The October 2025 Preliminary Economic Assessment for the Gabbs Project outlines a processing strategy that combines heap leaching for oxide material and a milling-flotation-cyanidation circuit for sulphide material, with shared crushing, SART, and ADR infrastructure.
Report context
The Gabbs Project Preliminary Economic Assessment NI 43-101 Technical Report, dated October 2025, presents the recovery methods proposed for the project. The report states that test work results have indicated the Gabbs mineralized material is amenable to both heap leaching and milling/flotation for the recovery of gold, silver, and copper.
Processing route
Heap Leach (Years 1-5)
The mine plan outlined in the report schedules a heap leach operation treating oxide material for the first five years of mine life. The heap leach material is estimated to contain an average of 0.44 g/t gold, 1.11 g/t silver, and 0.22% copper.
Material will be mined by standard open-pit methods, crushed using a three-stage crushing system incorporating a high-pressure grinding roll (HPGR) crusher as the tertiary stage, agglomerated with cement, and conveyor stacked on the heap leach pad in 8-metre lifts. The pad will hold approximately 80 million tonnes and will be constructed in four phases with a composite liner consisting of clay and textured HDPE geomembrane.
Heap material will be single-stage leached with a dilute cyanide solution for a total leach cycle of 150 days. Pregnant solution will be collected in the pregnant solution pond and pumped to the SART plant.
The heap leach material throughput will be 6,000,000 tonnes per year in Years 1 through 5, reduce to 5,000,000 tonnes per year in Years 5 through 14, and be 4,316,829 tonnes in Year 14.
SART Plant
Pregnant solution will be treated in a SART plant prior to entering the ADR plant. Copper and silver precipitation will occur in the SART circuit to produce a saleable copper-silver product. The process involves acidification of pregnant solution with 30 wt.% sulphuric acid to pH 4.0-4.5, precipitation of copper and silver with sodium hydrosulphide in agitated tanks, thickening, and filtration.
Sodium hydrosulphide will arrive in tanker truck at a 40 wt.% solution and diluted to 25 wt.%. Copper-silver precipitate filters will cycle manually every 4 to 8 hours, operating with a cake moisture of 40 wt.%, a filtration rate of 33 kg/m²/hr, and 10-16 bar operating pressure. Two filters will be installed.
Filter cake will be conveyed to drying pads where moisture will be reduced from 40 to about 20 wt.%. The dried cake will be sized to 100% minus 5 mm and loaded into 20-tonne containers for shipment.
Concentrated sulphuric acid, 98 wt.%, will arrive by truck in 20-tonne batches and be stored in a single carbon steel tank with 3 days storage capacity.
Solution Neutralization and Gypsum Handling
Solution from the Copper-Silver Thickener will overflow by gravity to the Neutralization Tank. The acidified thickener overflow will be neutralized with slaked lime and recycled gypsum thickener underflow slurry. Gypsum Thickener overflow solution will gravity flow to the ADR plant.
Recycle Gypsum Thickener underflow solids will be conditioned with slaked lime in the recycle mix tank to simulate a high-density sludge process. The recycle gypsum thickener underflow at 25 wt.% solids will be mixed with slaked lime at 20 wt.% solids. The Neutralization Tank is sized for a residence time of five minutes.
A gypsum slurry will be produced as a by-product of SART operations and disposed of on an unirrigated section of the heap.
Carbon Adsorption, Desorption, and Recovery (ADR)
The adsorption section will consist of a single train of five cascade-type open-top up-flow carbon adsorption columns. Approximately 1.1 tonnes of carbon per day are expected to be loaded and treated. Carbon stripping will occur about two to four times each week with each strip lasting approximately 18 hours.
The desorption circuit will use a Zadra pressure elution circuit. A five-tonne batch of carbon will be processed in an approximately 18-hour cycle. Barren strip solution containing sodium hydroxide and sodium cyanide will be pumped through heat recovery and primary heat exchangers and introduced to the elution vessel at 135°C and approximately 340 kPa (50 psig).
After desorption, half of the stripped carbon will be transferred to carbon reactivation dewatering screens and then to carbon regeneration. The other half will be screened and transferred to the carbon storage tank.
Electrowinning and Refining
Gold will be electrowon from the eluant in electrowinning cells using stainless steel cathodes with a current density of approximately 50 amps per square metre. Caustic soda in the eluate solution will act as an electrolyte. Loaded cathodes will be processed through a high-pressure washer, and the resulting sludge will be filtered, dried, and smelted with fluxes in an electric furnace to produce doré bullion.
Furnace fumes will pass through a bag house designed to remove over 99.5% of particulates present in the exhaust fumes.
Cyanide Destruction
The cyanide destruction circuit will use the air/SO₂ process to reduce WAD cyanide levels in the slurry. The circuit will consist of an agitated tank where air, lime, sodium metabisulfite, and copper sulphate are added.
Tails Filtration
Flotation tailings will be thickened, then direct cyanide leached. The leached solids will be washed in a CCD circuit. CCD tails will be treated in a cyanide destruction circuit, filtered using automated recessed plate filter presses, and conveyed to a dry stack storage facility.
Milling (Year 6 onwards)
Up until the mill becomes operational, all mined sulphide material will be stockpiled. Upon startup, stockpiled material will be fed to the mill along with mined sulphide material at approximately 5 million tonnes per year. ROM sulphide material will be crushed to P80 6.3 mm in a three-stage crushing circuit, with the third stage being an HPGR. The ore will be conveyed to a ball mill circuit to produce a primary grind P80 0.075 mm.
The milled sulphide product will be treated in a flotation plant to produce a copper concentrate. The flotation tailings will be thickened, then direct cyanide leached. The leached solids will be washed in a CCD circuit. Dissolved silver and copper will be recovered from CCD overflow solution in a SART plant. Gold in the SART plant barren solution will be recovered in an ADR plant.
The primary ball mill will operate in closed circuit with hydrocyclones to produce a 75-micron P80 overflow product. Cyclone overflow will be directed to flotation conditioning tanks.
Flotation and Regrind
Cyclone overflow will be directed to an agitated conditioning tank where collector and frother will be added. The rougher concentrate will be pumped to regrind cyclones classification. Coarse underflow will be directed to the regrind ball mill. Overflow from the regrind cyclones will flow to cleaner flotation cells to produce copper cleaner concentrate.
Copper concentrate from cleaner flotation will be dewatered by two horizontal recessed plate filter presses and stored for sale.
Reagent Handling
Sodium cyanide will be delivered as briquettes in 1,000 kg bulk bags stored in a covered storage area with approximately 30 days of storage. Cement will be delivered in bulk truckloads and stored in two 150-tonne silos. Estimated cement consumption is 66 to 110 tonnes per day depending on clay content, with a LOM average of 68 tonnes per day.
Pebble lime will be stored in a 150-tonne lime silo and fed to a lime slaker system. Slaked lime will be stored in an agitated tank with 12 hours residence time at 20 wt.% solids.
Sodium hydroxide will arrive as 50% solution in 10,000-litre containers and diluted to 20 wt.% for storage. Caustic will be distributed from the SART plant to the ADR in a 55-gallon drum or similar day tank.
Flocculant will arrive as dry powder in 25 kg bags, mixed to 0.5 wt.% solution, and diluted at the feed well to 0.02 wt.%. Antiscalant will be added at the barren and pregnant solution pump suctions.
Hydrogen peroxide, 10 wt.%, will be delivered to the SART plant in 10,000-litre containers with 5 days of storage. It will be fed to sump areas and sample points via a ring-main to destroy hydrogen cyanide and hydrogen sulphide during upset conditions.
Caustic Scrubber Systems
The Copper-Silver Recycle Mix Tank, Precipitation Tanks, Thickener, Filter Feed Tank, and NaHS storage areas will be ventilated to a caustic scrubbing system. The system will consist of two packed tower scrubbers with integral pumps, instrumentation, and fans on UPS and emergency power. The scrubber will remove hydrogen sulphide and hydrogen cyanide from vent gases using approximately 15 wt.% caustic solution.
The normal operating case will treat 6,800 Nm³/hr with assumed HCN and H₂S concentrations of 100 ppmv. The scrubber efficiency of 99.8% will discharge 0.2 ppmv HCN and H₂S. The emergency scrubber is designed to treat a burst of gas at 17,100 Nm³/hr with 76,800 ppmv H₂S and 61,300 ppmv HCN for 5 minutes, based on complete acidification of one precipitation tank.
On-Line Analysis
On-line analysis of Cu, Zn, Cd, Ag, pH, redox, and sulphuric acid will be determined from five streams: pregnant solution, acidified pregnant solution, and discharge from each precipitation tank. The system includes stream sampling with a multiplexer, sample filtration, XRF analysis, and automatic titration for pH, redox, and acid concentration.
Key reported parameters
| Parameter | Unit | Value | Basis |
|---|---|---|---|
| Heap leach feed grade (Au) | g/t | 0.44 | Mine plan average |
| Heap leach feed grade (Ag) | g/t | 1.11 | Mine plan average |
| Heap leach feed grade (Cu) | % | 0.22 | Mine plan average |
| Heap leach throughput Years 1-5 | t/yr | 6,000,000 | Design |
| Heap leach throughput Years 5-14 | t/yr | 5,000,000 | Design |
| Heap leach throughput Year 14 | t | 4,316,829 | Design |
| Crushing rate Years 1-5 | tpd | 24,658 | Design |
| Crushing rate Years 6-14 | tpd | 10,959 | Design |
| Crusher availability | % | 75 | Design |
| Crushing product size | mm | P80 6.3 | Design |
| Conveyor stacking availability | % | 75 | Design |
| Leach cycle | days | 150 | Testwork |
| Sodium cyanide consumption | kg/t | 1.11 | Design average |
| Cement consumption | kg/t | 7.47 | Design average |
| Gold recovery | % | 85 | Testwork |
| Silver recovery | % | 60 | Testwork |
| Copper recovery | % | 67 | Testwork |
| Mill throughput | t/yr | ~5,000,000 | Design |
| Primary grind size | mm | P80 0.075 | Design |
| Barren solution application | L/hr/m² | 8 | Design |
| Maximum barren solution flow | m³/hr | 2,200 | Design Years 1-5 |
| Barren solution flow Year 6+ | m³/hr | 1,000 | Design |
| Carbon elution temperature | °C | 135 | Design |
| Elution pressure | kPa | ~340 | Design |
| Carbon strip batch size | t | 5 | Design |
| Carbon strip frequency | per week | 2-4 | Design |
| Desorption cycle duration | hours | ~18 | Design |
| Electrowinning current density | A/m² | ~50 | Design |
| Carbon reactivation temperature | °C | ~750 | Design |
| Carbon reactivation time | min | 10 | Design |
| Filter cake moisture (pre-dry) | wt.% | 40 | Design |
| Filter cake moisture (post-dry) | wt.% | ~20 | Design |
| Filter cake sizing | mm | 100% minus 5 | Design |
| Filtration rate | kg/m²/hr | 33 | Design |
| Filter operating pressure | bar | 10-16 | Design |
| SART acid addition | wt.% | 30 sulphuric acid | Design |
| NaHS solution | wt.% | 25 | Design |
| Lime solids density | wt.% | 20 | Design |
| Neutralization tank residence | min | 5 | Design |
| Gypsum thickener underflow | wt.% | 25 | Design |
| Flocculant concentration | wt.% | 0.5 | Design |
| Flocculant diluted concentration | wt.% | 0.02 | Design |
| Caustic scrubber solution | wt.% | 15 | Design |
| Emergency scrubber gas flow | Nm³/hr | 17,100 | Design |
| Emergency H₂S concentration | ppmv | 76,800 | Design |
| Emergency HCN concentration | ppmv | 61,300 | Design |
| Scrubber discharge HCN/H₂S | ppmv | 0.2 / 0.2 | Design |
| Caustic storage | days | 3 | Design |
Project website: https://www.p2gold.com/projects/gabbs-project/
