TIMING OF MINERALIZATION AND DEFORMATION, PEAK-AU MINE, COBAR, NEW-SOUTH-WALES

The Ar-40/Ar-39 dating of alteration muscovite from the Peak Au mine i n the Early Devonian Cobar Basin, New South Wales, has distinguished t wo major episodes of mineralization. Veined (Pb-Zn-Cu-Au) mineralizati on was broadly synchronous with cleavage formation during the post-inv ersion, shortening deformation of the basin sedimentary rocks, and rep lacive Ag-Pb-Zn mineralization significantly postdates the latter even t. Veined base metals (Pb-Zn-Cu) and Au associated with silicification were coeval with three stages of cleavage formation (D1, D2 and D3) a fter basin inversion. The Cu-Au phase of mineralization at the Peak Au mine which was broadly contemporaneous with the culmination of the cl eavage-forming events (D3) and with the local development of high-stra in zones occurred at 401.5+/-1.0 Ma (Ar-40/Ar-39 on muscovite). This d ate is essentially coeval with known fossil constraints on the age of basin formation, and indicates that basin inversion and deformation ra pidly followed sedimentation. In contrast, replacive Ag-Pb-Zn minerali zation occurred at 384.0+/-1.4 Ma (Ar-40/Ar-39 on muscovite) during an extended period of relaxation characterized by normal faulting (D4) w hich followed the shortening deformation. This mineralization was asso ciated with desilicification and chlorite-muscovite replacement assemb lages which cross-cut the cleavages, and which may have been broadly c ontemporaneous with the deposition of part of the Mulga Downs Group wh ich uncomformably overlies the Cobar Supergroup. Rhyolite exposed in t he core of the Peak Au mine largely contains inherited zircons that ra nge in age from approximately 430-1500 Ma. A few euhedral zircons have an age of approximately 430 Ma and this is interpreted as a maximum d ate for the rock. Zircons from a syn-D3 chlorite-muscovite replacement zone within the deposit have Pb-206/U-238 ages of approximately 410-6 50 Ma and are apparently inherited.