The geology and mineralogy of mafic and ultramafic intrusives of the Mozambique belt in Mbeere and Tharaka-Nithi districts, Eastern Kenya

Abstract

The aim of this study was to investigate the detailed geology, the geochemistry and the mineralogy associated with the mafic and the ultramafic intrusives of the Mozambique Belt, eastern Kenya. To achieve this, a detailed geological mapping and sampling of the intrusives was done followed by laboratory analysis. The significance of the study was to highlight the nature, relative age and genesis of the intrusives and the potential for economic mineralization. This study has revealed that the intrusive rocks and the regional foliation trends are interrelated suggesting that the intrusives developed in vertical structures during regional metamorphism The elongated shapes of the bodies and their alignment with the enclosing regional structural grain of the gneisses suggest compressional deformation during the regional Mozambique Belt Orogeny. This is considered as evidence for at least a synorogenic emplacement of these intrusive rock bodies. The mafic and the ultramafic rocks represent a range of differentiated rocks from fairly primitive magma (Mg-number = 79) to highly fractionated magma (Mg-number = 36). However; the fact that the rocks are intrusive brings out the possibility that none of them may represent parental magma composition. The magmas have undergone modification by crystal fractionation processes either in high-level magir-c11ambersor during ascent from their mantle source regions. The gradual composition change in.thf feldspars, olivine and the pyroxenes is from high- to low-temperature members of their respective solid-solution series. This outlines the crystallization sequence. The whole rock chemical analysis shows that rocks rich in magnesium oxide are also rich in chromium and nickel and those rich in potassium oxide are also rich in rubidium and barium in conformity with the rules of partitioning. This implies that they originated from one parent magma. The rocks show iron enrichment trend and have high nickel and low aluminium oxide contents. This chemical evidence supports a tholeiitic magma affinity for the intrusive rocks An attempt ismade to construct the mineralogy of the gabbros based on the mineral chemistry. This has revealed that they are composed of magnesium-rich olivine (chrysolite) with approximately 80% forsterite. The pyroxene is both magnesium and calcium rich and has an enstatite molecular percentage of about 18%. It isan orthopyroxene (bronzite) inthe enstatiteferrosilite solid-solution series. The plagioclase feldspars show a range of composition from about 90-20% anorthite. This range shows that the magma crystallized slowly. Crystallization over a wide temperature has resulted in the formation of alkali feldspars and other minor constituents. Economically, the area has the potential for base metals. Chromium and nickel are present in concentrations higher than that found in average igneous rocks. Further geophysical work is recommended to reveal some more information in terms of the possibilities of economic deposits in this project area. Some of the pegmatite, particularly those near Muthanthara contains smallneedles of beryl. The beryl is blue green incolour but not of gem quality.