A FIELD ASSESSMENT OF THE GAMMA-RAY SPECTROMETER AND PORTABLE XRF ANALYZER IN DETERMINING POTASSIUM CONCENTRATIONS OF METASOMATIZED GRANITE GNEISS

Rare earth elements (REEs) are associated with historical iron deposits in the Adirondack Mountains of New York. The mineral deposits are typically hosted in the Lyon Mountain Granite Gneiss and are related to extensive Na and K metasomatism. The metasomatism process may hold the keys, not only to finding new REE deposits, but also to better understanding of the ore-forming process. It is critical to map alteration domains and yet, these domains are generally not optically identifiable in outcrop or hand specimen. Multiple techniques were used to characterize the alteration, including: handheld gamma-ray spectroscopic analysis, portable x-ray fluorescence analysis (pXRF), airborne geophysics, and thin section analysis. The gamma-ray spectrometer and pXRF proved to be successful at distinguishing the relative intensity of alteration in the granites based on K abundance. However, pXRF totals were not accurate when compared to whole rock high-resolution XRF geochemical data. Potassium radiation data from airborne geophysics could not be correlated with ground-based K data, probably because the scale of heterogeneity in the field is finer than can be resolved by the airborne geophysics flown at 250 m flight line spacing. Mineral modes and textures associated with K and Na metasomatism have been investigated using compositional electron microprobe maps of thin sections. The lowest K values given by the handheld devices correspond to rocks with pure albite and no K-feldspar. High-K outcrops are dominated by orthoclase. Medium K-values suggest either unaltered granite or partial alteration, and data from the full section maps show albite alteration rims on microperthite cores. Characterization of major and accessory minerals across mapped alteration gradients is critical for constraining the alteration/mineralization process(es), and the handheld instruments are essential for identifying alteration domains and gradients.