Rare Earth Element-Bearing Rocks in Mojave Desert - 2020

Rare Earth Deposits & Mining

Denton, and three co-authors, reported in early 2020 that Mountain Pass, California (USA), located in the eastern Mojave Desert, hosts one of the world’s richest rare earth element (REE) deposits in the world. The REE-rich terrane occurs in a 2.5-km-wide, northwest-trending belt of Mesoproterozoic (1.4 Ga) stocks and dikes, which intrude a larger Paleoproterozoic (1.7 Ga) metamorphic block that extends ∼10 km southward from Clark Mountain to the eastern Mescal Range. To characterize the REE terrane, gravity, magnetic, magnetotelluric, and whole-rock physical property data were analyzed.

Geophysical data reveal that the Mountain Pass carbonatite body is associated with an ∼5 mGal local gravity high that is superimposed on a gravity terrace (∼4 km wide) caused by granitic Paleoproterozoic host rocks. Physical rock property data indicate that the Mountain Pass REE suite is essentially nonmagnetic at the surface with a magnetic susceptibility of 2.0 × 10−3 SI (n = 57), and lower-than-expected magnetizations may be the result of alteration. However, aeromagnetic data indicate that the intrusive suite occurs along the eastern edge of a distinct northwest-trending aeromagnetic high along the eastern Mescal Range.

The source of this magnetic anomaly is ∼1.5–2 km below the surface and coincides with an electrical conductivity zone that is several orders of magnitude more conductive than the surrounding rock. The source of the magnetic anomaly is likely a moderately magnetic pluton.
 Combined geophysical data and models suggest that the carbonatite and its associated REE-enriched ultrapotassic suite were preferentially emplaced along a northwest-trending zone of weakness, which has potential implications for regional mineral exploration.

Carbonatite ore deposits are the primary source of rare earth elements (REEs), which are essential in modern civilian and military applications, healthcare and medical devices, and “green” technologies. Although REEs have crustal abundances similar to common industrial-grade metals (e.g., chromium, nickel, copper, zinc, tin, and lead), large economically viable REE deposits are uncommon (e.g., Haxel et al., 2002; Long et al., 2010; Verplanck and Van Gosen, 2011).

The largest-known carbonatite-related REE deposit is in the Bayan Obo region of Inner Mongolia, China, and has produced ∼97% of the global output of REEss (Haxel et al., 2002; Long et al., 2010; Hatch, 2012; Massari and Ruberti, 2013; U.S. Geological Survey, 2015). However, for several decades prior to the late 1980s, the Mountain Pass REE deposit in California, United States (Fig. 1), supplied much of the world’s demand for industrial-grade REEs (Haxel et al., 2002; Castor, 2008).

A Proterozoic carbonatite terrane at Mountain Pass hosts the largest resource of light REEs in North America with proven reserves of ∼16 million metric tons at an ore grade of ∼8 wt% rare earth oxide (Castor, 2008; Mariano and Mariano, 2012). Since the discovery of the REE-bearing mineral bastnäsite in the carbonatite ore body in 1949, many studies have been carried out to investigate the occurrence, extent, and genesis of this REE deposit (e.g., Olson et al., 1954; Haxel, 2007; Poletti et al., 2016). Despite decades of research, the subsurface geology of this ore-bearing terrane remains largely unknown because previous studies have focused on its surface geology, geochemistry, and geochronology (e.g., Olson et al., 1954; Hewett, 1956; Burchfiel and Davis, 1971, 1988; DeWitt et al., 1987; Miller et al., 2007a; Haxel, 2007; Poletti et al., 2016).

Although geophysical studies have proven useful in understanding and constraining the geologic framework associated with REE deposits and their subsurface extent (e.g., McCafferty et al., 2014; Drenth, 2014; Shah et al., 2017), prior geophysical studies have primarily focused on localities adjacent to or outside the Mountain Pass area (e.g., Carlisle, 1982; Hendricks, 2007; Langenheim et al., 2009). The authors presented new gravity (Denton and Ponce, 2016) and magnetotelluric (Peacock et al., 2019) data and interpretation for the Mountain Pass region.

By integrating these new data sets with existing regional aeromagnetic (Kucks et al., 2006; Roberts and Jachens, 1999) and local geologic and structural data (Olson et al., 1954; Hewett, 1956; Burchfiel and Davis, 1988; Fleck et al., 1994; Castor, 2008), they produced detailed geophysical maps and models of the subsurface related to the Mountain Pass REE terrane. The authors' contribution helps elucidate the regional geologic framework at Mountain Pass by characterizing regional geologic structures in the subsurface that may have influenced the emplacement of the REE-enriched intrusive suite.