OLD DATA – OLD TRICKS – MODERN INTERPRETATIONS; UNLOCKING THE SECRETS OF THE STILLWATER COMPLEX, MT

Abstract:

The Stillwater Complex in Montana hosts significant platinum-group element mineralization and has been mined since 1986. In 2000, a frequency-domain airborne electromagnetic (AEM) and magnetic survey was flown to aid in exploring for new deposits. The US Geological Survey (USGS) was given access to this dataset and has since made the data public (https://doi.org/10.5066/P9OLEU17).

The initial investigations into the AEM dataset utilized the raw data to create apparent resistivity maps, and visual inspection of individual data channels was used to analyze and determine anomaly locations and their shapes at depth. The USGS used GeoBiPy, a Monte Carlo inversion algorithm (https://doi.org/10.5066/P9K3YH9O), to invert these data and aid in interpretation of structure and the distribution of the mineralized regions. Information from these stochastic inversions include probabilities of layer interfaces and the resistivities associated with these layers. However, due to the low and negative amplitudes found throughout this survey it became clear that magnetic susceptibility plays a large role here and we utilized EM1DFM (https://gif.eos.ubc.ca/) to invert for both susceptibility and resistivity.

Inversion results from both programs support existing knowledge of the region and are helping guide and refine new interpretations of the geologic structure. These results enable us to map and connect known faults, both at the surface and at depth (~150 m), but also to identify unmapped faults. We found that the reef-type mineralization found here has minimal EM response due to thin sub-vertical rock units (<1 to several meters thick) whose low resistivities are largely masked due to the footprint of the AEM system. We also see conductive regions in known massive landslides and are currently working on understanding the nature of these anomalies. Through our modeling efforts we are able to interpret some conductive anomalies in poorly explored areas as having mineral potential.