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Chapter J 

Mineral Resources of the 

Trigo Mountains Wilderness Study Area, 

La Paz County, Arizona 

By DAVID R. SHERROD, RICHARD M. TOSDAL, 

ROBERT B. VAUGHN, DAVID B. SMITH, and 

M. DEAN KLEINKOPF 

U.S. Geological Survey 

ROBERT H. WOOD II 

U.S. Bureau of Mines 

U.S. GEOLOGICAL SURVEY BULLETIN 1702 

MINERAL RESOURCES OF WILDERNESS STUDY AREAS: 

SOUTHWESTERN AND SOUTH-CENTRAL ARIZONA 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR.·, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication 

is for descriptive purposes only and does not imply 

endorsement by the U.S. Government 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1989 

For sale by the Books and 

Open-File Reports Section, 

U.S. Geological Survey 

Federal Center, Box 25425 

Denver, CO 80225 

Ubrary of Congress Cataloging-In-Publication Data 

Mineral resources of the Trigo Mountains Wilderness Study Area, La Paz 

County, Arizona I by David R. Sherrod .. : [et al.]. 

p. em. - (U.S. Geological Survey bulletin; 1702) (Mineral resources of 

wilderness study areas-southwestern and south-central Arizona ; ch. J.) 

Includes bibliographical references. 

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

1. Mines and mineral resources-Arizona-Trigo Mountains Wilderness. 

2. Trigo Mountains Wilderness (Ariz.) I. Sherrod, David R. II. Series. Ill. Series: 

Mineral resources of wilderness study areas-southwestern and south-central 

Arizona ; ch. J. 

QE75.89 no. 1702-J 557.3 s-dc20 89--600283 

[TN24.A6] [553' .09791 '72] CIP 
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 of certain areas to determine the mineral values, if any, that 

may be present. Results must be made available to the public and submitted to the 

President and the Congress. This report presents the results of a mineral survey of 

part of the Trigo Mountains Wilderness Study Area (AZ-050-023B), La Paz County, 

Arizona. 

CONTENTS 

Summary Jl 

Abstract Jl 

Character and setting Jl 

Identified resources Jl 

Mineral resource potential J2 

Introduction J2 

Location, physiography, and access J2 

Previous and present investigations J2 

Acknowledgments J3 

Appraisal of identified resources J3 

Methods of investigation J3 

Production history and mining and leasing activity J4 

Mineral deposits J4 

Gold J4 

Manganese J4 

Silver, lead, and zinc JS 

Industrial minerals JS 

Assessment of mineral resource potential JS 

Geology JS 

Geochemical studies JS 

Methods of investigation JS 

Comparison with nearby mining districts JS 

Summary of results J9 

Geophysical studies J9 

Mineral and energy resources J9 

Gold and silver J9 

Manganese JlO 

Other commodities JlO 

References cited JlO 

Appendixes 

Definition of levels of mineral resource potential and certainty of 

assessment J14 

Resource/reserve classification JlS 

Geologic time chart J16 

FIGURES 

1. Index map showing location of Trigo Mountains Wilderness Study Area, La Paz 

County, Arizona J3 

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

Mountains Wilderness Study Area, La Paz County, Arizona J6 

Contents V 
Mineral Resources of the 

Trigo Mountains Wilderness Study Area, 

La Paz County, Arizona 

By David R. Sherrod, Richard M. Tosdal, Robert B. Vaughn, 

David B. Smith, and M. Dean Kleinkopf 

U.S. Geological Survey 

Robert H. Wood II 

U.S. Bureau of Mines 

SUMMARY 

Abstract 

The Trigo Mountains Wilderness Study Area 

(AZ-050-023B) is located in La Paz County, southwestern 

Arizona. At the request of the U.S. Bureau of Land Management, 29,095 acres of the Trigo Mountains Wilderness Study 

Area were evaluated for mineral resources (known) and 

mineral resource potential (undiscovered). Throughout this 

report, reference to the "Trigo Mountains Wilderness Study 

Area• or to the "study area• refers only to that part of the 

wilderness study area for which mineral surveys were requested by the U.S. Bureau of Land Management. 

An inferred subeconomic manganese resource of 

210,000 long tons, having a grade of 12.7 percent, is located 

in the central part of the study area. Placer gold occurs along 

the north margin of the study area, downstream from the 

nearby Hart mine. Common rock and sand and gravel in the 

study area have no unique qualities that make them more 

valuable than vast quantities in the surrounding areas. There 

is low mineral resource potential for gold and silver associated with quartz-bearing veins near the northeast corner of 

the study area and in the three areas underlain by crystalline 

rocks in the south-central part. The north-central part of the 

study area has a high resource potential for manganese in 

deposits of limited extent. There is no potential for geothermal energy or for oil resources; the potential for natural gas is 

low along the northwest border of the study area. 

Character and Setting 

The Trigo Mountains Wilderness Study Area, 

which includes a large part of the Trigo Mountains, is in 

the southwestern part of La Paz1 County, about 23 mi 

Manuscript approved for publication, July 24, 1989. 1

La Paz County was created by the Arizona Legislature effective 

January 1, 1983, from what was the northern half of Yuma County. 

north of Yuma, Ariz. Relief in the Trigo Mountains is 

locally as great as 2,300 ft, although it is less than about 

1,000 ft in the study area. The mountains are underlain 

by Proterozoic and Mesozoic gneiss and granitic rocks, 

and by Tertiary volcanic rocks that underwent a major 

episode of extensional deformation about 22 to 20 million years ago (Ma) (see geologic time chart in "Appendixes"). Sedimentary basins that bound the range were 

created during this time and subsequently filled by coalescing alluvial fans. An arm of the early Gulf of California briefly inundated the Palo Verde Valley northwest of 

the study area about 5 Ma, resulting in sandstone, 

siltstone, and limestone that abut the northwest edge of 

the Trigo Mountains. Faulting since that time enabled 

the Colorado River to establish its modern course in a 

trough along the west margin of the Trigo Mountains. 

Mines that have produced gold, silver, lead, and 

(or) zinc are located outside the northeast and southeast 

boundaries of the study area .. The associated mineralized 

rock, however, whose distribution is largely fault controlled, does not extend into the study area. Manganese 

has been mined in the study area and is limited to veins 

and fault breccia in Tertiary volcanic rocks. 

Identified Resources 

Manganese oxides occur as fissure fillings in faults 

and fractures that cut Tertiary volcanic rocks in the Trigo 

Mountains Wilderness Study Area. Inferred subeconomic manganese resources of about 100,000 long tons 

(It) of nearly 13 percent manganese and about 110,000 It 

of about 9 percent manganese were calculated for the 

Lopez Wash area. Of the 11 drainages in the northern 

part of the study area, 4 contain detectable amounts of 

gold In panned-concentrate samples; however, it is doubtful that a resource is present because the occurrences are 

scattered and the amount of gold in the samples is small. 

Mineral Resources of the Trigo Mountains Wilderness Study Area, .Arizona J1 
Sand and gravel, and rock suitable for crushing for 

road surfacing, are present in the study area. These 

materials, however, have no unique qualities that make 

them more valuable than those that are closer to markets 

in surrounding areas. 

Low resource potential for gold and silver is assigned to the northeastern corner and to three areas in 

the south-central part of the Trigo Mountains Wilderness 

Study Area, all of which are underlain by Proterozoic and 

Mesozoic granitic rocks and their gneissic equivalents. 

Vigorous prospecting may lead to the discovery of gold 

and silver in small veins of very limited extent and 

probably of limited grade. 

A high resource potential for manganese is assigned to th.e north-central part of the study area. Manganese (as manganese oxides) forms narrow veins and 

fracture-filling deposits. There is no resource potential 

for geothermal energy or for oil; the energy resource 

potential for natural gas is low along the northwest 

border of the study area. 

This mineral survey was requested by the U.S. 

Bureau of lLand Management and represents a cooperative effort by the U.S. Geological Survey and the U.S. 

Bureau of Mines. An introduction to the wilderness 

review process, mineral survey methods, and agency 

respollllsibilities is provided by Beikman and others 

{1983). The U.S. Bureau of Mines evaluates identified 

resources at indiv:idluall mines Mdl mineralized! areas by 

collecting data Olll current and! past mining activities and 

through. field! examination of mines, prospects, claims, 

and! mineralized! areas. Kdentified resources are classified 

according to a system that is a modification of that 

dlescribedl by McKeRvey (1972) and! the U.S. Bureau of 

Mines and U.S. Geological Survey (1980). Studies by the 

U.S. Geological Survey are designed! to provide a scientific basis for assessing the potentiall for llmdiscoveredl 

milllleral resollllrces by determimting geoiogic units Md 

structllllres, possilile ellllvironmelllltS of mnllllera! dleposntnon, 

presellllce of geoclhtemncall and! geoplhtysncall anomalies, alllldl 

appincalble ore-dleposit models. MilllleraU assessmellllt 

methodology alllldl terminology as tlhtey appUy to these 

surveys were dlnscllllssedl by Goudlarzn (1984). See "Appendixes" for tlhte dlefnnntnollll of ReveUs of mnlllleraU resource 

potelllltiaU alllldl certaillllty of assessmellllt alllldl for tlhte resourrce/lreserve dassnfncatnollll. 

The Trigo Mollllntailllls WliUdleme§S Sm~ Area 

(AZ-050-023B) is Uocatedl about 23 mn Irllorrfelln of Yuma, 

Ariz. (fig. 1 ), in the Basin and Range physiographic 

province of southwestern Arizona. The climate is arid, 

characteristic of the Sonoran Desert region of the southwestern United States and northern Mexico. 

The Trigo Mountains are barren and rocky. Winding washes in them drain to the Colorado River, which 

flows along the west and south margins of the range. 

Relief is moderate, with elevations ranging from about 

2,500 ft east of the Hart mine to about 200 ft at the 

Colorado River near Yuma Wash (fig. 1). 

Dirt roads approach the range from the north via 

the townsite of Cibola (SEE-bo-la) and from the south 

via Martinez lLake. Four-wheel-drive trails traverse some 

of the larger washes. An old jeep track along the west 

boundary near the Colorado River was destroyed during 

flooding in the early 1980's. This track also connected 

with a jeep trail down Clip Wash, a short part of which is 

a tortuous bedrock channel. Access by boat from the 

Colorado River is possible but difficult because broad 

thickets of tamarisk (salt cedar) and thorny shrubs crowd 

the shore. The eastern part of the Trigo Mountains is 

closed to public access because it lies within the Yuma 

Proving Ground, a training center of the U.S. Army. 

There have been few geologic studies of the Trigo 

Mountains, in part because of difficult access. Until 

recently, writers relied upon the important regional mapping by E.D. Wilson {1933; 1960) and a handful of mine 

summaries and university theses (for example, Wilson 

and others, 1934; Wilson, 1951; Farnham and Stewart, 

1958; Parker, 1966; Keith, 1978; JPietenpol, 1983; Bradley, 1986). The southern and northern frringes of the 

range were included illl water-resource investigations of 

the Yuma and Blythe areas (Metzger and others, 1973; 

Olmsted and others, 1973). Nearby parts of the Chocolate Mountains, across the Colorado River in California, 

were mapped by Crrowe (1978), Haxel (1977), and Morrton {1977). 

Knvestigations of the structlllra.l geology of the Trigo 

Mountains flourished in the nate 1970's and early 1980's 

whellll students and! facuity from San Diego State Univerrsity pooned theirr efforrts to umierrstcmdl tlhe distribution, 

geometrry, and! mil!llerralization of !arrge-scaie Tertiary extensionaU (norrmall) faunts nllll tlhle Rowerr Coiorrado Rnverr 

rregion (Weaverr, 1982; Garllllerr alllldl others, 1982). Howeverr, geoUogic units of the Trrigo Mountains rremained 

UarrgeUy Uirllmappedl. 

][rn 1986, the U.S. Burreau of Milllles condlucted a 

survey of the study arrea and! illllvestigatedl U.S. Bureau of 

lLandl Managemellllt mining danm and! millllerall lease 

recorrd!s. U.S. Bureau of Mines flie!dl studies werre dlirrectedl 

chiefly toward the examination of klllloWllll mines, prrospects, alllldl mineralized! areas illllsidle and! witlhtm about 1 mi 

of the study area lboulllldary. 
Field studies by the U.S. Geological Survey for this 

assessment were conducted mainly in 1986 and 1987 and 

included geologic mapping and geochemical studies. 

Rock and stream-sediment samples were collec.ted to 

supply background geochemical information and to identify areas containing anomalous concentrations of economic or indicator elements. 

Acknowledgments 

We gratefully acknowledge the advice and logistical 

support supplied by the managers and employees of the 

Cibola and Imperial National Wildlife Refuges (U.S. 

Fish and Wildlife Service) and the officers and civilian 

employees of the Yuma Proving Ground (U.S. Army) 

during the fieldwork for this study. 

APPRAISAL OF IDENTIFIED RESOURCES 

By Robert H. Wood II 

U.S. Bureau of Mines 

Methods of Investigation 

A total of 205 samples, including 194 rocks and 11 

panned concentrates, was collected from within and near 

the Trigo Mountains Wilderness Study Area for analysis 

,------------1 

I I 

I 

I 

I 

I 

YUMA PROVING GROUND nLA_!'AZCO_ 

YUMA CO 

Eureka 

~ mining 

Ou'V district 

~; Martine ~J' lAke 

32" 

45' 

0 10 20 30 MILES 

Figure 1. Index map showing location of Trigo Mountains Wilderness Study Area, La Paz County, 

Arizona. 

· Mineral Resources of the Trigo Mountains Wilderness Study Area, Arizona J3 
(Wood, 1989). Accessible mine workings were mapped 

using compass and tape, and rock samples were taken 

across veins and other suspected mineralized structures. 

Panned-concentrate samples were collected from drainages in an area of known gold placers (Johnson, 1972; 

Krason and others, 1982) in the northeastern part of the 

study area. A limestone sample from the Bouse Formation was analyzed by inductively coupled plasma-atomic 

emission spectroscopy for oxides of aluminum, calcium, 

iron, manganese, phosphorus, potassium, silica, sodium, 

and titanium.lLoss on ignition was determined gravimetrically, and sulfur was determined instrumentally. The 

remaining 204 samples were analyzed by fire assay for 

gold and silver. Of the samples from the central part of 

the -study area, 60 were analyzed by atomic absorption 

(AA) for manganese, and 11 of these samples were also 

analyzed by AA for iron and silver. All116 samples from 

the southern part of the area were analyzed by AA for 

lead, molybdenum, silver, vanadium, and zinc, and 17 of 

these samples were also analyzed gravimetrically for 

barium. (See Wood, 1989, for analytical details.) 

lP'Ir©©ll!.!l~n©U'i1 [}=(]fi~~©ff'W ~U11<dl MnU'i1nU'i1~ ~U'i1<dl 

ILM~nU'i1~ ~©~n~n~w 

The study area includes parts of the Trigo and 

Silver mining districts (fig. 1 ): the Trigo district in the 

central and northern part of the study area, and the Silver 

district extending into the southeastern part (Keith, 

1978). The Eureka mining district is 5 to 6 mi southeast 

of the study area along the Colorado River. 

Manganese oxides in the Trigo mining district 

occur as fracture fillings in andesitic volcanic rocks; 11 

occurrences are reportedly in the study area. Approximately 25,000 long tons (It) of 20- to 40-percent manganese ore were mined in the district during the 1950's, 

when the government was purchasing low-grade manganese ore (Krason and others, 1982). 

Mines in the Silver mining district are all outside 

the study rurea but as near as 0.5 mi. 'fhe mineral deposits 

are restricted to a narrow set of veins localized along 

north-trending fault zones (Bradley, 1986); the Eureka 

district has a similar setting (Keith, 1978). Production 

from the Silver and Eureka districts since about 1879, 

when the districts were organized, is reportedly about 1.6 

million ounces (oz) of silver, 1,457 short tons (st) of lead, 

940 oz of gold, 15 st of zinc, and 0.75 st of copper (Keith, 

1978). The largest mines in the Silver district are the Red 

Cloud, Clip, and! lBiaclk Rock (Wilson, 1951). Extensive 

drilling near the Clip, Black lRock, and Geronimo mines 

in the early 1970's delineated reserves in excess of 5 

miRHoll1l St COJrntrull1ling 4 OZ of silver/st and significant 

amou1rnts of fluorite, barite, lead, and zinc, which may be 

minable by open-pit methods (U.S. Bureau of Land 

Manageme1rnt, 1985). 

According to Bureau of Land Management 

records (as of January 1988), there are no patented 

claims in the study area, and unpatented mining claims 

are located only in the southern part. As of February 

1987, about 5,000 acres in the northern part of the study 

area were under lease for oil and gas. 

Wilnlill®fi"tal~ [Q)®p©~n~~ 

~old! 

Gold occurrences in the study area are limited to 

placer deposits along the north margin, downstream 

from mineralization in the nearby Hart mine area (figs. 1, 

2). Gold at the Hart mine occurs in narrow quartz veins 

in Jurassic dioritic gneiss. Major faults cut off the Mesozoic rocks 1.5 mi inside the study area's northeast 

corner (fig. 2). 

Panned-concentrate samples of stream sediments 

for this study were taken from dry washes in the area 

identified by Johnson (1972) and Krason and others 

(1982) as the locations of placer mining. No evidence of 

past or current placer mining was seen. Minor amounts 

of gold were detected in panned-concentrate samples 

from 4 of the 11 drainages sampled. The presence of gold 

in the panned-concentrate samples does not define a 

resource because of the small amount of gold detected, 

the absence of visible gold in the samples, and the 

scattered nature of the occurrences. 

Gold resources were calculated for the goldbearing quartz veins in two adits at the Hart mine, which 

is adjacent to the study area on the north (figs. 1, 2). ][n 

the upper adit, average gold concentrations are calculated to be 0.22 oz/st for about 150 st of material. ][n the 

lower adit, average gold concentrations are calculated to 

be 0.13 oz/st for about 10,000 st of material. The tonnages were calculated using one-half the strike length of 

each fault and a tonnage factor of 12.2 cubic feet per 

short ton. The resource is subeconomic. 

Manganese oxides, mostly pyrolusite and psilomelane, occur as fault and fracture fillings and between 

breccia clasts in volcanic rocks in the central and southern parts of the Trigo mining district (Farnham and 

Stewart, 1958; Keith, 1978). The manganese oxides were 

mined from surface cuts and underground stapes. Selective mining, crushing, screening, and commonly hand 

sorting were required to adequately concentrate the 

manganese prior to shipping. None of the manganese 

claims are currently on file with the lBuJreau of Land 

Management. 

Manganese concentrations in samples collected foJr 

this study range from less than 1 percent to 36 percent 
Exposures created by workings along two of the faults in 

the study area provided a basis for estimating the remaining manganese resource (see Wood, 1989, for calculations). The amount of the inferred subeconomic manganese resources along the two faults is about 100,000 It 

having 13 percent manganese and 110,000 It having 9 

percent manganese, or about 210,000 It of material 

containing manganese worth about $3 million, using an 

average manganese price of $1.27 !It unit (one It unit = 

22.4 lbs) (U.S. Bureau of Mines, 1988). Bulk sampling 

and metallurgical testing would be required to determine 

if the manganese can be. concentrated to the 46- to 

48-percent level required to make a salable product. 

Silver, Lead, and Zinc 

There are no identified resources of silver, lead, or 

zinc in the Trigo Mountains Wilderness Study. Area. 

Silver, lead, and zinc were mined from the Silver district, 

which borders the study area on the southeast. The ore 

minerals fill fractures along chiefly north-trending normal faults that extend into the study area north of the 

Clip and South Geronimo mines (fig. 2). Mineralization 

diminishes abruptly near the study area boundary, however. 

Discontinuous barite and calcite veins occur along 

the faults near the Clip mine. Workings sampled north of 

the Clip mine are mostly prospect pits that include three 

adits and five shafts. Concentrations of gold and molybdenum are low in the samples, whereas concentrations of 

barium, silver, vanadium, lead, and zinc vary. Rocks 

collected along faults near the mine contain gold (as 

much as 0.06 oz/st), barium (as much as 38 percent), 

silver (0.003 to 8.0 oz/st), molybdenum (2 to 14 parts per 

million, ppm), vanadium (26 to 465 ppm), lead (15 to 

1,900 ppm), and zinc (5 to 880 ppm). Wood (1989) gives 

details of sampling localities and analytical results. 

The fault at the South Geronimo mine contains 

calcite and quartz veins. Workings along the fault include 

two adits, two shafts, and several prospect pits and 

open cuts. Rocks collected along the fault contain gold (as 

much as 0.06 oz/st), silver (0.12 to 11.4 oz/st), molybdenum (4 to 610 ppm), vanadium (10 to 132 ppm), lead 

(560 to 42,000 ppm), and zinc (440 to 8,600 ppm). 

Inferred subeconomic silver resources of 70,000 st 

(silver grade of 5.0 oz/st), which may be mined by 

open-pit methods, were calculated for workings along the 

fault in the South Geronimo mine area (outside the study 

area). The in-place value of the silver resource is about 

$2 million, using the August 1988 silver price of $6.70 per 

oz (Wood, 1989). 

Industrial Minerals 

The washes and alluvial fans of the study area 

contain common-variety rock, sand, and gravel. Volcanic 

rocks in the study area are suitable for construction after 

crushing. From 1986 to 1988, sand, gravel, and rock 

(both volcanic and plutonic) were being quarried outside 

the northern boundary of the study area for use in road 

and levee construction along the Colorado River in the 

nearby Cibola National Wildlife Refuge. There are no 

unique qualities (other than distance to job site) that 

make the sand and gravel or rock in the study area more 

valuable than the vast quantities available in the surrounding area. Volcanic rock from the rip-rap quarry 

south of Hart Mine Wash (north edge of fig. 2) is highly 

fractured because of its location near a major Tertiary 

fault. The numerous fractures may have expedited the 

work of quarrying. 

Limestone of the Bouse Formation is exposed 

outside the northwest margin of the study area (fig. 2). 

Calcium carbonate (CaC03 ) content of the limestone. is 

69 percent, which is well below that needed for industrial 

applications. 

ASSESSMENT OF MINERAL RESOURCE 

POTENTIAL 

By David R. Sherrod, Richard M. Tosdal, 

Robert B. Vaughn, David B. Smith, and 

M. Dean Kleinkopf 

U.S. Geological Survey 

Geology. 

The Trigo Mountains are built of Tertiary volcanic 

and sedimentary rocks deposited nonconformably over 

Proterozoic. and Mesozoic crystalline rocks. The mountains are near the western edge of the southern Basin and 

Range physiographic province, where large-scale extensional deformation between about 22 and 20 Ma broke 

the region into a series of fault-bounded mountains and 

basins. Most mineralization in the study area is structurally controlled by the normal faults and is limited to 

vein-forming manganese oxides emplaced in Oligocene 

and Miocene rocks. Scarce gold-bearing quartz veins 

were emplaced in Mesozoic rocks outside the study area 

prior to Oligocene and Miocene volcanism and sedimentation. 

The oldest rocks in the study area are Proterozoic 

gneiss and amphibolite, which crop out near Lighthouse 

Wash (fig. 2). The gneiss and amphibolite were intruded 

by Mesozoic diorite and associated pegmatite and then 

were metamorphosed. Late Triassic monzonite crops out 

in the Hart mine area (Tosdal 1986, 1988), along the 

north boundary of the study area. Jurassic granitic rocks 

intruded the Proterozoic and Mesozoic gneisses and also 

the Late Triassic monzonite. The Jurassic granitic rocks, 

consisting of diorite, granodiorite, and granite that are 

locally converted to gneiss and mylonitic gneiss, are 

Mineral Resources of the Trigo Mountains Wilderness Study Area, Arizona JS 
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Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 

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' lithologically similar to and correlated with isotopically 

dated Middle and Late Jurassic plutonic rocks in adjacent ranges of southwestern Arizona and southeastern 

California (Tosdal, 1988; Tosdal and others, in press). 

The Proterozoic and Mesozoic crystalline rocks in the 

study area are part of the upper plate of the Chocolate 

Mountains thrust, a regional thrust fault of latest Cretaceous or early Tertiary age. The Chocolate Mountains 

·thrust and the Orocopia Schist, which occurs in the lower 

plate, are exposed 4 mi southeast of the study area. 

The Proterozoic and Mesozoic rocks are overlain 

by Oligocene(?) sedimentary rocks and Oligocene and 

Miocene volcanic rocks. From oldest to youngest, these 

strata consist of (1) a thin (generally less than 100-

ft-thick) discontinuous sequence of arkosic fanglomerate 

and tuffaceous sandstone too thin to show on figure 2, (2) 

andesitic to rhyodacitic lava flows with minor volcaniclastic strata-the Quechan volcanic rocks of Crowe (1978), 

and (3) silicic welded tuff with minor lava flows, domes, 

and tuff breccia (unit Tt on figure 2) . 

The Quechan volcanic rocks have potassium-argon 

(K-Ar) ages that range from about 32 to 27 Ma (Eberly 

and Stanley, 1978; Crowe and others, 1979; Weaver, 

1982). The few ages older than 32 Ma are considered 

unreliable because the rocks or minerals that were dated 

are moderately altered. The overlying sequence of welded tuff includes the ignimbrite of Ferguson Wash of 

Crowe (1978), which probably erupted near Picacho 

State Recreational Area, Calif. (3 mi south of the study 

area, fig. 1), and at least two ash-flow tuffs erupted from 

a caldera in the Kofa Mountains (30 mi east of the study 

area, fig. 1) (Grubensky and others, 1986). The ignimbrite of Ferguson Wash has K-Ar ages of about 25 to 24 

Ma (Crowe and others, 1979; Olmsted and others, 1973; 

Weaver, 1982). The youngest volcanic rocks in the study 

area, the tuffs from the Kofa Mountains sources, erupted 

about 22 Ma (L.G. Pickthorn, in Bagby and others, 

1987). 

Normal faulting and sedimentation occurred simultaneously, beginning sometime after about 22 Ma 

(Tosdal and Sherrod, 1985; Sherrod and others, 1987). 

The area was extended 10 to 40 percent, as indicated by 

strata that are tilted as much as 60°. Faults having the 

greatest throw (at least 1 mi) place the capping volcanic 

strata down against subjacent crystalline rocks, resulting 

in the repeated stratigraphic sequences seen on figure 2. 

A horst of crystalline rocks, exposed in the southwestern 

part of the study area from Lighthouse Wash to Black 

Rock mine, was uplifted with only minor rotation, as 

indicated by gently dipping Tertiary rocks preserved in 

depositional contact along its upper surface. Thick sequences of sand and gravel (in unit Ts) accumulated as 

large alluvial fans in isolated grabens and half grabens 

throughout the area. Lithologically, these rocks contain 

entirely Tertiary volcanic clasts at the base of each 

sequence but range upsection to mixtures of Tertiary 

Mineral Resources of the Trigo Mountains Wilderness Study Area, Arizona J7 
volcanic and pre-Tertiary crystalline clasts in their upper 

parts as a result of increasingly widespread exposure of 

crystalline rocks along moderately dipping faults. Tilting 

and large_-scale faulting had ended by 13 Ma, when 

subhorizontal basalt lava flows and fanglomerate were 

deposited at Black Mountain, 9 mi southwest of the study 

area (Crowe, 1978). 

Many of the faults in the study area probably merge 

at depth along a deep-seated regional detachment fault. 

Garner and others (1982), however, suggested that the 

detachment fault is arched up and breached by erosion 

wherever Proterozoic and Mesozoic crystalline rocks are 

expo"sed. In their model, the crystalline rocks (including 

the two plates of the Chocolate Mountains thrust) form 

the lower plate of the detachment and are everywhere in 

fault contact with Tertiary units. Our mapping, however, 

shows that the crystalline rocks are overlain depositionally by the Tertiary strata in many places. 

Marine water from the proto-Gulf of California 

invaded what is now the Palo Verde Valley about 5 Ma, 

creating a complex setting of estuaries and deltas that are 

recorded by the limestone, siltstone, and sandstone of the 

Bouse Formation (Metzger, 1968; Smith, 1970; Buising, 

1987). The Bouse Formation dips gently west and northwest along the northwest edge of the study area and 

underlies the Palo Verde Valley at depths at least as 

great as 400 ft, where it is penetrated by water wells. The 

Bouse Form~tion and the underlying sandstone and 

sedimentary breccia (unit Ts) in the Palo Verde Valley 

are downdropped against pre-Tertiary and Tertiary rocks 

along normal faults at the west margin of the Trigo 

Mountains. 

The Colorado River established itself through the 

Yuma-Blythe area sometime in the Pliocene (Dillon, 

1976; Buising, 1987). It originally exited the Palo Verde 

Valley area via Midway Well (approximately along the 

route of California Highway 78 and the Ogilby road, 10 

mi west of its present course; fig. 1) by following the 

previous Bouse seaway (Dillon, 1976). The river's modern course between the Trigo and Chocolate Mountains 

is probably the result of late Pliocene or Quaternary 

faulting that provided a new escape route for Colorado 

River water along the troughlike depression from Cibola 

Lake to Picacho State Recreational Area (fig. 1). Intermittent deposition from Pliocene to modern time in the 

study area h~s left poorly consolidated to unconsolidated 

sand and gravel (units QTf and Qa) as terrace-forming 

fanglomerate and alluvial fill. 

Geochemical Studies 

Methods of Investigation 

A reconnaissance geochemical survey was conducted in the Trigo Mountains Wilderness Study Area in 

1986--87. Minus-80-mesh stream sediments and heavymineral concentrates derived from stream sediments 

were selected as the primary sample media because they 

represent a composite· of rock and soil exposed in the 

drainage basin upstream from the sample site. Chemical 

analysis of the sediment samples helps to identify basins 

containing unu~ually high concentrations of elements 

that may be related to mineral occurrences. Heavymineral concentrates are a useful sample medium in arid 

environments or in areas of rugged topography, where 

mechanical erosion predominates over chemical erosion 

(Overstreet and Marsh, 1981; Bugrov and Shalaby, 1975; 

Zeegers and others, 1985). 

In this ·study, the nonmagnetic fraction of the 

heavy-mineral concentrates was analyzed. This fraction 

concentrates or(1 and ore-related minerals such as pyrite, 

galena, cassiterite, sphalerite, chalcopyrite, stibnite, free 

gold, barite, and scheelite, permitting determination of 

some elements that are not easily detected in bulk 

stream-sediment samples. 

.Stream-sediment samples and heavy-mineral con-

. centrates ~ere collected from active alluvium in firstorder (unbranched) and second-order (below the junction of two first-order) streams, as shown on U.S. 

Geological Survey topographic maps (scale 1:24,000). 

· Rock samples for geochemical analysis were collected 

both in and near the study area. Unaltered samples were 

analyzed to provide information on geochemical background concentrations, whereas samples from mines, 

prospects, and altered areas were analyzed to determine 

suites of elements associated with mineralization and 

alteration. All samples were analyzed for 31 elements 

including arsenic, antimony, barium, bismuth, cadmium, 

cobalt, chromium, copper, gold, iron, lead, magnesium, 

manganese, molybdenum, nickel, silver, thorium, tin, 

titanium, tungsten, vanadium, and zinc by semiquantitative emission spectroscopy. This method allows the rapid 

and economical analysis of many elements in numerous 

samples, but the lower limits of determination for a few 

elements are somewhat high. Therefore, the following 

elements were analyzed by atomic absorption, which 

lowers the determination limit (shown in parentheses for 

each element): antimony (2 parts per million, ppm), 

arsenic (10 ppm), bismuth (2 ppm), gold (0.05 ppm), and 

zinc (5 ppm). Rock samples were also analyzed for 

mercury (0.02 ppm), fluorine (100 ppm), and tungsten 

(0.5 ppm). Analytical data, sampling methods, sample 

preparation procedures, analytical techniques, and sampling sites are presented in Bullock and others (1989). 

Comparison with Nearby Mining Districts 

The Silver mining district, which comprises several 

mines that produced silver, lead, some gold, and minor 

zinc and copper (Keith, 1978) near the southeast edge of 

the study area, was sampled in order to compare its 

J8 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 
geochemical signature with that of the stream-sediment 

samples and heavy-mineral concentrates from within the 

study area. Rocks collected from mines and prospects 

within the district contain high concentrations of silver 

(as much as 500 ppm), lead (as much as 2 percent), zinc 

(as much as 1 percent), and molybdenum (as much as 

2,000 ppm). Heavy-mineral concentrates collected downstream from some of these mineralized zones also contain anomalous concentrations of metals: silver (as much 

as 70 ppm), lead (as much as 5 percent),'zinc (as much as 

7,000 ppm), and molybdenum (as much as 200 ppm). 

This geochemical· signature was not observed in any 

samples from within the study area, which supports the 

field evidence that this mineralization does not extend 

into the study area. 

Analyses of rock samples from the Hart mine area, 

which is located approximately 0.5 mi north of the study 

area, show high concentrations of gold (as much as 59 

ppm), mercury (as much as 2.4 ppm), and silver (as much 

as 5 ppm). No geochemical or geologic evidence for this 

type of mineralization was found ~ithin the study area. 

Summary of Results 

One area of geochemical anomalies was defined 

from the geochemical reconnaissance survey in the 

northern part of the study area. This area, which was 

delineated on the basis of anomalous manganese (1,500 

to 10,000 ppm) and silver (as much as 10 ppm) in the 

heavy-mineral concentrates, contains many small manganese mines and prospects (Farnham and Stewart, 1958; 

Keith, 1978). All the manganese deposits are hosted by 

Tertiary volcanic rocks. Rock samples collected from 

mines and prospects within this area contain concentrations of manganese as high as 5,000 ppm, the upper limit 

of determination. 

Smaller manganese anomalies (1,500 to 2,000 

ppm) defined by minus-80-mesh stream-sediment samples were noted from sites in the southern part of the 

study area. The area of these sites is geographically 

distinct from the area defined by the heavy-mineral 

concentrates, and the anomalies probably indicate low 

levels of manganese mineralization in calcite veins that 

cut the Tertiary volcanic rocks. 

Samples from several sites scattered throughout 

the study area have anomalous amounts of tin (generally 

100 to 300 ppm but locally greater than 2,000 ppm) in the 

heavy-mineral concentrates. Tin at the sites south of Clip 

Wash is associated with bismuth (mainly about 50 ppm 

but locally greater than 1,500 ppm). Wilson (1933, p. 51) 

noted minor showings of copper and bismuth in this same 

area, which lies north of the Silver mining district. The 

drainage basins containing these tin-bismuth anomalies 

are underlain primarily by Tertiary dacite and rhyodacite 

or rhyolite in the Quechan volcanic rocks (fig. 2), much 

of which was emplaced as shallow intrusions in that area. 

Tin (cassiterite) associated with Tertiary rhyolite was 

deposited at high temperatures in veins less than 3 in. 

wide and commonly only a few inches long (Reed and 

others, 1986). The tin in these types of deposits apparently was mobilized as the rhyolite cooled, as indicated by 

whole-rock analyses from similar occurrences elsewhere 

in the southwestern United States where vitrophyric 

(glassy, quickly cooled) rocks at the margins of a rhyolitic 

mass have slightly greater concentrations of tin than 

rocks in the interior have (W.A. Duffield, oral commun., 

1988). 

Geophysical Studies 

Limited gravity and magnetic data were utilized to 

determine the mineral resource potential of the Trigo 

Mountains Wilderness Study Area. The data consist of 

widely spaced surveys made for regional geologic studies 

and mineral resource evaluation (LKB Resources, Inc., 

1980; U.S. Geological Survey, Denver, Colo., unpublished gravity files, 1987). Fifteen gravity stations were 

located in and immediately adjacent to the study area. 

The magnetic data result from a survey flown 400 ft 

above terrain; north-south flightlines were spaced 3 mi 

apart, and east -west tielines cross the study area near its 

north and south ends. 

The Bouguer gravity anomaly map shows a northtrending high with relief of 10 to 14 milligals that 

generally corresponds to the physiographic Trigo Mountains. In the vicinity of Clip Wash (near lat 33°10' N.), low 

gravity values form a saddle across the regional gravity 

high. The central part of the saddle correlates with a 

closed negative magnetic anomaly of amplitude 40 nanoteslas. These anomalies reflect source rocks of lowert 

density and magnetization relative to adjacent rocks and 

may arise from lithologic contrasts within the Jurassic 

granitic rocks or from subsequent deformation and alteration. The detail of the gravity and magnetic anomaly 

data is insufficient to geophysically evaluate the extent or 

nature of mineralization in the study area. 

Mineral and Energy Resources 

Gold and Silver 

Low mineral resource potential for gold and silver, 

certainty level B, is assigned to Mesozoic intrusive rocks 

in the northeast corner of the Trigo Mountains Wilderness Study Area. This assignment is based on the proximity of vein mineralization at the Hart mine, the extension of Mesozoic rocks into the study area, and the 

localization of placer gold (Johnson, 1972; Krason and 

others, 1982). Major faults cut off the Mesozoic rocks 1.5 

mi inside of the study area's northeast corner (fig. 2). 

Mineral Resources of the Trigo Mountains Wilderness Study Area, Arizona J9 
The occurrence of gold- and silver-bearing quartz 

veins in pre-Tertiary rocks north and south of the study 

area indicates the possibility of similar mineralization. in 

those rocks within the study area. During the geologic 

mapping a few small exploration pits were noted, either 

along fault zones or where traces of secondary copper 

minerals had left minor stains on the crystalline rocks. 

Our geochemical studies failed to define any particular 

area for further investigation, but · the signs of limited 

exploration and alteration indicate a low mineral resource potential, certainty level B, for gold and silver in 

the pre-Tertiary rocks extending from Clip Wash to Red 

Cloud Wash in the south-central part of the study area 

and in a small inlier of pre-Tertiary rocks along each of 

those washes along the southeast side of the study area. 

Manganese 

There is high mineral resource potential, certainty 

level D, for fault-controlled deposits of manganese in the 

north-central part of the Trigo Mountains Wilderness 

Study Area. The area of high potential is centered on 

abandoned mines in the Lopez Wash area (part of the 

Trigo mining district) and is further delineated by the 

limited number of stream-sediment samples having 

anomalous manganese concentrations. 

Manganese production from the Lopez Wash area 

was economic only from 1953 to 1959, a period of U.S. 

Government price supports for the strategic metal. The 

manganese occurs in narrow vein- and fracture-filling 

deposits that are volumetrically limited and currently 

considered to be a subeconomic resource. 

other Commodities 

Sand and gravel are high-volume, low-unit-value 

commodities that require a local market for economic 

development. The occurrences in the Trigo Mountains 

Wilderness Study Area have no unique properties that 

make them more valuable than other deposits closer to 

markets. 

The study area was assigned a low to zero resource 

potential for oil and gas by Ryder {1983). Most of the 

study area is underlain by volcanic and crystalline rocks 

and has no hydrocarbon resource potential, certainty 

level of D. 

There is, however, low potential for natural gas, 

certainty level B, along the northwest border of the study 

area. The Palo Verde Valley northwest of the study area 

is a large Tertiary sedimentary basin. The Bouse Formation, an upper Miocene and Pliocene sequence of sandstone, siltstone, and limestone, forms a broad structural 

depression in the Palo Verde Valley. It is exposed as high 

as 500ft above sea level around the valley's margin; on 

the basis of water-well data, the top of the formation is at 

least 200ft below sea level in the valley center. Gravity 

data suggest that the Bouse Formation and other Tertiary sedimentary fill in the Palo Verde Valley are more 

than 3,000 ft thick. Gas conceivably may have been 

generated from organic debris in Bouse or older sedimentary strata. If so, the structural configuration of (at 

least) the Bouse and younger sedimentary rocks of the 

basin could be conducive to up-dip migration of gas 

toward the basin margin and up-structure migration 

along basin-bounding faults that coincide with the northwest boundary of the wilderness study area. 

Warm-water wells near Blythe (22 mi northwest of 

the study area) have temperatures of about 30 to 45 °C, 

defining an area "known or inferred to be underlain at 

shallow depth (less than 3,000 ft) by thermal water of 

sufficient temperature for direct heat applications" 

(Martin and Higgins, 1980). The geologic and hydrologic 

conditions are appropriate for deep circulation of ground 

water along the major basin-bounding faults of the Palo 

Verde Valley. Near the study area these faults separate 

Miocene and older volcanic and crystalline rocks of the 

Trigo Mountains from Quaternary and Tertiary sedimentary strata at the southeast end of the Palo Verde 

Valley (fig. 2). As the sedimentary basin of the Palo 

Verde Valley is adjacent to but outside of the study area, 

the Trigo Mountains Wilderness Study Area is considered to have no potential, certainty level D, for geothermal energy. 

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p. 

Pietenpol, D.J., 1983, Structure and ore deposits of the Silver 

district, La Paz County, Arizona: Tucson, University of 

Arizona, M.S. thesis, 67 p. 

Reed, B.L., Duffield, Wendell, Ludington, S.D., Maxwell, C.H., 

and Richter, D.H., 1986, Descriptive model of rhyolitehosted Sn, in Cox, D.P., and Singer, D.A., eds., Mineral 

deposit models: U.S. Geological Survey Bulletin 1693, p. 

168--171. 

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

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

wilderness land in the western United States: U.S. Geological Survey Circular 902-C, p. C1-C22. 

Sherrod, D.R., Pickthorn, L.B.G., Tosdal, R.M., Grubensky, 

M.J., and Koch, R.D., 1987, Major early Miocene extensional deformation in southwestern Arizona and southeastern California [abs.]: Geological Society of America 

Abstracts with Programs, v. 19, no. 7, p. 841. 

Smith, P.B., 1970, New evidence for Pliocene marine embayment along the lower Colorado River area, California 

and Arizona: Geological Society of America Bulletin, v. 

81, no.5,p. 1411-1420. 

Tosdal, R.M., 1986, Mesozoic ·ductile deformations in the 

southern Dome Rock Mountains, northern Trigo Mountains, Trigo Peaks, and Livingston Hills, southwestern 

Arizona, and Mule Mountains, southeastern California, 

in Beatty, Barbara, and Stone, Claudia, eds., Frontiers in 

geology and ore deposits of Arizona and the Southwest: 

Arizona Geological Society Digest, v. 16, p. 62-71. 

---1988, Mesozoic rock units along the Late Cretaceous 

Mule Mountains thrust system, southeastern California 

and southwestern Arizona: Santa Barbara, University of 

California Ph.D. dissertation, 365 p. 

Tosdal, R.M., and Sherrod, D.R., 1985, Geometry of Miocene 

extensional deformation, southeastern California and 

southwestern Arizona, in Papers presented to the Conference on Heat and Detachment in Crustal Extension 

on Continents and Planets: Houston, Texas, Lunar and 

Planetary Institute, p. 147-151. · 

Tosdal, R.M., Haxel, G.B., and Wright, J.E., in press, Jurassic 

geology of the Sonoran Desert region, southern AriZona, 

Mineral Resources of the Trigo Mountains Wilderness Study Area, Arizona J11 
southeastern California, and nothernmost Sonora [Mexico]: Construction of a continental-margin magmatic arc, 

in Jenny, J.P., and Reynolds, S.J., eds., Geologic evolution of Arizona: Tucson, Arizona Geological Society 

Digest, v. 17. 

U.S. Bureau of Land Management, 1985, Final Yuma District 

resource management plan and environmental impact 

statement: U.S. Bureau of Land Management, Yuma 

District, 310 p. 

U.S. Bureau of Mines, 1988, Mineral commodity summaries, 

1988: 193 p. 

U.S. Bureau of Mines and U.S. Geological Survey, 1980, 

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

Weaver, B.F., 1982, Reconnaissance geology and K-Ar geochronology of the Trigo Mountains detachment terrane, 

Yuma County, Arizona: San Diego, Calif., San Diego 

State University, M.S. thesis, 119 p. 

Wilson, E.D., 1933, Geology and mineral deposits of southern 

Yuma County: Arizona Bureau of Mines and Geology 

Bulletin 134, 234 p. 

---1951, Silver and Eureka districts, in Arizona zinc and 

lead deposits, part 2: Arizona Bureau of Mines Bulletin 

158, p. 83-97. 

---1960, Geologic map of Yuma County, Arizona: Arizona 

Bureau of Mines, scale 1:375,000. 

Wilson, E.D., Cunningham, J.D., and Butler, G.M., 1934, 

Arizona lode gold mines and gold mining: Arizona 

Bureau of Mines Bulletin 137, 254 p. 

Wood, R.H. II, 1989, Mineral resources of a part of the Trigo 

Mountains Wilderness Study Area (AZ-050-023B), La 

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

Report 20-89, 47 p. 

Zeegers, H., AI Shanfari, S.M., AI Muflehi, Y .A., and Letalenet, J., 1985, Aspects of regional geochemical prospecting in desert conditions, in Prospecting in areas of desert 

terrain: London, The Institution of Mining and Metallurgy, p. 131-140. 

J12 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 
APPENDIXES 
DEFINITION OF LEVELS OF MINERAL RESOURCE POTENTIAL 

AND CERTAINTY OF ASSESSMENT 

LEVELS OF RESOURCE POTENTIAL 

H HIGH mineral resource potential is assigned to areas where geologic, geochemical, and geophysical characteristics indicate a geologic environment favorable for resource occurrence, where interpretations of data 

indicate a high degree of likelihood for resource accumulation, where data support mineral-deposit models 

indicating presence of resources, and where evidence indicates that mineral concentration has taken place. 

Assignment of high resource potential to an area requires some positive knowledge that mineral-forming processes 

have been active in at least part of the area. 

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

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

indicate reasonable likelihood for resource accumulation, and {or) where an application of mineral-deposit models 

indicates favorable ground for the specified type{s) of deposits. 

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

define a geologic environment in which the existence of resources is permissive. This broad category embraces 

areas with dispersed but insignificantly mineralized rock, as well as areas with little or no indication of having 

been mineralized. 

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

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

moderate, or high level of resource potential. 

LEVELS OF CERTAINn' 

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

8 Available information only suggests the level of mineral resource potential. 

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

D Avail~ble information clearly defines the level of mineral resource potential. 

r 

-J 

< 

i= 

z 

UJ 

1-

0 

c.. 

UJ 

u 

CE:: 

::J 

0 

V') 

UJ 

~ 

u.. 

0 

....J 

UJ 

> UJ 

-J 

A 

U/A 

UNKNOWN POTENTIAL 

Abstracted with minor modifications from: 

B c 

H/B H/C 

HIGH POTENTIAL HIGH POTENTIAL 

M/B M/C 

MODERATE POTENTIAL MODERATE POTENTIAL 

UB uc 

LOW POTENTIAL LOW POTENTIAL 

LEVEL OF CERTAINn' 

D 

H/D 

HIGH POTENTIAL 

M/D 

MODERATE POTENTIAL 

UD 

LOW POTENTIAL 

N/D 

NO POTENTIAL 

Taylor, R.B., and Steven, T.A., 1983, Definition of mineral resource potential: Economic Geology, v. 78, no. 6, p. 1268-1270. 

Taylor, R.B., Stoneman, R.J., and Marsh, S.P., 1984, An assessment of the mineral resource potential ofthe 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. 

J14 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 
RESOURCE/RESERVE CLASSIFICATION 

IDENTIFIED RESOURCES UNDISCOVERED RESOURCES 

Demonstrated Probability Range 

Inferred 

Measured Indicated Hypothetical Speculative 

I Inferred I 

ECONOMIC Reserves Reserves 

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

MARGINALLY Marginal Inferred 

Marginal ECONOMIC Reserves Reserves ____ J_ ____ 1------------ I I -~-

SUB- Demonstrated Inferred 

Subeconomic Subeconomic 

ECONOMIC Resources Resources I 

_l I 

Majorelementsofmineral resource classification, excluding reserve base and inferred reserve base. Modified from McKelvey, V.E., 1972, Mineral 

resource estimates and public policy: American Scientist, v. 60, p. 32-40; and U.S. Bureau of Mines and U.S. Geological Survey, 1980, Principles 

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

Mineral Resources of the Trigo Mountains Wilderness Study Area, Arizona J15 
GEOLOGIC TIME CHART 

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

EON ERA PERIOD EPOCH 

Quaternary 

Holocene 

Pleistocene 

Neogene Pliocene 

Cenozoic Subperiod Miocene 

Tertiary Paleogene Oligocene 

Subperiod Eocene 

Paleocene 

Cretaceous Late 

Early 

Late 

Mesozoic jurassic Middle 

Early 

Late 

Triassic Middle 

Early 

Permian Late 

Phanerozoic Early 

Late 

Pennsylvanian Middle 

Carboniferous Early Periods 

Mississippian Late 

Early 

Late 

Devonian Middle 

Paleozoic Early 

Late 

Silurian Middle 

Early 

Late 

Ordovician Middle 

Early 

Late 

Cambrian Middle 

Early 

Late Proterozoic 

Proterozoic Middle Proterozoic 

Early Proterozoic 

Late Archean 

Archean Middle Archean 

Early Archean 

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

pre-Archean2 

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

21nformal time term without specific rank. 

j16 Mineral Resources of Wilderness Study Areas: Southwestern and South-Central Arizona 

AGE ESTIMATES OF 

BOUNDARIES IN 

MILLION YEARS (Ma) 

0.010 

1.7 

5 

24 

38 

55 

66 

- 96 

138 

205 

-240 

290 

-330 

360 

410 

435 

500 

1

-570 

900 

1600 

2500 

3000 

3400 

4550 
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