SULFUR LIBERATION FROM COUNTRY ROCKS AND INCORPORATION IN MAFIC MAGMAS

Abstract

A key step in the formation of many magmatic Ni-Cu-PGE sulfide deposits is the addition of crustal sulfur to mafic or ultramafic magmas. Sulfur addition has been proposed to take place via two different types of processes: (1) production of sulfurous fluids within the thermal aureole around an intrusion accompanying breakdown of sulfide- or sulfate-bearing minerals during devolatilization, followed by diffusive or advective transport of these fluids into the magma, and (2) by direct melting and assimilation of wall rock and xenoliths. We consider physical and chemical controls on the timescales of these processes and show that wall-rock and xenolith melting is by far the most efficient and quickest process for adding crustal sulfur, with melting processes taking place on a scale of minutes to years. In contrast, liberation of sulfur from a thermal aureole via diffusion is much slower and requires timescales of millions of years—two orders of magnitude longer than the time required for an intrusion to solidify by diffusion. We conclude that sulfur, which may be liberated in thermal aureoles (produced either via devolatilization reactions or dissolution involving hydrothermal fluids) and which must be transported via diffusional processes, has a negligible effect on the formation of magmatic Ni-Cu-PGE deposits.