罗布泊盐湖富钾卤水成因再探讨--碎屑层卤水蒸发实验分析

A further discussion on genesis of potassium_rich brine in Lop Nur:Evaporating experiments for brine in gypsum_bearing clastic strata

罗布泊盐湖钙芒硝岩孔隙中蕴藏有超大型规模的卤水钾矿,富钾卤水成因一直备受关注。罗北凹地从统一的罗布泊大湖区中分隔出来后,成盐过程中其湖水仍以南部大湖的补给为主,罗北凹地卤水化学演化与"大耳朵"湖水密切相关。"大耳朵"湖区含石膏碎屑层普遍储藏有卤水,应该是罗北凹地盐湖的"源卤水",钾离子(ρ(K^+)为3.12 g/L左右)已初步富集,平均矿化度为198.83 g/L。为了查明该卤水的化学演化趋势及析盐序列,笔者于2009年、2010年两次采集了大量卤水样品,分别进行室内等温蒸发和自然蒸发实验。蒸发实验结果表明:随着卤水浓缩首先析出(硬)石膏,随后析出大量石盐,最后出现少量钾石盐和光卤石,与EQL/EVP卤水蒸发模型模拟结果相似。将碎屑层卤水蒸发过程中化学组成变化与罗北凹地卤水进行对比,结果显示罗布泊古湖水蒸发至石膏沉积之后,在罗北凹地水化学组成明显发生变化,没有大量石盐沉积,而以钙芒硝沉积为主。推测应是受到深部"富钙水"的持续补给,而"大耳朵"湖起到"预备盆地"的作用,罗布泊古湖水经"大耳朵"湖蒸发浓缩后,钾离子得到初步富集,在流入罗北凹地后与深部"富钙"水混合,强烈蒸发浓缩,大量钙芒硝矿物析出,最后形成富钾卤水。

The ＂Great Ear＂ playa is located in the southern part of Lop Nur. The southern Great Lakes water was stillthe main supply to Luobei depression after the separation of Luobei depression from the unified Lop Nur region. Brine chemical evolution in Luobei depression is closely related to the ＂Great Ear＂. Brine in gypsum-bearing clastic strata in ＂Great Ear＂ is confined brine, and potassium （about 3.12 g/L） has been initially enriched witha salinity of 198.83 g/L. In 2009 and in 2010, the authors took a large amount of brines and carried out the isothermal evaporating and natural evaporating experiments in order to investigate the evolving tendency of the brine. The evaporating experimental results show that gypsum was first separated with the brine concentration, and then a large amount of halite and a small amount of sylvite and carnallite were separated finally. A compari- son of chemical composition during the clastic brine evaporation with the brine in Luobei shows that the composi- tion of ancient brine obviously changed after gypsum deposition. Huge amounts of glauberite minerals, instead of massive halite, were deposited. So it might be continuously recharged by the deep ＂calcium-rich water＂. There- fore, the ＂Great Ear＂ Lake acted as a ＂preparing basin＂. Potassium in the ancient lake water was preliminarily enriched through evaporation in the ＂Great Ear＂ Lake. Then the water might be mixed with the deep ＂calcium- rich＂ water in Luobei depression. Huge amounts of glauberite minerals were deposited through intense evapora- tion and finally the potassium-rich brine was formed.