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 (i.e., tellurobismuthite and tsumoite) 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 bitelluride, 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.

Recent exploration and development activities have discovered that economic gold grades can also be hosted in skarn-altered rocks. Skarnification occurs commonly within the Björkdal Mine, especially in the limestone/marble unit as discreet patches and lenses.

The Norrberget satellite deposit.
At Norrberget, the mineralization 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. The mineralization is primarily associated with amphibole alteration bands and veinlets. 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.

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.

OPEN PIT
The open pit has currently been halted and is planned to be restarted in 2023. The planned method is standard truck and shovel mining, as done historically. Details will be redefined closer to the restart date.

UNDERGROUND
The known Björkdal underground deposit lies within a footprint of approximately 1,200 m x 400 m and has a vertical extent of approximately 400 m.

The underground mine production rate is planned to be between 800,000 tpa and 900,000 tpa. The OVD will be carried out over approximately four years and stope production will be carried out over approximately seven years. A decrease in production is planned after the sixth year, when underground output reduces to a 243,000 tpa, 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 east wall will disrupt this access as well as the supply of other services such as emergency egress, electrical, ventilation, and mine drainage systems. An additional portal will be developed before open pit mining makes the old portals inaccessible, to ensure continuity of the supply of all necessary services.

The underground mining method used at the Björkdal 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 64 mm diameter up-holes connecting to the bottom of the overlying stope using Atlas Copco 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 hangingwall. The material is removed between blasts, which also creates void space for the following blast. Remotely operated scoops are used to muck the stopes to nearby re-handle areas or directly into trucks.

The majority of material mined underground is hauled to the surface as there is currently minimal underground waste disposal. As the mine continues to increase production from the Aurora Zone, some waste will be used as unconsolidated backfill. Otherwise, waste is only used where required to regain access over a mined out stope. If insufficient underground waste is available, open pit waste will be backhauled into the underground mine

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 provision for a 0.5 m overbreak on both the hangingwall and footwall sides of the stope.

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 taller 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.

The current top-down footwall to hangingwall retreat system results in the placement of ramp development outside the marble contact, while cross-cut pillars are within the ore zone.

A portion of remnant above cut-off grade material adjacent to previously mined out stopes is excluded from the underground Mineral Reserves, however, the extraction of these remnants warrants further evaluation. Some recovery of ore contained in pillar areas is planned at the end of the mine life, however, this is also excluded from the current Mineral Reserve estimate.

LOW GRADE STOCKPILES
Selective open pit mining at Björkdal commenced in 2009 with ore greater than 1.0 g/t Au being separated and milled as A-ore and material between 0.3 g/t Au and 1.0 g/t Au being stockpiled as B-ore.

Batch milling experience from these stockpiles has indicated that mill feed averages 0.65 g/t Au. Approximately 2.6 million tonnes of B-ore has been classified as an Indicated Mineral Resource at a grade of approximately 0.65 g/t Au.

This material forms the supply of ore for two phases of mine production. The mine is currently processing this ore and will do so until 2022. Stockpiled ore will not be processed when the open pit is restarted, except for periods of shortfalls in production. Once the open pit is depleted, the processing of the stockpile will recommence, up to the end of the life of mine.

The Norrberget satellite deposit.
The Norrberget open pit will be mined by a contractor with trucks and shovels, as part of the larger Björkdal contract. Norrberget will be mined from 2027, when ore from the main Björkdal pit begins to reduce.

Currently, the concentrator throughput is 1.3 Mtpa and the overall gold recovery is 88.3% of which 73% is obtained from the gravity processes and 17% from flotation.

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.

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 front end loader feeds a jaw crusher. Discharge from the jaw crusher is screened. The screen undersize is nominally minus 8 mm. The material is conveyed 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 i ........