Old data, New tricks: New insights into the Stillwater Complex

Abstract:

Geophysical data has been used for many years to aid exploration of platinum group elements (PGE) in layered mafic intrusions, such as the Stillwater Complex, Montana. Using modern processing schemes we glean new information from legacy data sets which include a Fugro DIGHEM V airborne electromagnetic (AEM) and aeromagnetic survey along with ground-based gravity data. We apply a modern stochastic inversion to image the AEM data. Stochastic modeling utilizes statistics to provide parameter distributions, such as number of layers, and resistivity and thickness of each layer. A variety of modern filters were applied to the magnetic and gravity data.

The complex’s gravity high continues >25 km north of its outcrop indicating that the complex is larger than its surficial expression. Xenoliths of the complex in Cretaceous intrusions have been found 25 km to the North, confirming the gravity. The layers of the complex have been grouped into five series: Upper Banded (highest), Middle Banded, Lower Banded, Ultramafic, and Basal series (lowest). The PGE zone of interest, the J-M Reef, occurs as cumulate layers in the Lower Banded series. The aeromagnetic data clearly discerns layering, including the J-M Reef, within the complex due to varying amounts of serpentinized olivine. The aeromagnetic highs can be traced under cover and correlate with subvertical conductors in the AEM models. The complex’s resistivities are generally high, while the Reef’s resistivity is expected to be slightly lower due to higher serpentinized olivine content. Our inversions show the overlying sediments, serpentinized ultramafic rocks, and the Basal series to be conductive. Forward modeling suggests that the AEM data should not be able to resolve the Reef; however, significant amounts of olivine exist above and below indicating a potential Reef zone. By using the lab resistivity measurements from specific stratigraphic units and the resistivity distributions from the AEM models we aim to tease out variations within the Lower Banded series. Through the use of legacy data and modern processing and imaging techniques we are able to better characterize the geometry and composition of the Stillwater Complex — approaches that can be applied to other layered mafic intrusions.