U-Pb geochronologic constraints on deposition of the Campbellrand Subgroup, Transvaal Supergroup, South Africa

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 U-Pb 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 Pb-Pb age of 2557 +/- 49 Ma for dolomites at the base of the Campbellrand Subgroup is broadly consistent with this U-Pb zircon age, new Pb-isotopic data for carbonate cements and massive sulfides lie along the same Pb-Pb array as the dolomites. This relationship implies that the 'isochron' is the result of post-depositional resetting of the U-Pb system in dolomite by fluids responsible for sulfide mineralization, and that the age which corresponds to its slope has no geological significance.