GENESIS OF THE PALEOPROTEROZOIC ROOINEKKE IRON AND MANGANESE ORE DEPOSIT OF THE TRANSVAAL SUPERGROUP IN SOUTH AFRICA WITH REFERENCE TO THE HISTORY OF FREE OXYGEN IN THE OCEAN AND ATMOSPHERE

The Rooinekke iron and manganese deposit, hosted by the ~2,4 Ga Rooinekke Iron Formation, comprises of high-grade (>60 wt % Fe) hematite iron ore with two interbeds of ferruginous manganese ore. The latter contains on average about 35 wt % Mn and 25-27 wt % Fe. It is composed of jacobsite, braunite, hausmannite and hematite with minor recent supergene Mn(IV) minerals.

Both the iron and manganese ores are of ancient supergene origin forming part of a lateritic weathering profile developed below an erosional unconformity covered by ~2,0 Ga Gamagara/Mapedi red beds. A comparison fresh unaltered drill core intersections of the Rooinekke Iron Formation and samples from the mineralized mining area, indicates that the high-grade hematite ores were derived from leaching of silica from the iron formation host rock and oxidative replacement of precursor sedimentary magnetite, iron carbonates and iron silicates by hematite. In contrast the ferruginous manganese ores originated from kutnahorite- and rhodochrosite-bearing sedimentary beds with grades of 11-37 wt % Mn and Fe/Mn ratios of 0,1-2,1. The manganese carbonates are finely intergrown with chert and hematite and contain excellent examples of ancient Liesegang weathering bands composed of jacobsite and hematite. It is suggested that the manganese carbonate beds were transformed to manganese wad in the paleoweathering profile before recrystallization to ferruginous manganese oxide ore. The manganese ores also contain abundant rhodonite, barite, celestite and anhydrite that are ascribed to a later ~1,1 Ga metasomatic overprint.

The sedimentary manganese carbonates are highly enriched in light carbon isotopes indicating that they were derived from coupled oxidation of organic carbon and reduction of Mn(IV) oxides during diagenesis. However, precipitation of Mn oxides requires at least micro-aerobic conditions; a conclusion supported by the presence of Ce anomalies in unaltered iron formation. It is important because the Rooinekke Iron formation predates the Great Oxidation Event (GOE) illustrating that that a flux of free oxygen was present in at least some part of the ocean water column prior to that event. Following the GOE, by the time the 2,0 Ga deep lateritic weathering profile developed, both the atmosphere and surface water on land must have been well oxygenated.