|Hold it right there, outlaw!|
This article is littered with spoilers, so I reckon you might ought'a mosey on down the road if you don't want to read any plot details.
The Bear Mountain Mine was started when Nate Harlow and his partner "Griff" discovered gold while prospecting on Bear Mountain. Due to being double-crossed by his partner, Nate and his family didn't have a chance to reap the bounty from the mine, however, which instead fueled the corrupt regime of Governor Griffon and his partner General Javier Diego.
Since the mine was under the sacred land of the Red Wolf Tribe, their enemies the Black Elk Tribe were provided guns and gold to keep the Red Wolves from interfering. Gold from the mine flows West to Brimstone through Sunset Canyon and South to Mexico on Diego's Railroad. The entrance to the mine is protected by Fort Diego.
The article title, "Bear Mountain Mine", is for convenience. It is not referred to by any particular name during the game, just as "the mine."
The mine is referenced from the beginning of the game, where Nate Harlow tells his family about the discovery of gold. The player first enters the mine in the mission "Jailbreak" and makes their way back out during the subsequent mission "The Mine".
The mine includes several jail cells for Diego's "guests" and any miners or soldiers who displease him. The central area of the mine includes a supply room and lounge area.
Sheriff Bartlett's Journal
The Jail is listed in Sheriff Bartlett's Journal on pages 320 - 321 under the Frontier Places section. The pages are unlocked by completing the chapter "Jailbreak". There is not a separate entry for the mine itself.