新疆北山地区两个含铜镍镁铁-超镁铁质岩体铂族元素地球化学研究

Platinum Group Elements Geochemistry of Two Mafic-Ultramafic Intrusions in the Beishan Block, Xinjiang, NW China

对北山地区坡一和罗东含铜镍的镁铁—超镁铁质岩体铂族元素研究表明,两个岩体的铂族元素(PGE)总量较低,PPGE较IPGE富集,原始地幔标准化模式呈正斜率,均较原始地幔亏损,具Ir和Rh的弱负异常。较低的Pd/Ir比值表明岩石主要受岩浆作用控制,后期热液作用影响不明显。两个岩体的原生岩浆均为MgO含量较高的PGE不亏损的拉斑玄武质岩浆,较高的Cu/Pd、Ti/Pd比值表明岩浆在演化过程中发生了硫化物的熔离。罗东岩体早期矿物相(橄榄石、铬铁矿)的分离结晶作用对岩浆中的硫达到饱和具有重要的贡献,而坡一岩体该作用对硫化物熔离的贡献不明显。坡一和罗东岩体的R值表明两岩体均具有达到中型Ni矿床的潜力。

PGE geochemistry of the Poyi and Luodong Ni-Cu sulphide-bearing mafic-ultramafic intrusions in the Beishan was analyzed in this study. The results show two intrusions have low total PGE contents, ranging from 2.15×10-9 to 18.66×10-9 in Poyi intrusion and 0.55×10-9 to 3.18×10-9 in Luodong intrusion, respectively, with PPGE higher than IPGE. The PGE primitive mantle-normalized patterns display a positive slop and low fractionation between IPGE and PPGE, and negative Ir and Rh anomalies. The PGE, Cu and Ni are depleted relative to the primitive mantle with low and narrow Pd/Ir ranges (3.47~7.35 and 9.73~20.67), indicating the magma contamination. The relationship between Ni/Cu and Pd/Ir indicates that the primary magma was high-magnesium tholeiitic. The Cu/Pd and Ti/Pd ratios of these rocks suggest that the primary magmas have undergone segragation of sulfide before their emplacement.The obviously positive correlation between Cr and Ni, Ir in the Luodong intrusion shows that fractionation crystallization of olivine and chromite made key contribution to sulfur saturation of the magma, with inconspicuous effect over sulfide segregation in the Poyi intrusion. Based on the PGE model calculations, calculated R values suggest that the two intrusions are of potential of medium-scale Ni deposit.