)/(Fe +Si + Al)], which inversely correlates with the normal probability for Cu grade. Areas of highest Cu grade (>1 wt %) spatially correlate with irregular zones having intermediate molar alteration indices: 0.34 < K/Al < 0.40, 20 < K/Na < 36, and HMSI < 0.98. Hematite breccias and Cu ore deposition developed after tilting of the host sequence into its present steep orientation, as indicated by geopetal structures within the breccia matrix. Thus, the economic mineralization occurred late in the deformational history of the region and after extrusion of the lower Gawler Range Voleanics. The formation of the Prominent Hill orebodies occurred during or after upthrusting of deeper seated rocks containing subeconomic Cu in skarns north of the fault. Faulting as well as ore formation may be related to orogenic processes in the central and northern part of the Mount Woods domain. Iron oxide introduction was decoupled from, and at least partly preceded, hydrothermal deposition of high-grade Cu. Geochemical and petrographic data indicate that economic Cu mineralization occurred together with mildly acidic hematitechlorite-sericite ± siderite alteration of originally carbonate-, illite-, and feldspar-bearing sedimentary rocks. The presence of copper enrichment wath an intermediate degree of cation leaching from the host rocks indicates that pH neutralization of initially highly acidic metal-transporting fluids was an essential factor causing Cu sulfide deposition. Distinct ranges in Na-K-Al ratios and low HMSI values offer potential as exploration indicators pointing toward higher ore grades. These results from Prominent Hill are consistent with recently published mineralogical studies at the giant Olympic Dam deposit, indicating similar ore depositional controls despite lithologically different host rocks. [ABSTRACT FROM AUTHOR]

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

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

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

Maureen is the largest among several U (-Mo-F) prospects occurring along a Late Devonian unconformity in the Georgetown area, northern Queensland, Australia. Mineralization is structurally controlled by the intersection of steep east-west fractures with an unconformity between a Proterozoic basement and a Paleozoic cover sequence of continental sedimentary rocks and abundant rhyolitic volcanic rocks. The mineralogical composition of high-grade ore (pitchblende + Fe-rich molybdenite + arsenopyrite + arsenian pyrite + fluorite + dickite + chlorite + goyazite ± graphite or hematite), postlithification brecciation, and quartz dissolution indicate a strong chemical gradient and disequilibrium during mineralization between the reduced basement and the largely oxidized cover sequence. Elemental and mineral zonation centered to faults, and fractures cutting the unconformity, indicates locally reducing and quartz-dissolving conditions during the formation of tabular U-Mo orebodies, which are surrounded by a halo rich in fluorite. A detailed pétrographie study, microthermometry, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses of quartz- and fluorite-hosted fluid inclusions provide concentrations of ore- forming components in fluids related to hydrothermal uranium mineralization. Three fluids were involved in the mineralizing process at Maureen: a likely oxidized saline (L3) fluid, an aqueous (LI) fluid, and a CH4-bearing carbonic vapor (cV). The oxidized saline (L3) fluids trapped in fluorite and dicldte-bearing hydrothermal quartz veinlets crosscutting detrital quartz show the highest average concentrations of U (10-47 ppm), Mo (489-888 ppm), and As (318-777 ppm), with element ratios close to those of high-grade mineralized rocks. We suggest that aqueous (LI) fluids mixed in the cover sequence with oxidized saline fluids (L3) to precipitate fluorite as a distal halo, surrounding the east-west fractures. Their mixture, an oxidized moderately saline fluid (L2), reacted with methane-bearing carbonic vapor (cV) to precipitate U and Mo by reduction, close to the intersection of the fractures with the unconformity. Geochemical analysis of coexisting vapor and liquid assemblages indicate mixing of oxidized saline fluids with reduced carbonic vapors to be the driving force for uranium precipitation. The Maureen deposit shares essential geologic characteristics and hydrothermal processes with Proterozoic unconformity-related U deposits, but probably owes its unusual element association of U with (equally redox sensitive) Mo and abundant F to a source region in the volcano-sedimentary cover sequence that was enriched in these elements, due to the presence of highly fractionated, easily leachable, and oxidizable felsic volcanics. [ABSTRACT FROM AUTHOR]

The formation of porphyry copper deposits requires a focused flux of magmatic fluid, expelled from a large reservoir of water-, metal-, and sulfur-rich magma. The dimensions of this usually hidden magma reservoir are difficult to determine but can be constrained by combining geophysical observations with thermal constraints and the mass balance imposed by the chemical enrichment of elements in the deposit. Here we show that an internally consistent scenario can be derived for the world-class Cu-Mo-Au deposit at Bingham Canyon (Utah, United States), which quantifies the essential characteristics, approximate dimension, and temporal evolution of a large pluton that generated the deposit. The mineralized district shows a distinct WSW-ENE-striking magnetic anomaly indicating a large intrusive body underlying the sedimentary host rocks of the Oquirrh Mountains. Modeling the deep body by geomagnetic methods is possible because of the high contrast in magnetic susceptibility between sedimentary host rocks and intrusive rocks and because a former volcanic edifice is largely eroded. Additional constraints from drilled geology and district-wide outcropping rocks, including partial demagnetization by hydrothermal alteration on the mine scale, restrict the range of possible solutions to a broadly laccolith-shaped intrusion with a volume of approximately 1,400 to 3,000 km³. From the roof of the laccolith, several smaller subvolcanic stocks and dikes protrude to the present surface, of which a major one is hosting the Bingham Canyon deposit. The roof of the laccolith probably lies between 2 and 3.5 km below the bottom of the present open-pit mine, and the average thickness of the laccolith is constrained between 2 and 3.5 km. Thermal modeling, using pluton dimensions derived from the geologic and geomagnetic modeling, indicates that a single laccolith with a magma volume of -2,000 km³ beneath Bingham would have solidified within about 230,000 years or less. Comparison of the thermal models with published high-precision geochronologic data and petrologic constraints suggests a scenario in which about 1,000 km³ of magma was encapsulated by inward crystallization of the pluton after the preore equigranular monzonite stocks solidified and extrusive volcanism was probably terminated. This encapsulated reservoir was close to water saturation and contained approximately 150 billion metric tons (Gt) of magmatic water for subsequent closed-system fractionation and eventual fluid expulsion driving porphyry copper mineralization. Chemical mass balance shows that the known metal endowment and mapped mass of vein quartz within the deposit can be advected and precipitated by a fluid mass that is slightly smaller than the available 150 Gt of water. A conservative estimate indicates that 115 Gt of water is sufficient to precipitate all the quartz associated with successive Cu-Au- and Mo-stage veins as well as their barren precursors. According to our thermal model, approximately 250 km³ of quartz monzonite magma with a temperature of about 690°C remained partially liquid some 215,000 years after initial intrusion of the laccolith. At that point, it expelled almost simultaneously the quartz monzonite porphyry and the main mass of accumulated fluid, generating most of the vein quartz in the quenched porphyry and the adjacent older rocks. Petrographic evidence indicates that the ore metals precipitated near the end of individual pulses of quartz veining that followed recurrent but waning pulses of porphyry intrusion. Considering published experimental solubility data as well as ore metal contents in fluid inclusions, a small fraction of the available fluid mass is sufficient to transport and precipitate all the ore metals after an initial fluid pulse precipitated most of the quartz. However, the total amount of sulfur present in the deposit, which includes Cu and Mo sulfides as well as a major addition of pyrite, would be facilitated by addition of a mafic magma input into the residual magma chamber that contained the evolved felsic magma. This magmatic injection probably triggered the emplacement of the mineralized porphyries, consistent with the more mafic composition of some of the latest porphyry dikes and the CO2-rich nature of ore-related fluid inclusions. [ABSTRACT FROM AUTHOR]

Numerical, multiphase pure-water simulations were performed to study the first-order geologic and physical parameters controlling the style and distribution of hydrothermal venting at Brothers volcano, southern Kermadec arc, New Zealand. By comparing the results for different permeability scenarios, we can show that the location of venting on the inner slopes of the caldera (e.g., at the NW caldera site) requires the presence of higher permeability faults. Venting at the top of the Upper cone develops naturally by hydrothermal flow in porous rocks above an underlying magma body. However, this magmatic reservoir cannot alone account for present-day hydrothermal venting at the NW caldera site, which implies that a larger magma chamber, which was responsible for caldera collapse, is still active. Modeled venting temperatures for scenarios with homogeneous host-rock permeability correspond well with formation temperatures determined by sulfate-sulfide mineral pairs from different vent sites at Brothers volcano. Direct measurements of vent fluids at the NW caldera site today, however, show higher temperatures than modeled. This may be due to rapid ascent of hot fluids in individual fractures that are not resolved in the simulations. At the cone sites, measured temperatures are lower than modeled, likely the result of mixing with ambient seawater in near-surface permeable rocks. The inferred presence of a constant magmatic fluid source underneath the volcanic edifice leads to a more rapid development of the hydrothermal circulation and stabilizes the system at higher temperatures. We suggest that the hydrothermal evolution and fluid-flow patterns at Brothers volcano are controlled by episodes of varying magmatic fluid input into a seawater-dominated convection system. [ABSTRACT FROM AUTHOR]

The giant Bingham Canyon porphyry Cu-Mo-Au deposit (Utah) is associated with Eocene sub volcanic intrusions. It shows a distinct metal zonation above a barren core, with dominantly shallow Cu-Au mineralization (Cu stage) following the early quartz monzonite porphyry (QMP) intrusion, and spatially deeper Mo mineralization (Mo stage) occurring in a separate vein set exclusively after a late quartz latite porphyry (QLP) intrusion that truncates earlier Cu-Au veins. To understand this metal separation and the geochemical process of molybdenite mineralization, we investigated fluid inclusions by microthermometry, Raman spectroscopy, and laser ablation inductively couple plasma mass spectrometry (LA-ICP-MS) microanalysis in low- and high-grade quartz veins of both mineralization stages. In deep, low-grade quartz veins interpreted to represent the root zone of the Cu stage we found high concentrations of Cu, S, and Mo in the fluid inclusions, whereas in low-grade Mo-stage veins, we found lower Cu, but similar concentrations of S and Mo, compared to the inferred input fluids to the Cu stage. Sulfur and copper concentrations were similar in intermediate-density-type fluid inclusions in deep low-grade Cu-stage samples, whereas intermediate-density-type inclusions in low-grade Mo-stage veins have S contents that exceed their Cu contents. In high-grade Mo-stage vein, we found large variations of Mo concentrations in coexisting brine and vapor inclusions. Compared to the P-T conditions of the Cu precipitation stage (90-260 bars and 320°-430°C), the Mo-precipitating fluids were trapped at higher pressures and temperatures of 140 to 710 bars and 360° to 580°C. Mass-balance calculation based on the compositions of intermediate-density inclusions and brine + vapor assemblages, interpreted to be derived by phase separation during decompression of the ascending single-phase intermediate-density fluid, indicate that the mass of vapor phase exceeded that of brine by about 9:1 in both mineralization stages. Combining this mass balance with the analyzed vapor/brine partitioning data indicates that more than 70% of Mo and S (by mass) in the deposit were deposited from the vapor phase. Earlier Cu-Au deposition was similarly dominated by vapor, but recently published data about post entrapment Cu diffusion in and out of fluid inclusions cast doubt on previous quantifications, suggesting that almost none of the copper was deposited by brine. Mo is less likely to be modified by selective diffusion, and high Mo contents (max 0.0054 Mo/Na in intermediate density; 380 µg/g Mo in brine) in the hydrothermal fluids were maintained from the early Cu stage to the late Mo stage. This indicates that Mo concentration was not the decisive (actor for separate precipitation of late Mo ore at Bingham Canyon. Instead, the metal separation may be explained by a reduction in redox potential and an increase in acidity in the evolving source region of the fluids, i.e., a large sub volcanic magma reservoir. This is indicated by the stoichiometry of chalcopyrite and molybdenite precipitation reactions, a tentative difference in the Fe/Mn ratio in fluids of veining stages, incipient muscovite alteration along high-temperature molybdenite veins, and an increasing tendency for Mo to fractionate from brine to vapor. We suggest that the early Cu-stage fluids were slightly more oxidized and neutral, allowing Cu-Fe sulfides to saturate first, while molybdenite saturation was suppressed and Mo was lost from the early ore stage. By contrast during the later Mo stage, the fluids were more reduced and acidic, thereby allowing selective saturation of molybdenite as the first precipitating sulfide in the cooling and expanding two-phase fluid, consistent with textural observations. This interpretation may imply more generally that small differences in redox potential and acid/base balance of the magmatic source of porphyry-mineralizing systems may be decisive in the temporal and spatial separation of the two metals. [ABSTRACT FROM AUTHOR]

Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) in combination with near-infrared microscopy of fluid inclusions hosted by ore minerals that are opaque to visible light can provide the composition of ore-precipitating fluids. We applied the two techniques to well-constrained fluid inclusion assemblages hosted by pyrite, enargite, and quartz to trace the sortie and evolution of the fluids in high-sulfidation epithermal veins overprinting a porphyry copper deposit at Rosin Poieni, Romania. Despite some analytical limitations caused by the sulfide host minerals, the data demonstrate that fluids trapped in apparently cogenetic quartz and ore minerals are chemically different. Systematic changes in major and trace element ratios between liquid-vapor, vapor-rich, and brine fluid inclusion assemblages in the three minerals record an evolving fluid source at the porphyry to epithermal transition. Regarding their Cs/(Na + K) ratios, most of epithermal quartz-hosted fluid inclusion assemblages form a well-defined cluster, which coincides with the narrow range of the porphyry-stage fluids trapped in early quartz of the porphyry stockwork veins. Their Cu/(Na + K) ratios are 10 to 100 times lower compared to the pyrite-hosted inclusions and correspond to the lowest Cu/(Na + K) ratios recorded for the porphyry-stage fluids. By contrast, pyrite-bested, vapor-rich fluid inclusions have the highest Cu/(Na + K) similar to the highest Cu/(Na + K) ratios measured in the porphyry-stage fluid inclusions. The results led to the conclusion that the gangue and ore minerals in the high-sulfidation epithermal veins at Rosin Poieni formed by successive pulses of chemically distinct hydrothermal fluids that were successively exsolved from residual melt batches of a progressively crystanizing magma at greater depth. These results are consistent with detailed textural observations, but petrography alone could not have led to this unambiguous conclusion. [ABSTRACT FROM AUTHOR]

Fluid inclusion microthermometry and laser-ablation ICPMS microanalysis are combined with geological and textural observations to reconstruct the spatial and temporal evolution of magmatic fluids that formed the subvolcanic porphyry Cu-Au(-Mo) ore deposit at Bingham Canyon, Utah. The Bingham Canyon orebody is exposed over ∼1.6 km vertically and has the shape of an inverted cup with distinct metal zoning. Fluid inclusions in the barren but highly veined and potassically altered deep center of the system have intermediate density (∼0.6 g cm-3) and a salinity of ∼7 wt percent NaCl equiv. They have subequal concentrations of Na, K, Fe, and Cu and contain minor CQ2. The intermediate-density fluids were trapped as a single phase, mostly at >500°C and >800 bars. The Au-Cu-rich center near the top of the orebody contains low-density vapor inclusions (-0.2 g cm-3) coexisting with brine inclusions containing ∼45 wt percent NaCl equiv. The vertical transition of different inclusion types indicates phase separation of the single-phase input fluid upon volume expansion associated with a pressure drop to 200 ± 100 bars. Mass-balance calculation based on all analyzed inclusion components indicates that the mass of the vapor phase exceeded that of the brine by ∼9/1. The vapor contained Cu as its dominant cation (∼1.5 wt %) and contributed about 95 percent of the total amount of copper transported to the base of the orebody. Bornite, chalcopyrite, and native gold were precipitated in a narrow temperature interval from 430° to 350°C, into secondary pore space created by local redissolution of vein quartz as a result of retrograde quartz solubility in the vapor-dominated fluid system. Intermediate-density fluid inclusions in the deepest parts of the peripheral copper ore zone have identical density and composition, including similar gold contents, as those in the deep center. Microthermometry and statistical estimation of phase proportions in the inclusions show that the vapor in the peripheral Cu-rich but Au-poor ore zone remained denser, and the separating brine was less saline (∑36 wt % NaCl equiv), compared to vapor and brine in the central Au-Cu ore zone. This indicates that the peripheral fluids experienced a lower degree of phase separation, due to slightly higher fluid pressure at equivalent temperature, compared to more strongly expanding fluids in the center of the system. The systematic zoning of Au/Cu within the ore shell, despite compositionally similar input fluids, is interpreted to have resulted from slightly different pressure-temperature-density evolution paths of magmatic fluids. Copper was selectively precipitated in the peripheral ore zone, in contrast to complete coprecipitation of Au and Cu in the central upflow zone of the vapor plume. The formation of particularly rich Cu-Au ore in the center of the upward-expanding fluid plume is consistent with published experimental data, showing that the solubility of metals in hydrous vapor decreases sharply with falling pressure, due to destabilization of the hydration shell around metal complexes in expanding vapor. Tiffs interpretation supports the classic vapor plume model for porphyry copper ore formation but additionally emphasizes the role of sulfur-bearing complexes as a key chemical control on magmatic-hydrothermal metal transport and the deposition of Cu and Au in porphyry ores. Our interpretation of selective Cu ± Au precipitation as a function of vapor density can explain the more general observation that most gold-rich porphyry copper deposits are formed in shallow sub-volcanic environments, whereas deeper seated porphyry Cu-(Mo) deposits are generally gold poor. [ABSTRACT FROM AUTHOR]

Copper, gold, and molybdenum in the Bingham Canyon deposit (Utah, United States) show a systematic distribution in grade and metal ratios. Most Cu-Au mineralization follows, both spatially and temporally, the emplacement of the quartz monzonite porphyry (QMP), a southwest-northeast-elongated thick dike intruding along the contact between the premineralization equigranular monzonite stock and surrounding sedimentary rocks. Copper ore grades define the shape of an inverted cup, which is centered on the QMP but has a much broader, near-circular footprint. Several deep root zones surround a barren core occupied by the same litho-logic units and intense potassic alteration but insignificant metal tenor. Throughout the deposit, gold to copper ratio is systematically zoned. The distribution of molybdenum resembles that of copper and partly overlaps with it, but the molybdenum ore shell is generally displaced inward and downward from the copper ore shell. Systematic measurement of the abundance and orientation of three major vein types obtained at the pit surface were complemented with unoriented vein density data from drill core logging. Quartz stockwork veins, the earliest and most abundant of the mapped vein types, are related to potassic alteration and major Cu-Au mineralization. Their greatest vein density follows the intrusion of the QMP but extends far beyond, into sedimentary rocks and especially into adjacent parts of the pre-ore equigranular monzonite. Their orientation is predominantly steep, with a variable strike. Quartz-molybdenite veins postdate all intrusions and are less abundant than quartz stockwork veins. They show variable orientation of strikes mad a weaker tendency to steep dips. Quartz-pyrite veins with sericitic alteration halos crosscut all intrusions and earlier veins. They are rare within the central high-grade part of the deposit and predominantly occur near and outside the northeastern and southwestern ends of the QMP. They have a strongly preferred orientation parallel to the porphyry (likes, with steep dips and strike directions fanning out radially from the center of the deposit. Repeated cycles of dike intrusions with distinct southwest-northeast orientation, followed by steep stockwork veins with variable strikes distributed over the broad ore shell and a final return to oriented postore veins are interpreted to result from alternation between two stress regimes. A regional, probably weakly transtensional regime controls the emplacement of dikes and postore veins. Ore vein formation and mineralization is controlled by active rock extension (increase in differential stress) in the roof area above a broad region of high fluid pressure in a subjacent magma chamber, rather than by local hydrofracturing caused by fluid exsolution from the porphyries (decrease in effective rock pressure and positive volume change upon magma crystallization). Shallow vein mineralization well above the lithostatic-to-hydrostatic transition is consistent with low (hydrostatic or even vapor-static) pressures of ore deposition indicated by a companion study of fluid inclusions. The process of pressure-driven roof extension favors the accumulation of metals in high-grade ore shells, compared to temperature-driven concepts according to which the porphyry mineralization is spread out vertically by following downward-retracting isotherms in a cooling magmatic-hydrothermal system. [ABSTRACT FROM AUTHOR]

Sediment-hosted gold deposits in Guizhou, China, are hosted in late Paleozoic and early Mesozoic sedimentary rocks along the southwest margin of the Precambrian Yangtze craton. They have characteristics similar to Carlin-type gold deposits in Nevada and are notably enriched in As, Sb, Hg, and Tl. The Shuiyindong and Yata deposits consist of disseminated, strata-bound sulfides in Permian bioclastic limestone and fault-con-trolled mineralization in Middle Triassic calcareous clastic rocks, respectively. Mineralization in both deposits consists of barren milky quartz veins, disseminated gold-bearing arsenian pyrite and arsenopyrite, stibnite, realgar, and orpiment. The barren milky quartz veins occur in the ore-controlling structures with an envelope of gold mineralization in the host rock consisting of disseminated gold-bearing arsenian pyrite and arsenopyrite and replacement-style quartz veinlets. Later drusy quartz, stibnite, realgar, and orpiment fill fractures and vugs on the periphery of gold mineralization. Petrography, microthermometry, laser Raman spectroscopy, and laser ablation inductively coupled plasma mass spectrometric (LA-ICP-MS) analyses of fluid inclusious are used to characterize the chemical evolution of ore fluids at Shuiyindong and Yata. Early barren milky quartz veins trapped aqueous fluid with moderate salinity (6.0 wt % NaCI equiv) and minor CO2 (<2.4 mol %) at a temperature of 230° ± 30°C. This fluid contains measurable B, Na, K, Cs, Sr, As, Sb, and Au (3.8 ± 0.5-5.7 ± 2.3 µg/g) determined by LA-ICP-MS and is interpreted to approximate the mineralizing fluid responsible for precipitation of early barren quartz veins. Low-salinity (0.9-2.3 wt % NaCI equiv), CO2-bearing (6.3-8.4 mol %), aqueous fluids are recorded in replacement-style quartz veinlets. They formed gold-beating arsenian pyrite and arsenopyrite and quartz veinlets at a temperature of 210° ± 20°C and pressures of 450 to 1,150 bars, corresponding to depths between 1.7 and 4.3 km under lithostatic conditions. Fluid inclusions in late stibnite-orpiment-realgar-quartz veins contain high CO2 (58-64 mol %) and N2 (19.2-23.7 mol %) with trace CH4 (up to 1.6 mol %). The increasing content of CO2 and decrease in the concentrations of Au and As in aqueous-carbonic inclusions are interpreted to result from carbonate dissolution at the level of the deposits during gold mineralization. Covariance of Au, As, Sb, and Sr concentrations in the fluids is interpreted to reflect deposition of As mad Sb sulfides and gold concurrent with carbonate crystallization. Iron is below detection limit (~400 µg/g) in all fluid inclusion types, suggesting that the ore fluids were Fe poor but possibly sulfur rich, possibly explaining their exceptionally high gold contents. Iron in sulfide minerals was probably derived from dissolution of ferroan minerals in the host rocks, sulfidized by H2S-rich fluids to precipitate arsenian pyrite, arsenopyrite, and gold. Pressure fluctuation induced by faulting resulted in local fluid immiscibility and led to late deposition of realgar, orpiment, stibnite, and calcite. Hydrogen and oxygen isotope compositions of ore fluids (δD(H2O) = -35 to -68‰, δ18O(H2O) = 4-16.5‰) indicate a metamorphic origin, possibly related to crustal thickening and prograde metamorphism during the late Yanshanian orogeny. [ABSTRACT FROM AUTHOR]

This paper addresses the question of whether or not there are fundamental differences in the composition of magmatic-hydrothermal fluids in barren versus variably mineralized systems, and if so, at which stage of magmatic-hydrothermal evolution the differences were generated. Combining microthermometry and laser ablation ICP-MS, we studied high-temperature, premineralization fluid inclusions in three barren granites, one Sn-W-mineralized granite, one porphyry Cu-mineralized intrusion, one porphyry Mo-mineralized intrusion, and one Th-U-REE--mineralized granite, and compared the results with published data from 10 other intrusive systems. Many of the fluid inclusions are paragenetically associated with melt inclusions, providing direct insights into the stage at which metals were transferred from the crystallizing magma into the exsolving aqueous fluids. In 10 out of 14 magma systems in which the physical and compositional properties of the magmatic fluid(s) could be determined, the exsolved aqueous fluid was a single phase of relatively low salinity (2-13 wt % NaCl equiv; avg 5 wt % NaCl equiv), confirming predictions based on indirect evidence and observations made in earlier studies. Furthermore, in magmas that crystallized at low pressure (< 1.3 kbars), the salinity of the magmatic fluid increased with increasing degree of crystallization, whereas in magmas that crystallized at higher pressure (>1.3 kbars) the opposite behavior occurred, as predicted by published numerical models of aqueous fluid-saturated magma crystallization. During the transition from magmatic to subsolidus conditions, both pressure and temperature within and adjacent to intrusions decreased, leading to widespread vapor-brine immiscibility. Due to the low salinity of the single-phase parent fluid the two-phase field generally was entered from the vapor side, resulting in condensation of subordinate amounts of brine from the low-density bulk fluid. If the transition from single- to two-phase aqueous fluid occurred in locally closed systems one can use the compositions of vapor and brine in subsolidus boiling assemblages to calculate the composition of the bulk fluid. This method was applied to those occurrences in which no single-phase parent fluid was found (i.e., where the earliest aqueous fluid found was already in the two-phase field). The metal content of the least fractionated low-salinity fluids at each location correlates positively with the type and amount of mineralization in the associated intrusions, with high Cu concentrations being observed in fluid related to porphyry Cu deposits, high Sn and W concentrations in fluids related to Sn-W mineralization and high Ce concentrations in fluids related to REE mineralization. The earliest fluids in barren intrusions are comparatively metal poor, except for two cases in which certain metal concentrations are as high as in mineralized systems. A significant portion of the geochemical signature of barren versus mineralized intrusions was thus inherited from earlier stages in the development of these magma systems. In the case of Mo, Sn, W, and Ce, the correlation between fluid composition and type and amount of mineralization is more distinct in high-temperature brines than in the least fractionated low-salinity fluids. This, together with other lines of evidence, suggests that brine condensates play a central role ill the formation of Sn, W, Mo, and REE deposits. In the case of porphyry Cu (-Au) systems it appears more likely that the greater proportion of metal precipitated from the vapor phase, although brines may have played a significant role as well. [ABSTRACT FROM AUTHOR]

The magmatic-hydrothermal evolution of the El Teniente porphyry Cu-Mo deposit in the Central Andes in Chile is reconstructed based on field relationships scanning electron microscopy cathodoluminescence petrography, and fluid inclusion analysis by microthermometry and laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS). Three major stages of Cu-Mo mineralization are observed. Following the barren hydrothermal stage 1 the stage 2 mineralization is characterized by quartz-anhydrite stockwork veins and breccias with chalcopyrite, bornite, and molybdenite. Both stages 1 and 2 are associated with pervasive potassic alteration. Quartz-anhydrite veins with chalcopyrite, bornite, and molybdenite associated with phyllic alteration represent stage 3 mineralization. Stage 4 mineralization, linked to the formation of the large Braden diatreme, is characterized by breccias and rare veins containing a lower temperature assemblage with tourmaline, sericite, and lesser tennantite, bornite, and chalcopyrite with late gypsum in local vugs. Ten fluid types are distinguished in this study based on petrographic and microthermometric criteria, such as phase proportions, daughter minerals, homogenization behavior, and salinity. The hydrothermal evolution across the stages of Cu deposition is characterized by the contraction of a vapor phase originating by phase separation during stage 2. Overall cooling of the system at pressures fluctuating around the two-phase fluid surface led to a transition from a two-phase fluid state dominated by vapor at ~410°C and 300 bars in stage 2, to a single-phase low-salinity fluid derived from cooling and contraction of magmatic vapor to a liquid, which dominates during stage 3 mineralization at <350°C and 200 bars. Copper mineralization mainly formed from the vapor phase and its low-salinity liquid derivatives, representing a large volume of fluid with an initially high Cu content (1.2 ± 0.4 wt % Cu). Copper sulfides precipitated upon cooling between 410° and 320°C, indicated by a drop of Cu/(Na + K + Mn + Fe) ratios of over four orders of magnitude through the evolution of the deposit from stage 2 to stage 4. The highest Mo concentrations occur in residual brines resulting from extreme boiling, as indicated by concurrent halite saturation. Recent geochronology (Cannell, 2005; Maksaev et al., 2004) suggests a relatively, long-lived magmatic- hydrothermal system at El Teniente. Our fluid chemical data show no evidence for major crystal fractionation in a large fluid-generating upper-crustal magma chamber, because Cs/(Na + K + Mn + Fe) is constant from pre-ore to all syn-ore fluids. However, the initiation of copper mineralization was associated with a 4- to 10-fold increase in the concentration of Cu, Mo, Li, and Fe in the inferred main ore-forming input fluid, compared with pre-ore fluids of intermediate salinity and otherwise very similar major and trace-element ratios. These data indicate that injection from depth of an exceptionally Cu, Mo, Li, and, probably, also S-rich volatile phase into an already actively evolving upper-crustal magmatic-hydrothermal system triggered the formation of this unusually large and rich copper deposit. [ABSTRACT FROM AUTHOR]

The breccia-hosted epithermal Au-Ag deposit of Roşa Montană is located 7 km northeast of Abrud, in the northern part of the South Apuseni Mountains, Romania. Estimated total reserves of 214.91 million metric tons (Mt) of ore at 1.46 g/t Au and 6.9 g/t Ag (10.1 Moz of An mid 47.6 Moz of Ag) make Roşia Montană one of the largest gold deposits in Europe. At this location, Miocene calc-alkaline magmatic and hydrothermal activity was associated with local extensional tectonics within a strike-slip regime related to the indentation of the Adriatic microplate into the European plate during the Carpathian orogenesis. The host rocks of the magmatic complex consist of pre-Mesozoic metamorphosed continental crest covered by Cretaceous turbiditic sediment (flysch), Magmatic activity at Roşia Montană and its surroundings occurred in several pulses and lasted about 7 m.y. Roşia Montană is a breccia-hosted epithermal system related to strong phreatomagmatic activity due to the shallow emplacement of the Montana dacite. The Montana dacite intruded Miocene volcaniclastic material (volcaniclastic breccias) and crops out at Cetate and Carnie Hills. Current mining is focused primarily on the Cetate open pit, which was mapped in detail, leading to the recognition of three distinct brecia bodies: the dacite breccia with a dominantly hydrothermal matrix, the gray polymict breccia with a greater proportion of sand-sized matrix support, and the black polymict breccia, which reached to the surface, contains carbonized tree trunks and has a dominantly barren elastic matrix. The hydrothermal alteration is pervasive. Adularia alteration with a phyllic overprint is ubiquitous; silicification and argillic alteration occur locally. Mineralization consists of quartz, adularia, carbonates (commonly Mn-rich), pyrite, Fe-poor sphalerite, galena, chalcopyrite, tetrahedrite, and native gold and occurs as disseminations, as well as in veins and fillings vugs within the Montana dacite and the different breccias. The age of mineralization (12.85 ± 0.07 Ma) was determined by 4th Ar39Ar dating on hydrothermal adularia crystals from vugs in the dacite breccia in the Cetate open pit. Microthermometric measurements of fluid inclusions in quartz phenocrysts from the Montana dacite revealed two fluid types that are absent from the hydrothermal breccia and must have leech trapped at depth prior to dacite dome emplacement: brine inclusions (32-5 wt % NaCl equiv, homogenizing at Th > 460°C) and intermediate density fluids (4.9-15.6 wt % NaCl equiv. Th between 345°--430°C). Secondary aqueous fluid inclusion assemblages in the phenocrysts have salinities of 0.2 to 2.2 wt percent NaCl equiv and Th, of 200° to 280°C, Fluid inclusion assemblages es in hydrothermal quartz from breccias and veins have salinities of 0.2 to 3.4 wt percent NaCl equiv and Th, from 200° to 270°C. The oxygen isotope composition of several zones ot an orerelated epithermal quartz crystal indicate a very constant δ18O of 4.5 to 5.0 per mil for the mineralizing fluid, despite significant salinity and temperature variation over time. Following microthermometry, selected fluid inclusion assemblages were analyzed by laser ablation-inductively coupled-plasma mass spectrometry (LAICPMS). Despite systematic differences in salinity between phenocryst-hosted fluids trapped at depth and fluids from quartz in the epithermal breccias, all fluids have overlapping major and trace cation ratios, including identical Na/K/Rb/Sr/Cs/Ba. Consistent with the constant near-magmatic oxygen isotope composition of the hydrothermal fluids, these data strongly indicate a common magmatic component of these chemically conservative solutes in all fluids.… [ABSTRACT FROM AUTHOR]

In most published hydrothermal ore deposit models, the main agent of metal transport is an aqueous liquid. However, there is increasing evidence from volcanic vapors, geothermal systems (continental and submarine), vapor-rich fluid inclusions, and experimental studies that the vapor phase may be an important and even dominant ore fluid in some hydrothermal systems. This paper reviews the evidence for the transport of metals by vapor (which we define as an aqueous fluid of any composition with a density lower than its critical density), clarifies some of the thermodynamic controls that may make such transport possible, and suggests a model for the formation of porphyry and epithermal deposits that involves precipitation of the ores from vapor or a vapor-derived fluid. Analyses of vapor (generally >90% water) released from volcanic fumaroles at temperatures from 500° to over 900°C and near-atmospheric pressure typically yield concentrations of ore metals in the parts per billion to parts per million range. These vapors also commonly deposit appreciable quantities of ore minerals as sublimates. Much higher metal concentrations (from ppm to wt %) are observed in vapor inclusions trapped at pressures of 20O to 1,000 bars in deeper veins at lower temperatures (400°-50°C). Moreover, concentrations of some metals, notably Cu and Au, are commonly higher in vapor inclusions than they are in inclusions of coexisting hypersaline liquid (brine). Experiments designed to determine the concentration of Cu, Sn, Ag, and Au in HCl-bearing water vapor at variable although relatively low pressures (up to 180 bars) partly explain this difference. These experiments show that metal solubility is orders of magnitude higher than predicted by volatility data for water-free systems, and furthermore that it increases sharply with increasing water fugacity and correlates positively with the fugacity of HCl. Thermodynamic analysis shows that metal solubility is greatly enhanced by reaction of the metal with HCl and by hydration, which results in the formation of species such as MeClm⋅nH2O. Nonetheless, the concentrations measured by these experiments are considerably lower than those measured in experiments involving aqueous liquids or determined for vapor fluid inclusions. A possible explanation for this and for the apparent preference of metals such as Cu and Au for the vapor over the coexisting brine in some natural settings is suggested by limited experimental studies of metal partitioning between vapor and brine. These studies show that, whereas Cu, Fe, and Zn all partition strongly into the liquid in chloride-bearing sulfur-free systems, Cu partitions preferentially into the vapor in the presence of significant concentrations of sulfur. We therefore infer that high concentrations of Cu and Au in vapor inclusions reflect the strong preference of sulfur for the vapor phase and the formation of sulfur-bearing metallic gas species. Phase stability relationships in the system NaCl-H2O indicate how vapor transport of metals may occur in nature, by showing a range of possible vapor evolution paths for the conditions of porphyry-epithermal systems. At the world-class Bingham Canyon porphyry Cu-Au deposit, evidence from fluid inclusions supports a model in which a single-phase fluid of intermediate to vapor-like density ascends from a magma chamber. On cooling and decompression, this fluid condenses a small fraction of brine by intersecting the two-phase surface on the vapor side of the critical curve, without significantly changing the composition of the expanding vapor. Vapor and brine reach Cu-Fe sulfide saturation as both phases cool below 425°C. Vapor, which is the dominant fluid in terms of the total mass of H2O, Cu, and probably even Cl, is interpreted to be the main agent in metal transport.… [ABSTRACT FROM AUTHOR]