U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2006–3072

2006

Copper-Silver Deposits of the Revett Formation, 

Montana and Idaho—Origin and Resource Potential

The Revett Formation of northern 

Idaho and western Montana contains 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 mineralresource assessment of Revett-type 

copper-silver deposits. The USGS 

assessment indicates that a large area 

of USFS-administered land in northwestern Montana and northern Idaho 

may contain signifi cant undiscovered 

Revett-type copper-silver deposits. 

The western Montana copper belt, 

which extends from northwestern Montana into northern Idaho, contains several 

large stratabound (confi ned to a single 

stratigraphic unit) copper-silver deposits 

in fi ne- to medium-grained quartzites 

(metamorphosed quartz sandstone) of the 

Revett Formation. A recent U.S. Geological Survey (USGS) assessment indicates 

that a large area of U.S. Forest Service 

(USFS)-administered land in northwestern 

Montana and northern Idaho may contain 

signifi cant undiscovered Revett-type copper-silver deposits. 

The Rock Creek-Montanore and Troy 

deposits, which are currently the most 

signifi cant undeveloped resources identifi ed in the western Montana copper belt, 

are among the Nation’s largest stratabound 

copper-silver deposits, containing about 

15 percent of the copper in such deposits 

in North America. Worldwide, stratabound 

copper-silver deposits contain 23 percent 

of all copper resources and are the secondmost important global source of the metal 

after porphyry copper deposits.

Production from the Revett Formation at the Troy Mine in Montana is 390 

million pounds of copper and 48 million 

troy ounces of silver, and reserves are estimated to be 156 million pounds of copper 

and more than 18 million troy ounces of 

silver. Undeveloped resources at nearby 

Rock Creek-Montanore in Montana are 

estimated to be more than 5 billion pounds 

of copper and 679 million troy ounces of 

silver. Numerous other stratabound copper-silver prospects and occurrences in 

western Montana and northern Idaho were 

located as part of the USGS study, although few have had recorded production.

Origin of the Copper-Silver Deposits

The Revett Formation is part of the 

1,401- to 1,470-million-year-old Belt 

Supergroup and was originally deposited 

during the Mesoproterozoic Era (1.6 to 1.0 

billion years ago). The formation consists 

of quartzite, siltite, and argillite (very fi ne 

grained to medium-grained, quartz-rich 

metamorphosed sedimentary rocks). The 

thick, clean quartzites of the Revett Formation, which are among the coarsest-grained 

sedimentary rocks in the lower part of the 

Belt Supergroup, acted as aquifers before 

consolidation of the sediment into rock. 

Metals were deposited where oxidized 

metal-bearing brines upwelled along faults, 

migrated laterally through the aquifers, 

and intersected preexisting pyrite-hematite 

interfaces in the sediment, where the metals 

were reduced and deposited. 

The deposits are characterized by mineral zones (characteristic ore and alteration 

U.S. Geological Survey scientists recently completed an assessment of copper-silver deposits in the Revett Formation in northwestern Montana and northern Idaho. This photograph shows an outcrop of Revett Formation 

quartzite beds in the Coeur d’Alene Mining District of northern Idaho. (USGS photograph by Jeffrey Mauk.)

Rippled surface of a block of white quartzite from the 

Revett Formation. The thickest, cleanest quartzite beds 

in the formation were aquifers through which metalbearing brines moved and became hosts for coppersilver deposits. (USGS photograph by Tim Hayes.)
EXPLANATION

Extent of copper-silver 

mineralization

Outcrop of the Revett 

Formation

Land managed by the 

U.S. Forest Service

Fault

Boundary of study area

Major deposit

Printed on recycled paper

minerals) that provide a useful guide for 

exploration for mineral deposits. Regional 

changes in the mineral zones, the thickness and grain size of sedimentary beds, 

and the overall geologic structure provide 

evidence for the direction of fl uid movement in the sediment during deposition of 

the Revett Formation and during formation 

of the copper-silver deposits and provide 

a framework to evaluate the potential for 

undiscovered deposits.

Results of the USGS Assessment

USGS scientists compiled and analyzed 

existing data on the distribution, stratigraphy, grain size, and primary and alteration 

mineralogy of the Revett Formation in 

western Montana and northern Idaho. The 

data—from 126 measured sections and 

diamond-drill holes, totaling more than 

150,000 feet of true thickness—were made 

available by Asarco, Inc., and Kennecott 

Exploration, Inc., after abandoning exploration programs for Revett copper-silver 

deposits. The USGS assessment was then 

For more information, contact:

Thomas P. Frost

U.S. Geological Survey

904 W. Riverside Ave., Room 202

Spokane, WA 99201

(509) 368-3103

http://minerals.usgs.gov/west/

For the complete assessment see:

 Boleneus, D.E., Appelgate, L.M., Stewart, J.H., 

and Zientek, M.L., 2005, Stratabound copper-silver 

deposits of the Mesoproterozoic Revett Formation in 

Montana and Idaho, US Geological Survey Scientifi c 

Investigations Report SIR 2005-5231, 66 p., 3 map pls. 

[http://pubs.usgs.gov/sir/2005/5231/].

This Fact Sheet and any updates to it are available 

online at http://pubs.usgs.gov/fs/2006/3072/

Thomas P. Frost and Michael L. Zientek

Edited by James W. Hendley II and George Havach

Graphic design by Susan Mayfi eld

COOPERATING ORGANIZATION

U.S. Forest Service

completed using these data plus descriptions of 57 Revett-type deposits and occurrences and geologic maps of the Revett 

Formation to determine areas permissive 

for the occurrence of other stratabound 

copper-silver deposits. This information 

is published as USGS Scientifi c Investigations Report 2005-5231 (http://pubs.

usgs.gov/sir/2005/5231/) and is also 

documented in ArcInfo® interchange fi les 

and spreadsheets to facilitate use in a geographic information system (GIS).

USGS scientists used the spatial distribution of copper-silver mineralization in 

the Revett Formation, as determined from 

an examination of measured sections and 

drill cores and an interpretation of the fl ow 

paths of brines in the sedimentary deposits, to conclude that approximately 556 

square miles of mostly USFS-administered 

lands is favorable for undiscovered copper-silver deposits. These interpretations 

in the report will serve as a useful guide 

for future mineral-resource exploration 

and assist the USFS in land-use decisions 

when mineral-resource development must 

be weighed against other land uses, such 

as recreation and habitat preservation. 

The new U.S. Geological Survey 

assessment of copper-silver deposits 

of the Revett Formation in northwestern 

Montana and northern Idaho will assist 

the U.S. Forest Service in making land-use 

decisions and serve as a useful guide for 

future mineral-resource exploration. This 

map of the area studied shows the outcrop 

of the Revett Formation and areas of known 

copper-silver mineralization. 

0 4 8 KILOMETERS

0 2 4 6 8 MILES

COEUR D’ALENE TROUGH

116°

48°

Troy

Rock Creek

Montanore

OSBURN FAULT

THOMPSON PASS FAULT

HOPE FAULT

Clark

Fork

Thompson

Falls

Lookout

Pass

Wallace

Kellogg

Hope

Pend Orielle Lake

Map area MONTANA

IDAHO

Extent of Belt Supergroup rocks

(Belt Basin)