The Bukura and Mbesa pyrite mineralisation of Western Kenya

Abstract

The predominantly pyrite - pyrrhotite Bukura and Mbesa mineralisation occurs in the Kakamega District of the Western Province of Kenya. The study area in which this mineralisation occurs covers approximately 244 km immediately to the south-west of Kakamega Town and is part of degree sheet 33 S.E. Quadrant. In studying the mineralisation, geological field mapping of the study area was carri d out at a scale of 1:50,000 and a geological map compiled at a scale of 1:25,000. This was followed by core logging of two drill cores DHYA2 and DHYA3c sunk into the Mbesa mineralisation zone. Laboratory work involving study of rock samples in thin section under the microscope, X-ray diffraction analyses, study of polished sections using the ore microscope and X-ray fluorescence element analyses pf samples from the two cores to find the major r. - elements present and also whether any traces of gold are associate- ted with the mineralisation, was subsequently carried out. The .. :: Bukura and Mbesa sulphide-mineralisation occurs mainly in steeply dipping.Kavirondian mudstones and to a lesser extent grits at the contact of the Mumias granite. The mineralisation is charar cterized by the sulphides; pyrite, pyrrhotite, sphalerite and chalcopyri te in order of abundance. Gangue minerals are cha.racterized mainly by~hornblende, chlorite, micas, epidote, clinozoisite and tremolite - actinolite. The mineralisation is of a hydrothermal nature and the water of the hydrothermal fluids in these contact zones is considered to have been primarily magmatic and metamorphic in origin .. A larger fraction of the sulphur, iron, zinc and copper of the sulphides is considered to have been derived from the host rocks and a lesser portion of these constituents from the Mumias granite itself. The energy required to drive the hydrothermal system responsible for the Bukura and