Remobilization and primary uranium genesis in the Damaran orogenic belt

The Damaran belt in Namibia represents a highly eroded orogenic area, the root zone of which is exposed between Swakopmund on the coast and Karibib in the west. Primary uranium deposits located in these structurally lower regions of the orogen, have been studied in an attempt to assess the importance of geological processes associated with remobilization in the genesis of the uraniferous Damaran granitoids. The pre-Damaran basement consists of a sequence of up to 1600m of shallow water clastic and carbonate rocks with interbedded metabasaltic and pyroclastic horizons. The 'meta-basalts' range in composition from basalt to andesite, and are characterized by chemical features diagnostic of a tholeiitic, and in some cases komatiitic affinity. This sequence, previously assigned to the Abbabis Formation, is intruded locally by 2.0 Ga gneisses which range in composition from diorites, through granodiorites to alkali-rich granites. Pb-isotope ratios of sulphides from the basement and the Damaran metasedimentary cover indicate that the basement was enriched in U/Pb at 1.7 Ga, and that the Damaran uranium province has been in existence for at least 1.7 Ga. The Damara Sequence consists of a lower Nosib Group and an upper Swakop Group. The former consists of fluviatile clastics (Etusis Formation), followed by shallow-water calcareous, feldspathic sandstones (Khan Formation). Uranium minerals, in the form of uranyl silicate inclusions, found within detrital constituents of the Etusis quartzites, provide direct evidence of radioactive material within the pre-Damaran basement prior to its erosion, transportation and deposition within the ii. Damara Sequence. The Swakop Group represents a typical geosynclinals sequence starting with carbonates, quartzites, conglomerates and pelites (Rossing Formation); followed by mixtites (Chuos Formation)and a dominantly calcareous succession (Karibib Formation); and ending with a monotonous series of biotite schists (Kuiseb Formation). Primary uranium minerals including uraninite and betafite located within schists and calc silicates of the Rossing Formation are considered to have recrystallized during the Damaran metamorphism from syngenetic uranium associated with stratabound copper deposits. The Damaran belt underwent regional deformation between about 650 and 550 Ma, and although early mafic granitoids were emplaced locally, the major granite forming event post-dates the major regional deformation. Between 540 and 46O Ma dome structures were developed by a process involving diapirism and the upward movement and subsequent ballooning of large volumes of granitic material. The Damaran granitoids may be broadly divided into syn- to post-tectonic Salem type granites and red granites, and late- to post-tectonic leucogranites and alaskites. Field relationships indicate that the Salem type granites are derived from a source deeper than the Damara Sequence, whilst the alaskites appear to be derived from migmatized basement and Damaran metasedimentary cover rocks. The earlier Damaran granitoids, which tend to be more mafic in character, show relatively low Sr-isotope ratios and contain chemical and mineralogical features in common with I-type granitoids. These various factors are considered to reflect derivation from the lower crust or upper mantle. In contrast the later granitoids are normally leucocratic in nature, commonly radioactive and occasionally mineralized. They display relatively high Sr-isotope ratios, and an affinity with S-type granitoids, and they are considered to be derived from basement and Nosib source rocks. The mineralized alaskites contain primary uranium minerals including uraninite, betafite and metamict thorite which crystallized from melts enriched in IJ and Th. Primary mineralizations are not restricted to alaskites, but also occur within red granites.Secondary uranium minerals within the mineralized Damaran granitoids include uranyl silicate, thorogummite, calciothorite, ferrothorite and uranophane. These minerals formed during a deuteric stage of alteration, and also during a recent stage of surface enrichment and secondary alteration.