Quantitative Geochemical Mapping of Ammonium Minerals in the Southern Cedar Mountains, Nevada, Using the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS)

Abstract

Imaging spectrometer (hyperspectral) data, field spectral measurements, and laboratory analyses were used to quantitatively map the concentration of mineral-bound ammonium (buddingtonite) in hydrothermally altered volcanic rocks in the southern Cedar Mountains, Esmeralda County, Nevada. Mineral-bound ammonium is a product of ion exchange in silicate minerals and has no visible distinguishing characteristics, however, diagnostic infrared spectral features are ideal for identification using both field and airborne/spaceborne spectrometers. Establishment of a laboratory- or field-based geochemical calibration is presently a prerequisite to quantitative mapping. For this study, ammonium content of rock samples was determined by chemical analysis and reflectance spectra were measured on whole-rock samples. A linear relation was found between ammonium concentration and the depth of a 2.12 μm ammonium absorption feature in buddingtonite. An image-map of ammonium concentration in ppm was derived from Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data by applying the linear calibration to the AVIRIS reflectance-calibrated spectrum at each pixel. Field spectral measurements, made with a PIMA field spectrometer on a 40 m grid, were used to create a ground-truth concentration map that confirmed the AVIRIS results. Field mapping, X-ray diffraction, and petrologic studies performed in conjunction with the AVIRIS analysis show that buddingtonite is the dominant ammonium mineral in the southern Cedar Mountains, that the ammonium is located along northeast-trending basin and range normal faults, and that it is restricted to two of four crystal-rich rhyolitic tuff units of Oligocene age. This study establishes that remote geochemical mapping using imaging spectrometer data is possible, and presents a methodology that could be extended to quantitatively map other minerals that have absorption features in the short-wave infrared.