Date of Award

Summer 8-16-2024

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Earth Sciences

Advisor

Alicia Cruz-Uribe

Second Committee Member

Chunzeng Wang

Third Committee Member

Martin Yates

Additional Committee Members

David Lentz

Amber Whittaker

Abstract

The Aroostook Manganese District in Northern Maine consists of northern, central, and southern subdistricts of Silurian age. Collectively, they represent the largest manganese reserve in the United States (Cannon et al., 2017). This study focuses on the northern and central manganese subdistricts, where cross-sectional characterization and sampling across four continuous exposures of the primary ore zone have been conducted. Analytical methods include pXRF, whole-rock geochemical analyses by WDXRF and ICP-OES, petrography, and mineral identification by electron dispersive spectroscopy (EDS) via electron microprobe. These methods provide a modern characterization of the stratigraphy and mineralogy of the deposits to better constrain the Fe-Mn metallogenesis. Field observations show that the central subdistrict, the Maple-Hovey deposit, is presented as a continuous but pinch-and-swell layer, and the primary ore zone consists of thinly laminated, very fine grained manganese ironstone within the lightly metamorphosed Maple Mountain Formation. The northern subdistrict Fe-Mn deposits (“New Sweden deposits”), hosted within the lightly metamorphosed New Sweden Formation, are presented as lenses or layers of thinly laminated, very fine grained manganese ironstone. In both subdistricts, the ore and surrounding host rock are dominated by a layered sequence with varying mineralogy. Petrographic observations and EDS spectra reveal chlorite-rich host rocks and iron-rich hematitic, apatite-rich, and Mn-rich carbonate and oxide laminae in both subdistricts. Mnsilicate (e.g., spessartine and rhodonite) rich laminae occur in the Maple-Hovey deposit. Minor crosscutting veinlets often contain barite, manganese oxides, manganese carbonate, and quartz. In the Maple-Hovey deposit, cross-cutting veinlets can also contain Mn-silicates. For selected Fe-Mn ore samples, the Mn content averages 11.23 wt. % (n = 16) in the Maple-Hovey deposit and 7.41 wt. % Mn (n = 19) for the New Sweden deposits. Rare earth element patterns normalized to PAAS show slight positive Ce/Ce* anomalies for both subdistricts. Geochemical discrimination, mineral assemblage, and stratigraphy suggest these deposits were formed by precipitation from seawater on the continental shelf (in a restricted forearc basin for the Maple-Hovey deposits and a broad foreland basin for the New Sweden deposits) at the redoxcline and were formed by dominantly hydrogeneous processes.

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