gold mine

noun
1.
a mine yielding gold.
2.
a source of great wealth or profit, or any desirable thing.
3.
a copious source or reserve of something required: a gold mine of information about antiques.

Origin:
1425–75; late Middle English

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Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
gold mine
 
n
1.  a place where gold ore is mined
2.  a source of great wealth, profit, etc
 
'gold-miner
 
n
 
'gold-mining
 
n

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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American Heritage
Idioms & Phrases

gold mine

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® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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Example sentences
If hydrogen fuel cells are the power source of the future, pee is a potential
  gold mine.
In the short term, this is a potential gold mine for foreign-affairs
  scholarship.
At the same time, the company also digs into a gold mine unique to this
  picturesque city.
The current crisis in the retail business has made his work a gold mine.
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