By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
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The important mineral deposits of Shoshone County are in the Coeur d'Alene Mountains, along its northeast boundary. This mineralized area covers about 500 square miles and includes about eight mining districts, known collectively as the Coeur d'Alene region.
The county's gold production from 1881 through 1942 was 393,088 ounces (Staley, 1946, p. 27) ; from 1943 through 1959 it was 41,113 ounces a total of 434,201 ounces for 1881-1959.
COEUR D'ALENE REGION
The Coeur d'Alene region is in northeastern Shoshone County between lat 47°25' and 47°40' N. and long 115°40' and 116°10' W., and includes the districts of Beaver, Eagle, Evolution, Hunter or Mullan, Lelande or Burke, Placer Center or Wallace, Summit, and Yreka. Wallace is its principal town.
The first mineral discoveries were lodes in 1878, but it was not until rich placers were discovered along Prichard and Eagle Creeks in the early 1880's that any concentrated interest was shown in the area. The town of Murray was founded in the placer area, and it soon became the county seat of Shoshone County. In 1885 rich deposits of lead and silver were discovered along the South Fork of the Coeur d'Alene River; at the same time the placers at Murray began to decline (Ransome and Calkins, 1908, p. 78-80).
A railway into the region was completed in 1887, and by that time many properties - among them the Bunker Hill and Sullivan, Mammoth, Tiger, Morning, Poorman, and Granite - were producing substantial amounts of ore. The succeeding decade was marked by strife between the miners' unions and the mine owners, and several times troops were called in to restore order (Ransome and Calkins, 1908, p. 81).
Meanwhile the Murray placers experienced a revival. The bench gravels were worked, and several new lode properties began producing. By 1911 the lead-zinc mines at Murray reached their peak, and many continued operating until 1933, when all of them closed (Shenon, 1938, p. 15). The region became slightly active again in the mid-1930's and remained so through 1959.
Most of the early gold production of the region was from placers and gold-quartz veins near Murray; the lead-zinc ores at Murray contained only negligible amounts of gold. A total of 227,890 ounces of gold was produced in the region from 1884 to 1905 (Ransome and Calkins, 1908, p. 82). From 1906 through 1934 the Murray area produced 66,092 ounces of placer and 3,154 ounces of lode gold (Shenon, 1938, p. 17), and from 1935 through 1951 a total of 16,275 ounces of placer and lode gold was produced (Hosterman, 1956, p. 740).
Mines in the Coeur d'Alene region, including the Murray district, produced a total of $7,180,151 (348,550 ounces) in gold from 1884 through 1931 (Ross, 1941, p. 85). Total gold production of the region through 1960 was about 439,000 ounces (Shenon, 1961, p. 1).
The geology of the region was described in detail by Ransome and Calkins (1908, p. 23-77), and the following was abstracted from their report. The oldest and most abundant rocks underlying the district are Precambrian sedimentary rocks of the Belt Series. These are subdivided into six formations, from the oldest to youngest: Prichard Formation, Burke Formation, Revett Quartzite, St. Regis Formation, Wallace Formation, and Striped Peak Formation.
All these formations, except the Striped Peak, are intruded along a northeast-trending belt by bodies of quartz monzonite. The intruded rocks, especially the calcareous types, are metamorphosed extensively along the contacts. In addition to quartz monzonite, other igneous rocks of the district are dikes of diabase and lamprophyre.
The age of the intrusions and deformation is not clearly revealed. The rocks were folded and then faulted, but the intrusions may have occurred at any time during or prior to the faulting, as some faults cut the monzonite.
The rocks are thrown into several large asym¬metrical folds which trend west-northwest, north, or northeast. Complicating the structure are smaller folds superimposed on the limbs of the major ones. The rocks are extensively dislocated by normal and reverse faults, the main group of which strikes nearly east and a smaller group, north to slightly west of north. The Osburn, Placer Creek, Big Creek, and White Ledge faults are examples of the first group; the Dobson Pass, Carpenter Gulch, and O'Neil Gulch faults are typical of the second.
Mineral deposits in the Coeur d'Alene region pri¬marily contain silver-lead-zinc ores, and some copper and gold are recovered as byproducts. Gold lodes have been mined near Murray. The silver-lead-zinc ores are mostly in the main part of the region, between Mullan and Kellogg. They are in veins and in tabular replacement bodies most of which are in the Revett and Burke Formations (Ransome and Calkins, 1908, p. 106).
Argentiferus galena, pyrite, pyrrhotite, chalcopyrite, sphalerite, and local tetrahedrite are the most abundant metallic minerals; siderite and locally occurring quartz are the dominant gangue minerals (Ransome and Calkins, 1908, p. 107-111).
Gold deposits occur in four different structural environments: (1) mineralized shear zones with steep dips, (2) quartz veins along bedding planes, (3) quartz veins along low-angle thrust faults, and (4) placers (Shenon, 1938, p. 18). Of these, the placers were the most productive, followed by the bedding-plane veins, then by the mineralized shear zones and thrust-fault deposits. All the gold lodes occur in beds of the Prichard Formation. Bedding-plane veins usually are found in argillite. They contain quartz and some ankerite, sericite, albite, chlorite, and apatite.
The ore minerals, which form as much as 5 percent of the vein material, are arsenopyrite, pyrite, galena, chalcopyrite, specularite, scheelite, and gold. Selenium is sparse (Shenon, 1938, p. 20). The shear-zone deposits are mineralogically similar to the bedding-plane veins except that they also contain pyrrhotite and sphalerite but no scheelite or specularite (Shenon, 1938, p. 19). The minable thrust-fault deposits are restricted to only one mine, the Wakeup Jim. The mineralogy also is similar to that of the bedding-plane veins.
Although copper deposits in this region are classified as a distinct type occupying an area in the southeast part of the district, they were worked by only one mine, and it was closed in 1915. The ores are disseminations of bornite, chalcocite, and chalcopyrite in the Revett Quartzite (Ransome and Calkins, 1908, p. 150). Gold is a minor constituent.
