Abstract
A volcanic ash bed that crops out 40–50 m below the contact between the Campbellrand Subgroup and the Kuruman Iron Formation, Transvaal Supergroup, South Africa, yields an UPb zircon age of 2521 ± 3 Ma. This new age demonstrates that the Campbellrand-Malmani carbonate platform was deposited during late Archean time, and is the oldest extensively preserved carbonate platform known. In addition, the age constrains deposition of the Kuruman, Griquatown, and Penge iron-formations to latest Archean to earliest Paleoproterozoic time. Although a previously reported PbPb age of 2557 ± 49 Ma for dolomites at the base of the Campbellrand Subgroup is broadly consistent with this UPb zircon age, new Pb-isotopic data for carbonate cements and massive sulfides lie along the same PbPb array as the dolomites. This relationship implies that the ‘isochron’ is the result of post-depositional resetting of the UPb system in dolomite by fluids responsible for sulfide mineralization, and that the age which corresponds to its slope has no geological significance.
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