罗布泊盐湖巨量钙芒硝沉积及其成钾效应分析

Sedimentation of glauberite and its effect on potash deposits formation in Lop Nur salt lake, Xinjiang,China

罗布泊盐湖位于新疆塔里木盆地东部,盐湖沉积的主要盐类矿物是钙芒硝,其资源量巨大,卤水钾矿赋存于钙芒硝岩晶间孔隙中,显然,钾盐的聚集与钙芒硝沉积有着密切的联系。文章通过对罗布泊盐湖的钙芒硝沉积特征研究,探讨了钾盐矿床的形成与钙芒硝的沉积关系。研究显示钙芒硝沉积时期,古气候具有“热”的特点,有利于钙芒硝沉积;盐湖物质来源有“富硫酸根”的地表水,也可能存在富钙水的补给。结晶试验显示,罗布泊现代卤水与含氯化钙水掺合后,蒸发析出石膏和水钙芒硝;同时,早期沉积的石膏被交代成为钙芒硝,这两种方式是钙芒硝形成的主要机制。罗布泊钙芒硝的成钾效应主要表现于3个方面:钙芒硝具有菱板状结构,在沉积和成岩过程中钙芒硝岩发育大量孔隙;钙芒硝孔隙储集了富钾卤水;大量钙芒硝的析出导致卤水中钾离子相对富集。研究认为,罗布泊盐湖巨量钙芒硝的沉积控制了钾盐的形成和分布,钙芒硝岩成储钾盐的认识在实践上推进了罗布泊盐湖的找钾工作。

The Lop Nur salt lake is located in the eastern part of Tarim basin, Xinjiang, China. From Mid-Pleis- tocene, the Lop Nur gradually evolved into a salt lake, and since 1995, brine-potash deposits have been discovered in this lake. The domination of glauberite in salt minerals constitutes the unique characteristics of this lake. The authors conducted a systematic research on the glauberite sedimentary feature and the relationship between glauberite sedimentation and potash-deposits formation. The results show that the paleoclimate was “hot” during the glauberite crystallization-sedimentation period, which was beneficial to the crystallization of glauberite, and that the lake was supplied with “sulfate-rich” surface water and probably “calcium-rich” hypergene brine. Crystallization experiments show that, if modern brine of the Lop Nur is mixed with “CaCl2” water, hydroglauberite and gypsum will crystallize from the mixture after evaporation. It is therefore inferred that if hydroglauberite loses water it will change into glauberite. Moreover, the rock-film study shows that gypsum often occurs as a remnant in the inner part of the glauberite crystal, or glauberite can replace gypsum in glauberite rocks, which also constitutes the mechanism of glauberite formation. A new chemical reaction of glauberite sedimentation is suggested in this paper. The effect of potash formation controlled by glauberite sedimentation covers three aspects： as glauberite had the rhombohedral-plate shape, lots of pores were developed in the glauberite rocks during sedimentation and diagenesis; pores of glauberite rocks hold brine; and glauberite crystallization in large amounts resulted in the accumulation of potassium in brine formed by the removal of Ca^2＋ and Na^＋ . It is hence considered that the crystallization and sedimentation of a considerable quantity of glauberite might have had an important effect on the formation and distribution of potash deposits in the Lop Nur. The understanding of the controlling role of glauberite in the formation of potash deposits can obviously promote the potash investigation in the Lop Nur salt lake.