Geochemistry and geochronology of the Shebandowan greenstone belt in the vicinity of the Moss Lake deposit, NW Ontario

Abstract

This thesis presents a detailed investigation of the geology, geochemistry, and geochronology of the Archean Shebandowan Greenstone Belt (SGB) in the vicinity of the Moss Lake gold deposit in Northwestern Ontario. The Moss Lake property is located in the western part of the SGB and consists primarily of rocks belonging to the Greenwater and Burchell assemblages. The study characterizes the geological attributes, tectonic setting, and timing of magmatism within the belt and provides regional context for ongoing mineral exploration. The research involved extensive fieldwork, including detailed lithological descriptions and structural analysis, supported by petrographic analysis of thin sections. Whole-rock geochemical analysis was performed on 56 samples to classify rock types, determine magmatic affinities, and evaluate element mobility, revealing distinct geochemical signatures between the Greenwater and Burchell assemblages and various intrusive bodies. The Greenwater assemblage mafic rocks exhibit tholeiitic affinities with flat HREE, enriched LREE, and negative Nb anomalies. The Burchell assemblage mafic rocks are calcalkaline, with moderately enriched LREE, flat HREE, and negative Nb-Ti anomalies. Similarly, the felsic and intermediate metavolcanic rocks show distinct geochemical signatures between assemblages. Greenwater rocks are more enriched in LREE and display stronger negative Ti anomalies compared to the less enriched LREE and weaker Ti anomalies observed in Burchell rocks. U-Pb zircon geochronology on four key samples yielded new ages: 2716.0 ± 0.45 Ma for the Moss Lake syenogranite stock, 2718.34 ± 0.14 Ma for the Obadinaw quartz syenite stock, 2711.80 ± 0.14 Ma for an intermediate metavolcanic rock, and 2707.35 ± 0.14 Ma for the Greenwater Lake quartz monzonite stock. These ages complement and refine the existing geochronological framework of the SGB. Neodymium isotope analysis of thirteen samples provided insights into the mantle source and crustal contamination processes. The Greenwater assemblage exhibits consistently positive εNd(t) values in both mafic and felsic-intermediate rocks (+1.6 to +2.7), indicating a dominantly juvenile mantle source with limited crustal involvement. In contrast, the Burchell assemblage shows a broader εNd(t) range (+0.01 to +3.2), suggesting a more heterogeneous source and greater influence from crustal assimilation, despite an overall juvenile magmatic character. The integration of geological, geochemical, and geochronological data supports models of a complex Neoarchean tectonic evolution involving distinct magmatic pulses and settings for the Greenwater and Burchell assemblages. Geochemical evidence suggested the Greenwater assemblage originated from an oceanic plateau evolving to a primitive arc, whereas the Burchell assemblage formed in a primitive arc environment. Intrusive bodies were classified as tonalite trondjhemite granodiorite, sanukitoids, and Archean hybrid granites, reflecting diverse sources and conditions of formation. The results confirm independent magmatic histories for the two assemblages and highlight a protracted crustal evolution involving juvenile mantle input and crustal assimilation.