Orogenic gold: is a genetic association with magmatism realistic?

Abstract Many workers accept a metamorphic model for orogenic gold ore formation, where gold-bearing aqueous-carbonic fluid is an inherent product of devolatilization across the greenschist-amphibolite boundary with majority deposits formed within seismogenic zone at depths 6–12 km. Fertile oceanic rocks that source and metal may be heated through varied tectonic scenarios affecting deforming upper crust (? 20–25 km depth). Less commonly, cover on downgoing slab release 25–50 travels up-dip along sealed plate until intersecting near-vertical structures facilitate migration deposition in crustal environment. Nevertheless, numerous world-class are alternatively argued to products magmatic-hydrothermal processes based upon equivocal geochemical mineralogical data or simply spatial association exposed hypothesized intrusion. Oxidized intrusions form porphyry epithermal ores 3–4 crust, but their ability economic resources mesozonal (? km) hypozonal > 12 limited. Although volatile saturation reached magmatic systems as deep 10–15 km, such doesn’t indicate release. Volatiles typically will channeled upward magma mush brittle apical roof zones epizonal levels < 6 before large pressure gradients rapidly focused fluid. Furthermore, sulfur solubility relationships favor relatively shallow formation systems; although from system below would generally diffuse, even if cases it was somehow better focused, unlikely contain substantial gold. Where reduced assimilation carbonaceous material, subsequent high pressures hydrofracturing have been shown lead development sheeted veins greisens 3–6 These systems, however, Sn W ores, low grade occurrences (< 1 g/t Au) being rare cases. Thus, whereas most moderate- high-T orogens host intrusions, there no genetic association.