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Placer deposits result from weathering and release of gold from lode deposits, transportation of the gold, and concentration of the gold dominantly in stream gravels. Unless preserved by burial, a placer subsequently may be eroded, and either dispersed or reconcentrated.
Placer Gold Deposits. Placer Deposits, or Secondary Gold Deposits, are the easiest and most cost-effective to mine. They are formed when gold is eroded by weather (temperature, wind, water) over millions of years from its source rocks.
Gold accumulations in an old stream bed that are high are called bench deposits. They can be found on higher slopes that drain into valleys. Dry stream beds (benches) can be situated far from other water sources and can sometimes be found on mountaintops. Today, many miners focus their activities on bench deposits.
Placer gold is gold that has broken loose from the main lode, as a result of weathering and erosion. Placers can be found both close to and far from the gold lode, depending on the forces at play. One example is placer gold that gets into a river, where it can travel thousands of miles downstream
Typical locations for alluvial gold placer deposits are on the inside bends of rivers and creeks; in natural hollows; at the break of slope on a stream; the base of an escarpment, waterfall or other barrier. Stream placers are the most economical and common types of placers and have provided many with riches in the past.
Placer gold was discovered in Park County in 1859, one of the earliest discoveries in Colorado. Most of the placer gold has come from the Fairplay, Tarryall, and the Alma districts in the northwest part of the county along the east slope of the Mosquito Range.
Known placer gold deposits were found along the southern and eastern flanks of the Ortiz Mountains; however, there could be additional placer gold deposits along the western flanks of the mountain range. Most placer deposits in New Mexico had been discovered by 1900.
California’s Sierra Nevada Mountain Range is by far the top gold region in the state. With well over 10,000 gold mines and thousands of active placer claims, this region has the state’s largest historical gold production totals and the most active modern placer mining districts.
Placer gold accumulates anywhere that gravity allows it to settle; below waterfall, behind rocks and boulders, or anywhere that the water velocity slows down. Black spots represent areas of gold deposition. Not only does gold go deep down in the gravel, the gold is also constantly moving downstream until it reaches an obstruction.
Placer deposits containing gold are areas that have highly concentrated accumulations due to stream/river erosional processes taking place around/over geologic terranes that contain gold, over a very long period of time.