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

: 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 ( d 1– d 4): 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 TiO 2 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 petˇr í ˇcekite; (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 Cu 2 Se 3 or Cu 5 Se 8 ), and possibly mandarinoite, mereheadite, orlandiite, and scotlandite as new species for this occurrence.


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][3][4][5][6][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, in the Mendoza Province.Sierra de Cacheuta is the type locality of achavalite FeSe, chalcomenite CuSeO  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.

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) 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 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.

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 (Figure 1).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 (Figure 2a).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 (Figure 1).Areas of the strongest foliation are stained greenish by malachite or brownish by goethite (Figure 2a).Optical polished-section analysis (Figure 2a,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 (Figure 3).Irregular black patches of bitumen (up to a few millimeters) occurred as fracture fillings, cementing the host rock fragments from the vein wall inwards (Figure 4).

Mineralogy and Structural Features
The selenium mineralization represents a strongly foliated, ductile-deformed, partially brecciated and cavernous fault filling that cuts the trachytic host rock.The main selenide minerals present in variable amounts are clausthalite, klockmannite, eucairite, eskebornite and naumannite, forming a banded or isoclinally folded tectonic mélange (Figures 1 and 5ad).Grain size ranges between 0.001 and 0.01 mm; grains that survived ductile deformation rarely approach 1 mm in diameter.Our investigation of the 1 to 2-cm-thick selenium veinlets reveals four types of selenide assemblages with regard to the modal abundance of the main selenides (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.3.2.The Cacheuta Vein-Type Se Mineralization

Mineralogy and Structural Features
The selenium mineralization represents a strongly foliated, ductile-deformed, partially brecciated and cavernous fault filling that cuts the trachytic host rock.The main selenide minerals present in variable amounts are clausthalite, klockmannite, eucairite, eskebornite and naumannite, forming a banded or isoclinally folded tectonic mélange (Figures 1 and 5a-d).Grain size ranges between 0.001 and 0.01 mm; grains that survived ductile deformation rarely approach 1 mm in diameter.Our investigation of the 1 to 2-cm-thick selenium veinlets reveals four types of selenide assemblages with regard to the modal abundance of the main selenides (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.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.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).
Minerals 2018, 8, x FOR PEER REVIEW 9 of 22 "spindle-shaped?exsolution lamellae" of eucairite and klockmannite, which also formed during stage (I) and make up Se vein Type 5 (cf.Table 1).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.

Micro-Structural Features
Figure 6 provides a summary of microtextural evidence for the relationship between selenide growth and the sequence of deformation, precipitation, and alteration.
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 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), petříčekite-(g3), krut'aite-(g3), native selenium-(g3) [7] (Figure 13), and two new phases (Figures 14 and 16).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 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), petříčekite-(g3), krut'aite-(g3), native selenium-(g3) [7] (Figure 13), and two new phases (Figures 14 and 16).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 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), petříčekite-(g3), krut'aite-(g3), native selenium-(g3) [7] (Figure 13), and two new phases (Figures 14 and 16).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.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 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 Cu 2 Se 3 or Cu 5 Se 8 .
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 trachytic host rock.Stage-(IV) of ongoing alteration in the oxidation zone gave rise to a plethora of secondary minerals of growth generation (g4): chalcomenite, molybdomenite (Figure 15), olsacherite, cobaltomenite, cerussite, malachite, native selenium, barite, and several incompletely characterized phases.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, from Cu > Fe to Cu = Fe to Cu < Fe.Possibly, these alteration products represent chalcomenite and mandarinoite, Fe 3+ 2(SeO3)3• 6H2O [14,15].Optical properties are shown in Figure 16.In plane-polarized incident light, the mineral is medium to dark grey.The decomposition 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 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 properties.
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 selenides with C-S-Cl-B-bearing fluids.One of those phases mainly consists of Pb, Cl, and Se, together with O and eventually H, and may represent orlandiite, Pb3(SeO3)(Cl,OH)4•H2O [16].In addition to these components, but devoid of Se, the second phase contains B and, if not constituting a new phase, may identify as mereheadite, Pb47O24(OH)13Cl25(BO3)2(CO3) [17].The third phase is a sulfite and probably resembles scotlandite, PbSO3 [18], the S-analogue of molybdomenite.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, from Cu > Fe to Cu = Fe to Cu < Fe.Possibly, these alteration products represent chalcomenite and mandarinoite, Fe 3+ 2 (SeO 3 ) 3 •6H 2 O [14,15].Optical properties are shown in Figure 16.In plane-polarized incident light, the mineral is medium to dark grey.The decomposition 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 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 properties.
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 selenides with C-S-Cl-B-bearing fluids.One of those phases mainly consists of Pb, Cl, and Se, together with O and eventually H, and may represent orlandiite, Pb 3 (SeO 3 )(Cl,OH) 4 •H 2 O [16].In addition to these components, but devoid of Se, the second phase contains B and, if not constituting a new phase, may identify as mereheadite, Pb 47 O 24 (OH) 13 Cl 25 (BO 3 ) 2 (CO 3 ) [17].The third phase is a sulfite and probably resembles scotlandite, PbSO 3 [18], the S-analogue of molybdomenite.

Mineral Chemistry
Representative results of the electron-microprobe analyses of the selected selenides and sulfides are listed in Tables 2 and 3. Formula proportions reported in the text refer to the main formulas.

Mineral Chemistry
Representative results of the electron-microprobe analyses of the selected selenides and sulfides are listed in Tables 2 and 3. Formula proportions reported in the text refer to the main formulas.
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 Cu 0.99 Fe 0.99 (S 1.59 Se 0.43 ) Σ2.02 .

Sources of Elements
Inferences regarding the possible sources of selenium and the accompanying elements could only be made from brief historic descriptions of the local geology and information that is provided by sample materials that unequivocally relate to the Cacheuta mineralization.
Despite these strong limitations in knowledge, the bituminous shales intercalated in various sedimentary rocks in the vicinity of the Se veins recognized by [9] are the prime candidates for the 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  [4,19,22,23], Pacajake [19,[22][23][24], and Hope's Nose [24,25].

Sources of Elements
Inferences regarding the possible sources of selenium and the accompanying elements could only be made from brief historic descriptions of the local geology and information that is provided by sample materials that unequivocally relate to the Cacheuta mineralization.
Despite these strong limitations in knowledge, the bituminous shales intercalated in various sedimentary rocks in the vicinity of the Se veins recognized by [9] are the prime candidates for the 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

Sources of Elements
Inferences regarding the possible sources of selenium and the accompanying elements could only be made from brief historic descriptions of the local geology and information that is provided by sample materials that unequivocally relate to the Cacheuta mineralization.
Despite these strong limitations in knowledge, the bituminous shales intercalated in various sedimentary rocks in the vicinity of the Se veins recognized by [9] are the prime candidates for the 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][27][28][29][30][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 (Se 2− ) became oxidized to selenate (Se 6+ ), 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.

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).

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 (Table 4, 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: (1) Occurrence of trachytic host rock fragments containing ferroselite, bitumen, and TiO 2 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.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   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.

Figure 1 .
Figure 1.Dispersed-light dark field image of Se-vein Type-4 (2.2-cm-thick, 6.6-cm-wide, embedded in resin, surface rolling grinded, with 1.200 mesh SiC abrasive), with layers and lenses of klockmannite (dark grey) and clausthalite (light grey) forming a strongly foliated and folded mélange (cf.Section 3.2.1).The spotty and lens-like aggregates of the trachytic host rock fragments are partly rotated; the vein selvage, host rock fragments and diagonal fractures are stained by various alteration products, such as goethite (dark brown), azurite and chalcomenite (blue), and malachite (green).

Figure 2 .
Figure 2. Details 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 products (malachite, azurite, or celadonite).Polished section, dispersed light dark-field image, width 2 mm.(b) Detail from Figure 2a (see black arrow) showing yellowish or brownish pseudomorphs consisting of a framework of oriented leucoxene lamellae originating in response to the alteration of titanomagnetite containing oriented ilmenite laths.Polished section, reflected light digital image, width 2 mm.

Figure 1 )
Figure 2. Details 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 products (malachite, azurite, or celadonite).Polished section, dispersed light dark-field image, width 2 mm.(b) Detail from Figure 2a (see black arrow) showing yellowish or brownish pseudomorphs consisting of a framework of oriented leucoxene lamellae originating in response to the alteration of titanomagnetite containing oriented ilmenite laths.Polished section, reflected light digital image, width 2 mm.

Figure 1 .
Figure 1.Dispersed-light dark field image of Se-vein Type-4 (2.2-cm-thick, 6.6-cm-wide, embedded in resin, surface rolling grinded, with 1.200 mesh SiC abrasive), with layers and lenses of klockmannite (dark grey) and clausthalite (light grey) forming a strongly foliated and folded mélange (cf.Section 3.2.1).The spotty and lens-like aggregates of the trachytic host rock fragments are partly rotated; the vein selvage, host rock fragments and diagonal fractures are stained by various alteration products, such as goethite (dark brown), azurite and chalcomenite (blue), and malachite (green).

22 Figure 1 .
Figure 1.Dispersed-light dark field image of Se-vein Type-4 (2.2-cm-thick, 6.6-cm-wide, embedded in resin, surface rolling grinded, with 1.200 mesh SiC abrasive), with layers and lenses of klockmannite (dark grey) and clausthalite (light grey) forming a strongly foliated and folded mélange (cf.Section 3.2.1).The spotty and lens-like aggregates of the trachytic host rock fragments are partly rotated; the vein selvage, host rock fragments and diagonal fractures are stained by various alteration products, such as goethite (dark brown), azurite and chalcomenite (blue), and malachite (green).

Figure 2 .
Figure 2. Details 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 products (malachite, azurite, or celadonite).Polished section, dispersed light dark-field image, width 2 mm.(b) Detail from Figure 2a (see black arrow) showing yellowish or brownish pseudomorphs consisting of a framework of oriented leucoxene lamellae originating in response to the alteration of titanomagnetite containing oriented ilmenite laths.Polished section, reflected light digital image, width 2 mm.

Figure 1 )
Figure 2. Details 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 products (malachite, azurite, or celadonite).Polished section, dispersed light dark-field image, width 2 mm.(b) Detail from Figure 2a (see black arrow) showing yellowish or brownish pseudomorphs consisting of a framework of oriented leucoxene lamellae originating in response to the alteration of titanomagnetite containing oriented ilmenite laths.Polished section, reflected light digital image, width 2 mm.

Figure 2 .
Figure 2. Details 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 products (malachite, azurite, or celadonite).Polished section, dispersed light dark-field image, width 2 mm.(b) Detail from Figure 2a (see black arrow) showing yellowish or brownish pseudomorphs consisting of a framework of oriented leucoxene lamellae originating in response to the alteration of titanomagnetite containing oriented ilmenite laths.Polished section, reflected light digital image, width 2 mm.

Figure 1 )Figure 3 .
Figure 2. Details 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 products (malachite, azurite, or celadonite).Polished section, dispersed light dark-field image, width 2 mm.(b) Detail from Figure 2a (see black arrow) showing yellowish or brownish pseudomorphs consisting of a framework of oriented leucoxene lamellae originating in response to the alteration of titanomagnetite containing oriented ilmenite laths.Polished section, reflected light digital image, width 2 mm.

Figure 5 .Figure 5 .
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 interspersed with irregularly shaped pores inside patchy aggregates of naumannite and klockmannite (dark grey).Clausthalite (light grey) occurs prevalently in thin layers at the vein wall.Notably, the ductile deformation with lineation of the selenides is limited to the vein selvage; (b) 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 interspersed with irregularly shaped pores inside patchy aggregates of naumannite and klockmannite (dark grey).Clausthalite (light grey) occurs prevalently in thin layers at the vein wall.Notably, the 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 Figure 1, 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).

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

Figure 11 .
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 multiple kinkbands in the klockmannite, which were later modified by recrystallization.Reflected light image, 500-μm-wide, plane polarized light.

Figure 11 .
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 multiple kinkbands in the klockmannite, which were later modified by recrystallization.Reflected light image, 500-µm-wide, plane polarized light.

22 Figure 12 .
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.

Figure 12 . 22 Figure 12 .
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.

Figure 14 .
Figure14.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.
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 trachytic host rock.Stage-(IV) of ongoing alteration in the oxidation zone gave rise to a plethora of secondary

Figure 15 .
Figure 15.Detail from Se vein Type-4.Dispersed-light dark field image of an acicular aggregate of molybdomenite (colorless transparent) showing internal reflections, partly stained by inclusions of goethite (red-brown,) and surrounded by chalkomenite (blue).Cacheuta is the type locality of chalcomenite and molybdomenite.Width 500 μm.

Figure 15 .
Figure 15.Detail from Se vein Type-4.Dispersed-light dark field image of an acicular aggregate of molybdomenite (colorless transparent) showing internal reflections, partly stained by inclusions of goethite (red-brown,) and surrounded by chalkomenite (blue).Cacheuta is the type locality of chalcomenite and molybdomenite.Width 500 µm.

Figure 16 .
Figure 16.Eskebornite (light-brown) and clausthalite (bright) largely replaced by unknown CuFe selenites (light-grey to dark-grey) of strongly varying Cu/Fe ratios, from Cu > Fe to Cu = Fe to Cu < Fe, possibly representing chalcomenite and mandarinoite.The darker the domains are, the more Fe-rich is the alteration mineral.Reflected light image, 500-μm-wide, plane polarized light.

Figure 16 .
Figure 16.Eskebornite (light-brown) and clausthalite (bright) largely replaced by unknown Cu-Fe selenites (light-grey to dark-grey) of strongly varying Cu/Fe ratios, from Cu > Fe to Cu = Fe to Cu < Fe, possibly representing chalcomenite and mandarinoite.The darker the domains are, the more Fe-rich is the alteration mineral.Reflected light image, 500-µm-wide, plane polarized light.

22 Figure 19
Figure19highlights 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.

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.