Overview

Deposit type

• Porphyry
• Metamorphic hosted
• Vein / narrow vein

Summary:

The Skouries deposit is centred on a small (less than 200 m in diameter), pencil-porphyry stock that intruded schist and gneiss of the Vertiskos unit. The mineralized porphyry intrusion plunges steeply to the south-southwest and obliquely crosscuts the moderate to steeply north-east dipping limb of a district-scale F2 antiform. Mineralization has been tested to a depth of 920 m from surface. Surface exposures and drill data indicate that the porphyry stock has a subtle north-east elongate geometry. The porphyry is characterized by at least four intrusive phases that are of probable quartz monzonite to syenite composition (Kroll et al. 2002; Frei, 1995) but contain an intense potassic alteration and related stockwork veining that overprints the original protolith. Potassic alteration and copper-gold mineralization also extend into the country rock; approximately two thirds of the Measured and Indicated Mineral Resources are hosted outside the porphyry with about a 50:50 split in gold-equivalent ounces. The potassic alteration is characterized by potassium feldspar overgrowths on plagioclase, secondary biotite replacement of igneous hornblende and biotite, and a fine-grained groundmass of K-feldspar-quartz with disseminated magnetite. Four main stages of veining are recognized:
• Early stage of intense quartz-magnetite stockwork (pre-ore stage).
• Quartz-magnetite veinlets with chalcopyrite ± bornite (initial ore stage).
• Quartz-biotite-chalcopyrite ± bornite-apatite-magnetite veinlets (main ore stage).
• Localized, late stage set of pyrite ± chalcopyrite-calcite-quartz veins (post-ore stage).

Skouries is typical of a gold-copper porphyry deposit. Mineralization occurs in stockwork veins, veinlets and disseminated styles typical of a porphyry, and has a subvertical, pipe-like shape. The multi-phase monzonite to syenite porphyries intruded into metamorphic basement rocks. Both igneous and metamorphic rocks contain high temperature potassic alteration (K-feldspar-biotite) and stockwork quartz-magnetite-chalcopyrite-bornite veins. The potassic zone in the surrounding country rock is surrounded by a high temperature inner propylitic alteration characterized by amphibole. The deposit, however, lacks extensive phyllic or argillic-advanced argillic zones typical of many porphyry systems. This may, in part, reflect a deeper level of erosion and the focused nature of the magmatic-hydrothermal system.

Mining Methods

• Truck & Shovel / Loader
• Sub-level open stoping (SLOS)

Summary:

The Skouries Project is designed as a two-phase mining operation. Phase 1 consists of a combined open pit and underground mine, operating over nine years. Phase 2 consists of mining from the underground mine only, for an additional 11 years. The total ore producing LOM is 20 years.

Phase 1 total mill feed rate is 8.0 Mtpa, consisting of a nominal 5.5 Mtpa from the open pit mine combined with 2.5 Mtpa from the underground mine. At the start of the mine life, during the initial two-year underground mine ramp-up period, the open pit feed rate is variable in order to maintain the 8.0 Mtpa mill feed. During Phase 1, 8.0 Mt of low-grade oxide ore is stockpiled to be rehandled for mill feed during Phase 2. Phase 1 is completed at the end of the open pit mine life in Year 9.

Phase 2 mine production, from Year 10 to the end of the LOM, is provided from the underground mine. Phase 2 mine development begins in Year 4 in order to allow a seamless ramp-up from the nominal Phase 1 production rate of 2.5 Mtpa. During the first four years of Phase 2, the mill feed rate of 8.0 Mtpa is maintained by reclaiming oxide ore stockpiled during Phase 1, at a rate which balances the mill feed to 8.0 Mtpa through Year 13. From Year 15, Phase 2 mill feed is maintained at a nominal feed rate of 6.5 Mtpa, solely from underground mine production, which tails off in Years 19 and 20.

Open pit mining will be by conventional truck-shovel operation. Sub-level open stoping (SLOS) has been confirmed as the most appropriate underground mining method. Production stopes will be backfilled with cemented paste fill. Shaft conveyance of ore will be utilized in Phase 2 to facilitate achievement of the projected production rate.

Open pit mining will be by conventional truck-shovel operation, with an ore production rate of approximately 5.5 Mtpa, at a waste to ore stripping ratio of 1.03 The mining sequence will consist of drilling, blasting, loading, and hauling of ore and waste materials for processing and waste disposal. Based on the modelled rock types, approximately 17% of the mined material is amenable to free digging; this material will not be blasted.

Drilling operations will be carried out continuously as part of the normal mining operation. Once full mine production is reached, drilling and blasting of approximately 1 Mt (dry) per month will be required to maintain production levels. Drilling and blasting activities will be carried out by Hellas Gold, with bulk explosives and associated blasting accessories being delivered to site as needed by an explosives contractor.

Waste material classified as red clay and overburden will not be drilled or blasted as it is considered free dig material. All other waste and ore material types (weak rock and hard rock) will be drilled and blasted.

The primary haul roads are designed at 25 m width, based on a 90 t haul truck. Other haul roads, to be used by contractor trucks, are designed for 55 t articulated haul truck with an overall roadway width of 15 m. Table 16.5 outlines the road widths for each truck class. Road grades are limited to 10% in-pit and ex-pit for the 90 t trucks and approximately 12.5% for the contractor trucks. Runaway lanes will be constructed as required for safe operations. The smaller 55 t haul truck has been selected for waste as the route which the waste needs to travel over to the various dump areas is winding along steep terrain.

The primary mining loading fleet will consist of a conventional 12 m3 diesel hydraulic excavator and two front-end loaders with 8.5 m3 and 12 m3 buckets.

Underground mining
The Skouries orebody that extends below the bottom of the open pit is amenable to a bulk underground mining method and has been evaluated under several different design approaches since the late 1990s, including block caving, sub-level caving (SLC), and SLOS. SLOS has been confirmed as the most appropriate underground mining method for a number of reasons.

The underground portion of the Skouries Project will begin from the existing ramp from the surface to 385 masl. The ramp is currently developed to 35 m above the first production level, 350L. Mining will proceed to the 350L to establish major infrastructure and services. The 350L will serve as the mucking horizon for two test stopes, which are situated in the Crown Pillar and within the mining limits to enable a mineralized and accurate representation of the mining to be completed in Phase 1.

Underground mining will be by conventional underground mining techniques. The mining sequence will consist of drilling, blasting, loading, and hauling of ore and waste materials. During Phase 1, ore will be hauled to the surface crusher by truck. During Phase 2 ore will be hoisted to surface by a shaft. In Year 4, the shaft headframe construction will commence, and shaft excavation will begin in Year 6. Excavation of the shaft will continue through Year 8, with the entire materials handling system projected for completion six months prior to the beginning of Phase 2 in Year 10.

The design of the Skouries mine includes provision for remote mining technology (RMT), which has an impact on the cycle times of stopes and the productivity of equipment. This technology includes tele-remote operation of mechanized equipment by an operator located on surface or in a remote area of the underground mine.

The Phase 2 materials handling will involve shaft hoisting of ore to surface. There are no vertical production nor development ore or waste passes included in the mine design; all broken rock will be loaded using load haul dumps (LHDs) and transported via the ramps in haul trucks. Shaft hoisting is critical to enable a ramp-up to full Phase 2 production of 6.5 Mtpa from the Phase 1 production of 2.5 Mtpa. In order to hoist the material by shaft, underground crushing will be implemented. During Phase 2, all stope ore and some late development ore will be hoisted to surface via the shaft. Development waste will continue to be hauled to surface via the dual ramp system, but these quantities are expected to be minimal.

Comminution

Crushers and Mills

Summary:

The ore is delivered by 90-t haul trucks from the open pit and 50-t haul trucks from underground to the primary crushing station dump pocket. The ore is then crushed by the gyratory primary crusher and the crushed ore is discharged via the primary crusher discharge feeder.

The transportation of the crushed ore from the primary crusher discharge to the covered, conical, crushed-ore stockpile is implemented using a belt conveyor system that is comprised of the primary crusher sacrificial conveyor and the primary crusher discharge conveyor. An over-belt magnet at the discharge of the sacrificial conveyor removes scrap metal before ore enters the ore stockpile.

An automatic dust suppression system will be installed at the crusher feeding point and at the crusher discharge feeder to prevent dust emission from the ore unloading and crushing operations.

The ore stockpile is covered and has a live storage capacity of 24,000 t, equivalent to one day of production. The stockpile area total storage capacity will be approximately 80,000 t, equivalent to slightly over three days of production.

The ore is extracted from the ore stockpile using three variable speed apron feeders, which are of 1,215 t/h total combined capacity and each one is driven by a 75 kilowatt (kW) motor. The feeders are located beneath the stockpile in a tunnel.

The feeders discharge onto a belt conveyor system, which is comprised of the ore reclaim sacrificial conveyor and the SAG mill feed conveyor, which reports to the SAG mill feed chute.

The primary grinding circuit is designed to reduce the feed ore with a particle size of 80% passing 150 mm to a product with a particle size of 80% passing 120 µm. The size reduction is achieved by a two-stage wet grinding circuit comprising a SAG mill driven by variable speed motors, a ball mill (with a fixed speed motor) and a pebble crushing circuit (SABC circuit).

The transfer size of the material between the SAG mill and the ball mill is in the range of 80% passing between 2.0 mm and 3.6 mm. The SAG mill has a diameter of 9.75 m and an effective grinding length of 4.57 m and is driven by two 4.8 MW motors with variable frequency drives (VFD) and including auxiliary lubrication circuits. The grinding media consists of 125 mm diameter balls made from high quality forged steel. The SAG mill liners are Cr-Mo alloy cast steel.

Two shorthead type cone crushers will be installed for the crushing of the oversize pebbles (+12 mm) produced by the SAG mill. Total crushing capacity is 254 t/h and each cone crusher will be driven by a 450 kW motor.

The primary ball mill has a diameter of 7.01 m and an effective grinding length 9.75 m and is driven by two 4.8 MW motors. The grinding media consists of 60 mm diameter balls made from high quality forged steel. The ball mill will be lined with rubber liners and lifters.

Both mills will be equipped with trunnion bearings and drive gear lubrication, drive protection and cooling systems. The SAG mill product flows to a vibrating screen with 12 mm openings. The oversize particles (> 12 mm) are transferred via the pebble conveyor No. 1 to the pebble crushers. The crushed product will be transferred back onto the SAG mill feed conveyor via the pebble conveyor No. 2.

The undersize particles (< 12 mm) flow to the concrete SAG and ball mill sump, where they are mixed with the ball mill product. In this sump, process water of controlled quantity is also added to control cyclone feed density and ball mill circulating load.

The slurry product of both mills is pumped to a 660 mm diameter hydrocyclone cluster. The hydrocyclone overflow slurry with solids content of, typically, 35% w/w and particle size of 80% passing 120 µm comprises the feed of the flotation circuit. The cyclone underflow slurry is directed to the ball mill feed chute. The grinding circuit operation is controlled by an automated control system to ensure that the product size for all types of ore is 80% passing 120 µm at the maximum daily ore feed of 24,000 t.

Concentrate regrinding is carried out by the regrind ball mill. The regrind ball mill has a diameter of 4.60 m and effective grinding length of 7.00 m. The mill is driven by a 2.25 MW motor and is equipped with lubrication, drive protection and cooling systems. The regrind mill has rubber lining and the regrinding media consists of forged steel balls of, typically, 25 mm diameter.

Processing

• Crush & Screen plant
• Flotation
• Dewatering
• Filter press

Summary:

For the first nine years of operation, the ore will be extracted from the open pit mine as well as from the underground mine for a total mill feed rate of 8.0 Mtpa. From the tenth year of operation until the depletion of Mineral Reserves, the plant will process ore extracted from the underground
mine at an average of around 6.5 Mtpa tailing off in Years 19 and 20. During years 10 to 14, previously stockpiled oxide ore will be re-handled to maintain mill feed at 8.0 Mtpa.

The plant will process the copper / gold ore at a projected LOM average head grade of 0.50% copper and 0.77 g/t gold. Anticipated LOM average payable recoveries are 87% for copper and 81% for gold. The mill will produce a flotation concentrate that contains an average of 26% copper and 27 g/t gold. Metallurgical tests have shown that the ore contains a small amount of palladium (Pd), which will be collected into the copper / gold concentrate during flotation.

The process plant design provides for a nominal 8.0 Mtpa of ore throughput. While gravity classification, secondary gravity classification and gold room circuits have been designed, installation has been deferred pending confirmation of the need for gravity concentration to meet designed overall gold recoveries.

The unit operations comprise of:
• Primary crushing and crushed ore stockpile.
• SABC grinding and pebble crushing.
• Flotation and regrinding.
• Flotation concentrate and tailings thickening.
• Flotation concentrate filtering, storage and loadout.
• Tailings filtration, conveying and paste fill production.
• Reagent preparation and services.

Flotation is carried out in six stages:
• Rougher.
• 1st cleaner.
• Cleaner scavenger.
• 2nd cleaner.
• 3rd cleaner.
• Additional cleaning, in order to produce a clean copper / gold concentrate while achieving
satisfactory recoveries.

Concentrate regrinding is carried out by the regrind ball mill.

The regrind ball mill operates in closed circuit with a cluster of 14 hydrocyclones (250 mm diameter), 10 duty and four on standby. The operation of the regrind circuit will be controlled by the central distributed control system (DCS) of the plant.

The regrinding circuit is fed with the concentrates of rougher and cleaner scavenger flotation as well as the regrind gravity concentration tailings and additional cleaner concentrate. These are directed to the hydrocyclone cluster. The regrind cyclone cluster overflow will flow to the 1st cleaning circuit. The regrind cyclone underflow feeds the regrind ball mill.

The final concentrate of the 3rd cleaning flotation stage is directed to a concentrate thickener of 12 m diameter. Flocculant solution is added to increase the solids sedimentation rate. The solids concentration in the thickener underflow is typically 60% w/w. The underflow is pumped to filter presses for filtration.

The concentrate thickener underflow is pumped to filter presses, for filtration to achieve the targeted moisture content. The filter cake is transferred with loaders to an adjacent covered concentrate storage space. There is also a second filter press in the building for spare capacity, as well as a concentrate bulk bag loading system.

Water Supply

Summary:

Process water system
A storage tank (6,100 m3 ) is used for storage of process water. The water comes mainly from boreholes as well as from the overflows of the tailing thickeners and the mine water clarifier. The water is distributed to the relevant locations via a pump station, with three process water pumps (two running / one standby) and a 610 mm distribution pipeline.

Utility water system
A storage tank (1,000 m3 ) is used for storage of utility / fresh water. Process water is recycled within the plant using a pond and thickeners. Make-up water is provided from boreholes and from the mine water clarifier. The water is then distributed via pumps to the various locations for system make-up, cooling systems, pumps gland seals, and dust suppression systems.

Firefighting water system
A storage tank (850 m3 ) is used for storage of firefighting water. The water comes mainly from the boreholes and is distributed where required via a main 254 mm pipeline. The firefighting water distribution is effected by the firefighting pump station, which is comprised of an electrically driven fire water pump, a diesel driven fire water pump and a fire water jockey pump.

Potable water system
A storage tank (25 m3 ) is used for storage of potable water. The water comes from the boreholes and is treated and distributed where required via the necessary pumps and a main 100 mm pipeline.

Water supply
Drinking water comes from boreholes, springs and a reservoir. The larger part of the local water supply network was reconstructed during the 1990s and serves almost the entire population of the area. At Skouries, a storage tank (1,000 m3 ) is used for storage of utility / fresh water. The water comes mainly from the boreholes during the filling of the tank and then from the mine water clarifier. A storage tank (25 m3 ) is used for storage of potable water. The water comes from the boreholes and is distributed via pumps and a main 100 mm pipeline. Maximized reuse of contact and non-contact water facilitates provision of a continuous supply for mine processing operations. A storage tank (850 m3 ) is used for storage of firefighting water. The water is distributed where required via a main 254 mm pipeline.

Production

Operational metrics

Personnel

Job Title	Name	Email	Profile	Ref. Date
Consultant - Mining	Mo Molavi			Jan 22, 2022
Consultant - Recovery Methods	Robert Chesher			Jan 22, 2022
Engineering Manager	Kostas Soultanis			Apr 28, 2024
General Manager Operations	Francisco Ballesteros			Apr 28, 2024
Head of Technical Services	Victor Vdovin			Apr 29, 2024
Mine Development Manager	Lefteris Kazatsanidis			Apr 28, 2024
VP, Technical Services	Peter Lind	peter.lind@eldoradogold.com		Apr 28, 2024