To be “sitting in a gold mine” is to be in possession of something of great value, often without knowledge or understanding of that value. The American idiom usually does not require actual sitting or an actual gold mine. Owners of land containing oil or natural gas (but not gold) can be said to be “sitting on a gold mine.” A baseball club with great players in the minor leagues can be said to be “sitting on a gold mine” of talent.
 
The expression possibly originated by someone literally sitting on a gold mine. In 1877 in Big Horn country (probably Lost Cabin, Wyoming), soldiers were looking for Indians, following the massacre the year before of troops led by George Armstrong Custer (1839-1876). A cavalry officer called a halt and sat down. A soldier spotted something and exclaimed, “My God, captain, you are sitting on a gold mine!” This story has been cited in print in an 1894 newspaper article and a 1904 book. The figurative use of “sitting on a gold mine” came into popular use by at least the 1910s.
 
   
The Free Dictionary
sitting on a gold mine
Fig. in control of something very valuable; in control of something potentially very valuable.
When I found out how much the old book was worth, I realized that I was sitting on a gold mine. Mary’s land is valuable. She is sitting on a gold mine.
 
(Oxford English Dictionary)
gold-mine, n.
Forms:  Also gold mine
A mine from which gold is obtained. Also fig. a source of wealth; esp. a source of abundant income or profit.
1483 Cath. Angl. 161/2   A Goldemyne.
1530 J. Palsgrave Lesclarcissement 226/1   Goldemyne, miniere a or.
 
UltimateWyoming.com
Lost Cabin
Named for a fabled gold mine where prospectors built a cabin. The cabin was never found after Indians scared them off. This little town grew up around the property of John B. Okie. Okie came to Wyoming as a penniless cowboy, then gradually got involved in the sheep industry, invested in a line of stores, and eventually became a millionaire. 
 
17 July 1894, The State (Columbia, SC), pg. 2, cols. 1-2:
FORTUNE THRUST ON THEM.
HOW WEALTH HAS COME TO MINERS BY CHANCE.
Strange Stories of “Dumb” Luck. Creede’s Experiences in Colorado. Sitting on a Gold Mine—A Lost Mountain of Nuggets.
Denver Sun.
(...)
A year after the Custer massacre in the Big Horn country, during a noon halt, a cavalry officer sat down to rest on a knoll of soft clay; near by was his orderly holding the bridles of both horses. They were smoking and admiring the grand mountain peaks about them.
 
After a bit the orderly’s eyes came down to the earth; they were glued to the knoll on which the captain sat. Suddenly he leaped to his feet and shouted out: “My God, captain, you are sitting on a gold mine!” and such proved to be the case, and these two men, who were out hunting Indians, stumbled into wealth.
   
Google Books
Secrets of the Rocks: or, the story of the hills and the gulches:
A manual of hints and helps for the prospector and miner
By Samuel Milligan Frazier
Denver, CO: Hall & Williams
1905, ©1904
Pg. 194:
In the fall of 1877, a little more than a year after the Custer massacre in the calley of the Little Big Horn, a cavalry officer of the regular army was scouting after hostiles, who led him a wild goose chase among the peaks of the Big Horn mountains. For days he followed them, but finally lost their trail and than became lose himself. One day about noon, after traveling since daylight, he called a halt, dismounted his men and ordered a rest for half an hour.
Pg. 195:
The soldier was momentarily paralyzed in all his faculties at what he saw. At last he could control himself no longer, and letting the horses go, he jumped to his feet, rushed over to the officer and shouted in his face: “Captain, you are sitting on a gold mine!”
Pg. 196:
It is possible, even probable, that the auriferous mound of the soldiers is an outcropping of the Lost Cabin mine. The latter is still hidden away somewhere among the Big Horn pile, and every indication points to this heap as being a part of the mystic ledge.
 
28 February 1906, The Evening Post (Charleston, SC), pg. 2, col. 4:
FACTS IN NEGRO REFORM.
(By the Orphanage Man.)
(...)
“I’ve been under the impression from what I’ve been hearing others say of you and your work that you were sitting on a gold mine and that somebody had given you the $15,000 to support the orphanage and reformatory.”
   
28 February 1914, The Daily Mail (London), “Mr. Sawyer and the Canteens,” pg. 8, col. 5: 
Mr. Sawyer: I wanted to know what my liabilities were. It was also a question of how much I was going to get. I wanted him to realize what was being done. I think he thought I was sitting on a gold mine.
     
Google Books
September 1917, Illustrated World, pg. 57 photo caption:
TWO “TOMMIES” SITTING ON A GOLD MINE
To make this heap of captured German grenades took one man’s labor for a year.
 
2 March 1923, Kingsport (TN) Times, pg. 7, col. 1:
Mr. Edwards said the farmers here were “sitting on a gold mine and did not know it.” This is the heart of the best tobacco belt in the United States, and no fear need be felt of a let-down in the market for years.
 
Google Books
The Magazine of Wall Street
Volume 35
1924
Pg. 1113:
Some stockholders now have the belief that they are sitting on a gold mine.
 
OCLC WorldCat record
Sitting on a gold mine.
Author: Julia May Shaw
Publisher: Mrs. J.M. Shaw, 1967.
Edition/Format:  Book : English
 
OCLC WorldCat record
Sitting on a goldmine : making the most of city-owned real estate
Author: Alan R Gianini; Ann Ludwig
Edition/Format:  Article : English
Publication: Western city. Vol. 66, no. 5 (May 1990)
 
OCLC WorldCat record
Sitting on a goldmine : a story of redemption and success
Author: Joe Race
Publisher: Victoria, BC : Trafford, ©2008.
Edition/Format:  Book : Fiction : English