Major types and occurrences of platinum-group minerals in the Jinchuan Ni-Cu-(PGE) sulfide deposit: Insights for PGE enrichment during hydrothermal alteration

Abstract

金川铜镍硫化物矿床是我国最主要的铂族元素（PGE）资源产地，其矿石受热液蚀变作用影响明显，并产出多种铂族矿物（PGM）。岩浆演化和热液蚀变过程中PGE的迁移富集机制和PGM的成因，一直是研究PGE地球化学行为非常关注的问题。本文对金川铜镍硫化物矿床中PGM的研究发现，其主要类型包括含PGE的硫砷化物（硫砷铱矿）和砷化物（砷铂矿），Pd的铋化物、碲化物和硒化物，以及少量其他铂族矿物。其中，硫砷铱矿可包裹于各种贱金属硫化物（镍黄铁矿、磁黄铁矿和黄铜矿）中，表明硫砷铱矿可能结晶于早期的含As硫化物熔体，随后被包裹于硫化物熔体冷凝分异产生的单硫化物固溶体（MSS）和中间硫化物固溶体（ISS）中。硫化物熔体中的As可能主要通过地壳混染作用加入幔源岩浆。大量铋钯矿（PdBi）呈微细乳滴状包裹于黄铜矿中，为晚期ISS冷凝形成黄铜矿过程中出溶的产物。少量铋钯矿（PdBi₂）呈不规则状充填于矿物裂隙，与次生磁铁矿脉紧密共生，并随矿石的蚀变程度增加，铋钯矿的化学成分由PdBi逐渐向PdBi₂转变，表明这部分铋钯矿为后期热液蚀变产物。铋碲钯矿和钯的硒化物则主要产出于镍黄铁矿裂隙且与次生磁铁矿紧密共生，指示明显的热液成因。钯的硒化物的出现表明，岩浆期后酸性、高盐度、高氧逸度的富Cl^-流体对金川铜镍硫化物矿床中Pd的迁移和富集起到了关键控制作用。;The Jinchuan Ni-Cu-(PGE) sulfide deposit hosts the major resource of platinum group elements (PGE) in China. Most Ni-Cu sulfide ores of the deposit have been altered by hydrothermal fluids and contain diverse types of platinum group minerals (PGM). The formation of PGM and enrichment of PGE are likely controlled by the partitioning behaviors of PGE during magmatic processes and late-stage hydrothermal overprints. In this study, main PGM types discovered in the ores include PGE-bearing arsenide (irarsite and sperrylite) and Pd-bearing bismuthide, telluride and selenide, as well as a few other types of PGM. Irarsite (IrAsS) is enclosed within pentlandite, pyrrhotite and chalcopyrite, indicating that irarsite may have crystallized earlier from the As-rich sulfide melt. During the fractionation of sulfide melts, the irarsite can be trapped in the initial monosulfide solid solution (MSS) or residual intermediate solid solution (ISS). Arsenic was likely incorporated into the mantle-derived mafic magmas by crustal contamination and then concentrated in sulfide melts. Most of the Pd bismuthide (PdBi) grains enclosed within chalcopyrite show obvious emulsion texture, indicating that they were exsolved from the ISS during subsolidus cooling. Some irregular Pd bismuthide (PdBi₂) grains are associated with secondary magnetite veins along micro-fractures in the ores. The Pd bismuthides have chemical compositions varying from PdBi to PdBi₂ with increasing degrees of hydrothermal alteration, indicating that they were likely produced by late hydrothermal alteration. Michenerite (PdBiTe) and Pd selenides grains mainly occur in the fractures throughout pentlandite and are associated with secondary magnetite, indicating a hydrothermal origin. Abundant Pd selenides in the ores indicates that the fluids are of acidic, saline and highly oxidized, which may play a key role in the PGE enrichment of the altered ores in the Jinchuan Ni-Cu sulfide deposit.