Geology, mineralogy, geochemistry and hydrothermal alteration of the Brandberg Alkaline Complex, Namibia

Abstract

Abstract provided by author:

The Brandberg Complex represents an anorogenic ring complex of the intra-plate type emplaced during Jurassic-Cretaceous times at a high level in the crust. At the present level of erosion the complex exhibits a number of subvolcanic, magmatic centres which are entirely granitic in composition. Metaluminous granites and quartz-monzonitic rocks were emplaced as cone-sheets outside the later caldera or as thick, ring-shaped sheets. Peraluminous granites occupy the central part of the complex and are followed by peripheral peralkaline granites. All rock types associated with the Brandberg Complex carry significant mineralogical and geochemical features of HHP, A-type granites with anomalous high levels for HFS- and LIL-elements

Several types of post-magmatic, hydrothermal alteration processes were recognised which are similar to those associated with anorogenic complexes in Nigeria. Depending on the initial chemical composition of the crystallising granite and its metasomatising hydrothermal fluids, processes of potassium metasomatism, sodium metasomatism, greisen-type alteration, tourmalinisation and chloritisation have been recognised. Cationic exchange processes led to the breakdown of the primary mineral assemblages in the granite and the generation of hydrothermal mineral parageneses, locally associated with deposition of Zn, Sn, Nb, Y and REE ore minerals. The spectrum of Subsolidus minerals in these assemblages is characterised by the occurrence of albite (An 0. 6-3. 2), reddened maximum microcline, the destabilisation of early crystallising biotite to micas of the siderophyllite-zinnwaldite series and, locally, concentrations of tourmaline. Fenitised granites carry hydrothermal zincian fluor-arfvedsonite and stanniferous aegirine as newly generated minerals and occur in layered, agpaitic rock series of the Amis Complex, a mineral stratified satellite intrusion in the southwestern periphery of Brandberg

The following types and styles of mineralisation were identified: * Dispersed Nb, Ta, REE-mineralisation; * Disseminated Sn-mineralisation; * Fissure and vein controlled suphide/oxide (Zn/Sn-mineralisation)

Mineralisation in the Brandberg Complex is generally limited to zones where post-magmatic rock-fluid interaction processes acted long and were intense enough to allow the generation of ore minerals. Alteration zones occur in the roof zones of the complex, along the contacts and margins of individual intrusions, and in adjacent country rock