Mineral Resources of the 

Signal Mountain Wilderness Study Area, 

Maricopa County, Arizona 

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Chapter C 

Mineral Resources of the 

Signal Mountain Wilderness Study Area, 

Maricopa County, Arizona 

By FLOYD GRAY, ROBERT J. MILLER, JERRY R. HASSEMER, 

DANIEL H. KNEPPER, JAMES A. PITKIN, and 

WILLIAM F. HANNA 

U.S. Geological Survey 

TERRY j. KREIDLER 

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 Sed ion 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Mineral resources of the Signal Mountain Wilderness Study Area, Maricopa 

County, Arizona. 

(Mineral resources of wilderness study areas-southwestern and southcentral Arizona ; ch. C)(U.S. Geological Survey bulletin ; 1702-C) 

Bibliography: p. 

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

1. Mines and mineral resources-Arizona-Signal Mountain Wilderness. 

2. Signal Mountain Wilderness (Ariz.) I. Gray, Floyd. II. Series. Ill. Series: 

U.S. Geological Survey bulletin ; 1702-C. 

QE75.B9 no. 1702-C 557.3 s 89-600105 

[TN24.A6] [553'.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 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 part of the Signal 

Mountain Wilderness Study Area (AZ-020-138), Maricopa County,.Arizona. 
CONTENTS 

Summary Cl 

Abstract Cl 

Character and setting Cl 

Identified resources and mineral resource potential Cl 

Introduction C3 

Location and physiography C3 

Procedures and sources of data C3 

Appraisal of identified resources C3 

Mining activity CS 

Mineral resource appraisal CS 

Energy resources CS 

Assessment of mineral resource potential CS 

Geology CS 

Geochemistry C6 

Methods C6 

Results C6 

Geophysics C6 

Gravity and aeromagnetic studies C6 

Aerial gamma-ray spectrometry C8 

Conclusions C8 

References cited C9 

Appendixes 

Definition of levels of mineral resource potential and certainty of 

assessment C12 

Resource/reserve classification C 13 

Geologic time chart C14 

FIGURES 

1. Index map showing location of Signal Mountain Wilderness Study Area, 

Maricopa County, Arizona C2 

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

Mountain Wilderness Study Area, Maricopa County, Arizona C4 

3. Bouguer gravity map of Signal Mountain Wilderness Study Area, Maricopa 

County, Arizona C6 

4. Aeromagnetic map of Signal Mountain Wilderness Study Area, Maricopa County, 

Arizona C7 

Contents V 
Mineral Resources of the 

Signal Mountain Wilderness Study Area, 

Maricopa County, Arizona 

By Floyd Gray, Robert J. Miller, jerry R. Hassemer, Daniel H. Knepper, 

James A. Pitkin, and William F. Hanna 

U.S. Geological Survey 

Terry j. Kreidler 

U.S. Bureau of Mines 

SUMMARY 

Abstract 

At the request of the U.S. Bureau of Land Management, 

approximately 15,250 acres of the Signal Mountain Wilderness Study Area (AZ-020-1 38) were evaluated. In this report, 

the area studied is referred to as the "wilderness study area" 

or simply "the study area"; any reference to the Signal 

Mountain Wilderness Study Area refers only to that part for 

which a mineral survey was requested by the U.S. Bureau of 

Land Management. Between 1 984 and 1 987 the U.S. Bureau 

of Mines and the U.S. Geological Survey conducted investigations to apprais~ the identified mineral resources (known) 

and assess the mineral resource potential (undiscovered) of 

the Signal Mountain Wilderness Study Area. These investigations revealed that basalt and sand and gravel may be identified mineral resources in the area. The mineral resource 

potential for copper, lead, silver, and gold is low along 2 

miles of the north border of the study area. The potential is 

also low for oil, gas, and geothermal energy resources 

throughout the study area. 

Character and Setting 

The Signal Mountain Wilderness Study Area is located 

approximately 30 mi southwest of Buckeye and 15 mi 

south of Wintersburg, Arizona (fig. 1). The area lies within 

the Sonora Desert section of the Basin and Range physiographic province, a region characterized typically by large 

mountain chains separated by deep, alluvium-filled valleys. 

The study area contains many steep-walled canyons, arroyos, ridges, and sharp peaks. Elevations range from 

Manuscript approved for publication February 16, 1989. 

2,182 ft at Signal Mountain to approximately 7(fJ ft along 

the southwest bOundary of the study area. The study area 

is underlain by a sequence of basalt to basaltic andesite, 

dacitic andesite, and minor rhyolite to rhyodacite flows, 

plugs, and air-fall tuffs of Miocene age (see geologic time 

chart in "Appendixes"). Pre-Tertiary rocks crop out less 

than 0.5 mi north and northeast of the study area but are not 

exposed in the study area. No prospects are located within 

the study area, but the Webb mining district is approximately 1 mi to the northeast. This district produced 27,000 

lb of copper, 200 troy oz of silver, and 50 troy oz of gold 

between 1935 and 1951. 

Identified Resources and Mineral Resource 

Potential 

Basalt and sand and gravel in the Signal Mountain 

Wilderness Study Area may be classified as identified mineral resources. Geochemical information from rock, 

stream-sediment, and panned-concentrate samples indicates 

that one area along the north border of the study area has 

low mineral resource potential for copper, lead, silver, and. 

gold in epithermal veins (fig. 2). These metals are suggested by geochemically anomalous concentrations of 

molybdenum, manganese, copper, barium, arsenic, lead, 

and silver found in samples collected 0.25 mi north of the 

study area along a west-northwest-trending fault. Although 

Tertiary volcanic rocks are exposed on the surface, the 

anomalies are probably derived from the underlying basement assemblage. No geochemically anomalous suite of 

elements was detected within the study area. 

The Webb mining district, located 1 mi northeast of 

the Signal Mountain Wilderness Study Area, contains 

Mineral Resources of the Signal Mountain Wilderness Study Area, Arizona Cl · 
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15'1 APPROXIMATE BOUNDARY 

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SIGNAL MOUNTAIN 

WILDERNESS STUDY AREA 

(AZ-020-138) 

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Figure 1. Index map showing location of Signal Mountain Wilderness Study Area, Maricopa County, Arizona. Dashed lines, unimproved roads. 

112°30' 

---, 
mainly copper with byproduct silver and gold in veins 

within metamorphic and igneous rocks of Proterozoic to 

Jurassic age and in scattered skarns formed from pendants 

of carbonate rocks. These rocks may be displaced downward by at least two large-displacement, northwest-trending faults northeast of the study area. Geologic data indicate that the rocks of Webb Mountain may extend into the 

study area below the exposed Tertiary volcanic section. 

Basalt from the study area could be used as aggregate 

and fill and may be classified as an inferred subeconomic 

resource, but other suitable rock is available closer to potential markets. Sand and gravel deposits in the area are 

small, and access to them is generally poor; sand and gravel 

deposits are also plentiful closer to potential markets. 

The entire study area has low potential for geothermal 

energy resources on the basis of hot springs in adjacent 

ranges. The potential for oil and gas resources in the study 

area is low because there is no evidence of favorable 

hydrocarbon source rocks. 

INTRODUCTION 

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

of Land Management (BLM) and is the result of a cooperative effort by the U.S. Bureau of Mines (USBM) and the 

U.S. Geological Survey (USGS). 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, prospects, and known mineralized areas 

by collecting data on current and past mining activities and 

through field examination. Identified resources are classified according to a system that is a modification of that 

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

Mines and U.S. Geological Survey (1980). USGS 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 assessment methodology 

and terminology as they apply to these surveys. See 

"Appendixes" for the definition of levels of mineral resource potential and certainty of assessment and for the 

resource/reserve classification. 

Location ·and Physiography 

The Signal Mountain Wilderness Study Area 

(AZr-020-138) in southwestern Arizona is in the northern 

Gila Bend Mountains (fig. 1). · The U.S. Bureau of Land 

Management requested a mineral survey of 15,250 acres of 

the study area in the steep, rugged mountains that include 

Signal Mountain, at an elevation 2,182 ft. The peripheral 

valley to the southwest has an elevation of approximately 

760ft. 

Low-lying areas adjacent to the study area are accessibleby dirt roads (fig. 1). The northern part of the study 

area can be reached from the Agua Caliente improved dirt 

road, which connects with several unimproved dirt roads 

that lead south into the study area. Southern access can be 

obtained through Woolsey Wash or an unimproved road 

parallel to the wash. 

The arid climate supports typical southern Arizona 

desert fauna and flora. Paloverde, saguaro, and creosotebush are found throughout the study area's alluvial slopes 

and upland areas, and many varieties of desert wildflowers 

bloom seasonally. The washes are lined with mesquite, 

ironwood, acacia, and paloverde trees. Bighorn sheep, rattlesnake, and desert tortoise inhabit the area. 

Procedures and Sources of Data 

The USBM conducted a literature search and examined 

mining claim information, land-status maps, and oil- and 

gas-lease maps acquired from the BLM to determine the 

extent of mining and prospecting activity within the study 

area. In 1986, USBM personnel did site-specific sampling 

in and around the study area and carried out laboratory 

analyses of these samples by several spectrographic techniques (Kreidler, 1986). 

From 1984 through 1987, the USGS conducted detailed field investigations within and adjacent to the Signal 

Mountain Wilderness Study Area. This work included 

geologic mapping at scales of 1:48,000 (Floyd Gray and 

R.J. Miller, unpub. data), geochemical sampling, and examining outcrops for evidence of mineralization. The geochemical survey utilized rock and stream-sediment (including fine-fraction and heavy-mineral-concentrate) samples 

that were analyzed for 33 elements by semiquantitative 

emission spectrography. Gold, arsenic, antimony, cadium, 

mercury, zinc, and bismuth were analyzed by atomic-absorption methods, and uranium and thorium were analyzed 

by delayed-neutron count. Geophysical studies, based on 

published data, include interpretations of gravity, aeromagnetic, and gamma-ray spectrometric data. Further details 

on analytical procedures undertaken for this resource assessment are given in the appropriate sections that follow. 

Generalized regional studies that include the wilderness study area appear in Wilson and others (1957, 1969); 

more recent investigations of the geology include a study of 

the Webb Mountain district (Cheeseman, 1974). 

APPRAISAL. OF IDENTIFIED RESOURCES 

By Terry j. Kreidler 

U.S. Bureau of Mines 

USBM personnel reviewed sources of mineral 

information that include published and unpublished 

literature, USBM files, and mining claim and oil- and gaslease records at the BLM State Office in Phoenix. 

Mineral Resources of the Signal Mountain Wilderness Study Area, Arizona Cl 
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Ag 

Au 

Cu 

Geo 

O,G 

Pb 

Qa 

QTa 

Tr 

" 10' 

APPROXIMATE BOUNDARY 

OF 

SIGNAL MOUNTAIN 

WILDERNESS STUDY AREA 

(AZ-020- 138) 

Area having low mineral resource potential (L) 

Levels of certainty of assessment Data only suggest level of potential 

Data give good indication of level of potential 

Commodities 

Silver 

Gold 

Copper 

Geothermal energy 

Oil and gas 

Lead 

Description of map units 

Alluvium (Quatemary}-Unconsolidated sand and 

gravel deposits 

Older alluvium (Quaternary and Tertiary}-

Includes older gravels and minor basalt and tuff 

outcrops 

Rhyolite flows and plugs (Miocene}-Includes 

minor tuffs 

Tt 

Tda 

Tb 

Trd 

Tba 

TKs 

E'g 

EXPLANATION 

Silicic tuff (Miocene}-Primarily unwelded air-fall 

tuff; may include ash-flow deposits 

Dacitic andesite flows (Miocene}-Biotite-, 

hornblende-, and pyroxene-bearing cliffforming flows 

Basalt flows and flow breccias (Miocene}-Thin 

flows and red-oxidized flow breccias; consists 

of olivine- and pyroxene-phenocryst-bearing 

basalts and minor tuffs that originated south of 

study area. Includes low-lying flows as well 

as mesa-forming basalt 

Rhyodacite flows and tuffs (Miocene}-Includes 

porphyritic volcanic rocks and minor thin silicic 

tuffs 

Basaltic andesite flows (Miocene}-Thin, olivinepyroxene-bearing flows underlying Tda unit 

and interbedded with Trd unit 

Sandstone and conglomerate (Tertiary to 

Cretaceous}-Pebbly conglomerate contains 

quartzite and chert fragments 

Granite (Proterozoic}-Leucocratic biotite-bearing 

granite 

Contact 

Fault- Dashed where approximately located 

Tb 

Tb 

112"55' 

Oa 

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CORRELATION OF MAP UNITS 

8 } QUATERNARY 

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a 

Unconformity 

B ]- CRETACEOUS 

Unconformity 

~ } PROTEROZOIC 

Figure 2. Mineral resource potential and generalized geology of Signal Mountain Wilderness Study Area, Maricopa County, 

Arizona. Geology mapped by Floyd Gray and Robert j. Miller intermittently from spring of 1984 through 1986. 

C4 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 
Discussions on the mineral resources of the study area were 

also held with BLM personnel at the Phoenix District 

Office. 

Fieldwork consisted of searching for mines and prospects-none were found within the study area-and sampling sediments in washes that drain the study area. Nine 

samples were taken, eight from within the study area. All 

were analyzed for gold and silver by fire assay and for 

copper, lead, and zinc by inductively coupled plasmaatomic emission spectroscopy. Complete analytical data 

are available for inspection at the U.S. Bureau of Mines, 

IFOC, Building 20, Denver Federal Center, Denver, CO 

80225. 

Mining Activity 

No evidence of mmmg or exploration was found 

within the study area. Mining has occurred in the Webb 

mining district 1 to 3 mi northeast of Signal Mountain. The 

district extends northwest from Webb Mountain to the 

Idazona mine. Between 1935 and 1951, copper (27,000 

lb), silver (200 troy oz), and gold (50 troy oz) were produced from six mines (Keith and others, 1983). The mines 

follow quartz veins in Proterozoic schist and gneiss that 

were intruded by granite, diorite, and porphyritic dikes of 

andesitic composition. The andesitic rocks at Webb Mountain, which are probably correlative with those in the study 

area, do not host any of the mineralization. 

In February and March 1986, Texasgulf Minerals and 

Metals Company drilled 10 exploration holes on a claim 

block 2 to 3 mi north of the study area near Black Butte. 

They were trying to delineate a gold trend following fractures and faults associated with a structural arch. The trend 

of the structure, N. 70° W., precludes it from intersecting 

the study area. The holes penetrated metamorphic and 

igneous rocks of probable Proterozoic age. 

Mineral Resource Appraisal 

Basalt and sand and gravel may be classified as inferred subeconomic resources; however, these are highvolume, low-unit-value materials that must be close to 

markets to be profitable. Andesite and basalt resembling 

that at Webb Mountain bear no signs of mineralization. 

Stream-sediment samples taken in the study area drainages 

contain insignificant amounts of gold, silver, copper, lead, 

and zinc (Kreidler, 1986). 

Energy Resources 

About 5,700 acres (37 percent) of the study area are 

covered by oil and gas leases; Ryder (1983), however, rates 

the oil and gas potential as low to zero because the rocks 

are volcanic. In the basins adjacent to the study area, the 

sedimentary rocks are no more than 5,000 ft thick, and 

conditions are not conducive to the accumulation of hydrocarbons. The leasing is probably based on speculation that 

the Wyoming overthrust belt, from which large quantities 

of hydrocarbons are produced, extends southward into 

Arizona (Keith, 1979). As of September 1986, results of 

all exploratory drilling to test this theory were negative. 

Leases in and near the study area, however, had not been 

tested. 

Hot springs occur in adjacent ranges to the south, but 

none are known within the study area. 

ASSESSMENT OF MINERAL RESOURCE 

POTENTIAL 

By Floyd Gray, Robert J. Miller, Jerry R. Hassemer, 

Daniel H. Knepper, James A. Pitkin, and William F. Hanna 

U.S. Geological Survey 

Geology 

The Signal Mountain Wilderness Study Area lies on 

the northern edge of a southward thickening sequence of 

Tertiary rocks (fig. 2) that becomes approximately 1 mi 

thick in the study area. Approximately 0.25 to 3 mi north 

and northeast of the study ·area, these rocks unconformably 

overlie a pre-Miocene basement sequence that probably 

extends beneath the volcanic rocks in the study area. Structure in the region is dominated by numerous large northwest-trending normal faults that bound fault blocks tilted as 

much as 50°. 

Basement rocks exposed just north of the study area, 

but not observed in it (fig. 2), consist of Proterozoic leucogranite and an overlying shallowly dipping sequence of 

Cretaceous to Tertiary coarse conglomerates and arkosic 

sandstones with minor interrcalated greenstone. 

A middle Miocene volcanic sequence deposited unconformably on the basement complex underlies the entire 

study area. The basal unit of this sequence consists of 

basaltic andesite. Minor reworked volcaniclastic sediments 

and bedded cinder deposits are locally present within the 

basal unit. The upper part of the unit is intercalated with 

a widespread rhyodacite unit Basalts probably originating 

from the Woolsey Peak area form the most extensive volcanic unit in the area. These rocks are overlain by more 

silicic rocks consisting mainly of porphyritic hornblendebiotite-bearing dacitic andesite flows and plugs and minor 

air-fall tuff. A major cliff-forming unit at the top of the 

section includes a crystal-rich dacitic tuff that underlies 

Signal Mountain. Rhyolite flows and plugs form the youngest Miocene unit in the area. 

Four unconsolidated Tertiary and Quaternary units are 

present in the area, but for simplification only the two most 

extensive units are shown on figure 2. Flanking some of 

the peaks are poorly sorted, semiconsolidated to unconsolidated, locally derived terrace gravels. Closer to some of 

Mineral Resources of the Signal Mountain Wilderness Study Area, Arizona C5 
the mountains are talus slopes that obscure underlying 

geology. Widespread pediment surfaces are covered by 

older alluvium, and major drainage channels contain Quaternary alluvium. 

Numerous high-angle, northwest-trending normal 

faults having large to moderate displacement are present 

throughout the study area. They caused significant tilting 

of fault-bounded blocks throughout this part of the Gila 

Bend Mountains. These faults are probably older than most 

basin-and-range faults in the region and form 'part of a 

prominent cluster of northwest-oriented structures in this 

part of the Basin and Range province. The faults trend 

principally N. 40° to 55° W., and the. fault blocks tilt as 

much as 50°. 

Geochemistry 

Methods 

Geochemical interpretation is based on analytical data 

from heavy-mineral-concentrate, stream-sediment, and 

whole-rock samples. In 1986 the USGS collected streamsediment samples from 77 sites within the study area. A 

panned, nonmagnetic, heavy-mineral concentrate was prepared from a portion of the sediment sample collected at 

each site. 

Stream-sediment samples represent material eroded 

from drainage basins upstream from the collection sites. 

The panned concentrate represents selectively concentrated 

minerals that may be related to mineralization. Chemical 

analyses of the minus-80-mesh and panned-concentrate 

fractions of stream sediments were used to identify areas of 

mineral resource potential. Samples were analyzed for 31 

elements using a six-step direct-current arc semiquantitative emission-spectrographic method (Grimes and Marranzino, 1968); samples were analyzed for gold by an 

atomic-absorption method and for uranium and thorium by 

the delayed-neutron-counting method of Millard (1976). 

Anomalous geochemical values were determined by 

inspection of histograms, percentiles, and enrichment of 

elements relative to crustal abundance. Commonly these 

anomalies reflect known mining activity, but in some cases 

they indicate the location of undisclosed or previously 

unrecognized metal concentrations. In general, the higher 

the analytical values from a sampling site, or the greater the 

number of anomalous elements, the more significant that 

site (or drainage basin) is in terms of mineral potential. 

Results 

The heavy-mineral-concentrate, rock, and stream-sediment samples collected within the study area contain no 

anomalous element concentrations and show little geochemical variation between rock types. The geochemical 

data give no indication of mineralization in the study area. 

Several samples collected about 0.25 mi north of the 

study area show anomalous multi-element suites. These 

anomalies include molybdenum, arsenic, lead, manganese, 

and copper in panned-concentrate samples. The suite of 

elements is similar to those found in the Webb mining 

district and near Black Butte. 

Geophysics 

Gravity and Aeromagnetic Studies 

The Signal Mountain Wilderness Study Area is covered by regional gravity surveys (Lysonski and others, 

1980a, b; Aiken, 1976; Aiken and others, 1981) and detailed aeromagnetic surveys (U.S. Geological Survey, 

1987) having sufficient resolution to define anomalies as 

small as a few square miles. Contours of complete (terraincorrected) Bouguer gravity anomalies are defined by 17 

observation points, all of which are at the periphery or 

outside of the area (fig. 3). Aeromagnetic anomalies are 

defined by horizontal gradiometer measurements made 

along 10 east-west flightlines spaced approximately 0.5 mi 

apart at a mean terrain clearance of 900 ft ((ig. 4). 

Most of the study area is covered by the south flank of 

a broad gravity high (fig. 3). To the northeast, north, and 

northwest, this high is bordered by a steep gradient that 

trends approximately east-west. The steepness of gradient 

and nearly linear trend of the contours suggest that the 

anomaly source has a faulted boundary to the north, a reFigure 3. Bouguer gravity map of vicinity of Signal Mountain 

Wilderness Study Area, Maricopa County, Arizona. Contour 

interval 1 milligal; hachures indicate closed area of lower 

gravity values. Small circles represent gravity observation 

points. 

C6 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 
lation commonly observed elsewhere in the Basin and 

Range province. To the southeast and southwest are areas 

of gravity lows having gentle gradients. The mushroomshaped high, which includes a saddle-shaped extension 

south of the study area, conforms remarkably well to the 

mapped distribution of volcanic rocks. This diverse assemblage of volcanic rocks presumably overlies metamorphic 

rocks. Thus, the high is known to be caused in part by 

volcanic rocks and is presumed to be caused in part by 

underlying metamorphic rocks, perhaps similar to those 

exposed north of the study area. On the basis of simple 

gravity modeling, the metamorphic and volcanic rocks may 

have similar average densities that contrast with that of the 

basin fill of alluvium by about 0.5 grams per cubic centimeter (g/cm3

). Such a density contrast, if applied to a 

steeply faulted structure, suggests that low-density basin fill 

is at least 1 mi thick several miles north of the study area 

and less than 0.3 mi thick southeast and southwest of the 

study area. 

The aeromagnetic anomaly map of the study area is 

characterized by magnetic anomalies of highly variable 

amplitude and wavelength (fig. 4), which correlate principally with volcanic rocks or their deeper seated equivalents 

of highly variable composition. Many of the magnetic lows 

appear to be generated by mafic rocks that show remanent 

magnetization of much greater magnitude than their induced magnetization. These remanent magnetic lows correlate spatially with their source rocks much more precisely 

than do other types of lows that commonly are paired with 

an adjacent anomaly, both being produced by a single 

source. These prominent anomalies correlate only in part 

with exposed rocks that are expected to be very strongly 

magnetic, especially the basalt. This lack of total correlation implies that subsurface hypabyssal feeders may underlie the basalt or that the basalt thickens preferentially over 

some ancient terranes. Finally, anomalies in the study area 

appear to coincide with those of a region where anomalies 

trending one direction change to a perpendicular direction. 

Areas of greatest geophysical significance are shown 

in figure 4. Region I is nonmagnetic, correlating mainly 

with a tract mapped as rhyodacite and related volcanic 

rocks. This region might serve as a window to magnetic 

basement rocks that underlie the volcanic rock pile, thus 

permitting an estimate of the volcanic rock thickness. We 

Figure 4. Aeromagnetic map of vicinity of Signal Mountain Wilderness Study Area, Maricopa County, Arizona (bold outline). 

Contour intervals 20 and 100 nanoteslas; hachures indicate closed areas of lower values. Numbered regions of geophysical 

significance discussed in text: I, nonmagnetic rocks; II, normally magnetized mafic rocks; Ill, rocks having short wavelengths; 

IV, reversely magnetized rocks. 

Mineral Resources of the Signal Mountain Wilderness Study Area, Arizona C7 
tentatively infer a volcanic rock thickness here of about 1 

mi. 

Region II is underlain by normally magnetized mafic 

rock and transects mapped contacts between basalt, a 

mixture of silicic tuff and basalt, and Quaternary units. It 

is not known how much of this feature is directly attributable to surficial basalt, but, because the feature trends 

across mapped boundaries, it may be caused mainly by a 

subvolcanic unit, possibly 0. 7 mi thick, most of which is 

buried to a depth of about 2,100 ft. This hypabyssal unit 

may be similar to the assumed basement rocks of region I. 

This prominent magnetic high has a distinctive east-west 

trend and separates northwest-trending anomalies to the 

northwest from northeast-trending anomalies to the southeast. 

Region III is characterized by short-wavelength features directly associated with surficial basalt. If the basalt 

of this region has magnetic properties similar to the basalt 

a few miles to the southeast, its thickness ranges from less 

than 30 to about 7 5 ft. Some of the small circular features 

occur directly over mapped basalt while others occur over 

an area of Tertiary and Quaternary alluvium mixed with 

silicic tuff and basalt. 

Region IV is underlain mostly by reversely magnetized 

mafic rock mapped as basalt. This region is represented by 

a northeast-trending belt of magentic lows that extends 

almost continuously northeastward to a small low associated with mapped J urrasic diorite at Webb Mountain, about 

. 1. 7 mi northeast of tthe map area. The lows within region 

IV may be accounted for by basalt about 75 ft thick if its 

magnetic properties are similar to those of basalt a few 

miles to the southeast. However, the apparent continuity of 

this belt of lows to the region of diorite at Webb Mountain 

suggests the possibility that the belt of lows might represent 

a narrow zone of weakness through which intrusion or extrusion, or both, has recurred throughout a long period of 

geologic time. 

Aerial gamma-ray spectrometry measures the nearsurface (0- to 20-in. depth} distribution of the natural radioelements potassium (K), equivalent uranium (eU), and 

equivalent thorium (eTill}. Because this distribution is 

controlled by geologic processes, aerial gamma-ray measurements can be used in geologic mapping, mineral exploration, and understanding of geologic processes. Calibration of aerial systems at sites of known radioelement concentrations permits survey results to be reported in percentage forK and paras per million (ppm) for eU and eTh. 

The spectrometry data discussed m iliis report were 

obtained 1by tthe NURE ]program of tthe U.S. Department of 

Energy (LKB Resoi\JlJrCes, 1979). NURE dam acquistion 

was keyed! oo 1 o 1by 2° to]!X>graplhic qanadrangles, and flightnine spacing was usually at 3-m:i ami! 6-m:i intervals. The 

coorse spacmg means that tthe dlata are suim1bRe for producing maps at scales of only 1:500,000 and smaller. All flight 

altitudes were 400ft above ground level, which permits an 

aerial gamma-ray system to effectively detect terrestrial 

gamma radiation from a swath 800 ft wide along the flightline. 

NURE data for Arizona and adjacent parts of California and New Mexico have been compiled and processed to 

produce sets of maps at scales of 1:500,000 and 

1: 1 ,000,000. They include contour maps and composite 

color-image maps, the latter produced as described by· 

Duval (1983). Spectrometric data described in this report 

were derived from the NURE report for the Phoenix 1° by 

2° quadrangle (LKB Resources, 1979). 

The Signal Mountain Wilderness Study Area is characterized by radio-element concentrations of 1 to 1.5 percent 

K, 2 to 3 ppm eU, and 4 to 6 ppm eTh. These concentrations are determined from three NURE flightlines that 

transect the study area and provide sufficient control for 

defining radioelement distribution for the area. The concentrations detected are not anomalous for the Tertiary and 

Quaternary sedimentary and volcanic rocks that occur in 

the study area. The sparse flightline coverage of the study 

area, however, precluded deriving information on mineral 

resource potential from the aerial gamma-ray data. 

Basalt and sand and, gravel in the Signal Mountain 

Wilderness Study Area may be classified as inferred subeconomic resources for aggregate and fill, but other suitable 

material is available closer to potential markets. 

There are no direct indications of mineral or energy 

resource potential, even though the area lies near an area 

characterized by hydrothermal vein or skarn (in carbonaterich rock) deposits that contain silver, copper, lead, manganese, and gold. Geologic studies, geochemical sampling, 

examination of mines and prospects, and review of mine 

production and ore types indicate that these deposits are 

mainly associated with northwest-trending, high-angle 

faults that cut the Webb Mountain area. Because multi-element suites of anomalous elements occur in rocks 0.25 mi 

north of the study area, a low mineral resource potential for 

copper, lead, silver, and gold resources, certainty level B, 

was asssigned along 2 mi of the north boundary of the 

study area (fig. 2). 

The potential for oil and gas resources in the study area 

is low, certainty level C. This evaluation is based on the 

absence of organic-rich rocks that might be a source of 

hydrocarbons and on the lack of suitable reservoir rocks. 

Although hot springs occur in adjacent ranges to the 

south, no evidenc~ of geothermal activity was seen in or 

near the studly area. No geothermal leases exist in the 

Signal! Mountain Wilderness Study Area. The area has low 

potential, certainly level B, for geothermal energy 

resources. 
REFERENCES CITED 

Aiken, C.L. V., 1976, Analysis of gravity anomalies in Arizona: 

Tucson, University of Arizona, Ph.D. dissertation, 127 p. 

Aiken, C.L.V., Lysonski, J.C., Sumner, J.S., and Hahman, W.R., 

1981, A series of 1:250,000 complete residual Bouguer gravity anomaly maps of Arizona: Arizona Geological Society 

Digest, v. 13, p. 31-37. 

Beikman, 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. 

Cheeseman, R.I., 1974, The geology of the Webb Mountain district, Gila Bend Mountains, Maricopa County, southwestern 

Arizona: Flagstaff, Northern Arizona University, M.S. thesis, 

69 p. 

Duval, J.S., 1983, Composite color images of aerial gamma-ray 

spectrometric data: Geophysics, v. 48, no. 6, p. 722-735. 

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. 

Keith, S.B., 1979, The great southwestern Arizona overthrust oil 

and gas play: Arizona Bureau of Geology and Mineral Technology, Fieldnotes, v. 9, no. 1, p. 10-14. 

Keith, S.B., Gest, D.E., DeWitt, Ed, Toll, N.W., and Everson, 

B.A., 1983, Metallic mineral districts and production in Arizona: Arizona Bureau of Geology and Mineral Technology 

Bulletin 194, 58 p. 

Kreidler, T.J., 1986, Mineral investigation of a part of the Signal 

Mountain Wilderness Study Area (AZ-020-138), Maricopa 

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

MLA 62-86, 9 p. 

LKB Resources, 1979, Aerial gamma-ray and magnetic reconnaissance survey, Colorado-Arizona area; Salton Sea, Phoenix, El Centro, Ajo, Lukeville quadrangles: U.S. Department 

of Energy, Report GJBX-12(80), v. 2. 

Lysonski, J.C., Sumner, J.S., Aiken, C.L.V., and Schmidt, J.S., 

1980a, Complete residual Bouguer gravity anomaly map of 

Arizona: Arizona Bureau of Geology and Mineral Technology, scale 1:500,000. 

---1980b, The complete residual Bouguer gravity anomaly 

map of Arizona (ISGN 71): Tucson, University of Arizona, 

Department of Geoscience, Laboratory of Geophysics, scales 

1:500,000 and 1: 1,000,000. 

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

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

Millard, H.T., Jr., 1976, Determination of uranium and thorium in 

USGS standard rocks by the delayed neutron technique, in 

Flanagan, F.J., ed., Descriptions and analyses of eight new 

USGS rock standards: U.S. Geological Survey Professional 

Paper 840, p. 61--65. 

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

Arizona: U.S. Geological Survey Circular 902-C, p. Cl-C22. 

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, 1987, Aeromagnetic map of the Woolsey 

Peak-Dendora Valley area, southwestern Arizona: U.S. 

Geological Survey Open-File Report 87-333, scale 1:50,000. 

Wilson, E.D., Moore, R.T., and Cooper, J.R., compilers, 1969, 

Geologic map of Arizona: Arizona Bureau of Mines and U.S. 

Geological Survey, scale 1:500,000. 

Wilson, E.D., Moore, R.T., and Pierce, H.W., 1957, Geologic map 

of Maricopa County, Arizona: Arizona Bureau of Mines, 

scale 1:375,000. 

Mineral Resources of the Signal Mountain Wilderness Study Area, Arizona C9 
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. 

r 

...J 

< 

i= 

z 

w 

t2 

w 

u 

~ 

::::> 

0 

V') 

w 

~ 

u.. 

0 

...J 

w 

> w 

...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. 

C12 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 
RESOURCE/RESERVE CLASSIFICATION 

IDENTIFIED RESOURCES UNDISCOVERED RESOURCES 

Demonstrated Probabi I ity Range 

Inferred - Measured Indicated Hypothetical Speculative 

I Inferred 

I 

ECONOMIC Reserves 

Reserves 

I I ----~---- ----------- -~-

MARGINALLY Marginal Inferred 

Marginal ECONOMIC Reserves Reserves 

I I - ____ T ____ ---------- -~-

SUB- Demonstrated Inferred 

Subeconomic Subeconomic 

ECONOMIC Resources Resources I I I 

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

resource estimates and public pol icy: 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 Signal Mountain Wilderness Study Area, Arizona C13 
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 OliRocene 

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 

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - (3800l)- - - - - - - - - - - - - - - - - -

pre-Archean2 

•Rocks older than 570 Ma also called Precambrian, a time term without specific rank. 21nformal time term without specific rank. 

C14 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 

AGE ESTIMATES OF 

BOUNDARIES IN 

MILLION YEARS (Ma) 

0.010 

1.7 

5 

24 

38 

55 

66 

r--- 96 

138 

205 

-240 

290 

-330 

360 

410 

435 

500 

1

-570 

900 

1600 

2500 

3000 

3400 

4550 --
r?Sii"~(Q)!dJ~C<dl~~ 

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Gray; Qthers-MINERAL RESOURCES, SIGNAL MTN. WILDERNESS. STUDY AREA, MARICOPA CO., ARIZ.-U.S. Geologrc

.-.--.- ---....... --------..-- ~ --..---- . ------------... .... --..----.----... ·----------------· 

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