Integrated Geochemical and Geophysical Approach to Mineral Prospecting – A Case Study on the Basement Complex of Ilesa Area, Nigeria - Advances in Data, Methods, Models and Their Applications in Geoscience - page 82

Fig. 6. The geochemical anomalous zone is enclosed by the rectangle with dashed lines on
the geological map (a) and the geophysical traverses and locations on the zone (b).
The real and imaginary data values were first plotted using the Microsoft Excel. Next was
the plot using the KHFFILT programme (Pirttijarvi, 2004) to obtain the Fraser - filtered plot
and the Karous - Hjelt filtered pseudo-section.
The Fraser and Karous - Hjelt filtering are the two methods widely used in processing VLF
-EM data (Fraser, 1969: Karous and Hjelt, 1983). The Fraser filter transforms the zero-
crossing points into positive peaks which indicate conductive structures. The Karous - Hjelt
filter is also used to obtain relative current density pseudo-sections, in which lower values
of relative current density correspond to higher values of resistivity (Benson et al., 1997).
The areas of high current density (represented in red colour) flow correspond to positive
values and low current density (in blue colour) flow to negative values on the
accompanying colour scale (for example see Figure 8).
5.3.1 The four west - east VLF (EM) profiles
The filtered response for the four West - East profiles are presented as in Figure 7 based on
their respective locations in Figure 6 to correlate and describe the fractures across the area
with the geochemical anomaly. The magnitude of the filtered response is varied along the
four profiles due to the nature of the conductivity of the underlying materials. It is clear that
the response along Olorombo to Ibode (7a) differs from those along Gada to Iwikun, Okeipa
to Eyinta and that of Itagunmodi to Aiyetoro which are similar (Figures (7 b- d)).
The positive peaks labeled F1 is seen to occur across the three profiles in Figures 7 b,7c and 7d.
It occurs at about station 1000m along each profile on the Amphibolite. This shows that the
linear feature, interpreted as mineralized fracture is consistent in occurrence in the
Amphibolite. The positive peaks labeled F2 and F3 are observed to cut across the three profiles
also on the Amphibolite as shown in Figures 7 c and 7 d. The linear feature labeled F4 is
observed to be consistent in occurrence in Figures 7c and 7d and actually lie near the
boundaries of the amphibolites and gneiss/migmatite Undifferentiated rocks (Figure 7c) and
amphibolites and quartzite/quartz schist rocks (Figure 7d). The linear feature labeled F4 is not
visible in Figure 7b due largely to the nature of the material hosted by the gneiss/migmatite
Undifferentiated and the schist/epidiorite complex rocks along this profile.