Auas Mountains

In the Auas Mountains south of Windhoek numerous occurrences of alkaline rocks are scattered over an area of about 25x20 km. Gevers (1934) identified several large vents, including Regenstein, 44 trachyte and breccia plugs, 35 trachyte dykes and sills and 13 sills and several dykes of phonolite. Rock types encountered include phonolite and tinguaite, analcimite, shonkinite, limburgite, alkali peridotite, biotite and aegirine trachyte and a range of tuffs, agglomerates and breccias. The largest occurrence is the Regenstein Vent which is 1.8x1.2 km with a small satellite vent to the north of it (Ferreira et al.,1979). The bulk of the vent is occupied by breccias of two types, one consisting essentially of angular blocks from 2 mm to 0.5 m in diameter of metamorphic rocks derived from the country rocks, and the second containing mainly trachyte and phonolite fragments. The matrix is a mixture of clay minerals and carbonate. The breccias are cut by numerous deeply weathered phonolite dykes. A prominent hill in the northwestern part of the vent marks a plug of alkaline rocks of 250x200 m with a smaller one east of it, while several other small bodies were encountered in boreholes (Ferreira et al., 1979). These rocks are highly heterogeneous with areas (?xenoliths) of medium-grained ijolite enclosed in a fine-grained porphyritic rock. The ijolite consists of up to 60% green clinopyroxene, sparse altered olivine, nepheline, analcime, nosean and hauyne; garnet and biotite are minor. The 'host' rock contains phenocrysts of diopsidic augite, altered olivine and in some varieties biotite, in a matrix of apatite, opaque minerals and tentatively identified clinopyroxene, nepheline and alkali feldspar. Other xenoliths in these rocks include wehrlite, pyroxenite, gabbro, syenite and anorthosite. In the southern part of the area around Aris two intrusions, which are expressed as hills, are composed of phonolite (Gevers, 1934). It contains phenocrysts of aegirine, nepheline and sanidine, while apparently leucite and hauyne have been identified in a few specimens, and varieties with a brown amphibole occur in other intrusions. Phonolite from the Aris quarries has been described by von Knorring and Franke (1987) who noted the presence of monazite, apophyllite and sphalerite while in veinlets eudialyte and makatite are encountered. Biotite trachytes are widespread, particularly in the northwestern part of the area (Gevers, 1934), some varieties containing aegirine. Detailed petrography of some of these rocks is given in a number of papers cited by Gevers (1934).