Tourmalinite mineralization in the late Proterozoic Kuiseb Formation of the Damara Orogen, central Namibia; evidence for a replacement origin

Abstract

Stratiform, strata-bound, and crosscutting tourmalinites occur in the Late Proterozoic Kuiseb Formation, a highly deformed and metamorphosed (mid-greenschist to upper amphibolite facies) flysch succession in the inland branch of the Pan-African Damara orogen, Namibia. Tourmalinites in biotite + or - muscovite schists are usually conformable with the most prominent enclosing mica-defined schistosity in central Namibia, but because of poor exposure and the complicated tectonic history of the region, the age of the mica-defined fabric can rarely be determined. The amount of tourmalinite at certain localities in central Namibia is volumetrically equivalent to parts of the Broken Hill district, N.S.W., Australia. Structural analysis of well-exposed, metaturbidite-hosted tourmalinites on the farm Ohere, 30 km northwest of the town of Omaruru, reveals that the stratiform tourmalinite lenses, although up to tens of meters in length, are confined to the hinge of a major F 2 fold. Elsewhere in the study area, crosscutting tourmalinites are oriented parallel to an axial planar S 2 fabric and commonly occupy kink bands. The subvertical S 2 cleavage in the aureole of a granitoid provided a channelway for boron-rich fluids, resulting in the selective replacement of pelitic schist and the formation of crosscutting tourmalinites. Contemporaneous replacement of the metaturbidites along the bedding-parallel S 1 fabric in the hinge zone of the fold led to the formation of stratiform tourmalinites. The tourmalinites contain significant scheelite mineralization (up to 2.59 wt % WO 3 ) and anomalous tin and fluorine contents. No tin- or fluorine-bearing phases (other than tourmaline) have been identified. Tourmalines in the Ohere tourmalinites have chemical compositions (Fe/[Fe + Mg] = 0.28-0.35; FeO/[FeO + MgO + MnO] = 0.41-0.49) that are marginally more dravitic than metamorphogenic tourmalines which are disseminated in the Kuiseb Formation metasediments (Fe/[Fe + Mg] = 0.35-0.47; FeO/[FeO + MgO + MnO] = 0.49-0.62). However, tourmalines from tourmalinites on the adjacent farm vary widely in chemical composition (FeO/[FeO + MgO + MnO] = 0.50-0.78)...