Abstracts

Abstract

Magmatic sulphide deposits fall into two major groups when considered on the basis of the value of their contained metals. There are one group in which Ni, and, to a lesser extent, Cu, are the most valuable products and a second in which the platinum-group element (PGE) are the most important. The first group includes komatiite-(both Archean and Paleoproterozoic), flood basalt-, ferropicrite-, and anorthosite complex-related deposits, a miscellaneous group related to high Mg basalts, Sudbury, which is the only example related to a meteorite impact melt, and a group of hitherto uneconomic deposits related to UralAlaskan type intrusions.PGE dominant deposits are mostly related to large intrusions comprising both an early MgO-and SiO 2 -rich magma and a later Al 2 O 3 -rich, tholeiitic magma, although several other intrusive types contain PGE in lesser, mostly uneconomic quantities. Most Ni-rich deposits occur in rocks ranging from the Late Archean to the Mesozoic. PGE deposits tend to predominate in Late Archean to Paleoproterozoic intrusions, although the limited number of occurrences casts doubt on the statistical value of this observation.A number of key events mark the development of a magmatic sulphide deposit. These are (i) partial melting of the mantle, (ii) ascent of the magma into the crust, (iii) development of sulphide immiscibility as a result of crustal interaction, (iv) ascent of magmazsulphides to higher crustal levels, (v) concentration of the sulphides, (vi) their enrichment through interaction with fresh magma (not always the case), and (vii) cooling and crystallisation.Factors governing this development include (i) partial mantle melting and how this affects the distribution of chalcophile metals between mantle and melt, (ii) the solubility of sulphur in silicate melts and how this varies as a function of partial mantle melting and subsequent fractional crystallisation, (iii) the partitioning of chalcophile metals between sulphide and silicate liquids, and how the results of this vary during fractional crystallisation and sulphide immiscibility (degree of crystallisation, R factor and subsequent enrichment), (iv) how effectively the sulphides become concentrated and the factors controlling this, and (v) processes that occur during the cooling of the sulphide liquid that govern aspects of exploration and mineral beneficiation. These topics will be discussed, first in general terms and then with specific reference to deposits at Noril'sk, Kambalda, and Voisey's Bay. With regard to Voisey's Bay, quantitative modelling is consistent with the very low PGE concentrations in this deposit being the result of some sulphide having been left behind in the mantle during partial melting. Both the Noril'sk and Voisey's Bay deposits are shown to be economic because of subsequent upgrading of the ores, which are located in magma conduits, through interaction with fresh, sulphide-unsaturated magma passing along the conduits.The Sakatti magmatic Ni-Cu-PGE deposit, northern Finland