内蒙古东升庙和炭窑口多金属硫化物矿床铁同位素地球化学特征及其成矿指示意义

摘要

本文报道了内蒙古狼山成矿带内两个最大的铅锌多金属硫化物矿床———东升庙矿床和炭窑口矿床中黄铁矿、黄铜矿单矿物的铁同位素研究结果。东升庙矿床绢云石墨片岩中不规则状黄铁矿的铁同位素组成δ56 Fe－IRMM值在＋0．04‰～＋1．11‰之间，呈现铁的重同位素富集，指示了海水中的铁以氧化态沉淀并在成岩期转化成黄铁矿的矿化过程。东升庙和炭窑口矿床富硫化物矿石中黄铁矿和黄铜矿的铁同位素组成δ56 Fe－IRMM 值的变化范围为－1．33‰～＋0．08‰，具有热液成矿特征，指示金属成矿物质来源于热液流体。另外，绢云石墨片岩中脉状黄铁矿的铁同位素组成δ56 Fe－IRMM 值的变化范围为－0．39‰～－0．04‰，处于矿石黄铁矿和围岩不规则状黄铁矿之间，指示脉状黄铁矿是热液矿化的产物，并在成矿过程中混入了围岩中早先形成的富集铁的重同位素的黄铁矿。绢云石墨片岩中广泛发育的不规则状黄铁矿与赋存在绢云石墨片岩中的富硫化物矿体具有完全不同的铁同位素组成，指示热液活动对不规则状黄铁矿没有明显成矿物质贡献，因此同沉积热液活动成矿的可能性不大。结合赋存在白云石大理岩中硫化物矿体的顶、底部常见硅化的白云石大理岩角砾，本文提出后生矿化是东升庙多金属硫化物矿体的主要成矿方式。另外，东升庙矿床和炭窑口矿床的矿石硫化物具有相似的铁同位素组成特征，指示两者的成矿物质来源具有相似性。%Here we report the iron isotopic compositions of pyrites and chalcopyrites from the Dongshengmiao and the Tanyaokou deposits in the Langshan area.Theδ56 Fe results of irregularly shaped pyrites from graphite-rich mica schists vary from + 0.04‰ to + 1.11‰,indicating partial oxidation of Fe2 + in an oxygen-limited environment and the conversion to pyrites during diagenesis.While the δ56 Fe results of pyrites and chalcopyrites from the ores vary from -1.33‰ to +0.08‰,indicating a hydrothermal origin. In addition,the δ56 Fe results of vein pyrites from graphite-rich mica schists vary from - 0.39‰ to-0.04‰,which lie between the values of irregularly shaped pyrites and ore pyrites,suggesting mixture of hydrothermal fluids and the early irregularly shaped pyrites. The totally different iron isotopic compositions between irregularly shaped pyrites from graphite-rich mica schists and ore sulfides in the Dongshengmiao deposit indicates that hydrothermal fluids didn’t contribute to irregularly shaped pyrite, therefore it is unlikely that synsedmentary hydrothermal activities account for the mineralization in the Dongshengmiao deposit.Combining with the common silicified dolomite marble fragments in the top and bottom part of dolomite marble-hosted sulfide orebody,we suggested that epigenetic mineralization is the main ore-forming form.Besides,the similiarity in iron isotopic characteristics of ore sulfides from the Dongshengmiao and the Tanyaokou deposits indicates the similarity of their metallogenic material sources.