Abstract

The Sinqeni Formation of the ~2.98 to 2.87 Ga Pongola Supergroup within the White Umfolozi Inlier in northern KwaZulu-Natal is a fluvial to shallow-marine sequence that has been affected by sea-level changes. Braided alluvial plain conglomerates of the basal Denny Dalton Member are overlain by shallow marine, trough cross-bedded, quartz arenites. A major marine transgression in the Pongola Basin is marked by a conglomerate lag representing a transgressive ravinement surface, which is overlain by banded iron formation, signifying sub-storm wave base deposition on a clastic sediment-starved shelf.

Conglomerates of the Sinqeni Formation are locally gold- and uranium-bearing with the ~1 m thick, laterally discontinuous “Mozaan Contact Reef” (MCR) hosting erratic mineralization. Where mineralized, the MCR contains rounded, detrital pyrite grains, derived primarily from erosion of sedimentary pyrites. Three generations of pyrite are recognised, with primary gold hosted as inclusions within compact, rounded detrital pyrite and gersdorffite grains. No uraninite grains are observed in the MCR, with allogenic uraniferous leucoxene and authigenic brannerite forming the major uraniferous component. The uppermost conglomerate of the Sinqeni Formation (CG 4) crops out ~30 m above the Denny Dalton Member conglomerates and hosts uranium precipitated as secondary uraninite and coffinite encrustations around detrital pyrite grains. The conglomerates form a placer deposit which has been subjected to post-depositional alteration. Heavy mineral assemblages as well as bulk geochemical data for conglomerates and quartz arenites suggest a granitoid-greenstone source. Palaeocurrent and mineralogical data suggest a proximal granitoid-greenstone provenance to the west of the White Umfolozi Inlier, whereas equivalent units in the main Pongola Basin have been derived from granitoid-greenstone successions to the north. The reef mineralogy and pyrite geochemistry however, suggest that detritus in the MCR was derived from a variety of sources including granitic basement, Palaeoarchaean greenstones and sulphidic sedimentary rocks of unknown affinity.

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