Geobotanical, biogeochemical and geochemical studies in the mosaic of savanna types in southern Ngamiland, with special reference to their use in mineral exploration in calcrete and sand covered areas

Abstract

A thesis written in four parts; part 1, the physical background of southern Ngamiland; part 2, geobotanical, geochemical and biogeochemical investigations in the Ngwakio pan area; part 3, geobotanical, geochemical and biogeochemical investigations in the Mawani area; part 4, conclusions and recommendations.

The area of principal study falls between Latitude 20° 30'N, 21° 00 'S and Longitude 22° 10'W, 23° 10'E.

The study in the Ngwako pan area was concentrated in the vicinity of known copper mineralization in Ghanzi series rocks to the north east of older Kgwebe series porphyries, sandstones and diabases.

The results of geobotanical, geochemical and bio-geochemical investigations demonstrated the applicability of these methods to prospecting here.

A close study of plant species distribution showed that Ecbolium lugardae distribution outlines an area within which a soil copper anomaly exists. These geobotanical studies also revealed a close relationship between high copper contents in plant tissue and soil, and the shrubs Anticharis linearis, and to a lesser extent Abutilon fruticosum.

The savanna vegetation of the area comprises a mosaic of vegetation associations, varying in structure and. life form with prevailing climatic and edaphic conditions and with the residual influences of geo-morphology. Distinctive vegetation associations are shown to distinguish various lithological units.

Biogeochemical studies showed a close correlation of high copper values in the soil with high copper content in the plant tissue. A correlation more easily recognised if the copper content in the plant tissue is expressed as number of standard deviations from the mean p.p.m. dry weight.

The biogeochemistry also indicated an area of anomalous copper values in plant tissue outside the geochemical anomaly, and which may be reflecting deeper seated mineralization.

The potential use of geobotanical and biogeochemical techniques in mineral exploration are pointed out.

Geochemical studies showed a dispersion of anomalously high copper values to surface through more than three metres of calcrete overlying minor copper mineralization. The best contrast of anomalous to threshold values was found in the -30 + 60 mesh fraction and in the -120 and finer mesh fractions. The highest absolute contrast was seen in the 270 mesh fraction, the finest fraction analysed.

In the Mawani area a search for kimberlite pipes proved unsuccessful. Distinctive vegetation associations are recognised over the different lithological and pedological units.