Zircon U-Pb dating of Early Palaeozoic monzonitic intrusives from the Goonumbla area, New South Wales

Abstract

Zircon U-Pb ages measured on four small intrusions into the succession of Ordovician volcanic rocks that hosts Northparkes Cu-Au mine northwest of Parkes, New South Wales, place limits on the age of the volcanic sequence. The basal Nelungaloo Volcanics are constrained by a cross-cutting monzodiorite to be ≥484.3 ± 2.9 Ma (Early Ordovician). Similarly, the overlying basal Goonumbla Volcanics are constrained by another cross-cutting monzodiorite to be ≥450.8 ± 4.2 Ma (Middle Ordovician). A later generation of monzonites intruded into the middle and upper Goonumbla Volcanics yield ages of 439.1 ± 4.5 and 438.9 ± 4.7 Ma (Siluro-Ordovician). These various ages are consistent with the ages of fossiliferous sediments within the volcanic sequence, and indicate that both the intrusive and volcanic rocks span an appreciable period of time--neither are the product of a single magmatic episode. Intrusion of the youngest monzonites and mineralisation was virtually contemporaneous.