Mother lode is a principal vein or zone of gold or silver ore. The term probably derives from a literal translation of the Spanish veta madre, a phrase common in old Mexican mining. It is also used colloquially to refer to the real or imaginary origin of something valuable or in great abundance.

In the United States, the California Mother Lode is a 190-kilometre-long (120 mi), 1.5-to-6-kilometre-wide (0.93 to 3.73 mi) alignment of hard-rock gold deposits in the Sierra Nevada of California, bounded on the east by the Melones Fault Zone. The deposits formed along the suture where the Smartville Block, a Jurassic oceanic terrane, accreted onto North America; hydrothermal fluids emplaced gold-bearing quartz veins during the Early Cretaceous. Discovered in the early 1850s during the California gold rush, the zone stretches from Georgetown in the north to Mormon Bar in the south and contains hundreds of mines. The Mother Lode district produced more than 13 million troy ounces of gold through 1959, making it one of the most productive gold-producing districts in the country.

The term has entered popular culture through films, video games, and other media. In mining terminology, the "mother lode" refers to the bedrock source from which placer gold erodes into streambeds.

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 an 11-kilometre-long (6.8 mi) 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 a long alignment of hard-rock gold deposits stretching northwest–southeast in the Sierra Nevada of California, bounded on the east by the Melones Fault Zone. It was discovered in the early 1850s, during the California gold rush. The California Mother Lode is a zone from 1.5 to 6 kilometres (0.93 to 3.73 mi) wide and 190 kilometres (120 mi) long, between Georgetown on the north and Mormon Bar on the south.

The Mother Lode coincides with the suture line of a terrane, the Smartville Block. 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 15 metres (49 ft) thick and a few thousand feet long. The California Mother Lode was one of the most productive gold-producing districts in the United States. Now it is known as a destination for tourism and for its vineyards.

As with most gold rushes, the California gold rush started with the discovery of placer gold in sands and gravels of streambeds, where the gold had eroded from 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".

The California Mother Lode lies along the Melones Fault Zone in the western Sierra Nevada foothills. This fault marks the boundary where the Smartville Block, a Jurassic-age oceanic island-arc terrane, accreted onto the North American Plate approximately 160–150 million years ago. The collision drove mineral-laden hydrothermal fluids through fissures along the contact zone. Gold-bearing veins were emplaced during the Early Cretaceous, approximately 127–108 million years ago, as the intrusion of the Sierra Nevada Batholith remobilized and concentrated gold from the oceanic terrane rocks.