The McArthur River lead-zinc-silver deposit of northern Australia is the largest known stratiform sediment-hosted base-metal deposit.

The mineralisation of the McArthur River area of the 'Here's Your Chance' deposit covers around two square kilometres and is 55 metres thick. It comprises eight recognisable and discrete zinc and lead ore zones. The natural movement of the earth changes the orientation and angle of each zone over time, which complicates our mining process.

Zinc-lead-silver mineralisation occurs predominantly as ultra-fine bedded parallel sphalerite and galena rich bands hosted by dolomitic and carbonaceous pyritic siltstones, graded beds and chaotic debris flow breccia.

The open pit excavation is being undertaken in six stages, with the first three targeting the shallowest ore. Conventional drilling, blasting, loading and haulage methods are used.

After blasting, the overburden is excavated in horizontal benches to expose the target ore, which we then remove for processing. Separating the ore and waste at the mining stage ensures a relatively pure ore is fed to the processing plant.

Excavators with up to 550 tonne capacity load 175-220 tonne haul trucks with ore or waste to be taken to stockpiles or the waste rock piles. The majority of the overburden rock from the open pit is stored at the North Overburden Emplacement Facility, or waste rock pile.

Refining the ore:
Ore is trucked from the open pit to the processing plant where it goes through three stages of crushing and screening, pre- concentration via heavy media separation followed by flotation, oxidative leaching and dewatering.

Crushing:
The ore is crushed down to rock sizes of 9 mm through three stages of crushing commencing with a jaw crusher, followed by secondary and tertiary crushing using cone crushers. After crushing, low grade ore moves through to the Heavy Medium Plant to upgrade the ore to the target process feed grades while high grade ore moves directly through to the primary grinding stage.

Primary grinding:
The ore then moves through a primary grinding circuit at a rate of around 550 tonnes per hour. This circuit consists of a 4 megawatt SAG Mill operating in open circuit followed by a 11.5 megawatt Ball Mill running in closed circuit with hydrocyclones. Through this circuit, the ore is slurried with recycled process water and ground to produce fine material of 80% less than 45 microns.

Flotation circuit:
The slurried ore is then passed onto the flotation circuit where the zinc and lead bearing minerals are separated from the non value minerals. Here, various reagents are introduced which attach themselves to the metal sulphide materials and enable them to be recovered from tanks as a froth. The zinc and lead bearing minerals are then recovered and upgraded to the desired product quality for the clients.

Pre-cycloning:
The concentrate material recovered from the first stage of flotation requires further upgrading. As such, this material is then pumped through a pre-cycloning stage to produce two products, a fine and a coarse material. The fine material is sent to conditioner tanks for further processing and the coarse material is sent to the regrinding circuit to obtain the target particle size prior to joining the fine product at the conditioner tank.

Regrinding:
The regrinding circuit consists of open circuit M10000 Isamills grinding the coarse precyclone underflow stream to 12microns followed by six M3000 Isamills operating in closed circuit to produce an ultra fine product of 7 microns. This material is returned to flotation for further upgrading of the metal grade.

Bulk concentrate:
The fine ground materials are then conditioned with reagents prior to passing through a six stage cleaner circuit to produce a final zinc- lead bulk concentrate. The concentrate and tailings products are pumped to different thickening tanks.

Oxidative Leach:
Some of the bulk concentrate is pumped to the oxidative leach circuit. Here the material is leached using acid, heat and air to separate the zinc bearing minerals from the lead bearing minerals. The leached product is returned to the flotation section to recover a saleable zinc concentrate, typically of 48% zinc grade and a lead concentrate of 50% lead grade. These concentrate products are pumped to different thickening tanks prior to filtration.

Thickening and filtering:
Thickened concentrate is sent to the stock tank prior to filtering through Larox pressure filters and then conveyed to the concentrate storage shed.

Tailings removal:
Thickened tailings are pumped to the tailings storage facility via a rubber lined steel pipeline. The tailings storage facility is located west of the mine and plays a key role in recycling water for use within the processing plant.

Handling of concentrate:
At the end of this process, the concentrates have a moisture content of around 13%, which reduces dust generation during handling. All trucking, loading and shipping operations are fully contained to protect against spillage and ensure environmental security. The lead concentrate is placed into large lined bulk bags which are then packed into steel containers for transport to Mt Isa Mines.