The geology, geochemistry, and magmatic evolution of the Legris Lake mafic–ultramafic complex, Ontario, Canada

Abstract

The Archean Legris Lake mafic–ultramafic complex (LLC) is spatially (~ 12 km east) and temporally (2690.59 ± 0.77 Ma) associated with the world-class Lac des Iles (LDI) palladium deposit. Modeling of whole-rock major-element compositions suggests that this complex, previously interpreted as gabbroic, is gabbronoritic in composition, adding to the likely association between LDI and the LLC. Mantle normalized trace-element profiles and modeled parental melt compositions from the cumulate lithologies indicate initial melt extraction from a metasomatized mantle source in an arc setting and subsequent crystallization of olivine, orthopyroxene, and plagioclase at depth following assimilation of SiO2 and S from tonalitic country rock. External S assimilation contributed to sulfide saturation at depth prior to or during emplacement, but sulfide melt entrainment in intruding magmas and its retention at the site of emplacement was variable. These interpretations are supported by (1) variations in Cu/Pd ratios between PGE-rich and PGE-poor lithologies, (2) the strongly negative Eu/Eu* of paragenetically early lithologies, (3) the overlap between whole-rock εNd(T) values of LLC lithologies and Wabigoon tonalite, and (4) the positive δ34S and uniformly non-zero ∆33S values of magmatic sulfides. During emplacement the intruding melts also assimilated metasedimentary country rock. This effect likely became more pervasive during later stages and resulted in additional melt fractionation and inheritance of the trace-element signature of the metasedimentary country rock by the intruding magmas. This assimilation does not, however, seem to have been an important driver of sulfide saturation in this system.