云南兰坪-思茅盆地和老挝钾盐矿床物质来源新认识

云南兰坪-思茅盆地勐野井钾盐矿与泰国—老挝的钾盐矿的成矿关系被认为可能具有同源性,尤其深部热液可能是其重要的物质来源之一。针对该观点氢氧同位素证据缺乏、且深部热液到底是哪种热液尚不清楚的问题,文中根据老挝钻孔ZK2893中石盐包裹体水的氢氧同位素分析结果:δ~(18)O为-2.3‰~9.5‰,平均值为2.9‰,δD的范围为-78‰~-150‰,平均值为-108.6‰,大部分小于-90‰;在δD-δ~(18)O关系图上,数据点均在交代热液范围内,因此推断,老挝钾盐成矿的深部热液为大气降水与围岩形成的交代热液,围岩提供了重要的成矿物质。此项分析还表明老挝的交代热液温度主要集中在150℃左右,即包裹体形成的温度可能也在150℃左右,云南兰坪—思茅石盐包裹体的捕获温度为145℃左右,最高达170℃,二者比较接近,这为两地钾盐矿同源的可能性提供了新的证据。

New Ideas on the Origins of Potash Deposits in Yunnan China and Laos

The Lanping-Simao basin in the southwestern China is located at the junction of the Eurasian and Indian Plates, and adjacent to a large potash deposit of Khorat Basin and Sakon Nakhon Basin in Thailand and Laos. Chinese scientists thought that the potash deposit in Lanping-Simao basin should be as large or important as that in Khorat Basin and Sakon Nakhon Basin. The relationship between Mengyejing potash in Lanping-Simao basin and potash deposits in the Sakon Nakhon Basin and Khorat Basins has long been discussed in many respects such as tectonic belt, depositional features, salt minerals, geochemical elements, and forming era. Many scientists thought that they had similar brine origins, and hydrothermal fluid was one important origin for the potash deposit. However, the ideas have received no supporting evidence. The type of hydrothermal fluid was not clear either. This study presents δ18O and δD of halite inclusions from a ZK2893 core in Laos to provide isotopic evidence for the idea. The values of δ18O are from -2.3 to 9.5‰ with an average of 2.9‰ , while those of δD are from -78 to -150‰ with an average of -108.6‰ . Most of δD was less than -90‰. The relationship between δ18O and δD shows that the hydrothermal fluid is metasomatic hydrothermal fluid which was originated from reactions between rainwater and surrounding rocks. Surrounding rocks are important origin for potash deposits. Most data were plotted near to the 150℃-curve in the δ18O-δD diagram. The possible forming temperature of halite inclusion in Laos could be about 150℃. On the other hand, the forming temperature of halite inclusion in Lanping-Simao Basin was about 145℃ with the highest value of 170℃. The similar temperatures provide new evidence for their similar brine origins of the two basins.