云南哀牢山金矿带墨江金镍矿床铂族元素(PGE)地球化学及其对矿床成因的制约

Platinum group elements (PGE) geochemistry of Mojiang Au-Ni deposit and its constraint on ore genesis

文章系统地测定了墨江金镍矿床中的金镍矿和镍矿矿石中的PGE含量,结果均较低,∑PGE为(2.58～109.66)×10-9,与超基性岩的∑PGE〔(14.58～50.48)×10-9〕相差不大,且矿石与超基性围岩PGE+Au的球粒陨石标准化模式基本一致,均为Pt和Ir相对亏损和Ru及Rh相对富集的M型,显示墨江金镍矿PGE来源较为一致,主要来自超基性岩体;墨江金镍矿Pd/Ir比值为0.18～10.0,远低于典型热液型镍矿的Pd/Ir值(>100),而与岩浆型镍矿相近,说明其中的镍主要为岩浆成因,后期热液改造并不是Ni成矿的主导因素,因此墨江应为早期岩浆型Ni矿和晚期热液型Au(Ni)矿叠加形成的复合矿床。墨江金镍矿的超基性围岩直接来自地幔,是由经历了基性岩浆抽提和交代作用形成的亏损地幔发生程度不同的部分熔融形成的,其原始岩浆中硫已达到饱和。

Mojiang is a lagre-size Au-Ni deposit occurring in the contact zone between ultramafic intrusions and metasedimentary rocks. It was previously considered to be a typical fault-controlled hydrothermal deposit. SystematicaUy sampling and analysis show that the PGE contents are quite low in both Au-Ni and Ni ores, and the ∑PGE is （2.58-109.66）×10^-9, quite similar to that of the ultramafic intrusions [（14.58 - 50.48） × 10^-9]. Besides, the ores and the ultramafic intrusions possess similar chondrite-normalized patterns, i.e., both of them belong to Pt and Ir depleted and Ru and Rh enriched type, suggesting that the Mojiang mine has an identical source of POE and most of them were derived from the ultramafic intrusions. The Pd/Ir ratios of the Mojiang ores are 0.18- 10.0, much lower than those of typical hydrothermal Ni deposits （ 〉 100） and similar to those of the typical magmatic type Ni deposits, implying that the Ni ores in the Mojiang mine mainly belong to the magmatic type, and that the post-magmatic hydrothermal alteration might have not played an important role in the Ni mineralization. It is thus concluded that the Mojiang deposit is a composite deposit composed mainly of early stage magmatic type Ni ores and late stage hydrothermal type Au-Ni ores. The ultramafic intrusions in the Mojiang mine were derived directly from the mantle, being products of partial melting of a depleted mantle in different degrees. The mantle in Mojiang area underwent extraction of marie magma and metasomatism, and the primary ultramafic magma was sulfur saturated.