Olivine compositions reveal an andesitic parent magma for the Archean palladium-mineralized Lac des Iles Complex of Ontario, Canada

Abstract

Feldspathic lherzolite and harzburgite are reported here for the first time in the southern Lac des Iles Complex; an ~ 2.69-Ga arcuate mafic intrusion that hosts world-class Pd mineralization within varitextured and brecciated gabbronoritic rocks. The olivine-bearing rocks (Mg#_75.9–80.8) are medium- to coarse-grained, weakly to strongly serpentinized, and bordered by variably altered norite. They possess relatively high Al₂O₃ contents (4.8–10.3 wt.%), pronounced negative Nb/Nb* (0.07–0.25) values, flat to shallow negatively sloping REE profiles (La/Yb_N 1.3–4.4), and variable Eu/Eu* (0.4–1.6) values. Weakly altered samples comprise subhedral olivine (Fo_78.6–81.8) with polymineralic melt inclusions and peritectic orthopyroxene rims, cumulus orthopyroxene, sub-poikilitic clinopyroxene, as well as plastically deformed and clustered plagioclase crystals. With increasing degrees of alteration, olivine is variably serpentinized or pseudomorphically replaced by an assemblage of talc, carbonate, magnetite, and Fe-sulfides. Sparsely disseminated pentlandite-chalcopyrite-pyrrhotite (± sphalerite) blebs with platinum-group minerals (zvyagintsevite, kotulskite, and sperrylite) are rare and commonly partially replaced by magnetite. Nickel concentrations are primarily controlled by olivine (1900–4200 ppm Ni), as supported by a positive correlation between whole-rock MgO and Ni contents. Sulfur, Cu, Pd, and Pt show positive correlations and Pd/Pt values range from 2.6 to 6.7. The whole-rock and mineral compositions can be replicated through the modeling of batch crystallization of a hydrous andesitic magma that has interacted with antecedent feldspathic cumulates. The parent magma was likely at or close to sulfide saturation upon emplacement and may have co-existed with a volatile-rich phase. The Lac des Iles Complex may serve as a type example of Archean continental arc-related magmatic sulfide deposits, fed by fertile andesitic parent magmas formed through the differentiation of primitive sub-arc mantle melts in the juvenile crust.