- Document: Report (3 MB pdf)
- Larger Work: This publication is Chapter C of Geophysical and geologic maps of Mountain Pass and vicinity, California and Nevada
- Data Release: Data Release - High-resolution airborne radiometric survey of Mountain Pass, California
- Open Access Version: Publisher Index Page
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Geophysical investigations of Mountain Pass and vicinity were begun 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, 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. 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. The Paleoproterozoic rocks were intruded by a Mesoproterozoic (1.4 Ga) ultrapotassic alkaline intrusive suite and carbonatite body. The intrusive rocks include, from oldest to youngest, shonkinite, mesosyenite, syenite, quartz syenite, potassic granite, carbonatite, carbonatite dikes, and late shonkinite dikes.
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.
A high-resolution radiometric survey of Mountain Pass was flown by helicopter over parts of the Clark Mountain Range, Mescal Range, and Ivanpah Mountains. 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.
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.
Ponce, D.A., and Denton, K.M. (D.A. Ponce, ed.), 2019, Airborne radiometric maps of Mountain Pass, California: U.S. Geological Survey Scientific Investigations Map 3412–C, scale 1:62,500, https://doi.org/10.3133/sim3412C.
ISSN: 2329-132X (online)
Additional publication details
|Publication Subtype||USGS Numbered Series|
|Title||Airborne radiometric maps of Mountain Pass, California|
|Series title||Scientific Investigations Map|
|Publisher||U.S. Geological Survey|
|Publisher location||Reston, VA|
|Contributing office(s)||Geologic Hazards Science Center, Geology, Minerals, Energy, and Geophysics Science Center|
|Description||1 Plate: 39 x 32 inches; Data Release|
|County||San Bernardino County|
|Online Only (Y/N)||Y|
|Google Analytic Metrics||Metrics page|