Different Methods of Mining Gold
- Placer mining, which reached the peak of its popularity in the 1850s, is an inexpensive method of mining gold. This process involves digging pay dirt, leaving it to dry in the sun and then pulverizing it into dust on a large canvas. The wind blows away lighter materials, leaving the heavier gold dust in the pan. Water is also used in panning. The light dirt and gravel are worked away by the water, leaving the heavier gold material in the bottom of the pan. Originally, simple tools including wooden hand tools, knives, shovels and picks were used, meaning that the process was much less efficient than many modern mining techniques.
- From the 1850s to the 1880s, hydraulic gold mining methods were used. This involved high-pressure nozzles squirting huge jets of water against large gravel banks. Gold was recovered after the gravel was washed through ditches and pipe lines. This method was banned in 1884 as the debris began to have a negative effect on agricultural interests.
- Dredging, as a method of mining gold, was at its most popular from the late 19th century until the 1970s. Early techniques used a chain of buckets to bring up gravel and mud from river beds and deposit it in sluice boxes on the boat. Screens separated the fine material from the coarse material and gold was caught by using mercury. Modern gold dredging methods use more sophisticated technology and while they still work on river bottoms, they also take ponds with them to work inland.
Open Pit Mining and Cyanide Leaching
- As of the early 21st century, gold is commonly mined through processes involving cyanide. In open pit mining, a pit is dug and cut to contain concentric, circular rings or ledges. This structure enables excavation equipment to move around the edge, pre-blasting the ore to dislodge it and using loading shovels to transfer the material onto hauling trucks and transport it to extracting facilities.
Low-grade gold is extracted from outdoor heaps of material through "heap leaching", or "cyanide leaching." Cyanide solution is sprinkled over the heap and within a few weeks, the gold will be dissolved, allowing it to run down an impermeable slope into a large sluice. The impermeable pad that lies on the slope prevents the solution, which carries poisons as well as gold, from being able to enter the groundwater.