mother lode

"important vein of an ore or mineral in rock," 1849, from mother (n.1) + lode (n.); said to be a translation of Mexican Spanish veta madre, a name given to rich silver veins. The American use is first in reference to a conspicuous vein of quartz rich in gold discovered during the gold rush in the Sierra Nevada of California. The colloquial or figurative sense of "richest source of something" is by 1916.