Explained by James E. Elliott
On the choice of deposit models
The descriptive model for polymetallic replacement deposits by Morris (Cox and Singer, 1986) was used for undiscovered deposits in Montana and northwestern Wyoming. These deposits are hydrothermal epigenetic deposits consisting of silver-, lead-, zinc-, and copper-bearing minerals in massive lenses, pipes, and veins in carbonate sedimentary rocks near igneous intrusions. Associated igneous rocks are commonly calc-alkaline and porphyritic. Types of alteration include dolomitization and silicification. On a district scale, the deposits are commonly zoned from a copper-rich central area, through a wide lead-silver zone, and to a zinc- and manganese-rich fringe.
On the delineation of permissive tracts
The permissive tract for polymetallic replacement deposits is made up of areas in northwestern, southwestern, and south-central Montana. It consists of those parts of the porphyry copper permissive tract 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 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 many known polymetallic replacement deposits or districts in southwestern Montana and numerous prospects in southwestern and south-central Montana and northwestern Wyoming. These have been important producers of silver and lead and were less important for zinc, copper, and gold. The known deposits are the Elkhorn mine, Elkhorn district (Klepper and others, 1957); the Hecla district (Karlstrom, 1948); the Castle Mountain district (Winters, 1968); and the Hope mine, Philipsburg district (Emmons and Calkins, 1913). The prospects occur in many districts where plutons of granitic, granodioritic, or dacitic composition are in contact with sedimentary carbonate rocks, especially those of Paleozoic age.
Ore bodies at the Elkhorn mine consisted of saddle-reefs, pipe-like bodies, and irregular masses in Cambrian limestone (Pilgrim Limestone) below the contact with a shale-limestone sequence (Red Lion Formation). The Elkhorn district is located near the margin of the Boulder batholith (Late Cretaceous), and was a large producer of lead and silver and small amounts of gold.
In the Hecla district, stratiform, pipe-like, and irregular-shaped ore bodies in Cambrian limestone and dolomite (Meagher and Pilgrim Limestones) were mined mainly for lead and silver. Small amounts of gold and copper were also produced. Ore zones in the Hecla district were controlled by anticlines and structural domes. The district is located near the northern margin of the Pioneer batholith, a granitic composite pluton of Late Cretaceous age.
Pipe-, pod-, and irregular-shaped ore bodies that are generally conformable to bedding were exploited in the Castle Mountain mining district. Host rocks for these deposits are limestones of Mississippian (Madison Group), Cambrian (Pilgrim Limestone), and Devonian (Jefferson
Limestone) age. The district is located along the margin of a granite pluton and ore bodies are commonly localized along Tertiary dacite porphyry intrusions that are younger than the granite. The district is a large producer of lead and silver.
The Philipsburg district is famous as a large producer of silver and manganese from vein and replacement deposits. One of the principal mines of the district, the Hope mine, is a polymetallic replacement deposit with ore bodies in saddle reefs and irregular stratiform masses that parallel bedding in Devonian limestone (Jefferson Limestone). The district is located near the margins of the Late Cretaceous Philipsburg batholith (granodiorite). The Hope mine was a large producer of silver with minor production of copper, manganese, and lead.
On the numerical estimates made
For the assessment of polymetallic replacement deposits, the grade-tonnage model of Mosier and others (Cox and Singer, 1986) was used. This set of 52 deposits has a median size of 1.8 million metric tons and median grades of: 5.2 percent lead; 3.9 percent zinc; 0.09 percent copper; 150 g/t silver; and 0.19 g/t gold. The known deposits in Montana are mainly lead-silver deposits and thus differ from the above median grades by having higher grades of lead and silver and lower grades of zinc, copper, and gold. For the 90th, 50th, 10th, 5th, and 1st percentiles, the team estimated 1, 4, 6, 8, and 12 or more districts consistent with the grade and tonnage model of Mosier and others (1986).
Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
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.
Karlstrom, T.N.V., 1948, Geology and ore deposits of the Hecla mining district, Beaverhead County, Montana: Montana Bureau of Mines and Geology, Memoir No. 25, 87 p.
Klepper, M.R., Weeks, R.A., and Ruppel, E.T., 1957, Geology of the southern Elkhorn Mountains, Jefferson and Broadwater Counties, Montana: U.S. Geological Survey Professional Paper 292, 82 p.
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.
Winters, A.S., 1968, Geology and ore deposits of the Castle Mountain mining district, Meagher County, Montana: Montana Bureau of Mines and Geology, Bulletin 64, 64 p.