吉林省大横路铜钴矿复杂的沉积-变质变形-热液作用演化过程及其对钴的赋存状态和富集成矿的制约

吉林省白山市大横路铜钴矿为国内少有的沉积-变质变形-热液等多种成因叠加改造的大型钴矿床, 是综合研究古元古代造山带钴复杂成矿作用过程的理想地区。目前, 有关该矿床中钴的赋存状态、富集迁移规律及复杂的成矿过程等方面的研究十分薄弱。本文采集矿床内典型钻孔样品, 通过精细的矿相结构观察、扫描电镜物相分析、X射线元素面扫描和矿物成分定量分析等综合研究, 发现大横路铜钴矿床经历了多期多阶段复杂的沉积-变质变形-热液作用, 钴主要以Co的独立矿物(硫镍钴矿和辉砷钴矿), 类质同象进入富Co黄铁矿和在磁黄铁矿中发育钴镍黄铁矿出溶体等形式存在。该矿床成矿作用分为以下5个阶段: 沉积阶段以黄铁矿-磁黄铁矿-黄铜矿呈极细粒包体零星分布为特点; 变质变形阶段以硫镍钴矿-磁黄铁矿-黄铜矿组合沿绢云母-黑云母-石英变质千枚理定向分布为特点, 变质程度达高绿片岩相; 早期热液作用主要沉淀粗大脉状磁黄铁矿及其钴镍黄铁矿出溶体; 中期热液作用以条带状自形富Co黄铁矿和硫镍钴矿为标志, 该阶段为Co富集成矿的主成矿期; 网脉状黄铜矿沉淀对应于最晚期热液作用, 同时发育极少量细粒辉砷钴矿。矿体的蚀变类型以硅化、碳酸盐化、钠长石化和绿泥石化为主, 次为钾长石化和绢云母化。其中, 硅化、铁白云石化和钠长石化蚀变稍早于金属硫化物形成, 为金属硫化物的沉淀与富集提供了有利条件; 辉砷钴矿的沉淀与钠长石的绿泥石化、钾长石化和绢云母化等蚀变作用关系密切。由此可见, 多期多阶段沉积-变质变形-热液事件对Co的迁移-富集起着关键的控制作用, 不同的变质变形-热液演化阶段均存在富Co硫化物的沉淀, Co的赋存形式也存在明显差异。富Co黄铁矿和硫镍钴矿为大横路铜钴矿Co最主要的赋存形式。综上所述, 在辽-吉古元古造山带形成演化过程中, 大横路大型铜钴矿钴的富集成矿经历了沉积-变质变形-多期热液叠加改造的复杂演变过程, 可称之为造山型铜钴矿床。原始的沉积作用为钴矿的预富集提供了重要的成矿条件, 古元古代造山过程及其伴随的区域变质变形事件对钴的初始活化-迁移起着关键的控制作用, 后期热液叠加改造事件主导了钴的迁移-超常富集成矿过程。

Complex evolution of the sedimentation,metamorphism-deformation and hydrothermal processes and their constraints on the occurrence,enrichment and mineralization of Co in the Dahenglu Cu-Co deposit,Jilin Province

The Dahenglu Cu-Co deposit in Baishan City, Jilin Province, is a large cobalt deposit in Northeast China. It is an ideal research area in investigating the multi-metallogenic processes within the Paleoproterozoic orogenic belt, whose genesis is controlled by multi-factors such as sedimentation, metamorphism and hydrotherm. At present, the occurrence, enrichment and migration of cobalt and the complex mineralizing processes of this deposit were poorly studied. In this study, representative drilling samples from the Dahenglu Cu-Co deposit were investigated in terms of detailed mineral texture observations, scanning electron microscope analysis, X-ray element mapping, and quantitative analysis of mineral compositions. It is revealed that the Dahenglu Cu-Co deposit underwent multiple stages of sedimentation, metamorphism and hydrothermal alterations. The cobalt in this deposit mainly exists in the form of independent minerals (i.e., siegenite, cobaltite and cobalt pentlandite), isomorphism in Co-enriched pyrite and exsolution of cobalt pentlandite in pyrrhotite. The pristine sedimentary stage is marked by the formation of very fine mineral inclusions of pyrite-pyrrhotite-chalcopyrite. The metamorphic stage is represented by the oriented siegenite-pyrrhotite-chalcopyrite aggregates that are distributed along the phyllitic foliation defined by high-greenschist-facies sericite-biotite-quartz assemblage. The precipitation of coarse-veined pyrrhotite and cobalt pentlandite exsolution within it defines the early-stage hydrothermal metasomatism. The subsequent hydrothermal stage is marked by the formation of banded euhedral Co-enriched pyrite and siegenite, which represents the main metallogenic epoch. The precipitation of chalcopyrite occurs during the late hydrothermal metasomatism, which is accompanied by the growth of fine-grained cobaltite. The alteration mainly includes silication, carbonatation, albitization, and chloritization, along with potassic feldspathization and sericitization. The silication, ankeritization, and albitization predate the metal sulfide hydrothermal. These alterations may not correlate to the Co mineralization, but would have provided the precipitate spaces for the metal sulfides. The precipitation of cobaltite is closely related to the chloritization, potassic feldspathization and sericitization. In conclusion, multiple metamorphic and hydrothermal events significantly controlled the migration and enrichment of the cobalt. The precipitations of Co-enriched sulfides with different occurrences developed throughout the different evolution stages of the metamorphism and hydrotherm. The Co-enriched pyrite and siegenite are the primary Co-occurrence states in the Dahenglu Cu-Co deposit. In summary, the Dahenglu Cu-Co large deposit in the Liao-Ji Paleoproterozoic orogenic tectonic regime underwent complex evolution processes of sedimentation-metamorphism-multiple hydrothermal overprinting that were related to the Co enrichment and mineralization. The Dahenglu Cu-Co deposit can be regarded as an orogenic Cu-Co deposit: the pristine sedimentation provided important ore-forming conditions for the Co mineralizaton; the Paleoproterozoic orogenic process and associated regional metamorphism and deformation controlled the original Co mobilization and migration; and the subsequent hydrotherm dominated the migration and ultra-enrichment of the Co in the Dahenglu Cu-Co deposit.