Genesis and Provenance of Chimanimani Diamonds in the Umkondo Basin: Insights from Geochemical Maturity

We report a novel discovery of a westward trend in geochemical maturity revealed by major oxides and SiO ₂ ratios in the Chimanimani diamondiferous sediments of the Umkondo Basin, eastern Zimbabwe. The analyses were carried out on diamond drill core samples obtained by the Zimbabwe Consolidated Diamond Company from the grits in the Lower Argillaceous Formation in Chimanimani. The origin of these sediments has been a subject of debate. Unlike the basal Marange diamondiferous deposits occurring basal to the Proterozoic Umkondo sediments, the Chimanimani grits occur within the sedimentary pile, overlying finer sandstones and underlying shales, which suggests a distinct depositional sequence. While some researchers have attributed the Chimanimani diamondiferous grits to eastward erosion and dispersal from the basal Marange diamondiferous deposits on the basin's western margin, others propose an origin from the older and metamorphosed Mozambique Facies thrust sheets in the east. This study favours a closer source for the diamonds, to the east of the current deposit, as revealed by the westward maturation of hosting grits. X-ray fluorescence (XRF) analysis of major and trace elements in the samples revealed ratios of major oxides to SiO ₂ ranging from 70 to 96%, indicative of mature sediments. This maturity could be attributed to a distant source for the sediments, or to hydraulic sorting within the basin. A striking westward increase in SiO ₂ /major oxide ratios, corroborated by a decrease in SiO ₂ /TiO ₂ , is observed across the deposit. This trend suggests a possible source for the sediments as being to the east of the Chimanimani deposit. This appears to contradict the hypothesis of derivation of the sediments from the west, similar to the provenance of the Marange deposits.