Explained by James E. Elliott
On the choice of deposit models
The descriptive porphyry copper model of Cox (1986) was used in the assessment of undiscovered Cretaceous deposits in Montana. Porphyry copper deposits consist of copper-bearing minerals in disseminated grains and in stockwork veinlets in hydrothermally altered porphyry and adjacent country rocks (Cox, 1986). These deposits generally occur in high-level intrusive rocks contemporaneous with abundant dikes, breccia pipes, and faulting and contemporaneous with or slightly younger than volcanism. The intrusive rocks are typically porphyritic rocks, commonly including epizonal or hypabyssal dacite, latite, quartz latite, and rhyolite porphyries but also including their equigranular plutonic equivalents such as quartz diorite, monzonite, quartz monzonite, and granite (Guilbert and Park, 1986).
Porphyry copper deposits commonly show concentric zoning both in the types and concentration of metals and in alteration facies. Alteration includes pyritic, argillic, phyllic, and potassic types. Porphyry deposits commonly show a spatial relationship to vein, replacement, and skarn deposits of base and precious metals.
On the delineation of permissive tracts
The criteria used to define the permissive tract in Montana are: (1) mapped areas of Cretaceous and Eocene volcanic, hypabyssal, and plutonic rocks as shown on the geologic map of Montana (Ross and others, 1955); (2) the predicted extent or presence of subsurface intrusive rocks based on gravity and magnetic data; (3) the presence of known mines, prospects, and occurrences of this deposit type.
The tract is located in western Montana, coincident with Cretaceous and Eocene volcanic and plutonic belts. Most of the known deposits and prospects are situated along a northeasterly-trending Idaho-Montana porphyry belt (Rostad, 1978), in southwestern Montana.
Important examples of this type of deposit
The only known deposit of Late Cretaceous age is the Continental deposit in the Butte district. This deposit and other prospects of this age are associated with plutonic and hypabyssal intrusive rocks of the Boulder, Pioneer, Idaho, and Tobacco Root batholiths and other intrusive bodies of southwestern Montana. Several prospects also exist, that have either low grade or are incompletely explored. They include Beaverton, Golconda, areas on the east flank of the Elkhorn Range, Rochester, Jackson Creek, West Fork of the Bitterroot, Cold Springs-Bannack, Argenta, Westside Tobacco Root, and Gold Hill.
The famous Butte district, in southwestern Montana, has been recognized as a porphyry copper district as well as one of the largest and most productive polymetallic vein districts in the world. The Continental porphyry copper-molybdenum deposit, presently being mined, is hosted by the Butte Quartz Monzonite (Boulder batholith) of Late Cretaceous age (Meyer and others, 1968; Ratcliff, 1973). Hypogene copper and molybdenum minerals occur as disseminations and in stockwork veinlets. Much of the ore-grade mineralized rock results from oxidation and supergene enrichment of hypogene minerals.
The Heddleston deposit, of Eocene age, is located in the west-central part of Montana near Lincoln. The deposit is associated with porphyritic intrusive rocks that invaded metasedimentary rocks of the Middle Proterozoic Belt Supergroup. These intrusions may be related to the nearby Lincoln andesitic and rhyolitic volcanic rocks. The Heddleston deposit consists of disseminated chalcopyrite and molybdenite and quartz-molybdenite veinlets in
Eocene quartz monzonite, quartz monzonite porphyry, and a breccia pipe. A blanket-like supergene deposit is superimposed on the hypogene mineralized zones. The host rocks for this deposit are metasedimentary and metaigneous rocks of Proterozoic age (Miller and others, 1973).
On the numerical estimates made
For the assessment, a North American subset of the porphyry copper tonnage and grade model of Singer and others (1986) was used (Hammarstrom and others, 1993; Mark3 index 81). This subset of 107 deposits, which range in age from Late Cretaceous through middle Tertiary, has a median size of 142 million metric tons and a median copper grade of 0.5 percent. Byproducts include silver, gold, and molybdenum. Montana deposits have relatively high contents of molybdenum and thus are distinctly different than most other porphyry copper deposits. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 0, 1, 3, 6, and 7 or more deposits consistent with the grade and tonnage model of Hammarstrom and others (1993).
Cox, D.P., 1986, Descriptive model of porphyry Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 76.
Guilbert, J.M., and Park, C.F., Jr., 1986, The geology of ore deposits: New York, W.H. Freeman and Company, 985 p.
Hammarstrom, J.M., Zientek, M.L., and Elliott, J.E., eds., 1993, Mineral resource assessment of the Absaroka-Beartooth study area, Custer and Gallatin National Forests, Montana: U.S. Geological Survey Open-File Report 93-207, 296 p., 19 plates.
Meyer, Charles, Shea, E.P., Goddard, C.C., Jr., Zeihen, L.G., Guilbert, J.M., Miller, R.N., McAleer, J.F., Brox, G.B., Ingersoll, R.G., Jr., Burns G.J., and Wigal, Thomas, 1968, Ore deposits at Butte, Montana, in Ridge, J.D., ed., Ore deposits of the United States, 1933–1967, v. 2: New York, American Institute of Mining, Metallurgical, and Petroleum Engineers, p. 1373-1416.
Ratcliff, M.W., 1973, Copper and molybdenum mineralization in the Continental deposit, East Butte district, Montana, p. I-1 - I-8, in Miller, R.N., ed., Guidebook for the Butte Field Meeting of Society of Economic Geologists: Society of Economic Geologists, Butte, Montana, 120 p.
Ross, C.P., Andrews, D.A., and Witkind, I.J., 1955, Geologic map of Montana: U.S. Geological Survey, scale 1:500,000.
Rostad, O., 1978, K-Ar dates of mineralization in the White Cloud-Cannivan porphyry molybdenum belt of Idaho and Montana—A discussion: Economic Geology, v. 73, no. 7, p. 1366-1368.
Singer, D.A., Mosier, D.L., and Cox, D.P., 1986, Grade and tonnage model of porphyry Cu, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 77-81.