ORIGIN OF PORT LEYDEN NELSONITE: NEW INSIGHTS FROM OXIDE CHEMISTRY, PETROGRAPHY, AND GEOPHYSICS

A small intrusion of nelsonite, an igneous rock comprised of magnetite+apatite±ilmenite, is exposed along the western margin of the Adirondack Highlands, near the boundary between Proterozoic rocks of the AMCG suite and sedimentary rocks of the Trenton Group to the west, at Port Leyden, NY. Deposits like this are typically associated with Proterozoic massif-type anorthosite or related rocks of gabbroic composition, and are thought to form by liquid immiscibility or fractional crystallization with gravitational settling. Rocks adjacent to the intrusion are metapelite gneisses; however, there are no known rocks within close proximity to the Port Leyden nelsonite which represent an appropriate parental magma. Nelsonite mineralogy and textures were examined by scanning electron microscopy, energy dispersive spectroscopy, and reflected light microscopy. Major and trace element concentrations of oxide minerals were determined by WD-EPMA and LA-ICP-MS, respectively. Primary magnetite and fluorapatite occur as equant grains. Ilmenite was observed exclusively as an exsolution phase where it occurs as trellis-textured lamellae within host magnetite or as large poikilitic grains, suggesting the primary magnetite was initially titanomagnetite with a high initial ulvöspinel content. These textures are consistent with cotectic crystallization of titanomagnetite and fluorapatite, with ilmenite forming during subsolidus oxy-exsolution of titanomagnetite. Sulfides, primarily pyrite, are disseminated throughout the rock, approaching a net texture in samples with sulfides approaching 20% modal abundance. Trace element analyses show ilmenite is enriched in Ti, Zr, Hf, W, Ta, Nb, Mg, and Mn, and magnetite is enriched in Al, Ga, Co, V, Ni, and Cr. Tin contents are high in both oxides (mt = 11.0-42.7 ppm; ilm = 31.7-88.5 ppm) compared to other Fe-Ti oxides ores, and variable Zn concentrations are likely influenced by the formation of sphalerite. A magnetic survey was conducted over the Port Leyden intrusion to constrain its lateral extent using a magnetometer. The dike-like ore body is about 35m wide and about 90m long and has a N-NW trend that correlates with the strike of lineaments in the region. The body splits into two linear features, possibly representing fracture systems which controlled magma emplacement.