柴达木盆地西部SG-1钻孔中白云石成因探讨

由于无机环境下不能沉淀白云石，该矿物的成因一直是学术争论的焦点。柴达木盆地西部钻孔SG-1（长938 m）中出现了大量白云石和铁白云石，白云石主要分布在钻孔下部500 m，而铁白云石主要分布在下部418 m。结合湖泊从淡水湖、咸水湖、盐湖至干盐湖的演化过程，文章分析了白云石和铁白云石的成因。白云石是盐类矿物的一种、且无机环境下不能沉淀，蒸发作用和微生物介导是白云石矿物形成的两个重要影响因素。蒸发作用为白云石的形成提供了足够浓度的Mg2+，微生物的介导作用帮助Mg2+克服动力学障碍进入碳酸钙晶格形成白云石。白云石是湖泊演化早期析出的一种碳酸盐类矿物，主要在咸水湖环境中沉淀，盐湖环境中主要沉淀硫酸盐类和氯化物矿物，在盐湖这种高盐度环境下能够生存的微生物非常少，白云石含量明显降低。铁白云石是白云石矿物的一种，是Fe2+替代白云石中的Mg2+形成的次生矿物。Fe2+有两种来源：粘土矿物转换过程中的释放和深部热液来源。Fe2+进入白云石的过程主要是在无机、高温环境下完成的，但不排除微生物的介导作用。

On the Origin of Dolomite in Laustrine Sediments of the Borehole SG-1 in Western Qaidam Basin

Since dolomite cannot be synthetized in inorganic conditions, the cause of formation of dolomite has long been a disputed topic. A 938 m-long borehole, located in Western Qaidam Basin, contains plenty of dolomite and ankerite, which were found in the intervals of 938-438 m and 938-520 m, respectively. This study is focused on investigating the formation of dolomite and ankerite based on the evolution of the paleolake. As an evaporative mineral, dolomite cannot be synthetized without microbial activities. Therefore, evaporation and microbial catalysis are necessary for the formation of dolomite. Evaporation makes Mg2+ become highly concentrated and microbial catalysis helps Mg2+ enter into the structure of CaCO3 to form dolomite crystals. As carbonate minerals are generally precipitated in the early stage of a lake’s evolution, the contents of dolomite in the Core SG-1 in brackish conditions are higher than those in salty conditions. However, ankerite, which is a type of dolomite, has different forming conditions. Fe2+ can be from deep hydrothermal fluid and was released during the transformation process between clays. Fe2+ is participated into the structure of dolomite to form ankerite, which is mainly finished in high temperatures and inorganic conditions with minor or without microbial activities.