KARST-HOSTED FRESH-WATER PALEOPROTEROZOIC MANGANESE DEPOSITS, POSTMASBURG, SOUTH-AFRICA

The Paleoproterozoic Postmasburg manganese field provides an excellent example of ancient karst-controlled manganese ore deposition. Ferrugi nous ores, composed of partridgeite, bixbyite, and braunite, and silic eous ores, composed of braunite and quartz, are present. The siliceous ores occur as small pods and lenses in the Wolhaarkop chert breccia w hich represents a solution collapse breccia that accumulated in a pale okarst cave system that developed in the dolomites of the Fairfield Fo rmation of the Late Archean Campbellrand Subgroup of the Transvaal Sup ergroup. The ferruginous manganese orebodies are irregularly shaped an d confined by the karst surface of the Reivilo Formation of the Campbe llrand Subgroup. They formed as infill of karst depressions and are co nformably overlain by a hematite pebble conglomerate and aluminous sha les of the Gamagara Formation of the Late Paleoproterozoic Olifantshoe k Group. Textural and geochemical similarities with unmetamorphosed ka rst-hosted manganese deposits suggest that the manganese ores in the P ostmasburg manganese field were deposited as residual manganese wad. T he ores underwent diagenetic compaction and lower greenschist facies m etamorphism that resulted in recrystallization of the manganese wad to braunite, partridgeite, hematite, and bixbyite. The wad was derived f rom Mn and Fe-rich dolomites of the Transvaal Supergroup during a peri od of erosion and intensive terrestrial weathering in the Late Paleopr oterozoic at the base of the Olifantshoek red-bed succession. The ores represent the oldest known oxidized terrestrial sediments and confirm the presence of an oxidizing atmosphere in the Paleoproterozoic at 2 to 2.25 Ga.