LEAD ISOTOPE STUDY OF THE LATE ARCHEAN LAC DES ILES PALLADIUM DEPOSIT, CANADA: ENRICHMENT OF PLATINUM GROUP ELEMENTS BY PONDED SULFIDE MELT

Abstract

Lead isotopic compositions of mineral separates are presented from the mineralized zones (Roby, Twilight, and High Grade zones) of the Lac des Iles palladium deposit (approximately 2.69Ga) and tonalitic country rocks (approximately 2.727 to 2.775Ga) in the Wabigoon subprovince of the western Superior Province of Canada. Plagioclase separates show Pb isotope ratios similar to those of the late Archean depleted mantle. Sulfide minerals coexisting with plagioclase have a more radiogenic Pb isotope composition, but very low concentrations of U (<0.2ppb U) and Th (<0.01ppb) compared to Pb (≫50ppb). The isotope data are consistent with a model involving the incorporation of radiogenic Pb from country rocks to sulfide melt near the base of the magma chamber or along the conduit of the parental magmas. The proposed interpretation is also supported by decreasing Cu/Pd ratios from earlier barren to late fertile gabbro intrusions in the complex. The ponding sulfide melt became enriched in platinum group elements, as it was interacting with a large magma volume during the evolution of the Lac des Iles igneous complex.