gold mine

Noun

gold mine (plural gold mines)

1. A mine for gold ore or metal.
   • South Africa has many goldmines.
2. (idiomatic) A very profitable economic venture.
   • This oil deposit is a regular gold mine. We make more and more money every year!
   • The possessors of Salisbury & Yeovil Railway shares had ample compensation for the anxiety which the first few years of the company's existence had given them, for the concern proved a veritable gold mine.1939 December, John D. Hewitt, “Some Notable British Main Lines: II. Salisbury and Exeter, S.R.”, in Railway Magazine, page 407
3. (idiomatic) A plentiful stockpile of something sought-after.
   • The Smithsonian Institution is a goldmine of knowledge, but like most mines, you have to dig for the good stuff.