Geobotany, biogeochemistry and geochemistry in exploration for stratiform copper deposits in low tree and shrub savanna in central South West Africa

Abstract

Stratiform copper mineralization was discovered in the Witvlei Area of South West Africa, within the Precambrian meta-sedimentary Tsumis Formation.

The region has a semi-arid climate and the vegetation is characterised by a low tree and shrub savanna. On a regional scale the classification and species composition of the vegetation is related to the depth and nature of the overburden. The three environments of near surface bedrock, sand covered and calcrete covered are recognised.

Geobotanical, biogeochenical and geochemical studies were carried out in areas of known mineralization in the various environments to determine the most effective use of these techniques in an integrated exploration program.

In the selected areas the geology and soil profiles are recorded from trench sections and wagon drilling, and the copper content of mineralized bedrock and soils determined. The distribution of plant species is recorded along continuous belt transects parallel to the trench sections so that a direct comparison of the geology, geochemistry and geobotany can be made. Geological and geobotanical mapping and geochemical soil surveys were extended over the areas of interest.

Plant samples of leaves, twigs and grasses were collected from the more common species along the transects for copper analyses. Investigation of the rooting habits and seasonal variation in copper content of selected tree and shrub species was also made.

In areas of near surface bedrock the horizons of copper mineralization are most clearly defined by anomalous soil values in the range 200--500 ppm., for the minus 80 mesh, and an anomalous vegetation zone dominated by the species Helichrysum leptolepis and Fimbristylis exilis, with the associated grass species Aristida congesta, Eragrostis denudata and Anthephora pubescence. Trees and shrubs and the common grass species of the area, Stipagrostis uniplumis, do not occur within this anomalous vegetation zone.

Leaf and twig samples from several species of trees and shrubs show a high copper content in areas of mineralization. The species Grewia flava and Phaeoptilon spinosum proved the most successful in the detection of biogeochemical anomalies because of their more regular distribution and more consistent values.

Areas suitable for detailed orientation studies could not be located in the regions of deeper overburden of sand and calcrete. No geobotanical indicators of mineralization were found in these areas and the use of biogeochemical techniques is very much restricted by the sporadic distribution of the deeper rooted species.