Placer or Alluvial Mining
Placer or alluvial mining (not including dredging) is the mining of stream beds, also known as alluvial deposits, for minerals. Placer mining is typically used to mine precious metal deposits, particularly gold, and gemstones. These deposits result from upstream erosion of mineral deposits with the eroded material transported downstream by the flow of the river and eventually settling out due to gravity. Gems and heavy metals like gold are considerably denser than the sand and gravel material they are found in, so they tend to accumulate at the base of placer deposits or in the curves of rivers.
The simplest technique to extract gold from placer ore is panning. This was the method first used by the miners in the California Gold Rush (1849 to 1855). In panning, potential mineral bearing material from a river bed is placed in a large metal or plastic pan, combined with a generous amount of water, and agitated so that the gold particles with higher density than the other materials settle to the bottom of the pan. The lighter material such as sand, mud and gravel are then washed over the side of the pan, leaving the gold behind. Pans are still used today by recreational miners, prospectors, and geologists.
Other, more productive methods were then developed by the Gold Rush Miners. A rocker box (or "cradle") could handle a greater volume of material than a gold pan. It was more portable and required less infrastructure than a sluice box, being fed by hand. It consisted of a box sitting on rockers, which when rocked, separated out the gold. Mercury was often used by miners to amalgamate the gold in the box.
Miners started mining on a larger scale by constructing a sluice box, with cross pieces along the bottom called riffles to trap the heavier gold particles as water washed them and the other material along the box. This method better suited more continuous excavation with shovels and to feed ore into the box. Flumes or pipes were often used to feed water to the box. Most of the easy placer deposits in the Sierra Nevada Mountains were mined out by 1865.
Due to the availability of water that could be used to mine, move, and separate the paying minerals from non-paying gangue, a high capacity method is known as hydraulic mining, hydraulic sluicing, or "hydraulicking" was used in the Sierra Nevada Mountains starting in 1853. From 1853 to 1884, hydraulicking of placers using water monitors (water cannon) removed enormous amounts of material from the gold fields, carried it downstream, and over time raised the level of portions of the Central Valley by some seven feet in affected areas and settled in long bars up to 20 feet thick in parts of San Francisco Bay. This caused occasional flooding of towns and farms and resulted in levee construction. These issued lead to formation of an opposition calling themselves the "Anti-Debris Association" and a lawsuit was filled to stop hydraulic mining activities in 1852. A major dam failure in 1853 (the English Dam) created a major downstream flood which was a final blow. In January 1884, the Sawyer Decision against the North Bloomfield Mining and Gravel Company case resulted in banning the flushing of debris into streams and the hydraulic mining in California's gold country came to an end.
The legacy of the Gold Rush and hydraulic mining in California from 1849 to 1884 (35 years) was the loss of mercury, used to amalgamate and remove gold from the placer deposits, into the watershed. It is estimated that about 10 to 30 percent of the mercury used in the gold extraction process of placer mining was lost, resulting in an estimated 10 million pounds of mercury being deposited into the Sierra Nevada Mountains watersheds and downstream tributaries (Churchill, 2000). However, just as gold mining created the problem, dredging today (which does not use mercury) is a viable way of removing and collecting this mercury so it can be recycled or disposed of in an environmentally responsible manner.
Modern placer mining today uses trommels. A trommel consists of a slightly-inclined rotating metal tube with a screen at its discharge end. Lifter bars, often bolted in angle iron, are attached to the interior. The ore is fed into one end of the trommel and water, often under pressure, is provided to mix with the ore. The combination of water and the mechanical action of the rotating trommel frees the valuable minerals from the ore. The freed ore passes through the screen and is further concentrated in smaller devices such as sluices and jigs. The larger pieces of material that do not pass through the screen are carried to a waste dump by a conveyor. Gold is separated by gravity and mercury is not used.
Placer mining includes both use of surface and underground mining techniques, depending on the geometry and depth of the deposit, as well as other factors. Where the ground is consolidated enough or frozen for mine safely using underground mining, mining by tunneling is possible. Both surface and underground placer mining is carried out in the Sierra Nevada Mountains today.
The Sierrans for Responsible Resource Development (the "Sierrans") believes that placer mining, using modern mining techniques, without the use of mercury, using gravity separation, properly designed settling ponds and water recycling, is a responsible and viable method of mining placer deposits in the Sierra Nevada Mountains. Today, these placer mines are required to have operating permits, reclamation plans, and reclamation bonds. Both surface and underground placer mining can provide jobs, support the local rural communities, and provide local, state, and federal tax dollar. Mining of these deposits may also provide recreational opportunities for the public that wish to view and operating gold mine in the Sierra Nevada Mountains.