176 / 2019-06-06 21:57:04

Hydrothermal Activities in the Middle Permian Maokou Formation, Tailai Area, Eastern Sichuan Basin

Eastern Sichuan Basin,Maokou Formation,Hydrothermal activities,In situ Sulfur isotope,Magnesium isotope

Abstract: The Middle Permian Maokou Formation in Sichuan Basin has been widely reported by the discoveries of abundant hydrothermal activities. However, it is still enigmatic and controversial about the hydrothermal compositions, fluid sources, and episodes of hydrothermal activities. Based on the observation of the outcrop and drilling cores, the different hydrothermal mineral assemblages and their geochemical features in Maokou Formation carbonate rocks in Tailai area, eastern Sichuan Basin, are described and discussed by using various testing methods. There are a large number of irregular tension fractures interweaved in Maokou limestones and dolomite, presenting a breccia-like structure. The fractures are usually full-filled or half-filled as veins by coarse dolomite and calcite crystals. Curved crystal faces and saddle-shaped twin crystals in veins dolomite are observed under microscope, other hydrothermal minerals including cryptocrystalline and microcrystalline quartz, pyrite, and sphalerite. They are either mineral assemblages or just single mineral clusters, indicating different sources of hydrothermal fluids. Al-Fe-Mn end-member diagram of chert nodules and thin layers of chert in Maokou carbonate rocks shows both hydrothermal and non-hydrothermal origins of the microcrystalline quartz. In situ sulfur isotopes of pyrite and sphalerite range between -3.01‰ and -6.87‰, indicating mixed sulfur sources from both bacterial sulfate reduction and leaching from basalt. The chondrite- normalized trace elements in carbonate minerals show obvious negative anomalies of Cerium, δCe varies from 0.204 to 0.975, average of 0.492; while δEu ranges between 0.525 and 5.344, average of 1.263, and some carbonate minerals show positive anomalies of Europium. Cerium and Europium anomalies in carbonate minerals are similar with those of hydrothermal deposits near mid-ocean ridge (Mills and Elderfield, 1995). The homogenization temperatures of fluid inclusions in veins dolomite range from 50 to 250℃, reflecting the results of multi-stages of fluid activities, which is consistent with the result of REEs. The δ26Mg of hydrothermal dolomite veins varies from 2.20‰ to -0.69‰, indicating mixed Magnesium sources from both crust and mantle. In summary, the mineralogical and geochemical features of hydrothermal mineral assemblages indicate that there existed different stages and mixed sources of hydrothermal activities in Middle Permian Maokou period in eastern Sichuan Basin.