Mineral Resources of the

Organ Mountains Wilderness Study Area,

Dona Ana County, New Mexico

U.S. GEOLOGICAL SURVEY BULLETIN 1735-D

Chapter D

Mineral Resources of the

Organ Mountains Wilderness Study Area,

Dona Ana County, New Mexico

By STEVE LUDINGTON, WILLIAM F. HANNA, and 

ROBERT L TURNER 

U.S. Geological Survey

RODNEY E. JESKE 

U.S. Bureau of Mines

U.S. GEOLOGICAL SURVEY BULLETIN 1735

MINERAL RESOURCES OF WILDERNESS STUDY AREASSOUTHWESTERN NEW MEXICO
DEPARTMENT OF THE INTERIOR 

DONALD PAUL MODEL, Secretary

U. S. GEOLOGICAL SURVEY 

Dallas L Peck, Director

UNITED STATES GOVERNMENT PRINTING OFFICE: 1988

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425

Denver, CO 80225

Any use of trade names in this report is for descriptive purposes only and does not imply 

endorsement by the U.S. Geological Survey.

Library of Congress Cataloging-in-Publication Data

Mineral resources of the Organ Mountains Wilderness Study Area, Dona Ana 

County, New Mexico.

(Mineral resources of wilderness study areas southwestern New Mexico ; 

ch. D) (U.S. Geological Survey bulletin ; 1735) 

Bibliography: p. 

Supt. of Docs, no.: I 19.3:1735-0

I. Mines and mineral resources New Mexico Organ Mountains 

Wilderness. 2. Organ Mountains Wilderness (N.M.) I. Ludington, Steve.

II. Series. III. Series: U.S. Geological Survey bulletin ; 1735. 

QE75.B9 no. 1735-D 557.3 s 88-600358 

[TN24.N6] [553'.09789'66]
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 the Organ Mountains Wilderness 

Study Area (NM-03G-074), Dona Ana County, New Mexico.

CONTENTS

Summary Dl 

Abstract Dl

Character and setting Dl 

Identified resources D2 

Mineral resource potential D2 

Introduction D2

Location and physiography D2 

Previous investigations D2 

Investigations by the U.S. Bureau of Mines D3 

Investigations by the U.S. Geological Survey D4 

Appraisal of identified resources D4

Mining and mineral-exploration history D4 

Mines, prospects, and mineral occurrences D5 

Assessment of mineral resource potential D9 

Geology D9 

Geochemistry Dll 

Geophysics D12 

Mineral and energy resources D15

Porphyry copper-molybdenum deposits D15 

Copper-bearing breccia-pipe deposits D15 

Skarn deposits D16 

Polymetallic replacement deposits D16 

Low-grade gold and silver deposits D16 

Vein fluorspar deposits D16 

Other commodities D16 

References cited D17 

Appendix D19

FIGURES

1. Index map showing location of the Organ Mountains Wilderness Study 

Area D3

2. Geology and mineral resource potential of the Organ Mountains Wilderness 

Study Area D6

3. Bouguer gravity anomaly map of the Organ Mountains Wilderness Study Area 

and vicinity D13

4. Total-intensity aeromagnetic anomaly map of the Organ Mountains Wilderness 

Study Area and vicinity D14

TABLE

1. Mining history and production near the Organ Mountains Wilderness Study 

Area, New Mexico D4

Contents

Mineral Resources of the

Organ Mountains Wilderness Study Area,

Dona Ana County, New Mexico

By Steve Ludington, William F. Hanna, and

Robert L Turner

U.S. Geological Survey

Rodney E. Jeske 

U.S. Bureau of Mines

SUMMARY 

Abstract

The Organ Mountains Wilderness Study Area 

(NM-030-074) is located in the Organ Mountains, Dona Ana 

County, southern New Mexico. At the request of the U.S. 

Bureau of Land Management, a total of 7,283 acres were 

studied (fig. 1). In this report, the area studied is referred to 

simply as "the study area." Field work for this report was 

conducted in 1984 and 1985. About 83 claims and prospects 

have been located within and adjacent to the study area; 

none are presently active. Inferred subeconomic resources of 

common variety carbonate rock and sand and gravel have 

been mined occasionally for limited local use. No other 

identified resources are located within the study area, 

although immediately adjacent to, and surrounded by the 

study area, copper resources are present at the Torpedo 

mine; silver, lead, zinc, and possible gold resources are 

present at the Stephenson-Bennett mine; and fluorspar 

resources are present at the Ruby mine (fig. 2).

. The mineral resource potential for copper, molybdenum, gold, and silver is high in an area along the northern 

edge of the study area, where a porphyry coppermolybdenum system has been emplaced at the margins of a 

granitic batholith of mid-Tertiary age (see geologic time chart 

in the Appendix for geologic time terms). The potential for 

lead, zinc, copper, silver, gold, and fluorspar resources is 

moderate near the western edge of the study area, where 

steeply dipping, faulted Paleozoic sedimentary rocks are

Publication approved by the Director, U.S. Geological Survey, 

March 15, 1988.

intruded by the batholith. Although the batholith is expressed 

by both gravity and magnetic anomalies, the subsurface 

extent of the mineralized phase of the batholith is not 

well-constrained by the geophysical anomaly data. Geochemical studies confirm the presence of known mineralized 

areas and help constrain the delineation of tracts with mineral 

resource potential. The mineral resource potential for oil and 

gas, and for geothermal energy is low throughout the study 

area.

Character and Setting

The Organ Mountains Wilderness Study Area is located 

in the northern end of the Organ Mountains, in Dona Ana 

County in southernmost New Mexico, about 15 mi (miles) 

northeast of Las Cruces, N. Mex. The study area straddles 

the mountain crest, which is about 8,000 ft (feet) high. Most 

of the central and eastern part of the study area is underlain 

by units of the Oligocene Organ batholith. In the western part 

of the study area, the rocks are primarily westward-dipping 

Paleozoic shales and carbonate rocks. The entire study area 

is part of an uplifted and west-tilted fault block. The 

topography is very rugged, with narrow ridges and needles 

of intrusive rock falling abruptly away into deep, narrow 

canyons. A broad pediment is developed on the Organ 

batholith on the east side of the range; on the west, the 

mountains are flanked by thick alluvial fans.

Mining and prospecting began at least as early as the 

1840's when silver, lead, and zinc ore was discovered at what 

is now the Stephenson-Bennett mine, although legends exist 

that describe much earlier mining activity, possibly in the 

18th century, when the area was part of New Spain. Mining 

activity peaked in the early part of the 20th century and had 

nearly ceased by 1935.

Organ Mountains Wilderness Study Area D1
Recent exploration activity has focused just north of the 

study area where extensive exploration of a porphyry 

copper-molybdenum system was carried out by several 

major mining companies throughout the 1970's (Newcomer, 

1984).

Identified Resources

Inferred subeconomic resources of carbonate rock and 

sand and gravel exist in the study area. No other identified 

resources are within the study area, although immediately 

adjacent to the study area, copper resources are present at 

the Torpedo mine; silver, lead, zinc, and possibly gold 

resources are present at the Stephenson-Bennett mine; and 

fluorspar resources are present at the Ruby mine (fig. 2).

Mineral Resource Potential

The northern edge of the wilderness study area has 

high mineral resource potential for copper, molybdenum, 

silver, and gold and the western edge has moderate resource 

potential for lead, zinc, copper, silver, gold, and fluorspar

(fig- 2).

Mineral deposits and occurrences in and near the study 

area are primarily related to intrusion of rocks of the Organ 

batholith, and are found in two major environments: (1) In the 

northern part of the area, within and near the probable top of 

the Sugarloaf Peak phase of the batholith, a porphyry 

copper-molybdenum system has developed, along with 

associated copper-bearing breccia pipe deposits, and (2) 

along the west side of the area, where steeply dipping 

Paleozoic rocks cut by fault zones are found on the flanks of 

the batholith, the geologic environment is permissive for 

skarn deposits, polymetallic replacement deposits, lowgrade precious metal deposits, and vein fluorspar deposits.

The northern area is defined by the presence of existing 

deposits, hydrothermally altered rocks, geochemical anomalies, and drilling results. This area has a high mineral 

resource potential for copper, molybdenum, gold, and silver 

in porphyry copper-molybdenum deposits, and a moderate 

potential for copper in copper-bearing breccia-pipe deposits.

The second area is defined by the presence of existing 

deposits, silicified, brecciated, and hydrothermally altered 

rocks, and geochemical anomalies. This area has moderate 

mineral resource potential for copper, silver, lead, and zinc in 

skarn deposits, and moderate potential for lead, zinc, 

copper, silver, and gold in polymetallic replacement 

deposits. In addition, altered rock and geochemical 

anomalies indicate a moderate mineral resource potential for 

silver and gold in large, low-grade deposits. The area also 

has moderate potential for fluorspar in vein deposits.

The potential for oil and gas resources and for geothermal energy resources is low.

INTRODUCTION

Location and Physiography

The Organ Mountains (NM-03Q-074) Wilderness 

Study Area is in Dona Ana County, N. Mex., about 15 mi 

northeast of Las Cruces, N. Mex. (fig. 1). At the request

of the U.S. Bureau of Land Management, a total of 7,283 

acres was studied. The study area is in the northern 

Organ Mountains, which are part of the Southern Rocky 

Mountains physiographic province. The physiographic 

province consists of a large west-tilted fault block that 

extends from El Paso, Tex., northward for 150 mi to 

central New Mexico. The central part of the study area 

consists of steep ridges and needles that reach elevations 

of more than 8,000 ft. On the east, the range is flanked by 

a broad pediment, and on the west, the mountain front is 

draped with thick alluvial fans. Elevations along the 

eastern and western boundaries of the study area are 

near 5,000 ft.

Road access is by paved highways adjoining the 

northern (U.S. 70 and 82) and eastern boundaries. Along 

the western margin of the area, several unimproved 

roads approach the mountain front from a gravel road 

that traverses the base of the alluvial fans (fig. 1).

Previous Investigations

This report presents an evaluation of the mineral 

endowment (identified resources and mineral resource 

potential) of the study area and is the product of several 

separate studies by the U.S. Bureau of Mines (USBM) 

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

resources are classified according to the system of the 

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

(1980) which is shown in the Appendix of this report. 

Identified resources are studied by the U.S. Bureau of 

Mines. Mineral resource potential is the likelihood of 

occurrence of undiscovered metals and nonmetals, 

industrial rocks and minerals, and of undiscovered 

energy sources (coal, oil, gas, oil shale, and geothermal 

sources). It is classified according to the system of 

Goudarzi (1984) which is shown in the Appendix. 

Undiscovered resources are studied by the U.S. 

Geological Survey.

The first comprehensive study of the geology and 

ore deposits of the Organ Mountains was by Dunham 

(1935). Earlier reports on the area were referenced by 

him. Albritton and Nelson (1943) provided further 

detailed descriptions of some of the mines in the area. 

Soul6 (1951) summarized results of core drilling at the 

Torpedo mine by the U.S. Bureau of Mines. Glover 

(1975) mapped the central part of the range. Macer 

(1978) interpreted the origin of fluorspar deposits in the 

region. In 1979, a minerals inventory and evaluation of 

the southern parts of the Organ mining district and parts 

of the eastern slopes of the Organ Mountains was 

prepared by McAnulty and McAnulty (1979). Seager 

(1981) published a detailed geologic map and description 

of the geology and mineral deposits of the area.

D2 Mineral Resources of Wilderness Study Areas Southwestern New Mexico
32°30'

32°15'

106°45' T~ 106°30' 

I

White Sands 

APPROXIMATE BOUNDARY Missle Range

OF THE ORGAN MOUNTAINS 

WILDERNESS STUDY AREA 

(NM-030-074)

Fort Bliss

Military

Reservation

10 MILES

Figure 1. Index map showing location of the Organ Mountains Wilderness Study Area, Dona Ana County, New Mexico.

Newcomer (1984) provided a detailed account of the 

porphyry copper-molybdenum system on the northern 

margin of the study area.

Investigations by the 

U.S. Bureau of Mines

The U.S. Bureau of Mines investigations included 

a literature search and a field investigation of the study 

area and adjacent areas. The literature search included 

the following sources: U.S. Bureau of Mines files; New 

Mexico Bureau of Mines and Mineral Resources mining 

files in Socorro, N. Mex.; U.S. Bureau of Land 

Management mining-claim records, oil and gas, and 

geothermal lease information from the U.S. Bureau of 

Land Management State Office in Santa Fe, N. Mex.; 

geothermal information from the New Mexico State 

University Energy Institute, Las Cruces; the Rio Grande

Historical Collection, New Mexico State University, Las 

Cruces; owners of patented claims, and the U.S. Bureau 

of Land Management Las Cruces District Office. There 

are no oil and gas leases within the study area.

The U.S. Bureau of Mines conducted field 

investigations in 1985. All known mines and prospects 

were visited. Foot traverses and a helicopter overflight 

were used to search for additional prospects. A total of 

199 rock samples was collected and analyzed (Jeske, 

1987). One hundred seventy-four samples were analyzed 

for 33 elements by ICP (inductively coupled plasmaatomic emission spectrography); the remaining samples 

were analyzed for 40 elements using semiquantitative 

optical emission spectrography. All samples were 

analyzed for silver and gold by fire assay/TCP, for arsenic 

by AA (atomic absorbtion), for tungsten by colorimetry, 

and for fluorine by chemical methods. In addition, 13 

samples were analyzed for mercury by a combustion AA 

method, 29 samples were analyzed by whole-rock

Organ Mountains Wilderness Study Area D3
Table 1. Mining history and production near the Organ Mountains Wilderness Study Area, New Mexico

Year discovered 

Patented mining

Last date of 

op era t Ion 

Total known

Stephenaon- Torpedo mine 

BenneCt mine

and zinc.

1849 1899

A 2 

1920 1921

ci I «;n nnn i «; ..4114*1.. ns,..~A~

Ruby mine Poor Man's 

Friend mine

1921 1926(?)

2 0 

1929 1927

Mo doc mine

1879

2

1905

<!9nn nnn

copper, some silver, 

small amount of 

lead, zinc.

to 1929; unknown amount. 

small amounts 

since then.

Nature of

workings 450-ft-deep shaft, Four shafts, deepest 120-ft-deep shaft, Partly filled Adit, 185-ft-deep 

scopes, drifts; to 500 ft; stopes, 600-ft crosscut, Inclined shaft, Inclined shaft, 

7 levels. drifts. trenches, adits. trench, pits. 100-ft-deep shaft,

50-ft-deep winze, 

drifts.

analysis using the American Society for Testing and 

Materials (ASTM) method, 13 samples were analyzed 

for calcium carbonate by the ASTM method, and 15 

samples were analyzed for fluorspar by the ASTM 

method. All analyses were performed at the Reno 

Research Center of the U.S. Bureau of Mines, Reno, 

Nev. Complete sample descriptions and analytical results 

can be found in Jeske (1987).

Investigations by the 

U.S. Geological Survey

U.S. Geological Survey personnel examined the 

study area in 1984 and 1985. Existing geologic maps were 

field checked, known mineral deposits were visited, and 

rock samples were collected and analyzed for 16 

elements using energy-dispersive X-ray fluorescence 

methods. Rock samples were slabbed in order to study 

their textures. Further geochemical studies were 

conducted using heavy-mineral concentrates derived 

from stream sediments collected from 17 sites. These 

samples were analyzed for 31 elements using directcurrent arc, semiquantitative spectrographic analysis.

Acknowledgments. We thank the Las Cruces 

office of the U.S. Bureau of Land Management for their 

cooperation. William Seager of New Mexico State 

University in Las Cruces provided valuable discussions of 

the geology of the area. Special thanks are extended to 

Daniel Ford, Anthony Juskiewicz, James Mundy, Audria 

and F.R. Palmer, Ben Schaberg, and John Stowe, who 

provided information on their mining claims.

APPRAISAL OF IDENTIFIED RESOURCES

By Rodney E. Jeske 

U.S. Bureau of Mines

Mining and Mineral-Exploration History

Five currently inactive mines (the StephensonBennett, Torpedo, Ruby, Poor Man's Friend, and 

Modoc) (table 1) of the Organ mining district are in and 

adjacent to the study area. Mining claims are concentrated along the northern and western boundaries (from 

the Torpedo mine to the Stephenson-Bennett mine), and 

near the Ruby mine. Eighty-three prospects consisting of 

13 adits, 11 shafts, and 59 pits are in or adjacent to the 

study area (Jeske, 1987).

Discovery of the Stephenson-Bennett mine in the 

mid-19th century ushered in mining in the Organ mining 

district, which includes the study area and surrounding 

areas. In 1881, completion of the railway through Dona 

Ana County stimulated prospecting, and by 1900, most of 

the larger orebodies had been discovered(Albritton and 

Nelson, 1943). Mining activity peaked in the district in the 

late 19th and early 20th centuries, then gradually 

dwindled from 1909 to about 1935, when mining virtually 

ceased. A second flurry of activity occurred during the 

first few years of World War II, but did not last (Seager, 

1981). Intermittent exploration in the Organ area and 

along the Torpedo-Bennett fault zone occurred from the 

1940's to about 1981 (Seager, 1981; Eveleth, 1983). 

Recently, the Sugarloaf Peak phase of the Organ batholith in the Organ mining district has been an attractive

D4 Mineral Resources of Wilderness Study Areas Southwestern New Mexico
porphyry copper-molybdenum exploration target. Most 

of this exploration has been north of the study area, 

where more than $1 million was spent for drilling in the 

1970's by several mining companies, including Texaco, 

Conoco, AMAX, Kerr-McGee, Phelps Dodge, Bear 

Creek, U.S. Borax, Leonard Resources, and Gulf 

Minerals. Gulf Minerals and Leonard Resources were 

exploring the area near and north of the town of Organ in 

1979 (U.S. Bureau of Land Management, 1981).

Mines, Prospects, and Mineral 

Occurrences

Four main mineralized areas and miscellaneous 

prospects were examined in and adjacent to the study 

area. These areas are, from northeast to south: the 

northern part of the Sugarloaf Peak phase of the Organ 

batholith and associated veins (including the Poor Man's 

Friend vein), the Torpedo mine, the Stephenson-Bennett 

mine, and the Ruby mine (Jeske, 1987).

The mineralized Sugarloaf Peak phase of the 

Organ batholith and associated veins occur in the 

northern Organ and southern San Andres Mountains. 

The distribution of metal values in the veins is spotty 

(Jeske, 1987, table 2). The veins, with one exception, are 

of limited extent. A 0.1- to 0.3-ft-thick quartz vein 

containing high silver values is exposed for approximately 

600 ft along strike in the study area. The vein strikes N. 

40° W., and dips 35°-72° SW. The nine samples taken 

along the vein had silver values ranging from 0.38 to 116 

ppm (parts per million) (0.1 to 3.4 troy ounces per ton) 

(Jeske, 1987, table 2). Surface samples indicate that the 

average grade is too low and mineralization too 

discontinuous to constitute a resource at this time.

Minor silver, gold, copper, molybdenum, lead, 

tungsten, and arsenic mineralization was associated with 

the Sugarloaf Peak phase of the Organ batholith. 

Newcomer (1984) reported similar copper and molybdenum occurrences, and he concluded that the area, at its 

present level of exploration, appears uneconomic.

The Poor Man's Friend mine, within the Sugarloaf 

Peak phase of the Organ batholith, is less than Vi mi 

north of the study-area boundary (fig. 2). A vein, which 

can be traced about V* mi, strikes N. 45°-50° W., and dips 

about 40° SW., and consists of quartz less than 4-in. 

(inch) wide (commonly only 0.5- to 1-in. wide) within 

sericitically altered envelopes extending as much as 10 ft 

away from the vein. Some shearing has occurred, and 

localized breccia zones are as much as 3-ft wide. Iron 

oxide staining is pervasive along the vein with localized 

concentrations of limonite and hematite veinlets. Sample 

analyses show silver, copper, molybdenum, lead, zinc, 

tungsten, and arsenic to be present in small amounts

(Jeske, 1987, table 3). The vein is too narrow and low 

grade to be considered a resource.

The Torpedo mine is Vs mi north of the study area 

(fig. 2). The surface reconnaissance located two open 

shafts, both unsafe to enter. No samples could be taken 

from underground workings. The deposit is in a silicified, 

kaolinized, and brecciated porphyry along the steep, 

east-dipping contact between the Sugarloaf Peak phase 

of the Organ batholith and Paleozoic sedimentary rocks. 

The deposit was explored 500 ft downdip along the 

contact, where it is 800 ft long and as much as 150 ft 

thick. The deposit and associated altered breccia are 

enclosed by two northerly trending faults, as much as 200 

ft apart, within the Torpedo-Bennett fault zone (Jeske, 

1987, fig. 4a). South of the shafts, the two faults converge 

and form a quartz rib that continues intermittently south 

across the hills to south of Baylor Canyon (Dunham, 

1935; Seager, 1981).

Primary minerals in the deposit are pyrite, chalcopyrite, and quartz as stockwork veinlets in the altered 

porphyry. The deposit has been oxidized to a depth of 

300 ft; ore in the oxidized zone is a mass of chrysocolla, 

kaolin, and native copper, with minor azurite, malachite, 

and silver-bearing minerals. Supergene chalcocite was 

reported below 300 ft. Primary chalcopyrite, sphalerite, 

and galena were noted in drill holes below 388 ft (Soul6, 

1951). Silver content appears to increase with sulfur 

content.

An ore shoot that averages 3 percent copper is 

exposed in the 200-ft level of the No. 4 shaft; it has a 

proven length of 275 ft and a thickness of 75 ft. Geophysical evidence suggests that the deposit continues 

south for hundreds of feet beyond the present working 

(Dunham, 1935) toward the study area. The indicated 

reserves of 17,000 tons of material contain between 1 and 

3 percent copper on the 200-ft level (Albritton and 

Nelson, 1943). A previously drilled U.S. Bureau of Mines 

angle drill hole (No. 1), southeast of shaft No. 4, cut a 

24-ft interval containing 4.6 percent copper, 3.2 percent 

zinc, and 0.5 percent lead about 200 ft below the 200-ft 

level (Soul6, 1951). The Torpedo mine reserves were 

estimated to be 600,000 tons averaging between 4 and 5 

percent copper (Weed, 1922).

The richer, easily accessible oxidized ore has been 

mined; a large amount of low-grade material remains 

between the mined ore shoots and on the 200-ft level of 

the No. 4 shaft (Jeske, 1987). The quartz outcrop that 

continues south is barren, but the outcrops over the 

known deposits were also commonly barren (Dunham, 

1935).

A more complete definition of reserves at the 

Torpedo mine and of possible extensions of ore into 

the study area would require a detailed drilling program 

and a thorough underground examination of the mine, 

as the old records are incomplete. Geological and

Organ Mountains Wilderness Study Area D5
106°32'30"

32°25'00

32°22'30

APPROXIMATE BOUNDARY OF 

THE ORGAN MOUNTAINS 

WILDERNESS STUDY AREA 

(NM-030-074)

Geology generalized from Seager (1 981)

Figure 2. Geology and mineral resource potential of the Organ Mountains Wilderness Study Area, Doha Ana County, 

New Mexico.

D6 Mineral Resources of Wilderness Study Areas Southwestern New Mexico
EXPLANATION OF 

MINERAL RESOURCE POTENTIAL

[The wilderness study area has low resource potential 

for oil and gas and geothermal energy, with certainty 

level B]

Geologic terrane having high mineral resource 

potential for copper, molybdenum, gold, 

and silver in (1) porphyry copper-molybdenum deposits (certainty level D) and 

moderate mineral resource potential for 

copper in (2) copper-bearing breccia-pipe 

deposits (certainty level D)

Geologic terrane having moderate mineral 

resource potential for copper, silver, lead, 

and zinc in (3) skarn deposits (certainty 

level D), having moderate mineral resource potential for silver, lead, zinc, copper, and gold, in (4) polymetallic replacement deposits (certainty level C), 

having moderate mineral resource potential for silver, and gold, in (5) low-grade 

gold and silver deposits (certainty level B), 

and having moderate mineral resource 

potential for fluorspar in (6) vein fluorspar 

deposits (certainty level C)

CORRELATION OF MAP UNITS

QUATERNARY AND 

TERTIARY

> TERTIARY

PALEOZOIC

EARLY PROTEROZOIC

DESCRIPTION OF MAP UNITS

Qta Alluvial fan deposits (Quaternary and Tertiary) Includes consolidated to unconsolidated boulders to sandy debris; includes 

Camp Rice Formation

Tob Baylor Peak phase of Organ batholith (Tertiary) Fine-grained, porphyrytic, subvolcanic rhyolite porphyry

Tos Sugarloaf Peak phase of Organ batholith 

(Tertiary) Coarse-grained, seriate to porphyritic quartz monzonite

Too Organ Needle phase of Organ batholith (Tertiary) Coarse-grained, equigranular quartz 

monzonite to quartz syenite

Tt Rhyolite Tuffs of Organ Caldera (Tertiary) 

Rhyolite ash-flow tuff

Ta Orejon Andesite (Tertiary) Andesite and dacite flows and lahars

Rzs Paleozoic sedimentary rocks (Paleozoic) 

Limestone, shale, and sandstone

Xg Granite (Early Proterozoic) Coarse-grained 

granite and pendants of metasedimentary 

rock

A A A

Contact

Fault Dotted where concealed

Thrust fault Dotted where concealed; 

teeth on upper plate

LEVELS OF CERTAINTY

Data indicate geologic environment and 

suggest level of resource potential

Data indicate geologic environment, indicate resource potential, but do not establish activity of resource-forming processes

Available information clearly defines the 

level of resource potential

Organ Mountains Wilderness Study Area D7
geophysical evidence suggest that additional mineralized 

rock occurs at depth and south of the present workings 

and suggest that silver may have been supergene 

enriched near the water table. The resources may extend 

into the study area, but drilling is needed to verify this.

The Stephenson-Bennett mine, which has been 

"cherry-stemmed" out of the study area, consists of three 

mined silver-lead-zinc orebodies: the Stephenson, the 

Bennett, and the Page (Jeske, 1987). All are subparallel 

replacement deposits in steeply dipping Paleozoic 

sedimentary rocks between two major strands of the 

Torpedo-Bennett fault zone. Porphyry dikes cut the 

sedimentary rocks and, locally, the mineralization 

followed the dike contacts. The primary ore minerals in 

all orebodies are sphalerite, argentiferous galena, and 

minor chalcopyrite.

The Stevenson and Page orebodies are approximately 300 ft long and 15 to 20 ft thick (Dunham, 1935). 

They are mostly mined out, except for ore left in pillars, 

samples from which contained as much as 0.036 troy 

ounces gold per ton, 14.2 troy ounces silver per ton, 25.2 

percent lead, 14.6 percent zinc, 0.36 percent copper, 550 

ppm molybdenum, 700 ppm tungsten, and 0.064 percent 

arsenic (Jeske, 1987, table 4).

The Bennett orebody is 500 ft long, as much as 20 

ft thick, and has been mined downdip for 250 ft and 

explored to 600 ft. Drilling during 1946-48 encountered 

mineralized rock at depths between 550 and 650 ft 

(Daniel Ford, owner, written commun., 1985). Lowgrade pillars of mineralized rock remain above 250 ft 

(Jeske, 1987). Upper parts of the Bennett orebody are 

oxidized; whereas, lower parts consist of sulfide minerals. 

The oxidized part has a low zinc content, about 25 

percent lead, and about 20 troy ounces silver per ton. The 

low zinc values suggest the leaching of this element from 

the upper zone. Assays that represent samples at 10-ft 

intervals on the 450-ft level contained 2.89 troy ounces 

silver per ton, 10.77 percent lead, and 12.96 percent zinc 

(Dunham, 1935).

A conservative estimate (Dunham, 1935) is that 

approximately 35,000 tons of mineralized rock, present 

between the 250-ft level and 50 ft below the 450-ft level, 

averages approximately 2.9 troy ounces silver per ton, 

10-11 percent lead, and 13 percent zinc. A small amount 

of mineralized rock also remains in pillars and in the 

walls of the slopes.

The downdip extent of the Bennett orebody is not 

known. Additional resources could be present within the 

Torpedo-Bennett fault zone and surrounding sedimentary rocks, and mineralized rock may be present 

along strike north or south of the Stephenson-Bennett 

mine inside the study area. Drilling would be needed to 

determine whether resources exist at depth.

Other samples from the Stephenson-Bennett mine 

were from underground, the quarry, and from the 

surface. They contain minor amounts of gold, silver, lead, 

zinc, copper, molybdenum, tungsten, and arsenic. Three 

samples taken from what appeared to be barren dolomite 

assayed 0.2-2.0 troy ounces silver per ton over 7- to 10-ft 

sample lengths (Jeske 1987, table 4). Thirteen of 14 

random samples taken from sedimentary rocks in the 

Stephenson-Bennett mine area contained 0.008-0.085 

ppm gold.

The Ruby fluorspar mine is a 40-acre inholding 

within the study area (fig. 2; table 1). The fluorspar 

occurs mainly as open-space fillings and to a lesser extent 

as replacements along several minor, north-trending, 

east-dipping faults that cut west-dipping Paleozoic rocks 

(Seager, 1981). A 500-ft-thick Tertiary andesite porphyry 

sill contains fluorspar veinlets less than 3 in. thick on the 

crosscut level.

Most of the fluorspar in the Ruby mine area occurs 

in six veins that are exposed on the surface (Jeske, 1987). 

Five of the veins are subparallel, strike approximately 

north and dip 55°-70° E., and are exposed for 369-615 ft 

along strike and have widths ranging from 1.5-15 ft. The 

sixth vein is a bedding-plane vein that strikes 

northwesterly and dips 45°-60° W. (Glover, 1975). 

Several additional mineralized fracture zones exposed in 

the underground workings contain varying but minor 

amounts of fluorspar.

Of the 15 samples collected from the Ruby mine 

area (Jeske, 1987, table 5), 9 samples from the six major 

veins contained from 2.4 to 45 percent fluorspar and had 

a weighted average of 18.3 percent fluorspar. The six 

sampled veins at the Ruby mine have an identified 

resource of 209,000 tons averaging 18.3 percent fluorspar. Exploration drilling would be needed to locate 

additional fluorspar resources in the area of the Ruby 

mine and to determine whether such mineralized rock 

extends into the study area.

A composite bulk sample for three veins taken by a 

consultant for the lessee contained an average of 52 

percent fluorspar. A chemical analysis in 1972 on fluorspar ore shipped to the Tuscaloosa Metallurgy Research 

Laboratory of the U.S. Bureau of Mines in Alabama 

contained 41.5 percent fluorspar. Flotation tests made by 

the U.S. Bureau of Mines showed that preparing acidgrade fluorspar ( > 97 percent CaF2 ) concentrates, with 

good recovery, is feasible (B.F. Schaberg, written commun., 1985).

Mining of fluorspar in the Western United States is 

currently (1986) uneconomic because of high freight 

rates, low-cost foreign supplies, and the depressed 

demand on the world market (U.S. Bureau of Mines, 

1986).

D8 Mineral Resources of Wilderness Study Areas Southwestern New Mexico
Samples from several prospects and occurrences 

along the western slope of the Organ Mountains, in and 

adjacent to the study area, contained minor amounts of 

silver, gold, arsenic, copper, fluorine, molybdenum, lead, 

zinc, and tungsten (Jeske, 1987, table 6). A high-grade 

select dump sample from an inaccessible shaft approximately V-i mi northwest of Baylor Pass in the study area 

contained 8.2 troy ounces silver per ton and 31.9 percent 

zinc.

Several elements were identified as geochemically 

anomalous in rock samples collected by McAnulty and 

McAnulty (1979) along the western slope of the Organ 

Mountains, from approximately 1 mi north of the 

Stephenson-Bennett mine to 1 mi south of the Ruby 

mine. The anomalous concentrations of elements 

included silver, arsenic, gold, barium, beryllium, cobalt, 

copper, fluorine, mercury, manganese, molybdenum, 

lead, strontium, titanium, tungsten, and zinc. These 

anomalies were associated with silicified limestone, 

skarn, and argillized zones. The skarn is characterized by 

disseminated epidote, garnet, and other calc-silicate 

minerals. Mercury anomalies were associated with most 

of the arsenic anomalies that occur in silicified limestone 

(McAnulty and McAnulty, 1979).

Extensive deposits of common variety carbonate 

rocks are an inferred subeconomic resource. Carbonate 

rock crops out along the western side of the study area 

(fig. 2, Pzs) and could be used in local markets for 

building material. Whole-rock analyses on 13 carbonate 

rock samples show that the calcium carbonate (CaCO3 ) 

content ranged from 29.27-85.32 percent and the magnesium carbonate (MgCO3 ) content ranged from 

1.65-48.95 percent (Jeske, 1987). The silica (SiO2 ) 

content (from chert and other impurities) ranged from 

6.4-39.1 percent. These results show that the carbonate 

rocks are suitable for agricultural lime or agricultural 

dolomite (greater than or equal to 85 percent combined 

CaCO3 and MgCO3 ). Bands of marble, about 2,000 ft 

long by 300-600 ft wide, north and east of the Ruby mine 

are estimated to have a total extractable quantity of about 

1.5 million tons (U.S. Bureau of Land Management, 

1981). Currently there is only little demand for this 

material.

Valley Transit Mix, Ltd., Las Cruces, N. Mex., has 

a use permit for about 10 acres of Federal land adjacent 

to the Stephenson-Bennett mine. The company is 

occasionally removing the carbonate rock that has been 

stockpiled in the study area since 1974 from the quarry at 

the Stephenson-Bennett mine (U.S. Bureau of Land 

Management, 1981). Small amounts of the rock are 

currently being used for landscaping and road metal. 

According to John Stowe, of Valley Transit Mix Ltd., also 

an owner of the Stephenson-Bennett mine (oral commun., 1985) material at the quarry are adequate to meet 

local demand for several years.

Large quantities of colluvium cover the pediment 

in the study area (fig. 2, QTa), and consist of pebbles, 

cobbles, and boulders in a matrix of silty clay. Rocks in 

the colluvium could be used for landscape decorative 

purposes, for preparing crushed stone, and as road metal 

and agglomerate. A local market for these inferred 

subeconomic resources exists, and the demand for them, 

though small, is continuous (U.S. Bureau of Land 

Management, 1981).

ASSESSMENT OF MINERAL 

RESOURCE POTENTIAL

By Steve Ludington, William F. Hanna, 

and Robert L. Turner 

U.S. Geological Survey

Geology

The Organ Mountains Wilderness Study Area is 

located in the Organ Mountains, which are part of a 

large, west-tilted fault block that extends for 150 mi from 

El Paso, Tex., northward to central New Mexico, along 

the eastern margin of the Rio Grande rift. In the 

immediate vicinity of the study area, the rocks range in 

age from Precambrian to Quaternary, and the older 

rocks have been complexly deformed at least twice. A 

detailed account of the geology of the region can be 

found in Seager (1981), and the description that follows 

is largely abstracted from his study.

Precambrian rocks are present as small remnants 

in the northwest portion of the study area. They consist 

primarily of brown or gray, coarse-grained, slightly 

porphyritic granite and a few small pendants of 

metamorphic rock. These rocks have not been dated 

radiometrically in this area, but, by analogy with other 

nearby Precambrian granites, they are believed to be 

about 1,400 Ma (million years before present).

Between 7,000 and 8,000 ft of Paleozoic sedimentary rocks are variously exposed in the vicinity of the 

study area. These include the Upper Cambrian Bliss 

Sandstone, the Lower Ordovician El Paso Group, the 

Middle and Upper Ordovician Montoya Group, the 

Lower and Middle Silurian Fusselman Dolomite, the 

Middle Devonian Canutillo Formation, the Upper Devonian Percha Shale, the Lower Mississippian Caballero 

Formation and Lake Valley Limestone, the Lower and 

Upper Mississippian Rancheria Formation, the Upper 

Mississippian Las Cruces and Helms Formations, the 

Pennsylvanian Lead Camp Limestone, the Lower 

Pennsylvanian La Tuna and Middle Pennsylvanian 

Berino Members of the Magdalena Formation, the

Organ Mountains Wilderness Study Area D9
Upper Pennsylvania!! and Lower Permian Panther Seep 

Formation, and the Lower Permian Hueco Limestone, 

Abo Sandstone, Yeso Formation, and San Andres 

Limestone. Carbonate rocks are prominent throughout 

this thick, complex section, and some evidence of skarn 

and polymetallic replacement mineralization is present in 

all of the carbonate strata. For the purpose of this 

mineral resource assessment, the Paleozoic strata are 

treated as one unit.

The study area lies on the northeast margin of the 

Cordilleran orogenic belt described by Drewes (1978), 

which was active during Laramide (Late Cretaceous and 

earliest Tertiary) time. The orogenic belt is characterized 

by large-scale northwest-trending thrust faults and 

asymmetric and overturned folds. The foreland to the 

northeast, of which the study area is a part, responded 

with the formation of broad, basement-cored uplifts 

flanked by monoclines and steeply dipping faults. The 

Organ Mountains, in the vicinity of the study area, are 

part of such a block, bounded on the east by the Organ 

Mountains fault, and on the west by a major monoclinal 

structure and the parallel and associated TorpedoBennett fault zone. In the study area, the fault zone 

separates a block of Precambrian granite on the east 

from vertical to overturned Paleozoic sedimentary rocks 

on the west (fig. 2). The zone consists in detail of many 

closely spaced, anastomosing and branching faults.

The Torpedo-Bennett fault zone is intruded and 

mineralized by the Oligocene Organ batholith. Though 

there is some evidence for post-emplacement movement 

on some faults, many details' of the batholith contact 

prove that it is intrusive, and it is clear that the batholith 

contact has been largely controlled by a pre-existing fault 

zone.

The Laramide structures of the study area were 

eventually covered by a great volume of lava flows and 

tuffs, and subsequently intruded by the Organ batholith, 

which is interpreted to represent the magma chamber 

that was the source of most of the volcanic rocks.

The volcanic rocks form two distinct groups. The 

lower group is andesitic to dacitic in composition and 

accumulated primarily as lava flows. These rocks, termed 

the Orejon Andesite (Tertiary), have not been radiometrically dated, and may be, in part, as old as Eocene. The 

upper group is mostly rhyolitic in composition, and 

accumulated primarily as ash-flow tuffs that fill the 

Organ caldera, a volcanic collapse structure south of the 

study area. These rocks are Oligocene in age and 

immediately predate the intrusion of the Organ batholith. Although the silicic ash-flow tuffs of the Organ 

caldera form an important part of the geologic history of 

the region, they do not occur within the study area, and 

are found only in the extreme southern part (fig. 2) 1-2 

mi southeast of the study-area boundary.

The Organ batholith is a complex body made up of 

multiple intrusions of medium- to coarse-grained, seriate 

to porphyritic granite, quartz syenite, and quartz monzonite. It underlies most of the study area, and intruded 

and deformed its overlying roof of Paleozoic sedimentary 

and Tertiary volcanic rocks. Volumetrically, the two 

principal phases are the quartz monzonite porphyry of 

Sugarloaf Peak, in the northern part of the area, and the 

quartz monzonite of Organ Needle, in the central and 

southern parts.

Subsequent to emplacement of the batholith, 

beginning at an unknown time, the study area has been 

uplifted as a horst. The block is bounded on the east by 

the Organ Mountains fault zone, several miles east of the 

study area, and on the west by the largely inferred 

west-side boundary fault, which is apparently covered by 

alluvial material and probably lies 1-2 mi west of the 

study-area boundary. In other parts of the Southern 

Rocky Mountains, this style of deformation began in late 

Oligocene to early Miocene time, continuing until the 

present. This uplift has been differential and has resulted 

in late Tertiary rotation of the Organ Mountains 

structural block by about 20° to the west.

Major erosion of the range in late Tertiary time has 

resulted in the deposition of the Camp Rice Formation, 

of late Pliocene to middle Pleistocene age (Seager, 1981). 

These rocks are primarily boulder conglomerates and 

finer sediments of continental origin and represent an 

erosional regime similar to that found today. The 

youngest rocks in the study area are largely alluvial fans 

that cover the lower slopes of the mountains.

The Organ batholith is interpreted to represent the 

magma that remained after the expulsion of the silicic 

ash-flow tuffs of the Organ caldera. Seager (1981) 

believed the intrusive rocks to be chemically related to 

the tuffs. They are distinctly less silica-rich, consistent 

with the hypothesis that they are the deeper, intrusive 

part of a chemically zoned magma chamber. However, an 

examination of the petrologic data of both Seager (1981) 

and Newcomer (1984) suggests that the two major 

phases of the batholith may not be comagmatic. The 

available data suggest that the quartz monzonite 

porphyry of Sugarloaf Peak is, compared to the slightly 

older quartz monzonite of Organ Needle, slightly more 

silica-rich (though both plutons span large compositional 

ranges), and distinctly poorer in alkalis and richer in 

calcium. The Sugarloaf Peak rocks are also richer in 

titanium and magnesium, relative to total iron. In short, 

it is more calc-alkaline in nature; the Organ Needle 

pluton shows alkalic affinities, similar to the volcanic 

tuffs of the Organ caldera.

Both plutons are closely associated with mineral 

deposits, the Sugarloaf Peak pluton with a porphyry 

copper-molybdenum system, with related skarn and

D10 Mineral Resources of Wilderness Study Areas Southwestern New Mexico
replacement deposits, and the Organ Needle pluton with 

skarn and possibly fluorspar deposits.

Geochemistry

A reconnaissance geochemical survey was 

completed in the Organ Mountains Wilderness Study 

Area in the fall of 1984. Heavy-mineral concentrates 

derived from stream sediments, and rocks were selected 

as the primary sample media. Seventeen sites were 

sampled for heavy-mineral concentrates and eight sites 

were sampled for rocks.

The heavy-mineral concentrate samples were 

collected at each site from active alluvium in the stream 

channel. Each sample was composited from several 

localities along a channel length of approximately 50 ft. 

The stream-sediment samples were sieved through a 

10-mesh screen and then panned until most of the quartz, 

feldspar, clay material, and organic matter were 

removed. The remaining light minerals were separated 

from the heavy minerals with a heavy liquid (bromoform, 

specific gravity 2.8). The material of specific gravity 

greater than 2.8 was further separated in an isodynamic 

magnetic separator. This sample medium represents a 

concentration of ore-forming and ore-related minerals 

that permits determination of some elements that are not 

easily detected in bulk stream sediments.

The rock samples were taken from mineralized and 

unmineralized outcrop and stream float. Samples that 

appeared fresh and unaltered were collected to provide 

information on geochemical background values. Altered 

or mineralized samples were collected to determine the 

suite of elements associated with the observed alteration 

or mineralization.

The nonmagnetic fraction of the heavy-mineral 

concentrates and the rocks were analyzed for 31 elements by direct-current arc, semiquantitative emission 

spectrographic analysis (Grimes and Marranzino, 1968; 

Crock and others, 1983). The rocks were also analyzed 

for arsenic, bismuth, cadmium, antimony, and zinc 

(O'Leary and Viets, 1986), for gold (Thompson and 

others, 1968), and for mercury (Koirtyohann and Khalil, 

1976). Analytical data and a description of the sampling 

and analytical techniques are given in Delaney and others 

(1987).

Three anomalous areas were identified, based on 

the heavy-mineral concentrate and rock data. The first 

area is at the southeast corner of the study area and 

contains heavy-mineral concentrates anomalous in lead 

(as much as 100 ppm), bismuth (as much as 20 ppm), and 

thorium (as much as 300 ppm), and rocks anomalous in 

copper (as much as 300 ppm) and molybdenum (as much 

as 5 ppm). Although no mineralized rock was observed,

these sample sites are from an area which drains the 

Organ batholith where it is cut by aplite dikes and 

overlain by epidote-akered Orejon Andesite.

The second anomalous area, in the north and 

northeast parts of the study area, was delineated by 

anomalous bismuth (150-500 ppm), tungsten (100-500 

ppm), silver (as much as 5 ppm), lead (as much as 100 

ppm), molybdenum (as much as 20 ppm), and thorium 

(as much as 300 ppm) in heavy-mineral concentrates. 

Rocks collected as float in the stream beds from this area 

are anomalous in silver (1-150 ppm), bismuth (12-700 

ppm), molybdenum (70-150 ppm), lead (700-20,000 

ppm), zinc (200-500 ppm), cadmium (1.7-2.3 ppm), 

arsenic (as much as 340 ppm), and antimony (as much as 

240 ppm). These anomalies are probably due to 

peripheral veins related to the porphyry copper system.

The third anomalous area in the study area is on 

the west flank of the mountains. This anomalous zone is 

represented by heavy-mineral concentrates with anomalous values of lead (100-1,000 ppm), molybdenum 

(15-30 ppm), bismuth (30-70 ppm), zinc (1,000-5,000 

ppm), cadmium (as much as 100 ppm), and silver, (as 

much as 7 ppm). These sample sites are downslope from 

the Torpedo-Bennett fault zone and the contact of the 

Organ batholith with Paleozoic sediments. There are 

several mines and prospects along this zone from the 

Stephenson-Bennett mine in the north to the Ruby mine 

in the south, as well as several areas of silicification and 

iron oxide alteration in the north to central part of this 

zone. Outcrop was sampled in the silicic and iron oxide 

alteration zone immediately to the south of the 

Stephenson-Bennett mine, but not at the mine site, and 

near the Ruby mine. The rocks sampled are anomalous 

in zinc (7-34,000 ppm), lead (less than 10-20,000 ppm), 

arsenic (16-1500 ppm), antimony (21-2,000 ppm), and 

silver (20-300 ppm).

A large body of geochemical data for rock samples 

is also available (McAnulty and McAnulty, 1979). 

Because the sample coverage is very uneven, these data 

are not very useful for elimination of unmineralized 

areas or for delineation of favorable tracts. Nevertheless, 

the data are useful to help characterize the nature of 

known mineralized areas, and the results are 

summarized here. The reader is referred to the original 

study for details about sample collection, preparation, 

and analysis (McAnulty and McAnulty, 1979).

Six of the geographic areas sampled are relevant to 

this study, the Stephenson-Bennett mine area, the Ruby 

mine area, the Modoc mine area, the San Augustin Pass 

area, the Baylor Pass area, and the area of The Needles.

In the vicinity of the Stephenson-Bennett mine, 

samples from near known mineralized rock are, as 

expected, anomalous in copper, molybdenum, lead, zinc, 

and silver. A weaker set of anomalous samples is from 

just northeast of the mine in argillized intrusive rock of

Organ Mountains Wilderness Study Area D11
the batholith at the mouth of Blair Canyon. In addition, 

arsenic and mercury were detected in anomalous 

amounts in several samples of silicified limestone, and 

two anomalous gold values were found (0.25 and 0.2 

ppm).

Near the Ruby mine, scattered skarn-mineralized 

rock and silicified limestone were noted, in addition to 

the known fluorspar deposits. The area with skarn 

minerals is characterized by copper, lead, molybdenum, 

and silver anomalies. North of the mine, arsenic, 

mercury, and scattered tungsten anomalies are coincident with silicified limestone. Here also, two samples 

were found to be anomalous in gold (0.2 and 0.02 ppm).

Near the Modoc mine, more weakly skarnmineralized rock and silicified limestone were found. The 

area is particularly high in barium, manganese, lead, and 

strontium; this geochemical signature may be a characteristic of the Hueco Limestone, which is host to. the 

mineralization in this area. Here also, arsenic, mercury, 

and tungsten were found to be anomalous in some 

samples of silicified limestone, as well as one gold 

anomaly (0.27 ppm).

In the San Augustin Pass area, between Baylor 

Peak and San Augustin Pass, copper and molybdenum 

anomalies, with some associated tungsten and arsenic, 

were found within areas of weak argillic and phyllic 

hydrothermal alteration. These results can probably be 

attributed to peripheral veins related to the porphyry 

copper-molybdenum system described by Newcomer 

(1984).

In the Baylor Pass area, at the head of Baylor 

Canyon, just southwest of Baylor Peak, a few molybdenum, lead, and silver anomalies were found in a zone 

of weakly altered rock.

In the area of The Needles, northeast of The 

Needles, within 1 mi of the study-area boundary, a few 

molybdenum, lead, silver, and barium anomalies were 

found associated with mineralized aplite dikes.

Geophysics

The Organ Mountains Wilderness Study Area and 

the surrounding area are covered by regional gravity and 

detailed aeromagnetic surveys having sufficient resolution to define anomalies of Vi or more square miles in 

areal extent. Gravity data are in the form of a partially 

terrain-corrected Bouguer gravity anomaly map (Keller 

and Cordell, 1983) and are based largely on about 35 

observation points within or adjacent to the study area. 

Aeromagnetic data are in the form of an anomaly map 

(Keller and Aiken, 1979) contoured from measurements 

made along 40 flight lines in an east-west direction, 

spaced 1/6 mi apart, at an average altitude of about 500 

ft above ground.

The gravity data (fig. 3 shown at the regional 

scale of fig. 1, index map) indicate that the rectangular 

area of figure 2 is characterized by short-wavelength 

anomalies surrounded by long-wavelength lows of basins 

to the east and west and by long-wavelength highs of 

elevated bedrock to the north and south. Two of the 

short-wavelength features are important: (1) a lowamplitude low over the northeastern part of the rectangular area, associated largely with the Organ batholith 

and related volcanic rocks, and (2) a higher amplitude 

high over the southwestern part of the rectangular area, 

associated largely with Paleozoic sedimentary rocks and 

Precambrian granite. The gravity high, which may be 

caused in part by hornfels and skarn at the margin of the 

batholith, suggests that no appreciable amounts of 

Tertiary intrusive rock underlie the Paleozoic and Precambrian rocks. At the periphery of the rectangular area, 

sublinear, steep gravity gradients mark steeply dipping 

faults that bound basins and ranges, such as the westtilted block forming the Organ Mountains.

The aeromagnetic anomaly data show a strong 

correlation of detailed features with both mapped 

geology and topography. The single most prominent 

feature is a high-amplitude high which trends northnorthwest across the southern part of the study area (fig. 

4). This high, which has a steep western gradient marking 

in part the Torpedo-Bennett fault zone, is associated 

almost exclusively with intrusive rocks of the Organ 

Needle phase of the Organ batholith at elevations above 

6,500 ft. This high becomes smaller in amplitude and 

somewhat discontinuous in the north, where the Sugarloaf Peak phase of the Organ batholith, a rhyolite 

porphyry and other rocks less magnetic than those to the 

south, form the exposed top of the batholith. The weak 

high over the central part of the rhyolite porphyry 

suggests that this part of the body is thin.

The anomaly data also suggest that other igneous 

rocks of the map area are magnetic, as anticipated from 

measurements made on rocks in nearby areas (Bath, 

1976; Klein and Wynn, 1984), but they are probably less 

magnetic than the rocks of the Organ Needle phase. This 

is especially evident in areas of high relief. For example, 

highly elevated parts of the Sugar loaf Peak stock and 

outcrops of rhyolitic tuffs in the southern part of the area 

generate large-amplitude highs of short wavelength. 

However, outcrop areas of these same rocks at lower 

elevations generate little, if any, magnetic effect, partly 

because of the departure from a draped survey at high 

altitudes and partly because of lateral variations of the 

total magnetization (sum of remanent and induced 

dipole moment per unit volume) in the rocks. Some, but 

not most, Precambrian granitic rocks are sufficiently 

magnetic to produce high-amplitude highs. For example, 

a single short-wavelength high, having a northwest trend, 

crosses the northernmost part of an exposure of PrecamD12 Mineral Resources of Wilderness Study Areas Southwestern New Mexico
106°45' 106°30' 106°15'

32°30' 

32°15'-^

-130

EXPLANATION

Gravity anomaly contours- Contour interval, 2 milligals; hachures indicate depression; 

X's indicate gravity observation points in neighborhood of rectangular area

Figure 3. Bouguer gravity anomaly map of the Organ Mountains Wilderness Study Area and vicinity, New Mexico. 

Small rectangular area corresponds to the area shown in figure 2. Contours are generalized from Keller and Cordell 

(1983).

brian rocks in the northwest part of the study area. Here, 

it is possible that the Precambrian terrane contains 

magnetic amphibolite in large enough quantities to 

produce the magnetic feature. Other igneous rocks, such 

as the Orejon Andesite, though undoubtedly magnetic, 

are generally too limited in volume to generate a 

significant magnetic effect. In regions of low topographic

relief, exposed or thinly covered areas of plutonic and 

volcanic rocks produce weak magnetic signatures in 

places that can be readily observed against a background 

field of low gradient.

Because many mineral deposits in the Organ 

district are associated with the Sugar loaf Peak stock, the 

subsurface extent of this stock assumes special

Organ Mountains Wilderness Study Area D13
106°35" 106°32'30'

32°25

32°22'30"

<]//-'. \ ' C, ( <'' p(HK t^OA^jjFOUNTAINS

'±'<^ ' >^t;cHVyrt^£j^S^S^DY AREA 

"^ x ""^\ )J ^)^S?hCfmn>4) \ ""

^m^my/i :^ax^n//-?f? ft i d

Figure 4. Total-intensity aeromagnetic anomaly map of the Organ Mountains Wilderness Study Area and vicinity, New Mexico 

(Keller and Aiken, 1979).

D14 Mineral Resources of Wilderness Study Areas Southwestern New Mexico
Qta

Tob 

Tos 

Too

Tt 

Ta 

Rzs 

Xg

EXPLANATION

Aeromagnetic anomaly contours Contour 

intervals, 20 and 100 gammas. Hachures 

show closed areas of lower magnetic 

intensity

Alluvial fan deposits (Quaternary and Tertiary) Includes consolidated to unconsolidated boulders to sandy debris; includes Camp Rice Formation

Baylor Peak phase of Organ batholith 

(Tertiary) Fine- grained, porphyry tic, 

subvolcanic rhyolite porphyry

Sugarloaf Peak phase of Organ batholith 

(Tertiary) Coarse-grained, seriate to 

porphyritic quartz monzonite

Organ Needle phase of Organ batholith 

(Tertiary) Coarse-grained, equigranular quartz monzonite to quartz syenite

Rhyolite Tuffs of Organ Caldera (Tertiary) Rhyolite ash-flow tuff

Orejon Andesite (Tertiary) Andesite and 

dacite flows and lahars

Paleozoic sedimentary rocks (Paleozoic) 

Limestone, shale, and sandstone

Granite (Early Proterozoic) Coarse-grained 

and pendants of metasedimentary rock

Contact

Fault Dotted where concealed

Thrust fault Dotted where concealed; teeth 

on upper plate

importance. Unfortunately, the Sugarloaf Peak stock is 

not sharply defined by either gravity or magnetic anomaly 

data. Rather, the stock is characterized by anomalies that 

suggest that it has an average density about the same as 

and a total magnetization less than the adjoining Organ 

Needle stock. Although the surface contact separating 

the two phases of the batholith is regular in form when 

viewed at the scale of the geologic map, its subsurface 

equivalent may undulate significantly, as evidenced by 

anomalies that crosscut the mapped contact. For 

example, immediately east of Baylor Peak, a cluster of 

magnetic lows is enigmatically nested over the Organ 

Needle phase of the batholith. Although this anomaly 

cluster may be caused by a hidden eastward extension of 

rhyolite exposed near Baylor Peak or an unknown nonmagnetic or altered phase of the Organ Needle stock, it 

seems more likely that a segment of the Sugarloaf Peak 

phase underlies the Organ Needle phase in this region.

Mineral and Energy Resources

Potential mineral resources in and near the study 

area are of diverse types (fig. 2). The characteristics

associated with each type and the assessment of potential 

are discussed successively.

Porphyry Copper-Molybdenum Deposits

Porphyry copper-molybdenum deposits consist of a 

mass of interconnecting veinlets (stockwork) of quartz, 

chalcopyrite, and molybdenite. They are commonly 

found in or near a shallow, porphyritic intrusion of 

tonalitic to monzongranitic composition (Cox, 1986).

Near the north end of the study area, widespread 

phyllic alteration (replacement of rock minerals by white 

mica) in the quartz monzonite of Sugarloaf Peak and 

related rocks indicates the presence of a porphyry 

copper-molybdenum system. The area has been 

extensively explored, both on the surface and by 

substantial amounts of diamond drilling (Newcomer, 

1984). The Poor Man's Friend vein (fig. 2) appears to be 

part of this porphyry system. No substantial amounts of 

ore-grade mineralized rock have been discovered. Thus, 

though it is almost certain that a porphyry coppermolybdenum system is present, the probability is low that 

a deposit of commercial grade exists. Accordingly, we 

assign a high potenial for the occurrence of copper, 

molybdenum, gold, and silver resources, with a certainty 

level of D, although development is unlikely based on 

current data. The area of high potential designated in 

figure 2 encloses areas of known hydrothermal alteration 

at the surface and known mineralized rock in the 

subsurface.

Copper-Bearing Breccia-Pipe Deposits

Typically associated with porphyry copper deposits 

are copper-bearing breccia-pipe deposits. They consist of 

brecciated rock (typically hydrothermal breccias), the 

interstices of which are filled with pyrite and copperbearing sulfides. Because of the porous nature of the 

bodies, supergene alteration (due to cool surface waters) 

is common, and much of the copper may be present as 

oxide and carbonate minerals. These deposits are 

commonly of higher grade than the associated porphyry 

deposits, but much smaller.

The Torpedo mine, southeast of the town of Organ, 

appears to be such a deposit, although field observations 

and Dunham's (1935) description do not exclude the 

possibility that the Torpedo is developed on a particularly 

rich portion of the porphyry copper deposit. Other 

deposits, similar to the one at the Torpedo mine, may 

occur within the area designated as having high potential

Organ Mountains Wilderness Study Area D15
for copper, molybdenum, and gold resources in a 

porphyry copper-molybdenum deposit. The potential for 

copper resources in copper-bearing breccia-pipe deposits 

is moderate, with a high degree of certainty, or certainty 

level D.

Skarn Deposits

Skarn deposits are commonly found where 

moderate- to high-level plutonic rocks intrude calciumrich sediments. They consist typically of replacement of 

limestone and dolomite by calc-silicate minerals, such as 

pyroxene and garnet, along with associated metal 

sulfides.

Skarn deposits may contain a variety of metals; 

both copper-rich and silver-lead-zinc-rich varieties are 

known (Cox and Singer, 1986). The average size of the 

skarn deposits known to exist in and near the study area 

is almost certainly smaller than most of those compiled 

by Cox and Singer (1986).

In and near the study area are skarn deposits that 

contain various concentrations of copper, silver, lead, and 

zinc. The Homestake, Modoc, Memphis, and Orejon 

deposits are shown in figure 2. They are all in the 

sedimentary rocks that surround the Organ batholith on 

the west and north. Some are probably related to the 

porphyry copper-molybdenum system just described. 

Others, such as the Modoc and Orejon, may be related to 

other phases of the Organ batholith. The distribution of 

known deposits, along with scattered geochemical 

anomalies that may be related to skarn mineralization, 

permit delineation of the tract of moderate mineral 

resource potential shown in figure 2. There is not enough 

information to separate areas with potential for copperrich skarn deposits from areas with potential for silverlead-zinc-rich skarn deposits. Though undiscovered 

examples of this type of mineralized rock probably exist, 

the mineral resource potential for copper, silver, lead, 

and zinc in skarn deposits is only moderate, with a 

certainty level D.

Polymetallic Replacement Deposits

Polymetallic replacement deposits consist of 

silver-, lead-, zinc-, and copper-bearing minerals in 

massive lenses, pipes, and veins in limestone, dolomite, 

and other soluble rock near igneous intrusions.

The Stephenson-Bennett mine (fig. 2) is a representative of this class of deposit. Many of the 

characteristics of polymetallic replacement deposits are 

similar to those of skarn deposits. However, polymetallic 

replacement deposits form at lower temperatures and 

are more strongly controlled by faults, fractures, and 

other favorable structural features. Commonly, polymetallic replacement deposits are found farther from the

plutons responsible for the mineralization than are skarn 

deposits.

The presence of the Stephenson-Bennett deposit, 

combined with the presence of favorable host rocks and 

scattered geochemical anomalies, permits designation of 

the same favorable tract as was designated for skarn 

deposits. Because of the favorable geologic characteristics and the low level of previous exploration, the 

potential for silver, lead, zinc, copper, and gold resources 

in polymetallic replacement deposits is moderate, with a 

certainty level of C. Physical exploration of favorable 

areas identified by geochemical methods would be 

necessary to raise the degree of certainty.

Low-Grade Gold and Silver Deposits

In addition, the presence of scattered areas of 

silicified limestone along the mountain front, from the 

Stephenson-Bennett to the Modoc mines, combined with 

the presence of rock samples anomalous in silver, 

arsenic, and antimony, suggests that the area may contain 

undiscovered large, low-grade deposits of gold and silver. 

The potential for silver and gold resources in such 

deposits is moderate, with certainty level B. Physical 

exploration with this model in mind would be necessary 

to raise the certainty level.

Vein Fluorspar Deposits

The Ruby mine (fig. 2) is a vein fluorspar deposit. 

Though there is no published grade-tonnage model for 

such deposits, the Ruby is of at least moderate size and is 

comparable to other such deposits that have been mined 

in the past (Greta Orris, U.S. Geological Survey, oral 

commun., 1987). The most reliable diagnostic criteria for 

the occurrence of such deposits are a combination of 

structurally favorable host rocks and proximity to late 

Tertiary rift environments. This description fits most of 

the study area, but, because faulting is best developed in 

the Paleozoic sedimentary rocks along the western edge 

of the study area, we designate the favorable tract for 

vein fluorspar deposits to be the one previously identified 

for polymetallic replacement, skarn, and low-grade 

precious metal deposits. This area has moderate 

potential for fluorspar resources with certainty level C.

Other Commodities

The study area is underlain primarily by highly 

metamorphosed sedimentary rocks and by igneous rocks, 

and the potential for oil and gas resources is therefore 

low, with certainty level B.

No young volcanism or significant hot springs are 

found in the area, and the potential for geothermal 

energy resources is low, with certainty level B.

D16 Mineral Resources of Wilderness Study Areas Southwestern New Mexico
REFERENCES CITED

Albritton, C.C., and Nelson, V.E., 1943, Lead, zinc, and copper 

deposits of the Organ district, New Mexico (with a 

supplement on the occurrence of bismuth): U.S. 

Geological Survey Open-File Report 43, 39 p.

Bath, G.D., 1976, Interpretation of magnetic surveys in intermontane valleys of Nevada and southern New Mexico: 

U.S. Geological Survey Open-File Report 76-440, 37 p.

Cordell, L.E., Keller, G.R., and Hildenbrand, T.G., 1982, 

Complete Bouguer gravity anomaly map of the Rio 

Grande rift, Colorado, New Mexico, and Texas: U.S. 

Geological Survey Geophysical Investigations Map 

GP-949, scale 1:1,000,000.

Cox, D.P., 1986, Descriptive model of Porphyry Cu-Mo, in 

Cox, D.P., and Singer, D.A., Mineral deposit models: 

U.S. Geological Survey Bulletin 1693, p. 115.

Cox, D.P., and Singer, D.A., 1986, Mineral deposit models: 

U.S. Geological Survey Bulletin 1693, 379 p.

Crock, J.G., Lichte, F.E., and Briggs, P.H., 1983, Determination of elements in National Bureau of Standards 

geological reference material SRM 278 obsidian and 

SRM 668 basalt by induction coupled argon plasmaatomic emission spectrometry: Geostandards Newsletter, 

no. 7, p. 335-340.

Delaney, T.A., Day, G.W., Turner, R.L., and Jones, J.L., 1987, 

Analytical results and sample locality map of heavymineral concentrate and rock samples from the Organ 

Mountains Wilderness Study Area (NM-030-074), 

Dona Ana County, New Mexico: U.S. Geological Survey 

Open-File Report 87-638.

Drewes, Harald, 1978, The Cordilleran orogenic belt between 

Nevada and Chihuahua: Geological Society of America 

Bulletin, v. 89, p. 641-657.

Dunham, K.C., 1935, The geology of the Organ Mountains: 

New Mexico Bureau of Mines and Mineral Resources 

Bulletin 11, 272 p.

Eveleth, R., 1983, An historical vignette: Stephenson-Bennett 

mine: New Mexico Geology, v. 5, p. 9-15.

Glover, T.J., 1975, Geology and ore deposits of the 

northwestern Organ Mountains, Dona Ana County, New 

Mexico: El Paso, University of Texas, M.S. thesis, 93 p.

Grimes, D.J., and Marranzino, A.P., 1968, Direct-current arc 

and alternating-current spark emission spectrographic 

field methods for the semiquantitative analysis of 

geologic materials: U.S. Geological Survey Circular 591, 

6 p.

Jeske, R.E., 1987, Mineral resources of the Organ Mountains 

Wilderness Study Area (NM-030-074), Dona Ana 

County, New Mexico: U.S. Bureau of Mines Open-File 

Report MLA 6-87, 81 p.

Keller, G.R., and Aiken, C.L.V., 1979, Geophysical investigations, Part III, in McAnulty, W.N., and McAnulty, 

Noel, Organ Mountains minerals inventory and 

evaluation project, Dona Ana County, New Mexico.

Keller, G.R., and Cordell, L.E., 1983, Bouguer gravity anomaly 

map of New Mexico: New Mexico State University 

Energy Institute, Geothermal Resources of New Mexico,

Scientific Map Series, scale 1:500,000 [available from 

National Geophysical Data Center, NOAA, Code 

E/GCX2, 325 Broadway, Boulder, Colo. 80303].

Klein, D.P., and Wynn, J.C., 1984, Density, porosity, and 

magnetic susceptibility of rocks from the Silver City 

1°X20 quadrangle, Arizona and New Mexico: U.S. 

Geological Survey Open-File Report 84-017, lip.

Koirtyohann, S.R., and Khalil, Moheb, 1976, Variables in 

determination of mercury by cold vapor atomic 

absorption: Analytical Chemistry, v. 48, p. 136-139.

Macer, R.J., 1978, Fluid inclusion studies of fluorite around the 

Organ cauldron, Dona Ana County, New Mexico: El 

Paso, University of Texas, M.S. thesis, 107 p.

McAnulty, W.N., and McAnulty, Noel, 1979, Organ Mountains 

minerals inventory and evaluation project, Dona Ana 

County, New Mexico: Bureau of Land Management 

Contract No. YA-512-CT9-110, Las Cruces District, 

148 p.

Newcomer, R.W., Jr., 1984, Geology, hydrothermal alteration, 

and mineralization of the northern part of the Sugarloaf 

Peak quartz monzonite, Dona Ana County, New Mexico: 

Las Cruces, New Mexico State University, M.S. thesis, 

108 p.

O'Leary, R.M., and Viets, J.G., 1986, Determination of 

antimony, arsenic, bismuth, cadmium, copper, lead, 

molybdenum, silver, and zinc in geologic materials by 

atomic absorption spectrometry using a hydrochloric 

acid-hydrogen peroxide digestion: Atomic Spectroscopy, 

v. 7, p. 4-8.

Seager, W.R., 1981, Geology of the Organ Mountains and 

southern San Andres Mountains, New Mexico: New 

Mexico Bureau of Mines and Mineral Resources Memoir 

36, 97 p.

Soul6, J.H., 1951, Investigation of the Torpedo copper 

deposit Organ mining district, Dona Ana County, New 

Mexico: U.S. Bureau of Mines, Investigation Report 

4791, 10 p.

Thompson, C.E., Nakagawa, H.M., and Van Sickle, G.H., 1968, 

Rapid analyses for gold in geologic materials, in 

Geological Survey Research 1968: U.S. Geological 

Survey Professional Paper 600-B, p. B130-B132.

U.S. Bureau of Land Management, 1981, Mineral resource 

inventory and assessment of the Organ Mountains 

Wilderness Study Area: Available from U.S. Bureau of 

Land Management, Las Cruces District Office, 1800 

Marquess, Las Cruces, New Mexico 88005, 41 p.

U.S. Bureau of Mines, 1986, Copper, fluorspar, lead, lime, 

silver, and zinc sections, in Mineral Commodity 

Summaries 1986: p. 42-43, 52-53, 88-89, 90-91, 

142-143, 178-179.

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.

Weed, W.H., 1922, International edition, The Mines 

Handbook, succeeding the copper handbook founded by 

Horace J. Stevens, 1900, describing the mining 

companies of the world: Tuckahoe, New York, The 

Mines Handbook Co., v. XV, 2248 p.

Organ Mountains Wilderness Study Area D17

APPENDIX
DEFINITION OF LEVELS OF MINERAL RESOURCE POTENTIAL 

AND CERTAINTY OF ASSESSMENT

Definitions of Mineral Resource Potential

LOW mineral resource potential is assigned to areas where geologic, geochemical, and geophysical characteristics define a geologic environment in which the existence of resources is unlikely. This broad 

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

or no indications of having been mineralized.

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

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

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

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

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

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

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

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

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

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

moderate, or high levels of resource potential.

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

area.

Levels of Certainty

U/A

UNKNOWN

POTENTIAL

H/B

HIGH POTENTIAL

M/B 

MODERATE POTENTIAL

L/B

LOW

POTENTIAL

H/C

HIGH POTENTIAL

M/C 

MODERATE POTENTIAL

L/C

LOW

POTENTIAL

H/D

HIGH POTENTIAL

M/D 

MODERATE POTENTIAL

L/D

LOW POTENTIAL

N/D

NO POTENTIAL

t

LU

2

LU 

O

cc 

o

CO

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.

40-42. 

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.
RESOURCE/RESERVE CLASSIFICATION

IDENTIFIED RESOURCES

Demonstrated

Measured Indicated

Inferred

UNDISCOVERED RESOURCES

Probability Range

 (or) 

Hypothetical i Speculative

ECONOMIC

MARGINALLY 

ECONOMIC

SUBECONOMIC

1

Reserves Inferred Reserves

Marginal Reserves

Demonstrated 

Subeconomic Resources

Inferred 

Marginal Reserves

Inferred

Subeconomic

Resources

+

Major elements of mineral resource classification, excluding reserve base and inferred reserve base. Modified from 

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.
GEOLOGIC TIME CHART 

Terms and boundary ages used in this report

EON

Phanerozoic

Proterozoic

Archean 

pre - At<

ERA

Cenozoic

Mesozoic

Paleozoic

Late Proterozoic

Middle Proterozoic

Early Proterozoic

Late Archean

Middle Archean

Early Archean 

:hean*

PERIOD

Quaternary

Neogene 

Subperiod

Tertiary 

Paleogene 

Subperiod

Cretaceous

Jurassic

Triassic

Permian

Pennsylvanian 

Carboniferous

Periods """"" 

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

L_ _

BOUNDARY AGE 

IN 

MILLION YEARS

-0.010

- 1.7

-5

- 24

IB

- 55

cc

- 96

uo

OAC

-A, 240 

- oon

- v 330

- 360

- 410

- 500

^~ 570'

- 900 

 1600

-2500

-3000

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

2 Informal time term without specific rank.

*U.S. GOVERNMENT PRINTING OFFICE: 1988-O-573-047/66071

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