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