新疆北山坡一镍矿床中钴的赋存状态、地球化学特征及其对钴成矿的启示

Occurrence and geochemical characteristics of cobalt of the Poyi nickel deposit in the Beishan region,Xinjiang:Insights into cobalt mineralization

钴作为当今社会必需的一种关键矿产资源,岩浆铜镍矿床是我国钴资源供应的重要矿床种类之一,该类矿床中钴元素通常赋存于镁铁质硅酸盐矿物、硫化物、类硫化物和铁氧化物中,然而镁铁质硅酸盐和铁氧化物中的钴难以分选利用,因此钴赋存状态的厘定是铜镍矿床中钴资源评价和利用的重要前提。新疆东天山-北山地区发育大量晚古生代、普遍伴生钴的岩浆铜镍矿床,其中位于北山地区的坡一镍矿床以赋含高镍硫化物为特色,是研究岩浆铜镍矿床中钴元素的赋存状态、分布特征以及钴与镍相互关系的理想对象。坡一镍矿的含矿岩相主要为单辉橄榄岩、纯橄岩和橄长岩,以稀疏浸染状矿化为主,钴主要以类质同象形式赋存于镍黄铁矿,其次为铬尖晶石、橄榄石和磁黄铁矿,少见独立钴矿物。矿物微区原位分析结果显示原生矿物中钴含量由高到低依次为镍黄铁矿(~8404×10^(-6))>>铬尖晶石(~475×10^(-6))>磁黄铁矿(高Ni含量)(~299×10^(-6))>橄榄石(~137×10^(-6))>黄铜矿(~13.0×10^(-6))>磁黄铁矿(低Ni含量)(~8.35×10^(-6))。坡一岩体的蚀变橄榄岩中发育三种产状的次生磁铁矿:蚀变橄榄石裂理中细脉状磁铁矿、铬尖晶石边部环状磁铁矿边、硫化物矿边部和裂隙中次生磁铁矿边。元素面扫描结果显示次生磁铁矿中Co含量明显高于原生硫化物、氧化物和硅酸盐矿物,且显示富Co贫Ni特征。坡一矿床中硫化物和硅酸盐矿物表现出明显的Co-Ni正相关,而铬尖晶石中Co和Ni显示负相关,表明Co^(2+)主要以类质同象形式替代Fe^(2+)进入硫化物和硅酸盐矿物晶格,以替代Fe^(2+)、Ni^(2+)的形式进入铬尖晶石。坡一矿床中Ni和Co的共生行为,主要归因于岩浆演化过程硫化物熔离和硅酸盐矿物分离结晶过程中相近的配分行为,而Ni和Co的离散特征主要受控于铬尖晶石的分离结晶和R值(岩浆和硫化物质量比)的变化、热液蚀变过程中次生磁铁矿Co富集作用。由于铬尖晶石与硅酸盐矿物中的钴难以分选利用,北山地区的钴矿资源勘查应关注贫铬尖晶石、橄榄石镍亏损的富硫化物超镁铁质岩体。

As a critical mineral,cobalt has a wide range of applications in battery manufacturing,aerospace and medical fields in today’s world.Magmatic Ni-Cu sulfide deposit is one of the most important cobalt sources in China.Cobalt in Ni-Cu deposits can be contained in various minerals such as mafic silicate minerals,sulfides,sulfide-like minerals and iron oxides.However,cobalt in mafic silicates and iron oxides is difficult to separate.Therefore,the determination of cobalt occurrence is important for evaluating and utilizing cobalt resources in Ni-Cu deposits.A large number of Late Paleozoic magmatic Ni-Cu sulfide deposits are developed in the East Tianshan-Beishan region of Xinjiang,in which cobalt commonly exists as an associated product.The Poyi magmatic nickel deposit is located in the Beishan region of the southern Central Asian Orogenic Belt,and is characterized by high-Ni tenor sulfides,making it is an ideal deposit to investigate the occurrence and distribution of cobalt in magmatic sulfide deposits,and the relationship between cobalt and nickel.The Poyi deposit is hosted in a small ultramafic body(mainly wehrlite and dunite)which contains disseminated sulfide mineralization.Cobalt mainly occurs in pentlandite in isomorphic state,followed by chrome spinel,olivine and pyrrhotite.Cobalt mineral is rare in this deposit.The LA-ICP-MS analyses results show that the sequence of concentrations of cobalt in the primary minerals is pentlandite(~8404×10^(-6))>>Cr-spinel(~475×10^(-6))>pyrrhotite(high Ni)(~299×10^(-6))>olivine(~137×10^(-6))>chalcopyrite(~13.0×10^(-6))>pyrrhotite(low Ni)(~8.35×10^(-6)).Three types of secondary magnetite are recognized in the altered ultramafic rocks:veinlet magnetite in the altered olivine fracture,ring-shaped magnetite at the edge of the Cr-spinel,and irregular magnetite at the edge of sulfides.Elemental scanning results show that the secondary magnetite is rich in Co and depleted in Ni,and its Co concentration is much higher than those of primary sulfide,oxide and silicate minerals.The sulfide and silicate minerals show positive Ni-Co correlations,while Co and Ni in the Cr-spinel show a negative correlation,indicating that Co^(2+)enters the sulfide and silicate mineral lattice mainly by replacing Fe^(2+),and enters the Cr-spinel by replacing Fe^(2+)and Ni^(2+).The consistent behavior of Ni and Co in the Poyi deposit is mainly attributed to the similar partitioning behavior during sulfide segregation and crystallization of silicates,while the discrete characteristics of Ni and Co are mainly controlled by fractional crystallization of spinel and variations of R factor(the ratio of silicate magma to sulfide melt),and Co enrichment in secondary magnetite during hydrothermal alternation.Since cobalt in spinel oxides and silicate minerals is difficult to separate and utilize,the selection of cobalt ore resources in the Beishan region should focus on sulfide-rich intrusions with low proportion of Cr-spinel and low Ni olivine.