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SIR 2004-5241
Remote Sensing for Environmental
Site Screening and Watershed Evaluation in Utah Mine Lands—East Tintic
Mountains, Oquirrh Mountains ,
and Tushar Mountains
By Barnaby W. Rockwell, Robert R. McDougal, and
Carol A. Gent
Prepared in cooperation with the United States Environmental Protection Agency
Version 1.2
Printable PDF (22 MB)
Keywords: remote sensing, imaging spectroscopy, reflectance spectroscopy, AVIRIS, Landsat 7 ETM+, mineral mapping, abandoned mine lands, acid rock drainage, Superfund Program, geoenvironmental assessment, environmental impacts of mining, Utah mines, mining in Utah, Bingham Canyon mine, Bingham mine, International Smelter, Camp Floyd mining district, Mercur mine, Stockton mining district, Bauer Mill, Tintic mining district, Eureka, Dragon mine, Burgin mine, Trixie mine, Marysvale volcanic field, Deer Trail mine, Big Rock Candy Mountain, jarosite, alunite, goethite, halloysite
Table of Contents
AVIRIS Data Acquisitions, Reflectance Calibration, and Georectification
The East Tintic Mountains and the Tintic Mining District
The Tushar Mountains/Marysvale Region
Appendix. X-Ray Diffraction Results
Abstract
Imaging spectroscopy—a powerful
remote-sensing tool for mapping subtle variations in the composition of
minerals, vegetation, and man-made materials on the Earth's surface—was applied
in support of environmental assessments and watershed evaluations in several
mining districts in the State of
The Landsat 7 ETM+ data were used for initial site screening and the planning of AVIRIS surveys. The AVIRIS data were analyzed to create spectrally defined maps of surface minerals with special emphasis on locating and characterizing rocks and soils with acid-producing potential (APP) and acid-neutralizing potential (ANP). These maps were used by the United States Environmental Protection Agency (USEPA) for three primary purposes: (1) to identify unmined and anthropogenic sources of acid generation in the form of iron sulfide and (or) ferric iron sulfate-bearing minerals such as jarosite and copiapite; (2) to seek evidence for downstream or downwind movement of minerals associated with acid generation, mine waste, and (or) tailings from mines, mill sites, and zones of unmined hydrothermally altered rocks; and (3) to identify carbonate and other acid-buffering minerals that neutralize acidic, potentially metal bearing, solutions and thus mitigate potential environmental effects of acid generation.
Calibrated AVIRIS surface-reflectance data were spectrally analyzed to identify and map selected surface materials. Two maps were produced from each flightline of AVIRIS data: a map of iron-bearing minerals and water having absorption features in the spectral region from 0.35 μm to 1.35 μm and a map of minerals (including clays, sulfates, micas, and carbonates) having absorptions in the spectral region from 1.45 μm to 2.51 μm. Several methods were used to verify the AVIRIS mapping results, including field checking of selected locations with a portable spectrometer, visual inspection of the AVIRIS reflectance spectra, and X-ray diffraction (XRD) analysis of field samples.
The maps of iron-bearing minerals
derived from analysis of the visible (VIS) and near-infrared (NIR) regions of
the electromagnetic spectrum were shown to be more consistently reliable in
indicating the presence of jarosite than were the maps generated from analysis
of the short-wave infrared (SWIR) region. When present in abundance,
phyllosilicate minerals tend to dominate the SWIR and mask the spectral features
of jarosite in that wavelength region. The crystal field absorptions of
jarosite in the
Large exposures of unmined hydrothermally altered rocks occur throughout the three study areas. These rocks commonly contain sulfide or sulfate minerals that produce sulfuric acid upon subaerial oxidation. The acid may be introduced into local surface and ground water and thus lower the baseline (that is, the premining) pH for a watershed.
The three study areas also have widespread exposures of rocks with acid-neutralizing potential. Lithologies containing carbonates and (or) other acid-buffering minerals—such as sedimentary limestones and dolomites and propylitically altered igneous rocks—were mapped with the AVIRIS data throughout the Oquirrh and East Tintic Mountains and locally in the Antelope Range and Tushar Mountains.
Because elevated levels of various
heavy metals in local soils and tap water have been identified by previous
USEPA studies, parts of the town of
In the
In the
Analysis of Landsat 7 ETM+ data can provide a very cost-effective screening tool for identifying mineralized and (or) mining-affected areas and guiding the planning of low-altitude imaging spectrometer surveys or field investigations. Then, when coupled with a geological understanding of a study area, the interpretation of mineral maps derived from imaging spectroscopy data can be an effective means of (1) evaluating potential environmental impacts associated with hydrothermally altered rocks and mine waste on a watershed or regional scale and (2) focusing field-sampling and remediation programs.
Introduction
This report is a summary of the
results obtained from analysis and interpretation of spectroscopic imagery
collected by the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) over
several mining districts in
This geophysical and mineralogical research was undertaken as a part of the United States Environmental Protection Agency (USEPA) and U.S. Geological Survey (USGS) Utah Abandoned Mine Lands (AML) Imaging Spectroscopy Project (U.S. Environmental Protection Agency and U.S. Geological Survey, 2002). An index map showing the study areas for this project is available on the project Web site (http://speclab.cr.usgs.gov/earth.studies/Utah-1/utahproj_large.jpg). The project had three primary goals:
- to remotely identify
and map natural and
anthropogenic sources of acid generation in and around several historic mining
districts in
- to seek evidence for downstream or downwind
transport of minerals associated with acid generation from mine sites and zones
of unmined, altered rock; and
- to identify and map carbonates and other
acid-buffering minerals that neutralize acidic, potentially metal bearing
solutions, thus mitigating their environmental effects.
Remotely sensed image data were used to screen and evaluate watersheds containing multiple sources of mining-related heavy metals because ground surveys using traditional methods of multimedia sampling and analysis are costly and time consuming. The geologic analysis of spectroscopic image data such as those acquired by AVIRIS enables the detection of specific materials and mixtures of materials on the land surface based on quantitative comparisons of spectral absorption features in the image data to libraries of standard reference spectra of minerals, water, vegetation, and man-made materials. Such detailed mapping allows an evaluation of the critical geochemical regimes and processes in an area, thus providing an objective, scientific means of prioritizing potential environmental hazards for the purposes of streamlining and focusing subsequent field-sampling and remediation programs.
Scientific Background
Mapping the Acid-Drainage Geochemical System
Using Imaging Spectroscopy
The iron sulfide mineral pyrite (FeS2) is a common gangue, or waste, mineral in precious and base metal deposits such as those in the mining districts studied in the Utah AML project. Because pyrite contains sulfur and is commonly unstable in moist, subaerial conditions, the mineral plays a key role in determining future geochemical regimes when it is exposed either by erosion or by mining. During mining operations, broken-up waste rock containing pyrite, carbonate minerals (for example, calcite and dolomite), and (or) phyllosilicate minerals (for example, clays and micas) associated with hydrothermal alteration was commonly dumped near the shafts, adits, and open pits of the mines. At ore-processing mills, slurries of tailings material containing pyrite and other gangue minerals were released into impoundments or directly into drainages. Through time, the pyrite will oxidize in the presence of atmospheric oxygen and water to form sulfuric acid (H2SO4) and various ferric and (or) ferrous iron sulfate-hydrate minerals including copiapite (Fe2+Fe3+4(SO4)6(OH)2·20H2O), and melanterite (Fe2+SO4·7H2O). As a part of the reaction process, thin coatings of sulfate salts such as copiapite may be precipitated on waste-rock surfaces as water from rain events evaporates. With time, most of the pyrite in the waste rock will oxidize, leaving behind coatings of fine-grained jarosite ((K,Na,H3O)Fe3+3(SO4)2(OH)6) that are more stable and less soluble than the hydrated iron sulfate salts that precipitate early in the process. These coatings may in turn break down to the metastable mineral ferrihydrite (approximately 5Fe3+2O3·9H2O), then to the ferric iron hydroxide mineral goethite (α-Fe3+O(OH)), and, with additional time, possibly to the ferric iron oxide mineral hematite (Fe2O3) (Swayze and others, 2000). In natural and anthropogenic exposures of jarosite formed from the oxidation of pyrite, a zoning pattern of iron-bearing minerals is commonly observed that reflects the pH of the waters from which the minerals precipitated (Swayze and others, 2000; Rockwell and others, 1999, 2000). This pattern consists of a central core of unaltered pyrite and (or) copiapite formed under low-pH conditions that grades outward into subconcentric and commonly discontinuous zones of jarosite, jarosite + goethite, goethite, and hematite formed under progressively more neutral pH conditions. The metastable secondary mineral schwertmannite (Fe3+16O16(OH)12(SO4)2) may also form in environments affected by acid drainage from undisturbed rocks, mine waste, and tailings (Ferris and others, 1989; Bigham and others, 1992; Desborough and others, 2000).
The sulfuric acid-bearing solutions generated by the oxidizing reactions can infiltrate downward through the waste-rock pile. The weathering process can also produce clay minerals such as smectites (for example, montmorillonite, (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·nH2O) and kaolinite (Al2Si2O5(OH)4) as alteration products of feldspars and micas in the waste rock; kaolinite forms under the most acidic conditions. If Ca concentrations in the acidic solutions are sufficiently high, gypsum (CaSO4·2H2O) may precipitate. The acidic solutions can also mobilize heavy metals (lead, cadmium, zinc, arsenic, etc.) present in the waste rock and potentially transport them into ground and surface water. If present in sufficient concentrations, these heavy metals can pose a health hazard, and they have been found to preferentially adsorb onto amorphous iron hydroxide minerals contained within mine waste in near-neutral pH environments where residence times can be long (Bowell, 1994).
Imaging spectroscopy has been used
since the mid 1990s to map surface minerals in abandoned mine lands for the
purposes of environmental site characterization (Farrand and Harsanyi, 1997;
Smith and others, 1998; Swayze and others, 2000; King and others, 2000; Dalton
and others, 2000). As copiapite, jarosite, goethite, and hematite are
characterized by distinct and diagnostic spectral absorption features in the
region of the electromagnetic spectrum measured by AVIRIS and other imaging
spectrometers (fig. 1) (
Figure 1. Reflectance spectra of minerals associated
with acid drainage.
with a high degree of accuracy by using imaging spectroscopy. In contrast, pyrite is difficult to detect through the use of remote spectroscopic mapping techniques because of its low overall albedo, weak (saturated) absorption features, and frequent masking by coatings of secondary iron sulfate minerals. Pyrite in very high concentrations, however, has been successfully identified and mapped by using AVIRIS data: both the Leadville, Colorado, mining district (Swayze and others, 2000) and the Bauer Mill site near Stockton, Utah (this report), have sufficient waste-rock pyrite to be identified and mapped by AVIRIS. In these cases, the mapped pyrite was surrounded by pixels in which jarosite was identified. Jarosite is thus an important indicator of the presence of rocks bearing pyrite, possibly other sulfide minerals such as chalcopyrite (CuFeS2), and (or) other sulfate minerals that are sources for the generation of acidic solutions. Copiapite is also an important indicator of sulfide minerals that can be reliably mapped with imaging spectroscopy data, but is highly soluble and is therefore less common than jarosite. Other highly soluble, sulfate-bearing salts such as alunogen (Al2(SO4)3·17H2O) and epsomite (MgSO4·7H2O) are also detectable with AVIRIS data, although the lack of narrow diagnostic absorption features in the spectra of these minerals makes remote detection less accurate. These soluble salts may be precipitated as thin, temporary crusts on exposed, pyrite-bearing rock after rain events (Cunningham and others, 2005). As the crusts dry, they can change in color (from yellowish orange to gray or white), suggesting possible mineralogic changes that could be identified and monitored by using spectroscopic data. Although ferrihydrite also has diagnostic electronic absorption features conducive to spectral identification and has been found to occur abundantly in acid-mine-drainage environments (Ferris and others, 1989), it has yet to be definitively identified through the use of imaging spectrometer data because of the apparent spectral dominance of jarosite, goethite, and hematite at the pixel scale and (or) the tendency of ferrihydrite to dissolve and reprecipitate as goethite (Bigham and others, 1992). Schwertmannite is stable at the Earth's surface only in low-pH and (or) aqueous environments, yet was spectrally identified in ferricretes associated with acid rock drainage in the Animas River watershed of the San Juan Mountains of Colorado (Dalton and others, 2000; Desborough and others, 2000).
The just-described background indicates that site-characterization plans developed on the basis of maps derived from remote-sensing surveys should focus subsequent field-sampling efforts on areas in which spectra signifying pyrite-, copiapite-, schwertmannite-, or jarosite-bearing mineral assemblages were identified, as it is likely that rocks in these areas contain the highest concentrations of acid-producing sulfide or sulfate minerals. However, it should be noted that goethite coatings on weathered rocks on the surface may mask abundant pyrite occurring in underlying rocks. On the basis of field studies and an understanding of the geochemical regimes discussed above, however, it can be generally assumed that there will be less pyrite at and near the surface in areas where goethite is prevalent than in areas where jarosite is the spectrally dominant mineral and, therefore, that surface runoff from goethite-coated areas will be of more neutral pH than that derived from jarositic areas.
Other Minerals with Acid-Producing Potential in
Unmined Mineralized Areas
Acid-producing minerals also occur on the Earth's surface in the natural environment, usually in rocks that have been altered and mineralized by hydrothermal solutions. Most sulfide and hydrous iron sulfate minerals have some acid-producing potential (APP), and the most commonly occurring of these can be ordered from high to low APP as follows: pyrite, copiapite, schwertmannite, and fine-grained secondary jarosite formed via pyrite oxidation. The APP of such jarosite is discussed by Desborough and others (1999). Alunite ((Na,K)Al3(SO4)2(OH)6) is a common mineral formed by acid-sulfate hydrothermal alteration and is found in abundance in the Tintic and Marysvale districts in Utah, the Silverton and Lake City calderas and the Summitville deposit in Colorado, and in the Goldfield and Cuprite districts in Nevada, among many others. Alunite (Rye and others, 1992) and some spectrally identifiable clay minerals, such as dickite (Al2Si2O5(OH)4) and pyrophyllite (Al2Si4O10(OH)2), commonly occur with pyrite in magmatic hydrothermal acid-sulfate systems. Therefore, although these minerals have never been associated with acid generation themselves, they can also be regarded as indicators of sources of potentially strong acid generation.
Not all jarosite is associated
with the presence of pyrite. Primary hypogene jarosite can form in steam-heated
acid-sulfate hydrothermal systems such as those associated with the Miocene
replacement alunite deposits in the Marysvale volcanic field,
When performing watershed-based environmental evaluations, researchers should consider that ground and surface runoff from unmined, but hydrothermally altered rocks might have a lower pH than runoff from unaltered rocks and that such low-pH runoff could effectively lower the premining pH baseline of a watershed. However, it can be generally assumed that pyrite-bearing waste-rock piles at mine sites and tailings near mill sites will have significantly higher APP than similarly sized exposures of naturally occurring, sulfur-bearing altered rock because (1) the acid-producing minerals in waste-rock piles and tailings are commonly in chemical disequilibrium with atmospheric conditions, (2) the surface area of sulfur-bearing minerals available for oxidation is increased because of the generally small grain size of the blasted or processed waste rock, and (3) the waste-rock piles are highly permeable, allowing for rapid penetration of oxidizing precipitation.
Acid-Buffering Minerals
The carbonate minerals calcite (CaCO3) and dolomite (CaMg(CO3)2) buffer acidic solutions on contact, causing an increase in fluid pH and the precipitation of some dissolved metals. "Free-hydroxyl" minerals such as chlorite ((Mg,Al,Fe)12[(Si,Al)8O20](OH)16) and the sorosilicate mineral epidote (Ca2(Al,Fe3+)3(SiO4)3(OH)) have more limited capacities for acid neutralization. The buffering capacity, or acid-neutralizing potential (ANP), of chlorite has been estimated to be an order of magnitude weaker than that of calcite (Desborough and others, 1998). If rocks bearing these minerals are located downstream from sources of acid generation, they can provide some buffering capacity.
AVIRIS Data Acquisitions, Reflectance Calibration, and Georectification
AVIRIS Data Acquisitions
Spectroscopic image data covering the East Tintic Mountains (fig. 2), the Oquirrh Mountains (fig. 3), and the Tushar Mountains/Marysvale region (fig. 4) of Utah were acquired on August 5, 1998, by the AVIRIS sensor from the high-altitude National Aeronautics and Space Administration (NASA) ER-2 aircraft flying at an altitude of ≈20 km (Vane, 1987). These data have a GIFOV (ground instantaneous field of view), or ground spatial resolution, of ≈17 m/pixel. Nongeorectified quicklook images of the high-altitude AVIRIS flightlines, or "runs," are accessible from the online quicklook index of 1998 data (http://aviris.jpl.nasa.gov/ql/list98.html) on the NASA Jet Propulsion Laboratory (JPL) AVIRIS Web site. The satellite-borne Landsat 7 ETM+ sensor acquired multispectral image data of these areas on October 17, 1999. The Landsat 7 data have a ground resolution of ≈30 m/pixel.
In 1999, a second phase of the
project focused more detailed mapping on intensely mined and (or) mineralized areas
identified by the high-altitude 1998 survey. On October 17-19, 1999, additional
flightlines (17 total) of low-altitude AVIRIS data were acquired over selected
parts of the study areas. These data were acquired from a Twin Otter aircraft
flying at ≈5.33 km altitude and have a ground resolution of 2-3 m/pixel.
Georectified quicklook images of these low-altitude AVIRIS flightlines are
accessible from the online quicklook index of 1999 low-altitude AVIRIS data (http://aviris.jpl.nasa.gov/ql/listla99.html) on
the NASA JPL AVIRIS Web site. Results obtained from the analysis of three of
these flightlines of low-altitude AVIRIS data are presented in this report.
Figure 4 shows the location of the flightline covering the Big Rock Candy
Mountain area near Marysvale, and figure 5 shows the locations of the two
flightlines covering the Tintic mining district. Results from analysis of the
Big Rock Candy Mountain flightline are also discussed by Cunningham and others
(2005).
Figure 2. Location map of East Tintic
Mountains-Cedar Valley region,
Figure 3. Location map of
Figure 4. Location map of Tushar Mountains/Marysvale region,
Figure 5. Location map of low-altitude AVIRIS data coverage over the
Tintic mining district,
Calibration of High-Altitude AVIRIS Data
The high-altitude AVIRIS data are
calibrated to reflectance by using a two-step process (Rockwell and others,
2002; King and others, 2000). In the first step, the data are corrected by
using an algorithm (ATREM, Gao and Goetz, 1990; Gao and others, 1992) that
estimates the amount of atmospheric water vapor in the spectrum of each pixel
independently, as compared with an atmospheric model. The algorithm uses this
information on a pixel-by-pixel basis to reduce the effects of absorptions
caused by atmospheric water vapor. This step also includes characterizing and
removing the effects of Rayleigh and aerosol scattering in the atmosphere (path
radiance) and a correction for the solar spectral response relative to
wavelength. The second step requires the on-site spectral characterization of a
ground-calibration site that is present within the AVIRIS data coverage. Table
1 lists the sites used for ground calibration of the high-altitude AVIRIS data
covering each of the three study areas. The spectra of these field sites are
used to smooth the AVIRIS data by removing residual atmospheric absorptions and
sensor artifacts. AVIRIS spectra smoothed in this way may be directly and
quantitatively compared to libraries of standard reflectance spectra. The
reflectance calibration of the 1998 high-altitude AVIRIS data covering the
Oquirrh and
Table
1. Ground sites used for reflectance calibration of high-altitude AVIRIS data.
Reflectance data derived from the
ground calibrations shown in table 1 contained substantial spectral artifacts
related to either residual absorptions of atmospheric gases and particulates
that were not removed by the ATREM and path-radiance corrections or sensor
noise in the 2.0- to 2.5-μm spectral region. Residual artifacts related to
atmospheric water (mainly 0.94 and 1.13 μm) and CO2 (2.01 and 2.06
μm) may become more pronounced for areas at elevations different from that of
the ground-calibration site. This effect is caused by the fact that the
ground-calibration process corrects the entire AVIRIS data coverage relative to
atmospheric conditions at the calibration site. As absorptions related to CO2
increase in depth with increased atmospheric path length, reflectance spectra
of pixels sampled from high elevations will show smaller CO2
absorption-feature depths than pixels sampled from lower elevations.
Overcorrection for CO2 will occur at elevations higher than the
calibration site, resulting in positive "humps" at the CO2
absorption-feature locations. Conversely, undercorrections for CO2
will occur for areas at lower elevations than the calibration site. The
reflectance data derived from the calibration site at the
To alleviate these deficiencies in the reflectance data, additional areas of known composition located near the average elevations for a study area were used to verify and further refine the accuracy of the calibrations and derive any residual corrections for path radiance. Reflectance spectra of bright (high surface albedo) areas of known composition were sampled from the calibrated high-altitude AVIRIS data and edited, or "polished," to identify and remove artifacts related to residual absorptions of atmospheric gases, particulates, and sensor noise. Corrections for the subtle artifacts identified in this way were incorporated into the data used for the original reflectance calibrations, and the AVIRIS radiance data were recalibrated to reflectance format by using this refined calibration data. Sites used for this secondary reflectance-based spectral polishing are also listed in table 1.
Calibration of
Low-Altitude AVIRIS Data
As no field spectra were obtained
during the low-altitude overflights and the flightlines did not cover the
calibration sites used for the high-altitude data, reflectance-calibrated
high-altitude AVIRIS spectra were used to simulate field spectra for the low-altitude
data calibration. The process of reflectance calibration described in Rockwell
and others (2002) was applied to the low-altitude AVIRIS data with the
exception that edited high-altitude AVIRIS spectra were used as simulated field
spectra of ground-calibration sites. Spectra of areas of bright soil and rock
covered by both the high- and low-altitude AVIRIS data were sampled from the
reflectance-calibrated high-altitude AVIRIS data, averaged, and edited to
remove residual atmospheric absorptions. This "boot-strapping"
procedure of using high-altitude AVIRIS data to calibrate overlapping
flightlines of low-altitude data is further described in Rockwell and others
(1999). For the two flightlines acquired over the Tintic mining district (figs.
6 and 7), a patch of bright soil in the
Figure 6. True-color composite image generated from the low-altitude
AVIRIS flightline over the
Figure 7. True-color composite image generated from the low-altitude
AVIRIS flightline over the East Tintic subdistrict,
Figure 8. False-color composite image generated from the low-altitude
AVIRIS flightline over the Big Rock Candy Mountain area of the Marysvale
volcanic field,
Georectification of AVIRIS Data
The high-altitude AVIRIS data were collected from a NASA ER-2 aircraft flying at an altitude of ≈20 km. Although the ER-2 was designed to simulate conditions on a stable satellite platform and is equipped with a roll-compensation system, geometric distortions related to variations in aircraft roll, pitch, yaw, and velocity are present in the AVIRIS data. These distortions must be removed prior to image georeferencing to a map projection if positional errors are to be minimized, especially in areas of significant terrain relief. The USGS AVRECGEN and AVRECTFY algorithms were used to remove these distortions; the algorithms are based on modeling the look-point equation for each AVIRIS pixel using the engineering and navigation data that are recorded simultaneously with the spectral image data (Clark and others, 1998).
The low-altitude AVIRIS data described here were acquired from a propeller-driven Twin Otter aircraft flying at 5.33-km altitude. The distortions caused by roll, pitch, yaw, and velocity variations are much more pronounced in data acquired by the low-altitude platform than in data from the ER-2. Therefore, a different algorithm was used to remove these distortions from the low-altitude data (Boardman, 1999). This method does not remove topography-induced image distortions, but does remove the aircraft-induced and scan mirror-induced distortions that dominate AVIRIS low-altitude data.
After distortion removal, the high-altitude data were georeferenced to the Universal Transverse Mercator map projection by using a second-order polynomial transformation with control points selected from USGS 1:24,000-scale Digital Raster Graphics. The low-altitude data were georeferenced by using rubber-sheeting functions (Watson, 1992).
Spectral Analysis
The USGS Tetracorder expert system
was used for spectral analysis of the AVIRIS data (Clark, Swayze, Livo, and
others, 2003). This semiautomated software system independently compared the
spectrum of each pixel in the AVIRIS data to a digital library of standard
laboratory reference spectra of minerals, mineral mixtures, water, snow,
man-made objects, and vegetation. The library reference spectra used by the
Tetracorder software are available in published spectral libraries (Rockwell,
2002; Clark, Swayze, Wise, and others, 2003). One or more diagnostic spectral
absorption features were analyzed according to a detailed set of rules for each
reference material. This analysis generated quantitative digital image maps of
(1) absorption-feature depth in the image spectra and (2) modified
least-squares fit of image spectra to library reference spectra across defined
spectral intervals (continua) for each reference material. In general,
absorption-feature depth is proportional to the spectral abundance of a
material in a pixel, given a constant grain size (
The spectrum of each pixel of AVIRIS data was analyzed separately for several groups of surface materials. These can be detected independently of each other because they have diagnostic absorption features in different wavelength regions of the electromagnetic spectrum. For every pixel, modified least-squares fit values were generated for each reference material belonging to a particular material group. The material with the highest fit value for that group was selected as the spectrally identified material within that group. The reliability of the mapping results is directly proportional to both high feature depths in the image spectra and high degrees of fit. Therefore, the image maps showing feature depth and feature fit are multiplied to generate a "fit x depth" image for each identified material (Clark, Swayze, Livo, and others, 2003). Pixels with high fit x depth values are most likely to be an accurate identification of a given material. Pixels not identified as a particular material in a group (that is, their fit and (or) depth values were below a user-defined threshold) were assigned a fit x depth value of zero for that group. Therefore, for each group of surface materials (for example, the iron-bearing mineral group or the clay, sulfate, mica, carbonate, and hydrous silica mineral group), a given pixel may have a positive value (representing an identification) for only one material, or it may not be identified as any material in that group. The fit x depth image map is used for the final interactive analysis of the mapping results. The Tetracorder system identifies only the material or mixture of materials that is spectrally dominant in a pixel, meaning that the absorption feature of the identified material is sufficiently unobscured by features of other materials to allow its recognition by spectral analysis. Therefore, identification of the spectrally dominant material in a spectrum does not imply that other materials do not also exist in that pixel.
A separate map can be generated for each material group. For this report, two types of maps were generated to show the distribution of the following materials: (1) those having absorption features in the 0.35- to 1.35-μm spectral region, such as iron-bearing minerals, snow, ice, and water; and (2) those having vibrational absorption features in the 1.45- to 2.50-μm spectral region, including such minerals as phyllosilicates (micas and clays bearing Al-OH or Mg-OH), sulfates, carbonates, amphiboles, hydrous quartz (chalcedony and opal bearing Si-OH bonds), and epidote (a sorosilicate mineral bearing calcium and Al-OH and (or) Fe-OH bonds). The AVIRIS-derived maps of surficial materials presented in this report consist of color-coded pixels identified as specific materials on a grayscale background image of a single AVIRIS band. In generating the final maps, each material is assigned a discrete color. The fit x depth image corresponding to a particular material may be digitally stretched so that pixels of all fit x depth values will be represented by a single color ("hard stretch"), or the image can be stretched so that the fit x depth values will be represented by a range of brightness levels for a given color ("continuous stretch"). For example, in the case of a continuous stretch, pixels with the highest fit x depth values will be represented by the color chosen for that material, and pixels with decreasing fit x depth values will be represented by successively darker shades of that color. Hard stretches are used more frequently than continuous stretches in making maps showing many different materials, as maps showing many shades of colors can be difficult to interpret. Minerals and mineral assemblages for which continuous stretches were used are marked with a "C" in the map explanations (legends).
Map Explanations
The explanations (legends) with the high-altitude AVIRIS mineral maps presented here have been designed to facilitate interpretation of the imagery. These explanations relate identified minerals and mineral assemblages to associated acid-producing potential (APP) and acid-neutralizing potential (ANP). The explanations for the maps of iron-bearing minerals are organized in order of decreasing APP from top (high APP) to bottom (low APP). APP can be considered to be inversely proportional to pH. In the explanations for the maps of clay, sulfate, mica, and carbonate minerals, minerals and mineral assemblages that either may occur with pyrite (for example, dickite) or have APP themselves (for example, jarosite) are indicated with an asterisk.
Verification of Spectral Analysis Results
The results of the mineral mapping were verified by field checking and (or) interactive comparison of AVIRIS spectra with standard library spectra. Selected mapping results were also verified by using X-ray diffraction (XRD) analysis of field samples. The appendix shows the XRD results of many field samples, along with sampling locations and other information. Appendix tables A1 and A3 include the Tetracorder mapping results for AVIRIS pixels in the vicinity of the sample collection locations. Tetracorder mapping results show several different minerals for a given location, meaning that either (1) mineral mixtures were directly identified in the AVIRIS data or (2) various individual minerals were spectrally identified in the area surrounding the location and the exact AVIRIS pixel corresponding to the sampling location could not be reliably identified. In cases where field checking and (or) laboratory analysis identified errors in the Tetracorder mapping, mapping rules were reviewed and modified and (or) new standards were added to the spectral library of reference materials. In the latter case, rock samples collected in the field were analyzed by XRD, and their reflectance spectra were measured in the laboratory. Mapping rules were then developed for one or more diagnostic absorption features present in the laboratory spectra, and the spectra were added to the spectral reference library. The Tetracorder expert system was then rerun on the AVIRIS data by using the modified mapping rules and expanded spectral library. Some of the field samples that were added to the spectral library are listed in blue in the appendix. Rockwell (2002) has documented the spectroscopic properties of these samples and has defined the absorption features in each sample spectrum that were analyzed by the Tetracorder expert system.
To exemplify a Tetracorder mapping
error that was remedied as a part of this research, maps of the southwestern
Because (1) the Tetracorder expert system is experimental and under constant development and revision and (2) the Earth's surface has inherent mineralogic complexity, 100 percent accuracy of mineral identifications cannot be guaranteed for each AVIRIS pixel. For many of the more common rock-forming minerals, the Tetracorder system is very robust, especially when the minerals occur abundantly in pure or nearly pure form. Although a great effort has been made to include many common mineral mixtures in the Tetracorder spectral library, it is not currently possible to include spectra of every combination of minerals. For this reason, and because Tetracorder only identifies the mineral or minerals that are spectrally dominant in a pixel, it can be assumed that the accuracy of the mineral maps will decrease as the number of constituent minerals in a rock increases. No legal or regulatory actions should be initiated on the basis of the mineral maps alone. Targets of potential significance identified by the mineral maps should be studied in detail in the field and (or) laboratory prior to decision-making regarding a site or watershed.
The East Tintic Mountains and the Tintic Mining District
Geologic Setting
The Tintic mining district is
located ≈95 km south-southwest of Salt Lake City, Utah, in the East Tintic
Mountains (see project index map at http://speclab.cr.usgs.gov/earth.studies/Utah-1/utahproj_large.jpg).
The town of
Figure 9. Geologic map of the Tintic mining district. Refer to figure 10 for explanation showing geologic units. Blue = Swansea Quartz Monzonite (Ts) stock and related intrusive rocks. Green = Sunrise Peak Monzonite Porphyry (Tsp) stock and related intrusive rocks (contemporaneous Gough and Dry Ridge sills, Tsps, are in black and white dot pattern). Red = Silver City Monzonite (Tsc) stock and related plutons. Modified from Morris and Mogensen (1978). View full-resolution file
Figure 10. Explanation for geologic map shown in figure 9. From Morris and Mogensen
(1978).
The second volcanic event resulted
in the deposition of the Tintic Mountain Volcanic Group and culminated with the
intrusion of the Sunrise Peak Monzonite Porphyry stock and many related plugs,
dikes, and extensive latite sills at the southern and eastern edges of the
district (fig. 9). Near the end of Oligocene volcanism (≈31.5 Ma), the Silver
City Monzonite stock and related dikes and plugs were intruded along a
north-northeast-trending zone extending from the northwest boundary of the
caldera; this igneous activity initiated circulation of hydrothermal fluids
through faults, fractures, and breccia zones in the Paleozoic sedimentary rocks
and older Oligocene volcanic rocks. These hot fluids pervasively altered the
country rock and ultimately resulted in deposition of ore minerals. The
youngest intrusive rock in the district is the quartz monzonite porphyry of Diamond
Gulch, which intruded the southern part of the
Ore
Lovering (1949) recognized the sequence of events involving hydrothermal alteration and mineralization in the Tintic district: (1) pervasive dolomitization of limestone beds, (2) propylitic alteration, (3) argillic alteration (including formation of alunite, kaolinite, etc.), (4) silicification, calcification, and pyritization (post-monzonite intrusion), and (5) ore deposition (quartz, barite, sericite, orthoclase, rhodochrosite, and ore minerals). Late-stage, high-temperature (≈257°-300°C) fluids created epigenetic polymetallic base and precious metal deposits as replacements in favorable carbonate beds, replacement veins, and fissure veins (Morris, 1990). Primary ore minerals include galena, sphalerite, argentite, tetrahedrite-tennantite, enargite, sulfosalts, native gold and silver, and secondary oxides. Alteration minerals include alunite, various clay and carbonate minerals, illite (sericite), and pyrite. Secondary gypsum and jarosite (after pyrite) are also present. Primary gangue (waste) minerals include quartz, barite, calcite, dolomite, and rhodochrosite. Table 2 lists the dominant gangue minerals as a function of ore type in the district.
Table 2. Dominant ore and gangue minerals and production figures, Tintic mining district.
[Modified
from Morris (1968) and Cox and Singer (1986); production data from Morris and
Mogensen (1978)]
Most ore deposits in the Tintic
district occur as replacement bodies, replacement veins, and fissure veins
(Morris, 1968). A majority of the metals produced from the Tintic district were
derived from ore bodies that have replaced favorable horizons in folded and
faulted Paleozoic carbonate rocks. Figure 11 is a map of the Tintic district
showing the principal mines and plan views of the major underground ore bodies
in the district. Metal production revealed strong patterns of horizontal
zonation across the district. Although lead and silver ores were common
throughout the district, figure 12 shows that zinc was mainly produced from the
northernmost sections of the main replacement ore zones, whereas gold (not
shown) and copper were common mainly in the southern part of the district in
the area surrounding the Silver City Monzonite stock. The central part of the
district (located between the lines showing copper and zinc occurrence limits
in fig. 12) was known chiefly for lead and silver production. In general, Pb/Zn
and Ag/Pb ratios decrease toward the north in the district.
Figure 11. Map of Main Tintic subdistrict
showing mines and plan views of ore bodies. For description of map units, see figure 10. From Morris (1968). View full-resolution file
Figure 12. Map of replacement
ore bodies, showing generalized compositional zonation. From Morris (1968).
Drilling at the southwestern edge of the Tintic district has identified the Southwest Tintic (SWT) porphyry copper deposit (fig. 9) (Krahulec, 1996). This deposit, which had not been mined as of summer 2004, has been characterized as a high-sulfide, low-copper porphyry system associated with the quartz monzonite porphyry stock of Diamond Gulch. The deposit is associated with intense and strongly zoned hydrothermal alteration, some of which is exposed at the surface. The richest copper grades are found in stockworks of quartz ± pyrite ± chalcopyrite ± magnetite ± molybdenite veins that are largely restricted to a biotite-rich zone at the core of the deposit. A shallow supergene chalcocite blanket is present beneath alluvium in Diamond Gulch 1-3 km to the southwest of Horseshoe Hill (fig. 2). Phyllic, or quartz-sericite-pyrite (QSP), alteration surrounds the core and is exposed in the vicinities of Horseshoe Hill, Treasure Hill, and Ruby Hollow. Within the QSP zone, clay minerals increase in abundance relative to quartz with increased distance from the potassic core of the deposit. A propylitic envelope consisting of an inner zone of actinolite and epidote and an outer zone of calcite and chlorite surrounds the QSP zone. The alteration zones are highly elongated along a northeast-trending structure, which has been interpreted as a tear fault formed during the Jurassic.
Mining History
The Tintic district was discovered
in 1869, and production of rich polymetallic ores steadily increased, peaking
in 1921. Production declined from 1921 until the mid 1950s, when new
discoveries in the East Tintic subdistrict prompted another burst of mining
that lasted until the 1990s. Metal production figures from 1869 to 1976 are
shown in table 2. Virtually no metallic ores have been produced from the Main
Tintic subdistrict since 1960. Natural caverns formed by dissolution of
carbonate rocks were used for mine dewatering, most notably in the Gemini and
Chief No. 1 mines adjacent to
The most recent mining activity in the Eureka/Tintic area has taken place in the East Tintic subdistrict. The main ore body of the Burgin mine was discovered in 1958 and was mined for Pb, Zn, Ag, minor Au and Cu, and high-silica flux ores until the late 1970s. The Trixie mine, located 2.5 km southwest of the Burgin, was discovered in the mid 1950s and mined for gold, copper, and silver until the 1990s (Morris, 1990). New gold discoveries were made in the late 1990s in the vicinity of the Trixie mine.
Because of elevated levels of Pb,
As, Sb, Cd, Hg, Ag, and other metals in the soil, parts of the town of
Mapping and Characterization of Mine Waste in the
Tintic Mining District
Maps of minerals and water were
generated from the high- and low-altitude AVIRIS data (figs. 13-18). Minerals
were identified in only a small percentage of processed AVIRIS pixels. Most of
the region within and surrounding the
Table 3 lists mines in the Tintic
district at which jarosite-bearing rocks were mapped by using AVIRIS data. For
the locations of these mines, refer to figures 11 and 13-19. The largest
exposures of jarosite-bearing tailings and waste-rock piles are marked with an
asterisk in table 3. For example, figure 20 shows the size of the waste-rock
piles at the
Table
3. Mine sites at which
jarosite-bearing waste rock was mapped by using AVIRIS data.
[Asterisks indicate the largest mapped
exposures of acid-generating minerals; mine locations shown in figs. 13-19]
Figure 13. Map of iron-bearing minerals and water in the
Figure
14. Map of clay,
carbonate, sulfate, and mica minerals in the
Figure 15. Map of iron-bearing minerals and water in the Silver City-Dragon
mine area, Main Tintic subdistrict,
Figure 16. Map of clay, carbonate, sulfate, and mica minerals in the Silver
City-Dragon mine area, Main Tintic subdistrict,
Figure 17. Map of iron-bearing minerals and water in the East Tintic
subdistrict,
Figure 18. Map of clay, carbonate, sulfate, and mica minerals in the East Tintic
subdistrict, Utah
Several large waste-rock piles
exist on the western and southwestern edges of the town of
Table 4.
Total production and average ore grades of several mines near town of Eureka Utah
[Data from Cook (1957); oz., ounces]
Figure 19. Subset of map of iron-bearing minerals and water derived from analysis of high-altitude 1998 AVIRIS data. The color explanation for the spectrally identified minerals is shown. Selected mines at which jarosite was spectrally identified in waste-rock piles are indicated in white. Spectrally identified exposures of jarosite not associated with mining activity are indicated with a red "J." Occurrences of chlorite-bearing, propylitically altered rocks having limited acid-neutralizing potential are indicated with the purple "C." These chloritic rocks are associated with the Sunrise Peak Monzonite Porphyry and related dikes and sills. This map is a detail of the map shown in figure 13. North is up, and the image width is approximately 12 km. View full-resolution file
Figure 20. Waste-rock piles with
yellowish and yellow-green coatings of jarosite at the Swansea Silver City
In the East Tintic subdistrict, jarositic waste-rock piles and tailings are located immediately southeast of the Burgin mine (see cover photograph in pdf version of report and figs. 7, 13, and 17). No strong spectral evidence for eolian deflation of the flat-lying tailings was identified with the AVIRIS mapping. Abundant jarositic waste rock exists at the Trixie mine and at other mine sites in the subdistrict. The high-altitude AVIRIS data indicated few carbonate-bearing rocks or soils downstream from the mines in the East Tintic subdistrict (fig. 14). However, the increased spatial resolution of the low-altitude AVIRIS data allowed the identification of numerous pixels of calcite and dolomite along roads and scattered throughout this subdistrict (fig. 18). In the hills between the Trixie and Apex Standard No. 1 mines, these spectrally identified carbonate-bearing rocks and soils correspond to field-mapped Middle Cambrian limestones and dolomites (figs. 9 and 10). Other carbonate-bearing soils in the area have not been field checked, but are probably related to eolian dusts, road metal, and (or) propylitically altered igneous rocks. The area downstream (northeast) of the Apex Standard No. 1 mine, including the Burgin mine, is underlain by Quaternary alluvium and Packard Quartz Latite, suggesting that little natural acid-neutralizing potential (ANP) is present.
The Dragon Halloysite Mine
The participants of the USEPA/USGS Utah Abandoned Mine Lands
Imaging Spectroscopy Project (U.S. Environmental Protection Agency
and U.S. Geological Survey, 2002) selected the Dragon mine as a site for
testing and comparing methods of mineral mapping with imaging spectroscopy
data. The mine is located in the Main Tintic subdistrict 4 km south of the town
of
Figure 21. True-color composite image of Dragon halloysite mine produced from
low-altitude AVIRIS data. Yellowish tones are indicative of rocks coated with
jarosite and (or) goethite. The horizontal distance from point A to point B is
≈1.4 km.
Figure 22. Detail of map
presented in figure 15 of iron-bearing minerals at the Dragon mine produced
from low-altitude AVIRIS data. Red = jarosite. Yellow = jarosite + goethite.
Green = goethite. Light and dark blue = hematite. See figure 15 for full
explanation of color coding. Pixels spectrally identified as thin coatings of
goethite have been omitted from this map.
The Dragon mine is situated along
the northern edge of the Silver City Monzonite stock near its contact with
Paleozoic metasedimentary rocks such as the Upper Cambrian
Although the waste-rock piles in
the central section of the Dragon halloysite mine are large (fig. 23), the
concentration of jarosite on the surfaces of these piles is considerably less
than is found on waste-rock piles elsewhere in the district at mines from which
base and precious metals ores were the primary extracted resource. The map of
iron-bearing minerals generated from the high-altitude AVIRIS data (fig. 13)
indicates that, at the 17 m/pixel resolution (ground instantaneous field of
view), the waste-rock piles at the Dragon mine are covered with both jarosite
and goethite (an "areal mixture" indicated in fig. 13 as
"goethite + jarosite"), whereas the surfaces of most of the other
waste-rock piles listed in table 3 are mainly composed of jarosite. However,
the low-altitude AVIRIS map of iron-bearing minerals of the mine (figs. 15, 22,
and 24A) resolves small patches of highly jarositic rocks within a large area
dominated by the jarosite + goethite areal mixture. For the Dragon mine's
waste-rock piles, AVIRIS spectra (figs. 24B and 25) show that both the pixels
spectrally identified as jarosite and the pixels spectrally identified as a
jarosite + goethite areal mixture have pronounced absorption features due to
jarosite (for example, at 0.43 μm), indicating substantial amounts of jarosite
at the surface. Field and XRD studies found that pyrite-bearing waste rock is
abundant in the areas of the piles where jarosite was mapped with the AVIRIS
data (appendix table A1, sample TN00-17B). Despite having only moderate
concentrations of jarosite when compared to other waste-rock piles in the
Tintic district, the large size of the pyrite- and (or) jarosite-bearing
waste-rock piles at the Dragon mine makes them an important potential source of
acid generation.
Figure 23. Pyrite-bearing waste
rock in the central section of the Dragon mine, looking east. The yellowish
color of the rocks in the foreground is indicative of the iron sulfate mineral
jarosite.
Figure 24. Spectral minitransect
across waste-rock piles in the central section of the Dragon mine. (A) Spectral
sampling locations on map of iron-bearing minerals derived from 1999
low-altitude AVIRIS data (refer to fig. 15 for location of sampling area). (B)
Average AVIRIS spectra sampled from these locations. Spectra shown are averages
of 16 AVIRIS pixels. Continuum-removed band centers for the ferric iron
absorptions near 0.93 μm due to crystal field effects are indicated. For
comparison, the continuum-removed band center is 0.9190 μm for reference
jarosite GDS99 and 0.9475 μm for coarse-grained goethite WS222. The absorption
occurs at longer wavelengths for goethite than for jarosite.
Figure 25 also shows that the Fe-OH
absorption feature of jarosite in the short-wave infrared (SWIR) spectral
region near 2.27 μm is not present in the AVIRIS spectra whose characteristics
in the visible (
Figure 25. Average low-altitude
AVIRIS spectra from minitransect across waste-rock piles at the Dragon mine
shown in figures 23 and 24. Here, the spectra are shown across the full AVIRIS
spectral range. Note that the Fe-OH absorption feature at 2.27 μm
characteristic of jarosite is not present in these spectra because of the
spectral dominance of phyllosilicates (clays and micas) at the AVIRIS pixel
scale. H,K = halloysite ± kaolinite. (S) = sericite (illite or muscovite).
Extensive exposures of rocks with
thin coatings of goethite were mapped with the 1999 low-altitude AVIRIS data in
and around the Dragon mine, especially in areas underlain by argillically and
(or) pyritically altered zones of the Swansea Quartz Monzonite and Silver City
Monzonite stocks (refer to fig. 9 for locations of stocks and figs. 16 and 18
to locate areas altered to alunite, kaolinite, and [or] kaolinite +
illite/muscovite). Rocks with goethite coatings were also spectrally identified
in drainages downstream from the Dragon mine, but these rocks may be derived
from the surrounding altered intrusive rocks and not exclusively from
downstream movement of pyrite- and jarosite-bearing rocks and (or) acidic
solutions from the Dragon mine. Most of the large exposures of goethite-coated
rocks outside of the Dragon mine were not mapped with the 1998 high-altitude
AVIRIS data, probably because these areas were masked with rangeland vegetation
when the data were acquired. A large range fire occurred in June 1999 (4 months
before the low-altitude AVIRIS data were acquired) that denuded the area
surrounding the Dragon mine of vegetation. The burn scar from this fire is
visible in reddish hues in figure 26, a Landsat 7 ETM+ color composite of the
Figure
26. Landsat 7 ETM+ color
composite (741/RGB) of the East Tintic Mountains, Utah. Green vegetation is
shown in green tones. A burn scar from a range fire that occurred in June 1999
is visible in reddish hues on the west flank of the range and in the
In addition to halloysite clay,
kaolinite + muscovite mixtures were mapped with the AVIRIS data in the central
and eastern parts of the Dragon mine (figs. 14 and 16). Rocks and soils
spectrally identified as kaolinite + muscovite mixtures within and just south
of the mine are most likely derived from argillically and pyritically altered
monzonite from the
Few carbonate-bearing rocks or soils were spectrally identified downstream from the Dragon mine area with the high-altitude AVIRIS data (fig. 14). From the low-altitude AVIRIS data, some soils containing calcite and dolomite were identified along roads in the Silver City area immediately southeast of the Swansea waste-rock piles and within the Silver City Monzonite just east of the contact with the Swansea Quartz Monzonite (fig. 16). These carbonate-bearing soils have not been field checked, but are probably related to eolian dusts, road metal, or propylitic alteration of the igneous rocks that underlie this area. Few OH-bearing minerals (for example, clays, sulfates, micas) were spectrally identified within the drainages downstream from the Dragon mine. Therefore, it appears as if little, if any, natural ANP exists downstream from the mine. Two important factors, however, reduce the potential for significant metal loading of soils and ground or surface water down-gradient from the waste-rock piles at the Dragon mine. First, the generally low amounts of annual precipitation in the area (on average, 35 cm/yr, Morris and Mogensen, 1978) reduce the overall rate and frequency of downstream flow of potentially metal bearing solutions from the mine. Second, geologic studies of the halloysite clay deposits at the Dragon mine have indicated that the widespread argillic and pyritic alteration of the sedimentary and intrusive rocks in the area occurred before the introduction of base and precious metals in the district and the hydrothermal solutions responsible for this alteration were in general barren of metals (Morris, 1968). The alteration events responsible for the deposits of hematite and halloysite clay are generally thought to be associated with the mid-barren (argillization) and late-barren (pyritization, calcification, and silicification) stages of alteration in the district (Lovering, 1949). On the basis of a geological understanding of the clay deposits at the Dragon mine, the waste rock associated with the deposits can be assumed to have relatively low concentrations of heavy metals.
Unmined Mineralized Rocks with Acid-Producing
Potential (APP)
Unmined areas underlain by
altered, mineralized rock can be clearly identified on AVIRIS-derived maps of
minerals having vibrational absorptions in the 1.4- to 2.5-μm spectral region
(fig. 27). Altered rocks in these unmined areas are characterized by alunite,
clay minerals, and micas associated with intense argillization, sericitization,
and local pyritization. Prominent altered areas include alunitic zones near
Figure 27. Subset of map of clay, carbonate, sulfate, and mica minerals derived from analysis of high-altitude 1998 AVIRIS data. The color explanation for the spectrally identified minerals is shown. Selected areas of argillic, advanced argillic, and quartz-sericite-pyrite (QSP) hydrothermal alteration are indicated in white. Pyrophyllite and hematite in an altered fracture zone cutting Cambrian Tintic Quartzite were field verified at location marked "Py." Dickite, quartz, and sulfides were field verified along an altered fracture zone at location marked "Dk." This map is a detail of the map shown in figure 14. North is up, and the image width is approximately 12 km. View full-resolution file
The mineral map generated from the
high-altitude 1998 AVIRIS data (figs. 14 and 27) shows a large area of alunite
+ pyrophyllite surrounded by pixels of alunite, pyrophyllite, and alunite +
kaolinite just north of
Altered and mineralized rocks,
including the ore deposits, occur along faults and fractures, usually within
several kilometers of exposures of intrusive igneous rocks (see fig. 9 for
principal locations of igneous rocks in the district). These fractured rocks
may serve as conduits for the transport and recharge of modern ground water.
Figure 28 shows the mineral maps derived from analysis of the high-altitude
AVIRIS data overlaid with faults and fractures digitized from published maps
(Morris and Mogensen, 1978). It is quite apparent that the waste-rock piles
from many mines, especially those associated with ore deposits in replacement
veins and fissure veins in the vicinity of the
Figure 28. Overlays of faults and fractures on mineral maps derived from spectral analysis of high-altitude AVIRIS data. Left: map of Fe-bearing minerals. Right: map of clay, carbonate, sulfate, and mica minerals. See figures 13 and 14 for color-coded explanations of mineral maps. Structure derived from Morris and Mogensen (1978). View full-resolution file
Published maps of hydrothermal
alteration in the East Tintic subdistrict (Lovering, 1960) indicate large
exposures of altered rock, mainly the Packard Quartz Latite, which contain
fine-grained disseminated pyrite. The AVIRIS-derived mineral maps show that
these rocks are characterized mainly by goethite on the surface (figs. 13 and
17). As an example, one of these outcrops of unmined, pyrite-bearing, altered
rock that is coated with goethite is located ≈1.5 km east-northeast of the
Burgin mine, immediately to the north of Route 6. Several pixels of jarosite
were also spectrally identified in this area within broad zones of goethite
(fig. 17). Pyritically altered rocks with similar goethite coatings were also
identified in the
Several other occurrences of
unmined, nonanthropogenic jarosite were mapped with the AVIRIS data in areas
underlain by altered rock that has been argillized, pyritized, and (or)
silicified. These occurrences of jarosite are shown on the maps of iron-bearing
minerals generated from the high-altitude and low-altitude AVIRIS data and are
marked with a red "J" in figure 19. These jarositic areas are
underlain by altered volcanic rocks including the alunite-bearing rocks and
silicified breccia zones located between Silver Pass and the Trixie mine (figs.
13-18) and by sericite-bearing rocks on Horseshoe Hill (figs. 19 and 27). XRD
analysis of rocks from these areas confirmed the presence of jarosite (appendix
table A1). The alteration of the rock exposed on Horseshoe Hill and on the low
hill 1.5 km to the south is most likely phyllic, or quartz-sericite-pyrite
(QSP), alteration associated with the unmined, subsurface Southwest Tintic
(SWT) porphyry copper system located 1-3 km southwest of Horseshoe Hill (figs.
9 and 27, and Krahulec, 1996). Through the use of the AVIRIS data, the
Tetracorder-based spectral mapping was able to differentiate these exposures of
natural, nonanthropogenic jarosite from jarosite in waste-rock piles. Jarosite
not associated with mining activity was spectrally identified exclusively as
coarse-grained jarosite (see map explanations), whereas jarosite-bearing waste
rock at mine sites was spectrally identified mainly as "fine-grained"
jarosite. Figure 29 shows the spectral variation between XRD-confirmed
jarosites sampled from pyrite-bearing waste rock in the Dragon mine (in blue)
and from unmined, altered rocks from the
Figure 29. Laboratory spectra of
jarosites associated with unmined argillic alteration (red, sample TN00-21) and
anthropogenic, pyrite-bearing mine waste from the Dragon mine (blue, sample TN00-17b).
These jarosites were differentiated by the AVIRIS mapping into
"coarse-grained" jarosite (Silver Pass
Where active sulfide oxidation is
still occurring, as is the case on pyrite-bearing waste-rock piles,
fine-grained coatings of relatively pure jarosite occur. Although no pyrite was
detected in the sample of jarosite and alunite from
Rocks with Acid-Neutralizing Potential (ANP)
Carbonate minerals (calcite and
dolomite) were detected in association with Paleozoic metasedimentary rocks and
propylitic alteration within and adjacent to intrusive rocks. These
carbonate-bearing rocks can provide ANP. The AVIRIS mineral map shows that the
Paleozoic carbonate-bearing lithologies are best exposed between the Dragon
mine and Eureka, north of Homansville Canyon on Pinyon Peak (fig. 2), and 1 km
northwest of the Trixie mine (figs. 14 and 18). Exposures of calcite and
epidote associated with propylitically altered igneous rocks were spectrally
identified around the Sunrise Peak Monzonite Porphyry at the south end of the
district. Exposures of epidote and (or) calcite located 2-3 km south of
Horseshoe Hill and in the vicinity of Treasure Hill (fig. 14) are most likely
propylitically altered andesitic crystal tuffs associated with the SWT porphyry
copper system located from 1 to 3 km southwest of Horseshoe Hill (Krahulec,
1996). Chlorite-bearing rocks, associated with propylitic alteration caused by
the intrusion of the Diamond Gulch stock and
Calcite, dolomite, and epidote all
have diagnostic absorption features near 2.3 μm. For calcite and dolomite,
these features are associated with combinational and overtone vibrational
absorptions related to the carbonate radical. For epidote, the feature is
possibly associated with Fe-OH vibrational absorptions (
The Oquirrh Mountains
Geologic Setting
The
Figure
30. Generalized geologic
map of the Oquirrh Mountains
In the northern part of the range,
the predominant bedrock is the
Overview of Ore Southern
Oquirrh
Silver was discovered in
Modern, large-scale open-pit mining began at Mercur in 1983 and also involved cyanide leaching for gold extraction. This new mining activity engulfed the old mill and existing tailings (Nash, 2002). Mining ended in 1998 after >41 million tons of ore containing 73.8 million g (2.6 million oz) of gold and 32.9 million g (1.2 million oz) of silver were recovered. By-product mercury production totaled 3,469 flasks (Mako, 1999).
Bedded replacement deposits of
gold and gold-mercury in the Great Blue Limestone are the major ore deposits in
the Mercur area. Fissure veins also contain replacement deposits of gold,
gold-mercury, silver, and silver-lead. The gold-bearing replacement deposits
contain pyrite, realgar (AsS), orpiment (As2S3), and cinnabar (HgS) (Koschmann
and Bergendahl, 1968). The area contains one of the principal deposits of
realgar and orpiment in the
Deposits of the
Overview of Ore
The
The Bauer Mill, located ≈2.5 km north of Stockton, processed ores from the Stockton mining district and other mines in the western United States from 1900 until 1957 by using selective flotation (Gilluly, 1932; John and Ballantyne, 1997). Deposits of tailings rich in sulfide minerals (mainly pyrite) accumulated at the mill site during this period.
The International Smelter and Refining mill, located 3 km east of Tooele (fig. 3), processed sulfide ores of copper, lead, and zinc from 1910 to 1972. The site was reclaimed in 1986. Federal, State, and local government agencies are currently interested in evaluating the effectiveness of the original reclamation efforts. The International Smelter and Refining site was included on the U.S. Environmental Protection Agency's National Priority List on July 27, 2000 (Steven Thiriot, Utah Department of Environmental Quality, written commun., 2001; U.S. Environmental Protection Agency, 2002b).
Mapping and Characterization of Mine Waste in the
Oquirrh Mountains
Maps of surface minerals and water
were generated from the high-altitude AVIRIS data covering the
Figure 31 (Above left). Map of iron-bearing
minerals and water in the western Oquirrh Mountains Utah Stockton Mercur Canyon
Figure 32 (Above right). Map of clay, carbonate,
sulfate, and mica minerals in the western Oquirrh Mountains Utah Stockton Mercur Canyon
Figure 33 (Above left). Map of iron-bearing minerals
and water in the central Oquirrh Mountains Utah Mercur Canyon
Figure 34 (Above right). Map of clay,
carbonate, sulfate, and mica minerals in the central Oquirrh Mountains Utah Mercur Canyon
Figure 35 (Above left). Map of iron-bearing
minerals and water in the eastern Oquirrh Mountains Utah Manning Canyon Bingham Canyon Bingham Canyon
Figure 36 (Above right). Map of clay,
carbonate, sulfate, and mica minerals in the eastern Oquirrh Mountains Utah Manning Canyon Bingham Canyon Bingham Canyon
Figure 37. Landsat 7 ETM+
continuous-tone map of mineral groups and vegetation centered on the southern
Interpretation
guide:
Red:
rocks and soils bearing hematite and (or) other ferric iron minerals.
Yellow: Fe3+ + clays, sulfates, and micas
(mostly hydrothermal alteration).
Green:
clays, sulfates, and micas, ± carbonates,
with possible bleaching.
Cyan:
green, moist vegetation.
Blue:
green, dry vegetation.
Mercur Canyon Manning Canyon
Preliminary
AVIRIS-derived mineral maps of the
Mineral-Distribution Patterns
The deposits of the
Figure 38. Tailings in the
Figure 39. Tailings at the remains of the ore-processing
mill in
Figure 40. Tailings extending down-gradient toward the
southeast from the mill site in
Figure 41. Open-pit clay mine in
AVIRIS-derived maps show iron
minerals, illite/muscovite, and mixtures of kaolinite and illite/muscovite
being transported out of the
Field and Laboratory Analyses
Field verification for
the
Eleven soil samples taken
from the
Although the tailings
deposits in the
Elevated levels of
arsenic, chromium, strontium, and lead were identified by the XRF field
measurements of the tailings at both locations (appendix table A2). Zinc was
also present in most of the samples at lower concentrations ranging from 90 to
250 ppm. In general, zinc values were higher in tailings from
Discussion
Although imaging
spectroscopy cannot directly detect elemental concentrations, the distribution
of metals can be inferred by the extent of iron hydroxides mapped with the
AVIRIS data. Previous studies of mine tailings similar to those examined here
indicate that elemental arsenic will preferentially adsorb to poorly
crystalline (amorphous) iron hydroxides in near-neutral pH environments
(Bowell, 1994). At the
Bauer Mill Site and Tailings near Ophir
Canyon
Of the sites investigated
in detail as a part of this study of the
Figure 42. Pyrite-bearing tailings and other waste at the site of the Bauer
Mill near
Pyrite was spectrally identified in
several pixels located in the southeastern part of the tailings near the site
of the mill (fig. 43). Generally, pyrite is difficult to detect through the use
of high-altitude AVIRIS data because of pyrite's low reflectance (albedo) and
weak absorption features. A pyrite-rich core area immediately adjacent to the
mill site grades spectrally toward the west and northwest into fine-grained
jarosite, jarosite + goethite areal mixtures, goethite, generic ferric iron,
and locally to hematite. This type of zonation pattern, attributed to the
oxidation of pyrite, was also identified in waste rock at mines near
Figure 43. Enlargement of map of
iron-bearing minerals and water (none detected) (fig. 31) over the site of the
Bauer Mill near Stockton
Figure 44. Enlargement of map of
clay, carbonate, sulfate, and mica minerals (fig. 32) over the site of the
Bauer Mill near Stockton
Areas dominated spectrally by
hematite and goethite were spectrally identified ≈3 km north-northwest of the
Bauer tailings (fig. 31). Much of the hematite in this area forms dark reddish
iron oxide coatings on rounded alluvial cobbles on the basin floor.
Fine-grained pyrite has been found in the soils associated with these cobbles
(Eric Dillenbeck, Colorado School of Mines, oral commun., 2002), suggesting
that the iron oxides in this area are related to oxidation of fine-grained
sulfide minerals transported northward from the Bauer Mill site by eolian
processes. The area that was spectrally identified as containing goethite
corresponds to an active gravel pit.
Two small deposits of
mill tailings were spectrally identified at the mouth of
International Smelter and Refining Site
There has been renewed
interest in evaluating previous reclamation efforts at the International
Smelter and Refining site. As growth from the town of
Mineral-Distribution Patterns
Down-gradient (northwest)
from the smelter slag and tailings pile, the AVIRIS maps (figs. 45 and 46) do
not indicate any substantial concentrations of minerals that might be
associated with elevated metal levels. However, field observations in this area
indicate that vegetation cover, mostly various types of grasses, is fairly
dense even during dry periods. This vegetation may be masking mineral
exposures. Areas in which iron-bearing minerals were detected by using the
high-altitude AVIRIS data are indicative of revegetation failure. Soils from
these areas should field checked for the presence of acid-producing sulfide or
sulfate minerals and tested in the laboratory or with a field XRF unit for the
presence of trace metals. Jarosite was spectrally identified in five pixels
located just west (down-gradient) from a dike formerly used to impound tailings
from the smelter (fig. 46). This jarosite was identified by the presence of a
strong band near 2.27 μm caused by a combination OH stretch and Fe-OH bend (
Figure 45. Enlargement of map of
iron-bearing minerals and water (none conclusively detected) (fig. 33) over the
International Smelter and Refining site east of Tooele. Approximate location of
smelter site: 40°33'9.87"N., 112°13'29.67"W. View full-resolution file
Figure 46. Enlargement of map of
clay, carbonate, sulfate, and mica minerals (fig. 34) over the International
Smelter and Refining site east of Tooele. Approximate location of smelter site:
40°33'9.87"N., 112°13'29.67"W. View full-resolution file
Several kilometers west-northwest
of the slag heaps, the AVIRIS mineral map also shows some small exposures of
calcite and dolomite that may provide some natural ANP (fig. 46). A possible
zone of argillic alteration occurs 3 km northeast of the smelter site, where
the AVIRIS data analysis identified kaolinite and calcite + kaolinite.
Kaolinite was spectrally identified along the flanks of the Oquirrh range just
east of the smelter. Illite/muscovite and mixtures of kaolinite +
illite/muscovite were spectrally identified in abundance along the flanks of
Discussion
In general, analysis of
the high-altitude AVIRIS data covering the area of the International Smelter
and Refining site did not identify any substantial evidence of minerals that may
be associated with elevated metal concentrations, although vegetation may be
masking surface minerals. It is possible that analysis of imaging spectrometer
data with increased spatial resolution may reveal small areas of mineral
concentrations that are unresolvable in the high-altitude data.
The Tushar Mountains/Marysvale Region
Geologic Setting
The Tushar
Mountains/Marysvale region is ≈280 km south of
Ore Northern
Tushar
Deposits of vein alunite
occur in the vicinity of
Small deposits of gold
and silver were mined between 1892 and 1937 from the Kimberly district on the
northern flanks of the
For a period of over one
hundred years (1878-1981), what is now called the Old Deer Trail mine, located
≈10 km south-southwest of the town of Marysvale (fig. 4), produced Pb, Zn, Ag,
Au, and Cu from underground workings beneath Deer Trail Mountain. Sulfide
minerals including galena, sphalerite, chalcopyrite, tetrahedrite, and pyrite
occur in strata-bound replacement deposits (mantos) in carbonate rocks adjacent
to feeder veins in faults and fractures. These deposits are similar in form and
genesis to those of the Tintic and
The early mining activity
in the Deer Trail area occurred at the Old Deer Trail and Lucky Boy mines
located 2 km east-northeast of the summit of
Ore Antelope Range
Deposits of gold, copper,
replacement alunite, and hydrothermal uranium occur in the
Figure 47. Landsat 7 ETM+ continuous-tone map of mineral groups and
vegetation centered on the
Interpretation
guide:
Red:
rocks and soils bearing hematite and (or) other ferric iron minerals.
Yellow: Fe3+ + CSM (mostly hydrothermal alteration).
Green:
clays, sulfates, and micas (CSM), with possible bleaching.
Cyan:
green, moist vegetation.
Blue:
green, dry vegetation.
After World War II, a mining boom
occurred in the
Mapping and Characterization of Mine
Waste in the Tushar Mountains/Marysvale Region
Material maps generated
from the high- and low-altitude data are shown in figures 48-53. Jarosite-bearing
rocks were spectrally identified at only one mine in the entire Tushar
Mountains/Marysvale region. At this mine, 1 km west-southwest of the (new) Deer
Trail mine, goethite and six pixels of coarse-grained jarosite were mapped.
This occurrence of jarosite has not been field verified because it is located
on private property. AVIRIS spectra sampled from these pixels show strong
electronic absorptions near 1.00 μm that are most likely related to ferric
iron, but spectral features diagnostic of jarosite at 0.43 and 2.27 μm are not
obvious. Waste-rock piles at most of the gold or silver mines in the eastern
Figure 48 (Above left). Map of iron-bearing
minerals in the central Tushar Mountains Utah
Figure 49 (Above right). Map of clay, carbonate,
sulfate, mica, and hydrous silica minerals in the central Tushar Mountains Utah
Figure 50 (Above left). Map of iron-bearing
minerals and water in the eastern Tushar Mountains Antelope Range Utah
Figure 51 (Above right). Map of clay,
carbonate, sulfate, mica, and hydrous silica minerals in the eastern Tushar Mountains Antelope Range Utah Marysvale
Figure 52. Map of iron-bearing
bearing minerals and water in the Big Rock Candy Mountain area of the Marysvale
volcanic field, Utah
Figure 53. Map of clay,
carbonate, sulfate, and mica minerals in the Big Rock Candy Mountain area of
the Marysvale volcanic field, Utah
The replacement alunite deposits
in the
Waste rock associated
with uranium mining (fig. 54) is characterized by goethite, montmorillonite
(smectite), illite/muscovite, and minor amounts of kaolinite (figs. 50 and 51).
Several pixels of coarse-grained goethite with possible trace amounts of
jarosite were spectrally identified in this area. Although pyrite is present in
the uranium-, molybdenum-, and fluorite-bearing ore deposits, the pyrite is not
present in large amounts in the waste-rock piles, which are composed mainly of
the weakly altered intrusive rocks (quartz monzonite and fine-grained granite)
that hosted the vein deposits. A suite of uranium minerals was not included in
the USGS spectral library used by the Tetracorder mapping software at the time
of this study; thus, the AVIRIS data were not queried for the presence of
uranium-bearing minerals. It is unlikely that uranium-bearing minerals in the
waste rock could be directly detected with high-altitude AVIRIS data.
Figure 54. View of the area from which uranium was extracted, looking east
toward
An occurrence of hematite,
goethite, and alunite was mapped with the AVIRIS data on the west bank of the
Sevier River immediately east of the town of
Small occurrences of
goethite were detected at the sites of the Old and (new) Deer Trail mines (fig.
50). This iron-bearing material at the site of the Old Deer Trail mine may be
waste rock and (or) tailings material from the mine and mill that used to exist
there, but the area was not field checked as it lies on private property. At
the (new) Deer Trail mine, the goethite occurs in very low spectral abundance
and corresponds to waste rock dumped immediately east of the main mine portal.
Talc (Mg3Si4O10(OH)2) and (or)
tremolite ([]Ca2Mg5Si8O22(OH)2)
were also detected with the AVIRIS mapping in this waste rock at the (new) Deer
Trail mine (fig. 51). Talc and tremolite commonly occur together and are
difficult to distinguish through the use of spectroscopic data having a
sampling interval and bandpass similar to those of AVIRIS; thus both minerals
were included on the explanations of the mineral maps. The presence of talc and
calcite in the waste rock was confirmed via laboratory spectroscopy and XRD
analysis (appendix table A3, sample MV-5-02-3). Although talc itself has not
been documented at the Deer Trail deposit, its occurrence is to be expected, as
talc and tremolite are the first minerals to form in the low-grade thermal
metamorphism of siliceous dolomites (Winkler, 1979). Tremolite-bearing hornfels
has been identified at the deposit, which is hosted by limestones, dolomites,
and calcareous shales (Beaty and others, 1986). Talc dominates the laboratory
and AVIRIS spectra of the waste rock in the 2.0- to 2.5-μm region, although
calcite was found to be present in significantly greater abundance
("major," >25 percent) than talc ("minor," 5-25 percent)
by the XRD analysis. Talc was also found to dominate the AVIRIS spectrum of
waste rock from carbonate-hosted polymetallic manto deposits in the
The tailings located in
the Kimberly district along Mill Creek (fig. 4) contain minor amounts of
goethite and clay minerals. Spectra of the tailings extracted from the
high-altitude AVIRIS data showed weak absorptions at 2.2 μm indicative of
phyllosilicates (most likely montmorillonite), but these features were too weak
to be identified by the Tetracorder spectral analysis. No spectral evidence of
significant amounts of oxidizing sulfide minerals was found in this area.
Unmined Mineralized Rocks with
Acid-Producing Potential (APP)
Occurrences of Acid-Sulfate Alteration
in the Tushar Mountains Antelope Range
Unmined areas of altered,
mineralized rock are clearly identified on the AVIRIS mineral maps. Altered
rocks in these areas are characterized by alunite, jarosite, clay minerals, and
micas formed by intense acid-sulfate hydrothermal alteration in the Miocene
Epoch at ≈23-14 Ma (Cunningham and others, 1984, 2005). Prominent
alunite-bearing mineralized areas include (1) altered rocks associated with the
replacement alunite deposits that occur around the peripheries of quartz
monzonite intrusive rocks in the Antelope Range (figs. 4, 47, and 51); (2)
argillic alteration along the northeast-striking trend between the Antelope
Range and Deer Trail Mountain associated with intrusions dated between 21 and
16 Ma; (3) argillic and advanced argillic alteration zones in the northern
Tushar Mountains north of Deer Creek Canyon and east of the Kimberly district
(fig. 4) within the Joe Lott Tuff Member of the Mount Belknap Volcanics (just
north of the northern rim of the topographic wall of the Mount Belknap
caldera); and (4) vein alunite deposits and associated wall-rock alteration in
the Alunite Ridge-Deer Trail Mountain area 10 km southwest of the town of
Marysvale (fig. 4). The alunite deposits spectrally identified in the northern
Occurrences of Jarosite in the Antelope Range
Several occurrences of
nonanthropogenic jarosite were mapped with the AVIRIS data in areas underlain
by altered rock. X-ray diffraction (XRD) analysis of rocks from some of these
areas confirmed the presence of jarosite (appendix table A3). In the
replacement alunite deposits of the
Figure 55. Genetic model for
replacement alunite deposits, Antelope Range Utah
Pyrite does not occur intimately
with the alunite or with the hypogene jarosite, hematite, and silica (Willard
and Proctor, 1946; Kerr and others, 1957; Rye and Alpers, 1997). This finding
was corroborated by the fact that XRD analyses of the coarse-grained, hypogene
jarosite from the Yellow Jacket cell did not detect the presence of pyrite
(appendix table A3, samples SW IH B1, SW IH B2, and MV01-1). Conversely, the
fine-grained coatings of natrojarosite that occur in several of the
hydrothermal cells in the
The Tetracorder-based
spectral mapping was able to differentiate jarosites formed under different
geochemical conditions within the hydrothermal systems associated with the
replacement alunite deposits. By using the high-altitude AVIRIS data,
coarse-grained, hypogene jarosite co-occurring with alunite in advanced
argillic alteration zones of the Yellow Jacket cell was differentiated
spectrally from fine-grained coatings of supergene natrojarosite found in the
pyrite-rich, propylitically altered feeder zones where exposed on Big Rock
Candy Mountain (fig. 50; Rockwell and others, 2000). In the White Horse cell,
however, this spectral differentiation was imperfect, as the AVIRIS mapping
detected both coarse- and fine-grained jarosite within pyrite-bearing rocks of
the sulfidized feeder zone that are coated with supergene jarosite (jarosite
composition has not been determined, although XRD analysis detected jarosite
and not natrojarosite). Compared to Big Rock Candy Mountain, the White Horse
area has undergone much less erosion and therefore has relatively low relief
and far less exposed pyrite-bearing rock. The relatively limited exposure of
pyrite-bearing rock has led to reduced acid generation and rates of jarosite
development. These factors have allowed thicker coatings of goethite to develop
from the jarosite, resulting in mixtures of jarosite and goethite. Analysis of
the high-altitude AVIRIS data detected two pixels of a goethite + jarosite
areal mixture on the northern flanks of the White Horse deposit (fig. 50). The
areas identified as coarse-grained jarosite near this deposit most likely
represent mixtures of goethite and jarosite that contain slightly more goethite
than the areas in which the goethite + jarosite mixture was directly detected.
Areas spectrally identified as goethite surround the areas identified as
coarse-grained jarosite. Throughout the Antelope Range and Tushar Mountains,
AVIRIS pixels directly detected as a goethite + jarosite areal mixture were
identified only within areas where active pyrite oxidation is occurring in the
propylitically altered feeder zones of the hydrothermal cells.
At the scale of
high-altitude AVIRIS pixels, variations in the VIS and NIR spectra of the
hypogene and supergene jarosites in the Antelope Range most likely relate to
increased amounts of goethite content and (or) coarser grain sizes in the
hypogene jarosite (Rockwell and others, 2000). The fine-grained, supergene natrojarosite
was a spectral match to a reference spectrum of synthetic jarosite (sample
GDS99, Clark, Swayze, Wise, and others, 2003), and the coarse-grained, hypogene
jarosite was a spectral match to a reference spectrum of a jarosite coating on
a rock rich in quartz and muscovite with additional coatings of what appear to
be manganese-iron oxides (sample BR93-34A2, Clark, Swayze, Wise, and others,
2003). Figure 29 shows spectra of jarosites from the Tintic mining district
that exhibit the VIS/NIR spectral variations between the reference jarosites.
The spectrum of sample TN00-21 is similar to the spectrum of reference sample
BR93-34A2, and the spectrum of sample TN00-17b closely resembles that of
reference sample GDS99. The spectrum of sample TN00-21 (and thus the spectrum
of reference sample BR93-34A2) has similarities in the VIS/NIR to goethite,
most notably a prominent feature at 0.66 μm. The continuum-removed centers of
the primary electronic crystal field absorption features caused by ferric iron
in the GDS99 and BR93-34A2 jarosite reference spectra are 0.9219 μm and 0.9466
μm, respectively. The continuum-removed band center of the feature in a
reference spectrum of goethite is 0.9475 μm (sample WS222, Clark, Swayze, Wise,
and others, 2003). The reference spectrum of the BR93-34A2 jarosite is thus
similar to that of goethite in some respects, suggesting that this jarosite
sample contains some amorphous goethite and (or) that it is characterized by
coarser jarosite crystal sizes than the GDS99 jarosite. As no goethite was
detected in the BR93-34A2 jarosite sample with XRD, pixels matching this
reference jarosite are labeled as coarse-grained jarosite on the maps of
iron-bearing minerals. It is unknown whether the coatings of
"manganese-iron oxides" present in the BR93-34A2 jarosite reference
sample are affecting its spectral shape.
Field samples of the
hypogene and supergene jarosites from the
In the Tintic mining
district, similar spectral variations were used to differentiate fine-grained
coatings of jarosite in waste-rock piles at mine sites from unmined,
nonanthropogenic occurrences of coarse-grained jarosite within zones of
advanced argillic alteration (fig. 29). In the Tintic district, all of the
spectrally identified jarosite was derived from supergene oxidation of pyrite.
Both the coarse-grained hypogene jarosite associated with the replacement
alunite deposits in the Antelope Range and the coarse-grained supergene
jarosite associated with rocks bearing alunite and pyrite in the Tintic
district have remained stable even though they have been exposed to natural
weathering for an extended time. The fact that jarosite remains on the surface
and has not broken down to ferrihydrite or goethite may be due to its
stabilization by the near-neutral pH conditions maintained by high concentrations
of K+ and SO2-4 (both of which are locally present in abundance within the
alunite-bearing rocks) or to slow reaction kinetics (Swayze and others, 2000).
However, abundant goethite was detected with the AVIRIS data around most
occurrences of jarosite in both study areas, which most likely indicates that
some breakdown of jarosite has occurred.
The laboratory-derived
spectra show subtle variations in the
Figure 56. Laboratory spectra of jarosite-bearing rocks formed by hypogene and
supergene processes. XRD results and sampling locations for these rocks are
given in appendix table A3. Absorption features caused by specific minerals are
indicated. J = jarosite. A = alunite. IL = illite. G = gypsum. (GOE) = possible
goethite.
Figure 57. Spectra of natrojarosite from Big Rock Candy Mountain and the
reference library. Because the jarosite is closely associated with other
minerals, jarosite composition is not discernible in the AVIRIS spectrum. After
the jarosite is separated from the other minerals, however, the diagnostic SWIR
spectral features of natrojarosite near 2.27 μm
are visible. The complete laboratory characterization of the jarosite from
Not all of the jarosite that
formed by supergene processes in the
The hypogene and supergene
jarosites most likely have different acid-producing potentials and, thus,
different implications regarding environmental impact. Leach studies are needed
to determine the relative APPs of jarosites of varying compositions
(substitutions of Na, K, and H3O) formed under different geochemical
conditions. Although rocks associated with either hypogene or supergene
jarosite will have some APP, it is likely that the exposures of jarosite formed
by supergene processes from pyrite-bearing rocks will have higher APP and thus
will represent a greater potential for heavy-metals release than the hypogene
jarosite that occurs with alunite and other argillic alteration minerals, but
not with pyrite, at higher levels in the hydrothermal systems (fig. 55). Most
of the hypogene jarosite associated with the replacement alunite deposits has
been eroded away, although it is still present and well exposed in the northern
and southwestern parts of the Yellow Jacket cell. It is unknown whether such
jarosite is exposed in the Al Kee Mee and White Hills cells, as these cells do
not lie within the coverage of the high-altitude AVIRIS data. The
pyrite-bearing rocks of the propylitically altered feeder zones are exposed in
the Big Rock Candy Mountain, Big Star, and White Horse hydrothermal cells
(figs. 47, 50, and 52). These pyrite-bearing rocks are especially evident along
Figure 58. View of Big Rock Candy Mountain looking south along Marysvale
Canyon. The
Figure 59. View of
Spectral Variations in Goethite-Bearing Rocks
Observed by AVIRIS
The maps of iron-bearing
minerals generated for this report from the AVIRIS data show areas identified
as goethite of varying grain sizes associated with altered rocks, soils, and
tailings material. The spectral variations responsible for this differentiation
of the goethite-bearing material are thought to be related to mineral
composition and abundance as well as grain size, as will be demonstrated by
research currently under way at the USGS (Rockwell, 2004). Previous studies
have shown that the distance (or path length) a photon may travel within a
mineral grain increases with effective grain size, as larger grains offer
increased volume-to-surface ratios and reduced chances for surface reflection
(Clark, 1999). Therefore, in the
Figure 60. Hematite grain-size series. All spectra are in continuum-removed
format. Larger grain sizes show broadening of the crystal field absorption at
0.9 μm caused by increased saturation, as well as the shifting of the apparent
reflectance minima to longer wavelengths. Modified from Clark (1999).
Figure 61. Goethite grain-size series. Plots show single-pixel reflectance
spectra sampled from high-altitude AVIRIS data at the southwestern edge of the
Big Star hydrothermal cell adjacent to the
Figure 61 shows single-pixel
high-altitude AVIRIS spectra sampled from altered rocks of the Big Star cell
across the Sevier River from Big Rock Candy Mountain (figs. 47 and 59). The
colors of the spectra in figure 61 are similar to those of the goethite mineral
classes on the mineral maps that they represent (figs. 50 and 52). Like the
spectra in the hematite grain-size series shown in figure 60, the goethite spectra
in figure 61 show a general broadening and shift of the absorption position to
longer wavelengths with increasing grain size. The spectrum identified as
coarse-grained goethite with trace jarosite (shown in magenta) shows a small
shift of the absorption minimum back toward shorter wavelengths and a small
feature at 0.43 μm, both indicative of the trace content of jarosite. However,
it is evident that the overall shape of the 0.9-μm absorption feature is quite
similar to that of the coarse-grained goethite spectrum shown in dark green
(spectrum no. 4). A positive spectral feature near 1.1 μm in the spectrum of
fine-grained goethite is a residual artifact caused by atmospheric water vapor.
On the AVIRIS mineral
maps, the number of pixels identified as thin coatings of goethite is far
greater than the number of pixels identified as coarse- or medium-grained
goethite. The areas identified as thin coatings of goethite correspond
generally to distal alluvial surfaces containing goethite-bearing rock fragments
in low abundance relative to soil components with low iron content and sparse
vegetation. Areas identified as coarser-grained goethite correspond to proximal
alluvial and outcrop surfaces bearing abundant coarser-grained goethite with
little to no soil component. Rock fragments that have been spectrally
characterized in the laboratory as coarse-grained goethite have been found in
areas identified with AVIRIS as coarse-grained goethite and as thin coatings of
goethite. For this reason, goethite abundance also appears to influence the
observed spectral variations in goethite-bearing areas. In future studies, both
abundance and grain-size variations should be considered when interpreting the
iron-mineral identification results generated by the expert system.
On pyrite-bearing
waste-rock piles at mine sites, the zonal pattern of jarosite and goethite
described here generally holds true, as both jarosite and coarser-grained
goethite tend to occur closer to the center of the piles where pyrite oxidation
and acid generation are still active (for example, figs. 13, 15, and 17). Where
tailings material is present, pixels identified as coarser-grained goethite on
the mineral maps may indicate areas in which goethite may be more abundant,
whereas thin coatings of goethite most likely indicate areas in which tailings
material is present only as a thin surface veneer, as is the case in the most
distal parts of the Mercur Canyon outwash deposits in Rush Valley (figs. 31 and
33). As goethite and hematite are very common on the Earth's surface, an
understanding of the spectral variations resulting from differences in
abundance and (or) grain size will enhance and facilitate the discrimination
and characterization of outcrop and alluvial surfaces, providing valuable
information for geologic, geomorphologic, mineral exploration, and
environmental assessment studies.
Acid-Sulfate Alteration on Alunite Ridge
and Deer Trail
Mountain
The acid-sulfate
hydrothermal alteration exposed on Alunite Ridge and Deer Trail Mountain in the
eastern Tushar Mountains was formed much later (at ≈14 Ma) than the replacement
alunite in the Antelope Range (23-21 Ma) and under contrasting geochemical and
geologic conditions. Geologic evidence including radial fracture patterns,
concentric ring fractures, and steeply tilted wall rocks points to the
existence of two unexposed intrusions of similar age underlying Alunite Ridge
and the summit of
On Alunite Ridge, veins of coarsely crystalline alunite up to 20 m wide are present within
larger envelopes of alunitized and argillized host volcanic rocks. Clays are not abundant
within the veins of pink alunite exposed in the local mines. The abundance of natroalunite and
clays in the altered wall rock appears to increase with vertical distance above the Mineral
Products mine, and these minerals were mapped pervasively on the northern part of Alunite Ridge
in the area surrounding the Christmas mine and on the south face of Mount Brigham by using the
high-altitude AVIRIS data (fig. 49). However, the wall-rock alteration is partly masked by
abundant forest vegetation that occurs on the ridge below the treeline located ≈60 m below the
Christmas mine. The alteration center on Deer Trail Mountain is characterized mainly by
replacement kaolinite with minor dickite and sericite at the summit. Primarily colluvial
dickite, kaolinite, and minor alunite are best exposed ≈1 km to the north-northeast and
south-southwest of the summit on the mountain flanks, suggesting that a
north-northeast-striking fracture trend exists that may have significance for base and precious
metal exploration lower down in the system.
Most of the vein alunite deposited above the present-day summit of Deer
Trail Mountain has been eroded away, and the kaolinite and dickite present near the summit
represent the roots of the hydrothermal system. Substantial amounts of alunite and clay
minerals have been shed eastward from these altered areas down into the Sevier River valley,
not only in the Close In landslide deposit but also within incised terraces deposited by the
ancestral Cottonwood Creek along the west bank of the Sevier River (figs. 4 and 51).
Jarosite (fig. 48, ≈260 m
south-southeast of Christmas mine) was found to occur with alunite along
fractures cutting illite in a hand sample and along fractures cutting alunite
at the Mineral Products mine. Jarosite was not mapped with the AVIRIS data at
the Mineral Products mine because the jarosite occurs along fractures that are
best exposed on the near-vertical walls of the open stopes of the mine and are,
therefore, not well exposed for mapping with high-altitude AVIRIS data. The
jarosite on Alunite Ridge was probably derived from the supergene oxidation of pyrite
in host rocks and aplite dikes near the uppermost parts of the alunite-bearing
veins. Exposures of "coarse-grained" jarosite were mapped with the
AVIRIS data in four main locations in the east-central Tushar Mountains and
Sevier River valley (figs. 48 and 50): (1) on Alunite Ridge, (2) in bedrock and
colluvium on the flanks of Deer Trail Mountain to the northeast and southwest
of the summit, (3) in colluvium underneath cliffs located east of Hennesy Point
≈2.5 km north-northeast of the summit of Deer Trail Mountain, and (4) locally
on pediment surfaces in the Sevier River valley immediately west of the river
(fig. 4). The jarosite occurrence on Alunite Ridge has been verified by field
observations. In all four places, the jarosite is associated with argillized
rocks bearing alunite, kaolinite, dickite, and (or) illite/muscovite. Hypogene
jarosite of the type found in the replacement alunite deposits in the
Rocks with Acid-Neutralizing Potential
(ANP)
The detection of
acid-buffering mineral assemblages containing calcite, chlorite, and (or)
epidote that are associated with either sedimentary rocks or propylitic
alteration within and adjacent to intrusive rocks is important in assessing
local acid-neutralizing potential. Some other scattered pixels of calcite
mapped with the AVIRIS data represent caliche in alluvial soils, such as in
Quaternary alluvium west of Joseph at the north end of figure 51, within the
Bullion Canyon Volcanics north of the Yellow Jacket cell, and within the Joe
Lott Tuff Member of the Mount Belknap Volcanics and alluvium to the northeast
of the Yellow Jacket cell. Pyrite-poor, propylitically altered intrusive rocks
located east of the Sevier River in the vicinity of
Conclusions
Interpretation of
watershed- or regional-scale maps of selected surface minerals derived from
analysis of imaging spectroscopy data such as AVIRIS can be an effective means
of (1) evaluating environmental factors associated with hydrothermally altered
rocks and mine waste and (2) focusing subsequent site-evaluation and
remediation efforts. An understanding of local geology is essential for
accurately interpreting geo-environmental impact from the mineral maps. The
mineral maps may be useful in directing the collection of ground samples that
can be tested for possible elevated metal levels. In assessing the
environmental condition of mining districts, the USEPA is mainly interested in
geochemical site screening in terms of mapping the sources and the transport
and fate of toxic trace metals that can adversely affect human health. So far,
remote-sensing methods can reliably target only sources of acid generation and
buffering, although laboratory characterization (for example, XRF [X-ray
fluorescence] and ICP-MS [inductively coupled plasma-mass spectrometry]) of
field samples can establish direct and indirect mineralogical relationships
between remotely sensed targets and trace metals (for example, the association
of goethite, kaolinite, and muscovite in the mill tailings of the Mercur and
Manning Canyon areas that is directly associated with elevated levels of trace
metals). It should be noted that this study did not attempt to identify any
radiological hazards associated with waste-rock piles in areas from which
uranium was extracted.
Large exposures of unmined,
hydrothermally altered rock exist throughout the
Mine tailings and waste rock
containing sulfide minerals and heavy metals were spectrally identified in the
Although many waste-rock
piles in the Tintic mining district contain oxidizing sulfide minerals and are,
thus, important potential point sources of acid generation and heavy-metals
release, little spectral evidence was found for downstream or downwind movement
of these minerals. In most cases, acid-producing rocks are confined to mine
sites, largely because of low amounts of annual precipitation. Owing to the
proximity of the Chief No. 1 and Centennial/Eureka mines to the town of
In the
In the Antelope Range
area of the Marysvale volcanic field in Sevier and Piute Counties,
pyrite-bearing rocks having high acid-producing potential (APP) were mapped by
using the AVIRIS data only in the propylitically altered feeder zones of the
Big Rock Candy cell, at the southwestern edge of the Big Star cell (both next
to the Sevier River), and at the White Horse cell. Significant amounts of
sediments containing jarosite, goethite, and possible pyrite are being shed
from the exposed eastern and northern slopes of Big Rock Candy Mountain in
Acknowledgments
We thank several U.S.
Geological Survey (USGS), U.S. Environmental Protection Agency (USEPA),
National Aeronautics and Space Administration (NASA), and U.S. Bureau of
Reclamation (USBOR) personnel for their assistance on this project: Robert O.
Green and the rest of the AVIRIS team at NASA Jet Propulsion Laboratory (JPL)
for providing the high-quality remote-sensing data that were the centerpiece of
the Utah AML project; Roger N. Clark, for his role in starting the Utah Imaging
Spectroscopy Project, his advice on calibration techniques, collection of
calibration spectra, and field assistance; Stephen J. Sutley, who helped
perform the X-ray diffraction analyses; Charles G. Cunningham, for background
geologic information, the photograph of Big Rock Candy Mountain, purifying the
natrojarosite from Big Rock Candy Mountain, field assistance, and other
valuable insights; Tony Selle, Ken Wangerud, and Luke Chavez (USEPA) for
initiating and funding the Utah Imaging Spectroscopy Project; K. Eric Livo for
the planning of the high-altitude AVIRIS flightlines, collection of calibration
spectra, and field assistance; Ron Pearson (USBOR) for field XRF operation and
field assistance; Raymond F. Kokaly for collection of calibration spectra; and
J. Sam Vance (USEPA) for his role in starting the Utah Imaging Spectroscopy
Project, providing contacts with State and Federal agencies and mining
companies, and field assistance. We also thank Daniel H. Knepper, Jr., Trude
V.V. King, and George A. Desborough of the USGS for their helpful reviews of
this manuscript. Mary Eberle also deserves special thanks for her thorough and
professional editing of the manuscript.
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Appendix. X-Ray Diffraction Results
Appendix tables A1-A3 give results of XRD analysis
for the Tintic mining district, the Mercur outwash area in the southwestern
Appendix Table 1. XRD analysis results-Tintic mining district, Utah.
Appendix Table 2. XRD analysis results and field XRF data-Mercur outwash,
southwestern Oquirrh Mountains, Utah.
