Placer

Secondary deposits of gold formed by erosion from veins or lodes in primary deposits are known as placers. They occur in alluvial or eluvial (residual) form, including the weathered and oxidized zones above gold-bearing sulphide lodes. Placers are also sometimes known as free gold. They comprise small particles or nuggets of metal which can be up to 998 fine but more usually are closer to 850 fine, with impurities of bismuth or arsenic that may render it brittle.

Placers are often found along the banks or in the beds of rivers and streams or in ancient drainage systems, as in the Yilgarn Block of Western Australia. Historically this is how gold was first found in the rivers of Africa, Asia Minor, China or South America with the gold being easily accessible and recoverable. The great gold rushes to California in 1848, to Australia in 1852 and to the Klondike in Canada in 1896 were all placer deposits (by contrast South Africa’s gold is essentially in primary deposits and was not so swiftly recognized).

Placer gold still attracts thousands of diggers through much of Africa (especially in Zaire), in Brazil, China, the Philippines, Russia and Venezuela. Worldwide placer output reached almost two hundred tonnes annually in the 1980s and still contributes around one hundred tonnes a year.