Abstract

Galena separates were collected from three Proterozoic supracrustal sequences comprising the Labrador Central mineral belt and also from minor occurrences in the Archean granitoid basement. Galena occurs in two environments: epigenetic quartz veins, and veins associated with uraninite occurrences. Fourteen samples from seven vein occurrences along the unconformity between shales of the ca. 2000-Ma Moran Lake Group and Archean basement and other spatially related rocks define a secondary isochron with a t 2 of 1163 Ma and a t 1 of 2473 Ma. These model ages suggest that Pb was mobilized from the Archean basement and deposited during the Grenvillian orogeny. Scatter in 208 Pb/ 206 Pb ratios reflects differing mixtures of source components. Separates from the Big Bight and Ford's Bight showings in coastal exposures of the 1860-Ma Upper Aillik Group have Pb isotope ratios which reflect a source region with higher U/Pb and Th/Pb values than source regions for the Moran Lake Group galenas. In uraninite-associated Pb occurrences linked to post-tectonic granites elsewhere in the Upper Aillik Group, Pb isotope ratios reflect mixtures of country-rock Pb from low U/Pb sources (similar to that of the Moran Lake Group) and uranogenic Pb. Mobilization of the Pb from uraninite may have been the product of Grenvillian tectonism. The Moran Lake Group and uraninite-associated leads were derived from Archcan crust in the Nain province and this crust isotopically resembles Nuk gneiss of southern Greenland. The Big Bight-Ford's Bight Pb isotope ratios indicate a more radiogenic Pb source to the southeast of the Central mineral belt and therefore the influence of another cratonic block outboard from the Nain province.

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