FROM THE MANTLE TO THE BANK: THE LIFE OF A NI-CU-(PGE) SULFIDE DEPOSIT

Abstract

The life of a magmatic Ni sulfide deposit can be envisaged as a series of stages: (1) birth of the magma in the source (mantle melting); (2) development of the magma (ascent into the crust); (3) fertilisation of the magma (interaction with the crust and the early development of immiscible sulfides); (4) delivery (ascent of the magma+immicible sulfides to a high level in the crust); (5) growth (concentration of the sulfides during magma emplacement); (6) nourishment (enrichment of the sulfides by further flowing magma) and (7) full maturity (cooling and crystallization of the host magma and related sulfides). In this paper the chemical and physical parameters constraining these stages are discused in theory and then with reference to three major Ni sulfide camps, Noril’sk, Voisey’s Bay and Kambalda. Modeling of partial melting, followed by magma ascent and early fractionation indicates that unless a magma interacts with its surroundings in a manner to change its SCSS (sulfur content at sulfide saturation) or acquires additional sulfur, it will not achieve sulfide saturation until much of its contained Ni has been removed in early crystallising olivine. In most cases (e.g. Noril’sk and Voisey’s Bay), it is apparent that external sulfur has been assimilated from country rocks. If, as has been the case at Noril’sk and Voisey’s Bay, too much sulfur has been assimilated (i.e. the initial R factor is low), modeling indicates that the deposits have only achieved economic viability as a result of subsequent, high temperature magmatic upgrading of the intially formed sulfides. Recent work has shown that the presence and packing density of phenocrysts in magma have a strong control on whether or not sulfides can settle in a magma as a result of their greater density. This places constraints on the origin of net-textured sulfides, such as those found at Kambalda. Wetting angles between silicates and sulfides in the presence of silicate magma are high which will prevent sulfides «leaking» out into country rock into permeable zones or adjacent structures when magma is present, but are low in the absence of magma, whereupon surface tension affects are likely to promote escape of magmatic sulfide into surrounding country rocks, as appears to have occurred at in the Reid Brook Zone of the Voisey’s Bay deposit. Once the sulfide melt starts to crystallise, its sulfur fugacity controls the composition of the mss and the pyrrhotite that forms from it, which in turn controls both the diffusion rate of Ni within the mss/pyrrhotite and the temperature at which pentlandite starts to exsove. In natural settings, these factors determine the concentration of Ni that will never exsolve from the pyrrhotite as pentlandite, and whether the pentlandite occurs as fine flames or larger, more easily separated, masses. Furthermore, it is only sulfur-rich pyrrhotites that are magnetic; this is a function of the sulfur/metal ratio of the original sulfide liquid, and subsequent alteration of the deposit by relatively oxidised fluids (e.g. grounwater). The Voisey’s Bay deposit is a classic example of how fortunes may, on rare occasions, be made in the mining industry, in which an investment of about a million dollars appreciated 4000-fold in less than three years. It is also significant that the database accumulated by geologists working on the deposit has been invaluable in stimulating subsequent research and providing criteria on which to base future exploration.