INTERIOR OF MILL
GOLD MINING
Gold is one of the most widely diffused of metals. Traces of it are found in practically all igneous and most sedimentary rocks. It occurs in sea-water, and quite frequently in beach-sands. Traces of it are also usually to be found in alluvial deposits and in the soils of most mountain-folds. In spite of its wide diffusion, however, all the gold that has been mined could be stored readily in the vaults of any large New York bank.
In all probability most of the gold now in use has been deposited by solution in quartz veins, the latter usually filling seams and crevices in granitic or volcanic rocks. Quartz veins seldom yield very great returns, but they furnish a steady supply of the metal. The rock must be mined, hoisted to the surface, and crushed. The gold is then dissolved by quicksilver (forming an amalgam from which the quicksilver is removed by heat), by potassium cyanide solution, or by chlorine solution.
In many instances the quartz veins have been broken and weathered by natural forces. In such cases the gold is usually carried off by swiftly running water and deposited in the channel lower down. In this way "placer" deposits of gold occur. Placer deposits are sometimes very rich, but they are quickly exhausted. The first gold discovered in California was placer gold.
Nearly all the gold mined in the United States has come from the western highlands. In 1900, Colorado, California, South Dakota (Black Hills), Montana, and Alaska yielded about seven-eighths of the entire product. The placer mines of Alaska are confined mainly to the beach-sands and the tributaries of Yukon River. Since 1849 the average annual yield of gold in the United States is about forty-three million dollars.
The Guinea coast of Africa, Australia, California, the Transvaal of South Africa, and Venezuela have each stood at the front in the production of gold. The aggregate annual production of the world has increased from one hundred and sixty million dollars in 1853 to more than three hundred million dollars in 1900.