Mining and Recovery

Gold has been mined and worked for almost 6,000 years. Although it has been found all over the globe, today the chief gold producing countries are South Africa, Russia, Canada, the U.S. Australia, and Brazil.

In the U.S., gold mining began around 1800. At that time, several methods were used to mine placer deposits. Panning was the simplest method. To increase production, miners began using rocker boxes, which had sloping bottoms and were mounted on a rocking base that resembled a child's cradle. Prospectors rocked gravel and water in the cradle, sifting out lighter material and isolating the gold.

A sluice box and miners in Alaska cira 1905. Paul J. Cry Collection, University of Alaska, Fairbanks.

A further improvement known as the long tom, was a though about three meters long with transverse cleats or riffles to entrap the gold particles more effectively. In turn, the long tom evolved into a series of sluice boxes through which a stream of water passed. In the bottom of each sluice box, the riffles were supplemented by blankets whose fibers entrapped even fine gold particles.

As more capital for investment become available, more advanced mining techniques were developed. In hydraulic mining, the gold-bearing gravel bank was washed with a jet of water that passed through a high-velocity nozzle. To build up the necessary pressure, the water from upstream was channeled into wooden flumes built along one wall of a valley and then down through iron pipes to the bottom of the valley. This powerful jet of water produced a slurry that passed through long lines of sluice boxes to separate and recover the gold. Unfortunately, the more effective this method became, the more destructive it was to the landscape. Court orders curtailed hydraulic mining in California in 1884.

Chinese miners work with rocker boxes. Hutching's California Magazine, Volunme V, no 3, Sept. 1860.

Once a prospector had located a gold-bearing stream by panning, he continued upstream until he found the "mother" lode. Most large deposits were discovered in this manner. These lode deposits were not amenable to the unsophisticated recovery techniques of the placer miners. The gold and other valuable minerals like silver, copper, and lead were bound with nonvaluable minerals in solid rock.

At first, lode despots were mined underground in much the same way as coal. Miners reached the gold by digging passages into the ground, following the direction of the vein. They drilled long holes into the rock, inserted explosives, and blasted the rock into pieces. These pieces were then transported to the surface.

The arrastra, a cheap and simple mill for crushing the blasted rock, relied on the abrasive action of heavy stones hitched to a horizontal arm. Originally, men and mules rotated the arm; later, water provided the force.

A woman uses a gold pan to separate gols from other heavy particles collected in the sluice box. Mary Jane Fate Collection, University of Alaska, Fairbanks

The stamp mill replaced the arrastra. Heavy metal stamps, or mallets, pounded the rock into a powder. The first stamp mills were water-powered. Later, larger mills relied on steam produced in boilers fired by burning wood or charcoal. Charcoal was the better fuel, as it burns hotter than wood.

Gold was isolated and concentrated using various techniques. In amalgamation, the pulverized rock was washed over plates covered with mercury. Because gold has a high affinity for mercury and bonds easily with it to form an amalgam (a mixture of elements), the gold would adhere to these plates, while other materials washed away: Workers would then scrape the amalgam off the plates and heat or smelt it to vaporize the mercury (which could be collected and reused) leaving the gold. Amalgamation works only with free-milling ores, in which the gold is merely mixed in with other minerals, not chemically combined with them.

Deeper ores, in which gold is bound to minerals, required that miners develop a different approach. In cyanidation, the crushed ore is ground into a fine powder that is treated with a chemical (cyanide) to dissolve the gold. The resulting solution then passes through a complicated series of filters, clarifying tanks, storage tanks, and precipitation plants.

Jeremy M. Brodie
Environmental Education and Volunteer Programs
Last Updated: May 1, 1996