aka Ashby Creek
The Potomac mining district is located in Missoula County in the Garnet Range south of the Blackfoot River. The mines are at the headwaters of Ashby Creek, tributary to Camas Prairie Creek which flows into the Blackfoot River. The Ashby Creek mines are just over the divide from mines at the headwaters of Wallace Creek in the Clinton district. The principal mine of the Ashby Creek area was the Charcoal, which produced silver, lead, gold and silver.
The valley of the Blackfoot around the small community of Potomac is a structural basin deeply filled with sediment that accumulated during Tertiary time. Granitic intrusions in the Garnet Range were emplaced along the big thrust faults that carried the Saphhire tectonic block east. Its north edge lies between Missoula and Ovando. Mineralization in the district occurred along large continuous shear zones that cut across the quartzitic sediments and the granodiorite stock indiscriminately. The ore bodies occurred on both sides of the igneous contact, but most of the production came from the sedimentary side of the contact. The mines of the Potomac district were valued chiefly for lead and silver (Sahinen 1957; Alt & Hyndman 1986).
BOUNDARIES OF THE DISTRICT
Pardee (1918) described the Clinton district as consisting of two major mining areas: along Wallace Creek east of the town of Clinton and along Ashby Creek toward the town of Potomac. Pardee wrote that
Most of the mines [of the Clinton district] are within an area 5 or 6 miles long and 1 or 2 miles wide that begins 2 miles east of Clinton and extends northeastward across the divide and part way down the Blackfoot River slope....The most extensively developed lodes are along Wallace Creek and its tributaries Trail and Woodville creeks, about 2 1/2 miles east- northeast of Clinton. Within a square mile there are 50 or more mines and prospects, on some of which the workings reach a depth of 500 feet vertically below the surface...Another numerous group [of mines] is in the northeastern part of the mineral-bearing area in the basin of Ashby Creek, and although prospects are by no means scarce in the rest of the area few of them have been worked to any extent.
Later geological researchers regarded the Clinton district as comprising only those areas along Wallace Creek.
Sahinen (1957) described the Potomac district as located "on the headwaters of Ashby Creek, about five or six miles south of Potomac post office..The Potomac mines are also accessible from Clinton." He also described the Charcoal mine as being located in the Wallace (Ashby Creek) district. Sanders & Winn (1994) believed that Pardee included the Ashby Creek mines in the Clinton district because of their geological similarity; most of the developed prospects followed the course of a well-defined exposure of granodiorite.
Figure 1 shows the Potomac district as defined by the AMRB (1994) with a smaller area delineated which reflects the definition of the Ashby Creek area by Sahinen (1957).
HISTORIES OF SELECTED MINES
Small lots of lead ore were shipped from the Adalin (or Adaline) mine in 1939. It was located within a half a mile southwest of the Charcoal mine, in Section 10, T12N, R16W (Rowe 1941; Sahinen 1957).
The Charcoal mine (also known as the Shawbut for its owner Lawrence Shawbut) was located on the East Fork of Ashby Creek, 6.5 miles south of Potomac, in Section 11, T12N, R16W. The mine was opened about 1889 and reportedly yielded $15,000 in lead and silver. In 1918 several shipments of lead ore containing considerable silver were made from the Royal property near Potomac. In 1919 the Royal Mining Company shipped a car of siliceous silver ore containing some lead. In 1928 the Charcoal mine produced 15 tons of silver-bearing lead- zinc ore, which was shipped to Butte for milling. Considerable development work was done in 1928 (Sahinen 1957; Pardee 1918).
In 1951 the mine produced 125 tons of ore yielding 3 ounces of gold, 137 ounces of silver, 9,300 pounds of lead, and 3,000 pounds of zinc valued at $2,384. The country rock is quartz diorite or granodiorite. An ore sample from the bin assayed 0.06 ounces gold; 20.7 ounces silver per ton: and 7.7 percent lead; 8.1 percent zinc; and 0.34 percent copper (Sahinen 1957).
The Daisy mine was located within 1/2 mile southwest of the Charcoal mine, in Section 10, T12N, R16W (Sahinen 1957).
The Gowrie mine was located in Section 6, T12N, R16W, a couple of miles northwest of the main group of mines in the district (Sahinen 1957).
In 1930 a test lot of lead ore was shipped from the Montana Center property south of Potomac (Sahinen 1957).
The Sumpter mine (also known as the Blackhawk) was located in Section 8, T12N, R16W, near Ashby Creek. The location consists of an adit, test pits, trenches and several residences probably occupied from 1911 to the 1930s (Sanders & Winn 1994; Sahinen 1957).
Abandoned Mine Reclamation Bureau (AMRB)
1994 Mining districts of Montana. Maps 1:100,000 and Map #94-NRIS-129. Compiled and edited by Joel Chavez. Prepared by Montana State Library Natural Resource Information System (NRIS) for Montana Department of State Lands. Helena
Alt, David, and Donald W. Hyndman
1986 Roadside Geology of Montana. Missoula: Mountain Press Publishing Company.
Pardee, J. T.
1918 "Ore Deposits of the Northwestern Part of the Garnet Range, Mont.", Contributions to Economic Geology (Short Papers and Preliminary Reports), 1917. Washington: U. S. Geological Survey.
Rowe, Jesse Perry
1941 Geography and Natural Resources of Montana. Missoula: Montana State University, 1933 (revised 1941).
Sahinen, Uuno M.
1957 "Mines and Mineral Deposits, Missoula and Ravalli Counties, Montana", Bulletin 8, Montana Bureau of Mines and Geology.
Sanders, Darrell, and Maureen Winn
1994 "Cultural Resource Inventory Report for the BLM-Plum Creek Land Exchange (Formerly BLM-Champion Land Exchange)", Prepared by the Bureau of Land Management.