3D Magnetotelluric Inversion and 3D Depth-to-Basement Gravity Inversion for Great Basin Minerals Exploration

Abstract:

The future of successful minerals exploration is in basins. Consider the distribution of gold deposits in Nevada. While almost 60% of Nevada is pediment, the pediment hosts less than 15% of the known gold deposits. Most existing gold mines are in outcrop. This means the real distribution of deposits has virtually nothing to do with the distribution of known mines. The probability of gold deposition in the pediment is the same as in outcrop. It's just that outcrop has been heavily explored while the basins are under explored. Conventional geologic exploration works well in outcrop but geology alone offers little in covered areas. Over the past several years we've developed a practical strategy for basin exploration. It consists of using 3D gravity depth-to-basement inversion to roughly track structures into the pediment then focused AMT data acquisition and 3D inversion to detect areas which exhibit substantial alteration. Typical earth resistivities range over ten orders of magnitude. This makes resistivity imaging supremely sensitive to small resistivity property changes. The subsurface resistivity distribution shows alteration zones which are frequently the conduits for mineralizing fluids. What we detect are these, hopefully mineralized, alteration zones. Consider the following set of images. No vertical exaggeration is used for these images. The top image is a 12 km by 8 km view of topography from an elevation of about 10 km. Mineralization was found in shallow drill holes at the north of the image. A gravity survey was conducted and the data inverted to remove the alluvium. The middle image shows the basement topography. The 3D depth-to-basement inversion clearly shows a set of N-S faults extending into the pediment. Knowledge of the location of these faults allowed us to focus our AMT survey and 3D inversion along these faults. An isosurface from the 3D inversion is shown as the lower image.