Structural control and metamorphic setting of the shear zone-related Au vein mineralization of the Adola Belt (southern Ethiopia) and its tectono-genetic development

Abstract

Structural study of the Adola Belt shows that most of its known and potential Au deposits occur in quartz veins which are localised within shear contacts between lithological units, and along major shear zones that divide the Adola Belt into different lithostructural domains. Analysis of the shear zone-related ore bodies and their host volcano-sedimentary succession and gneisses indicates that Au mineralization in the Adola Belt is pre-dated by two stages of deformation and a regional prograde metamorphism. The first deformation event (D 1) is a fold-and-thrust event which is characterised by low-angle thrusts, associated recumbent folds and axial planar S 1 foliation, and is related to nappe-style deformation. The second event (D 2) has folded and/or reactivated the thrust-related structures and formed upright folds and high-angle reverse shear zones and is related to the collision event. Gold mineralization occurred over a prolonged deformation history but is closely related to alteration, retrograde greenschistfacies assemblages and brittle-ductile deformation of late D 2 and D 3 transpressional shear zones that accommodate regional shortening both by crustal thickening and lateral displacement. The mineralization occurs in associated dilational jogs or bends that might have formed during the lateral or vertical expulsion. The Au-hosting shear zones are characterised by extensive development of heterogeneous mylonitic fault rocks which reveals that the accompanied deformation is characterised by processes that can increase the porosity and permeability of the rocks within the shear zones. This gave rise to further extensive dilatancy within the major dilational jogs and produced a suitable structural regime for vein-hosted Au mineralization. This implies that the Au mineralization is epigenetic in origin and that it resulted from precipitation from metamorphic hydrothermal fluids circulating through major shear zones and associated structures late during the deformation and metamorphic history of the shear zones, through a hydrodynamic process of vein formation. The association of Au with low temperature ore minerals like galena gives an independent evidence for the deposition of Au in low temperature retrograde conditions. The increase in Au concentration along the Megado greenschist-facies metavolcano-sedimentary terrane as compared to the gneissic rocks is related to its development in a rapid extensional zone and rapid subsidence environment in an inter-arc/back-arc setting, where a thicker amount of volcanic rocks containing trace amounts of massive sulphide-related Au can best be developed and rapidly buried. The occurrence of tellurides in association with the Au mineralization in the Megado Terrane confirms that the mineralization is genetically related to the subduction event.