The Eagle River mine occurs within the Mishibishu greenstone belt that is a broad arcuate syncline 55 km long east-west and 16 km wide north-south. This belt is part of the Wawa Subprovince of the Archean age Superior Province. Supracrustal rocks in the belt are dominated by greenschist facies mafic to intermediate volcanic rocks with lesser sedimentary rocks including iron formation and intermediate to felsic volcanic rocks. The belt is surrounded by Archean granitic rocks and includes two internal granitic batholiths occupying the central portion of the belt.
The northern limb of the belt, where the Mishi Mine is located, is dominated by an assemblage of clastic sedimentary rocks, felsic tuffs and mafic flows. The southern limb, where the Eagle River Mine is located, is dominated by tholeitic basalts and calc-alkaline andesites with minor interflow clastic sedimentary rocks and lean chert-magnetite iron formation.
Gold in the Mishibishu Lake greenstone belt occurs primarily in quartz vein deposits located within regional zones of deformation. The Mishibishu Deformation Zone follows a volcanic-sedimentary contact in the north limb of the belt hosting the Magnacon and Mishi deposits while the Eagle River Deformation Zone hosts the Eagle River deposit along the south limb of the belt. Late northeast striking and lesser northwest striking faults and fractures offset the greenstone stratigraphy and deformation zones.
Essentially all of the gold mineralization at the Eagle River mine occurs within a series of thick, white laminated quartz vein lenses. Gold bearing quartz veins at Eagle River are hosted primarily by subvertical to steeply north dipping east-west striking shear zones within an elliptical quartz diorite stock with dimensions of 2.4 km east-west and 0.5 km north-south. The quartz diorite stock intrudes a steeply dipping north-facing sequence of thin mafic to intermediate volcanic flows, flow breccias and interflow volcaniclastic rocks.
In general, the ore shoots mined to date occur at a spacing of 400 m along a 2.4 km strike length. They appear to be spatially related to an array of oblique 110º striking mafic dykes that are interpreted to post-date conjugate structures. A number of different ore zones have been distinguished that constitute different segments of the overall shear zone corridor and each has its own gold grade characteristic. The veins vary in thickness from one metre to three metres, averaging about 2.5 metres. Mineable portions of the individual zones form ore shoots that plunge steeply to the east. The bulk of the historic production has come from Zone 8 and Zone 6, which are entirely within the intrusive quartz diorite, while Zone 2 mineralization is hosted in sheared mafic volcanic rocks just east of the stock.
Recently, two new significant parallel zones have been discovered in the western portion of the mine diorite, namely the 300 and 7 Zones, located 200 and 400 metres north respectively of the main 8 Zone structure. Exploration is continuing to find the lateral extent of these parallel zones.
In addition, the most recent discovery of the 303 high grade lens is having a significant positive impact on the Eagle River mine production and mineral reserves, which is considerably higher grade and wider compared to previously mined ore. Locally the 303 lens is folded back on itself forming tight S-folds or “ballrooms” which form plunging, pipe-like bodies 12 to 15 metres in diameter often grading over 1 oz/t Au or 30 g/t Au. The 303 Lens has now been extended an additional 300 m down plunge to the 1,300 m-level, while continuing to illustrate above average widths and grades.
Falcon Zone
In 2018, surface drilling in the volcanics to the west of the mine diorite encountered two sub-parallel structures that broadly follow the stratigraphy within the mafic/felsic volcanic rocks. These could be extensions of the 300 and 7 zone structures which lie approximately 200 m to the east within the mine diorite. Both structures exhibit broad zones of deformed rock (3-20 m wide shear zones) affiliated with strong silicification over 20-50 m width. The two new mineralized zones identified by surface drilling define an area termed the Falcon Zones. The Falcon Zones occur near the contact between tholeiitic mafic volcanic and calc-alkaline felsic to intermediate volcanics, which are commonly sheared with biotite-sericite altered and laminated quartz veins with visible gold. Surface drilling has continued to expand the zone of mineralization to a depth of 4400 m elevation (i.e. 600 vertical m below surface) and over a strike of 200 m.
Falcon Zones Highlights:
- Hole ERM-2019-22: 18.5 g/t over 5.8 m core length (11.5 g/t Au cut, 3.3 m true width)
- Hole ERM-2019-36: 3,389.0 g/t Au over 0.3 m core length (60.0 g/t Au cut, 0.21 m width)
- Hole ERM-2019-15: 73.5 g/t Au over 1.3 m core length (34.7 g/t Au cut, 1.1 m true width)
- Hole ERM-2019-35: 51.1 g/t Au over 0.6 m core length (32.1 g/t Au cut, 0.4m true width)
- Assays capped at 60 g/t Au. True widths are estimated