Geodynamic Controls on Magmatic Ni-Cu-Platinum Group Element Sulfide Mineral Systems Through Time

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Abstract Most models for the formation of magmatic Ni-Cu-platinum group element (PGE) sulfide deposits invoke high-temperature events (e.g., plumes) to sufficiently melt enough peridotite mantle to produce MgO- and Ni-Cu-PGE–rich magmas, and the deposits are thus intrinsically linked to the emplacement of large igneous provinces (LIPs) in mostly extensional tectonic regimes. However, many Ni-Cu deposits are hosted by intrusions unrelated to LIP events, having formed during compressional orogenic events. We present a new conceptual framework for global magmatic sulfide deposits based on chalcophile element geochemistry, which reflects distinct combinations of mantle source composition (anhydrous peridotite vs. hydrous peridotite/pyroxenite) and the temperature and/or volume of melting. Low volumes of melting of hydrous mantle at low temperatures (<1,200°C) produce alkalic ultramafic-hosted, Ni-poor, Cu-Au–dominant, low-temperature Cu (LT-Cu) deposits, displaying relative enrichment in Cu, Au, and Te over the other chalcophile elements. Greater volumes of melting of a hydrous source produce mafic-ultramafic-hosted low-temperature Ni (LT-Ni) deposits defined by distinctly PGE-poor chemistries. High-temperature (>1,200°C) melting of more anhydrous mantle peridotite forms both mafic-ultramafic-hosted high-temperature Ni (HT-Ni) deposits enriched in all chalcophile elements and layered mafic intrusion-hosted high-temperature PGE (HT-PGE) deposits, which display relative PGE enrichment. Very-high-temperature melting of peridotite mantle forms komatiite-hosted very-high-temperature Ni (VHT-Ni) deposits with chalcophile element profiles close to primitive mantle ratios. Temporally and spatially, all VHT and HT deposits are inextricably linked to LIPs throughout Earth’s history. The major HT-PGE deposits are all >2.0 Ga and hosted within cratons, the VHT-Ni deposits are all >1.8 Ga and hosted in komatiites, and the HT-Ni deposits are distributed throughout geologic time and are mostly located at cratonic margins. Conversely, LT deposits are all younger than 2.0 Ga, are almost never associated with LIPs, and have a more varied spatial distribution, but are more commonly located within orogenic belts. These secular changes in the nature of magmatic sulfide systems through time are a function of (1) a cooling mantle from the Archean to the present, (2) varying compositions of mantle as a product of increased mantle metasomatic processes through time, (3) the mineralogical deportment of chalcophile metals in mantle rocks, and (4) the variety of thermal triggers to melt such assemblages. We present a revised mineral system framework for the source and geodynamic controls on magmatic sulfides that takes into account all these criteria and allows enhanced targeting capability and prediction of the metal fertility of potential deposits.

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