罗布泊杂卤石沉积特征及成因机理探讨

Sedimentary characteristics and origin of polyhalite in Lop Nur Salt Lake,Xinjiang

罗布泊是世界最大的盐湖之一。罗布泊第四纪地层中不仅蕴藏有超大型规模的卤水钾盐矿床,同时也沉淀了一定规模的固体钾盐矿,杂卤石是主要的固体钾矿物。罗布泊杂卤石主要分布于罗北凹地内部,分布区长轴走向与罗北凹地走向一致,面积约326km2;剖面上,杂卤石分布区呈不对称的扁透镜体状。杂卤石以杂卤石岩或含杂卤石钙芒硝岩等形式产出,杂卤石岩及含杂卤石岩呈薄层状分布,最多达12层,埋深10～38m,其KCl含量一般为3%～10%,最高达15.48%。研究表明,罗布泊杂卤石成因机制有2种:①盐湖(钙芒硝阶段)沉积期间,可能出现气候热波动事件,卤水化学体系演化到杂卤石相区,导致杂卤石结晶析出,主要形成方式为杂卤石交代钙芒硝,部分纯杂卤石岩可能直接从卤水中沉淀出来;②盐湖晚期或干盐湖阶段,石盐晶粒间卤水受地表水混合发生掺杂作用,导致杂卤石析出并沉积于石盐等晶粒间。罗布泊杂卤石等固体钾盐资源具有一定的成矿潜力,查明其成因和分布规律可为勘查评价罗布泊固体钾盐提供理论支持。

Located in the eastern part of Tarim Basin, Xinjiang, the Lop Nur Lake is one of the largest playa lakes m the world. In that lake, a superlarge brine potash deposit was discovered within Quaternary salt strata. There also exist some solid potash deposits in the strata, in which polyhalite is a dominant potash mineral, which is mainly distributed in the inner part of the Luobei Hollow within the Lop Nur Lake. The polyhalite is spread in an area of about 326 km^2, and its long axis is parallel to the axis of the Luobei Hollow, with its shape assuming an unsymmetrical lens in NE-WS direction. The polyhalite occurs as pure polyhalite rock or polyhalite-bearing rock, and there are twelve thin layers of polyhalite and polyhalite-bearing rock. The buried depth of polyhalite layers varies from 10 m to 38 m, and the KCI contents are generally from 3% to 10% wt., with the highest value up to 15.48%. Studies show that there exist two kinds of polyhalite precipitation mechanisms： （1） hot weather events might have occurred during the precipitation of glaubeirite, and polyhalite precipitated through the replacement of glauberite; meanwhile, some pure polyhalite beds probably precipitated directly from paleolake brine; （2） during the playa stage, the intercrystal brine enriched in K and Mg ions was mixed with Ca-rich surface water, polyhalite probably precipitated from the mixed water and occurred between halite grains. Investigation shows that solid potash resources in the Lop Nur Salt Lake deserve further exploration. The researches on the distribution and genesis of polyhalite can provide theoretical support to solid potash exploration in the Lop Nur Lake.