Primary gold deposits form deep in the Earth's crust when hot fluids containing carbon and sulphur move upwards, dissolving gold and other ore components from the rocks through which they pass. These fluids travel along faults, fractures and other weaknesses in the rocks, carrying the gold in solution as a gold-sulphur complex. Around 5 to 10 km beneath the Earth's surface, the gold-bearing fluids react with iron rich rocks, causing gold to precipitate within pyrite crystals and in quartz veins.

Secondary gold deposits are in the thick layer of weathered rocks blanketing much of Western Australia. Over the millions of years since primary gold deposits formed, the land has been uplifted and eroded in a continually changing climate. Under humid tropical conditions of 100 million years ago, primary gold was dissolved by rainwater and precipitated in horizontal layers just below the water table. About 15 million years ago, the climate became increasingly arid and the water table dropped. The

Total Quantity of Gold Mined in Western Australia:

Many decommissioned mines are retained by the companies for possible reworking should gold prices improve.

The slurry is filtered to remove non-soluble waste material, then mixed with carbon pellets to which the gold cyanide complex attach. The carbon pellets are removed from the process tanks and the gold stripped from them using a small volume of hot cyanide solution.

Open cut projects are best suited to secondary gold deposits or primary gold near the surface. The soft, weathered rocks are easier to mine, need less or no blasting, meaning gold grades as low as two grams per tonne can be economic.

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