High-salinity Fluid Inclusions Research Articles

Hydrothermal alteration zonation and fluid chemistry of the Southern Ridge and North deposits at Mt Tom Price

The Mt Tom Price deposit is a world-class high-grade hematite deposit in the Hamersley Province of Western Australia with an original resource of 900 Mt of almost pure hematite, averaging 63·9 wt-%Fe. Petrological and geochemical studies at both the Southern Ridge and the North deposit at Mt Tom Price have identified three hypogene alteration zones between unmineralised banded iron formation (BIF) and high-grade iron ore: distal magnetite-siderite-stilpnomelane; intermediate hematite-magnetite-ankerite-talc-chlorite; and proximal martite-microplaty hematite-magnetite-apatite alteration zones. Fluid inclusions trapped in siderite within the distal magnetite-siderite-stilpnomelane alteration zone at the North deposit revealed primary high salinity (25·5 eq.wt-%NaCl–CaCl2) inclusions that homogenised between 107 and 142°C into liquid. Fluid inclusions trapped in ankerite within ankerite-microplaty hematite veins in the intermediate hematite-ankerite-magnetite-talc-chlorite alteration zone at the North deposit revealed mostly H2O–CaCl2 pseudosecondary and secondary inclusions with salinities of 22·4–25·4 equivalent wt-%CaCl2 and 22·9–25·9 equivalent wt-%CaCl2 respectively. Pseudosecondary inclusions homogenised between 153 and 449°C and secondary inclusions homogenised between 103 and 157°C. Fluid inclusions trapped in apatite within talc-microplaty hematite veins in the intermediate hematite-magnetite-talc-chlorite-apatite alteration zone at Southern Ridge revealed that primary, medium salinity (7·0–9·0 equivalent wt-%NaCl) inclusions homogenised between 181 and 257°C and primary high salinity (22·8–25·9 equivalent wt-%CaCl2) fluid inclusions homogenised between 118 and 257°C into liquid. Microthermometric analysis of quartz from quartz-hematite veins from the Southern Batter Fault, Southern Ridge shows a complex fluid inclusion history. Primary fluid inclusions consist of: low and high salinity H2O–NaCI inclusions trapped at temperatures of approximately 140–230°C; vapour-rich inclusions of unknown compositions; and 'complex' salt-rich (Ca, Mg, K and Na inclusions). Secondary inclusions consist of medium-salinity fluid inclusions trapped at temperatures of about 140–280°C. A two stage hydrothermal model is proposed for the formation of both the Southern Ridge and North deposits. Early stage 1a hypogene alteration involved the release of hydrothermal NaCl-CaCl2-rich (25·5 equivalent wt-%) basinal brines (110–150°C) from the underlying Wittenoom Formation and directed upward along normal faults and focused within the silica-rich rocks of the Dales Gorge Member, by the shales of the underlying Mt McRae Shale Member and overlying Whaleback Formation. Within the Dales Gorge Member hydrothermal basinal brines migrated laterally within large-scale folds with permeability controlled by shale bands and the NW trending dolerite dyke sets. Fluid rock reactions transformed unmineralised BIF to magnetite-siderite-iron silicate BIF, with subsequent desilification of the chert bands. Stage 1b hypogene involved an increase in temperature of the hydrothermal, CaCl2-rich saline (24 equivalent wt-%) basinal brines (250–300°C) resulting in formation of hematite-ankerite-magnetite- talc-chlorite alteration and the crystallisation of microplaty hematite. Late stage 1c hypogene alteration involved the interaction of low-temperature (∼120°C) basinal brines with the hematite-ankerite-magnetite mineral assemblage. At the Southern Ridge deposit this process was very intense (i.e. with a high fluid flux), therefore resulting in the almost total removal of ankerite and resulting in the increased porosity of the ore. Stage 2 supergene enrichment, during the Tertiary, resulted in the removal of residual ankerite and apatite and the weathering of the shale bands to clay.

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

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

High salinity fluid inclusions in the Yinshan polymetallic deposit from the Le–De metallogenic belt in Jiangxi Province, China: Their origin and implications for ore genesis

A fluid inclusion investigation of the polymetallic mineralization at Yinshan from the Le–De metallogenic belt in Jiangxi Province of China has been carried out using petrographic and microthermometric techniques. The data obtained here indicate that three major types of fluids were involved during the formation of the deposit. They are type I vapor-rich, type II liquid-rich and type III halite-bearing inclusions within the H 2O–NaCl system. The high salinity fluids represented by type III inclusions, being unusual to the distal part of an intrusion-centered ore-forming system such as Yinshan, have been interpreted as the product of direct exsolution of a crystallizing magma, rather than a result of fluid immiscibility from a low salinity fluid. Evidence used to support such an interpretation includes the mode of homogenization of type III inclusions exclusively via halite dissolution, spatial separation of type I and type III inclusions on microscopic scale, the consistent phase ratios within the inclusions concerned, and considerable deviation in homogenization temperature for both type I and type III inclusions. Trapping conditions for type I inclusions were estimated to be around 440 °C and 260 bars, while type III inclusions were constrained to be trapped at least above 900 bars and > 500 °C. The formation temperatures for type II inclusions range from 270 to 390 °C if a lithostatic pressure of 260 bars is assumed. Pressure fluctuation determined by this fluid inclusion study coupled with decreases in salinity and temperature as result of the potential fluid mixing are supposed to have played an important role in triggering the precipitation of ore minerals from the hydrothermal solution.

Geologic Evolution of the Caylloma Epithermal Vein District, Southern Peru

The silver- and base metal-rich Caylloma epithermal district in the Tertiary volcanic belt of southern Peru has been worked intermittently since the Incaic period and has produced over 100 million ounces (Moz) of Ag. Intermediate-sulfidation mineralization is present in veins hosted by Miocene andesitic volcanic and volcaniclastic rocks, with minor ore in underlying folded Jurassic sedimentary basement. New 40 Ar/ 39 Ar dates give a host rock age of 20.30 ± 0.11 Ma (andesitic volcanic matrix), a hydrothermal alteration age of 18.35 ± 0.17 Ma (adularia in vein wall rock), and postmineralization ages of 11.8 ± 0.8 and 12.25 ± 0.07 Ma (sanidine and biotite from a rhyolite dome). Gangue minerals include quartz, calcite, rhodonite, rhodochrosite, pyrite and minor adularia, illite, barite, and helvite. Ore minerals include sphalerite, galena, chalcopyrite, and tetrahedrite. Hydrothermal alteration is pervasive in lava flows but weak and localized near veins in volcaniclastic rocks. Hydrothermal alteration types include silicification (quartz-adularia and quartz-illite) and propylitization (chlorite + calcite ± illite). Banded veins show four stages of mineral precipitation: (1) early sugary quartz, chalcedony, pyrite; (2) manganese minerals; (3) quartz + sulfides; and (4) late calcite + quartz. Cyclic bands in the manganese stage (early sulfides, coarse- to medium-grained quartz, late rhodonite + calcite + chalcedony) are a few millimeters to 5 cm thick and form ore bands up to 1 m thick. Veins occupy dextral normal faults (020°–050°, 45°–70° SE) and extension fractures (060°–090°, 70°–90° SE) and are 1 to 25 m wide and as much as several kilometers long. The veins display complex and multi-episodic filling with textures characteristic of open-space precipitation such as crustiform banding, symmetric banding, vugs, breccias, and cockade and comb textures. We present a structural model in which the principal veins formed in subparallel northeast-striking dextral-normal faults and related extension fractures within a northwest-striking structural corridor bounded by sinistral regional faults; second- and third-order veins formed by bookshelf sliding related to movement on higher-order, dominantly strike-slip faults. Slickenlines have low rakes (20°–50° SW), and fault-kinematic analysis suggests the extension axis was oriented 4°/330° and the shortening axis 55°/234°, in agreement with the strain field commonly proposed for the early-middle Miocene of southern Peru. Ore grade is discontinuous, with high-grade ore zones having strike lengths of tens to hundreds of meters and extending about 300 m downdip. Ore shoots in fault-hosted veins are narrow subvertical zones a few tens of meters wide (measured in the plane of the vein), separated by lower-grade ore zones. Ore shoots in extension fractures are shorter downdip and horizontally more continuous. Some ore shoots occur as half-cymoids that form along the main vein only in the hanging wall of synmineral transverse faults. Mineral composition and metal content are zoned horizontally and vertically; silver, base metals, and calcite and manganese minerals increase to the northeast and downward, and sulfide content increases gradually with depth. Shallower levels of the hydrothermal system are exposed to the west and southwest in the district. Evidence from sulfur isotopes and high-salinity fluid inclusions is consistent with a model in which magmatic fluids played a role in the hydrothermal system, introduced through channels of enhanced structural permeability. The cyclic nature of mineral precipitation with abrupt changes in mineralogy, textures, and fluid composition leads us to postulate that cyclic injections of magma-derived hydrothermal fluids occurred. Economic mineralization is estimated to have formed at a depth of about 650 m below the paleowater table, with a temperature of formation of ~270°C.

Fluids from the Neves Corvo massive sulphide ores, Iberian Pyrite Belt, Portugal

The Neves Corvo Cu–Sn–(Zn) mine is probably the richest VMS deposit in the world. The research was conducted in order to know if pristine volcanogenic fluids are present in the massive sulphide ores and proceeds to their characterization and interpretation. The samples were collected inside thick massive sulphide bodies that are undoubtedly of primary origin. Petrographic relations between quartz and sulphide phases (mainly pyrite + chalcopyrite) suggest that the quartz and the sulphides were formed simultaneously. Three types of fluid inclusions were observed: 1 – low salinity aqueous fluid inclusions either isolated, in clusters, or along trails; 2 – low salinity H 2O–CO 2–CH 4– NaCl fluid inclusions in intergranular arrays or intracrystalline trails (some in growth zones), some of which form clathrates on freezing; and 3 – (only in one sample) high-salinity fluid inclusions with halite, without any regular distribution in the crystals. The aqueous low salinity fluid is the most abundant, consisting of modified seawater resulted from convection through the basement. Homogenization temperatures lie between 140 and 370 °C. This wide range of Th is typical of VMS deposits from IPB and reflects the evolution of the hydrothermal system with time. Salinity ranges from 3 to 6 wt.% equiv. NaCl, except for a small group of the high-salinity fluid inclusions, which have around 30 wt.% eq. NaCl. These high-salinity fluids are interpreted as brines resulting from sporadic deep subsea-floor boiling that could originate either from seawater or from a magmatic fluid. The carbon dioxide and methane could have been derived from reactions with organic matter enclosed in the metasediments underlying the ores. However, a magmatic origin cannot be precluded.

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.

Significance of Alteration Assemblages for the Origin andEvolution of the Proterozoic Nabarlek Unconformity-Related Uranium Deposit,Northern Territory, Australia

The Proterozoic Nabarlek unconformity-related uranium deposit in the Alligator Rivers uranium field is hosted by Paleoproterozoic amphibolite-grade, metamorphosed semipelitic sedimentary rocks and amphibolite schist. High-grade ore is confined to the Nabarlek fault, a reverse fault/shear zone that crosscuts a series of interbedded muscovite-quartz-biotite schists and amphibolite. Petrographic studies on polished thin sections combined with electron microprobe analyses, X-ray diffraction, fluid inclusion data, O-H and U-Pb isotope values, as well as 40 Ar/ 39 Ar dating have identified up to eight significant fluid events beginning with the precipitation of early quartz veins during uplift of the Myra Falls Metamorphics at 1830 Ma and ending with limited uraninite recrystallization during reactivation of the Nabarlek fault between 1380 and 750 Ma. Quartz veins that likely formed toward the end of the Top End orogeny represent the earliest recorded fluid event. Fluid inclusion data and δ 18 O and δ D values indicate that these veins formed from basement-derived fluids that may have been heated by a cooling Nabarlek Granite during circulation through reverse faults/shear zones. The next fluid event, represented by fine-grained sericite and chlorite occurred when fluids passed into these faults and altered the metamorphic minerals following the exhumation of the basement and deposition of the Kombolgie Subgroup at ca. 1760 Ma. The intrusion of the Oenpelli Dolerite at ca. 1720 Ma resulted in the local remobilization of silica and the precipitation of quartz associated with minor pyrite and dolomite around the reverse faults. Uranium mineralization is associated with an inner and outer alteration halo that extends as far as 1 km from the Nabarlek fault. Alteration in the outer halo began as early as 1700 Ma and is dominated by chlorite and sericite, which formed when a 200°C fluid flowed into the Nabarlek fault from the overlying Kombolgie Subgroup. U-Pb and 207 Pb/ 206 Pb dating reveals that massive uraninite precipitated at ca. 1640 Ma and formed together with illite and hematite at ca. 200°C. Chlorite was not coeval with uraninite precipitation. Stable isotope values indicate that the pre- and synore alteration assemblage formed from basinal brines with δ 18 O fluid and δ D fluid values of 2 ± 2 and –25 ± 10 per mil, respectively. Reactivation of the Nabarlek fault at ca. 1360, 1100, and 900 Ma is indicated by U-Pb and 207 Pb/ 206 Pb dating of uraninite. These ages correlate with the intrusion of the Maningkorrirr phonolitic dikes and the Derim Derim Dolerite at ca. 1316 ± 40 and 1324 ± 4 Ma, respectively, the amalgamation of Australia and Laurentia during the Grenville orogen at ca. 1140 Ma, and the breakup of Rodinia between 1000 and 750 Ma. Fluid incursions associated with these events precipitated much of the chlorite that has previously been related to uraninite precipitation. Drusy quartz veins that host high-salinity fluid inclusions and sulfides, particularly galena, also formed after the initial uraninite-forming event. Finally, erosion of the Kombolgie Subgroup and subsequent weathering of the deposit resulted in the recent formation of kaolinite and numerous secondary uranium minerals. These data constrain individual events more precisely than previous studies and thus advance the current genetic model to a level that takes into account the multiple stages of fluid overprinting that occurred over a period of at least 800 m.y.

Origins, closed system formation and preservation of calcites in glaciated crystalline bedrock: evidence from the Palmottu natural analogue site, Finland

A study of the geochemistry of fracture filling calcite from the Palmottu research site, a U–Th deposit located about 100 km NW of Helsinki, Finland, was part of a comprehensive natural analogue research project considering the migration of U in the crystalline bedrock environment. Fracture filling minerals act as records of the paleohydrological and chemical conditions and therefore provide insight into magmatic processes, ore deposits and long-term stability of the crystalline bedrock environment in relation to deep disposal of radioactive waste. An earlier investigation of calcite and associated U minerals identified a recent origin for these fracture infillings. Uranium series disequilibrium data indicated that relatively recent, glacial water had penetrated to a depth of 62 m. A fluid inclusion study, combined with the isotope geochemistry of several generations of fracture calcite, was undertaken to further study this aspect and to better understand the thermal and fluid history in the crystalline rock environment in general. The study revealed that at least 3 fluids were recorded by fracture calcites: 1) A crystalline calcite precipitated at 139–238 °C from a low salinity, Na–Cl fluid of magmatic or metamorphic origin, 2) A massive calcite (with high salinity fluid inclusions) precipitated at 136–141 °C from a high salinity, Ca–Na–Cl fluid of magmatic or metamorphic origin and 3) A massive calcite (with low salinity fluid inclusions) precipitated at 43–286 °C from a low salinity, Na–Cl fluid of magmatic or metamorphic origin that underwent equilibrium fractionation during cooling in a rock dominated system. The preservation of high temperature infillings and the cooling trend, indicating isotopic re-equilibration of water due to a very low water/rock ratio, demonstrate that the downward percolation of geologically recent waters is limited, despite the reworking of highly soluble U compounds in the upper tens of metres of the bedrock that was shown by U series disequilibrium studies. The veins in which these calcites occur have not experienced a later phase of fluid activity that dissolved or altered the calcites. Therefore, it is likely that they have not been reactivated as transmissive fractures since that initial hydrothermal episode, even during the Quaternary history in which the formation was subjected to ice sheet loading and unloading.

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.

A fluid inclusion reconnaissance study of the Huanuni tin deposit (Bolivia), using LA-ICP-MS micro-analysis

Fluid inclusions have been studied in selected quartz-cassiterite veins of the Huanuni tin deposit, Bolivia. Fluid inclusion assemblages were found in quartz that bracket at least one stage of cassiterite deposition, and some were found in quartz crystallised during a later sulphide mineralisation stage. All fluid inclusions consisted of two phases only (liquid+vapour), which homogenised to a liquid upon heating and were classified to be of pseudosecondary or secondary in origin. Salinities in fluid inclusions related to the cassiterite deposition were up to 22 wt% NaCl equiv., and homogenisation temperatures of low and high salinity inclusions overlap in the range of 370 to 390 °C. Laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to determine element concentrations in selected fluid inclusions. The high salinity fluid inclusions show elevated contents of Li, B, Na, K, Mn, Fe, Zn, Ge, As, Rb, Sn, Sb, Cs, La and Pb compared with the lower salinity samples (wt% NaCl equiv. lower than 6%). High concentrations of incompatible lithophile elements including Li, B and Cs are characteristic for fluids derived from highly fractionated granitic systems. Fe, Zn, Pb, and Ge are the most important heavy metals transported with tin in the aqueous fluid. Sn, Fe, Zn, Pb are probably transported as chloride complexes, and the initial fluid-chemical data for successive fluid-inclusion generations indicates that SnO2 precipitation occurred by injection of a hot saline magmatic fluid into heated low-salinity waters of possible meteoric origin.

Drilling fluid system containing a combination of hydrophilic carbon black/asphaltite and a refined fish oil/glycol mixture and related methods

This investigation presents and interprets fluid inclusion data from different lithological units of the Cu skarn deposits at Mazraeh, north of Ahar, Azarbaijan, NW Iran. The results provide an assessment of the P–T conditions and mineral–fluid evolution and suggest new exploration parameters. Five types of inclusions are recognized from quartz and garnet. The temperature of homogenization of Type I inclusions with daughter minerals halite and sylvite ranges from 312° to 470 °C with total salinity of 52 to 63 wt.% NaCl equiv.; Type II and III inclusions with halite have homogenization temperatures of 230° to 520 °C and salinity of 31 to 50 wt.% NaCl equiv. The salinity of Types IV and V biphase (liquid + vapor) inclusions, based on their final ice melting temperature, varies between 10.2 to 20.8 wt.% NaCl equiv. Th vs. salinity plots of inclusions show that the salinity of the fluids correlates positively with temperature. The inclusions formed at low pressure. Changes in the temperature and salinity of the fluids can be reconstructed from the inclusions. Highly saline, high-temperature fluids were most abundant during the main chalcopyrite ore-forming phase in the skarn and mineralized quartz veins. Low-salinity aqueous fluids were abundant in barren veins, in which there is no evidence for early hot high-salinity brine, and might have resulted from late-stage dilution and mixing of hydrothermal fluids with meteoric water. Based on petrographic features and fluid-inclusion data, early-stage magnetite deposition is related to boiling of fluid at temperatures of about 500 °C. At a later stage, boiling at temperatures of around 320° to 400 °C favored the deposition of sulfides and Fe mobility was decreased at these lower temperatures. The following inclusion characteristics may be used as exploration parameters in the Mazraeh area. (i) Presence of high-temperature, salt-bearing inclusions, with Th between 300 and 500 °C; (ii) High-salinity fluid inclusions; and (iii) Inclusions showing evidence of boiling of the fluid. In addition, the presence of magnetite is an important exploration parameter.

Base Metal Skarns and Au Occurrences in the Southern GaspeAppalachians: Distal Products of a Faulted and DisplacedMagmatic-Hydrothermal System along the Grand Pabos-Restigouche Fault System

In the southern Gaspe Appalachians, base metal and Au occurrences are spatially associated with the Acadian dextral strike-slip Grand Pabos and Restigouche faults. Mineralization occurs as veins, replacement zones, and disseminations along the main structures and subsidiary faults with orientations compatible with the dextral strike-slip kinematics along the main faults. Limestone of the Upper Ordovician-Lower Silurian White Head Formation hosts numerous base metal skarns and Au occurrences that are surrounded by halos of argillaceous alteration. The Reboul prospect is a type example of base metal skarns and consists of three types of mineralization: (1) early strata-bound disseminated Cu-Zn-Ag associated with marble-hornfels and skarn, (2) northeast- and northwest-trending (Cu-Zn-Pb-Ag-(Au) and Ag-Cu-Zn-Pb-(Au-Sb)) veins, parallel and perpendicular to the regional folding, and (3) late Ag-Au-Zn-Pb mineralized zones in quartz breccia veins parallel to the Grand Pabos fault. Crosscutting relationships, fluid inclusion microthermometry, mineralogy of the three types of mineralization, and alteration patterns were used to define the timing of mineralizing processes which are (1) prograde metamorphism, (2) metasomatism and hydrothermal alteration with high-temperature skarn formation and Cu-Zn-Ag mineralization, (3) retrograde alteration with Cu-Zn-Pb-Ag-(Au) and Ag-Cu-Zn-Pb-(Au-Sb) veins, and (4) late Au precipitation at low temperature. The Saint-Andre-de-Ristigouche prospect is the best example of Au mineralization spatially associated with a base metal skarn in the area. It consists of Au-Sb-As extensional veins and stockwork hosted by limestones. Felsic dikes associated with base metal skarns and a magmatic-hydrothermal breccia at the Reboul prospect are evidence for magmatic activity coincident with the mineralizing events. Argillaceous alteration, high-temperature and high-salinity fluid inclusions, and the Pb isotope compositions of galena are compatible with a model linking the skarns and Au occurrence to Early to Middle Devonian magmatic activity at depth along the Grand Pabos and Restigouche faults. A palinspastic pre-Acadian strike-slip reconstruction of the southern Gaspe Appalachians shows that all mineral occurrences hosted by the White Head Formation belong to the same mineralized area that might represent distal components of a magmatic-hydrothermal system at depth.

Salt-rich aqueous fluids formed during eclogitization of metabasites in the Alpine continental crust (Austroalpine Mt. Emilius unit, Italian western Alps)

The metabasite bodies of the Mt. Emilius continental unit (western Italian Alps) underwent a stage of Alpine eclogite-facies metamorphism (1.1–1.3 GPa and 450–550°C) accompanied by polyphase deformation and recrystallization. The metabasites consist of two main rock types: (1) eclogites (omphacite+garnet+glaucophane+epidote+phengite/paragonite) preserving no relics of their precursors; (2) eclogitized granulites, i.e. rocks whose incomplete eclogitic recrystallization (clinopyroxene II+garnet II+epidote+amphibole+chlorite) allowed survival of textural and mineralogical relics of pre-Alpine granulitic assemblages (clinopyroxene I+garnet I+plagioclase+amphibole). In this latter case the pre-Alpine granulites were converted to eclogites as the result of infiltration of aqueous fluids during eclogitization. In both eclogites and eclogitized granulites hydrated high-pressure foliations are cut by eclogitic metamorphic veins. The bulk rock chemistry of the metabasites influenced the compositions of both the vein- and rock-forming clinopyroxenes and the compositional correlation between the vein- and rock-forming clinopyroxenes indicates that the syn-eclogitic fluids have re-equilibrated with the metabasite hosts. The predominant vein minerals (omphacite, epidote and garnet) contain primary high-salinity fluid inclusions. The fluids consist of two-phase (liquid+vapour) and of multiphase (liquid+vapour+salt+additional quartz) salty aqueous inclusions containing NaCl, CaCl 2 and MgCl 2 as the main chloride species. The vein inclusions show a salinity range from 17 to 45 wt.% salts in eclogites and from 20 to 50 wt.% salts in eclogitized granulites. In contrast, fluid inclusions in matrix minerals of the eclogitized granulites contain primary two-phase inclusions displaying a salinity range between 10 and 25 wt.% salts. The marked difference in fluid salinities recorded by the inclusions in the eclogitic veins and those in the partially re-equilibrated eclogitized granulites is interpreted in terms of progressive hydration during eclogitization of granulite-facies rocks, which caused an increase in the salt content of the residual inclusion fluids.

Micro-Beam XAFS Measurements of Zinc Complexes in Highly Saline Fluid Inclusions to 400°C Before and After Experimental Re-equilibration at High Hydrogen Pressure

Our current understanding of metal speciation in natural hydrothermai systems is based largely on experimental data derived from relatively simple H20-salt systems. However, complexing in natural multicomponent systems may be influenced by additional fluid components that are not considered in this experimental data base. Direct spectroscopic analyses of aqueous species in fluid inclusions at elevated temperatures may provide the most accurate representation of metal complexing in palaeohydrothermal solutions. However, it has been suggested that, in some cases, aqueous species in fluid inclusions may be influenced by the loss or addition of hydrogen by re-equilibration with an ambient fluid reservior (Mavrogenes and Bodnar, 1994; Sterner et al., 1995). In this study, the possible influence of hydrogen diffusion on zinc complexing in high salinity fluid inclusions from polymetallic sulphide quartz veins was investigated by analysing X-ray absorption fine structure (XAFS) spectra obtained from individual fluid inclusions at temperatures up to 400~ before and after experimental re-equilibration at high hydrogen pressures.

High salinity fluid inclusions formed from recycled seawater in deeply subducted alpine serpentinite

The origin of high-pressure brines has been investigated in the Erro-Tobbio peridotite (Western Alps), a mantle slice that was exposed to the seafloor of the Mesozoic Ligurian-Piedmontese Tethys and was later involved in Alpine subduction. Hydrothermal alteration by seawater-derived fluids led to replacement of the mantle assemblage by Cl-bearing serpentine (0.35 wt% Cl), brucite (0.2 wt%), Cl- and alkali-bearing phyllosilicates (0.2 wt% Cl; 0.2–0.55 wt% Na 2O; 1–5 wt% K 2O). Relics of these hydrous phases occur in olivine + titanian clinohumite + antigorite assemblages formed at 2.5 GPa and 550–600°C during partial devolatilization and veining of the hydrothermally altered peridotite. The high-pressure phases lack chlorine and alkalis and are coeval with fluid inclusions trapped in the syn-eclogitic veins. The inclusions are salt-saturated and contain up to 50 wt% Cl, Na, K, Mg and Fe. High fluid chlorinity was probably achieved during rehydration of relict mantle minerals and deposition of hydrous phases in the veins. The data presented suggest that the seafloor hydrothermal signature was inherited by the eclogitic fluid due to partitioning of chlorine and alkalis into the fluid phase. The presence of salty brines in eclogitized hydrous peridotites can indicate deep recycling of seawater-derived fluids. Hydrous ultramafic systems can therefore act as large-scale carriers of seawater into the mantle.

Activation of microglia and expression of heat shock protein during kainic acid-induced hippocampal seizure in the rat

The Late Cretaceous Hongniu–Hongshan porphyry–skarn copper deposit is located in the Zhongdian area of northwestern Yunnan Province, China. Garnets from the deposit have compositions that range from Adr14Grs86 to almost pure andradite (Adr98Grs2) and display two different styles of zoning. The garnets are predominantly of magmatic-hydrothermal origin, as is evidenced by their 18Ofluid (5.4–6.9‰) and low Dfluid (−142‰ to −100‰) values, both of which likely result from late-stage magmatic open-system degassing. Three generations of garnet have been identified in this deposit: (1) Al-rich garnets (Grt I; Adr22–57Grs78–43) are anisotropic, have sector dodecahedral twinning, are slightly enriched in light rare earth elements (LREEs) compared with the heavy rare earth elements (HREEs), have negative or negligible Eu anomalies, and contain high concentrations of F. Fluid inclusions within these Al-rich garnets generally have salinities of 12–39 wt.% NaCl eq. and have liquid–vapor homogenization temperatures (Th) of 272–331 °C. The Grt I are most likely associated with low- to medium-salinity fluids that were generated by the contraction of an ascending vapor phase and that formed during diffusive metasomatism caused by pore fluids equilibrating with the host rocks at low W/R (water/rock) ratios. These garnets formed as a result of the high F activity of the system, which increased the solubility of Al within the magmato-hydrothermal fluids in the system. (2) Fe-rich garnets (Adr75–98Grs25–2) have trapezohedral faces, and are both anisotropic with oscillatory zoning and isotropic. These second-generation Fe-rich garnets (Grt II) have high ΣREE concentrations, are LREE-enriched and HREE-depleted, and generally have positive but variable Eu anomalies. All of the Fe-rich garnets contain high-salinity fluid inclusions with multiple daughter minerals with salinities of 33–80 wt.% NaCl eq. Some of them show higher temperatures of halite dissolution (465–591 °C) than liquid–vapor homogenization temperatures (319–473 °C), and several Fe-rich garnets contain inclusion groups indicative of boiling. The Grt II are associated with high-temperature, hypersaline fluids that were segregated from magma at a depth of at least 5.6 km and reacted with carbonates at depths shallower than 2.0 km. (3) Al-rich garnet veins (Adr14–60Grs86–40) contain allotriomorphic crystals, have lower HFSE (high field strength element) and REE concentrations than the other garnets, and have HREE-enriched and LREE-depleted patterns with small Eu anomalies that are typical of the majority of garnets. The Grt III most likely formed from residual metasomatic fluids.

Physical, chemical, and mineralogical evidence for magmatic fluid migration within the Capitan pluton, southeastern New Mexico

The ≈300 km 3 , 28.8 Ma granitic Capitan pluton, located in southeastern New Mexico, was the source of high-temperature (up to 600°C), high-salinity (up to 80 wt%) fluids that formed small REE-bearing zones of mineralization in the pluton carapace. Erosional dissection of the pluton reveals zones in the granite that are characterized by bubble-like void spaces and fissures. These could be interpreted, on one hand, as original magmatic porosity resulting from the interconnection and/or coalescence of bubbles of magmatic volatile phase, or on the other hand, as void spaces resulting from postcrystallization dissolution. These porous zones grade upward into the brecciated portions of the pluton carapace that hosts mineralization. The zones are planar features, traceable across valleys, and even between adjacent valleys on the pluton9s flanks. The orientation of the zones is roughly concentric with the exposed shape of the pluton. Chemical analyses of samples collected across a porous zone reveal that some elements, including Fe, Al, Rb, and Sr are depleted in the entire area surrounding the porous zone, whereas Si, Na, Ba, and Ta are enriched. Other elements, including K, Sb, Zn, Zr, a number of rare earth elements (REE), Pb, and Th, exhibit a slight to strong depletion surrounding the porous zone and a strong enrichment roughly centered on the zone itself. Abundance of high-salinity fluid inclusions in igneous quartz is high within the entire 2.5-m-wide zone but is significantly higher in the central porous zone. The chemical and fluid inclusion abundance patterns could be produced by a two-step process with the wide depletion caused first, by fractionation of elements from the melt into a volatile phase at the time that volatile exsolution first occurred. This could be followed by the superimposed enrichment pattern for certain elements caused by deposition of these elements from trace-element-rich fluids, which evolved in a more central, deeper part of the pluton and were transported along the same zone (although now more crystallized) of fluid migration established earlier, depositing minerals as it cooled. Textural, petrographic, and geochemical evidence suggest that volatiles may have exsolved during crystallization, coalescing into bubble-rich zones that enhance the efficiency of volatile transport.

Contrasting styles of epithermal precious-metal mineralization in the southwestern Nevada volcanic field, USA

The southwestern Nevada volcanic field contains epithermal precious-metal deposits hosted by Miocene volcanic rocks and pre-Tertiary sedimentary rocks with production+reserves greater than 60 t of gold and 150 t of silver. The volcanic rocks consist predominantly of ash-flow tuffs erupted between 15 and 7 Ma during three major magmatic stages: the main stage (ca. 15-13 Ma); the Timber Mountain stage (ca. 13-9 Ma); and the late stage (ca. 9-7 Ma). Hydrothermal activity and precious-metal mineralization in the southern part of the field took place between ca. 13 and 8.5 Ma, coinciding with portions of all three magmatic stages. Regional extension during this period produced imbricate normal and detachment faulting that provided structural control for some of the mineralization. Contrasts in the style and geochemistry of mineralization, together with stratigraphic and radiometric age data and differences in geologic setting reflect the variable nature of hydrothermal activity during development of the southwestern Nevada volcanic field. During the main magmatic stage, silver-rich vein mineralization of the adularia-sericite type occurred in an intermediate volcanic center at Wahmonie. Secondary high-salinity fluid inclusions in felsic subvolcanic intrusions, a trace element suite that includes bismuth and tellurium, and geophysical data support the presence of a buried porphyry-type magmatic system at Wahmonie. Hydrothermal activity at Bare Mountain took place during the main magmatic stage, and may have continued into the Timber Mountain magmatic stage. Bare Mountain contains gold-rich, disseminated Carlin-type deposits with high arsenic, antimony, mercury and fluorine in sedimentary and igneous rocks. In northern and eastern Bare Mountain, mineralization is associated with felsic porphyry dikes that contain secondary high-salinity fluid inclusions. A genetic relationship between porphyry magmatism and shallow Carlin-type gold deposits seems likely at Bare Mountain. Sedimentary-rock-hosted mineralization at Mine Mountain is spatially associated with a thrust and was apparently deposited, in part, by a hydrothermal system active during the Timber Mountain magmatic stage. The silver:gold ratio is high and base-metal, arsenic, antimony, mercury and selenium contents are very high. Mine Mountain mineralization shares features with vein and disseminated silver deposits at Candelaria, Nevada. Gold-silver deposits in the areally extensive Bullfrog district comprise the largest known precious-metal resource in the volcanic field. They are mainly quartz-carbonate±adularia veins with alteration and mineralization styles similar to other adularia-sericite-type deposits in the Great Basin. Deposits in the Rhyolite area and at the Gold Bar mine have very low contents of arsenic and mercury compared to other epithermal deposits in the Great Basin, although copper and antimony are locally elevated. Similarities in mineralization style and assemblages, which include two occurrences of the rare gold-silver sulfide uytenbogaardtite, indicate deposition under similar conditions in different parts of the district. Hydrothermal activity in the Bullfrog district was coeval with extensional tectonism and may have continued from the Timber Mountain stage into the late magmatic stage. Mineralization at some deposits in the Bullfrog and Bare Mountain districts is spatially associated with, and, in part, structurally controlled by a regional detachment fault system. However, significant differences in age, mineralization style and geochemistry indicate that mineralization in the two districts is unrelated.

Alteration und die Rolle von Fluiden beim Absatz von Ni, Cu und Platingruppen-Elementen im Kontakt des Sudbury KOmplexes, Onaping-Levack-Gebiet, Ontario

A variety of alteration styles is associated with Cu, Ni and platinum-group element deposition in the Onaping-Levack area of the North Range of the Sudbury Structure. Two significant alteration assemblages are amphibole (actinolite) + epidote + chlorite + quartz ± albite ± K-feldspar ± calcite adjacent to Cu-rich veins in the Deep Copper Zone of Strathcona mine and the nearby Barnet property, and amphibole (actinolite to ferro-actinolite) + epidote + titanite + pentlandite + pyrite ±quartz ± magnetite in the Fraser mine Epidote Zone. The concentrically zoned Epidote Zone alteration has been further subdivided into facies that range from epidotized Footwall Breccia through actinolite-bearing rocks with few vestiges of original Footwall Breccia textures, to the most intensely altered magnetite-rich facies. Higher Fe3+ content in epidote in the most intensely altered rocks is compatible with the evolution of more oxidized fluids with time. Fe and Ni, in addition to Cu and PGE, have been mobilized. The 2- (L + V) and 3- (L + V + SC) phase fluid inclusions are similar in each of the deposit locations. Temperatures of first ice melting range from −64.9 to -42.3°C, indicating that the trapped fluids may be modelled in the system H2O-NaCl-CaCl2. Average salinities are high, between 20.8 to 34.4 NaCl wt.% eq., and show considerable overlap for each deposit. Isochores calculated from these data show a decrease in temperature range from the Epidote Zone (230 to 340°C) through Barnet (210 to 295°C) to the most distal Deep Copper Zone (175 to 280°C). The Cl-rich fluids deposited Cu and platinum-group elements in the Deep Copper Zone as a result of this thermal gradient. The high salinity fluid inclusions of the Sudbury North Range are similar to those from other high and intermediate temperature Cu-Ni-PGE deposits, and suggest that they were the result of late-stage hydrothermal activity within contact units of the Sudbury Igneous Complex.

Geologic setting, depths of emplacement, and regional distribution of fluid inclusions in intrusions of the central Wasatch Mountains, Utah

Nine mid-Tertiary calc-alkaline stocks, a subvolcanic porphyry system, and coeval volcanic rocks are exposed in a 45-km-long east-trending belt across the central Wasatch Mountains, Utah. The intrusions vary systematically from west to east in texture, style of emplacement, extent of contact metamorphism, hydrothermal alteration, and mineralization. Pressure-depth estimates based on metamorphic mineral assemblages, stratigraphic reconstructions, and fluid inclusion data indicate a regular variation in paleodepths ranging from about 11 km on the west to less than 1 km on the east. These data indicate that the central Wasatch Mountains have been tilted down to the east about 20 degrees during the late Cenozoic. Fluid inclusion populations in igneous quartz also vary systematically with paleodepth; high-salinity (halite-saturated) fluid inclusions are present in the eastern porphyry stocks and in the upper parts of the Alta and Clayton Peak stocks in the center of the belt but are absent in the deeper parts of the Alta and Clayton Peak stocks and in the Little Cottonwood stock on the west side of the belt. In the Alta and Clayton Peak stocks, nearly planar high-paleosalinity horizons, presently dipping 15 degrees to 20 degrees east, separate rocks containing high-salinity fluid inclusions (above the high-paleosalinity horizon) from those lacking such fluid inclusions. Comparison of fluid-inclusion populations in igneous and vein quartz in the Alta and Clayton Peak stocks indicates that high-salinity fluids predated most of the vein-forming hydrothermal activity and provide the earliest record of fluids to circulate in these stocks. High-salinity fluids probably formed either by boiling of fluids released during the late stages of crystallization in the parts of the intrusions where pressure was less than about 1,300 bars or by exsolution of immiscible high-salinity brines from the crystallizing magmas. Most hydrothermal mineralization associated with the intrusions, including Ag-Pb-Zn ores in the Park City district, are associated spatially with parts of the intrusions where high-salinity fluids were present. The major exception is the porphyry molybdenum system in the eastern part of the Little Cottonwood stock, which probably was at too great a depth (approximately 7 km) to form high-salinity brines and is dominated by low-salinity CO 2 -rich fluids.