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《Journal of Chengdu University of Technology(Science & Technology Edition)》 2012-04
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Authigenic noncarbonate minerals in hydrothermal dolomite of Middle Permian Qixia Formation in the west of Sichuan Basin,China

HUANG Si-jing,LI Xiao-ning,HUANG Ke-ke,LAN Ye-fang,LüJie,WANG Chun-mei State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, Chengdu University of Technology,Chengdu 610059,China  
The hydrothermal environment is an important diagenetic environment.MVT lead and zinc ore deposits and some dolomite reservoirs are related to hydrothermal environments.Based on thin section examination,scanning electron microscopy and the energy dispersion X-ray analysis,some authigenic noncarbonate minerals,including illite,fluorite,fluorapatite,quartz,barite and pyrite,and pyrobitumen are identified in the Middle Permian Qixia Formation on the west of Sichuan Basin.These minerals occur in the intercrystalline/intragranular pores and dissolution pores in dolomites with different textures,especially in the dissolution pores of nonplanar-saddle dolomite.Fluid inclusion homogenization temperatures of host dolomites mainly range from 80℃ to 180℃,up to 240℃ or higher.The occurrence of authigenic minerals taking Ca2+ as the cation,such as fluorite,fluorapatite and also calcite in the dissolution space of dolomite,reveals that the fall of temperature after the hydrothermal dolomite precipitation causes the change of fluid nature and dissolution of dolomite.The hydrothermal alternation also leads to the formation of illite owing to forced thermal evolution of clay minerals.Pyrobitumen existing between dolomite crystals represents the forced maturation and over maturation of organic matter due to hydrothermal alternation.The hydrothermal alternation of carbonate rocks from Qixia Formation in the west of Sichuan Basin occurred during Dongwu Movement about 259 Ma B.P.(the late stage of Middle Permian),associated with the thermal events related to the Emeishan basalt eruption with paleoheat flow up to or higher than 60~80 mW/m2 and duration time of about 6 Ma.The buried depth of the dolomite strata in Qixia Formation was less than 500 meters at that period,so the process of hydrothermal alteration had nothing to do with geothermal gradient function.Various authigenic minerals and pyrobitumen formed by abnormal thermal evolution of organic matter in the dolomite strata provide evidences for the hydrothermal environment diagenesis experienced by carbonates from Qixia Formation in the west of Sichuan Basin.
【Fund】: 国家自然科学基金资助项目(40839908 41172099)
【CateGory Index】: P619.234
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