Geochemistry of the Winneba-Mankoadze pegmatites in Southern Ghana: A clue to the petrogenesis of the pegmatites

Abstract

The Birimian Supergroup (2195–2135 Ma) of Ghana was affected by the Eburnean Tectono-thermal Event at approximately 2195 Ma. The latest stage of this event resulted in pegmatitic veining in the Birimian Supergroup of Ghana at 2072 Ma. Deposits of pegmatites have been reported by many researchers in the south-western coast of Ghana. However, their mineralogy and economic potential remain fully unexplored. This study determined the geochemical characteristics and mineralogical composition of the pegmatites in the Winneba-Mankoadze area using field observations, petrographic studies, and major and trace element data. From the field work and petrographic data, the pegmatites can be classified as rare-element type, LCT family. The modal composition of the major minerals are spodumene (0.0–30.0 %), muscovite (0.0–20.0 %), garnet (0.0–5.0 %), microcline (0.0–20.0 %), plagioclase (6.0–70.0 %), and quartz (20.0–85.0 %). The geochemical data indicates that the pegmatites are enriched in Rb (2.8–3465 ppm), Cr (22–1803 ppm), Sr (1.2–314 ppm), Ba (13–501 ppm) and the light rare-earth elements (LREE). The pegmatites are highly fractionated accounting for the enrichment in the LREE than the MREE and HREE. They are also depleted in K and Ti. The pegmatites are peraluminous with calc-alkaline trend and are late-orogenic to post-orogenic. Therefore, the source of the pegmatites is the Birimian meta-sedimentary rocks, and the source lithology is upper-to-middle crust supracrustal rocks.