Abstract

Pb-isotopic compositions are reported for 12 ore localities within the late Archean Abitibi greenstone belt. The studied samples carry massive or disseminated sulfides hosted in a variety of materials, including mafic–ultramafic igneous rocks, felsic lavas, porphyries, and sedimentary iron formations. Repeated leaching experiments on these sulfides frequently revealed the presence of a radiogenic Pb component, which is attributed to in situ decay of U and Th. The leaching experiments make it possible, in some cases, to separate the radiogenic Pb from the initial Pb included in the minerals. Six Pb–Pb isochrons formed by the analyses on leachates and residues show little evidence of secondary perturbations and yield ages that are, within error, similar to those determined for the supracrustal assemblage. This implies that the ores were concentrated synchronously with the main phases of magmatic activity, close to 2.7 Ga.The initial isotopic compositions of the sulfide specimens point to the existence of two different sources of metals: (i) juvenile, mantle-derived igneous rocks and (ii) older recycled supracrustal series. Ore formation frequently involves mixing of metals from these two sources in variable proportions. The initial isotopic composition of these two reservoirs is best evaluated by examining the composition of sulfides associated with komatiitic lava flows and with sedimentary iron formations, respectively.

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