Download Epigenetic deposit types include

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.
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
1:100,000 to 1:250,000 scale maps from USGS, IGS, MBMG sources
Aeromagnetic data (NAMAG,
2002) were investigated for
clues to basement composition
and structure
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
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
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
>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
~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
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)
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 metamorphicplutonic 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-andthrust 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
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
Idaho-Montana porphyry belt
Plutons of the same age,
texture, composition,
genesis, emplacement level
are barren outside of the
juvenile crust/metamorphicplutonic 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
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