Hydrothermal calcites from the Tianqiao Pb-Zn ore deposit, NW Guizhou Province, China, were analyzed using Continuous Flow Mass Spectrometry and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) for their carbon and oxygen isotope and rare earth elements (REE) compositions. The results show that the C and O isotopic compositions of the hydrothermal calcites are relatively homogeneous, and samples from different elevations show similar C and O isotopic compositions. The δ13CPDB and δ18OSMOW range from -5.3 per mil to -3.4 per mil and from 14.7 per mil to 19.5 per mil, respectively. In the δ13C vs. δ18O diagram, the hydrothermal calcite samples are all fall into the zone between igneous carbonatite and marine carbonate rocks. The ΣREE contents of the hydrothermal calcites are low (6.80×10-6 to 49.1×10-6), exhibiting enriched light rare earth elements, negative Eu anomalies (the Eu/Eu* ratios range from 0.30 to 0.55), and M-shape REE patterns, which are similar to those of the sulfides. Comparing the host rocks, unaltered rocks, and altered rocks with the hydrothermal calcites regarding C and O isotopic compositions and REE patterns, we believe that the ore-forming fluids were mixture of multiple source fluids, within which the C and REE were mainly sourced from carbonate wall-rocks, the sulfur from the thermo-chemical sulfate reduction of sulfates in the sedimentary strata, and the metamorphic water from underlying basement-rocks such as Kunyang Group, and affected by meteoric water.
Jinshachang Pb-Zn deposit,located in Yunnan province and the northwest of Sichuan-Yunnan-Guizhou(SYG) Pb-Zn-Ag multi-metal mineralization area,is mainly hosted in the Upper Neoproterozoic carbonate rocks of Dengying Group.The ore minerals mainly contain sphalerite and galena,and the gangue minerals mainly contain barite,fluorite and quartz.The δ34S values of sphalerite minerals are in the range between 3.6‰ and 13.4‰ with an average value of 5.7‰.The δ34S values of galena range from 6.0‰ to 9.0‰ with an average value of 7.1‰.Two δ34S values of barite are 34.8‰ and 34.5‰ and consistent with that of sulfate from the Lower Cambrian stratum,indicating that the sulfur of barite were derived from this stratum.Because the homogenization temperatures of fluid inclusions exceed the suitable temperature for bacterial sulfate reduction,therefore this process is not an efficient path for the production of reduced sulfur in this district.If sulfides were derived from the Cambrian stratum due to thermochemical sulfate reduction,sulfides in this deposit should preserve a minimum δ34S value of 14‰ which exceeding that of sulfide from Jinshachang deposit.Therefore,this process is not the only source of reduced sulfur.Considered the fact that the ore district is surrounded by basalts and δ34S values of sulfide related to magmatic activities are relatively low,the reduced sulfur of sulfide may be derived from magmatic activities.The δ34S values of sphalerite are higher than that of coexisting galena,indicating that sulfur isotopic composition in ore-forming fluid had partly reached equilibrium.The equilibrium temperatures calculated by using the δ34S values of surfur-bearing mineral pairs are consistent well with the homogenization temperature of fluid inclusions.
Abstract: Trace elements and rare earth elements (REE) of the sulfide minerals were determined by inductively‐coupled plasma mass spectrometry. The results indicate that V, Cu, Sn, Ga, Cd, In, and Se are concentrated in sphalerite, Sb, As, Ge, and Tl are concentrated in galena, and almost all trace elements in pyrite are low. The Ga and Cd contents in the light‐yellow sphalerites are higher than that in the brown and the black sphalerites. The contents of Ge, Tl, In, and Se in brown sphalerites are higher than that in light‐yellow sphalerites and black sphalerites. It shows that REE concentrations are higher in pyrite than in sphalerite, and galena. In sphalerites, the REE concentration decreases from light‐yellow sphalerites, brown sphalerites, to black sphalerites. The ratios of Ga/In are more than 10, and Co/Ni are less than 1 in the studied sphalerites and pyrites, respectively, indicating that the genesis of the Tianqiao Pb–Zn ore deposit might belong to sedimentary‐reformed genesis associated with hydrothermal genesis. The relationship between LnGa and LnIn in sphalerite, and between LnBi and LnSb in galena, indicates that the Tianqiao Pb–Zn ore deposit might belong to sedimentary‐reformed genesis. Based on the chondrite‐normalized REE patterns, δEu is a negative anomaly (0.13–0.88), and δCe does not show obvious anomaly (0.88–1.31); all the samples have low total REE concentrations (<3 ppm) and a wide range of light rare earth element/high rare earth element ratios (1.12–12.35). These results indicate that the ore‐forming fluids occur under a reducing environment. Comparison REE compositions and parameters of sphalerites, galenas, pyrites, ores, altered dolostone rocks, strata carbonates, and the pyrite from Lower Carboniferous Datang Formation showed that the ore‐forming fluids might come from polycomponent systems, that is, different chronostratigraphic units could make an important contribution to the ore‐forming fluids. Combined with the tectonic setting and previous isotopic geochemistry evidence, we conclude that the ore‐deposit genesis is hydrothermal, sedimentary reformed, with multisources characteristics of ore‐forming fluids.
Abstract: The Jinshachang lead–zinc deposit is mainly hosted in the Upper Neoproterozoic carbonate rocks of the Dengying Group and located in the Sichuan–Yunnan–Guizhou (SYG) Pb–Zn–Ag multi‐metal mineralization area in China. Sulfides minerals including sphalerite, galena and pyrite postdate or coprecipitate with gangue mainly consisting of fluorite, quartz, and barite, making this deposit distinct from most lead‐zinc deposits in the SYG. This deposit is controlled by tectonic structures, and most mineralization is located along or near faults zones. Emeishan basalts near the ore district might have contributed to the formation of orebodies. The δ 34 S values of sphalerite, galena, pyrite and barite were estimated to be 3.6‰–13.4‰, 3.7‰–9.0‰, −6.4‰ to 29.2‰ and 32.1‰–34.7‰, respectively. In view of the similar δ 34 S values of barite and sulfates being from the Cambrian strata, the sulfur of barite was likely derived from the Cambrian strata. The homogenization temperatures ( T ≈ 134–383°C) of fluid inclusions were not suitable for reducing bacteria, therefore, the bacterial sulfate reduction could not have been an efficient path to generate reduced sulfur in this district. Although thermochemical sulfate reduction process had contributed to the production of reduced sulfur, it was not the main mechanism. Considering other aspects, it can be suggested that sulfur of sulfides should have been derived from magmatic activities. The δ 34 S values of sphalerite were found to be higher than those of coexisting galena. The equilibrium temperatures calculated by using the sulfur isotopic composition of mineral pairs matched well with the homogenization temperature of fluid inclusions, suggesting that the sulfur isotopic composition in ore‐forming fluids had reached a partial equilibrium.
Abstract The Wufeng and Longmaxi organic-rich shales host the largest shale gas plays in China. This study examined the petrography, rare earth element (REE) and other trace-element geochemistry, Sm-Nd geochronology, and isotope geochemistry (87Sr/86Sr, δ18O, δ13C) of fracture-cementing minerals within core samples of the Wufeng and Longmaxi Formations from the Jiaoshiba shale gas field in order to (1) characterize the mineral phases occurring in the veins (mineralized fractures); (2) determine the ages of the calcite by the Sm-Nd isochron dating method; (3) understand the sources of calcite-precipitating fluids; and (4) explore the possible mechanisms responsible for calcite vein formation in shale gas systems. The fractures hosted in the Longmaxi Formation are mineralized with quartz as the predominant fracture cement, and calcite as an intracementation phase postdating the earlier quartz cement. In contrast, the fractures hosted in the Wufeng Formation are dominantly mineralized by calcite, which occurs either as the only cement present or as a cement phase predating later quartz cement. Calcite veins within the Longmaxi Formation have a Sm-Nd isochron age of 160 ± 13 Ma and δ13C values of –4.71‰ to –3.11‰, δ18O values of 17.1‰–17.4‰, and 87Sr/86Sr values of 0.72437–0.72869. Calcite veins within the Wufeng Formation yielded a Sm-Nd isochron age of 133 ± 15 Ma and are characterized by δ13C values of –2.29‰ to –1.03‰, δ18O values of 17.3‰–17.7‰, and 87Sr/86Sr values of 0.72202–0.72648. The similarity between 87Sr/86Sr values of the calcite and those of their respective surrounding host rocks (0.72670–0.72875 of the Longmaxi shales; 0.72030–0.72648 of the Wufeng shales), combined with relatively depleted δ13C and uniform fluid δ18O isotopic features, indicates that the calcite-precipitating fluids within the Wufeng and Longmaxi Formations were derived largely from their respective surrounding host-rock sources. REE data equally indicate that the distinguishable Eu anomalies (6.20–19.35; 4.45–11.91), Y anomalies (1.03–1.50; 1.44–1.70), and Y/Ho ratios (28.80–39.16; 38.86–45.18) of calcite veins within the Longmaxi and Wufeng Formations were controlled by their respective surrounding host rocks. The Sm-Nd isochron ages and fluid inclusion data of fracture cements suggest that fracture opening and calcite precipitation in composite veins within the Wufeng and Longmaxi Formations were triggered by gas generation overpressurization.
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