Airborne radiometric maps of Mountain Pass, California
<p>Geophysical investigations of Mountain Pass and vicinity were begun as part of an effort to study regional crustal structures as an aid to understanding…
Public-domain full text preserved in the Mountain Man Mining Library. Original source: pubs.usgs.gov.
Equivalent uranium concentration (eU), in parts per million Equivalent thorium concentration (eTh), in parts per million Potassium (K) concentration, in percent
MNP 4a REFERENCES CITED 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. International Atomic Energy Agency, 2003, Guidelines for radioelement mapping using gamma-ray spectrometry data: International Atomic Energy Agency, Technical document IAEA-TECDDOC-1363, 173 p. Miller, D.M., Miller, R.J., Nielson, J.E., Wilshire, H.G., Howard, K.A., and Stone, P., 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, ://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. Ponce, D.A., and Denton, K.M., 2019, High-resolution airborne radiometric survey of Mountain Pass, California: U.S. Geological Survey data release, ://doi.org/10.5066/P9ENLS6D. 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. Verplanck, P.L., Van Gosen, B.S., Seal, R.R., and McCafferty, A.E., 2014, A deposit model for carbonatite and peralkaline intrusion-related rare earth element deposits: U.S. Geological Survey Scientific Investigations Report 2010–5070–J, 58 p., ://doi.org/10.3133/sir20105070J. 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. EXPLANATION FOR MAPS A, B, AND C Location of radiometric anomaly discussed in text Outline of carbonatite body and associated alkaline instusive suite Carbonatite or alkaline intrusive dike Faults—Solid where location is accurate; dashed where location is inferred; dotted where location is concealed Fault, unspecified or unknown sense of slip Thrust fault—Sawteeth on upper plate Normal fault—Hachures on upper plate Boundary of Mojave National Preserve (MNP) INTRODUCTION Geophysical investigations of Mountain Pass, California, 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 encompasses Mountain Pass, which is host to one of the world’s largest rare earth element (REE) carbonatite deposits. The deposit is found along a north-northwest-trending, fault-bounded Paleoproterozoic 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, at about 1.7 Ga (Wooden and Miller, 1990). The Paleoproterozoic rocks were intruded by a Mesoproterozoic (1.4 Ga) carbonatite body and associated ultrapotassic alkaline intrusive suite (Olsen 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). METHODS A high-resolution radiometric survey of Mountain Pass was flown by CGG Canada Services Ltd. (CGG). This helicopter survey, which was flown at flightline spacings of 100 and 200 m, a flightline azimuthal direction of 70°, a nominal flightline elevation above ground of 70 m, and an average sampling distance of about 30 m, consists of about 1,814 line-kilometers (fig. 2). Tie lines, which were spaced at 1-km intervals, were flown in a flightline azimuthal direction of 160°. Closely spaced lines flown at low elevation are needed to resolve small-scale features and improve signal-to-noise ratio. Data were collected using a Radiation Solutions RS-500 spectrometer and processed by CGG, using standard radiometric-surveying techniques (see, for example, International Atomic Energy Agency, 2003) that include corrections for both aircraft and cosmic background radiation, radon background, Compton scattering effects, and variations in altitude. Aeroradiometric surveys measure the intensity and energy spectrum of gamma-ray radiation from the three most common naturally occurring radioelements: potassium (40K), thorium (232Th), and uranium (238U). For 232Th and 238U, the source of the gamma-rays comes from their thallium (208Tl) and bismuth (214Bi) decay products, respectively, and, thus, concentrations for Th and U are referred to as “equivalent concentration,” assuming radioactive equilibrium. The concentrations of these radioelements can be used together to estimate changes in geochemistry and lithology. Data, which were gridded at a 20-m interval, are expressed as percent K (Map A), parts per million (ppm) equivalent Th (eTh) (Map B), and ppm equivalent U (eU) (Map C). Although gamma rays are of high energy and frequency, they attenuate rapidly in rocks and soil, partly owing to Compton scattering, and they can only be detected from about the upper 50 centimeters (cm) of the Earth’s surface and mostly from the upper 30 cm (International Atomic Energy Agency, 2003, p. 114). DISCUSSION Carbonatite deposits typically have distinctive geophysical signatures because they are relatively dense, magnetic, and radiogenic. Specifically, the carbonatite and alkaline intrusive suite at Mountain Pass is ultrapotassic and contains relatively significant amounts of K, Th, and U, which can be delineated using airborne radiometric surveys. Values for K concentration range from −0.12 to 3.59 percent, with a mean of 1.22 percent; for eTh, from −0.74 to 180.20 ppm, with a mean of 8.16 ppm; and for eU, from −0.22 to 17.03 ppm, with a mean of 2.03 ppm. Negative concentrations were obtained over water or some alluvial deposits. Verplanck and others (2014) provided a deposit model for carbonatite- and alkaline-intrusion-related REE mineralization, and they described some of the geophysical tools used to assess these deposits. These radiogenic features are briefly discussed below (from northwest to southeast), and their locations are labeled on the maps and figure 1 (for example, locs. 1a, 3b): Very low concentrations of K, eTh, and eU correlate with Paleozoic dolomite, limestone, and other sedimentary rocks that are thrust against Proterozoic basement terrane along the western margin of the study area (locs. 1a, 1b, 1c) Moderate concentrations of K and low concentrations of eTh and eU delineate Mesozoic volcanic rocks in the Mescal Range (loc. 2) Moderate concentrations of K, eTh, and eU correlate with a small felsic body in the Clark Mountain Range (loc. 3a) and a Jurassic granite in the Ivanpah Mountains (loc. 3b) High concentrations of K, eTh, and eU correlate with the Birthday shonkinite (loc. 4a), the Sulphide Queen carbonatite body (loc. 4b), and the remainder of the alkaline intrusive suite composed of shonkinite, syenite, and granite (locs. 4c through 4h). High concentrations of K in the alkaline intrusive suite are due primarily to biotite mica, phlogopite, and potassium feldspars. High concentrations of eTh and eU are present because Th and U have the same valance and similar atomic radii as REEs and can substitute in various REE-related minerals Concentrations of K are variable across the Paleoproterozoic gneissic terrane along the central and eastern parts of the study area. Numerous linear, northwest-trending, moderate concentrations of K, eTh, and eU throughout the gneissic terrane (locs. 5a through 5i) reflect various changes in basement lithology or geochemistry, structure, or faults Prominent and mostly northeast-trending, moderate concentrations of K, eTh, and eU along the western margin of Ivanpah Valley (locs. 6a through 6f) reflect alluvial and eolian deposits derived from the Proterozoic basement terrane Moderate concentrations of K, eTh, and eU along Piute Valley (loc. 6g) are probably derived from alluvial and eolian deposits from a combination of Paleozoic metavolcanic and Proterozoic basement rocks Some high concentrations of K, eTh, and eU are associated with anthropogenic features such as tailings, dumps, and disturbed areas west of the carbonatite (locs. 7a, 7b, 7c) and an isolated eTh and eU anomaly southeast of the carbonatite body (loc. 7d). The diverse physical properties of rocks that underlie the study area are well suited to geophysical investigations. Contrasts in radiogenic signatures between Paleoproterozoic crystalline basement, rocks of the Mesoproterozoic carbonatite body and the associated alkaline intrusive suite, Paleozoic carbonate rocks, Mesozoic granitoids, Tertiary volcanic rocks, and unconsolidated alluvium, for example, produce a distinctive pattern of radiometric anomalies that can aid in understanding 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 aeroradiometric survey. We also thank Jared Peacock and Daniel Scheirer of the U.S. Geological Survey (USGS) for their reviews, and map editor Taryn Lindquist (USGS) for comments and suggestions. Figure 1. Map showing simplified geology of study area (modified from Olson and others, 1954; Miller and others, 2007). Black outline, area of radiometric survey; green line, boundary of Mojave National Preserve (MNP). Base map from U.S. Geological Survey 1:100,000-scale quadrangles: Ivanpah, 1985; Mesquite Lake, 1985; contour interval, 50 m; thin, red horizontal and vertical lines are township and range boundaries. Figure 2. Index map showing location of flightlines (blue lines) within area of radiometric survey (thick black outline). Green line, boundary of Mojave National Preserve (MNP). Base map from U.S. Geological Survey 1:100,000-scale quadrangles: Ivanpah, 1985; Mesquite Lake, 1985; contour interval, 50 m; thin, red horizontal and vertical lines are township and range boundaries. 35°35’ 115°30’ 115°35’ 115°25’ 35°30’ 35°25’ 1b 3b 7a 7b 7c 4a 4b 4d 4e 4f 4h 1a 4g 5a 5b 5d 5e 7d 6a 6b 6d 6e 6f 5f 5g 5h 3a 6g Mountain Pass Ivanpah Valley Clark Mountain Range la rk M ount ain Ra nge an pa h M ou nt ai n s
P iut e Va lle y
Mes cal Ra nge 4 KILOMETERS 2 MILES EXPLANATION Quaternary alluvium Quaternary gravel Granitic rocks, undivided Mesozoic volcanic rocks Mesozoic sandstone Paleozoic limestone Cambrian dolomite Cambrian to Precambrian sedimentary rocks Mesoproterozoic carbonatite Mesoproterozoic syenite and granite Mesoproterozoic shonkinite Paleoproterozoic granite and gneiss Location of radiometric anomaly discussed in text Carbonatite or alkaline intrusive dike Contact Faults—Solid where location is accurate; dashed where location is inferred; dotted where location is concealed Fault, unspecified or unknown sense of slip Thrust fault—Sawteeth on upper plate Normal fault—Hachures on upper plate 4a 35°35’ 115°35’ 115°30’ 115°25’ 35°30’ 35°25’ 4 KILOMETERS 2 MILES Map A—POTASSIUM (K) Map B—THORIUM (eTh) Map C—URANIUM (eU) 115°45' 30' 25' 30°25' 30' 30°35' 30°25' 30' 30°35' 25' 115°35' 30' 115°45' 30' 25' 30°25' 30' 30°35' 30°25' 30' 30°35' 25' 115°35' 30' 115°45' 30' 25' 30°25' 30' 30°35' 30°25' 30' 30°35' 25' 115°35' 30' Mountain Pass Piute Valley Clark Mountain Range Ivanpah Mountains Ivanpah Valley Mescal Range Clark Mountain Range Mountain Pass Piute Valley Clark Mountain Range Ivanpah Mountains Ivanpah Valley Mescal Range Clark Mountain Range Mountain Pass Piute Valley Clark Mountain Range Ivanpah Mountains Ivanpah Valley Mescal Range Clark Mountain Range 1b 3b 7a 7b 7c 4a 4b 4d 4e 4f 4h 1a 4g 5a 5b 5d 5e 7d 6a 6b 6d 6e 6f 5f 5g 5h 3a 6g 1b 3b 7a 7b 7c 4a 4b 4d 4e 4f 4h 1a 4g 5a 5b 5d 5e 7d 6a 6b 6d 6e 6f 5f 5g 5h 3a 6g 1b 3b 7a 7b 7c 4a 4b 4d 4e 4f 4h 1a 4g 5a 5b 5d 5e 7d 6a 6b 6d 6e 6f 5f 5g 5h 3a 6g 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 on-line 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 ://doi.org/10.3133/sim3412C Suggested citation: Ponce, D.A., and Denton, K.M., 2019, Airborne radiometric maps of Mountain Pass, California: U.S. Geological Survey Scientific Investigations Map 3412–C, scale 1:62,500, ://doi.org/10.3133/sim3412C. ISSN 2329-132X (online) ://doi.org/10.3133/sim3412C Airborne Radiometric Maps of Mountain Pass, California By D.A. Ponce and K.M. Denton CONTOUR INTERVAL 50 METERS APPROXIMATE MEAN DECLINATION, 2019 Data compiled in 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 April 19, 2019 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) U.S. Department of the Interior U.S. Geological Survey Scientific Investigations Map 3412–C