The mid-proterozoic Mporokoso basin, Northern Zambia: Sequence stratigraphy, tectonic setting and potential for gold and uranium mineralisation

Abstract

The Mporokoso Basin was formed on a dying silicic magmatic arc between 1800 and 1100 Ma. The basin consists of up to 5000 m of fluvial and shallow marine siliciclastic sediments. The stratigraphy and tectonic setting resembles that of mid-Proterozoic cratonic basins elsewhere. The Mporokoso Basin is the result of crustal extension, rather than compression. The gravitational collapse of crust thickened by tectonic and magmatic processes may have caused the extension and subsidence in this and other mid-Proterozoic basins. Fluvial sediments in the lower part of the basin fill are known to contain traces of gold; analysis has shown up to 1.3 ppm gold in conglomerates. Gold concentrations are most likely to occur along subtle unconformities which form the boundaries of depositional sequences. These sequence boundaries can be identified through sedimentary facies relationships. The underlying silicic igneous rocks are unlikely to have been the source of the gold. The Lupa goldfields of Tanzania lie to the east of the Mporokoso Basin and are the nearest probable source. However, only the lowermost sequence in the basin was derived from the east. The basement and sediments of the Mporokoso Basin are enriched in uranium. The critical factor missing for an uranium deposit of the unconformity type is a reducing lithology. The only known possible reductants are carbonaceous mudstone within the basin and graphite in the basement 200 km away.