Mother lode is a principal vein or zone of veins of gold or silver ore. The term probably came from a literal translation of the Spanish veta madre, a term common in old Mexican mining. Veta madre, for instance, is the name given to a seven-mile (11 km) long silver vein discovered in 1548 in Guanajuato, New Spain (modern-day Mexico).
In the United States, Mother Lode is most famously the name given to the long alignment of hard rock gold deposits stretching northwest to southeast in the Sierra Nevada of California. The California Mother Lode is a zone from one to four miles (6 km) wide and long, which stretches from Georgetown in El Dorado County on the north, through Amador, Calaveras, and Tuolumne counties, south to Mormon Bar in Mariposa County. It was discovered in the early 1850s, during the California gold rush. The zone contains hundreds of mines and prospects, including some of the best-known historic mines of the gold-rush era. Individual gold deposits within the Mother Lode are gold-bearing quartz veins up to thick and a few thousand feet long. The California Mother Lode was one of the most productive gold-producing districts in the United States, but is now given over to tourism.
The California gold rush, as with most gold rushes, started with the discovery of placer gold in sands and gravels of streambeds, where the gold had eroded out of the hard rock vein deposits. Placer miners followed the gold-bearing sands upstream to discover the source in the bedrock. This source was the "mother" of the gold in the river and so was dubbed the "mother lode".
A popular misconception is that small veins of gold or silver ore in a mining district are necessarily branches of a single rich and massive mother lode deep in the ground. This idea is contrary to modern theories of ore deposits.
The term is also used metaphorically to refer to the origin of something valuable or in great abundance.