Pb-Zn occurrences and their Pb-isotopic signatures bearing on metallogeny and mineral exploration; Paleozoic sedimentary rocks, Northern Appalachians, Quebec

Galena Pb-isotope ratios of epithermal vein and disseminated sulphide occurrences in the Taconian orogen and Siluro-Devonian basin cluster around 17.90-18.50 for (super 206) Pb/ (super 204) Pb. The major source of Pb in most, if not all, occurrences is a fairly common continental crust, a characteristic found in published analyses of Grenville feldspar Pb. A SW to NE increase in galena (super 206) Pb/ (super 204) Pb ratios is ascribed to the supply of several types of detritus from Grenville basement during the Cambro-Ordovician: Coarse-grained, K-feldspar-bearing in the SW, grading into fine-grained phyllitic, and relatively more highly radiogenic in the NE. Emplacement 1) of Pb-Zn-baryte veins and dissemination, commonly of homogeneous crustal Pb-isotopic signature, was late Taconian; 2) of Pb-Zn-quartz veins, of less homogeneous signature, was post-Taconian; and 3) of Pb-Zn- carbonate veins, relatively highly radiogenic and commonly homogeneous, was late or post-Acadian. Signatures of the first- mentioned group seem to be most useful in exploration.