甘肃北山黑山岩浆铜镍硫化物矿床橄榄石特征及成因意义

黑山铜镍硫化物矿床是近年在甘肃北山发现的大型岩浆铜镍硫化物矿床,含矿岩体主要由含矿橄榄岩相和南部边缘的角闪辉长岩相构成。研究发现含矿岩体中的橄榄石属贵橄榄石(Fo值为81.54~86.87),其w(Ni)介于(801.53~2 703.19)×10-6;利用橄榄辉长岩中最高Fo值和主量元素反演,表明原始岩浆属高镁玄武质岩浆,w(MgO)=11.65%,w(FeO)=10.12%;橄榄石分离结晶模拟计算结果表明,橄榄石结晶过程中伴随有0.12%~0.17%硫化物熔离,深部岩浆房中橄榄石分离结晶程度小于3%,橄榄石与硫化物最小质量比约14∶1;隙间硅酸盐熔浆和硫化物熔浆作用明显,是造成早期结晶橄榄石成分变化的重要原因。

Features and genetic significance of olivine from Heishan magmatic sulfide ore-bearing intrusion in Beishan area, Gansu Province

Heishan is one of the important sulfide-bearing mafic-ultramafic intrusions in Beishan area of Gansu Province. The intrusion is composed mainly of ore-bearing peridotite and amphibole gabbro distributed at the southern edge of the intrusion. Electron microprobe analyses of the olivine show that almost all olivine crystals are chrysolite with the forsterite content (Fo) ranging from 81.54 to 86.87 and Ni content varying from 801.53×10^-6 to 2 703.19×10^-6. The main components of the primitive magma are w(MgO)=11.65%, w(FeO)=10.12%, suggesting relatively high MgO basaltic magma. Based on modeling of olivine fractional crystallization and sulfide segregation, the authors hold that the sulfide segregation (about 0.12%~0.17%) and olivine fractional crystallization occurred almost at the same time during the magma evolution. The results also show that less than 3% fractional crystallization of olivine has been separated from magma and their minimum mass ratio is 14:1. In addition, the re-equilibration of the olivine crystals from the trapped silicate liquid or sulfide liquid seems to be an important factor for the variation of olivine composition.