Mother Lode Meaning

(idiomatic, by extension) Any source of valuable or useful material.

Example: 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.
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.