National mineral assessment tract NR37 (Sediment-hosted Cu, Revett)

Tract NR37
Geographic region Northern Rocky Mountains
Tract area 81,900sq km
Deposit type Sediment-hosted Cu, Revett
Deposit age Proterozoic


Confidence Number of
90% 9
50% 10
10% 15
5% 20
1% 30

Estimators: DCox, Whipple, Spanski, Zientek


Explained by Michael L. Zientek, Stephen E. Box, and Arthur A. Bookstrom
On the choice of deposit models
These syngenetic to diagenetic deposits are hosted in sedimentary rocks in epicratonic and intracratonic basins. The formation of sediment-hosted copper deposits depends on a redox reaction between an oxidized brine containing dissolved copper and a reductant. The brine, in equilibrium with hematite and free of sulfide, maintains the copper in solution as a stable complex ion. The source of the brines may be trapped seawater or fluids derived from evaporite basins. The copper in the brines may be derived from volcanic rock clasts and mafic clastic minerals in red beds, hydrous ferrous oxide cements in red beds, or subareal mafic volcanic rocks. Reductants include a wide variety of organic and inorganic material (see Kirkham, 1989).
Three subtypes of sediment-hosted copper deposits are distinguished bases on geologic setting, tonnage, and grade. Deposits of the reduced-facies model are found where oxidized continental clastic rocks (red beds) or basaltic to intermediate subaerial volcanic rocks are overlain by laterally extensive reduced marine or lacustrine shales or carbonate rocks. Deposits of the red-bed model are similar those of the reduced-facies model but lack laterally extensive reduced facies strata. Deposits of the Revett model include deposits in the Revett Formation in the Belt Supergroup. Cu-bearing oxidized brines migrated into reduced, permeable quartzites to form roll-front-like, mineralogically-zoned deposits at the redox boundary.
The Revett grade and tonnage model (Spanski, 1992) is based on deposits from the Belt Supergroup, and is clearly the model of choice.
On the delineation of permissive tracts
Permissive lithologic units in this area include the entire Middle Proterozoic Belt Supergroup above the Prichard Formation and the Yellowjacket Formation and Lemhi Group in Idaho. The Uinta Mountains Supergroup may contain similar rocks, but that area was not included in the tract because we lacked information. Although sandstone bodies that host known ore deposits are limited to those areas underlain by the Revett Formation in Idaho and western Montana (Spanski, 1992), reduced sandstone bodies large enough to host deposits comparable in size to Revett orebodies are present throughout the permissive area. For example, arenite-hosted silver-bearing copper deposits occur in Spokane Formation in the eastern part of the Belt basin (Lange and others, 1989) and sediment-hosted Cu-Ag mineralization has been reported to occur in the Yellowjacket Formation and Lemhi Group in Idaho (C.R. Allen, personal commun., 1994).
Important examples of this type of deposit
Numerous deposits and occurrences, including those that comprise the Revett grade and tonnage model are known from the Libby trough in western Montana and Idaho (Lange and Sherry, 1986; Harrison and others, 1986; Spanski, 1992). Examples include the Spar Lake deposit (Hayes and Einaudi, 1986) and the Montanore deposit (Adkins, 1993). Green-bed Cu-Ag deposits in the Spokane Formation in the Choteau 1°x2° quadrangle, Montana may have potential for Revett-type mineralization. Green bed-type mineralization is commonly found in the upper part of the Spokane Formation in a stratigraphic interval that represents a upwards transition from a predominately oxidized red-bed sequence to the reduced siliclastic-carbonate rocks of the Empire Formation. Earhart and others (1981) describe mineralized zones that range up to 3 m in thickness and may extend for 49 km on strike. Earhart and others (1981) estimate that one zone in the upper part of the Spokane Formation may contain about 35 million metric tons of submarginal resources that average 0.1 percent Cu and 7.1 g/metric ton Ag. This interval of mineralized Spokane Formation is also described by Lange and others (1989).
Although copper grades are lower than most Revett-type deposits, tonnages are comparable (Spanski, 1992). In Idaho, sediment-hosted Cu-Ag occurrences reported in a diamictite in the Lemhi Group in the Lemhi Range and at the Freeman Creek area in the Yellowjacket Formation in the Bitterroot Range are large enough to be considered as exploration targets (C.R.. Allen, personal commun., 1994). We do not have enough information on these deposits in Idaho to differentiate them into the Revett or red-bed models.
On the numerical estimates made
The estimate for Revett-type deposits was based on earlier quantitative estimates for the most prospective area in western Montana (Spanski, 1992) and from consultation with industry geologists with extensive knowledge of the geology and exploration activity of this region. In the Kootenai National Forest, which covers most, but not all, of the area favorable for Revett deposits, Spanski (1992) describes 26 occurrences of Revett mineralization. Of these, 13 are sufficiently explored to assure their status as deposits. Four of the 13 have reported grades and tonnages, leaving 9 as only partially determined. The estimation team used this number of partially explored deposits for their estimate for the 90th percentile. Spanski (1992) estimated the following distribution for undiscovered Revett-type deposits for one tract in the Kootenai Forest: 25 deposits at the 90th percentile, 50 at the 50th percentile and 85 at the 10th percentile. The team consulted with members of the estimation team for the Kootenai Forest and with exploration geologists in private industry. Based on these discussion, the team made a lower estimate for undiscovered deposits in the Belt basin than was earlier reported for the Kootenai Forest. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 9, 10, 15, 20, and 30 or more Revett-type deposits consistent with the grade and tonnage model of Spanski (1992).
Adkins, A.R., 1993, Geology of the Montanore stratabound Cu-Ag deposit, Lincoln and Sanders Counties, Montana: Belt Symposium III, Program and Abstracts, Belt Association, Inc., unpaginated.
Harrison, J.E., 1972, Precambrian Belt basin of northwestern United States—Its geometry, sedimentation, and copper occurrences: Geological Society of America Bulletin, v. 83, p. 1215-1240.
Harrison, J.E., Domenico, J.A., and Leach, D.L., 1986, Resource appraisal map for stratabound copper-silver deposits, in the Wallace 1°x2° quadrangle, Montana and Idaho: U.S. Geological Survey Miscellaneous Investigation Series Map I-1509-F.
Hayes, T.S., and Einaudi, M.T., 1986, Genesis of the Spar Lake strata-bound copper-silver deposit, Montana—Part I. Controls inherited from sedimentation and preore diagenesis: Economic Geology, v. 81, no. 8, p. 1899–1931.
Kirkham, R.V., 1989, Distribution, settings, and genesis of sediment-hosted stratiform copper deposits, in Boyle, R.W., Brown, A.C., Jefferson, C.W., Jowett, E.C., and Kirkham, R.V., eds., Sediment-hosted stratiform copper deposits: Geological Association of Canada Special Paper 36, p. 3-38.
Kirkham, R.V., 1989, Distribution, settings, and genesis of sediment-hosted stratiform copper deposits, in Boyle, R.W., Brown, A.C., Jefferson, C.W., Jowett, E.C., and Kirkham, R.V., eds., Sediment-hosted stratiform copper deposits: Geological Association of Canada Special Paper 36, p. 3-38.
Lange, I.M., Herberger, D., Whipple, J.W., and Krouse, H.R., 1989, Stratabound Cu-Ag and Pb-Zn mineralization in the Spokane and Helena Formations in the eastern part of the Belt Basin, Montana—A common origin?, in Boyle, R.W., Brown, A.C., Jefferson, C.W., Jowett, E.C., and Kirkham, R.V., eds., Sediment-hosted stratiform copper deposits: Geological Association of Canada Special Paper 36, p. 287-304.
Lange, I.M. and Sherry, R.A., 1986, Monmassive sulfide deposits in the Late Precambrian Belt Supergroup of western Montana, in Roberts, S.M., ed., Belt Supergroup—A guide to Proterozoic rocks of western Montana and adjacent areas: Montana Bureau of Mines and Geology Special Publication 94, p. 269-278.
Spanski, G.T., 1992, Quantitative assessment of future development of copper/silver resources in the Kootenai National Forest, Idaho/Montana: Part 1—Estimation of the copper and silver endowments: Nonrenewable Resources, v. 1, no. 2, p. 163-183.

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