mother lode

noun2

1. •
   A large or rich vein of gold or of another precious mineral from which other branches extend.

   • Undeterred, reasoning from the coarseness of the gold that it had not traveled far, they had set out in search of the mother lode. — 1912
2. •
   Any source of valuable or useful material.

   • It is also letting them probe the genomes of other organisms for DNA that could turn out to be a mother lode for medicine. — 2000