A review of geological controls on uranium mineralisation in sheeted leucogranites within the Damara Orogen, Namibia

Abstract

Uranium mineralisation is concentrated in alaskite sheets in the southern Central Zone of the Damara Orogen in Namibia; at Rössing, Valencia, the Ida Dome and Goanikontes. The alaskites in this area range in composition from tonalite to alkali-feldspar granite. There are six types (A–F): early pre-D3 types (A–C) are barren while post-D3 alaskites (D–F), which are more prospective for uranium, were emplaced post-D3 extensional fabric development and dome formation at upper-amphibolite facies. Alaskites form cross-cutting irregular, anastomosing dykes and concordant bodies generally following the trend of the gneissosity in the Damaran sediments. They are highly variable in texture, consisting essentially of quartz and feldspar with <5% mafic minerals, and range from a few centimetres in width to >100 m across. Uraninite, 30–50 μm in size, is the predominant primary mineral although locally betafite, a pyrochlore-group mineral with up to 30% UO2, is the major phase. Secondary minerals are reportedly dominated by betauranophane. There is a stratigraphical control on the location of the mineralised alaskites close to the Khan–Rössing Formation boundary or to the Khan–Chuos Formation boundary where the Rössing Formation is absent or highly attenuated. Structural controls on emplacement appear to be related to the transition from ductile to brittle deformation. There is no clear geochemical control or thermometric fluid inclusion difference between mineralised and barren sheets. However, fluid extraction analyses show that for Rössing, where the proportion of secondary minerals is higher than elsewhere, there is a greater amount of total trapped fluids (typically 20–40 μmole g−1), that are dominated by water and low CO2/non-condensibles. In contrast the fluid content in betafite-bearing samples from the SH area was consistently low, typically 3–5 μ mole g−1 with lower concentrations of NC phases and much higher CO2/H2O ratios (>3). Deposition of uranium is controlled by changes in redox conditions at the Khan–Rössing Formation boundary.