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Coeur d'Alene mining district: product of preconcentrated source deposits and tectonism within the Lewis and Clark Line.
Geologic field trips: Western Montana and adjacent areas, 2000, Roberts S, Winston D, eds. Missoula, MT: University of Montana, 2000 Jan; :95-101
The Lewis and Clark line (LCL) is a 50 km-wide structurally and topographically defined lineament that crosses the Mesoproterozoic Belt Supergroup and extends for at least300 km. No single style of tectonism is predominately responsible for the line; the LCL is the product of diverse, recurrent tectonism controlled by some underlying, crustal-scale structure at lease as old as the Belt rocks themselves, which has been reactivated at various times in response to new stress conditions. Most structures that define the LCL resulted from Cretaceous and Tertiary tectonism that was concentrated and intensified within the line. A particular tectonic event with a distinctive structural style involving formation of reverse faults and a dip-slip metamorphic shear zones is associated with formation of the silver-lead-zinc vein ore bodies of the Coeur d'Alene Mining District. Despite the great length of the LCL, the major concentration of silver-lead-zinc veins of the Coeur d'Alene district are limited to a short portion of the line. A reasonable explanation of this concentration is that it results from the intersection of the LCL with a second long-lived tectonic lineament, here called the Noxon line. During Belt sedimentation, the Noxon line was an arch that separated the Belt basin into two separate subbasins. During early Belt sedimentation, basement structures that controlled the Noxon arch probably localized Sullivan-type, syngenetic zinc-lead-silver deposits. Later, Revett-type, stratabound, copper-silver deposits in the Favalli Group were also concentrated within the Noxon arch. Redistribution of metals from these two separate sources during Late Cretaceous or early Tertiary tectonism within the LCL is a plausible explanation for the concentration of silver, lead, and zinc that from the veins of the Coeur d'Alene district.
Geologic strata; Geology; Geologic formations
Brian G. While, Spokane Research Laboratory, NIOSH, 315 Montgomery Ave., Spokane, WA 99207
Geologic field trips: Western Montana and adjacent areas.
University of Montana, Missoula, MT
Page last reviewed: March 11, 2019
Content source: National Institute for Occupational Safety and Health Education and Information Division