James E. Mungall

The formation of ore deposits of the platinum-group elements (PGE) requires that their concentrations be raised about four orders of magnitude above typical continental crustal abundances. Such extreme enrichment relies principally on the extraction capacity of sulfide liquid, which sequesters the PGE from silicate magmas. Specific aspects of PGE ore formation are still highly controversial, however, including the role of hydrothermal fluids. The majority of the world's PGE reserves are held in a handful of deposits, most of which occur within the unique Bushveld Complex of South Africa.

The Proterozoic Expo Intrusive Suite comprises a series of mafic to ultramafic intrusions crosscutting the Povungnituk Group of the Cape Smith Fold Belt in New Quebec. The intrusions are mainly in the form of blade-shaped dikes that penetrate a sediment-rich horizon in the middle of the Beauparlant Formation and terminate downward against massive basalts of the lower Beauparlant Formation. Significant accumulations of magmatic sulfide occur at the basal terminations of the dikes. At stratigraphic levels above the Beauparlant Formation the intrusions appear as broad dikes or sills within the Nuvilik Formation, below the mineralized lava flows and subvolcanic intrusions of the Raglan Formation. The Expo Intrusive Suite and the mineralized bodies of the Raglan Formation are probably coeval and comagmatic with the overlying Chukotat Group. Post-emplacement folding has exposed the Expo Intrusive Suite over about 5 km of structural relief, revealing the basal sulfide concentrations where dike segments terminate on the flanks of anticlines. The parent magma as preserved in chilled margins and narrow dikes was a picrite containing ∼17 wt % MgO (i.e. komatiitic basalt) and slightly depleted in Th, U and Nb relative to middle and heavy rare earth elements. The compositions of ultramafic cumulate rocks within the intrusions are strongly enriched in Th, U and Nb relative to heavy rare earth elements, reflecting assimilation of the enclosing basalts and metasediments. Modeling of the assimilation process suggests that the picritic magma was capable of assimilating masses of basalt or sediment up to 50% of the original mass of magma. Assimilation of ∼10% of a mixture of basalt and sediment caused the magma to become sulfide-saturated, and was accompanied by the crystallization of masses of ultramafic cumulates approximately equal to the mass of rock assimilated. The presence of dikes whose chilled margins resemble uncontaminated primary magmas but that contain abundant cumulates recording wholesale assimilation of host-rocks indicates that the process of assimilation and fractional crystallization required to produce continental tholeiites from picritic parent magmas may not require the presence of long-lived magma chambers, but can occur during transport along dikes and reaction with wall-rocks.

Understanding of the geochemistry of the chalcophile elements [i.e., Os, Ir, Ru, Pt, Pd (platinum-group elements), and Au, Cu, Ni] has been informed for at least 20 years by the common assumption that when crust-forming partial melts are extracted from the upper mantle, sulfide liquid in the restite sequesters chalcophile elements until the extent of partial melting exceeds ≈25% and all of the sulfide has been dissolved in silicate melt [Hamlyn, P. R. & Keays, R. R. (1985) Geochim. Cosmochim. Acta 49, 1797–1811]. Here we document very high, unfractionated, chalcophile element concentrations in small-degree partial melts from the mantle that cannot be reconciled with the canonical residual sulfide assumption. We show that the observed high, unfractionated platinum-group element concentrations in small-degree partial melts can be attained if the melting takes place at moderately high oxygen fugacity, which will reduce the amount of sulfide due to the formation of sulfate and will also destabilize residual monosulfide solid solution by driving sulfide melts into the spinel-liquid divariant field. Magmas formed at high oxygen fugacity by small degrees of mantle melting can be important agents for the transfer of chalcophile elements from the upper mantle to the crust and may be progenitors of significant ore deposits of Pt, Pd, and Au.

Deformation and melting of the crust during the formation of large impact craters must have been important during the Earth's early evolution, but such processes remain poorly understood¹. The 1.8-billion-year-old Sudbury structure² in Ontario, Canada, is greater than 200 km in diameter and preserves a complete impact section, including shocked basement rocks, an impact melt sheet and fallback material^3,4. It has generally been thought that the most voluminous impact melts represent the average composition of the continental crust⁴, but here we show that the melt sheet now preserved as the Sudbury Igneous Complex is derived predominantly from the lower crust. We therefore infer that the hypervelocity impact caused a partial inversion of the compositional layering of the continental crust. Using geochemical data, including platinum-group-element abundances, we also show that the matrix of the overlying clast-laden Onaping Formation represents a mixture of the original surficial sedimentary strata, shock-melted lower crust and the impactor itself.

Mass transfer between sulfide and silicate liquids is often discussed by reference to a mass ratio between hypothetical liquid reservoirs at equilibrium, defined as R. In an open system not at equilibrium the mass of silicate melt from which a growing sulfide droplet appears to have scavenged chalcophile elements will depend on diffusivities D_i and D_FeS of the element and of the FeS component in the silicate melt, respectively, on the degree of supersaturation of the melt with FeS C^FeS_o, on the radius a of the droplet and on the velocity v with which the droplet is advected through the melt, so that . Chalcophile elements are expected to show inter-element variations in D spanning several orders of magnitude in some melts, leading to large inter-element variations in effective R in a given sulfide body. These variations in effective R will lead to spatial separations between zones of maximum enrichment of chalcophile elements produced during fractional segregation of sulfide in layered intrusions. Whereas it has proven impossible to model the generation of spatially offset stratiform horizons in the Australian Munni Munni layered intrusion using equilibrium partition coefficients, the present kinetic model reproduces the observed patterns with great fidelity.