Since 2010, the global drive for clean energy and industrial metals led to the delineation and shortlisting of a number of “critical” metals by the European Union [1
]. Of the 26 metals listed in the 2017 report, high-tech metals, such as W, Nb and Ta, are of significant importance to the European manufacturing industry. Recent exploration for these metal deposits has been ongoing for more than two decades and has been predominantly focused around known prospective European Variscan belts, such as the Erzgebirge and Cornwall, and characteristic S-type granite provinces therein [2
]. A recent regional geochemical reconnaissance sampling campaign, however, identified I-type granites of the lesser known central Vosges Mountains near Sainte-Marie-aux-Mines (France) to be prospective for W and Li-Cs-Ta [3
]. Similarly, an investigation of Bureau de Recherches Géologiques et Minières (BRGM) public domain data [4
] led to the delineation of a distinct W anomaly in the Natzwiller Granite, which is located in the northern domain of the Vosges Mountains. The northern Vosges Mountains comprise a series of late Devonian–Permian intrusions which show a distinct development from primitive mantle to highly fractionated peraluminous melts and the emplacement of S and I–S-type granites. Detailed studies into the petrology and geochronology of the magmatic suites allowed the reconstruction of a complex magmatic system and evolution of tectonic processes during the Variscan Orogeny [6
]. The present study aims to investigate the granite-hosted W, Li, Nb and Ta mineralisation potential of the northern Vosges Mountains and provides a link between chemical and mineralogical analyses of regional stream sediment samples and magmatic processes outlined in previous studies. The application of automated mineralogical techniques (QEMSCAN®
) to stream sediment samples furthermore aims to demonstrate the usability of this technique in early stage mineral exploration and therefore adds to the limited literature available on the subject [8
]. Consequently, this paper provides an economic perspective to previous research carried out on the northern Vosges magmatic suite.
In order to obtain an independent dataset and a comprehensive geochemical analysis suite characterised by low detection limits, 20 stream sediment samples (samples 1–20) were collected from first and second-order streams in the Hohwald, Natzwiller, Schirmeck and Grendelbruch areas (Figure 1
b). The sample locations were principally designed to test distinctive structural and geochemical trends identified from previous BRGM investigations [5
] as well as to characterise the different intrusive units across the northern Vosges. The sampling strategy mirrors the workflow and orientation study described in [3
]. Samples were collected from stream traps, such as in the lee of large boulders or on point bars, and sieved to retain the <2 mm fraction in the field. The resulting material yielded average weights of 500 g per sample. Samples were stored in plastic bags, zip-tied and labelled with sample ID, coordinates and elevation information. The fine fraction was allowed to settle in the bag before excess water was poured back into the stream. After each sampling location, the equipment was thoroughly cleaned to prevent cross-contamination. A detailed list of stream sample attribute data (colour, grain and mesh size, contamination, trap type, etc.) was recorded on an iPad using ESRI’s ‘Collector for ArcGIS’ app (Version 19.0, ESRI, Redlands, CA, USA). Daily data quality checks and synchronisation with a master database ensured that the data quality was consistent throughout the sampling campaign. Detailed observations and comparisons of drainage sediment composition, outcropping adjacent lithologies and heavy minerals present in pans were noted and supported the lithological classification of samples, along with the identification of fractionated lithologies; for example, quartz or pegmatite-rich rock. Linking observations of stream sediments and adjacent outcrops confirmed that stream sediments appeared unweathered and accurately represent the overall bedrock geology of respective catchment areas, and therefore can be used for further representative lithogeochemical investigations. While glaciation occurred in central–western Europe and affected parts of the central–southern Vosges Mountains, the only glacial debris related to the last glacial event (Weichselian) are recorded in the Bruche River valley. No evidence of glacial sediment was encountered during fieldwork.
Samples were returned to Camborne School of Mines, University of Exeter (UK) laboratories and dried in laboratory ovens at a constant temperature of 40 °C. The samples were then sieved using a sieve stack and a Pascal Sieve Shaker to isolate the <75 µm fraction which was previously identified to be host to W, Li and Cu anomalies in the Vosges [3
]. Individual fraction weights were determined to assess if sample loss had occurred. All samples underwent ICP-MS analysis using a standard four acid digest (HCl-HF-HNO3
) and Agilent 7700 ICP-MS instrument (Agilent Technologies, Santa Clara, CA, USA), so that, compared to historic studies, a wider range of trace and pathfinder elements, including Nb, Ta, Li, Hf, could be obtained, aiding the determination of fractionation trends (Table 2
). Quality control was ensured by inserting silica blanks, duplicate samples and OREAS 147 and 751-certified reference materials into the sample stream, with no accuracy issues noted outside +/− 1 standard deviation of the certified mean value. In a subsequent geochemical interpretation, following a previously published approach in the central Vosges [3
] and southeast Ireland [30
], the additional ICP-MS assays supported the usage of multi-element major and trace geochemistry in classifying lithological populations and petrogenetic and mineralisation processes. The classification of lithological units was achieved by delineating population clusters in bivariate geochemical plots. Each sample point was assigned a lithology, which was refined using geological observations in upstream catchment areas, outcrops and float in order to better represent subtle nuances in geochemical composition, such as “Natzwiller/Belmont Granite with visible cassiterite”.
Of the 20 stream sediment samples (Samples 1–20) undergoing ICP-MS analysis, nine samples (2, 3, 14, 15, 16, 17, 18, 19, 20) of the <75 µm sample fraction were selected for mineralogical analysis. In addition, two rock samples (17A and 18A) were collected within a 10 m distance upstream of their corresponding stream sediment sample locations (17 and 18), and a mineralogical analysis (QEMSCAN®
) was performed on uncovered polished thin sections of these samples. Mineralogical analysis was conducted on the fine <75 um stream sediment fraction, which is considered homogeneous and allowed a direct comparison between mineralogy and geochemistry. The samples were prepared into 30 mm diameter epoxy resin mounts and mixed with pure graphite powder to reduce settling bias and separate particles. The sample surface of the cured mounts was carefully ground to expose the particles and polished to a 1 micron finish using diamond media, then carbon-coated to 25 nm thickness. Samples were analysed using a QEMSCAN®
] at Camborne School of Mines, University of Exeter, UK. Sample measurement used iMeasure version 4.2SR1 software for data collection and iDiscover 4.2SR1 and 4.3 software for data processing. The Particle Mineral Analysis (PMA) measurement mode was used to map particles at a resolution (pixel spacing) of 2 µm, field size of 600 µm (300 × 300 square, magnification of ×111), default of 1000 X-ray counts per analysis point and a target of 10,000 particles per sample. The final number of particles mapped per sample was higher than this (up to 14,556) due to the system completing the particles in the field it was on when it reached its 10,000-particle target. The number of analysis points per sample varied from 900,000 to 4 million.
The data collected during measurement were processed using a modified version of the standard LCU5 SIP (database), following and building upon details outlined in Section 7 of Rollinson et al. (2011) [35
]. Both mineral area-% and mineral mass-% (density weighted) data were produced, and it was decided to use the mineral mass-% data as they better reflect the economic mineral content of the samples. As these were stream sediments, the focus was on both the major minerals and trace or unusual minerals, with the SIP customised to reflect the mineralogy of samples. This included checking the mineral identification not just from the measured chemical spectra, but also against in-house mineral reference standards. Moreover, checks were also completed for possible Li minerals following the method developed at Camborne School of Mines during the FAME EU Horizon 2020 project [36
]. In the case of identified Ta-Nb minerals, independent SEM-EDS checks were also conducted.
The graduated point symbol and bivariate fractionation plots demonstrate a distinctive endowment of base metals and high field strength elements (HFSE) in the northern Vosges magmatic suite, with a particular emphasis on the Natzwiller and Kagenfels Granite suites. The geochemical trends observed in these plots are comparable with previous regional reconnaissance sampling [5
] and whole-rock geochemical data presented in Tabaud et al. (2014) [6
], particularly for Th, Sr, Rb and Ti, which provide a tool to distinguish intermediate and felsic magmatic rocks. The application of univariate anomaly mapping and the lithogeochemical classification of catchment sediments, therefore, supports the delineation of areas of increased enrichment of economically sought-after metals and corresponding magmatic lithologies. In particular, the present data confirm the known presence of Nb and Be and outline additional, previously unrecognised W anomalies in Kagenfels Granite.
Earlier empirical mineral and whole-rock geochemical studies have successfully demonstrated the use and application of magmatic fractionation ratios in defining late-stage magmatic melts prospective for Sn-W and Li-Cs-Ta mineralization [27
]. Decreasing K/Rb, Nb/Ta, and Zr/Hf ratios indicate the increasing fractionation of the granitic melt and a transition to hydrothermal alteration [27
]. Numerical changes in these ratios during late-stage magmatic fractionation are a result of the substitution of K with Rb in micas and feldspars [44
], fractionation of Nb over Ta due to secondary muscovitisation and hydrothermal sub-solidus reactions enriching Ta in F-rich residual melts [29
], and increasing Kd values of Hf in zircon, which are only weakly influenced by secondary fluid-related processes [28
]. Recent geochemical studies of the Leinster Granite (Ireland) and Central Vosges Mg-K granites [3
], along with mineralogical results of this study, have shown that these petrogenetic ratios are equally applicable to determine highly fractionated lithologies using geological materials affected by secondary dispersion processes, such as stream sediments. The lack of significant weathering of K, Rb, Sn, Nb and Zr-bearing silicate mineral phases in the analysed stream sediment fraction that were collected as well as the representativity of these stream sediments in relation to mapped and sampled outcrops of the catchment area confirm that these petrogenetic ratios can be employed to fingerprint fractionation patterns in the study area. In this context, the Natzwiller Granite shows fractionation ratios of 101 < K/Rb < 128.8, 12.3 < Nb/Ta < 12.7, and 33 < Zr/Hf < 36 (Figure 3
i–k), along with the generally highest Ti concentrations of 8640–10,500 ppm (Figure 5
) evidenced by abundant rutile, ilmenite and titanite in the samples. Of particular interest are the elevated concentrations of incompatible elements, such as Li (105.1 ppm), W (34.81 ppm), Ta (8.1 ppm), B (21.99 ppm) and Be (13.47 ppm) in sample 2 along with abundant tourmaline (8.48%). In addition, despite a relatively low concentration of Sn (17.98 ppm) being measured in sample 20, automated mineralogical techniques identified multiple cassiterite grains in the stream sediment sample. This evidence suggests that the Natzwiller Granite has experienced, at least locally around NE–SW trending fault zones, the introduction of a highly fractionated melt enriched in fluxing elements, which allows incompatible and HFS elements to remain in late-stage, low-temperature melts [29
]. Tabaud et al. (2014) [6
] previously described the Natzwiller Granite as sub-aluminous to weakly peraluminous in nature, resulting from partial melting of an enriched mantle source and subsequent interaction with subducted metasedimentary and metaigneous crustal source material. Therefore, despite the comparably lower fractionation grade, the Natzwiller Granite was able to retain incompatible elements of possible economic interest.
In contrast, the peraluminous S-type Kagenfels Granite typically displays low values of 64.5 < K/Rb < 119, 14 < Nb/Ta < 17.7, and 19 < Zr/Hf < 38 (Figure 3
i–k), and therefore demonstrates a high degree of magmatic fractionation and secondary muscovitisation, characteristic of peraluminous S-type granites [27
]. Geochemical fractionation trends (Figure 5
), PC analysis (Figure 6
) and mineralogical evidence in the form of tourmaline, muscovite, chlorite, wolframite, cassiterite, columbite and ilmenorutile imply a peraluminous evolution and confirm a highly fractionated and locally hydrothermally altered nature of the Kagenfels Granite. Strong fractionation of the melt, along with a predominant NE–SW structural control in the Kagenfels Granite, led to the emplacement of pegmatitic quartz-feldspar (-beryl) veins at “Grotte des Partisans” [23
] and the Barembach stream confluences observed in the present study. The same process also resulted in characteristic elemental concentrations of As (25.52–92 ppm), Cu (7.88–166.61 ppm) and incompatible elements, such as Be (10–21 ppm), Sn (7.19–26.29 ppm), Li (12.13–52.61 ppm), and W (6.94–102.56 ppm). However, distinctive high values of Nb/Ta > 17 and the general absence of Li concentrations of >45 ppm across the western part of the Kagenfels Granite (Figure 3
h,j) suggest that, locally, magmatic fractionation and hydrothermal alteration processes were not as pronounced as in the eastern part of the Kagenfels Granite, which yields higher concentrations of Be (24–34.22 ppm), and Li (49.35–54.77 ppm), at 16.4 < Nb/Ta < 16.8, 64.5 < K/Rb < 93, and 19 < Zr/Hf < 25 (Figure 3
h–k). The comparatively higher Nb/Ta ratios in the western Kagenfels Granite are a result of the predominant occurrence of Mn-columbite, euxenite-(Y), and Nb-rich titanium oxide/ ilmenorutile as evidenced in stream sediment sample 17 and 18. Consequently, the western part of the Kagenfels Granite predominantly produced a mineral assemblage with Nb > Ta-rich minerals, indicating the preferential fractionation of Nb over Ta-rich minerals and, therefore, the locally lower fractionation of the granitic melt. In a regional context, these observations imply that the Kagenfels Granite is the most fractionated granite suite in the northern Vosges Mountains, and consequently represents a prime target to explore for granite-hosted mineralisation.
The application of automated mineralogical techniques, such as QEMSCAN®
and manual SEM-EDS, supported the routine collection of stream sediment samples in a mineral exploration context. These techniques were not only able to identify the bulk mineralogical composition of the stream sediment samples and therefore link geochemical signatures to source mineralogy, but also provide potential information about element deportment characteristics (ilmenorutile and columbite as principal Nb hosts, wolframite as principal W host, cassiterite as principal Sn host) for mineral processing-related studies. Therefore, this study, along with previous investigations into heavy mineral ilmenite deposits in India [8
], demonstrates the usefulness of automated mineralogical techniques in early-stage exploration campaigns, which often predominantly involve the routine collection and analysis of stream sediment, soil and till samples and do not necessarily involve the link between sample geochemistry and mineralogical response and element deportment.