Probably a calque of Spanish veta madre (“mother lode”).
mother lode (plural mother lodes)
- (geology) A large or rich vein of gold or of another precious mineral from which other branches extend.
- 1912, Jack London, Smoke Bellew, ch. 7:
- Undeterred, reasoning from the coarseness of the gold that it had not traveled far, they had set out in search of the mother lode.
- (idiomatic, by extension) Any source of valuable or useful material.
- 2000, Dick Thompson, "The Feds Step Up the Pace," Time, 10 April:
- It is also letting them probe the genomes of other organisms for DNA that could turn out to be a mother lode for medicine.
large or rich vein of precious mineral