Low-salinity Fluids Research Articles

Magnetite as an indicator of mixed sources for W–Mo–Pb–Zn mineralization in the Huangshaping polymetallic deposit, southern Hunan Province, China

The Huangshaping polymetallic deposit, located in southern Hunan Province in China, hosts abundant W–Mo–Pb–Zn mineralization that is associated with a large-scale skarn system at the contact zone between late Mesozoic granitoids and Carboniferous carbonates. To better understand the processes of polymetallic mineralization in the Huangshaping deposit, trace element concentrations of magnetites from different skarn stages were obtained by in situ laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS). The results show that two groups of magnetites were distinguished based on their trace element signatures. Group-1 magnetites, found in medium-grained garnet and calcite, have relatively low concentrations of Na, K, Ca, Si, Ge, Sn, and W, and relatively high concentrations of Mg, Al, Ti, V, Zn, Ni, and Co. The opposite is true for the Group-2 magnetites, which occur in coarse-grained garnet, tremolite, and bulk iron ore. These findings suggest that two compositional endmembers of hydrothermal fluids may be responsible for mineralization in the Huangshaping deposit. Group-2 magnetites likely precipitated from evolved magmatic hydrothermal fluids from which other minerals, such as garnet and tremolite, precipitated during the early skarn stages. The high Na, K, Ca, and Si contents of these hydrothermal fluids can be attributed to dissolution of the host rocks, which include limestone, sandstone, and evaporite horizons. On the other hand, the Group-1 magnetites precipitated from hydrothermal fluids with low Na, K, Ca, and Si contents. These low-salinity fluids may have been subjected to large-scale circulation, extracting Mg, Al, and Zn from the underlying Zn-rich metamorphic basement and the Mg- and Al-rich strata in this region. The high Ti, V, Ni, and Zn concentrations of Group-1 magnetites also suggest that these hydrothermal fluids had lower oxygen fugacity than the fluids from which the Group-2 magnetites precipitated. The results of this study demonstrate that the trace element concentrations of magnetites can be used to infer the composition and physicochemical conditions of the hydrothermal fluids from which they precipitated. In the case of the Huangshaping polymetallic deposit, this indicates that the hydrothermal fluids responsible for the W–Mo–Pb–Zn mineralization were derived from mixed sources.

Diversity of Rare and Abundant Prokaryotic Phylotypes in the Prony Hydrothermal Field and Comparison with Other Serpentinite-Hosted Ecosystems.

The Bay of Prony, South of New Caledonia, represents a unique serpentinite-hosted hydrothermal field due to its coastal situation. It harbors both submarine and intertidal active sites, discharging hydrogen- and methane-rich alkaline fluids of low salinity and mild temperature through porous carbonate edifices. In this study, we have extensively investigated the bacterial and archaeal communities inhabiting the hydrothermal chimneys from one intertidal and three submarine sites by 16S rRNA gene amplicon sequencing. We show that the bacterial community of the intertidal site is clearly distinct from that of the submarine sites with species distribution patterns driven by only a few abundant populations, affiliated to the Chloroflexi and Proteobacteria phyla. In contrast, the distribution of archaeal taxa seems less site-dependent, as exemplified by the co-occurrence, in both submarine and intertidal sites, of two dominant phylotypes of Methanosarcinales previously thought to be restricted to serpentinizing systems, either marine (Lost City Hydrothermal Field) or terrestrial (The Cedars ultrabasic springs). Over 70% of the phylotypes were rare and included, among others, all those affiliated to candidate divisions. We finally compared the distribution of bacterial and archaeal phylotypes of Prony Hydrothermal Field with those of five previously studied serpentinizing systems of geographically distant sites. Although sensu stricto no core microbial community was identified, a few uncultivated lineages, notably within the archaeal order Methanosarcinales and the bacterial class Dehalococcoidia (the candidate division MSBL5) were exclusively found in a few serpentinizing systems while other operational taxonomic units belonging to the orders Clostridiales, Thermoanaerobacterales, or the genus Hydrogenophaga, were abundantly distributed in several sites. These lineages may represent taxonomic signatures of serpentinizing ecosystems. These findings extend our current knowledge of the microbial diversity inhabiting serpentinizing systems and their biogeography.

Multi-reservoir fluid mixing processes in rift-related hydrothermal veins, Schwarzwald, SW-Germany

Fluid mixing is an important process in the formation of many hydrothermal vein-type deposits. Here, we present evidence from hydrothermal fluorite-barite-quartz veins with Pb-Zn-Cu-(Ag)-sulfides and associated mineralization, indicating that mineral precipitation was initiated by mixing of fluids derived from multiple sources, including mixing between more than two end-member fluid compositions. Based on our observations, we relate the diversity of the hydrothermal veins of the Schwarzwald mining district in terms of mineral assemblage and fluid inclusion chemistry to the disturbed and transient geological environment during ongoing rifting. Literature data on the regional geology, current groundwater reservoirs, formation processes and hydraulic features are augmented by new fluid inclusion analyses from post-Cretaceous, hydrothermal vein minerals including microthermometry, crush leach, Microraman and LA-ICP-MS analyses of individual fluid inclusions.Petrography and microthermometry of fluid inclusions show complex sequences of alternating fluid signatures within different growth zones of one crystal. High (20–26wt% NaCl+CaCl2), moderate (5–20wt% NaCl+CaCl2) and low salinity (<5wt% NaCl+CaCl2), sulfate- and/or CO2-bearing primary fluids were trapped during crystal growth. Such variations are commonly observed in minerals from different localities. Bulk crush leach analyses show significant variations in major element composition of the trapped fluids, within the overall Na-Ca-Cl-SO4-HCO3-system. These variations are caused by mixing of fluids from different aquifers and in various proportions. Ancient fluids show chemical similarities to modern groundwater aquifers that are available for direct sampling, such as granitic basement, Lower Triassic sandstones or Middle Triassic limestones and evaporites. Analyses of individual fluid inclusions by LA-ICP-MS support this interpretation and document the multi-component fluid mixing processes at individual localities recorded on the scale of single crystal growth zones. The latter data are used in a diffusion model to obtain the duration of mineral growth (before the fluid is homogenized), which implies very short-lived fluid events on the order of seconds to hours.By defining end member fluids and their proportions, we show that nearly all fluid mixtures are saturated with respect to barite. By contrast, fluorite-saturated fluids can only be modelled by mixing of a basement brine with fluids from Triassic sandstones. All fluid mixtures are strongly undersaturated with respect to galena, chalcopyrite and sphalerite, the most commonly observed ore minerals in the hydrothermal veins. As the calculated fluid mixtures are typically relatively oxidized and contain high sulfate/sulfide ratios, precipitation of sulfides was probably related to short-lived reduction events caused by an influx of hydrocarbons, by reactions with graphitic wall rocks in fractures by sulfidation related to fluid-rock reaction with the surrounding host rocks or an external influx of hydrocarbon-bearing fluids. The multi-aquifer fluid mixing processes involving aquifers of different chemical and physical constitution were triggered by brittle deformation related to rifting of the Rhine graben. This appears to be essential for the formation of a large number of mineralogically diverse hydrothermal ore deposits.

Ore genesis of Badi copper deposit, northwest Yunnan Province, China: evidence from geology, fluid inclusions, and sulfur, hydrogen and oxygen isotopes

The Badi copper deposit is located in Shangjiang town, Shangri-La County, Yunnan Province. Tectonically, it belongs to the Sanjiang Block. Vapor–liquid two-phase fluid inclusions, CO2-bearing fluid inclusions, and daughter-bearing inclusions were identified in sulfide-rich quartz veins. Microthermometric and Raman spectroscopy studies revealed their types of ore-forming fluids: (1) low-temperature, low-salinity fluid; (2) medium-temperature, low salinity CO2-bearing; and (3) high-temperature, Fe-rich, high sulfur fugacity. The δ18O values of chalcopyrite-bearing quartz ranged from 4.96‰ to 5.86‰, with an average of 5.40‰. The δD values of ore-forming fluid in equilibrium with the sulfide-bearing quartz were from − 87‰ to − 107‰, with an average of − 97.86‰. These isotopic features indicate that the ore-forming fluid is a mixing fluid between magmatic fluid and meteoric water. The δ34S values of chalcopyrite ranged from 13.3‰ to 15.5‰, with an average of 14.3‰. Sulfur isotope values suggest that the sulfur in the deposit most likely derived from seawater. Various fluid inclusions coexisted in the samples; similar homogenization temperature to different phases suggests that the Badi fluid inclusions might have been captured under a boiling system. Fluid boiling caused by fault activity could be the main reason for the mineral precipitation in the Badi deposit.

The Relevance of Fluid Inclusions to Mineral Systems and Ore Deposit Exploration

ABSTRACT Fluid inclusions provide the only direct samples of palaeofluids that may be related to mineralisation processes. In order to apply fluid inclusion data to study fluid flow on a larger scale, we have used a mineral systems approach, which regards a mineral deposit as part of a much larger system and considers all the processes that are involved in mobilising ore components from a source, transporting and accumulating them in a more concentrated form and then preserving them throughout the subsequent geological history. This not only enables a better understanding of fluid flow processes but also enables fluid inclusions to provide an exploration target that is much larger than the ore deposit itself. As an example, a study of fluid inclusions associated with gold mineralisation in the Tanami Region of Northern Australia was used to determine the temperatures and compositions of the ore fluids. Once the parameters of the mineralising fluids were established, the study was then expanded to a region of central Australia covering almost 100,000 km2. It was concluded that a high temperature (320 – 360 °C), low salinity fluid containing CO2 and other gases was circulating in the northern part of this region at around 1720 Ma. This suggests the circulation of an orogenic gold style fluid and indicates that this region has potential for other orogenic gold deposits. In the southern part of this region, a lower temperature (120 to 190 °C), high salinity fluid with no detectable gases was present and appears to represent circulation of a basinal brine. In the second example, fluid inclusion data from Cu-U-Au-Ag-REE prospects in the Olympic Copper-Gold Province in South Australia were used to constrain geochemical modelling of the mineralisation processes. By combining the inversion-generated, 3-D geophysical maps with geochemical and magnetic susceptibility values derived from the modelling, it has been possible to divide the range of observed magnetic susceptibilities into divisions that represent the various alteration assemblages within this region. This approach allows us to use geochemical modelling to relate alteration assemblages, and hence, the predicted sites of mineralisation, to the geophysical expressions of the mineral deposit.

Isotope geochemistry of the Sarekuobu metavolcanic-hosted gold deposit in the Chinese Altay (NW China): Implications for the fluid and metal sources

The Late Triassic Sarekuobu lode gold (Au) deposit (reserves: 8.00 t Au at 3.68 g/t) is situated in the Kelan volcanic basin of the Chinese Altay Orogen, NW China. Gold mineralization is hosted in the deformed metavolcanic sequences of the Devonian Kangbutiebao Formation. Multi-isotope analyses were conducted to constrain the ore-forming fluid and metal sources of the Sarekuobu deposit. The measured δDV-SMOW values of the fluid inclusions from the mineralized quartz (stages II and III) are of −132.9‰ to −78.5‰. The measured δ18OV-SMOW values of the quartz range from 11‰ to 13.6‰ (the corresponding calculated δ18OH2O values for the ore fluids: 4.2–6.9‰), which are consistent with those of typical orogenic Au deposits worldwide and fall between the metamorphic water and meteoric water fields in the δ18OH2O vs. δDV-SMOW diagram. This implies that the ore-forming fluids were mainly derived from metamorphic devolatilization with some meteoric water input. Calcite in the late mineralization (Stage III) veinlets contains δ13CV-PDB of −5.2‰ to −1.2‰ and δ18OOV-PDB of −20.4‰ to −19.9‰, similar to the carbonates (limestone and marble) of the Kangbutiebao Formation, and plots near the marine/continental carbonate field(s) in the δ13CV-PDB vs. δ18OOV-PDB diagram. This suggests that the carbon in the late-stage hydrothermal fluids was originated from marine/terrestrial carbonates with organic sediment decarboxylation input. The δ34S values for the Au-hosting sulfides (1.8–8.2 ‰) overlap with the volcanic wall rocks of the Kangbutiebao Formation (−4.7–18.7‰), which suggests that the sulfur was most likely wall rock-derived. The Sarekuobu sulfides contain 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 18.071–18.874, 15.520–15.669, and 37.860–38.961, respectively. The sulfide Sr and Nd isotopes are featured by their narrow compositional ranges and crustal affinities (i.e., (87Sr/86Sr)i = 0.70959–0.72216 and calculated εNd(t) = −3.20000 to −1.86000). All the Pb, Sr and Nd isotope compositions of the Sarekuobu ore sulfides are similar to those of the Kangbutiebao Formation wall rocks, indicating that the metals were mainly wall rock-derived. The structurally-controlled nature, the wall-rock metamorphism and alteration styles (e.g., silicic, chlorite, epidote and carbonate), and the abundance of CO2-rich and low-salinity fluid inclusions of the Sarekuobu deposit are similar to typical orogenic Au deposits. We therefore propose that the Sarekuobu Au deposit is a typical metavolcanic-hosted orogenic Au deposit, and was likely formed in a Triassic collisional-related setting.

Physicochemical controls on bismuth mineralization: An example from Moutoulas, Serifos Island, Cyclades, Greece

The 11.6 to 9.5 Ma Serifos pluton intruded schists and marbles of the Cycladic Blueschist unit, causing thermal metamorphism, the development of magnetite Ca-exo- and endo-skarns and the formation of low-temperature vein and carbonate-replacement ores. Potentially, the most important ores occur in the Moutoulas prospect where the mineralization in retrograde skarn and quartz veins culminated with the deposition of native bismuth. A combination of fluid inclusion microthermometry and isotope geothermometry suggests that the Moutoulas mineralization formed at a hydrostatic pressure of ~100 bars, from moderate-to-low temperature (~190–250 °C), and low-salinity (1.3–5.6 wt% NaCl equivalent) fluids. The calculated δ 34 S H 2 S compositions are consistent with the ore fluids having been derived from the Serifos pluton. Bismuth mineralization is interpreted to have occurred as a result of wall-rock interaction and mixing of a Bi-bearing ore fluid with meteoric waters. Native bismuth and bismuthinite deposited at ~200 °C, near neutral pH (6.5), low f S 2 ( f O 2 (

Geology, U-Pb geochronology, and stable isotope geochemistry of the Tunca semi-massive sulfide mineralization, Black Sea region, NE Turkey: Implications for ore genesis

Upper Cretaceous volcano-sedimentary sequences of the Eastern Pontide orogenic belt, NE Turkey, are host to significant VMS mineralization, including near Tunca. The initial stages of felsic volcanism within the mineralized area are marked by the eruption of dacitic lavas and breccias of the Kızılkaya Formation. This was accompanied by the emplacement of domelike hematitic dacites. Autobrecciated and volcaniclastic rocks, both in situ and resedimented, were likely generated from extrusive portions of these dacite bodies. Basaltic volcanism is marked by the eruption of the lava flows and pillow lavas of the Çağlayan Formation. Hiatuses in basaltic activity are marked by thin horizons of volcaniclastics and mudstones. The uppermost felsic volcanic units were accompanied by resedimentation of autoclastic facies from previous volcanism and represent the latest phase of Upper Cretaceous volcanism in the area. The semi-massive sulfide mineralization is associated with a late stage of the initial felsic volcanism. U-Pb LA-ICP-MS zircon dating of a dacitic tuff breccia yielded an age of 88.1±1.2Ma (Coniacian-Upper Cretaceous), which is interpreted to be the age of the sulfide occurrences.A concentric zoned alteration pattern is observed in the footwall rocks. The alteration pattern is considered to have formed by lateral migration of hydrothermal fluids which had ascended along the discharge conduit. Fluid inclusion data indicate precipitation or mobilization processes within a relatively narrow temperature range of 152–255°C (avg. 200°C). The low-salinity fluids in the fluid inclusions, less than 5.9wt% NaCl equivalent, are consistent with typical modified seawater-dominant hydrothermal vent fluids. Sulfur isotope analysis of the Tunca sulfides yields a narrow range of 1.5–4.1per mil. These δ34S values are also typical of many VMS deposits. Most of the recorded δ18O values (+7.1 to +14.0permil) are greater than 9per mil. The most intensely hydrothermally altered rocks tend to have lower δ18O values relative to the less altered rocks. Collectively, the geologic relationships, mineralization style, and the lack of seafloor ore facies suggest that mineralization is principally of sub-seafloor origin. The most geologically reasonable interpretation of the genesis of the Tunca mineralization is the continuous interaction between the host rocks and seawater-derived fluids, without significant involvement of a magmatic fluid.

Geology, mineralization, and fluid inclusion characteristics of the Lermontovskoe reduced-type tungsten (±Cu, Au, Bi) skarn deposit, Sikhote-Alin, Russia

The Lermontovskoe deposit (∼48Kt WO3; average 2.6% WO3, 0.24% Cu, 0.23g/t Au) is situated in a W-Sn-Au metallogenic belt that formed in a collisional tectonic environment. This tungsten skarn deposit has a W-Au-As-Bi-Te-Sb signature that suggests an affinity with reduced intrusion-related Au deposits. The deposit is associated with an intrusion that is part of the ilmenite-series, high-K peraluminous granitoid (granodiorite to granite) suite. These rocks formed via mantle magma-induced melting of crustal sources.The deposit comprises reduced-type, pyroxene-dominated prograde and retrograde skarns followed by hydrosilicate (amphibole-chlorite-pyrrhotite-scheelite-quartz) and phyllic (muscovite/sericite-carbonate-albite-quartz-scheelite-sulfide, with abundant apatite) alteration assemblages. Fluid inclusions from the skarn assemblages indicate high-temperature (>500°C), high-pressure (1400–1500bars) and high-salinity (53–60wt% NaCl-equiv.) magmatic-hydrothermal fluids. They were post-dated by high-carbonic, methane-dominate, low-salinity fluid at the hydrosilicate alteration stage. These fluids boiled at 360–380°C and 1300–1400bars. The subsequent phyllic alteration started again with a high-temperature (>450°C), high-pressure (1000–1100bars) and high-salinity (42–47wt% NaCl-equiv.) fluid, with further incursion of high-carbonic, methane-dominated, low-salinity fluid that boiled at 390–420°C and 1150–1200bars. The latest phyllic alteration included the lower-temperature (340–360°C), lower pressure (370–400bars) high-carbonic, methane-dominated (but with higher CO2 fraction), low-salinity fluid, and then the low-temperature (250–300°C) H2O-CO2-CH4-NaCl fluid, with both fluids boiled at the deposit level. The high-salinity aqueous fluids are interpreted to have come from crystallizing granitoid magma, whereas the reduced high-carbonic fluids probably came from a deeper mafic magma source. Both of these fluids potentially contributed to the W-Au-As-Bi-Te-Sb metal budget. Decreasing temperatures coupled with high aCa2+ and fluid boiling promoted scheelite deposition at all post-skarn hydrothermal stages.The deposit is characterized by limited downdip extent of mineralized zones and abundant coarse-grained muscovite-quartz (+apatite, scheelite) aggregates that formed at the phyllic alteration stage. Together with presence of high-temperature, high-pressure and high-salinity fluids directly exsolving from crystallizing magma, this suggests a root level of the mineralized magmatic-hydrothermal system of reduced W skarn deposits.

Hydrothermal Fluid Origins of Carbonate-Hosted Pb-Zn Deposits of the Sanjiang Thrust Belt, Tibet：Indications from Noble Gases and Halogens

The Sanjiang metallogenic belt includes a variety of economically important carbonate-hosted Pb-Zn deposits that share some similarities with classic Mississippi Valley-type (MVT) ore deposits but are hosted within a thrust belt rather than an orogenic foreland. This study aims to clarify the origin of mineralizing fluids responsible for this style of mineralization. Fluid inclusions trapped in ore-stage carbonate and fluorite from these deposits have salinities of ~6 to 28 wt % NaCl equiv and homogenization temperatures of 70° to 370°C that extend to much higher values than are typical of MVT deposits. The majority of ore-stage samples have fluid inclusion molar Br/Cl ratios of between seawater (1.5 × 10 −3 ) and (2.86 ± 0.04) × 10 −3 , but low-salinity fluid inclusions in late calcite have lower Br/Cl of less than (0.55 ± 0.01) × 10 −3 . In contrast, fluid inclusion molar I/Cl ratios are uniformly greater than the seawater value of ~0.8 × 10 −6 and extend from (2.1 ± 1.1) × 10 −6 to (506 ± 12) × 10 −6 . This range of Br/Cl and I/Cl values is similar to what has been reported for fluid inclusions in other MVT districts and together with the fluid salinity implies the ore-forming fluids had a dominant origin from basinal brines (e.g., sedimentary formation waters) formed by the subaerial evaporation of seawater; all the fluids were influenced by addition of organic Br and I derived from the sedimentary host rocks and some fluids were locally modified by interaction with evaporites producing low Br/Cl ratios. The fluid inclusions have 40 Ar/ 36 Ar ratios of up to 441 that are higher than the atmospheric value of 296 and typical of carbonate sedimentary rocks. The fluid inclusions have high concentrations of atmospheric 36 Ar and variable 129 Xe/ 36 Ar and 84 Kr/ 36 Ar ratios that are outside the range expected from mixing air and air-saturated water. These data are likely to reflect a complex fluid history involving acquisition of atmospheric ( 36 Ar, 84 Kr, 129 Xe) and radiogenic (e.g., 40 Ar ✼ ) noble gases trapped in sedimentary rocks and fractionation of these gases between water and hydrocarbons. The 3 He/ 4 He ratios of fluorite fluid inclusions range from a typical crustal value of 0.061 ± 0.004 to values of >0.7 Ra, indicating a minor component of mantle-derived 3 He. The fluids with the highest 3 He/ 4 He also have 4 He/ 40 Ar ✼ close to the mantle value, suggesting the 3 He could have been introduced by a volumetrically minor fluid of either magmatic or deep metamorphic origin ( 40 Ar ✼ = radiogenic 40 Ar). The new halogen and noble gas data are consistent with a model in which regional Pb-Zn mineralization formed by mixing two modified basinal brines that were transported through independent aquifers and fluid pathways to the sites of mineralization. A low-temperature brine contained organic Br, I, and H 2 S, and a high-temperature metal-rich brine (>370°C) that included a volumetrically minor magmato-metamorphic component was channeled up deeply penetrating thrust structures.

Physical and Chemical Evolution of the Dabaoshan Porphyry Mo Deposit, South China: Insights from Fluid Inclusions, Cathodoluminescence, and Trace Elements in Quartz

The Dabaoshan polymetallic deposits in the Nanling Range, South China, consist of porphyry and skarn-type Mo mineralization genetically related to Jurassic porphyritic intrusions and adjacent strata-bound Cu-Pb-Zn mineralization hosted in mid-Devonian limestone. Porphyry Mo mineralization is characterized by the superposition of multiple generations of crosscutting quartz-bearing veins including: barren quartz veins (V1), quartz-molybdenite veins with K-feldspar alteration halos that host the bulk of the Mo mineralization (V2), quartz-pyrite veins with muscovite alteration (V3), and late base metal mineralization with argillic alteration (V4), as well as limestone-hosted strata-bound Cu-Pb-Zn mineralization (VS). Fluid inclusion petrography and microthermometry combined with cathodoluminescent textures and trace elements in quartz reveal changes in pressure and temperature of the hydrothermal system that formed the deposit. V1 and V2 veins are dominated by low-salinity (1–6 wt % NaCl equiv), CO 2 -bearing (4–10 mol %) two-phase inclusions with about 35 vol % bubble trapped in the one-phase field above the solvus of the fluid. V1 veins are dominated by CL-bright granular quartz mosaics with higher CL intensity than any other vein type. This quartz also contains more Ti than any other vein generation (24–89 ppm), while Al, Ge, and Li concentrations overlap with other vein generations. Molybdenum ore-hosting V2 veins are also dominated by CL-bright granular quartz mosaics, but V2 veins display slightly less CL intensity and correspondingly lower Ti concentrations (10–65 ppm). V3 veins have a broader spatial distribution than V1 and V2 veins, extending from inside the porphyries out into the adjacent limestone. These veins are also dominated by low-salinity (2–6 wt % NaCl equiv), CO 2 -bearing (4–7 mol %) two-phase inclusions, these containing about 45 vol % bubble trapped above their solvus. V3 veins are dominated by CL-dark quartz with euhedral growth zones of oscillating CL intensity and systematically lower Ti concentrations than previous vein generations (1.5–12 ppm). Minor V4 veins cut all the above vein generations and also occur as late infill in V3 veins. Low-salinity (4–7 wt % NaCl equiv), CO 2 -bearing (4–5 mol %) two-phase inclusions with about 20 vol % bubble prevail in V4 veins. V4 veins have the lowest CL-intensity quartz of all vein types, with euhedral growth zones of oscillating CL intensity and the lowest Ti concentration of all vein types (0.54–5.3 ppm). Intersections of fluid inclusion isochores with Ti-in-quartz isopleths indicate that the hydrothermal system evolved from near-magmatic pressures and temperatures of 2.7 ± 0.2 kbars and 650° ± 40°C for V1 veins to 1.9 ± 0.2 kbars and 530° ± 40°C for V2 veins to 0.65 ± 0.2 kbars and 400° ± 40°C for V3 veins. V4 veins, which lack rutile, formed as the system cooled to 250° to 300°C at maximum pressures of 0.40 to 0.65 kbar. Unlike nearly all other reported porphyry-type ore deposits, quartz-bearing veins from the Dabaoshan porphyry Mo deposit contain few halite-bearing or vapor-dominated fluid inclusions in any vein type. The dearth of such fluid inclusions, coupled with the abundance of two- and three-phase CO 2 -bearing low-salinity fluid inclusions in all vein types is evidence that the formation conditions of V1 to V4 veins remained above the V-L surface in the H 2 O-NaCl-CO 2 system, such that fluid unmixing rarely occurred in the Dabaoshan hydrothermal system. This is further supported by only rare evidence for quartz dissolution textures in all vein types, implying that pressures were dominantly higher than zone of retrograde quartz solubility. Taken together, the fluid inclusions, CL textures, and quartz trace element data indicate that the Dabaoshan porphyry Mo deposit is one of the deepest formed porphyry-type ore deposits, having formed at depths of 6 to 7 km below surface. The extreme depth and lack of fluid unmixing inhibited Cu precipitation in the porphyry system, and instead allowed Cu to remain in solution to precipitate with Pb and Zn upon interaction with the surrounding Devonian limestone.

Geology, mineralization, stable isotope, and fluid inclusion characteristics of the Vostok-2 reduced W-Cu skarn and Au-W-Bi-As stockwork deposit, Sikhote-Alin, Russia

The large (>180KtWO3 and at least 10–15 t Au) Vostok-2 deposit is situated in a metallogenic belt of W, Sn-W, Au, and Au-W deposits formed in late to post-collisional tectonic environment after cessation of active subduction. The deposit is related to an ilmenite-series high-K calc-alkaline plutonic suite that, by its petrologic signatures, is transitional between those at W-dominant and Au-dominant reduced intrusion-related deposits. Consistently, besides large W-Cu skarns of the reduced type, the deposit incorporates quartz stockworks with significant Au-W-Bi mineralization also formed in a reduced environment. The hydrothermal stages include prograde and retrograde, essentially pyroxene skarns, hydrosilicate (amphibole, chlorite, quartz) alteration, and phyllic (quartz, sericite, albite, apatite, and carbonate) alteration assemblages. These assemblages contain abundant scheelite associated with pyrrhotite, chalcopyrite and, at the phyllic stage, also with Bi minerals, As-Bi-Sb-Te-Pb-Zn sulfides and sulfosalts, as well as Au mineralization. The fluid evolution included hot, high-pressure (420–460°C, 1.1–1.2kbar), low-salinity (5.4–6.0wt% NaCl-equiv.) aqueous fluids at the retrograde skarn stage, followed by lower temperature cyclic releases of high-carbonic, low salinity to non-carbonic moderate-salinity aqueous fluids. At the hydrosilicate stage, a high-carbonic, CH4-dominated, hot (350–380°C) low salinity fluid was followed by cooler (300–350°C) non-carbonic moderate-salinity (5.7–14.9wt% NaCl-equiv.) fluid. At the phyllic stage, a high-carbonic, CO2-dominated, moderately-hot (330–355°C, 0.9kbar) low salinity fluid was followed by cooler (230–265°C) non-carbonic moderate-salinity (6.6–12.0wt% NaCl-equiv.) fluid. A homogenized magmatic source of water (δ18OH2O=+8.3 to +8.7‰), and a sedimentary source of sulfur (δ34S=−6.9 to −6.2‰) and carbon (δ13Cfluid=−20.1 to −14.9‰) at the hydrosilicate stage are suggested. A magmatic source of water (δ18O=+8.6 to +9.2‰) and a sedimentary source of sulfur (δ34S=−9.3 to −4.1‰) but a magmatic (mantle- to crustal-derived) source of carbon (δ13Cfluid=−6.9 to −5.2‰) are envisaged for fluids that formed the early mineral assemblage of the phyllic stage. Then, the role of sedimentary carbon again increased toward the intermediate (δ13Cfluid=−16.4 to −14.5‰) and late (δ13Cfluid=−16.3 to −14.7‰) phyllic mineral assemblages. The magmatic differentiation was responsible for the fluid enrichment in W, whereas Au and Bi could also have been sourced from mafic magma. The decreasing temperatures, together with elevated Ca content in non-boiling fluids, promoted scheelite deposition at the early hydrothermal stages. The most intense scheelite deposition at the phyllic stage was caused by CO2 removal due to boiling of CO2-rich fluids; further cooling of non-boiling fluids favoured joint deposition of scheelite, Bi and Au.

Geology and geochemistry of the sediment-hosted Cheshmeh-Konan redbed-type copper deposit, NW Iran

The several-hundred-m-thick Miocene Upper Red Formation in northwestern Iran hosts stratiform and fault-controlled copper mineralization. Copper enrichment in the percent range occurs in dm-thick carbonaceous sandstone and shale units within the clastic redbed sequence and consists of fine-grained disseminated copper sulfides (chalcopyrite, bornite, chalcocite) and supergene alteration minerals (covellite, malachite and azurite). The copper mineralization formed after calcite cementation of the primary rock permeability. Copper sulfides occur mainly as replacement of diagenetic pyrite, which, in turn, replaced organic matter. Electron microprobe analysis on bornite, chalcocite and covellite identifies elevated silver contents in these minerals (up to 0.12, 0.72 and 1.21wt%, respectively), whereas chalcopyrite and pyrite have only trace amounts of silver (<0.26 and 0.06wt%, respectively). Microthermometric data on fluid inclusions in authigenic quartz and calcite indicate that the Cu mineralization is related to a diagenetic fluid of moderate-to low temperature (Th=96–160°C) but high salinity (25–38wt% CaCl2 equiv.). The range of δ34S in pyrite is −41.9 to −16.4‰ (average −31.4‰), where framboidal pyrite shows the most negative values between −41.9 and −31.8‰, and fine-grained pyrite has relatively heavier δ34S values (−29.2 to −16.4‰), consistent with a bacteriogenic derivation of the sulfur. The Cu-sulfides (chalcopyrite, bornite and chalcocite) show slightly heavier values from −14.6 to −9.0‰, and their sulfur sources may be both the precursor pyrite-S and the bacterial reduction of sulfate-bearing basinal brines. Carbonates related to the ore stage show isotopically light values of δ13CV-PDB from −8.2 to −5.1‰ and δ18OV-PDB from −10.3 to −7.2‰, indicating a mixed source of oxidation of organic carbon (ca. −20‰) and HCO3− from seawater/porewater (ca. 0‰). The copper mineralization is mainly controlled by organic matter content and paleopermeability (intragranular space to large fracture patterns), enhanced by feldspar and calcite dissolution. The Cheshmeh-Konan deposit can be classified as a redbed-type sediment-hosted stratiform copper (SSC) deposit.

Thermobarometry in the Sarvian Fe-skarn deposit (Central Iran) based on garnet–pyroxene chemistry and fluid inclusion studies

The Sarvian Fe skarn deposit is located in the Urumieh–Dokhtar magmatic arc, western Iran. The Sarvian quartz diorite intruded the surrounding Permian to Tertiary limy formation, culminated in thermal metamorphism as well as skarnification in which the Sarvian deposit formed. Microthermometry studies in the Sarvian skarn deposit reveal two distinct inclusion groups; group A with medium–high temperature and hypersaline and group B with low–medium temperature and low salinity. Group A inclusions which are entrapped during formation of prograde are thought to be derived from the magmatic source. Fluid boiling and subsequent developing of hydraulic fracturing led to inflow and/or mixing of early magmatic fluids (group A) with circulating groundwater culminated in formation of low salinity and low temperature fluid inclusions (group B) during the formation of retrograde assemblage. Fluid inclusion thermometry reveals the formation temperature and the salinity of 300–370 °C and 31–33 wt% NaCl for the prograde stage and 180–230 °C and 1–15 wt% NaCl for the retrograde stage of skarnification at Sarvian skarn rocks. Fe-mineralization as well as hydrothermal minerals occurred during retrograde metasomatism. The estimated depth and pressure of occurrence for prograde stage are 1000–1200 m and 100–150 bars, and for retrograde stage, these are about 200 m and 50 bars, respectively. Garnet and pyroxene, as the main constituent minerals of prograde stage, are the most informative minerals offering a suitable tool to constrain the skarnification conditions. Garnets in the Sarvian deposit are mainly grossular and andradite, showing both normal and inverse zoning as the result of variation in their chemical composition. Such types of zoning represent alternation of high acidity oxidation and low acidity oxidation conditions that were prevailed on skarnification in the Sarvian prograde assemblage. Also, chemical composition of the Sarvian pyroxenes shows an alternation of high oxygen fugacity and low oxygen fugacity conditions for their formation. This is also supported by fluctuation of the ratios of andradite to grossular and diopside to hedenbergite, denoting to an obvious shifting that was prevailed between oxidizing and redox conditions during formation of prograde assemblage in the Sarvian. Garnet–pyroxene thermometry determines the formation temperature of prograde assemblage between 370 and 550 °C at Sarvian skarn rocks which is verified also by fluid inclusion thermometry. Decomposition of limestone by reaction of high-temperature hydrothermal fluids with carbonate host rock resulted in injection of CO2 into the Sarvian system that caused oxidation, changing Fe+2 to Fe+3 culminated in the magnetite deposition.

C-H-O Stable Isotope, Elements and Fluid Geochemistry of Uraniferous Leucogranites in Gaudeanmus Area, Southern Central Zone, Damara Orogen, Namibia

This paper focuses on the effect of the later hydrotherm on uraniferous leucogranites and the stages of uranium mineralization. Here, we review C-H-O stable isotope, elements and fluid geochemistry of uraniferous leucogranites in Gaudeanmus, Namibia. The results show that there is significant increasing amount of rare earth element from non-mineralized to uraniferous leucogra-nites, indicating the synchronization of REE enrichment and uranium mineralization. Uranium enrichment may have close relations with Pb, Th, Co, Ni, REE in this region, so REE and U evidently exist homology. There are at least two stages of uranium mineralization by later hydrothermal alteration: firstly, due to magnatic residual high temperature and low salinity fluid, the temperature of main metallogenetic epoch ranges from 470°C to 530°C, salinity ranges from 3.55% to 9.60% NaCleq, and C, H, O stable isotope is -23‰ - -13.6‰, -53.3‰ - -46.4‰, 7.71‰ - 8.81‰, respectively. Secondly, due to superim-posed hydrothermal fluid, the temperature, salinity, and C, H, O stable isotope is 150°C - 220°C, 4.65% - 19.05% NaCleq, -20.3‰ - -3.7‰, -64.7‰ - -53.6‰, 1.49‰ - 1.99‰, respectively. The fluid for reformation is derived from postmagmatic fluid, mixed with a number of meteoric water.

Identification of Two Types of Metallogenic Fluids in the Ultra-Large Huize Pb–Zn Deposit, SW China

This work investigates the ultra-large Huize Pb–Zn deposit, based on the results of preceding studies and detailed field geological surveys. The existing findings were reorganized and reinterpreted and supplemented with C–H–O isotopic measurements, which resulted in the identification of two different metallogenic fluids: a high temperature, low salinity, and acidic Fluid A, which originates from deep-seated fluids and is enriched in lighter C and O isotopes (−3‰&lt;δ13C‰&lt; −4‰; 10‰&lt;δ18O‰&lt; 17‰; −92‰&lt;δD‰&lt; −50‰), and a low temperature, high salinity Fluid B, which is a subsurface brine formed by atmospheric precipitation. Fluid B is characterized by heavier C–O–H isotopic compositions (−2‰&lt;δ13C‰&lt; 1‰; 2‰&lt;δ18O‰&lt; 24‰; −66‰&lt;δD‰&lt; −43‰) than Fluid A and cycles continuously within the strata. We hypothesize that the Huize Pb–Zn deposit is the result of large-scale fluid migration from deep regions of the crust. These upward-moving fluids extracted metallic elements from carbonate strata of various ages, forming a metal-rich metallogenic fluid (Fluid A). After higher-grade ores were precipitated from the fluid following decompression boiling, it then mixed with Fluid B and continued to precipitate sulfides.

Multistage Skarn-Related Tourmaline From the Galinge Deposit, Qiman Tagh, Western China: A Fluid Evolution Perspective

Abstract The Galinge skarn deposit, the largest iron polymetallic skarn deposit in the Qiman Tagh metallogenic belt (western China), was formed via multi-stage fluid-rock interactions. It is divided into six ore domains from east to the west. Skarn-related tourmaline is ubiquitous in the V ore domain of the Galinge deposit, occurring both in the altered basaltic andesite (Tour-I) and in the sandstone (Tour-II). The tourmaline composition in both rock types is within the dravite–uvite solid solution. Some Tour-I crystals show compositional growth zoning in which the early stage uvite cores (Gen-1) are overgrown by second-stage dravite rims (Gen-2). Some Tour-I crystals also show overgrowth rims and fracture-infilled textures (Gen-3). Some other Tour-1 tourmalines without clear growth zoning (Others) show an intermediate composition between Gen-2 and Gen-3. The varying composition of the zoned tourmalines records important information about the evolving hydrothermal fluids and host rocks. Gen-1 and Gen-2, displaying a narrow and high range of Fe 2+ /(Fe 2+ + Mg) ratios, are much more equilibrated with mafic host rocks. The alkaline (K + Na) content of tourmalines is associated with the salinities of the ore-forming fluids. The lowest Na + K content of Gen-3 indicates that it may have been equilibrated with a low-salinity fluid environment in which the concentration of metal-chlorite complexes decreased. The Gen-3 stage is considered to be the main ore-forming event. Tour-II have similar Ca/(K + Na + Ca) ratios with Gen-1 and Gen-2 ratios, which indicates that they are contemporarily formed by the same fluid as Tour-I. Through compositional comparison of the tourmalines with those from other hydrothermal deposit types, the Galinge skarn-related tourmalines are overwhelmingly controlled by the MgFe –1 substitution mechanism. This is different from the compositions of tourmalines in porphyry, VMS, and vein-greisen type deposits, which are, respectively, controlled by the Fe 3+ Al –1 , (CaMg)(NaAl) –1 and (NaMg)(□Al) –1 , and (Fe 2+ Fe 3+ )(MgAl) –1 substitution mechanisms. Different tourmaline compositions and substitution mechanisms could be used as guides for mineral exploration.

Fluid inclusion studies on Cu–Mo–Au bearing quartz–sulphide veins and veinlets in Qarachilar area, Qaradagh pluton (NW Iran)

The Qaradagh pluton is located in NW Iran. It is comprised of Eocene–Oligocene intrusive pulses and hosts hydrothermal alterations and vein-type Cu–Mo–Au mineralization, which is more manifested at Qarachilar area (central part of the Qaradagh pluton). This study aims to elucidate the physic–chemical characteristics of the ore-bearing fluid using microthermometric studies on quartz–sulphide veins/veinlets. Microthermometric analyses show that the salinity of ore-bearing fluid is about 15–65 wt% NaCl equiv , with the highest frequency between 25 and 45 wt% NaCl equiv . The homogenization temperature for primary 2-phase and multiphase inclusions ranges about 220–540°C. Most of the 2-phase inclusions homogenize by vapor disappearance with T H(L-V) values between 280 and 440°C. Most of the multiphase inclusions homogenize by simultaneous disappearance of vapor bubble and dissolution of halite daughter crystal, for which the T H value is 240–420°C. The data-points trend in T H(L-V) –Salinity plot indicates the occurrence of boiling of low-salinity fluids and dilution by superficial fluids. The minimum pressure at the time of fluid entrapment is estimated about 50 to 120 bars, equivalent to the hydrostatic depths of 500 to 1100 m.

Petrology, geochemistry and fluid inclusion analysis of altered komatiites of the Mendon Formation in the BARB4 drill core, Barberton greenstone belt, South Africa

The 3.33 to 3.26 Ga Mendon Formation in the Palaeoarchaean Barberton greenstone belt, South Africa, forms the uppermost unit of the Onverwacht Group. It is dominated by ultramafic volcanic rocks interbedded with thin layers of cherty sediments that show pervasive alteration, including widespread serpentinisation, silicification and chert and quartz veining. The BARB4 drill core of the ICDP Barberton drilling project exposes a unique section through the Mendon Formation in the Manzimnyama Syncline. The komatiites are pervasively altered to an assemblage comprising quartz, chlorite, carbonate, talc, biotite, and, locally, plagioclase, K-feldspar, muscovite, amphibole, stilpnomelane and ankerite. The overlying sediments are made up of banded iron formations and rare beds of siliciclastic rocks. Though the altered komatiites are pervasively silicified, SiO 2 contents do not exceed 58 wt.%, and their major and trace element geochemistry is similar to other komatiitic rocks of the Mendon Formation, particularly those of the M2v-member. Quartz veins and, less commonly, quartz-carbonate and quartz-carbonate-plagioclase veins are found throughout the core. Overall, composition and texture of the veins differ from primary and early diagenetic veins found in silicified komatiites elsewhere in the Barberton greenstone belt. In the BARB4 drill core, the veins are generally coarse-grained, and the immediate wall rocks are locally foliated along the vein margins. In addition, the δ 18 O values of vein quartz range from 14.1 to 15.3‰, significantly lower than the values typically found in veins on the modern seafloor that formed during low temperature hydrothermal seafloor alteration (~22 to 32‰). Fluid inclusions in vein quartz are homogeneous two-phase (L+V) aqueous inclusions that occur in trans- and intragranular trails and clusters. Intragranular and isolated fluid inclusions have a similar homogenisation temperature (T h ) of 130 to 200°C, with most data ranging between 145 and 175°C. Salinities cluster in three different groups of high (20 to 27wt.% NaCl equiv.), medium (10 to 15wt.% NaCl equiv.) and low salinity (0.3 to 1.5wt.% NaCl equiv.). The composition and microthermometric characteristics of the fluid inclusions analysed within the drill core show similarities to those found in quartz veins in silicified komatiites of the Mendon Formation, which are interpreted to have been entrapped during metamorphism. P-T calculations based on fluid inclusion microthermometry reveal conditions of 230 to 400 MPa and 250 to 400°C. Similar conditions of 240 to 270°C have been obtained using oxygen isotope thermometry, assuming a metamorphic fluid with a δ 18 O value of 6‰. Collectively, the δ 18 O values, together with the texture and composition of the veins, are interpreted to indicate a metamorphic origin of the veins. The presence of high salinity inclusions indicates the occurrence of a highly saline fluid that locally mixed with the dominant lower salinity fluids. The high salinity might have been derived from fluid circulation through evaporites.

Source of boron in the Palokas gold deposit, northern Finland: evidence from boron isotopes and major element composition of tourmaline

The recently discovered Palokas gold deposit is part of the larger Rompas-Rajapalot gold-mineralized system located in the Paleoproterozoic Perapohja Belt, northern Finland. Tourmaline is an important gangue mineral in the Palokas gold mineralization. It occurs as tourmalinite veins and as tourmaline crystals in sulfide-rich metasomatized gold-bearing rocks. In order to understand the origin of tourmaline in the gold-mineralized rocks, we have investigated the major element chemistry and boron isotope composition of tourmaline from three areas: (1) the Palokas gold mineralization, (2) a pegmatitic tourmaline granite, and (3) the evaporitic Petajaskoski Formation. Based on textural evidence, tourmaline in gold mineralization is divided into two different types. Type 1 is located within the host rock and is cut by rock-forming anthophyllite crystals. Type 2 occurs in late veins and/or breccia zones consisting of approximately 80% tourmaline and 20% sulfides, commonly adjacent to quartz veins. All the studied tourmaline samples belong to the alkali-group tourmaline and can be classified as dravite and schorl. The δ11B values of the three localities lie in the same range, from 0 to −4‰. Tourmaline from the Au mineralization and from the Petajaskoski Formation has similar compositional trends. Mg is the major substituent for Al; inferred low Fe3+/Fe2+ ratios and Na values (<0.8 atoms per formula unit (apfu)) of all tourmaline samples suggest that they precipitated from reduced, low-salinity fluids. Based on the similar chemical and boron isotope composition and the Re–Os age of molybdenite related to the tourmaline-sulfide-quartz veins, we propose that the tourmaline-forming process is a result of a single magmatic-hydrothermal event related to the extensive granite magmatism at around 1.79–1.77 Ga. Tourmaline was crystallized throughout the hydrothermal process, which resulted in the paragenetic variation between type 1 and type 2. The close association of tourmaline and gold suggests that the gold precipitated from the same boron-rich source as tourmaline.