Controls on the formation of komatiite-associated nickel-copper sulphide deposits.

Abstract

The Ni-Cu sulphide deposits associated with komatiites may be divided into two groups on the basis of host-rock composition and ore distribution: 1) stratiform, massive-matrix-disseminated sulphides hosted by cumulate komatiite lava flows, or strata-bound, coarse disseminated sulphides hosted by cumulate bodies near volcanic vents and 2) strata-bound, finely disseminated sulphides and more restricted massive sulphides hosted by subvolcanic komatiitic dunites. These represent an environmental continuum from distal to subvolcanic. It seems likely that these sulphides are magmatic in origin, though they are related to the distribution of specific country rocks. They probably exsolved from the komatiitic magmas in situ, in response to assimilation of the country rocks during emplacement. The model depends upon the coincidence of a heat source, ore metals and sulphur or components to induce sulphide separation. Rifting environments seem the most likely sites of ore emplacement, the degree of extension affecting the type of deposit. -R.E.S.