察尔汗盐湖成盐富钾物源联系：不同水体混合蒸发实验和同位素证据

There are both the thick halite layers and liquid beds of rich K+ and Mg2+ deposited in Qarhan Salt Lake. These halite layers mainly contained halite and poor gypsum minerals. The mineral components are also relatively simple. Most of brines in this region are enriched in K+, which could be divided into the chloride type and Mg-sulfate type water. Therefore, the study on original source of forming halite and enriching K+ in liquid brines is very significant to interpret the mechanism of forming potash evaporites in continental basins. Previous researches indicated that Qarhan Salt Lake was formed by the mixtures of surface river water and deep-source CaCl2 water. However, there is a lack of direct evidence from evaporation experiment and isotopic values of different water bodies and halite minerals to support this theory. Aiming to fill this gap, we will firstly collect both the deep-source CaCl2 water at the northern margin of Qarhan along a linear fault zone and river water at the southern Germu River, and complete isothermal evaporation experiment of mixed water with different proportion to obtain evaporation sequence, and then compare it with the mineral assemblages of halite minerals gain from drilling core. Secondly, we will collect lake water, river water, inter-crystal brines and CaCl2 water to determine their isotope ratio (δD, δ18O, 87Sr/86Sr and δ34S) and water chemistry feature. Thirdly, we will pick out the autogenic halite and gypsum minerals from drilling core and evaporation experiment, to determine 87Sr/86Sr and δ34S isotope values of these two minerals. Finally, we will compare and analyze all isotope values and water chemistry of water and halite minerals to discuss original source contact between different water and halite minerals. This project will offer important evidences to understand the chemical processing of forming halite and enriching K in liquid brines in Qarhan Salt Lake.

察尔汗盐湖沉积了厚层的盐类矿物和液体钾镁盐矿床。盐层主要沉积有石盐而相对贫石膏矿物，矿物组合简单；卤水富钾，有氯化物型和硫酸镁亚型两种水化学类型。察尔汗盐湖盐类和卤水沉积的成因研究，对于揭示内陆盆地成钾机理具有重要理论意义。以往研究表明，察尔汗盐湖是地表河水和深部来源的CaCl2型水混合掺杂形成的。对于其成盐理论，目前尚缺乏不同水体的混合蒸发实验和同位素证据。本项目拟以察尔汗盐湖北缘的CaCl2型水和南部河水为实验载体，完成不同比例混合蒸发实验，建立其析盐矿物序列并与钻孔盐层矿物组合对比；其次查明研究区河水、湖水、晶间卤水和CaCl2型水等不同水体的元素和同位素（氢、氧、锶、硫）地球化学特征。结合钻孔岩芯和蒸发实验析出盐类矿物（石盐和石膏）锶、硫同位素组成，提取盐类沉积的地球化学信息。多指标对比，探讨盐类沉积与不同水体的物源联系。本项目的实施，可为察尔汗盐湖成盐物源及富钾机理提供重要证据。

察尔汗盐湖是我国沉积最年轻（~50 ka）和最大液体钾镁盐矿床的干盐湖（张彭熹, 1987），其钾盐资源储量分别为5.40×108 t（张彭熹, 1987; 袁见齐等, 1995; 曹文虎和吴蝉, 2004），是我国最大的钾肥生产基地。察尔汗盐湖的形成原因一直存在着争论（Chen and Bowler, 1986; Lowenstein et al., 1989; Spencer et al., 1990; 张彭熹等, 1993; 朱允铸等, 1990, 1994; 袁见齐等, 1995; 刘兴起等, 2002）。例如：(1) Chen and Bowler (1985, 1986) 和陈克造等 (1990) 提出察尔汗盐湖是由柴达木古湖由西向东迁移浓缩形成的。这个解释是基于柴达木盆地残留的古湖岸沉积、地层和年代学对比、盆地中干盐湖或次级盆地的海拔高度（西高东低）、干盐湖盐类矿物的沉积学特征。(2)朱允铸等(1990, 1994)提出察尔汗盐湖是由新构造运动袭夺古湖水形成的。在东昆仑山在东昆仑山系存在着一系列的古湖，有的已发育成了盐湖。距今3万年左右的新构造运动使东昆仑山剧烈上升，导致那棱格勒河、格尔木河和香日德河等水系向南溯源侵蚀，使古湖水被袭夺并补给察尔汗地区，成为察尔汗盐湖盐类物质的一个重要来源。 (3)Lowenstein et al. (1989), Spencer et al. (1990)和张彭熹等(1993)提出察尔汗盐湖是由河水和CaCl2泉水混合形成的。同时，混合成矿理论得到了大量氯化物盐（钾石盐）沉积于达布逊湖的北岸（更接近CaCl2泉水）（袁见齐等, 1995）和混合河水Pitzer模拟结果的证实（刘兴起等, 2002）。直到目前，混合成矿理论能比较完整地解释矿物组合特征、不同水化学类型卤水和水体的空间分布，然而仍缺乏不同水体和盐类矿物锶同位素地球化学证据。基于河水、晶间卤水、泉水、石盐、石膏和贝壳的元素和锶同位素比值，结果表明不同水体和盐类沉积锶同位素比值从别勒滩到霍布逊区段呈现逐渐增加的趋势，证实了察尔汗盐湖混合成矿的理论框架。河水（低Sr和Sr同位素比值）和CaCl2泉水（高Sr和Sr同位素比值）两种水源基本控制着察尔汗盐湖水文地球化学，并分别表现出别勒滩、达布逊、察尔汗和霍布逊区段24:1, 5:1, 1:9 和1:2的混合比例。