a mine yielding gold.
a source of great wealth or profit, or any desirable thing.
a copious source or reserve of something required: a gold mine of information about antiques.
Origin of gold mine
late Middle English word dating back to 1425–75
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a place where gold ore is mined
a source of great wealth, profit, etc
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
A rich, plentiful source of wealth or some other desirable thing, as in That business proved to be a gold mine, or She's a gold mine of information about the industry. [First half of 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.