A review of some features potentially indicative of the presence of platinoid mineralization as deduced from the Stillwater Complex, Montana (USA)

Abstract

The Stillwater Complex of South Montana is a layered differentiated ultramafic intrusive body of late Archaean age. It crops out along strike over 48km and contains one of the richest zones of platinum group elements (PGE) in the world - the JOHNS-MANVILLE Reef. This reef is almost two metres thick and consists of 0.5 to 2.0% sulfide enclosed in noritic rocks. The sulfides are principally pentlandite, pyrrhotite and chalcopyrite with locally minor pyrite. Minerals of the braggite-vysotskite series contain the bulk of the platinum group elements. A relatively high Ni/Cu ratio of 1.3, and extremely high - 32 g/ton – concentration of platinum and palladium, a (Pt + Pd)/(Os + Ir + Ru) ratio of 230 and a high magma/sulfide or R ratio are quite different from what may be expected from sequential differentiation and crystal settling in an ultrabasic magma chamber. The most acceptable genetic model for the chemistry and geometry is one suggesting turbulent injection of a second buoyant magma into the original magma chamber to so alter the magma/sulfur ratios that the observed metal abundances and ratios are reached. A number of the petrological and geochemical features of the Stillwater Complex, and particularly their geometrical distributions are of significance for the understanding of such complexes and can be used in exploration. Some of these can be summarized as follows: If in a sulfide zone, 400-1000m above the first appearance of cumulus plagioclase, one finds - the presence of chromitite layers, - that olivine is the cumulus mineral in addition to plagioclase, - the presence of 'pegmatoid' zones (i.e. distinct orthocumulate texture), - that there is evidence of magma injection (slump textures etc.), - that the Ni/Cu ratio is greater than 1, - that there is no apparent nickel depletion of the associated reef olivine, - and that rare earth element abundances and ratios of plagioclase change in a way suggesting magmareplenishment, then it is highly probable that such a sulfide zone has an economically interesting platinoid content.