Thomas Cudahy

Tracing fluid pathways in fossil hydrothermal systems with near-infrared spectroscopy

We present a new method to detect and reconstruct fluid pathways in fossil hydrothermal systems that is based on systematic study of white micas in hydrothermally altered rock, using near-infrared spectroscopy. This method, developed in an Early Archean volcanic sequence in the Panorama district in Western Australia, uses new near-infrared spectroscopic data and published geological and geochemical data of fossil submarine hydrothermal systems in the area. Analysis of new near-infrared spectroscopic data revealed that the abundance of white mica and its Al content in altered volcanic rock vary systematically along fossil fluid pathways, from zones of low-temperature recharge to zones of high-temperature discharge, as a function of hydrothermal fluid chemistry, temperature of alteration, coexisting minerals, and composition of volcanic host rock. The abundance of white mica relative to that of chlorite and the Al content of white micas can be used to discriminate among the hydrothermal alteration facies along these fluid pathways, which permits detection and reconstruction of fossil fluid pathways.

Measurement of the hematite:goethite ratio using field visible and near‐infrared reflectance spectrometry in channel iron deposits, Western Australia

Empirical relationships and a field method have been developed for the measurement of the hematite:goethite ratio in Tertiary ooidal ironstones, locally named channel iron deposits, from the Hamersley region of Western Australia using visible to near‐infrared (400 to 1000 nm) refiectance spectrometry. The hematite:goethite ratio is important in the characterisation of these iron deposits as Al, P, water and Si are deleterious components commonly associated with goethite. The channel iron deposits typically comprise iron oxy‐hydroxides with less than 1% Fe2+ (present in maghemite or kenomagnetite), less than 8% Al3+‐substitution and with a mean crystal dimension of approximately 20 nm. The natural variations in the hematite:goethite ratio of the channel iron deposits were modelled using laboratory mixtures of pure hematite and goethite. The resultant spectral mixing trends produced consistent relationships with the hematite:goethite ratio, especially for the wavelength of the 6A1?4T1 crystal field absorpti...

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) after fifteen years: Review of global products

Abstract The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) is a 15-channel imaging instrument operating on NASA’s Terra satellite. A joint project between the U.S. National Aeronautics and Space Administration and Japan’s Ministry of Economy, Trade, and Industry, ASTER has been acquiring data for 15 years, since March 2000. The archive now contains over 2.8 million scenes; for the majority of them, a stereo pair was collected using nadir and backward telescopes imaging in the NIR wavelength. The majority of users require only a few to a few dozen scenes for their work. Studies have ranged over numerous scientific disciplines, and many practical applications have benefited from ASTER’s unique data. A few researchers have been able to mine the entire ASTER archive, that is now global in extent due to the long duration of the mission. Six examples of global products are described in this contribution: the ASTER Global Digital Elevation Model (GDEM), the most complete, highest resolution DEM available to all users; the ASTER Emissivity Database (ASTER GED), a global 5-band emissivity map of the land surface; the ASTER Global Urban Area Map (AGURAM), a 15-m resolution database of over 3500 cities; the ASTER Volcano Archive (AVA), an archive of over 1500 active volcanoes; ASTER Geoscience products of the continent of Australia; and the Global Ice Monitoring from Space (GLIMS) project.

Geologic and alteration mapping at Mt Fitton, South Australia, using ASTER satellite-borne data

The Japanese ASTER sensor on board the US Terra satellite was launched in December 1999 to establish a spaceborne capability for high spatial, multispectral visible-shortwave infrared and thermal infrared remote sensing data mapping of the Earths environment. The Mt Fitton test site in South Australia was chosen to test the ability of the ASTER instrument for geological mapping having been previously surveyed by several visible-shortwave IR and thermal IR airborne remote sensing instruments and several field campaigns collecting relevant spectral measurements. These previous airborne remote sensing surveys and field campaigns successfully mapped a suite of intrusives and sedimentary units with some greenschist metamorphic and localised hydrothermal alteration. Visible-NIR ASTER channels successfully mapped green vegetation and iron oxide information. ASTER SWIR data were spectrally unmixed into four spectrally recognizable endmembers that relate to areas rich in talc, chlorite, white mica and carbonate mineralogies. This result was confirmed using IRIS field spectra resampled to ASTER resolution wavelengths. Quartz, carbonate and talc-tremolite rich units at Mt Fitton were also discriminated using ASTERs thermal infrared data. These results from low level ASTER data products indicated that ASTER could discriminate mineral groups not achievable from Landsat TM, though more precise mineral species mapping is not possible.

The performance of the satellite-borne Hyperion hyperspectral VNIR-SWIR imaging system for mineral mapping at Mount Fitton, South Australia

Satellite-based hyperspectral imaging became a reality in November 2000 with the successful launch and operation of the Hyperion system on board the EO-1 platform. Hyperion is a pushbroom imager with 220 spectral bands in the 400-2500 nm wavelength range, a 30 meter pixel size and a 7.5 km swath. Pre-launch characterization of Hyperion measured low signal to noise (SNR<40:1) for the geologically significant shortwave infrared (SWIR) wavelength region (2000-2500 nm). The impact of this low SNR on Hyperions capacity to resolve spectral detail was evaluated for the Mount Fitton test site in South Australia, which comprises a diverse range of minerals with narrow, diagnostic absorption bands in the SWIR. Following radiative transfer correction of the Hyperion radiance at sensor data to surface radiance (apparent reflectance), diagnostic spectral signatures were clearly apparent, including: green vegetation; talc; dolomite; chlorite; white mica and possibly tremolite. Even though the derived surface composition maps generated from these image endmembers were noisy (both random and column), they were nonetheless spatially coherent and correlated well with the known geology. In addition, the Hyperion data were used to measure and map spectral shifts of <10 nm in the SWIR related to white mica chemical variations.

Applicability of the Thermal Infrared Spectral Region for the Prediction of Soil Properties Across Semi-Arid Agricultural Landscapes

In this study we tested the feasibility of the thermal infrared (TIR) wavelength region (within the atmospheric window between 8 and 11.5 μm) together with the traditional solar reflective wavelengths for quantifying soil properties for coarse-textured soils from the Australian wheat belt region. These soils have very narrow ranges of texture and organic carbon contents. Soil surface spectral signatures were acquired in the laboratory, using a directional emissivity spectrometer (μFTIR) in the TIR, as well as a bidirectional reflectance spectrometer (ASD FieldSpec) for the solar reflective wavelengths (0.4–2.5 μm). Soil properties were predicted using multivariate analysis techniques (partial least square regression). The spectra were resampled to operational imaging spectroscopy sensor characteristics (HyMAP and TASI-600). To assess the relevance of specific wavelength regions in the prediction, the drivers of the PLS models were interpreted with respect to the spectral characteristics of the soils’ chemical and physical composition. The study revealed the potential of the TIR (for clay: R2 = 0.93, RMSEP = 0.66% and for sand: R2 = 0.93, RMSEP = 0.82%) and its combination with the solar reflective region (for organic carbon: R2 = 0.95, RMSEP = 0.04%) for retrieving soil properties in typical soils of semi-arid regions. The models’ drivers confirmed the opto-physical base of most of the soils’ constituents (clay minerals, silicates, iron oxides), and emphasizes the TIR’s advantage for soils with compositions dominated by quartz and kaolinite.

Mapping surface mineralogy and scattering behavior using backscattered reflectance from a hyperspectral midinfrared airborne CO/sub 2/ laser system (MIRACO/sub 2/LAS)

Airborne, high-spectral resolution, thermal-infrared (TIR) MIRACO/sub 2/LAS reflectance data were evaluated for mapping surface mineralogy and scattering behavior for a variety of semi-arid, geological test sites in Australia. MIRACO/sub 2/LAS is a rapidly tuned, airborne CO/sub 2/ laser system that measures backscattered (bidirectional) reflectance at 100 wavelengths between 9.1 and 11.2 /spl mu/m for 2-m footprints in line profile mode. An operational methodology is described that permits reduction of the raw airborne signal-to-ground reflectance. This ground reflectance has two major properties, namely, wavelength-dependent mineralogical variations and reflection albedo variations related to surface roughness. Comparisons between the airborne MIRACO/sub 2/LAS spectra and laboratory directional hemispherical reflectance (DHR) spectra show the same spectral shapes, though differences in average reflectance (albedo) occur for some types of rocks. The minerals identified using MIRACO/sub 2/LAS include silicates (for example, quartz, microcline, plagiodase, almandine, spessartine, talc, tremolite, and kaolinite) and carbonates (dolomite and magnesite) as well as vegetation (dry and green). Many of the diagnostic spectral features that allow identification of these materials are narrow (<0.2 /spl mu/m), making them difficult to detect with broadband TIR systems, like the airborne TIMS and satelliteborne ASTER. Based on an empirical relationship between the minimum and maximum reflectance established using laboratory DHR spectra, a method is proposed that allows the use of MIRACO/sub 2/LAS data to identify surfaces that are characterized by Lambertian or specular scattering. The MIRACO/sub 2/LAS results show that Lambertian-type scatterers include soils and many types of isotropic rocks.

Airborne hyperspectral imaging of hydrothermal alteration zones in granitoids of the Eastern Fold Belt, Mount Isa Inlier, Australia

ABSTRACT Hyperspectral remote sensing data from the Eastern Fold Belt, Mount Isa Inlier, Australia were compared with petrographic and geochemical studies to map the spatial extension and compositional variations of Proterozoic granitoids and endoskarns as well as hydrothermal alteration patterns in adjoining metasedimentary successions. Detailed spatial analysis of spectral remote sensing data shows an almost circular alteration zoning in the Mallee Gap Granite, which was emplaced during a late phase of the Mesoproterozoic Williams event. A combination of hyperspectral images, such as white mica, kaolin and MgOH products, were used to map the alteration zoning. The formation of the endoskarn is presumably related to autometasomatism and interaction with fluids released from the country rocks during a late phase of the emplacement. The intrusion of the Mallee Gap Granite has only a local control on the hydrothermal alteration, but high potassic granites of the southern Mount Angelay Granite might have expelled oxidized mineralizing fluids and possibly had a major impact on regional scale alteration. Hyperspectral remote sensing data may be used to estimate the imprint of single igneous bodies on the Mesoproterozoic hydrothermal evolution of the Eastern Fold Belt and are important for the study of ore-forming hydrothermal processes in general.

Satellite-derived mineral mapping and monitoring of weathering, deposition and erosion

The Earth’s surface comprises minerals diagnostic of weathering, deposition and erosion. The first continental-scale mineral maps generated from an imaging satellite with spectral bands designed to measure clays, quartz and other minerals were released in 2012 for Australia. Here we show how these satellite mineral maps improve our understanding of weathering, erosional and depositional processes in the context of changing weather, climate and tectonics. The clay composition map shows how kaolinite has developed over tectonically stable continental crust in response to deep weathering during northwardly migrating tropical conditions from 45 to 10 Ma. The same clay composition map, in combination with one sensitive to water content, enables the discrimination of illite from montmorillonite clays that typically develop in large depositional environments over thin (sinking) continental crust such as the Lake Eyre Basin. Cutting across these clay patterns are sandy deserts that developed <10 Ma and are well mapped using another satellite product sensitive to the particle size of silicate minerals. This product can also be used to measure temporal gains/losses of surface clay caused by periodic wind erosion (dust) and rainfall inundation (flood) events. The accuracy and information content of these satellite mineral maps are validated using published data.

Investigations into Soil Composition and Texture Using Infrared Spectroscopy (2–14 m)

The ability of thermal and shortwave infrared spectroscopy to characterise composition and texture was evaluated using both particle size separated soil samples and natural soils. Particle size analysis and separation into clay, silt, and sand-sized soil fractions was undertaken to examine possible relationships between quartz and clay mineral spectral signatures and soil texture. Spectral indices, based on thermal infrared specular and volume scattering features, were found to discriminate clay mineral-rich soil from mostly coarser quartz-rich sandy soil and to a lesser extent from the silty quartz-rich soil. Further investigations were undertaken using spectra and information on 51 USDA and other soils within the ASTER spectral library to test the application of shortwave, mid- and thermal infrared spectral indices for the derivation of clay mineral, quartz, and organic carbon content. A nonlinear correlation between quartz content and a TIR spectral index based on the 8.62 μm was observed. Preliminary efforts at deriving a spectral index for the soil organic carbon content, based on 3.4–3.5 μm fundamental H–C stretching vibration bands, were also undertaken with limited results.