The Lac Des Iles Palladium Deposit, Ontario, Canada part I. The effect of variable alteration on the Offset Zone

Abstract

The recently discovered Offset Zone of the Mine Block Intrusion of the Lac des Iles Complex hosts palladium mineralization with unusually high Pd/Pt and Pd/Ir ratios in rocks that range from relatively unaltered norite to amphibolites and chlorite–actinolite–talc schist. Quantitative assessment of the effect of progressive alteration using mineral modes was done using total silicate H2O as a monitor of reaction progress (ξ = moles H2O added to form alteration minerals per 100 g of rock). Major mineral modal variations define three reaction regions: (1) ξ = 0.00–0.03, characterized by epidote/clinozoisite formation and some amphibole; (2) ξ = 0.03–0.23, characterized by formation of chlorite, amphibole, quartz muscovite/sericite, and calcite after plagioclase + pyroxene; and (3) ξ = 0.23–0.28, characterized by the formation of talc after earlier formed amphibole. Epidote occurs as an incongruent product from the destruction of plagioclase that is itself lost as the reaction proceeds. Pyroxene is altered at about twice the rate of plagioclase, resulting in pyroxene-rich protoliths to be more altered than those relatively enriched in plagioclase. Major elements variations largely reflect variations in the plagioclase/pyroxene ratio of the protolith, but compositional trends suggest a loss of Na with reaction progress. The base metal sulfides chalcopyrite, pyrrhotite, and pentlandite show decreasing abundance with reaction progress, forming pyrite (± magnetite) as an intermediate reaction product that also is lost as the reaction proceeds. Millerite is overall low but increases slightly. A more limited data set on the platinum-group minerals suggests that platinum-group element (PGE)-arsenides increase whereas PGE-sulfides and PGE-Bi-tellurides decrease with reaction progress. Assuming ore element concentrations in the protolith were constant and similar to relatively fresh norites, Pd increases modestly, by 5 %, whereas Pt decreases by about 65 % in the most altered rocks. Similarly, Cu, Au, and S decrease by 60, 82, and 94 %, respectively, in the most altered rocks. The antithetical behavior of Pd and the fact that Pd enrichment is not seen in altered dikes suggest that the Pd originally was enriched in the more melanocratic protoliths that are most extensively altered and that Pd also was lost in part. These results are consistent with the mineralization having been a high-temperature event that predated the amphibolite/greenschist alteration.