UTILIZING EARTH MRI GEOPHYSICAL DATA TO HIGHLIGHT RELATIONSHIPS BETWEEN ORE DEPOSITS AND FRAMEWORK GEOLOGY IN THE FLINT CREEK RANGE, WESTERN MONTANA

The USGS Earth Mapping Resources Initiative (Earth MRI) is a collaborative effort tasked with improving our understanding of the geologic framework of areas where undiscovered critical mineral resources may occur. To support this goal, airborne magnetic and radiometric surveys were flown over the Flint Creek Range, the Boulder batholith, and portions of the Pioneer Mountains in western Montana. These areas have been extensively mined in the past for base and precious metals and they may be favorable for future critical mineral production as a byproduct.

Although there are no known porphyry copper deposits in the Flint Creek Range, there are numerous skarn, replacement, and polymetallic vein deposits suggestive of formation from porphyry copper systems. Previous geologic mapping indicates most of these deposits are proximal to intrusive sources. New magnetic anomaly maps further corroborate this relationship and highlight a connection between the ore deposits and the more mafic plutonic units. Additionally, structural patterns obscured in the field are revealed by the juxtaposition of magnetic anomalies.

The radiometric data reflect the surface geology and generally agree with the lithologies shown on existing geologic maps. Hydrothermal alteration can often be identified using combinations of potassium, and ratios of potassium to thorium and uranium, signals; however, these signals do not indicate any hydrothermal alteration near known deposits in the Flint Creek Range. We interpret this to mean that although the ore deposits in the area likely involve hydrothermal alteration, they may be too small, and the data may not have a high enough spatial resolution to detect it.

This on-going work is being supported by new geochemistry, geochronology, and physical property measurements to further our understanding of the tectonic and metallogenic history of the Flint Creek Range.