CO2 H2O Inclusions Research Articles

Ore genesis of the Saridala gold deposit, Western Tianshan, NW China: Constraints from fluid inclusion, S-Pb isotopes and 40Ar/39Ar dating

The Saridala gold deposit is located in the Western Tianshan (Xinjiang, NW China), southwestern part of the Central Asian Orogenic belt (CAOB). The deposit is mainly hosted in Precambrian metamorphic rocks and Early Paleozoic 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 comprises three stages: quartz-pyrite veins (early stage), sulfide-quartz veins (middle stage), and quartz-carbonate veins/veinlets (late stage). Ore minerals and native gold were mainly formed in the middle stage. Four fluid inclusion types in quartz veins were identified: pure CO2 (PC-type), CO2-H2O (C-type), aqueous (W-type) and daughter mineral-bearing (S-type) inclusions. Hydrothermal fluids related to the mineralization are composed of CO2-H2O-NaCl±N2 fluids (115–307MPa) with Th(total) values of 256–475°C (peak at 320–380°C) and salinities <∼11 wt% NaCl equiv. (mostly 4–7 wt% NaCl equiv.). Ore minerals were likely deposited via fluid immiscibility and the subsequent fluid mixing. Except for the two negative outliners (−4.5 and −4.8‰), δ34S‰ values of the ore pyrite range from 11.0 to 17.9‰, suggesting that the ore-forming fluids contained heavier sulfur from marine sulfate-bearing rocks. Lead isotope compositions of the ore sulfides are characterized by being highly radiogenic and their relatively wide ranges: 18.01–19.62 (206Pb/204Pb), 15.58–15.78 (207Pb/204Pb) and 37.95–39.08 (208Pb/204Pb). These data indicate that the lead in the ores may have been derived from an orogenic reservoir with some crustal input. 40Ar/39Ar age dating on the sericite from the syn-mineralized quartz-sericite-sulfide alteration yielded a plateau age of 337.6±1.7Ma, an age coeval with the subduction–collision tectonic transition related to the closure of the North Tianshan Ocean. We concluded that the Saridala gold deposit shares many similarities with typical orogenic gold deposits, and was likely formed in an orogenic environment.

Carbonic fluids in the Hamadi gold deposit, Sudan: origin and contribution to gold mineralization

The Hamadi gold deposit is located in North Sudan, and occurs in the Neoproterozoic metamorphic strata of the Arabian–Nubian Shield. Two types of gold mineralization can be discerned: gold-bearing quartz veins and altered rock ores near ductile shear zones. The gold-bearing quartz veins are composed of white to gray quartz associated with small amounts of pyrite and other polymetallic sulfide minerals. Wall-rock alterations include mainly beresitization, epidotization, chloritization, and carbonatization. CO2-rich inclusions are commonly seen in gold-bearing quartz veins and quartz veinlets from gold-bearing altered rocks; these include mainly one-phase carbonic (CO2 ± CH4 ± N2) inclusions and CO2–H2O inclusions with CO2/H2O volumetric ratios of 30% to ∼80%. Laser Raman analysis does not show the H2O peak in carbonic inclusions. In quartz veins, the melting temperature of solid CO2 (Tm,CO2) of carbonic inclusions has a narrow range of −59.6 to −56.8 °C. Carbonic inclusions also have CO2 partial homogenization temperatures (Th,CO2) of −28.3 to +23.7 °C, with most of the values clustering between +4.0 and +20 °C; all of these inclusions are homogenized into the liquid CO2 state. The densities range from 0.73 to 1.03 g/cm3. XCH4 of carbonic fluid inclusions ranges from 0.004 to 0.14, with most XCH4 around 0.05. In CO2–H2O fluid inclusions, Tm,CO2 values are recorded mostly at around −57.5 °C. The melting temperature of clathrate is 3.8–8.9 °C. It is suggested that the lowest trapping pressures of CO2 fluids would be 100 to ∼400 MPa, on the basis of the Th,CO2 of CO2-bearing one-phase (LCO2) inclusions and the total homogenization temperatures (Th,tot) of paragenetic CO2-bearing two-phase (LCO2–LH2O) inclusions. For altered rocks, the Tm,CO2 of the carbonic inclusions has a narrow range of −58.4 to ∼−57.0 °C, whereas the Th,CO2 varies widely (−19 to ∼+29 °C). Most carbonic inclusions and the carbonic phases in the CO2–H2O inclusions are homogenized to liquid CO2 phases, which correspond to densities of 0.70 to ∼1.00 g/cm3. Fluid inclusions in a single fluid inclusion assemblage (FIA) have narrow Tm,CO2 and Th,CO2 values, but they vary widely in different FIAs and non-FIAs, which indicates that there was a wide range of trapping pressure and temperature (P–T) conditions during the ore-forming process in late retrograde metamorphism after the metamorphism peak period. The carbonic inclusions in the Hamadi gold deposit are interpreted to have resulted from unmixing of an originally homogeneous aqueous–carbonic mixture during retrogress metamorphism caused by decreasing P–T conditions. CO2 contributed to gold mineralization by buffering the pH range and increasing the gold concentration in the fluids.

Mechanism of mineralization in the Changjiang uranium ore field, South China: Evidence from fluid inclusions, hydrothermal alteration, and H–O isotopes

The Changjiang uranium ore field, which contains >10,000tonnes of recoverable U with a grade of 0.1–0.5%, is hosted by Triassic two-mica and Jurassic biotite granites, and is one of the most important uranium ore fields in South China. The minerals associated with alteration and mineralization can be divided into two stages, namely syn-ore and post-ore. The syn-ore minerals are primarily quartz, pitchblende, hematite, hydromica, chlorite, fluorite, and pyrite; the post-ore minerals include quartz, calcite, fluorite, pyrite, and hematite. The fluid inclusions of the early syn-ore stage characteristically contain O2, and those of the late syn-ore and post-ore stage contain H2 and CH4. The fluid inclusions in quartz of the syn-ore stage include H2O, H2O–CO2, and CO2 types, and they occur in clusters or along trails. Homogenization temperatures (Th) for the H2O–CO2 and two-phase H2O inclusions range from 106°C to >350°C and cluster in two distinct groups for each type; salinities are lower than 10wt% NaCl equiv. The ore-forming fluids underwent CO2 effervescence or phase separation at ∼250°C under a pressure of 1000–1100bar. The U/Th values of the altered granites are lowest close to the ore, increase outwards, but subsequently decrease close to unaltered granites. From the unaltered granites to the ore, the lowest Fe2O3/FeO values become lower and the highest values higher. The REE patterns of the altered granites and the ores are similar to each other. The U contents of the ores show a positive correlation with total REE contents but a negative correlation with LREE/HREE ratios, indicating the pitchblende is REE-bearing and selectively HREE-rich. The δEu values of the ore show a positive correlation with U contents, indicating the early syn-ore fluids were oxidizing. The δCe values show a negative correlation, indicating the later mineralization environment became reducing. The water–rock interactions of the early syn-ore stage resulted in oxidization of altered granites and reduction of the ore-forming fluids, and it was this reduction that led to the uranium mineralization. During alteration in the early syn-ore stage, the oxidizing fluids leached uranium from granites close to faults, and Fe2O3/FeO ratios increased in the alteration zones. The late syn-ore and post-ore alteration decreased the Fe2O3/FeO ratios in the alteration zones. The δ18OW–SMOW values of the ore-forming fluids range from −1.8‰ to 5.4‰, and the δDW–SMOW values range from −104.4‰ to −51.6‰, suggesting meteoric water. The meteoric water underwent at least two stages of water–rock interaction: the first caused the fluids to become uranium-bearing, and the second stage, which was primarily associated with ore-bearing faults, led to uranium deposition as pitchblende, accompanied by CO2 effervescence.

A Fluid Inclusion Study of Vein‐type Copper Mineralization in the East Wulasigou Pb–Zn–Cu Deposit, Altay, Northwestern China

AbstractThe Wulasigou Cu–Pb–Zn deposit, located 15km northwest of Altay city in Xinjiang, is one of many Cu–Pb–Zn polymetallic deposits in the Devonian Kelan volcanic‐sedimentary basin in southern Altaids. Two mineralizing periods can be distinguished: the marine volcanic sedimentary Pb‐Zn mineralization period, and the metamorphic hydrothermal Cu mineralization period, which is further divided into an early bedded foliated quartz vein stage (Q1) and a late sulfide‐quartz vein stage (Q2) crosscutting the foliation. Four types of fluid inclusions were recognized in the Q1 and Q2 quartz from the east orebodies of the Wulasigou deposit: H2O–CO2 inclusions, carbonic fluid inclusions, aqueous fluid inclusions, and daughter mineral‐bearing fluid inclusions. Microthermometric studies show that solid CO2 melting temperatures (Tm,co2) of H2O–CO2 inclusions in Q1 are from −62.3°C to −58.5°C, clathrate melting temperatures (Tm,clath) are from 0.5°C to 7.5°C, partial homogenization temperatures (Th,co2) vary from 3.3°C to 25.9°C (to liquid), and the total homogenization temperatures (Th,tot) vary from 285°C to 378°C, with the salinities being 4.9%‐15.1% NaCl eqv. and the CO2‐phase densities being 0.50–0.86 g/cm3. H2O–CO2 inclusions in Q2 have Tm,c02 from −61.9°C to −56.9°C, Tm,clath from 1.3°C to 9.5°C, Th,co2 from 3.4°C to 28.7°C (to liquid), and Th,tot from 242°C to 388°C, with the salinities being 1.0%‐15.5% NaCl eqv. and the CO2‐phase densities being 0.48–0.89 g/cm3. The minimum trapping pressures of fluid inclusions in Q1 and Q2 are estimated to be 260–360 MPa and 180–370 MPa, respectively. The δ34S values of pyrite from the volcanic sedimentary period vary from 2.3‰ to 2.8‰ (CDT), and those from the sulfide‐quartz veins fall in a narrow range of −1.9‰ to 2.6‰ (CDT). The δD values of fluid inclusions in Q2 range from −121.0‰ to −100.8‰ (SMOW), and the δ18OH2O values calculated from δ18O of quartz range from −0.2‰ to 8.3‰ (SMOW). The δD‐δ18OH2O data are close to the magmatic and metamorphic fields. The fluid inclusion and stable isotope data documented in this study indicate that the vein‐type copper mineralization in the Wulasigou Pb–Zn–Cu deposit took place in an orogenic‐metamorphic enviroment.

Genesis of the Haigou gold deposit, Jilin Province, NE China: evidence from fluid inclusions,40Ar/39Ar geochronology and isotopes

The Haigou gold deposit is one of the largest known lode gold deposits in the Jiapigou‐Haigou gold belt of the Yanbian area of NE China. Although this area contains a significant amount of gold mineralization, identifying new resources has been problematic. Here, we present the results of a systematic study of the ore geology and fluid inclusion characteristics of deep‐seated mineralization in this area, as compiled from previous research. We use these data to determine the genetic processes that formed these deposits and outline the key criteria that should be used for future exploration. The Haigou deposit is hosted by a monzonitic granite (monzonite) with mineralization present within quartz veins. The gold mineralization is associated with silicification, and four stages of mineralization have been identified: milky quartz, pyrite‐quartz, quartz‐polymetallic sulphides and quartz‐carbonate. The second and third stages host the majority of the mineralization. Three main types of fluid inclusion are present within the deposit: CO2–H2O, aqueous and pure CO2. The early milky quartz vein stage of mineralization hosts CO2–H2O and aqueous inclusions, whereas the main stages of mineralization are associated with all three types of fluid inclusions, which are randomly distributed but are especially well developed within pyrite‐quartz veins. These stages of mineralization are also associated with clusters of CO2–H2O inclusions that coexist with large but variable amounts of aqueous phase fluid inclusions. The only fluid inclusions associated with the later stages of mineralization are aqueous inclusions within calcite‐quartz veins. Microthermometric data indicate that the inclusions associated with metallogenesis homogenize at 145–410 °C and contain medium‐ to low‐salinity fluids (&lt;11.93 wt.% NaCl equivalent). The mineralization in this area formed during the Middle Jurassic (163–161 Ma), as evidenced by40Ar/39Ar dating of hydrothermal sericite from the main stage of mineralization. Integrated C‐H‐O‐S‐Pb isotope analysis indicates that the initial ore‐forming fluid was derived from CO2‐rich magmas produced by the anatexis of lower crustal material. This fluid is then mixed and underwent fluid immiscibility during ore formation. The Haigou gold deposit is a typical medium‐ to low‐temperature quartz lode gold deposit that formed after the collision of continental terranes during the transition from compressive to extensional tectonics. The data presented here indicate that this area has significant potential for deep‐seated mineralization; an area that should be the focus of future exploration. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Syntectonic fluids redistribution and circulation coupled to quartz recrystallization in the ductile crust (Naxos Island, Cyclades, Greece)

The presence of external fluids in metamorphic rocks has been shown to have a profound impact on rock rheology as high fluid pressure processes promote embrittlement and favor ductile deformation by recrystallization. Moreover, it has been proposed that brittle deformation guides fluid circulation and that intracrystalline deformation is responsible for fluid redistribution at the grain scale. Nevertheless, the amount of fluid present in the metamorphic ductile crust is debated and the nature of the interaction between fluids and recrystallization processes are not clearly identified. The aim of this study is to document the spatial distribution of fluid inclusions relative to microstructures in quartz grains and aggregates from veins sampled in amphibolite facies metamorphic rocks, exposed in the island of Naxos in the center of the Attic-Cycladic Metamorphic Complex in Greece. The veins, ranging from discordant structures with sharp contacts to totally transposed structures into the metamorphic foliation, display a large variety of microstructures and fluid evidences interpreted as recording exhumation processes through the ductile/brittle transition: (i) remnants of primary quartz grains contain abundant CO2-H2O fluid inclusions, decrepitated for the most part, distributed in clusters and in fluid inclusion trails, (ii) fluid inclusions with a similar composition are less abundant in recrystallized zones and in subgrains but are concentrated along grain boundaries indicating that grain boundary migration is responsible for redistribution of CO2-H2O fluids, (iii) subgrains of the last generation are almost devoid of fluid inclusions and are characterized by thick grain boundaries with abundant metamorphic fluids locally forming a continuous film. CO2-H2O fluid inclusions aligned in parallel, regularly spaced intragranular trails, locally rooted into grain boundaries, are interpreted as reflecting the spatial redistribution of these fluids in quartz slip planes owing to the increase of fluid pressure in grain boundaries. This proposition is corroborated by the parallelism between slip planes and fluid inclusion trails. Perpendicular sets of fluid inclusion trails formed at the junctions of subgrains suggest that redistribution of fluids along quartz slip planes contributes to the initiation of subgrain rotation recrystallization. Transgranular fluid inclusion planes oriented perpendicular to the regional mineral and stretching lineations of the host metamorphic rocks mark a transition from grain to rock scale brittle behavior associated with the infiltration of H2O fluid, mixing to various degrees with the initial CO2-H2O fluid already present in the metamorphic rocks. These features indicate that, in the ductile-metamorphic crust, fluid redistribution is intimately linked to recrystallization mechanisms allowing fluids circulation under lithostatic pressure.

Micro-textural and fluid inclusion data constraints on metallic remobilization of the Ashele VMS Cu-Zn deposit, Altay, NW China

The Ashele volcanogenic massive sulfide (VMS) Cu-Zn deposit is placed in the Devonian volcanic-sedimentary basin of the Chinese Altay Orogen, NW China. It occurs in the form of well-developed lensoid orebodies within the Devonian Ashele Formation. The orebodies display analogous deformational characteristics with respect to their host rocks. Two periods of ore formation are distinguishable in the Ashele VMS Cu-Zn deposit: the first one is related to the genesis of banded and massive ores, whereas the second one is characterized by the formation of earlier Cu-dominated- and later Pb-Zn dominated-polymetallic quartz veins. In most cases, the primary banded textures of ores are still preserved. However, some veins cross cut the earlier generated ores with brecciated, durchbewegung, recrystallization and pressure-solution textures produced, indicating that substantial remobilization of metals has occurred due to intense reworking of the VMS. Four types of fluid inclusions were recognized in Cu-dominated quartz veins encompassing CO2-H2O (C-type), aqueous water (W-type), daughter mineral-bearing (S-type) and carbonic (PC-type) fluid inclusions. The homogenization temperatures of fluid inclusions mainly vary between 220 and 280°C, whereas their salinities focus on 4 to 8wt.% NaCl equiv. Gases in fluid inclusions identified using Raman spectroscopy consist of CO2, CH4 and N2. Characteristics such as mesothermal temperature, low salinity and elevated CO2 content indicate that the ore-forming fluid might have been of metamorphic origin. Based on the major C-type FIs, the trapping pressures are greater than 8.5MPa, corresponding to minimum ore-forming depths are deeper than 3.1km. Taking into account the regional geodynamic evolution, it is concluded that the early formed ore layers were generated in an arc-related geotectonic setting; while deformation and metamorphism that were responsible for metal remobilization in the ore system happened in a post-subduction regime. Based on the micro-textural observation coupled with fluid inclusion data, we conclude that the Ashele Cu-Zn deposit is a typical remobilized VMS deposit intensely overprinted by subsequent deformation and metamorphism.

Genesis of the Huangshaping W–Mo–Cu–Pb–Zn polymetallic deposit in Southeastern Hunan Province, China: Constraints from fluid inclusions, trace elements, and isotopes

The Huangshaping polymetallic deposit is located in southeastern Hunan Province, China. It is a world-class W–Mo–Pb–Zn–Cu skarn deposit in the Nanling Range Metallogenic Belt, with estimated reserves of 74.31Mt of W–Mo ore at 0.28% WO3 and 0.07% Mo, 22.43Mt of Pb–Zn ore at 3.6% Pb and 8.00% Zn, and 20.35Mt of Cu ore at 1.12% Cu. The ore district is predominantly underlained by carbonate formations of the Lower Carboniferous period, with stocks of quartz porphyry, granite porphyry, and granophyre. Skarns occurred in contact zones between stocks and their carbonate wall rocks, which are spatially associated with the above-mentioned three types of ores (i.e., W–Mo, Pb–Zn, and Cu ores).Three types of fluid inclusions have been identified in the ores of the Huangshaping deposit: aqueous liquid–vapor inclusions (Type I), daughter-mineral-bearing aqueous inclusions (Type II), and H2O–CO2 inclusions (Type III). Systematic microthermometrical, laser Raman spectroscopic, and salinity analyses indicate that high-temperature and high-salinity immiscible magmatic fluid is responsible for the W–Mo mineralization, whereas low-temperature and low-salinity magmatic-meteoric mixed fluid is responsible for the subsequent Pb–Zn mineralization. Another magmatic fluid derived from deep-rooted magma is responsible for Cu mineralization.Chondrite-normalized rare earth element patterns and trace element features of calcites from W–Mo, Pb–Zn, and Cu ores are different from one another. Calcite from Cu ores is rich in heavy rare earth elements (187.4–190.5ppm), Na (0.17%–0.19%), Bi (1.96–64.60ppm), Y (113–135ppm), and As (9.1–29.7ppm), whereas calcite from W–Mo and Pb–Zn ores is rich in Mn (>10.000ppm) and Sr (178–248ppm) with higher Sr/Y ratios (53.94–72.94). δ18O values also differ between W–Mo/Pb–Zn ores (δ18O=8.10‰–8.41‰) and Cu ores (δ18O=4.34‰–4.96‰), indicating that two sources of fluids were, respectively, involved in the W–Mo, Pb–Zn, and Cu mineralization.Sulfur isotopes from sulfides also reveal that the large variation (4‰–19‰) within the Huangshaping deposit is likely due to a magmatic sulfur source with a contribution of reduced sulfate sulfur host in the Carboniferous limestone/dolomite and more magmatic sulfur involved in the Cu mineralization than that in W–Mo and Pb–Zn mineralization. The lead isotopic data for sulfide (galena: 206Pb/204Pb=18.48–19.19, 207/204Pb=15.45–15.91, 208/204Pb=38.95–39.78; sphalerite: 206Pb/204Pb=18.54–19.03, 207/204Pb=15.60–16.28, 208/204Pb=38.62–40.27; molybdenite: 206Pb/204Pb=18.45–19.21, 207/204Pb=15.53–15.95, 208/204Pb=38.77–39.58 chalcopyrite: 206Pb/204Pb=18.67–19.38, 207/204Pb=15.76–19.90, and 208/204Pb=39.13–39.56) and oxide (scheelite: 206Pb/204Pb=18.57–19.46, 207/204Pb=15.71–15.77, 208/204Pb=38.95–39.13) are different from those of the wall rock limestone (206Pb/204Pb=18.34–18.60, 207/204Pb=15.49–15.69, 208/204Pb=38.57–38.88) and porphyries (206Pb/204Pb=17.88–18.66, 207/204Pb=15.59–15.69, 208/204Pb=38.22–38.83), suggesting Pb206-, U238-, and Th 232-rich material are involved in the mineralization. The Sm–Nd isotopes of scheelite (εNd(t)=−6.1 to −2.9), garnet (εNd(t)=−6.8 to −6.1), and calcite (εNd(t)=−6.3) from W–Mo ores as well as calcite (εNd(t)=−5.4 to −5.3) and scheelite (εNd(t)=−2.9) from the Cu ores demonstrate suggest more mantle-derived materials involved in the Cu mineralization.In the present study we conclude that two sources of ore-forming fluids were involved in production of the Huangshaping W–Mo–Pb–Zn–Cu deposit. One is associated with the granite porphyry magmas responsible for the W–Mo and then Pb–Zn mineralization during which its fluid evolved from magmatic immiscible to a magmatic–meteoritic mixing, and the other is derived from deep-rooted magma, which is related to Cu-related mineralization.

Phase separation of ore forming fluid related to gold mineralization in Wynad Gold Field, Southern Granulite Terrain, India: Evidences from fluid inclusion studies

Fluid inclusion studies were carried out on auriferous quartz veins of Wynad Gold Field, Southern Granulite Terrain of India. Three types of primary fluid inclusions have been observed; Type-I: H2O–CO2 inclusions, Type-II: CO2 inclusions and Type-III: aqueous inclusions. The Type-I and Type-II inclusions are more abundant than Type-III inclusions. The coexistence of Type-I and Type-II inclusions are common within quartz grains in most of the samples studied. Variation in phase ratio and broad range of total homogenization temperature of Type-I and Type-III inclusions (i.e. 194°C to 300°C and 189°C to 282°C, respectively) indicate the entrapment of heterogeneous fluid in inclusions. This heterogeneity could be due to phase separation of original low saline H2O–CO2 ore fluid in response to drop in pressure and temperature. Gold along with other constituents could have precipitated in response to phase separation of the ore fluid.

An Early Paleozoic orogenic gold belt along the Jiang−Shao Fault, South China: Evidence from fluid inclusions and Rb–Sr dating of quartz in the Huangshan and Pingshui deposits

There are several gold deposits in the eastern section of the regional Jiang−Shao Fault between the Yangtze and Cathaysia Blocks in South China. Auriferous quartz veins in these deposits are strictly hosted in second-order NE-trending ductile shear zones. The ores generally contain low amounts of sulfide minerals (<5%), with pyrite as the most common sulfide mineral hosting native gold. Detailed fluid inclusion work and Rb–Sr dating were conducted on the auriferous quartz veins from the Pingshui and Huangshan deposits. H2O–CO2 inclusions (type I) and aqueous inclusions (type II) ubiquitously coexist in the main mineralization stage veins in the Huangshan and Pingshui deposits. Type I and II inclusions in the Huangshan deposit have similar homogenization temperatures at 214–282°C, but different salinities with 1.2–6.0 and 2.7–8.7wt.% NaCl equivalent, respectively. In the gold orebodies from the Pingshui deposit, type I and II inclusions also have similar homogenization temperatures ranging from 236 to 304°C, but different salinities ranging from 1.2 to 6.4 and from 3.2 to 9.8wt.% NaCl equivalent, respectively. Fluid inclusion observations and microthermometric results show that the ore fluids are low salinity and CO2-rich. Petrography and microthermometric results of fluid inclusions suggest that extensive fluid immiscibility occurred during the gold mineralization stage. Rb–Sr dating of quartz-hosted fluid inclusions (ca. 450Ma) for the gold mineralization at Pingshui, combined with previous radiometric age data (ca. 397Ma) of gold mineralization at Huangshan, suggest that the regional gold mineralization was formed in the Early Paleozoic. This study suggests that there is an Early Paleozoic orogenic gold belt in the eastern section of the Jiang−Shao Fault, formed in response to the coeval northward underthrusting of the Cathaysia Block beneath the Yangtze Block during the Caledonian Orogeny in South China.

Isotope and fluid inclusion geochemistry and genesis of the Qiangma gold deposit, Xiaoqinling gold field, Qinling Orogen, China

The Qiangma gold deposit is hosted in the >1.9Ga Taihua Supergroup metamorphic rocks in the Xiaoqinling terrane, Qinling Orogen, on the southern margin of the North China Craton. The mineralization can be divided as follows: quartz-pyrite veins early, quartz-polymetallic sulfide veinlets middle, and carbonate-quartz veinlets late stages, with gold being mainly introduced in the middle stage. Three types of fluid inclusions were identified based on petrography and laser Raman spectroscopy, i.e., pure carbonic, carbonic-aqueous (CO2–H2O) and aqueous inclusions.The early-stage quartz contains pure carbonic and CO2–H2O inclusions with salinities up to 12.7wt.% NaCl equiv., bulk densities of 0.67 to 0.86g/cm3, and homogenization temperatures of 280−365°C. The early-stage is related to H2O–CO2±N2±CH4 fluids with isotopic signatures consistent with a metamorphic origin (δ18Owater=3.1 to 5.2‰, δD=−37 to −73‰). The middle-stage quartz contains all three types of fluid inclusions, of which the CO2–H2O and aqueous inclusions yield homogenization temperatures of 249−346°C and 230−345°C, respectively. The CO2–H2O inclusions have salinities up to 10.9wt.% NaCl equiv. and bulk densities of 0.70 to 0.98g/cm3, with vapor bubbles composed of CO2 and N2. The isotopic ratios (δ18Owater=2.2 to 3.6‰, δD=−47 to −79‰) suggest that the middle-stage fluids were mixed by metamorphic and meteoric fluids. In the late-stage quartz only the aqueous inclusions are observed, which have low salinities (0.9−9.9wt.% NaCl equiv.) and low homogenization temperatures (145−223°C). The isotopic composition (δ18Owater=−1.9 to 0.5‰, δD=−55 to −66‰) indicates the late-stage fluids were mainly meteoric water.Trapping pressures estimated from CO2–H2O inclusions are 100−285 MPa for the middle stage, suggesting that gold mineralization mainly occurred at depths of 10km. Fluid boiling and mixing caused rapid precipitation of sulfides and native Au. Through boiling and inflow of meteoric water, the ore-forming fluid system evolved from CO2-rich to CO2-poor in composition, and from metamorphic to meteoric, as indicated by decreasing δ18Owater values from early to late. The carbon, sulfur and lead isotope compositions suggest the hostrocks within the Taihua Supergroup to be a significant source of ore metals. Integrating the data obtained from the studies including regional geology, ore geology, and fluid inclusion and C–H–O–S–Pb isotope geochemistry, we conclude that the Qiangma gold deposit was an orogenic-type system formed in the tectonic transition from compression to extension during the Jurassic−Early Cretaceous continental collision between the North China and Yangtze cratons.

Origin of REE mineralization in the Bastnäs-type Fe-REE-(Cu-Mo-Bi-Au) deposits, Bergslagen, Sweden

The Bastnas-type deposits, with mineral assemblages of Fe oxides, Ca-Mg silicates, rare earth element (REE) silicates, REE fluorocarbonates, and Cu-Fe-Mo-Bi sulfides, are associated with marble horizons in a strongly Na, K, and/or Mg altered, metavolcanic succession, over a distance of at least 80 km in a SW-NE trending zone in western Bergslagen. Two subtypes occur: (1) enriched (relative to the other type) in light REE (LREE) and Fe, exemplified by the Bastnas and Rodbergsgruvan deposits, and (2) enriched in heavy REE (HREE), Y, Mg, Ca, and F, represented by deposits in the Norberg district. Bastnasite hosts primary fluid H2O-CO2 inclusions with salinities of 6–29 eq. wt% CaCl2 and with total homogenization temperatures (Th tot) of ca. 300–400 °C. Subtype 2 has late-stage fluorite with fluid inclusions that show 1–16 eq. wt% NaCl and Th tot of ca. 90–150 °C. Molybdenite Re-Os ages obtained from three deposits are 1,904 ± 6, 1,863 ± 4, and 1,842 ± 4 Ma. Nd isotopic data from five different REE minerals yielded no defined isochron, but a range in eNd (1.88 Ga) of +0.2 to +1.6. The oxygen isotope values (δ18OSMOW) of dolomite and calcite from the associated REE-mineralized skarn and recrystallized carbonate assemblages lie in the range 6.1–8.6 ‰, overlapping with those of the host marbles. Carbon isotope values (δ13CPDB) show typical magmatic signatures of −6.7 to −4.4 ‰, while the host marbles group around ca. −2.4 ‰. The sulfur isotope (δ34SCDT) values of associated sulfides range between −10.8 and +0.2 ‰. The combined evidence suggests REE mineralization, beginning at 1.9 Ga, from mainly Svecofennian, juvenile magmatic (>400 °C) fluids carrying Si, F, Cl, S, CO2, and the REE in addition to other metals; mineralization occurred through reactions with dolomitic layers in the supracrustal units coevally with regional metasomatic alteration associated with fluid circulation through an extensive active volcano–plutonic complex.

Ore-forming process of the Huijiabao gold district, southwestern Guizhou Province, China: Evidence from fluid inclusions and stable isotopes

The Huijiabao gold district is one of the major producers for Carlin-type gold deposits in southwestern Guizhou Province, China, including Taipingdong, Zimudang, Shuiyindong, Bojitian and other gold deposits/occurrences. Petrographic observation, microthermometric study and Laser Raman spectroscopy were carried out on the fluid inclusions within representative minerals in various mineralization stages from these four gold deposits. Five types of fluid inclusions have been recognized in hydrothermal minerals of different ore-forming stages: aqueous inclusions, CO2 inclusions, CO2–H2O inclusions, hydrocarbon inclusions, and hydrocarbon–H2O inclusions. The ore-forming fluids are characterized by a H2O+CO2+CH4±N2 system with medium to low temperature and low salinity. From early mineralization stage to later ones, the compositions of the ore-forming fluids experienced an evolution of H2O+NaCl→H2O+NaCl+CO2+CH4±N2→H2O+NaCl±CH4±CO2 with a slight decrease in homogenization temperature and salinity. The δ18O values of the main-stage quartz vary from 15.2‰ to 24.1‰, while the δDH2O and calculated δ18OH2O values of the ore-forming fluids range from −56.9 to −116.3‰ and from 2.12‰ to 12.7‰, respectively. The δ13CPDB and δ18OSMOW values of hydrothermal calcite change in the range of −9.1‰ to −0.5‰ and 11.1–23.2‰, respectively. Stable isotopic characteristics indicate that the ore-forming fluid was mainly composed of ore- and hydrocarbon-bearing basinal fluid. The dynamic fractionation of the sulfur in the diagenetic pyrite is controlled by bacterial reduction of marine sulfates. The hydrothermal sulfides and the diagenetic pyrite from the host rocks are very similar in their sulfur isotopic composition, suggesting that the sulfur in the ore-forming fluids was mainly derived from dissolution of diagenetic pyrite. The study of fluid inclusions indicates that immiscibility of H2O–NaCl–CO2 fluids took place during the main mineralization stage and caused the precipitation and enrichment of gold.

Fluid immiscibility and gold deposition in the Xincheng deposit, Jiaodong Peninsula, China: A fluid inclusion study

The Xincheng gold deposit, located in west Jiaodong Peninsula in southeast North China Craton, is a representative mesothermal lode deposit hosted in Late Mesozoic granitoids in Jiaodong. Gold mineralization occurs as disseminated- and stockwork-style ores within the hydrothermal breccias and cataclastic zones controlled by the Jiaojia fault, whereas echelon tensile auriferous veins hosted in the NE- and NNE-trending subsidiary faults cutting the granitoids occur subordinately. According to crosscutting relationships and mineral paragenesis, four paragenetic stages were identified, which are pyrite–quartz–sericite (stage 1), quartz–pyrite (stage 2), quartz–polysulfide (stage 3) and quartz–carbonate (stage 4). Gold was deposited during the quartz–pyrite and quartz–polysulfide stages.On the basis of microthermometry and Raman spectroscopy on fluid inclusions contained within the quartz veins from stages 2 and 3, three types of fluid inclusions were recognized: (1) type 1 H2O–CO2 inclusions that show high temperatures (ca. 260°C), low salinities (2.4–8.9wt.% equiv. NaCl) and variable XCO2 (0.03 to 0.20), (2) type 2 aqueous inclusions with medium temperatures (ca. 220°C) and low to moderate salinities (3.1–13.3wt.% equiv. NaCl); (3) type 3 pure CO2 inclusions with a carbonic phase density of 0.712±0.03g/cm3. Types 1 and 2 inclusions appear in the same growth phase of the quartz grains from the breccias and tensile auriferous veins. These coexisting inclusions are likely formed by fluid immiscibility due to unmixing from a single homogeneous H2O–CO2 parent fluid at trapping P–T conditions of 221 to 304°C (average 261±19°C) and 780 to 2080bar. The fluid immiscibility is interpreted to be initiated by fluid pressure decrease at ca. 300°C.The ore-fluid P–T–X conditions of the Xincheng gold deposit are the same as those for mesothermal deposits. Gold was most probably transported as a Au(HS)2− complex at Xincheng. Fluid immiscibility over the temperature interval of 221–304°C resulted in significant H2S loss from the hydrothermal solution, thereby reducing Au(HS)2− solubility with concomitant deposition of gold. The mineralizing process of the granitoid-hosted Xincheng lode-gold deposit is likely related to the fluid immiscibility.

Geology, geochemistry and genesis of the Huachanggou gold deposit, western Qinling Orogen, central China

The large Huachanggou gold deposit, on the southern margin of the Mian‐Lue Suture in western Qinling Orogen (central China), is hosted in the Devonian Sanhekou Group composed of spilite, limestone and phyllite. The mineralization is considered to have occurred in three stages: (1) Early quartz–pyrite veins, (2) Middle quartz–sulphides veins, and (3) Late carbonate–quartz veinlets, with gold being introduced mainly in the second stage. Quartz formed in the two earlier stages contains four types of fluid inclusions, namely (1) Pure CO2, (2) CO2–H2O, (3) Solid‐bearing and (4) NaCl–H2O, whereas the late‐stage minerals contain only the NaCl–H2O inclusions. The inclusions in quartz formed in the early‐, middle‐ and late stages have yielded total homogenization temperatures of 272−385 °C, 232−395 °C and 118−228 °C, respectively, with salinities no higher than 12 wt.% NaCl equiv. For the middle‐stage, trapping pressures estimated from the CO2–H2O inclusions for the altered spilite‐type (123−326 MPa) and quartz vein‐type (132−342 MPa) orebodies correspond to mineralization depths of 12 km and 13 km, respectively. We suggest that fluid boiling and mixing may have caused rapid precipitation of metallic sulphides and native gold. Through boiling and influx of meteoric water, the ore‐forming fluid system has evolved from CO2‐rich to CO2‐poor and from metamorphic to meteoric, as indicated by the decreasing δ18Owatervalues from the early‐ to late stages. Integrating the evidence obtained from regional geology, ore deposit geology, fluid inclusion and C–H–O isotope geochemistry, we conclude that the Huachanggou gold deposit is best ascribed to be an orogenic‐type system formed in the tectonic setting of the north‐vergent oceanic plate subduction along the Mian‐Lue Suture during the Late Triassic. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Geology, geochemistry and ore genesis of the Wenyu gold deposit, Xiaoqinling gold field, Qinling Orogen, southern margin of North China Craton

The Wenyu giant gold deposit is hosted in the Precambrian Taihua Supergroup metamorphic rocks within the Xiaoqinling terrane (Qinling Orogen), on the southern margin of the North China Craton. The mineralization can be divided into three stages: quartz–pyrite veins early, quartz–sulfide veins middle (main), and carbonate–quartz veinlets late, with gold being mainly introduced in main stage. Quartz formed in two earlier stages contains three compositional types of fluid inclusions, i.e. pure CO2, CO2–H2O and NaCl–H2O, but the late-stage minerals only contain the NaCl–H2O inclusions. The inclusions in quartz formed in the early, main and late stages yield total homogenization temperatures of 262–417°C, 236–407°C and 114–239°C, respectively, with salinities no higher than 13wt.% NaCl equiv. Trapping pressures estimated from CO2–H2O inclusions are 139–399MPa and 111–316MPa in the early and main stages, corresponding to mineralization depths of 14km and 11km, respectively. Fluid boiling and mixing caused rapid precipitation of sulfides and native Au. Through boiling and inflow of meteoric water, the ore-forming fluid system evolved from CO2-rich to CO2-poor in composition, and from metamorphic to meteoric, as indicated by decreasing δ18Owater values from early to late. The carbon, sulfur and lead isotope compositions suggest the hostrocks within the Taihua Supergroup to be a significant source of ore metals. Integrating the data obtained from the studies including regional geology, ore geology, fluid inclusion and C–H–O–S–Pb isotope geochemistry, we conclude that the Wenyu gold deposit was an orogenic-type system formed in the tectonic transition from compression to extension during the Jurassic–Early Cretaceous continental collision between the North China and Yangtze Cratons.

Geology, fluid inclusion, and isotope constraints on ore genesis of the Neoproterozoic Jinshan orogenic gold deposit, South China

AbstractThe Jinshan gold deposit is located in the Neoproterozoic Jiangnan orogen between the Yangtze and Cathaysia blocks. Gold‐bearing disseminated ores are associated with the earlier stage of NWW‐trending ductile zone, and auriferous quartz vein‐type ores show an intimate relationship with the later stage of NE‐trending brittle‐ductile zones. Fluid‐inclusion studies were conducted on the quartz veins. Three types of fluid inclusions can be identified: H2O–CO2 inclusions (type I), CO2‐rich inclusions (type II), and aqueous inclusions (type III). The pre‐ore stage, quartz‐pyrite veins primarily contain type I inclusions with constant CO2 bubble volumetric proportions. The main gold mineralization‐stage veins have all three types of inclusions with variable gas‐phase ratios and CO2 contents. The post‐ore stage carbonate±chlorite veinlets only contain type III inclusions. Type I inclusions in the pre‐ore stage display homogenization temperatures (Th) of 285–340°C, with salinities of 1.4–6.1 wt.% NaCl equivalent. In the main gold mineralization stage, type II and III inclusions show similar Th at 208–277°C, but contrasting salinity values with 0.6–3.6 and 3.5–8.9 wt.% NaCl equivalent, and type I inclusions show variable CO2‐phase proportions and have Th of 241–292°C and salinities of 1.0–7.0 wt.% NaCl equivalent. In the post‐ore stage, type III inclusions yield Th of 109–201°C and salinities of 1.1–6.4 wt.% NaCl equivalent. Petrological observations and microthermometric results show that fluid immiscibility primarily occurred during the gold mineralization stages. The oxygen and hydrogen isotope compositions (δ18 O = +6.9‰ to +11.2‰, δD = −71‰ to −46‰) of inclusion water in quartz grains imply that ore fluids were principally metamorphic in origin. The sulfur and lead values of sulfide from the ores are analogous to those from the basement strata, suggesting a predominantly crustal source of the ore sulfides. The Jinshan deposit is a typical orogenic gold deposit.

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 8.03wt.% NaCl equivalent, bulk densities of 0.73 to 0.89g/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 9.59wt.% NaCl equivalent and bulk densities of 0.77 to 0.95g/cm3, with vapor bubbles composed of CO2, CH4, and N2. Fluid inclusions in the late-stage quartz are NaCl–H2O solution with low salinities (0.35–3.87wt.% 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 110–300MPa and 90–250MPa for the early and middle stages, respectively, suggesting that gold mineralization mainly occurred at depths of about 10km. 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.

P-T-X Conditions of Fluids in the Sunrise Dam Gold Deposit, Western Australia, and Implications for the Interplay between Deformation and Fluids

The Late Archean Sunrise Dam gold deposit (~10 Moz) is hosted within greenschist-facies rocks and is characterized by extreme structural complexity, resulting from a protracted deformation history with evidence of structural reactivation and multiple phases of gold mineralization. Early Group I orebodies are hosted within shallow- to moderately dipping northwest-trending shear zones and occur in foliation parallel veins within a strong penetrative fabric. Group II orebodies occur within steeply dipping shear zones and are characterized by veins and breccias, and Group III and IV orebodies comprise dominantly stockwork veins. The orebodies formed in response to multiple episodes of deformation. Many structures were created during D1 and D2 shortening whereas the bulk of the gold was deposited during D3 and D4 that marks the transition from a dominantly lithostatic pore fluid regime to hydrostatic conditions. Two main fluid inclusion types were recognized at Sunrise Dam: low-salinity CO2-H2O inclusions and CO2 inclusions. CO2-H2O inclusions are more common in the early Group I orebodies. These fluids were trapped under high P-T conditions during D3 (minimum estimates suggest conditions were likely >~300°C and >1 to 3 kbars predominantly within the one-phase field). CO2-H2O inclusions that occur in later Group II and III orebodies were trapped predominantly within the two-phase field at temperatures 1–3 kbars) at a temperature >300°C. Shear failure along these structures resulted in widespread precipitation of moderate-grade gold mineralization from the CO2-H2O fluid. Continued deformation and exhumation modified P-T-X conditions, and cooling of the host rocks below 300°C resulted in the CO2-H2O fluid entering the two-phase field. A combination of temperature decrease, a transition from lithostatic to suprahydrostatic and/or hydrostatic pressure conditions, fluid immiscibility, and the influx of a second CO2-rich fluid, resulted in fracture during D4 and precipitation of high-grade gold mineralization in steeply dipping structures forming veins, breccias, and stockworks.

Effects of temperature, pH and NaCl on protease activity in digestive tract of young turbot,Scophthalmus maximus

Isochemical conversion of garnet-biotite bearing paragneiss to charnockite in the Precambrian Khondalite belt of southern Kerala is described from Ponmudi area. Petrographic evidences indicate the formation of hypersthene by the breakdown of biotite in the presence of quartz following the reaction: Biotite + quartz → hypersthene + K-feldspar + vapour. The estimated pressure — temperature conditions of metamorphism are around 5–7 kbars and 750° ± 40°C. Presence of CO2-rich, mixed CO2-H2O and H2O-rich inclusions were noticed in gneiss as well as in charnockites. Charnockites contain abundant CO2-rich inclusions.