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. Ferruginous ores, composed of partridgeite, bixbyite, braunite, and siliceous ores, composed of braunite and quartz, are present. The siliceous ores occur as small pods and lenses in the Wolhaarkop chert breccia which represents a solution collapse breccia that accumulated in a paleokarst cave system that developed in the dolomites of the Fairfield Formation of the Late Archean Campbellrand Subgroup of the Transvaal Supergroup. The ferruginous manganese orebodies are irregularly shaped and confined by the karst surface of the Reivilo Formation of the Campbellrand Subgroup. They formed as infill of karst depressions and are conformably overlain by a hematite pebble conglomerate and aluminous shales of the Gamagara Formation of the Late Paleoproterozoic Olifantshoek Group. Textural and geochemical similarities with unmetamorphosed karst-hosted manganese deposits suggest that the manganese ores in the Postmasburg manganese field were deposited as residual manganese wad. The ores underwent diagenetic compaction and lower greenschist facies metamorphism that resulted in recrystallization of the manganese wad to braunite, partridgeite, hematite, and bixbyite. The wad was derived from Mn- and Ferich dolomites of the Transvaal Supergroup during a period of erosion and intensive terrestrial weathering in the Late Paleoproterozoic 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.