Ore formation in mantle peridotites before and after emplacement: review and new perspective

Abstract

Generation of ore deposits in and around mantle peridotite (serpentinite) massifs is reviewed and summarized, and future studies are discussed as well. Podiform chromitites are essentially formed by magma–peridotite (mainly harzburgite) reaction and subsequent magma mixing at the upper mantle. The UHP chromitite is, however, possibly formed by deep recycling of the low-pressure one formed in the upper mantle. Seawater, which contains Cl, is potentially able to dissolve and transport Cr and precipitate chromian spinel. Chromitite formation in the mantle wedge by slab fluids is possible but has not been well examined. Olivine in peridotites is potentially an important source of some elements (iron and related transition metals), which are liberated upon its breakdown during serpentinization. They are sometimes precipitated in situ as minute grains of oxides, sulfides or metals in serpentinite, but sometimes transported via hydrothermal solutions responsible for serpentinization and concentrated to form ores. Magnetite veins in serpentinite, possibly as well as Co–Ni arsenides along the serpentinite–diorite boundaries of the Bou-Azzer ophiolite, provide a good example. Precipitation of Mn nodules and other ores from the seafloor is possibly related to circulated seawater through peridotite emplaced in the crust. This issue should be examined thoroughly in the future.