Chromite deposits of the eastern part of the Stillwater complex, Stillwater County, Montana

Abstract

Chromite has been known for some years to occur in a belt of ultramafic rocks, composed chiefly of pyroxene and olivine, 27 miles long on the northeastern margin of the Beartooth Mountains, in Stillwater and Sweetgrass Counties, Mont. The outcrop of these igneous rocks, which form the lower part of the layered Stillwater complex, ranges in width from a quarter of a mile to a mile. Chromite occurs near the center of it in layers and lenses, parallel to the layering of the complex, throughout most of the length of the belt. . In 1939 a Geological Survey -party made a detailed study of the chromite deposits on the so-called Benbow claims, of the Chromium Products Corporation, on Little Rocky Creek, and a brief reconnaissance of the deposits on the west side of Stillwater Canyon. Trenching on the Benbow claims by the Bureau of Mines made it possible to trace segments of a tabular body of chrome ore with an aggregate length of 5,660 feet in a distance of 9,000 feet along the strike. The discontinuity is due in part to offsets on small faults approximately at right angles to the strike and in part to the occurrence of chromite in overlapping lenses. The average width of the ore is 5 to 7 feet, but on the eastern end of the claims a strip of disseminated ore 430 feet in length averages 29& feet In width. As all the ore is approximately half chromite, it will need to be concentrated. Considering the ore as proved for 50 feet below the mapped outcrop, the authors estimate the presence of 200,000 short tons of ore. The possible tonnage of ore above the Eclipse adit the lowest opening on the property is calculated to be nearly 1,500,000 short tons of massive ore and 345,000 tons of disseminated ore. The chromite horizon at each end of the property is covered, but geologic evidence strongly implies that it continues. On the west side of Stillwater Canyon chromite-rich bands of varying thickness alternate with bands of rock containing disseminated chromite for a distance of about 1,200 feet along the strike. Available data are insufficient for an estimate of tonnage, but the area appears promising for development of a large tonnage of millable ore. Although no high-grade ore is to be expected, it is clear from this study of a small part of the Stillwater complex that a large tonnage of submarglnal ore can be recovered In time of heed. 371