The airborne imaging spectrometer (AIS) has provided for the first time, the possibility to map mineralogical constituents in the Earth's surface and thus has enormously increased the value of remote-sensing data as a tool in the solution of geologic problems. The question addressed with AIS at Cuprite was how well could the mineral components at the surface of a hydrothermal alteration zone be detected, identified and mapped? The question was answered positively and is discussed. A relatively rare mineral, buddingtonie, that could not have been detected by conventional means, was discovered and mapped by the use of AIS

Goetz, A. F. H.

Srivastava, V.

English

NASA Technical Reports Server

N86-11619 
MINERALOGICAL MAPPING IN THE CUPRITE MINING DISTRICT, NEVADA 
ALEXANDEa F. H. GOETZ and VINAY SRIVASTAVA, Jet Propulsion 
Laboratory/California Institute of Technology, Pasadena, California 
91109, U.S.A. 
INTRODUCTION 
The Airborne Imaging Spectrometer (AIS) has provided for the first 
time, the possibility to map mineralogical constituents in the earth's 
surface and thus has enormously increased the value of remote-sensing 
data as a tool in the solution of geologic problems. 
The question addressed with AIS at Cuprite was how well could the 
mineral components at the surface of a hydrothermal alteration zone be 
detected, identified and mapped? The question has been answered posi-
tively as discussed below, and perhaps more importantly a relatively rare 
mineral, buddingtonite, that could not have been detected by conventional 
means, was discovered and mapped by the use of AIS. 
SETTING 
The Cuprite mining district straddles U.S. Hwy. 95 approximately 
17 km south of Goldfield in southwestern Nevada. The eastern half of the 
district studied with AIS is an area of extensive hydrothermal alteration 
within a sequence of rhyolitic welded ash flow and air fall tuffs. The 
major unit is the 7 my-old Thirsty Canyon Tuff whose source is the Black 
Mountain Caldera , 50 km to the southwest (Ashley and Silbermann, 
1976). The altered units consist of a central core of almost pure 
silica, a ring of opalized rocks containing alunite and kaolinite, and an 
outer argillized zone containing mainly'kaolinite and montmorillonite as 
well as opal and some limonite (Abrams, et a!., 1977). 
PREVIOUS STUDIES 
The Cuprite area has been the focus of remote-sensing studies for the 
last decade. Because of its varied mineralogy and relatively sparse 
vegetation cover, a number of spectral remote-sensing studies have been 
carried out there (Abrams, et al., 1977; Kahle and Goetz, 1983; Goetz, 
et al., 1985). Abrams, et al. (1977) showed that broad-band multispec-
tral imaging, including bands centered at 1.6 and 2.2~m, was far superior 
in mapping hydroxyl-bearing minerals than those restricted to the region 
short of 1.0~m, and provided the conclusive evidence that aided in the 
decision to include the seventh band (2.08-2.36~m) on the Landsat 
Thematic Mapper. 
Abrams, et al. (1977) found that color ratio composites utilizing the 
1.6 and 2.2~m bands, in combination with bands at shorter wavelengths, 
could be used to delineate the major boundaries in surface mineralogy 
very accurately. The mapping was verified by laboratory analysis of 
field samples. Although boundaries between units could be mapped accur-
ately, it was not possible to identify the surface mineralogy without 
resort to in-situ measurements. Laboratory-like spectral reflectance 
measuremen~~required to identify mineral constituents in the surface 
directly. The AIS provides image data at 9.3-nm intervals throughout the 
22 
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https://ntrs.nasa.gov/search.jsp?R=19860002152 2020-03-23T06:31:59+00:00Z
1.2-2.4"m spectral region and has sufficient spectral resolution to make 
direct mineral identification possible. 
AIS DATA ACQUISITION 
AIS data over Cuprite, Nevada were first acquired in August 1983. 
Two subsequent flights in 1984 covered the areas shown in Fig. 1. The 
1983 flight covered a small area extending from south to north across the 
southern boundary of the alteration zone. Prominent features are a 
bright hill in the center of the coverage shown in Fig. 2a and a bull-
dozed area which shows up very brightly in the northern end of the 
coverage. During 1984 the area was reflown, but in order to cover the 
same area exactly it was necessary to roll the aircraft approximately 
50. After overflying the bulldozed area, the horizontal attitude was 
resumed causing the dogleg seen in Fig. 1. 
DATA REDUCTION 
On-board data reduction consists of an automatic dark current 
subtraction before the data were recorded. On the ground the light 
transfer curve derived from laboratory calibration for each of the 1024 
detector elements is applied to produce the so-called "raw" data. The 
resulting spectrum for each pixel has the general shape seen in 
Fig. 2b. The major features are a fall-off in signal toward longer 
wavelengths, a function of irradiance, and the two major atmospheric 
water absorption features centered at 1.4 and 1.9lJm. The reduced curve 
shown in Fig. 2 was obtained by removing the solar spectrum and 
normalizing the data to an area ,overflown outside the Cuprite region, but 
during the same pass. The area was assumed to be spatially uniform and 
spectrally flat, or at least smoothly varying. All the data in one pass 
are normalized to this region. The normalization is important to remove 
residual stripping in the images caused by minor inaccuracies in the 
laboratory calibration data. 
The resulting reduced curve is shown in Fig. 2b. The regions within 
the atmospheric water bands are very noisy because of the lack of energy 
reaching 'the surface at these wavel~ngths. Mineral spectral features can 
be seen beyond 2.0lJm. 
RESULTS 
The results of th(~ 1983 overflight can be seen in Fig. 2c and 
Fig. 3. Figure 2c shows images in 32 contiguous channels from 2.03-
2.32lJm to which an equal area normalization has been applied (Goetz, 
et al., 1985). The result is an enhancement of spectral differences and 
suppression of bright.ness differences associated with topography. 
Figure 3 shows the result of extraction of spectra from individual 
3-by-3 pixel areas. The bright hill exhibits an absorption doublet at 
2.17 and 2.20lJm that is diagnostic of the clay kaolinite while the. 
bulldozed area exhibits a single, broad absorption minimum at 2.16lJm, 
diagnostic of alunite. Laboratory spectra for samples collected in these 
areas are shown as dashed lines. 
A ground traverse was carried out to identify the exact boundary of 
opa1ized and secondary quartz zones. Figure 1 shows the location of the 
traverse while Fig. 4 shows laboratory spectra of individual samples 
23 
Figure 1. Airphoto of the Cu prit e mining district showing the 
approximate AIS ground coverage. 
24 
ORIGINAL PAGt" f 
Of. POOR QUALITY 
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t 
a 120- ",-,.;------.,.;..---.,.2· b 210 
NEVADA 
w 
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w 
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WAVELENGTH, I'm 
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2.32 fLm 
Figure 2 . (a) Segment of an AIS ground track covering the southern portion of the Cuprite 
district. (b) Raw and normalized spectrum from a 3-by-3 pixel area. (c) Thirty-
two AIS images covering the wavelength range 2.03-2.32 ~m normalized by the equal 
energy technique so that each pixel spectrum contains the same area under the curve . 
0 0 
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Figure 3. 
- ..... LAB 
ALUNITE 
BEARING 
SAMPLE 
, I I I I I 
2.03 2.10 2.15 2.20 2.25 2.28 
An AIS image at 2.03 ~m of 
3-by-3 pixel spectra taken 
samples collected from the 
10 minutes to acquire, the 
WAVELENGTH, ~m 
the same area as shown 
from the image and in 
locations indicated. 
AIS spectra 10 ms. 
in Figure 2(a). Shown here are 
two areas laboratory spectra from 
The laboratory spectra required 
N 
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Cupr-ite C 
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Figure 4. Laboratory spectra from 36 samples taken at 10 m intervals in each of three 
traverses spaced 10 m apart at the location shown in Fig. 1. The predominant 
mineral is alunite with absorption at 2.16 and 2.32 ~m. At the top of the 
traverse the flat curves indicate almost pure secondary quartz. 
.~ 
''it~ 
~ 
lUI;; .1 ,II j &&1117 P j . J WiII:IlIIIllUIi .W. L J!IL IIHI_ _ HI; J L 1!2 .1_11. [ lin UIIIII. II JIIllIlllllIlllUlltJIl! ;; JULa 
collected at 10-m intervals along the traverse. The traverses A, Band C 
were spaced 10 m apart in an east-west direction. The dominant features 
are the 2.16 and 2.3211Ul absorption features of alunite. Several samples 
contain kaolinite and a mixture of the two minerals is also seen. The 
secondary quartz spectra seen at the top of Fig. 4 are almost featureless 
as expected for quartz. However, a 2.2511m feature is seen and can be 
attributed to an Si-OH overtone (Podwysocki, personal communication). 
The boundaries seen in the traverse spectra are also identified in the 
AIS images. 
The results of the July 1984 flight are seen in Fig. 5. The SPAM 
program (AIS User's Guide, 1985) was used to map mineral constituents 
that are plotted in red. In each case the upper spectrum is taken from 
the image and the lower is a laboratory spectrum of a sample collected in 
the mapped area. 
The mineral buddingtonite, an ammonium feldspar, although not at 
first identified, was detected through a 2.12~m absorption feature in the 
image spectra and subsequently sampled in the field in the area shown in 
red. The field sampling was blind because buddingtonite is not identi-
fiable in the field, and at the time the cause of the 2.12~m absorption 
feature was not known. The identification of the mineral was made by L. 
Rowan (personal communication) after samples were collected. Cuprite is 
now the fifth known location of buddingtonite in hydrothermal areas (D. 
Krohn, personal communication). 
The AIS is the first modern, albeit modest, imaging spectrometer to 
be flown over earthly targets. As the results in Cuprite have shown, 
direct identification of surface mineralog~ and mapping thereof, is feas-
ible. Proper calibration and normalization of the data are required to 
detect diagnostic absorption features and facilitate mineralogical 
mapping. 
ACKNOWLEDGMENTS 
This work was carried out at the Jet Propulsion Laboratory, 
California Institute of Technology, under a contract with the National 
Aeronautics and Space Administration. 
REFERENCES 
Abrams, M. 
A. B. 
mining 
2. 3511m 
J., Ashley, R. P., Rowan, L. C., Goetz, A. F. H. and Kahle, 
1977. Mapping of hydrothermal alteration in the Cuprite 
district, Nevada, using aircraft scanner imagery for the 0.46-
spectral region. Geology. vol. 5. pp. 713-718. 
AIS User's Guide - 1985, Jet Propulsion Laboratory, California Institute 
of Technology, Pasadena, California 91109. 
Ashley, R. P. and Silberman, M. L. 1976. 
mineralization of Goldfield, Nevada by 
track method. Economic Geology. vol. 
28 
Direct dating of 
potassium-argon and fission 
71. pp. 904-924. 
" 
.. 
I' 
I 
I 
I~ 
OR1Gt 
OF P C:-' 
KAOLINITE 
, , I I L I I I 
2.05 2.15 2.25 2.35 2.05 2.15 2.25 2.35 
WAVELENGTH~ I' m WAVELENGTH, I'm 
BUDDINGTONITE SECONDARY QUARTZ 
.. 
'1Z·1 ~~ I I I 
l ( 2.05 2.15 2.25 
I 
2.35 
I , , 
2.05 2.15 2.25 2.35 
WAVELENGTH, I'm WAVELENGTH, I'm 
Figur e 5. AIS coverage of Cuprite, with south at the top . covering th e 
t rack wi th a "dogleg" shown i n Fig . 1 . The upper curves ar e 
the J-by-J pixel spectra of th e red area in the respective 
i mages . Th e lower curves are laboratory spectra of samples 
collected in th e red areas. The i ndividual mineral maps were 
made us i ng the SPAM program by selecting representative regions 
for th e four spec tral types shown a nd searching the i mages for 
similar spec tral t ypes. In each i mage, the r egions of similar 
spec tra l t ypes are s hown i n red. 
29 
Goetz;' A. F. H., Vane, G., Solomon, J.E. and Rock, B. N. 1985. Imaging 
spectrometry for earth remote sensing. SCIENCE. in press. 
Kahle, A. B. and Goetz, A. F. H. 1983. Mineralogic information from a 
new airborne thermal infrared multispectral scanner. SCIENCE. 
vol. 222. pp. 24-27. 
31 


Mineralogical Mapping in the Cuprite Mining District, Nevada

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