MANGANESE MINERALIZATION IN AROOSTOOK COUNTY, MAINE: INSIGHTS FROM A 2021 EARTH MAPPING RESOURCE INITIATIVE [MRI] AIRBORNE MAGNETIC SURVEY

In 2021, the U.S. Geological Survey, under the Earth MRI flew a high-resolution airborne magnetic and radiometric survey over the Munsungun-Winterville belt of the Northern Maine Appalachians to aid evaluation of critical mineral resources. Manganese [Mn] deposits of the Aroostook County Northern District, located in the eastern part of the survey area, are hosted by sedimentary rocks of the Silurian New Sweden Formation. Mn is a critical mineral resource for which the U.S. is 100% reliant on imports. The new aeromagnetic data help (1) delineate areas likely to host previously unmapped Mn occurrences and (2) contribute to refining a genetic model for Mn deposition in similar geologic settings. Mn deposits in Aroostook County, which were first explored in the 1940’s, occur as layers or lenses interbedded with slate and argillaceous limestone deposited in a shallow marine environment. Low-grade Mn ores are found in both hematitic shale and siliceous limestone and have a significant amount of Fe. The Mn-Fe deposits in the northern district are part of the largest identified potential Mn resource in the US.

The new aeromagnetic data show a spatial correlation between two prominent sub-parallel, positive magnetic anomalies with known Mn-Fe occurrences, located near the contact between the New Sweden Formation to the northwest and the Ordovician Carys Mills Formation to the southeast. The relationship between the Mn deposits and magnetic anomalies was not previously recognized. The anomalies are narrow in width (<1 km) and small in amplitude (<30 nT), have an arcuate shape, and trend NE-SW in the north turning to a N-S direction and converging in the south. The continuation of the magnetic anomalies away from known Mn-Fe occurrences suggests an extension of the mineral system.. Moreover, other linear magnetic anomalies newly identified in this survey within the New Sweden Formation suggests additional Mn prospectivity. In addition to these sedimentary short-wavelength magnetic anomalies, the data also reveal a deeper magnetic interface between the New Sweden and Carys Mills formations. Recent bedrock mapping suggests this represents a fault contact rather than a simple lateral facies change. The Mn deposition may therefore be structurally controlled, but this has not been established with any other geological data and is still being explored.