Ultramafic and related mafic rocks; their classification and genesis with special reference to the concentration of nickel sulfides and platinum-group elements

Abstract

Ultramafic and related mafic rocks can conveniently be classified in terms of the tectonic environment in which they were emplaced. Among those bodies emplaced in an orogenic environment one can distinguish synvolcanic bodies, both intrusive and extrusive, emplaced before the peak of orogenesis; alpine bodies (ophiolites) emplaced during orogenesis; and Alaskan-type complexes emplaced in the waning stages of orogenesis. Large stratiform complexes (Bushveld type), plateau-type sills (Dufek type), and medium- and small-sized picritic intrusions are characteristic of a cratonic environment. Synvolcanic bodies belong to two distinct magma suites: (1) the highly magnesian komatiitic suite, characterized by spinifex-textured ultramafic flows, a low FeO/(FeO + MgO) ratio for a given Al 2 O 3 content, and relatively low TiO 2 contents; and (2) a less magnesian tholeiitic suite, lacking spinifex texture and having a higher FeO/(FeO + MgO) ratio and TiO 2 content.The Alaskan complexes differ from each of the other economically interesting types in the alkalic nature of their magma (Irvine, 1974) and therefore the total absence of orthopyroxene in the rocks. Stratiform complexes such as Bushveld and Stillwater are at the other end of the spectrum. They have a high normative orthopyroxene to clinopyroxene ratio and crystallize in this order: olivine, orthopyroxene, and plagioclase plus augite or plagioclase and then augite. Magmas responsible for some of the other important bodies fall between these two end members, although they are all tholeiitic and contain normative orthopyroxene.Platinum-group element concentrations occur as discrete platinum-group minerals or as solid solutions in sulfides, arsenides, and sulfarsenides associated with synvolcanic bodies, stratiform complexes, and picritic intrusions. Alpine and Alaskan-type rocks contain only very low grade concentrations of sulfides, but platinum-group element concentrations occur as placer deposits derived from Alaskan-type bodies and, to a lesser extent, from alpine bodies.In the sulfide ores of the Merensky, Pechenga, Sudbury, and Noril9sk deposits, the Pt/(Pt + Pd) ratio decreases systematically with an increasing Cu/(Cu + Ni) ratio. Since the latter ratio is thought to increase with progressive differentiation of the host silicate magma, it is suggested that the trend of the decreasing Pt/(Pt + Pd) ratio is also related to differentiation. Ores associated with komatiites are major exceptions to the trend exhibited by the tholeiites, the komatiite ores having both a low Cu/(Cu + Ni) and a low Pt/(Pt + Pd) ratio. It is suggested that this is due to the fact that komatiites originate at unusually great depths in the mantle, depths at which mantle sulfides (rich in Pd) have accumulated and are hence incorporated in the magmas.