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脚地铬铁矿的岩石学研究，建立了一套完整的铬铁矿成因模式。该铬铁矿呈透镜状，形成于晚侵入橄榄岩与晚侵入辉长岩块体的长英质部分熔体之间。虽然尖晶石呈足状，但其尖晶石呈完全自面体，质地细腻，基质为斜长石。尖晶石富含铬和全铁。这些特征与层状铬铁矿相似。总之，该铬铁矿具有介于足状和层状铬铁矿之间的中间性质。它是由两种岩浆混合形成的，一种是来自晚侵入岩体的玄武岩岩浆，另一种是来自辉长岩部分熔融的长英质岩浆。阿曼上部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岩浆混合形成的，即玄武岩岩浆和岩浆库壁部分熔融形成的硅质岩浆（Irvine（1975）首次提出）。结合Arai和Yurimoto（1994）对橄榄岩中脚状铬铁矿成因的解释，玄武岩岩浆和硅质岩浆两种岩浆的混合作用是形成任何类型铬铁矿不可或缺的因素。这一机制有利于通过玄武岩的PGE和硅熔体的S的供应来沉淀原生PGM。

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.