able structures where they pass

A Hecla Mining
through these strata.

Co. geologist
prepares to

Wall-rock alteration. Except  S@mple a stope

for hydrothermal bleaching that was hentpdaree
once thought to characterize Coeur oped Gold

d’Alene ore deposits, alteration of
wall rocks surrounding the ore bod-
ies has been regarded as subtle or
nonexistent. However, it has been
found that several types of wall rock
alteration do surround ore bodies
and clusters of ore bodies. Some
features of the alteration halos pro-
vide evidence concerning the devel-
opment of the ore bodies.

Hunter silver-
lead ore body.
Photo: Bri
White, NIOSH.

Hydrothermal bleaching. Hy-
drothermal bleaching in the Coeur
d’Alene district was first described
by Rasor (1934) at the Sunshine
Mine. The bleaching sometimes ex-
tended far beyond known ore bod-
ies. It was thought to provide a
rough exploration guide, so that
bleached strata became the focus of
much subsequent exploration and study (Weis, in
Fryklund, 1960). In time, investigators began to realize
that much of the presumed bleached strata extended far
beyond the bounds of the Coeur d’Alene district. This
indicated that the perception of the importance of hy-
drothermal bleaching had been exaggerated

At the Sunshine Mine, where hydrothermal bleach-
ing was first recognized, purplish, hematitic strata are
more prominent in the upper Revett Formation and
lower St. Regis Formation than in most of the remainder
of the district (personal observation). In this purplish
rock, hydrothermal bleaching typically extends 10 to 30
m (33 to 100 ft) out from ore bodies. A 0.5-m- (1.6-ft-)
wide zone of coarse, euhedral pyrite occurs at the edge of
the bleached zone along the redox boundary (Mitcham,
1952; reaffirmed by Sunshine Mine geologists). Thus,
hydrothermal bleaching is prominent only in a limited
rict where mineralizing structures inter-

Carbonate zoning. The earliest reports on district
geology (Ransome and Calkins, 1908) commented on
the presence of disseminated siderite near veins. But the
existence of other carbonates in the wall rocks has not
been considered diagnostic of Coeur d’Alene veins.
Shaw (1959) was apparently the first to identify a distal
carbonate zone containing ankerite and calcite around
ore bodies at the Bunker Hill Mine.

This discovery should have been immediately ap-
plied in exploration. But the practical problem of readily
distinguishing small amounts of disseminated ankerite,
calcite and siderite underground and in drill core pre-
vented these minerals from attracting attention.

Since then, Goers (unpublished progress reports to
Hecla Mining, 1969) identified similar zones around the
Star-Morning, Standard Mammoth and Lucky Friday
mines. Gitlin (1986) further investigated carbonate
zones about the Lucky Friday vein. With the application
of field-usable carbonate stains in the early 1980s, iden-

tification of wall rock carbonates became integrated into
exploration efforts at all active mines.

The zone of disseminated siderite, tens to hundreds
of meters wide that surrounds the veins, grades out into
a siderite-ankerite zone a few meters or tens of meters
wide. This zone is, in turn, surrounded by a zone up to
hundreds of meters wide containing both ankerite and
calcite. Out from the calcite-ankerite zone, disseminated
ankerite extends beyond the mineral belts and the dis-
trict. This indicates that carbonate is an original constitu-
ent of the sediments, probably diagenetic in origin. A
likely explanation for the concentric arrangement of
these carbonate zones is that the calcite component of
ankerite was dissolved and displaced out from the sider-
ite zone.

At the Lucky Friday Mine, the distribution of car-

FIG. 5

Major geologic features of the
Coeur d’Alene Mining District.

KEY
HB Upper Bett strata
Scale, km
| Prichard Formation 0 5
—— Fold 9 354-8
Scale, miles

— Fault

N

Mullan


emplaced much later. An
alternative to isotopic and
radiometric approaches
may be available through
conventional relative dating
of some of the structures.
found in the district. For
example, all parties agree
that veins postdate the
large, tight folds of the dis-
trict. But no such folds.
dated as Precambrian have
been identified in the sur-
rounding region.

On the other hand, two
younger sets of folds do ex-
ist and feature geometries
and orientations coincident
with district folds. These
folds are datable as Phan-
erozoic by virtue of their in-
volvement with Cambrian
strata northeast (Harrison
and Cressman, 1993) and
east (Harrison et al., 1974)
of the district and Mesozoic
by their development in the
higher grade metamorphic
zone adjacent to the Idaho
batholith (Harrison, 1986;
Hyndman ct al., 1988).
Contirmation that these are
the same sets of folds found
in the district would require
that the veins be Mesozoic
or younger.

Relative dating may be
able to more precisely iden-
tify the timing of vein formation. Two sets of folds occur
in the border zone of the Idaho batholith with geom-
ctries (Harrison, 1986) that make them likely to be the
folds that occur in the district. These folds evidently
formed while metamorphism related to formation of the
batholith was taking place but were intruded by the
batholith (Hietenan, 1984). ‘The veins of the district are
locally cut by thrust faults that logically correspond to
Laramide thrusts (Harrison et al., 1980). These appar-
ently developed most extensively in the early Tertiary
(White, 1978). Thus, veins formed after extensive meta-
morphism around the batholith had begun, but before
completion of the overthrust belt. This would bracket
the age of the veins as about equal to the time of the in-
trusion of the batholith, that is, late Cretaceous.

This scenario suggests that the batholith played a
role in generation of the dynamic metamorphism that
characterizes Coeur d’Alene veins and mineral belts.
Criss and Fleck (1990) support this interpretation. They
showed that oxygen isotopes found in Belt argillite are
found in zones in the northern part of the Idaho
batholith in an area extending all the way into the Coeur
@Alene district. They suggest that heat from the
batholith influenced fluid flow in the Coeur d’ Alene dis-
trict and contributed to formation of the veins.

ig Co.

The Longyear 44 is commonly used for surface explo-
n diamond drill holes. Photo: Lisa Hardy, Sunshine

Conclusions

As old ideas about the
Coeur d’Alene Mining Dis-
trict are discarded, modi-
fied or given new emphasis,
it is becoming evident that
as more is learned about
the district, the less unique
and exotic its deposits are
from other deposits around
the world, Advances in un-
derstanding Coeur d’Alene
ore deposits have aided the
discovery of new reserves.
But these advances make it
clear that Coeur d’Alene
ore bodies are more elusive
than had been first appreci-
ated. Many past discover-
ies resulted more from
perserverance and good
fortune than from compre-
hension of the geology. As
a result, it is certain that
many good ore bodies re-
main undiscovered.

As is true of ore depos-
its everywhere, the ore bod-
ies of the Coeur d’Alene
Mining District resulted
from locally unusual cir-
cumstances. In this case,
those circumstances were
extraordinary. However,
the basic processes that op-
erated during the forma-
tion of the ore bodies and
the general evolution of the
district are common to mining districts everywhere. As
a result, the Coeur d’Alene district is an outstanding
laboratory for discovering new things about ore bodies
and also about the tectonics and geologic history of the
region. However, much remains to be learned. The ideas
discussed in this article are also subject to modification
and/or replacement.

Acknowledgments

The geologic studies discussed here are part of ongo-
ing work by personnel of the Spokane Research Labora-
tory of the National Institute for Occupational Safety
and Health (NIOSH). These studies would not have
been possible without the continuing cooperation of
many mines in the Coeur d’Alene Mining District, in-
cluding those owned or operated by Bunker Hill, Day
Mines, Hecla, Sunshine, Coeur, Asarco and Silver Valley
Resources. The goal of this work is to better understand
the role of various geologic factors in the generation of
rock bursts and other ground-control hazards in mining.
Through these efforts, NIOSH is continuing the commi
ment of the former US Bureau of Mines to improving
the health and safety of the nation’s miners.

References
Anderson, R.A., 1970, “Structural control of the ore deposits in
the Coeur d'Alene District, Idaho,” Ph.D. dissertation, University of


may not be the case.

Recent papers (White, 1989, in
press; Bond et al., 1992; Waura et al.,
1994) show that movements during
mineralization were dip-slip or nearly
so. Evidence supporting this interpre-
tation arises primarily from identifi-
cation of a weak but pervasive
shearing lineation found on weakly
developed metamorphic foliation
paralleling the veins. Certain vein-
related textures and structural geom-
etries are all parallel to the shearing
lineation found in adjacent wall rocks.
These include ore shoots, lineated
mineral grains, ridges and grooves on
the walls of veins, and intersection
geometries of veins. Hobbs et al.
(1965) described many of these fea-
tures in detail. But they interpreted
the lineation as the result of intersect-
ing cleavage planes resulting from
transcurrent movements, rather than
as the result of dip-slip shearing.

Some indirect data also support
the interpretation that tectonism dur-
ing vein formation involved develop-
ment of the metamorphic foliation
and its associated shearing lineation.
For example, Landis et al. (1984) and
Leach et al. (1988) noted that fluid
inclusions in vein minerals contain
certain light hydrocarbons that are
best interpreted as products of meta-
morphism. In most parts of the dis-
trict, the shear-lineated foliation is
the only obvious evidence of meta-
morphism. This metamorphism sug-
gests that the lineated foliation repre-
sents the metamorphism responsible
for the metamorphic fluids found in
fluid inclusions.

The lineated metamorphic folia-
tion associated with the veins is not
always conspicuous. However, the fo-
liation is easiest to find in the known
mineral belts and is less conspicuous
or nonexistent outside the mineral
belts. This distribution suggests that
the mineral belts represent very low-
grade metamorphic shear zones and
provides an explanation for clustering
of the veins within these linear trends.

Recognition that ore shoots par-
allel the steep shearing direction was
partially responsible for Hecla
Mining’s discovery of major new ore
reserves at the old Gold Hunter Mine.
Prior exploration beneath the ex-
hausted deposits high in the mine had

always been done vertically beneath the old slopes to a
depth of 1,230 m (4,050 ft). Recognizing that the shear
lineation at the Gold Hunter actually rakes 75° west,
Hecla drilled exploratory holes west of the down-dip
projection of the old ore bodies on the 1,230-m (4,050-ft)

thickness of Belt
strata in Coeur d'Alene
Mining District (Modified
from Hobbs et al., 1965).

Formation Thickness, m

level of the Lucky Friday Mine. This
resulted in the discovery of a major
new ore body that approximates the
width, length and grade of the famous
Lucky Friday vein.

The Gold Hunter deposit pro-
vides good examples of textures and
fabrics common to shear-zone-hosted
gold deposits (Sibson and Poulsen,
1988). These include the metamor-
phic foliation, steeply raking shear
lineations and veins and veinlets that
either parallel foliation or are per-
pendicular to foliation and the linea-
tion (White, unpublished data). The
nearly horizontal veinlets at the Gold
Hunter Mine are of the crack-seal va-
riety in which quartz fibers parallel
the shearing lineation just as found in
the shear-zone-associated gold de-
posits. At other mines in the district,
these flat veinlets are more com-
monly represented by “ladder quartz”
veins that crosscut primary veins.

Stratigraphic control of ore.
Certain strata have been known to be
more favorable for ore than others.
For the most part, ore is associated
with sandier strata, now metamor-
phosed to quartzite. About 75% of
the total district ore production has
come from the Revett Formation
(Mitchell and Bennett, 1983). This
formation is particularly character-
ized by the presence of thick-bedded
quartzite. An additional 19% of pro-
duction has come from quartzitic
strata that span the Burke-Prichard
contact. All current production is as-
sociated with the Revett or the imme-
diately overlying St. Regis Formation.

Beginning with work at the Bun-
ker Hill Mine during the 1970s
(White and Winston, 1982), it has be-
come evident that ore bodies are par-
ticularly associated with limited
thicknesses of specific quartzitic
strata. At the Bunker Hill Mine, for
example, many ore bodies confined to
about 30-m- (100-ft-) thick quartzites
pinch out where the vein structure in-
tersects thick, argillitic strata. At
mines such as the Sunshine Mine,
most of the favorable strata are sim-
ply siltier than the unfavorable argil-
litic strata, where the ore bodies ab-
ruptly end (White, unpublished data).

During the past several years,
identification and mapping of spe-

cific, productive strata have led to the discovery of sig-
nificant new ore reserves, particularly in Silver Belt
mines. Once these favorable strata have been identified
and mapped at individual mines, a productive means of
exploration has been to locate and then explore favor-


A Cross section of the Lucky Friday vein show-
ing the distribution of disseminated carbonates
and galena in wall rocks. A narrow ankerite-
siderite zone at the boundary between the
erite and calcite-anki is not distinguished.

Large dot represents disseminated galena.

bonates is controlled by lithology and by the orientation
of bedding with respect to the Lucky Friday ore body
(Fig. 6). The siderite zone extends farther out in quartz-
itic strata than in argillitic strata and also extends farther
into strata that dip into the vein than in strata that dip
away from the vein. This establishes that ore fluids
flowed up and out from the ore body, rather than into the
ore body from the wall rocks, as some have speculated.

The carbonate zones around the Coeur d’Alene ore
bodies are similar to those that surround shear-zone-
hosted gold deposits. This is another major similarity be-
tween Coeur d’Alene deposits and another common
type of deposit.

Sulfide zoning. Disseminated galena, sphalerite,
pyrite and arsenopyrite are found around many of the
ore bodies in the district. Some have regarded these dis-
seminated deposits as likely source rocks for Coeur
d’Alene veins. At the Lucky Friday Mine, for example,
geologists have speculated that some of the disseminated
galena, known locally as “blue rock,” was physically iso-
lated from the ore body, as if it had been deposited inde-
pendently of and earlier than the vein.

Since the early 1980s, the distribution of blue rock at

the Lucky Friday Mine has become well established as a
result of diamond drilling and additional development
(White, unpublished data). Disseminated galena extends
out from the vein along selected bedding units for tens to.
hundreds of meters on the side of the vein where strata
dip into the vein. But it extends only 1 to 3 m (3.3 to 10
ft) on the side of the vein where strata dip away from the
vein (Fig. 6). This pattern mimics that of the carbonate
zones. It suggests that these sulfides came from the vein,
traveling up and out along available, permeable bedding
planes, not the reverse.

At other mines, disseminated sulfides are found only
near veins containing the same metal sulfides. Explora-
tion throughout the district has rarely discovered sul-
fides that cannot be explained by proximity to a nearby
vein that contains the same metal. The relationship is
consistent and frequent enough to provide evidence that
all of these wall rock sulfides came out of the veins and
represent alteration.

Vein zoning. Although Ransome and Calkins
(1908) and Mitcham (1952) thought they saw metal zon-
ing in some individual ore bodies, Fryklund (1960) recog-
nized only one vein that appeared to be zoned. How-
ever, continued consideration of metal ratios in many
Cocur d’Alene veins suggests that upward zonation of
sulfide minerals does exist.

Mitcham (1952) presented evidence that the Sun-
shine ore body contains increasing amounts of lead rela-
tive to silver higher in the vein. Since then, similar
patterns have been identified in other Silver Belt ore
bodies, including in the Sunshine and the Galena mines.
Thus, upward grading of tetrahedrite into galena in Sil-
ver Belt mines now seems to be an established pattern.

Outside of the Silver Belt, other consistent patterns
of metal zoning in veins have also been established. Pro-
duction statistics from the Lucky Friday vein, mined over
a vertical range of 1,500 m (4,920 ft), indicate that silver
and lead decrease slightly with depth, while the zinc con-
tent increases (Devoe, 1995, personal communication).
This pattern duplicates the zoning that Fryklund (1964)
recognized in the Tony ore shoot of the Page Mine.

Crosby (unpublished Hecla Mining data) compiled
production statistics for the Star-Morning and Tamarack
mines, nonSilver Belt mines that show similar trends.
Crosby (1989, personal communication) noted that sil-
ver-to-zine and lead-to-zinc ratios varied widely from
level to level in these mines. He speculated that geolo-
gists who worked at these large mines for only a few
years may not have become aware of systematic changes
in metal ratios with depth.

Tn smaller veins, systematic zoning may not have
been evident at all. However, over the 2,600-m (8,530-ft)
vertical range of the Star-Morning vein, the trend is un-
mistakable and shows increased zinc and decreasing lead
and silver with depth. In this same ore body, Hutchinson
(1984, personal communication) noted that pyrite was
more abundant with depth along the vein’s west end.
Some mine geologists consider that the veins of the Sil-
ver Belt also become pyritic or chalcopyrite-rich with
depth,

These observations establish that vein zoning is
present and widespread in the Coeur d’Alene district,
although not always obvious. The zoning patterns mostly
reflect the familiar sequences of sulfide minerals seen in


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Bond, W.D., et al., 1992, “Nature and genesis of ore shoots at the
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he Coeur d’ Alene Mining District of northern Idaho is remarkable for
its number of large ore bodies and for the amount of its historical pro-
duction of lead, zinc and silver (Fig. 1).

Since veins were first discovered in 1884, more than 127 Mt (140 million
st) of ore have been produced from narrow, high-grade veins. Mining contin-
ues at the Lucky Friday, Galena and Sunshine mines (Fig. 2). All have been
in production for decades. At these mines, more efficient mining and explo-
ration techniques have reduced costs while adding significant new reserves,
Even the old Bunker Hill Mine, where ore was first found in 1875, continues
operation on a limited basis, partly involving production of world-class pyro-
morphite specimens. Until recently, the Coeur Mine was also a profitable sil-
ver producer. Other large producers have included the Star-Morning, Hecla,
Page, Standard-Mammoth, Hercules and Gold Hunter mines (Fig. 2).

The first deposits were discovered by prospectors who spilled over from
the Murray gold mining district, 24 km (15 miles) to the north. The Bunker
Hill Mine was founded the year after the first veins were discovered. Bunker
Hill has now operated for more than 100 years. Production includes 2.3 Mt
(2.6 million st) of lead, 1.1 Mt (1.2 million st) of zine and 4.1 kt (132 million
oz) of silver.

Total district production through 1996 (Springer, 1997) amounted to 7.3
Mt (8.1 million st) of lead, 3 Mt (3.3 million st) of zinc, and 34.5 kt (1.109 bil-
lion oz) of silver. This record establishes the district as the largest recorded
producer of silver in the world, while also yielding major quantities of lead
and zinc. Only Potosi, Bolivia, may have produced more silver, although
records are unclear.

Despite the district’s longevity, historic production and current activity,
the published literature on the district lags behind with regard to recent con-
cepts of district geology. The most recent comprehensive US Geological Sur-
vey (USGS) bulletins (Hobbs et al., 1965; Fryklund, 1964) are dated. Much
research in the last half century has been short-lived or geographically re-
stricted because the district is large and its geology is complex — and even
this research tends to remain unpublished. Thus, it has been difficult for in-

hole being Brian 6, White is @
the Silver Belt geologist with the
Sugg ents re Wational Institute for
tation for new Occupational Safety and
a 4

shallow depths. Health, Spokane Re-
The drill is 0 search Laboratory, East
1000, a model 315 Montgomery Ave,
that is used Spokane, ‘WA 99207-
extensively in

district mines. 23.

Photo by Lars
Edling, Hagby,
USA.


ore bodies around the world. Thus, although the quan-
tity of metal in Coeur d’Alene veins is unusual, the pro-
cesses that formed the ore bodies appear to be no diff-
erent from the common ones that have formed ore bod-
ies elsewhere.

Metamorphism and genesis

Earlier writers speculated that Coeur d’Alene met-
als came from intrusive magmas, from the immediate
wall rock or deeper strata, or from a subcrustal source.
But an origin involving metamorphism of sediments now
seems likely.

Some papers describing Belt strata have commented
that the metamorphic grade of these rocks is low, but
that it increases toward the Idaho batholith. However, it
is actually only in the immediate border zone of the
batholith (Fig. 3) that metamorphism increases notably.
Elsewhere, burial metamorphism before development of
the batholith reached its highest grade in the deeper,
more central part of the Belt sedimentary basin north-
east of the district in Montana. As a result of these sepa-
rate areas of elevated metamorphism, the district sits in
a regional metamorphic low, where most strata had not
reached greenschist facies metamorphism before the ore
deposits formed.

Many authors describe wall rocks of the district as
being greenschist or upper greenschist in metamorphic
grade. This view, however, is not supported by field ob-
servations. Except locally, such as at the Atlas Mine, no
greenschist minerals associated with mineralization are
present.

Gitlin (1986) determined that diagenetic sericite in
Revett strata outside of mineral belts contains appre-
ciable iron and magnesium. This suggests that burial
metamorphism had not brought these rocks to green-
schist grade before formation of the Coeur d’Alene ore
bodies.

In contrast, at the Lucky Friday Mine, Gitlin found
that mica from equivalent strata contains significantly
less iron and magnesium and thus approaches a pure
muscovite. Her work suggests that mineralization was
superimposed onto subgreenschist-facies strata and that
conditions associated with mineralization approached
those of greenschist metamorphism.

The low-to-nonexistent burial metamorphism in the
district before the veins formed probably had conse-
quences that contributed to the formation of the ore
bodies. First, these sediments were probably not so ex-
tensively dewatered as higher-grade
rocks would have been. So even the
low-grade metamorphism associated
with the formation of the veins could
have exploited this water as ore fluids.
Second, the lack of burial metamor-
phism in the district left the pyrite in
the thick, black-shale Prichard For-
mation instead of the pyrrhotite that
characterizes this formation in the re-
gion surrounding the Coeur d’Alene
district.

Metamorphism of pyrite to pyr-
rhotite at depth within the metamor-
phic shear zones would have involved
the liberation of sulfur. This libera-
tion could have mobilized metals in

Sunshine Mine

diff

entiate
hydrothermally
bleached, green

strata by the
presence of py-
rite at hydrother-
mal redox

the form of bisulfide complexes that, in turn, could have
solubilized the anomalous metals that are found locally
in the lower Prichard Formation. The metals could then
have been transported up in the shear zones in the meta-
morphic waters. This hypothesis has not been widely dis-
cussed and its feasibility has not been assessed. How-
ever, the presence of the thick, pyritic, locally metal-
anomalous Prichard Formation at depth is one of the
more unique features of the district and could help ex-
plain the unusual wealth of its ore deposits.

Age

Interpretation of the age of the Coeur d’Alene ore
bodies has varied widely. Ransome and Calkins (1908)
thought the veins were related to an underlying batho-
lith that also formed the Mesozoic Gem stocks. Fryklund
(1964) and Hobbs et al. (1965) were once convinced that
the veins postdated the stocks. But Hobbs (Hobbs and
Fryklund, 1968) reconsidered this view in the face of per-
suasive arguments by USGS coworkers Gott and
Zartman (whose ideas involved the Precambrian isoto-
pic composition of Coeur d’Alene lead and new evi-
dence that the stocks postdated the veins). Conse-
quently, Hobbs became convinced (Hobbs and Fryklund,
1968) that a Precambrian date for origin of the veins was
more logical than a much younger date.

Uranium-lead dating of uraninite veins that postdate
folding but predate Silver Belt tetrahedrite ore bodies at
the Sunshine Mine (Eckelmann and Kulp, 1957) once
seemed to indicate that the uraninite veins and the fold-
ing that predates them are Precambrian. More recently
(Zortman and Smith, 1995), new and more reliable U-Pb
dates indicate that these veins are 136 ga or Mesozoic.

Fleck et al. (1991) analyzed strontium isotope com-
positions from siderite veins. They concluded that stron-
tium in the veins is too radiogenic to have possibly
formed in the Precambrian and must be no older than
Mesozoic. Leach (1998) reported that siderite and Silver
Belt tetrahedrite contain a trace amount radiogenic Pb.
This is in sharp contrast to the nonradiogenic, Precam-
brian Pb contained in Coeur d’Alene galena. Thus, evi-
dence is accumulating that Silver Belt tetrahedrite veins,
at least, formed in the Mesozoic. However, based on the
nonradiogenic character of galena Pb and on Ar-Ar
dates from sericite found in some veins in the district,
Leach and others (1998) propose that, while the galena
and sphalerite components of Coeur d’Alene veins were
emplaced in the Precambrian. the tetrahedrite was

pees



FIG. 1

Coeur d'Alene Mining District
in North America.

Mining [
reer

terested parties to get a good overall picture of the geol-
ogy and geologic history of the district.

The Coeur d’Alene district has a reputation for be-
ing exotic, one of a kind and so complex as to be beyond
reasonable comprehension. However, much has been
learned in the past 30 to 40 years since the last major
USGS reports were published.

Geologic framework of the Coeur d’Alene district
The Coeur d’Alene Mining District is one of three

world-class mining districts in this part of North

America. Others are Butte, MT, and the Sullivan mas-

FIG. 2

Mines in the Coeur d'Alene Mining District.

LEGEND
@ Page Mine Standard-Mammoth Mine
@® Bunker Hit Mine) Hecta wine
@ suretine Mine @ Star Moming Mine
@ coeur Mine
@ Galena mine @) Lucky Friday Mine
© Hercules Mine @@) Atlas Mine
@® Tamarack Mine) Mine mentioned in text
X Other mine, inactive

@ Gols Hunter Mine S 4

sive-sulfide deposit in British Columbia. All three dis-
tricts lie within a region underlain by Proterozoic sedi-
mentary strata of the Belt Supergroup (Fig. 3). Belt
strata are characterized by thick, uniform sequences of
slightly metamorphosed, predominantly fine-grained
sediments.

The Coeur d’Alene district lies in the Lewis and
Clark line (Fig. 3). This is a major tectonic and topo-
graphic lineament defined by subparallel normal faults,
reverse faults and strike-slip faults. Rapid facies and
thickness changes suggest that the line was active during
Belt sedimentation (Harrison, 1972; Winston, 1986). Suc-
cessive development of the other tectonic structures in-
dicates that the line was recurrently active in response to
various stresses (White, in press). Thus, the line appears
to represent a deep-seated basement structure that
formed in Precambrian time. Reverse faults that appear
to be part of the vein-forming tectonic event (White, in
press) partially define the line. This suggests that the
Coeur d’Alene district exists partly because of the Lewis
and Clark line.

The district is located north of the Idaho batholith
(Fig. 3). The northern lobe of the batholith and its asso-
ciated regional metamorphism are aligned parallel to the
Lewis and Clark line, as well as to many structures in the
Coeur d’Alene district. This alignment may be coinci-
dental but could also have bearing on the genesis of the
district.

Belt strata have been folded on a large scale and
abundantly faulted. Belt terrane is also found in the
Laramide overthrust belt (Harrison et al., 1980). Some
of these thrust faults have been mapped as extending
into the district from the north (Harrison, 1986).

The lower half of the Belt strata exposed in the dis-
trict (more than 4,000 m or 13,000 ft) is characterized by
pyritic black argillite of the Pritchard Formation and re-
flects deep-water sedimentation. Younger formations
reflect shallow-water sedimentation and are slightly
coarser grained (Fig. 4).

Strata equivalent to the deeper, unseen Prichard

Formation are host rocks for the

Sullivan massive sulfide deposit.

Where they are exposed in western

Montana, they are also known to lo-

cally contain anomalous lead and

zinc. The younger, generally siltier

and sandier strata above the Prichard

Formation include the Burke, Revett,

8 St. Regis and Wallace formations.

These sedimentary rocks have been

intruded by several stocks, primarily
the Gem stocks.

Strata in most mines have been
tightly folded on a large scale, creat-
ing steep bedding dips. These folds
are, in turn, cut by strike-slip, normal,
reverse and thrust faults of various
orientations, probably representing
separate tectonic episodes (White, in
press). The Osburn Fault is a major
tight-lateral, strike- slip fault. It has
separated two parts of the district by
about 25 km (16 miles).

Lindgren (1933) classified veins
of the Coeur d’Alene district as

2


mesothermal ore deposits. The veins consist of highly
variable proportions of sphalerite, galena and argentifer-
ous tetrahedrite in a gangue dominated by either quartz
or siderite.

The ore bodies are remarkable in their number and,
in many cases, in their large physical dimensions and ton-
nages. Strike lengths frequently exceed 300 m (985 ft),
dip lengths 1,000 m (3,300 ft) and thicknesses several
meters. The larger ore bodies have produced from 3.6 to
23 Mt (4 to 25 million st) of ore. Ore grades commonly
tun in the range of 685 to 857 g/t (20 to 25 oz/st) silver,
7% to 14% lead and 4% to 8% zinc. Average grades of
mined ore for all mines (Mitchell and Bennett, 1983) are
240 g/t (7 oz silver), 5.7% lead and 2.3% zinc. Average
grades, however, are not representative because many
veins are composed primarily of any one of these metals.

The ore bodies lie in west-trending linear clusters
known as mineral belts (Fig. 2). Most veins strike west-
erly, parallel to their mineral belts. Others strike north-
easterly and some change from westerly to northeasterly
in plan, forming S-shapes.

Most silver production has come from a mineral belt
south of the Osburn Fault, the eastern part of which is
known as the Silver Belt (Fig, 2). Other mineral belts
have primarily contributed lead or zinc, with lesser pro-
duction of silver, Disseminated copper-silver deposits at
the east ends of some of the mineral belts in the north-
east part of the district have occasionally been identified
as part of a separate, northwest-trending mineral belt
called the Copper Belt. This zone is defined by the up-
turned edge of Revett strata that contains stratiform
copper-silver deposits. This is the southwesternmost oc-
currence of this type of mineralization and is unrelated
to Coeur d’Alene-type veins.

Current ideas of district geology

Nearly all aspects of the geology of the Coeur
d'Alene district have been recxamined since the works
of Fryklund (1964) and Hobbs et al. (1965). In some
cases, well-known and once highly regarded ideas have
been significantly modified or replaced. A few old ideas
have received new emphasis. In the following sections,
some of the more significant and better-known ideas
about the district are reviewed and updated.

District tectonics. The interpretation of the basic
tectonics of the Coeur d’Alene district by Hobbs et al.
(1965) derived from the obvious patterns formed on geo-
logic maps by tight, large-scale folds. They noted that
northerly trending folds are prominent in the northern
part of the district, while only westerly trending folds are
obvious farther south (Fig. 5). They proposed that these
were a single set of folds that had been bent to their
present orientations by right-lateral transcurrent move-
ments in the Lewis and Clark line, They inferred that the
veins and various faults ultimately formed in response to
these movements and that the folds were finally trun-
cated and offset by the Osburn Fault. Following their
work, several graduate students also interpreted local
structures in district mines as being a reflection of trans-
current movements.

The author has mapped key areas where bending of
the supposed original fold set had been thought to be
most evident (shown in Fig. 5 as a heavy dashed line in
the northeastern part of district), He found no evidence

FIG. 3

Regional geology surroundi:

d'Alene Mining District (Modified after
Harrison, 1972; Hyndman et al., 1988; and
Griggs, 1973).

— Limit of Belt
outero|

Idaho batholith
terrane

to support this interpretation. The evidence suggested
that these are actually two separate fold sets (White,
1989, in press). This interpretation has also been ex-
pressed by Anderson (1970) and Juras (1982).

In the southern part of the district, the north-trend-
ing folds are evidenced by saddles and domes formed in
the westerly trending folds. Northward, these westerly
trending folds abruptly die out at the northern edge of
the fold belt that they define (Fig. 5).

The westerly trending folds become tighter in the
Lewis and Clark line, as if compressed against preexist-
ing structures within the line. This is another indicator
that the line is an old feature. It is reasonable to inter-
pret both fold sets as representing regional fold sets that
project toward the district from the north (Harrison,
1986), south (Wagner, 1949) and east (Harrison et al.,
1974).

White (in press) also argues that various faults in the
district represent separate episodes of reverse and nor-
mal faulting followed by strike-slip faulting. Thus, struc-
tures in the district are not mainly the result of trans-
current movements as Hobbs et al. (1965) believed. In-
stead, these structures reflect diverse kinds of move-
ments, again demonstrating a complex tectonic history in
the Lewis and Clark line.

Tectonics related to mineralization. The inter-
pretation of the tectonic history of the district by Hobbs
et al. led to a logical inference that the veins were also
the product of transcurrent movements. However, a re-
interpretation of the gross structure suggests that this