Reflectance Spectroscopy Applied to Exploration for

REFLECTANCE SPECTROSCOPY APPLIED TO EXPLORATION FOR
MINERAL DEPOSITS AND GEOTHERMAL SYSTEMS, AND TO THE
REMEDIATION OF MINED LANDS IN THE GREAT BASIN OF THE UNITED
STATES
James V. Taranik, Wendy M. Calvin, and Fred A. Kruse
Arthur Brant Laboratory for Exploration Geophysics
Department of Geological Sciences and Engineering
University of Nevada, Reno,
Reno, Nevada 89557-0172
Abstract
The Great Basin of the Western United States has thin continental crust and geologic
structures that have controlled the emplacement of significant precious and base metal
mineralization. Many of the mineral deposits in the Great Basin were emplaced by
hydrothermal processes that altered the mineralogy of their host rock assemblages and
facilitated concentration of metals. In the 1960’s and 1970’s research with laboratory
spectrometers determined that some alteration minerals associated with mineral deposits
have discrete spectral signatures that should permit their identification and mapping in
the field. Research in the 1970’s found that clays and iron oxides, associated with
mineralized systems, could be detected in multiband image data and mapped using their
broad spectral signatures. Beginning in the 1980’s, research with prototype airborne
imaging spectrometer data and ground-based spectrometers identified suites of alteration
minerals that are key indicators of mineralized systems. Recent airborne and satellite
systems have demonstrated that detailed mineral mapping is possible from aerospace
measurements, as verified by ground-based spectrometer measurements. These spectral
measurements document the zonation of low and high temperature mineral assemblages.
Minerals like chlorite, epidote, calcite were found as propylitic alteration farther from
igneous centers while quartz, quartz-alunite, and alunite were identified nearer the
centers. Clay mineral assemblages were located between the two alteration zones,
including: kaolinite, dickite, illite and montmorillinite. Iron oxide and iron sulfate
minerals (hematite, goethite and jarosite respectively) were also found associated with
alteration zones. There are now several reference spectral libraries and
airborne/spaceborne case histories available that illustrate the unique spectral character of
these distinctive mineral spectra and our ability to identify and map them from imagery.
The technology and methods developed are used in a variety of related areas; mineral and
geothermal exploration, detection of acid mine drainage, and the assessment of the
effectiveness of mined-land reclamation.
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The Great Basin of the Western United States
The Great Basin of the Western United States includes all of Nevada and portions
of Idaho, Utah and Arizona, Figure 1. The Great Basin includes the Carlin Trend that is
one of the five major precious metal districts in the world. It includes significant copper
and molybdenum production and North America’s largest open pit copper mine,
Bingham Canyon. The crust of the Great Basin is relatively thin and the structural
framework has allowed mineralizing fluids to emplace significant deposits. The
structural framework of the Great Basin, coupled with relatively shallow, high heat flow,
have combined to make the Great Basin the “Saudi Arabia” of geothermal energy. The
region is semi-arid, with vegetative cover frequently less than 30%, and sparse cloud
cover throughout much of the year. Therefore, the Great Basin became the physiographic
region of choice for testing new aerospace remote sensing techniques. When Landsat
(formerly ERTS-1) data became available in the early 1970’s the United States
Geological Survey (USGS) and the National Aeronautics and Space Administration
(NASA) investigators began to acquire aerospace image data over Goldfield and Cuprite,
Nevada and later over Virginia City, Nevada. Today, the Great Basin remains as one of
the most studied areas for evaluating the measurements made by spaceborne, airborne
and ground-based electromagnetic remote sensing sensors.
Figure 1. Relief map with outline of the Great Basin.
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History of Spectral Remote Sensing in the Great Basin
In the 1960’s laboratory research conducted mainly by Graham Hunt and John
Salisbury (Hunt and Salisbury, 1970) made the pioneering laboratory reflectance
measurements of reflectance variations from mineral and rocks. Their research at the Air
Force Cambridge Laboratory demonstrated the potential for remote detection of
important rock constituents and even specific minerals. They published their laboratory
spectral measurements in Modern Geology, a journal that is no longer in print. Most of
their laboratory spectra were measured under controlled conditions using dry, powdered
samples of relatively pure mineral substances from a wide variety of localities. Their
work laid the groundwork for the development of spectral libraries by the U. S.
Geological Survey and the Jet Propulsion Laboratory (JPL) of the California Institute of
technology (CIT).
In the 1970’s USGS and JPL research with aircraft and satellite multispectral
image data over Goldfield, Nevada found that mineral groups, (clays and iron oxides)
associated with hydrothermal alteration, could be detected and mapped (Rowan, et. al.,
1974). However, the laboratory spectral measurements by Hunt and Salisbury showed
promise of not only detecting the presence of clay and iron minerals, but also identifying
clay and iron minerals, and mapping their abundances throughout areas being
investigated. To understand dominant mineralogies and lithologies present in the field,
ground-base reflectance measurements became necessary. Initially, simple handheld
multiband instruments were developed for ground investigations of natural surface cover:
rocks, unconsolidated rock materials, soils, coatings on rocks and soil, vegetation, human
made materials and mixtures of the above. Many of these early instruments were first
tested in the Great Basin.
As the capabilities of airborne sensors increased in the 1970’s, and measurements
in more spectral bands became possible, the need increased for the development of field
instruments to measure solar reflectance from natural cover types in hundreds of spectral
bands. Field spectrometers were first developed by the Jet Propulsion Laboratory and a
private firm, Geophysical Environmental Research (GER). These spectrometers were
first tested in Goldfield and Cuprite, Nevada. Airborne radiometers (not imagers)
demonstrated the potential for measuring discrete spectra in hundreds of individual bands
from airborne platforms. (Chiu and Collins, 1978). In the late 1970’s, developments in
solid-state imaging technology made it possible to image terrain in tens of wavelength
bands with optical-mechanical scanners (Kruse, et. al., 1990).
In the early1980’s developments in solid-state imaging technology made possible
spectral image measurements in hundreds of wavelength bands. During the early 1980’s
the Jet Propulsion Laboratory, under the direction of Dr. Alex Goetz, developed the
Airborne Imaging Spectrometer (AIS) using the JPL Director’s discretionary funds. The
AIS measured reflectance in 128 spectral bands from 10 meter-sized areas. Some of the
earliest overflights with this new instrument were over Cuprite and Goldfield, Nevada in
the Great Basin.
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Geochemical Zonation in Great Basin Hydrothermal Systems
Figure 2. Elements of a Typical Volcanic-Hydrothermal system.
Great Basin mineral deposits and geothermal occurrences are intimately linked to
the evolution of volcanic-hydrothermal systems. Figure 2 illustrates linkages between
silic magmatic fluids derived from evolving porphyry intrusives, structural control of the
volcanic edifice, development of hot springs and low sulfidation base and precious metal
deposits. Structurally controlled high sulfidation systems tend to form closer to the
original volcanic center. The geochemistry of these systems dictates how the original
host rocks are altered to produce mineral suites that can be detected in discrete spectral
bands by aerospace measurements of solar reflectance, and subsequent earth emittance.
Figure 3 shows alteration mineral assemblages commonly found in lowsulfidation systems in the Great Basin. These systems are commonly characterized by
hot springs, associated opaline silica, chalcedony, alunite and kaolinite. Frequently,
native sulfur can be found in fractures near vents. Mixed layer clays, chlorite and epidote
are found at distance from the main volcanic center. Ore found in these systems can be
disseminated in permeable rocks surrounding the main fractures that delivered the
mineralizing fluids, and often, high-grade mineralization is found at depth below the
disseminated ore bodies. High-grade mineralization is often associated with quartz
alunite and quartz veins.
Figure 4 shows alteration mineral assemblages commonly found in highsulfidation systems found in the Great Basin. These systems have a distinct alteration
zonation from propylitic (chlorite, epidote, calcite) through argillic (montmorillionite,
illite and kaolinite) to, ultimately, quartz alunite and mineralized mineralized quartz
veins.
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Figure 3. Typical Mineralogy Found in Low-Sulfidation Systems in the Great Basin
Figure 4. Typical Mineralogy in High-Sulfidation Systems in the Great Basin.
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Table 1 – General Characteristics of epithermal precious metal deposits
Spectral Investigations with AVIRIS in the Great Basin
During the late 1980’s and throughout much of the 1990’s NASA and JPL
developed and perfected the AVIRIS system and began to acquire image data over
Cuprite, Goldfield and the Virginia City areas. This system was flown in an Earth
Resources U-2 high altitude aircraft, permitting coverage of large areas throughout the
United States and, later around the World. AVIRIS collects its hyperspectral data in 224
spectral bands, in 9.2 nanometer bandpasses, with 10 to 20 meter ground instantaneous
field of views, over about a 10 to 20-kilometer ground swath. Later in the 1990’s
AVIRIS was alternatively mounted in a Twin Otter to acquire higher spatial resolution
data. Development and flight of this hyperspectral system was paralleled by the
development of portable ground-based spectrometers for verification of the spectral
signatures of cover types. In the 1980’s Greg Swayze, of the U. S. Geological Survey’s
Spectral Laboratory in Denver, repeatedly acquired AVIRIS data over Cuprite and
Goldfield, Nevada. As the instrument was improved, Swayze was able to show that the
number of relatively pure mineral endmembers that could be detected in the Cuprite area
increased from 8 to 64 (Swayze, 1998). Alvaro Crosta, on a sabbatical at the Desert
Research Institute, collaborated with Chuck Sabine and Jim Taranik in a study of
hydrothermal alteration in the Bodie mining district using 1992 AVIRIS data, (Crosta,
1998). This study utilized two different image data analysis procedures; spectral angle
mapper (SAM), and Tricorder (Now Tetracorder) from the U. S. Geological Survey’s
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Spectroscopy Laboratory. Laboratory spectra was measured on a Beckman spectrometer
and compared with X-Ray Diffraction measurements, (Crosta, et. al., 1998).
Spectral Research Related to Mineral and Geothermal Systems in Nevada
Virginia City, Nevada
Figure 7. Mineralizing structure
Figure. 8. Location Map
Figure 9. Alteration minerals associated with the Comstock Fault, after Hudson, 1980.
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The Virginia City mining district is located 27 km southeast of Reno, (Figure 8). This
district produced over 225,000 kb of gold and 7 million kg of silver between 1862 and 1953.
The oldest rocks are Mesozoic metasediments and metavolcanic rocks that have been intruded
by Cretaceous granodiorite. These units are unconformably overlain by early Miocene silicic
ash-flow tuffs, thick andesite flows and associated breccias of the Alta Formation. Overlying
the Alta Formation, are andesite flows, breccias and accompanying dikes and stocks of the
Kate Peak formation that are the main host for mineralization for the district (Thompson, 1956).
Mineralizing fluids that emplaced ore deposits in the Comstock Lode district were
controlled by north-south trending listric faults, notably the Comstock and Occidental
faults (Figure 7). Argillic and propylitic alteration are pervasive throughout the district.
Occurrences of fine-grained alunite and kaolinite, resulting from supergene processes
(oxidation of pyrite and the formation of low pH fluids), are localized in the district.
Some of the areas described as “bleached” by earlier workers are hypogene quartz-alunite
and clay minerals produced by fluids having lower pH than those that formed the
Comstock deposits (Vikre, 1998). The main economic deposits of the district were found
along the Comstock fault, often at the intersection of cross-faults. The main alteration
assemblages noted in the district are: (1) Widespread propylitic assemblage not
associated directly with ore mineralization. Propylitized rocks typically have a greenish
color because of associated chlorite and epidote and these altered rocks commonly also
have fractures that are filled with calcite; (2) A zeolitic assemblage superimposed on the
propylitic zone; (3) Erratically distributed quartz-alunite alteration (of high-sulfidation
type) and quartz-sericite-montmorillonite-pyrite alteration associated with gold-silver
veins (Figure 9).
Because of its close proximity to the University of Nevada, Reno, Virginia City,
Steamboat and Geiger Grade became field laboratories for the Arthur Brant laboratory for
Exploration Geophysics (ABLE). AVIRIS data were acquired over Virginia City in 1995
as a part of a “group shoot,” a group of firms interested in using the data for exploration.
Initial data analysis was done by Joe Boardman and Jon Huntington. ABLE acquired the
data from this study from Fred Kruse and when Amer Smailbegovic arrived in 1998 he
began to analyze it with one of the first versions of the Environment for Visualizing
Images (ENVI). Within just a few weeks of his arrival, his analysis mapped some of the
mineral themes listed above and draped them on a false color image of AVIRIS data
(Figure 10). These same mineral themes were then draped on a USGS 1:24,000 scale
topographic map and the spectrally mapped themes from AVIRIS were then evaluated in
the field using an ASD Full-Range Spectrometer. The ASD field measurements were
used to collect samples which were, in turn, used for X-Ray Diffraction analysis in the
Nevada Bureau of mines and Geology. Based on these studies, the investigators
determined that the X-Ray Diffraction analysis was an unnecessary step to confirm the
presence of alteration mineral suites (Smailbegovic, 2000).
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Figure 10. ENVI Analysis of 1995 AVIRIS Data over Virginia City.
The HyperSpecTIR (HST) is an airborne hyperspectral imaging spectrometer
developed and operated by the SpecTIR Corporation. It measures solar reflected radiance
in 227 continuous spectral channels between 0.45 and 2.45 ⎧m (Table 2). It has an IFOV
of 1 mrad (0.057°) and a TFOV that is selectable from 0 to 1 rad (57°) (Watts et al.,
2001). The instrument is mounted on a tilting platform equipped with a "fast optical lineof-sight steering" system that allows for real-time compensation of aircraft motion (roll,
pitch and yaw variations) in heavy turbulence, and increases the dwell time over the
target (Watts et al., 2001). Images are acquired in a series of overlapping frames rather
than a single, continuous strip. HST data were acquired over Virginia City in June 2002
from a Cessna 310 aircraft at an altitude of 2.5 km AGL. The images cover a swath of
1.1 km and have a spatial resolution of 2.5 m. Figure 11 shows one of these swaths
which are scanned from left to right while the instrument collects spectra. The focal
plane is motion stabilized. Although the HyperSpecTIR instrument is no longer flown by
SpecTIR, it has been recently used for ground scans of road cuts and mine pits. Greg
Vaughan used image data from the HST to map spectral alteration themes in Virginia
City (Figure 11). In addition, Vaughan used the ASD to understand spectral mixing in
AVIRIS pixel data at Wheeler Reservoir in Virginia City (Figure 12). AVIRIS pixels
were plotted on an Orthophoto Quadrangle and then sampled with an ASD Spectrometer,
shown in use by Greg Vaughan in Figure 12.
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Figure 11. SpecTIR HyperSpecTIR (HST-3) sensor data processed with ENVI to
produce spectra of typical high-sulfidation alteration mineral assemblages, Vaughan and
Calvin, 2005.
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Figure 12. Spectral mixing analysis conducted by Greg Vaughan at Virginia City. Dr.
Vaughan shown at right with ASD Spectrometer.
Aurora, Nevada and Bodie, California
The Bodie Hills area is situated between the Wassuk Range on the North and
Mono Basin on the South, split roughly in half by the Nevada-California state line. The
area is mountainous with elevations ranging from 1,716 m (5,150 ft) in Fletcher Valley to
3,675 m (11,015 ft) on Potato Peak. The districts can be reached by improved gravel
roads from Hawthorne, Nevada and Bridgeport, California. The roads are treacherous at
times because they receive minimal or no maintenance and many have washed out over
the years. The United States Forest Service (Bridgeport station) manages most of the land
in the Bodie Hills, with the exception of Bodie (administered by the California State
Parks), and small areas of patented land, primarily near the old mining works of Aurora,
Paramount and Masonic.
The region is characterized by pre-Tertiary metavolcanic and granitic plutonic
rocks, Tertiary and Quaternary volcanic rocks and occasional sedimentary rocks (Figure
13). Tertiary volcanics rocks form about three quarters of the rock types encountered.
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Basement (pre-Tertiary) rocks are exposed only in a few isolated locales. Sedimentary
rocks are conglomerates localized on the fringes of the Bodie Hills, formed from the
erosion of (predominantly) Tertiary volcanic rocks.
The basement of the Bodie Hills is comprised of pre-Cretaceous metavolcanic
rocks and Cretaceous granitic plutons. The basement is exposed in the northern and
eastern portions of the Bodie Hills. The plutons are intruding pre-Cretaceoumetamorphic
rocks of the Excelsior Formation chiefly composed of hornfels and greenstones
(Smailbegovic, 2002). Two suites of Tertiary and Quaternary volcanics are present in the
area: Calc-Alkaline (26 Ma – 4 Ma) and Alkali-Calcic. The Calc-Alkaline suite is
dominated by porphyritic hornblende-pyroxene andesite and biotite-hornblende dacite.
Lying uncomformably on the Miocene assemblage are Pliocene and Pleistocene lava
flows of basaltic-andesite and basalt, ranging in age from 3.5 Ma – 250 Ka
(Smailbegovic, 2002).
Figure 13. Generalized Geologic map of Bodie
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Hyperspectral sensors used in this study (AVIRIS, HyperSpecTIR) are sensitive
to the absorptions associated with iron (0.4 – 1.0 µm ) and clay (2.1 µm to 2.4 µm)
minerals and are capable of accurately recording them with the 10 nm wide band passes.
By using hyperspectral imagery, the desire is to provide information on the type of
alteration observed in the Bodie Hills (particularly Bodie-Aurora trend) and where
possible differentiate high sulfidation from low sulfidation alteration assemblages. By
understanding observed alteration assemblages, conclusions can be drawn as to their
origin and possible temporal development.
Based on the available data and field observations, the types of hydrothermal
alteration encountered in the Bodie Hills fit into the category of Epithermal vein deposits
with two distinct subcategories: high sulfidation and low sulfidation. The minerals
targeted for characterization of the low sulfidation assemblages are: illite,
montmorillonite and sericite; for high sulfidation assemblages: alunite and kaolinite. It
should be noted that kaolinite occurs in both low and high sulfidation assemblages but it
is more pronounced in high sulfidation zones. A thematic mineral map produced from the
endmember-class regions of interest derived from AVIRIS and HST data is shown on
Figure 14. The mineral spectra directly associated with the alteration include kaolinitetype, alunite-type, illite/sericite-type (these two minerals are almost indistinguishable
spectrally in the hyperspectral imagery) and “propylitic-type” spectra. The alunite areas
dominate the ledge on top of the East Brawley Peak and also occur on top of the
Sawtooth complex. Kaolinite closely follows (and often fringes) the distribution of
alunite. However, kaolinite sometimes mixes with other clay minerals noted in the low
sulfidation areas.
Illite, sericite and montmorillonite (Ca/Na) all exhibit the same absorption
feature(s), hence the class should be thought of as a “mixed clay class.” The mixed clay
are spatially associated with the mineralized areas in Aurora and Bodie and do not extend
beyond the boundaries of the districts. On the high resolution imagery, it is narrowly
constrained to the strike of veins observed in the Aurora district. The “propylitic” spectral
endmember class has no apparent clay absorption bands and bears similarity to the
absorption spectra for the chlorite-class minerals (amesite, clinochlore, chamosite etc.).
The analysis of hyperspectral data suggests two types of hydrothermal alteration
present in the western portion of Bodie Hills. On the basis of spectral measurements –
low sulfidation and high sulfidation types of hydrothermal alteration can be distinguished
(Smailbegovic, 2002). Aurora and Bodie are very similar in their low-sulfidation system
characteristics, showing significant argillic/sericitic alteration, closely associated with the
vein systems and propylitic alteration in the general area.
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Figure 14. Results of the Analysis of AVIRIS and SpecTIR HyperSpecTIR data over the
Bodie-Aurora Area. Amer Smailbegovic, 2002.
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Cuprite, Nevada
The Cuprite mining district is located approximately 65 km (40mi) south of Tonopah,
Nevada (Figure 15). It consists of two spatially and temporally separate low-sulfur epithermal
acid-sulfate systems separated by US Highway 95 (Swayze, 1997). Host rocks include Tertiaryage volcanic ash flow/air fall tuffs, flows, sedimentary conglomerates and sandstones and
Cambrian-age carbonates, quartzites, siltstones (west side only). Abrams et al., 1977 mapped 3
alteration zones 1) Silicified (hydrothermal quartz +- calcite), 2) Opalized (opal, variable alunite
and kaolinite), and 3) Argillized (kaolinite and/or montmorillonite) (Figure 16). Cuprite has
been extensively used for over 30 years as a test site for remote sensing technology and
algorithms for mineral mapping. Specific examples include multispectral remote sensing in the
1970s and 1980s, the Airborne Imaging Spectrometer (1983 - 1986), AVIRIS (1987 – Present),
Hyperion (2001), and numerous other multispectral and hyperspectral sensors. It has been the
focus of many geology studies, however, the most comprehensive are those by Ashley and
Abrams (1980), and Swayze (1997). The U.S. Geological Survey has produced detailed mineral
maps using their “Tetracorder” expert system and verified with field mapping, VNIR/SWIR
spectral measurements, and other analytical methods.
Figure 15: Location map, Cuprite and Goldfield, Nevada
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Figure 16: Alteration Map after Abrams and Ashley, 1977
For the purposes of this study, AVIRIS data collected for the Cuprite site during 2002
were \run through the ENVI “hourglass” procedures. The data were corrected to apparent
reflectance using the ACORN atmospheric correction software, Mode 1 (calibrated hyperspectral
data). The atmospheric correction was further refined using a USGS-measured spectrum for
Stonewall Playa at Cuprite (Figure 17), used to “bootstrap” the correction, thus removing
residual artifacts from the atmospherically corrected AVIRIS data. The MNF Transform was run
on 48 SWIR bands from approximately 2.01 – 2.48 micrometers; the results are approximately
20 coherent MNF bands. These 20 MNF bands were used for 25,000 iterations of the PixelPurity-Index (PPI), producing an image band with all of the “pure” pixels ranging from 1 up,
with “hits” coded as a grayscale image. ENVI’s Region of Interest (ROI) utility was used to
extract only those pixels with more than 25 hits in the PPI image to form an ROI with
approximately 4000 pixels. These were loaded into the ENVI n-D Visualizer and interactive nDimensional scatterplot analysis was used in conjunction with spectral plots for the highlighted
pixels identify and extract the purest pixels (endmembers) for 12 spectral classes. An ROI was
created for each endmember and the mean spectrum was extracted from the atmospherically
corrected AVIRIS data for each class. In this case, the Spectral Analyst was used along with
expert knowledge of mineral spectroscopy and a new Plug-In to ENVI the “Spectral Expert”
(Kruse, 2003) to identify the endmembers at Cuprite from their reflectance spectra and
absorption features. Mixture-Tuned-Matched-Filtering (MTMF) was run on the 20 significant
MNF bands using the endmember ROIs (ENVI extracts MNF spectra automatically during
analysis). The output of the MTMF is a series of Matched Filter (MF) images and “Infeasibility”
images, one for each endmember. The MF images allow an estimate of mineral abundance
(Figure 18). The infeasibility images improve the MF results by causing improved rejection of
false alarms using mixture feasibility constraints. The two images (for each endmember) were
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used in ENVI’s interactive 2D scatterplotting function to extract pixels with high MF and low
Infeasibility scores for each endmember. The output is a ROI for each endmember showing
those pixels in the image that most closely match the spectral signature of each specific
endmember. The ROIs are then combined into a classified image using ENVI’s ROI-toClassification function, and can be overlain on a single band image for spatial location (Figure
19).
Figure 17: Photograph of Stonewall Playa at Cuprite, NV (Stop #2) and USGS Spectral Library
Reflectance Spectrum used for improved reflectance correction of Cuprite AVIRIS data.
Figure 18: MF filter images for selected endmembers at Cuprite, NV: (left: kaolinite, center: alunite, right: silica)
showing estimated abundance color coded from blue to red (low to high).
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Figure 19: Left: Cuprite 2002 AVIRIS Endmembers, Right: Cuprite 2002 AVIRIS MTMF Mineral Map: The minerals mapped were
extracted using 2D Scatterplotting of high MF score versus low Infeasibility Score for endmembers extracted from the AVIRIS
data. Endmember were identified by visual and numerical comparison to the USGS Spectral Library splib06. Only the best
mineral matches are shown.
Field investigations were conducted at Cuprite, NV during January and February 2009 to verify
imaging spectrometer analysis results. An ASD FieldSpec Pro spectrometer was used to make
reflectance measurements of selected sites illustrating the key alteration types and occurrences in
the Cuprite Mining District, Figure 20). These included (Figures 21 - 25) a silica pit showing
hydrothermal silica alteration (Stop #3), alunitic alteration (Stop #4), kaolinitic alteration (Stop
#5 – “Kaolinite Hill”), a second silica occurrence (Site #6), and buddingtonite (Stop #7,
“Buddingtonite Bump”). Unaltered volcanic rocks were also observed at Stop #7. Scaled,
printed image alteration maps were reviewed and compared to the on-the-ground alteration. The
ASD field spectrometer was used to verify selected mineralogies and to illustrate the effects and
scale of spectral mixing.
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Figure 20: Orthophoto showing location of and access to ASD measurement sites at Cuprite,
NV. Numbers refer to stops/ASD Spectra discussed in the text.
Figure 21: Stop #3: Silica Pit 1: Opaline Silica, Kaolinite, Alunite
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Figure 22: Stop #4: Alunite Hill
Figure 23: Stop #5: Kaolinite Hill
Figurer 24: Stop #6: Silica Pit 2
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Figure 25: Stop #7: Buddingtonite Bump: verify NH4 minerals, view unaltered Tuffs
Beginning in 2006, SpecTIR, LLC, based in Reno, Nevada began flying a dual
hyperspectral sensor, manufactured by Specim in Finland, and called ProSpecTIR. ProSpecTIR
uses two separate sensors, one for the Visible and Near-Infrared and one for the Short-Wave
Infrared. These sensors combined can make measurements at bandwidths of 5 nm, at spatial
resolutions of 2 meters, in over 350 spectral bandpasses. The sensors are easily mounted in a
light aircraft, Figure 26. ABLE began working with ProSpecTIR data beginning in 2007 and test
flights were flown over Cuprite, Nevada. A preliminary analysis of this data was done by Amer
Smailbegovic, Figure 27. In 2008, SpecTIR and Aerospace Corporation began co-mounting
both ProSpecTIR and SEBASS in the same stabilized mount in Aerospace’s Twin Otter aircraft.
Figure 26. SpecTIR’s ProSpecTIR instrument consists to two solid state imaging sensors
manufactured by Specim in Finland (left). Operator shown in Cessna 206 (right).
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Figure 27. Analysis of Cuprite ProSpecTIR image data by Dr. Smailbegovic.
Goldfield, Nevada
Goldfield, a historic mining district discovered in 1902, is located approximately 40 km
(25mi) south of Tonopah, Nevada (Previous Figure 15). It has been described as the type
locality for the Epithermal Bonanza deposit of the enargite-gold or quartz-alunite type (Berger
1992, Model 25e). Mineralization occurs primarily in Tertiary-age calc-alkalic volcanic rocks
(porphyritic trachyandesite, rhyodacite, quartz latite, and rhyolite). Some minor ore at depth
occurs in Ordovician argillite and Jurassic quartz-monzonite (Ashley, 1990). Goldfield displays
two ages of volcanism 1) Oligocene silicic ash flow tuffs, air-fall tuff, and flows associated with
development of an inferred caldera and associated ring-fracture zone followed by extensive
erosion and 2) emplacement of Lower Miocene flows, tuffs, and breccias with formation of
rhyolite and rhyodacite domes along the ring-fracture zone. Prominent early-Miocene normal
faults (mostly NE-trending, east dipping) formed late in the second episode of volcanism. Less
well-defined fault sets appear to follow the ring-fracture zone. Extensive hydrothermal
alteration, principally occurring in and around the ring-fracture zone is dated at about 20.6 Ma
(lower Miocene). The principal alteration consists of intense acid-sulfate alteration and
bleaching. Several recognized zones include: 1) Advanced argillic (quartz+-alunite+-kaolinite+pyrophyllite+-sericite+-diaspore+leucoxene+pyrite), 2) Phyllic-argillic
(quartz+kaolinite+sericite+-adularia+-opal+pyrite), 3) Argillic (quartz+montmorillonite+illite+kaolinite+-relict feldspar+pyrite), and 4) Propylitic (Chlorite+albite+-epidote+montmorillonite+-caclite+-zeolite+-pyrite), (Figure 28).
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Figure 28: Goldfield, NV, alteration map (from Ashley, 1990).
Figure 29: Left: Goldfield, NV, 2002 AVIRIS Endmembers, Right: Goldfield, NV, 2002 AVIRIS MTMF
Mineral Map: The minerals mapped were extracted using 2D Scatterplotting of high MF score versus low
Infeasibility Score for endmembers extracted from the AVIRIS data. Endmember were identified by visual
and numerical comparison to the USGS Spectral Library splib06. Only the best mineral matches are
shown.
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Field verification of the Goldfield, NV site was conducted during January and February
2009 (Figure 30). At an overlook point (Stop #1, Figure 30), topographic, geologic, and image
maps and perspective views were used to provide an overview of the geology and alteration at
Goldfield. The results of the AVIRIS mineral maps were compared to field ASD spectral
measurements for selected sites for comparison of on-the-ground mineralogy. A prospect pit
(Stop #2) was investigated and kaolinite, alunite, and jarosite alteration spectra measured (Figure
31) using an ASD Fieldspec Pro spectrometer. Stop #3 provided an overlook of the Florence Pit
and ASD measurements of extensive tailings piles confirmed principally kaolinite/jarosite
mineralogy (Figure 32). Stop #4 was at a road cut overlooking the Jumbo Pit, where host rocks
and alteration were measured and a variety of minerals identified including kaolinite and jarosite
(Figure 33). Stops #5 (northeast flank of Vindicator Mtn) and #6 (Ruby Hills) provided access
to alunitic alteration (Figure 34).
Figure 30: Orthophotograph showing the location of specific mineral alteration sites measured
using the ASD field spectrometer.
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Figure 31: Goldfield Stop #2, Prospect pit with alunite, kaolinite, and jarosite
Figure 32: Spectrum of typical Goldfield tailings material, Stop #3 near Florence Pit (Kaolinite
+ Jarosite)
Figure 33: Stop #4, Roadcut overlooking Jumbo Pit, Kaolinite and Jarosite
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Figure 34: Stop #6, Ruby Hills (Alunite, Goethite)
Spectral Studies of Geothermal Systems in the Great Basin
Figure 35. Brady’s/Desert Peak Geothermal Field
Hymap (hyperspectral mapper) image data were used to remotely map unique
geothermal indicator minerals over the Brady–Desert Peak geothermal fields.
Geothermal-related minerals and rocks such as sinter, tufa, and sulfates, display
diagnostic characteristics in the visible and shortwave infrared; their presence and
distribution can be used to guide more detailed field work for geothermal exploration.
The Brady–Desert Peak geothermal fields are located about 80 km east of Reno, Nevada
in the Hot Springs Mountains, (Figure 35). North–northeast-striking en-echelon faults
offset Tertiary volcanic and lacustrine rocks. Two geothermal power plants produce
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electricity from two separate geothermal systems, one with numerous fumaroles and
mudpots, the other showing no active surface expression of geothermal activity. Surface
occurrences of gypsum, calcium–carbonate, hematite, and opaline silica were identified
at both sites with the hyperspectral data; these minerals when considered together are
indicative of geothermal activity at both sites. Mapping results were synthesized with
other spatial data in a geographic information systems (GIS) database that was used to
help draw structural interpretations of faulting and fault controls at the Brady–Desert
Peak area, (Figures 36 and 37).
Figure 36. Brady Geothermal Field in the Western Great Basin
Figure 37. Cross-section of Brady Geothermal Field after Faulds, 2004
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Temperature zonation of alteration assemblages is well known in the exploration
of hydrothermal systems for economic metals. The use of these alteration facies in
geothermal systems has been less studied, particularly as it relates to borehole geology
and system temperature. Several studies have explored the use of both silica and chlorites
as geothermometers and alteration zonation, particularly of clay mineral type (illite,
montmorillonite, beidellite), can characterize the temperature of alteration in geothermal
systems. Infrared spectroscopy is particularly good at identifying a wide variety of
alteration minerals especially clays that are difficult to distinguish in hand samples.
Several promising pilot studies were performed that suggest the power of the technique to
sample continuously and provide mineral logs akin to geophysical ones (Kratt et al.,
2004; Calvin et al., 2005), (Figure 38). These studies demonstrated that core and drill
chips can be rapidly surveyed, acquiring spectra every few to tens of cm of section.
Preliminary work has helped to establish appropriate measurement techniques using the
ASD field spectrometer and we are actively developing core and chip spectral logs that
can be related to other borehole measurements (Figure 39).
Figure 38. ASD Spectrometer being used for spectral measurements of core.
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Figure 39. Incorporation of ASD measurements in core analysis at Bradys Geothermal
Field
Figure 40. ASD measurements of geothermal lithologies near Bradys Geothermal Field.
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Figure 41. Geothermal indicator mineral spectra from ASD measurements.
Conclusions
Beginning in the 1980’s, remote imaging spectroscopy emerged as a
transformational technology. Early experiments with the Airborne Imaging Spectrometer
(AIS) at Cuprite showed that it was not only possible to identify specific minerals using a
spectral library, but it was spectrally possible to measure unknown spectra and later
determine the minerals (Buddingtonite) associated with that spectra. Although studies of
laboratory spectra done a decade earlier on dry mineral powders indicated the potential of
identifying and mapping mineral spectra in the field, in situ measurements were
necessary to evaluate the effects of weathering, mineral coatings and the distribution of
different cover types within an imaged ground instantaneous field of view. Early field
spectrometers were often heavy and cumbersome to use in the field. The development of
the ASD series of portable spectrometers was a breakthrough that enabled one person to
collect the necessary ground spectral data that was important for image processing and
analysis of hyperspectral image data. Even when ground spectrometers were used to
confirm airborne spectral measurements, most traditional exploration geologists
questioned the measurements and insisted on X-ray analysis of field samples to
“confirm” correct identification of alteration mineral. X-ray analysis uses small samples
that are prepared in a laboratory setting and it makes a different measurement than
ground-based and airborne spectrometers. Over the past two decades, most mineralogists
now agree that X-ray analysis is not needed to confirm mineral compositions determined
through remote sensing techniques. ASD spectrometers are commonly used in the
laboratory to make measurements of whole-rock samples collected in the field. Precision
measurements of sub-centimeter samples are now done with a Nicolet spectrometer
system in ABLE’s spectroscopy laboratory at the University of Nevada, Reno.
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30
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Goldfield - Selected References
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Geothermal - Selected References
Kratt, C., W. M. Calvin, M. Coolbaugh, (200?) Mineral Mapping in the Pyramid Lake
Basin: Hydrothermal Alteration and Geothermal Energy Potential, submitted to
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Calvin, W. M., M. Coolbaugh, C. Kratt, and R. G. Vaughan, (2005), Application of
remote sensing technology to geothermal exploration, in Rhoden, H.N.,
Steininger, R.C., and Vikre, P.G., eds., Geological Society of Nevada Symposium
2005: Window to the World, Reno, Nevada, May 2005, p. 1083-1089, 2005.
Vaughan, R.G., S.J. Hook, W.M. Calvin and J.V. Taranik,(2005), Surface mineral
mapping at Steamboat Springs, Nevada, USA, with multi-wavelength thermal
infrared images, Remote Sensing of Environment, 99, (1-2), pp. 140-158, 2005.
Vaughan, R.G., W.M. Calvin, and J.V. Taranik, (2003), SEBASS hyperspectral thermal
infrared data: Calibrated surface emissivity and mineral mapping, Remote Sensing
of Environment, 85, (1), 48-63, 2003.
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remote sensing technology to geothermal exploration, in Rhoden, H.N.,
Steininger, R.C., and Vikre, P.G., eds., Geological Society of Nevada Symposium
2005: Window to the World, Reno, Nevada, May 2005, p. 1083-1089, 2005.
Calvin, W.M., C. Kratt, and J. E. Faulds, (2005), Infrared Spectroscopy for Drillhole
Lithology and Mineralogy, Proceedings, Thirtieth Workshop on Geothermal
Reservoir Engineering Stanford University, Stanford, California, January 31February 2, 2005 SGP-TR-176
Kratt, C., W. Calvin, and S. Lutz, (2004), Spectral Analyses of Well Cuttings from
Drillhole DP23-1, Desert Peak EGS area, Nevada – preliminary study of minerals
and lithologies by infrared spectrometry, Geothermal Resources Council,
Transactions, vol 28. 2004.
Kratt, C. M. Coolbaugh, and W. Calvin, (2003) Possible extension of Brady’s Fault
identified using remote mapping techniques, Geothermal Resources Council
Transactions. Vol. 27. 2003.
Calvin, W. M., Coolbaugh, and R. G. Vaughan, (2002), Geothermal site characterization
using Multi- and Hyperspectral Imagery. Geothermal Resources Council
Transactions, v. 26, p. 483-485, 2002.
Mark F. Coolbaugh, Ph. D. Geology, (2003),“The prediction and detection of geothermal
systems at regional and local scales in Nevada using a geographic information
system spatial statistics, and thermal infrared imagery”, University of Nevada,
Reno, 2003.
R. Greg Vaughan, Ph.D. (2004), Geology “Surface Mineral Mapping at Virginia City and
Steamboat Springs, Nevada with Multi-Wavelength Infrared Remote Sensing
Image Data”, University of Nevada, Reno, August 2004.
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Christopher B. Kratt, M.S. Geology, (2005), “Geothermal Exploration with Remote
Sensing from 0.45 – 2.5 μm over Brady-Desert Peak, Churchill County, Nevada”.
University of Nevada, Reno, May 2005.
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