Gold mine

South Africa has many goldmines.

This oil deposit is a regular gold mine. We make more and more money every year!

The possessors of Salisbury & Yeovil Railway shares had ample compensation for the anxiety which the first few years of the company's existence had given them, for the concern proved a veritable gold mine.

[Hal Varian] went to work helping Page, Brin, and Schmidt develop more efficient auction algorithms and build the auction model that became such a gold mine for Google.

The Smithsonian Institution is a goldmine of knowledge, but like most mines, you have to dig for the good stuff.