Hydrothermal activity temperature constrained by fractionation degree of sulfur isotope in symbiotic metal sulfide: a case study of the Lower Cambrian Longwangmiao Formation in central Sichuan Basin,China
LIU Dawei1, CAI Chunfang2, HU Yongjie3, JIANG Lei2, WANG Shi1, PENG Yanyan2, LI Yingtao1, LI Han'ao1
1 Petroleum Exploration and Production Research Institute,SINOPEC,Beijing 100083,China; 2 Key Laboratory of Cenozoic Geology and Environment,Institute of Geology and Geophysics, Chinese Academy of Sciences,Beijing 100029,China; 3 International Petroleum Exploration and Production Corporation,SINOPEC,Beijing 100029,China
Abstract：The study of sulphur cycle and sulphur isotope(δ34S)fractionation is significant for diagenesis of surface sphere,where the fractionation degree of sulphur isotope between multiple metal sulphides can be used to constrain the temperature of ore-forming hydrothermal fluid,which can act as the geothermometer of hydrothermal activity. The hydrothermal transformation in the reservoir of Longwangmiao Formation in the Sichuan Basin affects the heterogeneity of the reservoir. In this paper,the co-precipitation of pyrite(FeS2)-chalcopyrite(CuFeS2)associated with hydrothermal diagenetic dolomite in the target formation is studied in detail. The sulphur isotope distribution in the metal sulphides is measured based on the NanoSIMS. The mineralization temperature was calculated based on the degree of equilibrium fractionation between multiple sulphides driven by the thermodynamics,which in turn constrains the hydrothermal process of the specific formation. Results show that(1)the sulphur isotope distribution in the microzone implies that both the thermodynamic fractionation and kinetic fractionation occur during the co-precipitation of pyrite(FeS2)and chalcopyrite(CuFeS2),where the degree of kinetic fractionation can reach 40.1‰. The influence of kinetic fractionation can be effectively excluded based on the NanoSIMS to obtain thermodynamic equilibrium fractionation data;(2)there may exist different sulfur sources during the pyrite(FeS2)and chalcopyrite(CuFeS2)mineralization processes. The pyrite precipitation primarily utilizes the sulfur sources from the underlying Qiongzhusi Fm.,which is approximately equal to the δ34S value of solid bitumen modified without thermochemical sulfate reduction[TSR],and the fractionation degree is about 2.4‰ to 2.9‰,indicating the mineralization temperature is in the range of 98.2 to 135.0 ℃. The chalcopyrite precipitation mainly utilizes the sulphur source in the formation water,which is approximately equal to the δ34S value of carbonate association sulfate[CAS]),and the mineralization temperature calculation is insignificance;(3)the mineralization temperature of pyrite indicates that the fault activity and hydrothermal activity in studied formation occurred at the peak phase of oil production and it continued in the later burial process,resulting in the ankerite precipitation with high uniform temperature.
LIU Dawei,CAI Chunfang,HU Yongjie et al. Hydrothermal activity temperature constrained by fractionation degree of sulfur isotope in symbiotic metal sulfide: a case study of the Lower Cambrian Longwangmiao Formation in central Sichuan Basin,China[J]. JOPC, 2023, 25(1): 215-225.
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