⁴⁰Ar/³⁹Ar geochronological evidence for multiple postmetamorphic hydrothermal events focused along faults in the southern Abitibi greenstone belt

Abstract

Thermal histories in the Abitibi and Pontiac subprovinces vary for three general domains: (1) zones of impermeable rock with cooling rates of >8 °C/Ma; (2) zones of impermeable rock that are spatially associated with monzogranite plutons, with apparent cooling rates of 1–2 °C/Ma; and (3) permeable high-strain zones with apparent cooling rates of <l °C/Ma. Evidence for multiple hydrothermal events in fault zones includes (1) most age spectra having a disturbed form; (2) concave-down spectra for samples from subgreenschist-facies faults, indicating multiple episodes of white mica growth; (3) Cr-muscovite from a greenschist-facies segment of the Larder Lake–Cadillac fault yielding a plateau age of 2543 ± 8 Ma, postdating regional metamorphism by 120 Ma; (4) two discrete ages for postmetamorphic amphibole from the Larder Lake–Cadillac fault (2578 ± 10, 2421 ± 15 Ma); and (5) two discrete dates for white mica (2414 ± 9, >2594 Ma) from the volcanogenic massive sulphide-related sericite alteration, with the coarser mica yielding the younger date. Based on radiometric dating, impermeable rocks were affected weakly by postmetamorphic hydrothermal events. Minerals in zones of higher permeability were reset during the intrusion of monzogranite plutons. Highly permeable zones, such as the Larder Lake–Cadillac and Porcupine–Destor faults, were overprinted repeatedly. This produces a drawn-out series of dates from minerals with varying closure temperatures. Radiometric ages from fault zones, or ore deposits, cannot be used to interpret the cooling history and evolution of the Abitibi belt as a whole.