gems, corundum, USA.12, Montana.12, Camp Creek, Beaverhead County

The existence of the mineral corundum as the variety sapphire in the state of Montana has been known since the early 1870’s, and for a long time Montana ranked among the leading producing areas of gem-quality corundum. Sapphires first were noticed in gold placer operations, and probably many stones had been discarded before their value was discovered. The first published description of the occurrence of corundum in the state was by J. L. Smith (18) in 1873, who stated: “These pebbles are found on the Missouri River near its source, about 61 miles above (Fort) Benton; they are obtained from bars on the river, of which there are some four or five within a few miles of each other. Considerable gold is found on these bars, it having been brought down the river and lodged there; and the bars are now being worked for gold. The corundum is scattered through the gravel (which is about five feet deep) upon the rock bed. Occasionally it is found in the gravel and upon the rock bed in the gulches from forty to fifty feet below the surface, but it is very rare in such localities.” The purpose of this report is to describe the geology of a newly discovered corundum deposit that was found by the writer during the course of field work for the Montana Bureau of Mines and Geology in the summer of 1949 in the southern part of the Ruby Mountains southeast of Dillon, Beaverhead County, Montana. At the same time it seems desirable to present a brief summary of the geological characteristics of the other types of corundum deposits in Montana in order to be able to compare the new deposit with those previously recorded, descriptions of which are scattered widely throughout the geological literature.