Mineral Resources of the Fish Creek 

Mountains Wilderness Study Area, 

Imperial County, California

U.S. GEOLOGICAL SURVEY BULLETIN 1711-C

Chapter C

Mineral Resources of the Fish Creek 

Mountains Wilderness Study Area, 

Imperial County, California

By VICTORIA R. TODD, DAVID E. DETRA, JAMES E. KILBURN, 

ANDREW GRISCOM, and FRED A. KRUSE 

U.S. Geological Survey

HARRY W. CAMPBELL 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1711

MINERAL RESOURCES OF WILDERNESS STUDY AREAS:

SOUTHERN CALIFORNIA AND CALIFORNIA DESERT CONSERVATION AREA
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987

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 Fish Creek Mountains Wilderness 

Study Area, Imperial County, California

U.S. Geological Survey Bulletin 1711-C

Bibliography

Supt. of Docs. No. I 19.3:1711-C

1. Mines and mineral resources California Fish Creek

Mountains Wilderness. 2. Geology California Fish

Creek Mountains Wilderness. 3. Fish Creek Mountains

Wilderness (Calif.) I. Todd, Victoria R. (Victoria Roy). II.

Series.

QE75.B9 No. 1711-C 557.3 s 87-600093

[TN24.C2] [553'.09794'99]
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 part of the Fish 

Creek Mountains (CDCA-372) Wilderness Study Area, Imperial County, California.

CONTENTS

Summary Cl

Abstract 1

Character and setting 1

Identified mineral resources 1

Mineral resource potential 2 

Introduction 4

Area description 4

Previous and present investigations 5

Regional geologic setting 6

Acknowledgments 6 

Appraisal of identified resources 6

History and production 6

Mineral deposits 7

Appraisal of mineral deposits 7 

Assessment of mineral resource potential 8

Geology 8

Geochemical investigations 9

Geophysical investigations 10

Conclusions 10 

References cited 11

Appendix

Definition of levels of mineral resource potential and certainty of assessment 13 

Geologic time chart 14

FIGURES

1. Index map showing location of the Fish Creek Mountains Wilderness Study Area, 

Imperial County, California C2

2. Map showing mineral resource potential and generalized geology of the Fish Creek 

Mountains Wilderness Study Area, Imperial County, California 4

TABLE

1. Prospects in and adjacent to the Fish Creek Mountains Wilderness Study Area C3

MINERAL RESOURCES OF WILDERNESS STUDY AREAS:

SOUTHERN CALIFORNIA AND CALIFORNIA DESERT CONSERVATION AREA

Mineral Resources of the Fish Creek Mountains 

Wilderness Study Area, Imperial County, California

By Victoria R. Todd, David E. Detra, James E. Kilburn, 

Andrew Griscom, and Fred A. Kruse 

U.S. Geological Survey

Harry W. Campbell 

U.S. Bureau of Mines

SUMMARY 

Abstract

At the request of the Bureau of Land 

Management, 16,882 acres of the Fish Creek 

Mountains Wilderness Study Area (CDCA-372) were 

studied. In this report, the area studied is referred to 

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

area." Field work for this report was carried out 

between 1983 and 1985. There are no mining districts 

or mines in the study area. The U.S. Gypsum Company 

has 18 unpatented placer claims with an estimated 20 

million tons of gypsum reserves inside the study area, 

and oil and gas leases are held in and near the area. 

There is high potential for undiscovered resources of 

the nonmetallic mineral commodities gypsum and 

limestone in the northwestern and central parts of the 

study area, respectively. A northwest-trending zone 

that roughly bisects the study area has low potential 

for undiscovered resources of the metallic minerals 

lead, tungsten, and molybdenum.

Character and Setting

The part of the Fish Creek Mountains Wilderness 

Study Area that was studied for this report occupies 

approximately 16,882 acres in western Imperial 

County, Calif, (fig. 1). The study area encompasses 

most of the Fish Creek Mountains, one of the 

easternmost ranges of the Peninsular Ranges province. The steep, rugged northeast and east faces of 

the range rise about 2,400 ft above the Imperial 

Valley, which lies partly below sea level and is part of 

the Salton trough, the landward extension of the Gulf 

of California. The Fish Creek Mountains are 

characterized by an arid climate, extremely high 

summer temperatures, and a desert flora.

The study area is underlain by Cretaceous (138 to 

66 million years Before Present (Ma); see appendix for 

geologic time chart) plutonic igneous rocks of the

Peninsular Ranges batholith; these plutonic rocks 

comprise much of the crystalline bedrock of the 

Peninsular Ranges. The batholith is composed of a 

variety of syntectonic (deformed) and late-tectonic to 

posttectonic (relatively undeformed) plutons that 

intruded metamorphosed Paleozoic(?) (approximately 

570 to 240 Ma) sedimentary and volcanic rocks, 

remnants of which are exposed in tabular bodies, 

called screens, between the plutons. Neogene (24 to 

1.7 Ma) marine and alluvial deposits unconformably 

overlie the uplifted and deeply eroded batholithic 

rocks in the study area and faults of the San Andreas 

system cut the batholithic rocks and sedimentary 

deposits.

Identified Mineral Resources

There are no mining districts or mines in the Fish 

Creek Mountains Wilderness Study Area. The 

northwestern part of the study area is underlain by 

bedded gypsum deposits of Miocene age that are 

continuous with deposits mined by the U.S. Gypsum 

Company at the U.S. Gypsum quarry 1.5 mi west of the 

study area (fig. 2). The U.S. Gypsum Company has 18 

unpatented placer claims in the northwestern part of 

the study area. The gypsum reserves within the study 

area are estimated at more than 20 million tons. The 

Roberts Celestite mine, 3 mi northwest of the study 

area, produced 8,000 tons of strontium ore, mostly 

during World War II. Minor amounts of celestite and 

barite may occur within the gypsiferous strata within 

the study area, but no deposits of these minerals were 

identified.

Limestone is the major constituent of a large 

screen of Paleozoic(?) metasedimentary rocks in the 

central part of the study area. The Waters prospect 

(fig. 2, No. 2; table 1) encompasses large quantities of 

this limestone, which may be suitable for many 

commercial uses.

Skarns, or carbonate-rich rocks altered by the 

intrusion of granitic magma, and (or) clay alteration

Cl
zones occur locally in a northwest-trending shear zone 

that bisects the study area. The shear zone follows 

contacts between carbonate-bearing metasedimentary 

screens and granitic rocks. Seven prospects in the 

east-central part of the study area are located in this 

shear zone. Samples from these prospects contained 

minor amounts of zinc, copper, manganese, and 

barium. Anomalous concentrations of tungsten, lead, 

molybdenum, and silver are present in sediments from 

streams that drain the shear, zone (north half of the 

study area). Local metasomatism and (or) 

hydrothermal alteration of carbonate-bearing 

metasedimentary rocks in the shear zone were 

probably associated with the minor mineralization that 

produced the geochemical anomalies. A small 

quantity of manganese may have been produced from 

the Hanks Lost Mine prospect located 1,000 ft 

southwest of the study area (fig. 2, No. 12).

Although extensive sand and gravel deposits are 

present along the northeast and east boundaries of the 

study area, deposits outside of the study area are 

adequate to meet foreseeable needs.

Mineral Resource Potential

Geologic and geochemical studies indicate that 

the study area locally has high resource potential for 

gypsum and low resource potential for lead, tungsten, 

and molybdenum (fig. 2). Three unmined gypsum 

deposits are present in the northwestern part of the 

study area and additional deposits are inferred to 

underlie alluvium. The northwestern part of the study 

area therefore has high resource potential for 

gypsum. There is high resource potential for limestone 

in the central part of the study area in a large screen 

that includes the Waters prospect (fig. 2, No. 2; table 

1). The northwest-trending shear zone that contains 

skarn and (or) clay alteration zones associated with 

minor mineralization has been assigned low resource 

potential for lead, tungsten, and molybdenum.

No potential for energy resources was identified 

in the study area. Oil and gas leases are held in the 

eastern one-third of the study area, but no potential 

for hydrocarbon resources was identified during this 

study.

:REEK MOUNTAINS 

WILDERNESS STUDY 

AREA (CDCA-372)

Anza - Borrego Desert 

State Park

Recovery Parachute 

Carrizo Test Range

Impact r - 

Area i

IMPERIAL 

VALLEY

and Arizona Eastern Railroad

YUHA DESERT

10 MILES

Figure 1. Index map showing location of the Fish Creek Mountains Wilderness Study Area, Imperial County, 

California.

C2
TABLE 1. Prospects in and adjacent to the Fish Creek Mountains Wilderness Study Area

[*, outside the wilderness study area boundary]

Map No. 

(fig. 2) Summary

Workings and 

production Sample and resource data

Gypsum prospect

Waters prospect

Unnamed prospect

Unnamed prospect

Unnamed prospect

Unnamed prospect

Unnamed prospect

Unnamed prospect

Unnamed prospect

Unnamed prospect

Unnamed prospect

Hanks Lost Mine 

prospect

Bonanza Queen 

prospect

A bed of massive gypsum crops out in 

three significant bodies (1A, IB, 1C). 

Drill hole data show gypsum thickness 

of more than 100 ft. Interbedded shale, 

gypsum, and sandstone underlie 

massive gypsum bed.

A screen of carbonate rock about 3 mi 

long and 1 mi wide surrounded by 

granitic rocks. Exposed stratigraphic thickness of carbonate 

unit consists of about 1,500 ft of 

marble and lower 1,200 ft of dolomite 

with pegmatite dikes.

A narrow zone of alteration near 

marble-granitic rock contact. Altered 

zone contains mixture of clay and 

iron oxide minerals.

Small metasedimentary screens in 

granitic rocks. Metasomatic replacement 

formed skarns along fractures in 

carbonate rock in small screens. 

Skarn minerals are predominantly quartz, 

garnet, pyroxene, and epidote with iron 

and copper oxide staining. Some 

alteration near skarn and narrow 

shear zones.

A small screen exposed on ridge near 

head of Red Rock Canyon. Screen 

is mostly marble with narrow quartzite 

bed through center of screen. Small 

shear zones in marble are iron 

oxide stained and locally heavily 

altered. Epidote and siderite in 

shear zones.

A small marble screen has skarn containing 

quartz, garnet, epidote, actinolite, and 

pyroxene.

A shear zone strikes N. 10° W. and dips 

65° NE. along contact between gneissic 

granitic rocks and foliated dioritic 

rocks. Gneissic granitic rocks have 

numerous pegmatite dikes containing quartz, 

feldspar, garnet, and biotite.

A skarn zone trends N. 76° W. along 

marble screen-granitic rocks contact. 

Masses of garnet are in skarn; individual 

garnet crystals have diameters as much 

as 2 in. Skarn minerals mainly quartz 

and garnet with some epidote and pyroxene.

A marble screen with outcrop diameter of 

about 100 ft has skarn zone that strikes 

N. 10° W. and dips 15° NW. along 

fracture in marble. Skarn minerals 

are quartz, garnet, epidote, pyroxene, 

and idocrase.

A skarn zone strikes N. 80° E. and dips 

45° SE. along marble screen-granitic 

rocks contact.

A near-vertical fault zone about 10 ft 

wide strikes N. 65° E. for over 3/1 mi 

along contact separating granitic 

rocks on north from sedimentary rocks 

on south. Fault zone contains 

manganese and iron oxides.

A nearly vertical fault zone about 10 ft 

wide which strikes N. 65° E. has 

manganese oxide occurring as fracture 

fillings and disseminations in quartz 

veinlets.

A nearly vertical fault zone that strikes 

N. 65° E. has azurite and malachite in 

shear zone 3 ft wide.

No production or workings

One prospect pit

One pit 12 ft by 8 ft by 

1 ft deep.

Three shafts from 8 to 12 

ft deep and four prospect 

pits.

Five prospect pits

One prospect pit

One 23-ft shaft with 

30-ft crosscut at 

bottom.

Five prospect pits

One prospect pit

One prospect pit

Two shafts 15 and 17 ft 

deep and three prospect 

pits.

One shaft 70 ft deep and 

one 30-ft adit.

One shaft 25 ft deep

A chip sample from overlying weathered 

zone of gypsum body 1A had 91.0 pet 

gypsum. A grab sample from overlying 

weathered zone of body IB contained 92.1 

pet gypsum. A 10-ft vertical chip 

sample from gypsum body 1C averaged 89.0 

pet gypsum for upper 30 ft and 

58.5 pet gypsum and 37.1 pet 

anhydrite for lower 10 ft. Deposits 1A, 

IB, and 1C have 600,000 tons, 100,000 

and 20 million tons of inferred gypsum 

reserves, respectively.

Fourteen samples taken by U.S. 

Bureau of Mines and California Division 

of Mines and Geology had values that 

ranged from 51.2 pet CaO and 0.66 

pet MgO (limestone) to 29.9 

pet CaO and 21.7 pet MgO (dolomite).

One grab sample of altered material from 

dump contained no gold or silver.

Five chip and one select sample contained 

no gold, silver, or tungsten. One sample 

had 0.1 pet zinc and 0.05 pet 

copper. Spectrographic analysis of one 

sample showed minor amounts of barium.

Three chip samples contained no gold, 

silver, or tungsten. A spectregraphic analysis of one chip sample 

showed minor amounts of barium and 

manganese.

One chip sample contained no gold, 

silver, or tungsten.

Two chip and one select sample contained 

no gold, silver, tungsten, or molybdenum.

One chip and one grab sample contained no 

gold, silver, tungsten, or molybdenum.

One chip sample contained no gold, silver, 

or tungsten.

One chip sample contained no gold, silver, 

or tungsten.

Three chip samples had 0.1 oz/ton silver 

and one chip sample had a trace silver. 

Three chip samples contained 0.11 pet, 

0.70 pet, and 0.93 pet manganese.

One select sample contained 6.7 pet 

manganese.

One select sample had 1.9 pet copper.

C3
INTRODUCTION report occupies approximately 16,882 acres in western 

Imperial County, Calif, (fig. 1). One of the easternmost ranges of the Peninsular Ranges province, the 

Fish Creek Mountains rise above the broad, flat, 

The part of the Fish Creek Mountains Wilderness alluvium-filled Imperial Valley that lies to the 

Study Area (CDCA-372) that was studied for this northeast. The sedimentary deposits in the study area

Area Description

116°05 116°00'

Rpberts 

Cdestite\g 

Vnlne \§

Tsm\\ ! \£ 

Tic *^ &

BORREGO VALLEY

U.S. Gypsum" ' - Qs 

quarry.-^ Imperial Gypsum 

quarry ' 

v Ancient 

:. shoreline 

of

Gravel ) '' -..

APPROXIMATE BOUNDARY OF

FISH CREEK MOUNTAINS

WILDERNESS STUDY

AREA (CDCA-372)

VALLECITO - FISH 

CREEK BASIN

Figure 2. Mineral resource potential and generalized geology of the Fish Creek Mountains Wilderness Study 

Area, Imperial County, California. Geology of northeastern part of area by V.R. Todd; southeastern part from 

Morton (1977) and Woodard (1963).

C4
are part of a thick accumulation of Neogene clastic 

sediments that filled the Imperial structural 

depression, or Salton trough, which was once 

connected to the Gulf of California (Dibblee, 1954). In 

this region, an irregular erosion surface that developed 

on the crystalline rocks of the Peninsular Ranges

EXPLANATION

Area with high mineral resource potential

Area with low mineral resource potential

See appendix for definition of levels of mineral resource 

potential and certainty of assessment

Mo 

Pb 

w 

Gyp 

Ls

Qs

QTs

Tfc

Tal

Tsm

Kfc

Pb

X

11

Commodities

Molybdenum

Lead

Tungsten

Gypsum

Limestone

Geologic map units

Surficial deposits (Quaternary)

Sedimentary rocks (Quaternary and Tertiary) 

Palm Spring and Imperial Formations

Fish Creek Gypsum (Tertiary) 

Alverson Andesite (Tertiary)

Split Mountain Formation (Tertiary)

Plutonic rocks of Fish Creek Mountains 

(Cretaceous)

Prebatholithic metasedimentary rocks (Paleozoic?)

Contact

High-angle fault Dashed where approximately 

located or inferred; dotted where concealed. 

U on upthrown side, D on downthrown side

Brittle shear zone

Anticline

Syncline Dashed where concealed 

Stream-sediment sample site

Mine or quarry with recorded past production 

Prospect See table 1 for description

Figure 2. Continued.

batholith is overlain by Neogene clastic sediments 

derived both from the adjacent mountain ranges and 

from the area drained by the Colorado River to the 

east. These deposits represent virtually continuous 

deposition over the last 24 m.y. and probably attain 

thicknesses greater than 20,000 ft in the Imperial 

Valley.

The roughly triangular shape of the Fish Creek 

Mountains reflects the origin of the range as an 

upfaulted basement block. The northeast and southeast flanks of the range are bounded by faults of the 

San Andreas fault system along which the mountain 

block has been uplifted repeatedly and tilted gently to 

the southwest during the past 24 m.y. As a result of 

faulting, the northeast and southeast faces of the 

range are extremely steep and rugged. Relief varies 

from near sea level at the east boundary of the study 

area to 2,371 ft at Eagle Peak on the crest of the 

range. The climate of the region is arid with rainfall 

ranging between 3 and 4 in. per year (Morton, 1977), 

and summer daytime temperatures often exceed 

110 °F.

The study area is drained on the northeast by 

Carrizo Creek and Fish Creek Wash and their 

tributaries, all of which flow into the Salton Sea (fig. 

1). The study area is located about 2 mi east of the 

Anza-Borrego Desert State Park and is bounded on the 

southwest by the Carrizo Impact Area, an inactive 

bombing range that is administered by the AnzaBorrego Desert State Park. The study area overlaps 

the Recovery Parachute Test Range on the east. 

Access to the study area is by Split Mountain Road, 

which connects California Highway 78 in Ocotillo 

Wells with the U.S. Gypsum quarry at the northwest 

end of the range. An unimproved dirt road runs along 

the northeast flank of the range parallel to the narrow 

gauge Pacific Portland Cement Industrial Railroad, 

which runs from the U.S. Gypsum quarry to Plaster 

City.

Previous and Present Investigations

This mineral resource study is a joint effort by 

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

of Mines (USBM). The history and philosophy of such 

joint mineral surveys of U.S. Bureau of Land 

Management (BLM) study areas are discussed by 

Beikman and others (1983). Mineral assessment 

methodology and terminology are discussed by 

Goudarzi (1984). Studies by the USGS are designed to 

provide a reasonable scientific basis for estimating the 

possibilities for the existence of undiscovered mineral 

resources by determining geologic units and structures, 

possible environments of mineral deposition, and 

presence of geochemical and geophysical anomalies. 

The USBM evaluates identified resources at individual 

mines and known mineralized areas by collecting data 

on current and past mining activities and through field 

examination of mines, prospects, and mineralized 

areas.

The Fish Creek Mountains Wilderness Study Area 

includes parts of the Borrego Mountain SE, Harpers 

Well, Plaster City NW, and Carrizo Mountain NE 

7.5-minute quadrangles. Published geologic maps 

describe the geology of the Imperial Valley region

C5
(Loeltz and others, 1975; Morton, 1977), while 

unpublished mapping was provided by T.W. Dibblee, Jr. 

(written commun., 1943), for part of the Fish Creek 

Mountains. The U.S. Geological Survey carried out 

field investigations in the study area between 1983 and 

1985. The work included mapping, whole rock and 

stream sediment sampling for geochemical analysis 

and radiometric dating, and aeromagnetic and gravity 

surveys. Twenty-three panned heavy-mineral 

concentrates from stream-sediment samples were 

analyzed; the analytical data are given in Detra and 

Kilburn (1985).

The U.S. Bureau of Mines conducted a library 

search for information on mines and prospects in and 

adjacent to the study area. Mines and prospects near 

the study area were described by Tucker (1926) and 

Sampson and Tucker (1942). These data were 

supplemented by information from U.S. Bureau of Land 

Management mining claim and land status records and 

U.S. Bureau of Mines Mineral Industry Location 

System. Field studies by U.S. Bureau of Mines 

personnel were carried out in the fall of 1983 

(Campbell, 1986). Rock samples were checked for 

radioactivity and fluorescence and were analyzed for a 

variety of elements by fire-assay, inductively coupled 

plasma, chemical, and semiquantitative spectrographic 

methods. Detailed sample data are presented in 

Campbell (1986).

Regional Geologic Setting

The Fish Creek Mountains Wilderness Study Area 

is part of the Peninsular Ranges batholith of southern 

California, U.S.A., and Baja California, Mexico. The 

batholith is composed of hundreds of plutons of 

variable compositions that range in age from about 130 

to 80 Ma (Cretaceous Period) (Silver and others, 

1979). The original eastern limit of the batholith is 

uncertain, although it probably extended at least as far 

as northwestern Sonora, Mexico. Beginning in Miocene 

time, the eastern part of the batholith was disrupted 

by the San Andreas fault and the opening of the Gulf 

of California. In Imperial County, the batholith is 

overlapped by marine and continental sedimentary 

deposits that range in age from Miocene (24 to 5 Ma) 

to Holocene (10,000 yr to present).

Batholithic rocks in western Imperial County 

were intruded during and after the last stages of 

regional deformation (Todd and Shaw, 1979). 

Deformation structures in plutons include foliation, 

lineation, and tight isoclinal folds whose axial planes 

are coplanar with foliation. Contacts between these 

plutons, and between plutons and screens of 

metasedimentary rocks, are broadly concordant with 

these structures. Most contacts dip steeply (60° to 

75°).

The wallrock screens in Imperial County consist 

of metasedimentary rocks that were originally quartzrich sandstone, shale, and limestone deposited in a 

relatively shallow continental shelf sea (Hill, 1984; 

Gastil and Miller, 1984). The rocks are as yet undated, 

but contain thick carbonate sequences that resemble 

carbonate rocks in the Coyote Mountains about 10 mi 

to the south that were dated by Miller and Dockum 

(1983) as Ordovician.

Andesite and basalt were erupted in the central 

part of the batholith in California and in northern Baja 

California in Miocene time (Minch and Abbott, 1973). 

Volcanism was approximately contemporaneous with 

the inception of faulting on the San Andreas system 

and the opening of the Gulf of California. Folding, 

faulting, and localized subsidence and uplift related to 

these tectonic events continue to affect the regions 

marginal to the Gulf of California to the present 

time. In the study area, thick Miocene and Pliocene 

marine and continental clastic sediments that once 

filled the Imperial depression were largely eroded off 

the upfaulted Fish Creek Mountains block. 

Interbedded andesitic flows and marine and continental 

sediments of presumed Miocene age are exposed 2 to 3 

mi southwest of the study area.

Acknowledgments

The authors gratefully acknowledge R.J. 

Beckman, Senior Staff Geologist, United States 

Gypsum Company, Chicago, 111., for providing 

information concerning the Fish Creek Mountains 

gypsum deposits. We also thank personnel of the El 

Centro and Riverside district offices of the U.S. 

Bureau of Land Management for advice on access to 

the study area and for generously making their records 

available to us. The Anza-Borrego Desert State Park 

gave permission for helicopter landings in the state 

park adjacent to the study area.

APPRAISAL OF IDENTIFIED RESOURCES

By Harry W. Campbell, U.S. Bureau of Mines

History and Production

No mines or mining districts are present in the 

Fish Creek Mountains Wilderness Study Area. A large 

gypsum mine owned and operated by the U.S. Gypsum 

Company lies 1.5 mi west of the study area (the U.S. 

Gypsum quarry) (fig. 2). The Imperial Gypsum quarry 

(fig. 2) produced a small, unknown tonnage of gypsum 

that was marketed as a soil conditioner in the Imperial 

Valley (Morton, 1977). The U.S. Gypsum Company has 

18 unpatented placer claims inside the study area.

Completion of the San Diego and Arizona 

Eastern Railroad (fig. 1) in 1920 permitted the Fish 

Creek Mountains gypsum deposits to be mined. 

Imperial Gypsum and Oil Corporation acquired a large 

block of gypsum claims in the Fish Creek Mountains 

and constructed a narrow gauge railroad from the San 

Diego and Arizona Eastern track to the deposit in 

1922. The first shipment of crude gypsum was made in 

1922. In 1924, Pacific Portland Cement Company 

bought the Fish Creek Mountains gypsum operations 

and built a calcining plant (Plaster City plant) at the 

junction of the narrow gauge railroad with the main 

line. The Plaster City plant and quarry were sold in 

1945 to U.S. Gypsum Company who modernized and 

expanded the operation (Ver Planck, 1952, p. 71, 72; 

Pressler, 1984, p. 425; Burnett, 1984, p. 8). In 1971, 

the U.S. Gypsum quarry produced 742,000 tons of

C6
gypsum, making it the largest operating gypsum mine 

in the United States (Appleyard, 1975, p. 715).

About 8,000 tons of strontium ore (celestite) was 

shipped from the Roberts Celestite mine (fig. 2), 

mostly during World War II (Morton, 1977). Sand and 

gravel were quarried from a gravel pit east of the 

study area (fig. 2) and used for fill, aggregate, and 

ballast. A small quantity of manganese may have been 

produced from the Hanks Lost Mine prospect 

southwest of the study area (fig. 2, No. 12). Oil and 

gas leases are held in and near the study area (U.S. 

Bureau of Land Management, written commun., 

1986). No other mining claims or mineral leases are 

known inside the study area.

Mineral Deposits

Four types of deposits were examined by the U.S. 

Bureau of Mines: (1) bedded gypsum deposits; (2) 

limestone in a large screen; (3) small carbonate bodies 

with skarn and (or) clay alteration zones; and (4) sand 

and gravel in alluvial fan and ancient lacustrine 

shoreline deposits. In addition, a tungsten prospect 

and three manganese prospects (the latter outside the 

study area) were examined.

Three gypsum deposits are present inside the 

study area (fig. 2, Nos. 1A, IB, and 1C), although none 

of the gypsum has been mined. Gypsum outcrops in an 

area southeast of the U.S. Gypsum quarry (fig. 2) are 

remnants of a once-continuous bed (Ver Planck, 1957, 

p. 233). At this site, gypsum is preserved in the gently 

dipping limbs of a synclinal basin. The axis of this 

syncline lies under alluvium in the wash west of the 

quarry (fig. 2). Gypsum beds on the east and west 

synclinal limbs are exposed along the margins of this 

wash. Within the study area, gypsum in deposits 1A 

and IB is present as relatively small, isolated veneers 

that overlie a zone of thin, interbedded gypsiferous 

clay and sand layers. Massive gypsum more than 100 

ft thick is found in the study area at deposit 1C. The 

degree to which the deposit has been hydrated is 

unusually high (Appleyard, 1975, p. 715); however, 

there are lenses and layers of anhydrite in the gypsum 

unit. Most massive gypsum and anhydrite deposits are 

present as large, lenticular stratified bodies formed by 

the evaporation of lake or sea water in basins with 

restricted outlets. Results of a 174-ft hole drilled in 

the study area (fig. 2) by U.S. Gypsum Company in 

1985 showed 117 ft of massive gypsum overlain by 36 

ft of alluvium. The gypsum content of the unit ranges 

from 89.9 to 99.4 percent as determined by analyses of 

samples taken at 5-ft intervals (R.J. Beckman, written 

commun., 1985). Gypsum content is consistently more 

than 85 percent in the massive gypsum bed throughout 

the region. Deposits 1A, IB, and 1C (fig. 2) contain an 

estimated 600,000 tons, 100,000 tons, and 20 million 

tons, respectively, of gypsum. Resource estimates 

were based on data from surface samples, outcrops, 

aerial photographs, topographic maps, geologic 

inference, and the drill hole inside the study area.

Limestone is present inside the study area at the 

Waters prospect (fig. 2, No. 2). The prospect covers 

1,200 acres and is located near the crest of the Fish 

Creek Mountains. The limestone is in a large screen 

composed chiefly of metasedimentary rocks of

Paleozoic(?) age intruded by Cretaceous(?) granitic 

rocks. The maximum exposed stratigraphic thickness 

of the limestone at the Waters prospect is about 2,700 

ft. The lower 1,200 ft consists of gray- and brownweathered dolomite with numerous granitic dikes that 

diminish in number upsection. The upper 1,500 ft of 

carbonate rock is composed of light-gray, fine- to 

coarse-grained marble (Morton, 1977).

Prospects 3 through 6 and 8 through 10 (fig. 2) 

occur along a northwest trend in the east-central part 

of the study area. Pits and small shafts were dug on 

and near carbonate bodies with skarn and (or) clay 

alteration zones. Samples taken from these mineral 

occurrences did not contain significant amounts of 

economic elements, but minor amounts of zinc, 

copper, manganese, and barium were found. The 

nearly linear trend of these prospects suggests that 

mineralization may have been related to a northwesttrending fault zone.

Prospect 7 (fig. 2) in the west-central part of the 

study area lies within a northwest-trending shear zone 

that follows a contact between granitic and dioritic 

rocks. Numerous pegmatite dikes are present near the 

prospect. Morton (1977, pi. 1) designated this site as a 

tungsten prospect, however, three samples from this 

prospect did not contain significant amounts of 

tungsten, molybdenum, gold, or silver.

Prospects 11 through 13 (fig. 2) are south and 

west of the study area boundary. Shafts, pits, and an 

adit occur along a northeast-trending fault zone. A 

select sample from prospect 12 had 6.7 percent 

manganese, but manganese values were not consistent 

in the exposed part of the shear zone. A select sample 

from prospect 13 had 1.9 percent copper. 

Mineralization along this fault zone does not appear to 

continue into the study area.

Sand and gravel in alluvial fan and ancient 

lacustrine shoreline deposits are present along the 

northeast and east boundaries of the study area. These 

commodities were quarried just outside the east 

boundary of the study area (fig. 2) and used for fill, 

aggregate, and railroad ballast. This quarry was leased 

by the California Department of Transportation during 

the 1960's as a potential source of aggregate and fill 

for local road construction projects (California 

Department of Transportation, oral commun., 1984).

A brief description of mineral properties 

examined by the U.S. Bureau of Mines during this study 

is contained in table 1.

Appraisal of Mineral Resources

The gypsum resources within the study area (fig. 

2, Nos. 1A, IB, and 1C) are classified as "inferred 

reserves" (U.S. Bureau of Mines and the U.S. 

Geological Survey, 1980) because they are an extension 

of a gypsum bed that has been continuously mined 

since 1922, and their size, quality, and location are 

such that profitable extraction is possible under 

current economic conditions. The term "inferred" was 

applied because the size, shape, and mineral content of 

the gypsum deposits were not established well enough 

to assume continuity between points of observation. 

Although the general geologic character of the 

deposits is well understood, local folding and faulting

C7
introduce enough uncertainty that more accurate 

resource estimates cannot be made with present 

data. Also, subsurface sample data are not sufficient 

to reliably estimate the impurities within the gypsum 

bed, particularly the amount of anhydrite. However, 

the gypsum was deposited as one extensive 

sedimentary unit, and sample data from widely spaced 

sites show uniformly high purity of gypsum. This 

suggests that the deposits contain high-grade material 

that can be used with no beneficiation beyond 

crushing, grinding, and calcination. Inside the study 

area, and west of deposit 1C, are 25 acres of thickbedded gypsum with sand and gravel overburden that 

could be mined economically by open-pit methods. 

Most of the facilities required for transport and 

manufacturing of gypsum are already established 

adjacent to the study area by the U.S. Gypsum 

Company.

The Waters prospect in the central part of the 

study area (fig. 2, No. 2) has large quantities of 

limestone. The deposit is large, has little overburden, 

and could be quarried by the relatively low-cost 

method of level benches driven into the hillside. 

Preliminary sampling suggests the limestone is suitable 

for many commercial purposes, including cement 

manufacture, and gypsum and clay also necessary for 

cement production are nearby. However, the deposit 

has not been developed because more accessible 

limestone deposits have been mined 19 and 13 mi south 

of the study area in the Dos Cabezas mining district 

and in the Coyote Mountains. Furthermore, igneous 

dikes and silicified patches in the Waters prospect 

limestone would hinder mining, and access to the 

deposit on the crest of the Fish Creek Mountains is 

difficult. Much of the deposit is within 2 mi of the 

U.S. Gypsum Company narrow gauge railroad, and the 

deposit is near two major marketing centers, San 

Diego and Los Angeles.

Sand and gravel in alluvial fan and ancient 

shoreline deposits are abundant along the northeast 

and east boundaries of the study area. Because there 

are adequate deposits outside the study area, it is 

unlikely that development of the sand and gravel 

deposits in the study area would be necessary.

ASSESSMENT OF MINERAL RESOURCE POTENTIAL

By_ Victoria R. Todd, David E. Detra, James E. Kilburn, 

Andrew Griscom, and Fred A. Kruse

U.S. Geological Survey

Geology

Dibblee (1954) described the crystalline rocks of 

the Fish Creek Mountains as "a series of gneisses, 

gneissoid granites, and scattered lentils of marble"; 

"hornblende-rich diorite, with facies of quartz diorite 

and gabbro" were the dominant intrusive rocks. 

Geologic mapping for the present study shows that the 

range is underlain by a single large composite pluton, 

here informally called the Fish Creek Mountains

pluton, that probably is part of the late-tectonic to 

posttectonic intrusive sequence (Todd and Shaw, 

1979). A similar pluton 20 mi west of the study area 

has been dated by uranium-lead zircon method at 100 

Ma (L.T. Silver, personal commun., 1979). A large 

posttectonic pluton in the region has uranium-lead 

zircon ages of 98, 97, and 95 Ma (Silver and others, 

1979; Clinkenbeard and others, 1986). By analogy, the 

Fish Creek Mountains pluton is tentatively considered 

to be Late Cretaceous in age. The Fish Creek 

Mountains pluton ranges in composition from 

hornblende-biotite tonalite through biotite 

granodiorite to muscovite-biotite granite. Abundant 

pegmatite dikes probably are related to the late 

granite phase. Areas of relatively massive tonalite 

and granite are present locally, but much of the pluton 

was strongly deformed at metamorphic temperatures, 

probably during and closely following its 

emplacement. Much, but not all, of the strong 

deformation fabric probably resulted from flow against 

abundant wallrock screens during intrusion. Fabric 

elements, including mineral foliation in granitic rocks 

and wallrock inclusions and preferred orientation of 

wallrock screens and granitic dikes, compose a 

northwest-striking, northeast-dipping structural grain 

in the deformed parts of the pluton. Field relations 

suggest that the Fish Creek Mountains pluton intruded 

folded metasedimentary wallrocks and that some 

folding continued after intrusion.

Screens and inclusions of wallrocks in the Fish 

Creek Mountains pluton consist of high-grade 

metasedimentary rocks and lesser amounts of gabbro 

and related fine-grained mafic rocks. The 

metasedimentary rocks include metaquartzite, quartzrich pelitic schist and gneiss, and marble. These rock 

types represent quartz-rich sandstone, argillaceous 

sandstone, shale, and limestone deposited in a 

continental shelf environment. Gabbro in small 

deformed pods and sheet-like bodies may represent 

mafic magma that was coeval with the Fish Creek 

Mountains pluton. Some of these bodies are quartz 

dioritic in composition. Screens range in size from a 

few feet long to the one containing the Waters 

limestone prospect, which is about 16,000 ft long and 

5,000 ft thick. Limestone, now marble, is present in 

many smaller screens in the study area but is typically 

interlayered with other metasedimentary rocks and 

intruded by granitic dikes. Marble from a similar 

metasedimentary sequence near Carrizo Mountain, 

about 8 mi south of the study area, contains 

Ordovician fossils (Miller and Dockum, 1983). This age 

and lithologie similarities among the metasedimentary 

rocks of the Fish Creek Mountains, Carrizo Mountain, 

and Paleozoic formations of the Great Basin suggest 

that the metasedimentary rocks of the Fish Creek 

Mountains are probably Paleozoic in age.

The wallrock screens contain as much as 50 

percent concordant fine- to medium-grained granitic 

dikes that emanate from the surrounding Fish Creek 

Mountains pluton. The pluton and its dikes are 

heterogeneous because they were variably chilled and 

contaminated near metasedimentary and mafic 

screens. Leucocratic dikes commonly were emplaced 

at a later time into the contact zones. Skarns are 

present locally where limestone was intruded by the 

tonalite pluton and (or) associated granitic dikes.

C8
The eroded Paleozoic(?) and Cretaceous 

crystalline rocks of the Fish Creek Mountains are 

overlain on the northwest and southwest by Tertiary 

and Quaternary sedimentary deposits. These deposits 

are remnants of a thick (about 20,000 ft) sequence of 

terrestrial and marine sediments deposited in, and 

adjacent to, the Salton trough. The Salton trough is a 

complex rift valley that has been tectonically active 

since Miocene time, and is the landward extension of 

the Gulf of California. The Vallecito-Fish Creek 

basin, located southwest of the Fish Creek Mountains, 

is one of several pull-apart basins that formed in the 

rift valley (Gibson and others, 1984). Present-day 

tectonic activity is expressed by movement on the 

right-lateral San Jacinto fault zone, the most active 

strand of the San Andreas system, which includes the 

Coyote Creek fault (fig. 2).

The stratigraphic nomenclature for the Tertiary 

rocks in the study area is from Morton (1977). The 

Split Mountain Formation of Miocene age crops out in 

the northwestern part of the study area (fig. 2). The 

Split Mountain Formation formed as a series of 

coalescing alluvial fans during the early phase of uplift 

of the Fish Creek and adjacent ranges (Gibson and 

others, 1984). The formation inter fingers with 

andesite about 1.5 mi southwest of the study area. 

The Salton trough was flooded by marine waters in 

latest Miocene and earliest Pliocene time (BellCountryman, 1984). The Fish Creek Gypsum, here 

considered Miocene in age, depositionally overlies the 

Split Mountain Formation and formed as a lagoonal 

evaporite deposit near the shoreline of the advancing 

sea. Although the age of the Fish Creek Gypsum 

remains controversial, the formation is considered to 

be part of the Split Mountain Formation by Woodard 

(1974) and is interpreted as Miocene in age by Dibblee 

(1984). As the basin continued to subside, a shoaling 

series of alluvial fan, shoreline, and deep-water marine 

deposits were deposited rapidly. These deposits form 

the Imperial Formation of Pliocene age (BellCountryman, 1984), which includes silts and clays 

carried by the ancestral Colorado River from the 

continental interior. Gradationally overlying these 

predominantly marine deposits is the Palm Spring 

Formation of Pliocene and Pleistocene age (Quinn and 

Cronin, 1984), a sequence of brackish water to 

lacustrine sediments with marine interbeds. The 

Imperial and Palm Spring Formations, shown as the 

Quaternary and Tertiary sedimentary rocks unit in 

figure 2, form a southwest-dipping homocline 

southwest of the study area.

In late Pliocene(?) time, the Vallecito-Fish Creek 

basin underwent broad east-west folding and 35° 

clockwise rotation as a result of right-lateral shear in 

the San Andreas fault system (Gibson and others, 

1984). The homocline southwest of the study area is 

part of a large northwest-trending syncline that 

probably formed during late Pliocene folding. The 

smaller northwest-trending syncline that underlies the 

U.S. Gypsum quarry (fig. 2) probably formed during the 

same episode of folding. These folds are broken by 

high-angle faults with northwest and northeast 

trends. The Fish Creek Mountains probably began to 

rise as a large northwest-trending anticlinal uplift 

sometime during Pliocene time, and continued to be 

elevated by complex high-angle faults along the

northeast and southeast flanks of the range. The 

range-bounding faults may still be active, as suggested 

by movement on the Coyote Creek fault during the 

Borrego Mountains earthquake (magnitude 6.4 Richter) 

in 1968 (Clark, 1972).

One of the most recent geologic features in the 

region was ancient Lake Cahuilla whose high shoreline 

bounded the northeast flank of the Fish Creek 

Mountains. A thin coating of travertine on the 

mountain front, a gravel beach, and finer-grained, 

nearshore lake deposits are preserved in this area. The 

lake originated by periodic overflow and diversions of 

the Colorado River into the Salton trough (Morton, 

1977). Although it probably existed as recently as 

several hundred years ago, the lake's highest level 

occurred much earlier possibly during Wisconsin 

glacial time or shortly thereafter.

Geochemical Investigations

A reconnaissance geochemical survey of stream 

sediments was conducted in the study area by the U.S. 

Geological Survey. Heavy mineral concentrates were 

derived from twenty-three samples collected in major 

washes near the range fronts (fig. 2). The washes drain 

basins ranging in size from less than one to several 

square miles. Red Rock and Barrett Canyons (fig. 2), 

which drain the most heavily mineralized part of the 

brittle shear zone (prospects 3, 4, 5, 7, 8, and 9), were 

not sampled because they lie within the Carrizo 

Impact Area, a former Naval bombing range. Each 

sample was sieved through a No. 10-mesh screen to 

remove the coarsest material and divided into light 

and heavy fractions using a bromoform mineral 

separation technique. The heavy fraction was further 

separated using a magnetic separator. All samples 

were analyzed by a six-step semiquantitative emission 

spectrographic method (Grimes and Marranzino, 1968) 

for 30 elements. A complete listing of the analytical 

techniques and results can be found in Detra and 

others (1985). In addition, concentrates were analyzed 

for mineral identification.

Geochemical anomalies detected within and 

adjacent to the Fish Creek Mountains Wilderness Study 

Area may reflect diverse mineralizing processes. The 

collective trace-element suite includes barium, tin, 

tungsten, molybdenum, lead, strontium, and silver. 

The anomalies are limited to the north half of the 

study area with the exception of barium and tin. In 

addition, widespread scheelite and barite as well as 

localized occurrences of celestite, anglesite, and 

cerussite were identified optically in the concentrates.

Anomalous tungsten concentrations and scheelite 

are probably related to skarn deposits that are 

commonly present along contacts between 

metasedimentary rocks and intrusive phases of the 

Fish Creek Mountains pluton. Lead and molybdenum 

are found coincident with tungsten anomalies at two 

sites in the north-central part of the study area. 

These two elements are common constituents of 

tungsten-bearing skarns and may be related to skarn 

mineralization. Laser microprobe studies of metallic 

inclusions in dolomite grains from these sites, 

however, revealed anomalous concentrations of lead, 

zinc, and copper. Conceivably, the lead and

C9
molybdenum anomalies may also have originated from 

such metallic inclusions in dolomitic rocks of screens. 

Anomalous concentrations of lead and molybdenum are 

found in streams that drain the northwest corner of 

the Fish Creek Mountains. These differ from the 

previously mentioned lead and molybdenum anomalies 

by the absence of tungsten at both localities, and the 

presence of silver at the southern site. Anglesite and 

cerussite, both alteration products of galena, were 

identified optically in this sample. These geochemical 

anomalies and associated lead minerals may reflect 

minor hydrothermal systems and (or) local 

metasomatism.

Anomalous concentrations of strontium are found 

in the north-central and northwestern parts of the 

study area. Celestite is present in the samples and 

probably originates from known celestite deposits that 

occur as thin beds and erosional remnants that overlie 

massive gypsum deposits in the region (Morton, 1977).

Ubiquitous anomalous concentrations of tin 

appear to be a regional characteristic associated with 

the late-tectonic to posttectonic intrusive sequence. 

Although no mineralogic or lithologic host was 

determined, it is possible that much of the tin occurs 

in accessory minerals disseminated throughout the 

late-tectonic to posttectonic granitic rocks. Locally, 

skarns and pegmatites may have contributed to the 

anomalies.

The mineral host for the widespread anomalous 

concentrations of barium is barite, which was 

identified in most of the samples. The origin of the 

barite is uncertain. High barium values (more than 

10,000 ppm) coincide with the anomalous 

concentrations of strontium mentioned above and may 

indicate a sedimentary origin for barite in the northern 

part of the Fish Creek Mountains.

An isolated anomalous concentration of lead 

found on the east edge of the study area probably 

came from contaminating lead shot.

Geophysical Investigations

Five aeromagnetic and gamma-ray spectrometer 

profiles (High Life Helicopters, Inc., and QEB, Inc., 

1980a and 1980b; LKB Resources, Inc, 1980) cross the 

study area. The data were collected along east-west 

and north-south traverses by helicopter at a height of 

400 ft above ground. The magnetic data are nearly 

featureless because none of the rocks within the study 

area are particularly magnetic. One radioactivity 

profile (line 11 of High Life Helicopters, Inc., and 

QEB, Inc., 1980a) shows a uranium anomaly (anomaly 

3, Ibid., 1980a) that meets the minimum statistical 

requirements of the NURE program for a uranium 

anomaly. The anomaly is located near (but probably 

about 0.4 mi beyond) the west side of the study area in 

section 4 of T. 14 S., R. 9 E. The associated rocks are 

Tertiary sediments, and the cause of the anomaly is 

unknown. The anomaly does not significantly affect 

the mineral potential of the study area because the 

geology of the study area is here restricted almost 

entirely to pre-Tertiary crystalline rocks.

Gravity data available for the study area (Oliver 

and others, 1980; Roberts and others, 1981) represent 

only about 5 stations and do not contribute

significantly to the mineral resource appraisal of the 

area.

The limonite mineral group has unique spectral 

reflectance characteristics that can be detected on 

Landsat images using a color-ratio compositing 

technique described by Rowan and others (1974). This 

technique was used to look for areas of limonite in the 

Fish Creek Mountains Wilderness Study Area. 

Limonite occurrences in exposed bedrock are 

considered anomalous and may indicate the presence 

of hydrothermal alteration. Two large limonite 

anomalies were identified in the study area and one 

large area was identified just to the west of the study 

area. A linear anomaly that extends from the north 

boundary of the study area southeast to the head of 

Red Rock Canyon (just northwest of BM2371) 

corresponds to the northwestern part of the large 

metasedimentary screen that contains the Waters 

prospect (fig. 2, No. 2). A second anomaly with a total 

area of approximately 4 square miles occurs in granitic 

rocks containing abundant pegmatite dikes near the 

east end of the Fish Creek Mountains. A curvilinear 

alignment of limonite anomalies just west of the 

western boundary of the study area appears to be 

associated with exposures of Tertiary volcanic rocks.

Conclusions

Geologic, geochemical, and geophysical studies 

indicate that the Fish Creek Mountains Wilderness 

Study Area contains three main types of mineral 

occurrences, one associated with Tertiary sedimentary 

deposits and the other two with pre-Cenozoic plutonic 

and metasedimentary rocks.

Miocene strata in the region locally contain 

bedded gypsum deposits of high purity, such as those 

mined by the U.S. Gypsum Company at the U.S. 

Gypsum quarry 1.5 mi west of the study area. Known 

but unmined gypsum deposits and additional gypsum 

are present under alluvium in the northwest part of the 

study area. The northwestern part of the study area is 

assigned high resource potential, certainty level C, for 

gypsum (fig. 2). See appendix for definition of levels 

of mineral resource potential and certainty of 

assessment.

Metasedimentary strata of Paleozoic(?) age that 

are present as screens within the Fish Creek Mountains 

pluton contain relatively abundant carbonate (now 

marble). There is high resource potential, certainty 

level C, for limestone in the large northwest-trending 

metasedimentary screen in the central part of the 

study area that includes the Waters prospect (fig. 2). 

Limestone in smaller screens in the study area does 

not have resource potential.

Seven prospects in the east-central part of the 

study area are located in skarn and (or) clay alteration 

zones along a northwest-trending brittle shear zone. 

Rock samples from these prospects contained minor 

amounts of zinc, copper, manganese, and barium. The 

northwest-trending shear zone consists of a number of 

subparallel faults that follow northwest-trending 

Cretaceous structures. The zone is localized along the 

margins of the large central screen and can be traced 

to the northwest corner of the range where numerous 

smaller, carbonate-bearing screens are present (fig.

CIO
2). Local metasomatism and (or) hydrothermal 

alteration with minor mineralization of the 

metasedimentary rocks in this shear zone probably are 

responsible for the anomalous concentrations of lead 

(in cerussite and galena), tungsten (in scheelite), and 

molybdenum in stream sediments in the north half of 

the study area. Lead and molybdenum were also 

detected locally in metallic inclusions in dolomite 

grains. The part of the study area that is underlain by 

this shear zone has a low resource potential, certainty 

level B, for lead, tungsten, and molybdenum in skarns 

of metasomatic and (or) hydrothermal origin (fig. 2). 

Included in this area of low resource potential is a 

zone of north-trending faults and small carbonatebearing bodies in the eastern part of the study area.

Anomalous concentrations of strontium (in 

celestite) and barium (in barite) in stream sediments in 

the north-central and northwest parts of the study 

area probably are associated with the gypsiferous 

Miocene strata, as indicated by bedded celestite 

containing minor gypsum in the Roberts Celestite 

mine, 3 mi northwest of the study area. Barite and 

barium anomalies are ubiquitous in the Fish Creek 

Mountains, suggesting that the mineral may also have 

formed small replacement bodies in crystalline rocks 

from hydrothermal solutions moving through fault 

zones. However, no deposits of celestite or barite are 

present within the study area, and no resource 

potential is assigned for these minerals.

Oil and gas leases are held in the eastern onethird of the study area, but no potential for 

hydrocarbon resources exists because this area is 

underlain by crystalline rocks. Relatively abundant 

sand and gravel deposits extend into the study area 

from Borrego Valley and Carrizo Wash (fig. 2), but 

these deposits do not have resource potential.

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p. 71-85.

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.

Rowan, L.C., Wetlaufer, P.H., Goetz, A.F.H., 

Billingsley, F.C., and Stewart, J.H., 1974,

Discrimination of rock types and detection of 

hydro therm ally altered areas in south-central 

Nevada by the use of computer-enhanced ERTS 

images: U.S. Geological Survey Professional 

Paper 883, 35 p.

Sampson, R.J., and Tucker, W.B., 1942, Mineral 

resources of Imperial County: California Journal 

of Mines and Geology, v. 38, p. 105-145.

Silver, L.T., Taylor, H.P., Jr., and Chappell, B., 1979, 

Some petrological, geochemical, and 

geochronological observations of the Peninsular 

Ranges batholith near the International Border of 

the U.S.A. and Mexico, in Abbott, P.L., and Todd, 

V.R., eds., Mesozoic crystalline rocks: Guidebook 

for Geological Society of America Annual 

Meeting, San Diego, Calif., p. 83-110.

Todd, V.R., and Shaw, S.E., 1979, Structural, 

metamorphic and intrusive framework of the 

Peninsular Ranges batholith in southern San Diego 

County, California, in Abbott, P.L., and Todd, 

V.R., eds., Mesozoic crystalline rocks: Guidebook 

for Geological Society of America Annual 

Meeting, San Diego, Calif., p. 177-231.

Tucker, W.B., 1926, Imperial County: California State 

Mining Bureau, Report 22 of the State 

Mineralogist, p. 248-285.

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

PVer Planck, W.E., 1952, Gypsum in California: 

California Division of Mines Bulletin 163, 151 p.

___1957, Gypsum, in Wright, L.A., ed., Mineral 

commodities of California: California Division of 

Mines Bulletin 176, p. 231-240.

Woodard, G.D., 1963, The Cenozoic succession of the 

west Colorado Desert, San Diego and Imperial 

Counties, southern California: Berkeley, Calif., 

University of California, Berkeley, unpublished 

Ph.D. dissertation, 173 p.

__1974, Redefinition of Cenozoic stratigraphic 

column in Split Mountain Gorge, Imperial Valley, 

California: American Association of Petroleum 

Geologists Bulletin, v. 58, no. 3, p. 521-526.

C12
APPENDIX. 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 unlikely. 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 a 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 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.

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

A. 

B. 

C. 

D.

B C 

LEVEL OF CERTAINTY

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

Available information suggests the level of mineral resource potential. 

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

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.

40-42. 

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.

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

u

- 24

oo

oo 

_ cc

- 96

loo 

_ one

- -240

 OQfl

oon

_ QCf|

- 410

- 41R *too 

- 500

U/\J

yuu 

icnn

u. ocnn

L 3000

- 3400

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

'Informal time term without specific rank.

C14 GPO 785-048/77388