BULLETIN 77

Geology of the San Pedro Mountains,

Santa Fe County, New Mexico
by WILLIAM W. ATKINSON, JR.

1961

STATE BUREAU OF MINES AND MINERAL RESOURCES
NEW MEXICO INSTITUTE OF MINING & TECHNOLOGYCAMPUS STATIONSOCORRO, NEW MEXICONEW MEXICO INSTITUTE OF MINING & TECHNOLOGY
E. J. Workman, President
STATE BUREAU OF MINES AND MINERAL RESOURCES
Alvin J. Thompson, Director

THE REGENTS
Members Ex Officio

The Honorable .Edwin L . Mechem
Tom Wiley

Governor of New Mexico
Superintendent of Public Instruction

Appointed Members
William G. Abbott

Hobbs

Holm O. Bursum, Jr.

SocorroThomas M. Cramer.

CarlsbadFrank C. DiLuzio

Albuquerque

Eva M. Larrazolo (Mrs. Paul F . )Albuquerque

For sale by the New Mexico Bureau of Mines & Mineral Resources
Campus Station, Socorro, N. Mex. - - P r i c e $2.50

Contents
Page
ABSTRACT .………………………………………………………………………………1
INTRODUCTION .………………………………………………………………………..2
Location, accessibility, and climate…………………………………………………2
Topography and drainage … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … . . 2
Vegetation and animal life ……………………………………………………….....3
Scope and purpose of work .…………………………………………………………3
Acknowledgments .…………………………………………………………………...3
Previous investigations .……………………………………………………………..4
GENERAL GEOLOGY …………………………………………………………………………………………………………………………………………………………………..5
Sedimentary rocks ……………………………………………………………………………………………………………………………………………………………………...5
General statement ………………………………………………………………..5
Madera Limestone .……………………………………………………………….5
Abo Formation ……………………………………………………………………………………………………………………………………………………………………...6
Yeso Formation …………………………………………………………………..6
San Andres Formation .…………………………………………………………..7
Dockum Group .…………………………………………………………………..7
Igneous rocks …………………………………………………………………………………………………………………………………………………………………………………..8
General statement ……...………………………………………………………...8
Diabase porphyry ………………………………………………………….8
Monzonite …………………………………………………………………………8
Monzonite porphyry and latite porphyry ………………………………………9

iii

Page
Rhyolite porphyry ………………………………………………………………..12
Augite-bearing monzonite porphyry ………………………………………………………………………………………………………......12
Age relations of igneous rocks ………………………………………………...13
Metamorphic rocks ………………………………………………………………….13
Structure ……………………………………………………………………………..15
Regional setting …………………………………………………………………15
Igneous forms …………………………………………………………………...17
Stocks and pipes …………………………………………………………….17
Laccoliths ……………………………………………………………………17
Sills …………………………………………………………………………..19
Dikes ………………………………………………………………………..19
Faults …………………………………………………………………………….20
Folds ……………………………………………………………………………...21
Summary …………………………………………………………………………21
Geologic history …………………………………………………………………….22
ECONOMIC GEOLOGY ……………………………………………………………….24
Introduction ………………………………………………………………………….24
History and production ……………………………………………………………..24
Ore deposits …………………………………………………………………………29
Copper …………………………………………………………………………...29
Gold ……………………………………………………………………………...34
Veins and bedded deposits …………………………………………………34
Placer deposits …………………………………………………………………………………………… 36

iv

Page
Lead-zinc………………………………………………………………………...37
Iron ………………………………………………………………………………..38
Origin of ore deposits ………………………………………………………………..39
Nonmetallic deposits ………………………………………………………………...41
Quartz sand ……………………………………………………………………….41
Garnet ……………………………………………………………………………..41
Limestone …………………………………………………………………………41
Water supply ………………………………………………………………………….42
REFERENCES ……………………………………………………………………………43
INDEX …………………………………………………………………………………….47

Illustrations
FIGURES
1. Structure of the San Pedro Mountains (Keyes) …………………………………18

2 Chalcopyrite crystals from the San Pedro mine ……………………………......33
0

PLATES
1. Topographic map of the San Pedro Mountains, SantaFe County, New Mexico ……………………………………………………..pocket
2 Photomicrographs of samples grading from monzonite to latite
0porphyry
A Monzonite ......................................................................................10oV

B. Monzonite porphyry with 3-5% groundmass ………………………..………10
C. Monzonite porphyry with 37% groundmass ………………………………...11
D. Latite porphyry with 53% groundmass …………………………….............11
3. Metamorphism and hydrothermal zoning in the San
Pedro Mountains ………………………………………………………………pocket
4. Geologic map and structure of the San Pedro
Mountains ……………………………………………………………………pocket
5. Tectonic map of the San Pedro —Ortiz porphyry belt ……………………...pocket
6. Distribution of sills in the San Pedro Mountains ……………………………pocket
7. Distribution of dikes in the San Pedro Mountains …………………………..pocket
8. Fracture pattern in the San Pedro Mountains ………………………………..pocket
9. Pyrite pseudomorphs from the San Pedro mine ……………………………….....31
10.Unusually large chalcopyrite crystal from the San
Pedro mine ………………………………………………………………………33vi

Abstract
The San Pedro Mountains are one of a chain of four groups ofhills in north-central New Mexico. The chain consists of intrusions of
monzonite and related porphyry into sedimentary rocks ranging from
Pennsylvanian to Eocene. Igneous rocks in the northern two mountain
ranges erupted and were reworked to deposit the Espinaso volcanics,
which contain definite Oligocene vertebrate fossils.

The San Pedro Mountains are part of a horst in the Tijeras fault
system. The Dooley fault crosses the east end of the mountains. Many
faults in the mountains are either nearly parallel to the Tijeras fault systemor perpendicular to it, other faults radiate from intrusive centers. Fivestocks, two laccoliths, and many dikes and sills intruded the sedimentaryrocks. The main rock types are monzonite, monzonite porphyry, latite
porphyry, and rhyolite porphyry.

Sedimentary rocks have been extensively metamorphosed by the
intrusions,yielding
tactite,hornfels,
andmarble.
Important mineral deposits occur in veins and contact
metasomatic deposits. The San Pedro mine has been the most importantand has produced more than thirteen million pounds of copper since 1900,
from a contact-metasomatic deposit in limestone. Lead, zinc, and gold
have
been
producedfrommesothermalvein deposits. Gold placers have been among the richest in the State.
Nonmetallic deposits include quartz sand, garnet, and limestone. The
quartz sand is being developed at present. Water is scarce but available
from wells at an average depth of 500 feet.
igneousIntroduction
LOCATION, ACCESSIBILITY, AND CLIMATEThe San Pedro Mountains are in the southwestern corner of Santa Fe
County, New Mexico, in secs. 13-17, 21-24, and 26-28, T. 12 N®, R. 7 E.;
the northern edge of the area lies partly within the Ortiz Mine Grant andentirely within the New Placers mining district. The mountains lie onehalf mile southeast of Golden, which is 32 miles by paved road northeast ofAlbuquerque and 40 miles southwest of Santa Fe.
The most inaccessible parts of the mountains lie no more than onehalf mile from narrow dirt roads which connect with State Roads 10 and
344. State Road 344 parallels the mountains about one-half mile to thesouthwest of them. State Road 10 parallels the mountains one-quarter mile
to the west. Good dirt roads lead to the mines; however, they are apt to beslippery after the summer rains and in the winter when the snow partly
thaws during the daytime.
The climate is semiarid and somewhat cooler than Albuquerque. The
temperature seldom reaches 90 degrees in the summer, and snow commonly
lies on the northern sides of the mountains from December to March. The
annual precipitation is generally 16 to 20 inches, about half of which issnow. The remaining precipitation comes mostly in late July and earlyAugust as sudden afternoon cloudbursts. It may rain as much as an inch
during one of these cloudbursts.

TOPOGRAPHY AND DRAINAGE

The San Pedro Mountains are about four miles long in an easterly
direction and as much as two miles wide. They are dominated by twoeminences, Oro Quay Peak and San Pedro Mountain. These peaks reach
altitudes of about 8200 feet, rising 1300 feet above the surrounding
pediment surfaces. The slopes surrounding the mountains are similar innature and origin to those around the Ortiz Mountains, a few miles to thenorth, where they were termed a conoplain by Ogilvie (1905, p. 28) (pl.
1).
The San Pedro Mountains are part of a chain consisting of fourgroups of mountains which are aligned in a northerly direc-

GEOLOGY OF SAN PEDRO MOUNTAINS

3

tion. From north to south they are Los Cerr illo s, Ortiz Mountains, San
Pedro Mountains, and South Mountain.

VEGETATION AND ANIMAL LIFE
The mountains are covered with piñon and juniper trees, which
are sparse on the southern slopes and rocky places and thick on thenorthern slopes. In favorable places there are a few ponderosa pines, andscrub oak forms thickets in some valleys and on steep and rocky slopes. A
few spruce trees grow on the high peaks. On the southern sides of themountains desert plants, such as yucca and various cacti, are common. In
addition there are many varieties of thorny bushes.
Deer and rabbits are abundant, and bobcat tracks are frequent in
the snow in winter. Field mice and packrat nests are common. Porcupines
may be seen occasionally, and their quills and droppings indicate thatthey often sleep in abandoned mine tunnels. Unfortunately, rattlesnakes
are numerous and an ever-present danger in the summer.

SCOPE AND PURPOSE OF WORKThe geology was mapped on enlarged aerial photographs at a scaleof about 3.5 inches to one mile in order adequately to depict theconsiderable geologic detail. The mineral deposits were examined briefly
during the course of mapping. Twenty thinsections of igneous rocks werestudied to determine the rock types, al-though detailed petrographic
studies were not made.

ACKNOWLEDGMENTS
This bulletin is modified from a master's thesis submitted to the
Department of Geology of the University of New Mexico, June 1960. The
author wishes especially to thank Dr. V. C. Kelley for his supervision andguidance during the work. Thanks are due to Mr. C. F. Williams, owner of
the San Pedro mine, for permission to work on the mine property and formaking the many reports and maps of the mine available to me. Dr. C. B.
Read of the U. S.

4NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

Geological Survey permitted use of their equipment during preparation of
the maps. Financial aid from the New Mexico Geological Society is greatly
appreciated. I wish to thank Mr. James D. Knauer and Mr. Richard F.
Atkinson for their assistance in preparation of maps, and Sherman P. Marshfor use of his crystal drawings.

PREVIOUS INVESTIGATIONS

Although many authors have mentioned the general geology,
history, and production of the district, there have been very few thoroughgeological investigations of this area. Until this report, only two
generalized maps (Read et al., 1944, and Smith et a l . , 1945) had been
made of the entire group of mountains.
A number of papers published prior to 1900 deal with the San
Pedro mine in a general way. In 1903, M. B. Yung and R. S. McCaffery
published a report on the ore deposits of the San Pedro district, including adetailed map of the vicinity of the San Pedro mine, In 1916, Augustus
Locke and E. H. Perry prepared a private report to the owners of the San
Pedro mine, which contained maps and many suggestions for underground
exploration, most of which were successful and at least one of which led toa spectacularly rich stope. During the Second World War the U.S. Geological Survey carried out two investigations which eventually led to an
important report on the San Pedro mine (Smith et al., 1945). A plane-tablemap of the area was prepared and excellent maps of the mine showed thelocations of ore bodies. A rather hasty reconnaissance map of the entire
group of mountains was prepared on aerial photographs at a scale of 2
inches to one mile. This report was never published. In 1946, apparently
without the benefit of the federal report, L. P. Entwistle prepared a private
report on the San Pedro mine for the American Smelting and Refining
Company. This report contained another excellent plane-table map which
differs in some small respects from the Survey map. These four
investigations are the most important ones. The only published report isthat of Yung and McCaffery, which is inadequate in light of our present
knowledge. In addition to the most important general papers on the San
Pedro mine, several private reports have been prepared on specific aspectsof the mine.
Some very interesting historical articles, referred to under History
and Production, have been written on the mining district in general, and anumber of important geological papers mention the San Pedro Mountains.

General Geology

SEDIMENTARY ROCKS

General Statement

Igneous intrusions, faulting, and poor exposures in the San Pedro
Mountains make it impossible to describe the stratigraphy adequately.
Sections may be measured with difficulty in only a few places. In most of
the area, the sedimentary rocks have been metamorphosed until their
original color and texture have been obliterated.
The best unaltered stratigraphic section of beds equivalent to the
San Pedro Formation is in the nearby Hagan basin (Harrison, 1949).
Smith et al. (p. 6) believed that only the Madera Limestone andthe Abo Formation appeared in the area, but Dr. V. C. Kelley aided thewriter in identifying the San Andres, Ye so, and Abo Formations in thesemountains.

Madera Limestone
The Madera Limestone, which is the upper member of thePennsylvanian Magdalena Group, crops out in the western part of themountains. The Sandia Formation presumably lies directly on the
Precambrian in the subsurface but is not exposed in this area.
The limestone is light to dark gray, coarse to very fine grained,
and fossiliferous. Many intervals of 10 feet or less of gray and grayishgreen shale separate massive limestone beds. Emerick (1950, p. 18) foundthat the Madera Limestone is about 1000 feet thick in the vicinity . ofGolden; Harrison (p. 14) found that it is about 1260 feet thick in the
Hagan basin, eight miles west of San Pedro; Reynolds (1954, p. 16) foundthe thickness to be about 916 feet on Montezuma Mountain, eleven miles
west of San Pedro.
In Old Timer Gulch and on San Pedro Mountain, the lime-stone
has been extensively metamorphosed to garnet rock. In Old Timer Gulch
the garnet rock locally contains magnetite, quartz, specularite, andepidote. Some beds rich in limonite and hematite appear to have been
pyritized and later oxidized. On San Pedro

6

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

Mountain the garnet rock at places is ore. The mineralogy of these deposits
is discussed under Ore Deposits.
Abo Formation

The Permian Abo Formation normally consists of variegatedsandstone, shale, shaly sandstone, and siltstone, with some limestone beds.
The s a n d s t o n e is mostly deep red to purple, maroon, and brown. The
shale is greenish-gray, red, and brown. At places it grades into siltstoneand sandy shale (Harrison, p. 34). Harrison (p. 37) found the thickness of
the Abo Formation to be about 900 feet in the Hagan basin, and Reynolds (p.
18) found it was about 970 feet thick at Montezuma Mountain.
Approximately this general thickness is also present in the San Pedro
Mountains.
In the San Pedro Mountains the Abo Formation covers the central
third of the range. It has been considerably metamorphosed and consists ofblack, white, and gray hornfels together with one bed of arkose in the
middle part. A few small patches on the northern and the southern edges ofthe mountains have the normal color and texture of the unaltered Abo
Formation.
The lower contact with the Madera Limestone is taken as the topof the last fossiliferous marine limestone. In the San Pedro Mountains thisis represented by a massive garnet bed.

Yeso Formation

The Permian Yeso Formation consists of two members, the lower
Meseta Blanca Sandstone and the San Ysidro Member. In the Hagan basin,
Harrison (p. 46, 48) found the Meseta Blanca Member to be 73 feet thickand to consist of orange-red very fine-grained sandstone. He found the San
Ysidro Member to be 377 feet thick and to consist of dominantly brownishpink fine- to coarse-grained sandstone.
In the San Pedro Mountains no massive fine-grained sandstone isapparent at the base of the Yeso. Here the Yeso Formation consists of darkred, brown, and pink sandstone beds, much like Harrison's descriptions of
the San Ysidro Member. The Yeso Formation has not been subjected tomuch metamorphism, but in some areas it appears to have been bleached
and is buff to white. The outcrops of the Yeso Formation lie to the east of
the Abo Formation. A graphic calculation indicates that the Yeso Formation
is about 400 feet thick in the San Pedro area.

GEOLOGY OF SAN PEDRO MOUNTAINS

7San Andres Formation

The Permian San Andres Formation consists of three members,
the lower Glorieta Sandstone, a limestone member, and an upper member,which in turn may be divided into a lower sandstone and an upper
limestone.
The formation crops out in a narrow north-trending strip in theeastern part of the mountains. Graphic calculation showed the thickness tobe about 200 feet.
The San Andres Formation differs somewhat in the San Pedro
Mountains and the Hagan basin. The Glorieta is a white, pure, mediumgrained, and poorly cemented sandstone in the San Pedro Mountains, as
contrasted with a gray sandstone cemented by silica in the Hagan basin(Harrison, p. 55). The San Andres lime-stone member of the Formation in
the northern part of the San Pedro Mountains resembles Harrison's
description of a yellow to gray limestone which "weathers to a cavernous'worm-eaten' appearance." In the southern part of the mountains it has
been metamorphosed and is massive, finely crystalline, and dolomitic,
containing needles of tremolite up to half an inch long. The sandstone of
the upper member is mostly covered but appears to be a very sandy grayhornfels. The upper limestone bed of the member is present on the ridge
east of the Silica mine, but it pinches out a few hundred yards to the north.
It is a dark-gray limestone pebble conglomerate with a dark reddish-graymatrix.

Dockum GroupThe Triassic Dockum Group consists of the lower Santa Rosa
Sandstone and the upper Chinle Formation. The Santa Rosa Sandstone
near the base of the Dockum Group is fine to medium grained, cemented
with silica, and buff to white. It forms cliffs and is a good marker.
The Chinle Formation is generally poorly exposed, but along theeastern edge of the mountains it consists of a dark-purple shale which isslightly metamorphosed. In the extreme northeastern corner of themountains, the rocks are red shale and siltstone. Elsewhere the Dockum
Group has been metamorphosed to white, gray, and black hornfels, or to a
specularite-rich epidote rock. Approximately 1000 feet of the Dockum
Group crops out in the San Pedro Mountains, the upper part being coveredby alluvium to the east of the mountains.

8

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

IGNEOUS ROCKS

General Statement

Monzonite and rhyolite porphyry are the principal igneous rocktypes in the San Pedro Mountains. The monzonite grades into monzonite
porphyry and latite porphyry. Diabase porphyry dikes occur locally.
Thinsections were prepared from 20 of the numerous samples
collected in the field and were studied primarily to classify the rock types.
The rocks were classified according to Wah1strom (1955, p. 307320). A rock of monzonitic composition is called a monzonite porphyry
when the groundmass is greater than about 10 percent, and a latite
porphyry when the groundmass is more than 50 percent.

Diabase Porphyry

Several diabase porphyry dikes occur in the eastern part of themountains. In hand specimens the porphyry is dark green with prominent
black hornblende phenocrysts up to 1.5 cm long and 0.5 cm across.
Diopside phenocrysts of the same size as the hornblende occur in two of
the dikes. In some dikes the hornblende has been altered to chlorite, andone dike contains only diopside phenocrysts. In one place small feldspar
phenocrysts 1 x 3 mm occur. In most places, the groundmass is altered to apale cream color.
In thinsections the diabase porphyry consists of about 30 percent
hornblende phenocrysts and about 60 percent of fine-grained plagioclase
microlites, of approximate composition An 57 . Most of the remaining 10
percent is chlorite, with traces of magnetite and apatite.
Monzonite

The four stocks in the eastern part of the mountains and the small
stock in Old Timer Gulch are monzonite. In hand specimens the monzonite
i s gray, uniformly medium grained. In the outcrop it weathers to slabby,slightly rounded boulders. It is resistant to weathering and the stocks form
prominent peaks.

GEOLOGY OF SAN PEDRO MOUNTAINS

9

In thinsections the monzonite is composed of quartz, orthoclase,
plagioclase, hornblende, chlorite apparently derived from hornblende,
magnetite, apatite, sphene, and occasional sericite. The ratio of orthoclase
to plagioclase ranges from about 1:1 to 1:4, and the amount of quartz is
generally as much as 6 percent or 10 percent. The composition of theplagioclase ranges from An 38 to An 45 , and the centers of zoned plagioclase
crystals are as calcic as An 55 .
Monzonite Porphyry and Latite Porphyry
Monzonite porphyry and latite porphyry form dikes, sills, andlaccoliths. In addition, the porphyries commonly form shells around themonzonite stocks.
At the edges of some stocks a complete gradation was observed
from monzonite to monzonite porphyry to latite porphyry. Four samples
for thinsections were collected from a line 40 feet long, in which
monzonite with no groundmass at all graded into a latite porphyry with 53
percent groundmass (pl. 2). The size of the particles in the groundmass
varies from about 0.2 mm where the groundmass first appears to less than
0.01 mm,
The appearance and mineralogy of the two porphyries isessentially the same, except for the amount of groundmass. In hand
specimens the porphyries are bluish gray with white rounded to angularfeldspar phenocrysts 1 to 5 mm in diameter. In outcrops the porphyries
weather to spheroidal boulders. In most places the porphyries are not very
resistant to weathering, and in many places dikes and sills must be traced
by float at the surface.
In thinsections the porphyries are composed of the same minerals
as the monzonite described above. In some places the plagioclase
phenocrysts have been altered to epidote. In the coarser groundmass ofsome monzonite porphyries, the principal mineral is orthoclase, with somequartz and plagioclase. In the porphyry with fine-grained groundmass,
plagioclase predominates in the phenocrysts, and it is believed thatorthoclase is the main constituent of the groundmass. The composition of
the plagioclase phenocrysts varies from An 35 in some specimens to An 45 inothers. The amount of quartz in the porphyries ranges from none at all to
as much as 9 percent, the average being about 3 percent in the sections
examined.
Smith et al. (p. 8-9) described the rock as syenite, while Yung and
McCaffery (1903, p. 351) classified it as a syenite porphyry. Lindgren(Lindgren and Graton, 1906, p. 171) called it a granodiorite porphyry andstated that his specimens showed crystals of ande sine 2 to 3 mm indiameter and black prisms of horn-

10

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

A. Monzonite. Collected at 0 feet.

B. Monzonite porphyry with 3-5% groundmass.
Note zoned plagioclase. Collected at 17 feet.
Photomicrographs of samples grading from monzonite to latite porphyry.
Samples were collected in a 35-foot line at the edge of the stock northeast of
Oro Quay Peak.

Plate 2.

GEOLOGY OF SAN PEDRO MOUNTAINSC.

11

Monzonite porphyry with 37% groundmass.
Collected at 26 feet.

D. Latite porphyry with 53% groundmass. Compare
fineness of groundmass with B. Collected at 35 feet.
Photomicrographs of samples grading from monzonite to latite porphyry.
Samples were collected in a 35-foot line at the edge of the stock northeast of
Oro Quay Peak.

Plate 2.

12

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

blende set in a microcrystalline groundmass composed of quartz andorthoclase. He also gave the following partial analysis:
SiO 2

62.08 percent

CaO

4.62 percent

Na 2

4.76 percent

K20

2. 84 percent

Rhyolite Porphyry

Rhyolite porphyry sills are common in most parts of the mountains,
whereas rhyolite porphyry dikes are rather few in number and are confinedto the eastern part of the range. Rhyolite porphyry partly encircles severalof the monzonite stocks and appears to have risen from the same source.
In hand specimens, the rhyolite porphyry is white to light yellow
and very fine grained with sparse to moderately abundant phenocrysts oforthoclase and quartz. In one sill small euhedral orthoclase crystals may be
broken out of the rock. At some places the quartz phenocrysts are subhedral
corroded hexagonal dipyramids, representing former high-temperatures. In
outcrops the porphyry weathers to angular fragments.
In thinsections the rhyolite porphyry is composed of quartz,
orthoclase, and plagioclase, with traces of biotite, magnetite, sericite,
apatite, sphene, and hornblende. The amount of quartz ranges from 6 to 11
percent, the amount of orthoclase in phenocrysts from 16 to 25 percent, andthe amount of plagioclase from traces to 15 percent. The thinsection of themost representative sample showed only traces of plagioclase. The
groundmass is probably composed primarily of orthoclase and quartz.

Augite-bearing Monzonite Porphyry

A monzonite porphyry which occurs as a small pipe and a dike in
San Lazarus Gulch is so different from the other monzonite porphyries inappearance, texture, and mineralogy that it is considered separately.
In hand specimens the porphyry is white with conspicuous

GEOLOGY OF SAN PEDRO MOUNTAINS

13

glassy orthoclase phenocrysts averaging 2 cm in length and 1 cm in width,with abundant euhedral hornblende prisms 2 mm wide and 5 mm long. The
porphyry weathers to slightly rounded slabby boulders.
In thinsection the porphyry is composed of 15 percent orthoclase,
40 percent plagioclase, and no quartz. The composition of the plagioclase
is An 33 , more sodic than in any other igneous rock in the San Pedro
Mountains. In addition to 5 percent hornblende, 5 percent augite wasnoted. Sphene, apatite, and magnetite occur in minor amounts.

Age Relations of Igneous Rocks
Relative ages of different igneous rocks were determined in the field fromcrosscutting and gradational relationships.
The first intrusions in the San Pedro area were diabase porphyry
dikes. Next, early monzonite stocks were intruded and closely followedby rhyolite porphyry; then later monzonite stocks were emplaced. The
later stocks gave rise to most of the monzonite porphyry and latite
porphyry dikes and sills. It is not clear when the augite-bearing monzonite
porphyry was intruded, since it is not in contact with other igneous rocks.
But in Los Cerrillos (Disbrow, 1953, p. 15) and in the Ortiz Mountains(Peterson, 1958, p. 49-50) a similar augite-bearing monzonite porphyry
represented the last surge of intrusions.

METAMORPHIC ROCKS

Sedimentary rocks in the San Pedro Mountains have been
extensively metamorphosed by heat and fluids emanating from theintrusions. The intensity of metamorphism usually diminishes with
distance from the intrusion but it is also controlled by structural factors(pl. 3).
Metamorphic rocks were examined cursorily during the course of
field work. Minerals which could not be identified in the field were
determined by oil-immersion methods under the microscope.
Marmorized Madera Limestone occurs on the east side of Old
Timer Gulch adjacent to a monzonite porphyry dike and in the uppermost
beds of the limestone at the San Pedro mine, associated with tactite. At the
San Pedro mine marble is separated from the

14

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

nearest monzonite dike by tactite, and the sugary marble grades into finegrained limestone away from the tactite. Where the limestone contained
argillaceous layers, these are now resistant hornfels that may stand out inrelief on weathered surfaces.
The monzonite stock which forms Oro Quay Peak has metamorphosed the limestone of the San Andres Formation adjacent to thestock to a coarsely crystalline marble which grades into finer-grained
marble and finally into limestone. The coarse marble is exposed at the
Silica mine but is covered elsewhere. At one point about 1000 feet fromthe stock, abundant tremolite needles were noted in very fine-grained,
gray dolomitic marble.
Tactite has resulted from the complete replacement of parts of theMadera Limestone around the stock in Old Timer Gulch and in theuppermost beds in the limestone exposed on the west side of San Pedro
Mountain. The beds are above the exposed monzonite porphyry laccolith.
Replacement has taken place adjacent to a monzonite porphyry dike but
not at the roof of the laccolith. The replaced beds also dip toward aporphyritic monzonite pipe in San Lazarus Gulch and toward the Oro
Quay stock (pl. 4, A-A’). Replacement has probably resulted from fluids
emanating from the dike, pipe, or stock but not from the laccolith.
The tactite is composed primarily of garnet, with minor amountsof tremolite, epidote, orthoclase, chlorite, and quartz. The orthoclase ispresent in one place as a massive constituent of the tactite and is different
in appearance from the adularia present in vugs. Minor amounts ofspecularite, magnetite, pyrite, and chalcopyrite are also present locally.
The garnet in the upper part of the Madera Limestone is all birefringent,and its determination presents a special problem. The following tablesummarizes the properties:
Color

2V

brown

Sign

Minimum
index

Maximum
index

+

1.81

1.87+

green

750

+

1.81

1.81

brown

60°

-

1.77

1.825

brown

large

+

1.81

1.88

The indexes measured do not represent nx and nz for a single composition.
The garnets are usually zoned, with areas of various birefringences
and, therefore, different x and z indexes. Few of the samples examined
contained isotropic zones.

GEOLOGY OF SAN PEDRO MOUNTAINS

15

Since the indexes nx and nz cover a rather wide range, ny was
taken as the possible index of isotropic garnet of the same composition.
Although the indexes of the garnets are nearest that of almandite, 3FeO·
Al 2 O 3 ·3SiO 2 (n=1.830), their color suggests grossularite, 3CaO·Al 2 O 3 ·3SiO 2 (n=1.735), or andradite, 3CaO·Fe 2 O 3 ·3SiO 2 (n=1 . 8 9 5 ) . If thegarnet were almandite, all the calcium from the limestone would have been
removed. On the other hand, garnet with a composition between
grossularite and almandite would require the addition of only iron andsilica, the aluminum being present in , the argillaceous parts of thelimestone. Although the garnet of marble and tactite aurioles is known to
be dominantly grossularite with minor proportions of almandite andandradite, the positive confirmation requires either density or unit-celldata to supplement the optical data. Nonetheless, the garnet is tentatively
assigned to a grossularite-almandite composition.
Hornfels occurs throughout the San Pedro Mountains in the shaly
beds of the Madera Limestone, the Abo Formation, and the Dockum
Group. It ranges in color from white to black and various shades of green
and brown. In many places it is flinty and extremely hard, while adjacentto the large stocks it is soft and clay-like. Where redbeds have been
metamorphosed, the iron has been incorporated in finely disseminatedgreen minerals such as epidote and chlorite. Garnet nodules are abundantlocally. Red-brown garnet from one unaltered nodule was biaxial negative,
with minimum index 1.82 and maximum index 1.86. The garnet is probably
nearest to almandite. Hornfels is the principal host rock for gold veins
possibly because of the ease with which it fractures.
Any quartzite produced by contact metamorphism is not easily
distinguished from that produced by diagenetic sedimentary processes. No
contact metamorphic minerals were noted in quartzite in the San Pedro
Mountains,

STRUCTURE

Regional Setting
The San Pedro Mountains lie in the San Pedro—Ortiz porphyry
belt, a north-trending alignment of five intrusive centers about 25 miles
long and 5 miles wide. From north to south they are the Cienega center,
Los Ce r r i l lo s , Ortiz Mountains, San Pedro Mountains, and South
Mountain (p1, 5).

16

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

The porphyry belt lies to the east of the Rio Grande depression andforms an echelon with two north-trending mountain ranges, the Sandia to
the southwest and the Sangre de Cristo to the northeast. The Estancia
basin lies southeast of the belt.
Sedimentary rocks in the porphyry belt generally dip to the eastand northeast, in conformance with the general dip of the rocks in the Rio
Grande trough west of the belt to the Santa Fe embayment east of the belt.
Since this general easterly dip may be observed outside the areas affectedby the intrusions, tilting has probably resulted from faulting associatedwith the development of the Rio Grande trough.
In general, the intrusive centers fall on a nearly straight line. In
detail the arrangement is more complicated. The two principal intrusive
centers in the San Pedro Mountains do not lie on the straight line but are
equidistant to either side. A line of five centers of laccoliths in the Ortiz
Mountains trends north-northwest from the belt, and a small plug at the La
Bajada mine is several miles west of the belt.
The reason for the northerly straight trend of the porphyry belt isobscure. No folds or faults exposed at the surface connect the line of
intrusives. The trend may be controlled by a fracture or belt of fractures in
the Precambrian basement. A few of the intrusive centers show somerelationship to exposed faults, although these faults show no apparent
relationship to the porphyry belt as a whole. The five laccolith centers in
the Ortiz Mountains, mentioned above, lie directly in line with the La
Bajada fault. Intrusives in the San Pedro Mountains are cut by faults
related to the northeasterly-trending Tijeras fault system. The San Pedro
Mountains lie in a horst between the Dooley fault on the southeast and theGolden fault to the northwest. The largest stock in the San Pedro
Mountains, at Oro Quay Peak, is cut by the Dooley fault. Several faults
appear in the center of the horst and bifurcate to the north. The stock in Old
Timer Gulch is elongated parallel to a branch of one of these faults andlies adjacent to it.
Dr. V. C. Kelley (oral communication, 1960) has suggested that
South Mountain is offset from the line of the porphyry belt by strike-slip
movement in the Tijeras fault system. If a line is drawn through the
Cienega intrusive center, Cerro Bonanza in the Ce rr illos , the eastern partof the Ortiz intrusives, and the Oro Quay stock in the an Pedro Mountains,
South Mountain does indeed appear to be offset a few miles to thesouthwest along the Dooley fault (p1. 5). But this line misses Grand
Central Mountain in the Cer r illo s, the largest intrusive center in theporphyry belt. If the line is drawn through the center of Grand Central
Mountain. the central stock of the Ortiz Mountains, and the center of the
San Pedro Mountains, then the center of South Mountain falls directly on
the line. The author feels that straightness or offsets in lines

GEOLOGY OF SAN PEDRO MOUNTAINS

17

between intrusive centers is of little significance, since the pattern of
intrusions in the porphyry belt is rather irregular.
The San Pedro Mountains appear to be more closely related toSouth Mountain than to the Ortiz intrusives. A rhyolite sill was traced
from near San Pedro village to the west slopes of South Mountain, and thesource of a laccolith which pinches, out on the west slope of South
Mountain appears to have been the San Pedro Mountains. To the north the
San Pedro Mountains are separated from the Ortiz Mountains by a fault or
series of faults with a total vertical displacement of more than 5000 feetalong the East fault (Emerick, p. 54-55).

Igneous Forms
Stocks and Pipes
Five monzonite stocks and one augite-bearing monzonite
porphyry pipe have intruded the San Pedro Mountains. The stocks varyfrom about 1000 to 2000 feet in diameter and the pipe is about 100 feet indiameter. The stocks are roughly circular to elliptical, in plan and the pipe
is circular.
Several faults intersect at the Old Timer stock, and one dikewhich was fed by the stock followed a fault until the dike pinched out.
The pipe, which is called Castle Rock, has intruded near the intersectionof several major faults. The two largest stocks have intruded within a few
hundred feet of each other, and a line connecting the centers of the twotrends north-northeast. The larger of the two stocks, which forms Oro
Quay Peak, and the two remaining stocks lie in a straight line trending
east-northeast. The stocks decrease in size along the line toward the east,
with the largest on the west, the middle-sized stock in the middle, and thesmallest at the east end.

Laccoliths
The mountains of the San Pedro—Ortiz porphyry belt have beentermed "laccolithic mountains" since the earliest geologists examinedthem.
In 1898, Herrick (pc, 113) called San Lazarus Gulch "essentiallyan old andesite crater with more or fewer parasitic acid f l o w s . "18

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

Perhaps Yung and McCaffery (p. 353) believed that Oro Quay Peakwas the eroded conduit of the laccolith. But it appears that they did notrecognize the laccolith exposed on San Pedro Mountain when theydescribed it: "The intruded monzonite porphyry has entered like a wedge
under the western end of the mountain, tilting the strata about 13 degrees
to the e a s t . " They say nothing about this being a laccolith.
Lindgren is probably the first to have recognized the laccolith(Lindgren et a l . , 1910, p. 170).
Keyes (1909, p. 368) visited the San Pedro Mountains in 1909,
several years after Lindgren, but Lindgren's observations had not yet been
published. Keyes interpreted the structure of the San Pedro Mountains as in
Figure 1.

Figure 1. Structure of the San Pedro Mountains (Keyes)

When mapping was begun in June 1959, the author was skeptical ofthe many prospectors' reports of a laccolith, but mapping has conclusively
proved the laccolithic nature of these intrusions. In most places the roof isconcordant and dips away from the center of the laccolith (pl. 4). The baseof the laccolith usually follows the bedding, but it could not be seen
everywhere because at places the edges of the laccolith are cut by low-angle
normal faults.
The small monzonite porphyry stock in Old Timer Gulch isinterpreted as the feeder for the laccolith. On the north side of
the stock, a dike grades from coarse, dark monzonite to lightlatite porphyry. On the opposite side of the stock, the monzonite
grades into monzonite porphyry and latite porphyry in a small sill.
A long patch of monzonite porphyry crops out along the

GEOLOGY OF SAN PEDRO MOUNTAINS

19

western margin of the mountains. The top of this intrusive is concordant;
its bottom is not exposed. In preparing a north-south cross section throughSan Pedro Mountain (p1. 4, B-B'), it was noted that the beds are bent
downward at the south edge of the mountains beneath the exposedlaccolith. This bend in the beds and the concordant porphyry beneath theexposed laccolith were interpreted as belonging to another, largely buried,
laccolith. This is shown on sections A - A ' and B-B' (pl. 4).

Sills

Rhyolite porphyry and latite porphyry sills are common in the San
Pedro Mountains, They range in thickness from a few feet to more than
400 feet. The sills are usually extensive and were not observed to pinch
out along the strike in this small area (pl. 6).
The rhyolite sills seem to be somewhat more irregular than thelatite porphyry sills. In many places the rhyolite sills follow the bedding
to some joint or fault, cross the bedding for a short distance, then follow
the bedding again. A sill of this type is illustrated in Plate 4, section B–
B’.

Dikes

Dikes intrude all the sedimentary formations in the area. Rhyolite
p o r p h y r y a n d l a t i t e porphyry are the principal rock types, but a few
diabase dikes occur.
Most of the dikes follow nearly vertical fractures and are straightlines on the map, but some dip at angles as low as 50 degrees and describe
a rather irregular outcrop over rugged terrain. Other dikes appear to bevertical but are rather irregular and branching. These are near the stocks in
an apparently highly faulted area (pl. 7).
The augite-bearing monzonite porphyry dike presents a problem.
Jointing in the dike varies from almost horizontal to 10 degrees west. The
sedimentary rocks nearby dip 21 degrees east. At the top of the ridge
above Castle Rock, the same porphyry is very thin, rests on thesedimentary rocks, and may be lifted off in slabs a few inches thick. The
joints and the attitude of the slabs lying on the sedimentary rocks indicatethat the dike is probably a rather thin erosional remnant, but this is notcertain.
In most parts of the mountains the dikes are randomly oriented,
but on the small hill in the northeastern corner of the

20

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

area, dikes radiate from either the small monzonite stock or from a point at
the edge of the middle monzonite stock mentioned above (pl. 7) The
radiating dikes are rhyolite porphyry and latite porphyry. They are probably
more numerous on this hill because the intruded rock is Chinle shale, therather incompetent upper part of the Dockum Group.
Irregular, dikelike intrusions of rhyolite porphyry follow themargins of the stocks in many places (pl. 7) It appears that this was thesource of the numerous rhyolite porphyry sills and dikes.

FaultsThe San Pedro Mountains have been faulted extensively. It was
mentioned before that the San Pedro Mountains are part of the San Pedro
horst, which is a wedge-shaped block between two faults of the Tijeras fault
system (pl. 5) The Dooley fault on the south side of the horst cuts throughOro Quay Peak and is lost in the Recent sediments on the north side of themountains. The Golden fault passes somewhat to the northeast of the San
Pedro Mountains.
A pair of faults appears in the center of the San Pedro horst andcurves to the northeast, finally joining the Golden fault. The eastern fault
bifurcates about one-half mile north of San Pedro village and passes
through Old Timer Gulch. The Old Timer stock lies a few hundred feetwest of this fault.
Many of the larger faults are parallel to the Dooley fault or tobifurcations of the larger faults east of the mountains, but many other faults
are at right angles to the Tijeras fault system (pl. 8)
The faults are all normal faults and are believed to dip from 70 to
90 degrees. The attitude of the dipping faults was determined from theamount of deviation from a straight line on a map as the fault passes over ahigh point in the mountains.
Although a fault in , San Pedro Mountain was found to have a dip ofabout 70 degrees north by the method outlined above, its apparent dip on
section A-A’ . is about 20 degrees east, because it is nearly parallel to theline of the section.
Displacement on the faults is as much as 300 feet. The
largest displacement could be measured from the relative displacement of the San Andres Formation. Many sills served as
marker horizons from which dip separations could be measured.
Two ages of faulting are readily apparent in the area.
Faulting prior to the intrusions provided channelways for emplacement of dikes, and intersections of several faults apparently
served as loci for the emplacement of the stocks. Faulting sub-

GEOLOGY OF SAN PEDRO MOUNTAINS

21

sequent to intrusion has probably taken place largely along older faults, andthe intrusive bodies have been faulted.

Folds

The general dip of the sedimentary rocks is from 10 to 50 degrees
easterly. Folding has occurred on a minor scale associated with intrusion.
General doming is apparent around several stocks and over thelaccoliths. Doming perhaps is best developed around the stock in Old Timer
Gulch beneath the upper laccolith. Here the beds dip away from the stock on
all sides. In a general way, beds are domed above the lower laccolith, andthe beds above the upper laccolith dip rather uniformly to the east but are
turned downward somewhat sharply at the edges of the laccolith.
Beds are turned upward at steep angles around the principal stocks,
especially at Oro Quay Peak. Just east of this peak the beds dip outward
from it.
General doming is especially apparent around the larger of thestocks northeast of Oro Quay Peak, due to the directions of the dips and theradial dikes and faults.
Doming caused by intrusions combined with the easterly dip of thearea has given the San Pedro Mountains a general anticlinal structure.

Summary
Two periods of deformation are responsible for the major structuresin the San Pedro Mountains. Faulting and folding is associated with a period
of intrusion, while later deformation consists principally of faulting.
Three stocks at the eastern end of the mountains lie on a northeasttrending line. Although no single fracture connects all three stocks,
intrusion probably has taken place through a fracture in the Precambrianbasement, possibly caused by intrusion of the Oro Quay stock, the earliest
and largest of the three.
Doming and the general anticlinal structure of the mountains has
been caused by intrusion of laccoliths and stocks.
Post-intrusion faulting may be related to the uplift of the San Pedro
horst along the Dooley and Golden faults in the Tijeras fault system. The
Dooley fault is joined by a closely related fault

22

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

just north of the Oro Quay stock. The two largest north-trending faults
which pass through Old Timer Gulch are related to the bifurcating faults tothe west of the mountains, at the center of the Tijeras fault system.
Other post-intrusive faults are arranged in a rather random mannerand are difficult to interpret.

GEOLOGIC HISTORY

The geologic history of the San Pedro Mountains may be
determined best by study of adjacent areas. In the San Pedro Mountains
faulting has obscured the thicknesses of the formations; intrusion andmetamorphism have largely destroyed the original lithology; and depositionof gravels surrounding the mountains has covered the strata above andbelow the exposed section.
The Precambrian basement in the area consists of a complex seriesof metamorphic rocks intruded by granite. During the late Pennsylvanianperiod a thick sequence of limestone and shale was laid on the eroded
Precambrian rocks. After a period of nondeposition or slight erosion at theend of the Pennsylvanian, continental Permian rocks were deposited.
During San Andres time the sea returned and laid down sandstone andlimestone. Nondeposition and perhaps slight stripping intervened until late
Triassic when sandstone and shale were deposited under continental
conditions.
Sedimentation appears to have ceased during early Jurassic timeprior to the deposition of the Entrada Sandstone. Beds of Todilto limestone
and gypsum above the sandstone indicate the invasion of a lake or ashallow arm of the sea over most of northern New Mexico. Thicksandstone and variegated shale were deposited under continental
conditions later during the Jurassic.
Some broad stripping may have taken place through early
Cretaceous, before the seas .invaded the Rocky Mountain geosyncline inlate Cretaceous time. During the persistent subsidence of this geosyncline athick sequence of marine shale and sandstone was laid down.
Broad upwarping took place at the end of the Cretaceous, and the
Rocky Mountain area underwent considerable orogeny. During the Eocene
the Galisteo Formation was deposited unconformably on Upper Cretaceous
rocks by rivers flowing into and through a broad basin in the area of theporphyry belt, which was deepened and enlarged by warping contemporary
with deposition (Stearns, 1953, p. 467).

GEOLOGY OF SAN PEDRO MOUNTAINS23

The earliest period of "modern" deformation in the San Pedro
Mountains occurred at the time of the first intrusions. It is probable that
this was contemporaneous with intrusions in the other centers of theporphyry belt. Intrusion in the Cerrillos and possibly the Ortiz Mountains
was followed by eruption. Disbrow (p. 19-22) believed that the Espinaso
Volcanic Formation was de-rived from the Cerrillos eruptive center. In alater paper (Disbrow and Stoll, 1957, p. 11), Disbrow presented evidence
for dating the base of the Espinaso Formation as early Oligocene. He
further states that the extrusion of the formation may span Oligocene andearly Miocene time. The date of the San Pedro intrusions may, therefore,lie between early Oligocene and early Miocene.
Later deformation in the San Pedro Mountains probably occurred
during the same period as the major deformation of the Sandia uplift.
Reynolds (p. 53) stated that deformation in the Hagan—La Madera areaof the Sandia Mountains definitely began after deposition of the Espinaso
Volcanics, and that two phases of deformation took place prior to andafter deposition of the late Miocene and Pliocene Santa Fe Formation,
while some continuing deformation may have occurred during thedeposition of the Santa Fe Formation.
Uplift of the San Pedro Mountains probably took place from late
Miocene through Pliocene. Stearns (p. 473) stated that the Santa Fe
Formation contains fragments of contact metamorphic rocks which can
only have been derived from the Ortiz Mountains, San Pedro Mountains,and South Mountain.
Since the uplift of the porphyry belt, continual erosion has takenplace in the mountains, forming thick deposits of gravel at their bases.
The gravel deposits themselves have been subjected to erosion in
Quaternary time.

Economic geology
INTRODUCTION

The New Placers mining district has a long history of
production of base and precious metals. Gold placers encircle
the San Pedro Mountains, This gold was derived from veins andbedded deposits. Gold, lead, zinc, iron for fluxing, and somecopper have been produced from the veins. Copper and gold havebeen produced from contact metamorphic bedded deposits. Limestone has been quarried from the Madera Limestone, Recently,
pure quartz sand has been mined from the Glorieta Sandstone, anddevelopment work has prepared the sandstone for open-pit mining.
Other resources have been explored but not yet developed.
Magnetite occurs in contact deposits, and tungsten reserves havebeen investigated by the U.S. Geological Survey (Smith et al.).

HISTORY AND PRODUCTION

The porphyry belt has a long and colorful history of mining datingback to prehistoric times, The Indians of New Mexico mined turquoise in
the Cerrillos area for many years before the Spaniards came to New
Mexico, The Spaniards were especially eager to find mineral deposits.
Northrop (1959, p, 10) states that mineral deposits were discovered near
Cerrillos as early as 1581, and that assays of ore from the Cerrillos—Ortiz
region were made in 1583. Northrop (p. 14) also cites a reference whichclaimed that the Ortiz mine was a famous gold producer in 1680, Jones(1904, p. 30) tells that the Mina del Tiro was operated before 1680, andNorthrop (p, 17) says that a lead mine in the Cerrillos was granted byGovernor Hurtado in 1717 and that "gold was mined at La Mina de laTierra (or del Tiro) in the Cerrillos district in 1722." None of the New
Mexico historians refers to mining in the San Pedro M o u n t a i n s prior to1839, but Fray Dominguez mentions in his report (The Missions of New
Mexico, 1776) that the place name San Pedro, among others, was found in
the baptismal book of San Juan, In addition, he prepared an excellent map
which shows a village named San Pedro at the

GEOLOGY OF SAN PEDRO MOUNTAINS

25

western foot of the present San Pedro Mountains. It is probable that San
Pedro was even then a mining village. It is not an especially favorable
area for farming, Wells had to be dug for water, and it was many miles tothe nearest protection from Indian raids.
In 1828, gold was discovered in the Ortiz Mountains by aherder. Although many later writers simply state that it was a
sheepherder, Prince (1883, p. 241) states that it was discovered by acitizen of Sonora who was herding cattle. The herder saw some rockwhich resembled gold-bearing rock in his native state, and when he
examined the rock he found that it did contain gold. This discovery led tothe exploitation of the placer deposits at the foot of the Ortiz Mountains.
Lode gold was discovered in the Ortiz Mountains in 1833 by a man
named Ortiz. Legend has it that Ortiz knew nothing about mining andwent into partnership with an experienced Spaniard named Lopez. In
order to get rid of Lopez, the greedy Ortiz persuaded officials to deport
Lopez by an old edict which said that foreigners could not work in themines. But Ortiz and the corrupt officials did not obtain “one grain ofgold” (Jones, p. 23).
In 1839, the New Placers were discovered, and the village of
Tuerto, now Golden, rose into prominence as a business center. By 1845,
the town had 22 stores and transacted more business than Santa Fe (Prince,p. 243); by 1849, it had a population of 7000 with 35 stores and numerous
saloons and gambling halls. Jones (p. 25) stated that San Lazarus Gulch
was a "steady producer at this time." During this period the annual outputof the Old Placers and New Placers reached $250,000 and as many as 2000
men congregated there to work during the winter (Prince, p. 243).
Water was so scarce that winter was the "favorite time forworking the placers. Snow was melted in little reservoirs in the arroyos
with hot rocks, and the gold was then washed with wooden bateas, or handmade gold pans (Jones, p. 21).
Prince first stated that the annual production of the two placer
districts reached $25,000 per year in 1845, and Jones cited Prince asstating that the annual production of gold taken out of the Old Placers was$60,000 to $80,000 between 1832 and 1835; Wells and Wooton (1932, p.
18) report that in later years (after 1835) the production of the Old Placers
was not more than $30,000 to $40,000 per year. If Prince's original figure
is correct, the New P lac er s district produced $210,000 in 1845. This israther remarkable, considering that under the Mexican laws, each miner
held only one claim, ten paces in all directions from his discovery pit. If
2000 miners were there, this would be an average of $100 per miner for. a
winter's work. During the depression from 1932 to 1935, poverty-stricken
miners earned from $3 to $10 per week in the New Placers (James, 1955, p.
21), and present-day prospectors report that they recover from $2 to $4 ingold per day.

26

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

Several early travelers mention the San Pedro mine. Dr. Wislizenus(Bullock, 1952, p. 5) visited the San Pedro mine in 1846; it was not being
worked at the time. He reported that a Mr. Tournier had already sunk ashaft 40 varas (about 110 feet) and drifted 30 varas (about 80 feet). The ore
was carried by mules to the town of San Pedro and treated in an
amalgamation mill. Mr. Tournier told Dr. Wislizenus that he worked about
2-1/2 cargas (750 pounds) of ore each day with mule-driven mills, and heobtained about $l2 worth of gold per day.
In the same year Lieutenant J. W. Abert visited San Pedro. He
found the life of the placer miners very miserable and said he was happy he
did not own a gold mine. He also visited the San Pedro mine and collected
specimens of ore (Bullock, p. 5).
Perhaps the most accurate early history of the San Pedro mine wasrelated by M. A. Otero, former governor of New Mexico, in his book My
Life on the Frontier. The mine was rediscovered about 1840 by Antonio
Aguilar and Mariano Varela, both from Chihuahua. They were financed by a
Chihuahua lawyer, Don Carlos Jaquez. The ore was carried to Chihuahua on
the backs of burros. Later, Aguilar and Varela gave the deed to the mine to
Otero's uncle for payment of debts.
Just before the Mexican War, Jose Serafin Ramirez secured a small
land grant from the Mexican government. After the U.S. occupation, theland grant was recognized and a patent was granted in 1875. The grant,
called Canon del Agua, had no connection with the mine and had beenobtained for the purpose of establishing a sheep headquarters.
News of the Civil War depopulated the area overnight, and Golden
was deserted. Mining activity ceased completely, and it was many years
before activity resumed (Bullock, p. 6).
Otero tells us that sometime about 1880, the Santa Fe Ring, an
unscrupulous and powerful gang, found out about the San Pedro mine andarranged for Ramirez to have his grant resurveyed. The boundaries were
exchanged, and the east boundary of the grant was now the west boundary,so the: grant included the San Pedro mine property. Immediately the San
Pedro and Canon del Agua Mining Company was organized in Boston,where shares moved like wild-f ir e.
At this time Otero's uncle died and Otero's cousin discovered the deed to the mine in his father's papers. Otero's father
immediately took up the matter and consulted Carlos Jaquez, now
a judge in Chihuahua, whom he found still had an interest in themine. Judge Jaquez and the older Otero were able to identify theproperty easily and started legal proceedings to secure the mine.
When the Santa Fe Ring heard the news, the mining company
set up fortifications at the mine. They built stone breastworks, anarmory, and even installed a small French cannon. Notices were

GEOLOGY OF SAN PEDRO MOUNTAINS

27

posted at the mine and in the village and on numerous trees in the vicinity.
O t e r o ' s father had trouble prosecuting the case because theFederal Judge, L . B. Prince, was too soft and would not oppose the Santa
Fe Ring. Otero's father pressed for Prince's dismissal or resignation and he
finally resigned. Unfortunately, the Santa Fe Ring then secured theappointment of one of their own members.
At this tithe Otero's father died and all proceedings were stopped.
The Santa Fe Ring felt secure, but they had not reckoned with Otero
himself.
Otero discussed the situation with several of his prominent friends;
they decided that since they would be unable to obtain an injunction fromthe corrupt judge, they would be successful only by taking the mine byforce of arms. He bought 65 new Winchester rifles and shipped them to afriend in Golden. Otero and a dozen men took a roundabout way to the mineand climbed down the shaft on a rope. Each man had a rifle on his back, aColt pistol in his belt, and a flask of whiskey "in case of snakebite."
While they hid in the mine they counted 162 men who entered to go
to work. They waited until the same number left for lunch and then rushed
out the tunnel behind the miners, captured the mine, and forced the miners
to go back down to the village without lunch.
Dynamite charges were set in the tunnels. Otero threatened to blast
the mine if the company attempted to recapture the fortifications.
Unhappily, Otero was called back to Las Vegas on business, where
he was arrested and brought to trial. Although Federal Judge Samuel B.
Axte l l, the Ring's appointee, presided, the jury returned a verdict of not
guilty.
Because Judge Axtell was having trouble with the authorities inconnection with his corrupt legal practices, he was forced to give Otero's
men permission to surrender the mine to the court, and to appoint acustodian.
Several years later the case was decided in Otero's favor by anotherjudge, and the verdict was upheld by the Supreme Court. The boundaries of
the Canon del Agua Grant were declared fraudulent and the grant was
resurveyed and its size reduced greatly. The land was then thrown open formine location, but Otero's friends in Golden double-crossed him andclaimed the San Pedro mine for themselves.
Bullock (p. 6) filled in the remaining details of the history of the
San Pedro area. Before Otero interfered, the San Pedro and Canon delAgua company had begun extensive development work. Dams were built in
Los Tuertos and Madera Canyons in the Sandia Mountains, and 22 miles of
14-inch pipe were laid to bring water for placer mining. Stamp mills and a
60-ton smelter were28

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

built. General U. S. Grant was elected president of the company, but when
he heard that the stock jumped because his name was used, he resigned.
Perhaps he had heard of the company's crooked dealings.
Bullock (p a 6) mentioned the suit Otero's father initiated in 1881,
and told that litigation shut down the San Pedro mine and that a drouth shut
down the placer operations.
In 1887, a new gold strike was reported at San Pedro, and theGolden Hydraulic Company repaired the stamp mills, smelter, pipe lines,
and reservoirs. Placer mining and work at San Pedro mine resulted in 250
employees on the regular payroll.
The Lewisohn family of New York acquired control of the San
Pedro mine in 1889 and operated as the Santa Fe Gold and Copper
Company. A 100-ton concentrator was installed in 1891.
The U.S. Supreme Court decision in 1892, upholding the Territorial
Court ruling that the boundaries of the Canon del Agua Grant were
fraudulent, opened up 3300 acres of mineral lands, which attracted
numerous miners and prospectors.
During the 1890's an aerial tramway carried the ore from the San
Pedro mine to the smelter at the village below. In 1900, a new smelter was
built at the mine and operated until 1918 when it was destroyed in a fire
(C. F. Williams, oral communication, 1960, and Ellis, 1929, p. 98).
In 1938, the Lewisohns sold the San Pedro mine to the Raskob
Mining Interests, Inc. In 1940 and 1941, more than four million pounds ofcopper were mined, and then the mine was closed down. In 1945, the mine
was sold to Mr. C. F. Williams, the present owner. Mr. Torn Scartaccini
leased the mine in 1956 and has shipped a few carloads of ore since then.
Dr. V. C. Kelley of the University of New Mexico discovered
scheelite in the mine in 1944, which led to an investigation of the tungsten
reserves by the U. S. Geological Survey.
No records are available for the total production of the district.
Northrop (p. 31) stated that 40 to 50 tons of ore valued at $250 per ton
were produced in 1883. Lindgren et al, (p. 172) said that the production ofcopper from the San Pedro mine from 1889 to 1892 was several million
pounds and that gold production for the district was $2000 to $4000 per
year. Bullock (p. 9) stated that the San Pedro and Canon del Agua company
produced about $60,000 worth of bullion per month before 1881. Smith etal, (table 1) cited Johnson's (1943) production figures for the San Pedro
mine. From 1907 to 1918, about 8.5 million pounds of copper were
produced; from 1923 to 1939, one million pounds were produced; andduring 1940 and 1941, more than four million pounds were produced.

GEOLOGY OF SAN PEDRO MOUNTAINS

29

ORE DEPOSITS
Copper

The most important mineral deposits in the San Pedro Mountains
are the copper deposits, which have been extensively exploited at the San
Pedro mine. More than twelve million pounds of copper have been
produced since 1907 (Smith et a l . , p. 3).
Bedded copper deposits occur on the western side of San Pedro
Mountain and are limited to the upper beds of the Madera Limestone. The
beds lie immediately below a rhyolite sill and as much as fifty feet above
the monzonite porphyry laccolith. An easterly-trending monzonite
porphyry dike forms a ridge on the western side of San Pedro Mountain.
Some small copper deposits occur directly against the dike. The deposit at
San Pedro mine is about 1000 feet from the dike. A monzonite dike was
encountered in one drift in the mine, but it does not crop out at the surface
and its attitude with respect to the ore bodies is not known.
The ore bodies are generally tabular or pod-shaped, parallel to thebedding planes. The ore bodies are highly irregular in outline but remain
strictly confined to single favorable beds. The sizes of the ore bodies varyfrom patches of ore a few inches across to bodies 100 feet wide and 1000
feet long.
The upper limestone beds of the Madera Formation have been
extensively replaced by massive garnet rock, or tactite. The limits ofgarnetization are sharply defined by a highly irregular "marble line." On
the side of the " lin e " toward the ridge-forming monzonite porphyry dike,
the limestone has been replaced by garnet, while on the other side thelimestone has been metamorphosed to coarse, sugary marble. The marble
becomes progressively finer grained farther away from the "marble line"
and finally grades into unaltered limestone.
Smith et al. (p. 17-19) have discussed the relationship of the ore
to the "marble line" in detail. They found that the ore occurs in the tactite
adjacent to the "marble line," or not more than 200 feet from it in most
instances. No ore occurs in the marble.
The volume of the original limestone was somewhat greater thanthat of the replacing garnet. This difference in volume is represented byabundant cavities, in which the ore has been deposited. The smaller
cavities, up to several inches in diameter, were completely filled by ore
and gangue, but larger cavities, some many feet in diameter, were notfilled and permitted a second stage of deposition.

30

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

In the first stage of mineralization, chalcopyrite, bornite, pyrite,
pyrrhotite, calcite, specular hematite, quartz, and chlorite were depositedin the openings iii the tactite. Scheelite is also reported to be disseminatedin the tactite (Smith et a l . , p. 20). This ore is referred to as disseminatedore.
The second stage of mineralization is represented by coarsely
crystallized filling of the larger cavities. Quartz, pyrite, chalcopyrite, andabundant calcite are common; lesser amounts of siderite are present insome places. Some scheelite crystals were observed in the coarsely
crystallized material. In the weathered outcrop at the Spanish cut of the
San Pedro mine, coarsely crystallized adularia is abundant.
Although the paragenesis of the ores has not been completely
worked out, study of the coarsely crystallized material and a few polishedsections of the disseminated ore has yielded some information on thesequence of deposition. Chalcopyrite has replaced both pyrite andpyrrhotite in disseminated ore. In the later stages of deposition, quartz
crystals are enclosed by pyrite, chalcopyrite, and calcite. Pyrite crystals
are enclosed by chalcopyrite, and some calcite encloses chalcopyrite. The
calcite and chalcopyrite appear to have been deposited simultaneously,
since small crystals of chalcopyrite are not attached to earlier minerals
within the calcite. Striking pseudomorphs of pyrite after calcite give
evidence of later replacement of calcite by pyrite (pl. 9).
Many of these minerals are exceptionally well crystallized in the
San Pedro mine and deserve a brief description. Specular hematite isfrequently found in cavities as rosettes of extremely thin crystals, whichshow a deep red internal reflection. Calcite occurring with hematite
generally displays a rhombohedral form modified on the edges byscalenohedral faces. In one place these crystals were commonly twinnedon the rhombohedron face (1011), Calcite deposited directly on the
specularite often displays single scalenohedrons up to two inches longmodified by the rhombohedron (4041). Calcite in the coarsely crystallized
ore occurs as scalenohedrons usually about one inch long but occasionally
up to six inches long. The scalenohedrons are often twinned on (0001) or(1011). A remarkable cyclic twin composed of six scalenohedrons twinnedon (1011) and apparently on (1101) on a single central scalenohedron was
discovered near the water level.
An unusual phenomenon observed in the San Pedro Mountains is
the occurrence of quartz twinned according to the Japanese law, These twinsare fairly common in many places in the San Pedro mine, and one otheroccurrence was noted on a dump in Old Timer Gulch,
Mineral collectors in Albuquerque have reported the occurrence ofamethyst from a single vug in the Swan tunnel level of the San Pedro
mine. A light-purple amethyst labeled San Pedro mine

GEOLOGY OF SAN PEDRO MOUNTAINS

31

Plate 9. Pyrite pseudomorphs after calcite from the San
Pedro mine.

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NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

in the museum of the Geology Department of the University of New
Mexico is about two inches in diameter.
Chalcopyrite occurs in uncommonly large crystals, up to 3-1/2
inches across. Crystals whose principal form is the disphenoid (112) arecommonly twinned on (112) and modified by a tetragonal scalenohedron,probably (531). Sherman P. Marsh has measured the interfacial angles onsome highly modified crystals, 2 inches in length, which he found in asingle vug at the water level. These crystals were associated with calcite,
quartz, and scheelite. The measurements showed that the crystals included
positive disphenoid (112), negative disphenoid (112), tetragonal prism of
the first order (110), basal pinacoid (001) and the rare tetragonal
scalenohedron (101) (fig. 2; p1. 10).
The large chalcopyrite crystals usually formed around a pyrite
nucleus. These pyrite crystals, as a rule, are combinations of cube,
octahedron, pyritohedron, and diploid, about half an inch in diameter.
Simple octahedrons occur in the large cavities beneath the coarsely
crystallized minerals. Pyrite cubes up to one inch across are notuncommon. The pseudomorphs of pyrite after calcite, mentioned above, areup to one inch long and faithfully follow the outlines of the calcite
scalenohedrons. In all the cases observed, the pseudomorphs wereconnected to pyrite cubes from one-half to one inch across. Many of thecalcite crystals were only partially replaced, and the point of attachment to
the pyrite cube appears to have been the starting point of the replacement.
Euhedral scheelite crystals up to half an inch across were observedin the vuggy ore but are rare.
Siderite is found in several places in the San Pedro mine in two
types of occurrences. In some of the smaller vugs it occurs as columnar
aggregates of scalenohedral crystals growing vertically from the floor of
the vug. In the upper part of the mine siderite occurs with calcite andlimonite in partially oxidized ore as rhombohedrons half an inch across.
Adularia occurs as well-formed euhedral crystals up to 1-1/2 inches
long in the Spanish cut at the San Pedro mine.
Tetrahedrite (Northrop, p. 358) and molybdenite (Smith et a l . , p.
16, and Northrop, p. 358) have been reported by earlier workers but were
not observed by the present writer.
The copper deposits are oxidized to a depth of about 100 feet
(Smith et al., p. 17). Chrysocolla, malachite, and azurite are rather
common, and Smith et al. (p. 17) list melaconite (a powdery tenorite, CuO)
and cuprite. A specimen in an old collection at the University of New
Mexico, labeled San Pedro, Santa Fe Co., N. Mex., contained native copperin cuprite with quartz and malachite. The texture of the ore is typical ofthat from the San Pedro mine and the author has no doubt about the originof the ore. This is the first record of native copper from this locality.
LimoniteGEOLOGY OF SAN PEDRO MOUNTAINS

Figure 2. Chalcopyrite crystals from the San Pedro mine.

Plate 10. Unusually large chalcopyrite crystal from the
San Pedro mine.

33

34

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

is abundant, and chalcanthite is found as a coating on the walls of themines in many places. Melanterite was reported from diamond drill hole U2 in the San Pedro mine (Smith et al., appendix p. 7). Smith et al. (p. 17)
note that although chalcocite films coat chalcopyrite in several places, nonoteworthy bodies of secondarily enriched ore have been found. This isprobably due to the fact that the garnet rock is not sufficiently porous toallow free circulation of oxygen-charged ground water.
Minerals in the disseminated ore, especially specularite andpyrrhotite, indicate a rather high temperature of deposition. Later filling in
the large open cavities apparently fits Lindgren's mesothermal deposits.
This is indicated by abundant, coarse carbonates, especially siderite, andthe lack of characteristic features of epithermal deposits, such as delicate
crustified banding. The presence of the coarse adularia probably does notindicate low-temperature deposition. Adularia is usually regarded as a lowtemperature mineral, but Lindgren (1928, p. 692-694) classifies certain
veins in the Alps which contain coarsely crystallized adularia as
mesothermal.
Smith et al. (p. 16) stated that the ore at the San Pedro mineaverages about 3 percent copper, although locally much higher grade ore
has been mined. Gold content has averaged about 0.05 ounce per ton andsilver about 0.8 ounce per ton. High-grade pockets of gold have beenencountered, and specimens of gold in calcite have been sold to mineralcollectors.
In 1944, the underground workings of the San Pedro mine were
examined by J. R. Cooper and F. W. Farwell, of the U.S. Geological
Survey, f o l l o w i n g Dr. V. C. Kelley's discovery of scheelite. It was
found that the areas of tungsten concentration do not always occur in areasof copper concentration. The highest grade of tungsten ore was estimated to
be about 1 to 2 percent W0 3 , whereas the average grade of mineable bodies
was between 0.1 and 0.5 percent W0 3 (Smith et a l . , p. 20).

Gold

Veins and Bedded Deposits

Gold-bearing veins occur in all the metamorphosed areas and in all
the igneous rocks in the San Pedro Mountains.
At both ends of the mountains the gold veins are clustered about
the stocks. Veins are more numerous in the eastern end of the mountains
where there are several stocks.

GEOLOGY OF SAN PEDRO MOUNTAINS

35

In the western part of the mountains the gold veins are mostcommon in the latite porphyry and monzonite porphyry of the laccolith. In
the central and eastern parts of the mountains the veins are most common
in hornfels, although a few rich veins occur in monzonite. They do notoccur in the tactite and unmetamorphosed rocks. Veins in rhyolite
porphyry were apparently too poor to mine. In the laccolith and in Old
Timer Gulch, the veins belong to two sets, one striking from N. 75° E. to
N. 80° E., and the other striking from about N. to N. 25° E. In San
Lazarus Gulch most of the veins strike from S. 60° E. to S. 50° E . , andone vein strikes S. 30° E. In the vicinity of Oro Quay Peak, the veins
belong to two intersecting sets, one striking from N. 10° E. to N. 30° E.
and the other from N. 70° E. to N. 85°E. East of Oro Quay Peak, many ofthe veins are associated with dikes and these and other veins radiate fromthe largest of the eastern stocks.
Yung and McCaffery (p. 357) were the first to describe thecharacter of the veins in the San Pedro Mountains, and subsequent reports
have drawn mostly on their report. They describe the gold veins as being
either crushed and fissured zones or banded fissure veins. Most of theveins in the district are of the first kind.
Yung and McCaffery (p. 357) further write that the veins have no
definite walls. The gold and associated minerals have entered the cracks
in the shattered rock and form a network of veinlets. There are a great
number of these veins, but Yung and McCaffery say that they are "allsmall, isolated, pockety, and unreliable," and that there has been no
successful mine on these veins. Most of the veins cannot be traced formore than a few hundred feet on the surface.
Banded fissure veins are quite rare. In the course of mapping,some pieces of banded float were picked up, but none was seen in any
prospect. Yung and McCaffery (pa 361) gave the Ortiz mine in the Ortiz
Mountains as the only good example of a deposit of this type.
Gold occurs in metasomatized beds in many places. Yung and
McCaffery (p. 360) briefly mention this type of deposit. Lindgren et al.
(p. 172) stated that the San Lazarus mines were on a bed which contained
auriferous pyrite. Most of the bedded deposits observed were not morethan 3 feet thick and 30 feet long on the outcrop.
Gold is associated with pyrite, calcite, quartz, and rarely barite.
Yung and McCaffery (p. 358) stated that "the gold, which is usuallycoarse, occurs with these minerals, and occasionally furnishes finespecimens of wire and sheet gold." Ellis (p. 98) stated that in the Argomine (Old Timer mine), "the ore occurs as free gold in fissure veins inporphyry, and the assay value is said to be from $10 to $100,000 per ton."
Jones (p. 27) stated

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NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

that “some very fine specimens of leaf and wire gold have been taken fromthe various properties. Beautiful specimens enclosed in calcite have beenfound in the Gold Standard m i n e . ” In some places, the quartz contains freegold; at the Brooks mine and in the McKinley mine, free gold is associatedwith pyrite, quartz, and calcite (Lindgren et a l . , p. 172). Ellis believed thatpyrite probably contained the gold. The quartz observed in these deposits
ranges from massive white "bull" quartz to coarse crystals 1 mm long and
0.2 mm wide.
Some of the ores observed during the course of mapping were
mostly pyrite, with little quartz and no calcite. One small fissure branchingfrom a principal vein contained calcite, pyrite, and adularia crystals. The
veins are all oxidized at the surface, but oxidation does not reach the watertable in most places. In the oxidized portions of the veins the ore consistsof quartz boxwork filled with limonite.
In bedded deposits, the gold occurs in pyrite in impure hornfels
beds containing much garnet. In some places gold occurs in thin pyritized
lenses in tactite.

Placer Deposits

Placer deposits were not examined in detail during the course of
mapping. Yung and McCaffery (p. 361) described the placers in somedetail, as follows: "Gold has collected in fan-shaped placers at the foot of
each mountain arroyo. The gold is derived from the weathering of thenumerous small veins and…all the gravel carries gold in small quantities."
The gravel is coarsest at the edge of the mountains, where it contains
boulders up to one foot in diameter, and is 20 to 30 feet deep on the bedrock.
They state that "the richest streak is naturally next to the bedrock; and,judging from a great number of working-tests, this will run about $1 per cu.
yd., while the surface will not average over 30 or 40 cents per cu. yd."Wells and Wooton (p. 20) stated that several unsuccessful attempts atdredging have been made and that the most serious difficulty has been the
scarcity of water.
The gold is in the form of coarse and scarcely water-worn wire andflat scales, just as it came from the vein (Yung and McCaffery, p. 362).
Jones (p. 22) wrote that the largest nugget reported was worth $3400 but
warned that "the size of the nugget thus obtained is much to be doubted."
He stated that the fineness is about 918. Dr. F. A. Wislizenus analyzed
placer gold from the New Placers in 1846 and was cited by Northrop (p.
264):

GEOLOGY OF SAN PEDRO MOUNTAINS

37

Gold

SilverIron and. Silica

Total

92. 5

3.5

4.0

100.0

Northrop ( p 0 264) stated that the analysis was repeated by Blake (1856,
p. 94) and by Stevenson (1881, p. 399). Northrop added that "according toDale Car lson ( m e m o . , O c t. 5 , 1953), so me coarse nuggets in the New
Placers district have cavities lined with wires and crystals."

Lead-Zinc

Lead-zinc deposits are restricted to the southwestern corner of the
San Pedro Mountains and are found only in the Madera Limestone.
Most of the deposits are a few feet wide and as long as 1000 feet in
the outcrop. The ore appears to have replaced certain favorable beds, andthe deposits are usually no deeper than the thickness of the bed.
The small deposits have the shape of veins, but the large deposit at
the Carnahan mine (formerly Lincoln—Lucky mine) is an ore pipe. In theore shoots, the pipe is elliptical in cross section; elsewhere it is tabular.
The veins belong to a set which strikes from N. 75° E. to N. 90° E . , themost frequent strike being about N. 80° E.
At the surface many of the veins are capped with gossan, which
could be traced as far as 1000 feet in one instance. It is difficult todetermine the original condition of the veins from their present oxidized
condition, but several reports by early investigators are available.
The best lead-zinc deposit is at the Carnahan mine. Yung and
McCaffery (p. 357) wrote that the primary ore is composed of galena,
sphalerite, alabandite, marcasite and pyrite, with a small amount ofchalcopyrite. Lindgren et al. (p. 172) added that quartz and calcite are thegangue minerals. The quartz observed by this writer was fine grained andmassive, with vugs containing quartz crystals up to 1 mm long and 0.2 mm
across. Fluorite was observed at one prospect, and Northrop (p. 243)
mentioned that it was found at the village of San Pedro. Northrop also
listed traces of argentite from the Carnahan mine.
Northrop cited a number of oxidized minerals from the lead-zincd e p o s i t s. Anglesite, cerussite, botryoidal hematite,

38

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

limonite, possibly minium (Pb 3 O 4 ), pyrolusite, and native silver
are listed from the Carnahan mine, and cerargyrite is listed from theJackson mine. Northrop credited Dale Carlson for discovery of theminerals listed from the Carnahan mine. Abundant kaolin is also present in
the upper portions of this mine.
This type of deposit has not been very important commercially,
according to Yung and McCaffery (p. 357). The oxidized ores in the
Carnahan mine contained a little zinc as hemimorphite, but oxidized leadand silver minerals were most important in the upper parts of the veins.
Galena and sphalerite were important in the Carnahan mine in unoxidizedore below the water table. A few tons of manganese oxides were mined
after World War II.

Iron

Iron occurs in veins and bedded deposits, as magnetite andspecularite, and as hematite and limonite derived from the oxidation of
pyrite.
Most of the iron deposits have been prospected for gold, but someiron has been mined for a flux. The deposits are small and will probably
not be economically important in the future.
Deposits containing magnetite and specularite are clustered aroundthe stocks with but few exceptions. There is no apparent relation between
the distribution of the deposits and dikes and sills. Deposits containing
magnetite alone are generally closer to the stocks than those containing
specularite alone, whereas deposits containing both magnetite andspecularite occupy an inter-mediate position. Most of the deposits lie in anortheasterly-trending belt which contains the four stocks at the easternend of the mountains. A few deposits are adjacent to the stock in Old
Timer Gulch. Specularite occurs in the garnet beds in which the deposits
at the San Pedro mine occur. This is the principal occurrence ofspecularite away from the stocks.
Veins containing limonite and hematite derived from the oxidationof pyrite are important for their gold and have been discussed under gold
deposits. Most of these veins do not contain much iron.
Few of the veins containing specularite and magnetite are well
exposed, and the size and attitudes of most of the veins could not be
determined. One specularite vein which has been mined occupies a fault
which is within and follows a rhyolite dike. A stope in a swell in the veinwhich is accessible is approximately 6 feet wide, 15 feet long, and 10 feethigh.

GEOLOGY OF SAN PEDRO MOUNTAINS

39

The mineralogy of the veins is simple. Most of the veins containmagnetite, specularite, and quartz. Apparently pyrite was present in all theiron veins. Several prospects contain pyrite, while others contain goethite
pseudomorphs after pyrite, quartz boxwork, or limonite gossan, whichindicate the former presence of pyrite. Most of the magnetite is finegrained, but some very coarsely crystallized magnetite occurs in two veinsalong the northern side of the Oro Quay stock. The crystals range fromone-fourth inch to two inches across. The ore appears to have been about
50 percent pyrite and contains neither quartz nor specularite. The
specularite vein mentioned above contains specularite with goethite
pseudomorphs after octahedral pyrite crystals, some small quartz crystals,
and some malachite.
Bedded iron deposits have originated through contact metamorphism and metasomatism of certain favorable beds. These magnetite
deposits usually have an elongated lens shape and are limited to single
beds. Within a deposit the ore is usually spotty, with occasional richconcentrations of magnetite Specularite occurs in many metamorphosed
beds but is never concentrated enough to mine. Magnetite crystals with adodecahedral habit are peculiar to the metasomatic deposits.
Magnetite and specularite occur in medium-grained epidote-richhornfels, which appears to have been derived from an argillaceous
sandstone. Fine tremolite needles, and green and colorless chlorite arecommon accessory minerals.
A small magnetite deposit on the eastern side of Oro Quay Peak isof particular interest, since it appears to be of contact metamorphic origin.
The stock is in contact here with the lime-stone of the San Andres
Formation, which is somewhat dolomitic at this place. Fine-grained
magnetite is associated with pyrite, malachite, chrysocolla, serpentine,
finely fibrous tremolite, and crystalline calcite. The serpentine, tremolite,
and calcite have originated from the contact metamorphism of the
dolomitic limestone.

ORIGIN OF ORE DEPOSITSThe ores of the San Pedro Mountains have been deposited by fluids
emanating from the intrusions. The copper and magnetite deposits haveformed as a part of metasomatic processes, while the gold, lead, and zinc
have been deposited as fissure fillings. Localization of ore bodies is due tostructural controls and favorable properties of various rock types.

40

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

The specific intrusions responsible for certain ore deposits are notalways clear. The deposit at the San Pedro mine may have arisen from thedike cutting through San Pedro Mountain, from the stock in San Lazarus
Gulch, or perhaps from the stock at Oro Quay Peak.
Arching of the sediments by laccoliths has probably been the mostimportant structural control for ore deposition. The San Pedro mine deposit
occupies a slight anticline above the edge of a tilted laccolith. Other Jess
important, copper deposits lie in the broad arch above the laccolith and may
occupy the crests of other gentle anticlines.
Gold and lead-zinc veins occupy fractures which developed at thetime of intrusion or before. A few gold veins occupy fractures in the igneousrocks.
Favorable rock types have been at least as important as structuralcontrols. Limestone was particularly susceptible to replacement by ore andtactite. Rhyolite sills at the top of the Madera Limestone forced risingsolutions to pass through the limestone. Fracturing took place in hornfels
strong enough to hold fissures open, thus providing abundant channelways
for the deposition of gold ore. In contrast, the stronger rhyolite did not yieldmany open fissures. The pressures exerted during the late stages of intrusionof the monzonite stocks were sufficient to open within them some fissures
which were subsequently mineralized.
Areas in the immediate vicinity of the larger stocks were withinreach of the hydrothermal solutions but do not contain important mineraldeposits. Such areas did not have favorable conditions for the deposition ofore. The most important condition of all is probably an easily replaceable
limestone up-dip from a stock. Such a condition does exist where theMadera Limestone abuts the stock, but possible mineral deposits formed inthis manner are not exposed.
Lateral hydrothermal zoning outward from the stocks is evident in
the San Pedro Mountains (pl. 3), Magnetite veins are within or adjacent tothe stocks. The veins containing specularite are, in general, farther from thestocks. The metasomatic copper deposits are also farther from the stocks but
are somewhat closer than the lead-zinc deposits.

GEOLOGY OF SAN PEDRO MOUNTAINS41

NONMETALLIC DEPOSITS
Quartz Sand

The Glorieta Sandstone of the San Andres Formation on thesoutheastern slope of the San Pedro Mountains is unusually friable andclean. At present it is being mined in a small way.

Garnet

It has been stated above that certain garnet beds are more than 150
feet thick near the San Pedro mine. If demand were great enough, garnetmight be mined cheaply in surface cuts. In some places that are barren of
copper minerals, the garnet appears to be very pure, with little quartz or
calcite.
At present such garnet is used only as an abrasive. However,garnet mines in production elsewhere in the country are able to supply
industry with sufficient garnet for many years. Perhaps a new use will haveto be found before garnet is produced from San Pedro Mountain.

Limestone

Limestone has been quarried to a limited extent from several
places for use in smelters and as a building stone. The various beds of theMadera Limestone appear to be uniform in texture and color. Limestone
from any place in the stratigraphic section would probably be equallysuitable for future needs.

42

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

WATER SUPPLY

Water is rather scarce in the San Pedro Mountains. There. are no
springs and the streams are only ephemeral. Wells drilled in thesurrounding plains reach water at a depth of 500 to 800 feet, and it rises towithin 100 to 200 feet of the surface (Wells and Wooton, p. 20). A well at
the mouth of San Lazarus Gulch reached water at 400 feet, and it rose to alevel 125 feet below the well head (Lindgren et a l . , p. 175). The
groundwater level in the Carnahan mine is about at the top of the sulfide
ore, which is roughly 400 feet below the surface (Smith et a l . , plate 10).
Water may be obtained from several wells and three mineworkings, in addition to those listed. A well at the Baca ranch on thesouthern side of the mountains is being used at present. A well at theformer Perry camp, in the northeastern part of the mountains, has its
original casing, and water is in the-'bottom. At the Old Timer mine the wellstill has the original pump. One other well is located about 200 yards
southwest of the ruins of San Pedro village, but its condition is not known.
A shaft on the eastern side of San Lazarus Gulch reaches the water level.
On the eastern side of Old Timer Gulch, above the Old Timer mine, twotunnels reached the water table not more than 100 feet below the surface.
In one mine, a vein was stoped above and below the tunnel level about 200
feet from the portal. Water seeps from the back (roof) of the stope andstands in the stope within four feet of the tunnel level.

References
Blake, W. P. (1856) Report on the geology of the route: No, 1,
General report upon the geological collections (in Reports of
explorations and surveys, to ascertain the most practicable andeconomical route for a railroad from the Mississippi River to the
Pacific Ocean), U.S. 33d Cong., 2d. secs,, Senate Exec. Doc. 78 andHouse Exec, Doc. 91, v, 3, pt, 4,
Bullock, A. L. (1952) San Pedro exploration, New Mexico Prof. Eng., v.
4, n. 3, p. 5-6, 9-10.
Disbrow, A. E. (1953) The structural geology of the Cerrillos Hills area,
New Mexico, unpub. master's thesis, Univ. N. Mex.
--, and Stoll, W. C. (1957) Geology of the Cerrillos area,
Santa Fe County, New Mexico, N. Mex. Inst. Min, and Tech. , State
Bur. Mines and Mineral Res., Bull. 48.
Ellis, R. W. (1929) New Mexico mineral deposits, except fuels, Univ, N.
Mex. (Geol. S e r , ) Bull. 167, v. 2, n. 4,
Emerick, W. L. (1950) Geology of the Golden area, Santa Fe County , New
Mexico, unpub. master's thesis, Univ. N. Mex.
Entwistle, L. P. (1946) Private report on the San Pedro mine to American
Smelting and Refining Company.
Harrison, E. P. (1949) Geology of the Hagan coal basin, unpub, master's
thesis, Univ, N. Mex.
Herrick, C. L. (1898) The Geology of the San Pedro and Albuquerque
districts, Univ. N. Mex. B u l l . , v. 1, n. 4.
James, Henry (1955) Muleshoe gold (Golden), Sun Trails, v. 8, n. p. 2021.
Jones, F. A. (1904) New Mexico mines and minerals, Santa Fe: The New
Mexican Printing Co.
Johnson, C. H. (1943) San Pedro mine, Santa Fe Co., New Mexico, U.S.
Bur. Mines; War Minerals Rpt. 143.

44

NEW MEXICO BUREAU OF MINES & MINERAL RESOURCES

Keyes, C. R. (1909) Garnet deposits of copper and the depths at which
they are formed, Econ. Geol., v. 4, p. 365-372.
Lindgren, Waldemar (1928) Mineral deposits, New York: McGraw-Hill,--, and Graton, L. C. (1906) A reconnaissance of the mineraldeposits of New Mexico, U.S. Geol. Surv., Bull. 285,-- et al. (1910) The ore deposits of New Mexico,
Surv., Prof. Paper 68.

U.S. Geol.

Locke, Augustus, and P e r r y , E. H. (1916) Private report to the Santa FeGold and Copper Mining Company, San Pedro, New Mexico.
Northrop, S. A. (1959) Minerals of New Mexico, Albuquerque: Univ. N.
Mex. Press, revised edition.
Ogilvie, I. H. (1905) The high-altitude conoplain; a topographic form
illustrated in the Ortiz Mountains, Am. Geologist, v. 36, p. 27-34.
Peterson, J. W. (1958) Geology of the southern part of the Oritz
Mountains, Santa Fe County, New Mexico, unpub, master's thesis,
Univ. N. Mex.
Prince, L. B. (1883) Historical sketches of New Mexico, New York: Leggat Bros., and Kansas City: Ramsey, Millett, and Hudson.
Read, C. B. et al. (1944) Geologic map and stratigraphic sections ofPermian and Pennsylvanian rocks of part of San Miguel, Santa Fe,
Sandoval, Bernalillo, Torrance, and Valencia Counties, north-central
New Mexico, U.S. Geol. Surv., Oil and Gas Inv. Prelim. Map 21.
Reynolds, C. B. (1954) _Geology of the Hagan—La Madera area,
Sandoval County, New Mexico, unpub. master's thesis, Univ. N.
Mex.
Smith, J. F. et al. (1945) San Pedro and Carnahan mines, New Placers
mining district, Santa Fe County, New Mexico, U.S. Geol. Surv., File
Rpt.
Stearns, C. E. (1953)Tertiary geology of the Galisteo-Tonque area,
New Mexico, Geol. Soc. Am. Bull., v. 64, p. 459-508.

GEOLOGY OF SAN PEDRO MOUNTAINS

45

Stevenson, J. J. (188 . 1) Report upon geological examinations in southern
Colorado and northern New Mexico during the years 1878 and 1879,
U.S. Geog. and Geol. Surv. West of the 100th Meridian (Wheeler),
Suppl. 3.
Wah1strorn, E. E. (1955)Petrographic mineralogy, New York: John
Wiley & Sons, Inc.
Wells, E. H., and Wooton, T. P. (1932) Gold mining and gold deposits in
New Mexico, N. Mex. Inst. Min. and Tech., State Bur. Mines andMineral Res. , Circ. 5.
Yung, M. B. , and McCaffery, R. S. (1903) The ore deposits of the San
Pedro district, New Mexico, Am. Inst. Min. Eng. Trans., v. 33, p.
350-362.

IndexNumbers underscored indicate main references.

Abert, J. W., Lieutenant, 26
Abo Formation, 5, 6, 15
Adularia, 32, 34, 36
Aguilar, Antonio, 26
Alabandite, 37
Almandite, 15
American Smelting and Refining C o . , 4
Andesine , 9Andradite, 15
Anglesite, 37
Apatite, 8, 12, 13
Argo mine, 35
Axtell, Samuel B., 27
Azurite, 32
Baca ranch, 42
Bateas (wooden), 25
Bedded deposits, 34-36, 38
Biotite, 12Blake, W. P . , cited, 37
Bornite, 30
Botryoidal hematite, 37
Brooks mine, 36
Bullock, A. L . , cited, 26, 27, 28
Calcite, 30, 36, 39
Canon del Agua grant, 26, 27, 28
Carlson, Dale, 38; cited, 37
Carnahan mine, 37, 38, 42
Castle Rock, 17, 19
Cerrillos area, 24
Cerrillos—Ortiz region, 24
Cerro Bonanza, 16
C e r u s i t e , 37
Chalcanthite, 34
Chalcocite film, 34
Chalcopyrite, 14, 30, 32, 34, 37

Chinle Formation, 7, 20
Chlorite, 8, 9, 14, 30, 39
Chrysocolla, 32, 39
Cienega intrusive center, 15, 16
Cooper, J. R . , 34
Copper, 24, 39, 40
Cretaceous, 22
Cuprite, 32
Diabase, 8, 13, 19
Dikes, 9, 12, 13, 14, 17, 18, 19-20, 29
monzonite, 14, 29
monzonite porphyry, 13, 14, 29
rhyolite, 12
Diopside phenocrysts, 8Disbrow, A. E . , cited, 13, 23
and Stoll, W. C . , cited, 23Dockum Group, 7, 15, 20
Chinle Formation, 7
Santa Rosa Sandstone, 7
Doming, 21
Dooley fault, 16, 20, 21
East fault, 17
Economic geology, 24-42
Ellis, R. W., cited, 28, 35, 36
Emerick, W. L . , cited, 5, 17
Entrada Sandstone, 22
Entwistle, L. P . , 4
Eocene, 22
Epidote, 5, 9, 14
Espinaso Formation, 23
Estancia basin, 16
Farwell, F. W., 4
Faults, 16, 17, 20-21
Dooley, 16, 20, 21

East, 17Golden, 16, 2O, 21
La Bajada, 16
Tijeras fault system, 16, 20, 2l , 22Fluorite, 37
Foldsq 16, 21
Fray Dominguez, 24
Galena, 37, 38
Galisteo Formation., 22
Garnet, 5, 6, 14, 15, 41
Geologic history (of San Pedro Mountains), 22-23
Glorieta Sandstone, 7, 24, 41
Goethite, 39
Gold, 24, 25, 34-37, 39
lode, 25
placers, 24
veins, 40
Golden fault, 16, 20, 21
Golden Hydraulic C o . , 28Golden, N. Mex., 2, 5, 25, 27
Gold Standard mine, 36Grand Central Mountain, 16
Granodiorite porphyry, 9
Grant, U. S., General, 28
Graton, L. Co, see Lindgren, Waldemar
Grossularite, 15Hagan basin, 5, 6, 7
Hagan—La Madera area, 23Harrison, E. P., cited, 5, 6, 7
Hematite, 5, 38
Hemimorphite, 38
Herrick, C. L . , cited, 19
Hornblende, 8, 9, 12, 13
Hornfels, 15, 39
Hurtado, Governor, 24
Igneous rocks, 8-13
age relations, 13
Intrusive centers, 16
Iron, 15, 24, 37, 38-39
Jackson mine, 38James, Henry, cited, 25
Jaquez, Don Carlos, 26
Johnson, C. H., cited, 28
Jones, F. A . , cited., 24, 25; 35., 36
Jurassic, 22
Kaolin, 38
Kelley, V. C., 5, 16, 28, 34
Keyes, C. R., cited, 18
La Bajada fault, 16
La Bajada mine, 16
Laccolithic mountains, 17
Laccoliths, 9, 14, 16, 17-19, 21, 35, 40monzonite porphyry, 14
La Mina de la Tierra, 24
Latite porphyry, 9, 12, 13, 19, 35
sills, 19
Lead, 24, 39
Lead-zinc, 37-38
deposits, 40
veins, 40
Limestone, 6, 14, 24, 29, 39, 40, 41
Limonite, 5, 32, 38, 39
Lincoln—Lucky mine, 37
Lindgren, Waldemar, cited, 34
and Graton, L. C . , cited, 9
et al., cited, 18, 28, 35, 36, 37, 42
Locke, Augustus, 4
Los Cerrillos ( mountains ) , 3, 13, 15, 16 s
23
Los Tuertos Canyon, 27
Madera Canyon, 27
Madera Limestone, 5-6, 13, 14, 15, 24,
29, 37, 40, 41
Magnetite, 5, 8, l2, 13, 14, 24, 38, 39
veins, 40
Magdalena Group, 5, 6
Malachite, 32, 39
Marble, 13 2 14
“Marble line," 29Marcasite, 37
Marsh,Sherman P. , 32

INDEX
Pennsylvanian, 22
McCaffery, R. S., 4; see also Yung,
M. B.
Permian rocks, 22
Perry camp, 42
McKinley mine, 36
Perry, E. H., 4
Melaconite, 32Peterson, J. W., cited, 13
Melanterite, 34
Pipes, 17
Meseta Blanca Sandstone, 6Castle Rock, 17Metamorphic rocks, 13-15
monzonite porphyry, 14, 17
Mina del Tiro, 24
Placer deposits, 36-37
Minium, 38
Plagioclase, 9, 12
Miocene, 23
Pliocene, 23
Molybdenite, 32
Porphyry belt, see San Pedro—Ortiz
Montezuma Mountain, 5, 6
porphyry belt
Monzonite, 8-9
Dike, 14, 29
Precambrian, 5, 16, 21, 22
Monzonite porphyry, 8, 9, 12, 13, 18,9 35 Prince, L. B., 27; cited, 25
augite-bearing, 12-13
Pyrite, 14, 30, 32, 36, 37, 38, 39
dike, 19
Pyrolustie, 38
pipe, 17
Pyrrhotite, 30, 34
dike, 13, 14, 29
laccolith, 14
Quartz, 5, 9, 12, 14, 30, 36, 37, 39
pipe, 14
Quartzite, 15
stock, 17, 18
Quartz sand, 41
Quaternary, 23
New Placers mining district, 2, 24, 25 . 937
Ramirez, Jose Serafin, 26
history, 24
Raskob Mining Interests, Inc., 28
Northrop, S. A. cited, 24, 32, 36
Read, C. B. et a l . , cited, 4
Reynolds, C. B. , cited, 5, 6, 23
Ogilvie, I. H., cited, 2
Rhyolite, 8, 12, 19, 35
Oligocene, 23
dikes, 12
Old Placers, 25
sills, 12, 19
veins, 35
Old Timer Gulch, 5, 8, 13, 14, 16, 18,
20, 21, 22, 30, 35, 38, 42
San Andres Formation, 5, 7, 14, 20, 39,
Old Timer mine, 35, 42
41
Old Timer stock, 17, 2
Glorieta Sandstone, 7
Oro Quay Peak, 2, 14, 16, 17, 18, 20,
San Andres lime stone member, 7
21, 35, 39, 40
Sandia Formation, 5
Oro Quay stock, 14, 16, 2l , 22, 39
Sandia Mountains, 16, 23, 27
Orthoclase, 9, 12, 13, 14
Sandstone, 6, 7
Ortiz intrusive s, 16, 17
Sangre de Cristo mountain range, 16
Ortiz mine, 24
San Lazarus Gulch, 12, 14, 17, 25, 35,
Ortiz Mine Grant, 2
40, 42
Ortiz Mountains, 2, 3, 13, 14, 16, 17, 23,
San Lazarus mines, 35
25, 35
San Pedro and Canon del Agua Mining
Otero, M. A., 26
Co., 26, 27
San Pedro area, 6, 13, 24, 27, 32

INDEXSan Pedro Formation, 5
San Pedro horst, 21
San Pedro mine, 4, 13, 26, 28, 29, 30, 32,
34, 38, 40, 41
San Pedro Mountains, 2, 3, 6, 13, 15, 16,
17, 19, 20, 21, 22, 23, 24, 25, 29, 30,
34, 35, 37, 39, 40, 41, 42
accessibility, 2
climate, 2geologic history, 22-23
location, 2
San Pedro—Ortiz porphyry belt, 15-17
San Pedro village, 17, 20, 24, 25, 42
Santa Fe County, N. Mex., 2, 32
Santa Fe Formation, 23
Santa Fe Gold and Copper Co., 28
Santa Fe Ring, 26, 27
Santa Rosa Sandstone, 7
San Ysidro Member, 6
Scartaccini, Tom, 28
Scheelite, 30, 32, 34
Sericite, 9, 12
Serpentine, 39
Siderite, 30, 32
Silica, 15, 37
Silica mine, 14
Sills, 9, 12, 13, 14, 17, 19, 20
latite porphyry, 19
monzonite porphyry, 13, 14, 19, 29
rhyolite, 12, 19
Silver, 37, 38
Smith, J. F. et al., cited, 4, 5, 9, 24, 28,
29, 30, 32, 34, 42South Mountain, 3, 15, 16, 17, 23
Specular hematite, 30
Specularite, 5, 14, 34, 38, 39
Sphalerite, 37, 38
Sphene, 9, 12, 13
Stearns, C. E . , cited, 22, 23
Stevenson, J. J . , cited, 37
Stocks, 14, 17, 18, 20, 21, 38, 39, 40
monzonite porphyry, 17, 18
Old Timer stock, 17, 20
Oro Quay stock, 14, 16, 21, 22, 39
Stoll, W. C . , see Disbrow, A. E.

Swan tunnel, 30
Syenite, 9Tactite, 13, 14, 15, 30, 35
Tetrahedrite, 32
Tijeras fault system, 16, 20, 21, 22
Todilto limestone, 22Tournier, M r . , 26
Tremolite, 14, 39
Tuerto village (now Golden), 25
Tungsten ore, 34
Upper Cretaceous rocks, 22
Varela, Mariano, 26
Veins, 34-36, 38
banded fissure, 35
gold, 40
gold-bearing, 34
lead-zinc, 40
magnetite, 40
rhyolite, 35
Wah1strom, E. E . , cited, 8
Water, 42
Wells, E. H . , and Wooton, T. P . , cited,25, 36, 42
Williams, C. F . , 28; cited, 28
Wislizenus, F . A . , 26, 36
Wooton, T. P . , see Wells, E. H.
Yeso Formation, 5, 6
Meseta Blanca Sandstone, 6
San Ysidro Member, 6
Yung, M. B . , 4
and McCaffery, R. S., cited, 9, 18, 35,
36, 37, 38
Zinc, 24, 38, 39