The chemistry of concentrate minerals and diamond inclusions of the Dokolwayo Kimberlite, Swaziland

International Kimberlite Conference Extended Abstracts: 1986   unpublished
The Dokolwayo diatreme has the petrographic character of a Type II kimberlite (Smith, C.B. 1983) is diamondiferous and has a preferred emplacement age of 200 + 5 m.y. (Allsopp, H.L. and Roddick, J.C. 1985) It therefore just predates the Stormberg volcanism which in the vicinity of Dokolwayo commenced at + 190 m.y. Probably due to the extensive secondary alteration in the near-surface kimberlite sampled to date, only a few highly altered mantle xenoliths have been found. This study is therefore
more » ... onfined to mantle derived megacryst and macrocryst garnets (55%) and chromites (45%) and most significantly of diamonds with mineral inclusions. CHROME POOR GARNETS The majority of the garnets (+60%) have chrome poor compositions and are confined to a restricted range of high Mg/Fe ratios. MgO is within the range of 15-22 wt %, FeO 7-15 wt % and CaO 2.8-6 wt %. Two more calcic compositions extending this range were reported in an extensive earlier study of Dokolwayo garnets (Hawthorne et. al. 1979) Na20 lies in the range 0.04-0.0.22 wt % and shows a positive correlation with Ti02. Both megacryst and macrocryst garnets with similar compositional ranges and trends as have been established for the Monastery (Jakob, W.R. O. 1977) and Lekkerfontein (Robey, J.V.A. 1980) Cr-poor discrete garnet megacrysts are present. These have not previously been reported from Type II kimberlites. In addition garnet macrocrysts with chemistry similar to Group II eclogites from Roberts Victor (Hatton, C.J. 1978) have also been Identified. CHROME RICH GARNETS The chromlferous perldotitic macrocrysts are bimodal with respect to both CaO and Ti02, suggesting at least three populations are present. These are inferred to be derived from disaggregated garnet harzburgites, coarse garnet Iherzolite and high temperature deformed garnet Iherzolite. The presence of the latter in Type II kimberlite has also not been suspected previously. The sub-calcic (GlO) garnet population present (~1%) has been inferred to be derived from garnet harzburgite. It is consistent with the presence of diamonds in the kimberlite. (Gurney, J.J. 1984) . Green garnets form a very minor part of the macrocryst suite. Chromite Macrocrysts ^ The outstanding feature of the macrocryst chromites is the consistently high chrome content. More than 77% contain >60 wt % Cr203, with a highest determined concentration of 71,7 wt %. There is no correlation between the chromite compositions and grain size, texture or shape, although both euhedral and anhedral population groups are present. The presence of high chrome chromites is frequently observed in diamondiferous kimberlites, but this population is exceptional. The unusually high Cr203 content (70-75 wt %) in meteoritic chromites, above the stoichiometric 67,9 %
doi:10.29173/ikc1176 fatcat:y2lorovdw5bbhhqafhrhctwgw4