Mineral Resources of the 

Gibraltar Mountain and Planet Peak 

Wilderness Study Areas,, 

La Paz County, Arizona

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Chapter B

Mineral Resources of the 

Gibraltar Mountain and Planet Peak 

Wilderness Study Areas, 

La Paz County, Arizona

By ROBERT G. EPPINGER, JOCELYN A. PETERSON, 

H. RICHARD BLANK, JR., K. ERIC LIVO, 

DANIEL H. KNEPPER, JR., and JAMES A. PITKIN 

U.S. Geological Survey

JON E. SPENCER, STEPHEN J. REYNOLDS, and 

MICHAEL J.GRUBENSKY 

Arizona Geological Survey

TERRY J. KREIDLER and DAVID C. SCOTT 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1704

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

HAVASU REGION, ARIZONA
U.S. DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names

in this publication is for descriptive purposes only

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

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1990

For sale by the

Books and Open-File Reports Section

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Library of Congress Cataloging-in-Publication Data

Mineral resources of the Gibraltar Mountain and Planet Peak wilderness 

study areas, La Paz County, Arizona / by Robert G. Eppinger ... [et 

al.].

p. cm. (Mineral resources of wilderness study areas 

Havasu region, Arizona : ch. B) (U.S. Geological Survey bulletin : 

1704-B)

Includes bibliographical references.

Supt. of Docs, no.: I 19.3: 1704-B

1. Mines and mineral resources Arizona Gibraltar Mountain 

Wilderness. 2. Mines and mineral resources Arizona Planet Peak 

Wilderness. 3. Gibraltar Mountain Wilderness (Ariz.) 4. Planet 

Peak Wilderness (Ariz.) I. Eppinger, R.G. II. Series. III. Series: 

U.S. Geological Survey bulletin ; 1704-B

QE75.B9 no. 1704-B 557.3 s dc20 90-13943 

[TN24.A6] [553' .0979172] CIP
STUDIES RELATED TO WILDERNESS

Bureau of Land Management Wilderness Study Areas

The Federal Land Policy and Management Act (Public Law 94-579, October 21,1976) 

requires the U.S. Geological Survey and the U.S. Bureau of Mines to conduct mineral surveys 

on certain areas to determine the mineral values, if any, that may be present. Results must be 

made available to the public and be submitted to the President and the Congress. This report 

presents the results of a mineral survey of parts of the Gibraltar Mountain (AZ-050-012) and 

Planet Peak (AZ-050-013) Wilderness Study Areas, La Paz County, Arizona.

CONTENTS

Summary Bl

Abstract Bl

Character and setting B1

Identified resources and mineral resource potential of the Gibraltar

Mountain Wilderness S tudy Area B3 

Identified resources and mineral resource potential of the Planet Peak

Wilderness Study Area B3 

Introduction B3

Character and setting B3 

Previous investigations B6 

Present study B6 

Acknowledgments B6 

Appraisal of identified resources of the Gibraltar Mountain Wilderness

Study Area B6 

Methods of investigation B6 

Mining districts and history B6 

Appraisal of sites examined B7 

Mammon mine area B7 

Gray Eagle mine area B7 

Giers Mountain area B7 

Miscellaneous prospects B8 

Sand and gravel B8 

Conclusions B8

Appraisal of identified resources of the Planet Peak Wilderness Study Area B8 

Methods of investigation B8 

Mining history B8 

Appraisal of sites examined B8 

Pride mine area B8 

Planet Peak claim area B9 

Squaw Peak area B10 

Sand and gravel BIO 

Energy resources BIO

Comparison of study area to Copperstone mine B10 

Conclusions BIO

Assessment of mineral resource potential B11 

Geology fill

Regional setting B11 

Local geology B12

Lower-plate rocks B12 

Upper-plate rocks B12 

Postdetachment rocks B13 

Geochemistry B13

Introduction B13 

Methods B13 

Results B14 

Interpretation B18 

Geophysics B19

Aeromagnetic and gravity data B19 

Landsat thematic mapper images and interpretation B20 

Background B20 

Interpretive methods B21

Contents
Assessment of mineral resource potential Continued 

Geophysics Continued

Landsat thematic mapper images and interpretation Continued

Possible areas of hydrothermally altered rocks B22 

Aerial gamma-ray spectrometry B22 

General discussion of mineral and energy resources B24 

Deposits near detachment faults B24 

Energy resources B25 

Mineral and energy resources of the Gibraltar Mountain Wilderness

Study Area B25 

Mineral and energy resources of the Planet Peak Wilderness Study

Area B26

References cited B26 

Appendixes B29

Definition of levels of mineral resource potential and certainty of

assessment B30

Resource/reserve classification B31 

Geologic time chart B32

FIGURES

1. Index map showing location of Gibraltar Mountain and Planet Peak Wilderness Study 

Areas, Arizona B2

2. Map showing mineral resource potential and generalized geology of Gibraltar 

Mountain Wilderness Study Area, Arizona B4

3. Map showing mineral resource potential and generalized geology of Planet Peak 

Wilderness Study Area, Arizona B5

4. Map showing stream drainages having geochemical anomalies in and near Gibraltar 

Mountain Wilderness Study Area, Arizona B16

5. Map showing stream drainages having geochemical anomalies in and near Planet 

Peak Wilderness Study Area, Arizona BIT

6. Residual total-intensity aeromagnetic map of region including Gibraltar Mountain and 

Planet Peak Wilderness S tudy Areas, California and Arizona B20

7. Complete-Bouguer gravity anomaly map of region including Gibraltar Mountain and 

Planet Peak Wilderness Study Areas, California and Arizona B21

8. Map showing distribution of potentially hydrothermally altered rocks in Gibraltar 

Mountain Wilderness Study Area, Arizona; based on remote-sensing 

interpretation B23

9. Map showing distribution of potentially hydrothermally altered rocks in Planet Peak 

Wilderness Study Area, Arizona; based on remote-sensing interpretation B24

TABLES

1. Summary of production data for mining districts in and near the Planet Peak 

Wilderness Study Area, Arizona B9

2. Comparison of geochemical data from the Planet Peak Wilderness Study Area and the 

Copperstone mine, Arizona B11

3. Summary statistics for anomalous elements from stream sediments and heavy-mineral 

concentrates in and around the Gibraltar Mountain and Planet Peak Wilderness Study 

Areas, Arizona BIS

VI Contents
Mineral Resources of the 

Gibraltar Mountain and Planet Peak 

Wilderness Study Areas, 

La Paz County, Arizona

By Robert G. Eppinger, Jocelyn A. Peterson, H. Richard Blank, Jr., K. Eric Livo, Daniel H. 

Knepper, Jr., and James A. Pitkin 

U.S. Geological Survey

Jon E. Spencer, Stephen J. Reynolds, and Michael J. Grubensky 

Arizona Geological Survey

Terry J. Kreidler and David C. Scott 

U.S. Bureau of Mines

SUMMARY

Abstract

The parts of the Gibraltar Mountain (AZ-050-012) and 

Planet Peak (AZ-050-013) Wilderness Study Areas for which 

mineral surveys were requested by the U.S. Bureau of Land 

Management encompass 18,807 and 16,430 acres, respectively, in west-central Arizona. Throughout this report, reference to those specific areas, or to the "study areas" refers only 

to those parts of the wilderness study areas for which mineral 

surveys were requested. Field work was carried out during the 

winter and spring of 1988 by the U.S. Bureau of Mines and 

the U.S. Geological Survey to appraise the identified (known) 

resources and assess the potential for undiscovered mineral 

resources in the study areas.

The study areas lie within a region containing welldeveloped detachment faults, including the Buckskin-Rawhide detachment fault that is present in and adjacent to the 

study areas. Numerous small hematitic copper prospects are 

associated with this fault, but neither study area has identified 

mineral resources. The region currently is of interest for gold 

exploration. The mineral resource potential in the Gibraltar 

Mountain Wilderness Study Area is high for copper, iron, and 

gold and low for silver, barite, manganese, and fluorite in and 

near the Mammon mine along the southwest edge of the 

study area. In the northwest corner of that study area the 

mineral resource potential is moderate for copper, iron, and

Manuscript approved for publication, Aug. 27, 1990

gold and low for silver, barite, manganese, and fluorite. Basaltic rocks that cover much of the study area are probably 

not mineralized, but the underlying rocks may be; thus the 

underlying rocks have an unknown potential for copper, iron, 

gold, manganese, silver, barite, and fluorite resources. The 

mineral resource potential in the Planet Peak Wilderness Study 

Area is high for copper, iron, and gold and low for barite, 

manganese, and fluorite in those areas of exposed upperplate rocks and is moderate for copper, iron, and gold and 

low for barite, manganese, and fluorite in exposed lowerplate rocks. The mineral resource potential for silver around 

the Pride mine is high and elsewhere is low. Areas covered 

by Quaternary and Tertiary alluvium have an unknown potential for all these elements except in the alluvium-covered 

areas along the south, southeast, and northeast margins of the 

study area, which have a high resource potential for copper, 

iron, and gold in upper-plate rocks beneath the alluvium. The 

study areas have no potential for oil and gas or for geothermal energy.

Character and Setting

The Gibraltar Mountain (AZ-050-012) and Planet 

Peak (AZ-050-013) Wilderness Study Areas are in westcentral Arizona about 15 mi due east of Parker, Ariz., and 

about 25 mi southeast of Lake Havasu City, Ariz. (fig. 1). 

The areas lie within the Sonoran Desert section of the Basin 

and Range physiographic province, a region typically characterized by linear mountain ranges separated by wide, deep, 

alluvium-filled valleys. The Gibraltar Mountain Wilderness

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona 61
1 14°30' 

Lake Havasu City

CHEMEHUEVI 

INDIAN

, RESERVATION

Planet mineral^district 

Mineral Hill subdistrict

BUCKSKIN r. ' 

Planet 

Plane

Cienega 

mineral 

district

Gibraltar 

Mountain

\Cienega Springs 

Unnamed \ Mammon 

mineral 

district

Pride

mineral

district / APPROXIMATE BOUNDARY

OF

GIBRALTAR MOUNTAIN APPROXIMATE BOUNDARY 

WILDERNESS STUDY AREA 

(AZ-050-012)

OF

PLANETPEAK

WILDERNESS STUDY AREA

(AZ-050-013)

COLORADO RTVER 

INDIAN RESERVATION

Figure 1. Index map showing location of Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Ar

Welty (1989). Dashed line separates Cienega (left) and Santa Maria (right) mining districts. All boundaries are approximat
Study Area is underlain primarily by volcanic rocks of 

Tertiary age (see geologic time chart in "Appendixes") and 

by minor but important outcrops of older metasedimentary, 

mylonitic, and granitic rocks. Most of the Planet Peak Wilderness Study Area is underlain by mylonitic rocks, and 

there are small exposures of Paleozoic and Tertiary sedimentary and Tertiary intrusive rocks. Alluvium is present 

in low-lying areas.

Identified Resources and Mineral Resource 

Potential of the Gibraltar Mountain Wilderness 

Study Area

No mineral resources were identified in the Gibraltar 

Mountain Wilderness study area. Mineralized rock is exposed at few localities, and extensive areas of volcanic rocks 

cover potentially mineralized units. The area around the 

Mammon mine, which is known to be mineralized, has high 

potential for copper, iron, and gold resources and low potential for silver, barite, manganese, and fluorite resources 

(fig. 2). Rocks in the upper plate of the Buckskin-Rawhide 

detachment fault crop out in the northwest corner of the 

study area and have moderate potential for copper, iron, 

and gold resources and low potential for silver, barite, manganese, and fluorite resources. Other rocks adjacent to the 

Buckskin-Rawhide detachment fault lie beneath Tertiary 

postdetachment basaltic and trachytic rocks and are therefore 

inaccessible for evaluation. However, the local geology, the 

nearby mineralized rocks, and the geochemical anomalies 

found in the basaltic terrane but possibly derived from underlying rocks all suggest that the concealed detachmentrelated rocks have an unknown potential for copper, iron, 

gold, manganese, silver, barite, and fluorite resources.

Identified Resources and Mineral Resource 

Potential of the Planet Peak Wilderness 

Study Area

Mineral resources were not identified within the Planet 

Peak Wilderness Study Area, and quantifying any inferred 

resources outside the study area at the Pride mine was not 

possible due to the discontinuous nature of the mineralized 

rocks. The study area consists of upper-plate and lowerplate rocks of the Buckskin-Rawhide detachment fault and 

includes local exposures of the detachment surface itself; 

younger Tertiary and Quaternary sediments cover much of 

the periphery of the study area. Because of the close association of the mineralization and the detachment surface, 

the exposed upper-plate rocks have high mineral resource 

potential for copper, iron, and gold resources, and the lowerplate rocks have moderate resource potential for these elements; rocks of both plates have low potential for barite,

silver, manganese, and fluorite resources (fig. 3). The area 

directly surrounding the Pride mine, however, has high 

mineral resource potential for silver because of the known 

small production of this commodity from the mine. The 

mineral resource potential for all of these commodities is 

generally unknown where alluvium obscures detachmentrelated rocks, but it is likely to be similar to that of nearby 

exposed rocks. Along the south, southeast, and northeast 

borders of the study area, however, the alluvium covers 

upper-plate rocks that have high mineral resource potential 

for copper, iron, and gold. The study areas have no resource 

potential for oil and gas or for geothermal energy.

INTRODUCTION

This mineral survey was requested by the U.S. Bureau of Land Management (BLM) and is a joint effort by 

the U.S. Geological Survey (USGS) and the U.S. Bureau of 

Mines (USBM). An introduction to the wilderness review 

process, mineral survey methods, and agency responsibilities 

was provided by Beikman and others (1983). The USBM 

evaluates identified resources at individual mines and known 

mineralized areas by collecting data on current and past 

mining activities and through field examination of mines, 

prospects, and mineralized areas. Identified resources are 

classified according to the system that is a modification of 

that described by McKelvey (1972) and U.S. Bureau of 

Mines and tl.S. Geological Survey (1980). Studies by the 

USGS are designed to provide a reasonable scientific basis 

for assessing the potential for undiscovered mineral resources 

by determining geologic units and structures, possible environments of mineral deposition, presence of geochemical 

and geophysical anomalies, and applicable ore-deposit 

models. Mineral assessment methodology and terminology 

as they apply to these surveys were discussed by Goudarzi 

(1984). See "Appendixes" for the definition of levels of 

mineral resource potential and certainty of assessment and 

for the resource/reserve classification.

Character and Setting

The Gibraltar Mountain (AZ-050-012) and Planet Peak 

(AZ-050-013) Wilderness Study Areas in west-central Arizona encompass 18,807 and 16,430 acres, respectively (fig. 

1). The study areas are about 10 to 20 mi east of Parker, 

Ariz., the nearest municipality, and about 25 mi southeast 

of Lake Havasu City, Ariz. The Gibraltar Mountain Wilderness Study Area includes Gibraltar and Giers Mountains, 

prominent peaks in the central and northwestern parts of 

the study area; Planet Peak dominates the topography in the 

Planet Peak Wilderness Study Area. Both study areas are in 

the western part of the Buckskin Mountains, an east-trending range bounded by the Bill Williams River on the north

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona B3
and Cactus Plain and Butler Valley (about 20 mi southeast 

of the study area) on the south. The study areas are part of 

the Sonoran Desert section of the Basin and Range physiographic province, an area characterized by mountain ranges 

separated by broad valleys.

Access to the study areas is by numerous gravel roads 

that lead from State Highways 72 and 95 to the many mines 

and prospects peripheral to the study areas. Rugged topography within the study areas permits only foot traffic. The 

Central Arizona Project Canal further hinders access to the

34' 

15'

SAID'

114°05' 

 I 

M/C Cu. Fe, Au 

L/B Ag, Ba, Mn, F

\ YXc

Ts

APPROXIMATE BOUNDARY

OF

GIBRALTAR MOUNTAIN

WILDERNESS STUDY AREA

(AZ-050-012)

U/A Cu, Fe, Au, 

Ag, Mn, Ba, F

H/C Cu, Fe. Au

L/C Ag, Ba, Mn Tt>

L/BF

EXPLANATION

Geologic terrane having high mineral resource 

potential (H) for Cu, Fe, and Au and low mineral 

resource potential (L) for Ag, Ba, Mn, and F

Geologic terrane having moderate mineral resource 

potential (M) for Cu, Fe, and Au and low mineral 

resource potential (L) for Ag, Ba, Mn, and F

Geologic terrane having unknown mineral resource 

potential (U) for Cu, Fe, Au, Ag, Mn, Ba, and F

Levels of certainty of potential

Data are not adequate to determine level of

potential

Data only suggest level of potential 

Data give good indication of level of potential

Commodities

Ag Silver

Au Gold

Ba Barium

Cu Copper

F 

Fe

Mn

Fluorine

Iron

Manganese

Description of map units

QTat Alluvium and talus (Quaternary and Tertiary)

Tt Trachytic volcanic rocks (Tertiary)

Tb Basaltic volcanic rocks (Tertiary)

Ts Sedimentary rocks (Tertiary)

Tsv Sedimentary and volcanic rocks, undivided

(Tertiary)

MzS Metasedimentary rocks (Mesozoic) 

KXg Granitic rocks (Cretaceous to Proterozoic) 

YXc Crystalline rocks (Middle and Early Proterozoic) 

me Mylonitic crystalline rocks Protolith age is

Proterozoic to Tertiary

- - - Contact Dashed where approximately located

 Fault Dashed where approximately located

'm Area covered by patented and unpatented mining 

Esl claims

Adit

Prospect pit 

Shaft

Figure 2. Mineral resource potential and generalized geologic map of Gibraltar Mountain Wilderness Study Area, La Paz County, 

Arizona. Geology modified from Grubensky (1989) and Spencer (1989).

B4 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
114-02 -3 0" 114°00' 113-52 -3 0-

34° 

12' 

30'

34" 

ID34 0

07' | 

30'

Mineral H.ll am' >'i-in>:l 

mineral daim i

 ..

,j

APPROXIMATE BOUNDARY

OF

PLANET PEAK

WILDERNESS STUDY AREA

(AZ-050-013)

II/B Cu,Fe,Au 

U/A Ag. Ba, Mn. F

H/C Cu, Fe, Au

L/C Ba, Mn. Ag

L/BF

M/C Cu. Fe, Au 

L/C Ag, Ba, Mn

U/A Cu, 

Au, Ag, 

Ba, Mn

QTaH/C Cu, Fe, Au, Ag 

L/C Ba, Mn 

L/BF

New Standard 

mine » . ;/-i /-. r- M/CCu, Fe,

L/C Ag, Ba. Mn 

L/BF

3 MILES

I

FI/B Cu, Fc, Au 

U/A Ag, Ba, Mn, F

m

A 

B 

C

Ag 

Au 

Ba 

Cu

EXPLANATION

Geologic terrane having high (H) and unknown (U) mineral 

resource potential for commodities as noted in rocks below 

QTa

Geologic terrane having high (H) and low (L) mineral resource 

potential for commodities as noted

Geologic terrane having moderate mineral resource potential (M) 

for Cu, Fe, and Au and low mineral resource potential (L) for 

Ag, Ba, Mn, and F

Geologic terrane having unknown mineral resource potential (U) 

for Cu, Fe, Au. and (or) Ag, Ba, Mn, and F

Levels of certainty of assessment

Data are not adequate to determine level of potential

Data only suggest level of potential

Data give good indication of level of potential

QTa

Tbf

Tc

Ts

Tt

Ti

J p

Jv

ftc 

me

Commodities

Silver 

Gold 

Barium 

Copper

F 

Fe

Mn

Fluorine 

Iron

Manganese

Description of map units

Alluvium (Quaternary and Tertiary)

Basin-fill conglomerate and sandstone (Tertiary)

Replacement carbonate rocks (Tertiary)

Sedimentary rocks (Tertiary)

Air-fall tuff (Tertiary)

Intermediate to mafic intrusions (Tertiary)

Planet Volcanics of Reynolds and others (1987) (Jurassic) 

Silicic to intermediate volcanic and metavolcanic rocks 

Vampire Formation of Reynolds and others (1987)

(Jurassic?) Quartzite, metasandstone, and conglomerate 

Calcareous sedimentary rocks (Paleozoic) 

Mylonitic crystalline rocks Protolith age is Proterozoic to

Tertiary

Contact

Fault Dashed where approximately located

Buckskin-Rawhide detachment fault Barbs on side of lower plate

Area covered by patented and unpatented mining claims

Adit

Prospect pit 

Shaft

Figure 3. Mineral resource potential and generalized geologic map of Planet Peak Wilderness Study Area, La Paz County, 

Arizona. Geology modified from Spencer and others (1989) and Spencer (1989). B5
west side of the Planet Peak Wilderness Study Area, as 

only one road crosses it near the south end of the study area. 

Planet Peak, at an elevation of 3,141 ft, is the highest 

point within the study areas, whereas the alluvial plains 

southwest of the Gibraltar Mountain Wilderness Study Area 

have altitudes less than 1,000 ft. The climate is arid, and the 

region supports typical Sonoran Desert flora and fauna.

Previous Investigations

The earliest geologic reports of the region, in what 

was then northern Yuma County, described the geology 

and mineral deposits in the mining districts within and adjacent to the Gibraltar Mountain and Planet Peak Wilderness 

Study Areas (Bancroft, 1911; Blanchard, 1913). Until recently, detailed geologic mapping in the area had not been 

undertaken, but recognition of the significance of detachment faults and interest in mineralized areas associated with 

them prompted studies of the area, particularly by the University of Southern California and the Arizona Geological 

Survey (formerly Arizona Bureau of Geology and Mineral 

Technology). Shackelford (1976), working in the Rawhide 

Mountains east-northeast of the study areas, established the 

existence of what is now called the Buckskin-Rawhide detachment fault and confirmed its middle-Tertiary age. A 

l:100,000-scale geologic map that includes the Buckskin 

Mountains was compiled by Spencer (1989). Detailed mapping by Spencer and others (1989) and by Grubensky (1989) 

cover the northern part of the Planet Peak and southern part 

of the Gibraltar Mountain Wilderness Study Areas, respectively. The mineralized areas and mines peripheral to the 

study areas have been described in several publications, 

including Keith (1978), Welty and others (1985), and Spencer and Welty (1989). Spencer and Welty (1986a) provided 

a regional geologic context for the mineral deposits. Further 

details about the work in the region, particularly related to 

the understanding of detachment-fault tectonics, are summarized in Spencer and Reynolds (1989). Several reports 

for graduate studies conducted within the region are cited 

in Schnabel and Welty (1986).

In the early 1980's, the Arizona Bureau of Geology 

and Mineral Technology developed a classification system 

for metallic mineral occurrences based on geologic and 

metallogenic criteria (Keith and others, 1983a, b), which 

resulted in a regrouping of deposits and occurrences into 

"metallic mineral districts." These mineral districts are different from the traditional mining districts, which are based 

on geographic and political criteria. The USBM generally 

uses mining districts because published literature, claim location notices, and most production figures refer to these. 

This paper uses the designations of Keith and others (1983a, 

b) as modified by Spencer and Welty (1989). Table 1 shows 

the relations of the metallic mineral districts to the mining 

districts.

Present Study

USBM personnel reviewed pertinent literature (including published reports, USBM files, and mining claims 

and oil and gas leases filed with the BLM) on the geology, 

mineralized areas, and mining activity in the region of the 

study areas. Discussions on the mineral resources of both 

study areas were held with Chuck Botdorf, district geologist 

in the BLM Yuma District Office, and with Pat Green, 

wilderness coordinator of the BLM Resource Area Office 

in Lake Havasu City. Field work consisted of mapping accessible mine workings by the compass-and-tape method and 

sampling mines and prospects in and near the study areas.

The USGS examined the geology of the areas, using 

geologic maps of the Arizona Geological Survey (see "Geology" below). A geochemical survey was conducted in 

which stream-sediment, heavy-mineral-concentrate, and rock 

samples were collected and analyzed by quantitative and 

semiquantitative methods. Geophysical data for the areas 

are from published gravity and magnetic investigations and 

preliminary radioelement studies.

Acknowledgments

Stephanie L. Jones and Cliff D. Taylor capably assisted with the field work by the USGS.

APPRAISAL OF IDENTIFIED RESOURCES OF 

THE GIBRALTAR MOUNTAIN WILDERNESS 

STUDY AREA

By David C. Scott 

U.S. Bureau of Mines

Methods of Investigation

Accessible mine workings were surveyed, and 18 rockchip samples were collected. All samples were analyzed by 

inductively coupled plasma-atomic fluorescence spectroscopy (total digestion) for 24 elements and by fire assay/ 

atomic absorption spectroscopy for gold. All analyses were 

performed by Chemex Labs, Inc., Sparks, Nev. Data for all 

samples were discussed by Scott (1989). Further inquiries 

about sample analyses can be directed to the U.S. Bureau of 

Mines, Intermountain Field Operations Center, Box 25086, 

Denver Federal Center, Denver, CO 80225.

Mining Districts and History

The west side of the Gibraltar Mountain Wilderness 

Study Area is part of the Cienega mining district. It is adjacent to the Cienega mineral district and partly within the

B6 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
Mammon mineral district as defined by Spencer and Welty 

(1989). The Planet and Pride mineral districts are east of the 

study area (fig. 1). Copper, gold, and silver deposits were 

discovered in the districts as early as 1860 (Keith, 1978, p. 

129).

The Cienega mineral district is adjacent to the northwestern part of the study area. From 1870 to 1969, production from the district totaled 19,092 short tons (st) of ore 

that contained 1,713,533 Ib of copper, 1,596 oz of silver, 

and 12,011 oz of gold. Mineral deposits consist of replacement bodies of silicate, carbonate, and oxide minerals and 

concentrations of quartz and hematite in northwest-trending 

shear zones, both in the upper plate of the Buckskin-Rawhide detachment fault (Spencer and Welty, 1989, p. 224).

The Mammon mineral district lies along and inside 

the southwestern part of the study area. Production from 

1909 to 1955 totaled 841 st of ore containing 86,993 Ib of 

copper, 142 oz of silver, and 60 oz of gold. Mineral deposits consist of chrysocolla, malachite, hematite, and calcite 

in northwest-trending, steeply dipping shear zones in lowerplate mylonitic gneiss of the detachment fault (Spencer and 

Welty, 1989, p. 224).

The Planet mineral district is 3 to 7 mi northeast of 

the study area. Production from the district totaled 1,090,771 

st of ore that contained 19,503,012 Ib of copper, 270 oz of 

silver, and 320 oz of gold (Spencer and Welty, 1989, p. 

224). Copper carbonate, silicate, and sulfide minerals with 

quartz and calcite are present as disseminations and in veinlets and replacement bodies of hematite in the upper plate 

of the detachment fault and related northwest-trending shear 

zones (Spencer and Welty, 1986b, p. 196).

The Pride subdistrict within the Pride mineral district 

is about 2 mi east of the study area, partly within the Planet 

Peak Wilderness Study Area. Production from the district 

totaled 38 st of ore that contained 21 Ib of copper, 6 oz of 

silver, and 78 oz of gold (Spencer and Welty, 1989, p. 224). 

Malachite, chrysocolla, hematite, and quartz are found in 

both the lower and upper plates of the detachment fault in 

northwest-trending, high-angle shear zones (Spencer and 

Welty, 1986b, p. 196).

As of March 1989, 18 claims were on file with the 

BLM, but no evidence of any recent mining activity was 

found in the study area. Cyprus Mining Co. is currently 

evaluating the Mammon mine area (fig. 2) as a possible 

copper-leaching site (Jim Jones, owner, Mammon mine, 

Parker, Ariz., oral commun., Jan. 1989).

Appraisal of Sites Examined

No mineral resources were identified within the study 

area. Copper, gold, and silver occurrences are associated 

with both upper- and lower-plate rocks of the BuckskinRawhide detachment fault. Rock-chip samples were taken 

from lower-plate rocks exposed at the Mammon mine and

from upper-plate rocks exposed along the northwestern part 

of the study area. Sampled localities are discussed by geographic location.

Mammon Mine Area

Part of the Mammon mine (fig. 2) is on three patented 

mining claims. Several small adits and prospects were found 

at the mine; however, the main working is an adit about 

500 ft long. The adit was driven on a shear zone striking N. 

40° W. and dipping 40° SW. in an isolated outcrop of lowerplate mylonitic gneiss. The shear zone contains intensely 

sheared rocks that are heavily stained with limonitic iron 

oxide and copper carbonate, silicate, and oxide minerals; 

calcite and hematite are also present. Neither the shear zone 

nor the mylonitic gneiss can be traced into the study area; 

however, gneissic alluvial material was seen in drainages 

near the mine (R.G. Eppinger, written commun., 1989; see 

also "Geochemistry" below). Basalt that overlies the 

mylonitic gneiss is apparently not mineralized.

Two samples were collected in the study area east of 

the Mammon mine. A select sample, from the dump of a 

48-ft-deep shaft in basalt, contains veinlets of hematite 1 to 

4 in. thick. Gold was detected (5 part per billion, ppb), and 

high concentrations of copper (8,280 parts per million, ppm) 

and zinc (1,805 ppm) are present (Scott, 1989). The copper 

and zinc are probably associated with the hematite veinlets; 

no structural feature was visible in the shaft. The other 

sample, collected from a pit dug in oxidized basalt, contains no significant concentrations of any elements.

Gray Eagle Mine Area

A 41-ft-long adit, a shaft, and a prospect pit were 

found outside of but within 0.25 mi of the study area's west 

boundary, near the Gray Eagle mine (fig. 2). The workings 

are in upper-plate Paleozoic and Mesozoic limestone and 

shale. East- to northeast-trending high-angle faults and fractures in the limestone and shale contain azurite, calcite, 

chrysocolla, malachite, and hematite. These structures could 

not be traced outside of the workings; however, they trend 

toward the study area and may be present beneath the basaltic cover to the east.

Four samples were collected from the workings. Gold 

concentrations range from less than 5 to 1,560 ppb. The 

highest concentration was in a select sample from the dump 

of a prospect pit. Copper'concentrations range from 1,790 

to more than 10,000 ppm. All samples contain elevated 

lead (150-876 ppm), silver (2-140 ppm), and zinc (70- 

1,020 ppm) concentrations (Scott, 1989).

Giers Mountain Area

A cluster of workings on the northwest flank of Giers 

Mountain consists of four prospect pits, one shallow shaft,

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona B7
and one adit just inside the study area boundary (fig. 2) and 

one adit and one prospect pit just outside. The workings are 

on northwest- to northeast-striking faults in granite in the 

upper plate of the detachment fault. The fault gouge pinches 

and swells from 2 to 4 ft thick, but exposures are less than 

100 ft long. Azurite, malachite, and limonite are common 

in the gouge.

Samples collected from the faults contain gold concentrations ranging from less than 5 to 6,400 ppb. Copper concentrations range from 42 to more than 10,000 ppm. Lead, 

silver, and zinc contents are generally insignificant (Scott, 

1989). Similar mineralized faults containing these elements 

may be present beneath the basalt nearby in the study area. 

No resources were calculated because of the short strike 

lengths of the faults and sporadic element concentrations.

Miscellaneous Prospects

Two other pits within and adjacent to the southern 

part of the study area (fig. 2) are in basalt. No structural 

features were noted, and no significant metal concentrations 

were found in a sample from each pit (Scott, 1989).

Sand and Gravel

Sand and gravel deposits of Quaternary age are present 

in drainages in the study area; however, there is currently 

no local demand for them. Large, accessible volumes of 

similar sand and gravel are present outside the study area 

and these can satisfy any increase in local demand.

Conclusions

No mineral resources were identified in the study area. 

About 85 percent of the study area is covered by basalt and 

is apparently devoid of near-surface mineralized rock. Along 

the west boundary, high-angle faults and fractures in the 

upper and lower plates of the Buckskin-Rawhide detachment fault contain copper, gold, and silver occurrences.

APPRAISAL OF IDENTIFIED RESOURCES OF 

THE PLANET PEAK WILDERNESS STUDY AREA

*

By Terry J. Kreidler 

U.S. Bureau of Mines

Methods of Investigation

Accessible mine workings were studied, and 94 chip, 

grab, and select samples were analyzed for gold and 33 

other elements by neutron activation and for copper and 

lead by atomic absorption spectrometry by Bondar-Clegg,

Inc., Lakewood, Colo. Complete analytical data can be found 

in Kreidler (1989), available from the U.S. Bureau of Mines, 

Intermountain Field Operations Center, Box 25086, Denver 

Federal Center, Denver, CO, 80225.

Mining History

The Planet Peak Wilderness Study Area is near and 

includes parts of the Planet, Swansea, and Pride mineral 

districts of Spencer and Welty (1989). The study area also is 

about 1 mi northwest of the Green Streak subdistrict of the 

Midway mineral district (Kreidler, 1989, pi. 1). Table 1 

summarizes the production history of these districts. The 

Planet and Swansea districts produced gold, silver, and copper from ore present as massive to lensing replacement bodies 

of hematite in Paleozoic carbonate rocks in the upper plate 

of the Bucksin-Rawhide detachment fault. Production from 

the Planet district alone totaled 1,009,771 st of ore that 

included 9,752 st of copper, 270 oz of silver, and 320 oz of 

gold (Spencer and Welty, 1989, p. 224). The Pride mineral 

district produced gold, silver, copper, and minor lead from 

replacement pockets in partly metamorphosed Paleozoic and 

Mesozoic limestone, shale, and quartzite. The Midway mineral district contained gold, silver, and copper in sporadic 

deposits along faults and fractures in Proterozoic to Tertiary 

gneiss, schist, and foliated granitoid rocks and in Paleozoic 

limestone. As of February 1988, the mines within 5 mi of 

the study area were inactive. As of January 1988, blocks of 

mining claims had been staked along the north and east 

sides of the study area; all or part of about 36 claims were 

staked inside the study area (Kreidler, 1989, pi. 1).

Appraisal of Sites Examined

Mineralization in the Planet Peak Wilderness Study 

Area occurred primarily in three areas: the Pride subdistrict 

of the Pride mineral district on the west border, and two 

parts of the Squaw Peak subdistrict of the Swansea mineral 

district along a spur ridge of Planet Peak and in an area 

around Squaw Peak (fig. 1). In these areas, mineral occurrences along fault zones consist of veins, pods, and lenses 

of specular hematite and oxidized copper minerals, primarily chrysocolla, malachite, and azurite, that coat fracture 

surfaces. Gangue minerals include mainly hematite, quartz, 

and calcite. The mineralized rock is discontinuous, commonly found as near-surface pods no thicker than 2 or 3 ft. 

The discontinuous nature prevented quantifying of resources.

Pride Mine Area

In the Pride subdistrict of the Pride mineral district, 

mineralization occurred primarily at the Pride mine (fig. 3) 

and in an area about 1 mi to the southeast. Farther to the

B8 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
Table 1 . Summary of production data for mining districts in and near the Planet Peak Wilderness Study Area, La 

Paz County, Arizona

[Data from Keith, 1978. Mineral district names, in parentheses, are those used by Spencer and Welty (1989); NA, not applicable]

Production data

Mining district When active

Gold (oz) Silver (oz) Copper (tons) Lead (Ib) Other

Cienega (Cienega, 1880-1969 -----

Mammon and Pride 

mineral districts).

 - 11,707 3 364 Q17 A few

hundred.

NA

Santa Maria (Planet Intermittently 

and Swansea mineral since 1860's. 

districts).

Midway Early 1900'slate 1970's.

1,128

45

35,000

35

23,000

4.5

NA

NA

400 tons 

manganese 

ore.

NA

southeast, a third area of minor mineralization is defined by 

scattered prospects.

The Pride mine, just outside the study area boundary, 

is on the Buckskin-Rawhide detachment fault (Spencer and 

Welty, 1989, p. 246) at an elevation of about 1,250 ft. The 

upper-plate rocks consist of metamorphosed limestone of 

probable Paleozoic age; the lower plate is granitic gneiss, 

locally mylonitized, that underlies nearly the entire study 

area. Workings consist of two open cuts, an adit, two shafts, 

and a pit. The mine has been worked sporadically since the 

early 1900's, producing 38 st of ore including 78 oz of gold 

averaging 2.052 oz/st, 6 oz of silver averaging 0.158 oz/st, 

and 21 Ib of copper averaging 0.28 percent (Spencer and 

Welty, 1989, p. 224). In the larger open cut, a massive body 

of specular hematite is exposed over a horizontal distance 

of about 100 ft and vertically for 20 to 35 ft; abundant 

chrysocolla, malachite, and azurite coat fracture surfaces. 

The hematite appears to replace limestone, although the 

host rock has been so intensely altered that recognition of 

the original rock type is difficult. The geology in the adit 

and at the remainder of the workings is similar. Six of nine 

samples taken at these workings contain gold concentrations ranging from 11 to 150 ppb and averaging 56 ppb. 

Average gold content for these rock types worldwide is 4 

ppb (Levinson, 1980, p. 872). Copper concentrations are 

generally low, considering the abundance of copper minerals coating rock surfaces, ranging from trace amounts to 1.3 

percent and averaging 0.66 percent. Iron concentrations range 

from 3.2 to 49 percent and average 26 percent. Silver was 

not detected in any of the samples.

About 1 mi southeast of the Pride mine, inside the 

study area at elevations between 1,200 and 1,300 ft, hematite and copper minerals are present in lower-plate gneiss 

(locally mylonitic) as near-surface pods and lenses, probably replacing tectonic slivers of carbonate rock and as narrow veins along faults with widely variable dips. Limestone 

fragments are found locally in some fault zones. These 

occurences have been explored by several shafts and pits 

and one adit. Fractures having low-angle dips are possibly 

listric faults that may or may not be associated with the 

detachment fault (see Wilkins and others, 1986). Of the 18 

samples taken in this area, 8 contain gold above the detection limit of 5 ppb, ranging from 6 to 350 ppb, averaging 51 

ppb. This average is skewed by the value of 350 ppb; without it, gold averages 9 ppb. All samples but one contain 

copper above trace amounts, ranging from 0.02 to 1.5 percent and averaging 0.52 percent. Iron content ranges from 

2.2 to 42 percent, averaging 17.3 percent.

South and east of the Pride mine are several scattered 

prospects, shafts, and adits with mineral occurrences similar to those described above. Samples taken at these workings 

contain gold (6-23 ppb), copper (trace to 1.75 percent), and 

iron (5.1-62.7 percent).

Planet Peak Claim Area

Five prospect pits and three short adits are on a spur 

ridge on the southeast side of Planet Peak (fig. 3), at an 

elevation of about 2,200 ft in the Squaw Peak subdistrict of 

the Swansea mineral district. Mineral occurrences consist of 

narrow hematite veins along faults and of lenses and pods 

replacing slivers of carbonate rock from the upper plate; 

some occurrences contain secondary copper minerals. The 

fault zones have varying orientations and dips; fresh limestone is present in one working, and a highly altered mafic 

rock, possibly originally a diabase, was found in another. Of 

11 samples taken in this area, 5 contain gold above the detection limit (7-20 ppb, average 12.2 ppb). The samples 

also contain copper (trace to 3.76 percent, average 1.2 percent) and iron (1.9-58.7 percent, average 32.4 percent).

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona B9
Squaw Peak Area Comparison of Study Area to Copperstone Mine

Workings in the Squaw Peak area (fig. 3, Squaw Peak 

subdistrict of the Swansea mineral district) are less than 

0.5 to 4 mi east of the study area boundary and include a 

large group of prospects (Kreidler, 1989, pi. 1). Squaw 

Peak, underlain by the detachment fault, consists of upperplate Paleozoic sedimentary rocks, primarily limestones 

that are locally metamorphosed (Spencer and others, 1989). 

Northwest of Squaw Peak, scattered workings are found 

in lower-plate mylonitic gneiss. Minerals include hematite 

in pods and lenses and in veins in northwest- to west-striking faults having variable dips. Chrysocolla and malachite 

are also present locally. The discontinuous nature of 

the hematite veins and pods is well demonstrated in a decline north of Squaw Peak. At the surface the vein is 6 to 

12 in. wide. At the face of the first level the vein has pinched 

out. In the lower level, lenticular pods are separated by barren fault breccia. This characteristic of the mineralized rock 

in the study area precludes quantifying any resources. Of 

the 29 samples taken in the Squaw Peak area, only 5, 

of which 4 are from the same mine, contain gold above 

the detection limit (9-24 ppb, average 13.2 ppb), 

24 contain copper (0.038-4.08 percent, average 0.63 

percent), and all contain iron (2.6-65.8 percent, average 28.7 

percent).

Sand and Gravel

Large deposits of sand and gravel occur in the study 

area but have no unique qualities that make them more 

valuable than the vast quantities that blanket much of the 

desert in this region. Sand and gravel is a high-volume, 

low-unit-value material that must be located close to markets to be economic.

Energy Resources

About 9,500 acres of the study area are covered by 

oil and gas leases. However, Ryder (1983, p. C19) rates the 

oil and gas potential of the study area as low to zero because it is underlain by a thick sequence of metamorphic 

and igneous rocks that are not conducive to the formation 

of hydrocarbons. The leasing is probably a result of speculation that the hydrocarbon-rich overthrust belt, which contains large quantities of oil and gas in Wyoming, extends 

southward into Arizona (Keith, 1979, p. 10). All exploratory drilling in Arizona thus far, based on the overthrust 

model, has had negative results.

According to Witcher and others (1982), the study 

area has no known potential for geothermal resources; the 

nearest thermal well (130 °F) is about 4.5 mi to the northeast in Mohave County.

The Copperstone mine (Cyprus Gold Co.) is about 25 

mi southwest of the Planet Peak Wilderness Study Area, at 

the north end of the Moon Mountains in the hanging wall 

of the Moon Mountains detachment fault (Cyprus Gold Co., 

written commun., 1988). The host rock is a thick sequence 

of foliated to massive quartz latite tuffs of Jurassic age and 

Tertiary sedimentary breccias (Spencer and others, 1988). 

The main ore horizon lies along the contact of the breccia 

and the underlying quartz latite tuff. The gold occurs as 

small flakes ranging from 4 to 40 microns (0.00016-0.0016 

in.); the most common gangue minerals are specularite (hematite), chrysocolla, barite, amethyst, calcite, and fluorite.

Reserves are 6 million st of ore having an average 

gold grade of 0.075 oz/st (2.57 ppm); in mid-1988 they were 

being mined by open-pit methods. An additional 1 million st 

of ore at a grade of 0.17 oz/st of gold (5.83 ppm) have been 

blocked out for possible future underground mining (Bill 

Burton, Cyprus Minerals Co., oral commun., June 1988).

Geochemical data from the Copperstone mine are 

compared with data from the Planet Peak Wilderness Study 

Area and with the average abundance of each element in 

similar rock types (table 2). Although anomalies1 in the study 

area are not as high as those in rocks from the Copperstone 

mine, samples from the study area contain higher concentrations of the specified elements than is expected for these 

rock types (except for lead, in which the samples from the 

study area are notably deficient). These anomalies indicate 

that the study area may have been subject to mineralizing 

processes similar to those which created the Copperstone 

ore body. The higher concentrations of gold, however, are 

present only where the ground has been adequately prepared 

by faulting or where more reactive rock types occur. At the 

Copperstone mine, the breccia provided a conduit system 

for the circulation of hydrothermal fluids and deposition of 

mineral deposits. In the study area, the limestone and 

metacarbonate rocks of the upper plate, which have been 

eroded from all but a small part of the study area, supplied 

reactive rock for replacement.

Conclusions

With the possible exception of the Pride mine area, 

none of the mineralized areas in the Planet Peak Wilderness 

Study Area contain identified mineral resources. A detailed 

exploration program, including geologic mapping, geochemical and geophysical work, drilling, and trenching 

would be required to determine if any resources exist at the 

Pride mine. The large areas of sand and gravel in the study

'Any elemental concentration more than two times the average 

concentration for that rock type worldwide is considered anomalous.

B10 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
Table 2. Comparison of average USBM geochemical data from the Planet Peak Wilderness Study Area and the Copperstone 

mine, La Paz County, Arizona

Detection limits: gold (Au), 0.005 ppm; arsenic (As), 1 ppm; barium (Ba), 100 ppm; chromium (Cr), 50 ppm; copper (Cu), 1 ppm; iron (Fe), 0.5 percent; lead 

(Pb), 2 ppm; antimony (Sb), 0.2 ppm. <, less than detection limit; ND, no data. Number in parentheses beneath each element value is number of samples upon 

which the average is based]

Area

Copperstone mine l 

Pride mine 

Prospects 0.6 mi south of 

Pride mine. 2

Prospects 1.5 mi southeast of 

Pride mine and New 

Standard mine area. 2

Average for all samples from 

the Planet Peak Wilderness

Au

2.9

.056 

(6)

.009 

(7)

.012 

(7)

.012 

(5)

.013 

(5)

.0295 

(37)

As

11.0

9.2 

(9)

7.0 

(18)

3.7 

(17)

2.3 

(8)

4.3 

(28)

4.6 

(89)

Ba

3,300

510 

(8)

647 

(16)

1,111 

(12)

987 

(3)

3,443 

(23)

1,608 

(76)

Cr

(ppm)

48

76 

(1)

70 

(8)

148 

(3)

<50 

(11)

197 

(8)

128 

(23)

Cu

2,000

5,935 

(9)

4,934 

(18)

3,442 

(17)

10,544 

(11)

5,050 

(29)

5,938 

(94)

Pb

24.0

6.3 

(9)

3.2 

(5)

3.3 

(8)

3.0 

(2)

9.8 

(26)

7.6 

(62)

Sb

1.5

.6

(9)

.5 

(5)

.3

(9)

.4 

(ID

.7 

(25)

.5 

(67)

Fe

(percent)

6.7

26.0 

(9)

17.34 

(18)

22.68 

(17)

32.45 

(11)

28.81 

(29)

24.37 

(94)

Study Area.

Average for similar rock 

types. 3

.004 2.0 500 20 20 .2 ND

'Average element anomalies from Copperstone samples; data supplied by Bill Burton, Cyprus Minerals, June 1988. 

2See figure 3 in Kreidler (1989) for location of prospects. 

3From Levison (1980).

area have no more value than those that cover vast areas of 

the desert in this region, and thus are not likely to be developed. The area has been rated as having low to zero potential for oil and gas because it is underlain by crystalline 

rocks (Ryder, 1982, 1983). The area has no evidence of 

geothermal resources.

ASSESSMENT OF MINERAL RESOURCE 

POTENTIAL

By Robert C. Eppinger, Jocelyn A. Peterson, H. Richard Blank, 

Jr., K. Eric Livo, Daniel H. Knepper, Jr., anc/James A. Pitkin 

U.S. Geological Survey

Jon E. Spencer, Stephen J. Reynolds, and Michael J. Grubensky 

Arizona Geological Survey

Geology

Regional Setting

The geology of the Gibraltar Mountain and Planet 

Peak Wilderness Study Areas is best understood in the context of the regional tectonic setting of southeastern California and western Arizona, where metamorphic core complexes are commonly exposed in the lower plates of 

detachment faults that extend across entire mountain ranges 

and project into the subsurface. The Buckskin Mountains 

lie within a chain of metamorphic core complexes that 

stretches from the Santa Catalina-Rincon Mountains east 

of Tucson, Ariz., to the Sacramento Mountains in California (Coney, 1980; Anderson, 1988). The metamorphic core 

complexes are characterized by a metamorphic basement 

terrane that has been overprinted with a low-angle mylonitic 

and gneissic fabric. Above this mylonitic terrane are 

unmylonitized rocks that commonly contain many low-angle 

faults. These low-angle faults merge into a master 

subhorizonal detachment fault that separates the two terranes (Coney, 1980). Some mylonitization and faulting is 

of Tertiary age as evidenced by deformed Tertiary plutons, 

but at least some of the ductile deformation is significantly 

older (Reynolds and Spencer, 1989). Middle-Tertiary 

mylonitic deformation and low-angle extensional faulting 

followed Late Cretaceous and early Tertiary (Laramide) 

compressional tectonics (Coney, 1980). The drastic changes 

in deformation style can be correlated with changes in the

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona B11
style of convergence of the North American and Pacific 

plates; detachment tectonics was prevalent during the transition from a subducting continental margin to a strike-slip 

margin (Hamilton, 1988).

Several periods of metamorphism affected rocks within 

the Basin and Range province of Arizona during the Mesozoic and Cenozoic eras (Reynolds and others, 1988). The 

most recent took place in middle Tertiary time and was 

largely a retrograde metamorphic event associated with 

mylonitization and detachment faulting. Mylonitization occurred along deeper parts of the detachment-related shear 

zone, whereas discrete detachment faults and associated 

breccias formed at shallower levels (Spencer and Reynolds, 

1989; Reynolds and others, 1988). Chlorite-epidote alteration commonly accompanied brecciation. Eventually lowerplate rocks became unroofed. The Buckskin Mountains 

contain an unroofed mylonite that formed deep within the 

shear system (Rehrig and Reynolds, 1980; Spencer and 

Reynolds, 1989). That the Buckskin Mountains trend northeast rather than northwest, as is more typical in the Basin 

and Range of this region, is attributed to the development 

of transverse foliation arches within the huge metamorphic 

core complex (Spencer and Reynolds, 1989); the western 

one of these arches underlies the Planet Peak Wilderness 

Study Area (Bryant and Wooden, 1989).

Local Geology

The study areas are at the west end of the Buckskin 

Mountains and are geologically quite different from each 

other. The Planet Peak Wilderness Study Area has many 

excellent exposures of lower-plate mylonitic rocks and local 

occurrences of upper-plate rocks and the Buckskin-Rawhide 

detachment fault. In the Gibraltar Mountain Wilderness 

Study Area, however, these rocks are generally covered by 

postdetachment volcanic rocks. In the northwest corner of 

the Gibraltar Mountain study area, older granitic rocks within 

the upper plate are exposed. The following discussion is 

based on mapping by Spencer and Reynolds (see Spencer, 

1989), Grubensky (1989), and Spencer and others (1989) 

and on interpretations from aerial photographs of unmapped 

areas; these were all compiled at a scale of 1:100,000 by 

Spencer (1989). A combination of these maps was used to 

compile figures 2 and 3 of this report.

Lower-pi ate rocks

Lower-plate mylonitic rocks in the Planet Peak Wilderness Study Area are of three types: Early Proterozoic 

gneissic rocks, Cretaceous granite around and including 

Planet Peak, and a Tertiary plutonic suite on the northwest 

side of the study area (Bryant and Wooden, 1989; Spencer 

and others, 1989). These lower-plate rocks consist of variably pegmatitic amphibolite-grade gneiss and coarse- to finegrained intrusive rocks that are overprinted with

shallow-dipping middle-Tertiary mylonitic foliation. Calcsilicate rocks, quartzite, and dolomitic-marble sheets as thick 

as 10 ft are locally interlayered with the crystalline rocks. 

Directly below the detachment fault, the rocks have been 

brecciated and converted to chlorite-epidote breccia and silicified microbreccia. The Cenozoic intrusive rocks consist 

of homogeneous, dark-gray, fine-grained diorite(?) of late 

Oligocene(?) to middle Miocene(?) age. Their mylonitic 

fabric is generally weaker than that of older crystalline rocks, 

and locally the foliation and lineation are weak to absent. 

Low-angle faults mark some contacts of these intrusions 

with the older crystalline rocks. The intrusions generally 

form sill-like bodies along the northwestern part of the Planet 

Peak Wilderness Study Area (fig. 3).

Upper-plate rocks

Upper-plate rocks are composed of a variety of sedimentary and volcanic rocks ranging in age from Paleozoic 

through Tertiary and crystalline rocks of Proterozoic to 

Cretaceous age (Reynolds and Spencer, 1989). In the northwest corner of the Gibraltar Mountain Wilderness Study 

Area (fig. 2) variably foliated granitic rocks are strongly 

fractured in most places. They are fine to medium grained 

and relatively unaltered to somewhat chloritized and 

epidotized in hand specimen; locally they contain minor 

stains of secondary copper-oxide minerals.

The Paleozoic rocks in the Planet Peak Wilderness 

Study Area (fig. 3) are mostly calcareous strata dominated 

by brown dolomitic marble containing light- to dark-brown 

siliceous bands several centimeters thick. At the detachment-fault surface these rocks commonly form breccia fragments within a darker brown Tertiary hydrothermal 

replacement carbonate that is locally vuggy and veined and 

has siliceous selvages. Contacts between Tertiary replacement carbonate and Paleozoic marble are generally gradational.

Undifferentiated Mesozoic rocks on the west-central 

edge of the Gibraltar Mountain Wilderness Study Area are 

composed of quartzite and phyllite, presumably of the 

Buckskin Formation of Reynolds and others (1987). The 

northern part of the Planet Peak Wilderness Study Area has 

several small outcrops of the Jurassic Planet Volcanics of 

Reynolds and others (1987) and one outcrop of the Jurassic^) Vampire Formation of Reynolds and others (1987). 

The Planet Volcanics consist of schistose intermediatecomposition volcanic and metavolcanic rocks containing 

quartz, feldspar, muscovite, and magnetite. The Vampire 

Formation consists of light-colored quartzite that has a basal 

conglomerate.

Tertiary strata are uncommon in the upper-plate rocks 

in the study area, but several units are'present locally in the 

western parts of the study areas. The Gibraltar Mountain 

Wilderness Study Area has a unit of gently to steeply dipping, generally red to brown sandstone, siltstone, and conB12 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
glomerate in the extreme northwest corner. The same unit 

is exposed in the western part of the Planet Peak Wilderness Study Area, and one small outcrop of air-fall tuff is 

interbedded with sandstone on the west side of the study 

area. The northern part of that study area has several small 

klippen of hydrothermal replacement carbonate resting on 

mylonite.

Postdetachment rocks

The oldest postdetachment rocks crop out in the 

northern part of the Planet Peak Wilderness Study Area. 

They are poorly sorted, matrix-supported cobble to pebble 

fanglomerate and sandstone; tuffaceous rocks are present 

locally. Clasts are composed of several Tertiary volcanic, 

sedimentary, and granitic units and of lower-plate mylonite 

and carbonate rocks.

Most of the Gibraltar Mountain Wilderness Study Area 

is underlain by volcanic rocks correlated with the Miocene 

Osborn Wash Formation (Davis and others, 1980; 

Grubensky, 1989). They are as thick as 900 ft near the 

southwest corner of the study area and locally elsewhere 

but are generally less than 500 ft thick. These rocks in the 

northern two-thirds of the study area consist of nearly flatlying olivine-basalt flows with minor interbedded agglomerate, tuff, and sedimentary rocks. The southern one-third 

of the study area is composed of trachytic lava flows and 

pyroclastic deposits including air-fall and ash-flow tuffs and 

surge deposits. Erosion has exposed some plugs, and dikes 

are present locally. Grubensky (1989) believed that the entire trachytic sequence was likely emplaced along the flanks 

of a small stratovolcano, now eroded and buried. Basaltic 

and trachytic rocks are contemporaneous.

Alluvium ranging in age from latest Tertiary to Holocene surrounds Planet Peak and is adjacent to lava flows in 

the Gibraltar Mountain Wilderness Study Area. The larger 

stream beds also contain substantial alluvium. The alluvium 

consists of unconsolidated to poorly consolidated gravel, 

sand, and silt.

Geochemistry

Introduction

A reconnaissance geochemical survey was conducted 

in May 1988 as part of the mineral resource evaluation. The 

principal geochemical sampling media were stream sediments and heavy-mineral concentrates (HMC) collected at 

41 sites within and near the Gibraltar Mountain Wilderness 

Study Area and at 36 sites within the Planet Peak Wilderness Study Area. The stream-sediment sample represents a 

composite of rock and soil exposed in the drainage basin. 

The nonmagnetic fraction of an HMC sample is useful in 

detecting mineralized areas because primary and secondary

ore minerals are commonly found in this fraction. The concentration of ore and ore-related minerals in the HMC sample 

facilitates determination of elements that are not easily detected in bulk stream sediments. Rock samples were collected to provide information on background metal 

concentrations in unaltered bedrock and to identify anomalous metal suites in mineralized areas. Stream-sediment, 

HMC, and rock samples were also collected in mineralized 

areas near the study areas to supplement data from samples 

collected within the study areas.

This geochemical survey was designed to locate altered, mineralized, or geochemically anomalous areas, but 

not to find individual mineral deposits. Detailed geochemical studies to evaluate anomalies found in this reconnaissance 

study have not been undertaken.

Methods

Analytical results and a full description of sampling 

and analytical techniques are found in Eppinger and others 

(1990). Stream-sediment samples were collected from active alluvial channels, generally along first-order streams. 

The samples were sieved with 30-mesh screens, and the 

minus-30-mesh fractions were pulverized for chemical 

analysis. HMC samples were collected at stream-sediment 

sites and panned until most quartz, feldspar, clay, and organic 

matter were removed. These samples were then sieved using 

16-mesh screens. Light minerals, principally quartz and 

feldspar, remaining in the minus-16-mesh fractions were 

removed by heavy-liquid flotation (bromoform, specific 

gravity 2.85). The remaining portions of the HMC samples 

were separated with an electromagnet into magnetic, slightly 

magnetic, and nonmagnetic fractions. Nonmagnetic fractions 

were examined microscopically and by X-ray diffraction 

for mineral content. Both nonmagnetic and slightly magnetic fractions were pulverized for chemical analysis, 

whereas the magnetic fraction was archived. Rock samples 

were examined microscopically and then pulverized for 

chemical analysis.

Stream-sediment and rock samples were analyzed for 

35 elements, and the slightly magnetic and nonmagnetic 

HMC samples were analyzed for the same 35 elements plus 

platinum and palladium, all by semiquantitative emission 

spectrography. In addition, stream sediments and 

rocks were analyzed by more sensitive inductively coupled 

plasma atomic-emission sprectrometry for arsenic, 

bismuth, cadmium, antimony, and zinc. Stream-sediment and 

rock samples were analyzed for low-level gold (detection 

limit is 50 ppb for stream sediments and 2 ppb for rocks) 

by flame and graphite-furnace atomic absorption 

methods, respectively.

Histograms showing the general distribution and range 

of the data were constructed for selected elements. Boundaries between background and anomalous concentrations of 

these elements were chosen by visual inspection of the diaMineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona 813
grams and plots, by percentiles, and by average crustal 

abundances for elements given by Levinson (1980).

Results

Dominant lithologies in the two study areas are easily 

seen in the geochemical data. Abundant postdetachment 

Tertiary basalts in the Gibraltar Mountain Wilderness Study 

Area are reflected by elevated concentrations of the mafic 

elements nickel, cobalt, manganese, and iron. In the northwest corner of that study area, upper-plate Proterozoic granitic and Tertiary arkosic rocks are reflected by the more 

felsic elements lanthanum, niobium, and thorium. In the 

Planet Peak Wilderness Study Area, outcrops of lower-plate 

mylonitic granite and granitic to fine-grained felsic gneiss 

are common, and these rocks are reflected by the strong 

felsic elements lanthanum, yttrium, niobium, tin, beryllium, 

thorium, and barium. Sphene is the likely host mineral for 

most of the elements in this felsic suite, as it is known to 

contain lanthanum, yttrium, niobium, tin, and thorium (Deer 

and others, 1962) and is the dominant mineral phase, along 

with barite, in the HMC samples from the areas that are 

rich in these elements.

Generally low-level anomalies (meaning anomalous 

but near the threshold value given in table 3) possibly related 

to mineralization are found in several places within the study 

areas (see figs. 4 and 5). Metals found in anomalous concentrations in and around the study areas include silver, 

barium, bismuth, cadmium, cobalt, copper, iron, molybdenum, manganese, lead, antimony, tin, thorium, tungsten, 

and zinc. Gold was detected in only a few samples from 

near the Pride and Mammon mines, discussed below. Table 

3 lists metals found in anomalous concentrations, the ranges 

in concentration, and threshold values used in this study.

Samples from the Mammon mine area (fig. 4, area 1) 

contain anomalous concentrations of gold, cobalt, copper, 

and iron, and slightly anomalous concentrations of silver, 

barium, bismuth, molybdenum, lead, antimony, and zinc. 

One rock sample from this area contains 50 ppb gold. These 

data agree with analyses of rock samples from the Mammon mine by Scott (1989), who reported detectable gold, 

anomalous copper and zinc, and weakly anomalous silver 

and molybdenum.

Samples from one site in each of seven drainages in 

the Gibraltar Mountain Wilderness Study Area have weakly 

anomalous molybdenum and zinc (fig. 4, areas numbered 

2). Other elements are found in anomalous concentrations in 

one or more samples from these drainages (fig. 4). Anomalous concentrations for most of these elements are near the 

threshold value. However, concentrations in one HMC 

sample from a drainage in the southern part of the study 

area are highly anomalous for zinc (2,000 ppm), relatively 

strongly anomalous for copper and molybdenum, and less 

anomalous for cadmium, lead, and tungsten. In that sample, 

hematite, trace amounts of fresh pyrite, and a single grain of

sphalerite were identified. Ore and ore-related minerals were 

not found in any other HMC sample in areas numbered 2 on 

figure 4. Alluvium in all seven drainages is made up almost 

exclusively of postdetachment Tertiary basalt and trachyte. 

Lower-plate biotite-gneiss cobbles and pebbles were observed 

only in drainages near the Mammon mine.

Gold was found in only one sample outside of the 

Mammon mine area, in area 3 (fig. 4). Numerous calcite 

veins were observed cutting vesicular basalt in that area, 

and one vein sample contains 200 ppb gold. No other 

anomalous metal concentrations were found in that sample. 

One HMC sample from that drainage contains slightly 

anomalous manganese, and no ore or ore-related minerals 

were observed in that sample.

HMC samples from a drainage west of the study area 

(fig. 4, area 4) are highly anomalous in lead (15,000 ppm); 

anomalous in copper, iron, manganese, molybdenum, and 

antimony; and slightly anomalous in cobalt and tin. Minerals identified in the HMC samples include specularite, 

malachite, and fairly abundant malleable lead. 

Postdetachment Tertiary basalt, upper-plate metasedimentary 

and granitic rocks, and bull quartz with hematite-coated 

joint surfaces (no gold detected) were observed in stream 

cobbles in this area. Numerous prospects and adits are found 

immediately northwest of the sampled drainages (the Lion 

Hill mine group, Spencer and Welty, 1989).

Samples from the Pride mine area (fig. 5, area 5) 

contain anomalous concentrations of gold, barium, cobalt, 

copper, iron, and lead and slightly anomalous concentrations 

of silver, arsenic, manganese, molybdenum, and tungsten. 

Gold was found in two samples a stream sediment (50 

ppb) and a rock (200 ppb) in amounts similar to gold 

reported by Kreidler (1989). Malachite, chrysocolla, and a 

trace of galena were identified in an HMC sample from the 

Pride mine.

Samples collected in the New Standard mine and Little 

Golden prospect areas (fig. 5, area 6) have anomalous concentrations of gold, cobalt, copper, and iron, and slightly 

anomalous concentrations of silver, barium, bismuth, molybdenum, lead, antimony, and tungsten. Gold was detected 

in five rock samples (two at 10 ppb, one at 100 ppb, and 

two at 200 ppb) from this area.

Several localities within the Planet Peak Wilderness 

Study Area have low-level anomalous concentrations of one 

or more elements. Samples from area 7, west of Squaw 

Peak (fig. 5), contain anomalous concentrations of barium, 

copper, and manganese and slightly anomalous concentrations of silver and molybdenum. Traces of pyrite and fluorite were observed in HMC samples, and specularite, 

chrysocolla, and malachite were observed locally in alluvium. Bedrock in area 7 is composed of lower-plate 

mylonitic gneiss. The Buckskin-Rawhide detachment fault 

and upper-plate metasedimentary rocks have been mapped 

immediately east of area 7 on Squaw Peak (Spencer, 1989), 

and numerous prospects are located in the vicinity.

B14 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
Table3. Summary statisticsfor anomalous elementsfrom stream sediments and heavy-mineral concentrates 

collected by the USGS in and around the Gibraltar Mountain and Planet Peak Wilderness Study Areas, 

Arizona.

[ n, number of samples; N, not detected; L, detected but lower than determination limit; G, greater than upper determination limit. 

Analyses are by emission spectrography, except for elements having an "-a" suffix (flame atomic absorption) or an "-i" suffix 

(inductively coupled plasma-atomic emission spectrometry). Values are parts per million unless noted otherwise]

Element (Lower 

determination limit)

Level of concentrates

Minimum Maximum

Nonmagnetic heavy-mineral concentrates (n =

Ag (1)

Ba (50)

Be (2)

Co (20)

Cu (10)

Mo (10)

Nb (50)

Pb (20)

Sb (200)

Sn (20)

Th (200)

W (50)

Y (20)

Zn (500)

N

150

N

N

N

N

N

N

N

N

N

N

100

N

L N

G(10,000) 10,000

5 L

500 N

200 L

100 N

300 100

15,000 20

200 N

700 20

500 N

100 N

700 500

2,000 N

Weakly magnetic heavy-mineral concentrates (n

Co (20)

Cu (10)

Fe (0.1%)

La (100)

Mn (20)

Mo (10)

Ni (10)

Pb (20)

Sn (20)

Y (20)

Zn (500)

L

10

5

N

500

N

L

N

N

30

N

500 70

1,000 70

30 10

1,000 300

G( 10,000) 1,500

20 L

500 50

300 30

50 N

500 100

1,000 N

Threshold

77)

L

G(10,000)

5

200

30

L

200

100

L

100

300

L

700

L

= 77)

200

150

30

1,000

5,000

15

150

100

30

500

L

Number of 

samples above 

threshold

1

37

16

1

3

13

8

12

1

4

5

6

10

2

4

12

11

4

9

5

10

4

4

3

9

Stream sediments (n = 77)

Ag (0.5)

Au-a (SOppb)

Ba (20)

Bi-i (2)

Cd-i (0.1)

Co (10)

Cu (5)

Fe (0.05%)

La (50)

Mn (10)

Mo (5)

Nb (20)

Sb-i (2)

Y (10)

Zn-i (5)

N

N

100

N

0.1

L

L

1.5

N

100

N

L

N

10

20

0.7 N

50 N

2,000 500

5 N

1.2 0.25

150 20

500 20

10 3

200 100

700 300

5 N

50 L

7 N

100 30

180 52

L

50

1,500

5

1

100

70

7

150

700

5

30

2

70

100

6

1

2

1

4

1

4

4

17

6

2

10

3

7

2

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona B15
114° 10' 114°05' 

 I 

34°15' -

34°10'

APPROXIMATE BOUNDARY

OF

GIBRALTAR MOUNTAIN

WILDERNESS STUDY AREA

(AZ-050-012)

(Ag, Mo, Mn, Sn, Zn)

Pb, Cu, Fe, Mn, Mo, 

Sb (Co, Sn)

V / ! V ,

X'Au \ r 

/ (Mn) / / . ) \ 

,Mot Ai/"®IUv3 ^VN

An 7nt .£=-9- <' VM >^>J^ Gibraltar I I '

(Mo, Zn, Sn)

Zn, Mo, Cu (Cd, Pb, W)

3 MILES

Figure 4. Stream drainages having geochemical anomalies indicate that elements are weakly anomalous. Numbered areas 

identified from analyses of stream-sediment, heavy-mineral- are discussed in text. Ag, silver; Au, gold; Ba, barium; Bi, bisconcentrate, and rock samples and from ore-related minerals muth; Cd, cadmium; Co, cobalt; Cu, copper; Fe, iron; Mn, 

identified in nonmagnetic heavy-mineral concentrates collected manganese; Mo, molybdenum; Pb, lead; Sb, antimony; Sn, 

in and near Gibraltar Mountain Wilderness Study Area, La tin; W, tungsten; Zn, zinc. 

Paz County, Arizona. Large dots are sampling sites. Parentheses

B16 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
Several wide-spread samples from area 8 (fig. 5) 

southeast of Planet Peak have low-level anomalous molybdenum. A few scattered samples in area 8 have low-level 

anomalous concentrations of silver, cadmium, iron, antimony, and (or) tungsten. Traces of scheelite and fluorite 

were observed in HMC samples. Locally, specularite, malachite, chrysocolla, and epidotized and chloritized rocks were 

observed in alluvium, which consists principally of locally 

derived mylonitic, lower-plate, coarse- to fine-grained felsic 

to intermediate gneisses and granite. Prospects are located 

in area 8 south of Planet Peak (fig. 3).

Wide-spread samples from area 9 (fig. 5) are characterized by a low-level lead anomaly, and scattered samples 

exhibit low-level anomalies of silver, cadmium, copper, and 

antimony; one HMC sample in the northern part has highly 

anomalous concentrations of cobalt and iron. Ore and orerelated minerals identified in HMC samples include 

specularite, fluorite, chrysocolla, and trace amounts of vanadinite. A trace of acanthite (Ag2S) was identified in the 

cobalt-rich sample. In alluvium, specularite and epidote are 

common, and malachite was observed at two sites. Alluvial 

cobbles consist principally of mylonitic lower-plate felsic

114° 02'30" 114°00' 113° 52'30"

34° 

12' 

30-

APPROXIMATE BOUNDARY

OF

PLANETPEAK

WILDERNESS STUDY AREA

(AZ-050-013)

34° 

10'

34° 

07- 

30"

Pride mine

(Cu±Cd, Sb,

Fe.Mn, Mo)

Little Golden 

prospect

Au, Co, Cu, Fe (Ag, Ba, 

Bi, Mo, Pb, Sb, W)

3 MILES

Figure 5. Stream drainages having geochemical anomalies 

identified from analyses of stream-sediment, heavy-mineral-concentrate, and rock samples and from ore-related 

minerals identified in nonmagnetic heavy-mineral concentrates collected in and near Planet Peak Wilderness Study 

Area, La Paz County, Arizona. Large dots are sampling

sites. Parentheses indicate elements that are weakly anomalous. 

Numbered areas are discussed in text. Ag, silver; As, antimony; 

Au, gold; Ba, barium; Bi, bismuth; Cd, cadmium; Co, cobalt; 

Cu, copper; Fe, iron; Mn, manganese; Mo, molybdenum; Pb, 

lead; Sb, antimony; W, tungsten.

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona 817
to intermediate gneiss. Outcrops of mylonitic lower-plate 

gneiss are locally chloritized; joints are commonly 

coated with epidote; and specularite veinlets are found 

locally.

Samples from area 10 (fig. 5) contain weakly anomalous copper; a few scattered samples have low-level anomalous concentrations of cadmium and antimony and single-site 

anomalies for iron and manganese. One sample contains 

anomalous iron, copper, and molybdenum and slightly 

anomalous silver. Minerals identified in HMC samples from 

area 10 include abundant barite and anatase, small amounts 

of fluorite, and traces of scheelite. Cobbles in alluvium consist of mylonitic granite, biotite gneiss, and felsic gneiss. 

Specularite, epidote, malachite, and chrysocolla were observed locally in alluvium. Mylonitic-gneiss and granite 

bedrock is locally chloritized, coated with epidote on joints, 

and, in one area, cut by specularite veinlets. Hematite 

pseudomorphs after pyrite were observed locally along joint 

surfaces. Prospects in mylonitic gneiss and granite in the 

southern part of area 10 contain pods and lenses rich in 

specularite and chrysocolla.

Interpretation

A common suite of elements found in anomalous 

concentrations is evident in analyses of stream-sediment, 

HMC, and rock samples from around the Pride, Mammon, 

and New Standard mines, the Little Golden prospect, and 

prospects near Squaw Peak. Anomalies are relatively strong 

for copper and iron, relatively weak for silver, barium, bismuth, molybdenum, lead, and tungsten, and variable for 

gold, arsenic, cobalt, manganese, antimony, and zinc. East 

of the Planet Peak Wilderness Study Area, strongly to weakly 

anomalous concentrations, generally of this same suite of 

elements along with tin, vanadium, and thorium, were found 

in samples from mineralized areas in the Swansea Wilderness 

Study Area (Tosdal and others, 1990a). South of Cactus 

Plain, the same general suite, along with strontium, was 

found in samples from the Plomosa mining district (Tosdal 

and others, 1990b).

The numerous single-site anomalies for molybdenum 

and zinc with or without silver, copper, manganese, and tin 

in the Gibraltar Mountain Wilderness Study Area (fig. 4, 

areas numbered 2) are difficult to assess. Most of these are 

weak anomalies in samples from generally basaltic terrain 

where ore-related minerals were not found in HMC samples. 

Locally high manganese and iron content in samples 

throughout much of this study area suggests that adsorption 

of metals by manganese oxide and iron oxide minerals may 

be the reason for the anomalous metal concentrations. The 

adsorbed metals may have been derived from fluids leaking 

along fractures or joints in basalt from possible underlying 

concealed mineral deposits related to the detachment fault. 

Any concealed deposits would likely be similar to those in 

the Mammon mine area.

A notable exception to metal adsorption is the HMC 

sample from a drainage in the southern part of the Gibraltar 

Mountain Wilderness Study Area, where a suite of strongly 

anomalous zinc, molybdenum, copper, cadmium, lead, and 

tungsten is found in samples that contain traces of fresh 

pyrite and of sphalerite, the likely host mineral for the zinc 

and cadmium. This metal suite and the presence of fresh 

sulfide minerals suggest that exposed sulfide deposits may 

be present in the drainage. Follow-up work would be necessary to ascertain whether the metals in this drainage are 

from deposits that are related to the Buckskin-Rawhide detachment fault, which are common to the region, or from 

some other type of deposit.

Similarly, the gold-bearing calcite veins cutting basalt in area 3 (fig. 4) need more detailed study to determine 

whether the detection of gold in calcite can be repeated 

and, if so, to determine the extent and nature of the anomalous gold. The lack of associated anomalous metals in area 3 

is problematic and suggests that the 200-ppb gold value in 

calcite is spurious. However, further evaluation is warranted 

because many deposits in the Cienega mining district were 

relatively rich in gold (Spencer and Welty, 1986a, 1989).

The highly anomalous lead concentrations in area 4 

(fig. 4) probably have a human origin. The malleable lead 

fragments from the HMC sample were generally irregular 

pieces and fine shavings, a characteristic of lead shot found 

in HMC samples by Griffitts and others (1984). Anomalous 

antimony, a commonly used hardener in lead shot, was also 

found in one of the samples. However, other anomalous 

metals found in area 4 (copper, cobalt, iron, manganese, and 

molybdenum), the presence of malachite and specularite in 

HMC samples, and the presence of hematite-coated bull 

quartz in drainage cobbles are all favorable evidence for 

mineralization in the area. Any mineral deposits in area 4 

are most likely similar to known deposits immediately to the 

northwest of the area, where milky-quartz-bearing veins in 

adits and prospects are found locally along faulted upper-plate 

rocks (sites G, H, and I in fig. 2 of Spencer and Welty, 1989).

Samples from most of the Planet Peak Wilderness 

Study Area exhibit low- to moderate-level anomalies for 

many of the elements found in anomalous quantities (noted 

above). However, scattered single-site anomalies for one to 

three of the elements are most common; samples having 

large anomalous multi-elemental suites are rare. This information, coupled with numerous observations of cobbles, 

pebbles, and veins of specularite, chrysocolla, hematite after pyrite, epidote, and chlorite, along with fluorite and traces 

of pyrite, vanadinite, and acanthite in HMC samples 

throughout the study area, suggests that mineralizing 

proccesses were widespread in the area. Mines and prospects west and south of Planet Peak are attributed to mineralization related to the Buckskin-Rawhide detachment fault, 

in the lower-plate rocks and along the detachment fault 

(Spencer and Welty, 1989). The numerous samples having 

anomalous metals and the ore-related minerals in samples

B18 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
throughout the study area most likely reflect replacement or 

fracture-filling in lower-plate rocks along high-angle 

structures below and locally along the detachment surface, 

as depicted at sites A, B, and F in figure 2 of Spencer and 

Welty (1989). These deposits are typically small compared 

to deposits in upper-plate rocks (Spencer and Welty, 1986b). 

Thus, the geochemical anomalies in the Planet Peak 

Wilderness Study Area probably reflect numerous small 

sporadic copper-iron occurrences, rather than a large 

concealed deposit.

A subtle metal zonation, also exhibited in the Planet 

Peak Wilderness Study Area, results from slightly 

molybdenum-rich rocks to the south and slightly to 

moderately lead-rich rocks to the north. This zonation may 

reflect local chemical variation in the large-scale detachmentrelated mineralization of the region. Spencer and Welty 

(1989) reported that copper- and iron-sulfide minerals formed 

early in the genesis of many of the Buckskin-Rawhide 

detachment related deposits. A reduced chemistry for early 

mineralizing fluids is suggested by the presence of trace 

amounts of fresh pyrite (several occurrences), galena (Pride 

mine), and acanthite (north end of the Planet Peak Wilderness Study Area) and by the more common observation of 

hematite pseudomorphs after pyrite in samples throughout 

the study area.

Geophysics

Aeromagnetic and Gravity Data

Aeromagnetic and gravity data for a region encompassing the Gibraltar Mountain and Planet Peak Wilderness 

Study Areas have been compiled and examined for possible 

indications of geologic features that may have a bearing on 

mineral resource evaluations. No new geophysical field work 

has been performed for the present investigation.

Two sources of aeromagnetic data were used: a survey of the Prescott 1° by 2° quadrangle, Ariz., flown for the 

National Uranium Resource Evaluation (NURE) program, 

and a survey of the Needles 1° by 2° quadrangle, Calif, and 

Ariz., flown for the USGS. The Prescott traverses were 

made on east-west headings at spacings of 1 mi and at a 

nominal height of 400 ft above ground (Western Geophysical Company of America, Aero Service Divison, 1979). 

The Needles traverses were similarly on east-west headings 

but at 0.5-mi spacings and a nominal drape height of 1,000 

ft (U.S. Geological Survey, 1981). Gravity data were obtained from files of the Defense Mapping Agency (DMA) 

through the National Center for Solar-Terrestrial and Geophysical Data (Boulder, CO 80303) and from J.D. Hendricks 

of the USGS (Flagstaff, AZ 86001).

A map of the residual total-intensity aeromagnetic 

field was computed by removal of the International 

Geomagnetic Reference Field and merging of the two data

sets. This map (fig. 6) reveals a striking contrast in magnetic expression of the postdetachment- and the detachmentrelated rocks. Postdetachment volcanic rocks, chiefly basalts, 

produce intense short-wavelength anomalies, while the detachment-related mylonitic crystalline rocks produce 

anomalies that are much more subdued and of longer wavelength. The boundary between these two magnetic domains 

traces an arc of a circle centered near the southeast corner 

of the map (fig. 6) and suggests broad domal uplift. 

Antiforms and synforms of the folded (corrugated) detachment terrane correspond roughly to long-wavelength, northeast-trending anomaly highs and lows east of the boundary 

within the circle. The Planet Peak Wilderness Study Area is 

characterized by such anomalies, in contrast to the Gibraltar 

Mountain Wilderness Study Area, which is characterized 

by the short-wavelength postdetachment-related anomalies. 

Because gradients of anomalies over alluviated areas imply 

only slightly greater depths to source as compared to those 

over bedrock, the alluvium is most likely rather thin. The 

broad anomalies associated with the detachment terrane 

cannot be traced through the belt of short-wavelength 

anomalies produced by postdetachment volcanic rocks concealed beneath Cactus Plain, southwest of the study areas.

An area of strong field disturbances on the south margin of the map (fig. 6) includes outcropping Phanerozoic 

granitic rocks and extensive alluvium. These disturbances 

may reflect the presence of a largely concealed granitic 

pluton. Proterozoic crystalline rocks of the upper plate exposed in the northwestern and northeastern parts of the 

mapped region have an aeromagnetic expression intermediate between that of postdetachment volcanic rocks and the 

lower-plate mylonites in wavelength and intensity of 

anomalies. Upper-plate rocks such as these are probably 

present beneath the alluvium north of Bill Williams River 

near long 113°55'W.

The complete-Bouguer gravity anomaly field 

(fig. 7) was computed from about 235 stations of the 

DMA plus Hendricks regional set and additional stations of 

two detailed Pacific Arizona Crustal Experiment profiles. 

All data reductions and terrain corrections employed a standard Bouguer density of 2.67 grams per cubic centimeter 

and followed conventional USGS procedures (see, for 

example, Cordell and others, 1982).

The map reveals a non-uniform decrease in anomaly 

levels from a maximum of about -41 milligals (mGal) in 

the extreme southwest corner to a minimum of about -93 

mGal near the northeast corner. This overall gradient is 

associated with a first-order structural grain and is accompanied by a mild northeasterly increase in average topographic elevation. Superimposed on the regional gradient 

are broad lows of about 10-mGal amplitude that roughly 

correspond to areas of outcrop of postdetachment volcanic 

rocks. No lows of more than a few milligals amplitude are 

found anywhere on Cactus Plain. This confirms that no 

great thickness of alluvial fill is present and suggests that

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona B19
the buried postdetachment volcanic rocks inferred from 

magnetic data have thinned southward. Estimates of depth 

to basement made for this region on the basis of gravity 

data generally yield depths less than 1,000 ft except for a 

small area near the southwest corner of the map, where the 

depth of fill may be as great as 2,500 ft (Oppenheimer and 

Sumner, 1980).

Landsat Thematic Mapper Images and Interpretation

Background

A digitally processed Landsat Thematic Mapper (TM) 

color-ratio composite image was used to discern the location 

and distribution of spectral reflectance characteristics that 

suggest the presence of selected minerals commonly 

associated with hydrothermally altered rocks in the Gibraltar 

Mountain and Planet Peak Wilderness Study Areas (figs. 8, 

9). The spectral bands of the TM system are too broad to

allow individual mineral species to be identified; however, 

anomalous areas of two spectrally different groups of 

minerals (groups 1 and 2) can be distinguished (Knepper, 

1989).

Group 1 minerals consist of the common ferric iron 

oxide, hydroxide, and sulfate minerals hematite, goethite, 

lepidocrocite, and jarosite. These minerals commonly form 

during the weathering of pyrite and, consequently, are 

generally associated with weathered, hydrothermally altered 

rocks. The group 1 minerals are not, however, diagnostic of 

hydrothermally altered rocks because they can also form 

from the oxidation of iron-bearing minerals in unaltered 

rocks (Knepper, 1989, p. 13). Areas having concentrations 

of the group 1 minerals commonly indicate that the 

rocks have been altered and are said to have group 1 

anomalies.

Group 2 minerals are diverse, including hydroxylbearing and (or) hydrated minerals (clay minerals, micas, 

gypsum, alunite, and jarosite) and carbonate minerals (calcite

114°00' 113°45'

34°15'

34°00'

4 MILES 

I

Figure 6. Residual total-intensity aeromagnetic map of region including Gibraltar Mountain (GM) and Planet Peak (PP) Wilderness 

Study Areas, California and Arizona. Contour interval 20 nanoteslas; hachures indicate closed areas of lower values.

B20 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
and dolomite), all of which are spectrally similar in the TM 

bands. Jarosite shares common spectral characteristics with 

both group 1 and group 2 minerals and is included in both 

classes. Although the group 2 minerals are not restricted to 

altered rocks, they are commonly important constituents of 

altered rocks or are derived from the weathering of altered 

rocks (Knepper, 1989). Areas indicating group 2 minerals 

are said to have group 2 anomalies.

In some places, spectral characteristics indicate that 

areas of group 1 minerals overlap those of group 2 minerals 

such that both groups of minerals may be present. Such 

areas are said to have group 3 anomalies and to contain 

group 3 minerals.

Interpretive Methods

Interpretation of potentially hydrothermally altered 

rocks on the TM color-ratio composite image consists of

visually outlining distinct areas of unique colors related to 

group 1, group 2, or group 3 minerals (Knepper, 1989, p. 

17). Areas clearly related to unaltered rocks or sediments 

were excluded from the analysis. The remaining areas are 

considered anomalous and may reflect local exposures of 

hydrothermally altered rocks. Additional field studies would 

be necessary to document thoroughly the nature of any altered rocks associated with the anomalies and to eliminate 

possible false anomalies.

Light to moderate vegetation obscures parts of the 

study area. Because of the rugged topography, combined 

with the relatively low solar elevation angle at the time the 

TM data were acquired (25°), a significant part of the study 

area is in deep to moderate shadow on the TM image. The 

Mammon mining district in particular is partially covered 

by deep shadows. Vegetation and shadows, then, have obscured some areas of surface rocks and soils that could 

otherwise be spectrally analyzed for evidence of possible

114°00' 113°45'

34°15'

34°00'

GM

4 MILES 

I

Figure 7. Complete-Bouguer gravity anomaly map of region including Gibraltar Mountain (GM) and Planet Peak (PP) Wilderness 

Study Areas, California and Arizona. Contour interval 2 milligals; hachures indicate closed areas of lower values. Open circles 

represent gravity stations. ,

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona B21
alteration minerals. Even so, several anomalous areas were 

identified (figs. 8, 9); these anomalies, because of the masking effects of vegetation and shadows, may represent only 

parts of more extensive areas.

Possible Areas of Hydrothermally Altered Rocks

The Cienega mineral district west of the Gibraltar 

Mountain Wilderness Study Area is clearly anomalous in 

group 1, group 2, and group 3 minerals (fig. 8). The 

anomalies in this district appear to reflect altered host rock 

at least partly contained in mine dumps and tailings. Within 

the study area, two large areas of iron oxide (group 1) 

anomalies are present along with smaller local anomalies 

of group 1, group 2, and group 3 minerals. The large areas 

of iron oxide (group 1) anomalies appear to reflect unaltered bedrock. The large anomaly in the southern part of 

the study area is mostly associated with the trachytic rocks. 

The large anomaly along the northeast side of the study 

area has spectral characteristics that suggest iron oxide 

anomalies within basin sediments, but the area is primarily 

underlain by basalt These large anomalies are probably 

due to weathering of the underlying rock but could indicate 

subtle large-scale hydrothermal alteration, although such 

alteration was not seen in the field. The small anomalies 

labeled lv appear to be caused by iron oxide alteration 

formed through weathering of basalt outcrops. The remaining anomalies, representing hydrothermal alteration and 

mapped from the TM color-ratio composite image, are in a 

variety of rock types throughout the study area; these may 

mark small areas of previously unrecognized hydrothermally 

altered rocks. Note the group 3 anomaly in the southern 

part of the study area. All group 2 anomalies and the northernmost group 1 anomaly east of the study area possibly 

represent dispersed anomalies or disturbed ground. Further 

study, beyond the scope of this assessment, would be needed 

to evaluate these areas.

Near the north edge of the Planet Peak Wilderness 

Study Area are areas clearly anomalous in group 3 minerals (fig. 9). These anomalies appear to reflect altered host 

rock in and near the Mineral Hill and Planet subdistricts of 

the Planet mineral district. Within the study area, 

two moderate-sized and several small iron oxide (group 1) 

anomalies are very localized. The anomaly northwest of 

Squaw Peak lies just west of mined ground. One small 

group 3 anomaly is near the north border of the study area. 

No group 2 anomalies were detected, and no strong group 1 

anomalies were found in the study area by using Landsat TM 

data.

Aerial Gamma-ray Spectrometry

Natural radioelement distribution in the study areas 

was evaluated by examining available data from aerial 

gamma-ray Spectrometry surveys of the Needles and Prescott

1° by 2° quadrangles (Geodata International, Inc., 1979, 

and Western Geophysical Company of America, Aero Service Division, 1979, respectively). These surveys acquired 

the data along east-west flightlines 400 ft above ground 

level at intervals of 3 mi (Needles) and 1 mi (Prescott). The 

three flightlines that cross the Gibraltar Mountain Wilderness Study Area, which is wholly within the Needles quadrangle, provide about 5-percent coverage of the study area 

because an aerial gamma-ray survey at 400 ft above ground 

level effectively measures terrestrial gamma radiation from 

a swath 800 ft wide along each flightline. The Planet Peak 

Wilderness Study Area is approximately split between the 

Needles and Prescott quadrangles. No flightlines cross the 

Needles part of the study area, and seven flightlines cross 

the Prescott part. Resulting coverage for the Planet Peak 

study area is about 7.5 percent, all within the east half of 

the study area. For each study area, coverage represents a 

reconnaissance sampling of the near-surface (0- to 18-in. 

depth) distribution of the natural radioelements potassium 

(K), uranium (eU), and thorium (eTh). The prefix e (for 

equivalent) denotes the potential for disequilibrium in the 

uranium and thorium decay series.

Preliminary, relatively small-scale radioelement contour maps that include the Gibraltar Mountain Wilderness 

Study Area indicate that it is characterized by the relatively 

low concentrations of 1.2 percent K, 2.5 ppm eU, and 5 

ppm eTh. Generally, in terranes where igneous rocks 

predominate, higher radioelement concentrations relate to 

more silicic, more radioactive rocks and to detritus derived 

from them, whereas lower concentrations relate to less silicic, 

less radioactive rocks and their detritus. Low concentrations 

within the Gibraltar Mountain Wilderness Study Area, reflect 

the abundant Miocene olivine-basalt flows that dominate 

the northern two-thirds of the study area. Relatively more 

silicic and alkalic trachytic volcanic rocks, present in the 

southern one-third of the study area, would be expected to 

have higher radioelement concentrations than the basalts 

to the north; however, the reconnaissance coverage for 

the study area precludes detection of these lithologic 

contrasts.

Radioelement concentrations derived from relatively 

small-scale contour maps that include the Planet Peak Wilderness Study Area are 2 to 3 percent K, 4 to 5 ppm eU, 

and 10 to 15 ppm eTh. Higher concentrations of 3 percent 

K, 5 ppm eU, and 15 ppm eTh in the northern part of the 

study area may reflect the presence of silicic rocks of the 

Planet Volcanics and sedimentary rocks, whereas lower 

concentrations of 2 to 2.5 percent K, 4 ppm eU, and 10 to 

12.5 ppm eTh to the south are more characteristic of the 

mylonite. The concentrations measured in both study areas 

are thought of as normal (not anomalous) levels for the 

rocks present in these areas, and the widely spaced reconnaissance flightline coverage precludes deriving any direct 

information on mineral resource potential from the aerial 

gamma-ray data.

B22 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
114° 10'

34°15' -

34° 10'

114°05' 

I

APPROXIMATE BOUNDARY

OF

GIBRALTAR MOUNTAIN

WILDERNESS STUDY AREA

(AZ-050-012)

Figure 8. Distribution of potentially hydrothermally altered 

areas in and near Gibraltar Mountain Wilderness Study Area,

(see text): 1, group 1 minerals; 2, group 2 minerals; 3, groups 

1 and 2 minerals. Suffixes: v, probable iron oxide alteration

Arizona, based on interpretations of Landsat Thematic Mapper from weathering of basalt; s, iron oxide anomalies associated

color-ratio composite images. Numbered areas (1-3) contain with spectral characteristics of basin sediments; u, iron oxide

various combinations of spectrally different mineral groups anomalies associated with trachytic rocks.

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona B23
General Discussion of Mineral and Energy 

Resources

Deposits Near Detachment Faults

As indicated previously, several mineral districts lie 

adjacent to or partly within the study areas the Cienega,

Mammon, Midway, Planet, Pride, and Swansea. These districts and their subdistricts produced copper, gold, and silver and contain deposits characterized by specularite 

(hematite) and copper carbonate, silicate, and oxide minerals accompanied by quartz, barite, fluorite, chlorite, manganese oxides, and relict (oxidized) pyrite (Spencer and Welty, 

1989). Ore deposits are found in several settings in both

114° 02 "30" 114°00' 113°52'30"

Planet mineral 

3

district

34° 

12' 

30"

APPROXIMATE BOUNDARY

OF

PLANET PEAK

WILDERNESS STUDY AREA

(AZ-050-013)

34°

10'

34"

07' | 

30"

3 MILES 

J

Figure 9. Distribution of potentially hydrothermally altered 

areas in and near Planet Peak Wilderness Study Area, Arizona, based on interpretations of Landsat Thematic Mapper 

color-ratio composite images. Numbered areas (1-3) contain various combinations of spectrally different mineral groups 

(see text): 1, group 1 minerals; 2, group 2 minerals; 3, groups 

1 and 2 minerals.

B24 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
lower- and upper-plate rocks of the Buckskin-Rawhide detachment fault: (1) as massive replacements in carbonate 

slices within the lower plate, (2) as fracture fillings and 

possible local replacements in shear zones in lower-plate 

crystalline rocks, (3) as massive replacements in upper-plate 

metasedimentary rocks adjacent to the fault, (4) as fracture 

fillings and possible replacements in silicified, iron-stained, 

and brecciated rocks within and adjacent to the detachment 

fault, and (5) as fracture fillings and local replacements in 

high-angle shear zones in upper-plate rocks (Spencer and 

Welty, 1989). The ore deposits of the Cienega mineral district contain more gold than other deposits of the Buckskin 

Mountains, are more than 1,000 ft above the detachment 

fault, and are spatially associated with Mesozoic thrust faults.

Deposits similar to those in the Buckskin Mountains 

are found farther west in the Whipple Mountains (Ridenour 

and others, 1982; Wilkins and Heidrick, 1982). However, 

some deposits that may be associated with detachment faults 

contain primarily gold without copper or hematite (Frost 

and Watowich, 1987; Drobeck and others, 1986). Although 

both gold and copper-iron types of deposits are spatially 

associated with detachment faults, the exact relation between the two remains somewhat obscure. It is thought that 

the major function of the detachment fault was to serve as 

conduits for hydrothermal fluids. Most copper-hematite 

mineralization appears to have been contemporaneous with 

movement along the fault (Wilkins and others, 1986; Spencer and Welty, 1986b) and to have consisted of early development of copper and iron sulfide minerals, followed by 

deposition of hematite, and then by formation of copper 

oxide, silicate, and carbonate minerals (Spencer and Welty, 

1986b). The mineralizing process most likely resulted from 

the movement of metal-rich basinal brines out of the basins 

during detachment faulting (Wilkins and others, 1986). Frost 

and Watowich (1987) suggested that copper concentrations 

formed in deeper parts of a detachment system, whereas the 

gold seems to have been localized in the uppermost crustal 

levels where brittle deformation was predominant. These 

authors suggested that the copper deposits generally lack 

economic amounts of gold and that the two types of deposits 

(gold and copper-iron) probably represent end members of 

the types of deposits that could form along detachment faults. 

In other words, the deposits are zoned on a regional scale 

such that copper-iron deposits formed in the deepest parts of 

the detachment system, whereas gold deposits formed near 

the surface close to the headwalls of the detachment faults.

The above assertions about the distribution of gold 

and copper-iron deposits, if correct, and the occurrence of 

gold in the Cienega district well above the detachment 

surface could indicate that the copper-hematite deposits in 

the immediate vicinity of the study areas are unlikely to 

contain significant amounts of gold. However, the production 

of small amounts of gold from the Mammon and Pride 

mines and the presence of gold in several of the geochemical samples suggest that gold may indeed be present at the

detachment surface as well as in rocks well above the fault 

plane. Therefore, undiscovered copper-iron deposits containing small amounts of gold could be present, but in the 

Gibraltar Mountain Wilderness Study Area, except near the 

Mammon mine, they would be covered by Tertiary volcanic 

rocks correlated with the Osborn Wash Formation. Although 

the copper and iron in these deposits as well as the small 

amounts of barite, fluorite, and manganese-oxide minerals 

found locally are not currently of great interest as minable 

commodities, changes in the Nation's economy or in technology could make some of them attractive again in the 

future.

Energy Resources

Western Arizona has several localities designated as 

favorable for low-temperature geothermal resources within 

50 mi of the study areas (Witcher and others, 1982), and 

one thermal well is about 4.5 mi northeast of the Planet 

Peak Wilderness Study Area. The designated geothermal 

resource areas all lie within intermontane basins rather than 

in mountainous terrain like that of the study areas. Similarly, in California, known geothermal wells near the study 

areas are in basins, rather than in mountain ranges (Higgins, 

1980). Because inferred geothermal resources near the study 

areas are in a different geologic setting, it is doubtful that 

the study areas would have substantial, if any, geothermal 

resources.

Ryder (1982,1983) designated the region around the 

study areas as having low to zero potential for oil and gas 

because most of the mountain blocks are composed of crystalline and volcanic rocks rather than sedimentary rocks 

suitable for reservoirs. The Tertiary sedimentary rocks in 

the intervening basins are generally less than 5,000 ft thick 

but could provide traps for hydrocarbons. However, the study 

areas extend only a short distance basinward and are, thus, 

unlikely candidates for petroleum resources.

Mineral and Energy Resources of the Gibraltar 

Mountain Wilderness Study Area

The high mineral resource potential, certainty level 

C, for copper, iron, and gold in the Mammon mine area of 

the Gibraltar Mountain Wilderness Study Area is based on 

the known production of the commodities from the mine, 

on the abundant hematite in the case of iron, and on the 

geochemical anomalies for these elements in that area. The 

area also has low potential, certainty level C, for silver, 

barite, and manganese and low potential, certainty level B, 

for fluorite on the basis of the presence of these elements in 

geochemical samples from the area. Production of small 

quantities of silver is known from some deposits related to 

detachment faults. Although fluorine analyses were not performed during this study, the presence of fluorite in some

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona B25
geochemical samples and at nearby prospects suggest that 

some parts of the study area have low potential for fluorite 

resources.

The northwest corner of the study area, where upperplate Proterozoic rocks crop out, has moderate mineral resource potential, certainty level C, for copper, iron, and 

gold and low resource potential, certainty level B, for silver, barite, manganese, and fluorite because of small 

prospects in these rocks and because of the appropriate 

geologic environment for detachment-related deposits. 

USBM rock samples collected in the prospects of that area 

yielded anomalous gold and copper, but USGS stream 

sediments collected from the area do not contain any 

anomalous elements; therefore any mineralization probably 

is very sporadic and localized.

The remainder of the detachment-related rocks in the 

study area are buried below basalts and trachytes correlative with the Osborn Wash Formation or below Quaternary 

alluvium and, therefore, cannot be directly evaluated. The 

expected proximity of these buried rocks to the detachment 

surface and their presumed similarity to nearby exposed 

rocks suggest that they may contain copper, iron, and gold 

with accompanying silver, manganese, barite, and fluorite. 

Low-level geochemical anomalies in drainage samples from 

the basalts and trachytes may be derived from leakage halos from possible underlying mineralized rocks. These geochemically anomalous elements include gold, molybdenum, 

zinc, silver, manganese, lead, tin, tungsten, and cadmium. 

Without detailed examination, the mineral resource potential of these buried rocks is unknown, certainty level A, for 

copper, iron, gold, silver, manganese, barite, and fluorite.

The study area has no potential for energy resources 

such as geothermal waters or for oil and gas because the 

geological environment is unsuitable for the accumulation 

of these commodities.

Mineral and Energy Resources of the Planet Peak 

Wilderness Study Area

Exposed rocks of the Planet Peak Wilderness Study 

Area consist of lower- and upper-plate rocks near the 

Buckskin-Rawhide detachment-fault surface, which is 

exposed locally. Because rocks adjacent to the detachment 

surface are known to be mineralized, the mineral resource 

potential of the exposed upper-plate rocks is high and that 

of exposed lower-plate rocks is moderate, both certainty 

level C, for copper, iron, and gold. Although iron has never 

been mined from these deposits, its ubiquitous presence in 

hematite must be considered. The area directly around the 

Pride mine, where a small amount of silver was produced, 

also has a high resource potential, certainty level C, for 

silver, while the rest of the exposed upper- and lower-plate 

rocks have low resource potential, certainty level C, for 

silver.

Geochemical anomalies for manganese and barium in 

various parts of the study area and for manganese in nearby 

prospects indicate that the study area has low potential, 

certainty level C, for manganese and barite resources in the 

upper- and lower-plate rocks. Similarly, fluorite found in 

HMC samples from the study area and in nearby prospects 

suggests that the resource potential is low, certainty level 

B, for fluorite in the same areas.

Rocks of the lower and upper plates of the detachment 

fault continue beneath the alluvium along the edges of the 

study area. Along the northwest border the mineral resource 

potential is unknown, certainty level A, for copper, iron, 

gold, silver, barite, manganese, and fluorite because the rocks 

below the alluvium are part of the lower plate of the detachment fault. However, along the south, southeast, and northeast 

boundaries the mineral resource potential is high, certainty 

level B, for copper, iron, and gold and is unknown, certainty 

level A, for silver, barite, manganese, and fluorite because 

the rocks below the alluvium are part of the upper plate.

Because the geology of the study area is not conducive to the accumulation of oil or gas or for geothermal 

energy resources, the study area has no resource potential 

for these commodities.

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and U-Th-Pb geochronology of Triassic and Jurassic rocks in 

west-central Arizona, in Dickinson, W.R., and Klute, M.A., 

eds., Mesozoic rocks of southern Arizona and adjacent areas: 

Arizona Geological Society Digest, v. 18, p. 65-80.

Ridenour, James, Moyle, P.R., and Willett, S.L., 1982, Mineral 

occurrences in the Whipple Mountains Wilderness Study Area, 

San Bemardino County, California, in Frost, E.G., and Martin, 

D.L., eds., Mesozoic-Cenozoic tectonic evolution of the 

Colorado River region, California, Arizona, and Nevada: San 

Diego, Calif., Cordilleran Publishers, p. 69-75.

Ryder, R.T., 1982, Petroleum potential of wilderness lands, 

Arizona: U.S. Geological Survey Miscellaneous Geologic Investigations Map 1-1537, scale 1:1,000,000.

 1983, Petroleum potential of wilderness lands in Arizona, 

in Miller, B.M., ed., Petroleum potential of wilderness lands

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona B27
in the Western United States: U.S. Geological Survey Circular 902-C, p. C1-C22.

Schnabel, Lorraine, and Welty, J.W., 1986, Bibliography for metallic mineral districts in La Paz, Mohave, and Yuma Counties, 

Arizona: Arizona Bureau of Geology and Mineral Technology 

Circular 25, p. 1-16.

Scott, D.C., 1989, Mineral investigation of a part of the Gibraltar 

Mountain Wilderness Study Area (AZr-050-012), La Paz 

County, Arizona: U.S. Bureau of Mines Open-File Report 

MLA 18-89,17 p.

Shackelford, T.J., 1976, Structural geology of the Rawhide Mountains, Mohave County, Arizona: Los Angeles, University of 

Southern California Ph.D. dissertation, 176 p.

Spencer, I.E., compiler, 1989, Compilation geologic map of the 

Buckskin and Rawhide Mountains, west-central Arizona, in 

Spencer, J.E., and Reynolds, S.J., eds., Geology and mineral 

resources of the Buckskin and Rawhide Mountains, westcentral Arizona: Arizona Geological Survey Bulletin 198, plate 

3, scale 1:100,000.

Spencer, I.E., Duncan, J.T., Burton, W.D., 1988, The Copperstone 

mine: Arizona's new gold producer: Arizona Bureau of 

Geology and Mineral Technology Fieldnotes, v. 18, no. 2, p. 

1-3.

Spencer, I.E., and Reynolds, S.J., 1986, Some aspects of the middle 

Tertiary tectonics of Arizona and southeastern California, in 

Beatty, Barbara, and Wilkson, P.A.K., eds., Frontiers in geology and ore deposits of Arizona and the Southwest: Arizona 

Geological Society Digest, v. 16, p. 102-107.

 1989, Tertiary structure, stratigraphy, and tectonics of the 

Buckskin Mountains, in Spencer, I.E., and Reynolds, S.J., eds., 

Geology and mineral resources of the Buckskin and Rawhide 

Mountains, west-central Arizona: Arizona Geological Survey 

Bulletin 198, p. 103-155.

Spencer, I.E., Reynolds, S.J., and Lehman, N.E., 1989, Geologic 

map of the Planet-Mineral Hill area, northwestern Buckskin 

Mountains, west-central Arizona, in Spencer, I.E., and 

Reynolds, S.J., eds., Geology and mineral resources of the 

Buckskin and Rawhide Mountains, west-central Arizona: 

Arizona Geological Survey Bulletin 198, pi. 2, scale 1:24,000.

Spencer, I.E., and Welty, J.W., 1986a, Mid-Tertiary ore deposits 

in Arizona: Arizona Bureau of Geology and Mineral 

Technology Open-File Report 86-12,40 p.

 1986b, Possible controls of base- and precious-metal mineralization associated with Tertiary detachment faults in the 

lower Colorado River trough, Arizona and California: 

Geology, v. 14, p. 195-198.

 1989, Geology of mineral deposits in the Buckskin and 

Rawhide Mountains, in Spencer, I.E., and Reynolds, S.J., eds., 

Geology and mineral resources of the Buckskin and Rawhide 

Mountains, west-central Arizona: Arizona Geological Survey 

Bulletin 198, p. 223-254.

Tosdal, R.M., Blank, H.R., Jr., Eppinger, R.G., Erdman, J.A., 

Hanna, W.F., Pitkin, J.A., O'Leary, R.M., Watterson, J.R., 

and Kreidler, T.J., I990a, Mineral resources of the Cactus 

Plain and East Cactus Plain Wilderness Study Areas, La Paz 

County, Arizona: U.S. Geological Survey Bulletin 1040-D, 

32 p.

Tosdal, R.M., Eppinger, R.G., Blank, H.R., Jr., Knepper, D.H., 

Jr., Gallagher, A.J., Pitkin, J.A., Jones, S.L., and Ryan, G.S., 

I990b, Mineral resources of the Swansea Wilderness Study 

Area, La Paz and Mohave Counties, Arizona: U.S. Geological 

Survey Bulletin 1704-C, 24 p.

U.S. Bureau of Mines and U.S. Geological Survey, 1980, Principles 

of a resource/reserve classification for minerals: U.S. Geological Survey Circular 831,5 p.

U.S. Geological Survey, 1981, Aeoromagnetic map of the Needles 

1° by 2° quadrangle, California and Arizona: U.S. Geological 

Survey Open-File Report 81-85, scale 1:250,000.

Welty, J.W., Spencer, J.E., Alien, G.B., Reynolds, S.J., andTrapp, 

R.A., 1985, Geology and production of middle Tertiary 

mineral districts in Arizona: Arizona Bureau of Geology and 

Mineral Technology Open-File Report 85-1, 88 p.

Western Geophysical Company of America, Aero Service Division, 

1979, Airborne gamma-ray spectrometer and magnetometer 

survey, Prescott quadrangle (Arizona): U.S. Department of 

Energy, Grand Junction Office, Colo., Open-File Report 

GJBX-59(79), v. 1 and 2, scale 1:500,000.

Wilkins, Joe, Jr., Beane, R.E., and Heidrick, T.L., 1986, Mineralization related to detachment faults: a model, in Beatty, 

Barbara, and Wilkinson, P.A.K., eds., Frontiers in geology 

and ore deposits of Arizona and the southwest: Arizona 

Geological Society Digest, v. 16, p. 108-117.

Wilkins, Joe, Jr., and Heidrick, T.L., 1982, Base and precious 

metal mineralization related to low-angle tectonic features in 

the Whipple Mountains, California, and Buckskin Mountains, 

Arizona, in Frost, E.G., and Martin, D.L., eds., MesozoicCenozoic tectonic evolution of the Colorado River region, 

California, Arizona, and Nevada: San Diego, Calif., Cordilleran Publishers, p. 182-203-

Witcher, J.C., Stone, Claudia, and Hahman, W.R., Sr., 1982, Geothermal resources of Arizona: Tucson, Arizona Bureau of 

Geology and Mineral Technology, scale 1:500,000.

B28 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
APPENDIXES
DEFINITION OF LEVELS OF MINERAL RESOURCE POTENTIAL 

AND CERTAINTY OF ASSESSMENT

LEVELS OF RESOURCE POTENTIAL

H 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.

M 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 reasonable likelihood for resource accumulation, and (or) where an application of mineral-deposit models 

indicates favorable ground for the specified type(s) of deposits.

L 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 permissive. This broad category embraces 

areas with dispersed but insignificantly mineralized rock, as well as areas with little or no indication of having 

been mineralized.

N NO mineral resource potential is a category reserved for a specific type of resource in a well-defined area.

U UNKNOWN mineral resource potential is assigned to areas where information is inadequate to assign a low, 

moderate, or high level of resource potential.

LEVELS OF CERTAINTY

A Available information is not adequate for determination of the level of mineral resource potential.

B Available information only suggests the level of mineral resource potential.

C Available information gives a good indication of the level of mineral resource potential.

D Available information clearly defines the level of mineral resource potential.

LLJ 

LLJ

A

U/A

UNKNOWN POTENTIAL

B

H/B

HIGH POTENTIAL

M/B

MODERATE POTENTIAL

L/B

LOW POTENTIAL

C

H/C

HIGH POTENTIAL

M/C

MODERATE POTENTIAL

L/C

LOW POTENTIAL

D

H/D

HIGH POTENTIAL

M/D

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

LEVEL OF CERTAINTY

Abstracted with minor modifications from:

Taylor, R.B., and Steven, T.A., 1983, Definition of mineral resource potential: Economic Geology, v. 78, no. 6, p. 1268-1270.

Taylor, R.B., Stoneman, R.J., and Marsh, S.P., 1984, An assessment of the mineral resource potential of the San Isabel National Forest, south-central Colorado: U.S.

Geological Survey Bulletin 1638, p. 40-42. 

Goudarzi, G.H., compiler, 1984, Guide to preparation of mineral survey reports on public lands: U.S. Geological Survey Open-File Report 84-0787, p. 7, 8.

B30 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona
RESOURCE/RESERVE CLASSIFICATION

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RESOURCES

Demonstrated

Measured

Rese

Indicated

»rves

Marginal 

Reserves

Demonstrated 

Subeconomic

Resources

i

Inferred

Inferred 

Reserves

Inferred 

Marginal 

Reserves

Inferred 

Subeconomic 

Resources

UNDISCOVERED RESOURCES

Probability Range

Hypothetical Speculative

Major elements of mineral resource classification, excluding reserve base and inferred reserve base. Modified from McKelvey, V.E., 1972, Mineral 

resource estimates and public policy: American Scientist, v. 60, p. 32-40; and U.S. Bureau of Mines and U.S. Geological Survey, 1980, Principles 

of a resource/reserve classification for minerals: U.S. Geological Survey Circular 831, p. 5.

Mineral Resources of the Gibraltar Mountain and Planet Peak Wilderness Study Areas, La Paz County, Arizona B31
GEOLOGIC TIME CHART

Terms and boundary ages used by the U.S. Geological Survey in this report

EON

Phanerozoic

Proterozoic

pre-Archean2

ERA

Cenozoic

Mesozoic

Paleozoic

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean

PERIOD

Neogene

Subperiod

Tertiary

Paleogene

Jurassic

Triassic

Permian

Pennsylvanian 

Carboniferous

Mississippian

Devonian

Silurian

Ordovician

Cambrian

(iar\m\

EPOCH

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Pal eocene

Late

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

AGE ESTIMATES OF 

BOUNDARIES IN 

MILLION YEARS (Ma)

 0.010

 1.7

 5

1 A

1 R

 55

 66

96

1 1Q

205

^ )Af\

 290 

 -330

 360

 410

J V\J

** J / \J

J\J\J

 1600

 2500

 3000

j*twu

'Rocks older than 570 Ma also called Precambrian, a time term without specific rank. 

^Informal time term without specific rank.

B32 Mineral Resources of Wilderness Study Areas: Havasu Region, Arizona


SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

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Note.-Prices of Government publications listed in older catalogs, 

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prices charged may differ from the prices in catalogs, announcements, 

and publications.
cppinger; otners MlNtKAL KtbUUKLtb, L.IBKALIAK MUUNIAIN, PLAN11 PEAK WILDERNESS STUDY AREAS, ARIZONA U.S. Geological Survey Bulletin 1704-B

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