Placer Mining History
Placer mining is the mining of alluvial deposits (soil deposits made by rivers or other running water sources) for minerals. Excavation may be accomplished using water pressure (hydraulic mining), surface excavating equipment or tunneling equipment. The Hanging Flume was built to hydraulic mine the placer deposits at the Bancroft Claim. As the placer was blasted out of the earth, the placer rich water flowed over a sluice box. The sluice box was a long tough with a series of channels through which the gold-rich sediment was washed. Since gold is a heavy metal, the channels in the sluice box catch the gold and allow the non-valuable sediment to flow by.
Although not required, the non-valuable sediment and water may be recycled. While these recycling and reclamation processes are more common in modern placer mining operations they are still not universally applied.
In earlier times the process water was not generally recycled and the spent ore was not reclaimed. Environmental activists describe the hydraulic mining form of placer mining as environmentally destructive because of the large amounts of silt that it adds to previously clear running streams. Most placer mines today use settling ponds, if only to ensure that they have sufficient water to run their sluicing operations.