THE ORIGIN AND PETROGENESIS OF THE ULTRAMAFIC ENCLAVES AT UNKI MINE, SELUKWE SUBCHAMBER, GREAT DYKE, ZIMBABWE

Sinikiwe Ncube

A dissertation submitted to the Faculty of Science, University of Witwatersrand in the fulfillment of the requirements for the degree of Master of Science

Johannesburg

August, 2013

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Declaration I declare that this dissertation is my own, unaided work. It is being submitted for the Degree of Master of Science at the University of Witwatersrand, Johannesburg. It has not been submitted before for any degree or examination at any other University.

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30th day of August 2013 in Johannesburg

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Abstract The unique Selukwe Subchamber of the Great Dyke is bounded by the greenstone belt (SGB) on the west side for approximately 25 km and granitoids on the east side, as compared to other subchambers of the Great Dyke that are bounded on both sides by granitoids. It is also the narrowest section of the entire Great Dyke. The extensive xenolith suite is found on the western flank and the central zone of the subchamber. This study focuses on the PAR 11 borehole and the surface xenoliths in the Selukwe Subchamber (SSC). The PAR 11 core was drilled into an anomalous sequence of ultramafic rocks situated in the Mafic Succession of the SSC.

There are basically two rock types in the PAR 11 borehole: and . Comparison of the major and trace element geochemistry of the PAR 11 body with the MR 92 data of Coghill (1994) for the SSC reveals that they are similar but less evolved. The mineral assemblages and proportions of phases in the PAR 11 borehole samples are indicative of essentially the same composition as that which formed the layered sequence of the Great Dyke. Therefore, on the basis of the rock types and chemical compositions, the PAR 11 body and the Great Dyke cumulates appear to be petrologically and chemically similar and had the same petrogenesis.

There are three rock types in the xenolith suite that have been observed in the mafic succession of the Unki area: peridotites, pyroxenites and . Major and trace elements show a wide range of compositions that have CaO/Al2O3 ~ 1, which are dissimilar to both PAR 11 and MR 92 borehole data. REE patterns show depletion of LREE, with flat HREEs indicating a different to that which gave rise to the Great Dyke. Such flat patterns are typical of a primitive mantle source similar to that of komatiite magma. Stowe, (1974) describes and in the SGB and does not describe pyroxenites and gabbros. Therefore, it is not clear in the first instance that the xenoliths were derived from the SGB. It also does not necessarily mean that these rock types did not occur in the SGB and, if they did, maybe they were derived from an intrusion within the SGB that is at depth and never been seen iii before. The xenoliths do not have mineral compositions that are similar to the Great Dyke and therefore precludes them as having been derived from the Great Dyke Marginal Facies, a possible source of such rocks. Therefore, it is concluded from this study that they were inherited from another source which also does not appear to be the SGB because there is no report of such rock types (other than ) in the SGB. They are also not mantle derived.

The metasedimentary rocks that occur as xenoliths are banded iron formation and quartzites and are all clearly derived from the different formations of the SGB. The quartzites are from the Mont d’Or Formation and Wanderer Formation. The BIFs are from the Upper Greenstone and Wanderer Formation. The Shurugwi Greenstones were stripped off from the western flank w