GEOLOGICAL DISTRIBUTION
About one-half of the silver output of the world is obtained as a by-product in the winning of other metals, notably lead and copper. Such production comes from deposits of all geologic ages. The silver obtained from high-grade silver ores associated with minor amounts of gold or base metals is now derived chiefly from Tertiary deposits. Pre-Cambrian deposits are of some importance, e.g. Cobalt, Ontario; but the Tertiary is the great source of silver ores.
The Mexican mines, which are by far the most important as producers of silver ores, are associated with Tertiary volcanic rocks. A similar association occurs to the south, in Central America, Peru, and Bolivia. The deposits of the United States, Nevada, Colorado, Utah, and Montana, occur under similar conditions. This is also true of the chief worked-out deposits of recent silver-mining history—the Comstock Lode.
A great deal of silver has been mined in Europe in the past, chiefly from rich silver deposits which are now largely exhausted. Geologic conditions here are obscure, because the mining was completed so long ago.
In the future, Mexico and the Rocky Mountain-Andes Cordillera will be the chief region for the mining of rich silver ores. The pre-Cambrian areas of Canada may continue to be of some importance.