Liquid-rich Fluid Inclusions Research Articles

The evolution of volcano-hosted geothermal systems based on deep wells from Karaha-Telaga Bodas, Indonesia

Temperature and pressure surveys, fluid samples, and petrologic analyses of rock samples from deep drill holes at the Karaha - Telaga Bodas geothermal field on the volcanic ridge extending northward from Galunggung Volcano, West Java, have provided a unique opportunity to characterize the evolution of an active volcano-hosted geothermal system. Wells up to 3 km in depth have encountered temperatures as high as 353°C and a weakly altered granodiorite that intruded to within 2 to 3 km of the surface. The intrusion is shallowest beneath the southern end of the field where an acid lake overlies a nearly vertical low resistivity structure (<10 ohm-m) defined by magnetotelluric measurements. This structure is interpreted to represent a vapor-dominated chimney that provides a pathway to the surface for magmatic gases. Four distinct hydrothermal mineral assemblages document the evolution of the geothermal system and the transition from liquid- to vapor-dominated conditions. The earliest assemblage represents the initial liquid-dominated system generated during emplacement of the granodiorite between 5910 ± 76 and 4200 ± 150 y BP. Tourmaline, biotite, actinolite, epidote and clay minerals were deposited contemporaneously at progressively greater distances from the intrusive contact (assemblage 1). At 4200 ± 150 y BP, flank collapse and the formation of the volcano9s crater, Kawah Galunggung, resulted in catastrophic decompression and boiling of the hydrothermal fluids. This event initiated development of the modern vapor-dominated regime. Chalcedony and then quartz were deposited as the early low salinity liquids boiled (assemblage 2). Both vapor- and liquid-rich fluid inclusions were trapped in the quartz crystals. Liquid-rich fluid inclusions from the southern part of the field record salinities ranging from 0 to 26 weight percent NaCl-CaCl₂ equivalent and locally contain fluorite daughter crystals. We suggest, based on temperature-salinity relationships and evidence of boiling, that these fluids were progressively concentrated as steam was lost from the system. However, mixing with fluids derived from the underlying intrusion or generated during the formation of acid SO₄ water on the vapor-dominated chimney margins could have contributed to the observed salinities. As pressures declined, CO₂–and SO₄-rich steam-heated water drained downward, depositing anhydrite and calcite (assemblage 3) in the fractures, limiting further recharge. Fluid inclusions with salinities up to 31 weight percent NaCl equivalent were trapped in these minerals as the descending water vaporized. The final assemblage is represented by precipitates of NaCl, KCl and FeCl_x deposited on rock surfaces in portions of the vapor-dominated zone that boiled dry. Vapor-dominated conditions extend over a distance of at least 10 km and to depths below sea level. Deep wells drilled into the underlying liquid-dominated reservoir in the northern and central part of the volcanic ridge produce low salinity fluids representing recent recharge of meteoric and steam-heated water. The evolution of volcanic-hosted vapor-dominated geothermal systems can be described by a five stage model. Stage 1 involves the formation of an over-pressured liquid-dominated geothermal system soon after magmatic intrusion. In Stages 2 and 3, pressures progressively decrease, and a curtain of steam-heated water surrounding a magmatic vapor-dominated chimney at 350°C and 14 ± 2 MPa develops. The relatively low pressure near the base of the chimney causes liquid inflow adjacent to the intrusion and the development of a secondary marginal vapor-dominated zone. In Stage 4, the magmatic vapor discharge from the intrusion becomes small, vapor pressure declines, and the secondary vapor-dominated zone expands above the intrusion. In Stage 5, the vapor-dominated zone floods because heat from the intrusion is insufficient to boil all liquid inflow. A more common, liquid-dominated volcanic-hosted system then develops.

Genesis of the Wolverine Volcanic Sediment-Hosted Massive Sulfide Deposit, Finlayson Lake District, Yukon, Canada: Mineralogical, Mineral Chemical, Fluid Inclusion, and Sulfur Isotope Evidence

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

Experimental study of the behavior of copper and zinc in a boiling hydrothermal system

Vapor-liquid partitioning coefficients for Cu and Zn in aqueous fluid were determined under boiling conditions at 500–650 °C and 35–100 MPa in sulfur-bearing and sulfur-free systems. A synthetic fluid inclusion technique was used to sample the experimental system of coexisting vapor-rich and liquid-rich fluid inclusions; the Cu and Zn concentrations in individual fluid inclusions were then analyzed by synchrotron radiation X-ray fluorescence. The vapor-liquid distribution constant of Cu [K D m Cu(vapor) / m Cu(liquid) ], where = m denotes the molality of metal, is found to be strongly dependent on the sulfur content in the experimental solutions. In sulfur-bearing systems, Cu preferentially partitions into the vapor phase, whereas Zn preferentially fractionates into the hypersaline liquid. The K D values for Cu and Zn obtained in this study correspond well with those obtained from natural fluid inclusions in hydrothermal ore deposits. The results suggest that differential volatility of metals is an important factor affecting the enrichment of certain metals in different hydrothermal ore deposits.

Geology of the Sari Gunay Epithermal Gold Deposit, Northwest Iran

The Sari Gunay epithermal gold deposit is located within a mildly alkaline latitic to trachytic volcanic complex in central-northwest Iran. Intrusive and volcanic rocks that host the deposit have been dated at between 11.7 and 11.0 Ma (with one younger sample at 8.0 Ma; 40Ar/39Ar dating of igneous biotite and hornblende), whereas sericitic alteration associated with an early stage of hydrothermal activity occurred between ~10.8 and ~10.3 Ma (the best age estimate is 10.7 Ma obtained by 40Ar/39Ar dating of sericite). The main hydrothermal system was structurally localized by a diatreme breccia complex centered on the Sari Gunay hill. Earliest hydrothermal activity, only encountered at depth (>300 m below surface), produced weak potassic alteration and quartz-sulfide-magnetite veining with low grades of Cu and Au mineralization, reminiscent of shallow-level (≤ 1.5-km depth) porphyry gold systems. Paragenetically later quartz-tourmaline breccias and veins followed the south-southwest–north-northeast trend of the diatreme breccias and produced strong sericitic alteration and silicification in igneous clasts and wall rocks. Fluid inclusions indicate temperatures of 246° to 360°C and salinities of 34.4 to 46.1 wt percent NaCl equiv in hypersaline inclusions coexisting with low-density, CO2-bearing vapor-phase inclusions. These fluids do not appear to have deposited gold, but the breccias provided further structural permeability for flow of later mineralizing epithermal fluids. Invisible gold occurs in solid solution in fine-grained arsenical pyrite and minor arsenopyrite, deposited in the early stages of quartz-adularia-pyrite-stibnite veins. Late-stage minerals in these veins include realgar, orpiment, cinnabar, and extremely rare wire gold. Liquid-rich fluid inclusions in these veins have an average homogenization temperature (≈ trapping temperature) of 199° ± 24°C, salinities of 3.6 ± 1.1 wt percent NaCl equiv, and coexist with low-density CO2-bearing vapor-phase inclusions, suggesting low-pressure conditions. Phase separation may have been a contributory cause of gold deposition, but precipitation of sulfides in response to rapid cooling of the system may have been the main control on deposition of pyrite-hosted gold. The Sari Gunay gold deposit and its host volcanic system form part of the Takab belt of structurally controlled, middle Miocene, mildly alkaline volcano-plutonic complexes. The sediment-hosted Zarshuran gold deposit, located ~190 km to the north-northwest along the belt, is also related to middle Miocene magmatism. The Takab belt is marginally younger than, and displaced to the southwest of, the extensive Eocene-Miocene Urumieh-Dokhtar calk-alkaline volcanic arc, which hosts several large porphyry Cu deposits (e.g., Sar Cheshmeh, Meiduk). Rocks of the Takab belt are also distinctly more alkaline than the Urumieh-Dokhtar rocks. Viewed in the context of plate tectonic models for Neogene closure of a Neo-Tethys ocean between the Afro-Arabian and Eurasian plates, these differences may reflect a normal subduction zone origin for the Urumieh-Dokhtar arc and contained porphyry Cu deposits but a collisional origin for the slightly younger Takab belt volcanic rocks and epithermal gold deposits. The Sari Gunay deposit may thus be classified as a collision-related alkalic-type epithermal system, although it is less alkaline than classic deposits of this group such as Porgera, Emperor, or Cripple Creek (the latter apparently being related to postcollisional or back-arc extensional rather than syncollisional processes).

Hydrothermal evolution of the Sar-Cheshmeh porphyry Cu–Mo deposit, Iran: Evidence from fluid inclusions

The Sar-Cheshmeh porphyry Cu–Mo deposit is located in Southwestern Iran (∼65 km southwest of Kerman City) and is associated with a composite Miocene stock, ranging in composition from diorite through granodiorite to quartz-monzonite. Field observations and petrographic studies demonstrate that the emplacement of the Sar-Cheshmeh stock took place in several pulses, each with associated hydrothermal activity. Molybdenum was concentrated at a very early stage in the evolution of the hydrothermal system and copper was concentrated later. Four main vein Groups have been identified: (I) quartz+molybdenite+anhydrite±K-feldspar with minor pyrite, chalcopyrite and bornite; (II) quartz+chalcopyrite+pyrite±molybdenite±calcite; (III) quartz+pyrite+calcite±chalcopyrite±anhydrite (gypsum)±molybdenite; (IV) quartz±calcite±gypsum±pyrite±dolomite. Early hydrothermal alteration produced a potassic assemblage (orthoclase-biotite) in the central part of the stock, propylitic alteration occurred in the peripheral parts of the stock, contemporaneously with potassic alteration, and phyllic alteration occurred later, overprinting earlier alteration. The early hydrothermal fluids are represented by high temperature (350–520 °C), high salinity (up to 61 wt% NaCl equivalent) liquid-rich fluid inclusions, and high temperature (340–570 °C), low-salinity, vapor-rich inclusions. These fluids are interpreted to represent an orthomagmatic fluid, which cooled episodically; the brines are interpreted to have caused potassic alteration and deposition of Group I and II quartz veins containing molybdenite and chalcopyrite. Propylitic alteration is attributed to a liquid-rich, lower temperature (220–310 °C), Ca-rich, evolved meteoric fluid. Influx of meteoric water into the central part of the system and mixing with magmatic fluid produced albitization at depth and shallow phyllic alteration. This influx also caused the dissolution of early-formed copper sulphides and the remobilization of Cu into the sericitic zone, the main zone of the copper deposition in Sar-Cheshmeh, where it was redeposited in response to a decrease in temperature.

Fluid Inclusion Investigations of the Raigan Porphyry Copper System, Kerman-Bam, Iran

The Raigan porphyry copper system is associated with diorite/granodiorite to quartz-monzonite plutons of Miocene age that intruded Eocene volcano-sedimentary rocks. Several stages of hydrothermal alteration and associated mineralization occurred within the system. The main intense hydrothermal alterations are of potassic, phyllic, and propylitic types, especially within and adjacent to the diorite/granodiorite intrusive stock. Copper mineralization was accompanied by mainly phyllic and less potassic alteration. Early hydrothermal alteration produced a potassic assemblage (orthoclase-biotite) in the central part of the stock, propylitic alteration occurred contemporaneously with potassic alteration in peripheral parts, and phyllic alteration occurred later, overprinting all the earlier alterations. The early hydrothermal fluids are represented by high-temperature (487°C to 598°C), high-salinity (up to 61.1 wt% NaCl equiv.) liquid-rich fluid inclusions, and high-temperature (397°C to 401°C), low-salinity, vapor-rich inclusions. These fluids are interpreted to represent a very hot orthomagmatic fluid that boiled episodically; the brines evidently caused potassic alteration, producing the first generation of chalcopyrite. Propylitic alteration is attributed to a liquid-rich, lower-temperature (523°C to 298°C), Ca-rich, evolved meteoric fluid. Based on this research, the temperature of mineralizing fluids was too high to produce an economic porphyry copper deposit at Raigan.

Mineralogy and fluid inclusion investigations in the Reagan Porphyry System, Iran, the path to an uneconomic porphyry copper deposit

The Reagan Porphyry System in southeastern Iran (Kerman Province) is associated with diorite/granodiorite to quartz-monzonitic rocks of Miocene age. Three stages of hydrothermal alteration and associated mineralization have produced new minerals, created new textural relationships and in many cases obliterated the primary character of the rocks. Potassic, phyllic and propylitic are the main hydrothermal alteration types within the intrusive stock. The early hydrothermal alteration produced a potassic assemblage in the central part of the stock; propylitic alteration occurred contemporaneously with potassic alteration, but in the peripheral parts, phyllic alteration which contains the main copper mineralization, occurred later, overprinting the earlier alteration. Phyllic is also the major alteration in the Reagan system. The early hydrothermal fluids are represented by high temperature (487–598 °C), high salinity (up to 61.1 wt% NaCl equiv.) liquid-rich fluid inclusions, and high temperature (397–401 °C), low-salinity, vapor-rich inclusions, which are interpreted to originate from very hot orthomagmatic fluids. The brines are interpreted to have caused potassic alteration, producing first generation chalcopyrite. Propylitic alteration is attributed to a lower temperature, liquid- and Ca-rich, evolved meteoric fluid. Influx of meteoric water into the system, and mixing with magmatic fluid produced deep albitization, and shallow phyllic alteration. This influx also caused dissolution of early formed copper sulfides and remobilization of Cu into the phyllic zone. The following data indicate that the mineralizing fluid temperature was too high to produce an economic porphyry copper deposit at the studied area. Supergene alteration was restricted to a very thin blanket with uneconomic levels of Cu in the form of Cu-sulfides and oxides.

Geochronological Constraints Using40Ar/39Ar Dating on the Mineralization of the Hishikari Epithermal Gold Deposit, Japan

. Ages for thirty adularia samples collected from various veins were in the Hishikari gold deposit determined by 40Ar/39Ar dating to constrain the timing of adularia-quartz vein formation and to determine the temporal change in temperature of hydrothermal fluid. Plateau ages were obtained from all adularia samples, and significant excess 40Ar is not recognized from inverse isochrones. The duration of mineralization within individual veins was determined by adularia ages from the early and late stages of mineralization within the same vein. The durations of mineralization in the Daisen-1, Daisen-3, Hosen-2 and Keisen-3 veins in the Honko-Sanjin zone were 7,000, 140,000, 160,000 and 170,000 years, respectively. The durations of mineralization in the Seisen-2 and Yusen-1–2 veins in the Yamada zones were 360,000 and 320,000 years, respectively. Mineralization lasted for a relatively longer period in individual veins at the Yamada zone. Mineralization ages from the Honko-Sanjin zone range from 1.04 to 0.75 Ma, and most mineralization ages are concentrated in a short period from 1.01 to 0.88 Ma. In contrast, mineralization ages for the Yamada zone range from 1.21 to 0.64 Ma. These results indicate that fracturing and subsequent vein formation lasted for a longer period in the Yamada zone (about 570,000 years) compared with those events in the Honko-Sanjin zone (about 290,000 years). The homogenization temperatures of liquid-rich fluid inclusions in columnar adularia used for age determination were determined to be 223°C on average, and most of these temperatures range from 180 to 258dC. No significant temporal change in homogenization temperature is recognized in this study. However, adularia in the Keisen veins indicated higher homogenization temperatures compared with elsewhere in the deposit, suggesting that the principal ascent of mineralizing hydrothermal fluid was via the Keisen veins.

Evolution of the magmatic-hydrothermal acid-sulfate system at Summitville, Colorado: integration of geological, stable-isotope, and fluid-inclusion evidence

The Summitville Au–Ag–Cu deposit is a classic volcanic dome-hosted high-sulfidation deposit. It occurs in the Quartz Latite of South Mountain, a composite volcanic dome that was emplaced along the coincident margins of the Platoro and Summitville calderas at 22.5±0.5 Ma, penecontemporaneous with alteration and mineralization. A penecontemporaneous quartz monzonite porphyry intrusion underlies the district and is cut and overlain by pyrite–quartz stockwork veins with traces of chalcopyrite and molybdenite. Alteration and mineralization proceeded through three hypogene stages and a supergene stage, punctuated by at least three periods of hydrothermal brecciation. Intense acid leaching along fractures in the quartz latite produced irregular pipes and lenticular pods of vuggy silica enclosed sequentially by alteration zones of quartz–alunite, quartz–kaolinite, and clay. The acid-sulfate-altered rocks host subsequent covellite+enargite/luzonite+chalcopyrite mineralization accompanied by kaolinite, and later barite–base-metal veins, some containing high Au values and kaolinite. The presence of both liquid- and vapor-rich fluid inclusions indicates the episodic presence of a low-density fluid at all levels of the system. In the mineralized zone, liquid-rich fluid inclusions in healed fractures in quartz phenocrysts and in quartz associated with mineralization homogenize to temperatures between 160 and 390 °C (90% between 190 and 310 °C), consistent with the range (200–250 °C) estimated from the fractionation of sulfur isotopes between coexisting alunite and pyrite. A deep alunite–pyrite pair yielded a sulfur-isotope temperature of 390 °C, marking a transition from hydrostatic to lithostatic pressure at a depth of about 1.5 km. Two salinity populations dominate the liquid-rich fluid inclusions. One has salinities between 0 and 5 wt.% NaCl equivalent; the other has salinities of up to 43 wt.% NaCl equivalent. The occurrence of high-salinity fluid inclusions in vein quartz associated with mineralization, as well as in the deep stockwork veins, suggests that brines originating deep in the system transported the metals. The δ 34S values of sulfides in magnetite (−2.3‰) and of sulfate in apatite (5.4‰) in unaltered quartz latite indicate that δ 34S ΣS was near 0‰. The δ 34S values of coexisting alteration alunite and pyrite are 18.2‰ to 24.5‰ and −8.1‰ to −2.2‰, respectively. Deep in the system, most of the change in δ 34S values occurs in the sulfates, indicating that the fluids were initially H 2S-dominant, their redox state buffered at depth by equilibration with igneous rocks. However, in the main alteration zone, most of the change in δ 34S values occurs in pyrite, indicating that the fluids moved off the rock buffer and became SO 4 2−-dominant as pyrite precipitated and SO 2 disproportionation produced the sulfuric acid requisite for acid leaching. The δ 34S values of the late-stage barite and sulfides indicate that the system returned to high H 2S/SO 4 2− ratios typical of the original rock-buffered fluid. The δD H2O of alunite parent fluids was near −45‰ and their δ 18O ranged from 7‰ to −1‰, depending on the degree of exchange in the alteration zone at low water–rock ratio, or mixing with unexchanged meteoric water. The low δD values of some alunite samples are interpreted to result from postdepositional exchange with later ore fluids. Fluid exsolved from the magma at depth had δD H2O and δ 18O H2O values near −70‰ and 10‰, respectively. During and following migration to the top of the magma chamber, the fluid underwent isotopic exchange with the partially crystallized magma and its solid and cooler, but still plastic, carapace just below the transition from a lithostatic to hydrostatic pressure regime. These evolved magmatic fluids had δD H2O and δ 18O H2O values close to −40‰ and 5‰, respectively, prior to release into the superjacent hydrostatically pressured fracture zone, wherein the fluids separated into a low-density phase and hypersaline brine. The low-density phase rose to the level of acid-sulfate alteration, where disproportionation of SO 2 commenced upon condensation of the vapor plume causing the acid-sulfate alteration. Hydrogen and oxygen isotopic compositions estimated from deep kaolinitic wallrock alteration and fluid inclusions in quartz stockwork veins indicate the brine mixed with highly exchanged meteoric water at depth and rose to the ore zone, producing the Cu–Au–Ag mineralization. The composition of inclusion fluids in a single sample of enargite also indicates mixing between ore fluids and unexchanged meteoric water at shallow depths. The sulfate in fluids responsible for precipitation of most barite samples was of magmatic origin, but the δ 34S of a single sample suggests mixing with sulfate derived from the atmospheric oxidation of H 2S in an overlying steam-heated zone.

Telescoped porphyry Cu-Mo-Au mineralisation, advanced argillic alteration and quartz-sulphide-gold-anhydrite veins in the Thames District, New Zealand

Porphyry Cu-Mo-Au mineralisation with associated potassic and phyllic alteration, an advanced argillic alteration cap and epithermal quartz-sulphide-gold-anhydrite veins, are telescoped within a vertical interval of 400–800 m on the northeastern margin of the Thames district, New Zealand. The geological setting is Jurassic greywacke basement overlain by Late Miocene andesitic-dacitic rocks that are extensively altered to propylitic and argillic assemblages. The porphyry Cu-Mo-Au mineralisation is hosted in a dacite porphyry stock and surrounding intrusion breccia. Relicts of a core zone of potassic K-feldspar-magnetite±biotite alteration are overprinted by phyllic quartz-sericite-pyrite or intermediate argillic chlorite-sericite alteration assemblages. Some copper occurs in quartz-magnetite-chlorite-pyrite-chalcopyrite veinlets in the core zone, but the bulk of the copper and the molybdenum are associated with the phyllic alteration as disseminated chalcopyrite and as molybdenite-sericite-carbonate veinlets. The advanced argillic cap has a quartz-alunite-dickite core, which is enveloped by an extensive pyrophyllite-diaspore-dickite-kaolinite assemblage that overlaps with the upper part of the phyllic alteration zone. Later quartz-sphalerite-galena-pyrite-chalcopyrite-gold-anhydrite±carbonate veins occur within and around the margins of the porphyry intrusion, and are associated with widespread illite-carbonate (argillic) alteration. Multiphase fluid inclusions in quartz stockwork veins associated with the potassic alteration trapped a highly saline (50–84 wt% NaCl equiv.) magmatic fluid at high temperatures (450 to >600 °C). These hypersaline brines were probably trapped at a pressure of about 300 bar, corresponding to a depth of 1.2 km under lithostatic conditions. This shallow depth is consistent with textures of the host dacite porphyry and reconstruction of the volcanic stratigraphy. Liquid-rich fluid inclusions in the quartz stockwork veins and quartz phenocrysts trapped a lower salinity (3–20 wt% NaCl equiv.), moderate temperature (300–400 °C) fluid that may have caused the phyllic alteration. Fluid inclusions in the quartz-sphalerite-galena-pyrite-chalcopyrite-gold-anhydrite±carbonate veins trapped dilute (1–3 wt% NaCl equiv.) fluids at 250 to 320 °C, at a minimum depth of 1.0 km under hydrostatic conditions. Oxygen isotopic compositions of the fluids that deposited the quartz stockwork veins fall within the 6 to 10‰ range of magmatic waters, whereas the quartz-sulphide-gold-anhydrite veins have lower δ18Owater values (–0.6 to 0.5‰), reflecting a local meteoric water (–6‰) influence. A δ18O versus δD plot shows a trend from magmatic water in the quartz stockwork veins to a near meteoric water composition in kaolinite from the advanced argillic alteration. Data points for pyrophyllite and the quartz-sulphide-gold-anhydrite veins lie about midway between the magmatic and meteoric water end-member compositions. The spatial association between porphyry Cu-Mo-Au mineralisation, advanced argillic alteration and quartz-sulphide-gold-anhydrite veins suggests that they are all genetically part of the same hydrothermal system. This is consistent with K-Ar dates of 11.6–10.7 Ma for the intrusive porphyry, for alunite in the advanced argillic alteration, and for sericite selvages from quartz-gold veins in the Thames district.

Shotgun deposit: granite porphyry-hosted gold-arsenic mineralization in southwestern Alaska, USA

The Shotgun deposit (30 tonnes gold; inferred resource using 0.5 ppm cutoff) consists of stockwork Au-As mineralization in a Late-Cretaceous (69.7±0.3 Ma, 40Ar/39Ar) granite porphyry stock located approximately 150 km north of Dillingham, SW Alaska. Preliminary metallurgical results indicate >90% gold is recoverable by cyanide leaching. With average estimated concentrations of 1.4 ppm Au, 6.1 ppm Ag, 0.5% As, 0.07% Cu, 40 ppm Mo, and 8 ppm Bi, Shotgun is a low-fO2 porphyry Au-As deposit that shares characteristics with both intrusion-related gold and porphyry copper deposits. The bulk of gold occurs in intense quartz stockwork and breccias. It is texturally identical to Cu-Mo porphyries, but has different mineralization and alteration assemblages. Shotgun contains arsenopyrite>>pyrite>pyrrhotite>chalcopyrite, lacks K alteration and, instead, contains vein and pervasive albite-quartz±sericite±carbonate. As temperature decreased, early high-temperature, low-fS2 mineralization (arsenopyrite-loellingite±pyrrhotite) evolved to later, lower-temperature, moderately high fS2 conditions (pyrite-bornite-chalcopyrite). In addition, ores at Shotgun contain primary native gold, native bismuth, Bi-Te sulfides, cubanite, maldonite, scheelite, and sphalerite, as well as supergene covellite, chalcocite, marcasite, and native copper. Arsenopyrite geothermometry yields deposition temperatures of 470 to 630 °C. Fluid inclusion microthermometry and laser Raman spectroscopy indicate that vapor-rich inclusions (H2O>>CO2>CH4) homogenize to vapor at ~360 °C, and saline aqueous inclusions contain 40 to >60 wt% NaCl equiv. (probably NaCl-KCl) and homogenize to liquid at 280 to >600 °C. Coexisting vapor- and liquid-rich fluid inclusions in quartz suggest 'boiling' occurred. Stable-isotope data include δ18O of 16.4 to 17.1‰ (quartz), δD of –124 to –105‰ (sericite), and δ34S of –5.5 to –5.0‰ (asp, cpy). At 380 °C, fluid compositions of δ18O of 10.4 to 11.1‰, and δD of –95 to –74‰ are compatible with a magmatic fluid source. Gold correlates fairly well (r>0.60) with Ag, Bi, Mo, and Te. When combined with the evidence for high-temperature fluids, the fragmental morphology of the intrusion, stockwork veining, and exsolution of vapor-rich and aqueous saline-rich fluid inclusions indicate a direct relationship between early mineralization and magmatism. The Shotgun deposit shares similarities with porphyry Cu-Mo-Au deposits (stockwork, breccia) and reduced, nonporphyry intrusion-related Au-As-Bi deposits (sulfide mineralogy, geochemistry, and oxidation state). They appear to be products of the same style of magmatic-hydrothermal systems and differ only in depth and thus volatile-release history. The Shotgun deposit is essentially an intrusion-related Au-As-Bi deposit emplaced at pressures and temperatures typical of a porphyry system.

Corundum and zincian spinel from the Kakkonda geothermal system, Iwate Prefecture, northeastern Japan

Corundum and zincian spinel were found in a drill core of thermally metamorphosed rock at 2843 m in depth of the deep research well, WD-1, in the Kakkonda geothermal area, northeastern Japan. Corundum is surrounded by K-feldspar, and the occurrence suggests that corundum was formed by the decomposition of muscovite. Andalusite occurs in the same sample, without sillimanite. The petrography and previous works on phase relations between aluminous minerals suggest that corundum was formed at 600-700°C and 100 MPa at the peak of contact metamorphism. The ZnO content of spinel exceed 20 wt.%, and has a gahnitic composition. Zincian spinel is surrounded by K-feldspar as well as corundum, and it is assumed that spinel was formed by a similar reaction to the case of corundum. Two-phase vapor-rich and liquid-rich fluid inclusions occur in corundum, and they probably represent fluids formed at the metamorphic stage. A laser Raman microprobe analysis detected the presence of CO2, H2S and N2 in the fluid inclusions. The occurrence and analyses of the fluid inclusions indicate that the fluid produced during the thermal metamorphism at Kakkonda was low-salinity with gases and water.

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.

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.

MINERAL AND FLUID EQUILIBRIA IN Mo-BEARING SKARN AT THE ZENITH DEPOSIT, SOUTHWESTERN GRENVILLE PROVINCE, RENFREW AREA, ONTARIO, CANADA

Late-tectonic pegmatitic, aplitic and graphic granites intrude metasedimentary units of the Grenville Supergroup, Composite Arc Belt, in the Renfrew area of southeastern Ontario. Zoned molybdenite-bearing skarns occur at the contact of graphic pegmatites with mixed calcite–dolomite marble, para-amphibolite and gneiss at the Zenith molybdenite deposit. Distal skarns also are found, in the form of discordant veins within foliated marble. Mineral zoning in the skarn consists of diopside–microcline endoskarn plus exoskarn that grades, away from the intrusion, from a diopside-dominant to a tremolite-dominant metasomatic rock. Textural evidence indicates that tremolite was an earlier-formed skarn mineral in the diopside zone. Diopside is absent in the tremolite zone. These patterns are repeated in other similar occurrences of skarn in the area. Apatite grains formed during metasomatism contain primary high-salinity (≥30 wt.% NaCl eq.) liquid-rich fluid inclusions coexisting, along growth zones, with vapor-rich inclusions, indicative of the trapping of an immiscible fluid. The observed parageneses are interpreted to reflect metasomatic replacement of marble by infiltration of a silica-rich orthomagmatic fluid that evolved by the crystallization of the granitic pegmatite, at approximately 600°C and 300 MPa. Such conditions are not inconsistent with phase separation of the aqueous fluid, because the salinity of the fluid is known to drastically raise the critical temperature of the H 2 O–CO 2 miscibility gap. A decrease in temperature or in the chemical potential of silica in skarn furthest from the granites (or both) may account for formation of the tremolite zone. The Mo likely was transported in the metasomatic fluid as alkali oxyacidic complexes. A decrease in f (O 2 ) along with reduction of magmatic sulfur upon interaction of the fluid with graphite-bearing marble are inferred to have triggered the precipitation of molybdenite.

Controls of alteration and mineralization in the Sungun porphyry copper deposit, Iran; evidence from fluid inclusions and stable isotopes

The Sungun porphyry copper deposit is located in northwestern Iran (Azarbaijan province) and is associated with diorite-granodiorite to quartz monzonite of Miocene age which intruded Eocene volcano-sedimentary and Cretaceous carbonate rocks. Copper mineralization was accompanied by both potassic and phyllic alteration. Field observations and petrographic studies demonstrate that emplacement of the Sungun stock took place in several intrusive pulses, each with associated hydrothermal activity. Molybdenum was concentrated at a very early stage in the evolution of the hydrothermal system and copper somewhat later. Four main vein groups have been identified: (I) quartz + molybdenite + anhydrite + or - K feldspar with minor pyrite, chalcopyrite, and bornite; (II) quartz + chalcopyrite + pyrite + or - molybdenite; (III) quartz + pyrite + calcite + or - chalcopyrite + or - anhydrite (gypsum) + or - molybdenite; and (IV) quartz + or - calcite + or - gypsum + or - pyrite. Early hydrothermal alteration produced a potassic assemblage (orthoclase-biotite) in the central part of the stock, propylitic alteration occurred contemporaneously with potassic alteration, but in the peripheral parts of the stock, and phyllic alteration occurred later, overprinting the earlier alteration. The early hydrothermal fluids are represented by high-temperature (340 degrees - 500 degrees C), high-salinity (up to 60 wt % NaCl equiv) liquid-rich fluid inclusions, and high-temperature (320 degrees -550 degrees C), low-salinity, vapor-rich inclusions. These fluids are interpreted to represent an orthomagmatic fluid which boiled episodically; the brines are interpreted to have caused potassic alteration and deposition of group I and II quartz veins containing molybdenite and chalcopyrite. Propylitic alteration is attributed to a liquid-rich, lower temperature (240 degrees -330 degrees C), Ca-rich, evolved meteoric fluid. Influx of meteoric water into the central part of the system and mixing with magmatic fluid produced deep albitization (transition alteration) and shallow phyllic alteration. This influx also caused dissolution of early formed copper sulfides and remobilization of Cu into the sericitic zone where it was redeposited in response to a boiling-induced decrease in temperature. Supergene alteration was minor and restricted to a thin blanket of Cu sulfides below an argillic cap.

The formation and distribution of CO 2-enriched fluid inclusions in epithermal environments

Fluid inclusions from two geothermal systems associated with volcanic activity were studied to document the distribution of CO 2 in modern epithermal environments. The fluid inclusion data, combined with mineral distributions and chemical analyses of the production fluids from both systems, provide a record of steam and gas flux to depths in excess of 1 to 2 km and of transient variations in the gas contents of the reservoirs. The liquid-rich fluid inclusions can be grouped into two types on the basis of their CO 2 contents. Inclusions with CO 2 contents of less than about 4 wt% typically have calculated gas contents that are higher than the present-day reservoir fluids. However, the calculated pressures and temperatures of these inclusions are consistent with their depth of formation, indicating that they may have formed in response to boiling and mixing processes in the reservoir. Liquid-rich fluid inclusions with CO 2 contents between approximately 4 and 6 wt% are characterized by CO 2 clathrate dissociation temperatures greater than 0.0°C. These inclusions occur on the margins of the thermal systems where they define umbrella-shaped caps around the main zones of upwelling. The CO 2 contents of the inclusions require that they formed at pressures several tens of bars above hydrostatic. Elevated pressures and gas contents may have developed through compression and condensation of CO 2- enriched steam by tectonic stress.

Geochemical and temperature controls on ore mineralization at the Emperor gold mine, Vatukoula, Fiji

Emperor is a large epithermal gold-tellurium deposit which occurs at the margin of the Tavua caldera and is associated with high-level mineralized tuffs, breccias and silicious sinters. The deposit consists of mainly narrow, steep “shear” and shallow dipping “flatmake” mineralized vein structures. Mineralization spanned a period during which different veins and dyke intrusions occurred so that some early structures are offset by later ones. High Au values and high Au/Ag ratios, corresponding to the intersection of shear and flatmake structures, generally correlate with high Ag, Te and Ba values in some structures (Crown Shear) and high Hg in others (166 Flatmake). Other elements commonly anomalous in the high-grade ore zones are As, Mo and Cu. Thallium, Se and Sb are commonly high below ore zones while Hg may be high above (e.g., Crown Shear), in (166 Flatmake) or even below (e.g., Prince of Wales Shear) ore zones. Gold precipitated in a temperature interval of 180–210°C; higher temperatures commonly occur at depth in some flatmakes (166 N, 608) and lower temperatures nearer the surface. In the two steep shears analyzed, higher temperatures correspond to where they are intersected by flatmakes. Temperatures may decrease below such intersections in which case Hg, Sb, etc., commonly increase with depth. Evidence of boiling, characterized by coexisting gas- and liquid-rich fluid inclusions, is minor and spatially sporadic. Quartz and carbonate found in ore containing bonanza concentrations of Au contain few to no gas-rich fluid inclusions. The intensity of wall-rock alteration is unrelated to gold grades. Ore deposition is inferred to have been caused mainly in response to fluid mixing rather than extensive boiling and/or wall-rock alteration, although these processes occurred. The source of the Au, Ag, Te, etc. is inferred to be near a neutral, relatively reduced, bisulphide-rich, ore solution at near 300°C and derived in some way from the shoshonitic volcanics or associated monzonitic intrusions. The second non-ore solution necessary for mixing, is inferred to have been an acid, oxidized solution at ∼150°C and having a large meteoric component. The result of fluid mixing was ore genesis, and a telluride, minor sulphide and very minor sulphate (barite) and anhydrite ore assemblage.

Relationships between a porphyry Cu-Mo deposit, base and precious metal veins and Laramide intrusions, Mineral Park, Arizona

In the Wallapai mining district, porphyry-style copper-molybdenum mineralization occurs within and near Laramide granitoid stocks at the center of an elongate zone of polymetallic quartz veins. The principal mineralized veins form a well-defined paragenetic sequence, with a simplified mineralogy progressing through: (1) anhydrite-molybdenite (AM) veins of quartz + K-feldspar + anhydrite + pyrite + biotite + molybdenite, (2) quartz-molybdenite (QM) veins with quartz + molybdenite + pyrite, (3) anhydrite-chalcopyrite (AC) veins containing quartz + K-feldspar + anhydrite + pyrite + magnetite + chalcopyrite + chlorite + rutile, (4) quartz-pyrite (QP) veins of quartz + sericite + pyrite, and quartz + pyrite + chalcopyrite + epidote + chlorite, and (5) polymetallic quartz (PMQ) veins, with a gangue assemblage of quartz + sericite + calcite + kaolinite(?) + epidote + chlorite, and a variety of base and precious metal minerals.The coexistence of vapor- and liquid-rich fluid inclusions in vein quartz shows that the molybdenite-bearing anhydrite and quartz veins formed at 360 degrees to 410 degrees C from boiling, low-salinity brines. Chalcopyrite deposition in the anhydrite-chalcopyrite veins was at 380 degrees to 420 degrees C from nonboiling brines averaging 20 equiv wt percent NaCl and 25 equiv wt percent KCl. The quartz-pyrite veins formed at 320 degrees to 350 degrees C from brines of 1 to 13 equiv wt percent NaCl. The polymetallic quartz veins formed from dilute brines of 1 to 7 equiv wt percent NaCl. Temperatures in the first stage of mineralization of the polymetallic-quartz veins abruptly increased to 400 degrees to 450 degrees C in and near Mineral Park, but declined outward to less than 300 degrees C on the district periphery. In subsequent stages, fluid temperatures decreased across the district to less than 200 degrees C. Pressures were 250 to 300 bars, probably in a hydrostatic pressure regime, during boiling related to molybdenite mineralization, but they may have increased to about 1,100 bars lithostatic pressure during chalcopyrite deposition in the anhydrite-chalcopyrite veins. A depth during mineralization of 3 to 4 km is suggested.Copper and molybdenum mineralization occurred in a lithocap environment above a progenitor intrusion but was not directly related to the exposed Ithaca Peak stocks. The evolution from hypersaline fluids in the anhydrite-chalcopyrite veins to low-salinity, lower temperature fluids in the quartz-pyrite veins suggests an influx of meteoric water into the waning porphyry-style hydrothermal system. Brines up to 100 degrees C hotter appeared during the formation of the paragenetically later polymetallic quartz veins and constituted a new, and much larger, hydrothermal system. The heat source, presumably an unexposed intrusion, was much larger than the porphyry-style system. The zonation in base metal ratios of polymetallic quartz veins is symmetrical with respect to the proposed heat source.

Effects of glacial ice on subsurface temperatures of hydrothermal systems in Yellowstone National Park, Wyoming: Fluid-inclusion evidence

Hydrothermal quartz and fluorite crystals containing liquid-rich fluid inclusions (coexisting vapor-rich fluid inclusions were not observed) were found in drill cores from eight relatively shallow research holes drilled by the US Geological Survey in and near major geyser basins of Yellowstone National Park. Homogenization temperatures (T{sub h}) for mostly secondary fluid inclusions show variations in temperature that have occurred at give depths since precipitation of the host minerals. Within major hydrothermal upflow zones, fluid-inclusion T{sub h} values all were found to be equal to or higher (commonly 20-50 C and up to 155 C higher) than present temperatures at the depths sampled. During periods when thick glacial ice covered the Yellowstone National Park region, pore-fluid pressures in the underlying rock were increased in proportion to the weight of the overlying column of ice. Accordingly, theoretical reference boiling-point curves that reflect the maximum temperature attainable in a hot-water geothermal system at a given depth were elevated, and temperatures within zones of major hydrothermal upflow (drill holes Y-2, Y-3, Y-6, Y-11, Y-13, and upper part of Y-5) increased. The thicknesses of ice required to elevate boiling-point curves sufficiently to account for the observed fluid-inclusion T{sub h} values are within the ranges estimatedmore » by glacial geologic studies. At the margins of major hydrothermal upflow zones (drill holes Y-4 and Y-9), fluid-inclusion T{sub h} values at given depths range from 57 C lower to about the same as the current temperature measurements because of a previous decrease in the rate of discharge of warm water and/or an increase in the rate of recharge of cold water into the hydrothermal system.« less