No full-text available
To read the full-text of this research,
you can request a copy directly from the author.
Near real-time reconciliation of geochemical data acquired with handheld spectroscopic devices : Application to volcanogenic massive sulphide (VMS) deposit from the Iberian Pyrite Belt
Abstract
Mineral exploration focused on deeply concealed targets at depth requires effective techniques applicable in the field in order to identify ore-forming systems on a large scale and pathfinders to locate ore on a smaller scale. According to the rapid development of portable equipment in recent years, the importance of near real-time analysis in the field has been increasing by helping fast decision-making support before laboratory requests.Spectroscopic analysis using individual equipment has been widely used in the exploration of mineral resources, but it is rare to apply integrated data from several techniques to characterize “vectors”, which provide variations in lithology, geochemistry, mineralogy, and mineral chemistry. In addition, it is even rarer if the combination of spectral data is obtained from various portable instruments. Therefore, this study aims at reconciling geochemical data acquired from portable spectroscopic devices in order to determine the best geochemical information from each technique applied by combining the mineralogical and elemental information. Elemental and mineralogical data are provided in this study by six portable techniques: (i) elemental analyses such as XRF and LIBS for major, trace, and light elements, and (ii) mineralogical analyses such as Raman, VNIR-SWIR, MIR, and XRD to constrain rock-forming, ore, and alteration minerals. The final objective of this study is to identify vectors to the ore by applying the reconciled multi-spectral data obtained from the “real” sample in the Elvira volcanogenic massive sulfide (VMS) deposit. To achieve this, step-by-step procedures were carried out: (i) methodological understanding of each technique, (ii) establishment of a spectral database consisting of naturally monomineralic minerals, (iii) design of a decision tree to classify by mineral or mineral classes based on diagnostic bands, and mineral identification and quantification of (iv) carbonate and (v) phyllosilicate minerals (i.e., trioctahedral chlorites and dioctahedral micas), which are indicators of the target deposit.Several limitations of portable spectroscopy were confirmed based on the device itself and the geological environment in the Elvira deposit. Nevertheless, portable spectroscopy is effective in identifying the presence and compositional changes of various minerals from heterogeneous rock samples. Therefore, spectroscopic analysis on-site can be one of the vectoring tools to determine the implication for ore mineralization in hidden ore explorations.
... The advantages of pXRF include its versatility in handling various sample types (whole rock, powder, or pellet) and its semi-quantitative nature. However, it requires regular use of standardised samples for device calibration when precise quantitative results are necessary [3,4]. On the other hand, Laser-Induced 2 of 7 Breakdown Spectroscopy operates by focusing a laser pulse on the sample, generating a small plasma spark composed of excited ions, electrons, and neutral atoms. ...
... Proc. 2023, 15, 54 2 of 7 calibration when precise quantitative results are necessary [3,4]. On the other hand, Laser-Induced Breakdown Spectroscopy operates by focusing a laser pulse on the sample, generating a small plasma spark composed of excited ions, electrons, and neutral atoms. ...
Drill-core analysis is paramount for the characterization of deposits in mineral exploration. Over the past years, the use of hyperspectral (HS) sensors has rapidly increased to improve the reliability and efficiency of core logging. However, scanning drill-core samples of an entire mineral deposit entails several complex challenges, from transport logistics to large scale data management and analysis. Hence, academic studies on new applications of drill-core HS data at a mineral deposit scale remain rare.
We present a semi-automated workflow for large scale interpretation of HS data, founded on a novel approach of mineral mapping based on a supervised dictionary learning technique. This approach exploits the complementary information from scanning electron microscopy based automated mineralogy and hyperspectral imaging techniques for estimating mineral quantities along all boreholes. We propose that it is effectively possible to propagate the mineral quantification to the entire borehole from small samples with high resolution mineralogical information strategically selected throughout the deposit.
We showcase this approach on data acquired in the Elvira shale-hosted volcanogenic massive sulphide (VMS) deposit located at the Iberian Pyrite Belt (IPB), where 7000 m of drill-core were acquired along 80 boreholes. Resulting maps provide insights on the controls on the mineral assemblages and chemical composition of specific minerals across the whole volume at several spatial scales, from large scale variations within apparently homogeneous black shales to small scale mineral composition variations, of potential use as vectors towards mineralization. This approach adds value to the core data, allowing for a better understanding of the geological setting of the Elvira deposit and providing valuable insights for future exploration targeting in the region.
This approach based on machine learning can easily be transposed to different ore deposits with a limited input from a geologist.
In this work we have performed a detailed study of vectors to ore to a representative volcanic-rock-hosted replacive volcanogenic massive sulfide (VMS) deposit located in the northern Iberian Pyrite Belt (Spain), the Aguas Teñidas deposit. The investigated vectors include the following: (1) mineralogical zoning, (2) host sequence characterization and mineralized unit identification based on whole rock geochemistry discrimination diagrams, (3) study of the characteristics and behaviour of whole rock geochemical anomalies around the ore (e.g. alteration-related compositional changes, characteristics and extent of geochemical halos of indicative elements such as Cu, Zn, Pb, Sb, Tl, and Ba around the deposit), and (4) application of portable X-ray fluorescence (p-XRF) analysis to the detection of the previous vectors.
In the footwall, a concentric cone-shaped hydrothermal alteration zone bearing the stockwork passes laterally, from core to edge, from quartz (only local) to chlorite–quartz, sericite–chlorite–quartz, and sericite–quartz alteration zones. The hydrothermal alteration is also found in the hanging wall despite being tectonically allochthonous to the orebody: a proximal sericite alteration zone is followed by a more distal albite-rich one. Whole rock major elements show an increase in alteration indexes (e.g. AI, CCPI) towards the mineralization, a general SiO2 enrichment, and FeO enrichment as well as K2O and Na2O depletion towards the centre of the hydrothermal system, with MgO showing a less systematic behaviour. K2O and Na2O leached from the centre of the system are transported and deposited in more external areas. Copper, Pb, and Zn produce proximal anomalies around mineralized areas, with the more mobile Sb, Tl, and Ba generating wider halos. Whereas Sb and Tl halos form around all mineralized areas, Ba anomalies are restricted to areas around the massive sulfide body. Our results show that proposed vectors, or adaptations designed to overcome p-XRF limitations, can be confidently used by analysing unprepared hand specimens, including the external rough curved surface of drill cores.
The data presented in this work are not only applicable to VMS exploration in the Iberian Pyrite Belt, but on a broader scale they will also contribute to improving our general understanding of vectors to ore in replacive-type VMS deposits.
Lithium (Li) is one of the latest metals to be added to the list of critical materials in Europe and, thus, lithium exploration in Europe has become a necessity to guarantee its mid- to long-term stable supply. Laser-induced breakdown spectroscopy (LIBS) is a powerful analysis technique that allows for simultaneous multi-elemental analysis with an excellent coverage of light elements (Z < 13). This data paper provides more than 4000 LIBS spectra obtained using a handheld LIBS tool on approximately 140 Li-content materials (minerals, powder pellets, and rocks) and their Li concentrations. The high resolution of the spectrometers combined with the low detection limits for light elements make the LIBS technique a powerful option to detect Li and trace elements of first interest, such as Be, Cs, F, and Rb. The LIBS spectra dataset combined with the Li content dataset can be used to obtain quantitative estimation of Li in Li-rich matrices. This paper can be utilized as technical and spectroscopic support for Li detection in the field using a portable LIBS instrument.
Dataset:https://doi.org/10.24396/ORDAR-65; 17 May 2021.
Dataset License: CC-BY
A portable Raman device with a 532 nm excitation laser and a portable infrared spectrometer with ATR (Attenuated Total Reflection) mode were used to analyse the spectral features associated with the identification and compositional variation of Ca-Mg-Fe-Mn natural carbonate minerals with a calcite structure (calcite, ankerite, dolomite, siderite, rhodochrosite, and magnesite). A systematic study of the variations of the peak positions with various compositional ratios was carried out. Most of the band positions were shifted to lower wavenumbers with increasing ionic radius or atomic mass of the divalent cations but the band of the translational lattice (T) mode in Raman and the symmetric bending (ν4) band in the mid-infrared were the most sensitive. Therefore, the elemental variation of the Ca-Mg-Fe-Mn ratio in this carbonate series can be estimated from Raman and infrared band positions from spectra acquired with portable spectrometers.
Hyperspectral reflectance has the potential to provide rapid and low-cost mineralogical and chemical information that can be used to vector in mineral systems. However, the spectral signature of white mica and chlorite, despite numerous studies, is not fully understood. In this study, we review the mineralogy and chemistry of different white mica and chlorite types and investigate what mineralogical and chemical changes are responsible for the apparent shifts in the shortwave infrared (SWIR) spectroscopic absorption features. We demonstrate that the spectral signature of white mica is more complex than previously documented and is influenced by the Tschermak substitution, as well as the sum of interlayer cations. We show that an increase in the interlayer deficiencies towards illite is associated with a change from steep to shallow slopes between the wavelength position of the 2200 nm feature (2200 W) and Mg, Al(VI) and Si. These changes in slope imply that white micas with different elemental chemistry may be associated with the same 2200 W values and vice versa, contrary to traditional interpretation. We recommend that traditional interpretations should only be used in true white mica with sum interlayer cations (I) > 0.95. The spectral signature of trioctahedral chlorite (clinochlore, sheridanite, chamosite and ripidolite) record similar spectral relationships to those observed in previous studies. However, dioctahedral Al-rich chlorite (sudoite, cookeite and donbassite) has a different spectral response with Mg increasing with 2250 W, which is the opposite of traditional trioctahedral chlorite spectral interpretation. In addition, it was shown that dioctahedral chlorite has a 2200 W absorption feature that may introduce erroneous spectral interpretations of white mica and chlorite mixtures. Therefore, care should be used when interpreting the spectral signature of chlorite. We recommend that spectral studies should be complemented with electron microprobe analyses on a subset of at least 30 samples to identify the type of muscovite and chlorite. This will allow the sum I of white mica to be obtained, as well as estimate the slope of 2200 W absorption trends with Mg, Al(vi), and Si. Preliminary probe data will allow more accurate spectral interpretations and allow the user to understand the limitations in their hyperspectral datasets.
Portable X-ray fluorescence spectroscopy is now widely used in almost any field of geoscience. Handheld XRF analysers are easy to use, and results are available in almost real time anywhere. However, the results do not always match laboratory analyses, and this may deter users. Rather than analytical issues, the bias often results from sample preparation differences. Instrument setup and analysis conditions need to be fully understood to avoid reporting erroneous results. The technique’s limitations must be kept in mind. We describe a number of issues and potential pitfalls observed from our experience and described in the literature. This includes the analytical mode and parameters; protective films; sample geometry and density, especially for light elements; analytical interferences between elements; physical effects of the matrix and sample condition, and more. Nevertheless, portable X-ray fluorescence spectroscopy (pXRF) results gathered with sufficient care by experienced users are both precise and reliable, if not fully accurate, and they can constitute robust data sets. Rather than being a substitute for laboratory analyses, pXRF measurements are a valuable complement to those. pXRF improves the quality and relevance of laboratory data sets.
The late Paleozoic Iberian Pyrite Belt (IPB), southwestern Iberian Peninsula, hosts one of the largest concentrations of volcanogenic massive sulphide deposits on the Earth's surface. The ore-bearing sequence includes felsic-rock-hosted VMS deposits formed by host rock replacement in the northern area of the IPB, and shale-hosted deposits formed by direct sulphide precipitation on the seafloor in the southern area. The high-grade Elvira Cu-Zn-Pb deposit is the most recent discovery, and is located eastward of one of the largest orebodies in the southern IPB, the Sotiel-Migollas cluster. This deposit consists of a single massive sulphide lens located ca. 250-500 m below the surface and is hosted in an overturned and thrusted sequence dominated by dark shales dated to uppermost Devonian. The stratigraphic footwall includes a well-developed stockwork zone and pervasive chlorite-rich alteration. The massive sulphides show abundant sedimentary structures typical of deposition on the seafloor but also a large zone of sub-seafloor replacement of muds which marks the transition from the feeder zone to the exhalative massive sulphides.
Portable XRF is a rapid, mobile, high throughput, and potentially cost effective instrumental analytical technique capable of elemental assessment. It is widely used for environmental assessment of soils in a variety of contexts such as agriculture and pollution both in-situ and ex-situ, to varying levels of success. Portable XRF performance for soil analysis is often validated against wet chemistry techniques but a range of factors may give rise to elementally dependent disparities affecting accuracy and precision assessments. These include heterogeneity, analysis times, instrument stability during analyses, protective thin films, incident X-rays, sample thickness, sample width, analyte interferences, detector resolution, power source fluctuations and instrumental drift. Light elements comprising water and organic matter (i.e. carbon, oxygen) also negatively affect measurements due to X-ray scattering and attenuation. The often-overlooked phenomenon of variability in both soil organic matter and water can also affect soil density (e.g. shrink-swell clays) and thus sample critical thickness which in turn affects the effective volume of sample analyzed. Compounding this, for elements having lower characteristic fluorescence energy, effective volumes of analyses are lower and thus measurements may not be representative of the whole sample. Understanding the effects and interplay between determined elemental concentrations and soil organic matter, water, and critical thickness together with the subtlety of theoretical effective volumes of analyses will help analysts mitigate potential problems and assess the applicability, advantages and limitations of PXRF for a given site. We demonstrate that with careful consideration of these factors and a systematic approach to analysis which we summarize and present, PXRF can provide highly accurate results.
The mineral exploration industry requires new methods and tools to address the challenges of declining mineral reserves and increasing discovery costs. Laser-induced breakdown spectroscopy (LIBS) represents an emerging geochemical tool for mineral exploration that can provide rapid, in situ, compositional analysis and high-resolution imaging in both laboratory and field and settings. We demonstrate through a review of previously published research and our new results how LIBS can be applied to qualitative element detection for geochemical fingerprinting, sample classification, and discrimination, as well as quantitative geochemical analysis, rock characterization by grain size analysis, and in situ geochemical imaging. LIBS can detect elements with low atomic number (i.e., light elements), some of which are important pathfinder elements for mineral exploration and/or are classified as critical commodities for emerging green technologies. LIBS data can be acquired in situ, facilitating the interpretation of geochemical data in a mineralogical context, which is important for unraveling the complex geological history of most ore systems. LIBS technology is available as a handheld analyzer, thus providing a field capability to acquire low-cost geochemical analyses in real time. As a consequence, LIBS has wide potential to be utilized in mineral exploration, prospect evaluation, and deposit exploitation quality control. LIBS is ideally suited for field exploration programs that would benefit from rapid chemical analysis under ambient environmental conditions.
This paper focuses on handheld and top-of-hole techniques which have appeared since 2007 or have undergone major improvements, and discusses their benefits, challenges and pitfalls, why we use them and what to expect from them. There is an ongoing need to be innovative with the way we undertake mineral exploration. Recent technological advances that have been applied to successful mineral exploration include on-site or portable instruments, on-site laboratory technologies, various core scanners, and technologies for fluid analysis. Portable or field technologies such as pXRF, pXRD, pNIR-SWIR, µRaman and LIBS aid in obtaining chemical and mineralogical information. Spectral gamma tools, a well-known technology, recently took advantage of improved ground and airborne (drone) instruments, to complement hyperspectral imagery. At mine and exploration sites, top-of-hole sensing technologies, such as Lab-at-Rig® and various core scanners (both spectral- and XRF-based) have become useful tools to analyse metres of core as it is being drilled. Fluid analyses are not as common as analyses of solid materials, but there are advances in such technologies as anodic stripping voltammetry, polarography and ion-exchange electrodes aiming for analysis of commodity or environmentally important elements.
Field-portable geochemical techniques and on-site technologies now offer instant response and flexibility for most exploration tasks. By providing relevant data within minutes, they allow safer field decisions and focus on the most promising finds, while saving valuable resources in sampling grids or drilling. More efficient laboratory analysis programs are supported by sample screening and homogeneity checking on-site. Field analyses are not always as accurate as laboratory ones, but most of the time can be correlated with them, enabling reliable decisions. The level of confidence in field-made decisions needs to be compared between later and less numerous laboratory analyses, and less precise but more abundant and immediate field analyses. It may be demonstrated that, in many cases, the fit–for-purpose nature of the latter allows a better confidence level. Quality compromises associated with field analyses can be reduced by the application of better sample preparation and quality assurance/quality control (QA/QC) procedures. Most of the further development of on-site chemical analysis is expected to be based on its integration with lab methods and on sound QA/QC practice, allowing a precise evaluation of its confidence level and uncertainties. Mineralogical analyses are constrained by our ability to interpret the data in near-real time but offer promising approaches in both surface and drilling exploration campaigns.
Thematic collection: This article is part of the Exploration 17 collection available at: https://www.lyellcollection.org/cc/exploration-17
Raman spectrometry is a rapid, non-destructive alternative to conventional tools employed to assess the thermal alteration of organic matter (OM). Raman may be used to determine vitrinite reflectance equivalent OM maturity values for petroleum exploration, to provide temperature data for metamorphic studies, and to determine the maximum temperatures reached in fault zones. To achieve the wider utilisation of Raman, the spectrum processing method, and the positions and nomenclature of Raman bands and parameters, all need to be standardized. We assess the most widely used Raman parameters as well as the best analytical practices that have been proposed. Raman band separation and G-band full-width at half-maximum are the best parameters to estimate the maturity for rocks following diagenesis–metagenesis. For metamorphic studies, the ratios of band areas after performing deconvolution are generally used. Further work is needed on the second-order region, as well as assessing the potential of using integrated areas on the whole spectrum, to increase the calibrated temperature range of Raman parameters. Applying Raman spectroscopy on faults has potential to be able to infer both temperature and deformation processes. We propose a unified terminology for OM Raman bands and parameters that should be adopted in the future. The popular method of fitting several functions to a spectrum is generally unnecessary, as Raman parameters determined from an un-deconvoluted spectrum can track the maturity of OM. To progress the Raman application as a geothermometer a standardized approach must be developed and tested by means of an interlaboratory calibration exercise using reference materials.
Noachian surfaces on Mars exhibit vertical assemblages of weathering horizons termed as weathering profiles; this indicates that surface water caused alteration of the rocks that required a different, warmer climate than today. Evidence of this early Martian climate with CO2 vapor as the main component causing greenhouse warming has been challenged by the lack of carbonate in these profiles. Here we report the analysis of Compact Reconnaissance Imaging Spectrometer for Mars L‐detector data leading to the detections of carbonates using a spectral signature exclusively attributed to them. The carbonates are collocated with hydroxylated minerals in weathering profiles over the Martian surface. The origin of CO2 for the formation of carbonates could be the atmosphere. The widespread distribution of weathering profiles with carbonates over the surface of the planet suggest global interactions between fluids containing carbonate/bicarbonate ions with the surface of Mars in the presence of atmospheric water until around 3.7 billion years ago.
The compositions of phyllosilicates, with a focus on fluid-mobile elements, were evaluated as a means to fingerprint the Middle Ordovician metamorphosed (greenschist facies) volcanogenic massive sulfide deposits of the Bathurst Mining Camp (BMC), Canada. Ninety-five drill-core samples from six of the major deposits of the Bathurst Mining Camp (Brunswick No. 12, Heath Steele B zone, Halfmile Lake Deep zone, Key Anacon East zone, Louvicourt, and Restigouche) were analyzed using electron microprobe and laser ablation inductively coupled plasma-mass spectrometry. Typically, phyllosilicates (chlorite, white mica, and to a lesser extent biotite) are ubiquitous phases in the host rocks of the massive sulfide deposits of the BMC. Electron microprobe analysis results show a wide compositional variation in chlorite and white mica. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis was performed to measure fluid-mobile elements, showing that Tl is distinctly enriched in all white mica (up to 719 ppm) relative to chlorite (up to 50.1 ppm). Chlorite hosts Sn (up to 4600 ppm), Hg (up to 7.3 ppm), Sb (up to 35.4 ppm), As (up to 1320 ppm), In (up to 307 ppm), Cd (up to 83.2 ppm), and Se (up to 606 ppm). White mica hosts Sn (up to 1316 ppm), Hg (up to 93 ppm), Sb (up to 1630 ppm), As (up to 14,800 ppm), In (up to 1186 ppm), Cd (up to 98 ppm), and Se (up to 38.8 ppm). Limited LA-ICP-MS analysis on biotite indicates a higher overall concentration of Tl (mean = 14.6 ppm) relative to co-existing white mica (mean = 2.18 ppm). On average, biotite is also more enriched in Hg, Sn, and Ba relative to chlorite and white mica. Laser Ablation ICP-MS profiles of chlorite, white mica, and biotite demonstrate smooth time-dependent variations diagnostic of structural substitution of these elements. Compositional variation of chlorite-white mica pairs presented in the current study shows systematic variations as a function of distance from the mineralized horizons. This highlights the potential to use trace-element signatures in these phyllosilicate pairs to identify proximal (chlorite) and distal (white mica) footprints for volcanogenic massive sulfides exploration.
With modern field analysers, such as portable XRD and XRF devices with the support of field concentrator techniques, samples can already be analysed mineralogically and geochemically already in the field. Together with advanced electron optical methods, minerals can be identified fully or semi -automatically. Those methods speed up the exploration work process and improve the cost-efficiency of exploration. In this article, the first results of the project Indika (ERDF funded project) are presented. The use of Olympus Terra pXRD in critical mineral exploration is tested in two study areas in northern Finland: the Mäkärä Au-REE mineralization and the Sokli area (P-REE deposit), northern Savukoski. The first results show that the mineralogy analysed from the till concentrates using pXRD fits relatively well to the known mineralogy of the study areas. Furthermore, the identified minerals correlate well with the results produced by laboratory XRD and FE-SEM. This makes pXRD analysers useful in the mineralogical work and mineral exploration even directly in the field.
Four different spectrometer types were used to measure spectra in the library: (1) Beckman™ 5270 covering the spectral range 0.2 to 3 μm, (2) standard, high resolution (hi-res), and high-resolution Next Generation (hi-resNG) models of Analytical Spectral Devices (ASD) field portable spectrometers covering the range from 0.35 to 2.5 μm, (3) Nicolet™ Fourier Transform Infra-Red (FTIR) interferometer spectrometers covering the range from about 1.12 to 216 μm, and (4) the NASA Airborne Visible/Infra-Red Imaging Spectrometer AVIRIS, covering the range 0.37 to 2.5 μm. Measurements of rocks, soils, and natural mixtures of minerals were made in laboratory and field settings. Spectra of plant components and vegetation plots, comprising many plant types and species with varying backgrounds, are also in this library. Measurements by airborne spectrometers are included for forested vegetation plots, in which the trees are too tall for measurement by a field spectrometer. This report describes the instruments used, the organization of materials into chapters, metadata descriptions of spectra and samples, and possible artifacts in the spectral measurements. To facilitate greater application of the spectra, the library has also been convolved to selected spectrometer and imaging spectrometers sampling and bandpasses, and resampled to selected broadband multispectral sensors. The native file format of the library is the SPECtrum Processing Routines (SPECPR) data format. This report describes how to access freely available software to read the SPECPR format. To facilitate broader access to the library, we produced generic formats of the spectra and metadata in text files. The library is provided on digital media and online at https://speclab.cr.usgs.gov/spectral-lib.html. A long-term archive of these data are stored on the USGS ScienceBase data server (https://dx.doi.org/10.5066/F7RR1WDJ).
The suggested citation for the USGS Spectral Library version 7 is:
Kokaly, R.F., Clark, R.N., Swayze, G.A., Livo, K.E., Hoefen, T.M., Pearson, N.C., Wise, R.A., Benzel, W.M., Lowers, H.A., Driscoll, R.L., and Klein, A.J., 2017, USGS Spectral Library Version 7: U.S. Geological Survey Data Series 1035, 61 p., https://doi.org/10.3133/ds1035.
A broad suite of geological materials were studied a using a handheld laser-induced breakdown spectroscopy (LIBS) instrument. Because LIBS is simultaneously sensitive to all elements, the full broadband emission spectrum recorded from a single laser shot provides a ‘chemical fingerprint’ of any material – solid, liquid or gas. The distinguishing chemical characteristics of the samples analysed were identified through principal component analysis (PCA), which demonstrates how this technique for statistical analysis can be used to identify spectral differences between similar sample types based on minor and trace constituents. Partial least squares discriminant analysis (PLSDA) was used to distinguish and classify the materials, with excellent discrimination achieved for all sample types. This study illustrates through four selected examples involving carbonate minerals and rocks, the oxide mineral pair columbite-tantalite, the silicate mineral garnet and native gold how portable, handheld LIBS analysers can be used as a tool for real-time chemical analysis under simulated field conditions for element or mineral identification plus such applications as stratigraphic correlation, provenance determination and natural resources exploration. This article is protected by copyright. All rights reserved.
The analytical conditions including surface state, moisture effect, and device condition were investigated for
applying Short Wave Infrared (SWIR) spectroscopy to the field survey. Among the three surface state of samples (exposed surface, cutting face and powder), both spectra from the exposed surface and cutting face are almost identical whereas spectral variation was detected in powder sample. Over 24-hours-dryring of the wet sample at room temperature, the samples show a similar spectrum with that of dry condition. The result suggests that outcrop samples mighty be dried for 24 ~ 48 hours depending on the wetness of outcrop. The bright minerals could produce stable spectra with 10 times measurements as default value of the device under SWIR spectroscopy but the dark minerals would require about 10 seconds, which corresponds to 100 times measurements to get the reliable spectra. The position and shape 2,160 ~ 2,330 nm and/or other spectral features of hydrothermal alteration minerals by SWIR spectroscopy could be used for a classification of hydrothermal alteration zone in the field. Absorption peaks in 2,160 ~ 2180 nm are useful for identifying (advanced) argillic zone by spectral characteristics of kaoline, dickite, pyrophyllite, and alunite. Absorption peaks in 2,180 ~ 2,230 nm are able to define muscovite, sericite, and smectite, which are key alteration minerals in phyllic zone. Absorption peaks in 2,230 ~ 2,270 nm can be used to recognize prophylitic zone where chlorite and epidote occur. Absorption peaks of other principle minerals such as talc, serpentine, amphibole, and carbonate group are mainly detected within the wave length of 2,270 ~ 2,330 nm. This result indicates that the spectra of these minerals need to be carefully interpreted.
The 250 × 20–70 km Iberian Pyrite Belt (IPB) is a Variscan metallogenic province in SW Portugal and Spain hosting the largest concentration of massive sulphide deposits worldwide. The lowermost stratigraphic unit is the early Givetian to late Famennian-Strunian (base unknown) Phyllite-Quartzite Group (PQG), with shales, quartz-sandstones, quartzwacke siltstones, minor conglomerate and limestones at the top. The PQG is overlain by the Volcanic Sedimentary Complex (VSC), of late Famennian to mid-late Visean age, with a lower part of mafic volcanic rocks, rhyolites, dacites and dark shales, hosting VHMS deposits on top (many times capped by a jasper/chert layer), and an upper part, with dark, purple and other shales and volcanogenic/volcaniclastic rocks, carrying Mn oxide deposits. The VSC is covered by the thousands of meters thick Baixo Alentejo Flysch Group of late Visean to Moscovian age. The VSC comprises a bimodal submarine volcanic succession, with VHMS deposits spatially associated to dacites and rhyolites corresponding to effusive/explosive lava-cryptodome-pumice cone volcanoes. The lava/domes consist of coherent lithofacies surrounded by clast-rotated hyaloclastite breccia and minor autobreccia, with massive VHMS ore at the top of the felsic effusive units and stockworks in the autoclastic and pyroclastic breccias. The eastern IPB rocks are intruded by the voluminous Sierra Norte Batholith (tonalite-trondhjemite-granodiorite, TTG series).
Felsic volcanic rocks (dacite to high-silica rhyolite) predominating over basalts and dolerites, belong to the calc-alkaline series and plot mostly in the within-plate field in tectonic discriminative diagrams. Several periods of volcanism, from 384 to 359 Ma are recognized. Dacites and rhyolites exhibit Nd and Sr enrichment, typical of a crustal signature, and their overall geochemistry suggests generation by fractionation/partial melting of amphibolites at low pressure. Trace elemental modelling of the basic rocks, involving tholeiitic lavas and alkaline basaltic lavas and dolerites, points to mixing between E- and N-MORB and assimilation of crustal material. Variscan NW-SE/W-E-trending and SW- or S-verging folds (with NE- or N-dipping planar cleavage) and thrusts, occur in west-central and eastern IPB, respectively. In late to post-Variscan time strike-slip oblique faults formed, either N-S to NNW-SSE or NE-SW to ENE-WSW, dextral or sinistral (both extensional), respectively. The first set hosts late Variscan Cu-Pb-Ba veins and Mesozoic(?) dolerite dykes. IPB contains over 90 VHMS deposits, estimated before erosion at >1700 Million tonnes (Mt), with 14.6 Mt Cu, 34.9 Mt Zn, 13.0 Mt Pb, 46,100 t Ag, 880 t Au and many other metals, particularly Sn. Eight of these are giant (≥100 Mt) VHMS deposits, namely Rio Tinto, Tharsis, Aznalcóllar-Los Frailes, Masa Valverde, Sotiel-Migollas and La Zarza (Spain) and Neves Corvo and Aljustrel (Portugal). The VHMS deposits are of the felsic-siliclastic type and mostly of the Zn–Pb–Cu and Zn–Cu–Pb metal content types. The deposits range in thickness from 1 m to tens of meters (plus increase from tectonic stacking) and up to a few kilometers in extension, and many are underlain by large stockwork zones. Their age is either Strunian (palynological age) in the southern IPB or mostly Tournaisian in the northern IPB. The major massive ore minerals are pyrite, sphalerite, chalcopyrite, galena (and cassiterite at Neves Corvo), also present with dominant quartz-chlorite-sericite-carbonate in the stockwork ore. Sericite and chlorite were also formed from additional alteration in the hanging wall rocks. Metal zonation in most VHMS deposits consists of a Cu-rich stockwork and base of the massive ore, with Zn–Pb massive ore above and extending laterally. S-, O-, H- and C-isotope data indicate that ore-forming fluids contain predominant or exclusive modified seawater. A magmatic fluid contribution to the dominant seawater has been proposed for some deposits. The deposits are exhalative or formed by shallow subsurface replacement of either muds/shales or coherent felsic volcanic rocks.
The mean absolute percentage error (MAPE) is one of the most widely used measures of forecast accuracy, due to its advantages of scale-independency and interpretability. However, MAPE has the significant disadvantage that it produces infinite or undefined values for zero or close-to-zero actual values. In order to address this issue in MAPE, we propose a new measure of forecast accuracy called the mean arctangent absolute percentage error (MAAPE). MAAPE has been developed through looking at MAPE from a different angle. In essence, MAAPE is a slope as an angle, while MAPE is a slope as a ratio, considering a triangle with adjacent and opposite sides that are equal to an actual value and the difference between the actual and forecast values, respectively. MAAPE inherently preserves the philosophy of MAPE, overcoming the problem of division by zero by using bounded influences for outliers in a fundamental manner through considering the ratio as an angle instead of a slope. The theoretical properties of MAAPE are investigated, and the practical advantages are demonstrated using both simulated and real-life data.
A portable spectrometer should be small, lightweight, and capable of running on battery power for a reasonable duration of time. These instruments are intended for use in the field and, therefore, must perform well outside the climate‐controlled environment of a laboratory. When judged against its laboratory counterpart, a portable spectrometer, evaluated strictly on typical laboratory specifications for analytical performance, usually will not perform as well. For many of the spectroscopic techniques, the availability of portable spectrometers was driven by safety, security, terrorism, and military concerns. The major exceptions to this are handheld X‐ray fluorescence, laser‐induced breakdown spectroscopy, and near‐infrared spectroscopy systems, where field instrumentation has been driven almost exclusively by commercial reasons. Applications are critical for the success of portable spectrometers. This chapter also presents an overview of the key concepts discussed in this book.
The first part of this review describes laser-induced breakdown spectroscopy (LIBS) instrumentation for in situ measurement outside the laboratory, both prototype instruments and commercially available analyzers that can be operated by a single individual. Two types of devices are described – (i) field-portable type systems consisting of multiple units connected by an umbilical (fpLIBS) and (ii) handheld analyzers (hLIBS). The performance of
these two types of LIBS systems is compared with that of its primary competitor technology portable X-ray fluorescence spectroscopy (pXRF). The second part of the article reviews the results of both fpLIBS and hLIBS obtained in both research settings and for practical applications in various sectors where speed of analysis is often crucial. These include compositional screening of natural and manufactured materials, quality control of
manufacturing processes, identification of environmental contaminants, biomedical diagnostics, forensic analysis, and hazardous material identification in the industrial, environmental, geological, cultural heritage, agricultural, biological, nuclear and security sectors. The review concludes by highlighting the key trends and challenging future directions in making LIBS technology readily accessible for applications that demand easy
portability and fast analysis outside of the laboratory with instruments of compact dimensions and low weight but high performance, crucial features required for any portable device.
In this paper a review of different approaches for handling the self-absorption effect in Laser-Induced Breakdown Spectroscopy (LIBS) applications is presented. From an analytical point of view, self-absorption is a major issue affecting the accuracy of LIBS quantitative analysis. Self-absorption impacts emission line profiles and intensities, resulting in non-linear calibration curves and influencing the estimation of the sample composition. The main purpose of this work is presenting a summary of the experimental parameters giving raise to self-absorption and a thorough study of the different strategies for its estimation, compensation and exploitation in order to improve the analytical accuracy of the LIBS technique.
The review focuses on the most relevant advances and is reported in different sections relative to the analyzed objects (identification of rocks/minerals and sourcing; resources applications; slurry and drill cores; rare earth elements; light elements). Special sections report on the good practices for Laser Induced Breakdown Spectroscopy (LIBS) analysis and the most critical points that should be checked in order to validate any LIBS analysis on most common geological purposes. LIBS gives access to the most relevant elements in geosciences/geology and the typical detection limits fit usual requirements, with the advantage of permitting faster analyses than the other techniques classically used. Whether considering the case of metals of economic interest, that of critical elements, or that of light elements, LIBS has definitely been proved an adequate tool, and there is no need to do more on its evaluation in this field of application. Considering that LIBS measurements require limited sample pre-treatment, and considering also that LIBS is a fast all-optical multi-elemental technique, it is undoubtedly the optimal way to achieve a first quick screening and then provide valuable data prior to any further laboratory analyses. Therefore, the recent development of LIBS imaging should quickly lead to the implementation of LIBS imaging systems in the analytical laboratories worldwide in charge of analyzing geological samples.
Despite well-established advantages and a growing number of applications, laser-induced breakdown spectroscopy (LIBS) still fails to be recognized as a robust analytical technique, especially because many factors varying from an experimental setup to the other are preventing from any inter-comparison. The present paper is a guideline for increasing the quality of the LIBS analyses through a series of good practices, assessments and reporting along five key-steps being: i) monitoring of the LIBS signal, ii) optimization of the measurement conditions, iii) data filtering, iv) sorting, and v) quantification. This guideline offers an opportunity to increase the quality of the LIBS analysis and represents a significant step towards a standardization process. In addition, it allows comparing intra as well as inter-laboratory LIBS results.
As it is the case for any spectroscopic technique, laser-induced breakdown spectroscopy (LIBS) is strongly influenced by the signal fluctuations, and the LIBS spectra need to be normalized to obtain enhanced analytical performance. Nowadays, normalization in LIBS remains an open question and, in the present review, the normalization methods commonly applied to LIBS are presented and discussed, in particular those based on background, total area, internal standard, and Standard Normal Variate. We emphasize that the figures of merit, namely the coefficient of determination, the root-mean square error of prediction and the limit of quantification used to assess the advantages of processing normalized instead of non-normalized LIBS spectra, in a context of quantification, must be calculated in a rigorous way to be able to draw conclusions. We thus propose advices and good practices to achieve a rigorous comparison between quantitative models involving various normalization approaches, the final choice of the best normalization being ultimately driven by the analytical context. In order to take the best advantage from normalization in LIBS and thus increase the analytical performance of this technique, we encourage the analyst to thoroughly compare different normalization methods.
Hydrofluoric acid represents the majority of the industrial applications of fluorine in the world. It is synthetized
from fluorite, which is commonly purified by the froth flotation process to attain the high-grades required for
hydrofluoric acid production. Besides, in metallic ores such as tungsten and phosphate ores, fluorite does not
represent any added value compared to the extracted metals and is, therefore, considered as a gangue mineral
that has to be rejected. In both cases, the fluorine content has to be known precisely in the flotation process as
well as in all the industrial applications involving fluorine, to estimate the process efficiency and to optimise the
operations. Nevertheless, fluorine quantification is difficult using conventional techniques since it usually includes
heavy sample preparation such as dissolution. Though, Laser-Induced Breakdown Spectroscopy (LIBS)
provides a multi-element detection that has been successfully used to quantify fluorine using either elementary
or CaF molecular bands. Here, rock samples exhibiting a wide range of fluorine contents (from 1.48% to 40.73%)
were analysed, the fluorine being mainly comprised in fluorite. These samples corresponded to the products of
different flotation tests conducted on the same tungsten-skarn ore. The experimental conditions were optimised
to study the two CaF molecular bands, located between 529 and 543 nm, and between 590 and 606 nm, respectively.
Systematically, the LIBS emission intensities of the two studied bands were evaluated using the peak
areas, which were normalised, averaged over several ablated zones, and correlated with the fluorine content
determined by the fluoride-ion sensitive electrode method. The particle size played a key role as significant
differences in the LIBS intensities were exhibited between the 10–150 μm and the<20 μm powders, with no
discernible correlation between the size ranges and signal magnitude. Furthermore, the matrix effects strongly
impacted the LIBS intensities, which displayed a non-linear relationship with the fluorine contents: this induced
the development of non-linear univariate models that were calculated on 27 training samples and validated on
nine testing samples (3:1 ratio). Although non-linear models fitted adequately the experimental data, a multivariate
approach considering the two studied CaF bands was adopted to overcome the matrix effects. A formula
with linear, quadratic, and interaction terms was generated from the multivariate regression, predicting fluorine
contents with R2=0.94 and a mean average error of 2.18%F. The developed models demonstrated that a precise
and accurate quantification of fluorine is possible using a calibrated handheld LIBS, providing an on-line estimation
of the processes efficiency and a real-time adaptation.
This third edition of the Encyclopedia of Spectroscopy and Spectrometry provides authoritative and comprehensive coverage of all aspects of spectroscopy and closely related subjects that use the same fundamental principles, including mass spectrometry, imaging techniques and applications. It includes the history, theoretical background, details of instrumentation and technology, and current applications of the key areas of spectroscopy. The new edition will include over 80 new articles across the field. These will complement those from the previous edition, which have been brought up-to-date to reflect the latest trends in the field. Coverage in the third edition includes: Atomic spectroscopy Electronic spectroscopy Fundamentals in spectroscopy High-Energy spectroscopy Magnetic resonance Mass spectrometry Spatially-resolved spectroscopic analysis Vibrational, rotational and Raman spectroscopies The new edition is aimed at professional scientists seeking to familiarize themselves with particular topics quickly and easily. This major reference work continues to be clear and accessible and focus on the fundamental principles, techniques and applications of spectroscopy and spectrometry.
Until very recently, handheld spectrometers were the domain of major analytical and security instrument companies, with turnkey analyzers using spectroscopic techniques from X-ray fluorescence (XRF) for elemental analysis (metals), to Raman, mid-infrared, and near-infrared (NIR) for molecular analysis (mostly organics). However, the past few years have seen rapid changes in this landscape with the introduction of handheld laser-induced breakdown spectroscopy (LIBS), smartphone spectroscopy focusing on medical diagnostics for low-resource areas, commercial engines that a variety of companies can build up into products, hyphenated or dual technology instruments, low-cost visible-shortwave NIR instruments selling directly to the public, and, most recently, portable hyperspectral imaging instruments. Successful handheld instruments are designed to give answers to non-scientist operators; therefore, their developers have put extensive resources into reliable identification algorithms, spectroscopic libraries or databases, and qualitative and quantitative calibrations. As spectroscopic instruments become smaller and lower cost, “engines” have emerged, leading to the possibility of being incorporated in consumer devices and smart appliances, part of the Internet of Things (IOT). This review outlines the technologies used in portable spectroscopy, discusses their applications, both qualitative and quantitative, and how instrument developers and vendors have approached giving actionable answers to non-scientists. It outlines concerns on crowdsourced data, especially for heterogeneous samples, and finally looks towards the future in areas like IOT, emerging technologies for instruments, and portable hyphenated and hyperspectral instruments.
The chemical variation of white mica, in particular fluid-mobile elements typically associated with base-metal mineralization, in the host rock alteration haloes enveloping massive sulfide deposits of the Bathurst Mining Camp is constrained by LA-ICP-MS. White mica from examined deposits hosts Tl (up to 744 ppm), Sn (up to 1069 ppm), Hg (up to 524 ppm), Sb (up to 3650 ppm), As (up to 17100 ppm), and In (up to 524 ppm).. The occurrence of Tl, Sn, Sb, and probably As is predominantly controlled by crystal-chemical substitution, although the occurrence of micro- to nano-scale inclusions of other phases cannot be disqualified. White mica immediately adjacent (within 50 m) to the massive sulfides in both the hanging wall and footwall is enriched in Tl, Sb, Hg, and variably enriched in As, Sn, In, Se, Bi, and Cd and can be used to vector toward mineralization. In distal zones up to several hundred meters distant, the variation of fluid-mobile elements in white mica is recognized in lower concentrations, but detectable up to several hundred meters away from the ore horizons. Here, variation in the ∑ Tl+Sb+Sn+Hg can serve as a vectoring tool. This methodology can potentially serve as a complementary tool to other geochemical and geophysical exploration methods in prospecting for buried volcanogenic massive sulfide mineralization.
This book explains concepts of transmission electron microscopy (TEM) and x-ray diffractometry (XRD) that are important for the characterization of materials. The fourth edition adds important new techniques of TEM such as electron tomography, nanobeam diffraction, and geometric phase analysis. A new chapter on neutron scattering completes the trio of x-ray, electron and neutron diffraction. All chapters were updated and revised for clarity. The book explains the fundamentals of how waves and wavefunctions interact with atoms in solids, and the similarities and differences of using x-rays, electrons, or neutrons for diffraction measurements. Diffraction effects of crystalline order, defects, and disorder in materials are explained in detail. Both practical and theoretical issues are covered. The book can be used in an introductory-level or advanced-level course, since sections are identified by difficulty. Each chapter includes a set of problems to illustrate principles, and the extensive Appendix includes laboratory exercises.
For laser-induced breakdown spectroscopy (LIBS) quantitative analysis technique, baseline correction is an essential part for the LIBS data preprocessing. As the widely existing cases, the phenomenon of baseline drift is generated by the fluctuation of laser energy, inhomogeneity of sample surfaces and the background noise, which has aroused the interest of many researchers. Most of the prevalent algorithms usually need to preset some key parameters, such as the suitable spline function and the fitting order, thus do not have adaptability. Based on the characteristics of LIBS, such as the sparsity of spectral peaks and the low-pass filtered feature of baseline, a novel baseline correction and spectral data denoising method is studied in this paper. The improved technology utilizes convex optimization scheme to form a non-parametric baseline correction model. Meanwhile, asymmetric punish function is conducted to enhance signal-noise ratio (SNR) of the LIBS signal and improve reconstruction precision. Furthermore, an efficient iterative algorithm is applied to the optimization process, so as to ensure the convergence of this algorithm. To validate the proposed method, the concentration analysis of Chromium (Cr),Manganese (Mn) and Nickel (Ni) contained in 23 certified high alloy steel samples is assessed by using quantitative models with Partial Least Squares (PLS) and Support Vector Machine (SVM). Because there is no prior knowledge of sample composition and mathematical hypothesis, compared with other methods, the method proposed in this paper has better accuracy in quantitative analysis, and fully reflects its adaptive ability.
X-ray fluorescence spectrometry (XRF) is a powerful tool for the analysis of solid material. That is the reason why the technique was applied for the determination of rare earth elements (REEs) since about 1970. At present, energy-dispersive XRF and wavelength-dispersive XRF are used for the analysis of pressed powder pellets or fused Li-borate beads containing REEs. The production of reliable results can only be achieved by careful optimization of the parameter, in particular the selection of spectral lines. The quantification is based on a calibration realized by using reference samples. © 2017 Walter de Gruyter GmbH, Berlin/Boston. All rights reserved.
Brunswick No. 12 is a vent-proximal massive sulfide deposit that formed above a feeder pipe with associated hydrothermal alteration. Individual sulfide lenses show the following vertical and lateral zoning outward from the vent complex: massive pyrite-pyrrhotite-chalcopyrite core → interlayered pyrite-sphalerite-galena (Pb-Zn ore zone) → bedded pyrite. The feeder zone which underlies the Cu-rich core consists of an anastomosing network of pyrite-pyrrhotite-chalcopyrite-quartz veins with variable and generally minor contents of ferroan carbonate, Fe-rich chlorite, sphalerite, galena and arsenopyrite. Observations support a vent-proximal origin for the Brunswick No. 12 deposit and against a tectonic origin for the stringer zone. All of the evidence indicates that the feeder pipe formed during the ore forming hydrothermal event, but has been distorted and in some areas detached from the overlying massive sulfides during subsequent deformation. -from Authors
The Paleoproterozoic Flin Flon mining district is one of the world's most prolific volcanogenic massive sulfide (VMS) camps and includes a single stratigraphic interval that hosts the 85.5 million tonne (Mt) Flin Flon, 777, and Callinan Zn-Cu-(Au) deposits. Rapid seafloor burial of the VMS hydrothermal system by a thick succession of pillowed basalt resulted in the hanging-wall strata being affected to varying degrees by the still upward migrating fluids. This hanging-wall alteration hydrothermal fingerprint allows delineation of the regionally metamorphosed paleohydrothermal system, and its characterization has the potential to lead to discovery of buried, stacked, or structurally displaced mineralization. Evidence for the presence of continued seafloor hydrothermal activity above the Flin Flon-Callinan VMS horizon is observed in the pillowed flows, interlayered hyaloclastite-rich flow tops, and also within finely bedded interflow volcaniclastic sediment. A 30% to 60% metalliferous exhalative component was detected through geochemical and mineral analysis in interpillow volcaniclastic rocks, chert, and epidosite in the hanging-wall sequence. The regional distribution of Fe- and Mg-rich chlorite, epidote-clinozoisite, biotite-annite, actinolite-hornblende-ferrotschermakite, and stilpnomelane and albite-oligoclase modifies metamorphic isograds and defines discrete vertical fluid pathways controlled by synvolcanic growth faults and associated sill-dike swarms. Silica-enriched hanging-wall alteration zones are proximal to Fe-Ti basalt sills and occur as discrete hanging-wall zones parallel to the plunge of the 62 Mt Flin Flon deposit. Anomalous concentrations of Hg, Sb, Ag, Pb, Te, As, Au, and Bi form within these hanging-wall halo alteration zones, indicating migration of the more volatile metals present in the underlying VMS deposits. Synvolcanic depressions, dike swarms, and hydrothermal-metamorphic fluid corridors are detectable through trace element anomalies, trace mineral chemistry, and 18O isotope geochemistry. Oxygen isotope analysis of the Flin Flon-777-Callinan VMS hanging-wall strata defines a number of high δO18 anomalies extending 1,200 m above that indicate that <300°C subseafloor hydrothermal activity continued after burial of the massive sulfide deposits. Coupled with the geochemical and mineral chemical anomalies, this is indicative of the presence of continued, relatively low temperature hydrothermal fluid "leakage" from a robust seafloor hydrothermal event that generated the VMS deposits. A combination of techniques, including mineral chemistry, isotope, and trace element data, is demonstrated to be successful in identifying and delineating zones of hanging-wall hydrothermal alteration in greenschist- to amphibolite-grade metamorphic rocks of the Flin Flon mining camp. Use of these, coupled with mapping to define periods of quiescence as marked by horizons of sedimentary rocks in the hanging-wall basalts of the Hidden Formation, has the potential to lead to discovery of deeply buried deposits on the Flin Flon horizon or deposits at higher stratigraphic levels. Our findings indicate that the basaltic hanging wall on the Flin Flon-777-Callinan hydrothermal system was an efficient cap on the system, with vestiges of continued hydrothermal fluid flow detected in the interpillow and interflow components. These volumetrically minor components are critical sampling media and are pertinent to global exploration for detection of VMS mineralization buried beneath thick mafic volcanic sequences.
Three portable X-ray fluorescence (pXRF) methods were compared and tested in an exploration program using till in Sinoselkä, northern Finland. The use of one truck-mounted XRF unit and two handheld pXRF analysers were tested for basal till samples gathered using percussion drilling with a flow-through sampling bit. The datasets were compared to both conventional aqua regia based analyses and each other. The results prove that a correlation between the data generated by different pXRF methods was acceptable for some major (Ca, Fe) and most of the base metal elements (like As, Cr, Cu, Mn, Ni, Pb, Zn) in the Sinoselkä area. The pXRF analyses also correlatewell with the aqua regia geochemical data of the same elements. Distribution of the elements was comparable to the lithological changes in the underlying bedrock that indicates a short glacial transport distance. It is also demonstrated that more than absolute values, the relative values and their changes are those which should be considered and carefully examined. The results reported here emphasize the usefulness of pXRF analysers in till geochemical exploration and demonstrate that they involve easy and fast methods to collect geochemical data for tracing sources of multi-metal mineralization. Furthermore, pXRF is applicable in gold exploration, although indicator elements like As, Bi, Cu, Mn and Sb have to be used instead of Au. © 2016 The Author(s). Published by The Geological Society of London for GSL and AAG. All rights reserved.
This chapter is devoted to the theoretical aspects involved in the matter-radiation interaction process leading to the observed Raman spectra. These aspects include the description of the Raman effect as well as the principles of molecular vibrations. The first part is devoted to the Raman cross section which is the key parameter in understanding the spectral intensities and it is discussed in terms of the classical and quantum mechanical frameworks. The properties of the polarizability tensor are described in detail including polarization effects and selections rules. © 2012 the European Mineralogical Union and the Mineralogical Society of Great Britain & Ireland.
X-ray powder diffraction (XRD) is a well-established tool in the study of hydrothermal systems, as it allows for the identification and quantification of mineral assemblages. Through the quantification of alteration mineral assemblages geologists can characterize the geometry of a deposit or geothermal system and draw inferences regarding the fluid evolution and environmental conditions of deposition (e.g., pH, T). Traditionally, XRD devices have largely been restricted to laboratories; however, advances in XRD sample holders and X-ray sources have allowed for the development of portable XRD (pXRD) devices. This paper assesses the validity of the Olympus Terra pXRD instrument for qualitative and quantitative studies of hydrothermal systems through comparisons with data from laboratory XRD (Empyrean II diffractometer) and XRF techniques for both synthetic mixtures of natural minerals and a variety of samples from the Kulumadau epithermal gold deposit, Woodlark Island, Papua New Guinea. Diffractograms of synthetic mineral mixtures with known concentrations of quartz, kaolinite, muscovite, albite, and pyrite were analyzed quantitatively using the Rietveld-based Siroquant technique and showed good overall agreement for both devices, with the exception of muscovite, which encountered accuracy issues at concentrations <10% and a lower level of detection of around 5% for the pXRD. The Siroquant analyses of hydrothermally altered rocks based on XRD traces from the Empyrean II unit were confirmed by XRF data. Results from the pXRD unit for collection times of 5, 10, 20, and 40 minutes for representative samples spanning a range of rock types and alteration styles revealed that 5 minutes was sufficient for qualitative analysis, even of minor phases. Overall, the 5-minute collection time also yielded excellent quantitative results, but precision for minor mineral phases increased noticeably with increasing collection times. Quantitative mineral estimates for 20- and 40-minute data sets were compared directly to estimates made using the Empyrean II data and showed excellent correlation with R2 values of >0.92 for all major mineral phases (i.e., >5%). With the exceptions of pyrite and magnetite, minor to trace minerals (i.e., <5%) quantified using the Empyrean II XRD showed poor correlation with the pXRD data; however, the presence of these minerals was identified in greater than 50% of all cases. Due to its portability, robustness, minimal sample preparation, relatively fast collection times, and excellent correlation with laboratory-based XRD devices, the pXRD has been shown to be of great use for rapid acquisition of quantitative mineralogical data by the exploration geologist, allowing for more informed decisions during drilling programs.
The micas which are common rock-forming minerals include the white micas, muscovite, margarite, and paragonite, and the dark micas which may be broadly described as the biotites. These micas are extremely useful as petrogenetic indicator phases. This paper includes detailed discussions of the mineralogic and petrologic aspects of dioctahedral white micas and trioctahedral dark micas.