Summary:
Rainy River is interpreted to be an auriferous volcanogenic massive sulphide (VMS) deposit with a primary synvolcanic source and a secondary syn-tectonic mineralization event that deformed and enriched primary mineralization (Pelletier, 2016; Mercier-Langevin et al., 2015).
The initial stage of mineralization at Rainy River has been interpreted as coeval deposition of base metal and gold mineralization (Wartman, 2011; Pelletier, 2016) in a synvolcanic setting. Evidence for this includes the spatial correlation of gold, base metals, and hydrothermal alteration, the presences of pyrite with gold-rich cores and barren rims, and the stratiform distribution of mineralization within porous felsic volcaniclastic rocks.
Following the initial VMS-style mineralizing event, the area experienced protracted deformation, which eventually concentrated gold mineralization within shear zones, and folded, boudinaged, and ultimately transposed mineralization parallel the main southeast-plunging stretching direction (Hrabi and Vos, 2010; Pelletier 2016).
At Rainy River, gold and silver are the dominant metals. 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 multiple examples of goldrich VMS deposits with high ratios of gold to base metals, such as the Horne, Lemoine, Lalor deposits, etc. (MercierLangevin et al., 2015, and references therein).
Mineralization and alteration
The Rainy River Mine consists of gold-rich volcanogenic massive sulphide (VMS) mineralization occurring as sulphide and quartz-sulphide stringers hosted within a sequence of felsic volcaniclastic rocks. Primary VMS-style gold mineralization was later deformed by subsequent tectonic deformation, producing a secondary type of mineralization. The Rainy River deposit comprises multiple distinct zones of mineralization and alteration. The mineralized zones can be grouped into the Main Zone (ODM, 17, 433, HS, NW Trend, and Cap Zone), Intrepid Zone, and Other Zones (34 and other zones), which are minor and or peripheral to the deposit. Previous open-pit mining has focused on the ODM, 433, and HS zones.
ODM and 17 Zones
The ODM and 17 Zones form a series of continuous east-west trending, south-dipping lenticular domains, with ODME to the west and 17 to the east. They are hosted within calc-alkaline dacite of the felsic volcanic succession. To date, the ODM and 17 Zones have been collectively defined over a strike extent of 1,800 m and to depths of 1,200 m, with a true width of approximately 200 m. High-grade lenses plunge moderately to the southwest (aligned with the L2 stretching lineation). Mineralization in the ODM and 17 zones remains open at depth. These zones are considered part of the Main Zone.
Three styles of gold mineralization occur in the ODM and 17 zones:
• Low-grade intervals are characterized by tightly folded pyrite stringers and disseminated pyrite in sericite-quartz-chlorite-altered host rocks.
• Moderate-grade intervals are characterized by tightly folded and foliation-parallel pyrite ± sphalerite stringers, commonly associated with stronger silica and weak garnet alteration.
• High-grade gold mineralization is associated with deformed quartz-pyrite-gold veinlets that overprint other styles of mineralization.
433 Zone
The 433 Zone is located approximately 500 m north of the ODM Zone. It is hosted in strongly sericitized calc-alkaline dacite rocks and minor tholeiitic basalts. The 433 Zone forms a cigar-shaped lens which plunges steeply to the southwest. This zone has a strike length of 325 m, a vertical depth of approximately 1,000 m, and a true width of up to 125 m. It remains open at depth. The 433 zone is considered part of the Main Zone.
Gold mineralization is similar to that of the ODM and 17 zones but with minor differences:
• Host rocks are altered to chlorite in 433 Zone in contrast to sericite in the ODM and 17 zones.
• The 433 Zone includes the presence of altered heterolithic conglomerate.
• Chalcopyrite and chlorite are associated with high-grade quartz-pyrite-gold veinlets.
HS Zone
The HS Zone, located between the ODM and 433 zones, comprises a series of small, discontinuous southwestplunging and flattened shoots of mineralization. The overall zone has a current strike length of 700 m, extends to a vertical depth of approximately 1,000 m, and remains open at depth. The HS Zone is considered part of the Main Zone.
Discontinuous, irregular gold mineralization is hosted within the felsic volcanic rocks and is associated with < 2 cm thick sulphide-rich veinlets composed of pyrite and traces of chalcopyrite and iron-poor sphalerite. Veinlets are typically parallel to the main foliation and strongly deformed, showing flattening, folding, and transposition of veins parallel to the main foliation.
NW Trend
The NW Trend occurs west of the ODM Zone. It consists of a series of discontinuous 5 to 10 m wide mineralized lenses that strike approximately southeast and dips in average 50° to the southwest. Individual lenses encompass a strike length of between 50 and 500 m. Collectively these zones occur over an area of approximately 500 by 1,200 m and have been defined to depths of 60 to 500 m. The NW Trend mineralized lenses are considered part of the Main Zone.
Cap Zone
The Cap Zone, located approximately 200 m to the south of the ODM Zone, is hosted in both tholeiitic basalt and calc-alkaline volcanic rocks of the southern mafic volcanic succession. The Cap Zone has been defined over a strike length of 600 m, up to 150 m wide, to a depth of 750 m, is still open at depth. The Cap Zone is considered part of the Main Zone.
Typical Cap Zone gold mineralization occurs as sulphide bands, stockwork, and disseminations, with higher-grade gold mineralization associated with deformed quartz-ankerite-pyrite shear and extensional veins. Mineralization is hosted in quartz-ankerite-pyrite-altered mafic volcanic rocks. The Cap Zone has a higher pyrite and chalcopyrite content than the ODM, 17, and 433 zones.
Intrepid Zone
The Intrepid Zone is located approximately 800 m east of the ODM and 17 zones within dacitic tuff and breccia units of the felsic volcanic succession. The Intrepid Zone has been defined over a strike length of 410 m and to a depth of 650 m. The width of the zone is variable, ranging between 10 to 60 m. The Intrepid Zone is considered as its own zone.
Typical Intrepid gold mineralization occurs as sulphide bands, stockwork, and disseminations, with high-grade gold and silver mineralization associated with deformed quartz-pyrite veinlets that overprint other mineralization styles. Iron-poor sphalerite stringers are commonly associated to the high-grade gold mineralization.
34 Zone
The 34 Zone comprises magmatic nickel copper sulphide mineralization associated with precious metals (gold, platinum group metals) within a tubular, approximately 100 m thick, late-stage pyroxenite gabbro intrusion which crosscuts the ODM and 17 zones and postdates the main gold mineralization event. The host pyroxenite-gabbro intrusion is not metamorphosed but is locally altered to serpentine and talc. Magmatic sulphide textures vary from massive to net-textured to disseminated. Gold and silver mineralization occur within 5 to 50 m thick discontinuous northeast-trending pods over a strike length of 500 m and a down-dip plunge of 100 m. The 34 zone is considered part of the Other Zones.