Ta and Nb are considered critical raw materials; due to their properties and potential applications in wide sectors. This study deals with Sn-Ta-Nb minerals from the Penouta mine (Orense, Spain), the only active mine in Europe producing tantalum minerals. These are obtained from mining wastes accumulated during old mining jobs in tailing ponds. The industrial processing flowsheet is based on successive gravimetric stages followed by low intensity magnetic separation to reduce ferromagnetic contaminants. Sn-Ta-Nb concentrate, with grades between 35-45% Sn and 4-7% Ta₂O₅ and Nb₂O₅, is obtained in this stage with plant recoveries around 60-70% respectively. A chemical-mineralogical characterization by size fractions, FRX and DRX was carried out to implement a size classification stage using a circular vibrating screen in the processing plant. The finest fractions, containing higher grades of well liberated Sn, Ta, Nb minerals, were the feeding for dry high intensity magnetic separation (DHIMS) multifactorial tests, while, coarse fractions were re-grinded to maximize performance. The good results obtained in these tests demonstrate that two products with commercial quality could be obtained, a cassiterite concentrate with grades between 70-78% SnO₂ and a tantalite-columbite concentrate with grades ranging between 12 and 14% Ta₂O₅ and Nb₂O₅, also increasing the overall recovery of the plant.

The Hera Au-Ag-Pb-Zn deposit of central NSW, Australia with a total resource of 3.6 Mt @ 3.3g/T Au, 25g/T Ag, 2.6% Pb and 3.8% Zn occurs on the SE margin of the Cobar Basin. It is hosted by the shallow marine Mouramba Group and overlying turbiditic Amphitheatre Group. The siltstones comprise various mixtures of quartz, plagioclase, muscovite-phengite, biotite and clinochlore, along with relatively abundant accessory titanite and ilmenite. The deposit comprises a number of discrete lodes which are steeply west-dipping and strike NNW. Each lode has different abundances of the main ore minerals sphalerite, galena, chalcopyrite, pyrrhotite and gold. The North Pod and Far West lenses have the most diverse ore mineralogy in addition containing arsenopyrite, native silver, gudmundite, Ag-tetrahedrite, acanthite, dyscrasite, native antimony, nisbite and breithauptite. Electrum (continuous spectrum from Ag-rich to Au-rich) is associated with sulfides in the main ore lenses while native gold occurs in the host rocks along cleavages and away from the main ore. Most of the deposit has experienced greenschist facies metamorphism with pervasive green chlorite alteration, though the North Pod differs in being distinctly Ag and Sb rich and has reached at least amphibolite facies metamorphism with a garnet-wollastonite-vesuvianite-tremolite assemblage. Tremolite is relatively abundant throughout a number of the other lodes suggesting widespread low-T skarn alteration. Cross-cutting pegmatites comprise quartz, plagioclase (labradorite-andesine) and microcline. Scheelite is relatively abundant in the upper levels of the deposit, most commonly in cross-cutting quartz veins.

The classical Al-in-hornblende barometer has been very successful in determining the depth of intrusion of metaluminous cordilleran granitoid plutons that bear the buffering assemblage at near solidus conditions: hornblende-biotite-plagioclase-orthoclase-quartz-sphene-two Fe-Ti-oxides (or one Fe-Ti oxide + epidote)-melt-vapor (e.g., [1-3]).

Ridolfi et al. [4] and Ridolfi and Renzulli [5] derived empirical amphibole-only barometric expressions that could be potentially applied to a larger number of phenocrystic assemblages from volcanic rocks. However, Erdmann et al. [6] claimed that these barometers are inaccurate and can give untenable estimates.

A graphical barometer based on the partitioning of Al and Si between amphibole and plagioclase was derived by Fershtater [7] using amphibole-plagioclase compositional pairs of rocks from the Urals. More recently, Molina et al. [8] calibrated an empirical expression based on experimental data that can be applied to igneous and high-grade metamorphic rocks.

In order to compare the reliability of amphibole-only and amphibole-plagioclase barometry, in this work, we test the performance of the expressions of Ridolfi and Renzulli [5] and Molina et al. [8], using an experimental data set compiled from the literature that has been recently published by Molina et al. [9].

In accordance with Erdmann et al.[6], the test reveals unsustainable pressure estimates with the amphibole-only barometric expressions from Ridolfi and Renzulli [5]. By contrast, the amphibole-plagioclase barometer from Molina et al. [8] performs well and yields a precision better than 1.7 kbar for Qz-Amp-Pl and Ol-free-Cpx-Amp-Pl assemblages with amphibole compositions having > 1 apfu (23O; normalisation to 13-CNK) Al, 0.05-0.27 apfu Ti: and < 1.07 apfu Fe³⁺.

References

1. Hammarstrom and Zen, 1986, American Mineralogist 71, 1297–1313.

2 Schmidt, 1992, Contributions to Mineralogy and Petrology 110, 304–310.

3. Anderson and Smith, 1995, American Mineralogist 80, 549-449.

4. Ridolfi et al., 2010, Contributions to Mineralogy and Petrology 160, 45–66.

5. Ridolfi and Renzulli, 2012, Contributions to Mineralogy and Petrology 163, 877–895.

6. Erdmann et al., 2014, Contributions to Mineralogy and Petrology 167, 1016

7. Fershtater, 1990, Geokhimiya 3, 328–335.

8. Molina et al., 2015, Lithos 232 286–305.

9. Molina et al., 2020; American Mineralogist, in press, https://doi.org/10.2138/am-2020-7400

The intermediate-sulfidation epithermal Kylo deposit is part of the Drake Goldfield of north-eastern NSW. The mineralization is gold-dominant with minor silver and significant levels of zinc, copper and lead. Kylo has a resource of 2.298 Mt @ 1.23g/T Au and 1.35 g/T Ag. Mineralization mainly occurs in the form of vein stockworks. Petrographic and SEM analysis show that there are at least three mineralization events, with Au mineralization strongly associated with at least a deposit-scale alteration event. Quantitative XRD analysis shows a strong positive correlation between Au-mineralization and argillic-phyllic alteration. Electrum was found as inclusions in massive sphalerite in the main mineralization stage. Correlation analysis for the assay data indicates that Au has a strong relation with Ag and moderate correlation with Pb. Petrographic and geochemical analysis has identified three lithologies, rhyolite, rhyodacite/dacite and andesite, with Au mineralization more associated with the rhyodacites, while at deeper levels some of the andesites also show a relatively strong correlation with Au. Sr shows a significant strong depletion, due to the intense and pervasive alteration at Kylo. The andesitic volcanics show moderate LREE enrichment with small negative Eu anomalies, and relative depletion in Nb, Ta and Ti, indicating an island arc tectonic setting. The carbon and oxygen isotopes of late-stage vein carbonates suggest that the fluid was derived from a magmatic source with a significant influence of low-temperature alteration, while the sulfur isotopes indicate a magmatic sulfur source.

The newly-discovered Federation deposit with a resource estimate of 2.6Mt @ 7.7% Pb, 13.5% Zn, 0.8g/t Au and 9g/t Ag lies 10km south of the Hera deposit within the Cobar Basin of the Lachlan Orogen. Located just north of the Erimeran Granite contact and between the Lower Amphitheatre Group and underlying shallow marine Mouramba Group Roset Sandstone, the host siltstones and sandstones have been brecciated, intensely silicified and chloritized close to mineralisation. Orientated in an overall east-northeast strike and with a steep south-southeast dip, the siltstones mainly comprise quartz, clinochlore, biotite and muscovite. Federation also has highly fragmented zones with breccia and vein-fill of calcite. Main ore mineralisation includes sphalerite and galena with lesser chalcopyrite, pyrrhotite, pyrite and very rare boulangerite. Mineralisation occurs within silicified veins and breccias of Zn-Pb sulfides which generally grade to moderate veining of sulfides with Pb and Cu dominant sulfides as well as minor veining of base metals associated with visible gold. Mineralisation throughout the deposit is fairly simple with a low diversity of minerals. Iron concentration varies throughout the deposit, decreasing towards the centre. Observations of massive sphalerite with gradations of red to honey-comb yellow indicate the transition from high Fe (7-10%) to low Fe (2-5%) within the coarse sulfide assemblages. Within the main mineralised corridor, fibrous amphibole inclusions within galena/sphalerite assemblages are observed at 251m as well as epidote associated with sulfides at 573m. Other ore associated gangue minerals include siderite, scheelite, magnetite, apatite and rutile.

In this study, fuzzy logic modeling was applied to integrate the thematic layers produced by Principal Component analysis (PCA) technique for generating Pb-Zn mineral prospectivity maps using Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and WorldView-3 satellite remote sensing data. A spatial subset zone of the Central Iranian Terrane (CIT), Iran was selected in this study. For the processing of the satellite remote sensing datasets and producing alteration thematic layers, PCA techniques was implemented. The PCA4, PCA5 and PCA8 were selected as the most rational alteration thematic layers of ASTER for generating prospectivity map. The fuzzy gamma operator was used to fuse the selected alteration thematic layers. The PCA3, PCA4 and PCA6 thematic layers (most rational alteration thematic layers) of WV-3 were fused using fuzzy AND operator. Field reconnaissance, X-ray diffraction (XRD) analysis and Analytical spectral devices (ASD) spectroscopy were carried out to verify the occurrence of alteration zones and high prospective locations of Pb-Zn mineralization in the study area. Subsequently, mineral prospectivity maps of the study area were generated showing high potential zones of Pb-Zn mineralization.

The intermediate-sulfidation epithermal Strauss deposit is part of the Drake Goldfield of north-eastern NSW. The mineralisation in this deposit is gold-dominant with minor silver and significant levels of zinc, copper and lead. Strauss has a combined total resource of 2.66 Mt @ 1.5g/T Au and 1.9g/T Ag. Mineralisation mainly occurs in the form of vein stockworks of three main orientations. Structural analysis indicates these to be the result of two fault system events and one conjugate fault system. Petrographic and SEM analysis shows that there are two separate mineralisation styles, a pyrite dominant and sphalerite dominant style, with at least three mineralisation events. Correlation analysis for the assay data indicates that Au has a strong relation with Ag and moderate correlation with Pb. Petrographic analysis has shown the deposit to be made-up of varying layers of dacitic ash flow tuffs and a cross-cutting lamprophyre dyke though previous authors classified some of the tuffs as intrusive andesites. XRD analysis indicates widespread argillic-phyllic and propylitic alteration. Silver tellurides have been found as late-stage grains associated with sphalerite mineralisation. Sr shows a significant strong depletion, due to the intense and pervasive alteration at Strauss. The carbon and oxygen isotopes of vein carbonates suggest that the carbon from the late-stage carbonates was derived from a magmatic source with a significant influence of low-temperature meteoric water alteration, while the sulphur isotopes indicate a magmatic sulphur source.

Statistically, outliers are data points that are dissimilar to the whole dataset beyond stated limits. These existing outliers may give rise to misinterpretations in statistical and geostatistical analyses. To detect outliers two methods of (1) boxplot as a representative of statistical methods and (2) a combination of Mahalanobis Distance (MD) and network graph as a representative of geostatistical methods are applied. Variograms are the basis of geostatisticsal analysis which evaluate spatial variability. After application of variograms, pairs of data points are taken to draw H-scatter plots. In the H-scatter plots, data are illustrated through specific distances. In this case, lags of 20 meters are applied to the h-scatter plot. Then, mahalanobis distance and 97.5% confidence interval, taken from chi-square distribution, are applied to the h-scatter plot to detect pairs of outliers. In order to consider geospatial relation of each pair, a network graph is designed which counts the number of edges for each node. The number of edges demonstrates the outliers and their neighbouring nodes which the outlier detection is based on. The mentioned process, applied to the oxide zone of Sarigunay epithermal gold deposit in Iran, results in 286 data points detected as outliers throughout an 11945 sample dataset in which the ratio of outliers to raw data is 2.39%. The boxplot drawn for the raw data indicates the cut-off assay of 10 ppm Au. Substantially, combination of statistical and geostatistical outlier detection methods leads to robust variograms and more precise estimation.

In the present work, a chemical and mineralogical study of high purity bentonites has been done. The mineralogical characterization has been carried out by means of X-ray diffraction (XRD) and visible-near infrared and short wave spectroscopy (VNIR-SWIR). The chemical characterization has been conducted by Activation Laboratories Ltd., Ontario (Canada), using FUS-ICP to analyse mayor elements. The aim of this work was to determine the relationship between the spectral response and the chemical elements related to the isomorphic substitutions in the smectites’ sheets.

According to the X-ray study, the 7 samples studied are bentonites with high purity mainly composed of smectites (5 dioctahedral and 2 trioctahedral) and a very low proportion of impurities such as quartz or plagioclase. The VNIR-SWIR study showed the characteristic absorption features of the phyllosilicates. Furthermore, small differences between samples in the wavelength range studied were observed.

For the statistical analysis of the data, the second derivate of the spectra was calculated to obtain the position and the intensity of the absorption features. Wavelength-ranges were defined for each absorption feature. The statistical treatment of the intensity and position of the defined features together with the percentage of the oxides of the major elements of the samples showed significant relationships between the different absorption features and elements of octahedral sheet.

A detailed characterization of some minerals of the kaolinite group has been done. The
mineralogical and structural characterization has been conducted by X-ray diffraction (XRD)
together with the study of the spectroscopy response in the visible-near infrared and short wave
(VNIR-SWIR), and the main objective was the determination of kaolinite polytypes. For this
purpose, 13 samples were selected from a wide group of samples.

X-ray diffraction patterns confirm the high purity of all the samples which only present scarce
amounts of quartz or feldspar in certain cases. The detailed study of XRD patterns allows
identifying characteristic reflections and small features of each polytype as well as to obtain
crystallinity indices for each sample. The statistical analysis of the patterns groups the samples
according the kaolinite polytype in 5 kaolinites, 2 dickites and 6 halloysites.

The spectroscopic study (VNIR-SWIR) of kaolin samples shows different features in the
wavelength range studied with the ASD FieldSpec 4 Standard-Res. Groups of diagnostic peaks
for kaolinite, dickite y and halloysite polytypes have been identified in the second derivative of
the spectrum, of which position and intensity have been treated statistically with the aim of
classifying the spectra according to the polytype. In good agreement to the XRD results, the
statistical analysis of the spectroscopic data, both by cluster analysis and by principal
components analysis (PCA), allow an unequivocal classification of the samples according to the
polytype from their VNIR-SWIR spectra.