The Gruyere gold deposit is an Archaean orogenic gold deposit. This deposit type is widespread in the greenstone belts of the Yilgarn Craton and in other greenstone belts around the world including in Canada, Africa and India.

Archaean orogenic gold deposits share a number of similar geological characteristics including location in greenstone belts, strong structural control of orebodies, relative timing of gold mineralisation with respect to peak metamorphism, consistent metal association and broadly uniform hydrothermal fluid chemistry. However individual deposits display a diverse range of depositional site characteristics including host lithologies, structural setting, alteration and mineralisation styles, and various aspects of fluid and ore chemistry (oxidation state, gold fineness).

Most orogenic gold deposits are hosted in mafic-ultramafic extrusive and intrusive rocks whereas the Gruyere deposit is hosted in a granitoid. As the Gruyere deposit is a recent discovery and the first major deposit in the Yamarna area, insufficient studies on the mineralised system have been completed to indicate how closely Gruyere compares with major orogenic gold deposits in other locations such as the better known Kalgoorlie Terrane in the central Yilgarn Craton.

Gold mineralisation at Gruyere is characterised by varying intensity albite-sericite-chlorite biotite-calcite alteration, with associated pyrite-pyrrhotite disseminations, and coarse arsenopyrite proximal to high grade zones. Grade commonly increases with the intensity of albite-sericite-chlorite-biotite-calcite alteration; high-grade zones are commonly overprinted with limited porphyry textural retention. Intense alteration and pyrrhotite>pyrite+arsenopyrite mineralisation is often observed proximal to sheared, recrystallised south-east plunging quartz veins.

Lower grade gold mineralisation (commonly < 0.3 g/t Au) within the porphyry is characterised by reddened (hematite dusted) albite-quartz muscovite-biotite-oligioclase-magnetite alteration with only minor pyrite disseminations. Visible gold is commonly observed within brittle-ductile chlorite±pyrite bearing fractures, which are common throughout the porphyry.

While fractionated gold-only porphyry analogues are uncommon, the Canadian Malarctic deposit, hosted along the Cadillac Fault Zone – Abitibi Greenstone Belt, is similar in host lithologies, intrusive lithology, mineralised volume and primary structural features. Both deposits are hosted on major shear or fault zones along secondary dextral events, and include similar intrusive hosts - quartz monzonite porphyry (Gruyere) and quartz monzodiorite porphyry (Canadian Malarctic), intruding into volcanic/sedimentary country rock sequences.

Mineralisation within the Canadian Malarctic is primarily hosted in the host Pontiac group clastic sediments, and with the remaining mineralisation within the quartz monzodiorite porphyry, whereas mineralisation at Gruyere is entirely hosted within the quartz monzonite porphyry. Porphyry geometries vary, with the Malartic porphyry showing multiple dykes extending from a deeper pluton-stoping up into the Pontiac sediments, while the Gruyere porphyry has intruded the host volcanics as a singular intrusive, possibly as a dyke in a higher relative position to a deeper pluton.

Gold mineralisation at Gruyere has developed in response to a complex reverse shearing structural event. The porphyry, a more competent and brittle body compared to the relatively ductile host rocks, responded to deformation with significant cracking and fracturing resulting in increased permeability. Gold-bearing mineralising fluids were able to flow freely through the rock mass, resulting in uniform and disseminated gold mineralisation ubiquitous to the porphyry.

The entire Gruyere Porphyry is variably altered and gold grade can be related to variations in style and intensity, of alteration, structure, veining and sulphide species. Zones containing higher grade gold mineralisation above 1.2 g/t Au generally have strong albite ± sericite ± chlorite ± biotite alteration and are associated with a sulphide assemblage of pyrrhotite + pyrite ± arsenopyrite, weak to moderate foliation, common micro-fracturing and steeply dipping quartz veining.

Sulphides are common throughout the zone of gold mineralisation, with pyrite dominant in the upper areas and pyrrhotite-arsenopyrite increasing with depth. The total percentage of sulphide minerals is generally in the range 0.5-2%. Quartz ± carbonate vein sets observed in diamond core and optical televiewer surveys show multiple character: early shear veins parallel to the shear foliation; late tabular veins (0.01 to 1 metre thick) at high angle to foliation with variable albite alteration halos; veins with strong chlorite margins; chlorite fractures ± albite halos; and fine stock work veins in areas of intense alteration.

The Gruyere JV Mineral Reserve comprises five open pits plus ore stockpiles. The Mineral Resource includes seven open pits and one underground deposit. The Gruyere mine utilises mining contractors to mine the open pit using conventional drill, blast, load and haul activities. The Gruyere pit mined oxide and fresh rock material in 2021, allowing validation and optimisation of the geotechnical parameters. The pit is designed to be mined in stages over the LoM. Material was mined from Stage 1, Stage 2 and Stage 3 during 2021. The new LoM expands the pit from 3 to 7 stages. During 2021, mining consisted of predominantly fresh rock material mined, which was harder and more abrasive material. Crusher feed to the processing plant is provided by a combination of direct tip material from the mine and rock sourced from the RoM and long-term stockpiles.

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All ore mined is processed in the Gruyere plant, which consists of primary crushing, SAG/ball milling, gravity and carbon in leach (CIL) circuits. The processing plant was designed to 7.5Mtpa but subsequent work shows this can be increased to 9.262Mtpa for 2022 and then increased to 10Mtpa by 2024.

The TSF perimeter embankment is constructed in a downstream manner (in stages) to enclose a surface area of ~203ha at Stage 1 (starter) and 231ha at Stage 6 (final). The TSF has a High B ANCOLD consequence rating and a remaining LoM storage capacity of ~78Mt. Studies are in progress to extend the existing TSF to increase capacity by 29Mt – 34Mt

PLANT
Gravity Recovery
The gravity circuit will consist of four centrifugal concentrators treating a portion of the cyclone underflow. Gravity concentrate will be leached using a vendor supplied intensive leach reactor to yield a pregnant solution from which precious metals will be recovered by electrowinning.

The cyclone underflow launder will have three separate compartments. Two of these compartments will feed the gravity circuit. The number of cyclones servicing each gravity compartment can be adjusted as required. Each of the two gravity feed compartments will feed a dedicated 2.4 metres wide by 6 metres long, horizontal, wet vibrating screen. The screen deck panels will have alternating rows of 2.4 mm by 6 mm and 2.4 mm by 18 mm slots. Screen oversize will return to the ball mill feed. Screen undersize will feed the centrifugal gravity concentrators. Each screen will supply two 1.219 metre (48 inch) diameter centrifugal concentrators. The concentrators will operate in a staggered discharge cycle so that while one unit is flushing the other units are collecting concentrate. The gravity circuit has been designed for a 40-minute collection cycle followed by a standard flushing cycle.

The tailings from the gravity concentrators will return to the ball mill feed. Concentrate from the gravity concentrators will discharge to the intensive leach reactor. The batch leach process will be initiated on a daily basis. The leaching sequence will be controlled by a programmable logic controller (PLC). After leaching, the residue will be returned to the mill discharge hopper by a centrifugal slurry pump and the pregnant solution will be forwarded to electrowinning located in the gold room.

Electrowinning will be carried out in a dedicated 800 mm by 800 mm electrowinning cell fitted with 12 cathodes and 13 anodes. Electrical current will be supplied from a 1,200 A rectifier. The cathodes will be stainless steel and the precious metal precipitate will be removed by washing loaded cathodes in a cathode washing station and filtering the resulting sludge. The filter cake will be dried in an oven and then combined with fluxes and smelted to produce gold doré.

Leaching and Adsorption
After screening to remove trash, the cyclone overflow from the grinding circuit will be thickened using a 38 metre diameter Hi-rate thickener and then leached with cyanide in a hybrid CIL circuit that consists of a singlestage of leaching and six stages of leaching and adsorption. The total nominal pulp residence time in the hybrid CIL circuit will be 24 hours.

Pre-leach thickener feed will be dosed with flocculant and thickened in the 38 metre diameter Hi-rate thickener to 50% solids (w/w). The thickener underflow (leach feed) will be pumped by one of two centrifugal slurry pumps (14 by 12 inch) with 315 kW drives, arranged in a duty/ standby configuration, to the CIL tanks. Cyanide will be dosed into the suction of the duty thickener underflow pump, and oxygen will be injected into the leach feed line. The thickener overflow will gravitate to the process water pond via a sedimentation pond.

The leaching and adsorption circuit will consist of a 5,000 m³ leaching tank with a nominal pulp residence time for Fresh ore of four hours and six 4,200 m³ CIL tanks with a nominal 20 hour pulp residence time (leaching and CIL). For Oxide ore the residence time will be a total of 20.4 hours, for the Transition it will be 22.7 hours and for Fresh ore it will be 24 hours. The design will include the ability to bypass any tank in the train should this be required.

Each CIL tank will have two 20 m² mechanically wiped, inter-tank screens with 1 mm aperture stainless steel wedge wire to retain carbon. The design carbon concentration will be 9 g/L. Carbon will be advanced through the CIL circuit counter current to the pulp, on a batch basis, by recessed impeller pumps. Loaded carbon from the first stage of the CIL will be pumped to the loaded carbon screen. The loaded carbon screen will be a 1.5 metres wide by 3.6 metreslong, horizontal, wet vibrating screen. Loaded carbon from the loaded carbon screen will gravitate into the acid wash column. The design advance rate for the circuit is 15 t/d. Barren carbon from the kiln (or directly from the elution column) will be returned to the circuit via the barren carbon screen. The barren carbon screen will be a 1.5 metres wide by 3.6 metres long, horizontal, wet vibrating screen.

Elution and Gold Recovery
The carbon handling and gold recovery system will comprise the following:
- 18 t mild steel, rubber lined, acid wash column;
- 18 t stainless steel elution column;
- 6,500 kW elution heater;
- A split AARL elution system with two 249 m³ pregnant solution tanks and a 249 m³ barren solution tank;
- 1.5 tph carbon regeneration kiln and its associated quench tank;
- An eduction water system for carbon transfer including a recycle system with a settling cone to remove carbon fines from the circuit for bagging and subsequent treatment (by others);
- An electrowinning circuit with four 800 mm by 800 mm electrowinning cells with each cell fitted with 12 cathodes and 13 anodes and supplied by a 1,200 A rectifier;
- A cathode washing station and filter to recover precious metal precipitate;
- An A300 smelting furnace and crucible to produce gold doré;
- A secure gold room with a vault and safe for the storage of bullion.