Opinion ID: 445104
Heading Depth: 2
Heading Rank: 1

Heading: Placer Mining

Text: 2 A placer is an alluvial or glacial deposit containing particles of gold. Placer mining to remove the gold is often done on a large scale. The average mining operation requires an investment of $250,000 to $500,000 for bulldozers, a front-end loader, sluice box, airplane, airfield and buildings. Mining sites are usually in remote areas; many have no access by road. Miners remove surface soil and vegetation, then excavate the gold-bearing material (pay dirt) from the placer deposit. Next, the miners remove the gold from the pay dirt by a gravity separation process known as sluicing. Miners place the pay dirt in a sluice box, which is a channel having small submerged dams or riffles attached to its bottom. When rapidly moving water flows through the sluice box, the heavier particles, including gold, are caught in the riffles. Lighter materials, including sands, silts, and clays remain suspended in the waste water released from the end of the sluice box. 3 Untreated discharge water from these large-scale operations can kill fish and ruin their habitats before the suspended materials eventually settle out. The increased sediment load can alter stream flows or cause flooding and covering of historic sites. In addition, placer mining may release toxic arsenic and mercury into streams. Mercury is sometimes used in placer mining operations to increase the settling of gold-bearing materials. If mercury is added to the water flowing through the sluice box, it may contaminate the waste water. Arsenic apparently occurs in conjunction with some gold deposits in Alaska and may be released as a result of mining activities.