gold ‧ mine BrE AmE , gold mine /ˈɡəʊldmaɪn $ ˈɡoʊld-/ noun [countable]
1 . informal a business or activity that produces large profits:
The nightclub turned out to be a goldmine.
2 . a place where gold is dug out from a hole in the ground
3 . be sitting on a goldmine to own something very valuable, especially without realizing it