四川盆地西部栖霞组热液白云岩中的自生非碳酸盐矿物

热液环境是碳酸盐重要的成岩环境之一,MVT铅锌矿床和某些白云岩储层都与之有关。通过薄片观察、扫描电镜和能谱分析,在四川盆地西部中二叠统栖霞组白云岩地层中发现了一些自生非碳酸盐矿物和焦沥青,包括伊利石、萤石、氟磷灰石、石英、重晶石和黄铁矿等,它们赋存于具不同结构的白云石的晶间/晶内孔隙和溶解孔隙中,尤其是非平直晶面鞍形白云石的溶解孔隙中。赋存自生矿物的白云石主晶的包裹体均一化温度主要分布在80～180℃之间,最高可达240℃以上。白云石溶解空间中萤石、氟磷灰石(也包括方解石)等以Ca作为阳离子自生矿物的存在,反映热液白云石沉淀后温度降低造成的流体性质的改变和白云石的溶解,热液蚀变作用还造成了伊利石的形成和黏土矿物的强迫热演化,白云石晶间焦沥青的存在则代表了有机质因热蚀变而发生的强迫成熟和过成熟。川西栖霞组碳酸盐岩的热液蚀变作用发生在距今259Ma左右(中二叠世末)的东吴运动期间,与峨眉山玄武岩有关的热事件伴生。该期间古热流达60～80mW/m2,甚至更高,持续时间约6Ma,此时栖霞组白云岩地层的埋藏深度<500m,因而热液蚀变过程与地热增温无关。白云岩地层中各种自生矿物和非正常有机质热演化形成的焦沥青的存在为川西栖霞组碳酸盐岩所经历的热液环境成岩作用提供了依据。

Authigenic noncarbonate minerals in hydrothermal dolomite of Middle Permian Qixia Formation in the west of Sichuan Basin, 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.