Mineral Resources of the 

Elkhorn Wilderness Study Area, 

Broadwater and jefferson Counties, 

Montana 

U.S. GEOLOGICAL SURVEY BULLETIN 1805 

MONTANA 

Mineral Resources of the 

Elkhorn Wilderness Study Area, 

Broadwater and Jefferson Counties, 

Montana 

By WILLIAM R. GREENWOOD, STEVE LUDINGTON, 

WILLIAM R. Ml LLER, and WILLIAM F. HANNA 

With a section on URANIUM AND 

THORIUM POTENTIAL 

By KAREN J. WEN RICH, WILLIAM R. Ml LLER, 

VIVIAN J. SUITS, and JOHN B. MCHUGH 

U.S. GEOLOGICAL SURVEY BULLETIN 1805 

STUDIES RELATED TO WILDERNESS 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U. S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade names in this report is for descriptive purposes only and does 

not imply endorsement by the U.S. Geological Survey. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1990 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Ubrary of Congress Cataloging-In-Publication Data 

Mineral resources of the Elkhorn Wilderness Study Area, Broadwater and 

Jefferson Counties, Montana. 

(Studies related to wilderness) (U.S. Geological Survey bulletin ; 1805) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1805 

1. Mines and mineral resources-Montana-Elkhorn Wilderness. 

2. Elkhorn Wilderness (Mont.) I. Greenwood, William R. II. WenrichVerbeek, Karen J. Ill. Series: Studies related to wilderness-wilderness 

areas. IV. Series: U.S. Geological Survey bulletin; 1805. 

QE75.B9 no. 1805 557.3 s 87-600386 

[TN24.M9] [553' .09786'664] 
STUDIES RELATED TO WILDERNESS 

In accordance with the provisions of the Wilderness Act (Public Law 88-577, 

September 3, 1964) and the Joint Conference Report on Senate Bill4, 88th Congress, the 

U.S. Geological Survey and U.S. Bureau of Mines have been conducting mineral surveys of 

wilderness and primitive areas. Studies and reports of all primitive areas have been 

completed. Areas officially designated as ''wilderness," "wild," or "canoe" when the act was 

passed were incorporated into the National Wilderness Preservation System, and some of 

them are presently being studied. The Act provided that areas under consideration for 

wilderness designation should be studied for suitability for incorporation into the 

Wilderness System. The mineral surveys constitute one aspect of the suitability studies. This 

report discusses the results of a mineral survey of national forest land in the Elkhorn 

Wilderness Study Area, Montana, that is being considered for wilderness designation 

(Public. Law 94-557, October 19, 1976). The area studied is in the Helena and Deerlodge 

National Forests in Broadwater and Jefferson Counties. 

CONTENTS 

Summary 1 

Mineral resource assessment 2 

Introduction 2 

Previous studies 4 

Present investigation and acknowledgments 4 

Resource potential definitions 4 

Geologic setting 5 

Sedimentary rocks 5 

Middle proterozoic rocks 5 

Paleozoic rocks 5 

Mesozoic rocks 6 

Cenozoic rocks 6 

Igneous rocks 6 

Cretaceous Elkhorn Mountains Volcanics 7 

Cretaceous intrusive rocks related to Elkhorn Mountains 

Volcanics 7 

Cretaceous boulder batholith and related intrusive rocks 7 

Eocene quartz latite porphyry 7 

Tertiary felsic flows, tuffs, and intrusive bodies 7 

Tertiary basalt flows 8 

Structural geology 8 

Prevolcanic structural features 8 

Synvolcanic structural features 8 

Postbatholithic structural features 10 

Geophysical exploration 10 

Introduction 10 

Rock magnetic data ll 

Principal features of anomaly map U 

Interpretation of anomalies ll 

Anomalies 1, 2, and 3 15 

Anomaly 4 16 

Anomaly 5 16 

Anomaly 6 17 

Anomaly 7 17 

Anomalies 8, 9, and 10 17 

Anomaly 11 17 

Anomaly 12 17 

Anomalies 13 and 14 18 

Anomaly 15 18 

Anomaly 16 18 

Anomaly 17 18 

Anomaly 18 18 

Anomaly 19 19 

Anomaly 20 19 

Anomaly 21 19 

Anomaly 22 19 

Contents V 
Anomaly 23 19 

Anomaly 24 19 

Relation&hip of anomalies to mineralized ground 20 

Magnetic anomalies and strong geochemical anomalies 20 

Aeromagnetic anomalies possibly related to mineralized rock 20 

Geochemical exploration 20 

Geological and geochemical evaluation 23 

Introduction 23 

Mineral deposits in and near the study area 23 

East flank zone 23 

Elkhorn-Tizer Basin zone 23 

Jefferson City zone 25 

Porphyry deposits 25 

Golconda area 25 

Jackson Creek area 27 

Turnley Ridge prospect 29 

Breccia pipe deposits 30 

Skyline mine 30 

Blackjack mine 30 

Chicago and Last Hope mines 31 

Significance of geochemical anomalies 32 

Group 1-Precious- and base-metal anomalies 

(Au-Ag-As-Pb-Cu) 32 

Group 2-Rare-metal anomalies (Mo-W-Nb-La-Y-Th) 33 

Boron 34 

Uranium and thorium potential 34 

References cited 35 

PLATES 

[Plates are in pocket] 

1. Geologic map of the Elkhorn Wilderness Study Area 

2. Aeromagnetic anomaly map of the Elkhorn Wilderness Study Area 

3. Geochemical sample locality map of the Elkhorn Wilderness Study Area 

4. Geochemical anomaly map of the Elkhorn Wilderness Study Area 

5. Uranium and thorium anomaly map of the Elkhorn Wilderness Study Area 

FIGURES 

1. Map of mining districts in and near the Elkhorn Wilderness Study Area 2 

2. Map showing mineral resource potential of the Elkhorn Wilderness Study 

Area 3 

3. Aeromagnetic map of the Elkhorn Wilderness Study Area 12 

4. Generalized geologic map of the Elkhorn Wilderness Study Area 13 

VI Contents 
5. Map showing aeromagnetic highs and lows in and near the Elkhorn 

Wilderness Study Area 21 

6. Map showing location of major mines near the Elkhorn Wilderness Study 

Area 24 

7. Map of Golconda area, Elkhorn Mountains 26 

8. Ternary diagram showing modal compositions of intrusive rocks from the 

Golconda area 27 

9. Map showing copper and molybdenum content of rock samples collected in 

the Golconda area 28 

10. Map showing geology and rock sample localities in the Jackson Creek area, 

Elkhorn Mountains 29 

11. Map showing copper and molybdenum content of rock samples from the 

Jackson Creek area 30 

12. Map showing altered and mineralized zones in the Turnley Ridge stock, 

Elkhorn Mountains 31 

TABLES 

1. Magnetization and density data 14 

2. Correlation coefficients of two groups of elements 22 

3. Samples with anomalous amounts of precious and base metals 32 

Contents VII 

Mineral Resources of the 

Elkhorn Wilderness Study Area, 

Broadwater and Jefferson 

Counties, Montana 

By William R. Greenwood, Steve Ludington, 

William R. Miller, and William F. Hanna 

with a section on Uranium and thorium potential 

By Karen J. Wenrich, William R. Miller, 

Vivian J. Suits, and John B. McHugh 

SUMMARY 

The Elkhorn Wilderness Study Area in west-central 

Montana has a moderate to high potential for resources of 

porphyry-type copper and molybdenum in the western part 

of the area, and a moderate to high potential for resources of 

gold, silver, lead, and zinc in replacement and vein deposits 

in the eastern part of the area. No evidence of potential oil, 

gas, and geothermal resources was identified in this study. 

The study area includes about 87,000 acres {350 km~ 

of national forest lands in the Elkhorn Mountains about 15 

miles (24 km) southeast of Helena, Mont. The study area 

comprises about 82,000 acres (330 km2

) designated by 

Congress for wilderness study in 1976, plus about 5,000 

acres (20 km2

) studied at the request of the U.S. Forest 

Service. Some mineralized areas next to the study area are 

also discussed in this report. 

The study area includes parts of the Beaver Creek, 

Elkhorn, and Park mining districts, and all of the Tizer-Wilson 

mining district, none of which were producing ore at the time 

of the study (fig. 1). The study area also includes part of the 

Warm Springs mining district, within which only the White 

Pine mine, outside the study area, was producing ore during 

the study. 

The mineral resource potential of the study area and 

adjacent areas was evaluated by the U.S. Geological Survey 

and the U.S. Bureau of Mines in 1976 and 1977. The 

evaluation consisted of a mineral resource assessment by 

the U.S. Geological Survey based on geologic, geochemical, 

and geophysical surveys and an economic appraisal by the 

U.S. Bureau of Mines based on studies of existing mines and 

Manuscript approved for publication, March 6, 1977. 

prospects. The Bureau of Mines appraisal will be published 

separately. 

The mineral resource assessment by the U.S. 

Geological Survey was based principally upon geologic 

studies including chemical analyses of rocks, a detailed 

geochemical survey of stream deposits, and a detailed 

aeromagnetic survey. In the geochemical survey, bulk 

stream sediments and panned stream sediments were 

analyzed for 30 elements by semiquantitative spectrographic 

methods and for gold, zinc, and arsenic by atomic 

absorption methods. Geologic studies and rock sampling 

were then conducted in drainage basins from which streamsediment samples showed anomalously high metal content 

or in which evidence of hydrothermal alteration was known. 

The aeromagnetic survey served as a guide for collecting 

rock samples which were subsequently measured for remanent and induced magnetization for purposes of determining 

the range of physical properties of anomaly sources. 

Uranium and thorium resources were investigated by 

analyzing water and stream-sediment samples and by an 

airborne radiometric survey. Altogether, more than 1,400 

samples were analyzed in the assessment. 

The geologic and geochemical data indicate that the 

western part of the Elkhorn Mountains has a moderate to 

high potential for resources of porphyry-type copper and (or) 

molybdenum. Three deposits, Jackson Creek (fig. 2, loc. 1), 

Golconda (loc. 2), and Turnley Ridge (loc. 3), which are 

adjacent to the study area, have been explored by drilling, 

Golconda showing the highest potential for development. On 

the basis of geologic studies, other areas that have high 

potential for molybdenum and (or) copper resources include 

breccia pipes and dikes cemented by tourmaline, quartz, 

and sulfide minerals. The breccia bodies may overlie buried 

porphyry-type copper and (or) molybdenum deposits. These 

breccias occur at the south margin of the study area (near the 

Summary 1 
0 2 4 MILES 

f--+---r 

0 3 6 KILOMETERS 

EXPlANATION 

--- Approximate boundary of mining district 

Figure 1. Mining districts in and near the Elkhorn Wilderness Study Area, Montana. 

Skyline mine, fig. 2, Joe. 4) and the north margin (near and 

south of the Chicago mine, (Joe. 5). Geochemical data 

suggest a high potential for molybdenum and (or) copper 

resources at Nursery Creek (Joe. 6), Hidden Lake Ooc. 7), and 

South Fork Warm Springs Creek Ooc. 8), localities which 

show high contents of molybdenum, tin, and (or) tungsten in 

both panned concentrates and rock samples; and at 

Dutchman Creek (Joe. 9), which shows high contents of 

molybdenum, tin, and (or) tungsten in the panned 

concentrates alone. 

The geochemical data also suggest a moderate 

potential for precious- and base-metal resources in a broad 

northeast-trending zone in the southeast part of the study 

area. Several areas of high potential for these metals are 

indicated within this zone (fig. 2, locs. 10, 11, 12, 13, and 14). 

Of these, localities 10 and 14 include major mines, the 

workings from which have contributed to the high metal 

contents in the panned concentrates. The high metal 

contents in samples from localities 11, 12, and 13 (Longfellow 

Creek, an unnamed tributary to Crow Creek, and Staubach 

Creek, respectively) do not appear related to any major mine. 

Aeromagnetic anomaly and rock magnetic data 

suggest that relative negative anomalies, or lows, are in many 

places associated with subsurface zones of alteration and 

silicification in the Boulder batholith and in the Elkhorn 

Mountains Volcanics near the batholithic contact and that 

relative positive anomalies, or highs, are in many places 

associated with shallowly buried plutons in Elkhorn 

Mountains Volcanics farther from the contact. Both the lows 

and highs occur over many terranes which have been mined 

2 Elkhorn Wilderness Study Area, Montana 

or prospected and, therefore, delineate subsurface regions 

which merit consideration for future exploration. 

Anomalously high uranium was found in water samples 

at Badger Creek (fig. 2, Joe. 15), Muskrat Creek Ooc. 19), and 

Rawhide Creek (Joe. 17). The entire drainage of Warm 

Springs Creek (Joe. 8) had high concentrations of uranium in 

stream-sediment samples. Radon in water was anomalously 

high in samples from Dutchman Creek (Joe. 9), Warm 

Springs Creek, and Muskrat Creek. Anomalously high 

thorium was found in stream-sediment pan-concentrate 

samples from the following creeks: Rawhide, Nursery Ooc. 6), 

Weimer and Anderson Ooc. 2), Dutchman, Warm Springs, 

North Fork Warm Springs Ooc. 16), and McCarthy (Joe. 18) 

Creeks. 

MINERAL RESOURCE ASSESSMENT 

By William R. Greenwood, Steve 

Ludington, William R. Miller, and 

William F. Hanna 

Introduction 

The study area as described in this report includes 

the Elkhorn Wilderness Study Area designated by 

Congress in 1976, consisting of about 82,000 acres (330 

km2), and an additional area studied at the request of the 

U.S. Forest Service, consisting of about 5,000 acres (20 

km2). The study area is in parts of Broadwater and 

Jefferson Counties in western Montana, and includes 

parts of the Helena and Deerlodge National Forests. 

Some lands adjacent to the study area were also 

examined, as indicated in the report. 

The study area is accessible from the east by 

several secondary roads leading from U.S. Highways 12 

and 287. From the west it is reached by several secondary 

roads leading from U.S. Interstate Highway 15. Jeep 

trails provide access throughout Tizer Basin (an informal 

name for the basin at the head of Crow Creek-see pl. 3) 

and lower parts of the bordering slopes of the basin. 

From the south the study area is reached by secondary 

roads leading north from Montana State Highway 281 

and a secondary road connecting the towns of Elkhorn 

and Radersburg. U.S. Forest Service trails provide access 

by foot or horseback to most of the study area from June 

through October. During the winter and spring much of 

the study area is inaccessible to land vehicles due to 

snow. 

The study area includes the higher, central part of 

the Elkhorn Mountains, an isolated range in west-central 

Montana. Relief in the area is about 4,800 feet (1,500 m), 

ranging from an elevation of about 4,600 ft (1,400 m), 
1 . Jackson Creek 

2. Golconda 

3. Turnley Ridge 

4. Skyline rnine 

5. Chicago mine 

MONfANA 

~ELKHORN WILDERNESS 

STUDY AREA 

15 

• 

0 

0 

6. Nursery Creek 

7. Hidden Lake 

8. So. Fork Warm Springs Creek 

9. Dutchman Creek 

10. Park mining district 

5 MILES 

8 KILOMETERS 

LOCALITIES 

11 . Longfellow Creek 

12. Unnamed tributary to Crow Creek 

13. Staubach Creek 

14. Elkhorn mining district 

15. Badger Creek 

Figure 2. Mineral resource potential of the Elkhorn Wilderness Study Area, Montana. 

EXPlANATION 

POTENTIAL POR PORPHYRY 

COPPER AND MOLYBDENUM 

MINERALIZATION 

II High--May contain known deposits 

._-_- Moderate 

POTENfiAL FOR BASE-AND 

PRECIOUS-METAL 

MINERALIZATION 

High--May contain known deposits 

· · · · Moderate 

l!:l POTENTIAL FOR BRECCIAU PIPE MINERALIZATION 

e URANIUM AND TIIORIUM 

ANOMALIES 

~ MINE 

16. North Fork Warm Springs Creek 

17. Rawhide Creek 

18. McCarthy Creek 

19. Muskrat Creek 

where Warm Springs Creek leaves the study area, to 

9,414 ft (2,869 m) at Crow Peak, the highest peak in the 

study area. Although the maximum local relief is in the 

northern part of the study area, the scenery is more 

spectacular in the southern part because of the glacial 

cirques on the flanks of Crow and Elkhorn Peaks. 

The study area is in the Missouri River watershed 

and is drained on the east mainly by Crow Creek and 

Beaver Creek, on the northwest by Prickly Pear Creek, 

Dutchman Creek, Warm Springs Creek, and McClellan 

Creek, and on the southwest by Muskrat Creek, Elkhorn 

Creek, and Dry Creek. 

Mineral Resource Assessment 3 
The study area was glaciated during Pleistocene 

time and glacial features include cirques on the higher 

peaks. The upper parts of many valleys are U-shaped, 

and moraine and outwash deposits fill parts of Tizer 

Basin and most of the major drainages. 

The highest ridges and peaks of the area h3ve 

talus-covered slopes with a sparse cover of whitebark 

pine and alpine fir (vegetation data from U.S. Forest 

Service, 1976). Lower ridges and the upper slopes and 

basins generally are densely covered with ,lodgepole pine. 

Midslopes and valley bottoms have a mosaic of Douglasfir and lodgepole-pine woodlands and mountain 

grassland parks. On the lower slopes to the east and 

south, these parklands pass into open grasslands and 

shrub lands. 

The climate of the area is modified continental with 

large annual and daily temperature variations (U.S. 

Forest Service, 1976). Precipitation ranges from less than 

12 inches (30 em) per year in the valleys to over 30 in. (76 

em) per year at higher elevations. Average snowfall is 

over 100 in. (250 em) per year in the high country and 

makes up a large part of the annual precipitation of that 

part of the area. The area generally has very little rain 

during July and August. The temperature ranges from a 

minimum of well below 0°F (-18°C) in winter to a 

maximum of nearly 90°F (32°C) in summer and is quite 

variable from day to day. 

The study area is used principally as a livestock 

range and for recreational fishing and hunting. Logging 

in the area has yielded about 20 million board feet of 

timber and is a potential economic resource (U.S. Forest 

Service, 1976). Mineral exploration in the study area is 

being conducted by individual prospectors and by 

companies, and mine development work is being done on 

properties adjacent to the study area on Warm Springs 

Creek. 

Previous Studies 

The geology and mineral deposits of different parts 

of the study area have been discussed in many reports, 

which are listed in the references and cited elsewhere in 

this report. The geologic map of the study area (pl. 1) was 

compiled from the reports by Klepper and others (1957); 

Klepper, Ruppel, and others (1971); Becraft and others 

(1963); and Smedes (1966); details were added locally 

based on our mapping. The synopsis of geology 

presented here is drawn largely from these papers, 

supplemented by subsequent summaries and topical 

reports by these authors and their co-workers. The 

spirited exchange between Hamilton and Myers (1967, 

4 Elkhorn Wilderness Study Area, Montana 

1974a, b) and Klepper, Robinson, and Smedes (1971, 

1974) presented much useful information and opinion on 

the shape and manner of emplacement of the Boulder 

batholith. 

Present Investigation and Acknowledgments 

Investigations in the study area by the U.S. 

Geological Survey were made during November 1976 by 

W. R. Greenwood and W. R. Miller, and during June, 

July, and August 1977 by these geologists and S. D. 

Ludington, assisted in 1977 by Judy Melia Allen, Enid 

Bittner, and David Thompson. 

The fieldwork was conducted mainly by foot 

traverses through the area. Jeeps were used on several 

trails in Tizer Basin and to gain access to sample 

localities on the periphery of the study area. 

The mineral survey included reconnaissance 

geologic mapping aimed primarily at checking and 

updating existing maps. Most of the field effort, however, 

was devoted to collecting samples for geochemical 

analysis and searching for evidence of mineralization. 

Bulk stream-sediment samples and panned concentrates 

of stream sediments were collected from all first order 

(unbranched) streams. Water samples were also 

collected from these streams and from springs and seeps. 

The common rock types were sampled, as were visibly 

altered or mineralized rocks. Altogether, more than 

1,400 samples were collected and analyzed. The spectrographic analyses of these samples were made in a U.S. 

Geological Survey mobile laboratory by Jerry Motooka. 

Sample preparation and other chemical analyses were 

conducted in a mobile laboratory by John B. McHugh. 

The mineral survey benefited greatly from 

assistance extended by local residents, U.S. Forest 

Service officials, claim owners, and others. In particular 

we would like to acknowledge help received from James 

W. Whipple, formerly U.S. Forest Service geologist of 

the Helena National Forest, and Richard I. Walker, U.S. 

Forest Service, team leader for the Elkhorn Wilderness 

Study Area project. 

Resource Potential Definitions 

The following definitions of mineral resource 

potential, recently published by Goudarzi (1984), were 

used by the U.S. Geological Survey in this study. 

Geologic terranes considered unfavorable for the 

occurrence of mineral resources are generally classed as 

having low potential, recognizing that most of these areas 

still have a small likelihood of containing mineral 

resources. Areas with terrane considered favorable are 

subdivided into areas of moderate and high potential. 

Low mineral resource potential is assigned to areas 

where geologic, geochemical, and geophysical 
characteristics define a geologic environment in which 

the existence of resources is unlikely. This broad category 

embraces areas with dispersed but insignificantly 

mineralized rock as well as areas with few or no 

indications of having been mineralized. 

Moderate mineral resource potential is assigned to 

areas where geologic, geochemical, and geophysical 

characteristics indicate a geologic environment favorable 

for resource occurrence, where interpretations of data 

indicate a reasonable likelihood of resource 

accumulation, where an application of mineral deposit 

models indicates favorable ground for the specified 

type(s) of deposits. 

High mineral resource potential is assigned to 

areas where geologic, geochemical, and geophysical 

characteristics indicate a geologic environment favorable 

for resource occurrence, where interpretations of data 

indicate a high degree of likelihood for resource 

accumulation, where data support mineral deposit 

models indicating presence of resources, and where 

evidence indicates that mineral concentration has taken 

place. Assignment of high resource potential to an area 

requires some positive knowledge that mineral forming 

processes have been active in at least part of the area. 

Resources or deposits need not be identified for an area 

to be assigned high resource potential. 

Geologic Setting 

The Elkhorn Wilderness Study Area is underlain 

mostly by Elkhorn Mountains Volcanics and related 

intrusive rocks of Late Cretaceous age and by the 

Boulder batholith and satellitic stocks of Late Cretaceous 

age. A smaller part of the study area is underlain by 

strata of Paleozoic to Mesozoic age. Middle Proterozoic 

sedimentary rocks of the Belt Supergroup crop out 

extensively in the Elkhorn Mountains and presumably lie 

beneath the Paleozoic strata in parts of the study area. 

Mineral deposits occur in quartz veins, as replacement 

deposits, in breccia pipes, as disseminated deposits and 

stockworks in plutonic rocks, in skarns along intrusive 

contacts, and along faults that were active during 

Cretaceous plutonism. All these rocks were intruded by 

igneous dikes and stocks and locally are overlain by 

related volcanic rocks of middle Tertiary and late 

Tertiary age. 

The mountain range containing the study area is a 

tectonically uplifted block, bounded on the east and 

north and probably south and west by major faults. This 

uplifted block has been eroded to its present form by 

glaciers and by running water. 

The study area is located at the northeast apex of a 

trapezoidal structural block bounded on the north by the 

northwest -trending Proterozoic to Holocene Lewis and 

Clark lineament, on the east by north-trending 

Cretaceous thrust faults of the Townsend Valley and 

Paleocene thrust faults of the Montana disturbed belt, 

and on the south by the east -trending Precambrian and 

Cretaceous to Paleocene Willow Creek lineament. The 

western boundary of this block is not well defined but 

may be marked by north-trending Cretaceous thrust 

faults in the Deer Lodge valley west of the batholith. 

Cretaceous volcanics were deposited throughout this 

block; the Boulder batholith and related satellitic plutons 

underlying most of it comprise the east-most major 

Cretaceous batholithic unit of the Cordillera. The 

sedimentary and igneous history of the study area and the 

mineral deposits contained in it have been influenced or 

determined by the structural evolution of this block. 

Sedimentary Rocks 

Sedimentary rocks about 12,000 ft ( 4,000 m) thick 

and ranging in age from Middle Proterozoic to 

Cretaceous are exposed in or near the study area. These 

strata are overlain by a thick pile of Upper Cretaceous 

volcanic rocks. The Cretaceous and older rocks were 

strongly folded and faulted prior to and during the 

intrusion of the Boulder batholith and related Upper 

Cretaceous intrusive rocks. 

The sedimentary rocks and volcanic units in the 

study area have been fully described by Klepper and 

others (1957); Klepper, Ruppel, and others (1971); 

Smedes (1966); and Becraft and others (1963); 

accordingly, they are summarized here and in the 

explanation of plate 1. Only the details that are significant 

to resource assessment are discussed in this review. 

Middle Proterozoic Rocks 

Hornstone mapped as Belt Supergroup (Middle 

Proterozoic age), or specifically as Spokane(?) Shale, is 

exposed in the faulted core of an anticline in the Elkhorn 

mining district, adjacent to the southwest border of the 

study area (Klepper and others, 1957). The hornstone 

could be the Empire Formation which overlies the Spokane. The hornstone does not appear to be favorable for 

the occurrence of mineral deposits; only a few mines have 

been developed in the Belt rocks of this district. 

Paleozoic Rocks 

Paleozoic rocks in the study area comprise 

carbonate rocks and shale and include subordinate 

amounts of quartzite, phosphatic quartzite, and chert. 

These rocks have been mapped in 14 formations (pl. 1) 

having a combined thickness of about 5,000 ft (1,600 m). 

They crop out in the cores of two anticlines at the south 

edge of the study area and in up folded rocks several miles 

east and north of the study area. 

Mineral Resource Assessment 5 
Most of the mines of the Elkhorn mining district 

(Klepper and others, 1957) and some lead-zinc deposits 

in the Quartzite Ridge area southeast of the study area 

(Klepper, Ruppel, and others, 1971) have been 

developed in Paleozoic rocks. Many of these metal 

deposits formed as replacements of carbonate rocks n~ar 

contacts with overlying relatively impermeable shale. 

Such carbonate-shale contacts are attractive exploration 

targets and are shown in the southwestern and 

southeastern parts of the study area as well as just east of 

the study area (pl. 1). The phosphatic quartzite and chert 

of the Phosphoria Formation (Permian) locally contain 

one or two thin beds of phosphate rock as much as 30 in. 

(0.8 m) thick. Some phosphate rocks of the Phosphoria 

Formation of the region have anomalous uranium 

contents (Maughan, 1976), and such rocks are a possible 

exploration target. 

Mesozoic Rocks 

The Mesozoic sedimentary rocks in the study area 

(pl. 1) consist predominantly of sandstone and shale with 

subordinate limestone, tuff, and tuffaceous sandstone. 

They have a combined thickness from about 2,000 ft ( 610 

m) to as much as 3,840 ft (1,170 m). Marine and 

nonmarine sandstone and shale, partly calcareous or with 

limestone interbeds of the Swift, Morrison, and Kootenai 

Formations, are overlain by marine dark shale and 

sandstone of the Colorado Formation. North of Johnny 

Gulch in the southeastern part of the study area, these 

rocks grade upward through tuffaceous sandstone into 

tuff of the Slim Sam Formation that is conformably 

overlain by the Elkhorn Mountains Volcanics. South of 

Johnny Gulch, outside the study area, the Elkhorn 

Mountains Volcanics rest with angular unconformity on 

sandstone and shale of the Kootenai (Klepper, Ruppel, 

and others, 1971 ). 

The Mesozoic sedimentary rocks contain several 

carbonaceous units, and the upper part of the Slim Sam 

Formation contains carbonized and silicified wood in 

many places. Carbonaceous sedimentary rocks such as 

these commonly contain high concentrations of metals. 

Several mines and prospects in the southeastern part of 

the study area have been developed in or near those 

carbonaceous units, especially where they are cut by 

intrusive rocks. Leaching of the carbonaceous rocks by 

magmatic fluids may have released metals that were then 

concentrated in some of the ore deposits of the study 

area. The carbonaceous units are also of economic 

interest as possible sites of concentrations of uranium 

leached from the interbedded and overlying tuffs. 

8 Elkhorn Wilderness Study Area, Montana 

The Mesozoic sedimentary rocks also contain lenticular beds of titaniferous magnetite that appear to be 

lithified beach placers. Near Radersburg, about 6 mi (10 

km) southeast of the study area, such beds have been 

mined for use in cement. 

Cenozoic Rocks 

Cenozoic sedimentary rocks in and adjacent to the 

study area include thick Tertiary fluvial and lacustrine 

basin-fill deposits and generally thin, unconsolidated 

Tertiary and Quaternary deposits. 

Tertiary fluvial and lacustrine deposits in the 

Townsend Valley east of the study area contain tuff 

interbeds (Klepper, Ruppel, and others, 1971 ). Tuff 

interbeds in similar deposits in western Utah contain 

beryllium, lithium, and uranium; a few mines have 

produced beryllium and uranium from those deposits 

(Lindsey, 1977). 

Unconsolidated deposits of Tertiary and 

Quaternary age in the study area are grouped on plate 1 

according to the means and amount of transportation 

from their source. Surficial deposits originally mapped as 

fan deposits, mantle, hillwash pediment gravel, and masswaste deposits (such as landslides) are combined on plate 

1 into one map unit, regardless of age. These deposits 

generally are coarse grained, were derived by mechanical 

erosion, and most have been transported only short 

distances. Glacial deposits are combined in one unit, and 

alluvial deposits of several ages are combined in one unit. 

Igneous Rocks 

Igneous rocks of the study area include Cretaceous 

volcanic and hypabyssal rocks, predominantly of trachyandesitic to rhyodacitic composition; the Cretaceous 

Boulder batholith and related dikes, plugs, and stocks, 

predominantly of quartz monzonitic composition; quartz 

latite stocks and dikes of Tertiary, probably Eocene, age; 

rhyolite domes and tuff and basalt flows of Tertiary, 

probably late, age. 

Cretaceous volcanic and intrusive igneous rocks 

underlie most of the study area. Cretaceous volcanic and 

volcaniclastic rocks mapped as the Elkhorn Mountains 

Volcanics predominate in the east half of the study area 

and the Boulder batholith underlies the west half. The 

Elkhorn Mountains Volcanics are intruded by many sills 

and dikes of porphyritic igneous rock similar in 

composition and apparently related to the volcanics. 

These sills and dikes were intruded during and shortly 

following deposition of the volcanic rocks. The volcanic 

rocks and associated porphyritic intrusions are intruded 

by the Boulder batholith and by smaller, generally 

discordant plutonic bodies that are satellitic to the batholith. Eocene quartz latite dikes and related stocks were 
emplaced in the study area following uplift and erosion of 

the Elkhorn Mountains Volcanics and Boulder batholith. 

Late Tertiary rhyolite flows, volcanic sandstone, and tuff, 

and associated intrusive rhyolite domes and intrusive 

breccias and basalt flows occur on and near the west and 

east edges of the study area. 

The nomenclature used in this report for igneous 

rocks follows that of Klepper, Ruppel, and others (1971 ). 

Cretaceous Elkhorn Mountains Volcanics 

The Elkhorn Mountains Volcanics of Late 

Cretaceous age are divided into a lower heterogeneous 

volcanic member, a middle welded-tuff member, and an 

upper volcaniclastic member (Klepper and others, 1957). 

The lower member ranges in thickness from 2,500 to 

5,000 ft (760-1,520 m) and consists predominantly of 

complexly interbedded rhyodacitic and trachyandesitic 

tuff, tuff breccia, and breccia, with subordinate flows and 

thin welded-tuff units. Most of the finer grained tuff in 

the lower member is finely laminated and was deposited 

in shallow water with minor reworking (Klepper, Ruppel, 

and others, 1971 )'. The middle member is as much as 

7,500 ft (2,290 m) thick and consists of rhyolitic welded 

tuff inter layered with crystal and lithic tuff and with some 

less abundant volcanic breccia (Smedes, 1966, p. 26). The 

upper member is about 2,000 ft (610 m) thick in its 

thickest preserved section near High Peak in the 

northern part of the study area (Smedes, 1966, p. 26). 

This member consists of water-laid epiclastic mudstone 

to conglomerate, bedded tuff, and a few thin beds of 

volcanic breccia. 

Cretaceous Intrusive Rocks Related to Elkhorn 

Mountains Volcanics 

Several large laccolithic, lopolithic, and irregular 

bodies, consisting mainly of syenodiorite porphyry and 

granodiorite porphyry and many dikes, sills, plugs, and 

irregular bodies of trachyandesite porphyry and rhyodacite porphyry cut the Elkhorn Mountains Volcanics 

and older rocks. These intrusive rocks are compositionally similar to the volcanics and probably are products of 

the same magma or magmas (Klepper, Ruppel, and 

others, 1971, p. 20). Some of the larger intrusive bodies 

appear to have been emplaced as a series of sill-like 

sheets that coalesced to form laccolithic bodies, and 

locally the magma of these bodies breached the surface 

forming laharic or mud-flow deposits (Klepper, Ruppel, 

and others, 1971, p. 23). These larger intrusive bodies are 

concentrated along the axial zones of a major syncline in 

the center of the study area and along a major anticline in 

the eastern part of the study area, and they may have 

been emplaced during the early stages of folding 

(Smedes, 1966, p. 61; Klepper, Ruppel, and others, 1971, 

p. 33). 

Cretaceous Boulder Batholith and Related Intrusive Rocks 

The Upper Cretaceous rocks of the Boulder batholith are divided into Butte Quartz Monzonite; leucogranodiorite; and alaskite, aplite, and related rocks (pl. 1 ). 

The rocks of plutons satellitic to and apparently the same 

age as the Boulder batholith have been mapped as mafic 

rocks, granodiorite and related rocks, and quartz monzonite and granite. 

Butte Quartz Monzonite makes up most of the 

Boulder batholith in the study area and consists of 

granodiorite and quartz monzonite of many textural 

varieties. These textural varieties previously were 

mapped separately, but they have gradational boundaries 

(Smedes, 1966), are chemically similar, and apparently 

are the same age (Tilling, 1973). 

Leucogranodiorite is mapped near Jackson Creek 

at the northwest margin of the study area. This small, late 

intrusive body contains abundant disseminated pyrite 

and chalcopyrite. 

Aplite, alaskite, and related rocks are concentrated 

in a northeast-trending zone that also contains most of 

the major metalliferous veins in the northwestern part of 

the study area. 

Plutons satellitic to the Boulder batholith intrude 

the volcanic and sedimentary rocks in a north-trending 

zone that generally follows the axial zone of a major 

syncline east of the batholith. The satellitic mafic rocks 

range from gabbro to dark granodiorite and calcic syenite. The satellitic granodiorite and related rocks range 

from granodiorite to granite. 

Eocene Quartz Latlte Porphyry 

Dikes and stocks of quartz latite porphyry cut into 

rocks of the Boulder batholith and older rocks in the 

northern part of the study area. The quartz latite 

intrusions are at the northeastern end of a swarm that 

extends southwest 12.5 mi (20 km) to the quartz latitic 

Lowland Creek Volcanics (Smedes, 1962). These volcanics and the associated dike swarm have been dated as 

early Eocene (Smedes and Thomas, 1965). 

Tertiary Felsic Flows, Tuffs, and Intrusive Bodies 

Scattered outcrops of a flow-laminated felsitic rock 

occur along Crow Creek. The chemical analysis and 

norm of the rock (Klepper, Ruppel, and others, 1971, p. 

30), and spectroscopic analyses (Motooka and others, 

1978; samples 77BG049G through 77BG054G), indicate 

Mineral Resource Assessment 7 
it to be a quartz latite or rhyodacite. The rock is gray to 

black, and generally is dense to sparsely vesicular, but 

locally it is scoriaceous or lithophysal. It contains sparse 

phenocrysts of labradorite, pyroxene, and an opaque 

mineral, probably magnetite. The outcrops are remnants 

of flows (Klepper and others, 1957; Klepper, Ruppel, and 

others, 1971) and may, in part, be welded tuff that 

underwent secondary flowage. 

Rhyolite with a K-Ar age of 36 m.y. (Chadwick, 

1977) occurs just outside the northwestern boundary of 

the study area as funnel-shaped plugs, dikes, tuff, breccia, 

volcanic sandstone, and conspicuously laminated lava 

flows. This rhyolite has high contents of tin (10-15 ppm), 

niobium (30-100 ppm), and fluorine (0.2-0.5 percent). 

Phenocrysts of quartz, K-feldspar, and sodic plagioclase 

are present but sparse. Topaz is commonly found in 

miarolitic cavities. 

At Lava Mountain, silver-bearing galena, sphalerite, fluorite, and quartz occur in braided veinlets 

cementing brecciated rhyolite, as fillings in lithophysal 

cavities, and as local small replacement masses (Smedes, 

1966, p. 107-108). The extrusive rhyolite of this unit 

appears to occur as blocks that have partly foundered 

into the intrusive rhyolite during emplacement of the 

stocks (Smedes, 1966, p. 94). 

Tertiary Basalt Flows 

A few remnants of basalt flows crop out along and 

near Crow Creek from Sagebrush Gulch eastward in the 

southeastern corner of the study area. The basalt flows 

locally rest on sedimentary tuff considered to be of 

Oligocene age by Klepper, Ruppel, and others (1971, p. 

29-30). The basalt probably is late Miocene or Pliocene 

in age but could be as old as Oligocene (Klepper, 

Ruppel, and others, 1971, p. 30). 

Structural Geology 

The rocks east of the Boulder batholith in the study 

area are deformed by many folds and faults of 

Cretaceous and Tertiary age. These structural features 

strongly influenced the location of mineral deposits. 

The structural features are divided here into four 

groups listed by decreasing age: prevolcanic, synvolcanic, 

synbatholithic, and postbatholithic. Evidence for prevolcanic and synvolcanic structural features commonly is 

faint, because of later intrusion and deformation; 

however, these older structural features are important 

because they appear to have largely determined the 

trends and location of younger structures and related 

mineralization. 

Prevolcanlc Structural Features 

Prevolcanic folding or faulting is indicated by facies 

changes and erosional thinning across syndepositional 

anticlines (sec. 19, T. 9 N., R. 1 W.) in the Upper 

8 Elkhorn Wilderness Study Area, Montana 

Cretaceous Slim Sam Formation about 3 mi (5 km) 

northeast of the batholith contact in the north edge of the 

study area (Smedes, 1966, p. 96). These folds plunge 

20-30° SSW., have a wavelength of 0.3 mi (0.5 km), an 

amplitude of about 29.5 ft (9 m), and are asymmetric, 

being slightly. overturned to the east. Erosional thinning 

of this formation is also shown south of Johnny Creek on 

the east side of the study area (Klepper, Ruppel, and 

others, 1971). To the southwest, the Slim Sam was 

completely eroded before deposition of the Elkhorn 

Mountains Volcanics near the town of Elkhorn (Klepper 

and others, 1957). These data suggest that prevolcanic 

structural adjustments that induced erosion of the Slim 

Sam Formation were most intense near the present 

batholith contact and that uplift was most pronounced in 

the southwestern part of the study area. This suggests 

that prevolcanic inflation of the present batholith area 

produced Slim Sam age erosion in the west -central part 

of the study area. Preliminary interpretations of stratigraphic and structural data in the Boulder batholith 

region (Greenwood and others, 1979) support this 

interpretation and indicate that uplift of as much as 3,200 

ft (1,000 m) was widespread in the batholith area. Most 

of this uplift occurred after deposition of the Cretaceous 

Colorado Formation and prior to the deposition of the 

Elkhorn Mountains Volcanics. The syndepositional folds 

described in the Slim Sam Formation suggest that 

inflation was accompanied by east-west compression. 

These folds appear to be the result of regional thrusting 

which continued into early volcanic time as described in 

the following section. 

Synvolcanlc Structural Features 

Synvolcanic structural features include the 

following: early inflation of the batholith area 

accompanied by east-directed, north- to northeasttrending thrusting and related high-angle reverse faulting 

in the study area; eruption of a volcano and formation of 

one or possibly two volcanic caldrons, apparently in a 

northeast-trending fault zone that was subsequently 

intruded by the batholith; and depression of the study 

area and most of the batholith area concomitant with 

volcanism. 

Broad north- to northeast-trending structures in 

gently dipping, sedimentary rocks near the batholith 

contact in the west-central part of the study area pass into 

open-folded, moderately dipping, sedimentary rocks in 

the eastern part of the study area (pl. 1 ). To the east of 

the study area near the west side of the Townsend Valley, 

the folds are tight and cut by reverse faults in steeply 

dipping sedimentary rocks. The intensity of deformation 

increases away from the batholith contact, the regional 

folding and faulting, and thus is not directly related to 

batholith emplacement. The synvolcanic structural 
features appear to be the result of east-directed regional 

thrusting, the lead edge of which is partly exposed at the 

edge of the Townsend Valley and partly covered by 

valley-fill deposits. 

Reverse faulting mapped by Klepper, Ruppel, and 

others (1971) near the edge of the Townsend Valley 

about 2 mi (3 km) east of the study area (pl. 1) displaces 

the lower and middle members of the Elkhorn 

Mountains Volcanics against older rocks. As much as 0.6 

mi (1 km) stratigraphic offset was measured by Klepper, 

Ruppel, and others (1971, p. 31) along the Whitehead 

Ranch fault (sees. 4 to 27, T. 8 N., R. 1 W.) in this zone. 

They note that volcanic-related dikes both are cut by and 

intrude the Whitehead Ranch fault, which establishes the 

synvolcanic age of this fault zone. Reverse faults in this 

zone, named here the Indian Creek fault zone, parallel 

north-trending, locally tight folds with wavelengths of 

1.8-3 mi (3-5 km) and amplitudes of over 0.6 mi (1 km) 

(Klepper, Ruppel, and others, 1971, p. 1). East-, 

northwest-, and northeast-trending faults were offset and 

are intruded by volcanic-related dikes, and thus were 

developed at the same time as the north-trending reverse 

faults and folds. The northwest- and northeast-trending 

faults show lateral offsets of as much as 2,300 ft (700 m) 

and appear to be complementary shears related to eastwest compression (Klepper, Ruppel, and others, 1971, p. 

32). These writers also indicate that the northeasttrending faults served as tear faults during thrust 

movement and they relate to the Lombard thrust, an 

east-directed, post-batholith thrust, the lead edge of 

which is located to the east of the Townsend Valley 

(Freeman and others, 1958, p. 525-526). A tear fault of 

similar trend was mapped by Smedes (1966, pl. 1) near 

the Helena Valley north of the study area (secs.13, 14, T. 

9 N., R. 2 W.). Smedes related this tear to a near-bedding 

plane thrust fault he mapped in sec. 14, T. 9 N., R. 2 W., 

north of the study area. The thrust and tear faults 

mapped by Smedes (1966) have the same symmetry as 

the reverse and tear faults of Klepper, Ruppel, and 

others (1971 ). Movement in these fault zones may have 

been recurrent with both synvolcanic Cretaceous 

thrusting and postvolcanic Paleocene thrusting. 

We propose a synvolcanic origin for the fault zone 

mapped by Smedes (1966) between Mitchell Gulch and 

Corral Creek (sec. 22, T. 9 N., R. 2 W.) and named here 

the Mitchell Gulch fault zone. This structural feature was 

interpreted by Smedes (1966, p. 9) as a step fault with 

over 0.6 mi (1 km) up-to-the-west aggregate stratigraphic 

offset. He notes, however, that large south-plunging drag 

folds mapped in this zone indicate strong compression 

during faulting. Smedes (1966, p. 98) relates this fault 

zone to batholith emplacement. Although the location of 

these faults does suggest that they influenced the location 

of the eastern contact of the batholith, other evidence 

indicates an earlier origin for the zone. Because these 

faults offset the lower member of the Elkhorn Mountains 

Volcanics, are intruded by and do not offset rocks of the 

Boulder batholith, they are considered to be synvolcanic 

to early batholithic in age. Evidence for important synvolcanic up-to-the-west faulting in this zone is provided 

by the following: (1) boulders of Paleozoic limestone 

found during this study in conglomerate near the base of 

the lower member of the Elkhorn Mountains Volcanics 

on the ridge east of Corral Creek (sec. 23, T. 9 N ., R. 2 

W.), which indicate nearby exposed Paleozoic rocks 

during deposition; and (2) similar boulders and large 

blocks of Paleozoic limestone associated with thick (as 

much as 98 ft (30 m)) lenticular resedimented carbonate 

interbeds within lava flows at the base of the lower 

member of the volcanics on Elkhorn Peak (sec.1, T. 6 N., 

R. 3 W.). These resedimented carbonate beds and blocks 

probably were deposited in water ponded against a fault 

scarp that exposed Paleozoic limestone to erosion and 

mass wasting. We suggest that this scarp was produced by 

up-to-the-west reverse faulting on an extension of the 

Mitchell Gulch fault zone. These stratigraphic data 

suggest an early synvolcanic age for the first major 

movement on this zone of reverse faulting. We suggest 

that this movement was related to regional thrusting. 

Aligned along the proposed trace of this zone are a partly 

preserved volcano at Radersburg Pass just south of the 

study area; a probable ash-flow caldron extending south 

from a ring fault and dike at the Chicago mine (sec. 26, T. 

9 N., R. 2 W.), just north of the study area, to south of 

Crazy Peak (sec. 35, T. 8 N., R. 2 W.) in the center of the 

study area; and a possible ash-flow caldron extending 

south from Tizer Basin (sec. 28, T. 7 N, R. 2 W.) to near 

Crow Peak (sec. 7, T. 6 N., R. 2 W.) at the southern 

margin of the study area. These volcanic and structural 

features range in age from lower member to middle 

member and suggest a continued influence of the Mitchell Gulch fault zone during volcanism. Late volcanic 

influence of this zone is suggested by resurgence and 

emplacement of laccoliths during deposition of the 

predominantly volcaniclastic upper member in these 

proposed caldrons. Peperitic laccoliths contacts in the 

High Peak area (sec. 34, T. 8 N., R. 2 W.) indicate that 

the laccoliths intruded into unconsolidated wet upper 

member sediments during or shortly following 

deposition. Similar and probably coeval laccoliths were 

emplaced along the axis of the syncline west of the 

leading edge of the proposed thrust at Townsend VaJley. 

The largest synvolcanic structural feature is the 

major depression of the batholith area shown on the 

geologic map of Montana (Ross and others, 1955) and 

other small-scale geologic compilations of the area. 

These maps show predominantly Cretaceous layered 

rocks at and near the batholith contact and progressively 

older rocks in all directions away from the contact as 

observed by Hamilton and Myers (1967) and other 

Mineral Resource Assessment 9 
workers. This depression appears to have begun to form 

after the early volcanic inflation of the batholith area 

described above and continued during batholith 

emplacement. The synvolcanic Upper Cretaceous 

Golden Spike Formation (Gwinn and Mutch, 1965) in 

the Garrison area about 25 mi (40 km) west of the 

batholith consists of north- and northwestward-derived 

prevolcanic rocks and subordinate volcanic rocks similar 

to the lower member of the Elkhorn Mountains Volcanics. Rocks comparable to the middle memb~r are absent 

near Garrison as if a topographic barrier prevented 

westward flowage of these welded tuffs. This indicates 

that major depression of the batholith area began after 

inflation and by middle member time generally kept pace 

with volcanic deposition. At least 14,500 ft ( 4,400 m) of 

volcanic and volcaniclastic rocks were deposited in this 

depression. 

Synbatholith structural features in the study area 

include the following: major down-to-the-west normal 

faulting that controlled the eastern contact of the batholith by the Mitchell Gulch fault zone; localization of most 

satellite plutons in a syncline west of the Indian Creek 

fault zone; formation of faults and shear zones at the 

near intrusive contacts, mostly by reactivation of faults or 

weakness directions of volcanic age. Synbatholithic faults 

associated with intrusives were the main controls for 

mineralization in the eastern part of the study area. The 

batholith intrudes and is not cut by reverse faults of the 

Mitchell Gulch fault zone and therefore is entirely post 

thrusting. Rocks of the batholith intrude and are cut by 

normal faults in several places. Apparently extension and 

subsidence and not compression predominated during 

batholith emplacement. Late during batholith 

crystallization, a major northeast -trending weakness 

zone formed into which many alaskite-aplite, leucogranodioite, and metalliferous quartz veins were emplaced in 

the western part of the study area. Right-lateral and 

dip-slip offsets are shown on northeast-trending faults in 

this zone; east -trending faults in this zone show leftlateral and dip-slip offsets; and the few northwesttrending faults show right-lateral and dip-slip offsets 

(Smedes, 1966; Becraft and others, 1963). In the general 

vicinity of Jefferson City (sec. 6, T. 7 N., R. 3 W.), the 

northeast-trending zone and its constituent fractures, 

faults, veins, and igneous bodies turn east and pass 

generally north of the study area. Most of these 

structures and bodies are confined to the batholith itself. 

Postbatholithic Structural Features 

The northeast-trending weakness zone in the 

batholith also was site of Eocene volcanism and related 

plutonism (Smedes, 1966, pl. 1; Becraft and others, 

1963). Oligocene volcanism and plutonism occurred 

in but appear to cut across this weakness zone. 

10 Elkhorn Wilderness Study Area, Montana 

Range-front faults of late Tertiary to Holocene age 

are exposed along the northeast front of the Elkhorn 

Mountains (Klepper, Ruppel, and others, 1971, p. 32). 

The major faults of this system mostly are covered by fan 

gravel, but they probably bound the range on the north 

(Smedes, 1966), east (Klepper, Ruppel, and others, 

1971), and southwest (Klepper and others, 1957). These 

faults are part of a broad system of basin and range faults 

that produced the north- and northwest -trending valleys 

south of the Lewis and Clark line, which extends through 

the Helena Valley (Reynolds, 1977) to the north. 

Extensional faulting that produced these valleys, which 

are filled with Cenozoic sediments, is apparently related 

to major right-lateral faulting in the northwest-trending 

Lewis and Clark line. 

Geophysical Exploration 

By William F. Hanna 

Introduction 

Geophysical assessment of mineral resource 

potential in the Elkhorn Wilderness Study Area has been 

confined almost exclusively to an interpretation of 

recently acquired aeromagnetic anomaly data. Older 

reconnaissance gravity anomaly data were also examined, 

especially in light of recently measured rock densities, 

but were found to lack sufficient resolution to be of 

significant value in the overall assessment (Greenwood 

and others, 1978). Aeromagnetic data were obtained in 

two surveys: a regional survey flown by the U.S. 

Geological Survey in 1953 and 1955, reported in Johnson 

and others ( 1965) and generalized in figure 3, and a 

detailed survey flown for the U.S. Geological Survey in 

April1977, shown on plate 2. The trapezoid-shaped area 

(pl. 2) is bisected by part of the contact between two vast 

terranes of Upper Cretaceous igneous rock: the Boulder 

batholith and the Elkhorn Mountains Volcanics (fig. 4). 

The regional aeromagnetic data generalized in figure 3 

had indicated a close association of high-amplitude highs 

with the Elkhorn Mountains Volcanics and plutons 

intrusive into the volcanics, most prominently expressed 

near the contact between the volcanics and the Boulder 

batholith. The regional survey also showed that weaker 

highs and lows are associated with much of the Boulder 

batholith, the lows in places delineating regions of 

pronounced mineralization and ore deposition (Hanna, 

1969). The detailed survey made for this study (pl. 2) not 

only confirms the existence of these regional patterns but 

also discloses a number of low- and high-amplitude 

anomalies unseen in the original survey, permitting much 

improved correlations of individual anomalies with 

specific lithologic units. The purpose of the present 
investigation is to establish such correlations between the 

anomalies and their source rocks to help assess the 

mineral resource potential of the study area. 

Rock Magnetic Data 

An important aim of aeromagnetic anomaly 

interpretation is identification of the kinds of rocks 

responsible for causing the anomalies. Interpretation of 

anomaly source rocks is greatly aided where direct 

correlations of magnetic rock occurrences and measured 

anomalies are observed. For example, if there is a 

pronounced positive correlation of magnetic rock 

outcrops with anomalies, these rocks, or others allied in 

composition to them, may be inferred to extend into the 

subsurface, serving as the overall anomaly source. By 

measuring the magnetization of these exposed rocks, the 

subsurface volume and distribution required to produce 

the observed anomalies can be estimated. In the present 

study, total magnetizations of 51 outcrop samples were 

measured, the remanent component using a spinner 

magnetometer and the induced component using a 

magnetic susceptibility bridge. 

The measured remanent and induced 

magnetizations of the 51 samples, all of which were 

plutonic or volcanic rocks (table 1; pl. 2), range over five 

orders of magnitude, consistent with data from nearby 

collecting sites in Elkhorn Mountains Volcanics, Boulder 

batholith, and Lowland Creek Volcanics (Hanna, 1965, 

1967, 1969, 1973a, b, and 1977b). Of special interest are 

the magnitudes of remanent and induced magnetization, 

listed in table 1 under columns denoted "REM J" and 

"IND J," respectively. In the study area, rocks having 

magnitudes of either remanent or induced magnetization 

exceeding 100 x 10-5 emu/cm3 are capable of generating 

high-amplitude anomalies, depending upon their 

distribution in the subsurface, and are thus candidates 

for active sources of anomalies. Table 1 shows that 13 

samples have both remanent and induced magnetizations 

exceeding this value, 3 samples have remanent 

magnetizations alone exceeding the value, and 10 

samples have induced magnetizations alone exeeding the 

value. Of the remainder, 22 samples have magnetizations 

sufficiently weak to be considered nonmagnetic, most 

having been altered, silicified, or otherwise mineralized, 

and are thus candidates for passive sources of negative 

anomalies where surrounded by normally polarized 

rocks. The directions of remanent magnetization, 

denoted by values under columns "REM D" and "REM 

I" for the 14 oriented samples collected, indicate normal, 

reversed, and perhaps anomalous paleofield directions. 

Interpretations of the paleomagnetic significance of these 

data are limited by uncertainties about the possible 

overprint of secondary magnetization components 

acquired after the period of volcanism and plutonism and 

the structural history of tilting of layered volcanic rocks 

possessing stable remanent magnetization. 

Principal Features of Anomaly Map 

In the following discussion, unnecessary usage of 

numerical values of magnetizations will be avoided by 

using the terms "strong" magnetizations for those from 

100 to 1,000 x 10-5 emu/cm3 and "very strong" 

magnetizations for those exceeding 1,000 x 10-5 

emu/cm3

• 

The aeromagnetic anomaly map (pl. 2) has two 

principal features: steep magnetic gradients that largely 

coincide with the outcrop of the broadly folded, thick pile 

of Elkhorn Mountains Volcanics, which in the study area 

is intruded by a number of small plutons, and gentle 

magnetic gradients of the magnetic plateau associated 

with most of the Boulder batholith. Gradients are steep 

near the volcanic rock pile because total magnetizations 

of the volcanic and associated intrusive (unit Ki, pl. 1) 

rocks are generally strong to very strong and also because 

much of the volcanic rock terrane occurs at high 

topographic elevations, not far below the flight elevation 

at which magnetic field measurements were obtained. 

The effect of topography is in many places further 

enhanced by strong isothermal remanent magnetizations 

introduced by lightning discharges which are more 

numerous on high ground than on low. Gradients are 

gentle over most .of the Boulder batholith terrane 

because total magnetizations are generally weak to 

moderately strong and because lateral contrasts of rock 

types and corresponding magnetizations are relatively 

small, the batholith proper having a remarkably uniform 

composition. 

A notable exception consists of the steep magnetic 

gradients which extend westward across the Boulder 

batholith-Elkhorn Mountains Volcanics contact in the 

southwestern part of the map area. These gradients 

appear to coincide with an increase in the Bouger gravity 

anomaly field (Greenwood and others, 1978) signifying 

relatively dense, highly magnetic rocks in the shallow 

subsurface of this part of the Boulder batholith. Such 

rocks might belong to a westward-dipping tract of 

Elkhorn Mountains Volcanics or to highly mafic plutonic 

rocks of the Boulder batholith intruded during an early 

phase of plutonism. 

Interpretation of Anomalies 

Many individual anomalies within the study area 

are clearly associated with mapped lithologic units; 

others are only vaguely, if at all, related to exposed rocks. 

Because the number of magnetic features of possible 

economic interest is so large, they are labeled on plate 2. 

Mineral Resource Assessment 11 
0 5 10 MILES 

0 5 10 15 KILOMETERS 

Figure 3. Aeromagnetic map of the Elkhorn Wilderness Study Area and vicinity, Montana (modified from 

Johnson and others, 1965). Contours show total intensity magnetic field of the Earth in gammas relative to an 

arbitrary datum. Hachured to indicate closed areas of lower magnetic intensity; dashed where data are 

incomplete. Specifications of regional survey: flight spacing, 2 miles (3.2 km); flight elevation, 10,500 feet (3,200 

m) above sea level; compilation scale, 1 :250,000; contour interval, 100 gammas. Flown by U.S. Geological 

Survey, 1953 and 1955. Trapezoid-shaped outline shows area of detailed aeromagnetic survey discussed in text 

and shown on plate 2. 

12 Elkhorn Wilderness Study Area, Montana 
0 Helena 

0 5 

0 5 10 

EXPlANATION 

Eocene Lowland Creek Volcanics 

Cretaceous Boulder Batholith 

and related plutonic rocks 

Cretaceous Elkhorn Mountains Volcanics 

II Cenozoic to Proterozoic sedimentary 

L__j rocks and deposits 

Outline of Plate 2 

10 MILES 

15 KILOMETERS 

PALEOMAGNETIC 

SAMPLE SITE 

e Major 

• Minor 

Townsend0 

Figure 4. Generalized geologic map of the Elkhorn Wilderness Study Area and vicinity, Montana. Major 

terranes of igneous rocks highlighted (modified from Smedes and others, 1968). 

Mineral Resource Assessment 13 
Table 1. Magnetization and density data for selected rocks from the Elkhorn Wilderness Study Area and vicinity, Montana 

[Sample localities shown on plate 2. Leaders ~ ---), no data. Field no. indicates sample designation. Abbreviations: REM D, declination of remanent 

magnetization, degrees east of north; REM , inclination of remanent magnetization, degrees below horizontal, REM J, ma~tude of remanent 

magnetization, x to-5 emu/ems; K, intrinsi~ magnetii susceptibility based on sample-shape demagnetization factor of 4-tr/3, t - emu/ems; IND J, 

magnitude of induced m~etization, X w- emu/em ; Q, Koenigsberger ratio of magnitudes of remanent magnetization to rduced ~agnetization; 

p, dry bulk density, g/cm ; J-type of likely anomaly source, type of magnetization which is sufficiently strong (at least to- emu/em ) to setve as 

a source for high-amplitude anomalies: R-remanent magnetizations, 1-induced magnetization, C-combined remanent and induce~ 

magnetization, if similar in direction and sense, NON-relatively nonmagnetic; asterisks denote very strong magnetizations (greater than toemu/ems)] 

Sample 

locality Field REM REM REM INO 

J-type of 

likely anomaly 

No. No. D 1 3 K 3 a p Rock type source 

77lA081 794 28.9 16.7 47.5 2.35 Flow-banded rhyolite, R 

Strawberry Butte. 

2 GE049P 147 256 148 0.993 265 Biotite quartz monozonite. R,I 

3 GE036P 3.01 38.1 22.0 0.137 2.54 Weathered quartz monzo- NON 

nite; limonite bo:xworks. 

4 GE035P 15.2 66.5 38.4 0.3% 2.56 Granite dike. NON 

5 GE033P 79.6 242 140 0.568 2.61 Biotite quartz monzonite. I 

6 GE042P 15.2 25.1 14.5 1.05 2.36 Weathered porphyritic NON 

quartz monzonite. 

7 GE039P 0.415 1.90 1.90 0.381 2.51 Weathered biotite quartz NON 

monzonite. 

8 GE045P 34.8 207 119 0.292 2.61 Biotite quartz monzonite. 

9 GE046P 16.6 280 162 0.102 2.66 Porphyritic biotite 

granodiorite. 

10 77lA079 5.01 287 166 0.00302 2.64 Altered porphyritic 

granodiorite. 

11 BG126 275.5 21.0 259 271 156 1.66 2.66 Biotite granodiorite. R,I 

12 BG130 347.4 10.7 153 256 148 1.03 2.61 Biotite granodiorite. R,I 

13 77lA073 19.6 280 162 0.121 2.66 Altered granodiorite. I 

14 77lA077 27.3 176 102 0.268 2.61 Altered porphyritic quartz I 

monzonite. 

15 BG274 68.0 89.8 51.8 1.31 277 Andesitic sandstone, c 

Elkhorn Peak. 

16 BG097 1600 559 322 4.97 2.76 Breccia. R*, I 

17 GE013P 13.3 58.7 33.9 0.392 2.68 Altered pyrite-rich aplite. NON 

18 GE010P 0.377 1.30 0.750 0.503 2.50 Altered tourmaline-rich NON 

plutonic rock. 

19 GE011P 0.199 1.80 1.04 0.191 2.49 Altered aplite. NON 

20 GE012P 0.459 7.50 4.33 0.106 2.64 Altered plutonic rock. NON 

21 GE009P 451 269 155 291 273 Diorite. R,I 

22 GE033P 44.0 292 168 0.262 2.69 Biotite quartz diorite. I 

23 GE032P 15.0 727 41.9 0.358 284 Hornblende-rich basalt. NON 

24 GE024P 20.1 23.9 13.8 1.46 262 Weathered porphyritic NON 

volcanic intrusive (?) 

rock. 

25 BG155 81.3 325 188 0.432 269 Quartz-dioritic 

granodiorite. 

26 GE031P 57.4 319 184 0.312 278 Lithic fragmented tuff. I 

27 GE029P 10.1 49.7 28.7 0.352 263 Altered sandy volcanicL1Stic NON 

rock. 

23 GE025P 2.10 7.50 4.33 0.458 2.68 Pyrite-rich welded tuff; NON 

Ballard mines. 

29 GE027P 4.66 6.10 3.52 132 2.67 Altered silicified pyrite-rich NON 

welded tuff. 

30 GE018P 0.369 5.50 3.17 0.116 2.75 Silicified pyrite-rich NON 

andesite. 

31 GE017P 0.0689 2.90 1.67 0.0412 2.67 Silicified pyrite-rich NON 

volcanic(?) rock. 

32 GE016P 0.320 0.900 0.519 0.616 2.38 Altered pyrite-rich andesite; NON 

Cataloma mine. 

33 GE005P 0.298 1.80 1.04 0.286 272 Argillitic. NON 

34 GE003P 0.180 6.40 3.69 0.0488 2.73 Andesitic dike. NON 

35 GE001P 27.4 4.20 242 11.3 2.77 Andesite. NON 

36 GE002P 89.2 154 88.8 1.00 2.71 Porphyritic-lithic c 

fragmental volcanic 

(andesite). 

37 77lA073 346.2 -23.8 0.318 7.09 4.09 0.0778 2.92 Gabbro. NON 

38 77lA075 18.5 49.6 37.4 324 187 0.200 2.68 Porphyritic quartz NON 

monzonite. 

39 77lA070 302.3 -40.4 0.233 4.06 2.34 9.96 2.84 Hornfels. NON 

40 BG112 91.1 27.4 490 478 276 1.78 2.72 Slim Sam stock; R,I 

granodiorite. 

14 Elkhorn Wilderness Study Area, Montana 
Table 1. Magnetization 

Montana-Continued 

and density data for selected rocks from the Elkhorn Wilderness Study Area and vicinity, 

Sample 

locality Field REM REM REM 

No. No. 0 1 3 K 

41 80113 332.1 720 49.9 35.8 

42 77LA069 109.2 -47.3 174 24.0 

43 BG123 874 425 

44 BG120 334 675 

45 80125 9.3 -5.9 5700 363 

46 80118 330.5 56.1 115 513 

47 80119 2743 5.0 113 60.8 

48 80209 1000 469 

49 80117 24.8 -57.9 1790 1150 

50 80115 240.1 39.7 961 401 

51 80114 344.1 46.6 21.4 453 

A El-E5 460 8.30 

B E6-E11 86 3.10 

c E16-E20 80 2.40 

D E21-E25 50 2.90 

Major anomalies or groups of anomalies are numbered 

and parts of anomalies or individual anomalies in the 

groups are given letter symbols, starting first with highamplitude anomalies near the contact between the 

Boulder batholith and Elkhorn Mountains Volcanics and 

progressing to terranes of volcanic rocks to the southeast 

and batholithic rocks to the northwest. 

Anomalies 1, 2, and 3 

This northeast-trending group of magnetic highs, 

among those having the highest amplitude in the study 

area, is associated principally with topographically high 

exposures of intrusive rocks (Ki, pl. 1) associated with the 

Elkhorn Mountains Volcanics. Although magnetization 

data for these rocks are not available, magnetizations of 

similar intrusive rocks (table 1, locs. 44, 45, and 49) 

southeast and northeast of the topographic ridge are 

among the strongest in the area. Adding to the effects of 

the intrusive rocks are strong magnetizations of 

neighboring volcanic rocks which are also exposed at high 

elevations. For example, near the crest of anomaly 1, 

outcrops of amygdaloidal augite basalt (loc. 48) have very 

strong remanent and strong induced magnetizations; on 

the southwest flank of anomaly 3, exposures of volcanic 

breccia (loc. 16) have magnetizations even stronger than 

J-type of 

INO likely anomaly 

3 a p Rock type source 

20.6 2.42 2.70 South Fork stock; NON 

granodiorite. 

13.8 12.6 277 Diabasic flow. R 

245 3.57 2.83 Porphyritic andesitic flow or R,I 

CI}'Stal tuff. 

389 0.859 284 Porphyritic augite-rich R,I 

diorite. 

209 213 2.84 Hornblendic pL'lgioclase R*, I 

porphyty. 

296 0.388 2.66 Vosburg stock; quartz R,I 

monzonite porphyty. 

35.1 3.22 2.72 Hornfelsic welded tuff. R 

271 3.69 2.89 Amygdaloidal augite basalt. R*, I 

664 2.70 2.79 Diabasic intrusive. R*, I 

231 4.16 281 Orphan Boy intrusive; R,I 

diorite-gnbbro. 

261 0.0820 266 Antelope Creek Stock; 

granodiorite. 

4.78 96.2 285 Andesitic tuff; 5 samples R 

measured in 1962. 

1.78 48.2 284 Andesitic flow; 5 samples NON 

measured in 1962 

1.38 57.9 2.72 Andesitic tuff; 5 samples NON 

measured in 1962. 

1.67 29.9 2.71 Andesitic flow; 5 samples NON 

measured in 1962. 

the basalt. Although part of the magnetizations of these 

highly elevated rocks may be attributable to strong 

isothermal remanent magnetization induced by lightning 

discharges at the surface, the observed strong induced 

magnetizations indicate that these rocks are highly 

magnetic in the subsurface. 

On the northwest flank of anomaly 1, the negative 

indentation 1A is associated with plutonic rocks exposed 

in the upper reaches of Jackson Creek, probably a 

manifestation of relatively nonmagnetic altered 

subsurface rocks. Northeast of the anomaly, the elongate 

negative feature 1B appears to be the result of both a 

polarization low associated with anomaly 1 and a residual 

low nested between two high-amplitude positive 

anomalies. This negative feature widens over part of the 

upper drainage of Staubach Creek, indicating that 

magnetizations of subsurface rocks are weakest there. 

On the southeast flank of anomaly 1, the negative 

indentation 1 C appears to be caused by a small area of 

relatively nonmagnetic rocks that are not indicated by 

topography. In contrast, the positive indentation 10 and 

negative indentation 1E are clearly associated with 

topographic relief developed on volcanic rocks. Other 

features which are influenced by topography are saddles 

1F and 2A, negative feature 2B, positive noses 3A and 

3B, and negative indentation 3C. Features correlating at 

Mineral Resource Assessment 15 
least partly with topographic expression and partly with 

extrusive and intrusive rocks of the Elkhorn Mountains 

Volcanics are positive indentation 3D and negative 

indentation 3E. 

Anomaly 4 

This broad high-amplitude magnetic high, which 

tapers southwestward to the vicinity of Elkhorn Peak, 

occurs over a topographically high region of extrusive 

and intrusive rocks largely covered by Quaternary 

sediments. Of the five samples collected from an area 

beneath the broader part of the anomaly (locs. 25, 26, 27, 

28, and 29), altered or mineralized welded tuffs and 

volcaniclastic rocks are essentially nonmagnetic whereas 

granodiorite and lithic tuff are strongly magnetic, 

containing mainly induced magnetization. Near the 

tapered part of the anomaly, marked by the positive 

closure of contour lines near Elkhorn Peak (feature 4A), 

andesitic sandstone (loc. 15) has a combined remanent 

and induced magnetization, which if coincident with each 

other, are sufficiently strong to explain the anomaly. 

Although numerous lithologic units appear to serve 

collectively to produce the overall anomaly, buried 

intrusive rocks similar to those correlated with anomalies 

1, 2, and 3, may also help to produce the anomaly. Near 

Elkhorn Peak, which has an elevation approximately the 

same as that of the aeromagnetic flights, a significant 

effect may be caused by local occurrences of magnetiterich metamorphic rocks. Indeed, two bodies of such rock 

on the south and west slopes of Elkhorn Peak have been 

mined, its iron content giving it particular value as a 

smelter flux (Knopf, 1913a; Klepper and others, 1957). 

To the north of the broad part of the anomaly, 

features 4B, 4C, and 4D form closures over a region of 

low magnetic gradients. Of the 11 intrusive and extrusive 

rocks collected from this area (locs.17, 18, 19, 20, 21, 22, 

23, 24, 30, 31, and 32), all but two are nonmagnetic. 

Much of the magnetization assumed to have been 

present was probably diminished througn ieaching and 

other chemical alteration. The two exceptions are 

relatively magnetic diorite and biotite quartz diorite 

(locs. 21 and 22, respectively) which crop out beneath 

gradients on flanks of the local closures. The entire 

region covered by features 4B, 4C, and 4D is probably 

underlain by relatively nonmagnetic rock, much of which 

probably has been chemically altered. 

East of feature 4D, a steep north-trending 

magnetic gradient is indented by a negative feature 4E. 

This region between and near the margins of the lows has 

been extensively mined (Callahan, Pataloma, Big Tizer 

Wildcat, and Golden Age mines). Other minor variations 

of the commonly steep magnetic gradient, such as 4F, 

4G, 4H, and 41, appear to be appreciably influenced by 

topography developed on the extrusive and intrusive 

16 Elkhorn Wilderness Study Area, Montana 

rocks. Of greater interest, the negative closure 4J is 

centered near the site of the Little Tizer Wildcat mine, 

suggesting altered subsurface rocks. Farther south, the 

negative closure 4K and the dipole feature 4L, though 

not associated with previous mining, may also mark local 

terranes of altered subsurface rock. Another association 

of a negative magnetic feature with mineralized rock 

occurs near the Elkhorn Skyline mine marked by the 

indentation 4M; the dipole feature 4N may reflect 

another zone of altered volcanic rocks not exposed at the 

surface. The local high-amplitude negative closure 40, 

centered near the upper drainages of Black Canyon and 

Rabbit Gulch, reflects strong polarization associated with 

topographically high volcanic rocks near Elkhorn Peak, 

but the anomaly itself is developed over plutonic rocks of 

the Boulder batholith, which is relatively nonmagnetic in 

this region. North of anomaly 40, the prominent positive 

bulge 4P is influenced by high topographic relief over 

rocks of the Boulder batholith. 

Anomaly 5 

This prominent magnetic low, developed as a 

northwest- to west-trending trough near Elkhorn, is 

associated with an intensively mineralized zone which 

includes sites of the Heggen, Moreau, East Butte, Hardcash, and Carmody mines as well as molybdenum-rich 

rock of Turnley Ridge (the unnamed ridge in the Elkhorn 

mining district about 1 mi (1.6 km) south of the study 

area). Although the previously mined area is almost 

entirely restricted to the northern flank of this anomaly (a 

notable exception is the Boulaway mine site located on 

the anomaly axis), its westward extension to feature SA 

may reflect an almost continuous subsurface zone of 

alteration across the contact between the Boulder batholith and Elkhorn Mountains Volcanics. The westernmost 

extremity oL the anomaly, feature SA, also reflects a 

polarization effect developed north of the high-amplitude 

positive feature SB, the high presumably associated with 

relatively magnetic plutonic rocks of the Boulder batholith. Northeast of the magnetic trough, the southeasttrending positive nose of dipole anomaly SC and the 

positive bulge 50 are associated with mafic plutonic 

rocks which intrude Precambrian and Paleozoic 

sedimentary rocks. The negative indentation SE due 

north of Elkhorn is associated with intense 

mineralization marked by sites of the Louise, Golden 

Moss, C and D, and Union mines. 

Although magnetizations of rock samples were not 

determined in mineralized areas near Elkhorn, 

previously obtained data from Elkhorn Mountains Volcanics northeast of feature SE and south of feature 4N 

(locs. A, B, C, and D) provide representative values 
which are normal for this part of the volcanic rock section 

(Hanna, 1965, 1967). Data from 20 samples indicate the 

presence of weak to strong remanent magnetizations but 

only weak induced magnetizations (table 1 ). 

Anomaly 6 

Near the northeastern corner of the map area, in 

approximate alignment with anomalies 1, 2, and 3, a 

broad high-amplitude magnetic high occurs over a major 

exposed tract of the Antelope Creek stock, in a 

topographically low area drained by Antelope Creek. 

Although most of the anomaly is associated with granodiorite of the stock, which has an induced magnetization 

(loc. 51), part of the anomaly is generated by diabasic 

intrusive rocks (loc. 49) and such mafic rocks as dioritegabbro of the Orphan Boy intrusive (loc. 50), both of 

which have strong remanent and induced magnetizations. 

Northwest of the anomaly, the high-amplitude 

magnetic low 6A, only partially included in the detailed 

survey, appears to be associated with Elkhorn Mountains 

Volcanics containing mineralized breccia deposits. 

Anomaly 7 

At the eastern margin of the map area, a very high 

amplitude positive anomaly is associated with plutonic 

rocks of the Vosburg stock and surrounding volcanic 

rocks. Although the eastern half of the anomaly is largely 

attributable to quartz monzonite porphyry (loc. 46) of 

the stock, which has strong remanent and induced 

magnetization, it is also partially generated by a mixture 

of other rocks, including metamorphosed welded tuff 

(loc. 47) having strong remanent magnetization and 

hornblendic plagioclase porphyry (loc. 45) having very 

strong remanent and strong induced magnetization. The 

regular form of the anomaly suggests that plutonic rocks 

of the stock extend in the subsurface west of the mapped 

contact; however, it appears that rocks of the Elkhorn 

Mountains Volcanics also contribute to the western half 

of the anomaly. For example, porphyritic andesite (Joe. 

43) and porphyritic augite-rich diorite (loc. 44) both have 

strong remanent and induced magnetizations, and could 

contribute significantly to the feature. 

North of the anomaly, the prominent magnetic 

trough 7 A is developed over a tract of volcanic and 

plutonic rocks covering part of the upper drainage of 

Beaver Creek; either the quartz monzonite of the Moose 

Creek stock is relatively nonmagnetic, or the body is thin 

and extends to only shallow depth, in sharp contrast to 

similar plutonic rocks of the Antelope Creek and Vosburg stocks. Regardless of how much of the anomaly is a 

polarization effect linked with anomaly 7 and how much 

is a residual low developed between anomalies 6 and 7, 

the underlying tract of subsurface rocks is deficient in 

magnetite. 

On the northern flank of anomaly 7, the positive 

indentations 7B and 70 conform somewhat to 

topography but may in part represent subsurface lobes of 

plutonic rock extending from the core of the Vosburg 

stock. Negative indentations 7C and 7E are residual lows 

over relatively nonmagnetic rocks adjacent to the positive 

features. Positive feature 7F at the southwest margin of 

anomaly 7 is associated with an exposed quartz monzonitic body allied to the Vosburg stock; positive feature 

7H to the southeast is probably associated with a similar, 

though completely buried, small body of quartz monzonite. Negative indentation 7G south of the anomaly 

occurs over the trace of a major fault; the feature may in 

part be caused by gouge and chemical alteration of rocks 

within the fault zone. 

Anomalies 8, 9, and 10 

This trio of anomalies west of anomaly 7 probably 

represents the three expected polarization effects 

associated with the concave part of a bow-shaped outcrop 

pattern of Elkhorn Mountains Volcanics, convex to the 

north (fig. 3), having on the average a normal total 

magnetization. The low of anomaly 8 is associated mainly 

with the eastern part of the pattern; the low of anomaly 

10, with the western part. The residual positive anomaly 

9 results from the combined decrease in amplitude of 

both negative anomalies with increasing lateral distance 

from the sides or limbs of the pattern. Thus, these 

polarization effects cannot be correlated with local 

occurrences of mapped units; they are caused by the bulk 

effect of the volcanic rock pile. The local positivenegative interruption lOA of the western gradient of 

anomaly 10 is not part of the polarization effect; this 

feature appears to be a local east-west dipole anomaly of 

unknown source. 

Anomaly 11 

This very broad low-amplitude magnetic low is 

associated mostly with relatively nonmagnetic Mesozoic 

sedimentary rocks (for example, nonmagnetic shale of 

loc. 33) forming part of the core of the major anticline in 

that area. The negative lobes llA, llB, and llC along 

the northern flank of the anomaly coincide in large part 

with drainages of Crow Creek, Longfellow Creek, and 

Eureka Creek; intervening positive lobes appear to be 

related to exposures of intrusive rocks (unit Ki, pl. 1) of 

the Elkhorn Mountains Volcanics. 

Anomaly 12 

East of anomaly 11, at the eastern edge of the study 

area, a partially defined high amplitude magnetic high 

Mineral Resource Assessment 17 
appears to be associated with mapped exposures of mafic 

plutonic rocks and intrusive rocks (Ki, pl. 1) of the 

volcanic rock pile. One sample of diabasic lava flow (loc. 

42) has expectedly strong remanent magnetization, 

sufficient to serve as an anomaly source. To the north, 

negative feature 12A appears to be a polarization and 

residual low developed over rocks of the lower member 

of the Elkhorn Mountains Volcanics. To the south, 

negative indentation 12B is associated with a block of 

highly siliceous welded tuff of the middle member of the 

Elkhorn Mountains Volcanics; positive indentation 12C 

is associated with andesitic rocks of the lower member. 

Anomalies 13 and 14 

Southwest of anomaly 12, the saddle-shaped high 

13 is developed over quartz monzonite of the South Fork 

stock, the saddle interrupting an otherwise continuous 

magnetic trough at anomalies 11 and 14 that are 

associated with relatively nonmagnetic sedimentary rocks 

in the core of the major anticline. It is of considerable 

interest that, unlike most stocks which intrude the 

volcanic and sedimentary rocks within the study area, the 

South Fork stock has a very low amplitude magnetic 

signature. Granodiorite (loc. 41) from the stock has a 

weak remanent magnetization and a very weak induced 

magnetization. This indicates that the stock probably has 

a subsurface extent similar to those of other stocks in the 

study area; however, for an unknown reason, the rock is 

deficient in magnetite. 

Anomaly 15 

South and west of anomalies 13 and 14, anomaly 15 

is a high-amplitude magnetic high, partially within the 

surveyed area that occurs over the Slim Sam stock and 

neighboring Paleozoic sedimentary rocks. Granodiorite 

from the Slim Sam stock (loc. 40) exhibits strong remanent and induced magnetization, in sharp contrast with 

granodiorite from the South Fork stock, only 1 mi (1.6 

km) to the northeast. The anomaly suggests that granodiorite of the Slim Sam stock extends in the subsurface as 

much as 1 mi (1.6 km) northwest of the mapped contact 

beneath a thin veneer of sedimentary rocks. The anomaly 

abruptly terminates in a magnetically featureless region 

15A, marked by the Bonanza mine. The dipole feature 

15B to the west, developed in a polarity sense reversed to 

others in the study area, has an unknown source. 

Anomaly 16 

Northwest of anomaly 15, a broad positive nose is 

associated with quartz monzonite of a small exposure of 

plutonic rock, which probably has a total magnetization 

more similar to that of the Slim Sam stqck than the 

18 Elkhorn Wilderneaa Study Area, Montana 

nearby South Fork stock. The regular form of the 

anomaly suggests that the intrusive body may be nearly as 

large as the Slim Sam or South Fork stocks at depth, of 

possible significance to a geochemical anomaly observed 

in stream-sediment samples from Jenkins Gulch. Hornfels (loc. 39) which occurs at the intrusive rock contact, as 

well as andesite, gabbro, and quartz monzonite exposed 

farther to the north (locs. 34, 35, 36, 37, and 38), are 

relatively nonmagnetic except for a fragmental volcanic 

rock (loc. 36) which may have a combined remanent and 

induced magnetization sufficiently strong to serve as an 

anomaly source. The negative indentations 16A and 16B 

represent polarization and residual lows adjacent to the 

positive anomaly. 

Anomaly 17 

At the northwest corner of the study area, in the 

Boulder batholith, a broad low-amplitude magnetic high 

is developed over Strawberry Butte, Shingle Butte, and 

lower terrain extending eastward to McClellan Creek. 

The closure at Strawberry Butte and the prominent nose 

at Shingle Butte reflect topographically high exposures of 

rhyolite flows, which also create the broad positive nose 

17 A on Burnt Mountain, south of Strawberry Butte. 

Remanent magnetization of the rhyolite (loc. 1) is 

stronger than all but six of the volcanic and plutonic rocks 

sampled in the study area, a remarkable property 

considering its highly silicic composition. 

Southeast of Strawberry Butte, the positive nose 

17B appears to be associated with topographically high 

exposures of porphyritic quartz monzonite. South of this 

feature, the negative closure 17C bears no relation to 

mapped geology and therefore may represent a lowamplitude residual anomaly. Biotite quartz monzonite 

(loc. 2) from a region northeast of Burnt Mountain has 

strong remanent and induced magnetization assumed to 

be typical of much of the unaltered Boulder batholith and 

presumably accounts for feature 17A. 

Anomaly 18 

East of anomaly 17 and northwest of anomaly 1, a 

broad high-amplitude magnetic low extends over the 

contact between the Boulder batholith and Elkhorn 

Mountains Volcanics and is probably associated with a 

large tract of altered subsurface rocks covering much of 

the drainage of Jackson Creek. The elongate positive 

nose 18A to the north may be caused by a combination of 

unaltered plutonic rocks of the Boulder batholith and 

intrusive rocks (map unit Ki) of the Elkhorn Mountains 
Volcanics which crop out along the northwest gradient of 

the anomaly, 1 mi (1.6 km) southeast of Little Butte. 

Anomaly 19 

Southwest of anomaly 18, a northwest-trending 

low-amplitude magnetic trench forming closure at 19 and 

19A is developed over the Middle Fork Warm Springs 

Creek drainage. Surprisingly, the rhyolitic rocks of Lava 

Mountain produce no conspicuous anomaly, such as 

those associated with rhyolites of Strawberry Butte, 

Shingle Butte, and Burnt Mountain to the north. The 

rhyolites may be very thin at Lava Mountain or they may 

have been altered. Also surprising is that a number of 

plutonic rocks collected along Warm Springs Creek and 

Middle Fork Warm Springs Creek (locs. 5, 8, and 9) have 

strong induced magnetizations, ordinarily sufficiently 

strong to produce a magnetic high; others (locs. 3, 4, 5, 

and 7) which are highly silicic or intensively altered, are 

essentially nonmagnetic, as expected. Weathered porphyritic granodiorite (loc. 10) from an area beneath the weak 

gradient separating, 19C and 190 also possesses strong 

induced magnetization and is assumed to contribute to 

the anomaly source of the positive nose, which is also 

strongly influenced by topography. Despite the presence 

of some strongly magnetic plutonic rocks in the area 

underlying the magnetic trench, most rocks in the 

subsurface are probably relatively nonmagnetic, perhaps 

as a result of extensive alteration. The broad positive 

noses 19B and 19E reflect a constriction of the relatively 

nonmagnetic zone; thus the region most magnetitedeficient widens to include part of the drainage of South 

Fork Warm Springs Creek at the southern margin of 

anomaly 19 and to include the confluence of the North, 

Middle, and South Forks at anomaly 19A. The region 

circumscribed by anomaly 19 includes the sites of 

numerous inactive mines, and it contains a geochemical 

anomaly based on stream-sediment samples. 

Anomaly 20 

Southwest of anomaly 19, a broad magnetic low 

associated with biotite granodiorite of the Boulder batholith is flanked by the most gentle magnetic gradients 

observed on the aeromagnetic anomaly map. Samples of 

the granodiorite from two isolated outcrops (locs. 11 and 

12) have strong remanent and induced magnetizations, 

indicating that even in areas of insignificant magnetic 

relief, the magnetization of the batholith proper is 

significant. The observed magnetizations, about 10-3 

emu/cm3

, offer some measure of magnetization for 

"normal" batholithic rock. Inferred magnetizations of 

sources of anomalies elsewhere in the batholith can be 

referenced to this figure. 

Anomaly 21 

East of anomaly 20, conspicuous positive noses 21 

and 21A, separated by the negative indentation 21B, are 

significantly influenced by topography developed on granodiorite of the Boulder batholith, presumed to have 

strong magnetizations similar to the unaltered rock from 

locality 2 to the north. 

Anomaly 22 

Southwest of anomaly 20, a prominent magnetic 

trough extending fr9m features 22 to 22A is developed 

over the drainages of Prickly Pear and Golconda Creeks. 

Although the negative nose and closure of 22A appears 

to result from effects of high topography to the north and 

south, most of the elongate feature marks a zone of 

relatively nonmagnetic subsurface rocks that are perhaps 

hydrothermally altered. The highest amplitude portion of 

the negative anomaly overlaps a geochemical anomaly 

based on stream-sediment samples and covers a region of 

interest for possible porphyry copper deposits. 

Anomaly 23 

Southeast of anomaly 22, the east-west-trending 

magnetic ridge 23, as well as positive noses 23A and 23B, 

is associated with highly elevated plutonic rocks 

conforming closely to topographic configuration. 

Indentation 23C, which follows the steep upper drainage 

of Muskrat Creek, is a negative expression of this same 

topographic relief. 

Anomaly 24 

Southwest of anomaly 23, the high-amplitude 

broad positive nose 24 extending southwestward to the 

positive closure 24A is developed over altered plutonic 

rocks not conformable with topography. Altered porphyritic quartz monzonite from an outcrop underlying 

anomaly 24 (loc. 14) and altered granodiorite from an 

outcrop underlying feature 24A (loc. 13) have induced 

magnetizations sufficiently strong to serve as anomaly 

sources. Negative indentations 24B, 24C, 240, 24E, and 

24F are all developed over relatively nonmagnetic rocks, 

a number of them associated with mineralization and 

alteration. Feature 240, aligned with the principal 

drainage of Muskrat Creek, overlaps a geochemical 

anomaly based on stream-sediment samples. Features 

24E and 24F, following part of the drainage of Rawhide 

Mineral Resource Assessment 19 
Creek, may also reflect zones of subsurface 

mineralization. 

Relationship of Anomalies to Mineralized Ground 

This study of aeromagnetic anomalies shows that 

the data can reflect mineralized terrane in two ways; first, 

known mineralized ground associated with extensive 

altered areas show distinctive negative anomalies, and 

second, some intrusive rocks associated with mineralized 

areas show positive anomalies. In rare instances, 

anomalies may directly reflect magnetite-rich deposits, 

such as the skarn in replacement bodies near Elkhorn 

Peak; however, the resolution of the aeromagnetic 

anomaly data is insufficient to delineate other known 

magnetite-rich deposits such as stream placers and 

sedimentary lenses known to occur within and adjacent 

to the study area. 

Examples of negative anomalies which are 

associated with mineralized ground marked by mines and 

excavations in and near the western part of the study area 

include the following anomalies: north of 23A, at 19, 

surrounding SE, southeast of 5, north of 4A, at 4M, at 4J, 

and northwest of 4E. Examples of positive anomalies 

associated with mineralized ground in and near the 

eastern part of the study area include those east of 

anomaly 6, surrounding 7, north and south of 7H, north 

of 12, and west of 15. Near the center of the study area, 

magnetic gradients forming the flanks of anomalies are 

associated with mines southeast of 3D and north of 4A. 

Thus, within the Boulder batholith and Elkhorn 

Mountains Volcanics near the batholith contact, 

magnetic lows tend to correlate directly with mineralized 

ground; within the Elkhorn Mountains Volcanics and 

intruded plutons far from the contact, magnetic highs 

tend to be associated with mineralized ground. 

Magnetic Anomalies and Strong Geochemical Anomalies 

Most of the regions within the study area that have 

strong geochemical anomalies based on pan concentrate 

samples (pl. 4) are associated with the magnetic 

gradients forming the flanks of aeromagnetic anomalies 

rather than with the central parts of the anomalies. 

Regions of anomalous gold, silver, arsenic, lead, and 

copper, characteristic of hydrothermal vein-type 

mineralization determined by geochemical studies in the 

eastern part of the study area, are associated with 

gradients forming the saddle on the magnetic ridge 

connecting 4 and 4A; the high-low-high triplet of 7, 12A, 

and 12; the high-low-high triplet of 1, 1B, and 6; the 

southeastern margin of anomaly 9; and the eastern 

margin of anomaly 4F. Regions of anomalous 

molybdenum, tungsten, niobium, thorium, lanthanum, 

and yttrium, characteristic of molybdenite mineralization 

20 Elkhorn Wilderness Study Area, Montana 

in the Boulder batholith determined by geochemical 

studies, are associated with the northern flank of the 

magnetic trough connecting 19 and 19A; the 

northwestern flank of anomaly 3; the western flanks of 

anomalies 21 and 21B; the northern flank of anomaly 2; 

the southwestern flank of anomaly 4A; and the 

northwestern margin of anomaly 240. 

Geochemical anomalies indicate possible 

mineralization on or close to the surface and magnetic 

anomalies indicate possible mineralization at depth. In 

the study are~ surficial mineralization is strongest where 

there are lateral contrasts of magnetization. Some of 

these contrasts may indicate subsurface hydrothermal 

alteration. Because many of the geochemical anomalies 

cover regions having gradients connected with one or 

more positive anomalies over relatively high terrain, 

much surficial mineralization may be located on the 

upper flanks of these combined topographic and 

magnetic highs. 

Aeromagnetic Anomalies Possibly Related to 

Mineralized Rock 

More than 50 aeromagnetic anomalies (fig. 5) 

within and near the study area may be related to 

mineralized rock. Magnetic lows, even if only lowamplitude negative indentations of magnetic gradients, 

serve as reliable guides to subsurface alteration and 

associated mineralization in the terrane of Boulder 

batholith and on both sides of the Boulder batholithElkhorn Mountains Volcanics contact. One example of 

such a low in the Boulder batholith is anomaly 22; 

examples near the contact are negative closures 4J and 

4K. Magnetic highs associated with small plutonic rock 

bodies in the terrane of Elkhorn Mountains Volcanics 

serve as guides to possible subsurface mineralization 

within or at the margins of these bodies. Examples of 

such features are positive noses 7F and 7H and the 

northwestern part of high-amplitude magnetic high 15. 

The association of aeromagnetic anomalies with 

known mineralized ground and with other areas believed 

to be mineralized indicates that further exploration might 

well disclose additional deposits. 

Geochemical Exploration 

By William R. Miller, Steve Ludington, 

and William R. Greenwood 

Geochemical studies were made in the Elkhorn 

Wilderness Study Area, Montana, during November 

1976 and the summer of 1977 to help evaluate the 
EXPLANATION 

AEROMAGNETIC ANOMALIES 

+High 

-Low 

APPROXIMATE BOUNDARY OF ELKHORN 

WILDERNESS STUDY AREA -1A 

-1C 

-1F 

-19A -28 

-19C -2A -1E 

-3C 

-218 ·3E +7N 

-4C 

-22 -22A -4D -4E 

-40 +12 

-24C -23C -4J 

-24D 

-24E +12C 

+13 

Figure 5. Aeromagnetic highs and lows in and near the Elkhorn Wilderness Study Area, Montana. 

Anomalies keyed to plate 2. 

mineral resource potential of the area. A geochemical 

orientation survey was conducted during November 197 6 

to determine the optimal sampling medium and spacing. 

The results indicated that the nonmagnetic heavymineral fraction of panned concentrates from streamsediment samples was the most useful sample medium 

for delineating areas of anomalous metal concentrations. 

In addition, the sampling of small, unbranched (firstorder) streams provided the most efficient sampling 

density. 

During the fieldwork in 1977, rock, soil, and 

stream-sediment samples were collected in addition to 

panned concentrates. All samples were analyzed by semiquantitative emission spectrometry for 30 elements and 

by atomic absorption for gold, zinc, and arsenic on 

selected samples. Sample localities are shown on plate 3. 

The results of the analyses of all samples have been 

published by Motooka and others (1978). 

Approximately 187 panned concentrates were 

collected, predominantly from small, unbranched 

Mineral Resource Assessment 21 
streams above the confluence with major drainages. The 

samples were collected in active channels in the streams 

and panned at the sites. Duplicate samples of each 

sample type were collected at about 10 percent of the 

sample localities. Sample density averaged approximately 

one sample per 0.5 mi2 (1.5 km2

). The material sampled 

usually ranged from silt to sand and represents material 

that was introduced to the stream bed from bedrock and 

colluvium within the drainage basin. In this area, the use 

of nonmagnetic heavy-mineral concentrat~. samples is 

particularly useful for detecting molybdenum, tin, and 

tungsten. 

After panning, the sample was dried and sieved 

through a minus-18-mesh sieve. Magnetite and other 

highly magnetic minerals were removed using a hand 

magnet. The sample was then separated into two 

fractions using bromoform (specific gravity, 2.86). The 

light-mineral fraction was discarded, and the heavymineral fraction was passed through a Frantz Isodynamic 

Magnetic Separator with the current set at 0.2 amperes 

and the forward and side tilt settings at 15°. The magnetic 

fraction was discarded, and the nonmagnetic fraction was 

further separated at 0.6 amperes into a nonmagnetic and 

magnetic fraction. Each fraction was examined with a 

microscope and the mineralogy determined. The nonmagnetic fraction generally contained minerals such as 

zircon, monazite, apatite, sphene, and ore minerals such 

as sulfides and oxides. Scheelite was identified from 

several samples using an ultraviolet light. Both fractions 

were then pulverized to pass a minus-150-mesh sieve and 

analyzed for 30 elements by semiquantitative 

spectroscopic methods (Grimes and Marranzino, 1968). 

Based on the analysis of the duplicate samples, 

reproducibility is usually within one reporting interval. 

Because of higher total content and regional 

variation of trace-element concentration, the data on the 

nonmagnetic fraction were used for the geochemical 

interpretation. 

Histograms for elements of possible economic 

interest are shown on plate 4, one set representing 

streams draining the Boulder batholith, and another set 

representing all other streams. The histograms illustrate 

the geochemical differences between the two major rock 

terranes, and it is evident that rock type influences the 

background and threshold of some elements. 

Nevertheless, the differences are not large, and no 

distinction was made in the treatment of data from the 

two terranes. 

Statistical analysis of the geochemical data using 

the STATPAC (Van Trump and Miesch, 1977) statistical 

evaluation library indicates that groups of interrelated 

elements are present. The correlation coefficients (table 

2) form the basis for delineating two groups, called here 

Group 1 and Group 2. The two groups of interrelated 

elements are considered separately because they seem to 

22 Elkhorn Wilderness Study Area, Montana 

Table 2. Correlation coefficients of two groups of elements 

in geochemical samples from the Elkhorn Wilderness Study 

Area and vicinity, Montana 

(Circles indicate non-significant correlation at the 5 percent confidence 

level] 

Group I 

~ Correlation coefficients 

0. Ag Au As Pb Cu 

~ 

i Ag .90 .52 .70 

! Au .75 @ 

0 As .52 .53 

~ Pb 5 .52 E 

;:, 

z Cu 35 4 

Group II 

Correlation coefficients 

La Mo Nb w y Th 

~ 0.. La .68 .09 .84 .29 

~ Mo .56 .45 

·i Nb 143 .76 @ ! 

0 w 74 72 .27 .54 

~ y 167 134 144 .65 

E Th 62 61 62 ;:, 

z 

be related to different types, and perhaps different 

periods, of mineralization. A normalized sum was 

computed for each group, because an element with large 

values, like arsenic, could dominate a simple summation. 

Sums were normalized by dividing the analytical value for 

an element by the threshold value of that element. Thus, 

Normalized 

sum = Element 1 + Element 2 

Threshold of Threshold of 

element 1 element 2 

+ .... + Element n 

Threshold of 

element n. 

The threshold values were selected subjectively and 

are shown on plate 4. The normalized sums for each of 

the two groups were calculated for each sample and the 

results plotted as histograms on plate 4. 

Group·l consists of gold, silver, arsenic, lead, and 

copper. The normalized sum appears to represent 

hydrothermal-vein-type mineralization. This factor 

delineates the Park mining district in and near the 

eastern part of the study area (pl. 4, A-1). A slight 

anomaly is outlined near Lava Mountain (A-2), an area 

also containing several known mines. In area A-3, a 

sulfide-bearing vein was observed within a cirque. Other 

anomalies such as areas A-4 and A-5 probably represent 

small veins, in many cases associated with intrusives. 

Group 2 consists of lanthanum, molybdenum, niobium, tungsten, yttrium, and thorium. Because of its 

association with elements such as tin, and the greater 

contrast of molybdenum relative to copper, the 
normalized sum probably represents disseminated 

molybdenum mineralization. The largest anomalies 

appear within the Boulder batholith (pl. 4, B-1). This 

area appears to have a good potential for porphyry-type 

deposits of molybdenum. Another anomaly occurs just 

east of the study area (pl. 4, B-2) near the Vosburg mine. 

Small amounts of molybdenite have been reported from 

the mine (Klepper, Ruppel, and others, 1971). 

A third type of mineralization appears to be 

present within the study area and is represented by 

boron. Boron is anomalous along the contact of the 

volcanic rocks with rocks of the Boulder batholith (pl. 4). 

In this area, tourmaline and sometimes pyrite are 

present, but there is a scarcity of economic minerals. 

GEOLOGICAL AND GEOCHEMICAL 

EVALUATION 

By Steve Ludington and 

William R. Greenwood 

Introduction 

Several geologic and geochemical features within 

the Elkhorn Wilderness Study Area suggest the presence 

of mineral resources. To assist in evaluating these 

features, we first describe the geology of nearby ore 

deposits, and then describe the results of our geologic 

and geochemical investigations of individual areas. 

Together, these data provide the basis for our evaluation. 

Mineral Deposits In and Near the Study Area 

Data from major metal mines in and near the study 

area are summarized here to provide a background for 

later discussion of geologic, geochemical, and geophysical evidence of mineral resources. 

The total recorded value of metals produced from 

these mines is about $109 million (Klepper and others, 

1957; Klepper, Ruppel, and others, 1971; Smedes, 1966; 

Becraft and others, 1963). The mines within the study 

area are small, none having produced as much as 

$300,000, but several large historic producers are within 

a few miles of the study area boundary. The location of 

these larger mines is shown on figure 6. 

East Flank Zone 

Mines along the east flank of the Elkhorn 

Mountains have produced about $17 million worth of ore 

since 1866, primarily gold and silver with subordinate 

base metals. The zone lies mostly outside the study area. 

All data in this section of the report are abstracted from 

Klepper, Ruppel, and others (1971). 

Gold and silver occur principally in pyritic veins 

within or close to several granitoid stocks that intrude the 

axial zone of a major north-trending syncline. 

The veins include the following types: 

(1) Early quartz-rich veins north of Indian Creek 

associated with porphyritic felsic stocks. They have 

shallow dips and consist of pyrite, gold, a little molybdenite and chalcopyrite, and locally, sparse arsenopyrite, 

galena, and sphalerite. 

(2) Late quartz-rich veins north of Indian Creek 

associated with porphyritic felsic stocks. They dip steeply 

and consist of pyrite, gold, and various combinations of 

galena, sphalerite, arsenopyrite, chalcopyrite, and tetrahedrite. 

(3) Quartz-poor veins south of Indian Creek 

associated with nonporphyritic stocks of intermediate 

composition. They contain auriferous pyrite, sparse marcasite, very sparse chalcopyrite, and traces of pyrrhotite, 

galena, sphalerite, and chalcocite. These veins also dip 

steeply. 

In the Quartzite Ridge mining district, silver-leadzinc replacement deposits form pipes and lenses along 

east-trending faults in carbonate rocks. The deposits are 

in thin dolomite beds between quartzite beds of the 

Quadrant Formation, and a few also occur in the Mission 

Canyon Limestone. The ore is oxidized and consists of 

cerussite, pyromorphite, limonite, quartz, calcite, galena, 

and manganese oxides; locally it also contains smithsonite, wulfenite, sphalerite, pyrite hemimorphite, barite, 

cerargyrite, or fluorite. 

A small amount of gold occurs locally in calcsilicate skarns adjacent to a few stocks. Gold also occurs 

in placers south and west of Radersburg, along Indian 

Creek, and along Weasel and Beaver Creeks. 

Titaniferous magnetite has been mined near Radersburg from some lenticular beds in the middle of the 

Slim Sam Formation that probably are lithified beach 

concentrations. These deposits have been mined for use 

in making a special type of cement. 

Elkhorn-Tlzer Basin Zone 

Mines in the Elkhorn mining district and to the 

north in the vicinity of Tizer Basin, here called the 

Elkhorn-Tizer Basin zone, have produced metal with a 

value of about $16 million since 1875. Of this total, silver 

and gold accounted for about 75 percent and 12 percent, 

respectively. Data on mines in the Elkhorn-Tizer Basin 

zone cited in this portion of the report are from Klepper 

and others (1957) except as indicated. Much of this zone 

is in or immediately adjacent to the study area. 

Mines in the Elkhorn mining district, most of which 

lie just south of the study area, accounted for about 98 

percent of the metal produced in the zone, and exploited 

Geological and Geochemical Evaluation 23 
0 5 10 

0 5 10 KILOMETERS 

'APJ::JRCIXIMIATE BOUNDARY OF ELKHORN 

WILDERNESS STUDY AREA\ 

~ 

~ 

Figure 6. location of major mines (with production of $300,000 or more) near the Elkhorn Wilderness Study 

Area, Montana. 

replacement deposits in Paleozoic carbonate rocks. The 

deposits consist of argentiferous galena associated with a 

small amount of auriferous pyrite and sphalerite. Chalcopyrite, tetrahedrite, bournonite, and argentite in a 

quartz gangue also occur in some deposits. Much of the 

ore occurs as tabular siliceous bodies in the uppermost 

bed or beds of the Pilgrim Dolomite and as pods and 

irregular bodies of non-siliceous ore in lower beds of the 

Pilgrim. Ore also occurs in the Mission Canyon 

Limestone, Lodgepole Limestone, and Meagher 

Limestone. Most of these deposits formed along the crest 

or flanks of minor anticlines and beneath a hanging wall 

of silicified shale or argillite, though some deposits occur 

along steeply dipping shears that cut across bedding in 

the carbonate rocks. 

Several pipelike breccia deposits in the district have 

been explored. The deposits consist of clasts of volcanic, 

24 Elkhorn Wilderness Study Area, Montana 

sedimentary, and igneous rocks cemented or replaced by 

quartz, black tourmaline, galena, pyrite, sphalerite, and 

sparse arsenopyrite. Gold, silver, lead, and zinc have 

been produced from two of these deposits. 

About $220,000 worth of metal production has 

been reported from gold-bearing veins that occur at 

several places where volcanic rocks are exposed in and 

near Tizer Basin. The veins consist of vuggy quartz, 

sparse pyrite, galena, sphalerite, chalcopyrite, rare flecks 

of native gold, and possibly tetrahedrite. Gold-bearing 

veins in the basin strike east-northeast to northeast and 

dip moderately to steeply southeast. A pipelike breccia 

deposit cemented by pyrite and tourmaline adjacent to 

the basin has not been mined. 

Small non-mechanized placer operations have 

been conducted at several places along Wilson and Crow 
Creeks. Small amounts of placer gold also are reported in 

Little Tizer Creek, just below Tizer Lake. 

Jefferson City Zone 

The Jefferson City zone comprises mines 

developed in a large swarm of metalliferous quartz veins 

and chalcedony veins that trends northeast through the 

Jefferson City quadrangle and then east through the 

northern Elkhorn Mountains. The zone lies mostly 

outside of the study area. The data cited here come from 

Becraft and others (1963) and Smedes (1966). 

Most of the mines in the Jefferson City zone were 

developed on base- and precious-metal-bearing quartz 

veins that follow steeply dipping, east-trending shears. 

Recorded metal production from these quartz veins is 

about $67 miiiion, chiefly silver and to a lesser extent 

lead. The veins are quartz-rich, contain abundant pyrite, 

and locally contain galena, sphalerite, arsenopyrite, and 

chalcopyrite. Wall rocks along the veins show intense 

argiiiic and sericitic alteration. These veins are primarily 

older than the Lowland Creek Volcanics (Eocene) in the 

Jefferson City quadrangle; however, pyrite-bearing 

quartz veins are associated with volcano-tectonic breccia 

in the Lowland Creek Volcanics west of Alta Mountain. 

Reeflike outcrops of chalcedony and microcrystalline quartz veins follow and may cut the east-trending 

metaiiiferous quartz veins. The reeflike outcrops consist 

of many thin layers and stringers of chalcedony in 

silicified and argillically altered rocks. The veins locally 

contain pyrite and barite and sparse amounts of galena, 

sphalerite, arsenopyrite, ruby silver, argentite, molybdenite, chalcopyrite, and cinnabar. Small uranium deposits 

occur in some chalcedony veins and consist of primary 

uraninite and the secondary uranium minerals metaautunite, metatorbernite, phosphuranylite, uranocircite, 

vaglite, and rutherfordine (Wright and others, 1957). In 

the northern Elkhorn Mountains, chalcedony (!ements 

brecciated silver-bearing quartz veins and locally 

chalcedony veins cut Eocene quartz Iatite dikes (Smedes, 

1966, p. 106). 

Uranium ore was mined at the W. Wilson mine 

sporadically from 1951 to 1956 (Smedes, 1966). 

Brecciated Lowland Creek Volcanics in the Wickes 

mining district, about 5 mi (8 km) west of the study area, 

contain anomalous amounts of lead and zinc over an area 

larger than 2,400 ft by 1,200 ft (730 m by 365 m ). Samples 

from exploratory workings in these rocks contain 

disseminated pyrite in the breccia clasts and coarsely 

crystalline quartz, microcrystaiiine quartz, sphalerite, 

galena, and possibly ruby silver in the matrix. A pipelike 

body of mineralized breccia on the north side of the 

Boulder River has been explored at the Obelisk mine. 

The breccia consists of abundant large clasts of quartz 

monzonite and subordinate clasts of alaskite and 

carbonate veins in a sand-size matrix. This material is 

cemented by calcite and locally by sphalerite, galena, 

quartz, and sparse pyrite and chalcopyrite, forming 

steeply plunging pipe-like ore bodies as much as 30 ft by 

40 ft (9 m by 12 m) across. Siderite and elsewhere 

rhodochrosite occur in a few veinlets and vugs in and 

near the breccia. 

Small silver-lead-zinc deposits consisting of veinlet 

stockworks, lithophysal fillings, disseminations, and local 

replacements occur in rhyolitic tuff, breccia, and rhyolite 

at Lava Mountain. The ore consists of galena, sphalerite, 

fluorite, topaz, and quartz. 

Placer gold deposits in the Jefferson City zone are 

widespread. Extensive placer operations have been 

conducted along Prickly Pear and Clancy Creeks and to a 

lesser extent along the upper part of Mitchell Gulch. 

Porphyry Deposits 

At present, no porphyry-copper deposits or stockwork molybdenum deposits are being mined in the study 

area. Any such deposits would be of great interest 

because of their large potential value. Three possible 

porphyry deposits within or adjacent to the study area are 

described here. The probability is high that other such 

deposits may exist at depth in the western part of the 

study area, where the bedrock is Butte Quartz Monzonite, host to a large porphyry copper deposit at Butte, 

Mont. 

Golconda Area 

The Golconda area (fig. 7) is a porphyry coppermolybdenum deposit located astride the western study 

area boundary south of Prickly Pear Creek. It is 

significant not only for the value of any ore it may contain 

but also because it shows that the northern part of the 

Boulder batholith is favorable for the occurrence of this 

important type of ore deposit. In addition it can serve as 

a model to which other porphyry deposits may be 

compared. 

The deposit was discovered by geologists of the 

Exxon Corporation in the early 1970's. The corporation 

pursued a program of detailed geologic mapping and 

geochemical sampling, and in the summer of 1977 four 

diamond drill holes (fig. 8) of unknown depth were 

completed or in progress. 

The major rock type in the Golconda area is Butte 

Quartz Monzonite that has a wide range of composition 

(fig. 8) and grain size. Plagioclase in the rock forms 

sparse phenocrysts. Biotite is the principal mafic mineral, 

and hornblende is present locally; together they 

constitute 2 to 15 percent of the rocks; the mean is 8 

Geological and Geochemical Evaluation 25 
460 

21' 

0 

0 

QUATERNARY ALLUVIUM 

QUATERNARY GlACIAL DEPOSITS 

CREfACEOUS APLITE 

CREfACEOUS QUARTZ MONZONITE 

PORPHYRY 

CRETACEOUS QUARTZ MONZONITE 

1 KILOMETER 

EXPlANATION 

ROCK SAMPLE LOCALITY 

0 146 Unaltered 

Q 135 Moderate phyllic alteration 

0 159 5 . . trong phylhc alteration 

r·-----

j ---. 

1 MILE 

------ LIMtt OF OUI'PCR,OP 

CONfACf 

---- FAULT OR PHOTO LINEAMENT 

() DIAMOND DRILL HOLE 

====ROAD 

---- SI'UDY ARFA BOUNDARY 

Figure 7. Golconda area, Elkhorn Mountains, Mont., showing diamond drill holes, rock sample localities, and geology. All 

sample numbers prefixed by 77Sl. Blank areas not mapped in detail. 

26 Elkhorn Wilderness Study Area, Montana 
Quartz 

EXPlANATION 

e APLITE 

Quartz 

.& QUARTZ MONZONITE PORPHYRY 

+ QUARTZ MONZONITE 

Figure 8. Ternary diagram showing modal compositions of 

intrusive rocks from the Golconda area, Elkhorn Mountains, 

Mont. Between 100 and 500 points were counted for each 

slab; quartz and feldspar recalculated to total 100 percent. 

Color index for aplites ranges from 0 to 7 (average 3). Color 

index for quartz monzonite and quartz monzonite porphyry 

ranges from 2 to 15 (average 8). Solid lines denote fields of 

alaskite and Butte Quartz Monzonite as reported by Becraft 

and others (1963). Modal classification is that of Streckeisen 

(1973). 

percent. The quartz monzonite porphyry shown on the 

geologic map (fig. 7) is similar but contains more 

abundant plagioclase phenocrysts. 

Equigranular aplite, the other major rock type in 

the area, is host to most of the copper-molybdenum 

mineralization and hydrothermal alteration. The color 

index of the aplite ranges from 0 to 7 and averages 3. 

Post-mineral faulting is indicated by the abrupt 

termination of intensely hydrothermally altered aplite on 

the west and north along prominent topographic 

lineaments. Physical evidence for faulting along these 

lineaments was not observed. 

Intersecting quartz-sulfide veinlets and associated 

hydrothermal alteration are confined largely to the aplite. 

At the surface, intense leaching and oxidation of the rock 

have taken place, and no fresh sulfides other than pyrite 

were observed in any surface specimens. Iron and 

manganese oxides are, however, abundant in the veinlets. 

Samples of these rocks commonly contain anomalously 

high copper and molybdenum contents, the distribution 

of which is shown on figure 9. 

Hydrothermal alteration patterns shown in figure 7 

were determined by petrographic and X-ray diffraction 

studies. Strong phyllic alteration is here defined as nearly 

complete (90 percent) replacement of both potassium 

feldspar and plagioclase by a very fine grained aggregate 

of quartz and sericite, absence of biotite or possible 

sericite and iron oxide pseudomorphs after biotite, and 

many (1-5 percent) disseminations and veinlets of pyrite 

and (or) iron oxide pseudomorphs after pyrite. 

Weak and moderate phyllic alteration is similar to 

strong phyllic alteration, but more fresh feldspar 

remains, and the former presence of biotite is commonly 

indicated by chlorite. Rock on the fringes of the altered 

area contains quartz-sulfide veinlets with 0.5- to 2-in. 

(1-5 mm) selvages of strong phyllic alteration, suggesting 

that the strong phyllic zone is the result of coalescence of 

alteration envelopes around more closely spaced veinlets. 

The most strongly altered rocks generally have the 

highest copper and molybdenum values. Many of the 

rocks indicated as unaltered on figure 7 show partial 

replacement of biotite by chlorite, contain sparse 

disseminated pyrite, and probably constitute a propylitic 

alteration zone. Comparison of semiquantitative spectrographic analyses of altered and unaltered rocks shows 

that hydrothermal alteration of feldspars to quartz and 

sericite liberated significant amounts of calcium, but left 

barium and strontium largely fixed in the rock. 

The observed characteristics of the exposed 

portion of the Golconda prospect agree well with those 

ascribed to the upper part of a model porphyry copper 

system (Lowell and Guilbert, 1970; Gustafson and Hunt, 

1975) with the significant exception that molybdenumto-copper ratios at the Golconda prospect appear to be 

quite high. This may be partly because the surface 

samples are generally highly oxidized; copper is much 

more highly mobile than molybdenum in acid oxidizing 

waters, and so may have been significantly leached from 

the surface exposures. If this is true, the subsurface 

copper grade may be several times higher than surface 

sampling suggests, and a secondary enrichment blanket 

may be present at depth. However, even if copper has 

been removed, the absolute values of molybdenum seem 

high-three samples contain 500 ppm molybdenum. If 

such molybdenum contents were widespread, they would 

contribute materially to the value of the deposit and 

characterize it as a molybdenum-rich porphyry copper 

system. 

Jackson Creek Area 

Rocks of the Jackson Creek area, which lies just 

outside the study area, contain anomalous amounts of 

copper, molybdenum, and bismuth. However, Jackson 

Creek does not appear to resemble a typical porphyry 

copper system. More detailed studies would be needed to 

determine the nature and origin of this deposit. Our 

studies indicate the area has a low potential for the 

occurrence of metallic mineral resources. 

It is not known when exploration in the area first 

began, but the copper occurrence was known as early as 

Geological and Geochemical Evaluation 27 
460 

21 ' 

II 

II 

II 

Qal ; ====== II 

II 

I! 

f 

,f' 

\ 

I 

I 

r·-----

j ----

Kqm 

0 

-.... ___ ........... / 

I 

I 

I 

I 

I 

-::f' 

I I 

I I 

0 

0 

QUATERNARY ALLUVIUM 

QUATERNARY GlACIAL DEPOSITS 

CREfACEOUS APLITE 

CRETACEOUS QUARTZ MONZONITE 

PORPHYRY 

CRErACEOUS QUARTZ MONZONITE 

/ I 

I I 1 Kqm 1 

I I 

( 0 \ 

\ 0 \ 

\ I I I 

\ 0 ° I \ I \ ./// '--------

1 KILOMETER 

EXPlANATION 

ROCK SAMPLE LOCALITY, SHOWING 

MOLYBDENUM AND COPPER CONfENf 

IN PARTS PER MILLION 

o <5 ppm Mo; < 100 ppm Cu 

(JJ 5-30 ppm Mo;100-300 ppm Cu 

(jJ 50-200 ppm Mo; 500-1000 ppm Cu 

OJ > 300 ppm Mo; > 1000 ppm Cu 

1 MILE 

------ LIMIT OF OUTCROP 

---- CONfACf 

---- FAULT OR PHOTO LINFAMENf 

0 DIAMOND DRILL HOLE 

-===ROAD 

- • -- STIJDY ARPA BOUNDARY 

Figure 9. Copper and molybdenum content of rock samples collected in the Golconda area, Elkhorn Mountains, Mont. Area 

shown is same as on figure 7 (sample localities and geology). Left half of symbol refers to molybdenum content; right half to 

copper content. 

28 Elkhorn Wilderness Study Area, Montana 
1911 (Stone, 1911, p. 86). It was tested in the mid 1960's 

by Rio Amex Inc. by means of four shallow ( < 600 ft 

(200 m)) drill holes. That drilling was not followed by 

further exploration. 

The area occupies the central part of the Jackson 

Creek lobe of the Boulder batholith (Smedes, 1966), a 

wedge of intrusive rock separated from the main part of 

the batholith by a screen of strongly sheared and contactmetamorphosed Elkhorn Mountains Volcanics (fig. 10). 

Modal analysis of stained slabs of rock indicates the 

presence of quartz monzonite, monzonite, granodiorite, 

syenodiorite, diorite, and quartz diorite in the 

mineralized area. Outcrops are too few to determine 

whether the wide scatter of rock types represents an 

inhomogeneous pluton or a series of intrusions made up 

of many rock types. Accordingly, rock types are not 

differentiated in this discussion. 

The modal and chemical analyses reveal that the 

Jackson Creek lobe contains distinctly less potassium and 

more sodium than most of the Boulder batholith. The 

spectroscopic analyses indicate that the rocks are 

enriched in barium and strontium relative to the batholith. They possibly belong to the sodic magma series of 

Tilling (1973). 

Phyllic alteration in the Jackson Creek area is weak 

and irregularly distributed. Most specimens exhibit some 

sericite but none show strong phyllic alteration and most 

have fresh biotite. 

The copper and molybdenum content of the 

samples is shown on figure 11. Samples 77SL034, 

77SL035, and 77SL036 contain 700, 1,000, and 1,500 

ppm copper, respectively, but no other samples contain 

as much as 100 ppm. The copper in samples 77SL034 

and 77SL035 occurs in large (0.1 to 1.0 mm), isolated, 

anhedral blebs of chalcopyrite, which show no evidence 

of association with sporadic quartz veinlets that cut the 

rock. In 77SL056, the copper occurs in an azure blue 

mineral. 

There is a scattering of anomalous molybdenum 

values, the highest of which is 50 ppm. In addition to 

copper and molybdenum, one sample (77SL057) 

contains 1,000 ppm bismuth. The bismuth-bearing 

mineral is unknown, but it may be in the 5 to 10 percent 

disseminated pyrite in the rock. 

The Jackson Creek copper occurrence does not 

resemble any reported porphyry copper system. It may 

represent a poorly developed example of the sort of 

magmatic copper occurrence described by Knopf 

(1913b) from the Golden Curry mine (pl. 3) in the 

Elkhorn district. The likelihood that the area contains a 

copper deposit is low. 

Turnley Ridge Prospect 

The Turnley Ridge prospect is in the Elkhorn 

mining district about 1 mi (1.6 km) south of the study 

460 

28' 

0 1 MILE 

1 KILOMETER 

EXPlANATION 

:·.~ .. \·i·\··:·: :J ALLUVIUM (QUATERNARY)-And other surficial deposits 

~ INTRUSIVE ROCKS (CREfACEOUS)- Quartz 

monzonite,monzonite, syenodiorite, granodiorite, 

diorite, and quartz diorite G ELKHORN MOUNTAIN VOLCANICS (CREfACEOUS) 

---CONTACT 

o034 ROCK SAMPLES 

--- APPROXIMATE BOUNDARY OF ELKHORN 

WILDERNESS STUDY AREA 

Figure 10. Geology and rock sample localities in the Jackson Creek area, Elkhorn Mountains, Mont. Sample numbers 

prefixed by 77SL 

area. It was recognized in 1969 by the Siskon Mining 

Corporation of Reno, Nev. U.S. Borax completed two 

diamond drill holes to depths of 970ft (297m) and 690 

ft (210m) on the property in 1973 and 1974. The results 

were not encouraging, and the property was relinquished. 

All information presented here on the Turnley Ridge 

prospect is taken from Senter (1975). 

The prospect is located near the center of the 

Turnley Ridge stock, a quartz monzonite porphyry. 

Geological and Geochemical Evaluation 29 
0 1 MILE 

0 1 KILOMETER 

EXPlANATION 

SURFICIAL DEPOSITS 

BEDROCK 

--- CONI'ACT 

--- APPROXIMATE BOUNDARY 

OF ELKHORN WILDERNESS 

SIUDYAREA 

ROCK SAMPLES 

Q 700-1500 ppm molybdenum 

• 7-50 ppm molybdenum 

o Unmineralized 

Figure 11. Copper and molybdenum content of rock 

samples from the Jackson Creek area, Elkhorn Mountains, 

Mont. 

Modal analyses of four specimens showed the following 

averages: quartz, 35 percent; plagioclase, 28 percent; 

orthoclase, 30 percent; biotite, 2 percent; sericite, 2 

percent; other mafics, 2 percent; and accessory epidote, 

sphene, apatite, and zircon, 1 percent. The stock is 

believed to be an outlier of the Butte Quartz Monzonite. 

The distribution and characteristics of the hydrothermal alteration in the Turnley Ridge stock are shown 

on figure 12. Molybdenum and minor amounts of copper 

occur in quartz veins in the potassic and phyllic zones, 

most abundantly near the interface between the two 

zones. The best 10-ft (3-m) intercept in either drill hole 

30 Elkhorn Wilderness Study Area, Montana 

was 0.079 percent Mo, with intercepts of 60 to 320 ft 

(20--100 m) averaging 50 to 200 ppm Mo. The fact that 

the orthoclase and quartz-rich central portion of the 

system crops out at the surface argues against the 

presence of better mineralization at depth, as does the 

observation by Senter (1975, p. 72) that "only rarely in 

hand specimen or in thin sections are the quartz veinlets 

displaced and crosscut by other quartz veinlets." 

Apparently the Turnley Ridge deposit represents a single 

phase, relatively weak porphyry system. The grade of the 

rock sampled does not represent ore, but the presence of 

this deposit provides additional encouragement for 

prospecting for porphyry systems in this part of the 

Boulder batholith. 

Breccia Pipe Deposits 

Breccia pipe deposits have produced very little of 

the mineral wealth of the study area, but they merit 

description because they may indicate mineral deposits at 

depth. 

The two major breccia pipes-the Skyline and 

Blackjack mines-are shown on plate 3. They were 

mapped and sampled in detail during the present study. 

Skyline Mine 

The Skyline mine is in the cirque at the head of 

Queen Gulch, 1 mi (1.6 km) southeast of Crow Peak in 

the southern part of the study area. It was developed in a 

steeply plunging breccia pipe that has a roughly 

triangular horizontal section, with sides about 150 ft ( 46 

m) long at the surface. The pipe cuts gently dipping 

andesite and pyroclastic quartz latite near the 

intersection of several normal faults, some of which may 

be pre-breccia and related to cauldron subsidence. Brecciated andesite and quartz latite in the pipe are cemented 

by vuggy quartz and tourmaline, and locally they have 

been partially to totally replaced by pyrite, galena, 

sphalerite, and sparse chalcopyrite, arsenopyrite, and 

marcasite. All sulfides have been leached at the surface 

leaving a porous iron-stained gossan. Numerous samples 

of the breccia pipe contain trace amounts (5-100 ppm) of 

molybdenum, tin, and tungsten. The deposit has been 

explored through two short adits, one of which leads to a 

170-ft (51-m) winze and some short subdrifts, and by 

four diamond-drill holes. The most recent exploration (in 

1953) was partially supported by the Defense Minerals 

Exploration Administration. The results of that 

exploration indicated that the volume and grade of the 

ore discovered did not justify further exploration at that 

time (McWilliams and Weeks, 1954). 

Blackjack Mine 

At the Blackjack mine, about 2 mi (3 km) north of 

the Skyline mine in the southern part of the study area, 

four shallow prospect shafts are in a steeply plunging 
EXPlANATION 

---CONTACT 

---FAULT 

• DIAMOND DRILL HOLE 

Turnley Ridge Stock (Cretaceous) 

Showing Alteration 

Potassic-Stockwork of quartz veins as much as 1.5 

inches (4 em) wide; orthoclase moderately to entirely 

replaces plagioclase; sericite and tourmaline present; 

pyrite, in veinlets and disseminated, averages 0.5-1.0 

percent 

Phyllic-Pervasive replacement of all silicates by 

sericite; 0.02- to 0.05-inch (0.5- to 1.3-mm) selvages of 

orthoclase on sparse quartz veinlets; tourmaline 

present; pyrite averages 2-3 percent, chiefly as granular 

and euhedral disseminations 

Propylitic-Piagioclase generally fresh; biotite replaced 

by chlorite + secondary biotite + sphene; tourmaline 

and pyrite present but rare 

0111ERROCKS 

<:~· :_·: ·: :. Area within which rock chip samples contain > 10 ppm molybdenum 

0 1 MILE 

0 1 KILOMETER 

Figure 12. Altered and mineralized zones in the Turnley Ridge stock, Elkhorn Mountains, Mont. 

breccia pipe which cuts andesite. The pipe, elliptical in 

plan view, is elongated west-northwesterly and is about 

125ft (38m) long and about 60ft (18m) wide. The pipe 

is adjacent to a major fault (covered and not shown on pl. 

1) that separates the andesite from cauldron-fill-type 

welded tuffs to the east. Brecciated andesite in the pipe is 

cemented by black tourmaline and quartz and lesser 

amounts of epidote and pyrite. Two exploration pits 

about 240 ft and 300 ft (73 m and 90 m) south of the pipe 

are on north- and north-northwest-trending fractures 

that are partly filled and replaced by veins of quartz and 

tourmaline and small amounts of pyrite. Most of the 

pyrite in surface outcrops has been oxidized to hematite 

and limonite. 

Andesite, breccia, and vein samples show 

expectably high boron contents in and near the breccia 

pipe and in some veins. Molybdenum ranges from less 

than 5 ppm to 15 ppm, with a median of 5 ppm, in and 

near the breccia but is less than 5 ppm in the andesite. 

Copper and manganese are less abundant in the breccia 

than in andesite. Tin and tungsten were not detected in 

any samples. 

The breccia pipe at the Blackjack mine is smaller 

than the breccia pipe at the Skyline mine but is cemented 

by the same high-temperature minerals. Both breccias 

have anomalous molybdenum contents although the 

Blackjack breccia lacks the anomalous tin and tungsten 

contents found in the Skyline breccia. Both pipes may be 

the result of venting of a volatile-rich intrusive that might 

contain significant amounts of metals. The lack of 

precious- and base-metal minerals and the lack of tin and 

tungsten anomalies suggest that the breccia at Blackjack 

is less deeply eroded than the breccia at Skyline. 

Chicago and Last Hope Mines 

The Chicago and Last Hope mines, about 2 mi (3 

km) north of the study area (pl. 3), are described by 

Smedes (1966, p. 106) as being developed on veins that 

follow the contact between the lower and middle 

members ofthe Elkhorn Mountains Volcanics. However, 

detailed mapping indicates that near the Chicago mine 

these members are juxtaposed by a steep, down-tothe-south fault, which in most places is intruded by a ring 

Geological and Geochemical Evaluation 31 
dike of hornblende porphyry, which may mark the 

structural rim of a caldera. The Chicago and Last Hope 

mines and many unmapped prospect pits and shafts are 

developed at and near the contact of the hornblende 

porphyry dike with rocks of the middle member. The 

veins consist of quartz, carbonate minerals, pyrite, arsenopyrite, and small amounts of sphalerite and chalcopyrite. Gangue minerals are diopside, hedenbergite, garnet, 

tourmaline, epidote, and magnetite. The mines were 

worked for gold but also produced small amounts of 

silver, lead, copper, and zinc. The upper parts of the veins 

are oxidized and yielded high-grade gold ore. The 

mineralogy and structural setting at the Chicago and Last 

Hope mines is similar to that of the Skyline mine, 

suggesting that the Chicago and Last Hope veins may 

overlie one or more metal and volatile-rich intrusive 

bodies. 

Significance of Geochemical Anomalies 

This section attempts to place the geochemical 

anomalies discussed in an earlier section into a geologic 

perspective and to integrate the geochemical data with 

geologic observations. 

Group 1-Precious- and Base-Metal Anomalies 

(Au-Ag-As--Pb-Cu) 

Panned concentrate samples from eleven drainages 

are highly anomalous in the elements of this group. Seven 

of these drainages contain inactive precious-metal mines, 

as listed in table 3, and the anomalous amounts of metal 

in the samples were derived, at least in part, from these 

mines. 

The other four drainages (Longfellow Creek, 

sample no. 77SL009; unnamed tributary of Crow Creek, 

sample no. 77LA008; Staubach Creek, sample no. 

77SL023; cirque east of Elkhorn Peak, sample no. 

77WM104) contain no known deposits and no veins were 

found, but the anomalous metal content or the samples 

suggests the presence of concealed or inconspicuous 

veins that contain precious and base metals. 

All these drainages, except Staubach Creek, are 

found along or to the southeast of a line connecting the 

East Pacific mine and the town of Elkhorn. Thus, nearly 

all of the southeast part of the study area has a moderate 

potential (as defined in the Introduction) for preciousand base-metal deposits. The pattern is enhanced when 

zinc is considered along with other group 1 elements, 

even though zinc does not show a high overall correlation 

with these elements. The underlying nature of this 

pattern is not well understood. The Kleinschmidt, 

Champion, Marietta, Bullion King, Little Annie, Park, 

Eagle Basin, and St. Louis mines are all along or near the 

northeasterly trending Weasel Creek fault zone of Klep32 Elkhorn Wilderness Study Area, Montana 

Table 3. Samples with anomalous amounts of precious 

and base metals (group 1: Au-Ag-As-Pb-Cu) in drainages 

with known mines, Elkhorn Wilderness Study Area and 

vicinity, Montana 

Sample Drainage Mines 

no. 

77EB014 West fork Weasel Creek ... East Pacific. 

77SL017 Weasel Creek ............ Stray Horse. 

7701008 East fork Badger Creek ... Vosburg. 

77SL020 West fork Badger Creek ... Champion. 

77EB012 Indian Creek ............. Marietta, Bullion 

King, Park. 

77SL019 Tributary of Indian Creek . Little Annie. 

77SL002 West Fork Indian Creek ... St. Louis. 

per, Ruppel, and others (1971), as are a series of related 

quartz monzonite porphyry stocks, but this structure 

forms an acute angle with the aforementioned line, as 

does the contact of the Boulder batholith with the 

Elkhorn Mountains Volcanics. Perhaps this 

northeasterly trending alignment of geochemical 

anomalies, ore deposits, and stocks is associated with an 

inhomogeneity in the Precambrian basement which is not 

otherwise reflected in younger rocks and structures. 

The group of precious- and base-metal deposits in 

and near the Vosburg stock on the east-central boundary 

of the study area is of particular interest because Klepper, Ruppel, and others (1971) reported that molybdenite occurs in some of the veins of the area. In addition, 

anomalous amounts of tungsten and molybdenum were 

found in some panned concentrate samples from 

drainages surrounding the Vosburg area. Accordingly, 

the area was mapped and sampled in detail during the 

present investigation. 

Six of twelve vein samples from the area contained 

detectable ( > 5 ppm) molybdenum, and one sample 

contained 150 ppm tungsten. No molybdenum or 

tungsten-bearing minerals were recognized in hand 

specimen. Neither of these metals was detected in any 

samples of the stock. The observed occurrence of 

molybdenum and tungsten as accessory elements in the 

veins probably accounts for the geochemical anomalies; 

therefore the Vosburg is not likely to contain porphyrytype mineralization. 

Six streams that drain radially from the ridge near 

the head of Staubach Creek at the northern end of the 

study area provided panned-concentrate samples 

(77SL023, 77EB027, 77DT005, 77LA014, 77DT006, and 

77SL025) that are anomalous in a rather distinctive suite 

of elements: gold, silver, arsenic, bismith, niobium, 

thorium, and possibly yttrium and lanthanum. There are 

few prospects in the area, other than Jackson Creek. 

Rock samples collected from this prospect show no 

particular enrichment in this distinctive suite of elements, 
with the exception of one sample (77SL057), which 

contains > 1,000 ppm bismuth and 0.5 ppm silver. None 

of the other rock samples from these drainages are 

enriched in any of the elements under consideration. 

These anomalies may indicate undiscovered deposits of 

gold, silver, arsenic, bismuth, niobium, and thorium. 

Group 2-Rare-Metal Anomalies (Mo-W-Nb-La-Y-Th) 

The 21 panned-concentrate samples which were 

highly anomalous in rare metals are conveniently 

grouped into 6 areas: (1) Golconda, (2) South Fork 

Warm Springs Creek, (3) Dutchman Creek, (4) Nursery 

Creek, (5) Hidden Lake, and ( 6) Lava Mountain. 

The Golconda area, already described, appears to 

contain a copper-molybdenum porphyry system, and it 

seems reasonable to suspect that the rare-metal 

anomalies detected in the panned concentrates from that 

area may be related to the porphyry deposit. In the other 

areas, few geologic features suggestive of mineralization 

were seen. The designation of these areas as having 

mineral potential rests largely on the geochemistry of 

panned concentrates. 

Thirteen rock samples from the South Fork of 

Warm Springs Creek show molybdenum contents 

ranging from < 5 to 50 ppm and copper contents ranging 

from < 5 to 500 ppm. Tin and tungsten were not 

detected. Of four samples containing 5 ppm or more 

molybdenum or 70 ppm or more copper, two are from 

precious-metal veins. One of the others (77SL099) is of 

aplite; it contains 50 ppm molybdenum and 30 ppm 

copper. Molybdenite is common as an accessory mineral 

in the late-stage aplites of the Boulder batholith (Becraft 

and others, 1963). The fourth sample (77SL108) is of a 

pyritic metavolcanic rock; it contains 10 ppm 

molybdenum and 20 ppm copper. It is believed that the 

anomalous molybdenum in samples from this area is 

present as accessory molybdenite in the precious-metal 

veins, and possibly in aplite enriched in molybdenum. 

One of fifteen rock samples from the Dutchman 

Creek area in the west-central part of the study area 

(77SL120) contains detectable molybdenum. The quartz 

monzonite that underlies the area shows little hydrothermal alteration. The source of the geochemical anomaly is 

not known, but it may be the precious-metal veins in the 

area. 

Three rock samples (77SL079, 77SL080, and 

77SL082) from the Nursery Creek drainage, west of the 

study area, show detectable tin or molybdenum. Sample 

77SL082, which contains 50 ppm molybdenum, is stained 

by iron oxides and contains hydrothermal alteration 

sericite, whereas the other two samples, anomalous only 

in tin, appear to be fresh. Alteration is not widespread in 

the Nursery Creek area; however, the low but anomalous 

tin and molybdenum in rock samples, coupled with local 

hydrothermal alteration, suggest that the Nursery Creek 

area has a moderate mineral potential for molybdenum. 

Analysis of eleven rock samples from the Hidden 

Lake area in the southern part of the study area reveals 

three samples (77SL088, 77SL089, and 77BG152) with 

detectable molybdenum or tin. Two samples have high 

copper contents. A composite sample of brecciated 

andesite cemented by magnetite and stained by copper 

carbonates (77BG2~8) contains 15,000 ppm copper and 

2,000 ppm zinc. This rock also contains 200 ppm boron, 

although tourmaline was not visible in the hand 

specimen. The breccia is exposed in the walls of a shallow 

exploration pit and is undercut about 50ft (15m) below 

by one of the adits of the Iron mine. A sample (77SL087) 

of a similar rock was collected in talus just northeast of 

Elkhorn Peak. This sample consists of brecciated andesite cemented by what appears to be tourmaline, and it 

contains 300 ppm copper, 1,500 ppm lead, and 1,500 ppm 

zinc. Boron was not found to be high in this sample. 

The copper-rich breccias and molybdenumtin-tungsten anomaly in this area may indicate brecciapipe deposits similar to that at the Skyline mine. 

Samples of extrusive and intrusive rhyolite from 

the Lava Mountain area, about 1 mi (1.6 km) northwest 

of the study area, contain unusually high amounts of tin, 

niobium, beryllium, fluorine, and lithium. Beryllium 

contents of 18 samples range from less than 1 to 20 ppm 

(median, 10 ppm), and tin contents for the same samples 

range from less than 10 to 50 ppm (median, 18 ppm). 

Such high beryllium and tin contents appear to be 

characteristic of the Oligocene rhyolite in the northern 

part of the Boulder batholith (D. R. Shawe, written 

commun., 1977). Molybdenum content of the rhyolite is 

low, ranging from less than 5 to 10 ppm (median, less 

than 5 ppm). Therefore, the anomalously high 

molybdenum observed in stream-sediment and pannedconcentrate samples probably was not derived from the 

rhyolite. The molybdenum content of 12 samples of 

pre-rhyolite rocks in the Lava Mountain area ranges 

from less than 5 to 20 ppm (median, less than 5 ppm). 

The five samples of pre-rhyolite rocks that contain 5 ppm 

or more of molybdenum (77SL028, 77SL039, 77SL041, 

77SL042, 77SL114) consist of either vein material or 

altered Butte Quartz Monzonite adjacent to veins. These 

veins have high silver, lead, and zinc contents and are 

probably similar to the White Pine vein, south of the 

Middle Fork Warm Springs Creek. The anomalous 

molybdenum in this area is probably related to accessory 

molybdenite in the precious- and base-metal veins. 

Silver, lead, and zinc are present in four rhyolite 

samples (77SL036, 77SL037, 77SL038, 77SL040) from 

mine dumps on the south side of Lava Mountain. Silver 

in these samples ranges from 3 to 20 ppm, lead from 200 

to 5,000 ppm, and zinc from < 200 to 5,000 ppm. This 

mineralization appears to be similar to the local 

Geological and Geochemical Evaluation 33 
disseminations of galena and sphalerite in the rhyolite 

described by Smedes (1966). This lead-zinc-silver may be 

the result of a younger mineralization, but it may also be 

due to local assimilation and redistribution of sulfides in 

areas where rhyolite intruded pre-existing precious- and 

base-metal sulfide veins. 

The rhyolite of Lava Mountain is a highly 

differentiated rock of the sort that is associated with tin 

and molybdenum deposits in many parts of the world. If 

such a target exists in this area, no direct expression of it 

appears at the surface, and any potential mineralization 

would probably be at depths of at least a mile. 

Boron 

The panned-concentrate drainage samples which 

are anomalous in boron form a particularly coherent 

pattern (pl. 4 ). In all cases, these samples represent areas 

underlain by Elkhorn Mountains Volcanics in a band as 

wide as 2.5 mi (4 km) along the contact with the Boulder 

batholith. This is compelling evidence that boron was 

fixed in this band by reaction of hydrothermal fluids 

moving upward and outward from the cooling batholith 

with the surrounding wall rocks. In this region, 

tourmaline very commonly coats the joint faces of the 

volcanic rocks. 

The relationship of anomalous boron to economic 

mineral deposits is not so clear. Knopf (1913a) discussed 

at some length the significance of tourmaline in some of 

the precious- and base-metal deposits of the Greater 

Helena mining district. He felt that tourmaline-bearing 

deposits were earlier and formed at higher temperature 

than the other vein deposits of the area. This hypothesis 

fits well with the idea presented here that the batholith is 

the direct source of the boron, but a direct comparison 

cannot be made because none of the deposits Knopf 

discussed are near the study area, and no vein deposits in 

the study area are known to contain tourmaline. 

Three breccia-pipe deposits that contain 

tourmaline do occur in and near the study area: the 

Tourmaline Queen, the Skyline, and the Blackjack, 

discussed in an earlier section of this report. They all 

occur near the eastern edge of the band of anomalous 

boron. It would seem that those breccia pipes could have 

formed late in the intrusive history of the Boulder 

batholith. 

Several geologic traverses were made within 

drainages that panned-concentrate samples showed to be 

anomalous in boron. Some samples collected contain 100 

to 2,000 ppm boron. Tourmaline occurs chiefly as 

coatings on joint faces, but some pieces of float were 

found which indicate the presence of undiscovered 

tourmaline-bearing breccia pipes. Several samples 

contained detectable molybdenum. An area west of 

Bullock Hill contains a large number of small trenches 

34 Elkhorn Wilderness Study Area, Montana 

and prospect pits. Samples from these pits contained 1 to 

2,000 ppm silver, 300 to 20,000 ppm lead, and 200 to 

10,000 ppm· zinc. These samples, however, did not 

contain anomalous boron or molybdenum and 

apparently belong to a different geochemical system. 

Uranium and Thorium Potential 

By Karen J. Wenrich, William R. Miller, 

Vivian J. Suits, and John B. McHugh 

During the summer of 1977 an evaluation was 

made of the uranium and thorium potential in and 

adjacent to the Elkhorn Wilderness Study Area. 

Uranium-bearing deposits have been known since 1949 

in the area west of Clancy, Mont., 4 mi (6.5 km) 

northwest of the study area. Despite comprehensive 

studies of these deposits (Becraft and others, 1963; 

Roberts and Gude, 1953b ), no significant production of 

uranium has occurred. In the Clancy mining district, the 

uranium is present as pitchblende and secondary 

minerals located near silicified fracture zones in the 

quartz monzonite of the Boulder batholith or the 

younger alaskitic dike rocks. The uranium minerals are 

usually in silica stringers, along fractures, or in pore 

spaces of the altered host rock (Roberts and Gude, 

1953b ). According to Tilling and Gottfried (1969, p. E6), 

the Boulder batholith is higher in uranium and thorium 

than the surrounding wall rocks. Rocks of the batholith 

contain as much as 10 ppm uranium, and the wall rocks 

generally contain less than 5 ppm uranium. The batholithic rocks contain more than 15 ppm thorium and 

locally as much as 42 ppm, whereas the older rocks 

generally contain less than 15 ppm thorium. With a few 

exceptions, only the alaskitic dike rocks contain more 

than 20 ppm thorium. 

The study area was evaluated by means of a 

geochemical survey, using both surface waters and 

panned concentrates of stream sediments, and by an 

aerial gamma-ray survey. The water samples were 

collected only from streams draining the Boulder batholith, where the radio-element content was known to be 

considerably higher than in the older wall rocks. Both 

uranium and radon were determined in the water 

samples, uranium by extraction fluorometry and radon 

by an alpha-scintillation technique. Thorium was 

determined in the panned concentrates by semiquantitative emission spectroscopy. The aerial gamma-ray survey 

was flown over the northwest part of the study area, over 

the Clancy mining district, and over the intervening area. 

The data and results from the survey are presented by 

Duval and others (1978). The portions of flight lines from 

the aerial survey that show high eU-to-K and eTh-to-K 

ratios are shown on plate 5. 
The thorium content of the panned concentrates 

delineates the contact between the Boulder batholith and 

its wall rocks to the east. With few exceptions the thorium 

content of samples from the eastern part of the study 

area is less than the detection limit of 200 ppm, whereas 

panned concentrates from streams draining the Boulder 

batholith range from less than 200 to 3,000 ppm thorium. 

We consider thorium contents greater than 2,000 ppm to 

be anomalous, and areas where high thorium values were 

obtained have been outlined on plate 5. Two of these 

areas, one in Muskrat and Rawhide Creeks and the other 

south of Prickly Pear Creek, are coincident with an 

anomalously high radiometric eTh-to-K ratio. High 

eTh-to-K ratios were also found in the vicinity of Burnt 

Mountain, Strawberry Butte, and Lava Mountain, where 

young rhyolitic lavas cap the batholith. Two samples from 

the upper part of the South Fork Warm Springs Creek 

contain some of the highest thorium concentrations. 

Three small areas underlain by the Boulder batholith contain high amounts of radon in surface-water 

samples (pl. 5). The samples with the highest radon were 

collected from the upper part of Dutchman Creek-an 

area that straddles the study area boundary. The sample 

sites lie just downstream from a major northeasttrending fracture (pl. 5) identified by E. R. Verbeek (oral 

commun., 1977). The radon content of these surfacewater samples shows no correlation with uranium 

concentrations. In general, the radon content in this 

entire area is abnormally high; radon contents elsewhere 

in and near the study area are rarely greater than 50 pCi/L. 

A small area around the town of Alhambra (pl. 5) 

contains hot springs, hot flowing wells, and cold wells that 

show exceptionally high radon contents, 3,000-37,000 

pCi/L (Leonard and Janzer, 1977). These are among the 

highest radon contents found in the United States. 

No large areas of significantly high uranium in 

surface water were found, although anomalous values 

were measured on samples from Badger Creek (13 #lg/L ), 

Rawhide Creek (13 #lg/L), and an unnamed tributary of 

Clancy Creek (13 #lg/L ). All these drainages, except for the 

headwaters of Rawhide Creek, are outside the study 

area. In most areas except for Dutchman Creek and 

Prickly Pear Creek, where uranium concentrations are 

low, uranium in the surface waters ranged from 1 to 4 

#lg/L. These values are above average for uranium in 

surface water, particularly for small, first- and secondorder streams of moderately high U conductivity ratios. 

With the exception of one sample at 7.2 #lg/L, the wells of 

high radon content around Alhambra contain less than 1 

#lg/L uranium. High eU-to-K ratios determined in the 

aerial radiometric survey occur sporadically over the 

entire area (pl. 5) and do not correlate with uranium in 

surface water. 

Although the uranium content of surface water in 

the western part of the study area and adjacent land to 

the west is not extremely high, the overall radio-element 

content of the water and stream-sediment samples, as 

well as the eU-to-K and eTh-to-K ratios, is high and 

merits further study. The extremely high radon content in 

the Dutchman Creek area might possibly be explained by 

southeast movement, along fractures, of radon 

originating perhaps from uranium occurrences in the 

Clancy district to the northwest. Movement of this radon 

may then have been impeded by the major fracture (pl. 5) 

southeast of the anomaly. However, this high radon 

concentration in the surface water is more likely due to a 

localized source because of the short half life of Rn222 

and the large distance between the Clancy district (about 

6 mi (10 km)) and Dutchman Creek. The area around 

Muskrat and Rawhide Creeks is high in all radioelements and probably merits the most attention. 

Possibly, localized uranium occurrences are contributing 

to the high radon values in the wells and hot springs at 

Alhambra, as well as in surface water of areas outlined 

on plate 5. 

Time was not available to conduct the follow-up 

field studies necessary to evaluate the possible resource 

potential that could be indicated by the anomalous 

samples described above. These results are reported for 

future reference. 

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