The Mother Lode gold district of central California yielded millions of ounces of gold from placer and bedrock mines. Gold was produced from a region covering nearly 8000 square miles on the western margin of the Sierra Nevada Mountains; beginning with the historic 1848 discoveries, and continuing until the second world war, when mining activities were halted throughout the district. Today, only a very small amount of gold is produced from the district, mostly by amateur prospectors.

Bedrock mining in the Mother Lode district was concentrated in the vicinity of the structural and lithologic contact between the Mariposa slate and the adjacent igneous and metamorphic rocks to the west. This steeply west-dipping zone is considered to be the “mother lode” of gold for the region. Many of the early bedrock mines were developed in the black Mariposa slate, and typically followed quartz veins.

Several mines have been redeveloped during the last decade to produce the black, graphitic slate. Industrial Minerals Co., along with Horseshoe A Mining, has redeveloped several of these old mines near Plymouth and Drytown, Amador County. The bedrock surrounding the former drift mines is removed using open-pit techniques. The ore is crushed by Horseshoe A at the mine site, and is then ground by IMCO at its Sacramento mill. This slate flour is used as filler in asphalt seal coat; and is also blended with clay pigments for use in some novel plastics applications. The platy structure and its blue/black color make it ideal filler for these applications. Approximately 100,000 tons per year is processed for use in this expanding industry. Vein quartz is high-graded during mining, and gold is recovered as a valuable, but minor by-product.