Origin and evolution of the Mesoarchean Aouéouat greenstone belt and associated gold mineralisation

Abstract

This Aouéouat greenstone belt is situated in the Tasiast-Tijirit Terrane of the Reguibat Shield, part of the West African Craton. Petrographic, geochronological, geochemical and isotopic studies of the Aouéouat greenstone belt and surrounding rocks were undertaken in order to elucidate its geological evolution. The Aouéouat greenstone belt is comprised of three major stratigraphic units; the Temeimichat assemblage ultramafic to mafic volcanic rocks, the Aouéouat assemblage mafic to felsic volcanic rocks, and the Tasiast assemblage sedimentary rocks. The greenstone belt is bound on all sides by Tonalite-Trondhjemite-Granodiorite (TTG) intrusive rocks. The volcanic rocks of the Temeimichat assemblage are interpreted as mantle plume-derived juvenile rocks which formed at ~3.01 Ga, likely being extruded into a rift basin which developed on 3.32-3.17 Ga pre-existing continental crust. The Aouéouat assemblage formed between 3.00 Ga and 2.99 Ga, and contains a crustal component attained on its eruption through the pre-existing basement. Partial convective overturn of these volcanic rocks resulted in emplacement of the TTG suite between 2.97 Ga and 2.91 Ga, with the resulting tectonic displacement consequently leading to deposition of the Tasiast assemblage sedimentary rocks. The Tasiast assemblage clastic sedimentary rocks were sourced from all lithologies present in the region by ~2.95 Ga including the TTG rocks. A hydrothermal event at ~2.84 Ga utilised faults and shear zones in the Aouéouat greenstone belt as fluid pathways and traps, liberating gold from the juvenile Temeimichat assemblage, resulting in formation of the Tasiast gold deposit in the overlying Aouéouat and Tasiast assemblages.