Abstract
Geochemical studies of weathering profiles around Kitibi-Iwoye revealed redistribution of elements from parental pegmatite to the regolith. Geological mapping showed that the pegmatite intrudes diorite and migmatite gneiss. Weathered samples from B-horizon were air-dried, pulverised and analysed using X-ray Fluorescence Spectrometry (XRF) in the Department of Geological Sciences, University of Cape Town, South Africa. Chemical Index of Alteration (CIA), Weathering Index of Parker (WIP), Mass balance calculation and Al2O3 – CaO+Na2O – K2O (A–CN–K) ternary plot were employed to determine elemental mobility and distribution caused by weathering. SiO₂ (74.2 and 43.4 wt %), CaO (0.43 and 0.03 wt. %), Na2O (7.14 and 0.04 wt. %), K2O (1.90 and 0.67 wt. %), MnO (0.11 and 0.03 wt. %) and P2O5 (0.20 and 0.05wt. %) displayed depletion from parent rock to the regolith respectively. But Al₂O₃ (15.5 wt. % and 33.5 wt. %), Fe2O3 (0.39 and 3.40 wt. %), TiO2 (0.04 and 0.35 wt. %) and MgO (0.08 and 0.11 wt. %) showed enrichment from parent rock to the regolith respectively. Fe₂O₃ (3.19) is the most enriched whereas Na2O (-99.8), CaO (-98.9), P2O5 (-95.3), K2O (-89.5), SiO2 (-81.9), MgO (-73.1), MnO (-64.5) and Al2O3 (-23.6) are progressively depleted. Mean CIA value of 97.8 revealed that weathering has almost reached its completion whereas CIA of 62.1 for the pegmatite suggested that the parent rock is at incipient stage of weathering. Pegmatite had a WIP of 110.5 whereas the weathered samples with WIP ranging from 2.66, 3.88, 6.03, 6.23, 6.92, 8.08, 9.08, 9.76 and 14.6 respectively showed decreasing trend of weathering. This study confirmed contrasting behaviour of CIA and WIP. A-CN-K diagram suggested strongly weathered samples plotted at the apex of Al2O3 field whereas pegmatite plots along the A-CN line.
Highlights
The weathering profile of the study area which is directly underlain by pegmatite has been considerably altered through interaction between solution, rock and their weathering residues
Chemical analyses of major elements such as Si, Ti, Al, Fe, Mn, Mg, Ca, Na, K and P were done by X-ray Fluorescence Spectrometry (XRF) in the laboratory of Department of Geological Sciences, University of Cape Town, South Africa. 2 g of pulverized rock and weathered samples were weighed and put in the oven at 110 ̊C for at least 4 hours
Result of major oxide mobilization and redistribution in pegmatite and weathered samples of Kitibi-Iwoye is presented in table 1
Summary
Mobility of elements in a particular geological setting is mostly triggered by chemical reactions within a rock body after its crystallisation. Natural processes such as weathering, water-rock interaction, hydrothermal alteration, groundwater mixing, evolution and magmatic crystallisation can disrupt the pattern of mobility and fractionation of elements (Yusoff et al, 2013) Such elemental mobility and fractionation (redistribution) provide a veritable tool for comprehending processes that lead to the formation of weathered products and their abundance. Major and trace elements hosted by rock forming minerals are mostly liberated during chemical weathering (Van der Weijden and Van der Weijden, 1995) Their mobilization and redistribution during weathering is complicated because. Issues pertaining to mobilization and redistribution of the elements in the source rock and overlying regolith caused by weathering are addressed Various indices such as mass balance model, chemical index of alteration, weathering index of parker and Al2O3 – CaO+Na2O – K2O (A–CN–K) ternary diagrams were employed to interpret rate of weathering processes. The Chemical Index of Alteration (CIA) proposed and the Weathering Index of Parker (WIP) which was first introduced and developed are the two most commonly applied indices (Nesbitt and Young, 1982; Parker, 1970; Hamdan and Burnham, 1996)
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