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Influence of Basement Structure and Composition on Mineral Deposits in Montana and Idaho

This study aims to understand the geologic and metallogenic controls on mineral deposits in Montana and Idaho. Geologic maps, geochemical data, and aeromagnetic data were used to analyze the basement architecture and identify potential mineral deposit locations. The area has numerous economically significant deposits, including gold, silver, copper, lead, and zinc. Epigenetic deposit types and their associated minerals are discussed, as well as their distribution based on the basement geology. This study provides valuable information for mineral exploration and resource assessment in the region.

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Influence of Basement Structure and Composition on Mineral Deposits in Montana and Idaho

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  1. Influence of structure and composition of basement on mineral deposits across Montana and Idaho Lund, K., Klein, T.L, O’Neill, J.M., Sims, P.K., Taylor, C.D.

  2. USGS Headwaters Province project goal: develop understanding of fundamental geologic and metallogenic controls Geologic maps were produced, acquired, and compiled for a consistently formatted and integrated digital geologic map database Geochemical data (>60,000 samples) were compiled, integrated, and interpreted Mineral deposit studies included database, regional, and district scale Aeromagnetic data (NAMAG, 2002) were investigated for clues to basement composition and structure 1:100,000 to 1:250,000 scale maps from USGS, IGS, MBMG sources

  3. Information about basement architecture is very limited: Exposures of Archean or Proterozoic crystalline basement Isotopic data primarily from younger plutons Geophysical information

  4. Aeromagnetic anomaly map of northwestern United States Patterns in the aeromagnetic data delimit types of underlying basement. Data show terrane masses, terrane boundary structures, and major structures that deform the amalgamated basement terranes.

  5. Geologic map of concealed Archean and Paleoproterozoic crystalline basement, northern Rocky Mountain region Interpreted from exposures, isotopic geochemistry (Bickford and others, 1981; Toth and Stacey, 1992; Foster and Fanning, 1992; Doughty and others, 1998; O’Neill, 1999; Mueller and others, 2002), and aeromagnetic data

  6. Structures and components of the Paleoproterozoic, ~1.85 Ga, Great Falls tectonic zone (O’Neill, 1999; Dahl and others, 2002; Mueller and others, 2002)

  7. NE-striking Archean and Paleoproterozoic basement terranes and GFtz suture belts and structures are cut by conjugate N- and NW-striking wrench zones Both multiply reactivated

  8. Trace of Archean and Paleoproterozoic basement provinces, ~1.85 Ga GFtz, and ~1.5 Ga N-NW shear zones across Montana and Idaho Area evaluated for epigenetic mineral deposits relative to basement geology

  9. Mineral deposit studies Large quantities of metals have been produced from mineral deposits from 1860 to present. The area contains world class deposits and more than 80 other deposits considered economically significant Most of the deposits are epigenetic and associated with Cretaceous and Eocene granitoids. Debates raged and confusion reigned about the age and genesis of deposits

  10. Epigenetic deposit types include: Polymetallic vein Polymetallic carbonate replacement Epithermal gold veins Disseminated gold Tungsten vein Porphyry copper Porphyry molybdenum Hi-sulfidation Cu veins Breccia pipe precious metal Alkaline-intrusion precious metal Skarn and contact Metals include Au, Ag, Cu, Mo, Pb, Zn

  11. >21 million ounces of Au produced Gold production and resources are predominantly from rocks overlying juvenile magmatic crust and metamorphic-plutonic rocks of the Paleoproterozoic Great Falls tectonic zone Significant production from areas overlying edges of older terranes especially along N-NW ~1.5 Ga wrench zones

  12. ~1 billion ounces of Ag produced Siver production and resources are predominantly from rocks overlying juvenile magmatic crust and metamorphic-plutonic rocks of the Paleoproterozoic Great Falls tectonic zone

  13. 11 million tons of Cu produced Copper production and resources are predominantly from rocks overlying juvenile magmatic crust of the Paleoproterozoic suture zone (Great Falls tectonic zone)

  14. World class Cu deposits at Butte (which also contain significant Mo and other base and precious metals) are the most important of the Cu-bearing deposits. Butte lies at the junction between the Great Falls tectonic zone and the major NW-striking, reactivated basement shear system forming the Lewis and Clark line

  15. 1 million tons of Pb produced Lead production and resources are more widely distributed among the different sectors of the Great Falls tectonic zone

  16. 2.5 million tons of Zn produced Zinc production is similar to lead and more widely distributed among the different sectors of the Great Falls tectonic zone

  17. Epithermal and porphyry deposits are mostly confined to the juvenile crust and metamorphic-plutonic belts of the Great Falls tectonic zone Significant polymetallic deposits also align along N- and NW-striking basement structures Skarn and replacement deposits extend into foreland fold-and-thrust belt atop the Wyoming province Different distributions in deposit types

  18. Idaho-Montana porphyry belt was previously recognized by Armstrong and others (1978) and Rostad (1978)

  19. Porphyry Cu and Mo deposits range in age, spanning the major plutonic episodes that affected Idaho and Montana There is no apparent correlation with depth of melting, type of magmatism, from east to west across the region

  20. Regionally, porphyry Cu and Mo deposits clearly correlate with Great Falls tectonic zone Although hosted by plutons, there is no correlation with pluton age, texture, or composition Plutons of the same age, texture, composition, genesis, emplacement level are barren outside of the juvenile crust/metamorphic-plutonic belts of the ~1.85 Ga Great Falls tectonic zone or 1.5 Ga shear zones Idaho-Montana porphyry belt

  21. 1.85 Ga juvenile crust in the GFtz provided metals-endowed source terrain ~1.85 and 1.5 Ga shear zones provided rejuvenated fracture systems for younger plutonism and hydrothermal systems NW shear zones located sediment-hosted deposits--Coeur d’Alene, Blackbird

  22. Id-Mt belt of basement structure-controlled mineral deposits is comparable to the Colorado mineral belt of Tweto and Sims (1963) but has: 2X more Mo 3X more Pb 5X more Cu Distribution of different epigenetic deposit types in the Idaho-Montana “porphyry” belt

  23. Conclusions Great Falls tectonic zone continues across central Montana and Idaho Epigenetic mineral deposits are spatially related to the ~1.85 Ga GFtz, particularly to juvenile crust Metal production from Id-Mt porphyry belt is comparable to the Colorado mineral belt Epigenetic deposits are also related to intersections between the GFtz and ~1.5 Ga N- and NE-striking shear zones ~1.85 Ga juvenile crust in the GFtz formed metals-endowed source terrain ~1.85 and 1.5 Ga shear zones provided rejuvenated fracture systems for plutonism and hydrothermal systems

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