Geochemistry of Paleoproterozoic saprolite developed in diabase intruding the Hotazel Formation in the Avontuur deposit of the Kalahari Manganese Field, South Africa

Abstract

Abstract A diabase sill intersected in drill core GHEX-59 from the Avontuur deposit of the Kalahari Manganese Field of South Africa displays an 8 m thick paleosaprolitic weathering profile developed below the regional unconformity at the base of the ~2.0 Ga red-bed succession of the Gamagara Formation of the Elim Group (Keis Supergroup). Tracking element mobility in the weathered diabase relative to unaltered parent material assists the reconstruction of surface conditions and the nature of pore waters associated with weathering in the Paleoproterozoic. The upper part of the regolith profile (i.e., “E/A and upper B horizon” equivalents of a modern soil profile) was removed in core GHEX-59 by erosion before deposition of the Gamagara Formation. The preserved red upper paleosaprolite (probably corresponding to the “B horizon” of a modern soil profile) grades into a veined light grey-green and eventually a minimally veined dark green paleosaprolite (“C horizon”) with depth, and into little-altered diabase (“R horizon”) deeper still. The weathering is characterized by minimal loss of Si, suggesting limited mass loss and compaction, and large decreases in alkali and alkali earth elements such as Mg, Ca, and Na due to the breakdown of plagioclase and primary mafic minerals. K increases dramatically upwards in the weathering profile towards the unconformity. Various indices of weathering attest to high degrees of chemical weathering, which, combined with paleogeographic constraints, support weathering in humid sub-tropical conditions with mean annual precipitation possibly in excess of 1100 mm/yr. The behavior of Fe (FeT, Fe(II) and Fe(III)), Mn, V, Ce and Cu suggest that the weathering occurred under the influence of oxygenated ground waters below an oxygenated atmosphere. The likely presence of a well-developed terrestrial biosphere during weathering is reflected by the behavior of Cu and P in the paleosaprolite, which is in support of earlier studies that have suggested the widespread bacterial colonization of land in the Paleoproterozoic.