Aeromagnetic map of Mountain Pass and vicinity, California and Nevada

Ponce, David A.- ponce@usgs.gov, Denton, Kevin M.- kmdenton@usgs.gov · 2018 · Report

Magnetic investigations of Mountain Pass and vicinity were begun as part of an effort to study regional crustal structures as an aid to understanding the

This 2018 document, Aeromagnetic map of Mountain Pass and vicinity, California and Nevada, is preserved in the Mountain Man Mining Library for research and reference. Original source: pubs.usgs.gov.

MNP MNP MNP Mountain Pass 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 Primm Valley Mountains Valley MNP MNP MNP NEVADA CALIFORNIA 5 MILES 10 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 70o azimuth 800 m 305 m AG E-W 1,600 m 305 m AG E-W 115°45' 115°15' 35°17' 35°37' 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 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 CONTOUR INTERVAL 50 METERS APPROXIMATE MEAN 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' 115°15' 40' 35' 25' 20' 30' 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

Plates & figures from the original