Mineral Resources of the Mecca Hills 

Wilderness Study Area; 

Riverside County, California

U.S. GEOLOGICAL SURVEY BULLETIN 1710-C

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Chapter C

Mineral Resources of the

Mecca Hills Wilderness Study Area,

Riverside County, California

By DOUGLAS M. MORION, JAMES E. KILBURN, 

and ANDREW GRISCOM 

U.S. Geological Survey

HARRY W. CAMPBELL 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1710

MINERAL RESOURCES OF WILDERNESS STUDY AREAS:

SOUTH-CENTRAL CALIFORNIA DESERT CONSERVATION AREA, CALIFORNIA
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1988

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

Mineral resources of the Mecca Hills Wilderness Study Area, 

Riverside County, California.

U.S. Geological Survey Bulletin 1710-C

Bibliography

Supt. of Docs, no.: I 19.3:1710-C

1. Mines and mineral resources California Calif. I.

Morton, Douglas M. II. Series.

QE75.B9no. 1710-C

[TN24.C2]

557.3s 

[553'.09794'97]

87-600432
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 a part of the Mecca Hills Wilderness Study Area 

(CDCA-343), Riverside County, California.

CONTENTS

Summary Cl

Abstract 1

Character and setting 1

Identified resources 1

Mineral resource potential 1 

Introduction 2

Area description 3

Previous and present investigations 3 

Appraisal of identified resources 3

History and production 3

Mineral occurrences 3 

Assessment of mineral resource potential 6

Geology 6

Geochemical studies 6

Geophysical studies 7

Mineral and energy resources 7 

References cited 8 

Appendixes

Definition of levels of mineral resource potential and certainty of assessment 12

Resource/reserve classification 13

Geologic time chart 14

FIGURES

1. Index map showing the location of the Mecca Hills Wilderness Study Area, Riverside County, 

California C2

2. Map showing mineral resource potential and generalized geology of the Mecca Hills Wilderness 

Study Area, Riverside County, California 4

Contents V

MINERAL RESOURCES OF WILDERNESS STUDY AREAS:

SOUTH-CENTRAL CALIFORNIA DESERT CONSERVATION AREA, CALIFORNIA

Mineral Resources of the

Mecca Hills Wilderness Study Area,

Riverside County, California

By Douglas M. Morton, James E. Kilburn, and Andrew Griscom 

U.S. Geological Survey

Harry W. Campbell 

U.S. Bureau of Mines

SUMMARY

Abstract

The part of the Mecca Hills Wilderness Study Area (CDCA343) for which mineral surveys were requested encompasses 

9,490 acres in the northeast corner of the Salton Trough in 

southern California. Field work for this report was conducted in 

1983 and 1985 by the U.S. Bureau of Mines and U.S. Geological 

Survey to assess the identified mineral resources (known) and 

the mineral resource potential (undiscovered) of the area.

No metallic mineral resources were identified in the study 

area, nor were any current mining claims found. Oil and gas 

leases were present in 1983. The Skeleton Canyon mine, in the 

southwestern part of the study area, yielded clay to seal nearby 

irrigation canals, but does not currently constitute a resource.

Areas within the San Andreas fault zone extending northwest and southeast from the clay deposit have moderate resource potential for clay. Undiscovered clay resources in this 

area are likely to be of little economic interest. The area 

southwest of the San Andreas fault has low potential for oi I and 

gas resources and the entire study area has moderate potential 

for low-temperature (less than 90°C) geothermal resources. The 

study area has low potential for barite and strontium resources.

Character and Setting

The Mecca Hills Wilderness Study Area is a badlands 

area of low but extremely rugged relief located at the north end 

of the Salton Trough, 13 mi southeast of Indio, Calif, (fig. 1). 

It is underlain by a basement of Proterozoic (see appendix for 

geologic time chart) gneiss and related rocks and the Mesozoic Orocopia Schist. The basement rocks are overlain mainly 

by a deformed sequence of upper Pliocene and Pleistocene 

nonmarine sedimentary rocks. The San Andreas fault crosses 

the southwestern part of the study area, where it cuts the 

Pliocene and Pleistocene Palm Springs Formation.

Identified Resources

All prospects within and near the study area were examined, and eight samples were collected and analyzed. There 

are no identified mineral resources in the study area.

The only known mine within the study area is the Skeleton Canyon mine, which has a large tonnage of clay that could 

become a resource if a market were identified. This mine 

consists of bulldozer cuts from which clay was mined to seal 

irrigation canals (Crowell and Sylvester, 1979, p. 157). The 

clay is present as low hills of brick-red fault gouge within the 

San Andreas fault zone and contains fragments and blocks of 

siltstone and sandstone. The clay unit strikes N. 60° W. and 

dips about 30° ME. Exposures of the clay cover about 30 acres 

of which 20 acres are inside the study area. The clay zone is 

about 4,900 ft long and 50 to 100 ft thick. It pinches out to the 

southeast and is buried by alluvium on the northwest.

Mineral Resource Potential

There is no direct evidence that mineralizing events took 

place within the Mecca Hills Wilderness Study Area. The 

areas within the San Andreas fault zone northwest and

Mineral Resources of the Mecca Hills Wilderness Study Area, Riverside County, California C1
southeast of the clay occurrence at the Skeleton Canyon mine 

have a moderate potential for clay resources, although 

undiscovered clay resources are unlikely to be economic. The 

area southwest of the San Andreas fault has low potential for 

oil and gas resources and the entire study area has moderate 

potential for low-temperature (less than 90 °C) geothermal 

resources. The study area has low potential for barite and 

strontium resources.

INTRODUCTION

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

Land Management 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 U.S. Bureau of Mines

116°30' 116°00'

Twentynine Palms

_ San Bernardino County 

Riverside County

JOSHUA TREE 

NATIONAL MONUMENT

APPROXIMATE BOUNDARY OF 

MECCA HILLS WILDERNESS ~~- 

STUDY AREA (CDCA 343)

8000'

A

Santa Rosa 

Mtn

AREA OF MAP

Figure 1. Index map showing the location of the Mecca Hills Wilderness Study Area, Riverside County, California.

C2 Mineral Resources of Wilderness Study Areas: South-central California Desert Conservation Area
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 the system described by U.S. 

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

by the U.S. Geological Survey 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 appendix for the 

definition of levels of mineral resource potential, certainty of 

assessment, and classification of identified resources.

Area Description

The part of the Mecca Hills Wilderness Study Area 

(CDCA-343) for which mineral surveys were requested encompasses 9,490 acres. It is located in southern California, in 

the northeast side of the Salton Trough, 13 mi southeast of 

Indio and 4 mi north of the Salton Sea (fig. 1). The study area 

includes parts of the Thermal Canyon, Mecca, and Mortmar 

7.5-minute quadrangles and part of the Cottonwood Spring 

15-minute quadrangle. The climate is arid and the vegetation 

sparse. Access to the study area is by unimproved dirt roads 

from California Highway 195 and Interstate Highway 10 (fig. 

1) and by flat-bottomed sandy washes that can be traveled by 

off-road vehicles. The Mecca Hills have low relief but are 

deeply dissected by a labyrinth of narrow, steep-walled canyons forming a spectacular badlands topography. Elevations 

in the study area range from about 40 to 1,600 feet above sea 

level.

Previous and Present Investigations

Field work for this report was carried out between 1983 

and 1985. Geologic maps, principally by A.G. Sylvester, 

provided the geologic basis for the mineral resource potential 

assessment. The general geology of the study area was 

described by Dibblee (1954), Hays (1957), Ware (1958), 

Sylvester (1979), and Sylvester and Smith (1975,1976). Oil 

and gas potential of the study area was evaluated by Scott 

(1983).

The U.S. Bureau of Mines (USBM) collected information related to current and past m ining activities within and adjacent to the study area. Field work, conducted during 1983, 

consisted of an examination of all mines and prospects in and 

near the study area and collecting eight samples for chemical 

analysis. A scintillometer was carried during the field investigations to detect radioactive minerals. Quantitative amounts 

of gold and silver were determined by fire assay; quantitative

amounts for other elements were determined by atomicabsorption or induction-coupled plasma analysis. The 

samples were also analyzed by semiquantitative spectregraphic methods for 40 elements. In addition, a preliminary 

ceramic evaluation of one clay sample was done. Complete 

analytical results are on file at the U.S. Bureau of Mines, 

Western Field Operations Center, E. 360 Third Ave., 

Spokane, WA 99202.

The U.S. Geological Survey (USGS) conducted geologic 

mapping in the study area between 1983 and 1984 to augment 

existing geologic maps and made a reconnaissance geochemical survey of stream sediments in 1984. Geophysical interpretations were made from gravity and aeromagnetic maps in 

1985.

APPRAISAL OF IDENTIFIED RESOURCES

By Harry W. Campbell 

U.S. Bureau of Mines

There are no identified mineral resources in the Mecca 

Hills Wilderness Study Area. Clay, sand, and gravel occurrences are located inside the Mecca Hills Wilderness Study 

Area.

History and Production

Oil and gas leases covered much of the Mecca Hills 

Wilderness Study Area in 1983. The folds and faults near the 

San Andreas fault zone locally form discontinuous, complex 

structures that make small but favorable structural traps for 

hydrocarbons (Sylvester and Smith, 1976, p. 2081,2101). Oil 

and gas exploration in the Mecca Hills has occurred sporadically since 1921 without positive results. The only mining 

activity known in the study area was near the mouth of 

Skeleton Canyon. Clay was extracted there and used to seal 

nearby irrigation canals. Clay was also mined from the Red 

Top mine west of the study area. Sand and gravel were 

produced from an alluvial fan south of the study area and used 

as aggregate and fill for local construction projects. In 1983 

a commercial sand and gravel pit was operating south of 

Thermal Canyon 4 mi west of the area. A well northwest of 

the study area reportedly produced hot water (Oesterling and 

others, 1964, p. 195).

Mineral Occurrences

The Skeleton Canyon mine consists of bulldozer cuts 

from which clay was mined (Crowell and Sylvester, 1979, p. 

157). The clay forms low hills of brick-red fault gouge within 

the San Andreas fault zone and contains fragments and blocks 

of siltstone and sandstone. The clay zone strikes N. 60° W. and 

dips about 30° NE. Exposures of the clay are about 4,900 ft

Mineral Resources of the Mecca Hills Wilderness Study Area, Riverside County, California C3
116C00'

APPROXIMATE BOUNDARY OF

MECCA HILLS WILDERNESS 

STUDY AREA (CDCA-343)

L/B Ba, Sr (entire study 

M/C Geo

M/C Clay 

L/C 0,G

Skeleton Canyon

Suprise Canyon 

clay occurrence

Figure 2. Mineral resource potential and generalized geology of the Mecca Hills Wilderness Study Area, Riverside 

County, California. Geology from A.G. Sylvester (unpub. data, 1985).

C4 Mineral Resources of Wilderness Study Areas: South-central California Desert Conservation Area
long, 50 to 100 ft thick, and cover about 30 acres of which 

about 20 acres are inside the study area. The clay zone 

probably pinches out southeast of the mine and is buried by 

alluvium to the northwest.

The Skeleton Canyon mine has a large tonnage of bentonitic clay that could be a resource if a market were identified. 

Small quantities of clay were mined for transitory projects, but 

local demand is currently insufficient to justify commercial 

operation of a clay pit. Favorable characteristics of this 

occurrence are: (1) it is minable by relatively low-cost, 

openpit methods; (2) more than five million tons have no 

overburden; (3) the clay unit is 50 to 100 ft thick; and (4) it is 

accessible from paved roads and is near rail transportation. 

Unfavorable features of the occurrence are: (1) the clay is 

impure and requires processing (for example, washing and 

screening); (2) the complex deformation in this vicinity makes 

continuity of the clay unpredictable where it is not exposed; 

(3) water supplies are not readily available; (4) current commercial markets are relatively far away; and (5) other clay 

deposits are already being mined. A detailed sampling program involving trenching and drilling would be necessary to 

precisely determine the tonnage and quality of clay that may 

be present.

Suitability for most uses depends upon the physical 

properties of bentonite, and specialized tests mustbe made for 

each intended use. For this report, one sample was tested for

EXPLANATION

Area with moderate and low mineral resource 

potential

See appendix for definition of levels of resource potential (M, L) 

and certainty of assessment (B, C)

Commodities

Ba 

Sr

O, G 

Geo

Clay 

Barium 

Strontium 

Oil and gas 

Geothermal

Geologic map units

Qa Alluvium (Quaternary)

Qc Ocotillo Conglomerate (Quaternary)

QTp Palm Springs Formation (Quaternary and Tertiary)

Tm Mecca Formation (Tertiary)

Mo Orocopia Schist (Tertiary)

Eg Gneiss, migmatite, and intrusive rocks (Proterozoic)

 Contact

 Fault Dotted where concealed

Figure 2. Continued.

some common physical and chemical properties. As shown 

by X-ray diffraction analysis, montmorillonite is the major 

clay mineral, but kaolinite is also present in minor quantities. 

Impurities include quartz, feldspar, calcite, dolomite, and 

minor muscovite and gypsum. Preliminary ceramic testing 

suggests that the clay is unsuitable for structural clay products 

because it is too soft and has a short firing range (Campbell, 

1984, p. 13). A chemical analysis of the clay gave the 

following weight percent for oxides: 53.1,SiO2; 14.0,A12O3 ; 

4.5, Fe2O3 ; 0.6, TiO2; 7.1, CaO; 3.8, MgO; 2.8, Nap; 1.7, 

K^O; 0.08, MnO; 0.7, SO3'2; and 0.3, P2O5 . According to the 

USBM clay classification system, this clay is a low-swelling 

calcium bentonite (Ampian, 1979, p. 2, 3). Calcium bentonites have poor colloidal properties and have little value for 

drilling mud and other uses requiring thixotropic suspensions.

A second clay occurrence in the study area is along the 

San Andreas fault at Surprise Canyon. This prospect covers 

approximately 4 acres and is about 2,000 ft long and 50 ft 

thick. The quality of the clay here appears similar to that at the 

Skeleton Canyon mine.

Sand and gravel occur in alluvial fans and braided 

streams throughout the Mecca Hills Wilderness Study Area. 

A principal source of sand and gravel in southern California 

is from alluvial fans that have built up along mountain fronts 

(Goldman, 1968, p. 5). Present production, however, is 

primarily near rapidly expanding urban areas, where construction and industrial markets exist, and is intermittent in 

more remote areas where public projects (for example, highways, canals, dams) are being constructed. Because the unit 

price of sand and gravel is low, and transportation costs are 

high, it typically must be produced near potential markets; 

however, existing deposits near urban markets are being 

depleted, and zoning restriction and urban expansion preclude 

the development of some deposits. Therefore, it may become 

necessary to transport sand and gravel from distant deposits 

that are now uneconomic (Leighton, 1980, p. 168). Large 

deposits of sand and gravel near rail transportation in the 

Coachella Valley (fig. 1) make the region around the Mecca 

Hills a likely source of sand and gravel resources for the Los 

Angeles Basin, but because there are adequate deposits outside the study area, it is unlikely that the sand and gravel occurrences in the Mecca Hills Wilderness Study Area would be 

developed.

Chip samples were taken across quartz veins in gneiss 

and schist and across altered zones adjacent to felsic dikes and 

stocks. Assay results for these samples showed no significant 

metal concentrations.

Anorthosite occurs in canyons in the north-central part of 

the study area. Anorthosite is an igneous rock containing from 

26 to 35 percent alumina (aluminum oxide). The USBM is 

investigating methods to produce aluminum from anorthosite 

(Eisele and Bauer, 1983). The ultimate domestic aluminum 

resource is expected to be a low-grade, nonbauxitic material 

such as anorthosite; however, utilization of these large, lowgrade sources of aluminum awaits development of processes 

that are economically competitive with the Bayer process for

Mineral Resources of the Mecca Hills Wilderness Study Area, Riverside County, California C5
extracting alumina from bauxite. The anorthosite occurrence 

in the study area is advantageously situated near the Los 

Angeles metropolitan area and close to a rail and highway 

network; however, a shortage of water and the overlying 

sedimentary rocks are unfavorable features of this occurrence 

and would hinder any future mining. Further work is needed 

to determine the size and quality of this anorthosite occurrence. The presence of 30 billion tons of anorthosite in the San 

Gabriel Mountains, about 100 mi northwest of Mecca Hills, 

makes the mining of anorthosite in the study area unlikely in 

the foreseeable future.

ASSESSMENT OF MINERAL RESOURCE POTENTIAL

By Douglas M. Morton, James E. Kilburn, 

and Andrew Griscom 

U.S. Geological Survey

Geology

The Mecca Hills Wilderness Study Area is located in the 

northeast margin of the Salton Trough, which is the northernmost extension of a major structural zone in North America 

that includes the Gulf of California. The San Andreas fault is 

a major strike-slip fault that forms the east boundary of the 

Salton Trough and crosses the southwest margin of the study 

area (fig. 2). The Mecca Hills comprise a northeast-oriented 

elongate dome formed by compression associated with movement along the San Andreas fault.

The study area is underlain by two groups of crystalline 

rocks that are generally exposed only in canyons that have cut 

through overlying sedimentary rocks. The older group is 

Proterozoic gneiss and migmatite that was intruded by 

Proterozoic anorthosite, felsic dikes, and related rocks. This 

gneiss-anorthosite complex structurally overlies the younger 

basement rock group, the Orocopia Schist, a deformed thick 

sequence of greenschist facies metasedimentary rock and 

minor metavolcanic rock of probable Late Cretaceous age. 

The two basement complexes are separated by a major shear 

zone, which also is probably Late Cretaceous (Sylvester and 

Smith, 1976) in age. Quartz veins cut both the gneiss and 

schist.

The oldest sedimentary rock within the study area is the 

late Pliocene(?) Mecca Formation (Dibblee, 1954), which 

consists of thick-bedded, reddish-brown, nonmarine arkose, 

breccia, and conglomerate. These sedimentary rocks are 

largely derived from the basement upon which they unconformably rest. Overlying and in part interfingering with the 

Mecca Formation is the late Pliocene and Pleistocene Palm 

Springs Formation (Dibblee, 1954), which consists largely of 

nonmarine arkose and conglomerate. These two formations 

thicken from northeastto south west (Dibblee, 1954; Sylvester 

and Smith, 1976).

The Pleistocene Ocotillo Conglomerate overlies and 

interfingers with the upper part of the Palm Springs Formation

(Dibblee, 1954). Unconsolidated young alluvium occurs 

along canyon bottoms.

The dominant structures in the study area are four major 

northwest-striking faults. The northeasternmost fault, the 

Platform fault, forms the southwest boundary of the Orocopia 

Schist. The basement rocks offset by this fault have only a thin 

cover of sedimentary rocks of the Mecca and Palm Springs 

Formations (Sylvester and Smith, 1976). The amount and 

type of movement on this fault is unknown. The Painted 

Canyon fault is 1 mi southwest of the Platform fault (fig. 2). 

Between the Platform and the Painted Canyon faults the 

Mecca and Palm Springs Formations were deformed into a 

series of anticlines and synclines (Sylvester and Smith, 1976). 

One mile southwest of the Painted Canyon fault is the Skeleton Canyon fault, which is part of the San Andreas fault zone 

(Sylvester and Smith, 1976). It is about 0.25 mi northeast of 

the San Andreas fault. The San Andreas fault, the master 

strike-slip fault in California, is located near the southwest 

margin of the study area and is well marked by scarps and 

offset drainages (Clark, 1984).

Geochemical Studies

Nonmagnetic heavy-mineral concentrates from stream 

sediments are the most useful samples in arid parts of the 

western United States and were thus selected as the sample 

medium for the reconnaissance geochemical survey. Seventeen stream sediment samples were collected near the margins 

of the Mecca Hills Wilderness Study Area from drainages 

ranging from fractions of a mile to several square miles. The 

samples were analysed by a six-step semiquantitative emission-spectrographic method (Grimes and Marranzino, 1968) 

for 30 elements. A complete listing of the analytical techniques and results are in Detra and Kilburn (1985).

Samples for the nonmagnetic heavy-mineral concentrates were composited from active alluvium and sieved 

through a 10-mesh screen and panned to remove most of the 

less dense material such as quartz, feldspar, clay, and organic 

material. The resulting sample was further concentrated by a 

hand magnet and a magnetic separator to remove magnetite 

and other magnetically susceptible minerals. The remaining 

nonmagnetic concentrate, which may contain gold, silver, and 

ore-forming sulfide and oxide minerals, was split into two 

parts, one for spectrographic analysis and the other for microscopic study.

Microscopic study identified visible amounts of the 

minerals barite (BaSO4), sphene (CaTiSiO5), zircon (ZrSiO4), 

and rutile (TiO2) in all of the samples. Three of the samples 

contained lead shot. Widespread anomalous concentrations 

of barium, titanium, and zirconium were found in the geochemical analyses reflecting the presence of barite, sphene, 

zircon, and rutile. Anomalous concentrations of barium 

ranged from 5,000 to about 10,000 parts per million (ppm). 

Anomalous concentrations of titanium were greater than 

20,000 ppm. Anomalous concentrations of zirconium were

C6 Mineral Resources of Wilderness Study Areas: South-central California Desert Conservation Area
greater than 2,000 ppm. Those samples containing lead shot 

had anomalous concentrations of lead ranging from 7,000 to 

50,000 ppm, and one sample had an anomalous concentration 

of antimony (500 ppm). Analysis of the lead shot indicated the 

antimony was in the lead shot.

Three samples contained anomalous concentrations of 

molybdenum; two samples contained 20 ppm and the third 30 

ppm. Three samples contained anomalous concentrations of 

tin; two samples contained 100 ppm and the third 150 ppm. A 

single sample contained 10,000 ppm of strontium. Strontium 

is generally present in association with barium; barite forms a 

complete solid-solution series with the mineral celestite, 

(SrSO^. This single anomaly is probably due to strontium 

within the solid-solution series of barite-celestite.

Barite may be derived from veins within the basement 

and (or) overlying sedimentary rock. No parent mineral could 

be determined for the anomalous concentrations of tin and 

molybdenum. However, their small amounts and isolated occurrences indicate little, if any, mineralization. All of the titanium, zirconium, tin, and molybdenum concentrations 

found in the samples are to be expected in sedimentary 

deposits derived from a granitic terrain. None of the anomalous concentrations of elements other than strontium and 

barium are considered to be significant in relation to undiscovered mineral resources.

Geophysical Studies

An aeromagnetic map (U.S. Geological Survey, 1983) 

was prepared from an aeromagnetic survey of the Salton Sea 

region flown in 1981 by a private contractor and includes the 

Mecca Hills Wilderness Study Area. Comparison of the 

contoured aeromagnetic map with the geologic map indicates 

that the study area can be divided into four regions, each with 

a characteristic magnetic pattern. The central region, which 

includes about two-thirds of the study area, is characterized by 

an irregular-shaped magnetic high with maximum amplitudes 

of 80 to 120 gammas. The source rocks for this high are 

probably the Proterozoic crystalline rocks, which are exposed 

in a few canyons and are probably no deeper than a few 

hundred feet below the exposed sediments. In the north 

quarter of the study area, a magnetic low is probably caused 

by the nonmagnetic Orocopia Schist A northwest-trending 

curvilinear magnetic gradient between these aeromagnetic 

features marks the approximate location of the Platform fault 

separating the schist from the Proterozoic rocks.

The third aeromagnetic region is at the extreme north end 

of the study area, where a small magnetic high is possibly 

caused by Proterozoic rocks north of the Orocopia Schist. The 

concealed contact between these two units probably lies on 

the high side of the magnetic gradient approximately along the 

-20 gamma contour.

At the extreme south and southwest limits of the study 

area, a magnetic low coincides with thick nonmagnetic sediments in the Coachella Valley. On the northeast side of this

low, a linear northwest-trending magnetic gradient coincides 

with the main strand of the San Andreas fault, which truncates 

the magnetic Proterozoic rocks.

Gravity data that include the study area are available as 

complete Bouguer anomaly maps (Elders and others, 1972, 

Biehler and Rotstein, 1982; Oliver and others, 1980) and as 

residual isostatic gravity maps (Roberts and others, 1981). 

The area underlain by the Orocopia Schist has a local gravity 

high of at least 15 milligals. The cause of the gravity high is 

not evident because the density of the schist is probably about 

the same or less than that of the surrounding Proterozoic rocks. 

However, the basement rocks beneath the schist are probably 

oceanic crust because the schist contains small amounts of 

metabasalt and ultramafic rocks (Haxel and Dillon, 1978). 

The gravity high may indicate shallow oceanic crust beneath 

the schist (Griscom, 1980), but, if so, these presumably 

magnetic rocks must be too deep to show on the aeromagnetic 

map. A steep gravity gradient along the San Andreas fault 

slopes down to the southwest into a prominent low associated 

with the thick low-density sediments in the Coachella Valley. 

Analysis of this low (Biehler, 1964) suggests at least 15,500 

ft of sediments, the maximum thickness being along the 

northeast side of the valley near the San Andreas fault.

Mineral and Energy Resources

There is no evidence that mineralizing events took place 

within the Mecca Hills Wilderness Study Area. The results of 

geochemical studies yielded no suites of anomalous elements 

indicative of events that would concentrate metals. Anomalous barite and strontium concentrations indicate chemical 

processes, but these processes were not sufficient to create a 

resource. There are no known mineral deposit models that fit 

the limited data on metals, barite, and strontium in this area. 

There is low potential, certainty level B for barite and strontium resources in the study area, based primarily on geochemical data. There is moderate potential, certainty level C, 

for clay resources along the San Andreas fault extending 

northwest and southeast from a known clay occurrence and 

the Skeleton Canyon mine. Undiscovered clay would probably be impure, with narrow but deep deposits within the fault 

zone making development highly unlikely. The entire Mecca 

Hills Wilderness Study Area is part of a larger Salton Trough 

region that has "significant lateral extent favorable for discovery and development of local sources of low-temperature (less 

than 90 °C) geothermal water" (Muffler, 1979, Map 1). A 

thermal spring ha.ving a surface temperature of less than or 

equal to 50 °C is near or possibly within the study area 

(Muffler, 1979, map 1). Low-temperature geothermal resources may be used for direct heating or for drying agricultural or manufactured products. The heat source for these 

geothermal fluids is assumed to be a probable magmatic body 

at depth that helps heat fluids circulating along the fault zones 

(Sammel, 1979, p. 106-107). The potential for low-temperature (less than 90 °C) geothermal resources is moderate within

Mineral Resources of the Mecca Hills Wilderness Study Area, Riverside County, California C7
the study area, with certainty level of C. Roses are grown in 

Mecca using direct heat from geothermal wells (W. A. Elders, 

oral commun., 1987), however water from these wells may be 

related to geothermal systems south or wouthwest of Mecca, 

away from Mecca Hills.

An earlier, regional evaluation of the petroleum potential 

of certain lands in California found the Mecca Hills Wilderness Study Area to have low to zero potential for oil and gas 

(Scott, 1983). The area lacks marine source rocks, although 

such rocks may underlie the Coachella Valley west of the 

study area. One drill hole to 3,792 ft was drilled in R. 7 S., R. 

10 E., sec. 25 south of the Mecca Hills Wilderness Study Area 

in 1923. The driller's well log of the hold, Spindletop Oil 

Association Well No. 1, reported shows of gas and possibly 

heavy oil (D. Curtis, oral commun., 1987). There has been no 

known work in the intervening six decades to confirm this 

report. Oil and gas may have matured and been driven up 

along the flanks of the Salton Trough by geothermal processes, however, this is highly speculative. There are no direct 

indications that oil or gas may be present in the study area; 

however, the area southwest of the San Andreas fault has low 

potential for oil and gas resources, with a certainty of C.

REFERENCES CITED

Ampian, S.G., 1979, Clays: U.S. Bureau of Mines Mineral 

Commodity Profile, 16 p.

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.

Biehler, Shawn, 1964, Geophysical study of the Salton Trough 

of southern California: Los Angeles, California Institute of 

Technology, Ph.D. dissertation, 139 p.

Biehler, Shawn, and Rotstein, Yair, 1982, Bouguer gravity map 

of California, Salton Sea sheet: California Division of Mines 

and Geology, scale 1:250,000.

Campbell, H.W., 1984, Mineral resources of the Mecca Hills 

Wilderness Study Area (BLM No. CDCA-343), Riverside 

County, California: U.S. Bureau of Mines Open-file Report 

35-84, 17 p.

Clark, M.M., 1984, Map showing recently active breaks of the 

San Andreas fault and associated faults between Salton Sea 

and Whitewater River, Mission Creek, California: U.S. 

Geological Survey Miscellaneous Investigation Series Map 

1-1483, scale 1:24,000.

Crowell, J.C., and Sylvester, A.G., 1979, Tectonics of the juncture between the San Andreas fault system and the Salton 

Trough, southeastern California: University of California at 

Santa Barbara Guidebook, p. 157-161.

Detra, D.E., and Kilburn, J.E., 1985, Analytical results and 

sample locality maps of heavy-mineral-concentrate 

samples from the Morongo Valley (CDCA-218) and Mecca 

Hills (CDCA-343) Wilderness Study Areas, San Bernardino 

and Riverside Counties, California: U.S. Geological Survey 

Open-File Report 85-244.

Dibblee, T.W., Jr., 1954, Geology of the Imperial Val ley region, 

California, /njahns, R.H,ed., Geology of southern California: California Division of Mines Bulletin 170, p. 21-28.

Eisele, J.A., and Bauer, D.J., 1983, Recovery of alumina from 

anorthosite by an HCl-F-Gas sparging crystallization process: U.S. Bureau of Mines Report of Investigations 8806, 

11p.

Elders, W.A., Rex, W.R., Meidav, Tsvi, Robinson, P.T., and 

Biehler, Shawn, 1972, Crustal spreading in southern California: Science, v. 178, no. 4056, p. 15-24.

Goldman, H.B., 1968, Sand and gravel in California: California 

Division of Mines and Geology Bulletin 180-C, 56 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-currentspark emission spectrographic field 

methods for the semiquantitative analysis of geologic 

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

Griscom, Andrew, 1980, Salton Trough, in Oliver, H.W., ed., 

Interpretation of the gravity map of California and its 

continental margin: California Division of Mines and 

Geology Bulletin 205, p. 20-21.

Haxel, Gordon, and Dillon, John, 1978, The Pelona-Orocopia 

schist and the Vincent-Chocolate Mountain thrust system, 

southern California, in Howell, D.G., and McDougall, 

K.A., eds., Mesozoic: Society of Economic Paleontologists 

and Mineralogists, Pacific Section, Pacific Coast Paleogeography Symposium 2, p. 453-469.

Hays, W.H., 1957, Geology of the central Mecca Hills, Riverside County, California: New Haven, Conn., Yale University, Ph.D. dissertation, 324 p.

Leighton, F.B., 1980, Sand and gravel a largely untapped 

mineral resource of the California desert, m Fife, D.L., and 

Brown, A.R., eds., Geology and mineral wealth of the 

California desert: South Coast Geological Society, Dibblee 

Volume, p. 167-170.

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

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

790, 163 p.

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W.C., Oesterling, W.A., Spurck, W.H., and Tischler, M.S., 

eds., Minerals for industry: Southern Pacific Company 

Volume III, 242 p.

Oliver, H.W., Chapman, R.H., Biehler, Shawn, Robbins, S.L., 

Hanna, W.F., Griscom, Andrew, Beyer, L.A., and Silver, 

E.A., 1980, Gravity map of California and its continental 

margin: California Division of Mines and Geology, scale 

1:750,000 2 sheets.

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Preliminary isostatic residual gravity map of California: 

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1:750,000,5 sheets.

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Assessment of geothermal resources of the United States 

1978: U.S. Geological Survey Circular 790, 163p.

Scott, E.W., 1983, Petroleum potential of wilderness lands in 

California, in Miller, B.M., ed., Petroleum potential of 

wilderness lands in the western United States: U.S. GeoC8 Mineral Resources of Wilderness Study Areas: South-central California Desert Conservation Area
logical Survey Circular 902-A-P, p. D1-D12.

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faultzone, Mecca Hills, in CrowellJ.C., and Sylvester A.G., 

eds., Tectonics of the juncture between the San Andreas 

faultsystem and the SaltonTrough, southeastern California: 

University of California at Santa Barbara Guidebook, p. 

125-139.

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the San Andreas fault zone, Mecca, in Crowell, J.C., ed., 

San Andreas fault in southern California: California Division of Mines and Geology Special Report 118, p. 11 -118.

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Sea area, California: U.S. Geological Survey Open-File 

Report 83-664, scale 1:250,000.

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University of California at Los Angeles, M.S. thesis, 60 p.

Mineral Resources of the Mecca Hills Wilderness Study Area, Riverside County, California C9

APPENDIXES
DEFINITION OF LEVELS OF MINERAL RESOURCE POTENTIAL 

AND CERTAINTY OF ASSESSMENT

Definitions of Mineral Resource Potential

LOW mineral resource potential is assigned to areas where geologic, geochemical, and geophysical 

characteristics define a geologic environment in which the existence of resources is permissive. This 

broad category embraces areas with dispersed but insignificantly mineralized rock as well as areas with 

few or no indications of having been mineralized.

MODERATE mineral resource potential is assigned to areas where geologic, geochemical, and geophysical 

characteristics indicate a geologic environment favorable for resource occurrence, where interpretations 

of data indicate reasonable likelihood of resource accumulation, and (or) where an application of 

mineral-deposit models indicates favorable ground for the specified type(s) of deposits.

HIGH mineral resource potential is assigned to areas where geologic, geochemical, and geophysical 

characteristics indicate a geologic environment favorable for resource occurence, where interpretations of 

data indicate a high degree of likelihood for resource accumulation, where data supports 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.

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

moderate, or high levels of resource potential.

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

Levels of Certainty

U/A

UNKNOWN

POTENTIAL

H/B

HIGH POTENTIAL

M/B

MODERATE POTENTIAL

L/B

LOW

POTENTIAL

H/C

HIGH POTENTIAL

M/C

MODERATE POTENTIAL

L/C

LOW

POTENTIAL

H/D

HIGH POTENTIAL

M/D

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

t

o

a.

CJ

cc

D 

O 

C/5

B C 

LEVEL OF CERTAINTY

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

B. Available information 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.

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-^2. 

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: South-central California Desert Conservation Area
RESOURCE/RESERVE CLASSIFICATION

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

IDENTIFIED RESOURCES

Demonstrated

Measured Indicated

I 

Reserves

I

 | ______

I

Marginal 

Reserves

I

I 

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 U.S. Bureau of Mines 

and U.S. Geological Survey, 1980, Principles of a resource/reserve classification for minerals: U.S. Geobgical Survey Circular 831, p. 5.

Mineral Resources of the Mecca Hills Wilderness Study Area, Riverside County, California C13
GEOLOGIC TIME CHART 

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

EON

Phanerozoic

Proterozoic

Archean

pre- Arc

ERA

Cenozoic

Mesozoic

Paleozoic

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean 

hean 2

PERIOD

Quaternary

Tertiary

Neogene 

Subperiod

Paleogene 

Subperiod

Cretaceous

Jurassic

Triassic

Permian

Carboniferous 

Periods

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

 -(3800?)- -

EPOCH

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

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 Ma)

- 0.010 

- 1.7

f\M

- 24

oo

_ RR

- 96

one

- -240 

- 290 ^\j\j

*\*\r\ - ~330

- 410

- 435 A *if?

- 500

570

K- 1600

^ 2500

^ 3000 

- 3400

AKKH

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

Informal time term without specific rank.

C14 Mineral Resources of Wilderness Study Areas: South-central California Desert Conservation Area

GPO 585-045/78040