U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2007–1280

Metallogeny of Mesoproterozoic Sedimentary 

Rocks in Idaho and Montana—Studies by the

Mineral Resources Program, U.S. Geological

Survey, 2004–2007
Metallogeny of Mesoproterozoic Sedimentary 

Rocks in Idaho and Montana—Studies by the 

Mineral Resources Program, U.S. Geological 

Survey, 2004–2007

Edited by J. Michael O’Neill

Contributions by J. Michael O’Neill, Edward T. Ruppel, David A. Lopez,

Arthur A. Bookstrom, Craig A. Johnson, Gary P. Landis, Thomas P. Frost,

and John F. Slack

Volume comprises Chapters A, B, and C

Open-File Report 2007–1280

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2007

For sale by U.S. Geological Survey Information Services

Box 25286, Denver Federal Center

Denver, CO 80225

For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, 

natural hazards, and the environment:

World Wide Web: http://www.usgs.gov

Telephone: 1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

O’Neill, J.M., ed., 2007, Metallogeny of Mesoproterozoic sedimentary rocks in Idaho and Montana—Studies by the 

Mineral Resources Program, 2004–2007: U.S. Geological Survey Open-File Report 2007–1280, 28 p.; available at 

http://pubs.usgs.gov/of/2007/1280

Cover: Photomicrograph of a thin section, showing cobaltite grains (bluish gray) in biotite phyllite (yellowish to 

brownish green) with chloritoid porphyroblasts (pale green). The sample is from the Blackbird mine area, Lemhi 

County, Idaho. The thin section was photographed in plane-polarized transmitted light with incident light reflected 

from opaque cobaltite grains. The image was supplied by Stephen G. Peters, U.S. Geological Survey.

This publication has not been reviewed for stratigraphic nomenclature.
iii

Volume Contents

 Preface ...........................................................................................................................................v

 By J. Michael O’Neill

A. Great Divide Megashear, Montana, Idaho, and Washington—An Intraplate CrustalScale Shear Zone Recurrently Active Since the Mesoproterozoic.....................................3

 By J. Michael O’Neill, Edward T. Ruppel, and David A. Lopez

B. Blackbird Fe-Cu-Co-Au-REE Deposits.....................................................................................11

 By Arthur A. Bookstrom, Craig A. Johnson, Gary P. Landis, and Thomas P. Frost

C. Geochemical and Mineralogical Studies of Sulfide and Iron Oxide Deposits in the 

Idaho Cobalt Belt ........................................................................................................................21

 By John F. Slack

Conversion Factors

Units of measure

The following table lists conversion factors useful for this report: 

Multiply SI unit By To obtain English unit

centimeter (cm) 0.3937 inch (in.)

kilometer (km) 0.6214 mile (mi)

meter (m) 3.281 foot (ft) 

metric tonne 1.1025 English ton
iv

Abbreviations Used in This Report

Unit of measure

cm centimeter, centimeters

Ga giga-annum, billion years

g gram, grams

g Au/t grams of Au per metric tonne

km kilometer, kilometers

kt thousand metric tonnes

m meter, meters

Ma Mega-annum (millions of years ago)

Mt million metric tonnes 

mi mile, miles

m.y. million years (time interval)

ppb parts per billion

ppm parts per million

t metric tonne

vol percent volume percent

wt percent weight percent

Initialism

HREE Heavy rare-earth elements

ICP–MS inductively coupled plasma–

mass spectrometry

ICP–OES inductively coupled plasma–

optical emission spectrometry

INAA instrumental neutron activation 

analysis

IOCG iron oxide-copper-gold

LREE Light rare-earth elements 

pH potential of hydrogen; a

measure of acidity

REE Rare-earth elements

SEDEX sedimentary-exhalative

SEM scanning electron microscope

SEM-EDS scanning electron microscapeenergy dispersive spectrometry

SHRIMP sensitive high-resolution ion 

microprobe

VMS volcanogenic massive sulfide
v

Preface

By J.Michael O’Neill

The major emphasis of this project was to extend and refine the known Mesoproterozoic geologic and 

metallogenic framework of the region along and adjacent to the Idaho-Montana boundary north of the Snake 

River Plain. The Mesoproterozoic metasedimentary rocks in this part of east-central Idaho host important 

Cu-Co-Au stratabound mineral resources as well as younger, epigenetic hydrothermal, sulfide base-metal 

mineral deposits. Two tasks of this study were to more accurately understand and portray the character 

and origin of cobalt-copper-gold deposits that compose the Idaho cobalt belt and specifically to analyze ore 

mineralogy and metallogenesis within the Blackbird mining district in the central part of the belt. Inasmuch 

as the cobalt belt is confined to the Mesoproterozoic Lemhi Group strata of east-central Idaho, geologic 

investigations were also undertaken to determine the relationship between strata of the Lemhi Group and 

the more extensive, noncobalt-bearing, Belt-Purcell Supergroup strata to the north and northwest. 

Abrupt lateral differences in the character and thickness of stratigraphic units in the Mesoproterozoic 

Lemhi Basin may indicate differential sedimentation in contemporaneous fault-bounded subbasins. It is

suggested that northeast-trending basement faults of the Great Falls tectonic zone controlled development 

of the subbasins. O’Neill and others (chapter A, this volume) document a second major basement fault 

in this area, the newly recognized northwest-striking Great Divide megashear, a zone 1–2 km wide of 

left-lateral strike-slip faults active during Mesoproterozoic sedimentation and bounding the Cu-Co belt on 

the northwest. The megashear is a crustal-scale tectonic feature that separates Lemhi Group strata from 

roughly coeval Belt-Purcell strata to the north and northwest in Montana and northern Idaho.

The results of numerous geologic investigations of the Cu- and Co-bearing Mesoproterozoic rocks of 

east-central Idaho are integrated and summarized by Bookstrom and others (chapter B, this volume). In 

particular, their field investigations and analysis of evidence and previous arguments for synsedimentary 

versus epigenetic mineral deposit types, both of which have been postulated by earlier workers, led them 

to conclude that both processes were likely instrumental in forming the ore deposits of the Blackbird 

district.

Finally, this report supplies new data on isotopic ratios of sulfur, oxygen, carbon, and helium in minerals 

associated with cobalt-bearing ores of the cobalt belt. Slack (chapter C, this volume) identified several 

previously unrecognized rare-earth-element minerals in Blackbird ores: monazite (Ce,La,Y,Th)PO4, xenotime 

(YPO4

), allanite (CaCe)2(Al,Fe)3Si3O12(OH), and gadolinite (Be2FeY2Si2O10). Light rare-earth elements reside 

mostly in monazite, whereas yttrium and heavy rare-earth minerals reside mostly in xenotime. Dated monazite, 

which in the Blackbird district is interstitial to cobaltite, is Cretaceous. This date brings into question the 

otherwise geologically convincing interpretation of Blackbird ores as being of Mesoproterozoic age and 

synsedimentary origin.

This volume consists of three summary articles:

A. Great Divide megashear, Montana, Idaho, and Washington—An intraplate crustal-scale shear zone 

recurrently active since the Mesoproterozoic by J. Michael O’Neill, Edward T. Ruppel, and David A. Lopez

B. Blackbird Fe-Cu-Co-Au-REE deposits by Arthur A. Bookstrom, Craig A. Johnson, Gary P. Landis, and

Thomas P. Frost

C. Geochemical and mineralogical studies of sulfide and iron oxide deposits in the Idaho cobalt belt by

John F. Slack
Great Divide Megashear, Montana, Idaho, 

and Washington—An Intraplate CrustalScale Shear Zone Recurrently Active Since 

the Mesoproterozoic

By J. Michael O’Neill, Edward T. Ruppel, and David A. Lopez

Chapter A of

Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho

and Montana—Studies by the Mineral Resources Program,

U.S. Geological Survey, 2004–2007

Edited by J. Michael O’Neill

Open-File Report 2007–1280–A

U.S. Department of the Interior

U.S. Geological Survey
Contents

Introduction.....................................................................................................................................................3

Great Divide Megashear...............................................................................................................................3

Deadman and Bloody Dick Segments...............................................................................................4

Big Hole Segment .................................................................................................................................6

Painted Rocks Segment.......................................................................................................................8

Clearwater Segment ............................................................................................................................8

Summary..........................................................................................................................................................8

References Cited............................................................................................................................................9

Figures

1. Index map of northern Rocky Mountains and adjacent area showing the lateral

extent of the Great Divide megashear ......................................................................................4

2. Northwest-trending faults in the northern Rocky Mountains, including the trace

of the Great Divide megashear...................................................................................................5

3. Northern Rocky Mountains showing location of the Great Falls tectonic zone and

the left-lateral offset of the Dillon shear zone by the Great Divide megashear.................7
Introduction

A regional set of northwest-trending faults in southwestern Montana comprises more than 30 major subparallel crustal 

fractures that have been active since Mesoproterozoic time. 

Displacements along these faults have differed over time and 

include left-lateral strike-slip, reverse oblique-slip, and normal 

displacement. These faults first moved during the formation 

of the Mesoproterozoic Belt Basin (Winston, 1986; Sims and 

others, 2004), when left-lateral strike-slip faults caused extension, rifting, and subsidence. Along the southern margin of the 

basin these faults have been shown locally to have dip-slip displacement and to have controlled sedimentation patterns during deposition; Belt sedimentary rocks, where they cross these 

faults, display rapid facies and thickness changes (O’Neill and 

others, 1986; Schmidt and Garihan, 1986; O’Neill, 1995).

Neoproterozoic and early Paleozoic normal displacement along early Belt Basin faults has been demonstrated by 

Wooden and others (1978), Ruppel (1985), and Harlan and 

others (1990). Neoproterozoic dikes were intruded along the 

traces of these faults and provided conduits for syngenetic 

base metal mineralization in overlying Belt sedimentary 

rocks (Thorson, 1984). Cambrian rocks in the Tobacco Root 

Mountains and other mountain ranges in southwest Montana 

(Ruppel, 1985) display thinning of basal sands and shallowwater reefs on upthrown blocks.

Late Cretaceous Laramide oblique-slip displacement on 

northwest-trending faults is well documented (Reid, 1957; 

Foose and others, 1961; Swanson, 1970; Schmidt and Hendrix, 1981; Garihan and others, 1982; Schmidt and O’Neill, 

1982; Schmidt and Garihan, 1983; O’Neill and others, 1986; 

Schmidt and Garihan, 1986). Movement on the northwesttrending faults is responsible for 9–16 miles (15–25 kilometers 

(km)) of shortening of the foreland crust of southwest Montana by left-lateral reverse slip (Schmidt and Garihan, 1983).

The most recent movement on northwest-trending faults 

accommodated late Tertiary and Quaternary faulting and 

extension in southwestern Montana (Pardee, 1950; Reynolds, 

1979; Ruppel, 1982; O’Neill and others, 1986; Schmidt and 

Dresser, 1986; Stickney and Bartholomew, 1987). Recurrent 

movement along this older set of northwest-trending faults 

has controlled the location and orientation of basin and range 

block faults and has segmented Late Cretaceous overthrust belt 

structures; present day fault scarps, landslides, and displaced 

alluvial fans document ongoing movement along the traces of 

the older faults.

The Great Divide megashear of Montana and Idaho, 

newly recognized and described here, represents the westernmost fault zone of the regional set of northwest-trending structures in southwest Montana; it is also the most structurally 

complex. This megashear, unlike similar left-lateral strike-slip 

faults of the Rocky Mountain foreland to the east, separates 

two very different geologic terranes. These terranes were 

brought together by strike-slip faulting, rather than by thrust 

faulting as interpreted earlier (Skipp and Hait, 1977; Ruppel, 

1978; Ruppel and Lopez, 1984).

Great Divide Megashear

The Great Divide megashear extends northwest from the 

Snake River Plain into southwest Montana, thence northwestward, east of but parallel to the Continental Divide along the 

Montana-Idaho state line, across the West Fork of the Bitterroot River, and into adjacent Idaho (figs. 1, 2). Between the 

Snake River Plain and the Bitterroot River the megashear is 

a left-lateral strike-slip deformation belt that, at the surface, 

is characterized by a steeply dipping anastomosing array of 

faults, commonly with many bends and left-stepping offsets 

of individual fault strands. The megashear separates geologic 

terranes that have different and characteristic stratigraphic 

sequences, geologic structure, topography, and drainage 

patterns. No formations cross the megashear. Northwest of 

the Montana-Idaho state boundary, along the Continental 

Divide, the megashear merges with the Clearwater zone 

(Sims and others, 2005), interpreted as a deep-seated crustal 

tectonic zone marked by a strong negative linear aeromagnetic anomaly that can be traced across central Idaho into 

east-central Washington and whose geophysical signature is 

similar in size and intensity to that of the Lewis and Clark 

line in west-central Montana. Total length of the combined 

megashear exceeds 500 km. 

Great Divide Megashear, Montana, Idaho, and 

Washington—An Intraplate Crustal-Scale Shear Zone 

Recurrently Active Since the Mesoproterozoic

By J. Michael O’Neill, Edward T. Ruppel, and David A. Lopez
4 Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho and Montana

In extreme southwest Montana and adjacent Idaho the 

megashear is divided into four interconnected segments, two 

of which are left stepping (fig. 2). The Deadman and Bloody 

Dick segments are well exposed and include some individual 

faults that have been mapped previously. The Big Hole 

segment is concealed beneath Tertiary sediments in the Big 

Hole Basin; and the Painted Rocks segment, although well 

exposed, has not been recognized until now. The Clearwater 

segment has been recognized only as a through-going aeromagnetic negative anomaly (Sims and others, 2005).

Deadman and Bloody Dick Segments

The southernmost part of the megashear, the Deadman 

segment, incorporates some faults mapped by others along its 

trace, including the original Deadman fault of Scholten and 

others (1955), in a complex of steeply dipping, interlacing 

strike-slip faults. The Deadman segment trends northwest from 

the Snake River Plain for more than 60 km (fig. 2 map) and is a 

zone of interlacing faults, in places as much as 8 km wide.

The Deadman zone everywhere separates rocks and rock 

sequences of central Idaho from those of southwest Montana. 

West of the Deadman zone, the sequence of sedimentary rocks 

is dominated by thick quartzites and siltites of the Mesoproterozoic Lemhi Group and the Swauger Formation. Above the 

Mesoproterozoic rocks of east-central Idaho, and typically 

faulted against them, are basal Paleozoic formations of the 

Ordovician Summerhouse Formation, the massive white 

quartzite of the Kinnikinic Quartzite, and the dark dolomite of 

the Saturday Mountain Formation.

In southwest Montana Mesoproterozoic sedimentary 

rocks are absent northeast of the Deadman zone in the Tendoy 

Mountains. The basal Paleozoic rocks above the basement 

crystalline rocks of the Archean Wyoming Province are Cambrian Flathead Sandstone and overlying Cambrian, Devonian, 

and younger Paleozoic formations not found southwest of the 

Deadman segment.

The Deadman segment of the megashear steps left along 

east-trending faults on the south side of Horse Prairie Basin, 

creating a small, almost 4.6-km-deep, Tertiary pull-apart 

graben beneath Horse Prairie. The fault zone then continues 

northwest to the south end of the Big Hole Basin where it is 

cut by or concealed beneath detachment faults of early Tertiary 

age (O’Neill and others, 2004).

l l l l l l l l l l l

Rifted 

Continental 

Margin

WASHINGTON

OREGON

MONTANA

IDAHO

Belt 

Supergroup

Lemhi Group 

and

Yellowjacket 

Formation

Great Divide

Megashear

Rifted

Continental

Margin

49°

0 300 MILES

0 400 KILOMETERS

111°

42°

120°

120°

 l l l l l l 

Figure 1. Index map of northern Rocky Mountains and adjacent area showing the 

lateral extent of the Great Divide megashear. The megashear extends northwesterly 

from the Snake River Plain, parallel to the Idaho-Montana state line, gently curves 

west-northwest across central Idaho, and continues into eastern Washington, where it 

follows the trace of the Trans-Idaho discontinuity (modified from Burchfiel and others, 

1992). Rock units in colored areas discussed in text.
Figure 2. Northwest-trending faults in the northern Rocky Mountains, including the trace of the Great Divide megashear (screened band) and fault segments (modified from Sims and others, 2004). BC, Big Creek zone; CZ, Clearwater zone; LC, Lewis and Clark line; SF, Snake fault zone.

l 

l 

l 

 l l 

l

l 

 

l

 l 

 l

 

l

 

CANADA

MON

WYOMING

IDAHO

CZ

LC

SF

BC

0 150 MILES

0 200 KILOMETERS

Deadman

Big Hole

Bloody Dick

HorsePrairie

TendoyMountains

Painted Rock/Bitterroot River

Clearwater

Zone

Orofino #

Rifted

Continental

Margin

l

l

l

l

l

l

l

l

l

l

l

l

l

l

l

l

l

l

Lewis and

Clark

Line

Trans-Idaho Discontinuity

Snake River Plain

42°47°

115° 110° 10
6 Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho and Montana

The originally defined Deadman fault is indeed a down-tothe-west normal fault that follows the trace of the southernmost 

part, the Deadman zone, of the Idaho-Montana megashear. 

Archean rocks east of the normal fault are sheared, cataclasized, and intruded by numerous mafic to silicic dike and vein 

systems in a zone more than 1.5 km wide. Faults and shears in 

these crystalline metamorphic and igneous rocks lie within (or 

along the east side of) the northwest-trending Deadman zone. 

Those faults and shears predate Cenozoic normal faulting and 

postdate the assembly of the Archean metamorphic rocks of the 

Wyoming Province of which they are a part.

The Bloody Dick segment of the megashear, previously 

interpreted to be a thrust fault zone (Coppinger, 1974; Ruppel 

and others, 1993), is a vertical zone of mylonitic rock more 

than 1.5 km wide. Stretching lineations in the mylonite are 

subhorizontal, plunging gently both northwest and southeast. 

This segment of the megashear shows both transpression and 

transtensional structures related to mylonitization and strikeslip displacement. A strongly sheared, 1-km-wide horst of 

Paleoproterozoic and Archean basement rocks remains caught 

between two strands of the fault zone. North of the horst is a 

small, elongate, pull-apart basin now filled with weakly consolidated sedimentary rocks of the Miocene to Eocene Bozeman Group (Coppinger, 1974; Ruppel and others, 1993).

The ages of relatively recent movements (late Tertiary) 

along the Deadman fault are fairly clear, but the original age 

of the fault zone and its earliest movement is interpreted to be 

Mesoproterozoic, coeval with sedimentation of the Belt and 

Lemhi strata. Mesoproterozoic rocks of questionable affinity 

(in segments of the Deadman–Bloody Dick fault zone where it 

steps left at Horse Prairie) suggest initial movement along the 

zone in the Mesoproterozoic. These anomalous rocks north of 

Horse Prairie and east of the Bloody Dick fault segment include 

a thick sequence of feldspathic quartzite that combines lithologic and stratigraphic characteristics of both the Lemhi Group 

and Belt Supergroup (Coppinger, 1974; Tysdal and others, 

2005). We interpret these quartzites as a unique subfacies of the 

Missoula Group rocks—a syntectonic accumulation of clastic 

sediment deposited in a Mesoproterozoic pull-apart basin along 

a left-stepping releasing bend or accommodation zone of the 

megashear formed during Mesoproterozoic basin infilling.

Compelling evidence for large-scale left-lateral strike-slip 

displacement along the Deadman–Bloody Dick segment of the 

shear zone lies in the left-lateral offset of major throughgoing, crustal-scale basement structures. Both Paleoproterozoic 

and Archean rocks underlie the rugged ranges of southwest 

Montana. Exposures of these rocks reveal tectonic components of a Paleoproterozoic collisional orogen that underlies 

the 200-km-wide northeast-trending Great Falls tectonic zone 

(fig. 3) (O’Neill and Lopez, 1985; O’Neill, 1999). This 1.8-Ga 

collisional orogen extends from northeastern Montana to southcentral Idaho and reflects the suturing of the Archean Wyoming 

Province to the Medicine Hat Province of southern Alberta, 

British Columbia, and northwest Montana (O’Neill, 1999). 

The Tendoy Mountains (fig. 2), bounded on the southwest by 

the Deadman fault segment of the megashear, are underlain by 

several large exposures of crystalline basement rocks that are 

part of the Archean Wyoming Province. The Paleoproterozoic 

rocks on the southwest side of the main strand of the Deadman 

fault are not a part of the Archean Wyoming Province; the 

only known Paleoproterozoic rocks in this region comprise the 

accreted terrane within the 1.8-Ga Paleoproterozoic collisional 

orogen that bounds the Wyoming craton on the northwest. 

The northeast-trending Dillon shear zone (fig. 3), the tectonic 

boundary between cratonic and accreted terranes, lies 50 km 

northwest of this part of the Deadman fault zone. Based on the 

juxtaposition of accreted terrane within the collisional orogen 

against Archean cratonic rocks along the northwest-trending 

Deadman–Bloody Dick segment of the megashear, we interpret 

the strike-slip displacement along the megashear to be at least 

50 km of left-lateral offset.

Big Hole Segment

The northwest continuation of the Bloody Dick segment 

of the megashear into the Big Hole Basin is largely concealed 

by more than 5 km of Quaternary and Tertiary sedimentary 

and volcanic rock. High-angle normal faults in Quaternary 

deposits along the east side of the Beaverhead Mountains 

(Ruppel and others, 1993) may reflect recurrent movement 

that exploits weakness along the concealed megashear that 

underlies Big Hole Basin fill.

Faults related to the megashear are exposed in the Big 

Hole Pass area, northwest of the Big Hole Basin; in this area 

the subhorizontal detachment fault of the Chief Joseph metamorphic core complex (Ruppel and O’Neill, 2003; O’Neill 

and others, 2004) is exposed (Lopez and others, 2005) as are 

rocks beneath the detachment. Mesoproterozoic strata beneath 

the detachment can be divided into two groups that are juxtaposed along the Dahlonega fault (fig. 2), a broad, irregular, 

east-trending fault zone. Rocks found only in Idaho, the Yellowjacket and middle Lemhi Group strata, are exposed south 

of the fault zone, but north of the fault zone white to light-gray 

feldspathic quartzites of uncertain affinity crop out (Lopez, 

1982). These quartzites were mapped as Lemhi Group undifferentiated by Ruppel and others (1993) on the Dillon 2 degree 

quadrangle and more recently as undifferentiated Mesoproterozoic quartzite by Evans and Green (2003). Like the 

quartzite that underlies the left-stepping accommodation zone 

at Horse Prairie, rocks in the Big Hole Pass area represent a 

unique stratigraphic sequence that shares strong similarities 

with both Belt and Lemhi strata. One aspect of its uniqueness 

is the percentage of feldspar in these quartzites: these rocks 

carry both the “Belt” potassium feldspar suite intermixed with 

the “Lemhi” plagioclase suite (David Lindsey, U.S. Geological Survey, oral commun., 2007) These perplexing strata are 

restricted to the area north of a left-stepping accommodation 

zone; we interpret them to represent a hybrid Mesoproterozoic 

assemblage deposited in a pull-apart graben along a releasing 

bend of the Great Divide megashear.
Figure 3. Northern Rocky Mountains showing location of the Great Falls tectonic zone (screened) and the left-lateral offset of the Dilzone by the Great Divide megashear; Precambrian-cored basement uplifts are shaded tan (modified from Sims and others, 2004). ScreeGreat Falls tectonic zone.

Beartooth

Mountains

BighornMountains

Wind River

Mountains

FrontRange

SawatchRange

MONTANA

Colorado

Plateau

Uinta Mountains

Rocky Mountains

Outline of figure 2

Trans-Montana fold-and-thrust belt WYOMING PROVINCE Little Belt Mountains

Sevier thrust

front

Eastern limit of

Basin and

Range

extension

COLORADO

OROGEN

Dillon shear zone TRANS-HUDSON OROGEN

Western boundary

of continent

Great Falls shear zone

Extended

crust

45°

115° 105°

WYOMING

WASHINGTON

UTAH COLORA

IDAHO

OREGON

NEVADA

Dillon shear zone

Great Divide

megashear

0 300 MILES

0 400 KILOMETERS

40°
8 Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho and Montana

Painted Rocks Segment

Previous geologic investigations in the headwaters region 

of the West Fork of the Bitterroot River (fig. 2) did not recognize any strike-slip faulting (Berg, 1977; Fisk, 1969; Lund and 

others (1983). With the exception of Berg (1977), geologic 

maps of other reports did not distinguish between the various 

quartzites within the area. The Painted Rocks fault segment is 

well exposed only in the area of Painted Rocks Lake, where it 

separates Mesoproterozoic strata characteristic of the Lemhi 

Group rocks of Idaho on the southwest from Missoula Group 

quartzite and siltite on the northeast. The southeastern part 

of the fault places thick-bedded, light-gray quartzite of the 

Gunsight Formation of the Lemhi Group against middle and 

upper (?) Mount Shields Formation of the Belt Supergroup. 

Northwestward along the fault zone, the Gunsight and overlying Swauger formations are in fault contact with the Mount 

Shields Formation.

The Painted Rocks fault zone is 2–3 km wide; it records 

shearing, transposition, intrusion, and mylonitization of Mesoproterozoic sedimentary rocks. The rocks within the zone 

show variable amounts of shearing and subhorizontal stretching lineations on bedding plane surfaces and on shear surfaces. 

Intensity of shearing and mylonitization increases toward 

the central part of the shear zone. Shear zones are vertical 

near the middle of the zone and are more widely spaced and 

curviplanar away from the middle, where rock between the 

faults has been rotated and crushed.

Clearwater Segment

Most of the northwest-striking faults of the northern 

Rocky Mountains are coincident with narrow negative aeromagnetic anomalies (Sims and others, 2004; Sims and others, 

2005). In addition to these correlative anomalies and known 

faults, there are three larger northwest-trending negative 

magnetic anomalies, two of which can be correlated with the 

large Lewis and Clark line (fig. 2) of west-central Montana and 

adjacent Idaho and with the Snake River–Wichita fault zone, 

more than 2,000 km long, which originates east of Colorado 

(Sims and others, 2005). A third anomaly, similar in size and 

intensity, was named the Clearwater fault zone (fig. 2) by Sims 

and others (2005); it trends west-northwest across central Idaho 

from the Painted Rocks segment of the megashear to Orofino 

in west-central Idaho. At its west end this aeromagnetic 

anomaly is colinear with the Trans-Idaho discontinuity (fig. 

2), an inferred zone of transverse structures that Yates (1968) 

interpreted to represent a major continental-scale structure that 

offsets and separates sharply contrasting geologic terranes on 

the northeast and southwest. Yates (1968, p. 122) interpreted 

the discontinuity as a “strike-slip fault, or zone of faults, with 

left-lateral movement sense.” The existence and location of 

this discontinuity was more precisely defined by Armstrong 

and others (1977), who mapped it as an abrupt Sr87/Sr86 isotope 

boundary between an island-arc accreted terrane on the southwest and a continental craton on the northeast. They traced this 

isotopic boundary as a narrow north-trending zone in southwest 

Idaho that turns abruptly to the west near Orofino, Idaho; at 

Orofino, the boundary intersects the Trans-Idaho discontinuity 

of Yates (1968) and follows it westward into east-central Washington. Armstrong and others (1977) concurred with Yates that 

the discontinuity must represent a left-lateral transform fault 

that is a major crustal boundary in the northwest cordillera of 

the United States.

Summary

The Great Divide megashear is a regional-scale 

shear zone that can be traced for more than 500 km. It is a 

mappable, mylonitic shear zone as much as 3–8 km wide 

that extends northwestward from the Snake River Plain 

across extreme southwest Montana along and parallel to the 

Montana-Idaho state line, thence west-northwestward to connect with the Clearwater zone of Sims and others (2005) that 

crosses central Idaho, which in turn continues westward as 

the Trans-Idaho discontinuity of Yates (1968) into east-central 

Washington. Yates’ discontinuity shows left-lateral offset of 

the Laurentian continental margin in Idaho and Washington 

on the order of several hundred kilometers; the left-lateral offset of the discontinuity is in the same sense as but greater than 

the observed 50 km minimum left-lateral offset of piercing 

points within the northeast-trending Paleoproterozoic collisional orogen (Great Falls tectonic zone) that is offset by the 

Great Divide megashear adjacent to the Continental Divide in 

extreme southwestern Montana.

The megashear first formed in Mesoproterozoic time and 

is interpreted to have influenced Mesoproterozoic sedimentation in Montana and Idaho. Mesoproterozoic Belt Supergroup 

and Lemhi Group strata are restricted to areas northeast 

and southwest of the megashear, respectively. Correlations 

between the two are difficult and subject to several interpretations; correlations are based on the assumption that the two 

were deposited in the same depositional basin. Workers in 

the region do agree that unique quartzite sedimentary facies 

within or immediately adjacent to the megashear share similar 

sedimentary environments of deposition with both the uppermost Belt and Lemhi strata. Our work has shown that these 

unique facies are restricted to small extensional, left-stepping, 

pull-apart basins that formed along the strike of the Great 

Divide megashear, which everywhere separates Belt and 

Lemhi rocks. The difficulties in correlation between Belt and 

Lemhi stratigraphic successions across the megashear suggest 

that the Belt and Lemhi Basins were separate depocenters during the majority of basin infilling and that direct correlations 

between the two are not possible. The youngest rocks, the 

Mount Shields Formation of the Missoula Group to the north 

and northeast and the Swauger and Lawson Creek Formations 

overlying the Lemhi Group to the southwest, are interpreted to 

have interfingered across the megashear during the later stages 

of deposition and basin infilling.
Great Divide Megashear, Montana, Idaho, and Washington 9

References Cited

Armstrong, R.L., Taubeneck, W.H., and Hales, P.O., 1977, 

Rb-Sr and K-Ar geochronometry of Mesozoic granitic rocks 

and their isotopic composition, Oregon, Washington, and 

Idaho: Geological Society of America Bulletin, v. 88,

p. 397–411.

Berg, R.B., 1977, Reconnaissance geology of southernmost 

Ravalli County, Montana: Montana Bureau of Mines and 

Geology Memoir 44, 39 p.

Burchfiel, B.C., Cowan, D.S., and Davis, G.A., 1992, Tectonic 

overview of the Cordilleran orogen in the western United 

States, in Burchfiel, B.C., Lipman, P.W., and Zoback, M.L., 

eds., The Cordilleran orogen–Conterminous U.S.: Geological Society of America, The Geology of North America, 

v. G–3, Decade of North American Geology, p. 407– 480.

Coppinger, Walter, 1974, Stratigraphic and structural study 

of Belt Supergroup and associated rocks in a portion of the 

Beaverhead Mountains, southwest Montana and east-central 

Idaho: Oxford, Ohio, Miami University, Ph.D. dissertation, 

224 p.

Evans, K.V., and Green, G.N., 2003, Geologic map of the 

Salmon National Forest and vicinity, Idaho: U.S. Geological 

Survey Geologic Investigations Series Map I–2765, scale 

1:100,000.

Fisk, H.G., 1969, Painted Rocks Lake area, southern Ravalli 

County, Montana: Montana Bureau of Mines and Geology 

Special Publication 47, scale 1:48,000.

Foose, R.M., Wise, D.V., and Garbarini, G.S., 1961, Structural 

geology of the Beartooth Mountains, Montana and Wyoming: Geological Society of America Bulletin, v. 72,

p. 1123–1172.

Garihan, J.M., Schmidt, C.J., and Karasevich, L.P., 1982, 

Road log for the Ruby Range, part of the Highland Range, 

and adjacent intermontane basins, southwest Montana, 

with emphasis on recurrent history, in Beaver, Patricia, ed., 

Tobacco Root Geological Society, 7th annual field conference, Guidebook, p. 45–68.

Harlan, S.S., Snee, L.W., and Geissman, J.W., 1990, Paleomagnetic and 40Ar/39Ar results from mafic dikes and 

basement rocks, southwest Montana: EOS: Transactions of 

American Geophysical Union, v. 71, p. 1297.

Lopez, D.A., 1982, Reconnaissance geologic map of the Gibbonsville quadrangle, Lemhi County, Idaho and Beaverhead 

County, Montana: U.S. Geological Survey Miscellaneous 

Field Studies Map MF–1446, scale 1:24,000.

Lopez, D.A., O’Neill, J.M., and Ruppel, E.T., 2005, Preliminary geologic map of the Big Hole Pass–Lost Trail Pass 

area, southwestern Montana: Montana Bureau of Mines and 

Geology Open-File Report 522, scale 1:24,000.

Lund, Karen, Rehn, W.M., and Holloway, C.D., 1983, Geologic map of the Blue Joint Wilderness study area, Ravalli 

County, Montana, and the Blue Joint Roadless Area, Lemhi 

County, Idaho: U.S. Geological Survey Miscellaneous Field 

Investigations Map MF–1557–B, scale 1:50,000.

O’Neill, J.M., 1995, Table Mountain Quartzite and Moose Formation (new names) and associated rocks of the Middle Proterozoic Belt Supergroup, Highland Mountains, southwestern 

Montana: U.S. Geological Survey Bulletin 2121–A, 26 p.

O’Neill, J.M., 1999, The Great Falls tectonic zone, Idaho and 

Montana—An early Proterozoic collisional orogen beneath 

and south of the Belt Basin, in Berg, R.B., Belt Symposium 

III: Montana Bureau of Mines and Geology Special Publication 112, p. 222–228.

O’Neill, J.M., Ferris, D.C., Schmidt, C.J., and Hanneman, 

D.L., 1986, Recurrent movement along northwest-trending 

faults at the southern margin of the Belt Basin, Highland 

Mountains, southwestern Montana, in Roberts, S.M., ed., 

Belt Supergroup: Montana Bureau of Mines and Geology 

Special Publication 94, p. 208–216.

O’Neill, J.M., and Lopez, D.A., 1985, Character and regional 

significance of the Great Falls tectonic zone, east-central 

Idaho and west-central Montana: American Association of 

Petroleum Geologists Bulletin, v. 69, 437–447.

O’Neill, J.M., Lopez, D.A., and Ruppel, E.T., 2004, Newly 

recognized Chief Joseph metamorphic core complex and 

younger structures in the north-central Beaverhead Mountains, Idaho and Montana—Expanding the temporal limits 

of post-contractional extension in the northern Rocky 

Mountains [abs.]: Geological Society of America Abstracts 

with Program, v. 37, no. 6, p. 4.

Pardee, J.T., 1950, Late Cenozoic block faulting in western 

Montana: Geological Society of America Bulletin, v. 61,

p. 359–406.

Reid, R.R., 1957, Bedrock geology of the north end of the 

Tobacco Root Mountains, Madison County, Montana: Montana Bureau of Mines and Geology Memoir 36, 27 p.

Reynolds, M.W., 1979, Character and extent of Basin-Range 

faulting, western Montana and east-central Idaho, in Newman, G.W., and Goode, H.D., eds., RMAG/UGA Basin 

and Range Symposium and Great Basin Field Conference, Rocky Mountain Association of Geologists and Utah 

Geological Association, p. 185–193. [available from Utah 

Geological Association]

Ruppel, E.T., 1978, Medicine Lodge thrust system, east-central Idaho and southwest Montana: U.S. Geological Survey 

Professional Paper 1031, 23 p.

Ruppel, E.T., 1982, Cenozoic block uplifts in east-central 

Idaho and southwest Montana: U.S. Geological Survey 

Professional Paper 1224, 24 p.
10 Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho and Montana

Ruppel, E.T., 1985, The association of Middle Cambrian rocks 

and gold deposits in southwestern Montana: U.S. Geological Survey Open-File Report 85–207, 26 p.

Ruppel, E.T., and Lopez, D.A., 1984, The thrust belt in 

southwest Montana and east-central Idaho: U.S. Geological 

Survey Professional Paper 1278, 41 p.

Ruppel, E.T., and O’Neill, J.M., 2003, Early Tertiary detachment faulting in southwest Montana and east-central Idaho 

[abs.]: Northwest Geology, v. 32, p. 222–223.

Ruppel, E.T., O’Neill, J.M., and Lopez, D.A., 1993, Geologic 

map of the Dillon 1x2 degree quadrangle, Idaho and Montana: U.S. Geological Survey Miscellaneous Field Investigations Series Map I–1803–H, scale 1:250,000.

Schmidt, C.J., and Dresser, H.W., 1986, Late Cenozoic structural inversion in a region of small extensional strain, southwestern Montana: Geological Society of America Abstracts 

with Program, v. 16, no. 6, p. 646–647.

Schmidt, C.J., and Garihan, J.M., 1983, Laramide tectonic 

development of the Rocky Mountain foreland of southwestern Montana, in Lowell, J.D., and Gries, R., eds., Rocky 

Mountain foreland basins and uplifts: Rocky Mountain 

Association of Geologists Symposium, p. 271–294.

Schmidt, C.J., and Garihan, J.M., 1986, Middle Proterozoic 

and Laramide tectonic activity along the southern margin 

of the Belt Basin, in Roberts, S.M., ed., Belt Supergroup: 

Montana Bureau of Mines and Geology Special Publication 

94, p. 217–236.

Schmidt, C.J., and Hendrix, T.E., 1981, Tectonic controls for 

thrust belt and Rocky Mountain foreland structures in the 

northern Tobacco Root Mountains–Jefferson Canyon area, 

southwestern Montana, in Tucker, T.E., ed., SW Montana: 

Montana Geological Society, 1981 field conference and 

symposium, Guidebook, p. 167–180.

Schmidt, C.J., and O’Neill, J.M., 1982, Structural evolution 

of the southwest Montana transverse zone, in Powers, R.B., 

ed., Geologic studies of the Cordilleran thrust belt: Rocky 

Mountain Association of Petroleum Geologists, p. 167–180.

Scholten, Robert, Keenmon, K.A., and Kupsch, W.O., 1955, 

Geology of the Lima region, southwestern Montana and 

adjacent Idaho: Geological Society of America Bulletin, 

v. 66, p. 345–404.

Sims, P.K., Lund, Karen, and Anderson, E., 2005, Precambrian crystalline basement map of Idaho—An interpretation 

of aeromagnetic data: U.S. Geological Survey Scientific 

Investigations Map 2884, scale 1:1,000,000.

Sims, P.K., O’Neill, J.M., Bankey, Viki, and Anderson, 

E., 2004, Precambrian basement geologic map of Montana—An interpretation of aeromagnetic anomaly map: 

U.S. Geological Survey Scientific Investigations Map 2829, 

scale 1:1,000,000.

Skipp, Betty, and Hait, M.H., Jr., 1977, Allochthons along the 

northwest margin of the Snake River Plain, Idaho: Wyoming Geological Association, 29th annual field conference, 

Guidebook, p. 499–515.

Stickney, M.C., and Bartholomew, M.J., 1987, Seismicity and 

late Quaternary faulting of the northern Basin and Range 

Province, Montana and Idaho: Seismological Society of 

America Bulletin, v. 77, p. 1602–1625.

Swanson, R.W., 1970, Mineral resources of Permian rocks of 

southwest Montana: U.S. Geological Survey Professional 

Paper 313–E, p. 661–777.

Thorson, J.P., 1984, Suggested revisions of the lower Belt 

Supergroup stratigraphy in the Highland Mountains, southwestern Montana, in Hobbs, W.S., ed., The Belt: Montana 

Bureau of Mines and Geology Special Publication 90, 

p. 10–12.

Tysdal, R.G., Lindsey, D.A., Lund, K.I., and Winkler, G.R., 

2005, Alluvial facies, paleocurrents, and source of the 

Mesoproterozoic Gunsight Formation, east-central Idaho 

and southwestern Montana, chap. B, in O’Neill, J.M., and 

Tysdal, R.G., eds., Stratigraphic studies in southwestern 

Montana and adjacent Idaho—Lower Tertiary Anaconda 

Conglomerate and Mesoproterozoic Gunsight Formation: 

U.S. Geological Survey Professional Paper 1700–B, 

p. 21–39.

Winston, Don, 1986, Middle Proterozoic tectonics of the Belt 

Basin, western Montana and northern Idaho, in Roberts, 

S.M., ed., Belt Supergroup—A guide to the Proterozoic 

rocks of western Montana and adjacent areas: Montana 

Bureau of Mines and Geology Special Publication 94, 

p. 245–257.

Wooden, J.L., Vitaliano, C.J., Koehler, S.W., and Ragland, 

P.C., 1978, The late Precambrian mafic dikes of the southern 

Tobacco Root Mountains, Montana—Geochemistry, Rb-Sr 

geochronology, and relationships to Belt tectonics: Canadian 

Journal of Earth Science, v. 15. p. 467–479.

Yates, R.G., 1968, The Trans-Idaho discontinuity: International Geological Congress, 23rd, Prague, 1968, Proceedings, v. 1, p. 117–123.
Blackbird Fe-Cu-Co-Au-REE Deposits

By Arthur A. Bookstrom, Craig A. Johnson, Gary P. Landis, and Thomas P. Frost

Chapter B of

Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho

and Montana—Studies by the Mineral Resources Program,

U.S. Geological Survey, 2004–2007

Edited by J. Michael O’Neill

Open-File Report 2007–1280–B

U.S. Department of the Interior

U.S. Geological Survey
Contents

Introduction...................................................................................................................................................13

Mineral Deposit Models .............................................................................................................................16

Postmetamorphic Epigenetic Deposit Model ................................................................................16

Synsedimentary, Submarine Volcanic-Related Deposit Model..................................................16

Syngenetic Versus Epigenetic Mineralization ...............................................................................16

Geologic Setting...........................................................................................................................................16

Metasedimentary Strata....................................................................................................................16

Igneous Rocks.....................................................................................................................................16

Stratabound Deposits..................................................................................................................................17

In Biotite Rock .....................................................................................................................................17

In Silica Rock.......................................................................................................................................17

In Disrupted Strata..............................................................................................................................18

Other Types of Deposits..............................................................................................................................18

Vein Quartz...........................................................................................................................................18

Discordant Uncle Sam and South Idaho Ore Zones .....................................................................18

Late Discordant Dandy Zone ............................................................................................................18

Tourmaline Breccias ..........................................................................................................................18

Late Barren Quartz Veins...................................................................................................................19

Sulfur Isotope Data .....................................................................................................................................19

History of Mineralization, Deformation, and Metamorphism ...............................................................19

Acknowledgments.......................................................................................................................................19

References Cited..........................................................................................................................................19

Figures

1. Index map showing location of Blackbird mine in east-central Idaho..............................13

2. Blackbird mine and surrounding area, showing distribution of geologic units ...............14
Introduction

The Blackbird deposits of iron (Fe)-, copper (Cu)-, cobalt 

(Co)-, gold (Au)-, and rare-earth-element (REE)-bearing minerals are in east-central Idaho, about 34 kilometers (km) west 

of Salmon, Idaho; they lie near the midpoint of the Idaho cobalt 

belt, which extends at least 50 km N. 40°–45°W. across the 

Salmon River Mountains (fig. 1). Most mines and prospects are 

within or near the banded siltite unit of the Mesoproterozoic 

Apple Creek Formation, which crops out between northweststriking thrust faults of Cretaceous age (Slack, fig. 1, this 

volume).

The Blackbird mine complex consists of extensive underground workings on the Dandy, Idaho, Chicago, and Brown 

Bear ore zones and an open pit on the Blacktail ore zone (fig. 2). 

Past production from the Blackbird mine complex amounts to 

approximately 1.7 million metric tonnes (Mt) of high-sulfide 

Co-Cu ore, mined mostly between 1945 and 1966, from which 

about 10 thousand metric tonnes (kt) of Co and 32 kt of Cu 

were recovered (E.A. Magill, U.S. Bureau of Mines, unpub. 

data, 1966; Hughes, 1990). Known resources, including those 

of the Ram zone, discovered in 1994–95 by Formation Capital 

Corporation, are approximately 11.7 Mt of ore containing 77 

kt of Co and 120 kt of Cu at average grades of 0.7 percent Co 

and 1.1 percent Cu. About 3.7 Mt of ore in the Ram zone also 

contain about 2 metric tonnes (t) of Au at an average grade of 

0.55 g Au/t (Formation Capital Corporation, 2007). Most major 

ore zones extend below mining depths of 100 to 200 m, and 

exploration drilling indicates that ore zones continue downward 

to more than 100 m below that. On the basis of this and other 

information, it has been suggested that the Blackbird district 

may have an ultimate potential of about 50 Mt of ore (G.H. 

Hughes, Formation Capital Corporation, 2007). John Slack 

(2006) recently discovered locally abundant monazite and xenotime in cobaltite-rich ores and suggested that these minerals 

could be recovered for their light and heavy REE. 

Past production came mostly from massive to sulfide-rich 

and sulfide-arsenide-rich ores near the center of the Blackbird 

mine complex. Mine workings in the oxidizing vadose zone 

were backfilled with tailings containing iron-bearing sulfides. 

Continuing interaction of vadose water with these oxidizing 

tailings produces acidic mine-drainage waters with thousands 

of milligrams per liter of copper and sulfate and hundreds of 

milligrams per liter of iron and cobalt (Evans and others, 1995). 

Mine-drainage water is now being captured and treated to raise 

pH and remove dissolved metals. Proposed production would 

come from northern, relatively low-sulfide ore zones. Nearly 

all metal sulfides would be recovered, and the sand fraction of 

tailings would be placed underground as paste (Joe Scheuering, 

Formation Capital Corporation, oral commun., 2007).

Blackbird Fe-Cu-Co-Au-REE Deposits

By Arthur A. Bookstrom, Craig A. Johnson, Gary P. Landis, and Thomas P. Frost

Figure 1. Location of Blackbird mine in east-central Idaho.

Figure 2 (following page). Blackbird mine and surrounding area, 

showing distribution of geologic map units, garnet-bearing rocks, 

ore zones, and selected structural features, mines, prospects, and 

streams (after Vhay, 1948; Noranda Exploration, Inc., unpub. data, 

1985; Nash and Hahn, 1989; and Formation Capital Corporation, 

unpub. data, 2005). 

114°30' 114° 113°30' 

45°30' 

45°

44°30' 

0 

0 

MONTANA 

Area of 

this figure 

IDAHO 

IDAHO

93 

93 

28 

LOST 

SALMON 

RIVER 

MOUNTAINS 

Gunsight 

Peak 

Blackbird

Mine

Poison 

Peak 

LEMH I 

RANGE

MOU NT AINS 

BEAVERHEAD 

LEMHI 

VALLEY 

RIVER 

CLEARWATER 

MOUNTAINS 

River 

Panth er Creek 

Pahsimer oi River

Lemhi

H a yden

River 

Cr eek 

DeepCr 

S a lmon Fk 

N 

Salmon 

Yellow 

Lake 

Salmon 

River 

Challis 

Salmon 

North Fork 

Yellowjacket 

Leadore 

Cobalt 

Lemhi 

MILES 15 10 5 

10 20 KILOMETERS 

RANGE 

RIVER 

River 
14 Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho and Montana

Yg

Yag1

Ygr

Yab'

Yag2

Yab

Yag3

Yab'

Yab' 

Yab

Yag1 

Yab'

Yab

Little

Deer Cr.

Ygr

Td

Yd

1

4

5

7

8

6

2

9

3

10

11

12

1

1

1

1

1 1

1 1 1

b

e

d

c

a

f

BlackbirdCr.

Meadow Cr.

Big Deer Cr.

White

Ledge

shear Zone

Lookout 

structural 

block

Blackbird structural block

Haynes-Stellite

structural block

Meadow Cr. fault

Hawkeye Gulch fault

Slippery Cr. fault

114°20'W

45°7'30"N

45°10'N

0 500 1,000

Meters

1

1:30,000
Blackbird Fe-Cu-Co-Au-REE Deposits 15

EXPLANATION

 1. Blacktail

 2. Brown Bear

 3. Chicago

 4. Dandy

 5. Haynes-Stellite

 6. Idaho

 7. Merle

 8. Northfield

 9. Ram

10. St. Joe

11. Sunshine

12. Uncle Sam

Td

Yab''

Yd

Ygr

Yg

Yab

Yab'

Yag

Tertiary igneous rocks

Dikes, felsic

Mesoproterozoic igneous rocks

Granite, gneissic (1.37 Ga)

Dikes, mafic

Mesoproterozoic metasedimentary strata

Garnet-bearing rocks +/- chloritoid +/- cordierite

Gunsight Formation - Metasandstone and minor metashale

Apple Creek Formation and (or) Gunsight Formation 

Yag3 unit 3 -- Schistose metasandstone and minor schist

Yag2 unit 2 -- Banded schistose metasandstone and schist

Yag1 unit 1 -- Biotitic metasandstone

Apple Creek Formation, banded-siltite unit 

-- meta-siltite and meta-argillite

Banded siltite with many interlayers of biotite rock

Banded siltite with few interlayers of biotite rock

Adit

Ore Zones 

Ore-Zone Types

(prospect) 

1

Stratabound in silica-rock sequence

Stratabound in disrupted sequence

Stratabound in biotite-rock sequence

Stratabound and discordant

 Discordant, shear-zone hosted

Discordant, massive-sulfide breccia

Tourmaline breccia

(ore zone)

Structural Features

Contact

Normal fault -- Bar and ball

on downthrown side

Strike and dip

of bedding

Axis of syncline

Axis of anticline

Cultural Features

Shaft

Mine Workings

a Blackbird (6850 ft elev.)

Calera (7100 ft elev.)

Brown-Bear shaft

Blacktail pit

e New Hawkeye (7080 ft elev.)

d

c

b

f Tinkers Pride prospect

Strike-slip fault,

right-lateral offset

Streams

Perennial

Ephemeral

Open pit
16 Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho and Montana

Mineral Deposit Models

Postmetamorphic Epigenetic Deposit Model

Vhay (1948) mapped and described Blackbird ore zones 

as discontinuous veins and lenses in northwest-striking 

shear zones with moderate northeast dips and in north- to 

northeast-striking shear zones with steep dips. He also noted 

that some ore lenses plunge north, parallel to north-plunging 

folds. Anderson (1947), Vhay (1948), and Bennett (1977) 

interpreted the Blackbird deposits as postmetamorphic, 

epigenetic, hydrothermal vein and replacement deposits 

controlled by preore folds and shear zones. They suggested 

that ore-bearing hydrothermal fluids emanated from a nearby 

granite batholith, a part of which is shown in figure 2. They 

considered the batholith to be part of the Cretaceous Idaho 

batholith, which implied a Cretaceous age for the supposedly 

related Blackbird ores.

Synsedimentary, Submarine Volcanic-Related 

Deposit Model

Hughes (1983) recognized that northwest-striking, 

moderately dipping ore zones of the Blackbird mine area 

are stratabound in sequences of biotite-rich beds (fig. 2). 

He noted that the biotite-rich beds, which host much of 

the stratabound ore, are chemically similar to dikes of a 

coextensive swarm of mafic dikes, and he suggested that 

the biotite-rich rocks are metamorphosed submarine mafic 

tuffs. He described the Blackbird deposits as synsedimentary 

hydrothermal deposits, related to submarine volcanic activity 

along the northwest-trending axis of a tectonically active 

rift that was simultaneously receiving siliciclastic turbidites 

from its margins. This model was adopted and elaborated 

by Modreski (1985), Earhart (1986), Eiseman (1988), Nash 

(1989a), Nash and Hahn (1989), Hughes (1990), Nold 

(1990), Nash and Connor (1993), and Bending and Scales 

(2001).

Syngenetic Versus Epigenetic Mineralization

Eiseman (1988) cited cobaltite concentrations along 

bedding planes in silica-rich strata as evidence for syngenetic cobaltite mineralization by chemical-exhalative precipitation. However, Jerry Zieg (NovaGold Resources, Inc., 

oral commun., 2004) cited the lack of finely laminated 

beds of massive sulfides as evidence that most Blackbird 

ores formed not by chemical-exhalative precipitation 

from the water column but rather by selective epigenetic 

replacement of favorable host strata. Nash (1989a), and 

Nash and Hahn (1989) cited concentrations of cobaltite 

and chalcopyrite in disrupted strata as evidence of epigenetic mineralization of strata that were disrupted during 

burial and solidification.

Geologic Setting

The fault-bounded Blackbird structural block of Vhay 

(1948) contains the pervasively folded and sheared metasedimentary strata that host the stratabound and discordant ore 

zones of the Blackbird mine area (fig. 2). Host strata historically were assigned to the Yellowjacket Formation but are 

now assigned to the banded siltite unit of the Apple Creek 

Formation, which consists of about 2,000 m of gray siltite and 

argillite. In the Blackbird mine area, siltite layers commonly 

retain sedimentary structures, but argillitic layers generally are 

metamorphosed to biotitic phyllite or schist.

Metasedimentary Strata

Most of the 2,000-m-thick banded siltite unit consists of 

interlaminated siltite and dark-gray argillite. About 4,000 m of 

relatively thick-bedded quartzite, siltite, and gray argillite of 

the Gunsight Formation overlie the banded siltite unit, which 

is underlain about 8,000 m of gray siltite, argillite, quartzite, 

local diamictite, and minor calc-silicate rock of the Apple 

Creek, Hoodoo, and Yellowjacket Formations (Evans and 

Green, 2003). Correlation of this package of units with the 

upper Piegan Group and the presence in these formations of 

detrital zircons dated about 1450 Ma require very rapid deposition of these strata in about 10 m.y., between about 1455 and 

1445 Ma (Link and others, 2007). 

In the trough of a north-plunging syncline north of the 

Blackbird mine area, metasiltite and phyllite of the bandedsiltite unit are overlain by biotitic metasandstone, banded 

schistose metasandstone with subordinate garnet-schist 

interlayers, and thick-bedded schistose metasandstone with 

subordinate garnet-biotite schist interlayers (fig. 2). Evans and 

Green (2006) tentatively assigned these strata to the Apple 

Creek Formation, undivided, but we suggest that they are correlative with the Gunsight Formation, because they are sandy 

and they overlie banded siltite (fig. 2).

Igneous Rocks

Metamorphosed mafic dikes are common in and around 

the Blackbird mine area, but only a few of the larger dikes 

crop out. Vhay (1948) mapped many mafic dikes underground. Most cut ore zones, but some are cut by mineralized 

shear zones in the Uncle Sam zone (fig. 2). Some mafic dikes 

contain albite ± sodic amphibole, indicative of spilitization, 

which implies that some mafic dikes were emplaced into wet 

sediments saturated with saline connate water. Thin dikes and 

chilled margins of thicker dikes are commonly altered and 

metamorphosed to biotite phyllite or schist, similar to that of 

biotite-rock layers. Nash and Hahn (1989) noted that mafic 

dikes are compositionally similar to biotite-rock layers, and 

they suggested that mafic dikes represent subvolcanic mafic 

magmas that erupted to supply mafic tuff to biotite-rock 
Blackbird Fe-Cu-Co-Au-REE Deposits 17

layers, provided heat to drive the Blackbird submarine hydrothermal system, and may have contributed copper and cobalt 

to the hydrothermal system.

A large pluton of Mesoproterozoic granite and augen 

gneiss (1.37 Ga) with an associated mafic intrusive complex 

postdates the development of folds in its host metasedimentary strata north and east of the Idaho cobalt belt (Evans and 

Zartman, 1990; Doughty and Chamberlain, 1996; Evans and 

Green, 2003). Figure 2 shows a part of this granite pluton 

north of the Blackbird mine. Intrusions of OrdovicianCambrian, Cretaceous, and Eocene ages and volcanic rocks 

of Eocene age also crop out in the Salmon River Mountains 

(Evans and Green, 2003).

Stratabound Deposits

In Biotite Rock

Several major stratabound ore zones of the Blackbird 

mine area are hosted in sets of multiple biotite-rock layers, 

each less than about 30 centimeters (cm) thick, interlayered 

with banded siltite (fig. 2). Biotite rock consists mostly of 

iron-rich biotite (annite), which also is also rich in potassium 

and chlorine (Nash, 1989b). In addition, biotite rock contains 

variably minor amounts of quartz, albite, and microcline; trace 

amounts of apatite, zoisite, tourmaline, ilmenite, and zircon; 

and locally abundant scapolite spherules (Connor and others, 

1991; Nash and Connor, 1993). Much of the quartz in biotiterock layers is in subordinate beds of granular mosaic quartz, 

similar to that of the silica-rock layers that host the Sunshine 

and Ram zones.

Hughes (1993) interpreted biotite-rock layers as mafic 

tuffs rather than mafic sills, because they tend to have somewhat gradational contacts with overlying and underlying 

banded siltite. Nash (1989b) suggested that biotite rock 

formed by reaction of mafic volcaniclastic strata with 

potassium- and chlorine-rich fluid at temperatures appropriate for biotite formation. Nash and Hahn (1989) interpreted 

biotite rock as a metamorphosed hybrid of mafic tuff, clastic 

sediments, and hydrothermal constituents. Nash and Connor 

(1993) interpreted biotite rock as metamorphosed chemical 

sedimentary silicate-facies iron formation, rich in potassium 

and chlorine.

Zircon, ilmenite, and tourmaline grains in biotite rock 

probably are detrital. Nash (1989b) interpreted locally 

abundant spherules of sodium- and chlorine-rich scapolite 

as evidence for highly saline pore water during burial and 

lithification. Biotite rock is locally silicified or tourmalinitized (or both). Biotite rock that hosts stratabound ore zones 

contains cobaltite, chalcopyrite, and pyrite. Postore garnet, 

chloritoid, muscovite, and chlorite are locally abundant in 

metamorphosed biotite rock. Thermally metamorphosed 

biotite rock consists of randomly oriented biotite platelets, but 

dynamothermally metamorphosed biotite rock is phyllitic to 

schistose. 

Cobaltite-rich ore in biotite phyllite from the Brown Bear 

and Blacktail zones (fig. 2) is pervasively folded and sheared. 

In one ore sample, rod-like concentrations of cobaltite parallel 

fold axes, but on a face cut perpendicular to fold axes, cobaltite 

grains are equant and appear to be stacked perpendicular to 

layering. In another sample, cobaltite is concentrated in shear 

bands around augen-shaped clasts of biotite rock and silica 

rock. In an ore sample with incipient phyllonitic fabric, parallel 

layers of semimassive cobaltite are connected by S-shaped 

diagonal cobaltite stringers in the manner of S-C fabric, which 

is generated by intense ductile shearing according to Barker 

(1998).

Ore samples from the stratabound Chicago zone contain 

cobaltite in disseminations, layer-like concentrations, stringers, and pods, bounded by biotite phyllite. Some layer-like 

concentrations of ore minerals contain augen-shaped clasts 

of biotite phyllite, granular mosaic quartz, and vein quartz. 

Some cobaltite rims minor safflorite, and some chalcopyrite is 

interstitial to cobaltite. With increasing proximity to discordant massive sulfide breccias of the Dandy zone, ores hosted 

in biotite-rock strata contain increasing proportions of pyrite, 

pyrrhotite, and marcasite.

In Silica Rock

Silica-rock layers host the stratabound Sunshine and 

Ram ore zones and several stratabound prospects in the upper 

northern and western parts of the Blackbird mine area (fig. 2). 

These layers consist mostly of quartz with a sugary, granularmosaic texture, similar to that of metachert or metamorphosed 

siliceous exhalite and unlike that of metamorphosed feldspathic metasandstone. In silica-hosted ores of the Sunshine 

zone, Eiseman (1988) reported cobaltite, chalcopyrite, pyrite, 

and traces of native gold, silver, bismuth, bismuthinite, and 

millerite. She also described bedding-parallel laminae in 

silica-rock layers, which she interpreted as mineral accumulations on bedding planes. Examples are micaceous laminae 

with streaks of detrital ilmenite and zircon, delicate graphitic 

laminae, and laminae of cobaltite grains. She cited cobaltite 

grains on bedding planes in silica rock as evidence for precipitation and settling of submarine-exhalative hydrothermal 

cobaltite from the water column.

Cobaltite also occurs as sparse to semimassive disseminations in silica rock. In one sample, subrounded lumps of silica 

rock with disseminated cobaltite are surrounded by a matrix 

of silica rock with higher concentrations of disseminated 

cobaltite. This arrangement may indicate that mineralization occurred before and after the silica rock was disrupted. 

Another sample contains cobaltite in layer-parallel streaks and 

also in a zone of chevron-like crumples, which may indicate 

concentration of cobaltite into small-scale fold axes during 

postore dynamothermal metamorphism. Cobaltite inclusions are present in some dark-red iron- and aluminum-rich 

almandine garnets in the Sunshine zone, which indicates that 

cobaltite was deposited before garnets formed during postore 

metamorphism (Eiseman, 1988).
18 Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho and Montana

If biotite rock has a mafic tuff component and silica rock 

is hydrothermal siliceous exhalite, the close association of 

silica rock with biotite rock probably indicates that both are 

related to submarine volcanism. This hypothesis is supported 

by results from analysis of helium trapped in granular mosaic 

silica rock from the Sunshine zone, which yields helium 

isotope ratios indicative of remnant hydrothermal fluid that 

is highly enriched in mantle derived 3He, as is helium from 

modern mid-ocean ridge basalt, as characterized by Ballentine 

and others (2000).

In Disrupted Strata

Mineralized disrupted strata are common in drill core 

from the Blackbird, Blackpine, and Iron Creek areas, and 

disharmonic intrastratal folding, indicative of synsedimentary 

slumping, also is common in the vicinity of mineralized zones. 

Nash (1989a, p. 1) described disruption structures as “characterized by retextured sediment, loss of bedding features, 

lump breccia, and probably bedding-plane slips—all probably 

produced by liquefaction, fluidization, and submarine sliding 

of unconsolidated sediment.” Relatively angular fragments 

in disrupted strata at Blackpine may indicate that disruption 

also occurred in consolidated strata. Nash and Hahn (1989) 

described quartz and siderite in the matrix of cobaltite-bearing 

lump breccias of the Merle zone, and they reported that fluid 

inclusions in such quartz have filling temperatures between 

275° and 375°C and contain halite, sylvite, and unidentified 

daughter minerals, some of which did not dissolve at the filling temperature.

Samples of mineralized disrupted strata from the Merle 

zone contain disseminated cobaltite and chalcopyrite in breccia with lumps of biotite and sugary silica rock in a biotite-rich 

matrix that wraps around the lumps. In such breccia, cobaltite 

is disseminated in lumps and matrix but is concentrated along 

the rims of lumps and in braided stringers between lumps. We 

also recognize similar remnants of disruption features in some 

samples from the stratabound Chicago, Idaho, Brown Bear, 

Blacktail, and Sunshine zones.

Other Types of Deposits

Vein Quartz

Whitish vein quartz with conchoidal fracture and greasy 

luster probably was deposited before, during, and after 

deformation of stratabound ore zones hosted in biotite rock. 

Stratabound ores of the Chicago, Brown Bear, and Blacktail 

zones contain deformed augen-like fragments of vein quartz. 

Semiconcordant pods of vein quartz in the Hawkeye workings have irregular chalcopyrite masses in banded siltite 

along their margins. Pods of vein quartz in the Chicago zone 

cut stratabound cobaltite ore but contain irregular masses of 

chalcopyrite and siderite. Deformed strata are cut locally by 

planar, discordant, steeply dipping quartz veins and veinlets, 

containing cobaltite ± chalcopyrite ± siderite. Such undeformed planar veins and veinlets probably formed after folding and shearing of stratabound ores hosted in biotite-rock 

layers.

Discordant Uncle Sam and South Idaho Ore 

Zones

Mineralized shear zones of the Uncle Sam and South 

Idaho zones strike north to northeast, dip steeply, and crosscut 

bedding in tightly folded metasedimentary strata beneath and 

within the stratabound Idaho zone (fig. 2). These mineralized shear zones host lenses, veins, veinlets, and stringers 

that contain pyrite, chalcopyrite, and cobaltite (Vhay, 1948). 

Alteration envelopes around these mineralized shear zones 

also contain pyrite, chalcopyrite, and cobaltite in silicified and 

chloritized host rocks, consisting mostly of banded siltite and 

disrupted strata (S.G. Peters, U.S. Geological Survey, unpub. 

data, 1980).

Late Discordant Dandy Zone

Dike-like discordant massive sulfide breccias of the 

Dandy zone strike north to northeast, dip steeply, and 

cut the stratabound Chicago and Idaho zones from below 

(fig. 2). The massive sulfide matrix of these breccias, 

which composes about 90 percent of their mass, consists 

of pyrrhotite, pyrite, marcasite, minor chalcopyrite, and 

rare cobaltite. Semirounded clasts of vein quartz and 

biotite-rock compose about 10 percent of the mass of these 

breccias. Some of the biotite-rock clasts contain abundant 

disseminated cobaltite.

Near the intersection of the Calera drift with the Chicago 

zone, a steeply dipping dike or vein of Dandy-type massive 

sulfide breccia cuts stratabound cobaltite ore of the Chicago 

zone. Near the Dandy zone, ores of the Chicago and Idaho 

zones contain abundant pyrite, pyrrhotite, and marcasite, but 

the proportion of these minerals decreases laterally from the 

Dandy zone. Thus it appears that these minerals were added 

to preexisting stratabound cobaltite ores by hydrothermal 

fluids delivered by late massive sulfide breccias of the Dandy 

zone.

Tourmaline Breccias

Tourmaline breccias are composed of black microcrystalline tourmaline (schorl) and clasts of host rocks and vein 

quartz. They occur as discordant pipes and dikes but also as 

locally concordant bodies. Some tourmaline breccias contain local concentrations of finely disseminated cobaltite 

and chalcopyrite. The ore-bearing Haynes-Stellite breccia 

cuts sandy strata of the Gunsight Formation southeast of the 

Blackbird mine (fig. 2).
Blackbird Fe-Cu-Co-Au-REE Deposits 19

Late Barren Quartz Veins

Late barren quartz veins and pods with no associated 

ore minerals or alteration products cut metamorphosed ore 

zones and are widely distributed in siliciclastic country 

rocks (Roberts, 1953). Helium trapped in late vein quartz 

from the Sunshine zone is typical of radiogenic helium from 

continental crust, as characterized by Ballentine and others 

(2000).

Sulfur Isotope Data 

Sulfur isotope data collected for this study indicate that 

Blackbird ore minerals have delta 34SVCDT = 7–9 per mil. 

These isotopic compositions are consistent with an origin in 

a seafloor hydrothermal system, hosted either in sedimentary or volcanic rocks. Sulfides from such systems normally 

have delta 34S values between basaltic sulfur (0 per mil) and 

contemporaneous seawater sulfate (14–18 per mil during the 

Mesoproterozoic) (Strauss and Schrieber, 1990). Such isotopic 

compositions also would be consistent with a deep-seated 

ore-forming system that scavenged sulfur from deeply buried 

sedimentary sequences or from felsic igneous rocks. However, 

they are not consistent with a purely magmatic origin or with 

an origin by bacterial reduction of seawater sulfate.

Sulfur-isotope fractionations between pyrite and 

chalcopyrite are –1.0 to +0.2 per mil in samples from the 

Dandy and Blacktail zones, and fractionations between pyrrhotite and chalcopyrite are about –0.4 per mil in samples 

from the Chicago, Brown Bear, and Blacktail zones. Such 

small fractionations indicate sulfur-isotope mineral equilibration at temperatures higher than normal for seafloor hydrothermal deposits, either volcanogenic or sedimentary exhalative. 

This higher temperature equilibration lends support to the 

hypothesis that much of the stratabound ore formed by epigenetic mineralization of favorable host strata rather than by 

exhalative deposition at the seafloor surface.

History of Mineralization, Deformation, 

and Metamorphism

The present assemblage of stratabound and discordant 

ore deposits of the Blackbird mine area may have resulted 

from metamorphic-hydrothermal effects superimposed on 

submarine-volcanic-related chemical-sedimentary layers 

that were geochemically enriched in the constituents of ore 

and gangue minerals that are concentrated in and around 

layers of biotite rock and silica rock. We continue to gather 

information about the structural geology and geochronology 

of the Blackbird deposits and their host rocks in an effort to 

understand their history of mineralization, deformation, and 

metamorphism.

Acknowledgments

Greg Hahn donated his library of published and unpublished reports on Blackbird and the Idaho cobalt belt. Gordon 

Hughes provided polished ore slabs from each of the major 

Blackbird ore zones and notable prospects, and Mike Zientek 

donated samples from the Dandy zone. Bill Scales and George 

King (Formation Capital Corporation) made drill core available for inspection, discussion, and sampling. Dan Meyers 

(Noranda Inc.) gave permission for underground tours, led by 

Bruce Hull and Brett Riggan. Tom Nash, John Slack, Mike 

O’Neill, Jerry Zieg, Don Winston, Paul Link, Reed Lewis, and 

Virginia Gillerman provided information and insights based on 

their knowledge of the regional stratigraphy, structural geology, and mineral deposits of the Belt Basin. John Wallis digitized figure 2, and James Evans and Douglas Causey provided 

helpful technical reviews.

References Cited

Anderson, A.L., 1947, Cobalt mineralization in the Blackbird 

district, Lemhi County, Idaho: Economic Geology, v. 42, 

p. 22–46.

Ballentine, C.J., Burgess, R., and Marty, B., 2000, Tracing 

fluid origin, transport and interaction in the crust, in Porcelli, D., Ballentine, C.J., Wieler, R., eds., Noble gases in 

geochemistry and cosmochemistry: Reviews in Mineralogy 

and Geochemistry, vol. 48, p. 539–614.

Barker, A.J., 1998, Introduction to metamorphic textures and 

microstructures: Cheltenham, U.K., Stanley Thornes (Publishers), Ltd., 263 p.

Bending, J.S., and Scales, W.G., 2001, New production in the 

Idaho Cobalt belt: a unique metallogenic province: Transactions of the Institution of Mining and Metallurgy (Section 

B), Applied Earth Science, v. 110, p. B81–B87.

Bennett, E.H., 1977, Reconnaissance geology and geochemistry of the Blackbird Mountain–Panther Creek region, 

Lemhi County, Idaho: Idaho Bureau of Mines and Geology 

pamphlet, 167 p.

Connor, J.J., Evans, K.V., Briggs, P.H., Crock, J.G., Hopkins, R.T., Kennedy, K.R., O’Leary, R.M., Roushey, B.H., 

Slaughter, K.E., and Welsch, E.P., 1991, Chemical analyses 

of rock samples collected in east-central Idaho during a 

study of mineralization in the northern Lemhi Range, Lemhi 

County, Idaho: U.S. Geological Survey Open-File Report 

90–0655, 48 p.

Doughty, P.T., and Chamberlain, K.R., 1996, Salmon River arch 

revisited—New evidence for 1370 Ma rifting near the end of 

deposition in the Middle Proterozoic Belt Basin: Canadian 

Journal of Earth Sciences, v. 33, no. 7, p. 1037–1052.
20 Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho and Montana

Earhart, R.L., 1986, Descriptive model of Blackbird Co-Cu, in

Cox, D.P. and Singer, D.A., eds., Mineral deposit models: 

U.S. Geological Survey Bulletin 1693, p. 142.

Eiseman, H.H., 1988, Ore geology of the Sunshine cobalt 

deposit, Blackbird mining district, Idaho: Golden, Colorado 

School of Mines, M.S. thesis, 191 p.

Evans, K.V., and Green, G.N., 2003, Geologic map of the 

Salmon National Forest and vicinity, east-central Idaho: 

U.S. Geological Survey Geologic Investigations Series Map 

I–2765, 2 sheets, scale 1:100,000, pamphlet, 19 p.

Evans, K.V., Nash, J.T., Miller, W.R., Kleinkopf, M.D., 

and Campbell, D.L., 1995, Blackbird Co-Cu deposits, in

du Bray, E.A., ed., Preliminary compilation of descriptive 

geoenvironmental mineral deposit models: U.S. Geological 

Survey Open-File Report 95–831, p. 145–151.

Evans, K.V., and Zartman, R.E., 1990, U-Th-Pb and Rb-Sr 

geochronology of middle Proterozoic granite and augen 

gneiss, Salmon River Mountains, east-central Idaho: Geological Society of America Bulletin, v. 102, p. 63–73.

Formation Capital Corporation, 2007, Grade and tonnage of 

the Ram deposit: Formation Capital website (www.formcap.

com). Accessed Feb 26, 2007.

Hughes, G.H., 1983, Basinal setting of the Idaho cobalt belt, 

Blackbird mining district, Lemhi County, Idaho, in Ranta, 

D.E., Kamilli, R.J., and Pansze, A.G., eds., The Genesis of 

Rocky Mountain Ore Deposits—Changes with Time and 

Tectonics: Denver Region Exploration Geologists Symposium, Nov. 4–5, 1982, Proceedings, p. 21–27.

Hughes, G.H., 1990, The Blackbird mining district, Lemhi 

County, Idaho, in Moye, F.J., ed., Geology and ore deposits 

of the trans-Challis fault system/Great Falls tectonic zone: 

Tobacco Root Geological Society Field Conference, 15th, 

Guidebook, p. 5–29.

Link, P.K., Fanning, M.C., Lund, K.I., and Aleinikoff, J.N., 

2007, Detrital zircon populations and provenance of Mesoproterozoic strata of east-central Idaho, U.S.A.—Correlation 

with Belt Supergroup of southwest Montana, in Proterozoic 

geology of western North America and Siberia: SEPM Special Publication 86, p. 101–128.

Lund, K.J., and Tysdal, R.G., 2007, Stratigraphic and structural setting of sediment-hosted Blackbird gold-cobaltcopper deposits, east-central Idaho, U.S.A., in Proterozoic

geology of western North America and Siberia: SEPM

Special Publication 86, p. 129–147.

Modreski, P.J., 1985, Stratabound cobalt-copper deposits in 

the Middle Proterozoic Yellowjacket Formation in and near 

the Challis quadrangle, in McIntyre, D.H., ed., Symposium 

on the geology and mineral deposits of the Challis 1° x 2° 

quadrangle, Idaho: U.S. Geological Survey Bulletin 1658, 

p. 203–222.

Nash, J.T., 1989a, Geology and geochemistry of synsedimentary cobaltiferous-pyrite deposits, Iron Creek, Lemhi 

County, Idaho: U.S. Geological Survey Bulletin 1882, 33 p.

Nash, J.T., 1989b, Chlorine-rich biotites from the Blackbird 

mining district, Lemhi County, Idaho—Evidence for a 

saline hydrothermal brine: U.S. Geological Survey OpenFile Report 89–445, 25 p.

Nash, J.T., and Connor, J.J., 1993, Iron and chlorine as guides 

to stratiform Cu-Co-Au deposits, Idaho cobalt belt: Mineralium Deposita, v. 28, p. 99–106.

Nash, J.T., and Hahn, G.A., 1989, Stratabound Co-Cu deposits and mafic volcaniclastic rocks in the Blackbird mining 

district, Lemhi County, Idaho, in Boyle, R.W., Brown, A.C., 

Jefferson, C.W., Jowett, E.C., and Kirkham, R.V., eds., 

Sediment-hosted stratiform copper deposits: Geological 

Association of Canada Special Paper 36, p. 339–356.

Nold, J.L., 1990, The Idaho cobalt belt, northwestern United 

States—A metamorphosed Proterozoic exhalative ore district: Mineralium Deposita, v. 25, p. 163–168.

Roberts, W.A., 1953, Metamorphic differentiates in the 

Blackbird mining district, Lemhi County, Idaho: Economic 

Geology, v. 48, p. 447–456.

Slack, J.F., 2006, High REE and Y concentrations in CoCu-Au ores of the Blackbird district, Idaho: Economic 

Geology, v. 101, p. 275–280.

Strauss, Harald, and Schrieber, Jurgen, 1990, A sulfur isotope 

study of pyrite genesis—The Mid-Proterozoic Newland 

Formation, Belt Supergroup, Montana: Geochimica et Cosmochimica Acta, v. 54, p. 197–204.

Vhay, J.S., 1948, Cobalt-copper deposits in the Blackbird 

district, Lemhi County, Idaho: U.S. Geological Survey 

Strategic Minerals Investigation Preliminary Report 3–219, 

26 p., 3 plates. 
Geochemical and Mineralogical Studies 

of Sulfide and Iron Oxide Deposits in the 

Idaho Cobalt Belt

By John F. Slack

Chapter C of

Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho

and Montana—Studies by the Mineral Resources Program,

U.S. Geological Survey, 2004–2007

Edited by J. Michael O’Neill

Open-File Report 2007–1280–C

U.S. Department of the Interior

U.S. Geological Survey
Contents

Introduction...................................................................................................................................................23

The Idaho Cobalt Belt..................................................................................................................................24

Bulk Compositions of Mineralized Samples............................................................................................25

Recent Mineralogical Studies ...................................................................................................................25

Implications For Ore Genesis.....................................................................................................................25

Acknowledgments.......................................................................................................................................27

References Cited..........................................................................................................................................27

Figure

1. Simplified geologic map of the Idaho cobalt belt showing locations of major

sulfide and iron oxide deposits.................................................................................................23
Introduction

This chapter reports progress on geochemical and mineralogical studies of sulfide deposits in the Idaho cobalt belt 

(fig. 1) and continues the reconnaissance work presented in 

Slack (2006). Emphasis is on samples of stratabound CoCu-Au ore and mineralized rock from deposits in the Blackbird mining district, but data for other deposit types in the belt 

are also discussed: stratabound zones of pyrite-rich magnetite 

and massive magnetite in the Iron Creek area, at the southeast 

end of the belt, and a specular hematite vein from the McKim 

Creek prospect in the Lemhi Range southeast of the cobalt 

belt. Important microtextural and paragenetic relationships 

in the Co-Cu-Au samples that have a bearing on ore genesis 

are described, and possible ore deposit models for these CoCu-Au ores are evaluated.

Geochemical and Mineralogical Studies of Sulfide and 

Iron Oxide Deposits in the Idaho Cobalt Belt

By John F. Slack

Figure 1. Simplified geologic map of the Idaho cobalt belt showing the locations of major stratabound 

sulfide deposits and stratabound and discordant iron oxide deposits (modified from Slack, 2006). 

Geology from Tysdal and others (2003) and Lund and Tysdal (2007). Ages of the augen gneiss body and 

the Apple Creek Formation are from Lund and others (2004) and Link and others (2007), respectively. 

Locations of sulfide deposits are from Reed and Herdlick (1947), Nash and Hahn (1989), and Bookstrom 

and others (this volume); iron oxide deposits are from Modreski (1985) and Nash (1989). Other sulfide 

deposits not shown here but mentioned in the text (for example, Dandy, Brown Bear) are located 

between the Merle and Blacktail deposits (see Bookstrom and others, this volume). Note that the 

Haynes-Stellite deposit is a discordant, sulfide-bearing tourmalinized breccia pipe. The garnet 

isograd (after Nash and Hahn, 1989), shown by the open hexagon symbol, in this part of the district is 

coincident with the southern trace of the Iron Creek thrust fault of Lund and Tysdal (2007). EC, Elkhorn 

Creek Cu-Co anomaly (see Bookstrom and others, this volume); ICS, Iron Creek subplate. 

Idaho

BT

RAM

5 km IC

SS

MR

HS

EXPLANATION

Phanerozoic sedimentary,

volcanic, and intrusive rocks

Clastic metasedimentary

 rocks undivided

Apple Creek Formation

 (1445-1410 Ma)

Megacrystic augen gneiss

 (1380 Ma)

Biotite gneiss

Mesoproterozoic

Co + Cu ± Au deposits

IC Iron Creek (Jackass)

BT Blacktail

MR Merle

SS Sunshine

RAM Ram

HS Haynes-Stellite

Hematite veins and breccias

Magnetite ± Co ± Cu deposits

BP BP Blackpine

BB

BB Blackbird (Uncle Sam)
24 Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho and Montana

The Idaho Cobalt Belt

Stratabound mineral deposits of the Idaho cobalt belt 

of central Idaho (fig. 1) are strategically important because 

they contain the largest cobalt resource known in the United 

States. Bookstrom and others (this volume) summarize the 

past mining history and production in the region and estimate 

the sizes and grades of unmined metal concentrations. The 

stratabound deposits comprise both sulfide and oxide accumulations. All of these deposits occur in the siliciclastic Apple 

Creek Formation, of Mesoproterozoic age, mostly within the 

banded siltite unit as described by Tysdal and others (2003) 

and Lund and Tysdal (2007). A megacrystic augen gneiss 

body having a U-Pb zircon age of ~1380 Ma (Lund and others, 

2004) crops out to the north of the belt (fig. 1). The Mesoproterozoic rocks in the region are deformed by outcrop-scale 

folds and are cut by thrust faults of probable Cretaceous age; 

high-angle faults are mainly Tertiary (Lund and Tysdal, 2007). 

Older Proterozoic deformation in the cobalt belt cannot be 

ruled out, however, given the presence in wallrocks to the 

~1380 Ma megacrystic augen gneiss of folds with axial-planar 

fabrics that are cut by this body (see Bookstrom and others, 

this volume) and of a Lu-Hf age of 1194±4 Ma for the core of 

a garnet crystal from amphibolite in northern Idaho (Vervoort 

and others, 2005). Metamorphic grade in the study area ranges 

from lower greenschist in the southeast part of the cobalt belt, 

within the Iron Creek area, to middle or upper amphibolite in 

the northwest part, at and near the Salmon Canyon deposit. In 

the Blackbird district and vicinity, Vhay (1948) and Nash and 

Hahn (1989) recognized and mapped a garnet isograd, which 

Lund and Tysdal (2007) consider to be coincident with the 

southern trace of the Indian Creek thrust fault (fig. 1).

The stratabound sulfide deposits in the Blackbird district 

form stratiform to irregular lenses and subordinate veins 

within laminated siliceous rock and stratiform biotite-rich rock 

(Nash and Hahn, 1989; Bookstrom and others, this volume). 

The latter rock type, commonly termed “biotitite” by most 

previous workers, consists mainly of green biotite with minor 

chlorite, quartz, chloritoid, and Cl-rich scapolite in strata at 

middle greenschist grade; garnet and Al-rich silicate minerals 

including possible cordierite and sillimanite are present in the 

biotitite beds at higher metamorphic grade (see Lund and Tysdal, 2007). The author has also tentatively identified andalusite in biotitite beds from the Ram deposit. The origin of the 

stratiform biotitites is uncertain; they have variously been 

interpreted as altered mafic alkaline tuffs, metamorphosed 

hydrothermal alteration zones, and metamorphosed exhalative 

chemical sediments (Nash and Hahn, 1989; Nash and Connor, 

1993; Bookstrom and others, this volume).

Several nonstratabound sulfide deposits also occur in the 

district. One type is the discordant Dandy zone, consisting 

of massive pyrrhotite and marcasite with minor chalcopyrite 

and quartz, which Nash and Hahn (1989) suggested was a 

feeder to the adjacent stratiform Chicago and Idaho deposits. 

A second type occurs within tourmalinized breccia pipes that 

contain minor cobaltite and secondary Co minerals, such as at 

the Haynes-Stellite deposit (fig. 1; see Bookstrom and others, 

this volume).

Deformational features in the deposits include folds, 

faults, and shear zones. Folds are evident on a range of scales 

from meter-sized structures exposed in outcrops and underground mine faces to smaller structures seen in hand specimens and thin sections. In the Blacktail deposit, high-grade 

Co-Cu ore zones are aligned with north-plunging folds (Vhay, 

1948), providing evidence for a predeformational origin for 

at least some of the sulfide mineralization. On smaller scales, 

sulfide ore from the dump of the Sunshine deposit shows 

quartz and cobaltite layers that can be traced continuously 

around the nose of a 10-cm-amplitude fold, and drill core from 

the Ram deposit displays classic “durchbewegung” texture (for 

example, Vokes, 1969; Marshall and Gilligan, 1993) in which 

semimassive to massive cobaltite is infolded within schistose 

wallrock. Cobaltite has also been found in the cores of elongate, stretched garnets. In some parts of the district, apparent 

pretectonic features can be recognized, such as in the Chicago 

deposit where a 1- to 2-m-thick lens composed of massive 

chalcopyrite with minor pyrrhotite and cobaltite cuts bedding 

in the host siltite at a low angle (S.G. Peters, U.S. Geological 

Survey, oral commun., 2005). Possible soft-sediment deformational features are also reported to occur locally in some 

deposits (Bookstrom and others, this volume).

Stratabound magnetite-sulfide deposits in the Iron Creek 

area were studied by Nash (1989). The detailed geometry of 

these deposits is poorly known, owing to limited information 

from drill cores. However, at map scale the deposits appear 

to be broadly conformable with their metasedimentary host 

strata and hence are stratiform. Whether they are also bedded 

and stratiform is unclear. Biotite-rich rocks apparently are 

not associated with the magnetite-sulfide deposits in the Iron 

Creek area, in contrast to most of the deposits in the Blackbird 

district to the north.

The ore mineralogy of the sulfide deposits is dominated by cobaltite or chalcopyrite (or both) with generally 

minor pyrite, arsenopyrite, safflorite, pyrrhotite, and gold. 

Locally present in trace to minor amounts are native bismuth, bismuthinite, millerite, uraninite, and one or more 

unidentified Bi-Te phases (Anderson, 1947; Vhay, 1948; 

Bennett, 1977; Eiseman, 1988; Nash and Hahn, 1989). In a 

mineralogical study of the Sunshine deposit, Eiseman (1988) 

reported the presence of minor allanite, a mineral that contains major amounts of light rare-earth elements (LREE)—

[(Ca,LREE,Y)2

(Al,Fe3+)

3

Si3)

12(OH)]. Reconnaissance microbeam studies by Slack (2006), which used scanning electron 

microscope–energy dispersive spectrometry (SEM–EDS) on 

sulfide samples from the Sunshine and Merle deposits (fig. 1), 

discovered locally abundant monazite [(LREE,Y,Th)PO4

] and 

xenotime, the latter mineral containing major amounts of heavy 

rare earth elements (HREE)—[(Y,HREE)PO4

]. These REErich minerals tend to occur preferentially in Co-rich samples, 

as a matrix to disseminated cobaltite or interstitial to massive 

cobaltite. Allanite is also abundant in deposits such as Brown 

Bear, where it constitutes ~30 vol percent of some samples 
Sulfide and Iron Oxide Deposits in the Idaho Cobalt Belt 25

together with cobaltite and lesser epidote, apatite, chalcopyrite, 

and quartz. More recent SEM–EDS work by the author has 

identified abundant gadolinite [(Y,Ce)2Fe2+Be2Si2O10] in one 

sample from the Northfield deposit (fig. 1). Gangue minerals in 

the sulfide deposits comprise major quartz, and minor biotite, 

chlorite, muscovite, garnet, tourmaline, apatite, and siderite. 

Stratabound pyrite-rich magnetite deposits in the Iron 

Creek area (fig. 1) consist mainly of magnetite and cobaltiferous pyrite with trace to minor amounts of chalcopyrite in a 

gangue composed dominantly of quartz (Nash, 1989). 

Bulk Compositions of Mineralized 

Samples

Slack (2006) reported the results of a reconnaissance geochemical study of 15 samples of Co-Cu-Au ore and mineralized rock from the Blackbird district. This study was the first 

to use high-precision inductively coupled plasma–mass spectrometry (ICP–MS) on fused samples of sulfide-bearing rocks 

from the district. In addition to the known concentrations of 

Co, Cu, and Au, Slack (2006) discovered in 11 sulfide-rich 

samples previously unrecognized high contents of REE and Y, 

which average 0.53 wt percent ΣREE+Y oxides; some samples 

also have relatively high concentrations of As (maximum 17.3 

wt percent), Bi (maximum 2,953 parts per million (ppm)), 

and Ni (maximum 1,330 ppm). Since that study, 40 additional 

mineralized samples from the district have been analyzed by 

a combination of methods including ICP–MS, instrumental 

neutron activation analysis (INAA), fire assay–mass spectrometry, and inductively coupled plasma–optical emission 

spectrometry (ICP–OES). Data from these new analyses 

confirm the abundances of REE and Y in some Co-Cu-Au ores 

of the Blackbird district and indicate that several sulfide-rich 

samples have 2 to nearly 4 wt percent ΣREE+Y oxides. The 

new analyses show even higher Bi (maximum 9.2 wt percent) 

and Ni (maximum 6,780 ppm) concentrations; Sb (maximum 

429 ppm), Te (maximum 250 ppm), and U (maximum 124 

ppm) contents are elevated in a few samples. Significantly, in 

this larger dataset, selected samples also have concentrations 

of Sn (133 ppm) and W (142 ppm) that are anomalously high 

compared with those for most stratabound sulfide ores. One 

sulfide-rich rock from the Northfield deposit (fig. 1), which 

contains 4.2 wt percent Co, 3.7 wt percent Cu, and 0.9 ppm 

Au, is significant in also having 1,180 ppm Be. Chalcopyriterich ore from the Salmon Canyon copper deposit (one sample) 

has 2.2 wt percent Co, 5.9 wt percent Cu, 11.3 wt percent As, 

4.8 ppm Au, 2,110 ppm Bi, 2,220 ppm Ni, 78 ppm Sb, and 

516 ppm Y. Analysis of a subset of 21 sulfide-rich samples by 

fire assay–mass spectrometry indicates very low concentrations of Pt (maximum 1.0 ppb) and Pd (maximum 0.7 ppb). 

In the entire dataset for the Blackbird district (n = 55), Zn is 

slightly high in a few samples (maximum 929 ppm), but Pb 

and Mo are uniformly low (maximum 144 ppm and 25 ppm, 

respectively). 

Samples of stratabound pyrite-rich magnetite rock 

from the Iron Creek area (fig. 1) were also analyzed for bulk 

compositions. Results for three drill cores from the No-Name 

zone show elevated contents of several metals: the most pyritic 

sample, containing ~50 vol percent euhedral pyrite, also contains 1.3 wt percent Co, 255 ppm Cu, 74 ppb Au, 372 ppm As, 

1,010 ppm Bi, and 310 ppm Ni; the other two samples have 

lower Co (maximum 6,860 ppm) and very low Cu (maximum 

46 ppm) and Au (maximum 58 ppb) but elevated concentrations of other metals including as much as 1,290 ppm Bi, 

156 ppm Te, and 43 ppm W. Approximately 2 km south of 

the center of the No-Name zone, one roadside outcrop in the 

Jackass zone that consists of massive magnetite with minor 

disseminated sulfides contains 3.2 wt percent Co, 2,180 ppm 

Cu, 896 ppb Au, 4.3 wt percent As, 67 ppm Bi, 661 ppm Ni, 

and 420 ppm Y. A vein of massive specular hematite from the 

McKim Creek prospect, in the Lemhi Range southeast of the 

cobalt belt (see Bookstrom and others, this volume), contains 

355 ppm Co, 51 ppm Cu, 20 ppb Au, 2,940 ppm As, and 142 

ppm W.

Recent Mineralogical Studies

Sulfide-rich samples from the Idaho cobalt belt have 

been studied using a petrographic microscope and SEM–EDS 

analysis. Recently acquired SEM–EDS data confirm and 

expand the observations reported by Slack (2006), particularly with respect to the mineralogical residence of REE 

and Y within allanite, monazite, and xenotime. Important 

discoveries resulting from this new work are as follows: (1) 

large amounts of allanite (~20–30 vol percent) occur locally, 

generally with intergrown epidote and minor apatite, such as 

in the Brown Bear deposit; (2) allanite is volumetrically minor 

in the Sunshine deposit and may be rare in the Ram deposit; 

(3) monazite appears to be abundant only in the Sunshine and 

Ram deposits; (4) xenotime occurs rarely as small inclusions 

in cobaltite grains, but monazite inclusions in cobaltite have 

not been found; (5) allanite in the Sunshine, Ram, and Merle 

samples is commonly replaced by monazite and locally by 

xenotime; and (6) gadolinite is a volumetrically major mineral 

in one Co- and Cu-rich sample from the Northfield deposit, 

forming compositionally zoned grains both in the matrix to 

cobaltite and as inclusions within cobaltite.

Implications For Ore Genesis

Diverse genetic models have been proposed for the CoCu-Au deposits of the Blackbird district including formation 

from volcanogenic massive sulfide (VMS), sedimentaryexhalative (SEDEX), stratiform copper, and iron oxide–

copper–gold (IOCG) mineralizing systems (Modreski, 1985; 

Nash and Hahn, 1989, Nold, 1990; Bending and Scales, 2001; 

Slack, 2006). A carbonatite-related origin is also possible, 
26 Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho and Montana

given the high REE contents of some samples in the district, 

but this model is considered unlikely because REE-rich 

carbonatite deposits lack the Co-Cu-Au-Bi-Ni metal assemblage that characterizes the Blackbird sulfide samples. In the 

author’s opinion, the weight of the evidence obtained to date 

argues for an IOCG genetic model (see Williams and others, 

2005; Johnson and Barton, in press). This model encompasses 

several types of mineralizing systems but mainly involves 

magmatic-hydrothermal fluids.

The age of sulfide mineralization in the cobalt belt is 

enigmatic. Since the study by Bennett (1977), who proposed 

an epigenetic origin related to the augen gneiss body (then 

believed to be part of the Idaho batholith), most workers 

have considered the deposits to be broadly coeval with their 

Mesoproterozoic host rocks, having formed by either VMS or 

SEDEX processes. However, recent sensitive high-resolution 

ion microprobe (SHRIMP) U-Pb dating of 10 monazite grains 

from the Sunshine deposit by John Aleinikoff (Slack and others, 2006) yielded a 206Pb/238U age of 90.8±1.2 Ma. This Late 

Cretaceous age is close to a Lu-Hf age of 89.6±2.6 Ma for 

metamorphic garnet in northern Idaho (Vervoort and others, 

2005). Such similarity in ages raises the possibility that the 

monazite in the Sunshine deposit—and by implication the associated Co-Cu-Au ores—also formed during a Late Cretaceous 

metamorphic event. This hypothesis is undercut, though, by 

textural evidence showing that in the Sunshine and Ram deposits, cobaltite grains locally occur in the cores of garnet crystals 

(Eiseman, 1988; this study). A Cretaceous origin for the sulfide 

mineralization in the district cannot be ruled out, for example 

by multiple hydrothermal and metamorphic processes in which 

the Co-Cu-Au-REE-Y deposits formed early and the garnet and 

monazite grains formed late, all during the Late Cretaceous. 

However, in the author’s opinion, the presence of Cretaceous 

monazite in the Blackbird deposits is better explained by its 

formation during regional metamorphism from pre-Cretaceous 

allanite and apatite. Recent SEM studies by the author show 

that monazite replaces allanite in several samples from the Sunshine, Ram, and Merle deposits, and that allanite + apatite is a 

major assemblage in a few other deposits of the district such as 

Brown Bear and Merle. In pelitic schists metamorphosed under 

Barrovian-type high-P conditions, trace amounts of monazite 

form at the staurolite isograd by the breakdown of REE- and 

P-bearing garnet + muscovite + chlorite (Kohn and Malloy, 

2004). However, this process does not explain the abundance 

of monazite (and LREE) in some of the Blackbird district 

samples. More relevant is the work of Wing and others (2003), 

who proposed that under Buchans-type low-P conditions at the 

andalusite isograd, allanite + apatite + muscovite + andalusite 

+ quartz react to form monazite + biotite + Ca-plagioclase + 

water. This latter reaction does not appear to be completely 

applicable to the Blackbird district, because plagioclase has 

not been found together with monazite in the Sunshine and 

Ram deposits, even in sparse amounts. Other products of the 

breakdown of allanite + apatite may have formed with monazite, such as ankerite + siderite + muscovite; these and similar 

assemblages will be evaluated during the remainder of this 

study. Regardless of the metamorphic reaction or reactions 

involved, it seems likely that allanite and apatite, as well as 

xenotime, existed in the Blackbird deposits prior to Cretaceous metamorphism. This interpretation is supported by the 

apparent restriction of abundant monazite to the Sunshine and 

Ram deposits, which occur above the garnet isograd. It is also 

consistent with evidence for REE mineralization in the Mesoproterozoic in the Belt-Purcell Basin to the east and north: in 

the Lemhi Pass district along the Idaho-Montana border, where 

large Th-REE-Cu-Fe veins contain molybdenite with a Re-Os 

age of ~1100 Ma (Gillerman and others, 2002), and in the 

Sullivan deposit, southeastern British Columbia, where small 

Ti-Nb-REE veins contain polycrase with a SHRIMP 207Pb/206Pb 

age of 1415±5 Ma (Slack and others, 2004).

Geochemical data on the mineral deposits of the cobalt 

belt provide evidence that they formed by a common process. As outlined above, the Co-Cu-Au sulfide deposits in 

the Blackbird district have a strong accessory metal signature 

involving, in most samples, high Bi, Ni, REE, and Y. Abundance of the Y- and Be-rich mineral gadolinite in one sample 

from the Northfield deposit supports a link between this 

mineralization and formation of the other Y-rich minerals in 

the district. The presence of gadolinite grains within cobaltite 

crystals at Northfield further suggests—but does not prove—

that the Be concentration there was not introduced during or 

after Cretaceous metamorphism, but instead formed together 

with cobaltite during the Mesoproterozoic. Relatively high 

contents of Bi, Ni, and Y in the stratabound pyritic magnetite 

deposits of the Iron Creek area and elevated Co and As in the 

McKim Creek hematite vein provide a possible link to the 

Blackbird Co-Cu-Au deposits, and these data are consistent 

with a model in which all of these diverse deposits formed by 

similar mineralizing processes.

The source of the mineralizing fluids is unclear. High 

REE and Y concentrations in many of the sulfide deposits suggests a magmatic source, which is also supported by the presence of elevated Sn and W in several of the Blackbird deposits 

and by high Be in the Northfield deposit. However, high Sn, 

W, and Be concentrations are not unique to magmatically 

derived mineral deposits (for example, Slack and others, 2000; 

Barton and Young, 2002). Nevertheless, recent He isotope 

analysis of quartz gangue from the Sunshine deposit provides 

strong evidence for a magmatic component in the hydrothermal fluids. Bookstrom and others (this volume) report a large 

proportion of mantle-derived 3

He in this quartz, which may 

reflect the involvement of mantle-derived magmas such as 

carbonatites or granites. Although small Mesoproterozoic(?) 

carbonatite dikes and veins exist tens of kilometers or more 

to the north and east of the Blackbird district (for example, 

Anderson, 1960; Gibson and Wood, 1997), for the metallogenic reasons outlined above they are unlikely candidates for 

the magmatic fluid source. The large ~1380 Ma augen gneiss 

body along the northern border of the district is a more plausible source for the hydrothermal fluids, although the original 

granitic intrusion that now makes up this gneiss was emplaced 

~30–70 m.y. after deposition of the Apple Creek Formation.
Sulfide and Iron Oxide Deposits in the Idaho Cobalt Belt 27

A unified genetic model for the mineral deposits in the 

Idaho cobalt belt is still undefined and will require additional 

studies. Strictly exhalative to synsedimentary origins are problematic, because of the absence of key features that are characteristic of such deposits, such as laterally extensive stratiform 

sulfides and finely laminated ore minerals. However, laminated textures, if they existed, could have been destroyed by 

the postore deformation and metamorphism in the district. A 

late diagenetic origin is also possible but is difficult to evaluate for similar reasons. In the author’s current view, the most 

likely genetic model involves Mesoproterozoic replacement 

of permeable sediments or chemically favorable host strata (or 

both), either during late diagenesis or later following lithification of the sediment.

One mineralizing process that deserves consideration is 

epigenetic formation of the Co-Cu-Au ores, by which magmatically derived fluids selectively replaced biotitite beds 

(or their protoliths). Replacement of biotitite may have been 

promoted by the chemical reactivity of the Fe-rich biotitites, 

which could have reduced and precipitated the constituents 

of oxidized hydrothermal fluids. In an alternative model, the 

biotitites might have formed during Co-Cu-Au mineralization 

by the hydrothermal alteration of siliciclastic sedimentary wall 

rocks. Better constraints on the origin of the biotitite beds are 

needed before these hypotheses can be evaluated.

Acknowledgments

I thank Greg Hahn, formerly of Noranda Inc., for 

introducing me to the Idaho cobalt belt nearly 30 years ago, 

and Art Bookstrom, Tom Nash, Karen Lund, and Karl Evans 

of the U.S. Geological Survey for sharing their knowledge 

of the local geology and mineral deposits. I am indebted to 

Bill Scales and George King of Formation Capital Corporation for providing access to drill cores for sampling, and to 

Gordon Hughes, formerly of Noranda Inc., and Steve Peters of 

the U.S. Geological Survey, who supplied many of the rocks 

analyzed in this study; Art Bookstrom of the U.S. Geological Survey and Virginia Gillerman of the Idaho Geological 

Survey also donated samples for analysis. Harvey Belkin of 

the U.S. Geological Survey provided electron microprobe data 

that were critical for identifying gadolinite in the Northfield 

deposit. The manuscript was improved by the reviews of Art 

Bookstrom and Craig Johnson of the U.S. Geological Survey.

References Cited

Anderson, A.L., 1947, Cobalt mineralization in the Blackbird 

district, Lemhi County, Idaho: Economic Geology, v. 42, 

p. 22–46.

Anderson, A.L., 1960, Genetic aspects of the monazite and 

columbium-bearing rutile deposits in northern Lemhi 

County, Idaho: Economic Geology, v. 55, p. 1179–1201.

Barton, M.D., and Young, S., 2002, Non-pegmatitic deposits of 

beryllium—Mineralogy, geology, phase equilibria and origin: 

Reviews in Mineralogy and Geochemistry, v. 50, p. 591–691.

Bending, J.S., and Scales, W.G., 2001, New production in the 

Idaho cobalt belt—A unique metallogenic province: Transactions of the Institution of Mining and Metallurgy, v. 110, 

sec. B, p. B81–B87.

Bennett, E.H., 1977, Reconnaissance geology and geochemistry of the Blackbird Mountain–Panther Creek region, 

Lemhi County, Idaho: Idaho Bureau of Mines and Geology 

Pamphlet 167, 108 p.

Eiseman, H.H., 1988, Ore geology of the Sunshine cobalt 

deposit, Blackbird mining district, Idaho: Golden, Colorado 

School of Mines, M.S. thesis, 191 p.

Gibson, P.E., and Wood, S.A., 1997, Mineralogy, geochemistry and mobility of the REE and thorium at Lemhi Pass, 

Idaho [abs.]: Geological Society of America Abstracts with 

Programs, v. 29, no. 6, p. 50.

Gillerman, V.S., Jercinovic, M.J., and Stein, H.J., 2002, 

U-Pb and Re-Os geochronology suggests a multistage 

Precambrian-Mesozoic history for thorium and copper mineralization, Lemhi Pass, Idaho [abs.]: Geological Society of 

America Abstracts with Programs, v. 34, no. 6, p. 337.

Johnson, D.A., and Barton, M.D., in press, Iron-oxide ± CuAu-REE deposits and their by-products, in Orris, G.J., and 

Haxel, G.B., eds., Rare earth element resources—A basis 

for high technology: University of Arizona and U.S. Geological Survey Center for Mineral Resources, Monographs 

in Mineral Resource Science, v. 3.

Kohn, M.J., and Malloy, M.A., 2004, Formation of monazite via prograde metamorphic reactions among common 

silicates—Implications for age determinations: Geochimica 

et Cosmochimica Acta, v. 68, p. 101–113.

Link, P.K., Fanning, C.M., Lund, K.I., and Aleinikoff, J.N., 

2007, Detrital zircon populations and provenance of Mesoproterozoic strata of east-central Idaho, U.S.A.: Correlation 

with Belt Supergroup of southwest Montana, in Link, P.K., 

and Lewis, R.S., eds., Proterozoic geology of western North 

America and Siberia: Society of Economic Paleontologists 

and Mineralogists Special Publication 86, p. 101–128. 

Lund, K.I., Aleinikoff, J.N., Evans, K.V., and Kunk, M.J., 

2004, Proterozoic basins and orogenic belts of central Idaho 

[abs.]: Geological Society of America Abstracts with Programs, v. 36, no. 5, p. 271.

Lund, K.I., and Tysdal, R.G., 2007, Stratigraphic and structural setting of sediment-hosted Blackbird gold-cobaltcopper deposits, east-central Idaho, U.S.A., in Link, P.K., 

and Lewis, R.S., eds., Proterozoic geology of western North 

America and Siberia: Society of Economic Paleontologists 

and Mineralogists Special Publication 86, p. 129–147.
28 Metallogeny of Mesoproterozoic Sedimentary Rocks in Idaho and Montana

Marshall, Brian, and Gilligan, L.B., 1993, Remobilization, 

syn-tectonic processes and massive sulphide deposits: Ore 

Geology Reviews, v. 8, p. 39–64.

Modreski, P.J., 1985, Stratabound cobalt-copper deposits in 

the Middle Proterozoic Yellowjacket Formation in and near 

the Challis quadrangle: U.S. Geological Survey Bulletin 

1658, p. 203–221.

Nash, J.T., 1989, Geology and geochemistry of synsedimentary cobaltiferous-pyrite deposits, Iron Creek, Lemhi 

County, Idaho: U.S. Geological Survey Bulletin 1882, 33 p.

Nash, J.T., and Connor, J.J., 1993, Iron and chlorine as guides 

to stratiform Co-Cu-Au deposits, Idaho cobalt belt, USA: 

Mineralium Deposita, v. 28, p. 99–106.

Nash, J.T., and Hahn, G.A., 1989, Stratabound Co-Cu deposits and mafic volcaniclastic rocks in the Blackbird mining 

district, Lemhi County, Idaho, in Boyle, R.W., Brown, A.C., 

Jefferson, C.W., Jowett, E.C., and Kirkham, R.V., eds., 

Sediment-hosted stratiform copper deposits: Geological 

Association of Canada Special Paper 36, p. 339–356.

Nold, J.L., 1990, The Idaho cobalt belt, northwestern United 

States—A metamorphosed Proterozoic exhalative ore district: Mineralium Deposita, v. 25, p. 163–168.

Reed, G.C., and Herdlick, J.A., 1947, Blackbird cobalt deposits, Lemhi County, Idaho: U.S. Bureau of Mines Report of 

Investigations 4012, 37 p.

Slack, J.F., 2006, High REE and Y concentrations in CoCu-Au ores of the Blackbird district, Idaho: Economic 

Geology, v. 101, p. 275–280.

Slack, J.F., Aleinikoff, J.N., Belkin, H.E., and Ransom, P., 

2004, Mesoproterozoic polycrase-titanite veins in the Sullivan Pb-Zn-Ag deposit, southeastern British Columbia— 

Mineral chemistry, SHRIMP geochronology, and metallogenic significance [abs.]: Geological Society of America 

Abstracts with Programs, v. 36, no. 5, p. 445.

Slack, J.F., Aleinikoff, J.N., and Fanning, C.M., 2006, 

SHRIMP U-Pb geochronology of hydrothermal monazite in 

Co-Cu-Au ores of the Blackbird district, Idaho [abs.]:

Geological Society of America Abstracts with Programs, 

v. 38, no. 7, p. 372.

Slack, J.F., Meier, A.L., Malcolm, M.J., Fey, D.L., Doughten, M.W., and Wandless, G.A., 2000, Trace element and 

rare-earth element geochemistry of bedded and massive 

sulphides from the Sullivan Pb-Zn-Ag deposit, British 

Columbia—A reconnaissance, in Lydon, J.W., Höy, T., 

Slack, J.F., and Knapp, M.E., eds., The geological environment of the Sullivan deposit, British Columbia: Geological 

Association of Canada Mineral Deposits Division Special 

Publication 1, p. 720–735.

Tysdal, R.G., Lund, K.I., and Evans, K.V., 2003, Geologic 

map of the western part of the Salmon National Forest: 

U.S. Geological Survey Geologic Investigations Series Map 

I–2765, scale 1:100,000 [http://pubs.usgs.gov/imap/i-2765/]

Vervoort, J.D., McClelland, W.C., Oldow, J.S., Watkinson, 

A.J., and Sha, G.S., 2005, Grenville-age metamorphism on 

the western margin of Laurentia, northern Idaho: Evidence 

from Lu-Hf garnet geochronology [abs.]: Geological Society of America Abstracts with Programs, v. 37, no. 7, p. 89.

Vhay, J.S., 1948, Cobalt-copper deposits in the Blackbird 

district, Lemhi County, Idaho: U.S. Geological Survey 

Strategic Minerals Investigation Preliminary Report 3–219, 

26 p.

Vokes, F.M., 1969, A review of the metamorphism of sulphide 

deposits: Earth-Science Reviews, v. 5, p. 99–143.

Williams, P.J., Barton, M.D., Johnson, D.A., Fontboté, L., 

De Haller, A., Mark, G., Oliver, N.H.S., and Marschik, 

R., 2005, Iron oxide copper-gold deposits—Geology, 

space-time distribution, and possible modes of origin, in

Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J., and 

Richards, J.P., eds., Economic Geology 100th Anniversary 

Volume: Littleton, Colorado, Society of Economic Geologists, p. 371–405.

Wing, B.A., Ferry, J.M., and Harrison, T.M., 2003, Prograde 

destruction and formation of monazite and allanite during 

contact and regional metamorphism of pelites—Petrology 

and geochronology: Contributions to Mineralogy and 

Petrology, v. 145, p. 228–250. 

Published in the Central Region, Denver, Colo.

Manuscript approved for publication September 5, 2007

Graphics by author

Photocomposition by Mari Kauffmann (Contractor, ATA Services)

Edited by Mary-Margaret Coates (Contractor, ATA Services)