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.

Conventional open pit mining would be used at Perama Hill. The pit would operate one eight-hour shift, five days per week. The crushing circuit would operate 16 hours per day, seven days per week. The mining and crushing operating hours were reduced in consideration of their proximity to the local village. The processing plant would operate 24 hours per day.

The mine would use seven 33 tonne trucks and two matching backhoes. A front-end loader would be used for the ore stockpile at the crusher. The process plant would primarily use water from recycled sources, a local borehole as well as surface runoff where possible will supplement process water requirements. The TMF would have a structural fill embankment and filtered tailings near the process plant. The TMF would be a double lined system with impermeable GCL and HDPE membranes.

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 crushed ore bin will have a live capacity of approximately 4,000 tonnes. The storage bin will be furnished with two belt feeders discharging onto the mill feed conveyor.

Dust extraction will be installed as required to comply with the ETR limit for dust emissions of 30 mg/Nm3. Dust capture will be with cyclones and bag filters. In addition, dust suppression by water sprays will be employed at the discharge point into the fine ore silo.

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. The hard material has a Work Index of 20 kWh/t and the soft material has a Work Index of 15 kWh/t. To handle this variation a variable speed drive is proposed that will allow the mill to operate between 60% to 80% of its critical speed, Nc. Because of the increase in ore hardness as mining progresses the ball charge loading will increase from 27% v/v in the early years to 35% v/v for the harder ore in later years.

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.

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 ........