STRUCTURAL CONTROL AND METAMORPHIC SETTING OF THE SHEAR ZONE-RELATED AU VEIN MINERALIZATION OF THE ADOLA BELT (SOUTHERN ETHIOPIA) AND ITS TECTONO-GENETIC DEVELOPMENT

Structural study of the Adola Belt shows that most of its known and po tential Au deposits occur in quartz veins which are localised within s hear contacts between lithological units, and along major shear zones that divide the Adola Belt into different lithostructural domains. Ana lysis of the shear zone-related ore bodies and their host volcano-sedi mentary succession and gneisses indicates that Au mineralization in th e 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 napp e-style deformation. The second event (D-2) has folded and/or reactiva ted the thrust-related structures and formed upright folds and high-an gle reverse shear zones and is related to the collision event. Gold mi neralization occurred over a prolonged deformation history but is clos ely related to alteration, retrograde greenschist-facies assemblages a nd brittle-ductile deformation of late D-2 and D-3 transpressional she ar zones that accommodate regional shortening both by crustal thickeni ng and lateral displacement. The mineralization occurs in associated d ilational jogs or bends that might have formed during the lateral or v ertical expulsion. The Au-hosting shear zones are characterised by ext ensive development of heterogeneous mylonitic fault rocks which reveal s that the accompanied deformation is characterised by processes that can increase the porosity and permeability of the rocks within the she ar zones. This gave rise to further extensive dilatancy within the maj or dilational jogs and produced a suitable structural regime for vein- hosted Au mineralization. This implies that the Au mineralization is e pigenetic in origin and that it resulted from precipitation from metam orphic hydrothermal fluids circulating through major shear zones and a ssociated structures late during the deformation and metamorphic histo ry of the shear zones, through a hydrodynamic process of vein formatio n. The association of Au with low temperature ore minerals like galena gives an independent evidence for the deposition of Au in low tempera ture 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 extens ional zone and rapid subsidence environment in an inter-arc/back-arc s etting, where a thicker amount of volcanic rocks containing trace amou nts of massive sulphide-related Au can best be developed and rapidly b uried. The occurrence of tellurides in association with the Au mineral ization in the Megado Terrane confirms that the mineralization is gene tically related to the subduction event. Copyright (C) 1997 Elsevier S cience Ltd.