Isotopic Compositions Of Fluid Inclusions Research Articles

An Assessment of Deep Borehole Disposal Post-Closure Safety

Around the world, deep borehole disposal is being evaluated for intermediate level-waste (ILW), high-level waste, and spent nuclear fuel. To facilitate a disposal concept options analysis for ILW in Australia, desktop and lab-based geoscientific investigations, together with generic post-closure safety assessments of deep borehole disposal of long-lived ILW, have been undertaken. This paper reports on geoscientific data obtained on crystalline rock and rock salt as model rocks for geological disposal. Petrophysical and mineralogical properties for these rocks have been investigated to provide realistic data for evaluation and input to post-closure safety assessments. For crystalline rock samples originating from depths between 700 to 1900 m, very low hydraulic conductivity (2 × 10−12 to 3 × 10−11 m/s) and very low porosity (0.02% to 1.2%) were obtained. The noble gas isotopic composition of fluid inclusions from the same depth interval confirmed the rock had been devoid of recent interaction with meteoric water, thus providing potentially suitable conditions for geological disposal. Rock salt from a 802-m (heterogeneous sample with 40% halite) and a 1100-m (sample with 98% halite) depth also had a low hydraulic conductivity (5 × 10−10 to 5 × 10−9 m/s at 802 m and 10−11 to 2 × 10−10 m/s at 1100 m) and very low porosity (~0.8% for the heterogenous sample and ~0.2% for the pure halite sample). Post-closure safety assessments based on numerical modeling provided bounding conditions around the thermal evolution of the disposal environment in crystalline rock for low heat generating ILW (50 W per 180-L vitrified waste canister), including exploring the sensitivity of temperature evolution within the borehole and rock environment to parameters such as heat load, borehole depth, geothermal gradients, and rock thermal conductivity. The coupling of heat transport with radionuclide migration to account for buoyancy-driven transport was shown to have a limited impact on radionuclide migration. For a disposal borehole in crystalline rock, the radionuclide concentrations and annual dose rates from key radionuclides (99Tc and 79Se) for a 500-m, 1000-m, or 3000-m deep borehole were negligible (i.e., many orders of magnitude smaller than the threshold dose the International Atomic Energy Agency considers insignificant for humans, 0.01 mSv/year). For disposal in rock salt, a suite of numerical model scenarios explored the effectiveness of the engineered barriers, including the glass matrix, primary package, and overpack, assuming diffusion-dominated transport. These scenarios illustrated that the performance of the disposal system was insensitive to the presence or absence of engineered barriers, as dose rates at late time (>105 years) were nearly identical for all scenarios. These results indicate that the natural barrier provided by the salt is very effective at containing radionuclides, while the engineered barriers serve mainly to delay the arrival of the peak dose. While the results are preliminary, the post-closure safety assessments, supported by measured data from crystalline rock and rock salt, give confidence that deep borehole disposal of long-lived ILW would result in dose rates considered insignificant for humans within a few meters from the borehole.

He Isotopic Composition of Alkaline Intrusions of the Hovsgol Area, Northwestern Mongolia

The He isotopic composition of fluid inclusions in magnetite of alkaline and subalkaline intrusion of the Hovsgol region, Northwestern Mongolia, is studied. The measured 4He content varies from 6.6 × 10–7 to 114 × 10–7 cm3/g. The 3He/4He isotope ratio of most samples is 0.23–0.59 Ra indicating the presence of He from different sources in magmatic fluid. The maximum of mantle He (2.51 Ra) is observed in magnetite of subalkali gabbro. The mantle He component was ~40–60% during the generation of parental magma with possible involvement of subcontinental lithospheric mantle material or a plume-like reservoir less enriched in 3He. The mantle He component did not exceed 10–15% during the evolution of the foidoitic and alkali-syenitic melts because of mixing with crustal radiogenic He. We suggest the interaction of intrusions with fragments of the Precambrian accretionary–collision complexes of the Tuva–Mongolian Terrane. This mixing of He isotopes in melts and fluids can serve as indirect evidence of the development of mantle magmatism at the active continental margin.

Noble gas geochemistry of phenocrysts from the Ciomadul volcanic dome field (Eastern Carpathians)

Noble gas isotopic composition of fluid inclusions was analyzed in amphibole, plagioclase and clinopyroxene phenocrysts from the shoshonitic and dacitic volcanic products of the Ciomadul volcanic dome field, the youngest volcanic system within the Carpathian-Pannonian Region. The highest Rc/RA ratios (3.0–3.8 RA) were obtained for high-mg clinopyroxene of the Malnaş shoshonite. High-Al amphiboles from the Bixad dacitic pumices have Rc/RA ratios between 1.16 and 2.11 RA. These values overlap with the noble gas signature of the present-day CO2 emission. Thus, our new results reinforce the conclusion that the mantle component of the Ciomadul primitive magmas has relatively lower Rc/RA signature compared to the nearby Perşani lithospheric mantle. This is likely due to the thorough metasomatic nature resulting in elevated large ion lithophile elements and high water content of the Ciomadul magmas. On the other hand, the Rc/RA ratios from plagioclase-hosted fluid inclusions and those from low-Al amphiboles are 0.06–0.12 RA and 0.39–0.77 RA, respectively defining a dominant crustal origin (>90%) for the trapped fluids. Noteworthy, these minerals represent a low-temperature crystal mush assemblage that existed for protracted time in the magma reservoir. We tested the fraction of mantle contribution for different mantle end-member values, considering also the effect of magma aging on the R/RA ratios due to longer (up to 50 kyr) residence time. This resulted in a maximum of ~50–60% mantle fluid contribution for the high-Al amphibole-hosted fluid inclusions and a lower, ~25% mantle fluid contribution, for the low-Al amphiboles. The elevated mantle fluid contribution in the case of the high-Al amphiboles can be explained by a fresh magma recharge event and shorter residence time before the eruption. The results of this study imply that fluid inclusion in primitive clinopyroxene and amphibole phenocrysts could reflect the magmatic end-member, which is in the case of Ciomadul a strongly metasomatized lithospheric mantle with relatively low Rc/RA values. Thus, the noble gas signature of the lithospheric mantle could be heterogeneous even in a restricted area.

Episodic fluid evolution in the formation of the large scale Luoyang Fe deposit, Fujian, eastern China

The Luoyang Fe skarn deposit is located in the Yong’an–Meizhou depression belt in Fujian Province, eastern China. Skarn alteration and mineralization are concentrated in a zone adjacent to an early Yanshanian granite porphyry (146 Ma), within limestone of the upper Carboniferous Chuanshan and lower Permian Qixia formations. Petrographic observations reveal five paragenetic stages of skarn formation and ore deposition: Stage 1 (early skarn; andradite); Stage 2 (diopside + magnetite); Stage 3 (late skarn; actinolite + chlorite + tremolite + magnetite + pyrite); Stage 4 (quartz + calcite + fluorite + pyrite + hematite + sphalerite + galena + chalcopyrite); and Stage 5 (calcite + quartz). We analyzed fluid inclusions in garnet, diopside, calcite, and quartz from stages 1, 2, 4, and 5, and measured H–O isotopic compositions of fluid inclusions in garnet, magnetite, and quartz from stages 1, 2, and 4.Observations of fluid compositions and phase assemblages allow three types (and two subtypes) of fluid inclusions to be distinguished: halite-bearing (type 1a) and hematite-bearing (type 1b), vapor-rich (type 2), and liquid-rich aqueous inclusions (type 3). In stage 1 garnet contains type-1a, -1b, and -2 primary fluid inclusions, which comprise two immiscible phases that formed through boiling. In stage 2 diopside contains type-1a and -2 fluid inclusions that also record boiling, which resulted in the precipitation of magnetite. In stage 4 quartz contains type-3 primary inclusions that are related to sulfide mineralization. In stage 5 calcite and quartz contain type-3 fluid inclusions, which represent the post-sulfide mineralization fluids.Microthermometric and H–O isotopic data indicate that multiple episodes of fluid flow occurred in the Luoyang deposit. During the early skarn stage, an initial supercritical fluid that exsolved during crystallization of the parent granitic magma ascended and underwent a first boiling event at 488 °C–536 °C and 380–438 bar (lithostatic pressure), which produced two fluids with differing salinities, represented by type-1a and -2 inclusions. This stage of boiling resulted in minor magnetite precipitation and type 1b fluid inclusions associated with this stage contain hematite. The magnetite stage was associated with a second episode of fluid ascent as the parent magma cooled to 454 °C–498 °C, which triggered a second boiling event and abundant magnetite precipitation at 336–449 bar (lithostatic pressure). The sulfide stage is associated to a third period of fluid ascent and occurred at 220 °C–261 °C at the same pressure conditions as the second boiling event (~380 bar). This stage included the input of meteoric water, which resulted in sulfide precipitation. Late-stage carbonate crystallization involved the influx of further meteoric water at lower temperatures (144 °C–193 °C) and the same pressure (~380 bar), representing the collapse of the skarn system. These observations suggest that the Luoyang deposit formed at a shallower depth than the Makeng deposit, which is located within the same belt. Variations in skarn alteration and magnetite mineralization are ascribed to the difference in depth between the two skarn systems.

Helium and argon isotopic compositions of fluid inclusions and tracing to the source of ore-forming fluids for the southern Zhuguang uranium ore field in northern Guangdong Province

Helium and argon isotopic compositions of fluid inclusions and tracing to the source of ore-forming fluids for the southern Zhuguang uranium ore field in northern Guangdong Province

Genesis of the Jianzhupo Sb–Pb–Zn–Ag deposit and formation of an ore shoot in the Wuxu ore field, Guangxi, South China

The Jianzhupo Sb–Pb–Zn–Ag deposit in the southeastern Dachang Sn–polymetallic ore belt, Guangxi region, South China is the largest deposit in the Wuxu district and is hosted by Devonian limestone and mudstone. The deposit mainly comprises a stage I jamesonite–sphalerite ore shoot and stage II jamesonite–sphalerite–calcite–quartz (JSCQ) veins. We use homogenization temperatures, salinities, and the H–O isotopic compositions of fluid inclusions from the Jianzhupo deposit to constrain the genesis of the deposit and determine factors that controlled the formation of the ore shoot. Fluid inclusions from both stages of mineralization contain large volumes of CO2 and minor bitumen, C6H6, and CH4. Fluid inclusions from the ore shoot yield homogenization temperatures of 120 °C–290 °C (mainly 250 °C–270 °C) and salinities of 3.4–13.6 wt% NaCl equiv. Fluid inclusions from the stage II mineralization yield homogenization temperatures of 150 °C–350 °C (mainly 240 °C–310 °C) and salinities of 1.4–12.0 wt% NaCl equiv. Ore-forming fluids of the Jianzhupo deposit yield δ18OH2O and δDV-SMOW values of 4.9‰–9.9‰ and −78‰ to −111‰, respectively, and plot close to the composition of magmatic water on the δDV-SMOW vs. δ18OH2O diagram. CO2 in the ore-forming fluids was likely generated through deep skarnization beneath the Wuxu ore field rather than from metamorphism, thermal degradation, or the oxidization of organic matter. H–O isotopic compositions of ore-forming fluids that contain large volumes of CO2 (with minor bitumen and C6H6) indicate that they were derived from magmatic water that was mixed with basin brines. Ore-forming fluids of the stage I ore shoot yield lower δ18OH2O values (4.9‰–6.7‰) than those associated with the stage II mineralization (7.5‰–9.9‰). Moreover, ore-forming fluids of the ore shoot and the JSCQ veins yield strongly and weakly positive correlations between homogenization temperature and salinity, respectively. This difference suggests that rapid and large-scale mixing between magmatic water and basin brines was the key factor that controlled the formation of the ore shoot.

Age and metal source of orogenic gold deposits in Southeast Guizhou Province, China: Constraints from Re–Os and He–Ar isotopic evidence

The orogenic gold deposits in Southeast Guizhou are an important component of the Xuefeng polymetallic ore belt and have significant exploration potential, but geochronology research on these gold deposits is scarce. Therefore, the ore genetic models are poorly constrained and remain unclear. In the present study, two important deposits (Pingqiu and Jinjing) are investigated, including combined Re–Os dating and the He–Ar isotope study of auriferous arsenopyrites. It is found that the arsenopyrites from the Pingqiu gold deposit yielded an isochron age of 400 ± 24 Ma, with an initial 187Os/188Os ratio of 1.24 ± 0.57 (MSWD = 0.96). An identical isochron age of 400 ± 11 Ma with an initial 187Os/188Os ratio of 1.55 ± 0.14 (MSWD = 0.34) was obtained from the Jinjing deposit. These ages correspond to the regional Caledonian orogeny and are interpreted to represent the age of the main stage ore. Both initial 187Os ratios suggest that the Os was derived from crustal rocks. Combined with previous rare earth element (REE), trace elements, Nd–Sr–S–Pb isotope studies on scheelite, inclusion fluids with other residues of gangue quartz, and sulfides from other gold deposits in the region, it is suggested that the ore metals from Pingqiu and Jinjing were sourced from the Xiajiang Group. The He and Ar isotopes of arsenopyrites are characterized by 3He/4He ratios ranging from 5.3 × 10−4 Ra to 2.5 × 10−2 Ra (Ra = 1.4 × 10−6, the 3He/4He ratio of air), 40Ar*/4He ratios from 0.64 × 10−2 to 15.39 × 10−2, and 40Ar/36Ar ratios from 633.2 to 6582.0. Those noble gas isotopic compositions of fluid inclusions also support a crustal source origin, evidenced by the Os isotope. Meanwhile, recent noble gas studies suggest that the amount of in situ radiogenic 4He generated should not be ignored, even when Th and U are present at levels of only a few ppm in host minerals.

Genetic implications for the Damajianshan W‐Cu‐As polymetallic deposit in Lvchun, Southwest China: Constraints from H–O, He–Ar, S, and Pb isotopes

The Sanjiang Tethyan domain in SE Asia is one of the most important mineral belts in China. The Damajianshan (DMJS) W‐Cu‐As polymetallic deposit is located in the southern part of Sanjiang Tethyan domain, related to Triassic quartz porphyry. Detailed exploration thereafter shows that the reserves of W, Cu, and As are 0.09, 0.42, and 0.12 Mt, respectively. The W is a typical oxyphile element and always coexisted with Sn–Li–Be–Nb–Ta. The Cu–As are typical sulphophile elements and always coexisted with Au–Ag–Pb–Zn–Sb–Hg. It is rarely reported that a deposit preserves tungsten and copper–arsenic simultaneously in the southern Sanjiang region. In this study, the S–Pb isotopic ratios of sulphides, the H–O isotopic compositions of fluid inclusions in hydrothermal quartz, and the He–Ar isotopic ratios of fluid inclusions in sulphides were analysed to constrain the origin of the DMJS deposit. Studies have shown that the δ34S values of sulphides from ores are mainly between −6.17‰ and +0.02‰; the sulphur isotope compositions indicate that the ore‐forming materials might originate from deep sources. The Pb isotope characteristics indicate that the ore metals might originate from deep sources and be closely related to the quartz porphyry. The δ18Ofluid values of ore‐forming fluids calculated from hydrothermal quartz range from −5.5‰ to +7.5‰, and the δD values of the fluid inclusions in quartz are from −81.7‰ to −50.2‰. The H–O isotope systematics indicate that the ore fluids in the DMJS deposit were probably initially sourced from magmatic water and later gradually mixed with Mesozoic meteoric water. Fluid inclusions in pyrite, chalcopyrite, arsenopyrite, and pyrrhotite possess 3He/4He ratios of 0.02–0.05 Ra, and their 40Ar/36Ar ratios range from 562.47 to 4159.15, indicating a complete crustal fluids and with higher radiogenic 40Ar. The noble gas isotopic data, along with the stable isotopic data, suggest that the ore‐forming fluids have a deep source. The unique geologic structure of the Sanjiang Tethys tectonic metallogenic region and the evolution of ore‐forming fluids both promoted the formation of this rare DMJS deposit.

Lithium isotopes in fluid inclusions as tracers of crustal fluids: An exploratory study

Lithium isotopes are extensively used to trace fluid migration and fluid-rock interactions in the shallow Earth's crust. Although, Li isotope systematics might give important information about deep crustal fluids, this stable isotope system remains surprisingly under exploited in fluid inclusions. Here, twenty-three samples from a variety of deep environments, from diagenetic, metamorphic to magmatic-hydrothermal conditions (~150 to 600°C), have been investigated in order to provide the first exploratory overview of the range in Li isotopic composition of fluid inclusions. From 1 to 2g of hydrothermal quartz or dolomite, cation analysis (Na, K, Ca, Mg, Sr, Li), Li fraction recovery and Li isotopic analysis of bulk fluid inclusion leachates were carried out by using an innovative approach coupling crush-leach extraction, high performance ion chromatography and MC-ICPMS analysis. Reconstructed Li concentrations and δ7Li values of these fluid inclusions (respectively 12 to 653mg/l and −1.4±0.2‰ to +41.3±0.8‰) are compatible with previously proposed models for the origin of the fluids and fluid-rock interactions. δ7Li values are independent from other tracers (Br/Cl, I/Cl) and parameters such as temperature, salinity, and cation content. The results show that, in conjunction with other parameters, the Li isotopic compositions of fluid inclusions are potentially powerful source and process tracers of deep crustal fluids within a wide range of geological environments and related ore-forming systems, for example in from sedimentary basins, seafloor hydrothermal systems and subduction zones.

Noble gas data from Goldfield and Tonopah epithermal Au-Ag deposits, ancestral Cascades Arc, USA: Evidence for a primitive mantle volatile source

The He, Ne, and Ar isotopic composition of fluid inclusions in ore and gangue minerals were analyzed to determine the source of volatiles in the high-grade Goldfield and Tonopah epithermal Au-Ag deposits in southwestern Nevada, USA. Ar and Ne are mainly atmospheric, whereas He has only a minor atmospheric component. Corrected 3He/4He ratios (with atmospheric He removed) range widely from 0.05 to 35.8 times the air 3He/4He ratio (RA), with a median of 1.43 RA. Forty-one percent of measured 3He/4He ratios are ≥4 RA, corresponding to ≥50% mantle He assuming a mantle ratio of 8 RA. These results suggest that mafic magmas were part of the magmatic-hydrothermal system underlying Goldfield and Tonopah, and that associated mantle-sourced volatiles may have played a role in ore formation. The three highest corrected 3He/4He ratios of 17.0, 23.7, and 35.8 RA indicate a primitive mantle He source and are the highest yet reported for any epithermal-porphyry system and for the Cascades arc region. Compiled 3He/4He measurements from epithermal-porphyry systems in subduction-related magmatic arcs around the world (n=209) display a statistically significant correlation between 3He/4He and Au-Ag grade. The correlation suggests that conditions which promote higher fluid inclusion 3He/4He ratios (abundance of mantle volatiles and focused upward volatile transport) have some relation to conditions that promote higher Au-Ag grades (focused flow of metal-bearing fluids and efficient chemical traps). Results of this and previous investigations of He isotopes in epithermal-porphyry systems are consistent with the hypothesis posed in recent studies that mafic magmas serve an important function in the formation of these deposits.

The Sources of Ore‐forming Fluids from the Jinchang Gold Deposit, Heilongjiang Province, NE China: Constraints from the He–Ar Isotopic Evidence

AbstractThe Jinchang gold deposit (more than 40 tons of gold) is located in the easternmost portion of the Central Asian Orogenic Belt. The gold mineralization types include mainly breccia, quartz vein and alteration rock types. Geological and geochemical characteristics suggest that the Jinchang gold deposit formed by the superposition of epithermal gold deposits and porphyry gold deposits. In this study, the helium and argon isotopic compositions of fluid inclusions in pyrites from the different ores are analyzed. The results show that 3He/4He ratios of fluid inclusions in pyrites range from 0.13 to 2.42 Ra, 40Ar/36Ar ratios are within 308.9–336.7, indicating that the ore‐forming fluids of the Jinchang gold deposit were a mixture of crustal‐derived fluids and partial mantle‐derived fluids, and were contaminated by circum‐flow meteoric water. From the early to the late mineralization stages, Jinchang gold deposit had a fluid evolution trend from more mantle fluid in the central area to crustal fluid in the outer area. Jinchang gold deposit is the result of superimposition of mineralization of magmatic fluids at different phases and nature.

Geochemical and isotopic evidence for a magmatic-hydrothermal origin of the polymetallic vein-type Zn-Pb deposits in the northwest margin of Jiangnan Orogen, South China

Polymetallic vein-type Zn-Pb deposits are located in the Xiangxi–Qiandong zinc-lead metallogenic belt (XQMB) of the northwestern margin of the Jiangnan Orogen, South China. Ores are mainly found in fault-bounded quartz veins hosted in the upper part of the Banxi Group that consists of low-grade metamorphic sandstone, siltstone with minor tuff interbeds. The Zn-Pb deposits primarily contain sphalerite, galena, chalcopyrite and pyrite, accompanied by quartz and minor calcite. Zinc, lead, copper, indium and gallium are enriched in these ores. Investigation of the ore fluid reveals low temperature (87–262°C) with scattered salinity (range from 2.73 to 26.64wt% NaCleqv.). Hydrogen and oxygen isotopic compositions of fluid inclusions in quartz indicate mixing of magmatic hydrothermal fluid and meteoric water (δ18OH2O SMOW=0.2‰ to 4.2‰; δDH2O SMOW=−126‰ to −80‰). Carbon and oxygen isotopic composition of carbonate samples indicate the magmatic hydrothermal origin of CO32− or CO2 in ore-forming fluid (δ13CPDB=−6.9‰ to −5.7‰, δ18OSMOW=11.3‰ to 12.7‰). Sulfur and lead isotopic compositions (δ34SVCDT=8.8–14.2‰ and 206Pb/204Pb=17.156–17.209, 207Pb/204Pb=15.532–15.508, 208Pb/204Pb=37.282–37.546) demonstrate that sulfur sources were relatively uniform, and low radiogenic lead isotopic compositions indicate that ore metals were derived from a relatively unradiogenic source, probably by mixing of mantle with crust. Therefore, polymetallic vein-type Zn-Pb mineralization in this area probably arose from a magmatic-related hydrothermal system, and the deposition of sulfides occurred in response to cooling and boiling of magmatic hydrothermal fluids (high salinity, high δ18OH2O and δDH2O and metal-bearing), and is mainly the result of emplacement into open space and mixing with meteoric water (low salinity, low δ18OH2O and δDH2O). This study provides direct evidence that magmatism was involved in the ore-forming processes of the low temperature metallogenic district, South China, and it raises awareness about the presence of polymetallic vein-type Zn-Pb deposits in the northwest margin of Jiangnan Orogen and their potential as a source of zinc, copper, indium and gallium.

Lithium isotope traces magmatic fluid in a seafloor hydrothermal system

Lithium isotopic compositions of fluid inclusions and hosted gangue quartz from a giant volcanogenic massive sulfide deposit in China provide robust evidence for inputting of magmatic fluids into a Triassic submarine hydrothermal system. The δ7Li results vary from +4.5‰ to +13.8‰ for fluid inclusions and from +6.7‰ to +21.0‰ for the hosted gangue quartz(9 gangue quartz samples containing primary fluid inclusions). These data confirm the temperature-dependent Li isotopic fractionation between hydrothermal quartz and fluid (i.e., Δδ7Liquartz-fluid = –8.9382 × (1000/T) + 22.22(R2 = 0.98; 175 °C–340 °C)), which suggests that the fluid inclusions are in equilibrium with their hosted quartz, thus allowing to determine the composition of the fluids by using δ7Liquartz data. Accordingly, we estimate that the ore-forming fluids have a δ7Li range from −0.7‰ to +18.4‰ at temperatures of 175–340 °C. This δ7Li range, together with Li–O modeling , suggest that magmatic fluid played a significant role in the ore formation. This study demonstrates that Li isotope can be effectively used to trace magmatic fluids in a seafloor hydrothermal system and has the potential to monitor fluid mixing and ore-forming process.

Stable (H, O, C) and noble-gas (He and Ar) isotopic compositions from calcite and fluorite in the Speewah Dome, Kimberley Region, Western Australia: implications for the conditions of crystallization and evidence for the influence of crustal-mantle fluid mixing

In this study, the C-O-isotopic data from calcite at Yungul and Wilmott (Speewah. Western-Australia) are integrated with microthermometry, H2O-, CO2-content and H-He-Ar-isotopic data from fluid inclusions in genetically related calcite and fluorite to map the origin and crystallization paths of the fluids. In addition to the hydrogen isotopic compositions of fluid inclusions in fluorite, oxygen isotopic compositions were also determined by cavity ring-down spectroscopy. The geochemical data suggest mixing of a CO2-dominated mantle fluid and a H2O-domintated crustal brine. The fluid produced by this mixing is characterized by radiogenic (crustal-like) 3He/4He ratios, crustal-like δD values, relatively high salinity (19 − 24wt.% NaCl eq.), moderate homogenization temperatures (150 − 450 °C) and mantle-like CO2/3He ratios. Moreover, the large isotopic and elemental variations found in calcite indicate that its formation was accompanied by an extensive degassing (open system) leading to a decrease in δD and an increase in the CO2/3He values relative to the starting fluid composition. This degassing is consistent with the fluidal- and breccia-like texture of calcite observed in the field. In contrast, the fluorite which has coarse-grained banded to vughy textures formed in a passive aqueous system. Apparently the fluid that formed the fluorite has the same origin as the calcite, but the higher water content and the more radiogenic 3He/4He ratios reflect a greater involvement of crustal fluids. The historical description of the calcite-fluorite system in the Speewah area as “carbonatite” is now considered inappropriate because there is no evidence that crystallization is dominated by magmatic processes.

Metallogenic Model of the Trepča Pb-Zn-Ag Skarn Deposit, Kosovo: Evidence from Fluid Inclusions, Rare Earth Elements, and Stable Isotope Data

The Trepca Pb-Zn-Ag skarn deposit (29 Mt of ore at 3.45% Pb, 2.30% Zn, and 80 g/t Ag) is located in the Kopaonik block of the western Vardar zone, Kosovo. The mineralization, hosted by recrystallized limestone of Upper Triassic age, was structurally and lithologically controlled. Ore deposition is spatially and temporally related with the postcollisional magmatism of Oligocene age (2326 Ma). The deposit was formed during two distinct mineralization stages: an early prograde closed-system and a later retrograde open-system stage. The prograde mineralization consisting mainly of pyroxenes (Hd54100Jo045Di045) resulted from the interac- tion of magmatic fluids associated with Oligocene (2326 Ma) postcollisional magmatism. Whereas there is no direct contact between magmatic rocks and the mineralization, the deposit is classified as a distal Pb-Zn-Ag skarn. Abundant pyroxene reflects low oxygen fugacity (<1031 bar) and anhydrous environment. Fluid inclu- sion data and mineral assemblage limit the prograde stage within a temperature range between 390° and 475°C. Formation pressure is estimated below 900 bars. Isotopic composition of aqueous fluid inclusions hosted by hedenbergite (δD = 108 to 130‰; δ18O = 7.58.0‰), Mn-enriched mineralogy and high REE content of the host carbonates at the contact with the skarn mineralization suggest that a magmatic fluid was modified during its infiltration through the country rocks. The retrograde mineral assemblage comprises ilvaite, magnetite, arsenopyrite, pyrrhotite, marcasite, pyrite, quartz, and various carbonates. Increases in oxygen and sulfur fugacities, as well as a hydrous character of min- eralization, require an open-system model. The opening of the system is related to phreatomagmatic explosion and formation of the breccia. Arsenopyrite geothermometer limits the retrograde stage within the temperature range between 350° and 380°C and sulfur fugacity between 108.8 and 107.2 bars. The principal ore minerals, galena, sphalerite, pyrite, and minor chalcopyrite, were deposited from a mod- erately saline Ca-Na chloride fluid at around 350°C. According to the isotopic composition of fluid inclusions hosted by sphalerite (δD = 55 to 74‰; δ18O = 9.6 to 13.6‰), the fluid responsible for ore deposition was dominantly meteoric in origin. The δ34S values of the sulfides spanning between 5.5 and +10‰ point to a magmatic origin of sulfur. Ore deposition appears to have been largely contemporaneous with the retrograde stage of the skarn development. Postore stage accompanied the precipitation of significant amount of carbonates including the travertine deposits at the deposit surface. Mineralogical composition of travertine varies from calcite to siderite and all carbonates contain significant amounts of Mn. Decreased formation temperature and depletion in the REE content point to an influence of pH-neutralized cold ground water and dying magmatic system.

Inhomogeneous lithospheric thinning in the central North China Craton: Zircon U–Pb and S–He–Ar isotopic record from magmatism and metallogeny in the Taihang Mountains

The large scale Mesozoic magmatism and related metallogeny in the Taihang Mountains (TM) provide important clues for the lithospheric thinning of the North China Craton (NCC). Among the ore deposits, the vein gold mineralization of Shihu in the Fuping region and the skarn ore deposit of Xishimen in the Wu'an region represent typical Mesozoic metallogeny in the TM. In the Shihu gold mine, the Mapeng batholith is dominantly composed of monzogranite and granodiorite, whereas, the Wu'an pluton in the Xishimen iron mine mainly comprises monzonite and diorite. Here we present zircon LA–ICP-MS U–Pb data from 8 samples which reveal the timing of magmatism in the TM as ca. 130Ma, which is contemporaneous with the large-scale metallogeny in the margins of the NCC. The δ34S values recorded in the sulfide minerals from the Shihu gold deposit and the Xishimen skarn iron deposit show a range of 2.2‰–5.0‰, and 11.6‰–18.7‰, respectively. Helium isotopic compositions of fluid inclusions in pyrite from the Shihu gold deposit vary from 0.12 to 1.98Ra (where Ra is the 3He/4He ratio of air=1.39×10−6), with calculated mantle helium values of 1.4%–25%, whereas, those of the Xishimen skarn iron deposit range from 0.06 to 0.19Ra, with calculated mantle helium of 0.7%–2.2%. The S–He–Ar isotopic data suggest a lower crustal origin for the ore-forming components, with variable inputs of mantle source. The large population of inherited zircons in our samples, with 207Pb/206Pb ages ranging between 2500Ma and 1800Ma, also supports crustal participation. Our data reveal that the Shihu gold deposit witnessed greater mantle input than the Xishimen skarn iron deposit, suggesting that the continental lithosphere is markedly thinner under the Fuping region than that under the Wu'an region. Our interpretation is also supported by published data from two ultra-broadband high-precision magnetotelluric sounding profiles across the TM region showing a variation in the lithosphere thickness from 155km to 70km while moving from the south (Wu'an region) to the north (Fuping region). Our study suggests that inhomogeneous lithospheric thinning in the central NCC occurred at least as early as ca. 130Ma ago.

Crust and mantle contributions to gold-forming process at the Daping deposit, Ailaoshan gold belt, Yunnan, China

The Daping gold deposit contains one of the largest resources in the Ailaoshan gold belt, the most economically significant Cenozoic gold belt in China. Most previous researchers considered Daping to be a reworked type gold deposit, and the ore-forming fluid to have been a mixture of magmatic and meteoric water. Ore-forming materials were considered to be derived mainly from the diorite host rocks or metamorphic basement by water-rock interaction. Detailed observation and in-situ laser Raman analyses discovered a large amount of fine-grained, highly-crystallized graphite in the auriferous sulfide quartz veins with obvious O peaks and very weak D peaks in the laser Raman spectrum, which suggests that those graphite grains were formed under granulite facies conditions. Isotope analyses show that the ( 87Sr/ 86Sr) 0 and εNd (0) of the Daping scheelite, the earliest precipitated ore mineral in the paragenesis, vary between 0.7088 and 0.7112 and − 8.43 and − 6.20, respectively, and were projected in the lower crust field on a ( 87Sr/ 86Sr) 0 versus εNd (0) diagram. Noble gases isotopic compositions of fluid inclusions in Daping scheelites were performed by a high vacuum gas mass spectrum, and the results show that the 3He/ 4He ratios are (0.988–1.424) × 10 − 6 with an average of 1.205 × 10 − 6 , corresponding to R/Ra values of 0.706–1.018, with an average of 0.898. The 40Ar/ 36Ar ratios are between 1801.8 and 2663.8, much higher than that of the air (295.5); 20Ne/ 22Ne and 21Ne/ 22Ne are 9.600 to 11.56 and 0.028 to 0.0467, respectively, and the respective 134Xe/ 132Xe and 136Xe/ 132Xe are 0.394 to 0.692 and 0.301 to 0.462, demonstrating that ore-forming fluids and materials of the Daping mine derived mainly from the transitional zone between the lower crust and upper mantle. The δD compositions of fluid inclusions in Daping gold deposit are − 60.0‰ to − 85‰, with an average of − 74.5‰, whereas δ 18O H2O are 2.39‰ to 7.59‰, averaging 5.68‰, indicating that the ore-forming fluids consist predominantly of metamorphic fluid, with contributions from mantle-derived primary magmatic fluids. The δ 13C compositions of CO 2 in fluid inclusions of Daping gold deposit lie between − 3‰ to − 6.5‰, suggesting that most of the CO 2 in the ore-forming fluids was mantle-derived, and part came from the lower crust in the Ailaoshan gold belt. In addition, primitive mantle normalized platinum group elements (PGE) patterns for the auriferous ores are similar to that of the Cenozoic lamprophyre dykes in the gold deposit, but quite different from that of the diorite host rocks, also implying that the ore-forming materials in the Daping deposit derived mainly from the lower crust or even the upper mantle. The new data imply that crust–mantle interaction may have played an important role in the mineralization of the Daping deposit. At around 33 Ma, the middle and lower crust in the Daping area suffered high-temperature and high-pressure metamorphism because of ductile deformation and upwelling upper mantle magmas. The ore-forming fluids enriched in CO 2, 3He, 20Ne and 130Xe, generated by granulite facies metamorphism and baking by upwelling upper mantle magmas, were transported to the middle to upper crust along the ductile shear zone along with highly crystallized graphite grains, and finally precipitated auriferous sulfide-quartz veins in brittle structures because of declining temperature, pressure, and subsequent boiling. Daping is a typical ductile shear zone controlled orogenic gold deposit.

The genesis of sandstone‐type uranium deposits in the Ordos Basin, NW China: constraints provided by fluid inclusions and stable isotopes

Quartz from sandstone‐type uranium deposits in the east part of the Ordos Basin contains abundant secondary fluid inclusions hosted along sealed fractures or in overgrowths. These inclusions consist mainly of water with NaCl, KCl, CO2 (135–913 ppm) and trace amounts of CO (0.22–16.8 ppm), CH4 (0.10–1.38 ppm) and [SO4]2− (0.35–111 ppm). Homogenization temperatures of the studied fluid inclusions range from 90 to 210°C, with salinities varying from 0.35 to 12.6 wt‐% (converted to NaCl wt%), implying multiple stages of thermal alteration. Although high U is associated with a high homogenization temperature in one case, overall U mineralization is not correlated with homogenization temperature nor with salinity. The H and O isotopic compositions of fluid inclusions show typical characteristics of formation water, with δ18O ranging from 9.8 to 12.3‰ and δD from 26.9 to −48.6‰, indicating that these fluid inclusions are mixtures of magmatic and meteoric waters. The oxygen isotope ratios of carbonates in cement are systematically higher than those of the fluid inclusions. Limited fluid inclusion‐cement pairs show that the oxygen closely approaches equilibrium between water and aragonite at 150°C. Highly varied and overall negative δ13C in calcite from cement implies different degrees of biogenetic carbon involvement. Correlations between U in bulk rocks and trace components in fluid inclusions are lacking; however, high U contents are typically coupled with high [SO4]2−, implying pre‐enrichment of oxidized materials in the U mineralization layer. All these relationships can be plausibly interpreted to indicate that U (IV), [SO4]2− as well as Na, K were washed out from the overlying thick sandstone by oxidizing meteoric water, and then were reduced by reducing agents, such as CH4 and petroleum, likely from underlying coal and petroleum deposits, and possibly also in situ microbes at low temperatures.

Noble gases in South Indian carbonatites: Trapped and in situ components

We have studied noble gases in carbonates and apatites from three carbonatites of South India, namely Hogenakal (2400 Ma), Sevattur (770 Ma) and Khambamettuu (523 Ma) by vacuum crushing. Apatite has also been analysed by pyrolysis. Vacuum crushing mostly releases the trapped gas components. The ratios 21Ne/ 20Ne, 22Ne/ 20Ne and 40Ar/ 36Ar increase with progressive crushing due to preservation of different composition gases in smaller inclusions released in later steps. This heterogeneity of isotopic composition of fluid inclusions is a consequence of the involvement of magmas carrying different noble gas signatures. The inclusions with lower ratios suggest the presence of a subducted atmospheric component, while the higher 21Ne/ 20Ne, 22Ne/ 20Ne and 40Ar/ 36Ar can be attributed to the presence of an enriched lithospheric mantle component. In addition, very minor trapped gases from less degassed, deeper mantle may also be present but overprinted by lithospheric and/or nucleogenic components. We propose that these carbonatites were generated only in an advanced stage of magmatism when this lithospheric component overwhelmed any contribution from the deeper mantle source. The lithospheric mantle underwent enrichment during an ancient subduction process through mantle metasomatism manifested in nucleogenic/radiogenic isotopic ratios of 21Ne/ 20Ne, 22Ne/ 20Ne and 40Ar/ 36Ar. The apatites analysed by pyrolysis clearly show nucleogenic 21Ne from 18O(α,n) reaction. We have demonstrated the potential of using U,Th– 21Ne systematics as a thermo-chronometer in conjunction with the established U,Th– 4He and U– 136Xe clocks. While for Hogenakal, the U,Th– 21Ne age of 845 ± 127 Ma is in agreement with the age of emplacement of other adjacent younger carbonatites, syenites and alkali granites, for the Sevattur apatite (738 ± 111 Ma) it indicates the crystallisation age.

He‐Ar Isotopic Systematics of Fluid Inclusions in Pyrites from PGE‐polymetallic Deposits in Lower Cambrian Black Rock Series, South China

Abstract He‐Ar isotopic compositions of fluid inclusions trapped in pyrites from some representative PGE‐polymetallic deposits in Lower Cambrian black rock series in South China were analyzed by using an inert gas isotopic mass spectrometer. The results show that the ore‐forming fluids possess a low 3He/4He ratio, varying from 0.43×10−8 to 26.39×10−8, with corresponding R/Ra value of 0.003‐0.189. The 40Ar/36Ar ratios are 258–287, close to those of air‐saturated water (ASW). He‐Ar isotopic indicator studies show that the ore‐forming fluids were mainly derived from the formation water or basinal hot brine and sea water, while the content of mantle‐derived fluid or deep‐derived magmatic water might be negligible. The PGE‐polymetallic mineralization might be related to the evolution of the Caledonian miogeosynclines distributed along the southern margin of the Yangtze Craton. During the Early Cambrian, the formation water or basinal hot brine trapped in Caledonian basins which accumulated giant thick sediments was expelled and migrated laterally along strata because of the pressure generated by overlying sediments. The basinal hot brine ascended along faults, mixed with sea water and finally deposited ore minerals.