The role of komatiitic and picritic magmatism and S-saturation in the formation of ore deposits

Abstract

The introduction of high temperature, high magnesium komatiitic and picritic magmas into the Earth's upper crust has given rise either directly or indirectly to many of the world's major ore deposits. There is a direct association between komatiitic magmatism and Kambalda-style NiCuPGE deposits. The link between magma type and mineralization is not as clear cut in the case of the Noril'sk CuNiPGE deposits, but nevertheless there is an association between picritic magmatism and ore deposit development. Komatiitic and/or picritic magmatism may also have contributed to the formation of many types of hydrothermal ore deposits. Recent Nd isotope data generated by Kent et al. (1994) provide strong evidence for the hypothesis proposed by Keays in 1983 that the Au in Archaean Au deposits was derived either directly or indirectly from komatiites through the action of metamorphic fluids. The importance of komatiitic and picritic magmas in ore forming processes arises from the fact that they are among the few mafic/ultramafic magma types that are S-undersaturated at the time of magma formation. S-saturation of magmas leads to marked depletion in the chalcophile metals. For example, Czamanske and Moore (1977) estimated that a third of the original Cu and Ni contents of MORB were lost from the magmas due to extraction into magmatic sulphides. The depletion in Au and the PGE is much greater due to their much larger partition coefficients (Keays and Scott, 1976). Komatiitic and picritic magmas are S-undersaturated because they are high temperature magmas produced by large degrees of partial melting of upper mantle sourc regions that were already depleted in S through earlier partial melting events. Most mafic magmas, produced by less than 25% partial melting, are S-saturated and hence depleted in the ore forming chalcophile elements. In contrast, komatiitic and picritic magmas have their full complement of chalcophile metals. When they do become S-saturated, they form sulphides that are strongly enriched in Ni, Cu, Au, PGE and other strongly, chalcophile metals; these may accumulate to form ore deposits directly (e.g. magmatic NiCuPGE sulphides) or be dispersed in komatiitic/picritic rocks or their fractionation products making these source rocks for hydrothermal Au or VMS deposits.