SAPPHIRE DEPOSITS ALONG THE MISSOURI RIVER NEAR HELENA, MONTANA

Pleistocene strath terrace gravels currently being mined along the Missouri River near Helena, Montana yield gold and sapphires. Terraces were cut into metasedimentary rocks of the Belt Supergroup and range from 30 to 90 m above the level of Hauser Lake which now floods this section of the Missouri River Valley. Poorly sorted terrace gravel consists of cobbles and rare boulders in a matrix of finer material. Pebbles and cobbles of quartzofeldspathic gneiss found sparsely in the gravel indicate transport from sources to the south of the headwaters of the Missouri River. Sapphires are mined from the lower 1-2 m of the gravel deposits that may be as thick as approximately 5 m. The only known bedrock occurrence of sapphires in this area is a Tertiary trachybasalt sill less than 3 m thick exposed along the west side of Hauser Lake that contains rare corundum (variety sapphire) xenocrysts, obviously not a major source of the sapphires in these deposits. The occurrence of sapphires in alluvium in three gulches that enter Canyon Ferry Lake above Canyon Ferry Dam indicate unidentified bedrock sources in the Big Belt Mountains east of Canyon Ferry Lake. Sapphires are also found in a tributary to one of these gulches that is 250 m above Canyon Ferry Lake. Another possibility is that some of the Missouri River sapphires were derived from sources a long distance upriver from the present deposits. Sapphires have been recovered from the alluvium along Alder Gulch in the Jefferson River tributary system above the headwaters of the Missouri River, 200 km upriver from the Missouri River sapphire deposits at Hauser Lake.