STATUS OF MINERAL RESOURCE INFORMATION

FOR THE FORT YUMA AND COCOPAH INDIAN RESERVATIONS,

ARIZONA AND CALIFORNIA

By

Thomas M. Sweeney Robin Bradley

U. S. Bureau of Mines U. S. Geological Survey

Administrative Report BIA-85

1981
CONTENTS

SUMMARY AND CONCLUSIONS ................................................. 1

INTRODUCTION ................................................................ 1

Acknowledgments .......................................................... 1

Geographic Setting .......................................................... 2

Fort Yuma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Cocopah . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Map Coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

GEOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Previous Investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Introduction to Rock Units .................................................... 5

Pre-Phanerozoic Metamorphic Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Tumco Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Vitrifax Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Mafic Igneous Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Granitic Gneisses ..................................................... 7

Mesozoic Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Granitic and Related Rocks ............................................. 8

Metavolcanic and Metasedimentary Rocks of the McCoy Mountains Formation(?) . 8

Tertiary Nonmarine Sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Breccia, Conglomerate, and Redbeds ..................................... 8

Extrusive Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Older Andesite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Pyroclastic Rocks of Acidic to Intermediate Composition . . . . . . . . . . . . . . 10

Basaltic Andesite and Basalt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Silicic to Intermediate Intrusive Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Conglomerate of the Kinter Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Conglomerate of Bear Canyon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Tertiary Marine Sedimentary Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Older Marine Sedimentary Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Bouse Formation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Transition Zone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Tertiary and Quaternary Older Alluvium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Conglomerate of Chocolate Mountains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Deposits of Local Origin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Stream-Terrace and Piedmont Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Deposits of the Old Colorado and Gila Rivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Quaternary Younger Alluvium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Younger Deposits of the Colorado and Gila Rivers . . . . . . . . . . . . . . . . . . . . . . . . . 14

Wash and Sheetwash Deposits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Windblown Sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

STRUCTURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

ECONOMIC GEOLOGY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Fort Yuma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Cocopah . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

MINERAL RESOURCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Metallic Mineral Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Fort Yuma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Cargo Muchacho District . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Southeastern Chocolate Mountains District . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Cocopah . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Nonmetallic Mineral Resources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

ENERGY RESOURCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Uranium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Geothermal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Petroleum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

MINERAL LEASING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

RECOMMENDATIONS FOR FURTHER WORK . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 1

SUMMARY AND CONCLUSIONS

Metallic mineral resource potential for the Fort

Yuma Indian Reservation is promising. Lode and

placer gold are known to exist at two locations

within the reservation, and lode occurrences just

outside its boundary offer hope for future discovery of additional placer deposits. Copper occurrences indicated in the Cargo Muchacho Mountains may extend into the reservation.

The potential for a metallic mineral resource

within the Cocopah Indian Reservation is unlikely;

the only possibility seems to be in bars of the

Colorado River where placer gold may exist. No

metallic mines or prospects are in this river bottom

area and chance of metallic occurrence is extremely low. Some geothermal potential may exist

in this area as indicated by a hot water well four

miles to the east.

Both reservations have abundant alluvial

material which may be used for asphaltic and

concrete aggregate and road fill. The Fort Yuma

Indian Reservation has abundant gravels derived

from nearby mountains. Several deposits have

already been worked, and some are still in operation. Because of the mobile nature of uranium and

the permeability of local sediments, there is some

potential for discovery of uranium deposits. Anomalously high radiation readings in the east and

west highland areas indicate possible sources for

secondary deposits within the reservation.

Geothermal resources have not been recorded

in the reservation; however, known geothermal

areas west (Salton Sea), and east (Radium Hot

Springs) may indicate some local potential.

Further fieldwork on the Fort Yuma Reservation should include examination and sampling of

all nearby lode gold and copper deposits for possible extensions into the reservation. Well, spring,

and stream water should be sampled for uranium or

other metallic minerals. Then, if warranted, appropriate geophysical studies should be done to trace

extensions of mineralized structures into the

reservation.

INTRODUCTION

This report was prepared for the U.S. Bureau of

Indian Affairs (BIA) by the U.S. Bureau of Mines

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

The study was conducted to compile and summarize available information on geology, minerals,

energy resources, and the economic development

potential of certain Indian lands.

Information is from published and unpublished

sources as well as from personal communications.

No fieldwork was done.

Acknowledgments

Mr. James Crowther, Area Mining Engineer,

Phoenix Area Office, BIA, Phoenix, Arizona,

provided valuable maps, data, and general information.

Susan Marcus, Geologist, Bureau of Land

Management (BLM), California State Office,

Sacramento, California, supplied numerous related

reports; her cooperation is greatly appreciated.
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 2

Geographic Setting

The Fort Yuma and Cocopah Indian Reservations are located in the southern portion of the

Basin and Range physiographic province. This arid

region is characterized by isolated mountain ranges

rising abruptly from broad desert valleys. The Pilot

Knob Mesa, the Cargo Muchacho and Chocolate

Mountains are block-faulted ranges, elongated in

north or northwest directions. Although elevations

in these ranges do not exceed 649 m (2,129 ft--

Stud Mountain), topography of the mountain

masses is rugged and steeply sloped. Landforms in

the area include dissected old river deposits,

piedmont slopes, sand dunes, and river flood

plains.

The climatic conditions of this desert region

can vary from intense summer heat (118oF) to

severe cold (14oF) in winter months, with precipitation occurring as localized torrential thunderstorms or as gentle rain. Average annual precipitation is very low: long-term normal rainfall of the

Yuma area is 6.4 cm (2.5 in.). Practically all of the

rainfall evaporates; sheet flooding and subsurface

storage occur only during the wettest periods. The

natural vegetation is restricted to the lower

Sonoran life zone; sparse creosote bush, cacti, and

sage brush cover the higher terrain and non-irrigated lands.

The major drainage and only perennial flowing

water in the region is the Colorado River. The Gila

River and several ephemeral washes augment the

Colorado River with surface water after periodic

rain storms. The Colorado River provides water for

human needs as well as irrigation water for the

agricultural lands on the higher mesas and adjacent

to the Colorado and Gila Rivers. Plentiful irrigation water and the long growing season enable

several crops to be harvested each year in the

Yuma area.

Development of water management programs

has been mainly by the U. S. Bureau of Reclamation which authorized the construction of the

Yuma Project in 1904, and the Gila Project, reauthorized in 1947. The Yuma project occupies the

bottom lands between Yuma and the Mexican

border. Since 1941, water has been diverted at the

Imperial Canal and carried by the All American

Canal across the apex of the Colorado River delta

westward to Imperial County, Calif., and southward by canal to the siphon crossing the river to

southern Yuma County, Arizona. 

	

The Fort Yuma Indian Reservation includes

41,980 acres in California, T. 15 and 16 S., R. 21,

22, 23, and 24 E. SBBM, and 2,061 acres in Arizona, T. 8 S., R. 22 and 23 W. G & SRBM (Figure

1 and Figure 2). Tribal headquarters are in Yuma,

Arizona, directly across the Colorado River from

the southern reservation boundary.

Of the 1,752 Quechan Indians enrolled in the

tribe, 851 live on the reservation. Average annual

income is $5,000/person with a local labor force of

333.

Four commercial businesses, that employ 99

persons, have the potential to contribute $200,000

per year for the tribe. Other than oil and gas, two

mineral leases totaling 88 acres have been granted

by the tribe (BIA, 1979).
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 3

Yuma, Arizona (population 4,115), the largest

nearby city, is a center for truck, railroad, and

commercial airline travel. Reservation access is

generally good with a system of paved streets and

highways in the south and east and two secondary

roads and jeep trails in the northwest (Figure 2).

Several power lines cross the area, and Interstate

80 passes just inside the southern boundary. 


	


The Cocopah Indian Reservation includes four

separate parcels, the east and west reservation

parcels and two homestead areas, totaling 2,296

acres in T. 9 and 10 S., R. 24 and 25 W., G &

SRBM, near Somerton, Arizona (Figure 3). Tribal

headquarters are in the West Cocopah Reservation,

2.5 miles northwest of Somerton.

The tribe has 543 members of whom 465 live

on the reservation. The reservation has no mineral

industry or leases.(James Crowther, personal

commun., 1980).

Somerton, Arizona, is within three miles of all

parcels of the reservation; however, Yuma, Arizona, is the largest nearby city and the center for

truck, railroad, and commercial airline travel. The

reservation is crossed and surrounded by paved and

gravel roads.

The climate is similar to that of the Fort Yuma

Indian Reservation, 12 miles north. An extensive

irrigation network provides ample water to all four

parcels.

History

From the time of its earliest inhabitants, the

history of the lower Colorado area has been a

remarkable chronicle of man's social and economic

development in a challenging environment. Modern culture has evolved from the integration of the

heritage from the Indian, Spanish exploration and

American westward expansion periods.

The Yuma Indians have occupied the lands

near the Colorado River and engaged in agrarian

activities since ancient times. The principal crops

of corn, squash, beans, and cotton were nurtured by

irrigation waters perhaps as early as 500 A.D. This

stable economy and sedentary lifestyle stimulated

development of social, political, and religious

systems and advancement in arts and crafts.

The earliest contact with European civilization

occurred in 1540. Hernando de Alarcon was sent

as the captain of a Spanish flotilla carrying army

supplies to Vasquez de Coronado who was searching for the seven cities of gold. Alarcon discovered

the Colorado River and proceeded upstream to just

above the junction of the Colorado and Gila Rivers

in an unsuccessful attempt to meet with Coronado.

Other Spaniards passed through the area but no

new knowledge of the region was contributed until

the arrival of the evangelizing Father Eusebio

Francisco Kino, in 1700. Kino was a Jesuit missionary who crossed the lower Colorado to determine that Baja California was not an island, as was

believed in the Spanish settlements of Mexico. In

1701, Father Kino summarized his geographic

studies by drawing the first fairly accurate map of

the mainland and the Baja peninsula.
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 4

When Lieutenant-Colonel Juan Bautista de

Anza passed through Yuma in 1774 during his first

expedition to Monterey and San Francisco, he left

two missionaries to convert the friendly Yuma

Indians. Fray Francisco Graces and Padre Tomas

Eixarch stayed until de Anza's return two years

later. In 1780, the two fathers, accompanied by a

small army of Spanish soldiers and their families

returned to establish the Mission de la Purisima

Conception near the present site of Winterhaven.

The Yuma uprising of 1781 wiped out the establishment and closed de Anza's land route to San

Francisco. It was during the brief tenure of the

Spanish at Winterhaven that placer gold was first

mined near the Potholes, 16 km (10 mi) northeast

of the mission.

Although American trappers and traders frequented the area in the early 1800's, the regional

geography was not known to Americans until the

U.S. Army started to explore the western boundaries of the republic. In 1846, Lieutenant W. H.

Emory made a geographic record as he accompanied General S. W. Kearney and Kit Carson on a

reconnaissance survey from the Rio Grande to San

Diego. The first practical wagon route was established in the next year when Captain Phillip St.

George Cooke led the Mormon Batallion along the

old Spanish trail. With the construction of Fort

Yuma guaranteeing safe passage to California, the

Yuma area became the nerve center of the region

and a main stop on the southern transcontinental

mail routes.

Mining activity blossomed under the protective

arm of the U.S. Army and the arrival of steamboat

traffic on the Colorado in 1852. The discovery of

gold placers on the Gila River caused a brief

bonanza in the region, but declined abruptly due to

inadequate technology and insufficient water for

placer mining. However, the advent of a coast-tocoast railroad within hauling distance of the

Laguna, Picacho and Cargo Muchacho mining

districts revived mineral exploration and prospecting. Renewed activity included old workings in the

Cargo Muchacho Mountains and Potholes vicinity.

Peak production years were 1890-1910 and 1937-

1942. It is estimated that over $2 million was taken

out.

The construction of the irrigation canals and

dams on the Colorado River has provided water for

further development of the mining industries. The

expansion of the water control system and building

in the Yuma area has depended on the mining

industries to provide construction materials of

brick and sand and gravel, thus supporting a steady

growth and continued development of natural

resources.

Map Coverage

The topography of the Fort Yuma and Cocopah

Reservations is shown on Figure 4. Topographic

maps of the reservations may be ordered from the

U.S. Geological Survey, Denver Federal Center,

Denver, Colorado 80225 (Figure 5).

Small scale geologic map coverage is available

on a 1:500,000 map of the geology of Arizona

(Wilson, 1969); a 1:375,000 geologic map of

Yuma County, Arizona (Wilson, 1960); a

1:250,000 geologic map of California, San DiegoEl Centro sheet distributed by California Division

of Mines; and, a 1:125,000 geologic map of Imperial County, California (Morton, 1966).
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 5

Reservation boundary maps are available from

the U.S. Department of Interior, Bureau of Indian

Affairs (BIA), Phoenix Area Office, Phoenix,

Arizona.

GEOLOGY

Previous Investigations

This report summarizes the available published

and unpublished information concerning the

geology, development, and production of mineral

resources on the reservations. Past geologic mapping has been mainly speculative, consisting of

reconnaissance studies, either as part of groundwater assessment or in conjunction with California

and Arizona state mapping programs. A few

university theses have further described the vicinity

of the reservations, but the geologic history of the

lower Colorado River area has remained largely

unknown.

The most comprehensive geologic studies are

those published after 1970 and include: Olmsted

and others (1973) who described the geohydrology

of the Yuma area; a generalized geologic map and

description of the geology and mineral resources of

Imperial County, Calif., prepared for the California

Division of Mines and Geology by Morton (1977);

the geology of the basement rocks as described by

Haxel and Dillon (1973) and Haxel, (1974, 1977).

Structural relationships and tectonic activity along

the San Andreas fault system in southeastern

California have been discussed by Crowell (1973,

1975), Haxel and Dillon (1973), and Crowe (1973,

1978). Other reports concerning the geology and

mineral resources of the reservations lands are

listed in the bibliography.

Introduction to Rock Units

Rock units exposed on the Yuma and Cocopah

Indian Reservations vary in composition from hard

crystalline rocks in the mountains to extensive

unconsolidated alluvium covering the plains. The

rocks range in age from Precambrian (greater than

1.2 b.y. (billion years) old) to Holocene (less than

a few thousand years old). The precious-metal

deposits are found in crystalline rocks of preTertiary age especially along fault and schistose

planes in gneiss and metaconglomerate. The gneiss

seems to be the more favorable host as indicated by

the greater number of deposits it holds. The source

of the mineralization is not certain, but seems to be

related to the emplacement of granitic rocks in the

Mesozoic(?) period. Placer deposits are mostly

contained in Quaternary stream gravels, while the

only known occurrence of copper in the area is

confined to metavolcanics. A generalized geologic

map (Figure 6 and Figure 6a) shows the different

rock units described in this report. Stratigraphic

relationships are depicted in Figure 7.

Pre-Phanerozoic Metamorphic Rocks

The rocks of known and probable pre-Phanerozoic age consist of predominantly mafic, ultramafic meta-igneous rocks, metasedimentary laminated gneiss and schist intruded by granitic gneisses. Several lode gold deposits occur in these rocks.

These rocks crop out at Pilot Knob, the Picacho

and Cargo Muchacho Mountains. They have a
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 6

typically gray mottled color when viewed from a

distance. Topographic expression is characterized

by moderately steep rugged terrain.

The pre-Phanerozoic rocks are the oldest

exposed material in the area and are confined to

the upper plate of the Chocolate Mountain thrust.

They have been affected by at least one episode of

amphibolite-facies metamorphism; overprints from

numerous metamorphic events obscure most of the

original structures and textures. Although older

metamorphic features are preserved in a few

exposures, the foliation and lineation imposed by

the last metamorphic event is of regional extent

and parallels the Chocolate Mountain thrust which

lies to the northeast in the Chocolate Mountains.

	

The Tumco Formation crops out in the Cargo

Muchacho Mountains and Picacho area and is

composed of two lithologies: quartzofeldspathic

laminated gneiss of arkosic composition and

hornblende schist. Light-gray, medium-grained

laminated gneiss comprises approximately 95

percent of the Tumco Formation. Thin intervals of

quartzite, marble, amphibolite, and amphibolite

dikes occur in minor amounts. The compositional

laminations and bands are suggestive of sedimentary or volcanic origin. Supporting evidence for a

sedimentary origin is provided by fist-sized ovoid

bodies of calc-silicate minerals in the northern

Cargo Muchacho Mountains which are interpreted

as relict concretions. Amphibolites of metasedimentary or volcanic origin occur as thin beds

with banding and gradational contacts parallel to

the laminations in the surrounding quartzofeldspathic gneiss. Amphibolite also occur as

sharply bounded, locally discordant layers interpreted as metamorphic dikes and sills. The amphibolite dikes and sills cutting laminated gneiss in

the southeastern Cargo Muchacho Mountains and

at Pilot Knob are composed almost entirely of

hornblende and plagioclase. The dikes may have

originated from the mafic complex, as they are in

close proximity to more extensive outcrops of

amphibolite in the mafic complex.

		

The Vitrifax Formation is an unusual sheetlike

body of kyanitequartz rock found near the west end

of the American Girl canyon in the Cargo Muchacho Mountains. Some of the richest gold deposits

of the area are located in or near the Vitrifax

Formation. These rocks may have been produced

by leaching of the Tumco Formation by supracrustal fluids originating from the later Mesozoic

biotite granite intrusion. To the west and northwest, the Vitrifax Formation grades into laminated

gneiss of the Tumco Formation; to the south it is

partly bounded by the mafic complex. The Cenozoic American Girl and Padre-Madre-Araz faults

offset the cyanite-quartz rocks. The American Girl

fault may be a slightly offset contact between the

biotite granite and the Vitrifax Formation, thus

explaining the apparent localization of ore deposits

along the fault. Alternatively, the mineralization

resulting from the Mesozoic intrusion may have

been redistributed along the fault zone during

Tertiary activity.
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 7

	


The mafic igneous complex is composed of

mainly dark colored metamorphosed hornblende

quartz diorite to diorite with lesser amounts of

metagabbro, amphibolite, and ultramafic rocks.

Most of the meta-igneous complex is thought to be

pre-Phanerozoic, but portions may be younger.

Almost all of the complex is polymetamorphic,

and all but the ultramafic rocks have lineations

defined by hornblende and biotite. Internal contacts

between the dark lithologies are difficult to discern

because of manganese oxide and iron staining.

Gradational and crosscutting contacts occur, and

many contacts are further obscured by metamorphic features. Relic igneous textures are common,

especially in exposures of metagabbro at Pilot

Knob.

The most prevalent lithology is a slightly

schistose, lineated, medium-grained biotite-hornblende quartz diorite. The second most common

lithology is amphibolite which appears to grade

locally into metadiorite and metagabbro. Minor

amounts of metaclinopyroxenite and metaperidotite with relic cumulate textures are exposed

in a few outcrops at Pilot Knob.

	

Hornblende-biotite quartz monzonite gneiss

and augen gneiss intrude the Tumco Formation and

laminated gneiss similar to the Tumco Formation

at Pilot Knob, the Picacho area, and in the northern

Cargo Muchacho Mountains. The 1.7-b.y.-old

augen gneiss of Pilot Knob is texturally and chronologically different from other known gneisses of

southwestern California and Arizona; the augen

gneiss of the Cargo Muchacho Mountains is

presumed to be pre-Phanerozoic or Mesozoic

because in one exposure a metamorphic fabric is

cut by Jurassic biotite granite. The augen gneiss is

banded with banding, foliation, and lineation

defined by hornblende and biotite. The gneiss is

gradational into flaser gneiss near contact zones.

Contact phases also include mylonitic augen

gneiss, very coarse pegmatite, and hornblendebiotite granite.

The hornblende-biotite quartz monzonite

gneiss is generally coarse grained and includes

concentric zones rich in biotite and relict

potassium-feldspar phenocrysts. These may have

been inclusion-rich zones formed during magmatic

crystallization.

Mesozoic Rocks

The Mesozoic rocks include metavolcanic and

metasedimentary rocks of the McCoy Mountain

Formation and hornblende-biotite quartz

monzonite, biotite granite with associated aplitic

and pegmatite dikes, sill, and irregular masses and

minor quartz diorite dikes. The quartz monzonite

and biotite granite are Jurassic age (173 m.y. and

140-170 m.y. respectively); the dike rocks are

probably younger. The granitic rocks intrude the

pre-Phanerozoic gneisses and may represent an

extension of the Cordilleran batholitic belt. One or

more amphibolite-facies metamorphic events have

converted the granitic rocks into orthogneisses;

their metamorphic fabric parallels that of the prePhanerozoic rocks and the Chocolate Mountain

thrust. Deposits of gold, silver, copper, tungsten,
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 8

and uranium are genetically related to the emplacement of the Mesozoic intrusive rocks.

			

Hornblende-biotite quartz monzonite occurs in

the southern Cargo Muchacho Mountains and

consists of three facies. The most common is a

slightly blastoporphyritic facies which is found in

a complex contact zone with the Tumco Formation. It is in turn intruded by coarsely blastoporphyritic facies. Migmatites along the contact

zone with the pre-Phanerozoic mafic complex, are

the third facies. The quartz monzonites are

polymetamorphic, with relict igneous and

mylonitic textures.

Remobilization and recrystallization make

many contacts difficult to interpret. Foliations in

the quartz monzonite are intruded by biotite granite. Aplite and pegmatite rocks cut foliations in all

the older rocks except for the biotite granite.

Although some of the dike material may originate

from the biotite granite, field relations indicate that

several ages of intrusions are probably represented

and much of the aplite and pegmatite is younger

than the granite. The dike rocks are also orthogneisses but do not have apparent polymetamorphic textures since they lack minerals which

show characteristic metamorphic textures.

Chloritized dikes which intrude the granitic dikes

have locally disseminated iron sulfides ore.

		 	 		   



		 !"

Metavolcanic and metasedimentary rocks

tentatively correlated with the McCoy Mountain

Formation crop out in a small area west of Imperial

Dam. The pyrite-bearing rocks of this unit are

composed of metasandstone, metaconglomerate,

metavolcanics, mainly meta-andesite, and minor

limestone. The rocks are characteristically dark

gray or gray green and tend to have subdued

topography. The unit is probably late Paleozoic or

Triassic age. The Trio and Senator gold mines are

located in these rocks, although the genesis of

these deposits does not seem to be related to the

occurrence of the formation.

Tertiary Nonmarine Sediments

A great variety and thickness of Tertiary nonmarine sediments, pyroclastic and volcanic rocks

are exposed in the Yuma area; these rocks represent accumulation since Miocene time in a broad

basin that generally corresponds to the present

configuration of the area. The volcanic rocks may

be of economic significance as host rocks for

manganese deposits and source of volcanic rock

products.

#	$
	$	%

Breccia, conglomerates, and redbeds unconformably overlie gneissic rocks in the southern

Picacho area. The sequence of sedimentary and

associated volcanic rocks appear to have been

deposited prior to the time when the Colorado

River entered the Yuma area. The nonmarine

sediments are strongly to weakly indurated clastic

rocks and range from fine-grained mudstone and

shale to megabreccia and boulder conglomerate of
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 9

fanglomeratic and lacustrine origin. The conglomerate sequence lies unconformably upon the crystalline rocks and the metavolcanic and metasedimentary McCoy Mountain Formation and

underlies the older andesite volcanic sequence.

The basal part is composed of arkose, conglomerate, mudstone, fine-grained tuffaceous beds

and bentonitic ash which are a distinctive red

color. In the Picacho area, a breccia-conglomerate

sequence is the dominant rock type. It is composed

of heterogeneous material, and typically includes

angular to subangular fragments of all sizes as

much as many feet in diameter, composed of finegrained varicolored metasedimentary and metavolcanic rocks. The matrix is an earthy greenish-gray

color and consists of clayey sand and silt. The

sequence appears to have formed as mudflows,

talus, and colluvium as well as fanglomerates.

Bedding is obscure and truncated by poorly exposed faults which make thickness difficult to

estimate. Maximum exposed thickness probably

exceeds 152.4 m (5,000 ft). Both the redbeds and

the breccia conglomerate have been moderately

deformed; bedding generally dips 30 to 60 degrees

in west to southwesterly direction. The conglomerate rocks are probably pre-Miocene in age and may

be partially contemporaneous with the Kinter

Formation mapped by Olmsted in the nearby

Laguna Mountains. Unnamed coarse breccia and

conglomerate of probable Tertiary age also occur

in the east-central Cargo Muchacho Mountains.

&

Intermittent volcanism occurred concurrently

with deposition of nonmarine sediments during the

Tertiary. A thick section of the volcanic rocks

crops out in the Picacho area and includes several

compositional types: (1) Older andesite, (2) pyroclastic rocks of silicic to intermediate composition,

and (3) basaltic andesite or basalt. K-Ar dates of

the volcanic sequence range from 24.7± m.y. to

26.3± 1 m.y., late Oligocene to early Miocene age.

This relatively small spread of the apparent ages

suggests the volcanic rocks were extruded within

a short period of time. 

'(

The oldest volcanic rocks are andesitic flows,

pumiceous tuff, shallow intrusive bodies, agglomerate, flow breccia, and a basal breccia conglomerate (previously described). The flows lack pronounced primary structures such as flow banding;

they are cut by closely spaced irregular fractures.

Coloring ranges from dull gray to dull red. The

flows are predominantly very fine grained and

partly glassy; small phenocrysts of plagioclase,

pyroxene, or hornblende are present in small

quantities. The tuff is colored light gray, and

contains small amounts of biotite, hornblende, and

pyroxene. Inclusions of fine-grained andesitic and

glass fragments are scattered throughout the tuff.

The total thickness of these older rocks probably

does not exceed 305 m (1,000 ft). They are exposed in the western part of the Picacho area. An

apparent relationship exists between the volcanic

rocks and occurrence of manganese in surrounding

areas; the general distribution of manganese deposits coincides with that of the volcanic rocks
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 10

)	(	
*



Light-colored, friable, pumiceous ash-fall tuff

and red densely welded ash-flow tuff (ignimbrite)

overlie the older volcanic rocks, in some places

unconformably. The tuffs, both welded and

unwelded, contain mostly glass shards with occasional small pieces of fine-grained to glassy volcanic fragments. Several beds of water-lain tuff and

tuff breccia are also present. The pyroclastic

sequence attains a thickness of about 457 m (1,500

ft), although displacement along poorly exposed

faults renders estimates of unit thickness difficult.

Although the rocks have no present economic

value, the possibility exists that these rocks may

provide a source for construction materials.

#		(	#		

Resistant dark basaltic andesite and basalt form

prominent ridges and have a maximum exposed

thickness of approximately 305 m (1,000 ft). The

intermediate to mafic flows overlie breccia conglomerate with angular discordance, and also rests

unconformably over older volcanic rocks. In some

places they are overlain by silicic to intermediate

tuffs. These rocks are also truncated by poorly

exposed faults. The basalt rock is composed of

several distinct flows, colored medium gray to

darkish brown gray, except for weathered surfaces

which are coated with dark brown desert varnish.

The flows are more or less vesicular, and some are

brecciated, probably from flowage after the lava

had begun to solidify.

+	

Dacite or latite porphyry plugs and rhyodacitic

dikes intrude granitic and metamorphic rocks in

widely scattered occurrences. The intrusive rocks

are overlain by late Tertiary nonmarine sediments.

The mid-Tertiary intrusive rocks are probably

comagmatic with extrusive volcanic rocks since

both the intrusive and extrusive phases share

distributional, chronological, chemical, and petrologic similarities. The intrusive rocks are often a

patchy red color and are relatively resistant to

erosion. The dikes are generally small but abundant especially along arcuate faults in the Laguna

Mountains. Some ore deposits of copper, leadsilver, silver manganese, and uranium are localized

along dikes and faults in the surrounding areas.


	
,	

These rocks consist of predominantly coarse

grained nonmarine sedimentary rocks and minor

intercalated beds of tuff and ash. The unit is exposed in the Laguna Mountains to the southeast of

the Laguna Dam. The Kinter Formation underlies

the fine-grained deposits of the Bouse Formation.

The age of the Kinter Formation is well established

as Miocene on the basis of radiometric dating and

stratigraphic evidence. The Kinter Formation is

divided into two unnamed members: the lower is

composed of coarse unsorted breccia and tongues

of brown and gray arkosic sandstone and gray to

pink tuffaceous mudstone; and the upper is composed of yellowish-gray fanglomerate and soft

arkosic sandstone and mudstone.
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 11

The underlying volcanic rocks and the Kinter

Formation have generally been deformed by

normal faulting and tilting or folding with bedding

dips of less than 35o.


	#	
	

Middle to upper Miocene fanglomerate of Bear

Canyon consists of moderately indurated interbedded sandstone, heterolithic conglomerate, and

breccia. The unit has been folded and upended

locally, but in most places it is tilted between 5o

and 20o. It is exposed in the western Picacho area

in a few outcrops in the Cargo Muchacho Mountains. The fanglomerate unconformably overlies

mid-Tertiary volcanic rocks and older basement

rocks and is locally unconformably overlain by the

Bouse Formation. The Bear Canyon fanglomerate

may be laterally equivalent to older marine sedimentary rocks that occur in the subsurface south of

the Cargo Muchacho Mountains.

Tertiary Marine Sedimentary Rocks

Marine sedimentary rocks have been penetrated

by several oil test wells and water-test wells in the

Yuma area. These mostly fine-grained rocks

represent marine and possibly brackish-water

environments and are limited to the subsurface.

The marine deposits are divided into two units

which are separated by an angular unconformity:

(1) a more indurated older sequence, and (2) a

younger, generally finer grained sequence assigned

to the Bouse Formation by Olmsted (1973). A dip

log from an oil-test well shows the older marine

rocks to be conformable over underlying nonmarine deposits. The older marine sedimentary

rocks are more deformed than the Bouse Formation sequence, although both units are broadly

warped and faulted.

'	+	

These rocks consist of indurated light-gray

fine-grained sandstone with approximately subequal amounts of interbedded medium- to darkgray siltstone and claystone. Beds of ash or tuff are

also reported to be within the older marine sequence. Maximum thickness of these rocks may

exceed 1,000 ft. Fossil assemblages include

foraminifers and mollusks indicative of a marine

environment. The fossils are not age diagnostic,

but the stratigraphic position of these beds suggests

that they interfinger with nonmarine deposits of the

Kinter Formation of Miocene age.

#	

The younger sequence of marine sedimentary

rocks in the Yuma area is assigned to the Bouse

Formation by Olmsted (1973) on the basis of

similar lithology and stratigraphic position and an

identical foraminiferal fauna. These marine deposits consist of silt and clay with minor very fine to

fine sand, hard calcareous sandstone or sandy

limestone, tuff, and possible conglomerate of local

derivation. The clay and silt are pale greenish gray

to bluish gray, some strata are pink and brown. The

sand is well sorted and a light gray color. The basal

calcareous sandstone or sandy limestone is very

pale gray to grayish yellow with locally abundant

marine fauna including corals and mollusks.
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 12

Several species of foraminifera and organic remains, both plant and animal, are numerous in

some zones. These flora and fauna are said to be

representative of brackish to marine environment

but are not diagnostic as to age.

Only a few cuttings of a probable tuff bed

about 2.7 m (9 ft) thick were recovered during

drilling; this unit overlies the limestone/sandstone.

A conglomerate and arkosic sandstone composed

of granite and metamorphic clasts containing a 0.9

m (3 ft) bed of claystone and a 1.8 m (6 ft) stratum

of sandy limestone, underlies part of the upper

clay, silt, and fine-sand beds. The conglomerate

and sandstone beds may represent beach gravels or

offshore bars.

The Bouse Formation is wedge shaped and

attains a maximum thickness of 289 m (950 ft).

These younger marine sediments are limited to the

subsurface in the Yuma area but are most likely

more extensive than the older marine sequence.

They appear to be less deformed than the older

marine and nonmarine deposits, and were probably

deposited after the mountain masses took their

present configuration, but before they assumed

their present altitude.

Little additional evidence for the age of the

Bouse Formation is available in the Yuma area. A

lower limit of Miocene is well established since

the Bouse overlies the Kinter Formation with some

local angular discordance. Metzger (1968) has

concluded that the Bouse Formation in the ParkerBlythe-Cebola area, to the north, belongs within

the Pliocene; and, as in the Parker-Blythe-Cebola

area, no Colorado River alluvium occurs below or

within the younger marine sediments, so the Bouse

Formation antedates the entrance of the Colorado

River into the Yuma area.

	-

A transition zone of intertonguing marine and

nonmarine strata tops the Bouse Formation and

represents alternating marine and subareal deposition. This interval is of Pliocene age and reaches a

thickness of several hundred feet in the southwestern Yuma area. The top of the transition zone is

defined by the uppermost bed of fossiliferous gray

clay or silt, the base by the lowest most alluvial

bed of sand or gravel.

Tertiary and Quaternary Older Alluvium

The older alluvium consists of a great range of

material varying from clay to cobble-boulder

gravel, and originates from local streams as well as

the old Colorado and Gila Rivers. Dissected stream

terrace and piedmont deposits cover areas near the

mountain masses. These subareal sedimentary

rocks represent several cycles of alluvial filling

separated by degradation related to fluctuations in

sea level. Different alluvial fill cycles of the older

alluvium are commonly defined by unconformities

and are the most widely exposed deposits in the

Yuma area. Changes in sea level were associated

with glacial and interglacial stages as well as

regional and local warping of land surfaces. Unit

thicknesses range from 0 up to a maximum of 762

m (2,500 ft) in the southwest. This great thickness

of sediments indicates subsidence during deposition since the alluvium was deposited on the

surface that is now several hundred meters below
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 13

sea level. The age of the older alluvium ranges

from Pliocene to late Pleistocene and is linked to

the establishment of the Colorado River and later

Gila River as through flowing streams. McKee

(1967) has determined that the Grand Canyon was

cut to nearly its present depth by mid-Pleistocene,

which implies canyon-cutting and sediment transport by the Colorado River must have started well

before the Pleistocene. The older alluvium stratigraphically separates the transition zone-Bouse

Formation from the younger alluvium (most recent

sedimentary cycle).


	

		

The conglomerate of the Chocolate Mountains

unconformably overlies the late Miocene fanglomerate of Bear Canyon. The conglomerate is

probably middle to late Pliocene age and is nearly

continuous with the Bouse Formation. The conglomerate is the lowermost section of the older

alluvial cover, and is overlain by loosely consolidated deposits of late Pliocene to early Pleistocene

alluvium.

The conglomerate of the Chocolate Mountains

occurs as a deeply dissected alluvial apron and

contains clasts of local origin. The deposits range

from conglomerate to sandstone, siltstone, and

breccia; they are slightly to moderately consolidated. The conglomeratic beds have been tilted,

faulted, and eroded prior to deposition of overlying

alluvium.

./	'

The deposits of local origin were probably

deposited as alluvial fans. These poorly sorted

thick masses of debris form low hills and dissected

highlands along the margins of the area. They are

typically poorly bedded with scour and fill structures. Individual strata are poorly defined and

lenticular. The deposits consist of angular to

subangular gravel in a silty or clayey sand matrix.

Composition of the gravel reflects local rock types.

+	*		).

The stream-terrace and piedmont deposits

consist of ill-sorted gravel with much sand, silt,

and clay. They often overlie older fill in the older

alluvium from which they are difficult to distinguish. The piedmont deposits were formed by

cycles of sheetfloods and shifting ephemeral

streams; the stream terraces accumulated at different levels along the larger washes. Most exposures

of both types of deposits are covered with a mosaic

of stones termed "desert pavement." The stones are

often coated with "desert varnish" which consists

of oxides of manganese and iron.

.
'
			

By far the most abundant alluvial fill comes

from the ancestral Colorado and Gila Rivers. These

deposits differ significantly from deposits of local

origin. Sediments are better sorted, more rounded,

and less heterogeneous on a local scale. These

deposits are horizontally continuous beds which

can be traced for several miles.

The greatest bulk of the old river deposits

consists of sand, which is frequently crossbedded.

The sands are mostly feldspathic but contain an
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 14

amphibole-augite-garnet heavy-metal assemblage.

Gravel is the next most abundant material and is

also frequently crossbedded. Cementation, generally by calcium carbonate, is characteristic of the

old river gravels.

Clay and silt strata are locally conspicuous but

minor constituents compared to the sand and

gravel of the older river deposits. Abundant fossil

twigs, roots, and root fillings in the clay and silt

layers suggest a slack-water depositional environment, perhaps on broad flood plains. Some of the

clay and silt layers can be traced for several miles.

Quaternary Younger Alluvium

The younger alluvium represents sediments

from the most recent major cycle of deposition.

The deposits are divided by agents of deposition

into three categories: (1) deposits of the Colorado

and Gila Rivers, (2) wash and sheet wash deposits,

and (3) windblown sand. These recent sedimentary

units underlie the present river flood plains and the

washes. The soils of the younger alluvium generally lack mature profile development. They reflect

the dominant process of aggradation during the last

several thousand years, although some local degradation has occurred in the recent past.

.

			

The younger river deposits are mainly composed of sand and silt which have been laid down

on the pebbly sand and silt of present flood plains.

They are abundant in places. One extensive bed of

silt and clay has probably resulted from repeated

overbank flow during flooding. The younger river

deposits are similar to the older deposits except

they are generally less cemented. A basal gravel

unit identified by Metzger and others (1973) in the

Parker-Blythe-Cebola area has been dated as less

than 10,000 years. This unit does not occur as a

pervasive subunit in the Yuma area, although it

may be present in the western part of the area. The

gravel was probably the first clastic material to be

dumped by the Colorado River as sea level rose

after the retreat of the Wisconsin glacial stage.

0	
	+
1	
.

Deposits of sand and gravel with lenticular

beds of silt occur in washes and channels. These

deposits are generally thin and cut into older

alluvium and prealluvial rocks. The sheetwash

deposits resemble alluvial fans but generally

blanket older rocks in broad, poorly defined sheets

rather than thick wedges like true alluvial fans.

0%1+	

The windblown deposits are composed of wellsorted, fine to medium sand, probably derived

from old lacustrine or marine beaches. The sand

grains are subrounded to rounded and composed of

quartz with minor feldspar, rock fragments, and

heavy minerals. Many of the grains are frosted.

The sand deposits are extensive and form large

sinuous and arcuate dunes as well as thin sheets of

windblown material. One small area of low dunes

and sand sheets lies 5 to 6.5 km (3 to 4 mi) north

of Yuma and is adjacent to a Colorado River

meander channel which was cut off in historic

times.
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 15

STRUCTURE

The major structural features in the area can be

divided by the time of their development. Epochs

of deformation are limited to: 1) pre-Tertiary

crystalline rocks; 2) uplift of mountains and basins

to approximately their present outline by Tertiary

time; 3) subsidence of the Salton Trough during

Cenozoic time; 4) warping and tectonic activity

along the San Andreas and related fault systems

continuous throughout Tertiary and Quaternary

time.

Old fault and joint systems cut the pre-Tertiary

crystalline rocks; many of the faults are mineralized and filled with dikes and quartz veins. The

last major deformation to affect only the crystalline

rocks was the Laramide orogeny, which extended

from late Cretaceous into early Tertiary time.

Igneous activity, extensive folding, reverse and

normal faulting occurred during this mountainbuilding event. Precambrian gneissic rocks were

thrust over Mesozoic-age schists to the north of the

area by the Chocolate Mountain thrust (outside the

area shown on Figure 6); associated metamorphic

foliation and lineation were developed during the

emplacement of the upper plate of the thrust sheet.

The present configuration of the mountain

ranges in the Yuma area may be partially related to

block faulting and domal uplift that originated

during the Laramide orogeny; however, most of the

outlines of the ranges resulted from Tertiary Basinand-Range faulting. The present mountain fronts

are mostly erosional, with the frontal faults buried

by alluvium in the basins between the mountain

masses.

The southern part of the Salton Trough has

been filled with a thick sedimentary wedge. The

nonmarine and volcanic rocks of Tertiary age

unconformably overlie the crystalline rocks, and

are in turn separated by an angular discordance

from the overlying marine Bouse Formation,

transition zone, older and younger alluvium. The

Bouse Formation and younger sedimentary rocks

are substantially less deformed than the nonmarine

and volcanic rocks which have been considerably

affected by broad folding and warping. Nonetheless, most of the late Tertiary and Quaternary have

undergone some local faulting and broad warping

and tilting, probably related to subsidence in the

Salton Trough.

The northwest-trending branches of the San

Andreas fault system extend from the Salton

Trough southeastward to Mexico. Displacements

along the system have been right lateral and may

have amounted to as much as 160 miles since early

Miocene in southern California. Unaffected alluvium and windblown sand conceal traces of the

fault system, but geophysical data suggest the

system continues across the Yuma area. The last

significant movement along one of the major

traces, the Algodones fault, occurred before latest

Pleistocene and is concealed beneath unaffected

alluvial deposits in the Yuma Mesa and Yuma

Valley (Figure 6).

ECONOMIC GEOLOGY

Fort Yuma

Local mineral production has been from lode

and placer gold, copper, and sand and gravel
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 16

deposits. Geologic conditions favorable for these

minerals occur throughout the reservation area and

indicate some potential for future discovery of

presently unknown deposits.

Lode gold deposits just outside the northeast

corner of the reservation are associated with quartz

veins in porphyroblastic metagranites. Free gold

occurs in quartz breccia with secondary galena and

wolframite; the associated country rocks are schists

and metaconglomerates. Structures generally trend

northwest, but some strike northeast. Brecciation

appears to have been caused by renewed movement along a fault after mineralization occurred.

Coarse placer gold, indicating a local source, is

found near the Colorado River and on older stream

terraces. Dry placer mining occurred on what is

now reservation land as early as 1780.

A mineral deposit in the northwest corner of

the reservation yielded gold and copper from

quartz veins which trend N. 80oE. The veins are

composed of small lenticular bodies of quartz in

monzonite country rock. These bodies are reportedly only a few feet wide and a few tens of feet

long.

Sand and gravel occurs throughout the reservation, and many deposits have been developed

(Figure 2). This Holocene river alluvium is valuable as aggregate in building and as road fill. The

two sources of this material are the Colorado River

and the local mountains.

Uranium, though not previously indicated in

this area, could occur at depth in the coarse sediments. The nearby granitic mountains may be a

good source, and organic material within the

alluvial deposits could have concentrated the

uranium.

Cocopah

The reservation is entirely covered by Quaternary alluvium and no mines or prospects have been

located. This land appears to be barren of economic minerals.

MINERAL RESOURCES

Metallic Mineral Resources 

	

Several metallic mineral resources occur on or

near the reservation. Most of them are in the

northeast and northwest corners of the reservation

on the edges of the Chocolate and Cargo Muchacho Mountains, respectively. The rest of the reservation is covered by a thick layer of alluvium that

conceals the basement rocks.


	
	
.

This district is in the Cargo Muchacho Mountains, Imperial County, California, northwest of the

reservation.

The earliest known mining in the district

occurred in a brief period from 1780 to 1781 when

placer fields were worked. Completion of the

Southern Pacific Railroad sparked renewed discovery in 1877; by 1900 the bulk of the $5 million

worth of gold produced from the district had been

mined. Aside from gold, this area had small

amounts of copper, kyanite, sericite, and

wollastonite (Morton, 1977).
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 17

The district is predominantly underlain by preMesozoic crystalline rocks; lesser quartzite-sericite

schist, arkose and granitic intrusive rocks are

present (Morton, 1977). Ore deposits are generally

confined to the southwestern slopes of the mountains and show a common N. 35 W. trend (Morton,

1977).

+
	

		.

This area is 16 miles northeast of Yuma,

Arizona. Low-lying dissected terraces with intermittent highlands comprise most of the area, with

elevations ranging from 200 to 2,000 feet above

sea level (Morton, 1977).

The earliest California mineral production was

from this area; in 1779 local Indians worked the

Potholes placers. From 1850 to the present, the

district has produced $2 million to $3 million

worth of gold. Most mining was by dry placer

methods because of the lack of water on the terraces (Root, 1923). Area lode gold deposits are

pre-Tertiary in age and occur in metaconglomerate

and metagranite (Morton, 1977, Crawford, 1896,

and Tucker, 1942). There appears to be a relationship between gold occurrences and the Mesozoic

granitic intrusive; however, small amounts of gold

are also known to occur in schist and gneiss. A

potential exists for future discovery of subsurface

lode veins and placers in deeper areas in the terraces. 


	


Little mineral potential is evident on the

Cocopah Indian Reservation. If present, metallic

mineral deposits would probably be placer gold in

Colorado River bars to the west. No deposits are

known, and no production is recorded for this

section of the river.

Nonmetallic Mineral Resources

Geologic and topographic maps indicate

abundant alluvial material on both reservations

with concentrations of gravel in northern sections

of the Fort Yuma. The Fort Yuma has numerous

operating sand and gravel pits, the Cocopah has

none.

Gravel for building and concrete appears to be

the most marketable product, although the alluvium could also be a potential source for secondary

road fill. 

Table 1 describes the mines and prospects in or

near the reservations.

ENERGY RESOURCES

Uranium

No uranium occurrences have been recorded

within either of the reservations; however, radioactively anomalous areas to the east and in the Cargo

Muchacho Mountains of the Fort Yuma Reservation indicate some potential for local uranium

deposition (Fred Files, personal commun., 1980).

On the Fort Yuma reservation, alluvium containing organic debris has the potential for uranium

deposition. In a reducing environment, uranium in

ground water would precipitate and possibly

concentrate in small roll-front deposits.
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 18

Geothermal

No geothermal potential is indicated in the Fort

Yuma Indian Reservation even though it lies

between two confirmed geothermal areas. Fifty

miles west, several wells have been drilled in the

high potential area around the Salton Sea. The

Salton Sea area lies along the San Andreas Fault

system (Morton, 1977). Radium Hot Springs is

approximately 40 miles east. This thermal spring

group has temperatures of 140oF (Wilson, 1933).

There is one geothermal well in sec. 11, T. 20

S., R. 23 W., G & S.R.B.M., four miles east of the

East Cocopah Indian Reservation near the

Algadones Fault. No known extensions of the fault

occur on the reservation (Dutcher, 1972).

Petroleum

The reservations have no known petroleum

resources.

MINERAL LEASING

Permits to prospect definite areas of Indian

lands are available to the public (25 CFR 171.27a).

The duration of the lease for prospecting must be

stated in the application, and no ores may be

removed other than for testing or assaying purposes. Unless negotiated in advance, prospecting

permits do not include the right to leases or any

discoveries made on Indian lands. Further information concerning leases and permits can be obtained

from the California and Arizona offices of BIA.

RECOMMENDATIONS FOR FURTHER

WORK

The geology of the Yuma and Cocopah Reservations is incompletely known and should be

evaluated further for potential mineral resources.

Because of the extensive alluvial cover, the geologic evaluation should include detailed geochemical, gravity, and aeromagnetic surveys. The gravity

and aeromagnetic surveys can aid in (1) inferring

the geology underlying the sedimentary cover, (2)

determining thickness of sedimentary cover, (3)

locating intrusive bodies, (4) defining target areas

where alteration has reduced the magnetization of

the rocks, and (5) identifying anomalous concentrations of magnetic rocks. Geologic mapping of

the reservation lands should be directed toward

distribution of altered and mineralized rocks to

isolate areas of potential for mineral deposits,

industrial minerals, salines and lightweight aggregates.

More specifically, the following work is recommended for the Fort Yuma reservation:

1. Examine known lode deposits to determine

possible extensions into the reservation.

2. Sample well and spring water for subsurface

uranium or metallic minerals.

3. Undertake appropriate geophysical studies to

locate subsurface extensions of mineralized structures found outside the reservation.
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 19

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Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 20

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Bulletin 180-C, 55 p. 
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 21

Hadley, J. B., 1942, Manganese deposits in the

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Range province, southern California and Baja

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Angeles, Westernlore Press, 192 p. 
Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 22

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1933-1948. 

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area, Arizona and California: U.S. Geological

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map of parts of the Picacho Peak, Laguna,

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Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 23

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Status of Mineral Resource Information For the Fort Yuma and Cocopah Indian Reservations, Arizona and California

Thomas M. Sweeney and Robin Bradley

_________________________________________________________________________________________________

BIA Administrative Report 85 (1981) 24

U.S. Bureau of Reclamation, 1942, Imperial East

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307-322.




Figure 1. Location map for Fort Yuma and Cocopah Indian Reservations, Arizona and California.
Figure 2. Mineral occurrences of the Fort Yuma Indian Reservation and surrounding area, Arizona and California.
Figure 3. Cocopah Indian Reservation land parcels, Arizona.
Figure 4. Topographic map of the Fort Yuma and Cocopah Indian Reservations

 and surrounding area, Arizona and California.
Figure 5. Topographic map index of the Fort Yuma and Cocopah Indian Reservations, Arizona and California.
 Arizona and California. See Figure 6a for explanation.

Figure 6. Generalized geologic map of the Fort Yuma and Cocopah Indian Reservations,
Figure 6a. Explanation for Figure 6.
Figure 7. Stratigraphic column in the vicinity of the Fort Yuma and Cocopah Indian

 Reservations. From Olmsted and others (1973).