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Chapter E 

Mineral Resources of the 

Orocopia Mountains Wilderness Study Area, 

Riverside County, California 

By GORDON B. HAXEL, DAVID B. SMITH, 

CHARLES L. WHITTINGTON, ANDREW GRISCOM, 

DENNY V. DIVELEY-WHITE, and ROBERT E. POWELL 

U.S. Geological Survey 

TERRY J. KREIDLER 

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 HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade names and trademarks 

in this publication is for descriptive 

purposes only and does not constitute 

endorsement by the U.S. Geological Survey 

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 Orocopia Mountains Widerness 

Study Area, Riverside County, California. 

(U.S. Geological Survey Bulletin; 1710-E 

Bibliography: p. 

Supt. of Docs. no. : I 19.3:171 0-E 

1. Mines and mineral resources--California--Orocopia 

Mountains Wilderness. 2. Orocopia Mountains Wilderness 

(Calif.) I. Haxel, Gordon. II. Series. 

QE75.B9 no. 1710-E 557.3 s [553'.09794'97] 88-600428 

[TN24.C2] 
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 part of the Orocopia Mountains Wilderness Study Area (CDCA-344), Riverside County, California. 
CONTENTS 

Summary El 

Abstract El 

Character and setting El 

Identified mineral resources E2 

Mineral resource potential E2 

Introduction E3 

Location and physiography E3 

Previous and present investigations E4 

Appraisal of identified resources E4 

Methods of investigation E4 

Mining history ES 

Appraisal of sites examined ES 

Big Bullett claims ES 

Talc occurrences ES 

Orocopia mine (gold) E6 

Orocopia Fluorspar mine E6 

Oil and gas E7 

Geothermal resources E7 

Conclusions E7 

Assessment of mineral resource potential E7 

Geology E7 

Lithologic units E7 

Structure E8 

Geochemical studies E9 

Geophysical studies E12 

Mineral resource assessment El3 

Crystalline-rock-hosted disseminated gold E13 

Gold-bearing quartz veins El3 

Gold associated with propylitically altered mafic dikes E14 

Volcanogenic massive sulfide base metals and gold E14 

Talc E15 

Fluorite El5 

Evaporite deposits E15 

Porphyry copper-molybdenum E15 

Placer gold El6 

Geothermal energy E16 

Oil and gas El6 

References cited E16 

Appendixes 

Definition of levels of resource potential and certainty of assessment E20 

Resource/reserve classification E21 

Geologic time chart E22 

FIGURES 

1. Index map showing location of the Orocopia Mountains Wilderness Study Area, Riverside County, 

California E2 

2. Map showing mineral resource potential and generalized geology of the Orocopia Mountains 

Wilderness Study Area, Riverside County, California ElO 

TABLE 

1. Mineral deposits in and near the Orocopia Mountains Wilderness Study Area E6 

Contents V 
Mineral Resources of the 

Orocopia Mountains Wilderness Study Area, 

Riverside County, California 

By Gordon B. Haxel, David B. Smith, Charles L. Whittington, 

Andrew Griscom, Denny V. Diveley-White, and Robert E. Powell 

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 34,172 acres of the Orocopia Mountains 

Wilderness Study Area (CDCA-344) were evaluated for 

mineral resources (known) and mineral resource potential 

(undiscovered). In this report, the area studied is referred to 

as the "wilderness study area" or simply "the study area". 

Any reference to the Orocopia Mountains Wilderness Study 

Area refers only to that part of the wilderness study area for 

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

Land Management. 

The Orocopia Mountains Wilderness Study Area is located in southeastern Riverside County, California, about 25 

miles southeast of Indio. Fieldwork for this report was conducted between 1982 and 1986. In 1982, there were about 

20 unpatented mining claims within or adjacent to the wilderness study area. Identified resources in the Orocopia 

Mountains Wilderness Study Area include low-grade talc 

deposits of moderate size and a small, low-grade, low-tonnage quartz-vein gold deposit. Active exploration for gold, 

including geochemical sampling and exploratory drilling, was 

underway in the late 1980's, and a tract with high resource 

potential for disseminated gold in crystalline rocks is delineated in the central part of the Orocopia Mountains Wilderness Study Area. One small area in the southeast corner of 

the wilderness study area has moderate resource potential for 

gold in quartz veins associated with propylitically altered 

mafic dikes cut by fault zones. Other small tracts within the 

wilderness study area have low resource potential for gold in 

quartz veins, moderate resource potential for gold in placer 

Manuscript approved far publicatio:n, August 26, 1988. 

deposits, moderate resource potential for talc and fluorite, 

and moderate resource potential for gypsum and borates in 

evaporite strata. There is low resource potential for gold, 

copper, lead, zinc, and manganese in volcanogenic massivesulfide deposits and for copper and molybdenum in porphyry 

deposits. Moderate potential for geothermal energy resources 

is assigned to the southwestern two-thirds of the study area. 

There is no energy resource potential for oil, and gas in the 

study area. 

Character and Setting 

The Orocopia Mountains Wilderness Study Area is in 

southeastern Riverside County, about 25 mi southeast of 

Indio (fig. 1). The Orocopia Mountains comprise a small, 

rugged range on the northeast side of the northern Imperial 

Valley. The main part of the range lies about 6 mi northeast of the San Andreas fault zone. 

Major lithologic units in the Orocopia Mountains 

Wilderness Study Area are Early and Middle Proterozoic 

(see appendixes for geologic time chart) gneiss and migmatite; a Middle Proterozoic anorthosite-syenite complex; 

Proterozoic(?), Jurassic, and (or) Cretaceous granitoid 

rocks; the late Mesozoic Orocopia Schist; the Eocene 

marine Maniobra Formation (Crowell and Susuki, 1979); 

and the Oligocene(?) and early Miocene nonmarine Diligencia Formation (Crowell, 1975) (fig. 2). The principal 

structural features of the study area are the Late Cretaceous 

Orocopia thrust, which places the Proterozoic and Mesozoic plutonic and gneissic rocks over the Orocopia Schist, 

and the late Tertiary Clemens Well fault, a major right-slip 

fault. Both faults trend northwestward through the central 

part of the Orocopia Mountains. The Orocopia thrust was 

Mineral Resources of the Orocopia Mountains Wilderness Study Area, Riverside County, California El 
modified by middle to late Tertiary low-angle-normal and 

strike-slip faulting. 

Identified Mineral Resources 

Identified resources in the Orocopia Mountains Wilderness Study Area include low-grade talc deposits of 

moderate size and a small, low-grade, low-tonnage quartzvein gold deposit. Active exploration for low-grade, hightonnage disseminated gold deposits was underway during 

preparation of this report. 

Mineral Resource Potential 

In the central part of the study area, a northwest-trending strip, approximately 12 mi long and 0.75 to 1 mi wide, 

,.... 

i..--·-·-, 

i 

i 

116"00' 

L._."L.·-·-·-·-·-·-·, 

between the Orocopia thrust and the Clemens Well fault 

has high potential for disseminated gold in crystalline-rockhosted deposits. Geochemical sampling by industry indicates that rocks with anomalously high abundances of gold 

and arsenic (an alternative elemental indicator of gold 

mineralization) are widespread within this strip. A small 

area in the southeastern part of the study area between the 

Clemens Well fault and . the Diligencia Formation has 

moderate potential for disseminated gold in quartz veins 

and fault zones associated with propylitically altered mafic 

dikes (fig. 2). This area is underlain by Proterozoic gneiss. 

Small veins and pods of metamorphogenic white 

quartz are common in the late Mesozoic Orocopia Schist. 

Some of these veins are gold bearing; the Orocopia mine is 

developed in an unusually large gold-bearing quartz vein. 

The potential for gold in quartz veins within the Orocopia 

Schist is low. 

115'45' 

L_., 

33'45' 

33'30' 

Imperial 

Valley 

APPROXIMATE BOUNDARY 

OF 

i 

i 

I 

L.. • .., 

i L.-., 

i 

i 

i 

i. . ., 

OROCOPIA MOUNTAINS 

WILDERNESS STUDY AREA 

(COCA 344) 

L·-·-·-·-·-·"""' 

i 

i 

i 

i 

i 

i i 

i r·-·j 

i ! i JOSHUA TREE ,.-

't_ __ NATIONAL MONUMENT ( . .......... ___ . _____ ....... -........ _ /, 

. ...._. . ...._,_...,. 

Chiriaco Summit 

SALTON SEA 

RIVERSIDE COUNTY -------------- IMPERIAL COUNTY 

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

E2 Mineral Resources of Wilderness Study Areas: South-Central California Desert Conservation Area, California 
The Orocopia Schist is largely metagraywacke but 

includes minor metabasalt and ferromanganiferous metachert and siliceous marble. This oceanic assemblage is permissive for volcanogenic massive-sulfide deposits, including copper, lead, zinc, manganese, and possibly gold. 

However, regional and areal geologic and geochemical data 

indicate that the potential for such resources in the Orocopia Schist is low. 

Metamorphosed ultramafic bodies in the Orocopia 

Schist are composed of antigorite or actinolite and talc. A 

few are largely talc, and one of these contains low-grade 

talc of commercial quality. The Orocopia Schist has 

moderate potential for talc. 

There is moderate potential for fluorite in small veins 

in Mesozoic granitoid rocks. These granitic rocks are 

present only in the northernmost and central part of the 

study area. One such mineralized area is present about 

one-half mile north of the study area boundary at the 

Orcopia Fluorospar mine. 

Certain parts of the Oligocene(?) and early Miocene 

Diligencia Formation contain lacustrine evaporite beds; 

stream-sediment concentrates from these areas contain high 

boron, barium, and strontium. The evaporite-bearing parts 

of the Diligencia Formation (southeastern part of study 

area) have moderate potential for gypsum and borates in 

evaporitic deposits. 

Porphyry copper-molybdenum deposits in southern 

Arizona and adjacent regions are associated with Jurassic 

and, much more commonly, Late Cretaceous to early Tertiary granitoids. Although late Mesozoic granites crop out 

in the Orocopia Mountains, no geologic or geochemical 

evidence of porphyry copper mineralization or alteration is 

known; the potential for such copper and molybdenum is 

considered low. 

Drainages downstream from the strip having high 

potential for disseminated gold in crystalline-rock-hosted 

deposits are areas that have moderate potential for placer 

gold. No gold was detected in stream-sediment concentrates from this area. 

Two hot springs are known near the study area. Dos 

Palmas Spring, 2.5 mi south of the study area, has water 

temperature of 84 °F. Canyon Spring, about 0.5 mi east of 

the study area, has a water temperature of 97 °F. The 

southwestern two-thirds of the Orocopia Mountains Wilderness Study Area has moderate potential for geothermal 

energy. 

The high grade of metamorphism precludes oil and gas 

accumulation. There is no potential for oil and gas resources in the study area. 

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 and the U.S. Bureau of Mines. An 

introduction to the wilderness review process, mineral 

survey methods, and agency responsibilities was provided 

by Bei.kman 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 the system modified 

from that described by McKelvey (1972) and 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 are discussed by Goudarzi (1984). See appendixes for 

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

Location and Physiography 

The part of the Orocopia Mountains Wilderness Study 

Area (CDCA-344) requested for study encompasses 

34,172 acres in Riverside County, California, about 25 mi 

southeast of Indio and approximately 9 to 30 mi east of 

Mecca (fig. 1). The Orocopia Mountains are a small, 

rugged range on the northeast side of the northern Imperial 

Valley, about 6 mi northeast of the San Andreas fault zone, 

which runs along the northeastern side of the Imperial 

Valley. The highest peak of the Orocopia Mountains is 

3,815 ft and the range rises about 4,000 ft above the adjacent Salton Sea and Imperial Valley and about 3,000 ft 

above the southwestern base of the range. The Orocopia 

Mountains comprise two principal ridges. The larger of 

these ridges trends east-southeastward about 16 mi between 

Box Canyon and Salt Creek. The smaller ridge extends 

eastward about 11 mi from a point south of Chiriaco 

Summit. On some maps, this smaller ridge is considered a 

separate range-the Hayfield Mountains. Between these 

two ridges is a lower, hilly area, the northwestern part of 

which adjoins the Maniobra Valley. The rugged Mecca 

Hills adjoin the Orocopia Mountains to the northwest. 

Southeast of the Orocopia Mountains are the northernmost 

Chocolate Mountains. 

The Orocopia Mountains are surrounded by paved or 

maintained gravel roads. These roads are Interstate 10 to 

the north, the paved road (California Highway 195) in Box 

Canyon to the west, the gravel road along the Coachella 

Canal on the southwest, and the gravel road (Bradshaw 

road) in Salt Wash to the southeast and east. The low area 

between the two principal ridges of the Or<>eopia Mountains is accessible by several dirt roads or jeep trails that 

begin at Chiriaco Summit or branch off from the Bradshaw 

Mineral Resources of the Orocopia Mountains Wilderness Study Area, Riverside County, California E3 
Road. Numerous dirt roads or jeep trails, branching off 

from the various roads surrounding the range, ascend the 

pediments surrounding the two principal ridges that make 

up most of the Orocopia Mountains. However, the higher 

parts of the principal ridges are accessible only by walking. 

The Orocopia Mountains Wilderness Study Area includes most of the area of the higher, southwestern ridge of 

the Orocopia Mountains and about the west half of the hilly 

area between the two principal ridges. The south, southwest, and northwest boundaries of the study area generally 

follow the base of the steep part of the range. The northeast boundary follows the main dirt road that connects Salt 

Creek and Red Canyon with Maniobra Valley. The east 

boundary follows several section lines west of Red Canyon. 

Previous and Present Investigations 

Early studies of the geology of the Orocopia Mountains and surrounding areas include those by Brown (1923), 

Darton (1933), Miller (1944), and Dibblee (1954); the latter 

two reports define several of the stratigraphic names used 

in the region. The first detailed studies of the Orocopia 

Mountains were by J .C. Crowell and colleagues. Crowell 

and Walker (1962) studied a suite of distinctive Proterozoic 

(see appendixes for Geologic Time Chart) anorthositic and 

syenitic rocks, and Crowell and Susuki (1959) studied 

nearshore marine Eocene strata. These Proterozoic and 

Eocene rocks, the Mesozoic Orocopia Schist (Crowell, 

1974), and certain other geologic features and relations 

were important in Crowell's (1960, 1962) pioneering documentation of large strike-slip displacement along the San 

Andreas fault system. Proterozoic crystalline rocks in the 

Orocopia Mountains were included in regional uranium/ 

lead geochronologic studies by Silver (1966, 1968, 1971). 

Most of the Proterozoic and Mesozoic crystalline rock units 

in the Orocopia Mountains are closely related to terranes 

studied by Powell (1981, 1982) in ranges to the east. The 

Orocopia Schist and Orocopia thrust are discussed in the 

summary of the Orocopia Schist and related schist units by 

Jacobson and others (1988). Samples from the Orocopia 

Mountains were included in geochemical studies of the 

Orocopia Schist by Dawson and Jacobson (1986) and 

Haxel and others (1987). The Eocene, Oligocene, and 

Miocene strata of the central Orocopia Mountains were 

investigated by Cole (1958), Woodburne and Whistler 

(1973), Arthur (1974), Spittler (1974), Spittler and Arthur 

(1973, 1982), Squires and Advocate (1982), and Bohannon 

(1975, 1976). Paleomagnetic data from these Paleogene 

and Neogene strata were determined and analyzed by Terres (1984) and Carter and others (1987). The geology of 

the Orocopia Mountains is summarized by Crowell (1975). 

The geology of the Orocopia Mountains Wilderness 

Study Area and some adjoining areas was mapped by the 

U.S. Geological Survey during 1983-1986. Geochemical 

studies were conducted in 1982. Sieved and nonmagnetic 

heavy-mineral fractions were separated from samples collected from active stream channels. These separates were 

analyzed for 30 elements by semiquantitative emission 

spectroscopy and for gold by atomic-absorption methods. 

Previous geophysical investigations include an aerial radioactivity and magnetic survey by LKB Resources, Inc. 

(1980) for the U.S. Department of Energy, an aeromagnetic 

survey (U.S. Geological Survey, 1983), and gravity surveys 

(Mariano and others, 1986). These geophysical data were 

evaluated for this report. 

The U.S. Bureau of Mines conducted surveys of the 

Orocopia Mountains Wilderness Study Area from 1982 to 

1986, focusing on investigations of mines, prospects, and 

other mineralized areas. Mining files and claims were 

reviewed, large-scale maps of several mineralized areas 

were prepared, and samples of rocks and ores from mineralized areas were collected and analyzed by several methods. 

APPRAISAL OF IDENTIFIED RESOURCES 

By Terry j. Kreidler 

U.S. Bureau of Mines 

Methods of Investigation 

The U.S. Bureau of Mines reviewed various sources of 

mineral information, including published and unpublished 

literature, U.S. Bureau of Mines' files, and mining claim 

records at the county courthouse in Riverside, Calif., and at 

the Bureau of Land Management state office in Sacramento, California. In addition, claimants, prospectors, and 

industry geologists that have knowledge of the mining 

activity in the area were interviewed. Mines, prospects, 

and mineralized areas within the wilderness study area and 

those as close as 2 mi from the Orocopia Mountains Wilderness Study Area boundary were examined, mapped, and 

sampled in February 1982. 

U.S. Bureau of Mines personnel collected 35 samples 

in and near the study area. Accessible adits were sampled 

every 20 ft along their course and prospect pits and 

trenches were sampled across the mineralized structure. If 

no structure was present, or if a working was inaccessible, 

samples of the dump material were taken on a 10-ft-grid 

system. 

Gold and silver were determined by fire assay, copper 

and manganese by atomic-absorption analysis, uranium by 

fluorimetric analysis, and calcium fluoride (fluorite) by 

atomic-absorption and wet-chemical analysis. Two 

samples were analyzed for talc content and quality by 

inductively coupled plasma analysis, optical emission spectrography, and wet-chemical analysis. Selected samples 

were analyzed for 40 elements by optical emission spectrography. 

E4 Mineral Resources of Wilderness Study Areas: South-Central California Desert Conservation Area, California 
Complete sample data are available for public inspection at the U.S. Bureau of Mines, Intermountain Field 

Operations Center, P.O. Box 25086, Denver Federal Center, Denver, CO 80225. 

Mining History 

The earliest mining activity in the Orocopia Mountains 

was probably in the late 1880's. Miners, on their way to 

the La Paz gold field near Ehrenberg, Ariz., stopped at Dos 

Palmas spring and prospected in the nearby Orocopia 

Mountains. By 1894, two tunnels with nearly 300 ft of 

workings were located "6 mi northeast of Dos Palmas 

Spring" and were probably the Orocopia mine (Vredenburgh and others, 1981, p. 27). The main mining activity was between 1890 and 1920, and, except for some 

fluorspar and manganese mining in the 1940's and 1950's, 

very little mining or exploration occurred until the late 

1980's. 

No production records for mines within the Orocopia 

Mountains Wilderness Study Area are available; the only 

known mineral production near the study area was 10 tons 

of ore averaging 30 percent manganese from the Big Bu1-

lett claims in 1945 (Tucker and Sampson, 1945, p. 150) and 

about 50 tons of fluorite from the Orocopia Fluorspar mine 

in 1955 (Chesterman, 1957, p. 202) (fig. 2). 

Appraisal of Sites Examined 

Since late 1984, Utah International, Inc., Salt Lake 

City, Utah, and its joint venture partners, have been actively exploring for gold across the central part of the 

Orocopia Mountains Wilderness Study Area. As of November 1987, the company held a block of 583 valid 

mining claims and two prospecting permits. Exploration 

work to date has outlined four areas of anomalous gold 

concentrations (A.C. Ascencious, Utah International, Inc., 

written commun., 1986). Work for the winter of 

1987-1988 and the fall of 1988 concentrated on a wedge of 

land in the northwestern part of the study area bounded by 

the Clemens Well fault on the north and the Orocopia thrust 

on the south and includes detailed geologic mapping and 

geochemical sampling and drilling. Data collected as of 

December 1987 suggest a mineralization model similar to 

that for the Mesquite mine in the Chocolate Mountains 

about 50 mi to the south (Leigh Freeman, Orvana, Inc., 

Denver, Colo., oral and written commun., 1987). 

There are four other distinct mineralized areas, two 

within the study area and two within 2 mi of the boundary: 

manganese at the Big Bullett claims, talc at two areas 

southeast of Orocopia Canyon, gold at the Orocopia mine 

(within the wilderness study area) and fluorite at the Orocopia Fluorspar mine. Data on these mineralized areas and 

one other prospect are summarized in table 1. 

Big Bullett Claims 

Manganese was discovered near the west boundary of 

the study area (fig. 2, No. 1) sometime in the early 1940's. 

In 1945, 20 claims were located about 2.5 mi southeast of 

Shavers Well (Tucker and Sampson, 1945, p. 149) from 

which the previously mentioned manganese ore was produced. 

The manganese is present as irregular lenticular masses 

as much as 3 ft thick in a 4- to 12-ft-thick quartzite layer 

in the Orocopia Schist. The ore was apparently deposited 

as masses of radiating pyrolusite crystals in vugs and fissures (Tucker and Sampson, 1945, p. 150). The quartzite 

layer is exposed for 500 ft, strikes N. 35° W., and can be 

projected into the study area. 

The workings consist of an opencut 20 ft long by 8 ft 

deep and scattered prospect pits on a northwest-facing hillside. No pyrolusite was observed in any of the workings 

and the samples taken in the opencut and one prospect pit 

did not contain manganese concentrations above the background levels found in samples from other localities within 

and near the study area (table 2 of Kreidler, 1984). Pyrolusite was not seen anywhere else along the quartzite outcrop, 

which thins to the southeast in the study area and is buried 

under alluvium to the northwest. The known deposit 

appears to be mined out, but other lenticular masses of 

pyrolusite may be concealed within the quartzite. 

Talc Occurrences 

Impure talc is present at two prospects along the south 

boundary of the study area: at the Goat claims, inside the 

wilderness study area, and at an unnamed prospect 3/4 mi 

south of the study area (fig. 2, No. 2, 3). 

At the Goat claims, the talc occurs as a talcose chlorite-actinolite schist in a 50-ft-wide shear zone in the 

Orocopia Schist (R.B. Saul, C.H. Gray, and J .R. Evans, 

written commun., 1968). The shear zone trends N. 60°· W., 

dips 34 o SW ., and is traceable on the surface for about 600 

ft, beyond which it is buried by alluvium. 

Workings consist of several opencuts stair-stepping up 

the canyon wall. The claimant reports that "large reserves 

of subsurface talc were defined by a drilling program run 

by the previous claim holder." Further details and the 

drilling data were not available to the U.S. Bureau of Mines 

for examination and talc was not exposed in the workings. 

However, the deposit was examined in the winter of 

1982-1983 by two industrial minerals companies who 

found talc contaminated by serpentine, iron, and quartz 

(Jerry Stock, Cyprus Industrial Minerals Co., and Hans 

Ackerman, Pluess-Staufer Co., oral commun., 1982). 

About 2.75 mi southeast of the Goat claims, talc is 

exposed in two opencuts along a ridge. The talc occurs as 

pods and lenses in a broad shear zone in Orocopia Schist. 

The vertical shear zone strikes N. 50° E. Alluvial cover 

prevents tracing the shear beyond the workings. 

Mineral Resources of the Orocopia Mountains Wilderness Study Area, Riverside County, California ES 
Table 1. Mineral deposits in and near the Orocopia Mountains Wilderness Study Area 

[Numbers correspond to sample locations shown on fig 2. Mines or prospects labeled inactive have had no exploration or development work since 

1980] 

Map No. Name Location 

(fig. 2) 

2 

3 

4 

5 

Big Bullett NW. 1/4 sec. 31, 

claims T. 6 S., R. 11 E. 

Unnamed N. 1/2 sec .. 32, 

prospect T. 7 S., R. 12 E. 

Goat claims 

Orocopia 

mine 

Orocopia 

Fluorspar 

mine 

SW. 1/4 sec. 13, 

SE. 1/4 sec. 14, 

S.E. 1/4 sec. 23 

NW. 1/4 sec. 24, 

T. 7 S., R. 11 E. 

NW. 1/4 sec. 22 

T. 7 S., R. 12 E. 

NE. 1/4 sec. 25, 

T. 6 S., R. 12 E. 

Commodity Type of deposit 

Manganese Vug and fissure 

filling 

Talc Shear zone 

Talc Shear zone 

Gold Quartz veins 

Fluorite Vein 

The upper opencut is along the ridge crest and measures 50 ft by 15 ft. Pods of actinolite crystals are scattered 

through the talc body. The lower open cut is about 20 ft 

down the hillside and is about 30 ft by 15 ft. The talc pod 

exposed in the lower cut is brecciated and weathered, 

suggesting that talc formation preceded faulting. 

Analyses of two talc samples from this prospect are 

compared to talc analyses from several producing areas in 

the United States (table 3 of Kreidler, 1984). Talc from the 

prospect compares most closely to, and could be similarly 

utilized as, talc from the Cohutta Talc Co., Georgia, which 

is used for roofing granules (Chidester and others, 1964, p. 

30). 

Orocopia Mine (Gold) 

The Orocopia mine is within the wilderness study area 

about 4 mi west of Canyon Spring (fig. 2, No.4), and has 

also been known alternately as the Dos Palmas mine or 

Fish mine (Larry Vredenburgh, U.S. Bureau of Land 

Management, oral common., 1982). In February 1982, 

there were 462 ft of workings in three adits (upper, 66 ft; 

middle, 106ft; lower, 290ft); all that remained of the mill 

was a foundation, and tailings pile. The three adits are 

connected by stopes and an ore chute, by which the ore was 

moved to the lower adit and then to the mill. 

The adits were driven northeastward along a quartz 

vein that lies in a fault zone striking N. 55° E. and dipping 

45°-55° NW. in chloritic schist. The fault is traceable on 

the surface for over 0.75 mi to the northeast; it is abruptly 

Development 

Opencut; inactive 

Trenching and 

open-cut; inactive 

Prospect; 

inactive 

Three adits; inactive 

Trenching and 

shaft; inactive 

Description 

Pyrolusite in lenticular 

masses in fractured, 

contorted quartzite as 

thick as 3 ft. 

References 

Tucker and 

Sampson 

(1945). 

Faulted Orocopia Schist None. 

with talc and actinolite in 

shear zone. 

Talcose chlorite-actinolite 

schist in shear zone in 

Orocopia Schist. 

Quartz vein in shear, 

exposed for several 

thousand feet; three adits 

connected by stopes. 

Fluorite veins in shear 

zone in quartz monzonite 

cut by quartz-biotite schist. 

None. 

Tucker and 

Sampson 

(1945). 

Chesterman 

(1957). 

truncated by a wide shear zone a short distance to the 

southwest. 

The vein is as much as 5 ft wide, is locally fractured 

and brecciated, and contains locally abundant limonite 

pseudomorphs of pyrite; no other minerals were observed. 

However, 10 of 24 samples taken along the vein on the 

three levels contain gold concentrations ranging from 0.022 

to 0.250 troy oz/ton and averaging 0.125 troy oz/ton (table 

4 of Kreidler, 1984) (1 troy oz/ton == 34 ppm). An estimated 850 tons of ore averaging 0.107 troy oz/ton gold 

remain in the upper and middle adits; the lower adit is 

apparently mined out. More detailed work, including drilling and trenching would be needed to make an accurate 

estimate of resources. 

Orocopia Fluorspar Mine 

The Orocopia Fluorspar mine is north of the study area 

about 3.5 mi southeast of Chiriaco Summit (fig. 2, No. 5). 

The mine was opened in 1955 and has been worked intermittently since that time, although it was not worked in 

February 1982. In May 1983, four people held a single 

claim on the mine. 

The workings consist of a shaft, 35ft deep, centered in 

a trench 150ft long, 10ft wide, and as much as 5 ft deep 

along the strike of the main vein. The vein is exposed in 

prospect pits for another 75 ft beyond the ends of the 

trench. Several other prospect pits are scattered along 

smaller veins near the mine. 

E6 Mineral Resources of Wilderness Study Areas: South-Central California Desert Conservation Area, California 
The fluorite is in veins in fault and shear zones in 

Mesozoic quartz monzonite; at least nine veins were prospected. The quartz monzonite is cut by two well-developed 

joint sets and local, irregular massive quartz bodies. The 

veins are roughly parallel, strike N. 30° E., dip a few degrees from vertical, and are as much as 6 ft thick. The 

quartz monzonite is brecciated and silicified for several feet 

adjacent to the veins. 

The fluorite is usually colorless to light gray and, 

rarely, pale purple. The veins comprise crystalline aggregates with individual crystals as much as 0.25 in. across 

and colloform masses resembling chert. Gangue minerals 

are quartz, calcite, and iron oxide. Chesterman (1957, p. 

202) reported mine run ore of 91.7 percent CaF2 and a 

select sample contained 97.8 percent CaF2• Fluorite 

samples collected and analyzed by U.S. Bureau of Mines 

personnel ran as much as 94.9 percent CaF2 (table 2 of 

Kreidler, 1984). The fluorite is of metallurgical grade and 

was shipped to Kaiser Steel. Co. for use in their Fontana, 

Calif., blast furnaces. Assuming a depth equal to one-third 

the length of the outcrop of the largest vein, Chesterman 

estimated reserves of 5,000 tons of fluorite in 1955 (written 

commun., 1983). 

The veins, although striking toward the boundary, 

cannot be traced into the study area due to surface cover. 

Oil and Gas 

In 1923, an oil and gas test well was drilled about 3.5 

mi southwest of the wilderness study area boundary (fig. 

1); no other information is available. No other oil and gas 

wells or leases are reported in the Orocopia Mountains 

area. 

Geothermal Resources 

The Salton Trough, west of the San Andreas fault, is a 

Known Geothermal Resource Area containing many hotwater springs and wells. East of the San Andreas fault, two 

hot springs are known near the study area. Dos Palmas 

Spring, about 2.5 mi south of the study area, has a water 

temperature of 84 °F and Canyon Spring, about 0.5 mi east 

of the study area, has a water temperature of 97 °F (Higgins and Martin, 1980). 

No tracts of land are known to be leased for geothermal exploration in or near the study area; however, Calzia 

and others (1979) place the western part of the Orocopia 

Mountains Wilderness Study Area in a region designated as 

"prospectively valuable for geothermal resource." No heat 

flow data were included. 

Conclusions 

The Big Bullett manganese deposit appears to be 

mined out because no pyrolusite was seen in outcrop and 

no concentrations of mang~nese were found in ·any of the 

samples. 

Low-grade talc of commercial quality occurs at the 

unnamed prospect outside the southwest boundary and on 

the Goat claims within the study area. Industry geologists 

who have examined the Goat claims report the talc is 

contaminated with serpentine, iron, and quartz. Extensive 

subsurface exploration is needed to adequately evaluate this 

report. 

Along the Orocopia mine vein, gold concentrations are 

as high as 0.250 troy oz/ton and average 0.125 troy oz/ton 

in 10 of 24 samples. This constitutes a small-tonnage, lowgrade gold resource. 

An estimated 5,000 tons of fluorite is still in place at 

the Orocopia Fluorspar mine, about 1.5 mi north of the 

study area. 

The Orocopia Mountains are not underlain by any 

rocks which are considered possible reservoir rocks for oil 

and gas. No land within the study area has been leased for 

oil and gas. 

Although the westernmost part of the Orocopia Mountains Wilderness Study Area lies within an area designated 

as "prospectively valuable for geothermal resources," no 

known geothermal exploration or development has taken 

place. 

ASSESSMENT OF MINERAL RESOURCE 

POTENTIAL 

By Gordon B. Haxel, David B. Smith, 

Charles L. Whittington, Andrew Griscom, 

Denny V. Diveley-White, and Robert E. Powell 

U.S. Geological Survey 

Geology 

Two major faults divide the Orocopia Mountains into 

several structural blocks. These faults are the Late Cretaceous Orocopia thrust and the middle to late Tertiary Clemens Well fault system. The rocks of the Orocopia 

Mountains can be divided into three groups: (1) the 

Proterozoic and Mesozoic gneissic and granitoid rocks that 

form the upper plate of the Orocopia thrust; (2) the late 

Mesozoic Orocopia Schist, which forms the lower plate of 

the Orocopia thrust; and (3) unmetamorphosed Eocene to 

Miocene sedimentary and volcanic strata. The following 

summary of the geology of the Orocopia Mountains is 

based largely on Crowell (1975). 

Lithologic Units 

The oldest rocks in the Orocopia Mountains are Early 

Proterozoic (uranium-lead zircon age of about 1,670 Ma, 

million years ago; Silver, 1971) augen gneiss and migmatite exposed in the southeastern corner of the range. 

Most of these gneissic rocks are quartzofeldspathic; some 

quartzose and biotite-rich rocks are present. The next 

Mineral Resources of the Orocopia Mountains Wilderness Study Area, Riverside County, California E7 
oldest group of rocks is Middle Proterozoic (about 1,420 

Ma; Silver, 1971) banded gneiss that crops out in small 

bodies between the Orocopia thrust and the Clemens Well 

fault. The banded gneiss is characterized by distinctive 

blue or violet quartz. 

Intruding the Middle Proterozoic gneiss is a Middle 

Proterozoic anorthosite-syenite complex that consists of 

anorthosite, gabbro, diorite, syenite, alkali granite, and 

other less common rock types (Crowell and Walker, 1962). 

The uranium-lead isotopic age of this intrusive suite is 

about 1,220 Ma (Silver, 1971). This anorthosite-syenite 

complex makes up most of the structural block between the 

Orocopia thrust and the Clemens Well fault. 

Granitoid rocks of several compositions and ages are 

present in the Orocopia Mountains, and are most common 

in the hills south and southwest of Maniobra Valley and in 

the smaller, northern ridge of the Orocopia Mountains. The 

granitoid rocks intrude the Middle Proterozoic anorthositesyenite complex and the older gneisses. By analogy with 

nearby areas (Powell, 1981), granitoid rocks of Proterozoic, 

Jurassic, and Cretaceous age probably are present. None of 

the granitoid rocks intrude the Orocopia Schist. Although 

Triassic granodiorite and related granitoid rocks are exposed in several nearby areas (Ehlig, 1981), they do not 

crop out in the Orocopia Mountains. Augen gneiss and 

migmatite in the Canyon Spring area, east of the Clemens 

Well fault and west of the unconformable contacts with the 

Diligencia Formation, are cross cut by a swarm of propylitically altered, highly chloritic Mesozoic mafic dikes, 

commonly foliated. These dikes are in turn disrupted by 

numerous small faults that would go largely unrecognized 

were it not for the offset of the mafic dikes. 

The main (western), larger ridge of the Orocopia 

Mountains is underlain largely by the late Mesozoic Orocopia Schist. The structural thickness of the Orocopia 

Schist is more than 1 mi. The Orocopia Schist comprises 

chiefly quartzofeldspathic schist, derived from graywacke, 

and also includes minor interlayered metabasalt and ferromanganiferous metachert and siliceous marble (Haxel and 

Dillon, 1978; Ehlig, 1981; Haxel and others, 1987; Jacobson and others, 1988). Geochronologic data from elsewhere in southern California indicate that at least part of 

the protolith of the Orocopia Schist is Late Jurassic or older 

(Mukasa and others, 1984); and that metamorphism of the 

Orocopia Schist, and accompanying movement on the 

overlying Orocopia thrust, occurred in Late Cretaceous 

time (Haxel and Tosdal, 1986, p. 54). 

The oldest unmetamorphosed supracrustal rocks in the 

Orocopia Mountains make up the Eocene Maniobra Formation (Crowell and Susuki, 1959), which crops out in the 

area around the head of Red Canyon and in Maniobra 

Valley. These are marine strata about 4,900 ft thick, consisting of brown shale, sandstone, and conglomerate. The 

Maniobra Formation rests nonconfonnably on granitic 

basement rocks and is unconformably overlain by the Oligocene(?) and early Miocene Diligencia Formation (Crowell, 1975). The Diligencia Formation crops out over a wide 

tract between the Clemens Well fault, the main area of the 

Maniobra Formation, and Red Canyon. The Diligencia 

Formation consists of about 4,900 ft of nonmarine conglomerate, breccia, sandstone, and mudstone; basaltic to 

andesitic flows, sills, and dikes; and limestone and evaporites (Spittler and Arthur, 1982; Squires and Advocate, 

1982). Late Cenozoic clastic sedimentary rocks and deposits are widely exposed in the Mecca Hills, west-northwest 

of the Orocopia Mountains (Dibblee, 1954). Within and 

adjacent to the Orocopia Mountains Wilderness Study 

Area, the only strata younger than the Diligencia Formation 

are local accumulations of Pliocene to Quaternary fanglomerate and alluvium. 

Structure 

The Late Cretaceous Orocopia thrust places the 

Proterozoic gneiss and anorthosite-syenite complex and the 

Proterozoic and Mesozoic granitoid rocks atop the Orocopia Schist. The Orocopia thrust is synmetamorphic with 

respect to the Orocopia Schist. Where well preserved, the 

thrust is marked by a mylonitic zone, typically about 100ft 

thick, developed largely from the granitic upper-plate 

rocks. Along .much of its length, the Late Cretaceous 

Orocopia thrust was disrupted or modified by middle to late 

Tertiary low-angle normal (detachment) faulting characterized by cataclastic rocks. The Orocopia thrust may also 

locally have been affected by faulting related to the late 

Tertiary strike-slip Clemens Well fault system. Thus, the 

structure mapped as the Orocopia thrust evidently is actually a composite fault. This composite fault is exposed 

almost continuously for about 10 mi, along the northeastern 

margin of the outcrop area of the Orocopia Schist. 

The foliation of the Orocopia Schist defines a broad, 

northwest-trending, doubly plunging anticline of Tertiary 

age. The Orocopia thrust and its upper-plate crystalline 

rocks crop out on the northeast flank of this anticline; the 

upper-plate rocks, but not the thrust itself, are exposed on 

the southwest flank of the anticline. The Maniobra and 

Diligencia Formations occupy a broad structural trough 

between the two basement-rock ridges of the Orocopia 

Mountains. The Diligencia Formation is folded into several broad synclines and anticlines. The Maniobra Formation crops out on the northeastern flank of the trough, and 

dips southwestward beneath the Diligencia Formation; the 

Maniobra Formation is not exposed on the southwest side 

of the trough, where the Diligencia Formation was deposited directly on crystalline basement rocks. 

The late Cenozoic San Andreas fault zone (Crowell, 

1979, 1981), which consists of numerous fault segments 

and splays, lies about 6 mi southwest of the main part of 

the Orocopia Mountains, and is well exposed in the Mecca 

Hills (Sylvester and Smith, 1976). The Clemens Well fault 

passes northwesterly through the central part of the 

Orocopia Mountains. Along much of its trace, the Clemens 

EO Mineral Resources of Wilderness Study Areas: South-Central California Desert Conservation Area, California 
Well fault separates the Diligencia Formation to the 

northeast from crystalline basement rocks to the southwest, 

but along the southeastern part, crystalline rock is present 

on both sides of the fault. Right slip along the Clemens 

Well fault is probably no more than few tens of miles 

(Crowell, 1975). The Clemens Well fault apparently is part 

of a major late Tertiary right-slip fault that has been 

fragmented by the San Andreas fault system in late 

Cenozoic time (Powell, 1981). The Maniobra and 

Diligencia Formations are cut by numerous subvertical 

faults that strike northwest or northeast Several of the 

northeast-striking faults have minor left slip. South of the 

Orocopia Mountains, between the Orocopia Mountains and 

the northern Chocolate Mountains, the Salton Creek fault 

has uncertain but possibly substantial strike-slip 

displacement. 

Geochemical Studies 

A reconnaissance geochemical survey was conducted 

in the Orocopia Mountains Wilderness Study Area in 1982. 

The purpose of this type of survey is to evaluate a large 

area for evidence of mineralization. The survey was not 

designed to find individual mineral deposits; it was designed to delineate favorable geochemical provinces and 

mineralized districts that may be suitable for follow-up 

studies. 

Minus-80-mesh (less than 0.01 in.) stream sediments 

and nonmagnetic heavy-mineral concentrates derived from 

stream sediments were selected as primary sample media 

because they represent a composite of rock and soil exposed in the drainage basin upstream from the sample site. 

Chemical analysis of sediments provides information useful 

in identifying those basins that contain unusually high 

concentrations of elements that may be related to mineral 

deposits. 

Stream-sediment samples were collected from dry 

stream channels in and near the study area at 55 sites .. At 

all these sites both a minus-80-mesh sample and a heavymineral concentrate sample were collected. Samples were 

composited from what appeared to be the most recently 

active alluvium along 30 to 50 ft of channel length. 

Mter sieving, the minus-80-mesh fraction was ground 

to approximately minus-150-mesh in a vertical pulverizer 

with ceramic plates. The fraction coarser than 80 mesh was 

discarded. 

To produce the heavy-mineral concentrate, bulk stream 

sediment was first passed through a 10-mesh (0.08-in.) 

screen. Approximately 10 to 15 lb of the minus-10-mesh 

sediment were panned to remove most of the quartz, feldspar, organic materials, and clay-sized material. The 

panned concentrate was then separated into light and heavy 

fractions using bromoform (heavy liquid, density 2.8 gj 

cm3). The light fraction was discarded. The material of 

density greater than 2.8 g/cm3 was further separated into 

three fractions (highly magnetic, weakly magnetic, and 

nonmagnetic) using a Frantz Isodynamic Separator. The 

nonmagnetic fraction, or a split of it, was hand ground and 

saved for analysis. Whenever possible, an unground split 

was saved for visual examination· to determine mineralogy. 

These preparation procedures result in a concentrated 

mineral assemblage that may be rich in ore-forming or orerelated minerals such as pyrite, galena, cassiterite, sphalerite, chalcopyrite, stibnite, free gold, barite, and scheelite. 

This selective concentration of ore-related minerals permits 

determination of elements that are not easily detected in 

bulk stream-sediment samples. 

Samples were analyzed by a semiquantitative emission 

spectrographic method (Grimes and Marranzino, 1968) for 

29 elements in the heavy-mineral concentrates and for 31 

elements in the minus-80-mesh sediments. Minus-80-mesh 

stream sediments were also analyzed for gold by an atomicabsorption method (Hubert and Chao, 1985). A complete 

listing of all analyses was provided by B.M. Adrian and 

others (written commun., 1987). 

The reconnaissance geochemical survey delineated 

three anomalous tracts within the study area. The frrst tract 

consists of the northwest-trending band of gneiss, migmatite, and anorthosite-syenite complex that forms the 

upper plate of the Orocopia thrust. Nonmagnetic heavymineral concentrates from the southeastern part of this belt 

contain anomalous silver (as much as 2 ppm, parts per 

million), arsenic (as much as 1,000 ppm), bismuth (as much 

as 100 ppm), and tungsten (as much as 1,000 ppm). Rock 

samples collected during a current gold-exploration program along the northwestern part of the belt contain high 

values of gold (usually ranging from 10--800 ppb, parts per 

billion, with some values as high as 6 ppm) and arsenic (as 

much as 150 ppm) (Leigh Freeman, Orvana Resources 

Corp., oral and written commun., 1987). Eight rotary drill 

holes, each collared in crystalline rocks of the upper plate, 

were drilled in the summer of 1987. These holes intercepted zones containing anomalously high concentrations 

of gold (80--100 ppb), but did not penetrate any ore-grade 

material (Leigh Freeman, Orvana Resources Corp., oral and 

written commun., 1987). 

The second geochemically anomalous tract is the 

western part of the study area where one sample of nonmagnetic heavy-mineral concentrate contains 100 ppm gold 

and 30 ppm silver. The geochemical anomaly may indicate 

gold-bearing quartz veins within the schist. 

The third anomalous tract encompasses outcrops of the 

Diligencia Formation. Nonmagnetic heavy-mineral concentrates from this tract contain anomalous amounts of 

barium (greater than 10,000 ppm) and strontium (greater 

than 10,000 ppm). The nonmagnetic fraction of the concentrate contains 50--85 percent barite (BaSO J. Minus-80-

mesh stream sediments from the tract contain anomalous 

abundances of boron (300--2,000 ppm). The chemistry and 

mineralogy of the samples may reflect the presence of 

evaporites as previously reported by Crowell (1975). 

Mineral Resources of the Orocopia Mountains Wilderness Study Area, Riverside County, California E9 
l;2 0 2 3 4 5 MILES 

KKKK~==~====~==~====~==3 

Figure 2. Mineral resource potential and generalized geology of the Orocopia Mountains Wilderness Study Area, Riverside County, California. Geology from Crowell (1975). Contour interval, 200ft; 

supplementary interval, 100 ft. 

• 

~ 

[ill 

§ 

D 

B 

c 

D 

Au 

Pb 

Zn 

Cu 

Mo 

[ 

1 

2 

3 

4 

5 

6 

2 

3 

4 

5 

QTg 

Td 

Tm 

Mzo 

Mzob 

Mzg 

MzYg 

Ya 

YXg 

EXPLANATION 

Area having high mineral resource potential (H) for gold 

Area having moderate mineral resource potential (M) for commodities as 

indicated 

Area having moderate mineral resource potential (M) for gypsum and borate 

Area having moderate mineral resource potential (M) for fluorite 

Area having moderate mineral resource potential (M) for gold 

Area having low mineral resource potential (L) for commodities as indicated 

Level of certainty of assessment 

Data only suggest level of potential 

Data give good indication of level of potential 

Data clearly define level of potential 

Commodities 

Gold 

Lead 

Zinc 

Copper 

Molybdenum 

Mn 

G,B 

Fl 

T 

Geo 

Type of deposit or occurrence 

Manganese 

Gypsum and borate 

Fluorite 

Talc 

Geothermal 

Crystalline-rock-hosted disseminated gold 

Low-sulfide gold-bearing quartz veins 

Volcanogenic massive-sulfide base metals and gold 

Porphyry copper and molybdenum 

Gold associated with propylitically altered dikes 

Placer gold 

Mines, prospects, and claims 

Big Bullett claims 

Unnamed prospect 

Goat claims 

Orocopia mine 

Orocopia Fluorspar mine 

Geologic map units 

Fanglomerate and alluvium (Quaternary to Pliocene?) 

Diligencia Formation (Early Miocene and Oligocene?) 

Maniobra Formation (Eocene) 

Orocopia Schist (late Mesozoic)--Metamorphic age is Late Cretaceous; 

protolith age is Jurassic. Divided into: 

Late metagraywacke 

Metagraywacke with metabasalt, metachert, siliceous marble, and minor , 

ultramafic rock 

Granitoid rocks (Mesozoic) 

Granitoid rocks (Mesozoic to Middle Proterozoic) 

Anorthosite, syenite, and related rocks (Middle Proterozoic) 

Banded augen gneiss and migmatite (Middle and Early Proterozoic) 

Contact-Dashed where approximate 

Fault-Dashed where approximate; dotted where concealed 

~ Orocopia thrust-Dashed where extent uncertain. Sawteeth on upper plate 

Anticline-Dashed where approximate 

Syncline-Dashed where approximate 

'X' Mine 

~ Prospect or claim area-Red indicates identified resource 

ElO Mineral Resources of Wilderness Study Areas: South-Central California Desert Conservation Area, California Mineral Resources of the Orocopia Mountains Wilderness Study Area, Riverside County, California E11 
Geophysical Studies 

Geophysical evaluation of the mineral resources of the 

Orocopia Mountains Wilderness Study Area was based on 

interpretations of the three kinds of geophysical surveys: 

aeromagnetic, gravity, and aerial gramma-ray spectrometer. 

An aeromagnetic survey (U.S. Geological Survey, 

1983) was flown in 1981 by a private contractor. The 

aeromagnetic data were collected along parallel east-west 

flightlines spaced 0.5 mi apart at a nominal altitude of 

1 ,000 ft. The contractor subtracted the Earth's main 

magnetic field from the data, gridded the data set using a 

grid spacing of 650 ft, and machine contoured the gridded 

data at scales of 1:250,000 and 1:62,500 to produce the 

residual aeromagnetic map. Additional aeromagnetic data 

are available in the atlas on the Salton Sea quadrangle 

published for the Department of Energy (LKB Resources, 

Inc., 1980). These data consist of north-south profiles 

spaced at an interval of 3 mi and flown by helicopter at an 

average height of 400 ft above the ground surface; 5 profiles cross the Orocopia Mountains Wilderness Study Area. 

Variations in the Earth's magnetic field on a residual 

map are, in general, caused by variations in the amounts of 

magnetic minerals in different rock units, magnetite being 

the common magnetic mineral in this area. Magnetic 

minerals, where locally either concentrated or absent, may 

cause a high or low magnetic anomaly that can be a guide 

to mineral occurrences or deposits. Boundaries between 

magnetic and relatively less magnetic rock units are located 

approximately at the steepest gradient on the flanks of the 

magnetic anomaly because at these magnetic latitudes the 

inclination of the Earth's main magnetic field is relatively 

steep (59° below the horizontal). 

Comparison of the contoured aeromagnetic map with 

the geologic map indicates that the Orocopia Mountains 

Wilderness Study Area contains certain rock types that 

each have characteristic magnetic patterns or anomalies. 

The Orocopia Schist forms a regional magnetic low and is 

essentially nonmagnetic except for a small high (20 gammas in amplitude and about 1 mi across) that appears to be 

associated with the area of talc prospects at the Goat 

claims. Evidently, the associated chlorite-actinolite schist 

and serpentinite are moderately magnetic, a common observation in similar talc deposits elsewhere in the United 

States. The absence of other similar magnetic highs within 

the Orocopia Schist suggests that other similar talc occurrences are not present. 

Between the Orocopia thrust and the Clemens Well 

fault is a belt of Proterozoic rocks intruded locally by 

younger granitic rocks. The Proterozoic rocks include 

gneisses and substantial amounts of an anorthosite-syenite 

complex that also contains gabbro and diorite. One might 

expect such rocks to be relatively magnetic, yet only two 

magnetic anomalies are associated with this belt: one in 

the extreme southeast corner of the study area and one 

about 2 mi north of Orocopia Peak. These two highs are 

caused by rock masses about 2.5 and 1.5 mi across, respectively. This unexpected scarcity of strong magnetic anomalies in this belt may have significance in relation to the 

favorable geologic indications for crystalline-rock-hosted 

disseminated gold deposits that are described in the Mineral 

Resource Assessment section of this report. The indications include fracturing, brecciation, silica flooding, and 

oxidized zones containing iron-oxide staining. Such hydrothermal alteration can destroy the primary magnetite 

of fresh rocks and thus any favorable areas might not be 

expected to display magnetic variations. 

Other magnetic highs within the Orocopia Mountains 

Wilderness Study Area include small magnetic highs associated with the mafic volcanic rocks of the Diligencia 

Formation and a major, northwest-trending magnetic high 

associated with granitic rocks in the extreme northeast 

corner of the study area. These highs are normal and 

probably provide no information concerning possible mineral resources. 

Gravity data for the study area are available as complete Bouguer gravity maps (Elders and others, 1972; 

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

residual isostatic gravity maps (Roberts and others, 1981; 

Mariano and others, 1986). The area of Orocopia Schist is 

characterized by a local gravity high of at least 20 milliGal 

(mGal). The cause of the gravity high is not evident because the density of the schist probably is about the same 

or less than that of the surrounding Proterozoic rocks. The 

source of this anomaly may be fairly deep because the 

Clemens Well fault (assuming that it is steep) does not 

appear to influence the gradient on the north side of the 

high. The unknown basement rocks beneath the schist are 

probably former oceanic crust on the basis of the associated 

metabasalt and ultramafic rocks exposed in the Orocopia 

Schist (Haxel and Dillon, 1978). The gravity high may 

indicate uplifted former oceanic crust beneath the exposures of the schist (Griscom, 1980), but if so, these presumably magnetic rocks must be too deep to affect the 

aeromagnetic map pattern. The gravity field slopes down 

to the granitic rocks in the extreme northeastern part of the 

study area. Large masses of granitic rocks commonly 

produce gravity lows. 

Radioactivity data were collected and compiled by 

LKB Resources, Inc. (1980) for the National Uranium 

Resource Evaluation program of the Department of Energy. 

Aerial gamma-ray spectrometer measurements are available along 3-mi-spaced east-west flightlines. The recorded 

flight altitudes generally ranged from 200 to 400 ft. Recordings were made of gamma-ray flux from radioactive 

isotopes of uranium, thorium, and potassium or their decay 

products. Abrupt shifts in flux level were recorded over 

geologic contacts. The results indicate that statistically 

significant anomalies for uranium, potassium, and thorium 

were not found within the study area. One feature of 

possible economic significance is the presence of small 

potassium anomalies associated with the belt of rocks beE12 Mineral Resources of Wilderness Study Areas: South-Central California Desert Conservation Area, California 
tween the Orocopia thrust and the Clemens Well fault. 

Such anomalies might be caused by potassium metasomatism associated with hydrothermal alteration. 

Mineral Resource Assessment 

This assessment of the mineral resource potential of 

the Orocopia Mountains Wilderness Study Area is based 

upon geologic, geochemical, and geophysical data from 

previous literature; geologic and geochemical field studies 

by the authors and their colleagues; oral and written communication with geologists in the mining industry; and 

application of ore-deposit models (Cox and Singer, 1986; 

Eckstrand, 1984). These data and methods were used to 

delineate favorable tracts within the study area; an estimate 

was then made of the level of resource potential and degree 

of certainty for each tract for certain ore-deposit types. 

Selection of ore-deposit models was based on the geology 

of the Orocopia Mountains Wilderness Study Area and on 

mineral resource assessments of nearby or geologically 

similar areas (Powell and others, 1984a, b; Morton, 1977; 

Bagby and others, 1987; Smith and others, 1987; Peterson 

and others, 1987; Morton and others, 1988). The mineral 

resource potential refers to undiscovered mineral resources. 

In addition to the descriptive ore-deposit models in the 

compilation cited above, we have used models based on 

mineral occurrences in the Orocopia Mountains Wilderness 

Study Area and vicinity and, more importantly, models 

based on newly discovered gold deposits and occurrences 

recently recognized in southeasternmost California. 

The assessment is organized by deposit type. Characteristics of known deposits are briefly summarized and each 

tract is evaluated according to several assessment criteria. 

Definitions of levels of mineral resource potential and 

certainty of assessment are presented in the appendixes. 

Crystalline-Rock-Hosted Disseminated Gold 

Recent mineral exploration and development in southeastern California has resulted in recognition of a new type 

of gold deposit. The original or discovery example of this 

deposit type is the Mesquite deposit, at the southern tip of 

the Chocolate Mountains, about 60 mi southeast of the 

Orocopia Mountains (Manske and others, 1987; Willis and 

others, 1987; Smith and others, 1987; Tosdal and Smith, 

1987a, b). The Picacho deposit (Drobeck and others, 1986; 

Van Nort and Harris, 1984), also in southeasternmost California, may be another example of, or have affinities to, 

this deposit type. In crystalline-rock-hosted disseminated 

gold deposits, gold is generally hosted by gneiss or granitoid rocks and is localized or concentrated in intensely 

brecciated and oxidized zones. Gold and in some cases 

arsenic are the best geochemical indicators for this deposit 

type. Favorable geologic indications for this type of deposit include strong fracturing or shattering, silica flooding, 

and iron-oxide staining. Scattered anomalous concentrations of arsenic, antimony, mercury, tungsten, tellurium, 

and possibly thorium may also be present. The present 

understanding of this deposit type is insufficient to unequivocally characterize the environment of deposition; 

several models have been proposed (see references cited 

above). In southeasternmost California, the age of final, 

economic mineralization is probably middle or late Tertiary, but "precursor mineralization," that is, initial introduction of gold into the upper crust, may have occurred in 

Jurassic and (or) Cretaceous time (Tosdal and others, 1985; 

Keith, 1986). 

The northwest-trending strip of crystalline rocks between the Orocopia thrust and the Clemens Well fault is 

judged to have high resource potential, certainty level D, 

for gold in crystalline-rock-hosted disseminated gold deposits (fig. 2). This strip of mineralized ground is approximately 12 mi long and 0.3 to 1 mi wide. It is the surface 

expression of an irregular, downward-tapering wedge 

bounded on the northeast by the subvertical Clemens Well 

fault and on the southwest by the northeast-dipping Orocopia thrust. 

The following summary of the results of exploration 

geochemistry within the strip between the Orocopia thrust 

and Clemens Well fault is based on conversations in October 1987 with, and data shown to us by, Leigh Freeman of 

Orvana Resources Corp. in Golden, Colo. Surface rock 

sampling shows numerous samples containing anomalous 

arsenic concentrations from 20 to greater than 150 ppm. 

(The average upper crustal abundance of arsenic is approximately 1.5 ppm; Taylor and McLennan, 1985.) Anomalous 

gold concentrations range from 10 ppb to greater than 800 

ppb; a few samples contain as much as about 6 ppm gold. 

(The average upper crustal abundance of gold is about 2 

ppb (1 ppm == 0.03 troy oz/ton.) Anomalous concentrations of arsenic and gold are strongly correlated. Based on 

this geochemical sampling, eight rotary drill holes were 

drilled in the area about 6 mi southwest of Chiriaco Summit. Results of this drilling program were not available at 

the time this report was prepared. 

The geochemically anomalous whole-rock arsenic and 

gold concentrations, and the tract considered favorable for 

gold in crystalline-rock-hosted disseminated gold deposits, 

does not extend into the Orocopia Schist southwest of the 

Orocopia thrust nor into the Diligencia Formation and 

crystalline rocks northeast of the Clemens Well fault. 

Throughout southeastern California, the Orocopia Schist 

consistently appears to be an unfavorable host for disseminated gold mineralization. Stream-sediment analyses from 

the Orocopia Schist in the Orocopia Mountains Wilderness 

Study Area do not contradict this generalization. 

Gold-Bearing Quartz Veins 

This deposit type is characterized by the occurrence of 

gold in massive quartz veins, typically in regionally metamorphosed oceanic sedimentary or volcanic rocks (Berger, 

Mineral Resources of the Orocopia Mountains Wilderness Study Area, Riverside County, California E13 
1986). The veins are typically metamorphogenic and postmetamorphic in their fabric relations to their host metamorphic rocks. Native gold may be accompanied by minor 

sulfide and (or) carbonate minerals. In general, geochemical indicators include arsenic, gold, silver, lead, copper, 

zinc, and tungsten. 

Small veins and pods of white quartz are common 

throughout the Orocopia Schist; most are postmetamorphic, 

a few are synmetamorphic. The largest pods are typically 

no more than a few yards in largest exposed dimension. 

Typical quartz veins are few inches to about 3 ft wide and 

a few tens of feet long. Few of these quartz veins have 

been prospected. Geologic observations, absence of appreciable prospecting of these highly visible veins, and conversations with geologists actively engaged in mineral 

exploration in the southeastern California desert suggest 

that the metamorphogenic quartz veins in the Orocopia 

Schist have low potential, with a certainty level of B, for 

gold resources. A single nonmagnetic heavy-mineral concentrate from an area draining the Orocopia Schist contains 

anomalous gold and silver concentration, but the values are 

not necessarily derived from nor associated with the quartz 

veins. 

Gold Associated with Propylitically Altered Mafic Dikes 

In both the Chuckwalla and Eagle Mountains Wilderness Study Areas (east and northeast, respectively, of the 

Orocopia Mountains Wilderness Study Area), propylitically 

altered mafic dikes cut by quartz veins and small faults 

crop out in areas characterized by anomalously high abundances of a suit~ of elements including tungsten, molybdenum, silver, bismuth, and tin in the nonmagnetic fraction of 

stream-sediment samples (Powell and others, 1984a, b). 

Scheelite and fluorite are abundant in these samples. This 

geochemical and mineralogic signature is consistently associated with the altered mafic dikes, even though these dikes 

intrude a number of diverse Proterozoic gneiss units and 

Mesozoic plutonic units. In the Chuckwalla Mountains, 

almost all of the gold mines and prospects are in quartz 

veins and faults crosscutting the altered mafic dikes. This 

empirical geologic and geochemical association of gold 

occurrences with altered mafic dikes does not fit conveniently into any currently recognized ore-deposit model. 

However, the association did provide a framework for 

assessing mineral resource potential in the Chuckwalla and 

Eagle Mountains Wilderness Study Areas (Powell and 

others, 1984a, b). 

. In the study area, propylitically altered mafic dikes in 

the gneissic rocks of the Canyon Spring block, east of the 

Clemens Well fault and west of fault and unconformable 

contacts with the Diligencia Formation, are petrographically similar to the altered mafic dikes in the Chuckwalla 

and Eagle Mountains. Furthermore, the geochemical and 

mineralogic character of stream-sediment samples derived 

from the Canyon Spring crystalline block is similar to that 

of samples from the areas of altered mafic dikes in the 

Chuckwalla and Eagle Mountains. The propylitically altered mafic dikes in the Canyon Spring crystalline block 

are heavily prospected, and it is in that geologic setting the 

Orocopia (Dos Palmas) mine is located. The "chlorite 

schist" referred to in older descriptions of this mine is 

foliated, propylitically altered mafic dike rock. 

On the basis of these data and comparisons, the block 

of gneissic rocks between the Clemens Well fault and the 

Diligencia Formation is considered to have moderate potential, with certainty level C, for gold in quartz veins 

associated with and, possibly, in fault zones crosscutting 

the propylitically altered mafic dikes. Lacking an established ore-deposit model, grade-tonnage relations for such 

deposits are uncertain; but grade-tonnage relations for lowsulfide gold-bearing quartz veins (Berger, 1986) may be 

useful as a rough guide. The estimated tonnage (850 tons) 

and average grade (4 ppm) of the remaining ore at the 

Orocopia mine plor near the lower tenth-percentile end of 

the grade-tonnage models of Bliss (1986). This comparison, if valid, suggests that gold deposits in quartz veins 

associated with the altered mafic dikes are likely to be quite 

small. 

Volcanogenic Massive-Sulfide Base Metals and Gold 

This type of deposit consists of copper, lead, and (or) 

zinc sulfides, possibly accompanied by native gold, in 

submarine mafic or silicic metavolcanic or volcanic rocks. 

The geochemical signature commonly includes gold, arsenic, boron, and antimony. 

The metagraywacke of the Orocopia Schist contains 

sparse to locally abundant layers of metabasalt, metachert, 

and siliceous marble. Metachert and siliceous marble layers are typically 1 to 30 ft thick. This oceanic lithologic 

assemblage (Haxel and others, 1987) constitutes a permissive geologic setting for volcanogenic massive-sulfide 

deposits, possibly containing gold. However, six independent lines of evidence indicate that the potential for gold and 

(or) base metals (copper, lead, zinc) in massive-sulfide 

deposits within the Orocopia Schist is low, at certainty 

level C. (1) Extensive mapping of the Orocopia Schist and 

similar, related schist units in southern California by many 

geologists (see summaries by Haxel and Dillon, 1978; 

Ehlig, 1981; Jacobson and others, 1988) revealed no occurrences of metalliferous sediments or ma~sive-sulfide deposits. (2) Although metachert and siliceous marble in the 

Orocopia Schist ar~ commonly ferromanganiferous, as 

indicated by metamorphic magnetite and spessartite and 

(or) piemontite, the rocks typically contain no more than 2 

percent manganese oxide. (3) For nine samples of metachert and siliceous marble from the Orocopia Schist, typical abundances of copper, lead, and zinc are 170, 12, and 

·100 ppm, respectively. These copper and zinc concentraE14 Mineral Resources of Wilderness Study Areas: South-Central California Desert Conservation Area, California 
tions are several times greater than average upper-crustal 

abundances, but are far below any conceivable ore grade 

for these base metals. (4) All nine samples contain less 

than 50 ppm boron, less than 1 ppm tungsten, less than 10 

ppm arsenic, less than 100 ppb gold, less than 0.5 ppm antimony, and less than 1 ppm thallium. (5) Interpretation of 

trace-element systematics indicates that the metacherts and 

siliceous marbles in the Orocopia Schist are derived from 

fundamentally biogenic deposits (probably radiolarian 

cherts), in which the hydrothermal metalliferous component is minor (Haxel and others, 1987). Furthermore, this 

minor metalliferous component was not necessarily produced by in situ hydrothermal activity (compare with 

Dymond, 1981; Ruhlin and Owen, 1986). (6) Fuchsite 

(chromian muscovite) is an alteration mineral associated 

with some massive-sulfide deposits. Although the Orocopia Schist contains fuchsite, it is scattered through the 

metagraywacke rather than associated with the metabasalt, 

metachert, or siliceous marble. 

Manganese metachert layers in the Orocopia Schist 

commonly contain 0.2 to 2 percent manganese oxide 

(Haxel and others, 1987). The small amount of pyrolusite 

ore reportedly mined from the Big Bullett claims was 

apparently produced by remobilization of this manganese, 

possibly during Tertiary faulting. Available geologic and 

geochemical data suggest that the potential for manganese 

associated with metachert in the Orocopia Schist is low, 

certainty level C. 

Talc 

Pods and irregular masses of ultramafic rocks, ranging 

from several inches to 30 ft in largest exposed dimension, 

are sparsely scattered through the Orocopia Schist. In some 

areas, these masses are aligned along shear or fault zones. 

Some of the ultramafic masses are composed of antigorite 

serpentinite, but most are composed of variable proportions 

of actinolite (or tremolite) and talc, with or without minor 

chlorite, fuchsite, carbonate, serpentine, and (or) iron oxide. 

Ultramafic masses composed largely or entirely of talc are 

much rarer than those consisting of subequal amounts of 

talc and actinolite. The Goat claims are located in one such 

talc-rich mass. The Orocopia Schist of the study area has 

moderate potential, with certainty level C, for additional 

talc. Any such deposits would probably be small compared 

to those currently of commercial interest. 

Fluorite 

Based on the presence of a small vein-fluorite 

(fluorspar) deposit (the Or<>eopia Fluorspar mine, in Meso-

. zoic granite north of the Orocopia Mountains Wilderness 

Study Area), the north-central part of the study area is 

judged to have moderate potential, with certainty level B, 

for fluorite. Any such deposits would probably be small 

compared to those currently of commercial interest. 

Evaporite Deposits 

The middle part of the Diligencia Formation contains 

some thin-bedded lacustrine evaporite beds (Spittler and 

Arthur, 1982). These evaporite beds are restricted to the 

southwestern part of the outcrop area of the Diligencia 

Formation. They are generally composed of laminated 

gypsum; borate-bearing beds are present locally. The principal trace elements in stream-sediment samples from the 

southeastern part of the Diligencia Formation are boron, 

barium, and strontium. No other information on the composition of the evaporite beds is available. The southeastern and stratigraphically medial part of the Diligencia 

Formation has moderate potential, at certainty level B, for 

gypsum and borate in evaporite deposits 

Porphyry Copper-Molybdenum 

Porphyry copper deposits are associated with epizonal 

or shallow mesozonal granitoid plutons, typically including 

rocks with porphyroaphanitic textures (Titley, 1982; Cox, 

1986). Ore occurs in both the plutons and their altered 

country rocks. Such deposits and plutons are associated 

with convergent (subduction) plate margins. Disseminated 

and stockwork chalcopyrite and pyrite, with or without 

molybdenite, are typical. Porphyry copper mineralization 

is associated with characteristic types and patterns of alteration (Titley and Beane, 1981). Supergene alteration produces secondary copper carbonates, silicates, and sulfides. 

The geochemical signature of porphyry copper deposits 

includes, but is not limited to, copper, molybdenum, gold, 

silver, boron, strontium, tungsten, manganese, lead, zinc, 

sulfur, arsenic, antimony, selenium, technetium, and tellurium. 

The numerous porphyry copper deposits in southern 

and northwestern Arizona, northern Sonora, Mexico, and 

southwestern New Mexico are almost entirely associated 

with granitoid rocks. of Late Cretaceous to early Tertiary 

age. Only one deposit is associated with a Jurassic pluton. 

Furthermore, all but one of the Arizona-Sonora-New 

Mexico porphyry copper deposits lie along or northeast of 

a line from La Caridad, Mexico, to Kingman, Arizona. The 

single exception is Ajo, in southwestern Arizona. A small 

area of porphyry copper-type alteration is associated with a 

Jurassic granodiorite in the northern Kofa Mountains, Arizona (Bagby and others, 1987). Although southeastern 

California has been extensively explored for porphyry 

copper deposits, no such deposits have been found. 

Although the ages of the granitic rocks in the Orocopia 

Mountains Wilderness Study Area are not well known, 

both Late Cretaceous plutons and Jurassic plutons are 

probably present. The regional data summarized above 

indicate that the likelihood of a porphyry copper deposit in 

the Orocopia Mountains Wilderness Study Area is small. 

However, no geologic or geochemical evidence of porphyry copper-type mineralization or alteration is known in 

Mineral Resources of the Orocopia Mountains Wilderness Study Area, Riverside County, California El 5 
the study area. Therefore, those areas of the Orocopia 

Mountains Wilderness Study Area underlain by granitic 

rocks are assigned low potential, with certainty level B, for 

copper and molybdenum in porphyry copper deposits. 

Placer Gold 

Apparently, there has been no extensive, longstanding 

placer gold mining in the Orocopia Mountains. Nonetheless, the recognition of high potential for lode gold mineralization in the strip of crystalline rocks between the Orocopia thrust and the Clemens Well fault raises the possibility of placer gold in drainages downstream from this strip. 

No gold was detected in the stream-sediment concentrates. 

The areas that drain from the above-mentioned strip are 

considered to have moderate potential, with certainty level 

B, for placer gold occurrences. 

Geothermal Energy 

Available information pertaining to the possibility of 

geothermal energy in the Orocopia Mountains Wilderness 

Study Area is summarized in the Geothermal Resources 

section. The southwestern two-thirds of the Orocopia 

Mountains Wilderness Study Area is considered to have 

moderate potential, at certainty level B, for geothermal 

energy. 

Oil and Gas 

The high-grade metamorphic rocks in the study area 

prohibit oil and gas accumulation. The Orocopia Mountains Wilderness Study Area has no oil and gas potential, 

certainty level D. 

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Mineral Resources of the Orocopia Mountains Wilderness Study Area, Riverside County, California E17 
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. 

Peterson, J.A., Cox, D.P., and Gray, Floyd., 1987, Mineral resource assessment of the Ajo and Lukeville 1° by 2° quadrangles, Arizona: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-1834-B, scale 1:250,000. 

Powell, R.E., 1981, Geology of the crystalline basement complex, 

eastern Transverse Ranges, southern California: constraints 

on regional tectonics interpretations: Pasadena, Calif., California Institute of Technology, Ph.D. dissertation, 441 p. 

Powell, R.E., 1982, Crystalline basement terranes in the southern 

eastern Transverse Ranges, California, in Geologic Excursions in the Transverse Ranges, southern California: 

Anaheim, Calif., Cordilleran Section, Geological Society of 

America, Volume and Guidebook, p. 109-136. 

Powell, R.E., Watts, K.C., and McColly, R.A., 1984a, Mineral 

resource potential of the Chuckwalla Mountains Wilderness 

Study Area (CDCA-348), Riverside County, California, U.S. 

Geological Survey Open-File Report 84-674. 

Powell, R.E., Whittington, C.L., Grauch, V.J.S., and Lane, M.E., 

1984b, Mineral resource potential of the Eagle Mountains 

Wilderness Study Area (CDCA-334), Riverside County, 

California, U.S. Geological Survey Open-File Report 84-631. 

Roberts, C.W., Jachens, R.C., and Oliver, H.W., 1981, Preliminary isostatic residual gravity map of California: U.S. Geological Survey Open-File Report 81-573, scale 1:750,000, 5 

sheets. 

Ruhlin D.E., and Owen, R.M., 1986, The rare earth element 

geochemistry of hydrothermal sediments from the East Pacific 

Rise: examination of a seawater scavenging mechanism: 

Geochimica et Cosmochimica Acta, v. 50, p. 393-400. 

Silver, L.T., 1966, Preliminary history for the crystalline complex 

of the central Transverse Ranges, Los Angeles County, California [abs.]: Geological Society of America Program, Annual Meetings, p. 201. 

__ 1968, Pre-Cretaceous basement rocks and their bearing on 

large-scale displacements in the San Andreas fault system: 

Stanford University Publications in Geological Science, v. 11, 

p. 279-280. 

__ 1971, Problems of crystalline rocks of the Transverse 

Ranges [abs.]: Geological Society of America Abstracts with 

Programs, v. 3, p. 193-194. 

Smith, D.B., Berger, B.R., Tosdal, R.M., Sherrod, D.R., Raines, 

G.L., Griscom, Andrew, Helferty, M.G., Rumsey, C.M. and 

McMahan, A.B., 1987, Mineral resources of the Indian Pass 

and Picacho Peak Wilderness Study Areas, Imperial County, 

California, U.S. Geological Survey Bulletin 1711-A, 21 p. 

Spittler, T.E., 1974, Volcanic petrology and stratigraphy of nonmarine strata, Orocopia Mountains: their bearing on Neogene 

slip on the San Andreas fault, southern California: Riverside, 

Calif., University of California, M.S. thesis, 115 p. 

Spittler, T.E., and Arthur, M.A., 1973, Post early Miocene displacement along the San Andreas fault in southern California: 

Stanford University Publications in the Geological Science, v. 

13, p. 374-382. 

__ 1982, The lower Miocene Diligencia Formation of the 

Orocopia Mountains, southern California: stratigraphy, petrology, sedimentology, and structure, in Ingersoll, R.V., and 

Woodbume, M.O., eds., Cenozoic nonmarine deposits of 

California and Arizona: Pacific Section, Society of Economic 

Paleontologists and Mineralogists, p. 83-99. 

Squires, R.L., and Advocate, D.M., 1982, Sedimentary facies of 

the nonmarine lower Miocene Diligencia Formation, Canyon 

Spring area, Orocopia Mountains, southern California, in 

Ingersoll, R.V., and Woodbume, M.O., eds., Cenozoic nonmarine deposits of California and Arizona: Annual meeting, 

Anaheim, Calif., 1982, Pacific Section, Society of Economic 

Paleontologists and Mineralogists, p. 101-106. 

Sylvester, A.G., and Smith, R.R., 1976, Tectonic transpression 

and basement-controlled deformation in San Andreas fault 

zone, Salton Trough, California: American Association of 

Petroleum Geologists Bulletin, v. 60, p. 2081-2102. 

Taylor, S.R., and McLennan, S.M., 1985, The continental crust: 

its composition and evolution: Oxford, Blackwell, 312 p. 

Terres, R.R., 1984, Paleomagnetism and tectonics of the central 

and eastern Transverse Ranges, southern California: Santa 

Barbara, Calif., University of California, Ph.D. thesis, 323 p. 

Titley, S.R., 1982, Advances in the geology of the porphyry 

copper deposits of southwestern North America: Tucson, 

Ariz., University of Arizona Press, 560 p. 

Titley, S.R., and Beane, R.E., 1981, Porphyry copper deposits, pt. 

2: Hydrothermal alteration and mineralization: Economic 

Geology Seventy-Fifth Anniversary Volume, p. 235-269. 

Tosdal, R.M., Haxel, G.B., and Dillon, J.T., 1985, Lithologic 

associations of gold deposits, southeastern California and 

southwestern Arizona [abs.]: Geological Society of America 

Abstracts with Programs, v. 17, p. 414. 

Tosdal, R.M., and Smith, D.B., 1987a, Some characteristics of 

gneiss-hosted gold deposits of southeastern California, in 

Sachs, J.S., ed., U.S. Geological Survey Research on mineral 

resources-1987 program and abstracts: U.S. Geological 

Survey Circular 995, p. 71. 

__ 1987b, Gneiss-hosted kyanite gold and gneiss-hosted epithermal gold: A supplement to U.S. Geological Survey Bulletin 

1693: U.S. Geological Survey Open-File Report 87-272 B, 7 

p. 

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Riverside County: California Journal of Mines and Geology, 

v. 41, no. 3, p. 121-182. 

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, 1983, Aeromagnetic map of the Salton 

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

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

Vredenburgh, L.M., Shumway, G.L., and Hartill, R.D., 1981, 

Desert fever, an overview of mining in the California Desert: 

Canoga Park, Calif., Living West Press, 323 p. 

Van Nort, S.D., and Harris, Michael, 1984, Geology and mineralization of the Picacho gold prospect, Imperial County, California: Arizona Geological Society Digest, v. 15, p. 175-183. 

Willis, G.F., Tosdal, R.M., and Manske, S.L., 1987, The Mesquite 

mine, southeastern California: epithermal gold mineralization 

in a strike-slip fault system [abs]: Geological Society of 

America Abstracts with Programs, v. 19, p. 892. 

Woodbume, M.O., and Whistler, D.P., 1973, An Early Miocene 

Oreodont (Merychyinae, Mammalia) from the Orocopia 

Mountains, southern California: Journal of Paleontologists, v. 

74, p. 908-912. 

E18 Mineral Resources of Wilderness Study Areas: South-Central California Desert Conservation Area, California 
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. 

i .....J 

<( 

i= 

z 

UJ 

1-

0 

c.. 

UJ 

u 

~ 

:::J 

0 

V') 

UJ 

~ 

u.. 

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 

L/B L/C 

LOW POTENTIAL LOW POTENTIAL 

LEVEL OF CERTAINTY 

D 

H/D 

HIGH POTENTIAL 

M/D 

MODERATE POTENTIAL 

L/D 

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. 

E20 Mineral Resources of Wilderness Study Areas: South-Central California Desert Conservation Area, California 
RESOURCE/RESERVE CLASSIFICATION 

IDENTIFIED RESOURCES UNDISCOVERED RESOURCES 

Demonstrated Probability Range 

Inferred 

Measured Indicated Hypothetical Speculative 

I Inferred I 

ECONOMIC Reserves 

Reserves 

I - I ----~---- r---------- -~-

MARGINALLY Marginal Inferred 

Marginal ECONOMIC Reserves Reserves ____ _L ____ - I I r---------- -~-

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 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 Orocopia Mountains Wilderness Study Area, Riverside County, California E21 
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 

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - (3800?)- - - - - - - - - - - - - - - - - -

pre-Archean2 

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

21nformal time term without specific rank. 

AGE ESTIMATES OF 

BOUNDARIES IN 

Ml LLION 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 

E22 Mineral Resources of Wilderness Study Areas: South-Central California Desert Conservation Area, California 
Mineral Resources of 

Wilderness Study Areas: 

South-Central California 

Desert Conservation Area, 

California 

This volume was published as separate chapters A-E 

U.S. GEOLOGICAL SURVEY BULLETIN 1710 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 
CONTENTS 

[Letters designate the separately published chapters] 

(A) Mineral Resources of the Palen-McCoy Wilderness Study Area, Riverside 

County, California, by Paul Stone, Thomas D. Light, V.J.S. Grauch, Warren E. 

Yeend, and Russell A. Schreiner. 

(B) Mineral Resources of the Morongo Wilderness Study Area, San Bernardino 

County, California, by Jonathan C. Matti, Scott E. Carson, James E. Kilburn, 

Andrew Griscom, Douglas V. Prose, and Lucia Kuizon. 

(C) Mineral Resources of the Mecca Hills Wilderness Study Area, Riverside County, 

California, by Douglas M. Morton, James E. Kilburn, Andrew Griscom, and 

Harry W. Campbell. 

(D) Mineral Resources of the Santa Rosa Mountains Wilderness Study Area, 

Riverside County, California, by J.P. Calzia, D.J. Madden-McGuire, H.W. Oliver, 

and R.A. Schreiner. 

(E) Mineral Resources of the Orocopia Mountains Wilderness Study Area, Riverside 

County, California, by Gordon B. Haxel, David B. Smith, Andrew Griscom, 

Denny V. Diveley-White, Robert E. Powell, and Terry J. Kreidler. 
GPO 685-041/9776 
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Haxel and others-MINERAL RESOURCES, OROCOPIA MTS. WILDERNESS STUDY AREA, RIVERSIDE COUNTY, CALl F.-U.

....................... -- M - ---~- ,..n _.. -"'-- -- - - .. -"~• •- .. e -· ' " ' •