Full text: Mesures physiques et signatures en télédétection

Gregg Swayze, Roger N. Clark, and Fred Kruse 
U.S. Geological Survey MS 964 Box 2504b Federal Center., Denver, CO 80225 U.S.A. 
^Center for the Study of Earth from Space (CSES) 
Cooperative Institute for Research in Environmental Sciences (CIRES) 
University of Colorado, Boulder, CO 80309-0449 U.S.A. 
Analysis of the spectral properties of minerals can be used to derive 
information about their chemical composition and geologic history. When this 
laboratory information is used to create mineral maps with data acquired by 
imaging spectrometers, geologic relationships are revealed on a grand scale in 
unprecedented detail. 
Mineral abundance maps of 15 minerals were made of the Cuprite Mining Dis 
trict, Nevada using 1990 Advanced Visual and Infra-Red Imaging Spectrometer 
(AVIRIS) data and the Clark et al. (1990) Multiple Spectral Features Mapping 
Algorithm. This technique uses a modified least-squares fitting of isolated 
features from a laboratory reference spectrum to corresponding features in 
reflectance-calibrated AVIRIS data. Multiple features of each AVIRIS spectrum 
were simultaneously fit to multiple reference mineral spectra. Within the pro 
gram a decision process judging among the best fitting reference minerals was 
used to derive the mineral(s) present in the spectrum of each pixel. Informa 
tion from this selection process was then combined into images of each mineral 
found in the AVIRIS sc'ene. 
The Cuprite region consists of two "bull's eye" patterns of acid-sulfate 
hydrothermal alteration centers straddling U.S. Highway 95 in southwestern 
Nevada. Mineral abundance maps derived from AVIRIS data of the area show that 
the western center is zoned from sericite, at the exterior, to halloysite - 
dickite, kaolinite, Na-alunite, through K-alunite with halloysite in the inte 
rior. The eastern center is similarly zoned from halloysite, at the exterior, 
to kaolinite, intermediate (Na-K) alunite, through K-alunite, with a central 
core of siliceous sinter. Our study of the alunite distribution at Cuprite has 
lead to new insights on the spectral properties of alunite. 
Laboratory spectral analysis of natural and synthetic alunite samples show 
complex variations in the wavelength position of the 2.2-fim features thus ena 
bling us to map alunite solid-solution composition using AVIRIS data. Synthetic 
alunite endmember compositions have 2.2-^im spectral features which deepen and 
widen with increasing temperature. We theorize that, as alunite is heated, 
internal hydronium dissociates creating new hydroxyl bonds between sulfate oxy 
gens and the hydrogen left from the hydronium dissociation. Apparently the 
increase of hydroxyl bonds is proportional to the maximum temperature achieved 
by the alunite and is spectrally displayed as a strengthening of the 2.2-fj.m 
features. These changes can be measured with sufficient accuracy in AVIRIS data 
to create remotely sensed maps of hydrothermal paleotemperatures. 
Alunite hydrothermal temperature maps of Cuprite derived from AVIRIS data 
show that the western center was hotter, implying it was once originally deeper 
than the eastern center. Geologic field relations indicate that both alteration 
centers were originally a single hydrothermal conduit, which was later faulted 
by the collapse of the Stonewall Caldera around 7.6 million years ago. Faulting 
exposed the shallower portion of the hydrothermal system in the eastern center 
and the deeper root in the western center. This new understanding, gained by 
examining the AVIRIS data, may help in the search for gold and other precious 
metals in the Cuprite Area.

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