Preliminary report on the Comet area, Jefferson County, Montana

Trace Elements Investigations 282
This report concerns work done on behalf of the Division of Raw Materials of the U. S. Atomic Energy Commission



Several radioactivity anomalies and a few specimens of sooty pitchblende and other uranium minerals have been found on the mine dumps of formerly productive base-and precious-metal mines along the Comet-Gray Eagle shear zone in the Comet area in southwestern Montana. The shear zone is from 50 to 200 feet wide and has been traced for at least 5 1/2 miles. It trends N. 80° W. across the northern part of the area and cuts the quartz monzonitic rocks of the Boulder batholith and younger silicic intrusive rocks, as well as the pre-batholitic volcanic rocks, and is in turn cut by dacite and andesite dikes.

The youngest period of mineralization is represented by chalcedonic vein zones comprising one or more discontinuous stringers and veins of cryptocrystalline silica in silicified quartz monzonite and in alaskite that has not been appreciably silicified. In some places these zones contain no distinct chalcedonic veins, but are represented only by silicified quartz monzonite. These zones locally contain uranium in association with very small amounts of the following minerals: pyrite, galena, ruby silver, argentite, native silver, molybdenite, chalcopyrite, arsenopyrite, and barite. At the Free Enterprise mine, uranium has been produced from a narrow chalcedonic vein that contains disseminated secondary uranium minerals and local small pods of pitchblende and from disseminated secondary uranium minerals in the adjacent quartz monzonite.

Undiscovered commercial deposits of uranium ore may occur spatially associated with the base-and precious-metal deposits along the Comet-Gray Eagle shear zone, and chalcedonic vein zones similar to the Free Enterprise.

Study Area

Publication type Report
Publication Subtype USGS Numbered Series
Title Preliminary report on the Comet area, Jefferson County, Montana
Series title Trace Elements Investigations
Series number 282
DOI 10.3133/tei282
Edition Also issued as CIR 277
Year Published 1952
Language English
Publisher U.S. Geological Survey
Description Report: 20 p.; Plate 1: 21.96 x 21.55 inches; Plate 2: 17.03 x 22 inches
Country United States
State Montana
County Jefferson County
City Jefferson City
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