The arenaceous rocks and the pseudo-aplite of the Otavi mountain land, South West Africa

Abstract

Abstract provided by author

The lithological and structural features of the bodies of pseudo-aplite at Tsumeb West and at Kombat are identical with those at the Tsumeb mine, except that elliptical bodies of breccia with a sharp contact are apparently not associated with the intrusions present in the Kombat area

A quantitative and qualitative analysis of the heavy minerals present in the various arenaceous sedimentary beds which is found in the Otavi Mountain Land, shows very little, if any, resemblance to those obtained from the pseudo-aplite. The outstanding feature of the mineral grains obtained from the heavy concentrates of the pseudo-aplite is the very slight or complete lack of rounding exhibited by all these grains, whereas the grains from the sediments are generally well rounded. An analysis, of the X-ray fluorescence patterns, of the zircon -concentrates from the pseudo-aplite and the Mulden and Nosib quartzites, indicates that the zircon in these rocks has similar Zr/Hf ratios but that the yttrium content of the zircon from the sediments is consistently higher than those from the pseudo-aplite. Comparison of the characteristics of shape and elongation of zircon obtained from the pseudo-aplite with those obtained from the arenaceous sediments and from various fine-grained granitic rocks, indicate a closer correspondence between the pseudo-aplite and the latter, than between the pseudo-aplite and the sediments. The above features, as well as the very restricted suite of heavy minerals present in the psuedo-aplite, points to this rock being of, magmatic rather than of sedimentary origin

The origin of the pseudo-aplite is discussed at length, and evidence in favour of a sedimentary as well as an igneous origin is adduced. As the various structural and lithological features which are displayed by the bodies of pseudo-aplite cannot be explained on the assumption that it is a sediment, the conclusion is drawn that the pseudo-aplite is a fine-grained igneous rock of granitic composition. The term "pseudo-aplite" used to describe the intrusive-bodies of the Otavi Mountains is therefore incorrect, and should preferably be replaced by "aplogranite"