“A gold mine is a hole in the ground with a liar standing on top of it” is a quotation of credited to Mark Twain, the pen name of author Samuel Langhorne Clemons (1835-1910). There is no evidence that Twain ever said it.  The saying was attributed to an unnamed Kansan in 1903:
 
“A man in Atchison, Kan., has given utterance of a heartfelt definition of a gold mine, which tells its own story. He says that a gold mine is ‘a hole in the ground owned by a liar.’”
 
Had Mark Twain said it, the many 1903 newspapers that published this anecdote surely would have mentioned it. “Mark Twain once said, ‘A gold mine is a hole in the ground dug by a liar’” was cited in a 1916 newspaper.
 
The saying remains popular in financial discussions about gold mining companies.
 
 
25 August 1903, Rockford (IL) Morning Star, pg. 8, col. 5:
A man in Atchison, Kan., has given utterance of a heartfelt definition of a gold mine, which tells its own story. He says that a gold mine is “a hole in the ground owned by a liar.”
 
Chronicling America
12 September 1903, The Labor World (Duluth, MN), pg. 2, col. 3:
As Most of Us Have Found Out.
From the Philadelphia Ledger:
The Kansas definition of a gold mine is “a hole in the ground owned by a man who is a liar.”
 
27 March 1905, Denver (CO) Post, “Oily Gift Bitterly Attacked,” pg. 5, col. 5:
Rev. Harris G. Hale, pastor of the fashionable Leyden Congregational church in Beacon street, Brookline, said:
(...)
“A trust may be best defined as a thing not to be trusted, just as a gold mine is a hole in the ground owned by a liar.”
 
3 September 1916, San Diego (CA) Union, “U. of C. to Inaugurate Course in Prospecting,” pg. 9, col. 4:
Mark Twain once said, “A gold mine is a hole in the ground dug by a liar,” but it was the sturdy man whi, daring nature with his life, struck hard his pick into the frowning hill and mountain sides who brought to California its first wealth and fame, and much of her picturesque history.
 
5 July 1934, The Evening Tribune (San Diego, CA), “Sally’s Sallies” comic strip, pg. A7, col. 1:
A gold mine is a hole in the ground, and the man who owns it is a liar.
 
Google News Archive
12 October 1980, The Evening News (Newburgh, NY), “Gold Is Where You Find It” by John Sinor (Copley News Service), pg. 2B, col. 2:
A San Francisco newspaper pulled out an old Mark Twain quote to suggest the situation up there:
 
“A gold mine is a hole in the ground with a liar at the entrance.”
 
Google Books
The Complete Book of Gold Investing
By Jeffrey A. Nichols
Homewood, IL: Dow Jones-Irwin
1987
Pg. 113:
“A gold mine is a hole in the ground with a liar standing next to it.”
Mark Twain
 
The View from the Blue Ridge
A gold mine is a hole in the ground with a liar on top
By Christopher Pavese – October 7, 2009
Mark Twain once wrote “A gold mine is a hole in the ground with a liar on top.”
 
Kitco Forums 
Frank Matoka 
05-29-2010, 09:12 PM  
A gold mine is a hole in the ground with a liar standing next to it.
That’s a Mark Twain quote.
   
Business Insider
JIM ROGERS: Gold Mining Stocks Face Two Major Headwinds
Mamta Badkar Jul. 7, 2013, 8:16 AM
(...)
Jim Rogers: Mark Twain said the definition of a gold mine is ‘a hole in the ground with a liar standing at the top of the hole,’ because there’s just so many of them. Somebody once did a study, and I think he determined that more money has been lost in gold mining shares than any other industry in America, including airlines and railroads at one time. So miners are going to have a very difficult time ahead of them.