Pau Wawrzonkowski MSc Thesis Abstract
The Eagle River mine is a mesothermal orogenic gold deposit hosted within the Archean Mishibishu Lake greenstone belt, approximately 50 km west of Wawa, Ontario. The underground mine has been in constant production since 1995, mining multiple steeply dipping quartz-bearing, shear-hosted gold horizons that dip to the north. As of the end of 2021 the Eagle River Mine has produced a total of 7.4 Mt of ore averaging 9.86 g/t Au yielding 1.5 M oz of refined Au (SRK, 2022). The majority of the gold mineralization is hosted within an ellipsoidal quartz diorite, with the remainder hosted within the surrounding calc-alkaline to tholeiitic volcanic rocks. The Mishibishu Lake greenstone belt is surrounded by granitic units which include the Bowman Lake batholith to the north-east, the Floating Heart batholith to the south and the Central pluton to the northwest. The objective of the study was to correlate veining in the chaotic unit to mineralization, investigate alteration geochemistry, and to compare Eagle River to other Archean Au deposits. This was undertaken using a combination of geochronology, petrography, and whole rock lithogeochemistry. The thesis focused on four zones within the deposit, the 8, Falcon, Newt Lake, and Peek-a-Boo zones.
For the thesis, drill core within the four zones has been reexamined (154 drillholes) to have consistent logs between drillholes, and to focus on the chaotic and Laminated units. This constrained the extent of the units and how they are related to one another. Within the Falcon zone part of the mineralization is hosted within the Laminated unit that cuts the chaotic unit. Due to the association with the mineralization a vein paragenesis of the chaotic unit has been developed to determine how the veins are related to one another, and if there is any association with mineralization. To assess the tectonic history of the Eagle River area, geochronological, geochemical, and petrographical samples were collected to determine the ages of the surrounding batholiths and the diorite, to determine the geodynamic setting of the area, as well as to see how the rocks are geochemically related.
A total of 95 samples (geochronology=4, geochemistry=57, and thin sections=34) were selected from the four zones. The new U-Pb zircon ages from this study include the Central pluton (2656.38 ± 0.41 Ma), the Bowman Lake batholith (2658.35 ± 0.41 Ma), the Floating Heart batholith (2687.26 ± 0.42 Ma), and the mine diorite (2716.22 ± 0.41 Ma). The new age of the Bowman Lake batholith provides a tighter constraint on plutonism whereas the mine diorite age constrains the lower limit of mineralization.
Whole rock geochemistry was completed on the volcanic rocks, gabbro, feldspar porphyry, granite, and diorite, as well as the chaotic and Laminated units from the four zones. The Laminated unit has been subdivided into two subgroups (A and B) based on the volcanic classification of Pearce (1996). The volcanic rocks, gabbro, and Laminated Group B unit are characterized by La/Smpm ratios of 0.8-1.4, variable Ti anomalies (Ti/Ti* of 0.5-1.2), and negative Nb anomalies (Nb/Nb* of 0.5-0.9), consistent with a magmatic arc affinity and melts derived by slab dehydration. The feldspar porphyry, chaotic, and Laminated Group A units are characterized by enriched La/Smpm ratios of 2.1-6.1, variable Ti anomalies (Ti/Ti* of 0.3-1.0), and Zr anomalies (Zr/Zr* of 0.7-2.3), as well as negative Nb anomalies (Nb/Nb* of 0.3-0.6) consistent with a magmatic arc origin with a higher degree of crustal contamination. The granites are characterized by La/Smpm ratios of 2.8-7.1, generally negative Ti anomalies (Ti/Ti* of 0.1-0.6), and negative Nb anomalies (Nb/Nb* of 0.1-0.3), consistent with an arc origin possibly related to melting of the downgoing slab. The diorite is characterized by La/Smpm ratios of 1.7-3.5, variable Ti anomalies (Ti/Ti* of 0.6-1.8), and negative Nb anomalies (Nb/Nb* of 0.3-0.5), consistent with an arc origin with magmas derived from either a mix of slab melts and slab-dehydration or garnet-bearing source rocks. Based on the geochemical differences, the rocks from Eagle River area suggest a switch between slab melting and slab-dehydration in the arc source, but further geochronological dating is required to constrain the timing.
Eight different vein types were identified within the chaotic unit, as well as five different generations of quartz veining. The chaotic unit veins are similar between the zones, suggesting that the chaotic unit is continuous between the zones. The chaotic unit veins are variably striking, have variable infill and alteration halos, and the sequence of younging is dominantly unidirectional (with exception of Q3 which has multiple cross-cutting relationships, and Q4 which has poor constraints) based on cross-cutting relationships. From the vein types there were a total of six occurrences of gold found within the 8 and the Falcon zones, all of which are hosted within quartz veins (Q2 and Q5 veins) that are parallel to the bands within the Laminated unit. The gold can either be found as inclusions within pyrite, or along grain boundaries that include pyrite-pyrite, quartz-clinochlore, or quartz-pyrite. The alteration within the Eagle River complex is dominated by albitization, K-feldspar, chloritization and carbonatization. The potassic and hematite alteration commonly increases towards the quartz veining within the mine, but not consistently. Potassic and hematite alteration are associated with faulting and generally increases towards the fault. The sericite alteration is commonly associated with the Laminated unit, and can be categorized based on thickness, alteration intensity and associated minerals to point towards the center of the deformation zone, as well as potentially used as markers when cut by younger units or faults. The albite alteration is dominantly associated with the chaotic unit and the variable striking veining. In general, the alteration types and intensity are variable, but the alteration intensities do not consistently increase towards mineralization and consequently do not serve as reliable vectors towards it.