Browsing by Author "Muusha, Miracle"

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    A geological study of the River Ranch kimberlite pipe and associated diamonds and mantle minerals : Limpopo Mobile Belt, Zimbabwe
    (1997) Muusha, Miracle; Gurney, John J
    The River Ranch kimberlite is a 5.2 hectare diatreme from which the original surficial crater facies material has been removed by erosion. Proof of the prior existence of such a feature is provided by down rafted blocks of epiclastic and pyroclastic rocks exposed in the diatreme during open pit mining operations. Six intrusive kimberlite phases have been recognised in the diatreme by careful mapping and confirmed by petrographic observations, particularly variations in groundmass mineralogy. Subsequent to emplacement, the diatreme has been cut by intrusive tholeitic dolerites of apparently Karoo age. The River Ranch occurrence is classified as a Group I kimberlite although the definitive isotopic evidence is lacking due to pervasive alteration of the exposed rocks. The absence of megacrystic and groundmass ilmenite and the presence of groundmass diopside are unusual for the group of rocks. However the presence of monticellite and the low abundance of phlogopite argue against a Group II classification. The overwhelmingly peridotitic nature of the mantle mineral macrocrysts in the kimberlite is consistent with the observations of Kopylova et al (1995) that the diamonds at River Ranch are predominantly peridotitic and are likely to have formed in a single process. The approximate equilibration temperature for a small suite of coarse grained lherzolite from the kimberlites is 1200°C, suggesting a geothermal gradient rather higher than seen in the Kaap-Vaal craton. The diamonds at River Ranch are predominantly brown, strongly resorbed and have less than average value. It is predicted that a combination of kimberlite petrography, micro-diamond measurements and mantle macrocryst studies should be a valuable aid to grade control and mine planning.