Distribution patterns of trace metals in soils of Gbonganorileowu Area, southwestern Nigeria

Abstract

One hundred and eighty six (186) in situ soil samples of the B - horizon were collected around Gbongan- Orileowu area of Osun State, Southwestern Nigeria for a reconnaissance geochemical soil survey. The objective was to determine the distribution pattern of some trace metals in the area with a view to discovering any
indication of metallic mineralization and possible correlation with the underlying bedrock. The samples were air-dried and sieved to obtain the minus 230 micron fractions which were split into two portions for hot aqua regia acid digestion and pH determinations. The solutions obtained from the digestion were analyzed for Ag, As, Au, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Sn, and Zn using Atomic Absorption Spectrometric methods. The resulting data were used in preparing distribution maps of the elements in the study area, employing the location point symbol method. Simple statistical treatment was also carried out on the data to determine the mean and standard deviation for each element. With these, the spatial distributions of the trace elements in the area were discussed. The pH of the soils range from 4.70 to 7.59 and this reflects the geochemical distribution patterns of trace metals in the area. The spatial distribution maps of the elements showed similarity in the distributions of Co, Cr, Fe, Ni, Mn and Pb. This suggests close associations between these elements none of which showed any significant anomaly. The association might be due to the scavenging actions of Fe-Mn oxides on the other elements. Ag, Au and Sn on one hand and Zn on the other hand showed significant anomalies, which were thought to be due to mineralization and anthropogenic activities respectively. As, Co, Cu, Fe, Ni and Pb probably occur as sulphides within reported underlying ultramafic rocks or Au bearing quartz veins or both. It is probable that Au and Ag occur as disseminations within the ultramafic rocks, aside from their occurrence in the veins, while Sn is most probably from the pegmatites. The very high Zn in the area is probably due to contamination.

Keywords: Significant anomalies, Ultramafic, Anthropogenic, Mineralization