THE BULK CHEMICAL COMPOSITION AND H2O CONTENT OF MOUNT MICA PEGMATITE, OXFORD CO., MAINE, USA

Mount Mica pegmatite is the discovery site of the first tourmaline in North America and is famous for gem tourmaline production for nearly 200 years. The dike which ranges in thickness from 1 to 8 m and dips 20° SE, has a relatively simple zonal structure consisting essentially of a wall zone and core zone. The wall zone is unusual as it is essentially devoid of K-feldspar. The outer portion of the pegmatite consists of quartz, muscovite, albite (An 1.8) and schorl. Muscovite is the dominant K-bearing species in the outer portion of the pegmatite. K-feldspar only appears in the core zone of the pegmatite adjacent to pockets. We have devised a method to estimate the bulk composition of Mt. Mica that combines accurate mapping of the pegmatite as it was mined with a series of selected core samples across the pegmatite. A new mining operation that began in 2003 has now mined down dip for over 100 meters to a depth of 30 meters and produced a very detailed and accurately surveyed geologic map that shows the total area of pegmatite removed, the spatial distribution and aerial extent of pockets, massive lepidolite, microcline, and xenoliths. The map was vectorized in ArcGIS, scaled and imported into image analysis software which allows accurate calculation of the total volumes of pegmatite mined, lepidolite pods and all pockets found. Forty five drill cores were taken across the pegmatite from the hanging wall contact to the foot wall contact along a transect that avoided lepidolite masses and miaroles. Cores were pulverized and the powders were thoroughly mixed and homogenized to produce a sample that was representative of the pegmatite bulk composition sans lepidolite masses. The powder was then analyzed by ICP MS for major and trace elements and by DCP for boron. Fused pellets were analyzed for fluorine by EMPA and structural water was determined by loss on ignition. The lepidolite pods were analyzed separately and the wt. % calculated from the volume data was integrated into the result. H₂O wt. % was estimated by assuming that the measured open space of the miaroles was filled with H₂O (0.22wt.%). Oxide wt. %: SiO₂ 72.08, TiO₂ 0.07, Al₂O₃ 17.33, FeO t 1.21, MnO 0.04, MgO 0.15, CaO 0.48, Na₂O 5.35, K₂O 2.08, P₂O₅ 0.20, Li₂O 0.24, H₂O 1.16, F 0.25, subtotal 100.64, -O Ξ F 0.11, Total 100.53; element ppm Sr 44, Ba 35, Nb 23.5, Ta 6.3, Zr 17, Hf 1, Sn 84, Cs 99.9, Rb 636, B 287.