Eduardo Perez

B.Sc. (Honours) Thesis

A CASE STUDY OF CHROMITE AND ASSOCIATED MINERALS AS INDICATORS OF DIAMOND PRESERVATION IN THE BOTSWANAN ORAPA KIMBERLITE CLUSTE

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Kimberlites are deep mantle magmas and the primary source of diamonds. Ilmenite, perovskite, rutile, titanite and spinel group minerals are commonly found in matrixes of kimberlites. Prior research shows that changes in temperature, volatile content, oxygen fugacity, and degrees of country rock assimilation influence what minerals are formed and the textures which are created. Similarly, these processes influence diamond preservation in kimberlites. Presence and abundance of chromite and ilmenite in kimberlites has been long known to correlate with diamond potential. However, the nature of this correlation is not yet understood. In this study we examine assemblages of oxide minerals in different kimberlite facies aiming to constrain crystallisation conditions and their effect on the diamond population.

Two kimberlite bodies positioned in the Botswanan Orapa kimberlite cluster were examined. The first of these kimberlites consists of a single coherent kimberlite pipe, AK-15. BK-1, the second and more complex body is composed of two distinct coherent facies, CK-A and CK-B, with differing origins as well as the volcanoclastic MVK facies. Sixty-seven samples, made up of the 4 kimberlite facies, were investigated using optical microscopy to observe the textures, zoning and phases present in each sample. Twelve thin sections, three from each kimberlite facies, were examined using scanning electron microscope (SEM) with Back Scatter Electron imaging, X-ray mapping and Energy Dispersive Spectroscopic analysis to confirm the presence and relationship between the minerals of interest. CK-A showed heavily altered Ilmenite with clear exsolution textures and reaction products made up of titaniferous magnetite, rutile, and titanite indicating high fluid content which fluctuated during crystallization. Minor chromite was present which also showed exsolution textures and was typically rimmed by titanite. Perovskite was not observed in CK-A implying high silica activity. CK-B contained ilmenite macrocrysts displaying exsolution lamella and typically rimmed by perovskite and titaniferous magnetite. Titanite was often found in microcrysts throughout the sample rimmed by perovskite. Abundant perovskite in the groundmass indicates much lower silica activity than in CK-A magma as well as a much lower fluid content. Chromite and rutile were not observed. In MVK Ilmenite macrocrysts showed exsolution textures and were rimmed by intergrown titaniferous magnetite, rutile and titanite. Chromite was found in low quantities and was heavily altered and rimmed by titanite. This data is evidence for volatile exsolution and high silica activity possibly due to assimilation of crustal material. AK-15 contained ilmenite typically rimmed by titanite and titaniferous magnetite. Chromite was found throughout with alteration textures and titaniferous magnetite intergrowths. Rutile, perovskite and titanite were also found throughout. We propose that the observed difference in groundmass mineralogy between the four studied kimberlite lithology could be a result of difference in assimilation of crustal material, which would rise silica activity and trigger CO2 degassing with exsolution of fluid. Absence of fluid in CK-B lithology would explain corrosive surface features and high degree of kimberlitic resorption on CK-B diamonds.

Keywords: kimberlite, chromite, ilmenite, perovskite, titanite, resorption
Pages: 90
Supervisor: Yana Fedortchouk