ELECTRICAL RESISTIVITY CHARACTERIZATION OF BEDROCK FOR CONCEALED MINERAL DEPOSITS BENEATH BASIN FILL SOUTH OF THE BODIE-AURORA VOLCANIC FIELD, CALIFORNIA AND NEVADA

The contiguous United States is a mature exploration/mining terrane and has been heavily explored for large deposits in exposed bedrock. Future large discoveries are more likely to be found concealed by basin sediments or other types of rocks. Over half of the land surface in the Basin and Range Geologic Province is covered by Cenozoic basin fill and the potential for undiscovered mineral deposits in bedrock concealed by basin fill is large. Understanding bedrock lithology concealed by basin fill in this region is important for both finding concealed deposits and delineating permissive terranes required for regional mineral resource assessments.  

U.S. Geological Survey scientists are characterizing exposed and concealed bedrock by the use of textural classification of Earth’s magnetic field data. As an additional constraint to the delineation of concealed lithology, magnetotelluric data are being used to characterize the electrical properties of bedrock beneath basin fill in the Mono Basin area in Mono County, California, and Mineral County, Nevada. The magnetotelluric method is a passive surface geophysical technique that uses the Earth's natural electromagnetic fields to investigate the electrical resistivity structure of the subsurface from depths of tens of meters to tens of kilometers. In the upper crust, the resistivities of geologic units are largely dependent upon their fluid content, pore volume porosity, interconnected fracture porosity, and conductive mineral content. At greater depths, higher subsurface temperatures cause higher ionic mobility that reduces rock resistivities.  

Regional magnetotelluric soundings were collected in June 2016 primarily along four northwest-southeast profiles ranging in length from 15 to 25 kilometers. Sounding locations were chosen to cross known mineral alteration zones and to characterize bedrock beneath covered potential extensions of deposits. The magnetotelluric profile soundings were first inverted with a 2-D resistivity inversion for the initial analysis and later by a 3-D magnetotelluric inversion because all of the observed data indicated a 3-D electromagnetic response. The resistivity profiles obtained from these inversions both contribute to and constrain lithologic characterization derived from Earth’s magnetic field data.