LITHOFACIES OF THE PALEOPROTEROZOIC MOOIDRAAI FORMATION (SOUTH AFRICA) AND AN ALTERNATIVE MODEL FOR THE SETTING OF THE KALAHARI MANGANESE FIELD

As a result of limited exposure, the tectonic setting of the Paleoproterozoic Kalahari Manganese Field in the Northern Cape Province of South Africa is poorly understood, despite the economic importance of the Fe- and Mn-bearing Hotazel Formation. We addressed this problem by investigating the lithostratigraphy, lithofacies, and lateral facies distribution of the conformably overlying Mooidraai Formation, a 220-m-thick succession of deep to shallow marine carbonate rocks. The laterally repeating synchronous deposition of deep and shallow marine lithofacies types at the base of the Mooidraai Formation indicates deposition in a small-scale north-south striking half-graben. A rift-to-drift transition likely occurred during deposition of the upper Hotazel and lower Mooidraai formations, explaining the change in lithology from banded iron formation to carbonate rocks. The end of rifting permitted the establishment of extensive shallow marine carbonate environments during deposition of the upper Mooidraai Fm. Deposition of the Hotazel-Mooidraai succession might have occurred in small sub-rift basins perpendicular to a traditionally proposed back-arc spreading zone further to the west. The new model represents an important extension of the back-arc basin model; however, the new data could also be interpreted as indication for advanced rifting of a back-arc basin. The proposed rift setting corresponds well to a recent reconstruction of the supercraton Vaalbara. Timing and locus of the formation of the Kalahari Manganese Field corresponds to the estimated break-up-time and -location of Vaalbara 2.2 billion years ago at the northwestern edge of the Kaapvaal craton. Should the Kalahari Manganese Field represent a back-arc basin, without rifting, Vaalbara likely separated later during the bimodal Hartley igneous event.