MINERALOGY, GEOCHEMISTRY, AND METALLOGENESIS OF THE MAPLE-HOVEY MANGANESE DEPOSIT, NORTHERN MAINE

The Aroostook County Manganese District of northern Maine, which consists of northern, central, and southern sub-districts of Silurian age, hosts the largest manganese reserve in the United States. Despite its size, the district lacks detailed modern study, and the stratigraphic relationships and manganese metallogenesis remain to be solved. Funded by the Earth Mapping Resources Initiative (Earth MRI), this study focuses on the central sub-district, where the largest known deposit, the Maple-Hovey deposit, is located.

Detailed field mapping shows that the fine-grained clastic sedimentary rocks of the Maple Mountain Formation that hosts the deposit conformably overlie the volcanic arc sequence of the Spruce Top and Dunn Brook formations. The deposit lies at the top of the volcanic basin and consists of two units, a lower carbonate-rich unit and an upper continuous but pinch-and-swell unit, dominated by a layered sequence with varying mineralogy. The primary ore zone consists of thinly laminated, microcrystalline manganese ironstone. Petrographic observations and XRD, mXRF, WDXRF, pXRF and EDS spectra for major and minor elements, and ICP-OES-MS for trace and rare earth elements reveal a chloritic slate host rock and four major thinly-laminated layers within the primary mineralized zone – Fe-rich layers largely consisting of hematite, Mn-silicate-rich layers of largely spessartine with minor rhodonite (MnSiO₃), Mn-carbonate-rich layers of Ca-rhodochrosite (MnCO₃), and P-rich layers of apatite. Cross-cutting veinlets mirror the laminated mineralogy and contain rhodonite, bannisterite, rhodochrosite, potassium feldspar, and albite, indicating minor hydrothermal activity. Metamorphic minerals such as spessartine, magnetite, and stilpnomelane were produced during lower greenschist facies Acadian metamorphism. Mn/Fe ratios average at least 0.394 and Al/Ti ratios average 22.2 for the Fe-Mn laminae. These observations as well as enriched REE with distinctive positive Ce and negative Y anomalies, along with other geochemical discrimination diagrams, indicate the Maple-Hovey deposit formed via hydrogenous precipitation in a marine setting, likely in a restricted arc basin which allowed for the appropriate redox conditions for manganese and iron precipitation.