EARTH MRI GEOPHYSICAL DATA PROVIDE INSIGHTS INTO THE GEOLOGY AND MINERAL SYSTEMS OF THE RADERSBURG AND ELKHORN MINING DISTRICTS, MONTANA

The USGS Earth Mapping Resources Initiative (Earth MRI) is tasked with improving the Nation’s understanding of critical mineral resources and developing state-of-the-art geologic maps. To help address this goal, a high-resolution airborne magnetic and radiometric survey was flown over the Boulder batholith, including the Radersburg and Elkhorn mining districts in southwest Montana. The survey was flown along east-west flight lines spaced 200 m at a nominal terrain clearance of 100 m covering approximately 6,200 km². The mineral deposits in these districts are hosted in complexly folded Mesoproterozoic to Mesozoic sedimentary rocks that are intruded and overlain by Late Cretaceous volcano-plutonic complexes.

The magnetic data and maps of equivalent uranium, thorium, and potassium exhibit contrasting patterns between the sedimentary rocks, igneous rocks, and mineral deposits. The Devil’s Fence anticline exposes Paleozoic and Mesozoic carbonate-bearing sedimentary rocks that have low radiometric response relative to other rocks mapped within the structure. Derivative maps of magnetic anomalies show linear and circular patterns within the folded sedimentary rock package suggesting intrusions with pre-deformation sill and plug-like geometries, respectively. Similar linear features indicate folding continues in the bedrock beneath Quaternary sediments. The Elkhorn district contains skarn and carbonate replacement deposits related to diorite intrusions that exhibit strong magnetic response and low radiometric values; however, the deposits themselves are not apparent in the geophysical data. A combination of potassium and ratios of potassium to thorium and uranium highlight previously mapped hydrothermal alteration associated with porphyritic intrusions south of the skarn deposits. In the Radersburg district, a similar radiometric response having diameter ~1500 m near the Ohio-Keating mine is also indicative of hydrothermal alteration. Drilling in the 1970s indicated a gold-rich porphyry copper system in the district beneath 100-150 m of alluvium. Magnetic anomalies having diameter ~1600 m observed over Quaternary sediments south and east of the mine may be produced by granodiorite stocks that underlie reported granodiorite porphyry dikes and hydrothermally altered rocks.