Gold mine (slang)
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In colloquial English, the term gold mine is used to describe a very profitable economic venture. For example, a business or investment opportunity that provides (or could potentially provide) a considerable amount of money for the owners or shareholders as net earnings could be considered a gold mine. (A profitable physical gold mine could also be called a gold mine due to its profitable nature.)
An example sentence using the term gold mine in the idiomatic sense could be: "This oil deposit is a regular gold mine. We make more and more money every year!"