Summary:
The Mishibishu greenstone belt is a broad, arcuate syncline 55 km long in an east-west direction and 16 km wide in a north-south direction. 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. Minor intrusions include synvolcanic stocks and sills of intermediate to felsic composition and an array of northeast and northwest striking late Precambrian diabase dykes.
The southern limb, where the Eagle River Mine (ERM) is located, is dominated by tholeiitic basalts and calc-alkaline andesites with minor interflow clastic sedimentary rocks and lean, chert-magnetite iron formation. In this area, the supracrustal rocks form a steeply north-dipping and north-facing sequence displaying moderate to steep eastward plunges defined by minor fold axes and mineral lineations.
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.
The Eagle River deposits is mesothermal lode gold deposit hosted by Archean Greenstone Belts. This type of deposit is also known as orogenic, or shear zone hosted gold deposit.
Orogenic gold deposits are hosted by shear zones in orogenic belts, specifically in metamorphosed fore- arc and back-arc regions and were formed during syn- to late metamorphic stages of orogeny (Goldfarb and Groves, 2015). Formation of orogenic gold deposits is related to structural evolution and structural geometry of lithospheric crust, as hydrothermal fluids migrate through pre-existing and active discontinuities such as faults, shear zones, and lithological boundaries, generated by tectonic processes.
Gold bearing quartz veins at Eagle River are hosted primarily by subvertical to steeply north dipping eastwest striking shear zones within a broadly 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 mineralized 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. Gold mineralization is structurally concentrated within highly strained portions of the various quartz veins. Reflected light microscopy indicates that 60% of the gold occurs along quartz-sericite grain contacts, 32% along sulphide-gangue contacts and 1.4% within sulphide grains. The grains are generally less than 500 µm in diameter and grains less than 5 µm in diameter account for a negligible percent of the total gold. Free gold generally occurs as a multitude of fine grains, which result in a relatively low sub-sampling variance that generates very good assay precision for a vein type gold deposit.
Several mineralized zones have been distinguished that constitute different segments of the overall shear zone corridor, and each has its own gold grade characteristic. Mineable portions of the individual zones form 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.
Zone 8 is characterized by a series of thick, white, laminated quartz vein lenses. The veins vary in thickness from 1 m to 15 m, averaging approximately 2.5 m. Gold is concentrated in highly strained quartz of grey colour and in sericite-chlorite lamellae with accessory sulphide minerals including pyrite, pyrrhotite, galena, sphalerite, and chalcopyrite.
Zone 6 is a distinct and discrete shear zone that forms a splay off the shear hosting Zone 8 mineralization. The vein varies in thickness from 0.5 m to 2.0 m. Locally, the vein is folded back on itself forming tight S-folds or “ballrooms” which form plunging, pipe-like bodies 12 to 15 m in diameter. Zone 6 is high-grade averaging 12 to 18 g/t Au and has very competent wall rocks.
In the summer of 2013, two parallel structures of significance were newly identified and termed No.7 and No. 300 structures. They are located approximately 200 m and 400 m north of the No. 8 structure, respectively. These are now in production and are being actively explored. However, 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 m in diameter often grading over 30 g/t Au. The 300 East Zone, previously defined from the 750 m-level to 1,000 m-level, has now been extended to the 1,400 m-level. The down plunge extension is a relatively more tabular zone that now measures in excess of 100 m along strike with above average widths and grades; it remains open down plunge.
In 2018, surface drilling in the volcanic rocks to the west of the mine diorite encountered two sub- parallel structures that broadly follow the stratigraphy within the mafic/felsic volcanic rocks. Both structures strike approximately 245° and dip 70° - 80° to the north. The two mineralized zones newly identified by surface drilling define an area termed the Falcon Zones. The Falcon 7 Zone is now in production.
Recent drilling has discovered the Falcon 311 zone that is interpreted to extend from surface to at least the 900-metre level.