The June 2025 Feasibility Study for Osisko Development Corp's Cariboo Gold Project details a 4,900 tonnes per day processing plant designed around underground crushing, X-ray transmission ore sorting, and a flotation and gravity recovery circuit, with all mill tailings returned underground as paste backfill.
Article Body
The Cariboo Gold Project, located in Wells, British Columbia, will process 4,900 tonnes per day of run-of-mine material at a new mill concentrator. The plant flowsheet integrates underground crushing and screening with X-ray transmission ore sorting to reject waste early, followed by a conventional grinding, gravity, and flotation circuit. Final tailings from the process are dewatered and mixed with cement for underground paste backfill, while the flotation concentrate is filtered, stockpiled, and trucked offsite for sale. Gravity concentrate is further upgraded on a shaking table and then refined to produce doré bars.
Process design criteria establish an average feed grade of 3.6 grams of gold per tonne. The design specifies distinct availability targets for each major process area. Underground crushing is assigned a 65.0% availability, crushed ore reclaim and ore sorting 73.0%, grinding 91.0%, flotation 93.0%, and dewatering, concentrate handling, and tailings paste backfill each 92.0%. Gravity concentration has a target availability of 91.0%.
The design also defines specific mass pulls for each circuit. Underground crushing fines reporting to screening undersize account for 30.0% of feed, while the coarse fraction sent to ore sorting is 70.0%. Of the material fed to ore sorting, 30.0% is fine and bypasses sorting, with the remaining 70.0% processed by the sorters. The primary grinding circuit handles 100.0% of the feed, the regrind circuit receives 100.0%, and the secondary grinding stage takes 60.0% of the recirculating load. The gravity circuit is designed for a 30.0% mass pull to the concentrate, and the flotation circuit's final concentrate mass pull is set at 2.9%.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Average Feed Grade | 3.6 | g Au/t | Process design criterion |
| ROM Tonnage | 4,900 | tpd | Ramp-up capacity |
| Underground crushing availability | 65.0 | % | Design criterion |
| Crushed ore reclaim and ore sorting availability | 73.0 | % | Design criterion |
| Grinding availability | 91.0 | % | Design criterion |
| Flotation availability | 93.0 | % | Design criterion |
| Dewatering, concentrate handling, tailings paste backfill availability | 92.0 | % | Design criterion |
| Gravity concentration availability | 91.0 | % | Design criterion |
| Underground crushing fines (screening U/S) | 30.0 | % | Mass pull |
| Coarse fraction to ore sorting | 70.0 | % | Mass pull |
| Fine fraction bypassing ore sorting | 30.0 | % | Mass pull |
| Crushing product P80 | 10.6 | mm | Closed side setting of 13 mm |
| Regrind mill feed P80 | 190 | μm | Before regrind |
| Regrind mill product P80 | 25 | μm | After regrind |
| Gravity circuit feed rate | 561 | tph | Two parallel circuits, each 280 tph |
| Concentrate thickener underflow solids | 62 | % by weight | First stage dewatering |
| Clearer-scavenger cell volume | 15 | m³ | Two cells |
| Concentrator staffing | 88 | persons | 56 operations, 32 maintenance |
Overview
The Cariboo Gold Project processing facility is designed to treat 4,900 tonnes per day of underground-mined material. The flowsheet begins with underground crushing, followed by coarse ore storage and then a screening and ore sorting stage that uses X-ray transmission technology to reject barren rock. The sorted material and fines are then ground, with a portion of the recirculating load sent to a secondary grinding stage. Gold is recovered through a combination of primary and secondary gravity circuits and a flotation circuit. The final tailings are thickened, filtered, and mixed with cement to produce paste backfill for underground voids. Flotation concentrate is dewatered in a two-stage process and trucked offsite for sale.
The plant is designed with redundancy for critical equipment, including duty and standby compressors, blowers, and pumps. Process water is recirculated from the concentrate and tailings thickener overflows. Excess process water is directed to a water treatment plant. Reagent preparation and distribution systems include bagged reagent addition for solid reagents, agitated mixing tanks, and transfer pumps, configured to deliver PAX, flocculant, MIBC frother, and collector to their respective circuits.
Key Process Stages
The first stage of processing is underground crushing. Ore is crushed in an underground crusher set with a closed side setting of 13 mm, producing a product with a P80 of 10.6 mm. This crushed material is conveyed to a coarse ore storage silo, and from there it is reclaimed and fed to the surface screening and ore sorting plant.
At the screening and ore sorting stage, the material is screened to separate a coarse fraction from a fine fraction. The coarser material retained on the top deck is directed to Ore Sorter No.1, while the finer material retained on the second deck is sent to Ore Sorter No.2. The ore sorting area is serviced by an overhead crane and uses X-ray transmission technology to reject waste rock. A belt scale on the conveyor monitors throughput. The majority, or 70% (134 tph) of the retained material, is classified as coarse and routed to the sorting units.
The primary grinding circuit is fed by the crushed ore and operates in closed circuit with hydrocyclones. The circuit includes a primary gravity circuit, where a portion of the cyclone underflow is directed to two parallel gravity circuits, each handling 280 tph. One-third of the regrind cyclone cluster underflow is directed to the secondary gravity screen. The gravity concentrates are subsequently processed on a shaking table after regrinding to produce a final gravity concentrate.
The flotation circuit comprises rougher, cleaner, and cleaner-scavenger stages. Rougher flotation is carried out in tank cells, with air supplied by a dedicated blower set. The 1st cleaner, 2nd cleaner, and cleaner-scavenger cells share a second blower set. Reagents include PAX as collector, MIBC as frother, and a collector identified as Polyfloat 704. The flotation circuit operates at natural pH levels. Concentrate from the cleaner cells is collected in launders and sent to the final concentrate pump box before dewatering. The final flotation tailings are sampled and then transferred to the tailings thickener feed box.
Final concentrate dewatering is a two-stage process. The first stage uses a high-rate concentrate thickener to thicken the concentrate to 62% solids by weight in the underflow. The thickened concentrate is then fed to a concentrate filter for additional dewatering before stockpiling and truck shipment. The thickener overflow is sent to the process water tank for reuse.
Tailings dewatering involves thickening followed by filtration. The tailings thickener underflow is pumped to a tailings filter tank and fed to two tailings filters operating in parallel. Filtered tailings are combined with cement and water to produce a paste backfill that is transported underground. The tailings dewatering area includes dedicated air compressors, with standby units, supplying two disc filter air receivers.
The plant includes comprehensive air distribution systems. A high-pressure plant air compressor with a standby unit supplies instrument and plant air, along with a plant air receiver. Dedicated systems supply compressed air for the ore sorters, including an ore sorter air dryer with standby, and separate receivers for each sorter. Flotation air is provided by duty and standby blowers, and dedicated compressors supply air to the concentrate and tailings filters.
Reagent preparation and distribution systems are designed for the reagents used in the process. The PAX system has three operating pumps and one standby, the flocculant system has two operating and one standby, the MIBC system has six operating and two standby, and the Polyfloat 7040 system has four operating and two standby pumps. Reagent consumption estimates are based on laboratory test work.
Additional Interesting Data and Summary
The process design criteria specify availabilities that show the intended operating philosophy of the plant, with the underground crushing and ore sorting stages having lower availability targets due to the nature of the underground operations and the mechanical sorting equipment. The regrind mill is designed to reduce the material P80 from 190 μm to 25 μm to achieve sufficient liberation before flotation. The gravity circuit is designed to recover a significant portion of the gold, with a mass pull to concentrate of 30.0%, while the flotation circuit achieves a final concentrate mass pull of 2.9%.
The concentrator is planned to be staffed by 88 personnel, comprising 40 operations staff and 32 maintenance staff, with the remainder in supervision and support roles. The facility is designed for high availability in the grinding and paste plant operations, targeting 92% availability. The plant's water management system recycles process water from the thickener overflows, directing any excess to the water treatment plant.
Key Processes
- Underground crushing and screening
- X-ray transmission (XRT) ore sorting
- Primary and secondary grinding with hydrocyclone classification
- Primary and secondary gravity concentration
- Regrind milling to a P80 of 25 μm
- Rougher, cleaner, and cleaner-scavenger flotation
- Two-stage concentrate dewatering (thickening and filtration)
- Tailings dewatering and paste backfill production
Source: Cariboo Gold Project , 2025 Feasibility Study, June 2025.
Project website: Cariboo Gold Project, 2025 Feasibility Study
