,::_ ....... ,._.;•:. 

GEOLOGICAL SURVEY CIRCULAR 808 

Background Information to Accompany Folio of 

Geologic, Mineral Resource, Geochemical, 

Aeromagnetic, and Gravity Maps of the 

Hillsboro and San lorenzo Quadrangles, 

Sierra and Grant Counties, New Mexico 

Background Information to Accompany Folio of 

Geologic, Mineral Resource, Geochemical, 

Aeromagnetic, and Gravity Maps of the 

Hillsboro and San Lorenzo Quadrangles, 

Sierra and Grant Counties, New Mexico 

By D.C. Hedlund, with sections on 

Geochemistry by K. C. Watts and H. V. Alminas, 

and Geophysics, by J. C. Wynn and D. C. Hedlund 

GEOLOGICAL SURVEY CIRCULAR 808 

1979 
United States Department of the Interior 

CECIL D. ANDRUS, Secretary 

Geological Survey 

H. William Menard, Director 

Free on application to Branch of Distribution, U.S. Geological Survey, 1200 South Eods Street, Arlington, VA 22202 
CONTENTS 

Abstract------------- ·------------------- Introduction---------------------------------

Goal and method ---------------

Location and access ----------------

Mineral production----------------------

Previous geologic and mineral resource investigations--------

Present study ------------------------

Acknowledgments ----------------------

Descriptions of component maps of the HillsboroSan Lorenzo mineral resource assessment---------------

Geology (map MF-900A)--------------------

Mineral resources (map MF-9008)------------------

Geochemistry (maps MF-900C-K) by K. C. Watts 

and H. V. Alminas ------------------

Geophysics (maps MF-900L and M) by J. C. Wynn 

and D. C. Hedlund ---------------

Abstract 

Introduction---

Aeromagnetic data--·----·----

Collection and processing---------------

Description and analysis 

Gravity data----

Collection and processing·-------

Description and analysis 

Conclusions -------

Acknowledgments ---------· ·---------

Bibliography 

Geology 

Geochemistry 

Geophysics---------------

ILLUSTRATIONS 

FIGURE 1. Index map showing location and general geology of 

the Hillsboro and San Lorenzo quadrangles and 

adjacent areas 2. Locality map of major geographic features and 

major faults.-------------

3. Index map showing location of the Hillsboro and 

San Lorenzo quadrangles and the aeromagnetic 

map of garts of the Silver City and Las Cruces 

1° by 2 quadrangles, New Mexico----------

4. Parallel-surface-continued aeromagnetic map of 

the San Lorenzo-Hillsboro quadrangles--------

5. Map of magnetic anomaly in the Hillsboro district 

with north-south section showing the measured 

field, the calculated field, and the threedimensional model of the source body·--------

III 

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6. Map of magnetic anomaly of the Carpenter (Swartz) 

district with east-west section showing the 

measured field, the best remanent-fit field, the 

best susceptiblity-fit field, the best dipping-body 

susceptibility-fit field, and their respective 

source bodies---------------------------------- 14 

7. Complete Bouguer gravity map of the San Lorenzo 

and Hillsboro quadrangles-------------------------- 15 

8. Gravity map of the Emory Pass caldera---------------- 16 

9. Residual gravity map of the Emory Pass caldera, 

using an offset of +196.78 milligals-------------------- 17 

10. ralculated gravity map of the Emory Pass caldera 

from the three-dimensional model using -0.35 

gm/cm3 density contrast---------------------- 17 

11. Three-dimensional model of the Emory Pass caldera, 

using -0.35 gm/cm3 density contrast, and 0.5 

kilometer contour intervals------------------------- 18 

12. Three-dimensional model of the Emory Pass caldera, 

using -0.10 gm/cm3 density contrast, and 2.5 

kilometer contour intervals------------------- 18 

13. Calculated gravity map of the Emory Pass caldera, 

from the three-dimensional model using -0.10 

gm/cm3 density contrast--------------------------- 19 

14. Gravity map of the Copper Flat gravity anomaly--------- 20 

15. Residual gravity map of the Copper Flat gravity 

anomaly, using an offset of +197 .43 milligals--------- 21 

16. Calculated gravity map of the Copper Flat gravity 

anomaly, from the three-dimensional model using 

a density contrast of +0.40 gm/cm3---------------- 22 

17. Three-dimensional model of the Copper Flat gravity 

anomaly, using +0.4 gm/cm3 density contrast, 

and 0.25 kilometer contour intervals------------ 23 

TABLES 

TABLE 1. Component lllaps of the Hillsboro and San Lorenzo 

quadrangles mineral assessment-----

2-5. Semiquantitative spectrographic analyses of: 

2. Mineralized quartz monzonite and gold-bearing 

vein material from the Chance mine and from 

Copper Flat with supplemental atomic absorption 

analyses for gold and gravimetric analyses for 

silver 

3. Ore samples from the Carpenter (Swartz) mining 

district with supplemental gravimetric analyses 

for silver -----

4. Ore samples from the Kingston mining district 

with supplemental atomic absorption analyses 

for gold and gravimetric analyses for silver·-----

5. Ore samples from the Jap, Log Cabin, and 

Ingersoll mines with supplemental atomic 

absorption analyses for gold and gravimetric 

analyses for silver ------

6. Magnetic susceptibility and density data for some 

of the rock units described in the geophysical 

interpretation and accompanying maps ·-----

IV 

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7 

8 

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13 
Background Information to Accompany Folio of 

Geologic, Mineral Resource, Geochemical, Aeromagnetic, and 

Gravity Maps of the Hillsboro and San Lorenzo Quadrangles, 

Sierra and Grant Counties, New Mexico 

By D. C. Hedlund 

ABSTRACT 

The Hillsboro and San Lorenzo 15-minute quadrangles of southwestern New Mexico have been mapped at a 1:48,000 scale and selected mineralized areas 

within these quadrangles have been mapped in greater 

detail. This area of about 550 mi2 (1,424 km2) is 

within the southern part of the Black Range and includes much of the Mimbres Mountains and parts of 

the adjoining Black Range Primitive area. The region 

is highly mineralized with Laramide (75.1 ±2.5 m.y.) 

and middle Tertiary (about 32 to 35 m.y.) intrusions 

providing the source for much of the base- and 

precious-metal mineralization. The porphyry copper 

deposit at Copper Flat is a subvolcanic Laramide 

quartz monzonite stock that was intruded into a thick 

section (>2, 700 ft or >823 m) of Upper Cretaceous 

andesite. Numerous gold-bearing veins radiate from 

the central quartz monzonite stock, and locally offset 

a radial system of quartz latite dikes. Gold-bearing 

veins, chiefly along the south and east periphery of the 

stock, have provided the source for both gold lode and 

placer deposits. The middle Tertiary rhyolitic plugs, 

stocks, and dikes have intruded a dominantly carbonate 

section of Paleozoic sediments and bedding replacement deposits of zinc, lead, and copper are closely 

associated with tactite in the Carpenter (Swartz) 

mining district. The rich oxidized silver deposits of 

the Kingston district are largely fault controlled and a 

metallizing pluton is not present at the surface 

outcrop. Some of the silver-bearing base-metal veins 

in the Kingston district contain rhodochrosite, rhodonite, and alabandite and the highly oxidized parts of 

these veins have been mined for manganese ore. Very 

minor amounts of tungsten, as scheelite, occur in thin 

tactite zones along Tank Canyon, at the Silver Queen 

claims near Kingston, and at the Silver Tail group of 

mines along Pierce Canyon. 

INTRODUCTION 

Goal and Method 

This circular presents background information to 

accompany a folio of geologic, mineral resource, geochemical, aeromagnetic, and gravity maps for the 

1 

Hillsboro and San Lorenzo 15-minute quadrangles. 

This report represents a part of a larger mineral 

assessment study for southwestern New Mexico that 

began with the Black Range Primitive area report of 

G. E. Ericksen and others (1970); the Gila Wilderness 

map of J. C. Ratte and D. L. Gaskill (1975); and the 

gold placer studies of K. Segerstrom and J. C. 

Antweiler, III (1975). 

Most of the geologic mapping was done on aerial 

photos at a scale of 1:15,840 or on topographic maps at 

a scale of 1:48,000. The geologic mapping program 

was followed by a geochemical study that involved the 

extensive sampling of stream sediments and a geophysical survey that included aeromagnetic, ground magnetic, and gravity data. The field studies began in late 

1971 and were largely completed in early 1975. 

Location and Access 

The Hillsboro and San Lorenzo 15-minute quadrangles cover approximately 550 mi2 (1,424 km2) in 

southwestern New Mexico between lat 32°45' and 

33°00' N. and long 1 07°30' and l 08°00' W. The area 

includes the southern part of the Black Range, most of 

the Mimbres Mountains, and extends from the west 

margin of the Rio Grande Depression to the Santa Rita 

horst and Mimbres valley in the west. The crest of the 

Black Range and Mimbres Mountains forms the Continental Divide and streams flowing to the east comprise 

the tributary system of the Rio Grande which flows 

into the Gulf of Mexico. Streams that flow to the 

west of the divide form the tributary system of the 

Mimbres River which flows southward into a closed 

basin just south of Deming. The crest of the range is 

sinuous and ranges in elevation from about 7,400 ft 

(2,255 m) in the south to about 10,000 ft (3,048 m) in 

the north. Generally, the eastern side of the range is 

more deeply dissected than the western side and older 

Paleozoic and Precambrian rocks are commonly exposed in tilted fault blocks along the eastern side of the 

range, whereas the western side is more mantled with 

Tertiary volcanic rocks and surficial fan deposits. 

Access from east to west is by New Mexico State 

Highway 180 which is an all-weather road that crosses 

the Black Range at Emory Pass. Partial access along 

north-south roads is limited to New Mexico State 
Table 1.--Component maps of the Hillsboro and San Lorenzo quadrangles 

mineral resource assessment 

U.S. Geological Survey Miscellaneous 

Field Studies (MF) Map 

MF-900-A (Hedlund, 1977)---------------

B (Hedlund, 1977)---------------

C (Alminas, Watts, Siems, and 

Kraxberger, 1978)----------

D (Alminas, Watts, Siems, and 

Kraxberger, 1978)----------

E (Alminas, Watts, Siems, and 

Kraxberger, 1978)----------

F (Alminas, Watts, Siems, and 

Kraxberger, 1977)----------

G (Alminas and Watts, 1978)-----

H (Watts, Alminas, and 

Kraxberger, 1978)----------

I (Wattsm Alminas, Nishi, and 

Crim, 1978)----------------

J (Alminas, Watts, Nishi, and 

Crim, 1978)----------------

K (Watts, Alminas, Nishi, and 

Crim, 1978)----------------

L (Wynn, 1978)------------------

M (Wynn and Dansereau, 1978)----

Highway 61 along the Mimbres valley and to New 

Mexico State Highway 27 from Lake Valley to Hillsboro. Numerous secondary county and U.S. Forest 

Service roads provide access to the Royal John and 

Grandview Mines in the Carpenter (Swartz) district, to 

East Canyon along the McKnight road, to the Ingersoll 

mine north of Kingston and to the Log Cabin road 

along Tierra Blanca Creek. A population of about 400 

is chiefly confined to the small villages of Hillsboro, 

San Lorenzo, Mimbres, Kingston, and San JuanSherman. 

Mineral Production 

Silver-bearing base-metal deposits of the Kingston and Carpenter (Swartz) mining districts were 

chiefly mined from 1877 to 1893 and, more recently, in 

the early 1940's within the Carpenter (Swartz) district. In the Las Animas placer district (Hillsboro), the 

gold-bearing fissure veins and gold placers were chiefly mined from 1877 to 1893, 1906, 1918 to 1920, and 

1931 to 1933. From these three districts the total 

value of mineral production was about $15 million. 

The mineral deposits of the Las Animas placer 

district in order of their past importance are of three 

main types: (I) the gold placers east of Copper Flat, 

(2) the gold-bearing fissure veins within the andesite 

about the Copper Flat stock, and (3) the porphyry copper mineralization within the quartz-monzonite stock 

of Copper Flat. The gold-bearing fissure veins within 

the andesite have yielded about 51,000 ounces of gold 

(Harley, 1934; Segerstrom and Antweiler, 1975), 

whereas the gold placer deposits just east of Copper 

Flat have yielded about 98,000 ounces of gold. The 

2 

Subject 

Geology 

Mineral resources 

Silver in stream sediments 

Copper in stream sediments 

Molybdenum in stream sediments 

Zinc in stream sediments 

Interpretive geochemical map 

Detrital fluorite and cassiterite 

Tungsten and gold in stream sediments 

Lead in stream sediments 

Bismuth in stream sediments 

Aeromagnetic map 

Gravity map 

total value of the past gold production has been 

$6,900,000 (Harley, 1934). More recently, and beginning about 1953, the quartz-monzonite stock of Copper 

Flat has been extensively drilled and a relatively lowgrade porphyry copper deposit has been defined. The 

stock has an outcrop area of 0.4 mi2 (1.04 km2), but 

only about 0.1 mi2 (0.26 km2) of the stock is extensively mineralized. The copper reserves of the stock 

have not been completely evaluated but are probably 

in the range of 4 million tons of >0.5 percent copper to 

a depth of 800 ft (244 m) and about 15 million tons of 

>0.20 percent copper to a depth of 1,000 ft (305 m). 

Supergene enrichment is negligible. 

In the Kingston district, fault-controlled fissure 

veins and bedding-replacement deposits within dolomite beds of the Silurian Fusselman Dolomite and 

Ordovician Montoya Group underlying the Devonian 

Percha Shale have yielded an estimated 6 million 

ounces (186,600 kg) of silver. Scattered small silverbearing veins in the Tierra Blanca district yielded 

about 45,000 ounces of silver. Near-surface manganese deposits derived from the weathering of vein 

and bedding-replacement deposits in the Kingston district were mined in 1943, 1944, 1952, and 1953, and an 

estimated 5,689 tons of manganese ore that average 32 

percent manganese were shipped from the district 

(Farnham, 1961). 

In the Carpenter (Swartz) mining district, the 

sphalerite-rich base-metal ores are chiefly present as 

discontinuous bedding-replacement bodies within silicated cherty limestone beds within the upper part of 

the Ordovician El Paso Limestone. Past production 

figures suggest that as much as 60,000 short tons of 

ore with a value of about $1 million has been mined 
0 5 

I I I 

0 5 10 

32°30' 

108° 

10 

I I 

15 

15 MILES I 

I 

20 KILOMETERS 

Figure 2 

outline 

EXPLANATION 

Sediments and sedi~entary rocks--(Holocene to Pliocene) 

Volcanic rocks--(Tert1ary and Upper Cretaceous) 

Tertiary and Cretaceous intrusive rocks 

Cretaceous and Paleozoic sedimentary rocks 

---CONTACT 

----L- FAULT--Dashed where inferred; dotted concealed; 

Bar and ball on downthrown side -+- ANTICLINE 

_L_ STRIKE AND DIP OF BEDS 

INDEX MAP OF NEW MEXICO 

SHOWING THE LOCATION OF 

THIS REPORT AND MAJOR FAULTS 

FIGURE I.-Index map showing location and general geology of the Hillsboro and San Lorenzo quadrangles and 

adjacent areas. 

from the district. Generally, the silver values of the 

ore are much less than in the Kingston district and 

past production assay figures indicate 8 percent zinc, 4 

percent lead, 0.5 percent copper, and 3-4 ounces/ton 

of silver. 

A description of most of the mines in the various 

districts is largely dependent on past mining records 

and on an interpretation of various older mine maps. 

Only in the areas of more recent development such as 

at the Royal John, Ingersoll, and Black Colt mines and 

the Quintana adit has access been more favorable. 

There has been some reworking of the old mine dumps 

at the Opportunity mine but only small amounts of 

gold were recovered from the highly pyritic ores during 1975. 

3 

Previous Geologic and Mineral Resource 

Investigations 

The mining of rich silver deposits in the Kingston 

district from 1882 to 1896 and the discovery of gold 

deposits in the vicinity of Hillsboro in 1877 was followed by the geologic investigations of Lindgren, 

Graton, and Gordon (1910). Later Harley (1934) described the various mineral deposits in much greater 

detail and his descriptions of the various gold mines in 

the Las Animas placet'-Hillsboro districts remain the 

most comprehensive to date. With the economic decline of 1893, mining activity in the Kingston and 

Hillsboro districts greatly diminished, and silver and 

gold mining has been sporadic since then. The low-
33°30' 

33°00' 

1 ogooo· 

TREASURE 

MOUNTAIN 

FAULT \ (-DfAe('-

'\ AA.tvG~O 

108°00' 

( 

0 10 20 30 MILES 

Deming0 I I I I I I I I 

Las 

Cruces 

0 10 20 30 40 KILOMETERS 

MOUNTAINS 

FIGURE 2.--Locality map of major geographic features (long dashes) and major faults, dashed where uncertain; 

ball and bar on downthrown side. 

grade manganese deposits that formed from the 

weathering of the·silver-bearing base-metal deposits in 

the Kingston district were mined in 1943 and 1944 with 

some renewed mining in 1952 through 1955 (Farnham, 

1961 ). 

A strategic metals program conducted by the 

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

from 1942 to 1945 led to the mapping and drilling of 

various base-metal deposits in the Carpenter (Swartz) 

mining district (U.S. Geological Survey unpublished 

mapping and data by Griggs, Ellison, Kinney, Wagner, 

and Weissenborn, 1943, 1944; and the U.S. Bureau of 

Mines Reports of Investigations by Hill, 1946, and 

Soule, 1950). 

Interest in the porphyry copper deposit of Copper 

Flat, Las Animas placer mining district, began in the 

early 1950's and has continued to the present with ex4 

ploration at various times by the Newmont, Bear 

Creek, Inspiration, and Quintana mining companies. 

Numerous drill cores have been taken from the mineralized parts of the stock and a few holes have been 

drilled in the surrounding andesite in the search for 

concealed skarn deposits. Probably the first geologic 

map of the quartz monzonite of the Copper Flat stock 

and adjacent country rock was that of Harley (1934). 

Later, a more detailed geologic map of the stock with 

a report on copper assay values was published by F. J. 

Kuellm er (1956). 

Except for a geologic strip map (scale 1:31,680) 

along New Mexico State Highway 90 from Kingston to 

San Lorenzo by Kuellmer (1954), most geologic mapping of the Hillsboro-San Lorenzo quadrangles has been 

at reconnaissance scales. The Black Range Wilderness 

report of Ericksen and others (1970) included a geolog-
ic map with some coverage of the southern part of the 

Black Range south of lat 33° N. Kuellmer (1956) published a geologic map of the Hillsboro Peak 30-minute 

quadrangle, and other geologic maps have been presented by F. J. Kuellmer, F. E. Kottlowski, W. Elston, 

V. C. Kelley and 0. T. Branson in the Fourth Guidebook of the New Mexico Geological Society (1953). 

The regional geological setting of the Black Range is 

shown on the geologic map of southwestern New Mexico (Dane and Bachman, 1961). 

Numerous theses and some published studies of 

the Paleozoic and Mesozoic stratigraphy have facilitated the mapping program. A thesis with a geologic 

map by R. S. Lambert (I 973) describes the Paleozoic 

stratigraphy and geology of a small area southwest of 

San Lorenzo. Lauden and Bowsher (1949) made a detailed study of the Mississippian rocks within the 

Hillsboro quadrangle and recognized at least five 

members and two formations within the Mississippian 

section. The regional distribution of oolitic hematitebearing sandstone within the Ordovician and Cambrian 

Bliss Sandstone was described by Kelley (1951). 

Regional stratigraphic studies of the Paleozoic and 

Mesozoic strata of southwestern New Mexico were 

summarized by Flower (1953a,b) and by Kottlowski 

(1963). The paleogeography of the Cretaceous strata 

in the Silver City-Santa Rita areas was briefly summarized by Hayes (1970). 

Numerous descriptions of Upper Cretaceous and 

Tertiary volcanic rocks have been published by Elston 

(1957, 1968); Elston and others {1973, 1975); Jicha 

(1954); Ericksen and Wedow (1976); and Fodor (1975). 

Numerous isotopic age determinations of Tertiary volcanic rocks and a synthesis of the volcanic stratigraphy has been published by McDowell (1971) and by 

Elston and others (1973). Geologic maps of the surrounding quadrangles have greatly aided in the resolution of the volcanic stratigraphy; these include the 

maps and reports by Elston (1957) for the Dwyer quadrangle; Jicha (1954) for the Lake Valley quadrangle; 

Hernon, Jones, and Moore (1964) for the Santa Rita 

quadrangle; Moore (1948) for the Allie Canyon quadrangle; and Ericksen and others (1970) for the Black 

Range Primitive area. 

Special studies include the mapping and economic appraisal of the gold placer deposits of the Las 

Animas placer mining district by Segerstrom and 

Antweiler (1975). The high-temperature pegmatites 

within the Rabb Park intrusion were studied by Kelley 

and Branson (I 947). A new occurrence of helvite was 

described from the tactites of the Grandview mine by 

Weissenborn (1948). 

Present Study 

Detailed geologic reconnaissance mapping by the 

U.S. Geological Survey at a scale of 1:48,000 began in 

late 1971 and continued into 1975 as selected areas 

were chosen for more detailed mapping. The geochemical investigations began in 1972 and were completed in 1975. Numerous samples of stream sediments were taken within the Hillsboro and San Lorenzo 

quadrangles as a part of the geochemical surveys of 

the Cuchillo Range that began in 1966. The aeromagnetic map of parts of the Silver City and Las Cruces 1° 

5 

by 2° coverage was completed in 1973 and open-file 

copies of the aeromagnetic maps of the Hillsboro-San 

Lorenzo areas at a scale of I :62,500 were available in 

1974. 

Acknowledgments 

Both field and laboratory support by members of 

the U.S. Geological Survey contributed to the preparation of this report. Greater insight to the volcanic 

stratigraphy resulted from discussions in the field with 

James Ratte and Tommy Finnell. The four K-Ar age 

determinations of Laramide and Tertiary intrusive and 

extrusive rocks were made by R. F. Marvin, H. H. 

Mehnert, and V. Merritt. Open-file mine maps and reports by R. L. Griggs, D. M. Kinney, S. P. Ellison, H. 

C. Wagner, and A. E. Weissenbom fumished valuable 

subsurface information within the mineralized areas of 

the Carpenter (Swartz) mining district. Numerous fossil determinations by J. T. Oliver, W. J. Sando, R. C. 

Douglass, E. L. Y ochelson, and W. A. Oliver, Jr. aided 

in identifying some of the faunal zones within the 

Pennsylvanian limestones of the Magdalena Group. A 

special map and open-file report by K. Segerstrom and 

J. C. Antweiler, Ill, on the gold placers near Gold Dust 

provided an appraisal of the gold placer reserves in the 

Las Animas placer mining district. 

Numerous mine operators and company geologists provided access to their properties and, in certain 

instances, made available past and current mine records. Earl Burke of the Inspiration Development Co. 

and Peter Dunn of Quintana Minerals Co. allowed the 

examination and sampling of some drill core from the 

Copper Flat stock. Other mine operators that provided access to their mines include Oliver Reese, Jack 

Upton, E. H. Hale, Jr., Roy Tirey, and Donald Fingado. 

Of the many local ranchers that allowed access 

their land we wish to thank A. H. Latham, G. Whittenburg, S. A. Roberts, H. Bounds, J. Bason, F. Yates, 

R. 0. Anderson (owner) and A. Evans (foreman) of the 

Ladder Ranch, V. Cunningham, G. Miller, and G. 

Fowler. 

DESCRIPTION OF COMPONENT MAPS OF THE 

HILLSBORO-SAN LORENZO 

MINERAL RESOURCE ASSESSEMENT 

Geology (Map MF-900A) 

The Hillsboro and San Lorenzo 15-minute quadrangles are within the Mexican Highlands section of 

the Basin and Range physiographic province that is 

characterized by intensive block faulting. Geologically the area is along the southern extension of the 

Cuchillo and Black Range fault-block mountains with 

an overlap of volcanic rocks from the Datil-Mogollon 

volcanic field. Upper Cretaceous and Tertiary volcanic extrusives and ash flows were erupted onto essentially flat-lying Mesozoic and Paleozoic miogeosynclinal limestone, dolomite, sandstone, and shale sedimentary rocks that were later block-faulted in the 

middle Tertiary (about 20 m.y. ago). About 200ft (61 

m) of Upper Cretaceous sandstone, about 2, 700 ft (823 

m) of Paleozoic strata, and nearly 6,000 ft (1,800 m) of 
middle Tertiary volcanic rocks were faulted, til ted, 

and broadly arched over the crest of the Black Range. 

Generally, the fault blocks are til ted 20° to 35° east 

along the east side of the range, and are tilted 10° to 

25° to the west along the west side (geologic sections 

A-A' through D-D'). In places, large north-striking 

faults have brought Precambrian granitic and hornblendic rocks to the surface outcrop. 

The Paleozoic strata range in age from Cambrian 

to Permian with all systems represented. Carbonate 

rocks comprise about 75 percent of the sedimentary 

section and the strata are intensely faulted and locally 

intruded by stocks, plugs, and sills of Late Cretaceous 

and Oligocene age. Skarns are locally developed in the 

cherty limestone and dolomite beds of the El Paso 

Limestone, Montoya Group, and Fusselman Dolomite 

adjacent to middle Tertiary rhyolite plutons in the 

Carpenter district. Smaller skarn deposits in the Tank 

Canyon and Warm Spring Canyon area are related to 

Laramide intrusions. 

Erosional remnants of Lower Cretaceous Beartooth Quartzite with thin interbeds of shale uncon- formably overlie red beds of the Permian Abo Formation. A deltaic depositional environment is assumed 

for most of the sandstone. The maximum remnant 

thickness of the Beartooth Quartzite is about 200 ft 

(61 m). 

Seven small stocks and plugs of quartz monzonite, quartz monzodiorite, granite, quartz diorite, and 

diorite intrude Paleozoic rocks and (or) Upper Cretaceous andesite in the northeast quadrant of the Hillsboro quadrangle. The Copper Flat pluton, which is 

economically most important, was emplaced in the 

Late Cretaceous (75.1 ±2.5 m.y.) and probably represents the denuded core of a Laramide volcano because 

it has a radial dike system and intrudes Upper Cretaceous andesite flows that are at least 2, 790 ft (850 m) 

thick. The other plutons have not been dated but are 

assumed to be of Late Cretaceous age on the basis of 

lithology and the proximity to the Copper Flat stock. 

Except for the andesite at Copper Flat and the 

thin basaltic flows of Pliocene age within the Santa Fe 

and Gila Formations, most of the volcanic rocks in the 

region are of Miocene-Oligocene age (24-38 m.y.). The 

andesitic and latitic flows that underlie the Sugarlump 

and Kneeling Nun Tuffs over a wide area have been 

considered correlative with the Rubio Peak Formation 

(Jones and others, 1967). Two recent K-Ar age determinations on correlative andesitic rocks in the Hillsboro and San Lorenzo quadrangles indicate an Oligocene age of about 36.4 m.y. (R. F. Marvin, H. H. 

Mehnert, and Violet Merritt, written commun., 1975). 

The thick section of ash-flow tuff and tuffaceous sediments of the overlying Kneeling Nun and Sugarlump 

Tuffs are also of Oligocene age (33.4± 1.0 m.y ., 

Me Dowell, 1971) and probably represent nuee ardente 

eruptions from a nearby caldera-possibly the Emory 

caldera as proposed by Elston, Seager, and Clemons 

(1975). The margins of this caldera are still imprecise 

but the southern boundary is probably located in the 

north-central part of the San Lorenzo quadrangle. The 

ert1ption of the Kneeling Nun ash-flow tuff was closely 

followed by the intrusion of small rhyolite stocks and 

plugs such as the one at Rabb Park and those along 

Iron Creek. Locally, intraformational breccias within 

6 

the ash flow contain xenoliths of the rhyolite plutons 

suggesting that the rhyolite intrusions were contemporaneous with the ash-flow eruptions. These middle 

Tertiary rhyolite bodies are considered to be the 

metallizing source for the base-metal and associated 

skarn deposits of the Carpenter (Swartz) district. The 

eruption of the ash flows was followed by the viscous 

outpouring of the Mimbres Peak Rhyolite of Jicha 

(1954) and the nearly contemporaneous latite flows of 

the Pollack Quartz Latite of Jicha (1954). Some of the 

rhyolite was· erupted as mushroom-shaped intrusive 

domes that were alined along north-striking faults; for 

example, the rhyolite at Deer Hill, Tierra Blanca 

Mountain, Sherman Mountain, and Star Peak. After a 

short erosional interval, there followed the eruption of 

tuffs of the Caballo Blanco Rhyolite Tuff of Elston 

(1957), and the subsequent outpouring of basaltic andesite and latitic flows of the Razorback and Bear 

Springs Formation, both of Elston (1957). Intensive 

block faulting along aN. 5°-35° W.-strike followed the 

eruption of the Bear Springs Basalt and this faulting 

has been placed by Elston and others (1973) as about 20 

m.y. B.P. (before the present). Subsequent erosion 

and deposition within the numerous fault blocks produced thick deposits of fanglomerate that have been 

termed the Gila Conglomerate and the Santa Fe Formation. The fanglomerate deposits were probably as 

much as 2,000 ft (610 m) thick, but clastic deposition 

was interrupted about 6.4 m.y. B.P. when extensive 

flows of olivine basalt and alkali basalt covered much 

of the fanglomerate 

Mineral Resources (Map MF-900B) 

The numbered locations, with brief descriptions 

of known prospects, mineral occurrences, and inactive 

mines are shown on the mineral resource map (MF900B). In addition, potentially favorable areas for 

further investigative studies or prospecting are shown 

in their apparent order of decreasing potential. Grab 

and chip samples of vein and bedding-replacement deposits with a few of porphyry copper were analyzed by 

semiquantitative spectrographic and atomic absorption 

analyses (tables 2, 3, 4 and 5). .With the exception of 

the gold placer deposits, reserve estimates have not 

been adequately determined. Extensive drilling of the 

quartz monzonite stock of Copper Flat by Quintana 

Mineral Corporation has largely made the copper reserve estimates of this report obsolete. In the other 

districts, estimates of reserves have been made very 

difficult because of lack of access to old underground 

workings, the paucity of diamond-drill-core data, and 

the depletion of near-surface, oxidized and silverenriched base-metal ores. 

Alteration zones, generally silicification of limestone or dolomite, and areas of tactite development are indicated on the geologic map (map MF-900B). 

Areas of silicification are shown in a stippled pattern. In the Hillsboro district, some of the jasperoids 

contain limonite, cerussite, and plumbojarosite with 

minor inclusions of sulfide (T. G. Lovering, oral commun., 1974). The larger areas of tactite are shown in 

an x-pattern. The silicated limestone or dolomite may 

contain some or all of the following minerals: wollastonite, diopside, epidote, grossular garnet, tremolite, 
Table 2.--Semiquantitave spectrographic 

analyses of mineralized quartz monzonite 

and gold-bearing vein material from the 

Chance mine and from Copper Flat with 

supplemental atomic absorption analyses 

(A.A.) for gold and gravimetric analyses 

for silver 

[Results are reported in parts per million to the nearest number in 

the series l, 0.7, 0.5, 0.3, 0.2, 0.015, and so forth; about 30 

percent of the assigned groups for semiquantitative spectrographic 

results include the quantitative values. G, greater than 10 percent; N, not detected at limit of detection; L, looked for but not 

detected. Spectrographic analyses by L. A. Bradley, 1973. Silver 

analyses determined gravimetrically by A. W. Haubert; gold determined by atomic absorption by A. W. Haubert and Claude Huffman, 

Jr., 1974] 

Rock type----

Field No.----

Lab. No.-----

Be-----------

Mg-----------

Ca-----------

Sr-----------

Ba-----------

Zn-----------

Pb-----------

Cd-----------

Cu-----------

Ag-----------

Ag (chem.) ---

Au-----------

Au (A.A.)----

As-----------

Sb-----------

Bi-----------

Fe-----------

Mn-----------

Ti-----------

Zr-----------

Ni-----------

Co-----------

Cr-----------

Mo-----------

v------------

Nb-----------

Sc-----------

La-----------

Nd-----------

Yb-----------

Y------------

Sulfide Vein 

concentrate, material 

Sulfide 

concentrate, 

quartz 

monzonite 

vein 

HL-23-71 

Dl6l386 

1.5 

100 

1000 

15 

150 

10000 

5000 

100 

20000 

150 

(300) 

30 

(38) 

N 

N 

70 

200 

1500 

30 

30 

300 

150 

20 

20 

15 

15 

N 

20 

HL-23-7lA 

Dl61399 

HL-24-71 

Dl6l387 

Analyses (ppm) 

N 

300 

3000 

300 

1500 

N 

150 

N 

200 

50 

(50) 

N 

(0. 5) 

N 

N 

30 

70000 

100 

1500 

100 

L 

100 

150 

20 

L 

N 

N 

N 

15 

L 

500 

2000 

15 

30 

N 

30 

N 

G 

70 

(100) 

N 

(3. 5) 

N 

N 

50 

G 

150 

10000 

300 

50 

70 

15 

70 

150 

70 

15 

300 

700 

N 

100 

Mineralized 

quartz 

monzonite 

HL-24-7lB 

Dl61400 

3000 

20000 

300 

500 

N 

30 

N 

10000 

5 

(20) 

N 

(0. 2) 

N 

N 

N 

20000 

500 

2000 

150 

L 

10 

50 

70 

10 

10 

70 

70 

3 

30 

magnetite, talc, serpentine, helvite, fluorite, and 

phlogopite. 

The porphyry copper and gold deposits of the Las 

Animas placer mining district (Hillsboro) probably have 

the greatest resource potential. The porphyry copper 

deposit of Copper Flat probably contains as much as 4 

million metric tons of ore containing 0.5 percent copper and about 15 million tons of ore containing 0.20 

percent copper. Molybdenum and gold may be important byproducts of any future mining, although the 

molybdenum values are small (0.009 to 0.024 percent 

Mo) and the gold content of the sulfide concentrates is 

about 3.5 ppm (table 2). The gold-bearing veins that 

radiate outward from the Copper Flat stock have only 

been worked to shallow depths of 500 to 1,000 ft (152-

305 m) and the gold largely occurs in pockets along 

7 

brecciated veins in andesite. The gold placers within 

the same area have been extensively worked in the 

past and attempts were made in the early 1970's to 

further exploit the remaining placer gold. Segerstrom 

and Antweiler {1975) have estimated that the total 

reserves of near-surface placer gold are probably 

about 28,000 ounces, or about one-fourth of the past 

production. 

The near-surface silver-rich base-metal deposits 

of the Kingston district have largely been exploited, 

but there still remains a potential for more lower 

grade silver ores at greater depths in the lower Paleozoic carbonate strata. The chert-bearing limestone 

beds in the upper part of the El Paso Limestone should 

be favorable exploration target sites. Most past mining operations were in the Fusselman Dolomite near 

the contact with the Percha Shale, and mining was 

generally done at levels less than 460 ft (140 m) below 

the surface. It is generally conceded by most geologists that the impermeable Percha Shale served to dam 

the upward and laterally migrating ore fluids at the 

contact with the underlying Fusselman Dolomite. The 

silver values of the oxidized base-metal ore are very 

high and some spectrographic and atomic absorption 

analyses indicate as much as 790 ppm silver (table 4). 

Table 3.--Semiquantitative spectrographic 

analyses of ore samples from the Carpenter 

(Swartz) mining district with supplemental gravimetric analyses for silver 

[Results are reported in parts per million to the nearest number in the 

series 1, 0.7, 0.5, 0.3, 0.2, 0.015, and so forth; about 30 percent 

of the assigned groups for semiquantitative spectrographic results 

include the quantitative values. G, greater than 10 percent; N, not 

detected at limit of detection: L, looked for but not detected. 

Leaders ( ---) indicate not determined. Spectrographic analyses by 

J. c. Hamilton and L. A. Bradley, 1973. Silver analyses determined 

gravimetrically by G. D. Shipley and Claude Huffman, Jr., 1974] 

Mine------- Royal 

John 

Acklin Patsy Columbia Grandview Mineral 

Mountain 

Field No. - SL-5-RJ SL-143-73 SL-179-73 SL-167-73 SL-152-73 SL-303-73 

Lab. No.---

Be---------

Mg---------

Ca---------

Sr---------

Ba---------

Zn---------

Pb---------

Cd---------

Cu---------

Ag---------

Ag (chem.)-

Au---------

As---------

Sb---------

Bi---------

Fe---------

Mn---------

Ti---------

Zr---------

Ni---------

Co---------

Cr---------

Mo---------

v----------

Nb---------

Sc---------

La---------

Nd---------

Yb---------

Y----------

0161395 

N 

3000 

30000 

15 

1500 

700 

100 

50000 

10000 

150 

20 

70 

70 

10 

10 

0163856 

N 

2000 

30000 

15 

20 

7000 

7000 

L 

15000 

300 

(185) 

N 

N 

300 

70000 

3000 

150 

N 

70 

N 

N 

N 

N 

N 

N 

N 

0163857 0163858 

Analyses (ppm) 

20000 

G 

15 

5000 

3000 

N 

7000 

50 

N 

300 

5000 

1500 

30 

10 

70 

30 

150 

150 

10 

so 

N 

N 

50 

10 

7000 

G 

70 

10 

10000 

2000 

L 

15000 

N 

N 

N 

20 

G 

20000 

200 

20 

70 

N 

N 

N 

N 

N 

N 

0163859 

150 

20000 

70000 

15 

700 

2000 

150 

(93) 

N 

N 

200 

10000 

20000 

700 

30 

10 

10 

15 

70 

30 

N 

N 

N 

N 

10 

5163860 

5000 

G 

700 

70000 

70000 

300 

15000 

30 

N 

N 

70000 

20000 

30 

N 

10 

7 

N 

70 

N 

N 

N 

N 

N 

10 
Table 4.--Semiquantitative spectrographic 

analyses of base-metal ore samples from 

the Kingston mining district with supplemental atomic absorption analyses (A.A.) 

for gold and gravimetric analyses for 

silver 

[Results are reported in parts per million to the nearest number in the series 

1, 0.7, 0.5, 0.3, 0.2, 0.015, and so forth; about 30 percent of the assigned 

groups for semiquantitative spectrographic results include the quantitative 

values. G, greater than 10 percent; N, not detected at limit of detection; 

L, looked for but not detected. Spectrographic analyses by L. A. Bradley, 

1973. Silver analyses determined gravimetrically by A. w. Haubert, 1974. 

Gold determined by atomic absorption, by A. W. Haubert and Claude Huffman, 

Jr., 1974] 

Mine------- Black Black Blackie 

Colt Colt 

Base-metal ore 

Iron 

King 

Brush 

Heap 

Gray 

Eagle 

Gypsy 

Field No.-- 'Hb-90-73 Hb-1-BC Hb-93-73 HL-7-73 Hb-73-73 Hb-131-73 HI-46-72 

Lab. No.--- 0161388 0161389 0161390 0161397 0161391 0161394 0161393 

Be---------

Mg---------

Ca---------

Sr---------

Ba---------

Zn---------

Pb---------

Cd---------

Cu---------

Ag---------

Ag (chem.)-

Au---------

AU (A.A.)--

As---------

Sb---------

Bi---------

Fe---------

Mn---------

Ti---------

Zr---------

Ni---------

Co---------

Cr---------

Mo---------

v----------

Nb---------

Sc---------

La---------

Nd---------

N 

500 

3000 

G 

1000 

5000 

300 

(660} 

N 

(1.4) 

10000 

100 

70 

30 

500 

15 

30 

Analyses (ppm) 

N 

1500 

20000 

7 

10 

G 

30000 

1500 

10000 

300 

(790) 

N 

(0. 7) 

20000 

1000 

30 

20 

15 

100 

30 

15 

10000 

30000 

30000 

5000 

150 

1000 

150 

(130) 

N 

(0.2) 

15000 

20000 

150 

50 

10 

N 

1500 

50000 

7000 

3000 

N 

300 

70 

(60) 

N 

(0 .1) 

15000 

20000 

30 

L 

10 

N 

3000 

7000 

G 

50000 

1500 

10000 

500 

(740) 

N 

(0. 3) 

N 

1500 

70000 

3000 

200 

30 

1000 

1500 

N 

30 

70000 

100000 

150 

5000 

200 

(250) 

N 

(0.8) 

G 

1500 

150 

70 

20 

300 

30 

Yb--------- N 

Y-------------------------------~15_ 

7000 

1500 

5000 

1000 

N 

500 

1500 

(2650) 

N 

(21) 

7000 

700 

150 

30 

70 

15 

700 

15 

50 

N 

N 

15 

Both fi~;ure veins and tabular bedding-replacement bodies of sphalerite-rich base-metal deposits are 

localized along faults and sporadic skarns in the Carpenter (Swartz) mining district. The ore bodies are 

closely associated with a zone of thermal metamorphism that extends for about 6 mi (9. 7 km) along the 

strike of the faulted Paleozoic section. The silicated 

cherty limestone beds in the El Paso Limestone that 

are generally 20-70 ft (6.1-21 m) below the contact 

with the dolomite beds of the Montoya Group have 

been especially favorable host rocks for· mineralization 

throUghout the ·district. Past production records indicate. that the 60,000 short tons o.f zinc-rich base-metal 

ore mined from 1916 to 1949 had relatively low silver 

values that averaged about 140 ppm. Chip samples 

from· the bedding-replacement deposits in six mines 

were analyz·ed by $emiquantitative spectrographic and 

quantitative methods' and the results are shown in 

table 3. 

The manganese deposits of the Kingston district 

have largely been. depleted. Most of the manganese 

ore occurs in small pods, shoots, and replacement 

bodies along fractures and faults within dolomite beds 

8 

of the Fusselman Formation and Montoya Group. The 

manganese oxides have formed through the weathering 

and oxidation of silver-bearing base-metal veins that 

have rhodochrosite, alabandite and rhodonite gangue 

minerals. 

There are three areas of known tungsten deposits: (1) the Silver Tail property along Pierce Canyon, 

(2) the Silver Queen claims near Kingston, and (3) the 

small prospects within tactite along the south side of 

Tank Canyon. It is only from the Silver Tail group of 

mines that Dale and McKinney (1959, p. 66-67) report 

any appreciable tungsten ore production-about 55 

short tons of scheeli.te-bearing ore averaging 3-4 percent wo3• At this locality, the molybdenum-bearing 

scheelite occurs as disseminations and veinlets in 

highly fractured andesite. 

Table 5.--Semiquantitative spectrographic 

analyses of ore samples from the Jap, 

Log Cabin, and Ingersoll mines with 

supplemental atomic absorption analyses 

(A.A.) for gold and gravimetric analyses 

for silver 

[Results are reported in parts per million to the nearest number in 

the series 1, 0.2, 0.5, 0.3, 0.2, 0.015, and so forth; about 30 

percent of the assigned groups for semiquantitative spectrographic 

results include the quantitative values. G, greater than 10 percent; N, not detected at limit of detection; L, looked for but not 

detected. Spectrographic analyses by L. A. Bradley, 1973. Silver 

analyses determined gravimetrically by A. W. Haubert, G. D. Shipley, 

and Claude Huffman, Jr., 1974. Gold determined by atomic absorption 

by A. w. Haubert and Claude Huffman, Jr., 1974] 

Rock type---- Vein 

and mine sample, 

Jap mine 

Field No.---- S1-283-73 

Lab. No.----- 0163861 

Be-----------

Mg-----------

Ca-----------

Sr-----------

Ba-----------

Zn-----------

Pb-----------

Cd-----------

Cu-----------

Ag-----------

Ag (chem. )---

Au-----------

Au (A.A.)----

As-----------

Sb-----------

Bi-----------

3000 

30000 

30 

30 

G 

700 

50000 

300 

(230) 

N 

(n.d.) 1 

N 

N 

Fe----------- 70000 

Mn----------- 1500 

Ti----------- 1500 

Zr----------- 30 

Ni----------- 30 

Co----------- 30 

Cr----------- 10 

Mo----------- 50 

v------------ 50 

Nb----------- N 

Sc-----------

La----------- 70 

Nd----------- 100 

Yb-----------

Y------------ 70 

Ce----------- 200 

1Not determined. 

Sulfide Vein 

concentrate sample, 

from vein, Log Cabin 

Jap mine mine 

SL-1-JAP Hb-81-72 

0161396 0161398 

Analyses (ppm) 

N N 

700 10000 

10000 30000 

N 15 

2000 15 

G 

70000 

2000 

30000 

1000 

(1430) 

N 

{0.05) 

N 

N 

2000 

50000 

2000 

200 

N 

20 

150 

20 

500 

L 

L 

N 

300 

1500 

N 

100 

1000 

(1420) 

N 

(6) 

N 

N 

N 

1000 

1000 

30 

N 

L 

N 

N 

N 

N 

N 

N 

Sulfide 

concentrate 

from vein, 

Ingersoll mine 

HS-68-72 

0161392 

N 

70 

150 

N 

7 

G 

30000 

2000 

10000 

20000 

{17 ,800) 

N 

(11) 

1500 

10000 

N 

70000 

500 

30 

N 

15 

15 

30 

N 

L 

N 

N 

N 

N 

N 

N 
The vanadium, iron, and fluorspar deposits are all 

small and subeconomic. The sand and gravel deposits 

along Middle Percha Creek have been periodically 

quarried for road construction but are generally too 

small and too far from any major population center to 

be of value. Similarly the other nonmetallics such as 

limestone, dolomite, and perlite are all uneconomic 

deposits. 

Descriptions of the tungsten deposits were omitted on the mineral resources map (MF-900B.) 

Geochemistry (Maps MF-900C-K) 

By K. C. Watts and H. V. Alminas 

Geochemical field work in the Hillsboro and San 

Lorenzo quadrangles, New Mexico was completed in 

the fall of 1974. Some of the geochemical data within 

the Hillsboro quadrangle was published in Alminas and 

others (1975a,b,c) and includes data acquired before 

1973. Those publications include the entire Sierra 

Cuchillo as well as part of the data presented here for 

the Hillsboro quadrangle. Subsequent to the completion of the miscellaneous geologic investigations maps 

(Alminas and others, 1975a,b,c) additional geochemical 

sampling in the Hillsboro quadrangle was continued 

through the fall of 197 4 in order to better define the 

anomaly configuration along the crest of the Black 

Range between the two quadrangles; concurrent sampling occurred in the San Lorenzo quadrangle. 

The work consisted of systematic sampling of 

stream sediments primarily from first- and secondorder drainages. Small drainages were sampled because the area is a critical one from the standpoint of 

ore deposit potential and, in our estimate, it deserved 

more than a reconnaissance approach. Very often, in 

areas where closer control was desired, large welldeveloped drainages did not exist; sampling of very 

small drainages was therefore required in order to 

achieve the desired sample-site control. Large drainages also contribute prohibitively large amounts of 

barren material that dilute anomalies. We found that 

sampling on small drainages at angles oblique to the 

main streams showed anomalies having a very local 

bedrock source. Anomalies might otherwise be attributed to known mineralization derived from areas, like 

the Black Range crest, at the head of some large tributaries. The sampling scheme has shown that many 

anomalies are not derived from these areas of known 

mineralization. The sample design also provided for 

anomalies to be substantiated by more than one 

sample-often on opposite sides of a ridge, and often 

sampled by different individuals at different times. 

The geochemical maps have been constructed 

from data derived from pan-concentrated heavymineral samples collected from 1,102 sample sites. 

The principle sample material used for exploration 

purposes was the pan-concentrated heavy-mineral 

sample derived from stream sediment; but, in addition, 

the conventional -80 mesh sample was collected and 

analyzed over the same area for census and purposes 

of comparison. The -80 mesh sample does not provide 

9 

suitable contrast for use as an exploration sample 

medium in the area. Rocks displaying evidence of 

mineralization, such as gossan, veinlets, and stringers 

were sampled as well as rocks from favorable formation contacts and mine dumps, where these materials 

were located near the stream-sediment site. Many of 

these samples proved to be anomalous in at least OJle 

metallic element. Samples of fresh unaltered rock 

were also collected for the purpose of estimating 

background for each rock type. Analytical results 

indicate that lithology has less influence on the metal 

values obtained in heavy-mineral concentrates than it 

does with -80 mesh stream sediments. The reasons for 

this are the following: (1) Much of the trace-element 

variation due to lithology is removed by panning because of the loss of clays and feldspar; the trace elements related to mineralization are therefore more 

isolated. (2) The variation and level of anomalous 

metal content for heavy-mineral samples is often 

much greater than for the -80 mesh stream sediments 

and bulk rock. Anomalous metal values are therefore 

easier to distinguish from the background. 

Earlier work at Monticello Box (Griffitts and 

Alminas, 1968), in the San Mateo Mountains (Griffitts 

and others, 1972a-f), and in the Winston, Chise, and 

Priest Tank quadrangles (Alminas and others, 1972a-e, 

1973a-d), demonstrated that the use of heavy-mineral 

concentrate samples greatly enhanced the anomalybackground contrast and removed much of the "noise" 

caused by variations in lithology and dilution from 

barren material in these areas. The method seems 

particularly useful in those places where the concealed 

mineral deposits are overlain by possibly barren, but 

probably pre-ore, caprock. Near-surface deposits can 

sometimes be detected with conventional -80 mesh 

stream sediments, although the anomalies are less pronounced and frequently only the peaks are detected. 

To detect mineralization beneath caprock usually requires the enhancement of anomaly-background contrast, such as used here. 

Limonite and manganese oxides containing high 

trace-metal values occur in the magnetic (M-1) concentrate fraction and are derived from joint surfaces, 

fractures, and dispersions in caprock. This material is 

then mechanically transported to the stream bed. The 

trace-metal-rich limonite and manganese oxides are 

thought to result from processes associated with 

mineralization and later weathering. Much of the limonite is pseudomorphic after pyrite. In areas where 

the trace-metal-rich limonite is found, it may be derived from weathered primary halos of disseminated 

pyrite, possibly with other sulfides which may exist 

above blind mineral deposits. Perhaps ground water 

moving upward along the structural weakness has also 

deposited high trace-metal oxides from mineralization 

at depth; this mechanism is probably at work, but it is 

not regarded as a dominant factor except perhaps 

where late subvolcanic intrusions may have provided a 

mechanism for convection, such as may have occurred 

near Noonday Peak. 

High metal concentrations in the nonmagnetic 

(NM-1) concentrate fraction occur where primary and 

secondary ore minerals are exposed at the surface and 

mechanically enter the stream bed. 

Metals and combinations of metals shown on the 
geochemical maps were selected on the basis of the 

geochemical relation they may have to the occurrence 

of mineral deposits. The observed distribution of 

fluorite and cassiterite in heavy-mineral concentrates 

is also shown on map MF-900H. This data was derived 

from routine scans of all the samples by binocular 

microscope. Specific metals, such as anomalous lead 

in limonite (M-1 concentrate fraction), and bismuth 

(NM-1 concentration fraction) may indicate areas in 

which metal values are controlled by subsurface 

structural features because these metal anomaly zones 

sometimes occur as well-defined linear patterns both 

trending across mapped surface geology and occurring 

as surficial indications of mineralized structural features projected on strike into covered areas. 

Tungsten mainly defines skarn zones which have 

ore potential. Tungsten and other metallic elements 

also define at least one structural feature that apparently was a conduit for hydrothermal solutions 

(Pierce Canyon fault). A number of target areas contain several elements in anomalous concentrations. 

These multi-element anomalies, many supported by 

favorable geological conditions and geophysical 

evidence, may overlie blind mineral deposits. 

Geophysics (Maps MF-900L and M) 

By J. C. Wynn and D. C. Hedlund 

Abstract 

Gravity and aeromagnetic studies in the Hillsboro and San Lorenzo quadrangles of southern New 

Mexico are a part of a larger study by the U.S. Geological Survey that includes geologic mapping and 

geochemical stream-sediment sampling. There are 

several large structural features in the map area that 

have pronounced gravity and magnetic expressions. 

These include the Mimbres fault, which has a normal 

throw (east side downthrown) of nearly 2 km and the 

southern third of the Emory Pass caldera (a 34-milligal 

gravity low). This latter feature has been modeled as 

a 4-km-deep depression filled with a low density material, all of which may be Kneeling Nun Tuff. Zones of 

altered rock may also exist beneath this tuff layer in 

the margins of the caldera. 

!he p~incipal aeromagnetic anomaly in the map area IS a circular feature located just south of the 

Copper Flat stock near Hillsboro, which may be attributed to a small batholith that has a limited depth 

exten~ of ab~ut 7 km. Gravity data show a linear high 

associate? with the magnetic anomaly, but extending 

several kilometers to the south of it. This ridge may 

come to within a few hundred meters of the surface at 

a point about 5 km southeast of Hillsboro, and could be 

explained by an intrusion-related horst structure delimited on the west by the Berenda fault and on the 

east by another major north-south offset in the Skute 

St~ne Arro~o ~uadran~le.. The location of the high 

pomt of thiS ridge comc1des with local geochemical 

anomalies mapped in the area by the U.S. Geological Survey. 

Introduction 

The Hillsboro and San Lorenzo 15-minute quadrangles are located east of the Santa Rita (San Juan) 

mining district, between Silver City and Truth or 

Consequences, N. Mex. (fig. 3). The two quadrangles 

lie just south of the Gila Wilderness, and include the 

southern end of the Black Range in the Mexican Highland section of the Basin and Range province. Figure 1 

is a generalized geologic map with place names. 

Within the mapped area, volcanic and volcaniclastic rocks of the Datil-Mogollon field have been 

extruded and deposited over Paleozoic strata and 

remnant outcrops of Upper Cretaceous sandstone and 

andesite. Ash-flow tuffs of the Kneeling Nun Tuff 

(33.4 m.y.) are especially extensive and are thought to 

be derived from the Emory Pass caldera (Elston and 

others, 1975). This caldera is not completely defined, 

but the southern part appears to extend into the northern parts of the map area. 

Late Tertiary (about 20 m.y.) block faulting has 

tilted and arched about 820 m of Paleozoic strata and 

the 1800-2000 m of overlying volcanic rocks near the 

crest of the Black Range. Subsequent erosion has exposed the Paleozoic rocks and the economically important Laramide (75.1±2.5 m.y.) copper pqrphyry deposit 

of the Copper Flat stock, the middle Tertiary (34.8±1.2 

m.y.) skam deposits of the Carpenter (Swartz) district, 

and the middle Tertiary(?) silver and base-metal deposits of the Kingston district (Elston and others, 

1975). 

109° 

37° 

NEW 

0 

Santa Fe 

Tucumcari 0 

Albuquerque 

MEXICO 

103° 

FIGURE 3.-lndex map showing location of the Hillsboro and San Lorenzo quadrangles (solid), and the 

aeromagnetic map of parts of the Silver City and 

Las Cruces 1° by 2° quadrangles (patterned), New 

Mexico. 

10 
Gravity, ground magnetic, and aeromagnetic 

data were collected as part of a U.S. Geological Survey study that included geochemical stream-sediment 

sampling and geologic mapping (Hedlund, 1975a,b; 

Watts, Alminas, and Kraxberger, 1976; Watts and 

others, 1976). Using this data, several gravity and 

aeromagnetic models were calculated to give quantitative information about subsurface features of possible 

economic interest. It is well known that, under special 

circumstances, gravity and magnetic anomalies may be 

caused directly by deposits of metallic ores; for 

instance, massive sulfides or iron ore bodies. Generally, however, gravity and magnetic methods are 

used primarily as reconnaissance tools to isolate 

geologic settings that may be favorable for the occurrence of ores. Thus, the geophysical studies in this 

investigation mainly serve to outline parts of the 

Emory Pass caldera, to identify faults, and to isolate, 

among other things, possible buried tracts of rocks 

that might be hosts for ore deposits. 

Aeromagnetic Data 

Collection and processing 

In 1973, an aeromagnetic survey was flown and 

compiled by Aerial Surveys LTD. for the U.S. Geological Survey and the New Mexico Bureau of Mines and 

Minerals, as part of a cooperative program. The survey was flown at a constant barometric elevation of 

3.05 km above sea level, in east-west traverses spaced 

1.6 km apart. The original survey was compiled at a scale of 1:250,000 from individual maps of 1:62,500 

scale, with a contour interval of 10 gammas (U.S. Geological Survey, 1974). 

For the purposes of this investigation, the digitized data from this survey were gridded on a 0.5-km 

equal interval, using bicubic splines, and a regional 

quadratic surface (derived from the 1965 IGRF-International Geomagnetic Reference Field-updated to 

1973) was removed to facilitate subsequent processing. Because the topographic relief in the map area exceeds 1500 m, with elevations as high as the stated 

flight elevation, the data show strong topographic effects, especially in the central part of the twoquadrangle area, where magnetic rocks are exposed in 

high relief. 

In order to minimize these topographic effects, a 

new method of aeromagnetic data-processing called 

pal'allel-surface continuation was applied to the gri<}-

ded data set. This method was developed by Bhattacharyya and Chan (1977), and, in essence, calculates 

the magnetic field at a surface everywhere parallel to 

the topography. The effect is to simulate a perfect 

drape flight by the recording aircraft, where the 

effects of magnetized peaks are diminished until they 

contribute the same relative influence as the surrounding valleys. The results are a selective upward continuation of the aeromagnetic data set, where only 

data over peaks are continued. They are presented 

here in a contoured polyconic projection as the parallel-surface-continued aeromagnetic map of the San 

Lorenzo and Hillsboro quadrangles, New Mexico (fig. 4 

of this report; MF-900L). The mean distance between 

the continued maghetic surface and the ground was 1.5 

km. 

Description and analysis 

A comparative examination of the original aero- magnetic map (U.S. Geological Survey, 1974) and the 

parallel-surface-continued aeromagnetic map show 

that most of the topography-caused magnetic highs over the Kneeling Nun Tuff in the Black Range have 

been removed; these magnetic highs all appear to be 

economically uninteresting. The result is that several 

magnetic lows, one of which was almost totally obscured by topography, show up clearly along the south 

margin of the Emory Pass caldera. An extensive magnetic high over the Carpenter (Swartz) mining district 

was probably caused by intense but sporadic tactite 

development within the Paleozoic limestones, and in 

part by topgraphy; the crest of the magnetic high has 

been shifted several kilometers to the south on the 

new map. Most of the other magnetic features on both 

sides of the Black Range remain 'unaffected by the 

parallel-surface continuation. 

The parallel-surface-continued aeromagnetic 

map (hereafter called the aeromagnetic map or aeromagnetic data) is dominated by a very strong circular 

magnetic high on its east side, situated approximately 

over the southem part of the andesite of Copper Flat 

in the Hillsboro mining district (anomaly 1 on fig. 4). 

A series of north-south trending magnetic lows (anomalies 2, 3, and 4 on fig. 4) dominate the west side of 

the Hillsboro quadrangle, and are coincident with the 

outcrops of Paleozoic strata. The Pierce Canyon fault 

marks the westem edge of these lows. The San Loren- zo quadrangle contains the magnetic high (anomaly 5 

on fig. 4) with the greatest overall areal extent of any 

of the closed anomalies. The maximum amplitude 

point of the multi-armed feature is centered a few kilometers south of the Carpenter (Swartz) mining district, and auxiliary extensions of the anomaly are 

underlain by Sawyers Peak, Pine Flat Mountain, and 

Noonday Peak, which are also areas of geochemical 

anomalies. A magnetic high (anomaly 6 on fig. 4) 

centered approximately over the Rose Mine west of 

the Mimbres River, a string of magnetic lows (anomalies 7, 8, and 9 on fig. 4), and a magnetic high 

(anomaly 10 on fig. 4) in the northern part of the San 

Lorenzo quadrangle, complete the list of important 

aeromagnetic features. 

Anomaly 1 is important because of its proximity 

to the quartz monzonite stock of Copper Flat. It was 

necessary to use a three-dimensional model to fit this 

feature; the results are shown in figure 5. The anom- aly appears to be caused by a prism-shaped body which 

dips 45° .in a north-northwesterly direction. The apparent top of the body at the center of the anomaly is 

at roughly 1280-m elevation (300-600 m below the 

ground surface), and the vertical extent of the body 

attains roughly 8 km, plus or minus 2 km. This model, 

like most models, is probably a gross simplification of 

the true geologic configuration of the causative body. 

The calculation was made with the original constant 

(3.05 km) elevation data and the imperfect fit of 

model to data is largely due to the limitation of the 

modeling program to use only a single prism. The resul t, nevertheless, seems to clearly indicate a small 

pluton whose top is located beneath the southern mar- gin of the Hillsboro andesite field. The Copper Flat 

11 
32:55:00 

32:45:00 

0 

co 

0 

0 

0 

0 0 0 'l 'l 'l 

LJI .... ().4 

0 0 0 

0 0 0 

0 0 0 

FIGURE 4.-Parallel-surface-continued aeromagnetic map of the San Lorenzo-Hillsboro 

quadrangles, gridded at 600 m intervals. Individual features referred to in the text 

are located by the following symbols: H, Hillsboro; K, Kingston; C, Carpenter 

(Swartz) district; EC, Emory Pass caldera; R, Rose Mine; and SJ, San Juan. Anomalies 

1-10 are referred to in the text. Scale is 1:250,000. 

A A' 33° 107°35' 

00' c 

r---...., 

'-, MEASURED FIELD 

" ' CALCULATED FIELD)', 

(3-DIMENSIONAL MODEL) "-."-. 

........ ...._ __ 

--

SUSCEPTIBILITY: 0.0035? (EMU/cm3

) 

MILES 

CENTROID OF SOURCE+ 0 KILOMETERS 3 

FIGURE 5.-(A) Magnetic anomaly in the Hillsboro district; (B) north-south section 

showing the measured field, the calculated field, and; (C) the cross section of the 

source body. 

12 

107°30' 
stock is located above the north edge of the prism top, 

and unaltered monzonite from it is known to have a 

similar high magnetic susceptibility (see table 6). It is 

reasonable to believe that this stock may be a mineralized apophysis of the pluton. The identification of the 

pluton appears to be further verified by the presence 

of the Warm Spring Canyon stock and by two small 

plug-like bodies of quartz monzonite in sees. 10 and 

11, T. 16 S., R. 7 W. near its southern margin (Hedlund, 

1975a). The stock of Warm Spring Canyon includes 

components of granite, quartz monzonite, and quartz 

monzodiorite. Dioritic xenoliths in the Copper Flat 

stock and the presence of a nearby small quartz diorite 

pluton along Tank Canyon imply a dioritic composition 

for the buried pluton. 

Table 6.--Magnetic susceptibility and density 

data for some of the rock units described 

in the geophysical interpretation and 

accompanying maps. Measurements by W. C. 

Rivers (1974) and J. c. Wynn (1975) 

[NO, data not available] 

Rock unit on geologic map 

Basalt of Santa Fe---------

Formation. 

Basalt of Bear Springs-----

Rhyolite of Mimbres Peak---

Rhyolite porphyry----------

Kneeling Nun Tuff----------

Rubio Peak andesite--------

Andesite of Copper Flat----

Quartz monzonite of CopperFlat. 

Sericitized quartz---------

monzonite of Copper Flat. 

Sulfide ore from Copper----

Flat. 

Quartz latite porphyry-----

dike from Copper Flat. 

Hornblende latite sill-----

Hornblende latite (PollackQuartz Latite). 

Skarn from the Royal John--

Mine. 

Skarn from the Franklin----

Mine. 

Granite of Seven Brothers--

Mountain (pC). 

Quartz monzo-diorite of----

Warm Spring Canyon. 

Leuco-granite of Warm------

Spring Canyon. 

No. of Susceptibility 

samples (lo-6 emu/cm3) 

3723 

4480 

466 

8. 34 

88.3 

3. 34 

38 

293 

324 

332 

1000 

758 

947 

182 

733 

3399 

3680 

366 

370 

393 

15 

15 

21 

NO 

NO 

ND 

341 

205 

NO 

NO 

Specific 

gravity 

2. 92 

2.64 

2.43 

2.56 

2.50 

2.60 

2.64 

2.60 

2.60 

ND 

2.56 

2.60 

2.58 

ND 

ND 

ND 

2.76 

2.71 

There are several extensions of anomaly 1; one 

to the west under Warm Spring and North Percha 

Creek Canyons, where hot springs and altered rock 

may be found, another to the southwest underlying 

McClede Mountain, and a third to the south. It will be 

shown later that the southem extension coincides with 

a horstlike structure evident for the gravity data. 

Anomalies 2, 3, and 4 comprise a string of northsouth-trending magnetic lows in the western half of 

the Hillsboro quadrangle. These lows appear to closely 

follow outcrops of Paleozoic rocks; there are few intrusives, no metamorphic rocks, and the anomalies are, 

therefore, probably caused by a lack of magnetic 

rocks. 

Anomaly 5 is a mul tilobed magnetic high that 

reaches its peak expression in the San Lorenzo quadrangle just south of the Royal John Mine in the Carpenter (Swartz) mining district (Hedlund, 1975b). A 

two-dimensional iterative magnetic modeling program 

was used to model an east-west profile through the 

Royal John Mine, (located just southeast of point C in 

figures 4 and 7) and the results of the modeling effort 

are shown in figure 6. The better fit was obtained by 

assuming a body with purely remanent magnetization; 

however, induced magnetic effects are almost certainly involved (see table 1). All the models imply a semiinfinite dikelike body that reaches to within 200 m of 

the ground surface; considering the precision of the 

model, the dike must essentially outcrop. 

The anomaly is underlain by rhyolite porphyry on 

the northern end and rhyolite on the southern end. 

There are latite and rhyolite plugs and dikes within the 

Paleozoic strata in the central and northern part of 

the Carpenter (Swartz) district, and there are sporadic 

skarns present. The skarns, however, are only in the El 

Paso Limestone (therefore cannot exceed 150 m in 

thickness), and only beneath the northern part of the 

magnetic high. The mineralization in the Carpenter 

(Swartz) mining district is near-surface in origin, and 

was probably caused by thermal metamorphism in 

rocks beneath the volcanic cover. The source body for 

much of anomaly 5 might, therefore, be pneumatolitic 

magnetite (consistent with tactite development) in 

buried Precambrian rocks of granitic to monzonitic 

character, probably in association with overlying small 

and sporadic skam deposits. 

Anomaly 6 lies to the southwest of the Mimbres 

fault in the southwestern corner of the San Lorenzo 

quadrangle. The magnetic high is centered roughly 

over the Rose Mine, just south of New Mexico State 

Highway 180. The source for this magnetic high could 

be volcanic rocks upthrown to the southwest, but copper and molybdenum halos and the anomaly shape imply a possible buried porphyry system which, like the 

postulated pluton at anomaly 1, may have a good economic potential. 

The magnetic lows labeled 7, 8, and 9 are connected with each other, and together trend northeast 

across the northern half of the San Lorenzo quadrangle. These anomalies are not easily explained by 

surface geology. They connect on the northern edge of 

the map area with anomalies 2, 3, and· 4, and it is 

13 
A A' 

A 

145 

~DIPPING BODY 

\. FIT 

WEST •••••• EAST 

~------------~------~~~------~0 

A 0 

I 

\\ ·· .. I \' ·· ..... \ ' ·· .. iusCEPTIBILITY>,..., '--.. •• 

FIT ...... ", 

MEASURED FIELD 

KILOMETERS 

1 07°45' 

5 

I 

FLIGHT ELEVATION 10,000 FEET 

A' 

METERS 

3000 

2500 

FEET 

32"50' 

SAWYERS 

6. PEAK 

SUSCEPTIBILITY 

~ 

BODY 

I I 

I I 

ROYAL JOHN I I 

MINE ~: 

I I 

I I 

I I 

I I 

~ 

MAVERICK REMANENCE 

MOUNTAIN 6 BODY 

0 KILOMETERS 3 

2 Y• dimensional model 

FIGURE 6.-(A) Magnetic anomaly in the Carpenter (Swartz) district; (B) east-west 

section showing the measured field, the best susceptibility-fit field, the best 

remanent-fit field, and the best dipping-body susceptibility-fit field; and (C) the cross 

section of the source body. 

tempting to relate anomalies 7, 8, and 9 also to buried 

Paleozoic rocks, because of their spatial relationship 

to anomaly 2. The gravity data, however, clearly show 

that these anomalies cut across the gradational south 

margin of the Emory Pass caldera, and thus imply 

another source. Anomaly 8 is particularly intense-its 

source extending both close to the surface and to con- siderable depth. It is located over the head of Bear 

Trap Canyon, an area covered by Kneeling Nun Tuff. 

There is no apparent localized alteration, although 

low-grade regional metamorphism is known to have 

taken place in the pre-Kneeling Nun rocks. Anomalous 

amounts of bismuth and tungsten form characteristic 

halos around the Copper Flat, Hanover-Fierro, and 

Pinos Altos orebodies, and a low-level bismuth anomaly forms a halo around anomaly 8. The possibility 

exists that lows 8 and 9 (at least) are the result of hydrothermal alteration of tuffs or andesite flows in the 

buried rocks beneath the Kneeling Nun Tuff. These 

aeromagnetic lows lie on the apparent margin of the 

Emory Pass caldera, as inferred from the gravity data; 

14 

this type of geologic association is known to be favorable for mineralization. 

Anomaly 10 is a magnetic high that lies in the 

northwestem corner of the San Lorenzo quadrangle. 

This high lies over latitic rocks which contrast on the 

east with rhyolitic rocks and on the west with colluvium. 

Gravity Data 

Collection and processing 

One hundred and forty gravity stations were es- tablished in and around the Hillsboro and San Lorenzo 

quadrangles. The data were obtained using high-precision gravity meters, and all stations were tied to a U.S. Department of Defense (DOD) gravity base at Silver City, N. Mex. (979006.48 milligals). An additionAl 

18 stations were obtained from DOD sources and added 

to those obtained by the U.S. Geological Survey. 
Elevation control was taken from published topographic maps of the U.S. Geological Survey, and, 

where bench marks and survey points were not readily 

available, a theodolite was used to establish elevations 

from at least three nearby peaks of known elevation, 

or by running a level line from a known elevation 

point. Station elevation accuracy is, therefore, better 

than 3 m in all cases except those elevations obtained 

by referencing to three or more peaks. In the latter 

cases, only those stations with closure (for three 

measurements) within 4 m were used, giving a resulting maximum error for these stations of 0. 7 milligals 

due to elevation error. Terrain corrections to a distance of approximately 3 km were made for all stations. 

Powder densities for the volcanic rocks in the 

two quadrangles ranged from 2.43 gm/cm3 for Mimbres Peak Rhyolite to 2.92 gm/cm3 for basalt of the 

Santa Fe Formation (see table 1). The density varied 

around a mean of 2.67 gm/cm3, and the simple Bouguer and terrain corrections were made using this 

standard value. It should be noted, however, that if 

there are prominent topographic features in an area 

caused largely from surface rocks whose density differs greatly from 2.67 gm/cm3, small systematic errors will exist in the reduced gravity values. Such an 

error, in fact, may exist in the data around Hillsboro 

Peak. The complete Bouguer gravity values thus ob32:55:00 

32:45:00 0 0 

0 0 co " I.JI 0 0 

0 0 0 0 

tained were gridded on a 2-km equal interval and 

plotted with a polyconic projection (fig. 7; MF-900M). 

This was done using the gridding and contour computer 

programs of R. Codson (unpub. data). The polyconic 

projection was chosen to permit overlaying of the 

gravity onto the geology map (MF-900A). 

Description and analysis 

Regional gravity surveys, discussed by Ramberg 

and Smithson (1975), and by Eaton (Ericksen and 

others, 1975), place the San Lorenzo and Hillsboro 

quadrangles in a context of north-northwest trending 

structures that are oblique to the trend of the Rio 

Grande Rift located 45 km to the east. These oblique 

structures are probably related to tensional forces that 

broke open the rift along pre-existing fractures in the 

late Cenozoic (Ramberg and Smithson, 1975). 

Three major features dominate the gravity map 

(fig. 7): a gradient coincident with the Mimbres fault 

on the southwest corner, a deep gravity low which outlines the southern boundary of the Emory Pass caldera, 

and a north-trending gravity ridge in the Hillsboro district. In addition to these features, there is a broad 

north-trending gradient in the west half of the Hillsboro quadrangle that probably stems from uplifted 

basement and related magmatic intrusions in the Black 

0 

0 

0 0 ........ ........ 

~ Vol 0 0 

0 0 0 0 

FIGURE 7 .-Complete Bouguer gravity map of the San Lorenzo and Hillsboro quadrangles, gridded at 2,000 m intervals. Circles indicate gravity station localities; H, 

Hillsboro; K, Kingston; C, Carpenter (Swartz) district; EC, Emory Pass caldera; R, 

Rose Mine; and SJ, San Juan. Simple Bouguer and terrain corrections were made assuming an average rock density of 2.67 gm/cm3. Contour interval is 25 milligals; 

scale is 1:250,000. 

15 
Range, where a broad arching is apparent in the 

geology. 

The expression of the Mimbres fault (see MF900A) is difficult to separate from the southwestern 

edge of the Emory Pass caldera. This complication is 

not as serious near the town of San Juan (SJ in fig. 7) 

on the southern margin of the map area, however, and 

a two-dimensional gravity profile was modeled to calculate the apparently gradational or en echelon displacement on the Mimbres fault. For a density contrast of 0.4 gm/cm3 the (normal) cumulative throw on 

the Mimbres fault is 2.0±0.2 km, with the downthrown 

side to the northeast. The trace of the fault agrees 

closely with that mapped by Hedlund (1975b). 

The Emory Pass caldera, as inferred from the 

gravity map, (fig. 8) shows a 34-milligal gravity low 

having a rather sharp eastern boundary with more 

gradual southem and westem margins. The eastern 

edge at lat 33°00' N. is located about 3 km west of the 

juncture of the San Lorenzo and Hillsboro quadrangles. The southeast bulge of the caldera margin may 

be partly due to the 2.67 gm/cm3 Bouguer and terrain 

correction for the station atop Hillsboro Peak; the 

mountain consists mainly of Kneeling Nun Tuff overlain in part on the north by Pollack Quartz Latite. The 

Kneeling Nun Tuff has a density of 2.50 gm/cm3 and 

the quartz latite has a maximum density of 2.58 

gm/cm3. As a result, the complete Bouguer value for 

the Hillsboro Peak station may be 1 or 2 milligals low 

with respect to surrounding values; the exact value is 

32:5e:oo I~ 

I 

32:56:00 

32:54:00 

not obtainable, however, without precise additional 

information about the thicknesses and three-dimensional distribution. 

Elston, Seager, and Clemons (1975) thought that 

the Kneeling Nun Tuff filled the caldera after its collapse, and was subsequently raised and faulted by 

resurgent doming. A three-dimensional gravity modeling program (Cordell and Henderson, 1968) was used to 

obtain an estimate of the shape of the buried caldera, 

assuming the conceptual model of Elston, Seager, and 

Clemons (1975). The data were initially converted to a 

residual gravity map by adding +196.78 milligals to all 

values in the 9 by 15 grid (fig. 9). The program was 

then allowed to iterate three times, using a density 

contrast of -0.35 gm/cm3, and the resulting calculated 

field is shown in figure 10. 

The source model giving this result is a 4-kmdeep caldera, and is shown in figure 11, where the 

contour intervals are 0.5 km. The fit using this model 

is excellent (compare figs. 9 and 10) due to the limited 

number of points being calculated (only 135 grid points 

were possible due to the sparseness of the original 

data). If a density contrast of -0.1 gm/cm3 was used, 

a somewhat broader caldera shape was obtained, due 

to the caldera floor now extending to a depth of 20 km 

(fig. 12). The best-fit calculated model (fig. 13) is not 

nearly as close to the original field (fig. 9) as the calculated field from the p = -0.35 model (fig. 10). Both 

of these models show a steeper topographic gradient 

on the eastem edge, with more gradual thinning to the 

) ' 

~ 

I) I · I I (i 

~ 0 0 'I "-! 

-l>o -l>o 

en ~ 

0 0 0 0 

FIGURE 8.-Gravity map of the Emory Pass caldera, taken from figure 7. 

interval is 2.5 milligals;scale is 1:125,000. 

Contour 

16 
I 

32:58: oat 

i 

32:56:00 

32:54:00 

32:52:00 

0 

co 

0 

0 

0 

33: 0:00 

I 

32:58:001 

32:56:0J 

32:54:00 

~ 

0 

co 

0 

0 

0 

+ + 

~ 

0 0 0 0 0 0 0 0 'I 'I 'I 'I 'I 'I ~ 'I 

U1 U1 U1 U1 U1 A A A co "' A 1\) 0 co "' A 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

FIGURE 9.-Residual gravity map of the Emory Pass caldera, using an offset of 196.78 

milligals. Contour interval is 2.5 milligals; scale is 1:125,000. 

+ + 

~ 

0 0 0 0 0 0 0 0 'I 'I 'I ~ 'I 'I 'I "-.j 

U1 U1 U1 U1 U1 A A ..,. co "' A 1\) 0 co "' A 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

FIGURE 10.-Calculated gravity map of the Emory Pass caldera, from the threedimensional model using -0.35 gm/cm3 density contrast. Contour interval is 2.5 

milligals; scale is 1:125,000. 

17 
33: 0:00 

32:58:00 

32:56:00 

I 

\ ! 

+\~ 32:54:00 + 

Q 

32:52:00 + + I I 

33: 0:00 

32:58:00 

32:56:00 

32:54:00 

32:52:00 

0 

CD 

0 

0 

0 

0 

CD 

0 

~ ~ 

0 0 0 0 'I 'I 'I 'I 

1.11 1.11 1.11 1.11 CD en .J>, N 

0 0 0 0 0 0 0 0 

0 0 'I 'I 

1.11 .J>, 

0 CD 

0 0 0 0 

0 

"-..J 

.J>, 

en 

0 

0 

0 

'I 

.J>, 

.J>, 

0 

0 

FIGURE 11.-Three-dimensional model of the Emory Pass caldera, using -0.35 gm/cm3 

density contrast, and 0.5 km contour intervals, and datum is mean ground elevation over the map area. Scale is 1:125,000. 

0 0 0 0 'I 'I 'I 'I .. Ul Ul Ul Ul CD 0'1 .J>, N 

0 0 'I 'I 

Ul .J>, 

0 CD 

0 

'I 

.J>, 

0'1 

0 

'I 

.J>, 

.J>, 

.. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

FIGURE 12.-Three-dimensional model of the Emory Pass caldera, using -0.10 gm/cm3 

density contrast, and 2.5 km contour intervals, and datum is mean ground elevation 

over the map area. Scale is 1:125,000. 

18 

I 
33: 

32:58:00 

32:56:00 

32:54:00 

0 0 0 0 0 0 0 0 0 ro 'I 'I 'J 'I 'J 'J 'I 'J 

(.JI (.JI (.JI (.JI (.JI J>, J>, J>, 0 ro 0"\ J>, N 0 ro 0"\ J>, 

0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 

FIGURE 13.-Calculated gravity map of the Emory Pass caldera, from the threedimensional model using -0.10 gm/cm3 density contrast. Contour interval is 2.5 

milligals; scale is 1:125,000. 

south and southwest; the density on the -0.35 gm/cm3-

model is preferable because of its better fit. The caldera margin on the south would then be a gradually 

thinning wedge of Kneeling Nun Tuff, extending to 

perhaps lat 32050' N. 

One of the most important features on the gravity map (fig. 7) from an economic point of view is the 

gravity ridge that projects in from the Lake Valley 

mining district (off the southeast corner of the map) to as far as the andesite cone of Copper Flat on the 

northeast edge of the map (fig. 14). This gravity ridge 

crests at lat 32°55' N., and the nose of the ridge brackets the Copper Flat quartz monzonite stock almost 

exactly. The axis of the ridge, however, lies about 2 

km east of the center of the assumed prismatic body, 

which apparently caused the aeromagnetic high discussed and modeled earlier. 

There is no guarantee that the gravity ridge is 

caused by a single source. The geologic evidence, 

however, supports the hypothesis that the source could 

be a horstlike feature delimited by the Berenda fault 

(Hedlund, 1975a) on the west and another north-trending fault on the east in the Skute Stone Arroyo quadrangle. The geologic evidence indicates at least 250 m 

of displacement (west side down) on the Berenda fault, 

and that the sediments deepen rapidly to unknown 

depth on the east in the Skute Stone Arroyo quadrangle, as one moves towards the Rio Grande 45 km to the 

east. 

A residual gravity map was obtained (fig. 15) by 

adding +197.43 milligals to all the values in the 9 by 9 

grid. The computer program of Cordell and Henderson 

(1968) was used to obtain a good fit with the data with 

a three-dimensional model of the anomalous mass, us- ing a density contrast of +0.40 gm/cm3, a reference 

plane of -3.00 km, and thnee iterations. The reference 

plane is the level from which the model prisms are constructed in an upwards direction (in this case), 

subject to the constraint that they cannot exceed the 

level of the ground surface. The calculated field is 

shown in figure 16, and the source model is shown in 

figure 17. 

The results indicate a horstlike uplifted block, 

with cumulative displacements of more than 2 km. 

The top of the block comes to within a few hundred 

meters of the surface at a point about 5 km southeast 

of the town of Hillsboro. The density difference of 

0.40 gm/cm3 is caused by alluvium contrasted against 

the volcanics. There is a tungsten-gold anomaly, possibly indicative of skam in the vicinity of this high 

point, which is on the southern edge of the aeromagnetic anomaly described previously. The uplift may 

have been caused by the emplacement of a small ba,tholith beneath the gravity ridge. The quartz monzonite 

of the Copper Flat stock is probably a mineralized 

apophysis or hood cupola of a batholith of quartz 

dioritic composition. The aeromagnetic anomaly could 

then be caused by a dioritic intrusion in the subsurface 

enhanced possibly by tactite minerals. All of these 

lines of evidence taken together make the center of 

the gravity high an interesting exploration target, and 

there is a possibility that deep-electrical mining-geophysical methods might locate mineralized rocks below 

this gravity high. 

19 
32:58:00 

32:56:00 

32:54:00 

32:52:00 

0 

'I 

lN 

N 

0 

'I 

lN 

0 

0 

'I 

N 

co 

0 0 0 0 0 

0 0 0 0 0 

FIGURE 14.-Gravity map of the Copper Flat gravity anomaly, enlarged from figure 7. 

Contour interval is 2 milligals; scale is 1:125,000. 

Conclusions 

The Mimbres fault is a normal fault with a throw 

of about 2 km in the vicinity of San Juan, N. Mex. The 

Emory Pass caldera has a sharply defined eastern 

boundary, while the southern and western margins are 

more gradational; zones of hydrothermal alteration 

may be indicated in at least two places on the caldera 

margins. The Rose Mine may lie atop a discrete magnetized porphyry system, possibly similar to the 

Caballo Blanco porphyry discovery to the southwest. 

The Copper Flat stock near Hillsboro, N. Mex., and a 

small quartz diorite pluton along Tank Canyon, may be 

apophyses of a small dioritic batholith emplaced at 

shallow depth beneath and to the south of the Hillsboro 

andesite field. The uppermost parts of this batholith 

and its related horst may be within reach of mininggeophysical (electrical) methods to the southeast of 

Hillsboro, in an area already noted for anomalous geochemical measurements. 

Acknowledgments 

Several members of the U.S. Geological Survey 

contributed to the preparation of this section. Danny 

A. Dansereau and Gary Brougham both helped in the 

collection of the gravity data. In addition, Dansereau 

contributed a great deal towards the gravity data 

reduction and in obtaining the digitized aeromagnetic 

data. Tom Hildenbrand and John Cady provided modeling programs for the magnetic data, and Lindrith 

Cordell provided the gravity modeling program. 

20 
33: 0:00 

32:58:00 

32:56:00 

32:54:00 

32:52:00 

0 0 0 0 0 ""-~ ""-~ ""-~ ""-~ ""-~ .. .. .. ()II ()II ()II ()II N 0'1 ~ N 0 co ... 0 0 0 0 0 0 0 0 0 0 

FIGURE 15.-Residual gravity map of the Copper Flat gravity anomaly, using an offset 

of +197.43 milligals. Contour interval is 2 milligals; scale is 1:125,000. 

21 
33: 0:00 

32:58:00 

32:56:00 

32:54:00 

32:52:00 

0 0 0 0 0 'I 'J 'J 'I 'I .. .. .. (.N (,N (,N (,N N 0'1 ~ N 0 CD ... 0 0 0 0 0 

0 0 0 0 0 

FIGURE 16.-Calculated gravity map of the Copper Flat gravity anomaly, from the 

three-dimensional model using a density contrast of +0.40 gm/cm3. Contour interval 

is 2 gammas; scale is 1:125,000. 

22 
32:58:00 

32:56:00 

32:54:00 

32:52:00 

0 0 0 0 0 'J 'J 'J 'J 'J .. .. .. (.N t.N (.N (.N 1\) 0) .llo 1\) 0 CD .. .. 0 0 0 0 0 0 0 0 0 0 

FIGURE 17 .-Three-dimensional model of the Copper Flat gravity anomaly, using +0.4 

gm/cm3 density contrast and 0.25 km contour intervals. Datum is mean ground 

elevation over the map area. Scale is 1:125,000. 

23 
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24 

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Survey Misc. Geol. Inv. Map 1-886. 

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Geochemistry 

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* __ 1972d, Maps showing tungsten distribution in the 

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* __ 1972e, Maps showing fluorite distribution in the 

25 

Winston and Chise quadrangles and in the west 

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* 1973a,Maps showing barium distribution in the 

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* 1973b, Maps showing copper distribution in the 

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part of the Priest Tank quadrangle, Sierra 

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* 197 3c, Maps showing tin distribution in the 

--Winston and Chise quadrangles and in the west 

part of the Priest Tank quadrangle, Sierra 

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Field Studies Map MF-499. 

* __ 1973d, Maps showing zinc and antimony distribution in the Winston and Chise quadrangles and in 

the west part of the Priest Tank quadrangle, 

Sierra County, New Mexico: U.S. Geol. Survey 

Misc. Field Studies Map MF-500. 

*Alminas, H. V., Watts, K. C., Griffitts, W. R., Siems, 

D. L., Kraxberger, V. E., and Curry, K. J., 1975a, 

Map showing anomalous distribution of tungsten, 

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* 197 5b, Map showing anomalous distribution of 

--lead, tin, and bismuth in stream-sediment 

concentrates from the Sierra Cuchillo-Animas 

uplifts and adjacent areas, southwestern New 

Mexico: U.S. Geol. Survey Misc. Geol. Inv. Map 

1-882. 

* 197 5c, Map showing anomalous distribution of 

--molybdenum, copper, and zinc in streamsediment concentrates from the Sierra CuchilloAnimas uplifts and adjacent areas, southwestern 

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Map 1-882. 

*Griffitts, W. R., and Alminas, H. V., 1968, Geochemical evidence for possible concealed mineral 

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*Griffitts, W. R., Alminas, H. v., and Mosier, E. L., 

1972a, Zinc and antimony distribution in the 

Monticello and Sierra Fijardo 7-l/2-minute 

quadrangles, Socorro and Sierra Counties, New 

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* l972b, Niobium and gold distribution in the 

--Monticello and Sierra Fijardo 7-l/2-minute 

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* 1972c, Strontium distribution in the Monticello 

--and Sierra Fijardo 7-l/2-minute quadrangles, 

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* l972d, Tin distribution in the Monticello and 

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* __ 1972e, Lead distribution in the Monticello and 
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* 1972f, Lanthanum and silver distribution in the 

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tiGPO 1979-677..()26/64 

26 

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283-292. 

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and Leland, G. R., 1970, Mineral resources of the 

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* 1975b, Geologic map of the San Lorenzo quad-

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cassiterite and the localities of rock samples, 

Hillsboro and San Lorenzo quadrangles, exclusive 

of Black Range Primitive area, Sierra and Grant 

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exclusive of the Black Range Primitive area, 

Sierra and Grant Counties, New Mexico: U.s. 

Geol. Survey Misc. Field Studies Map MF-9001.