The first gold discovered in Montana was at Gold Creek near present-day Garrison in 1852. A series of major mining discoveries in the western third of the state starting in 1862 found gold, silver, copper, lead, and coal (and later oil) which attracted tens of thousands of miners to the area. The richest of all gold placer diggings was discovered at Alder Gulch, where the town of Virginia City was established. Other rich placer deposits were found at Last Chance Gulch, where the city of Helena now stands, Confederate Gulch, Silver Bow, Emigrant Gulch, and Cooke City. Gold output from 1862 through 1876 reached $144 million; silver then became even more important. The largest mining operations were in the city of Butte, which had important silver deposits and gigantic copper deposits.

Montana defines the boundary as extending from the modern intersection of Montana, Idaho, and Wyoming at:

The forty-fourth degree and thirty minutes of north latitude; thence due west along said forty-fourth degree and thirty minutes of north latitude to a point formed by its intersection with the crest of the Rocky Mountains; thence following the crest of the Rocky Mountains northward till its intersection with the Bitter Root Mountains; thence northward along the crest of the Bitter Root Mountains to its intersection with the thirty-ninth degree of longitude west from Washington; thence along said thirty-ninth degree of longitude northward to the boundary line of British possessions.

The Territory of Montana was an organized incorporated territory of the United States that existed from May 26, 1864, until November 8, 1889, when it was admitted as the 41st state in the Union as the state of Montana.

For fun, see  Montana Territory (film).

 Reprinted from Wikipedia

 Timeline of Pre-Statehood – Montana History

https://en.wikipedia.org/wiki/Timeline_of_pre-statehood_Montana_history