AIRBORNE MAGNETIC AND RADIOMETRIC DATA HIGHLIGHT THE GEOLOGIC FRAMEWORK OF CRITICAL MINERAL RESOURCES IN NORTHERN MAINE

The metallogenically important Munsungun-Winterville belt in northern Maine comprises Ordovician volcanic rocks that are unconformably overlain by Silurian and Devonian sedimentary rocks. The region is of interest for critical minerals within multiple deposit types, including occurrences of Co, Sb, Se, and Te in Ordovician volcanogenic massive sulfide deposits such as Bald Mountain, and Fe-Mn deposits in Silurian sedimentary rocks. Dense vegetation and varied amounts of glacial cover present a challenge to geologic mapping. As part of the USGS Earth Mapping Resources Initiative, an airborne magnetic and radiometric survey was flown over a ~9600 km² area in the region in 2021, complementing multi-year USGS STATEMAP geologic mapping efforts. Within the volcanic rocks, magnetic data show numerous 100-700 nT, < 500 m-wide lineaments representing Paleozoic structures, including imbricated fault slivers containing predominantly basalt, as confirmed by mapping. Most lineaments trend NE, but about 30 km south of Bald Mountain these begin to curve to the north and become oriented NNW near this deposit. Over the sedimentary rocks, magnetic anomalies are more subdued but show narrow (<500 m wide) lineaments, indicating shallow magnetic sources. East of the Munsungun-Winterville belt near Presque Isle, ~3-5 nT anomalies coincide with known Fe-Mn-rich deposits in Silurian sedimentary rocks. Northwest of the Munsungun-Winterville belt, 5-10 nT, NE-trending lineaments (parallel to the strata) coincide with Paleozoic trachyte dikes within the Silurian and Devonian rocks. This area also exhibits 5-10 nT NW-trending lineaments that cut across structures within both sedimentary and volcanic rocks, similar to Mesozoic dikes mapped elsewhere in the eastern U.S. Radiometric data are generally subdued, partly due to glacial cover. However, a prominent Th anomaly (~900 m x 400 m) lies within a slightly larger K anomaly. Geologic mapping shows that the K anomaly corresponds to a trachyte intrusion rich in potassium feldspar. Whole-rock analyses indicate that the Th anomaly is closely associated with pervasive REE-Nb-Zr mineralization in brecciated trachyte, revealing a previously unknown critical mineral deposit in Maine. See Wang et al. (this session) for additional details regarding this deposit.