Definition from Wiktionary, the free dictionary
Alternative forms 
- A mine for gold ore or metal.
- South Africa has many goldmines.
- (idiomatic) A very profitable economic venture.
- This oil deposit is a regular gold mine. We make more and more money every year!
- (idiomatic) A plentiful stockpile of something sought after.
- The Smithsonian Institute is a goldmine of knowledge, but like most mines, you have to dig for the good stuff.
See also 
mine for gold
very profitable economic venture
reserve of something sought after