A low-temperature hydrothermal cutoff: plagioclase .sup.40Ar/.sup.39Ar thermochronology of the Rustenburg Layered Suite, Bushveld complex

The mid-low-temperature thermal history of layered mafic intrusions is crucial to understanding the duration of hydrothermal systems and the formation of economically viable ore deposits within the large thermal anomaly of these magma bodies. Due to a lack of low-temperature chronometry, the mid-low-temperature thermal evolution of the world's largest layered intrusion, the Bushveld Complex, remains relatively unknown. We provide a comprehensive plagioclase .sup.40Ar/.sup.39Ar thermochronologic study throughout the Rustenburg Layered Suite (RLS) of the Bushveld Complex in South Africa. .sup.40Ar/.sup.39Ar step-heating analyses were applied to 20 plagioclase grains that encompass 12 stratigraphic levels and the entire ~ 7 km thickness of the Eastern Limb of the RLS. Individual plagioclase grains yield .sup.40Ar/.sup.39Ar plateau ages ranging from 1997.9 ± 5.0 Ma to 2051.4 ± 5.3 Ma, and closure temperatures of 151 ± 15 °C to 310 ± 12 °C (assuming a 10 °C Ma.sup.-1 cooling rate). Plagioclase .sup.40Ar/.sup.39Ar ages and closure temperatures, combined with .sup.40Ar/.sup.39Ar biotite ages from the same samples, suggest that RLS cooling rates decrease abruptly (from ~ 1000 °C Ma.sup.-1 to ~ 10 °C Ma.sup.-1) near the .sup.40Ar/.sup.39Ar closure temperature of biotite. The decrease in cooling rate is a product of the cessation of hydrothermal circulation (within < 1 Ma of zircon crystallization) associated with the emplacement of the RLS and is supported by numerical heat-loss simulations that produce the observed geochronology. The duration of hydrothermal circulation places age constraints on hydrothermal ore deposits within the host rocks (Transvaal and Witwatersrand Supergroups), as any ore deposits related to emplacement of the RLS must be coeval with .sup.40Ar/.sup.39Ar biotite ages.