mother lode

• mother-lode
• motherlode

Probably a calque of Spanish veta madre (“mother lode”).

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mother lode (plural mother lodes)

1. (geology) A large or rich vein of gold or of another precious mineral from which other branches extend.
   • 1912, Jack London, “The Little Man”, in Smoke Bellew:

     Undeterred, reasoning from the coarseness of the gold that it had not traveled far, they had set out in search of the mother lode.

2. (idiomatic, by extension) Any source of valuable or useful material.
   • 2000 April 10, Dick Thompson, “The Feds Step Up the Pace”, in Time‎:

     It is also letting them probe the genomes of other organisms for DNA that could turn out to be a mother lode for medicine.

   • 2014 March 4, Landon Thomas Jr., “Emerging, but No Longer a Mother Lode of Profits”, in New York Times‎:

     Emerging, but No Longer a Mother Lode of Profits [title]

• mother lode on Wikipedia.Wikipedia