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The running water in a river can cause deposits of gold to form: in the riverbed, on the neighboring inundation plains or the banks of the river. These are known as gold placer deposits, and they have been mined for hundreds, if not thousands of years.
The most basic kind of gold placer mining is the using a prospector’s pan to swirl sediment-filled water so that unwanted sediment spills out to leave bits of gold behind. This method is based on the fact that gold is heavier than typical sediment. This concept is the foundation for all placer mining techniques.
There are two different, but not distinct, kinds of gold placer deposits. Shallow gold placers are generally within or close to existing rivers and are not blocked by other deposits. Deep level placers are buried underneath debris or solid stone. The rivers that developed these deposits may have been pushed into other channels by substantial shifts in the geography of the surrounding area.
Gold in these deposits can appear near the surface, in river beds or spread throughout the thickness of a stratum. Usually, however, gold is found in the lowest portion of superficial beds, just above bedrock.
The Mining Process
Before a placer mining operation can begin, the first task is to identify where drainage has deposited gold into sediment layers. To locate a deposit, prospectors use pans to evaluate the surface materials. On dry land, surface material may be loosened with pick and shovel, and then washed for panning.
If panning yielded evidence of a placer in the past, prospectors may have tunneled horizontally to identify the richest area. This approach is known as drift mining. These tunnels were normally dug in winter because frozen ground was much less likely to collapse on miners. Then, in warm weather, a defrosting area of gold-rich gravel known as “paydirt” could be processed.
If water entered the shaft during drift mining, it was drawn out by a bucket. Unless it came in too fast, then the claim was abandoned.
More recently in California, miners would deflect rivers to access gold placer deposits. In Siberia and the Yukon jets of water or wood fires would be used to defrost ground so that it could be loosened with a pick and shovel.
Since the early 20th century, gold miners have increasingly turned to machines known as dredges, which are capable of making poor ground profitable. Using traditional techniques but on an industrial scale, dredges are capable of scooping thousands of cubic feet of gravel in a single day each day.
While a prospector’s pan can be used to separate gold from sediment, it isn’t the most effective approach.
A sluice box uses the power of flowing water to make for a faster process. Typically, around 3 meters long, 0.5 meters wide and less than a 25 cm deep, a sluice box has a series of riffles in the bottom that agitate a slurry of water and gravel to extract gold particles and nuggets. A stream of water is passed through the box to cause the lighter waste material to spill out one end.
A trommel is a rotating metal cylinder set at a slight incline with a screen at the bottom end. Lifter bars are located inside the tube. To start the process, materials from the placer are fed into the upper end of the device. Water, which is usually pressurized, is then fed into the apparatus, and the combination of flowing water and mechanical agitation works to free gold-containing bits of ore. The ore that passes through is then further refined in sluices and other devices.
To extract as much as possible, gold placer miners have added mercury to sluice boxes. Gold and mercury chemically bond to create an amalgam, which makes for a much higher extraction rate. After removing the amalgam from a sluice box, it is heated in a crucible, breaking the amalgam and turning mercury into a vapor. The gold can be liquified to get rid of impurities.
Due to ecological concerns, most legal, large-scale gold mining operations do not use mercury. However, illicit operations and those in developing countries are known to use it.
Sources and Further Reading