a gold mine
1. Literally, a mine where gold ore is found and excavated. Also written as "goldmine." More than 500 workers have become trapped in a gold mine after a minor earthquake cause the mineshaft to collapse.
2. An enterprise, opportunity, or resource that is or has the potential to be extremely profitable. The social networking site has since become a veritable gold mine, attracting millions of new users every month and generating staggering amounts of money from ad revenue. If we can secure the merger deal with the pharmaceutical giant in London, I'm sure it will prove to be a gold mine!
3. A person, place, or thing containing a plentiful amount of something valuable or desirable. Who knew that my grandfather's attic would turn out to be a gold mine of priceless antiques? This database is a gold mine for anyone interested in medical research trials. Our professor is a gold mine of information about medieval French literature.