Geology and Geochemistry of 

Synsedimentary. Cobaltiferou 

Depos.its, I ron .. reek,. ~~tl1hi 
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Geology and Geochemistry of 

Synsedimentary Cobaltiferous-Pyrite 

Deposits, I ron Creek, Lemhi County, Idaho 

By J. THOMAS NASH 

A description of cobaltiferous-pyrite lenses in Precambrian 

metasedimentary rocks that are partly stratiform and partly 

discordant in zones of soft-sediment deformation 

U.S. GEOLOGICAL SURVEY BULLETIN 1882 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U. S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1989 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Nash, J. Thomas (John Thomas), 1941-

Geology and geochemistry of synsedimentary cobaltiferous-pyrite 

deposits, Iron Creek, Lemhi County, Idaho I by J. Thomas Nash. 

p. cm.-(U.S. Geological Survey bulletin: 1882) 

"A description of cobaltiferous-pyrite lenses in Precambrian metasedimentary rocks that are partly stratiform and partly discordant in zones of 

soft-sediment deformation." 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1882 

1. Cobalt ores -Idaho-Iron Creek Region (Lemhi County) 

2. Pyrites-Idaho-Iron Creek Region (Lemhi County) 

3. Geology-Idaho-Iron Creek Region (Lemhi County) 4. Geology, 

Stratigraphic-Pre-Cambrian. I. Title. II. Series. 

QE75.B9 no. 1882 

[QE390.2.C58] 

557.3 s -dc19 

[553.4'8] 89-600025 

CIP 
CONTENTS 

Abstract 1 

Introduction 1 

Previous exploration and geologic studies 3 

Geophysics 3 

Geology 3 

Geochemical methods 5 

Geologic setting 5 

Stratigraphy 5 

Structure 6 

Jackass zone 6 

The No-N arne deposit 9 

Structure 9 

Lithology 9 

Soft -sediment deformation 11 

Ore mineralogy 14 

Alteration 14 

Geochemistry 15 

Discussion and interpretation 23 

Environment of formation 23 

Synsedimentary structural controls 23 

Geochemistry 24 

Similar ore deposits 24 

Metamorphism 25 

Resources and exploration 25 

Ore genesis 26 

References cited 26 

Appendix 1. Description of analyzed samples 30 

Appendix 2. Methods for geochemical analysis 32 

FIGURES 

1. Map showing location of Iron Creek area in Idaho cobalt belt 2 

2. Map showing generalized geology of Iron Creek area and location of 

mineralized zones 4 

3. Generalized lithologic logs of drill holes through the No-Name deposit showing 

distribution of disruption zones, cobalt, and copper 7 

4. Photographs of drill core from the Yellowjacket Formation showing disruption 

structures 8 

5. Photographs of thin sections showing textures of sedimentary rocks from the 

Yellowjacket Formation 10 

6. Photomicrographs of textures in thin sections of samples from the Yellowjacket 

Formation 12 

7. Scatterplots of geochemical data for the No-Name deposit 22 

Contents Ill 
TABLES 

1. Analytical results for samples of drill core from the No-Name deposit 16 

2. Summary of analytical results for ore and enclosing wallrocks, No-Name 

deposit 21 

3. Lower limits of determination in geochemical analyses of samples from the 

No-Name deposit 33 

IV Contents 
Geology and Geochemistry of 

Synsedimentary Cobaltiferous-Pyrite Deposits, 

Iron Creek, Lemhi County, Idaho 

By J. Thomas Nash 

Abstract 

Cobaltiferous-pyrite deposits at the No-Name prospect 

occur in weakly metamorphosed argillite and siltite of the 

Middle Proterozoic Yellowjacket Formation, about 28 km 

southwest of Salmon, Idaho. The No-Name deposit comprises several lenses of stratiform and discordant pyrite in a 

zone as much as 250 m thick and more than 1 ,500 m long. 

Cobalt grades typically are in the range 0.2-Q.7 percent, 

some intercepts being in excess of 1 percent, along with 

copper in the range 0.5-1 percent. Two stratabound zones of 

iron-rich mineralized rock are known in the area, of which the 

stratigraphically lower one contains chiefly magnetite having 

generally low sulfide and cobalt content. The No-Name 

deposit contains abundant magnetite at the bottom and 

pyrite and chalcopyrite in upper layers. Approximately equal 

amounts of fine-grained stratiform pyrite and coarse-grained 

epigenetic pyrite are present, and most of the cobalt is 

contributed by the epigenetic variety. The cobalt-copper 

zones are associated with synsedimentary disruption structures characterized by retextured sediment, loss of bedding 

features, lump breccia, and probable bedding-plane slipsall probably produced by liquefaction, fluidization, and submarine sliding of unconsolidated sediment. The pyrite 

deposits are enriched in Fe, S, As, Ba, Bi, Co, Cr, Cu, Ni, Pb, 

Se, and Zn, but the concentrations of Ag, Pb, and Zn are 

much lower than in most sediment-hosted base-metal 

deposits. Arsenic content is much lower than in the Blackbird 

deposits, 30 km to the northwest, which greatly simplifies the 

metallurgy of these sulfide concentrates. 

The cobaltiferous-pyrite deposits formed in a submarine geothermal system associated with rifting and volcanism in the Yellowjacket basin. Geothermal vents in this part 

of the rift basin are marked by lenses of magnetite ironformation, and the No-Name system evolved to one that 

deposited syngenetic and epigenetic iron and copper sulfide 

minerals. Syngenetic pyrite contains modest amounts of 

cobalt, but some growth zones in epigenetic pyrite are rich in 

cobalt. Cobalt- and copper-bearing fluids appear to have 

Manuscript approved for publication November 30, 1988. 

been generated during early diagenesis and to have 

migrated to sites of deposition in zones of soft-sediment 

deformation. These processes were probably promoted by 

submarine volcanism and sliding on the fan slope. The ore 

was deposited probably within 100 m of the seafloor, 

possibly due to rapid cooling or dilution of pore fluid by 

seawater. 

INTRODUCTION 

A significant volume of cobaltiferous pyrite (Copyrite) occurs in metasedimentary rocks of the Middle 

Proterozoic Yellowjacket Formation at the No-Name 

deposit, in the area of Iron Creek, about 28 km southwest 

of Salmon (fig. 1). This is the southernmost known 

deposit of those constituting the Idaho cobalt belt 

(Hughes, 1983), all of which are stratabound in the 

Yellowjacket Formation. The No-Name Co-pyrite 

deposit differs in many ways from the larger deposits at 

the Blackbird mine 30 km to the north. Possibly the most 

important differences are the low arsenic content and 

simple sulfide mineralogy of the No-Name deposit, both 

of which are favorable for conventional smelting rather 

than requiring hydrometallurgical refining not currently 

(1987) available in the United States. 

This study was undertaken as part of broader 

investigations of the geology and geochemistry of the 

Idaho cobalt belt by the U.S. Geological Survey under 

the Strategic and Critical Metals Program. A primary 

objective of the investigation of the Iron Creek deposits 

was to compare their geology and geochemistry with the 

better known cobalt deposits of the Blackbird mine area 

(Nash and Hahn, 1986). Goals of the projects were to 

describe the setting of the Co-pyrite deposits, controls on 

the distribution of cobalt, their geochemistry and mineralogy, and associated alteration, and to interpret conditions of formation and the potential for the occurrence 

of additional deposits. 

Introduction 
YELLOWJACKET FORMATION 

D PHANEROZOIC 

UNDIFFERENTIATED 

t;<',-~>J PROTEROZOIC 

GNEISS, GRANITE 

~ { t:~J}~~~:;:~~J UPPER UNIT ~.&~0 ············ ..JN 

~ffi [:\/·:.\~]MIDDLE UNIT 

-~ 

2~ ~ LOWER UNIT 

• COBALT 

DEPOSITS 

a.. 

Figure 1. Location of Iron Creek area in Idaho cobalt belt, Lemhi County, Idaho. Generalized 

geology of region is from Hahn and Hughes (1984). Iron Creek deposits (I C) are at southeastern 

end of the cobalt belt. 

This research was made possible by the cooperation of Noranda Exploration, Inc., who provided 

maps, geologic information, and access to drill core. 

Many Noranda geologists contributed important 

observations, in particular C.V. Chevillon, T.K. Stump, 

and T.A. Webster. I thank Vic Chevillon, Greg Hahn 

(formerly with Noranda Exploration, Inc.), and U.S. 

Geological Survey colleagues Jon Connor, Karl Evans, 

Dawn Madden, and Peter Modreski for helpful 

discussions. 

2 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
PREVIOUS EXPLORATION AND 

GEOLOGIC STUDIES 

The No-Name deposit was discovered during 

construction of a U.S. Forest Service road along the 

North Fork of Iron Creek in 1967 (Cohenour and others, 

1973). From 1968 several claim owners and mining 

companies explored the area and discovered several 

zones of mineralization (fig. 2). The No-Name deposit, 

cut at the site of the present east adit, was recognized as 

containing abundant pyrite along with scattered chalcopyrite and secondary copper minerals as disseminations and fracture fillings in argillite and quartzite. 

Geologic, geochemical, and geophysical surveys preceded more than 5,700 m of diamond drilling and about 

200m of underground drifting from two adits. The east 

adit on the No-Name deposit encountered rock averaging 0.76 percent Cu and 0.19 percent Co, and drilling 

intersected as much as 142 m of rock mineralized with 

sulfides (in a 248-m hole), containing 0.99 percent Cu 

equivalent (Co percent= 4 x Cu percent; Cohenour and 

others, 1973). The average combined thickness of sulfide 

zones was about 50 m at an average grade of about 0.83 

percent Cu equivalent. On the basis of geophysical, 

geologic, and drilling information in 1973, the No-Name 

deposit was estimated (Cohenour and others, 1973) to 

contain a resource of " ... a few 100,000 tons to millions 

[of tons]." 

Three other mineralized zones were identified in 

the Iron Creek area (Cohenour and others, 1973). (1) A 

zone rich in magnetite was found about 1.7 km southeast 

of, and parallel to, the No-Name zone (fig. 2). Called the 

"Jackass zone" for the canyon of that name, it seemed to 

contain only small amounts of copper and cobalt. The 

magnetite-pyrite deposit in the Jackass zone was 

apparently first discovered and staked in 1946; part of it 

is now well exposed in a roadcut of a logging road. (2) A 

zone of copper sulfate lies about 700 m north of the 

No-Name zone; geochemical and geologic studies prior 

to 1973 suggested this was a small, low-grade zone 

parallel to bedding and no further work has been done. 

(3) A zone of arsenopyrite containing traces of gold was 

found about 1.5 km north of the No-Name deposit and 

uncovered by a series of bulldozer trenches. The zone of 

veining is reported to be about 150 m wide, individual 

veins being as much as 2 m wide. The highest grade 

samples contained as much as 1 oz/ton Au, 0.4 percent 

Co, 0.4 percent Bi, and minor Ag (Cohenour and others, 

1973). 

Geophysics 

The geophysical studies described by Cohenour 

and others (1973) are summarized here because that 

report is difficult to obtain. An induced-polarization (IP) 

survey in 1968 around the No-Name discovery cut 

suggested the presence of an east -trending zone of 

sulfide minerals " ... several hundred feet wide and at 

least 1,700 feet long." All four holes drilled into this IP 

anomaly hit "significant sulfide mineralization." Subsequent "spontaneous polarization" measurements determined anomalies as large as 400 millivolts at sites of the 

strongest IP response. Additional IP lines run in 1971 

enlarged the anomaly to about 240 m wide by at least 

1,500 m long. Low resistivities were found in the IP 

anomaly. 

Ground and airborne magnetic surveys detected 

the No-Name zone. A regional airborne survey (area 26 

by 48 km, spacing 400 m, clearance 150m) was made of 

the area as far north as the Blackbird mine. A linear 

magnetic high was detected over the Iron Creek area, but 

the Blackbird deposits produced no magnetic signature 

in the 1972 survey. The magnetic trend was interpreted to 

reflect a faulted, buried "magnetic igneous rock" (Cohenour and others, 1973). It is more likely that the magnetic 

high is caused by magnetite-rich zones in the Yellowjacket Formation. The east-trending magnetic anomaly is 

terminated at its eastern end by a structure interpreted 

by Cohenour and others (1973) to be a left-lateral fault. 

Ground magnetometer surveys determined an anomaly 

having "significant relief' only along a line across the 

middle of the No-Name zone. The author has found a 

magnetic susceptibility meter to be a useful tool for 

reliable detection of magnetic minerals in drill core and 

outcrop. Magnetic methods seem useful for detecting 

zones of magnetite in the Iron Creek area. 

Geology 

Early geologic studies were aimed at defining the 

geometry of the Co-pyrite zone and its metallurgical 

character (Cohenour and others, 1973). The IP anomaly 

previously described and 25 drill holes were used to 

define the No-Name zone as about 250m wide and at 

least 1,500 m long. The early studies showed that the 

cobalt occurs in "lens-shaped bodies" interpreted by 

Cohenour and others (1973) to reflect " ... zones of 

brecciation and bedding plane fractures, possibly related 

to bedding plane faults." Electron microprobe analyses 

showed that "cobalt occurs chiefly in the matrix (lattice) 

of pyrite crystals and that none occurs in the chalcopyrite. 

These results indicate that essentially no cobalt can be 

recovered with the copper and that to recover cobalt a 

separate pyrite concentrate will have to be produced" 

(Cohenour and others, 1973). 

From 1977 to 1982, Noranda Exploration, Inc., 

undertook new studies of the Iron Creek area, focusing 

on the No-Name and Jackass zones. C.V. Chevillon, T.K. 

Previous Exploration and Geologic Studies 3 
44° 

57' 

30" 

~ 

~ 

Yyl 

,f' -:/ 

1' 

1' 

1000 2000 

3000 6000 FEET 

II 

II 

\\ ~' " ~ '\ 

65 .. \) 

r/\~~? 

:: Slump fold 

II 

II 

II 

II 

II 

II 

II 

II 

II 

)I 

II 

II 

\I 

II 

II 

II 

II 

II 

\\ 

II 

1\ 

\I 

II'. 

II 

Jl 

If 1: I;: ' .~ . 

J'45 .y?"~ 

3000 METERS 

EXPlANATION 

Volcanic rocks (Tertiary) 

Lower unit of Yellowjacket 

Formation (Middle Proterozoic) 

Contact-Approximately located 

Facies boundary-Approximately 

located 

5 Strike and dip of beds 

I

IC12 Drill-hole collar and projection 

= = = = = = = = Improved (logging) road 

Figure 2. Generalized geology of Iron Creek area and location of Jackass and No-Name mineralized zones. 

Also shown are locations of drill holes sampled in this study. Geology generalized from Chevillon (1979). Base 

from U.S. Geological Survey, 1:24,000, Degan Mountain, 1962. 

4 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
Stump, and T .A. Webster mapped the area in detail and 

relogged drill holes. Some of the work by Noranda at 

Iron Creek is alluded to by Hughes (1983), but important 

details remain hidden in unpublished maps and reports 

(Chevillon, 1979; Webster and Stump, 1980). The 

Noranda group traced several mineralized stratigraphic 

zones along strike for more than 1 km and recognized 

several types of soft-sediment deformation in those 

zones, including tight outcrop-scale folds and destruction 

of primary sedimentary textures. Noranda geologists 

recognized that the Co-pyrite is chiefly localized in zones 

of "fluidized" sediment that lack the fine lamination and 

many primary sedimentary structures of nearby Yellowjacket strata. They also documented the distribution of 

magnetite and pursued (without success) gold using the 

analogy of Tennant Creek, Australia: deposits similar to 

the No-Name prospect that are rich in magnetite, chlorite, and bismuth, as well as gold (Large, 1975). 

Regional mapping and studies of ore deposits are 

reported by Modreski (1985), who provides an excellent 

framework for understanding the Iron Creek and other 

cobalt deposits. Modreski reports centimeter-scale 

polygonal cracks in the Yellowjacket that he interprets to 

have formed during periods of emergence. 

GEOCHEMICAL METHODS 

This study started in 1984 with examination of 

surface rocks at Iron Creek and logging and sampling of 

drill core. The locations of the drill holes are shown on 

figure 2. Approximately 200 slices of drill core a few 

centimeters long were taken for petrographic study. 

Samples for chemical analysis were of two types. 

Generally, if there was sufficient core, a one-quarter 

portion 20-50 em long was sawn out. Some smaller 

samples were collected to focus on specific mineralized 

structures. Crushed core was used for places where the 

mineralized zones had been previously sampled more 

than once and for which very little core remained. The 

splits of crushed assay "reject" represent 2-5 ft (0.6-1.5 

m) of core. Where possible, a slice representative of 

unmineralized or mineralized rock was cut from the 

analyzed material for petrographic study. Fifty-five 

samples were prepared as thin sections (most polished), 

and an equal number of slabs were polished. Brief 

descriptions of many of the samples are in Appendix 1. In 

this report samples are identified by the drill-hole 

number and depth in the hole (in feet), such as 

IC10-300. 

Chemical analyses were made of 41 core samples; 

results are in table 1. Major elements were determined 

by X-ray fluorescence spectrometry (XRF), and minor 

and trace elements were determined by inductioncoupled plasma spectrometry (ICP) and wet chemical 

methods. The analytical methods and their limits of 

detection are described in Appendix 2. 

GEOLOGIC SETTING 

Stratigraphy 

The host rocks for the cobalt deposits are the oldest 

rocks in the area. These are fine-grained clastic rocks that 

have been metamorphosed to biotite grade of the greenschist facies. They probably are part of the Middle 

Proterozoic Yellowjacket Formation (Hughes, 1983). 

Another view (Modreski, 1985) is that these rocks are of 

Middle Proterozoic age but of uncertain correlation. 

Regional mapping reported by Hughes (1983) indicates 

that the Iron Creek rocks are part of the lower unit of the 

Yellowjacket (fig. 1). A similar correlation is proposed by 

Connor and Evans (oral commun., 1987) from their 

mapping in the Yellowjacket. The rocks in the Iron Creek 

area have a high proportion of argillite, possibly 

consistent with the interpretation of Hughes that the 

lower unit was deposited in a basin plain or fan fringe of 

a turbidite system but the small amount of graded 

bedding and other evidence for sedimentation from 

turbidity currents do not support such an interpretation. 

According to the reconstruction of Hughes (1983), the 

Iron Creek area was along the axis of a rift zone, possibly 

near a basin-margin fault. Paleocurrent measurements, 

chiefly on crossbedding and ripple crosslaminations, 

suggest sources on both the east and west margins of the 

rift basin (Hughes, 1983). Depositional conditions may 

have changed rapidly in the Iron Creek area. Local sand 

wedges suggest intermittent uplift or higher energy 

transport, whereas sequences of dark argillite suggest 

quiescence in a starved-basin stage (Hughes, 1983). 

Argillite containing abundant laminated fine-grained 

pyrite in the Iron Creek area supports the starved-basin 

stage. Bedded iron-formation at several localities in the 

Jackass Creek and No-Name areas suggests quiescence, 

the unusual concentrations of iron coming from seafloor 

vents. There are no red rocks in the Iron Creek area that 

might suggest oxidizing intertidal conditions, but mudchip conglomerate and sand-filled polygonal structures 

found in the Y ellowj acket about 10 km northeast of Iron 

Creek (Modreski, 1985) seem to be restricted to areas of 

strong currents, possibly shallow water and emergence. 

However, most of the evidence suggests that the lower 

part of the Yellowjacket was deposited below wave base. 

About 300m upsection (west) from the No-Name mine, 

sandier beds are more widespread and mark the 

approximate base of the middle unit of the Yellowjacket 

(Connor, oral commun., 1987). 

Geologic Setting 5 
Mafic igneous rocks are present in the area and 

seen in float, drill core, and rare outcrops. They are 

presumed to be of Proterozoic age because they are 

metamorphosed to the same grade as the Yellowjacket. 

Some mafic rocks retain a porphyritic fabric, but all are 

altered and metamorphosed. Details of their geometry 

and composition are not well known. In drill core the 

igneous rocks have massive fabric for a meter or more, 

which suggests they are intrusive. An outcrop near the 

Jackass prospect was interpreted as a bedded pyroclastic 

rock by R.L. Earhart (oral commun., 1986). Both mafic 

dikes and coeval pyroclastic rocks may be present, as they 

are in the Blackbird mine area (Hahn and Hughes, 

1984), but they are not abundant in the Iron Creek area. 

Tertiary volcanic rocks of the Challis volcanic field 

cap ridges in the area. These rocks were deposited on a 

surface having considerable relief. The Tertiary rocks are 

not known to be involved in processes forming or modifying the cobalt deposits, but they cover the south end of 

the No-Name zone. 

Structure 

Yellowjacket strata in the Iron Creek area 

generally dip about 50°-7SO to the northwest or northeast 

(fig. 2). Several north-striking faults cut Tertiary volcanic 

rocks, but faults are difficult to recognize in the Yellowjacket. Cohenour and others (1973) describe geophysical 

anomalies that they interpret to reflect faults, but these 

have not been confirmed on the ground. Local changes in 

dip suggest the presence of several broad folds in the 

Jackass Creek area (Chevillon, 1979), and some outcropscale folds are interpreted to be slump folds (Webster 

and Stump, 1980). In 1987 C.V. Chevillon showed me 

some of the tight folds, and I would agree that they seem 

to be soft-sediment rather than tectonic structures. 

Bedding in these folds is highly contorted, and the axial 

region of the folds is filled by material that resembles 

liquefied sand in a sandstone dike; there is very weak 

axial-plane cleavage. In the No-Name zone, strata consistently dip about 60° NW. and no major faults or folds 

have been recognized in drill core or outcrops. 

Cobalt deposits in the Iron Creek area are stratabound, and locally stratiform, but a major proportion 

of the ore minerals occurs within synsedimentary softsediment structures (fig. 3). These discordant structures, 

which will be described in a later section, produce lumpy 

to massive fabric (fig. 4B-4G) with few of the finely 

laminated beds that are typical of the Yellowjacket. 

Although the genesis of these fabrics is open to debate, 

the disruption structures and retextured fabrics are 

distinctive and diagnostic features of these mineralized 

zones. In drill core or good outcrop one can see a 

progression over tens of meters from beds having 

excellent lamination at millimeter scale to strata having 

no primary bedding features. Well-developed primary 

and secondary sedimentary structures are so widespread 

and typical of the Yellowjacket (Hughes, 1983; fig. SA, 

this report) that their absence is noteworthy and is 

presumed to reflect destruction or retexturing following 

sedimentation. Zones that lack characteristic primary 

and secondary sedimentary structures are generally 

above beds that display abundant evidence for dewatering: sand · dikelets, load casts, and wispy bedding. 

Noranda recognized the destruction of these sedimentary 

features in 1978 and called such beds "fluidized" in 

reference to the process believed to have caused the 

destruction of features. The nongenetic term "disruption" will be used in this report to describe beds that 

have partly or completely lost their primary bedding 

characteristics. Because cobalt deposits are localized in 

disruption structures and appear to have formed 

together, these structures will be discussed further in 

later sections. 

JACKASS ZONE 

The Jackass zone, about 300 m stratigraphically 

below the No-Name zone, contains abundant magnetite 

and pyrite in a 10-m-thick section of conspicuously 

orange-brown argillite and siltite that crops out in the 

upper part of Jackass Creek, about 1.7 km northeast of 

the No-Name adit (fig. 2). This gossan was staked in 

1946, later was deeply cut for a logging road, and now is 

called the "Jackass prospect." The zone of magnetic 

minerals has been traced on strike for several kilometers 

(Webster and Stump, 1980) and crops out 700 m east of 

the No-Name adit on the Iron Creek road. The roadcut 

through the Jackass prospect exposes about 5 m of 

metasedimentary rock containing abundant magnetite or 

magnetite and pyrite, now highly oxidized. Some of the 

magnetite is layered and could be called banded ironformation (BIF), but most of it has massive, swirled, or 

veinlike fabric. Some of the exposed ferruginous metasedimentary rock is moundlike in form, about 1 m high 

and 2-4 m wide. The exposures along the Iron Creek 

road contain thinner zones of magnetite and very little 

pyrite; the magnetite is partly bedded and partly in lumps, 

swirls, or thin crosscutting stringers. Much of the 

magnetite in the Jackass zone is inconspicuous in the 

dark argillite and siltite, but is easily detected with a 

magnetic-susceptibility meter. Concentrations of 1 

percent are common, and some parts contain more than 

50 percent magnetite across beds or structures 10-20 em 

thick. Some pods or disruption features in the magnetitebearing strata contain tourmaline that possibly is 

genetically similar to the tourmaline that cements breccia 

6 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
0 

200 

400 

600 

DIAMOND-DRILL HOLE IC18 

Total depth 

743 tt 

Cobalt Copper 

DIAMOND-DRILL HOLE IC12 

Sample 

• 

• 

.. 

' I 

• 

• 

I 

I 

• 

• 

DIAMOND-DRILL HOLE IC1 0 

Total depth 

1,303 tt 

Cobalt Copper 

DIAMOND-DRILL HOLE IC16 

Total depth 

608 tt 

EXPlANATION 

• 

;i~:i~~i:nd fine quartzite 

Disrupted lithology 

' 

Sample 

I 

• 

• 

I 

L 

• • 

• 

Figure 3. Generalized lithologic logs of drill holes through the No-Name deposit in the Yellowjacket 

Formation showing distribution of disruption zones, cobalt, and copper. Drill-hole locations shown on figure 2. 

Copper and cobalt values are generalized from assays by Coastal Mining Company. 

Jackass Zone 7 
north of Iron Creek and especially near the Blackbird 

mine (Modreski, 1985; Nash and Hahn, 1986). 

The Jackass zone strikes about N. 80° W., and the 

zones of magnetite±pyrite are generally conformable 

with enclosing unmineralized strata. However, in detail 

the magnetite-bearing strata show evidence for softsediment deformation that includes loss of primary 

bedding structures and redistribution in local dikelike 

features. These soft-sediment features resemble those in 

the No-Name zone, discussed in more detail in the next 

section. Siltite and fine quartzite about 100 m stratigraphically above the Jackass zone are involved in several 

outcrop-scale slump folds that possibly reflect the same 

general conditions of sediment instability as are evident 

in the Jackass zone. 

Gossan developed in areas of iron mineralization in 

the Jackass zone locally contains patches of pink or green 

minerals, suggesting the presence of some cobalt or 

8 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
Figure 4 (facing page). Drill core from the Yellowjacket 

Formation showing disruption structures. All drill core is 4.5 

em wide. A, Contrast in bedding character: two cores on right 

show typical good lamination of argillite and siltite, whereas 

two cores on left show disrupted bedding. Disruptions 

contain clasts of sediment similar to enclosing strata; epigenetic quartz and pyrite cement disruption fill. Some disruptions are aligned parallel to enclosing strata and others 

are discordant (IC1 0, 260- and 300-ft intercepts). B, Sawn 

drill core in disruption zone showing lump breccia, massive 

pyrite (Py), and discordant veinlike structures, some of which 

contain magnetite (Mt) (IC 18, interval approximately 1 ,069-

to 1 ,088-ft depth). C, Disruption zone showing clasts of siltite 

(S), magnetite (Mt), and injected clay (I) (IC1 0, parts of 

interval 809- to 827-ft depth). D, Clasts of siltite in disruption 

zone having rounded edges cemented by clay (I) that seems 

to have been injected between fragments (IC1D-824). E, Two 

zones of very rich massive-textured pyrite about 15 em thick 

that fill disruption structures; hangingwall is bedded argillite. 

Clasts of siltite float in pyritic matrix (IC1 0, 303- and 304-ft 

depth). F, Subround clasts of magnetite (Mt) and siltite (S) in 

disruption zone; polished slab of drill core (IC18-1090). 

G, Layer of fine-grained pyrite "mud" (Py, upper half of 

slabbed core) that is partly bedded and partly deformed 

(ragged interface with siliciclastic material); siltite (S) has 

lumpy breccia texture (IC12-305). 

copper in secondary minerals. However, assays of 

numerous rock-chip samples and core from two diamond 

drill holes reveal that only a few localities contain as 

much as 1,000 ppm Co (Webster and Stump, 1980). 

Current information suggests that the magnetite-rich 

parts of the Jackass zone are too thin and contain too 

little cobalt and copper to be a significant resource. 

However, these iron-rich deposits are an important 

geochemical feature of the Idaho cobalt belt and the 

evolving Yellowjacket basin. Many visitors to the Jackass 

prospect comment on the probable similarity of these 

deposits to ferruginous deposits formed on the seafloor 

by "black smokers" (for example, Mottl, 1983). 

THE NO-NAME DEPOSIT 

Structure 

Strata in the No-Name deposit have consistent dip 

where bedding is evident. No local abrupt changes in 

attitude are seen, such as would be expected at the hinge 

of a fold. Layering of ore minerals is generally parallel to 

layering of siliciclastic components, both in fine-grained 

layers thought to be clastic and in layers of coarsegrained minerals thought to be the result of epigenetic 

processes. Faults are neither evident in drill core nor 

required to explain any features, but bedding-plane slip is 

possibly indicated by smearing of layers parallel to 

bedding (fig. 4D, 4£). 

The most conspicuous structural feature of the 

No-Name deposit is zones that lack primary sedimentary 

textures (fig. 3). Generalized at deposit scale (fig. 3), 

these zones contain numerous centimeter-scale disruption structures that are mostly discordant, although 

some are concordant with bedding (where present). 

Some of the disruption structures are filled by veinlike 

material that is coarse grained and rich in pyrite, but the 

margins of the structures are not sharp like hard-rock 

veins and characteristically the enclosing rocks have 

lump-breccia1 textures believed to indicate formation 

from incompletely lithified sediment. 

Significant cobalt intercepts ( > 0.1 percent) are in 

every case in zones of disrupted bedding, as shown 

schematically on figure 3. Intervals of bedded fine pyrite 

or magnetite contain little cobalt, whereas intervals of 

discordant structures filled by pyrite, generally of coarser 

grain size, contain notable amounts of cobalt. The 

disruption structures are important at two scales: At bed, 

or centimeter, scale it is clear that Co-pyrite is confined 

to the disruption structures. Good examples are the thin 

zones 1-10 em thick, containing abundant coarse pyrite, 

that produce cobalt "kicks" greater than about 0.3 

percent for a 0.3- to 0.6-m interval of core. At deposit 

scale, the Co-pyrite zones of minable proportions are 

within a zone of numerous disruptions that is considerably thicker than the cobalt zone. For example, drill 

hole 10 encountered numerous disruptions over an 

interval of nearly 200 m. Within this interval the amount 

of disruption is highly variable: some parts consist of 

good bedding typical of the Yellowjacket, whereas in 

other parts almost no bedding can be recognized in the 

swirled mass. Not all disrupted zones (at centimeter 

scale) contain significant Co-pyrite, but Co-pyrite exists 

only in disruptions. Stratiform fine-pyrite layers contain 

as much as 3,600 parts per million (ppm) Co, but 

generally are too thin at this grade to constitute a 

significant contribution of cobalt. 

Lithology 

Yellowjacket strata in the Iron Creek area are 

generally very fine grained and rich in micas. Argillite 

and siltite beds a few millimeters to 1 em thick are most 

typical, and the strata include lesser amounts of finegrained quartzite. The rocks are generally medium gray 

green to charcoal gray on unweathered surfaces and tend 

to be lighter in weathered zones. Limonitic coatings are 

abundant in two gossan zones. Sedimentary features are 

conspicuous in drill core (fig. 4A) and in many large 

outcrops along Iron Creek. 

1

Term used for description only; no analogy to the "lumps" in 

carbonate rocks is implied. 

The No-Name Deposit 9 
,. :: 

. .. • J.. ;· •. . :-·. :) 

10 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
The No-Name deposit occurs within a sequence of 

argillite and siltite about 500 m thick that is distinct from 

underlying and overlying Yellowjacket strata, which are 

somewhat coarser grained and have better bedding 

features. Those footwall and hanging wall strata are 

micaceous fine quartzite. The No-Name zone contains 

very little quartzite. The general lack of bedding and 

other sedimentary structures in these fine-grained rocks 

is unusual for the Yellowjacket Formation. Close 

inspection of excellent outcrops, drill core, and thin 

sections indicates that the bedding features typical of the 

Yellowjacket (fig. 4A, SA), both regionally and nearby in 

Iron Creek canyon, were destroyed over scales of 1 em to 

several meters prior to complete consolidation. 

Proportions of quartz, white mica, biotite, chlorite, 

and feldspar vary from bed to bed. The argillite contains 

more white mica than it does biotite, and generally the 

white mica is very fine grained ( < 25 p.m; fig. 6B, 6E). 

Some biotite is fine grained, about 30-50 p.m, but much 

is 100 p.m or more and occurs in association with 

"vein1ike" quartz of similar grain size (fig. 6D, 6E). The 

"vein1ike" quartz is totally intergrown, has no rounded 

clastic form, and is coarser than in quartzite beds. White 

mica intergrown with the "veinlike" quartz also is coarse 

grained (75-200 p.m; fig. 6C) and could be termed 

muscovite. Erratic swaths of coarse quartz, biotite, and 

white mica in finer siltite or argillite (disruption 

structures) do not have the sharp walls of true veins. 

Plagioclase feldspar constitutes as much as 10 percent of 

some thin sandy layers. 

Ore minerals of the No Name deposit are present 

in four lithologic contexts: (1) Very fine grained pyrite, 

generally in clearly defined laminae, is interbedded with 

Figure 5 (facing page). Thin sections showing textures of 

sedimentary rocks from the Yellowjacket Formation. All 

about 4 em wide and photographed in transmitted light. 

A, Well-laminated sample of Yellowjacket Formation from 

near Blackbird mine showing well-preserved sedimentary 

features such as graded bedding, sand dikelets, and 

compaction features; these features are rarely seen in strata 

of No-Name zone. 8, Siltite showing weakly developed 

lamination, cut by sand dikelet that was folded during 

compaction; dark specks on margin of sand dikelet are 

grains of magnetite (Mt) (IC18-474). C, Magnetite (opaque) 

in wisps and sinuous injections in siltite (IC1 Q-821 ). 0, Siltite 

containing discontinuous microlenses of fine pyrite "mud" 

(truncated by bedding plane slip?), cut by injection structure 

containing abundant coarse pyrite (IC18-445). E, Siltite 

containing truncated beds of pyrite "mud" (opaque) and thin 

clasts of argillite; this texture seems to be caused by lowangle slip structures subparallel to bedding (IC12-305). 

F, Lump breccia of small pieces of argillite and siltite, 

cemented by quartz and pyrite; most clasts have round 

edges, but some are angular (IC12-323). G, Lump breccia of 

argillite and siltite clasts; the embayments in some clasts 

suggest some plastic deformation (IC10-829). H, Complex 

disruption texture that is a combination of tabular clasts, 

smear, and sand injection (I) (IC1 0-731 ). 

argillite and to a lesser extent with siltite (fig. 6A, 6E). (2) 

Coarse-grained pyrite is intergrown with the fine pyrite 

along bedding or within discordant disruption structures 

(fig. 6F, 6H ). (3) Chalcopyrite fills crosscutting stringers 

and surrounds coarse pyrite in disruption structures. ( 4) 

Magnetite occurs as fine grains that are bedded or in 

discordant veins, with or without pyrite (fig. 4B, 4F ). 

Where magnetite is abundant ( > 30 percent), it generally 

is in rounded or lumpy aggregates (fig. 4F, 5C ). Only 

rarely do the ore minerals fill structures having sharp 

margins typical of veins; rather, they occur in much the 

same setting as terrigenous clastic grains and recrystallized derivatives. 

Soft-Sediment Deformation 

A great variety of complex small structures are 

seen in drill core (fig. 4) and in some exceptional 

outcrops. These structures are discordant and have a 

variety of surfaces: curved to angular, sharp to 

gradational. Strata in and near the structures commonly 

contain rounded or bent fragments or have massive to 

swirled fabric. The structures and the unusual clastic 

fabrics are believed to have formed prior to consolidation. Nomenclature of these features is a problem: 

experts on the subject (for example, Elliott, 1965; Lowe, 

1975) do not seem to agree on terminology, much of 

which is genetic rather than descriptive. However, sedimentologists and soil scientists agree that (1) disruption 

of unconsolidated sediment is most likely in rapidly 

deposited sequences of clay, silt, and fine sand that have 

porosities of about 50 percent; and (2) cohesion is lowest 

for silt- and fine-sand-size grains. 

The following descriptions are based chiefly on 

drill core that does not permit determination of the strike 

of structures and bedding. Also, core provides excellent 

exposure of small features but no information about the 

rocks a few centimeters away. In the core that I inspected 

from five drill holes, the dip of bedding (where present) 

was remarkably consistent across tens or hundreds of 

meters. The following types of structures were observed, 

and seem to be distinct, although in many exposures 

several structures seemed to be combined. 

Contmted beds.-Bent, swirled, or erratic layering 

at scales from a few millimeters to several centimeters is 

common. Some features resemble convolute bedding. 

Most are probably developed in clay or silt beds by 

loading, but in the No-Name zone associated features 

suggest loss of cohesion. These structures are not folds 

such as seen in some clastic rocks that were deformed 

during diagenesis (for example, J onasson and Goodfellow, 1986); indeed, small, centimeter-scale folds are rare 

in the No-Name zone. 

The No-Name Deposit 11 
12 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
Injection structures.-These are crosscutting structures generally less than 1 em wide that contain chiefly 

sand-sized quartz, with or without pyrite or magnetite 

(fig. 4D, 4E, SB, SD). In thin section the quartz is coarser 

than is characteristic of adjacent sandy beds and is 

suspected to represent quartz cement added around sand 

grains. Micas are not abundant. Where these structures 

cut beds they are clearly visible, but texturally similar 

fabrics having concordant contacts might reflect injection 

parallel to,bedding or possibly liquefied or fluidized beds 

(described following). Where the crosscutting material is 

rich in clay, injection in a viscous or plastic state seems 

possible (fig. 4D). 

Massive fablics.-Rocks showing little or no 

internal fabric are common and clear exceptions to the 

normally well bedded or laminated Y ellowjacket strata. 

Many of these examples seem to be contained within 

undisturbed argillite or quartzite beds, although in some 

the enclosing rock has a lump-breccia texture (see 

following). Much of the rich magnetite or coarse-grained 

Co-pyrite occurs in2- to 10-cm swaths having this massive 

fabric (fig. 4C, 4£). 

Lump-breccia texture.-Rounded fragments of 

quartzite, siltite, or argillite are common in disrupted 

zones (fig. 4, 5). The lumps are generally 0.5-2 em in 

diameter and rarely are angular (fig. SF, SG, SH). Lump 

breccia generally is present adjacent to vein-like 

discordant structures. Fragments of argillite are tabular 

and commonly are bent. Very similar textures are 

described as "pelletoid conglomerate" by McNeil (1966). 

Figure 6 (facing page). Photomicrographs of textures in 

thin sections of samples from the Yellowjacket Formation. Q, 

quartz; Py, pyrite. A, Layers of fine pyrite "mud" (white cubes; 

Py) interbedded with clay (now argillite); reflected light 

(IC12-305). 8, Sand dikelet in argillite (Arg); fold probably 

was produced by compaction of original clay-rich sediment. 

Quartz grains are about 200 J.'m, coarser than in adjacent 

beds, probably explained by diagenetic grain growth; 

transmitted light, crossed polarizers (IC18-474). C, Quartz 

and mica (Mi) filling disruption structure; the quartz grains are 

larger than clastic sand grains in adjacent beds, and the mica 

(muscovite) is also much coarser than in beds-both types 

seem to reflect diagenetic growth; transmitted light, crossed 

polarizers (IC10-460). D, Clast of clay (Arg) in disruption 

structure that seems to have been squeezed into veinlike 

filling of coarse quartz (Q). Quartz and coarse flakes of biotite 

(Bi) cement clasts; transmitted light, crossed polarizers. 

E, Layered fine-grained pyrite "mud" in argillite, abutted by 

coarse grain of pyrite (Py2) in quartz-rich disruption; transmitted light, crossed polarizers (IC16-300). F, Same field as 

E, showing differences between early bedded pyrite and later 

coarse pyrite (Py2) in disruption; reflected light (IC16-300). 

G, Euhedral, zoned pyrite engulfed by massive pyrite; 

reflected light (IC12-323). H, Coarse-grained pyrite in which 

growth zones are revealed by tarnish. Dark zones are purple 

to blue, presumably reflecting an impurity in the pyrite lattice. 

This tarnish develops after several months of exposure to 

atmosphere; reflected light (IC18-861). 

A variant of lump breccia has crudely aligned 

fragments and a gneissic fabric (fig. SH); these are mostly 

dark-colored rocks that are not photogenic. The clasts 

are generally aligned with the plane of bedding in 

enclosing strata. This fabric seems to reflect shear or 

bedding-plane slip during or after brecciation, as might 

happen during slumping or sliding. 

Lenticular jab1ic.-This fabric is characterized by 

microlenses a few millimeters thick and less than about 3 

em wide that have feathery ends (fig. SD, 5E). The 

lenticular features are most easily seen in very fine pyrite 

or clay, possibly because they have high color contrast, 

and seem to be produced by shear or slip parallel or 

subparallel to bedding. 

Many of the secondary fabrics seem consistent with 

formation during downslope sliding of partly or 

completely liquefied sediment. The prevalent disturbed 

bedding, both within the No-Name deposit and 

elsewhere in the Yellowjacket, is commonly developed in 

interbedded clay, silt, and fine-sand sequences (see, for 

example, Lowe, 1975) and generally attributed to plastic 

deformation due to loading. The crosscutting sand dikelets also are commonly developed in such sequences and 

are attributed to liquefaction or fluidization by pore 

fluids and upward transport. My impression of the NoName zone is that vertical features related to dewatering 

are less common than subhorizontal features confined 

between beds. Also, retextured clean sandstone or 

quartzite is rare; thus, winnowing of clays seems to have 

been uncommon in the No-Name zone, suggesting that 

fluid velocities were generally not high or in the range 

characteristic of fluidization (Swarbrick, 1968; Lowe, 

1975). The majority of features tend to be aligned 

parallel to bedding and involve a high proportion of 

grains and clasts, similar to features in "slurry-slumps" 

and "slide-slumps" described by Elliott (1965), although 

documentation of horizontal displacement is difficult in 

drill core. Fluidized injections can form between beds 

(Lowe, 1975; Hiscott, 1979), but this does not seem to be 

as common elsewhere as more vertical geometries. At 

Iron Creek the parallelism of features with bedding 

seems more consistent with liquefaction-the " ... 

sudden loss of shearing resistance associated with grain 

framework collapse and a rapid but temporary increase 

in pore fluid pressure" (Lowe, 1975, p. 159). Sedimentologists note that liquefaction of unconsolidated sediment can cause mass movement on very gentle slopes 

(less than 10°) that in some ways resembles transport by 

turbidity currents, but does not produce graded bedding 

(Middleton and Hampton, 1973). The association of 

clastic dikes with slump folds has been noted (Dzulynski 

and Randomski, 1956; Hiscott, 1979) and demonstrated 

to occur at less than 60 m of burial. 

The No-Name Deposit 13 
Ore Mineralogy 

The No-Name deposit contains a limited suite of 

ore minerals. Pyrite, magnetite, and chalcopyrite are the 

common ore minerals, along with traces of marcasite, 

hematite, and various copper oxide minerals. 

Pyrite occurs in four forms: fine-grained, coarsegrained, anhedral, and corroded. Fine-grained pyrite, less 

than about 50 ~J.m in grain diameter, tends to be laminated and most abundant in argillite. The texture and 

association with argillite suggest that this was a syngenetic iron sulfide mud; recrystallization during diagenesis 

and metamorphism is likely, but does not seem to have 

produced many changes. Coarse-grained pyrite occurs 

both within laminae of fine pyrite, in which it probably is 

a recrystallization product, and in discordant structures. 

All of this pyrite takes a good polish, shows no 

replacement relations with other ore minerals, and is free 

of internal structures other than growth zones that 

tarnish to various shades of orange or purple (fig. 6G, 

6H). Coarse pyrite probably is slightly more abundant 

than is fine. Anhedral pyrite that formed late in the 

paragenesis surrounds other sulfide grains (fig. 6G). This 

variety tarnishes to shades of purple and blue and has no 

crystal form, although some grains seem to have 

cleavage. According to scanning electron microscopeenergy dispersive system (SEM-EDS) analysis, the 

general composition is the same as pyrite, but minor 

elements below about 1 weight percent could not be 

determined. It is optically isotropic and thus is tentatively 

identified as pyrite; the unusual morphology may be 

inherited by replacement of an earlier mineral. A minor 

variety of pyrite is characterized by porous texture and 

corroded outlines; this may have originally been pyrrhotite, as at the Blackbird mine, but lacks the concentric 

banding of that sulfide (Nash and Hahn, 1986). Some of 

this corroded iron sulfide is marcasite. 

Magnetite occurs in trace to major amounts as 

well-formed crystals about 10-200 ~J.m in diameter. It is 

generally euhedral, and no lamellae of ilmenite or other 

minerals have been seen. Microprobe analysis 

(Modreski, 1985) showed less than 0.02 percent Ti02 

and very low cobalt. There is no evidence to suggest that 

any of the magnetite grains are detrital. The magnetite 

was an essential part of the sediment from early in its 

history, like pyrite and quartz. Some is clearly bedded, a 

chemical sediment, and some is in disruption structures. 

Some very interesting occurrences are in sand dikelets, 

where the magnetite occurs along the margins of the 

dikelet (fig. 4B). The magnetite in discordant structures 

possibly was introduced in part as solid particles, along 

with quartz sand grains, but most grains are too large to 

have been in hydraulic equilibrium with silicates and thus 

are presumed to have grown larger in place. 

Chalcopyrite is a common mineral but rarely is as 

abundant as pyrite. It fills the matrix around pyrite 

crystals and tends to be most abundant in the upper part 

of cobalt zones (fig. 3). The paragenetic and spatial 

position of chalcopyrite suggests it formed late in 

mineralization. Films of oxidized copper minerals coat 

chalcopyrite in zones near the surface. 

Cobalt appears to reside in pyrite rather than in a 

distinct cobalt mineral, such as in cobaltite in the 

Blackbird deposits. My petrographic studies have found 

no cobalt-rich minerals in even the richest cobalt zones, 

those assaying greater than 1 percent Co. Coastal 

(Hanna) Mining Company and Noranda Mining 

Company found no cobalt minerals but determined 

2.5-4.5 percent Co by microprobe analyses in one variety 

of pyrite, and they concluded that cobalt is in the pyrite 

lattice (Webster and Stump, 1980). Modreski (1985) 

found 2-3 percent Co in pyrite. Reasons for the wide 

variation in cobalt content of the pyrite have not been 

established, but presumably relate to the times and 

conditions of pyrite growth. 

Arsenic is notably less abundant in the No-Name 

deposit than in the Blackbird ores, and no arsenic-rich 

minerals have been identified to date. Some preliminary 

microprobe analyses by Noranda Mining Company 

(Webster and Stump, 1980) indicate that 0.3-0.8 percent 

As is present in some pyrite growth zones. Incorporation 

of a few percent of cobalt or arsenic in the pyrite lattice, 

although not common, has been reported and accepted 

for other sulfide deposits (Ramdohr, 1980). 

Alteration 

Alteration of the host rocks in the No-Name 

deposit is subtle and possibly obscured by later regional 

metamorphism. Three types of alteration are developed 

in and near the Co-pyrite deposits: (1) Most typical and 

widespread is potassic alteration in which potassium 

micas were developed or remained stable. One example 

is the association of coarse white mica (muscovite) with 

coarse quartz and pyrite in disruption structures (fig. 

6C). Coarse biotite in association with muscovite, pyrite, 

and quartz is also probably an early-formed phase rather 

than a metamorphic product. High silica activities are 

suggested for the potassic alteration on the basis of 

strong silica cementation and formation of coarse grains 

of veinlike quartz. No selvages of micas or other minerals 

are noted around the disruption structures, their absence 

indicating that the fluid in the structures was in 

equilibrium with the surrounding sediment. (2) Formation of carbonate minerals is notable in many pyritic 

disruptions. Tan carbonate (ankerite?) cements the 

disruption fill and locally replaces biotite and pyrite. 

Similar iron carbonate cements some disruption struc14 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
tures in the Blackbird mine area (Nash and Hahn, 1986). 

(3) Tourmaline cements breccia and disruption zones 

near the No-Name deposit, in many places in association 

with magnetite. Tourmaline is present as a minor 

accessory mineral in some parts of the No-Name deposit 

and reflects introduced boron rather than clastic grains, 

but it is not abundant or obviously related to formation of 

Co-pyrite. These three types of alteration are compositionally distinctive and are evident despite metamorphism that probably caused recrystallization of the 

minerals associated with ore. 

Bright green chlorite showing anomalous blue 

birefringence is developed in and near many sulfide 

layers. This chlorite replaces brown biotite and especially 

favors the coarser grains of biotite in disruption 

structures. The texture of the chlorite indicates it is a 

post-metamorphic feature, a retrograde product not 

related to ore formation. 

Alteration associated with pyrite zones is not a 

simple guide to ore because the chemical and mineralogical changes are minor and not easily seen. Color of 

metamorphic rocks does not seem to be significantly 

different in zones of alteration associated with ore; the 

brighter shades of green in some zones seem to reflect 

retrograde alteration. Also, the altered rock is mostly 

concordant with bedding at deposit scale and is not as 

easily seen as discordant alteration along faults or veins. 

GEOCHEMISTRY 

Results of chemical analysis for major and minor 

elements are shown in table 1 and summarized in table 2. 

Major-element concentrations are typical of fine-grained 

metasedimentary rocks except for the unusually high 

concentrations of iron and sulfur in samples containing 

sulfides or magnetite. Considering the high to extremely 

high concentrations of iron, there is relatively little 

manganese. Mineralized samples contain as much as 

10,000 ppm (1 percent) Co and Cu, and some samples 

are slightly enriched in Ag, As, Au, Bi, Cr, Mo, Pb, and 

Zn (table 2). A comparison of mineralized and barren 

footwall or hanging wall samples (table 2) using discriminant function analysis (Davis, 1986) indicates that Fe, S, 

As, Ba, Bi, Co, Cr, Cu, Ni, Pb, Se, and Zn are significantly 

higher in Co-pyrite zones. These pyrite zones are slightly 

depleted in AI, Na, K, and Ti. Concentrations of most 

minor elements in mineralized samples are not significantly different from those in barren samples. 

The geochemistry of the cobalt -enriched zones is 

dominated by associations with sulfur as sulfide. Correlation analysis (Davis, 1986) and scatterplots (fig. 7A) 

indicate a direct and invariable association of cobalt and 

total sulfur. Cobalt is strongly associated with Fe, As, Se, 

Ni, Pb, and Bi and less so with Cu (fig. 7D). Some cobalt 

zones have notably low concentrations of Cu 

( < 20 ppm), such as in footwall strata containing magnetite. Correlation coefficients of cobalt with most rockforming elements such as Mg, Ca, K, and Na are weakly 

negative, indicating that Co-pyrite is not associated with 

any unusual mineral assemblage other than with sulfides. 

The general composition of the mineralized 

samples from the No-Name deposit is similar to ores 

from the Blackbird district in several aspects and 

different in others. The mineralized rock is similar in its 

high concentrations of Fe and some other "mafic" 

elements such as Bi, Cr, and Ni and notably low 

concentrations of Ag, Pb, and Zn compared to many 

sediment-hosted base-metal deposits. The No-Name 

deposit is enriched in As but in much lower concentrations than in the cobaltite ore at Blackbird, which 

typically contains more than 0.5 percent As. The low As 

content of the No-Name deposit relative to the Blackbird 

ore is a potentially significant economic factor because it 

allows more conventional smelting of sulfide concentrates. The No-Name deposit does not seem to be 

enriched in rare elements such as Nb, La, andY, as is the 

Blackbird (Nash and Hahn, 1986), and no significant 

zones of Au enrichment have been identified at the 

No-Name deposit. 

The composition of rocks containing fine-grained 

pyrite seems to differ in some fundamental ways from 

that of rocks containing coarse-grained pyrite. A test for 

differences between two groups of analyses grouped by 

general grain size of pyrite suggests that AI, Mg, and K 

concentrations are higher in the samples containing 

chiefly fine-grained pyrite, probably reflecting higher clay 

content, whereas total concentrations of S, Fe, Bi, Co, 

Cu, Ni, and Pb are higher in the samples containing 

chiefly coarse-grained pyrite. The suggestion that epigenetic pyritic samples contain higher concentrations of ore 

metals seems reasonable, but additional work is required 

to substantiate this generalization. 

The chemistry of mineralized and barren samples 

from the No-Name deposit is consistent with petrographic features described previously, albeit on the basis 

of only 41 samples. The major-element composition, like 

the mineralogy, suggests mild alteration within the 

mineralized zones. The modest enrichment in chalcophile elements is logical for the highly sulfidic environment and consistent with the generally simple sulfide 

mineralogy. The details of residence of Bi, Pb, and Zn 

have not been established, but these trace constituents 

might reside in pyrite rather than as distinct minerals. 

The modest anomaly of barium in mineralized samples is 

not apparent in the mineralogy; no barite has been 

recognized, but the barium might reside in white micas or 

biotite. 

Geochemistry 15 
Table 1. Analytical results for samples of drill core from the No-Name deposit, Lemhi County, Idaho 

[Major elements in percent (%);minor elements in parts per million (ppm); s, determined by ICP; Fe20s computed from total iron reported as 

FeTOs minus iron determined as FeO; N, not detected at level shown; <, detected but below limit of determination shown; --, not determined; 

H, interference, not determined; LOI, loss on ignition (900 °C)] 

Sample Si02% Al 203% Fe2o3% FeO% MgO% CaO% Na2o% K20% Ti02% P205% MnO % 

IC3-0064 65.7 13.50 3.84 4.79 2.21 0.10 0.56 3.42 0.50 0.12 <0.02 

IC3-0072 64.5 7.80 10.52 3.94 1. 78 .10 .35 1.04 .30 .09 <.02 

IC3-0083 60.4 12.30 5.26 6.70 2.36 .04 .63 3.76 .53 .08 .05 

IC3-0102 57.2 15.30 2.68 10.10 3.11 .12 .34 4.82 .56 .11 .09 

IC10-440 55.6 16.00 5.40 7.74 2.57 .09 .46 5.52 .63 .08 .06 

IC10-806 65.1 12.20 8.53 7.09 1.63 .09 .26 4.91 .30 .10 .04 

IC10-807 65.1 9.02 8.53 7.09 1.62 .09 <.15 3.91 .24 .07 .04 

IC10-809 .H .H 19.10 .H .H .H .H .H .H .H 

IC10-819 .H .H 12.10 .H .H .H .H .H .H .H 

IC10-824 54.0 8.23 15.91 9.18 1. 75 .14 .25 3.50 .29 .08 .05 

IC10-829 71.2 10.20 2.67 6.26 1. 70 .20 2.21 2.75 .30 .10 .04 

IC12-265 64.3 10.90 .H 1. 76 .17 <.15 4.32 .33 .10 .04 

IC12-280 62.8 10.40 7.41 5.93 1.81 .16 .19 4.32 .39 .10 .05 

IC12-285 55.6 9.41 12.53 5.73 1. 74 .04 .16 2.96 .26 .10 .05 

IC12-295 .H .H .H .H .H .H .H .H .H .H 

IC12-306 49.6 17.00 6.53 9.16 2.96 .15 1.18 4.86 .57 .15 .07 

IC12-325 59.3 14.30 .H 2.70 .22 <.15 5.66 .61 .12 .08 

IC12-328 64.4 14.60 2.61 6.14 2.17 .20 .60 5.41 .57 .16 .06 

IC12-488 60.9 9.45 .H 1.44 .38 1.51 2.02 .17 .22 .04 

IC16-236 56.9 14.30 4.09 8.20 2.77 1.44 2.14 3.81 1.17 .45 .14 

IC16-244 53.3 13.40 8.85 7.52 2.51 .16 • 45 4.86 .47 .11 .07 

IC16-271 66.5 11.60 4.32 5.83 2.02 .09 .21 4.56 .35 .08 .06 

IC16-281 61.7 9.43 8.46 5.98 1. 71 .05 .19 3.46 .33 .06 .05 

IC16-306 7.82 

IC16-311 68.0 9.55 4.75 6.80 2.08 .13 .76 2.03 .34 .09 .07 

IC16-336 64.8 11.70 .H 2.10 .17 1.26 2.96 .45 .09 .08 

IC16-351 57.8 9.68 .H 2.81 .19 <.15 2.08 .33 .09 .13 

IC16-542 59.4 10.50 .H 2.14 .16 .32 3.83 .37 .08 .11 

IC18-700 66.3 13.20 1.61 7.82 2.58 .12 1.96 1.76 .45 .10 .05 

IC18-762 63.1 12.50 3.71 6.29 2.21 .93 2.09 2.93 .89 .38 .07 

IC18-767 59.5 13.70 8.20 6.30 2.29 1.14 1.80 3.92 1.06 .41 .08 

IC18-768 59.5 13.70 4.55 6.80 2.29 1.14 1.80 3.41 1.06 .41 .08 

IC18-770 .H .H .H .H .H .H .H .H .H .H 

IC18-781 60.3 13.80 .H 2.84 .29 1.08 3.08 .60 .11 .09 

IC18-854 61.6 17.40 3.14 4.36 1.67 .14 1.30 5.86 .66 .10 .03 

IC181064 65.0 12.30 6.03 4.65 1.39 .20 .96 4.39 .47 .08 .04 

IC181068 71.9 10.60 4.03 4.04 1.26 .14 .49 3.96 .38 .08 .03 

IC181074 66.6 12.50 - 4.70 4.86 1.43 .19 1.01 4.30 .46 .09 .04 

IC181079 59.3 13.10 .H 2.15 .21 .93 4.47 .50 .10 .07 

IC181080 69.0 13.00 4.46 4.02 1.33 .22 .98 4.58 .46 .10 .04 

IC181081 68.5 13.00 3.99 4.14 1.31 .22 .98 4.58 .46 .09 .04 

16 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
Table 1. Analytical results for samples of drill core from the No-Name deposit, Lemhi County, Idaho-Continued 

Sample Total Total LOI Al Fe Mg Ca Na K Ti p 

C% S% 900C %-s %-s %- s %-s %-s %-s %-s %-s 

IC3-0064 <0.01 0.93 3.36 7.0 6.3 1.30 0.09 .36 2.80 .29 .050 

IC3-0072 .02 5.64 6.78 4.2 11.0 1.10 .09 .22 .77 .17 .030 

IC3-0083 <.01 3.36 4.84 6.6 9.0 1.40 .04 .44 3.10 .33 .030 

IC3-0102 <.01 .47 3.11 7.5 8.9 1.70 .10 .20 3.50 .31 .040 

IC10-440 <.01 2.66 4.06 8.8 10.0 1.50 .09 .30 4.50 .39 .030 

IC10-806 .01 2.50 3.30 6.6 12.0 .97 .09 .17 4.00 .19 .040 

IC10-807 .01 2.42 4.7 12.0 .95 .07 .04 2.90 .15 .030 

IC10-809 .01 7.82 .H 1.2 56.0 .17 .02 .02 .60 .02 <.005 

IC10-819 <.01 4.12 .H 3.4 28.0 .63 .07 .13 2.00 .12 .020 

IC10-824 <.01 6.19 5.18 4.4 19.0 1.00 .10 .12 2.60 .18 .030 

IC10-829 <.01 .93 1.40 5.5 6.9 1.00 .17 1.80 2.30 .19 .040 

IC12-265 <.01 3.46 4.02 5.4 8.4 1.00 .11 .07 2.70 .20 .030 

IC12-280 .02 4.46 4.69 5.4 9.7 1.00 .12 .07 3.20 .22 .040 

IC12-285 <.01 9.83 8.14 5.3 14.0 1.10 .05 .07 2.50 .17 .040 

IC12-295 <.01 19.30 .H 4.1 23.0 1.10 .11 .04 2.20 .26 .040 

IC12-306 <.01 3.83 5.15 9.8 13.0 1.80 .14 .95 4.20 .37 .070 

IC12-325 <.01 .77 2.62 7.2 9.0 1.60 .14 .07 3.50 .37 .040 

IC12-328 <.01 .38 2.16 7.8 6.7 1.30 .17 .37 4.30 .34 .060 

IC12-488 <.01 7.91 6.87 5.1 12.0 .89 .26 1.20 1.40 .12 .090 

IC16-236 .17 1.31 3.27 7.4 9.1 1.60 1.00 1.50 3.00 .67 .200 

IC16-244 <.01 6.11 5.99 7.9 13.0 1.60 .15 .30 4.30 .31 .050 

IC16-271 <.01 1.65 3.04 6.0 7.4 1.20 .07 .10 3.40 .21 .030 

IC16-281 <.01 6.05 5.98 5.2 11.0 1.00 .03 .06 2.70 .20 .020 

IC16-306 .03 5.13 8.5 13.0 1.60 .13 .97 3.60 .32 .060 

IC16-311 .05 2.24 3.64 5.0 8.5 1.20 .09 .53 1.50 .20 .040 

IC16-336 .07 1.22 3.32 5.9 8.6 1.20 .11 .97 1.90 .28 .030 

IC16-351 .20 4.16 6.43 4.7 14.0 1.60 .11 .05 1.30 .19 .030 

IC16-542 .23 2.17 3.99 5.3 13.0 1.20 .10 .24 2.60 .23 .030 

IC18-700 <.01 .15 2.61 7.1 7.3 1.50 .11 1.50 1.50 .27 .040 

IC18-762 .15 1.88 3.06 6.8 7.6 1.30 .72 1.70 2.50 .55 .170 

IC18-767 .20 4.95 6.11 7.3 11.0 1.30 .77 1.40 3.20 .62 .150 

IC18-768 .20 2.04 3.84 6.9 8.1 1.30 .77 1.40 2.30 .62 .150 

IC18-770 .17 24.00 .H 3.7 24.0 .64 .09 .22 1.40 .18 .030 

IC18-781 .03 .87 3.55 7.0 9.3 1.70 .19 .85 2.20 .37 .040 

IC18-854 <.01 .74 2.76 9.2 5.6 .96 .13 .97 4.70 .39 .040 

IC181064 <.01 2. 59 3.41 6.3 7.7 .81 .15 .64 3.30 .27 .040 

IC181068 <.01 1.32 2.07 5.7 6.0 .73 .13 .35 3.20 .23 .030 

IC181074 <.01 1.40 2.64 6.4 7.0 .82 .14 .70 3.30 .27 .040 

IC181079 <.01 2.69 3.68 6.5 10.0 1.20 .14 .70 3.00 .30 .040 

IC181080 <.01 1. 74 2.76 7.2 7.1 .84 .14 .75 4.00 .28 .040 

IC181081 <.01 .96 2.32 6.5 5.8 .74 .14 .75 1.40 .28 .040 

Geochemistry 17 
Table 1. Analytical results for samples of drill core from the No-Name deposit, Lemhi County, Idaho-Continued 

Sample Ag As B Ba Be Bi Ce Co Cr Cu 

ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s 

IC3-0064 <2 190 880 510 1 <10 17 540 52 2,700 

IC3-0072 2 610 3,860 150 <1 50 68 2,400 200 3,600 

IC3-0083 <2 90 500 200 1 <10 69 1,900 51 5,700 

IC3-0102 <2 <10 90 370 <1 <10 270 290 45 2,300 

IC10-440 <2 190 70 170 1 <10 130 1,200 74 140 

IC10-806 <2 190 30 360 2 <10 41 1,200 120 47 

IC10-807 <2 190 30 360 1 <10 27 1,200 120 47 

IC10-809 <2 420 60 190 1 120 <4 3,300 100 11 

IC10-819 <2 200 40 250 1 20 75 1,800 97 6 

IC10-824 <2 300 60 190 1 20 79 2,700 100 35 

IC10-829 <2 30 40 390 <1 <10 120 460 37 12 

IC12-265 2 170 23 <1 10 180 1,500 37 6,200 

IC12-280 <2 200 30 210 <1 <10 170 2,100 140 4,900 

IC12-285 3 560 40 380 <1 <10 150 4,800 33 10,000 

IC12-295 3 1,200 37 <1 70 110 10,000 48 5,100 

IC12-306 <2 280 120 630 1 <10 460 2,100 69 2,600 

IC12-325 <2 10 80 <1 <10 250 540 74 320 

IC12-328 <2 <10 50 530 <1 <10 230 250 57 110 

IC12-488 <2 530 13 <1 30 63 3,900 21 21 

IC16-236 <2 50 40 650 1 <10 160 760 69 410 

IC16-244 <2 330 40 910 <1 <10 150 3,600 55 3,000 

IC16-271 <2 90 40 360 <1 <10 200 1,000 120 980 

IC16-281 <2 390 40 210 <1 <10 77 3,300 150 6,800 

IC16-306 <2 370 120 330 <1 20 240 2,600 140 1,800 

IC16-311 <2 120 110 380 <1 10 170 1,200 130 980 

IC16-336 <2 60 71 <1 <10 110 770 57 350 

IC16-351 <2 240 20 <1 20 80 2,200 36 1,400 

IC16-542 <2 140 30 <1 10 58 1,000 42 11 

IC18-700 <2 <10 40 260 1 <10 37 70 40 1,100 

IC18-762 <2 90 30 100 1 <10 66 760 27 410 

IC18-767 <2 370 70 310 1 20 88 2,500 120 2,100 

IC18-768 <2 120 40 38 1 10 88 900 37 370 

IC18-770 5 1,700 10 <1 110 34 9,800 25 7,300 

IC18-781 <2 <10 190 <1 <10 190 400 72 410 

IC18-854 <2 30 50 190 1 <10 140 340 70 21 

IC181064 <2 140 30 280 <1 <10 120 1,300 130 12 

IC181068 <2 60 <20 52 <1 <10 110 650 39 2 

IC181074 <2 70 20 490 <1 10 110 690 120 5 

IC181079 <2 80 30 <1 10 120 1,200 54 27 

IC181080 <2 90 20 74 <1 <10 130 880 54 6 

IC181081 <2 60 20 68 <1 <10 110 490 54 3 

18 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
Table 1. Analytical results for samples of drill core from the No-Name deposit, Lemhi County, Idaho-Continued 

Sample Eu Ga La Li Mn Mo Nb Nd Ni Pb Sc 

ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s 

IC3-0064 <2 25 7 24 160 3 9 12 36 9 11 

IC3-0072 <2 18 34 9 270 10 7 30 76 110 9 

IC3-0083 <2 35 30 23 450 33 14 31 93 22 13 

IC3-0102 3 42 120 29 690 2 13 110 34 7 11 

IC10-440 <2 38 56 30 530 24 14 56 31 14 16 

IC10-806 <2 27 18 20 370 21 8 18 33 8 9 

IC10-807 <2 22 15 17 370 17 6 11 33 8 7 

IC10-809 <2 18 2 5 230 <2 <4 <4 94 48 6 

IC10-819 <2 18 39 13 250 11 5 35 51 13 6 

IC10-824 <2 20 39 15 420 22 8 40 51 16 6 

IC10-829 <2 18 57 15 390 3 6 53 22 8 6 

IC12-265 3 25 91 20 390 3 81 38 10 6 

IC12-280 3 25 84 19 450 7 6 78 40 11 6 

IC12-285 <2 23 63 21 450 3 9 68 93 22 6 

IC12-295 2 18 53 17 530 6 56 110 110 5 

IC12-306 5 37 190 33 650 22 13 190 64 14 13 

IC12-325 4 41 130 30 690 <2 120 44 8 12 

IC12-328 3 39 110 28 520 2 11 92 28 5 13 

IC12-488 <2 16 31 12 400 <2 29 25 40 4 

IC16-236 3 29 85 24 1,100 14 29 73 41 28 9 

IC16-244 <2 34 60 33 600 <2 12 60 75 37 9 

IC16-271 2 25 96 27 500 5 7 94 34 11 8 

IC16-281 <2 22 35 21 440 4 8 41 63 5 6 

IC16-306 3 33 110 26 560 15 12 120 87 15 11 

IC16-311 2 22 87 14 560 4 6 82 38 16 6 

IC16-336 <2 27 58 18 680 6 51 34 5 9 

IC16-351 <2 24 42 16 960 8 38 44 16 6 

IC16-542 <2 29 29 18 860 12 28 37 10 7 

IC18-700 <2 26 17 15 450 <2 9 15 28 6 10 

IC18-762 <2 22 31 18 610 6 25 31 23 7 7 

IC18-767 <2 33 46 22 640 11 10 44 49 11 8 

IC18-768 <2 24 46 19 640 9 44 28 8 7 

IC18-770 <2 18 17 11 490 12 16 290 91 3 

IC18-781 2 38 100 19 710 <2 85 54 <4 12 

IC18-854 <2 36 64 24 280 <2 15 51 7 6 18 

IC181064 <2 26 61 17 350 10 7 55 33 9 10 

IC181068 <2 22 50 16 310 9 8 45 17 6 7 

IC181074 <2 26 56 16 380 9 6 54 24 8 8 

IC181079 2 30 61 19 560 4 62 41 12 10 

IC181080 <2 28 55 20 340 16 8 56 27 10 8 

IC181081 <2 25 54 16 340 16 50 19 7 8 

Geochemistry 19 
Table 1. Analytical results for samples of drill core from the No-Name deposit, Lemhi County, Idaho-Continued 

Sample Se Sr Th v y Yb Zn Zr Au 

ppm ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s ppm-s ppm 

IC3-0064 29 12 53 9 1 53 254 0.05N 

IC3-0072 31.0 88 8 50 12 2 50 146 .05 

IC3-0083 28 12 48 14 2 69 269 .05N 

IC3-0102 24 16 55 20 2 71 236 .05N 

IC10-440 21 17 62 12 2 51 243 .05N 

IC10-806 2.8 18 12 34 22 3 34 138 .05N 

IC10-807 2.8 9 9 33 8 2 34 126 .05N 

IC10-809 43.0 5 9 47 <2 <1 17 21 .05N 

ICl0-819 19.0 12 9 28 8 <1 27 147 .05N 

ICl0-824 17.0 16 8 40 9 1 32 145 <.05 

IC10-829 69 9 23 12 1 26 116 .05N 

IC12-265 19.0 18 9 21 15 2 63 .05N 

IC12-280 19.0 20 7 21 10 1 80 168 .05N 

IC12-285 15 12 21 15 2 66 136 .05N 

IC12-295 70.0 14 12 24 6 1 60 .05N 

IC12-306 44 14 60 25 3 91 209 .05N 

IC12-325 9.3 19 14 55 20 3 51 .05N 

IC12-328 28 15 60 17 2 37 209 .05N 

IC12-488 24.0 31 8 18 12 2 31 .05N 

IC16-236 4.6 190 12 46 27 3 120 277 .05N 

IC16-244 31 13 37 24 3 76 184 .05N 

IC16-271 7.2 17 8 27 16 2 54 175 .05N 

IC16-281 27.0 12 7 21 10 1 67 159 .15 

IC16-306 26.0 38 12 51 15 2 81 196 .05N 

IC16-311 9.8 24 8 33 8 <1 140 150 .05N 

IC16-336 9.9 35 12 43 11 1 49 .05N 

IC16-351 17.0 15 8 47 14 2 68 .05N 

IC16-542 15.0 16 8 47 13 2 34 .05N 

IC18-700 30 12 42 11 2 55 203 .05N 

IC18-762 73 10 35 11 1 51 253 .05N 

IC18-767 18.0 76 14 41 12 2 57 272 .05N 

IC18-768 14.0 76 11 41 12 1 57 245 .05N 

IC18-770 87.0 15 9 18 4 <1 52 .05N 

IC18-781 7.7 32 12 63 14 2 58 .05N 

IC18-854 43 15 74 12 2 30 231 .05N 

IC181064 7.6 35 11 39 24 3 23 203 .05N 

IC181068 29 7 31 23 2 18 161 .05N 

IC181074 4.7 31 12 39 16 2 23 196 .05N 

IC181079 14.0 28 10 49 20 3 35 .05N 

IC181080 6.3 34 9 41 19 3 22 215 .05N 

IC181081 6.3 34 9 41 16 2 19 239 .05N 

20 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
Table 2. Summary of analytical results for ore and enclosing wallrocks, No-Name deposit, Lemhi County, Idaho 

[Oxides in percent, rest parts per million; n, number of valid determinations; n for many major elements determined by X-ray fluorescence is 

reduced by interference caused by high iron concentrations; L, less than value shown; FeTOs, total iron expressed as ferric iron, determined by 

ICP; **,not computed, too few valid determinations] 

Co-~lrite zones Barren hangi ngwa ll and footwall zones 

Element n min. max. geom. n min. max. geom. 

mean mean 

Si02 21 49.6 68.0 60.8 15 59.3 71.9 63.4 

Al 2o3 21 7.8 17.0 11.6 15 9.5 17.4 12.5 

FeT03 26 9.0 80 17.3 15 8.0 18.6 11.6 

MgO 21 1.4 3.1 2.1 15 1.3 2.6 1.9 

CaO 21 .04 1.4 .15 15 .04 1.1 .23 

Na2o 21 .16L 2.1 • 54 13 .15L 2.2 1.0 

K20 21 1.04 5.5 3.4 15 2.0 5.9 3.9 

Ti02 21 .24 1.2 .43 15 .17 1.1 .5 

P205 21 .06 .45 .11 15 .08 .41 .13 

S total 26 .15 24.0 3.1 15 • 38 7.9 1.6 

C total 12 .01L .20 .02 4 .03L .23 .02L 

Ag 4 2L 5 ** 1 2 2 ** 

As 24 50 1,700 242 13 10L 530 76 

Au 3 .05L .15 ** 0 .05L .05L ** 

B 22 20 3,860 68 8 20L 500 41 

Ba 26 10 910 221 15 13 530 79 

Bi 11 10L 120 13 5 10L 30 10L 

Co 26 70 10,000 1,586 15 250 3,900 774 

Cr 26 25 200 80 15 21 74 46 

Cu 26 5 10,000 532 15 2 6,200 70 

Mn 26 160 1,100 463 15 280 860 473 

Mo 23 2L 24 9 11 21 33 7 

Pb 26 5 110 16 14 5L 40 9 

Se 19 2 87 14 9 6.3 24 12 

v 26 18 62 37 15 18 74 40 

Zn 26 17 140 52 15 18 69 37 

Geochemistry 21 
30 

A fz 25- w + 

u 

0::: 

w 

0.. 20- + fJ: 

(!) 

ijj 15-

~ 

~ 

a: 10- + 

::> + + LL 

...J ++ ++ ::> 5 !- +:tr"'" (/) 

l/++ 

Q I I I I 

0 3000 6000 9000 12,000 15,000 

COBALT, IN PPM 

30 

B fz 25 !- w + u 

0::: 

w 

0.. 20 !- + fJ: 

(!) 

ijj 1 5 !-

~ 

~ 

ri 10 + 

::> 

LL 

...J +' + + ::> 5 - ++ + (/) + +' + + 

0 

t :t: ++I I I I 

0 3000 6000 9000 12,000 15,000 

COPPER, IN PPM 

2500r-------------------------------~ 

2000-

~ 0.. 

0.. 1500- z 

z 

~ 1000 f0::: 

<( 

500 fc 

+ 

+ 

+ 

6000 9000 12,000 1 5,000 

COBALT, IN PPM 

Figure 7. Scatterplots of geochemical data for the NoName deposit. A, Plot of cobalt against sulfur. The linear 

relation supports mineralogical evidence that cobalt 

substitutes for iron in pyrite rather than occurring as a 

distinct cobalt mineral. B, Plot of copper against sulfur. C, 

Plot of cobalt against arsenic. Although there is a linear 

relation of cobalt and arsenic, this relation actually reflects 

the control on both cobalt and arsenic by their incorporation 

15,000 

D 

12,000 !-

~ + 0.. 

0.. 9000 !-

~ 

ex:· 

w + + 

0.. 6000 !- + 0.. + 0 + u + 

+ 3000 + + + + ~ + +t 

0 ... ~. I I 

0 3000 6000 9000 1 2,000 1 5,000 

COBALT, IN PPM 

350 

E 

300 + 

~ 

250 

a.. 

0.. 

~ 200 

w ....~· 150 ~ 

u 

z + 100 + + + + + + + + 50 + + ""'" ~~+# + 

0 + 

0 3000 6000 9000 12,000 15,000 

COBALT, IN PPM 

30 

F fz 25 w + 

u 

0::: 

w 

0.. 20 f- + fJ: 

(!) 

ijj 15 f-

~ 

~ 10 !- a: + 

::> + + LL 

...J + + + + ::> 5 1- + + (/) + +1 

~Jf~ 0 +..t 

0 30 60 90 120 

SELENIUM, IN PPM 

in pyrite rather than their presence as a distinct cobaltarsenic mineral. D, Plot of cobalt against copper showing 

lack of linear relation at higher concentrations of cobalt and 

copper. E, Plot of cobalt against nickel. The occurrence of 

nickel, like cobalt, is controlled by pyrite. F, Plot of selenium 

against sulfur. The linear relation implies that selenium 

occurs in pyrite rather than as a distinct selenium mineral. 

22 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
DISCUSSION AND INTERPRETATION 

Environment of Formation 

The cobalt deposits of the Iron Creek area formed 

during and shortly after sedimentation in an unstable 

part of the Yellowjacket rift basin. Although there is 

some valid evidence for shallow-water sedimentation not 

far from the deposits (Modreski, 1985), strata that are 

mineralized are darker and were deposited below wave 

base. Some paleocurrent measurements indicate deposition was from the east, and the cobaltiferous zones at 

Iron Creek seem to have been in the central part of the 

northwest-trending rift basin (Hughes, 1983). Slump 

folds and synsedimentary deformation between undisturbed strata were produced by gravitational instability 

on the submarine fan. Mafic dikes and probable mafic 

volcaniclastic rocks suggest that volcanism accompanied 

sedimentation, but volcanic rocks are more abundant to 

the north in the vicinity of the Blackbird mine (Hahn and 

Hughes, 1984; Nash and Hahn, 1986). 

Synsedimentary Structural Controls 

Cobaltiferous pyrite zones favor disruption 

structures that formed prior to consolidation. Substantial 

amounts of magnetite and fine pyrite are stratiform or 

appear to have been part of the sediment prior to 

disruption; these early stages of mineralization, possibly 

syngenetic, contain only small amounts of cobalt. Thus, 

the synsedimentary structures provide key evidence for 

the timing and character of processes that introduced and 

deposited cobalt. 

Strata containing Co-pyrite also contain resedimented clastic grains and rounded clasts of partly 

consolidated sediment. As described earlier, most of the 

structures are concordant: they are confined between 

undisturbed beds, and the planar or linear features of 

their fabrics are aligned parallel to the dip of beds. In 

contrast, other mineralized structures are discordant: 

they cut across beds and indicate injection of unconsolidated material by upward-moving pore fluids that perhaps had sufficient velocity to meet the definition of 

fluidization (Lowe, 1975). Most of the transported 

material seems to have been only slightly suspended or 

liquefied and in that state was gravitationally unstable. 

Magnetite appears to have been especially unstable 

because very little of it is bedded, yet almost certainly it 

was originally a chemical sediment in beds a few 

millimeters to more than 1 m thick. Although details 

required to quantify specific behavior as plastic, 

liquefied, thixotropic, or fluidized are difficult to obtain 

and certainly are debatable, the qualitative conclusion 

that the deformation and associated mineralization 

occurred prior to consolidation seems valid. 

Cobalt enrichment was not strictly syngenetic but 

occurred early in the history of the host rocks. If clastic 

dikes form at depths of about 30-60 m (Dzulynski and 

Randomski, 1956; Hiscott, 1979), we can infer comparable conditions of very shallow burial for these 

structures at Iron Creek. Pore fluids clearly moved 

through these structures-indeed, were instrumental in 

producing them. The physical principles of liquefaction 

and fluidization (Lowe, 1975) imply geologically brief 

periods of elevated pore pressure and fluid injection. 

However, there is no way to count the number of fluid 

pulses or relate the specific time of formation of zones 10 

or 100 m apart. It is likely that the fluid pulses were 

episodic. Earthquakes are typically cited as causes 

(Lowe, 1975), but I would add slumping and volcanic 

eruptions as known Yellowjacket events that could have 

triggered fluid injection. In summary, this was a dynamic 

system of reactive unconsolidated sediment containing 

abundant pore fluid that was gravitationally unstable and 

was heated by a small volcanic system. 

There are few examples in the literature of similar 

synsedimentary controls on ore deposits. Descriptions of 

"quickstone" containing "pelletoid conglomerate," 

folded and "flowsheared" fabric, and gravity slides 

(McNeil, 1966) at the Orlando mine, Tennant Creek, 

Australia, seem similar to features of the No-Name 

deposit. (The Tennant Creek deposits also are rich in 

magnetite and provide a useful geochemical as well as 

structural analogy for the Iron Creek deposit.) Softsediment deformation of quartzite and argillite in the 

Belt Supergroup (Middle Proterozoic) at the Atlas mine, 

Coeur D'Alene mining district, Idaho, was reported by 

Chevillon (1977); the deformation was associated with 

synsedimentary movement of a major fault strand related 

to the regionally important Osborne fault. The faulting, 

soft-sediment deformation, and focus of fluids are part of 

the early stages of ore deposition. Sedimentary and early 

diagenetic structures are distinguished (J onasson and 

Goodfellow, 1986) from tectonic ones in the Howards 

Pass zinc-lead deposit, where disharmonic folds having 

no axial-planar cleavage are common and considered 

diagnostic of synsedimentary deformation; such structures were suspected in the No-Name deposit but small 

folds of any type are very rare. Well-exposed features 

such as ore clasts, ore breccia, folded beds, and gravityslide blocks in Kuroko massive-sulfide deposits indicate 

deformation penecontemporaneous with ore formation 

on the sea floor (Hashiguchi, 1983). This deformation is 

related to intrusion of rhyolite, doming, and phreatic 

explosions. On the other hand, McClay (1983) points out 

that many fold structures in sediment-hosted ore 

Discussion and Interpretation 23 
deposits that some think are synsedimentary are actually 

of tectonic origin as determined by microstructural 

criteria such as ductile deformation of sulfides. McClay 

(1983) does describe some synsedimentary structures 

that displace ore minerals and notes that one of the most 

reliable tests for this early age is crosscutting dewatering 

structures. 

Geochemistry 

The most notable chemical feature of the Iron 

Creek deposits is the high concentration of iron. Iron was 

deposited in several stages as chemical sediment interbedded with siliciclastic and volcaniclastic sediments. 

Regionally, this is but one cluster of iron-rich sedimentary deposits that trend northwesterly from the iron 

deposits at McKim Creek (Soregaroli, 1961), about 18 

km southeast of the No-Name deposit, through the Idaho 

cobalt belt (Hughes, 1983), with many occurrences of 

rocks rich in iron oxides or silicates, and possibly as far 

north as the bedded magnetite deposits at Woods Creek, 

Ravalli County, Mont. (about 90 km north of the study 

area). Introduction of iron into the Yellowjacket basin, 

probably from thermal vents, seems to be a regional 

manifestation of the rift system described by Hughes 

(1983). 

The ferruginous sediments accumulated during 

periods of mostly low energy sedimentation as beds a few 

millimeters thick to aggregates more than 3 m thick. The 

aggregated beds of magnetite outline local subbasins 

having strike lengths of more than1.5 km in the Jackass 

and No-Name zones; the width of these zones is not 

known in detail but must have been hundreds of meters. 

The surges of iron-rich hydrothermal fluids first caused 

the deposition of iron oxides and very little or no sulfide 

minerals. Concentrations of cobalt, copper, and other ore 

metals possibly were very low in the oxide stage; clearly, 

very small amounts of ore metals were deposited. The 

lack of sulfide minerals probably relates to lack of sulfur 

rather than to oxidation because there are only small 

amounts of hematite to suggest oxidizing conditions. 

Later in each period of ferruginous exhalation, sulfide 

became available. This caused the formation of ironsulfide muds, now fine-grained stratiform pyrite. Some 

cobalt was co-deposited with the sulfide mud. The ferruginous muds were unstable on the seafloor and were 

the focus of sliding, liquefaction, and occasional fluid 

injection into overlying beds. Diagenetic reactions caused 

recrystallization to fine well-formed grains of magnetite 

and pyrite. Pore fluids expelled from the compacting pile 

caused the liquefaction and carried iron, cobalt, and 

copper, but low concentrations of other metals. These 

processes of iron exhalation and sedimentation, buildup 

of sulfide, liquefaction and sliding, and deposition of 

Co-pyrite and chalcopyrite were repeated many times 

during the accumulation of 400 m of siliciclastic 

sediments in the No-Name zone. 

The Co-pyrite deposits are enriched in a limited 

suite of chalcophile elements. The most significant is 

copper, accompanied by very minor amounts of silver, 

lead, and zinc. Bismuth is notably concentrated, as is 

arsenic, but arsenic is not nearly as abundant as in the 

Blackbird deposits. These elements seem to have been 

introduced by diagenetic pore fluids, not by a fluid 

erupted on the seafloor. As explained above, episodic 

movement of pore fluid through the unconsolidated 

sediments seems likely. It is possible that the quiescent 

periods allowed leaching and equilibration with the 

sediment and also may have allowed a buildup of sulfide 

ion, perhaps by bacterial action. During periods of fluid 

flow through the synsedimentary structures, the chalcophile ions reacted with sulfide ions to form sulfide 

minerals, some of which were rich in cobalt. Details of 

the reactions are not known. The fluids appear to have 

been in equilibrium with the host rocks because no 

unusual alteration is found in association with the 

sulfides; thus, a mechanism such as change in pH does 

not seem to have caused sulfide deposition. Stable 

isotopic studies might provide constraints on the sulfur 

cycle and deposition mechanisms. 

Similar Ore Deposits 

The synsedimentary controls, mineralogy, and 

geochemistry of the Iron Creek deposits seem to be 

unusual among ore deposits, but similar features are 

known in some. Sediment-hosted deposits are major ore 

producers in many parts of the world (Stanton, 1972), but 

most tend to be rich in lead and zinc. Some that are rich 

in copper also contain significant amounts of cobalt. 

Most notable are the many important deposits of the 

Zambian Copperbelt, which is the major source of 

cobalt for the Western World (Annels and Simmonds, 

1984). Cobalt is not uniformly distributed at regional or 

deposit scale but is mostly confined to a belt about 100 

km long by 10 km wide, probably coincident with a rift 

zone (Annels and Simmonds, 1984). Cobalt in the Zambian deposits is always with copper-no cobalt-only 

deposits are known-and generally is most abundant in 

the central part of the copper-cobalt deposits. Much of 

the cobalt occurs in pyrite, some individual grains or 

growth zones containing more than 20 percent Co. Other 

hosts for cobalt include cobaltite and a variety of cobaltcopper-nickel sulfide minerals. A source of cobalt and 

copper in mafic igneous rocks in the rift zone is 

advocated (Annels and Simmonds, 1984). A major 

difference between the African and Idaho cobalt-copper 

deposits is the shallow-water sedimentary features in the 

24 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
Zambian Copperbelt sediments and the abundant diagenetic anhydrite that reacted to form sulfide minerals 

(Annels, 1974; Annels and Simmonds, 1984). 

The world-class base-metal deposits at Mount Isa 

and Broken Hill, Australia, thought by some to be 

synsedimentary in origin prior to severe deformation and 

metamorphism (Stanton, 1972), have some intriguing 

similarities to the deposits of the Idaho cobalt belt and 

contain local enrichments of cobalt. In the Mount Isa 

lead-zinc-copper deposits (Croxford, 1974) cobalt is 

locally concentrated to about 300 ppm and in a few rare 

places exceeds 1 percent. The cobalt is associated with 

zones rich in copper and is present as cobaltite and other 

cobalt-arsenic-sulfur minerals. The copper and cobalt 

are interpreted to have moved during metamorphism 

(Croxford, 1974), so their original residence is not known 

for comparison with the No-Name deposit. No significant 

cobalt has been found in the Mount Isa pyrite. 

The deposits of Tennant Creek, Australia (Large, 

1975, 1976), important producers of gold, may provide a 

useful geologic or geochemical analog. The Tennant 

Creek deposits are notably rich in magnetite (that forms 

as much as 80 percent of central ore zones), bismuth, and 

selenium, as well as gold. Although importance of syngenetic deposition of magnetite is a subject of debate 

(Dunn, 1976), the Tennant Creek deposits provide a 

useful exploration model for possible gold in the Iron 

Creek deposits. Assays to date have not found gold of 

significant concentration ( > 1 g/t) or volume. Gold at 

Tennant Creek (Large, 1975) is found in the central part 

of the zoned mineralized area in magnetite-chlorite rock, 

a bismuth zone being nearby; copper and sulfur are most 

abundant outside the gold zone. The gold-magnetitechlorite orebodies occur above chlorite-rich alteration 

pipes that probably were feeders; comparable alteration 

or likely feeder structures have not been identified in the 

Iron Creek deposits. Ore-forming conditions at Tennant 

Creek were more oxidizing than in typical sulfide 

deposits such as volcanogenic massive-sulfide deposits 

that are similar in many ways to Tennant Creek (Large, 

1976). Oxidizing conditions may be a requirement for 

that magnitude of gold transport and deposition. 

Although not emphasized by Large (1975, 1976), at least 

some deposits in the Tennant Creek district display 

evidence for synsedimentary soft-sediment deformation 

(McNeil, 1966) that seems similar to features in the Iron 

Creek area. 

Metamorphism 

Regional greenschist metamorphism seems to have 

had little or no effect on the Iron Creek mineralization. 

The silicate mineral assemblage is essentially the same 

within deposits as it is outside, and the fabric of the ore 

minerals seems consistent with,deposition or recrystallization in a shallow geothermal system. The late paragenetic position of chalcopyrite and the presence of a 

copper-rich zone abqve the Co-pyrite zone in the NoName deposit might reflect remobilization of copper 

during metamorphism, which commonly occurs, but also 

could be explained by later deposition of copper in the 

submarine system. The prominent cleavage in the area 

does not seem to influence ore minerals, and its age and 

origin is not known. 

Resources and Exploration 

The No-Name deposit has been a subeconomic 

resource in the recent cobalt market. If the price of 

cobalt were to increase substantially, or if cobalt cannot 

be supplied by foreign sources, the deposit could supply 

cobaltiferous-pyrite concentrate that could be smelted in 

existing United States facilities. The partially drilled 

resource is about 1 million tons and has a grade in excess 

of 0.5 percent Co (Webster and Stump, 1980), or about 

6 million kg cobalt metal. Depending upon economic 

factors, lower grade or thinner parts of the deposit, as 

well as downdip and on-strike extensions, might add 

substantially to this resource. 

The more general question is what additional Copyrite resources might exist and be discoverable. 

Certainly the very poor outcrop in the Idaho cobalt belt 

has prevented recognition of deposits. Although gossan is 

visible in the Jackass and No-Name zones, the zones 

attracted attention after being cut, quite by chance, by 

logging roads. Despite the poor exposure, several lenses 

of magnetite iron-formation are known in the cobalt belt, 

but only one has abundant Co-pyrite. The magnetite is a 

useful exploration feature because it is easily detected by 

magnetic surveys. The cobalt deposits also create strong 

geochemical anomalies in soil and in water (Erdman an.d 

Modreski, 1984) and should be detectable. Economic 

incentives are required to do the detailed exploration 

needed to locate and drill new prospects. 

There are several reasons to postulate the 

existence of many additional Co-pyrite zones similar to 

the No-Name. Many stratiform and discordant iron 

deposits are known along the more than 60-km strike 

length of the cobalt belt; these are indicators of hydrothermal vents along the rift zone. The starved-basin stage 

of the Yellowjacket basin (Hughes, 1983) is conceptually 

conducive to the accumulation of widespread pyritic 

sediment that would be a favorable component in making 

Co-pyrite deposits. Finally, the soft-sediment deformation and dewatering phenomenon that seem to be 

important in creating rich Co-pyrite zones are 

widespread in the lower and middle units of the Yellowjacket Formation and extend well. beyond areas ~here 

they are known to be associated With cobalt deposits. 

Discussion and Interpretation 25 
Exploration for gold in the Iron Creek area should 

be directed at identifying structures and alteration 

indicative of focused hydrothermal activity. According to 

the Tennant Creek example, gold might be expected to 

be richest in zones of magnetite, but in other types of 

iron-rich sedimentary sequences gold deposition favors 

the transition from oxide to sulfide facies (see, for 

example, Fripp, 1976). 

Ore Genesis 

The low-temperature transport of cobalt is not as 

well understood as that of most base metals in 

sedimentary environments (see, for example Lydon, 

1983). Thermochemical models (Haynes and Bloom, 

1987) predict that base-metal ratios can be explained by 

composition of the source rock, o:x)'gen fugacity of the 

transporting brine, and sulfur fugacity at the site of 

deposition. The very high cobalt: copper of the No-Name 

deposit, and low concentration of silver, lead, and zinc 

might be explained by a mafic source rock, moderate 

oxygen fugacity during leaching and transport, and high 

sulfur fugacity during deposition. There is experimental 

evidence that stable cobalt-chloride complexes form at 

temperatures of about 100-200 oc and are stronger than 

those of nickel-chloride (Susak and Crerar, 1985). This 

provides a mechanism for the separation of cobalt from 

nickel and explains the unusually high cobalt : nickel in 

the Co-pyrite as well as in the Blackbird deposits. The 

cobalt-chloride complexes weaken rapidly with decrease 

in temperature; thus, cooling should be a viable process 

for deposition as long as the requirements for reduced 

sulfur are met. 

Cobalt deposition might possibly have been 

triggered by mixing of a heated subsurface cobalt-bearing 

brine with cooler seawater. If cobalt was complexed by 

chloride in a saline brine, which could also have contained a limited amount of sulfide ion, mixing with 

seawater on the sea floor or in the disruption structures 

would have cooled the brine and also decreased the 

concentration of chloride-both processes would have 

destabilized cobalt-chloride complexes and permitted 

reaction with any available sulfide ions. Such a mechanism also would have promoted transport and deposition of iron and copper. 

Details of the hydrothermal plumbing system are 

lacking. Drill data suggest that much of the flow was 

subhorizontal in disruption zones confined between 

undeformed strata. No feeder faults or associated discordant alteration zones are known, although this might 

be explained by poor exposure and the small cross 

section encountered by drill holes oriented perpendicular 

to bedding. Also, it is possible that the deposit slid 

basinward from the vents. Given the present steep dip of 

beds and the likelihood of submarine slides (or aggregate 

of small slips), the concept of drilling "closer to the vent" 

in search of thicker Co-pyrite zones or higher gold 

concentrations is not easily accomplished. 

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et provinces cupriferes: Liege, Societe Geologique de 

Belgique, p. 235-254. 

Annels, A.B., and Simmonds, J.R., 1984, Cobalt in the Zambian Copperbelt: Precambrian Research, v. 25, p. 75-98. 

Chevillon, C.V., 1977, Tectonically induced, Proterozoic soft 

sediment deformation at the Atlas property, Coeur 

D'Alene mining district, Idaho: Geological Society of 

America Abstracts with Programs, v. 9, no. 6., p. 716. 

__ 1979, Iron Creek project progress report (1977-1978): 

Unpublished report, Noranda Exploration, Inc., 56 p. 

(geologic map, scale 1:4,800). 

Cohenour, R.E., Fox, R.C., and Robison, W.D., 1973, Geology 

and geophysics of the Iron Creek copper-cobalt deposit, 

Lemhi County, Idaho: American Institute of Mining, 

Metallurgical, and Petroleum Engineers, Pacific Northwest Metals and Minerals Conference, 1973, Coeur 

D'Alene, Idaho, 15 p. 

Croxford, N.J.W., 1974, Cobalt mineralization at Mount Isa, 

Queensland, Australia, with references to Mount Cobalt: 

Mineralium Deposita, v. 9, p, 105-115. 

Davis, J.C., 1986, Statistics and data analysis in geology: New 

York, John Wiley and Sons, 646 p. 

Dunn, P.G., 1976, Zonation of hydrothermal minerals at 

the Juno mine, Tennant Creek goldfield, central 

Australia-A discussion: Economic Geology, v. 71, 

p. 1614-1615. 

Dzulynski, S., and Randomski, A., 1956, Clastic dikes in 

Carpathian flysch: Rocznik Polskiego Towarzystwa Geologicznego, v. 26, p. 225-273. 

Elliott, R.E., 1965, A classification of subaqueous sedimentary 

structures based on rheological and kinematical parameters: Sedimentology, v. 5, p. 193-209. 

Erdman, J.A., and Modreski, P.J., 1984, Copper and cobalt in 

aquatic mosses and stream sediments from the Idaho 

cobalt belt: Journal of Geochemical Exploration, v. 20, 

p. 75-84. 

Fripp, R.E.P., 1976, Stratabound gold deposits in Archean 

banded iron formation, Rhodesia: Economic Geology, 

v. 71, p. 58-75. 

Hahn, G.A., and Hughes, G.J., Jr., 1984, Sedimentation, tectonism, and associated magmatism of the Yellowjacket 

Formation in the Idaho cobalt belt, Lemhi County, 

Idaho, in Hobbs, S.W., ed., The Belt, abstracts with 

summaries, Belt symposium II, 1983: Montana Bureau of 

Mines and Geology Special Publication 90, p. 65-67. 

Hashiguchi, Hironori, 1983, Penecontemporaneous deformation of Kuroko ores at the Kosaka mine, Akita, Japan, 

in Ohmoto, Hiroshi, and Skinner, B.J., eds., The Kuroko 

and related volcanogenic massive sulfide deposits: 

Economic Geology, Monograph 5, p. 167-183. 

26 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
Haynes, D.W., and Bloom, M.S., 1987, Stratiform copper 

deposits hosted by low energy sediments-TV. Aspects 

of sulfide precipitation: Economic Geology, v. 82, 

p. 875-893. 

Hiscott, R.N., 1979, Clastic sills and dikes associated with 

deep-water sandstones, Tourelle Formation, Ordovician, 

Quebec: Journal of Sedimentary Petrology, v. 49, p. 1-9. 

Hubbert, A.E., and Chao, T.T., 1985, Determination of gold, 

indium, tellurium, and thallium in the same sample 

digestion of geological materials by atomic absorption 

spectroscopy and two-step solvent extraction: Talanta, 

v. 32, p. 568-570. 

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

Blackbird mining district, Lemhi County, Idaho, in 

The genesis of Rock."Y Mountain ore deposits-Changes 

with time and tectonics: Denver Region Exploration 

Geologists Society, p. 21-27. 

Jackson, L.L., Brown, F.W., and Neil, S.T., 1987, Major and 

minor elements requiring individual determinations, 

classical whole rock analysis and rapid rock analysis, in 

Baedecker, P .A., ed., Methods for geochemical analysis: 

U.S. Geological Survey Bulletin 1770, p. G1-G23. 

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J.A., ed., Mineral deposits of northern Cordillera: Canadian Institution of Mining and Metallurgy Special 

Volume 37, p. 51-70. 

Large, R.R., 1975, Zonation of hydrothermal minerals at the 

Juno mine, Tennant Creek goldfield, central Australia: 

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___ 1976, Zonation of hydrothermal minerals at the Juno 

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Lichte, F.E., Golightly, D.W., and Lamothe, P.J., 1987, 

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p. B1-B10. 

Lowe, D.R., 1975, Water escape structures in course-grained 

sediments: Sedimentology, v. 22, p. 157-204. 

Lydon, J.W., 1983, Chemical parameters controlling the origin 

and deposition of sediment-hosted stratiform lead-zinc 

deposits, in Sangster, D.F., ed., Short course in sedimenthosted stratiform lead-zinc deposits: Mineralogical 

Association of Canada, v. 8, p. 175-250. 

McClay, K.R., 1983, Deformation of lead-zinc deposits, in 

Sangster, D.F., ed., Short course in sediment-hosted 

stratiform lead-zinc deposits: Mineralogical Association 

of Canada, v. 8, p. 283-309. 

McNeil, R.D., 1966, Geology of the Orlando mine, Tennant 

Creek, Australia: Economic Geology, v. 61, p. 221-242. 

Middleton, G.V., and Hampton, M.A., 1973, Sediment gravity 

flows-Mechanics of flow and deposition, in Turbidites 

and deep water sedimentation: Society of Economic 

Paleontologists and Mineralogists, Pacific Section Short 

Course, p. 1-38. 

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. 

Mottl, M.J., 1983, Metabasalts, axial hot springs, and the 

structure of hydrothermal systems at mid-ocean ridges: 

Geological Society of America Bulletin, v. 94, 

p. 161-180. 

Nash, J.T., and Hahn, G.A., 1986, Volcanogenic character of 

sediment-hosted Co-Cu deposits in the Blackbird mining 

district, Lemhi County, Idaho-An interim report: U.S. 

Geological Survey Open-File Report 86-430, 29 p. 

Ramdohr, Paul, 1980, The ore minerals and their intergrowths, 

2nd edition: New York, Pergamon Press, 1205 p. 

Soregaroli, A.E., 1961, Geology of the McKim Creek area, 

Lemhi County, Idaho: Moscow, University of Idaho, 

M.S. thesis, 53 p. 

Stanton, R.L., 1972, Ore petrology: New York, McGraw-Hill, 

713 p. 

Susak, N.J., and Crerar, D.A., 1985, Spectra and coordination 

changes of transition metals in hydrothermal solutionsImplications for ore genesis: Geochimica et Cosmochimica Acta, v. 49, p. 555-564. 

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and connate water: Sedimentary Geology, v. 2, 

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A.J., and Stewart, K.C., 1987, Analysis of geologic 

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geochemical analysis: U.S. Geological Survey Bulletin 

1770, p. E1-E19. 

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Lemhi County, Idaho: Unpublished report, Noranda 

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References Cited 27 

APPENDIXES 1 AND 2 
APPENDIX 1. DESCRIPTION OF ANALYZED SAMPLES 

[P, petrographic analysis; C, chemical analysis] 

IC3-6_. 

IC3-72 

IC3-83 

IC3..:_102 

IC3-113 

ICl0-440 

ICl0-448 

ICl0-460 

ICl0-496 

ICl0-806 

ICl0-807 

ICl0-809 

ICl0-814 

ICl0-819 

ICl0-821 

ICl0-824 

ICl0-829 

IC12-234 

IC12-265 

IC12-270 

IC12-280 

IC12-285 

IC12-295 

IC12-305 

IC12-325 

IC12-328 

IC12-488 

IC12-623 

IC16-236 

IC16-244 

[P,C] Weakly laminated siltite and fine quartzite; muscovite more abundant than biotite; contains knots of carbonate 

minerals that cut bedding. 

[C] Weakly laminated siltite and argillite; cobalt zone. 

[P,C] Partly bedded siltite and argillite containing sandy dikelets; some beds of fine pyrite (pyrite mud?). 

[C] Laminated siltite. 

[P] Weakly to nonbedded argillite and minor siltite containing sand dikelets; some beds of fine pyrite mud. 

[P,C] Laminated siltite above cobalt zone; cut by gash fractures filled by quartz. 

[P] Dismption 1 ft thick filled by coarse pyrite, quartz, and green mica (chlorite after biotite). 

[P] Dismpted nonbedded zone hard from silicification; quartz and muscovite are coarse (300,um) and recrystallized. 

[P] Greenstone (chlorite-epidote-plagioclase rock) containing weakly aligned crystals; altered mafic dike or 

pyroclastic. 

[C] Laminated argillite above magnetite zone. 

[C] Split of sample ICl0-806. 

[P,C] Dismption zone rich in magnetite; layers of fine and coarse magnetite, pyrite, or magnetite and pyrite. 

[P] Partly layered zone, mostly dismption-filling magnetite and pyrite, with minor siliciclastic component; quartz 

cements ore minerals. 

[C] Base of dismption zone, just above laminated siltite; magnetite and pyrite fill dismption. 

[P] Footwall to major dismption zone; sample is lumpy dismption with sausage-shaped infilling of magnetite that 

probably was folded during compaction; laminated siltite contains much more pyrite than magnetite, but infill 

contains much more magnetite than pyrite. 

[C] Disruption zone rich in magnetite and containing 10 percent pyrite; bedding appears stirred or smeared with 2-cm 

lumps of sediment. 

[P,C] Lumpy dismpted argillite with dikelets of sand-sized quartz and magnetite, cemented by quartz and biotite 

(retrograded to chlorite). 

[P] Dismption structure filled by coarse pyrite; wallrock siltite and argillite are recrystallized to coarse grains of quartz 

and mica; fabric is weakly gneissic (from slip?). This is at top of an 88-ft-thick sequence of mostly dismpted sediment. 

[P,C] Weakly laminated siltite and lumps of argillite; coarse veinlike quartz and pyrite fill openings. 

[P] Dismption zone filled by coarse pyrite and much less chalcopyrite, quartz, and chlorite (replaced biotite). No 

magnetite. Chalcopyrite engulfs pyrite. 

[C] Disrupted zone containing coarse pyrite and chalcopyrite. 

[C] Disrupted zone, abundant coarse pyrite and chalcopyrite. 

[C] Pyrite-rich (50 percent) filling of 1-ft-wide dismption. 

[P,C] Layered fine-grained pyrite in argillite; pyrite mud(?). 

[P,C] Footwall to dismption zone, laminated siltite containing little pyrite. 

[P,C] Footwall of ore zone, laminated argillite, scattered sand dikelets. 

[P,C] Laminated fine-grained pyrite in argillite; pyrite mud(?). 

[P] Massive textured rock from 619- to 657-ft depth; equal amounts of plagioclase, chlorite, and epidote, and 

abundant fine magnetite; phenocryst texture survives; mafic dike. 

[C] Laminated siltite and fine quartzite, low sulfide content, example of hangingwall. 

[C] Laminated siltite containing little sulfide, background reference. 

30 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
IC16-271 [C] Mostly laminated siltite and a few centimeter-size dismptions containing sulfides. 

IC16-281 [C] Moderately dismpted siltite, some bedding is normal; dismptions filled with pyrite. 

IC16-290 [P] Highly dismpted zone in which lumps of argillite are cemented by coarse (200 J.Lm) sand and chemical quartz; 

contains much more pyrite than chalcopyrite, chalcopyrite surrounds euhedral pyrite; no magnetite. 

IC16-296 [P] Dismption zone filled by coarse pyrite; crystals of pyrite are broken and corroded, cemented by quartz and biotite 

(altered to chlorite). Texture suggests shear or slip parallel to bedding. 

IC16-300 [P] Half of sample is fine-grained laminated pyrite, other half is mostly coarse pyrite, cemented by quartz and micas 

and by late marcasite. 

IC16-306 [P,C] Pyritic dismption zone, nonmagnetic. Wallrock is lumpy argillite and siltite. 

IC16-311 [C) Pyritic infilling of dismption zone, pyrite finer than in IC16-306; sulfides are layered parallel to adjacent bedding, 

caused by slump or slide(?). 

IC16-336 [C] Undisturbed laminated siltite and argillite footwall to dismpted sequence, generally low sulfide except for some 

laminae of fine pyrite (pyrite mud?). 

IC16-351 [C] Bedded siltite and argillite containing abundant disseminated and laminated fine pyrite. 

IC16-542 [C] Quartzite containing layers of magnetite, low sulfide content. 

IC18-700 [C] Thinly laminated siltite, collected for background reference. 

IC18-729 [P] Dismpted siltite and argillite having lumpy texture; carbonate-quartz-pyrite fill ovoids and replace argillite; 

muscovite and biotite (altered to chlorite) abundant in dismptions. 

IC18-762 [P, C] Thin dismption structure 3 em thick cuts lumpy clay-pyrite rock; pyrite replaces micas and is cemented by 

quartz. 

IC18-767 [C] Dismption zone having high sulfide content, averaging about 40 percent, some of which is veinlike. 

IC18-768 [C] Split of sample IC18-768. 

IC18-770 [P,C] Small sample very rich in pyrite, about 70 percent over a 0.6-m length, and assay indicated high arsenic content, 

about 0.25 percent; massive euhedral pyrite, minor marcasite, late anhedral pyrite (composition indicated by 

SEM-EDS analysis). 

IC18-776 [P] Laminated siltite and quartzite containing layers of fine pyrite and a phosphate mineral(?), well-crystallized 

biotite and muscovite, cemented by quartz. 

IC18-781 [P,C] Laminated argillite and siltite cut by thin dismptions filled by quartz-carbonate-pyrite. 

IC18-854 [C] Faintly laminated fine quartzite. 

IC18-861 [P] Siltite cut by 2-cm break filled by coarse pyrite and magnetite. 

IC18-862 [P] Siltite cut by 1-cm break filled by quartz-pyrite-magnetite; wallrock contains much more pyrite than magnetite, 

break contains about equal amounts of pyrite and magnetite. 

IC18-887 [P] Laminated argillite and siltite cut by 2-cm break filled by quartz-pyrite-magnetite; wallrocks contain more 

magnetite than pyrite. 

IC18-894 [P] Altered dike: carbonate and quartz alteration, elongate plagioclase phenocrysts, muscovite-biotite-chloritemagnetite matrix. 

IC18-1064 [C] Bedded siltite containing little sulfide, a few narrow dismptions. 

IC18-1068 [P,C] Bedded siltite and argillite above disruption zone, low sulfide content. 

IC18-1072 [P] Laminated siltite and argillite containing stringers of quartz-pyrite-magnetite and having no alteration selvage 

along stringers; quartz is sand-sized (100-200 J.Lm) mosaic of interlocked grains having no apparent detrital shapes. 

IC18-1074 [P,C] Thoroughly dismpted zone, rare beds survive; thick layers (as much as 25 em) of pyrite and magnetite; some 

layers of fine magnetite and pyrite have appearance of clastic beds. 

IC18-1079 [C] Laminated siltite below disruption zone, rich in fine-grained laminated pyrite. 

Appendix 1. Description of Analyzed Samples 31 
IC18-1080 

IC18-1081 

[C] Laminated siltite and fine quartzite in which there are only a few small disruptions; low sulfide content, 

moderately magnetic. 

[C] Sawn split of IC18-1080, above. 

APPENDIX 2. METHODS FOR GEOCHEMICAL ANALYSIS 

Rock samples were analyzed by chemists of the U.S. Geological Survey using standard methods described below. For more 

information consult the references cited. Limits of detection are summarized in table 3. 

X-ray fluorescence (XRF): Sample (0.8 g) is fused in lithium tetraborate, and the glass disk is analyzed for major elements by 

wavelength-dispersive X-ray spectrometry. J.E. Taggart and A.J. Bartel, analysts. The method is described by Taggart and others 

(1987). 

Induction-coupled plasma-atomic-emission spectrometry (ICP): Sample (0.2 g) is digested with mixed acids (HF, HCl-HN03, 

HC104) to dryness, then redissolved in HC1-HN03 and then analyzed by induction-coupled plasma-atomic-emission 

spectrometry for which lutetium is an internal standard (Lichte and others, 1987). A few refractory minerals are not dissolved 

by this acid attack; specifically, forB in tourmaline and Zr in zircon, the sample is sintered in Na202 prior to ICP analysis for 

B and Zr. Paul Briggs, analyst. 

Special element analysis: Six additional elements were analyzed on request as determined below. L.L. Jackson, E.E. Engleman, 

D.B. Hatfield, and E. Brandt, analysts. Methods are described by Jackson and others (1984) and Wilson and others (1987). 

Fe as FeO: Sample (0.5 g) is decomposed with HF and H2S04, and the digestate is immersed in a solution of boric, sulfuric, and 

phosphoric acids. Fe(II) is determined by potentiometric titration using potassium dichromate. 

H20: Sample (1.0 g) is heated at 110 oc for 1 hour, and H2o- is determined by weight loss. H20+ determined on 0.05-g sample 

heated with lead oxide and lead chromate, the evolved water analyzed by coulometric Karl Fischer titration. 

C, total: Sample (0.75 g) is mixed with Ah03 and combusted in an m -ygen .. atmosphere; total Cis determined as C02 by infrared 

absorption spectrometry. 

S, total: Sample (0.25 g) mixed with vanadium pentoxide and combusted at 1370°C in ox-ygenated atmosphere; totalS determined 

as so2 by infrared-absorption spectrometry. 

Au: Sample (10 g) is digested with HBr-Br2 and Au extracted with methyl-isobutyl ketone. After washing the organic phase with 

0.1M HBr to remove iron, Au is determined by flame atomic-absorption spectrometry (Hubbert and Chao, 1985). 

Se: Sample (0.3 g) is digested with K2S20 8 , HF, HN03, and HC104 followed by a second treatment by HN03 and HCl04 combined 

with H2S04. Solution is mixed with sodium borohydride, and resultant metal hydride is carried in an argon stream into a heated 

quartz glass tube and determined by atomic-absorption spectrometry. 

32 Geology and Geochemistry of Synsedimentary Cobaltiferous-Pyrite Deposits, Iron Creek, Lemhi County, Idaho 
Table 3. Lower limits of determination in geochemical 

analyses of samples from the No-Name deposit, Lemhi 

County, Idaho 

[FeTOs, total iron reported as Fe20s. (S) determined by inductioncoupled plasma-atomic-emission spectrometry] 

Major elements, in weight percent 

Si02 

Al 2o3 

FeT03 

FeO 

MgO 

CaO 

Na2o 

K20 

Fe (S} 

Mg (S) 

Ca (S) 

Ti02 

P205 

MnO 

H2o+ 

H2oC total 

S total 

Ti (S) 

Na (S} 

K (S) 

p (S} 

Minor elements, 

Ag 

As 

Au 

B 

Ba 

Be 

Bi 

Cd 

Ce 

Co 

Cr 

Cu 

Eu 

Ga 

Ho 

La 

L i 

Mn 

Mo 

Nb 

Nd 

Ni 

Pb 

Sc 

Se 

Sn 

Sr 

Ta 

Th 

v 

y 

Yb 

Zn 

Zr 

u U.S. GOVERNMENT PRINTING OFFICE: 1989---673-047/86,098 REGION NO. 8 

0.05 

.10 

.04 

.01 

.10 

.05 

.15 

.02 

.05 

.005 

.05 

.02 

.05 

.02 

.01 

.01 

.01 

.01 

.002 

.005 

.05 

.005 

in parts per million 

2 

10 

.05 

20 

1 

1 

10 

2 

4 

1 

1 

1 

2 

4 

4 

2 

2 

4 

2 

4 

4 

2 

4 

2 

.01 

10 

2 

40 

4 

2 

2 

1 

2 

8 

Appendix 2. Methods For Geochemical Analysis 33 



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