goldminegold·mine /ˈgəʊldmaɪnˈgoʊld-/ (also gold mine) noun [countableC]1informal a business or activity that produces large profitsPay-per-view television, under the right circumstances, is a proven gold mine.
2be sitting on a goldmine to own something very valuable, especially without realizing itThe three companies realized they were sitting on a gold mine: a computerized directory containing the names and addresses of most people in America
3a deep hole or system of holes under ground from which rock containing gold is takenThe Hayden Hill gold mine is expected to produce about 145,000 ounces of gold a year.