Gold
Gold is widely dispersed in the Earth’s crust in very low concentrations 0.001 grams per tonne. For mining to be viable, gold needs to be concentrated between 2000 and 10,000 times (2-10 g/tonne) to form a gold deposit. 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.
