Palaeoproterozoic gold mineralization of the Etéké Archaean greenstone belt (Gabon): its relation to the Eburnean orogeny

Abstract

The gold deposits of the Etéké district (Gabon) are hosted by metasediments and intermediate to ultramafic metavolcanites of the Etéké Archaean unit (2940 ± 6 Ma), a greenstone belt within the folded and metamorphosed Ogooué domain that was formed by the successive transport and stacking of nappes during Palaeoproterozoic (Eburnean) D1–D2 collisional tectonism—this lithotectonic pile, in the east of the Etéké district, rests on the Chaillu Archaean basement and palaeoproterozoic Francevillian foreland basin deposits. The gold is contained in quartz ± carbonate stockworks, ‘hydrothermalite’ lenses and mineralized quartz veins that indicate mineralization during a single D2 hydrothermal event extending from an early-D2 stage (stockwork) to a late-D2 stage (veins). The mineralization began during the early-D2 stage following a major phase of tangential tectonism (D1) when the Etéké unit, still in a shallow position (brittle deformation) in the outer (foreland) zone of the orogenic belt, would have received the repercussions of crustal thickening taking place in the inner zone of the belt (Ogooué series s.s.). These repercussions are reflected by the development of stockworks and ‘hydrothermalite’ lenses related to reverse faults that favoured the emplacement of tonalitic dykes and sills and channelled the first hydrothermal fluids. At the peak-D2 stage the Etéké unit, which by this time formed part of the outer zone of the orogenic belt, was caught up in nappe tectonism that imparted a major S2 foliation, with microfolding and foliation of the early stockworks. The early-D2 reverse faults evolved to synmetamorphic thrusts (epi- to mesozonal conditions) along which syntectonic granites and rare mineralized veinlets were emplaced, reflecting a dormancy of the hydrothermal event. The nappe tectonism ended with transport of the peak-D2 lithotectonic pile onto the Chaillu Archaean terrain and nearby Francevillian foreland basins, accompanied by a late-D2 emplacement of gold-bearing quartz veins cutting the S2 foliation and marking the end of the mineralizing hydrothermal event.