f Mapping the Metamorphic Grade of Volcanic Rocks of an Archean Granite-Greenstone Belt with Near-Infrared Spectroscopy

Near-infrared spectroscopy has been used for the identification and characterization of hydrothermal alteration zones associated with mineralization in a variety of lithologies and geologic settings. The aim ofthis study is to determine whether we can also use near-infrared spectroscopy for the estimating metamorphic grade in volcanic rock in Archean greenstone belts. The study was carried out on rock samples collected from selected Archean greenstone belts in in the East Pilbara Granite-Greenstone Terrane in Australia. Reflectance spectra in the range of 350nm to 2500nm of the 215 rock samples were measured in the ITC spectroscopy laboratory using ASD Fieldspec Pro spectrometer. The spectral minerals were interpreted and compared with thin sections and complementary geochemical and geological dataset obtained from published studies. The volcanic rocks contained the following spectrally detectable minerals: Chlorite (Fe-chlorite, intermediate and Mg-chlorite), amphiboles (hornblende and actinolite), white mica (illite, muscovite and phengite) and prehnite. Three metamorphic subfacies within the greenschist facies were interpreted based on the Mg-OH (2310-2360nm) absorption feature: 1) a zone containing Fe-chlorite; 2) a zone containing intermediate chlorite + epidote; and 3) a zone containing intermediate chlorite + actinolite and hornblende, in addition to the amphibolite facies. Prehnite-pumpellyite metamorphic facies could be identified from the 1470nm feature of prehnite. Near-infrared spectroscopy together with major elements lithogeochemistry appeared to be useful to characterize (low-grade) metamorphosed volcanic rocks in the East Pilbara Granite-Greenstone Terrane. The results are useful for the interpretation of near-infrared spectral data sets of for geological mapping and mineral exploration.