AN INVESTIGATION INTO THE CORRELATION BETWEEN NA-METASOMATISM AND ECONOMIC DEPOSTIS IN THE ADIRONDACK MOUNTAINS, NY

Rare earth elements (REEs) are fundamental in renewable energy resources, cell phones, magnets, and batteries. Future rare earth element (REE) availability is a concern due to the increasing demand, the difficultly finding deposits, and foreign control over the supply. In the Adirondack Mountains, NY, the hydrothermally altered Lyon Mountain Granite is known to contain historic iron ore deposits and significant REE concentrations. Valley et. al., 2011 interpreted that the Lyon Mountain Granite experienced early K-metasomatism and later Na-metasomatism, and that iron ore mineralization is closely associated with the Na-metasomatized granite. The goals of this study are: 1) to test the effectiveness of a gamma-ray spectrometer to identify the altered domains and 2) to investigate the hydrothermal alteration process that is associated with iron and REE mineralization. We mapped the Lyon Mountain Granite on Skiff Mountain near Ticonderoga, NY in order to delineate the geometry of alteration domains and to establish a sampling strategy. One key alteration indicator is the conversion of plagioclase/K-feldspar to pure albite. Therefore, outcrops that do not contain K radiation based on the gamma-ray spectrometer are most likely the sodium-altered rocks. We found distinct differences in gamma-ray intensity that persist along strike and parallel to ore deposits. The lowest levels are found immediately adjacent to the ore, intermediate levels are ~2-10 meters away from the ore, and standard levels are located in areas greater than 10 meters away. There are also areas with anomalously high intensity located at the locations farthest away from the ore. Thin sections from samples across the gradient were analyzed via electron microprobe (compositional mapping and quantitative analysis). The modal abundances of K-feldspar suggest that the gamma-ray spectrometer correctly revealed bulk K concentration. Quantitative analysis also shows that plagioclase is pure albite across the alteration gradient; no Ca-bearing feldspar is present in any domain. We propose that the gamma-ray spectrometer is a reliable resource to map the alteration domains, and that the alteration process involves systematic replacement of plagioclase and K-feldspar.