The United States emerged as a major economic power following the Gold Rush of 1849, during which miners extracted more than 750,000 pounds of gold, according to History.com. In those days, gold was uncovered by manual labor with a pick and shovel, a risky and dangerous proposition with uncertain profit even with a seemingly abundant supply. By 1853, hydraulic mining emerged, largely replacing independent miners. Today, gold is extracted from large scale open pit or underground mines.
In 2013, U.S. gold mine production was estimated at 227 tons and world total mine production was approximately 2,770 tons. However, the days of history-changing discoveries such as those that sparked the California Gold Rush are gone.
While automation and other modern technologies — like portable x-ray fluorescence (XRF) analyzers and cross-belt elemental analyzers — have significantly improved the speed, productivity, and safety of mining operations, gold remains difficult to find because it’s relatively scarce in the earth and occurs in a variety of rocks and geological environments.
The two primary types of gold deposits are lode (primary) deposits and placer (secondary) deposits. Gold, a report published by the U.S. Geological Survey, describes the main hypotheses geologists have proposed to explain the source of lode deposits.
- One popular hypothesis proposes that many gold deposits, especially those found in volcanic and sedimentary rocks, formed from circulating ground waters driven by heat from bodies of magma about 2 to 5 miles of the surface.
- Another theory suggests that gold-bearing solutions may be expelled from magma as it cools, precipitating ore materials as they move into cooler surrounding rocks. This hypothesis is applied to gold deposits located in or near masses of granitic rock, which represent solidified magma.
- A third hypothesis suggests that ore metals originate from sedimentary and volcanic rocks undergoing active metamorphism due to high temperatures and pressures.
Typical prospecting involves examining samples throughout the mine site to determine if the region is worth exploring. Gold occurs in very small concentrations, so it’s easy to miss. If gold is found, substantial capital investment is required to uncover these elusive deposits.
The last two years have added another layer of challenges: The declining price of gold is having a ripple effect in financing, exploration, and drilling decisions. With less money available, fewer exploration projects are being pursued. When deposits are located, few of them have large enough concentrations to be financially worthwhile.
Discoveries that actually do become mines can take up to two decades to come online and may require more expensive mining techniques. Efficient and cost effective practices are more important than ever.
To enhance their chances of finding gold, geologists may use geophysical methods to measure variations in the physical properties of rocks that may indicate the presence of gold, such as density, magnetism, electrical conductivity, or natural radioactivity. Although these geophysical methods can be crucial for gold exploration, geochemical methods – including portable x-ray florescence (XRF) – are the only methods that can measure concentrations of gold and other associated elements. Today, portable XRF is used in various stages of gold exploration and mining including grass-root exploration using pathfinder elements, finding source of gold in stream sediments, core logging, identification of lithologies, and even grade control.
Read An Uncertain Future for Gold? to learn about other important issues, and potential solutions, influencing the mining industry.