NEW AIRBORNE GEOPHYSICAL DATA ILLUMINATE ORDOVICIAN VOLCANIC ARC SYSTEMS OF NORTHERN MAINE

The Munsungun-Winterville belt is an inlier of Ordovician volcanic arc rocks in northern Maine. The arc rocks, though largely obscured by Devonian sedimentary cover, record the processes of arc creation, intra-arc extension, and arc evolution. The arc rocks host polymetallic volcanogenic massive sulfide (VMS) deposits and occurrences, including the large deposit at Bald Mountain that contains critical mineral resources of Co, Sb, Se, and Te. Geophysical data such as magnetics and gravity are capable of imaging buried basement rocks and can be used to identify patterns of magmatism, structural deformation, and hydrothermal alteration to help define the geologic controls on mineralization and assist in predicting the locations of other nearby VMS deposits.

In May-July 2021, the U.S. Geological Survey Earth Mapping Resources Initiative (Earth MRI) collected new airborne magnetic and radiometric data in northern Maine with the goal of advancing our understanding of the setting of volcanic arc terranes in the region and the processes associated with VMS mineral systems. The survey area, centered on the Munsungun-Winterville belt, is about 9,600 km². Strong positive magnetic anomalies over volcanic terranes suggest an association with large igneous complexes. An abrupt change in magnetic fabric to the west of the mapped extent of the Munsungun-Winterville belt may imply a fault-controlled contact with surrounding Devonian sedimentary rocks. Narrow (<500 m) linear anomalies within those sedimentary cover rocks are either parallel to the NE structural trend, consistent with the presence of magnetic minerals in these strata, or are E-SE-oriented, oblique to the structural trend, suggesting the presence of buried dikes. Preliminary radiometric data reveal a local (~700-m long and 300-m wide) Th-U anomaly associated with highly altered tuff. Future work will include analyses of derivative and frequency-filtered magnetic maps and construction of models that integrate bedrock mapping and borehole geology to better understand igneous and structural features. The geophysical data and methods used in this work will improve our understanding of the ancient processes of supra-subduction zone arc formation and accretion in this region of the Northern Appalachians.