滇西北羊拉铜矿床低δ34S黄铁矿的成因与地质意义

Genesis and geological significance of the low δ34S pyrite in Yangla Cu Deposit,Yunnan Province,SW China

滇西北羊拉铜矿床是'三江'成矿带金沙江成矿亚带内目前探明规模最大的铜矿床。前人分析该矿床硫化物的δ34S集中在-4.20‰^+2.60‰之间,塔式效应明显,指示成矿流体中的硫来自地幔或深部地壳。本文在成矿晚期的石英-方解石-硫化物脉型矿石中,获得一批黄铁矿的硫同位素组成,其δ34S明显低于前人的分析结果,在-40.38‰^-7.25‰之间,非塔式分布。笔者认为这种低δ34S黄铁矿的形成与成矿作用密切相关,为成矿晚期生物还原硫酸盐作用形成的富32S流体加入成矿流体,流体混合作用促使产物硫化物沉淀。本区低δ34S黄铁矿指示成矿作用晚期有有机流体加入,流体混合作用是成矿晚期石英-方解石-硫化物脉型矿石形成的重要机制。

The Yangla deposit in the northwestern Yunnan is the largest Cu deposit in the Jinshajiang subzone of the Sanjiang metallogenic belt at present. Previous works have shown that the δ34 S-values of sulfides are mainly concentrated in a range from-4. 20‰ to +2. 60‰,indicating the sulfur of the ore-forming fluid of the Yangla deposit could be derived from mantle or deep crust. However,this study has obtained a batch of δ34 S values,ranging from-40. 38‰ to-7. 25‰,of pyrites from the quartz-calcite-sulfide vein type ores formed in the late-ore stage. These δ34 S values are obviously lower than those of sulfides reported by previous studies. It is believed that these low-δ34 S pyrites are closely associated with the mineralization in the late-ore stage,and they were precipitated in the ore-forming fluid probably mixed with the32 S enriched fluid due to biological reduction of sulfate. The existence of low δ34 S pyrites shows that the organic-bearing fluid has been involved in the ore-forming fluid in the late-ore stage of the Yangla deposit. Therefore,we propose that the fluid mixing might be responsible for the formation of quartz-calcitesulfide vein type orebodies in the late-ore stage of the Yangla Cu deposit.