云南思茅盆地上白垩统勐野井组自形黄铁矿成因及地质意义

摘要

云南思茅盆地位于特提斯成矿带东南段,上白垩统勐野井组是该区域一套广泛分布的含盐地层,该组内赋存有中国目前唯一的前第四纪固体钾盐矿床.对思茅盆地江城地区勐野井组石膏和黄铁矿样品,利用电子探针和质谱仪进行了主、微量元素和硫同位素分析,结果表明黄铁矿的n(S)/n(Fe)平均值为2.02,为硫富集型黄铁矿;黄铁矿的w(Co)/w(Ni)值为3.01 ～10.74,变化范围较大;黄铁矿的δ34SV-CDT值为-8.36‰～-3.36‰,平均值为-6.33‰,而石膏的δ34SVCDT值为6.6‰ ～ 10.7‰,平均值为9.48‰.通过对黄铁矿n(S)/n(Fe)、w(Co) /w(Ni)、硫同位素以及石膏硫同位素进行分析,认为勐野井组石膏层在成岩期后受到了热液蚀变的作用,热液蚀变导致石膏被还原形成S2-,S2-与热液带来的Fe结合形成了粒状黄铁矿.通过对思茅盆地江城地区勐野井组黄铁矿、石膏的成因机制研究得出,受印度板块俯冲活动控制,深部热液对该区上白垩统勐野井组内原生蒸发岩有明显改造作用并有多种金属矿物伴生.思茅盆地晚白垩世以来热液活动频繁,多期次的热液作用对盆地金属成矿有积极影响,对勐野井组钾盐有不利的改造作用,使得原生钾盐发生蚀变,普遍发生重结晶作用.%The Simao Basin is located in the southeastern part of the Tethyan metallogenic belt.The Mengyejing Formation containing evaporite successions in the Upper Cretaceous is widely distributed in this region.The Mengyejing Formation is currently the only pre-Quaternary solid potash deposit in China.Samples of gypsum and pyrite were collected from the Mengyejing Formation in Jiangcheng County in Simao Basin,and the analysis of major and trace elements and sulfur isotopes of these samples were performed by electron probe micro-analyzer (EPMA) and mass spectrometer.The results show that the S/Fe value of pyrite is 2.02,showing that it belongs to a sulfur-concentration type pyrite.The Co/Ni values range from 3.01 to 10.74,and the varying range is larger.The δ34Sv-cDrvalues of pyrite are in the range of-8.36‰ to-3.36‰ with an average of-6.33‰c,while the δ34Sv.cDT values of gypsum are in the range of 6.6‰ to 10.7‰ with an average of 9.48‰.The values of S/Fe,Co/Ni and sulfur isotope of pyrites and the values of sulfur isotope of gypsums suggest that the pyrite in the Mengyejing Formation in Jiangcheng County is formed in hydrothermal processes.The hydrothermal alteration causes the gypsum to be reduced to form S2-.S2-formed the granulated pyrite combined with Fe from the hydrothermal fluid.Through this study of the genetic mechanism of pyrite and gypsum of the Mengyejing Formation,it comes to the conclusion that the primary evaporites of the Mengyejing Formation in the Upper Cretaceous might be experienced obvious alterations by deep hydrothermal fluids accompanied by several kinds of metals,which is controlled by the Indian and Asian collision,and the hydrothermal activities in Simao Basin recrystallized the potash minerals of the Mengyejing Formation,but might have favorable contribution to the metal deposition.