ARTICLE Lead isotope study of the late Archean Lac des Iles palladium deposit, Canada: enrichment of platinum group elements by ponded sulfide melt John G. Hinchey & Kéiko H. Hattori Received: 20 December 2005 / Accepted: 7 February 2007 / Published online: 27 February 2007 # Springer-Verlag 2007 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 2.69 Ga) and tonalitic country rocks (approximately 2.727 to 2.775 Ga) 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 (<0.2 ppb U) and Th (<0.01 ppb) compared to Pb (≫50 ppb). 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. Keywords Lead isotopes . Platinum group elements . PGE mineralization . Lac des Iles . Canada Introduction The Lac des Iles mine is hosted by the Neo-Archean Lac des Iles mafic–ultramafic igneous complex (Fig. 1) and represents the only primary Pd mine in Canada. It had proven and probable reserves of 88 million metric ton grading 1.51 g/t Pd, 0.17 g/t Pt, and 0.12 g/t Au before the major expansion of mining in 2002. As of December 2005, proven and probable reserves were 13.5 and 3.5 million metric ton grading 2.13 and 6.62 g/t Pd in the open pit and underground operations, respectively, (North American Palladium Annual Report, 2006). Unlike many other platinum group element (PGE) deposits that are hosted by Ni sulfides at the base of mafic intrusions (e.g., Sudbury and Noril’sk), the mineralization at Lac des Iles is characterized by low contents (<2 vol.%) of sulfides disseminated in altered gabbroic rocks. The host rocks formed by successive intrusions of comagmatic mafic magmas and show a variety of breccias containing earlier intrusions as fragments (Fig. 3). Later, fertile mafic intrusions are accompanied by abundant veins and pods of pegmatites, suggesting that they were volatile rich. In addition, the ore at Lac des Iles is highly enriched in Pd compared to other PGE (i.e., Pd/Pt ratios ∼10, Pd/Ir ratios ∼10,000; Hinchey et al. 2005). These characteristics had lead to various models for the mineralization, including hydrothermal/deuteric miner- alization (Watkinson and Dunning 1979) and zone-refining processes (Brügmann et al. 1989). Based on recent detailed mapping, Hinchey et al. (2005) proposed that the mineral- ization is essentially magmatic and that PGE were enriched by reworking of earlier formed sulfide melt by later magmas. Crustal rocks contain high concentrations of U and Th compared to mantle rocks, which makes Pb isotope compositions a sensitive indicator in detecting the contri- butions of crustal components to mantle-derived magmas Miner Deposita (2007) 42:601–611 DOI 10.1007/s00126-007-0129-3 Editorial handling: M. Chiaradia J. G. Hinchey (*) : K. H. Hattori Department of Earth Sciences, University of Ottawa, Ottawa, ON K1N 6N5, Canada e-mail: johnhinchey@gov.nl.ca Present address: J. G. Hinchey Department of Natural Resources, Geological Survey of Newfoundland and Labrador, P.O. Box 8700, St. John’ s, NL A1B 4J6, Canada