New lead isotopic and geochronologic constraints on mineralisation in the Macquarie Arc — insights from the Lake Cowal district, New South Wales AbstractArc of eastern Australia hosts several large to world-class Au and Cu deposits, including Cadia

and Northparkes. The Cowal district hosts the large Cowal gold mine (Endeavour 42) and other significant Au and Cu deposits including Endeavour (E) 41, E43 and Marsden. We present 37 new lead isotope results, including 31 high-precision Pb isotope analyses for the Cowal district, nearly all of which plot within or close to the field for Ordovician porphyry copper–gold deposits of Carr et al. (1995). The data are also consistent with mantle-like lead in the Tasmanides, suggesting that magmas and mineralising fluids did not interact with contemporaneous siliciclastic sediments such as those of the Adaminaby or Girilambone groups. Data for individual deposits mostly have lead model ages consistent with independent age constraints on mineralisation, including a new Re–Os date on molybdenite of 458 ± 2 Ma for porphyry-like mineralisation at Milly Milly. Results for Au-rich deposits in the Cowal district including E42 have discrete lead isotope signatures compared with porphyry-style copper–gold mineralisation. Overall, discrete lead isotope signatures associated with epithermal and/ or mesothermal gold mineralisation at E41 and E42 suggest that a different and/or more intense mineralising event occurred compared with porphyry-style prospects, including E43, E39 and Milly Milly. The results support studies which suggest that mineralisation in the Lake Cowal district occurred in the Ordovician around 450–460 Ma and not during the ‘alkalic’ porphyry–epithermal gold–copper event associated with the world-class Cadia and Northparkes deposits (~436–443 Ma). Decoy, the only VAMS-style prospect so far identified in the arc, has the least radiogenic lead isotope signature in the Lake Cowal district and is among the most primitive known to be associated with the Macquarie Arc — similar to that for Peak Hill and to some Cambrian MORB-like rocks. Such signatures may represent the substrate to the arc. We also