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 labile 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 red-bed model are found where oxidized continental clastic rocks (red beds) serve as a reductant, but no laterally extensive reduced-facies strata are found. They are relatively small. Deposits of the Revett model include deposits in the Revett Formation in the Belt Supergroup. 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. They can be quite large.
The reduced-facies model was chosen for assessment because of the widespread distribution of red-bed-type prospects and occurrences; this indicates that copper was mobile in oxidized parts of the section. Any reduced-facies beds, such as black shale or stromatolitic carbonate, in contact with the oxidized beds, would be a likely site for the occurrence of reduced-facies deposits.
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. Areas favorable for deposits of the reduced facies model would include thick sequences of laterally extensive reduced shales or carbonate rocks. One such favorable interval is the base of the middle Belt carbonate rocks. The transition from the Ravalli Group to the middle Belt carbonate rocks represents a major marine transgression that places reduced lithologies over oxidized, red-bed sequences (J.W. Whipple, personal commun., 1994).
Important examples of this type of deposit
Carbonate-type copper mineralization occurs throughout the middle Belt carbonate rocks (Empire, Helena, and Wallace Formations) where local accumulations of organic material (such as algal mats) appear to control metal deposition (see Harrison, 1972). No examples of identified deposits are known, but occurrences at Red Mountain and Wolf Creek, Montana have been described by Lange and Sherry (1986). Additional deposits that occur in this stratigraphic interval near these prospects are described by Elliott and others (1992). Laterally extensive, low-grade mineralization in this same stratigraphic interval has been described in Alberta and British Columbia (Binda and others, 1989). These prospects and occurrences indicate that the processes that generate reduced-facies deposits did operate in the Belt basin.
On the numerical estimates made
Although small examples of this mineralization type occur in the tract, no significant deposits of this type are known. A long exploration history for copper deposits in the region leads us to give a low estimate of the number of undiscovered deposits of this type. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 0, 1, 3, and 5 or more reduced facies deposits consistent with the grade and tonnage model.
Binda, P.L., Koopman, H.T., and Koopman, E.R., 1989, A stratiform copper occurrence in the Helikian Siyeh Formation of Alberta and British Columbia, 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. 269-285.
Elliott, J.E., Loen, J.S., Wise, K.K., and Blaskowski, M.J., 1992, Maps showing locations of mines and prospects in the Butte 1°x2° quadrangle, western Montana: U.S. Geological Survey Miscellaneous Field Investigations Series Map I-2050-C.
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.
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. 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.