Geochemistry of Birimian phyllites from the Obuasi and Prestea Mines, Southwestern Ghana: Implications for provenance and source-area weathering

Abstract

Phyllites from the Ashanti Belt of the Paleoproterozoic Birimian rocks of Ghana were analyzed for their major and trace element geochemistry in order to constrain their provenance and source area weathering. The metamorphosed sediments may be classified as immature by their high index of compositional variability values of mostly > 1. High field strength elements are generally depleted whereas transition metals are typically enriched relative to average Proterozoic upper crust. Such geochemical characteristics suggest that the source of the phyllites dominantly consisted of mafic rocks. Average Cr and Ni abundances and Cr/Ni ratios of average phyllites of the Birimian metasediments indicate that the source consisted of basaltic material. Major element data, plotted in Al2O3 (CaO+Na2O) K2O ternary diagram indicate that the phyllites have undergone significant post-depositional K metasomatism. Low to moderately high chemical weathering of the source terrane is indicated by pre-metasomatized chemical index of alteration values of 55 85. The geochemical data further suggest that the sediments were deposited within an active continental margin setting and mostly supplied by the adjacent volcanic belts.