A possible volcanic-exhalative origin for lenticular nickel sulfide deposits of volcanic association, with special reference to those in western Australia

Abstract

Archean lenticular nickel sulfide deposits of volcanic association bear a striking resemblance lo volcanic-exhalative Cu–Zn massive sulfide deposits. They have similar morphologies, show well developed mineralogical layering, and exhibit close stratigraphic relationships with intimately associated volcanics and intra-volcanic sediments of greenstone belts. The nickel sulfide deposits are associated with early ultramafic/mafic volcanics. and massive Cu–Zn sulfide deposits with intermediate to silicic volcanics that formed later in volcanic-sedimentary cycles.It is suggested that a magmatic volcanic-extrusive origin does not explain features of massive nickel sulfide deposits as well as an alternative volanic-exhalative origin, which provides a direct link between sulfide ores and intimately associated exhalative sediments. It also satisfactorily accounts for a number of important features, including mineralogical layering, a relative enrichment in pyrite, and the local abundance of millerite.Outpouring of extremely hot ultramafic lavas across top surfaces of deposits would partially melt the Ni–Cu sulfides, thereby generating the igneous textures observed. Later regional metamorphism has modified these earlier features.