Petrography and geochemistry of a 2000-2200-Ma-old hematitic Paleo-alteration profile on Ongeluk basalt of the Transvaal supergroup, Griqualand West, South Africa

A 4-m-thick paleo-alteration profile, thought to represent one of the oldest-known ferruginized (hematitic) paleoweathering zones, is described from a depth of 720 m in a drill core on the farm Drakenstein 263 some 20 km northwest of Hotazel in Griqualand West, South Africa. The paleo-alteration profile is developed on 2240 ± 57 Ma Ongeluk basalt along the angular unconformity that separates Transvaal strata from ; 2070±95 Ma basal red beds of the Olifantshoek Group. Basaltic Ongeluk lava was subjected to greenschist-facies metamorphism, represented by a mineral assemblage of Fe-pumpellyite, chlorite, albite, and quartz, before paleosol development. The subsequent paleo-alteration is characterized by concentration of Fe₂O₃, in the form of hematite, iron-rich chlorite and abundant sericite. The abundance of sericite results in a large excess of K₂O relative to the amount of Al₂O₃ that could have been derived from weathering of the parent basalt. The origin of the excess potassium is problematical and could have been derived from basin water, groundwater or metamorphic fluids. Field relationships favour introduction from basin water during deposition of overlying Olifantshoek red beds. Elements such as sodium, calcium and magnesium were leached from the alteration profile whereas REE became concentrated within it. The most interesting aspect of the REE distribution is the preferred concentration of the lighter lanthanides, La and Nd, in the middle part of the alteration profile relative to heavy REE and the development of a marked negative Ce anomaly in the paleo-weathered material. Hydrothermal vein alteration took place after deposition of the overlying Olifantshoek red beds.