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