Abstract

The importance of externally derived sulfur in the genesis of sulfide-rich, magmatic Ni-Cu-(platinum group element [PGE]) deposits remains a key, yet unresolved, issue. Calculations utilizing a variety of mafic magma types indicate that, in general, sulfide saturation by fractional crystallization occurs after Ni has been depleted due to olivine crystallization. Cu- and PGE-rich layers may form during relatively later stages of closed-system crystallization, but unless the collection of the cotectic proportion of immiscible sulfide is extremely efficient, the mass of sulfide is too small to produce an economic deposit. We show that there are numerous processes that may lead to early sulfide saturation in mafic/ultramafic magmas. Contamination of mantle-derived magmas by siliceous country rocks or their partial melts will lower the sulfur content needed to induce sulfide liquid saturation, typically by amounts ranging from 200 to 700 ppm. The mixing of magmas, particularly if the result is to lower the liquidus temperature of the mixed magma, may also lower the sulfur content needed to attain sulfide saturation by similar amounts. An increase in magma fO2 related to the addition of volatiles such as H2O and CO2 is less effective in decreasing the sulfur concentration needed to achieve sulfide liquid saturation. Contamination processes that lead to an increase in the activity of SiO2 in the melt, and hence may promote orthopyroxene rather than olivine crystallization, aid in generating relative Ni enrichment in remaining liquid as a result of the lower DNi (mineral – melt) value of orthopyroxene relative to olivine. Although contamination and magma mixing may produce early sulfide saturation without the addition of externally derived sulfur, Ni-rich sulfide deposits can form in such cases only from large-volume, open systems, where the efficiency of sulfide collection is high. No matter what the liquidus minerals may be, without the addition of country rock-derived sulfur, the mass of sulfide necessary to generate economic Ni-Cu-(PGE) concentrations requires efficient sulfide collection from large, but not necessarily unrealistic, volumes of magma. Small deposits (4–30 Mt of sulfide) may form from the collection of cotectic proportions of sulfide from less than 50 km3 of magma. Larger deposits such as those at Noril’sk could involve more than 200 km3 of magma; this volume of magma is not unreasonable, particularly in rift/plume-related settings. Despite such possibilities, sulfur isotope data clearly indicate that externally derived sulfur has been involved in the formation of many large deposits, and that collection of mantle-derived sulfide in sufficient quantities to produce orebodies is a rare process. We propose that magmatic Ni-Cu-(PGE) sulfide ore formation normally requires significant sulfide supersaturation, and that the addition of sulfur derived from xenoliths is the most viable mechanism for producing sulfide well above the cotectic proportion in mafic/ultramafic magmas.

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