The Lithology And Soil Geochemistry Of Ntumkumso And Its Surrounding Areas In The Ashanti Region Of Ghana

ABSTRACT

Rocks and soil covering approximately 81km2 in the Ntunkumso area in the Ashanti Region of Ghana were investigated in the present study. Petrographic studies of eighteen (18) rock samples collected suggest that the rocks are predominantly biotite schist, diorite, granodiorite, and granite. The biotite schist outcrops between Ntunkumso and Boiya and is made up of biotite, quartz, feldspar, iron oxides and opaque minerals. Diorite was encountered at Abesewase and Ofoase and is mineralogically composed of hornblende, plagioclase, microcline, biotite, quartz, iron oxides and opaque minerals. Granodiorite outcrops between Kokodie and Odoyefe. Major minerals in the rock are plagioclase, microcline, muscovite, quartz, garnet and opaque minerals. Granite in the study area outcrops at Apemso and Nkyerepoaso and consists of plagioclase, microcline, quartz, biotite, muscovite and opaque minerals.

Whole rock geochemical analysis of thirteen (13) representative rock samples using ICP-MS indicates that, SiO2 concentration in the biotite schist ranges from 71.14 to 73.17 wt%, Al2O3 also varies from 14.66 to 15.39 wt%, Fe2O3 4.45 to 5.40 wt% MgO 2.57 to 3.56 wt% .The contents of some minor elements are as follows; Cu 1.20 to 21.50 ppm, Ni 86.90 to 102.50 ppm. SiO2 in diorite varies from 59.87 to 61.64 wt%, Fe2O3 4.89 to 9.47 wt%, K2O 0 .53 to 1.49 wt%, TiO2 0.47 to 1.17 wt% and MgO 3.37 to 11.92 wt%. Cu and Ni concerntration range from 11.20 to 60.70 ppm, Ni 44.40 to 300.00 ppm. In granodiorite, SiO2 content ranges from 68.80 to 76.41wt%, Al2O3 13.77 to 14.97 wt%, Fe2O3 4.89 to 9.47 wt%, K2O 0 .53 to 1.49 wt%, TiO2 0.47 to 1.17 wt%, MgO 3.37 to 11.92 wt%. The contents of some minor elements are as follows; Cr 16.00 to 148.00 ppm, Sr 381.00 to 686.00 ppm. SiO2 content in granite varies from 69.38 to 83.93 wt%, Al2O3 11.19 to 13.60 wt%, K2O 2.72 to 3.26 wt%, MgO 0.02 to 0.32 wt%. The contents of some minor elements are as follows: Cu 0.90 to 4.50 ppm,

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Ba 10.00 to 240.00 ppm, Ni 0.90 to 4.90 ppm. XRF analysis of the thirty one soil samples (primary data) collected indicates that SiO2 concentrations in the soils range from 40.48 wt% to 81.01 wt%, Al2O3 12.70 to 38.77 wt%, Fe2O3 1.16 to 12.23 wt%, MgO 0.95 to 3.56 wt%, K2O 0.47 to 4.8 wt%. The trace element compositions are as follows, Cu 1.50 to 37.40 ppm, Ba 71.0 to 426.00 ppm, Ni 8.20 to 84.20 ppm, Rb 18.0 to 271.8 ppm. Soil sample data (secondary data) from the Ntunkumso area previously collected by the Geological Survey Department, Ghana, was also used for the present study. Multivariate statistical analysis was performed on the soil (primary and secondary data) and rock geochemical data. Comparing the mineralogical and geochemical compositions of the rock with the geochemistry of the soils, it has been established that soils overlying the biotite schist are representative of the biotite schist but have undergone moderate weathering due to the high concentrations of As, Ga, Zr, Cr and Sn in the soils than in the rock. The high concentrations of Zn, Ni and Cu in the biotite schist may also suggest hydrothermal alteration and leaching from the soil. CaO, MgO, Cr, Ni, Cu and Ba are highly concentrated in the diorite than in the soils overlying the diorite. This association may imply leaching of these elements from the soil. Also the diorite may have been affected by hydrothermal alteration. The granodiorite has experienced hydrothermal alteration. MgO and Fe2O3 show high concentration in the soils overlying the granodiorite than in the granodiorite implying probable transportation of mafic signatures to the area. The high concentration of Cu, Cr, Ni, Zr, MgO and Fe2O3 in the soil overlying the granite implies that the soil may not be in-situ because it has experienced some mafic contribution. The mineralogical potential of the area is not economically significant.