鄂尔多斯盆地东南部奥陶系马家沟组碳酸盐岩中黄铁矿成因

Genesis of pyrite in Ordovician Majiagou Formation carbonate in southeastern Ordos Basin

鄂尔多斯盆地东南部广泛发育黄铁矿,通过对黄铁矿的矿物学特征、元素组成、硫同位素等的分析,讨论和揭示了该黄铁矿的成因。岩石矿物学研究结果显示黄铁矿具有多种赋存形式,呈零星或集合体充填于膏溶孔和溶蚀孔洞中,或与层状硬石膏相伴生;硫同位素地球化学分析结果显示,黄铁矿的δ^34S值介于10. 50‰~24. 00‰之间,平均值为17. 33‰。盆地东南部具备薄层硬石膏、高温驱动和充足的烃类气体等热化学硫酸盐还原作用发生的条件,结合其产状证据认为黄铁矿形成于热化学硫酸盐还原作用。黄铁矿出现的地层中Fe含量较高,介于3 387. 50×10^-6~23 112. 50×10^-6之间,平均值为13 233. 33×10^-6,为黄铁矿的形成提供了物质来源。盆地东南部多产低含H2S天然气,研究认为黄铁矿的形成是造成该地区H2S含量较低的主要原因。

Pyrite is well developed in southeastern Ordos Basin,and occurs in anhydrite dissolved pores and karst caves,or is accompanied by anhydrite layers as dispersed grains or assemblages. The δ^34 S values of the pyrite vary in the range of 10. 50‰~ 24. 00‰,17. 33‰ on average,indicating its thermochemical sulfate reduction( TSR)origin. The basic condition for TSR is thin layer of anhydrite,high temperature and abundant hydrocarbon,which is all qualified in southeastern Ordos Basin. In combination with its modes of occurrence,the well-developed pyrite accompanied with anhydrite dissolution is considered to be the product of TSR. The formation with the existence of pyrite has high content of Fe,ranging from 3387.50×10^-6 to 23112.50×10^-6,averagely 13233.33×10^-6,which provided material source for the formation of pyrite. However,the H2S concentration is low in the Ordovician carbonate,which was probably caused by the precipitation of pyrite.