The Congo craton is an assemblage of Archean crustal blocks surrounded by Proterozoic belts. The evolution of the southern Proterozoic margin had direct influence on the development of a large mineral system in the Damara orogen resulting in uranium, tin, and lithium deposits. The southern Central Zone of the ~540-500 Ma Damara orogen host the world-class leucogranite-hosted Roessing and Husab deposits. Uranium mineralisation is associated with sheeted leucogranite intrusions, which Nex et al. (2001) classified into 6 types (A-F) based on field relationships, with types D and E being the most uraniferous type. New interpretation of existing work and new data demonstrate the importance of magma derived hydrothermal fluids in forming uranium mineralisation. The stratigraphy is dominated by metamorphosed Neoproterozoic clastic sediments with inherited magmatic zircon ages of 2 Ga and 1 Ga. Nd isotopy on zircons shows that clastic rocks of the southern Central Zone have undergone a prolonged crustal recycling. Due to the high concentration of radiogenic elements (U, Th, K) in these sedimentary packages, the metamorphic grade reached temperatures of ~800°C and pressures of 4 – 5 kbar resulting in widespread partial melting. Field evidence show that the leucogranite melts preferentially intrude the core of domes (i.e. Ida Dome) and minor anticlinal structures as evident at the Ongolo and MS7 deposits. The majority of leucogranite intrusions are similar in textural style and composition albeit show significant differences in relative ages across the Omahola area. The observed cross cutting nature between different sets of leucogranites and their relationship to its deformed host rocks is a feature of a highly ductile and inhomogeneous host rock that exhibits high competency contrast. Thus, the classification into different types of leucogranites regarding uranium mineralisation is thought to be irrelevant for the study area. Another significant observation is the spatially extensive occurrence (1-10 km’s) of skarns within the Rössing formation marbles. The distal and earlier assemblage is dominated by wollastonite, scapolite and clinopyroxene. The proximal garnet rich skarn is hosted in gneisses of the Khan formation and are spatially associated with uranium mineralisation (i.e MS7, Ongolo, Inca, Garnet Valley). The observations of pegmatite pods within fine-grained granite and quartz veins indicate the significant role of hydrothermal fluids during uranium mineralisation. The reaction of carbonates to various skarn minerals produced porosity within the Rössing formation and thus creating an outlet that focusses the regional fluid flow. The magmatic fluids within the leucogranite melt reacted to that pressure differential by percolating throughout the melt and concentrating uranium towards the Rössing-Khan boundary. At Inca, uranium mineralisation is part of an evolving hydrothermal system that is associated with skarn alteration including hedenbergite, Fe-actinolite, and magnetite. At an even larger scale, the outer most expression of this mineral system is the Shiyela magnetite/hematite deposit ca. 25 km south of the alaskite hosted Ongolo deposit.

Reference(s)

Nex, P. A. M., Kinnaird, J. and Oliver, G. J. H. (2001) Petrology, geochemistry and uranium mineralisation of post-collisional magmatism around Goanikontes, southern Central Zone, Damara Orogen, Namibia. African Earth Sciences, 33, 481-502.