The Lone Eagle mine is the only producing uranium deposit among the "base metal" veins in the Boulder batholith, Montana. Steeply dipping fracture fillings in quartz monzonite contain sparsely distributed pitchblende, together with pyrite, sphalerite, galena, and minor chalcopyrite and argentite in a gangue of quartz, calcite and siderite. Deposition of coarsely crystallized quartz and the sulfides was followed by extensive brecciation prior to the introduction of microcrystalline quartz, pitchblende, and minor fine-grained sulfides. The latter assemblage strongly resembles the "siliceous reef" uranium deposits in the batholith. Of the three varieties of pitchblende recognized, only one appears to represent primary introduction of uranium. Detailed data were obtained on the pitchblende varieties and an unusual, fine-grained galena associated with pitchblende.Study of the wall rock provided detailed information on the nature and composition of the feldspars, the percentage mineral composition and texture of the rock, and its chemical composition. The rock may be classified petrographically as a peraluminous granodiorite (Shand) or an adamellite (Johannsen).The alteration halo, extending as much as 30 feet from the vein, has a mineral assemblage--sericite, kaolinite, montmorillonite, chlorite, quartz, calcite, siderite, and pyrite--similar to that associated with the "siliceous reefs" and to the Butte alteration described by Sales and Meyer. Of the plutonic minerals, only quartz, apatite, and zircon are found adjacent to the vein. Study of the alteration zoning was inhibited by the lack of adequate samples more than a few feet from the vein.