Detectability of minerals on desert alluvial fans using reflectance spectra

We analyzed the visible and near-infrared reflectance spectra of soil samples collected from desert alluvial and colluvial surfaces in the Cuprite mining district, Nevada. These surfaces are downslope from hydrothermally altered volcanic rocks that contain spectrally characteristic minerals such as alunite and kaolinite. Coarse fractions of the soils on the alluvial fans are mineralogically variable and express the upslope lithologies; fine fractions are remarkably similar mineralogically and spectrally in all samples because of dilution of local mineral components by regionally derived windblown dust. Theoretical models for spectral mixing and for particle-size effects were used to model the observed spectral variations. Diagnostic mineral absorption bands in the spectra of fan materials were enhanced by computationally removing the spectrum of the homogenous fine-soil component. Detection limits for minerals were found to be controlled by the sensitivity of the instrument, the spectral contrast between the mineral and the background, and the relative brightnesses of the mineral and the background. These results show that spectral mixing models are useful for analyzing data with high spectral resolution obtained by field and aircraft spectrometers.

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