Bedded Precambrian Iron Deposits of the Tobacco Root Mountains, Southwestern Montana GEOLOGICAL SURVEY PROFESSIONAL PAPER 1187 Bedded Precambrian Iron Deposits of the Tobacco Root Mountains, Southwestern Montana By HAROLD L. JAMES GEOLOGICAL SURVEY PROFESSIONAL PAPER 1187 A geologic description and resource appraisal of the ironjormation found in several localities UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1981 UNITED STATES DEPARTMENT OF THE INTERIOR JAMES G. WATT, Secretary GEOLOGICAL SURVEY Doyle G. Frederick, Acting Director Library of Congress Cataloging in Publication Data James, Harold Lloyd, 1912- Bedded Precambrian iron deposits of the Tobacco Root Mountains, southwestern Montana. (Geological Survey professional paper ; 1187) Bibliography: p. 15-16 Supt. of Docs. no.: I 19.16:1187 1. Iron ores--Montana--Tobacco Root Mountains. 2. Geology, stratigraphic-­ Pre-Cambrian. I. Title. II. Series: United States. Geological Survey. Professional Paper 1187. TN403.M9J35 553.3'09786'663 81-607027 AACR2 For sale by the Superintendent of Documents, U.S. Go\'ernment Printing Office Washington, D.C. 20402 CONTENTS Page Page Plbstract -----------------------------------------------------1 Copper Mountain area-Continued Introduction __________________________________________________ ! Structure -----------------------------------------------11 General geology __________________ - _______________________ - ___ 2 Geologic history ----------------------------------------11 General features of iron-formation ----------------------------5 Exploration --------------------------------------------12 Description ________________ - ---____ - _______________ ---___ 5 Outlying areas of iron-formation -----------------------------12 Mineralogy and chemistry --------------------------------6 Carmichael Canyon (1) __________________________________ 12 Origin --------------------------------------------------7 Carmichael Creek (2) ____________________________________ 13 Copper Mountain area ----------------------------------------7 Boulder Lakes-Brannon Lakes (3) ________________________ 13 Quartzofeldspathic gneiss ---------------------------------7 Dry Georgia Gulch (4) -----------------------------------14 Metasedimentary sequence --------------------------------8 Currant Creek (5) _______________________________________ 14 Dolomite marble _____ -------------------_____________ 8 Other reported localities ---------------------------------14 Gneiss, quartzite, and schist --------------------------8 Indian Creek (6a) ____________________________________ l4 Iron-formation ---------------------------------------9 Johnson Creek (6b) ----------------------------------14 Quartzite and schist ----------------------------------9 South Fork of Mill Creek (6c) ________________________ 15 Plmphibolite --------------------------------------------10 Plntelope Creek (6d) _________________________________ 15 Ultramafic rocks _________________ ~ ______________________ 10 Resource appraisal ------------------------------------------15 Diabase ------------------------------------------------10 References cited ____________________________________________ 15 Quartz diorite and porphyry ------------------------------10 ILLUSTRATIONS Page PLATE 1. Geologic map of the Copper Mountain area, Madison County, Montana ____________________________________________ In pocket FIGURES 1-6. Maps of: 1. Part of southwestern Montana, showing distribution of older Precambrian rocks -------------------------------2 2. Tobacco Root Mountains, showing separate map areas and iron-formation localities ----------------------------3 3. Tobacco Root Mountains, showing outline of Ramshorn syncline and distribution of iron-formation _____________ _4 4. Copper Mountain area, showing principal structural elements ------------------------------------------------5 5. Carmichael Canyon area ----------------------------------------------------------------------------------13 6. Currant Creek area, showing distribution of Archean rocks --------------------------------------------------14 TABLES Page TABLE 1. Chemical analyses of iron-formation ____________________________________________________________________________________ 6 2. Minor-element content of iron-formation from the Tobacco Root Mountains ------------------------------------------------7 III BEDDED PRECAMBRIAN IRON DEPOSITS OF THE TOBACCO ROOT MOUNTAINS, SOUTHWESTERN MONTANA By HAROLD L. jAMES ABSTRACT the bounding valleys to nearly 11,000 ft in the highest Bedded deposits of iron-formation are minor components of the part of the range. Accessibility is fairly good; surfaced thoroughly metamorphosed and deformed Precambrian rocks that highways traverse the major valleys, and dirt roads make up the core of the Tobacco Root Mountains. The rocks are extend up most of the tributary streams. The rugged Archean in age; they predate a major Precambrian orogeny that uplands, however, can be reached only by foot, horse­ affected all of southwestern Montana about 2,750 m.y. ago. The prin­ back, or off-road vehicles. Most of the area is within or cipal bed of iron-formation occurs within a metasedimentary se­ quence that has dolomite marble at the base and rests on adjacent to Beaverhead National Forest. quartzofeldspathic gneiss of uncertain origin. The stratigraphic The present investigation, which has been carried on thickness of the preserved part of the metasedimentary group cannot intermittently since 1961, focuses on the distribution readily be established because of structural complexities, including and economic potential of banded iron-formation that both thickening and attenuation, but it probably does not exceed 300 m. The true (original) thickness of the iron-formation is even more occurs at several stratigraphic horizons within a com­ difficult to determine because of the structural incompetence of the plex sequence of metamorphic rocks. The principal rock, but it ranges from 15 to 30m. All the rocks, with the exception area of interest (fig. 2) is a belt about 11 km long on the of a few younger Precambrian (Proterozoic Y) diabase dikes, are west flank of the range, east of the town of Sheridan, metamorphosed to amphibolite or hornblende granulite facies. The Mont. (fig. 2). The southern part of this belt, centering iron-formation typically consists of quartz and magnetite, with sub­ on Copper Mountain in sec. 1, T. 5 S., R. 4 W., was ordinate amounts of iron silicates, mainly hypersthene, garnet, clinopyroxene, and grunerite. mapped by planetable (James and Wier, 1962); other The principal deposits of iron-formation are in the Copper areas were mapped or otherwise located on enlarge­ Mountain area, an area of about 13 km2 in the west-central part of ments of the Copper Mountain 1:24,000-scale topo­ the Tobacco Root range that has been mapped in some detail. The graphic base map. structure consists of an early set of tight isoclinal folds, trending north-south and overturned to the east, that are deformed by later The geology of the region· has been described in a crossfolds that trend and plunge northwest. The most prominent belt number of reports, notably those by Peale (1896), of iron-formation is on a tight anticlinal buckle within the north­ Tansley, Schafer, and Hart (1933), Reid (1957, 1963), south-trending Ramshorn syncline, a major structure of the first fold and Burger (1967, 1969). The occurrences of iron­ set. This belt of iron-formation is estimated to contain about 63 mil­ formation were described in summary fashion in a lion t of potential low-grade ore (taconite) to a depth of 100m. The rock contains about 35 weight percent Fe, mostly in the form of general study of Precambrian iron-formations of the magnetite. United States (Bayley and James, 1973), and a com­ Iron-formation occurs in many other localities in the region as prehensive report on the mineralogy of these rocks was distinctive but thin and discontinuous units. Some deposits probably presented by lmmega and Klein (1976). Data are stratigraphically equivalent to the iron-formation of the Copper significant to the present report have been obtained Mountain area, but others have quite different lithologic associ­ ations and probably different ages. from unpublished thesis studies, particularly those by Root (1965), Hess (1967), Gillmeister (1971), and Cor­ INTRODUCTION dua (1973). C. J. Vitaliano, under whose direction many of these thesis studies were made, and his col­ The Tobacco Root Mountains constitute one of sev­ leagues have prepared and recently published a gen­ eral block uplifts in southwestern Montana that have eralized map incorporating the available data, both been eroded to expose a core of older Precambrian rocks published and previously unpublished (Vitaliano and (fig. 1). The range occupies an area about 65 km long by Cordua, 1979; see also Vitaliano and others, 1979). 50 km wide, bounded on the north and west by the Acknowledgments.-! have received assistance and Jefferson and Ruby Rivers and on the east by the information from many sources. Particular thanks are Madison River. Elevations range from about 4,500 ft in due to: the Northern Pacific Railway Co., for making 1 2 BEDDED PRECAMBRIAN IRON DEPOSITS, SOUTHWESTERN MONTANA available the results of a regional survey by geologists brian sedimentary strata occur locally at the extreme E. E. Thurlow, L. C. Binon, D. W. Lindgren, and R. K. north margin of the range (McMannis, 1963). These Hogberg; the staff, visiting scientists, and students of strata presumably represent