As chemical weathering and erosion gradually break down the host rocks and lower the land surface, the quartz and gold veins are eventually exposed to the atmosphere. The veins provide far more resistance to chemical attack than the surrounding rocks, so that mechanical weathering is required to fragment the quartz, thereby releasing the gold. Because they are relatively heavy, particles of gold are more difficult to move and so become naturally concentrated in the soil or in adjacent gullies or streambeds. These concentrations are known as alluvial or placer deposits and have yielded incredible riches on some goldfields, such as those in California and central Victoria.
Alluvial deposits take many forms, including sands and gravels in the beds of modern-day streams, in old river valleys buried under lava flows or perched on hilltops due to uplift of the land surface. The terms shallow and deep leads are used in Victoria for gold-bearing gravels covered by younger sedimentary layers or lava flows. These were especially important in the Ballarat district. Because of its resistance to chemical attack, gold can be recycled from one type of alluvial deposit to another. Credits: By Dr Bill Birch, Senior Curator, Geosciences, Museum Victoria