Carbon dioxide as a proxy for orogenic gold source

Abstract:

After the term orogenic gold was defined a variety of sources of fluids has been distinguished, but in the end, two distinct sources have been accepted that allow for sufficient influx of fluids to the depositional sites: i. the lower/middle crust in orogenic settings and ii. the subducting slab . In both cases these aqueous fluids are released during progressive breakdown of minerals during metamorphic reactions and have some common characteristics through-out all deposits; they are: enriched in CO₂, sulphur-bearing and low salinity, Au-bearing fluids are released from the rocks during the metamorphic transition from greenschist- to amphibolite-facies (wide range of pressures – temperatures between 450 - 650°C) with minor/no fluids of other origins involved. Here we explore the devolatilization of a subducting slab material using coupled petrological thermo-mechanical numerical approach that takes into account dehydration and decarbonation processes. Recent phase equilibria calculations show that decarbonisation of the slab require much higher temperatures that normally occurring within subduction zones and the devolatilization site in the slab is spatially and temporally localised during slab roll-back or within a hot orogen when the slab material is incorporated into the overriding plate. We also investigate the plausibility of large gold sources in the collisional and back-arc environments.