Editorial for the Special Issue “Development of W-Sn and Rare-Metal Metallogenic Systems in an Orogenic Belt”

Tungsten, tin, and rare metals are regarded as “strategic resources” or “Critical Raw Materials” (CRMs) by the European Commission and the U.S. Department of Energy [1,2,3]. They have been listed according to their Supply Risk and their Economic Importance (EI) for the industry. They are essential to advanced technologies involved in the industrial development of the 21st century, more specifically for the defense, medicine, communication, and renewable energy sectors. In particular, the importance of tungsten applications in industry (lamps, transistors, diodes, drilling and milling tools, components for aircrafts, fusion reactors, etc.) and its low substitutability are such that this metal has the highest economic importance of all raw materials. Niobium’s high criticality resides essentially in the refining of ferroniobium, a high-strength, low-alloy steel with good corrosion resistance and low density, which represents most of the global niobium demand. It is used in gas and oil pipelines, chemical reactors, car and truck bodies, ship hulls, railroad tracks, and many other high technology applications. Tantalum is mostly used in capacitors, surface acoustic waves for smartphones, computers, tablets, and as sheets both for furnace parts, owing to its very high melting point, and for prosthesis, thanks to its high biocompatibility.

This Special Issue represents a cross-disciplinary contribution covering many of the processes involved in the formation of W–Sn and rare metal deposits spatially related to granites or pegmatites. These range from metallogenic system descriptions, through granite and enclosing rock geochemistry, mineral chemistry, trace element geochemistry, geochronology of ore and gangue minerals, to stable isotopes and fluid inclusion studies, with the ambition of tracing the sources of ore components and fluids, determining the physicochemical parameters controlling metal transport, as well as the mechanisms of metal accumulation during the genesis of ore deposits. Out of this lot, one paper is devoted specifically to the structural analysis of Shear Zone-Type Gold Deposits.

The paper by Marignac et al. [4] opens the issue by establishing the time–space relationships between the world-class Panasqueira W(Sn) deposit and the associated concealed granite system, thanks to the recent availability of drill core samples. Remarkably detailed petrographic, mineralogical, and geochemical studies allow for establishing that the apical part of the Panasqueira pluton represents a layered sequence of a succession of four granite injections of the high-phosphorus peraluminous rare-metal granite type, which underwent intense polyphase alteration. One of the specificities of this approach is the interpretation of evolutionary trends in mineralogical–geochemical cationic diagrams in tight relation with the observations of the magmatic and hydrothermal mineral paragenesis variations, and that of the trace elements. It is also shown that hydrothermal alteration provided rare metals to the coeval quartz–wolframite veins.

Zhu et al. [5] describe a recently discovered atypical and very large tungsten deposit (66,500 t at 0.23% WO₃), the Shangfang deposit, occurring within Paleoproterozoic amphibolites, away from any contact with granitic bodies, contrarily to what is commonly observed in other occurrences. This study is also remarkable by the very good reproducibility in the age determinations obtained by a variety of isotopic systems; thermal ionization mass-spectrometer Sm–Nd isotope analysis of scheelite from the orebody yielded similar ages as previously published U–Pb ages on zircon from nearby granites and Re-Os ages of molybdenite from the mineralization. These ages also coincide with a 160–150 Ma age period corresponding to a major W–Sn deposition event occurring in the world-class Nanling Range Metallogenic Belt in southeastern China. Despite the spatial disconnection between the location of the ore deposit and the granites, these age determinations and the H–O isotopes demonstrate a genetic connection between them. Interestingly, the negative εNd(t) value of −14.6 obtained on the Shanfang scheelite suggests the involvement of deep crustal material in connection with the contemporaneous subduction of the paleo-Pacific plate that caused an extensional tectonic setting associated with the formation of the Shangfang granites.

Monnier et al. [6] identify three episodes of wolframite deposition and relate them to distinct stages of the evolution of the Variscan orogeny. The three episodes occurred within a relatively restricted area, the Echassières district in central France, a well-known site, particularly for the occurrence of the Beauvoir rare-metal granite [7,8]. The first generation is interpreted to crystallize before the Barrovian metamorphism that affected these early wolframite veins and the host micaschists. The second and major wolframite deposition generation occurred after metamorphism and before the intrusion of the outcropping Colettes and Beauvoir granites. The last generation formed a smaller volume, related to the greisen alteration developed within the Beauvoir granite.

The contribution of Yang et al. [9] is devoted to the genesis of two types of skarn cassiterite polymetallic deposits, from the southern Great Khingan Range in northeast China, formed in the Lower Cretaceous (Late Yanshanian). Sulphur and lead isotopic signatures show that the Dashishan Sn–Pb–Zn deposit results from the percolation of magmatic–hydrothermal fluids deriving from a fine-grained syenogranite, which is more fractionated and more peraluminous than the syenogranite associated with the formation of the Damogutu Sn–Fe skarn deposit. The iron enrichment in the Damogutu deposit results from an increase in oxygen fugacity during the retrograde stage of skarn formation, inducing magnetite crystallization. The higher fractionation of the Dashishan granite led to a decrease in Fe content in the magma and an increase in Pb and Zn contents, explaining the Sn–Pb–Zn mineralization associated with this granite.

Roza Liera et al. [10] develop an interesting mineralogical and geochemical comparison of barren (Panceiros) and Li–Sn–Ta-bearing pegmatites (Presqueira) from central Galicia for application in exploration. Mineralized pegmatites are characterized by higher Rb, Cs, and Li contents in muscovite and crystallization of montebrasite, but they have surprisingly lower P contents, contrarily to what is usually observed in Li–Nb–Ta–rich highly peraluminous pegmatite or granite occurrences [11,12]. Ta–Nb oxides from the mineralized body show the classical trend from columbite-(Fe) to tantalite-(Fe) and tapiolite-(Fe) typical of F-poor rare-metal pegmatites. Petalite is the primary Li-aluminosilicate to be partially altered into a spodumene–quartz assemblage. It is suggested that the Li–Sn–Ta mineralized pegmatite would derive from magmatic differentiation of the spatially related granites, although this interpretation requires further studies to be fully validated. These authors also show that the geochemistry of mica from the pegmatites can be used as a favorable exploration indicator to identify rare-metal pegmatites, although wall-rock geochemistry alone does not allow for the differentiation of barren and mineralized occurrences.

Lastly, the study by Cheng et al. [13] is a thorough structural analysis from the field scale to a microstructural one, focusing on features of quartz and feldspar deformation in several shear zones and associated gold deposits. One of the interesting aspects of this contribution is the use of electron backscatter diffraction for the study of quartz crystallographic preferred orientations to detail the deformation mechanism, dislocation and slip system, pressure, and temperature of the deformation, combined with fractal analysis of dynamically recrystallized quartz grains. The kinematic characteristics of the Muping-Rushan metallogenic belt have been selected as the main target. It is shown that the different shear zones present in this area were continuously active during the exhumation of the crustal block from mid-crustal to subsurface levels under a NW–SE extensional regime for a very extended period of time, that is, late Jurassic to early Cretaceous. Gold deposition occurred within quartz veins during the brittle deformation stage in association with important pressure fluctuations and a flash vaporization mechanism.

Overall, I hope this special issue will constitute a step forward in the understanding of the cycle of rare metals and enhance the scientific debate.