Petrogenesis and potential for magmatic mineralisation of the mafic dykes of the northeastern part of the Irumide fold belt of Northeastern Zambia.
Date
2023
Authors
Musiwa, Sakwiba
Journal Title
Journal ISSN
Volume Title
Publisher
The University of Zambia
Abstract
The area of northeastern Zambia, between Chama and Lundazi towns, is underlain by rocks belonging to the Irumide Fold Belt, a Mesoproterozoic orogenic belt whose geology is dominated by biotite gneisses, which often show incipient migmatisation, that belong to the Kampemba Group. Less common unmigmatised rocks types are schists, quartzo-feldspathic gneisses, calc-silicates, quartzites and metamorphosed basic and aplitic sills. Porphyroblastic leucogneisses are represented as narrow strips and these have general NE-SW trending foliations, the general Irumide structural fabric, and dipping to the NW. A complex of a mafic dyke swarm that occupies a NE trending fracture zone, occurs mainly between Chama and
Lundazi. Individual dykes vary in size from about 100s of metres long by a few 10s of metres wide, to about 4 km long and about 100 metres wide. By nature of their emplacement, the dykes could contain potential for economic magmatic mineralisation. This study, therefore, looked at the petrogenesis of the mafic dyke swarm, through the application of whole rock geochemistry, geochronology and isotope geochemistry. The study also made an assessment of the potential for Cu-Ni-PGE sulphide mineralisation of the dyke swarm by applying chemical data and empirical evaluation methods. Fifteen fresh samples were sent to Actlabs in Canada for whole rock geochemical analysis. Three of the fifteen samples were analysed for Rb-Sr and Sm-Nd isotope geochemistry; and the same three samples were used in the K-Ar age determination of the rocks. The results received on age determination gave the ages, 1522 ± 90 Ma, 1067 ± 55 Ma and 608 ± 35 Ma. Sm and Nd concentrations range from 1.99 – 3.112 ppm and 6.666 – 9.88 ppm respectively. ENd were calculated by the laboratory at -5.23, -8.23 and -9.73. 87Sr/86Sr returned values of 0.710886, 0.714503 and 0.719612. From the study of the thin sections and photomicrographs, it has been concluded that the dykes studied are gabbroic in composition and exhibit differences in composition, textures, alteration and microstructures. They are composed mainly of plagioclase, clinopyroxene, hornblende with subordinate olivine, quartz, sericite, epidote and opaque phase minerals. The dyke rocks occur mainly in coarse and medium grained varieties as only two of the twelve samples examined, are fine grained. Several diagrams which have been applied in the evaluation of crystallisation fractionation of both types of rocks have revealed that fractional crystallisation affected the magma of both types of rocks studied, tholeiitics and calc-alkalics. This is further supported by the wide ranges of Mg#, MgO, Ni and Cr in both rock types, which indicate significant crystallisation fractionation either in magma chambers or during ascent. The low Mg# (29.7-58.0), Ni (80-720 ppm) and Cr (150-1400 ppm) are suggestive of evolved magmas that underwent fractional crystallisation. Since no two samples came from the same dyke, it can also be concluded that all the mafic dykes in the Study area experienced various levels of fractional crystallisation. Both types of dykes show enrichment of the source magma by addition. The enrichment must principally have come from the terrigenous felsic sediments rich in HFSE and LILE which were subducted at the mantle wedge. From the analysis of all the available information that would be critical to sulphide mineralisation and possible occurrence of deposits of Cu-Ni in the mafic dykes of the study area, it is concluded that the dykes exhibit no parameters that would have allowed such mineralisation to take place. Whilst
the study has revealed, from age dating perfomed on three samples, that the mafic dykes of the northeastern part of the Irumide Fold Belt of northeastern Zambia were emplaced in at
least three pulses, it is still recommended that high precision and more expensive age dating methods such as U-Pb Zr are carried out.
Description
Thesis of of Doctor of Philosophy in Geology