GEOLOGY AND GEOCHEMISTRY OF PIT ROCKS FROM THE OBUEM CONCESSION IN THE KONIYAW AREA - KUMASI

ABSTRACT Geological mapping and geochemical analysis of rock samples were made on pit rocks from Obuem Mining Concession found within the Birimian of Ashanti Belt of Ghana, in order to determine the nature of gold mineralisation. The study area consists of metavolcanics and metasediments with few intrusions of granitoids. Metasediments such as phyllites and schists as well as metavolcanics of andesitic and basaltic origin were sampled and analyzed for their major and trace element contents using ICP-MS and XRF methods in order to constrain their provenance and source area weathering. The analyses of the metavolcanics indicate a tholeiitic nature. The rate of alteration is fairly intense as indicated by the high loss of ignition values. Most of the primary minerals such as hornblende, biotite and plagioclase have been altered to sericite, epidote, chlorite and groundmass. These geochemical characteristics depict a mafic source for the phyllites dominantly consisted of. The stretched, contorted grain crystals and the presence of quartz veins in the rocks affirm the dynamic nature of the metamorphism as they indicate that the area has experienced some level of tectonic activity and fairly significant amount of metamorphism. The quartz grains show extreme deformational effects as deformation crenulation. Average Cr and Ni abundances and Cr/Ni ratios of average phyllites of the Birimian metasediments indicate that the source consisted of basaltic material. Geochemical data and tectonic discrimination diagram for the Birimian metasediments and metavolcanics suggest that few of the metasediments were deposited within a passive margin while majority of them in the passive continental margin.