PRELIMINARY RESULTS OF GEOLOGIC MAPPING IN THE LANDERS AND NORTH HALF OF THE YUCCA VALLEY NORTH 7.5' QUADRANGLES, SAN BERNARDINO COUNTY, CALIFORNIA

The California Geological Survey’s Mineral Resources Program mapped the Landers and north half of the Yucca Valley North 7.5’ quadrangles over the 2021-2022 USGS STATEMAP cycle. The project area is located about 50 miles east of Riverside in south-central San Bernardino County. Detailed mapping and new geochronology and geochemical results advance the understanding of the geologic history and critical mineral deposits in the quadrangles.

Bedrock within the project area consists primarily of Proterozoic metamorphic rocks intruded by a suite of Mesozoic plutons ranging from monzodiorite to granite. Proterozoic and Mesozoic rocks are also intruded by a suite of dikes including latite dikes, quartz porphyry dikes, pegmatite dikes, felsic dikes, and mafic dikes. Proterozoic rocks and Mesozoic intrusions are unconformably overlain by Tertiary basalts at Black Lava Butte and Flat Top Mesa. A Miocene age (8.62 +/- 0.02 Ma) at Flat Top Mesa was determined with new Ar/Ar geochronology. Geochronology on the Proterozoic and Mesozoic rocks is in progress.

Quaternary deposits within the project area are cut by Holocene faults including the Johnson Valley Fault Zone, the Kickapoo Fault, and the Homestead Valley Fault. The Johnson Valley Fault Zone ruptured near Landers in 1992, producing a M7.3 earthquake. Previously unmapped faults of various ages were mapped using LiDAR, aerial imagery, and field mapping.

Proterozoic and Mesozoic rocks within the project area host a variety of mineralization. The California Blue pegmatite deposit located in the southwestern corner of the Landers quadrangle hosts beryllium and rare-earth elements. New geochemical data indicate the metamorphic units have elevated concentrations of rare-earth elements, zirconium, thorium, and uranium compared to crustal abundances (Mason, 1966). Felsic dikes in the eastern Landers quadrangle have elevated concentrations of lithium, lead, bismuth, tantalum, rare-earth elements, gallium, and cesium compared to crustal abundances (Mason, 1966). Future electron microprobe analysis may identify which minerals host these elements.