A THEORETICAL GEOPHYSICAL ANALYSIS OF A SUBSURFACE CARBONATITE BODY AT MOUNTAIN PASS, CA

The Mountain Pass mine in California is one of the world's largest mass producers of light rare earth elements (REEs). These REEs are essential components for green energy and defense technologies. Neodymium and praseodymium are currently the most economically important REEs extracted from the mine and are used in high-strength magnets. A carbonatite stock hosts the REE ore at Mountain Pass and is an irregularly shaped intrusion that generally strikes north and dips steeply (30-60 degrees) to the west. The ore body is dissected by faults, including a large N50W left-lateral fault (Celebration fault) that bisects the stock, displacing the southern part of the ore body to the southeast and the northern part of the ore body to the northwest. It is unknown whether the ore body is fault-repeated elsewhere in the subsurface, which is vital for identifying minable resources. We are determining if it is possible to detect the existence of possible fault-offset portions of the carbonatite body near the Mountain Pass mine using gravity data. If it is possible, this would allow us to detect additional subsurface carbonatite bodies within the same area. For simplicity, we model the irregular-shaped carbonatite body as a sphere and vary the sphere’s size, depth, and density contrast with the surrounding country rock to understand relevant conditions for detection. Based on field experience, we estimate that a subsurface carbonatite body should be detectable if it produces a gravity anomaly with an amplitude of >+1 milligal (mGal). If the amplitude ranges between +0.5 to +1 mGal, it is possible but challenging to detect. Using estimates of the known carbonatite’s density and volume, we determined that additional carbonatite bodies with a density contrast of +0.3 g/cm³ and a radius of 200 m are detectable at a depth of 50 m, and possibly detectable at a depth of up to 150 m.