Identification of ore-deposition environment from trace-element geochemistry of associated igneous host rocks

Abstract

Synopsis The tectonic environment of formation of volcanogenic massive sulphide deposits and porphyry tin and copper deposits can be identified from the geochemical characteristics of the associated igneous rocks. The ‘stable’ trace-element geochemistry (involving Ti, Zr, Y, Nb, Cr and rare-earth elements) and geology of metabasalts related to 12 massive sulphide deposits indicate that the deposits studied fall into four distinct classes. (1) Cyprus-type, including Cyprus, Oman and Betts Cove, possibly formed during the early stages of back-arc basin development; (2) Løkken-type, including Løkken and York Harbour, possibly formed at back-arc basin spreading centres; (3) Joma-type, including Joma, Røros and Bidjovagge, possibly formed in a small ocean of Red Sea type; and (4) Gjersvik-type, including Gjersvik, Buchans, Noranda and Lynn Lake, possibly formed during an early stage of island arc evolution (Gjersvik), later during island arc evolution (the Buchans Kuroko-type deposit) or in a Precambrian setting (Noranda and Lynn Lake). Deposits related to major ocean ridge crests appear to be small and relatively uncommon, perhaps because relatively few favourable sites for ore deposition exist in such environments. The environment of intrusion of acid-intermediate igneous rocks can be deduced by use of diagrams based on the element Nb (e.g. SiO 2 versus Nb). On this basis, tinbearing granites can be classified either as within-plate magmas (Nigeria) or as magmas from evolved volcanic arc settings (e.g. Bolivia, Cornwall and Indonesia); the latter group can be further subdivided geologically into postorogenic and back-arc extensional settings. Although the continental crust may play a significant role in the formation of tin deposits, the geochemical data presented here suggest that partial melting of tin-enriched mantle above a subduction zone may be the single most important genetic factor.