The Perama Hill gold deposit is a stratabound, sediment hosted deposit of Eocene to Oligocene age, located at the eastern edge of the Maronia Graben at the contact point with a north northeast trending eastern graben fault. The Perama Hill deposit is described as an initial high sulphidation epithermal system overprinted by typical low sulphidation banded quartz chalcedony, barite, pyrite stockwork veins, and veinlets. Two key controlling factors on the mineralization are the porosity and rock competency contrasts between the volcanic sandstone and underlying andesite package.

The deposit extends 750 m in a north-south direction and up to 300 m in an east-west direction and the thickness varies from 15 m to 20 m at the flanks and up to 125 m at the centre. The gold mineralization has been fed into the system by structurally controlled feeders inside andesitic volcanic breccia and disseminated into overlying porous sandstone units. The central part of the deposit is mushroom-shaped. Eighty percent of the gold is hosted by sandstones and the rest is associated with andesitic volcanic breccia and conglomerates.

Mineralogical studies from the Perama Hill gold deposit indicate that gold mineralization is very uniform throughout the deposit and the gold grain size is small, generally less than 2 µm.

Gold mineralization occurred during the latest stage of silicification and gold bearing siliceous fluids were introduced after structural preparation by hydraulic fracturing and veining of the same hardened silicified rock. Gold is identified in the sulphide mineralization in association with very fine pyrite containing sub-micronic gold and gold silver telluride. Enargite and stannite group minerals (Cu, As, Sb sulphides) were also introduced in this event. Galena and sphalerite are also present in the sulphide ore but do not appear to correlate with gold.

The gold occurs as micron size particles in association with quartz, clay and hematite.

Mineralization in the sandstones accounts for more than 80% of the oxide deposit. Hydrothermal fluids, which travelled upwards through structurally controlled conduits in the andesite breccia were able to disseminate and travel laterally through a finely developed network of fracture systems developed in the porous sandstones.

Mineralization in the volcanic breccia unit is confined to the sulphide facies in the central part and the oxidized uplifted panel in the northern part of Perama Hill. Due to less supergene leaching in these less porous, quartz-free and strongly clay altered rocks, metals like Cu and Pb can be identified in outcrop in association with clay and strongly developed limonite minerals. Heterogeneity of the rock, allowing fluid circulation at the contact between clasts and matrix under hydraulic pressure, was a favourable factor.

The Perama Hill project has an orebody that is amenable to conventional open pit mining. The rate of production is 1.2 Mt of ore per year and an additional 350,000 tonnes of waste and low-grade material.

Using fracture spacing and hardness models a matrix of the two was made to identify the degree of blasting or ripping that will be required. The material is generally soft and fractured. Although a large percentage of the material is expected to be ripped and/or excavated without any need for ripping, a conservative 100% drilling and blasting mining method was selected for equipment requirements and costing. Light and medium blasting will be employed for most of the material with an average powder factor of 0.12 kg/t, hard blasting, which is a fraction of about 1% of the total material would use a powder factor of 0.20 kg/t.

The selected excavating equipment will be two Caterpillar 385C hydraulic backhoes each with 5.5 m³ bucket capacity loading into a fleet of six Mercedes Actros 4150 8x4 dump trucks each with a payload of 33 tonnes.

The Perama Hill process plant will be designed to treat 1,250,000 t/a of conventional oxide ore cap containing gold and silver bearing ore at a treatment rate of 3,400 tonnes per calendar day at an overall plant availability of 90%. The processing facilities include three stage crushing, followed by a single stage ball mill with a classification step to produce 80% passing 75 µm product size.

Milled ore will be thickened in a high rate thickener prior to pre-aeration and gold/silver dissolution in the CIL circuit. Hydrated lime will be added in the milling circuit. Precious metal dissolution will be by cyanide. Oxygen will be injected into the pre-aeration tank and leaching tanks to enhance the leach kinetics. The CIL circuit will consist of six tanks with a total retention time of 20 hours.

The CIL tailings will be detoxified to remove any remaining cyanide by the INCO SO2 Process. The Environmental Terms of Reference (ETR) requires a CNWAD of < 10 mg/L. Studies indicate a target concentration of <1 mg/L CNWAD should be attainable. Detoxified tailings will be thickened, pressure filtered and dry-stacked.

Carbon will be extracted from the CIL circuit and acid treated for calcium removal prior to elution on a daily basis. The gold and silver will be recovered by a split Anglo American Research Laboratories (AARL) elution system. Carbon will be reactivated and returned to the last stage CIL tank. Pregnant solution will be fed through the electrowinning circuit to recover the gold and silver metals. The resulting electrowinning sludge will be washed, filtered, dried and smelted to produce Doré metal. The barren solution will be pumped to the leaching circuit.

The crushing plant will be designed to treat more at a rate of 350 t/h to a product size of 100% passing 150 mm through a jaw crusher for a period of 16 h/d. The ROM bin, with a capacity of 150 tonnes, will be at least one and a half times the truck capacity.

The ROM bin will be equipped with a static grizzly and a permanently installed hydraulic rock breaker. Material will be discharged from the bin by a vibrating grizzly feeder. Minus 100 mm material scalped out by the grizzly will discharge onto the sacrificial collection conveyor. Oversize will pass to the primary crusher.

The crushing plant will be a conventional two stage crushing circuit comprising an open circuit secondary screen plus cone crusher followed by a closed circuit tertiary screen and cone crusher. The final product will be the tertiary screen undersize with a P80 of minus 8 mm. To ensure optimum choke feeding for the crushers feed surge bins of 15 minute holding capacity plus vibrating pan feeders will be provided at each crushing stage.

The milling circuit will consist of a single ball mill. A preliminary mill size of 4.88 m diameter x 7.62 m EGL is proposed powered by a 3.5 MW motor. The range of work Indices is high due to the variable nature of the ore.

Blending of ore feed may be required to ensure consistent mill throughput and grind size. Ore from different mine locations will be combined in a controlled manner ahead of the primary crusher and ROM stockpile. Equally, there is potential for an increase in feed throughput if the ore is slightly softer.

A 21 m high rate thickener will thicken the mill discharge prior to CIL. The installation of a thickener between milling and leaching will enable the pulp density within the milling circuit to be optimised for milling efficiency and the pulp density of the CIL to be maximised to reduce reagent costs and minimise tankage volume for the required residence time.

The CIL circuit residence time will be 20 hours. It will consist of 6 CIL tanks with a single preaeration tank of the same volume as the leach tanks. The carbon concentration will be 10 g/L to 15 g/L with a projected carbon loading of up to 8,000 g/t Au + Ag.

Oxygen will be injected into the slurry to enhance the leach kinetics. Oxygen injection will be into the pre-aeration tank and into any number of CIL Tanks 1 to 4. Injection will be via a Slamjet sparger.

Carbon will be advanced along the tank train for 20 h/d using vertical centrifugal pumps. Loaded carbon pumping to elution will operate for 10 h/d.

The flow of pulp between CIL tanks will be by Kemix pumped type MPS (P) interstage screens. These eliminate hydraulic drops between tanks allowing all tanks to be mounted at the same elevation. The tanks will be connected by launders, which will include bypass arrangements so that any single tank can be isolated for maintenance.

Under normal operations one batch of carbon will be removed per day. During mining peak ore grades, up to two carbon batches per day will be needed and the carbon concentration in the leach slurry can be increased to 20 g/L.

The fully automated elution circuit will treat a 7 tonne carbon batch size with a combined gold and silver loading of 8,000 g/t. This selection will be consistent with a maximum of two elutions per day, 6 d/wk while mining peak ore grades during Years 2 and 3 and up to one elution per day otherwise.

A mercury retort will be available as there is a possible presence of up to 120 g/t mercury on the final loaded carbon.

A split AARL elution system was specified for the Optimization Study by TGM. A conventional dilute acid wash stage will precede elution to remove calcium and magnesium.

The elution column will be operated at a normal stripping temperature of 115 to 120°C. However, to facilitate the elution of silver during periods of high silver to gold ratios the circuit will be designed to operate at up to 130°C. Maximum stripping times (including column filling and emptying) will be in the order of 8 hours.

The elution circuit will regenerate all of the activated carbon from each stripping cycle. The regeneration kiln will be an LPG fired rotary kiln with a variable speed dewatering screw feeder and a heat recovery arrangement that pre-heats the carbon with off gas. The 440 kg/h carbon kiln will treat up to 67 t/wk of carbon and a 1.5 Bed Volume hopper will be provided ahead of the kiln for surge capacity. An eluted carbon by-pass will be operated in emergency only.

Loaded carbon will be pumped from the CIL via the loaded carbon screen into a loaded carbon tank adjacent to the CIL circuit. From there the carbon will be pumped in batches into the acid wash column.

The loaded carbon tank will enable loaded carbon to be pumped while a previous batch is still being acid washed.

The electrowinning circuit will operate with a recirculating flow from the pregnant solution tank. The last cycle will be in open circuit to return the barren electrolyte to the CIL. This philosophy can be reviewed at the detailed engineering stage.

The hot pregnant solution will be pumped to the head of an electrowinning train consisting nominally of two 150 ft3 sludging cells in parallel incorporating 60 cathodes each 0.84 m2.

Filtered precious metal sludge will be dried and calcined in a mercury retort, mixed with crushed (recycled) smelting slag and fluxes and smelted in an induction furnace. Smelting will be performed on 2 d/wk.

The ETR requires that bag filters are installed not only on the furnace off gas but also on the electrowinning cells vent gas. Aker Solutions have replaced the latter with a scrubber because there will be no dust in that stream but there will be traces of ammonia.

A pressurised vent system will be provided for the Goldroom to prevent build-up of any mercury vapour. The retort will be installed within a separate room within the Goldroom with its own extraction fan.