金川铜镍硫化物矿床铂族元素地球化学差异及其演化意义

Distinction of platinum group elements geochemistry in Jinchuan Cu-Ni sulfide deposit and its implication for magmatic evolution

Ⅱ-2号矿体为金川铜镍硫化物矿床的第二大矿体,位于Ⅱ矿区30行以东。为了解Ⅱ-2号矿体母岩浆在演化过程中的特殊性,采用镍锍试金ICP-MS法分析金川矿床Ⅱ-2号矿体各类矿石的Cu、Ni、S及铂族元素(PGE)含量,研究其特征,并与Ⅱ-1号矿体矿石进行对比。结果表明:矿石ΣPGE含量变化较大(44.6×10-9~8526.7×10-9),Pd/Ir介于1.25~26.55之间,为典型的岩浆型成因。海绵陨铁状矿石和局部海绵陨铁状矿石为Ⅱ-2号矿体主要的矿石类型,其PGE配分模式极为相似,且为渐变过渡的接触关系,说明二者为同一期岩浆侵位的产物。富铜矿石与块状硫化物矿石、含细脉状矿石的配分模式有互补的特征,富铜矿石具有极高的ΣPGE(8526.7×10-9)和Cu/Ni(5.46),块状硫化物矿石和含细脉状矿石相对富集IPGE(ΣPPGE/ΣIPGE<1),这三者与海绵陨铁状及局部海绵陨铁状矿石的区别明显,为岩浆演化到后期的产物,而从相互的接触关系来看,块状硫化物矿先于富铜矿侵位。通过研究Ⅱ-2号矿体与Ⅱ-1号矿体海绵陨铁状矿石的铂族元素及Cu、Ni特征,发现二者母岩浆均为地幔中等熔融形成的高Mg O苦橄质玄武岩,具有相同的源区特征;由二者的铂族元素分异情况、Cu/Ni、Cu/Pd及Ir与Ru、Rh、Pt、Pd的相关性,研究二者的母岩浆在液态硫化物-硅酸盐体系和单硫化物固溶体-液态硫化物体系中的演化特征,认为Ⅱ-1号矿体和Ⅱ-2号矿体中的硫化物是先后从母岩浆中熔离,且分离后具有不同步的演化特征;通过Ⅱ-2号矿体的地球化学剖面及金川矿床的Cu、Ni品位纵剖面和XOY平面投影等值线图,证明这种先后熔离作用发生在深部岩浆房中,而后Ⅱ-2号矿体和Ⅱ-1号矿体的成矿岩浆分别进入两条相对独立的岩浆通道,最后由不同的岩浆通道中心侵位到最终的成矿空间,形成Ⅱ-2号矿体与Ⅱ-1号矿体。

The Ⅱ-2 orebody,which is located in the east of line Ⅱ30,is the second largest orebody of Jinchuan Cu-Ni(PGE) sulfide deposit.In order to find out the particularity of parental magma of Ⅱ-2 orebody during evolution,the contents of platinum group elements(PGE),Cu,Ni and S in Ⅱ-2 orebody were analyzed by using nickel sulphide fire assay technique ICP-MS.Its characteristics were researched and compared with that of Ⅱ-1 orebody.ΣPGE contents of the Ⅱ-2 orebody have large variation range(44.6×10-9-8526.7×10-9).The data of Pd/Ir,which ranges from 1.25 to 26.55,indicates that the Ⅱ-2 orebody is of magmatic origin.Sideronitic ore and partial sideronitic ore,which are the main ore types,have similar distribution patterns.Besides,sideronitic ore and partial sideronitic ore show gradual transition contact,which illustrates that the two ore types are products of the same emplacement period.Copper-rich ore,massive sulphide ore and vein interpenetration ore have complementary relations in the PGE distribution pattern diagram,and there is a clear distinction among these three ore types,sideronitic ore and partial sideronitic ore.The ΣPGE(8526.7×10-9) and Cu/Ni(5.46) of copper-rich ore are extremely high,and massive sulphide ore and vein interpenetration ore have the feature of relative accumulation of the IPGE(ΣPPGE/ΣIPGE<1).Therefore,copper-rich ore,massive sulphide ore and vein interpenetration ore are formed in the late stage of magmatic evolution,and the contact relationship shows that emplacement of massive sulphide ore precedes copper-rich ore.Based on comparison of sideronitic ores' PGE,Cu and Ni features between II-2 and II-1 orebodies,parental magmas of these two ore bodies belong to high-Mg O type topicritic basalt formed by moderate partial melting of mantle and have the same source characteristics.PGE distribution patterns,PGEs differentiation situation,Cu/Ni and Cu/Pd,and the relationship of Ir and Ru,Rh,Pt and Pd of sideronitic ores were used to study the parental magma of II-2 and II-1 orebodies in terms of sulfide liquid-silicate system and MSS-sulfide liquid system,it might be possible to deduce that sulfides of the two ore bodies are successively separated from the parental magma,with out-of-step evolution features.Based on the studies on genchemical section of II-2 orebody,the contour map of longitudinal section projection and XOY plane projection of Ni and Cu grade,the separation processes of the two orebodies sulfide occur in deep-seated magma chamber and intrude the pre-existing tensional rupture through different magma conduit systems,forming II-2 and II-1 orebodies.