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.

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

“…Also, there are no extensive iron and nickel depletion haloes surrounding ore within ultramafic units (see also Hudson 1972;Ross and Hopkins 1976;Barrett et al 1976). Further, the and our unpublished data) are virtually identical in terms of Fe, Ni, Co, and S: this would be an unusual coincidence if they had a separate origin as suggested by Lusk (1976).…”

“…This requires that any review of ultimate origin must first examine the effects of metamorphism, and then attempt to look back through these to determine the nature of preexisting deposits. Lusk (1976) has not done this, and he presents very little new information to support his hypothesis, so we believe that the magmatic model has not been criticaIly reviewed. Although be claims an overview, only a small proportion of the many nickel deposits of volcanic association are discussed by Lusk, and only a few relevant features of these deposits are considered.…”

“…Lusk (1976) has raised the interesting possibility that lenticular Fe-Ni sulfide deposits associated with Archean volcanic sequences have a volcanic-exhalative origin, asserting that this origin offers a better explanation for many of their features than the magmatic model (e.g. Naldrett and Gasparrini 1971 ;Ewers and Hudson 1972;Naldrett 1973;.…”

Boudinage on radial fractures: An alternative to magmatic models for the emplacement of nickel ores, Lunnon Shoot, Kambalda, Western Australia

“…Magmatic models (Figure 15b) are diverse, controversial and frequently contradictory, although the so-called 'consensus' merely explains these contradictions as 'the reflection of different mechanisms of emplacement in a variety of extrusive volcanogenic environments' (Hill & Gole 1990 p. 557). Diversity is shown by the variety of interpretations for the ores as: (i) volcanic exhalations equivalent to flanking sulfidic facies sediments (Lusk 1976 sidence and dislocation; sediments are absent due to active convection in the fissure environment (Ross & Hopkins 1975); (iv) komatiite flows infilling topographic depressions, with sediments and underlying basalt being assimilated ; (v) immiscible sulfide fractions resulting from sediment assimilation by lavas in scour channels (Huppert & Sparks 1985;Williams & Lesher 1996); and (vi) ribbons of ore formed at the base of constructional lava piles extruded on a flat surface-no embayments, no topographic depressions, but also by implication little or no assimilation of sediments (Cowden 1988).…”

Sulfur isotopes in Finnish nickel-copper occurrences

“…The discussion has recently turned to the origin of the massive Ni sulfide deposits associated with ultramafic extrusive rocks (e.g. Lusk, 1976;Seccombe et al 1977). Groves et al (1979) incline to the view that some, at least, of the nickel ores of Western Australia were formed when ultramafic magma was contaminated by sulfur from a crustal source.…”