SHRIMP U–Pb geochronology of magmatism and thermal events in the Archaean Marymia Inlier, central Western Australia
Zircons from porphyry and granitoid samples collected in and around the Marymia Gold Mine in the Marymia Inlier, Western Australia, record a complex history. The results of U–Pb studies confirm that the Plutonic Well greenstone belt, and the surrounding granitoid envelope (including a 2,721±6 Ma intrusion), represent an Archaean terrain, which was intruded by high-level, felsic to intermediate porphyries at 2,694±7 Ma and potentially also at 2,660±4 Ma. Zircon xenocrysts (≥ca. 3.35, 2.93 and 2.74 Ga) indicate that there was older crust within, or below, the greenstone belt at the time of porphyry emplacement. Zircons from the granitoid envelope and intrusions within the greenstone belt record subsequent metamorphism and/or hydrothermal activity coeval with magmatism in the Late Archaean (ca. 2.66–2.63 Ga), and peak metamorphism, magmatism and gold mineralisation in the Yilgarn Block. A later period of metamorphism and hydrothermal activity at ca. 1.72 Ga is coeval with orogenesis in the southern Capricorn Orogen. Both the Late Archaean and Palaeoproterozoic thermal events have altered zircons, redistributed trace elements and caused zircon recrystallisation, which is distinctive in its isotope chemistry (in particular Th/U ratios >1) and morphology (e.g. homogeneous in transmitted light and back-scattered electron images, but sector-zoned in cathodoluminescence).