AIRBORNE GEOPHYSICAL METHODS FOR EVALUATION OF CRITICAL MINERAL DEPOSIT POTENTIAL

Airborne geophysical methods provide subsurface geologic information over potentially large areas that can be used for a number of applications. Surveys that acquire magnetic field and radiometric measurements are the most frequently used for regional studies. Other commonly used methods include airborne gravity/gravity gradiometry and electromagnetic (EM) surveys, but these typically incur much higher costs, which commonly limits their use to much smaller areas. The objectives of magnetic, gravity, and EM methods are to image variations in the magnetic minerals, density, or resistivity of rocks, respectively. These variations are typically generated by differences in rock composition and/or the presence of structural features. For magnetic and gravity methods the maximum depth imaged depends on the survey configuration and the nature of the source rocks, whereas the range for EM methods is typically 100-500 m, depending on the system. Radiometric methods work as “remote geochemistry,” highlighting variations in K, Th, and U from natural radioactivity of materials in the upper ~1 m.

Success in identifying critical mineral deposits from airborne geophysical surveys depends highly upon the deposit type. In some cases airborne geophysical data can directly image deposits, whereas in others they are more important for providing a geologic context. For example, deposits associated with mafic minerals, such as the layered mafic intrusions hosting platinum group elements, or iron oxide-apatite deposits hosting rare earth elements (REEs), show strong correlations with magnetic and sometimes gravity anomalies. Similarly, radiometric data may image deposits typically associated with Th or U, such as sedimentary heavy mineral sands (Ti, Zr, REEs) or phosphate deposits (REEs), and EM data highlight areas rich in graphite or in sulfides. In contrast, certain basin brine or hydrothermal systems might not show anomalies over deposits, but geophysical data can assist with mapping structural or other geologic features related to the mineralizing system.

Here we present examples of airborne geophysical data collected over several critical mineral deposits, considering how these methods can assist evaluation and assessment as well as caveats to consider.