Kimberlite metasomatism at Murowa and Sese pipes, Zimbabwe

The Cambrian-aged Murowa and Sese kimberlites are located within the Archean Zimbabwe Craton just N of the boundary with the Limpopo Mobile Zone. The root zones of pipes are exposed at the current erosion level. Kimberlite lithologies present are hypabyssal macrocrystic kimberlite (“HMK”), HMK breccia, tuffisitic kimberlite breccia (“TKB”) and minor lithic tuffisitic kimberlite breccia (“LTKB”). Country rocks are Archean 2.6 Ga Chibi and Zimbabwe granite batholiths. Kimberlite metasomatic effects are widespread, and can be laterally as extensive as the kimberlites themselves. The porphyritic granites are composed of fresh K- feldspar, oligoclase, quartz, biotite and muscovite. During initial metasomatism the granites become spotted with green chlorite, needles of alkaline amphiboles (winchite, riebeckite, arfvedsonite) and pyroxenes (aegirine-augite) with minor carbonate and felts of talc. Plagioclase becomes extensively altered, dusted and reddened with hematite, whereas K- feldspar remains unaffected. The granites become converted to syenite through removal of quartz. Similar fenitisation has been recorded at Toubabouko lamproite (Ivory Coast) (Knopf, 1970), Lesueur Township kimberlite in Quebec (Watson, 1973), Bow Hill ultramafic lamprophyre dyke (Jaques et al., 1986) and adjacent to the Lissadell Road lamproite dykes, Western Australia (C.B. Smith, unpublished data). As metasomatism intensifies at Murowa and Sese, veins of green metasomatite cut and disrupt the granite. Progressive disruption entrains granite blocks, breaking down the granite still further and transporting needle-like feldspar slivers, so giving rise to LTKB. The metasomatite chemistry is intermediate between granite and kimberlite, having lower Si, Al and Y than granite, similar amounts of K, Na, Ti and Ba, higher Ca, Mg, Ni, Cr, Nb, Sr, P, CO2, H2O+ and a higher Fe3+/Fe2+ ratio. Ferguson et al. (1973) described similar chemical alteration of wall rock adjacent to De Beers kimberlite pipe, South Africa The metasomatic process is analogous to the fenitisation of granitic wall rock by carbonatite. Kimberlite metasomatism at Murowa and Sese appears to be formed by fluids from the rising but pent-up proto-kimberlite melt penetrating into cracks and the matrix of the granite country rock and reacting with it. These fluids are alkaline, hydrous and CO2-rich, perhaps with affinities to the alkaline carbonates erupted from the Oldoinyo Lengai carbonatite volcano