Mineral Resources of the 

Sierra EstreHa Wilderness Study Area, 

Maricopa Lt')unty, Ar:tzooa 

ARIZONA 

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and publications. 
Chapter I 

Mineral Resources of the 

Sierra Estrella Wilderness Study Area, 

Maricopa County, Arizona 

By WILLIAM j. KEITH, RICHARD j. GOLDFARB, 

VIKI L. BANKEY, and STEPHANIE L. JONES 

U.S. Geological Survey 

STANLEY L. KORZEB 

U.S. Bureau of Mines 

U.S. GEOLOGICAL SURVEY BULLETIN 1702 

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

SOUTHWESTERN AND SOUTH-CENTRAL ARIZONA 
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 : 1989 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

The Mineral resources of the Sierra Estrella Wilderness study area, Maricopa 

County, Arizona I by William j. Keith ... [et al.]. 

p. em.- (U.S. Geological Survey bulletin ; 1702-1) 

Bibliography: p. 

Supt. of Docs. no.: I 19.3:1702-1 

1. Mines and mineral resources-Arizona-Sierra Estrella Wilderness. 2. 

Sierra Estrella Wilderness (Ariz.) I. Keith, William j., 1933- II. Series. 

QE75.89 no. 1702-1 557.3 s-dc19 89-600046 

[TN24.A6] [5531.09791 '73] CIP 
STUDIES RELATED TO WILDERNESS 

Bureau of Land Management Wilderness Study Area 

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 of 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 the Sierra 

Estrella (AZ-020-160) Wilderness Study Area, Maricopa County, Arizona. 
CONTENTS 

Summary 11 

Abstract 11 

Character and setting 11 

Identified resources 11 

Mineral resource potential 11 

Introduction 13 

Area description 13 

Previous and present investigations 13 

Appraisal of identified resources 13 

Mining history 13 

Results 15 

Conclusions 15 

Assessment of mineral resource potential 15 

Geology 15 

Geochemical studies 16 

Geophysical studies 16 

Mineral and energy resource potential 18 

References cited 19 

Appendixes 

Definition of levels of mineral resource potential and certainty of 

assessment 112 

Resource/reserve classification 113 

Geologic time chart 114 

FIGURES 

1. Index map showing location of Sierra Estrella Wilderness Study Area, Maricopa 

County, Arizona 12 

2. Generalized geologic map of Sierra Estrella Wilderness Study Area showing areas 

of mineral resource potential 14 

3. Total intensity aeromagnetic anomaly map of Sierra Estrella Wilderness Study Area, 

Maricopa County, Arizona 17 

Contents V 
Mineral Resources of the 

Sierra Estrella Wilderness Study Area, 

Maricopa County,· Arizona 

By William j. Keith, Richard j. Goldfarb, Viki L. Bankey, and Stephanie L. Jones 

U.S. Geological Survey 

Stan ley L. Korzeb 

U.S. Bureau of Mines 

SUMMARY 

Abstract 

The Sierra Estrella (AZ-020-160) Wilderness Study Area 

is located in the Sierra Estrella of south-central Arizona. At 

the request of the U.S. Bureau of Land Management, 14,190 

acres of the Sierra Estrella Wilderness Study Area were studied. In this report, the area studied is referred to as "the 

wilderness study area" or simply "the study area." Geological, geochemical, geophysical, and mineral surveys were 

conducted by the U.S. Geological Survey and the U.S. Bureau of Mines in 1986 and 1987 to assess the identified 

mineral resources (known) and mineral resource potential 

(undiscovered) of the study area. No resources were identified in the study area, but the results of these surveys indicate 

that the northeast half of the study area has low resource 

potential for mica in small pods (probably not over 1 ton) 

with concentrations that may be as much as 20 percent. An 

area in the lower two-thirds of the study area on the east side 

(west of the range crest) has low potential for gold, silver, and 

associated copper resources in epithermal deposits. The 

southwest half of the study area, which is underlain by sediments, has low potential for gold resources in placer deposits. 

The southwest half also has moderate potential for low-temperature geothermal resources and low potential for oil and 

gas resources. 

Character and Setting 

Part of the study area is in the rugged Sierra Estrella, 

which is located in the Sonoran Desert part of the Basin 

and Range physiographic province, about 15 mi southwest 

of Phoenix (fig. 1). The topography is steep (over 2,700 ft 

rise in 1.5 mi) and deeply dissected. Elevations range from 

Manuscript approved for publication February 22, 1989. 

about 1,370 ft near the northwest comer of the study area 

to 4,119 ft on Butterfly Mountain in the northeastern part 

of the area. The study area is underlain by Precambrian 

(see geologic time chart in the appendixes) metamorphic 

rocks that are intruded by Precambrian granitic rocks. The 

metamorphic rocks contain pegmatite dikes of uncertain 

origin. The Sierra Estrella is bordered on the southwest by 

valley-fill sediments of Rainbow Valley (fig. 1). 

Identified Resources 

No mineral resources were identified during field studies. Local pegmatite dikes with high concentrations of 

muscovite occur within the metamorphic rocks of the study 

area, but because of their low tonnage and sporadic distribution they cannot be considered a resource for flake mica. 

Mineral Resource Potential 

Part of the Sierra Estrella lying within the lower twothirds of the study area has low potential for gold, silver 

and associated copper resources in epithermal deposits. 

The northeast half of the study area that is underlain by 

bedrock has low potential for mica. The southwest half of 

the study area has low potential for gold resources in placer 

deposits. The southwest half of the study area, which is 

underlain by sediments, also has moderate potential for 

low-temperature (less than 194 °F) geothermal resources 

and low potential for oil and gas resources, whose hypothetical source would be underlying and unexposed Tertiary sediments. 

Gold and silver were detected at background levels in 

quartz veins and in copper-stained areas, but gold reportedly occurs in anomalous amounts in the alluvium on the 

Mineral Resources of the Sierra Estrella Wilderness Study Area, Maricopa County, Arizona 11 
southwest side of the study area. Gold concentrations in 

the gravels are reported to be quite low (often background 

levels), but clay layers within the gravels may contain as 

much as 0.25 ounces/short ton (oz/st) of gold. The lack of 

anomalous gold and silver concentrations in the veins and 

dikes, and the low concentrations of placer gold suggest 

that the resource potential for these metals is low. 

, 12"25' 

The copper-stained areas are small, lenticular, and 

consist mostly of chrysocolla deposits on fracture surfaces. 

These areas are typically associated with the gold and silver 

concentrations. The potential for copper resources from 

these occurrences is low. 

Local pegmatite dikes in the metamorphic rocks contain higher concentrations of coarse-grained mica (muscoSOUTHERN PACIFIC RAILROAD 

.....---

F.\~--­

so\t ... ... ~ 

---- .. ~ 

/ - -F~-~ ,~ 

APPROXIMATE BOUNDARY 

OF SIERRA ESTREllA 

WILDERNESS STUDY AREA 

(AZ-020-160) 

8 

AREA OF MAP 

I ""' ""' ''-% I ., 

I GILARNER . 

~ INDIAN RESERVATION 

~ 

re, 

0 3 4 5 MILES 

Figure 1. Index map showing location of Sierra Estrella Wilderness Study Area, Maricopa County, Arizona. 

12 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 
vite). It is doubtful that these dikes contain enough usable 

mica to make an economic mining operation feasible either 

now or in the near future. Potential for mica resources is 

low. 

High heat flow and the presence of low-temperature 

geothermal systems within 50 mi of the study area suggest 

moderate potential for low-temperature geothermal energy 

resources in the southwest half of the study area that is 

underlain by sediments. 

The southwest half of the study area also has low 

potential for oil and gas resources. Tertiary source beds 

may underlie the sediments in this area or hydrocarbons 

may have migrated laterally into these sediments. 

INTRODUCTION 

This mineral survey was requested by the U.S. Bureau 

of Land Management and is the result of a cooperative 

effort by the U.S. Geological Survey and the U.S. Bureau 

of Mines. An introduction to the wilderness review process, mineral survey methods, and agency responsibilities is 

in Beikman and others (1983). The U.S. Bureau of Mines 

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, claims, and mineralized areas. Identified 

resources are classified according to a system that is a 

modification of that described by McKelvey (1972) and 

U.S. Bureau of Mines and U.S. Geological Survey (1980). 

U.S. Geological Survey studies are designed to provide a 

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. Goudarzi (1984) discussed 

mineral resource assessment methodology and terminology 

as they apply to these surveys. See the appendixes for the 

definition of levels of mineral resource potential and certainty of assessment and for the resource/reserve classification. 

Area Description 

At the request of the U.S. Bureau of Land Management, 14,190 acres of the Sierra Estrella (AZ-020-160) 

Wilderness Study Area were studied. The study area 

consists of sparsely vegetated basin-and-range terrain in 

southern Maricopa County, Arizona (fig. 1). Elevations 

range from 4,119 ft on Butterfly Mountain to about 1,370 

ft ncar the northwestern corner of the study area (fig. 2). It 

is in the northern part of the Sonoran desert and has geomorphological features typical of arid climates such as 

flash flood scars. Study area access on the west side is 

provided by unmaintained jeep roads and on the east by a 

gravel road through the Gila River Indian Reservation. The 

higher elevations are accessible only by helicopter or on 

foot. 

The vegetation of the study area is in the Lower Colorado River Valley subdivision of the Lower Sonoran Desertscrub Biotic Community (Brown, 1982). The major 

shrub species are creosotebush (Larrea tridentata), incienso (Encelia farinsa), and white bursage (Ambrosia 

dumosa). Sparsely distributed throughout the area are the 

small paloverde trees (Cercidium spp). Other trees include 

cat-claw acacia (Acacia greggii), mesquites (Prosopis spp.) 

and ironwood (Olneya tesota). The most spectacular cactus 

of the Sonoran Desert, the large columnar saguaro (Carnegiea gigantea), also grow in the study area, as do many 

common cactus species. 

Previous and Present Investigations 

A geologic map of the study area (fig. 2) prepared by 

the U.S. Geological Survey in 1986 at an approximate scale 

of 1:48,000 provided a base for the interpretation of geo- · 

chemical, geophysical, remote sensing, and mining data. 

The geologic map was generalized from those of Wilson 

(1969) and Spencer and others (1985). 

Geochemical data were obtained from analyses of 

stream-sediment and heavy-mineral-concentrate samples 

collected by the U.S. Geological Survey (Rose and others, 

1989) from streams draining the study area. Bedrock 

samples were also collected and analyzed to obtain background levels for many elements and to identify hydrothermally altered areas. 

Geophysical data interpreted are from an aeromagnetic 

survey from the National Uranium Resource Evaluation 

(NURE) Program (LKB Resources, 1980) and a regional 

gravity study by Peterson (1968). 

The U.S. Bureau of Mines studied the known prospects 

and claims within and adjacent to the study area to develop 

a model that would predict the grade and tonnage of mineral resources that might occur in the study area. 

APPRAISAL OF IDENTIFIED RESOURCES 

By Stanley L. Korzeb 

U.S. Bureau of Mines 

Mining History 

The study area is not within or adjacent to any established mining district. The Sierra Estrella was probably 

first explored prior to 1886, due to its proximity to transcontinental travel routes. Wilson (1969, p. 21) reported 

that several copper mines were present in the Sierra Estrella 

in the late 1800's, but the locations are unknown. In 1920, 

nine claims were developed by open cuts but appear to 

have produced little ore (Wilson, 1969, p. 21). 

Mineral Resources of the Sierra Estrella Wilderness Study Area, Maricopa County, Arizona 13 
The Crusher mica quarry, 0.1 mi outside the west-

. central boundary of the study area (fig. 2), was reported to 

be in operation in 1958 by the Arizona Department of 

Mines and Mineral Resources. An unrecorded amount of 

flake mica was produced from a pegmatite pod exposed in 

a 20- by 10- by 8-ft cut at the mine (Krason and others, 

33 •15"00" 

1982, p. 77). From 1980 to 1983, 74 placer claims were 

located within and adjacent to the west boundary of the 

study area (fig. 1). In 1980, a block of lode claims was 

located approximately 1 mi southeast of the Crusher mica 

quarry. No surface work is evident at or near any of these 

claims. 

1 12°15' 

\ 

\ 

112°12'30" 

L/C Mi 

\ 

' I 

p-£sg \~ 

I~ 

33°12'30" 

33•10'00" 

:~ \ 

\ 

p-£sg 

, _ BUTIER FLY MOUNTAIN 

',, _~ ----~19 

'-'rn I (f) 

I~ 

M/C Geo 

LIB O,G 

L/C Au 111 

L Au, Ag ,Cul21 \~ Mi I tp 

EXPLANATION 

J:::i!!i/:::::;:.:::::!:·:1 Arl!a having moderate resource potential (M) 

D Area having low mineral resource potential (L) 

Levels of certainty of assessment 

B Data only suggest level of potential 

C Data give good indication of level of potential 

Commodities 

Au Gold 

Ag Silver 

Cu Copper 

Mi Mica 

Geo Geothermal 

0 ,G Oil and Gas 

I I 

1 

2 

Deposit types 

Rlacer 

Epithermal 

Geologic map units 

Qs Unconsolidated surficial deposits (Quaternary)--

BM 3092 • 

APPROXIMATE BOUNDARY 

OF SIERRA ESTRELLA 

WILDERNESS STUDY AREA 

(AZ-020-160) 

lncludes silt, sand, gravel, and locally colluvium 

p-£g Granitoid rocks (Precambrian)--Locally includes 

pegmatite dikes 

p-£sg Schist and gneiss (Precambrian)--Locally includes 

pegmatite dikes 

--Contact 0 

---- Range crest 

2 MILES 

' ' \ 

\ 

I 

I 

I 

I 

I 

/I 

Figure 2. Generalized geologic map of Sierra Estrella Wilderness Study Area showing areas of mineral resource potential. 

14 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 
Results 

There are no identified resources in the study area, but 

it contains pegmatites similar to those mined for flake mica 

at the Crusher quarry. Most of the pegmatites are small 

vertical dikes 6 in. to 2 ft wide, but some are pods as much 

as 20 ft thick. Some dikes can be traced on the surface 

along strike for 100 ft. The pegmatites consist mostly of 

feldspar, quartz, and muscovite but include minor amounts 

of magnetite, garnet, schorl, and chrysocolla. Muscovite is 

confined to the outer edges of the pegmatites and occurs in 

books as much as 4 in. across and 2 in. thick (Korzeb, 

1988, p. 8). The muscovite books are bent, ruled, and 

discolored. Their condition makes them unsuitable for 

sheet mica but suitable for flake mica. Flake mica is 

ground to a mesh size ranging from less than 80 to greater 

than 325. The ground mica is used in the manufacture of 

asphalt shingles, joint cement, drilling mud, paint, rubber 

goods, plastic, and welding rods (Lesure, 1973, p. 416); its 

use depends on the mesh size to which it is ground. Flake 

mica is valued at about $48.00 per ton (Gene Reese, Pacer 

Corporation, Custer, South Dakota, oral commun., 1987). 

Locally, these pegmatites contain as much as 20 percent 

muscovite. The largest and highest grade muscovite zones 

may contain as much as 1 ton of mica; however, because of 

the sporadic distribution of these zones, the pegmatites 

cannot be considered a resource for flake mica. 

Muscovite schist hosting the pegmatites also contains 

flake mica. An analysis of this schist by Wilson (1969, p. 

23) showed that it contains as much as 21.8 percent muscovite. By comparison, mica schist mined for commercial 

use in South Dakota by the Pacer Corporation contains 

78-80 percent mica. The muscovite schist throughout the 

study area is too low grade to be considered a resource for 

commercial flake mica. 

Chemical analyses of the copper-stained pegmatites 

generally show background levels of 15 parts per billion 

(ppb) (0.0004 oz/st) to 40 ppb (0.001 oz/st) gold and 

weakly anomalous amounts of tantalum and niobium. 

They did, however contain up to 8,950 parts per million 

(ppm) copper. One quartz vein within a pegmatite contains 

23 ppm gold (.662 oz/st), 13.5 ppm silver, pyrite, and 

limonite (Korzeb, 1988, p. 9). Pyrite and limonite do not 

commonly occur in pegmatites. No gold resources were 

identified in the pegmatites, but they could be a source of 

gold in the placer deposits in the southwestern part of the 

study area. 

Alvin C. Johnson, Jr. (Sevenmile Mining Association, 

Tempe, Ariz., oral commun., 1987) reported that exposed 

gravels on the placer claims within and adjacent to the 

study area contain less than 0.1 oz/st gold; most samples 

from the gravels do not contain gold at detectable levels; 

beds of clay discovered by drilling under the gravels may 

contain as much as 0.25 oz/st gold. The placer deposits 

were estimated to contain 30 million yd3 or 45 million st of 

gravel from an assumed thickness of 10 ft. The thickness 

and extent of the underlying clay and the stratigraphic 

distribution of the gold are unknown and there are no 

identified resources as a result of this study. More detailed 

subsurface work such as trenching and drilling is necessary 

to verify the presence of gold. 

Sand and gravel in the study area have no unique 

properties and no local market. Sand and gravel are bulk 

commodities whose production cost depends on transportation costs. Sand and gravel at other locations, nearer 

markets, are better suited for local use. The sand and 

gravel in the study area, therefore, are not classed as an 

identified resource. 

Conclusions 

The Sierra Estrella Wilderness Study Area does not 

contain any identified resources. Pegmatites within the 

study area contain flake mica and weakly anomalous 

amounts of gold, but their low tonnage and sporadic distribution make them unsuitable as a resource. The muscovite 

schist is also too low grade to be considered a mica resource. Gold has been reported from the placer deposits in 

the study area, but the volume and concentration of the 

gold are unknown. Further investigation of the placer 

deposits in the southwest half of the study area, including 

trenching and drilling to bedrock, is necessary to determine 

if enough gold is present to represent a resource. 

ASSESSMENT OF MINERAL RESOURCE 

POTENTIAL 

By William j. Keith, Richard j. Goldfarb, and Viki l. Bankey 

U.S. Geological Survey 

Geology 

The study area is underlain mostly by muscovite schist 

and gneiss of various protoliths (sedimentary, volcanic, and 

granitic) that are not considered to be a metamorphic core 

complex (Spencer and others, 1985, p. 2). A rubidiumstrontium age of 1.38 billion years before .present (Ga) on 

the granitic rocks (Pushkar and Damon, 1974) intruding the 

metamorphic rocks suggests that both units are Precambrian. The metamorphic grade of these rocks is upper 

amphibolite to lower granulite facies (Sommer, 1982). The 

intruding granitic rocks consist largely of a porphyritic 

monzogranite but also include tonalite and quartz diorite. 

The sediments in the study area are typical valley-fill 

material. Rocks in the north end of the study area are cut 

by dioritic dikes that are probably of Tertiary age (Spencer 

and others, 1985, p. 10). Although these dioritic dikes 

appear to be randomly oriented, they are part of a regional 

northwest-trending system (Spencer and others, 1985, p. 

Mineral Resources of the Sierra Estrella Wilderness Study Area, Maricopa County, Arizona IS 
10). Rocks in the central to south-central part of the study 

area contain pegmatite dikes ranging from 6 in. to 20 ft in 

thickness. These pegmatite dikes consist mostly of quartz 

and feldspar but include lesser amounts of muscovite, 

magnetite, and schorl. The pegmatites could either be 

related to the metamorphism or to the Precambrian igneous 

activity. Also contained in the central to south-central part 

of the study area are rare quartz veins that may be part of 

the pegmatite system. These veins range in thickness from 

1 in. to 10 ft and are typically less than 20 ft in exposed 

length. The margins of these veins 'contain variable 

amounts of schorl. magnetite, muscovite, and garnet. The 

quartz veins appear to be randomly oriented, although 

Wilson (1969, p. 20) reports the occurrence of quartz veins 

in north-trending fractures. 

Very little evidence of hydrothermal activity was observed in the study area. The only alteration observed 

consists of locally copper-stained areas in quartz veins. 

pegmatites, and schist and minor limonite after pyrite in 

some quartz veins. The copper staining occurred after the 

formation of the pegmatites and quartz veins. Fluids that 

deposited the copper and associated elements probably 

used brecciated areas or the coarse pegmatites as conduits. 

Similar mineralization took place on the eastern slopes of 

the Sierra Estrella, but exploration of these areas by early 

prospectors yielded little ore (Wilson, 1969). 

Foliation is generally well developed in the study area 

and typically strikes northeast and dips vertically or steeply 

southeast. (Spencer and others. 1985). No faulting was 

observed in the study area during fieldwork, but the gravity data suggest a steeply dipping, northwest-trending fault 

about 3 mi southwest of the study area. 

Geochemical Studies 

The geochemical survey of the Sierra Estrella Wilderness Study Area consisted of the collection and analysis of 

26 stream-sediment and heavy-mineral-concentrate 

samples, and 5 rock samples from within and immediately 

adjacent to the study area boundary. Stream-sediment 

samples were air dried and sieved to minus-80-mesh-size 

fractions. All concentrate samples were panned from 1.5 to 

2 lb of minus-10-mesh stream sediment using a gold pan. 

In the laboratory. the concentrates were air dried, the 

lighter material was removed using bromoform, and the 

magnetic heavy-mineral fractions were removed using a 

magnetic separator. Each sediment and rock sample was 

analyzed for 35 elements and each concentrate sample was 

analyzed for 37 clements using an optical emission spectrograph according to the method outlined by Grimes and 

Marranzino (1968). In addition, all rock and sectiment 

samples were analyzed by atomic-absorption spectrometry 

for gold and all sediments were analyzed by inductively 

coupled plasma-atomic absorption spectrometry for arsenic, 

bismuth. cadmium. antimony. and zinc. Complete analytical data and a detailed discussion of the sampling and 

analytical mehods are given in Rose and others (1989). 

Most of the geochemical data show background levels 

for all elements studied. Gold was not detected in any of 

the concentrate, sediment, or rock samples, either by emission spectrography (lower detection limit of 10 ppm) or 

atomic-absorption spectrometry (lower detection limit of 

0.05 ppm). Concentrate samples from washes draining. the 

northeast side of the study area contain as much as 300 

ppm molybdenum and 1,000 ppm tungsten, and microscopic scheelite and powellite were identified in many of 

these samples. Such concentrations. however. represent 

normal background levels of these minerals for the Precambrian rocks underlying the watersheds. Values of 100 ppm 

lead detected in some of the concentrate samples correlate 

with abundant apatite. 

One sample from a quartz vein with visible azurite and 

malachite (Rose and others. 1989). collected near the 

Crusher mica quarry just outside the west-central boundary 

of the study area, contains 2 ppm silver, 20 ppm bismuth, 

and 10,000 ppm copper. A similar signature was obtained 

for a sample of a copper-stained pegmatite, collected 1.5 mi 

southeast of the quarry; anomalous metal concentrations 

consist of 15 ppm silver, 12 ppm bismuth, 20,000 ppm 

copper. and 7 ppm tellurium (W.J. Keith, unpub. data, 

1988). Five sediment samples, all from washes draining 

west within the southern half of the study area, contain 2-3 

ppm bismuth. Additionally, one concentrate sample in the 

same area contains 200 ppm copper. On the basis of these 

data, the portion of the study area south of BM 3092 and 

west of the Sierra Estrella range crest (fig. 2) has favorability for weak silver-bismuth-copper-tellurium mineralization. 

Geophysical Studies 

Bouguer gravity data were taken from a regional study 

by Peterson (1968). A. density of 2.67 g/cm3 was used and 

terrain corrections were applied to reduce each station to 

the complete-Bouguer gravity value. Gravity data reflect 

changes in the gravity field that are associated with varying 

rock densities or structural features. However, the lack of 

stations within the study area boundary gives these data a 

regional character only broad or deep features are enhanced 

and shallow. short-wavelength anomalies are missing. 

A broad regional gravity high is associated with the 

Sierra Estrella. The gravity data suggest a range-front fault 

near the southwest side of the study area with an adjoining, 

parallel basin that is deepest about 4 mi southwest of the 

fault. The fault continues as far as the south end of the Palo 

Verde Mountains; the Sierra Estrella gravity high includes 

those mountains, which suggests that they are part of one 

continuous feature. 

16 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 
Two other deep basins appear as gravity lows southeast 

and north of the Sierra Estrella. To the northeast of the 

study area, the gravity data suggest that the Gila River 

Valley is shallow between the Sierra Estrella and the South 

Mountains. Spencer and others (1985) discuss the lithology, structure, and gravity field in these two ranges and 

conclude that they are probably structurally continuous. 

However, the northeast-trending South Mountains constitute a metamorphic core complex (Spencer and others, 

1985) but the Sierra Estrella does not because it lacks 

evidence of post-Precambrian deformation. 

An aeromagnetic map (fig. 3) of the study area was 

compiled from an existing survey flown in 1979 as part of 

the National Uranium Resource Evaluation (NURE) Program. This survey (LKB Resources, 1980) was flown 

using a fixed-wing aircraft at a nominal ground clearance 

0 2 3 4 5 MILES 

Figure 3. Total intensity aeromagnetic anomaly map of Sierra Estrella Wilderness Study Area, Maricopa County, Arizona. 

Contour interval 50 nanoteslas; hachures indicate magnetic low. Anomalies labeled A-J; see text for discussion. 

Mineral Resources of the Sierra Estrella Wilderness Study Area, Maricopa County, Arizona 17 
of 400 ft, and east-west traverses were flown 3 mi apart. 

Aeromagnetic data are useful in delineating concealed intrusive rocks that could be associated with mineralization, 

. or they can be used to map buried extensions of magnetic 

rocks. In this area, Precambrian gneiss and schist as well 

as possible Tertiary intrusions are magnetic and have the 

same aeromagnetic expression. Therefore, these data can 

only suggest areas of interest that may be overlooked in 

geologic mapping, but they do not allow the distinction 

between rocks that have no mineral significance and intrusions that are more favorable for the development of mineralized zones. 

The aeromagnetic data show a magnetic high over 

most of the gneiss and schist of the Sierra Estrella, but the 

anomalies (labeled A-J, fig. 3) are not always contained 

within known structural boundaries. Positive anomalies 

A-D generally follow the trend of the mountains, and these 

local maxima probably reflect the varying composition of 

gneiss and schist across the study area. Anomaly D, south 

of the study area, is not definitively associated with the 

Sierra Estrella but trends east-west and crosses the rangefront fault. The source of this magnetic high is not reflected by the gravity data, but it may be buried Precambrian gneiss or schist, or perhaps a younger buried intrusion. 

Anomalies E1 and E2 are a dipole pair associated with 

magnetic rocks in the South Mountains, but positive anomaly E1 also outlines a magnetic source beneath the Gila 

River valley. Magnetic lows F and G separate the magnetic 

highs of the Sierra Estrella from the anomaly pair E1-E2, 

but these lows follow the topographic edge of the mountains rather than the valley, as would be expected. Together, these anomalies suggest, as do the gravity data, that 

the Sierra Estrella and South Mountains are structurally 

related or continuous. 

Tertiary intrusions that could affect the mineral resource potential are possibly buried beneath the study area 

and are delineated by any of anomalies D, E1, F, or G. 

However, these same anomalies, could also represent older 

buried metamorphic rocks. Broad magnetic lows H-J 

border the Sierra Estrella to the north and west, where 

gravity data suggest deep basins. 

Mineral and Energy Resou.rce Potential 

There is low potential (certainty level C) for gold and 

silver in epithermal deposits and associated copper resources in the lower two-thirds of the study area on the east 

side (west of the range crest). The northeast half of the 

study area has low potential (certainty level C) for mica in 

pegmatite dikes in Precambrian rocks underlying that area. 

There is low potential (certainty level C) for gold in placer 

deposits that would be co_ntained in the sediments of the 

southwest half of the study area. This area also has moderate potential (certainty level C) for low-temperature (less 

than 194 °F, Muffler, 1979) geothermal resources and low 

potential (certainty level B) for oil and gas resources. 

Combined data from the U.S. Bureau of Mines and the 

U.S. Geological Survey show that gold and silver occur in 

weakly anomalous amounts in veins and in copper-stained 

areas in veins, pegmatite dikes, and metamorphic rocks 

south of BM 3092 and. west of the range crest (fig. 2). 

Concentrations are typically less than .05 ppm for gold and 

15 ppm for silver (Korzeb, 1988, table 1; Rose and others, 

1989). The veins and copper-stained areas are typically 

small (inches to a few feet), randomly oriented, and sparse. 

Some of the stream sediments and heavy-mineral concentrates from the area south of BM 3092 and west of the 

range crest show anomalous concentrations of bismuth and 

copper, and some of the copper-stained areas also show 

anomalous concentrations of tellurium. Magnetic anomaly 

D south of the study area (fig. 3) suggests the possibility of 

a buried pluton. The scarcity and small size of the veins 

and copper-stained areas and the low concentrations of 

gold, silver, bismuth, copper, and tellurium suggest low 

potential (certainty level C) for gold-silver resources in 

small epithermal-type deposits and also for copper resources in the southern part of the study area. Similar 

mineralization on the east side of the range has been explored with poor results due to low grade and small quantity. Wilson (1969, p. 21) summed up the potential quite 

well: "Commercially, the copper-gold deposits in the Sierra 

Estrella range do not seem to offer much future promise; 

besides being difficult of access, they are small, lenticular, 

and not of high grade." 

The southwest part of the study area has low potential 

(certainty level C) for gold resources in small placer deposits; Clay layers in the sediments of this area reportedly 

contain concentrations of gold as high as 0.25 oz/st, although most of the gravel samples do not contain detectable gold (Korzeb, 1988, p. 9). The unknown lateral extent of the clay beds and the sporadic nature of gold concentrations are the basis for this potential assignment. 

Pegmatite dikes occurring sporadically throughout the 

metamorphic rocks in the study area typically contain mica. 

Some of these dikes form pods as much as 20 ft thick, 

where the concentration of mica can be as high as 20 

percent (Korzeb, 1988, p. 8). The muscovite in these pods 

occurs in books as large as 4 in. in diameter and 2 in. thick 

that are typically bent, ruled, and discolored (Korzeb~ 1988, 

p. 8). One of these pods (Crusher mica quarry) has been 

mined for flake mica. It is possible that other minable pods 

exist, but it is doubtful that enough of them exist to make 

an economic mining operation feasible, either now or in the 

near future. Low potential (certainty level C) 'for mica 

resources is assigned to the northeast half of the study area. 

Low-temperature geothermal systems occur within 5 to 

50 mi of the study area (Krason and others, 1982, p. 42; 

Muffler, 1979), with temperatures ranging from 77 to 104 

op (Sammel, 1979, p. 104). Also, heat flow in this area is 

18 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 
greater than 60 milliwatts/square meter (Muffler, 1979). 

On the basis of these data, the southwest half of the study 

area has moderate potential (certainty level C) for lowtemperature geothermal energy resources. 

The southwest half of the study area ~so has low 

potential (certainty level B) for oil and gas resources. 

Source beds may underlie the sediments in this area or 

hydrocarbons may have migrated laterally into the basin. 

Grabens in this part of Arizona typically contain Tertiary 

sediments less than 5,000 ft thick (Oppenheimer and 

Sumner, 1981) and Ryder (1983, p. C20) is of the opinion 

that "These shallow basins are the only part of cluster 11 

where commercial hydrocarbons could be found, (cluster 

11 includes the study area). However, no known oil shows 

occur in or near the sediments of the study area. Therefore, 

it is unlikely that there are any hydrocarbon deposits in the 

study area. The potential is only given a certainty level of 

B because of the unlikely possibility that Tertiary source 

rocks are buried beneath the study area. 

REFERENCES CITED 

Beikrnan, H.M., Hinkle, M.E., Frieders, Twila, Marcus, S.M., and 

Edward, J.R., 1983, Mineral surveys by the Geological Survey and the Bureau of Mines of Bureau of Land Management 

Wilderness Study Areas: U.S. Geological Survey Circular 

901, 28 p. 

Brown, D.E., ed., 1982, Biotic communities of the American 

Southwest-United States and Mexico, Desert Plants, v. 4, no. 

1-4, 342 p. 

Goudarzi, G.H., 1984, Guide to preparation of mineral survey 

reports on public lands: U.S. Geological Survey Open-File 

Report 84-787, 51 p. 

Grimes, D.J., and Marranzino, A.P., 1968, Direct-current arc and 

alternating-current spark emission spectrographic field methods for the semiquantitative analysis of geologic materials: 

U.S. Geological Survey Circular 591, 6 p. 

Krason, Jan, Wodzicki, Antoni, and Cruver, S.K., 1982, Geology, 

energy, and mineral resource assessment of the Maricopa 

area, Arizona: Geoexplorers International, Inc., Denver, 

Colorado, Prepared for U.S. Department of Interior, Bureau of 

Land Management, 94 p. 

Korzeb, S.L., 1988, Mineral investigation of the Sierra Estrella 

Wilderness Study Area (~020-160), Maricopa County, 

Arizona: U.S. Bureau of Mines Open File Report MLA 7-88. 

Lesure, F.G., 1973, Mica, in Brobst, D.E., and Pratt, W.P., eds., 

United States mineral resources: U.S. Geological Survey 

Professional Paper 820, p. 415-423. 

LKB Resources, 1980, Aerial gamma-ray and magnetic reconnaissance survey, Colorado-Arizona; Salton Sea, Phoenix, El 

Centro, Ajo, and Lukeville quadrangle: U.S. Department of 

Energy Open-File Report GJBX-12(80), v. 4, 107 p. 

McKelvey, V.E., 1972, Mineral resource estimates and public 

policy: American Scientist, v. 60, p. 32-40. 

Muffler, L.J.P., ed., 1979, Assessment of geothermal resources of 

the United States-1978: U.S. Geological Survey Circular 

790, 163 p. . 

Oppenheimer, J.M., and Sumner, J.S., 1981, Gravity modeling of 

the Basin and Range province, Arizona: Arizona Geological 

Society Digest, v. 13, p. 111-115. 

Peterson, D.L., 1968, Bouguer gravity map of parts of Maricopa, 

Pima, Pinal, and Yuma Counties, Arizona: U.S. Geological 

Survey Geophysical Investigations Map GP-615, scale 

1:250,000. 

Pushkar, Paul, and Damon, P.E., 1974, Apparent Paleozoic ages 

from southern Arizona: K-Ar and Rb-Sr geochronology: 

Isochron West, no. 10, p. 7-10. 

Rose, S.C., Adrian, B.M., Briggs, P.H., and Goldfarb, R.J., 1989, 

Analytical results and sample locality map of stream-sediment, heavy-mineral concentrate, and rock samples from the 

Sierra Estrella Wilderness Study Area, Maricopa County, 

Arizona: U.S. Geological Survey Open-File Report 89-17, 

17p. 

Ryder, R.T., 1983, Petroleum potential of wilderness lands in 

Arizona, in Miller, B.M., ed., Petroleum potential of wilderness lands in the western United States: U.S. Geological 

Circular 902-A-P, p. Cl-C22. 

Sammel, E.A., 1979, Occurrence of low-temperture geothermal 

waters in the United States, in Muffler, L.J.P., ed., Assessment of geothermal resources of the United States-1978: 

U.S. Geological Survey Circular 790, 163 p. 

Sommer, J.V., 1982, Structural geology and metamorphic petrology of the northern Sierra Estrella, Maricopa County, Arizona: Phoenix, Arizona State University, M.S. thesis, 127 p. 

Spencer, J.E., Reynolds, S.J., Anderson, Phillip, and Anderson, 

J.L., 1985, Reconnaissance geology of the crest of the Sierra 

Estrella, Central Arizona: Arizona Bureau of Geology and 

Mineral Technology Open-File Report 85-11, 20 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. 

Wilson, E.D., 1969, Mineral deposits of the Gila River Indian 

Reservation, Arizona: Arizona Bureau of Mines Bulletin 179, 

34 p. 

Mineral Resources of the Sierra Estrella Wilderness Study Area, Maricopa County, Arizona 19 
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. 

f 

..J 

< 

i= 

z 

UJ 

1-

0 

0.. 

UJ 

u 

~ 

:::> 

0 

V') 

UJ 

~ 

LL. 

0 

..J 

UJ 

> UJ 

..J 

A 

U/A 

UNKNOWN-POTENTIAL 

Abstracted with minor modifications from: 

B c 

H/B H/C 

HIGH POTENTIAL HIGH POTENTIAL 

M/B M/C 

MODERATE POTENTIAL MODERATE POTENTIAL 

UB uc 

LOW POTENTIAL LOW POTENTIAL 

LEVEL OF CERTAINTY 

D 

H/D 

HIGH POTENTIAL 

M/D 

MODERATE POTENTIAL 

UD 

LOW POTENTIAL 

N/D 

NO POTENTIAL 

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. 

112 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 
RESOURCE/RESERVE CLASSIFICATION 

IDENTIFIED RESOURCES UNDISCOVERED RESOURCES 

Demonstrated Probability Range 

Inferred 

Measured Indicated Hypothetical Speculative 

I Inferred 

I 

ECONOMIC Reserves 

Reserves 

____ l_ ____ - I ---------- -~- I Inferred MARGINALLY Marginal Marginal ECONOMIC Reserves Reserves ____ l_ ____ - I I ---------- -~-

SUB- Demonstrated Inferred 

Subeconomic Subeconomic 

ECONOMIC Resources Resources I I I 

Majorelementsofmineral 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 Sierra Estrella Wilderness Study Area, Maricopa County, Arizona 113 
GEOLOGIC TIME CHART 

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

EON ERA PERIOD EPOCH 

Quaternary 

Holocene 

Pleistocene 

Neogene Pliocene 

Cenozoic Subperiod Miocene 

Tertiary Paleogene Oligocene 

Subperiod Eocene 

Paleocene 

Cretaceous Late 

Early 

Late 

Mesozoic Jurassic Middle 

Early 

Late 

Triassic Middle 

Early 

Permian Late 

Phanerozoic Early 

Late 

Pennsylvanian Middle 

Carboniferous Early Periods 

Mississippian Late 

Early 

Late 

Devonian Middle 

Paleozoic Early 

Late 

Silurian Middle 

Early 

Late 

Ordovician Middle 

Early 

Late 

Cambrian Middle 

Early 

Late Proterozoic 

Proterozoic Middle Proterozoic 

Early Proterozoic 

Late Archean 

Archean Middle Archean 

Early Archean 

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - (3 800?}- - - ._ - - - - - - - - - - - - - -

pre-Archean2 

1Rocks older than 570 Ma also called Precambrian, a time term without specific rank. 

2 lnformal time term without specific rank. 

114 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 

AGE ESTIMATES OF 

BOUNDARIES IN 

Ml LLION YEARS (Ma) 

0.010 

1.7 

5 

24 

38 

55 

66 

>----- 96 

138 

205 

-240 

290 

-330 

360 

410 

435 

500 

1

-570 

900 

1600 

2500 

3000 

3400 

4550 

GPO 685-041/9795 
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