The Lithospheric Mantle Plays No Active Role in the Formation of Orthomagmatic Ore Deposits

Abstract

The hypothesis that the metals in certain orthomagmatic ore deposits come from a source in the subcontinental lithospheric mantle is evaluated in this paper. According to this hypothesis, parts of the mantle beneath the continents are metasomatically enriched in metals like Ni, Cu, and the platinum group elements (PGE). It is proposed that under some circumstances, these metals are transported into the crust where they become concentrated in orebodies. An examination of the compositions of xenoliths from the lithospheric mantle reveals little evidence, however, of components that could represent the source of metal-enriched magmas. In addition, the mechanism whereby metals are brought from the source to the surface is very unclear. The lithosphere is the coldest part of the mantle and it only melts under special circumstances. The normal product is a low-degree melt, an alkaline, Si-undersaturated magma of the type that only rarely contains ore deposits. Major magmatic orebodies normally form from high-volume, high-flux magmas that are produced by high-degree melting in deeper, hotter parts of the mantle—in the asthenosphere or a mantle plume. For melting to occur in the lithosphere, rather than in the hotter parts of the mantle, the melting point of the source must be drastically reduced by the presence of volatiles. On the other hand, there is ample evidence that the host magmas of ore deposits were abnormally rich in water or CO 2 as would have been the case if they came from a volatile-rich metasomatized source. Magmas from sublithospheric sources could have interacted with the lithospheric mantle as they ascended toward the surface and they could have picked up some metals through this interaction. This process could have contributed to the formation of some ores, a notable example being the PGE deposits in Bushveld Complex. There is ample geological and geochemical evidence, however, that the majority of magmatic deposits form when magmas from sublithosphere sources assimilate material from the continental crust and that the latter process is instrumental in the formation of the deposits.