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.
Citation
M.Sc (Geology)Sponsorhip
University of NairobiPublisher
Department of Geology, University of Nairobi
Description
Master of Science Thesis