Lead isotopes in the Grenville and adjacent Palaeozoic formations

Abstract

Lead-isotope ratios have been determined for a preliminary set of 16 galenas from the Grenville and some adjacent Palaeozoic formations in southern Ontario and Quebec. The 206Pb/204Pb ratios of the Grenville-hosted deposits range from 16.24 to 20.20. Six of the Grenville-hosted samples have a post-Ordovician age of mineralization, but may represent re-mineralized Grenville lead. With the five 'true' Grenville samples they can be used to define a Grenville 'secondary isochron' on a 207Pb/204Pb–206Pb/204Pb plot. The trend of this isochron appears to be consistent with published feldspar- and marble-lead data for nearby sections of the Grenville. Its slope (.120 ±.015) is compatible with a time of ~1300 Ma ago for inclusion of uranium in the sediments of the Grenville supergroup, followed by lead mineralization at ~1000 Ma. Calculations based on a simple lead-isotope evolution model give a similar age (1280 ± 15 Ma) for the stratabound Tétrault Pb–Zn deposit.The geologically similar New Calumet deposit has a quite different model age of 1115 ± 20 Ma. This is similar to the model age of the Balmat, New York ore deposits and it appears that New Calumet and Balmat may be members of a contemporaneous mantle (volcanic)–continental sequence.An apparent correspondence of the isotopic ratios of the Palaeozoic samples with the Grenville secondary isochron raises some critical questions regarding the development and migration of the Pb in these deposits.