Application of Trace Element Geochemistry to Magmatic Sulphide Exploration in Metamorphosed Terranes

Most magmatic Ni-Cu-(PGE)-sulphide deposits are interpreted to have formed from sulphide-undersaturated mafic or ultramafic magmas by the incorporation of crustal S and/or the assimilation of the adjacent country rock, resulting in sulphide-saturation and the formation of an immiscible sulphide liquid. As a result of such processes and the distinctive, yet complimentary, differences between the trace compositions of magmas derived by melting of a depleted mantle source and those of typical continental crustal rocks, many host rocks exhibit geochemical or isotopic signatures that are indicative of magma-crust interaction and/or chalcophile element depletion. These contrast with those expected from normal crystal fractionation and accumulation processes. Although the recognition of crustal contamination and chalcophile element depletion signatures within mafic and ultramafic rocks associated with magmatic sulphide deposits may be of considerable value in mineral exploration, many magmatic sulphide deposits and their country rocks have been metasomatized during regional metamorphism. As a consequence, many of the elements that have been enriched or depleted during contamination or sulphide segregation have been mobile during metamorphism. Although virtually all elements may be mobile during high grade metamorphism under high fluid:rock ratios, there are differences between the geochemical signatures of normal igneous fractionation, crustal contamination, and alteration that can be used to distinguish contamination and chalcophile element depletion signatures in all but the most strongly-metasomatized rocks. For example, the order of depletion of incompatible elements in depleted mantle (and high degree partial melts of depleted mantle) is broadly Cs > Rb > U > Th > Ba > K > Ta > Nb > LREE > Hf ~ Zr > MREE > HREE > Ti > Cu ~ PPGE > IPGE > Ni. In contrast, the order of enrichment during upper crustal contamination is broadly Cs > Ba > Th > U > LREE > Nb ~ Ta > MREE > HREE > Cu ~ PPGE > IPGE ~ Ni. The relative mobilities of these elements during metasomatism is controlled by the stabilities of the igneous and metamorphic phases and the solubilities of the elements in 'normal' metamorphic fluids, but is very broadly Cs > Rb > K > Na > Ba > Sr ~ Eu2+ ~ Ca > Cu(?) > LREE > MREE > HREE > PPGE > IPGE > Ti > Hf ~ Zr > Th > U4+ > Nb ~ Ta. As a consequence, although the alkalis (Cs, Rb, K) are most commonly enriched during contamination, they are also most commonly mobilized during metasomatism. The most reliable indicators of contamination are therefore Th, U, Ta, Nb, Hf, Zr, and Ti, and the most reliable indicators of chalcophile element depletion are Ni and PGE, all of which appear to be relatively immobile except under the most extreme degrees of metamorphism.