Portable Infrared Mineral Analyzer Research Articles

Characteristics of epithermal gold-base metals prospect in the Anggai block, northern Obi Island, Indonesia

The Obi island, located in the North Moluccas Au-Ag-Cu province, hosts several mineral prospects, one of which is gold-base metals in the Anggai block. Several companies have historically explored the area since the end of the 1900s. The ore bodies are white to smoky and brownish-white, banded, laminated, colloform, crustiform, and massive quartz vein structures or textures, trending north northwest to south southeast. The ore bodies consist of gold and base metals hosted in the Bacan Formation and are enveloped by dominantly argillic alteration and the addition of silicic alteration. In this study, we provide several analyses, namely petrography, measurements using Portable Infrared Mineral Analyser (PIMA), Atomic Absorption Spectrometry (AAS), and fluid inclusion analysis, to understand geological and geochemical characteristics related to formation of gold-base metals in the Anggai block. Under the microscope, gold (i.e., electrum) is freely observed and closely occurs with argentite. While galena, chalcopyrite, sphalerite, covellite, chalcocite, and pyrite are mainly observed together. The correlation coefficient between Au and other metals shows a strong positive correlation with Zn (r = 0.810188) and a weak positive correlation with Ag (r = 0.459857). Ag shows only a weak positive correlation to Au, while the other elements show a strong positive correlation. In addition, Cu, Pb, and Zn show a strong positive correlation with each other. The micro-thermometry analysis of fluid inclusions shows two modes of homogenization temperature (Th) and salinity: 180-200 °C and 240-280 °C, and 1.4-1.6 and 1.9-2.2 wt% NaCl eq., respectively, suggesting overprinting evidence to enrich the ore bodies with occurrences of gold and base metals in the quartz veins. Gold-base metals were probably carried by the sulfide complex, which later precipitated during the cooling of the hydrothermal fluid.

Sources and Significance of Pipestone Artifacts from Fort Ancient Sites

Abstract Red-stone artifacts, primarily pipes, have come from at least 18 Fort Ancient sites, plus many less-well-provenienced locations within Fort Ancient territory. Most appear to have been made of easily carved “pipestones.” Thanks to portable infrared mineral analyzer (PIMA) analysis by Thomas Emerson and colleagues at the Illinois State Archaeological Survey (ISAS), we now know the material source areas for over 80 museum-curated red pipestone artifacts from 16 Fort Ancient sites and related localities, as well as for a selection of nonred pipestone artifacts from the region. Only two material sources were identified: catlinite from southwestern Minnesota and Feurt Hill kaolinite flint clay from southern Ohio. Forms and disposition of the relatively few catlinite artifacts indicate interaction was taking place between Fort Ancient and Oneota peoples from at least the fourteenth to mid-fifteenth centuries onward. Red and speckled nonred Feurt Hill pipestone artifacts provide evidence of intraregional interaction.

Mapping and accuracy assessment of siltation of recharge dams using remote sensing technique

This study demonstrates the use of ASTER data to map the siltation of the Al-Khod Dam constructed at the lower reaches of Samail Catchment area, north Muscat, Sultanate of Oman. In this study, the decorrelated image of spectral bands 6, 3, and 1 clearly discriminated the distribution of siltation. The occurrence of siltation is confirmed by the detection of minerals using the VNIR-SWIR bands by the Spectral Angle Mapper (SAM) method. In addition, the siltation of the dam is studied for the years 1987 to 2018 using Landsat satellite images. Interpretation of images showed a gradual increase of the siltation. Mapping accuracy for the occurrence of siltation is assessed by a confusion matrix which produced an overall accuracy of 97.88% and Kappa coefficient of 0.98 in the Maximum Likelihood method. Results of image analyses are verified in the field and confirmed through laboratory analyses. The study of spectral absorption characters of field samples using a Portable Infrared Mineral Analyzer (PIMA) showed the presence of OH molecule bearing minerals (clays, serpentines, etc.) and carbonate minerals (calcite and dolomite) in the silt deposits. The occurrence of these minerals is confirmed further by X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) analyses.

Mapping advanced argillic alteration zones with ASTER and Hyperion data in the Andes Mountains of Peru

This study evaluates Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Hyperion hyperspectral sensor datasets to detect advanced argillic minerals. The spectral signatures of some alteration clay minerals, such as dickite and alunite, have similar absorption features; thus separating them using multispectral satellite images is a complex challenge. However, Hyperion with its fine spectral bands has potential for good separability of features. The Spectral Angle Mapper algorithm was used in this study to map three advanced argillic alteration minerals (alunite, kaolinite, and dickite) in a known alteration zone in the Peruvian Andes. The results from ASTER and Hyperion were analyzed, compared, and validated using a Portable Infrared Mineral Analyzer field spectrometer. The alterations corresponding to kaolinite and alunite were detected with both ASTER and Hyperion (80% to 84% accuracy). However, the dickite mineral was identified only with Hyperion (82% accuracy).

Mapping of Moho and Moho Transition Zone (MTZ) in Samail ophiolites of Sultanate of Oman using remote sensing technique

Moho and Moho Transition Zone (MTZ) of the Samail ophiolite of Sultanate of Oman are characteristic to potential occurrences of chromite deposit, hydrothermal mineralization and serpentinization. Mapping of Moho and MTZ, and discriminating them in between the mafic and ultramafic rocks in ophiolite sequence are more significant and important. The present study describes the remote sensing spectral characters of minerals and rocks of the Moho and MTZ and discriminates the Moho of Wadi Al Abyad of Nakhl massif, and Wadi Nidab and Wadi Abda regions of Sumail massif in the visible and near infrared (VNIR), and short wavelength infrared (SWIR) spectral regions using low-cost multispectral satellite data of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER). Results of this study show that the red–green–blue (RGB) color composite images of ASTER spectral bands 8, 3 and 1, and 8, 7 and 4 are able to delineate the Moho and MTZ of the regions. The RGB images of ASTER band ratios (4/8, 4/1, 3/2*4/3 and (1+3)/2, (4+6)/5, (7+9)/8) are capable to discriminate the mantle material (ultramafic harzburgites) and crustal rocks (mafic gabbros). The occurrence of such rock types is demonstrated by detection of their minerals using Spectral Angle Mapper (SAM) image processing method. The presence of Moho and MTZ, and associated lithologies are verified in field at Wadi Al Abyad, Wadi Nidab, Wadi Abda, Wadi Tayin, Wadi Fizh and several locations of Nakhl regions of Samail ophiolites. The laboratory study shows the occurrence of typical minerals namely olivine, orthopyroxene and clinopyroxene in the harzburgite and the minerals such as plagioclase, clinopyroxene, hornblende, orthopyroxene and olivine in the layered gabbro. The spectral properties of the rocks are studied using Portable Infrared Mineral Analyzer (PIMA) spectrometer and the occurrences of minerals are confirmed by X-ray diffraction (XRD) analyses. This study demonstrates the sensor capability of ASTER to map Moho and MTZ, and thus it is recommended to the geoscientists and geophysicists working on Moho and MTZ. The satellite image interpretations of Moho and MTZ, and the associated rocks and minerals are consistent with the field research studies carried out in these regions. Our images have potential to map the chromites mineralized zone and dunites (refractory materials) bearing MTZ.

EVALUATION OF INTER-INSTRUMENT VARIATIONS AMONG SHORT WAVELENGTH INFRARED (SWIR) DEVICES

In this study we investigate the inter-instrument variation of the alunite 1,480-nm OH (overall range ~1,475- 1,498 nm) and the white mica Al-OH (overall range ~2,186-2,225 nm) spectral absorption features by measuring the same sets of samples using two devices of each of the three major types of portable SWIR instruments: portable infrared mineral analyzer (PIMA), TerraSpec, and portable near-infrared spectrometer (PNIRS). All instruments have good short-term reproducibility. The ARC Centre of Excellence in Ore Deposits (CODES) PIMA was assessed for long-term reproducibility and it was found to be stable, with 0- to 1-nm variations over at least seven years. There are differences in the results measured by different instruments, even of the same type. The PIMA instrument from the Economic Geology Research Unit generates alunite results 1 to 2 nm lower than those of the CODES PIMA. The two TerraSpec instruments used in this study produced results very similar to each other, mostly with 0- to 1-nm differences in alunite 1,480-nm spectral absorption measurements, but the results are 1 to 3 nm lower than those of the CODES PIMA. The two PNIRS devices generated results that differed from each other by 1 to 4 nm, mostly with 2- to 3-nm differences in alunite measurements. Compared with the PIMA results, one PNIRS dataset is very similar, with 0- to 1-nm differences, whereas the other PNIRS dataset is 0 to 5 nm off, mostly with 3-nm differences, in alunite measurements. The white mica Al-OH spectral absorption feature wavelength measurements are more similar among the instruments, with the maximum difference being 2 nm. Despite the differences in absolute values, the ranking of the samples in those datasets for the two tested parameters is largely the same, which indicates that all the instruments will produce datasets showing the same trends. The implication is that one project should use the same instrument for all the measurements. If another instrument has to be used, or the original instrument is repaired or tuned, it is suggested that a set of samples covering most of the variation range should be measured with both instruments to evaluate the inter-instrument variation and to establish a correction formula to keep consistency in the database. © 2012 by Economic Geology, Inc.

Sourcing Red Pipestone Artifacts from Oneota Villages in the Little Sioux Valley of Northwest Iowa

The exchange of some Oneota red pipestone artifacts commonly, but sometimes incorrectly, identified as manufactured from catlinite from southwest Minnesota, likely created fictive kinship alliances between unrelated groups from ca. A.D. 1450 into the early 1700s. Researchers have determined, however, that red pipestone raw material occurs across a wide area within the United States and Canada. Determining the provenance of this red pipestone raw material is thus critical to understanding Oneota trade and alliance building. Using Portable Infrared Mineral Analyzer (PIMA) technology to identify the raw material sources of 84 red pipestone artifacts from seven Oneota villages in the Little Sioux valley of northwest Iowa, we demonstrate that while the local inhabitants had access to the catlinite quarries, they also used a wide range of pipestones from other sources. The possible implications of these multiple source areas are also discussed.

Evolution of the Girilambone regolith landscape, central-western New South Wales

A model of regolith and landscape evolution for the Girilambone region is derived from regolith–landform mapping and interpretation of drillhole information, the latter being underpinned by detailed petrography and Portable Infrared Mineral Analyser (PIMA) characterisation. The model is partly constrained by the interpretation of palynological, paleomagnetic, apatite fission-track, and airborne magnetic data. The model interprets the architecture of the three main sediment sequences and explains their derivation: a possible Late Jurassic fluvial sequence, an Early Cretaceous estuarine–shallow-marine and fluviolacustrine sequence (both sequences being part of the Surat Basin), and a Late Pliocene to Holocene colluvial–alluvial sequence. Implications of this model are that much, if not all, of the Cobar Uplands was covered by a shallow sea in the Early Cretaceous, and that neotectonic reactivation of north-trending Paleozoic faults caused the formation of the north-plunging depressed zone that contains the Mulga–Tindarey Paleovalley System.

Assessing the Influence of Reference Spectra on Synthetic SAM Classification Results

Spectral matching algorithms, such as the Spectral Angle Mapper (SAM), utilize the spectral similarity between individual image pixel spectra and a spectral reference library with known components. Here, we illustrate and quantify the effects that different sources of reference libraries have on SAM classification results. Synthetic images of three mineral endmembers were classified by using reference libraries derived from airborne hyperspectral imagery, ground spectra (Portable Infrared Mineral Analyser), and from a standard library (United States Geologic Survey). Results show that the source of the reference library strongly influences the classification results if all available wavelengths are used. This effect can be partially neutralized by using appropriate preprocessing methods. Two different types of spectral subsetting of the data, two types of continuum removal, and a combination thereof were tested. Best results were achieved by using a feature subset (i.e., limiting the input wavelengths to the diagnostic absorption features). This increased the average classification accuracy from 74% to 95% (ground spectral library) and from 68% to 94% (standard library).

Non-destructive infrared analyses: a method for provenance analyses of sandstones

Infrared spectroscopy (IR spectroscopy) is commonly applied in the laboratory for mineral analyses in addition to XRD. Because such technical efforts are time and cost consuming, we present an infrared-based mobile method for non-destructive mineral and provenance analyses of sandstones. IR spectroscopy is based on activating chemical bonds. By irradiating a mineral mixture, special bonds are activated to vibrate depending on the bond energy (resonance vibration). Accordingly, the energy of the IR spectrum will be reduced thereby generating an absorption spectrum. The positions of the absorption maxima within the spectral region indicate the type of the bonds and in many cases identify minerals containing these bonds. The non-destructive reflection spectroscopy operates in the near infrared region (NIR) and can detect all common clay minerals as well as sulfates, hydroxides and carbonates. The spectra produced have been interpreted by computer using digital mineral libraries that have been especially collected for sandstones. The comparison of all results with XRD, RFA and interpretations of thin sections demonstrates impressively the accuracy and reliability of this method. Not only are different minerals detectable, but also differently ordered kaolinites and varieties of illites can be identified by the shape and size of the absorption bands. Especially clay minerals and their varieties in combination with their relative contents form the characteristic spectra of sandstones. Other components such as limonite, hematite and amorphous silica also influence the spectra. Sandstones, similar in colour and texture, often can be identified by their characteristic reflectance spectra. Reference libraries with more than 60 spectra of important German sandstones have been created to enable entirely computerized interpretations and identifications of these dimension stones. The analysis of infrared spectroscopy results is demonstrated with examples of different sandstones used as dimension stones. All analyses have been done with the “portable infrared mineral analyser” (PIMA) of Integrated Spectronics Pty Ltd. at the Bundesanstalt fur Geowissenschaften und Rohstoffe in Berlin. This spectrometer has been designed to be light, portable and easy to handle. Investigations can be performed without any sample preparation for complete buildings as well as at small samples with diameters up to about one centimeter. Analyses of pulverized samples and granules are also possible. Consequently, infrared reflectance spectroscopy provides a reliable working, non-destructive technique to identify and characterize sandstones used for buildings and monuments.

Hydrothermal alteration mapping using ASTER data in the Infiernillo porphyry deposit, Argentina

The Infiernillo Cu–(Mo) porphyry deposit is located in the San Rafael Massif, in southern Mendoza Province, Argentina. It is hosted by and genetically related to Lower Permian Gondwanan volcanic rocks, which developed in a magmatic arc tectonic setting. The alteration zone has an oval shape about 3 km × 2 km in size, with a NNE–SSW strike. It consists of a small central quartz neck with appreciable hematite surrounded by an intense quartz-injected zone with local pervasive potassic alteration. Outwards there is a well-developed phyllic halo with intense bleaching which consists of pervasive and vein-type silicification, sericitization and pyritization. Cu and Mo anomalies are approximately coincident and are located between the silicified zone and the phyllic halo. In the outer part of the alteration zone, small polymetallic veins with pyrite, arsenopyrite, galena and minor chalcopyrite, sphalerite and electrum in quartz gangue crop out. An Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), with three visible and near infrared (VNIR) bands, six shortwave infrared (SWIR) bands and five thermal infrared (TIR) bands, was used to characterize the Infiernillo porphyry deposit alteration area. A circular zoning pattern was clearly observed in the VNIR + SWIR band combination. Spectral analysis of the surface reflectance SWIR detected 2.20 and 2.26 μm absorptions. Field data and sample spectroscopic analyses obtained using a GER 3700 spectral radiometer and a SWIR reflectance spectrometer (Portable Infrared Mineral Analyzer PIMA) showed a good correlation with the ASTER data. The 2.20 μm absorptions, due to Al-hydroxyl, reveal the presence of clay minerals (Al-illite, kaolinite) and sericite, whereas the 2.26 μm absorption due to Fe-hydroxyl, resulted from jarosite. The field samples were also analyzed with a TIR portable spectrometer Micro-FTIR model 102. The TIR was useful for detecting surface silica and potassic alteration through analysis of five-band surface emissivity data. ASTER images provided preliminary mineralogic information and geo-referenced alteration maps at low cost and with high accuracy. In this way ASTER has been proven to be a powerful tool in the initial steps of ore deposit exploration.

Mapping Fe-bearing hydrated sulphate minerals with short wave infrared (SWIR) spectral analysis at San Miguel mine environment, Iberian Pyrite Belt (SW Spain)

Short-wave infrared reflectance (SWIR) spectra obtained from a Portable Infrared Mineral Analyser (PIMA) were applied to map acidic mine soils at San Miguel massive sulphide deposit, Iberian Pyrite Belt, Spain. Field spectral measurements and laboratory analysis were performed on samples from 58 stations from two very polluted grounds. These analyses identified secondary and tertiary Fe-rich sulphate–hydrate minerals associated with the alteration of sulphide-bearing mine wastes and other associated infrared active minerals. The spectral absorption features for the pure salt types found in these areas allowed the preparation of a specific reference library for automatic mineral identification. Using this approach three separate zones around the sources of contamination have been discriminated: a “proximal” zone dominated by rozenite + hexahydrite, a “transitional” zone dominated by copiapite + coquimbite and a “distal” zone of hydronium–jarosite. An additional zone of “gypsum” was also recognized locally at the areas, where, last decade the neutralization of the acid soils with limestones was attempted. In addition to discriminating distinct mineralogical zones quantitative spectral data allowed contoured mineral maps to be produced that are comparable with the estimated mineralogical data obtained from conventional methods, such as XRD and field observations. These maps demonstrate that sulphate minerals were firstly formed by oxidation of the pyrite-rich ores followed by dissolution and precipitation involving a combination of oxidation, dehydration, and neutralization reactions. The study shows the potential of SWIR spectral analysis to identify and understand the distribution of efflorescent salts and other products of pyrite decomposition and provides a methodology to assist the research and monitoring of sources of environmental contamination.

Tremper Mound, Hopewell Catlinite, and Pima Technology

Archaeologists have long recognized that the Hopewell period was marked by the movement of exotic raw materials and artifacts across the eastern United States. The use of archaeometric techniques to trace the sources of these materials is well established. However, little work has been done on sourcing the pipestone used in Hopewell pipes. In this research we use Portable Infrared Mineral Analyzer (PIMA) technology to identify the presence of Minnesota catlinite as a raw material used for pipes recovered from the Tremper Mound (33SC4) caches in the Scioto Valley of Ohio. A new radiocarbon date places the deposition of these caches between B.C. 50 and A.D. 79. The presence of catlinite pipes in Ohio and in Wisconsin, but rarely in Illinois, suggests the existence of a direct west-to-east route from the catlinite quarries through Wisconsin to Ohio. This movement of catlinite appears to have been a short-term Hopewellian event and the material was not used again until late prehistoric times.

The Midcontinental Journal of Archaeology 2003–2005: A Retrospective

Archaeologists have long recognized that the Hopewell period was marked by the movement of exotic raw materials and artifacts across the eastern United States. The use of archaeometric techniques to trace the sources of these materials is well established. However, little work has been done on sourcing the pipestone used in Hopewell pipes. In this research we use Portable Infrared Mineral Analyzer (PIMA) technology to identify the presence of Minnesota catlinite as a raw material used for pipes recovered from the Tremper Mound (33SC4) caches in the Scioto Valley of Ohio. A new radiocarbon date places the deposition of these caches between B.C. 50 and A.D. 79. The presence of catlinite pipes in Ohio and in Wisconsin, but rarely in Illinois, suggests the existence of a direct west-to-east route from the catlinite quarries through Wisconsin to Ohio. This movement of catlinite appears to have been a short-term Hopewellian event and the material was not used again until late prehistoric times.

Short-wave infrared reflectance investigation of sites of paleobiological interest: applications for Mars exploration.

Rover missions to the rocky bodies of the Solar System and especially to Mars require lightweight, portable instruments that use minimal power, require no sample preparation, and provide suitably diagnostic mineralogical information to an Earth-based exploration team. Short-wave infrared (SWIR) spectroscopic instruments such as the Portable Infrared Mineral Analyser (PIMA, Integrated Spectronics Pty Ltd., Baulkham Hills, NSW, Australia) fulfill all these requirements. We describe an investigation of a possible Mars analogue site using a PIMA instrument. A survey was carried out on the Strelley Pool Chert, an outcrop of stromatolitic, silicified Archean carbonate and clastic succession in the Pilbara Craton, interpreted as being modified by hydrothermal processes. The results of this study demonstrate the capability of SWIR techniques to add significantly to the geological interpretation of such hydrothermally altered outcrops. Minerals identified include dolomite, white micas such as illite-muscovite, and chlorite. In addition, the detection of pyrophyllite in a bleached and altered unit directly beneath the succession suggests acidic, sulfur-rich hydrothermal activity may have interacted with the silicified sediments of the Strelley Pool Chert.

Using a Portable Spectrometer to Source Archaeological Materials and to Detect Restorations in Museum Objects

A team of archaeologists and geologists demonstrates how a portable infrared mineral analyzer (PIMA), first used by Australian geologists for mineral exploration, can be applied to provenance and authenticity studies. PIMA spectroscopy can be used to characterize both original materials and restored sections on stone and ceramic artifacts, including coatings such as shellac. The instrument works especially well in museum settings because it is portable and nondestructive. PIMA spectroscopy will become increasingly useful over time as reference databases of archaeological and restoration materials are accumulated.

Using a Portable Spectrometer to Source Archaeological Materials and to Detect Restorations in Museum Objects

AbstractA team of archaeologists and geologists demonstrates how a portable infrared mineral analyzer (PIMA), first used by Australian geologists for mineral exploration, can be applied to provenance and authenticity studies. PIMA spectroscopy can be used to characterize both original materials and restored sections on stone and ceramic artifacts, including coatings such as shellac. The instrument works especially well in museum settings because it is portable and nondestructive. PIMA spectroscopy will become increasingly useful over time as reference databases of archaeological and restoration materials are accumulated.

Composition, Diagenesis, and Weathering of the Sediments and Basement of the Callabonna Sub-Basin, Central Australia: Implications for Landscape Evolution

ABSTRACT The evolution of the Callabonna Sub-basin (central Australia) is described in terms of sediment provenance, stratigraphy, sedimentology, and postdepositional mineral and chemical alteration. Six mineral exploration drillholes in the Mundi Mundi Plain were studied using a combination of qualitative and quantitative XRD, PIMATM (portable infrared mineral analyzer), XRF, normative analysis, and SEM. The sedimentary cover consists of 160-210 m of diagenetically modified fluvio-lacustrine Tertiary clay, silt, and sand comprising sandy, mottled, and gray clay units overlain by Quaternary soil. The basement consists of weathered Proterozoic schist, gneiss, siltstone, and shale. Quartz, kaolinite, smectite, and muscovite are the dominant minerals within the sediments, whereas the weathered bedrock contains quartz, feldspar, kaolinite, illite, and muscovite. Characteristic wavelength absorption features from PIMATM show that the youngest sandy clay unit exhibits the lowest reflectance intensities because of its lower diagenetic maturity. Weathering of primary minerals within the basement produced an illite-kaolinite-rich saprolite whose clays exhibit strong reflectance intensities and well-developed crystallinity. The micromorphology of all secondary products reflects their environment of crystallization. Low porosity and anoxic conditions within the sediments resulted in poorly developed clays of variable textures, colors, and compositions. Conversely, the weathered bedrock developed over protracted periods of in-situ weathering resulting in better-developed secondary products. This study distinguishes between preburial weathering of the basement, detrital mineralogy, postdepositional alteration of the sedimentary cover, and subsequent weathering of previously formed mineral assemblages. This has widespread implications for paleotopographic(-weathering) reconstitution and landscape evolution thus: (1) the extent of weathering of basement relative to diagenesis and maturity of sediments and gradational nature of their unconformity suggests that most of the alteration of basement occurred prior to, and was sustained after, deposition of sediments, and (2) morphotectonic evolution of the region controlled the composition of transported materials by changing the style and patterns of fluvial systems, further influencing the distribution and preservation of sediments and weathered bedrock, and hence regional landscape morphology.

Geochemical signature of mineralization in weathered sediments and bedrock, Thunderdome prospect, Broken Hill region, westernNew South Wales, Australia: implications for mineral explorationunder cover

The stratiform Pb–Ag–Zn and stratabound Cu–Au mineralization of the Thunderdome prospect, located in the Mundi Mundi Plain 50 km NW of Broken Hill, NSW, is covered by c. 200 m of sediments. A downhole mineralogical and geochemical dispersion study was carried out on 245 regolith samples from six exploration drillholes to establish if geochemical dispersion from the mineralization could be identified in the overlying cover. This was achieved using a combination of qualitative and quantitative XRD, Portable Infrared Mineral Analyzer (PIMA) and XRF techniques. The stratigraphy is defined by a variable thickness of diagenetically modified and weathered Cenozoic sediments (and soil) overlying the weathered Proterozoic basement, which hosts the mineralization. The composition of the sediments and saprolite appears to have been influenced by chemical dispersion from underlying mineralization, as shown by anomalous concentrations of Cu (up to c. 13 times above background), Ni ( c. 9 times), As, Zn, Pb and Co (all c. 3 times) among others. Some of these anomalies are widespread and may be either clastic or related to adsorption on clay minerals, Fe-oxides and neoformed, poorly crystalline mineral phases. The precise transport mechanisms from the underlying mineralization need to be tested in order to gain further insights into geochemical dispersion processes under cover.

Spectral unmixing of hyperspectral imagery for mineral exploration: comparison of results from SFSI and AVIRIS

Hyperspectral image data sets acquired near Cuprite, Nevada, in 1995 with the Short-Wave Infrared (SWIR) Full Spectrum Imager (SFSI) and in 1996 with the Airborne Visible-Infrared Imaging Spectrometer (AVIRIS) are analysed with a spectral unmixing procedure and the results compared. The nominal pixel centre spacings are 1.0 by 1.5 m for SFSI and 16.2 by 18.1 m for AVIRIS across track and along track, respectively; the region imaged by SFSI is a small portion of the full AVIRIS scene. Both data cubes have nominal spectral band centre spacings of approximately 10 nm. The image data, converted to radiance units, are atmospherically corrected and converted to surface reflectances. Spectral end members are extracted automatically from the two data sets; those representing mineral species common to both are compared to each other and to reference spectra obtained with a field instrument, the Portable Infrared Mineral Analyser (PIMA). The full sets of end members are used in a constrained linear unmixing of the respective hyperspectral image cubes. The resulting unmixing fraction images derived from the AVIRIS and SFSI data sets for the minerals alunite, buddingtonite, kaolinite, and opal correlate well, with correlation coefficients ranging from 0.75 to 0.91, after compensation for shadowing and misregistration effects.