PRELIMINARY GEOLOGIC INTERPRETATIONS DERIVED FROM EARTH MRI GEOPHYSICAL DATA COLLECTED OVER THE BULL MOUNTAIN AREA, SOUTHWEST MONTANA

The USGS Earth Mapping Resources Initiative (Earth MRI) is tasked with improving our understanding of the geologic framework of areas where undiscovered critical mineral resources may occur. To support this goal, in 2022 an airborne magnetic and radiometric survey was flown over the Boulder batholith and Bull Mountain area in southwest Montana. Relative to its surroundings, Bull Mountain exhibits orientations and structures that are difficult to reconcile with the current understanding of the regional structural history. As well, the area lies at the intersection of multiple geologic maps of varying scale with conflicting interpretations. The new geophysical data enable comparison of local features across map boundaries and further afield, thus facilitating updated geologic interpretations.

Bull Mountain is a unique topographic range located just beyond and oblique to the eastern edge of the Boulder batholith. The core of the range consists of plutonic rocks ranging from gabbro to granite that intruded into and are surrounded by Late Cretaceous Elkhorn Mountain volcanic rocks. Mesoproterozoic metasedimentary and Paleozoic sedimentary rocks make up the southern end. Basin and Range normal faults bound the range to the east and west. Older structures consisting of west-directed thrust faults on the southern and eastern sides of the area are opposite to well-established, Late Cretaceous, east-directed thrust faults that define the eastward transport of the Lombard thrust sheet.

Aeromagnetic anomalies in the area suggest the presence of volcanic plugs, dikes, and flows beneath the Tertiary and Quaternary deposits that flank the range. Patterns of magnetic anomalies also reflect the structural history of the area and mimic regional fold and thrust orientations. The aeroradiometric data agree well with mapped lithologies and provide insight into areas of hydrothermal alteration, especially in the Golden Sunlight district. Preliminary analyses of the new geophysical data support revised geologic interpretations of the Bull Mountain area; however, they require further verification by additional field mapping and geochronologic studies to unify the currently disparate maps and create one holistic interpretation that fits into the greater region.