Definition of "gold mine"

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.

1939 December, John D. Hewitt, “Some Notable British Main Lines: II. Salisbury and Exeter, S.R.”, in Railway Magazine, page 407

[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.

2019, Rana Foroohar, Don't Be Evil: The Case Against Big Tech‎, Penguin UK