Mapping weathering and alteration minerals in Comstock and Geiger Grade areas using visible to thermal infrared airborne remote sensing data

To support research into both precious metal exploration and environmental site
characterization a combination of high spatial/spectral resolution airborne visible,
near infrared, short wave infrared (VNIR/SWIR) and thermal infrared (TIR) image
data were acquired to remotely map hydrothermal alteration minerals around the
Geiger Grade and Comstock alteration regions, and map the mineral by-products of
weathered mine dumps in Virginia City. Remote sensing data from the Airborne Visible
Infrared Imaging Spectrometer (AVIRIS), SpecTIR Corporation’s airborne
hyperspectral imager (HyperSpecTIR), the MODIS-ASTER airborne simulator
(MASTER), and the Spatially Enhanced Broadband Array Spectrograph System
(SEBASS) were acquired and processed into mineral maps based on the unique spectral
signatures of image pixels. VNIR/SWIR and TIR field spectrometer data were
collected for both calibration and validation of the remote data sets, and field sampling,
laboratory spectral analyses and XRD analyses were made to corroborate the
surface mineralogy identified by spectroscopy.
The resulting mineral maps show the spatial distribution of several important
alteration minerals around each study area including alunite, quartz, pyrophyllite,
kaolinite, montmorillonite/muscovite, and chlorite. In the Comstock region the mineral
maps show acid-sulfate alteration, widespread propylitic alteration and extensive
faulting that offsets the acid-sulfate areas, in contrast to the larger, dominantly acidsulfate
alteration exposed along Geiger Grade. Also, different mineral zones within
the intense acid-sulfate areas were mapped. In the Virginia City historic mining district
the important weathering minerals mapped include hematite, goethite, jarosite
and hydrous sulfate minerals (hexahydrite, alunogen and gypsum) located on mine
dumps. Sulfate minerals indicate acidic water forming in the mine dump environment.
While there is not an immediate threat to the community, there are clearly
sources of acidic drainage that were identified remotely.