昌都—兰坪—呵叻成盐带白垩纪成盐成钾过程

摘要

以昌都—兰坪—思茅盆地及呵叻盆地含盐带盐类沉积为研究对象,在分析总结前人研究成果的基础上,通过系统的大地构造学、矿床学、地球化学等研究方法,总结和研究了该区含钾蒸发岩矿床的矿床特征,提出了新的成矿模式.研究表明,兰坪—思茅盆地和呵叻盆地的含钾蒸发岩矿床为"海源陆相"成因,即在陆相环境下与海水的变异有关.该矿床缺失正常海水蒸发浓缩而形成的硫酸盐沉积,同时,矿床中少量菱镁矿等自生矿物的出现无法用正常海水蒸发理论解释.矿床中少量溢晶石等矿物的出现预示着该矿床富钙.兰坪—思茅盆地内广泛发育了白垩纪蒸发岩系(勐野井组或云龙组),江城地区甚至沉积了小型钾盐矿床.同位素和微量元素的研究表明,该区蒸发岩矿床为海相成因,Rb和Br的含量变化指示海水从北部兰坪侵入到呵叻.昌都及兰坪地区的白垩纪石膏硫、锶同位素研究表明,该区大套的硫酸盐沉积为海相成因,物源研究成果揭示思茅盆地的物源可能来自东羌塘地区,预示着昌都地区的硫酸盐沉积跟兰坪—思茅盆地的蒸发岩沉积具有同源性.通过上述研究,提出了多级盆地海水迁移变质成盐成钾模式.%In the salt-bearing zones of Changdu, Lanping-Simao and Khorat basin, evaporites are widely distributed.In this paper, we focus on these deposits to develop a new metallogenic model and processes of the Cretaceous potassium-bearing evaporites, based on the previous research results, tectonics, ore deposits and geochemistry.The results showed that the Cretaceous potassium-bearing evaporites involving Changdu, Lanping-Simao and Khorat basins are the genesis of "Marine source continental facies".This indicated that the formation of those evaporites is related to the intrusion and recharge of seawater in the continental environment.These deposits are lacking in the corresponding sulfate minerals formed by evaporation of normal sea water.Meanwhile, the occurrence of a small amount of authigenic minerals such as magnetite in those deposits cannot be explained by normal sea water evaporation theory.The occurrence of a small amount of tachyhydrite in the deposits indicates that the brine is rich in Ca.The Cretaceous evaporites (Mengyejing Formation or Yunlong Formation) were widely distributed in Lanping-Simao basin, especially small potash deposits have even been deposited in the Jiangcheng area.Studies of isotopes and trace elements indicate that the evaporite deposits in this area are marine origin, and the changes of Rb and Br contents indicate that the seawater intruded into Khorat from the north of Lanping.The S and Sr isotopes in gypsum from Changdu and Lanping-Simao basins indicate that the sulfate deposits in this area are marine origin, studies on provenance reveal that the source of sulfates in Simao basin may come from the east of Qiangtang area, indicating that the sulfates in Changdu area have the same origin with evaporite in Lanping-Simao basin.A new metallogenic model of seawater migration and metamorphism in multistage basins is developed.