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Bedded deposits of iron-formation are minor components of the thoroughly metamorphosed and deformed Precambrian rocks that make up the core of the Tobacco Root Mountains. The rocks are Archean in age; they predate a major Precambrian orogeny that affected all of southwestern Montana about 2,750 m.y. ago. The principal bed of iron-formation occurs within a metasedimentary sequence that has dolomite marble at the base and rests on quartzofeldspathic gneiss of uncertain origin. The stratigraphic thickness of the preserved part of the metasedimentary group cannot readily be established because of structural complexities, including both thickening and attenuation, but it probably does not exceed 300 m. The true (original) thickness of the iron-formation is even more difficult to determine because of the structural incompetence of the rock, but it ranges from 15 to 30 m. All the rocks, with the exception of a few younger Precambrian (Proterozoic Y) diabase dikes, are metamorphosed to amphibolite or hornblende granulite facies. The iron-formation typically consists of quartz and magnetite, with subordinate amounts of iron silicates, mainly hypersthene, garnet, clinopyroxene, and grunerite.
The principal deposits of iron-formation are in the Copper Mountain area, an area of about 13 km 2 in the west-central part of the Tobacco Root range that has been mapped in some detail. The structure consists of an early set of tight isoclinal folds, trending north-south and overturned to the east, that are deformed by later crossfolds that trend and plunge northwest. The most prominent belt of iron-formation is on a tight anticlinal buckle within the northsouth-trending Ramshorn syncline, a major structure of the first fold set. This belt of iron-formation is estimated to contain about 63 million t of potential low-grade ore (taconite) to a depth of 100 m. The rock contains about 35 weight percent Fe, mostly in the form of magnetite.
Iron-formation occurs in many other localities in the region as distinctive but thin and discontinuous units. Some deposits probably are stratigraphically equivalent to the iron-formation of the Copper Mountain area, but others have quite different lithologic associations and probably different ages.
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Bedded Precambrian iron deposits of the Tobacco Root Mountains, southwestern Montana