布什维尔德杂岩体Platreef矿床与金川铜镍硫化矿床微量元素地球化学特征对比及其意义

A comparison of trace element geochemical characteristics between the Platreef deposit of Bushveld Complex and the Jinchuan Cu-Ni-PGE sulfide deposit and its significance

采用ICP-MS方法分析了布什维尔德杂岩体Platreef矿床含矿岩石的主量及微量元素含量,并与金川铜镍硫化矿床进行对比研究。Platreef矿床含矿岩石与金川二辉橄榄岩的∑REE接近,球粒陨石标准化后的稀土元素(REE)均表现为轻稀土元素(LREE)富集、重稀土元素(HREE)平坦的右倾型配分曲线,但Platreef含矿岩石总体表现为正Eu异常,金川二辉橄榄岩与矿石样品则大部分表现为较强的负Eu异常,二者的差异实质上反映了成岩矿物斜长石与斜方辉石比率细微的变动。Platreef矿床岩(矿)石富集和高度分异不相容元素,Nb、Ta亏损,显示地幔柱岩浆受到地壳混染后,产生与弧苦橄岩相似的微量元素地球化学特征。金川含矿岩石富集Fe和Ti以及强不相容亲石元素,显示铁质玄武岩微量元素分布特征,虽然也可能受到地壳物质混染,但总体表现出含矿岩体母岩浆则可能来自EMI型富集地幔源区。Platreef矿层原始地幔标准化后Pt、Pd、Ru富集,Ir、Rh相对亏损的PGE配分型式以及Cs、Rb、Th、U、LREE等不相容元素和亲铜元素Cu、Ni的富集,暗示布什维尔德杂岩体岩浆源区残留地幔橄榄岩遭受含高度不相容元素的富矿流体的渗透混染即地幔交代,导致幔源岩浆发生PGE分异。

Major and trace element geochemistry of rocks and ores from the Platreef deposit of the Bushveld Complex was systemically studied in this paper by using new ICP-MS analytic method for the purpose of making a comparison with geochemistry of the Jinchuan Cu-Ni-PGE sulfide deposit and probing into the genesis of this type of deposits so as to guide their prospecting work. It is concluded that ∑REE values of the Platreef rocks are close to values of lherzolite from the Jinchuan Cu-Ni-PGE sulfide deposit, with very similar P, EE patterns. Chondrite-normalized REE patterns of the ore-bearing rocks from both deposits assuming rightinclined fractionation, with enrichment of LREE and smoothness of HREE; nevertheless, Platreef rocks have a positive Eu anomaly, whereas Eu negative anomalies are evident for most lherzolite and ores from the Jinchuan deposit. This might be a reflection of insignificantly varying plagloclase/orthopyroxene ratios during the crystallisation of the rocks, it is evident that, relative to the Jinchuan deposit, the Platreef deposit has higher contents of strongly ffactionated incompatible elements but shows a trough-shaped profile with pronounced depletion of Nb and Ta, suggesting a model of crustal contamination during mantle plume magma emplacement, evolution and ore formation that resulted in the geochemical features of trace elements similar to arc picrite. Jinchuan ore-beating magma is enriched in Fe, Ti and highly incompatible lithophile elements, exhibiting geochemical features of Ferropicrites trace elements; and hence it is thought to have been derived from conrrespondingly enriched mantle （EMI） sources even if there probably also existed some crustal substance contamination during the rock-forming and ore-forming process. The primitive mantle-normalized PGE patterns of platreef rocks are arch-shaped with the enrichment ofPt, Pd and Ru relative to Os, Ir and Rh, and incompatible elements Cs, Rb, Th, U and LREE as web as chalcophile dements Cu and Ni, suggesting that the residual mantle peridotite from which the Bushveld Complex derived has undergone infiltration and contamination-mantle ruetasomatism by external mineral-enriched fluids containing highly incompatible elements, which may be responsible for the fractionation of PGE from mantale-derived magma.