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 Eocene deposits in Montana and northwestern Wyoming. 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 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 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 south-central Montana and northwestern Wyoming are: (1) Mapped areas of Eocene intrusive and extrusive rocks of the Absaroka Volcanic Supergroup in the Absaroka ranges of Montana and Wyoming, as shown on the geologic map of Montana (Ross and others, 1955) and Wyoming (Love and Christiansen, 1985); (2) Predicted presence of subsurface intrusive rocks based on gravity and magnetic data; (3) Presence of known mines, prospects, and occurrences of this deposit type.
Permissive areas for porphyry copper deposits are located in south-central Montana and northwestern Wyoming, coincident with an Eocene volcano-plutonic belt. Most of the known deposits and prospects are situated within the northwesterly trending Absaroka-Gallatin volcanic province (Chadwick, 1970). The Kirwin deposit and 11 porphyry copper prospects are located along this belt (Hausel, 1982).
Important examples of this type of deposit
Eocene deposits include the Kirwin deposit in northwestern Wyoming near Meeteetse. The Kirwin deposit is located in the southern part of the Absaroka-Gallatin volcanic province (Chadwick, 1970) in northwestern Wyoming. It is associated with rhyolitic tuffs and breccias that intruded andesitic volcanic and volcaniclastic rocks. The deposit is contained within an area of intense hydrothermal alteration associated with a volcanic vent complex. The mineralized zone consists of stockworks with pyrite, chalcopyrite, and molybdenite in quartz-calcite veins and disseminated sulfides in altered rocks. A secondary enriched blanket containing chalcocite, digenite, and covellite overlies a part of the stockworks (Hausel, 1982).
Eleven other porphyry copper prospects are known and are located along two northwesterly-trending belts in the Absaroka-Gallatin province in northwestern Wyoming and south-central Montana. These prospects are New World, Independence, Emigrant, Silver Creek, Stinkingwater, Sunlight, Eagle Creek, Clouds Home Peak, Robinson Creek, Birthday, and Yellow Ridge.
On the numerical estimates made
A comprehensive analysis of part of the Absaroka-Gallatin volcanic province by Hammarstrom and others (1993) was used as a guide for the whole of that province. 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 1, 4, 6, 7, and 9 or more deposits consistent with the grade and tonnage model of Hammarstrom and others (1993).
Chadwick, R.A., 1970, Belts of eruptive centers in the Absaroka-Gallatin volcanic province, Wyoming-Montana: Geological Society of America Bulletin, v. 81, p. 267-274.
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.
Hausel, W.D., 1982, General geologic setting and mineralization of the porphyry copper deposits, Absaroka Volcanic Plateau, Wyoming: Wyoming Geological Association, Guidebook for Thirty-Third Annual Field Conference, p. 297-313.
Love, J.D., and Christiansen, A.C., 1985, Geologic map of Wyoming: U.S. Geological Survey, scale 1:500,000.
Ross, C.P., Andrews, D.A., and Witkind, I.J., 1955, Geologic map of Montana: U.S. Geological Survey, scale 1:500,000.
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.