Uraniferous leucogranites south of Ida Dome, central Damara Belt, Namibia: Morphology, distribution and mineralisation

摘要

Uranium deposits south of Ida Dome, Namibia, are hosted by networks of leucogranite dykes crosscutting lithostratigraphic units of the Damara Sequence. Examples are described that show a strong spatial relationship between uraniferous leucogranite and iron sulphide-rich schist containing pyrite, pyrrhotite, chalcopyrite and lesser bornite and molybdenite, situated between the Khan and Rossing or Karibib formations. Significant uranium deposition (above 400 ppm eU_3O_8) has occurred where this schist is thickest, e.g., in fold hinges, and where it has been infiltrated by large accumulations of leucogranite against a thick marble barrier. The marble played a mechanically important role by inhibiting magma migration, leading to ponding and increasing the degree of fluid-wallrock interaction. In this situation, the schist with high quantities of ferrous iron and reduced sulphides (pyrite, pyrrhotite and chalcopyrite) acted as an effective chemical trap for uranium transported in magmatic fluids. Recognition that the schist had a folded boudinage structure prior to leucogranite emplacement can help explain the partly strat-abound, yet highly irregular and pod-like nature of the orebodies.