Sudoite in the Archaean Witwatersrand Basin

Sudoite, with almost ideal composition of Mg₂Al₄Si₃O₁₀(OH)₈, has been found in the Khaki shale of the Witwatersrand Basin, South Africa. The sudoite occurs in aggregates of elongate bundles alternating with muscovite and rarely pyrophyllite, in an assemblage of sudoite-pyrophyllite-muscovite-quartz-rutile-pyrite. The stability of sudoite is attributed to a particularly Mg rich bulk rock composition, elevated sulphur activity, and metamorphic conditions just above the kaolinite to pyrophyllite transition. Nearby low variance chlorite, chloritoid-and pyrophyllite-bearing assemblages represent differing bulk rock compositions at potentially very similar metamorphic conditions. As the first find of sudoite in the Archaean, this occurrence in the Witwatersrand Basin extends the metamorphic setting and age in which sudoite is found to well beyond Phanerozoic belts of crustal thickening.