Petrological and geochemical constraints on the origin and nature of the Eagle’s Nest intrusion, McFaulds Lake Greenstone Belt, Ontario

Abstract

The Eagle’s Nest intrusion is a mafic-ultramafic, blade-shaped dike that hosts the only known economically significant orthomagmatic Ni-Cu-(PGE) mineralization in the Ring of Fire region of Ontario. It is part of the Koper Lake subsuite of the more voluminous Ring of Fire Intrusive Suite (~2736–2732 Ma) and occurs within the Meso- to Neoarchean in age McFaulds Lake Greenstone Belt. The Ring of Fire Intrusive Suite is host to chromite, Fe-Ti-V, and Ni-Cu-(PGE) mineralization. Previous studies investigated the orebody and mineralization hosted by the Eagle’s Nest intrusion, with limited attention to the unmineralized parts of the system. This study applied multidisciplinary petrological and geochemical techniques to evaluate the petrogenetic controls on the formation of the Eagle’s Nest intrusion through the examination of the less mineralized portions of the intrusion and genetically related mafic dikes. The Eagle’s Nest intrusion can be subdivided into the marginal and the inner zone. The marginal zone is composed of gabbroic rocks that exhibit the most evolved mineralogical and geochemical characteristics, with evidence of intense pseudomorphic alteration that often preserves primary magmatic textures. Contacts with the host tonalite vary, generally reflecting a prolonged high magma flux, but only rarely preserving evidence of rapid cooling and chilled margins. The marginal zone gradationally transitions into the inner zones, which consists of ultramafic ortho- to mesocumulate rocks. The inner zone is characterized by coherent linear geochemical trends that reflect olivine and chromite accumulation with variable proportions of intercumulus silicate phases and interstitial sulfides, consistent with petrographic observations. Most inner zone rocks are characterized by a strong positive correlation between MgO and Cr2O3, reflecting the crystallization of olivine and chromite in cotectic proportions. However, several of the mineralized peridotite samples deviate from this trend despite containing similar proportions of these minerals. Petrographic observations and intercumulus pyroxene mineral chemistry suggest that the deviation from the cotectic trend may be caused by sulfide percolation and displacement of a Cr-rich intercumulus silicate melt, rather than the presence of less than cotectic proportions of olivine and chromite. A new parental magma composition estimate was established using olivine and chromite mineral chemistry, as well as whole rock geochemistry of ultramafic cumulate rocks interpreted to reflect cotectic proportions olivine and chromite, with variable proportions of intercumulus silicate melt. The estimate yielded a parental magma composition that contained ~15 wt% MgO and ~11 wt% FeOt, consistent with a komatiitic basalt magma. The new composition is more evolved than previous estimates, however, it is in close agreement with the composition of identified chilled margins, associated mafic dikes, and olivine. Forward thermodynamic modeling simulations of the new parental magma, reproduce the petrographically determined crystallization sequence at low pressures, suggesting that the Eagle’s Nest intrusion formed at shallow crustal levels. Whole rock geochemistry and Sm-Nd isotopes show that the Eagle’s Nest magma was derived from a depleted mantle source, above the garnet stability field, which then underwent extensive crustal contamination from multiple sources that included both the host tonalite, and older supracrustal rocks. Crustal contamination by sulfur-bearing supracrustal rocks likely contributed to attaining sulfide saturation of the magma, as evidenced by Δ³³S values consistent with mass-independent fractionation. The distinctive petrological and metallogenic characteristics of the Eagle’s Nest intrusion in the Esker Intrusive Complex may be a result of several distinct processes involving both emplacement dynamics and parental magma composition, resulting in unique metal endowments relative to other intrusions in the McFaulds Lake Greenstone Belt.