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.

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

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

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.

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

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

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

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

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

Metals include Au, Ag, Cu, Mo, Pb, Zn 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

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 >21 million ounces of Au produced

~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

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

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

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

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

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

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

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

Idaho-Montana porphyry belt 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

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

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

Metal production from Id-Mt porphyry belt is comparable to the Colorado mineral belt 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 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 Conclusions