A history of our understanding of magmatic Ni-Cu sulfide deposits

Progress in the understanding of the origins of magmatic Ni-Cu sulfide deposits underwent a major acceleration with the advent of the 1960s. Prior to this decade, thinking had largely been influenced by observations on the Sudbury area, in Ontario. which was by far the dominant Ni producer. Discussion focused oil the nature of the Sudbury Igneous Complex. and whether the ores were gravitational segregates front the complex, or whether they had been introduced by hot aqueous fluids. During the 1960s the concept that Sudbury is all astrobleme was first proposed. the discovery of the Talnakh ore junction (Russia) elevated Noril'sk front minor to major status, and,I new class of deposit related to komatiitic volcanism was recognized at Kambalda (Australia). The 1960s were also,I turning point with respect to research funding. which led to all explosion both in the amount of research Conducted. and in its global nature. Thereafter, progress was stimulated by the new thinking about Sudbury. and the very different environments of ore deposition observed at Noril'sk and Kambalda. It has turned out that a number of broad themes have evolved under which much of the progress over the past 40 years call be grouped. (i) Magmas rising directly from the mantle are unlikely to reach crustal depths saturated in sulfide, and contamination with crustal rocks is required for sulfide immiscibility 10 Occur early in the crystallization process. (ii) With appropriate experimentally derived partition-coefficients. the relationship between the compositions of magma and sulfide call be modeled, and this modeling provides important constraints oil geologically based hypotheses. (iii) The development of sulfide immiscibility commonly leaves a mark on the composition of the source magma and the rocks crystallizing from it thin call act as a signpost for exploration. (iv) Sudbury is unique, probably because of the high degree of superheat that it experienced: sulfides have settled and accumulated over much of the base of the complex: in most other deposits. it has been the now of magma carrying immiscible sulfides that has caused the sulfides to concentrate in economically exploitable proportions. The physical environment represented by a given part of all igneous body is therefore important when considering its potential. (v) Once concentrated. sulfide magmas cool and fractionate. and the fractionated residual liquid may migrate away front the initial site of crystallization to form rich concentrations of Cu. Pt. Pd and Au elsewhere. Observations at Voisey's Bay (Canada) over the past 10 years have confirmed the importance of these five themes.