Geochemistry and petrography of beach sands along the western coast of Ghana: implications for provenance and tectonic settings

Abstract

The paleoweathering, provenance, and tectonic setting of sediments of the western coast of Ghana were unraveled usingthe geochemistry of 29 beach sands, which are characterized by coarse-, medium-, and fine-grained sizes. The coarse-grained beachsands contain higher SiO2 content (2.9–96 wt.%) than the medium-grained (4.9–94 wt.%) and fine-grained (16.1–90.7 wt.%) sands,implying that the increase in grain size is related to the increase in SiO2 content. Al2O3 and CaO concentrations are higher in thefine- and medium-grained sands than the coarse-grained sands. The beach sands are compositionally immature based on the index ofcompositional variable values (1.17–141) and Th/Sc versus Zr/Sc diagram although they have high SiO2/Al2O3 values. The high SiO2/Al2O3 is not indicative of the weathering conditions of the coastal sediments in the area. The sands are chemically unaltered clasticmaterials of first cycle regime that still have their labile minerals retained in them very close to the sediment source based on thechemical index of alteration values, plagioclase index of alteration values, and A–CN–K and A–CNK–FM ternary diagrams. The totalrare earth elements (ΣREE) content increases with decreasing grain size. However, there are some discrepancies where some coarsegrained samples have high ΣREE content. This suggests that apart from the grain size, the provenance of the sediments has direct controlover their geochemical composition. The correlation of the rare earth elements patterns of the beach sands with those of adjacentsource rocks points to felsic sources for their derivation. The felsic (granodioritic composition) igneous suites of the PaleoproterozoicBirimian highland rocks adjacent to the beach sediments in the western coast are possibly the source rocks of the beach sands. Riftingin the continents is the tectonic regime probably during the Paleoproterozoic times responsible for deposition of the beach sands in thewestern coast of Ghana.