Mineralogy of Skarn Ores from B ă it , a-Bihor, Northern Apuseni Mountains, Romania: A Case Study of Cu-, Bi-, and Sn-minerals

: The Antoniu, Antoniu North, and Blidar Contact orebodies from the B ă it , a-Bihor skarn deposit, Romania have been investigated using optical and electron microscopy. Electron probe microanalyses were acquired on samples from the Blidar Contact orebody. Bornite is the most abundant Cu-sulﬁde and hosts native bismuth, jos é ite-B, emplectite, and wittichenite. Kësterite and ferrokësterite were identiﬁed for the ﬁrst time in the B ă it , a-Bihor deposit; the occurrence of stannite was also conﬁrmed. Temperatures of ore deposition in the Blidar Contact orebody are constrained from the compositions of sphalerite-kësterite and sphalerite-ferrokësterite pairs at 287 ± 25 ◦ C to 310 ± 35 ◦ C, and 447 ± 17 ◦ C to 503 ± 68 ◦ C, respectively.


Introduction
The Băit , a-Bihor ore deposit is located in the northwestern part of the Bihor Mountains, Apuseni Mountains, on the upper course of the Cris , ul Negru river, 3.5 km ENE of Băit , a village. This deposit together with the Fe skarn deposits at Băis , oara (Mas , ca, Mas , ca Băis , oara, and Cacova) and the Valea Lita Pb-Zn-Cu vein deposit are the northernmost ore occurrences in the Banatitic Magmatic and Metallogenetic Belt (BMMB). Băit , a-Bihor is the most significant ore deposit in the North Apuseni Mountains (NA) [1] (Figure 1a). Băit , a-Bihor is a polymetallic skarn deposit consisting of tens of individual orebodies which can be considered altogether as distal to the fluid source, i.e., a granite to granodiorite-diorite at depth. At the ore deposit scale, however, there is clear zonation in terms of metals, whereby Mo-Bi-Cu-W, Cu-W, and Pb-Zn-B skarn ores are placed proximal to distal relative to the inferred intrusive fluid source.
The geological structure of the Băit , a-Bihor area reflects the tectonic structure of the Northern Apuseni Mountains. The Bihor Unit forms a basement that is unconformably overlain by several nappes that belong to the Codru nappe system and the Biharia nappe system [11,25,26] (Figure 1b,c).
The Bihor Unit consists of Jurassic -Lower Cretaceous detritic and calcareous formations. The overlying Codru nappe system is built up of the Codru unit, which is composed of several nappes (Bătrânescu, Următ, and Vetre) covered by the Aries , eni nappe [27,28]. The Codru unit comprises a complete Triassic sequence composed of sandstones, limestones, and dolomites and is unconformably overlain by Permian (Early Carboniferous?)-Lower Triassic detritic sedimentary rocks of the Aries , eni nappe [29]. The entire nappe system was subsequently pierced by Late Cretaceous magmatism [1,30], represented by a deep-seated granite to granodiorite-diorite batholith accompanied by subvertical basic to intermediate composition dykes.
Presently exhausted, exploitation of the Blidar Contact orebody started during the Middle Age [11,24,32] and continued on strike over 600 m [11]. This orebody has a tabular morphology with an NW-SE strike and a 45-50 • dip towards SW, similar to the Blidar thrust fault [11,32,33] ( Figure 1c). The Blidar Contact orebody hosts the most important Mo-Bi concentration within the Băiţa-Bihor deposit [33] and is associated with garnet-wollastonite skarn in the inner part of the orebody, and diopside-garnet-wollastonite skarns in the outer zone [11,33,34]. At the orebody-scale, there is a metal vertical zonation with Bi enrichment in the upper zone, and a horizontal zonation with an inner Mo-Bi zone and an outer Cu-W (-Mo-Bi) zone [11,32,34].
The exploitation of the Antoniu orebody began in the 19th century. This orebody is located in the western part of the deposit. It has a columnar morphology and occurs at the intersection of two major structures striking N60 and N300-310, respectively [11]. It is considered by Cook and Ciobanu [35] to be one of the most important orebodies from the entire Băit , a-Bihor deposit thanks to its dimensions, with a known mined-out vertical section of over 400 m from below the underground level 224 ASL to approximately 650 m ASL. Exploration programs executed so far have not intercepted the lower limit of the orebody.
The high-grade Cu(-Bi-W-B) ores of the Antoniu orebody [11,34] suggests a proximal position relative to the fluid source [36][37][38]. Skarn assemblages within the Antoniu ore pipe vary from diopside(±grandite)-bearing skarns in the inner zone to wollastonite/vesuvianite-humite-bearing skarns in the outer zones [8,11]. The formation of the Antoniu orebody has been accompanied by dedolomitisation, which formed an external halo marking the transition to the dolomite host rocks [8,11]. The orebody displays zonation in terms of metals with Cu enrichment in the upper part, Bi enrichment in the lower part, altogether with Pb-Zn [37,38], and B-bearing minerals in the peripheral zone [11,34,[39][40][41][42]. The Antoniu orebody has a northern apophysis (Figure 1c) that lies approximately 300 m north, known as Antoniu North [3,35]. The discovery and mining of Antoniu North started in 2002 and ceased in 2013. Underground mining is planned to resume during 2020. The morphology of the ore body is still poorly known in detail but seems to possess an irregular breccia dyke morphology, possibly merging with the Antoniu orebody at depth.

Materials and Methods
The studied material consists of ore samples collected from the lowermost central-western part of the Antoniu North orebody (Figure 2a) and the lowermost western part of the Antoniu orebody (Figure 2b-e). The deepest part of these orebodies is nowadays accessible by the underground mining works of level XVIII (224 m ASL), Băit , a mine. Other samples (e.g., Figure 2f) were selected from the "Valeriu Lucca" Ore Deposit Collection, Department of Geology, Babeş-Bolyai University, Cluj-Napoca. These samples were collected in 1971 from the underground works along the so-called Blidar Contact orebody by Professor Ioan Mârza. Optical microscopy, in both transmitted and reflected light, was carried out using a Nikon Eclipse LV100N POL diascopic/episcopic illumination polarizing microscope in the GET Laboratory (Géosciences Environnement Toulouse-Observatoire Midi-Pyrénées), Toulouse, France.
Semi-quantitative chemical data and back-scattered electron (BSE) images were obtained for samples from the Antoniu North and Blidar Contact orebodies using a JSM-6360 scanning electron microscope operated at 20 kV (GET Laboratory) as a preliminary step prior to quantitative electron probe microanalysis. Quantitative chemical analysis of samples from the Blidar Contact orebody was carried out at the UMS 3623-Centre de MicroCaractérisation Raimond Castaing, Toulouse University, France with a CAMECA SXFive electron microprobe using an acceleration voltage of 25 keV, and a beam current of 20 nA. Analytical conditions are presented in Supplementary Table S1. Only those EPMA data with totals between 97.77 and 101.50 (in wt%) were used for subsequent interpretation.  Wittichenite is frequently observed within bornite and occurs as grains, irregular patches, or filaments of various sizes that range from 10 to maximum 270 µm (Figure 3a,c-e). It was also observed as fine bleb-like exsolutions in bornite or as elongate grains apparently oriented along the main crystallographic axes of the host mineral.
Electrum occurs in Antoniu North orebody as grains of up to 15 µm in size hosted by bornite and intimately related to hessite (Figure 3e). Electrum was also observed as small grains of up to 5 µm included in chalcopyrite hosted by sphalerite (Figure 3f).
Galena occurs as small discontinuous rims of less than 50 µm in length along the margins of coarse bornite grains, and as elongated inclusions hosted by bornite (Figure 3d).
Digenite is quite abundant in the studied samples and occurs as a late replacement product of bornite and chalcopyrite (Figure 3a-c). Digenite occurs as rims, irregular replacement blebs, and veinlets within bornite, usually fissure-or void-controlled, or as regular network-like inclusions (Figure 3c). Digenite shows minor replacement by covellite showing a "dusty" texture with very small grains.

The Blidar Contact Orebody
Bornite and associated ore minerals occur as massive pods exceeding several centimeters across, as smaller nests composed of grains dispersed among skarn minerals, or as irregular veinlets. Bornite grains show common replacement by digenite along grain boundaries or crosscutting fissures. Digenite is locally replaced by covellite, which also replaces bornite ( Figure 4a). Wittichenite occurs as small worm-like intergrowth within bornite, small blebs, and grain aggregates of up to 200 µm across ( Figure 4b). It also occurs as up to 150 × 100 µm-sized grains associated with galena ( Figure 4c). Wittichenite is intergrown with bornite and contains bornite, digenite, and bornite-digenite blebs or irregular patches.
Galena occurs as inclusions in bornite, sphalerite, and in covellite replacing bornite/digenite. Galena is locally corroded by wittichenite (Figure 4c) or by Fe-oxides/hydroxides. Native bismuth, joséite-B, and emplectite are associated with wittichenite hosted by bornite ( Figure 4d). The occurrence of native bismuth is controlled by the presence of voids, cracks, and microfractures (Figure 4d,e).
Joséite-B occurs as sub-rounded and irregular grains up to 95 µm at contacts between wittichenite with native bismuth (Figure 4d) and as irregular inclusions of up to 50 µm hosted by bornite. When hosted by bornite, joséite-B is often associated with wittichenite.
Emplectite appears as a continuous border around native bismuth, along mutual grain boundaries between joséite-B and wittichenite (Figure 4d,e), as irregular inclusions within the bornite associated with the native bismuth, and as elongate grains in contact with Sn-bearing minerals. The contact between wittichenite and emplectite is frequently marked by Fe oxides/hydroxides. The presence of native bismuth inclusions within emplectite ( Figure 4e) indicates that native bismuth was partly replaced by emplectite.
Sphalerite is common within the ore. It was observed mostly as inclusions ranging in size from 70 to 300 µm, hosted by bornite. The contact zone between sphalerite and bornite is marked by a reaction zone composed of two sequences: (i) one formed on sphalerite, which contains small-sized sphalerite blebs surrounded by bornite and minor amounts of digenite and covellite, which also advances within sphalerite; and (ii) one formed on bornite which consists of Fe-oxides/hydroxides and digenite associated with minor covellite (Figure 4f). This reaction zone also formed along the sphalerite border even if the bornite-sphalerite contact is sealed by galena or wittichenite, with Fe-oxides/hydroxides concentrated in the adjacent bornite and bornite-sphalerite composite sequence in contact with wittichenite.
Kësterite and ferrokësterite were observed in association with sphalerite, bornite, emplectite, wittichenite, digenite, covellite, and Fe-oxides/hydroxides. These Sn-minerals occur as rounded or irregular grains up to 30 µm hosted by bornite. These two Sn-minerals are difficult to distinguish by optical microscopy alone. The sphalerite-kësterite/ferrokësterite contact is smooth, with no sign of replacement. Kësterite/ferrokësterite grains can display rims of galena that likely indicate the primary boundary of the Sn-minerals. Irregular grain boundaries between bornite and kësterite/ferrokësterite suggest that the kësterite/ferrokësterite grains were not formed in equilibrium with bornite, which partly replaced the outward portion of the Sn-mineral grains.
Stannite occurs as small irregular inclusions (up to 10 µm) within bornite in association with sphalerite and Fe-oxides/hydroxides. Kësterite(?) associated with native bismuth and related Bi-minerals shows an elongated morphology, with a maximum of 30 µm in length (Figure 4e), concentrated at emplectite-wittichenite grain boundaries.
The BSE images confirm the presence of electrum in Antoniu North orebody ( Figure 5a). Additionally, the presence of emplectite, joseite-B, and native bismuth was confirmed in the Blidar Contact ore (Figure 5b,c) with native bismuth inclusions in wittichenite and relict grains within emplectite and joséite-B. The BSE images also revealed the formation of Fe-oxides/hydroxides and digenite within bornite along the contact with sphalerite ( Figure 5d).
The existence of the Sn-minerals in Blidar Contact orebody, likely kësterite(?), and a mixture of secondary phases containing Cu, Sn, S, Fe, Bi, O have been also confirmed ( Figure 5e). Clear replacement relationships exist among these minerals as suggested among others by the presence of relict Sn-sulfide grains within emplectite (black spots).

Crystal Chemistry
EPMA data for bornite are listed in Table 1 and confirm a wide chemical variation range for the analyzed elements, likely related to the presence of associated and included ore minerals. Sphalerite, identified based on its optical properties, was confirmed by EPMA data on samples from the Blidar Contact orebody (Table 1). Quantitative EPMA data for bismuth minerals (Tables 2 and 3) are in agreement with the previously acquired semi-quantitative SEM-EDS chemical data and optical microscopy interpretation and confirm the occurrence of native bismuth, wittichenite, emplectite, and joséite-B.
EPMA data for Sn-minerals (Table 4) indicate the presence of kësterite, ferrokësterite, and stannite within the Blidar Contact orebody.  * Below detection limit values were obtained for As, Pb, Mn, Zn, Cd, in all measured points; additionally, below detection limit values were obtained for Te and Se for the native bismuth and emplectite. "bdl" = below detection limit.  * Below detection limit values were obtained for As, Ag, Pb, Sb, Bi in all measured points; additionally, below detection limit values were obtained for Te for stannite. "bdl" = below detection limit; "na" = not analyzed.
The continuous outer bornite border that envelops a Sn-mineral grain (right grain from Figure 5f; Table 1, rows 4-5) has Cu and Fe contents that decrease from the galena rim towards the Sn-mineral, i.e., from 58.14 to 49.39 wt%, and from 10.13 to 8.96 wt%, respectively. In contrast, Sn content increases from the galena rim towards the adjacent Sn-mineral from 2.83 to 9.80 wt%, reflecting the presence of small inclusions of Sn-mineral [3] which increase in abundance towards the Sn-mineral grain.
The morphologies of wittichenite and galena inclusions within bornite (Figure 3d,e) suggest the exsolution of Bi and Pb during cooling as the solid solution field in the system Bi-Cu-Fe-S contracts. The formation of these new minerals is frequently controlled by crystallographic directions, microfractures, and voids. The formation of wittichenite hosted by bornite indicates that the bornite was Bi-saturated [44]. However, bornite analyzed from Băit , a-Bihor shows a Bi content below the detection limit (0.40 wt%).
Bornite from the Antoniu North orebody also occurs as a replacement of chalcopyrite (Figure 3b), e.g., chalcopyrite-bornite grains with a chalcopyrite core and a bornite rim. In the studied samples, the bornite rim is followed by a digenite band. Bornite replacement by digenite and digenite replacement by covellite have been also observed for the Blidar Contact orebody. These texture relationships suggest that late fluids have remobilized copper, giving an overprint after the skarn formation.
The analyzed sphalerite contains 0.70-1.04 wt% Cd (Table 1, rows 7-9), slightly higher than the values mentioned by Cook et al. [46] and George et al. [37], i.e., 0.22 to 0.78 wt%, and 0.16 to 0.28 wt%, respectively. The presence of Cd is related to the isomorphous substitution of Cd 2+ for Zn 2+ [46]. The Mn values are slightly exceeding the content previously reported for the sphalerite from the Băit , a-Bihor deposit [37,46]. Its presence is due to the Zn 2+ ↔ Mn 2+ substitution [46]. EPMA data acquired for sphalerite grains in contact with Sn-minerals reveal Sn values ranging from 0.13 to 0.87 wt% (Table 1, rows 7-9). These values are higher than those reported previously [37,46] which showed smaller Sn concentrations of <5 ppm. Low Sn and Ag contents in sphalerite could be explained according to the substitution 2(Cu + ,Ag + )+Sn 4+ ↔ 3Zn 2+ [46]. Overall, concentrations of Cu, Fe, Mn, Cd, and Sn in sphalerite range between 13.01 and 15.8 wt%. These values are close to or slightly exceed the maximum amount of minor and trace elements that sphalerite typically hosts [46,47].
Textural relationships of wittichenite, galena, joséite-B, and native bismuth with bornite suggest Bi and Pb exsolution during the cooling of the ore. The formation of wittichenite, native bismuth, and joséite-B indicates that bornite was Bi-saturated [44] with Bi:X > 1, where X = Te,S,Se, suggesting the existence of a reducing environment at the time of formation.
Textural relationships observed among the Bi-minerals hosted by bornite from the Blidar Contact orebody and their chemical compositions suggest that native bismuth occurred first within bornite. It was subsequently partially replaced by joséite-B, as evidenced by the existence of native bismuth inclusions within joséite-B. Subsequently, the migration of Cu from bornite towards native bismuth and joséite-B in parallel with the Bi migration from native bismuth and joséite-B towards bornite produced wittichenite on the bornite side and emplectite on the native-bismuth and joséite-B side.

Cu-Sn-Zn(Fe)-S Minerals
The kësterite-stannite pseudobinary system has been continuously investigated since the 1960s, with a focus on the crystal structure and the chemical composition of the system members [48][49][50][51][52][53][54][55][56][57][58]. Stannite and kësterite are the end-members of the stannite-kësterite join system [48,49,51,52]. The two end-members form separate solid solutions and can be differentiated based on their chemical composition and crystal structure, kësterite being the Zn-dominant end-member crystallizing in the I4 space group, and stannite the Fe-rich member of the system with the I42m space group [51]. Between the two end members, there is an intermediate phase, namely ferrokësterite [53]. Ferrokësterite is the low-temperature polymorph of stannite [52], having a chemical composition similar to stannite and a crystal structure close to that of kësterite [53]. Therefore, ferrokësterite represents the Fe-rich analog of kësterite, with which it may form a solid solution [53].
Electron probe microanalysis of stannite from different occurrences allowed Springer [48] to point out a significant variation in the metal ratios, with an important Zn enrichment. This author stated that a complete solid solution exists between Fe-pure stannite and Zn-pure kësterite. Later, Springer [49] stated that the kësterite-stannite system is pseudobinary and proposed the first phase diagram for the system. He demonstrated that a complete solid solution exists between the two end-members above 680 • C and a structural change in the stannite structure occurs at 680 • C with the formation of a transition phase. A high-and a low-temperature stannite-kësterite phase consequently formed, being separated by a two-phase region. As the temperature decreases, an increase of zinc in the Fe-Zn site occurs and the composition is enriched in Zn and two stannite phases are formed, a β-high temperature phase, and an α-low-temperature phase. According to Moore and Howie [55], kësterite is isostructural with the β-high temperature phase. If the kësterite-stannite system is cooled in the two-phase region, ferrokësterite-kësterite exsolutions in stannite should appear in stannite.
Kissin and Owens [53] claimed the presence of a miscibility gap between the two end-members of the kësterite-stannite system; however, Corazza et al. [59] and Bernadini et al. [56] proposed the existence of a continuous solid-solution. Petruk [60] stated that the minerals sharing a stannite crystal structure might contain up to 55 at% Zn in the Fe-Zn site, and the minerals with a kësterite crystal structure might contain up to 55 at% Fe in the Fe-Zn site. This author also mentioned 'ferrian-kësterite', grouping minerals with a kësterite-like crystal structure, 25-55% Fe in the Fe-Zn site and significant concentrations of indium, and 'zincian-stannite', or minerals from the stannite field with a Zn-rich composition, with up to 40-55% Zn in the Fe-Zn sites, the latter also having been described by Springer [49] and Moore and Howie [55]. Zn-rich stannite was also studied from Otoge (Japan) by Watanabe et al. [57], who mentioned that the Zn-rich phase of stannite is characterized by Fe/(Fe + Zn) ratios in the 0.49-0.71 range. Kissin and Owens [53] demonstrated that the zincian-stannite described by Moore and Howie [55] is actually ferrokësterite and the ferrian-kësterite is, in fact, petrukite.
A plot of EPMA data acquired on Cu-Sn-Zn(Fe)-S minerals from Băit , a-Bihor (Table 4) on the Cu/(Cu + Sn) vs. Fe/(Fe + Zn) diagram of Petruk [60] allows for discrimination of the Cu-Sn rich minerals belonging to the stannite group, i.e., kësterite, ferrokësterite and stannite ( Figure 6). Fe/(Fe + Zn) after Petruk [60]. Published data from other occurrences are also given. The blue dashed lines separate the approximate stability field of kësterite, ferrokësterite, and stannite based on Fe/(Fe + Zn) ratio according to intervals proposed by Watanabe et al. [57].
The chemical composition of analyzed kësterite from Băit , a-Bihor is generally close to the standard composition described for the Këster deposit, Russia [61], and to that described from the Panagyurishte ore district, Bulgaria [62]. The Cu/(Cu + Sn) ratios for the kësterite from Băit , a-Bihor have an average value of 0.72, all greater than the 0.69 value obtained by Ivanov and Pyatenko [61], and by Bogdanov et al. [62].
Cadmium is the single element present at concentrations above the minimum detection limit; a negative correlation is noted between Fe vs. (Zn + Cd). The Fe-rich kësterite occurs in closer association with bornite, whereas Zn-rich kësterite is associated with sphalerite.
EPMA data (Table 4, rows 6-13) and position on the diagram of Cu/(Cu + Sn) vs. Fe/(Fe + Zn) ( Figure 6) allows for confirmation of the occurrence of ferrokësterite and kësterite, with ferrokësterite forming the inner part, and kësterite the outer part, of a given grain.
Ferrokësterite has a composition resembling that described from the Cligga mine, England, with the average chemical formula Cu 1 [53].
Considering the limit of the ferrokësterite stability field at Fe/(Fe + Zn) = 0.71 [57], all the EPMA points with Fe/(Fe + Zn) ratio values >0.71 are considered here as being stannite, the Fe end-member of the kësterite-stannite system. The possible occurrence of sub-micron stannite inclusions in chalcopyrite and bornite, respectively, in Băit , a-Bihor was proposed by Cioflică et al. [8] based on geochemical analyses, and later by Cook et al. [3] based on LA-ICP-MS data. The ideal formula of stannite is Cu 2 FeSnS 4 . However, Springer [48] proposed the formula Cu 2 (Fe,Zn)SnS 4 due to large variations in the relative proportions of Zn and Fe. The analyzed stannite is close to the ideal. Aside from Cd (up to 2.09 wt%), Mn (up to 0.09 wt%) and Se (up to 0.32 wt%), the average metal/sulfur ratio 0.99:1.02 remains close to stoichiometric [49]. The (Fe + Mn) values generally follow a negative trend, with (Zn + Cd) suggesting (Fe + Mn) → (Zn + Cd) substitution.
Overall, as shown in Figure 6, all the studied Sn-bearing minerals have higher Cu/(Cu + Sn) ratios compared to the data in the literature, due to a higher Cu content and/or a Sn deficit. Similarly, the chemical composition of the Sn-bearing minerals indicates a wide Fe-Zn substitution, with Fe/(Fe + Zn) ranging from 0.22 to 0.87.
According to Moore and Howie [55] and Nekrasov et al. [63], Sn-minerals associated with sphalerite are typically relatively enriched in Zn. Nekrasov et al. [63] claimed that kësterite-ferrokësterite may form starting from stannite, according to the equation Cu 2 FeSnS 4 + ZnS → Cu 2 ZnSnS 4 + FeS. It is worth noting that the analyzed sphalerite from the kësterite/ferrokësterite-sphalerite mineral assemblage indicates a deficit in Zn and related metals (Fe, Cu, Mn, Cd, Sn), i.e., a measured average 66.00 wt% vs. The ideal 67.10 wt%, which is likely related to Zn migration from sphalerite to stannite.
Taylor [64] points out that interaction between a complex Sn-Fe-Cu sulfide (e.g., stannite) and an acid solution (O 2 -rich) leads to remobilization of Cu, Fe, and Zn with the formation of Fe oxides/hydroxides and Cu minerals in the alteration zone. This process is likely responsible for the presence of finely-intergrown mixtures of emplectite-(kësterite/ferrokësterite)-Fe-oxides/hydroxides, and for the presence of digenite associated with minor covellite.
Kësterite-ferrokësterite and stannite are commonly associated with sphalerite [60]. This mineral assemblage represents a complex system, which apparently formed simultaneously, with no replacement or exsolution textures. The ferrokësterite-kësterite relationships and the chemical character of the Sn-minerals-bornites-phalerite mineral assemblage from the Blidar Contact orebody likely reflect the Zn enrichment/Fe depletion of ferrokësterite to form kësterite, and apparently also Sn migration from ferrokësterite/kësterite to bornite and sphalerite. Simultaneously, Zn migrated from sphalerite to ferrokësterite and contributed to the formation of kësterite, and Cu and Fe migrated from bornite to kësterite, favoring partial replacement of kësterite by bornite and the likely preservation of relict Sn-mineral grains within the newly formed bornite. This interpretation is based on the lack of crystal-structural evidence suggesting that bornite can accommodate concentrations of Sn approaching 10 wt%.
Ferrokësterite, or the so-called zincian-stannite of Springer [49], was considered by this author to be a possible geothermometer if this mineral phase coexists with stannite. According to Shimizu and Shikazono [65], EPMA data on samples that show subsolidus phenomena and a disequilibrium texture are unsuitable for geothermometry use. Co-existing Sn-bearing minerals and sphalerite have been used as a reliable geothermometer based on Fe and Zn partitioning [63,[65][66][67] according to the reaction Cu 2 FeSnS 4 (in stannite) + ZnS (in sphalerite) ↔ Cu 2 ZnSnS 4 (in stannite) + FeS (in sphalerite), which is valid for all the minerals from the stannite group. For this reaction a Kd partition coefficient is calculated by the formula Kd = (Fe/Zn)stannite/(Fe/Zn)sphalerite [63,66]. The temperature estimation of the stannite-sphalerite system was experimentally determined and approximated according to the Equations (1) (by Nekrasov et al. [63]) and Equation (2) (by Nakamura and Shima [66]): log Kd = −1274/T + 1.174 (1) log Kd = −2800/T + 3.5 (2) where T is the temperature in Kelvin. Osadchiy et al. [68] state that accurate results can be obtained when the data used for the temperature estimation are acquired from mineral grains located at a maximum of 10 µm away from the sphalerite-stannite contact. In contrast, Shimizu and Shikazano [65] claimed that reliable results are obtained by the analyses performed on mineral grains located at least 30 µm away from the stannite-sphalerite contact. Equations (1) and (2) have been used to calculate the temperatures of both the co-existing ferrokësterite and sphalerite and kësterite and sphalerite pairs. No EPMA data is available for the sphalerite associated with stannite. The kësterite, ferrokësterite, and sphalerite EPMA data points considered further for the temperature calculations are located in the vicinity of the kësterite/ferrokësterite-sphalerite contact at a distance ranging between 3-13 µm for kësterite; 6-9 µm for ferrokësterite; and 4-5 µm for sphalerite in the polished section plan. The calculated temperature for the kësterite-sphalerite pair is 503 ± 68 • C according to Equation (1), and 447 ± 17 • C according to Equation (2). The calculated temperature for the co-existing ferrokësterite-sphalerite is 287 ± 25 • C according to Equation (1), and 310±35 • C according to Equation (2).

Fe-oxides/Hydroxides
The mineral assemblage from the Blidar Contact orebody also contains Fe-oxides/hydroxides. These minerals mainly occur along fissures and mutual grain boundaries between different minerals, e.g., bornite, wittichenite, sphalerite, and galena. They likely formed during the retrograde skarn stage with the release of abundant fluids into the mineralizing system [2]. Iron-oxides/hydroxides are hosted by almost all identified sulfides, with a higher frequency for bornite and wittichenite (Figure 4c). Fe-oxides/hydroxides are also present at sphalerite-bornite contacts (Figures 4f and 5d) and are located along the boundary of bornite grains associated with digenite and minor covellite, and locally, also kësterite-ferrokësterite. In an acid environment, Fe ions released from chalcopyrite may migrate toward the grain border and form a superficial magnetite layer [71], or can be released from the system [72]. Due to iron migration, the remaining bulk particles possess an iron deficit with a composition close to bornite or even covellite [72].

Conclusions
Bornite is the main sulfide in the Blidar Contact, Antoniu, and Antoniu North orebodies from the Băit , a-Bihor skarn deposit. New EPMA data confirm the occurrence of stannite and support the first mention of kësterite and ferrokësterite in the deposit. Bismuth-bearing minerals, i.e., wittichenite, emplectite, joséite-B, and native bismuth are relatively common, especially in the Blidar Contact orebody. Locally, the Cu-Bi sulfosalts and tellurides are concentrated around native bismuth and occur according to the outward zonality: native bismuth/joséite-B → emplectite → wittichenite. Part of the wittichenite and emplectite formed as a result of the reaction between bornite and native bismuth.
Native gold and electrum were microscopically identified in the Antoniu North ore body in association with hessite and wittichenite hosted by bornite and also in association with chalcopyrite hosted by sphalerite.
The use of ferrokësterite-sphalerite geothermometry constrains the temperature of ore formation in the range of 287 ± 25 • C to 310 ± 35 • C. Application of the kësterite-sphalerite geothermometer yields a somewhat higher temperature (447 ± 17 • C to 503 ± 68 • C).