Geology of Northern Idaho and the Silver Valley

By Victoria E. Mitchell and Reed S. Lewis, Idaho Geological Survey and Joe Larsen, Idaho State University

...Geology, Photos and History of Mining in the Silver Valley ...

Geology of Northern Idaho

Coeur d'Alene Mining District

Geology of the Idaho Panhandle    

PDF Slideshows: North Idaho Rocks by Jim Cash
Flythroughs: Clark Fork, Salmon River, Clearwater River, Big Wood River, Middle Fork Salmon

Vocabulary Words

orogeny

foreland basin

mylonite

terrane

decollement

anticlinorium

 

 

Geology of Northern Idaho

Northern Idaho contains rocks of both the Omineca crystalline belt and the Foreland fold belt of the Cordilleran orogenic system (Figure 1). The Foreland Belt rocks are found on the eastern and western sides of the Idaho panhandle and consist of Middle Proterozoic Belt Supergroup strata; these are covered in Module 2 on the Digital Geology of Idaho and they also host the ore deposits of the Coeur d’Alene district.

Figure 1: Northern Idaho contains rocks of both the Omineca crystalline belt and the Foreland fold belt of the Cordilleran orogen.

Figure 2: Geologic map of Priest River core complex in northern Idaho (adapted from Doughty et al., 1997; Doughty and Price, 2000).

Coeur d’Alene Mining District

Idaho's Silver Valley, the Coeur d'Alene mining district, is in the South Fork of the Coeur d'Alene River in the panhandle of northern Idaho (Figures 3 and 4). The Coeur d'Alene mining district has one of the world's largest concentrations of silver metal. Since 1884, the district has produced over 1.18 billion ounces of silver, along with major amounts of lead, zinc, copper, and antimony. In total, more than $2.89 billion worth of metal has been produced from veins in the district.

Figure 3. Location of Coeur d'Alene district.

Figure 4. Generalized map of the Coeur d’Alene district, showing the locations of some of the major mines.

Gold was discovered on the South Fork of the Coeur d’Alene River in 1878. The gold rush lasted only a few years. In 1884 the first major lead-zinc-silver discovery was staked, and all of the major mines were found within a year. Of 90 mines historically in the district, only two were still operating in 2007 (the Lucky Friday and the Galena), and another (the famous Sunshine mine) was being targeted for reopening. The Coeur d'Alene district contains the largest underground mine in the United States (the Bunker Hill Mine, with over 150 miles of workings), the deepest mine (the Star-Morning, which is over 7,900 feet deep), and the richest silver mine (the Sunshine, which has produced over 367 million ounces of silver).

Figure 5. Generalized geology of the Coeur d’Alene district. Yp, Yra, Yc, Ymil,

Heavy lines are faults. Normal faults have ball and bar on down-thrown block. Thrust faults have sawteeth on upper plate (from Shoshone County map of the Digital Atlas of Idaho).

Figure 5 shows the geology of the Coeur d’Alene district. The mines are in the Proterozoic rocks of the Belt Supergroup. Intrusive rocks include the Cretaceous Gem and Dago Peak stocks and diabase and lamprophyre dikes of uncertain age. The district lies within the Lewis and Clark fault zone or shear zone, which is composed of many separate northwest-trending faults.

Stratigraphy: Host Rocks of the Coeur d’Alene District

 

The mines of the Coeur d’Alene district are hosted by the Proterozoic Belt Supergroup (see Module 2) that is about 1.4 billion years old. Figure 6 is a stratigraphic column with descriptions of the Belt Supergroup of the Coeur d’Alene district. Most of the ore is in the Revert-St. Regis transition zone, the Prichard-Burke transition zone, and the middle Prichard Formation quartzite (Bennett, et. al, 1989; Winston, 2000).

Figure 6. Stratigraphic column of the Belt Supergroup in the Coeur d’Alene district (Winston, 2000).

 

Structure: Faults & Folds

The structural geology of the area is complicated by a variety of folds and faults of diverse ages and movements. The district is at the intersection of the west-northwest-trending Osburn fault and a north-trending anticlinal uplift called the Noxon arch (White, 2000; Figure 7).

Figure 7. Sketch map showing location of the Coeur d'Alene district in relation to the Noxon arch (Noxon line@) and the faults of the Lewis and Clark shear zone (White, 2000).

Complex fold and fault structures are present, and pronounced facies and thickness changes in the Belt sedimentary host strata suggest that tectonism affecting the district began early (White, 2000). Abrupt southward stratigraphic thickening across the Coeur d’Alene district in the Belt Supergroup strongly suggests prolonged down-to-the-south faulting during most of the Belt deposition (Winston, 2000). The Cretaceous to Paleocene compressional folding and thrusting, accompanied by metamorphism, and the later Tertiary normal and strike slip faults are narrowly focused and superimposed, making the structural geology of this region even more complex (Hobbs and others, 1965).

Figure 8. Map showing locations of some of the faults in the Coeur d’Alene district. Circles, diamonds, and triangles mark the locations of most of the mines that have produced ore (Bennett, 1984).

The Lewis and Clark shear zone is a 50 km. wide, structurally and topographically defined lineament that crosses the Belt Supergroup and extends for at least 300 km. The zone includes the Placer Creek fault and the Osburn fault (Figure 8). Right-lateral strike-slip displacement on the Osburn fault was 16 miles on the segment east of the Dobson Pass fault and 12 miles on segment west of Dobson Pass fault, a dip-slip fault. Map elements can be rearranged to show what the district would have looked like before the faults moved (Figure 9).

Figure 9. Map showing Coeur d'Alene district before movement along the Osburn and Dobson Pass faults (Bennett, 1984). Also see Bennett and Venkatakrishnan (1982).


Geology of the Idaho Panhandle

By Joe Larsen, Idaho State University

The scope of this section will focus on rocks found on the central-western portion of the panhandle, namely the Priest River core complex and associated rocks. Though poorly defined it is implied that the Priest River core complex is comprised of the Kootenay Arc Terrane (Orr and Orr, 2002), the Belt Supergroup, and the underlying Hearne province (see Module #2, Figure #3, referred to there as Medicine Hat province or Figure 10 here) (Doughty et al., 1997).

Figure 10. Hearne province or as Medicine Hat province.

The Hearne/Medicine Hat Province is 2.5 Ga basement with the same age basement material found in the Spokane dome region of the core complex (Figure 2) (Doughty et al., 1997). The Archean basement is located southwest of the Eocene Wrencoe pluton, in the northern area of the Spokane gneiss dome. Doughty et al. (1997) have mapped a metaquartzite unconformably above the Archean basement that is believed to be equivalent to the Prichard Formation of the Belt Supergroup. It is named the Gold Cup Mountain quartzite, after the locality of the same name.

The Kootenay Arc is the easternmost terrane of the Omineca Belt and is believed to have begun docking with North America between 190 and 160 Ma (Orr and Orr, 2002). Subsequent collisions of other terranes to the west further compressed the Kootenay Arc which consists largely of sedimentary strata. The Spokane dome is mainly comprised of the Hauser Lake gneiss, a mylonitized migmatitic paragniess. Peak metamorphism occurred around 72 Ma (Doughty et al., 1997) recording the highest pulse of compression during the Mesozoic orogenic event. Deformation led to thickening and ultimately to melting and subsequent injection of plutons (Orr and Orr, 2002). This is evidenced by the Cretaceous Kaniksu Batholith, which make up the north south trending Selkirk Mountains, located in the northern portion of the core complex (Figure 2). The Kaniksu Batholith is a mixture of granite and granodiorite (Doughty and Price, 2000).

During Mesozoic deformation the Purcell Trench and Purcell Anticlinorium formed, separating the highly metamorphosed Priest River core complex to the west and the relatively undeformed Belt Supergroup to the east (Figure 2). This relationship produced the argument that the Purcell Trench marks the western extent of the North American Craton (Orr and Orr, 2002). If the Purcell Trench marks the North American boundary and because Hauser Lake gneiss is to the west of the Purcell Trench it would make the Hauser Lake gneiss part of the Kootenay Arc.

However the presence of Belt Supergroup rocks in the hanging wall west of the core complex is an argument that core is probably the western most edge of North America. An implication of this is that the Kaniksu Batholith is intruding North America, not the Kootenay Arc Terrane. Doughty et al. (1997) suggests the mylonite zone above the metaquartzite is the décollement of the Rocky Mountains (thrusting occurred in mid-Cretaceous to Paleocene time) making Gold Cup Mountain a window through the metaquartzite (metamorphosed Belt Supergroup) into the crystalline Archean basement (2.5 Ga Medicine Hat Province) underlying the Belt Supergroup. The Purcell Trench is obscured by Quaternary glacial fill making relationships across the trench hard to characterize.

Eocene Extension

During the Cenozoic, 52 Ma, compression ceased and the rock in northern Idaho began to relax. The tensional strain created a series of grabens, thinning the crust. The reduction of overburden created increased magmatism and pluton emplacement as evidenced by the Wrencoe, Silver Point, and other plutons (Doughty et al., 1997; Doughty and Price, 2000; Orr and Orr, 2002).

Continued extensional stresses caused the occurrence of low-angle normal faults, also called detachment faults. Tens of kilometers of displacement along these faults is common, allowing the removal of overlying, younger rock; permitting glimpses into the “core” of the crust (see Module #1, Digital Geology of Idaho). Displacement along the Newport Fault to the North and the Purcell Trench Fault to the east have created the Priest River core complex allowing deep crustal material to be exposed on the surface (Doughty et al., 1997; Doughty and Price, 2000; Orr and Orr, 2002).

The Purcell Lobe of the large Cordilleran Ice Sheet covered the region gouging out what is now Pend Oreille Lake (see Module #13, Digital Geology of Idaho).

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References

Bennett, E.H., 1984, A hypothesis concerning the genesis of orebodies in the Coeur d'Alene mining district, Idaho: Idaho Geological Survey Technical Report TR-84-7, 39 p.

Bennett, E.H., 2006, The fabulous Coeur d’Alene district: Presentation for North Idaho Chamber of Commerce Legislative Tour, November 2006, Post Falls, Idaho, 35 p.

Bennett, E.H., P.L. Siems, and J.T. Constantopoulos, 1989, The geology and history of the Coeur d’Alene mining district, in V. E. Chamberlain, Roy M. Breckenridge, and Bill Bonnichsen, editors, Guidebook to the Geology of Northern and Western Idaho and Surrounding Area: Idaho Geological Survey Bulletin 28, p. 137-156.

Bennett, E.H., and Venkatakrishnan, 1982, A palinspastic reconstruction of the Coeur d’Alene mining district based on ore deposits and structural data: Economic Geology, v. 77, p. 1851-1866.

Doughty, P.T., and Price, R.A., 2000, Geology of the Purcell Trench rift valley and the Sandpoint Conglomerate: Eocene en echelon normal faulting and syrift sedimentation along the eastern flank of the Priest River metamorphic complex, northern Idaho: Geological Society of America Bulletin, v. 112, no. 9, p. 1356-1374.

Doughty, P.T., Price, R.A., and Parrish, R.R., 1997, Geology and U-Pb geochronology of Archean basement and Proterozoic cover in the Priest River complex, northwestern Unite States, and their implications for Cordilleran structure and Precambrian continent reconstructions: Canadian Journal of Earth Science, v. 35, p. 39-54.

Fleck, R.J., R.E. Criss, G.F. Eaton, R.W. Cleland, C.S. Wavra, and W.D. Bond, 2002, Age and origin of base- and precious-metal veins of the Coeur d’Alene mining district, Idaho: Economic Geology, v.97, no.1, p. 23-42.

Hobbs, S.W., A.B. Griggs, R.E. Wallace, and A.B. Campbell, 1965, Geology of the Coeur d'Alene district, Shoshone County, Idaho: U.S. Geological Survey Professional Paper 478, 139 p., scale 1:24,000.
Orr, W.N., and Ore, E.L., 2002, Geology of the Pacific Northwest: 2nd Edition, McGraw-Hill Higher Education, 337.

Ransome, F. L., and F. C. Calkins, 1908, The geology and ore deposits of the Coeur d'Alene district, Idaho: U.S. Geological Survey Professional Paper 62, 203 p.

White, B.G., 2000, Coeur d'Alene mining district: product of preconcentrated source deposits and tectonism within the Lewis and Clark line, in Sheila Roberts and Don Winston, eds., Geologic field trips, western Montana and adjacent areas: Rocky Mountain Section of the Geological Society of America, University of Montana, p. 95-101.

Winston, Don, 2000, Belt Supergroup stratigraphy, sedimentology, and structure in the vicinity of the Coeur d’Alene mining district, in Sheila Roberts and Don Winston, eds., Geologic field trips, western Montana and adjacent areas: Rocky Mountain Section of the Geological Society of America, University of Montana, p. 85-94.

Wyld, S.J., Umhoefer, P.J., and Wright, J.E., 2006, Reconstructing northern Cordilleran terranes along known Cretaceous and Cenozoic strike-slip faults: Implications for the Baja British Columbia hypothesis and other models, in Haggart, J.W., Enkin, R.J., and Monger, J.W.H., eds., Paleogeography of the North American Cordillera: Evidence for and against large-scale displacements: Geological Association of Canada, Special Paper 46, p. 277-298.

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