A stream sediment geochemical survey of the area around Acturus Mine, Zimbabwe

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Tembo, Francis
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This study illustrates the use of stream sediment in mineral exploration in a semi-arid environm.ent area in central Zimbabwe. The distribution of As, Cu, Zn, Fe and Mn in different size fractions of both sorted and ur.sorted stream sediments were investigated and compared to bedrock and soil contents of the same elements-Also determined were the -relative proportions of sediment in five size fractions (-20 + 40 mesh, -40 + 80 mesh, -80 + 120 mesh, -120 + 200 mesh and -200 mesh) in the two types of stream sediment samples and the /v^senic content of a stream draining some mine dumps. The geology consists of Archaean basaltic greenstones, amphibolites, ultramafic rocks, sulphide facies iron-formations with minor limestones and nhyllites. Gold mineralization occurs in the investigated area.The -80 + 120 mesh size fraction is the finest fraction offering enough material for analysis and consistent analytical results. This has been attributed to a deep weathering profile in the study area leading to the development of a lateritic horizon, hence the olemonts are concentrated in the coarse resistant iron-manganese nodules.The -120 + 200 mesh chiefly consists of quartz fragments and therefore only acts as a diluent in the chemical analysis. Unsorted stream sediment sample material approximates the bedrock composition nx3re closely than sorted sediment material and also occurs in adequate amounts for analysis.All the elements, Cu, Zn, As, Fe and Mn are enhanced in the stream sediment relative to bedrock owing to surficial weathering processes and physico-chemical conditions in the environment.Both sorted and unsorted sediment exhibit a lognormal distribution of particle size with two populations,the boundary being at the -120 + 200 size fraction. The same general trend is observed in the element content of the different size fractions. The decrease in -it-concentration at -120 + 200 mesh is a result of dilution. In general the fine fraction of the sediment increases downstream. This is accompanied by a decrease in the contents of Cu, and Zn.Arsenic increases downstream because it is more strongly adsorbed by iron-manganese oxides. Arsenic concentrations in the contaminated drainage are lowest in the active channel owing to low pH.The combined sampling and analytical errors for Cu, Zn and As were found to be in the range ± 40%, ± 61% and ± 53% respectively.
Geochemistry -- Zimbabwe , Mineral science