Metallogeny of a Pan-African oceanic arc: VHMS and gold deposits in the Ariab-Arbaat belt, Haya terrane, Red Sea Hills (Sudan)

Abstract

Gold in the Ariab district (Red Sea Hills, Sudan) is mined in gossans developed over volcanogenic massive sulphide (VHMS) deposits formed in the context of a Tonian ensimatic arc of the Haiya terrane, in the Neoproterozoic Arabo-Nubian Shield. All the primary gold occurrences of the Ariab belt, whatever their style (VHMS-, quartz lode- or shear-zone-hosted), appear, however, to have been syn-to late-kinematic, i.e., formed in the context of the final Pan-African collision leading to the Arabian-Nubian Shield (D3 event), and therefore are post-VHMS. Gold is thus found associated with either galena or bismuth telluride in late syn- to post-metamorphic sets of (micro)cracks, mostly within earlier pyrite or arsenopyrite. Gold deposition as native gold or electrum occurred when an intense fluid circulation of metamorphic CO2-H2O rich fluids underwent significant decompression from lithostatic to hydrostatic pressures at temperatures decreasing from 400 to 430 °C at around 8 km depth down to 300 °C. Although no clear evidence of volatile unmixing was observed, the decompression, together with a temperature decrease due to dilution by aqueous rich fluids, favoured gold deposition.The late-D3 gold event in the Ariab belt has all the makings of an orogenic gold system, notably similar to the Late Carboniferous gold event in the West European Variscan belt. Thus, it is possible to propose that, at the end of the collision events, fluids released from the newly formed terrane root were channelled towards the upper crust through the major D3 shear zones such as the Oko shear zone. The fluids outpouring from these major drains were then conveyed through fault plays and damaged zones, up to specific chemical traps within massive sulphide ore bodies or quartz veins systems. Thus, the Ariab gold district could be classified as a representative of the class of orogenic gold deposits.