地理情報学と地球統計学: 概要

Statistical Studies of Soil Geochemical Data from Gbongan-Odeyinka Area, Southwestern Nigeria

Adewale Adesiyan, Adeyinka Adekoya, Akin Akinlua and Nelson Torto

Statistical Studies of Soil Geochemical Data from Gbongan-Odeyinka Area, Southwestern Nigeria

In order to follow up some geochemical anomalies discovered in a regional stream sediment survey in parts of southwestern Nigeria, a medium density geochemical soil survey was carried out in Gbongan-Odeyinka area of Osun State, southwestern Nigeria. As a result, soil geochemical data on 13 elements (Ag, As, Au, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Sn, and Zn) obtained from 186 samples from the study area were subjected to statistical analysis involving frequency distribution, Pearson correlation and R-mode varimax rotated factor analysis techniques. The frequency distribution plots revealed that all the elements investigated except Ag and Fe, which display approximately normal distribution, are positively skewed with Au and Zn being the strongest. The Pearson correlation study and factor analysis enabled separation of the 13 elements into the following three factors or metal associations: 1) Pb-Co-Ni-Fe-Cr- As-Mn-Cu; 2) Sn-Ag-Au-Cd; and 3) Zn-Cd, which are interpreted to reflect the influence of both environmental and mineralization factors on the soil of the study area. Of the three associations only the Sn-Ag-Au-Cd is considered related to metallic mineralization in the study area based on the existing knowledge of Au and Sn mineralization in the adjacent Ife-Ilesa area. The gold is believed to be hosted by schists and the tin by pegmatites. The other metal associations are interpreted in terms of environmental factors. On the basis of the presence of Fe and Mn in the Pb-Co-Ni-Fe-Cr-As- Mn-Cu association, it is inferred that the scavenging action of iron and manganese oxides was at least partly responsible for forming such trace elements association in the soil. In the case of the Zn-Cd association, both metals are considered to be anthropogenic, being possibly introduced to the soil through the application of pesticides on cocoa and kolanut plantations of the study area, although the possibility of Zn being derived from other sources cannot be ruled out.