Mineral composition and reconstruction of the source areas of manganese-bearing alluvial deposits in the Ventersdorp area, South Africa

Abstract

The paper presents the results obtained from petrographic and lithogeochemical studies of Tertiary alluvial deposits in the Buchansvale 61 IQ farm around the Ventersdorp region. These are unconsolidated manganese-bearing sediments of alluvial origin, currently making up the West-wits alluvium which overlies the manganiferous dolostones of the Transvaal Basin in a weathering crust. Mineralogically, the alluvial deposits are composed mainly of detrital quartz, feldspars, and grains of zircon and traces of ilmenite. Authigenic mineral phases are predominantly composed of clays and iron oxides, cryptomelane (KMn8O16), hollandite/romanechite (BaMn8O16/10), spinel-galaxite (Mn2+Al2O4) and other manganese oxides/oxyhydroxides. Quartz modal content makes up an average of 70% of all detrital framework grains. The content of feldspar, cement minerals and clays seldom exceeds 20%. An extensive diagenetic replacement of detrital grains and early formed authigenic mineral and cements by Mn oxides is evident. The study applied the method of modal composition analytical study to count the percent ratios of framework grains (grains of quartz, feldspar, pyroxene, amphibole, mica, etc.) in the samples in order to reconstruct their source regions. According to the observations made from the data interpretation, the probable sources of the sediments were inferred from old recycled orogens and cratonic interiors. It was further noticed that the sediments are characterized by arkoses to subarkoses and subordinate quartz arenite. These observations indicate the near source regions, most probably a combination of Pretoria Group sediments and Black Reef Quartzite. Sedimentation occurred in a low-lying plain characterized by tropical-moderately humid and subhumid climates. Here, in combination with increased pH of the basinal waters which is attributable to surficial weathering of the Malmani Dolostones and intensive oxidation, a replacement of detrital grains with manganese dioxide precipitates became prominent.