GEOLOGICAL AND GEOCHEMICAL EXPLORATION STUDIES OF DAGBALA-ATTE DISTRICT IN IGARRA SCHIST BELT, SOUTHWESTERN NIGERIA

Abstract

Anomalous concentrations of metals in systematically collected stream sediment samples can often be indicative of mineralization, assuming there is no source of contamination. An integrated geological and geochemical exploration study of Dagbala-Atte District was carried out with a view to investigating possible sources of anomalous concentrations of some metallic elements found in a previous reconnaissance geochemical stream sediment survey. Geological study involved lineament extraction from Landsat-8 imagery, geological mapping on a scale of 1:25,000 and petrographic study of rocks. Geochemical study comprised systematic geochemical sampling of soils and bedrocks, trace-element analysis of 49 B-horizon soil samples and forty-five rock chip samples using ICP-MS method after modified aqua-regia digestion and data interpretation using histograms, dispersion box plots, correlation and factor matrices, soil geochemical maps and line plots. Regional N-S, NE-SW and NW-SE trending major lineaments characteristic of the Basement Complex of Nigeria were observed with a few minor E-W trending structural features. Field and Petrographic studies showed that granitic gneiss, quartz biotite schist, garnet biotite schist and marble are the main rock types in the District. Four small isolated bodies of ore-bearing silicified sheared rock constitute minor rock. Weathering of these basement rocks resulted in the formation of duricrust profile consisting of top-soil, laterite, mottled zone and saprolite. Quartz, plagioclase feldspars, microcline, and biotite are the essential minerals in the rocks. Recrystallized quartz, altered plagioclase, perthite, sericite, myrmekite, and actinolite are some of the alteration minerals identified in the silicified sheared rock and granitic gneiss. The metallic minerals present in the silicified sheared rock are pyrite, chalcopyrite and inclusions of gold in the chalcopyrite. The trace elements in the soils of the District are log-normally distributed with some revealing mixture of populations. At 95 % confidence level and above, the best correlations (r ≥ 0.75) are between each of the following pairs of elements Fe-Cu, Mo-Pb and U-Th, very good correlation (0.65 ≤ r < 0.75) exists between each of Ag-Zn, Cu-Cr and Mo-Th, good correlation (0.55 ≤ r < 0.65) occurs between each of Ag-Pb, As-Cr, Co-Sr, Cr-Cu and Cu-Zn, and fairly good correlation (0.45 ≤ r < 0.55) exist between Au-Bi. Four factor models similar to correlation groups were variously interpreted to reflect possible U-Th-Mo-Hg-La-Pb mineralization; polymetallic sulphide mineralization with which minor gold is associated; coprecipitation or adsorption of Ba, Co, Ga, Ni, Sr and Tl with Fe and Mn oxides; and Ag-Pb-Sb-Zn mineralization. Common strong anomalous localities for Au, Bi, Cr, Cu and Hg lie in the northern part of the District underlain by the northernmost body of the silicified sheared rock. The common strong anomalous areas for U, Th and La exist in the northeastern and eastern part underlain by the granitic gneiss. The anomalous areas for Co, Ni, Sb, Tl, V and Zn fall around the southwestern part underlain by the schists. The trace elements in the rocks of the District are log-normally distributed and positively skewed. Elements such as Ag, As Ba, Bi, Cd, Cr, La, Mn, Mo, Pb, Se, Sr, Th, U and W have mixed populations. Pearson correlation analysis for the rocks reveal that the best correlations are between Co-Cr, Fe-Ga, Co-Zn, Th-La, U-Tl and Fe-Ni; very good correlation exists between V-Zn, Fe-Sc, La-Th, Ga-Mn and Ni-Zn; good correlation occurs between each of Ag-Pb, Bi-Cu, Ba-Ga and Mn-Sc; and fairly good correlation exist between Ag-Au, Au-Bi, Ni-W, U-Th and Ag-Pb. Three factor models obtained have been interpreted to indicate the nature of the dominant lithologies of schist and granitic gneiss; the presence of sulphide mineralization in the silicified sheared rock and La- Th mineralization in the granitic gneiss. There is a general depletion of Ba, Cd, Co, Cr, Fe, Ga, Ni, Sb, Sc and W in all the rock types. There is also an enrichment of Ag, As, Au, Bi, Cu, Mo and Pb in the silicified sheared rock, La, Mo, Th and U in the granite gneiss; and Tl, V and Zn in the schists. The sources of anomalous concentrations of some metals are suspected to be possible metallic mineralization types and high original contents of some trace elements in the rocks that underlie the District.