Arc-related gold-copper mineralization, basement domes and crustal extension

In western Pacific magmatic arcs, many of the largest low-sulphidation epithermal gold vein deposits are situated on basement antiforms which often include ophiolite. Analogy with metamorphic core complexes, some of which also host low-sulphidation gold, implies that the antiforms result from elevation of pre-existing nappe stacks in response to subduction-induced extensional stress and crustal extension on low-angle detachment faults. Porphyry copper (-gold) deposits in the western Pacific are mostly situated away from antiform axes, in shallow stocks, with their tops at stratigraphic levels similar to these of the deeper parts of low-sulphidation epithermal veins. We suggest that epithermal systems develop where tension fractures on antiforms permit descent of meteoric fluids to depths of at least several km; here, they mix with ascending deep fluid, perhaps heated by magmatic sills rather than by the shallow stocks required for generation of porphyry-type deposits. Basement antiforms without associated magmatic arc rocks lack associated gold but, in some cases, host mercury and antimony mineralization. The association of gold with ophiolite appears to be related to the structure of the underlying crust; most ophiolite nappes themselves are too high-level to have formed a source of gold. The overall field relationships suggest that porphyry-type and low-sulphidation epithermal gold mineralization take place at specific stages in the evolution of a magmatic arc; with continued extension some arcs undergo intra-arc rifting, “back-arc” spreading and eventually renewed subduction.