Missouri River placers, Helena Mining District (Spring Hill Mining District), Lewis and Clark County, Montana, USA
In the 1880s, small crystals of sapphire, partially suitable for jewelry purposes, were found in gold-bearing alluvial placer deposits of the Missouri River basin in Montana. Since that time, the sapphire has been extracted with gold as a by-product. Some placer deposits were mined solely for gem production. The primary sources of sapphire in placer deposits are assumed to be corundum-bearing lamprophyres, about 150 km to the east-northeast, or biotite trachytes from the Helena region.Ref: E.Ya. Kievlenko (2003) Geology of gems, p. 69