U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5029

Detailed Interpretation of Aeromagnetic Data from the 

Patagonia Mountains Area, Southeastern Arizona
Cover: Aeromagnetic anomaly map of the Patagonia Mountains study areas, 

southeastern Arizona.
Detailed Interpretation of Aeromagnetic 

Data from the Patagonia Mountains Area, 

Southeastern Arizona

By Mark W. Bultman

Scientific Investigations Report 2015–5029

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director 

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Bultman, M.W., 2015, Detailed interpretation of aeromagnetic data from the Patagonia Mountains area, southeastern 

Arizona: U.S. Geological Survey Scientific Investigations Report 2015-5029, 25 p., http://dx.doi.org/10.3133/

sir20155029.

ISSN 2328-0328 (online)
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Geology of Patagonia Mountains Region ..................................................................................................3

1996 Patagonia Aeromagnetic Survey .......................................................................................................7

Total Field Magnetic Data...........................................................................................................................10

Magnetic Susceptibility of Rock in the Patagonia Mountains Area ...................................................13

Measured Susceptibility, Remanence, and Geology Incorporated into Forward 

Magnetic Modeling .......................................................................................................................14

Geologic Model............................................................................................................................................16

Complete Bouguer Anomaly Map.............................................................................................................18

Euler Deconvolution Depth Estimates......................................................................................................19

Conclusions...................................................................................................................................................23

References Cited..........................................................................................................................................23

Figures

1. Map showing location of the Patagonia Mountains study area, southeastern 

Arizona............................................................................................................................................2

2. Map showing simplified geology and locations of mines, mining districts, and

geographic features in the northern part of the Patagonia Mountains study area,

southeastern Arizona...................................................................................................................4

3. Geologic map of the Patagonia Mountains study area and surrounding region,

southeastern Arizona...................................................................................................................5

4. Decorrugated aeromagnetic anomaly map of the Patagonia Mountains study

area, southeastern Arizona.........................................................................................................8

5. Map showing results of inversion of 0.5 (x) x 0.5 (y) by 1.0 (z) kilometer blocks in

the Red Mountain area based on reduced to the pole Earth’s magnetic field data,

Red Mountain area, southeastern Arizona ..............................................................................9

6. Graph showing susceptibility of common rocks ...................................................................11

7. Graph showing Koenigsberger ratio of common rocks .......................................................12

8. Diagram showing contribution to the Earth’s magnetic field total magnetic

intensity from the natural remanent magnetic field based on the angle between

the magnetic field and the NRM field......................................................................................13

9. Histogram of the base 10 logarithm of susceptibility for samples from the

Patagonia Mountains, southeastern Arizona ........................................................................13

10. Stereonet plot of samples collected from the Patagonia batholith with reduced

total magnetic intensity natural remanent magnetic field and the present-day

inductive field of the Earth, Patagonia Mountains study area, southeastern Arizona ...15

11. Geologic cross section for profile 1, Patagonia Mountains study area,

southeastern Arizona.................................................................................................................16

12. Geologic cross section for profile 2, Patagonia Mountains study area,

southeastern Arizona.................................................................................................................17
iv

13. Complete Bouguer anomaly map of the northern part of the Patagonia Mountains

study area, southeastern Arizona............................................................................................19

14. Map showing Euler deconvolution depth estimates for a structural index of 

0 plotted over aeromagnetic data in the Patagonia Mountains study area,

southeastern Arizona.................................................................................................................20

15. Map showing Euler deconvolution depth estimates for a structural index of 

1 plotted over aeromagnetic data in the Patagonia Mountains study area,

southeastern Arizona.................................................................................................................21

16. Map showing Euler deconvolution depth estimates for a structural index of 

2 plotted over aeromagnetic data in the Patagonia Mountains study area,

southeastern Arizona.................................................................................................................22

Figures—Continued

Tables

1. Map units and physical properties used to develop a geologic model of the

Patagonia Mountains, southeastern Arizona ..........................................................................6

2. Patagonia granodiorite remanent magnetization measurements, Patagonia

Mountains study area, southeastern Arizona........................................................................14

Conversion Factors

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Mass

metric ton 1.102 ton

Acceleration

milligal (mGal) 3.281 x 10-5 feet per second squared (ft/s2)

Magnetic field strength

nT (nanoTesla) 1.000 x 10-5 gauss (G)

Magnetic field intensity

amperes/meter (A/m) 1.257 x 10-2 oersteds (Oe)

Datums

Vertical coordinate information is referenced to the North American Vertical Datum of 1929 

(NAVD 29).

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27).

Altitude, as used in this report, refers to distance above the vertical datum.
Detailed Interpretation of Aeromagnetic Data from the 

Patagonia Mountains Area, Southeastern Arizona

By Mark W. Bultman

Abstract

The induced magnetic field and the remanent magnetic 

field of rock masses are important to geologic modeling based 

on Earth’s magnetic field data. The orientation of the induced 

magnetic field is approximately parallel to the orientation of 

Earth’s geomagnetic field and its intensity can be derived from 

measured magnetic susceptibilities of rocks in a study area. 

The orientation and intensity of the natural remanent magnetic 

field is much harder to determine; therefore, few investigators 

have included magnetic remanence as a contributing factor 

to studies of continental magnetic anomalies. All rocks have 

remanent magnetism and, in intrusive or volcanic rocks, 

this component of the total magnetic intensity of the Earth’s 

magnetic field can be as large as or larger than the induced 

component.

The Patagonia Mountains in southeastern Arizona 

were selected to produce a subsurface geologic model from 

aeromagnetic data by incorporating physical properties of 

rock including measured magnetic susceptibilities, estimated 

remanent magnetic field orientations and intensities, a 

known association of intrusive events, and information from 

existing geologic mapping. The result is a model of geology 

at depth that may better represent reality than previous poorly 

substantiated cross sectional models. This new model includes 

concealed intrusive rocks and defines areas where concealed 

mineral deposits may be found. It also shows that volcanic 

rocks might occupy basins at relatively shallow depths in 

basins with low aeromagnetic anomalies.

Euler deconvolution depth estimates derived from 

aeromagnetic data with a structural index of 0 show that 

mapped faults on the northern margin of the Patagonia 

Mountains generally agree with the depth estimates in the new 

geologic model. The deconvolution depth estimates also show 

that the concealed Patagonia Fault southwest of the Patagonia 

Mountains is more complex than recent geologic mapping 

represents. Additionally, Euler deconvolution depth estimates 

with a structural index of 2 locate many potential intrusive 

bodies that might be associated with known and unknown 

mineralization.

Introduction

Since at least 1938, geoscientists have known that 

remanent magnetism in rocks (termed natural remanent 

magnetism) can be an important or dominant component 

of Earth’s magnetic field measured at the surface of the 

Earth (Koenigsberger, 1938a, 1938b). The natural remanent 

magnetization of igneous, volcanic, and sedimentary rocks is a 

vector magnetic field that the rock acquires when it is formed 

whose direction can be used to help interpret rotation since 

that time by comparing it to Earth’s current (2014) magnetic 

field (Koenigsberger, 1938a, 1938b).Vine and Matthews 

(1963) associated patterns of magnetic striping emanating 

from ocean spreading ridges to reversals of the Earth’s 

geomagnetic field, which helped lead to an understanding 

of both geomagnetic reversals and plate tectonics. Clearly, 

natural remanent magnetism in rocks is an important concept 

in modeling the geology responsible for anomalies in the 

Earth’s magnetic field. Few investigators, however, have 

included natural remanent magnetism as a contributing 

factor to studies of continental magnetic anomalies (Morris 

and others, 2007). Considering that many rock masses have 

natural remanent magnetism recorded with a field direction 

significantly different from the direction Earth’s current 

magnetic field, this is startling and may reduce the accuracy of 

many published interpretations.

Caution must be used when modeling geologic anomalies 

in the Earth’s magnetic field in areas that include rocks with 

a natural remanent magnetism with a field direction different 

from the direction of the Earth’s current magnetic field. 

Interpretations based on transformations often performed 

on aeromagnetic survey data, including reduction to the 

pole, various forms of filtering, and some depth-to-source 

estimation techniques are not valid for magnetic fields with 

natural remanent magnetization directions other than that of 

the inducing field (Gettings, 2002). Additionally, Saltus and 

Blakely (2011) argued that geophysical interpretations become 

more constrained when information such as geological 

mapping, rock physical property measurements, and results 

from other geophysical methods are included in a geophysical 

interpretation.
2 Detailed Interpretation of Aeromagnetic Data from the Patagonia Mountains Area, Southeastern Arizona

The Patagonia Mountains area of southeast Arizona 

(fig. 1) is a geologically diverse region with intrusive, 

extrusive, and sedimentary rocks and a complicated 

geologic history. In 1996, the USGS contracted to acquire 

an aeromagnetic map over this region and the adjacent Santa 

Cruz Valley and Santa Rita Mountains (U.S. Geological 

Survey, 2000; Gettings, 2002). These aeromagnetic data 

have been used by several studies to make interpretations of 

the Patagonia Mountains and surrounding areas including 

Gettings (2002), Phillips (2002), Rystrom and others (2002), 

and Berger and others (2003). Gettings (2002) notes “... that 

some of the reversely polarized rocks indicate paleomagnetic 

directions that are not parallel (or anti-parallel) to the current 

induction direction, it is clear that in much of the survey area, 

remanent magnetization is an important, sometimes dominant, 

component of the magnetic anomaly.” Additionally, Hagstrum 

(1994) presents measured paleomagnetic vectors on some of 

the intrusive and extrusive rocks in this area, many of which 

include directions not aligned with the current magnetic field 

of Earth. Because of the importance of remanent magnetism in 

this region and the availability of age and remanent magnetic 

vector information for some rocks, the Patagonia Mountains 

area was selected to study the use of incorporating detailed 

geologic information, including measured susceptibilities and 

published remanence vectors, into an interpretation of the 

1996 aeromagnetic survey of the Patagonia Mountains area.

Santa Cruz River

Santa Cruz River

Sonoita River

Cienega Creek

Pantano Wash

Josephine Canyon

MT

WRIGHTSON

RED

MOUNTAIN

SANTA

RITA

MOUNTAINS

PATAGONIA

MT

NS

SAN RAFAEL VALLEY

Patagonia

Nogales

Tucson

SANTA CRUZ

PIMA

ARIZONA, U.S.A.

SONORA, MEXICO

Base map modified from U.S. Geological Survey and other digital 

data sources, various scales. Coordinate reference system: 

USA_Contiguous_Albers_Equal_Area_Conic_USGS_version

WKID: 102039 Authority: ESRI

111° 110°50' 110°40'

32°

31°

50'

31°

40'

31°

30'

31°

20'

0 5 10 MILES

0 5 10 KILOMETERS

Phoenix

Tucson

Nogales

ARIZONA

Study area

Figure 1. Location of the Patagonia 

Mountains study area, southeastern 

Arizona.
Geology of Patagonia Mountains Region 3

This report provides an overview of the geology of the 

Patagonia Mountains region and reviews several geophysical 

analyses in the region. The known geology of the region was 

used to create a model of the geology in cross section for two 

aeromagnetic profiles. It proposes that the large aeromagnetic 

low anomaly in the study area is caused by a concealed 

intrusion with a natural remanent magnetism direction that 

is not aligned to the direction of the Earth’s current magnetic 

field. It concludes by looking at Euler depth estimates (which 

are not affected by rocks with nonaligned natural remanent 

magnetism) in the region to see if they corroborate the findings 

in the model and if they provide any new insight into the 

geology of this region.

Geology of Patagonia Mountains 

Region

The geology of the Patagonia Mountains area is shown 

in figure 2 (general geology with mines and other geographic 

labels; Berger and others, 2003) and figure 3 (geologic map; 

Drewes, 1980; Drewes and others, 2002). An explanation 

of map units in figure 3 is provided in table 1. Precambrian 

basement rocks include biotite-quartz monzonite and 

hornblende diorite to the east of the Patagonia Mountain range 

and biotite-hornblende quartz monzonite and hornblende 

gabbro on the west side (Simons, 1974). Small outcrops of 

Precambrian pelitic schist, diorite, and gabbro also can be 

found in the area (Simons, 1974; Drewes, 1980). Paleozoic 

sedimentary rocks are present in two areas: (1) on the east 

side of the Patagonia Mountains where quartzites, limestones, 

and dolomites outcrop, and (2) in small brecciated blocks of 

shear-bounded limestone that crop out at the Flux Mine and 

are enclosed in late Triassic to lower Jurassic volcanic and 

volcaniclastic rocks (Simons, 1974, Drewes, 1980). Several 

similar limestone blocks are north of the Chief Mine. These 

limestone blocks are thought to be Permian in age and were 

interpreted by Lipman and Sawyer (1985) to be fragments of 

a Jurassic caldera complex. Throughout most of the Patagonia 

Mountains, Paleozoic carbonate rocks are intruded either by 

granitic rocks or in tectonic contact with younger sedimentary 

rocks (Berger and others, 2003).

Mesozoic rocks in the study area include Triassic to 

Jurassic, predominantly silicic, volcanic, and sedimentary 

rocks, including quartzite, sandstone, arkose, and shale 

(Simons, 1972). These rocks generally are present in a northnorthwest-trending belt on the east side of the main range and 

as Jurassic granite intruding the Precambrian rocks on the west 

flank of the range. Cretaceous sedimentary rocks of the Bisbee 

Formation occur southwest of the Harshaw Creek Fault (map 

unit Kbu, fig. 3) and consist of siltstone and mudstone with 

intercalated limestone, sandstone, and conglomerate (Simons, 

1974).

A Laramide-age, predominantly quartz monzonite, 

batholith is present as the core of the central and southern part 

of the Patagonia Mountains (map unit Ti, fig. 2; map unit Tlg, 

fig. 3). Vikre and others (2009) defined a succession of six 

events that formed this batholith, and various mineral deposits 

that are associated with it, over a period of approximately 

17 million years (74–58 Ma). The first event in this sequence 

is the intrusion of the Washington Camp stock at 74 Ma. The 

skarn deposits at the Washington Camp mining area (copper 

[Cu]-lead [Pb]-zinc [Zn]-silver [Ag]-gold [Au]-arsenic [As]; 

fig. 2) are spatially associated with this stock, but some or 

all of the deposits could be younger. Laramide silicic tuffs, 

tuff breccias, ash-flow tuffs, and a latite dome are mapped 

on the southwest side of Red Mountain. Stratigraphically 

overlying these is the second event in Vikre and others (2009) 

succession, a widespread 71 Ma trachyandesite event in the 

area of current Red Mountain and extending to Meadow 

Valley to the east. Potassium rich mica dated in a vein at the 

Flux Mine carbonate replacement deposit (Cu-Pb-Zn-Ag-Aumanganese [Mn]) are approximately 71 Ma, but uncertain 

paragenetic relations preclude association of the Flux Mine 

to the trachyandesite volcanism. The Ventura molybdenum 

(Mo)-Cu breccia deposit (fig. 2) is related to a 65 Ma 

granodiorite stock, one of several 65–63 Ma small-volume 

intrusions associated with the Patagonia batholith, which 

comprises the third event of the succession. Additionally, 

deposits from the Blue Nose and Morning Glory mines 

were associated with similar small volume intrusions. At 

62 Ma, the volcanic rocks of Red Mountain were intruded 

by a mineralized (chalcopyrite-bornite) quartz monzonite 

porphyry copper deposit (~0.6 billion metric ton [Gt] and 

0.6 percent Cu, 0.01 percent Mo) and with a near surface 

secondary enriched chalcocite-enargite resource associated 

with advanced argillic alteration that is paragenetically 

younger but not firmly dated. This marked the fourth event 

in the succession. The Cu-Mo breccia deposit at Red Hill 

(Four Metals Mine), in the southern part of the pluton, may 

mark a degassing conduit in a large-volume granodiorite 

event at 60 Ma, the fifth event. The Sunnyside porphyry 

copper system consists of a deep chalcopyrite resource 

(~1.5 Gt and 0.3 percent Cu, 0.01 percent Mo), and a nearsurface chalcocite–enargite–tenorite resource is dated at 

about 61–58 Ma and marks the last event of the succession 

(Graybeal, 1996; Vikre and others, 2009).

To the east, west, and north of these rocks is a thick 

sequence of Quaternary and Tertiary gravels (Simons, 1974; 

Drewes, 1980). Minor tuffs and limestone are included in this 

unit. The Holocene alluvium that is present in the modern 

stream channels also is included in this latter unit.
4 Detailed Interpretation of Aeromagnetic Data from the Patagonia Mountains Area, Southeastern Arizona

men12-7690_fig 02

Blue Nose

Morning Glory

Washington

Camp

Four Metals

i

San 

Rafael 

Basin Red 

Hill 

Lead

Queen 

i i

i

Meadow

Valley

Sonoita

Basin

i

U.S.

Miocene to Holocene basin 

deposits, undifferentiated

Tertiary limestone

Tertiary Red Mountain 

intrusion

Tertiary intrusive rocks

Tertiary volcanicrocks

Cretaceous volcanic rocks

Creataceous intrusions

Mesozoic rocks, undivided; ls, 

limestone breccia blocks

Jurassic intrusive rocks

Permian rocks, undivided

Precambrian rocks, 

undivided

EXPLANATION

Generalized geologic

units

Contact, approximately 

located

Fault, dashed where 

inferred; dotted where 

concealed

Mine, selected mines named

Boundary of mineral 

district, approximately 

located

Stream channel

110°45'

31°

30'

0 1 MILE

0 1 KILOMETER

Figure 2. Simplified geology and locations of mines, mining districts, and geographic features in 

the northern part of the Patagonia Mountains study area, southeastern Arizona. Simplified geology 

after Berger and others (2003). 
Geology of Patagonia Mountains Region 5

Guajolote Fault

Patagonia Fault

Harshaw Creek Fault

Sycamore Canyon

Providencia Canyon

RED

MOUNTAIN

Yg

Jvs

P

Tv

Trm

Єs

EXPLANATION

Contact, approximately 

located

Fault, dashed where 

inferred; dotted where 

concealed

Tuc

Tlv

Tlg

Kbu Bisbee formation

Limestone

Yg

Jg

Jvs

Trm

P

Basin fill, unconsolidated

Nogales Formation conglomerate

Rhyolite to andesite

Biotite-hornblende granodiorite to 

quartz monzonite

Comoro Canyon granite

Siliceous volcanic and 

sedimentary rocks

Mount Wrightson Formation

Hornblende-rich granite

Important map units and description

Qtg

N

Profile 1

Profile 2

0 1 2 3 4 5 MILES

0 1 2 3 4 5 KILOMETERS

Figure 3. Patagonia Mountains study area and surrounding region, southeastern Arizona (after Drewes and others, 2002). 

Geologic map units are described in table 1. Only units used in modeling are included.
6 Detailed Interpretation of Aeromagnetic Data from the Patagonia Mountains Area, Southeastern Arizona Table 1. Map units and physical properties used to develop a geologic model of the Patagonia Mountains, southeastern Arizona.

[Map unit: From Drewes (1980) and Drewes and others (2002). Unit color: Colors shown in figures 10 and 11. Q: Koenigsberger ratio from figure 7 (Koenisberger, 1938a, 1938b). A/m, amperes per meter]

 Map

Description

Mean 

susceptibility

(SI)

Q 

Remanent magnetic field

Source or reference for susceptibility 

and remanent vector Unit Model unit Unit color Intensity (A/m) Normal/reverse polarity Declination Inclination

QTgu, QTg Qtg Basin fill, unconsolidated 0.000000 Normal

Tuc Tuc Nogales Formation conglomerate 0.000105 Normal Susceptibility from Gettings (1999).

Tgr Biotite-hornblende granodiorite to 

monzonite, concealed

0.035000 0.6 0.8066 Reversed 14.4 -62.3 Susceptibility estimated from figure 6, remanent 

direction estimated from Hagstrum (1994) sample PG-6.

Tv Extrusive vent, Patagonia Mountains 0.000050 Normal Susceptibility from Gettings and Bultman (2014).

Tgc Monzonite, concealed 0.050000 0.6 1.1500 Normal 341.8 49.1 Susceptibility estimated from figure 6, remanent 

direction estimated from Hagstrum (1994).

Tlg Tlg Biotite-hornblende granodiorite to 

quartz monzonite

0.0200 0.6 0.4600 Normal 341.8 49.1 Susceptibility from Gettings and Bultman (2014).

Tlv Ka Trachyandesite, Red Mountain area 0.000050 Normal Susceptibility from Gettings and Bultman (2014).

Trachyandesite east of Red 

Mountain 

0.012300 Normal Susceptibility from Gettings and Bultman (2014).

Kuvs Kuvs Undifferentiated volcanic rock, 

Sonoita Basin

0.020000 1.4 1.0800 Reversed 14.4 -62.3 Susceptibility from figure 6, remanent direction 

estimated from Hagstrum (1994).

Undifferentiated volcanic rock, 

San Rafael Basin

0.000500 Normal Susceptibility from figure 6.

Jg Jg Comoro Canyon graniteequigranular granite 0.004200 Normal Susceptibility from Gettings and Bultman (2014).

J^vs J^vs Silicious volcanic and sedimentary 

rocks

0.000200 Normal Susceptibility from Gettings and Bultman (2014).

Corral Canyon member of J

^vs, 

directly east of Harshaw Creek fault

0.040000 0.6 0.9200 Normal 326 51 Susceptibility measured, remanent direction 

estimated from Hagstrum (1994).

^

m

^

m Mount Wrightson Formation 0.000200 Normal Susceptibility from Gettings and Bultman (2014).

Yg Yg Hornblende rich granite 0.016000 Normal Susceptibility from Gettings and Bultman (2014).
1996 Patagonia Aeromagnetic Survey 7

The most prominent faults in the Patagonia Mountains 

strike northwest, northeast, and east-northeast and likely 

originated in the Jurassic period. These faults likely had a leftlateral component of displacement during the Jurassic period 

that changed to reverse and then right-lateral displacement 

during the latest Cretaceous to early Tertiary periods (Davis, 

1981, Berger and others, 2003). Berger and others (2003) 

show that strike-slip faulting in the Santa Rita Mountains (to 

the north of the study area) continues south into the Patagonia 

Mountains (fig. 1). Several parallel strike-slip fault zones with 

small stepovers mechanically linking them are referred to as 

the Sawmill Canyon Fault zone (fig. 2) which dominated shear 

strain accommodation in the region (Berger and others, 2003). 

Strike-slip zones to the southwest of the Sawmill Canyon Fault 

zone became linked into it through northeast-striking faults 

and were locally obscured by thick accumulations of volcanic 

rocks in structural basins east of Red Mountain within zones 

of transtension (Berger and others, 2003). In the Patagonia 

Mountains, the Paleocene Patagonia biotite- hornblende 

granodiorite batholith (map units Ti, fig. 2 and Tlg; fig. 3) is 

elongated northwest and bounded by a linear, subparallel fault 

zone known as the Guajolote Fault (fig. 2). These relations 

together with cross cutting, northeast-striking faults and veins 

suggest that the batholith (or related higher level apophyses 

or cupolas) accommodates extensional strain in-line along the 

northwest-striking fault zones, and might have been emplaced 

in an extensional stepover (Berger and others, 2003).

In the Red Mountain topographic block at the north 

end of the range, there are northeast- striking normal faults 

and silicified lenses as well as a northeast-striking Tertiary 

intrusion (map unit Ti; fig. 2). These relations in conjunction 

with the outcrop of middle Mesozoic sedimentary and 

volcanic rocks on the southwest side of the Harshaw Creek 

Fault opposite Red Mountain and their absence to on the 

northeast side of the fault and beneath the Red Mountain 

topographic block suggest the possibility that these Late 

Cretaceous to early Tertiary volcanic rocks and intrusions were 

localized in an extensional stepover between the Harshaw 

Creek Fault zone and parallel northwest-striking strike-slip 

faults to the northeast (Berger and other, 2003).

An area of intense alteration associated with the Red 

Mountain porphyry copper deposit was mapped by Simons 

(1974). The area contains the minerals pyrophyllite, sericite, 

alunite, and (or) kaolinite. In outcrop, much of this area 

appears strongly bleached and iron-stained as a result of the 

breakdown of the dark rock-forming minerals and pyrite 

(Simons, 1974, Corn, 1975).

1996 Patagonia Aeromagnetic Survey

The U.S. Geological Survey (2000) contracted with 

Sial Geosciences, Inc. for a detailed aeromagnetic survey 

of the Santa Cruz Basin and Patagonia Mountains area of 

south central Arizona in 1996 (U.S. Geological Survey, 2000; 

Gettings, 2002). The aeromagnetic data were collected along 

east-west flight lines spaced 250 m apart at a terrain clearance 

of 230 m (Phillips, 2002). Data also were collected along 

several widely spaced north-south tie lines that were used to 

level the flight-line data. The aeromagnetic map gridded from 

the contractor's flight-line data shows significant streaking 

or "corrugation" along the flight line direction resulting 

from level shifts between adjacent flight lines, probably due 

to the large changes in elevation over the survey area. This 

corrugation can be corrected for by a technique developed by 

Urquhart (1988) and implemented by Phillips (1997, 2007). 

The gridded decorrugated aeromagnetic data from the 

aeromagnetic survey over the Patagonia Mountains study 

area are shown in figure 4. Only data from the region are 

displayed and gridded with a cell size of 50 m, decorrugated, 

and displayed as a color shaded relief image. Additionally, 

geologic line work from Drewes (1980) and Drewes and 

others (2002) (fig. 3) is shown by thin black lines on figure 4. 

Also shown in figures 3 and 4 are the locations of profiles 1 

and 2 which are parts of aeromagnetic flight lines that were 

used to build geologic models.

Three interpretations of the 1996 Patagonia aeromagnetic 

data were published in 2002. Gettings (2002) provided a 

thorough study and included a comparison of the aeromagnetic 

data with geologic map data demonstrating correlation of the 

magnetic anomaly field with mapped geology and shows that 

numerous map units of volcanic and intrusive rocks from the 

Jurassic, Cretaceous, and Middle Tertiary have significant 

natural magnetic remanence. Gettings also performed a 

textural analysis of the aeromagnetic data, which delineated 

areas of consistent magnetic anomaly texture for individual 

exposed lithologies to be extended under sediment cover in 

Cenozoic basins. Textural measures of the aeromagnetic data 

(Gettings, 2002) included a number of peaks and troughs per 

kilometer and Euclidean length per kilometer.

Phillips (2002) used analytic techniques on the 1996 

Patagonia aeromagnetic map to delineate contacts and make 

depth estimates based on three methods of depth estimation. 

They include the horizontal gradient method, the analytic 

signal method, and the local wavenumber method. The 

horizontal gradient method (Cordell and Grauch, 1985; 

Blakely and Simpson, 1986) is dependent on performing 

a “reduction to the pole” operation that recalculates total 

magnetic intensity data as if the inducing magnetic field had 

a 90-degree inclination. This transformation converts dipolar 

magnetic anomalies to monopolar anomalies centered over 

the geologic bodies responsible for the anomaly. Although 

reduction to the pole can make interpretation easier when 

the magnetization of the causative bodies is due to magnetic 

susceptibility, remanent magnetization is not dealt with 

properly if the direction of remanence is different from the 

direction of the Earth's magnetic field. The horizontal gradient 

method has a low sensitivity to noise and its maxima are 

highly continuous and generally parallel to the contours of 

the reduced-to-pole aeromagnetic field. Phillips (2002) used 

the horizontal gradient method to determine the locations of 
8 Detailed Interpretation of Aeromagnetic Data from the Patagonia Mountains Area, Southeastern Arizona

physical property (magnetization) boundaries during terracing. 

Violations of the many assumptions required in the horizontal 

gradient method can result in displacement of the contacts 

away from their true locations. The displacement is typically 

down dip from the true contact location (Grauch and Cordell, 

1987). The analytic signal method and the local wavenumber 

method are very sensitive to noise in the data, but provide 

contacts between units of contrasting susceptibility that are 

less continuous than the horizontal gradient method. Rystrom 

and others (2002) performed a similar analysis as Phillips 

(2002) and were able to show that many geologic map 

units displayed unique magnetic properties. Phillips (2002) 

and Rystrom and others (2002) successfully used the 1996 

Patagonia aeromagnetic data to delineate known geologic map 

features. Because they used methods that are affected by rocks 

with natural magnetic remanence, caution should be used on 

estimates of depth to source in these analyses.

In another analysis of the 1996 Patagonia aeromagnetic 

data, Berger and others (2003) presented a three-dimensional 

inversion for magnetic susceptibility of total field 

aeromagnetic data that used ModelVision ProTM software 

to estimate the magnetic susceptibility of the near-surface 

materials near Red Mountain. The inversion was based on 

506 vertical prisms each having dimensions of 0.5 (x) × 

0.5 (y) × 1.0 (z) km. Using prisms of that size assumed that 

all the magnetic material producing observed anomalies 

-104.775

-124.033

-137.675

-149.136

-159.728

-169.557

-178.965

-187.961

-197.014

-206.063

-215.270

-224.269

-233.303

-242.694

-252.273

-261.793

-271.562

-282.000

-292.542

-303.349

-314.905

-326.574

-339.275

-352.163

-366.125

-380.776

-396.981

-414.527

-433.133

-452.977

-474.770

-499.781

-528.107

-561.372

-602.467

-654.147

-717.963

-808.292

Magnetic anomaly, in 

nanoteslas (nT)

EXPLANATION

Profile 1

Profile 2

Region of

figure 5

Hypothesized

concealed

intrusion

RED

MOUNTAIN

505000 510000 515000 520000 525000 530000

3490000 3485000 3480000 3475000 3470000

N

Figure 4. Decorrugated aeromagnetic anomaly map of the Patagonia Mountains study area, 

southeastern Arizona. Overlain geologic linework (black lines) from Drewes and others (2002).
1996 Patagonia Aeromagnetic Survey 9

was in a layer 1 km thick, having its top at the topographic 

surface. The magnetization boundaries that were estimated 

from reduced-to-pole aeromagnetic data using the horizontal 

gradient method were included (Berger and others, 2003). 

Additionally, low and negative susceptibilities produced 

by the inversion (fig. 5) for rock masses south of Red 

Mountain are indicative of nonmagnetic basin-fill material 

overlying weakly magnetized basement rocks adjacent to the 

range (Berger and others, 2003). There are magnetization 

boundaries within the region of low magnetic susceptibility 

as it crosses the Patagonia Mountains between Trench Camp 

and the Three R Mine indicating the rocks within the range 

are relatively weakly magnetized (fig. 5). According to 

Berger and others (2003, p. 1,009), “Because intrusive rocks 

at the Three R, European, and Ventura mines are the same 

composition as those with high magnetic susceptibility to the 

west and southwest in figure 5, it is assumed that some of 

the low susceptibility on the west side of the range is due to 

hydrothermal alteration.”

Greater than or 

equal to 0.001

0.00078

0.00056

0.00033

0.00011

-0.00011

-0.00033

-0.00056

-0.00078

Less than or

equal to -0.001

EXPLANATION

Magnetic susceptibility, 

dimensionless centimeter-gramsecond (cgs) units 

Magnetization boundary

Mine

N

0 2 Miles

0 2 Kilometers

3489

3487

3485

3483

3481

3479

521 519 523 525 527 5 110°45' 29

31°30'

Figure 5. Results of inversion of 0.5 (x) x 0.5 (y) by 1.0 (z) kilometer blocks in the Red Mountain area 

based on reduced to the pole Earth’s magnetic field data, Red Mountain area, southeastern Arizona. 

Data from Berger and others (2003).
10 Detailed Interpretation of Aeromagnetic Data from the Patagonia Mountains Area, Southeastern Arizona

Total Field Magnetic Data

Total field magnetometers used in magnetic surveys 

of the Earth as discussed here measure the total intensity 

(without regard to direction) of the vector sum of the 

following: (1) the magnetic field generated internally by the 

Earth and approximated by the International Geomagnetic 

Reference Field (IGRF); (2) the diurnal or transient magnetic 

field generated by interaction of the internal magnetic field 

with solar wind; (3) the induced magnetic field generated 

by the interaction of the Earth’s internal magnetic field 

and the magnetic susceptibility of the rock masses near the 

magnetometer; and (4) the natural remanent magnetism field 

component of rock masses near the magnetometer (Morris and 

others, 2007). The IGRF is known (International Geomagnetic 

Reference Field, 2010) and is usually removed from the 

acquired data by the contractor (as was case for the 1996 

Patagonia aeromagnetic survey). Additionally, the contractor 

made the necessary transient magnetic field corrections based 

on a stationary magnetometer in the location of the survey 

during the acquisition of survey data. Because the Earth's 

internal field is dominant, the measured values are assumed 

to be in the Earth's field direction at the time of measurement. 

Induced magnetization is the component of magnetism in a 

material produced in response to an external applied magnetic 

field; it vanishes when the applied field is removed. Remanent 

magnetization is the permanent magnetization that remains 

in a material when the applied external field is removed and 

is essentially unaffected by weak magnetic fields (Clark, 

1997). Neel (1955), supported by the work of Nicholls (1955) 

and Runcorn (1955), showed that for most rocks, especially 

those with thermoremanent magnetization, the remanent 

magnetization is stable over geologic time scales. The induced 

and remanent magnetic field properties of rock masses 

give rise to magnetic anomalies that are used for geologic 

interpretation. These anomalies are created by contrasts in 

magnetic susceptibility and remanence at contacts between 

adjacent rock masses of differing physical properties.

The magnetization of a rock mass, J (magnetic dipole 

moment per unit volume), is equal to the vector sum of the 

induced field, Ji

, and the natural remanent magnetism field, 

Jr, that is, J = Ji + Jr. For sufficiently weak fields, such as the 

Earth’s magnetic field, the induced magnetization, Ji, of a 

material is approximately proportional to and parallel to the 

applied field with the constant of proportionality known as 

the magnetic susceptibility, k (Clark, 1997). Therefore, the 

magnetization component due to the susceptibility (SI) of 

rock mass is equal to Ji = kF, where F is the magnitude of the 

Earth’s internal field in amperes per meter (A/m), and can be 

approximated by IGRF. Figure 6 shows susceptibilities for 

several types of rock lithologies.

The natural remanent magnetism vector, Jr

, is a quasipermanent magnetization inherent in the rock mass and 

composed of primary and possibly secondary magnetizations 

(Butler, 1998). The primary remanent magnetization for 

igneous rocks is thermoremanent magnetization, acquired 

by cooling and solidification of an igneous rock from 

greater than the Curie temperature. For sedimentary rocks 

it is detrital remanence, produced by magnetic sediments 

(composed of minerals) aligning themselves with the Earth’s 

magnetic field before being incorporated into a sedimentary 

rock. Secondary remanent magnetizations include chemical 

remanent magnetization, which records the magnetization of 

the Earth’s field from the crystallization of minerals less than 

the Curie temperature, viscous remanent magnetization, which 

is acquired by ferromagnetic minerals after spending a great 

deal of time in a magnetic field with a stable orientation, and 

several generally less important components.

The Koenigsberger ratio, Q (Koenigsberger, 1938a and 

1938b), is the ratio of the remanent magnetization to the 

induced magnetization for a rock mass, |Jr

| / |Ji|. Figure 7

shows Q values ranges for several types of rock lithologies. 

The total magnetization of a rock mass is the vector sum of Jr

and Ji and the direction of Ji is always approximated by the 

direction of the Earth’s current magnetic field. The direction of 

Jr is dependent on the direction of the Earth’s magnetic field at 

the time the remanence was incorporated into the rock and any 

tectonic movement that the rock mass has experienced since 

recording the past Earth’s field. If aligned near the present-day 

magnetic field, Jr will increase the magnetization of the rock 

mass; if Jr differs from the current alignment by more than 90 

degrees it will lower the observed magnetization of the rock 

mass or possibly create a rock dominated by the remanent 

magnetization vector (if Q > 1). If Jr points 90 degrees from 

Ji, Jr will not contribute to the measured magnetic field 

component in the Earth field direction. The natural remanent 

magnetism vector cannot have any possible orientation; rather 

the orientation must be compatible with the expected age of 

remanence acquisition (Morris and others, 2007) and with 

tectonic activity since that time.

For geologic anomalies, a total field magnetometer 

measures the intensity |J| = |Ji

| + |Jr|. If the susceptibility 

and remanence vector of a rock mass are both known, then 

the complete anomaly due to a rock mass can be defined. 

Generally, however, neither susceptibilities nor remanences 

are known. Susceptibility often is estimated by lithology from 

tables such as figure 6. Remanence is often ignored. Most of 

the total magnetic field interpretations of the Earth use only 

susceptibility, and few investigators have included magnetic 

remanence as a contributor to studies of continental magnetic 

anomalies (Morris and others, 2007).
Total Field Magnetic Data 11

men12-7690_fig06

IGNEOUS ROCKS

Basalt/dolerite

Metamorphism

Metamorphism

Serpentinised

Amphibolite Greenschist

Metabasic

Gabbro/norite

Ilmenite series Magnetite series

Quartz porphyries

Quartz-feldspar porphyries

Granulite

Granulite

Granite/granodiorite/tonalite

Eclogite

Andesites

Phonolites

S-type I-type

Acid volcanics

Trachyte/syenite

Monzonite/diorite

Unserpentinised Peridotite (including dunite)

Pyroxenite/hornblendite

(Alaskan type)

METAMORPHIC ROCKS

Metasediments

Acid granulite

Banded Iron Formation (anisotropic)

Haematite-rich Magnetite-rich

SEDIMENTARY ROCKS

Sands/silts

WEATHERED ROCKS

Fe-rich chemical sediments

Laterite

Skarn

Sediments

Magnetite skarn

Carbonates, shales Detrital magnetite

Haematite/goethite-rich Maghaemite-bearing

Spilites

Amphibolite/basic granulite

0.00001 0.0001 0.001 0.01 0.1 1 10

Susceptibility (SI)

EXPLANATION

Bar length is range

Range of most rocks

Figure 6. Susceptibility of common rocks (after Clark, 1997). Bar represents range, shaded area 

includes most rocks.
12 Detailed Interpretation of Aeromagnetic Data from the Patagonia Mountains Area, Southeastern Arizona

men12-7690_fig07

INDUCTION DOMINANT REMANENCE DOMINANT

Rocks and ores

Gabbro/norite

Peridotite including dunite (serpentinised

Granite/granodiorite/tonalite

Acid volcanics

Diorite/monzonite/syenite

Pyroxenite/hornblendite (Alaskan type)

Massive pyrrhotite Disseminated pyrrhotite

Banded Iron Formation

Haematite-bearing

Meta-igneous

Laterite

Skarn

Pyrrhotite skarn

Pyrrhotite-bearing

Sediments/metasediments

Magnetite skarn

Magnetite-bearing

Spilites Dykes/sills Flows

Pillow lavas

Basalt/dolerite

Andesites/intermediate volcanics

Chilled margins

0.1 1 10 100 1,000

Koenigsberger ratio (JNRM/JIND) Q

Bar length is range

Range of most rocks

EXPLANATION

Figure 7. Koenigsberger ratio of common rocks (after Clark, 1997). Bar represents range, shaded area includes most rocks.

In this report, two terms are used to describe remanent 

magnetization directions in rocks (suggestion from Mark 

Gettings, U.S. Geological Survey, written commun., 2013). 

The first, enhanced total magnetic intensity remanence 

describes natural remanent magnetism that contributes or adds 

to the intensity of the induced field (fig. 8). These are remanent 

field directions that are between 0 to just less than 90 degrees 

from the direction of the current induced field. The second, 

reduced total magnetic intensity remanence describes natural 

remanent magnetism that reduces the intensity of the induced 

field (fig. 8). These remanent field directions are from slightly 

more than 90 degrees to 180 degrees from the direction of 

the induced field. Generally, rocks that have a reduced total 

magnetic intensity natural remanent magnetism inherited 

their remanent field when the Earth’s magnetic field had a 

polarity opposite that of the current geomagnetic field. Some 

rocks with reduced total magnetic intensity natural remanent 

magnetism may have been rotated tectonically to a position 

where their remanent field vector is more than 90 degrees from 

the Earth’s current magnetic field direction.
Magnetic Susceptibility of Rock in the Patagonia Mountains Area 13

men12-7690_fig08

90

90 80

100 70

60

110 120 50

130 40

140

30

150

20

160

10

170 0180

180

0 170

10

160

20

150

30

140

40

130

50

120

60

110

70

100

80

90°

0

161.8°

-0.95

120°

-0.5

18.2°

0.95

Reduced TMI 

60.0°

0.5

41.4°

0.75

138.6°

0.75

75.5°

0.25

104.5°

-0.25

Aligned NRM Reversed NRM

0°

1.0

180°

-1.0

Enhanced TMI

Figure 8. Contribution to the Earth’s magnetic field total magnetic intensity (TMI) from the natural 

remanent magnetic (NRM) field based on the angle between the magnetic field (induced) and the NRM 

field. Angles between the two fields are labeled for contributions to the induced field of 1.0, 0.95, 0.75, 

0.5, 0.25, 0, -0.25, -0.5, -0.75, -0.95, and -1.0 times the remanent intensity. The angle between the remanent 

field used for reduced TMI concealed intrusions in this report is 120.6 degrees (red triangle) which 

reduces the TMI by 0.51 times the NRM field intensity. Diagram created with suggestions from Mark 

Gettings (U.S. Geological Survey, written commun., 2013).

Magnetic Susceptibility of Rock in the 

Patagonia Mountains Area

Magnetic susceptibility (SI) measurements of 

outcropping rock were acquired at 45 sites in the Patagonia 

Mountains region and were published in Gettings and Bultman 

(2014). These data were acquired with a ZH Instruments 

SM-30 magnetic susceptibility meter. Outcrop susceptibility 

was estimated by making 7 to 20 readings taken at random 

locations with minimum spacing of approximately 20 cm on 

the outcrop face. The mean susceptibility is 0.0076 SI and 

only one sample has a negative susceptibility. Six samples 

have susceptibilities greater than 0.02 SI. Figure 9 displays a 

histogram of the susceptibility data.

Paleomagnetic data for the Patagonia batholith were 

presented by Hagstrum (1994) and are presented in table 2. 

Samples were collected from Sycamore and Providencia 

Canyons (fig. 3) on the west central part of the stock and three 

sites (PG4, PG5, and PG6; table 2) showed the presence of 

reduced total magnetic intensity natural remanent magnetism 

(Hagstrum, 1994).

-4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5

Number of samples

Log base 10 of susceptibility (SI)

0

1

2

3

4

5

6

7

Figure 9. Histogram of the base 10 logarithm of 

susceptibility for samples from the Patagonia Mountains, 

southeastern Arizona. One sample with negative 

susceptibility was excluded.
14 Detailed Interpretation of Aeromagnetic Data from the Patagonia Mountains Area, Southeastern Arizona

Table 2. Patagonia granodiorite remanent magnetization 

measurements, Patagonia Mountains study area, southeastern 

Arizona.

[After Hagstrum (1994)]

Patagonia batholith Inclination Declination Number of 

samples

PG1 41.3 355.1 6

PG2 54.4 334.9 8

PG3 48.3 343.7 8

PG4 52.5 343.9 7

-47 334.7 11

PG5 47.3 329 2

-72.4 27.9 7

PG6 -62.3 14.4 9

Average, PG1 through 

PG4

49.1 341.8 5

Measured Susceptibility, Remanence, 

and Geology Incorporated into Forward 

Magnetic Modeling 

Berger and others (2003), presented a geologic 

model of the region near Red Mountain (figs. 4 and 5) 

based on 506 vertical prisms 0.5 (y) × 0.5 (x) × 1.0 (z) km 

in size. This simple model, in an area with exceedingly 

complex geology, produced results that relied on negative 

susceptibilities to reproduce the observed magnetic field. 

Eighteen measurements of rock susceptibility in the area 

of this geologic model produced no samples with negative 

susceptibility (Gettings and Bultman, 2014).

Geologic information was incorporated into new 

modeling using measured magnetic susceptibilities (Gettings 

and Bultman, 2014), estimated natural remanent magnetism 

based on lithology, magnetic field strength and direction of the 

Earth, estimated Koenigsberger ratios, measured remanence 

directions based on Hagstrum (1994), geologic mapping 

(Simons, 1974; Drewes, 1980; 1996; Drewes and others, 

2002), and succession of events that led to the formation of the 

Patagonia batholith (Vikre and others, 2009). By incorporating 

known geology and measured physical properties of rock in 

the region into the model, a new geologic model was produced 

that is strikingly different from the model produced by Berger 

and others (2003).

Two-dimensional forward modeling was accomplished 

with the GM-SYS modeling package in Oasis Montaj 

(Geosoft, 2013). GM-SYS allows the inclusion of remanent 

magnetism if the remanent intensity, inclination, and 

declination are known. When possible, both enhanced 

and reduced total magnetic intensity natural remanent 

magnetizations were included in the geologic modeling of the 

Patagonia Mountains study area.

Based on Hagstrum (1994) the Patagonia batholith (map 

unit Tlg; table 1, fig. 3) has a natural remanent magnetic field 

with an average inclination of 49.1 degrees and declination of 

341.8 degrees based on the data from five samples collected 

from the west side of the batholith (table 2). Additionally, 

three samples collected from that general area and within 

the batholith show a natural remanent magnetism direction 

quite different from the current induced field direction (those 

with negative inclinations, table 2). A main component of 

the new modeling presented here suggests the existence of 

a late concealed stock emplaced at the northern end of the 

Patagonia Mountains region that corresponds with a low in 

the aeromagnetic anomaly map (fig. 4). One of the natural 

remanent magnetic field samples, PG6 (Hagstrum, 1994, 

table 2), was selected to represent the reduced total intensity 

natural remanent magnetic field in the study area at the time of 

the emplacement of the concealed stock. The natural remanent 

magnetic field direction of PG6 is 120.6 degrees from the 

current magnetic field of the Earth (fig. 10). Vikre and others 

(2009) have shown the main large volume emplacement of 

the Patagonia batholith is associated with the 60 Ma Cu-Mo 

breccia deposit at Red Hill (Four Metals Mine, fig. 2) in the 

southern part of the pluton. The estimated emplacement of 

the northern concealed stock is 61–58 Ma (Vikre and others, 

2009); therefore, the natural remanent magnetism vector 

direction for this stock is plausible.

Remanent intensities were estimated by using Q values 

for specific lithologies based on figure 7 (Clark, 1997) and 

on the relation Jr = Ji × Q = k × F × Q. At the time of the 

survey, F = 38.41 A/m. For example, map unit Tlg (Drewes 

and others, 2002), has measured k = 0.0200 SI (table 1). 

Based on the lithology of map unit Tlg (granodiorite to quartz 

monzonite, table 1) the estimated Koenigsberger ratio (Qe) 

for this composition of rock varies from 0.1 to 10 with most 

values ranging from 0.2 to 0.6 (fig. 7). Qe was selected to be 

0.6 because the composition of map unit Tg is on the more 

mafic end of the rock compositions used in figure 6. This 

gives an estimated remanent intensity of Jr as 0.46 A/m. Other 

remanent intensities were calculated in a similar fashion and 

are given in table 1.

Two intrusive units that are entirely concealed are 

hypothesized here and used in the modeling. Their existence 

is required to fit forward magnetic models of the geology to 

the aeromagnetic data when measured rock physical properties 

are used in the models. The intrusive rocks are late stages in 

the emplacement of the Patagonia batholith, during the fifth 

and sixth events of the succession of events defined by Vikre 

and others (2009). Their composition is estimated to be the 

same as the composition of the Patagonia batholith, diorite to 

quartz monzonite. The estimated susceptibility and remanent 

intensity for these concealed intrusive rocks was based on that 

composition and data from figures 6 and 7.
Measured Susceptibility, Remanence, and Geology Incorporated into Forward Magnetic Modeling 15

men12-7690_fig10

Present day

magnetic field

PG6

PG4

PG5

Figure 10. Stereonet plot of samples collected from the Patagonia batholith with 

reduced total magnetic intensity natural remanent magnetic field (table 2) and the 

present-day inductive field of the Earth, Patagonia Mountains study area, southeastern 

Arizona. Solid symbols indicate projection from the upper hemisphere and open symbols 

indicate projection from the lower hemisphere. The natural remanent magnetic field for 

hypothetical intrusions with reduced total magnetic intensity natural remanent magnetic 

field in this report (map unit Tgr, table 1) uses sample PG6, which is 120.6 degrees from the 

current (2014) magnetic field in the Patagonia Mountains.
16 Detailed Interpretation of Aeromagnetic Data from the Patagonia Mountains Area, Southeastern Arizona

Geologic Model

Geologic modeling was based on the aeromagnetic 

survey flight-line data whose locations are displayed in 

figures 3 and 4. Modeling began by placing all the data 

from the survey flight lines into GM-SYS and adding all 

contacts in the proper locations at the surface of the model 

(fig. 3; Drewes, 1980; Drewes and others, 2002). Map units 

(column 2, table 1) used for modeling were based on Drewes 

(1980) and Drewes and others (2002). Average magnetic 

susceptibilities as measured in the field (or from the literature 

as cited in table 1) were assigned to each map unit lithology 

and remanent magnetisms were estimated for lithologies with 

susceptibility greater than 0.02 SI. Adding remanence to map 

units with susceptibilities less than 0.02 did not significantly 

affect the model in this study. Two concealed intrusive rocks 

were inserted at depth to allow the model to fit the data. Then 

model geologic boundaries were moved until the model 

achieved a reasonable geology and a forward magnetic model 

(black lines, figs. 11 and 12) that matched the measured 

magnetic anomaly (black dots, figs. 11 and 12). The model 

results are shown in figures 11 and 12.

Profile 1, a south to north profile of the Patagonia 

Mountains is shown in figure 11 (figs. 3 and 4 show 

profile location). This profile contains mostly outcropping 

bedrock with measured susceptibilities and is therefore very 

constrained. Colors for map units in the geologic model 

were selected to be similar to those used in the geologic map 

shown in figure 3. Precambrian granite (Yg; fig. 11, table 1) 

outcrops on the south end of the profile and likely is underlain 

by Jurassic granite (Jg; fig. 11, table 1), which surrounds it 

at the surface. Using measured susceptibilities and remanent 

intensities calculated from estimated Q values (table 1, 

fig. 7), it was not possible to obtain the adjacent high and low 

anomaly values seen in profile 1 (fig. 11) at kms 8.5 and 14 

without introducing a high susceptibility concealed intrusive 

(map unit Tgc, table 1) and an intrusive with reduced total 

magnetic intensity natural remanent magnetism (map unit 

Tgr, table 1) into the model. Map unit Tgc is assumed to be 

of a similar composition to Tlg (table 1) but was assigned 

a higher susceptibility (and therefore remanence) than Tlg 

to produce the aeromagnetic high at the km 9 mark of the 

profile. Based on figures 6 and 7 map unit Tgc was arbitrarily 

assigned k = 0.05 (SI) and Qe = 0.6; therefore, the intensity of 

Jr = 1.15 A/m. These values of SI and Q are well within the 

Kilometers

0 5 10 15 20

0

1

0

100

-200

-100

-300

Nanoteslas

Kilometers

SOUTH NORTH Crossing point for profiles 1 and 2 

Aeromagnetic

survey data Modelled

magnetic

field

Ka

Tgr

Tgr

Jg Tlg Tlg

Tgc

Jvs

Jvs

QTg

Jg

Tgr

m

Yg

Kuvs

Faults

Figure 11. Geologic cross section for profile 1, Patagonia Mountains study area, southeastern Arizona. Profile locations 

are shown in figures 3 and 4. Black dots, aeromagnetic survey line data; black line, model results. Hypothesized concealed 

intrusions are labeled as map units Tgr and Tgc. Map unit labels are shown in table 1. 
Geologic Model 17

range of values that are in rocks of diorite to quartz monzonite 

composition (figs. 6 and 7). Map unit Tgc is assumed to be an 

apophysis of very late stage intrusion in the emplacement of 

the main body of the Patagonia batholith and may be related 

to outgassing and mineral deposits (for example, Four Metals 

mine) formed near Red Hill (fig. 2; Vikre and others, 2009).

The aeromagnetic anomaly low at km 14 in the profile 1 

(fig. 11) could not be modeled without including a concealed 

intrusive having a reduced total magnetic intensity remanence, 

map unit Tgr (table 1). This intrusive, the last phase of the 

formation of the Patagonia batholith may be responsible for 

the Sunnyside porphyry copper deposit that lies just 1 km to 

the east of this profile at the location of the lowest anomaly 

value, near km 14. Although the exact age of this concealed 

body is not known, there are large periods of reversed 

magnetic field polarity that occurred during its formation (61–

58 Ma, Vikre and others, 2009) associated with chrons 27 and 

26 (Butler, 1998). This intrusive is therefore given a reduced 

total magnetic intensity natural remanent magnetic field 

with a direction estimated from the Patagonia batholith (~62 

Ma) sample PG6 (table 2; Hagstrum, 1994) and a remanent 

intensity estimated from figures 6 and 7 (table 1).

The Patagonia Fault is located at km 20 in the profile, and 

the Sonoita Basin lies to the north of the fault. Betts and others 

(1998) used electromagnetic data to model the Sonoita Basin 

and determined that it contained about 200 m of basin fill just 

north of the Patagonia Fault where Alum Gulch enters the 

Sonoita Basin (fig. 2), near the location of profile 1. The model 

displayed in profile 1 also shows about 200 m of basin fill at 

this location. Volcanic rocks (Kuvs; table 1) with a relatively 

high SI (0.02), a high Q (1.4), and a low total magnetic 

intensity natural remanent magnetic field (table 1) underlie 

the basin fill. This concealed volcanic unit could be map 

unit Jg (table 1), which is exposed to the north of the basin 

as the Squaw Gulch volcanic rocks (Gettings, 2002) and has 

reduced total magnetic intensity natural remanent magnetism. 

Based on the aeromagnetic anomaly low shown in figure 4, 

map unit Kuvs in this location extends to the southwest to an 

easting of 519000 and to the northeast to the edge of the Red 

Mountain block. A reduced total magnetic intensity natural 

remanent magnetic field is required for this map unit in order 

that the magnetic model to fit the aeromagnetic data. Map unit 

Kuvs may be related to Cretaceous “Bathtub” volcanic rocks 

mapped just to the north of this profile, at least some of which 

Kilometers

0 5 10 15 20

0

1

0

-150

-300

Nanoteslas

Kilometers

WEST EAST

Tuc

Jg

Tgc

Tlg

Kuvs

Kuvs

Tgr

Jg

QTg

Jvs

Jvs

m Jvs

Yg

Ka

Jvs

QTg

Ka

Ka Tv

Faults

Faults

Trace of Harshaw

Creek fault

Patagonia

fault

Crossing point for

profiles 1 and 2 

Aeromagnetic

survey data

Modelled

magnetic

field

Figure 12. Geologic cross section for profile 2, Patagonia Mountains study area, southeastern Arizona. Profile locations 

are shown in figures 3 and 4. Black dots, aeromagnetic survey line data; black line, model results. Hypothesized concealed 

intrusions are labeled as map units Tgr and Tgc. Map unit labels are shown in table 1. 
18 Detailed Interpretation of Aeromagnetic Data from the Patagonia Mountains Area, Southeastern Arizona

are known to have a reduced total magnetic intensity natural 

remanent magnetic field (Gettings, 2002). The inclination and 

direction of that remanent field was selected to coincide with 

sample PG 6 (table 2; Hagstrum, 1994). All physical properties 

of map unit Kuvs are estimated and the exact structural 

relations, especially depth, could vary based on the selection 

of those physical properties. If the assumptions made here 

are valid, it may show, as suggested by Gettings and Gettings 

(1996) for the Tombstone area, that some Cenozoic basins in 

Arizona with large aeromagnetic anomaly lows may contain 

relatively shallow basin fill underlain by volcanic or intrusive 

rocks with nonaligned remanent magnetic fields and not thick 

accumulations of low susceptibility sediments. This could 

have significant water resources implications. When models 

are used in basins with aeromagnetic data, it is important to 

simulate adjacent ranges also to determine if the low values in 

basins can be accurately simulated when lithologies of known 

susceptibility are included in the model in the ranges.

Figure 12 displays an east to west geologic model along 

profile 2 (figs. 3 and 4). As in figure 11, the dotted black 

line on top of the cross section is the aeromagnetic data, the 

thick black line is the aeromagnetic anomaly produced by the 

forward model due to the geology in the cross section. This 

model was started at the crossing point of the two models 

where Jurassic granite (Jg, table 1; fig. 12) is exposed at the 

surface and a hypothesized concealed Tertiary granite with 

reduced total magnetic intensity natural remanent magnetism 

(Tgr, table 1) is found at depth. The concealed granite was 

initially given the same depth on both profiles at this location 

(about 600 m). The final model in figure 12 has the body 

deeper, about 800 m, but the contact is dipping steeply at this 

location. Concealed lithologies under map units Tuc and QTg 

(table 1) in the western part of the model are based on Simons 

(1974), Drewes (1980), and Drewes and others (2002). A 

concealed intrusive unit (Tg; table 1) was required at depth at 

km 5–7 in the profile to create the aeromagnetic anomaly high 

that occurs between kms 5 and 7 in the profile (fig. 12).

The concealed Tertiary granite with reduced total 

magnetic intensity natural remanent magnetism, map unit 

Tgr (table 1) plays a large role in modeling the aeromagnetic 

profile data of figure 12. The profile is dominated by a major 

magnetic low anomaly centered at km 12 and this anomaly 

is part of the same low seen at km 14 in figure 11. As in the 

model for profile 1 (fig. 11), this anomaly could only be 

modeled correctly by using a lithology with a reduced total 

magnetic intensity natural remanent magnetic field. Profile 2 

directly crosses the Sunnyside porphyry copper deposit, near 

the km 14.5 mark. Although the Harshaw Creek Fault at this 

location is shown as concealed by a Tertiary flow by both 

Simons (1974) and Drewes (1980), field evidence indicates 

this is not a flow but an extrusive vent that may be associated 

with the emplacement of the main Patagonia batholith (Tlg; 

table 1), or an earlier intrusive in this area. The eruptive event 

may have followed the fault, which is known to be at least 

Jurassic in age. This volcanic neck (Tv, fig. 12 and table 1) 

cannot be modeled with a reduced total magnetic intensity 

remanent magnetic field, so is not directly related to Tgr 

(table 1). However, it may be associated with some earlier 

phase of the intrusion here and with the Sunnyside porphyry 

copper deposit. Moving farther east, there is a mapped fault 

separating map units Ka and J^vs at the surface with the 

hanging wall on the west side. Near km 19, a modeled range 

bounding fault drops the bedrock to basin depths. 

The model cross sections displayed in figures 11 and 

12 indicate that the large aeromagnetic low to the south 

of Red Mountain (figs. 2 and 5) can be simulated only by 

incorporating a concealed intrusive body with a reduced 

total magnetic intensity natural remanent magnetic field at 

that location in the model. This intrusive is coincident in 

timing and location with the sixth and last event in Vikre and 

others (2009) succession of events that formed the composite 

Patagonia batholith. It is also coincident with the Sunnyside 

porphyry copper deposit, at least in location, but the extrusive 

event located structurally above map unit Tgr , map unit Jg, 

cannot possess a reduced total magnetic intensity natural 

remanent magnetic field and be accurately simulated. The 

approximate location of this concealed intrusive can be seen in 

figure 4 superimposed on the aeromagnetic data with a thick 

yellow line. It is truncated to the northwest and east by basin 

bounding faults. Additionally, the bedrock in the Sonoita Basin 

along Sonoita Creek (fig. 2) at the northern end of profile 1 

(fig. 11) can be simulated as volcanic rock with a reduced total 

magnetic intensity natural remanent magnetic field concealed 

by relatively shallow basin fill.

Complete Bouguer Anomaly Map

The complete Bouguer anomaly map (fig. 13) of the 

northern part of the study area is based on data from Gettings 

and Houser (2000). The white crosses on the map indicate the 

locations of gravity stations from which the map was made. 

Figure 13 does not have a sufficient density of gravity stations, 

especially in the area of the hypothesized concealed intrusive 

(yellow line, fig. 13), to be used for the model. Although a 

distinct gravity high is associated with the proposed concealed 

intrusive, especially on the southeast side, it is controlled by 

only a few gravity stations. The model cannot conclusively 

prove or disprove the existence of this proposed intrusive 

body. This gravity anomaly does indicate that map unit Tgr 

may be cut off to the east and to the north by faults as was 

proposed in by the model simulations and that the proposed 

intrusive may extend slightly farther to the northeast than 

indicated by the magnetic anomaly data in figure 4. This 

supposition is based on the small number of gravity stations.
Euler Deconvolution Depth Estimates 19

men12-7690_fig13

Hypothesized

concealed

intrusion

RED

MOUNTAIN

Complete Bouguer

anomaly in milligals

-140.621

-142.926

-144.255

-145.248

-145.981

-146.764

-147.547

-148.118

-148.570

-149.003

-149.336

-149.633

-149.969

-150.389

-150.843

-151.212

-151.644

-152.161

-152.678

-153.137

-153.661

-154.181

-154.688

-155.213

-155.773

-156.385

-157.063

-157.833

-158.637

-159.427

-160.121

-160.844

-161.670

-162.837

-164.978

-166.977

-168.494

-170.167

EXPLANATION

N

Gravity station

500000 505000 510000 515000 520000 525000 530000 535000 54000 545000 550000

3490000 3495000 3500000 3485000 3480000 3475000 3470000

Figure 13. Complete Bouguer anomaly map of the northern part of the Patagonia Mountains study area, southeastern Arizona. 

Overlain geologic linework (black lines) from Drewes and others (2002).

Euler Deconvolution Depth Estimates

Euler’s inhomogeneity equation can be used for 

making depth estimates with Earth’s total field magnetic 

data (Yaghoobian and Boustead, 1993). This technique has 

numerous advantages over other types of depth estimates 

because no particular geological model is assumed, and it 

is not sensitive to magnetic inclination, declination, and 

remanence because these become a part of the constant in the 

anomaly function of a given model (Yaghoobian and Boustead 

1993; Hsu 2002). Although the method can yield rather 

inaccurate estimates of depth (Casto, 2001), it still gives an 

approximation of anomaly depth and is ideal for estimating the 

spatial locations of the sources of observed anomalies. Depth 

estimates using the Euler equation are dependent on a scaling 

exponent selected at the start of the analysis. These scaling 

exponents are called structural indices and relate the distance 

dependence of the magnetic field to the shape of a geologic 

feature.

Depth estimates were obtained using Euler deconvolution 

in the Oasis Montaj program (Geosoft, 2013). Estimates 

for a structural index of 0, which locates depth solutions 

for contacts and steps are shown in figure 14. The Euler 

deconvolution package in Oasis Montaj (Geosoft, 2013) 

allows the user to adjust several parameters including window 

size, depth tolerance, and distance. All settings were adjusted 

to allow for numerous solutions in order to produce a large 

number of depth estimates. The estimates that form linear 

patterns are used to identifying linear and elongate features 

in the data. The Patagonia Fault is clearly visible and labeled 

in figure 14. In the northeast part of the study area, the fault 

separates the Red Mountain block from volcanic rocks 

modeled with a reduced total magnetic intensity natural 

remanent magnetic field and concealed by basin fill to the 

northwest. Most of the depth estimates were about 200 m on 

the northeast part of the fault, which agrees with the depth 

of the volcanic rock in the Sonoita Basin northwest of the 

northeast striking Patagonia Fault presented in figure 12 and 
20 Detailed Interpretation of Aeromagnetic Data from the Patagonia Mountains Area, Southeastern Arizona

men12-7690_fig14

Patagonia Fault

Patagonia Fault

Sycamore Canyon Fault

Harshaw Creek Fault

Mt. Benedict Fault?

N

Red

Mountain 

UTM, NAD 27, zone 12 north 

Estimated depth of

anomaly, in meters

-135.357

-151.254

-164.201

-174.125

-181.836

-189.482

-196.984

-202.833

-209.226

-215.666

-222.168

-228.506

-235.370

-241.842

-248.399

-254.862

-262.533

-269.756

-277.694

-285.827

-293.889

-302.321

-311.059

-319.961

-329.449

-339.554

-349.580

-359.779

-369.813

-379.374

-391.853

-405.642

-423.167

-444.587

-470.020

-504.217

-544.631

-606.129

EXPLANATION

510000 515000 520000 525000 530000

3490000 3485000 3480000 3475000 3470000 3465000

Figure 14. Euler deconvolution depth estimates (circles) for a structural index of 0 plotted over aeromagnetic 

data in the Patagonia Mountains study area, southeastern Arizona. Overlain geologic line work (black lines) from 

Drewes and others (2002).

reported in Betts and others (1998). To the southwest, the 

fault becomes obscured by intrusive rocks near Red Mountain 

and is lost in rocks with a reduced total magnetic intensity 

natural remanent magnetic field west of Red Mountain. 

Farther southwest, it deepens, then shallows and crosses from 

the south to north side of the Sonoita Basin. This had been 

unrecognized previously. The Harshaw Creek Fault is clearly 

visible on the eastern side of the cross section (fig. 12). It 

is truncated to the north where it meets low aeromagnetic 

anomaly values due to the proposed intrusion with reduced 

total magnetic intensity natural remanent magnetism. To the 

south, the fault shows a sharp kink just before the southeast 

edge of the map (fig. 14). The Guajolote Fault (fig. 2) is not 

visible in data shown in figure 14, but an unmapped northwest 

to southeast fault, which appears to be the western margin of 

the Patagonia batholith is clearly visible and intersects the 

Sycamore Canyon Fault, which is visible. Several faults or 

contacts are visible in the Patagonia batholith, which might 

differentiate rock of slightly different composition or be 

related to intrusive sub-bodies within the stock. The concealed 

fault along the Santa Cruz River Valley (southwest corner of 

figure 14) is displaced to the east of its mapped location in this 

model. Several other concealed faults also can be seen in the 

west area of figure 14.

A structural index of 1 displays depth solutions derived 

from the edges of sills and dikes and other bodies that are 

1 plus dimensional and depth estimates for these features are 

shown in figure 15. The faults examined in figure 14 are much 

less visible in figure 15 but the Guajolote Fault is visible. 

This may indicate that thick alteration or shear zones are 

associated with the fault. Few other linear features are visible 

in figure 15.
Euler Deconvolution Depth Estimates 21

men12-7690_fig15

Red

Mountain 

Guajolote Fault

N

UTM, NAD 27, zone 12 north 

Estimated depth of

anomaly, in meters

EXPLANATION

-214.252

-233.177

-247.715

-261.735

-269.729

-279.604

-291.864

-302.145

-310.376

-319.562

-328.496

-336.979

-345.128

-355.521

-365.187

-373.881

-380.977

-390.061

-398.417

-406.815

-417.007

-426.700

-436.210

-446.806

-458.034

-471.299

-483.784

-498.126

-511.141

-523.785

-537.556

-550.204

-569.270

-588.345

-615.809

-645.475

-720.416

-805.058

515000 520000 525000 530000

3485000 3480000 3475000 3470000 3465000

Figure 15. Euler deconvolution depth estimates (circles) for a structural index of 1 plotted over aeromagnetic 

data in the Patagonia Mountains study area, southeastern Arizona. Overlain geologic line work (black lines) from 

Drewes and others (2002).

Euler depth estimates with a structural index of 2 produce 

solutions that are depth estimates of cylinder and pipe shaped 

sources. These sources are often indicated when a several 

depth estimates are stacked nearly on top of each other, which 

may be due to an apophysis or cupola of an intrusive body. 

Several types of solutions shown in figure 16 include: (1) 

porphyritic intrusive of Red Mountain at a depth of more than 

400 m; (2) possible intrusive near Lead Queen Mine (fig. 2); 

(3) possible intrusive near U.S. Mine (fig. 2); (4) possible 

intrusive that may be associated with the Washington Camp 

(fig. 2) and Duquesne mineral districts (south of area shown 

in figure 2); (5) possible intrusive that may be associated with 

Morning Glory Mine (fig. 2); (6) possible intrusive associated 

with Santo Nino Mine (south of area shown in figure 2); and 

(7) possible intrusive associated with Four Metals Mine, 

which may be related to map unit Tgc (figs. 2 and 11). Several 

potential intrusive rocks can be seen west of the Patagonia 

batholith (map unit Tlg; fig. 3). Euler depth estimates with 

a structural index of 3 display depths of spheres and barrel 

shaped sources. The depth estimate map produced for a 

structural index of 3 was nearly identical to figure 16.
22 Detailed Interpretation of Aeromagnetic Data from the Patagonia Mountains Area, Southeastern Arizona

men12-7690_fig16

Red

Mountain 

N

UTM, NAD 27, zone 12 north 

1

2

3

4

5

6

7

Estimated depth of

anomaly, in meters

EXPLANATION

-290.288

-315.792

-342.202

-358.981

-373.340

-386.959

-403.507

-416.153

-427.640

-436.463

-445.053

-456.403

-466.789

-476.091

-486.682

-496.575

-505.926

-516.518

-528.327

-540.375

-551.283

-563.220

-575.001

-587.289

-601.870

-617.548

-631.603

-647.273

-663.080

-680.063

-696.833

-713.800

-734.418

-757.010

-785.616

-811.706

-843.580

-885.303

515000 520000 525000 530000

3485000 3480000 3475000 3470000 3465000

Figure 16. Euler deconvolution depth estimates (circles) for a structural index of 2 plotted over aeromagnetic data in 

the Patagonia Mountains study area, southeastern Arizona. Overlain geologic line work (black lines) from Drewes and 

others (2002). Numbers depict depth estimates for: (1) porphyritic intrusive of Red Mountain at a depth of more than 

400 meters; (2) possible intrusive near Lead Queen Mine; (3) possible intrusive near U.S. Mine; (4) possible intrusive 

that may be associated with the Washington Camp and Duquesne mineral districts; (5) possible intrusive that may 

be associated with Morning Glory Mine; (6) possible intrusive associated with Santo Nino Mine; and (7) possible 

intrusive associated with Four Metals Mine, which may be related to map unit Tgc (fig. 2, table 1). Locations of mines 

are shown in figure 2.
References Cited 23

Conclusions

An infinite number of solutions are available for all 

geophysical problems, but there is only one correct answer. 

Producing the best geophysical modeling requires that all 

known geologic information be incorporated into the model. 

Included is the knowledge that when looking at the Earth as a 

whole and possibly excluding some very old rocks, about onehalf of the rocks near the Earth’s surface were formed when 

the Earth’s magnetic field was in a state of reversed polarity. In 

small areas, this may not be the case. Likewise, it may be true 

in a small area that most of the rocks may have formed in a 

reversed magnetic field. Furthermore, tectonic events, such as 

repeated faults from extension, can rotate the natural remanent 

magnetism to arbitrary orientations.

In the Patagonia Mountains study area in southeastern 

Arizona, geological knowledge was incorporated into 

modeling by using rock physical properties, using known 

association of intrusive events, and incorporating information 

from existing mapped geology. Using this geological 

knowledge, a model of geology at depth was produced that 

may better represent actual conditions than previous models. 

This new model has major implications for undiscovered 

mineral resources in the study area. Contacts at the top of the 

concealed intrusive rocks are a likely area where concealed 

mineral deposits may be found, especially in regions where 

the overlying rock is a host rock for disseminated, or skarnrelated deposits, as in Mesozoic and Paleozoic rocks east of 

the Harshaw Creek Fault and continuing into the San Rafael 

Basin.

This new geologic model also may have implications for 

water resources. Although not the focus of this study, volcanic 

rocks in the Sonoita Basin can be simulated with reduced total 

magnetic intensity natural remanent magnetic field underlying 

relatively thin basin fill instead of the thick basin fill that is 

assumed to exist in the basin.

The complete Bouguer anomaly map of the Patagonia 

Mountains study area indicates that the proposed intrusive 

does have a positive gravity anomaly and is cut off on the 

east and north by faults. It also shows that this intrusive might 

extend farther to the northeast than its magnetic anomaly 

would indicate.

Euler depth estimates with a structural index of 0 along 

the Patagonia Fault generally agree with the depth estimates 

simulated by the model. They also show that the concealed 

Patagonia Fault southwest of the Patagonia Mountains 

is more complex than mapped. Additionally, Euler depth 

estimates with a structural index of 2 located many potential 

intrusive bodies that may be associated with both known and 

undiscovered mineralization.

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Publishing support provided by the U.S. Geological Survey

 Science Publishing Network, Tacoma Publishing Service Center

For more information concerning the research in this report, contact the

Director, Geology, Minerals, Energy, and Geophysics Science Center

U.S. Geological Survey

520 N. Park Avenue

Tucson, AZ 85719

http://geomaps.wr.usgs.gov/gmeg/
Bultman—Detailed Interpretation of Aeromagnetic Data from the Patagonia Mountains Area, Southeastern Arizona—Scientific Investigations Report 2015–5029

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155029