东川式铜矿的成矿作用及后期叠加改造:来自硫化物原位硫同位素的制约

Mineralization and subsequent overprint history of Dongchuan-type copper deposits:Constraints from in situ sulfur isotope of sulfides

沉积岩型层状铜矿床(SSC型)的成因争论聚焦在成矿作用主要集中在沉积成岩期并可能叠加有后期成矿作用,还是形成于成岩后盆地闭合过程和造山作用有关。产于扬子板块西缘的东川式铜矿是中国SSC型矿床的典型代表,这些矿床赋存在晚古元古界东川群岩石中,主要呈层状矿体产出,但也存在少量脉状矿体。文章选择东川铜矿田内因民、汤丹和滥泥坪3个典型矿床的层状和脉状矿体中硫化物(黄铁矿、黄铜矿、斑铜矿和辉铜矿)开展原位硫同位素组成的对比研究。实验结果表明,这些矿床的硫化物原位硫同位素组成分布范围较广:因民矿床层状硫化物的δ^(34)S值分布于4.7‰~22.1‰,汤丹和滥泥坪矿床层状硫化物的δ^(34)S值为-3.3‰~3.1‰;因民矿床脉状硫化物的δ^(34)S值分布于21.0‰~30.7‰,汤丹和滥泥坪矿床脉状硫化物的δ^(34)S值为-19.4‰~3.5‰。层状矿体和脉状矿体的硫化物硫同位素组成明显不同,表明形成2种产状矿体的硫来源不同。层状矿体较大的硫同位素组成差异指示了海相硫酸盐不同程度的热化学还原作用,表明初始成矿流体中的硫来源于循环盆地卤水中溶解的海相蒸发岩。脉状矿体的硫同位素组成则强烈受控于矿区的赋矿围岩,因民矿床硫化物中极高的硫同位素组成表明硫的来源为地层中的海相蒸发岩,而汤丹和滥泥坪矿床中亏损^(34)S的特征则表明硫的来源为富含生物还原硫的碳质板岩。结合野外地质关系和前人研究成果,文章认为层状矿体和脉状矿体是2期独立成矿事件的产物,层状矿体形成于成岩作用时期,脉状矿体形成于后期独立的局部构造热成矿事件,也即SSC型矿床的成矿作用主要发生在成岩期,但普遍遭受后期热液活动的叠加,并且在不同的成矿期中可能存在着多阶段的成矿作用。

The genesis of sedimentary rock-hosted stratiform copper deposits(SSC deposits)has long been highly controversial.There are two prevailing perspectives on genetic models,i.e.,①ore mineralization was formed during sedimentary diagenesis but may have undergone multiple post-mineralization overprint events;②the formation of SSC deposits was related to basin inversion and subsequent orogeny.The Dongchuan-type copper deposits are widespread on the western margin of the Yangtze Block,which are typical SSC deposits in China.These deposits consist mainly of bedding-parallel stratiform orebodies with minor discordant vein-type orebodies hosted in weak ly metamorphosed rocks of the late Paleoproterozoic Dongchuan Group.Representative SSC deposits in this region,i.e.,Yinmin,Tangdang,and Lanniping deposits,were selected for a comprehensive study on in situ sulfur isotopic compositions of various sulfides to better understand the ore genesis.The results reveal a large range of sulfur isotopic compositions of sulfide minerals from the three deposits.Specifically,the sulfides from stratiform ores from the Yinmin deposit have δ^(34)S values from 4.7‰to 22.1‰,whereas the δ^(34)S values of the sulfides in stratiform ores from the Tangdan and Lanniping deposits have a relatively narrow range from-3.3‰ to 3.1‰.On the other hand,the sulfides of vein-type ores from the Yinmin deposit are unusually enriched in ^(34)S,with δ^(34)S values varying from 21.0‰to 30.7‰,while the δ^(34)S values of sulfides in vein-type ores from the Tangdan and Lanniping deposits extend from-19.4‰to 3.5‰.The significant differences in sulfur isotopic compositions of sulfides for the two types of ores imply different sources of sulfur for them.Sulfur of stratiform ores from all deposits were sourced from variable degrees of thermochemical reduction of sulfate derived from the circulating basinal brines.However,sulfur of vein-type ores was controlled by host rocks in the ore district.The Yinmin deposit was sourced from marine evaporites within the strata,whereas the sources of the Tangdan and Lanniping deposits were provided by organic-enriched carbonaceous slates.In combination with field relationships and previous studies,the two types of ores are considered to represent two discrete mineralization events.The stratiform ores were formed during diagenetic or slightly late hydrothermal processes,whereas the vein-type ores were formed during much younger tectono-thermal events.The authors hence propose that mineralization for SSC deposits occurred mainly during the diagenesis of host rocks and might have been overprinted by subsequent hydrothermal events.