Sapphire Deposits of the Missouri River Near Helena, Montana
Route
Start point
Helena, MT
End point
Castle Mine, MT
Roads
I-12, Montana 284, Jim Town Road, York Road, Nelson Road
Total distance
81.6 miles
Geology
Summary
Sapphires occur with gold, both in terrace deposits along the Missouri River and in the active river channel. Both of these minerals have been recovered from these deposits beginning with recovery of gold in the latter part of the 1800's. More recently, up until the early 1940's, sapphires were also recovered from these operations for industrial uses. In recent years recovery of sapphires by recreationists for faceting into gem stones has become important. Five deposits open for fee digging will be visited during this field excursion. They are situated on the French, Eldorado, Spokane and Gruell’s bars. Also, exposures of a sapphire-bearing andesite sill close to the Missouri River and considered a source for some of the sapphires found in the gravels will be examined.
Key Lithologic Features
- Spokane Formation argillite
- Grayson Formation
Structures
Landforms
Other Features
- Korizek mine
- Lovestone mine
- Guffey mine
- Castle mine
Reference
Zeihen, L.G., and Jones, K., 1987, Road Log No. 2, Sapphire Deposits of the Missouri River Near Helena, Montana,
in Berg, R.B., and Breuniger, R., eds.,
Guidebook of the Helena Area, West-Central Montana: Guidebook for the 12th Annual Field Conference , Montana Bureau of Mines and Geology Special Publication 95, p. 59-60.
Availability
Order from the Tobacco Root Geological Society c/o the Montana Bureau of Mines and Geology. Go to the
TRGS publications page for ordering information.
