Paper No. 257-5
Presentation Time: 2:40 PM
GEOLOGIC MAPPING AND EXPLORATION FOR LI-BEARING PEGMATITES OF THE RUMFORD SERIES IN THE OXFORD COUNTY PEGMATITE FIELD, ME, USA
FELCH, Myles1, EUSDEN Jr., J.2, BRADLEY, Dwight3, KOTEAS, Christopher4, WHITTAKER, Amber H.5, MARVINNEY, Robert5, HOLM-DENOMA, Chris6, PIANOWSKI, Laura S.6, SMITH, Steven M.6, KINNEY, Sean7, TIBBETTS, David8 and FREEMAN, Gary9, (1)Maine Mineral & Gem Museum, 99 Main St., Bethel, ME 04217, (2)Earth and Climate Sciences, Bates College, Carnegie Science, 44 Campus Ave, Lewiston, ME 04240, (3)U.S. Geological Survey, 4210 University Drive, Anchorage, AK 99508, (4)Norwich UniversityDepartment of Earth and Env. Sciences, 158 Harmon Dr, Northfield, VT 05663-1000, (5)Maine Geological Survey, 93 State House Station, Augusta, ME 04333, (6)U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver Federal Center, Denver, CO 80225, (7)Columbia University, Palisades, NY 07631-3150, (8)Columbia UniversityEarth and Environmental Sciences, 240 E Palisade Ave Apt 4F, Englewood, NJ 07631-3150, (9)Freeman Resources, LLC, 48 Lovejoy Rd., Paris, ME 04281
Recent discovery of large (>11m) spodumene crystals from the Plumbago North Pegmatite have renewed exploration interests in western Maine. This exploration survey was conducted around Plumbago Mtn., Newry, ME to better characterize the distribution of Li-bearing pegmatites, understand their temporal and structural relations in the tectonic framework, and assess potential exploration strategies for Li-bearing pegmatites in this region. This study included bedrock mapping; stream sediment sampling; handheld XRF analyses for K and Rb in micas and feldspar and comparative LIBS analyses of the same samples for Li, K, Rb; and U-Pb geochronology of pegmatites, other plutons, and detrital geochronology of metasedimentary rocks.
Spodumene is the primary lithium ore mineral in the study area; lepidolite, montebrasite, triphylite, and elbaite are also present in smaller volumes. Lithium pegmatites are concentrated along the perimeter of the Plumbago Mountain Pluton (ca. 405±2 Ma, zircon), a metagabbro, and within an adjacent mafic metavolcanic(?) actinolite schist. The Plumbago North pegmatite, with its mega-crystals of spodumene, is hosted within the regional schist and quartzite metasedimentary sequence but is within a few 100 m of the Plumbago Mountain Pluton. Almost all pegmatites are 10-100 m thick sills oriented parallel to the local layering and foliation of the gabbro and metasedimentary units.
Cassiterite U-Pb ages were determined for three Li-bearing pegmatites of the Rumford Series (Plumbago North, Crooker Quarry, and Black Mtn. Quarry, ~330Ma), one from the northern Phillips Series (Bemis Stream, 360-346 Ma), one from the Greenwood Series (Emmons 270-265 Ma), and one from the Paris series (Mt. Mica, 257-250 Ma). Ages from the Paris series are consistent with previously published ages. Ages for the Rumford and Phillips series are older and reveal at least three generations of lithium pegmatites in the Oxford County Pegmatite Field.
The origin of the pegmatites around Plumbago Mtn. is still uncertain. Geochronology has revealed that there are no spatially associated parental sources. It is conceivable that the Newry pegmatites may be of anatectic origin, like what has been proposed for the Li-bearing pegmatites in the Paris series (Simmons et al. 2016).
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