A study of the pseudo-aplite of the Otavi mountains and the rock alteration in the adjoining dolomite

Abstract

Abstract provided by author:

The texture of the pseudo-aplite is considered to be essentially sedimentary, and as such militates strongly against any form of mobilisation or anatexis having occurred which would, upon reconsolidation, have been reflected in the fabric of the rock. The absence of any chilled or [?] -grained selvages about the periphery of the main bodies, or in the intrusive veins, or apophyses, affords further support for this view. Distinctly waterworn pebbbles of quartzite, argillaceous sandstone and leucogranite were found in the pseudo-aplite at Tsumeb West, and are considered to be of allogenic origin and a criterion favouring sedimentary origin

The absence of any source-magma in the near proximity which could be construed as an earlier phase of the pseudo-aplite, coupled with the lack of associated pegmatities, is not in accord with established geological occurrence of true eruptive aplite bodies. Furthermore, comparison of the chemical analyses of the pseudo-aplite with Vogt's (1930:47) range of aplite analyses indicates that the former are approximately 2 percent too high in silica, 5 percent deficient in alkalies and 6 percent deficient in alumina

The conclusion is reached that the pseudo-aplite bodies were formed by the emplacement of undoubted arenaceous sediments into the carbonate-rocks of the Otavi system. Speculation concerning the mode of emplacement is treated at length in an effort to reconcile abstract geological principles with positive field observations. A subaqueous fillling of sink-holes and solution-channels with clastic arenaceous material from above, is favoured in preference to some form of sediment mobilisation in depth followed by upward injection into higher-lying strata. The undoubtedly intrusive veins of pseudo-aplite which reticulate from the main body at Tsumeb West into the surrounding country rock were probably occasioned by gentle flexuring of strata after the sink-hole was filled with water-saturated sediment. Alternately, or in combination with the former view, the hydrostatic head of the mobile sand-filling may have been sufficient to result in a gradual walling of such material into pre-existing fractures surrounding the sink-hole. It is to be emphasized that the pseudo-aplite occurring in vein form constitues only a small fraction of a percent of that forming the central plug-like mass

The pseudo-aplite as such cannot be the source of the metallisation in the Tsumeb Mine but has actually been replaced, together with surrounding dolomites and limestones, to a greater or lesser degree by later hydrothermal emanations. The close association of the pseudo-aplite and hysrothermal replacement products is not due to any genetic relationship, but is resultant upon the determination of the loci of deposition of both along earlier formed zones of intense structural deformation. The presence of pseudo-aplite bodies is held to be indicative of the existence of such deformed zones which are unuasually susceptible to hydrothermal replacement and metallisation, and thus indicate geographically where structural conditions favourable to the deposition of economic minerals can be expected to occur