Remobilisation and primary uranium genesis in the Damaran Orogenic Belt, Namibia

Abstract

Abstract provided by author:

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 U 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