The geologic setting, kinematics and deformation mechanisms of the Eagle River gold deposit, Northern Ontario

Abstract

The Neoarchean Eagle River orogenic gold deposit in the Mishibishu greenstone belt of northern Ontario, Canada, was investigated through a multidisciplinary approach integrating field mapping, lithogeochemistry, isotope analysis, EBSD mapping, CVA analysis and geothermometry techniques. Near mine host rocks comprise two distinct suites: tholeiitic and calc-alkaline volcanic rocks. When normalized to primitive mantle values, the tholeiitic basalts and gabbros have negative Nb and Ti anomalies, near flat LREE (La/Smpm = 0.94-1.25) to weakly fractionated HREE patterns (Gd/Ybpm = 0.88-1.62) and εNd T(2700) values of +1.93 to +2.50, indicating little crustal contamination and formation within a primitive intra-oceanic plateau setting analogous to the Phanerozoic Ontong Java plateaus. The second suite (calc-alkaline) comprises intermediate and felsic samples, including diorites, andesites, granodiorites, and granites. The intermediate samples exhibit negative Nb and Ti anomalies, with moderate LREE patterns (La/Smpm = 1.29-5.82) and HREE fractionation (Gd/Ybpm = 0.70-4.28). The felsic samples have moderate LREE enrichment (La/Smpm = 1.29-5.82), HREE fractionation (Gd/Ybpm = 0.70-4.28), and εNd(2700 Ma) values of +1.53 to +2.56, suggesting evolution in a closed system through subduction-driven fractional crystallization, similar to the modern-day western Canadian Cordillera orogeny. The Eagle River deformation zone is a ~4 km-long, curvilinear, along-strike brittle–ductile shear zone characterized by oblique dip-slip deformation. It records two dominant kinematic regimes: a pure shear–dominated component associated with oblique crustal shortening, and a simple shear component reflecting dextral reverse transpression. Sheared quartz veins have been fully recrystallized and exhibit non-coaxial deformation with triclinic flow geometry. Gold mineralization preferentially occurs at vein selvages where fractured plagioclase networks provide a brittle contrast with dynamically recrystallized quartz veins and veinlets. Sheared quartz grains show evidence of grain boundary migration within the dislocation creep regime, with gold grains, sulphides, and other impurities concentrating at the quartz vein selvage. These findings support a tectonic model in which subduction-related, calc-alkaline rocks were emplaced on primitive oceanic plateau crust in an intra-oceanic island arc environment and underwent ductile deformation, with gold concentration facilitated by crystal plastic deformation, creating favorable structural traps and fluid pathway arrays. This research enhances our understanding of the formation and tectonic framework of this high-grade gold deposit, providing insights into exploration prospectivity within similar geological settings.