Magnetotelluric Imaging of a Carbonatite Terrane in the Southeast Mojave Desert, California and Nevada

Abstract

SUMMARYThe southeast Mojave Desert hosts one of the world's largest rare earth element (REE) deposits at Mountain Pass, California. Although surface geology has been studied, a full understanding of the carbonatite and associated intrusive suite complex requires subsurface geophysical characterization. In this study, a combination of geophysical methods, including magnetotelluric (MT), magnetics, and gravity are used to create a two-dimensional (2D) geophysical model to a depth of about 10 km. An electrically conductive body is found 2-3 km below and west of the deposit that is associated with a magnetic high that could be connected to a deeper (10 km) conductive body related to possible intrusions or hydrothermal systems. The carbonatite body coincides with a steep magnetic gradient and a bench or terrace in the gravity data that may reflect relative lower-density intrusive rocks. Although carbonatite rocks are typically magnetic, the carbonatite rocks, associated intrusive suite, and host rocks in this area are essentially non-magnetic. Combined geophysical data indicate that the enriched REE deposit may be related to a regional extensive hydrothermal alteration event.

Geophysical characterization of a Proterozoic REE terrane at Mountain Pass, eastern Mojave Desert, California, USA

“…Multiple starting models and resistivity structures were tested to ensure robustness. The preferred model has a normalized root-mean-square error of 1.73 using only the TM mode with an error floor of ~20% for the apparent resistivity and an error floor of 1.4° for the impedance phase (Peacock et al, 2016). The data and model response fits are shown in Figure 5A, where the model represents the data well with only a few exceptions.…”

Geophysical Characterization of a Rare Earth Element Enriched Carbonatite Terrane at Mountain Pass, California Eastern Mojave Desert

“…Fluid-filled interconnected pore spaces can increase the overall bulk rock conductivity of the subsurface features (Olhoeft, 1985;Peacock et al, 2016). Electrically conductive features are especially pronounced when compared to competent resistive basement rocks, which can differ by several orders of magnitude.…”

“…Moreover, conductive properties of the subsurface can be calculated from the relationship between components of the measured electric and magnetic field variations, using transfer functions (Cagniard, 1953;Berdichevsky and Dmitriev, 2008;Chave and Jones, 2012;Peacock, 2016).…”

“…Multiple starting models and resistivity structures were tested to insure model robustness. A preferred model has a normalized root-mean-square error of 1.73 using only the transverse magnetic model with an error floor of ~20% for the apparent resistivity and an error floor of 1.4 degrees for the impedance phase (Peacock, 2016).…”