Igwisi Hills

Three small volcanic craters up to 370 m across, and aligned northeast-southwest, have rims of tuff rising to about 70 m. The largest crater is breached on its east-northeastern side by a lava flow which extends for nearly 1 km. The tuffs contain very abundant rounded 'pebbles' of forsteritic olivine and irregularly shaped rock fragments that are cemented by carbonate with many veins and patches of carbonate. The lavas are olivine porphyries, the olivine being well rounded with numerous carbonate-filled amygdales. Less common inclusions in the olivine include rare pyrope garnet, pale-green chromium diopside, orthopyroxene, phlogopite and spinel. The matrix includes minor perovskite and apatite, but calcite, dolomite and serpentine, occasionally as sub-parallel laths, are dominant. Most investigators have concluded that these rocks are extrusive kimberlites e.g. Reid et al. (1975a) and Dawson (1994), but they concede, however, that the high carbonate content, high Sr and very low Rb, more closely resemble carbonatite than kimberlite. Mitchell (1970), in contrast, on the basis of a single whole-rock analysis, anomolously high in Al2O3, considers that these rocks are not kimberlites. In many respects the flow mineralogy is very similar to that of the calcite-rich kimberlite of the Benfontein sills, Kimberley, South Africa (South Africa No. 151-00-036) (Dawson and Hawthorne, 1973; J.B. Dawson, pers. comm., 1989).