Identifi cation 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 Barnaby W. Rockwell* Albert H. Hofstra U.S. Geological Survey, Box 25046, MS 973, Denver Federal Center, Denver, Colorado 80225, USA ABSTRACT posed primarily of other minerals, they were carbonate mapping demonstrated here can inferred to be hydrothermal in origin and be used in well-studied or frontier areas to ASTER (Advanced Spaceborne Thermal compared to known occurrences of hydro- verify the accuracy of existing geologic maps, Emission and Refl ection Radiometer) ther- thermal alteration and mineralization. guide future detailed stratigraphic and struc- mal infrared imagery over a 389 km × 387 The ASTER-based quartz mapping iden- tural mapping in lithologically complex ter- km area in northern Nevada (38.5°–42°N, tifi ed thick sequences of quartzite, bed- ranes and allochthons, and identify hydro- 114°–118.5°W) was analyzed to evaluate its ded radiolarian chert, quartz sandstone, thermal features for exploration and resource capability for accurate and cost-effective conglomerates with clasts of quartzite and assessment purposes. identifi cation and mapping of quartz and chert, silicic and/or altered rhyolites, and carbonate minerals at regional to local scales. silicic welded tuffs. Alluvial fan surfaces, Keywords: remote sensing, ASTER, thermal The geology of this area has been mapped at sand dunes, and beach deposits composed infrared, quartz, carbonate. a wide range of scales and includes a diver- of quartz and/or carbonate are prominent, sity of rock types and unconsolidated surfi - well-mapped features. Quartz was also iden- INTRODUCTION cial materials, many of which are composed tifi ed in smaller bodies of jasperoid, quartz- primarily of quartz and carbonate minerals. alunite, and quartz-sericite-pyrite alteration, The Great Basin physiographic province is This area is also endowed with a wide vari- hot spring silica sinter terraces, and several the world’s second leading producer of gold ety of economically and scientifi cally impor- diatomite and perlite mines and prospects. and is also host to a wide variety of ore deposits tant ore deposit types that contain an array The ASTER-based carbonate mapping iden- that contain large resources of silver, base met- of commodities (Au, Ag, Pb, Zn, Cu, Mo, W, tifi ed thick sequences of dolomite, limestone, als, and other important metallic and industrial Sn, Be, F, Mn, Fe, Sb, Hg, and barite). The and marble, as well as small hot spring trav- minerals (Hofstra and Wallace, 2006). Because hydrothermal systems that generated these ertine deposits. Eolian carbonate was identi- hydrothermal silicifi cation accompanies miner- deposits frequently deposited large amounts fi ed in several playas. Dolomite exhibited a alization in many metal deposits, the identifi ca- of quartz where fl uids cool, and generally stronger carbonate response than calcite, as tion and mapping of quartz in rocks composed smaller amounts of calcite or dolomite by predicted based on their thermal spectral mainly of other minerals is of great value for other mechanisms. characteristics. Quartz was detected at lower exploration and assessments of resource poten- To identify and map quartz and carbon- concentrations than carbonates because of tial. Identifi cation of quartz and carbonate in ate minerals, band ratioing techniques were the greater strength of the quartz reststrahlen rocks and unconsolidated surfi cial materials developed based on the shapes of laboratory features in the thermal infrared compared to across large areas holds promise for assessing the reference spectra and applied to ASTER the bending-related spectral features of car- potential for industrial rock-mineral resources Level 2 surface emissivity products of 108 bonates. The 90 m ground pixel size of the including aggregate, sand, gravel, caliche, and overlapping scenes. These mineral maps ASTER thermal imagery prevents the iden- calcrete. The regional mapping of these miner- were mosaicked into a single coverage that tifi cation of small-scale features. Despite this als also facilitates evaluation of existing geo- was overlain with published, vector-format limitation, numerous bodies of hydrothermal logic maps and recognition of locales in need of geologic maps of various scales to determine quartz were detected in or near known Car- more refi ned mapping or topical investigation. which geologic terranes, formations, and lin-type gold deposits, distal disseminated Data from spaceborne remote sensing systems geomorphic features correspond to identifi ed Au-Ag deposits, high- and low-sulfi dation have been applied to prospecting in terranes quartz or carbonate. Where quartz or car- epithermal Au-Ag deposits, and geothermal with requisite geologic and tectonic frameworks bonate minerals were mapped in rocks com- areas. Detection of hydrothermal carbonate for decades, but the Advanced Spaceborne was rare and mainly in geothermal areas. Thermal Emission and Refl ection Radiometer The ASTER-based thermal quartz and (ASTER) sensor aboard the Earth Observing *[email protected]. Geosphere; February 2008; v. 4; no. 1; p. 218–246; doi: 10.1130/GES00126.1; 17 fi gures; 3 tables; 3 plates, 2 supplemental index maps. 218 For permission to copy, contact [email protected] © 2008 Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/4/1/218/3341029/i1553-040X-4-1-218.pdf by guest on 29 September 2021 Remote identifi cation of quartz and carbonate minerals, Nevada System (EOS) Terra satellite launched in 2000 TABLE 1. MEASURED SPECTRAL emissivity “absorption” features. As the labora- provides revolutionary new capabilities for cost- PERFORMANCE OF ASTER THERMAL tory spectra were measured in biconical, rather INFRARED BANDS effective mineral and landcover mapping over than hemispherical, refl ectance, the generated Central large areas. The ASTER sensor acquires multi- ASTER Band width emissivity spectra can be used to predict only wavelength band (µm) spectral data in 14 bands, including 5 bands in (µm) the spectral shapes, and not absolute emissiv- the mid-infrared, or thermal (8–14 µm), region 10 8.291 0.344 ity values, of the mineral spectra. In both Fig- of the electromagnetic spectrum, in addition to 11 8.634 0.347 ures 1 and 2, the emissivity spectra convolved another band with backward-looking geometry 12 9.075 0.361 to ASTER sampling and bandpass are shown in the near-infrared (NIR, 0.76–0.86 µm) to pro- 13 10.657 0.667 in red. In Figure 1, the emissivity absorption 14 11.318 0.593 vide capability for stereoscopic observation and Note: Data from ASTER (Advanced features of quartz at ASTER bands 10 and 12 digital elevation model generation. The multi- Spaceborne Thermal Emission and are related to fundamental asymmetric Si-O spectral thermal infrared (TIR) data of ASTER Reflection Radiometer) User’s Guide (Earth stretching vibrations (reststrahlen bands). The are vital for detecting nonhydrous varieties of Remote Sensing Data Analysis Center, reststrahlen bands of quartz are the strongest quartz that are not spectrally identifi able in the 2005). of any silicate mineral (Salisbury and D’Aria, visible, near-infrared, and shortwave-infrared 1992b). In Figure 2, the emissivity absorption (SWIR, 1.4–2.5 µm) spectral regions because features of calcite and dolomite at ASTER band of a lack of diagnostic absorption features. The shows the wavelength centers and band widths 14 are related to out-of-plane bending modes of ASTER TIR data also provide capability for the of the fi ve ASTER thermal bands, which have the CO3 ion (Clark, 1999). Note that dolomite remote identifi cation and mapping of areally a ground instantaneous fi eld of view (GIFOV), exhibits a greater decrease in emissivity between extensive occurrences of other minerals, includ- or ground spatial resolution, of 90 m. Figures bands 13 and 14 than calcite. This characteristic ing carbonates, although the ASTER SWIR data 1 and 2 exhibit the spectral features of quartz, is caused by the greater width and shorter wave- are more sensitive at detecting hydrous quartz calcite, and dolomite in this spectral region at length position of the bending feature of dolo- (e.g., opal and chalcedony) and differentiating original laboratory resolution and convolved to mite at 11.15 µm relative to the bending feature carbonate mineral species. Despite its poten- ASTER sampling and bandpass (Table 1). The of calcite at 11.27 µm. tial utility, the mineral mapping capabilities of emissivity spectra shown in these fi gures were In the 8–11 µm spectral region measured by ASTER TIR data are often underutilized in sci- created by applying Kirchoff’s Law, E = 1 – R, ASTER, some fabricated construction materials entifi c studies. where E and R are emissivity and refl ectance, such as paving concrete and asphalt (macadam) This study evaluates the utility of band ratio to refl ectance spectra from a mid-infrared labo- have absorption features that are similar to that analysis techniques applied to ASTER thermal ratory spectral library (Salisbury et al., 1991). of quartz at ASTER sampling and bandpass infrared emissivity data for the identifi cation Kirchoff’s Law inverts refl ectance peaks into (ASTER Spectral Library, 2002), possibly due and mapping of quartz and carbonate minerals emissivity troughs commonly referred to as to quartz sand in the aggregate used to make across a 3.5º × 4.5º (389 km × 387 km) area of the central Great Basin in northern Nevada. The quartz and carbonate mineral maps were over- lain 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 investi- gations. Of particular relevance to metallogeny was evidence of hydrothermal quartz, includ- ing jasperoids or other quartz associated with argillic and phyllic alteration, in areas of known mineralization.