Fluid inclusion characteristics of intrusion-related gold mineralization, Tombstone–Tungsten magmatic belt, Yukon Territory, Canada

Intrusion-hosted, low sulfide, sheeted vein systems are common within many plutons and stocks of the middle Cretaceous Tombstone-Tungsten magmatic belt, Yukon Territory, and host significant gold mineralization. Fluid inclusion characteristics of five such systems, namely Emerald Lake, Dublin Gulch, Scheelite Dome, Mike Lake, and MacTung, constrain the vein-forming fluid composition, formation temperatures and pressures, hydrothermal fluid processes, and potential fluid sources. The veins contain a wide range of fluid inclusion types. Ubiquitous type 1A and 1B inclusions are low salinity (1A: XNaCl<0.02; 1B: XNaCl<0.03), CO₂-rich (1A: XCO₂=0.18-1.00; 1B: XCO₂=0.02-0.33). Laser Raman studies indicate that type 1A and 1B inclusions commonly contain minor CH₄ (XCH₄<0.09) and N₂ (XN₂<0.12). Type 2 inclusions are H₂O-rich (XH₂O=0.94-0.99), of low to moderate salinity (XNaCl=0.01-0.06), and were common at Emerald Lake and Dublin Gulch, in addition to localized type 3A halite-bearing inclusions (XNaCl=0.12-0.16). Both inclusion types post-dated the CO₂-rich inclusions. Sheeted veins in the Mike Lake pluton contained coexisting type 1A and 3A and 3B (halite + sylvite) inclusions. Type 1A inclusions in all studied systems homogenized between 208 and 362 °C, type 1B ranged between 205 and 329 °C, and type 2 between 154 and 261 °C. Type 3A and 3B inclusions homogenized between 217 and 355 °C. A predominantly magmatic source for both CO₂-rich and saline H₂O-rich fluids is favored, with variations in trapping pressure (<1 kbar at Mike Lake, >1 kbar at Emerald Lake and Dublin Gulch, and >2 kbar at MacTung and Scheelite Dome) that potentially control fluid composition and evolution. A variety of fluid processes may have been responsible for gold precipitation including immiscibility and/or release of an evolving magmatic fluid.     

Proterozoic low-sulfidation epithermal Au-Ag mineralization in the Mallery Lake area, Nunavut, Canada

The Mallery Lake area contains pristine examples of ancient precious metal-bearing low-sulfidation epithermal deposits. The deposits are hosted by rhyolitic flows of the Early Proterozoic Pitz Formation, but are themselves apparently of Middle Proterozoic age. Gold mineralization occurs in stockwork quartz veins that cut the rhyolites, and highest gold grades (up to 24 g/t over 30 cm) occur in the Chalcedonic Stockwork Zone. Quartz veining occurs in two main types: barren A veins, characterized by fine- to coarse-grained comb quartz, with fluorite, calcite, and/or adularia; and mineralized B veins, characterized by banded chalcedonic silica and fine-grained quartz, locally intergrown with fine-grained gold or electrum. A third type of quartz vein (C), which crosscuts B veins at one locality, is characterized by microcrystalline quartz intergrown with fine-grained hematite and rare electrum. Fluid inclusions in the veins occur in two distinct assemblages. Assemblage 1 inclusions represent a moderate temperature (Th=150 to 220 °C), low salinity (~1 eq. wt% NaCl, with trace CO₂), locally boiling fluid; this fluid type is found in both A and B veins and is thought to have been responsible for Au-Ag transport and deposition. Assemblage 2 inclusions represent a lower temperature (Th=90 to 150 °C), high salinity calcic brine (23 to 31 wt% CaCl₂-NaCl), which occurs as primary inclusions only in the barren A veins. Assemblage 1 and 2 inclusions occur in alternating quartz growth bands in the A-type veins, where they appear to represent alternating fluxes of dilute fluid and local saline groundwater. No workable primary fluid inclusions were observed in the C veins. The A-vein quartz yields '¹⁸O values from 8.3 to 14.5‰ (average=10.9ǃ.7‰ [1C], n=30), whereas '¹⁸O values for B-vein quartz range from 11.2 to 14.0‰ (average=13.0ǂ.9‰, n=12). Calculated '¹⁸O_H2O values for the dilute mineralizing fluid from B veins range from -2.6 to 0.2‰ (average=-0.8ǂ.9‰, n=12) and are consistent with a dominantly meteoric origin. No values could be calculated for the brine, however, because all A-vein quartz samples contain mixed fluid inclusion populations. However, the fact that A-vein quartz samples extend to lower '¹⁸O values than the B veins suggests that the brine had a lighter isotopic signature relative to the dilute fluid. Hydrogen isotopic ratios of fluid inclusion waters extracted from eleven quartz samples of both vein types range from 'D_FI=-56 to -134‰, but show no particular correlation with vein type. In most respects, the mineralogical and fluid characteristics of the Mallery Lake system are comparable to those of Phanerozoic low-sulfidation deposits, and although the presence of high salinity brines is unusual in such deposits, it is not unknown (e.g., Creede, Colorado). In addition, one of the few other examples of well-preserved, Precambrian, low-sulfidation epithermal deposits, from the Central Pilbara tectonic zone, Australia, contains a similarly bimodal fluid assemblage. The significance of these saline brines is not clear, but from this study we infer that they were not directly involved with Au-Ag transport or deposition.     

Fluids in granulites of the Southern Marginal Zone of the Limpopo Belt, South Africa

The Southern Marginal Zone of the Limpopo Belt in South Africa is characterised by a granulite and retrograde hydrated granulite terrane. The Southern Marginal Zone is, therefore, perfectly suitable to study fluids during and after granulite facies metamorphism by means of fluid inclusions and equilibrium calculations. Isolated and clustered high-salinity aqueous and CO₂(-CH₄) fluid inclusions within quartz inclusions in garnet in metapelites demonstrate that these immiscible low H₂O activity fluids were present under peak metamorphic conditions (800-850 °C, 7.5-8.5 kbar). The absence of widespread high-temperature metasomatic alteration indicates that the brine fluid was probably only locally present in small quantities. Thermocalc calculations demonstrate that the peak metamorphic mineral assemblage in mafic granulites was in equilibrium with a fluid with a low H₂O activity (0.2-0.3). The absence of water in CO₂-rich fluid inclusions is due to either observation difficulties or selective water leakage. The density of CO₂ inclusions in trails suggests a retrograde P-T path dominated by decompression at T<600 °C. Re-evaluation of previously published data demonstrates that retrograde hydration of the granulites at 600 °C occurred in the presence of H₂O and CO₂-rich fluids under P-T conditions of 5-6 kbar and ~600 °C. The different compositions of the hydrating fluid suggest more than one fluid source.     

Geology and genesis of Variscan porphyry-style gold mineralization, Petráčkova hora deposit, Bohemian Massif, Czech Republic

A large number of Variscan mesothermal gold deposits are located in the central part of the Bohemian Massif, close to the Central Bohemian Plutonic Complex. The Petrá)kova hora deposit has many features that distinguish it from other deposits in the region and suggest its mineralization is closely related to the late magmatic processes associated with the Petrá)kova hora granodiorite. The gold ores occur as sheeted arrays of quartz veins and veinlets hosted by the small Petrá)kova hora granodiorite stock. Gold is found mainly as free grains of >900 fineness, and is accompanied by abundant pyrrhotite and chalcopyrite, and accessory pyrite, arsenopyrite, loellingite, and molybdenite. Molybdenite from the Petrá)kova hora deposit has been dated by the Re-Os method at 344.4DŽ.8 Ma. Hydrothermal alteration in the Petrá)kova hora deposit exhibits a distinct temporal paragenesis. Selectively pervasive, early K-alteration and silicification are the oldest hydrothermal phases. These were followed by early quartz veins (Q₁ to Q₄) that contain most of the gold mineralization. Late quartz veins (Q₅) and fracture-controlled silicification are gold-poor or barren. Barren calcite veins are the youngest hydrothermal product. Extensive low-temperature, meteoric-water dominated alteration, as is typical of classic porphyry deposits, is absent. However, the lower '¹⁸O whole rock values for Petrá)kova hora granodiorite and aplite (+2.4 to +5.1‰ SMOW) compared to other intrusions in the region reflect either interaction with isotopically light external fluids or magma assimilation of small volumes of hydrothermally altered country rock. The '¹⁸O isotopic compositions for quartz, scheelite and hornblende (7.7 to 13.4‰ SMOW) and the '³⁴S compositions for sulfide minerals (-1 to +3.5‰ CDT) from early, gold-rich quartz veins indicate formation at high temperatures (590 to 400 °C) from fluids with a magmatic isotopic signature ('¹⁸O_FLUID of 5.7 to 7.2‰). Fluids related to late quartz veins (Q₅) suggest the presence of a significant component of non-magmatic water ('¹⁸O_FLUID: +2.5 to +4.0‰). The '³⁴S values of post-Q₅ sulfide minerals (-4.5 to -3.5‰) reflect at least partial derivation of late-stage sulfur from a source external to the intrusions. Aqueous, aqueous-carbonic and nitrogen-bearing fluid inclusions were identified in hydrothermal and igneous quartz, with the aqueous inclusions being the most common. In hydrothermal vein quartz, the salinity of primary aqueous inclusions falls into ranges 6 to 23 and 33 to 41 equiv. wt% NaCl; in igneous quartz, populations in salinity were observed between 5 to 16, 35 to 40 and 62 to 70 equiv. wt% NaCl. The salt component of these fluids is best, and minimally, approximated by the NaCl-KCl-CaCl₂ system. Low- and high-salinity aqueous-carbonic inclusions are accessory in many of the analyzed samples. Three large successive pulses of fluids are recognized. Each pulse begins with a high-salinity (>30 equiv. wt% NaCl) magmatic fluid and evolves toward a lower salinity (~5 equiv. wt% NaCl) fluid. Data suggest that external (meteoric?) water(s) were significant for only the third fluid pulse, which formed the late Q₅ quartz veins and the calcite veins. Polyphase fluid inclusions hosted by igneous quartz of the Petrá)kova hora granodiorite indicate minimum trapping conditions of about 3 kbar and 550 °C. The gold-rich Q₁ to Q₄ veins may have formed along a quasi-isobaric cooling path at 2.5 to 1.5 kbar and 590 to 400 °C. This was followed by uplift, and formation of late Q₅ quartz veins (0.5 to 1.5 kbar; ~300 °C) and post-ore calcite veins (<0.5 kbar; 100 to 140 °C). The characteristics of the Petrá)kova hora deposit suggest that it may represent a position intermediate between intrusion-related gold systems (e.g., Fort Knox deposit, Alaska) and gold-rich, copper-poor porphyry deposits (e.g., Maricunga Belt in Chile). As such, the Petrá)kova hora deposit might be an example of the reduced gold sub-type of porphyry deposit.     

Compositional characteristics of fluid inclusions as exploration tool for Au-mineralization at Larafella, Burkina Faso

The Larafella Au-prospect (Burkina Faso) lies within dacitic rocks of the Palaeoproterozoic Birimian greenstone belts. Gold mineralization is intimately associated with zones of cataclastic deformation. Whilst the lode-vein mineralization is closely associated with CO₂-rich fluid inclusions, the barren quartz veins are characterized by H₂O ± salt-bearing inclusions. Geochemical studies on the immediate wall-rock of the quartz veins have shown an increase of As in zones of gold enrichment, while alteration overprints such as carbonatization and chloritization cannot be correlated unequivocally with Au-mineralization. Consequently, fluid inclusion studies of quartz veins and As-anomalies constitute important exploration tools for mesothermal gold mineralization, since Au-rich zones can be distinguished from Au-depleted zones.     

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 '¹⁸O_water values (-0.6 to 0.5‰), reflecting a local meteoric water (-6‰) influence. A '¹⁸O versus 'D 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.     

The origin and significance of non-aqueous CO2 fluid inclusions in the auriferous veins of Bin Yauri,northwestern Nigeria

Non-aqueous CO₂ and CO₂-rich fluid inclusions are found in the vein quartz hosting mesothermal gold-sulphide mineralization at Bin Yauri, northwestern Nigeria. Although mineralizing fluids responsible for gold mineralization are thought to be CO₂-rich, the occurrence of predominantly pure to nearly pure CO₂ inclusions is nevertheless unusual for a hydrothermal fluid system. Many studies of similar CO₂-rich fluid inclusions, mainly in metamorphic rocks, proposed preferential loss (leakage) of H₂O from H₂O-CO₂ inclusions after entrapment. In this study however, it is proposed that phase separation (fluid immiscibility) of low salinity CO₂-rich hydrothermal fluids during deposition of the gold mineralization led to the loss of the H₂O phase and selective entrapment of the CO₂. The loss of H₂O to the wallrocks resulted in increasing oxidizing effects. There is evidence to suggest that the original CO₂-rich fluid was intrinsically oxidized, or perhaps in equilibrium with oxidizing conditions in the source rocks. The source of the implicated fluid is thought to be subducted metasediments, subjected to dehydration and devolatilization reactions along a transcurrent Anka fault/shear system, which has been described as a Pan-African (450–750 Ma) crustal suture.     

Very high-density carbonic fluid inclusions in sapphirine-bearing granulites from Tonagh Island in the Archean Napier Complex, East Antarctica: implications for CO2 infiltration during ultrahigh-temperature (T>1,100 °C) metamorphism

The ultrahigh-temperature (UHT) metamorphism of the Napier Complex is characterized by the presence of dry mineral assemblages, the stability of which requires anhydrous conditions. Typically, the presence of the index mineral orthopyroxene in more than one lithology indicates that H₂O activities were substantially low. In this study, we investigate a suite of UHT rocks comprising quartzo-feldspathic garnet gneiss, sapphirine granulite, garnet-orthopyroxene gneiss, and magnetite-quartz gneiss from Tonagh Island. High Al contents in orthopyroxene from sapphirine granulite, the presence of an equilibrium sapphirine-quartz assemblage, mesoperthite in quartzo-feldspathic garnet gneiss, and an inverted pigeonite-augite assemblage in magnetite-quartz gneiss indicate that the peak temperature conditions were higher than 1,000 °C. Petrology, mineral phase equilibria, and pressure-temperature computations presented in this study indicate that the Tonagh Island granulites experienced maximum P-T conditions of up to 9 kbar and 1,100 °C, which are comparable with previous P-T estimates for Tonagh and East Tonagh Islands. The textures and mineral reactions preserved by these UHT rocks are consistent with an isobaric cooling (IBC) history probably following an counterclockwise P-T path. We document the occurrence of very high-density CO₂-rich fluid inclusions in the UHT rocks from Tonagh Island and characterize their nature, composition, and density from systematic petrographic and microthermometric studies. Our study shows the common presence of carbonic fluid inclusions entrapped within sapphirine, quartz, garnet and orthopyroxene. Analysed fluid inclusions in sapphirine, and some in garnet and quartz, were trapped during mineral growth at UHT conditions as 'primary' inclusions. The melting temperatures of fluids in most cases lie in the range of -56.3 to -57.2 °C, close to the triple point for pure CO₂ (-56.6 °C). The only exceptions are fluid inclusions in magnetite-quartz gneiss, which show slight depression in their melting temperatures (-56.7 to -57.8 °C) suggesting traces of additional fluid species such as N₂ in the dominantly CO₂-rich fluid. Homogenization of pure CO₂ inclusions in the quartzo-feldspathic garnet gneiss, sapphirine granulite, and garnet-orthopyroxene gneiss occurs into the liquid phase at temperatures in the range of -34.9 to +4.2 °C. This translates into very high CO₂ densities in the range of 0.95-1.07 g/cm³. In the garnet-orthopyroxene gneiss, the composition and density of inclusions in the different minerals show systematic variation, with highest homogenization temperatures (lowest density) yielded by inclusions in garnet, as against inclusions with lowest homogenization (high density) in quartz. This could be a reflection of continued recrystallization of quartz with entrapment of late fluids along the IBC path. Very high-density CO₂ inclusions in sapphirine associated with quartz in the Tonagh Island rocks provide potential evidence for the involvement of CO₂-rich fluids during extreme crustal temperatures associated with UHT metamorphism. The estimated CO₂ isochores for sapphirine granulite intersect the counterclockwise P-T trajectory of Tonagh Island rocks at around 6-9 kbar at 1,100 °C, which corresponds to the peak metamorphic conditions of this terrane derived from mineral phase equilibria, and the stability field of sapphirine + quartz. Therefore, we infer that CO₂ was the dominant fluid species present during the peak metamorphism in Tonagh Island, and interpret that the fluid inclusions preserve traces of the synmetamorphic fluid from the UHT event. The stability of anhydrous minerals, such as orthopyroxene, in the study area might have been achieved by the lowering of H₂O activity through the influx of CO₂ at peak metamorphic conditions (>1,100 °C). Our microthermometric data support a counterclockwise P-T path for the Napier Complex.     

CO<Subscript>2</Subscript>-rich fluid inclusions with chalcopyrite daughter mineral from the Fenghuangshan Cu–Fe–Au deposit,China: implications for metal transport in vapor

CO₂-rich fluid inclusions containing opaque mineral crystals were found in the Fenghuangshan skarn-porphyry Cu–Fe–Au deposit in Tongling, Anhui, China. These inclusions show variable CO₂ contents and are accompanied by aqueous inclusions, both occurring as secondary inclusions in quartz and being locally associated with chalcopyrite mineralization. Laser Raman microspectroscopic analyses confirm the predominance of CO₂ in the vapor and the presence of H₂S as high as 8 mol%, and identify the opaque mineral with yellow reflectance color in the inclusions as chalcopyrite. More than half of the CO₂-bearing inclusions contains chalcopyrite, whereas few of the associated aqueous inclusions do so. The chalcopyrite, occupying less than 1% (volume) of the inclusions, is interpreted to be a daughter mineral, and calculated Cu concentrations in the inclusions range from 0.1 to 3.4 wt%. Copper is inferred to have been transported in CO₂-dominated fluids as HS⁻ complexes. The occurrence of chalcopyrite daughter crystals in CO₂-rich fluid inclusions indicates that CO₂-rich vapor has the capacity of transporting large amounts of Cu, and possibly Au. This finding has significant implications for metal transport and mineralization in hydrothermal systems enriched in CO₂, such as orogenic-type and granitic intrusion-related gold deposits.     

Tin–polymetallic sulfide deposits in the eastern part of the Dachang tin field (South China) and the role of black shales in their origin

The Dafulou and Huile vein and stratabound cassiterite-sulfide deposits and sheeted ore veins at the Kangma cassiterite-sulfide deposit are located in the eastern part of the Dachang tin field. These deposits are hosted in a sedimentary sequence containing significant concentrations of organic matter in the form of Lower Devonian calcareous black shales and hornfels. These rocks together with the younger intrusion of Longxianggai granite (91DŽ Ma) actively participated in the formation of Sn-polymetallic deposits. The following three major stages have been distinguished in stratiform and vein-type orebodies at Dafulou, Huile and Kangma: stage I (cassiterite, pyrrhotite, arsenopyrite, tourmaline, carbonate), stage II - main sulfide stage (quartz, cassiterite, arsenopyrite, pyrrhotite, sphalerite, stannite, pyrite, carbonates) and stage III (native Bi, galena, electrum, sulfosalts). Stage IV (post-ore), recognized at Huile is represented by barren carbonates and zeolites. Whole rock geochemistry has revealed that at Dafulou, Bi and Cu correlate strongly with S, whereas V and Pb correlate well with C_org (organic carbon). The similar distribution patterns of selected elements in average slightly mineralized low-Ca black shales indicate a fluid composition similar for all deposits studied. Studies of graphitization of the organic matter in black shales adjacent to orebodies indicate that d₍₀₀₂₎ and FWHM (full width in half maximum)/peak height values gradually decrease in the following sequence: Dafulou deposit M Kangma deposit M Huile deposit. The pyrolysate of wall rocks at the Dafulou deposit is relatively enriched in asphaltenes and maltenes (55.6-72.0% of the pyrolysate) comparable with pyrolysate obtained from more distal black shales (19.2-28.5%). Typical GC-MS spectra of pyrolysate from distal black shales are dominated by alkanes in the n-C15 to n-C25 range, aromatic molecules being represented mostly by alkyl-naphthalenes. In contrast, only traces of aliphatic hydrocarbons in the n-C14 to n-C18 range and elemental sulfur were identified in pyrolysates from pyrrhotitized wall rocks. The earliest fluid inclusions of the studied system occur in the quartz-tourmaline-cassiterite assemblage of stage I at Dafulou. These inclusions are H₂O-CO₂-CH₄-rich, with 10 to 20 vol% of aqueous phase. P-T conditions of the trapping of inclusions are estimated to be up to 400 °C and 1.3 to 2.0 kbar (between 5.0 and 7.5 km under lithostatic pressure). In contrast, the presence of a low density gaseous CO₂-CH₄ phase indicates relatively low pressures during the formation of the breccia-type quartz-calcite-cassiterite-sulfide mineralization (stage II), when P-T conditions probably reached approx. 380 to 400 °C and 0.6 kbar (up to 6 km under hydrostatic pressure). Fluid inclusion data and oxygen isotope thermometry indicate that cassiterite-sulfide ores of the main sulfide stage (stage II) formed from aqueous-carbonic fluid (CO₂/CH₄ =ᄺ) at temperatures of up to 390 °C at Dafulou and in a temperature range of 250 to 360 °C at Huile and 260 to 370 °C at Kangma. The '³⁴S values of sulfides from Dafulou range mostly between -1 and -6‰, whereas sulfides from the Kangma and Huile deposits are characterized by more negative '³⁴S values (between -8 and -11‰, and between -9 and -12‰, respectively). These data suggest that bacteriogenic sulfides of black shales were a dominant source of reduced sulfur for epigenetic (vein and replacement) mineralization. Oxygen isotopic compositions of five quartz-cassiterite pairs from Dafulou and Huile show a relatively narrow range of calculated oxygen isotope temperatures (250-320 °C, using the equation of Alderton 1989) and high '¹⁸O_fluid values between +8 and +10‰ (SMOW), which are in agreement with fluid derivation from and/or high temperature equilibration with the Longxianggai granite. The carbon and oxygen isotope composition of carbonates reflects variable carbon sources. Stage I calcite is characterized by narrow ranges of '¹³C (-7.0 to -9.5‰ PDB) and '¹⁸O (+15.0 to +17.5‰ SMOW). This calcite shows ubiquitous deformation, evidenced by intense development of twins. Fluid compositions calculated at 330 °C for the Dafulou and Huile deposits and at 270-300 °C for the Kangma deposit ('¹⁸O_fluid between +10.0 and +11.5‰ SMOW, '¹³C_fluid between -5.5 and -7.5‰ PDB), agree with fluid derivation from and/or equilibration with the peraluminous, high-'¹⁸O Longxianggai granite and suggest a significant influence of contact metasedimentary sequences (carbon derived from decomposition and/or alteration of organic matter of calcareous black shales). The '¹³ C values of organic matter from the Lower to Upper Devonian host rocks at the Dafulou deposit (-24.0 and -28.0‰) fit with a marine origin from algae. However, organic matter adjacent to the host rock-ore contact displays a slight enrichment in ¹³C. The organic carbon from the Huile and Kangma deposits is even more ¹³C enriched (-24.6 to -23.5‰). The most heavy '¹³ C values (-16.5‰) were detected in hornfels sampled at the contact of the Upper Devonian sediments with the Longxianggai granite. The '¹³C data broadly correlate with the degree of structural ordering (degree of graphitization) of organic matter, which indicates that both variables are related to thermal overprint.     

Evidence for fluid phase separation in high-grade ore zones at the Porgera gold deposit, Papua New Guinea

Coexisting, liquid-rich and vapor-rich primary fluid inclusions in quartz provide direct evidence for fluid phase separation in high-grade quartz–roscoelite–gold veins and breccias from the Porgera alkalic-type gold deposit. Vapor-rich fluid inclusions are CO₂-rich, and sometimes contain liquid CO₂ at room temperature. The close spatial and paragenetic relationship between these “boiling assemblage” fluid inclusions and gold suggests that gold was precipitated by phase separation, at least locally. Additionally, the occurrence of carbonate and sulfate minerals in high-grade veins (reflecting pH increase and oxidation of the boiled fluid) and the appearance of hydrothermal breccias, are consistent with the process of fluid phase separation. Liquid CO₂-bearing fluid inclusions are rare in near-surface epithermal deposits, and indicate that the Porgera vein system was formed at greater depths and pressures (our estimates suggest pressures between 250 and 340 bars). It is suggested that alkalic-type gold deposits may be distinguished from other epithermal deposit types by the more gaseous nature of the ore-forming fluids, in addition to their association with alkalic magmas. Received: 24 February 2000 / Accepted: 6 April 2000     

Origin of CO2-rich fluid inclusions in synorogenic veins from the Eastern Mount Isa Fold Belt, NW Queensland, and their implications for mineralization

Synorogenic veins from the Proterozoic Eastern Mount Isa Fold Belt contain three different types of fluid inclusions: CO₂-rich, aqueous two-phase and rare multiphase. Inclusions of CO₂ without a visible H₂O phase are particularly common. The close association of CO₂-rich inclusions with aqueous two-phase, and possibly multiphase inclusions suggests that phase separation of low- to -moderate salinity CO₂-rich hydrothermal fluids led to the selective entrapment of the CO₂. Microthermometric results indicate that CO₂-rich inclusions homogenize between –15.5 and +29.9 °C which corresponds to densities of 0.99 to 0.60 g.cm⁻³. The homogenization temperatures of the associated aqueous two-phase inclusions are 127–397 °C, with salinities of 0.5 to 18.1 wt.% NaCl equivalent. The rarely observed multiphase inclusions homogenize between 250 and 350 °C, and have salinities ranging from 34.6 to 41.5 wt.% NaCl equivalent. Evidence used to support the presence of fluid immiscibility in this study is mainly derived from observations of coexisting H₂O-rich and CO₂-rich inclusions in groups and along the same trail. In addition, these two presumably unmixed fluids are also found on adjacent fractures where monophase CO₂-rich inclusions are closely related to H₂O-rich inclusions. Similar CO₂-rich inclusions are widespread in mineral deposits in this region, which are simply metal-enriched synorogenic veins. Therefore, we argue that fluid immiscibility caused volatile species such as CO₂ and H₂S to be lost from liquid, thus triggering ore deposition by increasing the fluid pH and decreasing the availability of complexing ligands. Received: 28 April 1997 / Accepted: 4 January 1999     

Geology and Metallogenesis of the Sawayaerdun Gold Deposit in the Southwestern Tianshan Mountains, Xinjiang, China

The Sawayaerdun gold deposit, located in Wuqia County, Southwest Tianshan, China, occurs in Upper Silurian and Lower Devonian low‐grade metamorphic carbonaceous turbidites. The orebodies are controlled by a series of NE‐NNE‐trending, brittle–ductile shear zones. Twenty‐four gold mineralized zones have been recognized in the Sawayaerdun ore deposit. Among these, the up to 4‐km‐long and 200‐m wide No. IV mineralized zone is economically the most important. The average gold grade is 1–6 g/t. Gold reserves of the Sawayaerdun deposit have been identified at approximately 37 tonnes and an inferred resource of 123 tonnes. Hydrothermal alteration is characterized by silicification, pyritization, arsenopyritization, sericitization, carbonatization and chloritization. On the basis of field evidence and petrographic analysis, five stages of vein emplacement and hydrothermal mineralization can be distinguished: stage 1, early quartz stage, characterized by the occurrence of quartz veins; stage 2, arsenopyrite–pyrite–quartz stage, characterized by the formation of auriferous quartz veinlets and stockworks; stage 3, polymetallic sulfide quartz stage, characterized by the presence of auriferous polymetallic sulfide quartz veinlets and stockworks; stage 4, antimony–quartz stage, characterized by the formation of stibnite–jamesonite quartz veins; and stage 5, quartz–carbonate vein stage. Stages 2 and 3 represent the main gold mineralization, with stage 4 representing a major antimony mineralization episode in the Sawayaerdun deposit. Two types of fluid inclusion, namely H₂O–NaCl and H₂O–CO₂–NaCl types, have been recognized in quartz and calcite. Aqueous inclusions show a wide range of homogenization temperatures from 125 to 340°C, and can be correlated with the mineralization stage during which the inclusions formed. Similarly, salinities and densities of these fluids range for each stage of mineralization from 2.57 to 22 equivalent wt% NaCl and 0.76 to 1.05 g/cm³, respectively. The ore‐forming fluids thus are representative of a medium‐ to low‐temperature, low‐ to medium‐salinity H₂O–NaCl–CO₂–CH₄–N₂ system. The δ³⁴S_CDT values of sulfides associated with mineralization fall into a narrow range of ?3.0 to +2.6‰ with a mean of +0.1‰. The δ¹³C_PDB values of dolomite and siderite from the Sawayaerdun gold deposit range from ?5.4 to ?0.6‰, possibly reflecting derivation of the carbonate carbon from a mixed magmatic/sedimentary source. Changes in physico‐chemical conditions and composition of the hydrothermal fluids, water–rock exchange and immiscibility of hydrothermal fluids are inferred to have played important roles in the ore‐forming process of the Sawayaerdun gold–antimony deposit.     

Petrological significance of manganese carbonate inclusions in spessartine garnet and relation to the stability of spessartine in metamorphosed manganese-rich rocks

Idioblastic spessartine garnet pervasively developed in Mn-rich rocks and impure manganese carbonate ore at the Lower Proterozoic Nsuta manganese deposit, Western Ghana, contains abundant inclusions of micritic and microconcretionary carbonates and, to a lesser extent, quartz. Detailed mineralogical and microprobe studies indicate all the carbonate phases (i.e. carbonate inclusions in garnet, carbonates coexisting with garnet and carbonates not directly in contact with garnet, the latter hereafter referred to as matrix carbonates) lie within the rhodochrosite-kutnahorite solid solution series, i.e. ~Mn_55-80(Ca + Mg)_20-45CO₃ to Ca₄₂(Mn + Mg)₅₈(CO₃)₂. Minor compositional differences occur in the various carbonate phases, but partition of major elements among coexisting phases indicate most carbonate minerals strongly fractionate Ca and Mg over coexisting spessartine. The nature, composition and textural relationship of coexisting minerals and inclusions in porphyroblastic spessartine indicate that the latter formed from metamorphic reactions in which rhodochrosite and/or kutnahorite and quartz were consumed, in part corroborating earlier observations on a rhodochrosite precursor for spessartine. Spessartine formation is thus envisaged to have taken place when the predominantly Mn carbonate-quartz assemblage became unstable in the presence of minor amounts of an unknown aluminous phase. Because all the carbonates appear to be low-temperature phases with no indications of significant recrystallisation or homogenisation, it could be argued that the spessartine + rhodochrosite - kutnahorite - quartz - pyrite assemblage stabilised during very low-grade greenschist facies metamorphism under relatively low but uniform f_O2 conditions. These observations also suggest the stability field of spessartine could extend to relatively lower temperatures than currently envisaged.     

Fluid inclusion characteristics of mesothermal gold deposits in the Xiaoqinling district, Shaanxi and Henan Provinces, People's Republic of China

Fluid inclusions were studied in quartz samples from early (stage I) gold-poor quartz veins and later (stage II) gold- and sulphide-rich quartz veins from the Wenyu, Dongchuang, Qiangma, and Guijiayu mesothermal gold deposits in the Xiaoqinling district, China. Fluid inclusion petrography, microthermometry, and bulk gas analyses show remarkably consistent fluid composition in all studied deposits. Primary inclusions in quartz samples are dominated by mixed CO₂-H₂O inclusions, which have a wide range in CO₂ content and coexist with lesser primary CO₂-rich and aqueous inclusions. In addition, a few secondary aqueous inclusions are found along late-healed fractures. Microthermometry and bulk gas analyses suggest hydrothermal fluids with typically 15–30 mol% CO₂ in stage I inclusions and 10–20 mol% CO₂ in stage II inclusions. Estimates of fluid salinity decrease from 7.4–9.2 equivalent wt.% NaCl to 5.7–7.4 equivalent wt.% NaCl between stage I and II. Primary aqueous inclusions in both stages show consistent salinity with, but slightly lower Th _total than, their coexistent CO₂-H₂O inclusions. The coexisting CO₂-rich, CO₂-H₂O, and primary aqueous inclusions in both stage I and II quartz are interpreted to have been trapped during unmixing of a homogeneous CO₂-H₂O parent fluid. The homogenisation temperatures of the primary aqueous inclusions give an estimate of trapping temperature of the fluids. Trapping conditions are typically 300–370 °C and 2.2 kbar for stage I fluids and 250–320 °C and 1.6 kbar for stage II fluids. The CO₂-H₂O stage I and II fluids are probably from a magmatic source, most likely devolatilizing Cretaceous Yanshanian granitoids. The study demonstrates that gold is largely deposited as pressures and temperatures fall accompanying fluid immiscibility in stage II veins. Received: 15 May 1997 / Accepted: 10 June 1998     

Geochemistry of Mesozoic plutons, southern Death Valley region, California: Insights into the origin of Cordilleran interior magmatism

Mesozoic granitoid plutons in the southern Death Valley region of southeastern California reveal substantial compositional and isotopic diversity for Mesozoic magmatism in the southwestern US Cordillera. Jurassic plutons of the region are mainly calc-alkaline mafic granodiorites with )_Ndi of -5 to -16, ⁸⁷Sr/⁸⁶Sr_i of 0.707-0.726, and ²⁰⁶Pb/²⁰⁴Pb_i of 17.5-20.0. Cretaceous granitoids of the region are mainly monzogranites with )_Ndi of -6 to -19, ⁸⁷Sr/⁸⁶Sr_i of 0.707-0.723, and ²⁰⁶Pb/²⁰⁴Pb_i of 17.4-18.6. The granitoids were generated by mixing of mantle-derived mafic melts and pre-existing crust - some of the Cretaceous plutons represent melting of Paleoproterozoic crust that, in the southern Death Valley region, is exceptionally heterogeneous. A Cretaceous gabbro on the southern flank of the region has an unusually juvenile composition ()_Ndi -3.2, ⁸⁷Sr/⁸⁶Sr_i 0.7060). Geographic position of the Mesozoic plutons and comparison with Cordilleran plutonism in the Mojave Desert show that the Precambrian lithosphere (craton margin) in the eastern Mojave Desert region may consists of two crustal blocks separated by a more juvenile terrane.     

Geochemical Characteristics of the Ore-forming Fluids of the Haigou Gold Deposit, Jilin

The quartz in the Haigou gold deposit contains a great abundance of three-phase CO2-NaCl-H2O and two-phase CO2-rich inclusions, which are associated with two-phase NaCl-H2O ones. The ore-forming fluids, which were rich in CO2, are classified into two types with two different sources: the high-salinity CO2-rich NaCl-H2O fluid derived from magmatic hydrothermal solution, and the low-salinity NaCl-H2O fluid from ancient meteoric water. The optimum conditions for gold mineralization are 220-300℃ for the temperature, 4-20 MPa for the fluid static pressure, 1-3 km for the mineralization depth, 2-7 w (NaCl)/10-2 for the fluid salinity, and 0.644 g/cm3 for the total density. The fluid was in a critical or supercritical state at the initial stage of mineralization, and it boiled and was unmixed with CO2 and NaCl-H2O in the climax of mineralization, leading to the decomposition of Au-chlorine complexes and the bulk precipitation of Au.The type, association, homogenization temperature and composition (CO2/H2O val     

Genesis of itabirite-hosted Au–Pd–Pt-bearing hematite-(quartz) veins, Quadrilátero Ferrífero, Minas Gerais, Brazil: constraints from fluid inclusion infrared microthermometry, bulk crush-leach analysis and U–Pb systematics

Fluid inclusions hosted in quartz and specular hematite from auriferous (jacutinga) and barren veins in the Quadrilátero Ferrífero (QF) have been studied using conventional and near infrared microscopy, respectively. The mineralization consists of veins that cross-cut metamorphosed iron formation (itabirite) of the Paleoproterozoic Itabira Group. The sample suite comprises hematite from veins from the low-strain domain in the W and SW of the study area, as well as hematite samples from the eastern high-strain domain in the central and NE parts of the QF. Halogen ratios of fluid inclusions in quartz and hematite from all studied deposits are consistent with a fluid evolved from dissolving and reprecipitating halite that was subsequently diluted. Fluid inclusions hosted in quartz and hematite are characterized by consistent Na/K ratios and considerable SO₄ contents, and suggest similar formation conditions and, perhaps, fluid origin from a common source. Na/K and Na/Li fluid mineral geothermometers indicate water–rock interaction at approximately 340±40°C. Hematites from the high-strain domain contain fluid inclusion assemblages of high-temperature aqueous-carbonic and multiphase high-salinity, high-temperature aqueous inclusions probably due to fluid immiscibility in the system H₂O–NaCl–CO₂. Fluid inclusions hosted in hematite from barren veins in the low-strain domain, as well as in hematite from jacutinga-type mineralization from the central part of the QF, only host multiphase aqueous fluid inclusions all showing narrow ranges of salinity (7.2–11.7 wt.% NaCl equiv.) and homogenization temperatures (148 to 229°C). Lower homogenization temperatures and the absence of CO₂-rich inclusions in specular hematite from these occurrences are attributed to carbonate precipitation and/or CO₂ escape due to cooling during fluid migration from the high- to the low-strain domain. Pb–Pb and U–Pb systematics of gold, hematite and hematite-hosted fluid inclusions in combination with geochemical evidence indicate distinct sources for Pd, Au, and Pb. The formation of specular hematite veins may be related to retrograde metamorphic fluids being released during the Brazilian orogenic cycle (600–700 Ma). The Pb isotopic characteristics of all samples are readily reconciled in a simple model that involves two different Paleoproterozoic or Archean source lithologies for lead and reflects contrasting depths of fluid percolation during the Brasiliano orogeny.     

Genetic Environment of the Intrusion-related Yuryang Au-Te Deposit in the Cheonan Metallogenic Province, Korea

Abstract. The Yuryang gold deposit, comprising a Te‐bearing Au‐Ag vein mineralization, is located in the Cheonan area of the Republic of Korea. The deposit is hosted in Precambrian gneiss and closely related to pegmatite. The mineralized veins display massive quartz textures, with weak alteration adjacent to the veins. The ore mineralization is simple, with a low Ag/Au ratio of 1.5:1, due to the paucity of Ag‐phases. Ore mineralization took place in two different mineral assemblages with paragenetic time; early Fe‐sulfide mineralization and late Fe‐sulfide and Au‐Te mineralization. The early Fe‐sulfide mineralization (pyrite + sphalerite) occurred typically along the vein margins, and the subsequent Au‐Te mineralization is characterized by fracture fillings of galena, sphalerite, pyrrhotite, Te‐bearing minerals (petzite, altaite, hessite and Bi‐Te mineral) and electrum. Fluid inclusions characteristically contain CO₂ and can be classified into four types (Ia, Ib, IIa and IIb) according to the phase behavior. The pressure corrected temperatures (≥500d?C) indicate that the deposit was formed at a distinctively high temperature from fluids with moderate to low salinity (<12 wt% equiv. NaCl) and CH₄ (1?22 mole %). The sphalerite geo‐barometry yield an estimated pressure about 3.5 ?2.1 kbar. The dominant ore‐deposition mechanisms were CO₂ effervescence and concomitant H₂S volatilization, which triggered sulfidation and gold mineralization. The measured and calculated isotopic compositions of fluids (δ¹⁸O_H2^O = 10.3 to 12.4 %o; δD_H2^O = ‐52 to ‐77 %o) may indicate that the gold deposition originated from S‐type magmatic waters. The physicochemical conditions observed in the Yuryang gold deposit indicate that the Jurassic gold deposits in the Cheonan area, including the Yuryang gold deposit are compatible with deposition of the intrusion‐related Au‐Te veins from deeply sourced fluids generated by the late Jurassic Daebo magmatism.     

Geology and fluid evolution of the Wangfeng orogenic-type gold deposit,Western Tian Shan,China

The Wangfeng gold deposit is located in Western Tian Shan and the central section of the Central Asian Orogenic Belt (CAOB). The deposit is mainly hosted in Precambrian metamorphic rocks and Caledonian granites and is structurally controlled by the Shenglidaban ductile shear zone. The gold orebodies consist of gold-bearing quartz veins and altered mylonite. The mineralization can be divided into three stages: quartz–pyrite veins in the early stage, sulfide–quartz veins in the middle stage, and quartz–carbonate veins or veinlets in the late stage. Ore minerals and native gold mainly formed in the middle stage. Four types of fluid inclusions were identified based on petrography and laser Raman spectroscopy: CO₂–H₂O inclusions (C-type), pure CO₂ inclusions (PC-type), NaCl–H₂O inclusions (W-type), and daughter mineral-bearing inclusions (S-type). The early-stage quartz contains only primary CO₂–H₂O fluid inclusions with salinities of 1.62 to 8.03 wt.% NaCl equivalent, bulk densities of 0.73 to 0.89 g/cm³, and homogenization temperatures of 256 °C–390 °C. Vapor bubbles are composed of CO₂. The middle-stage quartz contains all four types of fluid inclusions, of which the CO₂–H₂O and NaCl–H₂O types yield homogenization temperatures of 210 °C–340 °C and 230 °C–300 °C, respectively. The CO₂–H₂O fluid inclusions have salinities of 0.83 to 9.59 wt.% NaCl equivalent and bulk densities of 0.77 to 0.95 g/cm³, with vapor bubbles composed of CO₂, CH₄, and N₂. Fluid inclusions in the late-stage quartz are NaCl–H₂O solution with low salinities (0.35–3.87 wt.% NaCl equivalent) and low homogenization temperatures (122 °C–214 °C). The coexistence of inclusions of these four types in middle-stage quartz suggests that fluid boiling occurred in the middle-stage mineralization. Trapping pressures estimated from CO₂–H₂O inclusions are 110–300 MPa and 90–250 MPa for the early and middle stages, respectively, suggesting that gold mineralization mainly occurred at depths of about 10 km. In general, the Wangfeng gold deposit originated from a metamorphic fluid system characterized by low salinity, low density, and enrichment of CO₂. Depressurized fluid boiling caused gold precipitation. Given the regional geology, ore geology, fluid-inclusion features, and ore-forming age, the Wangfeng gold deposit can be classified as a hypozonal orogenic gold deposit.