This technical report presents the proposed processing route for the GSS Project based on metallurgical testwork conducted at SGS.
Report context
The 2018 technical report for the GSS Project describes a proposed mineral processing plant design with a nominal throughput of 6.0 Mtpa. The design is based on metallurgical testwork conducted at SGS Vancouver and Lakefield during 2017 and 2018. The report includes the process design criteria, unit process operations, infrastructure requirements, and plant control philosophy for the proposed treatment facility.
Processing route
Overview
Metallurgical testwork confirmed that GSS and Wadi Doum material is amenable to gold and silver recovery via conventional cyanidation techniques and carbon adsorption across a wide range of ore domains and lithologies. A trade-off study comparing hybrid CIL, leach CIP and leach pumpcell circuits showed that the leach pumpcell circuit had the lowest operating costs and the best economics over the life of mine, despite hybrid CIL having the lowest capital cost. Therefore, a leach pumpcell circuit has been adopted for design.
The process plant design is based on a robust metallurgical flowsheet designed for optimal precious metal recovery and removal of deleterious elements such as mercury. The flowsheet includes unit operations that are well proven in the industry.
Crushing and ore stockpile
The crushing circuit consists of single stage primary crushing with a gyratory crusher to produce a crushed product size P80 of approximately 110 mm. A vibrating grizzly jaw crusher installation was considered high risk due to the abrasive and competent nature of some fresh ores, so a primary gyratory crusher with an open side setting of 140 mm has been selected.
A crushed ore stockpile with a nominal live capacity of 10,000 t will be provided, equivalent to 13 hours of mill feed at 6 Mtpa. Three apron feeders will be installed underneath the stockpile in the reclaim tunnel to deliver ore to the milling circuit. Quicklime for pH control will be added directly onto the SAG mill feed conveyor.
Grinding and classification
The grinding circuit is configured as a two stage circuit with a SAG mill and closed circuit ball mill (SAB). The SAG mill will be a 9.14 m x 5.30 m EGL variable speed mill fitted with a 9.3 MW motor. The ball mill will be a 6.70 m x 11.3 m EGL fixed speed overflow ball mill fitted with a 9.3 MW motor.
The circuit will produce a P80 grind size of 53 µm for GSS oxide and Wadi Doum ores, approximately 75 µm for GSS transition ore and 90 µm for GSS fresh ores. In later years, pebble crushing will be required due to increasing ore competency; provision for this equipment has been allowed in the design. Pebble crushing will only be required for the more competent ores processed towards the end of the mine life. Cyclones of 500 mm in diameter have been selected for classification duty.
Pre-leach thickening
A high rate pre-leach thickener ahead of the leach circuit has been selected to thicken trash screen undersize to the design leach circuit feed density of 55% w/w solids. Rheology testwork confirmed that this density is achievable even for the oxide blends, as there are only trace amounts of clay in the ore.
Leach and CIP circuit
Head assays and metallurgical testwork indicated that the Block 14 ores do not show preg-robbing characteristics, therefore a leach CIP circuit can be considered for design. A dedicated pre-aeration tank has been included for passivation of cyanide consuming minerals, as testwork indicated that four hours of pre-aeration reduced cyanide consumption for a number of ore types.
The leach circuit incorporates 10 leach tanks in series to provide 45 hours leach residence time. Testwork indicated that for fresh and transition ores, leaching is generally complete within 32 hours, but oxide and Wadi Doum ores show benefit from longer leach times. Each leach tank will have a live volume of approximately 4,000 m³. The design is based on air addition rather than oxygen, as metallurgical testwork demonstrated no gold recovery benefit from using oxygen compared with air.
A Kemix Pumpcell CIP circuit has been selected for recovery of gold onto carbon, to minimise carbon inventory, gold in circuit and operating costs. The CIP circuit will operate as a carousel system in which the feed and discharge points of each tank can be changed, and carbon will only be moved when transferring loaded carbon to the elution circuit or returning barren carbon from regeneration. Carbon loading testwork indicated that good CIP performance can be expected, with a design carbon loading for gold and silver of 6,133 g/t Au + Ag used for sizing the CIP and elution circuit.
Elution, electrowinning and gold recovery
A 12.5 t split AARL elution circuit has been selected, including optional cold cyanide strip for removal of copper from carbon prior to elution. Based on calculated carbon movements, a total of seven elution cycles are required each week. Three parallel 33 cathode electrowinning cells are proposed for the gold room. Due to the high silver content of the ore, the electrowinning capacity is relatively large for the plant throughput.
A mercury retort will be provided for removal of mercury prior to smelting to produce doré. Electrowinning cell sludge will be filtered in a pressure filter prior to transfer to the mercury retort. Off-gases from electrowinning cells will be passed through an activated carbon filter for mercury removal. Barren carbon will be regenerated in a horizontal rotating kiln incorporating scrubbing of off-gases for mercury removal.
Tailings thickening and pumping
A high rate thickener has been selected to thicken the CIP tails to maximise process water recovery and reduce the volume of tailings. The tailings thickener also allows residual free cyanide in the process water to be recovered for re-use in the leaching circuit. Tailings thickener underflow will be pumped to the tailings storage facility using two stages of centrifugal pumping.
Tailings storage facility
The proposed tailings storage facility is located within a valley north of the unnamed Wadi running north of the process plant. The facility has been designed as a two cell paddock facility to allow alternate deposition and subsequent upstream raising. A partial HDPE basal liner will be employed over the base of the valleys with an extensive underdrainage system and subliner drainage. Seepage analysis and stability analysis were conducted on the embankments to support the design.
At the final stage of construction, the total embankment length of the East Cell would be approximately 2.8 km, with a maximum embankment height of 38 m. The total embankment length of the West Cell would be approximately 1.3 km, with a maximum embankment height of 31 m. The tailings beach surface at full capacity will cover an area of approximately 280 ha.
Water supply
The groundwater resource required to operate a 6.0 Mtpa plant is a constant water supply of an average of 145 l/s, or 4.6 million cubic metres of water per annum. Exploration drilling of 17 boreholes in Area 5 and four test production boreholes commenced in April 2017 and was completed in September 2017. Boreholes were drilled between 90 and 200 m deep, with groundwater intersected at an average depth of 75 m below surface. A numerical model was constructed using Visual Modflow Pro and calibrated to match pump tests. The aquifer contains 100 Mm³ in the Measured Resource category and a further 40 Mm³ in the Indicated Resource category.
Power supply
The power supply will be an on-site heavy fuel oil (HFO) fuelled generation plant under a Build Own Operate Transfer (BOOT) arrangement, transferred after five years. The load profile estimates a 24.3 MW peak and 23.5 MW average demand, serviced using eight 5.2 MW Anglo Belgian Corporation HFO generators. A power cost of US$0.136/kWh was used in the operational cost analysis based on a HFO380 price of $0.525/l.
Key reported parameters
| Parameter | Units | Oxide & Wadi Doum | Transition | Fresh | Source |
|---|---|---|---|---|---|
| Plant Capacity | tpa | 6,036,000 | 6,036,000 | 6,036,000 | Orca |
| Gold Head Grade (max) | g Au/t | 2.04 | 1.84 | 1.62 | Orca |
| Silver Head Grade (max) | g Ag/t | 3.45 | 4.33 | 2.44 | Orca |
| Design Gold Recovery (Leach) | % | 90.5 | 85.5 | 83.7 | Testwork |
| Design Silver Recovery (Leach) | % | 37.2 | 45.3 | 56.7 | Testwork |
| Plant Availability | % | 91.3 | 91.3 | 91.3 | Lyco |
| Crushing Work Index (CWi) | kWh/t | 4.4 to 6.7 | 5.0 to 11.2 | 6.8 to 11.3 | Testwork |
| Bond Ball Mill Work Index (BWi) | kWh/t | 8.5 to 12.7 | 8.4 to 13.3 | 12.3 to 19.3 | Testwork |
| Abrasion Index (Ai) | 0.04 to 0.11 | 0.11 to 0.34 | 0.40 to 0.91 | Testwork | |
| Grind Size (P80) | µm | 53 | 75 | 90 | Orca |
| Pre-aeration Time | hrs | 4 | 4 | 4 | Testwork |
| Leach Circuit Residence Time | hrs | 45 | 45 | 45 | Orca |
| Leach Slurry Density | % w/w | 55 | 55 | 55 | Orca |
| Number of Leach Tanks | 10 | 10 | 10 | Lyco | |
| Number of Adsorption Tanks | 7 | 7 | 7 | Kemix | |
| Cyanide Consumption – Plant | kg/t | 0.44 | 0.49 | 0.51 | Testwork / Lyco |
| Quicklime Consumption – Plant | kg/t | 1.31 | 1.09 | 0.72 | Testwork / Lyco |
| Elution Circuit Type | Split AARL | Split AARL | Split AARL | Lyco | |
| Elution Circuit Size | t | 12.5 | 12.5 | 12.5 | Kemix |
| Frequency of Elution | strips/wk | 7 | 7 | 7 | Kemix / Lyco |
| Tailings Thickener U/F Density | % w/w | 58 | 62 | 65 | Testwork / Lyco |
Source notes: "Orca" refers to advice from Orca Gold Inc. "Testwork" refers to metallurgical testwork conducted at SGS or vendor testwork. "Lyco" refers to Lycopodium calculations and modelling. "Kemix" refers to modelling by Kemix. "OMC" refers to advice from Orway Mineral Consultants.
Technical qualifications
The metallurgical testwork conducted at SGS has provided the basis for the plant design. The complete OMC Report No. 7889-02 Rev 2, March 2018, should be referenced for detail of comminution modelling and complete comminution data set. Reagent consumption in the plant includes an allowance for residual cyanide and quicklime supply at 90% active CaO.
The process plant design was based on a nominal capacity of 6.0 Mtpa of ore and head grade of 1.84 g/t Au and 4.33 g/t Ag, with overall gold recovery 86% and silver recovery 45%, showing the highest pregnant solution grade over life of mine.
The reported ranges for comminution parameters show variation across ore types and ore domains. Process design criteria were developed from multiple sources including metallurgical testwork conducted at SGS, Orca advice, comminution modelling by Orway Mineral Consultants, CIP modelling by SGS and Kemix, and Lycopodium calculations and modelling.
Source: GSS Project 2018 Technical Report, Sections 1.16, 1.17, 17.1, 17.2, 17.3, 17.4, 17.5, 17.6.
Consolidated report summaries
The following former standalone pages about this operation were consolidated here on October 25, 2026. They are retained for source traceability; dates and figures reflect each report’s own effective date and may be superseded by the current information above.
GSS/WD Project — Recovery Methods Section
Metallurgical testwork has validated that GSS and WD material types are amenable to gold recovery via cyanidation, with a carbon-in-leach (CIL) process selected as the most economically effective scheme.
Report context
This section presents the recovery methods for the GSS/WD Project based on a technical report (SGS Report No. TS537-0000-T-REP-003 Rev 0). The report describes metallurgical testwork conducted to date, including Gold Deportment and Diagnostic Leach Test Work. The proposed design is based on a nominal capacity of 1.8 Mtpa for the comminution circuit and recovery plant.
Processing route
Comminution circuit
Oxide, transitional and fresh Run-of-Mine (ROM) material from GSS and WD is fed into a primary crusher and secondary and tertiary crusher circuit. Material is reduced from a top size of 600 mm to 80% passing 8 mm before being fed into the ball mill. The material is then ground to 80% passing 75 µm.
Results from comminution testwork were used to develop a comminution circuit simulation. The current design is based on the results of the circuit simulation utilising a single primary mill with nominal dimensions of 16.0 ft x 28.5 ft with installed power of 3,900 kW to produce the required 80% passing 75 µm discharge size.
The cyclone underflow stream returns to the ball mill for further grinding.
Crushing and stockpiling
ROM material will be trucked from local GSS open pits and from the WD open pit. Material will be tipped from trucks into a ROM bin and withdrawn at a controlled rate by an apron feeder with a nominal feed rate of 400 tph. The feeder discharges into a primary jaw crusher that reduces material top size from 600 mm to 80% passing 125 mm. This material is then stockpiled on the coarse stockpile. Material is reclaimed under the stockpile tunnel by variable speed vibrating feeders and delivered onto a conveyor to feed the secondary cone crusher. A magnet is installed over the secondary cone crusher feed conveyor for removal of tramp steel.
Coarse material is fed onto a double deck screen: plus 40 mm material is fed into the secondary cone crusher and the minus 40 mm plus 10 mm size fraction material reports to the tertiary crusher circuit. The discharge from the secondary cone crusher feeds two tertiary cone crushers. Products from the tertiary crushers are combined and conveyed to a vibrating dry screen. Screen oversize is returned to the tertiary crushers and the minus 10 mm undersize material is fed onto the fine material stockpile.
Ore is reclaimed from under the fine stockpile by vibrating feeders. Material is fed onto a conveyor where lime is proportionally dosed before being fed into the mill.
Milling and classification
Ore from the fine material stockpile is fed to the ball mill. Inlet dilution water is added to the mill feed chute to control mill slurry density. The ball mill discharges slurry via a trommel screen into a launder. Oversize from the trommel, mostly ball scats, drops into a bunker. Dilution water is added to the mill discharge launder. Slurry is pumped from the ball mill sump to the cyclone cluster for size classification.
At the cyclone clusters, coarse particles pass through the cyclone spigots as dense slurry that gravitates to the underflow distributor and fine particles pass through the cyclone overflow at about 40% solids slurry to the CIL. The two outlets of the cyclone underflow distributor divert slurry where about 75% is returned to the mill and the remainder flows to the CIL circuit.
Spillage pumps are provided in the mill bunded spillage area located in a drop out sump arrangement. Coarse solids settle out, and the spillage pump returns only excess water and slimes to the mill sump.
Carbon-in-leach (CIL)
Slurry from the cyclone overflow gravitates to the CIL section for cyanidation. A splitter box is installed equipped with a plug valve. Slurry overflows from the mechanically agitated leach tank and then flows through six subsequent mechanically agitated CIL tanks, to ensure complete dissolution of gold as a cyanide complex and adsorption onto activated carbon.
Cyanide solution is added from a ring main to the leach tank. Compressed air is injected into the leach tank with the facility to add air to the first two CIL tanks and fourth tank, should it be required, to provide oxygen for the cyanidation reaction. All tanks are equipped with two launder valves on the outlet. The normally open valve passes slurry to the next tank downstream. The second valve allows diversion of slurry to the successive tank downstream, to bypass any single tank if needed.
Each CIL tank is equipped with an inter-stage screen mechanism, with a cylindrical basket-type stainless steel wedge-wire screen surface. The mechanism drive turns wiper blades mounted on the outside of the basket. The screen has a pumping mechanism to allow slurry in the tank to flow through the screen surface whilst the tanks are on the same level. Tailings slurry from the last CIL tank gravitates to the tailings section.
A recessed impeller type pump located in the first CIL tank transfers slurry containing loaded carbon from the first CIL tank to the loaded carbon screen. The screen separates and rinses the loaded carbon from the slurry and discharges it into the elution section.
Interstage screens are cleaned regularly using high-pressure water sprays. A spare interstage screen and a maintenance bay are provided.
Cyanide detoxification and filtration
The discharge from the final CIL tank flows by gravity through a carbon catchment screen to the cyanide detoxification tanks. The cyanide detoxification tanks are mechanically agitated tanks with airflow to assist with the detoxification reaction. Copper sulphate is pumped from a dedicated copper sulphate storage and mixing plant and added to the pulp. Sodium metabisulphite is pumped from a dedicated sodium metabisulphite storage and mixing plant and added to the pulp. Caustic is used to increase pH as required.
The residue is pumped to a thickener. The water is reused as process water and the thickened slurry is sent to the final tails dam.
Elution
The elution section uses a pressurised 4 tonne Zadra system. Loaded carbon is eluted by pumping a hot caustic cyanide solution, typically 1.0 to 2.0% NaOH and 0.2 to 0.6% NaCN, through the column at 120°C under pressure. Gold adsorbed onto the loaded carbon is eluted off the carbon and collected in the eluate solution. The eluate is passed through electrowinning cells to remove gold from the circulating eluate stream.
The column operates under a pressure of typically 300 to 350 kPa. Solution exiting the column passes through the recuperative heat exchanger to cool the hot eluate and preheat eluate entering the column. Eluate solution is reused for a number of elution cycles until the level of contamination becomes unacceptable.
Regeneration and electrowinning
Carbon is withdrawn from the eluted carbon tank to feed the kiln by a screw feeder, which discharges the carbon into the rotary kiln for thermal regeneration. Regenerated carbon from the kilns is quenched in the quench pan and passed over a screen to remove fines, before discharge into the CIL to replace loaded carbon transferred out of the adsorption circuit.
Gold in the eluate is plated out onto cathodes in the electrowinning cells. Periodically the cathodes are removed and washed with a high pressure water gun. The resultant slurry is dropped into the laboratory filter press. The filter press cake is stripped out and packed into trays for calcining. The calcined material is smelted with fluxes into gold doré bullion in a furnace.
Reagents
Dry powdered lime is added onto the mill feed belt from a silo by a variable speed rotary vane and screw feeder arrangement. Hydrochloric acid stored in drums is transferred to an acid storage tank and then to the acid soak tank in the regeneration area. Sodium cyanide stored in bags is discharged through a bag breaker into a cyanide mixing tank equipped with a mechanical mixer. Caustic soda stored in bags is discharged through a bag breaker into a caustic mixing tank equipped with a mixer. SMBS stored in bags is discharged through a bag breaker into an SMBS mixing tank equipped with a mixer.
Key reported parameters
| Parameter | Value | Unit | Basis |
|---|---|---|---|
| Nominal plant capacity | 1.8 | Mtpa | Design |
| ROM top size | 600 | mm | Design |
| Crusher product size | 80% passing 8 | mm | Design |
| Ball mill product size | 80% passing 75 | µm | Design |
| Ball mill dimensions | 16.0 x 28.5 | ft | Design |
| Ball mill installed power | 3,900 | kW | Design |
| Apron feeder nominal feed rate | 400 | tph | Design |
| Primary crusher product size | 80% passing 125 | mm | Design |
| CIL feed slurry flow | 433 | m3/h | Design |
| CIL total residence time | 30 | h | Design |
| CIL total tank volume required | 12,981 | m3 | Design |
| Number of CIL tanks | 7 (1 leach + 6 CIL) | – | Design |
| CIL tank diameter | 13.00 | m | Design |
| CIL tank height | 16.300 | m | Design |
| Cyanide consumption | 0.75 | kg/t | Design |
| Detox free cyanide target | less than 50 | ppm | Design |
| Elution method | Zadra | – | Design |
| Design carbon loading | 1,900 | g/t | Design |
| Design barren carbon loading | 50 | g/t | Design |
| Strip batch size | 4.0 | t | Design |
| Eluant strength (design) | 1% NaCN and 2% NaOH | – | Design |
| Carbon bulk density | 0.50 | t/m3 | Design |
| Elution column diameter | 1.1 | m | Design |
| Elution column height (tan-tan) | 5.3 | m | Design |
| Flow through elution column | 20.0 | m3/h | Design |
| Regeneration kiln capacity | 400 | kg/h | Design |
| Eluant solution electrowinning cell capacity | 500 | l/min | Design |
| Eluant solution electrowinning cycle type | 20 | h | Design |
| Tailings storage facility footprint (estimate) | 57 | hectares | Cost estimate basis |
Project website: https://www.wd-deo.gc.ca/eng/20204.asp
Technical qualifications
The report notes that no tailings storage facility design has been completed at this stage of the project. An existing design for a conventional surface thickened tailings storage facility with an approximate footprint of 57 hectares was used to estimate costs.
The report identifies upward potential in gold recovery using flash flotation followed by ultra-fine grinding and cyanide leaching of the flash flotation concentrates to recover some of the more refractory gold associated with pyrite. This option has not been included in the current design and will be addressed in the Pre-Feasibility Study.
Source: GSS/WD Project, Recovery Methods Section, SGS Report No. TS537-0000-T-REP-003 Rev 0.
