MAGNETIC ANOMALY AND SUSCEPTIBILITY ANALYSIS OF ORE BODIES IN THE WESTERN HUDSON HIGHLANDS, NEW YORK

Five abandoned mines are present within a northeast striking dextral shear zone in the Western Hudson Highlands, Harriman State Park, NY. Hogen Camp Mine and Bradley Mine are the two end members of this dextral shear zone. Varying bedrock was observed at the two localities; to understand the diverse geology between the two mines, both a magnetic and petrological analysis were conducted. A high precision Overhauser magnetometer and gradiometer were used to discern the magnetic field anomaly (nT) deviations locally and between localities. To assess precision of the magnetic field anomaly, magnetic susceptibility of surrounding wall rock, and of the transition from wall rock to massive ore was quantified. A petrological analysis was then adopted to ascertain the formation and composition of the massive ore deposits within the fractures produced by the dextral shearing. At Hogen Camp Mine, layers of mafic and intermediate metavolcanic gneiss were observed, with local calc-silicate gneiss. The mafic layers of metavolcanic gneiss are supersaturated in iron, which was conclusively determined by mineralogy and concentration of iron oxides. The massive ore deposit is primarily clinopyroxene and magnetite with minor constituents of amphibole and biotite. At Bradley Mine, surrounding quartzofeldspathic gneiss and calc-silicate gneiss were observed. The massive ore deposit is primarily composed of magnetite, clinopyroxene, and calcite. It is interesting to note that the country rock present at these two mines are responsible for the variance in the magnetic field anomaly. Hogen Camp Mine has an overall higher magnetic field anomaly, which is in direct correlation to the surrounding metavolcanic gneiss. Comparatively, Bradley Mine has a lower magnetic field anomaly, ultimately produced by the calc-silicate gneiss.