An integrated genefical model and exploration/evaluation system for chromittite and platinum-group element ores

通过对阿曼蛇绿岩中发现的上地壳足壳铬铁矿的岩石学研究，建立了一个完整的铬铁矿成因模式。其铬铁矿为透镜状，形成于晚期侵入橄榄岩与辉长岩部分熔融交界处，形成于晚期侵入岩块中。尽管有豆荚状的特征，但它的尖晶石是完美的正面体和细小的，并有斜长石的基质。尖晶石富铬，全铁。这些特征与层状铬铁矿相似。简而言之，该铬铁矿的性质介于豆荚状和层状铬铁矿之间。它是由两种岩浆混合而成的，一种是来自晚期侵入体的玄武岩，另一种是来自辉长岩部分熔融的长英质岩浆。阿曼的上地壳铬铁矿与层状铬铁矿的化学相似性可能意味着后者也是由岩浆、玄武岩岩浆和由欧文(1975)首次提出的岩浆储藏壁部分熔融形成的硅质岩浆混合而成的。结合Arai和Yurimoto(1994)对橄榄岩中豆荚状铬铁矿成因的解释，认为玄武岩和硅质两种岩浆的混合对铬铁矿的成因是不可缺少的。这种机制有利于从玄武岩中提供PGE，从硅质熔体中提供S，从而有利于原生PGM的沉淀。

An integrated model for chromitite genesis has been established through the petrological research of upper crustal podifarm chromitite that we found from the Oman ophiolite. Ihis chromitite is lenficular and was formed at the boundary between the late-intrusive peridotite and felsic partial meltfrom gabbroic blocks captured by the late intrusion. Despite the podiform character, its spinel is perfectly euhedral and fine and has a matrix of plagioclase. Spinel is rich in Cr and total Fe. These features are similar to those of the stratiform chromitite. In short, this chromitite has an intermediate character between the podiform and stratiform chromitites. It was formed by mixing of two kinds of magma, a basaltic one from the late intrusive body and a felsic one from the partial melt of gabbro. The chemical similarity between the upper crustal chromitite from Oman and the stratifrom chromitite may mean that the latter is also formed by mixing of magmas, a basaltic magma and a silicic one formed by partial melting of wall of the magma reservoir as first suggested by Irvine(1975). Combined with an interpretation of Arai and Yurimoto(1994) on the origin of podiform chromitite in peridotite, the mixing of two kinds of magma, basaltic and silicic ones, is indispensable to genesis of chromitite irrespective of type. This mechanism favors the precipitation of primary PGM by supplies of PGE from basalt and of S from a silicic melt.