The petrography, genesis and associated mineralization of the Erongo granite, Omaruru hills, SWA/Namibia

Abstract

Abstract provided by author

The Omaruru Hills pluton, situated lO km south-west of Omaruru, forms part of the Erongo Complex, one of the many post - Karoo subvolcanic ring complexes in central Namibia. Three granite types have been classified: a coarse-grained granite, a medium-grained granite and a quartz-feldspar porphyry. Textural evidence classifies them as subsolvus granites. Crystallization occurred at shallow depths from melts relatively poor in dissolved water. F and B are therefore thought to be important in the production of the subsolvus mineralogy. The crystallization sequence is as follows: albite - oligoclase - orthoclase - perthite - quartz - biotite. Development of the granophyric texture is suggested to be mainly as a result of K and Na metasomatism

A solid crystalline carapace formed due to the rapid cooling of the quartz-feldspar porphyry and the medium-grained granite. Continuing crystallization of entrapped melt led to the formation of the coarse-grained granite. Two stages of B-rich vapour phases were produced which were later responsible for the formation of tourmaline nests and veins. Boron metasomatism was involved in the reaction of boron with biotite and feldspars to produce tourmaline and quartz

Petrological and geochemical criteria suggest that the tin mineralization is related to Erongo Granite. Borofluoric hydrothermal fluids acted as transporter of Sn and W to the adjacent Krantzberg area