MINERALOGICAL AND PETROLOGICAL STUDY OF AN UNNAMED MAGNETITE PROSPECT, OWLS HEAD MOUNTAIN, FRANKLIN COUNTY, NEW YORK

An unnamed underground magnetite prospect is located in the northeastern Adirondack Highlands on the southern flank of Owls Head Mountain, 11 km southeast of Malone, Franklin County, New York, and 16 km west of the Lyon Mountain magnetite deposit. Excavated and possibly mined between 1820-1860, the prospect measures 9.7m long x 2.5m wide (avg) x 1.9m high and exposes both fine- and coarse-grained/pegmatitic meta-granite, and two sub-parallel ≤12cm-wide basalt dikes. Magnetite is fine-grained and disseminated throughout all rock types but is especially abundant as large crystalline aggregates, masses, and pods within the coarser-grained portions of the meta-granite and was probably the target of the initial prospect work. A sharp contact between the fine- and coarse-grained meta-granite may suggest two generations of magma emplacement; cross-cutting relationships reveal the two basalt dikes clearly post-date the meta-granite + magnetite.

Fourteen samples of magnetite ore, meta-granite, and basalt were collected in situ; transmitted and reflected light microscopy, electron microprobe analysis, and AA, ICP, XRF, and LECO analytical methods were utilized to determine the mineralogical and geochemical composition of the magnetite ore and rock types. The low-Ti magnetite ore contains 1.64 wt% TiO₂ and is comprised mostly of magnetite with very minor ilmenite (+Mn) and possibly pyrophanite; isolated, rounded cpx grains with Na-feldspar rims occur rarely in the ore. The meta-granite probably represents the microcline/perthite facies of the Lyon Mountain pyroxene granite gneiss and consists chiefly of Ca>Mg,Fe cpx, sphene, quartz, and feldspar and contains the largest pods of magnetite. The fine-grained portion of the meta-granite exhibits foliation. The basalt consists of cpx, Ca-amphibole, Ti-magnetite, and Na-feldspar with pyrite, chalcopyrite, fluorite, glass, and K-feldspar and quartz as probable xenocrysts derived from the meta-granite.

The mineralogy of the rocks and composition of the magnetite ore within the unnamed Owls Head Mountain prospect indicates that the geological setting and magnetite ore formation at the Owls Head Mountain prospect is probably similar to that of the magnetite deposit at nearby Lyon Mountain.