Summary:
The Rainy River Mine lies within the Rainy River Greenstone Belt, part of the larger Late Archean age Wabigoon Subprovince of komatiitic to calc-alkaline metavolcanics overlain by clastic and chemical sediments and intruded by granitoid batholiths. The intrusions deformed their host rocks into synformal fold structures, often producing shear zones along the axial planes. Rocks within the immediate area of the mine comprise a series of tholeiitic mafic rocks structurally overlain by calc-alkalic intermediate to felsic metavolcanic rocks. Rocks of intermediate dacitic composition host most of the gold mineralization.
Four main styles of gold and silver mineralization have been identified at Rainy River: gold-bearing sulphide ± quartz stringers and veins in felsic quartz-phyric rocks; quartz-ankerite-pyrite shear veins in mafic volcanic rocks; sulphide-bearing silver-enriched quartz veinlets in dacitic tuffs and breccias and copper-nickel-platinum group metals mineralization hosted in a small younger mafic-ultramafic intrusion situated within the main cluster of gold and silver deposits. All deposits show some degree of deformation, excepting the copper-nickel-platinum-bearing type. Most of the gold mineralization identified to date occurs in the sulphide-bearing stringers and veins within the felsic quartz-phyric rocks.
At Rainy River, gold and silver are the dominant metals and the base metal (Cu-Pb-Zn) sulphides, although good indicators of the presence of gold, represent less than 10%, by volume, of the host rock. This is in contrast with other VMS systems that generally contain large amounts of base metals. However, there are exceptions, i.e., gold-rich VMS deposits that often contain modest amounts of base metals relative to gold (MercierLangevin et al. 2015 and references therein).
Deposit geology and mineralization
The Rainy River deposit comprises eight distinct zones of gold and silver mineralization. The bulk of the gold mineralization at Rainy River is contained in sulphide and quartzsulphide stringers and veins hosted by felsic quartz-phyric rocks.
ODM/17 Zone
The ODM/17 Zone is a series of east-west trending, south dipping lenticular sheets hosted within calc-alkaline dacites of the intermediate fragmental volcanic succession. The zone is cut by numerous NNE trending faults. The ODM/17 Zone has presently been defined over a strike extent of 1,600 m and to depths of 975 m. The true width of the zone is approximately 200 m. High grade lenses plunge south-west (aligned with the L2 stretching lineation). Mineralization in the ODM/17 Zone is open below the modelled depth.
Three styles of gold mineralization are observed in the ODM/17 Zone. Low grade intervals are characterized by tightly folded pyrite stringer veins and disseminated pyrite in sericite-quartz-chlorite altered host rocks. Moderate-grade intervals are characterized by tightly folded and foliation parallel pyrite-sphalerite and pyrite stringer veins, commonly associated with stronger silica and weak garnet alteration. High grade gold mineralization is associated with deformed quartz-pyrite-gold veinlets that overprint other mineralization styles.
433 Zone
The 433 Zone is located approximately 500 m north of the ODM/17 Zone and hosted within strongly sericitized calc alkaline dacite rocks and lesser tholeiitic basalts. The 433 Zone comprises a cigar-shaped lens which plunges steeply south-west. This zone has a strike length of 325 m, a vertical distance of approximately 820 m, and a true width of up to 125 m.
Footwall Silver Zone
The Footwall Silver Zone occurs in altered dacitic tuffs and tuff breccias immediately adjacent to a high strain zone at the northern contact of the ODM/17 Zone. This zone plunges to the south-west in similar orientation to the ODM/17 Zone. It is hosted by centimetre scale sulphide-bearing quartz veinlets with common millimetre scale fracture filling to dendritic native silver inclusions. Sulphides contained within these veinlets, in order of frequency, comprise pyrite, sphalerite, chalcopyrite, and galena. Localized spessartine garnets have been noted.
HS Zones
Several subsidiary zones of gold mineralization occur between the ODM/17 Zone and 433 Zone.
The HS Zones comprise a series of small, discontinuous south-west plunging, flattened shoots of mineralization. Discontinuous, irregular low-grade gold mineralization is associated with chlorite-pyrite replacement of matrix in flattened, albitized, heterolithic pebble conglomerates. The zone has a strike length of 200 m and extends to a vertical distance of approximately 700 m. The full extent of the HS Zone has not been defined by drilling to date.
The Western Zone
The Western Zone occurs near surface approximately 500 m north-west of the western extent of the ODM/17 Zone. It is composed of stockwork of discrete centimetre scale anastomosing, folded to linear quartz and quartz-carbonate veinlets.
The Western Zone comprises a series of discontinuous 5 to 10 m wide zones of mineralization which strike approximately south-east and dip south-west at approximately 50°. Individual zones encompass a strike length of between 50 and 500 m. Collectively these zones occur over an area of approximately 500 x 1,200 m. They have been defined to down-dip depths of approximately 60 to 500 m.
The CAP
Zone The CAP Zone is located approximately 200 m to the south of the ODM/17 Zone in both tholeiitic basalts and calc-alkaline dacite of the upper diverse mafic volcanic succession. The CAP Zone has been defined over a strike length of 400 m, up to 120 m wide and with a down-dip extent of 750 m. Mineralization in the CAP Zone is open below the modelled depth.
Intrepid Zone
The Intrepid Zone is located approximately 800 m east of the ODM/17 Zone within dacitic tuffs and breccias of the intermediate fragmental volcanic succession. The Intrepid Zone has been defined over a strike length of 410 m and to 450 m down-dip. The width of the zone is variable ranging between 10 m to 60 m.
34 Zone
The 34 Zone comprises magmatic nickel copper sulphide mineralization associated with precious metals (gold, platinum group metals) within a tubular, ~100 m thick, late-stage pyroxenite gabbro intrusion which cross cuts the ODM/17 Zone and post-dates the main gold mineralization event. The host pyroxenite-gabbro intrusion is unmetamorphosed, but locally altered into serpentine and talc. Magmatic sulphides vary from massive to nettextured and disseminated. Gold and silver mineralization occur within 5 to 50 m thick dislocated (and therefor discontinuous) north-east oriented pods over a strike length of 500 m with a down-dip plunge of 100 m.