The 2025 technical report presents a feasibility-level design for a 15 ktpd gold processing facility at the Mt Todd Gold Project in the Northern Territory, Australia.
Report context
This technical report, dated 2025, describes the processing facility design for the Mt Todd Gold Project, which is based on treating hard ore from the Batman open pit. The report presents a feasibility study design for a nominal throughput rate of 15,000 tonnes per day (5.325 Mtpa). Historical testwork campaigns from 2011 and 2012, together with additional metallurgical and process testing conducted in 2016, 2017, 2018, and 2019, were used to develop the process design criteria, process flowsheet, and mass balance presented in the report.
Processing route
Crushing circuit
The crushing circuit has been designed at a nominal treatment rate of 868 tph based on 6,134 operating hours per year, representing a 72% utilization rate. Run-of-mine ore will be trucked using 190 tonne class rigid-frame haul trucks and tipped into a dual-tip primary ROM bin. The primary crusher is a 50-65 gyratory crusher equipped with a 375 kW motor, with a nominal top size capacity of 1,270 mm, operating with an open-side setting of 140 mm. Oversize material in the dump hopper will be broken using a fixed rock breaker.
Primary crusher discharge feeds onto a sizing screen via conveyors and a feed bin. Oversize from the sizing screen is conveyed to a secondary cone crusher operating in closed circuit with the sizing screen. The secondary crusher reduces the sizing screen oversize F80 of 137 mm to a product P80 of 47.8 mm, with a closed-side setting of 40 mm. The final crushed product from the sizing screen undersize is conveyed to the coarse ore stockpile (COS). The stockpile has a nominal live storage capacity of 32,600 tonnes, equivalent to 48 hours of milling time at the 5.325 Mtpa reclaim rate.
Coarse ore storage, HPGR circuit, and ore sorting
Reclaim from the coarse ore stockpile uses two apron feeders that discharge onto the HPGR feed conveyor. The HPGR has a 2.0 m roll diameter with a 1.5 m face width, running at 1.3 m/s. Feed top size of 31.5 mm is reduced to a P80 of 3.25 mm. HPGR product discharges onto screens: double-deck units 4.2 m wide by 8.5 m long with upper deck aperture of 16 mm and lower deck aperture of 4.5 mm. Screen undersize at P80 of 3.25 mm reports to the primary cyclone feed hopper.
X-ray transmission (XRT) ore sorting is applied to the HPGR product screen top deck oversize fraction (29.5+16 mm). Two primary XRT ore sorters are fed from a bifurcated bin arrangement. Rejects from ore sorting are conveyed to a rejects stockpile, while the product stream returns to the HPGR surge bin combined with bottom deck screen oversize. The rejects stream discards approximately 55 tph, reducing mill circuit feed to 624 tph.
Grinding and classification
Primary grinding uses a 7.16 m diameter by 9.75 m long overflow ball mill with dual-pinion variable-speed drives totalling 11,000 kW installed power. The ball mill operates in closed circuit with hydrocyclones (six 800 mm diameter cyclones, three operating and three standby) to produce a cyclone overflow P80 of 250 µm. The ball mill is charged with 75 mm grinding balls at a charge of 28-29%.
Secondary grinding utilises four Vertimill VTM4500 mills operating in parallel, each equipped with a 3,350 kW motor. The secondary grinding circuit has an overall installed power of 13,422 kW and draws nominally 11,164 kW at the 5.325 Mtpa throughput requirement. Secondary classification uses two clusters of Cavex 250 mm diameter cyclones (22 operating and 4 spares per cluster) operating at 130 kPa to produce a cyclone overflow P80 of 40 µm for the leaching circuit.
Pre-leach thickening
Trash screen undersize from the secondary grinding circuit is pumped to a pre-leach thickener designed to increase solids concentration from 34% to a design underflow of 62% solids suitable for leach circuit feed.
Conditioning, leaching, and adsorption
The flowsheet includes two agitated pre-oxidation/conditioning tanks of 3,750 m³ nominal capacity each, providing 7.5 hours of retention. The testwork programme included four hours of pre-conditioning with lead nitrate at 50 g/t. Slurry is pre-treated with lime and oxygen prior to cyanide addition to form an insoluble ferric hydroxide layer on iron sulfide particle surfaces.
The hybrid CIL circuit comprises two leach tanks followed by six adsorption tanks, each of 3,750 m³ capacity. Total nominal residence time is 30 hours at the 5.325 Mtpa treatment rate. Cyanide is stage-dosed into the two leach tanks and the first three CIL tanks. High-shear pumps and reactors improve dissolved oxygen levels, with oxygen supplied to leach tank reactors and injected down agitator shafts in the first and second CIL tanks. Each CIL tank has two mechanically wiped intertank screens with 1 mm aperture stainless steel wedge wire to retain carbon. Design carbon concentration is 8 g/L. Carbon advance rate is 12 tonnes per day based on expected loaded carbon grade of 1,927 g Au/t, 1,410 g/t Ag, and 195 g/t Cu.
Carbon handling and gold recovery
The elution circuit uses a split AARL system comprising one acid wash/cold cyanide column and one elution column, both of 12 tonnes capacity. The elution system is designed for six elution cycles per week. A cold cyanide pre-strip step is included due to the possibility of high copper loading on carbon from secondary copper minerals in some ore zones.
The elution sequence includes: cold cyanide stripping with 2% cyanide and 2% caustic for 30 minutes, followed by a 120-minute water rinse; acid washing with 3% hydrochloric acid over 25 minutes; water rinsing with preheating to 90°C; carbon transfer to the elution column; and hot treatment with 3% cyanide and 3% caustic solution at 120°C.
Electrowinning uses three cells (800 mm x 800 mm) each with 9 stainless steel cathodes and 10 anodes, supplied by an 1,800 A rectifier. Gold doré is produced via an A200 smelting furnace. The carbon regeneration circuit includes a 0.75 tonne per hour kiln with quench tank.
Tailings detoxification
Detoxification of leach tailings is by the Air/SO₂ process. Final tailings are transferred to the tailings storage facility (TSF) for deposition, with supernatant water recovered from the TSF surface for re-use in the process plant.
Key reported parameters
| Parameter | Unit | Design value | Basis |
|---|---|---|---|
| Annual ore feed rate (ROM) | Mtpa | 5.325 | Design |
| Daily ore feed rate (ROM) | tpd | 15,000 | Design |
| Crushing rate | tph | 868 | Design (6,134 h/a) |
| HPGR new feed rate | tph | 679 | Design (7,838 h/a) |
| Milling rate | tph | 624 | Design (7,838 h/a) |
| Ore sorting rate | tph | 121 | Design |
| ROM feed F80 to primary crushing | mm | 711 | Design |
| Primary grind P80 | µm | 250 | Design |
| Secondary grind P80 | µm | 40 | Design |
| Gold head grade (ROM feed) – Design | g Au/t | 1.29 | Design |
| Gold head grade (ROM feed) – LOM average | g Au/t | 0.97 | LOM average |
| Gold head grade (Mill feed) – Design | g Au/t | 1.38 | Design |
| Silver head grade | g Ag/t | 1.58 | Design |
| Copper head grade | ppm | 550 | Design |
| Cyanide soluble copper | ppm | 65 | Design |
| Design ore specific gravity | t/m³ | 2.76 | Design |
| Design crushing work index | kWh/t | 20.0 | Design |
| Design ball mill work index | kWh/t | 24.5 | Design |
| Leach slurry density | % solids w/w | 45 | Design |
| Total leach and adsorption time – Design | hours | 30 | Design |
| Overall recovery (LOM average) | % Au | 88.5 | LOM average |
| Maximum gold extraction – Design | % | 91.6 | Design |
| Ball mill installed power | kW | 11,000 | Design |
| Secondary grinding installed power | kW | 13,422 | Design |
Project website: https://www.vistagold.com/mt-todd/overview
Technical qualifications
The process design criteria and flowsheet were developed using historical testwork results from 2011 and 2012 campaigns and additional metallurgical and process testing conducted in 2016, 2017, 2018, and 2019. The report presents a feasibility study design only and does not include operational data from an existing plant at this throughput configuration. The processing facility has been designed to process hard ore from the Batman open pit, with ore characteristics including a design crushing work index of 20.0 kWh/t, ball mill work index of 24.5 kWh/t, and an abrasion index of 0.23. The report notes that initial years of mining may be subject to higher cyanide-soluble copper in the feed due to the presence of secondary copper minerals in the upper ore zones.
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*Source: Vista Gold Corp. NI 43-101 Technical Report, Mt Todd Gold Project, Northern Territory, Australia, 15 ktpd Feasibility Study, 2025, Sections 17.1–17.7*
