甘肃黑山铜镍硫化物含矿岩体的地质特征及橄榄石成因探讨

黑山铜镍硫化物含矿岩体位于中亚造山带南缘北山褶皱带东部,对该岩体的研究有助于正确理解造山带型铜镍硫化物的成矿作用。详细的野外基础地质特征表明黑山矿区仅出露单一的新元古代青白口系地层,无寒武纪地层出露。黑山含矿岩体由早期的方辉橄榄岩、二辉橄榄岩和橄榄辉长苏长岩,以及稍晚期的辉长岩脉组成。黑山岩体位于黑山背斜南翼,其初始产状应为一水平或近水平的岩床状岩体,因后期褶皱作用,岩体发生了倾斜形成现今的产状。黑山岩体中橄榄石Fo值为80.5~87.0,其Ni含量为1070×10^-6~3461×10^-6。借助"MELTS"软件,我们进一步厘定了黑山母岩浆为高镁玄武岩浆 (12.81% MgO, 10.84% FeO, 430×10^-6 Ni)。研究表明,黑山橄榄石的主要控制因素为:1)母岩浆的成分;2)后续新鲜岩浆的补充混合作用;3)硫化物熔离作用;4)晶间硅酸盐熔浆作用;5)橄榄石与硫化物之间发生的Fe-Ni交换。模拟计算表明,硫化物熔离与橄榄石结晶几乎同时发生,橄榄石与硫化物的最小质量比约为20:1。1号矿体及矿化体的橄榄石比4号矿体更富Mg和Ni,并且还呈现非常好的负相关关系,表明这些橄榄石与硫化物发生了不同程度的Fe-Ni交换,而与其发生Fe-Ni交换的硫化物更富Ni以及共生的岩浆更为原始。这很可能是因为黑山岩体形成于开放的岩浆通道系统,早期融离的硫化物与后续的新鲜的硫不饱和岩浆发生反应,使得该类硫化物含量减少但硫化物中的金属元素含量增高从而形成1号矿体及矿化体。

Geology and olivine geochemistry of the Heishan Ni-Cu-(PGE) sulfide deposit, Gansu, NW China

The Heishan intrusion hosting Ni-Cu-(PGE) sulfide deposit is located in the eastern of the Beishan Fold Belt at the southern margin of the Central Asian Orogenic Belt (CAOB). Studying the intrusion is helpful for learning the genesis of the orogen-type Ni-Cu-(PGE) sulfide deposit. The detailed field geology indicates that only Neoproterozoic strata but no Cambrian occur in the Heishan area. The dominant rocks of the intrusion are harzburgite and lherzolite. Olivine gabbronorite and gabbro dyke occur only along the southern margin of the intrusion. The Heishan intrusion was originally sill-like and was tilted to the south east on the south limb of an anticline formed during following collisional orogeny. The Fo of olivine from the Heishan samples is from 80.5 to 87.0 with 1070×10^-6~3461×10^-6 Ni. We further improve the Heishan calculated parent magma is a high-Mg basaltic magma (12.81% MgO, 10.84% FeO, 430×10^-6 Ni). The olivine data of Heishan indicate that they are controlled by 1) the composition of the parent magma; 2) mixed melt by latter ascending fresh magma; 3) the sulfide segregation; 4) the trapped liquid shift; 5) Fe-Ni exchange between olivines and sulfide liquids. Our calculation implies that sulfides began to segregate as the same time as olivine crystallization, and the minimum ratio of olivine:sulfide was 20:1. Olivines from No.1 ore body and mineralization bodies contain richer Mg and Ni than those from No.4 ore body, and show negative correlations between Ni and Fo. It indicates that not only the olivines from No. 1 ore body and mineralization bodies occur Fe-Ni exchange with sulfide liquids which contain more enriched Ni, but also the concomitant magmas with the sulfide liquids were more primitive. It is because the Heishan intrusion was formed in a magmaic plumbing system, the early sulfides reacted with the latter fresh magma and occurred upgrading and enrichment and formed the No.1 ore body and mineralization bodies.