夏日哈木镍钴硫化物矿床镍钴赋存状态及其地质意义

东昆仑造山带夏日哈木超大型镍钴硫化物矿床伴生钴资源, 对其钴富集机制的认识离不开矿物中钴含量及其赋存状态方面的制约。本文采用场发射-扫描电子显微镜-能谱分析和电子探针对夏日哈木Ⅰ号镁铁-超镁铁质岩体中不同类型的矿物进行了成矿元素含量测定及表征, 在矿物层面探讨了钴含量变化和赋存状态。确认钴镍的赋存状态(含量)与矿物类型、岩浆结晶阶段(演化时间)有关: 硫(砷)化物的钴含量高于硅酸盐造岩矿物, 辉砷钴矿等砷化物钴独立矿物具有最高的钴含量。镁铁质岩浆硫不饱和时, 早期结晶橄榄石中镍含量较高, 中期结晶辉石岩的辉石晶体结构中的钴发生晶体化学分散; 岩浆硫饱和-硫化物熔离过程中钴和镍优先进入硫化物熔体而富集, 单硫化物固溶体(MSS)早期结晶的磁黄铁矿中钴含量大于镍含量, 钴与镍含量正相关; MSS中期结晶的镍黄铁矿优先富集钴, 钴与镍含量负相关, 钴可能与镍类质同象替换进入镍黄铁矿而优先富集; MSS早期钴与砷形成辉砷钴矿等钴独立矿物, 钴富集程度高、但总量有限。最后, 本文认为硫化物熔离是夏日哈木矿床钴富集的主要机制。

The Ni-Co occurrence modes and geological significance of the Xiarihamu Ni-Co sulfide deposit,western China

The Xiarihamu giant Ni-Co sulfide deposit in the East Kunlun orogenic belt, western China is a typical world-class magmatic deposit with economic cobalt resources in an orogenic belt setting. The cobalt enrichment mechanism needs to be constrainted from its concentration and occurrence in minerals. In this study, metallogenic elements (Co and Ni, etc.) in different types of minerals of different lithofacies in the No.Ⅰ mafic-ultramafic intrusion of Xiarihamu deposit have been determined and characterized by field emission scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and electron microprobe analysis (EMPA). The content variation and occurrence modes of cobalt and nickel are discussed at a mineral level. The results show that the occurrence (contents) of cobalt is related to the mineral type, crystallization stage (consolidation time) of mafic magma. The cobalt contents of sulphide (including arsenide) minerals are higher than that of silicate rock-forming minerals, among which the cobaltite has the highest cobalt content. The nickel is enriched in olivine crystallized early of mafic magma under sulfur unsaturation, while the cobalt is enriched in pyroxene in the crystallization stage of pyroxenite. Cobalt preferentially enters the monosulfide solid solution (MSS) together with Ni during the process of sulfide saturation and segregation. The content of cobalt is higher than that of nickel in the pyrrhotite during early crystalline of MSS, and they are positively correlated with each other. Furthermore, the cobalt is preferentially enriched in pentlandite in the middle stage of MSS crystalline, and its content is negatively correlated with nickel, since the cobalt could be replaced by isomorphic substitution with nickel into pentlandite. In the early stage, cobalt and arsenic can form independent cobalt minerals (e.g., cobaltite etc.), which has a high degree of cobalt enrichment but with a limited total amount. In conclusion, sulfide saturation and segregation were the main mechanism of cobalt enrichment in the Xiarihamu deposit.