Co-Cu-Au Deposits in Metasedimentary Rocks— 

A Preliminary Report

By J.F. Slack, J.D. Causey, R.G. Eppinger, J.E. Gray, C.A. Johnson, K.I. Lund, and K.J. Schulz

Open-File Report 2010–1212

U.S. Department of the Interior

U.S. Geological Survey
ii

U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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Suggested citation:

Slack, J.F., Causey, J.D., Eppinger, R.G., Gray, J.E., Johnson, C.A., Lund, K.I., and Schulz, K.J., 2010, 

Co-Cu-Au deposits in metasedimentary rocks—A preliminary report: U.S. Geological Survey 

Open-File Report 2010–1212, 13 p. (Available at http://pubs.usgs.gov/of/2010/1212/.) 

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 material contained within this report.
iii

Contents

Abstract ......................................................................................................................................................................... 1

Introduction.................................................................................................................................................................... 1

Tonnage and Grade Variations...................................................................................................................................... 3

Regional Environment ................................................................................................................................................... 5

Deposit Features ........................................................................................................................................................... 6

Mineralogy..................................................................................................................................................................... 6

Hydrothermal Alteration................................................................................................................................................. 6

Geochemical Characteristics ......................................................................................................................................... 7

Metasedimentary Host Rocks........................................................................................................................................ 7

Petrology of Related Igneous Rocks.............................................................................................................................. 8

Genetic Models.............................................................................................................................................................. 9

Geophysical Characteristics .......................................................................................................................................... 9

Geoenvironmental Features and Anthropogenic Mining Effects...................................................................................10

Acknowledgments ........................................................................................................................................................11

References Cited..........................................................................................................................................................11

Figures

1. Plot of tonnage versus average Co grade for Co-Cu-Au deposits in metasedimentary rocks .......................... 3

2. Plots of (A) average Co-Cu grade and (B) average Co-Au grade of Co-Cu-Au deposits in metasedimentary 

rocks ................................................................................................................................................................. 4

Table

1. Selected features of Co-Cu-Au deposits in metasedimentary rocks................................................................. 2
iv

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Length

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

meter (m) 1.094 yard (yd) 

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

cubic meter (m3) 35.31 cubic foot (ft3)

cubic meter (m3) 1.308 cubic yard (yd3) 

Mass

gram (g)

tonne

tonne

0.03527

1.10231

2204.62 

ounce, avoirdupois (oz)

ton, short

pound (lb)

Density

gram per cubic centimeter (g/cm3) 62.4220 pound per cubic foot 

(lb/ft3) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32
v 

Abbreviations Used in This Report

Units of Measure

μg/L micrograms per liter

mg/L milligrams per liter

permil parts per thousand

Symbols

δD delta deuterium (H-2) 

δ18O delta O-18

δ34S delta S-34

eK equivalent potassium 

eTh/K equivalent thorium/potassium ratio 

eU/Th equivalent uranium/thorium ratio 

Initialisms

Ga giga-annum, billion years

REE rare earth elements

SEDEX sedimentary-exhalative

SHRIMP sensitive high-resolution ion microprobe 

Ag silver

As arsenic

Au gold

Be beryllium

Bi bismuth

Cl chlorine

Co cobalt

CO2 carbon dioxide

Cu copper

Fe iron

Hf hafnium

H2S hydrogen sulfide

K potassium

Mn manganese

Nb niobium

Ni nickel

Pb lead

S sulfur

Ta tantalum

Th thorium

U uranium

W tungsten

Y yttrium

Zn zinc

Zr zirconium

1 

Co-Cu-Au Deposits in Metasedimentary Rocks— 

A Preliminary Report

By J.F. Slack,1 J.D. Causey,2 R.G. Eppinger,3 J.E. Gray,

3 C.A. Johnson,4 K.I. Lund,3 and K.J. Schulz1

Abstract

A compilation of data on global Co-Cu-Au deposits in metasedimentary rocks refines previous 

descriptive models for their occurrence and provides important information for mineral resource 

assessments and exploration programs. This compilation forms the basis for a new classification of such 

deposits, which is speculative at this early stage of research. As defined herein, the Co-Cu-Au deposits 

contain 0.1 percent or more by weight of Co in ore or mineralized rock, comprising disseminated to 

semi-massive Co-bearing sulfide minerals with associated Fe- and Cu-bearing sulfides, and local gold, 

concentrated predominantly within rift-related, siliciclastic metasedimentary rocks of Proterozoic age. 

Some deposits have appreciable Ag ± Bi ± W ± Ni ± Y ± rare earth elements ± U. Deposit geometry 

includes stratabound and stratiform layers, lenses, and veins, and (or) discordant veins and breccias. The 

geometry of most deposits is controlled by stratigraphic layering, folds, axial-plane cleavage, shear 

zones, breccias, or faults. Ore minerals are mainly cobaltite, skutterudite, glaucodot, and chalcopyrite, 

with minor gold, arsenopyrite, pyrite, pyrrhotite, bismuthinite, and bismuth; some deposits have 

appreciable tetrahedrite, uraninite, monazite, allanite, xenotime, apatite, scheelite, or molybdenite. 

Magnetite can be abundant in breccias, veins, or stratabound lenses within ore or surrounding country 

rocks. Common gangue minerals include quartz, biotite, muscovite, K-feldspar, albite, chlorite, and 

scapolite; many deposits contain minor to major amounts of tourmaline. Altered wall rocks generally

have abundant biotite or albite. Mesoproterozoic metasedimentary successions constitute the 

predominant geologic setting. Felsic and (or) mafic plutons are spatially associated with many deposits 

and at some localities may be contemporaneous with, and involved in, ore formation. Geoenvironmental 

data for the Blackbird mining district in central Idaho indicate that weathering of abundant Fe, S, As, 

Co, and Cu in sulfide minerals of the deposits produces acidic waters, especially in pyrite-rich deposits; 

mine runoff has high concentrations of Fe, Cu, and Mn that exceed U.S. drinking water or aquatic life 

standards.

Introduction

This preliminary report provides an improved model for a specific type of Co-Cu-Au deposit 

that will be evaluated in the next U.S. Geological Survey (USGS) assessment of undiscovered mineral 

resources in the United States. Emphasis is on describing, in a modern context, a unified deposit model 

that includes both geologic and geoenvironmental aspects. The new model presented here supersedes 

previous USGS models by Earhart (1986) and Evans and others (1995), which are based solely on 

 

1 U.S. Geological Survey, National Center, MS 954, Reston, VA 20192

2 U.S. Geological Survey, 904 West Riverside Ave., Spokane, WA 99201

3 U.S. Geological Survey, Denver Federal Center, Box 25046, MS 973, Denver, CO 80225

4 U.S. Geological Survey, Denver Federal Center, Box 25046, MS 963, Denver, CO 80225
2 

Table 1. Selected features of Co-Cu-Au deposits in metasedimentary rocks. Data sources are available from the U.S. Geological Survey. 

[Mt, million metric tonnes; g/t, grams per tonne; NWT, Northwest Territories; NA, data not available; m, meter; %, percent; --, none identified]

Deposit name Country State/Province Size (Mt) Co (%) Cu (%) Au (g/t) Major metals Minor associated metals

Blackbird-ICB1 USA Idaho 14.343 0.58 1.24 0.58 Co, Cu, Fe Y, REE, Bi, As, Ni, U

Cobalt Hill Canada Ontario NA NA NA NA Co, Cu, Au Ni, As, Hg Te

Contact Lake 

Belt Canada NWT NA NA2 NA2 NA2

Cu, Au, Ag, Co, 

U Bi, Zn, As

Werner Lake Canada Ontario NA 1.68 0.35 3.22 Cu, Co, Au Mo, Zn, Ag, Ni

NICO Canada NWT 330.986 0.12 0.04 0.91 Co, Au, Bi As, W, Fe, Ba, P, Co, F, Be

Modum

(Skuterud) Norway 1.0 0.26 2.0 1 Cu, Co, Au As, Ni, U, Th

Gladhammar Sweden 0.006 NA NA NA Co, Cu, Fe, Au Zn, Pb, Mo, Bi

Solstad Sweden 0.009 NA NA NA Cu, Co, Au Ni, Pb, Zn

Haarakumpu Finland 4.680 0.17 0.34 NA Co, Cu Au, Bi, Ni

Hangaslampi Finland 0.176 0.1 NA 6 Au, Co Ag, Cu, REE, Ni, U, Mo, Pb, W

Jouhineva/Pöllä Finland 0.450 0.18 0.81 0.88 Co, Cu, Au, Ag Mo, Ni, REE, U

Juomasuo Finland 1.800 0.2 NA 3 Au, Co Cu, Ag, Mo, Ni, REE, U, Bi, Te, Pb, Zn

Kouvervaara Finland 1.580 0.1 0.2 0.4 Co, Cu, Au Zn, Mo, Bi, W

Kuumaso Finland NA 0.14 2.0 2.2 Co, Cu, Au --

Lemmonlampi Finland 0.090 0.3 0.4 0.15 Co, Cu, Au --

Meurastuksenaho Finland 0.284 0.25 0.28 2.3 Co, Au, Cu Mo, U, REE

Sirkka Finland 0.250 0.1 0.38 0.8 Co, Ni, Au, Cu U, Zn, Ag, As

Mount Cobalt Australia Queensland 0.060 40.05 0.33 NA Cu, Co, W As, Ni, Au, REE

Kendekeke China Qinghai NA NA NA NA Co, Bi, Au, Cu Bi, As, Zn, Pb, Te

Dahenglu China Jilin NA NA NA NA Co, Cu Zn, Pb, As

1

ICB, Idaho cobalt belt; includes numerous separate deposits.

2

0.27% Cu, 0.10% Co, 1.31% As, 0.15 g/t Au over 24 m.

3

Does not include 6.5 Mt of marginal subeconomic resource.

4

From 1919–1934 produced 20,000 tonnes of ore at an average grade of 4% Co (Nisbet and others, 1983).
3 

Co-Cu deposits in the Blackbird mining district of central Idaho. This report is a broader synthesis of 

information on numerous Co-Cu-Au deposits in metasedimentary rocks worldwide (table 1) that share 

common geologic, mineralogical, and geochemical features. With one exception, deposits in the 

database contain on average ≥0.1 percent Co; Mount Cobalt, Australia, is included because it has past 

Co production from higher-grade ore zones (Nisbet and others, 1983). Six deposits that lack data for 

average Co grades are also included because each reportedly contains abundant Co. Many of the 

deposits are noteworthy as possible resources of Ag, Bi, W, Ni, Y, rare earth elements (REE), and U. 

Significantly, the grouping of Co-Cu-Au deposits in metasedimentary rocks presented in this report 

includes deposits that other workers have previously classified in different ways. 

Tonnage and Grade Variations

Most of the Co-Cu-Au deposits described in this report comprise resources of 100 to 10,000 

tonnes of contained Co (fig. 1). Sizes of the Co-Cu-Au deposits overlap those of sedimentary Cu 

deposits of the central African copperbelt in terms of total Co content. With the exception of the 

Blackbird district in central Idaho, the Co-Cu-Au deposits represent smaller Co resources than Co-rich 

volcanogenic massive sulfide (VMS) deposits (Windy Craggy, Canada; Outokumpu, Finland), and large 

magmatic Ni-Cu deposits (for example, Noril’sk-Talnakh, Russia). Some large cobaltiferous iron oxidecopper-gold (IOCG) deposits such as Olympic Dam and Ernest Henry in Australia contain significant 

Co resources (fig. 1), although Co is not currently produced from either deposit. Despite the smaller Co 

contents relative to other deposit types, Co-Cu-Au deposits in metasedimentary rocks can be important 

national resources—particularly deposits or groups of deposits as large as Blackbird—because the 

United States has few known Co resources of likely economic viability. 

Figure 1. Plot of tonnage versus average Co grade for Co-Cu-Au deposits in metasedimentary rocks. Shown for 

comparison are data for other cobaltiferous deposit types including iron oxide-copper-gold (IOCG), volcanogenic 

massive sulfides (VMS), sedimentary Cu, and magmatic Ni-Cu. Data sources are available from the 

U.S. Geological Survey.
4 

Average Co grades are plotted versus average Cu and average Au grades in figure 2A and 2B, 

respectively. In general, the metasedimentary Co-Cu-Au deposits considered in this report have more 

Co and less Cu, and higher Co/Cu ratios, than other deposit types, except for several Co-rich 

sedimentary Cu deposits of the central African copperbelt. Relatively high Au grades are a key 

distinguishing feature of the metasedimentary Co-Cu-Au deposits. Most of the Finnish deposits (table 1) 

have more than 2 grams per metric tonne (g/t) Au; some have as much as 6 g/t Au. By comparison, 

cobaltiferous sedimentary Cu, VMS, and IOCG deposits have less than 1 g/t Au, excluding the Guelb 

Moghrein (Mauritania) IOCG deposit that contains an average of nearly 1.5 g/t Au. The lack of 

associated redbed sedimentary rocks, or their metamorphosed equivalents (for example, magnetitebearing quartzite), further distinguishes the Co-Cu-Au deposits considered here from Co-rich 

sedimentary Cu deposits of the central African copperbelt.

Figure 2. Plots of (A) average Co-Cu grade 

and (B) average Co-Au grade of Co-Cu-Au 

deposits in metasedimentary rocks. 

Abbreviations for other cobaltiferous deposit 

types: EH, Ernest Henry (Australia); GM, Guelb 

Moghrein (Mauritania); LW, Luiswishi 

(Democratic Republic of Congo); MK, Mukondo 

(Democratic Republic of Congo); NT, Noril’skTalnakyh (Russia); OD, Olympic Dam 

(Australia); OK, Outukumpu-Keretti (Finland); 

OV, Outukumpu-Vuono (Finland); WC, Windy 

Craggy (Canada). Data sources are available 

from the U.S. Geological Survey.
5 

Regional Environment

Host rocks for Co-Cu-Au deposits in the Blackbird district (Lund and Tysdal, 2007) and for 

similar deposits elsewhere (table 1) are metasedimentary sequences that were deposited predominantly 

within intracontinental extensional basins. Most basins are of Proterozoic age, but several are Archean 

or Paleozoic. The basins are commonly filled by thick (multiple kilometer), supracrustal successions of 

fine- to medium-grained siliciclastic rocks. Although the tectonic environment was extensional, 

synsedimentary igneous rocks are rare in the stratigraphic sequences.

Subsequent intracontinental extension was superimposed on the sedimentary basins and resulted 

in post-depositional, volumetrically minor, extension-related, mafic to felsic igneous magmatism. In the 

region surrounding the Blackbird district, three post-depositional, intracontinental extensional events 

produced A-type, within-plate, igneous rocks about 30, 700, and 900 million years after 

Mesoproterozoic sedimentation but prior to Cretaceous formation of the structural fabrics that give the 

mineral deposits their present form (Lund and Tysdal, 2007; Aleinikoff and others, 2010). 

Mineral deposits in this compilation occur near, but not within, the extensional igneous rocks. In 

the Blackbird district, the oldest extensional igneous event is the same age, within SHRIMP U-Pb 

analytical uncertainty, as xenotime (a Y-REE phosphate mineral) that occurs locally in the ores (Slack, 

2006; Aleinikoff and others, 2010), suggesting that the extensional magmatism (1) was responsible for 

metal emplacement, (2) modified preexisting deposits, or (3) produced ground preparation (early rock 

alteration zones, rheology boundaries, and so on) for later mineralization.

Younger contractional deformation, caused by continental collision and translation of stress 

inboard, deformed the country rocks as part of regional thrust belts or other deep-crustal structures. The 

resulting structural inversion of the sedimentary basins produced deformation and regional 

metamorphism of the preexisting sedimentary and igneous rocks. Mineral deposits and districts are 

typically clustered along minor structures related to major, through-going, deep-crustal structures. The 

deep-crustal origin of the major structures is paired with generally medium to high metamorphic grades 

of the host rocks. The contractional phase was the final element in establishing the regional settings of 

the mineral deposits, providing a heat source, dehydration-derived fluids, and structural pathways for 

processes that either led to metal emplacement or modified preexisting deposits.

The ages of many of these mineral deposits are debated or undetermined. Several factors are 

responsible, including (1) poorly known ages of sediment accumulation due to the difficulty of precisely 

dating the mainly Precambrian sedimentary host rocks, (2) degree and complexity of metamorphic and 

tectonic overprint(s), and (3) large time intervals separating sedimentation and tectonic events. 

Likewise, the duration of mineralization is uncertain but may include a multi-stage, complex tectonic 

history spanning a significant time interval. An important factor common to several deposits appears to 

be a tectonic setting involving multiple episodes of reworking of continental crust, including (1) 

intracontinental extension resulting in crustal-scale structures and thick, syntectonic basin fill; (2) 

younger extension to give superposed, bimodal within-plate magmatism; and (3) compression resulting 

in deep-seated structures and circulation of metamorphic fluids through a thick crustal section. In the 

Blackbird district, Y and REE (and probably Be) were introduced in the Mesoproterozoic 

contemporaneous with the first episode of A-type granite emplacement (Aleinikoff and others, 2010). 

The Co-Cu-Au-Bi component of the deposits also may have formed during the Mesoproterozoic or 

possibly much later during regional Cretaceous orogenic events, a 1.3 billion year time span (Lund and 

Tysdal, 2007; Aleinikoff and others, 2010). 
6 

Deposit Features

The deposits are predominantly hosted by siliciclastic metasedimentary rocks, but a few deposits 

have lithologically different wall rocks, such as NICO (Canada), which is locally within metarhyolite 

(Goad and others, 2000). No deposits in our database are hosted by plutons, regardless of age or 

composition. 

Mineralized zones vary greatly in form and geometry. Most are less than a few meters thick and 

several hundred meters in length, but some zones are much larger, such as those in the Blackbird 

district, which are up to 1 km long (Nash and Hahn, 1989; Bookstrom and others, 2007), and the 

Skuterud (Modum district) in Norway, which is about 2 km long (Grorud, 1997). The Bowl zone of the 

NICO deposit in Yukon, Canada, is up to 70 m thick and 1.9 km in length (Goad and others, 2000). 

Mineralized zones are known to be stratabound or discordant and typically are elongate parallel to local 

structures such as faults, shear zones, fold axes, and intersections of axial plane cleavage with bedding. 

A few deposits, such as some in the Blackbird district, are localized along lithologic contacts. Breccias 

are common hosts. 

Mineralogy

The ore mineralogy of the deposits includes predominant Co ± As ± Ni sulfides such as

cobaltite, glaucodot, skutterudite, safflorite, smaltite, carrollite, linnaeite, and gersdorffite. 

Accompanying ore minerals typically are chalcopyrite and gold, with generally minor arsenopyrite, 

pyrite, pyrrhotite, marcasite, millerite, tetrahedrite, bismuthinite, bismuth, uraninite, monazite, allanite, 

xenotime, apatite, and (or) molybdenite. Some deposits contain sparse to trace amounts of bornite, 

tellurobismuthite, gadolinite, Co-selenides, sphalerite, or stannite. Gangue minerals are chiefly quartz 

and muscovite, with locally abundant tourmaline, albite, biotite, chlorite, K-feldspar, and scapolite. 

Most deposits have appreciable magnetite and (or) hematite within the mineralized zones or in 

surrounding country rocks. Minor gangue constituents include apatite, barite, carbonate, fluorite, 

carbonates, garnet, pyroxene, amphibole, and graphite. 

Discerning mineral paragenesis in the deposits considered herein is difficult owing to effects of 

varying recrystallization and remobilization during post-ore deformation and metamorphism. However, 

a general characteristic is paragenetically early magnetite veins, lenses, or breccias that are cut or 

replaced by later sulfides, gold, and other minerals. Tourmaline, where present, is also typically early in 

the paragenesis, although in some deposits such as at Blackbird it appears to be contemporaneous with 

ore deposition (Slack, 2007). Bismuth concentrations, commonly in native Bi and bismuthinite, tend to 

be spatially associated with gold, but in some cases gold appears to be texturally separate from, and 

paragenetically unrelated to, Bi minerals.

Hydrothermal Alteration

Hydrothermally altered rocks spatially associated with the deposits extend tens to hundreds of 

meters from the mineralized zones. Three distinctive types of altered rocks have been recognized, which 

consist of stratabound and locally stratiform lenses composed mainly of (1) Fe-rich biotite, such as the 

Fe- and Cl-rich biotite that occurs in wall rocks to the Blackbird deposits (Nash and Hahn, 1989); (2) 

Fe-rich tourmaline, especially well developed in the Blackbird district (Nash and Hahn, 1989; Slack, 

2007); and (3) albite, such as in host rocks to the Skuterud deposit in the Modum district of Norway 

(Grorud, 1997). Scapolite also occurs in the wall rocks and country rocks surrounding several of the 
7 

Co-Cu-Au deposits listed in table 1, such as Modum, but the spatial relationship and paragenesis of this 

scapolite to the mineralized zones are unclear.

Geochemical Characteristics 

In addition to Co, Cu, and local Au, some deposits have appreciable Ag ± Bi ± W ± Ni ± Y ±

REE ± U. In a few deposits such as NICO in Canada and several of those in the Blackbird district, Bi 

concentrations in mineralized zones are locally as much as several percent (Goad and others, 2000; 

Slack, 2007) and hence constitute a possible byproduct commodity from mining. In some deposits, other 

possible economically recoverable metals include Y and REE (Slack, 2006).

Fluid inclusions have been identified in quartz from Co-Cu-Au deposits, but the relationship of 

the inclusions to the sulfide assemblages is difficult to determine due to changes in rock textures that 

commonly occurred during syn- or post-mineralization deformation or metamorphism. Multiple 

inclusion types are typically present and record complex fluid histories. Heating and freezing 

experiments on inclusions from several deposits have revealed both high salinity fluids (Nash and Hahn, 

1989), with salt contents up to 10 times that of seawater and low salinity fluids containing CO2. The 

roles that these two fluid types may have played in transporting metals to sites of ore deposition, or 

redistributing preexisting metal accumulations, is uncertain. Also uncertain is whether the measured 

homogenization temperatures for the inclusions, which typically are in the range 250 to 400°C, reflect 

sulfide precipitation or later metamorphic or hydrothermal events.

Available sulfur isotope analyses of sulfide minerals show positive δ34S values, with means for 

individual deposits ranging from 1 permil for Werner Lake (Pan and Therens, 2000) to 21 permil for 

Modum (C.A. Johnson, U.S. Geological Survey, unpublished data). Variations in δ34S are small at 

Blackbird and Werner Lake, for which standard deviations about the mean are about 1 permil, but data 

for Modum show a broad δ34S range of approximately 16 permil. High mean δ34S values at Blackbird—

about 8 permil—(Johnson and others, 2007) and Modum—about 18 permil—suggest that sulfur in the 

ores was derived predominantly from sedimentary sources. A lower mean δ34S value at Werner Lake 

may reflect a magmatic component at this deposit, although a sedimentary source is not precluded. 

Limited variation in δ34S at Blackbird and Werner Lake contrasts with sulfur isotope values for most 

sedimentary exhalative (SEDEX) Pb-Zn deposits, for example, and precludes sulfide formation from 

H2S that was produced by bacterial sulfate reduction during sedimentation or diagenesis. 

Limited hydrogen and oxygen isotope analyses reveal hydrothermal effects in metasedimentary 

wall rocks to the ores. Decreases in δD and δ18O have been observed within meters to perhaps a few 

hundred meters of the Co-Cu-Au deposits, consistent with fluid-rock isotope exchange at elevated 

temperatures. Carbon isotopes in carbonate minerals show evidence for multiple carbon sources, 

including sedimentary organic matter (or mobile hydrocarbons derived therefrom), and magmatic or 

metamorphic CO2. 

Metasedimentary Host Rocks

The depositional environment of the metasedimentary host rocks to most of the deposits 

considered herein is obscured by metamorphism and deformation. However, at Blackbird, the host rocks 

probably originated as turbidite flows and, to a lesser extent, as fine-grained marine sandstones (see 

discussion in Lund and Tysdal, 2007). Near Blackbird, a separate, thrust-bounded package of shallowwater siliciclastic rocks includes scapolite-bearing metacarbonate layers that originated as evaporates 

(Lund and Tysdal, 2007). In some analogous Co-Cu-Au deposits elsewhere, evaporative or exhalative 

rocks also are present regionally in strata that are broadly correlative with the mineralized sequence. 
8 

Metamorphic grade of the Blackbird host rocks is middle greenschist to lower amphibolite 

facies. Host rocks in several similar deposits are at granulite grade. The metamorphic mineralogy is 

similar to that of the unmetamorphosed, stratigraphically equivalent rocks (muscovite, biotite, feldspar, 

quartz), but the original sedimentary grains were coarsened by metamorphism, producing fine- to 

medium-grained schist or gneiss. 

The metamorphic fabrics change across the region and the Blackbird district from fold-related 

cleavage in middle greenschist-facies rocks, to transposed layering and axial-planar foliation in upper 

greenschist-facies rocks, and to intrafolial foliation and metamorphic compositional layering in lower 

amphibolite facies rocks (Lund and Tysdal, 2007). Because of the range in metamorphic grades, the 

geometries of resultant mineralized zones range from bedding-oblique cleavage zones to metamorphictectonite-parallel zones. The complex sequence of sulfide and gangue minerals invaded the regional 

metamorphic fabrics, and the resultant mineralized zones were deformed further during continued 

tectonism. The range of metamorphic mineral assemblages and fabrics across the Blackbird district 

includes variations in metamorphic grade and fabric of country rocks; grain size, metamorphic mineral 

assemblage, and complexity of mineralized zones; and the proportions of different metals in the 

mineralized zones. From the textural and fabric evidence, the structurally concordant ore assemblages 

formed during peak to post-peak metamorphic and syntectonic to late syntectonic phases (Lund and 

Tysdal, 2007). In previous studies, the compositional layer-parallel geometry of the deposits led to 

interpretations that these were originally stratiform deposits and that the original metal deposition was 

synsedimentary or synvolcanic in origin (Earhart, 1986; Nash and Hahn, 1989; Evans and others, 1995). 

Late-metamorphic, brittle, discordant structures controlled late-phase mineralization. Analogous 

deposits also display evidence of late-stage mineralization during late-tectonic or retrograde 

metamorphism.

Petrology of Related Igneous Rocks

Intrusive igneous rocks temporally associated with Co-Cu-Au deposits range in composition 

mostly from diorite, granodiorite, tonalite, monzodiorite, and quartz monzonite to granite and 

syenogranite. Intrusions may show evidence for mixing and mingling of granitoid melts with more basic 

magma (for example, Pollard and others, 1998), such as in the Blackbird region where coeval granite 

and gabbro were emplaced about 1.37 Ga (Doughty and Chamberlain, 1996; Aleinikoff and others, 

2010). Quartz monzonites and granites locally have well-developed, rapakivi-textured K-feldspar. 

Textures range from medium- to coarse-grained hypidiomorphic granular to porphyritic. Chemically, 

the intrusive rocks are mostly subalkaline to alkaline, high-potassium to shoshonitic, metaluminous to 

weakly peraluminous, I- to A-type granitoids. The intrusions occurring in Proterozoic provinces are 

mainly ilmenite series (reduced) and contain abundant highly incompatible trace elements such as Th, 

U, Nb, Ta, Zr, Hf, and light REE, plotting in within-plate fields on tectonic discrimination diagrams 

(Pollard and others, 1998; Pollard, 2006). Their geochemical and isotopic characteristics suggest that 

they are mainly products of partial melting of older crust, possibly as a result of underplating and (or) 

intrusion into the lower crust of mantle-derived magmas.

Mafic rocks, generally in the form of dikes and sills, are typically present in proximity to 

Co-Cu-Au deposits and have been suggested as possible sources of some ore components, such as Ni 

and Co (Pollard, 2000). Although the timing of mafic magmatism generally is not well defined, in some 

cases field relationships suggest commingling of felsic and mafic magma and that the mafic magmatism 

was comagmatic with the felsic plutons (Doughty and Chamberlain, 1996). In the Blackbird district, 

mafic intrusions have within-plate, alkali basalt-like compositions (Nash and Hahn, 1989), but in other 

areas they are continental tholeiitic basalts (Frost and Frost, 1997). 
9 

The importance of igneous rocks to Co-Cu-Au deposits is unclear, in part because many deposits 

show a clear temporal, but not spatial, relationship to major magmatic intrusions like the settings of 

many IOCG deposits (Groves and others, 2010). However, abundant isotopic data, including both 

radiogenic and stable isotopes, and fluid inclusion data from a number of IOCG deposits suggest that 

magmatic fluids were a major contributor to ore formation (Pollard, 2000; 2006; Williams and others, 

2005). Similar studies are needed on Co-Cu-Au deposits in metasedimentary rocks in order to better 

evaluate possible genetic links between magmatism and mineralization.

Genetic Models

The broadly stratabound character of mineralized zones in most of the Co-Cu-Au deposits 

considered herein led many previous workers to invoke syngenetic mineralizing processes like those 

that formed VMS and SEDEX deposits. However, reevaluation of these deposits in the present study 

instead suggests an origin involving epigenetic hydrothermal processes long after sedimentation and 

diagenesis. The presence in most of the Co-Cu-Au deposits of magnetite-rich rocks, abundant K-rich 

(biotite) minerals or Na-rich (albite) minerals in altered wall rocks, and in some cases high 

concentrations of Y, REE, and U, provides permissive evidence for a link to IOCG-type deposits (for 

example, Davidson and Large, 1998; Hitzman, 2000; Williams and others, 2005; Corriveau, 2007). In 

this speculative model, the metasedimentary-hosted Co-Cu-Au deposits described herein may represent 

a Co-rich variant of IOCG deposits, similar to the Cloncurry-type category of Corriveau (2007). It is 

important to note, however, that the highly deformed and metamorphosed character of most of the 

Co-Cu-Au deposits obscures or masks primary features of hydrothermal mineralogy, texture, and 

alteration, such that discerning the nature and timing of original mineralization is difficult at best. 

Testing an IOCG model for the deposits included in this report will require more detailed geological, 

geochemical, and mineralogical studies, including high-precision U-Pb dating and (or) other 

geochronological methods in order to better evaluate the age of mineralization in each deposit relative to 

the history of local sedimentation, diagenesis, metamorphism, and plutonism. 

Geophysical Characteristics 

Owing to the presence of magnetite in many metasediment-hosted Co-Cu-Au deposits, regional 

geophysical surveys that include magnetics and gravity can be useful in identifying potentially 

mineralized zones. Significantly, deposits are not necessarily coincident with a discrete magnetic 

anomaly but typically occur in areas having “magnetically active” signatures (Smith, 2002). 

Aeromagnetic data have been obtained for the Idaho cobalt belt where the Blackbird deposit lies on the 

southwest flank of a prominent magnetic trough that parallels the belt (Evans and others, 1995; Lund 

and others, 1989). Aeromagnetic data, together with very-low-frequency (VLF) electromagnetic 

surveys, aid in identifying regional-scale, structural lineaments, faults, and shear zones that may be 

important in localizing deposits. The presence of plutonic bodies, however, can have a strong control on 

regional geophysical patterns (for example, the Idaho cobalt belt; Lund and others, 1989), potentially 

complicating interpretations.

Because highly potassic (biotite or K-feldspar) and (or) sodic (albite) alteration commonly forms 

halos around the deposits, regional radiometric surveys can be useful in recognizing alteration in areas 

where unweathered rocks are exposed at the surface (Wellman, 1999; Goad and others, 2000). For 

example, in the Lou Lake area, Northwest Territories, Canada, the NICO deposit is associated with 

discrete positive eK and negative eTh/K anomalies that are coincident with positive magnetic anomalies 
10

(Goad and others, 2000). Areas having high U concentrations also are characterized by increased eU/Th 

anomalies.

Deposit-scale geophysical signatures are typically more complex than regional features. 

Abundant magnetite will produce characteristically strong positive magnetic anomaly (values up to 

16,000 nanoteslas (nT) above background); however, a predominance of hematite tends to yield only 

low-intensity anomalies (Goad and others, 2000). Because iron oxides and related iron-rich silicate

minerals have relatively high densities (5.2 g/cm3 and 3.0 to 3.4 g/cm3, respectively) compared with

those of typical continental crust (2.5 to 2.7 g/cm3

), the resulting density contrast may generate subtle to 

distinct Bouguer-gravity anomalies (Goad and others, 2000; Smith, 2002). A range of electrical methods 

has been used for direct detection of these deposits, because most deposits are at least weak conductors. 

Transient electromagnetic methods (TEM) and induced polarization (IP) have been generally successful 

in detecting these deposits, but they also respond to the presence of both iron oxides and barren sulfides, 

which commonly are more laterally extensive than the target mineralized zone. Standard 

electromagnetic geophysical methods (EM) have generally not been as successful, probably because 

massive, continuous mineralization is rare in many of the deposits considered herein. 

Geoenvironmental Features and Anthropogenic Mining Effects

The environmental geochemistry of Co-Cu-Au deposits was evaluated using published data for 

mine waste, stream sediment, soil, mine runoff water, and stream water collected in the Blackbird area. 

No published environmental geochemical data are known for other, similar Co-Cu-Au deposits 

elsewhere; therefore the descriptions herein are limited to data for deposits in the Blackbird district. 

Because host rocks to these deposits are mostly fine-grained siliciclastic metasedimentary strata with 

little carbonate, surrounding rocks have little acid neutralizing capacity. As a result of mining, greater 

than 3,500,000 m3 of tailings are estimated to be present in the Blackbird Creek area (Beltman and 

others, 1993). Weathering of Blackbird deposits and mine wastes results in a high potential to generate 

acidic water in mine runoff, and stream water of pH 2 to 4 has been reported (Beltman and others, 

1993). Acid water generation is primarily due to the oxidation of sulfide minerals such as pyrite and 

pyrrhotite. Metal concentrations in mine runoff water and sediment are highly elevated relative to 

regional baseline concentrations; several studies have indicated that the most problematic contaminants 

of concern are Co, Cu, As, Fe, and Mn (Rocky Mountain Consultants, 1995; Mebane, 1997; CH2MHill, 

2001; Eppinger and others, 2003; Giles and others, 2009). 

Analyses of unfiltered stream water in the Blackbird area indicate that concentrations of Co, Cu, 

Fe, and Mn generally exceed drinking water or chronic aquatic life standards (intended to be protective 

of the majority of the aquatic communities), whereas concentrations of As in water are generally below 

established standards. Concentrations of Cu in stream water collected below mines vary from 18 to 

49,000 μg/L; most samples exceed the U.S. Environmental Protection Agency (USEPA) drinking water 

standard for Cu of 1,000 μg/L and the chronic aquatic life standard of 2.7 μg/L (at a water hardness of 

25 mg/L). No water standards have been established for Co, but concentrations of Co are as high as 

1,470,000 μg/L in mine runoff water in the Blackbird area, whereas Co contents are much lower in 

stream water collected from regional uncontaminated sites, ranging from <0.02 to 6.3 μg/L. 

Concentrations of As in water are a potential human health concern because As is highly toxic and a 

known carcinogen. Stream water samples contain As that varies from <1 to 18 μg/L, most being below 

the USEPA drinking water standard for As of 10 μg/L; all are below the chronic aquatic life standard of 

190 μg/L with the exception of one water seep containing As of 930 μg/L. Mine water runoff and 

streams proximal to mines contain Fe concentrations varying from <6.8 to 159,000 μg/L. Most water 

samples exceed the USEPA secondary (maximum contaminant limit goal) drinking water standard for 
11

Fe of 300 μg/L and the USEPA chronic aquatic life standard of 1,000 μg/L for Fe. The only established 

water standard for Mn is the USEPA secondary drinking water standard of 50 μg/L. Concentrations of 

Mn vary widely in mine water runoff and streams in the Blackbird area, ranging from 2.5 to 

35,000 μg/L, many of which exceed the drinking water standard for Mn. 

Acknowledgments

We thank A.A. Bookstrom and M.W. Hitzman for constructive reviews that improved the 

manuscript.

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