MULTIPLE SULPHUR SOURCES FOR ARCHAEAN KOMATIITE-HOSTED NICKEL SULFIDES CONFIRMED BY S AND FE ISOTOPES: EXPLORATION IMPLICATIONS

Understanding the source of sulfur in Archaean komatiite-hosted Fe-Ni sulfide deposits provides important insights into the operation of the Earth System prior to 2.5 billion years ago as well as a key to exploration for one of the world's most important mineral resources. The source of the sulfur in these deposits has long been debated. Because the sulfur content of the mantle is low and komatiites result from high degrees of melting, it is generally accepted that that komatiites were sulfide undersaturated at the time of separation from mantle residue and that assimilation of sulfur either during ascent through the crust or during emplacement on the sea-floor is the key process that triggers sulfide saturation and immiscibility, and thus ore genesis. However, the spectrum of mineralization styles may reflect more than one sulfur source and saturation mechanism, with assimilation of sulfur from sea-floor sediments generally regarded as a key process in ore genesis. Because the δ³⁴S values of sulfides in Archean sedimentary rocks vary much less than those in younger sediments, existing isotope data can not unambiguously resolve the source of sulfur.

Multiple sulfur and iron isotope data indicate that komatiite-hosted Ni sulfide systems incorporated sulfur from local low temperature hydrothermal sulfide accumulations and/or sulfidic sediments during magma emplacement. A range of negative and positive Δ³³S values indicate a contribution from sulfur produced by atmospheric photochemical reactions to komatiite-hosted magmatic Fe-Ni sulfide systems. Hydrothermal sulfide accumulations in felsic volcanic and associated sedimentary rocks were the major sulfur source for most world-class Late Archean komatiite-hosted massive Fe-Ni sulfide deposits in Western Australia and Canada. Interaction of very high temperature komatiite magmas with submarine hydrothermal systems which contained accumulations of Fe sulfides explains the abundance of world-class massive Ni sulphide deposits at a time when the oceans and sediments were sulfur limited. Multiple isotope studies thus provide an important new exploration tool for komatiite-hosted Fe-Ni sulphide deposits.