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Identification of quartz and carbonate minerals across northern Nevada using ASTER thermal infrared emissivity data—Implications for geologic mapping and mineral resource investigations in well-studied and frontier areas

¹ U.S. Geological Survey, Box 25046, MS 973, Denver Federal Center, Denver, Colorado 80225, USA

	ABSTRACT

TOP ABSTRACT INTRODUCTION METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES CITED

 
ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) thermal infrared imagery over a 389 km x 387 km area in northern Nevada (38.5°–42°N, 114°–118.5°W) was analyzed to evaluate its capability for accurate and cost-effective identification and mapping of quartz and carbonate minerals at regional to local scales. The geology of this area has been mapped at a wide range of scales and includes a diversity of rock types and unconsolidated surficial materials, many of which are composed primarily of quartz and carbonate minerals. This area is also endowed with a wide variety of economically and scientifically important ore deposit types that contain an array of commodities (Au, Ag, Pb, Zn, Cu, Mo, W, Sn, Be, F, Mn, Fe, Sb, Hg, and barite). The hydrothermal systems that generated these deposits frequently deposited large amounts of quartz where fluids cool, and generally smaller amounts of calcite or dolomite by other mechanisms.

To identify and map quartz and carbonate minerals, band ratioing techniques were developed based on the shapes of laboratory reference spectra and applied to ASTER Level 2 surface emissivity products of 108 overlapping scenes. These mineral maps were mosaicked into a single coverage that was overlain with published, vector-format geologic maps of various scales to determine which geologic terranes, formations, and geomorphic features correspond to identified quartz or carbonate. Where quartz or carbonate minerals were mapped in rocks composed primarily of other minerals, they were inferred to be hydrothermal in origin and compared to known occurrences of hydrothermal alteration and mineralization.

The ASTER-based quartz mapping identified thick sequences of quartzite, bedded radiolarian chert, quartz sandstone, conglomerates with clasts of quartzite and chert, silicic and/or altered rhyolites, and silicic welded tuffs. Alluvial fan surfaces, sand dunes, and beach deposits composed of quartz and/or carbonate are prominent, well-mapped features. Quartz was also identified in smaller bodies of jasperoid, quartz-alunite, and quartz-sericite-pyrite alteration, hot spring silica sinter terraces, and several diatomite and perlite mines and prospects. The ASTER-based carbonate mapping identified thick sequences of dolomite, limestone, and marble, as well as small hot spring travertine deposits. Eolian carbonate was identified in several playas. Dolomite exhibited a stronger carbonate response than calcite, as predicted based on their thermal spectral characteristics. Quartz was detected at lower concentrations than carbonates because of the greater strength of the quartz reststrahlen features in the thermal infrared compared to the bending-related spectral features of carbonates. The 90 m ground pixel size of the ASTER thermal imagery prevents the identification of small-scale features. Despite this limitation, numerous bodies of hydrothermal quartz were detected in or near known Carlin-type gold deposits, distal disseminated Au-Ag deposits, high- and low-sulfidation epithermal Au-Ag deposits, and geothermal areas. Detection of hydrothermal carbonate was rare and mainly in geothermal areas.

The ASTER-based thermal quartz and carbonate mapping demonstrated here can be used in well-studied or frontier areas to verify the accuracy of existing geologic maps, guide future detailed stratigraphic and structural mapping in lithologically complex terranes and allochthons, and identify hydrothermal features for exploration and resource assessment purposes.

Keywords: remote sensing • ASTER • thermal infrared • quartz • carbonate

	INTRODUCTION

TOP ABSTRACT INTRODUCTION METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES CITED

 
The Great Basin physiographic province is the world's second leading producer of gold and is also host to a wide variety of ore deposits that contain large resources of silver, base metals, and other important metallic and industrial minerals (Hofstra and Wallace, 2006). Because hydrothermal silicification accompanies mineralization in many metal deposits, the identification and mapping of quartz in rocks composed mainly of other minerals is of great value for exploration and assessments of resource potential. Identification of quartz and carbonate in rocks and unconsolidated surficial materials across large areas holds promise for assessing the potential for industrial rock-mineral resources including aggregate, sand, gravel, caliche, and calcrete. The regional mapping of these minerals also facilitates evaluation of existing geologic maps and recognition of locales in need of more refined mapping or topical investigation.

Data from spaceborne remote sensing systems have been applied to prospecting in terranes with requisite geologic and tectonic frameworks for decades, but the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) sensor aboard the Earth Observing System (EOS) Terra satellite launched in 2000 provides revolutionary new capabilities for cost-effective mineral and landcover mapping over large areas. The ASTER sensor acquires multi-spectral data in 14 bands, including 5 bands in the mid-infrared, or thermal (8–14 µm), region of the electromagnetic spectrum, in addition to another band with backward-looking geometry in the near-infrared (NIR, 0.76–0.86 µm) to provide capability for stereoscopic observation and digital elevation model generation. The multi-spectral thermal infrared (TIR) data of ASTER are vital for detecting nonhydrous varieties of quartz that are not spectrally identifiable in the visible, near-infrared, and shortwave-infrared (SWIR, 1.4–2.5 µm) spectral regions because of a lack of diagnostic absorption features. The ASTER TIR data also provide capability for the remote identification and mapping of areally extensive occurrences of other minerals, including carbonates, although the ASTER SWIR data are more sensitive at detecting hydrous quartz (e.g., opal and chalcedony) and differentiating carbonate mineral species. Despite its potential utility, the mineral mapping capabilities of ASTER TIR data are often underutilized in scientific studies.

This study evaluates the utility of band ratio analysis techniques applied to ASTER thermal infrared emissivity data for the identification and mapping of quartz and carbonate minerals across a 3.5° x 4.5° (389 km x 387 km) area of the central Great Basin in northern Nevada. The quartz and carbonate mineral maps were overlain with geologic maps of a wide range of scales (1:500,000–1:6000) to evaluate their reliability and to identify previously unknown occurrences of quartz and carbonate to guide future investigations. Of particular relevance to metallogeny was evidence of hydrothermal quartz, including jasperoids or other quartz associated with argillic and phyllic alteration, in areas of known mineralization. Also evaluated was the utility of the mineral maps for identifying quartz and/or carbonate anomalies within complex, deformed terranes for which only coarse-scale geologic mapping is available. Such anomalies can enhance understanding of these terranes by delineating stratigraphic marker horizons and structural features in need of more detailed geologic mapping.

	METHODS

TOP ABSTRACT INTRODUCTION METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES CITED

 
Quartz and carbonate minerals are spectrally characterized by strong vibrational absorption features within the 8–14 µm atmospheric window (Salisbury and D'Aria, 1992a; Hook et al., 1999) that is measured by the five thermal infrared bands of the ASTER sensor. Table 1 shows the wavelength centers and band widths of the five ASTER thermal bands, which have a ground instantaneous field of view (GIFOV), or ground spatial resolution, of 90 m. Figures 1 and 2 exhibit the spectral features of quartz, calcite, and dolomite in this spectral region at original laboratory resolution and convolved to ASTER sampling and bandpass (Table 1). The emissivity spectra shown in these figures were created by applying Kirchoff's Law, E = 1 – R, where E and R are emissivity and reflectance, to reflectance spectra from a mid-infrared laboratory spectral library (Salisbury et al., 1991). Kirchoff's Law inverts reflectance peaks into emissivity troughs commonly referred to as emissivity "absorption" features. As the laboratory spectra were measured in biconical, rather than hemispherical, reflectance, the generated emissivity spectra can be used to predict only the spectral shapes, and not absolute emissivity values, of the mineral spectra. In both Figures 1 and 2, the emissivity spectra convolved to ASTER sampling and bandpass are shown in red. In Figure 1, the emissivity absorption features of quartz at ASTER bands 10 and 12 are related to fundamental asymmetric Si-O stretching vibrations (reststrahlen bands). The reststrahlen bands of quartz are the strongest of any silicate mineral (Salisbury and D'Aria, 1992b). In Figure 2, the emissivity absorption features of calcite and dolomite at ASTER band 14 are related to out-of-plane bending modes of the CO₃ ion (Clark, 1999). Note that dolomite exhibits a greater decrease in emissivity between bands 13 and 14 than calcite. This characteristic is caused by the greater width and shorter wavelength position of the bending feature of dolomite at 11.15 µm relative to the bending feature of calcite at 11.27 µm.

View larger version (12K): [in this window] [in a new window]   Figure 1. Plot of reference laboratory spectrum of quartz from Salisbury et al. (1991), converted to qualitative emissivity using Kirchoff's Law. The spectrum at original resolution is shown in blue. The spectrum convolved to ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) spectral resolution (Table 1) is shown in red. ASTER band centers are also shown. Note quartz reststrahlen absorption features at bands 10 and 12.

View larger version (17K): [in this window] [in a new window]   Figure 2. Plot of reference laboratory spectra of calcite (blue) and dolomite (green) from Salisbury et al. (1991), converted to qualitative emissivity using Kirchoff's Law. The spectra convolved to ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) spectral resolution (Table 1) are shown in red. ASTER band centers are also shown. Note carbonate bending mode absorption features at band 14.

Salisbury and D'Aria (1992b) described how grain size, soil moisture, and mineral mixtures can have significant effects on the shape of the TIR emissivity spectra of rocks and soils. In quartz-bearing soils, coarse grain sizes show much larger increases in emissivity between ASTER bands 12 and 13, and between ASTER bands 10 and 11, than do finer grain sizes. Dry quartz-bearing soils show larger increases in emissivity between ASTER bands 12 and 13 than do moist soils. Mixtures of quartz with other minerals can also affect spectral shape. Figure 3 shows emissivity spectra of several minerals common in rocks and soils that, if present in sufficient amounts, can obscure the presence of quartz or carbonate. The presence of these minerals in sufficient concentrations will decrease the prominence of the diagnostic quartz emissivity peak at band 11 or create an emissivity absorption feature at those wavelengths. The presence of montmorillonite, kaolinite, or muscovite, among other minerals, will result in a substantial decrease in emissivity between bands 10 and 12. Minerals such as phyllosilicates (clays and micas), carbonate, and sulfates (including gypsum) can be accurately identified using ASTER data of the SWIR spectral region (1.4–2.5 µm; Rowan et al., 2005; Rockwell et al., 2006). Therefore, ASTER SWIR data (30 m GIFOV) can be used to identify many rock-forming minerals that will affect the TIR spectral response.

View larger version (18K): [in this window] [in a new window]   Figure 3. Plot of laboratory spectra of mont-morillonite, kaolinite, gypsum, and muscovite from Salisbury et al. (1991), converted to qualitative emissivity using Kirchoff's Law. These minerals commonly occur in rocks and soils and have spectral features in the 8–14 µm atmospheric window measured by ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) that can substantially affect TIR (thermal infrared) spectral response. Where these and other minerals occur in mixtures with quartz, the presence of quartz can be spectrally obscured, as discussed in text.

Based on the TIR emissivity signatures of quartz and carbonate minerals shown in Figures 1 and 2, indices using compound band ratios were developed to identify these mineral groups.

Equation 1 is the index formula for quartz, and equation 2 is the index formula for carbonate.

Equation 1 (quartz index):

Equation 2 (carbonate index):

This quartz index exploits the emissivity absorption features at ASTER bands 10 and 12 relative to bands 11 and 13. Expression (a) of the index will yield bright values for areas that are high in band 11 relative to the sum of bands 10 and 12, and expression (b) will yield bright values for areas that are high in band 13 relative to band 12. The quartz index is the product of these two expressions, both of which yield high values for quartz-bearing surfaces. The utility of the type of compound band ratio used in expression (a), in inverted form, for isolating specific absorption features in imaging spectrometer data was described by Crowley et al. (1989). A different quartz index that exploits the same spectral features as expression (a), and the carbonate index of equation 2, was described by Ninomiya et al. (2005) for application to ASTER TIR radiance-at-sensor data. High digital number (DN) values for these indices indicate spectral signatures similar to those of the particular mineral group they were designed to map, and should indicate the presence of quartz or carbonate if the ASTER spectral signatures are unambiguous with those of other surface materials. Given the greater decrease of emissivity between bands 13 and 14 for dolomite relative to calcite, dolomitic rocks should exhibit higher carbonate index values than limestone and other calcite-bearing surfaces. The indices were applied to the 108 scenes of ASTER emissivity data on a scene-by-scene basis using automated batch processing.

The resultant mineral index maps were then histogram matched and mosaicked into single coverages, which were then georeferenced to the Universal Transverse Mercator (UTM) projection using a rubber-sheeting transformation and control points from an orthocorrected coverage of Landsat Thematic Mapper (TM) imagery acquired from the U.S. Geological Survey Seamless Data Distribution system. Control points from vector geology coverages (Ludington et al., 2005; Crafford, 2007) were used in some areas where more detailed control was required. ASTER scenes acquired with significantly off-nadir viewing geometries (>6°) have higher positional errors in the ephemeris data than do scenes acquired with near-nadir geometries, requiring the use of the rubber-sheeting, rather than polynomial, transformation. Rubber sheeting is based on triangle-based finite element analysis (Watson, 1992). Because of the variable positional accuracies of the satellite ephemeris data, the 90 m GIFOV of the ASTER thermal data that made control point selection difficult, and the nature of the rubber-sheeting transformation, geometric distortions may be locally present in the mineral index maps. Several small and isolated cumulus clouds were masked from the carbonate mineral index map.

To determine the minimum DN levels corresponding to accurate detections of quartz or carbonate by the mineral indices, the georeferenced indices were overlain with 1:24,000 scale vector-based geologic map data of the Alligator Ridge and Bald Mountain gold districts in White Pine County (Nutt, 2000; Nutt and Hart, 2004). These districts are known to contain large jasperoids of multiple ages, hydrothermal alteration, and a wide range of lithologies including abundant carbonate rocks (Nutt and Hofstra, 2003, 2007). Minimum DN value thresholds were chosen for each index at the level at which pixels began to occur outside of geologic units known to contain the indexed mineral. To verify the accuracy of the thresholds, ASTER emissivity spectra of surfaces with high index DN values (Fig. 4) were extracted from the original data to determine their similarity with the reference laboratory spectra (Figs. 1 and 2). Figure 4 shows that the mid-infrared spectral features of quartz and carbonate are present in the selected spectra, and that there is little consistent variation in spectral shape between quartz-bearing units of sedimentary and hydrothermal origin (e.g., jasperoids). The populations of index pixels selected as accurate mineral detections showed strong spatial contiguity with each other. Spatial contiguity thus served as a secondary, qualitative means of choosing index detection thresholds. ASTER emissivity spectra of sand dunes and hot springs that were identified as either quartz or carbonate bearing elsewhere in the study area were also extracted to determine if the thresholds were accurate across the entire study area (Fig. 5). Such spectra also corresponded well with reference laboratory spectra. Index pixels with DN values below the detection thresholds were set to zero, and color tables were applied to index pixels above the thresholds. Pixels with the highest index values for quartz and carbonate were set to brightest red and green, respectively. Successively darker shades of these colors were assigned to pixels with decreasing DN values. The gradational color tables enable the interpretation of the index level for each pixel selected as a mineral detection. The index values of a pixel are affected by abundance of that particular mineral or mineral group, degree of exposure relative to soil and vegetation cover, carbonate mineral type (e.g., calcite or dolomite), grain size distribution, moisture content, and/or areal or intimate mixtures with other minerals or materials.

View larger version (69K): [in this window] [in a new window]   Figure 4. Plot of ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) emissivity spectra of surfaces in the Alligator Ridge–Bald Mountain area identified as containing either quartz or carbonate using ratio-based indices. Spectra of carbonate-bearing lithologies are shown at top. Jasperoids formed by hydrothermal processes are indicated in red at right. ASTER band centers are also shown. Note correspondence of spectral shapes with the reference spectra shown in Figures 1 and 2.

View larger version (55K): [in this window] [in a new window]   Figure 5. Plot of ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) emissivity spectra of geomorphic and hot springs features identified as containing either quartz or carbonate using the ratio-based indices. Spectra of carbonate-bearing travertine terraces are shown at top. Hot springs features formed by hydrothermal processes are indicated in red at right. ASTER band centers are also shown. Note correspondence of spectral shapes with reference spectra shown in Figures 1 and 2. The spectrum of the Winnemucca Dunes was identified as quartz bearing based mainly on the sharp increase in emissivity between ASTER bands 12 and 13. The shape of the spectrum from bands 10–13 shows influence of a mixture with other minerals, possibly gypsum and muscovite.

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES CITED

View larger version (90K): [in this window] [in a new window]   Figure 6. Overview of quartz and carbonate mineral index maps. Areas shown in detail in subsequent figures are indicated by white rectangles. A—Wood Hills and northeastern East Humboldt Range. B—Ruby Mountains and East Humboldt Range. C—Kinsley mining district. D—Winnemucca Dunes. E—Easy Junior mining district. F—Independence Mountains. G—Beowawe hot springs. H—hot springs, Ruby Valley. I—Diana's Punchbowl hot springs, Monitor Valley. J—Illipah mine area. K—Paradise Peak mine area. L—Coffin Mountain area, southern Piñon Range. M—Bald Mountain and Alligator Ridge mining districts. Major mineral trends, other districts, and mines are indicated in yellow. IN: Independence Trend. GT—Getchell Trend. CN—Carlin Trend. BM/EK—Battle Mountain–Eureka Trend. JB—Jarbidge district. RH—Rawhide district. RO—Robinson district. CP—Colado/Perlite diatomite mine. FC—Florida Canyon mine. LT—Lone Tree mine. MG—Marigold mine. MD—McDermitt deposit, Cordero mine. RM—Round Mountain mine. PC—Palisade Canyon perlite prospect. PP—Pamela Placer perlite prospect. Other features mentioned in the text are shown in cyan. DM—Diamond Mountains. GQ—quartz anomaly near Gold Quarry mine, Carlin Trend. QS—quartz-bearing soils. BV—carbonate soils in Buena Vista Valley. CS—carbonate soils in Carson Sink. Counties are indicated on index map of Nevada at lower left. H—Humboldt County. P—Pershing County. C—Churchill County. M—Mineral County. L—Lander County. K—Eureka County. E—Elko County. W—White Pine County. N—Nye County. Mineral index maps in GeoTIFF: quartz map, carbonate map (see footnote 1). To access a full-resolution PDF of this figure, please visit http://dx.doi.org/10.1130/GES00126.S6 or the full-text article on www.gsajournals.org.

View larger version (112K): [in this window] [in a new window]   Plate 1. Map of study area showing vectorized quartz and carbonate index maps, overlain on Landsat Thematic Mapper derived from U.S. Geological Survey Seamless Data Distribution system, relative to hot springs (black Xs), significant ore deposits (colored symbols), and major roads (gray lines). Quartz detected using the ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) TIR (thermal infrared) data is shown with red polygons, and detected carbonate minerals with green polygons. The mineral index maps were simplified prior to vectorization using a 3 x 3 majority filter. Unfiltered maps are shown in raster format in Plates 2 and 3, and in other figures. The Landsat image is a color composite of bands 2, 3, and 4 displayed in red, green, and blue, respectively. In this treatment, green vegetation is displayed in hues of blue. Hot spring locations are from Shevenell and Garside (2005). The ore deposit symbols indicate deposit type, whereas the color indicates the primary commodity (see legend). Displayed ore deposit locations were selected from the Mineral Resources Data System (2007), an international database of mineral site records with related geologic, commodity, and deposit information. The deposits shown are the economically significant Au, Ag, Cu, Pb and Zn deposits as defined by Long et al. (2000), or listed in Wallace et al. (2004), as well as the largest known examples of the Fe, Mn, Mo, W, Sn, Be, F, Sb, Hg, and barite deposits (Lowe, 1985; Sherlock et al., 1996). If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00126.S1 or the full-text article on www.gsajournals.org to access the full-resolution file of Plate 1.

Map of Mineral Indices Overlain on 1:500,000 Scale Geology
Plate 2 shows the mineral index maps in the context of the statewide geology of Ludington et al. (2005). Color coding of the geologic units is based on the primary lithology attribute of the vector geology coverage, and secondary lithologies from the coverage are listed in the explanation in parentheses. Quartz identified using the ASTER data corresponds well with units described as primarily quartzite (colored gold) and chert (colored black), and in alluvium and/or colluvium derived from them. Identified carbonate corresponds well with both limestone (colored blue) and dolomite (colored cyan) units, with the highest index values most often corresponding with dolomitic units, as expected given the greater decrease in emissivity between ASTER bands 13 and 14 for dolomite (Fig. 2). Many of the mapped quartz and carbonate anomalies are also located within units described as primarily shale or felsic volcanic rocks. Most, but not all, of these anomalous occurrences of quartz and carbonate reflect the coarse scale and generalized descriptions of the primary lithologies of the geologic units in the statewide geology coverage. Units described as primarily chert, especially within the Roberts Mountain allochthon, contain units of quartzite, chert, and shale, and the mapped quartz most often corresponds to the quartzite units within these complex terranes. The quartz-rich Diamond Peak Formation has been grouped with associated shale units in the statewide geology coverage (e.g., along the west flank of the Diamond Mountains mentioned above). Several occurrences of identified carbonate within units described as felsic volcanic rocks also reflect the broad nature of the described primary lithologies. For example, within the Sand Springs Range in Churchill and Mineral Counties east and north of the Rawhide Ag district (RH, Fig. 6), mapped carbonate corresponds to shelf limestones intercalated with volcaniclastic rocks. The mineral maps demonstrate both the merits and problems inherent in such small-scale lithologic maps with heterogeneous units. Hence, lithostratigraphic (rather than chronostratigraphic) geologic maps of the largest possible scale should be used when interpreting mineralogic information derived from remote sensing data.

View larger version (91K): [in this window] [in a new window]   Plate 2. Map of study area showing quartz and carbonate index maps overlain on background of 1:500,000-scale geology and faults from Ludington et al. (2005) and a shaded-relief digital elevation model (DEM) having 90 m spatial resolution. U.S. Geological Survey DTED-1 3-arc second DEM data were used to create the shaded-relief image (Reheis, 1999). Detected quartz is shown red, carbonate minerals in green. The geology polygons are partially transparent so that topography from the elevation model is visible. The explanation at right indicates the primary lithologies from the vector geology coverage. Secondary lithologies are also shown, in parentheses. Main roads are shown in white. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00126.S2 or the full-text article on www.gsajournals.org to access the full-resolution file of Plate 2. ASTER—Advanced Spaceborne Thermal Emission and Reflection Radiometer.

View larger version (82K): [in this window] [in a new window]   Plate 3. Map of study area showing quartz and carbonate index maps overlain on background of 1:250,000-scale geology from Crafford (2007) and a shaded-relief digital elevation model (Reheis, 1999). Geology polygon outlines have been color coded relative to included quartz-and carbonate-bearing lithologies or geologic features (see explanation of color coding at top right). The polygon color coding shown here is included only as an approximate guide to included lithology, and the geology coverage and county-level maps should be consulted to obtain detailed and accurate information about geologic units. Main roads are shown in white. If you are viewing the PDF of this paper or reading it offline, please visit http://dx.doi.org/10.1130/GES00126.S3 or the full-text article on www.gsajournals.org to access the full-resolution file of Plate 3. ASTER—Advanced Spaceborne Thermal Emission and Reflection Radiometer.

View larger version (79K): [in this window] [in a new window]   Figure 7. Subset of Plate 3 showing mineral mapping results in the Wood Hills and northeastern East Humboldt Range, Elko County (A, Fig. 6). Quartzites indicated in magenta, carbonate-bearing units in blue. Refer to text and Tables 1 and 2 for unit explanations. Interstate 80 extends across the top of the maps. Map at left is from Coats (1987). ASTER—Advanced Spaceborne Thermal Emission and Reflection Radiometer. To access a full-resolution PDF of this figure, please visit http://dx.doi.org/10.1130/GES00126.S7 or the full-text article on www.gsajournals.org.

View larger version (108K): [in this window] [in a new window]   Figure 8. Subset of Plate 3 showing variation in geologic mapping detail between Ruby Mountains and East Humboldt Range (B, Fig. 6). Quartzite and quartzitic schist units can be delineated within undifferentiated metamorphic units using the quartz mineral index map. Note small patches of identified carbonate within calcite marble in Ruby Mountains. Interstate 80 extends across the upper left of the maps. Map at left is from Coats (1987). ASTER—Advanced Spaceborne Thermal Emission and Reflection Radiometer. To access a full-resolution PDF of this figure, please visit http://dx.doi.org/10.1130/GES00126.S8 or the full-text article on www.gsajournals.org.

View larger version (93K): [in this window] [in a new window]   Figure 9. Subset of Plate 3 over the Independence Mountains, Elko County (F, Fig. 6). The quartz index map highlights the McAfee Quartzite within the undifferentiated unit Ds of Crafford (2007) that composes the Roberts Mountain allochthon (Mississippian Antler orogeny) in this area. Small outcrops of cherts and jasperoids were also identified as quartz bearing within this allochthon. The mapped jasperoid is shown in Figure 10. Units of chert and/or argillite within the Devonian–Permian Schoonover Sequence (part of the undifferentiated Golconda terrane, GC, of Crafford, 2007) in the northwestern mountains were also mapped as quartz bearing. Exposures of identified quartz in the Golconda terrane parallel the general southwest-northeast strike of the many faults in this area (Miller et al., 1984) that were created during thrusting of the Golconda allochthon during the Permian–Triassic Sonoma orogeny. JC—Jerritt Canyon mine. CM—California Mountain mine. ED—East Dash mine. SV—Saval mine. BW—Burns West mine. ASTER—Advanced Spaceborne Thermal Emission and Reflection Radiometer. To access a full-resolution PDF of this figure, please visit http://dx.doi.org/10.1130/GES00126.S9 or the full-text article on www.gsajournals.org.

View larger version (130K): [in this window] [in a new window]   Figure 10. Google EarthTM image of the large jasperoid (yellow arrow) in the Jerritt Canyon district (F, Fig. 6) identified as a small quartz anomaly (Fig. 9). View is toward the north. Note scale at bottom left. Pointer 41°23'16.31''N, 115°58'17.10''W. Elevation 7774 ft. Eye altitude 8562 ft. Scale bar is 497 ft (151.5 m). Image © 2007 DigitalGlobe; © 2007 Europa Technologies; © 2007 TerraMetrics. The jasperoid is localized by the Bidart anticline and is hosted in unit 1 of the Hanson Creek Formation (Silurian–Ordovician) that was silicified during the Eocene (Hofstra et al., 1999). Inset shows field photograph of jasperoid from Eliason and Wilton (2005).

Hydrothermal Quartz and Carbonate
Plate 3 shows that there is a high correlation between identified quartz and jasperoids of uncertain age and undefined relation to ore deposits (outlined in orange). Such correlation is especially high in White Pine County, where jasperoids were well mapped and differentiated by Hose et al. (1976). Below, mineral indices are compared to geologic maps of various scales (1:250,000–1:6000) from mining districts and geothermal areas that are known to contain large exposures of hydrothermal quartz. In a few areas, detected quartz prompted literature investigations of known mineralization. Sizable occurrences of quartz formed by hydrothermal processes were identified that are associated with Carlin-type Au (Hofstra and Cline, 2000; Cline et al., 2005), distal-disseminated Au-Ag (Cox, 1992; Theodore, 1998), and epithermal high- and low-sulfidation Au-Ag (John, 2001) deposits. Siliceous sinters associated with modern hot springs (Shevenell and Garside, 2005) were also mapped (Fig. 11).

View larger version (51K): [in this window] [in a new window]   Figure 11. Subsets of Plate 3 showing modern hot springs at which siliceous or travertine sinter deposits were identified using the ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data. (A) Quartz identified at hot springs along east flank of Ruby Mountains in Ruby Valley, Elko County (H, Fig. 6). (B) Carbonate identified in travertine deposits at Potts Ranch and Diana's Punchbowl hot springs in Monitor Valley, Nye County (I, Fig. 6). (C) Quartz identified at the Geysers and Beowawe hot springs, Eureka County (G, Fig. 6). Geology polygons are not shown in B and C for simplicity. Significant quartz was identified between the Geysers and Beowawe that represents outflow deposits from the Geysers spring. Sinters and/or tufa deposits (QThs) from the geology coverage of Crafford (2007) are outlined in cyan in Plate 3. Quartz or carbonate was not identified at several of these deposits. ASTER emissivity spectra of the Beowawe, Diana's Punchbowl, and Potts Ranch hot springs are shown in Figure 5. To access a full-resolution PDF of this figure, please visit http://dx.doi.org/10.1130/GES00126.S10 or the full-text article on www.gsajournals.org.

Carlin-Type Au Deposits
Many Carlin-type gold deposits are associated with jasperoids formed by pervasive silicification of carbonates, shales, or conglomerates (Hofstra and Cline, 2000). Some of the largest jasperoids are associated with gold deposits in the Alligator Ridge trend (Nutt and Hofstra, 2003) and neighboring deposits in east-central Nevada (Maher, 1997). In these deposits, the largest jasperoids are stratabound and are frequently present in the Chainman Shale, Joana Limestone, Pilot Shale, and Devonian Guilmette Formation (Dg) limestone, and especially at contacts between these formations. The Jerritt Canyon district, in the Independence Mountains, also has some large jasperoids, especially in the lower part of the Silurian and Devonian Roberts Mountains Formation and upper part of the Ordovician and Silurian Hanson Creek Formation (Hofstra et al., 1999). In the largest gold trends (Carlin, CN, Fig. 6; Battle Mountain–Eureka, BM/EK; Getchell, GT), jasperoids are generally smaller, but the largest jasperoids present also are typically developed in certain limestone units and along shale-limestone contacts. Consequently, quartz identified using ASTER TIR data in the above-mentioned units is likely to be jasperoid. Several examples from these trends and districts are described below.

In the adjacent Bald Mountain and Alligator Ridge districts (Fig. 12), jasperoids of four different ages have been recognized: (1) small stratabound jasperoids hosted in Cambrian–Ordovician Formations that predate Jurassic intrusions, (2) small discordant jasperoids associated with Jurassic reduced intrusion-related gold deposits hosted in Cambrian–Mississippian Formations, (3) large stratabound jasperoids associated with Eocene Carlin-type gold deposits hosted in Devonian–Mississippian Formations, and (4) large jasperoids hosted in a chaotic breccia unit of Eocene–Miocene age (Nutt and Hofstra, 2003, 2007; Nutt and Hart, 2004). Of these, only the latter two are large enough to discern between areas disturbed by mining using the ASTER TIR data. Those associated with Carlin-type deposits, such as Winrock (W, Fig. 12), Galaxy (G), Horseshoe (H), Saga (S), and others [JB(1–3) and J1, Fig. 12], are localized by north-striking faults along Mooney Basin and intersecting northwest-striking faults of the Bida trend (BT, Fig. 12). The Bida trend also served to localize the Jurassic Bald Mountain stock and related deposits. Jasperoids exposed in and adjacent to the Carlin-type deposits are hosted in Chainman Shale (Mch of Nutt and Hart, 2004), Joana Limestone (Mj and jasperoid Mjj), and Pilot Shale (MDp). In the Bida trend, between the Eocene Horseshoe deposit and the Jurassic Top deposit (T, Fig. 12), quartz was identified on a low ridge in an area underlain by Pilot Shale and Chainman Shale (J2, Fig. 12). ASTER SWIR data indicate that these rocks have been highly argillized to kaolinite. The large size and typical host of the J2 quartz anomaly suggest that it may be a remote manifestation of the Eocene Carlin-type hydrothermal system that extends into the older Bald Mountain district. Farther south, between the Vantage and Yankee deposits, a large jasperoid (J3, Fig. 12) hosted in the Guilmette Formation (Dg, limestone and dolomite), Pilot Shale, Joana Limestone, and Eocene conglomerate (not shown) was identified. ASTER SWIR data show that some of the jasperoids in the Pilot Shale are accompanied by significant argillization (mainly kaolinite), while in adjacent areas sericite (most likely muscovite) was identified. A large quartz anomaly was mapped on the west side of the Bald Mountain district in the chaotic breccia unit [JB(4), Fig. 12]. Although some of this quartz could be due to brecciated Diamond Peak Formation and silicified Chainman Shale, jasperoid in the hanging wall of the north-northwest–striking Ruby listric normal fault is likely Miocene in age (C. Nutt, 2007, personal commun.). An ASTER emissivity spectrum for an exposure of the jasperoid in Water Canyon shows a strong quartz signature (Fig. 4).

View larger version (96K): [in this window] [in a new window]   Figure 12. Subset of Plate 3 over the Bald Mountain and Alligator Ridge districts, White Pine County (M, Fig. 6). Quartz-bearing units are indicated in magenta, carbonate-bearing units in blue, and jasperoids in orange (Crafford, 2007). Mdp—Diamond Peak Formation. Dd—Devonian Guilmette Formation limestone (labeled Devils Gate Formation in Crafford, 2007). Dn—Devonian Nevada Formation (mainly dolomite), Simonson Dolomite, and/or Sevy Dolomite. DOd—Devonian–Ordovician dolomites, undivided. P c—Riepe Spring and Ely Limestones, undivided. Oe—Ordovician Eureka Quartzite, Ordovician Pogonip Group (silty limestone ± sandstone), and/or Cambrian Windfall Formation (limestone with local interbedded chert and clastics). Bald Mountain district: BT—Bida trend; MB—Mooney Basin; Ji—Jurassic stock; SS1—sandstone beds and/or lenses of Pogonip Group on Big Bald Mountain; SS2—Permian Rib Hill Sandstone; J1—possible jasperoid in Pilot Shale along Mooney Basin trend; J2—quartz in argillized Pilot Shale and Chainman Shale, southeast of Top Pit area (T); W—Winrock deposit, jasperoid in Joana Limestone, Mjj of Nutt and Hofstra (2004) (Fig. 4); G—Galaxy deposit; H—Horseshoe deposit; S—Saga deposit; JB—jasperoid breccia (see text for explanation) from Crafford (2007). The Horseshoe and Galaxy deposits have been significantly disturbed by mining. Alligator Ridge district: V—Vantage deposit; Y—Yankee deposit; J3—possible jasperoids in locally argillized Joana Limestone, Pilot Shale, and Guilmette Formation. ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) emissivity spectra of several of the units and features in this figure are shown in Figures 4 and 5. To access a full-resolution PDF of this figure, please visit http://dx.doi.org/10.1130/GES00126.S11 or the full-text article on www.gsajournals.org.

View larger version (86K): [in this window] [in a new window]   Figure 13. Subset of Plate 3 over the Illipah Au mine in the northern White Pine Range, White Pine County (J, Fig. 6). In the map at top, quartz-bearing units are indicated in magenta, carbonate-bearing units in blue, and jasperoids in orange (Crafford, 2007). Note correspondence of identified quartz with the Illipah mine area, jasperoids, and the Diamond Peak Formation (Mdp). Mj—Joana Limestone. Pc—Pennsylvanian–Permian Riepe Spring and Ely Limestones. Psc—Permian Arcturus Formation (siliciclastics, limestones, and dolomites) and/or Rib Hill Sandstone. Also note identified carbonate within the limestone-bearing units. Image at bottom shows perspective view from Google EarthTM looking southeast over the mine toward the jasperoid, which forms the flat top of the ridge in the background (Nutt and Hofstra, 2003). Cyan arrow in the map at top indicates the look direction of the perspective view. ASTER—Advanced Spaceborne Thermal Emission and Reflection Radiometer. To access a full-resolution PDF of this figure, please visit http://dx.doi.org/10.1130/GES00126.S12 or the full-text article on www.gsajournals.org.

View larger version (84K): [in this window] [in a new window]   Figure 14. Subset of Plate 3 over the Easy Junior mining district in White Pine County (E, Fig. 6). Railroad Valley is at lower right. Quartz-bearing units are indicated in magenta, carbonate-bearing units in blue, and jasperoids in orange (Crafford, 2007). Note correspondence of identified quartz with the Nighthawk Ridge mine area, jasperoids (br), and the Diamond Peak Formation (included here within the Mcl unit of Crafford, 2007). JP—identified quartz occurrences not included in the geology coverage of Crafford (2007) that are likely to be jasperoids. These quartz bodies mainly occur within the MDcl unit of Crafford (2007) that includes the Pilot Shale, Joana Limestone, and Chainman Shale, and correspond well to mapped jasperoids indicated in red on the map at left, which has been modified from Carden (1991). Dc—Devils Gate Limestone (note correspondence with identified carbonate). ASTER—Advanced Spaceborne Thermal Emission and Reflection Radiometer. To access a full-resolution PDF of this figure, please visit http://dx.doi.org/10.1130/GES00126.S13 or the full-text article on www.gsajournals.org.

In the Carlin Trend, quartz was mapped in the Silurian–Lower Devonian Roberts Mountain Formation 3.3 km north of the main open pit of the Gold Quarry mine along the northeast side of Maggie Creek Canyon (GQ, Fig. 6; Plates 1–3). This northwest-southeast–trending zone of quartz appears to have high albedo suggestive of bleaching, and corresponds in part to hydrothermal alteration mapped along a thrust fault of similar strike (Fig. I-10 in Harlan et al., 2002). The composition of the Roberts Mountain Formation ranges from a silty limestone to a calcareous siltstone, but no carbonate was detected on this outcrop using the ASTER data. Where decalcified, the rocks consist mainly of fine-grained, platy siltstone with high quartz content. Therefore, these outcrops with low to moderate quartz index values may be decalcified and/or silicified. Smaller quartz-bearing areas with low index values also were detected in an area of mapped alteration 1 km to the east, in the vicinity of the Rainbow deposit (Harlan et al., 2002).

Pluton-Related Distal-Disseminated Deposits
Silicification was generally not detected along pluton contacts (Plate 2) or near proximal pluton-related ore deposits (Plate 1). However, small jasperoids are frequently present in or near open pit mines that produce gold from distal-disseminated deposits hosted in sedimentary rocks (e.g., Robinson district, Maher, 1996; Lone Tree and Marigold mines in the Battle Mountain district, Theodore, 2000; Bald Mountain district, Nutt and Hofstra, 2007). Quartz was detected in each of the above mining areas, but it was difficult to find exposures of jasperoid in adjacent areas that were undisturbed. For example, Maher (1996) mapped hydrothermal silicification within the marble halo at the Robinson porphyry Cu-Au deposit near Ely in White Pine County, but we were unable to spatially discriminate quartz in outcrop from quartz in the pits and tailings. At Lone Tree (LT, Fig. 6) and Marigold (MG), the deposits are hosted in Havallah Sequence, Edna Mountain Formation, Battle Formation, and Valmy Formation that contain quartz sandstones, siltstones, and chert, making it nearly impossible to distinguish hydrothermal quartz. At Bald Mountain (Fig. 12), most of the discordant jasperoids associated with the Jurassic distal disseminated deposit are small, difficult to resolve, and only a few occur outside areas disturbed by mining. Quartz with moderate index values (SS1, Fig. 12) was identified on the summit ridge of Big Bald Mountain several kilometers north of the Top Pit area within the Ordovician Pogonip Group. At this locality, the Pogonip is composed primarily of silty limestone with thin beds and lenses of resistant quartz sandstone (C. Nutt, 2007, personal commun.). Hence, the identified quartz likely corresponds to the sandstone, although silicification and/or decalcification may also be present.

The small Kinsley mining district in eastern Elko County (C, Fig. 6) has small exposures of jasperoid that occur both within and outside the pits. Comparison with the geologic map from Lapointe et al. (1991) shows that both mined and undisturbed jasperoids were successfully identified using the ASTER data (Fig. 15). This example also shows that tiny jasperoids, such as those located 2–3.5 km north-northeast of the main mining area ("Main Zone"), are too small to be resolved with the ASTER TIR data. Quartz was also identified in a heap leach pile located 2 km east of the main mining area. The Eocene pluton inferred to be responsible for the mineralization is located 3.2 km south of the main mining area along the southern edge of the maps shown in Figure 15.

View larger version (77K): [in this window] [in a new window]   Figure 15. Subset of quartz and carbonate index maps over Kinsley mining district, Kinsley Mountains, Elko County (C, Fig. 6). Maps are overlain on background of Landsat Thematic Mapper data. Antelope Valley is at right. Map at left is from Lapointe et al. (1991). Note identification of quartz corresponding to both mined and undisturbed jasperoids, and heap leach pile. Patchy jasperoids in upper right of geologic map were too small to be identified using the ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data. The Tertiary intrusion responsible for the distal-disseminated mineralization in the district is located at the south edge of the maps, near the Phalen mine. Abundant carbonate was identified within limestones of the Ordovician Pogonip Group (Op) that underlie most of the range to the north of Au deposits, and within limestones and dolomites of the Cambrian Windfall Formation to the south. To access a full-resolution PDF of this figure, please visit http://dx.doi.org/10.1130/GES00126.S14 or the full-text article on www.gsajournals.org.