High resolution maps- Maps of Potassium, Thorium and Uranium concentrations with (K/eTh), (U/eK) ratios with ternary map of K, Th and U were created from radiometric datasets as presented in Figs. 4–8.
4.1 Potassium concentration
The magnetic anomalies have a regional gradient with an increasing field intensity from South-West to North-East with values ranging from 33405.6 to > 33717.3 nT. The study area is characterized by low magnetic amplitude ranging from 33405.6 nT to 33476.5 nT, intermediate magnetic amplitude ranging from > 33476.5 to 33556.6 nT and high magnetic amplitude > 33556.5 nT. The variations in the magnetic field intensity across the TMI, is ascribed to the differences in magnetic mineral content between the different rock units and the varying depths
of the underlying rocks (Oladele et al., 2016). The lower total magnetic intensity is observed around Kuje, Gwagwalada, Gwarimpa, Kubwa, Mpape and Lafia. The intermediate magnetic amplitude is observed around Bwari, Jere, Kwoi, Nok, Masaka and Keffi while the high magnetic amplitude is observed around Akwanga, Kwakwasa, Kafanchan and Kachia (Fig. 3) shows moderate potassium region (MPI), MP2 and MP3) as observed at central part of the study area. Figure 4. The intrusion of schist by igneous rock increases K content due to alteration, the increase is majorly as a result of the present of quartzite, Quazo-feldspatic vein and muscovite within the rock unit. LP1 and LP2 are regions with low potassium activity (Fig. 4) these regions have the lowest potassium rates in the study area which are interpreted as granite gneiss while LP3 represents low potassium count or activity within the sedimentary basin around Lafia (Fig. 4)
4.2 Thorium concentration
Figure 5 represents the thorium concentration grid covering the study area, the thorium concentration varies between the ranges of 0 < 6.4ppm to > 31.9ppm in rocks within the area. HP1 recorded moderate to low concentration of thorium suggesting HP1 to be migmatite and not granite gneiss. HP2 recorded moderate concentration of thorium and a very high concentration of potassium, this suggest that HP2 could be a porphyritic granitic rock. HP3 recorded much very high concentration of thorium in the South-eastern part of the study area (Fig. 5) indicating rocks likely to be granite and a member of the younger granite unit of the central basement complex of Nigeria. HP4 has an average amount of thorium concentration (higher that HP2 but lower than HP3). The varying high concentration within the study area mapped as HP1, HP2, HP3, HP4 and HP5 is interpreted to have resulted from the variation of potassium concentration within different granitic rock units. High potassium region HP6 East of the study area (Fig. 5) is an indicative of remobilization of potassium content in the rock unit, hence indicative of rock body made of Biotite granite, rhyolitic and syenite.
4.3 Uranium concentration
In the uranium concentration map (Fig. 6) HP1 has high amount of uranium, thorium and also potassium which is an indication of felsic intrusion within the schist, it also further suggest that HP1 could contain minerals such as pegmatite, quartz and monzonite (Boadi et al, 2013).
LP3 and LP4 which recorded low thorium and potassium concentration also recorded low uranium concentration (Fig. 6). This further suggest that LP3 and LP4 as mafic to ultra-matic volcanic units. The low amount of uranium at MP1 and LP2 is an indicative of quartzite rich altered schist while the high amount of uranium around MP1 and HP2 is indicative of gneissic rock.
From the K/eTh ratio map Fig. 7a, it is seen that the area affected by the hydrothermal process is featured by pink color with a high value of about 0.17%ppm to 4.874%ppm of K/eTh ratio correlating this to geology, areas with alteration of interest are observed at the Northern and Western part of the study area. The massive high K/eTh ratio observed at the Southern part of the study area in Fig. 6a is interpreted as felsic igneous rock as they generally have high K/eTh ratio (Shives et al, 1985).High eU/eTh ratio is observed at the Western part of the study area (Fig. 7b) which is interpreted as a pegmatite quartz vein, while the lower and intermediate concentration are observed at the central and Southern part of the study area, it is interpreted as granite gneiss. The ratio of eTh to eU for normal continental crust is approximately 3.0 which shows that no significant fractionalization occur during weathering and no involvement in metasomatic activity of the radio element (Okeyode et al, 2018).Hanson, 1980; Wollenberg and Smith, 1987, define the eU/K ratio in unaltered schist rock as 1.14ppm% and that of granite as < 0.76ppm% comparing this with the study area, altered schist rock are interpreted as < 1.14 eU/K ratio and this zones concides with the altered zone mapped with the K/eTh ratio, low eU/k ratio observed at the Southern part of the study area (Fig. 7c) interpreted granite, the low eU/K ratio is indicative of potassium enrichment in the granite rock.
(Fig. 8) displays a ternary image by combining the three (3) radio-elements. It usually give a superior image of the geology.The radioactive element and the colours are as follow:
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Red high potassium with low uranium and thorium
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Blue is high uranium with low potassium and thorium
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Green is high thorium with low potassium and uranium
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Cyan is high thorium and uranium with low potassium
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Magenta is high potassium and uranium with low thorium
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Yellow is high potassium and thorium with low uranium
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Black is low potassium and thorium and uranium
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White is high potassium, thorium and uranium
Going by the colours from the ternary map, HP1 has higher concentration of Uranium (U), Potassium(K) and Thorium (Th) depicted by white colours. High concentration of all the three radioactive elements are linked with thermal alteration of the schist.
The formation, Mp1 interpreted to be a schist rock is dominated by thorium with occasional intrusion of uranium, HP1 has high amount of thorium with distribution of white patches indicating high concentration of all three elements, LP3 and LP4 has consistently recorded very low concentration of potassium, uranium and thorium in all the three concentration maps of radioactive elements and the ratio maps indicating mafic to ultra-mafic rocks. The formation Mti has high concentration of thorium and uranium in Fig. 5 and Fig. 6 respectively and moderate potassium in Fig. 4. In the Ternary map Fig. 8: MP1 recorded high concentration of thorium and uranium with thorium slightly above the uranium, indicating that MP1 is magmatic in nature as interpreted in Table 1.
The geology of the area deduced from the airborne radiometric data as presented in Fig. 3, revealed five different types of rock associations; they includes granite gneiss, porphyritic granite rock, felsic granitic rock, schist and migmatite gneiss.
Table 1
Summary of the results and deductions made based on the ternary concentration grid for the study area.
Region on Concentration Map | Tentative Interpretation | Interpretation |
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HP1 | Felsic and granitoid intrusions in schist | Felsic granitic rock within schist |
HP2 | Porphyritic Granite Granite Gneiss | Porphyritic Granite Granite Gneiss |
HP3 | Gneiss rock | Migmatite gneiss |
MP1 | Quartzite rich altered schist | Quartzite rich altered schist/ Granite Gneiss |
MP2 | Gneiss rock | Migmatite gneiss |
MP3 | Quartzite rich schist | Quartzite rich schist |
LP1 | Quartzite rich altered schist | Quartzite rich altered schist |
LP2 | Mafic to ultra-mafic volcanic units | Mafic to ultra-mafic volcanic units |
LP3 | Mafic to ultra-mafic volcanic units | Mafic to ultra-mafic volcanic units |
From these analyzed airborne and ground radio-element result, it is evident that the rock have undergone structural deformation that produced varying degree of fracturing due to metamorphism and intrusions. This probably in turn most have resulted in some of the earth tremor experienced in 2016 at Kwoi and Nok within Kaduna and Mpape area in Abuja (Fig. 8; (Dickson and Scott, 1997 Brempong et al, 2019).