Follow-up soil survey to evaluate regional Pb and Zn soil anomalies in Modimolle, Limpopo Province, South Africa

Abstract

The Bushveld Complex was sampled in 1989 as part of the Council for Geoscience (CGS) national geochemical mapping programme. This programme was conducted in order to produce geochemical data to update the geological maps, undertake mineral exploration and determine geochemical baseline concentrations. About 5kg of surficial soil samples were collected using a sample density of one sample per 1 km2. Soil samples were collected by means of a helicopter and analysed by a simultaneous wavelength-dispersive X-ray fluorescence spectrometer for TiO2, MnO, Fe2O3T, Sc, V, Cr, Co, Ni, Cu, Zn, As, Rb, Sr, Y, Nb, Zr, Ba, W, Pb, Th and U. These results produced elevated concentrations of several elements and this report focuses on Pb and Zn, which showed elevated concentrations of up to 429 ppm and 1 064 ppm, respectively. Most of these elevated concentrations were localised in areas such as Modimolle, Thabazimbi, Mabopane, Pretoria and Groblersdal among others. A further follow-up survey was conducted north of Modimolle area to verify these elevated Pb and Zn concentrations, determine whether they indicate potential mineralisation and generate target areas of potential mineralisation. Soil samples were collected at 200 m intervals with a depth of 10–60 cm, prepared and analysed for SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, P2O5, As, Ba, Ce, Co, Cr, Cu, Ga, Hf, Mo, Nb, Nd, Ni, Pb, Rb, S, Sb, Sc, Sn, Sr, Ta, Th, U, V, W, Y, Zn, and Zr by X-ray Fluorescence Spectrometry. The results showed Pb and Zn ranging between 16–1043 ppm and 28–4564 ppm, respectively, and average concentrations (i.e. 179 ppm and 284 ppm, respectively), which are significantly higher than the regional rock averages of 11 ppm Pb,147 ppm Zn and regional soil averages of 10 ppm Pb and 50 ppm Zn. About 65% of Zn concentrations lie in the background range of 0–300 ppm and 35% within the high concentration range of 300–4650 ppm. Lead showed 11.5% of concentrations in the background range of 0–100 ppm and 88.5% within the high concentration range of 100–1100 ppm. High Pb and Zn concentrations (i.e. Pb>100 ppm and Zn>300 ppm) coupled with high variance values (Zn = 30613 and Pb = 9186), high maximum concentrations (Zn= 4564 ppm and Pb = 1043 ppm), high mean concentrations (Zn = 284 ppm and Pb = 179 ppm) and positive skewness of Pb and Zn, statistically reflected anomalous characteristics indicating potential mineralisation. Multivariate analysis produced element associations of Zn-Pb-Cu-Mn which characterises the potential mineralisation of the area. Spatial distribution of Zn, Pb, Cu and Mnanomalies produced target zones mainly along the Swaershoek anticline, indicated a possible structural controlon the anomalies. The potential type of mineralisation associated with these Zn-Pb-Cu-Mn anomalies is characterised by Zn-Pb-Cu-Ag (veins, fissure lodes, polymetallic sulphide and Irish style mineralisation), and the potential ore-group is characterised by element assemblages of Zn-Pb-Cu/Zn-Cu-Pb, Pb-Zn-Cu/Pb-Cu-Zn and Cu-Pb-Zn which indicate an ore group rich in Zn, Pb and Cu.