U–Pb systematics in carbonates of the Postmasburg Group, Transvaal Supergroup, South Africa: Primary versus metasomatic controls

Abstract

Petrography, geochemistry and U–Pb geochronology of a section through the Mooidraai carbonates which overly the Hotazel iron formation in the Postmasburg Group of the Transvaal Supergroup, and comparison with previous work, demonstrate different degrees of localized metasomatic overprints causing chemical changes and affecting the U–Pb isotopic features of the rocks. The lower parts of the section in drillhole OLP-2, at the transition from iron formation to carbonates, exhibit extensive formation of riebeckite in concordant seams and layers, probably crystallized in response to addition of Na from early pore water or brines. Mooidraai limestone samples throughout the section are characterized by the extensive development of stylolites, which are enriched in U with respect to intervening massive carbonate laminae. The latter preserve Pb–Pb systematics close to those documented previously in dolomitized Mooidraai Formation, whereas the stylolites have strongly discordant U–Pb systems. The U–Pb data define an upper intercept age of 2392±23Ma, which confirms the previous Pb–Pb age of the formation, and a lower intercept age of 588±31Ma, which is interpreted to reflect a metasomatic overprint, also recorded elsewhere in the wider region by Ar–Ar dating of hydrothermal minerals. The combined available geochronological record indicates the occurrence of several extensive episodes of fluid-controlled metasomatism, apparently representing the far-field response to distal orogenic processes. The initial Pb composition of the Mooidraai carbonates is much less radiogenic than that in carbonates associated with the stratigraphically lower Makganyene diamictite Formation, reflecting a provenance of the Pb from erosion of older Archean crust rather than a typical sea-water signal as recorded in the Mooidraai carbonates.