XRD AND INFRARED SPECTROSCOPIC VALIDATION OF WEATHERING SURFACES FROM ULTRAMAFIC AND MAFIC LITHOLOGIES EXAMINED USING HYPERSPECTRAL IMAGERY, CROSS LAKE AREA, CAPE SMITH BELT, NORTHERN QUEBEC, CANADA

The study area is located at the tectonic boundary between the Povungnituk and Chukotat Groups in the Early Proterozoic Cape Smith Belt near Cross Lake, in northern Quebec. This zone is characterized by elongate intrusive bodies of mafic-ultramafic rocks which have been interpreted as comagmatic with the Fe-and Mg-rich basalts of the lower Chukotat Group. Such ultramafic sills are often Ni-Cu-PGE bearing in the area. Probe-1 Hyperspectral imagery is being used to differentiate the lithologies in the study area. In an applied field setting such imagery records the spectrum of the surface weathering product rather than the signal from a fresh rock surface. The objective of this investigation was to generate a calibrated database of the hyperspectral signatures of the surficial weathering products of each of the local lithologies. Validation of lithologies and surficial weathering characteristics was carried out during two field campaigns, as well as in subsequent laboratory evaluation using infrared spectroscopy and mineralogical analyses. Field ground-truthing indicated significant visual compositional and textural differences between surficial weathering products for study area lithologies, found to be related to their composition, grain size and most commonly associated vegetation (lichen). In-situ and subsequent laboratory and powdered sample infrared analyses of fresh and weathered surfaces confirmed that infrared absorption features observed in hyperspectral imagery signatures for the lithologies was solely associated with the weathered mineral oxides. Infrared spectroscopy and X-ray diffraction analyses of surficial weathering products confirmed that the more olivine-rich ultramafic rocks are predominately characterized by serpentine and chlorite, whereas the more pyroxene-rich ultramafics are dominated by chlorite and minor mica. The abundance of iron oxide was observed to be related to the weathering of sulfides. Weathering products of mafic rocks, in contrast, are void of serpentine, and may contain epidote as well as chlorite and mica, with minor iron minerals. The weathering products of the associated volcanogenic sediments are dominated by mica and iron oxide. Identification of the mineral composition of weathered products and their diagnostic infrared signatures for the study area lithologies, along with field-truthing of hyperspectral imagery helps to understand the characteristic signatures of these minerals in a mixed-mineral lithologic setting. This information provides critical information for further work on airborne hyperspectral data unmixing studies of mixed-mineral lithologies.