Abstract

The lead isotopic compositions of sulphide samples (mainly galenas) from a representative set of mineral deposits within the Central Metasedimentary Belt of the Grenville Province have been determined by high-precision solid-source mass spectrometry. Data for 13 sites of Proterozoic age (including Tétrault mine, which lies outside the belt) reveal an isotopic fine structure that can be correlated with the various geological and regional groupings of the deposits. In particular, sulphides in the Balmat–Edwards mining district are seen to be isotopically atypical of Central Metasedimentary Belt Pb–Zn mineralizations, and those in the Mont Laurier Basin are distinct from those in the Hastings Basin. Collectively, the data suggest that the lead in the sulphides was derived from an environment with the relatively low U/Pb characteristics of oceanic mantle, and therefore that the Central Metasedimentary Belt is a segment of "proto-crust" formed at approximately 1.3–1.0 Ga. This precludes its derivation from older segments of the Canadian Shield. Leads in ores of the Balmat–Edwards district, however, seem to be derived at least partly from sources of a more continental character.Galenas from 11 post-Ordovician fissure veins define a secondary isochron that shows the Grenville basement to be the only likely source of the vein lead. The slope of the isochron indicates that the radiogenic component of the lead in the veins could not have been derived from the uraniferous deposits dated at 970–950 Ma by Fowler and Doig (1979). The isotopic variations in the vein galenas are thought to reflect a general uniformity of Th/Pb and variability of U/Pb in the Grenville gneisses.

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