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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.