Source-to-Sink Analysis of the Gold-Hosting Mesoarchean Main and Mondeor Formations (Central Rand Group) in the Area South of Johannesburg, Witwatersrand Basin, Kaapvaal Province, South Africa

Abstract

To date, all source-to-sink research has focused on the Phanerozoic and Proterozoic stratigraphic record. This study attempts to apply source-to-sink principles to the 2.8–3.0 Ga placer gold–hosting Witwatersrand Supergroup in South Africa, for which the source area is eroded such that a reconstruction of source parameters is based on the preserved stratigraphic record. The paradigm tested in this study is that bankfull channel depth is a function of drainage basin area and discharge and that an understanding of these parameters can constrain paleogeographic models including source area location and age. Bankfull depth estimates based on thicknesses of fluvial architectural elements are 3.5 and 6.2 m for the older Main and younger Mondeor Formations of the Central Rand Group, respectively. Drainage basin area and discharge estimates are based on empirical relationships developed for different climatic settings. Evidence from facies associations and mudstone geochemistry suggests a warm paleoclimate with relatively high rainfall that constrains drainage basin areas to 3900–19,000 km2 for the Main Formation and 17,000–106,000 km2 for the Mondeor Formation. Estimates for both formations imply an increase in drainage basin area through time, and detrital zircon age spectra reveal a significantly older source area component for the Mondeor than for the Main Formation. The likely sources of the older zircon grains are the Buhwa Quartzite and Tokwe Segment in southern Zimbabwe, some 500 km north of the preserved Witwatersrand sedimentary rocks, consistent with the larger drainage basin area estimate for the Mondeor Formation (106,000 km2). An enlarged drainage basin area is compatible with tectonic models that infer collision of the Kaapvaal and Zimbabwe cratons during the late stages of Witwatersrand sedimentation.