ALTERATION MAPPING USING GEOPHYSICAL AND GEOCHEMICAL INSTRUMENTATION: IMPLICATIONS FOR IOA AND REE EXPLORATION

The Adirondack Mountains of New York contain historical iron-oxide apatite (IOA) deposits with variable rare earth element (REE) concentrations. The deposits are typically hosted in the Lyon Mountain Granite Gneiss and associated with extensive Na and K metasomatism. This correlation suggests that metasomatic zones are key not only in finding new REE deposits, but also in better understanding the mineralization process. Therefore, it is critical to map alteration domains, however, these domains are generally not visually identifiable in outcrop or hand specimen. Here we report on the integration of multiple techniques to characterize the alteration near known REE-bearing and REE-poor deposits, including handheld gamma-ray spectroscopy, portable x-ray fluorescence (pXRF), airborne geophysics, and petrography.

The gamma-ray spectrometer and pXRF proved to be successful at distinguishing the relative intensity of alteration in the granites based on K abundance. The potassium-poor values occur as haloes immediately adjacent to the deposits. Away from seams, potassium content increases such that the magnitude and width of the gradient is similar along strike of the deposits. Elevated U and Th values are present in host rocks adjacent to REE-bearing deposits. The pXRF and gamma-ray potassium values are remarkably consistent with laboratory-based whole rock XRF geochemical data and, therefore, useful for semi-quantitative analysis. Potassium radiation data from airborne geophysics could not be correlated with ground-based K data, as the scale of heterogeneity is finer than can be resolved by the 250m wide flight line spacing. Mineral modes and textures associated with K and Na metasomatism were evaluated from compositional maps of thin sections. Lowest K values detected by the handheld devices correspond to rocks with pure albite and no K-feldspar. High-K outcrops are dominated by K-feldspar that may reflect K-metasomatism or unaltered granite. Medium K-values may suggest partial alteration. This study demonstrates the utility of handheld instruments for identifying otherwise cryptic alteration gradients and mineral deposits. Characterization of major and accessory minerals across mapped gradients will be critical for further constraining the alteration/mineralization process(es).