Geochemistry of Soils from the Bosumtwi Impact Structure, Ghana, and Relationship to Radiometric Airborne Geophysical Data

Abstract

In a high-resolution aerogeophysical survey across the Bosumtwi impact structure, a pronounced high radiometric signal (high equivalent concentration of potassium) was found around the crater rim and to the north of the crater rim. Here we report data on major and trace element chemical compositions of soils from the structure, which have been investigated to determine the provenance, influence of source area weathering, and possible cause of the high equivalent concentration of potassium in the airborne gamma radiation data. Bosumtwi, an impact crater of 1.07 Ma age and a diameter of similar to10.5 km, is located in early Proterozoic Birimian-Tarkwaian rocks of the semi-equatorial climatic region of Ghana, West Africa. The area forms part of the tropical rainforest environment, where warm climate, high rainfall, and high organic activity prevail, and chemical weathering is intense, leading to the formation of lateritic soils, which can be up to tens of meters thick. Fifty-four soil samples, including forty-four from 30 cm depth and ten from 100 cm depth, were used in the study. The concentrations of both major and trace elements in the soil samples show considerable variation, as is expected from a chemical weathering environment. Compared to the underlying parent rocks, the soils are generally enriched in SiO2, Fe2O3, Zr, Nb, Hf, Ta, W and Sb, and depleted in Na, K, and the rare earth elements (REE). Compared to average upper continental crust composition, the Bosumtwi soils are more siliceous and ferruginous. At this advanced stage of weathering, with an average chemical index of alteration (CIA) of 83,the soils have a rather simple mineralogical composition that is dominated by quartz, Fe- and Al-oxides and kaolinite, in addition to a few refractory minerals (e.g., zircon). Pronounced positive Ce anomalies, associated with high Fe2O3, are noted in the soils. A positive correlation of REE and Al2O3 contents in the soils suggests that REE are contained principally in phyllosilicates and clays. Discrimination based on abundances of Co, Ni, and Cr, elemental ratios (Th/Sc, Cr/Zr, Zr/Hf, Co/Th, La/Sc), and La-Th-Sc and Th-Hf-Co plots, shows the importance of mafic rock components in the soils. Soils collected from areas that show a high radiometric potassium signal in the aerogeophysical maps are characterized by relatively higher K2O contents. This may reflect K mobilization due to the impact event, or source rocks for these sediments with variable concentrations of K.