MNP

MNP

MNP

Mountain

Pass

I-15

Nipton

Ivanpah

Cima

Jean

McCullough Range

Clark Mountain Range

Shadow

Ivanpah

Mid Hills

New York Mountains

Mesquite Valley

Spring Mountains

Lucy Gray Mountains

Mescal

Range

Cima

Dome

PrimmValley

Mountains

Valley

MNP

MNP

MNP

I-15

NEVADA

CALIFORNIA

0 5 0 5 MILES10 KILOMETERS 

35°10’

35°15’

35°30’

35°45’

35°50’

115°50’ 115°45’ 115°30’ 115°15’ 115°05’

EXPLANATION

Quaternary alluvium

Cenozoic sedimentary rocks

Cenozoic volcanic rocks

Tertiary basalt

Granitic rocks, undivided

Cretaceous rocks

Paleozoic rocks

Proterozoic rocks

Figure 1. Index map showing simplified geology of eastern Mojave Desert (modified from 

Jennings and others, 1977; Stewart and Carlson, 1978). Red outline, study area; black outline, 

area of detailed aeromagnetic survey; green line, boundary of Mojave National Preserve (MNP); 

gray lines, roads.

Figure 2. Index map showing areas of regional (pink shading, California; gray shading, Nevada; 

Roberts and Jachens, 1999; Kucks and others, 2006) and detailed (black outline; red lines show 

flightlines) aeromagnetic surveys. White transparent boxes highlight flight specifications of 

surveys, listed from top to bottom: spacing (for example, 1,600 m); elevation (for example, 305 m 

above ground [AG]); direction (for example, east-west [E–W]; and year flown (for example, 1979). 

Base map from U.S. Geological Survey 1:100,000-scale quadrangles: Ivanpah, 1985; Mesquite 

Lake, 1985.

100–200 m

70 m AG

70

o

 azimuth

2016 800 m

305 m AG

E–W

1981

1,600 m

305 m AG

E–W

1979

115

°

45’ 115

°

15’

35

°

17’

35

°

37’

0 5 KILOMETERS 

INTRODUCTION 

Magnetic investigations of Mountain Pass and vicinity were conducted as part of an effort to 

study regional crustal structures as an aid to understanding the geologic framework and mineral 

resources of the eastern Mojave Desert. The study area, which straddles the state boundary between 

southeastern California and southern Nevada (fig. 1), encompasses Mountain Pass, which is host to 

one of the world’s largest rare earth element carbonatite deposits. The deposit is found along a 

north-northwest-trending, fault-bounded block that extends along the eastern parts of the Clark 

Mountain Range, Mescal Range, and Ivanpah Mountains (fig. 1). This Paleoproterozoic block is 

composed of a 1.7-Ga metamorphic complex of gneiss and schist that underwent widespread 

metamorphism and associated plutonism during the Ivanpah orogeny (Wooden and Miller, 1990). The 

Paleoproterozoic rocks were intruded by a Mesoproterozoic (1.4 Ga) ultrapotassic alkaline intrusive 

suite and carbonatite body (Olson and others, 1954; DeWitt and others, 1987; Premo and others, 2016). 

The intrusive rocks include, from oldest to youngest, shonkinite, mesosyenite, syenite, quartz syenite, 

potassic granite, carbonatite, carbonatite dikes, and late shonkinite dikes (Olson and others, 1954).

MAGNETIC METHODS

REGIONAL MAGNETIC MAP

The regional aeromagnetic map was derived from statewide aeromagnetic maps of California and 

Nevada that were compiled from numerous surveys flown at various flightline altitudes and spacings 

(Roberts and Jachens, 1999; Kucks and others, 2006). Aeromagnetic data were corrected for diurnal 

variations of the Earth’s magnetic field, upward- or downward-continued to a constant elevation of 

305 m above the ground surface, adjusted to a common datum, and merged to produce a uniform map 

(Roberts and Jachens, 1999; Kucks and others, 2006). This compilation, although composed of surveys 

acquired using different specifications, allows seamless interpretation of magnetic anomalies across 

survey boundaries.

Regional aeromagnetic surveys in the study area range in flightline spacing from 800 to 1,600 m, 

and they have a flightline altitude of 305 m (fig. 2). Because of the moderate spacing of the surveys, 

shallow magnetic sources may not be well resolved in some parts of the regional aeromagnetic map.

DETAILED MAGNETIC MAP

A detailed aeromagnetic survey of the inset area (black outline on map and on fig. 1) was flown 

by CGG Canada Services Ltd. This high-resolution helicopter survey, which was flown at flightline 

spacings of 100 and 200 m, a flightline azimuthal direction of 70°, and a nominal flightline elevation 

above ground of 70 m (fig. 2), consists of about 1,814 line-kilometers of data. Tie lines, which were 

spaced at 1-km intervals, were flown in a flightline azimuthal direction of 160° (fig. 2). Scintrex CS-3 

cesium magnetometers were used throughout the airborne survey and also for the ground base-station 

survey. Data were corrected by the contractor for diurnal variations of the Earth’s magnetic field, were 

tie-line leveled, and were microleveled, and then an International Geomagnetic Reference Field of the 

Earth was removed (Ponce and Denton, 2018a).

DISCUSSION

Generally speaking, magnetic anomalies reflect lateral changes in subsurface magnetization that 

can be used to infer subsurface geologic structure, provided that a magnetization contrast is present 

across geologic boundaries. Magnetic anomalies can, for example, reveal variations in lithology and 

delineate geologic features such as faults, plutons, volcanic rocks, calderas, and sedimentary basins, all 

of which play an important role in defining the geologic framework of the eastern Mojave Desert.

REGIONAL MAGNETIC MAP 

A broad, regional magnetic high near the southwest corner of the map area (R1) reflects the 

weakly magnetic “Teutonia batholith” of Beckerman and others (1982); the high can be used to infer 

the horizontal extent of the batholith. A magnetic low in Shadow Valley (R2) reflects nonmagnetic 

basin-fill material in a relatively shallow basin, which is about 1- to 2-km deep as defined by gravity 

data (see Ponce and Denton, 2018b). This magnetic low extends eastward across the Mescal Range, 

where it reflects nonmagnetic Paleozoic sedimentary rocks, then continues southeastward across the 

Ivanpah Mountains, where it reflects the (informally named) Ivanpah granite of Beckerman and others 

(1982), which is essentially nonmagnetic. A small, circular magnetic high in the northeastern part of 

Shadow Valley and also along the western margin of the Clark Mountain Range (R3) probably reflects 

a buried magnetic pluton, part of which may be exposed in small outcrops there (Miller and others, 

2007). Owing to its circular nature, a magnetic high along the north edge of the map area near Ivanpah 

Lake (R4) is inferred to reflect a buried pluton. Another broad magnetic high in the northeast corner of 

the map area (R5) reflects a large pluton, part of which is exposed (Jennings and others, 1977) in the 

northern part of the Lucy Gray Mountains (fig. 1).

DETAILED MAGNETIC MAP 

The detailed magnetic survey (black outline on map and on figs. 1, 2) shows a prominent 

magnetic high along the western part of the Clark Mountain Range near exposures of nonmagnetic 

undivided Mesoproterozoic gneiss and Paleozoic rocks (D1). This anomaly probably represents a 

moderately magnetic granitic body, the top of which is at a depth of about 2 to 4 km on the basis of 

matched filtering (Phillips, 2001); the granitic body also is expressed as a terrace or area of flattening 

in the gravity-anomaly map (Ponce and Denton, 2018b) owing to its relatively low density. The 

“Mountain Pass carbonatite and alkaline intrusive suite” is present along the eastern margin of this 

anomaly, and the intrusive suite may have preferentially followed the source of the magnetic feature 

(or vice versa). Although carbonatites usually have distinctive magnetic signatures because they (or 

their associated alteration zones) commonly contain magnetic minerals, physical-property data indicate 

that the Mountain Pass carbonatite (D2) is essentially nonmagnetic, having an average magnetic 

susceptibility of 0.18×10–3 SI units (Denton and Ponce, 2016). Many rocks exposed at the surface, 

which also are only weakly magnetic to essentially nonmagnetic, include shonkinite, syenite, granite, 

granitic gneiss, and dolomite (Denton and Ponce, 2016). For example, a large shonkinite body, the 

“Birthday stock,” which is essentially nonmagnetic, is expressed as a large magnetic low (D3). 

Another shonkinite body, the “Wheaton stock” (or “Pop’s pluton”) and its associated dikes (Haxel, 

2005), in the central part of the area of detailed magnetic mapping, is weakly magnetic. The “Wheaton 

stock” is associated with a low-amplitude magnetic high (D4), which indicates that this stock is more 

extensive in the subsurface than its outcrop pattern suggests. In the eastern part of the area of detailed 

magnetic mapping, a long, narrow amphibolite dike (Miller and others, written commun., 2018) 

coincides with a prominent linear magnetic high (D5). On the basis of detailed magnetic and geologic 

data, this anomaly likely represents one of a number of radiating dikes that extend to the northnorthwest from a large, inferred (buried) amphibolite intrusion, which is expressed by a pronounced 

magnetic high (D6).

The diverse physical properties (Denton and Ponce, 2016) of rocks that underlie the study area are 

well suited to geophysical investigations. The contrasts in magnetic properties between Proterozoic 

crystalline basement, Mesozoic granitoids, Cenozoic volcanic rocks, and Cenozoic unconsolidated 

alluvium, for example, produce a distinctive pattern of magnetic anomalies that can be used to infer 

subsurface geologic structure, which in turn aids in the understanding of the geologic framework and 

mineral resource potential of the eastern Mojave Desert.

ACKNOWLEDGMENTS

We thank David Grenier of CGG Canada Services Ltd. for coordinating and facilitating the 

detailed aeromagnetic survey. We also thank Dan Scheirer and Geoff Phelps of the U.S. Geological 

Survey (USGS) for their reviews, and map editor Taryn Lindquist (USGS) for comments and 

suggestions.

REFERENCES CITED

Beckerman, G.M., Robinson, J.P., and Anderson, J.L., 1982, The Teutonia batholith—A large intrusive 

complex of Jurassic and Cretaceous age in the eastern Mojave Desert, California, in Frost, E.G., 

and Martin, D.M., eds., Mesozoic-Cenozoic tectonic evolution of the Colorado River region, 

California, Arizona, and Nevada: San Diego, Calif., Cordilleran Publishers, p. 205–220.

Denton, K.M., and Ponce, D.A., 2016, Gravity and magnetic studies of the eastern Mojave Desert, 

California and Nevada: U.S. Geological Survey Open-File Report 2016–1070, 20 p., 

https://doi.org/10.3133/ofr20161070. 

DeWitt, E., Kwak, L.M., and Zartman, R.E., 1987, U-Th-Pb and 40Ar/39Ar dating of the Mountain Pass 

carbonatite and alkalic igneous rocks, southeastern California: Geological Society of America, 

Abstracts with Programs, v. 19, no. 7, p. 642.

Haxel, G.B., 2005, Ultrapotassic mafic dikes and rare earth element- and barium-rich carbonatite at 

Mountain Pass, Mojave Desert, southern California—Summary and field trip locations: U.S. 

Geological Survey Open-File Report 2005–1219, 55 p., https://pubs.usgs.gov/of/2005/1219/.

Jennings, C.W., Strand, R.G., and Rogers, T.H., 1977, Geologic map of California: California Division 

of Mines and Geology, scale 1:750,000.

Kucks, R.P., Hill, P.L., and Ponce, D.A., 2006, Nevada magnetic and gravity maps and data—A 

website for the distribution of data: U.S. Geological Survey Data Series 234, accessed April 13, 

2012, at http://pubs.usgs.gov/ds/2006/234.

Miller, D.M., Miller, R.J., Nielson, J.E., Wilshire, H.G., Howard, K.A., and Stone, Paul, 2007, 

Geologic map of the East Mojave National Scenic Area, California, plate 1 in Theodore, T.G., ed., 

Geology and mineral resources of the East Mojave National Scenic Area, San Bernardino County, 

California: U.S. Geological Survey Bulletin 2160, 265 p., 6 pls., scale 1:125,000, 

http://pubs.usgs.gov/bul/b2160/.

Olson, J.C., Shawe, D.R., Pray, L.C., and Sharp, W.N., 1954, Rare-earth mineral deposits of the 

Mountain Pass District, San Bernardino County, California: U.S. Geological Survey Professional 

Paper 261, 75 p., 13 pls.

Phillips, J.D., 2001, Designing matched bandpass and azimuthal filters for the separation of 

potential-field anomalies by source region and source type: Australian Society of Exploration 

Geophysicists, 15th Geophysical Conference and Exhibition, Brisbane, Australia, August 2001, 4 

p.

Ponce, D.A., and Denton, K.M., 2018a, High-resolution aeromagnetic survey of Mountain Pass, 

California: U.S. Geological Survey data release, https://doi.org/10.5066/P92XVOOF.

Ponce, D.A., and Denton, K.M. (Ponce, D.A., ed.), 2018b, Isostatic gravity map of Mountain Pass and 

vicinity, California and Nevada: U.S. Geological Survey Scientific Investigations Map 3412–A, 

scale 1:62,500, https://doi.org/10.3133/sim3412A.

Premo, W.R., Miller, D.M., Moscati, R.J., Holm-Denoma, C., Neymark, L., and Ponce, D.A., 2016, 

Searching for the aerial extent of the Mountain Pass carbonatite event—Evidence from U-Pb 

zircon geochronology of Proterozoic rocks in southwestern United States [abs.]: Geological 

Society of America, Abstracts with Programs, v. 48, no. 7. 

Roberts, C.W., and Jachens, R.C., 1999, Preliminary aeromagnetic anomaly map of California: U.S. 

Geological Survey Open-File Report 99–440, 14 p., https://pubs.usgs.gov/of/1999/0440/.

Stewart, J.H., and Carlson, J.E., 1978, Geologic map of Nevada: Nevada Bureau of Mines and 

Geology Map, scale 1:500,000.

Wooden, J.L., and Miller, D.M., 1990, Chronologic and isotopic framework for Early Proterozoic 

crustal evolution in the eastern Mojave Desert region, SE California: Journal of Geophysical 

Research, v. 95, p. 20,133–20,146.

D1

−255 −240 −225 −210 −195 −180 −165 −150 −135 −120 −105 −90 −75 −60 −45 −30 −15

15

30

45

60

75

90

105

120

135

150

165

180

195

210

225

240

255

270

285

300

>300

<−255

0

15

EXPLANATION

Residual magnetic intensity, in nanoteslas (nT)

Boundary of Mojave National Preserve 

(MNP)

Area of detailed aeromagnetic survey

Location of magnetic anomaly 

discussed in text

R1

R2

R3

R4

R5

D1

D2

D3

D4

D5

D6

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government

This map or plate is offered as an online-only, digital publication. Users should be 

aware that, because of differences in rendering processes and pixel resolution, 

some slight distortion of scale may occur when viewing it on a computer screen or 

when printing it on an electronic plotter, even when it is viewed or printed at its 

intended publication scale

Digital files available at https://doi.org/10.3133/sim3412B

Suggested citation: Ponce, D.A., and Denton, K.M., 2018, Aeromagnetic map of 

Mountain Pass and vicinity, California and Nevada: U.S. Geological Survey Scientific 

Investigations Map 3412–B, scale 1:62,500, https://doi.org/10.3133/sim3412B.

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3412B

Aeromagnetic Map of Mountain Pass and Vicinity, California and Nevada

By

D.A. Ponce and K.M. Denton

2018

APPROXIMATE MEAN CONTOUR INTERVAL 50 METERS DECLINATION, 2018

Base map from U.S. Geological Survey 

1:100,000-scale quadrangles: Ivanpah, 1985; 

Mesquite Lake, 1985

Universal Transverse Mercator projection, Zone 

11N, North American Datum of 1983 (NAD 83)

Data compiled in 2017–2018

GIS database and digital cartography by D.A. 

Ponce and K.M. Denton

Edited by Taryn A. Lindquist; digital cartographic 

production by Katie Sullivan

Manuscript approved for publication July 20, 2018

115°45’ 115°15’

35°17’

35°37’

40’ 35’ 25’ 20’

20’

25’

30’

115°45’ 40’ 35’ 30’ 25’ 20’ 115°15’

30’

35’

35°17’

35°37’

20’

25’

30’

35’

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3412–B