Abstract

Karoo-age diamond-bearing conglomerates, located to the southwest of Gweru in central Zimbabwe, have long presented an enigma in terms of their mode of deposition, and the primary source of the diamonds. Although conglomerate units occur throughout the Somabula Karoo succession, the diamonds and associated varied heavy mineral suite are concentrated in a discontinuous basal unit, with large subangular to subrounded clasts, supported by a clay-rich or arkosic matrix, which fills irregularities in the palaeo-floor. The concentration of diamonds and heavy minerals in this extremely immature basal deposit is not readily explained in terms of simple single-stage fluvial processes. Further, the occurrence in the gravels of well formed crystals of soft Pt-group minerals, as well as a small proportion of idiomorphic crystals of staurolite (the dominant heavy mineral phase, with a distal provenance), is difficult to reconcile with fluvial transport over any significant distance. It is therefore suggested that the basal conglomerate clasts and associated heavy mineral suite, including diamonds, represent the winnowed and concentrated lag products of a former Permian till, with a distal provenance. This implies a major erosional break between the deposition of the basal till and overlying Triassic (upper Karoo) sediments. The latter are dominated by immature arkoses with subordinate rudaceous and argillaceous units, interpreted to reflect deposition in a braided river system. The initial pulse of upper Karoo sedimentation swamped the residual clast/heavy mineral lag left by winnowing of the former Permian till, with only minor reworking of this material. The result was an assemblage of clasts and heavy minerals in marked hydraulic disequilibrium with the supporting matrix.

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