Gold-rich mineralisation in Izu Bonin fore-arc at ODP site 786B: evaluation of magmatic input into oceanic crust hydrothermal system
Gold and other metals variably enriched in volcanogenic massive sulphide (VMS) deposits are mobilised in the oceanic crust by two main processes: hydrothermal alteration of the oceanic crust’s lower sheeted dykes and exsolution of metal-rich fluids into the hydrothermal system from differentiating magmas. The extent to which each process contributes to metal enrichment in VMS deposits, and the style of mineralisation produced varies between different tectonic settings. Oceanic Drilling Program (ODP) Hole 786B recovers the upper oceanic crust of a supra-subduction zone including a 30 m-wide mineralised transitional zone at the base of the hole which shows evidence for magmatic fluid input. This study uses in-situ trace element, S-isotope analyses in sulphide minerals and whole-rock data to characterise the metal endowment of the mineralised zone, the sources of the trapped metals and the specific signature of magmatic fluid inputs in the hydrothermal system. Magmatic fluid exsolution most likely provided most of the Au, Se, S, Mo, Bi and Te enriched in the mineralised zone whilst As, Sb could have been mobilised by rock-buffered hydrothermal alteration. Little base metal mobilisation occurred in Hole 786B leading to significant metal fractionation and promoting Au-rich mineralisation.