Matilda Thomas

Application of Hyperspectral Infrared Analysis of Hydrothermal Alteration on Earth and Mars

An integrated analysis of both airborne and field short-wave infrared hyperspectral measurements was used in conjunction with conventional field mapping techniques to map hydrothermal alteration in the central portion of the Mount Painter Inlier in the Flinders Ranges, South Australia. The airborne hyperspectral data show the spatial distribution of spectrally distinct minerals occurring as primary minerals and as weathering and alteration products. Field spectral measurements, taken with a portable infrared mineral analyzer spectrometer and supported by thin-section analyses, were used to verify the mineral maps and enhance the level of information obtainable from the airborne data. Hydrothermal alteration zones were identified and mapped separately from the background weathering signals. A main zone of alteration, coinciding with the Paralana Fault zone, was recognized, and found to contain kaolinite, muscovite, biotite, and K-feldspar. A small spectral variation associated with a ring-like feature around Mount Painter was tentatively determined to be halloysite and interpreted to represent a separate hydrothermal fluid and fluid source, and probably a separate system. The older parts of the alteration system are tentatively dated as Permo-Carboniferous. The remote sensing of alteration at Mount Painter confirms that hyperspectral imaging techniques can produce accurate mineralogical maps with significant details that can be used to identify and map hydrothermal activity. Application of hyperspectral surveys such as that conducted at Mount Painter would be likely to provide similar detail about putative hydrothermal deposits on Mars.

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.

Hydrothermal mineral alteration patterns in the Mount Isa Inlier revealed by airborne hyperspectral data

High-resolution mineral maps derived from hyperspectral imaging (4.5 m pixel) enable the recognition of various types of hydrothermal alteration and the identification of fluid pathways. Airborne hyperspectral images from the Eastern Fold Belt of the Mount Isa Inlier were tested as a new tool for the detection of Fe-oxide Cu–Au (IOCG) related alteration. Four different types of hydrothermal alteration were identified with the hyperspectral mineral maps: (1) Metasomatic 1: white mica mineral maps show the spatial distribution of regional sodic–calcic alteration in metasedimentary successions of the Soldiers Cap Group in the Snake Creek Anticline. (2) Metasomatic 2: alteration zonation is evident from albitised granites assigned to the Williams–Naraku Suite along the Cloncurry Fault. These show characteristic absorption features in the shortwave infrared range (SWIR) which are depicted on the white mica mineral maps (white mica composition, white mica content, white mica crystallinity index). Alteration zonation in gabbros of the Cloncurry District was detected by a combination of MgOH and Fe2+ mineral maps (MgOH content, MgOH composition, amphibole/chlorite and Fe2+ and MgOH) combined with white mica mineral maps (white mica composition and white mica content). (3) Fluid channels 1: major fault zones, such as the Mt Dore fault zone in the Selwyn Corridor, are interpreted as important fluid pathways, where gradual changes in the mineral chemistry are highlighted with mineral maps (e.g. white mica content, white mica composition, white mica crystallinity index). (4) Fluid channels 2: MgOH and Fe2+ mineral maps were used to map breccia pipes in the northern Cloncurry District north of the Saxby Granite (Suicide Ridge). The MgOH and Fe2+ mineral maps were also used to distinguish various mafic rocks from amphibolites, which are host rocks for some of the IOCG deposits in the Eastern Fold Belt (e.g. Mount Elliott), and calcsilicate breccias pipes (e.g. Suicide Ridge).

Using the Geoscience Australia-CSIRO ASTER maps and airborne geophysics to explore Australian geoscience

This study evaluated the geological mapping potential of the recently released Australian CSIRO-GA ASTER satellite geoscience products in providing mineral abundance and compositional information. A range of environments was examined by using test sites including the temperate cultivated New South Wales area of Wagga Wagga, and the semi-arid rangeland Mt Fitton of South Australia. Data integration of the ASTER derived products was undertaken with geophysical data, digital elevation models and fractional vegetation cover information. The study demonstrated that these products can successfully assist geological mapping within semi-arid areas and, to a lesser extent, within temperate open woodland environments.

Mapping Soil Surface Mineralogy at Tick Hill, North-Western Queensland, Australia, Using Airborne Hyperspectral Imagery

The use of airborne hyperspectral imagery for mapping soil surface mineralogy is examined for the semi-arid Tick Hill test site (20 km2) near Mount Isa in north-western Queensland. Mineral maps at 4.5 m pixel resolution include the abundances and physicochemistries (chemical composition and crystal disorder) of kaolin, illite–muscovite, and Al-smectite (both montmorillonite and beidellite), as well as iron oxide, hydrated silica (opal), and soil/rock water (bound and unbound). Validation of these hyperspectral mineral maps involved field sampling (34 sites) and laboratory analyses (spectral reflectance and X-ray diffraction). The field spectral data were processed for their mineral information content in the same way as the airborne HyMap data processing. The results showed significant spatial and statistical correlation. The mineral maps provide more detailed surface composition information compared with the published soil and geological maps and other geoscience data (airborne radiometrics and digital elevation model). However, there is no apparent correlation between the published soil types (i.e. Ferrosols, Vertosols, and Tenosols) and the hyperspectral mineral maps (e.g. iron oxide-rich areas are not mapped as Ferrosols and smectite-rich areas are not mapped as Vertosols). This lack of correlation is interpreted to be related to the current lack of spatially comprehensive mineralogy for existing regional soil mapping. If correct, then this new, quantitative mineral-mapping data have the potential to improve not just soil mapping but also soil and water catchment monitoring and modelling at local to regional scales. The challenges to achieving this outcome include gaining access to continental-scale hyperspectral data and models that link the surface mineralogy to subsurface soil characteristics/processes.

Continent-scale mineral information from ASTER multispectral satellite data

Continent-scale digital maps of mineral information of the Earths land surface are now achievable using geoscience-tuned remote sensing systems. Multispectral ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data and mineral information derived from it provide the opportunity for characterization of geological and soil processes, including the nature of soils and regolith (weathered) cover and the alteration footprints of hydrothermal ore deposits. This paper describes work from the Western Australian (WA) Centre of Excellence for 3D Mineral Mapping, which is part of CSIROs Minerals Down Under Flagship and supported by the WA State Government, Geoscience Australia and other Australian Geological Surveys, to generate a series of ASTER mineral group maps (both content and composition) for the whole Australian continent at a 30 m pixel resolution.

The acquisition and processing of voluminous spectral reflectance measurements of soils and powders for national datasets

The acquisition and collation of large numbers of spectral measurements of samples requires the strict adherence to set protocols to ensure the integrity of the data. Errors can occur in the measurement or processing that may not initially be detected and would require the time-consuming process of re-measuring of samples. This paper outlines the development of a national spectral library, and, the associated protocols for the systematic acquisition, quality control and processing of the spectral measurements of a collection of over 1300 soil and powders from across Australia using portable field spectroradiometers and laboratory Fourier transform infrared spectrometers. The processed spectra were uploaded to the CSIRO Data Access Portal, along with the metadata of the samples and measurements. These data are now publicly available as a national spectral library for the benefit of the research and user community.

Regolith research in Australia

Regolith science is a multidisciplinary, comparatively recent field, having evolved from several older disciplines: geology, geomorphology, soil science, and geography. Initially, practitioners drew on terms from these varied sources, not always understanding them fully or using them accurately, leading to persistent misunderstandings and confusion around terminology. The term “regolith” was introduced by Merrill (1897), who wrote of the incoherent mass of varying thickness covering the underlying rocks, later clarified and redefined in The Regolith Glossary (Eggleton 2001) as: “the entire unconsolidated or secondarily recemented cover that overlies more coherent bedrock, that has been formed by weathering, erosion, transport and/or deposition of the older material. The regolith thus includes fractured and weathered basement rocks, saprolites, soils, organic accumulations, volcanic material, glacial deposits, colluvium, alluvium, evaporitic sediments, aeolian deposits and groundwater”. Or, in simple terms: “everything from fresh rock, to fresh air.” Most of the continents, which were once part of Gondwana (Africa, India, Australia and South America), have old land surfaces and deep regolith. However, the extent of regolith and its preservation in Australia are exceptional. The Australian landscape is remarkable for its relative flatness, dominated by low elevations (Pain, Pillans, Roach, Worrall, & Wilford, 2012) with some areas exposed at the surface since the Precambrian, some since the Paleozoic and the rest since the Mesozoic or Cenozoic. This results in ancient weathering mantles that, may be over 100 m thick. Significant denudation is predominantly limited to the continental margins, resulting in old landforms and weathering profiles that are much more likely to persist in Australia than in other Gondwanan continents (Scott & Pain, 2008).