Wt Percent NaCl Equiv Research Articles

AN EVALUATION OF FLUID INCLUSION MICROTHERMOMETRIC DATA FOR ORPIMENT-REALGAR-CALCITE-BARITE-GOLD MINERALIZATION AT THE BETZE AND CARLIN MINES, NEVADA

Carlin-type deposits contain gold in association with main-stage quartz-pyrite-kaolinite mineralization and late-stage orpiment-realgar-calcite-barite mineralization. Fluid characteristics for main-stage mineralization are well documented by fluid inclusion and stable isotope studies on quartz. In contrast, fluid characteristics for late-stage mineralization are not well constrained because of large ranges in fluid inclusion microthermometric data. These ranges could represent real variations in fluids or be a result of the reequilibration of fluid inclusions. Microthermometric analyses were conducted on fluid inclusions in samples of barite, calcite, realgar, and orpiment from the Betze and Carlin mines, Nevada. Petrographic studies of individual crystals and cleaved sections reveal that fluid inclusions in realgar and barite have negative crystal shapes, in contrast to elongate and rounded inclusions in orpiment and calcite. Point-count data document that one-phase liquid inclusions (type 1) are the dominant type in barite and realgar, relative to two-phase, vapor-poor inclusions (type 2) in calcite and orpiment. Type 2 inclusions in realgar and barite commonly reequilibrate (e.g., stretch) during analysis and exhibit ranges in homogenization temperatures (T h ) of 100° to 250°C and 110° to 300°C, respectively. In contrast, type 2 inclusions in orpiment and calcite have T h of 108° to 182°C, which could be repeated to within 1°C. Based on these results, fluid inclusions in barite and realgar are most susceptible to reequilibration, with T h of ~100° to 110°C most representative. Fluid salinities for orpiment and calcite are 1.7 to 5.4 wt percent NaCl equiv, relative to 1.1 to 2.9 wt percent NaCl equiv for barite and realgar. The lower T h and salinity for fluid inclusions in barite and realgar suggest fluid cooling and dilution, following the deposition of paragenetically earlier orpiment and calcite.

Reduced and Oxidized Au-Cu-Bi Iron Oxide Deposits of the Tennant Creek Inlier, Australia: An Integrated Geologic and Chemical Model

Iron oxide-associated Au-Cu-Bi deposits of the Tennant Creek gold field range from magnetite-pyrrhotite-pyrite (reduced) Cu-rich deposits, through sulfide-poor, magnetite-rich high-grade Au-Bi (intermediate- f O2) deposits, to hematite-rich high-grade Au (oxidized) deposits. Discordant magnetite-hematite-chlorite-quartz ironstone bodies that host the ore formed hydrothermally during the ~1840 to 1860 Ma Barramundi orogeny at temperatures up to ~350° to 400°C and ~2.5 to 5(?) kbars. Deposition of ironstone minerals was localized by dilatency that was generated during D1 reverse shearing and folding. A separate Au-Cu-Bi hydrothermal stage overprinted selected ironstones during D2 at or before ~1825 to 1830 Ma, where favorable D1 structures and fluid pathways were reactivated. The extremes of the reduced- to oxidized-deposit spectrum represent divergent chemical conditions that cannot be reconciled by the action of a single fluid. We have identified both reducing and oxidizing fluids that helped to form the Au-Cu-Bi deposits. At the West Peko deposit, which represents the reduced end-member subtype, constraints from oxide-silicate-sulfide mineral stabilities, fluid inclusions, and chlorite thermochemistry indicate that Au-Cu-Bi ore was introduced into ironstone at 300° to 340°C by a reducing (pyrrhotite-stable), weakly acidic, sulfur-rich and N2-CH4–rich aqueous fluid containing 3 to 10 wt percent NaCl equiv. An ore fluid source outside the relatively oxidized metasedimentary host sequence (Warramunga Formation) is implied. We suggest similar ore fluids for other reduced Cu- and sulfide-rich deposits in the district. Chemical mass transfer modeling of reactions between reducing fluid and ironstone, incorporating estimated thermodynamic properties for aqueous bismuth complexes, indicates that highly efficient gold and native Bi deposition was predominantly in response to desulfidation and oxidation of the fluid. Chalcopyrite precipitated mainly in response to increased pH. Gold-bismuth ore in the Eldorado deposit, an oxidized and hematitic end member, is interpreted to have involved mixing of an oxidizing (hematite-stable) Ca-Na-Cl brine with an intermediate- f O2, low- to moderate-salinity ore fluid, as well as reaction of both fluids with ironstone. Furthermore, the mass transfer modeling suggests that mixing of reducing and oxidizing fluids in the absence of ironstones could have resulted in high-grade Au deposits lacking Fe oxides, a subtype that has received comparatively little exploration attention in the Tennant Creek district.

The Waihi Epithermal Gold-Silver-Base Metal Sulfide-Quartz Vein System, New Zealand: Temperature and Salinity Controls on Electrum and Sulfide Deposition

The large, andesite-hosted quartz vein system at Waihi contains a major epithermal Au-Ag deposit (total production of 6.4 Moz Au and 39.9 Moz Ag, Ag:Au = 6.2). It is notable for a large vertical extent (up to 575 m) of economic Au-Ag ore. A pervasive adularia-quartz-illite-chlorite-pyrite alteration zone bounds the larger quartz veins and is associated with veinlets of quartz + calcite + pyrite ± adularia ± illite. This zone is enveloped by an illite-quartz-adularia-chlorite-calcite-pyrite alteration zone, which grades out through a transition zone characterized by interlayered illite-smectite into a smectite-chlorite-calcite alteration zone in the least altered andesites. The formation of the adularia, quartz, chlorite, calcite, pyrite, and some illite in these alteration assemblages was associated with an initial stage of vein filling by platy calcite and quartz. This was followed by a main stage of quartz and sulfide vein filling, with crustiform quartz at deeper levels and colloform quartz plus minor adularia at shallow levels. Deposition of main-stage quartz and sulfides was accompanied by illite overprinting adularia in the wall rock and filling fractures in the quartz-calcite-pyrite ± adularia veinlets. Sulfide-bearing bands associated with the main-stage quartz show a zonation with decreasing depth from a coarse-grained sphalerite-galena-pyrite-chlorite assemblage at deep levels to a finer grained sphalerite-galena-pyrite-chalcopyrite-acanthite-electrum assemblage at medium to shallow levels. Fluid inclusion studies indicate that the main-stage quartz was deposited from fluids which ranged in temperature and composition from up to 295°C and 1.7 wt percent NaCl equiv at depth, to 190°C and 0.2 wt percent NaCl equiv at shallow levels. These are true salinities, because the fluids extracted from the fluid inclusions have low CO 2 contents (0.05–0.24 wt %). Quartz within electrum-poor Zn-Pb-Fe sulfide bands was deposited at higher temperatures (295°–242°C) and salinities than quartz in electrum-bearing Zn-Pb-Fe-Cu-Ag sulfide bands (260°–190°C). Temperature versus depth profiles for fluid inclusions with evidence of boiling indicate that the paleowater table was located about 160 m above the present erosion level during deposition of the main-stage quartz. Minor amethyst as the final stage of vein filling was locally preceded by inesite and was not associated with sulfides and electrum. Amethyst was deposited at 253° to 170°C from fluids containing up to 4.5 wt percent NaCl equiv. These higher salinities are attributed to concentration of salts in the residual liquid by localized, open-system boiling. Chloride-depth, chloride-enthalpy, and δ 18 O water -enthalpy plots indicate mixing of deep fluids (up to 1.7 wt % NaCl for the main-stage quartz) with very low salinity, steam-heated ground water. Oxygen isotope δ 18 O values for vein quartz and amethyst range from 6.1 to 11.6 per mil and for vein calcite from 2.29 to 9.4 per mil. Calculated δ 18 O water values define a trend from –2.9 per mil at shallow levels to +0.6 per mil at depth. Extrapolation of the dilution trend on a δ 18 O water -enthalpy plot indicates that the deep fluid ( δ 18 O water +0.6‰) was mixed with heated (160°–180°C) ground water of local meteoric water composition (–5.5‰). Dilution of a deep chloride fluid (1–2 wt % NaCl equiv), carrying zinc and lead as chloride complexes, with steam-heated ground water is inferred to have been responsible for deposition of the electrum-poor Zn-Pb-Fe sulfide assemblage from 300° to 240°C at a paleodepth below about 500 m. Deposition of this assemblage may also have been promoted by decrease of sulfur in solution through loss of H 2 S gas by boiling during episodic fracturing. The decrease in salinity following dilution favored the transport of gold, silver, and copper as hydrosulfide complexes and led to the deposition of the electrum-bearing Zn-Pb-Fe-Cu-Ag sulfide assemblage over a temperature range of 260° to less than 190°C at paleodepths of 500 to less than 250 m. In this assemblage, the decrease of sulfur in solution as a result of deposition of the associated chalcopyrite, pyrite, and galena is a likely mechanism for electrum deposition.

INFRARED MICROTHERMOMETRY AND CHEMISTRY OF WOLFRAMITE FROM THE BAIA SPRIE EPITHERMAL DEPOSIT, ROMANIA

The chemical and microthermometric characteristics of wolframite crystals from the Baia Sprie Au-Ag-Pb-Zn-Cu-W ore deposit (Baia Mare district, Romania) have been investigated using electron probe microanalysis (EPMA), X-ray elemental mapping, and infrared microscopy. The chemical composition of wolframite changes from a nearly pure huebnerite (MnWO4) core to nearly pure ferberite (FeWO4) rims. This zonation correlates with a decrease in infrared transmittance from core to rims of wolframite crystals as demonstrated by Fourier transform infrared spectra. The Fe content of wolframite may control its infrared transparency, just as the Fe2+⇌Mn2+ substitution controls its transparency in visible light. Microthermometric measurements, performed with an infrared microscope, indicate salinities ranging from 4.6 to 6.4 wt percent NaCl equiv and homogenization temperatures of 154° to 240°C for primary fluid inclusions, all trapped in the Mn-rich zones of the studied crystals. Secondary fluid inclusions showed similar microthermometric characteristics (4.6–6.4 wt % NaCl equiv; temperature of homogenization, 171°–232°C). These microthermometric data agree well with data from previous studies on transparent minerals (quartz, sphalerite) from the Baia Sprie ore deposit. However, it is notable that wolframite appears to have precipitated after scheelite and at lower temperatures. A possible magmatic contribution to ore has been invoked for epithermal gold deposits that contain wolframite, including Baia Sprie. The unexpected low temperatures and low salinities of ore-forming fluids responsible for forming wolframite at Baia Sprie may indicate a mixing of magmatic fluids with a dominant meteoric water component.

Fluid Characteristics of Granitoid-Hosted Gold Deposits in the Birimian Terrane of Ghana: A Fluid Inclusion Microthermometric and Raman Spectroscopic Study

Fluid inclusions in vein quartz from 10 granitoid-hosted gold deposits and prospects in the Birimian terrane of Ghana, as well as from the Sansu mine (Ashanti shear zone type) and quartz veins in the nonmineralized Princess Town granodiorite, were studied by microthermometry and Raman microspectrometry. Fluid inclusions from the granitoid-hosted gold deposits are dominated by aqueous H2O-CO2-NaCl type 1 and liquid CO2-N2 ± CH4 type 2, with minor (<10%) aqueous H2O-NaCl type 3. Type 1 inclusions show large variations in CO2 phase volume proportions (10–90%) at 25°C and have salinities commonly between 0 and 6 wt percent NaCl equiv. Their bulk densities fall in a major range from 0.62 to 1.11 g/cm3. Type 2 inclusions show no visible H2O phase at room temperature and have bulk densities between 0.30 and 0.92 g/cm3. Type 3 inclusions, containing 10 to 20 vol percent H2O vapor, have salinities of 1 to 8 wt percent NaCl equiv and bulk densities of 0.77 to 1.03 g/cm3. In most cases, the three types of fluid inclusions coexist as groups in individual quartz grains of the samples studied. These fluid inclusions are interpreted to be trapped during phase separation of an originally homogeneous H2O-CO2 fluid, with low salinity (<6 wt % NaCl equiv) and moderate to high density (0.65–0.95 g/cm3). The type 1 fluid inclusions are suggested to be heterogeneous mixtures of the two end members of type 2 and 3 inclusions. Fluid immiscibility is documented by petrographic characteristics and microthermometric results of the three types of inclusions. Trapping temperatures and pressures, estimated from microthermometry of type 3 inclusions, equation of state, and the P-T-X nature of the H2O-CO2-NaCl system, are mainly between 200° and 350°Cand 1 and 3 kbars for the gold deposits. By contrast, fluid inclusions in the Sansu mine at Ashanti mostly comprise the liquid CO2-N2 ± CH4 type 2. The inclusions are considered to represent postentrapment modifications of trapped fluids. In addition, fluid inclusions in barren vein quartz from the Princess Town granodiorite comprise the low-salinity (commonly <6 wt % NaCl equiv) H2O-NaCl type 3. Raman microspectrometry shows that gaseous compositions of fluid inclusions from both granitoid- and shear zone-hosted gold deposits are mainly composed of CO2 (80–95 mol %), with significant amounts of N2 (2–20 mol %) and CH4 (0–10 mol %). The distinct low-salinity H2O-CO2-rich fluids from the granitoid-hosted gold deposits are comparable in composition to those from the major Ashanti and Tarkwaian types of gold deposits in the Birimian terrane of Ghana. These fluids are most likely to be metamorphic in origin and associated with the waning stages of the regional Birimian orogeny. Gold deposition within the Birimian granitoids was related to fluid phase separation and sulfidization of host rocks during hydrothermal alteration and mineralization. The present study, together with previous publications, suggests that fluid inclusions are characterized by H2O-CO2-NaCl and/or CO2-N2 ± CH4 types in mineralized areas, whereas H2O-NaCl fluid compositions dominate in barren areas. Fluid inclusion characteristics may, therefore, be a useful tool for regional gold exploration in the Birimian terrane of Ghana.

Origin of the Gacun Volcanic-Hosted Massive Sulfide Deposit inSichuan, China: Fluid Inclusion and Oxygen Isotope Evidence

The Gacun polymetallic, Ag-rich, volcanic-hosted massive sulfide deposit occurs in a Triassic submarine calc-alkaline volcanic belt that forms part of the Yidun collisional orogenic zone of southwestern China. The deposit is hosted in felsic volcanic rocks associated with an underlying mafic unit (a bimodal suite), which formed in an intra-arc rift basin at about 1,000 m water depth. The volcanic rocks underwent regional lower greenschist facies metamorphism and related deformation during the Yanshan-Himalayan orogeny, resulting in folding and shearing of the ore lenses. The deposit is made up of three mineralized zones: a sheet-like upper massive sulfide zone with exhalite (barite, chert, and jasper), a middle stringer-stockwork strata-bound zone hosted in rhyolitic volcanic rocks, and an underlying lower stringer strata-bound zone in dacitic volcanic rocks. Fluid inclusion studies indicate that two-phase primary fluid inclusions in quartz in the lower stringer ore zone homogenized between 299° and 319°C (average temperature 308°C), whereas those in sphalerite yielded a temperature range from 185° to 260°C (average temperature 241°C). Homogenization temperatures of fluid inclusions in quartz from the middle stringer-stockwork zone ranged from 150°to 350°C and averaged 243°C. The homogenization temperature of fluid inclusions in quartz and sphalerite from the massive sulfide zone show a bimodal distribution corresponding to temperatures from 250° to 150°C for quartz and from 200° to 140°C for sphalerite. Fluid inclusions in barite fragments from an interpreted submarine vent recorded higher homogenization temperatures ranging from 208° to 358°C, whereas those in fined-grained barite from the upper gray-white massive baritic ore gave lower homogenization temperatures ranging from 98° to 125°C. Salinities of fluid inclusions show a wide range from 4.2 to 21.3 wt percent NaCl equiv in the three mineralized zones. Fluid inclusions in quartz from the lower stringer zone have the highest salinities (17.1–21.3 wt % NaCl equiv), whereas those in barite from gray-white massive baritic ores have the lowest salinity (average 4.0 wt % NaCl equiv), close to that of normal seawater. Salinities of fluid inclusions in sphalerite ranged from 5.1 to 14.5 wt percent NaCl equiv, and the high-salinity end member was found mainly in the massive sulfide zone. Preliminary laser Raman spectroscopic analysis of fluid inclusions from the three mineralized zones identified CO 2 , CH 4 , and H 2 S. Oxygen isotope compositions of eleven quartz samples separated from the stringer and stockwork ores and of two silica breccias in the basal sulfide massive ores yielded a limited range from 13.7 to 16.4 per mil. Fourteen host felsic volcanic rock samples and seven quartz samples separated from several alteration zones display a wide range in bulk rock δ 18 O values from 8.0 to 17.1 per mil. For the volcanic rocks in various alteration zones, a significant increase in δ 18 O value was recorded in the deposit outward from the orebodies and downward from upper to lower volcanic units. The most strongly altered quartz-hyalophane zone gave a bulk rock δ 18 O value of 11.3 to 14.4 per mil. The moderately altered sericite-quartz zone yielded slightly higher bulk rock δ 18 O values of 12.5 to 15.4 per mil. The weakly altered lower volcanic unit has much higher bulk rock δ 18 O values ranging from 15.1 to 17.1 per mil; these values are comparable to those measured for the zeolite zone in the Kuroko deposits of Japan. The hydrothermal system that formed the Gacun deposit was relatively high temperature (up to 350°C); it had a relatively high gas content and high salinity and was enriched in 18 O. These data suggest that the dominant 18 O-rich hydrothermal fluid was derived from a felsic magma chamber at depth. The high-salinity fluid inclusions in the massive sulfide zone suggest that ore formed in a sea-floor brine pool filled by episodic venting of mixed subsea-floor fluids. Fluid inclusions with salinities close to that of normal seawater and homogenization temperatures around 250°C in sphalerite and barite within the ore zone suggest that seawater was heated to that temperature by felsic intrusions. Near the mineralized center and main path of hydrothermal fluid discharge at Gacun, fluid inclusions in the strata-bound zone recorded that a minor amount of cold seawater mixed with magmatic fluid. In comparison, away from the mineralized center, a large amount of cold seawater mixed with magmatic-derived fluid and heated seawater in the strata-bound zone.

The Geology of the Nifty Copper Deposit, Throssell Group, Western Australia: Implications for Ore Genesis

The Nifty syndeformational, sediment-replacement Cu deposit is located approximately 450 km southeast of Port Hedland, Western Australia, in subgreenschist-grade rocks of the Neoproterozoic Paterson orogen. The resource of 99 million metric tons (Mt) @1.7 percent Cu is hosted in a syncline within carbonaceous and dolomitic shales of the upper Broadhurst Formation. The Cu mineralization occurred during Neoproterozoic DY2 deformation and formed at the center of a zoned hydrothermal alteration system. Interbedded dolomitic and carbonaceous shale of the upper and lower units of the Nifty member was altered S in two stages. Early siliceous S alteration with minor chlorite, pyrite, hematite, sericite, and stilpnomelane, which was replaced by syntectonic black quartz in the core of the deposit. On the fringes of the alteration, framboidal pyrite in the carbonaceous shale was replaced by quartz, chalcopyrite, euhedral pyrite, sphalerite, and galena. Inward is a sequence of interlayered hydrothermal quartzdolomite and unaltered shale beds. The hydrothermal quartz-dolomite first appears as spots and 2- to 4-mm-thick veins with light gray diffuse margins. In areas of intense spotty alteration, dolomitic alteration forms bedding-parallel zones. Farther inward and proximal to the highest ore grade is silicified dolomitic shale, which grades into fine-grained quartz and chalcopyrite, with minor fluorapatite and carbonaceous material. Synmineralization fluid inclusions are small (10 microns) liquid-vapor inclusions with homogenization temperatures between 200 degrees to 370 degrees C (median 270 degrees C) and salinity of 8 to 27 wt percent NaCl equiv (median 15 wt % NaCl equiv). Sulfur isotope data from framboidal pyrite range between delta 34 SCDT of -27 and +16 per mil, chalcopyrite delta 34 SCDT ranges between -6 and +6 per mil, and delta 34 SCDT for euhedral, synmineralization pyrite ranges between -12 to +12 per mil. Carbon and oxygen isotope results from synmineralization vein stage carbonates and dolomitic alteration overlap with delta 13 CVPDB values from +2 to -10 per mil and delta 18O VSMOW values from 14 to 21 per mil. It is likely that decarbonation of host rock caused the large shift in delta 13 CVPDB values. The formation of the Nifty Cu deposit is linked to deformation (tectonic loading) associated with the Miles orogeny driving fluids from deep in the Yeneena basin. Hydrothermal fluids were focused along DY2 thrust faults. During tightening of the Nifty syncline, thrust faulting penetrated carbonaceous and dolomitic shales, allowing hydrothermal fluids to react with the Nifty member. Early fluid pulses were weakly oxidized and contained low concentrations of base metals. These fluids altered the host rock and precipitated green quartz. The fluid evolved to a hotter, more saline, reduced and moderately acidic fluid with significant Cu concentration. These fluids reacted with the dolomite-dominated Nifty member, forming the dolomitic alteration and siliceous replacement of carbonaceous shale and dolomitic mudstone. Chalcopyrite precipitation was controlled mainly by changes in pH accompanied by a slight decrease in temperature. The mineralization and alteration styles at Nifty are similar to the copper orebodies at Mount Isa.

Hydrothermal Alteration and Hydrologic Evolution of theGolden Cross Epithermal Au-Ag Deposit, New Zealand

Golden Cross, located in the Coromandel peninsula, New Zealand, is a classic example of a volcanic rock-hosted, low-sulfidation epithermal gold-silver deposit. Gold and silver ore is confined to the Empire vein system and shallow-level stockwork. The veins are hosted by Miocene to early Pliocene andesites and dacites of the Waipupu Formation and the Waiharakeke Dacite that are unconformably overlain by the postore Whakamoehau Andesite. Hydrothermal alteration minerals display distinct spatial and temporal zonation around veins, as defined along three cross sections (1,000 m long × 450 m deep), located 200 m apart, which transect the Empire vein system. Along these sections replacement quartz, chlorite, and pyrite are ubiquitous with the abundance of quartz veinlets increasing toward major veins. Replacement adularia envelops the Empire vein system and shallow stockwork in each section. It is coextensive with, and is variably replaced by, illite that progressively grades upward and laterally into a zone of interstratified illite-smectite that mantles the deposit. Replacement calcite and minor siderite formed contemporaneously with, and also overprint, the above minerals, whereas late barren calcite veins crosscut mineralized quartz veins. Kaolinite ± pyrite veinlets, together with rare, very local alunite, formed during late-stage hydrothermal activity. Clay mineral zonation is well developed. Illite occurs at depth and close to the veins, grading outward and upward into illite-smectite, with minor smectite occurring ~600 m east of the Empire vein system. This overall zonation reflects paleothermal gradients of ~150°C on the periphery to >220°C near the veins, consistent with the observed T h range of 150° to 240°C for fluid inclusions in quartz, platy calcite, and late barren calcite veins. Final ice-melting temperatures for inclusions mostly range from 0.0° to –1.4°C, corresponding to apparent salinities of less than 2.4 wt percent NaCl equiv. Ice-melting temperatures combined with vapor bubble expansion on crushing indicate the presence of dissolved CO 2 in some platy calcite and late-stage barren calcite. The CO 2 content is estimated to range from 0.35 to 3.5 wt percent, with the lower limit set by fluid inclusion vapor expansion during crushing and the upper limit by the absence of any observable clathrates. Depth estimates based on inclusions in platy calcite suggest that the shallow-level stockwork zone formed about 100 m below the paleowater table under hydrostatic conditions. Veins and alteration minerals at Golden Cross formed in the shallow part (<400 m) of a hydrothermal system analogous to geothermal systems in the nearby Taupo Volcanic Zone. The assemblage of quartz, adularia, chlorite, illite, calcite, and pyrite reflects the upflow of near-neutral pH to weakly alkaline chloride waters. Contemporaneously, steam-heated CO 2 -rich waters formed on the margins and reacted with the wall rock to form the assemblage of calcite, siderite, smectite, illite-smectite, and kaolinite. Eventual thermal collapse and invasion of these peripheral CO 2 -rich waters into the former upflow zone produced late-stage barren calcite veins and an overprint of clay-carbonate alteration minerals. At the same time, localized steam-heated acid-sulfate waters, generated above the water table, descended to form the late overprint of kaolinite, pyrite, and alunite.

The Fluid Inclusion and Mineralogic Record of the Transitionfrom Liquid- to Vapor-Dominated Conditions in The Geysers GeothermalSystem, California

The Geysers, an active vapor-dominated geothermal system related to a hypabyssal granitic pluton, was liquid dominated from its inception at 1.2 to ~0.3 Ma when the modern vapor-dominated regime formed. As the hydrothermal liquids boiled off, a vapor-dominated heat pipe developed in which the flow of steam was balanced by downward percolation of condensate. A record of the transition from liquid- to vapor-dominated conditions is contained within late-stage quartz and calcite crystals that are common in the upper part of the steam reservoir and overlying cap rock. Samples from three localities were examined in detail. Quartz from these veins is characterized by corroded crystal faces, solid inclusions of contemporaneous calcite that are generally etched, coeval high- and low-salinity fluid inclusions, primary low-salinity liquid-rich inclusions, and abundant vapor-rich inclusions. Primary vapor-rich inclusions up to 2 mm across were trapped mainly on rhombic and prism faces of skeletal quartz crystals during periods of rapid growth. These inclusions nucleate crystals of CO2 that disappear by sublimation rather than melting, at temperatures ranging from –92.9° to –56.7°C. Liquid-rich fluid inclusions from veins near the top of the modern steam reservoir yield homogenization temperatures ranging from 304° to 188°C. These temperatures commonly exceed boiling point to depth relationships based on the paleoground surface, suggesting that pressures were initially greater than hydrostatic. Temperature-salinity relationships indicate that the early, shallow liquids had salinities of 1.5 to 2 wt percent NaCl equiv and that boiling began by ~300°C. As the fluids boiled off, salinities of the residual liquid increased to 3 to 4 wt percent NaCl equiv. Trapping of low-salinity fluids (0.0–0.5 wt % NaCl equiv) interpreted as condensate in primary liquid-rich inclusions suggest vapor-dominated conditions had begun to develop by the time temperatures had dropped to 290°C. The primary origin of these inclusion fluids indicates that they were capable of depositing quartz and calcite. During the evolution of the vapor-dominated system, the hydrothermal fluids were at different times, but at the same location, undersaturated, in equilibrium, or oversaturated with respect to quartz and calcite. The hydrothermal fluids initially deposited quartz and calcite as they boiled off. Dissolution of these minerals occurred as the condensate, which was initially undersaturated with respect to quartz and calcite, percolated down. Subsequent deposition of these minerals occurred as the condensate, or mixtures of condensate and boiled liquid that leaked back into the fractures, vaporized during periods of intermittent pressure reduction. The top of the vapor-dominated system migrated downward with time as the overlying fractures were progressively sealed off. Today, pressures within the steam reservoir are vaporstatic and temperatures range from 240° to at least 342°C.

Preliminary Fluid Inclusion Constraints on Fluid Evolution in the Bayan Obo Fe-REE-Nb Deposit, Inner Mongolia, China

The Bayan Obo Fe-REE-Nb deposit, Inner Mongolia, China, is the world’s largest REE resource and is also exploited as a major Fe oxide deposit. The deposit consists of replacement bodies hosted in dolomite marble and of magnetite, hematite, and REE minerals associated with apatite, aegirine, amphibole, fluorite, calcite, and barite. Fluid inclusions from monazite-(Ce), bastnasite-(Ce), apatite, aegirine, fluorite, and barite were studied from eight samples to provide new constraints on ore genesis. During early disseminated monazite mineralization the fluids are best represented by the system H 2 O-CO 2 -NaCl, with salinities ranging from 1 to 5 wt percent NaCl equiv and X (CO 2 ) values from 0.3 to 0.45, at temperatures of >280° to 330°C and pressures of >0.7 kbar. Fluids in apatite and bastnasite from the main-stage banded ores were initially aqueous, with salinities ranging from 6 to 10 wt percent NaCl equiv. They developed significant CO 2 contents (X (CO 2 ) = 0.1–0.3) due to carbonate dissolution at temperatures of >400° to ~300°C and pressures of >0.9 to 1.4 kbars. Evidence for phase separation from coexisting brine and CO 2 -rich fluid inclusions in vein fluorite indicates pressures of 0.8 to 1.0 kbar during vein formation. These pressures can be used to infer temperatures of 430° to 460°C for aegirine alteration and of 340° to 240°C for apatite, bastnasite, and fluorite vein fill. Fluids responsible for aegirine alteration were dominantly aqueous, with salinities from 5 to 15 wt percent NaCl equiv, whereas those responsible for apatite and bastnasite mineralization had salinities of 1 to 6 wt percent NaCl equiv and X (CO 2 ) values from 0.25 to 0.75. Inclusions in late-stage fluorite and barite have homogenization temperatures from 240° to 150° and 200° to 130°C, respectively. The general salinity trend in these inclusions suggests dilution from around 15 to 5 wt percent NaCl equiv down to very low salinities (<1 wt % NaCl equiv) during barite precipitation. Halite-bearing inclusions may indicate episodes of boiling or aqueous-carbonic fluid immiscibility. These fluids are similar to those in some carbonatitic, alkaline igneous, and other igneous-related REE- and Fe-rich magmatic hydrothermal systems. However, they are different from fluids in igneous-related Fe oxide-Cu-Au-U-REE deposits, which have previously been compared to those at Bayan Obo.

Porphyry Gold Deposits of the Refugio District, Maricunga Belt, Northern Chile

The porphyry gold deposits of the Refugio district and similar deposits in the Maricunga belt contain the lowest known copper to gold ratios (% Cu/ppm Au = ~0.03) of any porphyry-type deposit. The gold deposits are associated with subvolcanic andesitic to dacitic intrusions emplaced into coeval volcanic rocks. Both the Verde and Pancho deposits are zoned in space from a deeper zone of banded quartz veinlets associated with chlorite-magnetite-albite and/or pyrite-albite-clay alteration to a shallow zone of pyrite-albite-clay and local quartz-alunite ledges. Pancho contains an additional, deepest, porphyry copperlike zone, with quartz veinlets (A-veinlets) and potassic alteration. Relative to Verde, Pancho is telescoped, with all three zones present within a 400-m-vertical interval. The porphyry copperlike zone at Pancho is characterized by A-veinlets and pervasive potassic alteration, both restricted to intrusive rocks. A-veinlets range from hairline streaks of magnetite ± biotite with minor quartz and chalcopyrite, and K feldspar alteration envelopes to sugary quartz veinlets <1 cm in width with magnetite and chalcopyrite and no alteration envelopes. Hypersaline liquid inclusions coexisting with vapor-rich inclusions indicate temperatures above 600°C and salinities as high as 84 wt percent NaCl equiv. A pressure estimate of 250 bars indicates a depth of 1,000 m, assuming lithostatic pressure. Potassic alteration consists of a central zone of magnetite-K feldspar-oligoclase that changes outward to a biotite-rich zone. Total sulfide content, predominately as chalcopyrite, is generally <1 vol percent, whereas magnetite content is 2 to 5 percent. Where A-veinlets and potassic alteration predominate, grades are typically 0.1 wt percent hypogene copper and 0.5 to 1 ppm gold. Banded quartz veinlets are present at both Verde and Pancho, where they occur mostly above A-veinlets and cut A-veinlets where they overlap. They are less than 2 cm in thickness and lack alteration envelopes. Dark gray bands, whose color is due to abundant vapor-rich fluid inclusions and micrometer-sized grains of magnetite, commonly occur as symmetric pairs near the vein walls. The bands are commonly botryoidal and are continuous through quartz grains, suggesting that the quartz recrystallized from a silica gel. Rare liquid-rich fluid inclusions in quartz indicate temperatures <350°C and salinities <35 wt percent NaCl equiv. Estimated pressures are <200 bars, suggesting depths of 190 to 1,500 m under hydrostatic pressure. Gold occurs both in the dark bands with magnetite and outside the dark bands with pyrite, chlorite, illite, and K feldspar. Banded veinlets occupy steeply dipping radial and shallowly dipping concentric fractures. Zones of abundant banded veinlets without early A-veinlets generally contain 0.5 to 2 ppm gold and <0.05 wt percent hypogene copper. Most of the differences between porphyry gold deposits at Refugio and porphyry copper deposits can be attributed to shallower depths of formation—less than 1 km compared to 1.5 to 4 km that is typical for porphyry copper deposits. Shallower depths resulted in lower sulfide concentrations, local garnet veinlets, widespread albite-bearing alteration, and most importantly banded quartz veinlets, which are unique to porphyry gold deposits. Banded quartz veinlets are a direct result of episodic intrusion of magma to within 1 km of the surface and exposure of high-temperature magmatic fluids to hydrostatic pressures. Episodic rupturing of a brittle-ductile boundary surrounding the intrusive centers at Verde and Pancho led to flashing of magmatic fluids, loss of sulfur to vapor, and low sulfide/gold ratios in ore.

Round Hill Shear Zone-Hosted Gold Deposit, Macraes Flat, Otago,New Zealand: Evidence of a Magmatic Ore Fluid

The 0.8-Moz Round Hill shear zone-hosted gold deposit is one of several located on, or near, the Hyde-Macraes shear zone, Macraes Flat, New Zealand. Field relationships show stockwork veins are the oldest part of the hydrothermal system, followed by flat and ramp veins. Hanging-wall shear veins formed early but have been modified by late deformation. Fluid inclusion microthermometric results for dominant type I inclusions show that flat and ramp veins have the same average wt percent NaCl equiv values (2.2± 0.6%) and were precipitated from the same fluid. Salinity data for these veins overlap with those from hanging-wall veins (average 1.3 ± 0.3%) and stockwork veins (1.6 ± 0.6% for vein margins; 0.9 ± 0.3% for vein centers), although both of the latter trend toward lower overall wt percent NaCl equiv values, suggesting mixing between relatively dilute and more saline end members. All of the above veins have similar temperatures of homogenization (T h ), ranging between 110° and 188°C. Fluid inclusion trapping temperatures are considered to be ~300°C, equating to a ~10-km depth of formation. Carbon dioxide dissolved in the hydrothermal fluid is responsible for up to one third of fluid inclusion apparent salinities. Stockwork and hanging-wall veins thus have true salinities of <1 wt percent NaCl equiv (<0.17 molal) and flat and ramp veins have true salinities of Fluid inclusion volatiles from Round Hill and other Macraes deposits are dominated by H 2 O (99.02–99.69 mol %), with lesser CO 2 (0.14–0.76 mol %), N 2 (0.03–0.32 mol %), and CH 4 (0.06–0.17 mol %). Noticeable concentrations of hydrocarbons occur in many of the samples. Fluid inclusion cation and anion data show few components of relatively low concentration are dissolved in the hydrothermal fluid, consistent with CO 2 -corrected microthermometric measurements. A range in fluid inclusion δ D H 2 O values obtained for the Round Hill deposit is coincident with the different vein types and also indicates mixing between two fluids. One fluid is considered to be O isotope shifted meteoric water, as trapped by the early stockwork veins, and has the lowest δ D H 2 O values of –80 per mil. The other fluid is represented by flat and/or ramp and hanging-wall veins that have δ D H 2 O values up to –40 per mil. The Macraes δ D H 2 O data show a trend toward the magmatic water end member for geothermal systems associated with convergent plate boundaries and are consistent with a 25 to 50 percent magmatic component in the Macraes hydrothermal system.

Vein and Karst Barite Deposits in the Western Jebilet ofMorocco: Fluid Inclusion and Isotope (S, O, Sr) Evidence for Regional FluidMixing Related to Central Atlantic Rifting

Numerous vein and karst barite deposits are hosted by Hercynian basement and Triassic rocks of the western Jebilet in Morocco. Sulfur, oxygen, and strontium isotope analyses of barite, combined with fluid inclusion microthermometry on barite, quartz, and calcite were used to reveal the nature and source of the ore-forming fluids and constrain the age of mineralization. The δ 34S values of barite between 8.9 and 14.7 per mil are intermediate between the sulfur isotope signatures of Triassic evaporites and Triassic-Jurassic seawater and lighter \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(SO_{4}^{2{\mbox{--}}}\) \end{document}, probably derived from the oxidation of dissolved H2S and leaching of sulfides in the Hercynian basement. The 87Sr/86Sr ratios of barite between 0.7093 and 0.7130 range between the radiogenic strontium isotope compositions of micaceous shale and sandstone and the nonradiogenic isotopic signature of Triassic to Jurassic seawater and Cambrian limestone. The δ 18O values of barite between 11 and 15 per mil (SMOW) support mixing between two or more fluids, including Late Triassic to Jurassic seawater or a water dissolving Triassic evaporites along its flow path, hot basinal, or metamorphic fluids with δ 18O values higher than 0 per mil and/or meteoric fluids with δ 18O values lower than 0 per mil. The general trend of decreasing homogenization temperatures and initial ice melting temperatures with increasing salinities of H2O-NaCl ± CaCl2 fluid inclusions trapped in barite, quartz, and calcite indicates that a deep and hot basinal fluid with salinities lower than 6 wt percent NaCl equiv might have mixed with a cooler surficial solution with a mean salinity of 20 wt percent NaCl equiv. Calcium was leached from the Cambrian limestone and the clastic and mafic volcanic rocks of the Hercynian basement. Alkali feldspars and micas contained in the Cambrain sandstones provided most of the Ba to the hydrothermal system. Vein and karst deposits are modeled as a two-component mixing process in which the temperature and the S and Sr isotope composition of the end members changed during the 220 to 155 Ma interval. The hot basinal fluid remained volumetrically dominant during the entire mineralization process. Differences in mean S, O, and Sr isotope compositions among the barite families are interpreted as reflecting differences in mineralization age. Most barite deposits formed before the Kimmeridgian, except for north-south-oriented vein barite, karst barite, and barite cement in the conglomeratic Upper Jurassic, which were deposited later, possibly around 155 Ma. Similar genetic processes have been described for late Paleozoic to Mesozoic F-Ba vein deposits in western Europe. The vein and karst barite in the western Jebilet of Morocco reveals a wide-scale regional mineralization event related to Central Atlantic rifting.

A Fluid Inclusion Study of Au-Bearing Quartz Vein Systems inthe Central and North Deborah Deposits of the Bendigo Gold Field, CentralVictoria, Australia

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.

The Genesis of Hydrothermal Fluorite-REE Deposits in the Gallinas Mountains, New Mexico

Breccia-hosted fluorite-bastnaesite deposits associated with epizonal quartz syenite intrusions in the Gallinas Mountains, New Mexico, provide an important example of hydrothermal rare earth element (REE) mineralization, and an excellent opportunity to investigate the transport and deposition of the REE under hydrothermal conditions. Fluid inclusion studies show that mineralization commenced with the deposition of quartz at a temperature of approximately 400°C from sulfate-rich NaCl-KCl brines having a salinity of ~15 wt percent NaCl equiv. These fluids are interpreted to have been of orthomagmatic origin. Fluorite and bastnaesite deposition occurred at slightly lower temperatures, and coincided with the introduction of separate CO2-bearing and sulfate-poor NaCl brines into the system. These fluids are considered to have been of external origin, the latter being a formation water that circulated in surrounding Permian sandstones and limestones. Evaluation of the available thermodynamic data on the aqueous speciation of REE, and the preliminary data on the chemistry of the fluids obtained in this study, suggest that the REE were dissolved primarily as fluoride complexes. A model is proposed in which the REE were transported by the orthomagmatic fluids, and deposited as bastnaesite owing to destabilization of REE-fluoride complexes. The latter occurred in response to the sharp reduction in F– activity that accompanied large-scale deposition of fluorite as a result of mixing of the orthomagmatic fluids and formation waters. This mixing also led to the deposition of anhydrite and barite owing to the interaction of externally introduced calcium and barium ions with orthomagmatic sulfate ions. The later stages in the evolution of the Gallinas Mountains hydrothermal system were marked by increased incursion of external fluids, and the alteration of bastnaesite to Ca bastnaesite and minor deposition of Ca bastnaesite and parisite owing to lowered F–/\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(CO_{3}^{2{\mbox{--}}}\) \end{document} ratios and/or increased Ca2+ activity. The study suggests that Ca-free, REE fluoride-bearing fluids are an important ingredient for REE mineralization, that mixing of such fluids with Ca-, carbonate-bearing fluids or interaction with carbonate rocks is a major control of REE-fluorocarbonate mineral deposition, and that the occurrence of fluorite could provide an important guide in the exploration for economic REE deposits.

Metallogenesis of Zn-Pb Carbonate-Hosted Mineralization in theSoutheastern Region of the Picos de Europa (Central Northern Spain)Province: Geologic, Fluid Inclusion, and Stable Isotope Studies

The Zn-Pb deposits of the southeastern area of the Picos de Europa province display an epigenetic hydrothermal mineralization of moderate temperature hosted by intensely dolomitized limestones of Carboniferous age. They are structurally and lithologically controlled since the three phenomena of fracturing, dolomitization, and mineralization are interrelated. Two types of mineralization have been found: type I, granular dark brown-colored sphalerite, galena, and dolomite, and type II, toffee-colored sphalerite, galena, and calcite. Type II mineralization is later than type I. The Aliva and Andara deposits are the two largest in the Picos de Europa area, and both contain type I and type II styles of mineralization. Fluid inclusion studies indicate that the mineralizing solutions for both stages were similar and had trapping temperatures between 170° and 200°C and salinities around 15 wt percent NaCl equiv. However, microthermometric data from sphalerite (I and II), dolomite I, calcite II, and fluorite samples show various stages of circulation and trapping of brines with spatial and temporal variations in salinity and temperature. Calcite and dolomite from the mineralization have δ 13 C PDB values between 1 and 2.9 per mil ( δ 13 C Σ CO2 = –0.8–1.1‰), and δ 18 O SMOW values between 11.5 and 16.2 per mil ( δ 18 O H 2 O varied from 0.7–1.9‰). Sulfides show δ 34 S values between 12.9 and –16.8 per mil ( δ 34 S fluid for the Aliva deposit ranged between 13.4 and –2.5‰). The combined stable isotope data indicate evolved marine waters. Isotope evidence suggests that the mineralizing fluids were probably formation waters of the Palentine zone and the Picos de Europa province sediments. During the Permian, possibly during extensional tectonic phases, these brines moved toward and through the Picos de Europa province along late Hercynian fractures giving rise to the Zn-Pb mineralization.

Sedimentary exhalative nickel-molybdenum ores in South China

Unique bedded Ni-Mo ores hosted by black shales were discovered in localized paleobasins along the Yangzte platform of southern China in 1971. Textural evidence and radiometric dates imply ore formation during sedimentation of black shales that grade into readily combustible beds, termed stone coals, which contain 10 to 15 percent organic carbon. Studies of 427 fluid inclusions indicate extreme variation in hydrothermal brine salinities that were contained by Proterozoic dolostones underlying the ore zone in Hunan and Guizhou. Variations of fluid inclusion salinities, which range from 0.1 to 21.6 wt percent NaCl equiv, are attributed to differences in the compositions of brines in strata underlying the ore bed, complicated by the presence of sea-water and dilute fluids that represent condensates of vapors generated by boiling of mineralizing fluids or Cambrian meteoric water. The complex processes of ore deposition led to scattered homogenization temperatures ranging from 100 degrees to 187 degrees C within the Hunan ore zone and from 65 degrees to 183 degrees C within the Guizhou ore zone. While living organisms probably did not directly accumulate metals in situ in sufficient amounts to explain the unusually high grades of the deposits, sulfur isotope ratios indicate that bacteria, now preserved as abundant microfossils, provided sufficient sulfide for the ores by reduction of seawater sulfate. Such microbiota may have depended on vent fluids and transported organic matter for key nutrients and are consistent with a sedex origin for the ores. Vent fluids interacted with organic remains, including rounded fragments of microbial mats that were likely transported to the site of ore deposition by the action of waves and bottom currents prior to replacement by ore minerals.

Epithermal gold mineralization in an old volcanic arc; the Jacinto Deposit, Camagueey District, Cuba

The Jacinto vein system in the Camaguey district of central Cuba is a low-sulfidation, adularia-sericite epithermal gold deposit. It is hosted by volcanic and volcaniclastic rocks of Cretaceous age along the margin of the Camaguey batholith. Jacinto is unusually old and much more closely associated with a large batholith than would be expected for an epithermal vein deposit in an island arc. This study was undertaken to determine how Jacinto could have been preserved for so long. The Jacinto system includes at least five veins that range in length up to 1 km, have maximum widths of 60 m, and extend to depths of at least 150 m. The veins consist almost entirely of quartz, with minor amounts of adularia, calcite (commonly bladed), pyrite, and gold, which form at least 20 growth zones that can be divided into five main stages and can be correlated from one vein to another. Gold ore shoots are restricted vertically and are more continuous horizontally. Their present configuration suggests that the entire system has been tilted about 5 degrees to the northeast. Wall-rock alteration is limited and consists of minor K feldspar and sericite. Fluid inclusions in vein quartz homogenize at temperatures of 200 degrees to 320 degrees C and freeze at temperatures characteristic of salinities of about 0 to 1 wt percent NaCl equiv. The presence of bladed calcite and silica pseudomorphs after bladed calcite suggests that the hydrothermal fluids boiled. Stable isotope analyses of quartz, calcite, and inclusion fluids indicate that the veins were deposited by waters with delta 18 O and delta D values that range from values just above those of magmatic water to those typical of Cretaceous meteoric water. 40 Ar- 39 Ar analyses of adularia in the Jacinto veins indicate an age of about 72 Ma for mineralization, and geologic relationships suggest that the vein system is related to La Sierra rhyolite flows and domes of that approximate age. La Sierra rhyolite was extruded onto an erosion surface that exposed the Camaguey batholith and related volcanic rocks, indicating that erosion rates were relatively rapid immediately prior to formation of the deposit. Shortly after the deposit formed, however, it was tilted and then buried beneath Late Cretaceous to Eocene sediments of the Maraguan basin, which are being eroded to expose the system today. This fortuitous sequence of volcanism, batholith emplacement, rapid erosion to deep batholith levels, extrusion of rhyolite, and local sedimentation appears to be the result of collision between the Cuban arc and the Bahamas platform. Similar collisions between arcs and continental masses could have formed other epithermal deposits.

Basement-hosted quartz-barite sulfide veins in the French Alps; a record of alpine tectonic fluid expulsion in the external crystalline massifs; structural, fluid inclusion, and isotope (S and Sr) evidences

Quartz-barite sulfide veins hosted by a late Variscan monzonite in the polymetamorphic basement of the Aiguilles Rouges massif, western Alps, France, have been the subject of a combined structural, microthermometric fluid inclusion, and sulfur and strontium isotope study. The mineralized veins have a maximum age of 40 Ma and were emplaced along vertical tension fractures formed, or possibly reopened, during the Mesoalpine thrust phase. Two mineralization stages have been recognized: a major stage of quartz-barite veins with minor and irregularly distributed sulfides and sulfosalts, and a minor crosscutting stage of quartz-stibnite veins. Fluid inclusion microthermometry, homogeneous 87 Sr/ 86 Sr ratios of the barite, and the texture and mineralogical composition of the polymetallic quartz-barite veins indicate that they were formed mainly due to the cooling of a single aqueous fluid with a salinity around 9 wt percent NaCl equiv. Contemporaneous pressure variations cannot be excluded. Sulfur isotope thermometry yields formation temperatures between 255 degrees and 270 degrees C, and combined with the fluid inclusion isochores they indicate pressures of 0.45 to 1.55 kbars, corresponding to a depth of 3.6 (super +2.3) (sub -1.9) km assuming lithostatic conditions. Sulfur in the fluid was dominated by H 2 S which was oxidized at the ore site due to cooling of the fluid. The vein barites are enriched in 87 Sr with respect to the host rocks and indicate that the strontium and the barium were leached from deeper lithologies. After the emplacement of the quartz-barite veins, the hydrothermal system was opened toward shallower structural levels, allowing the ingress of meteoric water. The late quartz-stibnite veins are associated with a distinct and warmer CO 2 -bearing fluid. We conclude that the H 2 O-NaCl ore-bearing fluid originated at a depth of about 15 km by dewatering of Mesozoic sedimentary rocks during the collisional-thrust Mesoalpine phase. Subhorizontal thrust planes between the basement crystalline rocks and overlying sedimentary units inhibited mixing or regeneration of the system with superficial fluids. Subvertical fluid migration was restricted to faults in the polymetamorphic basement. Late Alpine extension tectonics allowed the upward migration of deep CO 2 -bearing fluids and also favored the influx of surface fluids that had remained isolated from the H 2 O-NaCl hydrothermal system.

Stable isotope evidence for the origin of the Mesoproterozoic Starra Au-Cu deposit, Cloncurry District, Northwest Queensland

New stable isotope data are consistent with a metasomatic origin for the controversial Mesoproterozoic ironstone-hosted Au-Cu deposit at Starra in the Cloncurry district of northwest Queensland. This supports textural and mineralogical evidence that the ore-bearing ironstones formed after the peak of metamorphism and were strongly controlled by brittle-ductile deformation. Three dominant paragenetic stages are recognized. These are: (1) early, widespread Na-Ca metasomatism (quartz-albite-scapolite-actinolite), (2) localized K-Fe metasomatism (biotite-magnetite-hematite-quartz-pyrite), and (3) mineralization (quartz-anhydrite-barite-hematite-calcite-gold-pyrite-chalcopyrite-bornite-chalcocite-chlorite-muscovite). Crosscutting anhydrite veins associated with carbonate + or - hematite are rare in the upper parts of the system but more common at depths greater than 700 m. Stage 3 minerals demonstrate that the ore fluid was highly oxidized.A restricted range of Fe oxide delta 18 O implies isotopically similar fluids were responsible for the magnetite ironstones and later hematite alteration associated with gold and sulfides (between -0.2 to +3.3ppm except for one magnetite at 5.4ppm). Sulfur isotope compositions suggest that pyrite (-0.2 to +3.9ppm), chalcopyrite (-5.8 to -0.7 and -14.6ppm), bornite (-4.7ppm), chalcocite (-0.1ppm), and anhydrite (1.1-2.4ppm) all formed from the same sulfur source. Anhydrite delta 34 S falls within the sulfide range, suggesting that the sulfate inherited its delta 34 S through hydrolytic processes similar to those of some porphyry environments. delta 13 C of ore-stage carbonates range from -7.3 to -2.2 per mil. Temperatures based on stage 2 quartz-magnetite pairs imply 400 degrees to 500 degrees C for ironstone formation and stage 3 calcite-hematite pairs indicate 180 degrees to 330 degrees C for mineralization. Stage 2 fluid inclusions have homogenization temperatures between 345 degrees and 615 degrees C and salinities of 34 to 52 wt percent NaCl equiv. Stage 3 fluid inclusions homogenized between 225 degrees and 360 degrees C and have salinities of 30 to 42 wt percent NaCl equiv. Calculated delta 18 O fluid compositions for magnetite-quartz and hematite-calcite average 7.8 per mil and 9.5 per mil, respectively. Fluid compositions (for stages 2 and 3) fall within the range for a magmatic or metamorphic fluid. However, combined stable isotope and fluid inclusion data and thermodynamic considerations suggest a magmatic-metasomatic origin for both the magnetite ironstones and the highly oxidized mineral assemblage associated with high-grade gold deposition.