Metal fluxes during magmatic degassing in the oceanic crust: sulfide mineralisation at ODP site 786B, Izu-Bonin forearc

Abstract

Volcanogenic massive sulfide deposits are enriched in metals that are either derived from hydrothermal alteration of the basement rocks or supplied by exsolution of metal-rich volatiles during magmatic differentiation. The extent to which each process contributes to metal enrichment in these deposits varies between different tectonic settings. Ocean Drilling Program Hole 786B recovered > 800 m of upper oceanic crust from a supra-subduction zone setting and includes a 30-m-thick mineralised zone. In situ S isotopic compositions of pyrite decrease from 5.9 ± 2.9‰ in the upper mineralised zone down to − 3.3 ± 2.1‰ in the extensively altered central mineralisation zone, potentially indicating strong magmatic fluid input in this area. Whole rock data and in situ trace element analyses in sulfide minerals show enrichment of Ag, As, Au, Bi, Mo, S, Se, Sb and Te in the mineralised zone. Evaluation of metal behaviour during magmatic differentiation and primary metal fertility of basement rocks suggests that degassing melt is the main source for the high Au, Se and S enrichment observed in the mineralised zone. Magmatic volatile exsolution occurred late during the magmatic differentiation (~ 2 wt.% MgO), concomitant with oxide crystallisation and metal depletion in the melt. Comparison of Ocean Drilling Program Hole 786B with volcanogenic massive sulfide deposits hosted by boninitic volcanic successions, such as in the Semail ophiolite, the Newfoundland Appalachians and the Flin Flon Belt, suggests that magmatic fluid exsolution could be a common mechanism for Au enrichment in bimodal mafic volcanogenic massive sulfide deposits.

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