Petrography and geochemistry of manganiferous rocks in the Mankwadzi area in the southern Kibi-Winneba metavolcanic belt, Ghana: Implications for genesis of Mn formations in the Birimian of West Africa

Manganiferous rocks in the Mankwadzi area in the southernmost portion of the Kibi-Winneba metavolcanic belt, one of several Mn occurrences in the Paleoproterozoic Birimian of Ghana, are hosted in hornblende schist and amphibolite. These rocks are, in places, intruded by hornblende dyke. In outcrop, the manganiferous rocks appear to be conformable with the host schist and amphibolite, are macroscopically dark, fine-grained and structurally massive to distinctly banded. Observed alternating light and dark occasionally macro-folded bands suggest post-depositional deformation of both light and dark bands. Microscopic observations revealed that the light bands are dominantly Si-rich and the dark bands mainly of opaque minerals. Whole rock analyses of the manganiferous rocks show high contents of MnO (16.75-27.4 wt%) suggesting that the opaque minerals are likely rich in Mn. The analyzed rock samples show moderate to strong enrichments in light rare earth elements compared to heavy rare earth elements. Whereas the manganiferous rocks show perceptibly negative Eu anomaly, host hornblende schist and hornblende dyke do not. Eu anomaly in amphibolite samples is, however, uncertain as the three samples analyzed gave positive, negative and no Eu anomalies. Based on the field characteristics, microscopic and geochemical features, we suggest that the Mn occurrence in the Mankwadzi area originated via sedimentary deposition and was later modified by metamorphism, hydrothermal and/or supergene processes similar to manganiferous occurrences at Nsuta and Tambao in the Birimian of West Africa.