罗布泊盐湖钙的来源及钙芒硝结晶作用对富钾卤水形成的约束

The Lop Nur playa exits the largest sulfate-type-brine potash deposits in the World. Due to the large-scale development, finding reserve resources has been put on the agenda. Genesis study on potassium-rich brine is significant to search for reserve resources in the Lop Nur playa. Huge amounts of glauberite minerals were deposited, accompanied by large-scale potash-bearing brines gathered. However, some key issues, such as the genesis of glauberite minerals and the relationship between potash-bearing brines gathered and huge amounts of glauberite minerals, remain less well known. It is very important to identify sources of calcium ions during the glauberite formation and obtain fluid inclusions composition in glauberite minerals for the further study. The project plans to study the crystal habit of glauberite and analyze fluid inclusions. The chemical composition and temperature of the ancient lake will be determined during the glauberite rock formation. Then glauberite crystallization simulation test will be conducted according to the result above. Sources of calcium will be studied and the relationship between massive accumulation of glauberite minerals and potassium-rich brine formation will be discussed eventually from the perspective of crystallography. There will be an important and theoretical significance to search for reserve resources in the Lop Nur playa. It will also provide sedimentary mineralogy evidence for the study on aridification in Central Asia.

罗布泊盐湖卤水钾矿进入大规模开发后，寻找后备资源已提上日程。正确认识罗布泊富钾卤水成因，对于该地区扩大寻找后备资源具有重要现实意义。罗布泊富钾卤水资源主要赋存于巨量钙芒硝岩晶间孔隙中，巨量钙芒硝如何形成一直是阻碍富钾卤水成因研究的重要问题，查明钙芒硝沉积时期大量钙离子的来源、获取钙芒硝流体包裹体化学组成及均一温度是进一步研究的关键。本项目拟开展钙芒硝矿物结晶习性及流体包裹体研究，反演钙芒硝结晶时的水化学体系，并结合相图分析进行钙芒硝结晶模拟实验，利用同位素地球化学研究钙离子的来源，查明钙芒硝的结晶条件和形成机理，从结晶学角度探讨钙芒硝沉积对富钾卤水形成的约束。该项目的实施不仅对罗布泊扩大找钾及古代盐湖成钾远景研究具有重要指导意义，也可为亚洲内陆干旱化研究提供沉积矿物学证据。

罗布泊盐湖位于亚洲大陆干旱中心，是世界上最大的干盐湖之一，沉积了十分罕见的巨量钙芒硝，其孔隙中赋存了超大规模卤水钾盐矿床。然而，钙芒硝晶间富钾卤水成因仍未完全查明。研究巨量钙芒硝成因及其对富钾卤水的约束，对丰富罗布泊盐湖成钾理论、指导深部找钾以及寻找后备资源均具有重要意义。项目通过对LDK02孔岩芯进行系统的盐类矿物鉴定，研究了蒸发岩层段岩石学及主要盐类矿物矿物学特征。开展了盐类矿物流体包裹体成分分析、均一温度测试、塔里木河流域浅层地下水的蒸发演化模拟、同位素测试等，重点探讨了罗布泊盐湖钙芒硝的成因。.在晚更新世末期钙芒硝沉积停止，石盐开始沉积。流体包裹体均一温度测试结果显示，晚更新世末期沉积的石盐原生流体包裹体最大均一温度35.6℃～43℃，根据盐田中石盐包裹体最大均一温度与最高气温的差值（2.3℃），推算晚更新世末期最高气温达37.9℃～45.3℃，指示了极度干热的气候条件。包裹体成分结果显示钙芒硝沉积时期古湖水中钾含量高于正常海水浓缩到石盐开始沉积时的钾离子含量，钙离子含量也很高，说明罗布泊盐湖可能存在深部富钾和富钙水的补给。晚更新世末期极度干热的气候条件促使罗布泊盐湖卤水快速、强烈蒸发，最后形成富钾卤水。.塔里木河流域浅层地下水可能代表了罗布泊盐湖的补给水源，用EQL/EVP程序模拟出来的矿物析出序列和实际观测到的罗布泊盐湖沉积矿物序列有很好的一致性。模拟结果表明，在开放系统下可以形成少量的钙芒硝矿物，而在封闭系统下可以沉淀出大量的钙芒硝矿物，其含量比开放系统高出100倍，这表明封闭系统更有利于钙芒硝矿物的形成。但是，即使在封闭系统中，通过模拟蒸发析出的钙芒硝量也远远小于罗布泊盐湖中实际观测到的钙芒硝沉积量。因此，推断罗布泊盐湖肯定还有富含钙离子的深部水补充。以塔里木河流域和深部富钙水为补给源，在干旱气候条件下，罗布泊盐湖湖水蒸发浓缩，经过漫长时间的物质积累，最终形成大量钙芒硝，为罗布泊提供富钾卤水的储存空间。