Copper-Silver Deposits of the Revett Formation, Montana and Idaho—Origin and Resource Potential

he Revett Formation of northern TIdaho and western Montana con- tains major stratabound copper-silver deposits near Troy, Rock Creek, and Rock Lake, Montana. To help the U.S. Forest Service (USFS) meet its goal of integrating geoscience information into the land-planning process, U.S. Geological Survey (USGS) scientists recently completed a compilation of regional stratigraphy and mineralogy of the Revett Formation and a mineral- resource assessment of Revett-type copper-silver deposits. The USGS assessment indicates that a large area of USFS-administered land in north- western Montana and northern Idaho may contain signifi cant undiscovered Revett-type copper-silver deposits.

U.S. Geological Survey scientists recently completed an assessment of copper-silver deposits in the Revett For- mation in northwestern Montana and northern Idaho. This photograph shows an outcrop of Revett Formation The western Montana copper belt, quartzite beds in the Coeur d’Alene Mining District of northern Idaho. (USGS photograph by Jeffrey Mauk.) which extends from northwestern Mon- tana into northern Idaho, contains several The Rock Creek-Montanore and Troy located as part of the USGS study, al- large stratabound (confi ned to a single deposits, which are currently the most though few have had recorded production. stratigraphic unit) copper-silver deposits signifi cant undeveloped resources identi- in fi ne- to medium-grained quartzites fi ed in the western Montana copper belt, Origin of the Copper-Silver Deposits (metamorphosed quartz sandstone) of the are among the Nation’s largest stratabound The Revett Formation is part of the Revett Formation. A recent U.S. Geologi- copper-silver deposits, containing about 1,401- to 1,470-million-year-old Belt cal Survey (USGS) assessment indicates 15 percent of the copper in such deposits Supergroup and was originally deposited that a large area of U.S. Forest Service in North America. Worldwide, stratabound during the Mesoproterozoic Era (1.6 to 1.0 (USFS)-administered land in northwestern copper-silver deposits contain 23 percent billion years ago). The formation consists Montana and northern Idaho may contain of all copper resources and are the second- of quartzite, siltite, and argillite (very fi ne signifi cant undiscovered Revett-type cop- most important global source of the metal grained to medium-grained, quartz-rich per-silver deposits. after porphyry copper deposits. metamorphosed sedimentary rocks). The Production from the Revett Forma- thick, clean quartzites of the Revett Forma- tion at the Troy Mine in Montana is 390 tion, which are among the coarsest-grained million pounds of copper and 48 million sedimentary rocks in the lower part of the troy ounces of silver, and reserves are esti- Belt Supergroup, acted as aquifers before mated to be 156 million pounds of copper consolidation of the sediment into rock. and more than 18 million troy ounces of Metals were deposited where oxidized silver. Undeveloped resources at nearby metal-bearing brines upwelled along faults, Rock Creek-Montanore in Montana are migrated laterally through the aquifers, estimated to be more than 5 billion pounds and intersected preexisting pyrite-hematite Rippled surface of a block of white quartzite from the of copper and 679 million troy ounces of interfaces in the sediment, where the metals Revett Formation. The thickest, cleanest quartzite beds silver. Numerous other stratabound cop- were reduced and deposited. in the formation were aquifers through which metal- bearing brines moved and became hosts for copper- per-silver prospects and occurrences in The deposits are characterized by min- silver deposits. (USGS photograph by Tim Hayes.) western Montana and northern Idaho were eral zones (characteristic ore and alteration

U.S. Department of the Interior USGS Fact Sheet 2006–3072 U.S. Geological Survey 2006 116° Extent of Belt Supergroup rocks (Belt Basin)

Map area MONTANA Hope Troy HOPE FAUL

e k a L T e ll e ri Clark O nd Fork Pe Rock Creek IDAHO Montanore

48°

EXPLANATION Extent of copper-silver mineralization Outcrop of the Revett Formation 0 2 4 6 8 MILES Land managed by the U.S. Forest Service 0 4 8 KILOMETERS Fault Boundary of study area Major deposit

Thompson Falls The new U.S. Geological Survey THOMPSON PASS FAULT OSBURN F assessment of copper-silver deposits AULT of the Revett Formation in northwestern Kellogg Montana and northern Idaho will assist the U.S. Forest Service in making land-use COEUR D’ALENE TR Wallace decisions and serve as a useful guide for OUGH future mineral-resource exploration. This Lookout map of the area studied shows the outcrop Pass of the Revett Formation and areas of known copper-silver mineralization. minerals) that provide a useful guide for completed using these data plus descrip- when mineral-resource development must exploration for mineral deposits. Regional tions of 57 Revett-type deposits and oc- be weighed against other land uses, such changes in the mineral zones, the thick- currences and geologic maps of the Revett as recreation and habitat preservation. ness and grain size of sedimentary beds, Formation to determine areas permissive and the overall geologic structure provide for the occurrence of other stratabound Thomas P. Frost and Michael L. Zientek copper-silver deposits. This information evidence for the direction of fl uid move- Edited by James W. Hendley II and George Havach ment in the sediment during deposition of is published as USGS Scientifi c Investi- Graphic design by Susan Mayfi eld the Revett Formation and during formation gations Report 2005-5231 (http://pubs. of the copper-silver deposits and provide usgs.gov/sir/2005/5231/) and is also COOPERATING ORGANIZATION U.S. Forest Service a framework to evaluate the potential for documented in ArcInfo® interchange fi les undiscovered deposits. and spreadsheets to facilitate use in a geo- graphic information system (GIS). For more information, contact: USGS scientists used the spatial distri- Thomas P. Frost Results of the USGS Assessment U.S. Geological Survey USGS scientists compiled and analyzed bution of copper-silver mineralization in 904 W. Riverside Ave., Room 202 existing data on the distribution, stratigra- the Revett Formation, as determined from Spokane, WA 99201 an examination of measured sections and (509) 368-3103 phy, grain size, and primary and alteration http://minerals.usgs.gov/west/ mineralogy of the Revett Formation in drill cores and an interpretation of the fl ow paths of brines in the sedimentary depos- For the complete assessment see: western Montana and northern Idaho. The Boleneus, D.E., Appelgate, L.M., Stewart, J.H., data—from 126 measured sections and its, to conclude that approximately 556 and Zientek, M.L., 2005, Stratabound copper-silver diamond-drill holes, totaling more than square miles of mostly USFS-administered deposits of the Mesoproterozoic Revett Formation in lands is favorable for undiscovered cop- Montana and Idaho, US Geological Survey Scientifi c 150,000 feet of true thickness—were made Investigations Report SIR 2005-5231, 66 p., 3 map pls. available by Asarco, Inc., and Kennecott per-silver deposits. These interpretations [http://pubs.usgs.gov/sir/2005/5231/]. in the report will serve as a useful guide Exploration, Inc., after abandoning explo- This Fact Sheet and any updates to it are available ration programs for Revett copper-silver for future mineral-resource exploration online at http://pubs.usgs.gov/fs/2006/3072/ deposits. The USGS assessment was then and assist the USFS in land-use decisions

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