Grundmann, G., Förster, H.-J

Originally published as:

Grundmann, G., Förster, H.-J. (2018): The Sierra de Cacheuta Vein-Type Se Mineralization, Mendoza Province, Argentina. - Minerals, 8.

DOI: http://doi.org/10.3390/min8040127 minerals

Article The Sierra de Cacheuta Vein-Type Se Mineralization, Mendoza Province, Argentina

Günter Grundmann 1 and Hans-Jürgen Förster 2,* 1 Eschenweg 6, DE-32760 Detmold, Germany; [email protected] 2 Helmholtz Centre Potsdam German Research Centre for Geosciences GFZ, DE-14473 Potsdam, Germany * Correspondence: [email protected]; Tel.: +49-0331-288-28843

Received: 14 February 2018; Accepted: 15 March 2018; Published: 22 March 2018

Abstract: The Sierra de Cacheuta vein-type Se mineralization in the Mendoza Province predominantly consists of clausthalite, klockmannite, eskebornite, eucairite, and naumannite. These primary selenides formed in a fault zone, cutting through fine-grained trachytic host rock. Cross-sections perpendicular to the veinlets, polarized light microscopy, and scanning-electron microscopy, combined with electron-microprobe analysis, provide a record of the relationship between different crystallization and deformation events. Mineralization encompasses four episodes of fault formation (d1–d4): early zonal selenide crystallization (stage (I)); ductile deformation of the selenides (stage (II)); fault re-opening, fluid-mediated metal mobilization, metalliferous-fluid infiltration, and mineral precipitation (stage (III)); and subsequent alteration (stage (IV)). The Se vein originated from multiple injections of highly oxidized, metal-rich fluids. These low-T solutions (estimated max. temperature ◦ 100 C, max. pressure 1 bar) possessed high to exceptionally high Se fugacities (log f Se2 between −14.5 and −11.2) that prevailed for most of the evolution of the deposit. The source of the Se and the accompanying metals (Cu, Ag, Pb, and Fe) is probably the neighboring bituminous shale. The deposition of Se minerals occurred when the oxidized metal-bearing solutions came in contact with a reductant, which caused the reduction of mobile selenate to immobile selenide or elemental Se. We identified several features that permit us to safely distinguish samples from Cacheuta from Argentinian Se deposits in the Province of La Rioja: (I) trachytic host rock fragments containing bitumen and TiO2 pseudomorphs after titanomagnetite; (II) early Co-rich and Ni-poor krut’aite (Co < 6.7 wt %, Ni < 1.2 wt %) partly replaced by clausthalite, umangite, klockmannite, eskebornite, Ni-poor tyrrellite (Ni < 2.7 wt %), Ni-poor trogtalite (Ni < 1.2 wt %), and end-member krut’aite and petˇríˇcekite;(III) lack of calcite gangue; and (VI) Se-bearing alteration minerals comprising chalcomenite, molybdomenite, cobaltomenite, an unnamed Cu selenide (for which the ideal formula may be either Cu2Se3 or Cu5Se8), and possibly mandarinoite, mereheadite, orlandiite, and scotlandite as new species for this occurrence.

Keywords: selenium mineralization; trachytic host rock; tyrrellite; trogtalite; krut’aite; petˇríˇcekite; genetic sequence; Sierra de Cacheuta; Argentina

1. Introduction and Geological-Historical Background In the South American Andes, the provinces of La Rioja and Mendoza in Argentina host several mineralogically important selenium occurrences [1]. Modern research on Argentinian selenium deposits has mainly focused on the study of rare or new Se species, the determination of their chemical composition, and the deﬁnition of their crystal structure [2–7]. Minor effort has been spent studying in detail the origin of the various selenide occurrences, notably the relationship between the crystallization and deformation of the selenium mineralization and its host rock. This particularly holds true for the selenium mineralization in the Sierra de Cacheuta district, also known as el Cerro de Cacheuta,

Minerals 2018, 8, 127; doi:10.3390/min8040127 www.mdpi.com/journal/minerals Minerals 2018, 8, 127 2 of 22 in the Mendoza Province. Sierra de Cacheuta is the type locality of achavalite FeSe, chalcomenite CuSeO3·2H2O, cobaltomenite CoSeO3·2H2O, and molybdomenite PbSeO3. Nowadays, the location of the Sierra de Cacheuta Se occurrence is not precisely known; it has never been recovered [1,8]. It was discovered sometime between 1860–1862 [9]. In the earliest and most detailed description of the area, the following information was provided [10]: The occurrence encompasses several small selenide veinlets ranging in thickness from 1–2 mm to a maximum of 4 cm. The main selenium minerals were clausthalite and copper selenides. Near the surface, the ore veins contained Ag contents of up to 21 wt %. At depth, the Ag content decreased rapidly down to almost zero at only 12 m depth—the reason why the mining activity had been terminated. Geographically, the occurrence was located on the right bank of the Rio de Mendoza, where it came from the Sierra de Cacheuta in the West [9,10]. The mines were on the southern slope of the mountain range, a few leguas (miles) from the river. The slope was built up by andesitic to trachytic rocks, which had vesicular cavities locally filled with calcite or agate. The volcanic host rocks varied in texture from stratified to massive and were irregularly jointed. Stelzner examined the post-volcanic selenium veinlets in several small galleries, which were mined at different heights on the slope. On the surrounding dumps, he found only two little veinlet fragments, consisting mainly of granular ankerite and minor ore minerals. At the bottom of the mountain range, sandy, calcareous, or marly sediments occurred, with intercalations of bituminous shale and local asphalt enrichment. Brackebusch (1893) [11] also mentioned bitumen (“Erdpech” in German) as a constituent of the Se veins. Ramdohr (1975) [12] identified the following main ore minerals: klockmannite, clausthalite, eukairite, umangite, naumannite, accessory ferroselite, and two unspecified cobalt selenides. The sulfides were chalcopyrite and pyrite. The first black and white photomicrographs and mineral reflectance and compositional data, as well as a crystallization sequence of selected selenides from Cacheuta, were erroneously published under the locality name “Los Llantenes, La Rioja, Argentina” [2]. This mistake was later noted by [1], who also provided new color photomicrographs and mineral-chemical data for the same sample (No. #49). Confusion regarding the Argentinian Se occurrences may be frequent, considering that even the newest online encyclopedias (e.g., mindat.org; Mineralienatlas; Wikipedia) and books bear inconsistencies in terms of the mineral content, macrostructure, microstructure, and chemical composition for the individual Se occurrences. Our examination of 16 polished sections prepared from 11 “historic” Se-vein fragments (Collection F.N. Keutsch, Harvard University, Cambridge, MA, USA; labeled as “Cacheuta”, or “Cerro de Cacheuta”, or “Sierra de Cacheuta”) permits us to elaborate upon a comprehensive paragenetic scheme for this mineralization. Application of special sample preparation techniques permitted an in-depth study of the internal structure of the different types of Se-veinlets discriminated at Cacheuta. We report new microstructural and chemical data for the selenium vein mineralization, also addressing its host rock. We provide a genetic model that considers the relationship between the crystallization and deformation of the primary and secondary minerals, and we discuss possible mechanisms for the formation of that unique Se occurrence. Finally, we discuss diagnostic features that allow us unequivocally to distinguish samples from Cacheuta from those of the Se occurrences in the Province of La Rioja in Argentina.

2. Methods Petrographic and microstructural studies were mainly performed by optical microscopy (Leitz DMRM polarizing microscope, Type 301-371-010, Leitz/Leica, Wetzlar, Germany). Information on microstructure was obtained from Se-vein fragments cut and polished perpendicular to the vein wall and selvages. Sample preparation involved the following machinery and materials: lapping and polishing machine type “Buehler Metaserv”, Buehler UK Ltd., Coventry, UK; grinding and polishing machine type “Kent MK 2A”, Engis Ltd., UK; “Pellon” polishing chemotextile type PAN-W, Hartfeld & Co., Allerod, Denmark; diamond suspension, water soluble (grain sizes 6 µm, 3 µm, 1 µm, 0.25 µm) Minerals 2018, 8, 127 3 of 22 type Gala-Tec GmbH, Kaiserslautern, Germany; SiC grinding powder (600, 800, 1000, 1200 mesh), ﬁxed abrasives are steel discs, consisting of diamond in steel-binding (grain sizes 600, 800, 1200 mesh). For the surface treatment of the vein sections, two different preparation methods were used:

(a) Rolling grinding (loose abrasive grains). In this case, the surface is velvet. In dispersed-light dark field illumination, the internal structures (like foliation and fine folding) of the opaque metallic minerals are clearly visible. (b) Polishing (bonded abrasive grains). In this case, the surface is polished. In dispersed-light dark field illumination, the polished opaque metallic minerals appear black.

Selenides and sulﬁdes were routinely checked for concentrations of Cu, Ag, Pb, Hg, Fe, Co, Ni, Zn, As, Sb, Bi, S, and Se. Quantitative chemical analyses were conducted in WDS mode, using a JEOL electron probe X-ray microanalyzer JXA-8230 (JEOL, Akashima, Japan). The probe was operated at 20 kV, 20 nA; the beam size was 1–2 µm. The counting time on peak was 20 s, with half that time on background on both sites of the peak. The following standards, emission lines and analyzing crystals (in parentheses) were used (detection limits in ppm in brackets): Cu [200]—natural eskebornite, Kα (LIFL); Ag [300]—natural bohdanowiczite, Lα (PETJ); Pb [500]—natural clausthalite, Mα (PETH); Hg [800]—cinnabar, Lα (LIFL); Fe [150]—pyrite, Kα (LIFL); Co [180]—cobaltite, Kα (LIFL); Ni [200]—pentlandite, Kα (LIFL); As [300]—cobaltite, Lα (TAP); Sb [300]—InSb, Lα (PETJ), Bi [450]—synthetic Bi2Se3, Mα (PETH); S [100]—chalcopyrite, Kα (PETJ); and Se [700]—natural naumannite, Kα (LIFL). The CITZAF routine in the JEOL software, which is based on the φ(ρZ) method [13], was used for data processing. Selenates were studied by semi-quantitative EDS analysis using the same device.

3. Results

3.1. Petrography of Trachytic Host Rock Lens-like trachytic host rock fragments of up to 2 cm in size are present inside the Se veinlets, forming a rubble breccia and/or boudinage (Figure1). The altered fragments are generally stained greenish, bluish, reddish, and/or brownish by finely dispersed secondary minerals, such as malachite, celadonite, azurite, chalcomenite, hematite, and goethite (Figure2a). The grain size of the fine-grained, non-oriented silicate matrix ranges between 0.1 and 2 mm. The groundmass shows an intersertal texture with twinned feldspar laths and interstices filled with greenish alteration products (celadonite pseudomorphs?) of pre-existing mafic phenocrysts (amphiboles). Compression caused brittle deformation, producing a finely branched network of fractures and foliation planes throughout the matrix, occasionally forming lens-like or boudinaged fragments (Figure1). Areas of the strongest foliation are stained greenish by malachite or brownish by goethite (Figure2a). Optical polished-section analysis (Figure2a,b) revealed the following modal mineralogy (estimated volume percentages in parentheses): feldspar (~85), clay (~10), celadonite (~4), and accessory components (~1), including reddish or yellowish pseudomorphs after titanomagnetite phenocrysts (average size 70 µm). These pseudomorphs consist of a framework of oriented leucoxene or hematite as alteration products, likely formed by oxidation of the original titanomagnetite containing oriented ilmenite laths. Single crystals and twinned or radially grown crystal aggregates of ferroselite were rare and confined to larger rock fragments (Figure3). Irregular black patches of bitumen (up to a few millimeters) occurred as fracture fillings, cementing the host rock fragments from the vein wall inwards (Figure4). Minerals 2018, 8, 127 4 of 22 MineralsMinerals 2018 2018, ,8 8, ,x x FOR FOR PEER PEER REVIEW REVIEW 44 of of 22 22

FigureFigure 1.1. DispersedDispersed--lightlight darkdark fieldfield imageimage ofof SeSe--veinvein TypeType--44 (2.2(2.2--cmcm--thick,thick, 6.66.6--cmcm--wide,wide, embeddembeddeded Figure 1. Dispersed-light dark ﬁeld image of Se-vein Type-4 (2.2-cm-thick, 6.6-cm-wide, embedded in inin resin, resin, surface surface rolling rolling grinded, grinded, with with 1.200 1.200 mesh mesh SiC SiC abrasive) abrasive), , with with layers layers and and lenses lenses of of resin, surface rolling grinded, with 1.200 mesh SiC abrasive), with layers and lenses of klockmannite klockmanniteklockmannite (dark (dark grey)grey) and and clausthalite clausthalite (light (light grey)grey) forming forming a a strongly strongly foliated foliated and and folded folded (dark grey) and clausthalite (light grey) forming a strongly foliated and folded mélange (cf. mélangemélange (cf.(cf. SectionSection 3.2.1)3.2.1). . TheThe sspottypotty andand lenslens--likelike aggregatesaggregates ofof thethe trachytictrachytic hosthost rockrock fragmentsfragments Section 3.2.1). The spotty and lens-like aggregates of the trachytic host rock fragments are partly areare partly partly rotated; rotated; thethe veinvein selvage, selvage, host host rockrock fragmentsfragments and and diagonal diagonal fractures fractures are are stained stained by by rotated; the vein selvage, host rock fragments and diagonal fractures are stained by various alteration various alteration products, such as goethite (dark brown), azurite and chalcomenite (blue), and products,various alteration such as goethite products (dark, such brown), as goethite azurite (dark andchalcomenite brown), azurite (blue), and and chalcomenite malachite (green). (blue), and malachitemalachite (green). (green).

Figure 2. Details of a trachyte host rock fragment within a strongly foliated selenide veinlet (cf. Figure 2.2. DDetailsetails of a trachyte host rock fragment within a strongly foliated selenide veinlet (cf. Figure 1). (a) Interstices and heavily altered phenocrysts filled with greenish or bluish alteration Figure1 1)).. ((a)) I Intersticesnterstices and heavily altered phenocrysts filledfilled with with greenish greenish or or bluish bluish alteration alteration products (malachite, azurite, or celadonite). Polished section, dispersed light dark-field image, width productsproducts (malachite,(malachite, azurite,azurite, oror celadonite).celadonite). PolishedPolished section,section, dispersed lightlight dark-fielddark-field image,image, widthwidth 2 mm. (b) Detail from Figure 2a (see black arrow) showing yellowish or brownish pseudomorphs 22 mm.mm. (b) DetailDetail from Figure2 2aa (see(see black black arrow) arrow) showing showing yellowish yellowish or or brownish brownish pseudomorphs pseudomorphs consisting of a framework of oriented leucoxene lamellae originating in response to the alteration of consistingconsisting ofof aa frameworkframework ofof orientedoriented leucoxeneleucoxene lamellaelamellae originatingoriginating in responseresponse to thethe alterationalteration ofof titanomagnetite containing oriented ilmenite laths. Polished section, reflected light digital image, titanomagnetitetitanomagnetite containing containing oriented oriented ilmenite ilmenite laths. laths. Polished Polished section, section, reflected reflected light digital light digital image, image, width width 2 mm. 2width mm. 2 mm.

Minerals 2018, 8, 127 5 of 22 Minerals 2018, 8, x FOR PEER REVIEW 5 of 22 Minerals 2018, 8, x FOR PEER REVIEW 5 of 22

FigureFigure 3. 3. Single and twinned euhedral grains and radially grown crystal aggregates of ferroselite Figure 3. Single and twinned euhedral grains and radially grown crystal aggregates of ferroselite (bright(bright cream) cream) growngrown insideinside aa hosthost rockrock fragment fragment (cf.(cf. Figure Figure1 ). 1). NextNext to to them them (upper (upper right) right) are are (bright cream) grown inside a host rock fragment (cf. Figure 1). Next to them (upper right) are titanomagnetitetitanomagnetite pseudomorphspseudomorphs (cf. (cf. Figure Figure2b). 2b) Polished. Polished section, section, plane plane polarized polarized reﬂected reflected light digital light titanomagnetite pseudomorphs (cf. Figure 2b). Polished section, plane polarized reflected light image,digital widthimage, 200 widthµm. 200 μm. digital image, width 200 μm.

Figure 4. Cont.

Minerals 2018, 8, 127 6 of 22 Minerals 2018, 8, x FOR PEER REVIEW 6 of 22

FigureFigure 4. 4. (a(a)) Irregular Irregular dark-brown dark-brown patches patches of bitumen of bitumen (see white (see arrows)white arrows) as fracture as fillings fracture cementing fillings cementingthe host-rock the fragments host-rock from fragments the vein from wall the inwards vein wall (cf. Figureinwards5d). (cf. The Figure bright 5d). grains The are bright pseudomorphs grains are pseudomorphsof TiO2 after titanomagnetite. of TiO2 after titanomagnetite. Polished section, Polished plane polarized section, reflectedplane polarized light digital reflected image, light width digital 200 image,µm; (b ) width Goethite 200 (g) μm; pseudomorph (b) Goethite after (g) ankerite pseudomorph (?) cemented after by ankerite acicular malachite (?) cemented (m) embedded by acicular in malachiteclausthalite (m) (cl, embedded white). Polished in clausthalite section, reflected(cl, white). light Polished image, section, width 1 reflected mm, plane light polarized image, light.width 1 mm, plane polarized light. 3.2. The Cacheuta Vein-Type Se Mineralization 3.2. The Cacheuta Vein-Type Se Mineralization 3.2.1. Mineralogy and Structural Features 3.2.1. TheMineralogy selenium and mineralization Structural Features represents a strongly foliated, ductile-deformed, partially brecciated andThe cavernous selenium fault mineralization filling that cuts representsthe trachytic a host strongly rock. Thefoliated main, selenideductile-deformed, minerals presentpartially in brecciatedvariable amounts and cavernous are clausthalite, fault filling klockmannite, that cuts the eucairite,trachytic eskebornitehost rock. The and m naumannite,ain selenide minerals forming presenta banded in orvariable isoclinally amounts folded are tectonicclausthalite, mélange klockmannite, (Figures1 andeucairite5a–d)., eskebornite Grain size rangesand naumannite between , forming0.001 and a 0.01 banded mm; or grains isoclinal thatly survived folded ductiletectonic deformation mélange (Fig rarelyures approach1 and 5a 1d mm). Grain in diameter. size ranges Our betweeninvestigation 0.001 of and the 10.01 to 2-cm-thick mm; grains selenium that surviv veinletsed ductile reveals deformation four types of rarely selenide approach assemblages 1 mm with in diameterregard to. Our the modalinvestigation abundance of the of 1 the to main2-cm- selenidesthick selenium (Table veinlets1, Types reveals 1–4), in four addition types toof aselenide Type 5 assemblagescalculated from with published regard to data the [ 1modal,2]. Vein abundance types 1–4 areof the shown main in selenides Figure5a–d. (Table 1, Types 14), in addition to a Type 5 calculated from published data [12]. Vein types 1–4 are shown in Figure 5ad. Table 1. Estimated modal percentages of selenides in the five types of Se veinlets. Table 1. Estimated modal percentages of selenides in the five types of Se veinlets. Mineral Type 1 Type 2 Type 3 Type 4 Type 5 Mineral Type 1 Type 2 Type 3 Type 4 Type 5 Eucairite 70 30 <1 <1 7 ClausthaliteEucairite 570 4030 <1 75 <1 507 25 KlockmanniteClausthalite 205 1540 75 5 50 4525 25 NaumanniteKlockmannite 420 1315 5 2 45 <125 <1 UmangiteNaumannite <14 <113 <12 <1 <1<1 40 Eskebornite 1 2 15 2 2 TyrrelliteUmangite <1<1 <1<1 <1 2 <1 <140 <1 Krut’aiteEskebornite <11 <12 15 1 2 22 1 TrogtaliteTyrrellite <1<1 <1<1 <12 <1 <1<1 <1 PetˇríˇcekiteKrut’aite <1<1 <1<1 <11 2 <11 ? Trogtalite <1 <1 <1 <1 <1 Petříčekite <1 <1 <1 <1 ?

Minerals 2018, 8, x FOR PEER REVIEW 7 of 22

The mineralization consists of strongly foliated, lineated selenide assemblages that precipitated in discrete ductile shear zones. The selenides show evidence of dominant ductile flow and strain accumulation that is higher than that of the adjacent trachytic host rock (Figures 1 and 5a–d). Feldspar grains and larger host rock fragments inside the Se veins have generally survived ductile deformation, in contrast to the selenides. Carbonate minerals (calcite and dolomite), which constitute the gangue in many selenide deposits, were not observed in any of our samples. However, a carbonate gangue may have initially been deposited in the Cacheuta Se veins as implied from a single fragment of a pseudomorph of goethite after dolomite (?), ankerite (?), or siderite (?), embedded in a matrix of clausthalite (white) and malachite (grey) (Figure 4b). The complete list of minerals identified during this study is provided in Figure 6. From the species previously reported in Cacheuta, both achavalite and berzelianite were not detected, corroborating the observations of modern studies of this occurrence [1]. As inferred from this list, Cacheuta hosts several undiscovered, extremely rare, or even new species among the alteration mineral assemblage (highlighted by question marks), which deserve special attention in future studies. The small grain size of these species, together with an intimate intergrowth with neighboring minerals, precluded their collection of XRD-structural data (see below Section 3.2.2). The formation of the Cacheuta Se occurrence involved grain-size reduction of the coarser-grained precursor selenides. The shapes and arrangements of isoclinal folds reflect the sense of shear. Rarely, small crystal pockets of very early formed selenides in host rock fractures remained unaffected from the subsequent ductile deformation. Post-selenide cataclasis cut randomly through all the previous mineralization and deformation structures and opened spaces between the selenium vein and the trachytic host that were partly or completely filled with copper sulfides. All of the selenium veinlets experienced strong overprinting, withMinerals goethite,2018, 8, 127 malachite, azurite, chalcomenite, and molybdomenite as the most common alteration7 of 22 minerals.

Figure 5. (a) Dispersed-light dark field image of Se-veinlet Type-1 (1.7-cm-thick, 2.3-cm-wide, surface Figure 5. (a) Dispersed-light dark field image of Se-veinlet Type-1 (1.7-cm-thick, 2.3-cm-wide, surface rolling grinded, with 1200 mesh SiC abrasive) with eucairite (olive brown) as the main selenide rolling grinded, with 1200 mesh SiC abrasive) with eucairite (olive brown) as the main selenide interspersed with irregularly shaped pores inside patchy aggregates of naumannite and interspersed with irregularly shaped pores inside patchy aggregates of naumannite and klockmannite klockmannite (dark grey). Clausthalite (light grey) occurs prevalently in thin layers at the vein wall. (dark grey). Clausthalite (light grey) occurs prevalently in thin layers at the vein wall. Notably, the Notably, the ductile deformation with lineation of the selenides is limited to the vein selvage; (b) ductile deformation with lineation of the selenides is limited to the vein selvage; (b) Dispersed light dark field image of Se-veinlet Type-2 (1-cm-thick, 2.1-cm-wide, embedded in resin, surface rolling grinded, with 1200 mesh SiC abrasive) with eucairite (olive brown) showing a strongly foliated and folded mélange of naumannite and klockmannite (dark grey), clausthalite (light grey) and eucairite (olive brown). The vein selvage contains remnants of chalcopyrite (bright yellow) and as alteration products goethite (dark brown) and malachite (green); (c) Dispersed-light dark field image of Se-veinlet Type-3 (1.5-cm-thick, 2.5-cm-wide, embedded in resin, surface rolling grinded, with 1200 mesh SiC abrasive) with a strongly foliated and folded mélange of eskebornite (light brown), klockmannite (dark grey), clausthalite (light grey), and naumannite. The vein selvage contains goethite as the main alteration product (brown). Also shown are partly rotated spotty aggregates of trachytic host-rock fragments; (d) Dispersed-light dark field image of Se-veinlet Type-4 (detail from Figure1, 2.2-cm-thick, 4-cm-wide, embedded in resin, surface rolling grinded, with 1200 mesh SiC abrasive) with layers and lenses of klockmannite (dark grey) and clausthalite (light grey) showing a strongly foliated and folded mélange. Vein selvage, host-rock fragments, and diagonal fractures are stained by goethite (dark brown), azurite and chalcomenite (blue), and malachite (green). Bitumen rarely occurs as irregularly distributed cement impregnating host-rock fragments from the vein wall inwards (see white arrows).

The mineralization consists of strongly foliated, lineated selenide assemblages that precipitated in discrete ductile shear zones. The selenides show evidence of dominant ductile ﬂow and strain accumulation that is higher than that of the adjacent trachytic host rock (Figures1 and5a–d). Feldspar grains and larger host rock fragments inside the Se veins have generally survived ductile deformation, in contrast to the selenides. Carbonate minerals (calcite and dolomite), which constitute the gangue in many selenide deposits, were not observed in any of our samples. However, a carbonate gangue may have initially been deposited in the Cacheuta Se veins as implied from a single fragment of Minerals 2018, 8, 127 8 of 22 a pseudomorph of goethite after dolomite (?), ankerite (?), or siderite (?), embedded in a matrix of clausthalite (white) and malachite (grey) (Figure4b). The complete list of minerals identiﬁed during this study is provided in Figure6. From the species previously reported in Cacheuta, both achavalite and berzelianite were not detected, corroborating the observations of modern studies of this occurrence [1]. As inferred from this list, Cacheuta hosts several undiscovered, extremely rare, or even new species among the alteration mineral assemblage Minerals 2018, 8, x FOR PEER REVIEW 9 of 22 (highlighted by question marks), which deserve special attention in future studies. The small grain size“spindle of these-shaped? species, exs togetherolution with lamellae an intimate” of eucairite intergrowth and klockmannite, with neighboring which minerals, also formed precluded during their collectionstage (I) ofand XRD-structural make up Se vein data Type (see 5 (cf. below Table Section 1). 3.2.2).

Figure 6. Paragenetic sequence and crystallization/deformation diagram for the Sierra de Cacheuta Figure 6. Paragenetic sequence and crystallization/deformation diagram for the Sierra de Cacheuta selenium mineralization. selenium mineralization.

The formation of the Cacheuta Se occurrence involved grain-size reduction of the coarser-grained precursor selenides. The shapes and arrangements of isoclinal folds reﬂect the sense of shear. Rarely,

Minerals 2018, 8, 127 9 of 22 small crystal pockets of very early formed selenides in host rock fractures remained unaffected from the subsequent ductile deformation. Post-selenide cataclasis cut randomly through all the previous mineralization and deformation structures and opened spaces between the selenium vein and the trachytic host that were partly or completely ﬁlled with copper sulﬁdes. All of the selenium veinlets experienced strong overprinting, with goethite, malachite, azurite, chalcomenite, and molybdomenite as the most common alteration minerals.

3.2.2. Micro-Structural Features Figure6 provides a summary of microtextural evidence for the relationship between selenide growth and the sequence of deformation, precipitation, and alteration. Three stages of selenide precipitation (stage (I), stage (II), and stage (III)) and one stage of alteration (stage (IV)), triggered by three brittle deformational events (deformation (d1), deformation (d3), and deformation (d4)) and one major ductile deformational event (deformation (d2)), could be distinguished and are described hereafter according to its inferred age sequence (Figure6):

• Deformation (d1): First vein opening and brecciation of the trachytic host rock. Stage (I) corresponds to the zonal and granular growth generation (g1) cementing host rock fragments. • Deformation (d2): Compression and subsequent shearing of stage (I) selenides causing foliation and isoclinal folding. Stage (II) corresponds to the crystal-plastic ﬂow, rotation, dissolution, replacement, and/or recrystallization of stage (I) selenides. • Deformation (d3): Second vein opening, fragmentation of stage (I) and (II) selenides Stage (III) corresponds to the growth generation (g3) of Cu–Fe sulﬁdes and the formation of major shrinking cracks in selected selenides. • Deformation (d4): Fourth vein opening, fragmentation of stage (I)–(III) aggregates. Stage (IV) corresponds to the intensive alteration of pre-existing selenides and the formation of various alteration minerals, resulting in growth generation (g4), cementing all previous structures.

Brittle deformation (d1) caused the opening of the fracture zone and subsequent fluid flow into the open spaces of the trachytic host rock, giving rise to stage (I) selenide precipitation. The earliest generation of krut’aite-(g1), trogtalite-(g1), clausthalite-(g1), eskebornite-(g1), umangite-(g1), eucairite-(g1), naumannite-(g1), and klockmannite-(g1) partly cemented and/or corroded the fine-grained feldspar and hematite fragments of the host rock along the grain boundaries (Figure7). Pseudomorphs after cubic crystal forms (cubes, octahedra, and cuboctahedron) occur prevalently in klockmannite-, eskebornite-, and clausthalite-rich layers and lenses. These forms (ca. 20 vol %) are frequently outlined by isolated relics of krut’aite and trogtalite and newly grown tyrrellite (Figures8–10). We interpret these structures as the earliest krutaite-(g1) generation (with up to six optically distinguishable zones of max. 20 µm thickness) that was deposited in oscillating alternation with trogtalite-(g1) and crystallized as euhedral to subhedral grains or coatings grown from the vein wall inwards, followed by the other (g1) selenides (Figures8 and9). In addition to the observations made during this study, Paar et al. (2016) [1] reported inclusions of umangite with oriented “spindle-shaped? exsolution lamellae” of eucairite and klockmannite, which also formed during stage (I) and make up Se vein Type 5 (cf. Table1). Minerals 2018, 8, 127 10 of 22 Minerals 2018,, 8,, xx FORFOR PEERPEER REVIEWREVIEW 10 of 22

FigureFigure 7.7. DetailDetail fromfrom SeSe veinvein TypeType 1:1: AA granulargranular aggregateaggregate ofof eucairiteeucairite (eu), klockmanniteklockmannite (kl),(kl),, andand naumannite (nau) (nau) is is partially partially replacing replacing a host a rock host fragmentrock fragment alongthe along grain the boundaries grain boundaries of K-feldspar of (black)K--feldsparfeldspar and (black)(black) hematite andand (he, hematitehematite grey). Reﬂected ((he, grey). light Reflected image, light 500- image,µm-wide, 500- plane-μm--wide, polarized plane light. polarized light.

Figure 8. Detail from Se vein Type 3: Few preserved remnants of former subhedral krut’aite-(g1) Figure 8. Detail from Se vein Type 3: Few preserved remnants of former subhedral krut’aite--(g1)(g1) crystals (k) project into a cavity (black) of the host rock. The originally rectangular shapes of the crystals (k) project into a cavity (black) of the host rock. The originally rectangular shapes of the krut’aite crystals are traced by thin zones of trogtalite-(g1). Patchily distributed replacement and krut’aite crystals are traced by thin zones of trogtalite--(g1).(g1). Patchily Patchily distributed distributed replacement replacement and and alteration products are klockmannite-(g2) (kl), eskebornite-(g2) (es), and clausthalite-(g2) (cl). Reﬂected alteration products are klockmannite--(g2)(g2) (kl), (kl), eskebornite eskebornite--(g2)(g2) (es), (es), and and clausthalite clausthalite--(g2)(g2) (cl). (cl). light image, 500-µm-wide, plane polarized light. Reflected light image, 500--μm--wide, plane polarized light.

Minerals 2018, 8, x FOR PEER REVIEW 11 of 22 Minerals 2018, 8, 127 11 of 22 Minerals 2018, 8, x FOR PEER REVIEW 11 of 22

Figure 9. Detail from Se vein Type 3: Replacement relics of krut’aite-(g1) (kr). The original Figurerectangular 9. 9.Detail Deta shapesil from from of Sethe vein Se krut’aite vein Type Type 3:crystals Replacement 3: are Replacement outlined relics by of relics krut’aite-(g1)patch i ofly distributed krut’aite (kr).- The(g1) trogtalite original (kr). The-(g1) rectangular original(tr) and shapestyrrelliterectangular of the (g2) shapes krut’aite (ty). of the Othercrystals krut’aite replacement are outlined crystals byare and patchily outlined alteration distributed by patch productsily trogtalite-(g1) distributed are klockmannite trogtalite (tr) and tyrrellite-(g1)-(g2) (tr) (kl),(g2)and (ty).eskebornitetyrrellite Other (g2) replacement-(g2) (ty).(brown Other andspots alteration replacement) and clausthalite products and -(g2) are alteration klockmannite-(g2) (cl). Reflected products light (kl), are image, eskebornite-(g2) klockmannite 500-μm-wide,-(g2) (brown plane (kl), spots)polarizedeskebornite and light. clausthalite-(g2)-(g2) (brown spots (cl).) and Reﬂected clausthalite light image,-(g2) (cl). 500- Reflectedµm-wide, light plane image, polarized 500-μ light.m-wide, plane polarized light.

Figure 10. 10.Detail Detail from from Se-vein Se-vein Type-3: Type Originally-3: Originally cubically cubically shaped and shaped oscillatory and grown oscillatory krut’aite-(g1) grown crystal traced by thin zones of trogtalite-(g1) (see white arrows). Other replacement and alteration krut’aiteFigure 10.-(g1) Detail crystal fromtraced Se by-vein thin Type zones-3: of Originally trogtalite -(g1) cubically (see white shaped arrows) and. Other oscillatory replacement grown products are klockmannite-(g2) (kl), eskebornite-(g2) (es), clausthalite-(g2) (cl), and chalcopyrite-(g3) andkrut’aite alteration-(g1) crystal products traced are by klockmannite thin zones- (g2) of trogt (kl),alite eskebornite-(g1) (see- (g2) white (es), arrows) clausthalite. Other- (g2) replacement (cl), and (ch). Reﬂected light image, 200-µm-wide, plane polarized light. chalcopyriteand alteration-(g3) products (ch). Reflected are klockmannite light image,-(g2) 200 (kl),-μm - eskebornitewide, plane- (g2)polarized (es), clausthalitelight. -(g2) (cl), and chalcopyrite-(g3) (ch). Reflected light image, 200-μm-wide, plane polarized light. Subsequent compressional stress and shearing ( d2d2)) caused a strong local ductile deformation, givingSubsequent rise to foliation,foliation, compressional isoclinalisoclinal stress folding,folding and, andand shearing boudinageboudinage (d2) caused ofof thethe selenides. selenides.a strong local Formerly ductile brecciated deformation, and cementedgiving rise mineralmineral to foliation, aggregatesaggregates isoclinal were were folding partitioned partitioned, and andboudinage and rotated. rotated. of Plastic Plasticthe selenides. ﬂow flow of theof Formerlythe grains grains initiated brecciatedinitiated stage stage and (II) selenide(cementedII) selenide overprint mineral overprint involvingaggregates involving dissolution, were dissolution, partitioned replacement, replacementand rotated. and/or, Plastic recrystallizationand/or flow recrystallization of the reactions. grains reactions.initiated Rich eucairite stage Rich eucairite(II) selenide mineralization overprint involving of Se vein dissolution, Type 1 generally replacement suffered, and/or flattening recrystallization of granularly reactions. textured Rich eucairite mineralization of Se vein Type 1 generally suffered flattening of granularly textured

Minerals 2018, 8, x FOR PEER REVIEW 12 of 22 eucairite (eu), showing a network of extensional fractures outlined by klockmannite (kl), which partially replaced the eucairite along the grain boundaries (Figure 11). Subsequent compression also caused multiple kinkbands in the klockmannite, which were later altered by recrystallization. A syn-tectonic phase was followed by a post-tectonic phase associated with widespread alteration of the earlier mineralization and subsequent growth of post-tectonic mineral aggregates. Fine-grained polygonal aggregates of clausthalite-(g2), eskebornite-(g2), umangite-(g2), eucairite-(g2), naumannite-(g2), and klockmannite-(g2) precipitated. In contrast to previous reports [1,2], the samples studied in this paper contain only tiny, armored relics of umangite-(g1) in hematite-(g1). Due to the intensive ductile overprint, most of the umangite-(g1) became unstable and Mineralswas replaced2018, 8, 127by klockmannite-(g2) and clausthalite-(g2). 12 of 22 During stage (II), eucairite was replaced by naumannite, klockmannite, and/or clausthalite. Trogtalite was replaced by tyrrellite, eskebornite, clausthalite, and/or klockmannite. Umangite was mineralizationreplaced by hematite, of Se vein klockmannite Type 1 generally, and/or suffered tyrrellite. ﬂattening Eskebornite of granularly was replaced textured by klockmannite, eucairite (eu), showingclausthalite a network, and/or oftyrrellite. extensional Notably, fractures large outlined portions by of klockmanniteumangite and (kl), klockmannite which partially often replacedshowed thefinely eucairite branched along shrinkage the grain cracks, boundaries giving (Figure strong 11 evidence). Subsequent that these compression selenides also suffered caused significant multiple kinkbandsloss of volume in the [1]. klockmannite, which were later altered by recrystallization.

Figure 11. Detail from Se vein Type-1: Flattening of granularly textured eucairite (eu) showing a Figure 11. Detail from Se vein Type-1: Flattening of granularly textured eucairite (eu) showing a network of extensional fractures outlined by klockmannite (kl). Subsequent compression created network of extensional fractures outlined by klockmannite (kl). Subsequent compression created multiple kinkbandskinkbands inin the the klockmannite, klockmannite, which which were were later later modiﬁed modified by recrystallization.by recrystallization. Reﬂected Reflected light image,light image, 500-µ 500m-wide,-μm-wide, plane plane polarized polarized light. light.

Brittle deformation (d3) generated multiple micro-fractures and micro-brecciation preferentially parallelA syn-tectonicto the selvages phase and wasperpendicular followed byto the a post-tectonic Se vein (Figure phase 12). associated Subsequently, with Cu widespread–Fe–S-rich alteration of the earlier mineralization and subsequent growth of post-tectonic mineral aggregates. fluids infiltrated the Cacheuta fault system, giving rise to another event of widespread alteration of Fine-grainedthe earlier mineralization. polygonal aggregates Late-stage of ( clausthalite-(g2),III) sulfides comprising eskebornite-(g2), chalcopyrite umangite-(g2),-(g3), chalcocite eucairite-(g2),-(g3), and naumannite-(g2), and klockmannite-(g2) precipitated. In contrast to previous reports [1,2], the samples covellite-(g3) postdate the (g1)–(g2) selenides. Chalcopyrite, mostly as thin colloform coatings of studied“blister copper” in this paper, symmetrically contain only covered tiny, and armored cemented relics the of micro umangite-(g1)-fractures in(Figure hematite-(g1). 12). Due to the intensive ductile overprint, most of the umangite-(g1) became unstable and was replaced by klockmannite-(g2) and clausthalite-(g2). During stage (II), eucairite was replaced by naumannite, klockmannite, and/or clausthalite. Trogtalite was replaced by tyrrellite, eskebornite, clausthalite, and/or klockmannite. Umangite was replaced by hematite, klockmannite, and/or tyrrellite. Eskebornite was replaced by klockmannite, clausthalite, and/or tyrrellite. Notably, large portions of umangite and klockmannite often showed finely branched shrinkage cracks, giving strong evidence that these selenides suffered significant loss of volume [1]. Brittle deformation (d3) generated multiple micro-fractures and micro-brecciation preferentially parallel to the selvages and perpendicular to the Se vein (Figure 12). Subsequently, Cu–Fe–S-rich fluids infiltrated the Cacheuta fault system, giving rise to another event of widespread alteration of the earlier mineralization. Late-stage (III) sulfides comprising chalcopyrite-(g3), chalcocite-(g3), and covellite-(g3) postdate the (g1)–(g2) selenides. Chalcopyrite, mostly as thin colloform coatings of “blister copper”, symmetrically covered and cemented the micro-fractures (Figure 12). At the end of stage (III), the existing shrinkage cracks, micro-fractures, interstices, and porous selenide aggregates were successively invaded by strongly oxidizing fluids, which also attacked the newly precipitated sulfide cement to form goethite-(g4). Pre-existing clausthalite that crystallized at Minerals 2018, 8, x FOR PEER REVIEW 13 of 22

Minerals 2018, 8, 127 13 of 22 the margins and outside the complex selenide structure was easily accessible for these ﬂuids, which triggered its partial or complete replacement by a complicated mixture of extremely rare, ﬁne-grained selenides-(g3) including klockmannite-(g3), petˇríˇcekite-(g3),krut’aite-(g3), native selenium-(g3) [7]

(FigureMinerals 201813),, 8 and, x FOR two PEER new REVIEW phases (Figures 14 and 16). 13 of 22

Figure 12. Brittle deformation (d3) caused detachment and fragmentation of fine-grained klockmannite (blue-grey) and clausthalite (bright) perpendicular to the vein selvage and flattening of the selenides. Notably, large portions of klockmannite show finely branched shrinkage cracks (black), providing evidence for significant loss of volume. Reflected light image, 1-mm-wide, plane polarized light.

At the end of stage (III), the existing shrinkage cracks, micro-fractures, interstices, and porous selenide aggregates were successively invaded by strongly oxidizing fluids, which also attacked the newly precipitated sulfide cement to form goethite-(g4). Pre-existing clausthalite that crystallized at Figure 12. Brittle deformation (d3) caused detachment and fragmentation of fine-grained the marginsFigure 12. andBrittle outside deformation the complex (d3) caused selenide detachment structure and was fragmentation easily accessible of fine-grained for these klockmannite fluids, which klockmannite (blue-grey) and clausthalite (bright) perpendicular to the vein selvage and flattening of triggered(blue-grey) its partial and clausthalite or complete (bright) replacement perpendicular toby the a vein complicated selvage and mixture flattening of of the extremely selenides. rare, the selenides. Notably, large portions of klockmannite show finely branched shrinkage cracks fine-grainedNotably, large selenides portions-(g3) of klockmannite including klockmannite show finely branched-(g3), petříčekite shrinkage- cracks(g3), krut’aite(black), providing-(g3), native evidence(black), providing for significant evidence loss offor volume. significant Reflected loss of light volume. image, Reflected 1-mm-wide, light planeimage, polarized 1-mm-wide, light. plane seleniumpolarized-(g3) light.[7] (Figure 13), and two new phases (Figures 14 and 16).

Figure 13. Clausthalite (cl) partly replaced by end-member krut’aite (blue), end-member petříčekite Figure 13. Clausthalite (cl) partly replaced by end-member krut’aite (blue), end-member petˇríˇcekite (blue-violet), molybdomenite (dark grey), and native selenium (light grey), forming a myrmecitic (blue-violet), molybdomenite (dark grey), and native selenium (light grey), forming a myrmecitic texture [7]. Reflected light image, 500-μm-wide, plane polarized light. texture [7]. Reﬂected light image, 500-µm-wide, plane polarized light.

Figure 13. Clausthalite (cl) partly replaced by end-member krut’aite (blue), end-member petříčekite (blue-violet), molybdomenite (dark grey), and native selenium (light grey), forming a myrmecitic texture [7]. Reflected light image, 500-μm-wide, plane polarized light.

Minerals 2018, 8, x FOR PEER REVIEW 14 of 22

One of those new species is a Cu selenide forming an angular network-like intersertal texture of lath-shaped thin plates with solitary inclusions of end-member krut’aite and end-member petříčekite replacing clausthalite, naumannite and eskebornite. Optical properties are shown in Figure 14. In plane-polarized incident light, the mineral is greenish-grey in color, slightly bireflectant and weakly pleochroic from greenish-grey to light grey, showing no internal reflections. Under crossed polarizers, the mineral is weakly anisotropic, with light-brown to dark-grey rotation tints. The selenide always contains several weight percentages of Ag and Pb, which are negatively Mineralscorrelated.2018, In8, 127the case that it represents a discrete phase and not a nano-scale mixture of two or14 more of 22 phases, its ideal formula may be either Cu2Se3 or Cu5Se8.

Figure 14.14. ReflectedReﬂected light light images images (each (each 200 200--μmµm-wide)-wide) of of the the unnamed unnamed Cu Cu selenide selenide (white (white mineral mineral = =clausthalite) clausthalite).. (a) ( aPlane) Plane polarized polarized light; light; (b ()b ) crossed crossed polarizers polarizers;; un un = = unname unnamedd Cu Cu selenide, pet == end-memberend-member petˇrpetříčekiteíˇcekite.For. For further explanations see text.text.

OneModerate of those post new-selenide species and is a post Cu selenide-sulfide cataclasis forming an (d4 angular) cut through network-like all existing intersertal mineralization texture of lath-shapedand deformation thin platesstructures with and solitary opened inclusions space between of end-member the selenium krut’aite ve andin sections end-member and the petˇr trachyticíˇcekite replacinghost rock. clausthalite,Stage-(IV) of naumanniteongoing alteration and eskebornite. in the oxidation Optical zone propertiesgave rise to are a plethora shown inof secondary Figure 14. In plane-polarized incident light, the mineral is greenish-grey in color, slightly bireﬂectant and weakly pleochroic from greenish-grey to light grey, showing no internal reﬂections. Under crossed polarizers, the mineral is weakly anisotropic, with light-brown to dark-grey rotation tints. The selenide always contains several weight percentages of Ag and Pb, which are negatively correlated. In the case that it represents a discrete phase and not a nano-scale mixture of two or more phases, its ideal formula may be either Cu2Se3 or Cu5Se8. Minerals 2018, 8, 127 15 of 22

Moderate post-selenide and post-sulﬁde cataclasis (d4) cut through all existing mineralization and deformation structures and opened space between the selenium vein sections and the Minerals 2018, 8, x FOR PEER REVIEW 15 of 22 trachytic host rock. Stage-(IV) of ongoing alteration in the oxidation zone gave rise to a plethora ofminerals secondary of growth minerals generation of growth (g4): generation chalcomenite, (g4): chalcomenite, molybdomenite molybdomenite (Figure 15), (Figure olsacherite, 15), olsacherite,cobaltomenite, cobaltomenite, cerussite, malachi cerussite,te, native malachite, selenium, native barite, selenium, and several barite, incompletely and several characterized incompletely characterizedphases. phases.

Figure 15. DetailDetail from from Se Se vein vein T Type-4.ype-4. Dispersed Dispersed-light-light dark dark field ﬁeld image image of ofanan acicular acicular aggregate aggregate of ofmolybdomenite molybdomenite (colorless (colorless transparent transparent)) showing showing internal internal reflections, reﬂections, partly partly stained stained by byinclusions inclusions of ofgoethite goethite (red (red-brown,)-brown,) and and surrounded surrounded by by chalkomenite chalkomenite (blue). (blue). Cacheuta Cacheuta is is the the type type locality of chalcomenite and molybdomenite. Width 500500 µμm.m.

One of the phases yet unknown from Cacheuta is a Cu–Fe selenite, representing an alteration One of the phases yet unknown from Cacheuta is a Cu–Fe selenite, representing an alteration product of eskebornite (Figure 16). These grains are characterized by strongly varying Cu/Fe ratios, product of eskebornite (Figure 16). These grains are characterized by strongly varying Cu/Fe from Cu > Fe to Cu = Fe to Cu < Fe. Possibly, these alteration products represent chalcomenite and ratios, from Cu > Fe to Cu = Fe to Cu < Fe. Possibly, these alteration products represent mandarinoite, Fe3+2(SeO3)3·6H2O [14,15]. Optical properties are shown in Figure 16. In chalcomenite and mandarinoite, Fe3+ (SeO ) ·6H O[14,15]. Optical properties are shown in plane-polarized incident light, the mineral2 3 is3 medium2 to dark grey. The decomposition of the Figure 16. In plane-polarized incident light, the mineral is medium to dark grey. The decomposition eskebornite grains increasingly developed from bottom to top and showed the state of of the eskebornite grains increasingly developed from bottom to top and showed the state of transformation along the strong cleavage of eskebornite parallel (0001). The direction of the cleavage transformation along the strong cleavage of eskebornite parallel (0001). The direction of the cleavage indicates the spatial position of the individual eskebornite grains. The close subparallel intergrowth indicates the spatial position of the individual eskebornite grains. The close subparallel intergrowth of the eskebornite relics and alteration products prevents a more detailed description of their optical of the eskebornite relics and alteration products prevents a more detailed description of their properties. optical properties. Three other not clearly identifiable species, which are commonly associated with cerussite, Three other not clearly identifiable species, which are commonly associated with cerussite, constitute different alteration products of clausthalite and attest to the late-stage interaction of the constitute different alteration products of clausthalite and attest to the late-stage interaction of the selenides with C–S–Cl–B-bearing fluids. One of those phases mainly consists of Pb, Cl, and Se, selenides with C–S–Cl–B-bearing fluids. One of those phases mainly consists of Pb, Cl, and Se, together together with O and eventually H, and may represent orlandiite, Pb3(SeO3)(Cl,OH)4·H2O [16]. In with O and eventually H, and may represent orlandiite, Pb (SeO )(Cl,OH) ·H O[16]. In addition addition to these components, but devoid of Se, the second phase3 contains3 B4 and,2 if not constituting to these components, but devoid of Se, the second phase contains B and, if not constituting a new a new phase, may identify as mereheadite, Pb47O24(OH)13Cl25(BO3)2(CO3) [17]. The third phase is a phase, may identify as mereheadite, Pb47O24(OH)13Cl25(BO3)2(CO3)[17]. The third phase is a sulfite sulfite and probably resembles scotlandite, PbSO3 [18], the S-analogue of molybdomenite. and probably resembles scotlandite, PbSO3 [18], the S-analogue of molybdomenite.

Minerals 2018, 8, 127 16 of 22 Minerals 2018, 8, x FOR PEER REVIEW 16 of 22

Figure 16.16. Eskebornite (light-brown)(light-brown) and clausthalite (bright) largely replaced by unknown Cu–FeCuFe selenitesselenites (light-grey(light-greyto to dark-grey) dark-grey) of of strongly strongly varying varying Cu/Fe Cu/Fe ratios, ratios, from from Cu Cu > Fe> Fe to Cuto Cu = Fe = toFe Cuto Cu < Fe, < possiblyFe, possibly representing representing chalcomenite chalcomenite and mandarinoite. and mandarinoite The darker. The thedarker domains the domains are, the more are, the Fe-rich more is theFe-rich alteration is the alteration mineral. Reﬂected mineral. lightReflected image, light 500- image,µm-wide, 500-μ planem-wide, polarized plane polarized light. light.

3.2.3. Mineral Chemistry 3.2.3. Mineral Chemistry Representative results of the electron-microprobeelectron-microprobe analyses of the selected selenides and sulfides sulﬁdes are listed in Tables2 2 and and3 .3. Formula Formula proportions proportions reported reported in in the the text text refer refer to to the the main main formulas. formulas.   Krut’aite-(g1)Krut’aite-(g1) (cf. Figures Figures 88 9and) is9 ) moderately is moderately rich rich in Co in (4.9 Co (4.9–6.76.7 wt %) wt and %) andcontains contains minor     minorconcentrations concentrations (in wt %) (in of wt Ag %) (0.5 of Ag3.8), (0.5–3.8), Ni (0.4 1.2), Ni (0.4–1.2), and Fe (0.5 and1.2) Fe (Table (0.5–1.2) 2, ana#(Table 12,3) ana#. The concentrations of the other elements sought are below 0.1 wt %. Its mean formula is 1–3). The concentrations of the other elements sought are below 0.1 wt %. Its mean formula is (Cu0.71Co0.22Ag0.04Ni0.03Fe0.03)1.03Se1.98 (n = 6). In the course of later replacement, cobalt, copper and (Cu0.71Co0.22Ag0.04Ni0.03Fe0.03)Σ1.03Se1.98 (n = 6). In the course of later replacement, cobalt, copper andnickel nickel liberated liberated during during dissolution dissolution of ofkrut’aite krut’aite-(g1)-(g1) were were concent concentratedrated in in tyrrellite tyrrellite.. The The low low Ni content was insufﬁcientinsufficient to crystallize independent nickel selenides as, for example, in El Dragón,Dragón, Bolivia [19]. Late-formed krut’aite-(g3) (Table 2, ana# 4) has almost end-member composition Bolivia [19]. Late-formed krut’aite-(g3) (Table2, ana# 4) has almost end-member composition (Cu0.99Se2.00) (n = 3). The only non-ideal cation measured in klockmannite [(Cu0.98Ag0.02)1.00Se1.00; n = 4] (Cu0.99Se2.00) (n = 3). The only non-ideal cation measured in klockmannite [(Cu0.98Ag0.02)Σ1.00Se1.00;   0.99 1.01 1.99 nis Ag = 4] (0.7 is Ag2.2 wt (0.7–2.2 %) (Table wt % 2), ana# (Table 52,6) ana#. Near 5–6).-ideal Near-idealeskebornite eskebornitehas the mean has formula the mean Cu Fe formulaSe (n = 4), with only Ag being detectable as non-stochiometric element (Table 2, ana# 78). Late-stage Cu0.99Fe1.01Se1.99 (n = 4), with only Ag being detectable as non-stochiometric element (Table2, ana# petříčekite has also near-end-member composition, possessing the mean formula 7–8). Late-stage petˇríˇcekitehas also near-end-member composition, possessing the mean formula (Cu0.98Fe0.01)0.99Se2.00 (n = 7). It occasionally hosts minor concentrations (in wt %) of Ag (00.5), Pb (Cu0.98Fe0.01)Σ0.99Se2.00 (n = 7). It occasionally hosts minor concentrations (in wt %) of Ag (0–0.5),   Pb(0 0.4), (0–0.4), and and Fe (0 Fe (0–0.5).0.5). Eucairite, Cu1.00Ag1.00Se0.99 (n = 4), is also very poor in non-ideal elements (Table 3, ana# 12). Eucairite, Cu1.00Ag1.00Se0.99 (n = 4), is also very poor in non-ideal elements (Table3, ana# 1–2). Naumannite has the average formula (Ag0.98Cu0.02)1.00Se1.00 (n = 7), with some Cu (0.11.4 wt %) Naumannite has the average formula (Ag0.98Cu0.02)Σ1.00Se1.00 (n = 7), with some Cu (0.1–1.4 wt %) possibly substituting for Ag (Table 3, ana# 34). Clausthalite, (Pb0.97Cu0.04)1.01Se0.99 (n = 8), may as well possibly substituting for Ag (Table3, ana# 3–4). Clausthalite, (Pb 0.97Cu0.04)Σ1.01Se0.99 (n = 8), may as have incorporated minor Cu (0.11.7 wt %) (Table 3, ana# 5). well have incorporated minor Cu (0.1–1.7 wt %) (Table3, ana# 5).

Minerals 2018, 8, 127 17 of 22

Table 2. Representative results of the electron-microprobe analyses of krut’aite, klockmannite, eskebornite, and petˇríˇcekite.

Element/# 1 2 3 4 5 6 7 8 9 10 11 Cu (wt %) 18.37 20.19 20.78 28.48 42.87 44.09 22.66 22.67 28.32 27.78 28.11 Ag 3.80 1.50 0.89 b.d 2.24 0.73 0.19 0.16 b.d b.d 0.53 Pb 0.06 0.06 0.09 b.d b.d b.d b.d b.d 0.11 0.40 0.20 Fe 0.64 0.50 1.15 0.05 b.d b.d 20.33 20.45 b.d 0.21 0.48 Co 5.99 6.69 6.11 0.11 b.d b.d b.d b.d b.d b.d b.d Ni 1.22 0.45 0.47 b.d b.d b.d b.d b.d b.d b.d b.d S 0.04 0.04 0.05 0.03 b.d b.d b.d b.d b.d 0.06 0.06 Se 70.73 70.78 70.97 71.17 55.59 55.67 56.40 56.67 71.33 71.61 70.99 Total 100.85 100.21 100.51 99.84 100.70 100.50 99.58 99.92 99.76 100.06 100.34 Notes: b.d. = below detection limit; 1–4 = krut’aite, 5–6 = klockmannite, 7–8 = eskebornite, 9–11 = petˇríˇcekite.

Table 3. Representative results of electron-microprobe analyses of eucairite, naumannite, clausthalite, tyrrellite, and trogtalite.

Element/# 1 2 3 4 5 6 7 8 9 10 11 Cu (wt %) 25.44 25.53 0.74 1.36 1.72 12.86 12.90 12.75 4.17 7.48 6.20 Ag 43.59 43.27 72.43 71.74 0.09 b.d b.d b.d 0.08 0.95 0.35 Hg 0.14 b.d. b.d. 0.28 b.d. b.d. b.d. b.d. b.d. b.d. b.d. Pb b.d. 0.08 b.d. b.d. 70.19 b.d. b.d. 0.08 b.d. b.d. b.d. Fe b.d. b.d. b.d. b.d. b.d. 0.09 0.12 0.08 0.38 0.78 0.31 Co b.d. b.d. b.d. b.d. 0.06 22.38 22.33 20.83 21.80 19.82 21.66 Ni b.d. b.d. b.d. b.d. b.d. 0.74 1.19 2.72 1.23 b.d. b.d. S b.d. b.d. b.d. b.d. b.d. 0.31 0.22 0.09 0.17 0.10 0.09 Se 31.35 31.25 27.02 27.01 28.46 63.19 63.16 63.33 72.01 71.33 71.74 Total 100.52 100.14 100.19 100.39 100.52 99.57 99.92 99.89 99.84 100.46 100.35 Notes: b.d. = below detection limit; 1–2 = eucairite, 3–4 = naumannite, 5 = clausthalite, 6–8 = tyrrellite, 9–11 = trogtalite.

Non-ideal components present in tyrrellite comprise Ni (0.5–2.7 wt %) and S (0.1–0.3 wt %) (Table3, ana# 6–8). Silver, Hg, Pb, and Fe were occasionally measured at low concentrations. It has the mean formula Cu1.00(Co1.87Ni0.11Fe0.01)Σ1.99(Se3.98S0.03)Σ4.01 (n = 18). The replacement mineral trogtalite, (Co0.79Cu0.19Fe0.02Ag0.01Ni0.01)Σ1.02(Se1.97S0.01)Σ1.98 (n = 9), is moderately rich in Cu (4.1–7.5 wt %) and notoriously contains minor Ag (0.1–1.0 wt %), Fe (0.1–0.9 wt %) and S (0.1–0.2 wt %) (Table3, ana# 9–11). A single spot was conducted on a ﬁnal-stage sulﬁde (i.e., the chalcopyrite grain included in the krut’aite pseudomorph shown in Figure 10). This grain is rich in Se (16.4 wt %) and exhibits the formula Cu0.99Fe0.99(S1.59Se0.43)Σ2.02. Plotting the compositions of trogtalite, krut’aite, and tyrrellite in the Co–Ni–Cu ternary diagrams representing the systems (Co,Ni,Cu)Se2 and (Co,Ni,Cu)3Se4 reveals chemical signatures that mostly deviate from those reported from other Se occurrences (Figures 17 and 18) and, thus, constitute diagnostic features for this site. Minerals 2018, 8, 127 18 of 22 Minerals 2018, 8, x FOR PEER REVIEW 18 of 22 Minerals 2018, 8, x FOR PEER REVIEW 18 of 22

FigureFigure 17.17 17. Co–Ni–Cu.Co Co–Ni–Ni––CuCu ternaryternary ternary diagramdiagram diagram (mole(mole (mole %)%) %) ofof of thethe the systemsystem system trogtalite–penroseite–krut’aite.trogtalite trogtalite–penroseite–penroseite––krut’aite.krut’aite. DataData Data sources:sources: Cacheuta Cacheuta Cacheuta ([ 1 ([1,2],,2([1,2],], this this paper),this paper paper Tumiñico),) , Tumiñico Tumiñico [1,2], Trogtal [1 [1,2],,2], Trogtal [2 Trogtal,20], Petrovice [2,20], [2,20], Petrovice [ Petrovice21], El Drag [21], [21],ón [ El4 El,19 Dragón, Dragón22,23], Pacajake[4,19,22[4,19,22,23],, [23],19 ,Pacajake22 Pacajake–24], and [19,22 [19,22 Hope’s––24],24], Noseand and Hope’s [Hope’s24,25]. Nose Nose [24 [24,25].,25].

Figure 18. CoNiCu ternary diagram (mole %) of the system (Co,Ni,Cu)3Se4. FigureFigure 18. 18Co–Ni–Cu. CoNiCu ternary ternary diagram diagram (mole (mol %)e %) of of the the system system (Co,Ni,Cu) (Co,Ni,Cu)3Se3Se4.4. 4. Discussion 4.4. Discussion Discussion

4.1.4.14.1. . Sources Sources of of Elements Elements InferencesInferencesInferences regarding regarding thethe the possiblepossible possible sources sources sources of of of seleniumselenium selenium andand and thethe the accompanying accompanying accompanying elements elements elements couldcould could onlyonly bebe be mademade made fromfrom from briefbrief brief historic historic historic descriptions descriptions descriptions of of theof the the local local local geology geology geology and and and information information information that that that is providedis is provided provided by samplebyby sample sample materials material material thats sthat that unequivocally unequivocally unequivocally relate relate relate to the to to the Cacheuta the Cacheuta Cacheuta mineralization. mine mineralization.ralization. DespiteDespite these these strong strong limitations limitations limitations in in in knowledge, knowledge, knowledge, the the bituminousbituminous bituminous shales shales intercalated intercalated intercalated in in variousvarious various sedimentarysedimentary rocks rocks in in the the vicinity vicinity of of the the Se Se veins veins recognized recognized by by [[9] 9[9]] are are thethe the primeprime prime candidates candidates forfor for thethe the majormajor source source of of Se Se and and the the accompanying accompanying metals. metals. No Notete that that bitumen bitumen is is also also noted noted as as constituent constituent of of thethe Se Se veins veins ([11], ([11], this this paper). paper). Suggesting Suggesting these these shales shales as as elemental elemental source source is is speculative, speculative, because because we we

Minerals 2018, 8, 127 19 of 22 major source of Se and the accompanying metals. Note that bitumen is also noted as constituent of the Se veins ([11], this paper). Suggesting these shales as elemental source is speculative, because we know practically nothing about their extent, composition, and mineralogy. However, black shales and coals are typically rich in framboidal pyrite and organic matter and are globally known as important carriers of a large suite of metals, including selenium [26–31]. It is our suspicion that black shales in the vicinity of the Se veins likely acted as the major host rock for the elements that are immobilized in the various crystallized selenides at Cacheuta. Processes involving highly oxidized fluids have likely (a) oxidized and dissolved Se-containing sulfide minerals and (b) mobilized absorbed and organic-bound selenium. During interaction with these fluids, reduced selenide (Se2−) became oxidized to selenate (Se6+), capable of migrating and depositing outside of the host. Selenate was again reduced to selenite or even native selenium when the migrating fluids lost their oxidation potential through interaction with reductants, triggering the deposition of the primary Se species at this site.

4.2. Origin of Se Mineralization Considering the thermodynamic properties of binary selenides at T = 100 ◦C and elevated oxygen fugacities [32], the absolute and relative oxygen, selenium, and sulfur fugacities of the fluids during the different stages of formation of the Cacheuta mineralization could have been constrained. The presence of hematite in the earliest stage (I) implies that the f O2 was above the hematite–magnetite buffer. Moreover, formation of ferroselite suggests that the oxygen fugacity was still considerably higher at the onset of Se mineral precipitation, at least about 5.8 log units above the hematite–magnetite buffer [32]. At such strongly elevated oxygen fugacity, the absence of berzelianite indicates that the log f Se2 was above that defined by the berzelianite–umangite univariant reaction, (i.e., exceeded −14.5) which also permitted the crystallization of Co-rich krut’aite. The lack of pure krut’aite and the crystallization of klockmannite implies that the Se fugacity in the fluids remained below the klockmannite–krut’aite buffer (i.e., log f Se2 was less than −11.2). This range in Se fugacity prevailed until the end of stage (II). Accordingly, the lack of sulfides implies that log f S2 was a maximum of −22 (i.e., the Se/S fugacity was greater than unity throughout both stages). At the beginning of stage (III), the short-term precipitation of Se-rich chalcopyrite indicates that the S fugacity in the fluid was a little higher than in the preceding stages, possibly amounting to log f S2 between −22 and −19. As evidenced by the formation of stoichiometric krut’aite and native selenium, the Se fugacity remained high, exceeding ◦ the value defined by the krut’aite–native selenium univariant reaction (log f Se2 of −11.2 at T = 100 C) (i.e., the Se/S fugacity was still greater than unity until the end of the deposition of the fracture-filling selenides at Cacheuta).

4.3. Diagnostic Features of Cacheuta, Mendoza, Compared to the Se Deposits of the Province of La Rioja As pointed out in the Introduction, there is occasionally confusion about the correct assignment of Se-bearing samples from the Mendoza and La Rioja Provinces in Argentina (Table4, Figure 19). Figure 19 highlights the characteristic differences in the vein structure between the Se-mineralization of Sierra de Cacheuta, Mendoza Province, and of the La Millionara Mine, Sierra de Cacho District, which is representative for the Se occurrences of the Province of La Rioja. Based on historic descriptions, the investigation of historical reference samples, and the most recent review of Argentinian Se deposits [1], we propose the following features as diagnostic for the Cacheuta selenide samples:

(3) The selenides show striking evidence of dominant ductile ﬂow, recrystallization, and strain accumulation exceeding that in the adjacent trachytic host rock that only suffered brittle deformation. (4) Early, Co-rich krut’aite (originally comprising up to 20 vol % of the selenides) is largely replaced by clausthalite, umangite, klockmannite, eskebornite, Ni-poor tyrrellite (Ni < 2.7 wt %) and trogtalite (Ni < 1.2 wt %), as well as krut’aite and petˇríˇcekiteof end-member composition. (5) Calcite gangue, Hg–selenides (tiemannite), berzelianite, and native gold are lacking. (6)MineralsFormation 2018, 8, x FOR of PEER numerous REVIEW Se-bearing alteration minerals, possibly including mandarinoite,20 of 22 mereheadite, orlandiite, and scotlandite. Figure 19 highlights the characteristic differences in the vein structure between the (7) Presence of an unnamed Cu selenide (ideal formula either Cu Se or Cu Se ). Se-mineralization of Sierra de Cacheuta, Mendoza Province, and of2 the3 La Millionara5 8 Mine, Sierra de Cacho District, which is representative for the Se occurrences of the Province of La Rioja.

Figure 19.19. ((aa)) Dispersed Dispersed-light-light dark dark field ﬁeld image image of of Se Se-veinlet-veinlet Type Type-4-4 from Sierra Sierra de de Cacheuta Cacheuta (2.2-cm-thick,(2.2-cm-thick, 6.6-cm-wide, 6.6-cm-wide, embedded in resin, resin, polished polished surface). surface). The opaque metallic selenides selenides appear black. black. Vein Vein selvage, selvage, host host-rock-rock fragments fragments and and diagonal ffracturesractures are stained by various various alterationalteration products products like like goethite goethite (dark (dark brown), brown), azurite azurite and chalcomenite and chalcomenite (blue), (blue), and malachite and malachite (green). ((green)b) Dispersed-light. (b) Dispersed dark-light ﬁeld dark image field of aimage representative of a representative Se-veinlet sampleSe-veinlet from sample the La from Millionara the La Mine,Millionara Sierra Mine, de Cacho Sierra District,de Cacho Province District, ofProvince La Rioja of (3.8-cm-thick, La Rioja (3.8-cm 7.1-cm-wide,-thick, 7.1-cm polished-wide, polished surface). Thesurface). undeformed The undeformed Se mineralization Se mineralization (black) consists (black) of umangite, consists of berzelianite umangite, and berzelianite klockmannite. and Theklockmannite. selenides cement The selenides the euhedral cement crystals the euhedral (average grain crystals size (average 2 mm) and grain crystal size aggregates 2 mm) and of crystalcalcite (white),aggregates which of calcite are intensively (white), whichstained are by intensively chalcomenite stained (blue) by and chalcomenite malachite (green). (blue) Collection and malachite F.N. Keutsch,(green). Collection Harvard University, F.N. Keutsch Cambridge,, Harvard MA, Univ USA.ersity, Cambridge, MA, USA.

Based on historic descriptions, the investigation of historical reference samples, and the most recent review of Argentinian Se deposits [1], we propose the following features as diagnostic for the Cacheuta selenide samples:

(1) Occurrence of trachytic host rock fragments containing ferroselite, bitumen, and TiO2 pseudomorphs after titanomagnetite. (2) Several thin zones of trogtalite in Co-rich krut’aite-(g1) (Co <6.7 wt %) pseudomorphs, representing several stages of selenide deposition. (3) The selenides show striking evidence of dominant ductile flow, recrystallization, and strain accumulation exceeding that in the adjacent trachytic host rock that only suffered brittle deformation.

Minerals 2018, 8, 127 21 of 22

Table 4. Compilation of the most important Se occurrences of the provinces of La Rioja and Mendoza, Argentina [1]. Mineral species documented in color images by [1] are given in italics.

(1) La Rioja Province

(a) Los Llantenes district—most important mines: El Chire (naumannite, tiemannite, native gold, chrisstanleyite, jagüétite, calcite); San Pedro (naumannite, eucairite, tiemannite, calcite, clausthalite, hematite); El Portezuelo; Luis; La Ramada (b) Sierra de Cacho district (former, “Sierra de Umango“)—most important mines: El Quemado (umangite, ferroselite, hematite); Las Asperezas (native selenium, eucairite, naumannite, ﬁschesserite, native gold); El Tolar; El Hoyo; La Millionara (berzelianite, klockmannite, hematite, calcite) (c) Cerro Cacho (northern part of the Sierra de Cacho district)—most important mines: Tumiñico (native selenium, native gold, tiemannite, umangite, klockmannite, clausthalite, eskebornite, brodtkorbite, crookesite, hakite, bucovite, chaménaite, cadmoselite, ferroselite, graphite, chalcomenite, tyrrellite, calcite, hematite, malachite); Pichanas (umangite, klockmannite) (d) Sañiogosta district—most important mines: Santa Brigida (stilleite, umangite); La Piedra Pintada (e) Sierra de Famatina—most important mine [1]: San Francisco (umangite, eldragónite, calcite)

(2) Mendoza Province

(a) Sierra de Cacheuta (clausthalite, eucairite, umangite, tyrrellite–bornhardtite, eskebornite, klockmannite, trogtalite, krut’aite)

Acknowledgments: We are indebted to Frank N. Keutsch for loaning the specimens from Cacheuta and sharing his scientific expertise on this occurrence. Two anonymous reviewers provided valuable suggestions. Author Contributions: Günter Grundmann manufactured the polished sections, performed the microstructural analysis, and determined the genetic sequence of the mineral deposition and alteration. Hans-Jürgen Förster conducted the electron-microprobe analyses. Hans-Jürgen Förster and Günter Grundmann wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

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