Björkdal
The Björkdal gold deposit is a lode-style, sheeted vein deposit hosted within the upper portions of the Skellefte Group sediments. Gold is found within quartz veins that range in thickness from less than a centimetre to over several decimetres. These veins are usually observed as vertical to sub-vertical dipping veins that strike between azimuth 000° and azimuth 090°, with the majority of veins striking between azimuth 030° and 060°. The veining is locally structurally complex, with many cross-veining features observed and thin mineralized quartz veinlets in the wall rocks proximal to the main quartz veins.

Gold-rich quartz veins are most often associated with the presence of minor quantities of sulphide minerals such as pyrite, pyrrhotite, marcasite, and chalcopyrite alongside more common non-sulphide minerals such as actinolite, tourmaline, and biotite. Scheelite and bismuth-telluride compounds are also commonly found within the gold-rich quartz veins and are both excellent indicators of gold mineralization. Gold occurs dominantly as free gold, however, gold mineralization is also associated with bismuthtelluride minerals, electrum, and pyroxenes. Silver is seen as a minor by-product of the Björkdal processing plant, however, very little is known about its deportment within the deposit, although it is assumed to be associated with electrum.

Skarnification occurs commonly within the Mine, especially in the limestone/marble unit where it occurs as discreet patches and lenses. However, similar calc-silicate alteration has taken place in areas where local shearing has affected the volcanoclastic host rock. Shearing is a known mechanism of skarnification. The skarnification here is most likely due to fluid influx where shears and faults interacted with the limestone/marble unit or calcite banded volcanoclastic rocks. The limestone/marble unit is predisposed to accommodate strain and be exploited by structures due to the rheological difference between the limestone/marble and the surrounding volcanic and volcaniclastic rocks.

The most striking feature of the Björkdal Au system is the anastomosing, sheeted quartz-vein network in which the majority of gold is hosted. This epigenetic vein network appears structurally controlled, consisting of more than a thousand sub-parallel quartz veins (typically striking 030° to 090° from truenorth). Such strong structural-geological influences over geometry of any quartz vein hosted mineralization clearly suggest a strong spatial and temporal relationship with orogenic/tectonic processes (i.e., mesothermal/greenstone gold systems). However, the mineralogical associations with gold mineralization, and the larger alteration signature of the Björkdal area, could also suggest that alternative depositional mechanisms are responsible for the mineralization at Björkdal as there are some similarities with skarn and/or porphyry systems.

The main type of mineralization found in the Björkdal gold system is dominated by vertical to sub-vertical dipping quartz-filled veins. Common accessory minerals contained within these veins are (in approximate order of occurrence): tourmaline, calcite, biotite, pyrite, pyrrhotite, actinolite, scheelite, chalcopyrite, bismuth-tellurides (pilsenite and tsumoite), gold, and electrum. Gold mineralization is most closely related to the bismuth-telluride minerals, and is also more reliably encountered in veins with high abundances of pyrrhotite, pyrite, scheelite, and/or chalcopyrite. In general, veins of pure quartz and free of the accessory minerals listed above are generally quite poor hosts for significant quantities of gold mineralization. As such, the informal terminology of “clean veins” and “dirty veins” has been adopted at the mine site in order to quickly describe vein-fill characteristics. Structural analysis of these two distinct vein-fill types from the Main Zone -325 and -340 levels suggests that the “cleaner” veins will more often strike between 030° and 040° from true north, while the “dirty”, inclusion-rich veins are more likely to strike between 050° and 090° from true north. This structural-geochemical relationship suggests that vein development in the Björkdal deposit occurred as more than a single “vein-forming” event, and that the fluids responsible for the vein-fill and mineralization were evolving with time.

Norrberget
The primary mineralization at Norrberget is observed to be associated with amphibole alteration bands and veinlets, and where mafic tuffs and volcaniclastic rocks are interbedded, contrary to what is observed at Björkdal. The mineralization is preferentially emplaced where there is a structural change to the rock such as at lithological contacts, altered bands and where shearing interacts with the interbedded sequences, due to the changing in the rheological characteristics of the unit. Zones where pyrrhotite and pyrite occur and are absent appear to be lithological controlled within the volcaniclastic package which can indicate a differing redox based upon temperature change and fluid evolution.

The mineralization at Norrberget is limited spatially to 50 m stratigraphically below the lower marble contact, which is believed to be a result of the cooling and redox changes of the fluid as it passes through the units. The gold is very fine grained and rarely visible. Where gold grains have been observed, they are found to be on the boundary or in the interstitial material between grains. High grade gold is mostly found in areas with low to no pyrite.

The major controls on the mineralization at Norrberget include the large scale shear zone that marks the base of the marble unit, the rheological differences between different stratigraphic units, the variation in the lithological and porosity of the volcanic package, and the development of the fluid system which utilized the shear zone. These large scale shear zones run extensively through the area along the base of the marble unit which extends beyond the Mine and across Norrberget. The mineralization occurs principally within a package of heterogeneous volcaniclastics containing interbedded ash falls, flows, and tuffs which have varying composition along with differing porosity and rheological characteristics. Where the Norrberget volcaniclastics are not sheared, they are packaged conformably between metasedimentary rocks and mafic volcanic rocks above and medium grained subvolcanic intrusions and volcanic rocks below.

The mineralization at Norrberget is stratabound within an interbedded altered volcaniclastic package that sits unconformably below a 30 m to 40 m thick marble unit. Gold mineralization has been observed up to 50 m below this contact. Gold mineralization is principally hosted in an amphibole-albite banded alteration and is also common where volcaniclastics are interbedded with crystalline tuff units. Theses alteration bands vary between one centimetre and 50 cm in thickness, are typically fine to medium grained, and appear to be sheared. Trace sulphides and minor quartz/carbonate are associated with the bands.

Prior to 2020, production was split between the underground mine and the open pit. In 2019, mining of the open pit was stopped for economic reasons and this production was replaced with ore from the low grade stockpile and an increase in underground production. The remaining open pit material remains economically viable, however, the low grade stockpile realizes more value, so open pit mining has been deferred for several years until the stockpile is run-down and open pit production is needed to offset reducing underground production. The current production strategy is to maximize the underground extraction with the remaining ore coming from stockpiles. Underground production for 2021 is planned to be 960,000 tonnes which is derived from the updated Resource Model and Mineral Reserves. The current 2021 Mining Budget includes additional material and was derived from the previous Resource Model. No production from the open pit is planned in 2021. Instead, 340,000 tonnes of ore will be drawn from the stockpile to make up the balance of the mill feed. As presently envisaged in the LOM plan, open pit pre-stripping and production will be restarted in 2024 to supplement the decrease in production from the underground mine.

The open pit has currently been halted and is planned to be restarted in 2024, however this could be delayed further in the event of additional underground reserves being identified. The planned method is standard truck and shovel mining, as done historically. Details will be redefined closer to the restart date.

The known Björkdal underground deposit lies within a footprint of approximately 1,600 m x 600 m and has a vertical extent of approximately 400 m. The long-term LOM underground production rate is planned to average 975,000 tpa up until 2023, and 775,000 tpa thereafter until production tails off in 2027. On vein development (OVD) will be carried out over approximately three years and stope production will be carried out over approximately seven years. A decrease in production is planned after 2023, when underground output reduces, with the balance being made up with open pit and stockpile tonnes.

Primary access to the underground operation is via ramp systems originating from two portals located in the wall of the existing open pit. Open pit mining and removal of the crown pillar in the north/east wall will disrupt this access as well as the supply of other services such as emergency egress, electrical, ventilation, and mine drainage systems. Open pit ore mining will therefore commence at Nylunds initially until underground operations cease, with limited pre-stripping only above the underground access and infrastructure in the crown pillar area.

The underground mining method used at the Mine is longhole stoping with a sub-level spacing of 15 m to 20 m, depending on the zone. Cross-cuts are established perpendicular to the vein system. Veins are then developed by drifting on each sub-level from the cross-cut. All pre-production vein, cross-cut, and ramp development is drilled and blasted using conventional trackless mining equipment.

Stoping blocks are currently drilled with approximately 15 m long 70 mm diameter up-holes connecting to the bottom of the overlying stope using Epiroc Simba drill rigs. When production drilling is completed, initial slot raises are developed and drill lines blasted in groups of three to five rings using a burden of 1.5 m and retreating towards the hanging wall. The material is removed between blasts, which also allows a void for the following blast.

The objective of the current materials handling strategy is as follows:
• Development material from cross-cuts and ramps above a grade of 0.32 g/t Au is hauled to a Bore stockpile at the mill.
• OVD material is either hauled to the OVD production ROM stockpile where it is classified as waste or ore and sent to the appropriate location.
• All stope production, regardless of grade, is hauled to the stope production stockpile.

In consideration of the variable vein geometry and existing equipment configuration, 3.5 m has been measured as the average minimum mining width. This includes a base 2.5 m minimum width plus an allowance for 0.5 m of overbreak on both the hanging wall and footwall sides of the stope. An additional 10% dilution is added for planning purposes.

Most of the mined out stopes are left open without any backfill, however the relatively new Aurora Zone will have stopes that will be both wider, longer, and higher than in other areas. In these areas, the stopes are planned to be mined and backfilled with unconsolidated fill. This will allow pillars to be reduced and will increase the extraction ratio.

Ore is delivered to a series of small stockpiles that are utilized to campaign ore through the processing facility in order to provide reconciliation data for various parts of the mines. From the stockpiles, a frontend loader feeds a jaw crusher. Discharge from the jaw crusher is screened. The screen undersize is nominally minus 8 mm. The material is conveyed to a 5,000-tonne fine ore bin or to an emergency stockpile. Screen oversize is stored in a 400-tonne stockpile. Ore is reclaimed from the stockpile and fed to a secondary cone crusher. Discharge from the cone crusher is conveyed to a second screen. Undersize from the screen is combined with the undersize from the first screen and stored in the fine ore bin or the emergency stockpile. Oversize from the second screen is fed to a tertiary cone crusher. The discharge from the tertiary crusher is combined with the discharge from the secondary cone crusher and fed to the second screen. Thus, the ore is recirculated through the tertiary cone crusher until it meets required product size (i.e., minus 8 mm).

Crushed ore is reclaimed from the fine ore bin and passed across a series of two screens prior to being fed to the primary grinding circuit. The screen oversize is directed to an oversize material stockpile. The screen undersize is split and fed to the primary ball mill and primary rod mill that are operated in parallel. Discharge from the primary mills is fed to a classifying screen. The screen oversize is returned to the primary ball mill for additional grinding. Screen undersize has a particle size of approximately 80% passing (P80) 560 µm. The slurry is pumped to hydrocyclones for additional classification.

The concentrator includes primary, secondary, and tertiary crushing, primary, and secondary grinding, a series of gravity concentration steps, regrinding, and flotation to produce three gravity concentrates and a flotation concentrate.

The cyclone underflow (P80 800 µm) is fed to rougher spiral concentrators. Tailings from the rougher and cleaner spirals are returned to the secondary ball mill number 3 with a discharge P80 475 µm. From the discharge of mill number 3, the slurry is pumped to combine with the discharge from ball mills 1 and 2. The discharge from the three mills is pumped to the classifying screen. Concentrate from the rougher spirals is fed to the cleaner spiral classifiers. Tailings from the cleaner spirals are combined with the tailings from the rougher spirals and processed in the regrind secondary ball mill number 3 circuit. Concentrate from the cleaner spirals is cleaned on shaking tables. Tailings from the shaking tables are fed to a Knelson centrifugal ........