Structural setting of Neoproterozoic mineralization, Asmara district, Eritrea

Late Neoproterozoic collision between East and West Gondwana concentrated transpressional deformation in the juvenile crust of the Nubian Shield in Eritrea along at least two steep, curvilinear crustal-scale belts, the Augaro-Adobha Belt (AAB) and the Asmara-Nakfa Belt (ANB). Volcanosedimentary rocks dominantly metamorphosed at greenschist-facies conditions characterize the belts. Each of these belts comprises a complex network of syn-metamorphic shear-fold structures. Steep strike-slip shear zones and accompanying vertical to steeply plunging folds dominated the latest phase of deformation. Quartz vein-hosted gold ± sulphide type and volcanic-hosted massive sulphide type deposits and occurrences are either deformed or hosted by these steep shear zones and folds. The deposits are broadly grouped into three major mineral districts, Asmara, Augaro and Bisha. The Asmara district, the main focus of this study, is located where the southern part of the Asmara-Nakfa Belt changes in strike from NNE–SSW to NNW–SSE. Combined field, micro-structural, and magnetic fabric studies are conducted in the sheared host rocks of a series of the mineral deposits and/or occurrences of the Asmara mineral district. These combined studies revealed that the Asmara area was subjected to a transpressional deformation accommodated in a complex and curved flower structure. Both the quartz vein and massive sulphide types of deposits are sheared, folded and generally spatially associated. The ore-bearing quartz veins are often concentrated along dilatant-extensional en-echelon fracture arrays in reverse and normal sense shear zones, and they either cut through or structurally overlie, the massive sulphide deposits. The massive sulphides that formed at the same time as the Neoproterozoic volcanosedimentary rocks were later deformed and metamorphosed with them. This study, along with previous investigations, further implies that the Asmara area represents an intra-arc, palaeo-oceanic trough or basin located over a west-northwestward dipping subduction zone that subsequently underwent transpression. The transpressional belts track the general locations of such oceanic basins into which ore-bearing fluids that resulted in various phases of vein type deposits were channeled. This study can help to locate new prospects and develop existing ore deposits and/or occurrences in Neoproterozoic Eritrea and elsewhere in areas of similar structural setting.