思茅盆地下白垩统蒸发岩硫同位素地球化学特征及其钾盐成矿意义

关于思茅盆地下白垩统勐野井组蒸发岩主要物源为海水的认识争议很少,但是关于其成矿时代和成矿模式的认识还有争议,关于陆源淡水对蒸发岩物质成分的影响还缺乏探讨。本文主要通过分析盆地内L2井27件蒸发岩样品的化学成分和硫同位素地球化学特征,结合邻区已发表的硫同位素数据,探讨了蒸发岩的物质来源、陆源淡水对蒸发岩物质成分的影响、成盐时代以及可能的钾盐成矿模式。结果表明:(1)思茅盆地蒸发岩受陆源淡水和火山热液补给,其中陆源淡水补给使蒸发岩硫同位素明显低于同一地质时期的其他海相样品;(2)海水可能自现今盆地北西方向补给,一级周期上海水补给存在两次,二级周期上海水补给至少存在七次;(3)物源海水的时代为中侏罗世,沉积析盐的时代为早白垩世晚期,可能的钾盐成矿模式为中侏罗世海水侧向迁移成矿。这些结果对解释思茅盆地及邻区海相蒸发岩异常低的硫同位素值、高硫同位素值与中侏罗世海水相当以及钾盐成矿缺失"碳酸盐岩相和硫酸盐岩相"有重要的意义。

Sulfur Isotope Geochemistry of the Lower Cretaceous Evaporite and Its Significance for Potash Mineralization in the Simao Basin,Southwest China

Scientists believe that primary material sources of the Mengyejing potash deposit are seawater but there are still disputes about the metallogenic epoch and metallogenic model. In addition, the influence of terrestrial freshwater on the composition of evaporite is still lacking of discussion. In order to explore the material source of evaporite, the effect of freshwater on the evaporite composition, the age of material source, the metallogenic epoch, and the probable potassium metallogenic model, the authors analyzed composition and sulfur isotope of 27 evaporite samples from well L2 in the Simao basin, and also considered the published sulfur isotope data of evaporite from adjacent basins. Some conclusions have been reached: (1) Material sources of the evaporite in the Simao basin were recharged by freshwater and volcanic hydrothermal fluids except seawater. The data of sulfur isotope of the evaporite were significantly lower than that of other marine samples in the same geological epoch, it was caused by freshwater flowing into basin. (2) Seawater flowed northwestward into the Simao basin. There existed two times of seawater recharge in the first-order period and at least seven times in the second-order period. (3) The epoch of material source seawater is the Middle Jurassic, and the salting out age is the late Early Cretaceous. The probable potassium mineralization model was as follows: "Middle Jurassic seawater of lateral migration evaporated and crystallized in the late Early Cretaceous". These results are of great significance in explaining the abnormally low sulfur isotope values and high sulfur isotope values of marine evaporites in the Simao basin and its adjacent areas, which are comparable to those of Middle Jurassic seawater, and the absence of " carbonate facies and sulphate facies" in potassium mineralization.