MINERALOGY, GEOCHEMISTRY, AND METALLOGENESIS OF THE NORTHERN AROOSTOOK MANGANESE SUB-DISTRICT, AROOSTOOK COUNTY, NORTHERN MAINE

The Aroostook County Manganese District in Northern Maine consists of northern, central, and southern sub-districts of Silurian age. Collectively, they represent the largest manganese reserve in the United States. This presentation focuses on the northern manganese sub-district, where cross-section characterization and sampling were performed across two continuous exposures of the primary iron-manganese ore zone at the Dudley and Ryan pits. Portable-XRF analyses, whole-rock geochemical analyses by WDXRF and ICP-OES, petrographic observations, and mineral identification by EDS via electron microprobe have been conducted on the samples to provide a modern characterization of the lithogeochemistry, chemostratigraphy, and mineralogy of the deposit to better constrain its metallogenesis. Field observations show that the manganese 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 mudstone and shale of the Upper-Silurian New Sweden Formation. The ore and surrounding host rocks are dominated by a layered sequence with varying mineralogy. Petrographic observations and EDS spectra reveal a chlorite-rich host rock and hematitic iron-rich, apatite-rich, and carbonate and oxide manganese-rich laminae. Cross-cutting veinlets often contain barite, manganese oxides, manganese carbonate, and quartz. Albitized volcanic or volcaniclastic rocks occur near the middle of the deposit. Geochemical discrimination for manganese and iron metallogenesis will be discussed.