Old data — Modern interpretations understanding the value of inverse modeling on the Stillwater Complex, Montana

Abstract

The Stillwater Complex in Montana hosts significant platinum-group element mineralization; one deposit, the J-M Reef 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 recently 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. In addition, individual data channels were analyzed to determine the location of anomalies along flight line and their shapes at depth. After receiving the AEM data, we used GeoBiPy, a modern stochastic inversion method (https://doi.org/10.5066/P9K3YH9O), to aid in interpretation of structure (layering and faulting) and the extent and distribution of the mineralized masses of rock. Information from these new inversions include probabilities of layer interfaces and the resistivities associated with these layers. One drawback is that this stochastic inversion is unable to account for magnetic susceptibility, in which elevated values are expected in this area. The effects of higher magnetic susceptibilities result it lowered data amplitudes, especially in the in-phase components. Our dataset exhibits these lowered amplitudes, so much so, that much of the in-phase data are negative. More recent work using an older deterministic inversion program that is able to account for magnetic susceptibility (EM1DFM, https://gif.eos.ubc.ca/) shows added utility in inverse modeling for both resistivity and susceptibility. These inversion results support existing knowledge of the region but are also helping guide and refine new interpretations of the geologic structure. These results enable us to map and connect known faults, both laterally at the surface and at depth, but also to identify unmapped faults. These models are also able to provide strike and dip information for these structures. We found that stratiform, disseminated sulfide mineralization (reef-type mineralization) have minimal EM response because the rock units are thin (<1 m to several meters thick), elongated, sub-vertical structures; their lower resistivities are washed out with the footprint of the AEM system. While we cannot detect these reef-type mineralization bands, we are able to image the sepentinized olivine package surrounding them. The magnetic susceptibly results allow us to image mafic dikes, the iron formation, and peridotite zones. We also see conductive regions in known massive landslides and are currently working on understanding the nature of these higher conductivities. Known areas with contact-type mineralization (large volumes of rock with disseminated sulfide minerals) are characterized by lower resistivities. We use our interpretations of both lower resistivities and elevated susceptibilities to extend areas with mineral potential to regions that are poorly explored and characterized.