Timing of mineralization and deformation, Peak Au mine, Cobar, New South Wales∗

Tables 3 and 4 (indicated with an asterisk [∗] in the text) are Supplementary Papers lodged with the National Library of Australia (Manuscript Section); copies may be obtained from the Business Manager, Geological Society of Australia

The ⁴⁰Ar/³⁹Ar dating of alteration muscovite from the Peak Au mine in the Early Devonian Cobar Basin, New South Wales, has distinguished two major episodes of mineralization. Veined (Pb‐Zn‐Cu‐Au) mineralization was broadly synchronous with cleavage formation during the post‐inversion, shortening deformation of the basin sedimentary rocks, and replacive Ag‐Pb‐Zn mineralization significantly postdates the latter event. Veined base metals (Pb‐Zn‐Cu) and Au associated with silicification were coeval with three stages of cleavage formation (D₁, D₂ and D₃) after basin inversion. The Cu‐Au phase of mineralization at the Peak Au mine which was broadly contemporaneous with the culmination of the cleavage‐forming events (D₃) and with the local development of high‐strain zones occurred at 401.5 ± 1.0 Ma (⁴⁰Ar/³⁹Ar on muscovite). This date is essentially coeval with known fossil constraints on the age of basin formation, and indicates that basin inversion and deformation rapidly followed sedimentation. In contrast, replacive Ag‐Pb‐Zn mineralization occurred at 384.0 ± 1.4 Ma (⁴⁰Ar/³⁹Ar on muscovite) during an extended period of relaxation characterized by normal faulting (D₄) which followed the shortening deformation. This mineralization was associated with desilicification and chlorite‐muscovite replacement assemblages which cross‐cut the cleavages, and which may have been broadly contemporaneous with the deposition of part of the Mulga Downs Group which unconformably overlies the Cobar Supergroup. Rhyolite exposed in the core of the Peak Au mine largely contains inherited zircons that range in age from ∼430–1500 Ma. A few euhedral zircons have an age of ∼430 Ma and this is interpreted as a maximum date for the rock. Zircons from a syn‐D₃ chlorite‐muscovite replacement zone within the deposit have ²⁰⁶Pb/²³⁸U ages of ∼410–650 Ma and are apparently inherited.

Key words:

• ⁴⁰Ar/³⁹Ar age
• Cobar
• Peak Au mine
• syndeformational mineralization
• U‐Pb determination

Notes

Tables 3 and 4 (indicated with an asterisk [∗] in the text) are Supplementary Papers lodged with the National Library of Australia (Manuscript Section); copies may be obtained from the Business Manager, Geological Society of Australia

Present address: Australian Geological Survey Organization, GPO Box 378, Canberra, ACT 2601, Australia.