Geology of an Ordovician Stratiform Base-Metal 

Deposit in the Long Canyon Area, Blaine County, 

Idaho 

By Bruce R. Otto and Gerald A. Zieg


U.S. Geological Survey Bulletin 2064-KK


Prepared in cooperation with the Idaho Geological Survey, 

Idaho State University, and the University of Idaho 

U.S. Department of the Interior 

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

Gale A. Norton, Secretary 

U.S. Geological Survey 

Charles G. Groat, Director 

Version 1.0 2003 

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


Published in the Central Region, Denver, Colorado


Manuscript approved for publication November 26, 2003


Graphics by Dave Walters and Norma J. Maes 


Photocomposition by Norma J. Maes

Contents 

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


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


Geology ................................................................................................................................................. 1


Regional Geology .......................................................................................................................... 1


Long Canyon Area Geology ......................................................................................................... 4


Paleozoic Stratigraphy ......................................................................................................... 4


Footwall Stratigraphy ........................................................................................... 4


Mineralized Sedimentary Rocks ......................................................................... 6


Hanging-Wall Stratigraphy .................................................................................. 7


Age Correlation ..................................................................................................... 8


Tertiary Rocks ....................................................................................................................... 8


Structure ................................................................................................................................. 8


Folds......................................................................................................................... 8


Faults ....................................................................................................................... 9


Discussion ............................................................................................................................................. 9


Acknowledgments........................................................................................................................ 10


References. ........................................................................................................................................... 10


Figures 

1. Index map showing location of the Long Canyon area, Blaine County, Idaho ............. 2


2. 	 Map showing structural geology of the Fish Creek Reservoir window, 


Blaine County, Idaho............................................................................................................... 3


3. Geologic map of the Long Canyon area, Blaine County, Idaho....................................... 5


4. Composite stratigraphic column, Long Canyon area, Blaine County, Idaho................. 6


5. 	 Geologic cross-sections A-A′, B-B ′, and C-C ′, Long Canyon area, 


Blaine County, Idaho............................................................................................................... 7


METRIC CONVERSION FACTORS 

Multiply By o obtain 

Miles 1.609 

Feet 0.3048 

Inches 2.54 

Tons 1.016 

Short tons 0.907 

Troy ounces 31.103 

Ounces 28.35 

T

Kilometers 

Meters 

Centimeters 

Metric tons 

Metric tons 

Grams 

Grams 

III

Geology of an Ordovician Stratiform Base-Metal 

Deposit in the Long Canyon Area, Blaine County, 

Idaho 

By Bruce R. Otto and Gerald A. Zieg 

Abstract 

In the Long Canyon area, Blaine County, Idaho, a stratiform base-metal-bearing gossan is exposed within a complexly 

folded and faulted sequence of Ordovician strata. The gossan 

horizon in graptolitic mudrock suggests preservation of bedded 

sulfides that were deposited by an Ordovician subaqueous 

hydrothermal system. Abrupt thickness changes and geochemical zoning in the metal-bearing strata suggest that the gossan is 

near the source of the hydrothermal system. 

Ordovician sedimentary rocks at Long Canyon represent a 

coarsening-upward section that was deposited below wave base 

in a submarine depositional environment. The lowest exposed 

rocks represent deposition in a starved, euxinic basin and overlying strata represent a prograding clastic wedge of terrigenous 

and calcareous detritus. The metalliferous strata are between 

these two types of strata. 

Strata at Long Canyon have been deformed by two periods 

of thrust faulting, at least three periods of normal faulting, and 

two periods of folding. Tertiary extensional faulting formed five 

subhorizontal structural plates. These low-angle fault-bounded 

plates truncate Sevier-age and possibly Antler-age thrust faults. 

The presence of gossan-bearing strata in the four upper plates 

suggests that there was only minor, although locally complex, 

stratigraphic displacement and rotation. The lack of correlative 

strata in the lowest plate suggests the displacement was greater 

than 2000 ft. 

The metalliferous strata were exposed to surface weathering, oxidation, and erosion prior to and during deposition of the 

Eocene Challis Volcanic Group. The orientations of erosional 

canyons formed during this early period of exposure were 

related to the orientations of Sevier-age thrust faults, and 

stream-channel gravel was deposited in the canyons. During this 

and subsequent intervals of exposure, sulfidic strata were oxidized to a minimum depth of 700 ft. 

Introduction 

The Long Canyon area is along the northern margin of the 

Snake River Plain, approximately 8 mi northeast of the community of Carey, in secs. 20 and 29, T. 1 N., R. 22 E., Blaine 

County, Idaho (fig. 1). Closest road access to the property is via 

Road Canyon and a single track dirt road into Long Canyon. 

The Long Canyon area is not in a formally recognized mining district and no recorded production has occurred in the area. 

Three short adits and a few shallow prospect pits are the only 

visible records of historic exploration activity. These prospects 

explored sulfidic bull quartz veins. Several impressive stratabound gossans exposed in the area show no evidence of past 

prospecting. 

The present study resulted from exploration efforts by 

Cominco American Resources Incorporated and included geologic mapping, soil and rock-chip geochemistry, geophysics, 

and drilling of three core holes totalling 1,664 ft. 

Geology 

Regional Geology 

Strata exposed near Long Canyon occupy a structurally 

compressed part of a lower and middle Paleozoic sequence of 

hemipelagic, siliciclastic, and calcareous sedimentary rocks 

(Skipp and Sandberg, 1975) (fig. 2). Strata that can be correlated 

stratigraphically are exposed in places from the northern margin 

of the Snake River Plain on the south to the Clayton, Idaho area 

on the north (Umpleby and others, 1930; Anderson and others, 

1950; Sandberg and others, 1975; Skipp and Hall, 1975; Skipp 

and Sandberg, 1975; Carter and Churkin, 1977; Batchelder and 

Hall, 1978; Carstensen, 1983; Hall, 1985). Hall (1985) named 

the part of this belt from Bellevue area north to the Salmon 

River area the central Idaho black-shale mineral belt. Rocks 

exposed in the Long Canyon area have been dated paleontologically as Middle to Late Ordovician (Claire Carter, oral commun.,1985). This indicates a partial stratigraphic correlation 

with lower Paleozoic strata described by Dover and others 

(1980) in the Trail Creek area 30 mi to the northwest. The location of the Long Canyon area near the northern margin of the 

Snake River Plain southeast of Bellevue logically makes it part 

of the central Idaho black-shale mineral belt described by Hall 

(1985). 

Other Paleozoic sedimentary rocks in the Fish Creek Reservoir area include flysch deposits of the Mississippian Copper 

Basin Formation, which is a thick sequence of carbonate and 

siliciclastic rocks (Nilsen, 1977a, b), and the Middle Pennsylvanian-Lower Permian Wood River Formation (Skipp and Hall, 

1975; Skipp and Sandberg, 1975). The lower Paleozoic strata 

are in structural contact with middle and upper Paleozoic rocks 

and are exposed in an erosional window which Skipp and Sandberg (1975) refer to as the Fish Creek Reservoir Window. Volcanic and volcaniclastic rocks of the Challis and Snake River Plain 

are in both depositional and structural contact with Paleozoic 

strata. 

1
Figure 1. Index map showing location of the Long Canyon area, Blaine County, Idaho. 

2 Ordovician Stratiform Base-Metal Deposit, Long Canyon, Idaho 
Figure 2. Map showing structural geology of the Fish Creek Reservoir window, Blaine County, Idaho. 

Geology 3 
In the Trail Creek area and the adjacent Warm Springs and 

Mineral mining districts, a thick basinal siliciclastic and clastic 

carbonate section represents the time interval from Ordovician 

through early Late Devonian (Carter and Churkin, 1977; Dover 

and others, 1980; Sandberg and others, 1975; Otto and Turner, 

1987; Turner and Otto, 1988). These strata, which are lithologically similar to and are time correlative with the strata in the 

Long Canyon section, have a significant history of exploration 

for and production of base- and precious-metals. At the Triumph 

mine, the largest past producer in the Warm Springs district, bedded sulfide in argillite, calcareous argillite, limestone, and 

quartzite of the Lower to Upper Devonian Milligen Formation 

produced almost two million tons of ore at a grade of 11 percent 

combined zinc and lead, 0.08 ounces per ton of gold, and 7.5 

ounces per ton of silver (Turner and Otto, 1995). 

Other significant past producers and prospects in the Warm 

Springs and Mineral mining districts include the Mascot, Minnie 

Moore, and Homestake mines, all hosted in Devonian strata, and 

the Phi Kappa mine, hosted in Ordovician strata (Umpleby and 

others, 1930). Many of these mines and prospects are stratabound zinc-lead-silver deposits hosted by basinal strata that 

are lithologically similar to those exposed at Long Canyon. 

Long Canyon Area Geology 

Rocks of the Long Canyon area (fig. 3) consist of a structurally complex sequence of lower Paleozoic argillite, limestone, 

and sandstone and Tertiary and Quaternary volcanic rocks. 

Skipp and Hall (1975) and Link and others (1988) showed the 

upper Paleozoic Wood River Formation in structural contact 

with the lower Paleozoic sequence (fig. 2). The lower Paleozoic 

rocks, originally deposited in a continental slope environment of 

the early Paleozoic North American craton, were folded in Late 

Devonian to Early Mississippian time during Antler deformation 

(Turner and Otto, 1988). The rocks were folded and thrust 

faulted during the later Sevier tectonic event (Skipp and Hall, 

1975; Skipp and Hait, 1977; Dover, 1981) and were normal 

faulted during the Tertiary. The combination of rapid facies 

changes, intersecting fold systems, complex fault geometries, 

and poor exposures has hindered measurement of an accurate 

stratigraphic section; however, figure 4 shows a measured section drawn on the basis of available field data. 

Paleozoic Stratigraphy 

Rapid facies changes recorded in the Paleozoic section 

exposed at Long Canyon make it difficult to correlate the tightly 

folded rock units across low- and high-angle faults. The few recognized stratigraphic directional indicators suggest that the 

rocks generally are not overturned but units have been repeated 

by folds and thrust faults. The preliminary stratigraphic section 

shown on figure 4 is based on the presence of similar rock 

sequences in each of the stratigraphically discordant structural 

plates (fig. 3). We interprete the repetitions in the section to be 

bedding-subparallel thrust faults. The low-angle normal faults 

shown on the geologic map were mapped on the basis of brittle 

gouge, slickenslides, and rock unit juxtaposition (fig. 3). 

The base of the stratigraphic section is dolomitic siltstone 

and sandstone (unit Os), which is overlain depositionally by 

cherty argillite and chert (unit Oca), which in turn grades vertically to argillite, siltite, and channelized quartzite (units Osh and 

Oq), and then to limestone and calcareous diamictite (unit Ol). 

The stratabound metal system at Long Canyon is at the transition 

from argillite and quartzite (units Osh and Oq) to the calcareous 

rocks (units Ol and Os) (figs. 3, 5). 

Footwall Stratigraphy 

The exposed base of the lower Paleozoic section consists of 

a sequence (unit Os) of buff-colored dolomitic and calcareous 

sandstone, siltstone, and silty dolomite that weather to rusty colors (figs. 3, 4). These strata are characterized by relatively abundant bedding structures, including convolute beds, ripples, and 

planar laminations. We interpret these beds what Bouma (1962) 

called turbidites (Tbcde). 

Black chert and cherty argillite (unit Oca) are in sharp, conformable contact with the underlying unit (Os). Unit Oca is 

exposed in the bottom of Long Canyon in the lowest structural 

plate as well as isolated thrust fault segments in higher structural 

plates (figs. 3, 5). The argillite is marked by few bedding structures and commonly has a near-bedding-parallel cleavage. Unit 

Oca contains as much as 50 percent structureless black chert. 

A highly foliated argillite cataclasite in unit Oca (unit Ofa) 

is well exposed in the bottom of Long Canyon. In drill hole LC3, the cataclasite is intersected between 165 and 200 ft (figs. 3, 

5), indicating a minimum thickness of 35 ft. Contacts with the 

overlying and underlying cherty argillite (unit Oca) are gradational over intervals of several feet. The cataclasite exhibits a 

distinctive, intensely developed cleavage with abundant coplanar 

millimeter- to centimeter-scale tight, isoclinally folded quartz 

veins. Limbs of the isoclinal folds are parallel to the cleavage 

that defines the unit. The cleavage is parallel with a regional fabric in lower Paleozoic units of the Central Idaho black-shale belt. 

Cherty argillite (unit Oca) grades upward to graptolitic, carbonaceous mudstone and siltstone (unit Osh). The latter strata 

are thin bedded and lack bedding structures; they were originally 

very carbonaceous, as indicated by the unoxidized carbonaceous 

centers of some of the larger pieces of float and from unoxidized 

parts of drill core. Poorly preserved graptolites, were collected 

by Cominco workers from the carbonaceous mudstone. The 

samples were analyzed by Claire Carter of the U.S. Geological 

Survey, who identified three species, Climacograptus, Glyptograptus, and Orthograptus and defined the age of as the mudstone and siltstone as Middle to Late Ordovician. The gossanhosting section (unit Og) and correlative strata (units Ofc, Oc) 

are correlative with the upper parts of unit Osh (fig. 4). 

Locally abundant coarse-grained massive quartzite beds 

and lenses (unit Oq) occur in the upper part of unit Osh (figs. 3-

5). Some of the quartzite lenses are at and near the stratigraphic 

4 Ordovician Stratiform Base-Metal Deposit, Long Canyon, Idaho 
Figure 3. Geologic map of the Long Canyon area, Blaine County, Idaho. 

Geology 5 

Explanation of lithologic map units shown on figure 4. 
Figure 4. Composite stratigraphic column, Long Canyon area, Blaine 

County, Idaho. 

position of the metal-bearing strata of units Ofc and Og. This 

observation is based on the map distribution of quartzite and 

gossan lenses and the presence of centimeter-scale laminations 

of coarse-grained quartzite that are interbedded with the cherty 

ironstone of units Ofc and Og. The beds of quartzite range from 

a few centimeters to several meters in thickness. Grain size of 

the quartz sand is as large as 1 mm, but generally averages 0.5 

mm. Bedding orientation criteria and bedding structures are 

generally absent in the quartzite. 

Mineralized Sedimentary Rocks 

Metal-bearing sedimentary strata, mapped as three spatially separate but temporally equivalent units (units Ofc, Oc, 

Og), correlate laterally with the upper part of the carbonaceous 

mudstone of unit Osh. 

Thin- to thick-bedded chert (unit Oc) forms the stratigraphic footwall of the gossan (unit Og) and laterally equivalent 

parts of unit Ofc (figs. 3, 4). In general, thicker beds and a 

thicker section of chert underlie the thickest and best developed 

gossan. Thinner sections of unit Oc with thinner chert beds 

underlie ferruginous cherty ironstone of unit Ofc. The chert is 

internally textureless in hand specimen and contains very 

uneven and distinctive load structures. Thicker bedded chert has 

load structures as much as 1 ft in diameter and 6 in. thick, 

whereas the thinner bedded chert contains load structures as 

much as a few inches in diameter. Immediately below the gossan, white to flesh-colored, medium- to thick-bedded chert contains load casts on bedding surfaces. Downsection, the chert 

grades to darker colors and the abundance of load structures in 

chert beds decreases. A hydrothermal exhalative origin is suggested for unit Oc by the changes in lateral thickness, the occurrence of chert beds and load structures, the light color, and the 

spatial association with metal-enriched strata. 

Gossans (unit Og), in some places more than 50 ft thick, 

are underlain by chert of unit Oc and overlain by limestone of 

unit Ol. Oxidation of high-sulfide cherty strata likely produced 

the cherty ironstone and gossan of units Ofc and Og. At some 

gossan outcrops, millimeter-scale laminations of quartz sand, 

silt, and clay are present in massive and laminated hematite. At 

other outcrops, the processes of oxidation of sulfides, dissolution, and in situ reprecipitation of iron oxides have destroyed 

bedding structures. 

A tremolite- and magnetite-bearing carbonaceous argillite, 

weathering to silvery colors, is structurally below the gossan of 

unit Og on the west side of Long Canyon near a buried granodiorite intrusion. Its exact stratigraphic position with respect to 

the gossan is unknown because of structural complexities and 

poor exposure; at some outcrops argillite directly underlies the 

gossan strata. The presence of tremolite suggests that the protolith was calcareous argillite, unlike strata nearer the thicker 

gossan to the northwest (fig. 3). A granodiorite intrusion at the 

200 ft depth in the nearby LC-3 drill hole likely caused the 

calcsilication.

6 Ordovician Stratiform Base-Metal Deposit, Long Canyon, Idaho
Surface rock-chip sampling and D-horizon soil sampling 

were conducted in gossans of unit Og. Samples collected near 

the gossans were analyzed for Cu, Pb, Zn, Au, Ag, and Ba, and 

show anomalous concentrations of Pb and Zn. Although scattered silver and copper values were detected, they are generally 

associated with epigenetic quartz veins in and near normal faults 

and probably not associated with the metalliferous strata of 

units Ofc or Og. 

Lead concentrations in soil collected downslope from the 

gossans range from 150 to 1,000 ppm. These values decrease 

with distance from the gossan to a background level of 20-150 

ppm. Lead concentrations in rock-chip samples collected from 

gossan exposures range from 2,000 to 3,000 ppm; locally they 

are higher. The lead in rock-chip samples is primarily plumbojarosite. 

Background concentrations of zinc in the Paleozoic strata 

range from 80 to 250 ppm. In soil samples, elevated levels of 

zinc form a broader dispersion halo than do elevated levels of 

zinc which are associated with the ferruginous chert of unit Ofc 

rather than the gossans. Concentrations of zinc in soil samples 

associated with unit Ofc typically range from 250 to 1,000 ppm; 

locally higher concentrations were measured. Soils developed 

on unit Og have zinc concentrations of 250-500 ppm. The area 

of elevated zinc coincides with the ferruginous cherty strata and 

the low-angle faults that cut this part of the section indicating 

the possibility that zinc from the metal-bearing strata was remobilized along the structures. 

Cherty and carbonaceous argillite of unit Osh grades laterally and upward to ferruginous chert and argillite (unit Ofc). 

The ferruginous chert contains a clastic component, including 

siltstone and fine- to medium-grained sandstone. Although unit 

Ofc is dominantly ferruginous chert, it includes gradations from 

massive, bedded silica ironstone to ferruginous mud-rich clastic 

rock. Local erratic distribution of the ferruginous chert and 

argillite (unit Ofc) may reflect structural dismembering of a single stratum or multiple and separate depositional lenses of those 

lithologies. Unit Ofc grades laterally into unit Og and represents 

a distal facies of the oxidized sulfidic strata. 

Hanging-Wall Stratigraphy 

Massive gray limestone, silty limestone, and limy diamictite (unit Ol) overlie carbonaceous argillite (unit Osh) and gossanous strata (units Og, Ofc) (figs. 3, 4). The limestone and silty 

limestone consist of a shelf-derived accumulation of carbonate 

Figure 5. Geologic cross-sections A-A′, B-B′, and C-C′, Long Canyon area, Blaine County, Idaho. 

Geology 7 
and abundant organic debris and fossil hash. Bedding structures 

are generally lacking in these strata, probably because of overprinting by S1 cleavage. Limy mud-matrix diamictite locally 

constitutes much of the limestone section and consists of mudsupported, rounded siliciclastic and carbonate clasts in a limemud matrix. All of the clasts are lithologies similar to nearby 

strata, suggesting local derivation. Thickness of the unit is 

unknown because of structural dismemberment. 

Age Correlation 

Skipp and Hall (1975) and Skipp and Sandberg (1975) 

mapped the Long Canyon area as part of the Devonian Milligen Formation. Although similar in lithology and vertical lithological progression to the Milligen, Middle to Late Ordovician 

graptolites (unit Osh) also exhibits a partial correlation with the 

Lower Ordovician to Middle Silurian Phi Kappa Formation of 

the Trail Creek area (Claire Carter, oral commun.,1995). The 

Phi Kappa Formation, as described by Carter and Churkin 

(1977), is an “unbroken succession from late Early Ordovician 

through latest Ordovician” that is only 200 m (656 ft) thick. 

Although some unit Osh strata are clearly Ordovician in age, 

higher parts of the section may also correlate with younger 

units, such as the Trail Creek or Milligen Formations, or with 

the unnamed Silurian and Devonian unit of Dover (1980). 

Because the mineralized strata of units Og, Ofc and Oc correlate laterally to unit Osh, we believe that the gossan strata are 

likely Ordovician in age. 

Tertiary Rocks 

Andesite flow rocks, welded rhyolite lapilli ash-flow tuff, 

and epiclastic sedimentary rocks (unit Tv) exposed in the Long 

Canyon area are part of a regionally extensive sequence of the 

Challis Volcanic Group that erupted from nearby sources (figs. 

3-5). The andesite flow rocks are both aphyric and plagioclaseporphyritic. Individual flows, as thick as 50 ft, covered the entire 

area prior to late Tertiary erosion. The rhyolite lapilli ash-flow 

tuff is a single cooling unit approximately 20 ft thick that is 

interlayered with andesitic lavas. The ash flow deposits likely 

erupted from a vent 4 mi west of Long Canyon near the Little 

Wood River. A sequence of epiclastic sedimentary rocks of 

unknown thickness interbedded with the andesite lavas represents deposition and emplacement in a Challis-age fluviolacustrine drainage system. These poorly indurated strata include 

tuffaceous sandstone, siltstone, and mudstone. 

A laterally extensive sequence of well-rounded, coarsegrained, gravel beds and local accumulations of fanglomerate 

underlie and are interbedded with the Challis lavas (unit Tal, 

figs. 3, 4). The temporal and spatial relations between the coarse 

gravel deposits and the epiclastic sedimentary rocks are unclear. 

The gravel deposits generally contain volcanic detritus, suggesting that deposition occurred after the first eruption of Challis 

lavas, although in places gravel deposits are found at the base of 

the volcanic pile. Extensive gravel and conglomerate deposits of 

similar age are present north and northeast of Long Canyon in 

the upper Fish Creek drainage. 

High-energy streams that drained a source area to the north 

deposited the Tertiary conglomerate. Similar and thicker clastic 

deposits of the same age are known to occur in upper Fish Creek 

Valley to the north and northeast. The position of the gravel with 

respect to surrounding bedrock exposures indicates deposition 

in an area of high relief, similar to that in the region today. Surface oxidation coincidental with this erosional event or a later 

events reached a minimum depth of 700 ft, as seen in drill hole 

DDH C-2. 

Crystalline rocks exposed at Long Canyon include dikes of 

quartz-porphyry rhyolite and andesite. A Tertiary or younger age 

for the dikes is demonstrated by their presence in low-angle fault 

breccia that cuts Eocene-age volcanic rocks. 

A diamond drill hole collared on the floor of Long Canyon 

intersected and drilled 200 ft into an altered and mineralized 

porphyritic granodiorite stock (fig. 4). The age of this stock is 

unknown; however, its texture and mineralogy are similar to 

those of Late Cretaceous stocks in the Wood River area to the 

north. The only surface expression of the intrusion is minor 

development of a calc-silicate mineral assemblage in nearby calcareous strata. The granodiorite has a porcellaneous hypabyssal 

texture, with phenocrysts of hornblende, plagioclase, and potassium feldspar in an aphanitic groundmass. Alteration of the 

stock includes chloritization of hornblende and white clay alteration of feldspars and groundmass. Pyrite, chalcopyrite, sphalerite, and molybdenite are disseminated throughout the rock in 

trace amounts; locally samples from the stock yield assays of 

more than 2,100 ppm Zn, 1,600 ppm Cu, 200 ppm Mo, and 5 

ppm Ag. 

Structure 

Two folding events and several generations of thrust faulting and normal faulting complicate the geology of Long Canyon. Poor exposure hinders an understanding of the structural 

geology. 

Folds 

Argillite exposed at Long Canyon records two folding 

events. A refolded, intensely developed S1 cleavage characterizes the older of these. The folded cleavage, described by Sandberg and others (1975), Skipp and Hall (1975), and Turner and 

Otto (1988), cuts bedding at a low angle and locally forms axial 

planes to tight, isoclinal folds in bedding. Although pervasive in 

the argillite, the cleavage is best developed in the cataclastic 

zone that defines the foliated argillite of unit Ofa. Regional-scale 

folds associated with the early cleavage are not documented but 

may be present. The pervasive S1 cleavage is similar in style and 

occurrence to cleavage described by Otto and Turner (1987) and 

Turner and Otto (1988), attributed by them to the early Paleozoic 

Antler tectonic event. 

The second folding event resulted in tight folds with S1 

cleavage and bedding. Locally, the later folding event created an 

S2 fracture cleavage that crosses S1 at a high angle. The F2 folds 

are commonly seen in outcrop, but regionally the F2 folds lack 

8 Ordovician Stratiform Base-Metal Deposit, Long Canyon, Idaho 
definition primarily because of superimposed faulting and poor 

exposure. One such regional anticline is on the floor of Long 

Canyon (fig. 3); it repeats the stratigraphic section over a limbto-limb distance of 3,000 ft. The inverse repetitions of strata and 

fluctuations in the attitude of bedding and cleavage suggest the 

presence of other megascopic F2 folds. This style of folding and 

related thrust faulting has been described previously and is generally recognized as Mesozoic in age (Skipp and Hait, 1977; 

Dover, 1980; Otto and Turner, 1987; Burton and Link, 1989; 

Link and Mahoney, 1989). 

Faults 

Thrust faults of probable Mesozoic age, low-angle normal 

faults of Tertiary age, and high-angle faults of Tertiary age are 

present in the long Canyon area. Thrust faults of early Paleozoic 

age may be present, but their presence is not documented. 

Thrust faults of probable Mesozoic age, present throughout 

the area, have multiple orientations. They generally strike north 

to northwest and dip 30°-45° to the west. The faults form boundaries between stratigraphic domains of different bedding and 

cleavage orientation. The northwest-trending faults bound strata 

that have a northwest-southeast structural strike, whereas the 

north-trending faults bound strata that have a north-south structural strike. The difference in orientation of these structures may 

be a function of one of the following: (1) two temporally unique 

faulting events, (2) later folding of one fault set, or (3) differential rotation by Tertiary low-angle normal faults. Geologic mapping suggests that there was rotation of thrust fault segments 

across Tertiary low-angle normal faults (fig. 3). The thrust faults 

are locally identified by indurated fault gouge and by repetition 

of the stratigraphic section. Some structures bifurcate, forming 

fault-bounded duplexes that are particularly common in units Og 

and Ol. 

The thrust faults cut the sulfidic strata of units Og and Ofc 

(figs. 3, 5) and controlled development of a pre-Challis-age 

drainage system. The canyons that formed along the traces of the 

structures served as sites of deposition for Challis-age gravel. 

Many of the gravel fragments are rock types that are present in 

the Long Canyon area, suggesting that the gravel was locally 

derived. 

Thrust faults of early Paleozoic age have not been conclusively identified. The cataclastic zone (unit Ofa, figs. 3-5) is an 

intensely developed area of S1 cleavage that is believed to have 

formed during the earliest recorded deformation. Other isolated 

outcrops of S1-parallel cataclastic argillite are in areas of stratigraphic repetition. Those cataclastic rocks may be the products 

of contractional movement during Antler tectonism, as described 

by Turner and Otto (1988). 

Tertiary-age high-angle and low-angle normal faults are 

present throughout the area. Low-angle normal faults dominate 

the structural style and generally are older than the high-angle 

faults. 

Four low-angle extensional faults separate five subhorizontally oriented and laterally continuous structural plates (fig. 3). 

The lowest structural plate crops out only in the bottom of Long 

Canyon, and the higher plates crop out on adjacent hillslopes 

and ridgetops (figs. 3, 5). The low-angle faults typically dip 

2°-10° to the west and southwest; however, their present dip may 

result from downwarping of the Snake River Plain. In areas 

where detailed geologic maps are available, mapped low-angle 

structures demonstrate a southeast-northwest direction of transport similar to that described in the Hailey area by Turner and 

Otto (1988). The smallest of the known low-angle faults has a 

minimum of 600 ft displacement; the larger faults likely moved 

greater distances. The lowest exposed low-angle fault juxtaposes 

dissimilar rocks throughout the area, suggesting a total displacement of more than 2000 ft. Intraplate, low-angle faults of small 

displacement are present locally. One such fault cuts the gossan 

on the west side of Long Canyon and has a hanging-wall displacement of approximately 200 ft to the southwest (fig. 3). 

Gouge zones with subhorizontal slickenslides across which 

rocks show lithological discontinuity are visible in low-angle 

faults. Breccia zones of the low-angle faults are typically rubbly 

and unconsolidated; sample recovery is poor in drill hole intersections and recessive weathering occurs along surface exposures. Gouge zones of the larger faults are as thick as 50 ft. The 

breccia zones serve as ground-water courses, as demonstrated by 

the loss of drilling fluids and by differential oxidation between 

the fault breccia and wallrock. 

High-angle normal faults cut the low-angle faults and have 

two orientations, north-south and east-west. The north-south 

faults predate the east-west structures. The best example of a 

north-south fault separates Paleozoic strata from Tertiary volcanic rocks along the western margin of the Paleozoic outcrop belt 

(fig. 3). This fault has a west-side-down geometry of unknown 

displacement. The north-south faults are Eocene or younger in 

age because they cut Eocene volcanic rocks. These faults are 

geometrically similar to Neogene faults that bound the Wood 

River graben to the north. 

East-west structures identified in the southern part of the 

area define the northern margin of the Snake River Plain. These 

faults juxtapose Paleozoic and Tertiary stratigraphy, cut northsouth faults, and show a south-side-down sense of movement. 

Discussion 

A coarsening- and thickening-upward lower Paleozoic 

stratigraphic section at Long Canyon in Blaine County, Idaho is 

believed to be a remnant of a prograding, sub-wave-base depositional system. The following preliminary interpretation fits available data and published models of prograding submarine fan 

complexes (Walker, 1981). 

The exposed base of the Long Canyon section consists of 

an unknown thickness of dolomitic sand and siltstone (unit Os). 

Chert and cherty argillite (unit Oca) record the initiation of 

pelagic and siliceous chemical sedimentation in a starved, euxinic, sub-wave-base depositional environment. Carbonaceous 

argillite (unit Osh) overlying the cherty section records 

encroachment of a distal, active depositional environment. This 

environment is suggested by thick accumulations of thin, planarbedded mudstone lacking current-generated bedding structures 

and by the presence of graptolites. We interpret these strata to be 

similar to distal Tde turbidites described by Bouma (1962). 

Higher in the section, the distal Tde turbidites are interspersed 

Discussion 9 
with sandstone of unit Oq, which represents a feeder system to a 

prograding clastic wedge. The sandstone strata include channel 

sandstone lenses and local overbank accumulations of thin-bedded sandstone laminae in the argillite. 

Deposition of calc-turbidites of unit Ol demonstrates the 

end of the low-energy sedimentation represented by unit Osh. 

The shelf-derived limestone was deposited as channel diamictite 

and as laterally continuous sheets. Where bedding structures are 

preserved, the limestone units are Tbcde calc-turbidites (Bouma, 

1962) that were deposited in an active high-energy environment. 

The calc-turbidites of unit Ol are exposed at the top of the Long 

Canyon section. 

Gossan horizons at Long Canyon are between the top of the 

argillaceous strata of unit Osh and the bottom of the calcareous 

turbidites of unit Ol; they form unit Og and parts of unit Ofc. 

Bedding structures and clastic-bedding thicknesses in the metalliferous strata suggest that the depositional environment during 

formation of the strata was similar to that attributed to unit Osh. 

The metal-bearing parts of the units are stratabound in some 

places and stratiform in other places. The metals may have been 

introduced as original chemical sediment, during later diagenesis, or as replacement of lithified rocks during emplacement of 

the granodiorite stock. Association of mud-matrix diamictite, 

hydrothermal chert, and stratiform textures in metal-enriched 

rocks suggests mineralization was syngenetic or early diagenetic. The lateral correlation of metalliferous strata with unit 

Osh suggests an Ordovician age for the mineralization. 

Acknowledgments 

We would like to thank Curt Hogge and Burke Williams, 

the property owners, for access to the study area and for their 

cooperation. In addition we would like to thank Betty Skipp, 

who provided help and insight about the regional geology, Claire 

Carter for her support and analyses, and Paul Link for his 

encouragement. We would also like to thank Cominco American 

Resources Incorporated for their encouragement during the 

course of this study and for their permission to publish the 

results. 

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References 11