Precise age of the platiniferous Merensky reef, Bushveld Complex, South Africa, by the U-Pb zircon chemical abrasion ID-TIMS technique

The Bushveld Complex, South Africa, is the world's largest layered intrusion and host to the majority of the known resources of platinum group elements chromium, and vanadium. Despite intensive study, determining the precise crystallization age of the Bushveld Complex has proven to be difficult. In this study, zircon and rutile were identified in thin section and separated from a sample of pegmatitic feldspathic orthopyroxenite from the PGE-rich Merensky reef, Nest mine (Townlands shaft), Rustenburg section. Low U (21-105 ppm) zircon occurs with interstitial biotite and is locally in direct contact with sulfide. Rutile occurs mainly as inclusions within or overgrowths on chromite, especially in the chromitite stringers that bound the reef, and also as acicular grains with biotite. A U-Pb crystallization age of 2054.4 +/- 1.3 Ma was determined from single-crystal chemical abrasion ID-TIMS U-Pb dating of zircon and multigrain fractions of rutile yielded a U-Pb cooling age of 2055.0 +/- 3.9 Ma. Integration of the Merensky reef crystallization age with existing U-Pb age determinations for intrusions that pre- and postdate emplacement and crystallization of the layered rocks of the Bushveld Complex indicate that magmatism occurred at ca. 2054 Ma and is contemporaneous with related magmatic activity across the northern Kaapvaal craton of South Africa and Botswana.