An oxygen-free world
The Yilgarn Craton is a vast, ancient piece of Earth's crust, covering approximately 650,000 square kilometers of Western Australia. Much of this continental block formed during the Archean Eon, with some parts containing zircon crystals dated as old as 4.4 billion years. The craton has remained above sea level for about 2.7 billion years, making it one of the oldest preserved regions on the planet.
Its most famous feature is its enormous gold deposits, which were emplaced between 2.66 and 2.63 billion years ago. These deposits are classified as orogenic, meaning they formed during mountain-building events. This process occurred in a world starkly different from today's. Atmosphere and oceans were anoxic, containing virtually no free oxygen. In this oxygen-poor chemical environment, gold could dissolve in hydrothermal fluids. These hot, metal-rich waters circulated through deep fractures in the crust, created by intense tectonic activity that was assembling the craton from various smaller crustal fragments.
The gold-bearing fluids moved through networks of faults and shear zones within "greenstone belts"—sequences of metamorphosed volcanic and sedimentary rocks. As the fluids ascended, they cooled and depressurized, causing the dissolved gold to precipitate, forming veins of quartz and gold, often alongside sulfide minerals like pyrite. The majority of the craton's gold is hosted within these greenstone belts.
Chemical history
The period for this type of large-scale gold mobilization closed around 2.45 billion years ago with the Great Oxidation Event (GOE). The rise of photosynthetic microorganisms began releasing enormous quantities of oxygen, which first accumulated in the oceans and then in the atmosphere. This event permanently altered Earth's surface chemistry.
Oxygen's presence made gold significantly less soluble in near-surface fluids, effectively halting the chemical processes that had concentrated it in the Yilgarn Craton. The gold deposits were locked in place, recording the planet's anoxic past. The conditions that allowed for their formation on such a scale have not been repeated since.
The scale of these deposits is immense. The Yilgarn Craton accounts for about 4% of the world's economically recoverable gold reserves. Since the first discoveries in the 1890s, the region has produced staggering amounts of gold. The famous Golden Mile at Kalgoorlie alone, once considered the richest square mile on Earth, has produced over 1,457 tonnes of gold. The gold here is primarily hosted within a specific rock unit called the Golden Mile Dolerite, a 600 to 750-meter-thick sill of igneous rock.