Hydrothermal activities in the middle Permian Maokou Formation in eastern Sichuan Basin
Li Hong1, Wang Liang-Jun2, Liu Yi-Qun1, Zeng Tao2, Zhang Dong-Dong1, Li Wen-Hou1, Zhou Ding-Wu3, Yang Kang1, Dong Yang-Kun1, Peng Yi-Feng1
1 Department of Geology/State Key Laboratory of Continental Dynamics,Northwest University,Xi'an 710069,China; 2 SINOPEC Exploration Company,Chengdu 610041,China; 3 College of Earth Science and Engineering,Shandong University of Science and Technology,Shandong Qingdao 266590,China
Abstract：The abundant hydrothermal activities have been found in the Maokou Formation of middle Permian in the Sichuan Basin. Based on the field investigation and core observation, many kinds of measured methods are applied to discuss the hydrothermal activity characteristics of the carbonate rocks in the Maokou Formation of middle Permian in the Sichuan Basin. Two types of hydrothermal activities in Maokou Formation are identified in eastern Sichuan Basin: one is hydrothermal silica, formed as thin layered silicalite or nodules; the other is featured by hydrothermal carbonate minerals, including coarse calcite and dolomite that are filled in the tensile fractures or formed as the “snow flake” aggregates, sometimes they occur as few metal sulfides including the pyrite and sphalerite. The Al-Fe-Mn end-member diagram of cherts and correlation diagrams between silica and other main elements indicate the hydrothermal origin, and the silica comes from both the hydrothermal and terrestrial sources. The in situ sulfur isotopes of pyrite and sphalerite range between -3.91‰ and -6.87‰, implying the mixed sulfur sources of microbial and basic magma. Both coarse calcite and saddle dolomite in the veins have similar geochemical features, such as higher CaO content than those in stoichiometry calcite and dolomite, rich in Sr and U, poor in REE and Ti, obvious negative anomalies of Ce/Ce*, and positive anomalies of Eu/Eu*, and very positive Y anomalies, illustrating the hydrothermal origin and the mixed sources of fluids related with volcanic activity and the Ca-bearing underground water. High concentration of Ge and Cd, and ratios of Zn/Cd and Zn/Fe in sphalerite reflect the medium mineral-forming temperatures. The homogenization temperature ranges between 54.7℃ and 294.3℃ measured from fluid inclusions in microcrystalline quartz, calcite and dolomite, which are consistent with the features illustrated by the REEs and trace elements. The results show that the first occurrence of chert nodules and chips penetrating into the rocks is the indicant of the beginning of hydrothermal activities in Maokou Formation. The tensile fractures and the hydrothermal dolomitization caused by carbonate-rich fluid in host rocks significantly improve the reservoir properties of dolomite.
Li Hong,Wang Liang-Jun,Liu Yi-Qun et al. Hydrothermal activities in the middle Permian Maokou Formation in eastern Sichuan Basin[J]. JOPC, 2021, 23(1): 153-174.
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