Symposium on the Geology an 

Mineral Deposits of the Challis 

1°x2° Quadrangle, Idaho

U. S. GEOLOGICAL SURVEY BULLETIN 1658 A-S

Symposium on the Geology and 

Mineral Deposits of the Challis 

1°x2° Quadrangle, Idaho

Edited by D. H. MclNTYRE

Papers presented at the 

Northwest Mining 

Association Convention, 

Spokane, Washington, 

Dec. 1-2,1983

U. S. GEOLOGICAL SURVEY BULLETIN 1658 A-S
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1985

For sale by the Branch of Distribution 

U.S. Geological Survey 

604 South Pickett Street 

Alexandria, VA 22304

Library of Congress Cataloging in Publication Data

Symposium on the Geology and Mineral Deposits of the Challis I°x2° Quadrangle, Idaho (1983 : Spokane, Wash.)

(U.S. Geological Survey bulletin ; 1658A-S)

"Papers presented at the Northwest Mining Association Convention, Spokane, Washington, Dec. 1-2, 1983."

Bibliography: p.

Supt. of Docs. No.: I 19.3:1658A-S

1. Geology Idaho Challis Region Congresses. 2. Mines and mineral resources Idaho Challis Region Congresses.

I. Mclntyre, David H. II. Northwest Mining Association (U.S.). Convention (1983 : Spokane, Wash.) III. Title. 

IV. Series.

QE75.B9 no. 1658A-S 557.3 s

[557.95'7] 85-600112

[QE104.C46]
CONTENTS

[LETTERS INDICATE CHAPTERS]

(A) Summary of the Geology, Mineral Deposits, and Resource Potential for 

Selected Commodities in the Challis Quadrangle, by Frederick S. 

Fisher. 1

(B) Plutonic Rocks of Cretaceous Age and Faults in the Atlanta Lobe of the

Idaho Batholith, Challis Quadrangle, by Thor H. Kiilsgaard and Reed S. 

Lewis. 29

(C) Eocene Cauldron-Related Volcanic Events in the Challis Quadrangle, by E. 

B. Ekren. 43

(D) Precambrian and Paleozoic Sedimentary Terranes in the Bayhorse Area of

the Challis Quadrangle, by S. Warren Hobbs. 59 

(E) Regional Geophysical Studies in the Challis Quadrangle, by Don R. Mabey

and Michael W. Webring. 69 

(F) Tertiary Plutons and Related Rocks in Central Idaho, by Earl H. Bennett

and Charles R. Knowles. 81 

(G) The Twin Peaks Caldera and Associated Ore Deposits, by R. F.

Hardyman. 97 

(H) Ore Deposits Related to the Thunder Mountain Caldera Complex, by B. F.

Leonard. 107 

(I) Epithermal Gold-Silver Mineralization Related to Volcanic Subsidence in the

Custer Graben, Custer County, Idaho, by D. H. Mclntyre and K. M.

Johnson. 109 

(J) Stratigraphy and Mineral Deposits in Middle and Upper Paleozoic Rocks of

the Black-Shale Mineral Belt, Central Idaho, by Wayne E. Hall. 117 

(K) Structural and Stratigraphic Controls of Ore Deposits in the Bayhorse Area,

Idaho, by S. Warren Hobbs. 133 

(L) A Case for Plants in Exploration Gold in Douglas-Fir at the Red Mountain

Stockwork, Yellow Pine District, Idaho, by J. A. Erdman, B. F. Leonard,

and D. M. McKown. 141 

(M) Mineral Deposits in the Southern Part of the Atlanta Lobe of the Idaho

Batholith and Their Genetic Relation to Tertiary Intrusive Rocks and to

Faults, by Thor H. Kiilsgaard and Earl H. Bennett. 153 

(N) Rhyolite Intrusions and Associated Mineral Deposits in the Challis Volcanic

Field, Challis Quadrangle, by R. F. Hardyman and Frederick S.

Fisher. 167 

(O) Lead-Isotope Characteristics of Ore Systems in Central Idaho, by Bruce R.

Doe and M. H. Delevaux. 181 

(P) Light-Stable Isotope Characteristics of Ore Systems in Central Idaho, by

Stephen S. Howe and Wayne E. Hall. 183 

(Q) Depositional Controls of Breccia-Fill and Skarn Tungsten Deposits in the

Challis Quadrangle, by Theresa M. Cookro. 193 

(R) Stratabound Cobalt-Copper Deposits in the Middle Proterozoic Yellowjacket

Formation in and Near the Challis Quadrangle, by Peter J.

Modreski. 203 

(S) Comments on the Development of Resource Assessment Models, Analogy,

and Metaphor, and Their Use in Resource Evaluation, Challis

Quadrangle, by Frederick S. Fisher. 223

Contents

INTRODUCTION

The symposium on the geology and mineral 

deposits of the Challis 1°X2° quadrangle, presented at 

the Northwest Mining Association convention in Spokane, 

Wash., on December 1-2, 1983, was the seventh public 

meeting in which the results of studies made under the 

Conterminous United States Mineral Assessment Program 

(CUSMAP) have been discussed. The chapters in this 

volume are slightly expanded, written versions of the talks 

presented as part of the symposium.

CUSMAP was initiated in 1977 to provide an upto-date assessment of the mineral resource potential of 

the lower 48 States. Under CUSMAP, more than 66,000 

square miles in 12 states have been assessed to date, and 

additional assessments will be completed on another 

60,000 square miles in the near future.

One of the major objectives of CUSMAP is the 

development and application of new concepts for the identification of mineral resource potential in heretofore 

untested but possibly mineralized areas. CUSMAP is 

providing new information on present and potential 

mineral supplies and is producing important data to guide 

our national minerals policy, for minerals exploration, and 

for land-use planning by Federal, State, and local governments. The results of the CUSMAP assessments are 

published in folio format. In addition to a mineral

resource appraisal report and map, a folio may include 

geologic, geochemical, geophysical, and other maps and 

reports. A summary report for each quadrangle is published as a US. Geological Survey Circular. Basic data 

developed during the studies are published in a wide range 

of other technical reports.

The Challis CUSMAP project began in 1979. At 

that time, modern geologic mapping was available for 15 

percent of the quadrangle. Major emphasis, therefore, was 

placed on improvement of the geologic data base, a 

necessary prerequisite to a meaningful mineral resource 

appraisal. A wide range of geologic settings is present; 

41 mineral commodities have been produced or occur in 

anomalous but subeconomic quantities.

The chapters in this volume are summaries of parts 

of the work done on the Challis project prior to December 

1983. In many respects, the chapters should be considered 

progress reports because work on some of the topics 

discussed is continuing. And, because the articles originated as scripts for oral presentation, not all the 

customary conventions for formal written presentation 

could be followed. For example, not all geographic terms 

mentioned in the text are shown on the figures. However, 

the text provides sufficient information about their location to guide the interested reader.

Introduction

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter A

Summary of the Geology, Mineral Deposits, and 

Resource Potential for Selected Commodities in 

the Challis Quadrangle

By FREDERICK S. FISHER

CONTENTS

Abstract 3 

Introduction 3 

Geologic setting 3 

Resource investigations 3 

Metallogenic events 5

Precambrian 5

Paleozoic 11

Cretaceous 14

Tertiary 14 

Spatial distribution of selected mineral commodities 15

Fluorspar 15

Mercury 15

Gold 15

Silver 18

Molybdenum 18

Base metals 18

Metal production in relation to geologic provinces 18

Resource potential of selected commodities based on recent geologic studies 19 

References cited 26

Summary of the Geology, Mineral Deposits and Resource Potential
FIGURES

Al. Map showing selected geologic features 4

A2. Simplified geologic map 6

A3. Map showing the trans-Challis fault system 9

A4. Map showing mines and prospects 9

A5. Diagram showing number of mineral deposits arranged by commodity 10

A6. Chart comparing significance and chance of discovery of mineral

commodities 10 

A7. Diagram showing temporal relationships of the major geologic and tectonic

features and metallic mineral deposits 11

A8. Map showing mining districts and selected principal mines 12 

A9-A16. Maps showing mines and prospects for: 

A9. Fluorspar 15

A10. Mercury 16

All. Gold 18

A12. Silver 19

A13. Molybdenum 20

A14. Zinc 21

A15. Lead 22

A16. Copper 23

A17. Graph showing production of gold and silver 24 

A18. Graph showing production of copper, lead, and zinc 25

TABLE

Al. Mineral commodities produced from or present in anomalous concentrations 

in the Challis quadrangle 9

Challis 1°x2° Quadrangle, Idaho
Abstract

At least four major metallogenic events are represented 

in the Challis quadrangle. The earliest of these was syngenetic 

deposition of cobalt, copper, iron, gold, and minor silver arid 

lead in Precambrian sedimentary rocks about 1.7 to 1.5 b.y. 

(billion years) ago. The second event, syngenetic deposition of 

silver, barium, lead, vanadium, zinc, and lesser amounts of gold, 

copper, molybdenum, tungsten, and tin, took place in Cambrian to Permian time. The third event was introduction of 

niobium, tantalum, thorium, uranium, and the rare-earth 

elements in pegmatites and primary minerals of the Idaho 

batholith, which was emplaced in several phases during the 

Cretaceous between 112 and 70 m.y. (million years) ago. The 

final event was the introduction of beryllium, uranium, thorium, 

molybdenum, tin, gold, silver, mercury, antimony, and fluorine 

during Tertiary igneous activity about 47 to 29 m.y. ago. Blacksand and gold placer deposits accumulated during Pleistocene 

and Holocene time.

Previously deposited metals were remobilized during igneous and tectonic activity in the Precambrian, Cretaceous, 

and Tertiary. Ore minerals occur in veins, replacements, 

disseminations, stockworks, and skarns associated both with 

the Idaho batholith and its satellite plutons and with many types 

of Tertiary intrusive bodies. Most metals in these deposits are 

believed to have been derived from crustal sources, most likely the metalliferous Precambrian and Paleozoic sedimentary 

rocks.

Much of the area along the trans-Challis fault system has 

moderate to high resource potential for silver and gold, as does 

the northwestern side of the Twin Peaks caldera. The southwestern and western sides of the Thunder Mountain cauldron 

complex have a high resource potential for silver, gold, mercury, molybdenum, and tungsten. Roof pendants of metamorphosed Paleozoic rocks in the Idaho batholith have a high 

resource potential for tungsten. The greatest resource potential in the Challis quadrangle is in Paleozoic sedimentary rocks, 

which in the Bayhorse area have a high potential for silver, gold, 

copper, fluorspar, lead, and zinc. Elsewhere in the quadrangle, 

argillite and micritic limestone beds in the Devonian Milligen 

Formation and the Paleozoic Salmon River assemblage have 

a high resource potential for silver, barite, molybdenum, lead, 

antimony, vanadium, tungsten, and zinc in veins, replacements, 

and stratabound syngenetic deposits.

INTRODUCTION

The Challis quadrangle is in central Idaho north 

of the Snake River Plain (fig. Al). The largest community in the quadrangle is Challis, with a population 

of 794. Other settlements are Stanley, Lowman, Garden 

Valley, and Yellow Pine, all with populations of less than 

100.

More than 95 percent of the quadrangle is federally owned land, encompassing parts of the Salmon, 

Challis, Boise, Sawtooth, and Payette National Forests. 

Nearly one-third of the area is within the River-Of-NoReturn Wilderness and the Sawtooth National Recreation 

Area (fig. A4). Extensive tracts, chiefly in the southeastern

part of the quadrangle, are managed by the U.S. Bureau 

of Land Management.

Most of the quadrangle is mountainous. Elevations 

range from a low of 3,000 ft (feet) in the valley of the 

South Fork of the Payette River where it exits the quadrangle in the southwest to a high of 11,815 ft at the summit of Castle Peak in the White Cloud Peaks (fig. A2). 

Local relief of 3,000-4,000 ft is common throughout the 

area, and canyons and valleys are steep sided.

Much of the terrane is moderately to heavily 

forested and generally below timberline; however, parts 

of the White Cloud Peaks and Sawtooth Range and other 

scattered peaks rise well above timberline and have 

subalpine climates.

Principal industries in the area are farming, ranching, lumbering, mining, and recreation.

GEOLOGIC SETTING

The rocks in the quadrangle can be subdivided into five main groups (fig. A2). Precambrian sedimentary 

and metamorphic rocks crop out mainly in the northern 

and eastern parts of the area. Paleozoic sedimentary rocks 

are mostly in the south-central and east with scattered exposures through the central parts of the quadrangle. The 

Cretaceous Idaho batholith occupies the western half of 

the quadrangle, and the Tertiary Challis volcanic field 

covers most of the eastern half. Tertiary intrusive 

batholiths and stocks are mainly in the central part of the 

quadrangle.

Four major structural features predominate: first, 

a northeast-trending structural grain expressed by extensive high-angle faults, fracture systems, dike swarms, and 

grabens; second, a north- and northwest-trending system 

of high-angle faults; third, caldera and collapse features 

associated with the Challis volcanic field; and fourth, extensive thrust faults in the Paleozoic formations.

RESOURCE INVESTIGATIONS

At the start of the Challis CUSMAP (Conterminous 

United States Mineral Assessment Program) project in 

1979, only about 15 percent of the quadrangle had 

published geologic maps less than 50 years old, and about 

half of the quadrangle had never been mapped geologically. Regional geophysical data were incomplete, and 

geochemical studies had been conducted only in parts of 

the River-Of-No-Return Wilderness and the Sawtooth National Recreational Area.

Field studies made during the Challis project have 

resulted in modern geologic maps for the entire quadrangle (Fisher, Mclntyre, and Johnson, 1983), gravity and 

aeromagnetic maps for the entire quadrangle (Webring 

and Mabey, 1981; Mabey, 1982; Mabey and Webring, chap. 

E, this volume), and extensive geochemical surveys

Summary of the Geology, Mineral Deposits and Resource Potential
116°

40 MILES

40 KILOMETERS

EXPLANATION

 CALDERA, CAULDRON, AND CAULDRON-COMPLEX

BOUNDARIES

_1___ FAULT Bar and ball on downthrown side 

_.__t_ THRUST FAULT Sawteeth on upper plate

~L_~~_T- DIKE SWARM

Figure A1. Map showing selected geologic features in the Challis quadrangle.

(Callahan and others, 1981a, 1981b; Shacklette and Erdman, 1982; Erdman and others, 1983; Fisher and May, 

1983; Fisher and others, 1983).

Some of the highlights of these field studies are 

summarized here. The discovery and delineation of the 

caldera complexes and development of the framework 

stratigraphy of the Challis volcanic field (Mclntyre and

others, 1982; Fisher, Mclntyre, and Johnson, 1983) 

resulted from studies spearheaded by E. B. Ekren, D. H. 

Mclntyre, R. F. Hardyman, and B. F. Leonard, all with 

the U.S. Geological Survey, Denver, Colo. Detailed mapping and petrological studies by Thor Kiilsgaard (U.S. 

Geological Survey, Spokane, Wash.) and Earl Bennett 

(Idaho Bureau of Mines and Geology, Moscow, Idaho)

Challis 1°x2° Quadrangle, Idaho
in the western half of the quadrangle have shown that the 

Atlanta lobe of the Idaho batholith is composed of six rock 

types emplaced in at least three stages during a considerable 

time span (Kiilsgaard and Lewis, chap. B, this volume). 

Kiilsgaard and Bennett also discovered, delineated, and 

described a previously unrecognized bimodal group of 

epizonal granitic and dioritic Tertiary plutons, some of 

batholithic size, within areas thought to be composed of 

Cretaceous rocks of the Idaho batholith (Bennett, 1980). 

Mapping in the central and south-central parts of the quadrangle by W. E. Hall and S. W. Hobbs (Fisher, Mclntyre, 

and Johnson, 1983) unraveled a highly complicated sequence of thrust faults bounding large areas of allochthonous Paleozoic terranes. Also, the stratigraphic studies 

in these terranes have described host rocks for major ore 

deposits including both veins and replacements and also 

newly recognized stratabound syngenetic occurrences (Hall, 

chap. J, this volume; Hobbs, chaps. D, K, this volume). 

Together these field studies have led to the recognition of 

a major northeast-trending fault zone more than 100 mi 

(miles) (160 km or kilometers) long, marked by dike 

swarms, grabens, faults, and calderas, that transects the 

quadrangle and extends well into Montana (fig. A3).

These newly discovered features the fault zone, the 

caldera complexes, the six phases of the Idaho batholith, 

the Tertiary plutons, and the thrust faults and allochthons have had a profound influence on the character, 

type, and location of ore deposits in the quadrangle and 

are the subjects of the succeeding chapters in this volume.

The evaluation of the mineral resource potential of 

the Challis quadrangle is still in progress, and final resource appraisal maps will be completed when the project is concluded at the end of 1985. The resource appraisal 

method being used is best described as "simple subjective;" it relies mainly on the opinions of highly qualified 

economic geologists and is firmly grounded on a solid 

geologic-map base. In a broad sense, we are following 

many of the principles recently outlined by Taylor and 

Steven (1983).

Forty-one commodities either have been produced 

or are present in anomalous but subeconomic quantities 

within the quadrangle (table Al), and hundreds of mines 

and prospects are scattered across the quadrangle 

(fig. A4). Descriptions and locations of all the mines and 

prospects in the Challis quadrangle are given in Mitchell 

and others (1981).

The number and diversity of deposits containing the 

12 most abundant metallic commodities in the quadrangle 

are shown in figure A5. Precious-metal deposits make up 

the largest group, followed in order by lead, copper, and 

zinc; but the remaining 7 commodities account for more 

than 300 deposits. The other 12 commodities (table Al) 

have been produced from fewer than 10 deposits each, or 

as byproducts, and are not shown on figure A5.

Because of the large number and variety of mineral

deposits within the quadrangle, an attempt was made to 

identify the most important commodities and to focus efforts on them. First, a list of all commodities known to 

occur in the area was compiled (table Al); second, those 

commodities were ranked into four groups according to 

significance (loosely defined here as national importance) 

(fig. A6), using the net import reliance tables as determined by the U.S. Bureau of Mines (U.S. Bureau of Mines, 

1983). Finally, 10 geologists having more than 100 years 

of combined experience in the area were asked to rank 

the listed commodities into one of five groups based on 

their beliefs as to the likelihood of new deposits of a given 

commodity occurring within the quadrangle. Their 

responses were averaged, and each commodity was then 

plotted on figure A6 according to significance and 

likelihood of occurrence. Those commodities in the 

shaded area have received generally more geologic study 

by the Challis CUSMAP project.

METALLOGENIC EVENTS

At least four major metallogenic events are represented in the Challis quadrangle. These occurred during 

Precambrian, Paleozoic, Cretaceous, and Tertiary time. 

The relationship of these events to major igneous, sedimentary, and tectonic features in the Challis quadrangle 

are shown on figure A7.

Precambrian

The earliest metallogenic event occurred in the Precambrian. Nearly 1 b.y. of sedimentary history are represented 

by argillic rocks of the Middle Proterozoic Yellowjacket 

Formation and quartzite and siltite of the Lemhi Group 

and the Hoodoo Quartzite, Swauger, Lawson Creek, and 

the Late Proterozoic Wilbert Formations. Precambrian igneous activity consisted of intrusion of granitoid plutons 

about 1.5 to 1.3 b.y. ago (Evans, 1981). Mafic tuffs and 

dikes dated at about 1.7 b.y. are also present within the 

Yellowjacket sequence (Hughes, 1982). Rocks of the 

Yellowjacket Formation range from argillaceous quartzite 

to argillite. They have been interpreted as having been 

deposited in a rift basin bordering the craton and representing depositional environments ranging from basinal 

to deepwater clastic fan deposits and turbidites (Modreski, 

chap. R, this volume; Hughes, 1982). Metals deposited 

during Yellowjacket time were probably formed by some 

type of sea-floor hot springs related to the mafic volcanic 

activity. The principal metals were cobalt, copper, and 

iron, lesser amounts of gold, and probably some silver 

and lead. The metals are mostly stratabound and were 

probably deposited syngenetically with the enclosing 

sediments.

Summary of the Geology, Mineral Deposits and Resource Potential
0 20 MILES

0 10 20 KILOMETERS 

Figure A2. Simplified geologic map of the Challis quadrangle (modified from Fisher, Mclntyre, and Johnson, 1983).

6 Challis 1°x2° Quadrangle, Idaho
115°

Figure A2. Continued.

Summary of the Geology, Mineral Deposits and Resource Potential 7
CORRELATION OF MAP UNITS

EOCENE CHALLIS VOLCANICS AND RELATED ROCKS

**$

!ft* :!;

QUATERNARY

TERTIARY

IDAHO BATHOLITH AND RELATED ROCKS

Kg*

Klg*

Kmg*

Kgd*

Kgdp*

Kt*

CRETACEOUS

PALEOZOIC

Pu

Mu

DSu

OCu

OGZI

Yu

d*

PERMIAN

MISSISSIPPI

DEVONIAN AND 

SILURIAN 

ORDOVICIAN AND 

CAMBRIAN

ORDOVICIAN, CAMBRIAN, 

AND LATE PROTEROZOIC

MIDDLE PROTEROZOIC

UNCERTAIN AGE

*lmrusive rocks

t Rocks with both intrusive and extrusive characteristics

DESCRIPTION OF MAP UNITS

QUATERNARY DEPOSITS, UNDIVIDED Includes floodplain, terrace, landslide, and glacial deposits. Materials 

include boulders, cobbles, pebbles, sand, silt, clay, 

peat, and travertine

CHALLIS VOLCANICS, UNDIVIDED (EOCENE)

YOUNGER PYROCLASTIC ROCKS AND SEDIMENTS 

Associated with development of Twin Peaks caldera. 

PantherCreek graben, and late stages of Thunder Mountain cauldron complex. Welded and nonwelded ashflow and ash-fall tuffs, lavas, pumice flows, flow breccias, 

caldera-slump breccias, and reworked tuffaceous sediments. Mostly rhyolite and rhyodacite

OLDER PYROCLASTIC ROCKS AND SEDIMENTS Associated with development of Van Horn Peak cauldron 

complex, Custer graben, and early stages of Thunder 

Mountain cauldron complex. Welded and nonwelded 

ash-flow and ash-fall tuffs, lavas, flow breccias, and reworked tuffaceous sediments. Mostly thyolite, quartz 

latite, and rhyodacite, with lesser amounts of dacite and 

andesite

LAVAS, FLOW BBRECCIAS, AND AGGLOMERATES Principally andesite, dacite, and latite, with lesser amounts 

of basalt. Phenocryst rich to phenocryst poor, varying 

combinations of plagioclase, biotite, amphibole, clinopyroxene, orthopyroxene, and less common olivine and 

quartz

RHYOLITE INTRUSIVE BODIES Dikes, plugs, domes, 

and stocks. Phenocryst rich to phenocryst poor. Typically 

contain quartz, alkali feldspar, and sparse plagioclase 

and biotite phenocrysts

VOLCANICLASTIC AND SEDIMENTARY ROCKS Includes 

subaqueously deposited pyroclastic flows, mudflow 

breccia and conglomerate, volcanic sandstone and mudstone. Volcanic components of these rocks contain 

plagioclase, biotite, and amphibole. Quartz and pyroxene are sparse as phenocrysts

POTASSIUM-RICH ANDESITE, LATITE, AND BASALT 

LAVAS Predominantly aphyric, reddish-brown, blocky 

to platy lavas that contain interbedded oxidized breccia. Olivine and pyroxene phenocrysts in some samples 

along with lesser amounts of plagioclase 

GRANITE Pink to gray, medium- to coarse-grained granite 

characterized by pink perthitic feldspar that commonly 

forms phenocrysts. May contain miarolitic cavities locally 

lined with smoky-quartz crystals

DIORITE COMPLEX Complex suite of rocks ranging from 

nonporphyritic diorite to porphyritic granodiorite. which 

is more common. Characterized by abundance of hornblende, euhedral biotite, and magnetite, which together 

make up as much as 35 percent of rock

IDAHO BATHOLITH AND RELATED ROCKS (CRETACEOUS) Granitic rocks, undivided

Leucocratic granite Light-gray to white, fine- to medium- grained, with distinctive anhedral texture. Principal minerals are quartz (33 percent), potassium feldspar (29 percent), and plagioclase (An2 g.3o 33 percent). Biotite may constitute as much as 2 percent of 

rock. Garnet is common. Occurs as dikes and irregular 

stocks that are resistant to erosion and tend to form 

high points and ridges

Muscovite-biotite granodiorite Gray to light-gray, medium to coarse-grained, equigranularto porphyritic; contains books of muscovite visible in hand specimen and 

comprising as much as 5 percent of rock

Biotite granodiorite and granite Gray to light-gray, 

medium- to coarse-grained, equigranularto porphyritic. 

Plagioclase (An22-36) is chief component of rock, with 

lesser quantities of quartz and potassium feldspar; biotite, 

as much as 5 percent, is principal mafic mineral; hornblende is absent

Porphyritic biotite granite and granodiorite Coarsely 

porphyritic granitoid rock with metacrysts of pink potassium feldspar (microcline) 3-10 cm long in medium- to 

coarse-grained matrix that contains roughly equal 

amounts of microcline, plagioclase, and quartz. Rock 

generally contains 5-15 percent biotite and variable 

amounts of hornblende

Tonalite Gray to dark-gray, medium- to coarse-grained, 

equigranular to porphyritic. Plagioclase is dominant 

mineral of rock, which may contain biotite or hornblende 

and ranges from massive to highly foliated 

PERMIAN ROCKS, UNDIVIDED Includes carbonaceous 

argillite, siltite, limestone, calcareous sandstone, and 

fine-grained quartzite of the Dollarhide and Grand Prize 

Formations. Parts of sequence are graded, have abundant crossbeddmg, convolute structures, and prominent banding. Thickness about 2,400 m

8 Challis 1°x2° Quadrangle, Idaho
SALMON RIVER ASSEMBLAGE PALEOZOIC Medium- to 

dark-gray, thin- to thick-bedded, well laminated argillite, siltstone, calcareous siltstone, and fine-grained 

calcareous sandstone. Faintto prominent lamination in 

most places, with cross lamination, current bedding, 

and sole structures in many fine-grained, thin sandstone 

beds. Dark colors related to amount of carbonaceous 

material, which ranges from none to a few impure coaly 

seams. Thickness unknown, but in places 1,400 m are 

present

MISSISSIPPIAN ROCKS, UNDIVIDED Includes limestone, 

cherty limestone, mudstone, siltstone, sandstone, and 

conglomerate. In places highly fossiliferous, containing crinoidal debris, corals, and brachiopods. Thickness about 3,600 m

DEVONIAN ANDSILURIAN ROCKS, UNDIVIDED Includes 

dolomite, siltstone, sandstone, and quartzite. Dolomite 

units moderately to highly resistant and form blocky 

ledges and locally cliffs. Thickness about 1,800 m

ORDOVICIAN AND CAMBRIAN ROCKS. UNDIVIDED Includes shale, argillite, phyllite, dolomite, and quartzite. 

Locally deformed and recrystallized. Thickness unknown 

but probably more than 3,500 m

INTERBEDDED QUARTZITE, DOLOMITE, AND ARGILLITE 

OF LEATON GULCH (ORDOVICIAN, CAMBRIAN, AND 

LATE PROTEROZOIC) Predominantly quartzitic strata 

containing subordinate dolomite interbeds and some 

thin to thick (100 m) interbeds of argillite. Ripple marks, 

flute casts, and magnetite are locally abundant. Many 

argillite layers are metamorphosed to phyllite close to 

thrust faults. Thickness unknown but probably several 

hundred meters

MIDDLE PROTEROZOIC, UNDIVIDED Includes quartzite, 

phyllite, argillite, siltite, and sandstone. Locally common ripple marks, cross laminations, load casts, and 

mud cracks. Thickness unknown but probably greater 

than 1 0,000 m

ROOF PENDANTS AND XENOLITHS OF METAMORPHIC 

ROCK IN IDAHO BATHOLITH, UNDIVIDED (UNCERTAIN AGE) Includes schist, quartzite, and calc-silicate rocks. At some localities graphite, garnet, and 

andalustite are common constituents

DIORITE, QUARTZ DIORITE, AND SYENITE (UNCERTAIN 

AGE) Composite masses and mixed sequences of intrusive rocks of uncertain age. Rocks range from fresh 

to altered; where altered, epidote, chlorite, sericite, and 

calcite are common

CONTACT 

 FAULT 

. THRUST FAULT Sawteeth on upper plate

, CALDERA BOUNDARY Dashed where inferred: bar and 

ball on downthrown side

Figure A2. Continued.

Table A1. Mineral commodities produced from or present in 

anomalous concentrations within the Challis quadrangle.

PRODUCED

Silver

Arsenic

Gold

Barium

Bismuth

Cadmium

Cobalt

Copper

Fluorspar

Semiprecious gemstones

Mercury

Molybdenum

Niobium

Lead

Rare-earth elements

Sand and gravel

Antimony

Selenium

Stone

Tantalum

Tellurium

Thorium

Uranium

Tungsten

Zinc

ANOMALOUS CONCENTRATIONS

Beryllium 

Clay 

Iron

Garnet

Graphite 

Limestone

Mica

Manganese

Nickel 

Sulfur 

Silica

Tin

Titanium 

Vanadium

Zirconium

Zeolites

116° 

45°

114°

PANTHER CREEK GRABENTWIN PEAKS 

CALDERA

, Challis

KNAPP CREEK GRABEN

CUSTER GRABEN

0 40 MILES

I I I 

0 40 KILOMETERS 

EXPLANATION

-j FAULT Bar and ball on 

downthrown side

Figure A3. Principal features of the trans-Challis fault system 

(modified from Fisher, Mclntyre, and Johnson, 1983).

Rocks of the Yellowjacket and Hoodoo Formations 

were complexly folded and metamorphosed to greenschist 

facies probably between 1.4 and 1.2 b.y. ago (Hughes, 

1982). The cobalt-copper-iron ores in the Yellowjacket Formation were remobilized and recrystallized at this time. 

The Blackbird deposit, north of the Challis quadrangle, 

and the Iron Creek deposit (fig. A8) are examples of these 

remobilized Precambrian deposits.

Summary of the Geology, Mineral Deposits and Resource Potential 9
116°

20 40 MILES

^\ 

20 40 KILOMETERS 

EXPLANATION

QUATERNARY ALLUVIUM Includes flood plain, terrace, 

and alluvial-fan deposits

TERTIARY INTRUSIVE VOLCANIC ROCKS Dikes, stocks, 

and batholiths of dioritic, granitic, or rhyolitic composition

TERTIARY EXTRUSIVE VOLCANIC ROCKS Tuffs, lavas.

and volcanic sedimentary rocks of rhyolite, rhyodacite, 

dacite, and andesite

CRETACEOUS INTRUSIVE ROCKS OF THE IDAHO BATHOLITH Tonalite, granodiorite, granite, muscovitebiotite granite, and leucocratic granite

PALEOZOIC SEDIMENTARY ROCKS Limestone, dolomite, 

shale, siltstone, argillite, quartzite, sandstone, and 

slate

MIDDLE PROTEROZOIC METAMORPHIC AND SEDIMENTARY ROCKS Argillite. slate, quartzite, sandstone, 

siltite, schist, and phyllite

ROCKS OF UNCERTAIN AGE Argillite, carbonate, quartzite, and phyllite

CONTACT 

FAULT

* * *- THRUST FAULT Sawteeth on upper plate

 CALDERA BOUNDARY 

 MINE OR PROSPECT

AREA CONTAINING ABUNDANT MINES AND PROSPECTS

APPROXIMATE BOUNDARY OFTHE RIVER OF NO RETURN 

WILDERNESS

APPROXIMATE BOUNDARY OF THE SAWTOOTH NATIONAL 

RECREATION AREA

Figure A4. Map showing locations of mines and prospects, and wilderness and recreation areas in the Challis quadrangle.

10 Challis 1°x2° Quadrangle, Idaho
500 

450

400

350

300

O 250 

cc

HI 

CD

Z> 200

150

100

50

487

384

239

198

133

72

 45 44

34

22

Au Ag Pb Cu Zn F W Mon Sb Mo U Hg 

COMMODITY

Figure A5. Number of mineral deposits arranged by commodity, Challis quadrangle. Mon., monazite.

Rocks of the Yellowjacket and Hoodoo Formations 

were complexly folded and metamorphosed to greenschist 

facies probably between 1.4 and 1.2 b.y. ago (Hughes, 1982). 

The cobalt-copper-iron ores in the Yellowjacket Formation 

were remobilized and recrystallized at this time. The 

Blackbird deposit, north of the Challis quadrangle, and 

the Iron Creek deposit (fig. A8) are examples of these 

remobilized Precambrian deposits.

Paleozoic

During the Paleozoic Era (fig. A7), silver, barium, 

lead, vanadium, and zinc were deposited syngenetically in 

many types of sedimentary rocks (Fisher and May, 1983; 

Skipp and others, 1979; Ketner, 1983). These metals, along 

with lesser amounts of gold, copper, molybdenum, and 

tungsten, are present in lower Paleozoic quartzite, limestone, dolomite, siltstone, and shale in the Bayhorse district 

(Hobbs, chap. K, this volume) and in middle and upper 

Paleozoic dark limestone, argillite, shale, and siltstone of 

the Milligen Formation, Copper Basin, Grand Prize, and 

Dollarhide Formations, and in the Paleozoic Salmon River

o

O 

O

o

u_ 

O 

O 

O

o

Mo

Sand and

Stone

Th

Yt

Limestone

Silica

Zr

Fe

S 

Garnet 

Mica

Clay

Gemstone

U

-Cu

1 Sb; 

} Zrt

Pb ;

Cd

Te

Se

Rare- earth

elements

Ba 

Graphite

As ;

Alt

W

1

=vf

Mb

Hg

Bi

Ti

Ta

Mn 

Be 

As

,

?! I

i ;C9 S I

* :

Sn

Ni

Less Greater

SIGNIFICANCE

Figure A6. Chart comparing the significance as defined by the 

net import reliance tables (U.S. Bureau of Mines, 1983) and 

chance of discovery of mineral commodities in the Challis 

quadrangle.

assemblage and the Wood River Formation in the southern 

parts of the quadrangle (Hall and others, 1978; Skipp and 

others, 1979). The only Paleozoic igneous activity recorded 

in the quadrangle was the intrusion of some diorite and 

gabbro stocks in the north near the \ellowjacket region 

about 400 m.y. (million years) ago. Tectonic activity in the 

region was associated with the Late Devonian-Early 

Mississippian Antler orogeny, during which the Paleozoic 

rocks were widely folded and thrust eastward (Dover, 1980). 

Presumably, some refolding of the Precambrian rocks also 

occurred at that time.

Examples of syngenetic Paleozoic deposits are the 

zinc and barite ores at the Hoodoo mine (fig. A8) in the 

Paleozoic Salmon River assemblage. Other examples of 

syngenetic mineral deposits in the Salmon River assemblage 

are the vanadium-silver concentrations described by Fisher 

and May (1983) and possibly the lead-silver ores at the Livingston mine (fig. A8). The Lower Permian fine-grained 

carbonaceous basinal sedimentary rocks have potential

Summary of the Geology, Mineral Deposits and Resource Potential 11
Co, Cu, Fe Co, Cu, Fe

GS 

/WVAA

Tuffs 

? ?-? 

YJ H LEMHI GROUP, SW, LC, W

Aft Ba, Pb, Zn

Quartz, limestone, dolomite 

siltstone, shale

,CB, SRA 

Argillite, shale, siltstone ML

GP DH

A

1800 1600 1400 1200 1000 800 600 

MILLION YEARS

A lA

600 550 500 450 400 350 300 250

MILLION YEARS 

i PALEOZOIC l

SL TC

Mon (Nb, Ta, Th, REE)

Aft Au, Ba, Cu, Mo, Pb, W, Zn (skarns) 

? i

SB GC RM LF 

Aft Au, Cu, Mo, Pb, Sb, Sn, U, W, Zn

 ?

Tuffs

VHP

Arkose Volcanic sediments and mudflows 

?'? Lavas

120 100 80

MILLION YEARS 

 CRETACEOUS 

60 60 55 50 45 40

MILLION YEARS

 TERTIARY 

35 30 25

Co, Cu, Fe 7 -

EXPLANATION

TIME INTERVAL DURING WHICH

METALS WERE DEPOSITED Queried where uncertain

Co,Cu,Fe TIME INTERVAL DURING WHICH 

"-^ - ^ METALS WERE REMOBILIZED 

W\ ~v PERIOD OF METAMORPHISM

PERIOD OF THRUST FAULTING 

PERIOD OF FOLDING

GS, green-schist facies

YJ, Yellowjacket Formation

H, Hoodoo quartzite

SW, Swauger Formation

LC, Lawson Creek Formation

W, Wilbert Formation

CB. Copper Basin Formation

SRA, Salmon River assemblage

ML, Milligen Formation

GP, Grand Prize Formation

DH, Dollarhide Formation

SL, Skylark vein

TC, Thompson Creek mine

SB, Sunbeam mine

GC, General Custer vein

RM, Red Mountain prospect

LF, Little Falls molybdenum deposit

TP, Twin Peaks caldera

TM, Thunder-Mountain caldera

CG, Custer graben

VHP, Van Horn Peak cauldron complex

REE, Rare-earth elements

Mon, Monazite

Figure A7. Diagram showing the temporal relationships of the major geologic and tectonic features and metallic mineral deposits 

in the Challis quadrangle.

PERIOD OF ROCK FORMATION 

Abbreviations explained at right

PERIOD OF HIGH-ANGLE FAULTING 

PERIOD OF CALDERA FORMATION 

DIKE

STOCK

12 Challis 1°x2° Quadrangle, Idaho
20 40 KILOMETERS 

EXPLANATION

QUATERNARY ALLUVIUM Includes flood plain, terrace, 

and alluvial-fan deposits

TERTIARY INTRUSIVE VOLCANIC ROCKS Dikes, stocks, 

and batholiths of dioritic, granitic, or rhyolitic composition

TERTIARY EXTRUSIVE VOLCANIC ROCKS Tuffs. Isvas.

snd volcanic sedimentsry rocks of rhyolite, rhyodscite,

dacite, and sndesite 

CRETACEOUS INTRUSIVE ROCKS OF THE IDAHO BATHOLITH Tonalite, granodiorite, granite, muscovitebiotite grsnite, and leucocratic granite

PALEOZOIC SEDIMENTARY ROCKS Limestone, dolomite, 

shale, siltstone, argillite, quartzite, sandstone, and 

slate

MIDDLE PROTEROZOIC METAMORPHIC AND SEDIMENTARY ROCKS Argillite, slate, quartzite, sandstone, 

siltite, schist and phyllite

ROCKS OF UNCERTAIN AGE Argillite, carbonate, quartzite, and phyllite

40 MILES

CONTACT

FAULT

THRUST FAULT Sswteeth on upper piste

CALDERA BOUNDARY

MINE OR PROSPECT

1. Iron Creek mine

2. Hoodoo mine

3. Livingston mine

4. Thompson Creek mine

5. Little Boulder Creek deposit

6. Skylark vein

7. Sunbeam mine

8. General Custer vein

9. Red Mountain deposit

10. Little Falls deposit

11. Meyers Cove district

12. Boulder Creek district

13. Lucky Boy mine

14. Yellow/jacket mine

15. Mammoth mine

16. Golden Day mine

17. Parker Mountain deposit

18. Snowshoe Creek deposit

19. Singheiser mine

20. Sunnyside mine

21. Dewey mine

22. West End deposit

23. Yankee Fork placer deposits

24. Loon Creek placer deposits

25. Stanley Creek placer deposits

26. Valley Creek placer deposits

27. Clayton Silver mine

28. Bayhorse district

29. Virginia Beth deposit

30. Hall-Interstate mine

31. Ramshorn vein

32. Seafoam district

Figure A8. Map showing mining districts and selected principal mines in the Challis quadrangle.

Summary of the Geology, Mineral Deposits and Resource Potential 13
deposits of zinc, lead, and barite in the Grand Prize and 

Dollarhide Formations (Hall, chap. J, this volume; Skipp 

and others, 1979). All of these syngenetic ores show varying degrees of remobilization during Late Cretaceous and 

Tertiary igneous activity.

Cretaceous

The intrusion of several phases of the Idaho 

batholith and satellite stocks from about 112 to 70 m.y. 

ago dominated the Cretaceous Period (fig. A7). Folding 

and eastward thrusting of Paleozoic rocks were 

widespread before and during batholith intrusion and 

culminated during the Sevier orogeny in Late Cretaceous 

time (Skipp and Hall, 1975). Some normal faulting also 

occurred during this time, and contact metamorphism was 

locally intense (Anderson, 1948).

Intrusion of various phases of the batholith probably remobilized yet again the Precambrian cobalt-copperiron deposits; certainly the Paleozoic syngenetic silver, 

barium, lead, and zinc deposits were also remobilized to 

varying degrees. Skarn deposits containing silver, gold, 

barium, copper, molybdenum, lead, tungsten, and zinc 

were formed at this time in roof pendants and adjacent 

to the batholith, as were the Thompson Creek and Little 

Boulder Creek porphyry molybdenum deposits (fig. A8). 

The most widely accepted date for the Thompson Creek 

deposit is 88 m.y. (Marvin and others, 1973). Vein and 

replacement lead-silver deposits in the Bayhorse district, 

specifically the Skylark vein (fig. A8), have been dated 

at 95 m.y. (Mclntyre and others, 1976; recalculated). The 

source of the metals in the skarns, the porphyry 

molybdenum, and the vein-replacement deposits is a matter of conjecture. In my opinion, the metals in these 

deposits were derived from the metalliferous Paleozoic and 

Precambrian rocks by hydrothermal systems associated 

with the various intrusive phases of the batholith, and the 

batholith itself contributed little metallic content to the 

ore deposits of the region. The batholith did, however, 

contribute euxenite and columbite in pegmatites, and 

monazite as a primary disseminated mineral. These 

minerals contain valuable concentrations of niobium, tantalum, thorium, uranium, and the rare-earth elements, and 

accumulated in placer deposits as a result of late Tertiary 

weathering followed by concentration during the 

Pleistocene and Holocene (Schmidt and Mackin, 1970).

Tertiary

Tertiary time was characterized by the deposition 

of the Eocene Challis Volcanics, the development of the 

Challis volcanic field, and the emplacement of several 

batholiths and many stocks (fig. A7). Prior to the igneous 

activity, however, the Idaho batholith was being eroded,

and arkosic sediments were deposited in local areas. These 

sediments eventually were the traps for stratabound 

uranium deposits in the Stanley region (Choate, 1962). 

Volcanism began with the widespread eruption of lavas 

of intermediate composition about 51 m.y. ago (Mclntyre 

and others, 1982). Lavas were intermittently erupted for 

another 5 to 6 m.y. About 48 m.y. ago the character of 

volcanic activity changed and ash-flow eruptions 

predominated. These eruptions led to the development of 

the cauldron complexes, starting with the Van Horn Peak 

cauldron complex, which was closely followed and probably partly concurrent with the formation of the Custer 

graben, and the Thunder Mountain and Twin Peaks 

calderas (Ekren, chap. C, this volume). The size and shape 

of the calderas were closely related to pre-existing grabenbounding northeast-trending faults. Northeast and northwest-trending high-angle faults began to form at least 51 

m.y. ago, as dated by lavas that ponded against pre-existing 

fault scarps (Mclntyre and others, 1982).

Intrusive activity from 50 to about 43 m.y. ago 

resulted in the emplacement of the Casto pluton (more 

accurately called a batholith), the Sawtooth batholith, and 

many other smaller stocks and countless dikes (Bennett, 

1980). From 43 to 39 m.y. ago, extensive rhyolitic intrusions and high-level domes and dikes were emplaced 

(Hardyman and Fisher, chap. N, this volume). "Highlevel" is a term used in this report for rhyolite intrusive 

bodies emplaced at or very near the Earth's surface. Some 

of these bodies may have breached; in plan view their 

shapes may be circular, linear, or very irregular. Limited 

age data suggest that at least some of the intrusive activity in the Boise Basin dike swarm occurred as recently as 

29 m.y. ago (Kiilsgaard and Bennett, chap. M, this 

volume).

A great diversity of deposits is associated with Tertiary igneous activity (fig. A7). The oldest date we have 

so far in the Tertiary is 45 m.y. for the Sunbeam gold-silver 

deposit (Johnson and Mclntyre, 1983). The General 

Custer epithermal gold-silver vein deposit is dated at 43 

m.y., the Red Mountain gold-molybdenum deposit in 

high-level rhyolite at 39 m.y., and the Little Falls veinfracture molybdenum deposit at 29 m.y. (Johnson and 

Mclntyre, 1983). To the best of our knowledge, then, most 

of these metals were deposited in a minimum time span 

of 18 m.y. A broad paragenetic sequence of three stages 

within that time period can be postulated: a base-metal 

stage, overlapped and followed by a fluorite and possibly 

mercury episode, in turn overlapped and followed by 

zeolite development. This sequence is strictly conjecture 

at present, as we do not have detailed dating of the different stages. However, the major fluorspar and zeolite 

deposits and possibly also the mercury deposits all are 

spatially separate from the main areas of base- and 

precious-metal deposits.

Hundreds of deposits within the quadrangle are examples of Tertiary mineralization. A few examples are:

14 Challis 1°x2° Quadrangle, Idaho
the lead-silver-antimony-tin veins in the Wood River Formation in the southern part of the quadrangle; tungstenantimony-gold deposits at Yellow Pine; stratabound and 

vein uranium deposits near Stanley; vein fluorspar 

deposits near Meyers Cove, Challis, and Stanley; mercury 

vein-replacement deposits in roof pendants near Yellow 

Pine; and many of the epithermal gold-silver vein deposits 

in the quadrangle.

The source of the metals for the extensive Tertiary 

mineralization is still an enigma. Did the Tertiary plutons, 

much as the Idaho batholith, simply provide a driving 

mechanism for convecting hydrothermal systems, or were 

the plutons themselves metalliferous? Geochemical 

evidence suggests that the Tertiary granites were enriched 

in beryllium, uranium, thorium, molybdenum, and tin 

(Bennett, 1980). However, as suggested by Howe and Hall 

(chap. P, this volume), the sulfur isotopes indicate a 

crustal source for the sulfur.

SPATIAL DISTRIBUTION OF 

SELECTED MINERAL COMMODITIES

Examination of the distribution of the major commodities shows certain definite spatial associations with 

major structural and stratigraphic features. These associations are discussed here by commodity, in order of 

fluorspar, mercury, gold, silver, molybdenum, zinc, lead, 

and copper.

Fluorspar

Fluorspar deposits are present in three major areas 

in the Challis quadrangle, the Meyers Cove, Stanley, and 

Challis-Bayhorse districts (fig. A9). Elsewhere throughout 

the quadrangle fluorspar is a common gangue mineral 

associated with both base- and precious-metal deposits.

In the Meyers Cove district (Anderson, 1943; Cox, 

1954), the fluorspar deposits are tabular veins paralleling 

fracture zones, with some stringers and lenses cutting 

obliquely across the fracture zones. The deposits form 

well-defined lodes along the fractures, generally with some 

minor replacement. Individual lodes may be 8 to 30 m 

long. The ore minerals are fluorite with or without minor 

stibnite in a gangue of barite, calcite, and chalcedony. The 

deposits are within a zone 1 km wide by 3 km long. Host 

rocks are the 47-m.y.-old tuffs of Camas Creek.

In the Stanley district (Choate, 1962; Tschanz and 

others, 1974) fluorspar is present in tabular zones containing veins and stringers; individual veins are as much 

as 1 m wide, and the composite width of veins and 

stringers is as much as 5 m. All of the deposits are 

associated with faults and shears and are spatially related 

to rhyolite dikes. The veins are mostly open-space fillings 

with minor replacement. Two main assemblages are

present: a quartz-pyrite-gold-fluorite assemblage, and 

simply quartz and fluorite. The host rock for all of the 

fluorspar deposits near Stanley is the Idaho batholith.

Fluorspar in the Bayhorse-Challis district is widely 

distributed in dolomitic rocks and is controlled in part 

by faults and in part by breccia zones (Snyder, 1978; 

Hobbs, chap. K, this volume). Two types of fluorspar 

deposits are present. The first type is fissure veins ranging from small veinlets measured in centimeters to zones 

several meters wide containing lodes tens of meters long; 

the second type is stratabound zones of breccia a few tens 

to several meters wide in a stratigraphic interval as much 

as 7 m thick within the Ordovician(?) Bayhorse Dolomite. 

The stratabound deposits have been interpreted as openspace filling of collapse breccia (Snyder, 1978), and alternatively as Mississippi Valley/Appalachian-type deposits 

modified by later igneous and tectonic events (Hoagland, 

1979). The mineralogy of both the vein and stratabound 

deposits is grossly similar; fluorite is present along with 

earlier formed base-metal sulfides and sulfosalts of silver 

and copper in a silica gangue.

Thus fluorspar is a fairly common gangue mineral 

in both base- and precious-metal deposits throughout the 

quadrangle, but, more important, the main fluorsparproducing deposits are spatially separate from the major 

base-metal deposits, as in the Meyers Cove and Stanley 

districts. In the Bayhorse district base metals are common 

but are markedly earlier than the fluorspar. Therefore, 

fluorspar mineralization may have occurred separately and 

later than the base- and precious-metal mineralization. 

This event was probably the result of the emanations of 

volatile elements late in the crystallization history of the 

Tertiary magmas.

Mercury

Mercury occurs in several areas in the quadrangle, 

including the Boulder Creek district where it is associated 

with base- and precious-metal deposits, and the Stanley 

area where trace amounts are associated with the uranium 

vein deposits (fig. A10). However, the only district containing mercury of economic significance is the Yellow 

Pine area (Bailey, 1964) where cinnabar occurs as veins, 

fracture coatings, and replacements in Precambrian rocks 

adjacent to the Idaho batholith in ring fractures of the 

Thunder Mountain caldera (Leonard, chap. H, this 

volume).

Gold

The location and production of gold mines in the 

quadrangle are shown on figure All. The Custer graben 

(Johnson and Mclntyre, 1983) and the Thunder Mountain caldera (Leonard, chap. H, this volume) have produced the bulk of the gold from the area. Gold is present

Summary of the Geology, Mineral Deposits and Resource Potential 15
0

'I I I f0 2040 KILOMETERS 

EXPLANATION

QUATERNARY ALLUVIUM Includes flood plain, terrace, 

and alluvial-fan deposits

TERTIARY INTRUSIVE VOLCANIC ROCKS Dikes, stocks, 

and batholiths of dioritic, granitic, or rhyolitic composition

TERTIARY EXTRUSIVE VOLCANIC ROCKS Tuffs, lavas, 

and volcanic sedimentary rocks of rhyolite, rhyodacite, 

dacite, and andesite

CRETACEOUS INTRUSIVE ROCKS OFTHE IDAHO BATHOLITH Tonalite, granodiorite, granite, muscovitebiotite granite, and leucocratic granite

PALEOZOIC SEDIMENTARY ROCKS Limestone, dolomite, 

shale, siltstone, argillite, quartzite, sandstone, and 

slate

MIDDLE PROTEROZOIC METAMORPHIC AND SEDIMENTARY ROCKS Argillite, slate, quartzite, sandstone, 

siltite, schist, and phyllite

ROCKS OF UNCERTAIN AGE Argillite, carbonate, quartzite, and phyllite

40 MILES

CONTACT

FAULT

THRUST FAULT Sawteeth on upper plate

CALDERA BOUNDARY

FLUORSPAR MINE OR PROSPECT

Figure A9. Fluorspar mines and prospects in the Challis quadrangle.

in several types of occurrences in a wide variety of geolog- are the Lucky Boy and General Custer mines in the Custer 

ical settings. The common deposit types (fig. A8) are: (1) graben, the Yellowjacket mine in the Yellowjacket district,

gold-silver veins containing subordinate base metals; these 

occurrences are numerous in the quadrangle, and examples

the Mammoth mine in the Grimes Pass district, and the 

Golden Day mine near Stanley; (2) stockwork veins and

16 Challis 1°x2° Quadrangle, Idaho
20

20 40 KILOMETERS 

EXPLANATION

QUATERNARY ALLUVIUM Includes flood plain, terrace, 

and alluvial-fan deposits

TERTIARY INTRUSIVE VOLCANIC ROCKS Dikes, stocks, 

and batholiths of dioritic, granitic, or rhyolitic composition

TERTIARY EXTRUSIVE VOLCANIC ROCKS Tuffs, lavas, 

and volcanic sedimentary rocks of rhyolite, rhyodacite, 

dacite, and andesite

CRETACEOUS INTRUSIVE ROCKS OF THE IDAHO BATHOLITH Tonalite, granodiorite, granite, muscovitebiotite granite, and leucocratic granite

PALEOZOIC SEDIMENTARY ROCKS Limestone, dolomite, 

shale, siltstone, argillite, quartzite, sandstone, and 

slate

MIDDLE PROTEROZOIC METAMORPHIC AND SEDIMENTARY ROCKS Argillite, slate, quartzite, sandstone, 

siltite, schist and phyllite

ROCKS OF UNCERTAIN AGE Argillite, carbonate, quartzite, and phyllite

40 MILES

CONTACT

FAULT

THRUST FAULT Sawteeth on upper plate

CALDERA BOUNDARY

MERCURY MINE OR PROSPECT

Figure A10. Mercury mines and prospects in the Challis quadrangle.

disseminated gold and silver in high-level rhyolites, such graben; (3) stratabound gold-silver replacement deposits 

as at the Sunbeam mine in the Custer graben, the Parker in volcanic rocks, the best example of which is the SunMountain and Snowshoe Creek deposits in the Twin Peaks nyside mine in the Thunder Mountain caldera; (4) 

caldera, and the Singheiser mine in the Panther Creek stratabound gold-silver replacement deposits in calderaSummary of the Geology, Mineral Deposits and Resource Potential 17
filling sediments, such as at the Dewey mine, also in the 

Thunder Mountain caldera; (5) vein-replacement deposits 

in argillized metasedimentary Precambrian rocks, such as 

the West End deposit in the Yellow Pine district; and (6) 

gold placer deposits, examples of which are the Yankee 

Fork placers in the Custer graben, the Loon Creek placers, 

and the Stanley Creek and Valley Creek placers near 

Stanley.

mine, replacement deposits in the Seafoam district, and 

the Hall-Interstate vein deposit (fig. A8).

Copper occurrences are widely scattered and appear 

to be associated with the Paleozoic sedimentary rocks and 

also with the Precambrian sedimentary rocks (fig. A16). 

The main producing mines were vein deposits in the 

Bayhorse district and the Hall-Interstate mine, and also 

vein replacement deposits in the Seafoam district.

Silver

As would be expected, the distribution of silver 

within the quadrangle strongly resembles that of gold 

(fig. A12). The main difference is in the relative proportions of the two metals in a given deposit. Like gold, silver 

occurs in a wide range of deposit types. In addition to 

the types previously mentioned for gold, two other significant occurrence types for silver are replacement deposits 

in carbonate rocks containing predominant silver and 

lead, the best example of which is the Clayton Silver mine; 

and stratabound syngenetic deposits in argillic rocks, examples of which are the Livingston mine and the 

vanadium-silver occurrences in the Salmon River assemblage in the Bayhorse district.

Molybdenum

Molybdenum is present in at least five types of occurrences in the Chain's quadrangle (figs. A8 and A13). 

These are: (1) Cretaceous porphyry deposits in 

granodioritic to granitic rocks at the Thompson Creek 

mine; (2) Cretaceous porphyry vein stockworks in 

sedimentary rocks at the Little Boulder Creek deposit; (3) 

skarn deposits with tungsten in roof pendants and adjacent to the Idaho batholith, such as the Virginia Beth occurrence; (4) Tertiary vein-fracture systems associated with 

dike swarms, such as the Little Falls deposit; and (5) 

disseminations and stockworks with gold and silver in 

high-level rhyolites.

Base metals

Zinc in the quadrangle (fig. A14) occurs in replacements in carbonate rocks, in veins, and in partly remobilized stratabound syngenetic deposits. The remobilized 

stratabound syngenetic deposits at the Livingston mine 

and the Hall-Interstate vein were the two largest producing mines.

Lead occurrences are somewhat more scattered than 

zinc but are strongly associated with the Paleozoic 

sedimentary rocks (fig. A15). The largest producing mines 

were the Ramshorn vein deposit in the Bayhorse district, 

the remobilized stratabound deposit at the Livingston

METAL PRODUCTION IN RELATION 

TO GEOLOGIC PROVINCES

The production of gold and silver from the major 

structural and stratigraphic provinces within the quadrangle is shown on figure A17. Production figures are plotted on a logarithmic scale, and the total amount produced 

is shown by the value at the top of each column. As can 

be seen, the Thunder Mountain caldera (TMC, fig. A17) 

has produced the most gold, followed by the Custer 

graben (CG), and then roof pendants (RP). The total 

amount of gold produced from volcanic sources within 

the Challis volcanic field, the first four columns on figure 

A17 totaled, is shown by the column labeled TV.

The production of silver shows a somewhat different 

pattern. Mines within the Paleozoic sedimentary rocks 

have produced by far the greatest amount of silver. The 

combined total of the four volcanic provinces, the Panther Creek graben, the Twin Peaks caldera, the Thunder 

Mountain caldera complex, and the Custer graben, is 

about 10 times less than the amount produced from the 

Paleozoic sedimentary rocks.

Production figures thus suggest that gold and lesser 

amounts of silver are associated with volcanic provinces, 

and silver with lesser amounts of gold are associated with 

the Paleozoic sedimentary rocks.

Production data for copper, lead, and zinc from the 

main geologic provinces in the quadrangle are shown on 

figure A18. Three patterns are obvious: first, the greatest 

production by far has come from the Paleozoic sedimentary rocks (PS), by several orders of magnitude; second, 

the roof pendants (RP) have produced significant 

amounts of base metals; and third, production of base 

metals from the volcanic provinces (PCG, TPC, TMC, 

and CG) is insignificant. The relative proportions of base 

metals produced from the volcanic provinces is interesting 

to note. In the Paleozoic rocks, production from greatest 

to least is lead, zinc, and copper; in the roof pendants the 

order is zinc, lead, and copper; and in the Precambrian 

sedimentary and metamorphic rocks the order is copper, 

lead, and zinc. Copper has been produced from all of the 

major volcanic provinces except the Twin Peaks caldera, 

lead from all but the Panther Creek graben and the Twin 

Peaks caldera, and zinc from all but the Panther Creek 

graben, Twin Peaks caldera, and Thunder Mountain 

caldera.

18 Challis 1°x2° Quadrangle, Idaho
] 20 40 MILES

I

. i i i 1 |

20 40 KILOMETERS 

EXPLANATION

QUATERNARY ALLUVIUM Includes flood plain, terrace, 

and alluvial-fan deposits

TERTIARY INTRUSIVE VOLCANIC ROCKS Dikes, stocks, 

and batholiths of dioritic, granitic, or rhyohtic composition

TERTIARY EXTRUSIVE VOLCANIC ROCKS luffs, lavas, 

and volcanic sedimentary rocks of rhyolite, rhyodacite, 

dacite, and andesite

CRETACEOUS INTRUSIVE ROCKS OF THE IDAHO BATHOLITH Tonalite, granodiorite, granite, muscovitebiotite granite, and leucocratic granite

PALEOZOIC SEDIMENTARY ROCKS Limestone, dolomite, 

shale, siltstone, argillite, quartzite, sandstone, and 

slate

MIDDLE PROTEROZOIC METAMORPHIC AND SEDIMENTARY ROCKS Argillite, slate, quartzite, sandstone, 

siltite, schist and phyllite

ROCKS OF UNCERTAIN AGE Argillite, carbonate, quartzite, and phyllite

 CONTACT

 FAULT

 - THRUST FAULT Sawteeth on upper plate

 CALDERA BOUNDARY

GOLD MINE SHOWING PRODUCTION IN OUNCES 

> 100,000 

10,000-100,000 

1000-10,000 

<1000

Figure All. Gold mines and prospects in the Challis quadrangle showing production to 1982.

RESOURCE POTENTIAL OF SELECTED 

COMMODITIES BASED ON 

RECENT GEOLOGIC STUDIES

In the following discussion the terms "moderate" 

and "high" resource potential are used as defined by

Taylor and Steven (1983). "A high mineral resource potential is deemed to exist where geologic, geochemical, and 

geophysical characteristics favorable for resource accumulation are known to be present, or where enough of 

these characteristics are present to give strong support to 

genetic models favorable for resource accumulation and

Summary of the Geology, Mineral Deposits and Resource Potential 19
'l I 'l I 1 ' I I 

20 40 KILOMETERS 

EXPLANATION

QUATERNARY ALLUVIUM Includes flood plain, terrace, 

and alluvial-fan deposits

TERTIARY INTRUSIVE VOLCANIC ROCKS Dikes, stocks, 

and batholiths of dioritic, granitic, or rhyolitic composition

TERTIARY EXTRUSIVE VOLCANIC ROCKS Tuffs, lavas,

and volcanic sedimentary rocks of rhyolite, rhyodacite,

dacite, and andesite 

CRETACEOUS INTRUSIVE ROCKS OF THE IDAHO BATHOLITH Tonalite, granodiorite, granite, muscovitebiotite granite, and leucocratic granite

PALEOZOIC SEDIMENTARY ROCKS Limestone, dolomite, 

shale, siltstone, argillite, quartzite, sandstone, and 

slate

MIDDLE PROTEROZOIC METAMORPHIC AND SEDIMENTARY ROCKS Argillite, slate, quartzite, sandstone, 

siltite, schist, and phyllite

ROCKS OF UNCERTAIN AGE Argillite, carbonate, quartzite, and phyllite

40 MILES

CONTACT

FAULT

THRUST FAULT Sawteeth on upper plate

CALDERA BOUNDARY

SILVER MINE SHOWING PRODUCTION IN OUNCES

> 1,000,000

100,000-1,000,000

10,000-100,000

< 10,000

Figure A12. Silver mines and prospects in the Challis quadrangle showing production to 1982.

where evidence shows that mineral concentration 

mineralization in the broad sense has taken place. * * * 

Moderate mineral resource potential exists where geologic, 

geochemical, and geophysical characteristics favorable for 

resource accumulation are known or can reasonably be

interpreted to be present but where evidence for mineralization is less clear cut or has not yet been found. A reasonable 

possibility for the discovery of valuable mineral deposits 

should exist in all areas rated as having moderate mineral 

resource potential" (Taylor and Steven, 1983,p. 126).

20 Challis 1°x2° Quadrangle, Idaho
20 40 MILES

20 40 KILOMETERS 

EXPLANATION

QUATERNARY ALLUVIUM Includes flood plain, terrace, 

and alluvial-fan deposits

TERTIARY INTRUSIVE VOLCANIC ROCKS Dikes, stocks, 

and batholiths of dioritic, granitic, or rhyolitic composition

TERTIARY EXTRUSIVE VOLCANIC ROCKS Tuffs, lavas,

and volcanic sedimentary rocks of rhyolite, rhyodacite,

dacite, and andesite 

CRETACEOUS INTRUSIVE ROCKS OF THE IDAHO BATHOLITH Tonalite, granodiorite, granite, muscovitebiotite granite, and leucocratic granite

PALEOZOIC SEDIMENTARY ROCKS Limestone, dolomite, 

shale, siltstone, argillite, quartzite, sandstone, and 

slate

MIDDLE PROTEROZOIC METAMORPHIC AND SEDIMENTARY ROCKS Argillite, slate, quartzite, sandstone, 

siltite, schist and phyllite

ROCKS OF UNCERTAIN AGE Argillite, carbonate, quartzite, and phyllite

CONTACT 

FAULT

-* * *- THRUST FAULT Sawteeth on upper plate

 CALDERA BOUNDARY

 MOLYBDENUM MINES OR PROSPECTS

Figure A13. Molybdenum mines and prospects in the Challis quadrangle.

Much of the area along the trans-Challis fault the southeastern side. Metals associated with the fault 

system (fig. A3) has moderate to high resource potential zone may be present in the rocks in a wide range of occurfor gold and silver. Gold and silver mines and prospects rence types, including epithermal gold-silver veins, highare closely associated with the fault zone, especially along level rhyolites, and disseminated gold-silver stockworks.

Summary of the Geology, Mineral Deposits and Resource Potential 21
20

..I ________

I I 

20 40 KILOMETERS

EXPLANATION

QUATERNARY ALLUVIUM Includes flood plain, terrace, 

and alluvial-fan deposits

TERTIARY INTRUSIVE VOLCANIC ROCKS Dikes, stocks, 

and batholiths of dioritic, granitic, or rhyolitic composition

TERTIARY EXTRUSIVE VOLCANIC ROCKS Tuffs, lavas, 

and volcanic sedimentary rocks of rhyolite, rhyodacite, 

dacite, and andesite

CRETACEOUS INTRUSIVE ROCKS OF THE IDAHO BATHOLITH Tonalite, granodiorite, granite, muscovitebiotite granite, and leucocratic granite

PALEOZOIC SEDIMENTARY ROCKS Limestone, dolomite, 

shale, siltstone, argillite, quartzite, sandstone, and 

slate

MIDDLE PROTEROZOIC METAMORPHIC AND SEDIMENTARY ROCKS Argillite, slate, quartzite, sandstone, 

siltite, schist, and phyllite

ROCKS OF UNCERTAIN AGE Argillite, carbonate, quartzite, and phyllite

40 MILES

CONTACT

FAULT

THRUST FAULT Sawteeth on upper plate

CALDERA BOUNDARY

ZINC MINE SHOWING PRODUCTION IN POUNDS 

>1,000,000

100,000-1,000,000 

10,000-100,000 

< 10,000

Figure A14. Zinc mines and prospects in the Challis quadrangle showing production to 1982.

Many areas in this zone are covered by forest, particular- yield information on the precious-metal potential of that 

ly in the north along the bounding faults of the Panther part of the fault zone.

Creek graben (figs. Al, A2). Biogeochemical techniques Within the Challis volcanic field, the newly mapped

using the wood of Douglas-fir trees (Erdman and others, bounding structures of the various calderas must be conchap. L, this volume; Leonard and Erdman, 1983) should sidered favorable areas for the discovery of additional

22 Challis 1°x2° Quadrangle, Idaho
o / ru 1 

Yellow/ >> 

Pine [-/

20

20 40 KILOMETERS 

EXPLANATION

QUATERNARY ALLUVIUM Includes flood plain, terrace, 

and alluvial-fan deposits

TERTIARY INTRUSIVE VOLCANIC ROCKS Dikes, stocks, 

and batholiths of dioritic, granitic, or rhyolitic composition

TERTIARY EXTRUSIVE VOLCANIC ROCKS Tuffs, lavas, 

and volcanic sedimentary rocks of rhyolite, rhyodacite, 

dacite, and andesite

CRETACEOUS INTRUSIVE ROCKS OF THE IDAHO BATHOLITH Tonalite, granodiorite, granite, muscovitebiotite granite, and leucocratic granite

PALEOZOIC SEDIMENTARY ROCKS Limestone, dolomite, 

shale, siltstone, argillite, quartzite, sandstone, and 

slate

MIDDLE PROTEROZOIC METAMORPHIC AND SEDIMENTARY ROCKS Argillite, slate, quartzite, sandstone, 

siltite, schist and phyllite

ROCKS OF UNCERTAIN AGE Argillite, carbonate, quartzite, and phyllite

40 MILES

CONTACT

FAULT

THRUST FAULT Sawteeth on upper plate

CALDERA BOUNDARY

LEAD MINES SHOWING PRODUCTION IN POUNDS

>1 ,000,000

100,000-1,000,000

10,000-100.000

<10,000

Figure A15. Lead mines and prospects in the Challis quadrangle showing production to 1982.

mineral resources; plumbing systems consisting of faults, these areas. New mapping (Hardyman, 1981) indicated

fracture zones, joints, and permeable rocks are well de- that the northwestern side of the Twin Peaks caldera is

veloped, and many of these systems have been mineral- favorable for gold, silver, and molybdenum resources, and

ized. Moderate to high resource potential is assigned to the central part contains zeolites along the 12-km length

Summary o! the Geology, Mineral Deposits and Resource Potential 23
20 40 KILOMETERS 

EXPLANATION

QUATERNARY ALLUVIUM Includes flood plain, terrace, 

and alluvial-fan deposits

TERTIARY INTRUSIVE VOLCANIC ROCKS Dikes, stocks, 

and batholiths of dioritic, granitic, or rhyolitic composition

TERTIARY EXTRUSIVE VOLCANIC ROCKS Tuffs, lavas, 

and volcanic sedimentary rocks of rhyolite, rhyodacite, 

dacite, and andesite

CRETACEOUS INTRUSIVE ROCKS OF THE IDAHO BATHOLITH Tonalite, granodiorite. granite, muscovitebiotite granite, and leucocratic granite

PALEOZOIC SEDIMENTARY ROCKS Limestone, dolomite, 

shale, siltstone, argillite, quartzite, sandstone, and 

slate

MIDDLE PROTEROZOIC METAMORPHIC AND SEDIMENTARY ROCKS Argillite, slate, quartzite, sandstone, 

siltite, schist and phyllite

ROCKS OF UNCERTAIN AGE Argillite, carbonate, quartzite, and phyllite

A A A

40 MILES

CONTACT

FAULT

THRUST FAULT Sawteeth on upper plate

CALDERA BOUNDARY

COPPER MINES SHOWING PRODUCTION IN POUNDS 

>100,000 

10,000-100,000 

1000-10,000 

<1000

Figure A16. Copper mines and prospects in the Challis quadrangle showing production to 1982.

of the medial fracture zone (Hardyman, chap. G, this 

volume). The southwestern and western sides of the 

Thunder Mountain caldera have a high potential for 

resources of gold, silver, tungsten, mercury, molybdenum,

and the rare-earth elements, particularly in silicified and 

argillized zones (Leonard, chap. H, this volume).

It is interesting to speculate about the roots of the 

high-level rhyolites. Mapping (fig. A2) showed that these

24 Challis 1°x2° Quadrangle, Idaho
106

105

LLJ 

O

Z

D

O

Z

z 10*

O

d13 

Q

O

DC

Q.

103

1O2

GOLD

CN 

CN

ID

CD

CO »

co in

CN CN

06 N

in

o>

00

I

i

'

 i

u

'1 i

CN 

O

co CD"

to

/

 =

'.

CN

O) 

CN 

CN

CN

CN

CN

s s

00

to

co

to

; !:

i u'r

106

105

10*

ma

SILVER

.._

.._

CO

CN

05 

 00

0)

co

co

8CN

O

CNin

5>

CD"

; U

 ' ^ *

: j§p

: ID

'. S*fc

f-x

! IS: :'

: «s ;

'c5-

.V*

>

/

/

co

CO

00 

00

in

CN

i 

co

1

(

GEOLOGIC

w' PCG TPC TMC CG TV PS PCS BB RP 

PROVINCE

Figure A17. Graph showing the production of gold arid silver from the main geologic provinces 

in the Challis quadrangle. Panther Creek graben (PCG), Twin Peaks caldera (TPC), Thunder Mountain caldera (TMC), Custer graben (CG), Paleozoic sedimentary rocks (PS), Precambrian sedimentary rocks (PCS), Boise Basin dike swarm (BB), roof pendants and skarns (RP), total (TV) from 

volcanic provinces (PCG+TPC+TMC+CG).

rhyolites (unit Tr) occur within the trans-Challis fault 

system, suggesting the likelihood of linear feeder systems. 

All of these high-level rhyolites contain anomalous 

amounts of molybdenum. Analogy by attribution suggests 

a comparison with the Little Falls molybdenum deposit 

(Rostad, 1966) in the Boise Basin dike swarm at the southern end of the trans-Challis fault system (fig. A3). Our 

assessment model is based on the likelihood of a Little 

Falls-type molybdenum deposit at depth beneath the highlevel rhyolites. This possibility makes the central part of 

the Van Horn Peak cauldron complex, which contains 

many high-level rhyolites (Hardyman and Fisher, chap. 

N, this volume), an area of moderate resource potential 

for molybdenum.

Sedimentary rocks filling the Thunder Mountain 

cauldron complex contain gold deposits at the Dewey mine 

(Leonard, chap. H, this volume). Mapping showed that 

thick sections of caldera-fill sediments are present near 

mineralized structures (Fisher, Mclntyre, and Johnson, 

1983). Moderate resource potential exists for preciousmetal deposits in the fill of the Panther Creek graben, 

Custer graben, Twin Peaks caldera, and the Van Horn 

Peak cauldron complex (figs. Al, A2).

Roof pendants of metasedimentary rocks along the 

eastern side of the Idaho batholith are considered to be 

favorable areas for the occurrence of tungsten deposits 

as discussed by Cookro (chap. Q, this volume), and the 

area has a high resource potential.

The greatest resource potential in the quadrangle is 

within Paleozoic sedimentary rocks. In the ChallisBayhorse region, horizons of solution breccia are present 

in the Bayhorse Dolomite (Snyder, 1978; Hobbs, chap. K, 

this volume). The entire Bayhorse Dolomite is a potential host for breccia-filling fluorspar deposits and has a 

high resource potential. Elsewhere in the Bayhorse district, 

fault structures mapped by Hobbs (chaps. D, K, this 

volume) near the margins of the Bayhorse anticline have 

a high potential for the occurrence of replacement baseand precious-metal deposits.

Ketner (1983) described mineralized rocks 

throughout large areas of the western United States that 

occur near the Ordovician-Silurian boundary. This time 

interval is represented in the Challis quadrangle by the 

upper units of the Saturday Mountain Formation, which 

are mineralized in the Bayhorse-Challis region. These units 

have a moderate resource potential for base metals.

Summary of the Geology, Mineral Deposits and Resource Potential 25
108

107

106

o

0105 

Q.

103

109

COPPER

00 Zl

108

107

106

104

108

LEAD

 'or 

M*>.

(D 

3

107

106

10!

10-*

103

ZINC

PCG TPC TMC CG PS PCS BB RP PCG TPC TMC CG PS PCS BB RP PCG TPC TMC CG PS PCS BB RP

GEOLOGIC PROVINCE

Figure A18. Graph showing the production of copper, lead, and zinc from the main geologic provinces in the Challis quadrangle. Panther Creek graben (PCG), Twin Peaks caldera (TPC), Thunder Mountain caldera (TMC), Custer graben (CG), Paleozoic 

sedimentary rocks (PS), Precambrian sedimentary rocks (PCS), Boise Basin dike swarm (BB), roof pendants and skarns (RP).

Finally, a high resource potential is assigned to black 

argillite and micritic limestone beds mapped and described 

by Hall (chap. J, this volume) in the Devonian Milligen 

Formation and the Paleozoic Salmon River assemblage. 

These rocks are particularly favorable for epigenetic vein 

replacement deposits in areas below regional thrust faults 

and near Cretaceous and Tertiary stocks. Possible deposit 

types include lead-silver-zinc vein deposits, lead-zincantimony-silver-tin vein deposits similar to Bolivian-type 

occurrences, skarn tungsten deposits, and porphyry 

molybdenum stockwork deposits. Stratabound syngenetic 

deposits of vanadium, silver, zinc, barite, and lead are also 

present in both the Milligen Formation and the Salmon 

River assemblage and may contain large resources.

REFERENCES CITED

Anderson, A. L., 1943, The antimony and fluorspar deposits 

near Meyers Cove, Lemhi County, Idaho: Idaho Bureau 

of Mines and Geology Pamphlet 62, 20 p.

_____1948, Role of the Idaho batholith during the Laramide 

orogeny: Economic Geology, v. 43, no. 2, p. 84-99.

Bailey, E. H., 1964, Mercury, in Mineral and water resources 

of Idaho: U.S. Congress, 88th, 2d session, Committee Print, 

p. 119-123.

Bennett, E. H., 1980, Granitic rocks of Tertiary age in the Idaho 

batholith and their relation to mineralization: Economic 

Geology, v. 75, no. 2, p. 278-288.

Callahan, J. E., Cooley, E. E, and Neuerburg, G. J., 1981a, 

Selected Mo and W concentrations from stream sediments 

and panned heavy-mineral concentrates, Challis, Idaho 2° 

quadrangle: U.S. Geological Survey Open-File Report 

81-1344, 9 p.

_____1981b, Using the minus 200 mesh fraction in regional 

stream sediment surveys for Mo-W results from the 

Thompson Creek drainage, Idaho [abs.]: Mining Engineering, v. 33, no. 10, p. 1523.

Choate, Raoul, 1962, Geology and ore deposits of the Stanley 

area: Idaho Bureau of Mines and Geology Pamphlet 126, 

122 p.

Cox, D. C., 1954, Fluorspar deposits near Meyers Cove, Lemhi

26 Challis 1°x2° Quadrangle, Idaho
County, Idaho: U.S. Geological Survey Bulletin 1015-A, 

21 p.

Dover, J. H., 1980, Status of the Antler orogeny in central 

Idaho clarification and constraints from the Pioneer 

Mountains, in Fouch, T. D., and Magathan, E. R., eds., 

Paleozoic paleogeography of the west-central United 

States: Society of Economic Paleontologists and 

Mineralogists, Rocky Mountain Section, Rocky Mountain paleogeography symposium 1, Denver, Colo., 1980, 

Proceedings, p. 1-15.

Evans, K. V., 1981, U-Th-Pb zircon geochronology of Proterozoic Y granitic intrusions in the Salmon area, eastcentral Idaho [abs.]: Geological Society of America 

Abstracts with Programs, v. 13, no. 4, p. 195.

Fisher, F. S., and May, G. D., 1983, Geochemical characteristics of the metalliferous Salmon River sequence, central Idaho: U.S. Geological Survey Open-File Report 

83-670, 28 p.

Fisher, F. S., May, G. D., Mclntyre, D. H., and Johnson, F. 

L., 1983, Mineral resource potential, geologic, and geochemical maps of part of the White Cloud-Boulder 

Roadless Area, Custer County, Idaho: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1580, scale 

1:62,500.

Fisher, F. S., Mclntyre, D. H., and Johnson, K. M., 1983, 

Geologic map of the Challis 1°X2° quadrangle, Idaho: 

U.S. Geological Survey Open-File Report 83-523, 39 p., 

2 oversize sheets, scale 1:250,000.

Hall, W. E., Rye, R. O., and Doe, B. R., 1978, Wood River 

mining district, Idaho intrusion-related lead-silver 

deposits derived from country rock source: U.S. Geological Survey Journal of Research, v. 6, no. 5, p. 

579-592.

Hardyman, R. F, 1981, Twin Peaks caldera of central Idaho, 

in Tucker, T. E., ed., Montana Geological Society Field 

Conference and Symposium Guidebook to southwest 

Montana: Billings, Mont., Montana Geological Society, p. 317-322.

Hoagland, A. D., 1979, Geology of the Bayhorse fluorite 

deposit, Custer County, Idaho A discussion: 

Economic Geology, v. 74, no. 1, p. 164-165.

Hughes, G. J., Jr., 1982, Basinal setting of the Idaho cobalt 

belt, Blackbird mining district, Lemhi County, Idaho, 

in The genesis of Rocky Mountain ore deposits 

Changes with time and tectonics: Denver Region Exploration Geologists Society Symposium, Denver, Colo., 

1982, Proceedings, p. 21-27.

Johnson, K. M., and Mclntyre, D. H., 1983, Disseminated 

gold-silver deposit in a rhyolite dome at the Sunbeam 

Mine, Custer County, Idaho [abs.]: Geological Society 

of America Abstracts with Programs, v. 15, no. 5, p. 324.

Ketner, K. B., 1983, Strata-bound, silver-bearing iron, lead, 

and zinc sulfide deposits in Silurian and Ordovician 

rocks of allochthonous terranes, Nevada and northern 

Mexico: U.S. Geological Survey Open-File Report 

83-792, 6 p.

Leonard, B. F., and Erdman, J. A., 1983, Preliminary 

report on geology, geochemistry, geochemical exploration, and biogeochemical exploration of the Red Mountain stockwork, Yellow Pine district, Valley County, 

Idaho: US. Geological Survey Open-File Report 83-151, 

49 p.

Mabey, D. R., 1982, Gravity and magnetic features along the 

eastern margin of the Idaho batholith in central Idaho: 

Geological Society of America Abstracts with Programs, v. 14, no. 6, p. 320.

Marvin, R. F, Tschanz, C. M., and Mehnert, H. H., 1973, 

Late Cretaceous age for molybdenite mineralization 

in Custer County, Idaho: Isochron/West, no. 7, p. 1.

Mclntyre, D. H., Ekren, E. B., and Hardyman, R. R, 1982 

[1984], Stratigraphic and structural framework of the 

Challis Volcanics in the eastern half of the Challis 

I°x2° quadrangle, Idaho, in Bonnichsen, Bill, and 

Breckenridge, R. M., eds., Cenozoic geology of Idaho: 

Idaho Bureau of Mines and Geology Bulletin 26, p. 

3-22.

Mclntyre, D. H., Hobbs, S. W, Marvin, R. F, and Mehnert, 

H. H., 1976, Late Cretaceous and Eocene ages for 

hydrothermal alteration and mineralization, Bayhorse district and vicinity, Custer County, Idaho: 

Isochron/West, no. 16, p. 11-12.

Mitchell, V. E., Strowd, W. B., Hustedde, G. S., and Bennett, E. H., 1981, Mines and prospects of the Challis 

quadrangle, Idaho: Moscow, Idaho, Idaho Bureau of 

Mines and Geology, Mines and Prospects Map Series, 

47 p.

Rostad, O. H., 1966, Geochemical case history at the Little 

Falls molybdenite prospect, Boise County, Idaho, in 

Cameron, E. M., ed., Proceedings, symposium on 

geochemical prospecting, Ottawa, 1966: Geological 

Survey of Canada Paper 66-54, p. 249-252.

Schmidt, D. L., and Mackin, J. H., 1970, Quaternary geology 

of Long and Bear Valleys, west-central Idaho: U.S. 

Geological Survey Bulletin 1311-A, 22 p.

Shacklette, H. T, and Erdman, J. A., 1982, Uranium in 

spring water and bryophytes of Basin Creek in central 

Idaho: Journal of Geochemical Exploration, v. 17, no. 

3, p. 221-236.

Skipp, Betty, and Hall, W. E., 1975, Structure and Paleozoic 

stratigraphy of a complex of thrust plates in the Fish 

Creek Reservoir area, south-central Idaho: U.S. Geological Survey Journal of Research, v. 3, no. 6, p. 671-689.

Skipp, Betty, Sando, W. J., and Hall, W. E., 1979, The 

Mississippian and Pennsylvanian (Carboniferous) 

systems in the United States Idaho: U.S. Geological 

Survey Professional Paper 1110-AA, 42 p.

Snyder, K. D., 1978, Geology of the Bayhorse fluorite deposit, 

Custer County, Idaho: Economic Geology, v. 73, no. 

2, p. 207-214.

Taylor, R. B., and Steven, T. A., 1983, Definition of mineral 

resource potential: Economic Geology, v. 78, no. 6, p. 

1268-1270.

NOTE. THIS COPY IS PART OF THE LIST OF 

REFERENCES CITED IN CHAPTER A AND 

SHOULD FALL BETWEEN PAGES 26 AND 27.

Summary of the Geology, Mineral Deposits and Resource Potential

TCchanz, C. M., Kiilsgaard, T. H., and Seeland, D. A., 1974, 1983: Washington, D.C., U.S. Government Printing Office,

Mineral resources of the eastern part of the Sawtooth 183 p.

National Recreation Area, Custer and Blaine Counties, Webring, M. W., and Mabey, D. R., 1981, Principal facts

Idaho: U.S. Geological Survey Open-File Report 74-1100, for gravity stations in the Challis, Idaho I°x2° quad314 p. rangle: U.S. Geological Survey OpenFile Report 81-652,

U.S. Bureau of Mines, [1983], Mineral commodity summaries 25 p.

Summary of the Geology, Mineral Deposits and Resource Potential 27

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter B

Plutonic Rocks of Cretaceous Age and Faults 

in the Atlanta Lobe of the Idaho Batholith, 

Challis Quadrangle

By THOR H. KIILSGAARD and REED S. LEWIS

CONTENTS

Abstract 31 

Introduction 31

The Cretaceous Atlanta lobe plutonic rocks 31 

Lithology 31 

Tonalite 31

Hornblende-biotite granodiorite 32 

Porphyritic granodiorite 32 

Roof pendants 33 

Biotite granodiorite 34 

Muscovite-biotite granite 35 

Leucocratic granite 35

Geochemical composition of Atlanta lobe rocks 36 

Age relations of Atlanta lobe rocks 36

Major regional faults in plutonic rocks of the Challis quadrangle 38 

Faults trending in a northerly direction 38 

Northwest-trending faults 38

Northeast-trending faults the trans-Challis fault system 40 

Effect of faults on emplacement of Tertiary plutonic rocks

and dikes 40 

Age of faulting 41 

References cited 41

Atlantic Lobe of the Idaho Batholith 29
FIGURES

Bl. Index map showing the Atlanta lobe of the Idaho batholith 31 

B2. Map of the western part of the Challis quadrangle showing rocks of

the Atlanta lobe and roof pendants 33 

B3. Triangular diagrams showing modal analyses of rocks of the Atlanta

lobe 34 

B4. Photograph of a leucocratic granite dike cutting porphyritic

granodiorite 35 

B5. Map of the western part of the Challis quadrangle, showing silicic rocks of

the Atlanta lobe 36 

B6. Map showing principal structural features 39

TABLES

Bl. Mean values of major-element oxides of Atlanta lobe plutonic rocks 37 

B2. Potassium-argon ages of Atlanta lobe plutonic rocks 37

30 Challis 1°x2° Quadrangle, Idaho
Abstract

Rocks making up the Atlanta lobe of the Idaho batholith, 

in the western part of the Challis quadrangle, include tonalite, 

hornblende-biotite granodiorite, porphyritic granodiorite, biotite 

granodiorite, muscovite-biotite granite, and leucocratic granite. 

The rocks were emplaced during the Late Cretaceous.

Tonalite, the oldest rock type, crops out near the western 

and eastern margins of the batholith. Closely associated with 

the tonalite are hornblende-biotite granodiorite and porphyritic 

granodiorite, the latter characterized by unusually large 

phenocrysts of potassium feldspar. Biotite granodiorite is 

younger, is the most common rock, and is widely exposed. 

Muscovite-biotite granite forms the core of the Atlanta lobe and 

is mineralogically similar to the leucocratic granite, which intrudes the other Cretaceous plutonic rocks.

The Cretaceous plutonic rocks are intensively faulted and 

intruded by numerous dikes and plutonic rocks of Tertiary age. 

Principal faults strike north, northwest, and northeast. Northwest-trending faults terminate against the trans-Challis fault 

system, a major set of northeast-trending faults that extends 

across the quadrangle and beyond.

INTRODUCTION

The western part of the Challis quadrangle is 

underlain by the Idaho batholith, an enormous body of 

granitic rock that underlies most of central Idaho. The 

batholith consists of two lobes, the northern, Bitterroot, 

lobe and the southern, Atlanta, lobe, which are separated 

by a narrow, northwest-trending belt of Precambrian 

metamorphic rocks, the Salmon River arch (Armstrong, 

1975a). Rocks of the Atlanta lobe were emplaced in Late 

Cretaceous time, were extensively faulted, and were intruded by Tertiary plutonic rocks and swarms of Tertiary 

dikes.

Although much has been written about various 

aspects of the Idaho batholith, very little mapping of the 

rock types that compose the Atlanta lobe (fig. Bl) has 

been done. Schmidt (1964), working in an area extending 

from near Cascade east to Warm Lake, in the northwestern 

part of the Challis quadrangle, separated Atlanta lobe 

rocks into four types. Anderson (1947) separated Tertiary 

plutonic rocks from older plutonic rocks of the Atlanta 

lobe in the area south of the South Fork Payette River, 

in the southwestern corner of the Challis quadrangle. Reid 

(1963) and Kiilsgaard and others (1970) mapped part of 

the.Sawtooth Range in the south-central part of the 

Challis quadrangle.

As part of the Challis quadrangle study, we mapped 

the southwestern quarter of the quadrangle to identify exposed rock types and geologic structures, and to appraise 

the mineral-resource potential of the area. We also used 

mapping of other Challis quadrangle workers: Earl Bennett and associates mapped 15-minute quadrangles north 

of the southwestern quarter of the Challis quadrangle, 

Wayne Hall mapped southeast of Stanley, and Paul Weis

50 MILES

I T

0 50 KILOMETERS

Figure Bl. Index map of central Idaho showing location of the 

Challis quadrangle with respect to the Atlanta lobe of the Idaho 

batholith. Cretaceous plutonic rocks exposed within the quadrangle are light shaded; Tertiary plutonic rocks are dark shaded.

mapped along the western margin of the Challis quadrangle (Fisher and others, 1983).

The part of the Atlanta lobe studied in the Challis 

quadrangle project consists generally of six rock types. 

The rock types are tonalite, hornblende-biotite granodiorite, porphyritic granodiorite, biotite granodiorite, 

muscovite-biotite granite, and leucocratic granite. Batholithic rocks exposed in a small area in the northwestern 

part of the quadrangle were not separated into specific 

types and are referred to on the geologic map of the quadrangle as mixed rocks (Fisher and others, 1983).

THE CRETACEOUS ATLANTA LOBE 

PLUTONIC ROCKS

Lithology

Tonalite

Tonalite grading to granodiorite is exposed along 

the western edge of the quadrangle and in the central part 

near the headwaters of Loon Creek (fig. B2). Smaller

Atlantic Lobe of the Idaho Batholith 31
exposures of tonalite and granodiorite cap parts of some 

of the high ridges north and northeast of Deadwood 

Reservoir and east of Stanley. The rock is gray to dark 

gray, medium to coarse grained, and equigranular to porphyritic. Andesine is the predominant mineral in the rock. 

As much as 20 percent biotite gives the rock a dark color 

and defines a foliation that varies from faint at some 

localities to intense at others. Variable amounts of hornblende occur in the rock, and yellow-brown sphene as long 

as 20 mm is common. Shredded flakes of secondary 

muscovite are in some altered feldspars. Apatite, magnetite, allanite, and zircon are the principal accessory 

minerals. Modal plots of 12 stained slabs and thin sections of the rock (fig. B3) show a mean composition of 

30 percent quartz, 6 percent potassium feldspar, and 64 

percent plagioclase.

The quartz diorite gneiss of Donnelly (Schmidt, 

1964) is included in the tonalite unit. Other exposures of 

quartz diorite and gneissic quartz diorite near the western 

boundary of the Atlanta lobe (Anderson, 1934b, 1952; 

Moore, 1959; Ross, 1963; Taubeneck, 1971) probably 

would have been classed as tonalite had the classification 

of Streckeisen (1973) been used (fig. B3). The gneissic 

character of tonalitic rocks in marginal facies of the 

batholith is discussed by Anderson (1942). Hyndman 

(1983) described the western border zone of the Atlanta 

lobe as being largely foliated tonalite or trondhjemite, in 

which steep foliation parallels the inferred country-rock 

contact.

Field evidence indicates that the tonalite is the oldest 

of the batholithic rocks. Xenoliths of the foliated tonalitic 

rock have been found in younger biotite granodiorite at 

several localities in the western and north-central parts 

of the quadrangle.

Hornblende-biotite granodiorite

Small, localized exposures of hornblende-biotite 

granodiorite (fig. B2) are on the high ridges on both sides 

of the upper reaches of the Deadwood River, and on high 

ridges to the east. The rock is characterized by hornblende, 

some of which occurs in such small crystals that it is 

difficult to recognize in hand specimen. The rock varies 

from gray to dark gray, depending on the biotite content, 

which ranges from less than 5 percent to about 20 percent and which defines a weak to strong foliation in the 

rock. Sphene is abundant and allanite is common. The 

study of 11 stained slabs and thin sections of the rock 

show that it has an average modal composition of 27 percent quartz, 20 percent potassium feldspar, and 53 percent plagioclase.

Some exposures of the rock were mapped as tonalite 

in the field, but subsequent microscopic study of the rock 

showed it to be granodiorite. Nevertheless, the mineral 

assemblage, foliation, and general appearance indicate 

that the rock is closely associated to the tonalite unit.

Porphyritic granodiorite

Although granitic rocks of the Idaho batholith are 

commonly porphyritic, containing feldspar phenocrysts 

as long as 15 mm (millimeters), only granodiorite containing unusually large phenocrysts (megacrysts) was 

mapped as porphyritic. The porphyritic granodiorite is 

shown as one unit on the geologic map of the Challis 

quadrangle (Fisher and others, 1983), but it may be subdivided into two varieties (fig. B2).

The most striking and by far the most extensive 

variety of porphyritic granodiorite is a coarsely porphyritic granitoid rock, characterized by large feldspar 

megacrysts as long as 10 cm (centimeters), which crops 

out in a northwest-trending belt that extends more than 

125 km (kilometers) across the quadrangle. The rock is 

dark gray, medium to coarse grained, typically foliated, 

and contains variable quantities of hornblende. The 

megacrysts are poikilitic microcline, generally pink but 

white in some places. They are euhedral in hand specimen; 

under the microscope the grain boundaries are irregular. 

Inclusions include anhedral embayed plagioclase and 

subordinate quartz and biotite. Carlsbad twinning is well 

developed in the megacrysts and clearly visible in outcrop. 

Plagioclase is anhedral, has a composition of An 23-37, 

shows weak normal or reverse zoning, has bent lamellae, 

and has undergone moderate to extreme sericitization. 

Epidote locally is present. Quartz is anhedral with welldeveloped undulatory extinction. The rock contains as 

much as 15 percent biotite that is pervasively altered to 

chlorite. Biotite defines the foliation. Hornblende is as 

much as 8 mm in maximum dimension, and in rare instances has cores of clinopyroxene. Sphene is the most 

common accessory mineral; other accessories include 

apatite, allanite, zircon, and opaque minerals.

The foliated, hornblende-bearing, porphyritic 

granodiorite contains many features of the previously 

described tonalite and hornblende-biotite granodiorite; it 

differs mainly in containing microcline megacrysts, the 

distribution of which are so variable that modal estimates 

are not consistent (fig. B3). The rock appears to grade 

into biotite granodiorite and hornblende-biotite 

granodiorite, but spatially it is mostly associated with the 

latter rock.

The other variety of porphyritic granodiorite (fig. 

B2) is light gray, contains less biotite, rarely contains hornblende and then only trace amounts, and the rare foliation, where seen, tends to be weak. The light-gray porphyritic granodiorite crops out chiefly on high ridges in 

the southwestern part of the quadrangle.

The origin of the microcline megacrysts is controversial. The similarity of megacrysts over large areas 

and the beltlike distribution and extent of the hornblendebearing, foliated, porphyritic granodiorite suggests that 

the megacrysts are of primary origin but formed late in 

the crystallization sequence. Most evidence, however,

32 Challis 1°x2° Quadrangle, Idaho
116° 114°30'

DARK-GRAY PORPHYRITIC 

GRANODIORITE

HORNBLENDE-BEARING 

GRANODIORITE

LIGHT-GRAY PORPHYRITIC 

GRANODIORITE

10 MILES

10 KILOMETERS

Figure B2. Map of the western part of the Challis quadrangle, showing hornblende-bearing and porphyritic rocks of the Atlanta 

lobe, and roof pendants.

indicates that they formed from potassium metasomatism. 

This origin is suggested by their poikilitic texture and 

embayed inclusions. More convincing evidence is 

numerous potassium feldspar-rich veinlets that cut the 

rock and appear to have been a source of potassium-rich 

fluid for the metasomatism. Megacrysts appear to have 

grown from the source veinlets and diminish in number 

away from the veinlets. Some megacrysts have grown 

across the veinlets, indicating that they grew after formation of the veinlets under subsolidus conditions (Lewis, 

1984). Along certain leucocratic granite dikes that have

intruded the porphyritic granodiorite, arrays of megacrysts 

are crowded along the dike contacts (fig. B4). The megacrysts appear to have grown most intensely near the dikes, 

and their number diminishes sharply away from the dike.

Roof pendants

Isolated roof pendants of metamorphosed sedimentary rocks trend discontinuously northwest across the 

quadrangle (fig. B2). Generally, they are near the eastern

Atlantic Lobe of the Idaho Batholith 33
Figure B3. Modal analyses for samples of plutonic rocks of the Atlanta lobe of the Idaho batholith, Challis quadrangle. Analyses 

normalized to 100 percent quartz (Q), potassium feldspar (A), and plagioclase (P). Open circles show mean values. A, rock classification from Streckeisen (1973); B, tonalite; C, porphyritic granodiorite; D, biotite granodiorite; E, muscovite-biotite (two-mica) granite; 

F, leucocratic granite.

limits of the Idaho batholith. Many are on ridge tops 

where they commonly are surrounded by hornblendebiotite granodiorite or by porphyritic granodiorite. Common rock types of the roof pendants include biotite schist, 

quartzite, carbonate-rich quartzite, quartz-diopside marble, and calc-silicate gneiss. In a few places where foliated 

granitic rocks could be seen in contact with schist, the 

foliation of the two rock types is conformable. The conformable contacts and the location of pendants on ridge 

tops suggest that the roof pendants mark the top or near 

top of the batholith.

Biotite granodiorite

Biotite granodiorite is the most common rock type 

of the Atlanta lobe, and it is exposed over a wide area 

(fig. B5). Included in the unit is the granodiorite of Gold

Fork (Schmidt, 1964), the quartz monzonite of Bear Valley 

(Swanberg and Blackwell, 1973), and part of the rock 

mapped by Anderson (1947) as quartz monzonite. The 

rock is light gray, medium to coarse grained, and 

equigranular to porphyritic. Plagioclase (oligoclase) is the 

chief component of the rock, with lesser quantities of 

quartz and potassium feldspar (fig. B3). Biotite averages 

about 5 percent of the rock, although locally it is more; 

it is pervasively altered to chlorite. Hornblende and 

muscovite are present only in trace amounts. Sericite, 

altered from feldspars, is widespread throughout the rock. 

Accessory minerals include minute quantities of sphene, 

allanite, zircon, monazite, and opaque minerals. Foliation 

is rare in the rock, commonly only near the contacts. The 

rock weathers readily, and fresh exposures, even on bluff 

faces, are hard to find. Most of the surface area is covered 

by grus.

34 Challis 1°x2° Quadrangle, Idaho
.\.<- 

Figure B4. Dike of leucocratic granite cutting porphyritic 

granodiorite. Note how megacrysts of potassium feldspar appear to have grown out of the dike. Away from the dike, the 

megacrysts decrease in number. Base of hammer is 4 cm wide. 

Photograph by Earl Bennett, Idaho Geological Survey.

Field relations indicate that the biotite granodiorite 

is younger than the tonalite and the hornblende-biotite 

granodiorite. In the western part of the quadrangle, biotite 

granodiorite was intruded into older tonalite, and in the 

60-90 m (meters)-wide contact zone it has primary foliation as well as xenoliths of tonalite. Xenoliths of hornblende-biotite granodiorite in biotite granodiorite are 

exposed on the high ridges east and west of the upper 

reaches of the Deadwood River south of Landmark 

(fig. B2).

Muscovite-biotite granite

Muscovite is widespread in the Idaho batholith 

(Larsen and Schmidt, 1958), but in granitic rocks of 

the Challis quadrangle it occurs principally as secondary 

muscovite formed from alteration of feldspar minerals. 

In the western part of the quadrangle, however, there is 

an irregular, north-trending belt of granite grading to 

granodiorite that contains notable quantities of primary 

muscovite. The quartz monzonite of Warm Lake 

(Schmidt, 1964) is part of this unit. We mapped this rock 

as muscovite-biotite (two-mica) granite (fig. 5B), using 

laminated plates or books of muscovite large enough to 

see in hand specimen as the principal field criterion. We 

consider this rock to be the core of the Atlanta lobe.

Muscovite-biotite granite is light gray, medium to 

coarse grained, equigranular to porphyritic rock that consists chiefly of oligoclase, quartz, and potassium feldspar. 

Modal values show a range in composition of the rock, 

the mean value within the granite field but very close

to the granodiorite field (fig. B3). Muscovite makes up 

as much as 5 percent of the rock but probably averages 

about 2 percent in the specimens that were studied. The 

biotite content averages less than 5 percent. Other 

characteristics of the rock are local occurrences of small 

crystals of garnet, the absence of hornblende, and the rare 

occurrence of sphene. The rock is nonfoliated.

Muscovite-biotite granite is transitional with biotite 

granodiorite through a zone as wide as 2 km or more. At 

one locality near the western boundary of the quadrangle, 

the muscovite-biotite granite has an aphanitic texture in 

a border zone about 7 m thick at the contact with biotite 

granodiorite. The texture could have been caused by intrusion into cooler biotite granodiorite. The location of 

the muscovite-biotite granite in the central part of the 

batholith (fig. B5) and the transitional contact of the rock 

with biotite granodiorite also suggests that muscovitebiotite granite is younger.

Leucocratic granite

Leucocratic granite is light gray to nearly white, is 

fine to medium grained, and has a distinctive anhedral 

texture. It consists of roughly equal amounts of quartz, 

plagioclase, and potassium feldspar (fig. B3). The 

plagioclase (An 25-30) commonly is altered to sericite, 

as is the potassium feldspar, which is chiefly perthitic 

microcline. Biotite may constitute as much as 2 percent 

of the rock and generally is extensively altered to chlorite. 

Garnets as large as 2 mm in diameter are locally present, 

as are minute amounts of muscovite. Magnetite is a common accessory mineral, but sphene was found only rarely. 

Hornblende was not found in the rock.

Leucocratic granite, described as leucocratic quartz 

monzonite by Reid (1963), as aplite by Cater and others 

(1973), and as aplitic quartz monzonite by Anderson 

(1947), occurs as dikes, sill-like masses, and irregular 

stocks that are resistant to erosion and tend to form high 

points on ridges. Rubbly weathered scree from the high 

exposures may be extensive on lower slopes and may give 

the impression of being derived from larger exposures than 

actually exist. Exposures of larger masses of the rock are 

in the southwestern part of the quadrangle on high ridges 

in upthrown blocks between regional faults. Near Tenmile 

Creek, west of the Sawtooth Range, the basal contact of 

a large sill-like mass at least 790 m thick dips east at 6-8° 

(Kiilsgaard, 1983).

The leucocratic granite intrudes biotite granodiorite, 

commonly along joints, which indicates that the biotitegranodiorite was solidified prior to intrusion. Leucocratic 

granite, in turn, is intruded by plutonic rocks of Eocene 

age. The relation of the leucocratic granite to muscovitebiotite granite is not clear. The two rocks are comparable 

mineralogically and chemically, differing largely in texture 

and grain size. Leucocratic granite may represent a more 

rapidly cooled variety of the muscovite-biotite granite.

Atlantic Lobe of the Idaho Batholith 35
114°30'

MUSCOVITE-BIOTITE GRANITE 

BIOTITE GRANODIORITE

LEUCOCRATIC GRANITE 

FAULT

lit

o 10 MILES

0 10 KILOMETERS

figure B5. Map of the western part of the Challis quadrangle, showing the more silicic rocks of the Atlanta lobe.

Geochemical composition of Atlanta lobe rocks

Major-element oxide analyses of 65 samples taken 

across the southwestern quarter of the Challis quadrangle 

are shown in table Bl. The six rock types listed in the table 

are paired by mineralogical and chemical similarities.

Tonalitic rocks on the west side of the quadrangle 

are low in SiO2 and K2O and high in A12O3 , Fe2O3 , MgO, 

CaO, TiO2, and P2O5 with respect to leucocratic granite 

in the east (table Bl, figs. B2, B5). The increase in SiO2 

and K2O content from older tonalite to younger leucocratic granite, the postulated age differences of which are 

based on field relations, supports the findings of Tilling

(1973, 1974) in the Boulder batholith of Montana. SiO2 

and K2O contents do not increase to the east across the 

batholith, however, as tonalite and hornblende-bearing 

porphyritic granodiorite are exposed in eastern parts of 

the batholith, and the geochemical content of those rocks 

is comparable to that of similar rocks on the western side 

(table Bl).

Age relations of Atlanta lobe rocks

Radiometric dates obtained from a wide range of 

plutonic rocks of the Atlanta lobe show a range in age from

36 Challis 1°x2° Quadrangle, Idaho
Table B1. Mean values of major-element oxides of plutonic rocks of the Atlanta lobe, Challis quadrangle

[Values in weight percent. Analytical determination by X-ray fluorescence. Analysts: J. E. Taggart, J. S. Wahlberg, A. J. Bartel, J. D. Baker, L.

L. Jackson, G. R. Mason, D. B. Hatfield, F. E. Lichte, and H. G. Neiman, U.S. Geological Survey. <, less than]

Rock type

Leucocratic granite 

Muscovite-biotite

granite.

Biotite

granodiorite.

Light-gray

porphyritic

granodiorite.

Hornblende-biotite

granodiorite.

Tona 1 ite 

No. of 

Samples

11

12

22

6

3

11

Si02

75.3

73.8

71.9

71.4

66.5

64.0

A1 2°3

14.0

14.6

15.2

15.3

15.5

16.9

^e

0

1

1

3

4

2°3

.73

.92

.55

.66

.79

.40

MgO

0.1

.3

.4

.5

1.70

1.6

CaO

0.78

1.41

1.91

2.46

3.98

4.64

Na20

4.3

3.8

4.1

3.95

3.32

3.7

K20

4.11

3.47

3.30

2.76

2.97

2.36

Ti02

0.05

.10

.21

.25

.63

.77

P2 05 MnO

<0.1 <0.02

<.l .03

<.l .02

.07 .03

.21 .07

.26 .07

iron reported as Fe20-j.

135 m.y. (million years) (Larsen and others, 1958) to 43 

m.y. (McDowell and Kulp, 1969). This spread in reported 

ages has been narrowed by the efforts of subsequent 

workers. Armstrong (1975b) pointed out that some of the 

earlier uranium-lead dates are too high. Geologic mapping 

has shown that some plutonic rocks previously considered 

as components of the Atlanta lobe are younger (Reid, 

1963; Kiilsgaard and others, 1970). Potassium-argon 

dating of these younger intrusive rocks show them to be 

of Eocene age (Armstrong, 1975b; Criss, 1980; Kiilsgaard, 

1983; Bennett and Knowles, chap. F, this volume). Vast 

areas of the Atlanta lobe have been affected by widespread 

Eocene plutonism that reset the potassium-argon system 

in older batholithic rocks (McDowell and Kulp, 1969; 

Criss and others, 1982). Armstrong (1975b) has summarized data on age determinations of Idaho rocks, and Armstrong and others (1977) noted that the Atlanta lobe of 

the Idaho batholith was emplaced during the culmination of Cretaceous igneous activity about 75 to 100 m.y. 

ago. The oldest ages have been obtained from rocks along 

the western and eastern edges of the Atlanta lobe and the 

youngest ages from rocks in the central part (Armstrong 

and others, 1977; Criss and others, 1982).

In the Challis quadrangle, Cretaceous plutonic rocks 

of the Atlanta lobe may be grouped into three categories, 

each of which appears to have been emplaced at a different time.

1. A batholithic border facies of foliated tonalite grading 

to hornblende-bearing granodiorite, xenoliths of which 

are in contiguous biotite granodiorite and on high 

ridges across the lobe in the west-central part of the 

quadrangle.

2. A major central part of the batholith that consists of 

biotite granodiorite in transitional contact with muscovite-biotite granite that forms the core of the batholith. 

Crosscutting relationships of these two rock types have 

not been found. The core location, transitional contact, and potassium-argon ages (table B2) suggest that 

the muscovite-biotite granite may be slightly younger.

Table B2. Potassium-argon ages of Cretaceous plutonic rocks 

of the Atlanta lobe, Challis quadrangle

[Dated by R. F. Marvin, U.S. Geological Survey; except for four samples 

dated by R. J. Fleck, U.S. Geological Survey]

Rock type

Muscovite-biotite granite 

Tonalite and porphyritic

granodiorite.

No. of

Samples

6

8

10

6

Age

(m.y.)

72-64

75-66

QO (LQ

73-97

3. Stocks and dikes of leucocratic granite that intrude 

all older rocks. Leucocratic granite crosscuts pegmatite dikes in biotite granodiorite, which indicates that 

it was emplaced after crystallization of the biotite 

granodiorite. Dikes of leucocratic granite along joints 

in biotite granodiorite also indicate solidification of 

the latter rock prior to dike intrusion.

The range in age of the rock types (table B2) indicates overlapping time of emplacement, although the 

time span for the different types is in general agreement 

with geologic mapping results. Three of the leucocratic 

granite samples are from hydrothermally altered areas, and 

biotite from the samples gave ages ranging from 64 to 66 

m.y, ages that may be too young due to argon loss. Biotite 

from the other three leucocratic granite samples gave an 

age of about 72 m.y., an age that may be more representative of the emplacement age of the rock. Muscovitebiotite pairs in six samples of muscovite-biotite granite 

gave concordant ages. Only biotite was dated in the 10 

biotite granodiorite samples. Biotite also was dated in the 

six tonalite and porphyritic granodiorite samples.

Zircon from a sample of porphyritic granodiorite 

taken at milepost 201, U.S. Highway 75 east of Stanley, 

lat 114°50' N., long 44°20' E., was analyzed by Lynn 

Fischer, U.S. Geological Survey, and the uranium-lead age 

obtained was 88 ±6 m.y. This zircon age agrees with a 

potassium-argon age of 87.7±2.7 m.y. obtained from

Atlantic Lobe of the Idaho Batholith 37
hornblende collected at the same locality. Biotite from this 

locality gave a potassium-argon age of 79.8 ±2.7 m.y. The 

zircon age also agrees with a potassium-argon age of 90 ±3 

m.y. obtained from hornblende from quartz diorite collected near Donnelly on the western side of the Idaho 

batholith (Armstrong, 1975b). Field evidence and radiometric ages indicate that the oldest plutonic rocks are in 

the western and eastern border zones of the Atlanta lobe.

MAJOR REGIONAL FAULTS IN PLUTONIC ROCKS 

OF THE CHALLIS QUADRANGLE

The plutonic rocks of the Atlanta lobe are extensively faulted. Displacement along faults commonly cannot be measured because of the absence of horizon 

markers; thus, some estimates are based on topographic 

relief. Some regional faults extend across the Challis quadrangle, and appear to have guided emplacement of Tertiary plutonic rocks and dikes. In a general way, the 

regional faults may be grouped in three sets: northerly 

trending, northwest trending, and northeast trending.

Faults trending in a northerly direction

A set of major northerly trending faults crosses the 

western part of the Challis quadrangle (Fisher and others, 

1983). Two of these faults cross the upper reaches of 

Scriver Creek north of Crouch; the western one, the Middle Fork Payette fault, extends north along the Middle 

Fork Payette River, and the eastern one, the South Fork 

Salmon fault, extends along the South Fork Salmon River 

(fig. B6). Surface traces of both faults are clearly displayed 

on LANDSAT imagery and are shown on the ground surface by alignment of topographic features and local exposures of sheared and crushed rocks. Muscovite-biotite 

granite has been offset .by left-lateral movement along 

both faults (fig. B5).

The north-trending Boise Ridge fault (fig. B6) was 

identified by Anderson (1947), the location determined 

from the scarp along the east side of Boise Ridge and the 

displacement of well-defined datum planes, including 

basalt flows of the Miocene Columbia River Group. The 

fault displacement proposed by Anderson was checked 

by examining the 110-m-thick exposure of west-dipping 

basalt that caps Hawley Mountain 2.5 km southwest of 

the southwestern corner of the Challis quadrangle and 

west of the fault. Exposures of basalt east of the fault 

also were examined. Normal magnetic polarity measurements were obtained from most of the basalt test sites 

on both the eastern and western sides of the fault. Samples 

of basalt from different exposures on both sides of the 

fault were analyzed in the laboratory of Peter Hooper, 

Washington State University, Pullman. Analytical data 

resemble those of the Weiser Basalt, which overlies the

Grande Ronde Basalt of the Columbia River Basalt 

Group, (Hooper, oral commun., 1985). The Grande Ronde 

Basalt is no older than 16.5 m.y. (Hooper, 1982). Magnetic 

polarity and chemical data indicate that the same basalt 

is on both sides of the fault. Accepting the base of the 

basalt as a datum plane, measurements show that basalt 

east of the fault has been downdropped a minimum of 

490 m with respect to basalt west of the fault. The age 

of initial movement along the fault cannot be determined 

from the data on hand, but displacement of the basalt 

cannot be older than 16.5 m.y.

The Boise Ridge fault enters Garden Valley east of 

Crouch and probably extends north along the valley of 

the Middle Fork Payette River, although near Crouch the 

fault is concealed by alluvium. North of Crouch, sedimentary rocks of the Payette Formation terminate against the 

fault. Floral assemblages, collected from the probable upper part of the Payette, are of middle to early late Miocene 

age (W. C. Rember, University of Idaho, written commun., 

1983) or about 13 m.y. old. The sedimentary rocks have 

been tilted west by fault movement, and as they terminate 

against the fault, displacement along the fault can be no 

older than about 13 m.y., a fault-displacement age that 

is probably the same for the basalt displacement east of 

Hawley Mountain.

The Deadwood fault zone (fig. B6) is well exposed 

along the west side of Deadwood River, north from its 

junction with the South Fork Payette River. To the north, 

near Deadwood Reservoir, the fault splits into several 

strands that merge near the Deadwood mine and continue 

north along Johnson Creek in Deadwood canyon (Fisher 

and others, 1983). The fault zone has a distinct 

topographic expression; the west side is upthrown. Foliated 

hornblende-biotite granodiorite east of the fault zone and 

near the Deadwood mine is about 500 m lower than 

similar rock on the high ridge west of the fault, which 

suggests fault displacement of at least that much.

Northwest-trending faults

Three major northwest-trending faults, Sawtooth, 

Bear River, and Deer Park (fig. B6), are in the southern 

part of the Challis quadrangle and terminate against the 

trans-Challis fault system.

The Sawtooth fault extends more than 60 km along 

the eastern side of the Sawtooth Range. Throughout most 

of its projected length, the fault is masked by glacial 

moraine or alluvium, but it is recognized by the linear 

front of the range, truncated ridges, the towering rise of 

the Sawtooth Range above Sawtooth Valley, and a linear 

gradient in the aeromagnetic pattern (Kiilsgaard and 

others, 1970, pi. 1; Mabey and Webring, chap. E, this 

volume, fig. E5). South of the Challis quadrangle, along 

the western side of the Salmon River, the fault juxtaposes 

Paleozoic metasedimentary rocks and Eocene Challis

38 Challis 1°x2° Quadrangle, Idaho
EXPLANATION

FAULT Bar and ball 

on downthrown side; 

arrows show lateral 

movement

10 20 KILOMETERS

Figure B6. Map of the Challis quadrangle, showing principal faults, grabens, cauldron complexes, and caldera.

Volcanics. Minimum displacement along the fault was 

estimated by Reid (1963) as 1,200 ft (feet), the upthrown 

side to the west.

The Bear River fault extends from the North Fork 

Boise River to the South Fork Payette River, about 22 km 

(Kiilsgaard, 1983). It is best exposed on the ridge northwest of Bear River, where it forms a broad zone, tens of 

meters wide, of crushed, sheared, and intensely altered 

granite of Tertiary age (fig. M3, chap. M, this volume), 

and southeast of Bear River where Tertiary granite is 

displaced against biotite granodiorite of the Atlanta lobe. 

The Bear River fault may be the offset segment of the 

Montezuma fault, which extends more than 40 km along 

the western flank of the Sawtooth Range south of the 

Challis quadrangle. The Montezuma fault was first 

mapped near Atlanta by Anderson (1939), who estimated 

vertical displacement along the fault to be as much as 

2,000 ft, the downdropped side to the southwest. The main 

part of the Sawtooth Range was considered by Reid (1963) 

to be a horst bounded on the west by the Montezuma fault 

and on the east by the Sawtooth fault. The Montezuma

fault has been mapped northwest to the North Fork Boise 

River, where it may terminate or be offset to the southwest, 

continuing as the Bear River fault (fig. M3).

The Deer Park fault is generally parallel to the 

Sawtooth and Bear River-Montezuma faults (figs. M3, B6) 

and extends at least 40 km southeast of the southern 

boundary of the Challis quadrangle. Anderson (1934a) 

traced the fault southeast from the South Fork Payette 

River chiefly by the displacement of an old summit erosion surface. We do not think that the fault reaches the 

South Fork Payette River but terminates about 1.5 km 

southeast of the river against a strand of the trans-Challis 

fault system (Fisher and others, 1983). Southeast of the 

southern boundary of the Challis quadrangle, the fault 

scarp forms steep slopes along the southwestern sides of 

Goat Mountain and Swanholm Peak. Vertical displacement in that locality, based on topographic relief, is at 

least 2,600 ft, the downdropped block on the southwestern 

side (Kiilsgaard, 1983).

A significant aspect of the northwest-trending 

faults, Sawtooth, Bear River-Montezuma, and Deer Park,

Atlantic Lobe of the Idaho Batholith 39
is that although they have strong topographic expression 

and extend over considerable strike-length distances, they 

terminate at the trans-Challis fault system. They do not 

appear to have been truncated and displaced by the transChallis fault system, as no evidence of them has been 

found northwest of the fault system. The Sawtooth Range 

appears to be a large upthrust block bounded on the east 

by the Sawtooth fault, on the west by the MontezumaBear River faults, and on the northwest by the transChallis fault system. The topographic linearity of the 

northwest-trending faults and the vertical displacement 

along them suggests that they are deep faults, and initial 

movement along them may predate the Idaho batholith. 

Measurable displacement along them, however, postdates 

the Idaho batholith. Tertiary gravel is displaced along the 

Montezuma fault, which suggests post-Pleistocene fault 

movement. Steep gradients along the North Fork Boise 

River and Bear River across the scarp of the Deer Park 

fault (fig. M3) also suggest recent fault movement. Movement quite likely has recurred along the faults up to the 

present time. Recurrence is suggested by displacement 

along a similar northwest-trending fault during the Idaho 

earthquake of Oct. 28, 1983. That fault extends along the 

western base of the Lost River Range, and appears to be 

a member of the above-described northwest-trending set 

of faults.

Northeast-trending faults the trans-Challis 

fault system

A broad zone of northeast-trending high-angle 

regional faults crosses the southwestern quarter of the 

Challis quadrangle and aligns with the Panther Creek 

graben (fig. B6). Only near the Van Horn Peak cauldron 

complex (Ekren, chap. C, this volume) is there a break 

in fault continuity, and even there the Custer graben, Twin 

Peaks caldera, and part of the Van Horn Peak cauldron 

complex are parallel to and presumably part of the same 

tectonic feature. The faults and grabens make up a 

regional fault system herein named the trans-Challis fault 

system.

The trans-Challis fault system extends beyond the 

Challis quadrangle. Faults of the system extend southwest 

of the southern boundary of the quadrangle to beyond 

Idaho City (fig. M5), where at least one of the faults passes 

beneath the Holocene and Pleistocene Snake River Group. 

The Panther Creek fault, part of the system, has been 

mapped more than 30 km northeast of the northern 

boundary; thus the length of the fault system in the Challis 

quadrangle region is at least 220 km. The fault system is 

very well aligned and appears to be continuous with the 

Great Falls lineament, as proposed by O'Neill and Lopez 

(1983), who projected the lineament northeast from Idaho 

across Montana. The trans-Challis fault system therefore 

appears to be part of a regional tectonic feature.

In the southwestern quarter of the Challis quadrangle, the trans-Challis fault system is best exposed in 

an area about 30 km west-northwest of Stanley, along the 

upper reaches of Warm Springs Creek and on the divides 

at the head of Eightmile Creek and the East Fork of Eightmile Creek (Luthy, 1981; Fisher and others, 1983). In that 

area are several northeast-striking faults characterized by 

white to tan, nonresistant fault gouge in bleached and 

altered granitic rocks; these sheared zones commonly are 

devoid of vegetation. Some zones of bleached and crushed 

rock are more than 500 m wide, and within them the rocks 

are so pulverized that few fragments are more than 30 cm 

in diameter. Rocks in the more intensely sheared zones 

are so altered that their original composition is difficult 

to determine. Dike rocks in the sheared zones are not as 

altered as the granitic host rock, but they generally are 

broken by faults, which indicates recurrent movement 

along the fault zones.

Faults of the trans-Challis fault system in the central and northeastern part of the Challis quadrangle are 

discussed by Bennett and Knowles (chap. F, this volume), 

Mclntyre and Johnson (chap. I, this volume), Hardyman 

(chap. G, this volume), and Ekren (chap. C, this volume).

Effect of faults on emplacement of Tertiary plutonic rocks 

and dikes

The trans-Challis fault system has guided emplacement of Tertiary dikes and Tertiary plutonic intrusive 

rocks. This guidance is particularly evident in the northern part of Boise Basin and along the South Fork Payette 

River (fig. M5). The dike swarm in Boise Basin trends 

more or less parallel to major faults of the trans-Challis 

fault system, although dikes within the swarm strike in 

various directions. The dikes range from diabase to 

rhyolite in composition, most are porphyritic, and they 

range from 0.5 to 60 m thick and to as long as 2 km. A 

dioritic stock in the northern part of Boise Basin reaches 

widths of more than 1.5 km and is more than 16 km long 

(fig. M5). Biotite from a similar plutonic stock within the 

trans-Challis fault system (fig. M5) gave a potassiumargon age of 49.2 m.y. (Kiilsgaard and Bennett, chap. M, 

this volume), and zircon from a rhyolite dike at the Little 

Falls molybdenum deposit (fig. M5) gave a fission-track 

age of 29.3 m.y. (F. S. Fisher, oral commun., 1983).

Pink granite, intrusive into biotite granodiorite of 

the Idaho batholith, is exposed along the trans-Challis 

fault system in an area about 30 km west-northwest of 

Stanley. The granite is lithologically comparable to Eocene 

pink granite exposed in the Sawtooth Range and is considered to be of the same age. The exposure of granite 

is elongated along the trans-Challis trend and is in part 

fault bounded, but at the northeast and southwest ends, 

it is displaced along northeast-trending faults. Swarms of 

rhyolite and rhyodacite dikes, elongated along the transChallis system, intrude the pink granite and the older

40 Challis 1°x2° Quadrangle, Idaho
biotite granodiorite to the southwest and northeast. Near 

the north end of Copper Mountain, in the Knapp Creek 

graben (fig. B6), pink granite intrudes overlying intrusive 

rhyolite in what appears to be an instance where a shallow 

batholith has intruded its own hypabyssal-equivalent rock. 

Hornblende from the pink granite gave a potassium-argon 

age of 45.3±0.4 m.y. (R. J. Fleck, oral commun., 1983). 

Intrusion of the rhyolite by pink granite is indicated by 

(1) a chilled zone of fine-grained pink granite as much 

as 9 m thick beneath the contact; (2) an uneven pink 

granite-rhyolite contact, with pink granite protruding as 

much as 30 m into the rhyolite, a small dikelet of pink 

granite cutting the rhyolite, and foundered blocks of the 

overlying rhyolite engulfed in the pink granite; (3) the 

rather flat roof contact, which may be traced along a canyon wall for almost 1 km beneath the overlying rhyolite. 

The intrusive character of the rhyolite is proven by the 

way in which it cuts across the bedding of metasedimentary roof pendants and by the low-angle contact it makes 

with overlying biotite granodiorite of the Atlanta lobe that 

forms the top of Copper Mountain. Xenoliths of biotite 

granodiorite are in the intrusive rhyolite.

To the northeast, still within the Knapp Creek 

graben, the intrusive rhyolite is in unconformable contact 

with extrusive volcanic rocks ranging from flow-banded 

rhyolite to rhyolitic lapilli tuff and vitrophyre. The rhyolitic rocks are characterized by phenocrysts of sanidine. 

Well-developed flow banding is defined by alternating 1-2 

mm reddish-white laminations composed of spherulites 

(Lewis, 1984). A fossilized wood fragment was found in 

the tuffaceous rocks. Sanidine from the extrusive rhyolite 

gave a potassium-argon age of 39.0 m.y. (R. J. Fleck, oral 

commun., 1984).

The extrusive rhyolitic rocks form a northeast-trending ridge about 10 km long, which rises more than 470 

m above flanking valley floors, yet extrusive rocks are not 

found beyond the flanking valleys. There is no surface 

evidence that suggests a northeast-trending valley that 

might have been filled by extrusive rocks, the remnants 

of which account for the present ridge. Instead, the rocks 

are preserved in the northeast-trending Knapp Creek 

graben. The graben block apparently has been tilted to 

the northeast because the volcanic rocks in the northeastern part of the graben give way to the shallow intrusive 

rhyolite and pink granite exposed to the southwest.

Vertical displacement along the trans-Challis fault 

system was not measured as there are no horizon markers. 

There are few topographic breaks along the faults such 

as the scarps along the northwest-trending faults and some 

of the north-trending faults. Left-lateral displacement 

along faults of the trans-Challis system is indicated at 

several localities. Along the northwestern side of the 

Knapp Creek graben, Eocene granite appears to be offset 2.9 km (Fisher and others, 1983). Southwest of the 

Knapp Creek graben, an exposure of Tertiary granite is

offset about 800 m by left-lateral displacement along one 

of the strands of the trans-Challis fault system (Luthy, 

1981). Left-lateral movement along a trans-Challis fault 

south-southwest of the Knapp Creek graben has displaced 

Tertiary granite about 535 m.

Age of faulting

The age of initial movement along the trans-Challis 

fault system is obscure, but evidence indicates that adjustment along the system could have recurred from Precambrian to Holocene time. In the southwestern quarter of the 

Challis quadrangle the indicated timespan of faulting, 

based on field evidence, is rather short. Fault movement 

displaced Cretaceous plutonic rocks of the Atlanta lobe 

and guided emplacement of Eocene granitic rocks and 

younger dikes. Subsequent movement along the faults 

displaced the Eocene granitic rocks and the dikes. Many 

Eocene rhyolites of the Challis Volcanics in the northeastern part of the quadrangle are elongated along the 

trans-Challis system (Fisher and others, 1983), and their 

emplacement obviously has been guided by faults of the 

system. Holocene fault movement has disrupted stream terraces that contain glacial erratics of presumable Pleistocene 

age. The densest distribution of recent earthquake epicenters in Idaho is in the Challis quadrangle (Roy Breckenridge, Idaho Geological Survey, written commun., 1983), 

most of them plotting within the trans-Challis fault system.

Near the northern boundary of the Challis quadrangle the Middle Proterozoic Yellowjacket Formation is 

displaced against Eocene volcanic rocks of the Challis 

Volcanics in the Panther Creek graben, but about 16 km 

farther northeast the Panther Creek fault offsets the contact between Yellowjacket Formation and 1.4-b.y. (billion 

years)-old plutonic rock (Lopez, 1981). At that locality, the 

Panther Creek fault is marked by tight isoclinal folds in 

the Yellowjacket Formation, and truncation of them by the 

1.4-b.y-old plutonic rocks indicates Precambrian movement 

along the fault (J. M. O'Neill and D. A. Lopez, written 

commun., 1984).

The remarkable linearity of the trans-Challis fault 

system for at least 220 km in the Challis quadrangle vicinity, the grabens and calderas within the system, and the 

linearity of intrusive and extrusive rocks along the system 

all indicate crustal extension and deep faulting and rifting 

that may have existed before intrusion of the Idaho batholith. Fault-movement evidence now visible in the quadrangle 

represents recurrent adjustment along faults of the system.

REFERENCES CITED

Anderson, A. L., 1934a, A preliminary report on recent block 

faulting in Idaho: Northwest Science, v. 8, no. 2, p. 17-28.

_____1934b, Geology of the Pearl-Horseshoe Bend gold belt, 

Idaho: Idaho Bureau of Mines and Geology Pamphlet 41,

36 p.

Atlantic Lobe of the Idaho Batholith 41
__1939, Geology and ore deposits of the Atlanta district, 

Elmore County, Idaho: Idaho Bureau of Mines and 

Geology Pamphlet 49, 71 p.

__1942, Endomorphism of the Idaho batholith: Geological Society of America Bulletin, v. 53, no. 8, p. 1099-1126.

__1947, Geology and ore deposits of the Boise Basin, 

Idaho: U.S. Geological Survey Bulletin 944-C, 319 p.

__1952, Multiple emplacement of the Idaho batholith:

Journal of Geology, v. 60, no. 3, p. 255-265.

Armstrong, R. L., 1975a, Precambrian (1500 m.y. old) rocks of 

central Idaho The Salmon River arch and its role in Cordilleran sedimentation and tectonics: American Journal of 

Science, v. 275-A, p. 437-467.

_____1975b, The geochronometry of Idaho: Isochron/West, 

no. 14, 51 p.

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

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

and their Sr isotopic composition, Oregon, Washington, 

and Idaho; Geological Society of America Bulletin, v. 88, 

p. 397-411.

Cater, F. W., Pinckney, D. M., Hamilton, W. B., Parker, R. L., 

Weldin, R. D., Close, T. J., and Zilka, N. T, 1973, Mineral 

resources of the Idaho Primitive Area and vicinity, Idaho: 

U.S. Geological Survey Bulletin 1304, 431 p.

Criss, R. E., 1980, An 18O/ 16O, D/H and K-Ar study of the 

southern half of the Idaho batholith: Pasadena, California Institute of Technology Ph.D. dissertation.

Criss, R. E., Lanphere, M. A., and Taylor, H. P., Jr., 1982, Effects of regional uplift, deformation, and meteorichydrothermal metamorphism on K-Ar ages of biotites in 

the southern half of the Idaho batholith: Journal of 

Geophysical Research, Sen B, v. 87, no. 8, p. 7029-7046.

Fisher, F. S., Mclntyre, D. H., and Johnson, K. M., 1983, 

Geologic map of the Challis 1°X 2° quadrangle, Idaho: U.S. 

Geological Survey Open-File Report 83-523, 41 p., 2 maps, 

scale 1:250,000.

Hooper, P. R., 1982, The Columbia River basalts: Science, v. 

215, no. 4539, p. 1463-1468.

Hyndman, D. W., 1983, The Idaho batholith and associated 

plutons, Idaho and western Montana: Geological Society 

of America Memoir 159, p. 213-240.

Kiilsgaard, T. H., 1983, Geologic map of the Ten Mile West 

Roadless Area, Boise and Elmore Counties, Idaho: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-1500-A, scale 1:62, 500.

Kiilsgaard, T. H., Freeman, V. L., and Coffman, J. S., 1970, 

Mineral resources of the Sawtooth Primitive Area, Idaho: 

U.S. Geological Survey Bulletin 1319-D, p. D1-D174.

Larsen, E. S., Jr., Gottfried, D., Jaffe, H. W, and Waring, C. 

L., 1958, Lead-alpha ages of the Mesozoic batholiths of 

western North America: U.S. Geological Survey Bulletin 

1070-B, p. 35-62.

Larsen, E. S., Jr. and Schmidt, R. G., 1958, A reconnaissance 

of the Idaho batholith and comparison with the southern 

California batholith: U.S. Geological Survey Bulletin 

1070-A, 32 p.

Lewis, R. S., 1984, Geology of the Cape Horn quadrangle, southcentral Idaho: University of Washington M.S. thesis, 91 p.

Lopez, D. A., 1981, Stratigraphy of the Yellowjacket Formation 

of east-central Idaho: U.S. Geological Survey Open-File 

Report 81-1088, 219 p.

Luthy, S. T, 1981, The petrology of Cretaceous and Tertiary intrusive rocks of the Red Mountain-Bull Trout Point area, 

Boise, Custer, and Valley Counties, Idaho: Missoula, 

University of Montana M.A. thesis.

McDowell, F. W, and Kulp, J. L., 1969, Potassium-argon dating 

of the Idaho batholith: Geological Society of America 

Bulletin, v. 80, no. 11, p. 2379-2382.

Moore, J. G., 1959, The quartz diorite boundary line in the 

western United States: Journal of Geology, v. 67, no. 2, 

p. 198-210.

O'Neill, J. M., and Lopez, D. A., 1983, Great Falls lineament, 

Idaho and Montana [abs.]: American Association of 

Petroleum Geologists Bulletin, v. 67, no. 8, p. 1350-1351.

Reid, R. R., 1963, Reconnaissance geology of the Sawtooth 

Range: Idaho Bureau of Mines and Geology Pamphlet 129, 

37 p.

Ross, C. P., 1963, Evolution of ideas relative to the Idaho 

batholith: Northwest Science, v. 37, no. 2, p. 45-60.

Schmidt, D. L., 1964, Reconnaissance petrographic cross 

section of the Idaho batholith in Adams and Valley 

Counties, Idaho: U.S. Geological Survey Bulletin 1181-C, 

50 p.

Streckeisen, A. L., 1973, Plutonic rocks Classification and 

nomenclature recommended by the IUGS Subcommission 

on the Systematics of Igneous Rocks: Geotimes, v. 18, 

p. 26-30.

Swanberg, C. A., and Blackwell, D. D., 1973, Areal distribution and geophysical significance of heat generation in the 

Idaho batholith and adjacent intrusions in eastern Oregon 

and western Montana: Geological Society of America 

Bulletin, v. 84, no. 4, p. 1261-1282.

Taubeneck, W. H., 1971, Idaho batholith and its southern extension: Geological Society of America Bulletin, v. 82, 

no. 3, p. 1899-1928.

Tilling, R. L, 1973, Boulder batholith, Montana A product of 

two contemporaneous but chemically distinct magma 

series: Geological Society of America Bulletin, v. 84, no. 

12, p. 3879-3899.

_____1974, Composition and time relations of plutonic and 

associated volcanic rocks, Boulder batholith region, Montana: Geological Society of America Bulletin, v. 85, no. 

12, p. 1925-1931.

42 Challis 1°x2° Quadrangle, Idaho
Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter C

Eocene Cauldron-Related 

Volcanic Events in the 

Challis Quadrangle

By E. B. EKREN

CONTENTS

Abstract 45 

Introduction 45

Stratigraphy and sequence of Eocene volcanic events 45 

Van Horn Peak cauldron complex and adjacent areas 45 

Intermediate-composition and mafic lavas 45 

Cauldron-related rocks 48

Tuff of Corral Creek 48

Tuff of Ellis Creek 48

Tuff of Eightmile Creek and tuffs of Camas Creek and Black 

Mountain 49

Tuffs in the Van Horn Peak vent 50

Quartz-biotite tuff 50

Tuff of Castle Rock 53

Tuff of Challis Creek 53 

Thunder Mountain cauldron complex 53 

Chemical composition of the Eocene volcanic rocks 57 

References cited 58

Eocene Cauldron-Related Volcanic Events 43
FIGURES

Cl. Map showing areas of Eocene volcanic rocks 46

C2. Map showing structural and volcanic features of the eastern part of the

Challis quadrangle 47 

C3. Histograms showing amounts of and components of phenocrysts in

volcanic rocks of the Van Horn Peak cauldron complex 48 

C4. Map showing principal structural and volcanic features in the northeastern

part of the Challis quadrangle 49

C5. Photograph showing mudflow in the Corral Creek cauldron segment 50 

C6. Generalized geologic map of the Camas Creek area 51 

C7. Photographs showing the tuff of Ellis Creek 52 

C8. Photographs showing rocks of the Van Horn Peak tuff vent 52 

C9. Photograph showing the tuff units making up Castle Rock 53 

CIO. Alkali-lime diagram for analyses of rocks from the Thunder Mountain

and Van Horn Peak cauldron complexes 56 

Cll. AFM diagram of volcanic rocks from the Van Horn Peak and Thunder

Mountain cauldron complexes 57 

C12. Classification of principal volcanic rocks of the Challis quadrangle using

peralkaline-peraluminous-metaluminous divisions of Shand 57

TABLES

Cl. Major rock units of the Van Horn Peak cauldron complex 49 

C2. Chemical analyses and norms for rocks from the Thunder Mountain 

cauldron complex 54

44 Challis 1°x2° Quadrangle, Idaho
Abstract

Volcanism started in the Challis quadrangle about 51 m.y. 

(million years) ago when intermediate-composition lavas were 

erupted from numerous vents in the eastern and north-central 

parts of the quadrangle. Succeeding voluminous eruptions of 

tuff caused two cauldron complexes to form. The easternmost, 

termed the Van Horn Peak cauldron complex, began with the 

eruption of the 48.4 m.y.-old rhyodacite tuff of Ellis Creek, 

following minor activity in the Corral Creek segment. Cauldron 

formation ended with the eruption of the 45-47 m.y.-old rhyolite 

tuff of Challis Creek. The north-central complex, termed the 

Thunder Mountain cauldron complex, also started with the 

eruption of a rhyodacite tuff about 47-48 m.y. ago and ended 

with the eruption of a 45-46 m.y.-old rhyolite tuff informally 

called the Sunnyside rhyolite. The two cauldron complexes lie 

on the southeastern and northwestern flanks of the granitic 

Casto pluton and probably reflect the evolution of separate 

cupolas on the rising magma body that formed the pluton.

INTRODUCTION

This chapter briefly describes the principal volcanic 

events of Eocene age in the Challis quadrangle, stressing 

the relationships between the principal ash-flow tuffs and 

the various cauldrons or cauldron segments that formed 

as the result of these tuff eruptions. More detailed lithologic descriptions of the volcanic units, especially the 

rocks that are not associated with cauldrons, are in 

Mclntyre and others (1982) and Fisher and others (1983). 

Descriptions of the granitic rocks of Eocene age are in 

the chapter by Bennett and Knowles (chap. F, this 

volume).

The Challis Volcanics of Eocene age were first 

named and described by Ross (1933, 1934), who noted the 

variety of rock types and thick sections of volcanic rocks 

preserved locally in central Idaho (fig. Cl); the concept 

of calderas was poorly understood at that time. Calderas 

were first recognized at Thunder Mountain by B. F. 

Leonard (in Cater and others, 1973, p. 45-51). The present study, done under the Conterminous United States 

Mineral Assessment Program (CUSMAP), has provided 

a more complete understanding of the Thunder Mountain cauldron complex and led to the recognition of the 

Van Horn Peak cauldron complex and two volcanicrelated graben east of the Thunder Mountain area.

The term "cauldron complex" as used in this 

chapter, denotes a volcanic terrane within which many 

related cauldrons occur. Although the cauldrons originally 

were basin-shaped topographic depressions (calderas) 

bounded by arcuate faults, subsequent events have greatly modified them.

Prior to deep erosion, the outflow sheets from the 

calderas probably covered large areas surrounding their 

sources. Outflow from the Challis volcanic field may have 

overlapped Eocene volcanic fields in adjacent states (fig. 

Cl), which in turn overlapped extensive fields in southern

British Columbia (for example, see Souther, 1970). The 

area affected by this Eocene volcanic episode was huge, 

and the volume of volcanic rock extruded was immense. 

The cause of this volcanic activity, however, is debated. 

According to Lipman and others (1972) and Lipman 

(1983), the ultimate cause was the subduction of the 

Farallon plate beneath the North American continent. Fox 

(1983), on the other hand, believed that the ultimate cause 

was intracontinental rifting triggered by collision of the 

Pacific plate with the North American plates.

Northwest- and northeast-trending faults existed 

prior to volcanism (Mclntyre and others, 1982), as indicated by regional geologic mapping for the CUSMAP 

study of the Challis quadrangle. More than 1,000 ft (feet) 

(300 m (meters)) of structural and topographic relief 

existed along some northwest- and north-striking normal 

faults in the southeastern part of the quadrangle (D. H. 

Mclntyre, oral commun., 1983; Hays and others, 1978). 

The rugged topography of the prevolcanic surface was 

recognized by Ross (1937). Furthermore, along the northern boundary of the quadrangle, several northwest and 

northeast-striking faults are present that are primarily 

strike-slip faults; lateral displacements along these faults 

also largely predate the earliest volcanic activity.

STRATIGRAPHY AND SEQUENCE OF EOCENE 

VOLCANIC EVENTS

The volcanic rocks are preserved mostly in the 

eastern half and north-central part of the quadrangle. The 

preserved remnants in the eastern half are mostly within 

the Van Horn Peak cauldron complex, the Panther Creek 

graben, and the Custer graben (fig. C2). In addition, a 

considerable thickness of intermediate-composition lavas 

and remnants of outflow tuff sheets are preserved outside of these structures. The volcanic rocks in the northcentral part, on the other hand, are preserved only within 

the Thunder Mountain cauldron complex (fig. C5) and 

an early segment of the Van Horn Peak cauldron complex that extended into the Thunder Mountain region.

Except for the earliest volcanic eruptions, the two 

cauldron complexes and their eruptive volcanic centers 

were independently active, producing .a series of distinctive ash-flow tuff sheets.

VAN HORN PEAK CAULDRON COMPLEX AND 

ADJACENT AREAS

Intermediate-composition and mafic lavas

Volcanic activity started about 51 m.y. ago with the 

eruption of intermediate and mafic lavas from numerous 

vents (fig. C2) scattered throughout a large part of the eastern half of the quadrangle (Mclntyre and others, 1982).

Eocene Cauldron-Related Volcanic Events 45
CANADA 

UNITED STATES

MONTANA

WYOMING

150

EOCENE MARINE AND SUBAERIAL BASALTS AND SEDIMENTARY ROCKS Allochthonous

300 KILOMETERS 

EXPLANATION

EOCENE SUBAERIAL TUFFS AND 

LAVAS Autochthonous

300 MILES

_J

STRUCTURAL CONTACT Generalized from Fox (1983)

Figure C1. Map of the northwestern United States showing areas within which remnants of volcanic rocks of Eocene age 

are preserved.

The eruption of these lavas continued for about 2 m.y. 

and was accompanied by only minor eruptions of tuff. 

Most of the intermediate lava is dacite and rhyodacite in 

composition, but the lava sequences commonly contain 

potassium-rich basalt, andesite, latite, and locally quartz 

latite.

The dacite and rhyodacite rocks typically contain 

30 percent conspicuous phenocrysts (fig. C3) consisting 

of plagioclase, 3-10 mm long, and various combinations 

of mafic phenocrysts including biotite, hornblende, 

clinopyroxene, orthopyroxene, and altered olivine. Quartz 

phenocrysts are locally present.

The potassium-rich basalt and andesite commonly 

occur together, and both are rich in K2O and MgO 

(Mclntyre and others, 1982). They are mostly aphanitic 

gray rocks that weather brownish gray and black. They 

commonly contain altered olivine and xenocrystic quartz 

with reaction rims.

Potassium-rich basalt, andesite, and latite lavas are 

present in several of the older lava sequences that are 

about 51 m.y. to about 49 m.y. old. These lavas also form 

a regionally extensive and volumetrically large younger 

stratigraphic unit as much as 400 m thick erupted about 

48 m.y. ago, after the initial subsidence of the Van Horn 

Peak cauldron complex. Both the older and younger lavas 

form tabular flows of considerable areal extent. These 

flows were obviously less viscous than the dacite and 

rhyodacite and appear to have been fed from long, narrow dikes and from plugs.

In both stratigraphic horizons the potassium-rich 

basalt, andesite, and latite flows are megascopically and 

microscopically very similar. Most flows are brown and 

reddish brown, weathering gray to black. Most contain 

only sparse phenocrysts of clinopyroxene that locally are 

as long as 1 cm. More rarely, they contain hornblende in 

addition to clinopyroxene and also minor plagioclase.

46 Challis 1°x2° Quadrangle, Idaho
115° 114°

&

WALL OF CAULDRON OR CALDERA Bar and£| 

ball on downthrown side. Dotted where ob-»-Sf , 

literated by Casto pluton lT(

FAULT Dashed where approximately located; ^~v'\ 

dotted where concealed. Bar and ball on down- :i*it 

thrown side; arrows show relative movement yjj

BROAD ZONE OF FAULTS e^Jv

SOURCE VENTS FOR INTERMEDIATE-COMPO- J^T^gy 

SITION AND MAFIC ROCKS ^*>Pi §^£

fi\ y-£

%# ' M £ -6--.!' 0 10 MILES

I 

10 KILOMETERS

*s -* ' ^ ,vr s-^>w^{ * *- * *^< * c*

;\\* ^'f

44°

Figure C2. Map of the eastern part of the Challis quadrangle showing vents for intermediate-composition and mafic 

volcanic rocks and other structural and volcanic features. Predominant volcanic-rock compositions also shown. 

Generalized from Mclntyre and others (1982). Base from U.S. Geological Survey, Idaho state map, 1968, scale 

500,000.

Eocene Cauldron-Related Volcanic Events 47
EXPLANATION

QUARTZ

ALKALI FELDSPAR

PLAGIOCLASE

BIOTITE

PYROXENE

HORNBLENDE

TOTAL PHENOCRYSTS

100

TUFFS OF

VAN HORN PEAK

AND 60

u

TABLE MOUNTAIN u. 4Q

Q.

20

TUFFS OF 

CAMAS CREEK

AND 

BLACK MOUNTAIN

Section at 

Camas Creek

3000 m. 

or more thick

VAN HORN PEAK 

Outer ring

Lower

Lower

100-

TUFF OF 

ELLIS CREEK

Section at

Morgan Creek

200 m. or

more thick

Age 48.4 m.y.

LAVAS OF 

INTERMEDIATE 

COMPOSITION

Age 51 -48.6 m.y.

VAN HORN PEAK 

Inner core

Middle

Middle

TABLE 

MOUNTAIN

Upper

Upper

Figure C3. Volume percent of phenocrysts and abundance of six phenocryst mineral components of principal volcanic rocks 

in the Van Horn Peak cauldron complex, central Idaho.

Mclntyre and others (1982) reported that some black 

glassy latite flows contain conspicuous altered olivine.

Cauldron-related rocks 

Tuff of Corral Creek

The first significant ash-flow unit erupted from the 

Van Horn Peak cauldron complex was a dacite or 

rhyodacite tuff rich in lithic fragments called the tuff of 

Corral Creek (table Cl). This tuff is intercalated with 

dacite, latite, and minor potassium-rich andesite flows and 

is older than the tuff of Ellis Creek, which is associated 

with the major collapse of the cauldron complex. The tuff 

of Corral Creek contains as much as 50 percent lithic 

fragments of dacite and latite. It is generally densely

welded and contains about 10 percent phenocrysts of 

plagioclase, 5 percent phenocrysts of hypersthene, and 

sparse hornblende. The eruption of this tuff caused the 

Corral Creek cauldron segment, in the northeastern part 

of the Van Horn Peak cauldron complex (fig. C4), to collapse along pre-existing northeast- and northwest-striking 

faults. The collapsed block rapidly filled with mudflows 

and lava flows of dacite and latite (fig. C5).

Tuff of Ellis Creek

The rhyodacite tuff of Ellis Creek, erupted about 

48.4 m.y. ago, was, before erosion, the most voluminous 

volcanic unit in the Challis quadrangle. Some remnants 

of the outflow sheet are more than 300 m thick, and the 

unit is at least 1,500 m thick where preserved within the 

cauldron complex (figs. C4, C6). Remnants of the unit

48 Challis 1°x2° Quadrangle, Idaho
Table CI. Major rock units of the Van Horn Peak cauldron 

complex, Challis quadrangle 

[Leaders (--), no K-Ar age]

Unit Lithology

K-Ar 

age (m.y.)

Tuff of Challis Creek 

(caldera forming and 

filling).

Tuff of Castle Rock 

(caldera filling).

Quartz-biotite tuff 

(caldera forming).

Tuff of Table Mountain

Tuffs of Camas Creek 

and Black Mountain 

(caldera filling).

Tuff of Ellis Creek 

(caldera forming).

Tuff of Corral Creek

Alkali rhyoliteRhyo 1 i te 

Rhyolite and 

quartz latite.

Quartz latite and 

rhyolite.

Quartz latite and 

rhyodacite.

Rhyodacite - 

Rhyodacite 

45-46.5

48.4

are preserved as far as 25-35 km both south and east of 

the cauldron-complex boundary. The outflow sheet has 

been eroded from other areas, but if it once extended 30 

km in all directions from the cauldron complex, it would 

have had an original volume on the order of at least 400 

km3 (cubic kilometers). Of course, this figure does not 

include the uncertain but surely very large volume of tuff 

trapped within the cauldron complex itself. The eruption

of this enormous ash-flow sheet is inferred to have caused 

collapse of a northwest-trending, roughly elliptical area 

about 70 km long and 40 km wide (fig. C4). The subsided 

source area of the cauldron is now in part occupied by 

the Casto pluton. The intracauldron tuff on both sides 

of the granite pluton tends to be darker colored than the 

outflow sheet, because of abundant magnetite dust, and 

it contains ubiquitous epidote. In some places within the 

cauldron complex, however, the tuff is virtually the same 

color as the outflow sheet (fig. C7).

The tuff of Ellis Creek is all rhyodacite. There is 

no obvious mineralogic change from base to top, although 

there are more phenocrysts at the top (fig. C3). Pumice 

is typically darker than the matrix, and the pumice tends 

to contain larger plagioclase phenocrysts than the matrix. 

In places the plagioclase in the pumice is as long as 1 cm, 

whereas the plagioclase in the matrix rarely exceeds 4 or 

5 mm. Quartz phenocrysts are sparse but distinctive 

because they are so strongly resorbed. Biotite is the principal mafic mineral; hornblende is present everywhere and 

in some exposures is nearly as abundant as biotite (fig. 

C3). Trace amounts of clinopyroxene were observed in a 

few thin sections.

Tuff of Eightmile Creek and tuffs of Camas Creek and Black 

Mountain

Following the eruption of the voluminous tuff of 

Ellis Creek, a series of eruptions produced ash flows that

EXPLANATION

BIOTITE GRANITE OF CASTO 

PLUTON

AREA INFERRED TO HAVE SUBSIDED AS A RESULT OF EXTRUSIONS OF TUFF OF ELLIS CREEK

CAULDRON-BOUNDARY FAULTOR 

NORMAL FAULT WITH LARGE 

DISPLACEMENT Bar and ball 

on downthrown side; dotted 

where concealed or projected

WELDED-TUFF VENT AT VAN HORN 

PEAK

T

20 KILOMETERS

Figure C4. Map of the northeastern part of the Challis quadrangle showing principal structural and volcanic features. Shaded 

area is inferred cauldron that originated with the voluminous eruptions of the tuff of Ellis Creek.

Eocene Cauldron-Related Volcanic Events 49
Figure C5. Mud flow or lahar in Corral Creek cauldron segment. These flows were cold because they engulfed standing 

trees that are now petrified and show little evidence of charring.

filled the subsiding cauldron complex. The earliest of these 

is the tuff of Eightmile Creek (Mclntyre and others, 1982). 

This unit is megascopically very similar to the Ellis Creek 

but contains alkali feldspar, more quartz, and less biotite 

and hornblende. The tuff of Eightmile Creek is thin in 

outflow sections where it is generally mapped with other 

volcanic units, but, in the northern part of the Custer 

graben (Foster, 1982) (fig. C4) and in the Meyers Cove area 

(fig. C6) in the north-central part of the Van Horn Peak 

cauldron complex, it is 600-800 m thick. The large 

thicknesses in these two localities suggest that an elongate 

area, extending northward through the northern Custer 

graben to Meyers Cove and perhaps as far to the north 

as the northern boundary of the Van Horn Peak cauldron 

complex, probably subsided during the eruption of the 

tuff of Eightmile Creek.

The eruption of the tuff of Eightmile Creek was 

followed, with little or no time lapse, by eruption of the 

tuffs of Camas Creek and Black Mountain. Where best 

exposed along Camas Creek and at Black Mountain, the 

tuffs are intracauldron rocks that consist of several cooling units. The lowest cooling units are megascopically and 

mineralogically identical to rock in the outer part of the 

welded-tuff vent at Van Horn Peak (Ekren, 1981). The 

densely welded parts of the tuffs throughout the Camas 

Creek-Black Mountain section commonly are black 

because abundant magnetite dust is distributed 

throughout the groundmass. Where the rocks are not 

densely welded, they are not impregnated with magnetite 

and are various shades of gray. The pumice lapilli are 

chloritized and are dark green or brownish green, darker 

than the matrix that encloses them. Modes of samples of

the tuffs in the Camas Creek-Black Mountain sequence 

are similar to the Eightmile Creek, but the phenocrysts 

are smaller and fewer. In contrast to the tuffs of Ellis and 

Eightmile Creeks, whose principal mafic mineral is biotite, 

and hornblende is a secondary mafic constituent, the 

Camas Creek-Black Mountain sequence contains altered 

pyroxene as the principal mafite, and biotite and hornblende are secondary.

Tuffs in the Van Horn Peak vent

The Van Horn Peak welded-tuff vent (Ekren, 1981) 

is a prominent landmark in the Challis area (fig. C8). Two 

distinct tuff units occur within the cone-shaped vent: the 

older tuff forms a discontinuous outer ring or rim that 

is inferred to comprise remnants of an early vent-filling 

tuff that correlates with the lower tuffs in the Camas 

Creek-Black Mountain sequence; the younger tuff forms 

a circular (in plan view) inner core that is about 1.5 km 

in diameter. This inner core is mineralogically identical 

to the tuff of Table Mountain (Mclntyre and Hobbs, 1978; 

fig. C3, this chapter) that caps Table Mountain within the 

Corral Creek segment of the Van Horn Peak cauldron 

complex (figs. C4, C6). The tuff of the inner core intrudes 

the outer-ring remnants and also many other country 

rocks, including the tuff of Eightmile Creek, calderafilling sedimentary rocks, breccia, and rhyolite. Eutaxitic 

foliation defined by flattened pumice fragments parallels 

the flanks of the conical plug and dips consistently inward 60-80° (fig. C8, A and E). The tuffs in the vent and 

adjacent rocks are riddled with small intrusive masses of 

both rhyolitic and intermediate composition.

Quartz-biotite tuff

The eruption of the quartz-biotite tuff (table Cl) 

probably caused the initial collapse of the Castle Rock 

half-moon or trapdoor cauldron segment (figs. C4, C6). 

The tuff is thickest within this cauldron segment and appears to have been hottest there. The tuff consists of at 

least two megascopically indistinguishable cooling units 

separated by a thin zone of bedded tuff a few meters to 

possibly as much as 30 m thick. Neither cooling unit was 

systematically sampled from base to top during this study. 

Thin sections of samples from widely scattered outcrops 

show about 30 percent phenocrysts consisting of quartz, 

alkali feldspar, and plagioclase in subequal amounts, and 

4-18 percent biotite and lesser hornblende. The quartz 

grains commonly are as large as 4 mm (locally as large 

as 6 mm) and mostly quite smoky. Some of the smoky 

quartz is optically anomalous, having optic angles as large 

as 15° or 20°. These anomalous grains are not strained, 

and they do not show inclusions. The tuff contains allanite 

as a common accessory mineral.

Evidence of high temperatures near Castle Rock includes extensive flow layering and local thick zones of

50 Challis 1°x2° Quadrangle, Idaho
114°30'

CORRELATION OF MAP UNITS

OTa | }

Eocene

QUATERNARY 

AND TERTIARY

\- TERTIARY

CRETACEOUS

MIDDLE 

PRDTEROZOIC

PRECAMBRIAN

QUATERNARY 

QTa Alluvium and colluvium filling grabens

EOCENE

Tck Tuff of Castle Rock

Tqb Quartz-biotite tuff and other tuffs younger 

than tuffs of Camas Creek and Black 

Mountain

Granite rocks- of Castro pluton 

Tuffs of Camas Creek and Black Mountain 

Tuff of Ellis Creek

Volcanic rocks of Corral Creek cauldron 

segment, undivided

CRETACEOUS 

Kgd Grandodiorite

MIDDLE PROTEROZOIC 

Yh Hoodoo Quartzite 

Yy Yellowjacket Formation

PRECAMBRIAN 

pGm Metamorphic rocks

1C KILOMETERS

DESCRIPTION OF MAP UNITS

 CONTACT Approximately located

Tg 

Tc 

Te 

Tu

FAULT Dashed where approximately 

located. Arrows show relative movement. Bar and ball on downthrown 

side

CAULDRON-BOUNDARY FAULT Bar

and ball on downthrown side 

WELDED-TUFFVENT

STRIKE AND DIPOF FOLIATION IN WELDED TUFF 

Inclined

Vertical

Figure C6. Generalized geologic map of the Camas Creek area.

Eocene Cauldron-Related Volcanic Events 51
 :**?!^Wi< ^m.-r tf

r'Jf^-^rl.:' - vf/.'rWV5!t '. ; I "

Figure C7. The tuff of Ellis Creek. >\, outflow sheet just east 

of Salmon River on Highway 93 between Challis and Salmon; 

the tuff is about 100 m thick in this area, and despite the 

soft-weathering aspect is densely welded; B, densely welded, 

steeply dipping intracauldron tuff near Camas Creek; note that 

the pumice lapilli (best seen near compass) are darker than the 

matrix; a dark aphyric apophysis is visible in upper right.

flow brecciation. These features are similar to those 

described in the extremely hot ash-flow tuffs of Owyhee 

County, Idaho, by Ekren and others (1984).

Figure C8. Rocks of the tuff of Van Horn Peak vent. A, columnar 

jointing on the west flank of the vent; foliation within the jointed 

zone dips about 70° toward the east, central, part of the vent; 

B, flattened pumice (light patches) on the north flank of the vent 

at the contact with country rock; foliation as defined by the 

pumice dips 65° toward the central part of the vent.

52 Challis 1°x2° Quadrangle, Idaho
Tuff of Castle Rock

This sequence of cauldron- and graben-filling tuff 

(unit Tck, fig. C6) is recognized with certainty only within 

the Castle Rock cauldron segment and within the Panther 

Creek graben. Although the rocks are mostly poorly 

welded, they are almost totally preserved, due to rapid subsidence and subsequent deep burial in both structures. The 

sequence resembles the tuff of Challis Creek (Hardyman, 

chap. G, this volume) so closely that it was originally correlated with that tuff (for example, see Mclntyre and 

others, 1982). Both tuff sequences are red or reddish, and 

both contain predominant alkali feldspar. They differ in 

two important ways: (1) All of the Castle Rock cooling 

units are quite rich in small volcanic lithic fragments from 

base to top, and the Challis Creek cooling units are nearly 

free of lithic fragments. (2) All of the Castle Rock cooling units contain sparse but ubiquitous flakes of biotite; 

the Challis Creek cooling units, on the other hand, do 

not contain enough biotite to be commonly observed 

either in hand specimen or thin section. Furthermore, the 

tuff of Castle Rock is generally only partly welded, 

whereas the tuff of Challis Creek is generally densely 

welded.

The tuff within the Castle Rock cauldron segment 

consists of five cooling units with thin interbeds of ashfall tuff; the entire sequence is 630 m thick. It consists 

of a partly welded to nonwelded, slope-forming cooling 

unit, rich in lithic fragments, about 190 m thick at the 

base, that rests on the quartz-biotite tuff. This cooling 

unit is overlain by a cliff-forming, moderately welded unit 

about 150 m thick, which is overlain by two thin, partly 

welded, slope-forming cooling units, rich in lithic 

fragments, each about 30 m thick. The uppermost cooling unit forms cliffs and is about 230 m thick (fig. C9). 

These units are less welded northward in the Panther 

Creek graben where only one unit consistently forms cliffs. 

Most of the units in the graben are virtually nonwelded 

and form tepee-shaped monoliths and varicolored 

hoodoos. Some of these monolith-forming units are glassy 

and pumice fragments are silky remarkable preservation 

for tuffs at least 45 m.y. old. All the units contain 20-30 

percent phenocrysts consisting of 15-45 percent quartz; 

45-78 percent alkali feldspar; 1-4 percent plagioclase; a 

trace to 3 percent clinopyroxene, hornblende, and biotite; 

and trace amounts of accessory allanite as well as 

ubiquitous zircon and apatite.

Tuff of Challis Creek

The eruption of the tuff of Challis Creek was the 

final important volcanic event in the Van Horn Peak 

cauldron complex. Its eruption created the Twin Peaks 

caldera (fig. C4; Hardyman, 1981, chap. G, this volume). 

This tuff, as stated previously, is very similar to the tuff of 

Castle Rock. Hardyman recognized two thick (each 300 m

Figure C9. Castle Rock, which is made up of two moderately 

welded, cliff-forming rhyolitic cooling units of the tuff of Castle 

Rock, separated by two thin, partly welded, slope- and benchforming cooling units rich in lithic fragments.

or more) cooling units within the Twin Peaks caldera. The 

inference that the tuff of Challis Creek is younger than 

the tuff of Castle Rock is based on the fact that no tuff 

of Challis Creek is now recognized in either the Castle Rock 

cauldron or the Panther Creek graben. Surely some remnants of tuff of Challis Creek would have been preserved 

there if it were the older unit.

THUNDER MOUNTAIN CAULDRON COMPLEX

Mapping of the part of the Thunder Mountain 

cauldron complex that lies in the Challis 1°X2° quadrangle 

by Ekren and G. D. May during this study (in Fisher and 

others, 1983) resulted in a somewhat different interpretation and outline of the complex than that deduced earlier 

by Leonard and Marvin (1982) on the basis of reconnaissance traverses. Nevertheless, there were more important agreements than disagreements.

Ekren and May concluded that the sequence of 

events in the Thunder Mountain cauldron complex was 

very similar to that at the Van Horn Peak cauldron complex. The first volcanic activity was the eruption of mostly latite lavas (table Cl) and minor intercalated latite tuffs, 

starting about 49-51 m.y. ago (Leonard and Marvin, 1982). 

This activity was followed by eruption of tuffs that are 

characterized by abundant, well-flattened pumice lapilli the 

size of dimes and quarters. These tuffs have the same kinds 

of pumice and the same mineralogy as the lower parts of 

the tuffs of Camas Creek and Black Mountain (table C2). 

In fact, the sequences of tuffs from the two cauldron complexes may actually interfinger in some localities within 

each of the cauldron areas; however, interfingering was not 

proven conclusively during this study. Available chemical 

data (fig. CIO) indicate that the dime- and quarter-lapilli 

tuff ranges in composition from rhyodacite to rhyolite, a 

chemical range inferred also for the tuffs of Camas Creek 

and Black Mountain (Mclntyre and others, 1982).

Eocene Cauldron-Related Volcanic Events 53
Table C2. Chemical analyses and CIPW norms for volcanic rocks

[Samples collected by B. F. Leonard. Analyses by U.S. Geological Survey analysts: H. F. Phillips, P. L. D. Elmore, K. E. White, E. L. Munson,

Sample No. 

Field No. 

Lab No. 

N. latitude 

W. longitude12345 

Y-54-582 83L57 MS-262 L-64-2639 L-62-2150 

140642 D-256196 D101856 D100631 1-4100 

45°00'28" 45°00'42" 44°58'00" 44°48'12" 45°14'42" 

115°15'12" 115°15'54" 115°08'00" 115O 18'21" 115°15'12"

6 

Y-54-565 

140641 

45°01'45" 

115°16'30"

789 

L-56-1361 Y-54-597 Y-54-598 

149464 140643 140644 

45°10'40" 45°05'06" 45°04'24" 

115°22'50" 115°21'36" 115°19'45"

Chemical analyses

Si09

A1 203 _____ 

Fe203 ______ 

FeO

MgO

r a n . . _ .___.

V Q » -» 

H n+ H20 

Hn~ _ _ __

TiO 

P2o5 

C 0 o ~" " "" ~* 

/- I

T7 t- «-

Sum 

n_-> ^ __--_ 

Q _L~G R -"-M - -.

di-fs 

hy-fs 

ol-fo 

ol-fa ~ 

ap

57.9 

17.3 

2.0 

3.9 

3.0 

7.4 

2.7 

2.2 

1.6 

.12 

.98 

.39 

.11 

.74 

.04 

.01

100.39

15.04

13.02 

22.60

28.71 

.31 

7.47 

4.10

.00 

.00 

.00 

.00

2.90 

.00 

1.86 

.93 

1.68

3+

58.0 

17.3 

1.40 

3.85 

2.56 

6.65 

2.74 

2.42 

1.87 

.15 

.85 

.28 

.09 

1.56 

<.01 

.06

99.78

19.43 

2.45 

14.83 

24.04

22.09 

.00 

.00 

.00

6.61 

2.39 

.00 

.00

2.11 

.00 

1.67 

.69 

3.68

4+

58.18 

16.20 

1.71 

4.82 

3.13 

5.52 

3.43 

3.03 

1.08 

.35 

.89 

.59 

.11 

.1 

.05 

.13

99.32

10.33

17.90 

28.92

19.88 

1.26 

7.76 

5.36

.00 

.00 

.00 

.00

2.48 

.00 

2.64 

1.41 

.00

Felsic-mafic

4.2

58.61 

15.42 

2.29 

3.71 

3.74 

5.14 

2.69 

3.38 

2.44 

.68 

.85 

.31 

.11 

.44 

.01 

.07

99.89

CIPW

14.44

20.65 

23.54

20.70 

.29 

.19 

.08

9.44 

3.77 

.00 

.00

3.43 

.00 

1.67 

.76 

1.03

ratio

4.3

58.72 

14.79 

1.25 

4.77 

5.67 

5.97 

2.62 

2.08 

2.23 

.48 

.78 

.20 

.12 

.15 

.00 

.05

99.88

norms

13.67

12.65 

22.82

23.12 

2.11 

1.34 

.63

13.20 

6.23 

.00 

.00

1.87 

.00 

1.53 

.49 

.35

59.0 

17.5 

2.1 

3.4 

2.4 

6.2 

3.1 

2.5 

1.8 

.05 

.92 

.36 

.11 

1.1 

.01 

.04

100.59

17.20 

1.89 

14.74 

26.22

21.34 

.00 

5.97 

3.20

.00 

.00 

.00 

.00

3.06 

.00 

1.75 

.84 

2.50

61.9 

17.3 

3.2 

2.7 

1.4 

3.9 

3.5 

3.2 

1.0 

.2 

.96 

.30 

.09 

<.05 

.01 

.10

99.76

19.63 

2.0 

18.92 

29.62

16.58 

.00 

3.48 

.90

.00 

.00 

.00 

.00

4.63 

.00 

1.82 

.71 

.00

62.6 

16.2 

2.3 

2.6 

2.2 

4.1 

3.3 

2.9 

2.0 

.2 

.74 

.32 

.09 

.26 

.01 

.00

99.82

20.98 

1.49 

17.14 

27.89

16.78 

.00 

5.48 

1.82

.00 

.00 

.00 

.00

3.33 

.00 

1.41 

.77 

.59

63.1 

15.5 

2.0 

2.4 

2.7 

3.6 

3.5 

3.5 

2.0 

.09 

.70 

.34 

.07 

.76 

.00 

.03

100.29

19.87 

2.01 

20.70 

29.62

10.77 

.00 

6.73 

1.73

.00 

.00 

.00 

.00

2.90 

.00 

1.34 

.80 

1.73

( Si02+K20+Na20 ) / (FeO+Fe 203+MgO+CaO )

3.6 4.6 6.1 6.1 6.6

SAMPLE

1. Latite lava: 43 percent phenocrysts consisting of 65 percent plagioclase (0.1-2.0 mm), 21 percent clinopyroxene (0.1-0.5 mm), 13 percent chloritized pyroxene; dense microcrystalline, structureless groundmass.

2. Latite lava: 33 percent phenocrysts consisting of 71 percent plagioclase (0.1-3.0 mm), 22 percent chloritized pyroxene (0.1-0.5 mm), 3 percent oxidized biotite or hornblende, 2 percent quartz, 1.5 percent apatite; dense microcrystalline, structureless groundmass.

3. Latite lava: 60 percent unoriented plagioclase microlites (0.1-0.3 mm), 5 percent altered pyroxene (0.1 mm), 25-30 percent black opaque groundmass.

4. Latite lava: 34 percent phenocrysts consisting of 60 percent plagioclase (0.2-5.0 mm), 23 percent clinopyroxene (0.2-2.5 mm), 17 percent chloritized pyroxene; dense, 

weakly trachytic groundmass.

5. Latite lava: 36 percent phenocrysts consisting of 42 percent plagioclase (as long as 3.0 mm), 45 percent chloritized pyroxene (glomerocrysts and clots as long as 4 

mm), 13 percent clinopyroxene (as long as 4 mm); dense trachytic felty groundmass.

6. Latite intrusion: 64 percent phenocrysts consisting of 80 percent plagioclase (as long as 6.0 mm), 15 percent chloritized pyroxene; 3.7 percent oxidized biotite, 1.5 percent 

fresh clinopyroxene; microgranular groundmass.

7. Dacite lava: about 20 percent phenocrysts consisting of 70 percent fresh plagioclase (0.1-10.0 mm), and 30 percent chloritized pyroxene; pilotaxitic groundmass.

8. Dacite lava: about 20 percent phenocrysts consisting of 70 percent plagioclase (0.1-5.0 mm), 30 percent mostly chloritized clinopyroxene, trace of biotite; pilotaxitic 

groundmass.

9. Dacite lava: 19 percent phenocrysts consisting of 57 percent plagioclase (as long as 5 mm), 8.5 percent quartz (as long as 3 mm), 17 percent chloritized, oxidized 

hornblende, 17 percent chloritized, calcitized pyroxene; pilotaxitic groundmass.

54 Challis 1°x2° Quadrangle, Idaho
from the Thunder Mountain cauldron complex, Challis quadrangle

C. L. Parker, A. J. Bartel, E. L. Brandt, D. F. Powers, S. M. Berthold, P. W. Scott, and V. C. Smith. N., north; W., west; <, less than]

10

83(34105

D-256194 

45°03'00"

11

83(34101

D-256193 

45°02'48" 

1 115°20'54"

12

83L69

D-256199 

45°03'54" 

115°19'27

13

L-61-229

W3559 

45°02'24"

14

L-74-7

D103789 

44°58'54" 

115°05'00"

15

TM-71-2

D103790 

44°52'00" 

115°02'00"

16

Y-54-599

140645 

45°03'36" 

115°18'48"

17

83L64

D-256198 

45°03'00" 

115°21'42"

18

Y-54-599A

140646 

45°03'36" 

115°18'48"

19

H-54-63

140640 

45°03'45" 

115°15'48"

20

L-61-106

H3558 

45°03'30" 

115°24'12"

Chemical analyses

63.6

15.4

1.33

2.54

1.65

3.77

3.30

3.70

1.58

.10

.57

.18

.06

1.48

<.01

.06

99.32

23.58

3.05

22.40

28.61

8.37

.00

.00

.00

4.21

2.80

.00

.00

1.98

.00

1.11

.44

3.45

7.6

63.9

15.0

1.34

2.44

1.50

3.30

3.33

3.72

1.82

.32

.54

.17

.06

1.78

<.01

.04

99.26

25.90

4.15

22.64

29.02

4.13

.00

.00

.00

3.85

2.67

.00

.00

2.00

.00

1.06

.41

4.17

8.3

67.2

14.9

1.76

1.56

1.92

2.05

3.78

3.63

1.43

.09

.54

.20

.05

.05

<.01

.03

99.19

25.08

1.66

21.97

32.75

8.75

.00

.00

.00

4.90

.63

.00

.00

2.61

.00

1.05

.49

.12

10.2

69.57

14.94

1.45

1.25

.75

1.03

3.86

4.46

1.18

.58

.32

.07

.05

.12

.01

.05

99.69

CIPW

27.53

2.39

26.93

33.37

3.98

.00

.00

.00

1.91

.68

.00

.00

2.15

.00

.62

.17

.28

Felsic-maf ic

17.4

71.02

14.23

2.09

.58

.38

1.47

3.15

5.15

.58

.68

.40

.09

.04

.00

.00

.07

99.93

norms

29.94

1.03

30.86

27.03

6.80

.00

.00

.00

.96

.00

.00

.00

.85

1.53

.77

.22

.00

71.59

14.57

1.22

.76

.36

1.15

3.17

5.01

1.22

.32

.30

.09

.04

.00

.00

.04

99.84

31.78

2.09

30.13

27.30

5.21

.00

.00

.00

.91

.00

.00

.00

1.74

.04

.58

.22

.00

74.4

14.0

1.6

1.0

.00

.38

3.7

5.1

.52

.07

.11

.09

.03

.27

.02

.00

101.29

33.33

2.43

30.10

31.14

.00

.00

.00

.39

.00

.00

.00

.00

2.32

.00

.21

.20

.48

74.7

13.6

.58

.80

.12

.16

3.44

5.02

.72

.03

.09

<.05

<.02

<.01

<.01

.03

99.29

35.10

2.25

30.11

29.55

.81

.00

.00

.00

.30

.85

.00

.00

.85

.00

.17

.00

.00

75.1

14.0

1.1

.30

.08

.45

3.4

5.2

.79

.11

.10

.07

.02

<.05

.00

.00

100.72

34.58

2.14

30.72

28.73

1.76

.00

.20

.00

.00

.00

.00

.00

.75

.59

.19

.17

.00

76.7

13.6

.91

.08

.02

.12

3.1

5.1

.77

.06

.11

.01

.02

<.05

.01

.00

100.61

38.94

2.80

30.10

26.22

.50

.00

-.05

.00

.00

.00

.00

.00

.00

.91

.21

.00

.00

76.88

13.14

.24

.20

.06

.04

2.84

5.27

.65

.26

.10

.01

.00

.00

.00

.03

99.72

40.54

2.75

31.53

24.33

.13

.00

.00

.00

.15

.00

.00

.00

.35

.00

.19

.02

.00

ratio (Si0 2+K2 0+Na2 0)/(FeO+Fe 2 03 +MgO+CaO)

17.5 22.9 27.9 50.1 43.4 75.1 157.4

DESCRIPTIONS

10. Rhyodacite tuff (contains dime- and quarter-size pumice lapilli): about 15 percent phenocrysts consisting of 60 percent plagioclase (0.1-3.0 mm), 10 percent quartz 

(0.5-2.0 mm), 25 percent altered pyroxene, 5 percent altered hornblende; shards visible in groundmass.

11. Rhyodacite tuff (contains dime- and quarter-size pumice lapilli): about 10-15 percent phenocrysts consisting of 60 percent plagioclase (0.1-3.0 mm), 5 percent quartz 

(0.5-2.0 mm), 35 percent altered pyroxene, 10 percent altered hornblende(?); shards visible in groundmass.

12. Rhyodacite lava: about 40 percent phenocrysts consisting of 65 percent plagioclase (0.1-8.0 mm), 35 percent oxidized, chloritized biotite and hornblende, possible 

sparse altered pyroxene; dense trachytic groundmass.

13. Rhyolite tuff (contains dime- and quarter-size pumice lapilli): 11 percent phenocrysts consisting of 46 percent plagioclase (as long as 2 mm), 22 percent alkali feldspar 

(as long as 3 mm), 9 percent quartz (as long as 2 mm), 11 percent chloritized pyroxene, 7 percent altered hornblende, 5 percent oxidized biotite; considerable epidote 

in altered feldspar and groundmass.

14. Rhyolite tuff ("lower Sunnyside"): 48 percent phenocrysts consisting of 39 percent alkali feldspar (as long as 4 mm), 28 percent plagioclase (as long as 4 mm), 

23 percent quartz (as long as 3 mm), 4.8 percent oxidized biotite, 4.8 percent altered hornblende; probable lower part of a cooling unit.

15. Rhyolite tuff ("lower Sunnyside"): 34 percent phenocrysts consisting of 46 percent plagioclase (as long as 4 mm), 22 percent quartz (as long as 3 mm), 13 percent 

alkali feldspar (as long as 4 mm), 11 percent chloritized biotite, 5 percent chloritized hornblende, 2 percent possible altered pyroxene; strong propylitic alteration; 

feldspars albitized and partly replaced by zeolite.

16-20. Rhyolite (buff rhyolite): few phenocrysts; flow laminated, some laminae contain abundant tiny vapor-phase crystals. Rhyolite contains 3-5 percent small (0.5-2.0 

mm) feldspar crystals that are principally plagioclase but are so replaced by albite and sericite that they are not easily distinguished from albitized sanidine; all 

contain sparse altered hornblende and traces of altered biotite; patches of secondary quartz suggests silicification as well as albitization.

Eocene Cauldron-Related Volcanic Events 55
ROCKS FROM THUNDER MOUNTAIN CAULDRON COMPLEX 

Correlations are tentative 

Tbr, buff rhyolite 

Tsl, lower Sunnyside tuff

Tdq, dime-and quarter-size lapilli

tuff 

Til, lower latite and dacite lavas

SiO2 , IN WEIGHT PERCENT 

EXPLANATION

ROCKS FROM VAN HORN PEAK

CAULDRON COMPLEX 

Tcr, tuff of Challis Creek 

Ttm, tuff of Table Mountain (same

as inner core. Van Horn Peak) 

Tc, tuff of Camas Creek-Black

Mountain

Tern, tuff of Eightmile Creek 

Te, tuff of Ellis Creek 

Tdf, intermediate lavas at base

of volcanic sequence

Figure C10. Alkali-lime diagram for analyses of rocks from Thunder Mountain and Van Horn Peak cauldron complexes. Solid 

squares, rocks from Thunder Mountain cauldron complex; Tbr, buff rhyolite; Tsl, lower Sunnyside tuff; Tdq, dime- and quartersize lapilli tuff; Til, lower latite and dacite lavas. Open circles, rocks from Van Horn Peak cauldron complex; Tcr, tuff of Challis 

Creek; Ttm, tuff of Table Mountain (same as inner core, Van Horn Peak); Tc, tuff of Camas Creek and Black Mountain; Tern, 

tuff of Eightmile Creek; Te, tuff of Ellis Creek; Tdf, intermediate lavas at the base of the volcanic sequence. Correlations of "lower 

Sunnyside tuff" are tentative.

The third major event in the Thunder Mountain 

cauldron complex was the eruption of the "lower Sunnyside tuff1 " which consists of at least three compositionally zoned cooling units of rhyolite and quartz latite 

tuff. Each cooling unit grades upward from a rhyolite 

base, rich in quartz and alkali feldspar, to a quartz latite 

top, rich in plagioclase and mafic minerals (principally 

biotite) and low in quartz and alkali feldspar. This is the 

classic zonation of cauldron-forming tuffs (for example, 

compare with the Miocene Timber Mountain Tuff of 

Byers and others (1976), and with Lipman and others 

(1966). These three units are each 100-300 m thick. They 

are slightly to moderately hydrothermally altered.

The final major pyroclastic activity in the Thunder 

Mountain cauldron complex was the emplacement of 

megabreccia followed by the eruption of "upper Sunnyside tuff!' The breccia (at least 100 m thick) was

'This informal unit, with the "upper Sunnyside tuff,' comprise the 

informal Sunnyside rhyolite of Leonard and Marvin (1982).

recognized only in the western part of the complex where 

it consists of large blocks (in places as large as a small 

house) of "lower Sunnyside tuff in a matrix of "upper 

Sunnyside tuff!' This unit may have been formed by 

landsliding concomitant with the early eruptions of "upper Sunnyside tuff or it may have been extruded as tuff 

extremely rich in lithic fragments. The "upper Sunnyside 

tuff is a multiple-flow compound cooling unit that is 

the counterpart in the Thunder Mountain cauldron complex of the tuffs of Castle Rock and Challis Creek in the 

Van Horn Peak cauldron complex. The "upper Sunnyside 

tuff is rhyolitic and mostly red, reddish gray, and red 

brown. It shows no obvious mineralogical zoning except 

that the basal vitrophyre contains as many as four grains 

of hornblende and eight grains of pigeonite per thin section, whereas the upper devitrified part of the unit contains only sparse biotite as a mafic constituent. The tuff 

contains essentially no plagioclase, and the ratio of quartz 

to alkali feldspar, although variable, does not show any 

systematic variation from base to top.

56 Challis 1°x2° Quadrangle, Idaho
CHEMICAL COMPOSITION OF THE 

VOLCANIC ROCKS

Chemical analyses of the principal volcanic rocks 

in the eastern half of the Challis quadrangle have been 

published previously by Mclntyre and others (1982). Additional analyses of the tuff of Challis Creek and related 

rhyolite intrusive rocks are presented by Hardyman (chap. 

G, this volume). Chemical analyses of 20 volcanic rocks 

from the Thunder Mountain cauldron complex are shown 

in table C2. The mineralogy and available chemical 

analyses of rocks from the Van Horn Peak and Thunder 

Mountain cauldron complexes show that the rocks are a 

calc-alkaline suite, although a small amount of alkali 

basalt is present in the eastern part of the quadrangle 

(Mclntyre and others, 1982, table 1). The volcanic rocks 

have an alkali-lime index (Peacock, 1931) of between 56 

and 57 (fig. CIO) and fall along the calc-alkaline trends 

of Ringwood (1974) (fig. Cll).

No peralkaline rocks were found, although the composition of some of the intrusive rhyolites of Hardyman 

(chap. G, this volume) approaches the peralkaline field of 

Shand (1947). Both Al2O3 /Na2O+K2O and Al2O3 /Na2O 

+K2O+CaO for those rocks nearly equal unity. The 

buff rhyolite and related rocks in the Thunder Mountain 

cauldron complex (table C2, samples 16-20) are alkali rhyolite and extreme alkali rhyolite according to the classification of Young (in Segerstrom and Young, 1972) but, nevertheless, are not peralkaline (fig. C12). These rhyolites are 

inferred to be older than the informal Sunnyside rhyolite 

(Leonard and Marvin, 1982). Nevertheless, they are richer 

in silica and alkalis than the "lower Sunnyside tuff (table 

C2, samples 14, 15). No chemical analyses are available 

for the "upper Sunnyside tuff!' It contains more quartz 

and alkali feldspar phenocrysts than the "lower SunnysideT suggesting that it is richer in silica and alkalis and, 

therefore, is chemically very similar to the buff rhyolite.

METALUMINOUS PERALUMINOUS

FeO + Fe2 O3

Tholeiitic 

trends

Skaergaard, 

Greenland

-Thingmuli, 

Iceland

Til 

(Dacite

, Calc-alkaline trends 

Te of Ringwood (1974)

Na2 O + K2O

EXPLANATION

ROCKS FROM THUNDER MOUNTAIN CAULDRON COMPLEX 

Correlations are tentative 

Tbr, buff rhyolite

Tdq, dime and quarter-size lapilli

tuff

Til, lower latite and dacite lavas 

ROCKS FROM VAN HORN PEAK

CAULDRON COMPLEX 

Tcr, tuff of Challis Creek

Tc, tuff of Camas Creek - Black 

Mountain

Tern, tuff of Eightmile Creek 

Te, tuff of Ellis Creek 

Tdf, intermediate lavas at base 

of volcanic sequence

MgO

Figure Cll. AFM plot of volcanic rocks from the Van Horn 

Peak and Thunder Mountain cauldron complexes. Tholeiitic 

trends from Skaergaard, Greenland, and Thingmuli, Iceland, 

and calc-alkaline trends of Ringwood (1974) are also shown.

2.0

1.9

1.5

1.4

O 1.3 

2

O

£

+ 1 0

O S1 z

1.1

1.0

0.9

1 1 

0Tdf

^^^^^

-

-

-

PERALKALINE

1 1

I I

0Te 

A83L69 (Rhyodacite lava

°Tem

ATM-71-2(Tsl)

A L-61-229(Tdq) 

A L-74-7(Tsl) 

H-54-63 (Tbr)A ~ 

L-61-106(Tbr) A 

Y-54-599A(Tbr) A 83L64 ( Tbr)

A Y-54-599(Tbr) ~

0Tcr

I I

0.7 0.8 0.9 

A12 0 3

Na2O + K2O + CaO

EXPLANATION

1.0 1.1 1.2 

MOLECULAR RATIOS

1.3

A ROCKS FROM THUNDER

MOUNTAIN CAULDRON 

COMPLEX Correlations 

are tentative 

Tbr, buff rhyolite 

Tsl, lower Sunnyside tuff 

Tdq, dime-and quarter-size 

lapilli tuff

o ROCKS FROM VAN HORN PEAK 

CAULDRON COMPLEX

Tcr, tuff of Challis Creek

Tc, tuff of Camas Creek-Black 

Mountain

Tern, tuff of Eightmile Creek

Te, tuff of Ellis Creek

Figure C12. Classification of principal volcanic rocks of the 

Challis quadrangle using peralkaline-peraluminousmetaluminous divisions of Shand (1947). Modified from Pallister 

(1982). Sample numbers refer to analyses in table C2.

Eocene Cauldron-Related Volcanic Events 57
REFERENCES CITED

Byers, F. ML, Jr., Carr, W. J., Orkild, P. P., Quinlivan, W. D., 

and Sargent, K. A., 1976, Volcanic suites and related 

cauldrons of Timber Mountain-Oasis Valley caldera complex, southern Nevada: U.S. Geological Survey Professional 

Paper 919, 70 p.

Cater, F. W., Pinckney, D. M., Hamilton, W. B., Parker, R. L., 

Weldin, R. D., Close, T. J., and Zilka, N. T., 1973, Mineral 

resources of the Idaho Primitive Area and vicinity, Idaho: 

U.S. Geological Survey Bulletin 1304, 431 p.

Ekren, E. B., 1981, Van Horn Peak a welded tuff vent in central Idaho: Montana Geological Society, Field Conference 

and Symposium Guidebook to Southwest Montana, 

p. 311-315.

Ekren, E. B., Mclntyre, D. H., and Bennett, E. H., 1984, Hightemperature, large-volume, lavalike ash-flow tuffs without 

calderas in southwestern Idaho: U.S. Geological Survey 

Professional Paper 1272, 76 p.

Fisher, F. S., Mclntyre, D. H., and Johnson, K. M., 1983, 

Geologic map of the Challis 1°X2° quadrangle, Idaho: US. 

Geological Survey Open-File Report 83-523, 41 p., 2 sheets, 

scale 1:250,000.

Foster, Fess, 1982, Geologic map of Mount Jordan and vicinity, Custer County, Idaho: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1434, scale 1:24,000.

Fox, K. F, Jr., 1983, Melanges and their bearing on Late 

Mesozoic and Tertiary subduction and interplate translation at the west edge of the North American plate: U.S. 

Geological Survey Professional Paper 1198, 40 p.

Hardyman, R. F, 1981, Twin Peaks caldera of central Idaho: 

Montana Geological Society, Field Conference and Symposium Guidebook to Southwest Montana, p. 317-322.

Hays, W. H., Mclntyre, D. H., and Hobbs, S. W., 1978, Geologic 

map of the Lone Pine Peak quadrangle, Custer County, 

Idaho: U.S. Geological Survey Open-File Report 78-1060, 

scale 1:62,500.

Leonard, B. F, and Marvin, R. F, 1982 [1984], Temporal evolution of the Thunder Mountain caldera and related features, 

central Idaho, in Bill Bonnichsen and R. M. Breckenridge, 

eds., Cenozoic geology of Idaho: Idaho Bureau of Mines 

and Geology Bulletin 26, p. 23-41.

Lipman, P. W., 1983, Tectonic setting of the Mid to Late 

Tertiary in the Rocky Mountain region A review, in 

The genesis of Rocky Mountain ore deposits Changes 

with time and tectonics: Denver Region Exploration Geologists Society Symposium, Denver, 1982, Proceedings, 

p. 125-131.

Lipman, P. W., Christiansen, R. L., and O'Connor, J. T., 1966, 

A compositionally zoned ash-flow sheet in southern 

Nevada: U.S. Geological Survey Professional Paper 524-F, 

p. F1-F47.

Lipman, P. W., Prostka, H. J., and Christiansen, R. L., 1972, 

Early and Middle Cenozoic, Part 1 of Cenozoic volcanism 

and plate-tectonic evolution of the western United States, 

in A discussion on volcanism and the structure of the Earth: 

Royal Society of London Philosophical Transactions, sen 

A, v. 271, no. 1213, p. 217-248.

Mclntyre, D. H., Ekren, E. B., and Hardyman, R. F, 1982 [1984], 

Stratigraphic and structural framework of the Challis 

Volcanics in the eastern half of the Challis 1°X2° quadrangle, Idaho, in Bill Bonnichsen and R. M. Breckenridge, 

eds., Cenozoic geology of Idaho: Idaho Bureau of Mines 

and Geology Bulletin 26, p. 3-22.

Mclntyre, D. H., and Hobbs, S. W., 1978, Geologic map of the 

Challis quadrangle, Custer County, Idaho: U.S. Geological Survey Open-File Report 78-1059, scale 1:62,500.

Pallister, J. S., 1982, Reconnaissance geology of the Jabal Al 

Ilman quadrangle, sheet 18144A, Kingdom of Saudi 

Arabia: Saudi Arabian Deputy Ministry for Mineral 

Resources Open-File Report USGS-OF-02-90, 61 p.

Peacock, M. A., 1931, Classification of igneous rock series: Journal of Geology, v. 39, no. 1, p. 54-67.

Ringwood, A. E., 1974, The petrological evolution of island arc 

systems: Geological Society of London Journal, v. 130, pt. 

3, p. 183-204.

Ross, C. P., 1933, The Thunder Mountain mining district, Custer 

and Camas Counties, Idaho: Idaho Bureau of Mines and 

Geology Pamphlet 33, 26 p.

_____1934, Geology and ore deposits of the Casto quadrangle, Idaho: U.S. Geological Survey Bulletin 854, 135 p.

_____1937, Geology and ore deposits of the Bayhorse region,

Custer County, Idaho: U.S. Geological Survey Bulletin 877, 

161 p.

Segerstrom, Kenneth, and Young, E. J., 1972 [1973], General 

geology of the Harms Peak and Farwell Mountain quadrangles, Routt County, Colorado, with a discussion of Upper Triassic and pre-Morrison Jurassic rocks, by G. N. 

Pipiringos: U.S. Geological Survey Bulletin 1349, 63 p.

Shand, S. J., 1947, Eruptive rocks, their genesis, composition, 

classification, and their relation to ore deposits, with a 

chapter on Meteorites (2d ed.): New York, John Wiley, 

444 p.

Souther, J. G., 1970, Volcanism and its relationship to recent 

crustal movements in the Canadian Cordillera: Canadian 

Journal of Earth Sciences, v. 7, no. 2, pt. 2, p. 553-568.

58 Challis 1°x2° Quadrangle, Idaho
Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter D

Precambrian and Paleozoic Sedimentary 

Terranes in the Bayhorse Area of the 

Challis Quadrangle

By S. WARREN HOBBS

CONTENTS

Abstract 60

Introduction 60

Terranes of Middle Proterozoic and Paleozoic rocks 60

Terrane A 61

Terrane B 64

Terrane C 64

Terrane D 65

Terrane E 66

Terrane F 67

The Salmon River lineament 67 

Summary 68 

References cited 68

FIGURES

Dl. Map showing outcrop areas of pre-Tertiary sedimentary rocks and

Cretaceous-Tertiary plutonic rocks 61 

D2. Map showing structural-stratigraphic terranes 62 

D3. Geologic map and columnar section showing distribution of Proterozoic

rocks 63

Sedimentary Terranes in the Bayhorse Area 59
Abstract

Limited outcrops of pre-Tertiary sedimentary rocks of Proterozoic to Permian age are exposed mainly in the eastern half 

of the Challis quadrangle, in the Bayhorse region, and extend 

south into the Wood River region. They provide the only information on the early sedimentary history and structural complexity of a great stack of allochthonous terranes. These rocks 

indicate a long and varied sedimentary record from the Early 

Proterozoic through the Paleozoic. Sedimentation was interrupted by regional folding and faulting in the Cambrian and 

Ordovician and was followed by extensive early Mesozoic folding, by extensive middle and late Mesozoic west-to-east thrust 

faulting, and by late Mesozoic normal faulting. The Idaho 

batholith intruded and engulfed the allochthonous sedimentary sequences in the mid-to-Late Cretaceous, and the Challis 

Volcanics blanketed the region in Eocene time.

The Bayhorse region contains six fault-bounded structuralstratigraphic terranes, each of which is made up of a sedimentary sequence that differs significantly from that of adjacent terranes. Four of the six terranes include stratigraphic units older 

than Middle Ordovician that have not been definitely correlated 

with rocks beyond the Bayhorse area, but Middle Ordovician 

and younger units of three terranes are widespread far to the 

east and south.

The physical properties, stratigraphic succession, and composition of the rocks in each terrane have profoundly influenced 

the origin and localization of ore deposits. The Salmon River 

lineament, a major northeast-trending fault or fault zone, 

separates two of the terranes, and may have regional 

significance for the distribution of mineral occurrences.

INTRODUCTION

The Bayhorse region in central Custer County, 

Idaho, includes the rugged Salmon River Mountains 

highland in its western part and the northern part of the 

east-central Idaho Basin and Range Province to the east. 

More than 35,000 ft (feet) of predominantly marine strata 

of Paleozoic age or older occur within the area. These 

strata were folded and thrust faulted during the late 

Paleozoic and early Mesozoic by northeasterly directed 

compressional forces and were subsequently cut by steep 

normal faults during the late Mesozoic and Tertiary. A 

major north-northeast-trending structural break, called 

the Salmon River lineament, divides the area with significant differences in lithology and structure on each side.

The pre-Tertiary sedimentary rocks now exposed in 

the Challis 1°X2° quadrangle are but bits and pieces of 

a once very extensive and locally very thick stratigraphic 

record that was largely destroyed in the western half of 

the quadrangle by the Cretaceous Idaho batholith and was 

extensively buried in the eastern half by the Eocene Challis 

Volcanics. These sedimentary rocks, or their metamorphosed equivalents, are exposed over less than 15 percent 

of the Challis quadrangle, and their distribution and 

general ages are shown on figure Dl. The areas shown as 

Precambrian contain Middle Proterozoic rocks but

include some that are probably older and certainly some 

that grade into the Late Proterozoic and maybe even the 

early Paleozoic. Cambrian to Devonian strata include a 

great diversity of rocks of various ages and facies that are 

discussed in more detail later in this chapter. The 

Mississippian through Permian strata make up two general 

groups, one along the southern part of the eastern border 

of the Challis 1°X2° quadrangle that represents the 

predominantly carbonate cratonic facies and one farther 

west in the drainage basin of the mainstem Salmon River 

that comprises western continental margin-deep sea blackshale facies. These groups are juxtaposed by extensive 

thrust faulting. The Mesozoic and Tertiary plutonic rocks 

make up the mid- to Late Cretaceous Idaho batholith and 

the Eocene plutons and dike swarms that together largely engulf the main mass of the sedimentary rocks in the 

western half of the Challis 1 °X2° quadrangle. Only scattered roof pendants of the sedimentary rocks remain in 

this area. Eocene Challis Volcanics, and Quaternary 

deposits of great variety, bury extensive areas of both the 

sedimentary rocks and the batholith.

The sedimentary rocks of the Challis 1°X2° quadrangle and adjoining areas are of specific interest because 

of the close relation of the stratigraphic units to evaluation of the mineral resource potential of the quadrangle. 

Of equal importance are the physical characteristics and 

the composition of the sedimentary section porosity, 

permeability, carbonate content, geochemistry, and other 

factors that can be directly related to known ore deposits 

and, consequently, are factors to be used in locating 

deposits as yet unknown. Structure is also intimately 

related to the origin and localization of ore deposits, and 

only by careful geologic mapping can we hope to understand the sedimentary and structural history. And most 

exciting is the mounting documentation that points to the 

origin of many ore deposits by the remobilization of 

metals from sedimentary source rock and their concentration in favorable structural settings. Details of some 

of the source rocks are described by Hall (chap. J, this 

volume).

TERRANES OF MIDDLE PROTEROZOIC 

AND PALEOZOIC ROCKS

The Bayhorse area has been divided into five units 

that are here termed structural-stratigraphic terranes, for 

purposes of stratigraphic and structural description and 

analysis (fig. D2). These terranes are generally bounded 

by major faults, have consistent internal structural styles, 

and consist of rocks that differ in significant ways from 

those of other terranes. Although each terrane is an integral part of the geologic patchwork that is the whole 

area, each shows a locally unique combination of geologic 

features that raises serious and as yet unresolved problems 

in relating one terrane to another and in the possible

60 Challis 1°x2° Quadrangle, Idaho
116'

Area of figure D3

(Challis 1 5-minute

quadrangle)

10 20 30 KILOMETERS 

EXPLANATION

QUATERNARY DEPOSITS AND EOCENE 

CHALLIS VOLCANICS

ROOF PENDANTS OF PRE-TERTIARY 

SEDIMENTARY ROCKS Clusters of 

small pendants outlined by short dashes

TERTIARY AND MESOZOIC PLUTONIC 

ROCKS

PERMIAN TO DEVONIAN SEDIMENTARY 

ROCKS

DEVONIAN TO CAMBRIAN SEDIMENTARY ROCKS

PROTEROZOIC SEDIMENTARY ROCKS 

CONTACT

Figure Dl. Map showing outcrop areas of pre-Tertiary sedimentary rocks and Cretaceous-Tertiary plutonic rocks in the Challis 

1°X 2° quadrangle.

correlation of the geology with that of areas outside the 

Bayhorse region.

The original geographic and stratigraphic relations between some of these terranes have not been accurately determined, but by describing and delimiting the components 

of each terrane our present knowledge will be defined and 

the remaining problems outlined. The following discussion

of the terranes emphasizes the major elements of the structure and stratigraphy of each.

Terrane A

This terrane comprises rocks of Proterozoic age that 

are best exposed in the Challis 15-minute quadrangle to the

Sedimentary Terranes in the Bayhorse Area 61
114°30'

O

ft)

Q.

EXPLANATION

TERRANE F (MISSISSIPPIAN TO ORDOVICIAN)

TERRANE E(UPPER MISSISSIPPIAN, DEVONIAN, AND UPPER CAMBRIAN)

TERRANE D (LOWER PALEOZOIC) 

TERRANE C (LOWER PALEOZOIC ?)

TERRANE B (ORDOVICIAN AND 

OLDER)

TERRANE A (PROTEROZOIC)

CONTACT OF TERRANE AND

YOUNGER ROCKS 

HIGH-ANGLE FAULT Bar and ball on

downthrown side; arrows show relative

movement 

THRUST FAULT Sawteeth on upper

plate

GENERALIZED CROSS SECTION ALONG LINE A-A'

0 5 10 MILES

0 5 10 KILOMETERS

Figure D2. Generalized map showing structural-stratigraphic terranes in the Bayhorse area. Area of map shown on figure D1. Unpatterned areas include 

Eocene volcanic rocks.
north of the Bayhorse area (Mclntyre and Hobbs, 1978) 

and across the northern edge of the Challis 1 °X 2° quadrangle (figs. Dl, D3). Those rocks across the northern edge 

of the 1°X2° quadrangle are, for the most part, the 

Yellowjacket Formation, which is the lowest and most 

widespread recognized formation in the Middle Proterozoic in central Idaho. Yellowjacket strata that consist 

of more than 10,000 ft of argillite, siltite, and fine-grained 

sandstone are discussed in more detail by Modreski (chap. 

R, this volume). These rocks are of interest because of 

the stratabound cobalt deposits within them in the 

Blackbird district just north of the Challis 1°X2° quadrangle boundary. Related cobalt-bearing sulfide deposits 

are in the northeastern corner of the Challis 1°X2° 

quadrangle.

Within the Challis 15-minute quadrangle, segments 

of younger Proterozoic units are exposed (fig. D3). The 

oldest of these is the Apple Creek Formation, which is 

a thin-bedded, well-laminated, medium-greenish-gray to 

grayish-red-purple siltstone containing irregular streaks 

and lenses of light-gray, pinkish to pale-brown, finegrained sandstone cemented by ferrodolomite. No exposures of the overlying Gunsight Formation are within 

this area. The Swauger Formation is mainly pinkishpurple to nearly white, medium- to coarse-grained, slightly 

feldspathic quartzite at least 10,000 ft thick. Beds range 

from 1-6 ft thick but generally are 3-5 ft, and many show 

prominent cross-lamination. Ripple marks are well 

developed in places. Both the Apple Creek and Swauger 

Formations are widely distributed in the Lemhi Range east 

of the Challis 1°X2° quadrangle (Ruppel and others, 

1975; Ruppel, 1980). The Swauger Formation is overlain 

by the Lawson Creek Formation, which is the youngest 

known unit of the Middle Proterozoic in this part of Idaho 

(Hobbs, 1980). This unit grades upward from massive 

quartzite of the Swauger through a transition zone into 

a red-purple, thin-bedded siltstone-argillite sequence that 

is more than 4,000 ft thick. The top is concealed by Challis 

Volcanics.

Scattered exposures in the southern part of the 

Challis 15-minute quadrangle are a heterogeneous mixture of green phyllite; argillite; siltstone; massive- to

44° 45' 114

44° 30'

EXPLANATION

CONTACT

GENERAL STRIKE AND 

DIP OF BEDDING

THRUST FAULT 

Dotted where 

concealed

o

Challis

5 MILES

5 KILOMETERS

Sequence of 

Leaton Gulch

Lawson Creek Formation 

I 4200 feet

\- LATE PROTEROZOIC

Swauger Formation 

10,000 feet

Gunsight Formation 

6000 feet 

(not exposed)

Apple Creek Formation 

2500 feet

- MIDDLE PROTEROZOIC

Figure D3. Generalized columnar section and geologic map showing distribution of Proterozoic rocks in the Challis 

15-minute quadrangle.

Sedimentary Terranes in the Bayhorse Area 63
thin-bedded, white to pink quartzite with some breccia 

and conglomerate; and local layers of pure, massive, 

crystalline dolomite. These strata, called informally the 

sequence of Leaton Gulch, are of equivocal age but are 

probably Late Proterozoic and could likely include some 

Early Cambrian. They may be related to comparable strata 

in the southern part of the Lemhi Range to the southeast 

(McCandless, 1982).

Terrane B

Terrane B consists of less than 0.5 sq mi (square 

mile) of exposed strata of Ordovician and Middle Cambrian age and older(?) within a window in a major thrust 

sheet on upper Squaw Creek in the western part of the 

Bayhorse area (fig. D2). The sequence comprises more 

than 2,500 ft of strata. The upper unit is a medium- to 

thick-bedded, sandy and silty, light-colored dolomite at 

least 500 ft thick, whose age, based on poorly preserved 

fossils, is most probably Ordovician. The dolomite rests 

disconformably on a sequence of strata that consists of: 

(1) an upper red and green shale and siltstone unit of 

unknown thickness that contains unequivocal basal 

Middle Cambrian fossils; (2) 1,300 ft of clean, mediumbedded, slightly feldspathic, medium-pinkish-gray quartzite named the Lower or Middle Cambrian Cash Creek 

Quartzite; (3) a very distinctive carbonate zone, about 600 

ft thick, that includes massive dolomite at the top and 

thinly laminated, calcareous siltstone in the lower 500 ft; 

and (4) a basal, very massive, cross-laminated white 

quartzite, the quartzite of Boundary Creek, more than 400 

ft thick whose base is covered (Hobbs and others, 1968). 

The total sequence in terrane B is known only from this 

one area of outcrop, and nothing comparable has been 

identified in eastern Idaho. The nearest possible correlatives are some of the units described by McCandless 

(1982) in the southern Lemhi Range.

Several diverse structural features are important 

elements of terrane B: (1) the terrane is bounded at the 

top by a major thrust fault that brings quartzites of terrane D over the Ordovician carbonate of terrane B; (2) 

within terrane B the unconformity between the Ordovician carbonate and the lower Middle Cambrian siltstone 

documents uplift, steep faulting, and planation of the 

Cambrian section before deposition of the Ordovician carbonate; and (3) the base of the quartzite of Boundary 

Creek is not exposed, but the lowest outcrops show much 

brecciation that may relate to a thrust fault not far below.

Anticlinal folding followed the major thrusting of 

terrane D over terrane B and produced the present configuration in which the thrust plate is folded nearly 

parallel to the bedding. The possible extension of this fold, 

and the strata within it, beyond the known exposures is 

uncertain because of masking by overlying allochthons 

and the Challis Volcanics.

Terrane C

Terrane C consists of a block of undated strata of 

probable Early Ordovician and Late Cambrian age that 

includes four lithologic units and forms the core of the 

massive Bayhorse anticline that trends north across the 

entire Bayhorse area (fig. D2). Numerous mines and 

mineral occurrences of the central Bayhorse mining 

district are within this anticline. Strata that make up this 

structure form a generally concordant sequence that is 

more than 5,000 ft thick and comprises the Ramshorn 

Slate (Ordovician?), Bayhorse Dolomite (Ordovician?), 

and Garden Creek Phyllite (Cambrian?) of C. P. Ross 

(1934), and the informally named lower dolomite of 

Bayhorse Creek that was discovered by D. P. Wheeler 

(Umont Mining, Inc., oral commun., 1964).

The basal unit is massive dolomite that is barely 

exposed in the narrow valley of Bayhorse Creek, but which 

is known from drill-hole data to be underlain at a depth 

of about 200 ft by an intrusive porphyry. Overlying the 

dolomite is the Garden Creek Phyllite, a black, carbonaceous, internally folded argillaceous rock having welldeveloped axial-plane cleavage and whose original 

thickness can only be estimated at about 750 ft. Narrow 

quartz veins that cut this unit have yielded small amounts 

of lead, zinc, and silver. Its upper part grades into the thinbedded impure black limestone of the basal Bayhorse 

Dolomite that overlies the Garden Creek.

The Bayhorse Dolomite is of indeterminate original 

thickness. Its upper contact is an erosional disconformity 

or low-angle unconformity, and the unit ranges from 

several hundred to more than 1,200 ft thick. The lower 

one-half to two-thirds is thin- to medium-bedded, darkgray impure limestone; the upper part is massive, mediumto thick-bedded, cream-colored dolomite. At least two 

intervals near the top contain several thin-bedded, platy 

siltstone beds, silty dolomite, and dolomitic siltstone. 

Oolitic (pisolitic) zones, where they have been preserved, 

are distinctive marker horizons in the upper part of the 

section. In many places the dolomite, at its contact with 

the overlying Ramshorn Slate, is marked by collapse 

breccias, cavern fillings, and other features related to a 

karst topography that was developed during the interval 

of exposure before deposition of the Ramshorn. The 

structural and stratigraphic significance of this break in 

the depositional record is unknown, but this paleokarst 

horizon was very important in the localization of mineral 

deposits.

The uppermost unit of terrane C is the Ramshorn 

Slate. This unit is predominantly a thin-bedded, characteristically laminated argillite, siltstone, and sandy siltstone 

containing thin sandstone and pebble conglomerate near 

the top. Near Bayhorse and Garden Creeks a very thick, 

massive pebble and cobble channel conglomerate at the 

base marks a major stream valley that crossed the old 

eroded surface. This basal conglomerate, locally more

64 Challis 1°x2° Quadrangle, Idaho
than 400 ft thick, feathers out to the north and south in 

to sandy fades of the basal Ramshorn.

Most of the Ramshorn Slate has a very well 

developed slaty or axial-plane cleavage that gives the rock 

a deceptively simple structural grain or false bedding. The 

real bedding in the slate is deformed into irregular wavy 

folds and locally well-developed isoclinal drag folds, all 

of which are cut by the pervasive axial-plane cleavage. The 

complex internal structure of the unit precludes accurate 

measurement of thickness, but it probably totals at least 

2,400 ft.

No part of terrane C is definitely dated, but small 

shell fragments from several localities in the Bayhorse 

Dolomite strongly suggest an early Paleozoic age, and this 

evidence, together with general stratigraphic considerations, makes an age of Early Ordovician or Late Cambrian for all of the section most probable. Furthermore, 

none of the four formations that make up terrane C has 

been conclusively identified outside of the Bayhorse area, 

although Baldwin (1951), Ross (1937, 1947), and McCandless (1982) suggested possible correlations of certain units 

in the Lost River Range and the southern part of the 

Lemhi Range with the Ramshorn Slate, Bayhorse 

Dolomite, and Garden Creek Phyllite.

Terrane D

Rocks of terrane D, although including units that 

differ widely in lithology and age, make up a generally 

conformable sequence of a major allochthon whose upper contact is with the thrust sheet of terrane E, and whose 

lower contact, wherever seen, is with terrane B or the 

Ramshorn Slate of terrane C.

Two contrasting groups of strata are within terrane 

D: a lower, undated thick quartzite sequence, and an upper, well-dated Ordovician carbonate-quartzite sequence. 

The lower quartzite of this terrane comprises more than 

3,000 ft of heterogeneous, predominantly quartzitic strata 

that are parts of at least two and possibly more separate 

stratigraphic sequences that have been juxtaposed during the complex structural history of the area. The most 

prominent, thickest, and most widespread member is the 

Ordovician Clayton Mine Quartzite (Hobbs and others, 

1968) that can be traced or correlated with a fair degree 

of certainty from its type section near Clayton over much 

of the western half of the Bayhorse area (fig. D2).

Most of the Clayton Mine Quartzite is composed 

of poorly sorted, coarse- to medium-grained feldspathic 

quartzite that includes conglomerate layers, pebbly quartzite in the upper part, and a few thin, discontinuous 

dolomite layers near the base. Several siltstone intervals 

and scattered shale partings are distributed in the lower 

part of the section. Many of the thick quartzite layers are 

cross-laminated. The color of these rocks ranges from 

light gray, light yellowish orange, or light pink in the

western part to reddish gray, red, and purple to the east. 

Most of the quartzite is medium to thin bedded with some 

interbeds of siltstone. Feldspar is abundant, and much of 

it is very coarse. This quartzite sequence is underlain by 

the major thrust fault that overlies terranes B and C.

Two other quartzite units that are included in terrane B are probably related to the Clayton Mine Quartzite. One of these units is a quartzite sequence on upper 

Cash Creek, east of terrane B, that has many of the attributes of the Clayton Mine Quartzite but has pronounced differences in lithology and structure that 

preclude definite correlation. Most of the quartzite is thin 

to medium bedded with thin interbeds of siltstone and 

one siltstone interval at least 300 ft thick. Feldspar is 

abundant, and much of it is very coarse. This quartzite 

unit is underlain by a major thrust fault that is folded 

into a broad antiform; the quartzite rests on Ordovician 

carbonate at the exposures in upper Cash Creek and on 

Middle Cambrian shale at the exposures west of Squaw 

Creek.

A quartzite unit that is more questionably correlated 

with the Clayton Mine Quartzite is a sequence that is 

informally called the siltstone, sandy siltstone, and quartzite of the Rob Roy mine area. These strata along Kinnikinic Creek near the Clayton Silver mine make up a thick, 

heterogeneous sequence of various quartzite and siltstone 

layers and, in its lower half, scattered beds of pure 

dolomite.

The Clayton Mine Quartzite has been identified 

beyond the Bayhorse area only in the central Pioneer 

Mountains about 40 mi (miles) to the south-southwest. 

Possibly the Proterozoic Wilbert Formation, as described 

in the southern Lemhi Range by Ruppel and others (1975) 

and as further described by McCandless (1982), may correlate with some facies of the Clayton Mine Quartzite, but 

direct evidence is lacking.

A disconformity separates the Clayton Mine 

Quartzite from the overlying Ella Dolomite and represents 

an unknown time interval. It is marked by a sharp, drastic 

change in depositional regimes and by an apparent lowangle unconformity.

The Middle Ordovician Ella Dolomite, the Middle 

Ordovician Kinnikinic Quartzite, and the Middle Ordovician and younger (possibly lowermost Silurian) Saturday 

Mountain Formation lie above the disconformity (Hobbs 

and others, 1968). The Ella Dolomite is a massive, 

medium- to thick-bedded, laminated dolomite at least 

700 ft thick. The Kinnikinic is a fine-grained, pure, wellindurated quartzite, and the Saturday Mountain comprises 

more than 3,000 ft of impure dolomite and limestone with 

some thick zones of black shale and a few quartzite beds. 

These are all well-dated formations whose type localities 

are within the Bayhorse area (Ross, 1937; Hobbs and 

others, 1968). Both the Saturday Mountain Formation and 

the Kinnikinic Quartzite correlate with units far to the 

east and south of the Bayhorse area. Scattered roof

Sedimentary Terranes in the Bayhorse Area 65
pendants of metamorphosed calcareous and quartzitic 

rocks in the Idaho batholith to the west and south of the 

Bayhorse area are considered to be detached parts of these 

formations.

The allochthon that consists of the D terrane is 

folded in general conformity with the major folds of the 

underlying B and C terranes. In detail, however, the strata 

within the allochthon are locally very intensely folded and 

deformed. The allochthon appears to terminate eastward 

at the Salmon River lineament, and this boundary may 

represent an uplifted block against which the allochthon 

stalled or over which it rode. However, no extension of 

this thrust plate has been identified east of the lineament.

Terrane E

Rocks that make up terrane E, in the southwestern 

corner of the Clayton 7 1/2-minute quadrangle, are but 

a very small part of an extensive allochthon of stratigraphically and tectonically complex strata that has been 

transported for an unknown distance from the west to its 

present position where it overlies the Ordovician units of 

terrane D (fig. D2). These strata, described by Nilsen 

(1977) as the Salmon River sequence and redesignated the 

Salmon River assemblage in this chapter, consist of 

generally dark-gray to nearly black argillite, calcareous 

siltstone, siltstone, turbidite, and lesser amounts of finegrained quartzite with some relatively pure limestone. 

Much of the assemblage in the Bayhorse area is thin to 

medium bedded and consists of interlaminated siliceousfacies argillite, siltstone, fine-grained sandstone, and local 

limestone turbidites. Small-scale current structures, sole 

markings, and local contortion produced by syndepositional slumping are abundant. Farther to the west and 

south the allochthon includes less sand and silt and more 

laminated siliceous-facies black argillite with interlayered 

calcareous siltstone and local fine-grained quartzite. Most 

of the beds dip steeply to the west, and many have internal structures that indicate that tops are to the east. The 

structure within the allochthon, however, is very complex, 

and the great apparent thickness that is exposed westward 

from the Bayhorse area can most reasonably be explained 

by isoclinal folding, thrust faulting, and the attendant 

duplication of section that has not everywhere been 

recognized. The western edge of the allochthon is intruded 

by the Idaho batholith, overlapped by higher thrust plates 

of Pennsylvanian and Permian strata, or covered by 

Challis Volcanics (Hall, chap. J, this volume).

Although the strata of terrane E have much in common with the Lower Mississippian Copper Basin Formation of Ross (1962a), they include lithologies and faunas 

that contrast quite significantly in some places with those 

of the Copper Basin Formation. The Late Mississippian 

age of the Salmon River sequence (Nilsen, 1977) was based 

on two collections of fossils derived from float material

discovered at two localities. One of these faunas was in 

a single, semirounded float block that was collected in 

Deerspring Gulch, an informally named drainage on the 

northern side of the Salmon River opposite the mouth 

of Slate Creek, near the southern edge of the Thompson 

Creek 71/2 minute quadrangle. This material (USGS loc. 

no. 24408-PC) was examined by J. T. Dutro, Jr., in 

consultation with W. J. Sando, who reported (written 

commun., 1971) that "This collection is Mississippian in 

age, most probably middle or Late Mississippian. The 

large fragment of the spiriferoid brachiopod, Anthracospirifer, is most useful because that genus is now known 

to range from late Early Mississippian to Middle Pennsylvanian. Other elements of the fauna, though not 

diagnostic, are compatible with the Mississippian age 

assignment!'

Conodonts from the same material were studied by 

J. W. Huddle (written commun., 1971) who reported that 

several diagnostic species "* * * occur with Apatognathus 

in the upper part of the St. Louis Limestone in the 

Mississippi Valley. This collection could be as old as late 

St. Louis time or as young as early Chesterian time!' An 

abundant calcareous Formaniferal fauna was reported by 

Betty Skipp (written commun., 1972) to be "* * * a middle Visean (late Meramecian) fauna which represents F.Z. 

13-15 of the Mamet Foraminiferal zonation. These zones 

are present in the St. Louis and Ste. Genevieve Limestones 

of the midcontinent region; the fauna is typical of the 

lower part of the Scott Peak Formation of south-central 

Idaho (Mamet and others, 1971)!' Although the stratigraphic section on the ridge above this specimen locality 

contains widely scattered carbonate beds in the 

predominantly argillite-siltite sequence, no fossils were 

found in place. The closest known fossil-bearing carbonate 

section of comparable age is the Scott Peak Formation, 

a part of terrane F where exposed in the southeastern corner of the Bayhorse area (fig. D2).

Material from a second fauna! locality, in the lower 

part of Mill Creek near the northern boundary of the Livingston Creek 7 1/2-minute quadrangle, comprises a large 

angular float block that contains an abundant megafauna 

(USGS loc no. 27036-PC). This block, found about 1 mi 

southeast of the Deerspring Gulch locality, is roughly at 

the same stratigraphic horizon and of similar age. It has 

also been reported on by J. T. Dutro, Jr. (written commun., 1979), who stated, "The excellent silicified fossils 

are identifiable as an assemblage characteristic of Upper 

Mississippian strata in the northern Rocky Mountains. 

The assemblage is of probable late, but not latest, 

Chesterian age, approximately a correlative of Mamet 

Foraminiferal Zone 18. Beds of about this age occur also 

in the Surrett Canyon Formation of Idaho, the Horseshoe 

Shale Member of the Amsden Formation of Wyoming, 

and the Big Snowy Group of central Montana. The most 

diagnostic species is Anthracospirifer shawi sham Gordon, which occurs in Zone 18 equivalents in Wyoming

66 Challis 1 ° x 2° Quadrangle, Idaho
(Gordon, 1975)!' No similar material has been found in 

undisturbed outcrops anywhere else in terrane E.

Subsequent sampling of several well-defined carbonate and calcareous sandstone and siltstone horizons 

at five localities within the predominantly siltstone and 

calcareous siltstone parts of terrane E has produced conodonts whose ages range from Late Cambrian to Late 

Devonian. Two of these localities have been collected from 

by F. G. Poole, W. E. Hall, and S. W. Hobbs in the lower 

reaches of Thompson Creek in the Thompson Creek 7 

1/2-minute quadrangle. The first locality (USGS loc. no. 

11020-SD), on the west bank of Thompson Creek about 

2 mi above its mouth, was reported by J. E. Repetski to 

contain conodonts of Late Devonian age. Material from 

the second locality, about 0.5 mi south of the first and 

on the same side of Thompson Creek, includes three 

samples, two of which were collected by Poole and Hall 

(USGS loc. nos. 9851-CO and 9852-CO), and the third 

by Hobbs (USGS loc. no. 9618-CO). Conodonts from 

these three collections were studied by J. E. Repetski, who 

reported that two were of Late Cambrian age and the third 

"Most likely latest Cambrian to earliest Ordovician" 

Samples from outcrops in a roadcut on State Scenic Route 

75 about 1.1 mi west of the mouth of Thompson Creek 

were collected by F. G. Poole (USGS loc. nos. 10736-SD 

and 10737-SD) and examined by A. G. Harris, who 

reported them to be Middle Devonian. The remaining two 

known localities are in the lower valleys of Sheephead 

Creek and Last Chance Creek, which are west-side 

tributaries to Slate Creek about 4 mi and 5 mi, respectively, above its junction with the Salmon River, and in 

the west-central part of the Livingston Creek quadrangle. 

The sample from Sheephead Creek was collected by Poole 

and Hall (USGS loc. no. 11021-SD) from detrital 

limestone float blocks on the north side of the valley and 

less than 0.5 mi above its junction with Slate Creek, and 

reported by J. E. Repetski to have an age of "Middle Ordovician to Middle Devonian". Similar material from the 

north side of Last Chance Creek about 1,000 ft above its 

junction with Slate Creek was reported by Repetski to be 

Late Devonian.

Several aspects of the terrane E allochthon, including the widely different ages of strata, the documented 

structural complexity, and the variations in lithologic 

character, make the use of the term "sequence" inappropriate, as applied to these strata. It is here proposed 

that the strata that make up terrane E be called the Salmon 

River assemblage. No connotation as to origin is implied.

Terrane F

All the Paleozoic strata east of the Salmon River 

lineament (fig. D2) are included in this terrane, which is 

but a small part of the very extensive east-central Idaho 

Basin and Range Province. In the Bayhorse area the strata

are largely of middle and late Paleozoic age, are 

predominantly carbonate, and represent parts of the 

stratigraphic sections that are mostly absent in the terranes to the west. The stratigraphic section exposed here 

is an extension of the stratigraphy studied in the Lost River 

Range by Ross (1947) and Mapel and others (1965), and 

across southeastern Idaho and southwestern Montana by 

numerous other workers.

The Kinnikinic Quartzite of Ordovician age is the 

oldest unit in terrane F, and both it and the overlying 

Saturday Mountain Formation consist of significantly different facies and thicknesses than those described in terranes west of the lineament. In terrane F, the Saturday 

Mountain Formation grades stratigraphically upward into rocks that are equivalent to the Roberts Mountains 

Formation of Nevada. Succeeding strata make up the 

predominantly carbonate succession of the upper 

Paleozoic eastern facies that includes the Silurian Laketown Dolomite, the Devonian Beartooth Butte Formation, 

Grand View Dolomite, Jefferson Dolomite, and Three 

Forks Formation, and the Mississippian McGowan Creek, 

Middle Canyon, and Scott Peak Formations.

The structural pattern of terrane F, although 

containing many elements of the prevailing patterns in 

terranes B, C, and D, is dominated by an extensional 

regime of post-Eocene age that produced well-developed 

block-fault ranges with wide intermontane basins. The 

N.-30 0 W. regional trend of the basins and ranges and internal structures in the major fault blocks is distinctly different from the predominantly northern trends of fold 

axes and steep faults of the other terranes to the west.

THE SALMON RIVER LINEAMENT

A major fault or fault zone that I have called the 

Salmon River lineament marks the boundary between terrane F and terranes C and D, just west of it (fig. D2). This 

structure merits special attention in a description of the 

geology of the Bayhorse area because it stands out as a 

consistent, linear structural and stratigraphic boundary. 

Although poorly exposed and ill defined, the pronounced 

differences in structural trends, deformation, and stratigraphic facies across the lineament are strong evidence for 

its possible regional significance. It can be clearly identified only in the north-central part of the map area (fig. 

D2) where a north-northeast-trending zone of very complex faulting that separates terrane D from terrane F is 

present. The structure is buried by Challis Volcanics and 

Quaternary deposits to the north and south.

Of the three main differences that distinguish the 

geology on one side of the lineament from that on the 

other, the stratigraphic record is the most compelling. The 

heterogeneous Ordovician and older lithologies to the west 

contrast markedly with the very thick and predominantly carbonate Ordovician, Silurian, Devonian, and

Sedimentary Terranes in the Bayhorse Area 67
Mississippian sections that predominate east of the lineament. The Saturday Mountain Formation and the Kinnikinic Quartzite, however, are present as a part of the 

stratigraphic section on both sides, and notable facies 

changes within these units across the boundary are further cogent evidence of the structural significance of this 

break. West of the lineament the Kinnikinic Quartzite, 

although folded and faulted, is estimated to be 500 to 800 

ft thick. It contains partings and interbeds of black shale 

and siltstone, anomalous irregularities of bedding and 

grain size, and impurities that impart an uncharacteristic 

dark color to much of the unit. Kinnikinic Quartzite east 

of the lineament is significantly thicker, more massive, and 

very pure.

The Saturday Mountain Formation on opposite 

sides of the lineament shows an even more striking contrast both in composition and in thickness. The western 

facies consists of a thick heterogeneous section of massive 

dolomite, sandy dolomite, silty limestone, calcareous siltstone, black shale, and a few beds of dolomitic quartzite. 

The total thickness of the western facies of the Saturday 

Mountain Formation near Squaw Creek is estimated to 

be about 3,000 ft. The eastern facies that spans the same 

age range consists of a continuous section of relatively 

pure, medium- to thin-bedded dolomite that is less than 

1,000 ft thick.

Although the regional significance of the Salmon 

River lineament remains obscure, these substantial facies 

changes, which occur within a distance of a few miles, 

together with compelling evidence for tectonic shortening and structural discontinuities across the structure, 

indicate that the Salmon River lineament is a major tectonic feature. One notable aspect of the feature is the near 

absence of ore deposits east of this line in the Bayhorse 

area, as well as throughout the entire Lost River Range 

from Challis to Arco; this absence contrasts sharply with 

the numerous mines and prospects to the west.

SUMMARY

The six terranes as described above include strata 

of widely diverse source areas, environments of deposition, and, to some extent, structural style. Terranes A, B, 

and C have no exposed basal thrust faults, but work in 

many of the surrounding areas provided strong evidence 

that all rocks in central Idaho from Middle Proterozoic 

through Permian have been thrust from the west as a series 

of imbricate allochthons, and that the resulting structural 

pattern in the Bayhorse area has been further complicated 

by younger steep normal faults and the enigmatic Salmon 

River lineament. These various structural-stratigraphic terranes that have a complex geologic history and varied 

composition and that have been invaded by a series of 

igneous rocks of several ages provide a prime environment 

for the generation and localization of ore deposits, not 

only in the Bayhorse area, but throughout central Idaho.

REFERENCES CITED

Baldwin, E. M., 1951, Faulting in the Lost River Range of Idaho: 

American Journal of Science, v. 249, no. 12, p. 884-902.

Gordon, Mackenzie, Jr., 1975 [1976], Brachiopoda of the 

Amsden Formation (Mississippian and Pennsylvanian) of 

Wyoming: U.S. Geological Survey Professional Paper 

848-D, p. D1-D86, 13 pis.

Hobbs, S. W., 1980, The Lawson Creek Formation of Middle 

Proterozoic age in east-central Idaho: U.S. Geological 

Survey Bulletin 1482-E, 12 p.

Hobbs, S. W., Hays, W. H., and Ross, R. J., Jr., 1968, The Kinnikinic Quartzite of central Idaho redefinition and subdivision: U.S. Geological Survey Bulletin 1254-J, p. J1-J22.

Mamet, B. L., Skipp, Betty, and Sando, W. J., 1971, Biostratigraphy of upper Mississippian and associated Carboniferous rocks in south-central Idaho: American 

Association of Petroleum Geologists Bulletin, v. 55, no. 

1, p. 20-33.

Mapel, W. J., Read, W. H., and Smith, R. K., 1965, Geologic 

map and sections of the Doublespring quadrangle, Custer 

and Lemhi Counties, Idaho: U.S. Geological Survey 

Geologic Quadrangle Map GQ-464, scale 1:62,500.

McCandless, D. O., 1982, A re-evaluation of Cambrian through 

Middle Ordovician stratigraphy of the southern Lemhi 

Range: University Park, Pa., The Pennsylvania State 

University, M.S. thesis, 157 p.

Mclntyre, D. H., and Hobbs, S. W., 1978, Geologic map of the 

Challis quadrangle, Custer County, Idaho: U.S. Geological Survey Open-File Report 78-1059, scale 1:62,500.

Nilsen, T. H., 1977, Paleogeography of Mississippian turbidites 

in south-central Idaho, in Stewart, J. H., Stevens, C. H., 

and Fritsche, A. E., eds., Paleozoic paleogeography of the 

western United States: Society of Economic Paleontologists and Mineralogists, Pacific Section, Pacific Coast 

Paleogeography Symposium 1, p. 275-299.

Ross, C. P., 1934, Correlation and interpretation of Paleozoic 

stratigraphy in south-central Idaho: Geological Society of 

America Bulletin, v. 45, no. 5, p. 937-1000.

_____1937, Geology and ore deposits of the Bayhorse region, 

Custer County, Idaho: U.S. Geological Survey Bulletin 877, 

161 p.

_____1947, Geology of the Borah Peak quadrangle, Idaho: 

Geological Society of America Bulletin, v. 58, no. 12, 

p. 1085-1160.

_____1962a, Stratified rocks in south-central Idaho: Idaho 

Bureau of Mines and Geology Pamphlet 125, 126 p.

_____1962b, Upper Paleozoic rocks in central Idaho:

American Association of Petroleum Geologists Bulletin, 

v. 46, no. 3, p. 384-387.

Ruppel, E. T, 1980, Geologic map of the Patterson quadrangle, 

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

Quadrangle Map GQ-1529, scale 1:62,500.

Ruppel, E. T., Ross, R. J., Jr., and Schleicher, David, 1975, 

Precambrian Z and lower Ordovician rocks in east-central 

Idaho, in Precambrian and lower Ordovician rocks in eastcentral Idaho: U.S. Geological Survey Professional Paper 

889-B, p. 25-34.

68 Challis 1°x2° Quadrangle, Idaho
Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter E

Regional Geophysical Studies in the 

Challis Quadrangle

By DON R. MABEY and MICHAEL W. WEBRING

CONTENTS

Abstract 70

Introduction 70

Physical properties of the rocks 70

Gravity and magnetic features in the Challis quadrangle 70

Gravity and magnetic profiles 72

Conclusions 78

References cited 79

FIGURES

El. Regional gravity contour map of Idaho 71

E2. Residual gravity anomaly map of southern Idaho 73

E3. Complete Bouguer gravity anomaly map of the Challis quadrangle 74

E4. Residual gravity anomaly map of the Challis quadrangle 75

E5. Residual magnetic anomaly map of the Challis quadrangle 76

E6.-E8. Profiles of magnetic and gravity data:

E6. Line A-A 77

E7. Line B-B' 78

E8. Line C-C" 79

Regional Geophysical Studies 69
Abstract

Regional aeromagnetic and gravity data in the Challis 

quadrangle can be used to define several crustal blocks with 

distinctive lithologies and different geologic histories. The 

geophysical data confirm the general outlines of the Thunder 

Mountain and Van Horn Peak cauldron complexes. The part 

of the Idaho batholith that occurs within the quadrangle is a 

low-density mass that is weakly magnetic. The buried parts of 

Eocene intrusive masses appear more extensive than the parts 

exposed at the surface. A regional magnetic high in the northeastern corner of the quadrangle is produced by magnetite-rich 

zones in Precambrian sedimentary rocks. The high average 

topographic altitude of the quadrangle is an isostatic response 

to a mass deficiency at depth.

INTRODUCTION

The Challis quadrangle contains examples of most 

of the major geologic structures of the northern Rocky 

Mountains: a large Cretaceous batholith, Tertiary igneous 

intrusive bodies in a variety of sizes and shapes, Tertiary 

volcanic fields, including large caldera complexes, Paleozoic sedimentary rocks juxtaposed by major overthrusts, 

Precambrian sedimentary and metamorphic rocks, basinand-range structures, and an active seismic zone.

The U.S. Geological Survey compiled and interpreted regional gravity and aeromagnetic maps of the 

quadrangle as part of the CUSMAP (Conterminous 

United States Mineral Assessment Program) study of the 

Challis 1°X2° quadrangle. Geophysical data were 

available from several earlier studies. Gravity data had 

been obtained in earlier surveys by the Geological Survey 

(unpub. data) primarily in the intermontane valleys and 

along roads. The U.S. Department of Defense had obtained data using an approximate 15-km (kilometer) grid 

for the entire quadrangle. As part of the CUSMAP study, 

the data from these earlier surveys were merged into a 

single data set. Additional gravity stations were established 

to complete an approximate minimum 5-km grid over the 

quadrangle (Webring, 1981). The data were reduced to the 

Bouguer anomaly using a density factor of 2.67 g/cm3 

(grams per cubic centimeter) and computer-calculated terrain corrections to a distance of 167 km from each station. The Bouguer anomaly values for nearly all stations 

are considered to be accurate to within 1 mGal (milligal).

The aeromagnetic coverage is based primarily on 

earlier surveys by and for the Geological Survey, having 

flight lines 1.6 and 3.2 km (1 and 2 miles) apart and flight 

levels 2,740 and 3,650 m (meters) (9,000 and 12,000 feet) 

above sea level. One additional survey was flown in the 

southeastern part of the quadrangle to complete the 

coverage. The data from the several surveys were adjusted 

to a common datum using the International Geophysical 

Reference Field of 1975 and analytically continued to 

3,650 m.

Davis (in Cater and others, 1973) has qualitatively 

analyzed the aeromagnetic field in the north-central section of the quadrangle as part of the Idaho Primitive Area 

study. In addition to the aeromagnetic data presented in 

this report, data are available from the U.S. Department 

of Energy's NURE (National Uranium Resource Evaluation) program. Flight lines for these data are 9.66 km (6 

miles) apart and about 122 m (400 feet) above the land 

surface. The NURE data are useful in detailed analysis 

along the flight lines.

PHYSICAL PROPERTIES OF THE ROCKS

Most of the rocks in the Challis quadrangle can be 

divided into six groups based on density and magnetization. The most dense rocks, averaging about 2.7 g/cm3 , 

are of Precambrian age. Paleozoic sedimentary rocks have 

a lower average density, about 2.6 g/cm3 . The granodiorite and two-mica granite that make up the main mass 

of the Idaho batholith in the Challis quadrangle are 2.58 

g/cm3 . Tonalite and related rocks in the western margin 

of the Idaho batholith are more dense, averaging about 

2.67 g/cm3 . The larger Tertiary intrusive bodies are of 

two densities: the granite of the Sawtooth batholith 

averages about 2.56 g/cm3 , but the Tertiary diorite is 

more dense, about 2.7 g/cm3 . The Tertiary volcanic rocks 

and the Cenozoic sedimentary rocks are the least dense; 

the volcanic rocks range widely in density and magnetization. The Cenozoic and pre-Cretaceous rocks are generally 

weakly magnetic, but Precambrian sedimentary rocks 

locally contain magnetite-rich zones that are very strongly 

magnetic. Most of the rocks of the Idaho batholith are 

weakly to moderately magnetic. The Tertiary intrusive 

rocks are more strongly magnetic than the Idaho 

batholith. Some of the Tertiary volcanic rocks are very 

strongly magnetic, and remanent magnetization, both normal and reverse, is commonly greater than induced 

magnetization.

GRAVITY AND MAGNETIC FEATURES IN 

THE CHALLIS QUADRANGLE

The regional anomalies in the Challis quadrangle 

are related to the geographic setting of the quadrangle. 

Although the highest point in Idaho, Mount Borah, is 

about 24 km east of the quadrangle, the quadrangle has 

the highest average topography in Idaho. The quadrangle 

is largely within a broad regional Bouguer gravity low 

centered over central Idaho (fig. El). Excluding low 

Bouguer anomaly values in a local gravity low over Lemhi 

Valley east of the quadrangle, which reflect very thick 

basin fill, the Bouguer anomaly values in the Challis quadrangle are the lowest in the northwestern United States. 

The Bouguer anomaly, both regionally and within the

70 Challis 1°x2° Quadrangle, Idaho
117°

EXPLANATION

MILLIGALS

-80.0-

-95.0- 

-110.0

100 MILES

> :ISJiH!ii

nr

Figure E1. Regional gravity contour map of Idaho. Wavelengths shorter than 64 km removed during processing of data.

Challis quadrangle, shows a strong inverse correlation with 

regional elevation; the Bouguer anomaly values decrease 

toward the highest topography in the Challis quadrangle.

This broad gravity low reflects the approximate isostatic 

balance of the region, as a negative-mass anomaly underlies and supports the high topography. Much of this

Regional Geophysical Studies 71
regional negative-mass anomaly likely results from a 

thickened crust under central Idaho. Superimposed on this 

broad regional low are more local anomalies that reflect 

mass anomalies in the upper crust.

The local anomalies and departures from a 

calculated isostatic anomaly can be best identified if the 

broad regional field is removed. The regional field can 

be derived in several ways. One way is to correct the freeair anomaly value for each station for the gravity effect 

of a layer of rock having a thickness equal to the difference between the altitude of the station and the average 

altitude over an area around the station. The residual 

anomaly shown in figure E2 was generated using a 64-km 

averaging radius. The resulting value is about equal to the 

free-air anomaly that would be measured at a station at 

the average altitude and corrects the free-air anomaly for 

the strong dependence on the altitude of the individual 

stations. The resulting anomaly is a form of isostatic 

anomaly called the average free-air or Faye anomaly 

(Mabey, 1966). On the residual-gravity anomaly map of 

the region (fig. E2), the values in eastern Idaho are 

generally more positive than those to the west, and the 

major gravity high in the eastern part of the Challis quadrangle is a prominent regional feature. With values near 

zero, the area of the Idaho batholith appears to be more 

nearly consistent with the residual anomaly than the area 

to the east and was chosen to represent the residual gravity 

anomaly from which the interpreted cross sections (figs. 

E6-E8) were derived.

The complete Bouguer anomaly map of the Challis 

quadrangle (fig. E3) shows the strong inverse correlation 

with regional elevation; the trough of lowest values is over 

the Sawtooth batholith in the south and over the Van 

Horn Peak cauldron complex in the north. The residual 

gravity anomaly (fig. E4), from which the correlation with 

regional altitude is largely removed, better illustrates the 

local gravity anomalies. Gravity lows are produced by the 

low-density sedimentary rocks underlying the intermontane valleys. The largest low is over the Sawtooth Valley 

with a smaller low over Round Valley. Large gravity lows 

are over the Van Horn Peak and Thunder Mountain 

cauldron complexes. Gravity highs and lows are over the 

Cretaceous and Tertiary plutons. The largest is the low 

over the Idaho batholith. The crest of the large 

north-trending gravity high in the southeastern part of 

the quadrangle approximately coincides with the Bayhorse 

anticline in Paleozoic sedimentary rocks (Hobbs, chap. 

D, this volume).

The residual magnetic field (fig. E5) shows a broad 

low over the Idaho batholith. Most of the local highs 

within this low reflect Tertiary intrusions. In the west the 

older phases of the batholith are marked by increased 

magnetic intensity. The Sawtooth batholith and the Casto 

pluton both produce large magnetic highs. Several areas 

of Tertiary volcanic rocks produce positive or negative 

anomalies depending on the direction of remanent

magnetization. A very large magnetic high occurs over 

volcanic rock at the southeastern edge of the Van Horn 

Peak cauldron complex. An extensive magnetic high occurs in the area of the Bayhorse anticline. The magnetic 

high in the northeastern corner of the quadrangle is part 

of a 110-km-long high"trending northwest across Precambrian quartzite and phyllite. This regional anomaly trends 

toward the Blackbird cobalt mine a few kilometers north 

of the quadrangle but terminates short of the mine.

Several linear or nearly linear features are apparent 

on the gravity and magnetic maps (figs. E2-E5). The 

clearest feature, here called the Yankee Fork lineament (fig. 

E3), is one of a series of northeast-trending lineaments 

in eastern Idaho; the most prominent is the eastern Snake 

River Plain. The Yankee Fork lineament forms the northern limit of abundant Paleozoic rock and the southeastern 

limit of the Van Horn Peak cauldron complex; it approximately coincides with the trans-Challis fault system 

(Kiilsgaard and Lewis, chap. B, this volume). Another less 

well defined lineament is here called the Mayfield lineament (fig. E3). These two lineaments appear to define 

blocks of crust having contrasting physical properties that 

have responded differently to the regional stress affecting 

central Idaho.

GRAVITY AND MAGNETIC PROFILES

The major regional gravity and magnetic anomalies 

in the Challis quadrangle are shown by three profiles (figs. 

E4-E8); two that trend generally east across the quadrangle and one that trends southeast across the Van Horn 

Peak cauldron complex. Structural cross sections shown 

with the profiles (figs. E6-E8) were produced by a Marquardt (1963) nonlinear inversion technique operating on 

both the gravity and magnetic data (Webring, 1985).

At the western end of the northern east-west profile (A-A, fig. E6), both the Bouguer and average freeair gravity anomalies increase westward toward the border 

phases of the Idaho batholith. The Thunder Mountain 

cauldron complex is indicated by a gravity low and strong 

magnetic relief over part of the complex. Gravity and 

magnetic highs occur over the Casto pluton; these highs 

match the higher density and susceptibility of the rocks, 

measured from a limited number of samples, compared 

to the Idaho batholith. The gravity low over the Panther 

Creek graben indicates subsurface structure that correlates 

well with the structure inferred from the surface geology. 

The magnetic high at the eastern end of the profile is over 

the magnetite-rich zone in the Precambrian sedimentary 

rocks.

Profile B-B' crosses the Van Horn Peak cauldron 

complex (fig. E7). It shows anomalies over the Thunder 

Mountain cauldron complex and the Casto pluton that 

are similar to profile A-A. The broad gravity low over 

the Van Horn Peak cauldron complex (fig. E4) contains

72 Challis 1°x2° Quadrangle, Idaho
116'

on o o

s- 42'

EXPLANATION

MILLIGALS

50.0

45.040.0

35.030.025.020.0

15.0

10.0

5.00.0

Q. n' Figure E2. Residual gravity anomaly map of southern Idaho.
EXPLANATION

30 MILES

0 10 20 30 KILOMETERS

Figure E3. Complete Bouguer gravity anomaly map of the Challis quadrangle. Reduction density is 2.67 g/cm3 . ML, Mayfield lineament; YFL, Yankee Fork lineament.
116°

EXPLANATION

MILLIGALS 

25.0

a.

]] 10

101

1 

20

20

1 

30 KILOMETERS

30 MILES

1

Figure E4. Residual gravity anomaly map of the Challis quadrangle, showing lines of profiles shown in figures E6-E8.
EXPLANATION

NANOTESLAS 

2250.0

0 30 MILES0 10 20 30 KILOMETERS

Figure E5. Map of residual magnetic anomaly of the Challis quadrangle measured at 12,000 feet. Lines A-A', B-B', and C-C' show locations for profiles shown in 

figures E6-E8.
WEST

60 70 80 90 100 

DISTANCE, IN KILOMETERS

110 120 130 140 150 160

KliriMFTFRS THUNDER MOUNTAIN 

IDAHO BATHOLITH CAULDRON COMPLEX 

? 5 -i

SEA LEVEL 0.0 -

o o

X ^ ' / \ x 

,\ , 3 \ 4 Xx 5

VERTICAL EXAGGERATION X 2

Figure E6. Profiles of magnetic and gravity data, and model 

showing variations in physical properties of rocks along line AA', Challis quadrangle. Line of profiles shown in figures E4 and 

E5. Dashed lines indicate poorly resolved boundaries.

an interior low on the southeast, over the Twin Peaks

Body 

no.

1 

2 

3

Densitv 

(gm/cm )

2.66 

2.55 

2.70

PANTHER CREEK 

CASTO PLUTON GRABEN

6 1 \ \ 10 \ B

Magnetic 

susceptibility Rock 

(cgsxlO ) characteristics

1.0 Idaho batholith; tonalite. 

.5 Idaho batholith; granodiorite. 

3.8 Precambrian

caldera. This anomaly is caused partly by the low-density 

fill in the cauldron complex but may also be caused partly by underlying low-density intrusive bodies. The highest 

amplitude magnetic anomaly in the Challis quadrangle 

lies near the southeastern edge of the Twin Peaks caldera. 

The anomaly can be produced by a tabular, steeply dipping mass that is modeled as a ring-fracture intrusive 

body.

Profile C-C (fig. E8) shows a magnetic high over 

the Sawtooth batholith. Modeling of this anomaly indicates that part of the Sawtooth batholith is downfaulted 

under Sawtooth Valley. The gravity and magnetic lows that 

coincide approximately with the valley appear to reflect 

both the low-density basin fill and a weak reverse 

magnetization of the Sawtooth batholith. East of 

Sawtooth Valley, at 110 km from the western end of

metasedimentary rocks.

4 2.46 2.2 Thunder Mountain cauldron complex;

volcanic rocks more magnetic than 

no. 5.

5 2.46 1.19 Thunder Mountain cauldron complex;

volcanic rocks.

6 2.58 1.81 Casto pluton; granitic rocks.

7 2.32 .04 Van Horn Peak cauldron complex;

volcanic rocks.

8 2.23 1.10 Panther Creek graben; lower layer

rocks of unknown composition.

9 2.13 -.77 Panther Creek; volcanic rocks of

upper layer.

10 2.60 3.0 Precambrian metasedimentary rocks;

same as no. 3 at surface; unknown 

at depth.

11 2.48 -1.0 Volcanic rocks.

Regional Geophysical Studies 77
50 60 70 80 90 100 110 116

DISTANCE, IN KILOMETERS

KILOMETERS 

2.7-

SEA LEVEL 0.0-

THUNDER MOUNTAIN 

CAULDRON COMPLEX CASTO PLUTON

TWIN

PEAKS

CALDERA ,BAYHORSE ANTICLINE

VERTICAL EXAGGERATION X2

Figure E7. Profiles of magnetic and gravity data, and model showing variations in physical properties of rocks along line 6-6', Challis 

quadrangle. Line of profiles shown in figures E4 and E5. Dashed line indicates poorly resolved boundary.

profile C-C (fig. E8), a prominent magnetic high occurs 

over the White Cloud Peaks stock. The large magnetic 

low over the adjoining outcrop of Eocene volcanic rocks 

indicates reverse remanent magnetization. Gravity and 

magnetic highs coincide approximately with the Bayhorse 

anticline where it is intersected by the profile. The gravity high is offset slightly to the west and the magnetic high 

to the east. The axis of the gravity high closely follows 

the axis of the anticline, but the magnetic high does not. 

The eastern edge of the gravity high coincides approximately with the Salmon River lineament of Hobbs (chap. 

D, this volume).

CONCLUSIONS

The regional gravity and magnetic data for the 

Challis quadrangle indicate that the part of the Idaho 

batholith in the quadrangle is a low-density mass and is 

relatively weakly magnetic. Eocene intrusive masses appear to be more extensive in the subsurface than at the 

ground surface. The geophysical data generally confirm

Magnetic

Body Density susceptibility 

no. (gm/cm ) (cgsxlO )

Rock 

characteristics

1 2.47 1.18 Thunder Mountain cauldron complex; 

volcanic rocks.

2 2.56 1.89 Casto pluton; granitic rocks.

3 2.50 -.56 Twin Peaks caldera; weak reverse 

remanent magnetization.

4 2.35 -1.10 Twin Peaks caldera; surface layer, 

reverse remanent magnetization.

5 2.39 4.7 Twin Peaks caldera; ring-fracture 

intrusion, highly magnetic.

6 2.67 .1 Bayhorse anticline; Paleozoic

sedimentary rocks.

7 2.45 .1 Combination of alluvial valley fill 

and volcanic rocks.

the structures defined by surface mapping of the Thunder 

Mountain and Van Horn Peak cauldron complexes. A 

regional magnetic high in the northeastern corner of the 

quadrangle is produced by magnetite-rich zones in 

Precambrian sedimentary rocks. The high average 

topographic altitude of the quadrangle is an isostatic 

response to a mass deficiency at depth.

78 Challis 1°x2° Quadrangle, Idaho
75 100 125 

DISTANCE, IN KILOMETERS

150 170

SAWTOOTH

BATHOLITH 

KILOMETERS IDAHO BATHOLITH 7

SEA LEVEL 0.0- 1 \ 2 \ "

WHITE CLOUD PEAK 

3 /T°CK 9

BAYHORSE 

ANTICLINE

3 \ 4 ^^^ 5 ^ -i:

VERTICAL EXAGGERATION X2

Figure E8. Profiles of magnetic and gravity data, and model showing variations in physical properties of rocks along line C-C, 

Challis quadrangle. Line of profiles shown in figures E4 and E5. Dashed lines indicate poorly resolved boundaries.

The geophysical data can be used to define several 

blocks of crust within the Challis quadrangle. These 

blocks appear to have distinctive lithologies and different 

geologic histories.

The regional geophysical data can be used to 

develop exploration strategies and to identify broad targets 

for more intensive exploration. For example, the magnetic 

and gravity anomalies can be used to infer the location 

and three-dimensional geometry of some of the large intrusive bodies that are related to mineral occurrences, and 

subtle magnetic lows appear to indicate areas of alteration.

REFERENCES CITED

Cater, F. W., Pinckney, D. M., Hamilton, W. B., Parker, R. L., 

Weldin, R. D., Close, T. J., Zilka, N. T., 1973, Mineral 

resources of the Idaho Primitive Area and vicinity, Idaho, 

with a section on Aeromagnetic interpretation, by W. E. 

Davis: U.S. Geological Survey Bulletin 1304, 431 p.

Mabey, D. R., 1966, Relation between gravity anomalies and 

regional topography in Nevada and the eastern Snake River 

Plain, Idaho: U.S. Geological Survey Professional Paper 

550-B, p. B108-B110.

Marquardt, D. W., 1963, An algorithm for least-squares estimation of nonlinear parameters: Journal of the Society of 

Industrial and Applied Mathematics, v. 11, no. 2, p. 

431-441.

Magnetic

Body Density susceptibility 

no. (gm/cm3 ) (cgsxlO~J )

Rock 

characteristics

9

10

11

2.55

2.46

2.55

2.74

2.67

2.68

2.10

2.52

2.31

2.41

2.27

.1 

.1

-3.2

-4.0

.5

2.4

Idaho batholith; granodiorite. 

Sawtooth batholith; granite.

White Cloud Peaks stock; 

 granodiorite.

Bayhorse anticline; Paleozoic 

sedimentary rocks.

Bayhorse anticline; Precambrian 

metasedimentary rocks.

Paleozoic sedimentary rocks. 

Alluvial fill in Sawtooth Valley.

Combination of Paleozoic sedimentary 

rocks and Tertiary volcanic rocks.

Volcanic rocks. 

Volcanic rocks. 

Alluvial valley fill.

Webring, M. W., 1985, SAKI A FORTRAN program for 

generalized linear inversion of gravity and magnetic profiles: US. Geological Survey Open-File Report 85-122, 

104 p.

Webring, M. W., and Mabey, D. R., 1981, Principal facts for 

gravity stations in the Challis I°x2° quadrangle, Idaho: 

U.S. Geological Survey Open-File Report 81-652, 24 p., 1 

sheet, scale 1:250,000.

Regional Geophysical Studies 79

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter F

Tertiary Plutons and Related Rocks 

in Central Idaho

By EARL H. BENNETT1 and CHARLES R. KNOWLES1

CONTENTS

Abstract 82

Introduction 82

Previous work 82

Lithology of the plutons and related rocks 82

Monzodiorite and related rocks 85

Granite and related rocks 86

Other volcanic rocks 88 

Structural setting of the Tertiary plutons 89

The trans-Challis fault system and related features 89

Northwest-trending structures 90 

Anorogenic origin of the Tertiary plutonic rocks 92 

References cited 93

FIGURES

Fl. Map and table showing location of Tertiary and Cretaceous plutonic rocks 83 

F2. Classification diagram showing composition of samples of Tertiary plutonic

rocks 85

F3. Map showing trans-Challis fault system 86

F4. Map showing structural features in central Idaho and western Montana 91 

F5. Generalized cross section of the trans-Challis fault system 92 

F6. Simplified geologic map of the southern Atlanta lobe 93

TABLES

Fl. Correlation of Tertiary plutonic, hypabyssal, and volcanic rocks 87

F2. Microprobe analyses of selected elements in biotite 88

F3. Whole-rock chemical analyses of samples from Tertiary plutons 88

F4. Values of rubidium and strontium in plutonic rocks 90

1 Idaho Geological Survey, Moscow, Idaho 83843

Tertiary Plutons and Related Rocks in Central Idaho 81
Abstract PREVIOUS WORK

The Tertiary plutonic rocks in the Challis quadrangle are 

a bimodal suite consisting of 42- to 45-m.y. (million years)-old 

epizonal granite and 45- to 50-m.y.-old quartz monzodiorite. 

Both rock types are best exposed in the central one-third of 

the quadrangle. Each plutonic type has an associated hypabyssal 

intrusive equivalent, which collectively are known as the Idaho 

porphyry belt, and other dike swarms. Areal distribution and 

similarity of rock type suggest that each of the hypabyssal units 

has a volcanic equivalent that is part of the Eocene Challis 

Volcanics.

The Tertiary granite and quartz monzodiorite complexes 

are generally found near each other. These rocks have many 

of the characteristics of anorogenic (A-type) granite and are 

probably genetically related to intracontinental rifting or 

extension.

Many of the intrusive rocks in the north-central and 

eastern parts of the map area are along major northeast-trending faults that are related to Eocene extension. One of these 

structures is named the trans-Challis fault system. The system 

extends across Idaho and possibly Montana. Major northwest-trending basin-and-range structures in the southern Atlanta 

lobe terminate against the trans-Challis fault system. The system 

and similar northeast-trending structures are complicated by 

regional uplift that has been more extensive in the western part 

of the Idaho batholith, where plutonic rocks of Cretaceous are 

forming the core of the Atlanta lobe are exposed.

The Tertiary plutons, including the Sawtooth batholith and 

others in the Hailey 1°X2° quadrangle, south of the Challis 

quadrangle, are exposed in a series of rhomboid-shaped fault 

blocks formed by the intersection of major northwest- and northeast-trending faults. The northwest-trending faults are basinand-range structures that produced a series of horsts and 

grabens. Erosion of the horst blocks has exposed the Tertiary 

plutons; grabens contain older plutonic units of the Idaho 

batholith.

The pioneering study in the Challis quadrangle was 

of the Casto 30-minute quadrangle by C. P. Ross (1934). 

He mapped and named the Casto pluton (fig. Fl, area 

12) and several diorite bodies (fig. Fl, areas D, F). He also 

described and subdivided the Challis Volcanics and the 

Casto Volcanics. Cater and others (1973) mapped part of 

the diorite along the Rapid River (fig. Fl, area H) and 

noted that the Casto Volcanics are probably altered units 

of the Challis Volcanics; he abandoned the name "Casto 

Volcanics!'

Olson (1968) first described the Idaho porphyry belt, 

a series of hypabyssal dikes that extends from the Boise 

Basin to the Casto quadrangle. Hyndman and others 

(1977) recognized that the dike swarm appeared to truncate northwest-trending basin-and-range structures in 

eastern Idaho. The Sawtooth batholith (fig. Fl, area 26) 

was mapped by Reid (1963). Tschanz and others (1974) 

described Tertiary units in the Sawtooth National Recreation Area (fig. Fl, areas 32, 33, Q, R).

Studies that were made of the Tertiary plutonic 

rocks in general include those by Anderson (1952) and 

Ross (1928, 1963). Armstrong (1975) published numerous 

age dates on the Tertiary plutonic rocks and interpreted 

these dates. Swanberg and Blackwell (1973) summarized 

the state of knowledge about the Idaho batholith and 

noted the high radioactivity associated with the Tertiary 

granite. Also, they first described and named the Twin 

Springs pluton (fig. Fl, area 29). Bennett (1980a) described 

the Tertiary granite in Idaho and noted the similarities 

between numerous individual granite bodies. Criss (1981) 

and Criss and Taylor (1983) interpreted oxygen isotope 

data and age dates in the Tertiary granite in the Atlanta 

lobe of the Idaho batholith.

INTRODUCTION

Geologic mapping and related laboratory studies 

conducted as part of the Challis 1°X2° quadrangle 

CUSMAP (Conterminous United States Mineral Assessment Program) project have produced new information 

about the plutonic, hypabyssal, and volcanic rocks of Tertiary age in central Idaho. The bimodality of the plutonic 

suite and the close relationship between the plutonic rocks 

and equivalent hypabyssal and volcanic units is a major 

finding of the study (table Fl).

Acknowledgments. The following University of 

Idaho students assisted in the field work for this study: 

W. B. Strowd, R. S. Lewis, M. J. Barber, E. A. Kaufman, 

D. J. Shiveler, Ted Cammarata, P. C. Rogers, T. M. 

Scanlan, and D. A. Cockrum.

LITHOLOGY OF THE PLUTONS AND 

RELATED ROCKS

The Tertiary plutonic rocks in central Idaho are a 

bimodal suite of granite and quartz monzodiorite, as indicated by the modes and norms calculated from studies 

of samples of the rocks in the Challis quadrangle 

(fig. F2). This bimodality is similar to other Tertiary igneous systems in the western United States (Lipman and 

others, 1972). Rock names used are from the IUGS (International Union of Geological Sciences) classification 

(Streckeisen, 1976).

In many places the monzodiorite bodies are intimately mixed with numerous hypabyssal rhyodacite-dacite 

bodies. These mixed rocks are called monzodiorite in

82 Challis 1°x2° Quadrangle, Idaho
EXPLANATION

TERTIARY GRANITE 

TERTIARY MONZODIORITE

CRETACEOUS INTRUSIVE 

ROCKS

Figure F1. Map and table showing location of Tertiary and Cretaceous plutonic rocks. Modified from Bennett (1980a).

Tertiary Plutons and Related Rocks in Central Idaho 83
Tertiary granite plutons in Idaho 

[Do., ditto]

Map 

No. Name Age and dating method Reference

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

Bungalow stock- - 

Lolo batholith 

Whistling Pig pluton 

Running Creek pluton 

Painted Rocks pluton

(Painted Lake stock?). 

Carmen stock 

Bobcat Gulch 

Whimstick Creek stock 

Logan Creek stock -

43 m.y., K-Ar 

Age unknown 

51 m.y., K-Ar (hornblende) 

49 m.y., K-Ar 

Age unknown 

Hietanen (1969).

Nold (1974).

Toth (1983); Greenwood and Morrison (1973).

Toth and others (1983); Greenwood and

Morrison (1973). 

Toth and others (1983); Fisk (1969).

 do. 

49 m.y., K-Ar 

Age unknown 

38 m.y., K-Ar 

Ruppel (1978).

Sunning and Burnet (1981).

Bennett (unpub. data).

B. F. Leonard, U.S. Geological Survey, written

commun., 1979. 

Small unnamed stocks in Age unknown Cater and others (1973).

Idaho Primitive Area.

Crags pluton 44-48 m.y., K-Ar Cater and others (1975). 

Casto batholith 43 m.y., K-Ar Cater and others (1973); Ross (1934). 

Casto batholith, Age unknown Ross (1934).

Tincup outlier.

Knapp Peak stock do. Fisher and others (1983). 

Little Eightmile stock do. Ruppel (1978). 

Alder Creek stock do. Do. 

Falls Creek stock do. Do. 

North Fork stock do. Do. 

Ima stock (concealed) 48 m.y., K-Ar Ora Rostad, AMAX Corp., written commun.,

1978.

Blue Jay stock 51 m.y., K-Ar Ruppel (1978). 

Park Fork stock Age unknown Do. 

Big Timber stock do. Do. 

Gilmore stock 49 m.y., K-Ar (dike) Do. 

Sawmill Canyon stock Age unknown Do. 

North Sawtooth batholith do. Fisher and others (1983). 

Sawtooth batholith 45 m.y. (about), K-Ar Fisher and others (1983); Kiilsgaard

and others (1970); Reid (1963). 

Sawtooth batholith 43.6 m.y., K-Ar Kiilsgaard (1983).

(Tenmile).

Unnamed pluton Age unknown Fisher and others (1983). 

Twin Springs pluton 38-39 m.y., K-Ar Swanberg and Blackwell (1973); Criss

(1981).

Dismal Swamp stock 44 m.y., K-Ar Bennett (1980a); Criss (1981). 

Steel Mountain stock Age unknown Bennett (1980a). 

Ibex Canyon stock do. Tschanz and others (1974). 

Boulder stock do. Do. 

Prairie Creek stock do. C. M. Tschanz, U.S. Geological Survey,

written commun., 1979.

Summit Creek stock do. Dover and others, (1976). 

Pioneer Mountain stock 45 m.y., K-Ar Do. 

Mackay stock 40-50 m.y., zircon Nelson and Ross (1968). 

Lake Creek pluton 48 m.y., K-Ar Paull and others (1972). 

Shaw Mountain stock Age unknown C. M. Tschanz, U.S. Geological Survey,

written commun., 1979. 

Possible stock beneath do. Hall and others (1978).

Triumph mine. 

Clarendon Hot Springs do. Bennett (unpub. data).

stock.

Possible Tertiary do. Ross (1931); Anderson (1929). 

intrusion in Lava Creek 

mining district.

Soldier Mountain stock do. Bennett (unpub. data). 

Hall Gulch stock do. R. F. Delong (University of Idaho,

unpub. data).

Figure Fl Continued.

this chapter. Numerous rhyolite dikes in the Challis map 

area are believed to be related to the Tertiary granitic 

intrusions.

In general, the monzodiorite and the granite plutons 

are closely associated spatially. The Casto pluton, for example, is ringed by several monzodiorites (fig. Fl, areas 

D, E, F, H, I). The granite plutons and diorite are very

similar in age also (fig. Fl, table), and they may possibly 

be comagmatic. The ages of the three monzodiorites in 

the Bitterroot lobe of the Idaho batholith (fig. Fl, areas 

A, B, C) are highly speculative as there are no dates 

available for these units, but they are lithologically similar 

to the Tertiary diorites in the Atlanta lobe (Toth, 1983). 

As noted, each plutonic rock type has a hypabyssal

84 Challis 1°x2° Quadrangle, Idaho
Tertiary monzodiorite-dacite-rhyodacite complexes and plutons in Idaho

[Do., ditto]

Map 

No.

A 

B 

C 

D

E 

F 

G

H

I 

J 

K

L

M 

N 

0

P

Name

Unnamed monzodiorite 

-.-.-.-.-.^.-.-.-.A n ~. 

_ ______ . __ uu. J _ ___ ____ ^ ___

Lake Mountain stock 

Monzodiorite along the 

Rapid River. 

Monzodiorite at Pinyon 

Peak. 

Monzodiorite at 

Monumental Peak. 

Diorite at Cape Horn 

Lakes .

Unnamed monzodiorite 

Diorite at Jackson Peak 

Diorite at Horseshoe 

Bend. 

Diorite of Boise Basin 

Age and dating method

Age unknown - ---

_ _/4^\ __ _-__-____

47.2 m.y., K-Ar 

47.1 m.y., K-Ar 

Age unknown 

46-50 m.y., K-Ar 

Age unknown 

44-48 m.y., K-Ar 

Reference

Toth and others (1983). 

Do. 

Do. 

Ross (1934); Fisher and 

others (1983). 

Fisher and others (1983). 

Do. 

Do. 

Do.

Bennett (unpub. data) .

Robert Fleck, written 

commun. , 1983. 

Fisher and others (1983); 

Armstrong (1975); Olson (1968); 

Lewis (1984). 

Fisher and others (1983). 

Do. 

Do. 

Anderson (1934).

Fisher and others (1983); Anderson

Q Diorite of Ibex Canyon 

R Diorite of Silver Peak 

S Diorite of Trinity

Mountain. 

T Quartz diorite of Hailey

Age unknown------

 do . 

44 m.y., K-Ar 

(1947); Olson (1968); Percious and

others (1967). 

Tschanz and others (1974).

Do. 

Bennett (1980a); Criss (1981).

Hall and others (1974).

Figure F1 Continued.

equivalent (granite-rhyolite, monzodiorite-rhyodacite to 

dacite). Each type also has volcanic equivalents that are 

described later.

Monzodiorite and related rocks

Typical monzodiorite in the Challis quadrangle is 

coarse-grained, hornblende-bearing, quartz monzodiorite 

that contains mafic restite inclusions in places. A good 

example of this rock type is exposed at Jackson Peak (fig. 

Fl, area N). Similar occurrences are at Monumental Peak 

(area J), Lake Mountain (area G), below Pinyon Peak 

(area I), and at Cape Horn Lakes (area K). These monzodiorite plutons, although generally distinctive in the 

field, can be confused with tonalite of Cretaceous age in 

the quadrangle (Kiilsgaard and Lewis, chap B, this 

volume), as in the Loon Creek area, where both occur. 

The monzodiorite and tonalite can generally be distinguished in thin section, because plagioclase zoning in the 

older tonalite is simple, whereas plagioclase zoning in the 

Tertiary monzodiorite is quite complex (Lewis, 1984).

The diorites within the Challis quadrangle are 

typified by the large body along Rapid River (fig. Fl, area 

H). The diorite of Rapid River consists of small monzodiorite stocks and thousands of rhyodacite-dacite dikes, 

which intrude the older rocks of the Idaho batholith.

QUARTZ

PLAGIOCLASE

EXPLANATION

QUARTZ MONZODIORITE

 Modes 

D Norms 

GRANITE

 Modes 

O Norms

Figure F2. Classification diagram showing composition of 

samples from several Tertiary granite plutons and diorite com-- 

plexes, Atlanta lobe of the Idaho batholith, centra! Idaho. Composition determined from modes and norms of samples. 

Classification diagram from Streckeisen (1976).

These exposures are probably equal in size to the Sawtooth 

batholith. Rapid River and the Middle Fork of the Salmon 

River dissect the diorite, and the best exposures of it are

Tertiary Plutons and Related Rocks in Central Idaho 85
116° 115°

;;^JPANTHER CREEK 

'' GRABE

Approximate boundary

of the trans-Challis EXPLANATION

fault system

RHYODACITEDACITE TUFF

CONTACT

FAULT

MINE

20 MILES

J

20 KILOMETERS

Figure F3. Map showing the trans-Challis fault system and Tertiary plutonic and volcanic units. Units not described are shown 

on figure A2 (chap. A).

along the river valleys. The Middle Fork of the Salmon 

River at Indian Creek makes a U-shaped bend, following 

the contact between the diorite and older batholithic rocks 

to the north (Fisher, chap. A, fig. A2, this volume).

The rhyodacite-dacite dikes in the diorite of Rapid 

River are good examples of hypabyssal equivalents of the 

monzodiorite bodies in the Challis quadrangle. The dikes 

generally consist of fine-grained, gray to green rock that 

is porphyritic in places. Phenocrysts where present consist of hornblende, biotite, and plagioclase in varying proportions. These dikes are widespread throughout the 

batholith and, with rhyolite dikes that are the hypabyssal 

equivalent of the granite, make up most of the Tertiary 

dike swarms in the Atlanta lobe.

The probable volcanic equivalents of the plutonic 

(monzodiorite) and hypabyssal (rhyodacite-dacite) units 

are the tuff of Ellis Creek, the tuff of Eightmile Creek, 

and the tuffs of Camas Creek and Black Mountain 

(formerly called the Casto Volcanics) (table Fl; Ekren, 

chap. C, this volume). The map patterns of these units

closely conform to the geologic map pattern of the Casto 

Volcanics as shown by Ross (1934). The composition and 

age of these volcanic rocks are similar to the diorite stocks 

and dikes, and they are spatially related.

Granite and related rocks

Tertiary granite in the Idaho batholith is more 

widespread than monzogranite or diorite and is present 

in the central part of the Challis quadrangle (fig. Fl). 

However, lack of recognition of diorite in the field may 

have resulted in mapping of less diorite than is actually 

present.

Most of the Tertiary granite plutons in Idaho have 

the following characteristics, based on studies by Bennett 

(1980a) and Criss and Taylor (1983). These are similar to 

the characteristics of anorogenic granites as described by 

Loiselle and Wones (1979), Collins and others (1982), 

White and Chappell (1983), and Anderson (1983).

86 Challis 1°x2° Quadrangle, Idaho
Table F1. Tentative correlation of plutonic, hypabyssal, and volcanic rocks of Tertiary age in the Challis quadrangle, Idaho 

[Based in part on Mclntyre and others (1982), Ekren (chap. C, this volume), and Hardyman (chap. G, this volume)].

Plutonic unit Hypabyssal unit Volcanic unit

Pink granite 

Monzodiorite-----

UnknownRhyolite dikesDacite-rhyodacite dikes 

(green and gray).

Rhyodacite-andesite sills

Rhyolite flows and tuffs; includes rocks related to the Twin 

Peaks caldera. Explosive origin (Hardyman, chap. G, this 

volume).

Tuff of Eightmile Creek, tuff of Bills Creek, tuffs of Camas 

Creek and Black Mountain; centered on Van Horn Peak cauldron 

complex. Explosive origin (Ekren, chap. C, this volume).

Dacite, andesite, rhyodacite; mostly flows from localized 

vents. Nonexplosive origin (Mclntyre and others, 1982).

A major component of the granite is potassium feldspar, which imparts a distinctive pink color to the rocks. 

The composition is in the granite field of the IUGS 

classification system (fig. F2). 

Most granite has vertical jointing that results in rugged 

topography such as on the Sawtooth batholith (fig. Fl, 

area 26).

Miarolitic cavities indicate epizonal emplacement (probably within a few kilometers or less of the surface). 

Most rocks average two to three times more uranium, 

thorium, and potassium-40 than the Cretaceous Idaho 

batholith, as measured with a gamma-ray spectrometer. 

Smoky quartz crystals that line miarolitic cavities are 

a product of this high radioactivity. Many areas of 

granite are clearly delineated on the National Uranium 

Resource Evaluation (NURE) airborne gamma-ray spectrometry surveys in Idaho.

Most granite has a high content of large-cation elements, 

including tin, tungsten, uranium, thorium, molybdenum, and other elements such as niobium, zirconium, 

some lead, zinc, and beryllium. Beryl crystals were found 

in the Sawtooth batholith, the Dismal Swamp stock, and 

the Crags pluton (fig. Fl), according to Reid (1963) and 

Pattee and others (1968). Anomalous amounts of these 

elements are common in stream-sediment samples collected from Tertiary granite terranes (Kiilsgaard and 

others, 1970; Cater and others, 1979; Coxe and Toth, 

1983; Kiilsgaard, 1983a).

Microprobe analyses for fluorine in biotite from selected 

Tertiary granite and Cretaceous plutonic rocks (table F2) 

show a high fluorine content in the Tertiary granite. 

High fluorine indicates that these rocks formed from 

dry melts that were able to rise to within a few kilometers 

of the Earth's surface before crystallizing. Fluorite and 

topaz occur in miarolitic cavities in the Sawtooth 

batholith and other Tertiary granite plutons. 

Analyses of biotite show high iron-magnesium ratios 

(table F2). Biotite in the Tertiary granite is much closer 

in composition to annite, the iron-rich end member of 

the biotite series, than to phlogopite, the magnesiumrich end member. Biotite in the Knapp Peak stock (fig. 

Fl, area 14) is almost pure annite.

The high iron content may contribute to the 

positive aeromagnetic expression of the Tertiary granite.

Fayalite, the iron-rich end member of the olivine series, 

occurs as inclusions in the Sawtooth batholith and the 

Knapp Peak stock.

Perthitic potassium feldspar seen in thin section indicates that the granite has, in part, hypersolvus texture. Well-developed intergrowths of myrmekite and 

granophyric textures are common. Some phases of the 

Tertiary granite plutons contain two kinds of feldspar 

and no indication of hypersolvus texture. In others, 

large perthitic potassium feldspar grains are in a finer 

grained groundmass of plagioclase and quartz, indicating quenching of the magma by release of 

volatiles. In general, textures indicate high-temperature 

crystallization complicated by quenching. 

Oxygen isotope studies indicate that large hot-water 

convective systems composed of meteoric water, rather 

than magmatic water (based on oxygen and hydrogen 

isotope studies), circulated around the Tertiary granite 

bodies (Criss and Taylor, 1983). 

The Tertiary granite in Idaho is slightly peraluminous 

by Shand's (1927) classification, having A/CNK 

(aluminum/alkali) ratios of greater than 1.0 (table F3). 

The granite is probably alkali-calcic to calc-alkaline 

according to Peacock's (1931) classification, although 

this classification is difficult to apply as the Tertiary 

granite does not form a differentiated suite. The 

Reynolds and Keith (1982) scheme classifies 

peraluminous granite as alkaline or subalkaline using 

KjOCaO/SiC^ or CaO/SiO2 variation diagrams. The 

boundary between the two fields approximates the calc 

alkalic-alkali calcic boundary of Peacock (1931). Idaho 

Tertiary granite plots in the alkaline field on both variation diagrams.

Whole-rock chemical analyses (table F3) show 

that the Tertiary granite contains less A12O3 , MgO, 

and CaO; more K2O and SiO2 ; and about the same 

Na2O as the Cretaceous granite and Australian 

S- and I-type granitoid rocks (table F3). This composition is probably typical of anorogenic granite 

worldwide as indicated by the similarity to the average 

composition of Australian A-type granite. 

Preliminary data indicate that the Tertiary granite is 

lower in strontium and higher in rubidium than 

Cretaceous granitic rocks (table F4).

Tertiary Plutons and Related Rocks in Central Idaho 87
Table F2. Microprobe analyses of iron, magnesium, fluorine, 

and chlorine in biotite samples from Tertiary and Cretaceous 

intrusive rocks and stoichiometric annite and phlogopite 

[Values in percent; leaders (--), amounts less than 0.1 percent detection limit]

Sample source

Phlogopite (stoichiometric) - -- 

Diorite of Rapid River (Tertiary) ----- 

Two-mica granite (Cretaceous) --- -

Pat Hughes (Thompson Creek) stock 

(Cretaceous).

Fe

17.1 

12.9 

21.6 

29.0

33.9 

.0 

16.5 

19.8

18.7

14.7 

14.6

Mg

3.8 

8.3 

3.5

.1

.0 

18.3 

6.9 

2.8

4.8

6.2 

7.9

F

3.7 

4.2 

1.7 

1.2

.4 

.7

.9

1.0 

1.2

Cl

0.43 

.41

The hypabyssal equivalent of the granite is 

numerous rhyolite dikes. These dikes are throughout the 

Challis map area but are concentrated in the trans-Challis 

fault system.

The Knapp Peak stock (fig. Fl, area 14) is interesting 

because it contains features of both granite and rhyolite. 

Most of the stock is very fine grained and in places looks 

like rhyolite. Its upper part, however, is typical Tertiary 

granite. The fine-grained part of the stock is similar to 

other rhyolite plugs and stocks that intrude faults in the 

trans-Challis fault system (fig. F4). These intrusive bodies, 

in turn, connect to rhyolite flows that are part of the

Challis Volcanics (Hardyman, chap. G, this volume). Also 

on Copper Mountain at the north end of the Sawtooth 

batholith, granite becomes intrusive rhyolite through a 

short distance (Kiilsgaard and Bennett, chap. M, this 

volume). The intrusive rhyolite is overlain by rhyolite flows 

between Beaver and Knapp Creeks, in the Knapp Creek 

graben (fig. A2). The graben is also within the transChallis fault system.

Other volcanic rocks

Major units of the Challis Volcanics, including the 

tuff of Ellis Creek, the tuffs of Camas Creek and Black 

Mountain (formerly called the Casto Volcanics), and 

rhyolite in the trans-Challis fault zone, are related to the 

diorite stocks and dikes and granite plutons. The lower 

units of the Challis Volcanics, dacite, rhyodacite, and 

andesite, do not appear to be related to the bimodal Tertiary plutonic rocks. These rocks are called the rhyodaciteandesite unit in this chapter. Ekren (chap. C, this volume) 

describes the rhyodacite-andesite unit and identifies 

numerous small volcanic centers for these rocks. These 

rocks are mostly flows and are not related to explosive 

volcanism, as are the volcanic rocks that are related to 

the bimodal plutonic rocks. The rhyodacite-andesite unit 

has a different composition and covers a much broader 

area than the other units of the Challis Volcanics. The 

rhyodacite-andesite unit has not been related to any major plutonic bodies.

Table F3. Whole-rock chemical analyses of samples 

[Leaders (-), no data; A/CNK,

A1 2 03 -

Fe2°3 

FeO -

MgO 

CaO 

Na20 

K20 ~ 

Ti02 

P2 o5 

MnO -

A/CNK 

1

75.22

13.07

.67

.79

.25

.69

3.54

4.70

.15

.07

.03

1.19

2

77.11

12.54

.21

.25

.24

.35

3.78

4.52

.09

.06

.05

1.07

3

74.83

13.50

.47

.53

.12

.63

3.46

4.93

.04

.05

.02

1.06

4

75.20

12.90

1.55

 

.20

.55

3.34

5.02

.14

.10

.02

1.08

5

73.31

13.50

.89

1.04

.20

.57

3.14

5.01

.24

.05

.06

1.17

6

73.22

14.17

.96

.91

.30

1.30

3.42

5.20

.24

.08

.04

1.12

7

77.33

12.75

.35

.41

.26

.13

3.58

4.87

.10

.02

.02

1.12

8

75.69

13.01

.24

.28

.23

.49

3.50

5.01

.04

.03

.02

1.08

9

75.88

13.22

.27

.24

.27

.21

4.15

4.99

.09

.04

.05

1.05

10

76.30

12.81

.23

.20

.10

.24

3.89

4.71

.05

.01

.03

1.07

1. Sawtooth batholith, average of nine analyses.

2. Dismal Swamp stock, average of two analyses.

3. Granite of Cape Horn Lakes (Lewis, 1984), average of six analyses.

4. Running Creek pluton (Coxe and Toth, 1983), average of 52 analyses.

5. Whistling Pig pluton, (W. E. Motzer, University of Idaho, written commun., 1985), average of 12 analyses.

6. Painted Rock pluton, northern part (W. E. Motzer, University of Idaho, written commun., 1985), average of six analyses.

7. Lolo batholith, one analysis.

8. Bungalow stock, one analysis.

9. Twin Springs pluton, two analyses.

10. Steel Mountain stock, one analysis.

SAMPLE

88 Challis 1°x2° Quadrangle, Idaho
In the Loon Creek area the rhyodacite-andesite 

rocks are the extrusive equivalent of andesite sills and 

dikes. Hundreds of sills intrude roof pendants of 

Paleozoic rocks and fill joints in batholithic rocks just 

west of the General, a 10,300-ft (foot)-high peak west of 

Mayfield Creek. These sills and a vent complex described 

by Foster (1982) probably fed rhyodacite-andesite flows 

south of the General in the Mt. Jordan area.

STRUCTURAL SETTING OF THE 

TERTIARY PLUTONS

Several Tertiary plutons and related units are within 

the trans-Challis fault system, a major structural feature 

that crosses the Challis quadrangle. South of this northeast-trending fault system, Tertiary granite is exposed in 

upthrown blocks between major northwest-trending basinand-range faults.

The trans-Challis fault system and related features

The trans-Challis fault system is a major northeast-trending structural zone that crosses the Challis quadrangle (figs. A2, F4-F6; chaps. B, M). A northeast-trending structural zone was first suggested by Hyndman and 

others (1977), who recognized that the Idaho porphyry 

belt appears to truncate northwest-trending basin-andrange mountains like the Beaverhead Mountains and

Lemhi and Lost River Ranges in eastern Idaho. The fault 

zone was delineated by mapping done as part of the 

CUSMAP study. The fault zone includes the Custer 

graben, the Panther Creek graben, and the Knapp Creek 

graben (figs. A2, F6). Confinement of rhyolitic volcanic 

rocks to the Custer graben is apparent. The rhyodaciteandesite volcanic units, Paleozoic rocks, and underlying 

batholithic rocks are exposed in upthrown fault blocks 

near the margins of the trans-Challis fault system.

The trans-Challis fault system is a southwestern 

extension of the Great Falls lineament, a major structural 

zone that trends northeast across central Idaho, from the 

Panther Creek area in the northeast corner of the Challis 

quadrangle to the Canadian border and perhaps beyond. 

O'Neill and others (1982) and O'Neill and Lopez (1983) 

described the lineament and showed that it passes just west 

of the Boulder batholith in Montana (fig. F5). The lineament is not only a significant regional structure but also 

largely controlled emplacement of Tertiary plutonic and 

volcanic rocks and related mineralization (Kiilsgaard and 

Bennett, chap. M, this volume).

The tuffs of Camas Creek and Black Mountain and 

the tuff of Ellis Creek, related to the monzodiorite complexes, and eruptive centers in the Van Horn Peak 

cauldron complex (Ekren, chap. C, this volume), are mostly confined to the trans-Challis fault system. Likewise, 

many rhyolite intrusions and related rhyolitic volcanic 

rocks, including the Twin Peaks caldera (Hardyman, chap. 

G, this volume), are in this fault system.

from Tertiary plutons in Idaho and comparative rocks 

alumina-alkali ratio]

11

73.74

14.17

1.13

1.29

.73

.95

3.72

4.85

.27

.08

.05

1.08

12

75.22

13.39

.49

.56

.22

.25

3.37

5.27

.09

.01

.03

1.14

13

73.60

12.69

.99

1.72

.33

1.09

3.54

4.51

.33

.09

.06

1.03

14

67.98

14.49

1.27

2.57

1.75

3.78

2.95

3.05

.45

.11

.08

0.97

15

69.08

14.30

.73

3.23

1.82

2.49

2.20

3.63

.55

.13

.06

1.25

16

70.42

15.44

.92

1.42

.87

2.56

3.70

3.19

.39

.13

.04

1.09

17

72.00

15.30

1.35

 

.43

1.80

4.01

3.80

.17

.08

.03

1.07

18

65.57

15.27

1.61

2.35

2.12

3.22

3.81

3.84

.62

.22

.07

0.94

19

70.13

15.36

1.05

1.21

1.13

2.76

3.87

3.75

.36

.15

.05

1.00

20

59.30

18.30

1.62

4.71

5.53

7.70

3.34

3.14

.91

.38

.12

0.80

DESCRIPTIONS

11. Mackay stock, one analysis.

12. Crags pluton, one analysis.

13. A-type granite (White and Chappell, 1983), average of 31 analyses.

14. I-type granite (White and Chappell, 1983), average of 532 analyses.

15. S-type granite (White and Chappell, 1983), average of 316 analyses.

16. Granodiorite of Atlanta lobe, average of 45 analyses.

17. Granodiorite of Bitterroot lobe (Cox and Toth, 1983), average of 69 analyses.

18. Diorite of Jackson Peak, average of four samples.

19. Diorite of Trinity Mountain, one analysis.

20. Diorite of Cape Horn Lakes (Lewis, 1984), average of six analyses.

Tertiary Plutons and Related Rocks in Central Idaho 89
Table F4. Values of rubidium and strontium in samples of 

plutonic rocks in the Challis quadrangle and the Bitterroot lobe 

of the Idaho batholith

[Values are in parts per million. Challis quadrangle data is preliminary, 

from Robert Fleck (U.S. Geological Survey, written commun., 1983); 

Bitterroot lobe data is from Hyndman (1984); Do., ditto]

Sample Rubidium Strontium

Tertiary granite:

Sawtooth batholith (Tenmile) 167.7 83.9 

D o. 143.2 345.0 

Red Mountain stock (near Knapp

Peak stock). 160.0 129.0 

Sawtooth batholith (near 154.0 133.0

Stanley Lake). 

Atlanta lobe rocks (average):

Leucogranite (2 samples) 

Bitterroot lobe rocks (average):

 /H.I 

(36-154) 1 

on o

 86.3

QO 1

(66-99) 1

(610-1, 517) 1 

785.0 

750.0

896.9 

(583-1, 080) 1

Range of values in samples.

West of the trans-Challis fault system are many Tertiary intrusive bodies. A broad area of northeast-trending dike swarms, including the Pistol Creek swarm, 

together form the Idaho porphyry belt. This name should 

perhaps be used only for the area west of the fault system, 

as there are numerous other dike swarms throughout the 

southern part of the Atlanta lobe, outside of the Idaho 

porphyry belt. The Thunder Mountain cauldron complex 

(fig. A2), the Casto batholith (fig. Fl, area 12), the diorite 

of Rapid River (fig. Fl, area H), and other related intrusive bodies are also just west of the fault system. The 

Idaho porphyry belt and the Tertiary intrusions represent 

a deeper zone of exposure than that in the fault system, 

because the two-mica granite core of the Atlanta lobe west 

of the fault system has been uplifted relative to the fault 

system (Kiilsgaard and Lewis, chap. B, this volume).

Regional structural elements in central Idaho include lineaments parallel to the trans-Challis fault system 

bounding the northern side of the eastern Snake River 

Plain (Mabey and Webring, chap. E, this volume) and 

extending from Sun Valley, Idaho, to just north of Dillon, 

Mont., and beyond. The Sun Valley-Dillon lineament may 

have formed the straight eastern side of the Boulder 

batholith (Ruppel, 1982).

Northwest-trending structures

Well-known northwest-trending structures in the 

Challis quadrangle include the faults bordering the 

Beaverhead Mountains and the Lemhi and Lost River 

ranges (Ruppel, 1982). As noted, these faults abruptly terminate at the trans-Challis fault system. In addition, other

faults parallel to these major ranges cut through the 

southern Atlanta lobe, to the west. They include the 

Sawtooth, Montezuma, Deer Park, Trinity Mountain, 

Boise Front, and several unnamed faults (fig. F5). These 

faults produced a series of horsts and grabens in the Atlanta lobe rocks similar to the basin-and-range type horsts 

and grabens to the east. These faults also terminate against 

the trans-Challis fault system; for example, the Deer Park 

fault ends abruptly at the South Fork of the Payette River 

(Kiilsgaard, 1983b). The Tertiary granite and monzodiorite 

are exposed in horsts between the northwest-trending 

faults. Intervening grabens contain older batholithic rocks 

that were mineralized during the Tertiary (Kiilsgaard and 

Bennett, chap. M, this volume).

The intersection of northwest-trending basin-andrange structures and northeast-trending Eocene extensional structures may be important loci for earthquake 

activity. The intersection of the Sun Valley-Dillon lineament and the Lost River Range fault marks the epicenter 

of the magnitude-7.2 earthquake of Oct. 28, 1983, just 

north of Mackay, Idaho.

The simple horst and graben structure in the Atlanta 

lobe could be related to either basin and range extension 

or to a scheme of block faulting, along basement-rockcored block faults proposed by Ruppel (1982). We believe 

that exposures of Tertiary rocks in the southern Atlanta 

lobe are a result of possible Precambrian structures reactivated during basin-and-range extension. In the southern 

part of the Atlanta lobe, the simple horst-and-graben 

structure is complicated by a series of poorly exposed 

northeast-trending fault segments that parallel the transChallis fault system. The Atlanta dike swarm that lies just 

north of Atlanta, Idaho, may be in one of these fault 

zones, as may the Middle Fork of the Boise River. The 

southern end of the Sawtooth batholith may be truncated 

by one of these zones (fig. F7). The northeast-trending 

faults combined with the northwest-trending faults that 

produced the horst-and-graben structure have broken the 

southern Atlanta lobe into rhomboid-shaped blocks that 

have moved up and down relative to each other. The 

Sawtooth batholith is the most elevated block, and Atlanta 

Hill is the most downthrown block recognized so far.

Although the northwest-trending faults have not 

been observed in the western part of the Atlanta lobe in 

the Challis quadrangle, and in places terminate against 

the trans-Challis fault system, they may possibly have extended farther to the northwest at one time. Altered porphyritic granodiorite trends northwest across the Atlanta 

lobe (fig. F5; Kiilsgaard and Lewis, chap. B, this volume). 

These rocks may be related to ancestral northwest-trending basement faults. Traces of most of the northwest-trending faults in the western half of the quadrangle 

may have been destroyed by the uplift of the Atlanta lobe 

and related faulting, such as the Deadwood fault, plus 

the development of the trans-Challis fault system and 

related volcanic activity.

90 Challis 1°x2° Quadrangle, Idaho
50 100 KILOMETERS 

EXPLANATION

Ki

QUATERNARY SURFICIAL DEPOSITS, UNDIVIDED 

TERTIARY VOLCANIC ROCKS 

CRETACEOUS IDAHO BATHOLITH 

PALEOZOIC SEDIMENTARY ROCKS

PRECAMBRIAN SEDIMENTARY AND

METAMORPHIC ROCKS 

MEGACRYSTIC GRANODIORITE (METASOMATIZED

TONALITE)

CONTACT 

FAULT

Figure F4.

strong and

Map showing selected structural features in central Idaho and western Montana. Strontium isotope line from Armothers (1977).

Tertiary Plutons and Related Rocks in Central Idaho 91
WEST

-CUSTER GRABEN

EAST

Rhyolite volcanic 

and intrusive rocks

EXPLANATION

Rhyolitic 

volcanic

and

intrusive 

rocks

An de site 

lavas

Tonalite

Granodiorite

Paleozoic rocks

CONTACT Queried where inferred

FAULT Showing direction of movement 

queried where inferred

EOCENE

CRETACEOUS

Figure F5. Generalized cross section of the trans-Challis fault system from Loon Creek to east of the Salmon River, Challis quadrangle. Horizontal scale about 1:250,000; no vertical scale.

ANOROGENIC ORIGIN OF THE 

TERTIARY PLUTONIC ROCKS

The bimodal Tertiary plutonic rocks in the Challis 

quadrangle are similar to anorogenic suites in other parts 

of the world. The main characteristics of A-type, 

anorogenic granite are very similar to the characteristics 

of the Tertiary granite, listed previously in this chapter. 

Loiselle and Wones (1979) described the characteristics of 

A-type or anorogenic granite. More recently, Collins and 

others (1982) and White and Chappell (1983) described 

anorogenic granite in Australia, compared it to other Atype granite in the world, and contrasted the A-type with 

the more familiar orogenic S- and I-type granite. Anderson (1983) summarized the characteristics of Proterozoic 

anorogenic granite.

The source and genesis of anorogenic granite are 

debated. Collins and others (1982) and Loiselle and Wones 

(1979) believed that the magmas from which anorogenic 

granite was formed came from a granulite terrane from 

which an earlier magma had been produced. The magma 

that formed the Cretaceous Idaho batholith must have

had a major crustal component because the core of the 

batholith consists of S-type, two-mica granite (Kiilsgaard 

and Lewis, chap. B, this volume). Anderson (1983) suggested an alternative source for anorogenic granitic 

magmas, fusion of the lower crust containing I-type 

granite. All these authors agreed that these bimodal rocks 

formed by some type of continental extensional force, 

perhaps related to intracontinental rifting.

The diorite stocks and dikes, the other half of the 

anorogenic suite, are the more mafic analog of the granite. 

The origin of the diorite is not clear, although bimodal 

volcanism, generally producing basalt and rhyolite, is 

widely recognized as an important result of basin-andrange extension that postdates the Eocene plutonicvolcanic activity in Idaho. The diorite was probably 

formed from hybrid magmas as described by Hamilton 

(1981), "Arc magmas generated at depths near 100 

kilometers by dehydration of subducting slabs are olivine 

rich mela-basalts, but the magmas that reach the surface 

in mature continental magmatic arcs have an average composition near that of rhyodacite. Depth-varying fractionations and equilibrations profoundly modify the initial

92 Challis 1°x2° Quadrangle, Idaho
116° 115°

44C

EXPLANATION

TERTIARY GRANITE

TERTIARY QUARTZ

MONZODIORITE 

DIKE SWARM

FAULT

Generalized ojjtline 

of Atlanta lobe of 

Idaho batholith

20 MILES

20 KILOMETERS

Figure F6. Simplified geologic map of the southern Atlanta lobe of the Idaho batholith. Modified from Bennett (1980b).

magmas!' The diorite complexes in Idaho may be contaminated mafic magmas generated during extension. 

These magmas provided heat to melt the lower crust, 

forming anorogenic granite.

REFERENCES CITED

Anderson, A. L., 1929, Geology and ore deposits of the Lava

Creek district, Idaho: Idaho Bureau of Mines and Geology

Pamphlet 32, 70 p. 

_____1934, Geology of the Pearl-Horseshoe Bend gold belt,

Idaho: Idaho Bureau of Mines and Geology Pamphlet 41,

36 p. 

_____1947, Geology and ore deposits of Boise basin, Idaho:

U.S. Geological Survey Bulletin 944-C, p. 113-319. 

_1952, Multiple emplacement of the Idaho batholith:

Journal of Geology, v. 60, no. 3, p. 255-265.

Anderson, J. L., 1983, Proterozoic anorogenic granite plutonism 

of North America: Geological Society of America Memoir 

161, p. 133-154.

Armstrong, R. L., 1975, The geochronometry of Idaho: 

Isochron/West, no. 14, p. 1-50.

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

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

and their Sr isotopic composition, Oregon, Washington, 

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

p. 397-411.

Bennett, E. H., 1980a, Granitic-rocks of Tertiary age in the Idaho 

batholith and their relation to mineralization: Economic 

Geology, v. 75, no. 2, p. 278-288.

_____1980b, Reconnaissance geology and geochemistry of 

the Trinity Mountain-Steel Mountain area, Elmore County, Idaho: Idaho Bureau of Mines and Geology Open-File 

Report 80-11, 56 p.

Tertiary PIutons and Related Rocks in Central Idaho 93
Bunning, B. B., and Burnet, F. W., 1981, Copper-molybdenum 

mineralization in the Bobcat Gulch porphyry system, 

Lemhi County, Idaho: Northwest Mining Association, annual meeting, 1981, Spokane, Wash.

Cater, F. W., Pinckney, D. M., Hamilton, W. B., Parker, R. L., 

Weldin, R. D., Close, T. J., and Zilka, N. T., 1973, Mineral 

resources of the Idaho Primitive Area and vicinity, Idaho: 

US. Geological Survey Bulletin 1304, 431 p.

Cater, F. W., Pinckney, D. M., and Stotelmeyer, R. B., 1975, 

Mineral resources of the Clear Creek-Upper Big Creek 

study area, contiguous to the Idaho Primitive Area, Lemhi 

County, Idaho: U.S. Geological Survey Bulletin 1391-C, 

41 p.

Collins, W. J., Beams, S. D., White, A. J. R., and Chappell, B. 

W., 1982, Nature and origin of A-type granites with particular reference to southeastern Australia: Contributions 

to Mineralogy and Petrology, v. 80, no. 2, p. 189-200.

Coxe, B. W, arid Toth, M. L, 1983, Geochemical maps of the 

Selway-Bitterroot Wilderness, Idaho County, Idaho, and 

Missoula and Ravalli Counties, Montana: U.S. Geological Survey Miscellaneous Field Studies Map MF-1495-C, 

scale 1:125,000.

Criss, R. E., 1981, An18O/16O, D/H and K-Ar study of the 

southern half of the Idaho batholith: Pasadena, California Institute of Technology Ph.D. thesis, 401 p.

Criss, R. E., and Taylor, H. P., Jr., 1983, An 18O/16O and D/H 

study of Tertiary hydrothermal systems in the southern half 

of the Idaho batholith: Geological Society of America 

Bulletin, v. 94, no. 5, p. 640-663.

Dover, J. H., Hall, W. E., Hobbs, S. W., Tschanz, C. M., Batchelder, J. N., and Simons, F. S., 1976, Geologic map of 

the Pioneer Mountains region, Blaine and Custer Counties, Idaho: US. Geological Survey Open-File Report 76-75, 

scale 1:62,500.

EG&G Geometries, 1979, Aerial gamma ray and magnetic survey, 

Idaho project, Hailey quadrangle of Idaho final report: 

US. Department of Energy Open-file Report GJBX-1080, 

731 p.

Fisher, F. S., Mclntyre, D. H., and Johnson, K. M., 1983, 

Geologic map of the Challis 1°X 2° quadrangle, Idaho: U.S. 

Geological Survey Open-File Report 83-523,41 p., 2 maps, 

scale 1:250,000.

Fisk, H. G., 1969 [1970], Painted Rocks Lake area, southern 

Ravalli County, Montana: Montana Bureau of Mines and 

Geology Special Publication 47, 1 sheet.

Foster, Fess, 1982, Geologic map of Mt. Jordan and vicinity, 

Custer County, Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF-1434.

Greenwood, W. R., and Morrison, D. A., 1973, Reconnaissance 

geology of the Selway-Bitterroot Wilderness area: Idaho 

Bureau of Mines and Geology Pamphlet 154, 30 p.

Hall, W. E., Batchelder, J. N., and Douglass, R. C., 1974, 

Stratigraphic section of the Wood River Formation, Blaine 

County, Idaho: U.S. Geological Survey Journal of 

Research, v. 2, no. 1, p. 89-95.

Hall, W. E., Rye, R. O., and Doe, B. R., 1978, Wood River mining district, Idaho Intrusion-related lead-silver deposits 

derived from country rock source: US. Geological Survey 

Journal of Research, v. 6, no. 5, p. 579-592.

Hamilton, Warren, 1981, Crustal evolution by arc magmatism, 

in The origin and evolution of the Earth's continental crust:

Royal Society of London Philosophical Transactions, sen 

A, v. 301, no. 1461, p. 279-291.

Hietanen, Anna, 1969, Distribution of Fe and Mg between 

garnet, staurolite, and biotite in aluminum-rich schist in 

various metamorphic zones north of the Idaho batholith: 

American Journal of Science, v. 267, p. 422-456.

Hyndman, D. W., 1984, A petrographic and chemical section 

through the northern Idaho batholith: Journal of Geology, 

v. 92, p. 83-102.

Hyndman, D. W, Badley, Ruth, and Rebal, Donald, 1977, 

Northeast-trending early Tertiary dike swarm in central 

Idaho and western Montana [abs.]: Geological Society of 

America Abstracts with Programs, v. 9, no. 6, p. 734.

Kiilsgaard, T. H., 1983a, Geochemical map of the Ten Mile West 

Roadless Area, Boise and Elmore Counties, Idaho: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-1500-B, scale 1:62,500.

_____1983b, Geologic map of the Ten Mile West Roadless 

Area, Boise and Elmore Counties, Idaho: US. Geological 

Survey Miscellaneous Field Studies Map MF-1500-A, scale 

1:62,5000.

Kiilsgaard, T. H., Freeman, V. L., and Coffman, J. S., 1970, 

Mineral resources of the Sawtooth Primitive Area, Idaho: 

U.S. Geological Survey Bulletin 1319-D, 174 p.

Lewis, R. S., 1984, Geology of the Cape Horn Lakes quadrangle, 

south-central Idaho: Seattle, University of Washington M.S. 

thesis, 90 p.

Lipman, P. W, Prostka, H. J., and Christiansen, R. L., 1972, 

Cenozoic volcanism and plate-tectonic evolution of the 

western United States. Part I, Early and middle Cenozoic: 

Royal Society of London Philosophical Transactions, 

v. 271-A, p. 217-248.

Loiselle, M. C., and Wones, D. R., 1979, Characteristics and 

origin of anorogenic granites [abs]: Geological Society of 

America Abstracts with Programs, v. 11, no. 7, p. 468.

Mclntyre, D. H., Ekren, E. B., and Hardyman, R. F., 1982, 

Stratigraphic and structural frameworks of the Challis 

Volcanics in the eastern half of the Challis 1°X2° quadrangle, Idaho, in Bonnichsen, Bill, and Breckenridge, R. 

M., eds., Cenozoic geology of Idaho: Idaho Bureau of 

Mines and Geology Bulletin 26, p. 3-22.

Nelson, W. H., and Ross, C. P., 1968, Geology of part of the 

Alder Creek mining district, Custer County, Idaho: U.S. 

Geological Survey Bulletin 1252-A, p. 1-30.

Nold, J. L., 1974, Geology of the northeastern border zone of 

the Idaho batholith: Northwest Geology, v. 3, p. 47-52.

Olson, H. J., 1968, The geology and tectonics of the Idaho porphyry belt from the Boise basin to the Casto quadrangle: 

University of Arizona Ph.D. thesis, 154 p.

O'Neill, J. M., and Lopez, D. A., 1983, Great Falls lineament, 

Idaho and Montana [abs.]: American Association of 

Petroleum Geologists Bulletin, v. 67, no. 8, p. 1350-1351.

O'Neill, J. M., Lopez, D. A., and Desmarais, N. R., 1982, Recurrent movement along and characteristics of, northeast-trending faults in part of east-central Idaho and westcentral Montana [abs.]: Geological Society of America 

Abstracts with Programs, v. 14, no. 6, p. 345.

Pattee, E. C., Van Noy, R. M., and Weldin, R. D., 1968, Beryllium 

resources of Idaho, Washington, Montana, and Oregon: 

US. Bureau of Mines Report of Investigation 7148,169 p.

94 Challis 1°x2° Quadrangle, Idaho
Paull, R. A., Wolbrink, M. A., Volkman, R. G., and Grover, 

R. L., 1972, Stratigraphy of the Copper Basin Group, 

Pioneer Mountains, south-central Idaho: American 

Association of Petroleum Geologists Bulletin, v. 56, no. 8, 

p. 1370-1401.

Peacock, M. A., 1931, Classification of igneous rock series: Journal of Geology, v. 39, no. 1, p. 54-67.

Percious, J. K., Damon, P. E., and Olson, H. J., 1967, 

Radiometric dating of Idaho batholith porphyries: U.S. 

Atomic Energy Commission Annual Progress Report, No. 

COO-689-76, Appendix A-X, p. 1-6.

Reid, R. R., 1963, Reconnaissance geology of the Sawtooth 

Range: Idaho Bureau of Mines and Geology Pamphlet 129, 

37 p.

Reynolds, S. J., and Keith, S. B., 1982, Geochemistry and mineral 

potential of peraluminous granitoids, in Arizona Bureau 

of Geology and Mineral Technology, Field Notes, v. 12, 

no. 4, p. 4-6.

Ross, C. P., 1928, Mesozoic and Tertiary granitic rocks in Idaho: 

Journal of Geology, v. 36, no. 8, p. 673-693.

_____1931, A classification of the lode deposits of southcentral Idaho: Economic Geology, v. 26, no. 2, p. 169-185.

_____1934, Geology and ore deposits of the Casto 

quadrangle, Idaho: U.S. Geological Survey Bulletin 854, 

135 p.

_____1963, Evolution of ideas pertaining to the Idaho

batholith: Northwest Science, v. 37, p. 45-63.

Ruppel, E. T., 1978, Medicine Lodge thrust system, east-central 

Idaho and southwest Montana: U.S. Geological Survey Professional Paper 1031, 23 p.

_____1982, Cenozoic block uplifts in east-central Idaho and 

southwest Montana: U.S. Geological Survey Professional 

Paper 1224, 24 p.

Shand, S. J., 1927, The eruptive rocks: New York, Wiley, 488 p.

Streckeisen, A. L., 1976, To each plutonic rock its proper name: 

Earth Science Review, v. 12, p. 1-33.

Swanberg, C. A., and Blackwell, B. D., 1973, Areal distribution 

and geophysical significance of heat generation in the Idaho 

batholith and adjacent intrusions in eastern Oregon and 

western Montana: Geological Society of America Bulletin, 

v. 84, no. 4, p. 1261-1282.

Toth, M. I., 1983, Reconnaissance geologic map of the SelwayBitterroot Wilderness, Idaho County, Idaho, and Missoula 

and Ravalli Counties, Montana: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1495-B, scale 

1:125,000.

Toth, M. I., Coxe, B. W., Zilka, N. T., and Hamilton, M. M., 

1983, Mineral resource potential map of the SelwayBitterroot Wilderness, Idaho County, Idaho, and Missoula 

and Ravalli Counties, Montana: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1495-A, scale 

1:125,000.

Tschanz, C. M., Kiilsgaard, T. H., Seeland, D. A., Van Noy, R. 

M., Ridenour, James, Zilka, N. T., Federspiel, F. E., Evans, 

R. K., Tuchek, E. T, and McMahan, A. B., 1974, Mineral 

resources of the eastern part of the Sawtooth National 

Recreation Area, Custer and Blaine Counties, Idaho, with 

sections on Aeromagnetic survey and tentative interpretation, by D. R. Mabey and C. M. Tschanz, and Electromagnetic surveys, by F. C. Frischknecht: U.S. Geological Survey open-file report, 2 v., 648 p.

White, A. J. R., and Chappell, B. W., 1983, Granitoid types and 

their distribution in the Lachlan Fold Belt, southeastern 

Australia: Geological Society of America Memoir 159, p. 

21-34.

Tertiary Plutons and Related Rocks in Central Idaho 95

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter G

The Twin Peaks Caldera and 

Associated Ore Deposits

By R. F. HARDYMAN

CONTENTS

Abstract 98

Introduction 98

Geologic setting 98

Structural elements of the caldera 98

Earliest collapse segment 98

Southeastern structural block 99

Northwestern structural block 100 

Associated hydrothermally altered rocks 101

Alteration along the southern ring fracture 102

Medial zeolite zone 103

Altered intrusive rhyolites in the northern structural block 104 

Associated ore deposits 104 

Conclusions 105 

References cited 105

FIGURES

Gl. Map showing structural elements of the central part of the Challis quadrangle 99

G2. Bouguer gravity map of the Twin Peaks area 100

G3. Generalized geologic map of the Twin Peaks caldera 101

G4. Histograms showing average modal phenocryst abundances in intracaldera

tuffs 102

G5. Diagrammatic cross section across the Twin Peaks caldera 103

G6. Photograph showing zeolitized ash-flow tuffs 103

G7. Photograph showing altered volcanic breccia 104

G8. Diagrammatic geologic cross section at the Williams and Parker mines 105

TABLE

Gl. Average values of chemical analyses and normative values of intracaldera tuffs 

of Challis Creek 102

Twin Peaks Caldera and Associated Ore Deposits 97
Abstract

The Twin Peaks caldera formed about 45 m.y. ago with 

the eruption of large volumes of ash-flow tuff that formed an 

extensive outflow sheet and ponded within the collapse structure to a thickness of more than several hundred meters. Following this initial caldera-forming cycle, additional collapse took 

place in the northern half of the structure, concomitant with 

renewed eruption of ash-flow tuff. An extensive megabreccia, 

consisting of caldera-wall slump debris, is intercalated with this 

tuff, and together these rocks have buried much of the collapsed 

northern structural block. The youngest magmatic event to affect the caldera was the emplacement of many rhyolite intrusions within the failed northern structural block and along the 

northern ring-fracture system.

Hydrothermal activity was localized along the caldera ringfracture system and within the northern structural block. Rocks 

of the southern structural block of the caldera, however, were 

little affected by hydrothermal solutions. Hydrothermal solutions have zeolitically altered an extensive zone of intracaldera 

rocks separating the two structural blocks of the caldera. In addition, hydrothermal solutions, that may in part be temporally 

related to intrusive rhyolites in the northern structural block, 

have altered some of these intrusions. Precious-metal deposits 

have been mined from the Parker Mountain district on the 

western margin of the caldera.

INTRODUCTION

The Twin Peaks caldera, approximately 13 km (kilometers) northwest of Challis, Idaho, is a Valles-size caldera 

with dimensions of 20 by 14 km. It formed about 45 m.y. 

ago with the eruption of approximately 320 km3 (cubic 

kilometers) of alkali-rhyolite ash-flow tuff, the tuff of 

Challis Creek. It is the youngest recognized collapse structure of the Van Horn Peak cauldron complex, which was 

the source area for most of the pyroclastic rocks of the 

Challis volcanic field in the eastern half of the Challis 

quadrangle.

The caldera was first recognized through geologic 

mapping in 1979 as part of the CUSMAP (Conterminous 

United States Mineral Assessment Program) study of the 

Challis quadrangle. A brief description of the caldera and 

the intracaldera pyroclastic rocks, together with an interpretation of its collapse history, has been presented 

elsewhere (Hardyman, 1981, 1983). This chapter reviews 

the structural setting of the caldera and discusses the 

precious-metal mineral deposits and hydrothermally 

altered rock associated with the caldera.

GEOLOGIC SETTING

The Twin Peaks caldera is a roughly elliptical collapse structure, about 20 km in longest dimension, which 

lies at the northeastern end of the Custer graben and along 

the southern margin of the Van Horn Peak cauldron complex (fig. Gl). These structures lie within a regionally

extensive, northeast-trending zone of normal faults, 

grabens, and caldera segments of the Van Horn Peak 

cauldron complex that collectively are elements of the transChallis fault system (Kiilsgaard and Lewis, chap. B, this 

volume; Fisher, chap. A, this volume, fig. Al). The fairly 

linear southern margin, and especially the northern margin 

of the caldera may have formed by collapse along preexisting northeast-trending faults in the pre-Tertiary basement rocks (Hardyman, 1981, p. 322).

The caldera is approximately centered on Twin Peaks, 

two similar glaciated peaks more than 10,300 ft (feet) (3,139 

meters) in elevation (lat 44°36'N., long 114°27'W.) that 

were carved from intracaldera ash-flow tuff. The tuffs of 

Challis Creek are the youngest pyroclastic rocks of the 

Challis Volcanics in this region and cap many of the high 

peaks near the caldera to the north and west. Erosional 

remnants of the tuff of Challis Creek are preserved on the 

crest of the Pahsimeroi Mountains, about 24 km east of 

the Twin Peaks caldera, and remnants have been identified 

as far as 40 km southeast of the caldera along the Salmon 

River (Mclntyre and others, 1982) and as far as 20 km west 

of the caldera.

Extreme and abrupt thickness changes of the rhyolite tuffs of Challis Creek, together with arcuate faults that 

juxtapose very dissimilar ash-flow tuffs and other volcanic 

rocks, define the caldera margins (Fisher and others, 1983). 

In addition, intermediate to silicic intrusive rocks and localized deposits of caldera-wall slump debris clearly delineate 

the caldera margins. Older rhyodacitic lavas and intrusions, 

for the most part, bound the caldera on the south and east, 

and older pyroclastic rocks of the Van Horn Peak cauldron 

complex bound the caldera on the west and north. PreTertiary basement rocks are exposed about 5 km east of 

the caldera and about 2 km south. Plutonic rocks of the 

Idaho batholith crop out about 10 km west of the western 

margin of the caldera.

STRUCTURAL ELEMENTS OF THE CALDERA

The Twin Peaks caldera is divisible into three parts:

(1) an earliest collapse segment at the southwestern end;

(2) a southeastern structural block; and (3) a northwestern 

structural block. Together these structural elements record 

a complex history of collapse and concomitant volcanism. 

Geologic relations just west of the southwestern margin 

of the caldera proper suggest a collapse segment that I interpret to record the earliest subsidence of the Twin Peaks 

caldera. The southeastern and northwestern structural 

blocks within the caldera are separated by a northeast-trending line along the northern flanks of Twin Peaks.

Earliest Collapse Segment

A 10-mGal (milligal) negative gravity anomaly is 

associated with the Twin Peaks caldera and is one part

98 Challis 1°x2° Quadrangle, Idaho
115°

CASTLE

ROCK

CAULDRON

SEGMENT

VAN HORN

PEAK

CAULDRON 

COMPLEX

Challi"

o Stanley

EXPLANATION

TERTIARY GRANITE

CRETACEOUS ROCKS OF IDAHO 

BATHOLITH

FAULT Dashed where approximately located; dotted where concealed. 

Bar and ball on downthrown side; 

arrows show direction of movement

BOUNDARY OF DIKE SWARM

VAN HORN PEAK FISSURE VENT

CORRAL CREEK CAULDRON 

SEGMENT

0 20 MILES

0 10 20 KILOMETERS

Figure G1. Structure map of the central part of the Challis quadrangle.

of a peanut-shaped gravity-anomaly pattern having an 

18-mGal low centered near the western margin of the 

caldera (fig. G2; Mabey and Webring, chap. E, this 

volume). The 18-mGal gravity low coincides with an area 

containing exposures of rhyolitic ash-flow tuff and overlying volcaniclastic sedimentary rocks that are intruded by 

younger rhyolites. The moderately welded rhyolite ashflow tuff is at least 215 m (meters) thick and grades upward into air-fall tuff. These rocks are similar in modes 

to the tuffs of Challis Creek and certainly are products 

of the same magmas that produced the tuffs of the Twin 

Peaks caldera. The ash-flow to air-fall tuff section in this 

18-mGal gravity-low area is overlain by at least 120 m of 

coarse pebble conglomerate and volcaniclastic sandstone. 

Clasts of pyroclastic rocks of older units in the Challis 

volcanic field occur in these sedimentary rocks, but clasts 

of densely welded tuff of Challis Creek from the Twin 

Peaks caldera, which presumably once formed an outflow 

sheet in this area, are absent.

Details of the stratigraphic relations between rocks 

of these exposures and intracaldera tuffs just to the east 

are obscure due to intrusive rhyolites that pervade the

rocks in this area. The above relations, however, suggest 

that the rhyolite ash-flow tuff and overlying sedimentary 

rocks infill an early collapse segment of the Twin Peaks 

caldera and that continued eruption of tuffs of Challis 

Creek and subsequent caldera collapse shifted slightly to 

the east-northeast.

Southeastern Structural Block

The southeastern half of the Twin Peaks caldera is 

a simple intact structural block composed of two major 

intracaldera-filling ash-flow-tuff cooling units (map units 

Tcr, and Tcr3 , fig. G3) that total at least 800 m in thickness. The lower cooling unit is 500 m thick, and the upper cooling unit (the top of which has been removed by 

erosion) is at least 310 m thick. At least two thin ash flows 

and discontinuous epiclastic sedimentary rocks separate 

the two thick cooling units. These cooling-break tuffs and 

sedimentary rocks are about 33 m thick.

Rocks of the two thick cooling units are very similar 

petrographically and in hand specimen, and collectively

Twin Peaks Caldera and Associated Ore Deposits 99
EXPLANATION

 --220j~: BOUGUER GRAVITY CONTOUR 

Contour interval 2 mGal

 BOUNDARY OF TWIN PEAKS CALDERA

 BOUNDARY OF EARLIEST COLLAPSE

SEGMENT 

H GRAVITY HIGH

L GRAVITY LOW

0 5 1D KILOMETERS

Figure G2. Bouguer gravity map of the Twin Peaks area, Challis quadrangle. From Mabey and Webring (chap. E, this volume).

with the cooling break tuffs, they compose the intracauldron tuff of Challis Creek. These tuffs contain 

abundant phenocrysts of sanidine and quartz and only 

minor plagioclase (fig. G4). The tuff of Challis Creek is 

distinctive in its paucity of plagioclase and near lack of 

any ferromagnesian minerals, especially biotite. Clinopyroxene is preserved in vitrophyres, and pseudomorphs 

are observable in devitrified tuff, but biotite is rare. 

Chemically, the rocks of the tuff of Challis Creek, as their 

modes suggest, are alkali rhyolites (table Gl).

The tuffs in the southeastern half of the caldera dip 

gently 10-20° south-southeast and are little disrupted by 

faulting (fig. G5). This half of the caldera apparently 

represents the intact part of the caldera fill that was little 

affected by subsequent renewed collapse and explosive 

volcanism.

Northwestern Structural Block

In contrast to the structurally simple southeastern 

half of the caldera, geologic relationships in the

northwestern half are complex and record additional 

pyroclastic volcanism and associated structural collapse. 

Much of the northern half of the caldera is filled with 

ash-flow tuff and megabreccia deposited during further 

collapse of the northern block (fig. G3). Collapse of this 

structural block, which probably resembled the present 

southeastern half of the caldera, was apparently piecemeal 

and catastrophic, leaving nearly the entire northern margin 

of the caldera exposed as a topographic wall. Collapse 

of this wall during continued eruption of pyroclastic 

material resulted in a megabreccia deposit intermixed with 

ash-flow tuff (unit Tsd, fig. G5). Ash-flow eruptions during collapse of the north wall probably issued from a major medial fracture (now buried) and perhaps from around 

foundering segments of the original northern half of the 

infilled caldera. Preliminary petrography of the ash-flow 

tuff (unit Term, fig. G4) enclosing megabreccia blocks indicates that plagioclase is significantly more abundant in 

this megabreccia tuff than in the major intracaldera tuff 

cooling units.

The megabreccia deposit is exposed over an area 

of about 29 km2 (square kilometers) and contains an

100 Challis 1°x2° Quadrangle, Idaho
114°30'

PARKER 

T Pc MINE

5 10 KILOMETERS 

EXPLANATION

TERTIARY (EOCENE) 

INTRUSIVE RHYOLITE

INTRUSIVE ROCKS OF PATS CREEKUPPER EDDY CREEK 

UWA OF RED BUTTE

MEGABRECCIA 

INTERMEDIATE INTRUSIONS

TUFF OF CHALLIS CREEK 

Upper cooling unit

Cooling break tuffs 

Lower cooling unit 

Earliest tuff

Figure G3. Generalized geologic map of the Eocene

estimated 24 km3 of material ranging from bedded, 

coarse-grained talus to unsorted boulder- and blockdominated debris (Hardyman, 1983). Megabreccia blocks 

range from cobble to house size (more than 65 meters in 

length) and include intermediate lava, rhyolite, and nearly all of the older pyroclastic units of the Van Horn Peak 

cauldron complex that are exposed just north of the 

caldera. Blocks of tuff of Challis Creek are also abundant in this deposit. The megabreccia unit laps against and 

locally over both cooling units of the tuff of Challis Creek

-A'

PRECALDERA VOLCANIC ROCKS 

PRE-TERTIARY SEDIMENTARY ROCKS

CONTACT

FAULT Dashed where approximately 

located; bar and ball on 

downthrown side

MINE

APPROXIMATE LINE OF CROSS SECTION 

SHOWN ON FIGURE G5

ZONE OF ZEOLITIC ALTERATION

Twin Peaks caldera.

that form the escarpment marking the northern boundary 

of the southeastern caldera block.

ASSOCIATED HYDROTHERMALLY 

ALTERED ROCKS

Hydrothermally altered rocks associated with the 

Twin Peaks caldera occur along the southern ring-fracture 

system, along the medial zone marking the boundary

Twin Peaks Caldera and Associated Ore Deposits 101
£ 80

LU

O 60 

cc

LU

°- 40

LU

^

^ 20

O

> 0

Tcrn Tcr, Tcr-,

<2.5

Tcr3 Tcr,

2-3 <2 2-5 

AVERAGE PHENOCRYST SIZE, IN MILLIMETERS

<2.5

Figure G4. Average modal phenocryst abundances of intracaldera tuffs of the Twin Peaks caldera. Tcr0/ tuff of earliest collapse 

segment; Tcrr lower cooling unit, tuff of Challis Creek; Tcr2, cooling break tuffs; Tcr3, upper cooling unit, tuff of Challis Creek; 

Tcr , megabreccia-matrix tuff.

Table G1. Average values of nine chemical analyses and CIPW 

normative values of intracaldera tuffs of Challis Creek, Challis 

quadrangle.

Chemical analyses

S102 

17_ Fe2f) 03 ____

CaO 

Na20

P2°5 

MnO 

co2 

Sum

75.33

1.43

no

.26

5.10

.51

.47 

.13

.00

.01

.00

98.66

Normative

q 

hy-en 

hm 

j i _______

values

38.11

30.85

1.32

.20

.24 

1.30

.25

between the two intracaldera structural blocks, and 

associated with rhyolitic intrusions that occur primarily 

within the northern half of the caldera (fig. G3). Rocks 

of the southeastern structural block are generally unaltered, 

except along the southern ring fracture, but hydrothermal 

alteration has been locally pervasive within the structurally more disrupted northwestern half of the caldera.

Alteration Along The Southern Ring Fracture

The southern and southeastern margin of the caldera 

coincides with Challis Creek from near Pats Creek (shown 

on the Twin Peaks 15-minute quadrangle topographic map) 

southwest and west to above Mosquito Flat. West of

Mosquito Flat the caldera margin is defined by a fault that 

trends across the ridge just north of Summit Rock, near 

Mill Creek Summit, and extends into the uppermost Yankee 

Fork drainage. This fault juxtaposes densely welded tuff 

of Challis Creek on the north against older tuffs and 

rhyolite lavas on the south. Below Mosquito Flat, tuff of 

Challis Creek forms the canyon walls on the north side of 

Challis Creek, and older rhyodacitic lavas and intrusions 

are exposed on the south side of the creek. Intermediatecomposition intrusive rocks occur locally along this part 

of the caldera ring fracture. These rocks are propylitically 

altered and contain disseminated pyrite. Plagioclase and 

ferromagnesian minerals are altered to calcite and chlorite.

One small porphyry intrusion, containing distinctive 

plagioclase phenocrysts as long as 6 cm, and exposed on 

Challis Creek road about 6.4 km northeast of Mosquito 

Flat Reservoir, is highly sheared and has quartz-sericite 

alteration. This rock contains narrow quartz-carbonatelimonite veinlets. This intrusion is partly buried by calderawall slump debris that is locally exposed along the 

southeastern margin of the caldera (fig. G3). Rocks of this 

caldera-wall slump debris are also altered to a carbonateclay assemblage.

Intrusive rocks along the northeastern margin of the 

caldera (unit Til, fig. G3) cut tuff of Challis Creek, but 

this intrusion and adjacent tuffs are little altered. A similar 

pluglike intrusive mass along the north margin of the 

caldera contains abundant gas cavities containing rosettes 

of gypsum and minor calcite.

Intracaldera tuffs of the southeastern structural block 

are essentially unaltered except for one zone of weak, 

zeolitically altered tuff exposed on the upland erosion surface about 5 km north of Mosquito Flat Reservoir (fig. 

G3). Here the altered tuff adjoins a small dikelike plug of 

intrusive rhyolite (unit Tir), which is the only rhyolite intrusion that has been observed within the southeastern half 

of the caldera.

102 Challis 1°x2° Quadrangle, Idaho
A

NORTHWEST

METERS

SEA LEVEL

A'

SOUTHEAST

FEET 

10.000

8000

_

-GOOD

-4000

-2000

- SEA LEVEL

Jsd

EXPLANATION

INTRUSIVE RHYOLITE 

MEGABRECCIA 

INTERMEDIATE INTRUSIONS

TUFF OF CHALLIS CREEK 

Megabreccia matrix tuff

Upper cooling unit 

Cooling break tuffs ' 

Lower cooling unit 

Probable earliest tuff 

PRECALDERA VOLCANIC ROCKS

3 MILES

J

0 2 3 KILOMETERS

CONTACT Queried where interpretation is less certain 

FAULT Arrows show direction of movement; dashed 

where fault has been intruded by intermediate to felsic 

intrusive rocks or buried by megabreccia

Figure G5. Diagrammatic cross section across the Tertiary Twin Peaks caldera. Line of section shown on figure G3.

Medial Zeolite Zone

An extensive northeast-trending zone of zeolitic 

altered volcanic rocks coincides with the medial line that 

separates the two structural blocks of the Twin Peaks 

caldera (fig. G3). Zeolitized rocks in this zone include ashflow tuff of the lower cooling unit of the tuff of Challis 

Creek, megabreccia matrix tuff, and some megabreccia 

blocks within the megabreccia deposit. Zeolites are visible 

petrographically in most samples from this zone and replace glass shards and pumice fragments. X-ray analysis 

of samples collected across and along strike of this zone 

shows mordenite and clinoptilolite. Zeolitically altered rocks 

are easily recognized in outcrop because they are bleached 

to pale green or buff and generally do not support vegetation; this lack of vegetation is probably the result of nutrient deficiencies in these altered rocks (fig. G6). Locally 

within this zone, pyrite-bearing, limonitically stained, 

northeast-trending rhyolite dikes cut the zeolitized rock. 

The zone of zeolitized rock extends at least 12,000 m along 

strike and is locally at least 1,000 m wide. Patches of 

zeolitized rock occur sporadically along strike to the southwestern caldera margin, and zeolitized rock occurs outside 

the caldera in tuffs within the earliest collapse segment.

The amount of zeolitized tuff indicates that large 

volumes of hydrothermal fluids permeated the rocks along

White 

Mtn

Figure G6. Zeolitized ash-flow tuff of Challis Creek (lightcolored, treeless outcrops) along medial structural boundary 

within the Twin Peaks caldera. View to the northwest along 

strike of the zeolite zone from near the southwestern margin 

of the caldera.

this zone. Presumably a medial fault or faults, now mostly buried and marking the southern boundary of the 

foundered northwestern half of the caldera, served as conduits for these low-temperature hydrothermal waters.

Twin Peaks Caldera and Associated Ore Deposits 103
Altered Intrusive Rhyolites in the 

Northwestern Structural Block

Intrusive rhyolites that postdate caldera collapse are 

abundant in the northwestern half of the caldera where 

they crosscut intracaldera tuff of Challis Creek and the 

megabreccia deposit (fig. G3). The ring fracture along the 

northern margin of the caldera served as the conduit for 

emplacement of some of the rhyolites, but they are not 

restricted to the ring-fracture zone. The northeastern trend 

of many of the linear dikes within this half of the caldera 

probably indicates northeast-trending structures in the 

precaldera basement rocks.

The rhyolites are similar in appearance and mineralogy to intrusive rhyolites found elsewhere in the Van 

Horn Peak cauldron complex and adjacent Custer "and 

Panther Creek grabens (Hardyman and Fisher, chap. N, 

this volume). These rocks are aphyric to porphyritic and 

generally contain sanidine or sanidine and quartz.

Many of the rhyolite dikes are stained by limonite 

and contain disseminated pyrite. Alteration of these dikes 

and other rhyolite masses ranges from argillic to quartz 

sericitic and locally strongly silicic. In some intrusive 

bodies, the rhyolite is brecciated. Brecciation may occur 

along intrusive contacts or may occur as isolated breccia 

bodies within massive rhyolite. One such breccia body, 

within the intrusive rhyolite mass (unit Tir) along the 

northern ring fracture (fig. G3), resembles a hydrothermal or explosion breccia. Fragments in this breccia are 

monolithologic and consist of angular rhyolite fragments 

set in a matrix of finer grained breccia. Quartz-lined 

cavities occur in the breccia matrix, and hairline quartz 

veins crosscut fragments and the breccia matrix. Commonly, breccias, such as this example, are weakly to 

strongly altered, are limonitically stained, and contain 

pyrite.

ASSOCIATED ORE DEPOSITS

The only known mineral deposits associated with 

the Twin Peaks caldera occur along the northwestern 

margin near Parker Mountain. Gold-silver deposits were 

intermittently mined at the Parker mine and adjacent 

Williams mine from about 1905 to 1926 (Ross, 1934). Total 

reported production from these mines is 819 ounces of 

gold and 4,520 ounces of silver (A. Leszcykowski, U.S. 

Bureau of Mines, written commun., 1983). Brief descriptions of these mines were reported by Umpleby (1913) and 

Ross (1934).

Precious metals were produced at the Parker mine 

(Parker Mountain mine of Ross, 1934, p. 127) from vein 

quartz and silicified volcanic rock in a shear zone 3-5 ft 

wide that I interpret to be associated with the ring-fracture 

system along this segment of the caldera margin. At the 

Williams property, about 500 m to the north-northeast

along the ring-fracture zone, ore was mined from silicified 

breccia that contains fragments of petrified wood, finegrained tuffaceous sediment, and dark carbonaceous 

material (fig. G7). These fragments are interpreted to 

come from surficial deposits accumulated in depressions 

along the caldera ring-fracture zone at the time of calderawall collapse and accumulation of the megabreccia 

deposit.

The Parker and Williams mines are both near the 

northwestern contact of a large rhyolite intrusive mass that 

extends inward from the caldera margin (unit Tir, fig. G3). 

My interpretation of the geology at these deposits is shown 

in figure G8. Aphyric to porphyritic rhyolite was emplaced 

along the ring fracture and intruded megabreccia. 

Precious-metal-bearing fluids ascended along this conduit 

and along associated sympathetic fractures, silicifying and 

mineralizing older tuffs (and possibly caldera-related 

tuffs) at the Parker mine, and silicifying breccia, of similar 

collapse origin as the megabreccia deposit, at the Williams 

property. I do not interpret the breccia exposed at the surface at the Williams property to be an intrusive breccia,

Figure G7. Altered fine-grained volcanic breccia collected at 

the ground surface near the Williams mine on the northwestern 

side of the Twin Peaks caldera. Light-colored fragments are very 

fine grained tuffaceous rock; dark fragment is carbonaceous 

material. Note subtle layering of the breccia.

104 Challis 1°x2° Quadrangle, Idaho
WEST Parker Mtn 

(Just to north) EAST

EXPLANATION

INTRUSIVE RHYOLITE

MEGABRECCIA DEPOSIT

OLDER PYROCLASTIC ROCKS OUTSIDE 

OF TWIN PEAKS CALDERA

TUFF OF CHALLIS CREEK (UNDIFFERENTIATED) WITHIN CALDERA 

CONTACT

HYDROTHERMALLY ALTERED ROCK 

ASSOCIATED WITH 

RING-FRACTURE SYSTEM

1 

1000 METERS

Figure G8. Diagrammatic cross section of geologic relations of Tertiary rocks at the Williams and Parker mines, Twin Peaks 

caldera. Line of section is through the Williams mine; the Parker mine is toward the reader from the section and along the 

zone of sympathetic fractures associated with the caldera ring fracture.

although narrow veins of hydrothermal breccia probably 

occur along fractures in the underlying intrusive rhyolite.

CONCLUSIONS

Subsequent to the initial caldera-forming volcanism, 

the northern half of the TAvin Peaks caldera failed along 

structures, now mostly buried, into which postcollapse intrusive rhyolites were emplaced. Many of these rhyolites 

intruded to near-surface levels, and some contain spatially 

associated hydrothermal- or explosion-breccia bodies.

The structural disruption of the northern half of 

the caldera provided the most favorably prepared ground 

for the circulation of hydrothermal fluids. These fluids 

extensively altered the intracaldera tuffs along a northeast-trending medial zone through the caldera and locally altered intrusive rhyolites and tuffs elsewhere in the 

northern structural block. Hydrothermal activity along 

the western ring-fracture zone produced precious-metal 

deposits at the Parker and Williams mines. The northern 

half of the caldera has the most potential for mineral 

deposits.

REFERENCES CITED

Fisher, F. S., Mclntyre, D. H., and Johnson, K. M., 1983, Geologic map of the Chain's 1°X2° quadrangle, Idaho: U.S. 

Geological Survey Open-File Report 83-523,41 p., 2 maps, 

scale 1:250,000.

Hardyman, R. F, 1981, TAvin Peaks caldera of central Idaho, 

in TUcker, T. E., and others, Guidebook to southwest Montana: Montana Geological Society, Field Conference and 

Symposium, p. 317-322.

_____1983, Multiple collapse of the Twin Peaks caldera, 

Challis volcanic field, Idaho: Geological Society of 

America Abstracts with Programs 1983, v. 15, no. 5, p. 433 

[combined Cordilleran-Rocky Mountain section meeting].

Mclntyre, D. H., Ekren, E. B., and Hardyman, R. F, 1982 [1984], 

Stratigraphic and structural framework of the Challis 

Volcanics in the eastern half of the Challis 1°X2° quadrangle, Idaho; in Bonnichsen, Bill, and Breckenridge, R. 

M., eds., Cenozoic geology of Idaho: Idaho Bureau of 

Mines and Geology Bulletin v. 26, p. 3-22.

Ross, C. P., 1934, Geology and ore deposits of the Casto quadrangle, Idaho: U.S. Geological Survey Bulletin 854, 

p. 127-130.

Umpleby, J. B., 1913, Geology and ore deposits of Lemhi County, 

Idaho: US. Geological Survey Bulletin 528, 182 p.

Twin Peaks Caldera and Associated Ore Deposits 105

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter H

Ore Deposits Related to the Thunder Mountain 

Caldera Complex

By B. F. LEONARD

Abstract

Mined deposits of gold, silver, mercury, antimony, and 

tungsten are structurally related to subsidence of the Eocene 

caldera complex and to superposed regional strain shown by 

north-striking right-lateral shear zones and northeast-striking extension fractures. Gold deposits of the Thunder Mountain mining district are in the pyroclastic and volcaniclastic filling of 

Challis Volcanics in the central, youngest caldera. All the other 

mined deposits are outside the youngest caldera, within or close 

to silicified zones that cut plutonic and metamorphic rocks of 

the Idaho batholith. The extra-caldera deposits are near the 

intersection of discontinuous ring and radial fractures of the 

subsided complex. Ring fractures at %, 1/2 , %, %, and % of the 

30-km radius of the cauldron (superstructure bounding the 

caldera complex) are favored loci for the extra-caldera deposits,

most of which are xenothermal, virtually lacking in regional 

zoning of metals, and derived from metal sources only broadly 

referable to Tertiary magmatic processes. The distribution of 

many prospects fits the hypothesis that relates extra-caldera mined deposits primarily to large subsidence structures and superposed shear zones. Though cauldron subsidence (about 47 m.y. 

ago) and regional strain (superposed about 44 m.y. ago) are 

datable events, and though extensive silicification is by inference 

not older than about 44 m.y., the age of metallization remains 

uncertain. Interpretation of the Eocene structures and the 

mineral deposits within them is further complicated by 

widespread block faulting that has affected rocks of Miocene 

and Holocene age. Despite these uncertainties and complexities, the relations of large structures of Eocene age, together 

with local patterns of wallrock alteration, help narrow the search 

for concealed ore deposits related to the caldera complex.

Ore Deposits, Thunder Mountain Caldera Complex 107

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter I

Epithermal Gold-Silver Mineralization 

Related to Volcanic Subsidence in the 

Custer Graben, Custer County, Idaho

By D. H. MclNTYRE and K. M. JOHNSON

CONTENTS

Abstract 110

Introduction 110

Stratigraphy 110

Structure 112

Mineralization and alteration 113

Ore genesis 114

Ore preservation 114

Conclusions 114

References cited 115

FIGURES

11. Geologic map of the part of the Custer graben that includes the Yankee Fork 

mining district 111

12. Cross section through part of the Custer graben 112

13. Map showing principal mines and prospects 113

14. Diagram showing value of total production from mines in the Yankee Fork 

mining district 114

Gold-Silver Mineralization in the Custer Graben 109
Abstract

The Custer graben is a 13-by-32 km (kilometer) northeast-trending volcano-tectonic graben in the Challis volcanic 

field in central Idaho. The Yankee Fork mining district, within 

the graben, has produced in excess of $12,000,000 in gold, 

silver, copper, lead, and zinc since the early 1880's.

Four major rock types are present within the graben: 

Paleozoic sedimentary rocks and Tertiary andesite, pyroclastic 

rocks, and rhyolite. Graben subsidence began about 48 m.y. 

(million years) ago with the eruption of the tuff of Eightmile 

Creek. Hydrothermal alteration and epithermal mineralization took place about 45 m.y. ago in parts of the graben where 

there was both silicic intrusive activity and persistent fault 

movement.

The andesites, pyroclastic rocks, and rhyolites are hosts 

for vein and disseminated mineral deposits. Ores are localized 

along a few discrete northeast- and northwest-trending fracture 

zones. Hypogene alteration consisted of pervasive propylitization and silicification followed by less extensive oxidation. Ore 

minerals are electrum, native gold and silver, chalcopyrite, and 

various sulfosalt minerals in a gangue of pyrite and fine-grained 

quartz.

Volcano-tectonic subsidence features are good exploration targets because young high-level mineral deposits are 

preserved in this environment. The Custer graben was an important host for epithermal mineral deposits because it had a 

favorable combination of source rocks, hydrothermal fluids, and 

suitably permeable host rocks. The processes that formed the 

mineral deposits were genetically related to structural and intrusive events accompanying the development of the graben.

INTRODUCTION

The Custer graben is a major northeast-trending 

volcano-tectonic depression in the Challis volcanic field 

in the east-central part of the Challis quadrangle (fig. Al, 

chap. A, this volume). It is part of the trans-Challis fault 

system (Bennett and Knowles, chap. F, and Kiilsgaard and 

Lewis, chap. B, this volume). The graben is an Eocene 

structural feature that has little modern-day topographic 

expression. The part of the graben southwest of the Yankee 

Fork mining district has been outlined by Choate (1962) 

and Siems and others (1979); within and northeast of the 

mining district it has been mapped by Mclntyre (unpub. 

data, 1975-82) and by Foster (1982). The Yankee Fork 

mining district, most of which is within the graben, has 

produced more than $12,000,000 in gold and silver since 

the 1880*s (Anderson, 1949; Koschmann and Bergendahl, 

1968). Most of this production was from mines along or 

near major faults and fractures within the graben and near 

the graben margin.

STRATIGRAPHY

The oldest rocks that occur in and near the Custer 

graben are dark-gray siltite, argillite, quartzite, and

limestone of the Lower Permian Grand Prize Formation 

(Hall, chap. J, this volume). These rocks are intruded by 

granitic rocks of the Cretaceous Idaho batholith (fig. II). 

The Grand Prize Formation and granitic rocks of the Idaho 

batholith are the two principal rock types that underlie 

volcanic rocks within the Custer graben. The exposures of 

Grand Prize Formation within the graben in the Yankee 

Fork mining district and the local occurrence of Grand 

Prize clasts in a breccia pipe that crosscuts rocks of the 

graben fill suggest that much of the graben in and northeast of the mining district is underlain by rocks of this unit. 

The part of the Custer graben southwest of the mining 

district is underlain chiefly by rocks of the Idaho batholith.

The oldest volcanic rocks in and around the Custer 

graben are andesite lavas and associated pyroclastic rocks 

that were emplaced about 50 m.y. ago, prior to graben formation. These rocks were erupted from several centers, two 

of which are within the area of figure II, one near Mt. Jordan (Foster, 1982) and the other east of Mt. Greylock. The 

total thickness of andesite lavas exceeds 1 km (fig. 12).

Initial subsidence of the Custer graben accompanied 

eruption of a quartz latitic ash-flow tuff, the tuff of Eightmile Creek, which is in part as old as about 48 m.y. Continued graben subsidence trapped a sequence more than 

300 m thick of quartz latitic to rhyolitic pyroclastic rocks 

and minor rhyolite lava that were derived from a variety 

of sources. This sequence is no younger than about 47 m.y.

Patterns of deposition within the graben were influenced by the irregular topography on the underlying 

andesite and by different subsidence rates of the various 

fault-bounded blocks within the graben. Subaqueous deposition occurred in blocks bounded by faults having most 

rapid or most frequently recurrent movement. The most 

significant fault blocks of this kind are those in the Sunbeam mine-Estes Mountain-Loon Creek Summit area (fig. 

II). Movement on these faults persisted during and after 

the time of mineralization in this area; the faults were important ore controls. Elsewhere in the graben, deposition 

was wholly subaerial.

Four kinds of Tertiary intrusive rocks are within and 

near the Custer graben. Andesite intrusions occur in the 

vent areas from which the andesite lavas-were erupted. A 

granodiorite-quartz monzonite stock is southeast of Custer. 

Exposures there are of the roof zone of the stock; small 

dikes and plugs, an aeromagnetic high, and the distribution of altered rocks indicate a buried extension of this 

stock at least 3 km west of the area where it crops out. 

Many dikes, domes, and irregular masses of rhyolite occur throughout the graben. The stock southeast of Custer 

and some of the rhyolite masses have genetically related 

ore deposits. An irregular mass of intermediate composition intrudes the rhyolite southwest of the Sunbeam mine 

(fig. II). It was emplaced following ore deposition.

Foster (1982) suggested that some of these late 

intermediate-composition intrusions may have fed a lava 

field that once buried the rocks now exposed. Some such

110 Challis 1°x2° Quadrangle, Idaho
114°35'

Area of L 

this figure

4 MILES

CORRELATION OF MAP UNITS

QUATERNARY

Eocene TERTIARY

4 KILOMETERS

Qs 

Tdi

Tr 

Tgi 

Trl 

Tt 

Td 

Kg

Pg

CRETACEOUS

PERMIAN .25

LIST OF MAP UNITS

SURFICIAL DEPOSITS

INTERMEDIATE-COMPOSITION INTRUSIVE

ROCKS

INTRUSIVE RHYOLITE 

GRANITIC INTRUSIVE ROCKS 

RHYOLITIC LAVA 

SILICIC PYROCLASTIC ROCKS 

ANDESITE LAVAS 

GRANITIC ROCKS 

GRAND PRIZE FORMATION

CONTACT Dashed where approximately

located 

FAULT Dashed where approximately located;

dotted where concealed. Bar and ball on

downthrown side; queried where uncertain;

arrows show direction of movement 

STRIKE AND DIP OF BEDS

Figure 11. Geologic map of the part of the Custer graben that includes the Yankee Fork mining district. Faults that bound the 

graben are near Mt. Jordan on the northwest, and north of Bonanza Peak on the southeast. Section A-A' is shown on figure 12. 

Generalized from D. H. Mclntyre (unpub. data, 1975-82) and Foster (1982).

Gold-Silver Mineralization in the Custer Graben 111
GOOD

T?

A?

1 2 KILOMETERS 

EXPLANATION

INTERMEDIATE-COMPOSITION INTRUSIVE 

ROCKS (EOCENE)

INTRUSIVE RHYOLITE (EOCENE) 

SILICIC PYROCLAST1C ROCKS (EOCENE) 

ANDESITE LAVAS (EOCENE)

CONTACT

 FAULT Arrows show direction of 

movement T, toward; A, away; 

queried where uncertain

Figure 12. Cross section through part of the Custer graben, along line A-A' (fig. II). Rainbow Ridge is an informal name used 

in this report for the ridge formed by a rhyolite dome west of the Sunbeam mine.

cover appears necessary to account for the preservation 

of the currently exposed graben fill (fig. 12).

The ages of the intrusive rocks within the graben 

are only incompletely known. Zircon from hydrothermally 

altered rhyolite at the Sunbeam mine gave a fission-track 

age of 45.8 ±2.3 m.y. This is an age for the hydrothermal 

alteration, which presumably is not significantly younger 

than the intrusion. A potassium-argon age of 47.4±1.8 

m.y. has been obtained for biotite from the rhyolite dome 

at Mt. Greylock. Age analysis of this sample is not yet 

complete, so this result should be considered provisional 

(R. F. Marvin, oral commun., 1984).

juxtaposes andesitic lava and the Grand Prize Formation. 

This fault is the outermost in the fault zone at the southeastern margin of the graben.

Comparable displacements, both dip slip and strike 

slip, may have taken place along the fault zone at the 

northwestern margin of the graben as well as along faults 

within the graben fill. Detailed mapping at the Sunbeam 

mine shows lateral and vertical fault displacements of tens 

of meters on minor faults subsidiary to nearby major fault 

zones. Movement on both major and minor faults began 

before and continued after mineralization at the Sunbeam 

mine (Mclntyre and Johnson, 1983).

STRUCTURE

The most prominent structures within the graben 

are northeast-trending faults that parallel the graben 

margins. Most of the faults have a predominant dip-slip 

component. Some also have a measurable strike-slip component; about 2 km of left-lateral strike-slip displacement 

is demonstrable along the northeast-trending fault that 

follows the Yankee Fork near Custer. About 0.4 km of 

dip-slip displacement (down to the northwest) also occurred along this fault. These displacements postdate the 

mineralization. The fault now is the most obvious one 

near the southeastern margin of the Custer graben. 

Farther southeast is another northeast-trending fault that

MINERALIZATION AND ALTERATION

Mineralization in the Yankee Fork mining district 

resulted in high-grade gold-silver veins and associated 

chimneylike deposits, disseminated gold-silver deposits, 

and gold-silver veins having relatively high base-metal contents. Anomalous concentrations of gold, silver, copper, 

and molybdenum were found in stream-sediment geochemical surveys (McDanal and others, 1984). The 

deposits occur within andesite, silicic tuff, and rhyolite 

and are localized along northeast- and northwest-trending fractures.

The deposits are concentrated in two areas, the 

Sunbeam mine-Estes Mountain area and the Charles

112 Challis 1°x2° Quadrangle, Idaho
Dickens-Lucky Boy mines area (fig. 13). Both areas are 

surrounded by broad haloes of propylitic alteration. The 

rocks near the deposits commonly are intensely altered 

to clay minerals and quartz. Supergene leaching caused 

by weathering of pyrite adds to the bleached appearance 

of rocks exposed near the ore deposits.

The rocks in large zones encompassing the two 

mineralized areas are characterized by depletion of 

oxygen-18 (18O). Ratios of 18O to 16O are lowest near 

clusters of ore deposits and increase systematically away 

from the deposits; the areas affected are larger than those 

having strong visible alteration (Criss and others, 1985). 

The 18O depletion occurred during alteration and 

mineralization when the rocks interacted with meteorichydrothermal fluids of moderate temperature and low 

18O content that were circulating around shallow-level 

silicic intrusions (Criss and others, 1985).

Most of the early-day production in the district 

came from the high-grade gold-silver veins. Typically, as 

at the General Custer and Sunbeam mines, the ore 

minerals in these veins are so fine grained as to make identification of ore minerals difficult or impossible, although 

bunches of coarse-grained gold and silver-bearing minerals 

were found in the early-day operations at some of the 

mines (Anderson, 1949). The gangue most commonly is 

fine-grained quartz, generally white but often gray, blue, 

or bluish gray because of the finely disseminated, dark 

metallic minerals it contains. One exception is the Lucky 

Boy mine, where the gangue is about half calcite and half 

quartz. Pyrite is ubiquitous and often is the only identifiable metallic mineral in the veins. Anderson (1949) 

identified under the microscope pyrite, chalcopyrite, 

sphalerite, tetrahedrite, arsenopyrite, enargite, galena, 

stephanite, miargyrite, pyrargyrite, argentite, aguilarite, 

gold, and electrum.

The largest producing mine in the Yankee Fork 

district (fig. 14) was the General Custer mine, where erosion stripped most of the hanging wall from a northwest-trending vein, vastly reducing mining costs. The vein 

filling at the General Custer is fine-grained quartz in 

which the ore minerals occur as dark-colored blotches. 

The gold was largely free, and silver occurred as native 

silver, argentite, and cerargyrite. The ore is reported to have 

contained 80 to 90 ounces of silver for each ounce of gold 

(Anderson, 1949).

Production from other properties in the district 

was small compared to that from the General Custer 

mine. Other mines in the district reporting much more 

silver than gold include the Lucky Boy, McFadden, and 

a host of small producers. The Montana and Sunbeam 

produced more gold than silver. Sunbeam ore is said to 

have had a gold-silver ratio of 2.5 to 1 (Lockard and Rice, 

1971).

A broad zone of disseminated gold and silver surrounds the high-grade veins at the Sunbeam mine. The 

host rocks are variably argillized and silicified rhyolite and

114°45'

B A . ESTES MOUNTAIN \o 

,TSX X x X MONTANA MINE ", 

X ^ 

X

A MOUNT GREYLOCK

CHARLES DICKENS x 

MINE

x X ^,,^w BACHELOR 

* X BOY MOUNTAIN 

MINE

A

BONANZA 

PEAK

4 MILES

J

I ' I I

0 2 4 KILOMETERS

EXPLANATION

X MINE OR PROSPECT

Figure 13. Map showing principal mines and prospects, Yankee 

Fork mining district, Custer graben.

silicic tuff. A similarly altered zone in andesite surrounds 

the General Custer vein. The zone is controlled in part 

by many small faults known from mining exploration in 

the area but not shown on figure II. Both deposits were 

being explored in 1983.

The principal gold-silver-base metal occurrence in the 

district is at the Charles Dickens mine (fig. 13). The Dickens 

produced more silver than gold; only minor amounts of 

copper, lead, and zinc were produced. The base metals occur in chalcopyrite, covellite, galena, and sphalerite. The 

veins rich in base metals may have been deposited at greater 

depth than those containing only gold and silver.

ORE GENESIS

Exceptionally long-lived movement along two northeast-trending fault zones, one in the Sunbeam mine-Estes 

Mountain area and the other in the Charles Dickens-Lucky 

Boy mines area, was a controlling factor for ore localization within the Custer graben. Movement on these faults 

began with initial graben subsidence, and, as noted above, 

locally influenced depositional environments within the 

graben. These and other fault zones within the graben 

localized intrusion of rhyolite and the granodiorite-quartz

Gold-Silver Mineralization in the Custer Graben 113
o

7

tf>

cc. 3 

o

Q

LL.

O

« 5

O

_l _l

5

z 4

z 

o

h

PRODUi ,Lu

i 2

(-

1

n

-

-

-

-

-

-IP

*gi

;« ! 

: . .:. .: :A. ! !

; *£ i

ill!

" : ^J|: ':

5

h^

Uj -j

o b OQ iy

1 co i

. 

- , 1 ii sS

G : ':.: 0 0 

!' '. " Q ^

^ ^ ^ ^ Q ^ ^

g 3 1 1 1 ^

m Mill!

. ]: ':'- '' ' ':' : T | V-' '.. 'J [!:" :'" """""I

 : ' . ' : ; : : i [' !K T J' [: '':' '.': ' I H"^7 1

.;..-..: .,. ... ,. ; : 1. : : t ; ;. ; 3 1: .' ... . . ] . ..»... .« .< <...[ . .-.....,...,. .

 :. : -. ... :....:.. ',.; F ,. .. .A I:..; ..:. :.. .1 t. . : .J I; ;. ..... 1 !: : .;- ...J

CO

c5

o 

3

a.

^

ki2

2:

§

^

O

1

^

DREDGE

*:

§

u.

YANKEE

Figure 14. Value of total production of precious metals from 

mines during 1882 to 1959 in the Yankee Fork mining district, 

Custer graben.

monzonite stock. Continued fault movement and repeated 

renewal of fracture permeability in these fault zones 

permitted establishment of extensive, long-lived hydrothermal convection cells within each fault zone. 

Hydrothermal convection was driven by heat from the 

silicic intrusions. These processes continued long enough 

to permit formation of ore-grade deposits. Cooling of the 

silicic intrusions brought hydrothermal circulation to an 

end. Fault movement continued after hydrothermal circulation and ore deposition had ceased.

The source of the metal ions is not known with certainty. The Grand Prize Formation, which underlies the 

graben within the mining district, is a likely source for base 

metals and silver because, elsewhere in the region, it is known 

to be intrinsically rich in these metals (Hall, chap. J, this 

volume). The contribution of the silicic magmas to the metal 

content of the ore deposits in the graben is not known.

postmineralization cover rocks may have been is speculative 

because no trace of them now remains.

There is some evidence that not all the deposits 

originally formed have been preserved. At Mount Jordan, 

at the northwestern margin of the graben, there is an occurrence of anomalous copper and other base metals in the 

volcanic rocks (Foster and Cooley, 19S2). This occurrence 

may represent a lower level of a hydrothermal system that 

deposited precious metals at a higher level, now removed 

by erosion. Vertical zonation, with base metals below 

precious metals, occurs within the graben at Estes 

Mountain.

Not all the precious metals from eroded lode deposits 

escaped the area. Rich placer deposits in Quaternary gravels 

along Jordan Creek and Yankee Fork were mined extensively before 1952 (Choate, 1952). Several small-scale placer 

operations were underway in 1983.

CONCLUSIONS

The Custer graben contains epithermal ore deposits 

because of a combination of favorable characteristics. 

Paleozoic sedimentary rocks that form the basement 

beneath the mining district are a potentially rich source 

of a variety of metals. Rhyolite and granodiorite-quartz 

monzonite magmas also may have been sources for metals. 

The heat that accompanied intrusion of these silicic 

magmas drove extensive hydrothermal circulation systems 

in which meteoric water was the predominant component. 

Faults that remained active during and after graben 

formation provided pathways for the fluids and sites for 

ore deposition. In parts of the graben where there was little 

faulting or no silicic intrusive activity, there also are no 

known ore deposits and few prospects.

Another critical factor for the presence of epithermal ore deposits is preservation. This is one of the major 

reasons why Tertiary volcano-tectonic subsidence structures 

are good exploration targets. Subsidence and infilling 

of a graben or caldera preserves ore deposits that have 

been formed within them. Otherwise, epithermal deposits, 

especially those as old as Eocene, would long since have 

been lost to erosion, which is probably what happened 

to deposits that formed in areas adjoining the graben 

margin but on the upthrown side of the graben-bounding 

faults.

ORE PRESERVATION

Graben subsidence and further burial after ore 

deposition helped protect the deposits from erosion, which 

probably has continued with little interruption from late 

Eocene to the present. As indicated above, the area may 

have been covered by a blanket of lavas of intermediate 

composition. How thick or how extensive these or other

REFERENCES CITED

Anderson, A. L., 1949, Silver-gold deposits of the Yankee Fork 

district, Custer County, Idaho: Idaho Bureau of Mines and 

Geology Pamphlet 83, 37 p.

Choate, Raoul, 1962, Geology and ore deposits of the Stanley 

area [Idaho]: Idaho Bureau of Mines and Geology Pamphlet 126, 121 p.

114 Challis 1°x2° Quadrangle, Idaho
Criss, R. E., Champion, D. E., and Mclntyre, D. H., 1985, Oxygen isotope, aeromagnetic, and gravity anomalies associated with hydrothermally altered zones in the Yankee Fork 

mining district, Custer County, Idaho: Economic Geology, 

v. 80, no. 5, p. 1277-1296.

Foster, Fess, 1982, Geologic map of Mt. Jordan and vicinity, 

Custer County, Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF-1434.

Foster, Fess, and Cooley, E. F., 1982, Semiquantitative spectrographic analyses of rocks from the Mt. Jordan vicinity, 

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

Report 82-467, 13 p.

Koschmann, A. H., and Bergendahl, M. H., 1968, Principal goldproducing districts of the United States: U.S. Geological 

Survey Professional Paper 610, 283 p.

Lockard, D. W., and Rice, W. L., 1971, Preliminary investigation of a low-grade gold deposit in Custer County, Idaho: 

U.S. Bureau of Mines Open-File Report 5-71, 40 p.

McDanal, S. K., Cooley, E. F., and Callahan, J. E., 1984, 

Analytical results of stream-sediment and nonmagnetic 

heavy-mineral-concentrate samples with sample locality 

map from portions of the Chain's 1°X 2° quadrangle, Idaho: 

U.S. Geological Survey Open-File Report 84-0634, 389 p.

Mclntyre, D. H., and Johnson, K. M., 1983, Geologic map of 

part of the Sunbeam mine area, Custer County, Idaho: US. 

Geological Survey Open-File Report 83-329, [scale 1:2,400].

Siems, P. L., Albers, D. F, Malloy, R. W., Mitchell, V. E., and 

Perley, P. C., 1979, Uranium potential and geology of the 

Challis Volcanics of the Basin Creek-Yankee Fork area, 

Custer County, Idaho: U.S. Department of Energy Report 

GJBX-33(79), 200 p.

Gold-Silver Mineralization in the Custer Graben 115

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter J

Stratigraphy of and Mineral Deposits in 

Middle and Upper Paleozoic Rocks of the 

Black-Shale Mineral Belt, Central Idaho

By WAYNE E. HALL

CONTENTS

Abstract 118 

Introduction 118 

Geology 118

Sedimentary rocks 118

Ordovician to Devonian rocks, undivided 118 

Milligen Formation 120 

Copper Basin Formation 120 

Salmon River assemblage 120 

Wood River Formation 121 

Dollarhide Formation 124 

Grand Prize Formation 124 

Plutonic rocks 125 

Structure 125

Boulder Mountains stack 126 

Pioneer Mountains stack 126 

Ore deposits 126

Lead-silver-zinc vein deposits 127 

Molybdenum stockwork deposits 129 

Stratabound deposits 129 

References cited 131

FIGURES

Jl. Simplified geologic map of the black-shale mineral belt 119

32. Index map showing the study area and mining districts 120

J3. Simplified geologic map of the northern part of the black-shale mineral belt 121

J4. Simplified geologic map of the southern part of the black-shale mineral belt 122

J5. Maps showing localities of rock samples containing anomalous concentrations of metals 128

J6. Photograph of Stratabound zinc-lead-silver ore from the Triumph mine 130

J7. Photograph of jamesonite ore from the Livingston mine 130

TABLES

Jl. Spectrographic analyses of lead-silver-tin ore from Boulder Basin 127 

J2. Geochemical data from the Salmon River assemblage 129

Black-Shale Mineral Belt 117
Abstract

A belt of highly mineralized, black, siliceous-facies sedimentary rocks of Late Cambrian to Permian age crops out on the 

eastern side of the Idaho batholith in central Idaho. All of the 

Paleozoic rocks are allochthonous and occur in imbricated structural plates separated by regional thrust faults. The structural 

plates are stacked, younger formations over older, in most places. 

The value of past production of metals from this belt is $65 

million; a conservative estimate of the total known resources is 

$6 billion. The most intensely mineralized zones are in black 

argillite and micritic limestone beds of the Milligen Formation 

(Devonian) and the Salmon River assemblage (Upper Mississippian, Upper Devonian, and Upper Cambrian) and the Dollarhide 

Formation (Permian) below regional thrust faults near biotite 

granodiorite stocks of Cretaceous age.

Mineral resources include: (1) many small to moderate-size, 

high-grade lead-silver-zinc vein deposits; (2) an area containing 

tin associated with lead-silver vein deposits in the Galena and 

Boulder Basin districts; (3) skarn tungsten veins; (4) two large 

molybdenum stockwork deposits; and (5) stratabound deposits 

of zinc, barite, vanadium, and possibly lead and silver. As much 

as 6 weight percent tin occurs in ore at Boulder Basin in a mineralogic and chemical assemblage (Pb-Zn-Sb-Ag-Au-Sn-Se) similar 

to Bolivian-type tin deposits. These deposits occur in limy sandstone of the Pennsylvanian and Permian Wood River Formation 

near Eocene hypabyssal intrusive bodies; all other lead-silver vein 

deposits are in black argillite sequences near Cretaceous stocks.

The black shale beds contain and probably are the source 

of metals for the vein deposits. Also, some of the productive 

deposits are stratabound and are syngenetic. Recent mining activity has concentrated on exploration for the stratabound 

syngenetic deposits in the black-shale mineral belt and on 

development and bringing into production the Thompson Creek 

molybdenum stockwork deposit.

INTRODUCTION

A belt of highly mineralized, black siliceous-facies 

sedimentary rocks, 145 km (kilometers) long in a northnorthwest direction and 15 to 45 km wide, of Late Cambrian to Permian age crops out on the eastern side of the 

Idaho batholith, from the Salmon River in the southcentral part of the Challis quadrangle south to Bellevue, 

Idaho, in the Hailey quadrangle (fig. Jl). This area is called 

the central Idaho black-shale mineral belt in this report. 

The belt extends east to the Bayhorse district, which is 

discussed by Hobbs (chaps. D and K, this volume). This 

chapter presents a preliminary overview of the stratigraphy, 

structure, and ore deposits of the belt.

The value of past production of metals, mostly lead 

and silver, from the central Idaho black-shale mineral belt 

is $65 million; a conservative estimate of the total known 

resources, most of which are in two molybdenum stockwork deposits, is $6 billion. Mining districts within the 

belt include, from north to south, the Slate Creek, Little 

and Big Boulder Creeks, Germania Creek, Fourth of July 

Creek, East Fork, Galena, Boulder Basin, Carrietown,

Triumph-Parker mineral belt, and Wood River 

(fig. J2).

Most of the production from the black-shale mineral 

belt was from lead-silver deposits mined during the late 

19th century and World War II. Early descriptions of the 

geology and ore deposits were by Anderson and others 

(1950), Ross (1937), Umpleby (1915), and Umpleby and 

others (1930). Recently, the geology of the black-shale 

mineral belt has been reinterpreted; all the fine-grained, 

carbonaceous Paleozoic formations are now recognized to 

be allochthonous, and many of the ore deposits are 

syngenetic. A partial listing of recent references includes: 

Dover (1969, 1981), Hall and Czamanske (1972), and Hall 

and others (1978, 1984).

The geology of the central Idaho black-shale mineral 

belt (figs. J3 and J4) was mapped by the author in the 

south-central part of the Challis quadrangle south of the 

Salmon River and in the Wood River area, and by S. W. 

Hobbs north of the Salmon River (Fisher and others, 1983; 

Hobbs and others, 1975). The geology (fig. Jl) in both the 

northern and southern parts of the belt is simplified from 

detailed mapping. The geology in the central part of the belt, 

in the Galena and Boulder Basin mining districts, is based 

on only three east-west traverses made to trace the Paleozoic 

formations and thrust faults between the areas of detailed 

mapping. Many Tertiary porphyry intrusions in the 

Paleozoic rocks were not mapped in making these traverses.

GEOLOGY

The sedimentary rocks in the central Idaho blackshale mineral belt are predominantly black, fine-grained, 

siliceous-facies argillite, siltite, limy sandstone, shale, and 

siltstone of Late Cambrian to Permian age. All of the 

Paleozoic rocks are allochthonous and occur in imbricated 

structural plates separated by major thrust faults. The 

structural plates are stacked with predominantly younger 

Paleozoic sequences over older ones. All of the formations 

are tectono-stratigraphic units. Different names are applied 

to essentially coeval rock units on separate thrust plates 

if the lithology, internal structures, degree of sorting and 

roundness of clasts, and composition of clasts make each 

unit lithologically distinctive. The Paleozoic rocks are intruded by granitic plutons of Late Cretaceous and Eocene 

age and by dikes and sills of porphyritic dacite and 

rhyodacite and rhyolite of Eocene age. Thick sequences of 

the Eocene Challis Volcanics unconformably overlie the 

Paleozoic and Cretaceous rocks.

Sedimentary Rocks

Ordovician to Devonian Rocks, Undivided

The undivided Ordovician to Devonian rocks (fig. 

J3, map unit DSOu) include the Phi Kappa (Ordovician)

118 Challis 1°x2° Quadrangle, Idaho
114°30' 114°00'

CORRELATION OF MAP UNITS

QUATERNARY AND 

TERTIARY 0

TERTIARY

CRETACEOUS

PALEOZOIC

LIST OF MAP UNITS

SURFICIAL UNITS, UNDIVIDED 

HYPABYSSAL ROCKS 

CHALLIS VOLCANICS

BIOTITE GRANODIORITE AND 

GRANITE

SEDIMENTARY ROCKS OF BLACKSHALE MINERAL BELT

CHALLIS QUADRANGLE

HAILEY QUADRANGLE

PIONEER MOUNTAINS 

GNEISS DOME

Figure J1. Simplified geologic map of the black-shale mineral belt, central Idaho.

and Trail Creek (Silurian) Formations, which were art unnamed Silurian and Devonian unit described by

originally named by Umpleby and others (1930) and were Dover (1981, p. 34). Dover showed that the Phi Kappa

described and redefined by Dover and others (1980), and and Trail Creek Formations consist of an intricately

Black-Shale Mineral Belt 119
115° 114°

44°

_Salmon

SLATE 

CREEK

BIG BOULDER

CREEK

LITTLE BOULDER 

CREEK

CHALLIS QUADRANGLE

GERMANIA 

CREEK

WASHINGTON 

BASIN

HAILEY QUADRANGLE

* TRIUMPH-PARKER 

MINERAL BELT

o Hailey

Bellevue

20 MILES

I ' ' ' ' I I

0 10 20 KILOMETERS

Figure J2. Index map showing the study area and mining 

districts in the black-shale mineral belt, central Idaho.

imbricated series of thrust slices that are composed of 

black argillite and shale imbricated with thrust slices of 

metasiltstone, fine-grained quartzite, and siliceous mudstone. Much of the shale and mudstone is finely laminated 

and contains internal structures such as scour, crossbedding, and convolutions, indicating deposition of these 

transitional deposits on a slope, which is interpreted from 

facies models to have been near the continental margin.

Milligen Formation

The Devonian Milligen Formation (fig. J4, map unit 

Dm) crops out in a northwest-trending belt about 50 km 

long and 11 km wide in the Wood River area. The formation consists of interbedded dark-gray to black siliceous 

argillite, siltite, and micritic limestone but includes minor 

dolomitic siltstone, quartzite, and granule conglomerate 

in the upper part. The formation is divided into two informal members a lower member at least 900 m (meters) 

thick of dark siliceous argillite and an upper member as 

much as 300 m thick that is much less argillitic and more 

heterogeneous in lithology (Sandberg and others, 1975). 

The lower unit is a deep-water siliceous oceanic facies, and 

the upper unit was deposited during uplift that presaged

the Antler orogeny. A more complete description of the 

lithology and metamorphic grade of the Milligen is given 

in Sandberg and others (1975).

Copper Basin Formation

The Mississippian Copper Basin Formation (fig. J4, 

map unit Me) crops out along the eastern side of the study 

area, mostly on the north side of the Pioneer Mountains 

gneiss dome. The Copper Basin is a deep-sea fan turbidite 

sequence at least 3,000 m thick that was deposited in a 

foreland basin between the Antler highland on the west 

and the craton to the east (Nilsen, 1977). The formation 

consists of dark-gray argillite and siltite in the lower part, 

thinly bedded, fine-grained limestone turbidites in the 

middle, and siliceous pebble conglomerate and quartzite 

in the upper part.

Salmon River Assemblage

The Salmon River assemblage (fig. J3, map unit 

Esr) was named informally by Nilsen (1977) for a sequence composed of limestone turbidites interbedded with 

dark-gray argillite, siltite, shale, and fine-grained quartzite. The assemblage is well exposed along the Salmon River 

in the northern part of the study area (fig. J3). It is intensely deformed, so the thickness is difficult to measure, 

but it is estimated to be more than 2,000 m thick.

The Salmon River assemblage is a tectonostratigraphic mixture of Late Cambrian, Late Devonian, 

and Late Mississippian fine-grained, carbonaceous rocks. 

The fossil reports indicating the mixture of ages in the 

Salmon River assemblage are given in Hobbs (chap. D, 

this volume). Late Cambrian fauna have been identified 

from silty, carbonaceous limestone interbedded with 

argillite on the eastern margin of the Salmon River 

allochthon on the western side of Thompson Creek (fig. 

J3). The limestone is interpreted by Hobbs and Hall to 

be a tectonic slice at the base of the allochthon. Conodonts of Late Devonian age have been identified by 

Repetski (in Hobbs, chap. D, this volume) from samples 

of carbonaceous limestone that were collected by F. G. 

Poole and George Desborough and by Poole and Hall between tributary canyons on the west side of Slate Creek 

on the southwest to Thompson Creek on the northeast 

(fig. J3). The limestone is interbedded throughout a large 

part of the stratigraphic thickness of the assemblage. Two 

specimens of float from the same terrane as the collections of Poole and Hall contained a megafauna that is 

Late Mississippian (Hobbs, chap. D, this volume). This 

part of the Salmon River assemblage, consisting of 

graded, fine-grained carbonaceous rocks, was deposited 

on a slope, probably near the edge of the continental slope. 

The Salmon River assemblage rocks are weakly metamorphosed and have a fracture cleavage that is almost parallel

120 Challis 1°x2° Quadrangle, Idaho
114°30'

PARTIAL LIST OF MINES 

AND PROSPECTS

1. Tungsten Jim

2. Thompson Creek

3. Crater Lake

4. Livingston

5. Hoodoo

6. Silver Rule

7. Patty Flynn

8. Silver Dollar

9. Deer Trail

10. Meadow View

11. Empire

12. Black Rock

13. Little Boulder Creek W/.:.;.y.;.H^.ij^JJfc j^

r

5 10 KILOMETERS 

EXPLANATION 

CORRELATION OF MAP UNITS

QUATERNARY AND 

TERTIARY (?)

TERTIARY 

CRETACEOUS

CONTACT

FAULT Bar and ball on downthrown side, dotted where concealed

THRUST FAULT Sawteeth on 

upper plate

MINES OR PROSPECTS

\ PERMIAN AND 

J PENNSYLVANIAN

~ MISSISSIPPI (? 

TO CAMBRIAN

~\ DEVONIAN TD 

/ORDDVICIAN

QTu

%5

PPw

LIST OF MAP UNITS

SURFICIAL DEPOSITS 

CHALLIS VOLCANICS

BIOTITE GRANODIORITE AND 

GRANITE

GRAND PRIZE FORMATION 

WOOD RIVER FORMATION 

SALMON RIVER ASSEMBLAGE

SEDIMENTARY ROCKS, 

UNDIVIDED

Figure J3. Simplified geologic map of the northern part of the black-shale mineral belt, central Idaho.

to bedding. They differ from the Milligen Formation, 

which has a penetrative shear cleavage that is at a large 

angle to bedding, has a much higher metamorphic grade, 

and has a phyllitic sheen.

Wood River Formation

The Pennsylvanian-Permian Wood River Formation 

(figs. J3 and J4, map unit Pw) crops out in a northwestBlack-Shale Mineral Belt 121
Mixed Ti and Pd

Shaw 

Mountain'

x

43°30'

Figure J4. Simplified geologic map of the southern part of the black-shale mineral belt, central Idaho.

122 Challis 1°x2° Quadrangle, Idaho
PIONEER MOUNTAINS 

GNEISS DOME

CORRELATION OF MAP UNITS

QUATERNARY AND

TERTIARY(?)

TERTIARY

CRETACEOUS

PERMIAN

\PERMIAN AND 

J PENNSYLVANIAN

\MISSISSIPPIAN 

DEVONIAN

DEVONIAN TO

QTu

.-.. ..Pd-.V.

PIPw

"flmV" 

LIST OF MAP UNITS

SURFICIAL DEPOSITS, UNDIVIDED

HYPABYSSAL INTRUSIVE ROCKS

CHALLIS VOLCANICS

BIOTITE GRANODIORITE AND GRANITE

DOLLARHIDE FORMATION

GRAND PRIZE FORMATION

WOOD RIVER FORMATION

COPPER BASIN FORMATION

MILLIGEN FORMATION

SEDIMENTARY ROCKS, UNDIVIDED

CONTACT

THRUST FAULT Sawteeth on

upper plate 

MEASURED SECTION

MINES AND PROSPECTS

PARTIAL LIST OF MINES 

AND PROSPECTS

14. Peace of Mine

15. Pole Group

16. Darlin

17. Stoneboat

18. Black Carbonate

19. Golden Glow

20. Independence

21. Triumph

22. North Star

23. Deer Creek barite

24. Red Cloud

25. Red Elephant

26. Liberty Gem

27. Eureka

28. Snoose

29. Minnie Moore

30. Silver Star Queen

Figure J4. Continued.

Black-Shale Mineral Belt 123
trending belt 100 km long and 40 km wide throughout 

the Wood River area north to White Cloud Peaks. The 

formation is about 3,000 m thick. It consists of a basal 

siliceous conglomerate as thick as 120 m, and thick sequences of fine-grained, light-gray, limy sandstone and 

interbedded fine-grained quartzite and silty limestone. The 

formation was subdivided into seven units by Hall and 

others (1974). It ranges in age from Middle Pennsylvanian (Desmoinesian) to Early Permian (Wolfcampian and 

Leonardian?).

Dollarhide Formation

Dollarhide Formation (Permian; fig. J4, map unit 

Pd) is the name given herein for a sequence of interbedded, dark-gray carbonaceous limestone, siltite, 

fine-grained quartzite, sandstone, and minor granule conglomerate that is exposed along the ridge between 

Dollarhide Summit and Dollarhide Mountain in the 

Dollarhide Mountain 7 1/2-minute quadrangle, 32 km 

west of Ketchum; this is the type locality for the formation. The Dollarhide Formation, formerly mapped as part 

of the Wood River Formation, crops out over an area of 

about 100 sq km (square kilometers). It was intruded by 

the Idaho batholith.

The formation is characterized by highly carbonaceous limestone, fine-grained limy sandstone, siltstone, and siltite that is banded dark-gray and brown in 

layers 1 to 4 cm (centimeters) thick. Graded bedding and 

convolute structures are common. The limestone, siltstone, 

and siltite of the Dollarhide Formation are more poorly 

sorted and have less rounded and larger sand grains than 

Wood River Formation limy sandstone and quartzite of 

the same age.

A partial section of Dollarhide Formation 653 m 

thick was measured on the eastern side of Wolftone Creek 

in the Mahoney Butte 7 1/2 minute quadrangle, 12 km 

west of Hailey, in sec. 8, T. 2 N., R. 17 E., on the north 

side of the Red Cloud mine (fig. J4). This was selected 

as a reference locality because it contains the only fossils 

that have been found in the Dollarhide Formation.

Measured partial section of Dollarhide Formation

[Measured by W. E. Hall on ridge north of Red Cloud mine 

in sec. 8,,T. 2 N., R. 17 E.]

Thrust-fault contact with overlying Wood River Formation 

unit 5

Thickness

Dollarhide Formation: (meters) 

Limestone, silty, gray, carbonaceous, finegrained .................................. 22

Siltite, dark-gray, carbonaceous; and quartzite, gray, 

fine-grained .............................. 15

Limestone, gray, fine-grained, carbonaceous, thinbedded .................................. 15

Measured partial section of Dollarhide Formation Continued

Thickness

Dollarhide Formation: Continued (meters)

Quartzite, light- to dark-gray, fine-grained; and siltite, 

gray, carbonaceous ...................... 191

Siltstone, limy, dark-gray, carbonaceous; and quartzite, fine-grained; shattered .................. 27

Quartzite, gray, fine- to medium-grained, cross 

bedded; and siltstone, limy, gray ............ 15

Siltstone, limy, gray; and quartzite, medium-gray, 

fine-grained, carbonaceous; beds 15-30 cm thick 92

Limestone, silty, gray, carbonaceous; and siltite, 

gray, thin-bedded ......................... 15

Siltite, gray, carbonaceous; some parts limy; beds 

15-30 cm thick; graded bedding ............ 22

Siltstone, limy, gray; and siltite, gray, laminated; 

beds 1-2 cm thick ......................... 53

Sandstone, limy, gray, grain size 1-2 mm 

(millimeters); and siltite, gray, shattered ...... 15

Limestone, silty, gray, laminated; beds 1-2 cm thick 7

Sandstone, limy, gray, medium-grained; and siltstone, 

limy, gray; convolute structures in fine-grained 

siliceous beds; graded beds, 5-15 cm thick ... 7

Limestone, silty, gray, carbonaceous; and siltstone, 

limy, gray on fresh surfaces, weathers dark brown; 

beds 10-20 cm thick; graded beds ........... 53

Siltstone, gray, limy; and limestone, silty; contains 

abundant fusulinids (collection P-134); age is 

Early Permian (R. C. Douglass, written commun., 

1978) .................................... 7

Total thickness incomplete Dollarhide Formation .............................. 653

Base not exposed

R. C. Douglass (written commun., 1978) reported 

the following on the age of fusulinids collected in 

Wolftone Creek in the lowest unit of the measured section: "Collection P-134, Blaine County, Idaho * * * the 

fusulinids present have had most of the gross structures 

destroyed. Again the carbonaceous matter in the wall of 

the outer volution displays the detailed structure of the 

keriotheca beautifully. The wall structure indicates a Late 

Pennsylvanian to Permian age. The critical gross structures are not preserved, but one section includes an 

oblique equatorial that suggests highly arched chambers characteristic of the Early Permian Pseudofusulina. I cannot make a positive statement about the age 

of this material, but the odds are now better that this 

is Early Permian rather than Late Pennsylvanian. In 

any event, I would show this unit as more likely Early 

Permian!'

Grand Prize Formation

Grand Prize Formation (figs. J3 and J4, map unit 

Pg) is the name given herein for a thick sequence of interbedded light- to medium-gray limy siltstone; fine-grained

124 Challis 1°x2° Quadrangle, Idaho
quartzite; dark-gray, banded siltite; and medium- to darkgray, carbonaceous, silty to sandy limestone that is well 

exposed in the Horton Peak 7 Vi -minute quadrangle, 

Custer County, on the northern side of Pole Creek at the 

confluence with Grand Prize Creek; this is the type locality for the formation. The formation was previously 

mapped as part of the Wood River Formation by Ross 

(1937) and Tschanz and others (1974), but it overlies the 

Wood River Formation in thrust-fault contact and is lithologically distinct from the Wood River Formation. It crops 

out over an area of 800 sq km from 2 km north of Galena 

Summit north to the Yankee Fork of the Salmon River. 

The formation is divided into four informal units 

at the type locality on the north slope of Pole Creek canyon at the confluence with Grand Prize Creek, 6.4 km 

south of the southern boundary of the Challis quadrangle.

Measured partial section of Grand Prize Formation 

[Section measured by W. E. Hall]

Top not exposed 

Grand Prize Formation:

Thickness 

(meters)

4. Siltite, dark-gray, banded, weathers dark red and 

reddish brown; and limestone, silty, fine-grained, 

gray and bluish-gray, crossbedded, contains softsediment deformation .................... 150

3. Siltstone, limy, gray, banded, medium- to thickbedded, in part contact metamorphosed and 

bleached to light-gray hornfels and calc-hornfels; 

and dark-gray carbonaceous siltite, banded, 

crossbedded, contains convolute structures .. 580

2. Limestone, silty, bluish-gray, fine-grained; contains abundant fossil debris but no diagnostic 

fossils for age determination .............. 30

1. Siltite, carbonaceous, dark-gray, prominently 

banded, limy in upper part; limestone, gray, sandy, abundant crossbedding and convolute structures in lower part ....................... 690

Total thickness units 1-4, Grand Prize

Formation ......................... 1,450.

The age of the Grand Prize Formation is Early Permian (Leonardian), and it could be as old as Early Permian (Wolfcampian) or Pennsylvanian. C. M. Tschanz 

collected a sample from the base of the type locality at 

the confluence of Grand Prize and Pole Creeks from 

which two conodonts were separated. B. R. Wardlaw (written commun., 1979) identified the conodonts as Neogondolella idahoensis (Youngquist, Hawley, and Miller)-2, and 

said: "Neogondolella idahoensis indicates a LeonardianRoadian age (middle-upper Leonardian of U.S. Geological Survey usage). This age is equivalent to the uppermost 

part of the Pequop Formation, the Kaibab and Grandeur 

Formations, and the lower part of the Meade Peak 

Phosphatic Shale Member of the Phosphoria Formation 

in southern Idaho, Utah, and Nevada!'

A sample of limestone of the Grand Prize Formation was collected by S. W. Hobbs from the SunBeam

7 Vi -minute quadrangle, Custer County, on the east side 

of Peach Creek (lat 44°17'43" N.; long 114°38'27" W). 

The sample was processed for conodont extraction and 

examined by A. G. Harris (written commun., 1981), who 

reported:

10 Pa elements of Adetognathus sp.

6 Pa elements ofHindeodus cf. H. Minutus (Ellison)

9 Pa elements of Idiognathodus sp.

2 M elements

2 Pb elements 

31 indeterminate bar, blade, and platform fragments.

These conodonts are poorly preserved, stretched, 

corroded, and mostly incomplete, so that species determination is uncertain to impossible. Nonetheless, the 

species that these specimens probably represent are longranging common elements of shelf faunas in the Pennsylvanian through earliest Permian (late Morrowan to late 

Wolfcampian).

Plutonic Rocks

The Paleozoic rocks are intruded by many plutons 

of biotite granodiorite and biotite granite of Cretaceous 

age, and by small Eocene hypabyssal biotite granite 

plutons and dacite, rhyodacite, and rhyolite dikes and sills 

(figs. J3 and J4). Two molybdenum stockwork deposits 

are associated with the Cretaceous calc-alkaline Thompson Creek biotite granodiorite-biotite granite stock and 

the White Cloud Peaks and Little Boulder Creek biotite 

granodiorite stocks (fig. J3). Many of the lead-silver-zinc 

vein deposits are localized near small plutons of Cretaceous biotite granodiorite. Most of these plutons are sills 

or laccoliths that intruded into and along the brecciated 

zones of thrust faults, and most vein deposits are localized in steep shear zones below the thrusts. At the Livingston mine and in the Galena and Boulder Basin 

districts, the vein deposits are localized next to dikes and 

sills of Eocene rhyolite, rhyodacite, and dacite porphyry.

Structure

All of the Paleozoic formations are allochthonous 

and are separated by regional thrust faults. The central 

Idaho black-shale mineral belt (figs. J3 and J4) can be 

described as two separate, imbricated stacks of thrust 

plates. The northern stack, which includes the Salmon 

River sequence and Grand Prize Formation, is herein 

called the Boulder Mountains stack. It extends from the 

Galena district north to the Tungsten Jim mine (fig. J3), 

and east from the study area into the western Bayhorse 

district (Hobbs, chap. D, this volume). The southern stack 

is called the Pioneer Mountains stack. It extends south 

and southeast from the Boulder Basin. The only 

allochthon that transcends both stacks is the Wood River

Black-Shale Mineral Belt 125
allochthon, which extends north along the southeastern 

margin of the Boulder Mountains. The allochthons have 

all been thrust to the east or northeast for distances that 

range from a few kilometers to as much as 200 km.

Boulder Mountains Stack

The Boulder Mountains stack includes three 

allochthonous sequences of late Paleozoic age in the 

Boulder Mountains (figs. J3 and J4). The structurally 

lowest unit is the Salmon River assemblage of Late Cambrian, Late Devonian, and Late Mississippian age. The 

base of the Salmon River assemblage is a thrust fault that 

brought it over Devonian and older rocks (fig. J3) (Hobbs 

and others, 1975). The Wood River Formation overlies the 

southeastern part of the Salmon River assemblage south 

of the Livingston mine. The Grand Prize Formation structurally overlies both of the above units and was thrust 

450 m upward and toward the northeast over the 

topographically high Salmon River assemblage. This relation is well exposed on the north slope of Washington 

basin, where the thrust fault is exposed in the basin floor 

rising eastward to the basin rim. This upward and 

eastward transport of the Grand Prize Formation over the 

Salmon River assemblage caused intense brecciation, 

forming a tectonic breccia as much as 300 m thick. This 

zone of breccia follows the thrust fault and locally cuts 

across bedding in both the overlying and underlying 

allochthons. This tectonic breccia is what Thomasson 

(1959) called Hailey Conglomerate Member of the Wood 

River Formation in his measured sections in the Boulder 

Mountains, at Slate Creek and Fourth of July Creek; it 

has been described by Winsor (1981, p. 25).

Both the Salmon River assemblage and the Grand 

Prize Formation are highly deformed into tight isoclinal 

folds; axial planes of the folds are steep and are truncated 

by the thrust faults separating the two allochthons. This 

truncation indicates that folding was prior to or during 

early stages of thrusting.

Pioneer Mountains Stack

The Copper Basin Formation is the structurally 

lowest element in the Pioneer Mountains stack of thrust 

plates in the study area (fig. J4). It crops out only along 

the eastern edge of the study area north of the Pioneer 

Mountains gneiss dome, and it extends to the east into 

Copper Basin where it has been described by Paull and 

others (1972), Skipp and Hall (1975), and Nilsen (1977).

The Copper Basin Formation is structurally overlain 

by undivided Devonian, Silurian, and Ordovician rocks 

(fig. J4, map unit DSOu), which are an imbricated complex of thrust slices shown in figures J3 and J4 as a single 

tectono-stratigraphic unit. This Devonian-to-Ordovician 

unit is interpreted to be structurally overlain by the

Milligen Formation, which is widespread through the 

Wood River area and is the favorable host rock for ore 

deposits in the Wood River lead-silver district (fig. J2). 

The Milligen Formation is highly deformed, with axial 

planes of tight folds nearly horizontal, and the dark-gray 

Milligen argillite beds have a shear cleavage with oriented 

sericite that cuts across bedding and gives the rock a 

phyllitic sheen. This cleavage, phyllitic sheen, and internal deformation is unique to the Milligen. The author 

believes that these features formed during tectonic 

transport of the Milligen during the Antler orogeny, 

possibly while being obducted across a subduction zone, 

in the latest Devonian and Mississippian. Davis (1984) 

described microstructures in the Milligen that he interprets formed during the Antler orogeny.

The Milligen is overlain in thrust-fault contact by 

the Wood River Formation. The Wood River Formation 

is deformed into broad, open folds and is much less 

metamorphosed than the Milligen. As much as 1,500 m 

of the Wood River Formation have been cut out in the 

troughs of synclines by the Wood River thrust fault at the 

base of the allochthon.

In the western part of the Pioneer Mountains stack, 

near the Red Cloud mine and west of Bellevue at the Silver 

Star Queen mine, the Milligen Formation structurally 

overlies the Dollarhide Formation, and the Dollarhide 

Formation is the host rock for the lead-silver deposits west 

of the Wood River. The Dollarhide Formation is only 

weakly metamorphosed. The Dollarhide Formation is 

overlain in thrust-fault contact by the Wood River Formation near the Deer Creek barite deposit (fig. J4, no. 

23).

ORE DEPOSITS

The central Idaho black-shale mineral belt is a 

highly mineralized area containing deposits of antimony, 

barite, lead, molybdenum, silver, tin, tungsten, vanadium, 

and zinc. Mines that have a recorded production or have 

had extensive exploration work are shown in figures J3 

and J4. The most favorable hosts for ore are the carbonaceous black argillite and micritic limestone of the 

Devonian Milligen Formation, carbonaceous argillite, 

siltite, and silty limestone of the Upper Cambrian, Upper Devonian, and Upper Mississippian Salmon River 

assemblage, and the carbonaceous micritic limestone and 

siltite of the Dollarhide Formation near granitic plutons 

of Cretaceous or Eocene age. They contain abundant 

high-grade lead-silver-zinc vein deposits, tungsten skarn 

deposits, and syngenetic deposits of barite, sphalerite, and 

vanadium. The Wood River Formation is unfavorable for 

ore deposits in the Bellevue-Hailey-Sun Valley area (fig. 

J4) but contains many small lead-silver-zinc-tin veins in 

the Galena and Boulder Basin districts. The lead-silverzinc vein deposits and the syngenetic deposits are

126 Challis 1°x2° Quadrangle, Idaho
discussed here; the tungsten skarn deposits are discussed 

by Cookro (chap. Q, this volume).

Lead-Silver-Zinc Vein Deposits

Most of the past mining in the central Idaho blackshale mineral belt has been from lead-silver-zinc vein 

deposits (figs. J3 and J4). Vein deposits in the Dollarhide 

Formation are present in the Wood River district near 

Bellevue and Hailey and on the west side of Dollarhide 

Summit (fig. J2); in the Milligen Formation in the 

Triumph-Independence mine area (fig. J4, nos. 20, 21) 

east of Sun Valley; in the Wood River Formation in the 

Galena and Boulder Basin districts; and in the Salmon 

River assemblage in the Boulder Mountains. The following conditions are common to all the vein deposits:

1. The host rock is black shale.

2. The veins lie under but close to regional thrust 

faults, especially where the thrust faults are arched or 

domed, as, for example, at the Triumph mine (fig. J4, 

no. 21).

3. The veins are close to contacts of granitic intrusive 

rocks, predominantly of Cretaceous but also of Eocene 

age.

4. The most productive deposits have a sideritequartz gangue rather than a calcite-quartz gangue.

These lead-silver veins have a regional mineralogical 

zoning pattern. In the southern part of the black-shale 

belt near Hailey and Bellevue, the principal sulfide 

minerals are galena, bournonite (PbCuSbS3), sphalerite, 

and pyrite (Hall and Czamanske, 1972). Siderite is the 

principal gangue mineral; calcite and quartz are present 

but less abundant. The galena is argentiferous; for each 

percent of lead, the ore averages 1.25 ounces silver per 

ton. The galena was analyzed by electron microprobe by 

G. K. Czamanske (Hall and Czamanske, 1972) and contains abundant antimony and silver and small amounts 

of copper, iron, manganese, selenium, and tin. Seven 

purified mineral separates of galena contained 2,100 to 

5,000 ppm (about 65-150 oz) silver and 3,100 to 4,700 ppm 

antimony. About one-half of the antimony and silver is 

in solid solution in the galena; the rest is present in minute 

inclusions, predominantly as diaphorite (Pb2Sb3Ag3S8) 

(Hall and Czamanske, 1972).

Emission spectrographic analyses of the lead-silver 

ore from the Galena and Boulder Basin districts in the 

central part of the black-shale mineral belt indicate that 

the ore contains lead, silver, gold, antimony, tin, zinc, copper, and selenium. Ore minerals are galena, stannite, zincian stannite, cassiterite, tellurian canfieldite, and 

sphalerite (B. F. Leonard, written commun., 1973, in 

Tschanz and others, 1974). Silver-bearing tetrahedrite and 

a silver telluride mineral were identified by Robert Felder 

and Robert Oscarson (written commun., 1984). These 

silver-tin-lead vein deposits resemble the productive

tin-silver deposits of Bolivia in many respects. The veins 

are narrow (a few centimeters to 2 m wide), steeply dipping lenticular structures in the Wood River Formation 

and are closely associated with rhyodacite and dacite porphyry dikes and sills of Tertiary age. Tschanz and others 

(1974) recognized the widespread occurrence of tin in these 

lead-silver veins through a systematic regional geochemical 

sampling program. Analyses of 92 mineralized rock 

samples from dumps and in place within this belt contained from 0.05 to 6 percent tin (Tschanz and others, 

1974). Semiquantitative spectrographic analyses of two 

grab samples taken from dumps in the Boulder Basin 

district are given in table Jl. The samples contain abundant silver, gold, antimony, tin, and selenium as well as 

lead, zinc, and copper. These two samples apparently are 

representative of some shoots of high-grade ore that was 

mined from small ore bodies in the Boulder Basin district. 

Umpleby (1915), who visited the district in 1912, reported 

ore shoots that averaged 360 ounces of silver and 3 ounces 

of gold per ton.

Table J1. Semiquantitative emission spectrographic analyses 

of two samples of lead-silver-tin sulfide ore from mine dumps 

in the Boulder Basin mining district, Blaine County, Idaho

[>, more than; ~, about; NA, not analyzed for]

Element

Ag 

As 

Au - 

Cu 

Pb 

Sb 

Sn 

Se 

Sample; 1

(ppm)

5,000

1,000

20

20,000

MOO, 000

20,000

300

NA

(oz/ton)

"160

NA-0.6

NA

NA

NA

NA

NA

Sample

(ppm) (o

15,000

3,000

200

>20,000

>50,000

>50,000

15,000

3,100

2

z/ton)

"500

NA~6

NA

NA

NA

NA

NA

The localities of all mineralized rock samples collected by Tschanz and others (1974) that contained more 

than 0.05 percent tin are shown in figure J5A. Anomalous 

concentrations of tin occur in a north-trending belt 50 

km long and 5 km wide that extends from the Boulder 

Basin district to the Big Boulder Creek district.

The greatest concentrations of deposits containing 

anomalous tin are in the Galena and Boulder Basin 

districts. The mineralogy and resource potential of the 

deposits in the Boulder Basin district are currently being 

investigated. Preliminary mineralogical studies indicate 

that the tin is present in stannite; silver is present in 

tetrahedrite and a small amount of silver telluride. The 

localities of mineralized rock samples containing 

anomalous concentrations of lead (more than 14,500 

ppm), silver (more than 100 ppm), and antimony (more 

than 3,000 ppm) are shown in figures J5/?, J5C, and 35D, 

respectively. Lead, silver, tin, and antimony occur together 

throughout the northern half of the black-shale mineral 

belt (figs. J3, J4, J5).

Black-Shale Mineral Belt 127
114°45' 114°3D' 114°45' 114°30f

BLACK SHALE BELT

>

Grand 

Prize ^s 

(Creek \ 

EXPLANATION

X LOCALITY OF ROCK SAMPLE 

CONTAINING MORE THAN 

14.500 PPM LEAD

EXPLANATION

LOCALITY OF ROCK SAMPLE 

CONTAINING MORE THAN 

500 PPM TIN

BLACK SHALE BELT

EXPLANATION

LOCALITY OF ROCK SAMPLE 

CONTAINING MORE THAN 

100 PPM SILVER

X LOCALITY OF ROCK SAMPLE 

CONTAINING MORE THAN 

3000 PPM ANTIMONY

10 KILOMETERS

Figure J5. Maps showing localities of rock samples containing anomalous amounts of selected metals, black-shale mineral belt, 

central Idaho.

128 Challis 1°x2° Quadrangle, Idaho
In the northern part of the black-shale mineral belt, 

at the Livingston and Crater mines, the principal ore 

mineral is jamesonite (Pb4FeSb6S14), which is present 

from surface outcrop to the deepest mine workings, which 

are 580 m below the ground surface at the Livingston mine 

(Kiilsgaard, 1949). Galena, sphalerite, chalcopyrite, pyrite, 

pyrrhotite, and arsenopyrite are also common sulfide 

minerals in the ore. The ore has a ratio of about 1 oz silver 

to 2 percent lead, but the mode of occurrence of silver 

in the ore is not known. The ore also contains a trace of 

gold (Kiilsgaard, 1949).

Molybdenum Stockwork Deposits

Most of the value of the known mineral resources 

in the central Idaho black-shale mineral belt is from two 

large, extensively explored molybdenum stockwork 

deposits. The molybdenum stockwork deposit at Thompson Creek is in a biotite granodiorite and biotite granite 

stock of Late Cretaceous age that intruded the Salmon 

River assemblage (Hall and others, 1984). This mine began 

operation in August 1983. The Little Boulder Creek 

deposit in the White Cloud Peaks is a molybdenum 

stockwork in contact-metamorphosed limy sandstone of 

unit 6 of the Wood River Formation on the east side of 

the Little Boulder Creek stock (fig. J3). The deposit contains scheelite as well as molybdenite. As one of the 

molybdenum deposits has been described in detail elsewhere (Hall and others, 1984), they are not described here.

Strata bound Deposits

Much mineral exploration in central Idaho during 

the past decade has been concentrated on the potential 

for stratabound syngenetic ore bodies in the Paleozoic 

black shale and argillite beds, using the Selwyn basin in 

western Canada as a model (Carne and Cathro, 1982). A 

geochemical study of the concentration and distribution 

of metals in the Salmon River assemblage was made by 

Fisher and May (1983). They found that the elements most 

consistently anomalous include silver, barium, copper, 

molybdenum, vanadium, and zinc, as compared to average 

values for black shale in the United States determined by 

Vine and Tourtelot (1970) (table J2).

The samples of Fisher and May (1983) showed that 

highly carbonaceous bands in the Salmon River 

assemblage contain as much as 10,000 ppm vanadium, 

and the assemblage may be a possible future resource for 

vanadium. A similar study was made of the Milligen and 

Phi Kappa Formations in the Boulder and Pioneer Mountains by Simons (1981). He found that these formations 

contained anomalous amounts of metals but in concentrations lower than those of the Salmon River assemblage; 

the values were about the same as for the average of black

Table |2. Anomalous concentrations of selected elements in 

samples from the Salmon River assemblage, central Idaho.

[<, less than; >, more than; ~&, more than or equal to]

Element

Ag 

Ba - 

Cu 

Mo 

V 

Zn 

Range of

concentration

(ppm)

1-15

20-5,000

10-7,000

<5-100

30-10,000

<200->10,000

Threshold values for

anomalous concentrations

Vine and Tourtelot (1970)

(ppm)

>5

>3,000

MOO

>50

>1,000

>200

Percent of

anomalous

samples

7

9

25

11

26

28

shales of the United States as given in Vine and Tourtelot 

(1970).

Most of the ore deposits that have been mined, 

which are principally epigenetic lead-silver-zinc vein 

deposits, occur only in the black-shale sequences. The 

veins are localized close to contacts with granitic stocks 

of Cretaceous and Eocene age just below regional thrust 

faults. Hall and others (1978) showed that the hydrothermal fluid that deposited the ore in the Wood River area 

was mainly of meteoric origin and that the sulfur and lead 

had a shallow, crustal source. They showed that the 

geology, isotope, and fluid-inclusion data are consistent 

with an environment of hydrothermal systems of predominant meteoric water in faulted and shattered Paleozoic 

rocks near granitic stocks. This environment permitted 

deep circulation of hydrothermal fluids, which dissolved 

the metals and sulfur from the Paleozoic black shale and 

deposited ore in favorable beds and shear zones under 

regional thrust faults. The thrust faults form structural 

traps for the ore fluids.

It is now generally recognized that not only are the 

black shales hosts and probable sources of metals for the 

vein deposits, but some of the productive deposits are 

stratabound and are probably of syngenetic origin. These 

deposits include the zinc and barite ore bodies at the 

Hoodoo mine, some stratabound zinc ore bodies at the 

Triumph mine, and the barite deposits at Deer Creek, 10 

km west-northwest of Hailey (fig. J4). These deposits contain very fine grained stratabound ore similar in texture 

to that at known syngenetic deposits. All the deposits are 

close to plutonic bodies, and the ore is partly recrystallized and locally remobilized. This contact metamorphism 

makes identification of original sedimentary textures and 

structures difficult in many places.

Stratabound lead-zinc ore at the Triumph mine (fig. 

J6) was originally called "complex bedded ore high in 

zinc" by Kiilsgaard (in Anderson and others, 1950) and 

was considered to have formed as the replacement of a 

favorable limestone bed. It is reinterpreted now by 

Kiilsgaard and the author as syngenetic sphalerite ore. The 

largest shoot of stratabound zinc ore was 200 m along 

strike, 2 to 15 m thick, extended 50 m down dip, and

Black-Shale Mineral Belt 129
Figure J6. Polished slab of stratabound zinc-lead-silver ore 

from the Triumph mine, central Idaho. The ore contains disseminated grains of sphalerite, galena, pyrite, and arsenopyrite 

concentrated along bedding in a micritic limestone of the 

Milligen Formation (Devonian). Pyrite is finely disseminated 

throughout the rock, and arsenopyrite was identified by microscopic examination of the ore. gn, galena; si, sphalerite.

averaged 5.9 percent lead, 13 percent zinc, and 6.5 ounces 

of silver per ton (Kiilsgaard, in Anderson and others, 1950, 

p. 50).

The ore from the Triumph mine shown in figure J6 

is a fine-grained, thinly bedded, dark-gray, carbonacous, 

micritic, silty limestone containing minute grains of 

sphalerite, galena, arsenopyrite, and pyrite disseminated 

along the bedding. The grain size of the micritic limestone 

is 0.05 to 0.1 mm. The limestone is locally recrystallized 

into veinlets and lenses that are mostly parallel to bedding. The sulfide minerals occur as disseminated, anhedral 

grains concentrated in specific beds and are in part 

recrystallized and remobilized into minute pods or 

veinlets. The recrystallization and mobilization of the 

sulfide minerals are due to an inferred underlying granitic

stock that has metamorphosed micritic limestone to 

tremolite-bearing limestone and calc-hornfels, and argillite 

to hornfels in the mine area.

The existence of stratabound syngenetic lead-silver 

ore is more nebulous. The lead-silver ore is more coarse 

grained than the syngenetic sphalerite ore, and much of 

it obviously crosscuts bedding. At the Livingston mine, 

most of the ore has come from the Livingston vein, which 

strikes about west and dips 28-44° N. The vein is parallel 

to bedding of the host Salmon River assemblage and lies 

a short distance below the thrust-fault contact with the 

overlying Wood River Formation (fig. J3). High-grade 

lead-silver ore shoots are localized close to the intersections of Eocene rhyolite porphyry dikes with the stratabound Livingston vein (Kiilsgaard, 1949). Kiilsgaard 

resampled the Livingston vein in 1983 on the 1,800-, 

2,000-, 2,200-, and 2,400-ft (foot) levels. The samples collected on the 1,800-, 2,000-, and 2,200-ft levels were all 

from an ore shoot that is parallel to bedding. On the 

2,400-ft level, galena was collected from veinlets cutting 

the rhyolite porphyry Blumenthal dike (T. H. Kiilsgaard, 

written commun., 1984).

Ore from the Livingston vein (fig. J7) consists of 

highly deformed black, siliceous argillite with bedded 

sphalerite, pyrite, and minor galena and jamesonite, which 

are interpreted to be syngenetic sulfide minerals that were 

deposited with the black argillite. The syngenetic sulfide 

ore and black-shale host rock were subsequently shattered 

and displaced by steep, north-striking faults (Kiilsgaard, 

1949). The shattering probably also was related to intrusion of rhyolite porphyry dikes adjacent to the Livingston

Figure J7. Polished slab of jamesonite ore from the 2,200-ft 

level of the Livingston mine, central Idaho. The ore contains 

fine-grained pyrite, sphalerite, jamesonite, and galena 

disseminated along bedding in tightly folded and shattered 

siliceous argillite (a) of the Salmon River assemblage (Paleozoic). 

The folded, mineralized argillite is part of the Livingston vein 

near a rhyolite porphyry dike. The fractures and matrix between 

argillite clasts are filled with pods and lenses of jamesonite (b), 

galena (c), and sphalerite (d) that have been remobilized from 

the mineralized argillite.

130 Challis 1°x2° Quadrangle, Idaho
vein. At the time of intrusion of the dikes, the lead-silver 

minerals were locally remobilized, forming crosscutting 

veinlets and filling or replacing the matrix between the 

shattered argillite fragments (fig. J7). The remobilized 

lead-silver minerals formed some of the highest-grade ore 

bodies at the intersections of the rhyolite dikes and the 

Livingston vein. Brecciation continued after intrusion of 

the rhyolite porphyry dikes, and veinlets of galena cut 

rhyolite. Fluid-inclusion and isotopic evidence for a syngenetic origin of the stratabound ore is given by Howe 

and Hall (chap. P, this volume).

REFERENCES CITED

Anderson, A. L., Kiilsgaard, T. H., and Fryklund, V. C., Jr., 1950, 

Detailed geology of certain areas in the Mineral Hill and 

Warm Springs mining districts, Blaine County, Idaho: Idaho 

Bureau of Mines and Geology Pamphlet 90, 73 p.

Carne, R. C., and Cathro, R. J., 1982, Sedimentary exhalative 

(sedex) zinc-lead-silver deposits, northern Canadian Cordillera: Canadian Institute of Mining Bulletin 75, p. 66-78.

Davis, Steven, 1984, Antler erogenic microstructures in the Devonian Milligen Formation, Pioneer Mountains, central Idaho: 

Geological Society of America Abstracts with Programs 

v. 16, no. 4, p. 220.

Dover, J. H., 1969, Bedrock geology of the Pioneer Mountains, 

Blaine and Custer Counties, central Idaho: Idaho Bureau 

of Mines and Geology Pamphlet 142, 61 p.

_____1981, Geology of the Boulder-Pioneer Wilderness Study 

Area, Blaine and Custer Counties, Idaho, chap. A of U.S. 

Geological Survey and U.S. Bureau of Mines, Mineral 

resources of the Boulder-Pioneer Wilderness Study Area, 

Blaine and Custer Counties, Idaho: U.S. Geological Survey 

Bulletin 1497-A, p. 1-75.

Dover, J. H., Berry, W. B. N., and Ross, R. J., Jr., 1980, Ordovician and Silurian Phi Kappa and Trail Creek Formations, 

Pioneer Mountains, central Idaho Stratigraphic and structural revisions, and new data on graptolite faunas: U.S. Geological Survey Professional Paper 1090, 54 p.

Fisher, F. S., and May, G. D., 1983, Geochemical characteristics 

of the metalliferous Salmon River sequence, central Idaho: 

U.S. Geological Survey Open-File Report 83-670, 30 p.

Fisher, F. S., Mclntvre, D. H., and Johnson, K. M., 1983, Geologic 

map of the Chain's 1°X2° quadrangle, Idaho: U.S. Geological Survey Open-File Report 83-523, 41 p., 2 maps, scale 

1:250,000.

Hall, W. E., Batchelder, J. N., and Douglass, R. C., 1974, 

Stratigraphic section of the Wood River Formation, Blaine 

County, Idaho: U.S. Geological Survey Journal of Research, 

v. 2, no. 1, p. 89-95.

Hall, W. E., and Czamanske, G. K., 1972, Mineralogy and trace 

element content of the Wood River lead-silver district, Blaine 

County, Idaho: Economic Geology, v. 67, no. 3, p. 350-361.

Hall, W. E., Rye, R. O., and Doe, B. R., 1978, Wood River mining district, Idaho intrusion-related lead-silver deposits 

derived from country rock source: U.S. Geological Survey 

Journal of Research, v. 6, no. 5, p. 579-592.

Hall, W. E., Schmidt, E. A., Howe, S. S., and Broch, M. J., 1984, 

The Thompson Creek, Idaho porphyry molybdenum

deposit an example of a fluorine-deficient molybdenum 

granodiorite system: Sixth Quadrennial International 

Association on the Genesis of Ore Deposits, Tbilisi, USSR, 

1982, Proceedings; E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, v. 1, p. 349-358.

Hobbs, S. W, Hays, W. H., and Mclntyre, D. H., 1975, Geologic 

map of the Clayton quadrangle, Custer County, Idaho: US. 

Geological Survey Open-File Report 75-76, 23 p., 1 map, 

scale 1:62,500.

Kiilsgaard, T. H., 1949, The geology and ore deposits of the 

Boulder Creek mining district, Custer County, Idaho: Idaho 

Bureau of Mines and Geology Pamphlet 88, 28 p.

Nilsen, T. H., 1977, Paleogeography of Mississippian turbidites 

in south-central Idaho, in Pacific Coast Paleogeography 

Symposium, 1st, Bakersfield, Calif., 1977, Paleozoic paleogeography of the western United States: Los Angeles, Society 

of Economic Paleontologists and Mineralogists, Pacific Section, p. 275-299.

Paull, R. A., Wolbrink, M. A., Volkmann, R. G., and Grover, 

R. L., 1972, Stratigraphy of the Copper Basin Group, Pioneer Mountains, south-central Idaho: American Association of Petroleum Geologists Bulletin, v. 56, no. 8, 

p. 1370-1401.

Ross, C. P., 1937, Geology and ore deposits of the Bayhorse 

region, Custer County, Idaho: U.S. Geological Survey Bulletin 877, 161 p.

Sandberg, C. A., Hall, W. E., Batchelder, J. N., and Axelsen, 

Claus, 1975, Stratigraphy, conodont dating, and paleotectonic interpretation of the type Milligen Formation (Devonian), Wood River area, Idaho: U.S. Geological Survey Journal of Research, v. 3, no. 6, p. 707-720.

Simons, F. S., 1981, A geological and geochemical evaluation of 

the mineral resources of the Boulder-Pioneer Wilderness 

Study Area, Blaine and Custer Counties, Idaho, chap. C 

of U.S. Geological Survey and U.S. Bureau of Mines, Mineral 

resources of the Boulder-Pioneer Wilderness Study Area, 

Blaine and Custer Counties, Idaho: U.S. Geological Survey 

Bulletin 1497-C, p. 87-180.

Skipp, Betty, and Hall, W. E., 1975, Structure and Paleozoic stratigraphy of a complex of thrust plates in the Fish Creek Reservoir area, south-central Idaho: U.S. Geological Survey Journal of Research, v. 3, no. 6, p. 671-689.

Thomasson, M. R., 1959, Late Paleozoic stratigraphy and 

paleotectonics of central and eastern Idaho: Madison, 

University of Wisconsin Ph. D. thesis, 244 p.

Tschanz, C. M., Kiilsgaard, T. H., Seeland, D. A., Van Noy, R. 

M., Ridenour, J., Zilka, N. T, Federspiel, F. E., Evans, R. 

K., Tuchek, E. T, and McMahan, A. B., 1974, Mineral 

resources of the eastern part of the Sawtooth National 

Recreation Area, Custer and Blaine Counties, Idaho: US. 

Geological Survey Open-file report, 2 v., 648 p.

Umpleby, J. B., 1915, Ore deposits in the Sawtooth quadrangle, 

Blaine and Custer Counties, Idaho: U.S. Geological Survey 

Bulletin 580, p. 221-249.

Umpleby, J. B., Westgate, L. G., and Ross, C. P., 1930, Geology 

and ore deposits of the Wood River region, Idaho: US. Geological Survey Bulletin 814, 250 p.

Vine, J. D., and Tourtelot, E. B., 1970, Geochemistry of black 

shale deposits a summary report: Economic Geology, 

v. 65, no. 3, p. 253-272.

Winsor, H. C., 1981, The paleogeography and paleoenvironments 

of the Middle Pennsylvanian part of the Wood River Formation in south-central Idaho: San Jose, Calif., San Jose 

State University M.S. thesis, 81 p.

Black-Shale Mineral Belt 131

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter K

Structural and Stratigraphic Controls of 

Ore Deposits in the Bayhorse Area, Idaho

By S. WARREN HOBBS

CONTENTS

Abstract 134

Introduction 134

Mineral deposits near the Ramshorn Slate-Bayhorse Dolomite contact 134

Mineral deposits in terrane D 137

Relationship of mineral deposits to intrusive rocks 139

References cited 140

FIGURES

Kl. Map and cross section of structural-stratigraphic terranes 135

K2. Columnar sections of terranes C and D 136

K3. Geologic maps of the Mill Creek-Garden Creek and Bayhorse Creek areas 138

K4. Geologic maps of the Clayton Silver mine and Squaw Creek areas 139

Controls of Ore Deposits in the Bayhorse Area 133
Abstract

Major mineral commodities produced from the Bayhorse 

region include lead, zinc, silver, gold, molybdenum, tungsten, 

and fluorspar. Mineral commodities of unknown potential include tin, copper, vanadium, uranium, and barite.

Three factors dominate the placement of ore deposits in 

this region: (1) the sedimentary sections their depositional 

history, composition, physical characteristics, and general 

chemical composition; (2) the structural evolution of the area 

and the resulting pattern of folds and faults; and (3) the 

associated intrusive rocks that supplied some of the ore-forming 

elements but mainly supplied energy to mobilize hydrothermal fluids throughout the stratigraphic section.

Deposition of most of the fluorite and some of the leadzinc-silver minerals in occurrences that flank the Bayhorse 

anticline was largely controlled by an extensive zone of solution, brecciation, and collapse structures that makes up a 

paleokarst horizon developed beneath an ancient erosion surface cut on Bayhorse Dolomite, which is now capped by the 

impervious Ramshorn Slate. Steep faults that intersect this zone 

were channels for ore solutions from some subjacent source 

into a porous, reactive environment of deposition. Similar steep 

faults that cut other carbonate units at the Clayton Silver mine 

and mines on Squaw Creek also have been channels to replacement ore bodies of considerable size. At the Ramshorn and 

Skylark mines, steep faults in the Ramshorn Slate controlled 

the location of sulfide-bearing quartz-siderite veins that are in 

places controlled by bedding as well.

Intrusive rocks served both as the source of mineralizing 

solutions in contact-metamorphic aureoles and, more significantly, as the source of energy to circulate connate and meteoric 

waters that mobilized sparse syngenetic metals and carried them 

to favorable sites of deposition.

INTRODUCTION

The Bayhorse area in the east-central part of the 

Challis quadrangle (fig. Dl, Hobbs, chap. D, this volume) 

is a small, economically important area in which a variety of mineral deposits and occurrences have been mined 

or are known. The earliest recorded mineral discovery in 

the Bayhorse area was in 1887 at the Riverview mine on 

Bayhorse Creek. Other discoveries followed quickly, and 

by 1900 most of the major deposits in the area had been 

developed. Continued interest in mineral deposits led to 

geological investigations by many workers, from the earlier 

studies of Umpleby (1913), Ross (1937), Anderson (1954), 

and others, to the more recent work of the U.S. Geological Survey (Hays and others, 1978; Hobbs, 1980; Hobbs 

and others, 1975; Mclntyre and Hobbs, 1978; Mclntyre 

and others, 1982), Idaho Bureau of Mines and Geology, 

and many students, and the very detailed and concentrated 

exploration activities of numerous geologists with mining companies.

The major mineral commodities of the Bayhorse 

area and environs include lead, zinc, silver, tungsten, gold, 

molybdenum, and fluorspar. Secondary commodities that

are known either within the Bayhorse area or nearby, and 

that may have potential for future mining development 

include tin, copper, barite, uranium, and vanadium.

Two periods of mineralization, one related to the 

middle Late Cretaceous Idaho batholith and the other to 

the Eocene Challis Volcanics and their coeval granitic 

plutons, were recognized by Ross (1931) during his early 

studies of the Bayhorse area and adjacent areas. More recent studies have identified ore deposits related to other 

periods of mineralizing activity. Accumulating evidence 

indicates that the mineralizing processes and the plumbing systems that controlled the localization and deposition of ore deposits were active for a long time.

Many of the basic concepts of ore genesis and control of ore deposition in central Idaho are presented by 

Hall (chap. J, this volume). This chapter (K) expands these 

concepts, emphasizing the localization of ore deposits in 

the Bayhorse area and describing several modes of occurrence that have been recognized in this mining district that 

could serve as guides for future discovery of mineral 

deposits.

Three factors that seem to control the location of 

ore deposits in central Idaho are:

1. The sequence of sedimentary rocks; its depositional history, composition, physical characteristics, stratabound and syngenetic ore-mineral content, and minorelement chemical composition.

2. The structural features; the pattern of folds and 

faults that resulted from a long history of deformation.

3. The intrusive rocks; the possible source of certain ore elements and the source of energy for the movement of metal-bearing fluids through the country rock.

One or more of these factors has influenced the 

localization of the major ore deposits in the Bayhorse 

area.

MINERAL DEPOSITS NEAR THE RAMSHORN 

SLATE-BAYHORSE DOLOMITE CONTACT

One of the stratigraphic terranes that comprise the 

basic geologic elements of the Bayhorse area (Hobbs, 

chap. D, this volume) includes terrane C, a sequence of 

lower Paleozoic strata that is folded into a major 

north-trending anticline in the central part of the area (fig. 

Kl). This sequence is composed of the Ramshorn Slate 

at the top, underlain by the Bayhorse Dolomite, both of 

Ordovician(?) age, the Cambrian(?) Garden Creek 

Phyllite, and a lower dolomite also of Cambrian(?) age 

(fig. K2A). The stratigraphy and sedimentary history that 

is illustrated by these strata influenced the localization of 

deposits of lead, zinc, and fluorspar in the Bayhorse area. 

Several major mines and numerous prospects are in the 

Bayhorse Dolomite along and near its contact with the 

overlying Ramshorn Slate. This contact is a major erosional disconformity, and an unknown thickness of

134 Challis 1°x2° Quadrangle, Idaho
oo&>

o

EXPLANATION

TERRANE F (MISSISSIPPIAN TO ORDOVICIAN)

TERRANE E(UPPER MISSISSIPPIAN, DEVONIAN, AND UPPER CAMBRIAN)

TERRANE D (LOWER PALEOZOIC) 

TERRANE C (LOWER PALEOZOIC ?)

TERRANE B (ORDOVICIAN AND 

OLDER)

TERRANE A (PROTEROZOIC)

CONTACT OF TERRANE AND

YOUNGER ROCKS 

HIGH-ANGLE FAULT Bar and ball on

downthrown side; arrows show relativemovement 

THRUST FAULT Sawteeth on upper

plate

GENERALIZED CROSS SECTION ALONG LINE A-A'

0 10 MILES

J

0 5 10 KILOMETERS 

Figure K1. Generalized map and cross section showing structural-stratigraphic terranes in the Bayhorse area, and the location of figures K3 and K4»
Disconformity

DC sc->>;>->;->

Garden Creek 

Phyllite (750 ft)

Lower dolomite 

(250± ft)

Porphyry intrusion

Covered

Thrust

Kinnikinic Quartzite 

(700± ft)

Ella Dolomite 

(700± ft)

Clayton Mine Quartzite 

(2000 - 3000 ft)

TERRANE C

LOWER PALEOZOIC 

(Intrusion is probably 

Cretaceous; lower dolomite 

and Garden Creek are 

probably Cambrian; Bayhorse and Ramshorn are 

Ordovician)

TERRANE D

LOWER PALEOZOIC 

(Mostly Ordovician)

QUARTZITE

FELDSPATHIC QUARTZITE 

PEBBLY QUARTZITE 

SLATE AND PHYLLITE

EXPLANATION

SILTSTONE

CALCAREOUS SILTSTONE 

LIMESTONE 

SILTY LIMESTONE

DOLOMITE 

SILTY DOLOMITE 

CHERTY DOLOMITE

Figure K2. Columnar sections of terranes C and D, Bayhorse area.

Bayhorse Dolomite was eroded before the Ramshorn Slate 

was deposited. The wide thickness range of the Bayhorse 

Dolomite (500-1,500 ft (feet)) reflects this erosional event, 

as does the drastic change in depositional regime in Ramshorn time that produced the Ramshorn siltstone, sandstone, and locally a very thick basal conglomerate.

Beneath this contact, the Bayhorse Dolomite shows 

very striking evidence of this disconformity and prolonged 

exposure to extensive weathering. Different parts of the 

dolomite are present below the Ramshorn at different 

places; nearly everywhere the carbonate strata show deep 

weathering that consists of carbonate dissolution and 

local accumulation of residual sand and clay, brecciated 

and silicified carbonate, and local cavern filling of 

transported material. At least one breccia pipe, possibly

a solution-collapse feature, has been recognized, but its 

origin is as yet unproved. Part of this disturbed zone at 

the contact is most certainly related to the Ordovician erosion that produced the disconformity and to the development of what is now a paleokarst horizon; the zone 

appears to have been modified and probably enlarged by 

deformation during folding.

The geologic evolution of terrane C as described 

above has produced an extensive stratigraphically and 

structurally controlled permeable zone that is capped by 

a thick, impermeable slate sequence (Ramshorn Slate). 

The zone was a channel for subsequent circulating 

hydrothermal waters that probably modified the channel 

by solution and further brecciation. These or subsequent 

hydrothermal fluids introduced the ore-forming elements

136 Challis 1°x2° Quadrangle, Idaho
that were precipitated to form the ore deposits in or near 

this permeable horizon. Generally north-trending, steeply 

dipping faults and fractures have served as channels that 

fed mineralizing solutions into the brecciated paleokarst 

zone. In some places these same fractures and solutions 

have localized vein-filling or replacement ore deposits of 

several types at other stratigraphic horizons.

Many mines and prospects, mostly for fluorspar but 

some for lead and silver, are localized along or near the 

Bayhorse Dolomite-Ramshorn Slate contact in the northern part of the Bayhorse anticline (fig. K3A). The fluorspar prospect on Mill Creek (fig. K3A) on the west limb 

of the Bayhorse anticline at its north end is in weathered 

and brecciated zones of the Bayhorse Dolomite at or very 

near the contact with overlying Ramshorn Slate. At this 

place, the disconformity between these two units has been 

cut into the dolomite to the level of one of several pisolite 

horizons that have become, in part, host rocks for the 

sparse fluorspar deposits that have so far been discovered 

here. On the east limb of the anticline about 2 mi (miles) 

to the southeast, the Chalspar No. 1 vein is in a steeply 

dipping fault zone in Bayhorse Dolomite whose relation 

to the disconformity is unknown. This fault and its vein 

material may have been a channel that fed ore-bearing 

fluids into parts of the paleokarst horizon that have been 

removed by erosion here. The Chalspar No. 2 and No. 5 

prospects on the west limb of the anticline are in brecciated zones related to both stratabound breccias and to 

steeply dipping shear and fault zones that parallel the major fold axis and that offset the contact between the 

Bayhorse Dolomite and the Ramshorn Slate. Although 

the geologic relations are not completely clear, the combined effects of both of these ore controls certainly contributed to the concentration of fluorspar at these two 

locations. At the Westspar and Troxspar localities, fluorspar and some minor sulfide minerals are in brecciated 

dolomite close below the basal conglomerate of the Ramshorn Slate on opposite sides of the Bayhorse anticline. 

As with the other deposits, the brecciated and mineralized 

rock along or near the disconformity strongly suggests 

a genetic relationship. At the Keystone Mountain property 

on the gently south-plunging nose of the Bayhorse anticline, the disconformity is alternately exposed and 

covered for more than 6,000 ft across the axis of the fold 

and well down both flanks. Brecciation, silicification, and 

fluorspar occur along most of this distance either at the 

contact or in a horizon several hundred feet below the contact. Some sulfide minerals on the lower part of the east 

limb are probably related to the same structural setting.

The main Bayhorse mining district (fig. K3/?) is on 

the Bayhorse anticline just south of the area of figure 

K3A. The district has the same stratigraphic and structural setting that the northern district has. The Pacific 

mine ore body is near the crest of the anticline, within 

a highly brecciated zone in the Bayhorse Dolomite, and 

below the contact of the dolomite with a small patch of

nearly flat lying Ramshorn Slate. This major deposit contains both base metals and fluorspar that were probably 

deposited in two separate metallogenic episodes (Snyder, 

1978). Several hundred feet of essentially undisturbed 

dolomite separates the main ore bodies of the Pacific mine 

from the Ramshorn contact, and the dolomite adjacent 

to the contact is less mineralized and brecciated than at 

the mine. The development of breccia at two levels within 

the dolomite below the Ramshorn contact may have 

resulted from a complex event involving the disconformity, 

stratigraphy, structure, and compositional differences of 

the carbonate strata, all of which could have controlled 

the rate of solution and brecciation.

The Beardsley and Riverview deposits are on the east 

flank of the Bayhorse anticline where the dolomite dips 

from gently eastward to vertical, but both are near the disconformity and undoubtedly were controlled by the character of the weathered horizon. Several small occurrences of 

base metals to the south are in comparable structural settings (fig. K3/?). One of these, the Turtle mine, is barely 

exposed in a very small erosional window of Bayhorse 

Dolomite within an extensive area of the overlying Ramshorn Slate. All of the mines and prospects described above 

and shown on figure K3 are very close to the disconformity, which was probably critical to their origin.

Mineralizing solutions that followed the near-bedding-plane brecciated and porous zones related to the disconformity were undoubtedly channeled along the steeply 

dipping, north-trending normal faults that postdate thrusting and folding. Examples of these faults are the major 

steep faults that bracket the deposits at Keystone Mountain and the Bayhorse district (fig. K3). The Beardsley mine 

in the Bayhorse district (fig. K3/?) reportedly includes 

replacement deposits in the dolomite (below the breccia 

zone described above) that could be related to the adjacent steep fault that is, itself, mineralized in places.

The control of ore deposition by the north-trending 

anticline and faults is more obvious at the well-known silverbearing veins in Ramshorn Slate at the Ramshorn and 

Skylark mines on upper Bayhorse Creek (fig. K3.fi). These 

veins, consisting of galena and tetrahedrite in a quartzsiderite-crushed slate gangue, are in complex north-trending shear and fault zones that are controlled in part by the 

west-dipping slaty cleavage of the Ramshorn Slate. Relation of these veins to the small Juliette stock to the south 

is suggested by a potassium-argon age of the stock of 98.1 

m.y. (million years) and of sericite from a selvage along 

the Skylark veins of 95.1 m.y. (Mclntyre and others, 1976; 

recalculated using currently accepted constants).

MINERAL DEPOSITS IN TERRANE D

Terrane D (Hobbs, chap. D, this volume) includes 

a stratigraphic sequence consisting of two quartzite units 

and two carbonate units (fig. K2.6). These units are best

Controls of Ore Deposits in the Bayhorse Area 137
B. BAYHORSE CREEK AREA

2 MILES

2 KILOMETERS

A MILL CREEK-GARDEN CREEK AREA

EXPLANATION

BIOTITE GRANODIORITE (CRETACEOUS) Juliette stock 

[ CLAYTON MINE QUARTZITE (ORDOVICIAN OR OLDER) 

RAMSHORN SLATE (ORDOVICIAN?) 

BAYHORSE DOLOMITE (ORDOVICIAN?) 

GARDEN CREEK PHYLLITE (CAMBRIAN?)

CONTACT

STEEP FAULT Dashed where approximately located;

bar and ball on downthrown side 

 * THRUST FAULT Sawteeth on upper plate 

| ANTICLINE

VEINS

MINE OR PROSPECT

Figure K3. Geologic maps of the Mill Creek-Garden Creek and Bayhorse Creek areas. Location of maps shown on figure K1.

exposed in the western half of the Bayhorse area and especially in the valleys of Squaw and Kinnikinic Creeks. As 

in terrane C, the major stratigraphic control of ore 

deposits is the reactive carbonate units, which, in this area, 

are the Ordovician Ella Dolomite and the Ordovician 

Saturday Mountain Formation. The major structural controls are both earlier low-angle thrust faults and later 

steeply dipping normal faults. The sequence in terrane D

contains no notable disconformities such as the one that 

controls ore deposition in terrane C.

At the Clayton Silver mine on Kinnikinic Creek (fig. 

K44), a relatively small block of Ordovician Ella Dolomite, bounded by two steep normal faults and capped 

by a flat thrust fault, contains a replacement ore body 

of lead, zinc, and silver that has been mined for many 

years. The steep boundary faults were the channels for

138 Challis 1°x2° Quadrangle, Idaho
hydrothermal solutions, and the structurally disturbed and 

reactive dolomite was an ideal location for deposition of 

ore. Although the effect of the overlying thrust plate cannot be documented, it probably had some control over 

the containment of the hydrothermal solutions.

The Red Bird and Dryden mines (fig. K4B) are at 

opposite ends of a major fault or series of faults that 

parallel the general course of Squaw Creek. Both mines 

have replacement ore bodies in carbonates of the Saturday Mountain Formation. Analyses of soil samples taken 

along traverses perpendicular to the fault between the Red 

Bird and Dryden deposits showed anomalous amounts of 

lead and zinc, indicating that the fault was a channel for 

mineralizing solutions along much of its length. The 

South Butte property is along a side fault that splits from 

this major fault zone and cuts carbonate beds that could 

be either Saturday Mountain Formation or Ella Dolomite. 

Several other deposits on the west side of Squaw Creek 

are along or near other steeply dipping faults. An intriguing common factor of all these deposits in the western 

part of this area is their proximity to an overlying thrust

plate or to the projection of such a plate. These relations 

could well have important genetic implications, as proposed by Hall (chap. J, this volume), who has documented 

in areas to the south and west of the Bayhorse area that 

the intersection of steeply dipping normal faults and 

younger low-angle thrust faults provides an ideal environment for the formation of ore deposits.

RELATIONSHIP OF MINERAL DEPOSITS 

TO INTRUSIVE ROCKS

Intrusive rocks of several ages were undoubtedly involved directly or indirectly in the origin of many of the 

ore deposits. The contact of the main body of the midCretaceous Idaho batholith is a few miles to the west of 

the Bayhorse area (fig. Dl), and minor offshoots and 

outliers occur within the Bayhorse area. Small Tertiary 

intrusive bodies related to the Challis Volcanics are scattered through the area and may underlie some parts of 

it at shallow depth. The control that these plutonic rocks

A CLAYTON MINE AREA 

EXPLANATION

SALMON RIVER ASSEMBLAGE (PALEOZOIC) 

SATURDAY MOUNTAIN FORMATION (ORDOVICIAN) 

ELLA DOLOMITE (ORDOVICIAN)

CLAYTON MINE QUARTZITE (ORDOVICIAN OR OLDER) 

I RAMSHORN SLATE (ORDOVICIAN?)

 CONTACT

 j STEEP FAULT Bar and ball on downthrown side

 * * *- THRUST FAULT Sawteeth on upper plate 

K MINE OR PROSPECT

B. SQUAW CREEK AREA

2 MILES

J

2 KILOMETERS

Figure K4. Geologic maps of the Clayton Silver mine and Squaw Creek areas. Locations of maps shown on figure K1.

Controls of Ore Deposits in the Bayhorse Area 139
have on ore deposits relates in large part to the stratigraphy 

and structure with which they react or interact.

The direct contact of the batholith with carbonate 

rocks in the Lower Permian Grand Prize Formation, 

which is slightly west of the Bayhorse area, resulted in 

one productive scheelite-bearing skarn deposit and several 

prospects. These deposits and many others are discussed 

by Cookro (chap. Q, this volume). These same plutonic 

rocks elsewhere possibly supplied magmatic fluids and 

gases that followed structures of various kinds and transported metals into favorable sites of deposition. Furthermore, it is becoming increasingly apparent (Hall, chap. 

J, this volume) that igneous rocks provided the energy to 

activate connate and meteoric waters that permeate crustal 

rocks, leach the entrapped metals, and carry these elements to sites of deposition. Such sedimentary source 

rocks, good examples of which are in the Challis quadrangle, could well be the ultimate stratigraphic control on 

the formation of ore deposits.

REFERENCES CITED

Anderson, A. L., 1954, A preliminary report on the fluorspar 

mineralization near Challis, Custer County, Idaho: Idaho 

Bureau of Mines and Geology Pamphlet 101, 12 p.

Hays, W. H., Mclntyre, D. H., and Hobbs, S. W., 1978, Geologic 

map of the Lone Pine Peak quadrangle, Custer County, 

Idaho: U.S. Geological Survey Open-File Report 78-1060, 

scale 1:62,500.

Hobbs, S. W., 1980, The Lawson Creek Formation of Middle 

Proterozoic age in east-central Idaho: U.S. Geological 

Survey Bulletin 1482-E, p. 1-12.

Hobbs, S. W., Hays, W. H., and Mclntyre, D. H., 1975, Geologic 

map of the Clayton quadrangle, Custer County, Idaho: U.S. 

Geological Survey Open-File Report 75-76, 23 p., 1 map, 

scale 1:62,500.

Mclntyre, D. H., Hobbs, S. W., Marvin, R. F., and Mehnert, 

H. H., 1976, Late Cretaceous and Eocene ages for 

hydrothermal alteration and mineralization, Bayhorse 

district and vicinity, Custer County, Idaho: Isochron/West, 

no. 16, p. 11-12.

Mclntyre, D. H., Ekren, E. B., and Hardyman, R. F., 1982, 

Stratigraphic and structural framework of the Challis 

Volcanics in the eastern half of the Challis 1°X2° quadrangle, i/i Bill Bonnichsen and R. M. Breckenridge, eds., 

Cenozoic geology of Idaho: Idaho Bureau of Mines and 

Geology Bulletin 26, p. 3-22.

Mclntyre D. H., and Hobbs, S. W., 1978, Geologic map of the 

Challis quadrangle, Custer County, Idaho: U.S. Geological Survey Open-File Report 78-1059, scale 1:62,500.

Ross, C. P., 1931, A classification of the lode deposits of southcentral Idaho: Economic Geology, v. 26, no. 2, p. 169-185.

_____1937, Geology and ore deposits of the Bayhorse region, 

Custer County, Idaho: U.S. Geological Survey Bulletin 877, 

161 p.

Snyder, K. D., 1978, Geology of the Bayhorse fluorite deposit, 

Custer County, Idaho: Economic Geology, v. 73, no. 2, 

p. 207-214.

Umpleby, J. B., 1913, Some ore deposits in northwestern Custer 

County, Idaho: U.S. Geological Survey Bulletin 539,104 p.

140 Challis 1°x2° Quadrangle, Idaho
Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter L

A Case for Plants in Exploration Gold in 

Douglas-Fir at the Red Mountain Stockwork, 

Yellow Pine District, Idaho

By J. A. ERDMAN, B. F. LEONARD, and D. M. McKOWN

CONTENTS

Abstract 142

Introduction 142

Geologic setting 142

Vegetation and soils 143

Field methods 145

Analytical methods 146

Results of analyses 146 

Gold distribution 146 

Molybdenum distribution 148 

Tungsten distribution 148

Geomagnetic anomalies 149

Conclusions 152

References cited 152

FIGURES

LI. Index map of Red Mountain and vicinity. 142

L2. Generalized structure map of the Thunder Mountain caldera complex. 143 

L3. Generalized geologic map of the Red Mountain stockwork area. 144 

L4. Map showing sample localities and concentrations of gold in the ash of

douglas-fir wood. 147

L5. Histogram of gold in the ash of douglas-fir wood. 148 

L6. Map showing sample localities and concentrations of molybdenum in the ash

of beargrass and elk sedge. 149 

L7. Map showing sample localities and concentrations of tungsten in the ash of

beargrass. 150 

L8. Map showing traces of geomagnetic anomalies. 151

Gold in Douglas-Fir, Red Mountain Stockwork 141
Abstract

Exploration targets for concealed deposits of gold, 

molybdenum, and tungsten exist outside the Red Mountain 

stockwork, based on evidence from this study. The targets are 

new and, to a considerable degree, unexpected from other surface evidence. The biogeochemical anomalies that help define 

the targets are extensive, and the deposits that might be sought 

are presumably of low grade.

Red Mountain, which lies on the western edge of the ringfracture zone of the Eocene Quartz Creek cauldron (Leonard 

and Marvin, 1982), has been prospected for gold and silver for 

at least 50 years. A biogeochemical study was conducted in 

1980-81 in an attempt to better assess the mineral potential 

of the stockwork area. Bedrock contacts are concealed by colluvium, glacial deposits, and forest cover. Soil and plant samples 

were collected on 200-yd (yard) centers of a sampling grid over 

an area of 3,600 ft (feet) by 8,400 ft. The wood of douglas-fir 

(Pseudotsuga menziesii) and the leaves of beargrass (Xerophyllum 

tenax) were used because they concentrate gold and 

molybdenum, respectively. Results of the soil sampling were 

insignificant, although they did indicate a tungsten anomaly 

south of the stockwork. Analysis of ashed wood by instrumental neutron activation yielded gold values of 0.07-14.2 ppm 

(parts per million) and revealed two distinct gold populations. 

More important, the highly anomalous samples (more than 4 

ppm) are concentrated in the southern quarter of the sample 

grid in an area that has no anomalous gold in the sampled soils, 

has not been prospected for gold, and lies within inclusionbearing granodiorite, not stockwork. Beargrass samples, which 

typically contain 20 ppm molybdenum, contained less than 5 

to more than 500 ppm. A belt of localities of samples containing above-median values of molybdenum transects some part 

of every geologic unit except the quartz body at the summit 

of Red Mountain. The great extent and the continuity of this 

belt require some comparably extensive bedrock source of the 

molybdenum. The location, shape, and molybdenum content 

of the bedrock source remain conjectural, but the source must 

be large. Subsequent geomagnetic traverses confirmed the belt 

configuration.

INTRODUCTION

Biogeochemical studies were conducted in 1980-81 

in an attempt to better assess the mineral resource potential of the Red Mountain stockwork area. The Red Mountain stockwork is 3.5 mi (miles) north-northeast of Yellow 

Pine, a small settlement near the northwestern corner of 

the Challis quadrangle. Red Mountain, so called from the 

reddish-brown color of its soil, is an informal name in 

local use for the land mass between Quartz Ridge and the 

forks of Quartz and Vein Creeks (fig. LI). The stockwork 

has been prospected for gold and silver for at least half 

a century with little success. Recently, however, the area 

has received renewed interest by several claim holders, in 

large measure owing to our geochemical studies.

Acknowledgments. ]. G. Brophy, William Lynch, 

and D. K. Swanson served as field assistants in 1979-81. 

Spectrographers L. A. Bradley, E. F. Cooley, and M. J.

115°30'

5 KILOMETERS

Figure L1. Index map of Red Mountain and vicinity, Yellow 

Pine mining district. From U.S. Geological Survey, Challis and 

Elk City quadrangles, 1:250,000.

Malcolm, chemist J. G. Crock and his associates, and 

plant analysts Thelma Harms and Clara Papp provided 

the analytical data essential to this report. Mary Lou 

Tompkins processed the analytical data for computer 

treatment. A. T. Miesch introduced us to the Gapmap program used for part of the statistical analysis. All the 

workers listed above are or were members of the U.S. Geological Survey. C. W. Erdman and Eleanor Leonard helped 

with surveying and sample collecting.

GEOLOGIC SETTING

The Red Mountain stockwork is a geologic anomaly 

in the region for the following reasons: (1) it is a fine-scale 

stockwork, instead of a system of subparallel quartz veins 

with sporadic apophyses; (2) it is cut by small radial dikes, 

rather than by a few large dikes or by dike swarms; (3) 

it is locally dissected by radial faults; (4) it contains 

molybdenite, rarely seen in the Yellow Pine gold-mining 

district (Koschmann and Bergendahl, 1968), as well as 

scheelite and stibnite, which are common ore minerals in 

the district; (5) it contains sparsely disseminated pyrrhotite 

in addition to ubiquitous pyrite and arsenopyrite; and (6) 

its envelope of clay-mineral alteration is by far the largest

142 Challis 1°x2° Quadrangle, Idaho
known in the region. Because the stockwork is geologically anomalous, it is the kind of feature that exploration geologists are always looking for and hoping to find 

ore bearing.

The stockwork is near the western margin of the 

Quartz Creek cauldron (fig. L2), the very large Eocene 

subsidence feature that contains the Cougar Basin and 

Thunder Mountain calderas of Challis Volcanics (Leonard 

and Marvin, 1982). At the latitude of Red Mountain, 

north-striking ring fractures and attendant silicified zones 

of the cauldron appear to be offset along an ill-defined, 

east-trending zone that is partly occupied by the Red 

Mountain stockwork. The Johnson Creek-Quartz Creek 

silicified zone cannot be traced farther northward up 

Quartz Creek, and the Profile-Smith Creek silicified zone, 

passing through Profile Summit (Profile Gap), cannot be 

traced farther southward down Quartz Ridge. During subsidence of the Quartz Creek cauldron, it is reasonable to 

suppose that the mechanical energy ordinarily released 

along radial or other faults linking discontinuous ring 

fractures may have accumulated at the site now occupied 

by the stockwork. The Red Mountain block, severely 

strained, yielded partly by internal deformation and partly 

by intense crackling. Later, the silica-bearing solutions that 

produced the quartz veins and lodes of the great ringfracture-controlled silicified zones penetrated the crackled 

ground of the Red Mountain block and formed the 

stockwork.

Because contacts of the rock units at Red Mountain 

are concealed at the surface by colluvium or locally by 

other Quaternary deposits, Leonard mapped the contacts 

by float. Details of the geology and ore minerals observed 

at Red Mountain are given in Leonard and Erdman (1983). 

For this report the geology (fig. L3) has been simplified 

from unpublished mapping of Leonard as follows:

1. Undifferentiated Quaternary deposits (unit Qs, 

fig. L3).

2. A virtually barren quartz body on the summit 

of Red Mountain, the Summit quartz body (unit Tsq, 

fig. L3).

3. A quartz stockwork surrounding the quartz body 

and underlying the upper slopes of the mountain (unit 

Tst, fig. L3). This stockwork contains quartz veins and 

veinlets, commonly intersecting, and replacement aggregates of quartz. It locally contains relics of granite.

4. A small, isolated part of a silicified zone, within 

granite, exposed low on the northwestern flank of Red 

Mountain (unit Tsz, fig. L3). The north end of the Quartz 

Creek silicified zone, here consisting of minor quartz 

veins, veinlets, and replacement patches in granite or in 

the mixed-rock unit, is poorly exposed southwest of Red 

Mountain.

5. A Cretaceous granite surrounding most of the 

stockwork and underlying the lower slopes (unit Kig, fig. 

L3). This unit is variably deformed, locally argillized, and 

sporadically replaced by muscovite and quartz.

115°

Boundary of -T" 

\ Quartz Creek I

cauldron

/(queried where ' 

J assumed) /

Boundary of 

Cougar Basin 

v caldera

20 MILES

I 

0 10 20 KILOMETERS

Figure L2. Generalized structure map of the Thunder Mountain caldera complex, Challis quadrangle. Heavy lines, faults. 

From Leonard and Marvin (1982).

6. A biotite-muscovite granodiorite of the Idaho 

batholith exposed at the northern edge of the area (unit 

Kigd, fig. L3). The unit is similar to the regional 

batholithic rock type; it is porphyritic to subporphyritic 

and is generally little altered.

7. A mixed-rock unit at the southeastern corner of 

the study area (unit Kim, fig. L3). It is a mass of granodiorite and alaskite of the Idaho batholith suite, here containing sparse inclusions of quartzite, biotite schist, and 

amphibolite of the Middle Proterozoic Yellowjacket 

Formation.

Taken as a whole, the following features suggest that 

a Tertiary intrusion is concealed beneath Red Mountain: 

(1) the presence of radial dikes and faults; (2) the presence 

and gross shape of map units Tsq, Tst, and Kig (fig. L3); 

(3) the drainage pattern; (4) the zonal distribution of clay 

minerals in soil; (5) the subelliptical shape of the molybdenum anomaly in beargrass, described below; and (6) 

the crudely oval pattern of magnetic anomalies. The gold 

anomaly, described below, is in the area underlain, for 

the most part, by the inclusion-bearing granodiorite (unit 

Kim, fig. L3), not the stockwork.

VEGETATION AND SOILS

The study area is characterized by fairly rugged terrain that ranges from 6,000 ft at Quartz Creek on the

Cold in Douglas-Fir, Red Mountain Stockwork 143
115°27'3D'

EXPLANATION

QUATERNARY 

SURFICIAL DEPOSITS, 

UNDIVIDED

TERTIARY 

SUMMIT QUARTZ BODY

QUARTZ STOCKWORK

SILICIFIEDZONE 

CRETACEOUS 

GRANITE

GRANODIORITE 

MIXED-ROCK UNIT

CONTACT

FAULT Dashed where

inferred 

ADIT

05 1 KILOMETER 

CONTOUR INTERVAL 400 FEET

Figure L3. Generalized geologic map of the Red Mountain stockwork area. Base from U.S. Geological Survey, 1:24,000, Yellow 

Pine and Profile Gap quadrangles.

western edge to about 8,000 ft at Quartz Ridge on the east 

(fig. LI).

The forest zone is mainly upper montane and is 

dominated by douglas-fir (Pseudotsuga menziesii [Mirbel]

Franco), although lodgepole pine (Pinus contorta Dougl.) 

and limber pine (P. flexilis James) are locally common. 

At higher elevations and on the slopes with northern exposures, elements of the subalpine forest, engelmann

144 Challis 1-2 Quadrangle, Idaho
spruce (Picea engelmannii Parry) and subalpine fir (Abies 

lasiocarpa [Hook.] Nutt.), either partly or totally replace 

the douglas-fir. Douglas-fir is also absent along many of 

the streams; these areas are ideal habitats for blue spruce 

(Piceapungens Engelm.). The proximity of the sampling 

grid to the riparian and subalpine zones, therefore, leaves 

several gaps in the plant-sampling design, described 

below.

The forest understory consists of many species of 

small trees or shrubs, such as aspen (Populus tremuloides 

Michx.), alder (Alnus sp.), chokecherry (Prunus virginiana 

L.), and serviceberry (Amelanchier alnifolia Nutt.). Common ground-cover species are beargrass (Xerophyllum 

tenax [Purshl Nutt.), grouseberry (Vaccinium scoparium 

Leiberg), and elk sedge (Carex geyeri Boott), plus a fairly rich assemblage of other herbaceous plants.

The soils of the Red Mountain area are mainly 

residual or very nearly so. Most of the clay minerals in 

soil developed on bedrock reflect the hypogene alteration 

of the parent bedrock. The clay minerals in soil of the 

study area (Leonard and Erdman, 1983) are distinctively 

zoned. The sericite and kaolinite zones are crudely central to the stockwork, and the montmorillonite and 

chlorite zones are peripheral.

FIELD METHODS

A gridded sampling design with centers 200 yds 

apart was used in collecting soil and vegetation samples 

from the stockwork area in 1980. Based on the initial 

analytical results, the sample grid was extended in 1981 

to the south and east, for a total of 124 localities. Even 

the extended grid did not enclose the area of anomalous 

samples.

A soil sample, weighing 0.5-1 kg (kilogram), was 

taken at each grid intercept at a depth of 4-8 in. (inches) 

after the site was scraped free of forest litter or plant 

growth. The sample was screened to minus 10 mesh, 

mixed, and split into two parts, one for chemical analysis 

and the other for clay studies. Organic debris that passed 

the stainless-steel screen was later floated off with water 

in the laboratory.

Douglas-fir was selected as the main sampling 

medium for the biogeochemical survey for two reasons. 

First, although it does not grow everywhere in the study 

area, it is more common than the several other species 

of conifer that occur there. And second, ". . . wherever 

douglas-fir occurs it affords an excellent biogeochemical 

tool" (Warren, 1980, p. 361). Warren and Delavault (1950) 

were the first to report a gold content in douglas-fir, 0.65 

ppm, in the ash of "fresh new growth" from a mineralized area in British Columbia.

Results from a biogeochemical survey in 1979 of 

many mineral prospects in the Challis quadrangle (Erdman, unpub. data) pointed to douglas-fir as a likely

concentrator of gold, specifically its wood tissue. In a 

study of the Empire gold district in Colorado, Curtin and 

others (1968) concluded that gold is most concentrated 

in tree roots. However, in our judgment, wood from the 

tree trunk is more easily sampled than is that from the 

roots. Our results do not agree with those of Khotamov 

and others (1966), who found gold most concentrated in 

the leaves of plants.

We selected a medium-size douglas-fir tree (about 

18 inches in trunk diameter) that was closest to each sample point at the grid intercept. Some trees were not close 

to the intercept and soil-sample pit, but all of the sampled 

trees were within 100 ft of the grid point. In some places, 

the only tree available for sampling was either somewhat 

smaller or, more commonly, considerably larger than 

moderate size.

About 50 g (grams) of wood were extracted from 

the trunk using a brace and bit. This amount filled a small 

cloth sample bag and provided sufficient ash for analyses 

by emission spectroscopy and neutron activation. The ash 

yield of wood is 0.1-0.3 percent, so most samples yielded, 

at minimum, 50 mg (milligrams) of ash.

Two types of wood can be clearly recognized when 

coring, a commonly thin zone of almost white sapwood 

and an interior zone of red heartwood. We sampled the 

sapwood, each sample requiring about six borings, on the 

average.

Analytical precision was estimated from analyses of 

duplicate samples taken from nine trees scattered throughout the 1980 study area. Separate bags were filled from 

alternate sapwood borings. We found the reproducibility 

of analytical results to be satisfactory for the purposes 

of this study.

In addition to sampling douglas-fir mainly for its 

gold content, we also sampled two kinds of herbaceous 

plants, beargrass and elk sedge, because of their ability 

to concentrate molybdenum. Molybdenum is often a 

pathfinder element in gold occurrences and is of special 

interest at Red Mountain because molybdenite occurs 

there. Beargrass, a member of the lily family, is a large, 

clump-forming herb common to the area; samples of the 

leaves were collected from 96 of the grid-sample locations. 

The samples were taken from three to five mature, healthy 

plants within 100 ft of the soil-sample pit and grid intercept, and the samples of leaves from the individual 

plants were combined into one sample weighing several 

hundred grams. Eighteen samples of elk sedge, each sample a composite of leaves of three or more plants near the 

soil-sample pit, were collected along the three southernmost traverse lines where beargrass was absent (14 locations) or where beargrass and elk sedge were collectible 

as paired samples (4 locations). Based on the small 

number of paired samples from a restricted area, elk sedge 

may be the better concentrator of molybdenum, but 

beargrass is more quickly recognized, identified, and 

collected.

Gold in Douglas-Fir, Red Mountain Stockwork 145
ANALYTICAL METHODS

The minus-10-mesh fraction of the soil samples was 

pulverized, then analyzed for 40 elements by semiquantitative emission spectroscopy and for gold, mercury, 

tungsten, arsenic, and antimony by special methods: 

atomic absorption for gold and mercury, inductively 

coupled plasma for tungsten, and hydride generationatomic absorption for arsenic and antimony.

The plant samples were first oven dried at 40 °C 

(Celsius), then pulverized to pass a 1.3-mm (millimeter) 

sieve in a Wiley1 mill, and finally ashed in a muffle furnace having controls that permitted a slow increase in 

temperature to a maximum of 500 °C. The heating and 

cooling cycle was 24 hours. Small aliquots of ash (about 

10 mg) were submitted for analysis of 29 elements by semiquantitative emission spectroscopy.

Gold determinations were made on samples of about 

30-40 mg by neutron-activation analysis, a technique considered to be ideal for samples of small mass and containing low element concentrations (Warren, 1980; Brooks 

and others, 1981; Hoffman and Brooker, 1982). Instrumental neutron-activation analysis provides a relatively 

convenient, precise measurement of low-level gold in ashed 

plant samples. In the analysis of the sapwood of douglasfir, the method yielded a detection limit of about 0.008 

ppm gold in the ash, which corresponds to 0.05 ppb (parts 

per billion) in the dried raw wood. Based on replicate 

measurement of a single sample, the precision was within 

10 percent at the 0.1-ppm level. Briefly, the technique is:

Aliquots of 30-40 mg of wood ash were weighed into precleaned 2/5-dram polyvial irradiation containers. 

Elemental standards were prepared by weighing similar 

volume aliquots of U.S. Geological Survey gold-quartz 

standard GQS-1 (Millard and others, 1969) and by evaporating aliquots of a multielement standard solution containing gold, arsenic, and antimony onto Specpure SiO2 . 

Samples and standards were simultaneously irradiated in 

the U.S. Geological Survey Triga reactor (at Denver) for 

8 hours at 5X1012 neutrons per square centimeter per 

second. Irradiated sample vials were cleaned externally with 

water and loaded into appropriate automatic samplechanger vials (2-dram polyvials) for counting. Data were 

acquired automatically from a system consisting of multiple germanium (lithium) detectors with sample changers 

coupled to a Nuclear Data ND-6620 computer-based 

multichannel analyzer. Samples and standards were successively counted for 20 minutes each after a decay period 

of 4-6 days, and a direct comparison of sample and standard peak area was made after appropriate normalization 

for differences in decay time and dead time.

Tungsten was determined colorimetrically (Quin and 

Brooks, 1972) in samples of ashed beargrass leaves. The

!Trade names used in this chapter are for descriptive purposes only 

and do not imply endorsement by the U.S. Geological Survey.

molybdenum content determined by semiquantitative emission spectroscopy was confirmed colorimetrically on a 

representative suite of these samples.

RESULTS OF ANALYSES

Metal anomalies in soils sampled from the study area 

are mostly subtle and not extensive, although the samples 

did indicate a tungsten anomaly south of the stockwork. 

The only gold anomaly, 1.1 ppm, was in a sample from 

a site on the stockwork. Gold concentrations in most soil 

samples were less than 0.05 ppm. Specifics of the soil results 

are given in Leonard and Erdman (1983, p. 21-29).

In contrast to the metals in soil, the metals in plant 

ash showed anomalies so strong and associated areas of 

above-median values so large that Red Mountain would 

be an attractive area for exploration even if the local 

geology were unknown. Few of the metal anomalies in 

plants are in samples from the stockwork. Rather, they are 

from geologically puzzling terrane the valley fill of Quartz 

Creek and the inclusion-bearing granodiorite. The anomalies are consistent with the inference of a concealed elliptical feature that may be hoodlike and may contain more 

than one mineralized zone.

Gold distribution

Gold concentrations in the ash of douglas-fir sapwood ranged from 0.07 to 14.2 ppm, more than two orders 

of magnitude. More important, the highly anomalous 

samples (containing more than 4 ppm) were concentrated 

in the southern part of the sampling grid (fig. L4) where 

no anomalous gold was found in the soils. This area has 

not been prospected for gold and is underlain by inclusionbearing granodiorite, not stockwork.

The mean of the gold values is 0.85 ppm, and the 

standard deviation is 1.72. Because the frequency distribution is strongly skewed (skewness, 5.2), we plotted the 

frequency distribution on a log scale and transformed the 

data to logarithms to compute the geometric means (fig. 

L5). The histogram (fig. L5) shows a bimodal distribution, 

which suggests the presence of two populations. This possibility was tested by using a statistic recently proposed by 

Miesch (1981). The statistic is a standardized gap which, 

if significant, can be taken as the separation between two 

geochemical populations. The results indicate a significant 

gap (fig. L5) whose probability of occurrence if no anomaly were present is 0.086 (confidence level, 91.4 percent). The 

gold values for 53 of the 114 samples are to the right of 

the gap (fig. L5), whose center is at 0.35 ppm. Therefore, 

two populations of approximately equal size are indicated, 

the upper one being anomalous.

Another measure of the remarkably high concentrations of gold in the anomalous sample population at

146 Challis 1°x2° Quadrangle, Idaho
115°27'30'

^l^Sfem

EXPLANATION

QUATERNARY 

SURFICIAL DEPOSITS, 

UNDIVIDED

TERTIARY 

SUMMIT QUARTZ BODY

QUARTZ STOCKWORK

SILICIFIEDZONE 

CRETACEOUS 

GRANITE

GRANODIORITE 

MIXED-ROCK UNIT

CONTACT

FAULT Dashed where 

inferred

SAMPLE GRID BOUNDARY

ADIT

LOCALITY OF SAMPLE CONTAINING

GOLD 

More than 4 ppm

1 - 4 ppm 

0.4 - 1 ppm 

Less than 0.4 ppm 

No data

1 KILOMETER

CONTOUR INTERVAL 400 FEET

Figure L4. Map showing sample localities and concentrations of gold in the ash of douglas-fir wood, Red Mountain area. The 

threshold between background and anomalous concentrations is 0.4 ppm.

Red Mountain is the gold in a suite of 20 samples of 

douglas-fir wood from the Basin Creek watershed, which 

is northeast of Stanley but in an area of no known gold 

mineralization. There the gold concentrations ranged from 

0.04 to 0.31 ppm, which correspond with the statement by

Shacklette and others (1970, p. 2) that the amounts of gold 

in plant ash are usually much smaller than 1 ppm.

The only reasonable conclusion we can reach is that 

gold levels are unusually high in the southern quarter of 

the sampled area. Lakin and others (1974) stated, "The

Gold in Douglas-Fir, Red Mountain Stockwork 147
Normal population

Geometric mean 0.15

61 samples

Anomalous population

Geometric mean, 1.0

53 samples

15 i 

0.05 0.1 0.5 1.0 2 

GOLD, IN PARTS PER MILLION

10

J 

20

Figure L5. Histogram of gold in the ash of douglas-fir wood, showing two populations, Red Mountain area.

relative insolubility of gold drastically limits the portion 

of the total gold under the plant that is available to the 

plant roots!' Therefore, we would expect to find such unusual concentrations of gold in plants only in an area where 

gold is relatively abundant. The high concentrations observed in the ash of douglas-fir wood, but not in soil from 

the same sample sites and those nearby, suggest that the root 

system of the trees has taken up gold from a source deeper 

than that which commonly contributes gold to the soil.

Molybdenum distribution

Biogeochemical sampling for possible polymetallic 

occurrences may require the use of more than one type of 

plant. We sampled douglas-fir mainly for its ability to concentrate gold, but we sampled beargrass, and to a limited 

extent elk sedge, because of their ability to concentrate 

molybdenum.

Molybdenum concentrations in beargrass sampled 

from the Challis quadrangle in 1979 were less than 5 to 

20 ppm; these were in areas that were considered unmineralized for molybdenum.

The molybdenum content of 96 samples of ashed 

beargrass leaves from the study area ranged from less than 

5 to more than 500 ppm; that of 18 samples of elk sedge 

leaves ranged from 5 to 100 ppm. The 50th and 95th 

percentiles were the same from both plants, 20 and 100 

ppm, respectively.

Normal concentrations of molybdenum in elk sedge 

appear to be about 20 ppm, based on 27 samples collected 

from nine prospects or mines that represented various kinds 

of mineral deposits in the region. Anomalous concentrations of 100 to 200 ppm were observed in samples from 

the Sunnyside gold mine in the Thunder Mountain mining district, the new Thompson Creek mine (a world-class 

molybdenum deposit), and several tungsten occurrences.

With few exceptions, all above-median values were

from samples in a belt that extends around the western side 

of the grid area and is peripheral to the exposed stockwork 

(fig. L6). We were unable to close off the anomaly by 

means of the sampling design that we used.

The belt of above-median values of molybdenum in 

beargrass and elk sedge transects some part of every 

geologic map unit in the area except the summit quartz 

body (unit Tsq, fig. L3). Much of the belt is on Quaternary deposits, either alluvial, glacial, or colluvial, and is 

therefore most difficult to interpret.

We can draw one inference with assurance; the great 

extent and the continuity of the belt of above-median 

molybdenum values in beargrass and sedge requires some 

comparably extensive bedrock source of molybdenum. The 

location, shape, and molybdenum content of the bedrock 

source remain conjectural, but the source must be large.

Tungsten distribution

In his book on biogeochemical exploration for 

mineral deposits, Kovalevskii (1979, p. 87) stated, "The 

most important indicators of molybdenum and tungsten 

deposits are the elements themselves!' Yet very little is 

known of the absorption of tungsten by plants (Kovalevskiy, 1966). Quin and Brooks (1974) reported that even 

though soil sampling was generally satisfactory, tree-trunk 

sampling was effective in locating extensions of known 

scheelite-bearing reefs in New Zealand.

Previous studies by Leonard (in press) on the response of beargrass to tungsten occurrences encouraged 

us to submit our samples for analysis by colorimetry. 

At Red Mountain, the tungsten anomaly in beargrass 

is clearly subtle; ashed leaves of beargrass contain less than 

1 to 2 ppm tungsten. Nevertheless, at this low level the area! 

distribution of the 1- and 2 ppm values shows a pattern 

generally resembling that of the major belt of abovemedian values of molybdenum in beargrass (fig. L7).

148 Challis 1°x2° Quadrangle, Idaho
115°27'30'

A*"~*

\\ *. * *v £- <.

" " - //s>-''*C;^N^kV^x>''>^ !T^^' ' ? - \

." ." . "^^HmHp" t \

i ^ ^ '&&-;j£-J' * * c\ \ .

\v_ -r-i/ ^' ' sf--~*<, iifc J- v r;

1 MILE

0.5 1 KILOMETER 

CONTOUR INTERVAL 400 FEET

EXPLANATION

QUATERNARY 

SURFICIAL DEPOSITS, 

UNDIVIDED

TERTIARY 

SUMMIT QUARTZ BODY

QUARTZ STOCKWORK

SILICIFIEDZONE 

CRETACEOUS 

GRANITE

GRANODIORITE 

MIXED-ROCK UNIT

CONTACT

FAULT Dashed where

inferred 

ADIT

LOCALITY OF SAMPLE CONTAINING

MOLYBDENUM 

More than 200 ppm 

50 - 200 ppm 

20 - 50 ppm 

Less than 20 ppm 

No data

Figure L6. Map showing sample localities and concentrations of molybdenum in the ash of beargrass and elk sedge, Red Mountain area. Isopleths (dashed lines) enclose localities of samples having values above the median (20 ppm).

GEOMAGNETIC ANOMALIES

The biogeochemical data provide evidence of an apparent subsurface structure that was not obvious from 

detailed surface mapping. It is discordant with the

bedrock structure. A subsequent packborne geomagnetic 

survey tended to confirm this conclusion.

An aeromagnetic map of part of the Elk City 1°X2° 

quadrangle (U.S. Geological Survey (1972), contour interval 20 gammas), shows a distinct crowding of contours

Gold in Douglas-Fir, Red Mountain Stockwork 149
115°27'30'

EXPLANATION

QUATERNARY 

SURFICIAL DEPOSITS, 

UNDIVIDED

TERTIARY 

SUMMIT QUARTZ BODY

QUARTZ STOCKWORK

SILICIFIEDZONE 

CRETACEOUS 

GRANITE

GRANODIORITE 

MIXED-ROCK UNIT

CONTACT

FAULT Dashed where

inferred 

ADIT

LOCALITY OF SAMPLE CONTAINING

TUNGSTEN 

2ppm 

1 ppm

Less than 1 ppm 

No data

KILOMETER

CONTOUR INTERVAL 400 FEET

Figure 17. Map showing sample localities and concentrations of tungsten in the ash of beargrass, Red Mountain area. Isopleths 

(dashed lines) enclose localities of samples containing detectable tungsten.

in the Red Mountain area but no isolated magnetic high 

or low. Therefore, in making a ground survey by precession 

magnetometer, we did not expect to learn much about the 

gross structure of the area. We did hope that a ground survey 

would show the distribution of disseminated ferromagnetic

pyrrhotite within stockwork or granite, and that the pattern of its distribution would be useful in interpreting local 

structures. We found, instead, that Red Mountain itself is 

an area of uninterpretable magnetic noise, that magnetic 

anomalies having some continuity are present north, west,

150 Challis 1°x2° Quadrangle, Idaho
115°27'30'

EXPLANATION

QUATERNARY 

SURFICIAL DEPOSITS, 

UNDIVIDED

TERTIARY 

SUMMIT QUARTZ BODY

QUARTZ STOCKWORK

SILICIFIEDZONE 

CRETACEOUS 

GRANITE

GRANODIORITE 

MIXED-ROCK UNIT

CONTACT

FAULT Dashed where

inferred 

ADIT

CENTERLINES OF MAGNETIC

ANOMALIES 

100 gammas or more 

40 - 50 gammas

 30 to 70 gammas

 100 gammas or more

KILOMETER

CONTOUR INTERVAL 400 FEET

Figure L8. Map showing traces of geomagnetic anomalies, Red Mountain area. Centerlines of magnetic anomalies indicate surface 

traces of sharply defined increases (or decreases) of magnetic intensity measured on the ground with a portable magnetometer. Camma values indicate heights (depths) of the peaks above (below) background level.

and south of the mountain, and that the northern and 

southern anomalies are discordant to the rock structure as 

mapped. More detailed work would be required to choose 

between near-surface alteration and deep-seated structure 

as

the probable cause of these magnetic anomalies.

Details of the packborne magnetometer survey and 

of the specific anomalies are described in Leonard and Erdman (1983). Collectively, the magnetic anomalies of the Red

Gold in Douglas-Fir, Red Mountain Stockwork 151
Mountain area show the features of an oval (fig. L8). Its 

major axis is nearly north-south, its north end is flattened, 

its south end is tapered, its west side is rectilinear, and its 

east side is barely sketched. The mappable part of the oval 

resembles the shape of the curved belt of above-median 

values of molybdenum in beargrass, and the two features 

are superposed along the western and southern sides of 

the grid.

Our best guess is that the sources of the magnetic 

anomalies are somehow related to wallrock alteration attendant on mineralization. The anomalies discordantly 

overprint the east-trending part of the belt of abovemedian values of molybdenum in beargrass and the 

swatch of high values of gold in douglas-fir, but they are 

confined to these biogeochemical domains.

CONCLUSIONS

Evidence obtained from this study indicates possible exploration targets for concealed deposits of gold, 

molybdenum, and tungsten at Red Mountain. The targets 

are new, the biogeochemical anomalies that help define 

the targets are extensive, and the deposits that might be 

sought are presumably of low grade but perhaps large.

The distribution of metals in plants, of clay minerals 

in soil, and of low-intensity magnetic anomalies at the 

limits of the sampling grid strongly suggest that the subsurface structure of the area is different from the structure that can be mapped. The subsurface structure may 

be hoodlike, relatively flat or gently undulating beneath 

the exposed quartz stockwork, but draped downward and 

covered by Quaternary deposits along Vein Creek and 

Quartz Creek.

Several of the targets would be unsuspected if our 

study did not show that plants concentrate gold, 

molybdenum, and tungsten. The advantages of sampling 

plants in lieu of or as a complement to sampling soils are 

especially clear in searching for gold. Two of these advantages are the circumvention of the particle sparsity effect (Harris, 1982) and the ability of plants to extract 

elements from a large volume of soil and underlying 

weathered bedrock.

REFERENCES CITED

Brooks, R. R., Holzbecher, J., and Ryan, D. E., 1981, Horsetails 

(Equiseturri) as indirect indicators of gold mineralization: 

Journal of Geochemical Exploration, v. 16, p. 21-26.

Curtin, G. C, Lakin, H. W., Neuerburg, G. J., and Hubert, A. 

E., 1968, Utilization of humus-rich forest soil (mull) in 

geochemical exploration for gold: U.S. Geological Survey 

Circular 562, 11 p.

Harris, J. F., 1982, Sampling and analytical requirements for 

effective use of geochemistry in exploration for gold, in 

Levinson, A. A., ed., Precious metals in the northern Cordillera: Association of Exploration Geochemists Symposium, Vancouver 1981, Proceedings, p. 5367.

Hoffman, E. L., and Brooker, E. J., 1982, The determination 

of gold by neutron activation analysis, in Levinson, A. A., 

ed., Precious metals in the northern Cordillera: Association of Exploration Geochemists Symposium, Vancouver 

1981, Proceedings, p. 69-77.

Khotamov, Sh., Lobanov, E. M., and Kist, A. A., 1966, [The 

problem of the concentration of gold in organs of plants 

from ore districts]: Akademiya Nauk Tadzhikskoy SSR 

Doklady, v. 9, no. 11, p. 27-30 (in Russian).

Koschmann, A. H., and Bergendahl, M. H., 1968, Principal goldproducing districts of the Unted States: U.S. Geological 

Survey Professional Paper 610, 283 p.

Kovalevskii, A. L., 1979, Biogeochemical exploration for mineral 

deposits: New Delhi, Amerind Publishing Company Pvt. 

Ltd.; also available from National Technical Information 

Service, Springfield, Va., 22170, as Report TT 76-52029; 

136 p.

Kovalevskiy, A. L., 1966, Biogeochemistry of tungsten in plants: 

Geochemistry International, v. 3, p. 555-562.

Lakin, H. W., Curtin, G. C., and Hubert, A. E., 1974, 

Geochemistry of gold in the weathering cycle: U.S. Geological Survey Bulletin 1330, 80 p.

Leonard, B. E, in press, The Golden Gate tungsten deposit and 

metal anomalies in nearby soils and plants, Yellow Pine 

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

Open-File Report 83-835.

Leonard, B. F., and Erdman, J. A., 1983, Preliminary report on 

geology, geochemical exploration, and biogeochemical exploration of the Red Mountain stockwork, Yellow Pine 

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

Open-File Report 83-151, 49 p.

Leonard, B. F., and Marvin, R. F., 1982 [1984], Temporal evolution of the Thunder Mountain caldera and related features, 

central Idaho, in Bonnichsen, Bill, and Breckenridge, R. 

M., eds., The Cenozoic of Idaho: Idaho Bureau of Mines 

and Geology Bulletin 26, p. 23-41.

Miesch, A. T., 1981, Estimation of the geochemical threshold 

and its statistical significance: Journal of Geochemical Exploration, v. 16, p. 49-76.

Millard, H. T., Jr., Marinenko, John, and McLane, J. E., 1969, 

Establishment of gold-quartz standard GQS-1: U.S. Geological Survey Circular 598, 6 p.

Quin, B. F, and Brooks, R. R., 1972, The rapid determination 

of tungsten in soils, stream sediments, rocks and vegetation: Analytica Chimica Acta, v. 58, p. 301-309.

_____1974, Tungsten concentrations in plants and soils as 

a means of detecting scheelite bearing ore bodies in New 

Zealand: Plant and Soil, v. 41, p. 177-188.

Shacklette, H. T, Lakin, H. W., Hubert, A. E., and Curtin, G. 

C., 1970, Absorption of gold by plants: U.S. Geological 

Survey Bulletin 1314-B, 23 p.

U.S. Geological Survey, 1972, Aeromagnetic map of part of the 

Elk City l°by 2° quadrangle, Idaho-Montana: U.S. Geological Survey Geophysical Investigations Map GP-841, 

scale 1:250,000.

Warren, H. V, 1980, Biogeochemistry, trace elements, and 

mineral exploration, in Davies, B. E., ed., Applied soil trace 

elements: New York, Wiley, p. 353-380.

Warren, H. V, and Delavault, R. E., 1950, Gold and silver content of some trees and horsetails in British Columbia: Geological Society of America Bulletin, v. 61, p. 123-128.

152 Challis 1°x2° Quadrangle, Idaho
Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter M

Mineral Deposits in the Southern Part of the 

Atlanta Lobe of the Idaho Batholith and their 

Genetic Relation to Tertiary Intrusive 

Rocks and to Faults

By THOR H. KIILSGAARD and EARL H. BENNETT1 

CONTENTS

Abstract 155 

Introduction 155 

Sawtooth Wilderness 155

Geology 155

Anomalous metal concentrations in pink granite of the Sawtooth 

atholith 155

Gold-silver deposits and their relation to the Montezuma fault 156 

Ten Mile West Roadless Area 157

Geology 157

Geochemical analyses 159

Mineral deposits related to faults 159

Mineral resource potential 159 

Mineral deposit relationship to the Montezuma and Bear River faults, Sawtooth

Wilderness and Ten Mile West Roadless Area 160 

Boise Basin area 160

Geology 160

Mineral deposits 160

Gold-silver deposits along or near the trans-Challis fault system 163 

References cited 164

1 1daho Geological Survey, Moscow, Idaho.

Mineral Deposits in the Atlanta Lobe of the Idaho Batholith 153
FIGURES

Ml. Index map showing location of geographic and structural features described 

in this chapter 155

M2. Map of the Sawtooth Wilderness showing geologic features, mines, and 

prospects 156

M3. Simplified geologic map of the Ten Mile West Roadless Area 158

M4. Map of the Sawtooth Wilderness and Ten Mile West Roadless Area, showing mines and prospects 161

M5. Map of Boise Basin showing gold-silver veins of the trans-Challis fault 

system 162

M6. Map showing the location of mines and prospects with respect to the transChallis fault system 163

154 Challis 1°x2° Quadrangle, Idaho
Abstract

Most mineral deposits in the Atlanta lobe of the Idaho 

batholith are genetically related to Tertiary intrusive rocks; 

regional faults have controlled location of the deposits. Pink 

granite of the Sawtooth batholith, in the Sawtooth Wilderness, 

about 45 m.y. (million years) old, contains more beryllium, 

uranium, fluorine, and molybdenum than does rock of the 

Idaho batholith. Cold-silver vein deposits along the western side 

of the Sawtooth batholith are concentrated in hydrothermally 

altered rocks of the Idaho batholith on the downthrown side 

of the Montezuma fault. Many of the veins follow or crosscut 

Tertiary dikes.

Pink granite of the Sawtooth batholith and nearby dioritic 

rocks about 47 m.y. old crop out in the Ten Mile West Roadless 

Area, as do swarms of associated dikes. Geochemical sampling shows silver, gold, and molybdenum concentrated near the 

intrusive Tertiary rocks along northwest-trending faults that cross 

the area.

A major northeast-trending fault system controls the location of vein deposits in the Boise Basin. Tertiary plutonic rocks 

and dikes have intruded along the faults, and the veins cut both 

the Tertiary plutonic rocks and the dikes. Two molybdenum 

deposits in the area occur in and along rhyolitic dikes, and a 

fission-track date from zircon taken from rhyolite at one of the 

deposits gives a date of 29.3±1.7 m.y. Veinlets containing 

molybdenite cut the rhyolite and are younger. The mineralization probably relates to waning phases of the Tertiary plutonic 

activity. Mineral deposits in Tertiary rocks also occur along 

northeast-trending faults and grabens in the northeastern part 

of the Challis quadrangle.

INTRODUCTION

Most mineral deposits in the Atlanta lobe of the 

Idaho batholith are genetically related to intrusive rocks 

of Tertiary age and are structurally controlled by regional 

faults. Granitic rocks of the Atlanta lobe of the Idaho 

batholith (fig. A2, chap A; and fig. Bl, chap. B, this 

volume), now considered to be Cretaceous in age, and 

principal faults in the western part of the Challis quadrangle are described by Kiilsgaard and Lewis (chap. B, this 

volume), and Tertiary intrusive rocks of Eocene age are 

described by Bennett and Knowles (chap. F, this volume). 

The epigenetic character of mineral deposits in Tertiary 

intrusive rocks was first recognized by Lindgren (1898) in 

a description of mineral deposits near Boise Basin, Idaho. 

Anderson (1951) believed that mineral deposits in the 

southern part of the Idaho batholith were not genetically related to the batholith but were associated with early 

Tertiary igneous activity. Characteristics of the Tertiary 

intrusive rocks and their relation to mineral deposits have 

been discussed by Bennett (1980).

Four areas in or near the Challis quadrangle, in 

which mineral deposits, Tertiary intrusive rocks, and faults 

are present, are discussed in this chapter. The areas are: 

the Sawtooth Wilderness, the Ten Mile West Roadless 

Area, the Boise Basin, and a northeast-trending belt, the

trans-Challis fault system, that extends across the Challis 

quadrangle (fig. Ml).

SAWTOOTH WILDERNESS 

Geology

The Sawtooth Wilderness, formerly called the Sawtooth Primitive Area, covers most of the Sawtooth Range 

southwest of Stanley (fig. Ml). The most conspicuous rock 

type in the range is pink granite identified by Reid (1963, 

p. 8) as making up the Sawtooth batholith. The granite 

is distinguished by coarse granitoid texture and abundant 

perthitic potassium feldspar, which gives the rock its 

characteristic pink color. Biotite from three samples of 

the rock, analyzed by the potassium-argon method, gave 

ages of 43.7, 44.1, and 44.4 m.y., respectively (Armstrong, 

1975); these ages were recalculated to 44.9, 45.7, and 45.6 

m.y., using the decay constants of Steiger and Jaeger 

(1977), and identify the rock as Tertiary (Eocene). The 

pink granite intrudes rocks of the Idaho batholith, which 

are principally granodiorite. Both the younger Sawtooth 

batholith and the older Idaho batholith are intruded by 

innumerable dikes that range from diabase to rhyolite in 

composition. The more common dikes range from 1.5 to 

15 m (meters) in thickness and from a few hundred meters 

to more than 1 km (kilometer) in length. Structurally, the 

Sawtooth Range is a large horst, bounded on the northeastern side by the Sawtooth fault and on the southwestern 

side by the Montezuma fault (fig. M2).

Anomalous metal concentrations in pink 

granite of the Sawtooth batholith

Pink granite of the Sawtooth batholith contains 

more anomalous concentrations of metals than do rocks

116° 114°

Jen Mile West Roadless Area 

o Stanley

Sawtooth Wilderness 

CHALLIS QUADRANGLE

HAILEY QUADRANGLE

Boise Basin area

0 40 MILES

I 1 \

0 20 40 KILOMETERS

Figure Ml. Index map showing the location of the Sawtooth 

Wilderness, Ten Mile West Roadless Area, and Boise Basin, and 

the approximate trace of the trans-Challis fault system, Idaho.

Mineral Deposits in the Atlanta Lobe of the Idaho Batholith 155
115°

O Stanley

Approximate 

boundary of 

the Sawtooth 

Wilderness

Atlanta

5 MILES

0 5 KILOMETERS 

EXPLANATION

PINK GRANITE (EOCENE)

CONTACT

REGIONAL FAULT Bar 

and ball on downthrown 

side

MINE OR PROSPECT

Figure M2. Map of the Sawtooth Wilderness showing major 

geologic features, and mines and prospects.

of the Idaho batholith. This conclusion is based on an 

appraisal of the mineral resoures of the former Sawtooth 

Primitive Area (Kiilsgaard and others, 1970) during which 

more than 1,100 samples of stream sediments, rocks, and 

mineralized deposits were collected and analyzed.

Beryllium is distinctly anomalous in rocks of the 

Sawtooth batholith, as determined by analyses of rock 

and stream-sediment samples (Kiilsgaard and others, 1970, 

fig. 9). The median beryllium content of unaltered granitic 

rocks of the Sawtooth batholith is 6 ppm (parts per 

million), in contrast to a median content of 1 ppm 

beryllium in rocks of the Idaho batholith.

Molybdenum is concentrated in pink granite of the 

Sawtooth batholith. Forty-three samples of altered granite, 

taken largely from northeast-trending altered zones, 

ranged from 2 to 1,500 ppm molybdenum but averaged 

122 ppm. In contrast, 49 samples of altered granitic rock 

of the Idaho batholith contained an average of 13 ppm 

molybdenum (Kiilsgaard and others, 1970, table 1, pis. 

1, 2). Samples from obviously mineralized localities, of 

both types of rock, were excluded from the comparison. 

The clustered distribution of molybdenum in the Sawtooth granite batholith is readily apparent (Kiilsgaard and 

others, 1970, fig. 8). Other mineralized localities shown 

in that figure as outside of the Sawtooth batholith are 

chiefly quartz veins that are explored by various prospect 

workings and are known to contain minor quantities of 

molybdenite. None of the large altered zones of Sawtooth 

granite batholith have been explored sufficiently to determine whether they contain molybdenum deposits of commercial significance.

Uranium also is concentrated in the pink granite of 

the Sawtooth batholith. Uranium content of 10 samples 

of the rock ranged from 10 to 190 ppm. The pink granite 

consistently has been found to be two to three times more 

radioactive than rocks of the Idaho batholith. Sediment 

samples from streams that drain areas underlain by pink 

granite of the Sawtooth batholith, particularly samples 

rich in organic debris, contained appreciable uranium. 

Stream-sediment samples that contained 100 ppm or more 

uranium were collected from a large area in the southern 

part of the Sawtooth batholith (Kiilsgaard and others, 

1970, table 4, fig. 10). Despite the widespread, anomalously high uranium content in stream-sediment and rock 

samples, no concentrated deposits of uranium were found 

in rocks of the area.

High background values in uranium, molybdenum, 

and beryllium are characteristic of anorogenic granite (Atype) worldwide. The Sawtooth batholith and other 

similar granitic plutons in Idaho are believed to consist 

of anorogenic granite and are described by Bennett and 

Knowles (chap. F, this volume).

Gold-silver deposits and their relation to 

the Montezuma fault

A conspicuous geologic feature near the western 

border of the Sawtooth Wilderness is the concentration 

of gold-silver deposits along the Montezuma fault, chiefly 

along the western side of the fault in the downthrown 

block (fig. M2). The largest and most productive of these 

deposits is the Atlanta lode, which was discovered in 1864 

and was worked intermittently through several mines until the early 1950's. For several years, the Atlanta lode was 

the largest producer of gold in Idaho.

The gold-silver deposits along the Montezuma fault 

have many common characteristics. They are quartz veins

156 Challis 1°x2° Quadrangle, Idaho
from which gold has been the most valuable metal produced, most of which was native gold mined from the oxidized zone near the surface. Ballard (1928, p. 17) described 

auriferous pyrite at deeper levels of mines in the Atlanta 

lode, where the grade of ore was lower and where silver 

sulfide minerals also were produced, as were minor quantities of lead-zinc minerals. Native gold in the oxidized 

zone probably resulted from oxidation of the auriferous 

pyrite and other gold-bearing sulfide minerals. Although 

more valuable, gold has been exceeded by silver in total 

weight of metal produced. Anderson (1939, p. 23) noted 

that in early bonanza deposits in the Atlanta district, the 

ratio of silver to gold by weight was as much as 200 to 

1. Granitic host rock of the gold-silver deposits is hydrothermally altered, with extensive sericite developed from 

feldspar minerals. Even away from the deposits, the country rock shows the effects of strong hydrothermal alteration. The quartz veins commonly extend along the walls 

of Tertiary dikes. Fragments of dike rock are found in 

some quartz veins; some mineralized quartz veins cut 

across the dikes, but in other places dikes cut across the 

veins. Parallel quartz veins have been found along both 

walls of an intervening dike, much like two pieces of bread 

on either side of a slice of salami in a sandwich. The 

spatial relationship of the veins and dikes indicates that 

both were deposited more or less contemporaneously. 

Comparable dikes intrude the Sawtooth batholith and 

therefore are younger, which indicates that veins associated 

with such dikes also are younger than the batholithic 

rocks.

Gold-silver deposits along the Montezuma fault are 

structurally controlled by the fault (fig. M2). The extent, 

linearity, and amount of displacement along the fault indicate that it is a deep fault that originated before the 

mineral deposits. Downward displacement of the western, 

hanging-wall, block of the fault created fractures in the 

block that made it more permeable, both to intruding 

dikes and mineralizing solutions. Field evidence, however, 

suggests recurrent movement along the fault after mineral 

deposition. Anderson (1939, p. 17) observed that the 

Montezuma fault cut and displaced the Atlanta lode. 

Gravel of possible Pleistocene age caps the ridge northwest of Atlanta in the downdropped block of the 

Montezuma fault. Rock types in the gravel are the same 

as those that crop out east of the fault, in the uplifted 

horst, but similar gravel was not found in that area, which 

suggests that rapid erosion has stripped the gravel from 

the uplifted terrane and that movement along the fault 

is Pleistocene or younger. Erosion also may account for 

the scarcity of vein outcrops in the uplifted block east of 

the Montezuma fault.

Recurrent movement along preexisting faults in central Idaho may be more commonplace than has been 

previously assumed. An excellent example of such movement is that associated with the earthquake of Oct. 28, 

1983, which occurred on a fault along the western front

of the Lost River Range just southeast of the Challis 

quadrangle. That fault appears to be of the same pattern 

as the Montezuma fault. It strikes northwest, dips 

southwest, and is marked by an ancestral crushed and 

sheared zone. During the October 1983 earthquake, terrane along the western side of the fault dropped as much 

as 3.5 m. Multiple fractures formed in the downdropped 

block, extending at various angles away from the main 

fault. Fractures in the downdropped block of the Montezuma fault, some of which subsequently were mineralized, may have formed from similar recurrent fault 

movement.

TEN MILE WEST ROADLESS AREA

Geology

The Ten Mile West Roadless Area adjoins the 

western side of the Sawtooth Wilderness (fig. Ml). The 

geology of the two areas is similar. Granitic rocks of the 

Idaho batholith underlie most of the roadless area. The 

batholithic rock is intruded by dioritic rocks and by pink 

granite, both of Tertiary age. All of the plutonic rocks are 

intruded by Ibrtiary dikes, and all rocks are displaced 

along northwest-trending faults (Kiilsgaard, 1983a). The 

largest exposure of dioritic rocks is along the ridge leading southeast from Jackson Peak (unit Tdc, fig. M3). The 

rocks are dark, are characterized by abundant hornblende, 

biotite, and magnetite, and range from quartz monzodiorite to granite in composition, although porphyritic 

granodiorite is the more common rock type. Biotite from 

two samples of the porphyritic granodiorite, from near 

Jackson Peak, gave potassium-argon ages that averaged 

46.7 m.y.; hornblende from the two samples averaged 49.3 

m.y. (R. J. Fleck, written commun., 1981). The 

granodiorite thus is Eocene in age.

Pink granite in the roadless area is identical to pink 

granite of the Sawtooth Wilderness. Parts of two samples 

of the rock from exposures northeast of Goat Mountain 

gave whole-rock rubidium-strontium ages of 43.6 m.y. 

(R. J. Fleck, written commun., 1983), which agree with 

previously mentioned dates for the Sawtooth batholith 

and which indicate that the pink granite is slightly younger 

than the dioritic rocks.

Innumerable dikes that range from diabase to 

rhyolite in composition intrude the plutonic rocks. Most 

of the dikes are porphyritic. Rhyolite and quartz latite are 

the more common dike types, and they tend to cluster near 

and show a close affinity to the Tertiary pink granite. 

Rhyolite dikes, like the pink granite, tend to be more 

radioactive than other dikes. By contrast, the more dacitic 

dikes tend to be near or in line with exposures of the 

dioritic intrusive rocks and probably represent offshoots 

from those intrusions.

Mineral Deposits in the Atlanta Lobe of the Idaho Batholith 157
44° DO 1

EXPLANATION

PINK GRANITE (EOCENE) 

DIORITE COMPLEX (EOCENE)

CONTACT Dashed where approximately located

REGIONAL FAULT Dashed where approximately 

located; bar and ball on downthrown side

LOCALITY OF STREAM-SEDIMENT SAMPLE 

Contains 3 ppm or more silver 

Contains 10 ppm or more molybdenum

LOCALITY OF ROCK SAMPLE CONTAINING 

5 PPM OR MORE GOLD AND (OR) 30 

PPM OR MORE SILVER 

MINES AND PROSPECTS 

Lone Cabin 

Gold Bug 

Valley Chief 

Graham

I

Approximate 

boundary of 

the Ten Mile 

West Roadless 

Area

1 1 1 1 1 

} 5 KILOMETERS 1

Figure M3. Simplified geologic map of the Ten Mile West Roadless Area, showing sample localities.

158 Challis 1°x2° Quadrangle, Idaho
The Montezuma fault was traced from the Sawtooth 

Wilderness northwest to the North Fork Boise River (fig. 

M3). It may continue to the northwest or may be offset 

to the southwest by an inferred northeast-trending fault 

and then continue northwest as the Bear River fault 

(Kiilsgaard, 1983a). The southwest side of the roadless 

area is bounded by the Deer Park fault, the western side 

of which is downdropped like the Montezuma fault.

The age of initial movement along the northwest-trending faults is uncertain. The Bear River fault 

trends parallel to the largest exposure of dioritic rocks, 

which suggests that the fault or a parallel companion fault 

could have guided intrusion of those rocks. The pink 

granite also is exposed along the Bear River fault, but it 

is not clear if this location reflects intrusive guidance by 

the fault, as the fault has displaced pink granite northeast of Goat Mountain. Exposure of the pink granite 

could result from uplift between the Deer Park and Bear 

River faults and resulting accelerated erosion. Elsewhere 

in the area, the displacement of Quaternary gravel along 

faults indicates recent fault movement, as does the rapid 

downcutting of streams across fault scarps.

Geochemical analyses

A total of 584 rock and stream-sediment samples 

from the Ten Mile West Roadless Area were collected and 

analyzed. Stream-sediment samples that contained detectable silver (0.5 ppm), as determined by semiquantitative 

spectrographic analysis, came from many localities 

(Kiilsgaard, 1982), but stream-sediment samples that contained 3 ppm or more silver were largely from sites near 

the pink Tertiary granite and along the Bear River and 

Montezuma faults. Many stream-sediment samples also 

contained concentrations of molybdenum, and some 

samples that contained 10 ppm or more molybdenum were 

from streams that drain areas that also yielded higher 

values in silver. Sediments of some streams draining the 

northwestern part of the roadless area are enriched only 

in molybdenum (Kiilsgaard, 1983b).

Mineral deposits related to faults

Mineral deposits in the roadless area are concentrated in hydrothermally altered rocks along the western 

side of the Montezuma fault and on both sides of the Bear 

River fault (Kiilsgaard and others, 1983). The Lone Cabin 

prospect, just west of the Montezuma fault (fig. M3), consists of four adits, all caved, and several pits and trenches 

that explore a vein over a strike length of more than 

305 m. Of eight samples taken at the property in 1966, 

the richest one, a selected sample from a dump, contained 3.5 oz (ounce) gold per ton (Kiilsgaard and others, 

1970, p. D75). Twelve samples taken at the prospect in

1981 contained none to 0.80 oz gold and none to 0.90 oz 

silver per ton (Kiilsgaard and others, 1983, p. 8).

The Gold Bug prospect (fig. M3) also is west of the 

Montezuma fault. The prospect has been explored by an 

adit and several pits and trenches. Of 18 samples taken of 

iron-stained and altered granitic rock at the prospect, the 

richest sample contained 3.20 oz gold per ton (Kiilsgaard 

and others, 1983, p. 5).

The Valley Chief prospect (fig. M3) is another mineralized locality near the Montezuma fault. Two adits at 

the prospect, both caved at the portal, explore a vein whose 

outcrop may be traced for about 550 m. The richest of ten 

samples of vein outcrop and dump material contained 14.6 

oz silver per ton (Kiilsgaard and others, 1983, p. 8).

The Graham mine (fig. M3) is on the east side of 

the Bear River fault. It consists of several adits that explore quartz veins and dikes, samples from which yielded 

low values in silver and gold. Northwest-striking quartz 

veins that cross the ridge between Big and Little Silver 

Creeks north of the Graham mine also are on the Graham 

property, and samples from these veins and from adjoining hydrothermally altered granitic rock contained as much 

as 43 ppm gold and ranged from 15 to 120 ppm silver 

(Kiilsgaard, 1983b, fig. 4). Some stream-sediment samples 

from tributaries both downstream and upstream from the 

quartz veins were enriched in silver and molybdenum.

Mineral resource potential

A northwest-trending belt that has a potential for 

resources of one or more of the metals gold, silver, molybdenum, lead, and zinc crosses the Ten Mile West Roadless 

Area. The belt is about 4.8 km wide and is structurally controlled by the Montezuma and Bear River faults (Kiilsgaard 

and others, 1983). In addition to the faults, the concept 

of a mineralized belt is supported by widespread hydrothermally altered granitic rocks, mineralized lode deposits, the 

pattern of stream-sediment samples that contained anomalously high metal content, some of which may indicate undiscovered mineral deposits, and by identified placer-gold 

deposits. The clustering of dikes and the location of intrusive plutonic rocks of Tertiary age at and near the 

mineralized localities also are distinguishing marks of the 

belt. Altered granitic rocks of the Idaho batholith along 

the Bear River fault may be underlain at shallow depth 

by Tertiary intrusive rocks, an observation supported by 

alteration features and by geophysical studies (Mabey, in 

press).

At the Graham mine (fig. M3), a mineralized quartz 

vein cuts across a rhyolite dike, which indicates that the 

vein is younger than the dike. If the dike is related to 

a nearby exposure of pink granite of Tertiary age, as is 

likely, both the dike and the vein can reasonably be assumed to be genetically related to the Tertiary plutonic 

activity.

Mineral Deposits in the Atlanta Lobe of the Idaho Batholith 159
MINERAL-DEPOSIT RELATIONSHIP TO 

THE MONTEZUMA AND BEAR RIVER 

FAULTS, SAWTOOTH WILDERNESS AND 

TEN MILE WEST ROADLESS AREA

The spatial relationship of mineral deposits to the 

Montezuma and Bear River faults along the western side 

of the Sawtooth Wilderness and across the Ten Mile West 

Roadless Area is shown in figure M4. Most of the deposits 

are in downthrown blocks of the faults, in or near fractures created by movement along the faults. Hydrothermal alteration of rocks in and near the mineralized 

localities is pervasive, part of a regional alteration feature 

that resulted from Tertiary plutonic activity (Criss and 

Taylor, 1983). Mineral assemblages and characteristics of 

the gold-silver veins indicate that they are epithermal and 

may not extend to great depths. The crosscutting relationship of veins and some of the dikes suggests that 

mineralization occurred during waning stages of Tertiary 

plutonic activity. The belt along the western side of the 

Montezuma fault and along both sides of the Bear River 

fault has a potential for mineral resources.

BOISE BASIN AREA

The northern part of the Boise Basin area extends 

into the Challis quadrangle (fig. Ml). Placer gold was 

discovered in Boise Basin in 1862, and lode deposits were 

discovered soon thereafter. One of the lode deposits, the 

Gold Hill mine, was located in 1863 and was worked 

almost continuously from its discovery until 1938 (Anderson, 1947, p. 176). The total amount of gold produced 

from the basin is disputed because of incomplete earlyday records, the difficulty of separating gold production 

of Boise Basin from production of other localities in Boise 

County, and because some of the gold that was mined 

was not reported. From recorded production statistics, a 

total of about 2,891,000 ounces of gold was produced from 

Boise Basin, of which about 74 percent came from placer 

deposits. The accumulated output makes Boise Basin the 

largest gold-producing district in Idaho and one of the 

larger gold-producing districts in the United States.

Geology

Boise Basin is underlain by granitic rocks of the 

Idaho batholith that have been intruded by Tertiary stocks 

and dikes. More or less parallel northeast-trending faults, 

members of the trans-Challis fault system, cross Boise 

Basin and appear to have guided emplacement of the dikes 

and possibly the stocks as well. Gold-bearing quartz veins 

crop out in the district, the more productive of which trend 

more or less parallel to Tertiary dikes. Some of the quartz 

veins extend along dike walls, some are within dikes, but

some veins are cut by dikes. Erosion of the veins has produced the placer deposits of the basin.

A northeast-trending belt of dikes and stocks intrudes the Idaho batholith in the northern part of the 

Boise Basin (fig. M5). Lindgren (1898) and Ballard (1924) 

first described the rocks, but Ross (1934, p. 250) first 

classed the intrusives as Tertiary (Miocene). Anderson 

(1947) mapped the area, identified different types of dikes, 

and separated the dikes from the stocks. He identified 

diorite, granodiorite, and quartz-hornblende-biotite monzonite porphyry as common rock types of the stocks and 

noted that the largest of the stocks was more than 1 mile 

wide and more than 10 miles long. Rocks of the large stock 

are similar to those of the dioritic complex mapped in the 

Ten Mile Roadless Area and are characterized by 

phenocrysts of zoned andesine, hornblende, fresh biotite, 

and magnetite in a pinkish-gray matrix that consists chiefly of potassium feldspar. Biotite from a somewhat similar 

rock exposed in a stock that is crossed by the South Fork 

Payette River (fig. M5) gave a potassium-argon age of 48 

m.y. (Percious and others, 1967), but a recalculated age, 

using the decay constants of Steiger and Jaeger (1977) 

showed the rock to be 49.2 ±1.4 m.y. old. Dacite porphyry 

and rhyolite porphyry are the more common types of dike 

rock; rhyolite porphyry is particularly common along the 

western side of the northeast-striking dike swarm. Many 

of the dacitic dikes are similar in composition to the Tertiary stocks and appear to be offshoots of those larger 

masses.

The dikes and stocks are aligned along the transChallis fault system, which extends northeast across the 

Boise Basin. The westernmost fault of the system projects 

into the vicinity of Quartzburg, where Ballard (1924, 

p. 53) described a broad zone of shearing 0.5 mile or more 

wide, which is believed to be part of the system. Large, 

more-or-less parallel, northeast-trending faults occur near 

Pioneerville, Centerville, and Idaho City (fig. M5), and 

some of them are mentioned by Lindgren (1898, p. 667) 

and Anderson (1947, p. 168). All of the faults are 

southwestern extensions of the broad trans-Challis fault 

system described by Kiilsgaard and Lewis (chap. B, this 

volume).

Mineral Deposits

Veins at 78 mines and prospects in the Boise Basin 

are shown in figure M5. Anderson (1947, p. 180-191) 

divided these veins into two groups, early Tertiary goldquartz deposits, most of which occupy fissure and fracture zones trending west-northwest, and Miocene gold 

deposits confined to the belt of porphyry dikes. Ballard 

(1924, p. 40-42) described the relation of quartz veins to 

dikes and regional shearing and noted that the more 

productive lode deposits form at the intersections of shear 

zones and rhyolite dikes. In mines that Ballard examined,

160 Challis 1°x2° Quadrangle, Idaho
115°

EXPLANATION

PINK GRANITE (EOCENE) 

DIORITE COMPLEX (EOCENE)

CONTACT Dashed where approximately 

located

REGIONAL FAULT Dashed where approximately located; bar and ball on downthrown side

MINE OR PROSPECT

Stanley

Approximate 

boundary of 

The Ten Mile 

West Roadless 

Area

'Approximate 

boundary of 

the Sawtooth 

Wilderness

10 MILES

Atlanta x> 

\

v * 

\

10 KILOMETERS

Figure M4. Map of the Sawtooth Wilderness and Ten Mile West Roadless Area, showing location of mines and prospects with 

respect to the Montezuma and Bear River faults.

the shear zones were well defined within hydrothermally 

altered granitic rock, but at the more brittle, acidic dikes, 

the shearing tended to become countless seams and fractures, all of which made the dike rock more permeable 

to subsequent mineralizing solutions (Ballard, 1924, 

p. 42). Ore bodies mined in the rhyolitic dikes tended to 

pinch out in the enclosing granitic rock. Some ore bodies 

in rhyolitic dikes were found to be cut by still younger

rhyolite dikes, which indicates a close relationship between 

the age of mineralization and the age of dike intrusion. 

Gold was the principal metal mined from the Boise 

Basin lode deposits, but minor amounts of other metals 

also were produced. Deposits of lead-bismuth sulfides 

were found along the Quartzburg mineral zone as far 

northeast as Mineral Hill, according to Ballard (1924, 

p. 47), as was argentiferous galena, and lesser amounts of

Mineral Deposits in the Atlanta Lobe of the Idaho Batholith 161
CHALLIS QUADRANGLE

HAILEY QUADRANGLE /

EXPLANATION

EOCENE GRANITIC ROCKS

EOCENE DIORITIC STOCKS

TERTIARY DIKES

CONTACT

REGIONAL FAULTS Dashed 

where approximately located;

bar and ball on downthrow i 

side

GOLD-SILVER VEINS

Figure M5. Map of Boise Basin showing location and strike of gold-silver veins with respect to regional faults of the trans-Challis 

fault system, stocks of dioritic rocks, granitic rocks, and Tertiary dikes. Modified from Anderson (1947).

sphalerite, chalcopyrite, and tetrahedrite. Anderson (1947, in the late 1970's and early 1980's by AMAX Corp. At

p. 191) noted that at several deposits silver was the most that property, veinlets and disseminations of molybdenite

valuable metal. occur in rhyolitic dikes, although at least one rhyolite dike

The Cumo deposit was explored for molybdenum cuts across the mineralized body. The Little Falls prospect

162 Challis 1°x2° Quadrangle, Idaho
(fig. M5) on the South Fork Payette River is another 

molybdenum deposit at which stockwork veinlets of molybdenite cut across rhyolite dikes. Zircon from a rhyolite 

dike at the Little Falls deposit gave a fission-track age of 

29.3 ±1.7 m.y (F. S. Fisher, oral commun., 1983).

Accumulated field evidence indicates that vein deposits of the Boise Basin are in fractures related to regional 

northeast-trending faults. The mineralized deposits are in 

dikes but in some places are cut by dikes, thereby indicating 

a close genetic relationship. If the zircon dating at the Little Falls deposit may be taken as a measure, the deposits 

are younger than 29 m.y., or no older than Miocene, an 

age proposed by Ballard (1924, p. 41).

GOLD-SILVER DEPOSITS ALONG OR NEAR 

THE TRANS-CHALLIS FAULT SYSTEM

The broad zone of northeast-trending regional faults 

described by Kiilsgaard and Lewis (chap. B, this volume) 

as the trans-Challis fault system, a name proposed by Earl 

Bennett, exerts a control on the location of many goldsilver deposits within or near the Challis quadrangle (fig. 

M6). The previously described Boise Basin deposits are 

within the fault system. The Banner mine (fig. M6, no. 28), 

east of the Boise Basin district and within the trans-Challis

fault system, produced silver ore valued at about $3,000,000. 

Anderson and Rasor (1934, p. 372) described the northeast-striking Banner veins as accompanied by porphyry 

dikes. They noted that one of the veins passes through a 

rhyolite dike, but a lamprophyre dike is in a fissure with 

the Banner vein and is altered but not mineralized. The 

unmineralized character of the lamprophyre dike indicates 

that it was intruded shortly after vein formation, soon 

enough to be altered by thermal solutions passing through 

the mineralized zones. Granodiorite host rock near the 

Banner veins is intensely altered and thoroughly sericitized. 

The Miller Mountain gold mine (fig. M6, no. 23) in 

sec. 17, T. 9 N., R. 8 E., about 8 km northeast of Lowman, 

was the only lode mine producing gold in the southwestern 

quarter of the Challis quadrangle in 1978-80. A limited 

tonnage of ore was being mined by A. W. Josue, Lowman, 

Idaho, and was processed through a small stamp mill. The 

mineralized material was highly altered granodiorite of 

the Idaho batholith in which pyrite was the only sulfide 

mineral seen. The mineralized zone extends along the hanging wall of an andesite dike that strikes N. 80° E., and fault 

gouge on the dike hanging wall suggests that a fault older 

than the dike guided emplacement of the dike and the gold 

mineralization. A prospect 1.2 km west appears to be the 

strike extension of the Miller Mountain gold deposit.

FAULT Bar and ball on

downthrown side 

MINE OR PROSPECT

Yellowjacket

Rabbit Foot

Singheiser

Parker Mountain

Lost Packer

McFadden

Montana

Golden Sunbeam

Charles Dickens

General Custer

Lucky Boy

Morrison

Red Mountain

Valley Creek

Buckskin

Blue Jay

Copper Mountain

Golden Day

Iron Crown

Mountain Girl

Prospect

Idaho Birthday 

Miller Mountain 

Specimen 

Prospect 

Prospect 

Eno Prospect 

Banner 

Little Falls 

Hole in the Wall 

Sloper

Golden Cycle 

Golden Chariot 

Mammoth 

CUMO 

Golden Age 

Come Back 

Missouri

PANTHER CREEK GRABEN

TWIN PEAKS CALDERA

Figure M6. Map showing the location of mines and prospects with respect to the trans-Challis fault system, Challis quadrangle.

Mineral Deposits in the Atlanta Lobe of the Idaho Batholith 163
The Specimen gold mine (fig. M6, no. 24), about 

2.4 km southwest of the Miller Mountain gold mine, is 

in highly altered granodiorite of the Idaho batholith. An 

abandoned mill at the mine site processed gold ore from 

the mine, but the amount of gold produced is not known, 

and the mine workings were inaccessible in 1979.

The Branson mine, formerly known as the Idaho 

Birthday mine (fig. M6, no. 22), is south of the mouth 

of Eightmile Creek, about 19 km east of Lowman. Gold 

production from the mine reportedly was stopped in 1942 

by War Production Board Order L-208, and the mine has 

been nonproductive since that time. At least three moreor-less parallel quartz veins crop out at the mine. The veins 

have been explored by several short adits, the northern 

one about 366 m higher in elevation than the southern 

one. A gold-bearing vein at an altitude of 1,585 m is intersected by three andesite dikes and is offset by two 

parallel northeast-trending faults. Pyrite was the only 

sulfide mineral seen in the faulted vein, two samples from 

which contained 7 and 5 ppm gold, respectively. A vein 

intersected in the No. 8 adit, at altitude 1,451 m, strikes 

N. 53° E., dips 64° NW., and follows the hanging wall 

of a large northeast-trending fault. The Branson mine is 

between two major strands of the trans-Challis fault 

system, one of which extends along Eightmile Creek and 

the other along East Fork Eightmile Creek. Adjustment 

along these faults appears to have created fractures in the 

granodiorite that subsequently were mineralized.

The Lost Packer gold-copper deposit (fig. M6, 

no. 5) is near the trans-Challis fault system. Umpleby 

(1913, p. 42-45) described the northeast-striking Lost 

Packer vein as having been explored for about 2,000 ft 

(feet) along strike and 1,000 ft along dip. The vein follows 

a well-defined fault, and it cuts across what Umpleby 

classed as granite dikes but which currently are recognized 

as Tertiary dikes, similar in characteristics to those in Boise 

Basin and elsewhere along the trans-Challis fault system. 

The vein-dike-fault relationship at the Lost Packer mine 

is similar to other relationships along the trans-Challis 

fault system.

Several productive gold-silver mines are in the 

Custer graben, a structure considered to be part of the 

trans-Challis fault system. Some of these deposits are 

described by Mclntyre and Johnson (chap. I, this volume) 

and others by Umpleby (1913, p. 80-90) and Anderson 

(1949, p. 21-27). Gold-silver deposits in the Twin Peaks 

caldera, northeast of the Custer graben, are described by 

Hardyman (chap. G, this volume).

The Singheiser, Rabbit Foot, and Yellowjacket gold 

deposits (fig. M6, nos. 3, 2, and 1) in the northeastern 

part of the Challis quadrangle are within or near the transChallis fault system. The Leesburg gold district, which 

is about 31 km northeast of the Challis quadrangle, is 

directly on strike with the Panther Creek graben 

(fig. M6), and the gold-bearing veins in the district, like 

those previously discussed, are within the probable

northeastern continuation of the trans-Challis fault 

system.

Gold-silver deposits from Boise Basin to Leesburg 

occur within or near the trans-Challis fault system. Most 

of the deposits have similar characteristics. They are Tertiary in age and either follow or cut across Tertiary dikes 

or other intrusive bodies. The mineralized deposits occupy fissures that are related to regional faults. Gold and 

silver are the metals of principal value in the deposits. 

These features suggest that the gold-silver deposits are 

products of mineralizing activity that was regionally controlled by adjustment along the trans-Challis fault system.

REFERENCES CITED

Anderson, H. L., 1939, Geology and ore deposits of the Atlanta district, Elmore County, Idaho: Idaho Bureau of Mines 

and Geology Pamphlet 49, 71 p.

_____1947, Geology and ore deposits of Boise Basin, Idaho: 

US. Geological Survey Bulletin 944-C, 319 p.

_____1949, Silver-gold deposits of the Yankee Fork district, 

Custer County, Idaho: Idaho Bureau of Mines and Geology 

Pamphlet 83, 37 p.

_____1951, Metallogenic epochs in Idaho: Economic

Geology, v. 46, no. 6, p. 592-607. 

Anderson, A. L., and Rasor, A. C., 1934, Silver mineralization

in the Banner district, Boise County, Idaho: Economic

Geology, v. 29, no. 4, p. 371-387. 

Armstrong, R. L., 1975, The geochronometry of Idaho:

Isochron/West, no. 14, 51 p. 

Ballard, S. M., 1924, Geology and gold resources of the Boise

Basin, Boise County, Idaho: Idaho Bureau of Mines and

Geology Bulletin 9, 103 p.

_____1928, Geology and ore deposits of the Rocky Bar quadrangle: Idaho Bureau of Mines and Geology Pamphlet 26,

41 p. 

Bennett, E. H., 1980, Granitic rocks of Tertiary age in the Idaho

batholith and their relation to mineralization: Economic

Geology, v. 75, no. 2, p. 278-288. 

Criss, R. E., and Taylor, H. P., Jr., 1983, An 18O/ 16O and D/H

study of Tertiary hydrothermal systems in the southern half

of the Idaho batholith: Geological Society of America

Bulletin, v. 94, no. 5, p. 640-663. 

Fisher, F. S., Mclntyre, D. H., and Johnson, K. M., 1983,

Geologic map of the Challis l°x 2° quadrangle, Idaho: US.

Geological Survey Open-File Report 83-523, 41 p., 2 maps,

scale 1:250,000. 

Kiilsgaard, T. H., 1982, Analytical determinations from samples

taken in the Ten Mile West Roadless Area, Boise and

Elmore Counties, Idaho: U.S. Geological Survey Open-File

Report 82-1099, 34 p. 

_____1983a, Geologic map of the Ten Mile West Roadless

Area, Boise and Elmore Counties, Idaho: U.S. Geological

Survey Miscellaneous Field Studies Map MF-1500-A, scale

1:62,500. 

_____1983b, Geochemical map of the Ten Mile West

Roadless Area, Boise and Elmore Counties, Idaho: US.

Geological Survey Miscellaneous Field Studies Map

MF-1500-B, scale 1:62,500.

164 Challis 1°x2° Quadrangle, Idaho
Kiilsgaard, T. H., Benham, J. R., and Avery, D. W., 1983, Mineral 

resource potential map of the Ten Mile West Roadless Area, 

Boise and Elmore Counties, Idaho: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1500-C, scale 

1:62,500.

Kiilsgaard, T. H., Freeman, V. L., and Coffman, J. S., 1970, 

Mineral resources of the Sawtooth Primitive Area, Idaho: 

U.S. Geological Survey Bulletin 1319-D, 174 p.

Lindgren, Waldemar, 1898, The mining districts of the Idaho 

basin and the Boise Ridge, Idaho: U.S. Geological Survey 

Eighteenth Annual Report, pt. 3, p. 617-719.

Mabey, D. R., in press, Gravity and magnetic anomalies in the 

Ten Mile West Roadless Area, Boise and Elmore Counties, 

Idaho: U.S. Geological Survey Miscellaneous Field Studies 

Map MF-1500-D.

Percious, J. K., Damon, P. E., and Olson, H. J., 1967, Radiometric 

dating of Idaho batholith porphyries: U.S. Atomic Energy 

Commission Annual Progress Report No. COO-689-76, Appendix A-X.

Reid, R. R., 1963, Reconnaissance geology of the Sawtooth Range: 

Idaho Bureau of Mines and Geology Pamphlet 129, 37 p.

Ross, C. P., 1934, Some lode deposits in the northwestern part 

of the Boise Basin, Idaho: U.S. Geological Survey Bulletin 

846-D, p. 239-277.

Steiger, R. H., and Jaeger, E., compilers, 1977, Subcommission 

on geochronology Convention on the use of decay constants in geo- and cosmochronology: Earth and Planetary 

Science Letters, v. 36, no. 3, p. 359-362.

Umpleby, J. B., 1913, Some ore deposits in northwestern Custer 

County, Idaho: U.S. Geological Survey Bulletin 539, 104 p.

Mineral Deposits in the Atlanta Lobe of the Idaho Batholith 165

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter N

Rhyolite Intrusions and Associated Mineral 

Deposits in the Challis Volcanic Field, 

Challis Quadrangle

By R. F. HARDYMAN and FREDERICK S. FISHER

CONTENTS

Abstract 168

Introduction 168

Structural setting 168

Ages 168

Compositions 169

Field characteristics 172

Fabrics and internal textures 173

Alteration and mineralization 174

Conclusions 176

References cited 177

FIGURES

Nl. Index map showing principal rhyolite intrusions 169

N2. SiO2 K2O variation diagram showing classification of rhyolite samples 172 

N3-N6. Photographs showing:

N3. Younger porphyritic rhyolite cutting older devitrified to perlitic 173

N4. Glassy to perlitic rhyolite in part intrusive into bedded sediments 173

N5. Breccia fabrics in rhyolite 174

N6. Intrusive rhyolite breccia resembling a pebble dike 175

N7. Photomicrograph showing blotchy texture of matrix of porphyritic rhyolite 175 

N8. Photomicrograph showing micromyrmekitic texture in matrix of rhyolite intrusion 176 

N9. Geologic map of the Snowshoe Creek area 176 

N10. Maps showing sample localities 177

TABLE 

Nl. Analyses of intrusive rhyolites of the Van Horn Peak cauldron complex 170

Rhyolite Intrusions, Challis Volcanic Field 167
Abstract

Intrusion and local extrusion of rhyolite magma within the 

Van Horn Peak cauldron complex and associated Custer and 

Panther Creek grabens spanned the entire time (about 48 to 

45 m.y. (million years)) of pyroclastic volcanism within the 

Challis volcanic field. The intrusive rhyolite bodies are similar 

in chemical composition to many of the Tertiary granite bodies 

in the Challis quadrangle and to the youngest ash-flow tuffs of 

the volcanic field, which they intrude. The rhyolites were 

emplaced at high crustal levels within the volcanic pile and occur primarily as a belt of intrusive bodies that coincide with 

the regionally extensive, northeast-trending trans-Challis fault 

system. This system includes a series of subparallel high-angle 

faults that extends northeast from the Idaho batholith terrane 

in the southwestern part of the Challis quadrangle to the Van 

Horn Peak cauldron complex in the northeast. The Custer and 

Panther Creek grabens, extending southwest and northeast, 

respectively, from the cauldron complex, are also part of the 

trans-Challis fault system. Dikes, small pods and pipelike bodies 

of rhyolite, and multiple intrusive rhyolitic complexes are scattered through the grabens, and this belt of rhyolites extends 

across the cauldron complex between these two structures.

Locally, rhyolites within the Custer and Panther Creek 

grabens and the rhyolite belt of the cauldron complex are 

hydrothermally altered and mineralized. Alteration of these 

rocks ranges from weakly argillic through moderately strongly 

quartz sericitic, to strongly silicic. Disseminated pyrite locally 

is common in unfaulted altered rhyolite, and fine-grained 

granular pyrite locally is concentrated along structures in some 

faulted rhyolite bodies. Fluorite, calcite, and quartz are common in vugs and veins in brecciated rhyolite and in altered 

country rock.

Epithermal precious-metal mineral occurrences are spatially associated with several rhyolite bodies within the rhyolite belt. 

The known precious-metal occurrences include deposits at the 

Rabbit Foot and Singheiser mines in the Panther Creek graben 

and deposits and prospects in the Custer graben, including 

those at Estes Mountain, the Sunbeam mine, and Red Mountain. No precious-metal deposits or prospects were known 

within or associated with rhyolites of the rhyolite belt within 

the Van Horn Peak cauldron complex between the Panther 

Creek and Custer grabens. Reconnaissance geologic mapping 

and geochemical sampling of rhyolites in this intracauldron 

region, however, indicate that some of these rhyolites may have 

potential for spatially related epithermal mineral deposits.

INTRODUCTION

Intrusive rhyolitic rocks are locally abundant within 

the Challis quadrangle. They are particularly numerous 

within the volcanotectonic structures of the eastern part 

of the quadrangle where the thickest accumulations of 

pyroclastic rocks of the Challis volcanic field occur. 

Rhyolites are present within the Custer graben (fig. Nl) 

(Mclntyre and Johnson, chap. I, this volume) and within 

the Panther Creek graben in the northeastern part of the 

quadrangle. Precious-metal deposits spatially associated 

with some of the rhyolites within these two areas have been

exploited in the past, and mineral exploration in these 

areas continues to the present. Geologic mapping in the 

Van Horn Peak cauldron complex and the Twin Peaks 

caldera has delineated a belt of previously unrecognized 

intrusive rhyolite bodies that lies between the Custer and 

Panther Creek grabens. This chapter summarizes the 

preliminary results of studies of chemical composition, 

petrography, and field characteristics of these rhyolitic 

rocks, along with preliminary studies of local hydrothermal alteration and mineralization. Field relationships and 

alteration and mineralization of rhyolites in the Twin 

Peaks caldera are further discussed by Hardyman (chap. 

G, this volume).

STRUCTURAL SETTING

Rhyolitic rocks that postdate cauldron-related 

pyroclastic rocks of the Challis volcanic field crop out in 

a northeast-trending belt that extends from the Stanley 

Basin area to the south, northeastward to near the northeastern corner of the Challis quadrangle (fig. Nl). This 

so-called rhyolite belt conforms to a regional northeast-trending series of faults named the trans-Challis fault 

system (Kiilsgaard and Lewis, chap. B, this volume). Dike 

rocks of similar rhyolitic compositions intrude Idaho 

batholith rocks along this fault zone southwest from the 

Stanley Basin and, southwest of the Challis quadrangle, 

trend into the Boise Basin dike swarm that extends northeast from the western Snake River Plain.

Rhyolites northeast of Stanley Basin occur as narrow dikes and small plugs and domes (less than 2.5 sq 

km (square kilometers)) scattered throughout the northeast-trending Custer graben, across the Van Horn Peak 

cauldron complex, and into the Panther Creek graben. 

One large rhyolitic complex is exposed over an area of approximately 129 sq km in the upper East Fork Mayfield 

Creek region (fig. Nl). This areally extensive unit, the 

rhyolitic complex of East Fork Mayfield Creek, is at the 

triple juncture of the Custer graben, the Van Horn Peak 

cauldron complex boundary, and the Twin Peaks caldera 

ring fracture, and it almost completely engulfs the area 

interpreted to be the earliest collapse segment of the Twin 

Peaks caldera.

AGES

Intrusion and local extrusion of rhyolite magma occurred during the entire time (about 48-45 m.y.) of 

caldera-forming pyroclastic eruptions within the Challis 

volcanic field. Within the Custer graben-Van Horn Peak 

cauldron complex-Panther Creek graben area, rhyolite 

flows in the northeastern part of the Custer graben predate 

the tuff of Eightmile Creek (46.9 ± 1.6 m.y., Mclntyre and 

others, 1982) and mark an early episode of rhyolitic 

magmatism. These flows, the rhyolite flows of Mill Creek

168 Challis 1°x2° Quadrangle, Idaho
116° 115° 114°

EFMC

RM 

SB 

EM 

SC 

SH 

RF 

CR 

CCK

A 

X

EXPLANATION

RHYOLITE INTRUSIVE BODIES

FAULT Bar and ball on downthrown

side, where known 

RHYOLITE OF EAST FORK MAYFIELD

CREEK

RED MOUNTAIN 

SUNBEAM MINE 

ESTES MOUNTAIN 

SNOWSHOE CREEK 

SINGHEISER MINE 

RABBIT FOOT MINE 

CASTLE ROCK CAULDRON SEGMENT 

CORRAL CREEK CAULDRON

SEGMENT

LOCATION OF FIGURE N3 

LOCATION OF FIGURE N4

20 MILES

20 KILOMETERS

Figure N1. Index map of the Challis quadrangle showing the trans-Challis fault system, cauldron complexes, grabens, and outcrops of the principal intrusions of the rhyolite belt.

Summit (Hobbs and others, 1975), are dated at 48.5 ±1.2 

m.y. (Marvin and Dobson, 1979, p. 17).

A rhyolite flow or possible sill that correlates with 

a younger generation of rhyolites within the Custer graben 

and is exposed on upper Tenmile Creek about 10 km west 

of Mill Creek Summit yielded a date of 46.5 ±1.7 m.y. (R. 

F. Marvin, written commun., 1984). In this same area a 

rhyolite dike cuts an intrusive rhyodacite body dated at 

46.0±1.7 m.y. (R.F. Marvin, written commun., 1983). Rhyolites elsewhere in the Custer graben, such as at Mt. Greylock and the Sunbeam mine, are 45-47 m.y. (Mclntyre and 

Johnson, chap. I, this volume). The youngest rhyolite intrusion associated with the rhyolite of Red Mountain in 

the southwestern part of the Custer graben is 39.1+2.0 

m.y. (C. W. Naeser, written commun., 1982).

At least two discrete intrusive events can be demonstrated by field relations within the rhyolitic complex of 

East Fork Mayfield Creek. This rhyolitic complex is composed of multiple rhyolite intrusions that in part must be 

younger than about 45 m.y., as rhyolite intrudes outflow 

tuff of the tuff of Challis Creek. Likewise, rhyolite dikes 

and plugs to the northeast in the Van Horn Peak cauldron 

complex intrude all of the intracauldron ash-flow-tuff 

units, including the youngest tuff, the tuff of Challis 

Creek. Intrusive rhyolite is abundant in the northern half

of the Twin Peaks caldera (the youngest caldera of the Van 

Horn Peak cauldron complex), and here, too, these rocks 

postdate the tuff of Challis Creek and younger calderawall slump-debris deposits.

COMPOSITIONS

The intrusive rhyolites of this study are mostly porphyritic-aphanitic rocks containing generally less than 10 

percent phenocrysts of sanidineand quartz. Sanidine phenocrystsareas large as 1 cm (centimeter) in places, but phenocrysts are typically less than 2 mm (millimeters) in size. 

Some rhyolites contain phenocrysts of sanidine only, some 

contain sanidine and minor plagioclase only, and a few 

dike rocks contain sanidine, quartz, and minor plagioclase 

and biotite. Augite is present in some places, especially 

in some vitrophyric rhyolites, but hornblende is conspicuously rare. Aphyric varieties of rhyolite, containing only 

traces of phenocrystic sanidine and (or) quartz, are locally 

common. Within some intrusions and along some intrusion contacts, aphyric rhyolite grades into phenocrystbearing rhyolite.

Major-element-oxide analyses of selected samples 

of intrusive rhyolites, primarily from the northeastern end 

of the Custer graben and the East Fork Mayfield Creek 

are shown in table Nl. Regardless of their phenocryst

Rhyolite Intrusions, Challis Volcanic Field 169
Table N1. Analyses of intrusive rhyolites of 

[Major-element analyses by X-ray spectroscopy by A. J. Bartel, K. Stewart, J. E. Taggart, J. W. Baker, J. S. Wahlberg; FeO, H2O+ ,

Analysis No.- 1 

Field No. RH-CH-845 

Lab No. D-245296 

N. latitude 44°32' 

W. longitude- 114°39'

OlU o~~~ "~~ « / Q

A12°3 10

* 62U3 l 

FeO 

(J0Q__.. ________

J^-J ()_________._,,__! ____ .._, 2

H

n"*"

u n"" « 

T-t r\ ________

P2°5 

MnO 

Sum 98

CIPW

or 26 

££)________ _-.__ 2 4

di-en 

di-fs 

hy-en 

hv f s~ 

ol fo~ 

ol-fa 

nm *~ - i

ap 

Phenocryst e 

mineralogy.

.80 

.20 

.44 

.04 

.00 

.17 

.85 

.43 

.48 

.36 

.09 

.00 

.00 

.00

.86

norms

.83 

.42 

.72 

.61

.86 

.00 

.00 

.00

.00 

.00 

.00 

.00

.00 

.47 

.09 

.00 

.00

>q

2 3 

RH-CH-848 RH-CH-851 

D-245297 D-245298 

44°31- 44°31' 

114°39'50" 114°40'

76.90 

12.40 

.91 

.54 

.00 

.13 

3.71 

4.74 

.56 

.10 

.09 

.00 

.00 

.00

100.08

(weight

37.08 

.94 

28.17 

31.58

.65

.00 

.00 

.00

.00 

.09 

.00 

.00

1.33 

.00 

.17 

.00 

.00

s,q

76.20 

12.70 

1.41 

.09 

.00 

.16 

3.81 

4.72 

.41 

.05 

.08 

.00 

.00 

.00

99.63

4 

RH-CH-867 

D-245304 

44°29'30" 

114°31'30

Major

78.90 

10.70 

.83 

.09 

.14 

.53 

2.46 

4.29 

.83 

.10 

.08 

.00 

.00 

.00

98.95

5 67 8 9 10 

RH-CH-881 RH-CH-886 RH-CH-891 RH-CH-894 RH-CH-901 RH-CH-927 

D-245310 D-245313 D-245314 D-245316 D-247772 D-247778 

44°35' 44°35'30" 44°30' 44°30' 44°29' 44°30' 

" 114°42'30" 14°40' 114°37'30" 114°37' 114°39' 114°42'30"

oxides (weight

75.20 

12.50 

1.21 

.11 

.22 

.25 

2.57 

5.16 

.95 

.32 

.10 

.00 

.00 

.04

98.63

percent; calculated using major oxide

35.93 

1.04 

28.13 

32.51

.80 

.00 

.00 

.00

.00 

.00 

.00 

.00

.06 

1.38 

.15 

.00 

.00

s

47.78 

1.07 

25.86 

21.24

2.68 

.00 

.00 

.00

.36

.00 

.00 

.00

.06 

.81 

.16 

.00 

.00

Aphyric

40.83 

2.39 

31.32 

22.34

1.01 

.00 

.00 

.00

.56

.00 

.00 

.00

.07 

1.20 

.20 

.00 

.09

s,p, s 

b(altered) 

h( trace, 

altered).

percent)

72.60 

14.00 

1.17 

.08 

.10 

.81 

3.74 

5.05 

.44 

.15 

.23 

.00 

.00 

.00

98.37

analyses

30.35 

.93 

30.56 

32.41

4.12 

.00 

.00 

.00

.26 

.00 

.00 

.00

.00 

1.20 

.17 

.00 

.00

>q>p> 

b

(trace).

73.90 

13.20 

1.34 

.12 

.00 

.40 

3.83 

5.29 

.49 

.00 

.17 

.00 

.00 

.00

98.74

normalized

31.08 

.45 

31.83 

33.00

2.02 

.00 

.00 

.00

.00 

.00 

.00 

.00

.00 

1.36 

.26 

.00 

.00

s,

py

(trace).

79.20 

10.10 

1.39 

.03 

.00 

.07 

3.24 

4.00 

.46 

.08 

.07 

.00 

.00 

.00

98.64

to 100

45.78 

.32 

24.10 

27.96

.35

.00 

.00 

.00

.00 

.00 

.00 

.00

.00 

1.42 

.06 

.00 

.00

s,q

77.90 

11.10 

1.32 

.12 

.00 

.13 

3.53 

4.56 

.22 

.20 

.10 

.00 

.00 

.00

99.18

percent H^O

40.14 

.12 

27.28 

30.24

.65

.00 

.00 

.00

.00 

.00 

.00 

.00

.10 

1.27 

.19 

.00 

.00

s,q

80.50 

10.30 

1.25 

.10 

.00 

.07 

3.30 

4.17 

.45 

.10 

.10 

.00 

.00 

.00

100.34

free)

45.29 

.23 

24.69 

27.98

.35

.00 

.00 

.00

.00 

.00 

.00 

.00

.03 

1.23 

.19 

.00 

.00

s,q

SAMPLE

1. Autobrecciated porphyritic rhyolite, rhyolitic complex of East Fork Mayfield Creek; west of Sherman Peak.

2. Microvesicular porphyritic rhyolite, rhyolitic complex of East Fork Mayfield Creek; west of Sherman Peak; quartz-lined flattened gas cavities.

3. Porphyritic intrusive rhyolite, rhyolitic complex of East Fork Mayfield Creek; contains numerous diktytaxiticlike cavities; East Fork Mayfield Creek.

4. Aphyric rhyolite intrusion; junction of McKay Creek and Yankee Fork, northeastern part of Custer graben.

5. Porphyritic intrusive rhyolite, locally autobrecciated and spherulitic, some hairline quartz veinlets, clay-chlorite alteration; Ibex Creek.

6. Porphyritic intrusive rhyolite; south of Mid Cottonwood Point lookout.

7. Faintly flow-foliated, porphyritic intrusive rhyolite with minor miarolitic cavities; Twelvemile Creek, northeastern part of Custer graben.

8. Spherulitic, locally autobrecciated porphyritic intrusive rhyolite with opal-lined gas cavities; ridge east of Twelvemile Creek, northeastern part of Custer graben.

9. Crystal-poor, porphyritic rhyolite lava or possible sill with autobrecciated base; ridge between Tenmile and Eightmile Creeks; northeastern part of Custer graben

This rhyolite dated at 46.5 ±1.7 Ma (see text). 

10. Massive, locally flow-laminated or autobrecciated, porphyritic rhyolite, silicified, with tridymite and opal in matrix; rhyolitic complex of East Fork Mayfield Creek,

uppermost East Fork Mayfield Creek.

170 Challis 1°x2° Quadrangle, Idaho
of the Van Horn Peak cauldron complex area.

H2O~, CO2 analyses by H. C. Nieman and C. Mason, s, sanidine; q, quartz; p, plagioclase; b, biotite; h, hornblende; py, pyroxene]

11

RH-CH-931

D-247779 

44°28'30" 

114°41'30"

80.10

10.30

.19

.12

.00

.12

2.50

5.14

.68

.00

.10

.00

.00

.00

99.25

C1PW norms

46.29

.41

30.81

21.46

.60

.00

.00

.00

.00

.00

.00

.00

.10

.12

.19

.00

.00

s,q

12

RH-CH-932

D-247780 44°29' 

114°41'30

75.80

11.90

1.81

.18

.12

.07

3.88

4.90

.44

.03

.13

.00

.00

.00

99.26

(weight

34.57

.09

29.31

33.23

.35

.00

.00

.00

.30

.00

.00

.00

.21

1.69

.25

.00

.00

s,q

13

RH-CH-932A

D-247781 44°29' 

" 114°41'30"

72.00

11.80

.67

.90

.10

.39

2.76

5.82

4.20

.16

.12

.00

.00

.00

98.92

14

RH-CH-933

D-247782 44°29' 

114°41'30'

Major

70.90

12.20

1.41

.40

.19

.72

2.10

4.44

5.42

1.14

.12

.00

.03

.00

99.07

15

RH-CH-955

D-247786 44°38' 

1 114°49'

16 17 18 19

RH-CH-958 RH-CH-960 RH-CH-2035 RH-CH-885

D-247788 D-247789 D-254774 D-245312 

44°38' 44°38' 44°45' 44°34' 

114°53' 114°53' 114°45' 114°42'30"

20

oxides (weight percent)

74.80

12.40

1.47

.03

.00

.29

3.36

5.41

.47

.04

.15

.00

.00

.00

98.42

69.00 74.20 74.60 75.30

12.50 13.80 12.90 12.40

1.38 .57 .55 1.54

.34 .13 .74 .49

.11 .10 .24 .00

.72 .59 .78 .13

3.03 3.18 3.13 3.65

5.11 6.26 5.24 4.83

4.40 .49 .29 .60

1.99 .06 .03 .03

.20 .10 .14 .14

.00 .00 .06 .00

.03 .00 .00 .00

.00 .00 .00 .01

98.81 99.48 98.70 99.12

percent; calculated using major oxide analyses normalized to 100 percent K^O

34.14

.27

36.37

24.70

2.05

.00

.00

.00

.26

.95

.00

.00

1.03

.00

.24

.00

.00

Aphyric

43.10

2.84

28.36

19.21

3.86

.00

.00

.00

.51

.00

.00

.00

1.12

.75

.25

.00

.00

s,q

34.70

.50

32.69

29.07

1.47

.00

.00

.00

.00

.00

.00

.00

.00

1.50

.06

.00

.00

s

32.58 30.66 34.74 35.88

.73 .73 .82 .97

32.67 37.39 31.47 28.98

27.74 27.20 26.92 31.36

3.86 2.96 3.53 .59

.00 .00 .00 .00

.00 .00 .00 .00

.00 .00 .00 .00

.30 .25 .61 .00

.00 .00 .68 .00

.00 .00 .00 .00

.00 .00 .00 .00

.66 .13 .81 1.19

1.03 .49 .00 .74

.41 .19 .27 .27

.00 .00 .14 .00

.00 .00 .00 .02

s, s,p Not s,q 

p(trace), b(oxi- availpy(altered). dized) . able.

75.33

12.08

1.43

.18

.08

.26

3.08

5.10

.51

.47

.13

.00

.01

.00

98.66

free)

38.11

1.04

30.85

26.68

1.32

.00

.00

.00

.20

.00

.00

.00

.24

1.30

.25

.00

.00

s,q

DESCRIPTIONS

11. Massive and locally flow-banded intrusive rhyolite, silicified; rhyolitic complex of East Fork Mayfield Creek; uppermost East Fork Mayfield Creek.

12. Younger porphyritic rhyolite dike, vertically flow laminated with stretched vesicles, vitrophyric at margins; rhyolitic complex of East Fork Mayfield Creek, uppermost East Fork Mayfield Creek.

13. Vitrophyre of 12 above.

14. Older, glassy to perlitic, porphyritic rhyolite intruded by 12 and 13 above.

15. Flow-laminated, porphyritic rhyolite grading to vitrophyric autobrecciated rhyolite; ridge east of Grouse Creek Peak.

16. Glassy and perlitic, flow-laminated porphyritic rhyolite; west of Grouse Creek Peak.

17. Devitrified porphyritic rhyolite, vitrophyric along intrusive contact; northeast of Pinyon Peak.

18. Pink granite (Casto pluton-type granite); near Falconberry Ranch on Loon Creek.

19. Quartz porphyry intrusion with quartz, sanidine, and altered ferromagnesian phenocrysts in a granophyric matrix; Ibex Creek.

20. Tuff of Challis Creek (unit Tcr, fig. N9); average of 9 analyses of intracaldera tuffs of Twin Peaks caldera.

Rhyolite Intrusions, Challis Volcanic Field 171
mineralogy, these rocks are all high-potassium rhyolites 

(fig. N2, table Nl). Likewise, using the felsic-mafic ratio 

(SiO2 +K2O+Na2O/FeO+Fe2O3 +MgO+CaO) of Segerstrom and Young (1972), these rocks are alkali or extreme 

alkali rhyolites.

In addition to the seventeen rhyolite analyses, 

chemical analyses of a granite sample from the Casto 

pluton and of intracaldera tuff samples from the Twin 

Peaks caldera are shown in table Nl. These rocks are also 

classified as high-potassium rhyolite (granite) or alkali 

rhyolite (granite). An average of 10 analyses of Tertiary 

granite of the Sawtooth Range (E. H. Bennett, Idaho Geological Survey, unpub. data and written commun., 1983), 

together with the analyses of table Nl, are plotted on 

figure N2. The plotted intrusive rhyolite compositions 

show considerable scatter in this diagram, but, eliminating 

the analyses of vitrophyric or perlitic rhyolite samples and 

analyses of the obviously silicified and probably silicified 

samples (although not obvious in hand specimen or thin 

section), they cluster about the plotted compositions of 

the Tertiary granite samples and the pyroclastic-rock 

samples from the Twin Peaks caldera. These similarities 

in composition are probably to be expected because many

of these intrusive rhyolites are undoubtedly high-level 

equivalents of the Tertiary granite. Some intrusions may 

also represent residual rhyolitic magma emplaced after the 

explosive eruptions that formed the Twin Peaks caldera.

FIELD CHARACTERISTICS

Rhyolite bodies within the northeast-trending rhyolite belt appear to be intrusive, for the most part. Rarely 

can unequivocal flow origins be demonstrated by field 

relations. These rocks typically form linear dikes several 

meters wide or irregular pods and pipelike intrusive 

bodies. In outcrop these rocks are commonly massive to 

flow laminated. Steep to vertical flow foliation is common, and where flow folds are observed, their axial planes 

are invariably near vertical. Columnar joints, where 

developed, are typically subhorizontal. Contacts with 

older volcanic rocks are discordant and steep. Linear 

northeast-trending dikes and strings of dikelike bodies can 

be shown locally to have been emplaced along pre-existing 

faults, and not uncommonly postemplacement faulting 

has displaced the rhyolites.

EXPLANATION

 MICROCRYSTALLINE RHYOLITE 

I O VITROPHYRIC-PERLITIC RHYOLITE 

D SILICIFIED RHYOLITE

* AVERAGE OF 1 7 RHYOLITE 

ANALYSES (TABLE N1)

A AVERAGE OF 10 ANALYSES OF ( 

SAWTOOTH GRANITE 

BATHOLITH-From E.H. Bennett 

(unpub. data, 1984)

x GRANITE OF CASTO PLUTON

+ AVERAGE OF 9 ANALYSES OF 

TUFFOFCHALLIS CREEK

,17

HIGH-POTASSIUM 

DACITE

CALC- ALKALINE 

DACITE

LO W-POTASS UIM 

DACITE

CALC-ALKALINE RHYOLITE

LOW-POTASSIUM RHYOLITE

63 69 75 80

Si02 , IN WEIGHT PERCENT

Figure N2. SiO2-K2O variation diagram showing classification of rhyolite samples analyzed for this study.

172 Challis 1°x2° Quadrangle, Idaho
Figure N3. Younger porphyritic rhyolite intrusive bodies (dark 

outcrops) cutting older devitrified to perlitic rhyolite. Ridge between upper East Fork Mayfield Creek and Yankee Fork 

drainage. View to the west.

In some areas, especially in the large rhyolitic complex of East Fork Mayfield Creek, the rhyolite consists 

of multiple intrusive bodies. At least two intrusive events 

can be identified in exposures within the rhyolitic complex of East Fork Mayfield Creek in the ridge separating 

the upper East Fork Mayfield Creek from the Yankee Fork 

drainage. In this locality (fig. N3), narrow dikes and irregular bodies of light-gray, reddish-brown-weathering 

porphyritic rhyolite intrude an earlier, buff-devitrified to 

gray-perlitic phase of intrusive rhyolite.

The younger rhyolite phase (analysis 12, table Nl) 

is locally vitrophyric (analysis 13, table Nl) at contacts 

with the older perlitic rhyolite (analysis 14, table Nl). Away 

from its contacts, the younger phase shows vertical flow 

lamination and local autobrecciation and contains 

stretched gas cavities, some infilled with amorphous silica. 

The older rhyolite intrusive phase might be the eroded core 

of a bulbous dome that may have breached the surface.

Intrusive rhyolites in other localities within the 

rhyolitic complex of East Fork Mayfield Creek may have 

breached the ground surface, as suggested by local lenses 

of coarse epiclastic sediments, possibly representing domeapron deposits, isolated within intrusive rhyolite and adjacent to bulbous rhyolite intrusions. One such example, 

exposed southeast of the junction of Ibex Creek and East 

Fork Mayfield Creek (locality of fig. N4 on fig. Nl), consists of bedded, coarse-grained, tuffaceous sandstone 

grading up into coarse sedimentary breccia containing 

abundant porphyritic rhyolite pumice lapilli. Lenses of 

pyroclastic rhyolite, thin beds of cryptocrystalline silica, 

and thin rhyolite lava flows or sills are interbedded in this 

sedimentary section. These sedimentary rocks dip moderately to steeply away from a vitrophyric to perlitic, 

porphyritic rhyolite plug that in part intrudes the sedimentary rocks (fig. N4).

Other patches of coarse volcaniclastic sediments 

isolated within the rhyolitic complex of East Fork

Figure N4. Glassy to perlitic rhyolite (dark outcrops, left 

foreground) in part intrusive into bedded coarse epiclastic 

sediments. Rhyolite of East Fork Mayfield Creek near Ibex Creek. 

View to the southeast.

Mayfield Creek are not apparently related to the intrusive 

rhyolites but correlate with sedimentary rocks overlying 

the tuff of Challis Creek that infills the earliest collapse 

segment of the Twin Peaks caldera (Hardyman, chap. G, 

this volume).

FABRICS AND INTERNAL TEXTURES

Although they are generally massive or flow 

laminated, the intrusive rhyolites show a wide variety of 

fabrics in outcrop. Autobrecciated rhyolite is common, 

especially along intrusive contacts, but pods or pipelike 

bodies and irregular zones of breccia occur isolated within 

massive to flow-laminated rhyolite. Monolithologic autobreccia clasts range in size from less than 1 cm to 25 cm 

(fig. N5>4). Some breccias contain as much as 40 percent 

rounded to subangular fragments (ranging in size from 

less than 3 mm to about 0.3 m) cemented by interstitial 

rhyolite or, locally, amorphous silica (fig. N55).

Other intrusive rhyolite breccias, containing 

rounded aphyric or porphyritic rhyolite fragments in a 

phenocryst-bearing matrix, resemble pebble dikes (fig. 

N6).

Some pipelike breccia pods are perlitic to devitrified 

and are locally frothy and lithophysal and grade outward 

into rock that is like tuff breccia along their intrusive contacts. Some rhyolite breccias are pervasively silicified, and 

drusy cavities, some with infillings of later amorphous 

silica, are common, regardless of other textures or fabrics 

present. Miarolitic cavities, irregular diktytaxiticlike cavities, hairline quartz veinlets, and vein breccias are commonly present in massive to flow-laminated rhyolite bodies 

and in breccias.

Groundmass textures in the rhyolitic rocks are as 

varied as their macroscopic fabrics. Textures range from 

glassy or perlitic to microcrystalline, where grain sizes

Rhyolite Intrusions, Challis Volcanic Field 173
Figure N5. Breccia fabrics in rhyolites of the Challis volcanic 

field. A, brecciated rhyolite containing angular rhyolite clasts 

as large as 25 cm with interstitial devitrified rhyolite matrix or 

locally amorphous silica matrix. Pencil is 14 cm long. B, rhyolite 

breccia containing abundant rounded to subrounded, flowlaminated, porphyritic rhyolite clasts in a matrix of the same 

rhyolite.

rarely exceed 0.2 mm. Microcrystalline textures are locally pilotaxitic to felted, but allotriomorphic granular and 

typically blotchy or snowflake textures prevail (fig. N7). 

Micrographic to micromyrmekitic textures (fig. N8) commonly dominate the groundmass; spherulitic devitrification textures are also common.

ALTERATION AND MINERALIZATION

Rhyolites in the rhyolite belt are varicolored, in gray 

to light brown or pinkish red. Where hydrothermally 

altered, these rocks are bleached and weakly to intensely 

stained by limonite or hematite. Alteration of these rocks 

ranges from weakly argillic through moderately strongly 

quartz-sericitic to locally strongly silicic. Disseminated 

pyrite (as much as 2 percent observed) is locally common 

in unfaulted altered rhyolite, and fine-grained granular

pyrite is locally concentrated along structures in some 

faulted rhyolite bodies. Fluorite as disseminated grains, 

replacements, veins, and open-space fillings in brecciated 

rhyolite and in altered country rock is locally common. 

Calcite, gypsum, or quartz commonly fills open spaces 

in brecciated altered rhyolite and in veins.

Epithermal precious-metal mineralization has affected several rhyolite bodies throughout the rhyolite belt. 

Known mineral occurrences include deposits at the Rabbit Foot mine (116 oz Au, 136 oz Ag, reported production) and Singheiser mine (317 oz Au, 8,415 oz Ag, 

reported production), in the Panther Creek graben; and 

deposits or prospects in the Custer graben, including those 

at Red Mountain, the Sunbeam mine, and Estes Mountain (Mclntyre and Johnson, chap. I, this volume).

The Snowshoe Creek area, about 6 km northwest 

of the Twin Peaks caldera (fig. Nl), is one example of a 

favorable location for rhyolite-associated mineralization 

that lies within the area of the Van Horn Peak cauldron 

complex. In this area (fig. N9) dikes and plugs of rhyolite 

and rhyolite breccia intrude intracaldera pyroclastic rocks 

related to the Van Horn Peak cauldron complex that are 

unconformably overlain by outflow ash-flow tuff from 

the Twin Peaks caldera.

Northeast-trending faults displace the pyroclastic 

rocks, and in part rhyolite appears to intrude at least one

174 Challis 1°x2° Quadrangle, Idaho
Figure N6. Intrusive rhyolite breccia resembling a pebble dike, 

Challis quadrangle. A, rounded, devitrified to glassy, porphyritic 

rhyolite fragments in a breccia matrix containing quartz and 

sanidine phenocrysts. 6, photomicrograph of thin section from 

the hand specimen in A.

of these structures. Locally, the rhyolite and older 

pyroclastic rocks are moderately to strongly argillized and 

sericitically altered. Brecciated rock along structures is 

silicified and contains fluorite, calcite, and quartz in open 

vugs and veins. Disseminated pyrite is present along faults 

and in rhyolite and adjacent pyroclastic rocks. Stockwork 

hairline quartz veinlets are common in the rhyolites and 

in the country rock.

Sixty rock-chip samples were collected for geochemical study from 50 localities in the Snowshoe Creek area 

(fig. N10). Anomalous values of arsenic (5-2,000 ppm), 

molybdenum (7-700 ppm), and silver (0.5-10 ppm) were 

obtained from some samples (fig. N10). In addition, one 

sample contained 0.05 ppm gold and another contained 

2,000 ppm lead. Trace amounts of beryllium (1-10 ppm), 

antimony (1-18 ppm), zirconium (50-500 ppm), lanthanum (20-150 ppm), niobium (20-100 ppm), and 

barium (50-2,000 ppm) were also present in some samples.

The intrusive rhyolite in the Snowshoe Creek area is 

only one example of rhyolite that may have potential for 

spatially associated precious-metal deposits. Other rhyolites (some intrusions are too small to show on fig. Nl)

*- -

Figure N7. Photomicrograph of blotchy or snowflake-textured 

microcrystalline matrix of porphyritic rhyolite, Challis volcanic 

field.

Rhyolite Intrusions, Challis Volcanic Field 175
Figure N8. Photomicrograph of micromyrmekitic texture in 

matrix of porphyritic rhyolite intrusive body, Challis volcanic field.

114°37'30'

within the area of the Van Horn Peak cauldron complex, 

including the East Fork Mayfield Creek multiple intrusion, 

are altered and have associated structures that are similar 

to the Snowshoe Creek area and to rhyolites in the Panther Creek and Custer grabens.

CONCLUSIONS

Intrusive and minor extrusive rocks of rhyolitic composition that postdate cauldron-related pyroclastic rocks 

of the Challis volcanic field crop out in a northeast-trending belt that is essentially coincident with the trans-Challis 

fault system. Rhyolites within this belt are aphyric to porphyritic and have textures and field relations that indicate 

emplacement at shallow levels of the crust. Some of the 

intrusive rhyolites appear to have breached the surface and 

formed flow-dome complexes.

Hydrothermal alteration has affected some of the 

high-level rhyolite bodies within the Van Horn Peak cauldron complex, and rhyolite breccias are common. The 

presence of favorable permeable zones in these rhyolites 

combined with the presence of hydrothermally altered rock 

suggests that these rhyolites are favorable targets for mineral 

exploration. Preliminary investigation of the Snowshoe 

Creek area indicates that this is a favorable prospect for 

precious-metal deposits spatially associated with intrusive 

rhyolite.

___ EXPLANATION

[ '.Ql's'.-j QUATERNARY LANDSLIDE DEPOSITS 

TERTIARY

INTRUSIVE RHYOLITE Pattern 

indicates intrusive breccia

INTERMEDIATE INTRUSIONS 

TUFF OF CHALLIS CREEK 

QUARTZ-BIOTITE TUFF

TUFFS OF CAMAS CREEKBLACK MOUNTAIN

GRAY PORPHYRY 

CONTACT

Tcr

Tc

-T FAULT Dashed where approximately located; bar and ball on 

downthrown side

J? STRIKE AND DIP OF COMPACTION 

FOLIATION

0 2 MILES

0 1 2 KILOMETERS

Figure N9. Geologic map of the Snowshoe Creek area, Challis quadrangle.

176 Challis 1°x2° Quadrangle, Idaho
114°37'30'

EXPLANATION

| Qls | QUATERNARY LANDSLIDE DEPOSITS 

___ TERTIARY

[Tir | INTRUSIVE RHYOLITE Pattern 

indicates intrusive breccia

INTERMEDIATE INTRUSIONS 

TUFF OF CHALLIS CREEK 

QUARTZ-BIOTITE TUFF

Tii

Tcr

Tqb

Tc TUFFS OF CAMAS CREEKBLACK MOUNTAIN

GRAY PORPHYRY 

CONTACT

., FAULT Dashed where approximately located; bar and ball on 

downthrown side

-20 LOCALITY OF ROCK-CHIP SAMPLE 

 Anomalous value (in parts per 

million) of arsenic shown where 

found in sample. Values range 

from 5 to 2000 ppm. Analyses by 

atomic absorption

A. ARSENIC

2 MILES

J

2 KILOMETERS

Figure N10. Maps of the Snowshoe Creek area, Challis quadrangle, showing localities of rock-chip simples analyzed for 

arsenic, gold, silver, and molybdenum.

REFERENCES CITED

Ewart, A., 1979, A review of the mineralogy and chemistry of 

Tertiary-Recent dacitic, latitic, rhyolitic, and related salic 

volcanic rocks, in Barker, R, ed., Trondhjemites, dacites, 

and related rocks: Amsterdam, Elsevier, p. 13-121.

Hobbs, S. W., Hays, W. H., and Mclntyre, D. H., 1975, Geologic 

map of the Clayton quadrangle, Custer County, Idaho: U.S. 

Geological Survey Open-File Report 75-76, 23 p., 1 pi., 

scale 1:62,500.

Marvin, R. R, and Dobson, S. W., 1979, Radiometric agescompilation B, U.S. Geological Survey: Isochron/West, 

no. 26, 32 p.

Mclntyre, D. H., Ekren, E. B., and Hardyman, R. R, 1982 [1984], 

Stratigraphic and structural framework of the Challis 

Volcanics in the eastern half of the Challis I°x2° quadrangle, Idaho, in Bonnichsen, Bill, and Breckenridge, R. 

M., eds., Cenozoic geology of Idaho: Idaho Bureau of 

Mines and Geology Bulletin 26, p. 3-22.

Segerstrom, Kenneth, and Young, E. J., 1972 [1973], General 

geology of the Hahns Peak and Farwell Mountain 

quadrangles, Routt County, Colorado, with a discussion 

of Upper Triassic and pre-Morrison Jurassic rocks, by 

G. N. Pipiringos: U.S. Geological Survey Bulletin 1349, 

63 p.

Rhyolite Intrusions, Challis Volcanic Field 177
114°37'30'

Tii

Tcr

Tqb

Tc

EXPLANATION

QUATERNARY LANDSLIDE DEPOSITS 

TERTIARY

INTRUSIVE RHYOLITE Pattern 

indicates intrusive breccia

INTERMEDIATE INTRUSIONS 

TUFF OF CHALLIS CREEK 

QUARTZ-BIOTITE TUFF

TUFFS OF CAMAS CREEKBLACK MOUNTAIN

GRAY PORPHYRY 

CONTACT

r FAULT Dashed where approximately located; bar and ball on 

downthrown side

r5 LOCALITY OF ROCK-CHIP SAMPLE 

 Anomalous value (in parts per 

million) of silver shown where 

found in sample. Silver values 

range from 0.05 to10 ppm. Analyses by semiquantitative emission spectroscopy 

Locality of sample containing 

anomalous gold (0.05 ppm)

,0.5

B. GOLD AND SILVER

0 2 MILES

J

2 KILOMETERS

Figure N10. Continued.

178 Challis 1°x2° Quadrangle, Idaho
114°37'30'

C. MOLYBDENUM

2 MILES

2 KILOMETERS

EXPLANATION

Qis | QUATERNARY LANDSLIDE DEPOSITS 

_^ TERTIARY

Tir I INTRUSIVE RHYOLITE Pattern 

^^^ indicates intrusive breccia

Tn INTERMEDIATE INTRUSIONS

Tcr TUFF OF CHALLIS CREEK

Tqb QUARTZ-BIOTITE TUFF

Tc TUFFS OF CAMAS CREEK-

 BLACK MOUNTAIN

Tgp | GRAY PORPHYRY

 CONTACT

 FAULT Dashed where approximately located; bar and ball on 

downthrown side

20 LOCALITY OF ROCK-CHIP SAMPLE 

 Anomalous value (in parts per 

million) of molybenum shown 

where found in sample. Values 

range from 7 to 700 ppm. Analyses 

by semiquantitative emission 

spectroscopy

Figure N10. Continued.

Rhyolite Intrusions, Challis Volcanic Field 179

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter O

Lead-Isotope Characteristics of Ore Systems in 

Central Idaho

By BRUCE R. DOE and M. H. DELEVAUX

Abstract

About 60 new lead-isotope analyses were made on ore 

mineral, igneous-rock, and shale samples from central Idaho. 

The analyses were made to test the use of lead-isotope studies 

in mineral-resource evaluation of Cretaceous and Tertiary 

magmatothermal deposits.

The lead-isotope pattern is complex, but the analyses can 

be separated into two groups, A and B. Group A analyses have 

many values of 206Pb/204Pb more than or equal to 18.0, but 

some values exceed 21. All Group B analyses have values of 

18.5 or more, and most are between 19 and 20. For Group A 

and B analyses that have similar values of 206Pb/204Pb, 

208Pb/204Pb values of Group A analyses are greater than Group 

B analyses. Tin granites can be in either Group A or B and cannot be distinguished from molybdenum deposits by lead isotope 

ratios.

Group A values are similar to those from samples from 

the Rocky Mountains; Group B values resemble those from the

Great Basin. Large molybdenum deposits generally have lower 

values of 206Pb/204Pb, but values are in both isotopic groups. 

For example, Group A values were found in samples from the 

Thompson Creek deposit, and Group B values were found in 

samples from the White Cloud deposit. Analyses from the 

Clayton Silver mine are also in Group A, but 206Pb/204Pb here 

range between 18 and 19.

Group B analyses are from samples from a horn-shaped 

band opening to the east along the boundary between the 

Challis and Hailey quadrangles. Group A galena samples are 

from areas of positive gravity anomalies, where the gravity is 

not dominated by volcanic rocks, indicating relatively shallow 

Precambrian crystalline basement. Their lead-isotope apparent 

age is 1,620+100 m.y. Group B samples are from areas that do 

not have positive gravity anomalies, indicating that the 

crystalline basement is deep or absent. The primary source of 

these lead isotopes seems to be material deep under the Great 

Basin, rather than shallow Paleozoic or Middle Proterozoic Belt 

Supergroup rocks.

Lead-Isotope Characteristics of Ore Systems in Central Idaho 181

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter P

Light-Stable-Isotope Characteristics of 

Ore Systems in Central Idaho

By STEPHEN S. HOWE and WAYNE E. HALL

CONTENTS

Abstract 184

Introduction 184

Analytical techniques 186

Source of water in the hydrothermal systems 186

Source of sulfur in the hydrothermal systems 187

Conclusions 189

References cited 189

APPENDIX

Sulfur-isotope analyses of sulfide- and sulfate-mineral samples, and sulfide-pair 

temperatures of mineralization 191

FIGURES

PI. Map showing sample localities 185

P2. Diagram showing distribution of d 18O values of quartz and feldspar from

the Atlanta lobe 187 

P3. Graph of D values of biotite plotted against d 18O values of coexisting

feldspar, Atlanta lobe 188 

P4. Histogram showing <5 34S values of sulfide- and sulfate-mineral samples of

syngenetic origin 188 

P5. Histograms showing <5 34S values of sulfide- and sulfate-mineral samples from

many host rocks 188 

P6. Diagram showing variation with time in d34S of seawater sulfate. 189

TABLES

PL Localities sampled for study of sulfur isotopes 184

P2. Hydrogen-isotope analyses of water in white mica and in fluid inclusions,

and oxygen-isotope analyses of quartz and siderite 186 

P3. Comparison of sulfide-pair and fluid-inclusion geothermometry of ore

samples 188 

P4. Comparison of homogeneity in sulfur-isotopic compositions of selected

districts and deposits 189

Light-Stable-Isotope Characteristics of Ore Systems 183
Abstract

Light stable isotopes in mineral deposits and their host 

rocks were investigated to place constraints on the sources of 

water and sulfur in hydrothermal systems of central Idaho. Analyses of inclusion fluids extracted from crushed sulfide and 

gangue minerals from Cretaceous porphyry and vein deposits 

give 6D values of -130±25 per mil, indicating the presence of 

meteoric water. Meteoric water, which continued to predominate in hydrothermal systems around Tertiary plutons within 

the Idaho batholith, created zones extremely depleted in 

deuterium and 18O in areas larger than 15,000 km2 (square 

kilometers) through water-rock interactions. The d 34S values of 

sulfide minerals from most of the Cretaceous and Tertiary 

mineral deposits are quite high, generally from +4 to +13 per 

mil; several samples from deposits considered to be of 

syngenetic or remobilized-syngenetic origin have d 34S values 

as high as +23 per mil. No correlation between commodity 

and sulfur-isotopic composition exists. Mineral deposits in black 

shale, however, generally have d 34S values higher than those 

in carbonate rocks, quartzite, or granite. Although many of the 

mining districts are isotopically inhomogeneous, individual 

deposits commonly have very similar sulfur-isotopic 

compositions.

These isotopic data support a model in which meteoric 

water, set into convective circulation by heat from granitic intrusions, leached 34S-enriched sulfur from crustal sources, such 

as syngenetic sulfide minerals in sedimentary rocks, evaporite 

deposits, or pre-Cretaceous sulfide deposits. The sulfur then 

mixed locally in varying proportions with 34S-depleted sulfur 

from the magmatic reservoirs of adjacent plutons. This hybrid 

sulfur was then homogenized isotopically before reaching the 

sites of ore deposition.

INTRODUCTION

An objective of the CUSMAP (Conterminous 

United States Mineral Assessment Program) study of the 

Challis quadrangle was to develop models for the formation of a variety of mineral deposits that occur within the 

quadrangle. The origin of metals, water, and sulfur in the 

hydrothermal systems of the deposits is of paramount importance in the formulation of these models. This chapter 

summarizes the hydrogen- and oxygen-isotope data from 

earlier studies by Hall and others (1978, 1984) and Criss 

and Taylor (1983) and their conclusions concerning the 

source of water in certain hydrothermal systems. It also 

contains a tabulation of all of the sulfur-isotope data 

available to 1984 and a preliminary interpretation of the 

source of sulfur in 45 pre-Cretaceous, Cretaceous, and Tertiary mineral deposits in the Challis quadrangle and in 

parts of the adjoining Hailey and Elk City 1°X 2° quadrangles (fig. PI, table PI). The deposits include lead-zincsilver-tin-antimony-bismuth veins and breccia filling, 

molybdenum stockworks, lead-zinc-silver and barite stratabound deposits, and cobalt-copper stratabound and vein

Table PI. Localities sampled for study of sulfur isotopes, Elk 

City, Challis, and Hailey 1°x2° quadrangles

[do., ditto; leaders ( ), unknown, unnamed, or undefined mining district]

Loc. No. Mining 

(fig. PI) Name district County

Elk City quadrangle

1 Blackbird mine Blackbird Lemhi

Challis quadrangle

2 Quartz Creek prospect Yellowpine 

8 Beardsley Gulch mine do. 

prospect.

13 Rob Roy mine do. 

15 Roadcut between Buckskin 

and Thompson Creek mines. - do. 

19 Little Boulder

20 Silver Dollar prospect East Fork -

Valley 

Custer 

Boise 

Do. 

Do.

Custer 

Do. 

Do. 

Do.

Do.

Do. 

Do. 

Do.

Do. 

Custer

Do. 

Do.

Do. 

Do.

Hailey quadrangle

23 Golden Glow mine do. 

32 Panther Gulch prospect 

33 Red Cloud mine Wolftone 

38 Silver Star Queen mine Wood River 

39 Minnie Moore mine do. 

42 Triumph mine do. 

prospect.

Blaine 

Do. 

Do. 

Do. 

Camas

Blaine 

Do. 

Camas 

Do. 

Do.

Do. 

Blaine 

Do. 

Do. 

Do.

Do. 

Do. 

Blaine 

Do. 

Do.

Do. 

Do. 

Do. 

Do. 

Do.

deposits. We have concentrated our discussion on mineral 

deposits within the extensive black-shale belt east of the 

Idaho batholith (Hall, chap. J, this volume) due to the 

interest of the mining industry in this terrane.

Acknowledgments. J. N. Batchelder, W. J. 

Pickthorn, and L. D. White assisted various phases of the 

study, in the laboratory of J. R. O'Neil, U.S. Geological 

Survey, Menlo Park, Calif.

184 Challis 1°x2° Quadrangle, Idaho
21 y'^ i >

A \ '-'Boulder Mountains

24x SAMPLE LOCALITY LISTED 

IN TABLE P1

Figure PI. Localities of samples collected for sulfur-isotope study in the Elk City, Challis, and Hailey 1°x2° quadrangles. A description of the localities is in table P1.

Light-Stable-Isotope Characteristics of Ore Systems 185
ANALYTICAL TECHNIQUES

Sulfide and gangue minerals were crushed in 

evacuated stainless-steel tubes to liberate inclusion fluids. 

The water in these fluids was reacted with uranium at 

700 °C (Celsius), and the evolved hydrogen was collected 

on charcoal in glass sample bottles, similar to the method 

of Friedman (1953). Water released from hydrous silicates 

decomposed at 1,300^ by an RF induction furnace (Godfrey, 1962) was reacted in the same manner.

Quartz and feldspar were powdered and reacted with 

BrF5 or C1F3 at 600^ for 16 hours in nickel reaction 

tubes heated by electric resistance furnaces, using the 

methods of Clayton and Mayeda (1963) and Borthwick 

and Harmon (1982). The liberated oxygen was converted 

to CO2 by a heated carbon rod. Siderite was reacted with 

H3PO4 at 5Q°C for 6 days to evolve CO2 (McCrea, 1950). 

The CO2 was collected in glass sample bottles.

Powdered sulfide minerals were mixed with excess 

Cu2O and placed in silica tubes. The tubes were heated 

at 1,150°C for 10 minutes in a resistance furnace. The 

SO2 generated was purified of O2 , CO2 , and H2O by 

vacuum distillation and then collected in glass bottles or 

6-mm glass tubes.

The hydrogen-, oxygen-, and sulfur-isotopic compositions of the collected H2 , CO2 , and SO2 were analyzed using three different mass spectrometers. The 

hydrogen- and oxygen-isotopic compositions are reported 

in the standard 6 notation as per mil deviations from 

SMOW (Standard Mean Ocean Water), and the sulfurisotopic compositions are reported as per mil deviations 

from CDT (Canon Diablo Troilite). The precision of the 

analyses is ±1 per mil for dD and ±0.1 per mil for d 18O 

and d34S.

SOURCE OF WATER IN THE 

HYDROTHERMAL SYSTEMS

The most direct evidence concerning the source of 

water in the hydrothermal systems of two types of mineral 

deposits in central Idaho comes from the hydrogenisotopic compositions of inclusion fluids extracted from 

sulfide and gangue minerals from the Cretaceous Thompson Creek stockwork molybdenum deposit and the 

Cretaceous Wood River lead-zinc-silver vein deposits 

(localities 12 and 34-45, fig. PI). Samples of minerals 

thought to have been deposited during main oreformation episodes were identified after measurements of 

the homogenization temperatures and salinities of fluid 

inclusions were made (Hall and others, 1978, 1984). 

Highly depleted D (deuterium) compositions of -130±25 

per mil and -115 ±5 per mil for samples from the 

molybdenum and lead-zinc-silver deposits, respectively 

(table P2), indicate that the ore fluids in these deposits 

were derived predominantly from meteoric water (Taylor,

Table P2. Hydrogen-isotope analyses of water in white mica 

and in fluid inclusions in sulfide and gangue minerals, and 

oxygen-isotope analyses of quartz and siderite, from central 

Idaho

[Values in per mil; d 18OH o values for quartz calculated from Matsuhisa and others (1979) and for siderite from Becker and Clayton (1976), 

with T=280°C and T-270°C for Thompson Creek and Wood River 

deposits, respectively, from fluid-inclusion homogenization temperatures 

(Hall and others, 1978, 1984); do., ditto; leaders ( ), no analyses or 

calculations]

Sample 

No.

S-27-770 

S-27-789 

WH-81-5 

S-51-13 

S-89-2 

Mineral

Thompson

~~d0 *

Quar t z 

 do. 

o T\ e * °n 8DH 0 S 0 

' mineral

Creek deposit

-58 

11.2 

-108 10.7

-121 10.9

»108 

-155

-123

Calculated 

8T S20 at T

3.6 

3.1

3.3

~

Wood River deposit

WH-70-8A 

WH-70-13B 

WH-70-15 

WH-70-16 

Galena and

sphalerite. 

Siderite 

 do. 

Galena and

sphalerite.

-110

14.8

-120 13.3

-120

-115 16.4

7.8

6.3

8.4

1979). A dD value of -58 per mil for water in hydrothermal white mica that formed during the earliest period of 

potassic alteration at the Thompson Creek deposit (Hall 

and others, 1984) suggests that magmatic water was present initially in the hydrothermal system of this deposit.

Oxygen isotopes commonly help indicate the extent 

to which hydrothermal fluids have undergone isotopic exchange with surrounding country rocks. Quartz from the 

Thompson Creek stockwork deposit has a d18O value of 

+ 11.0±0.3 per mil, whereas quartz and siderite from the 

Wood River deposits have d 18O values of +14.9±1.6 per 

mil (table P2). The calculated oxygen-isotopic compositions of the water that deposited these gangue minerals 

are about +3.3 and +7.5 per mil, respectively. For the 

Thompson Creek deposit, a d 18O value of +3.3 per mil 

for the water is that of a fluid in approximate isotopic 

equilibrium with granitic rocks. However, the heavier 

d 18O values for the fluids that deposited the Wood River 

ores suggest that the fluids had partly exchanged 18O 

with 18O-enriched shale host rocks 1 .

Meteoric water continued to dominate hydrothermal systems during the Tertiary. Huge zones, areas larger

lTwo analyses of the host rock, the Devonian Milligen Formation, 

gave high d 18O values of +18.8 and +20.7 per mill (Hall and others, 

1978).

186 Challis 1°x2° Quadrangle, Idaho
than 15,000 square kilometers extremely depleted in D and 

18O, developed around Tertiary plutons within the Idaho 

batholith due to water-rock interactions. Criss and Taylor 

(1983) showed the distribution of 18O values for coexisting quartz and feldspar from rocks of the Atlanta lobe 

of the Idaho batholith (fig. P2). Feldspar is much more 

susceptible to isotopic exchange during hydrothermal 

alteration than is quartz (Taylor, 1979; Cole and others, 

1983). The strongly skewed d 18O values for the feldspar 

clearly show the pronounced depletion in 18O due to interactions with meteoric water. Deuterium in biotite and 

18O in feldspar are also extremely depleted in altered 

rocks of the Atlanta lobe (fig. P3; Criss and Taylor, 1983). 

Only repeated circulation of low-D, low-18O meteoric 

water through these rocks could lower the dD and d 18O 

values so greatly. Criss and Taylor (1983) suggested that 

many epithermal gold-silver deposits related to Tertiary 

plutonism within the Idaho batholith are associated with 

these altered rocks and that low-18O halos may be useful 

targets for mineral exploration.

SOURCE OF SULFUR IN THE 

HYDROTHERMAL SYSTEMS

The sulfur-isotopic compositions of the deposits 

sampled are generally enriched in 34S (fig. P4; appendix). 

Although the values range from 3.5 to +22.9 per mil, most 

are between +4 and +13 per mil. Four sulfide-mineral 

samples and one sulfate-mineral sample from preCretaceous deposits considered to be syngenetic or 

remobilized syngenetic, as discussed by Hall (chap. J, this 

volume), have values of +11.3, +11.3, +13.2, +15.6, and 

+22.9 per mil.

Within most individual deposits, the order of 

enrichment of 34S in coexisting epigenetic sulfide minerals is galena<bournonite<sphalerite<pyrite<molybdenite,

precisely the fractionation trend expected for deposition 

under conditions of isotopic equilibrium, even though few 

of the minerals were deposited contemporaneously. For this 

reason, we hoped that isotopic fractionation between two 

coexisting sulfide minerals could be used to calculate geologically reasonable pressure-independent temperatures of 

mineralization. Sulfide pairs (table P3; appendix) from 

several lead-zinc-silver vein deposits apparently did precipitate at geologically reasonable temperatures of 191° to 

375*^, which, for the Wood River deposits, match fluidinclusion homogenization temperatures fairly closely (Hall 

and others, 1978).

Rye and Ohmoto (1974) have shown that similar 

types of deposits commonly have somewhat different 

sulfur-isotopic compositions, and that for some deposits 

these differences are a result of incorporating sulfur from 

different sources in the deposit. The absence of correlation between commodity or type of deposit and sulfurisotopic composition is also true for most of the deposits 

examined in central Idaho (appendix). Some deposits are 

relatively enriched in 34S, such as the Thompson Creek 

stockwork molybdenum deposit (average <J34S is +10.6 

per mil), and the Livingston lead-zinc-silver vein deposit, 

(average <5 34S is +11.9 per mil). Also, some deposits are 

relatively depleted in 34S, such as the Little Boulder Creek 

stockwork molybdenum deposit, (average d34S is +6.4 per 

mil) and the Dryden lead-zinc-zilver vein deposit (average 

d34S is +5.0 per mil).

When the d34S values are grouped by lithology of 

the host rocks for the deposits (fig. P5), an interesting 

distinction is seen. Deposits within the Ordovician(?) Ramshorn Slate, the Devonian Milligen Formation, and the 

Cambrian to Mississippian Salmon River assemblage2, all 

predominantly black shale, have <J34S values averaging 6

formations described in detail by Hobbs (chap. D, this volume) 

and by Hall (chap. J, this volume).

<

O

1.1,1.1,1

QUARTZ

i i i i i n i n n FI

FELDSPAR

pj rrj

t'-l LI 1 11 1 R'PI , Ns'l 1! t>;| IT-1 1 H 1 M f-i-l I -1 l-l

-8 -6 -4 -2 0

~\ JT w

n :

j""t'"l IN 1 !4r 1 1 j:j 1"

2 4

i

6

1

n rrfi 1

1

fe:

i :

5

?-l [7-]

- : .. : 

~~ " d:~ Q

LE:_, p : :-,; ,,:^T- n H

*3h ,

8 10 12

8 18O IN PER MIL

Figure P2. Distribution of <J 18O values of samples coexisting quartz and feldspar from rocks of the Atlanta lobe of the Idaho batholith. 

From Criss and Taylor (1983).

Light-Stable-Isotope Characteristics of Ore Systems 187
-60

-80

LLI

t -100

O

co -120

Q

co . 14Q

-160

-180

-8-6-4-20246 

81*0 IN FELDSPAR

8 10 12

Figure P3. Graph of dD values of samples of biotite (with or 

without chlorite) plotted against the d 18O values of coexisting 

feldspar in altered rocks of the Atlanta lobe of the Idaho 

batholith. Original composition for dD was about -65 to -75 

per mil, and for d 18O, +8 to +11 per mil. From Criss and 

Taylor (1983).

-4 -2 6 8 10 12 14 16 18 20 22 24 

§34S IN PER MIL

Figure P4. Distribution of d 34S values of 100 sulfide-mineral 

samples and one sulfate-mineral sample. Unpatterned areas 

show samples considered to be of syngenetic or remobilizedsyngenetic origin.

Table P3. Comparison of sulfide-pair and fluid-inclusion 

geothermometry of ore samples from central Idaho 

[Sulfide-pair temperatures calculated from appropriate fractionation 

equations of Ohmoto and Rye (1979), ±45°C due to uncertainties in 

the fractionation equations and in the analyses of the sulfur-isotopic 

compositions of the sulfides. Fluid-inclusion temperatures are homogenization temperatures from Hall and others (1978). Leaders (-), no 

analyses]

Deposit or district

Little Boulder Creek 

Sulfide-pair 

temperature (°C)

322 

284 

355 

375, 269 

191-291

Fluid-inclusion 

temperature (°C)

270±30

per mil greater or heavier than those deposits within carbonate rocks, quartzite, and granite. This gap of 6 per mil 

appears to cause the bimodal nature of the distribution 

of all of the <J 34S data (fig. P4). The d34S values for 

deposits in Precambrian metasedimentary rocks fall in between. Although analyses of shales barren of ore are 

needed, the data suggest that the large positive d 34S

CRETACEOUS PLUTONS INTRUDED 

BY TERTIARY DIKES

ii

EARLY PERMIAN DOLLARHIDE FORMATION

n

MIDDLE PENNSYLVANIAN TO EARLY 

PERMIAN WOOD RIVER FORMATION

CAMBRIAN TO LATE MISSISSIPPIAN 

SALMON RIVER ASSEMBLAGE

_TL n

DEVONIAN MILLIGEN FORMATION

i i i i i i i

MIDDLE ORDOVICIAN KINNIKINIC QUARTZITE 

AND ELLA DOLOMITE

i I i I i I i I i I i I i I i

ORDOVICIAN (?) RAMSHORN SLATE

i i i i i

PRECAMBRIAN METASEDIMENTARY ROCKS

JH_

-4-202 6 8 10 12 14 

834SIN PER MIL

16 18 20 22 24

Figure P5. Distribution of <$4S values in samples of sulfide 

and sulfate minerals from host rocks of many ages and 

lithologies. Unpatterned areas show samples considered to be 

of syngenetic or remobilized-syngenetic origin.

values for the deposits in the black shale reflect the 34Senriched sulfur-isotopic composition of the black shale 

itself. Seawater sulfate is the only source of sulfur that 

has contained such highly 34S-enriched sulfur throughout 

geologic time (fig. P6; Claypool and others, 1980).

Some of this seawater sulfate was reduced to H2S 

in the depositional environments of the black shales. 

Syngenetic-diagenetic sulfides, disseminated within the 

sediments or concentrated in lenses, likely had heavy 

sulfur-isotopic compositions that reflected the isotopic 

composition of the original sulfate. Disseminated pyrite 

is known from shale in the Challis and Hailey quadrangles, but no sulfur-isotopic analyses of this pyrite have 

been made. However, sedimentary pyrite from similar 

lower Paleozoic host rocks in central Nevada have heavy

188 Challis 1°x2° Quadrangle, Idaho
 TERTIARY 

- -CRETACEOUS

JURASSIC

TRIASSIC

PERMIAN

PENNSYLVANIAN

MISSISSIPPI

DEVONIAN

SILURIAN

ORDOVICIAN

CAMBRIAN

PRECAMBRIAN

10 15 20 25 

8 34SIN PER MIL

30

Figure P6. Variation with time in the <J 34S of sulfate in 

seawater. Dashed lines show the range of relatively few analyses. 

The heavy line is the best estimate for d 34S of sulfate mineral 

in equilibrium with the world ocean surface sulfate of that date. 

The shaded area is the estimate of the uncertainly of this curve. 

From Claypool and others (1980).

<5 34S values, as much as +12 per mil (Rye and others, 

1974). As noted earlier, sulfide lenses from syngenetic 

deposits have d34S values of +11 to +23 per mil. The 

seawater sulfate that was not reduced to H2S may have 

been incorporated into evaporites and lenses of barite. 

Although there is no evidence of evaporites in the Challis 

and Hailey quadrangles, they were deposited on the 

cratonic shelf to the east. The single sulfate analysis is 

of a sample of stratabound barite from the Deer Creek 

deposit (fig. PI, no. 34; table PI) in the Early Permian 

Dollarhide Formation. Its <5 34S value of +13.2 per mil is 

nearly identical to that of Permian seawater sulfate 

(Claypool and others, 1980).

The relative depletion of 34S in the deposits within 

carbonate rocks and quartzite probably indicates contributions of sulfur from sources more depleted of 34S. 

These sources may be the carbonate and quartzite themselves, or they may be more deep seated, possibly 

magmatic, sources, which would typically have lighter 

sulfur-isotopic compositions (Ohmoto and Rye, 1979). 

Note the correspondence between the <5 34S values (fig. 

P5) of the deposits within the carbonate and quartzite, 

and those of the deposits within granite. Intermediate 

sulfur-isotopic compositions probably resulted from mixing of seawater sulfate with magmatic sulfur.

The d 34S values trend lower as the age of the host 

rock decreases from Late Mississippian to Pennsylvanian 

to Early Permian (fig. P5). This trend is very similar to 

that of <5 34S values along the seawater sulfate curve (fig. 

P6) during this same period. This correspondence may

indicate the decrease in <J34S for seawater sulfate during 

the upper Paleozoic.

Some mining districts are isotopically inhomogeneous, although individual deposits within them 

commonly have very similar sulfur-isotopic compositions 

(table P4). This relationship suggests that the sulfur was 

homogenized isotopically before it was incorporated into individual deposits.

Table P4. Comparison of the homogeneity in sulfur-isotopic 

compositions of samples from selected districts and deposits 

[Values in per mil]

District Range in 834S

Boulder Creek 15.4

I[

Deposit

Livings tonRange in 834S

1.4 

1.5

CONCLUSIONS

Our preliminary interpretation of isotopic data from 

central Idaho indicates the following model. Cretaceous 

and Tertiary hydrothermal systems in central Idaho were 

dominated by meteoric water set into convective circulation by heat from granitic intrusions. Most of the sulfur 

in the mineral deposits associated with such systems was 

34S-enriched sulfur leached from shallow sedimentary 

sources, such as syngenetic-diagenetic sulfide minerals in 

sedimentary rocks, evaporite deposits, or pre-Cretaceous 

sulfide deposits. The sulfur then mixed locally in varying 

proportions with 34S depleted sulfur from the magmatic 

reservoirs of adjacent plutons. This hybrid sulfur was then 

homogenized isotopically before reaching the sites of ore 

deposition. This model suggests that a wide variety of undiscovered mineral deposits may exist in central Idaho 

because of the large reservoir of sulfur available to the 

hydrothermal fluids. The most favorable targets appear 

to be those areas where granitic stocks intrude the extensive black-shale belt in the region.

REFERENCES CITED

Decker, R. H., and Clayton, R. N., 1976, Oxygen isotope study 

of a Precambrian banded iron-formation, Hamersley 

Range, western Australia: Geochimica et Cosmochimica 

Acta, v. 40, p. 1153-1165.

Borthwick, James, and Harmon, R. S., 1982, A note regarding 

C1F3 as an alternative to BrF5 for oxygen isotope analysis: 

Geochimica et Cosmochimica Acta, v. 46, p. 1665-1668.

Claypool, G. E., Holser, W. T., Kaplan, I. R., Sakai, Hitoshi, 

and Zak, Israel, 1980, The age curves of sulfur and oxygen isotopes in marine sulfate and their mutual interpretation: Chemical Geology, v. 28, p. 199-260.

Clayton, R. N., and Mayeda, T. K., 1963, The use of bromine 

pentafluoride in the extraction of oxygen from oxides and 

silicates for isotopic analysis: Geochimica et Cosmochimica 

Acta, v. 27, p. 43-52.

Light-Stable-Isotope Characteristics of Ore Systems 189
Cole, D. R., Ohmoto, Hiroshi, and Lasaga, A. C, 1983, Isotopic 

exchange in mineral-fluid systems. I. Theoretical evaluation of oxygen isotopic exchange accompanying surface 

reactions and diffusion: Geochimica et Cosmochimica Acta, v. 47, p. 1681-1693.

Criss, R. E., and Taylor, H. P., Jr., 1983, An 18O/16O and D/H 

study of Tertiary hydrothermal systems in the southern half 

of the Idaho batholith: Geological Society of America 

Bulletin, v. 94, p. 640-663.

Friedman, 1.1., 1953, Deuterium content of natural water and 

other substances: Geochimica et Cosmochimica Acta, v. 4, 

p. 89-103.

Godfrey, J. D., 1962, The deuterium content of hydrous minerals 

from the east-central Sierra Nevada and Yosemite National 

Park: Geochimica et Cosmochimica Acta, v. 26, 

p. 1215-1245.

Hall, W. E., Rye, R. O., and Doe, B. R., 1978, Wood River mining district, Idaho intrusion-related lead-silver deposits 

derived from country rock source: U.S. Geological Survey 

Journal of Research, v. 6, p. 579-592.

Hall, W. E., Schmidt, E. A., Howe, S. S., and Broch, M. J., 1984, 

The Thompson Creek, Idaho, porphyry molybdenum 

deposit an example of a fluorine-deficient molybdenum 

granodiorite system, in Janelidze, T. V., and Tvalchrelidze, 

A. G., eds., Proceedings: Sixth Quadrennial International

Association on the Genesis of Ore Deposits, Tbilisi, USSR 

1982: E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, v. 1, p. 349-357.

Matsuhisa, Yukihiro, Goldsmith, J. R., and Clayton, R. K, 1979, 

Oxygen isotopic fractionation in the system quartz-albiteanorthite-water: Geochimica et Cosmochimica Acta, v. 43, 

p. 1131-1140.

McCrea, J. M., 1950, On the isotopic chemistry of carbonates 

and a paleotemperature scale: Journal of Chemical Physics, 

v. 18, p. 849-857.

Ohmoto, Hiroshi, and Rye, R. O., 1979, Isotopes of sulfur and 

carbon, in Barnes, H. L., ed., Geochemistry of hydrothermal ore deposits, 2d ed.: New York, John Wiley and Sons, 

p. 509-567.

Rye, R. O., Doe, B. R., and Wells, J. EX, 1974, Stable isotope 

and lead isotope study of the Cortez, Nevada, gold deposit 

and surrounding area: U.S. Geological Survey Journal of 

Research, v. 2, p. 13-23.

Rye, R. O., and Ohmoto, Hiroshi, 1974, Sulfur and carbon 

isotopes and ore genesis a review: Economic Geology, 

v. 69, p. 826-842.

Taylor, H. P., Jr., 1979, Oxygen and hydrogen isotope relationships in hydrothermal mineral deposits, in Barnes, H. L., 

ed., Geochemistry of hydrothermal ore deposits, 2d ed.: 

New York, John Wiley and Sons, p. 236-277.

190 Challis 1°x2° Quadrangle, Idaho
APPENDIX

Sulfur-isotope analyses of 100 sulfide-mineral samples and one sulfate-mineral sample, and sulfide-pair temperatures 

of mineralization, from 45 deposits in the Elk City, Challis, and Hailey 1°x2° quadrangles 

[d 34 S values in per mil; A 34 S=d34 Sa-d34 Sb ; temperatures in ° C calculated from appropriate fractionation equations of Ohmoto 

and Rye (1979), ±45°C due to uncertainties in the fractionation equations and analyses of the sulfur-isotopic compositions 

of the sulfides. do., ditto; leaders ( ), no calculations]

Sample No. Location Mineral 8 34S A S Temperature

Elk City quadrangle

Blackbird mine, Blackbird district, Lemhi County Chalcopyrite 7.3 

8.1

 

Challis quadrangle

QC-138i82-

QC-170-82- 

QC-187-82A 

QC-187-82B

CT2-78 

79T22M 

79T23M 

79T24M 

H08 

SWH52-76 

Buckskin 

SWH152-76-

CT3-78 

CT4-78 

CT4-78 

S-27-747 

S-27-770 

S-76-895 

SWH1 20-79- 

SWH154-76-

SWH1 54-76- 

Buckskin 

drill road. 

D863A 

T135L-1 

T135L-1 

R558 

D851 

D851-1 

Ul OA_

W312C 

T70 

Quartz Creek prospect, Yellowpine district, Valley 

County .

 do.

ao.

Seafoam Lake prospect, Seafoam district, Custer 

County .

Little Falls prospect, Grimes Pass district, Boise 

County .

do.

Carlsqn Gulch mine, Grimes Pass district, Boise 

County . 

Comeback mine, Grimes Pass district, Boise County 

Ramshorn mine, Bayhorse district, Custer County 

Beardsley Gulch mine, Bayhorse district, Custer 

County.

Rattlesnake Creek prospect, Bayhorse district, 

Custer County.

Thompson Creek mine, Bayhorse district, Custer 

County.

Rob Roy mine, Bayhorse district, Custer County 

Dryden mine, Bayhorse district, Custer County 

Roadcut between Buckskin and Thompson Creek 

mines, Bayhorse district, Custer County. 

Hoodoo mine, Boulder Creek district, Custer CountyLi vingston mine, Boulder Creek district, Custer 

County .

Willow Lake prospect, Boulder Creek district, 

Custer County. 

Little Boulder Creek prospect, Boulder Creek 

district, Custer County.

do.

-do.

do.

Silver Dollar prospect, East Fork district,

Sphalerite 

 do. 

 do. 

 do. 

Tetrahedrite 

Chalcopyrite 

Galena 

 do. 

Py r i t e 

Ar senopyrite( ? )-

Pyrite 

Galena 

Galena, coarse 

Sphalerite 

JamesoniteC?) 

Galena+minor 

jamesonite.

Molybdenite 

Sphalerite, black

Pyrite 

Sphalerite, redSphalerite, black

Holybdenite 

Pyrite 

9.1

11.0 

9.0 

8.9 

7.0

5.5

7.5 

4.1 

6.7

.8

11.5 

10.1 

11.3 

11.4

10.1 

4.2

6.8 

11.4

9.6 

9.9

10.4 

11.3 

12.1 

10.3 

11.9

8.7 

22.9* 

11.3* 

6.6 

5.4

4.6 

8.7

15.6* 

12.5

11.4 

11.1

12.6 

8.2

8.7 

6.1

5.7 

4.0 

6.0 

0.2 

5.1

5.9 

7.0 

6.4 

8.6 

8.3

 

 

 

 

 

 

 

 

 

2.04 322

 

Custer County.

Light-Stable-Isotope Characteristics of Ore Systems 191
APPENDIX-Continued.

Sulfur-isotope analyses of 100 sulfide-mineral samples and one sulfate-mineral sample, and sulfide-pair temperatures

of mineralization, from 45 deposits in the Elk City, Challis, and Hailey 1°x2° quadrangles

[d 34 S values in per mil; A 34S=d34 Sa-d34 Sb; temperatures in °C calculated from appropriate fractionation equations of Ohmoto

and Rye (1979), ±45^ due to uncertainties in the fractionation equations and analyses of the sulfur-isotopic compositions

of the sulfides. do., ditto; leaders ( ), no calculations]

Sample no. Location Mineral AS Temperature

Hailey quadrangle

79T565M 

D807A 

D807D 

T302M 

76T31 

E758B 

T49M ----- -

T9T10M - 

79T10AM 

7QT1 ftAU«»_

D784 

D784 

E880-1 

Tft 1 M_ _.

T68M 

E684 

E684A -----

D831A 

WH-70-13BA-176 

WH-70-6 

WH-70-6 

UH 7O 7 . 

WH-70-1 1 

WH-70-16 

WH-70-1 7 

WH^70-17 

A

1£. 

WH-70-12 

WH-74-2B-- 

WH-74-2B--

WH-74-1B 

D247 

Unnamed prospect, Galena district, Blalne County 

Unnamed prospect, Boulder Ba"sln district, Blalne 

County.

do.

Golden Glow mine, Boulder Basin district, Blalne 

County.

Webfoot mine, Vienna district, Blalne County 

GAF prospect, south of Vienna district, Camas County

Rooks Creek prospect, Boyle Mountain district, 

Blalne County.

Unnamed prospect, Boyle Mountain district, Blalne 

County. 

Upper prospect, Carrietown district, Camas County 

Dump, Carrietown district, Camas County 

 do. - --- ------- - - -- 

Horn Sliver mine, Carrietown district, Camas County

__ J _ _ _ _ __

Carrie Leonard mine, Carrietown district, Camas 

County.

Red Cloud mine, Wolf tone district, Blaine County 

Deer Creek mine, Wood River district , Blaine County

Liberty Gem mine. Wood River district, Blaine County

Treasure Vault mine, Wood River district, Blaine 

County,

Silver Star Queen mine, Wood River district, Blaine 

County 

Silver Star Queen mine, Wood River district, Blaine 

County.

Minnie Moore mine, Wood River district, Blalne 

County . 

Dennlson mine, Wood River district, Blalne County 

North Star mine, Wood River district, Blaine County

Independence mine, Wood River district, Blaine 

County . 

Trail Creek prospect, Wood River district, Blalne 

County. 

Leke Creek-Trail Creek prospect, Wood River 

district, Blaine County.

Galena 

St annite 

Sphalerite 

Galena 

 do. 

 do . - 

Mo lybdeni t e 

Galena 

Pyrite --- -- 

 do.'* 

Galena 

 ""~d O " ~-~~i~"~ 

Sphalerite 

 "~ d o . *~> -. «« 

Sphalerite 

Barite 

 do. 

Chalcopyrite 

Galena - 

Bournonlte 

Galena 

Sphalerite 

Ga lena 

Sphalerite 

 do. 

 do. 

 do. 

 do. 

7.6 

4.5 3.29

6

1) 

** « 

4.2 1.83

6

f\ ___

0.3 

4.7

5.0

2.8 

3.4 

61 .»_..

-0.4 2.43

2.0 

4.3 

1.2 

-0.1 2.46

2.4 

8.8 

-1.0 

t c »«

13.2* 

60 «.» 

2.2 2.17

4.4 

4.3

4.0 3.20

10.8

12.6 

11.2

12.0 2.66

14.6 

10.7 3.36 

14.0 

11.0 3.11

14.1 

10.9 

7.2 2.29

9

e _

H

?*

11.4 

6.7 

0.4 

284

 

355

 

375 

269

 

 

243 

291

248

191 

209

289

 

* Analyeis of eample from deposit thought to be of syngenetic or remobilized-syngenetlc origin.

192 Challis 1°x2° Quadrangle, Idaho
Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter Q

Depositional Controls of Breccia-Fill and 

Skarn Tungsten Deposits in the 

Challis Quadrangle

By THERESA M. COOKRO

CONTENTS

Abstract 194 

Introduction 194 

Breccia-fill deposits 194 

Skarn deposits 197 

References cited 199

FIGURES

Ql. Map showing tungsten mines and prospects 195 

Q2. Map showing breccia-fill and skarn tungsten deposits 196 

Q3-Q5. Photographs of tungsten ore from:

Q3. The Quartz Creek mine 197

Q4. The Yellow Pine mine 198

Q5. The Golden Gate mine 199

Q6. Map showing tungsten skarn deposits east of Stanley, Idaho 200 

Q7. Photograph of tungsten ore from the Tungsten Jim mine 201

Depositional Controls of Breccia-Fill and Skarn Tungsten Deposits 193
Abstract

Breccia-fill and skarn tungsten deposits within the Challis 

quadrangle have been mined for tungsten, antimony, gold, and 

silver. The Yellow Pine mine, with characteristic breccia-fill textures, produced 800,000 units of WO3 , 100,000 ounces of 

gold, and 600,000 ounces of silver. The ore had an average 

grade of 1.6 percent WO3 . The Springfield mine, a skarn 

deposit, produced 6,000 units of WO3 from an unknown 

grade of ore. Knowledge of several of the recognized depositional controls can be used in exploration for new deposits.

The breccia-fill deposits are within the Idaho batholith and 

localized where regional shear zones trending north to 

N. 20° E. intersect local faults trending N. 30°-60° E. that 

branch from the main shear zone. The mineralized fracture fillings reveal continued brecciation of the rock throughout the 

time of metal deposition. The breccia was mineralized in three 

stages which produced: early dissemination of arsenopyrite in 

the batholith, then quartz-scheelite veins, and later stibnite 

veins. The three stages are not distinct but grade into one 

another. The veins are fillings in either extensional fractures or 

between the breccia clasts of the batholith, or in cavities 

resulting from the dissolution of feldspar grains. Where the 

quartz-scheelite veins are in contact with carbonate inclusions 

in the batholith, replacement textures are prominent. The 

precious-metal history of these deposits is uncertain. Gold may 

have been deposited continually throughout the three stages 

of mineralization, and some silver is reported in stibnite, a lateforming mineral.

Skarn deposits formed at the contact of Upper Cretaceous 

biotite granodiorite plutons with carbonate-rich Paleozoic and 

possibly Precambrian metasedimentary rocks. The granodiorite 

related to the skarn is typical of plutons of the Idaho batholith 

and White Cloud stock and not typical of the younger Tertiary 

intrusive rocks. The skarn deposits in the south-central part of 

the quadrangle are structurally just below thrust faults that are 

cut by the Idaho batholith. Scheelite in the skarns occurs along 

bedding planes, within high-angle fault zones, as disseminations, and locally associated with pyrrhotite-bearing veins and 

replacement sulfide minerals.

INTRODUCTION

The Challis quadrangle contains about 50 tungsten 

mines and prospects (fig. Ql) that can be grouped into 

at least three types of occurrences: breccia-fill, skarn, and 

stockwork. Breccia-fill deposits are within north-striking 

regional shear zones in the Idaho batholith where minor 

faults branch from the main shear zones. The granodiorite 

of the batholith locally contains metasedimentary inclusions; remobilized calcite from these inclusions is common 

in the breccia-fill deposits. Scheelite is more concentrated 

where quartz veins containing Scheelite are in contact with 

the carbonate, especially the remobilized calcite. The 

Yellow Pine and Quartz Creek mines are breccia-fill 

deposits with carbonate-rich metasedimentary inclusions. 

The Yellow Pine mine produced tungsten, antimony, gold, 

and silver (Cooper, 1951), and the Quartz Creek mine 

sporadically produced tungsten until 1982. Gold, silver,

arsenic, mercury, and antimony have been found in several 

of the deposits, and manganese oxides are ubiquitous.

Skarn deposits are abundant within the Challis 

quadrangle, and several have produced small amounts of 

tungsten. The tungsten skarns are at the contact of 

carbonate-rich Paleozoic and possibly Precambrian 

metasedimentary rocks with the Idaho batholith. Thrust 

faults are structurally just above most of the deposits; 

higher grades of ore are associated with steep faults within 

the deposits. The skarns are characterized by diopsidehedenbergite, garnet, scheelite, quartz, and feldspar. The 

sulfide-mineral content of the deposits varies but pyrrhotite is very common. The Springfield, Tungsten Jim, 

and Meadowview mines, the Emma and Flintstone 

groups, and an occurrence in Hennessy Meadows are examples of tungsten skarns.

The Red Mountain occurrence, north of the town 

of Yellow Pine, and the Little Boulder Creek deposit in 

the White Cloud Peaks are two stockwork deposits with 

associated tungsten. They were described by Leonard 

(1983) and Cavanaugh (1979), respectively. The stockworktype deposit is not discussed in this chapter. Judging from 

the mining history of the quadrangle, the breccia-fill and 

skarn deposits (fig. Q2) have the best potential for future 

discovery and development of tungsten resources.

BRECCIA-FILL DEPOSITS

The breccia-fill deposits are within a major northto northeast-trending system of shear zones in the Idaho 

batholith. The shear zones are several kilometers wide and 

extend approximately from northeast of Garden Valley 

through the Yellow Pine area and north of the quadrangle. 

The shear zones are prominent features of the western 

one-fourth of the Challis quadrangle. The deposits are 

in fault breccias where more local faults striking N. 

30°-60°E. branch from the north-striking shear zones. 

In the brecciated areas of granodiorite, the rock is vuggy 

and highly silicified.

The north- to northeast-trending shear zones are 

shown on the geologic map of the Challis quadrangle 

(Fisher and others, 1983) in two ways:

1. In the northwestern part of the quadrangle the 

shear zones are mapped as unit Tsi, which, according to 

B. F. Leonard (written commun., 1984) is "Silicified zones 

in granitic rocks of the Idaho batholith. Silicified zones 

are related to ring fractures of the Thunder Mountain 

cauldron complex and to attendant superposed northstriking shear zones!' In some regions the batholithic rocks 

were replaced almost completely by silica considered by 

Leonard to be Tertiary in age.

2. Elsewhere on the map the regional shear zones 

are shown as prominent north-striking faults. The northstriking faults in the batholithic terrane are zones of 

intense shearing and silicification.

194 Challis 1°x2° Quadrangle, Idaho
115 3 114 C

HU

44°

. pi^A2

. . ' ^/>°r\460

C

0 542«"'

724 o

77R* 725 726 , . .

434,436

445,446

 

. * 647

Bwf-V

655'

 

.

l't

 

/ " « . 90 1 

728° 735 Stanley a

. 

 >

.

 * *?4^ ,fi?1

9 1 K 7 I 

^ .&88282"824

830,83/845 ." [

/986» 

382,383x987^393*. .

*. 959

* » 964

i 1 i

] 20

MINES AND

105 Peterson prospect

112 Yellow Pine mine

117 Sulfide No. 10 prospect

215 Bonanza mine

243 McClure prospect

291 McClure No. 4

382 Fourth of July deposit

383 Deer Trail mine

388 Red Robin No. 3 prospect

393 Meadowview prospect

0

o

20

1

3880v/o 994

394-5^406,407

40 MILES

I

40 KILOMETERS

PROSPECTS

654

647

655

724

725

726

728

732

735

818

394 Washington Peak mine

406 Reconstruction vein

407 Washington Basin depositEmpire

422 Warner Gold ( W group)

423 Quartz Creek, Cinnabar

425 Golden Gate occurrence

429 Johnson Creek area

431 Emma group

434 Copper Mountian prospect

436 Big Chief Scheelite

445 Old Faithful Group mine

446 Springfield mine

460 Mule Train claims

542 Tungsten Point

821

822

824

827

829

830

831

845

959

964

986

987

994

1030

Muskeg Creek

Chuck Creek

Soldier Creek

Horsefly prospect

Merry Blue prospect

Wilson Creek

White Hawk Meadows

White Hawk Mountain

BE&L

Scheelite Nellie, Nelvinny

Nos. 1 - 4

Tungsten Jim mine

Buckskin claims

Greyhound Ridge

Salmon River Scheelite

Peach Creek No. 2

Patricia Ann occurrence

Peach Creek W claims

Flintstone Group

Cramer Lakes prospect

Hidden Lake prospect

Sheelite occurrence

McClure No. 6 claim

Boulder Creek Prospect

Hennessy Meadows occurrence

Figure Q1. Map showing localities of tungsten mines and prospects (open circles) 

and stream-sediment samples containing more than 50 ppm tungsten (solid circles), 

Challis quadrangle. Modified from Mitchell and others (1981) and Callahan and 

others (1981), respectively.

Whether or not the regional shear zones are mapped as 

silicified zones or regional faults, they are broad areas of 

shearing that contain large amounts of secondary quartz. 

The silica enrichment of the batholith is especially present where local faults branch off the north-trending shear 

zones. These intersections commonly form a topographic 

bulge or mound and contain breccia-fill tungsten deposits. 

The intersecting faults may be second-order faults that

formed in dilated zones, similar to the structure at the 

Henderson 2 mine in Quebec (Guha and others, 1983). 

The Idaho batholith, consisting mostly of biotite 

granodiorite, is strongly altered and brecciated in the regions of shearing. It is vuggy and appears finer grained 

than other parts of the batholith due to the influx of finegrained secondary silica; it contains secondary feldspar 

veins and sericite is common. The alteration of biotite to

Depositional Controls of Breccia-Fill and Skarn Tungsten Deposits 195
20 40 KILOMETERS 

EXPLANATION

QUATERNARY ALLUVIUM Includes flood plain, terrace, 

and alluvial-fan deposits

TERTIARY INTRUSIVE VOLCANIC ROCKS Dikes, stocks, 

and batholiths of dioritic, granitic, or rhyolitic composition

TERTIARY EXTRUSIVE VOLCANIC ROCKS Tuffs, lavas, 

and volcanic sedimentary rocks of rhyolite. rhyodacite, 

dacite, and andesite

CRETACEOUS INTRUSIVE ROCKS OF THE IDAHO BATHOLITH Tonalite, granodiorite, granite, muscovitebiotite granite, and leucocratic granite

PALEOZOIC SEDIMENTARY ROCKS Limestone, dolomite, 

shale, siltstone, argillite. quartzite, sandstone, and 

slate

MIDDLE PROTEROZOIC METAMORPHIC AND SEDIMENTARY ROCKS Argillite. slate, quartzite, sandstone, 

siltite, schist and phyllite

ROCKS OF UNCERTAIN AGE Argillite, carbonate, quartzite. and phyllite

CONTACT

FAULT

THRUST FAULT Sawteeth on upper plate

CALDERA BOUNDARY

BRECCIA-FILL DEPOSITS

SKARN DEPOSITS

Figure Q2. Map showing locations of breccia-fill and skarn deposits in the Challis 

quadrangle. Each symbol represents one or more tungsten occurrences or mines. 

Modified from Fisher (fig. A2, chap. A, this volume).

chlorite and fine-grained oxide minerals, and of feldspars 

to clay gives the normally speckled granodiorite a uniform 

light-brown to cream-colored appearance. The primary igneous texture was gradationally lost due to silicification. 

The batholith contained roof pendants and large inclusions of Paleozoic and Precambrian metasedimentary 

rocks that were incorporated in the breccia of the shear 

zones. Remobilized calcite forms veins and broken blocks 

in the breccia, probably having originated from those inclusions or roof pendants of carbonate metasedimentary 

rocks within the batholith before brecciation. The remobilized calcite is often extensively replaced by scheelite.

The breccia is both clast and matrix supported; the 

matrix-supported breccia has higher grades of tungsten. 

The breccia clasts are not sorted, and the size of the 

fragments is highly variable. Variations in structure of the 

breccias can be seen in samples from the Quartz Creek,

Yellow Pine, and Golden Gate mines. The intense 

silicification tends to mask the fragmental structure of 

the breccias, and in many samples only ultraviolet light 

shows the brecciated texture, with scheelite and quartz filling in between and outlining the breccia fragments (figs. 

Q3-Q5). At the Quartz Creek mine the batholith has been 

mostly crackled, meaning the rock is broken but with only 

slight rotation of clasts. The Yellow Pine mine has more 

strongly rotated clasts, but in many places these can be 

pieced back together in a jigsaw-puzzle fashion. Breccia 

at the Golden Gate mine is more chaotic and has randomly oriented clasts.

Vugs in areas of silicified batholithic rocks range 

widely in size, depending on their origin. Several processes formed the vugs: (1) extensional fracturing provided open spaces that range from about 0.3 to 1 m long and 

that may be partly or wholly filled by secondary materials;

196 Challis 1°x2° Quadrangle, Idaho
 - - -* W *s^ - 

 .^ o\ 5l--V»r«. R jfc. 1 -# >- tSi J* - ' ^»-"^ , ****"

Figure Q3. Sample of tungsten ore from Quartz Creek mine, 

Challis quadrangle. A, sample in ordinary light showing crackle

breccia with infillings of remobilized calcite, quartz, and scheelite. B, sample in ultraviolet light showing fluorescing scheelite.

(2) leaching of feldspar grains (especially potassium 

feldspar megacrysts) formed smaller cavities, averaging 5-10 

mm across, that later were filled; and (3) leaching of 

materials between breccia clasts formed vugs of variable 

sizes that now contain quartz and scheelite.

Cooper (1951) recognized three stages of mineralization at the Yellow Pine mine that could have been partly 

true for other breccia-fill deposits: (1) dissemination of 

arsenopyrite and gold; (2) formation of scheelite-quartz 

veins; and (3) formation of stibnite-silver veins. Cooper 

favored a replacement origin for most of the scheelite, 

although he recognized that the scheelite was most abundant in material between rock fragments or breccia clasts. 

I think that the primary mechanism for introduction of 

scheelite was in quartz that filled openings within the breccia. Where quartz-scheelite veins are in contact with carbonate rocks or remobilized calcite, scheelite deposits 

developed by replacement of the carbonate.

At the Quartz Creek mine, Petersen (1984) also found 

similar evidence for the three stages of mineralization. Gold 

at Quartz Creek may have been deposited throughout all 

the stages. In addition to the gold in arsenopyrite at Quartz 

Creek, Petersen found gold within a late-stage stibnite vein.

At least two generations of both scheelite-quartz and stibnite veins show evidence of continued fracturing during 

mineralization.

Three types of scheelite occurrences are in the 

breccia-fill deposits: (1) quartz-scheelite veins, (2) replacement of carbonate clasts and remobilized vein carbonate, 

and (3) fine, powdery coatings on manganese oxides. At 

the contacts of quartz-scheelite veins and carbonate-rich 

clasts, some euhedral to subhedral scheelite crystals replace 

carbonate on the margins of the clasts. At the contacts of 

quartz-scheelite veins and remobilized calcite, there is 

massive replacement of calcite with scheelite. The replacement crystals of scheelite commonly have better defined 

crystal form and are larger than the scheelite crystals in 

the quartz veins. In some places tiny clay-size crystals of 

scheelite have formed on the manganese oxides within vugs 

and on fracture surfaces.

SKARN DEPOSITS

The tungsten-bearing skarn deposits are dark green 

to light gray, medium to coarse grained, granoblastic, and

Depositional Controls of Breccia-Fill and Skarn Tungsten Deposits 197
Figure Q4. Sample of tungsten ore from the Yellow Pine mine, 

Challis quadrangle. A, in ordinary light. B, in ultraviolet light,

showing fluorescing scheelite. Breccia fragments (black) can be 

seen more clearly in ultraviolet light than in ordinary light.

slightly foliated. Minerals are xenoblastic and include 

diopside-hedenbergite, plagioclase, quartz, potassium 

feldspar, biotite, sphene, calcite, garnet, chlorite, epidote, 

and sericite. Scheelite is the tungsten-bearing mineral. Pyrrhotite, pyrite, arsenopyrite, chalcopyrite, sphalerite, 

magnetite, ilmenite, galena, jamesonite, stannite, gold, 

bismuth, and bismuthinite also may be present. Scheelite 

occurs as disseminations throughout the skarn; it is also 

controlled by bedding planes and fracture zones. The 

pyrrhotite-rich zones of sulfide-bearing veins and replacement deposits commonly contain scheelite.

The skarn deposits are in carbonate-bearing beds 

of Paleozoic and Precambrian metasedimentary rocks 

where they are in contact with Cretaceous plutons. In the 

south-central part of the Challis quadrangle the principal 

Paleozoic host-rock units are the Salmon River assemblage 

(Mississippian to Cambrian), the Grand Prize Formation 

(Permian), and the Wood River Formation (Pennsylvanian and Permian) (fig. Q6). A detailed description of 

these rocks is given by Hall (chap. J, this volume). Because 

of its small scale, figure Q6 shows skarn deposits as

occurring within the batholith or volcanic rocks. Those 

deposits are either in roof pendants of the batholith or 

in erosional windows in the volcanics where Paleozoic 

rocks are exposed. In the northwestern part of the quadrangle the skarns are in Precambrian metasedimentary 

rocks mapped by B. F. Leonard (unpub. mapping, 1982). 

The specific Precambrian metasedimentary units are 

uncertain.

Tungsten skarns in the south-central part of the 

quadrangle are structurally just below thrust faults that 

are cut by the Upper Cretaceous Idaho batholith. The 

thrust faults seem to have controlled late-phase intrusions 

of the batholith (W. E. Hall, oral commun., 1982). Breccias within high-angle fault zones commonly have greater 

amounts of scheelite than unfaulted areas. Also, some 

mine raises follow the steep faults, suggesting former highgrade ore pockets along them. The high-angle faults are 

not necessarily related to thrust zones.

At the contact with the metasedimentary rocks, the 

Cretaceous intrusive rocks, including the Idaho batholith 

and the White Cloud stock, range in composition from

198 Challis 1°x2° Quadrangle, Idaho
 v-<* ,,-><;?. je &-_^-r >->'&

I" fi « * ' T. i ..;?

I ^"--st * ** * Jilft-

^ 

f*^: * * ~V -r'; : < * *v

'S^rv- .'v-"< ** .*. ^-* * v. *^%-"- ^"*vr* *r^ ^ 

, ;;j^;"% . *K-*>v, j? -^"xN.^ *

m4* J* * sw-"-» 1^

riv^ --

^ * m. 

% . *^v. »>v , \ ; ' A

\>>, "- kV.*-'/

% X:;, .» ., jjr * *" V

. ,v v **.^ <" ' O 

V*-~tr, -\-v.3rS

granite to granodiorite. Biotite within the intrusive bodies 

near the skarn is often completely altered to a mixture 

of chlorite and oxide minerals. Most of the tungstenbearing skarns are heavily chloritized. The Cretaceous 

intrusions locally contain potassium feldspar veins in contact with potassium feldspar megacrysts. The 1-3 cm long 

megacrysts are porphyroblasts that incorporate minerals 

formed earlier such as biotite, quartz, and primary 

feldspars. In a few areas metasomatism seems to have 

enhanced the ore grade.

Skarns in the central part of the Challis quadrangle 

are the Tungsten Jim mine (fig. Q7), the Meadowview 

prospect, and the Flintstone claim group on Beaver Creek. 

In the northwestern part of the quadrangle the Springfield 

mine produced 6,000 units of WO3 from ore of unknown 

grade (Cater and others, 1973). Other skarn deposits are 

the Hennessy Meadows occurrence and the Emma group. 

See figures Ql and Q6 for location of deposits.

REFERENCES CITED

Cater, F. W., Pinckney, D. M., Hamilton, W. B., Parker, R. L., 

Weldin, R. D., Close, T. J., and Zilka, N. T., 1973, Mineral 

resources of the Idaho Primitive Area and vicinity, Idaho,

to see in hand sample. A, in ordinary light. B, in ultraviolet light, 

which shows the breccia clasts not seen in ordinary light.

with a section on the Thunder Mountain district, by B. 

F. Leonard and a section on Aeromagnetic interpretation, 

by W. E. Davis: U.S. Geological Survey Bulletin 1304, 

431 p.

Callahan, J. E., Cooley, E. F., and Neuerburg, G. J., 1981, 

Selected Mo and W assays from stream-sediment samples 

and panned heavy-mineral concentrates, Challis, Idaho 2° 

topographic quadrangle: U.S. Geological Survey Open-File 

Report 81-1344, 13 p.

Cavanaugh, Patrick, 1979, Geology of the Little Boulder Creek 

(White Cloud) molybdenum deposit, Custer County, 

Idaho: Missoula, University of Montana Ph.D. dissertation, 100 p.

Cooper, J. R., 1951, Geology of the tungsten, antimony and gold 

deposits near Stibnite, Idaho: U.S. Geological Survey 

Bulletin 969-F, p. 151-197.

Fisher, F. S., Mclntyre, D. H., and Johnson, K. M., 1983, 

Geologic map of the Challis 1°X 2° quadrangle, Idaho: US. 

Geological Survey Open-File Report 83-523, 39 p., 2 maps, 

scale 1:250,000 (also available from the Idaho Bureau of 

Mines and Geology).

Guha, Jayanta, Archamibault, G., and Leroy, J., 1983, A correlation between the evolution of mineralizing fluids and the 

geomechanical development of a shear zone as illustrated by 

the Henderson 2 mine, Quebec [Canada]: Economic Geology, 

v. 78, no. 8, p. 1605-1618.

Depositional Controls of Breccia-Fill and Skarn Tungsten Deposits 199
114°30'

EXPLANATION

SURFICIAL DEPOSITS

(QUATERNARY) 

CHALLIS VOLCANICS

(TERTIARY) 

GRANITIC ROCKS

(CRETACEOUS) 

GRAND PRIZE FORMATION

(PERMIAN) 

WOOD RIVER FORM/VRON

(PERMIAN and

PENNSYLVANIAN) 

SALMON RIVER ASSEMBLAGE (MISSISSIPPIAN

to CAMBRIAN) 

ORDOVICIAN ROCKS

CONTACT 

NORMAL FAULT

THRUST FAULT Sawteeth 

on upper plate

TUNGSTEN SKARN DEPOSITNumbers are identified 

on figure Q1

CHALLIS 

QUADRANGLE

Area of 

this figure

4 MILES

J

4 KILOMETERS

Figure Q6. Simplified geologic map showing the south-central cluster of tungsten skarn deposits east of Stanley, Idaho. Modified 

from Fisher and others (1983) and unpublished mapping of W. E. Hall and S. W. Hobbs (1983).

200 Challis 1°x2° Quadrangle, Idaho
Figure Q7. Sample of ore from the Tungsten Jim mine, Challis 

quadrangle. A, in ordinary light. B, in ultraviolet light, showing 

fluorescing scheelite.

Leonard, B. E, 1983, The Golden Gate tungsten deposit and metal 

anomalies in nearby soils and plants, Yellow Pine district, 

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

Report 83-835, 26 p.

Mitchell, V. E., Strowd, W. B., Hustedde, G. S., and Bennett, E. 

H., 1981, Mines and prospects of the Challis quadrangle,

Idaho: Idaho Bureau of Mines and Geology, Mines and Prospects Map Series, 47 p., 1 map, scale 1:250,000. 

Petersen, M. A., 1984, Geology and mineralization at the Quartz 

Creek tungsten mine, Yellow Pine, Idaho: Kent, Ohio, Kent 

State University M.S. thesis, 80 p.

Depositional Controls of Breccia-Fill and Skarn Tungsten Deposits 201

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter R

Stratabound Cobalt-Copper Deposits in the 

Middle Proterozoic Yellowjacket Formation 

in and Near the Challis Quadrangle

By PETER J. MODRESKI

CONTENTS

Abstract 205

Introduction 205

Regional geology 205

Lithology of the Yellowjacket Formation 208

Mineral deposits 209

Cobalt-copper-magnetite deposits in the Iron Creek area 209

Tourmaline-quartz veins and breccias 211

Lead and copper deposits at the Twin Peaks mine 214

Stratigraphic and structural framework of the Twin Peaks mine area 214

Minor copper occurrences 215

Specular hematite occurrences 215

Cobalt and other metals in the Blackbird district and vicinity 215

Other copper, gold, and base-metal deposits 217 

Geochemical composition of rocks 217

Genesis of the Blackbird-type deposits and comparison to other districts 218 

References cited 219

FIGURES

Rl. Generalized geologic map of parts of the Challis, Elk City, Dillon, and

Dubois quadrangles 206 

R2. Stratigraphic column showing metasedimentary rock formations of

Middle Proterozoic age 207

R3. Generalized Stratigraphic columns of the Yellowjacket Formation 209 

R4-R7. Photographs showing:

R4. Penecontemporaneous deformation and disruption of bedding 

in quartzite and argillite of the Yellowjacket Formation 210 

R5. Bedding-plane polygonal structures in the Yellowjacket

Formation 210 

R6. Breccia of sericite-quartzite with tourmaline and quartz,

Yellowjacket Formation 214 

R7. Specular hematite veins in quartzite, Ifellowjacket Formation 215

Cobalt-Copper Deposits in the Yellowjacket Formation 203
TABLES

Rl. Concentrations of selected elements in metavolcanic rocks of the

Yellowjacket Formation 209 

R2. Contents of cobalt and other metals in mineralized and unmineralized

rocks 212 

R3. Strontium content of rocks of the Yellowjacket Formation and Lemhi

Group 218

204 Challis 1°x2° Quadrangle, Idaho
Abstract

Stratabound cobalt-copper-iron deposits occur within the Middle Proterozoic Yellowjacket Formation in the northeastern corner of the Challis 1°x2° quadrangle. These deposits form the 

southern end of the northwest-trending Idaho cobalt belt, which 

contains the currently inactive Blackbird mine, the major cobalt 

deposit in North America. The deposits in this belt consist of 

Stratabound sulfide and arsenide minerals (pyrite, chalcopyrite, 

cobaltite, pyrrhotite, and arsenopyrite) in argillaceous quartzite. 

The ore minerals were recrystallized and remobilized to varying degrees during metamorphic and erogenic events that are 

inferred to have taken place mainly in Precambrian time. The 

metamorphic grade of the Yellowjacket Formation is lower 

greenschist facies within most of the Challis quadrangle but increases to the north and west. The metals appear to have been 

incorporated into the sediments syngenetically and represent a 

sea-floor exhalative, distal-volcanogenic process of mineralization. The source of the metals was probably related to the 

volumetrically minor basaltic volcanism in the basin.

The mineralized rock within the Challis quadrangle is along 

the North Fork of Iron Creek, where the Little No-Name mine 

explored strata containing chalcopyrite and cobaltiferous pyrite. 

Adjacent strata, which are high in magnetite and contain minor 

disseminated sulfide minerals, appear to have been deposited 

in a more oxidized environment, possibly farther from the source 

vents of the metal-bearing fluids. Veins and breccias of black finegrained quartz-tourmaline rock occur sporadically throughout 

the district; they are not directly associated with ore in the Iron 

Creek area, although some occurrences near the Blackbird mine 

contain cobaltite. Specular hematite veins, which are essentially cobalt free, occur locally in the Yellowjacket Formation, 

Hoodoo Quartzite, and quartzite of the overlying Lemhi Group; 

they probably formed during metamorphism. The Twin Peaks 

mine, in the extreme northeastern corner of the Challis quadrangle, produced a small amount of lead and copper; the ore, 

with high silver values but essentially no cobalt, is apparently 

Stratabound but concentrated mainly in shear zones. Its host rock 

may (questionably) be the Yellowjacket Formation.

Northwest of the Challis quadrangle, the cobalt belt includes the inactive Blackpine mine, the Blackbird mine and nearby prospects, several recently discovered cobalt occurrences 

within the Special Mining Management Zone-Clear Creek of the 

River of No Return Wilderness area, and the Salmon Canyon 

Copper Co. mine. Several Stratabound copper deposits in the 

Salmon River Mountains and Lemhi Range west and south of 

Salmon may or may not be genetically related to the cobalt 

deposits. The gold and lead-zinc-silver deposits in the region, 

including the the Yellowjacket mining district, the Mackinaw 

district near Leesburg, and the Mineral Hill district near Shoup, 

probably originated through Tertiary igneous activity.

INTRODUCTION

This chapter describes the cobalt-copper deposits 

in metamorphosed sedimentary rocks of the Middle Proterozoic Yellowjacket Formation in the Salmon River 

Mountains in the northeastern part of the Challis quadrangle. The deposits are Stratabound and appear to have

formed during a syngenetic, volcanogenic-exhalative 

mineralizing event on the sea floor. They occur along a 

northwest-southeast trend, the so-called Idaho cobalt belt. 

Most of the cobalt belt and its principal deposit, the 

Blackbird mine, lie just north of the Challis quadrangle; 

for this reason, the scope of this chapter has been 

broadened to include the Precambrian rocks in parts of 

the adjoining Elk City, Dillon, and Dubois 1°X2° quadrangles. Field work by the author, principally in the Degan 

Mountain and Taylor Mountain 7 Vi -minute quadrangles, 

was done in the summers of 1980-83; mines and mineral 

deposits in the adjacent areas were also examined during 

this time. E. B. Ekren also mapped parts of the Degan 

Mountain, Taylor Mountain, and adjacent quadrangles. 

In addition to field mapping, emission spectrographic 

analyses were obtained of mineralized and unmineralized 

rocks, and some samples were studied in thin section, 

polished section, with the electron microprobe, and with 

X-ray diffraction.

Acknowledgments. I was fortunate to be able to 

draw on the extensive stratigraphic work done on the 

Yellowjacket Formation by D. A. Lopez. I thank G. A. 

Hahn and G. J. Hughes, Jr., of Noranda Exploration, Inc., 

for discussions about their detailed studies of the 

Blackbird deposit and adjacent areas, and for the opportunity to examine mining properties and rock samples. I 

also thank E. B. Ekren, K. V. Evans, S. W. Hobbs, B. F. 

Leonard, D. A. Lopez, Karen Lund, and E. T. Ruppel, 

all of the U.S. Geological Survey; Terry Webster and S. 

G. Peters, formerly of Noranda; and D. W. Peters of the 

U.S. Forest Service, for valuable discussions about the area 

and its geology. The collaboration with E. B. Ekren in 

mapping the Precambrian rocks within the Challis quadrangle is particularly appreciated. I thank R. A. Milne and 

E. F. Waterman for allowing access to their mining 

properties.

REGIONAL GEOLOGY

Precambrian rocks of Middle Proterozoic age occur mainly in the northern and northeastern part of the 

Challis quadrangle (fig. A2, chap. A, this volume; fig. Rl). 

They include, from oldest to youngest, the Yellowjacket 

Formation, the Lemhi Group and the Swauger Formation (fig. R2). The Yellowjacket Formation consists principally of thin-bedded argillaceous quartzite; Shockey 

(1957) estimated its total thickness to be 10,000 m (meters). 

The top and base of the Yellowjacket Formation have 

nowhere been recognized in the field. The Hoodoo 

Quartzite of Ross (1934) is a clean medium-grained, 

feldspathic quartzite, which, according to recent mapping and interpretation by E. B. Ekren (written commun., 

1984), forms a unit about 1,100 m thick within the upper 

part of the Yellowjacket Formation. The Hoodoo crops 

out in a broad area in and around Taylor Mountain and

Cobalt-Copper Deposits in the Yellowjacket Formation 205
114°45' 113°45'

t - 'N XL i' "' y.'

rf"-/'V5.

S^^lPg

MINING DISTRICT

114°OD'

20 MILES

20 KILOMETERS

45°

00'

ELK CITY QUADRANGLE

CHALLIS QUADRANGLE

DILLON

QUADRANGLE

DUBOIS

QUADRANGLE

Figure R1. Generalized geologic map showing outcrop areas of Precambrian rocks and locations of selected mines and prospects in parts of the Challis, Elk City, Dillon, and Dubois quadrangles. Geology modified from Bennett (1977), Evans (1981), Fisher 

and others (1983), Lund and others (1983), Mitchell and Bennett (1979), Rember and Bennett (1979), Ruppel and others (1983), 

and unpublished mapping by E. B. Ekren and P. J. Modreski.

farther west in the Challis and Elk City quadrangles (fig. 

Rl); it was earlier interpreted to overlie the Yellowjacket 

(Ross, 1934).

The Lemhi Group (Ruppel, 1975) consists of five 

formations. In ascending order, they are the Inyo Creek,

West Fork, Big Creek, Apple Creek, and Gunsight Formations (fig. R2). Only the upper three have been 

recognized with certainty within the Challis quadrangle. 

The Lemhi Group lies in thrust contact on the Yellowjacket wherever it has been studied (Ruppel, 1975, 1978,

206 Challis 1°x2° Quadrangle, Idaho
EXPLANATION 

PHANEROZOIC

VOLCANIC AND SEDIMENTARY ROCKS Includes small

areas of Paleozoic sedimentary rocks 

INTRUSIVE ROCKS, MAINLY EOCENE AND LATE CRETACEOUS

MIDDLE PROTEROZOIC 

LEMHI GROUP Includes Swauger Quartzite in Lemhi

Range and Belt Supergroup in Beaverhead Mountains

METASEDIMENTARY ROCKS OF UNCERTAIN CORRELATION Includes fine grained clastic rocks of Apple Creek 

or Yellowjacket Formations and quartzite of Big Creek, 

Hoodoo, or Yellowjacket Formations

PLUTONIC ROCKS Includes granite, augen gneiss, and 

migmatite

HOODOO QUARTZITE

YELLOWJACKET FORMATION

-OWJACKET FORMATION Metamorphosed to gneiss 

_nd schist

CONTACT Dashed where approximately located

e) 26 MINES AND PROSPECTS Circled numbers show deposits 

	that are part of the cobalt belt

1. Little No-Name mine (Cu, Co)

2. Twin Peaks mine (Cu, Pb )

3. Specular hematite prospect near Moyer Creek (Fe )

4. Yellowjacket mine, Yellowjacket mining district (Au, Ag, Cu, Pb)

5. Black Eagle mine, Yellowjacket mining district (Au)

6. Musgrove mine (Au)

7. Blackpine mine (Cu, Co)

8. Unnamed prospect at head of French Gulch (Cu, Co)

9. Patty B prospect Blackbird mining district (Co)

10. Haynes -Stellite mine. Blackbird mining district (Co)

1 1 . Blackbird mine. Blackbird mining district (Co, Cu)

12. Sweet Repose mine. Blackbird mining district (Co)

1 3. Tinker's Pride (Cu, Co ) and Bonanza Copper (Cu) mines. Blackbird

	mining district

1 4. Cobalt occurrences near Garden Creek (Co)

1 5. Cobalt occurrences near Elkhorn Creek (Co )

1 6. Salmon Canyon Copper Co. mine (Cu, Co )

17. Clipper Bullion and other mines. Mineral Hill mining district (Au)

18. Copper King mine, Mackinaw mining district (Cu)

1 9. Mayflower mine, Mackinaw mining district (Cu, Au, Ag )

20. Haidee mine, Mackinaw mining district (Au)

21. Italian mine, Mackinaw mining district (Au)

22. Blue Jay mine. Mackinaw mining district (Pb, Ag )

23. Ringbone Cayuse mine, Makinaw mining district (Pb, Ag)

24. Gold placers near Leesburg, Mackinaw mining district (Au)

25. U. P. and Burlington mine. Eureka mining district (Au)

26. Bowman mine. Eureka mining district (Cu)

27. Queen of the Hills mine. Eureka mining district (Cu)

28. Silverton prospect near Bob Moore Creek, Eureka mining district (Pb )

29. Tormey mine. Eureka mining district (Cu)

30. Pope-Shenon mine (Cu)

31. Harmony mine (Cu)

32. Thorium prospects near Diamond and Wallace Creeks (Th, U, rare 

	earths)

33. Mines near Meyers Cove, Gravel Range mining districtfAu, Pb, 

	fluorite )

34. Singheiser mine. Gravel Range mining district (Au)

35. Rabbitfoot mine. Gravel Range mining district (Au, Cu)

36. Specular hematite prospects north of McKim Creek (Fe)

1980). The Apple Creek Formation resembles the Yellowjacket in many respects, and distinction of the two formations is difficult. Much of the upper part of the Yellowjacket(?) Formation north of Taylor Mountain and around

Swauger Formation

Gunsight Formation 

Apple Creek Formation

Big Creek Formation

/West Fork Formation 

--Inyo Creek Formation

Lemhi Group 

(Ruppel, 1975)

Thrust fault

Hoodoo Quartzite

Yellowjacket

Formation

(4000 - 10,000 m

thick)

Figure R2. Stratigraphic column showing metasedimentary rock formations of Middle Proterozoic age in the 

Challis quadrangle and vicinity.

Degan Mountain (fig. Rl) may actually be the Apple 

Creek Formation and is shown as rock of questionable 

identity on figure Rl. The Lemhi Group is overlain by reddish quartzite of the Swauger Formation (Ruppel, 1975; 

Hobbs, 1980). The Middle Proterozoic Lawson Creek Formation overlies the Swauger south of the area shown in 

figure Rl (Hobbs, 1980). The only Paleozoic rocks observed in the northeastern corner of the Challis quadrangle are in fault contact with the Yellowjacket(?) 

Formation in a very restricted area near Rattlesnake Creek 

and the Twin Peaks mine.

The Yellowjacket Formation is exposed for about 

80 km (kilometers) northwestward from the Iron Creek 

drainage in the Challis quadrangle (fig. Rl). Yellowjacket 

rocks also crop out to the east in the northern Lemhi 

Range and the Beaverhead Mountains. Cobalt occurrences, however, are restricted to a belt about 54 km long, 

extending northwestward from Iron Creek to the Salmon 

Canyon Copper Co. mine on the northern side of the 

Salmon River. Rocks within this cobalt belt strike generally northwest. They commonly dip to the northeast, but 

detailed mapping within the Challis quadrangle indicates 

that the Yellowjacket is extensively folded; in places it is 

isoclinally folded. The fold axes trend north-northwest. 

In areas of tight folding, axial-plane cleavage is prominently developed in the more argillaceous (phyllitic) beds 

of the Yellowjacket. Deformation is more intense in

Cobalt-Copper Deposits in the Yellowjacket Formation 207
the Elk City quadrangle to the north (Bennett, 1977; 

Evans, 1981; Maley, 1974). Because of the relative paucity 

of observable bedding-top criteria and the lack of distinctive, traceable beds, considerable repetition of strata 

through faulting or isoclinal folding is possible.

The absolute age of the Yellowjacket Formation is 

not well constrained. Evans (1981) dated one pluton that 

intrudes the Yellowjacket near the Salmon River at 1.37 

b.y. (billion years), setting a lower limit on the age of the 

formation. Hughes (1983) reported a 1.7-b.y. potassiumargon age for rock interpreted to be mafic tuff within the 

Yellowjacket, but the limits of error on this age may be 

large (Karl Evans, oral commun., 1984). The possibility 

has long been considered (for example, Ruppel, 1975) that 

the Yellowjacket Formation correlates with the Prichard 

Formation of the Belt Supergroup, or at the very least that 

it represents a synchronous deposit in a similar environment in a separate depositional basin.

The metamorphic grade in the Precambrian rocks 

within the northeastern part of the Challis quadrangle is 

principally lower greenschist facies. The argillaceous 

quartzite and interbedded argillite commonly contain both 

green and brown biotite, sericite, and chlorite. Calc-silicate 

beds contain amphibole, scapolite, epidote, clinozoisite, 

and phlogopite. As mentioned earlier, metamorphic grade 

increases to the north and west, with amphibolite-grade 

rocks containing garnet, staurolite, and chloritoid occurring near the Blackbird mine, and sillimanite-grade rocks 

to the northwest, closer to the Precambrian augen gneiss 

plutons and migmatites along the Salmon River (Bennett, 

1977; Evans, 1981). No detailed study of the metamorphic petrology of this region has ever been made. The 

regional metamorphism probably took place during the 

Precambrian (Tucker, 1975; Evans, 1981; Snee, 1983), with 

the possibility of thermal overprinting during Paleozoic, 

Cretaceous, and Eocene intrusive events.

Rocks of the region were thrust eastward during the 

Antler orogeny (mid-Paleozoic) and more particularly 

during the Sevier and Laramide orogenies (Cretaceous, 

and Late Cretaceous to early Tertiary) (Skipp and Hait, 

1977; Ruppel, 1978, 1982). To the east in Idaho and adjacent Montana, Ruppel (1978) described as the "Medicine 

Lodge thrust system" the structure on which the Lemhi 

Group and younger rocks have been carried eastward over 

Yellowjacket and older basement rocks. The region was 

subsequently broken by predominantly north-south-trending, basin-and-range type normal faulting and (or) block 

uplift (Skipp and Hait, 1977; Ruppel, 1982), continuing 

today.

LITHOLOGY OF THE YELLOWJACKET 

FORMATION

The Yellowjacket Formation, which contains the ore 

deposits, consists of orthoquartzite (rare) to argillaceous

quartzite, siltite, sandy or silty argillite, and argillite. The 

quartzites are mostly arkosic, containing both plagioclase 

and potassium feldspar, plus detrital tourmaline, zircon, 

garnet, sphene, and magnetite-hematite. Some of the 

quartzite is laminated and some is cross-laminated; alternating or interbedded quartzite and siltite-argillite are 

common. The grains making up the quartzite are typically 

poorly sorted. The most abundant sediment was probably 

very fine grained sand, but sediment sizes ranged from 

clay and silt through very fine to fine, medium, and some 

coarse sand. The original clay matrix has been recrystallized to sericite, biotite, chlorite, and iron oxide.

The Yellowjacket Formation was originally described by Ross (1934) from his type section along Yellowjacket Creek as consisting of 2,676 m of quartzite and 

argillaceous and calcareous quartzite. In the Leesburg and 

Blackbird Mountain 15-minute quadrangles (Shockey, 

1957; Purdue, 1975; Bennett, 1977), the formation has 

classically been divided into a lower phyllite member and 

an upper quartzite member (fig. R3). Lopez (1981) divided 

the Yellowjacket into five members, generalized in figure 

R3, having an aggregate thickness of 7,100-8,100 m. Near 

the Blackbird mine, Hughes (1983) and Sobel (1981,1982), 

distinguished three members totalling 6,000 m, coarsening upward from predominantly silty and sandy argillite 

(member A) to argillaceous quartzite and sandy argillite 

(member B) to quartzite (member C). The generalized 

maps of Hughes (1983) and Hahn and Hughes (1984) 

show most cobalt occurrences to be in or near their unit 

A, along what was inferred to be the axis of the depositional basin. Within the northeastern corner of the Challis 

quadrangle, the lithology varies upsection from argillaceous quartzite along Iron Creek, to more siltite and 

sandy argillite in the northern part of the Degan Mountain and Taylor Mountain 7 Vi -minute quadrangles, to 

quartzite in the south-eastern part of the Leesburg 

15-minute quadrangle. This sequence may represent as 

much as 7,000 m of section and probably corresponds to 

members B through E of Lopez (1981) and to members 

A through C of Hughes (1983).

Calcareous beds (now predominantly or entirely 

calc-silicate rocks) occur locally in the Yellowjacket 

section. Ross (1934) and Carter (1981) described calcareous 

beds or lenses near the base of the Yellowjacket section 

in the Yellowjacket Creek area. Leonard (1962) and Cater 

and others (1973) described calcareous beds in the Big 

Creek area further to the west. Calc-silicate and calcareous 

beds occur along Moyer Creek northwest of Taylor Mountain (fig. Rl); some of these calcareous beds overlie the 

specular hematite deposit discussed in the section on 

"Specular Hematite Occurrences!'

Rocks of igneous origin in the Yellowjacket Formation appear to be volumetrically more abundant north of 

the Challis quadrangle, where Hahn and Hughes (1984) 

mentioned finding tuffs and tuffaceous sediments, dikes, 

sills, and diatremes. The metamorphosed igneous rocks

208 Challis 1°x2° Quadrangle, Idaho
Shockey(1957)

Leesburg 15-minute

quadrangle

"Impure 

. , gray 

" quartzite"

3

5 s

' PHylliie \

Lopez(1981) 

Composite of 1 2 

sections, various 

areas

Quartzite E

Argillaceous jj 

quartzite

(Jouplets Ci

Quartztte/ g 

sittite s

£uartzrfe A

Hughs (1983) 

Blackbird area

Quartzite G

Sarid-^iit s8

Mud-sand fA

Figure R3. Comparison of informal stratigraphic units 

of the Yellowjacket Formation showing tentative correlation of rocks exposed in different areas near the 

northeastern corner of the Challis quadrangle.

can be distinguished by their high content of magnesium, 

titanium, vanadium, chromium, iron, nickel, and copper 

(table Rl).

Depositional sedimentary structures are locally well 

preserved in the Yellowjacket Formation. Structures seen 

include parallel laminations and cross-laminations, 

oscillatory and current ripples, load casts, fluid-escape and 

sand-injection structures (fig. R4), soft-sediment slump 

folds, graded bedding, and others (Lopez, 1981).

Local bedding-plane polygonal structures that 

appear to be subaerial desiccation cracks in the Yellowjacket Formation (fig. R5) have been observed by the 

author at several locations north of Iron Creek in the 

Degan Mountain quadrangle, and by E. B. Ekren in three 

locations in and near the Yellowjacket mining district to 

the west (written commun., 1984). Ekren and I concur that 

these structures are distinct from the "pseudo-mudcracks" 

noted by Lopez (1981) as having been produced by the 

intersection of cleavage surfaces with deformed bedding 

surfaces in laminated rocks. If they are true subaerial 

desiccation cracks and not synaeresis (shrinkage) cracks 

formed underwater in buried sediments due to dewatering 

and changes in pore-fluid electrolyte balance, they indicate 

that at least locally and periodically the surface of deposition was emergent. This conclusion agrees with the 

interpretations of Ross (1934) and Carter (1981) that the 

Yellowjacket Formation was of shallow marine origin. Recently, however, Lopez (1981), Sobel (1981, 1982), and 

Hughes (1983) interpreted much of the Yellowjacket 

section to the north as consisting of distal, deep-water turbidites. This lithology may indicate a transition from the 

margin of the basin of deposition to the center.

MINERAL DEPOSITS

The most important ore deposits within the PrecamTable Rl. Contents of selected elements in the metavolcanic 

rocks of Yellowjacket Formation from the Challis quadrangle 

[1, metamorphosed volcanic rock from the Yellowjacket Formation at 

about lat 44°56'46" N., long 114°06'07" W., about 1.8 km westsouthwest of Degan Mountain. 2, metamorphosed volcanic rock (mafic 

tuff?) from the Yellowjacket Formation at about lat 44°56'47" N., long 

114° 7'113" W., along North Fork of Iron Creek, about 3.2 km westsouthwest of Degan Mountain (K. V. Evans, U.S. Geological Survey, unpub. data, 1984). 3, typical values for clastic sedimentary rocks of the 

Yellowjacket Formation, data from Lopez (1981) and this study. Values 

are in parts per million except where noted; >, greater than]

Element 1 2 3

In percent

Mg 

Ti _______

Cr ________

j-i_ _. _

CU ________

3 

1.5 

.7 

5

In

200 

300 

700

50 

150 

2,000

5 

7 

>1 

15

parts per million

500 

200 

1,500

70 

150 

70

0.3-1.0 

.05-.7 

.1-.5 

.7-10

30-150 

15-100 

50-1,500

5-15 

5-30 

5-50

brian rocks in the study area are the cobalt-copper 

deposits of the belt centered about the Blackbird mine. 

The southeastern end of this cobalt belt lies within the 

Challis quadrangle, in the Iron Creek area. The Twin 

Peaks lead-copper deposit, near the extreme northeastern 

corner of the Challis quadrangle, may be largely unrelated 

to the stratabound Blackbird-type deposits. Other small 

copper, gold, and base-metal deposits within the Precambrian rocks are likely to have been derived mainly as a 

result of post-Precambrian igneous activity.

Cobalt-Copper-Magnetite Deposits in the 

Iron Creek Area

Strata of the Yellowjacket Formation, mineralized 

with chalcopyrite and cobaltiferous pyrite, dip moderately 

(40-60°) to the northeast and strike northwest across the 

North Fork of Iron Creek (fig. Rl) in the northeastern 

corner of the Challis quadrangle. This copper-cobalt 

deposit was discovered in 1967-68 during construction of 

a logging road up the creek. Several hundred feet of drifts 

and crosscuts, the Little No-Name mine, were driven into the hillsides from adits on both sides of the creek by 

Sachem Prospects Corp. in 1970-71 (Cohenour and 

others, 1973). Additional work by Sachem and succeeding companies included core drilling and induced 

polarization, resistivity, and ground magnetic geophysical 

surveys, soil and stream-sediment geochemical sampling, 

and additional drift construction (Cohenour and others, 

1973; Peters, 1979). The patented claims along the North 

Fork of Iron Creek are currently (1985) held by Noranda 

Mining, Inc.

Cobalt-Copper Deposits in the Yellowjacket Formation 209
Figure R4. Laminated very fine grained argillaceous quartzite 

and silty argillite showing penecontemporaneous deformation 

and disruption of bedding by injection of sand during differential 

compaction and dewatering. Yellowjacket(?) Formation, Swan 

Peak, Leesburg 15-minute quadrangle.

Cohenour and others (1973) described the deposit 

as "fracture and cavity fillings as veins and veinlets and 

commonly as fine-grained disseminations'' "concentrated 

in what appear [s] to be distinct lens-shaped bodies" and 

"associated with zones of brecciation and bedding plane 

fractures, possibly related to bedding plane faults!' Roadcuts along the creek show that this mineralized zone is 

separated from a magnetite-rich zone about 500 m to the 

south and about 400 m stratigraphically lower by largely 

barren argillaceous quartzite and siltite. Microprobe 

analysis of pyrite in ore from the Little No-Name mine 

revealed as much as about 2-3 weight percent cobalt; no 

separate cobalt mineral phases have been recognized. 

Cohenour and others (1973) reported that systematic 

sampling from the adits and outcrops showed average 

metal contents of 0.76 percent copper, 0.19 percent cobalt 

(along 233 feet of the east adit); 0.87 percent copper, 0.30 

percent cobalt (along a 124-foot drift off the east adit); 

1.08 percent copper, 0.21 percent cobalt (120 feet along 

outcrops); and 0.16 percent copper and 0.08 percent cobalt 

(along 375 feet of the west adit). Samples analyzed by the 

U.S. Geological Survey (table R2, no. 8) showed as much 

as 2 percent copper and 0.3 percent cobalt.

Erdman and Modreski (1984) have described the 

utility of aquatic moss as a concentrator of and prospecting guide for cobalt and copper, based on a study done

A

Figure R5. Bedding-plane polygonal structures inferred to be 

mudcracks of subaerial origin. A, undeformed polygons in

argillaceous siltite from an outcrop about 1 km east of Yellowjacket Creek, within the Casto quadrangle, studied by Ross 

(1934). 6, deformed polygons in argillaceous siltite of Yellowjacket^) Formation north of Badger Creek in the Iron Creek 

area, Degan Mountain 7!/2-minute quadrangle.

210 Challis 1°x2° Quadrangle, Idaho
in the North Fork Iron Creek drainage. They reported as 

much as 2,000 ppm cobalt and 35,000 ppm copper in dryash weight of aquatic mosses at and downstream from the 

deposits.

Cohenour and others (1973) distinguished four socalled "trends" or "zones" of mineralized rock across the 

North Fork of Iron Creek (see also Erdman and Modreski, 

1984). From south to north these are a "magnetite-coppercobalt trend;" a "copper-cobalt trend" that includes the 

Little No-Name mine; a "copper sulfate zone" named for 

copper sulfate bloom on surface exposures, said to overlie 

rock containing disseminated chalcopyrite; and an "arsenopyrite-gold zone" said to contain gold- and cobalt-bearing 

arsenopyrite veins that cut across bedding.

The strata rich in magnetite also locally contain some 

disseminated pyrite and chalcopyrite, and some tourmalinequartz veins. Magnetite in these beds is in thin laminae 

along bedding planes, in irregular streaks and veinlets, and 

in layers composed entirely of massive magnetite. The magnetite is essentially pure Fe3O4 and is not particularly 

cobaltiferous; microprobe analysis showed 0-0.02 weight 

percent MgO, 0.13-0.18 percent A12O3 , 0.01 percent CaO, 

0-0.02 percent TiO2, 0-0.1 percent V2O3 , 0-0.02 percent 

MnO, and 0-0.03 percent CoO. Emission spectrographic 

analysis of a fairly pure sample of magnetite showed only 

30 ppm cobalt.

These magnetite-bearing strata are best exposed at 

three places along the road that follows the North Fork of 

Iron Creek and then circles eastward around the south side 

of Degan Mountain: (1) about 4.0 km north of the confluence of the North Fork of Iron Creek with Iron Creek; 

(2) about 3.4 km north-northeast of this confluence, on the 

north side of Jackass Creek; and (3) about 2.5 km northeast of this confluence, southeast of Jackass Creek. A prominent red gossan is near Jackass Creek, and the roadcuts 

on the northern side of the creek expose mineralized rocks 

that develop prominent efflorescences of blue chalcanthite 

(copper bloom) and pink to white and yellow efflorescences 

which have been taken to be erythrite (cobalt bloom), but 

which chemical analysis and X-ray diffraction studies actually show to be cobalt-bearing pickeringite (hydrous 

magnesium-iron aluminum sulfate containing about 0.5-1.0 

weight percent CoO). At outcrops southeast of Jackass 

Creek and north of the road, black veinlets of fine-grained 

intergrown tourmaline and quartz cut the magnetite-bearing 

strata. Prominent tourmaline-quartz veins and breccias are 

also present closer to Iron Creek, and are discussed in the 

section below. This magnetite-rich horizon, and possibly additional strata containing minor amounts of magnetite more 

distant from the ore deposits, probably accounts for the 

strong, positive magnetic anomaly that trends northwest 

across this corner of the Challis quadrangle (Mabey and 

Webring, chap. E, this volume). Cohenour and others (1973) 

also suggested that sharp, local discontinuities in magnetic 

anomalies in this area may be due to a northwest-trending 

fault that offsets igneous rock at depth.

The "arsenopyrite-gold zone" of Cohenour and others 

(1973) is poorly exposed in bulldozer cuts on the hillside 

west of the North Fork of Iron Creek, about 2.0-2.2 km 

north of the Little No-Name mine. No specific cobalt 

minerals have been identified from there, but the arsenopyrite is cobalt bearing, and pale-pink cobalt bloom of 

cobaltoan pickeringite is on weathered samples. Cohenour 

and others (1973) reported that high-grade vein samples from 

the "arsenopyrite-gold zone" showed as much as 1 ounce 

of gold per ton, 0.4 percent cobalt, and 0.4 percent bismuth. 

Our analyses (table R2, no. 8) have shown as much as 0.3 

percent cobalt, 0.15 percent bismuth, and 150 ppm nickel; 

we made no assays for gold.

Tourmaline-Quartz Veins and Breccias

Bodies and veins of black, fine-grained tourmaline 

in quartz occur in several places near the strata mineralized 

with cobalt and copper near Iron Creek, but not within the 

mineralized strata. They range from breccias composed of 

angular quartzite clasts in a fine-grained, flinty matrix of 

quartz-tourmaline (fig. R6), to spiderwebs of thin, black 

veinlets permeating the rock, to solid veins of fine-grained 

quartz and tourmaline as thick as 6 cm (centimeters). Prominent outcrops of breccia and veins occur just northeast of 

the confluence of Iron Creek and its North Fork, about 

240-300 m (800-1,000 feet) above the creek level. Quartzite 

within and adjacent to the breccias is pale and bleached, 

and contains only sericite with no dark mica or chlorite. 

Similar quartz-tourmaline veins are present as outcrops and 

float in at least six other places in the Degan Mountain and 

Taylor Mountain quadrangles. A few exposures show thin 

veins containing quartz, tourmaline, and specular hematite. 

Tourmaline-quartz veins and breccias also occur near the 

Blackbird mine, particularly at the Patty B and HaynesStellite prospects, which contain fine-grained, silicified, tourmalinized breccia that also is mineralized with disseminated, 

fine-grained cobaltite. In the Iron Creek area, the tourmaline-quartz rock contains only near-background-level concentrations of cobalt and copper (table R2, no. 15).

Tourmaline, specifically magnesian tourmaline, has 

been noted as a prominent associate of massive sulfide 

deposits, notably at the Sullivan mine, British Columbia, 

and at several Appalachian sulfide deposits in volcanic terranes (Slack, 1982). The tourmaline at Iron Creek is intermediate in composition; preliminary microprobe analyses 

show atomic Mg:Mg+Fe to be about 0.35, and about 0.50 

in coarser grained tourmaline in quartz near the Blackbird 

mine. At the Sullivan mine, extensive footwall tourmalinization is interpreted to represent boron metasomatism by fluids 

related to those that transported the metals to the deposit 

(Ethier and Campbell, 1977; Campbell and Ethier, 1983). 

In the Blackbird mine and Iron Creek areas, the boronbearing fluids may have been laterally distant from those 

that carried the cobalt, copper, and iron, or they may have

Cobalt-Copper Deposits in the Yellowjacket Formation 211
Table R2. Contents of cobalt and other metals in mineralized

[Analyses by six-step semiquantitative emission spectrography, except zinc in 68 Yellowjacket Formation samples by atomic 

Conklin, M. Malcolm, U.S. Geological Survey. Value given as arithmetic mean and median (in parentheses) for each data 

in parts per million; iron in weight percent. Limits of detection are as indicated except zinc is 5 ppm for samples analyzed 

equal to; <, less than or equal to; ~, about]

1.

2.

3.

4.

5. 

6. 

7.

8.

9.

10. 

11. 

12. 

13. 

14.

15. 

16.

Yellowjacket Formation 

(Lopez, 1981);

Yellowjacket Formation, 

unmineralized;

Lemhi Group;

Blackbird mine (C. M. 

Tschanz, unpub. data, 

1979); 10 samples 

Blackbird mine;

Blackpine mine;

Haynes Stellite mine 

and Patty B prospect;

Iron Creek Area:

Arsenopyrite zone; 

5 samples 

Little No-Name 

mine; 3 samples 

Magnetite-rich 

rock; 5 samples 

Sulfide-bearing 

rock; 5 samples 

Minor copper prospects 

and showings;

Musgrove mine;

Pope-Shenon mine;

Salmon Canyon mine;

Sweet Repose mine

Specular hematite 

prospects:

McKim Creek; 

2 samples 

Moyer Creek;

Tourmaline-quartz veins 

and breccias;

Twin Peaks mine;

Co 

5

9 

(10)

9 

(8)

>6,000 

(6,000)

16,000 

(18,000)

58 

(20)

11,000 

(1,000)

780 

(150)

2,800 

(2,000)

92 

(30)

690 

(500)

46 

(7)

10

52 

(70)

2,700 

(700)

400

5

8

13 

(5)

8 

(7)

B 

20

70 

(30)

92 

(30)

63 

(70)

<10

75 

«20)

<20 

«20)

5,000 

(7,000)

20 

«20)

320 

(200)

650 

(200)

20 

(20)

100 

«20)

 

5,000 

(2,000)

V 

7

82 

(30)

42 

(30)

53 

(30)

1 

«D

10 

(12)

25 

(30)

38 

(30)

88 

(70)

20 

(15)

57 

(70)

22 

(10)

42 

(25)

20

15 

(15)

50 

(30)

110

30 

165

45 

(40)

11 

(10)

Cr

1

52 

(50)

35 

(25)

28 

(20)

5 

(5)

7 

(6)

16 

(15)

32 

(30)

18 

(30)

13 

(10)

1 

(1)

7 

(5)

-28 

(20)

15

13 

(7)

30 

(30)

120

4 

7

21 

(18)

4 

(2)

Mn

1

370 

(200)

450 

(250)

73 

(100)

490 

(355)

150 

(110)

170 

(150)

60 

(70)

750 

(200)

230 

(300)

360 

(110)

220 

(300)

390 

(300)

70

210 

(300)

600 

(500)

250

83 

83

100 

(100)

230 

(300)

Fe 

0.001

4 

(3)

3 

(2.5)

1 

(1)

>20 

>8

>7 

(7)

4 

(3)

6 

(7)

>10

>10

6 

(3)

2 

(2)

5

6 

(7)

>9 

(10)

7

>10 

>10

3 

(3)

7 

(7)

Ni 

5

21 

(20)

13

(10)

9 

(7)

1,300 

(1,200)

2,000 

(425)

77 

(70)

280 

(100)

64 

(70)

150 

(150)

15 

(15)

21 

(20)

17 

(12)

20

24 

(15)

420 

(150)

70

13

10

22 

(15)

10 

(7)

Analysis by atomic absorption; limit of detection 5 ppm. 

9 Analysis by quantitative emission spectrography; limit of detection 10 ppm.

212 Challis 1°x2° Quadrangle, Idaho
and unmineralized rocks, Challis quadrangle and vicinity

absorption (Lopez, 1981), and 10 Blackbird mine samples by quantitative emission spectrography. Analysts: L. Bradley, N. M. 

set. Only one value is given if data set was two samples or fewer, or if all values are below or above detection range. Values 

by atomic absorption (group 1). Leaders (--), value less than limit of detection; <, less than; >, more than; >, more than or

Cu Zn As Mo Ag Sn Pb Bi 

1 300 1,000 3 0.5 10 10 10

17 *48 1 2? 9 

«5) (30) «5) «10) (5)

23

13

M7.000 1 280 20,000 2 9 10 10 240 

(20,000) (285) (15,000) «6) (10) «10) (9) (190)

17,000 ~50,000 4 920 

(18,000) (4) (320)

28,000 >300 23 10 _<10 33 

(10,000) «300) (15) «10) «10) (30)

80 27,000 4 20 10 

(70) (7,000) (5) (30) (10)

120 40,000 1 9 330 

(150) «1,000) «0.5) «10) «10)

18,000 >500 >3 8 70 38 

(20,000) «1,000) «3) (10) (30) (50)

110

110 13 10

(70) (10) (10)

16,000 <3 10 110 XL4

(4,000) «3) (2) (30) «10)

23,000 6 28 15

(20,000) (7) (15) (15)

28,000 300 40,000 4 34 7 13 220

(30,000) (300) (15,000) (5) (50) (10) (10) (150)

1,500 1,000

65

110 >6

(22) «10)

33,000 3 63 5,000

(3,000) «3) (30) (2,000)

Cobalt-Copper Deposits in the Yellowjacket Formation 213
Figure R6. Breccia of sericite-quartzite with matrix of black, 

fine-grained tourmaline and quartz, cut by later veinlets of white 

quartz in tension cracks. Yellowjacket Formation, north of Iron 

Creek, Degan Mountain /V^-minute quadrangle.

been the same fluids at an earlier or later stage in their 

time-temperature-depth evolution. It is also possible that 

the fluids that formed these tourmalinites were exhaled 

from magma in unexposed Cretaceous or Eocene plutons 

(Purdue, 1975).

Lead and Copper Deposits at the Twin Peaks Mine

The inactive Twin Peaks mine, southwest of Rattlesnake Creek near the extreme northeastern corner of the 

Challis quadrangle (fig. Rl), consists of four principal 

adits and drifts that were worked for lead and copper. The 

mine produced galena (with silver values) and copper 

(chalcopyrite-malachite-cuprite-pyrite), according to Stair 

(1955) and Peters (1979). Starr reported that the ore from 

the "lead adits" contained about 10 percent lead, 6 ounces 

of silver per ton, and 0.01-0.02 ounces of gold per ton, 

and ore from the "copper adit" contained about 10-12 

percent copper and 9 ounces of silver per ton. Only a few 

hundred tons of ore were shipped. Starr's (1955) mine 

mapping showed that the minerals were mainly in shear 

zones that cut across bedding, with some disseminated 

minerals in the massive rock. The host rock is mainly gray 

or greenish-gray siltite and argillaceous quartzite. 

Although it is generally similar in appearance to the

Yellowjacket sequence along the North Fork of Iron 

Creek, it is more argillaceous overall. The mine may actually be in strata of the Apple Creek Formation of the 

Lemhi Group. A possible indication that the rock is not 

Yellowjacket is the relative absence of cobalt and nickel 

from this deposit (table R2, no. 16).

Stratigraphic and Structural Framework of the Twin Peaks 

Mine Area

Just east of the mine, the Yellowjacket(?) beds 

terminate against a clean, massive, medium-grained, 

feldspar-free white quartzite. Much of the quartzite has 

a distinctly bimodal size distribution of quartz grains, 

resembling that described by McCandless (1982) in the 

Lower Ordovician Summerhouse Formation. The contact 

is not well exposed; some of the quartzite is fractured near 

the contact, but attitudes of the massive quartzite and the 

argillaceous quartzite and siltite beds are essentially conformable; both dip about 30-50° to the west. About 

200 m of this massive quartzite is exposed in the steep 

canyon of Rattlesnake Creek, where a thin (50 m at most, 

generally much less) layer of gray, fine-grained dolomite 

lies atop the quartzite and appears to grade into the quartzite. The dolomite is poorly exposed for about 1 km north 

from Rattlesnake Creek. Landreth (1964) reported fossil 

corals in the dolomite and suggested that it and the 

underlying quartzite correlate with the Ordovician Kinnikinic Formation. A sample of the dolomite collected 

during the present study contained fragmentary brachiopods and conodonts, which indicate a probable Middle 

Ordovician age (J. T. Dutro, Jr., and John Repetski, U.S. 

Geological Survey, written commun., 1983, USGS sample no. 9666-CO). The dolomite and quartzite may be 

equivalent to the Middle Ordovician Ella Dolomite and 

the Lower Ordovician Summerhouse Formation (Hobbs 

and others, 1968; McCandless, 1982; Ruppel and others, 

1975) or to the basal part of the Middle and Upper Ordovician Saturday Mountain Formation (Hobbs, chap. D, 

this volume) and an underlying quartzite such as the 

Kinnikinic.

These relations indicate that, despite a 30-50° dip 

to the west of both the dolomite-quartzite sequence and 

the adjacent argillaceous Yellowjacket(?), the two sequences are separated by a major fault. The trace of this 

fault contact indicates an average 60° dip to the west 

(Fisher and others, 1983). The dolomite-quartzite sequence appears to rest conformably on a thin- and thickbedded sequence of green to red, feldspathic, sericitic 

quartzite and interbedded phyllite that most closely 

resembles quartzites of the Big Creek Formation of the 

Lemhi Group. These relations likewise suggest that, 

despite the apparent conformity of dip in the two sequences, the dolomite-quartzite sequence is separated 

from the underlying quartzite-phyllite sequence by a 

major fault. Both faults parallel the bedding, but the

214 Challis 1°x2° Quadrangle, Idaho
direction of displacement is unknown. Landreth (1964) 

concluded that the fault above the dolomite-quartzite sequence was a high-angle reverse fault dipping about 60°. 

The fault may have been reactivated in early Tertiary time 

as a right-lateral strike-slip fault.

Minor Copper Occurrences

Several small showings of copper are present in the 

Yellowjacket Formation in the northeastern part of the 

Challis quadrangle. These showings are mostly malachite 

along bedding planes; a few include chalcopyrite associated with quartz pods. Prospect pits exposing minor 

amounts of malachite and chalcopyrite are in the Yellowjacket(?) Formation south of Rattlesnake Creek, west of 

the Twin Peaks mine. Several small pits and adits were dug 

into showings in Badger Basin, just north of Iron Creek 

and south of Degan Mountain (fig. Rl). Other showings 

are along the North Fork of Iron Creek, along the eastfacing cliffs about 2.8 and 3.8 km north-northeast of Iron 

Lake, and about 0.9 km east-southeast of Moyer Peak. 

Analyses (table R2, no. 9) show only minor amounts of 

cobalt in these occurrences, and no cobalt bloom was seen.

Specular Hematite Occurrences

Veins of specular hematite in rocks of the Yellowjacket Formation occur on the west side of Moyer Creek 

west-northwest of Moyer Peak (fig. Rl). The hematite 

veins (fig. R7) are in a zone just beneath a series of calcsilicate beds; the quartzite containing the hematite is also 

somewhat calcareous. The zone is exposed by bulldozer 

cuts, known as the Iron King claims (Peters, 1979). Most 

of the hematite occurs as veins having various attitudes, 

but some layers in the calcareous quartzite appear to have 

been partly replaced by fine-grained flakes of hematite. 

The exposure is erosionally truncated and covered by 

Challis Volcanics.

A similar deposit occurs north of McKim Creek, 

just east of the Salmon River in the Lemhi Range (fig. 

Rl, Dubois quadrangle). Here specular hematite veins 

were mined on a small scale from open cuts at the Black 

Angus deposits (Soregaroli, 1961). Some of the ore is said 

to have been sold around 1982 for use in heavy drilling 

mud (D. W. Peters, oral commun., 1983). The host rock 

of this iron deposit is a micaceous quartzite, commonly 

green (chloritic) near the deposit; it locally has a pinkishwhite, bleached appearance near the veins. Its formational 

affinity is uncertain; Rember and Bennett (1979) have indicated that rocks near this area are part of the Lemhi 

Group. Anderson (1961) described additional hematite 

vein deposits (with minor associated magnetite and veins 

of white quartz) farther to the east in the Lemhi Range, 

in rocks he ascribed to his "Apple Creek Phyllite" and 

Swauger Quartzite.

Figure R7. Specular hematite veins cutting calcareous (calcsilicate) quartzite. Yellowjacket Formation, west of Moyer Creek, 

Opal Lake /'/"-minute quadrangle.

The specular hematite veins at both the above locations are essentially barren of cobalt and most other 

metals, although those at McKim Creek contain a notable 

amount of tin (table R2, no. 14). They show no unusual 

abundance of rare-earth elements; the presence of these 

elements would have suggested a relationship to the 

quartz-thorite-hematite-copper vein deposits of the Lemhi 

Pass thorium district of the Beaverhead Mountains 

(Staatz, 1979); those veins occur in rocks of the Yellowjacket Formation (and Lemhi? Group) but are probably 

related to Late Cretaceous-Tertiary plutons, according to 

Staatz. Bennett (1977) reported masses of hematite as float 

northwest of Leesburg along the projection of the Leesburg fault, and hematite is associated with some of the 

gold ores in the Mackinaw (Leesburg) district. Small 

veinlets of specular hematite or hematite and quartz are 

widely distributed throughout the Yellowjacket Formation 

and Hoodoo Quartzite in the Degan Mountain-Taylor 

Mountain-Opal Lake area. The hematite veins appear to 

be largely a product of metamorphism and intrusion of 

the Phanerozoic plutons.

Cobalt and Other Metals in the Blackbird District 

and Vicinity

The Blackbird mining district is about 30 km west 

of Salmon, in the Elk City l°x 2° quadrangle (Anderson,

Cobalt-Copper Deposits in the Yellowjacket Formation 215
1943b, 1947; Bennett, 1977; Umpleby, 1913; US. Bureau 

of Mines, 1943; Vhay, 1948). It comprised many nearby 

small mines and prospects, including the Belielle, Bohannon, Brown Bear, Bryan-Columbus, Chicago, Copper 

Queen, Dusty, Ella, Hawkeye, Haynes Stellite, High Five, 

Katherine, Ludwig, Patty B, St. Joe, Sunshine, Tom Jefferson, and Uncle Sam mines. Some of these were 

originally operated as separate mines but were later consolidated and connected through their underground workings as the Calera mine, and then as the Blackbird mine. 

Workings and ore horizons in the Blackbird mine (including the Idaho, Dandy, Chicago, and Brown Bear zones 

and the Blacktail open pit) strike N. 40° W. and dip 

50-60° to the northeast (Bennett, 1977). The cobalt belt 

extends northwest and southeast from the Blackbird mine. 

To the northwest are additional cobalt occurrences recently 

found within the Special Mining Management Zone-Clear 

Creek of the River of No Return Wilderness, and one inactive mine containing copper and cobalt, the Salmon 

Canyon Copper Co. mine. To the southeast are a few 

small, inactive mines and prospects, including the Blackpine mine, and at the extreme southeastern end of the 

trend is the Iron Creek area previously discussed in this 

chapter. In addition to the deposits discussed in more 

detail below, the Tinker's Pride mine about 6 km north 

of the Blackbird mine was reported to contain minor 

cobalt (Bennett, 1977; Vhay, 1948), and an unnamed prospect at the head of French Gulch about 6 km south of 

the Blackbird mine was reported to have been worked for 

cobalt and copper (Bennett, 1977), although Vhay (1948) 

found no cobalt there.

Blackbird mine. The inactive Blackbird mine is on 

the largest identified cobalt deposit in North America. 

The ore consists of cobaltite, chalcopyrite, pyrite, pyrrhotite, arsenopyrite, and minor amounts of other sulfides 

and arsenides, in biotite- to garnet-grade metamorphic 

rocks of the Yellowjacket Formation (Anderson, 1947; 

Roberts, 1953; Vhay, 1948; Purdue, 1975; Bennett, 1977; 

additional, unpublished, reports about the district are 

listed by Lund and others, 1983). The mine consisted of 

the Blacktail open pit and extensive underground workings. Preliminary work to reopen the underground mine, 

begun by Noranda Mining, Inc. in 1978, was suspended 

in 1982 due to a decline in the price of cobalt; reserves 

are said to be on the order of 4 million tons of ore containing 0.6 percent cobalt and 1.2 percent copper (Engineering and Mining Journal, 1980; table R2, nos. 4, 5). 

The mine produced about 14 million pounds of cobalt 

during the 1950's. The ore at the Blackbird mine is stratabound and appears to have been syngenetic (Lopez, 1981; 

Hahn and Hughes, 1984), but it has been extensively 

recrystallized and remobilized along shear zones and folds. 

Evident metamorphic changes include segregation of 

chalcopyrite into quartz pods at fold crests and the 

localization of the principal ore horizons along major 

zones of shearing or deformation (Purdue, 1975; Bennett,

1977). As was pointed out by Lund and others (1983), some 

of the gold and silver mineralization noted by Anderson 

(1947) may be a product of veins associated with Tertiary 

igneous activity rather than a part of the original Precambrian deposit.

The textural evidence for a syngenetic origin of the 

deposits is by no means as clearcut as it is in some of the 

world-famous volcanogenic massive-sulfide deposits, such 

as the Kuroko deposits of Japan. The actual preservation 

of syngenetic structures in the ore at the Blackbird mine 

has not been documented. Thin laminae of fine-grained 

cobaltite parallel to bedding may represent relict syngenetic layering. However, indirect evidence exists: some 

siliceous rocks in and near the ore zones may be exhalative 

cherts, silica that was vented into seawater by hot springs 

and subsequently precipitated (Hahn and Hughes, 1984). 

Black biotite-rich strata that occur close to ore horizons 

were interpreted as metamorphosed basaltic tuffs or 

sediments having an originally high tuffaceous component 

(Hahn and Hughes, 1984).

Hughes (1983) and Hahn and Hughes (1984) interpreted the basin into which the "Yellowjacket sediments were 

deposited as a northwest-trending rift basin bordering the 

Archean craton to the east. They interpreted the "Yellowjacket sediments to be a submarine fan complex that prograded into a deepwater basin, with the Blackbird and 

associated cobalt deposits lying near the axis of the basin.

Haynes Stellite mine. This mine, about 2 km eastsoutheast of the main Blackbird mine (fig. Rl), produced 

cobalt during 1917 to 1921 (Vhay, 1948). It was originally 

known as the Haynes Stellite mine and later as the Cobalt 

mine, and the adits are labeled "Cobalt Mine" on the current (1950) Blackbird Mountain IVi -minute topographic 

quadrangle. The rocks at this deposit (table R2, no. 7) include silicified and tourmalinized breccia containing 

disseminated cobaltite and mineralized biotitic strata that 

may represent former tuffaceous beds.

Patty B prospect. This prospect, about 3.6 km 

southeast of the Blackbird mine (Vhay, 1948), includes a 

prominent outcrop of silicified, tourmalinized breccia containing disseminated cobaltite (table R2, no. 7).

Sweet Repose mine. This privately held claim is on 

one of several small cobalt occurrences peripheral to the 

Blackbird deposit. It is about 6.5 km northeast of the 

Blackbird mine (fig. Rl) (Anderson, 1943; Lopez, 1981). 

It contains fairly coarse grained cobaltite in biotite schist 

(two analyses are summarized in table R2, no. 13) that may 

represent mafic tuffaceous rock.

Blackpine mine. This inactive deposit, about 14 km 

southeast of the Blackbird mine (fig. Rl), contains pyritechalcopyrite-arsenopyrite ore similar to that of the Blackbird mine but lacks cobaltite (Shockey, 1957; Bennett, 

1977). Ore samples collected on the dump had a low cobalt 

content (table R2, no. 6). Ore occurs as veinlets and 

stringers, grossly stratabound within chlorite-biotitemuscovite siltite-quartzite.

216 Challis 1°x2° Quadrangle, Idaho
Salmon Canyon Copper Co. mine. This deposit, 

in high-grade garnet-muscovite-biotite schist and gneiss, 

represents the farthest known extension of the cobalt belt 

to the northwest. It is at about 45°18' N., 114°33' W., on 

the north side of the Salmon River, 26 km northwest of 

the Blackbird mine (fig. Rl). The minerals here include 

rather coarsely crystalline chalcopyrite, pyrite, 

arsenopyrite, and cobaltite, and are associated with quartz 

stringers and pods. Analyses of three ore samples are summarized in table R2, no. 12.

Cobalt occurrences in the Special Mining Management Zone Clear Creek. About 15-18 km northwest of 

the Blackbird mine, within the River of No Return Wilderness, geochemical cobalt anomalies were found in several 

drainages, principally along Garden Creek and Elkhorn 

Creek (Lund and others, 1983). This area is within the 

special mining management area designated by the U.S. 

Congress to permit continued development of cobalt 

resources. Outcrops containing cobaltite were found at one 

or more locations, near Elkhorn Creek.

Other Copper, Gold, and Base-Metal Deposits

The inactive Pope-Shenon copper mine is south of 

the town of Salmon, at the northern end of the Lemhi 

Range (fig. Rl, Dillon quadrangle) (Anderson, 1943a). 

The ore was predominantly chalcopyrite, pyrite, and 

delafossite (CuFeO2), with magnetite, hematite, actinolite, epidote, and quartz, and was said to occur mainly 

in east-trending shear zones within dark-green biotitesericite-chlorite quartzite. Bodies of breccia, mineralized 

with pyrite and chalcopyrite, were probably associated 

with Tertiary dikes; the mine is near a Tertiary caldera 

mapped by Ruppel and others (1983). The ore contained 

mainly copper (2-15 percent) with only minor silver (0.1 

ounce/ton or less) and a trace of gold (0.01 ounce/ton or 

less) (Anderson, 1943a). Our analyses showed minor 

cobalt (15-70 ppm, table R2, no. 11). The quartzitic host 

rock resembles Yellowjacket Formation, and a preliminary 

geologic map of the Dillon 1°X2° quadrangle (Ruppel 

and others, 1983) shows it as such. The possible relationship of this deposit to the cobalt belt is problematical; 

it may be solely a product of later events.

Similar copper deposits (the Harmony mine and 

others) occur elsewhere within the Lemhi Range (Anderson, 1943, 1956, 1961). Also, the Tormey, Bowman, Copper King, and Bonanza Copper mines are in the Salmon 

River Mountains west of Salmon (Umpleby, 1913; Vhay, 

1948; Anderson, 1956; Shockey, 1957; Bennett, 1977; 

Peters, 1979).

The Musgrove mine, predominantly a gold deposit 

with quartz veins and limonitic breccia, is about 9.6 km 

south of the Blackbird mine (fig. Rl; Umpleby, 1913). It 

is within rocks of the Yellowjacket Formation but is probably related to Tertiary intrusive activity. Analysis of one

sample from the dump shows only a trace of cobalt (table 

R2, no. 10), though some prospects containing copper and 

cobalt occur elsewhere along Musgrove Creek (Vhay, 1948; 

Bennett, 1977).

The mines of the Yellowjacket district (fig. Rl), 

about 21 km southwest of the Blackbird district, contain 

gold and some copper, lead, and silver. The deposits are 

in rocks of the Yellowjacket Formation but are probably 

related to the Crags and Casto Tertiary plutons (Anderson, 

1953; Bennett, 1977; Carter, 1981; Peale, 1982). Likewise, 

lead-silver deposits (Ringbone Cayuse and Blue Jay mines) 

in the Mackinaw mining district near Leesburg, and gold 

deposits near Leesburg and in the Mineral Hill mining 

district near Shoup (all within the Elk City 1°X2° quadrangle, fig. Rl), are all probably related to Tertiary (or 

possibly Precambrian) intrusions (Bennett, 1977; Shockey, 

1957).

The gold, copper, and fluorite deposits of the Gravel 

Range mining district near Meyers Cove (Anderson, 1954; 

Ross, 1927; Umpleby, 1913) in the southwestern part of 

the area of figure Rl (nos. 33-35) are within the Challis 

Volcanics. The Eureka mining district, west of Salmon 

(fig. Rl, nos. 26-29) includes gold and copper deposits, 

some near a Precambrian pluton and some lying within 

the Yellowjacket Formation (Peters, 1979; Shockey, 1957; 

Anderson, 1956, 1943a; Umpleby, 1913). Also within this 

mining district are some small wolframite-quartz veins 

reported west of Baldy Mountain (Peters, 1979; Shockey, 

1957; near no. 26, fig. Rl), and thorium-uranium-rare 

earth veins near Diamond and Wallace Creeks (Peters, 

1979; Anderson, 1958; no. 32, fig. Rl) that appear to represent the southwesternmost extension of the Lemhi Pass 

thorium-rare earth district (Staatz, 1979). Other goldmining districts, not shown on figure Rl, include the 

Carmen Creek mining district, north of Salmon (Anderson, 1956; Umpleby, 1913), and the Indian Creek mining 

district, east of Shoup (Umpleby, 1913).

GEOCHEMICAL COMPOSITION OF ROCKS

Lopez (1981) noted that the Yellowjacket Formation 

as a whole is characterized by anomalously high values 

of cobalt, copper, arsenic, antimony, bismuth, and strontium. Because of relatively high limits of detection, 

the analyses reported in this chapter (table R2) do not 

include useful new data for arsenic, antimony, and 

bismuth in unmineralized rocks. My very limited data set 

(table R3) suggests that the high strontium content of the 

Yellowjacket Formation (about 100-150 ppm (parts per 

million)) may be useful in distinguishing it from lithologically similar units of the Lemhi Group (15-50 ppm 

strontium). The high strontium content may reflect the 

more calcareous and feldspathic nature of the Yellowjacket 

Formation, because strontium geochemically replaces 

calcium in calcite and feldspar. However, mineralized

Cobalt-Copper Deposits in the Yellowjacket Formation 217
Table R3. Strontium content in mineralized and unmineralized 

rocks of the Yellowjacket Formation and Lemhi Group, Challis 

quadrangle and vicinity

[Analyses by semiquantitative emission spectroscopy. Values are in parts 

per million. Limit of detection 5 ppm, except 100 ppm for the 68 samples 

from the Yellowjacket Formation (Lopez, 1981). Although some ore 

minerals were present in the mineralized rocks, all the samples in this 

table were composed predominantly of clastic sedimentary or volcanic 

material. <, less than]

No. of 

Source of sample analyses

Yellowjacket

Quartzite, siltite, and argillite 

Argillaceous quartzite and

Blackpine mine -- - 

Iron Creek area mineralized rocks:

North Fork of Iron Creek 

Southeast of Jackass Creek 

Rocks with tourmaline

Salmon Canyon Copper Co.

Formation

68

5 

2 

2 

3 

3

6 

4 

5 

4

3 

2 

1

3

2

Mean 

(median)

100 (100)

120 (150) 

420 

650 

22 (20) 

20 (30)

160 (60) 

40 (40) 

5 (5) 

88 (90)

80 (70) 

135 

15

29 (30) 

225

Range 

of data

<100-300

30-150 

150-170 

300-1000 

15-30 

<5-30

<5-700 

30-50 

<5-7 

20-150

70-100 

70-200 

15

7-50 

150-350

Lemhi Group

Ami 1 1aoon,io n.tar-t-^-lf-a . . .- - ^ 98 On} 1 R "ifl

Uncertain affiliation, either Yellowjacket Formation 

or Lemhi Group

Argillaceous quartzite and

siltite 5 60 (70) 30-100 

McKim Creek hematite prospect 2 42 15-70 

Pope-Shenon mine 2 11 7-15 

Twin Peaks mine 5 5 «5) <5-15

quartzite-argillite samples from the Blackbird mine and 

several others, firmly believed to belong to the Yellowjacket Formation, also show very low strontium contents 

(table R3), so more work needs to be done before this relationship can be verified. J. J. Connor (oral, commun., 

1984) has begun a detailed survey of the chemical composition of the Yellowjacket Formation, with the goal of 

using statistically significant chemical parameters as a 

means of distinguishing the Yellowjacket from other formations and as indicators of the provenance of its 

sediments, as has been done for the St. Regis, Grinnell, 

and Spokane Formations of the Belt Supergroup (Connor, 1984).

GENESIS OF THE BLACKBIRD-TYPE DEPOSITS 

AND COMPARISON TO OTHER DISTRICTS

The Blackbird district deposits were originally interpreted to be hydrothermal, related to the Idaho 

batholith (Anderson, 1947; Roberts, 1953; Purdue, 1975). 

It now appears that the deposits represent original 

syngenetic, stratabound mineralization that has been

modified by recrystallization, differentiation, and remobilization during regional metamorphism, and possibly 

by hydrothermal alteration or thermal metamorphism 

during subsequent intrusive events. Because of the present limited knowledge of the details of the process that 

formed Blackbird-type deposits, their formation can be 

modeled only in general terms. Metal-bearing fluids could 

have been derived either from cooling intrusions or from 

circulation set in motion by intrusive heat sources. Sulfide 

and arsenide minerals would have precipitated as the fluids 

cooled during venting on the sea floor. Silica in the fluids 

formed the exhalative chert, while quartz-tourmaline veins 

may have been forming at depth. The magnetite-rich 

deposits may have formed at a location laterally distant 

from the sulfides, in a more oxidized environment.

Compared to other deposits, the Blackbird-type 

cobalt-copper deposits appear to be partway between two 

extremes. One extreme (Franklin and others, 1981) is the 

volcanogenic, sea-floor massive sulfide deposits, such as 

the Kuroko, Besshi, and Cyprus-type base-metal and copper deposits, and the metal deposits presently forming at 

midocean spreading centers, all of which are in predominantly volcanic host rocks. The other extreme (Gustafson and Williams, 1981) is ore deposits within shallowwater clastic sedimentary rocks, such as the copper-silver 

deposits in Belt Supergroup rocks of Montana (Hayes, 

1984), the copper and copper-cobalt deposits of Zambia 

and Zaire (Annels, 1974; Annels and others, 1983), and 

the deposits of the Kupferschiefer of eastern Europe. 

Many of these sedimentary deposits are thought to have 

formed during diagenesis of organic-rich, shallow-water 

sediments (Renfro, 1974; Brown, 1978), although the 

ultimate source of the metals may have been at least partly 

magmatic (Annels and others, 1983). Metamorphosed 

deposits that seem to have particular affinities to those 

of the Blackbird district include the cobalt-gold-copper 

deposit of the Standard mine, Quartzburg district, Oregon 

(Vhay, 1959), the copper-zinc-iron-cobalt deposit at 

Outokumpu, Finland (Koistinen, 1981), and several Appalachian sediment-hosted, stratabound massive sulfide 

copper-lead-zinc deposits such as those of Ore Knob, N.C.; 

Black Hawk, Maine; and Elizabeth, Vt. (Slack, 1982); 

Ducktown, Tenn. (LeHuray, 1984); and the Gossan lead 

district, Virginia (Gair and Slack, 1984). The Sullivan, 

B.C., lead-zinc deposit resembles the Idaho cobalt-copper 

deposits in several important ways: it is within a finegrained sedimentary host rock (the Aldridge Formation, 

which is a possible though distant correlative of the 

Yellowjacket), it has indirect volcanic affinities, and it has 

tourmalinized footwall rocks (Hamilton, 1984; Hamilton 

and others, 1982). A unique and unmatched feature of 

the Blackbird district, however, is the high concentration 

of cobalt relative to other metals, particularly copper, 

nickel, and silver.

Better understanding of the genesis of the Idaho 

cobalt deposits may come from future mineral-chemistry

218 Challis 1°x2° Quadrangle, Idaho
studies; trace-element geochemistry studies of the ore, 

chert, and sedimentary and igneous rocks; sulfur and 

oxygen isotopic measurements; fluid-inclusion studies; 

laboratory studies of the partitioning of cobalt, copper, 

and nickel between minerals and hydrothermal fluids; and 

continued detailed stratigraphic study and geologic 

mapping.

REFERENCES CITED

Anderson, A. L., 1943a, Copper mineralization near Salmon,

Lemhi County, Idaho: Idaho Bureau of Mines and Geology

Pamphlet 60, 15 p., 2 pi. 

_____1943b, A preliminary report on the cobalt deposits in

the Blackbird district, Lemhi County, Idaho: Idaho Bureau

of Mines and Geology Pamphlet 61, 34 p. 

_____1947, Cobalt mineralization in the Blackbird district,

Lemhi County, Idaho: Economic Geology, v. 42, no. 1,

p. 22-46. 

_____1953, Gold-copper-lead deposits of the Yellowjacket

district, Lemhi County, Idaho: Idaho Bureau of Mines and

Geology Pamphlet 94, 41 p., 9 pi. 

_____1954, Fluorspar deposits near Meyers Cove, Lemhi

County, Idaho: Idaho Bureau of Mines and Geology

Pamphlet 98, 34 p., 12 pi. 

_____1956, Geology and mineral resources of the Salmon

quadrangle, Lemhi County, Idaho: Idaho Bureau of Mines

and Geology Pamphlet 106, 102 p., 3 pi. 

_____1958, Uranium, thorium, columbium, and rare earth

deposits in the Salmon region, Lemhi County, Idaho: Idaho

Bureau of Mines and Geology Pamphlet 115, 81 p. 

_____1961, Geology and mineral resources of the Lemhi

quadrangle, Lemhi County, Idaho: Idaho Bureau of Mines 

and Geology Pamphlet 124, 111 p., 2 pi.

Annels, A. E., 1974, Some aspects of the stratiform ore deposits 

of the Zambian copperbelt and their genetic significance, 

in Bartholome, Paul, Gisments stratiformes et provinces 

cupriferes: Liege, Belgium, Societe Geologique de Belgique, 

p. 235-254.

Annels, A. E., Vaughan, D. J., and Craig, J. R., 1983, Conditions of ore mineral formation in certain Zambian copperbelt deposits with special reference to the role of cobalt: 

Mineralium Deposita, v. 18, p. 71-88.

Bennett, E. H., 1977, Reconnaissance geology and geochemistry 

of the Blackbird Mountain-Panther Creek region, Lemhi 

County, Idaho: Idaho Bureau of Mines and Geology 

Pamphlet 167, 108 p.

Brown, A. C., 1978, Stratiform copper deposits evidence for 

their post-sedimentary origin: Minerals Science Engineering, v. 10, no. 3, p. 172-181.

Campbell, F. A., and Ethier, V. G., 1983, Environment of deposition of the Sullivan orebody: Mineralium Deposita, v. 18, 

p. 39-55.

Carter, C. H., 1981, Geology of part of the Yellowjacket mining district, Lemhi County, Idaho: Moscow, Idaho, University of Idaho M.S. thesis, 131 p.

Cater, F. W., and others, 1973, Mineral resources of the Idaho 

Primitive Area and vicinity, Idaho: U.S. Geological Survey 

Bulletin 1304, 431 p.

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, 1973 Pacific 

Northwest Metals and Minerals Conference, Coeur d'Alene, 

Idaho 1973, 15 p.; available at U.S. Bureau of Mines, 

Western Field Operations Center, East 360 3rd Ave., 

Spokane WA, 99202.

Connor, J. J., 1984, Geochemistry of the Middle Proterozoic 

Spokane, Grinnell, and St. Regis Formations of the Belt 

Supergroup, in Hobbs, S. W., ed., The Belt: Montana 

Bureau of Mines and Geology Special Publication 90, 

p. 102-103.

Engineering and Mining Journal, 1980, Reopening of the 

Blackbird mine a possible comeback for US cobalt: v. 181, 

no. 1, p. 29, 31.

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.

Ethier, V. G., and Campbell, F. A., 1977, Tourmaline concentrations in Proterozoic sediments of the southern Cordillera 

of Canada and their economic significance: Canadian 

Journal of Earth Science, v. 14, p. 2348-2363.

Evans, K. V., 1981, Geology and geochronology of the eastern 

Salmon River Mountains, Idaho, and implications for 

regional Precambrian tectonics: University Park, Pa., Pennsylvania State University Ph.D. thesis, 222 p.

Fisher, F. S., Mclntyre, D. H., and Johnson, K. M., 1983, 

Geologic map of the Challis 1°X 2° quadrangle, Idaho: U.S. 

Geological Survey Open-File Report 83-523, 41 p., 2 maps, 

scale 1:250,000.

Franklin, J. M., Lydon, J. W., and Sangster, D. F., 1981, Volcanicassociated massive sulfide deposits, in B. F. Skinner, ed., 

Economic Geology, 75th Anniversary Volume, 1905-1980: 

El Paso, Texas, Economic Geology Publishing Co., p. 485-627.

Gair, J. E., and Slack, J. F, 1984, Deformation, geochemistry, 

and origin of massive sulfide deposits, Gossan lead district, 

Virginia: Economic Geology, v. 79, no. 7, p. 1483-1520.

Gustafson, L. B., and Williams, Neil, 1981, Sediment-hosted 

stratiform deposits of copper, lead, and zinc, in B. F. Skinner, ed., Economic Geology, 75th Anniversary Volume, 

1905-1980: El Paso, Texas, Economic Geology Publishing 

Co., p. 139-178.

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: Montana Bureau of Mines 

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

Hamilton, J. M., 1984, The Sullivan deposit, Kimberley, British 

Columbia a magmatic component to genesis?, in Hobbs, 

S. W., ed., The Belt: Montana Bureau of Mines and Geology Special Publication 90, p. 58-60.

Hamilton, J. M., and others, 1982, Geology of the Sullivan 

orebody, Kimberley, B.C., Canada: Geological Association 

of Canada Special Paper 25, p. 597-665.

Hayes, T. S., 1984, The relation between stratabound coppersilver ore and Revett Formation sedimentary facies at Spar 

Lake, Montana, in Hobbs, S. W., ed., The Belt: Montana 

Bureau of Mines and Geology Special Publication 90, 

p. 63-64.

Cobalt-Copper Deposits in the Yellowjacket Formation 219
Hobbs, S. W., 1980, The Lawson Creek Formation of Middle 

Proterozoic age in east-central Idaho: U.S. Geological 

Survey Bulletin 1482-E, 12 p.

Hobbs, S. W., Hays, W. H., and Ross, R. J., Jr., 1968, The Kinnikinic Quartzite of central Idaho redefinition and subdivision: US. Geological Survey Bulletin 1254-J, 22 p.

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

Blackbird mining district, Lemhi County, Idaho, in The 

genesis of Rocky Mountain ore deposits changes with 

time and tectonics: Wheat Ridge, Colo., Denver Region Exploration Geologists Society, p. 21-27.

Koistinen, T. J., 1981, Structural evolution of an early Proterozoic 

stratabound Cu-Co-Zn deposit, Outokumpu, Finland: 

Royal Society of Edinburgh Transactions; Earth Sciences, 

v. 72, no. 2, p. 115-158.

Landreth, J. O., 1964, Geology of the Rattlesnake Creek area, 

Lemhi County, Idaho: Moscow, Idaho, University of Idaho 

M.S. thesis, 51 p.

LeHuray, A. P., 1984, Lead and sulfur isotopes and a model for 

the origin of the Ducktown deposit, Tennessee: Economic 

Geology, v. 79, no. 7, p. 1561-1573.

Leonard, B. F, 1962, Old metavolcanic rocks of the Big Creek 

area, in central Idaho, in Short papers in geology, 

hydrology, and topography: U.S. Geological Survey Professional Paper 450-B, p. B11-B15.

Lopez, David, 1981, Stratigraphy of the Yellowjacket Formation 

of east-central Idaho: U.S. Geological Survey Open-File 

Report 81-1088, 206 p.

Lund, Karen, Evans, K. V., and Esparza, L. E., 1983, Mineral 

resource potential map of the Special Mining Management 

Zone, Clear Creek, Lemhi County, Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF-1576-A, 

scale 1:50,000.

Maley, T. S., 1974, Structure and petrology of the lower Panther Creek area, Lemhi County, Idaho: Moscow, Idaho, 

University of Idaho Ph.D. thesis, 130 p.

McCandless, D. O., 1982, A reevaluation of Cambrian through 

Middle Ordovician stratigraphy of the southern Lemhi 

Range [Idaho]: University Park, Pa., Pennsylvania State 

University Ph.D. thesis, 157 p.

Mitchell, V. E., and Bennett, E. H., 1979, Geologic map of the 

Elk City quadrangle, Idaho: Idaho Bureau of Mines and 

Geology, scale 1:250,000.

Peale, R. N., 1982, Geology of the area southeast of Yellowjacket, 

Lemhi County, Idaho: Moscow, Idaho, University of Idaho 

M.S. thesis, 129 p.

Peters, D. W., 1979, Geology, mineral, and energy resources, in 

Source Document, Taylor Mountain Planning Unit, July 

1979: unpublished report available at Forest Supervisor's 

Office, Salmon National Forest, Salmon, Idaho 83467.

Purdue, G. L., 1975, Geology and ore deposits of the Blackbird 

district, Lemhi County, Idaho: Albuquerque, University of 

New Mexico M.S. thesis, 49 p.

Rember, W. C., and Bennett, E. H., 1979, Geologic map of the 

Dubois quadrangle, Idaho: Idaho Bureau of Mines and 

Geology, scale 1:250,000.

Renfro, A. R., 1974, Genesis of evaporite-associated stratiform 

metalliferous deposits a sabkha process: Economic 

Geology, v. 69, no. 1, p. 33-45.

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

Blackbird mining district, Lemhi County, Idaho: Economic 

Geology, v. 48, p. 447-456.

Ross, C. P., 1927, Ore deposits in Tertiary lava in the Salmon 

River Mountains, Idaho: Idaho Bureau of Mines and 

Geology Pamphlet 25, 21 p.

_____1934, Geology and ore deposits of the Casto quadrangle, Idaho: U.S. Geological Survey Bulletin 854,135 p.

Ruppel, E. T, 1975, Precambrian Y sedimentary rocks in eastcentral Idaho, Chap. A in Precambrian and Lower Ordovician rocks in east-central Idaho: U.S. Geological Survey 

Professional Paper 889, p. 1-23.

_____1978, Medicine Lodge thrust system, east-central Idaho 

and southwest Montana: U.S. Geological Survey Professional Paper 1031, 23 p.

_____1980, Geologic map of the Patterson quadrangle, 

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

Quadrangle Map GQ-1529, scale 1:62,500.

_____1982, Cenozoic block uplifts in east-central Idaho and

southwest Montana: U.S. Geological Survey Professional 

Paper 1224, 24 p.

Ruppel, E. T, O'Neill, J. M., and Lopez, D. A., 1983, Preliminary 

geologic map of the Dillon I°x2° quadrangle, Montana: 

U.S. Geological Survey Open-File Report 83-168, 1 map.

Ruppel, E. T, Ross, R. J., Jr., and Schleicher, David, 1975, 

Precambrian Z and Lower Ordovician rocks in east-central 

Idaho, chap. B in Precambrian and lower Ordovician rocks 

in east-central Idaho: U.S. Geological Survey Professional 

Paper 889, p. 25-34.

Shockey, P. N., 1957, Reconnaissance geology of the Leesburg 

quadrangle, Lemhi County, Idaho: Idaho Bureau of Mines 

and Geology Pamphlet 113, 42 p., 1 pi.

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

northeast margin of the Snake River Plain, Idaho, in 

Heisey, E. L., and others, eds., Rocky Mountain thrust belt 

 geology and resources: Wyoming Geological Association, 

29th Field Conference, Guidebook, no. 29, p. 499-515.

Slack, J. F, 1982, Tourmaline in Appalachian-Caledonian 

massive sulphide deposits and its exploration significance: 

Transactions of the Institution of Mining and Metallurgy, 

Section B, Applied Earth Science, v. 91, p. B81-B89.

Snee, L. W., 1983, Effects of thermal history on mineralization, 

in Lund, Karen, Evans, K. V., and Esparza, L. E., Mineral 

resource potential of the Special Mining Management 

Zone Clear Creek, Lemhi County, Idaho: U.S. Geological Survey Miscellaneous Field Studies Map MF-1576-A, 

Pamphlet, p. 7.

Sobel, L. S., 1981, A stratigraphic model for Precambrian-Y turbidites of central Idaho [abs.]: Geological Society of 

America Abstracts with Programs, v. 13, no. 7, p. 557.

_____1982, Sedimentology of the Blackbird mining district, 

Lemhi County, Idaho: Cincinnati, Ohio, University of Cincinnati M.S. thesis, 235 p.

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

Lemhi County, Idaho: Moscow, Idaho, University of Idaho 

M.S. thesis, 53 p.

Staatz, M. H., 1979, Geology and mineral resources of the Lemhi 

Pass thorium district, Idaho and Montana: U.S. Geological Survey Professional Paper 1049-A, p. A1-A90.

220 Challis 1°x2° Quadrangle, Idaho
Starr, R. B., 1955, Geology of the TWin Peaks mine, Lemhi 

County, Idaho: Ithaca, N.Y., Cornell University M.S. thesis, 

38 p.

Tucker, D. R., 1975, Stratigraphy and structure of Precambrian 

Y (Belt?) metasedimentary and associated rocks, Goldstone 

Mountain quadrangle, Lemhi County, Idaho, and 

Beaverhead County, Montana: Oxford, Ohio, Miami 

University Ph.D. thesis, 221 p.

Umpleby, J. B., 1913, Geology and ore deposits of Lemhi County, 

Idaho: U.S. Geological Survey Bulletin 528, 182 p.

U.S. Bureau of Mines, 1943, Blackbird district, Lemhi County, 

Idaho Supplement to War Minerals Report 78: War 

Minerals Report 131 Copper, Cobalt, 20 p.

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

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

Strategic Mineral Investigations Preliminary Report 3-219, 

26 p., 4 pis.

_____1959, Preliminary report on the copper-cobalt deposits 

of the Quartzburg district, Grant County, Oregon: US. 

Geological Survey Open-File Report, 20 p., 3 pis.

Cobalt-Copper Deposits in the Yellowjacket Formation 221

Symposium on the Geology and Mineral Deposits of the 

Challis 1°x2° Quadrangle, Idaho

Chapter S

Comments on the Development of 

Resource Assessment Models, Analogy, and 

Metaphor, and Their Use in 

Resource Evaluation, Challis Quadrangle

By FREDERICK S. FISHER

CONTENTS

Abstract 224 

Introduction 224 

Models 224

Analogy and metaphor 224 

Examples of models 225 

References cited 226

Resource Assessment Models, Analogy, and Metaphor 223
Abstract

Modeling of ore deposits and other geological phenomena 

is a common practice, yet we seldom examine the underlying 

thought processes used in constructing and comparing models. 

Models are sets of information containing both facts and deductions that are based on our perceptions and on incomplete data. 

As abstractions, they are distorted from reality by deletion, inference, generalization, and the bias of our own experience. 

Knowledge of how models differ from reality is important in 

critically examining existing models and constructing new ones.

Analogy is a process by which we extend our ideas into 

new physical and conceptual territory, and may be classified 

as analogy by proportionality, by attribution, and by metaphor; 

all three types are commonly used in ore-deposit studies. 

Analogy by metaphor is particularly useful for introducing novel 

concepts, for working with uncertain products and processes, 

and for explaining ideas to nongeologists. Metaphor, by focusing attention on new combinations of facts and processes, helps 

to increase our understanding and perception. Increased 

understanding and perception reduces the bias of our own 

experiences and aids the thought process by consolidating information. Metaphor assists us in formulating ideas and extending those ideas into new areas. Metaphor is thus useful in 

creating new models and introducing them into the general 

body of geological knowledge.

Models of syngenetic stratabound vanadium-silver occurrences in the Paleozoic Salmon River assemblage and of 

mineralized high-level Eocene rhyolites in the Challis volcanic 

field illustrate the use of analogy and metaphor in their development as exploration and assessment guides.

INTRODUCTION

This chapter presents a summary of some of the 

conceptual processes underlying the study of ore deposits 

for the resource appraisal in the Challis l°x 2° quadrangle. 

Models of ore deposits are used for a variety of purposes 

in geology, including exploration guides, assessment 

methods, financial calculations, and program planning 

(Peters, 1978, p. 162, 360-362, 542-550). Much has been 

written about the types of data that should be included 

in ore-deposit and resource-appraisal models (Harris and 

Agterberg, 1981; Bailly, 1981; Singer and Mosier, 1981; 

Botbol and others, 1978; Wolf, 1976; and Cameron, 1975). 

The focus here is on what a model is, and how models 

are constructed and compared.

The use of models, analogies, and metaphors is 

inherent in the science of mineral-resource appraisal, 

but only models are commonly identified as such, and 

the thought processes involving analogies and metaphors 

are rarely if ever described. Models of ore-deposit types 

are created by selecting common factors from an 

assemblage of data on a number of ore deposits and 

occurrences, but those who create the models generally 

present the finished product and do not describe the process of selection and synthesis. The term "analogy" is

sometimes used to describe the comparison of the model 

with data from a new area but again without explaining 

the type of comparison being made. The term "metaphor" 

is rarely seen in geologic literature, even though the use 

of metaphor in resource appraisal and elsewhere in 

geology is widespread. It is useful to examine some of the 

basic features of these concepts and how they are applied 

in geology.

MODELS

The models we construct are based on our perceptions and on incomplete data; as abstractions they differ 

and are distorted from reality. In general, our models can 

be thought of as differing from reality in three ways 

(Handler and Grinder, 1975): through deletion, inference, 

and generalization. We delete because if we put all available facts into our models we are overwhelmed with detail, 

much of which would be valueless for the purposes of 

our model. We select the pertinent information by a process of deletion and simplification.

A second difference between models and reality is 

inference or distortion, which occurs when we shift from 

observed facts to inferred possibilities. For example, from 

a study of fluid inclusions, we determine the daughter products and measure filling and freezing temperatures, and 

then make inferences about the chemistry of the parent ore 

fluids; from stable-isotope measurements we postulate the 

sources of the metals in a given ore deposit. Our personal 

experience biases our thinking so that we select, delete, and 

evaluate data according to our own experience.

Ore-deposit models also differ from reality through 

generalization. Generalization is the process by which a 

model of a given ore-deposit type, based on a selected 

smaller group of deposits, comes to represent a much 

larger group of deposits. Generalization allows us to 

recognize deposits we have never seen before and to 

classify deposits into selected groups, facilitating the use 

and understanding of the model.

The most efficient model contains the least number 

of elements while still maintaining its usefulness. The effectiveness of exploration models is generally increased 

by focusing the model more toward genetic process than 

descriptive content. An accurate genetic model of ore 

deposition is a more powerful prospecting tool than a 

model based solely on empirical observations because it 

enables prediction of resource possibilities from less data 

and permits extension of knowledge into unexplored 

environments.

ANALOGY AND METAPHOR

We use a process of analogy to refine existing 

ore-deposit models, to extend them into new physical and

224 Challis 1°x2° Quadrangle, Idaho
conceptual territory, and to develop new models. 

Hypotheses "are always suggested through analogy. 

* * * Analogic reasoning suggests that the desired 

explanation is similar in character to the known, and this 

suggestion constitutes the production of a hypothesis" 

(Gilbert, 1896).

Webster's Third New International Dictionary of the 

English Language Unabridged (1964) defined analogy as 

"a similarity of ratios or properties; inference that if two 

or more things agree with one another in one or more 

respects they will probably agree in other respects!' The 

dictionary indicated that analogy may be divided into 

analogy by proportionality, by attribution, and by 

metaphor. Examples of the use of all three of these types 

in conjunction with ore-deposit modeling are not difficult 

to find. An example of analogy by proportionality is 

Lasky's cumulative tonnage-grade relationship (Lasky, 

1950), whereby the tonnage of ore increases exponentially 

as the grade decreases arithmetically. Examples of analogy 

by attribution are widespread. In the simplest case we 

compare the attributes of an ore-deposit model with those 

of unexplored ground to assess its resource potential. 

Analogy by metaphor is fertile ground for developing and 

explaining new ideas. The use of metaphor is widespread 

in geology; examples include the description of intrusive 

bodies as "heat engines" driving hydrothermal convection cells, of certain kinds of faults as "trap-door" structures, and of veins as "plumbing systems" recognizing that 

the word "vein" itself in geologic usage started as a 

metaphor.

Metaphors introduce novel concepts by using 

familiar terms to make new ideas easier to understand. 

By focusing attention on new combinations of facts and 

processes, metaphor helps to increase our understanding, 

reduces the bias of our own experiences, and aids the 

thought process by consolidating information, making it 

easier to use and remember. Metaphor can be as simple 

as the substitution of a single word for another word in 

a given model. Most often, however, the use of metaphor 

is more general, involving numerous substitutions of fact 

and process and containing considerable unspecified 

implications (Black, 1962).

Commonly the most obvious implication or idea 

expressed is the underlying comparison of similar concepts 

in a metaphor. For example, in the phrases "trap-door 

fault" and "ladder vein," the similarity is the geometry 

of the structures; the shape of the ladder is compared to 

the vein and the trap door to the fault. In the phrase "heat 

engine^' the comparison is thermodynamic; the production of heat by the engine is compared to that of the intrusive body. But all of these metaphors carry many more 

unspecified implications or implicit ideas; fluid movement, element dispersal, cooling rates, and convection cells 

are all associated with the heat-engine metaphor, and the 

trap-door-fault metaphor implies different structural styles 

associated with the hinge zone verses the sides of the trap

door. These examples illustrate the power of metaphor to 

aid creative thought by leading our thinking into new areas 

and also illustrate the general idea of the importance of 

the unspecified implications or implicit information contained in a metaphor.

In short, metaphors depend very much on the user, 

each user possibly deriving a somewhat different array 

of ideas from the same metaphor. Metaphor is most 

effective when searching for new ideas and thinking 

broadly.

In the context of mineral exploration, models convey a sense of measurement, precision, scale, detail, 

boundaries, standards, and pattern. In contrast, when 

using metaphor, flexibility increases, precision decreases, 

unknowns are more common and acceptable, scale 

becomes less material, and details and boundaries are 

vague. Metaphor assists formulation of ideas and the 

extension of those ideas into new areas. Metaphor thus 

is useful for creating new models and for the introduction of those models into the general body of geological 

knowledge. The models themselves are required when 

evaluating and judging ideas and when focusing on practical application.

EXAMPLES OF MODELS

About 55 ore-deposit types requiring models for 

assessment and exploration have been identified in the 

Challis quadrangle. Two examples illustrating some applications of analogy and metaphor are a syngenetic 

stratabound vanadium-silver model for the Paleozoic 

Salmon River assemblage, and a high-level rhyolite model 

useful within the Challis volcanic field. High-level rhyolites are rhyolite intrusive bodies emplaced at or very near 

the Earth's surface; some may have breached the surface. 

In plan, shapes may be circular, linear, or very irregular.

Remobilized lead-silver deposits (Hall and others, 

1975) and stratabound zinc deposits in rocks of the Devonian Milligen Formation (Hall, chap. J, this volume) are 

known south of the Challis quadrangle. The recognition 

of rocks in the Challis quadrangle that have gross 

similarities in lithology, color, structural setting, and 

possibly age to the Milligen (analogy by attributes) suggested a search for stratabound and remobilized deposits 

in all similar rocks in the Challis quadrangle, including 

the Paleozoic Salmon River assemblage. Geochemical data 

gathered from reconnaissance rock sampling to test this 

idea suggested vanadium and silver rather than lead and 

silver in the Salmon River assemblage. Thus a new assessment model (changed by substitution) was developed.

The deposits in the Salmon River assemblage are 

in a thick accumulation of dark-gray to black turbidites 

composed of argillite, siltstone, calcareous siltstone, 

quartzite, sandstone, and carbonate beds. These rocks are 

allochthonous, thrust eastward during the Sevier orogeny

Resource Assessment Models, Analogy, and Metaphor 225
(Armstrong, 1968), and are now exposed along the eastern 

side of the Idaho batholith (Fisher and others, 1983). Individual beds are generally 1-2 m thick but may be as thick 

as 10 m. Flute and groove casts, cross laminations, and 

graded bedding are common. The rocks have been highly 

deformed, and isoclinal folds are common. The exact 

thickness of the sequence is unknown but is estimated 

to be more than 2,000 m. Silver, barium, copper, molybdenum, vanadium, and zinc are all present in anomalous 

concentrations in these rocks (Fisher and May, 1983). 

Rock sampling in selected areas in the Salmon River 

assemblage indicated that stratabound concentrations 

of vanadium and silver are present and generally are 

associated with carbonaceous units (Fisher and May, 

1983). Zinc and barite may also be present. Geologic mapping suggests that such units are relatively widespread. 

Details of the geological characteristics and settings of 

these and other deposits in upper Paleozoic rocks are 

discussed by Hall (chap. J, this volume). This assessment 

model indicates that much of the Salmon River assemblage may be favorable for syngenetic vanadium and silver 

resources.

Epithermal precious-metal deposits are associated 

with high-level Eocene rhyolites in the Challis volcanic 

field. A general model for mineral deposits associated with 

rhyolites (Berger, 1982; Silberman, 1982) was modified to 

fit data from three mineralized areas in the Challis quadrangle, the Sunbeam, Singheiser, and Parker Mountain 

gold-silver deposits (Fisher, chap. A, this volume). Many 

features are presently poorly defined in this model; the 

term "high-level" is useful because we do not know how 

close some of these intrusions came to the surface some 

breached, others did not. Fracture patterns and directions 

of hydrothermal-fluid flow markedly control the zoning 

of ore deposits in these high-level intrusions (Peterson and 

others, 1977). The cooling history, hydrothermal-fluid 

flow, and mineralization of rhyolites that breached are 

probably different from those that did not breach. The 

rhyolite bodies are mostly domes and dikes that formed 

late in the volcano-tectonic sequence, probably less than 

45 m.y. ago, but the exact time cannot be specified. Preliminary geochemical sampling suggests that silver, gold, 

arsenic, molybdenum, and zirconium are present in anomalous concentrations (Hardyman and Fisher, chap. N, this 

volume). These rhyolites are aphyric to porphyritic and are 

associated with the trans-Challis fault system (Kiilsgaard 

and Lewis, chap. B, this volume). Application of this 

exploration model to favorable areas identified by geological mapping by Hardyman (Fisher and others, 1983) has 

led to the discovery of new areas having high resource 

potential for gold, silver, and possibly molybdenum (Hardyman and Fisher, chap. N, this volume).

Awareness of the basic aspects of models, analogy, 

and metaphor focuses attention on some of the thought 

processes involved in the development of exploration and 

assessment models and in the search for new mineral

resources. It helps avoid the mental traps of rigidly defined 

models that stifle the innovative thinking needed to see ore 

possibilities where some may see only barren rock.

REFERENCES CITED

Armstrong, R. L., 1968, Sevier erogenic belt in Nevada and Utah: 

Geological Society of America Bulletin, v. 79, no. 4, 

p. 429-458.

Bailly, P. A., 1981, Today's resource status tomorrow's resource 

problems The need for research on mineral deposits, in 

Mineral resources Genetic understanding for practical applications: Washington, D.C., National Academy Press, 

p. 21-32.

Handler, Richard, and Grinder, John, 1975, The structure of magic 

I: Palo Alto, Calif., Science and Behavior Books, Inc., 

225 p.

Berger, E R., 1982, The geological attributes of Au-Ag base-metal 

epithermal deposits, in Erickson, R. L., compiler, Characteristics of mineral-deposit occurrences: U.S. Geological 

Survey Open-File Report 82-795, p. 119-126.

Black, Max, 1962, Models and metaphors: New York, Cornell 

University Press, 267 p.

Botbol, J. M., Sinding-Larsen, R. S., McCammon, R. R, and 

Gott, G. R, 1978, A regionalized multivariate approach to 

target selection in geochemical exploration: Economic 

Geology, v. 73, no. 4, p. 534-546.

Cameron, E. M., ed., 1975, Conceptual models in exploration 

geochemistry, the Canadian cordillera and Canadian shield: 

Journal of Geochemical Exploration, v. 4, no. 1, 213 p.

Fisher, F. S., and May, G. D., 1983, Geochemical characteristics 

of the metalliferous Salmon River sequence, central Idaho: 

US. Geological Survey Open-File Report 83-670, 28 p.

Fisher, F. S., Mclntyre, D. K, and Johnson, K. M., 1983, Geologic 

map of the Challis l°x 2° quadrangle, Idaho: US. Geological Survey Open-File Report 83-523, 39 p. and 2 oversize 

sheets, scale 1:250,000. Also available from Idaho Bureau 

of Mines and Geology, Moscow, Idaho 83843.

Gilbert, G. K., 18%, The origin of hypothesis, illustrated by the 

discussion of a topographic problem: Science, new series, 

v. 3, no. 53, p. 1-13.

Hall, W. E., Rye, R. O., and Doe, B. R., 1975, Wood River mining district, Idaho Intrusion related lead-silver deposits 

derived from country rock source: US. Geological Survey 

Journal of Research, v. 6, no. 5, p. 579-592.

Harris, D. P., and Agterberg, E P., 1981, The appraisal of mineral 

resources, in Skinner, B. J., ed., Economic Geology, 75th 

anniversary volume, 1905-1980: Economic Geology 

Publishing Co., p. 897-938.

Lasky, S. G., 1950, How tonnage and grade relations help predict 

ore reserves: Engineering and Mining Journal, v. 151, no. 4, 

p. 81-85.

Peters, W. C, 1978, Exploration and mining geology: New York, 

John Wiley and Sons, 696 p.

Peterson, Ulrich, Noble, D. C., Arenas, M. J., and Goodell, D. 

C., 1977, Geology of the Julcani mining district, Peru: 

Economic Geology, v. 72, p. 931-949.

226 Challis 1°x2° Quadrangle, Idaho
Silberman, M. L., 1982, Hot-spring type, large tonnage, mineral resource assessment methods: Economic

low-grade gold deposits, in Erickson, R. L., compiler, Geology, v. 76, no. 5, p. 1006-1015.

Characteristics of mineral-deposit occurrences: Wolf, K. H., 1976, Conceptual models in geology, in Wolf,

U.S. Geological Survey Open-File Report 82-795, K. H., ed., Handbook of strata-bound and stratiform

p. 131-143. ore deposits, v. 1, Principles and general studies:

Singer, D. A., and Mosier, D. L., 1981, A review of regional New York, Elsevier Scientific Publishing Co., p. 11-78.

*u.s. GOVERNMENT PRINTING OFFICE: 1986-676-0^7:2601^ Resource Assessment Models, Analogy, and Metaphor 227