Evidence for a nonmagmatic component in potassic hydrothermal fluids of porphyry cu-Au-Mo systems, Yukon, Canada

The Tagun‐Khin‐Dan gold deposit in the Mogok‐Mandalay‐Mergui Belt, Central Myanmar, is characterized by an array of quartz‐veins hosted in mudstone of the Kogwe Formation of the Carboniferous Mergui Group. Two major deformational stages were recorded in the area; (1) N‐S shortening and (2) uplifting and emplacement of various dykes and quartz veinlets. The N‐S shortening within the area lead the development of km‐scale faults, determined largely by the presence of a zone of major WNW‐ESE trending dextral strike‐slip faulting. Quartz veins in the deposit include: (1) type‐A quartz veins, parallel to the dextral NW‐SE trending major fault; and (2) type‐B quartz veins which occur as isolated parallel veins. Gold in the type‐A quartz vein is present as native gold and electrum locked within pyrite and associated with pyrite and galena and in the type‐B quartz veins as electrum associated with sulfide minerals such as pyrite, chalcopyrite, galena and sphalerite. The mineralization stages can be classified into the type‐A quartz vein stage and the type‐B quartz vein stage. Two type of fluid inclusions; liquid‐rich aqueous inclusions (L‐type) and vapor‐rich aqueous inclusions (V‐type) are identified in the type‐A quartz veins. The homogenization temperature of L‐type fluid inclusions of the type‐A quartz veins ranges from 203 to 321°C and salinity of the fluid inclusions varies from 0.4 to 1.6 wt% NaCl equiv. The homogenization temperature of V‐type fluid inclusions of type‐A quartz veins ranges from 290 to 340°C with a salinity ranging from 0.4 to 1.9 wt% NaCl equivalent. In the type‐B quartz veins, only liquid‐rich aqueous inclusions (L‐type) are identified. The type‐B quartz veins yielded low homogenization temperatures from 160 to 220°C, with low salinities from 0.2 to 1.9 wt% NaCl equiv. compared with those of the type‐A veins. The depth range of ore formation is estimated to be a shallow depth of less than 0.2 km based on fluid inclusion microthermometry. Fluid boiling is evident during the type‐A quartz vein stage, and fluid cooling and mixing in the later type‐B quartz vein stage. Precipitation of pyrite in the ore zone occurred as four recognized types: arsenic‐rich pyrite‐1, 2, 3 in the type‐A quartz veins and pyrite‐4 in the type‐B quartz veins. A positive relation between Au and As contents of pyrites suggests that the gold is present together with arsenic in the structure of pyrites of the type‐A quartz veins as solid solution in addition to as nanoparticle inclusions. The high Co and Ni contents of pyrites of both the type‐A and the type‐B quartz veins, with no evidence of CO2 in the system indicate that the ore‐forming fluids were epizonal magmatic‐hydrothermal fluids rather than metamorphic fluid. The hydrothermal fluids of the Tagun‐Khin‐Dan deposit were driven by faulting to form the mudstone‐hosted epithermal gold mineralization and related to continuing northwards movement of the Indian Plate that initiated the displacement on the strike‐slip Sagaing Fault.

Hydrothermal alteration and fluid chemistry data of the early Cretaceous Endako porphyry molybdenum deposit, British Columbia, provide new information on the hydrothermal fluids associated with low-fluorine molybdenite mineralization. Molybdenite mineralization and hydrothermal alteration occur as early quartz ± molydenite stockwork veins with K feldspar-bearing selvages and paragenetically later quartz-molybdenite ribbon veins with sericite-bearing selvages. Late hydrothermal alteration is associated with the development of kaolinite and postore (Tertiary age) calcite veins. Fluid inclusions in early-formed quartz ± molybdenite stockwork veins with K feldspar-bearing alteration assemblages are dominated by moderate-salinity (5 to 15 wt % NaCl equiv), liquid-rich (type 1) and rare high-salinity (30 to 45 wt % NaCl equiv), halite-bearing (type 3) fluid inclusions. Type 1 and type 3 fluid inclusions in early veins homogenize between 390° and 430°C and 375° and 420°C, respectively. Secondary fluid inclusions (type 2) of low salinity (1 to 5 wt % NaCl equiv) in these early veins are minor, and homogenize between 130° and 285°C. Fluid inclusions in quartz-molybdenite ribbon veins with sericite-bearing alteration assemblages are dominated by moderate-salinity, liquid-rich (type 1) inclusions, with minor type 2 fluid inclusions. Type 1 fluid inclusions of ribbon veins homogenize between 360° and 400°C. Fluid inclusions in postore calcite veins are of only type 2 fluid inclusions, which homogenize at 209°C. Hydrothermal fluids recorded by type 1 and type 3 fluid inclusions in early veins were trapped under lithostatic to hydrostatic conditions between 0.3 and less than or equal to 2.0 kbar, and 360° and 560°C. Postore fluids recorded by type 2 fluid inclusions were trapped under conditions less than or equal to 0.5 kbar, and between 190° and 300°C. Quartz stockwork and ribbon veins possess δ 18O values of 8.4 ± 0.2 ( n = 9) and 8.4 ± 0.6 ( n = 13), respectively. Hydrothermal K feldspar and biotite from K feldspar alteration assemblages possess δ 18O values of 6.8 ± 0.4 ( n = 7) and 3.5 ± 0.8 ( n = 8), respectively. Oxygen isotope geothermometry of quartz-biotite and quartz-K feldspar pairs from K feldspar alteration assemblages yield temperatures between 200° and 490°C, which is similar to the trapping temperatures of hydrothermal fluids determined from fluid inclusion studies associated with molybdenite mineralization, the development of kaolinite, and calcite veins. The oxygen isotope temperatures of the quartz-biotite and quartz-K feldspar pairs suggest that K feldspar and biotite either record the approximate 18O composition of hydrothermal fluids associated with K feldspar alteration or have undergone 18O exchange with late-stage hydrothermal fluids. Hydrogen isotope composition of quartz stockwork and ribbon veins fluid inclusion waters range between –105 and –173 per mil. Solute chemistry studies of fluid inclusion waters indicate that ore-forming fluids from Endako have low Br/Cl and Br/Na ratios, and high I/Cl and I/Br ratios in comparison to Porgera (epithermal), Babine Lake (porphyry Cu), and St. Austell, Capitan Pluton (vein) deposits associated with magmatic processes. Na/K ratios of fluid inclusion waters yield temperatures (308° to 429°C) similar to those determined from type 1 and type 3 fluid inclusions and stable isotope thermometry. Results from fluid inclusion and solute chemistry studies indicate the involvement of hydrothermal fluids exsolved from a crystallizing melt in the formation of the Endako molybdenum deposit. However, oxygen and hydrogen isotope values deviate from the generally accepted magmatic compositions, which suggests the early involvement of meteoric water in the ore-forming fluids and ore genesis.

The Central and North Deborah gold deposits, with a production of about 7 tons Au, are located in one of 15 domes in the Bendigo gold field, central Victoria, Australia, an important mesothermal lode gold province. The gold-bearing quartz vein systems are hosted predominantly by reverse faults in Lower Ordovician turbidites, which underwent lower greenschist facies metamorphism. In the Central and North Deborah mines, there are six stages of quartz veins that developed in the following sequence: (1) laminated veins, with a metal association of Au-As-Sb-Pb-Zn-Ni; (2) spurs (Au); (3) massive barren veins (main dilation stage); (4) brecciated veins, with Au-As; (5) quartz-ankerite veinlets; and (6) late, pure quartz or calcite veinlets. Of the three mineralized vein stages (1, 2, and 4), only the first one predated or was coeval with the main deformation event, whereas stages 2 and 4 postdated the main deformation. The vein structures indicate that vein development was controlled by repeated fluctuations of fluid pressure inducing hydraulic fracturing. Fluid inclusions in vein quartz contain C-O-H fluids of variable compositions. Three main types of fluid inclusions are recognized at room temperature: type I, two-phase, primary (CH4)V + (H2O)L fluid inclusions; type II, two- or three-phase, primary-pseudosecondary (CO2)V or L + (CH4)V or L + (H2O)L fluid inclusions, including IIa, CH4-CO2-H2O, and IIb, CO2-H2O fluid inclusions; and type III, (H2O)V + (H2O)L fluid inclusions. Type III fluid inclusions are further divided into three groups: IIIa, primary-pseudosecondary, vapor-rich; IIIb, primary-pseudosecondary, liquid-rich; and IIIc, secondary, liquid-rich. Data from fluid inclusion distribution, microthermometry, and Raman spectroscopy indicate that fluids associated with Au mineralized quartz veins (stages 1, 2, and 4) have moderate salinity ranging from 0 to 12 wt percent NaCl equiv (modeled salinities around 7–8 wt % NaCl equiv). These veins formatted at temperatures from 325° to 375°C, and pressures of 200 to 300 MPa, with emplacement depths of about 8 to 12 km. Fluids associated with barren quartz veins (stages 3, 5, and 6) have a low salinity of about 0 to 3 wt percent NaCl equiv (modeled salinities about 2–3 wt % NaCl equiv) and lower temperatures. There is evidence of fluid immiscibility in all vein stages and mixing between fluids of the same souce, but differing fO2 may also have influenced gold deposition (Cox et al., 1995). Hydrothermal ore-forming fluids responsible for gold mineralization are in the CH2-CO2-H2O-NaCl system, whereas those of massive barren veins (stage 3) and veinlets (stages 5 and 6) are in the CO2-H2O-NaCl and H2O-NaCl systems, respectively. The carbonic phases of type I and II fluid inclusions range from pure CH4, through mixed CH4 and CO2, to pure CO2. Gold ore-forming fluids were weakly acidic, with f O2 estimated to have been 10–25 to 10–37 bars, and generally above the CO2/CH4 and below the SO2/H2S buffer boundaries throughout gold mineralization. The relatively reduced mineralizing fluids are similar to those of other turbidite-hosted lode gold deposits and counterparts in volcanic-plutonic terranes.

Basic technique for the extraction and isotopic analysis of fluid inclusions in halite was investigated using synthetic single crystals of halite and three natural halite samples from China. Vacuum ball-mill, vacuum decrepitation and vacuum melting methods were examined for the extraction of fluid inclusions. Results of analyses on δD and δ18O of the water in fluid inclusions extracted by the ball-mill method from synthetic single crystals agreed with those of the mother solution from which the single crystals were formed. Although the δ18O value of the water extracted by the melting method agrees well with that of the mother solution, the δD value was about 7‰ more negative than that of the mother solution. There was a great difference in both δD and δ18O of the water extracted by the two methods applied to the identical natural halite samples; the melting method gave consistently more negative values for both δD and δ18O compared with those by the ball-mill method. The difference was interpreted as a result of the hydrolysis of NaCI with H2O combined with the formation of oxides of alkaline-earth elements in brine inclusions in natural halite samples in the process of the melting method. Although the melting method has an advantage of complete recovery of volatiles in halite samples, the chemical and isotopic compositions of volatiles can not be retained owing to a variety of thermal reactions which occur at high temperatures. It was concluded that the ball-mill method is much superior to the melting method in order to obtain isotopic and chemical information on fluid inclusions in halite, though the recovery of fluid inclusions by the ball-mill method is not 100%, and high concentrations of Mg2+ and Ca2+ in fluid inclusions require correction for the δD and δ18O values of the water.

The presence of fluid inclusions in quartz veins is crucial to reconstruct fluid migration pathways in the subsurface. In this study, we provide an innovative approach to analyse  the hydrogen and oxygen isotopic composition of water fluid inclusions using cavity ring-down spectroscopy (CRDS). The CRDS is connected to a mechanical crusher in order to release fluid inclusion water from the host mineral. The evaporated fluid inclusion water from the crushed sample is added to a moistened background of nitrogen gas. For this purpose, we designed a temperature-regulated evaporation unit at Earth Science Stable Isotope Laboratory at the Vrije Universiteit Amsterdam (VU) to ensure that the isotopic composition and concentration of the background water vapour remains constant. The isotopic compositions of the fluid inclusions are calculated by subtracting the isotopic and concentration of the ‘wet’ background. This newly designed setup allows for reliable measurements of the oxygen and hydrogen isotopic compositions of fluid-inclusions in quartz minerals. The objective of this study is to analyse the isotopic compositions of fluid-inclusions in quartz veins from distinct regions in Europe (Germany and Portugal), which are both linked to the Variscan orogeny. The isotopic data align with the modern Global Meteoric Water Line, providing evidence for the presence of meteoric fluids in the examined fold-and-thrust belts of the Variscan orogeny. Complementary microthermometry data, isotopic signatures of silicon and oxygen of  the quartz host mineral further document the cooling of hydrothermal systems under the influence of meteoric water at various geological events. This interpretation concords with the 40Ar/39Ar dating fluid rich fraction of quartz vein minerals.

Contrasting paleofluid systems in the continental basement: a fluid inclusion and stable isotope study of hydrothermal vein mineralization, Schwarzwald district, Germany

Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relationship to District-Wide Zn, Pb, and Cu Mineralization

One of the most common methods for estimating paleohydrothermal δ D values in epithermal quartz veins is hydrogen isotope analysis of H 2 O extracted from fluid inclusions in quartz by thermal decrepitation. The validity of the method is questioned. The δ D values of water extracted from fluid inclusions in clear euhedral quartz from active geothermal systems are up to 30 per mil more negative than the δ D value (–42‰) of the geothermal water. Measured δ D values of the fluid inclusion water are dependent on the temperature at which the water in the quartz was extracted. Water extracted at 800°C has δ D values about 10 to15 per mil lower than fluid inclusion water extracted at 500°C (–53 to –61‰). Fluid inclusion water in calcite from an active geothermal well has δ D values that match those of the geothermal water. Calcite is thus potentially a more suitable mineral for estimating paleohydrothermal δ D values.

Precious metal veins of the National district, Humboldt County, Nevada, occur in strongly altered Miocene volcanic rocks. Hydrothermal assemblages in Buckskin Mountain display a marked zonation from the paleosurface downward that correlates with the elevation and physical properties of enclosing rhyolites. Light stable isotope analyses indicate that meteoric water from two distinct sources comprised the ore fluid. The delta D values in fluid inclusions from deep-level quartz vein segments, >1,250 ft (>379 m) below the palcosurface, range from -115 to -129 per mil. Fluid inclusion waters from near-surface quartz vein segments, 0 to 1,250 ft (0-379 m) below the paleosurface, have delta D = -86 to -125 per mil and are generally more depleted than local Miocene ground water whose delta D is calculated at -85 per mil. The delta D range in inclusion waters from near-surface quartz veins has resulted from mixing of the fluids present in deep quartz veins with local ground water and condensed steam derived from boiling of both fluids. The fluids present in deep quartz veins probably originated as precipitation on distal highlands with elevations approaching 6,000 ft (1,818 m). Oxygen isotope data from vein and wall-rock minerals record increasing depletion as veins are approached, indicating that quartz veins were the major conduits of fluid flow. The degree of oxygen exchange between hydrothermal fluid and wall rocks, water/rock mass ratios, and the abundance of hydrothermal phases increase toward the paleosurface. The progressive influx of local ground water with increasing elevation is also indicated by chloride dilution.The estimated duration of the Buckskin Mountain hydrothermal system, 50,000 to 200,000 yrs, is limited by the volume of high-temperature alteration assemblages and the heat budget of a proposed intrusion. Thermal constraints and isotopic data provide an internally consistent range in lifetimes which is several times shorter than that proposed for some active hydrothermal systems. These long-lived modern systems apparently require multiple thermal and/or intrusive events.Based on geologic, isotopic, and floral data, the palcogeography of ancestral Buckskin Mountain was dominated by forested volcanic terrane having considerable relief. An intermontane basin, remnants of which comprise the summit of Buckskin Mountain today, contained scattered thermal pools and patches of steaming ground. The basin covered several square miles and stood at 3,000 ft above sea level, while adjacent uplands were several thousand feet higher in elevation. Floras included subalpine spruce and fir, mixed conifer-hardwood slope forests, and basin-floor hydrophytes (willow and marsh grasses). Buckskin Mountain during the Miocene closely resembled present landforms in Yellowstone National Park, Wyoming.

The ~20-million-ounce (Moz) Porgera gold deposit, Papua New Guinea, is hosted by 6-m.y.-old alkalic intrusions and Cretaceous sedimentary rocks in which the intrusions were emplaced. Gold-bearing veins occur in three stages: (1) magnetite-sulfide-carbonate ± quartz veins with minor gold (prestage I), (2) base metal-sulfide-carbonate ± quartz ± Au veins (stage I), and (3) quartz-roscoelite-pyrite-gold veins and breccias (stage II). Stage II veins are economically the most significant. Quartz-roscoelite-pyrite-gold veins form high-grade zones associated with the Roamane fault (a late normal fault that crosscuts the intrusive complex) and in the footwall to the fault (the North zone). The North zone mineralization is the main focus of this study. The quartz-roscoelite-pyrite-gold assemblage occurs in three texturally distinct styles: (1) thin (1–5 mm) veinlets in which roscoelite-pyrite-gold are more abundant than quartz and in which roscoelite and gold also occur in the wall rock; (2) veins (5 mm to 10 cm) in which roscoelite-pyrite-gold with minor quartz form a band at the vein edges, followed by coarse-grained quartz and the vein centers commonly filled with anhydrite and carbonate; and (3) breccia veins and breccias in which wall-rock fragments are rimmed by roscoelite-pyrite-gold and minor quartz, followed by vuggy quartz infilling. Fluid inclusions from quartz in these veins and breccias are mostly liquid rich, and average salinities in individual samples range from 7.5 ± 1.0 to 9.6 ± 0.2 wt percent NaCl equiv. In five of 27 samples, an additional cluster of salinities between 4.4 and 6.2 wt percent NaCl equiv was observed. These relatively low salinity inclusions occur toward the vein center and are less abundant than high-salinity inclusions that occur toward the vein margins. Three samples exhibit a continuous salinity trend from 4.5 to 10.2 wt percent NaCl equiv. For samples where CO 2 analyses were available average corrected salinities range from 5.1 to 8.0 wt percent NaCl equiv. Average homogenization temperatures (T h ) of individual samples range from 127° ± 12° to 167° ± 25°C. The average T h of the low-salinity inclusions (145° ± 9°C) is marginally lower but overlaps with that of all high-salinity inclusions (152° ± 17°C). Gas chromatographic analyses showed that the high-salinity fluid contains up to 2 mol percent CO 2 , 0.11 mol percent CH 4 , 0.065 mol percent N 2 , and traces of C 2 H 4 , C 2 H 6 , and COS. Concentrations of Cl – (310–609 mM/l), Br – (0.28–0.75 mM/l), Li + (1.25–8.80 mM/l), Na + (462–1126 mM/l), K + (0–81 mM/l), Mg 2+ (0–7.0 mM/l), and Ca 2+ (0–185 mM/l) were determined by ion chromatography. The δ 18 O values of quartz range from 13.9 to 18.3 per mil, and calculated δ 18 O H 2 O values range from –1.2 to 4.1 per mil. The δ D H 2 O values lie between –77 and –52 per mil. The calculated isotopic composition of the hydrothermal fluid lies between that of magmatic and that of meteoric water. The δ 18 O values of carbonates range from 15.2 to 16.6 per mil. Carbon isotopes were analyzed on carbonates ( δ 13 C = –3.3 to –2.4‰), altered and unaltered sedimentary rocks (–5.4 to –4.0 and –1.5 to 0.0‰, respectively), and organic carbon in shales (–23.6 to –16.8‰). The δ 34 S pyrite values range from –9.4 to 6.1 per mil, and δ 34 S anhydrite values range from 12.4 to 17.5 permil. The petrographic and analytical results suggest that an ascending fluid interacted with the sedimentary rocks and/or fluid hosted by the sedimentary rocks at depth (rather than at the site of ore deposition). This interaction is suggested by the presence of organic-derived volatiles and high \(NH_{4}^{+}\) concentrations in inclusion fluid. In some samples, two fluids were involved in stage II vein formation. Vapor-rich fluid inclusions indicate that the hydrothermal fluid boiled locally, and continuous salinity trends suggest that fluid mixing occurred in some stage II veins. The analytical data and paragenetic information were used to estimate element concentrations in the Porgera hydrothermal fluid that were used for thermodynamic reaction path modeling using the software CHILLER. This fluid was subjected to boiling, mixing with sedimentary formation water, cooling, and fluid-rock reaction. During boiling, quartz, pyrite, gold, and K-feldspar formed; mixing resulted in the deposition of pyrite, gold, and mica; reaction with diorite produced the observed wall-rock alteration assemblage; and cooling formed quartz plus minor mica, pyrite, gold, and kaolinite. Observations, analytical data, and modeling results suggest that more than one process was involved in stage II vein formation in different locations and at different times. Boiling, mixing, and fluid-rock reaction, all accompanied by cooling, occurred intermittently over the entire depth extent of the North zone, and the size of Porgera may be a result of all of these processes occurring at the deposit.

Fluid inclusion and stable isotope constraints on the heavy rare earth element mineralisation in the Browns Range Dome, Tanami Region, Western Australia

The origin and evolution of heavy rare earth element mineralisation in the Browns Range area, Northern Australia

An indication of high‐sulfidation epithermal–porphyry transition was observed in the Kumbokarno prospect, East Java, Indonesia. The prospect is composed of two Middle Miocene intrusions with tonalitic and dioritic compositions. Tonalite, the main host‐rock was subjected to argillic, advanced argillic, and vuggy quartz alteration, whereas the juxtaposing diorite was subjected to peripheral propylitic alteration. Three types of vein exist in the research area, which are massive quartz, comb quartz, and stockwork vein. In addition, supergene alteration represented by goethite and hematite pervasively superimposed both the hydrothermal alteration and mineralization. Fluid inclusion petrography and microthermometry analysis on three types of quartz veins distinguished primary fluid inclusions into two groups, that is, V30 group composed of vapor (30 vol%)–liquid (70 vol%), and the second group V30H group composed of vapor (30 vol%)–halite (30 vol%)–liquid (40 vol%). The homogenization temperatures of both the groups show a similar range of ca. 350–480°C, but the V30H group has significantly higher salinity (35–50 wt% NaCl eq.) compared to the V30 group (10–20 wt% NaCl eq.). In terms of the vein types, the massive quartz vein has the highest homogenization temperatures, followed by the comb quartz vein and lastly the stockwork veins. The presence of alunite and its sulfur isotope compositions, δ34S = 19.6 (σ = 2.1‰), indicate acidic pH and presence of SO42− in the hydrothermal fluids. The prospect is an intrusion‐centered magmatic‐hydrothermal system reflecting the porphyry‐epithermal transition. The fluid inclusions with high homogenization temperatures up to 480°C and high salinity up to 50 wt% NaCl eq. also support the transition of porphyry to high‐sulfidation epithermal mineralization. The presence of two different types of primary fluid inclusions suggests that boiling process occurred and separated the original magmatic fluid into the liquid and vapor phases. More, this fluid underwent dilution and mixing with meteoric waters. The migration of both the fluids were likely unrelated to the formation of the advanced argillic–argillic alteration halo because the quartz veins cut across this alteration. The prospect was later subjected to intensive weathering process that altered most of the sulfides into iron oxides and hydroxides. Small amounts of copper and minor gold were detected, especially in the iron oxides and hydroxides ones with colloform and bladed textures. The Kumbokarno prospect evidences the potential for high‐sulfidation to porphyry deposits at the Southern Mountain Arc, Indonesia.

Origin and evolution of hydrothermal fluids in the Taochong iron deposit, Middle–Lower Yangtze Valley, Eastern China: Evidence from microthermometric and stable isotope analyses of fluid inclusions

The Ashanti belt of Ghana is the key district of gold mineralization in the Paleoproterozoic terrane of West Africa. The area considered in southwest Ghana is covered by lithologies of the volcanic-sedimentary Birimian Supergroup and the overlying elastic sedimentary Tarkwaian Group which were jointly folded and metamor- phosed under greenschist facies conditions during the Eburnean teetonothermal event at about 9..1 Ga. Regional fotiation md subparallel shear zones hosting mesothermal gold mineralization developed during deformation coeval with metamorphism. Four major tsoes of primary gold mineralization are present in the Ashanti belt: (1) mesothermal, generally steeply dipping quartz veins in shear zones mainly in Birimian sedimentary rocks, (9,) sulfide ores with auriferous arsenopyrite and pyrite, spatialty dosely associated with the quartz veins, (3) sulfide disseminations and stockworks in granitoids, and (4) paleoplaeers of the Tarkwaian Group. This study concentrates on types (1) and (9,) of the hydrotherlnal gold mineralization. Stable isotope analyses of host-rock and ore components were performed with the ailn of obtaining parameters relevant to the origin and evolution of the fluids that produced gold mineralization. Carbonaeeous matter in the Birimian metasediments displays 93C values ranging from -11.4 to -9,8.3 per mil relative to PDB, indicating an organogenie origin. Carbonates display a unimodal distribution of 9aC values ranging from -9.9 to -17.0 per rail relative to PDB. COz extracted from fluid inclusions in the auriferous quartz veins has 93C values ranging from -9.5 to -15.7 per mil relative to PDB. It is proposed that these carbon isotope compositions of carbonates and COz reflect extensive interaction of the CO,2-rieh hydrothermal fluids with reduced carbon in Birimian sediments in the deeper parts of the hydrothermal systems. Carbonates and auriferous vein quartz have 6So values ranging from 19,.9 to 9,9,.9, and 19,.8 to 15.6 per mil relative to SMOW, respectively. Carbonates and quartz were deposited in near isotopic equilibrium with respect to 6So, indicating fluid-dominated conditions during ore formation, from fluids of metamorphic or magmatie origin. Such an origin is corroborated bySD values of water extracted from fluid inclusions in vein quartz (-37 to -53% relative to SMOXV). Pyrite of synsedimentary-diagenetie origin in Birimian schists displays sulfur isotope compositions ranging from +7.3 to -9,0.9 per rail (median ca. -10% relative to CDT). Similar compositions and wide ranges are usually attributed to sulfide generation by bacterial sulfate reduction from seawater. Arsenopyrite and eogenetie pyrite from the sulfide ores generally have 534S values in the range -5.3 to -10.9, per rail relative to CDT. The tight unimodal distribution of 534S values indicates a large, homogeneous fluid reservoir. The low (534S values are interpreted as source-inherited, not related to unusual pH, Eh, temperature, or depositional conditions. Sulfides in Birimian sediments represent the most likely sulfur reservoir tapped by the fluid systems. The C, O, H, and S isotope compositions of ore-related hydrothermal minerals and fluid inclusion compo- nents indicate that the mineralizing fluids interacted extensively with the Paleoproterozoie rocks, especially Birimian sediments, at deeper crustal levels and at high temperatures. The isotopic compositions are most compatible with the formation of fluids from devolatilization reactions invoMng Birimian strata during pro- grade metamorphism at depth (metamorphic fluids).