Precious Metals

Placer gold is derived from gold-bearing rock from which the metal has been freed by weathering. Gravity and running water then combine to separate the dense grains of golds from the much lighter rock fragments. Rich concentrations of gold can develop above deeply weathered gold veins as the lighter rock is washed away. The "Welcome Stranger" from the gold fields of Victoria, Australia, is a spectacular 71.3 lb (32.4 kg) example of this type of occurrence.

Gold washed into mountain streams also forms placer deposits where the stream's velocity diminishes enough to deposit gold. Stream placers form behind boulders and other obstructions in the streambed and where a tributary stream merges with a more slowly moving river. Placer gold is also found in gravel bars where it is deposited along with much larger rocky fragments.

The discovery of place gold set off the California gold rush of 1849 and the rush to the Klondike in 1897. The largest river placers known are in Siberia, Russia. Gold-rich sands there are removed with jets of water, a process known as hydraulic mining. A fascinating byproduct of Russia's hydraulic mining is the unearthing of thousands of woolly mammoths, many with flesh intact, locked since the Ice Age in frozen tundra gravel.

Stream placer deposits have their giant ancient counterparts in paleoplacers, and the Witwatersrand district in South Africa outproduces all others combined. Gold was reported from the Witwatersrand (White Waters Ridge) as early as 1834, but it was not until 1886 that the main deposit was discovered. From that time until today, it has occupied the paramount position in gold mining history. Witwatersrand gold was deposited between 2.9 and 2.6 billion years ago in six major fields, each produced by an ancient river system.

Placer and paleoplacers are actually secondary gold deposits, their gold having been derived from older deposits in the mountains above. The California 49ers looked upstream hoping to find the mother lode, and that's exactly what they called the system of gold veins they discovered.