Explained by James E. Elliott
On the choice of deposit models
The model used for the assessment of gold-bearing skarns is based on the descriptive model by Theodore and others (1991). The main criteria used for this model are: (1) The deposits must have an average gold grade of at least 1 g/t, and (2) the mineral assemblage of the deposit must be representative of a skarn environment. Pyroxene and garnet are the most important diagnostic minerals. Gold-bearing skarns are commonly the result of large-scale metasomatic transfer of components between hydrothermal fluids and predominately carbonate rocks. They are generally calcic exoskarns associated with intense retrograde hydrosilicate alteration. The igneous rocks act as the heat source and, in most cases, the source of components for the hydrothermal fluids. Gold-bearing skarns can be associated with porphyry copper or copper-molybdenum, polymetallic replacement, and polymetallic vein deposits.
On the delineation of permissive tracts
The permissive tract for gold-bearing skarns is the same as that for polymetallic replacement deposits (C07); these areas are located in northwestern, southwestern, and south-central parts of Montana. The tract is made up of those parts of the porphyry copper permissive tracts that have sedimentary carbonate rocks at the surface or at shallow depths (less than 1 km) below the surface, based on geologic maps of Montana (Ross and others, 1955) and Wyoming (Love and Christiansen, 1985). The more favorable parts of these areas are where sedimentary carbonate rocks are near igneous contacts, especially near margins of Late Cretaceous or Eocene granite, granodiorite, or dacite porphyry. The most favorable environment for the occurrence of gold-bearing skarns is at or near an intrusive-sedimentary carbonate contact whereas polymetallic replacement deposits can form at some distance from an intrusive contact.
The most favorable sedimentary carbonate rocks are Paleozoic, and include the Meagher and Pilgrim Limestones of Cambrian age, the Jefferson Formation of Devonian age, and the Madison Group of Mississippian age. Less favorable carbonate rocks are Mesozoic (Lower Cretaceous Kootenai Formation and Jurassic Ellis Group) and Middle Proterozoic in age (Belt Supergroup: Newland, Empire, Helena, and Wallace Formations).
Important examples of this type of deposit
There are several important gold-bearing skarn deposits and districts in southwestern and south-central Montana. The most important ones are the Bannack district (Loen and Pearson, 1989; Theodore and others, 1991); the Butte Highlands district (Sahinen, 1950); the Silver Star district (Loen and Pearson, 1989); the Cable mine in the Georgetown district (Emmons and Calkins, 1913); the New World district (Elliott and others, 1992); and the Diamond Hill mine. All of the gold-bearing skarns are located at contacts of intrusive rocks. Two (Butte Highlands and Silver Star) are located at contacts of the Late Cretaceous Boulder batholith with Cambrian sedimentary carbonate rocks, three (Bannack, Cable, and Diamond Hill) are located along margins of small Late Cretaceous stocks, and several deposits in the New World district are located at contacts with Eocene dacitic porphyritic intrusive complexes. Host rocks for these deposits are mostly Paleozoic sedimentary carbonate rocks. The most favorable host rocks are the Cambrian Meagher Limestone(or Silver Hill Formation) and Pilgrim Limestone(or Hasmark Formation). The Diamond Hill mine is unusual in that no carbonate rocks are present; the ore bodies occur in garnet-diopside-epidote skarn that replaced Cretaceous volcanic rocks.
On the numerical estimates made
Within the permissive tract for gold-bearing skarns, six subtracts or areas are favorable for the occurrence of undiscovered gold-bearing skarn deposits. Separate estimates were made for undiscovered gold-bearing skarns in each of these areas. A seventh estimate was made for undiscovered deposits in the remaining less-favorable permissive area. The estimates considered the presence of known deposits, prospects, and favorability of geologic setting. The favorable areas are: (1) the Absaroka-Gallatin volcanic province (Chadwick, 1970) in south-central Montana and northwestern Wyoming; (2) the Dillon area including the eastern margin of the Pioneer batholith and southern margin of the Boulder batholith in southwestern Montana; (3) the Elkhorn area, along the eastern side of the Boulder batholith; (4) the Helena area, along the northern side of the Boulder batholith; (5) the southern Flint Creek Range in southwestern Montana, and (6) the Garnet area, east of Missoula. The individual estimates are:
percentile 90th 50th 10th 5th 1st
Absaroka-Gallatin 2 5 8 10 12
Dillon 0 2 3 4 5
Elkhorn 1 1 2 3 4
Helena 1 1 2 3 4
Flint Creek Range 1 2 3 4 5
Garnet 0 1 1 2 3
Other areas 0 0 1 2 3
Combining these estimates, for the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 5, 12, 20, 28, and 36 or more gold skarn deposits consistent with the grade and tonnage model of Theodore and others (1991).
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., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Elliott, J.E., Kirk, A.R., and Johnson, T.W., 1992, Field guide; gold-copper-silver deposits of the New World district, in Elliott, J.E., ed., 1992, Guidebook for the Red Lodge-Beartooth Mountains-Stillwater Area, Tobacco Root Geological Society Seventeenth Annual Field Conference: Northwest Geology, v. 20/21, p. 1-19.
Emmons, W.H., and Calkins, F.C., 1913, Geology and ore deposits of the Philipsburg quadrangle, Montana: U.S. Geological Survey Professional Paper 78, 271 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.
Loen, J.S., and Pearson, R.C., 1989, Map showing locations of mines and prospects in the Dillon 1° x 2° quadrangle, Idaho and Montana: U.S. Geological Survey Map I-1803-C, scale 1:250,000.
Sahinen, U.M., 1950, Geology and ore deposits of the Highland Mountains, southwestern Montana: Montana Bureau of Mines and Geology Memoir 32, 63 p.
Theodore, T.G., Orris, G.J., Hammarstrom, J.M., and Bliss, J.D., 1991, Gold-bearing skarns: U.S. Geological Survey Bulletin 1930, 61 p.