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Infrared Imaging Spectrometer (AVIRIS)
Quantitative Geochemical Mapping of Ammonium Minerals in the Southern Cedar Mountains, Nevada, Using the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) William M. Baugh,* Fred A. Kruse,†‡ and William W. Atkinson, Jr.‡
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maging spectrometer (hyperspectral) data, field spectral measurements, and laboratory analyses were used to quantitatively map the concentration of mineral-bound ammonium (buddingtonite) in hydrothermally altered volcanic rocks in the southern Cedar Mountains, Esmeralda County, Nevada. Mineral-bound ammonium is a product of ion exchange in silicate minerals and has no visible distinguishing characteristics, however, diagnostic infrared spectral features are ideal for identification using both field and airborne/spaceborne spectrometers. Establishment of a laboratory- or field-based geochemical calibration is presently a prerequisite to quantitative mapping. For this study, ammonium content of rock samples was determined by chemical analysis and reflectance spectra were measured on whole-rock samples. A linear relation was found between ammonium concentration and the depth of a 2.12 lm ammonium absorption feature in buddingtonite. An image-map of ammonium concentration in ppm was derived from Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data by applying the linear calibration to the AVIRIS reflectance-calibrated spectrum at each pixel. Field spectral measurements, made with a PIMA field spectrometer on a 40 m grid, were used to create a groundtruth concentration map that confirmed the AVIRIS re* National Center for Atmospheric Research (NCAR), Boulder, Colorado † Analytical Imaging and Geophysics LLC, Boulder, Colorado ‡ Department of Geological Sciences, University of Colorado, Boulder Address correspondence to William M. Baugh, NCAR, 1850 Table Mesa Dr., Boulder, CO. E-mail: [email protected] Received 20 May 1996; revised 10 December 1997. REMOTE SENS. ENVIRON. 65:292–308 (1998) Elsevier Science Inc., 1998 655 Avenue of the Americas, New York, NY 10010
sults. Field mapping, X-ray diffraction, and petrologic studies performed in conjunction with the AVIRIS analysis show that buddingtonite is the dominant ammonium mineral in the southern Cedar Mountains, that the ammonium is located along northeast-trending basin and range normal faults, and that it is restricted to two of four crystal-rich rhyolitic tuff units of Oligocene age. This study establishes that remote geochemical mapping using imaging spectrometer data is possible, and presents a methodology that could be extended to quantitatively map other minerals that have absorption features in the short-wave infrared. Elsevier Science Inc., 1998
INTRODUCTION Mineral-bound ammonium (NH⫹ 4 ) was discovered by the U.S. Geological Survey in the southern Cedar Mountains of Esmeralda County, Nevada in 1989 (Fig. 1). At 10 km in length, this site is 100 times larger than any previously known occurrence in volcanic rocks (Krohn, 1989). Naturally occurring mineral-bound ammonium is fairly rare; however, it has been found to occur in gold-bearing hydrothermal deposits (Krohn et al., 1988a). Because of this association, it is thought that ammonium may be a useful tool in exploration for gold and other metal deposits (Klyakhin and Levitskiy, 1969; Bottrell and Miller, 1990). Mineral-bound ammonium incorporates an organically-formed ion into an inorganic rock crystal structure. It is produced when an ammonium ion (NH⫹ 4 ) substitutes for alkali cations (usually K⫹) in the crystal structures of silicate minerals such as feldspars, micas, and 0034-4257/98/$19.00 PII S0034-4257(98)00039-X
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Figure 1. Index map of southern Cedar Mountains, Esmeralda County, Nevada.
clays (Krohn et al., 1988a). The ammonium originates in buried organic matter and is transported to the host rock by hydrothermal fluids (Hallam and Eugster, 1976). Ammonium-bearing rocks are not visually distinct from other altered rocks, and the X-ray diffraction pattern of buddingtonite can be easily mistaken for the K-feldspar pattern. Ammonium minerals are clearly identified, however, by absorption features in short wave-infrared (SWIR) wavelengths (2.0–2.5 lm), shown in Figure 2. This study is based on applying a laboratory method developed by Felzer (1990) for measuring ammonium concentration with reflectance spectroscopy to airborne imaging spectrometer data. Felzer (1990) quantitatively measured ammonium concentration in powdered rock samples from Cuprite, Nevada. He found that, over a limited range (i.e., 4000–11,000 ppm), ammonium concentration varies linearly with the depth of an ammonium absorption feature at 2.11 lm; and that this linear relationship could be used to make quantitative measurements. [Note: Felzer used 2.11 lm rather than the commonly accepted 2.12 lm.] This study extends Felzer’s work for use with remotely sensed data and whole rock samples. Previous studies have shown that quantitative
measurements based on remotely sensed data are possible (Hapke, 1981; Johnson et al., 1985; Mustard and Pieters, 1987, Sabol et al., 1992). Effects due to spectral mixing and particle size variation were also shown to hinder quantative measurements. In this study we implement a simple empirical approach because the site presents a unique, pure and relatively unmixed exposure of ammonium minerals. Although the ammonium zone in the Cedar Mountains is by far the largest known occurrence in volcanic rocks, it was only discovered in the last decade (Krohn, 1989). The ammonium distribution at this site has not previously been mapped in detail. Ammonium minerals, in general, have not been quantitatively mapped using remotely sensed data. The objectives of this study were first, to map ammonium concentrations in the southern Cedar Mountains using Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data; second, to determine the mineralogical and structural controls on the distribution of the ammonium and its relationship with the sequence of volcanic rocks; and, third, to determine the relationship of the ammonium to a hot springs system and any precious or base metal mineralization.
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Figure 2. Buddingtonite spectrum from the Cedar Mountains showing a fitted continuum and a continuum-removed spectrum.
The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) is a state-of-the-art remote sensing device flown by NASA on the ER-2 aircraft at 20 km altitude. It has 224 contiguous spectral bands 10 nm wide between 0.4 lm and 2.45 lm, a spatial resolution of 20 m, and produces near laboratory quality spectra (Vane et al., 1993). MINERALOGY AND OCCURRENCE OF AMMONIUM MINERALS Source and Occurrence of Ammonium Minerals Mineral-bound ammonium is part of a biologically assisted nitrogen cycle, analogous to the carbon cycle. Atmospheric nitrogen is fixed by plants and accumulates in some sedimentary rocks as organic matter. Diagenesis and metamorphism of the sediments releases NH3 and N2. These gasses may escape back to the atmosphere or be bound as ammonium in silicate or sulfate rocks. Nitrogen can be released from rocks by thermal decomposition, dehydration, or cation exchange reactions, and may recombine with other rocks or return to the atmosphere, thus completing the cycle (Hallam and Eugster, 1976). Ammonium minerals are associated with hydrothermally altered rocks, hydrocarbons in oil shales, and coal-bearing formations (Krohn et al., 1993). Organicrich shales may have ammonium contents up to 1000 ppm, while most igneous and sedimentary rocks contain less than 100 ppm. In some hydrothermal systems, however, mineral bound ammonium may become much more concentrated. Examples of these sites include: The Geysers, in Sonoma County, California (1400 ppm), Sulfur Bank, California, (almost 50,000 ppm), and Ivanhoe, Nevada (30,000 ppm) (Krohn et al., 1993). In the south-
ern Cedar Mountains ammonium values in hydrothermally altered rocks range from 0 to 8700 ppm (reported here). The charge and radius of ammonium (NH⫹ 4 ) make the substitution for alkali cations easy. In K-feldspars ammonium ions have a radius of 1.43 A˚ and substitute for potassium ions (K⫹), which have a radius of 1.33 A˚ (Klock and Lamothe, 1986). Other alkali cations, such as sodium, can also occupy analogous positions. Buddingtonite, an ammonium feldspar with zeolitic water (water that is not fixed at definite places in the crystal structure), is closely related to the K-feldspars orthoclase and sanidine (Erd et al., 1964). Buddingtonite is less dense than orthoclase or sanidine because the larger (but lighter) ammonium ion expands the cell dimensions while reducing the mass. To accept ammonium, host rocks must have ample minerals with potassium substitution sites. In unaltered volcanic rocks, rhyolite is the most favorable ammonium host because of its abundant K-feldspar. Basalt, on the other hand, should be the least receptive to NH⫹ 4 because of its lack of K-minerals (Ridgway et al., 1990). For sediment-hosted deposits, pelitic rocks, graywakes and arkoses should provide sufficient K-sites, but pure carbonates and quartz sandstones will not. Alteration assemblages will also affect the availability of substitution sites. Basalt, for example, has a mineralogy that is unstable in a hot springs environment, and could easily alter to montmorillonite that would accept ammonium (Atkinson, personal communication, 1994). Potassic and sericitic alteration, and advanced argillic alteration (with alunite) will also create K substitution sites. Conversely, kaolinitization, where K-bearing minerals are removed, and silicification will tend to prevent high ammonium values (Ridgway et al., 1990).
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Because of its lack of distinguishing features, naturally occurring mineral-bound ammonium was only recently discovered. The first description of natural mineral-bound ammonium was by Erd et al. (1964). They found buddingtonite as plagioclase pseudomorphs in hydrothermally altered andesitic rocks at the Sulfur Bank mercury mine in Lake County, California. Other naturally occurring ammonium minerals discovered since then include tobelite, an ammonium mica in hydrothermally altered rhyolitic tuff in Japan (Higashi, 1982); ammonium illite in black shales of a Pb-Zn-Ag deposit in northwestern Alaska (Sterne et al., 1982); ammonioalunite in the Ivanhoe mining district in Elko County, Nevada (Altaner et al., 1988); and ammonioleucite (Hori et al., 1986). Detection of Ammonium Minerals Mineral-bound ammonium is difficult to detect. The substitution of ammonium for alkali cations is subtle and does not produce visible changes in the rock. Bound ammonium has been measured, directly or indirectly using wet chemical methods, X-ray diffraction, combustion techniques, and, more recently, using infrared spectroscopy. With many methods total nitrogen is measured and assumed to be from bound ammonium. Organic material, however, can contribute ammonium or nitrogen and increase the amounts measured. Determining relative proportions of organic and inorganic (bound) ammonium is difficult. [For discussions of these methods see Klock and Lamothe (1986), Kydd and Levinson (1986), Krohn et al. (1988a,b) and Hall (1993).] LOCATION AND GEOLOGY The southern Cedar Mountains are located in, and affected by two large-scale structural features in the western United States: the basin and range province and the Walker Lane belt. The basin and range province is a zone of crustal extension and thinning, characterized by north northeast-trending horst and graben ranges and basins. The Walker Lane belt is a transition zone between the basin and range and the Sierra Nevada to the west. It is characterized by northwest-trending strike-slip faults and extensive areas of altered rock. The mineralbound ammonium zone is about 55 km northwest of Tonopah in T5N.-T6N., R38E. and is restricted to the western edge of the southern Cedar Mountains. The ammonium is located approximately between an unnamed 6927 ft peak and a radio tower about 8 km to the south (Figs. 1, 8, and 11). A single small area of ammonium concentration was selected for detailed study (ammonium ridge area, Fig. 1) based on the occurrence of ammonium minerals shown on the processed AVIRIS images. This area is slightly less that a quarter of a square kilometer, is well exposed, and contains the highest ammonium concentrations.
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Geologic mapping was done on U.S. Geological Survey 7.5-min topographic maps at a scale of 1:24,000 (Outlaw Springs and Cole Spring quadrangles). A geologic map by Whitebread and Hardyman (1987), at a scale of 1:62,500, identifies geologic units and structural features in good detail and was used as a base map for geologic mapping. Current field observations differ from this map in some places, however. Field mapping results are shown in Figure 3 (color map). The southern Cedar Mountains are composed of both pre-Tertiary clastic rocks and Tertiary volcanic rocks (Figs. 3, 4). Rock units consist of the pre-Tertiary Mina (Pm) and Dunlap Formations (Jd); and the Tertiary Tuff of Cirac Valley (Tcv), an unnamed andesite unit (Ta), the Tuff of Royston Hills (Trh), the Tuff of Cedar Mountains Upper (Tcmu) and Lower (Tcml) units and an unnamed sedimentary unit (Ts). The Tertiary section unconformably overlies the pre-Tertiary rocks (Fig. 4). Four types of faults are observed in the southern Cedar Mountains: 1) northeast- and 2) northwest-trending high angle faults, 3) a small thrust fault, and 4) minor east striking high angle faults. A northeast-trending normal fault (basin and range), northwest of the unnamed 6972 ft peak, offsets Tertiary units approximately 100 m down to the north (Fig. 3). Another northeast-trending normal fault down-dropped the ammonium ridge several tens of meters. Northweststriking Walker Lane (?) faults occur dominantly through the center of the field area. In the Tertiary rocks, on the western side of the mountains, three directions of faulting (northeast, northwest, and east) are superimposed and densely fracture the rock. Pieces of unfractured rock larger than several square meters are scarce. GEOCHEMICAL AND INFRARED SPECTRAL ANALYSIS Chemical Nitrogen Determinations The concentration of ammonium in rock samples was determined by two laboratory methods: a combustion (Dumas) technique that measures nitrogen (Tabatabai and Bremner, 1991), and with an ion chromatograph that deion. Sample preparation consisted of tects the NH⫹ 4 crushing the rock with a mortar and pestle to a fine texture. Rock chips were chosen that were near, and included, the weathered surface so that concentrations measured would represent those imaged by AVIRIS. The Dumas method is an indirect measure of ammonium in which total nitrogen is assumed to represent ammonium. Samples were combusted in a furnace to release NH4 as N2 and NOX. The nitrogen combustion products were reduced to N2 and measured on a gas chromatograph. Nitrogen values were converted to ammonium concentration with a multiplicative scaling factor of 1.286 (Kydd and Levinson, 1986). This method was
Figure 3. A) Geologic map of the southern Cedar Mountains, Esmeralda County, Nevada. UTM coordinates (Zone 11) of corner locations are shown in parentheses. Contour interval is 100 ft. Names refer to ammonium zones discussed in the text. B) Map legend, compiled from field observations and Whitebread and Hardyman (1987).
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Figure 4. Composite columnar section in the southern Cedar Mountains, Esmeralda County, Nevada. Compiled from field observations, Whitebread and Hardyman (1987), and Albers and Stewart (1972).
chosen because it is quick and relatively inexpensive. Fifty-four samples were analyzed using a Carlo Erba Model 1500 Nitrogen Analyzer. Calculated ammonium concentrations ranged from 0 to 8700 ppm (0.87%). To confirm the presence of ammonium 13 samples were analyzed with a Dionex ion chromatograph (IC). The method is adapted from a technique described by Klock and Lamothe (1986). The samples were decomposed in hydrofluoric and hydrochloric acids in capped plastic bottles for 36 h, then diluted, and injected directly into the IC. The results of the IC analysis prove the existence of ammonium (since the method detects ammonium directly), and generally match the Dumas measurements. Table 1 shows the results of the IC analysis and the corresponding Dumas ammonium values for comparison. Several IC concentrations are very low in comparison to
Table 1. Comparison of IC and Dumas Ammonium Concentration Results Sample CM92-3 CM92-30nl CM92-37 CM92-A1 CM92-B1 CM92-C1 CM92-I7 CM92-I12 CM92-K8 CM93-1G CM93-1I CM93-G16 CM93-K18
IC NH4 (ppm) 1120 4950 897 1038 943 6396 1179 ⬍4 4200 ⬍4 16,132 6483 118
Dumas NH4 (ppm) 1800 3858 257 7201 6815 6815 5787 1028 4372 N/A 5915.6 8744.8 8744.8
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Figure 5. XRD data showing the shift of the 020 buddingtonite peak toward lower values of 2h with increasing NH4 substitution. Data were acquired with CuKa radiation.
the corresponding Dumas measurements. This may be due to ammonium escaping as gas during the lengthy digestion process. It could also be due to the Dumas method measuring nitrogen in the rock samples in a form other than ammonium. Other sources of nitrogen may include organic material adhering to the rock surface. If the low readings are excluded, then the remaining data points form a linear relationship with a slope close to 1 (y⫽1.1x⫹336, R2⫽0.44). X-Ray Diffraction Analysis X-Ray Diffraction X-ray diffraction (XRD) was used to identify the type of ammonium minerals present and to identify other minerals in the samples. XRD allows the most direct measurement of ammonium substitution by distinguishing changes in the crystal structure. The sensitivity of XRD to ammonium substitution is low, however, because the change in crystal size due to the larger ammonium cation is small. Additionally, the presence of smaller substituting cations, such as sodium, can average out the effect of the ammonium cation (Krohn et al., 1988a). To create a known buddingtonite XRD pattern, a buddingtonite sample from Sharon Heights in Menlo Park, CA was crushed and analyzed (NHB2301). A pure orthoclase crystal was also crushed and analyzed for a nonammonium feldspar pattern. Analysis of these samples shows the expected shift of 2h peak position to lower values with increasing ammonium concentration (Fig. 5). A similar shift is seen in the Cedar Mountains samples, indi-
cating ammonium substitution. Distinctive buddingtonite peaks were identified at 15.0⬚ and 22.3⬚ (5.9 A˚ and 4.0 A˚). The only other minerals identified are: orthoclase (K-feldspar), albite (plagioclase feldspar), and quartz. Two nonammonium samples also contain biotite and sericite. Laboratory Spectral Analysis Infrared spectral analysis, the backbone of this research, is a direct measure of NH4 bonds within minerals. The ammonium absorption features in the near-IR and SWIR (0.7–2.5 lm) are believed to be caused by N–H vibrational modes and are analogous to hydroxyl (O–H) vibrational modes, only shifted slightly in wavelength (Krohn et al., 1988a). Buddingtonite absorption features in these wavelengths occur at 1.56 lm, 1.9 lm, 2.02 lm, and 2.12 lm (Fig. 2). (The feature at 1.9 lm is caused by water in the buddingtonite structure.) PIMA Spectrometer Infrared reflectance spectra were measured in the laboratory on all of the samples using an Integrated Spectronics Portable Infrared Mineral Analyzer (PIMA) spectrometer (Kruse, 1996). The PIMA is a hand-held field spectrometer that measures reflectance in wavelengths from 1.3 lm to 2.5 lm. It has a spectral resolution of 0.007–0.010 lm, and a 0.002 lm sampling interval. An internal light source is used, and calibration is provided by an internal reflectance standard. Spectra were also standardized with respect to a National Bureau of Standards halon spectrum (Weidner and Hsia, 1981). For all samples both fresh and weathered surfaces were measured.
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members in the spectrum. Felzer (1990) encountered this situation in the Cuprite, Nevada area where buddingtonite and NH4 smectite spectra were significantly mixed.
Figure 6. Cedar Mountains buddingtonite reflectance spectrum compared with USGS library spectra of buddingtonite (GDS85 D-206), ammonium-jarosite (SCR-NHJ), ammoniumillite (GDS87), and ammonium-alunite (NMNH145596) (Clark et al., 1993).
Measurements taken with the PIMA spectrometer show that buddingtonite is the dominant ammonium mineral present in the southern Cedar Mountains. Buddingtonite absorption features are evident in many of the PIMA spectra; and the spectra are commonly dominated by the strong, asymmetrical feature at 2.12 lm. Figure 6 shows USGS library spectra of several types of ammonium minerals (Clark et al., 1993). The typical Cedar Mountains NH4 spectrum clearly matches the known buddingtonite spectrum. This is the strongest evidence that the ammonium mineral in the southern Cedar Mountains is truly buddingtonite. Other ammonium minerals identified in Cedar Mountains spectra include ammonium montmorillonite or illite–smectite, and possibly ammonium–alunite. The depth of the buddingtonite absorption feature can be reduced by the presence of some minerals, including jarosite, pyrite, and smectite (Felzer, 1990; Krohn, 1988a). Fortunately, field observations and XRD results showed that none of these are present in significant quantities at the study site. Only ammonium smectite was found, but in small enough quantities to be discounted. In locations where mixed spectral features are significant, synthetic mixture experiments would be necessary to determine the proportion of the mixed end
Ammonium Calibration Curve To extract quantitative information from the AVIRIS data, it was necessary to find a function that describes the relationship between ammonium concentration and the depth of an ammonium absorption feature. Felzer (1990) showed that the relationship between ammonium concentration and the buddingtonite absorption feature at 2.12 lm is linear. To see if this hypothesis applies in the southern Cedar Mountains, the laboratory ammonium concentrations and depths of the 2.12 lm absorption features were plotted. A linear relationship was also observed (Fig. 7). The depth of the buddingtonite absorption feature was measured on continuum-removed PIMA spectra. A continuum is the slowly varying reflectance component found in many spectra, and continuum removal normalizes the spectrum. Continuum removal was accomplished using a program that ratios the spectrum with the lowest convex curve lying above the spectrum (Green and Craig, 1985). Figure 2 illustrates this concept. The resulting “continuum-removed” spectrum appears as a horizontal line at 1.0 interrupted by absorption features extending downward. Small absorption features are enhanced and band depths of absorption features can be quantitatively compared (Kruse, 1988). The continuum-removed band depths and corresponding ammonium concentrations (measured in the laboratory) were plotted, and a good linear fit (R2⫽0.862) was observed (Fig. 7). The equation of the best-fit line through the data is Y⫽13865X⫺348,
(1)
where X is the continuum-removed band depth and Y is the concentration of ammonium in ppm. This relationship is used to convert continuum-removed AVIRIS spectra to ammonium concentration. The two dashed lines in Figure 7 show the 95% prediction intervals about the regression line. IMAGE PROCESSING AND ANALYSIS AVIRIS data were acquired over the southern Cedar Mountains on 25 July 1990. The spectral data were processed to remove the effects of the solar irradiance curve and atmosphere so that only surface reflectance remained. The surface reflectance spectrum is the same type of information gathered by a laboratory spectrometer. Ammonium and clay minerals were initially located using simple display techniques. More accurate mineral identification and location were achieved with a spectral matching program called the Spectral Angle Mapper (SAM). Continuum removal was done for every pixel in the image and ammonium absorption band depths were
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Figure 7. Plot of ammonium concentration vs. band depth, best fit regression line, and 95% prediction intervals. Data points represent 54 southern Cedar Mountains buddingtonite samples.
measured. The linear calibration determined by the laboratory analysis was then applied to convert the band depths to ammonium concentration, and concentration maps were produced. The SAM output was used as a mask to eliminate spurious false positives. Correction to Apparent Reflectance To compare AVIRIS data with laboratory spectra it must first be converted to reflectance. This was accomplished using the empirical line method (Farrand et al., 1994). The radiance measured by AVIRIS is dominated by the solar irradiance curve, atmospheric attenuation and scatter, and viewing geometry effects. Useful mineral absorption features are lost in these factors. The empirical line method calculates a gain and offset for each band based on a least-squares best fit between measured field reflectance spectra and raw image radiance values (Kruse et al., 1990). Spectra for the empirical line correction were collected with a Geophysical and Environmental Research, Inc. (GER), single-beam visible/infrared intelligent spectroradiometer (SIRIS). It measures wavelengths from 0.35 lm to 2.5 lm, and was necessary because the PIMA spectrometer only scans wavelengths from 1.3 lm to 2.5 lm. The SIRIS is a single beam instrument, so the halon reference and sample radiance were measured separately. For the empirical line calibration, light and dark ground target areas that are large enough to cover multiple AVIRIS pixels were selected in the field. A playa was chosen as a light target and a basalt flow as a dark target. Five spectra were measured with the SIRIS at each site. Ten additional measurements of each target were made in the laboratory with the SIRIS. Great care was taken to preserve the upper exposed surfaces of the samples so that representative spectra could be measured. The 15 spectra were averaged and resampled to match AVIRIS
spectral bands. AVIRIS pixels corresponding with the field locations were selected and the empirical line calibration computed. After correcting to apparent reflectance, quick interactive viewing of the image was done to locate areas of ammonium enrichment. The red, green, and blue display colors were used to bracket the ammonium absorption feature at 2.12 lm. Bands slightly below and above 2.12 lm were displayed in red and blue respectively, and the band at 2.12 lm was displayed in green. Where ammonium features occur, low green and high red and blue result in a magenta or purple color. With this method, the major zones of ammonium were quickly identified. Spectral Angle Mapper The location and relative concentration of ammonium minerals were mapped using a spectral matching program called the Spectral Angle Mapper (SAM) (Boardman, unpublished data; Kruse et al., 1993a). This program determines the similarity of a test spectrum (from an AVIRIS pixel) to a reference spectrum (laboratory spectrum) by calculating the “angle” between them, treating each as a vector in a space with dimensionality equal to the number of bands (in this case 37). Output from the SAM is a gray scale image where lower values denote a better match with target spectra. The gray-scale SAM image of ammonium in the southern Cedar Mountains is presented in Figure 8. It was created using a laboratory spectrum of buddingtonite from 2.0 lm to 2.4 lm and AVIRIS Bands 181–217. The output SAM values were inverted so that bright pixels indicate ammonium occurrences. Inspection of the image reveals six general areas of ammonium concentration. These zones were used as targets for field research, and the accuracy of their boundaries was confirmed in the field. For example, the SAM image shows a small
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Figure 8. Grayscale SAM image of the southern Cedar Mountains. Light gray and white areas indicate buddingtonite occurrences. Names refer to ammonium zones discussed in the text.
occurrence of ammonium, on the south side of the 6927 ft peak, in the Tcml unit. This unit is known from field and laboratory work to not contain ammonium. In the field, it was found that a landslide of boulders from the overlying, ammonium-bearing, Tcmu unit corresponded exactly with this anomaly. A relative concentration map of buddingtonite in the ammonium ridge area was created by contouring the SAM output (not shown). Based on comparison with the field mapping and ground sampling, the method of contouring SAM output to create relative concentration maps appears to be effective and could be applied to some other clay
minerals that occur in hydrothermal systems. Relative concentration maps that include several types of minerals could be useful in the study of hydrothermal systems and in prospecting for metals that occur in these systems. Concentration Maps The method that allows the creation of quantitative concentration maps from AVIRIS data is based on work by Felzer (1990) and depends on a linear relationship between the ammonium concentration and the depth of the ammonium absorption feature at 2.12 lm. The first step is to remove the continuum from each of the 300,000⫹
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Figure 9. AVIRIS buddingtonite spectrum from the Cedar Mountains showing a fitted continuum, a continuum-removed spectrum, and the 2.12 lm band depth.
spectra in the AVIRIS cube. Continuum removal was achieved with a prototype expert system called the General Use Expert System for Spectra (GUESS) (Kruse et al., 1993b). A continuum was defined for each spectrum (each pixel) by finding the high points and fitting straight line segments between these points. Dividing the continuum into the original spectrum normalizes the absorption bands to a common reference, or datum of 1.0. [See Clark and Roush (1984) for a discussion of division versus subtraction of the continuum.] Figure 9 shows an AVIRIS buddingtonite spectrum from the Cedar Mountains and a continuum-removed buddingtonite spectrum. The second step is to measure the depth of the 2.12 lm ammonium absorption feature on the continuum-removed data. AVIRIS Band 190 is located at 2.119 lm and is used for this purpose. The continuum-removed Band 190 value was subtracted from 1.0 resulting in a banddepth value between 0 and 1 with larger values corre-
sponding to deeper bands. Figure 9 (top spectrum) shows this process graphically. The result is a single-band image of continuum-removed band depth values. The third step is to convert the continuum-removed band depth values to ammonium concentration by applying the linear calibration that was developed in the laboratory [Eq. (1)]. In this case, X is the input band depth “DN” value and Y is the output ammonium concentration in ppm. Applying this function to the AVIRIS data converts each pixel’s DN value to ammonium concentration in ppm. A concentration map was made at this point by contouring the quantitative AVIRIS data, but a problem was observed. The concentration map indicated some anomalously high ammonium values in regions that were known from field work to be barren. Inspection of several AVIRIS spectra from these regions showed no buddingtonite features. Instead, the spectra simply exhibited very low reflectance and continuum removal produced extreme differences between spectral highs and lows (Fig. 10). Where a low happened to fall on the band used to measure the buddingtonite absorption feature, an extremely deep “absorption” was recorded and an anomalously high ammonium value was produced. The anomalous ammonium values were masked using the SAM results. A threshold SAM value was chosen by inspection to decide where buddingtonite spectra were present. The SAM values were evaluated at each pixel in the concentration image. If the SAM value was above the threshold, the concentration value was kept; but if it was lower, the concentration value was set to zero. This process effectively masked false high-ammonium values. The masked ammonium concentration image was used to create the final ammonium concentration maps. Figure 11 shows the entire 11 km by 8 km field area with colors corresponding to ammonium concentration. Contours are omitted for clarity. Figure 12a
Figure 10. AVIRIS false ammonium spectrum (lower), continuum-removed spectrum (upper), and the false 2.12 lm band depth.
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Figure 11. AVIRIS NH4 concentration map of the southern Cedar Mountains. The grayscale image is AVIRIS Band 28. Colors denote the following (in ppm): blue (2000– 2999), green (3000–3999), yellow (4000–4999), orange (5000–5499), and red (5500–8000). Names refer to ammonium zones discussed in the text.
shows an enlargement of the ammonium ridge area and Figure 12b the ground truth map that is described in the next section. Ground Truth Sampling on Ammonium Ridge The ammonium ridge area was sampled for the purpose of creating a ground-truth concentration map. A regular grid with 40 m spacing was measured using a Brunton compass and a tape. Several distinctive features on the ground were tied into the grid so that it could be located exactly on a topographic map. Using a PIMA spectrometer, spectra of fresh and weathered surfaces were taken
at the 154 surveyed locations. The area covered corresponds to about 620 AVIRIS pixels and accounts for most of the exposed ammonium-bearing rock on the ammonium ridge. Ammonium concentrations were derived from the spectral measurements using the linear calibration described earlier [Eq. (1)]. Finally, the ground-truth concentration data were contoured for comparison with the AVIRIS concentration map of the same area (Fig. 12b). For the purpose of final verification, three samples of ammonium-bearing rock were collected from precisely known locations on this grid. The samples were analyzed for ammonium with the Dumas method, and compared
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Figure 12. AVIRIS (A) and ground-truth (B) ammonium concentration maps of the ammonium ridge area. Contour interval is 1000 ppm, dashed contour is 5500 ppm.
with the corresponding AVIRIS pixels. Table 2 presents this data. The AVIRIS values are approximately half of the laboratory-measured values. This difference may be explained by differences in sampling scale and spectral mixing (see the Discussion section). DISCUSSION AND INTERPRETATION OF RESULTS This site clearly represents a best-case situation for mapping mineral concentration with absorption band depth. The altered rock is well exposed with little vegetation, soil, or talus cover. The alteration mineral of interest, buddingtonite, occurs in a nearly pure state. Other minerals, whose absorption features might interfere with the buddingtonite features, occur in such low concentrations as to be negligible. Spectroscopic measurement of mineral concentration requires consideration of linear and nonlinear effects of particle size and spectral mixing (Hapke, 1981; Mustard and Pieters, 1987; Shipman and Adams, 1987; Sabol et al., 1992). These effects were neglected for this study, however, for the following reasons. First, the goal of this study was to develop a simple, empirical method for measuring ammonium concentration Table 2. Comparison of AVIRIS-Predicated and GroundSampled NH4 Concentrations Sample No.
Rock NH4 ppm
AVIRIS NH4 ppm
CM-93-1l CM-93-G18 CM-93-K18
5915 8744 8744
3342 4467 4565
at this site with AVIRIS data. Second, the characteristics of the site minimize the impacts of particle size (solid rock outcrop) and spectral mixing (pure mineralogy, continuous outcrop at ammonium ridge). Nonetheless, the effects of particle size, spectral mixing, and scaling are evident in the poor match between the three rock samples taken on the ground and corresponding AVIRIS pixel concentrations (Table 2). All three samples were collected from locations that are dominated by talus or alluvium. Both large particle size and spectral mixing (some vegetation, talus, and soil) result in reduced band depth and thus lower ammonium concentration measurements. Finally, the 20 m pixel size causes a scaling effect that plays a role in the mismatch between ground and AVIRIS samples. The pixels contain the average spectra for a 400 m2 area (includes buddingtonite and other material). The three rock samples, and all of the ground-truth grid samples, are pinpoint measurements representing an enormously different scale. Valuable further study would include looking at the variability of the ammonium concentration over scales of several meters, and increasing the density of the points in the groundtruth grid to 10 m or 5 m spacing. Evaluation of Ammonium Mapping Despite the effect of neglecting spectral mixing and particle size, good results are observed by comparing the AVIRIS concentration map and the ground-truth map of the ammonium ridge (Fig. 12). The AVIRIS map indicates that the region is dominantly greater than 4000 ppm and increases to 5500 ppm in the center. The groundtruth map shows less of the highest concentrations, yet it
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confirms that the region is dominantly greater than 4000 ppm and increases to 5500 ppm in the center. The high quality of the AVIRIS concentration map is shown by a small zone of 0% ammonium that appears on both maps (Fig. 12). It was masked by the SAM filter, on the AVIRIS image and showed no ammonium on the ground survey. Field work revealed a 20 m by 5 m piece of a lower tuff unit (Trh-middle unit) faulted in place. This anomaly is one fourth the size of the AVIRIS pixel. Another pixel in the ammonium ridge area was also masked by the SAM filter but the cause of this was not determined. Geology of the Ammonium Occurrence Field studies and AVIRIS images showed that the ammonium minerals occur in two of four exposed volcanic tuff units in the southern Cedar Mountains and form isolated zones along the western mountain front (Figs. 3 and 11). The zones of ammonium correspond with northeast-trending basin and range normal faults and also show a northeast internal grain. Because of the relationship with the northeast-trending structures, it is believed that these fractures were the path used by ammonium-bearing hydrothermal fluids. If this is true, then the time of the ammonium mineralization can be estimated based on the age of the basin and range faults. Stratigraphic control of the ammonium mineralization was first observed on AVIRIS color composite and SAM images. Comparison between AVIRIS images and a geologic map showed that ammonium occurs primarily in the Tcmu unit, and, to a lesser extent, in the Trh unit. Field and laboratory work confirmed these observations, and also showed that ammonium occurs only in the upper unit of Trh. The ground-truth concentration map of the ammonium ridge shows a north- to northeast-trending ammonium concentration pattern (Fig. 12). The zone of the highest ammonium concentration, in the center of the image, trends about 15⬚. A line with a similar trend can also be drawn through concentration peaks on the left side of the image. These patterns may be due to the fairly wide spacing of the sampling points (40 m) and the north–south pattern of sampling. But, more likely, these patterns are caused by the dominant northeast structural grain of the area and may be evidence that the ammonium-bearing hydrothermal waters traveled up the northeast-trending faults and fractures. Age of Ammonium Alteration There is no direct way to find the age of the ammonium alteration. The age of closely related geologic events can be used, however, to estimate the timing of the alteration. If we assume that the ammonium-bearing fluids traveled up basin and range extensional faults that are less than 7 million years old (Kleinhampl and Ziony,
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1985), then the alteration must be younger. Also, in this time period, a heat source can be identified that might have driven the hydrothermal system. Basalt flows and rhyolite domes of Pleistocene age (1.6–5.3 my) occur within 2 km south and southwest of the southern Cedar Mountains (Albers and Stewart, 1972). The intersection of these two time periods is between 1.6 and 5.3 million years ago, and is thus the most likely time of ammonium alteration. Source of Ammonium No obvious source for the ammonium was found in the southern Cedar Mountains. The nearest rock that could be expected to produce ammonium is 30 km to the southwest, near the town of Coaldale (Fig. 1). Four coal beds, totaling about 10 m of thickness, outcrop in Tertiary sedimentary rocks at this site (Albers and Stewart, 1972). The organic-rich sediments of coal beds would be an excellent source for ammonium. The coal outcrops, however, are separated from the Cedar Mountains by the Monte Cristo Range and are quite far away. Yet, the existence of the coal indicates that organic-rich sediments may occur elsewhere, closer to the mineral-bound ammonium. Relationship of Ammonium to Mineralization In the southern Cedar Mountains there appears to be no relationship between ammonium alteration and heavy or precious metal mineralization. The area has been heavily prospected for over 100 years (Krohn, 1989), yet there is little evidence of successful mineral recovery near the ammonium zones. The recorded mineral production is limited to a small amount of antimony that was mined on the east side of the southern Cedar Mountains, and uranium anomalies reported within 1 km south and southwest of the ammonium zone (Albers and Stewart, 1972; Garside, 1973). No drill holes were observed near any of the ammonium zones, however, and nothing is known about what lies below the ammonium. Since ammonium has been linked with heavy metal mineralization, it would be interesting to see if any metals lie at a greater depth in the hydrothermal system and what happens to the ammonium below the surface. Applicability of the Linear Ammonium Calibration The method developed here should be applicable to mapping ammonium minerals at other sites and to mapping other minerals that have absorption features in the short-wave infrared wavelengths. At other sites, however, a new quantitative calibration will be necessary because the calibration itself is site-specific (due to the mineralogical conditions in the southern Cedar Mountains). At other sites the type, or mixture, of ammonium minerals may be different; and minerals such as illite, jarosite, or pyrite may suppress the depth of the ammonium feature.
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For example, consider Ivanhoe, NV, where NH4–alunite and buddingtonite occur together (Krohn et al., 1988a). This combination may increase or decrease the depth of the 2.12 lm ammonium absorption feature for a given ammonium concentration as compared to southern Cedar Mountains rocks. If pyrite or jarosite are also present at Ivanhoe, then the depth of the feature will be further altered for a given ammonium concentration. Making quantitative measurements in other locations, without collecting field samples, would be quite involved. Spectral mixing would have to be considered. A method would need to be developed that takes into account the band suppression due to other minerals in the rock. Band-suppressing minerals could be identified on the imaging spectrometer data, however. CONCLUSIONS A comparison between ground-truth data and processed AVIRIS data shows that AVIRIS is capable of quantitatively mapping the concentration of mineral-bound ammonium in the southern Cedar Mountains, Nevada. The quantitative map was created by producing a calibration equation based on laboratory analysis and applying it to appropriately processed AVIRIS data. Interpretation of AVIRIS data, and field and laboratory observations have lead to the following conclusions: 1. The depth of the 2.12 lm ammonium absorption band varies linearly with ammonium concentration in whole rock samples from the southern Cedar Mountains, Nevada. Ammonium concentrations range from 0 to 8700 ppm in these rocks. 2. The depth of the 2.12 lm ammonium absorption band also varies linearly with ammonium concentration in continuum-removed apparent reflectance data from AVIRIS. Ammonium concentrations are derived from AVIRIS data by applying the linear relationship developed in the laboratory. An AVIRIS concentration map and a detailed groundtruth concentration map of the ammonium ridge area were compared and are similar in both magnitude and pattern of ammonium concentration. 3. Based on laboratory SWIR spectral analysis and X-Ray diffraction, buddingtonite is the dominant ammonium mineral in the southern Cedar Mountains. A small amount of ammonium illite and ammonioalunite also occur. Other minerals, such as illite, jarosite, and organic carbon, that interfere with the ammonium spectral features, are rare or absent in the rocks, thus making this location an ideal site for a remote sensing study. 4. The location of the ammonium minerals is both structurally and stratigraphically controlled. The ammonium was delivered to the rock by hydrothermal fluids that traveled in northeast-trending basin and range normal faults. The hydrothermal
system may have been driven by nearby extrusive igneous activity between 1.6 and 5.3 million years ago, and the rock was made permeable to hydrothermal fluids by three directions of superimposed fracturing. The source for the ammonium is not known, but is suspected to be related to coal beds that outcrop 30 km to the southwest. 5. There is no connection between the ammonium alteration and visible mineralization at the surface. The possibility of mineralization at depth was not ruled out, however, because there is no subsurface information. 6. Quantitative measurement of other minerals with absorption features in the SWIR may be possible. The approach, methods, and concepts described here are adaptable to analysis of many minerals and should allow detailed mapping of fossil hot springs systems, resulting in a better understanding of hydrothermal systems and possibly a new prospecting tool. The method of quantitative mapping developed here may be applicable to other minerals that contain absorption features in the near-IR and SWIR and are also found in hydrothermally altered rocks (if the minerals occur in relatively pure form or consistent mixture proportions). Besides buddingtonite, examples include alunite, kaolinite, and jarosite. If these can be quantitatively mapped, not only can we determine the geometry of fossil hot spring systems, but we can infer much about the chemistry of hydrothermal fluids that affect different parts of such systems. Alunite would indicate low pH and more oxidizing fluids, while buddingtonite denotes high pH and more reducing fluids. Kaolinite is formed by oxidizing fluids of an intermediate pH (Krohn et al., 1988a). Knowledge of fluid chemistry in fossil hot springs obtained using imaging spectrometry and the techniques described here can contribute valuable insight to the nature and geologic history of hot springs systems as well as providing a new, cost effective means of exploration. This research was conducted while the first author was a graduate student at the University of Colorado, Boulder. Partial funding was provided by NASA Grant NAGW-1601. Additional support was provided by the Center for the Study of Earth from Space (CSES) Program for Industrial Excellence (PIERS) and by the Department of Geological Sciences Minerals Applied Research Consortium (EMARC), both at the University of Colorado. Geochemical analysis support was provided by John Drexler, Jeff Swope, and Paul Boni of the Department of Geological Sciences. Field assistance was provided by Chris Dye and Chris Baugh. The authors gratefully acknowledge the advice of M. Dennis Krohn of the U.S. Geological Survey throughout this research.
REFERENCES Albers, J. P., and Stewart, J. H. (1972), Geology and mineral deposits of Esmeralda County, Nevada. Nevada Bureau of Mines and Geology Bulletin, Reno, Nevada, Vol. 78, 80 pp.
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Altaner, S. P., Fitzpatrick, J. J., Krohn, M. D., et al. (1988), Ammonium in alunites. Am. Mineral. 73:145–152. Bottrell, S. H., and Miller, M. F. (1990), The geochemical behaviour of nitrogen compounds during the formation of black shale hosted quartz-vein gold deposits, north Whales. App. Geochem. 5:289–296. Clark, R. N., and Roush, T. L. (1984), Reflectance spectroscopy: quantitative analysis techniques for remote sensing applications. J. Geophys. Res. 89(B7):6329–6340. Clark, R. N., Swayze, G. A., Gallagher, A., King, T. V. V., and Calvin, W. M. (1993), The U. S. Geological Survey Digital Spectral Library: Version 1: 0.2 to 3.0 mm, Open File Report 93-592, U. S. Geological Survey, Washington, DC, 1340 pp. Erd, R. C., White, D. E., Fahey, J. J., and Lee, D. E. (1964), Buddingtonite, an ammonium feldspar with zeolitic water. Am. Mineral. 49:831–850. Farrand, W. H., Singer, R. B., and Merenyi, E. (1994), Retrieval of apparent surface reflectance from AVIRIS data: a comparison of empirical line, radiative transfer, and spectral mixture methods. Remote Sens. Environ. 47:311–321. Felzer, B. S. (1990), Quantitative reflectance spectroscopy of buddingtonite from the Cuprite Mining District, Nevada, M.S. thesis, University of Colorado, Boulder, 137 pp. (unpublished). Garside, L. J. (1973), Radioactive mineral occurrences in Nevada. Nevada Bureau of Mines and Geology Bulletin, Reno, Nevada, Vol. 81, 121 pp. Green, A. A., and Craig, M. D. (1985), Analysis of aircraft spectrometer data with logarithmic residuals. In Proceedings of the Airborne Imaging Spectrometer Data Analysis Workshop (G. Vane and A. F. H. Goetz, Eds.), JPL Publication 85-41, Jet Propulsion Laboratory, Pasadena, CA, pp. 111–119. Hall, A. (1993), Application of the indophenol blue method to the determination of ammonium in silicate rocks and minerals. Appl. Geochem. 8:101–105. Hallam, M., and Eugster, H. P. (1976), Ammonium silicate stability relations. Contr. Mineral. Petrol. 57:227–244. Hapke, B. (1981), Bidirectional reflectance spectroscopy 1. Theory: J. Geophys. Res. 86(B4):3039–3054. Higashi, S. (1982), Tobelite, a new ammonium dioctahedral mica. Mineral. J. 11(3):138–146. Hori, H., Nagashima, K., Yamada, M., Miyawaki, R., and Marubashi, T. (1986), Ammonioleucite, a new mineral from Tatarazawa, Fujioka, Japan. Am. Mineral. 71:1022–1027. Johnson, P. E., Smith, M. O., and Adams, J. B. (1985), Quantitative analysis of planetary reflectance spectra with principal components analysis. In Proceedings of the Fifteenth Lunar and Planetary Science Conference, Part 2, J. Geophys. Res. 90:C805–C810. Kleinhampl, F. J., and Ziony, J. I. (1985), Geology of northern Nye County, Nevada, Nevada Bureau of Mines and Geology Bulletin, Reno, Nevada, Vol. 99A, 172 pp. Klock, P. R., and Lamothe, P. J. (1986), Determination of ammonium in a buddingtonite sample by ion-chromatography. Talanta 33:495–498. Klyakhin, V. A., and Levitskiy, N. F. (1969), Possible role of in the hydrothermal process. Geochem. Int. 6(1): NH⫹ 4 193–198. Krohn, M. D. (1989), Preliminary description of a mineralbound ammonium locality in the Cedar Mountains, Esmeralda County, Nevada, Open File Report #89-637, U.S. Geological Survey, Washington, DC, 12 pp.
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Krohn, M. D., Altaner, S. P., and Hayba, D. O. (1988a), Distribution of ammonium minerals at Hg/Au-bearing hot springs deposits: initial evidence from near-infrared spectral properties. In Proceedings of the Bulk Minable Precious Metal Deposits of Western United States Symposium (R. W. Schaffer, J. J. Cooper, and P. G. Vikre, Eds.), Geological Society of Nevada, Reno, pp. 661–679. Krohn, M. D., Evans, J., and Robinson, G. R., Jr. (1988b), Mineral-bound ammonium in black shales of the Triassic Cumnock formation, Deep River Basin, North Carolina. In Studies of the Early Mesozoic Basins of the Eastern United States (Froelich and Robinson, Eds.), USGS Bulletin, Vol. 1776, U.S. Geological Survey, Washington, DC, pp. 86–98. Krohn, M. D., Kendall, C., Evans, J. R., and Fries, T. L. (1993), Relations of ammonium minerals in several hydrothermal systems in the western U.S. J. Volcanol. Geotherm. Res. 56:401–413. Kruse, F. A. (1988), Use of airborne imaging spectrometer data to map minerals associated with hydrothermally altered rocks in the northern Grapevine Mountains, Nevada, and California. Remote Sens. Environ. 24:31–51. Kruse, F. A. (1996), Identification and mapping of minerals in drill core using hyperspectral image analysis of infrared reflectance spectra. Int. J. Remote Sens. 17:1623–1632. Kruse, F. A., Kierein-Young, K. S., and Boardman, J. W. (1990), Mineral mapping at Cuprite, Nevada with a 63channel imaging spectrometer. Photogramm. Eng. Remote Sens. 56:83–92. Kruse, F. A., Lefkoff, A. B., Boardman, J. W., et al. (1993a), The spectral image processing system (SIPS)—interactive visualization and analysis of imaging spectrometer data. Remote Sens. Environ. 44:145–163. Kruse, F. A., Lefkoff, A. B., and Dietz, J. B. (1993b), Expert system-based mineral mapping in northern Death Valley, California/Nevada, using the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) Remote Sens. Environ. 44:309–336. Kydd, R. A., and Levinson, A. A. (1986), Ammonium halos in lithogeochemical exploration for gold at the Horse Canyon carbonate hosted deposit, Nevada, U.S.A.: use and limitations. Appl. Geochem. 1:407–417. Mustard, J. F., and Pieters, C. M. (1987), Abundance and distribution of ultramafic microbreccia in Moses Rock Dike: quantitative application of mapping spectroscopy. J. Geophys. Res. 92(B10):10,376–10,390. Ridgway, J., Appleton, J. D., and Levinson, A. A. (1990), Ammonium geochemistry in mineral exploration—a comparison of results from the American cordilleras and the southwest Pacific. Appl. Geochem. 5:475–489. Sabol, D. E., Adams, J. B., and Smith, M. O. (1992), Quantitative subpixel spectral detection of targets in multispectral images. J. Geophys. Res. 97:2659–2672. Shipman, H., and Adams, J.B. (1987), Detectability of minerals on desert alluvial fans using reflectance spectra. J. Geophys. Res. 92(B10):10,391–10,402. Sterne, E. J., Reynolds, R. C., and Zantop, H. (1982), Natural ammonium illites from black shales hosting a stratiform base metal deposit, DeLong Mountains, northern Alaska. Clays Clay Min. 30(3):161–166. Tabatabai, M. A., and Bremner, J. M. (1991), Automated instruments for determination of total carbon, nitrogen, and sulfur in soils by combustion techniques. In Soil Analysis
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Modern Instrumental Techniques, 2nd ed. (K. A. Smith, Ed.), Marcel Dekker, New York, pp. 261–286. Weidner, V. R., and Hsia, J. J. (1981), Reflection properties of pressed polytetrafluoroethylene powder. J. Opt. Soc. Am. 71:856–859. Whitebread, D. H., and Hardyman, R. F. (1987), Preliminary geologic map of part of the Cedar Mountains and Royston
Hills, Esmeralda and Nye Counties, Nevada, Open File Report 87-613, scale 1:62,500, U.S. Geological Survey, Washington, DC. Vane, G., Green, R. O., Chrien, T. G., Enmark, H. T., Hansen, E. G., and Porter, W. M. (1993), The airborne visible/infrared imaging spectrometer (AVIRIS). Remote Sens. Environ. 44:145–163.
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maging spectrometer (hyperspectral) data, field spectral measurements, and laboratory analyses were used to quantitatively map the concentration of mineral-bound ammonium (buddingtonite) in hydrothermally altered volcanic rocks in the southern Cedar Mountains, Esmeralda County, Nevada. Mineral-bound ammonium is a product of ion exchange in silicate minerals and has no visible distinguishing characteristics, however, diagnostic infrared spectral features are ideal for identification using both field and airborne/spaceborne spectrometers. Establishment of a laboratory- or field-based geochemical calibration is presently a prerequisite to quantitative mapping. For this study, ammonium content of rock samples was determined by chemical analysis and reflectance spectra were measured on whole-rock samples. A linear relation was found between ammonium concentration and the depth of a 2.12 lm ammonium absorption feature in buddingtonite. An image-map of ammonium concentration in ppm was derived from Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data by applying the linear calibration to the AVIRIS reflectance-calibrated spectrum at each pixel. Field spectral measurements, made with a PIMA field spectrometer on a 40 m grid, were used to create a groundtruth concentration map that confirmed the AVIRIS re* National Center for Atmospheric Research (NCAR), Boulder, Colorado † Analytical Imaging and Geophysics LLC, Boulder, Colorado ‡ Department of Geological Sciences, University of Colorado, Boulder Address correspondence to William M. Baugh, NCAR, 1850 Table Mesa Dr., Boulder, CO. E-mail: [email protected] Received 20 May 1996; revised 10 December 1997. REMOTE SENS. ENVIRON. 65:292–308 (1998) Elsevier Science Inc., 1998 655 Avenue of the Americas, New York, NY 10010
sults. Field mapping, X-ray diffraction, and petrologic studies performed in conjunction with the AVIRIS analysis show that buddingtonite is the dominant ammonium mineral in the southern Cedar Mountains, that the ammonium is located along northeast-trending basin and range normal faults, and that it is restricted to two of four crystal-rich rhyolitic tuff units of Oligocene age. This study establishes that remote geochemical mapping using imaging spectrometer data is possible, and presents a methodology that could be extended to quantitatively map other minerals that have absorption features in the short-wave infrared. Elsevier Science Inc., 1998
INTRODUCTION Mineral-bound ammonium (NH⫹ 4 ) was discovered by the U.S. Geological Survey in the southern Cedar Mountains of Esmeralda County, Nevada in 1989 (Fig. 1). At 10 km in length, this site is 100 times larger than any previously known occurrence in volcanic rocks (Krohn, 1989). Naturally occurring mineral-bound ammonium is fairly rare; however, it has been found to occur in gold-bearing hydrothermal deposits (Krohn et al., 1988a). Because of this association, it is thought that ammonium may be a useful tool in exploration for gold and other metal deposits (Klyakhin and Levitskiy, 1969; Bottrell and Miller, 1990). Mineral-bound ammonium incorporates an organically-formed ion into an inorganic rock crystal structure. It is produced when an ammonium ion (NH⫹ 4 ) substitutes for alkali cations (usually K⫹) in the crystal structures of silicate minerals such as feldspars, micas, and 0034-4257/98/$19.00 PII S0034-4257(98)00039-X
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Figure 1. Index map of southern Cedar Mountains, Esmeralda County, Nevada.
clays (Krohn et al., 1988a). The ammonium originates in buried organic matter and is transported to the host rock by hydrothermal fluids (Hallam and Eugster, 1976). Ammonium-bearing rocks are not visually distinct from other altered rocks, and the X-ray diffraction pattern of buddingtonite can be easily mistaken for the K-feldspar pattern. Ammonium minerals are clearly identified, however, by absorption features in short wave-infrared (SWIR) wavelengths (2.0–2.5 lm), shown in Figure 2. This study is based on applying a laboratory method developed by Felzer (1990) for measuring ammonium concentration with reflectance spectroscopy to airborne imaging spectrometer data. Felzer (1990) quantitatively measured ammonium concentration in powdered rock samples from Cuprite, Nevada. He found that, over a limited range (i.e., 4000–11,000 ppm), ammonium concentration varies linearly with the depth of an ammonium absorption feature at 2.11 lm; and that this linear relationship could be used to make quantitative measurements. [Note: Felzer used 2.11 lm rather than the commonly accepted 2.12 lm.] This study extends Felzer’s work for use with remotely sensed data and whole rock samples. Previous studies have shown that quantitative
measurements based on remotely sensed data are possible (Hapke, 1981; Johnson et al., 1985; Mustard and Pieters, 1987, Sabol et al., 1992). Effects due to spectral mixing and particle size variation were also shown to hinder quantative measurements. In this study we implement a simple empirical approach because the site presents a unique, pure and relatively unmixed exposure of ammonium minerals. Although the ammonium zone in the Cedar Mountains is by far the largest known occurrence in volcanic rocks, it was only discovered in the last decade (Krohn, 1989). The ammonium distribution at this site has not previously been mapped in detail. Ammonium minerals, in general, have not been quantitatively mapped using remotely sensed data. The objectives of this study were first, to map ammonium concentrations in the southern Cedar Mountains using Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data; second, to determine the mineralogical and structural controls on the distribution of the ammonium and its relationship with the sequence of volcanic rocks; and, third, to determine the relationship of the ammonium to a hot springs system and any precious or base metal mineralization.
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Figure 2. Buddingtonite spectrum from the Cedar Mountains showing a fitted continuum and a continuum-removed spectrum.
The Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) is a state-of-the-art remote sensing device flown by NASA on the ER-2 aircraft at 20 km altitude. It has 224 contiguous spectral bands 10 nm wide between 0.4 lm and 2.45 lm, a spatial resolution of 20 m, and produces near laboratory quality spectra (Vane et al., 1993). MINERALOGY AND OCCURRENCE OF AMMONIUM MINERALS Source and Occurrence of Ammonium Minerals Mineral-bound ammonium is part of a biologically assisted nitrogen cycle, analogous to the carbon cycle. Atmospheric nitrogen is fixed by plants and accumulates in some sedimentary rocks as organic matter. Diagenesis and metamorphism of the sediments releases NH3 and N2. These gasses may escape back to the atmosphere or be bound as ammonium in silicate or sulfate rocks. Nitrogen can be released from rocks by thermal decomposition, dehydration, or cation exchange reactions, and may recombine with other rocks or return to the atmosphere, thus completing the cycle (Hallam and Eugster, 1976). Ammonium minerals are associated with hydrothermally altered rocks, hydrocarbons in oil shales, and coal-bearing formations (Krohn et al., 1993). Organicrich shales may have ammonium contents up to 1000 ppm, while most igneous and sedimentary rocks contain less than 100 ppm. In some hydrothermal systems, however, mineral bound ammonium may become much more concentrated. Examples of these sites include: The Geysers, in Sonoma County, California (1400 ppm), Sulfur Bank, California, (almost 50,000 ppm), and Ivanhoe, Nevada (30,000 ppm) (Krohn et al., 1993). In the south-
ern Cedar Mountains ammonium values in hydrothermally altered rocks range from 0 to 8700 ppm (reported here). The charge and radius of ammonium (NH⫹ 4 ) make the substitution for alkali cations easy. In K-feldspars ammonium ions have a radius of 1.43 A˚ and substitute for potassium ions (K⫹), which have a radius of 1.33 A˚ (Klock and Lamothe, 1986). Other alkali cations, such as sodium, can also occupy analogous positions. Buddingtonite, an ammonium feldspar with zeolitic water (water that is not fixed at definite places in the crystal structure), is closely related to the K-feldspars orthoclase and sanidine (Erd et al., 1964). Buddingtonite is less dense than orthoclase or sanidine because the larger (but lighter) ammonium ion expands the cell dimensions while reducing the mass. To accept ammonium, host rocks must have ample minerals with potassium substitution sites. In unaltered volcanic rocks, rhyolite is the most favorable ammonium host because of its abundant K-feldspar. Basalt, on the other hand, should be the least receptive to NH⫹ 4 because of its lack of K-minerals (Ridgway et al., 1990). For sediment-hosted deposits, pelitic rocks, graywakes and arkoses should provide sufficient K-sites, but pure carbonates and quartz sandstones will not. Alteration assemblages will also affect the availability of substitution sites. Basalt, for example, has a mineralogy that is unstable in a hot springs environment, and could easily alter to montmorillonite that would accept ammonium (Atkinson, personal communication, 1994). Potassic and sericitic alteration, and advanced argillic alteration (with alunite) will also create K substitution sites. Conversely, kaolinitization, where K-bearing minerals are removed, and silicification will tend to prevent high ammonium values (Ridgway et al., 1990).
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Because of its lack of distinguishing features, naturally occurring mineral-bound ammonium was only recently discovered. The first description of natural mineral-bound ammonium was by Erd et al. (1964). They found buddingtonite as plagioclase pseudomorphs in hydrothermally altered andesitic rocks at the Sulfur Bank mercury mine in Lake County, California. Other naturally occurring ammonium minerals discovered since then include tobelite, an ammonium mica in hydrothermally altered rhyolitic tuff in Japan (Higashi, 1982); ammonium illite in black shales of a Pb-Zn-Ag deposit in northwestern Alaska (Sterne et al., 1982); ammonioalunite in the Ivanhoe mining district in Elko County, Nevada (Altaner et al., 1988); and ammonioleucite (Hori et al., 1986). Detection of Ammonium Minerals Mineral-bound ammonium is difficult to detect. The substitution of ammonium for alkali cations is subtle and does not produce visible changes in the rock. Bound ammonium has been measured, directly or indirectly using wet chemical methods, X-ray diffraction, combustion techniques, and, more recently, using infrared spectroscopy. With many methods total nitrogen is measured and assumed to be from bound ammonium. Organic material, however, can contribute ammonium or nitrogen and increase the amounts measured. Determining relative proportions of organic and inorganic (bound) ammonium is difficult. [For discussions of these methods see Klock and Lamothe (1986), Kydd and Levinson (1986), Krohn et al. (1988a,b) and Hall (1993).] LOCATION AND GEOLOGY The southern Cedar Mountains are located in, and affected by two large-scale structural features in the western United States: the basin and range province and the Walker Lane belt. The basin and range province is a zone of crustal extension and thinning, characterized by north northeast-trending horst and graben ranges and basins. The Walker Lane belt is a transition zone between the basin and range and the Sierra Nevada to the west. It is characterized by northwest-trending strike-slip faults and extensive areas of altered rock. The mineralbound ammonium zone is about 55 km northwest of Tonopah in T5N.-T6N., R38E. and is restricted to the western edge of the southern Cedar Mountains. The ammonium is located approximately between an unnamed 6927 ft peak and a radio tower about 8 km to the south (Figs. 1, 8, and 11). A single small area of ammonium concentration was selected for detailed study (ammonium ridge area, Fig. 1) based on the occurrence of ammonium minerals shown on the processed AVIRIS images. This area is slightly less that a quarter of a square kilometer, is well exposed, and contains the highest ammonium concentrations.
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Geologic mapping was done on U.S. Geological Survey 7.5-min topographic maps at a scale of 1:24,000 (Outlaw Springs and Cole Spring quadrangles). A geologic map by Whitebread and Hardyman (1987), at a scale of 1:62,500, identifies geologic units and structural features in good detail and was used as a base map for geologic mapping. Current field observations differ from this map in some places, however. Field mapping results are shown in Figure 3 (color map). The southern Cedar Mountains are composed of both pre-Tertiary clastic rocks and Tertiary volcanic rocks (Figs. 3, 4). Rock units consist of the pre-Tertiary Mina (Pm) and Dunlap Formations (Jd); and the Tertiary Tuff of Cirac Valley (Tcv), an unnamed andesite unit (Ta), the Tuff of Royston Hills (Trh), the Tuff of Cedar Mountains Upper (Tcmu) and Lower (Tcml) units and an unnamed sedimentary unit (Ts). The Tertiary section unconformably overlies the pre-Tertiary rocks (Fig. 4). Four types of faults are observed in the southern Cedar Mountains: 1) northeast- and 2) northwest-trending high angle faults, 3) a small thrust fault, and 4) minor east striking high angle faults. A northeast-trending normal fault (basin and range), northwest of the unnamed 6972 ft peak, offsets Tertiary units approximately 100 m down to the north (Fig. 3). Another northeast-trending normal fault down-dropped the ammonium ridge several tens of meters. Northweststriking Walker Lane (?) faults occur dominantly through the center of the field area. In the Tertiary rocks, on the western side of the mountains, three directions of faulting (northeast, northwest, and east) are superimposed and densely fracture the rock. Pieces of unfractured rock larger than several square meters are scarce. GEOCHEMICAL AND INFRARED SPECTRAL ANALYSIS Chemical Nitrogen Determinations The concentration of ammonium in rock samples was determined by two laboratory methods: a combustion (Dumas) technique that measures nitrogen (Tabatabai and Bremner, 1991), and with an ion chromatograph that deion. Sample preparation consisted of tects the NH⫹ 4 crushing the rock with a mortar and pestle to a fine texture. Rock chips were chosen that were near, and included, the weathered surface so that concentrations measured would represent those imaged by AVIRIS. The Dumas method is an indirect measure of ammonium in which total nitrogen is assumed to represent ammonium. Samples were combusted in a furnace to release NH4 as N2 and NOX. The nitrogen combustion products were reduced to N2 and measured on a gas chromatograph. Nitrogen values were converted to ammonium concentration with a multiplicative scaling factor of 1.286 (Kydd and Levinson, 1986). This method was
Figure 3. A) Geologic map of the southern Cedar Mountains, Esmeralda County, Nevada. UTM coordinates (Zone 11) of corner locations are shown in parentheses. Contour interval is 100 ft. Names refer to ammonium zones discussed in the text. B) Map legend, compiled from field observations and Whitebread and Hardyman (1987).
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Figure 4. Composite columnar section in the southern Cedar Mountains, Esmeralda County, Nevada. Compiled from field observations, Whitebread and Hardyman (1987), and Albers and Stewart (1972).
chosen because it is quick and relatively inexpensive. Fifty-four samples were analyzed using a Carlo Erba Model 1500 Nitrogen Analyzer. Calculated ammonium concentrations ranged from 0 to 8700 ppm (0.87%). To confirm the presence of ammonium 13 samples were analyzed with a Dionex ion chromatograph (IC). The method is adapted from a technique described by Klock and Lamothe (1986). The samples were decomposed in hydrofluoric and hydrochloric acids in capped plastic bottles for 36 h, then diluted, and injected directly into the IC. The results of the IC analysis prove the existence of ammonium (since the method detects ammonium directly), and generally match the Dumas measurements. Table 1 shows the results of the IC analysis and the corresponding Dumas ammonium values for comparison. Several IC concentrations are very low in comparison to
Table 1. Comparison of IC and Dumas Ammonium Concentration Results Sample CM92-3 CM92-30nl CM92-37 CM92-A1 CM92-B1 CM92-C1 CM92-I7 CM92-I12 CM92-K8 CM93-1G CM93-1I CM93-G16 CM93-K18
IC NH4 (ppm) 1120 4950 897 1038 943 6396 1179 ⬍4 4200 ⬍4 16,132 6483 118
Dumas NH4 (ppm) 1800 3858 257 7201 6815 6815 5787 1028 4372 N/A 5915.6 8744.8 8744.8
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Figure 5. XRD data showing the shift of the 020 buddingtonite peak toward lower values of 2h with increasing NH4 substitution. Data were acquired with CuKa radiation.
the corresponding Dumas measurements. This may be due to ammonium escaping as gas during the lengthy digestion process. It could also be due to the Dumas method measuring nitrogen in the rock samples in a form other than ammonium. Other sources of nitrogen may include organic material adhering to the rock surface. If the low readings are excluded, then the remaining data points form a linear relationship with a slope close to 1 (y⫽1.1x⫹336, R2⫽0.44). X-Ray Diffraction Analysis X-Ray Diffraction X-ray diffraction (XRD) was used to identify the type of ammonium minerals present and to identify other minerals in the samples. XRD allows the most direct measurement of ammonium substitution by distinguishing changes in the crystal structure. The sensitivity of XRD to ammonium substitution is low, however, because the change in crystal size due to the larger ammonium cation is small. Additionally, the presence of smaller substituting cations, such as sodium, can average out the effect of the ammonium cation (Krohn et al., 1988a). To create a known buddingtonite XRD pattern, a buddingtonite sample from Sharon Heights in Menlo Park, CA was crushed and analyzed (NHB2301). A pure orthoclase crystal was also crushed and analyzed for a nonammonium feldspar pattern. Analysis of these samples shows the expected shift of 2h peak position to lower values with increasing ammonium concentration (Fig. 5). A similar shift is seen in the Cedar Mountains samples, indi-
cating ammonium substitution. Distinctive buddingtonite peaks were identified at 15.0⬚ and 22.3⬚ (5.9 A˚ and 4.0 A˚). The only other minerals identified are: orthoclase (K-feldspar), albite (plagioclase feldspar), and quartz. Two nonammonium samples also contain biotite and sericite. Laboratory Spectral Analysis Infrared spectral analysis, the backbone of this research, is a direct measure of NH4 bonds within minerals. The ammonium absorption features in the near-IR and SWIR (0.7–2.5 lm) are believed to be caused by N–H vibrational modes and are analogous to hydroxyl (O–H) vibrational modes, only shifted slightly in wavelength (Krohn et al., 1988a). Buddingtonite absorption features in these wavelengths occur at 1.56 lm, 1.9 lm, 2.02 lm, and 2.12 lm (Fig. 2). (The feature at 1.9 lm is caused by water in the buddingtonite structure.) PIMA Spectrometer Infrared reflectance spectra were measured in the laboratory on all of the samples using an Integrated Spectronics Portable Infrared Mineral Analyzer (PIMA) spectrometer (Kruse, 1996). The PIMA is a hand-held field spectrometer that measures reflectance in wavelengths from 1.3 lm to 2.5 lm. It has a spectral resolution of 0.007–0.010 lm, and a 0.002 lm sampling interval. An internal light source is used, and calibration is provided by an internal reflectance standard. Spectra were also standardized with respect to a National Bureau of Standards halon spectrum (Weidner and Hsia, 1981). For all samples both fresh and weathered surfaces were measured.
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members in the spectrum. Felzer (1990) encountered this situation in the Cuprite, Nevada area where buddingtonite and NH4 smectite spectra were significantly mixed.
Figure 6. Cedar Mountains buddingtonite reflectance spectrum compared with USGS library spectra of buddingtonite (GDS85 D-206), ammonium-jarosite (SCR-NHJ), ammoniumillite (GDS87), and ammonium-alunite (NMNH145596) (Clark et al., 1993).
Measurements taken with the PIMA spectrometer show that buddingtonite is the dominant ammonium mineral present in the southern Cedar Mountains. Buddingtonite absorption features are evident in many of the PIMA spectra; and the spectra are commonly dominated by the strong, asymmetrical feature at 2.12 lm. Figure 6 shows USGS library spectra of several types of ammonium minerals (Clark et al., 1993). The typical Cedar Mountains NH4 spectrum clearly matches the known buddingtonite spectrum. This is the strongest evidence that the ammonium mineral in the southern Cedar Mountains is truly buddingtonite. Other ammonium minerals identified in Cedar Mountains spectra include ammonium montmorillonite or illite–smectite, and possibly ammonium–alunite. The depth of the buddingtonite absorption feature can be reduced by the presence of some minerals, including jarosite, pyrite, and smectite (Felzer, 1990; Krohn, 1988a). Fortunately, field observations and XRD results showed that none of these are present in significant quantities at the study site. Only ammonium smectite was found, but in small enough quantities to be discounted. In locations where mixed spectral features are significant, synthetic mixture experiments would be necessary to determine the proportion of the mixed end
Ammonium Calibration Curve To extract quantitative information from the AVIRIS data, it was necessary to find a function that describes the relationship between ammonium concentration and the depth of an ammonium absorption feature. Felzer (1990) showed that the relationship between ammonium concentration and the buddingtonite absorption feature at 2.12 lm is linear. To see if this hypothesis applies in the southern Cedar Mountains, the laboratory ammonium concentrations and depths of the 2.12 lm absorption features were plotted. A linear relationship was also observed (Fig. 7). The depth of the buddingtonite absorption feature was measured on continuum-removed PIMA spectra. A continuum is the slowly varying reflectance component found in many spectra, and continuum removal normalizes the spectrum. Continuum removal was accomplished using a program that ratios the spectrum with the lowest convex curve lying above the spectrum (Green and Craig, 1985). Figure 2 illustrates this concept. The resulting “continuum-removed” spectrum appears as a horizontal line at 1.0 interrupted by absorption features extending downward. Small absorption features are enhanced and band depths of absorption features can be quantitatively compared (Kruse, 1988). The continuum-removed band depths and corresponding ammonium concentrations (measured in the laboratory) were plotted, and a good linear fit (R2⫽0.862) was observed (Fig. 7). The equation of the best-fit line through the data is Y⫽13865X⫺348,
(1)
where X is the continuum-removed band depth and Y is the concentration of ammonium in ppm. This relationship is used to convert continuum-removed AVIRIS spectra to ammonium concentration. The two dashed lines in Figure 7 show the 95% prediction intervals about the regression line. IMAGE PROCESSING AND ANALYSIS AVIRIS data were acquired over the southern Cedar Mountains on 25 July 1990. The spectral data were processed to remove the effects of the solar irradiance curve and atmosphere so that only surface reflectance remained. The surface reflectance spectrum is the same type of information gathered by a laboratory spectrometer. Ammonium and clay minerals were initially located using simple display techniques. More accurate mineral identification and location were achieved with a spectral matching program called the Spectral Angle Mapper (SAM). Continuum removal was done for every pixel in the image and ammonium absorption band depths were
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Figure 7. Plot of ammonium concentration vs. band depth, best fit regression line, and 95% prediction intervals. Data points represent 54 southern Cedar Mountains buddingtonite samples.
measured. The linear calibration determined by the laboratory analysis was then applied to convert the band depths to ammonium concentration, and concentration maps were produced. The SAM output was used as a mask to eliminate spurious false positives. Correction to Apparent Reflectance To compare AVIRIS data with laboratory spectra it must first be converted to reflectance. This was accomplished using the empirical line method (Farrand et al., 1994). The radiance measured by AVIRIS is dominated by the solar irradiance curve, atmospheric attenuation and scatter, and viewing geometry effects. Useful mineral absorption features are lost in these factors. The empirical line method calculates a gain and offset for each band based on a least-squares best fit between measured field reflectance spectra and raw image radiance values (Kruse et al., 1990). Spectra for the empirical line correction were collected with a Geophysical and Environmental Research, Inc. (GER), single-beam visible/infrared intelligent spectroradiometer (SIRIS). It measures wavelengths from 0.35 lm to 2.5 lm, and was necessary because the PIMA spectrometer only scans wavelengths from 1.3 lm to 2.5 lm. The SIRIS is a single beam instrument, so the halon reference and sample radiance were measured separately. For the empirical line calibration, light and dark ground target areas that are large enough to cover multiple AVIRIS pixels were selected in the field. A playa was chosen as a light target and a basalt flow as a dark target. Five spectra were measured with the SIRIS at each site. Ten additional measurements of each target were made in the laboratory with the SIRIS. Great care was taken to preserve the upper exposed surfaces of the samples so that representative spectra could be measured. The 15 spectra were averaged and resampled to match AVIRIS
spectral bands. AVIRIS pixels corresponding with the field locations were selected and the empirical line calibration computed. After correcting to apparent reflectance, quick interactive viewing of the image was done to locate areas of ammonium enrichment. The red, green, and blue display colors were used to bracket the ammonium absorption feature at 2.12 lm. Bands slightly below and above 2.12 lm were displayed in red and blue respectively, and the band at 2.12 lm was displayed in green. Where ammonium features occur, low green and high red and blue result in a magenta or purple color. With this method, the major zones of ammonium were quickly identified. Spectral Angle Mapper The location and relative concentration of ammonium minerals were mapped using a spectral matching program called the Spectral Angle Mapper (SAM) (Boardman, unpublished data; Kruse et al., 1993a). This program determines the similarity of a test spectrum (from an AVIRIS pixel) to a reference spectrum (laboratory spectrum) by calculating the “angle” between them, treating each as a vector in a space with dimensionality equal to the number of bands (in this case 37). Output from the SAM is a gray scale image where lower values denote a better match with target spectra. The gray-scale SAM image of ammonium in the southern Cedar Mountains is presented in Figure 8. It was created using a laboratory spectrum of buddingtonite from 2.0 lm to 2.4 lm and AVIRIS Bands 181–217. The output SAM values were inverted so that bright pixels indicate ammonium occurrences. Inspection of the image reveals six general areas of ammonium concentration. These zones were used as targets for field research, and the accuracy of their boundaries was confirmed in the field. For example, the SAM image shows a small
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Figure 8. Grayscale SAM image of the southern Cedar Mountains. Light gray and white areas indicate buddingtonite occurrences. Names refer to ammonium zones discussed in the text.
occurrence of ammonium, on the south side of the 6927 ft peak, in the Tcml unit. This unit is known from field and laboratory work to not contain ammonium. In the field, it was found that a landslide of boulders from the overlying, ammonium-bearing, Tcmu unit corresponded exactly with this anomaly. A relative concentration map of buddingtonite in the ammonium ridge area was created by contouring the SAM output (not shown). Based on comparison with the field mapping and ground sampling, the method of contouring SAM output to create relative concentration maps appears to be effective and could be applied to some other clay
minerals that occur in hydrothermal systems. Relative concentration maps that include several types of minerals could be useful in the study of hydrothermal systems and in prospecting for metals that occur in these systems. Concentration Maps The method that allows the creation of quantitative concentration maps from AVIRIS data is based on work by Felzer (1990) and depends on a linear relationship between the ammonium concentration and the depth of the ammonium absorption feature at 2.12 lm. The first step is to remove the continuum from each of the 300,000⫹
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Figure 9. AVIRIS buddingtonite spectrum from the Cedar Mountains showing a fitted continuum, a continuum-removed spectrum, and the 2.12 lm band depth.
spectra in the AVIRIS cube. Continuum removal was achieved with a prototype expert system called the General Use Expert System for Spectra (GUESS) (Kruse et al., 1993b). A continuum was defined for each spectrum (each pixel) by finding the high points and fitting straight line segments between these points. Dividing the continuum into the original spectrum normalizes the absorption bands to a common reference, or datum of 1.0. [See Clark and Roush (1984) for a discussion of division versus subtraction of the continuum.] Figure 9 shows an AVIRIS buddingtonite spectrum from the Cedar Mountains and a continuum-removed buddingtonite spectrum. The second step is to measure the depth of the 2.12 lm ammonium absorption feature on the continuum-removed data. AVIRIS Band 190 is located at 2.119 lm and is used for this purpose. The continuum-removed Band 190 value was subtracted from 1.0 resulting in a banddepth value between 0 and 1 with larger values corre-
sponding to deeper bands. Figure 9 (top spectrum) shows this process graphically. The result is a single-band image of continuum-removed band depth values. The third step is to convert the continuum-removed band depth values to ammonium concentration by applying the linear calibration that was developed in the laboratory [Eq. (1)]. In this case, X is the input band depth “DN” value and Y is the output ammonium concentration in ppm. Applying this function to the AVIRIS data converts each pixel’s DN value to ammonium concentration in ppm. A concentration map was made at this point by contouring the quantitative AVIRIS data, but a problem was observed. The concentration map indicated some anomalously high ammonium values in regions that were known from field work to be barren. Inspection of several AVIRIS spectra from these regions showed no buddingtonite features. Instead, the spectra simply exhibited very low reflectance and continuum removal produced extreme differences between spectral highs and lows (Fig. 10). Where a low happened to fall on the band used to measure the buddingtonite absorption feature, an extremely deep “absorption” was recorded and an anomalously high ammonium value was produced. The anomalous ammonium values were masked using the SAM results. A threshold SAM value was chosen by inspection to decide where buddingtonite spectra were present. The SAM values were evaluated at each pixel in the concentration image. If the SAM value was above the threshold, the concentration value was kept; but if it was lower, the concentration value was set to zero. This process effectively masked false high-ammonium values. The masked ammonium concentration image was used to create the final ammonium concentration maps. Figure 11 shows the entire 11 km by 8 km field area with colors corresponding to ammonium concentration. Contours are omitted for clarity. Figure 12a
Figure 10. AVIRIS false ammonium spectrum (lower), continuum-removed spectrum (upper), and the false 2.12 lm band depth.
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Figure 11. AVIRIS NH4 concentration map of the southern Cedar Mountains. The grayscale image is AVIRIS Band 28. Colors denote the following (in ppm): blue (2000– 2999), green (3000–3999), yellow (4000–4999), orange (5000–5499), and red (5500–8000). Names refer to ammonium zones discussed in the text.
shows an enlargement of the ammonium ridge area and Figure 12b the ground truth map that is described in the next section. Ground Truth Sampling on Ammonium Ridge The ammonium ridge area was sampled for the purpose of creating a ground-truth concentration map. A regular grid with 40 m spacing was measured using a Brunton compass and a tape. Several distinctive features on the ground were tied into the grid so that it could be located exactly on a topographic map. Using a PIMA spectrometer, spectra of fresh and weathered surfaces were taken
at the 154 surveyed locations. The area covered corresponds to about 620 AVIRIS pixels and accounts for most of the exposed ammonium-bearing rock on the ammonium ridge. Ammonium concentrations were derived from the spectral measurements using the linear calibration described earlier [Eq. (1)]. Finally, the ground-truth concentration data were contoured for comparison with the AVIRIS concentration map of the same area (Fig. 12b). For the purpose of final verification, three samples of ammonium-bearing rock were collected from precisely known locations on this grid. The samples were analyzed for ammonium with the Dumas method, and compared
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Figure 12. AVIRIS (A) and ground-truth (B) ammonium concentration maps of the ammonium ridge area. Contour interval is 1000 ppm, dashed contour is 5500 ppm.
with the corresponding AVIRIS pixels. Table 2 presents this data. The AVIRIS values are approximately half of the laboratory-measured values. This difference may be explained by differences in sampling scale and spectral mixing (see the Discussion section). DISCUSSION AND INTERPRETATION OF RESULTS This site clearly represents a best-case situation for mapping mineral concentration with absorption band depth. The altered rock is well exposed with little vegetation, soil, or talus cover. The alteration mineral of interest, buddingtonite, occurs in a nearly pure state. Other minerals, whose absorption features might interfere with the buddingtonite features, occur in such low concentrations as to be negligible. Spectroscopic measurement of mineral concentration requires consideration of linear and nonlinear effects of particle size and spectral mixing (Hapke, 1981; Mustard and Pieters, 1987; Shipman and Adams, 1987; Sabol et al., 1992). These effects were neglected for this study, however, for the following reasons. First, the goal of this study was to develop a simple, empirical method for measuring ammonium concentration Table 2. Comparison of AVIRIS-Predicated and GroundSampled NH4 Concentrations Sample No.
Rock NH4 ppm
AVIRIS NH4 ppm
CM-93-1l CM-93-G18 CM-93-K18
5915 8744 8744
3342 4467 4565
at this site with AVIRIS data. Second, the characteristics of the site minimize the impacts of particle size (solid rock outcrop) and spectral mixing (pure mineralogy, continuous outcrop at ammonium ridge). Nonetheless, the effects of particle size, spectral mixing, and scaling are evident in the poor match between the three rock samples taken on the ground and corresponding AVIRIS pixel concentrations (Table 2). All three samples were collected from locations that are dominated by talus or alluvium. Both large particle size and spectral mixing (some vegetation, talus, and soil) result in reduced band depth and thus lower ammonium concentration measurements. Finally, the 20 m pixel size causes a scaling effect that plays a role in the mismatch between ground and AVIRIS samples. The pixels contain the average spectra for a 400 m2 area (includes buddingtonite and other material). The three rock samples, and all of the ground-truth grid samples, are pinpoint measurements representing an enormously different scale. Valuable further study would include looking at the variability of the ammonium concentration over scales of several meters, and increasing the density of the points in the groundtruth grid to 10 m or 5 m spacing. Evaluation of Ammonium Mapping Despite the effect of neglecting spectral mixing and particle size, good results are observed by comparing the AVIRIS concentration map and the ground-truth map of the ammonium ridge (Fig. 12). The AVIRIS map indicates that the region is dominantly greater than 4000 ppm and increases to 5500 ppm in the center. The groundtruth map shows less of the highest concentrations, yet it
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confirms that the region is dominantly greater than 4000 ppm and increases to 5500 ppm in the center. The high quality of the AVIRIS concentration map is shown by a small zone of 0% ammonium that appears on both maps (Fig. 12). It was masked by the SAM filter, on the AVIRIS image and showed no ammonium on the ground survey. Field work revealed a 20 m by 5 m piece of a lower tuff unit (Trh-middle unit) faulted in place. This anomaly is one fourth the size of the AVIRIS pixel. Another pixel in the ammonium ridge area was also masked by the SAM filter but the cause of this was not determined. Geology of the Ammonium Occurrence Field studies and AVIRIS images showed that the ammonium minerals occur in two of four exposed volcanic tuff units in the southern Cedar Mountains and form isolated zones along the western mountain front (Figs. 3 and 11). The zones of ammonium correspond with northeast-trending basin and range normal faults and also show a northeast internal grain. Because of the relationship with the northeast-trending structures, it is believed that these fractures were the path used by ammonium-bearing hydrothermal fluids. If this is true, then the time of the ammonium mineralization can be estimated based on the age of the basin and range faults. Stratigraphic control of the ammonium mineralization was first observed on AVIRIS color composite and SAM images. Comparison between AVIRIS images and a geologic map showed that ammonium occurs primarily in the Tcmu unit, and, to a lesser extent, in the Trh unit. Field and laboratory work confirmed these observations, and also showed that ammonium occurs only in the upper unit of Trh. The ground-truth concentration map of the ammonium ridge shows a north- to northeast-trending ammonium concentration pattern (Fig. 12). The zone of the highest ammonium concentration, in the center of the image, trends about 15⬚. A line with a similar trend can also be drawn through concentration peaks on the left side of the image. These patterns may be due to the fairly wide spacing of the sampling points (40 m) and the north–south pattern of sampling. But, more likely, these patterns are caused by the dominant northeast structural grain of the area and may be evidence that the ammonium-bearing hydrothermal waters traveled up the northeast-trending faults and fractures. Age of Ammonium Alteration There is no direct way to find the age of the ammonium alteration. The age of closely related geologic events can be used, however, to estimate the timing of the alteration. If we assume that the ammonium-bearing fluids traveled up basin and range extensional faults that are less than 7 million years old (Kleinhampl and Ziony,
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1985), then the alteration must be younger. Also, in this time period, a heat source can be identified that might have driven the hydrothermal system. Basalt flows and rhyolite domes of Pleistocene age (1.6–5.3 my) occur within 2 km south and southwest of the southern Cedar Mountains (Albers and Stewart, 1972). The intersection of these two time periods is between 1.6 and 5.3 million years ago, and is thus the most likely time of ammonium alteration. Source of Ammonium No obvious source for the ammonium was found in the southern Cedar Mountains. The nearest rock that could be expected to produce ammonium is 30 km to the southwest, near the town of Coaldale (Fig. 1). Four coal beds, totaling about 10 m of thickness, outcrop in Tertiary sedimentary rocks at this site (Albers and Stewart, 1972). The organic-rich sediments of coal beds would be an excellent source for ammonium. The coal outcrops, however, are separated from the Cedar Mountains by the Monte Cristo Range and are quite far away. Yet, the existence of the coal indicates that organic-rich sediments may occur elsewhere, closer to the mineral-bound ammonium. Relationship of Ammonium to Mineralization In the southern Cedar Mountains there appears to be no relationship between ammonium alteration and heavy or precious metal mineralization. The area has been heavily prospected for over 100 years (Krohn, 1989), yet there is little evidence of successful mineral recovery near the ammonium zones. The recorded mineral production is limited to a small amount of antimony that was mined on the east side of the southern Cedar Mountains, and uranium anomalies reported within 1 km south and southwest of the ammonium zone (Albers and Stewart, 1972; Garside, 1973). No drill holes were observed near any of the ammonium zones, however, and nothing is known about what lies below the ammonium. Since ammonium has been linked with heavy metal mineralization, it would be interesting to see if any metals lie at a greater depth in the hydrothermal system and what happens to the ammonium below the surface. Applicability of the Linear Ammonium Calibration The method developed here should be applicable to mapping ammonium minerals at other sites and to mapping other minerals that have absorption features in the short-wave infrared wavelengths. At other sites, however, a new quantitative calibration will be necessary because the calibration itself is site-specific (due to the mineralogical conditions in the southern Cedar Mountains). At other sites the type, or mixture, of ammonium minerals may be different; and minerals such as illite, jarosite, or pyrite may suppress the depth of the ammonium feature.
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For example, consider Ivanhoe, NV, where NH4–alunite and buddingtonite occur together (Krohn et al., 1988a). This combination may increase or decrease the depth of the 2.12 lm ammonium absorption feature for a given ammonium concentration as compared to southern Cedar Mountains rocks. If pyrite or jarosite are also present at Ivanhoe, then the depth of the feature will be further altered for a given ammonium concentration. Making quantitative measurements in other locations, without collecting field samples, would be quite involved. Spectral mixing would have to be considered. A method would need to be developed that takes into account the band suppression due to other minerals in the rock. Band-suppressing minerals could be identified on the imaging spectrometer data, however. CONCLUSIONS A comparison between ground-truth data and processed AVIRIS data shows that AVIRIS is capable of quantitatively mapping the concentration of mineral-bound ammonium in the southern Cedar Mountains, Nevada. The quantitative map was created by producing a calibration equation based on laboratory analysis and applying it to appropriately processed AVIRIS data. Interpretation of AVIRIS data, and field and laboratory observations have lead to the following conclusions: 1. The depth of the 2.12 lm ammonium absorption band varies linearly with ammonium concentration in whole rock samples from the southern Cedar Mountains, Nevada. Ammonium concentrations range from 0 to 8700 ppm in these rocks. 2. The depth of the 2.12 lm ammonium absorption band also varies linearly with ammonium concentration in continuum-removed apparent reflectance data from AVIRIS. Ammonium concentrations are derived from AVIRIS data by applying the linear relationship developed in the laboratory. An AVIRIS concentration map and a detailed groundtruth concentration map of the ammonium ridge area were compared and are similar in both magnitude and pattern of ammonium concentration. 3. Based on laboratory SWIR spectral analysis and X-Ray diffraction, buddingtonite is the dominant ammonium mineral in the southern Cedar Mountains. A small amount of ammonium illite and ammonioalunite also occur. Other minerals, such as illite, jarosite, and organic carbon, that interfere with the ammonium spectral features, are rare or absent in the rocks, thus making this location an ideal site for a remote sensing study. 4. The location of the ammonium minerals is both structurally and stratigraphically controlled. The ammonium was delivered to the rock by hydrothermal fluids that traveled in northeast-trending basin and range normal faults. The hydrothermal
system may have been driven by nearby extrusive igneous activity between 1.6 and 5.3 million years ago, and the rock was made permeable to hydrothermal fluids by three directions of superimposed fracturing. The source for the ammonium is not known, but is suspected to be related to coal beds that outcrop 30 km to the southwest. 5. There is no connection between the ammonium alteration and visible mineralization at the surface. The possibility of mineralization at depth was not ruled out, however, because there is no subsurface information. 6. Quantitative measurement of other minerals with absorption features in the SWIR may be possible. The approach, methods, and concepts described here are adaptable to analysis of many minerals and should allow detailed mapping of fossil hot springs systems, resulting in a better understanding of hydrothermal systems and possibly a new prospecting tool. The method of quantitative mapping developed here may be applicable to other minerals that contain absorption features in the near-IR and SWIR and are also found in hydrothermally altered rocks (if the minerals occur in relatively pure form or consistent mixture proportions). Besides buddingtonite, examples include alunite, kaolinite, and jarosite. If these can be quantitatively mapped, not only can we determine the geometry of fossil hot spring systems, but we can infer much about the chemistry of hydrothermal fluids that affect different parts of such systems. Alunite would indicate low pH and more oxidizing fluids, while buddingtonite denotes high pH and more reducing fluids. Kaolinite is formed by oxidizing fluids of an intermediate pH (Krohn et al., 1988a). Knowledge of fluid chemistry in fossil hot springs obtained using imaging spectrometry and the techniques described here can contribute valuable insight to the nature and geologic history of hot springs systems as well as providing a new, cost effective means of exploration. This research was conducted while the first author was a graduate student at the University of Colorado, Boulder. Partial funding was provided by NASA Grant NAGW-1601. Additional support was provided by the Center for the Study of Earth from Space (CSES) Program for Industrial Excellence (PIERS) and by the Department of Geological Sciences Minerals Applied Research Consortium (EMARC), both at the University of Colorado. Geochemical analysis support was provided by John Drexler, Jeff Swope, and Paul Boni of the Department of Geological Sciences. Field assistance was provided by Chris Dye and Chris Baugh. The authors gratefully acknowledge the advice of M. Dennis Krohn of the U.S. Geological Survey throughout this research.
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