37 CRITICAL MINERAL DEPOSITS AND ORE SYSTEMS OF THE NORTHEAST

This session will showcase new findings pertaining to critical mineral resources and hydrothermal-magmatic ore systems across the northeast. We welcome submissions involving fieldwork, geochemical, petrologic, geochronologic, and other approaches from micro- to macroscopic scales.

Tuesday, 24 March 2026
8:00 AM - 12:00 PM
CCC, Room 25

Newly Recognized Devonian Sedimentary Manganese in the Lower Seboomook Group, Northern Maine: A Marine Oxidation-Mineralization Event During Early Acadian Basin Development

Paper number: 37-9
Program: Technical Sessions
Time: 10:40 AM – 11:00 AM

Authors

Abstract

The Lower Devonian Lochkovian Seboomook Group is a major part of the Connecticut Valley–Gaspé Trough in northern–northwestern Maine and has long been known as a flysch sequence of turbiditic slate and sandstone deposited in a foreland basin related to advancing Acadian deformation. Recent new mapping demonstrates that the Seboomook rests unconformably on highly mineralized Cambrian–Silurian Munsungun, Winterville, Lunksoos Lake–Weeksboro, Number Nine Mountain, and East Branch inliers, with this regional unconformity marking the end of the Salinic orogeny and the onset of Acadian basin development. The lower Seboomook comprises a laterally continuous “trinity” of basal conglomerate, fossiliferous limestone and calcareous sandstone, and maroon–red (locally purplish) slate-siltstone. The trinity locally contains multiple basalt flows erupted in a continental-arc setting, recording the onset of Acadian arc volcanism in northern Maine. These units, previously mapped as separate or Silurian formations, are now dated by conodonts as Lochkovian and are included within the Seboomook Group; for example, the Frost Pond Formation of red slate northeast of Ripogenus Lake is reassigned to the maroon–red slate-siltstone unit. The maroon–red slate-siltstone unit forms an hematitic iron formation typically 10–15 m thick (locally up to near 100 m), indicating a critical early Devonian marine oxidation event during Acadian foreland basin development. In 2025 field season, a nearly 2-m-thick manganese-rich interval was discovered within this unit in a borrow pit located northwest of Saddle Pond, near Hay Brook, in T7 R9 WELS township. The interval consists of alternating red, pink, and black Fe-Mn bands or laminae of very fine-grained minerals; iron phases are predominantly hematite and minor magnetite and manganese phases dominated by Ca-rhodochrosite, with abundant cross-cutting rhodonite veinlets (generally < 2 mm) and surficial black manganese-oxide films produced by supergene weathering processes. Whole-rock analyses of two samples yield very high MnO content (45.8 and 47.1 wt.%) and low Fe2O3T content (3.09 and 2.97 wt.%). The high Mn content is confirmed by microXRF scanning. A slightly positive Ce anomaly, negative Y anomaly, and convex-upward PAAS-normalized REY distribution pattern support a hydrogenous origin for manganese mineralization. This occurrence, referred to “Hay Brook manganese occurrence” in our study, represents the first documented Devonian sedimentary-type manganese mineralization in the Northern Appalachians.

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411335

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