Mineral Resources of the Jacumba (In-ko-pah) 

Wilderness Study Area, 

Imperial County, California

U.S. GEOLOGICAL SURVEY BULLETIN 1711-D

Chapter D

Mineral Resources of the

Jacumba (In-ko-pah) Wilderness Study Area,

Imperial County, California

By VICTORIA R. TODD, JAMES E. KILBURN,

DAVID E. DETRA, ANDREW GRISCOM, and FRED A. KRUSE

U.S. Geological Survey

EDWARD L McHUGH 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1711

MINERAL RESOURCES OF WILDERNESS STUDY AREAS:

SOUTHERN CALIFORNIA AND CALIFORNIA DESERT CONSERVATION AREA
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1987

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center, Box 25425

Denver, CO 80225

Library of Congress Cataioging-in-Publication Data

Mineral resources of the Jacumba (In-ko-pah) Wilderness 

Study Area, Imperial County, California.

U.S. Geological Survey Bulletin 1711-D 

Bibliography:

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

1. Mines and mineral resources California Jacumba (Inko-pah) Wilderness. 2. Jacumba (In-ko-pah) Wilderness 

(Calif.) I. Todd, Victoria R. (Victoria Roy) II. U.S. 

Geological Survey. III. Series.

QE75.B9 No. 1711-D 557.3 s 87-600129 

[TN24.C2] [553'.09794'99]
STUDIES RELATED TO WILDERNESS 

Bureau of Land Management Wilderness Study Areas

The Federal Land Policy and Management Act (Public Law 94-579, October 21, 

1976) requires the U.S. Geological Survey and the U.S. Bureau of Mines to conduct 

mineral surveys on certain areas to determine the mineral values, if any, that may be 

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

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

Jacumba (In-ko-pah) (CDCA-368) Wilderness Study Area, Imperial County, California.

CONTENTS

Summary Dl

Abstract 1

Character and setting 1

Identified mineral resources and resource potential 1 

Introduction 3

Area description 3

Previous and present investigations 3

Acknowledgments 3 

Appraisal of identified resources 5

History and production 5

Mineral deposits 5

Appraisal of mineral deposits 6 

Assessment of mineral resource potential 7

Geology 7

Geochemical studies 8

Geophysical studies 8

Conclusions 9 

References cited 9 

Appendixes

Definition of levels of mineral resource potential and certainty of 

assessment 13

Geologic time chart 14

FIGURES

1. Index map showing location of the Jacumba (In-ko-pah) Wilderness Study 

Area, Imperial County, California D2

2. Map showing mineral resource potential and simplified geology of the 

Jacumba (In-ko-pah) Wilderness Study Area 4

TABLE

1. Prospects in and adjacent to the Jacumba (In-ko-pah) Wilderness Study 

Area D15

MINERAL RESOURCES OF WILDERNESS STUDY AREAS:

SOUTHERN CALIFORNIA AND CALIFORNIA DESERT CONSERVATION AREA

Mineral Resources of the

Jacumba (In-ko-pah) Wilderness Study Area,

Imperial County, California

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

Andrew Griscom, and Fred A. Kruse 

U.S. Geological Survey

Edward L. McHugh 

U.S. Bureau of Mines

SUMMARY

Abstract

This report presents the results of a mineral 

survey of part of the Jacumba (In-ko-pah) Wilderness 

Study Area (CDCA-368) that was designated by the 

Bureau of Land Management as suitable for mineral 

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

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

area." The study area encompasses approximately 

28,443 acres in southwestern Imperial County, Calif. 

Field work for this report was carried out between 

1983 and 1985. Numerous mines, prospects, and claims 

are present in the study area, but none are being 

actively mined or prospected. Three sites inside the 

study area have identified limestone resources that 

total about 1.8 million tons. Four areas in the 

northwestern, northern, and eastern parts of the study 

area have high potential for undiscovered limestone 

resources. Two north-trending belts in the western 

and eastern parts of the study area have low resource 

potential for tungsten.

Character and Setting

The Jacumba (In-ko-pah) Wilderness Study Area 

is located between the Peninsular Ranges province on 

the west and the Yuha Desert on the east (fig. 1). The 

United States-Mexico border forms the south boundary 

of the study area. Much of the area is characterized 

by rugged topography. Maximum relief in the study 

area is about 3,400 ft. A semiarid climate and 

chapparal vegetation are found at the higher 

elevations, whereas an arid climate and desert plant 

communities are characteristic of lower areas.

The study area is underlain by Cretaceous (66 to 

138 million years before present (Ma); see appendix for 

geologic time chart) plutonic rocks of the Peninsular 

Ranges batholith (informal name). The batholith is 

composed of a variety of plutons that intruded 

metamorphosed Paleozoic and Mesozoic sedimentary 

and volcanic rocks. Neogene (24 to 1.7 Ma) marine and 

alluvial deposits unconformably overlie the uplifted 

and deeply eroded batholithic rocks in the study area, 

and faults of the Elsinore fault zone cut both the 

batholithic rocks and the sedimentary deposits. 

Evidence of mining or prospecting activity was found 

at 26 sites in, or just outside of, the Jacumba (In-kopah) Wilderness Study Area. In 1983, 106 claims were 

held in 13 claim groups. At that tim£, none of the 

claims or sites was being actively mined or 

prospected.

Identified Mineral Resources and Resource Potential

Geologic and geochemical studies, and an 

evaluation of known mineral occurrences indicate 

locally high potential for undiscovered limestone 

resources in three areas in the northern part of the 

study area and in one area in the eastern part. In 

addition, a northwest-trending belt in the western part 

of the study area and four irregular areas that define a 

north-trending belt in the eastern part have low 

potential for tungsten resources (fig. 2).

Metasedimentary strata of Paleozoic and 

Mesozoie(?) age that are present as steeply dipping 

tabular bodies, or screens, within granitic plutons in 

the study area locally contain abundant limestone 

(metamorphosed to marble). Known deposits in the 

study area contain identified resources of about 1.8

D1
RECOVERY PARACHUTE 

TEST RANGE

ANZA-BORREGO 

DESERT STATE PARK

IMPERIAL 

VALLEY

ar\d Arizona Eastern Railroad

MOUNTAINS 

Ocotillo

e Wells

JACUMBA (IN-KO-PAH)

WILDERNESS STUDY

AREA(CDCA-368)

YUHA DESERT

I I I I I

5 MILES

j

Figure 1. Index map showing location of the Jacumba (In-ko-pah) Wilderness Study Area, Imperial County, 

California.

D2
million tons of limestone, although economic 

conditions are unfavorable for the development of 

these small deposits at the present time. Abundant 

deposits of high-quality limestone are present in the 

Coyote Mountains to the north.

Tungsten-bearing skarn deposits are present 

between marble-bearing metasedimentary screens and 

surrounding plutons. These small, irregular, and 

sporadically mineralized skarn deposits are probably 

the source of widespread anomalous concentrations of 

tungsten, molybdenum, and tin, and localized gold, 

silver, bismuth, and zinc anomalies in stream 

sediments in the study area. Rock samples from 

several skarn deposits contain significant amounts of 

tungsten. Although known tungsten occurrences inside 

the study area are too small or low grade to be 

considered resources, undiscovered resources may 

exist. Thus, areas of skarn deposits and metasedimentary screens in the study area have low resource 

potential for tungsten. No potential for geothermal 

energy, uranium, or oil and gas resources was 

identified in the study area.

INTRODUCTION 

Area Description

The Jacumba (In-ko-pah) Wilderness Study Area 

(CDCA-368) occupies 28,443 acres in the southwest 

corner of Imperial County, Calif, (fig. 1). The study 

area is located in the southeastern part of the 

Jacumba Mountains, which forms the eastern faulted 

escarpment of the Peninsular Ranges immediately 

north of the international border with Mexico. The 

Jacumba Mountains rise above the Yuha Desert, which 

is the western part of the Imperial Valley, a broad 

alluvial valley that lies partly below sea level. The 

study area is roughly bisected by north-northwesttrending Davies Valley (fig. 1), which was eroded along 

a major fault zone that separates high-standing 

batholithic rocks on the west from low hills that 

expose metasedimentary rocks and Neogene volcanic 

and sedimentary rocks on the east. Other northtrending faults cross the eastern and western parts of 

the study area, and part of the Elsinore fault zone 

bounds the study area on the east (fig. 2). These faults 

have produced a rough stair-step topography in which 

fault-bounded blocks are progressively lower to the 

east. Elevations range from about 4,000 ft in the 

western part of the area to about 600 ft near the 

eastern front of the range. The climate of the region 

is arid, with rainfall ranging between 3 and 4 in. per 

year (Morton, 1977) and summer temperatures often 

exceeding 100 to 110 °F at lower elevations.

Access to the area is chiefly from California 

Highway 98, which extends south from Interstate 8 to 

Calexico. Several dirt roads maintained by the Bureau 

of Land Management lead southwest from California 

Highway 98 into the eastern part of the area. On the 

west side of the study area, a jeep trail leads from 

Jacumba Valley to the PK mine (fig. 2, No. 26).

Previous and Present Investigations

This mineral resource study is a joint effort by 

the U.S. Geological Survey and the U.S. Bureau of 

Mines. The history and philosophy of such joint 

mineral surveys of U.S. Bureau of Land Management 

Wilderness Study Areas were discussed by Beikman and 

others (1983). Mineral assessment methodology and 

terminology were discussed by Goudarzi (1984). See 

appendix for the definition of levels of mineral 

resource potential and certainty of assessment. 

Studies by the U.S. Geological Survey are designed to 

provide a reasonable scientific basis for assessing the 

potential for undiscovered mineral resources by 

determining geologic units and structures, possible 

environments of mineral deposition, presence of 

geochemical and geophysical anomalies, and applicable 

ore-deposit models. The U.S. Bureau of Mines 

evaluates identified resources at individual mines and 

known mineralized areas by collecting data on current 

and past mining activities and through field 

examination of mines, prospects, claims, and 

mineralized areas. Identified resources are classified 

according to the system described by U.S. Bureau of 

Mines and U.S. Geological Survey (1980).

The Jacumba (In-ko-pah) Wilderness Study Area 

includes parts of the In-ko-pah Gorge and Coyote Wells 

7.5-minute quadrangles. Published geologic maps by 

Brooks and Roberts (1954) and Morton (1977) describe 

the geology of the region. Unpublished mapping by 

T.W. Dibblee, Jr. (written commun., 1943) covers part 

of the Jscumba Mountains. The U.S. Geological 

Survey carried out field investigations in the study 

area between 1984 and 1985. The work included 

geologic mapping and whole-rock and stream-sediment 

sampling for geochemical analyses and radiometric 

dating. A total of 52 panned heavy-mineral 

concentrates from stream sediment samples were 

analyzed; the analytical data are given in Detra and 

Kilburn (1985).

The U.S. Bureau of Mines conducted a library 

search for information on mines and prospects in and 

adjacent to the study area. Mines and prospects in and 

near the study area were described by Sampson and 

Tucker (1942) and Morton (1977). These data were 

supplemented by information from U.S. Bureau of Land 

Management mining claim and land status records. 

Claim owners were contacted when possible. Field 

studies by U.S. Bureau of Mines personnel were carried 

out in the fall of 1983. Rock samples collected from 

outcrops, prospects, and mines were analyzed for a 

variety of elements by fire-assay, atomic absorption, 

colorimetr c, X-ray fluorescence, plasma 

spectrographic, and semiquantitative spectrographic 

methods. Detailed sample data are presented in 

McHugh (1985).

Acknowledgments

The authors gratefully acknowledge Sean 

Hagerty and Steve Nelson of the Bureau of Land

D3
32° 

45

116°05' 116°00'

Ancient shoreline / 

of 

Lake Cahuilla f

APPROXIMATE BOUNDARY 

OF

JACUMBA (IN-KO-PAH)

WILDERNESS STUDY

AREA (CDCA-368)

-' QTs

32°

40'

115°55'

I

YUHA 

DESERT

Gravel pit b x

H/D Ls

Qs

QTs___.

2 MILES

Figure 2. Mineral resource potential and simplified geology of the Jacumba (In-ko-pah) Wilderness Study Area, Imperial County, California.
Ls 

W

Ql

Qs 

QTs

Tj 

Tsm

Kip 

Kgm

pb

EXPLANATION

Area with high mineral resource potential Area with low mineral resource potential

Mine with recorded past production See table 1 for 

description

See appendix for definitions of mineral resource potential and 

levels of certainty

Commodities

Limestone 

Tungsten

Geologic map units

Lacustrine deposits of Lake Cahuilla (Quaternary) 

Surficial deposits (Quaternary) Sedimentary rocks (Quaternary and Tertiary) 

Jacumba Volcanics of Minch and Abbott (1973) (Tertiary) 

Split Mountain Formation of Woodard (1974) (Tertiary) 

Tonalite of La Posta (Cretaceous) Tonalite of Granite Mountain (Cretaceous) 

Prebatholithic metasedimentary rocks (Mesozoic and 

Paleozoic?)

Contact

Fault Dashed where approximate; dotted where concealed. 

Bar and ball on downthrown side

Geochemical sample site 

Prospect

Mines, prospects, and deposits See table 1 for 

descriptions. Underline indicates identified resource.

1 Mountain Spring limestone deposit 

Identified resource extends into 

study area

2 Myer Valley prospect3 Unnamed prospect4 Chain prospect

5 Unnamed prospect

6 White Christmas deposit (Bronko claims)7 Stone quarry

8 Unnamed9 Unnamed prospect

10 Easy Pickins prospect

11 White Hope limestone deposit12 Red Schist prospect

13 Weaver deposit

14 Hematite prospect15 Unnamed prospect

16 Unnamed prospect

17 Unnamed prospect18 Unnamed prospect

19 Unnamed prospect

20 Golden Alter prospect21 Unnamed prospect22 Roark mine (Graphite claim)

23 North star prospect

24 Gal-White limestone deposit

25 Unnamed prospect

26 PK mine (Elliott mine)

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Mineral Deposits

Two principal deposit types were examined by

the U.S. Bureauof Mines: (i) limestone (metamorphosed to marble) in screens consisting ofmetasedimentary rockssurrounded byplutonic and (2) bodies oftungsten-bearing skarnassociated with contacts between limestone-bearing screens and

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Diatomite attheWhite Christmas deposit (fig. 2,

No. 6) was first claimed in1941; no production isknown. Pyroclastic basaltfrom theWeaver deposit (fig. 2,No. 13) was mined duringthe 1950's foruse asroofing granulesnd decorative gravel (Morton, 1977,

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pegmatite dikes and scheelite-bearing skarn zone at the Mica Gemmine, 1.6miwest of thesudy area, were prospected between 1936 and1940 (Weber, 1963,

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other prospects inside thesudy area wre developed for tungsten during government subsidy programs between 1951 and1958. Originally prospected for gold(Morton, 1977, p. 96), the PKmine produced 596short ton units oftungsten trioxide (11,920 IbWO 3) during 1954 and 1955. TheRoark mine may have produced a small aount of tungsten. Coarse muscovite in
plutonic rocks. Resources were identified on the basis 

of geologic evidence and sample data.

Limestone resources were identified at three 

sites in the study area. The Mountain Spring and White 

Hope limestone deposits are in the northern part of the 

study area (fig. 2, Nos. 1 and 11), and the Cal-White 

deposit is in the southeastern part (fig. 2, No. 24). The 

limestone is present as irregular lenses that dip 

moderately to steeply. The thickness of the lenses 

probably varies where concealed, and overburden 

increases rapidly with depth as a result of the steep 

dips. Resource projections are limited because of 

these factors. Geochemical analyses of samples from 

the deposits (table 1) suggest that the limestone is 

generally suitable for portland cement and agricultural 

uses.

Limestone was opencut mined at the Mountain 

Spring deposit just outside the study area. The main 

lens contains about 210,000 tons of limestone and 

extends inside the study area. At the White Hope 

deposit, two lenses of limestone are aligned parallel to 

foliation in enclosing gneissic tonalite. One lens 

contains approximately 175,000 tons of limestone. The 

second lens is not well exposed, but is estimated to 

contain 170,000 tons of limestone. At the Cal-White 

limestone deposit, lenses of limestone are intercalated 

with metasedimentary rocks and gneissic tonalite. 

This deposit contains an estimated 1.4 million tons of 

limestone resources.

Two prospects and an inactive mine in the east 

half of the study area (fig. 2, Nos. 18, 22, and 23) may 

have tungsten resources associated with skarn 

deposits. The diopside-tremolite-garnet skarn is in 

thinly laminated limestone and biotite schist along 

contacts with tonalite. Irregular quartz-tourmaline 

pegmatite dikes transect both the host rocks and the 

skarn. The calcium tungstate mineral scheelite 

(CaWO^) is disseminated in the skarn zones as blebs 

and in stringers. Although the scheelite-bearing zones 

are too sparsely mineralized or too small to constitute 

resources, additional sampling in unexposed parts of 

the zones might reveal tungsten resources.

Two scheelite-bearing skarn zones are exposed at 

the unnamed prospect 1 mi east of Davies Valley (fig. 

2, No. 18). A limestone lens at the Roark mine 

(Graphite claim, fig. 2, No. 22) is poorly exposed but is 

at least 25 ft thick; a weakly silica ted zone in the bed 

contains disseminated scheelite (Morton, 1977, p. 96). 

Segments of six skarn zones are exposed in workings 

and outcrops at the North Star prospect (fig. 2, No. 

23). One chip sample from a 3.2-ft-thick skarn zone 

contained 0.02 oz/ton gold and 1.66 percent tungsten 

trioxide (WOg); an 8-ft-thick shear zone in schist is 

enriched in manganese oxides. Samples from other 

workings and outcrops suggested only minor 

mineralization.

The PK mine (Elliott mine), located immediately 

outside the study area about 1,000 ft north of the 

international border (fig. 2, No. 26), is in a small roof 

pendant in tonalite that contains an irregular tungstenbearing skarn body. Resource estimates based upon 

sampling of mineralized skarn at the 3,990, 4,000, and 

4,040 ft levels of the mine and upon an assumed 

downdip width of the skarn body of 60 ft indicate 

30,000 tons of low-grade ore with an average grade of 

0.19 percent WO3 . One 9,000-ton segment of the

deposit also contains 0.06 oz/ton gold. At the 

estimated average 1985 price of $68/t WO3 (metric 

ton unit of WO3, which contains 7.93 kg W) or $3.89/lb 

W (U.S. Bureau of Mines, 1986, p. 168), the value of 

tungsten ore in the deposit is $11.71/ton. The average 

grade of the deposit is low, and the roof pendant that 

contains the deposit is small. Underlying tonalite is 

exposed in the lower workings, ruling out downward 

extension of the mineralized zone. No associated 

tungsten occurrences are exposed in the study area.

The White Christmas diatomite deposit (Bronko 

claims) is located about 3 mi southwest of Ocotillo, 

just outside the study area (fig. 2, No. 6). Diatomite 

beds as much as 12 ft thick are present in Quaternary 

lake sediments in two erosional remnants. Two 

diatomite samples collected by the Bureau of Mines 

contained abundant sand, silt, and broken diatom tests, 

which limits use of the material for most filter 

applications. Loomis (1965, p. 4) estimated 3,000 to 

5,000 tons of material averaging 79 to 80 percent SiO2 

in the diatomite deposits, and possibly 30,000 to 50,000 

tons of similar material covered by unconsolidated 

sand near the deposit. No identified diatomite 

resources are inside the study area.

Appraisal of Mineral Deposits

The Cal-White, Mountain Spring, and White Hope 

limestone deposits contain indicated limestone 

resources of about 1.4 million, 210,000, and 175,000 

tons, respectively. The White Hope and Cal-White 

deposits are at the range front along the edge of the 

study area, making access relatively simple. The 

Mountain Spring deposit is in In-Ko-Pah Gorge along 

Interstate 8, where access might require construction 

of an interchange. Large, partially developed deposits 

of high-quality limestone are present 6 mi north of 

Ocotillo in the Coyote Mountains, where Bowen and 

others (1973) describe three deposits, each with more 

than 20 million tons of limestone resources. These 

deposits would probably be developed before the 

smaller deposits in the study area.

Known tungsten occurrences inside the study 

area are too small or low grade to be considered 

resources. The character of the known mineralized 

skarn bodies suggests that other such occurrences, if 

present, would be small, irregular, and only 

sporadically mineralized. Low prices have resulted in 

periodic, temporary closing of domestic tungsten 

mines in recent years. Tungsten is a strategic and 

critical commodity, and net reliance on imports in 

1985 was about 68 percent. Although U.S. demand for 

tungsten is expected to increase at an average annual 

rate of 16 percent through 1990 (U.S. Bureau of Mines, 

1986), development of such small deposits as might be 

expected to occur in the study area is not probable in 

the foreseeable future.

Diatomite deposits do not extend into the study 

area, and no large-scale development of the White 

Christmas deposit is expected. Very large deposits of 

relatively pure diatomite in other parts of California 

and in Nevada dominate domestic markets and provide 

exports to foreign markets.

Sand and gravel in alluvial fan deposits are 

mainly outside the study area along the east side of

D6
the range. In the northern part of the area, fan 

deposits mantle bedrock both inside and outside the 

study area. Development of sand and gravel deposits 

inside the study area is unlikely because similar 

deposits are available closer to major population 

centers.

ASSESSMENT OF MINERAL RESOURCE POTENTIAL

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

Andrew Griscom and Fred A. Kruse 

U.S. Geological Survey

Geology

The southern part of the Jacumba Mountains is 

underlain by relatively undeformed plutons that are 

considered to belong to a late-tectonic to posttectonic 

intrusive sequence in the Peninsular Ranges batholith 

(Todd and Shaw, 1979). Similar plutons to the west in 

San Diego County have uranium-lead zircon ages that 

range from 95 to 100 Ma (Silver and others, 1979; 

Clinkenbeard and others, 1986). Thus, the plutons in 

the southern Jacumba Mountains are tentatively 

considered to be middle Cretaceous in age. Two 

plutons of this intrusive sequence crop out in the 

Jacumba (In-ko-pah) Wilderness Study Area. Strongly 

foliated late-tectonic tonalite forms most of the 

crystalline bedrock east of Davies Valley, whereas 

weakly foliated to massive posttectonic tonalite 

underlies much of the block west of Davies Valley (fig. 

2). Both plutons range in composition from 

hornblende-biotite tonalite to biotite granodiorite; the 

western pluton also includes leucocratic muscovitebiotite granodiorite. Pegmatite dikes are abundant in 

both western and eastern tonalite plutons.

Screens and inclusions of wallrocks in the study 

area consist of high-grade metasedimentary rocks of 

unknown age including metaquartzite, semipelitic 

schist, and marble, the last containing the limestone 

deposits mentioned in an earlier section of this 

report. These rocks may include two units deepwater arkosic and graywacke sandstones and shale 

(turbidites) with scarce interbedded carbonate rocks 

that crop out in San Diego County to the west (the 

Julian Schist) and a continental shelf sequence of 

quartz-rich sandstone, shale, and abundant carbonate 

that is present in the eastern part of the batholith. 

The Julian Schist is presumed to be Triassic and (or) 

Jurassic in age (Donnelly, 1934), while the eastern 

prebatholithic rocks are at least in part Paleozoic 

(Miller and Dockum, 1983). Therefore, the 

prebatholithic rocks in the study area are tentatively 

considered to be Paleozoic and Mesozoic(?) in age.

The wallrock screens contain concordant fine- to 

medium-grained granitic dikes that emanate from the 

surrounding plutons. Pegmatite dikes commonly were 

emplaced into contact zones between screens and 

tonalite plutons. Skarns developed locally where 

limestone was intruded by tonalite and (or) by granitic 

dikes. Retrograde alteration and oxidation of highgrade skarn minerals such as garnet and diopside led to 

local concentrations of epidote, clinozoisite, 

tremolite-actinolite, chlorite, hematite, limonite, and

calcite. Skarn minerals form irregular scattered 

lenses as much as 300 ft across that commonly are 

associated with isoclinally folded metasedimentary 

rocks. Scheelite occurs locally in skarn as 

disseminated blebs, stringers, and pods locally as much 

as 2 ft long but typically less than a few inches long.

The eroded crystalline basement of the southern 

Jacumba Mountains is overlain on the north and east 

by Miocene through Pleistocene sedimentary and 

volcanic deposits; remnants of Miocene deposits 

overlie the crystalline rocks on both sides of Davies 

Valley (fig. 2). These deposits are remnants of a thick 

(locally 20,000 ft) sequence of terrestrial and marine 

sediments and intercalated volcanic rocks deposited in, 

and marginal to, the Salton Trough. The Salton Trough 

is a complex rift valley that has been tectonically 

active since Miocene time (Gibson and others, 1984).

The Miocene Split Mountain Formation of 

Woodard (1974) crops out mainly east of Davies Valley 

(fig. 2). Patches of this formation are also preserved 

beneath the Jacumba Volcanics of Minch and Abbott 

(1973) in fault blocks west of Davies Valley. The Split 

Mountain Formation formed as a thickening wedge of 

coalescing alluvial fans during the early phase of uplift 

of the Peninsular ranges (Gibson and others, 1984). 

The formation underlies andesitic pyroclastic rocks in 

the northeast part of the study area. These volcanic 

rocks locally form a very thin, extensively faulted 

veneer over the Split Mountain Formation and possibly 

also over Quaternary sediments. The volcanic rocks 

may have been partly or wholly displaced by 

posteruptive gravitational gliding or large-scale 

landsliding during uplift of the Peninsular Ranges.

The Salton Trough was flooded by marine waters 

in early Pliocene time (Bell-Countryman, 1984). As 

the basin continued to subside, a shoaling series of 

alluvial fan, shoreline, and deep-water marine deposits 

was rapidly deposited conformably on the Split 

Mountain Formation and volcanic rocks. Gradationally 

overlying these predominantly marine deposits is a 

sequence of Pliocene and Pleistocene brackish-water 

to lacustrine sediments with marine interbeds, the 

Palm Spring Formation. The Imperial and Palm Spring 

Formations crop out east of the study area in the 

down-faulted Yuha Desert block, and in the badlands 

that lie north of the study area between the Coyote 

Mountains and the northern Jacumba Mountains. An 

undated sequence of alluvial fan sands and gravels 

overlies the Jacumba Volcanics in the northern part of 

the study area; these sediments may correlate with the 

Pliocene and younger Imperial and Palm Spring 

Formations, or may consist of Miocene strata.

The southern Jacumba Mountains probably began 

to rise as an uplifted fault block sometime during 

Pliocene time and then continued to be elevated during 

Quaternary time by north- and north-northwesttrending high-angle faults in Davies Valley and along 

the east flank of the range. A mosaic of north- and 

east-trending dip-slip faults in the northeastern part of 

the study area has produced a triangular graben 

composed chiefly of Miocene rocks; this graben is 

bounded on the west by a fault, or faults, under the 

alluvium of Davies Valley (fig. 2). The Laguna Salada 

fault, which lies about I mi east of the southern 

Jacumba Mountains, cuts Quaternary alluvium 

suggesting that the range-bounding faults may still be

D7
active. All of the above faults are part of the active 

Elsinore fault zone, which has undergone chiefly dipslip motion south of 33° N. lat (Todd and Hoggatt, 

1979).

The most recent geologic feature adjacent to the 

southern Jacumba Mountains is the high shoreline of 

ancient Lake Cahuilla, which lies less than 1 mi east of 

the range. The lake originated by periodic flooding 

and overflow of the ancestral Colorado River into the 

Salton Trough (Morton, 1977). Although it probably 

existed as recently as several hundred years ago, the 

lake has probably formed and dried up repeatedly since 

at least late Pleistocene time. Fine-grained lake 

sediments in the northern part of the study area (the 

White Christmas diatomite deposit) (fig. 2, No. 6) and 

patchy lacustrine sediments east of the study area 

suggest that the earliest lakes may have extended 

westward into the valleys of the southern Jacumba 

Mountains (Morton, 1977).

Geochemical Studies

A reconnaissance geochemical survey of stream 

sediments was conducted in the study area in 1984 by 

the U.S. Geological Survey. Heavy-mineral 

concentrates were derived from fifty-two samples 

collected from streams that drain areas ranging from a 

fraction to several square miles within the study 

area. Each sample was sieved through a minus-80- 

mesh screen to remove the coarsest material and 

divided into light and heavy fractions using a 

bromoform mineral separation technique. The heavy 

fraction was further separated using a magnetic 

separator. All samples were analyzed for 30 elements 

by a six-step semiquantitative emission spectrographic 

method (Grimes and Marranzino, 1968). A complete 

listing of the analytical results can be found in Detra 

and Kilburn (1985). In addition, concentrates were 

examined optically for mineral identification.

Spectrographic analysis of the concentrates 

revealed widespread anomalous concentrations of 

tungsten and tin as well as numerous anomalous 

concentrations of barium, lead, and molybdenum. In 

addition, isolated local anomalous concentrations of 

gold, silver, bismuth, zinc, and strontium were noted. 

Mineralogically, the concentrates are characterized by 

abundant barite and scheelite coupled with an apparent 

absence of ore-forming sulfide and oxide minerals. 

Optical studies also identified gold at one sample 

site. Contaminating lead shot observed in the samples 

coincides with five of the nine anomalous lead 

concentrations.

The ubiquitous scheelite and accompanying 

tungsten anomalies are undoubtedly mineralogical and 

chemical manifestations of the numerous skarn 

deposits that are present in the study area. Anomalous 

concentrations of one or more of the elements 

molybdenum, tin, gold, silver, bismuth, and zinc, often 

constituents of tungsten-bearing skarns, are associated 

with the tungsten anomalies throughout the eastern 

part of the study area and in the western part near the 

PK (Elliott) mine. A skarn-related multiple-element 

anomaly is found in a minor drainage below the PK 

mine (Elliott mine) in the southwest corner of the 

study area. The anomalous trace-element suite here

included tungsten, tin, molybdenum, bismuth, gold, and 

silver. The sample contained a substantial amount of 

scheelite (50 percent) and several gold flakes. The PK 

mine, initially a gold prospect but later mined for 

tungsten, is situated within an irregular skarn body 

near the contact between a metasedimentary roof 

pendant and enclosing tonalite (Morton, 1977).

The tin anomalies appear to be a regional 

geochemical signature of the late-tectonic to 

posttec tonic intrusive sequence. Although no 

mineralogic or lithologic host was determined, it is 

possible that much of the tin may be derived from an 

accessory mineral or minerals disseminated throughout 

these granitic rocks. Additionally, skarns may have 

contributed in some measure to the tin anomalies.

The source of the barium and barite anomalies is 

uncertain. Although barite was identified as a minor 

component in most of the concentrates, the majority 

of barium anomalies are restricted to the region east 

of Davies Valley. This may represent structural 

control by the numerous faults that bound the valley 

and the eastern range front and (or) lithological 

control by the Miocene sedimentary rocks that occur 

mainly east of Davies Valley. Anomalous 

concentrations of strontium and barium are present 

jointly near the northeast boundary of the study area; 

the strontium may be a constituent of barite, having 

replaced barium in a solid solution series.

Most, and perhaps all, of the anomalous lead 

concentrations can be attributed to contamination 

rather than natural sources. Evidence includes lead 

shot fragments found in over half the anomalous 

samples and the failure to identify a lead-related 

mineral in any of the concentrates.

Geophysical Studies

Three aeromagnetic and gamma-ray 

spectrometer profiles (High Life Helicopters, Inc., and 

QEB, Inc., 1980) cross the study area. The data were 

collected along east-west and north-south traverses by 

helicopter at a height of 400 ft above ground. The 

magnetic data are nearly featureless because none of 

the rocks within the study area are particularly 

magnetic, except for some weakly magnetic volcanic 

rocks. The radioactivity data for these profiles show 

no anomalous amounts of uranium or thorium within, 

the study area.

Gravity data available for the study area (Oliver 

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

only of about 15 stations and do not contribute 

significantly to the mineral resource appraisal of the 

area.

The limonite mineral group has unique spectral 

reflectance characteristics that can be detected on 

Landsat images using a color-ratio compositing 

technique described by Rowan and others (1974). This 

technique was used to map areas of limonite in the 

Jacumba (In-ko-pah) Wilderness Study Area. Limonite 

occurrences in exposed bedrock are considered 

anomalous and may indicate the presence of 

hydrothermal alteration. The largest limonite 

occurrence is in the southeast corner of the study area 

and covers roughly 1 mi . This and several other 

limonite anomalies in the area seem to be associated

D8
with faulted parts of the tonalite of Granite Mountain 

(fig. 2). These areas are favorable for the occurrence 

of hydrothermally altered rocks. Scattered limonite 

occurrences within the study area coincide closely 

with the eastern of two zones of low tungsten resource 

potential (fig. 2) and, to a lesser extent, with the 

western zone. However, these occurrences are small 

and their areal extent does not indicate a large 

hydrothermal system if indeed they are associated 

with hydrothermally altered rocks. Other limonite 

occurrences are associated with volcanic rocks that 

also may be hydrothermally altered.

Conclusions

Geologic studies, geochemical sampling, 

examination of prospects and mines, and a review of 

the history of mine production in and near the 

Jacumba (In-ko-pah) Wilderness Study Area indicate 

that the study area contains two main types of mineral 

occurrences, both associated with pre-Cenozoic 

crystalline rocks. Metasedimentary strata of 

Paleozoic(?) age that are present as screens within 

tonalite plutons locally contain abundant limestone. 

Limestone resources were identified at three sites 

within the study area. The study area has a high 

potential, certainty level D, in four areas for 

additional low- to moderate-tonnage limestone 

resources in and near these deposits (fig. 2). See 

appendix for definition of levels of mineral resource 

potential and certainty of assessment.

The second type of mineral occurrence in the 

study area consists of relatively abundant skarn 

deposits. Rock samples from four localities in the 

study area contain significant amounts of tungsten and 

minor amounts of gold. Samples from the PK mine 

(Elliott mine) outside the study area contain tungsten, 

gold, and silver. Widespread occurrences of the 

tungsten mineral scheelite in stream-sediment 

concentrates in the study area probably originated in 

skarn deposits. Local anomalous concentrations of 

molybdenum, tin, gold, silver, bismuth, and zinc also 

probably are derived from tungsten-bearing skarns. No 

deposits of ore minerals containing these elements 

were found in the study area.

The parts of the study area that are underlain by 

known skarn deposits and (or) metasedimentary screens 

have low resource potential, certainty level C, for 

tungsten in skarns of metasomatic origin (fig. 2). 

Known tungsten occurrences inside the study area are 

too small or low-grade to be considered resources, but 

the widespread development of skarns in the study 

area indicates that other skarn bodies may be present 

at depth. Geologic evidence suggests that such 

occurrences will be small, irregular, and only 

sporadically mineralized. No potential for other 

mineral resources or for geothermal energy, uranium, 

or oil and gas resources was determined in the study 

area.

REFERENCES CITED

Beikman, H.M., Hinkle, M.E., Frieders, Twila, Marcus, 

S.M., and Edward, J.R., 1983, Mineral surveys by 

the Geological Survey and the Bureau of Mines of

Bureau of Land Management Wilderness Study 

Areas: U.S. Geological Survey Circular 901, 28

PBell-Countryman, Pat, 1984, Environments of 

deposition of the Pliocene Imperial Formation, 

Coyote Mountains, southwest Salton trough, in 

Rigsby, C.A., ed., The Imperial Basin tectonics, 

sedimentation, and thermal aspects: Pacific 

Section, Society of Economic Paleontologists and 

Mineralogists, v. 40, p. 45-70.

Bowen, O.E., Gray, C.H., Jr., and Evans, J.R., 1973, 

The mineral economics of the carbonate rocks, in 

Bowen, O. E., ed., Limestone and dolomite 

resources of California: California Division of 

Mines and Geology Bulletin 194, p. 13-60.

Brooks, Bay lor, and Roberts, Ellis, 1954, Geology of 

the Jacumba area, San Diego and Imperial 

Counties, in Jahns, R.H., ed., Geology of 

southern California: California Division of Mines 

and Geology Bulletin 170, map sheet 23, scale 

1:62,500.

Clinkenbeard, J.P., Walawender, M.J., Parrish, K.E., 

Wardlaw, M.S., and Smith, B., 1986, The 

geochemical and isotopic composition of the La 

Posta granodiorite, San Diego County, California 

[abs.]: Geological Society of America Abstracts 

with Programs, v. 18, no. 2, p. 95.

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

and sample locality maps of heavy-mineral 

concentrate samples from the Jacumba (In-kopah) (CDCA-368) Wilderness Study Area, 

Imperial County, California: U.S. Geological 

Survey Open-File Report 85-272.

Donnelly, Maurice, 1934, Geology and mineral deposits 

of the Julian district, San Diego County, 

California: California Journal of Mines and 

Geology, v. 30, no. 4, p. 331-370.

Gibson, L.M., Malinconico, L.L., Downs, T., and 

Johnson, N.M., 1984, Structural implications of 

gravity data from the Vallecito-Fish Creek 

Basin, western Imperial County, California, in 

Rigsby, C.A., ed., The Imperial Basin tectonics, 

sedimentation, and thermal aspects: Pacific 

Section, Society of Economic Paleontologists and 

Mineralogists, v. 40, p. 15-29.

Goudarzi, G.H., 1984, Guide to preparation of mineral 

survey reports on public lands: U.S. Geological 

Survey Open-File Report 84-787, 51 p.

Grimes, D.J., and Marranzino, A.P., 1968, Directcurrent arc and alternating-current spark 

emission spectrographic field methods for the 

semiquantitative analysis of geologic materials: 

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

High Life Helicopters, Inc., and QEB, Inc., 1980, 

Airborne gamma-ray spectrometer and 

magnetometer survey, San Diego quadrangle*, 

California [ 1° by 2 ] sheet; National Uranium" 

Resource Evaluation program, GJBX-214 (80) ; 

Grand Junction, Colo., U.S. Department of 

Energy, variously paged.

Logan, C.A., 1947, Limestone in California: California 

Journal of Mines and Geology, v. 43, p. 175-357.

Loomis, T.H.W., 1965, Mineral patent application of 

Andrada Desert Enterprises, Inc., for the White 

Christmas, White Christmas No. 2, and Le 

Plateau placer mining claims: U.S. Bureau of

09
Land Management Mineral Report Ser. No. R 

02425, 5 p.

McHugh, E.L., 1985, Mineral resources of the Jacumba 

Wilderness Study Area, Imperial County, 

California: U.S. Bureau of Mines Open-File 

Report MLA 55-85, 27 p.

Miller, R.H., and Dockum, M.S., 1983, Ordovician 

conodonts from metamorphosed carbonates of 

the Salton Trough, California: Geology, v. 11, p. 

410-412.

Minch, J.A., and Abbott, P.L., 1973, Post batholithic 

geology of the Jacumba area, southeastern San 

Diego County, California: San Diego Society of 

Natural History, Transactions, v. 17, no. 11, p. 

129-135.

Morton, P.K., 1977, Geology and mineral resources of 

Imperial County, California: California Division 

of Mines and Geology County Report 7, 104 p.

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

S.L., Hanna, W.F., Griscom, A., Beyer, L.A., and 

Silver, E.A., 1980, Gravity map of California and 

its continental margin: California Division of 

Mines and Geology, scale 1:750,000, 2 sheets.

Roberts, C.W., Jachens, R.C., and Oliver, H.W., 1981, 

Preliminary isostatic residual gravity map of 

California: U.S. Geological Survey Open-File 

Report 81-573, scale 1:750,000, 5 sheets.

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

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

Discrimination of rock types and detection of 

hydrothermally altered areas in south-central 

Nevada by the use of computer-enhanced ERTS 

images: U.S. Geological Survey Professional 

Paper 883, 35 p.

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

resources of Imperial County: California Journal 

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

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

Some petrological, geochemical and 

geochronological observations of the Peninsular 

Ranges batholith near the international border of 

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

Todd, V.R., eds., Mesozoic Crystalline Rocks: 

Geological Society of America Annual Meeting, 

San Diego, Calif., 1979, Guidebook, p. 83-110.

Todd, V.R., and Hoggatt, W.C., 1979, Vertical 

tectonics in the Elsinore fault zone south of 

33°7'30" [abs.]: Geological Society of America 

Abstracts with Programs, v. 11, p. 528.

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

metamorphic, and intrusive framework of the 

Peninsular Ranges batholith in southern San 

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

Todd, V.R., eds., Mesozoic Crystalline Rocks: 

Geological Society of America Annual Meeting, 

San Diego, Calif., 1979, Guidebook, p. 177-231.

U.S. Bureau of Mines, 1986, Mineral Commodity 

Summaries 1986: U.S. Bureau of Mines, 185 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.

Weber, F.H., Jr., 1963, Geology and mineral resources 

of San Diego County: California Division of 

Mines and Geology County Report 3, 309 p.

Woodard, G.D., 1974, Redefinition of Cenozoic 

stratigraphic column in Split Mountain Gorge, 

Imperial Valley, California: American 

Association of Petroleum Geologists Bulletin, v. 

58, no. 3, p. 521-526.

D10
APPENDIXES

Definition of levels of mineral resource potential and 

certainty of assessment

Definitions of Mineral Resource Potential

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

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

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

few or no indications of having been mineralized.

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

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

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

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

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

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

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

models indicating presence of resources, and where evidence indicates that mineral concentration has 

taken place. Assignment of high resource potential to an area requires some positive knowledge that 

mineral-forming processes have been active in at least part of the area.

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

moderate, or high levels of resource potential.

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

Levels of Certainty

Z

LU

LU 

CJ 

DC

O

tt>

LU 

DC

O

U/A

UNKNOWN

POTENTIAL

H/B

HIGH POTENTIAL

M/B 

MODERATE POTENTIAL

L/B

LOW

POTENTIAL

H/C

HIGH POTENTIAL

M/C 

MODERATE POTENTIAL

L/C

LOW

POTENTIAL

H/D

HIGH POTENTIAL

M/D 

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

B C 

LEVEL OF CERTAINTY

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

B. Available information suggests the level of mineral resource potential.

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

D. Available information clearly defines the level of mineral resource potential.

Abstracted with minor modifications from:

Taylor, R. B., and Steven, T. A., 1983, Definition of mineral resource potential: Economic Geology,

v. 78, no. 6, p. 1268-1270. 

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

of the San Isabel National Forest, south-central Colorado: U.S. Geological Survey Bulletin 1638, p.

40^2. 

Goudarzi, G. H., compiler, 1984, Guide to preparation of mineral survey reports on public lands: U.S.

Geological Survey Open-File Report 84-0787, p. 7, 8.

D13
GEOLOGIC TIME CHART 

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

EON

Phanerozoic

Proterozoic

Archean 

pre -Arc

ERA

Cenozoic

Mesozoic

Paleozoic

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean 

;hean 2

PERIOD

Quaternary

Tertiary

Neogene 

Subperiod

Paleogene 

Subperiod

Cretaceous

Jurassic

Triassic

Permian

Carboniferous 

Periods

Pennsylvanian

Mississippian

Devonian

Silurian

Ordovician

Cambrian

 -(3800?)- -

EPOCH

Holocene

Pleistocene

Pliocene

Miocene

Oligocene

Eocene

Paleocene

Late 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

Late 

Middle 

Early

 _

AGE ESTIMATES 

OF BOUNDARIES 

(in Ma)

- 0.010 

- 1.7

* » A

JO

*J\J

_ cc

- 96 

 no

1

one

- ~240

 9QO

oou 

ocn

- 410

 d?R too

- 500

_ ^, ~D/UC"7/"\1

 . Qnn

- 1600

£*j\j\j

OUVJU

- 3400

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

informal time term without specific rank.

D14
TABLE 1. Mines, prospects, and deposits in and adjacent to theJacumba (In-ko-pah) Wilderness Study Area 

[*, outside study area]

Map No. Workings and

(fig. 2) Name Summary production ______ ____Sample and resou

1* Mountain Spring Coarsely crystalline, white to gray lime- An open cut in limestone Two samples of limestone 

limestone deposit stone in a 200-ft-wide, 2,500-ft-long about 70 ft across and 35 0.60 pet AljO^, 51.8 pe

steeply dipping tabular body, or screen ft high. Three small Fe2 02, 1-6 pet Mgo, les

composed of metasedimentary rocks in tona- prospect pits in skarn ^25' atu' "*^ P ct ^ ' ^2" 

lite. Screen also contains biotite schist zones. A few hundred tons from skarn zones contai

and narrow pegmatite dikes along bedding. of limestone mined in Lens contains 210,000-t

Limestone forms 1,600-ft-long lens as 1940 and 1959 (Morton, 1977; subeconomic limestone r

much as 30 ft thick that strikes p. 66). 

N. 65° W. and dips 45° SW. Limestone 

contains small inclusions of schist and 

some disseminated graphite. Irregular pods 

of garnet-epidote-quartz skarn along 

margins of limestone lens; pods are 

as much as 5 ft thick and 15 ft long.

2 Myer Valley Banded epidote-diopside-garnet skarn Three small prospect pits and Four samples of skarn co

prospect along contacts between small marble and several bulldozer cuts. significant mineral val

schist screens and tonalite. Screens 

are as much as 240 ft long and 80 ft wide. 

Pegmatite dikes and sills common.

3 Unnamed prospect Irregular veins of coarsely crystalline One 4-ft-deep, 5-ft-square Veins are too impure and 

white calcite in deeply weathered pit along road. fractured to be calcite 

tonalite. Veins are as thick as 25 in. source; one sample cont

and strike N. 55° E., dipping steeply significant mineral val

northwest.

4 Chain prospect Quartz-garnet-diopside skarn is in biotite Four shallow pits in One of four samples of s

schist and marble near contacts with skarn zones in area 0.01 pet WO^; remainder 

tonalite. Skarn in irregular, scat- 1,000 ft across. no significant mineral 

tered lenses as much as 300 ft across.

5 Unnamed prospect A garnet-quartz-diopside skarn zone lies A 37-ft-long inclined shaft Three samples from skarn 

along bedding in marble. Zone was driven S. 56° W. and contained no significan

exposed 40 ft downdip and as thick down at 37°. values.

as 4 ft; parts heavily limonitestained. Ultraviolet light disclosed no

scheelite.

6* White Christmas Diatomite beds as much as 12 ft thick in The diatomite beds are ex- Samples of diatomite wer

deposit (Bronko Quaternary (?) lake sediments. Diato- posed along stream-cut T. Loomis in 1965 for p

claims) mite mixed with volcanic ash and with channels, and in several cation: 7 ft channel s

sand- and silt-sized grains of quartz, pits and trenches. southern exposure conta

feldspar, and biotite. Diatomite beds rest Si02 - Clay in Quaterna

on older alluvium, including fanglomerate also sampled by Loomis 

and montmorillonite clay, and over- but found to be unsuita

lain by unconsolidated sand. Beds drilling mud or sealing 

flat-lying and exposed in two erosional because of poor gelling 

remnants. Northern exposure 150 ft and excessive sand cont

long and as much as 60 ft wide; southern diatomite samples colle

exposure is 200 ft long and as much as 50 U.S. Bureau of Mines in 

ft wide. high content of sand, s

	tests, which limits use 

	for most filter applica

7* Stone quarry Intrusive rock of tonalite composition One opencut, several vats Claimants proposed to sc

has mantle of decomposed rock from for treatment of decomposed micaceous, decomposed i

0 to 5 ft thick. material. Apparently only remove coarse particles 

material sufficient for biotite-chlorite-silt 

testing was removed. drywall and paint indus

treatment would greatly 

of product (Loomis, 196

samples were taken.

o_JL 

O1
o_JL

o> TABLE 1. Mines, prospects, and deposits in and adjacent to the Jacumba (In-ko-pah) Wilderness Study Area-Continued

Map No. 

(fig. 2) Summary

Workings and 

production Sample and resour

10

Unnamed prospect

Easy Pickins 

prospect

Fracture zone in volcanic mudflow debris 

strikes N. 15° E. and dips 60° NW.

Fragments of chalcedony and common opal 

in andesite tuff breccia.

Area of unconsolidated Quaternary alluvium, mainly sandy gravel, listed as 

gold prospect by Morton (1977, p. 51).

None (prospect shown on 

In-ko-pah Gorge 1:24,000 

sheet not found. Fracture 

zone at location was sampled).

One pit and one 3-ft-wide, 

10-ft-long, 1.5-ft-deep 

trench.

One 15-ft-deep shaft, caved

One chip sample contained 

mineral values.

One chip sample contained 

mineral values.

No minerals of economic v

no samples were collecte

White Hope lime- Two parallel beds of coarse-grained white 

stone deposit gray limestone strike N. 10°- 35° W. and 

dip 35°-55° SW. along foliation in 

gneissic tonalite. One bed 8 to 21 

ft thick, averaging about 14 ft, and 

exposed along strike for about 1,500 ft. 

Second bed 300 to 400 ft to the east, 

about 20 ft thick, and exposed for 

1,000 ft along strike. Limestone 

contains sparsely disseminated graphite. 

Granitic sills intrude limestone 

in places and follow segments of beds.

One small opencut Four samples collected fr

sure averaged 0.30 pet A

CaO, 0.10 pet Fe2 0 3 , 0.7

less than 0.02 pet P2 05 , 

SiO, Western bed conta

ton indicated limestone 

eastern bed inferred to 

ton inferred limestone r

Red Schist 

prospect

Weaver deposit

14 Hematite prospect

15 Unnamed prospect

Intensely sheared and folded biotite 

schist pervasively hematite-stained. 

Schist bounded on south and west 

by tonalite and overlain by andesitic 

volcanic rocks and alluvium on 

north and east. Schist contains bands 

of marble and veins of quartz. Pegmatite 

dikes to 10 in. thick contain coarse 

garnet, apatite, and sphene crystals.

Vesicular, deeply weathered, dark-gray 

basalt exposed at the mouth of a dry 

wash.

A 2-ft-thick, hematitic quartz vein extends for 22 ft along shear zone in 

chloritic schist; it strikes N. 58° W. 

and dips 14° SW. Isolated quartz lenses 

contain blebs of chalcopyrite. An 8-ftthick bed of montmorillonite-rich clay 

underlies fanglomerate in area.

Quartz-garnet-actinolite skarn zones follow bedding in interlayered quartzite 

and biotite schist. Strike about 

N. 15° W., dip 30°-60° NE (Morton, 

1977, p. 97).

A small production of roofing granules during the 

1950's (Morton, 1977, 

p 84). Several bulldozer 

cuts, and opencut about 

130 ft square and 12 ft 

deep

Two prospect pits: one on 

quartz vein, one on 

weathered pegmatite dike.

Two small prospect pits

Seven chip samples of sch

pegmatite dikes, and qua

veins contained no miner

economic significance. 

stream sediment sample c

0.00046 oz/yd3 gold in m

mesh flakes.

Although possible source 

tive gravel, degree of w

and proximity of more su

rial 8 mi north in the C

(Morton, 1977, p. 83) ma

of deposit unlikely.

Two chip samples of quart

weathered pegmatite cont

icant mineral values. A 

bed had 13.3 pct A12 03 , 

4.3 pct Fe2 03 , 2.4 pct K

1.1 pct Na2 0, 0.038 pct 

60.5 pct Si02 .

One grab sample of actino

contained 0.19 pct WO,. 

of one skarn zone (2.6 

tained 0.01 pct W03 .
TABLE 1. Mines, prospects, and deposits in and adjacent to the Jacumba (In-ko-pah) Wilderness Study Area-Continued

Map No. Workings and

(fig. 2) Name Summary production Sample and resour

16 Unnamed prospect An 18-in.-thick marble bed in biotite A 44-ft-long incline (25°) Two chip samples of skarn 

schist contains weakly developed (20 ft underground) was and grab sample of limon

quartz-diopside-garnet skarn zone. driven S. 61° W. A 10- from site of vertical sh

Ultraviolet light disclosed no scheelite. ft-deep vertical shaft 800 no significant mineral v

ft southwest of incline

(Morton, 1977, p. 96) has

apparently been filled.

17 Unnamed prospect A limonitic pegmatite dike in biotite- One caved adit about 15 ft One sample of pegmatite c

muscovite schist strikes N. 20° W. and long. significant mineral valu

dips 35° SW.

18 Unnamed prospect Fine- to medium-grained diopside-tremolite- Southern working con- Four chip samples of skar

garnet skarn in zones along contacts sists of 61-ft-long, pegmatite contained less 

where tonalite intrudes thinly laminated curved trench as much as WO.,. Scheelite-bearing 

limestone and biotite schist. Irregular 8 ft deep, at crest of from north and south wor

quartz-tourmaline pegmatite dikes transect isolated knob. Northern use of ultraviolet light

schist and skarn. Scheelite disseminated working, about 1,100 ft and 1.46 pet WO.,, respec

in skarn zones and in blebs as much as N. 18° W. from trench is samples contained detect

0.5 in. across and in stringers as 25-ft-long incline driven silver, 

much as 0.25 in. thick and 3 in. long. S. 80° W. 

Scheelite-bearing skarn zone at southern 

working exposed for about 65 ft in 

northerly direction and averages about 

4 ft wide. Scheelite found in one 

apparently isolated, 2-ft-long pod at 

northern working.

19 Unnamed prospect Hematitic biotite schist has narrow dikes of One 5-ft-diameter, 2-ft-deep One chip sample from zone 

biotite tonalite along foliation. pit. no significant mineral v

20 Golden Alter Two quartz veins strike N. 30° to 65° W. Eight pits and two shafts, Five chip samples from qu

prospect in gneissic tonalite and biotite 14 and 15 ft deep, on contained no significnt 

schist; dip 65° NE to vertical. One quartz veins. One A select sample from man

vein exposed intermittently for 1,800 pit in tonalite. pod had 11 pet manganese 

ft and averages 3.4 ft thick; other Jeep trails and bulldozer zinc 

50 ft long and 1 .4 ft thick. Both cuts lead to most workings, 

stained in places by iron and 

manganese oxides.

21 Unnamed prospect Narrow stringers of siliceous hematite One 6-ft-deep, 3-ft-square One chip sample from hema

along biotite schist-pegmatite pit. contact zone contained 0

contact. Strike is N. 80° E., dip is 

45° NW.

22 Roark mine A limestone lens at main shaft poorly A 30-ft-deep vertical A chip sample of quartz-g

(Graphite claim) exposed but at least 25 ft thick; it shaft has 80 ft of hori- in prospect pit containe

strikes N. 60° W. and dips 55° SW; a zontal workings at Chip sample of pegmatite 

weakly silicated zone in lens contains 30-ft level (Morton, 1977, pit contained no signifi

disseminated scheelite (Morton, 1977, p. 96). Two prospect pits values. A grab sample 

p. 96). A 5- to 10-ft-thick felsic dike southeast of shaft. of gray, medium-grained 

forms footwall of bed and schist forms some diopside-quartz ska

hanging wall. Tonalite crops out nearby, 0.29 pet WO^. 

and biotite-tourmaline pegmatite dikes 

transect both metasedimentary and intrusive rocks. Scheelite sparsely disseminated in quartz-garnet skarn zone in 

interlayered limestone and biotite schist 

600 ft southeast of shaft.
o

00

TABLE 1. Mines, prospects, and deposits in and adjacent to the Jacumba (In-ko-pah) Wilderness Study Area-Continued

Map No. 

(fig. 2) Summary

Workings and 

production Sample and resourc

North Star 

prospect

24 Cal-White limestone deposit 

(Cal-White, Little 

White, Kimberly, 

and Ray claims)

25 Unnamed prospect

PK mine (Elliott 

mine)

Interlayered limestone and biotite schist 

near contact with underlying tonalite 

have been altered to tremolite hornfels and, in places, to garnet-quartzdiopside-calcite skarn. Subparallel 

foliation and banding in metasedimentary rocks strike N. 20 to 55° W. and 

dip 20 to 75° SW. Skarn zones are 

irregular and controlled by host rock 

composition and proximity to intrusive 

rocks. Segments of six skarn zones 

exposed in workings and outcrops are 0.7 

to 7 ft thick and as long as 60 ft. 

Pegmatite dikes as thick as 6 ft transect 

both metasedimentary and intrusive rocks. 

An 8-ft-thick sheared zone in schist 

is enriched in manganese oxides.

Bedded, white to gray, medium to coarsely 

crystalline limestone intercalated 

with schistose metasedimentary rocks and 

gneissic tonalite. Limestone occurs 

in lenses of variable orientation, apparently having been thinned in places by 

folding. Graphite is disseminated in 

minor amounts through parts of limestone. A distinct HnS odor is released 

from freshly broken rock. Chlorite and 

coarse hornblende hornfels bands 

sporadically included in limestone 

bodies near margins. One irregular 

body essentially continuous for 1,550 ft 

and ranges from 6 to 50 ft thick. Other 

lenses 100 to 300 ft long and 16 to 

30 ft thick.

Chloritic, limonite-stained tonalite contains veinlets of calcite.

An irregular, tungsten-bearing skarn body 

in small roof pendant in tonalite. Roof 

pendant 350 ft long and 300 ft wide and 

exposed along the crest of ridge. Pendant 

consists of biotite schist with thin beds 

of interlayered limestone. Pegmatite 

dikes as much as 20 ft thick transect 

both tonalite and rocks of roof pendant. 

Skarn consists of pyroxene (diopside), 

garnet (mainly adradite), and scheelite. 

Retrograde alteration and oxidation of 

skarn resulted in formation of epidote, 

clinozoisite, tremolite, and hematite. 

Scheelite both finely disseminated 

and in blebs to 1 in. across, concentrated in zone 200 ft long and about 

30 ft wide.

One 52-ft-long adit 

mainly in biotite schist 

and pegmatite dikes 

crosscuts manganesebearing sheared zone. 

Ten prospect pits are in 

alluvium, marble, schist, 

or tonalite. Skarn zones 

exposed in three pits.

A 14-ft-high, 30-ft-wide 

opencut and two shallow 

trenches in limestone 

where road ends at 

foot of range.

Sloughed pit(?)

Scheelite-bearing skarn 

exposed in four adits with 

108, 164, 169, and 253 ft 

of workings. Stopes in 

three adits open to 

surface. A fifth adit 

is 79 ft long entirely in 

tonalite. At least four 

opencuts in roof pendant; 

largest is 40 ft in diameter 

and 25 ft deep. During 

1954 and 1955, 1,950 tons 

of ore yielded 11,920 Ib W03 

(596 short ton units).

Twelve samples collected 

and outcrops. Four sampl

and nearby pits contained 

cant mineral values. One 

from 3.2-ft-thick skarn z

above, and 600 ft north o

contained 0.02 oz/ton gol

WOo. Two of three chip 

skarn zone in pits 1,300 

contained 0.01 pet W03 ; 

no significant mineral va

mineral values were detec

other samples from skarn 

and pegmatite northwest o

Twelve samples taken acros

bedding in three limeston

contained 0.12 to 1.7 pet 

to 55.9 pet CaO, 0.12 to 

0.8 to 10.6 pet MgO, less 

0.27 pet P2 °5» and 1-0 to 

SiOo* Three exposures co

million tons of indicated 

limestone resources.

One chip sample from acros

contained no significant 

Of 23 chip samples from s

tained 0.01 to 0.91 pet W

tained a trace to 0.22 o

select samples of scheel

contained 0.75 pet and 1

of three samples of tona

0.02 oz/ton gold and 0.1 

Scheelite-bearing zone c

tons of indicated subecon

with average grade of 0.1

including 9,000 ton block 

0.06 oz/ton gold.

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