A review of Pb-Sb(As)-S, Cu(Ag)-Fe(Zn)-Sb(As)-S, Ag(Pb)-Bi(Sb)-S and Pb-Bi-S(Te) sulfosalt systems from the Boranja orefield, West Serbia

Recent mineralogical, chemical, physical, and crystallographic investigations of the Boranja orefield showed very complex mineral associations and assemblages where sulfosalts have significant role. The sulfosalts of the Boranja orefield can be divided in four main groups: (i) Pb-Sb(As)-S system with ±Fe and ±Cu; (ii) Cu(Ag)-Fe(Zn)-Sb(As)-S system; (iii) Ag(Pb)-Bi(Sb)-S; (iv) and Pb-Bi-S(Te) system. Spatially, these sulfosalts are widely spread, however, they are the most abundant in the following polymetallic deposits and ore zones: Cu(Bi)-FeS Kram-Mlakva; Pb(Ag)-Zn-FeS2 Veliki Majdan (Kolarica–Centralni revir–Kojići); Sb-Zn-Pb-As Rujevac; and Pb-Zn-FeS2-BaSO4 Bobija. The multi stage formation of minerals, from skarnhydrothermal to complex hydrothermal with various stages and sub-stages has been determined. All hydrothermal stages and sub-stages of various polymetallic deposits and ore zones within the Boranja orefield are followed by a variety of sulfosalts.


Introduction
Sulfosalts are complex sulfide minerals with the general formula: A m B n S p ; where A represents a metal such as Cu, Pb, Ag, Fe and rarely Hg, Zn, V; B usual-ly represents semi-metal such as As, Sb, Bi and rarely Ge, or metals like Sn and rarely In; and S is S or rarely Se or/and Te (ANTHONY et al. 1990;MOËLO et al. 2008).Formerly, it was believed that the sulfosalts were salts of complex hypothetical thioantimonic or A review of Pb-Sb(As)-S, Cu(Ag)-Fe(Zn)-Sb(As)-S, Ag(Pb)-Bi(Sb)-S and Pb-Bi-S(Te) sulfosalt systems from the Boranja orefield, West Serbia thioarsenic acids (e.g., HSbS 2 , H 18 As 4 S 15 , H 3 AsS 3 ).X-ray diffraction (XRD) analyses indicate that the crystal structures of Pb-Sb-As-S sulfosalts are based on structural fragments of simpler compounds such as galena (lead sulfide; PbS) blocks and stibnite (antimony trisulfide; Sb 2 S 3 ) sheets (WERNICK 1960).No encompassing theory has been evolved to rationalize many of these curious compounds.The complexity of many of the structures evidently results from them having crystallized at low temperatures and the consequent high degree of ordering of the metal atoms.Syntheses of such compositions at higher temperature usually result in structures simpler than the complicated low-temperature forms.There are about 200 known sulfosalts (MOËLO et al. 2008).These minerals were formed under the mutual influence of different sulfantimonide, sulfarsenide, sulfstanate, sulfbismuthinide, etc. anions (e.g.SbS 2 1-, As 4 S 15 18-, AsS 3 3-, Bi 2 S 6 6-), with metal ions (e.g., Cu 1+; 2+ , Ag 1+ , Fe 2+ , Pb 2+ ).These reactions occur only in mineralized solutions with increased alkalinity and high concentration of H 2 S. Deposition of miargirite AgSbS 2 , pyrargyrite Ag 3 SbS 3 and stephanite Ag 5 SbS 3 occur during mutual influence of sulfantimonide anions with Ag 1+ .Interactions between already deposited sulfides (galena, chalcopyrite, etc.) and mineralized solutions (ascendant and/or descendent) may result in the formation of younger sulfosalts.The typical example of the reaction crystallization is contact between galenachalcopyrite with sulfantimonide solutions when bournonite PbCuSbS 3 was deposited.The corrosive reaction is characteristic for influence of sulfantimonide solutions along galena surfaces thus creating wool-like variety boulangerite Pb 4 Sb 3 S 11 and/or semseyite Pb 9 Sb 8 S 21 (RAMDOHR 1980;ANTHONY et al. 1990).Moreover, decrease of temperature and pressure led to decomposition of high-temperature solid solutions when two or more stable sulfosalts phases were formed.This is particularly visible in Pb-Ag-bearing sulfbismuthinide when complex exsolutions with lamellae structures were deposited (i.e., phases along the lillianite-gustavite solid solution Pb 3 Bi 2 S 6 -AgPbBi 3 S 6 ) (COOK 1997).
Although under exceptional circumstances some sulfosalts may constitute Ag ores (i.e., proustite, pyrargyrite, and stephanite), and other species have constituted Ag ores (in minor amounts), Hg, Tl, As, and Sb (i.e., boulangerite, livingstonite, enargite, and tennantite-tetrahedrite groups), their economic importance is sometimes significant (Ag in the Pb-concentrate) and sometimes trivial.Aside from mineralogical curiosities, the sulfosalts are of interest because their electronic properties are related to those of semiconductors (CHVILYOVA et al. 1988).
The Boranja orefield (BOF) is well-known since the Roman Empire and Medieval times and is still important factor of modern mining in this part of Serbia.Significant research began during the second half of the 20 th century.Comprehensive mineralogical, crystallographic, geochemical and petrological studies yielded important results in defining mineral compositions of the mineralizations and surrounding rocks (e.g.KARAMATA 1955;RADUKIĆ 1960;TOMIĆ 1962;BORODAEV 1978;JANKOVIĆ 1978;RADOSAVLJEVIĆ et al. 1982).This study shows a synthesis of previous research of sulfosalts with revisited and new data.
Minerals of the BOF were deposited in several successive stages, which together correspond to a single regional-scale mineralization event that is related to the subvolcanic-plutonic intrusions of the Boranja magmatic complex.This is well demonstrated by the Fig. 1.Detailed geological and metallogenic map of the BOF (modified according to Basic Geological Map of Serbia, 1:100,000).Upper left corner shows exact location of BOF within Serbia (MONTHEL et al. 2002).
The Bobija polymetallic barite-sulfide deposit is situated in East slopes of Sokolske Mnt., which is some 15 km to the NE away from Ljubovija.The deposit itself consists of complex geological composition mainly built of Paleozoic and Mesozoic sediments (JANKOVIĆ 1990).The Bobija deposit is composed of massive barite and sulfide FeS 2 -Pb-Zn-Cu elongated lens-like ore bodies.Massive barite ore bodies are consisted of 50 to 90 wt% of BaSO 4 (RADO-SAVLJEVIĆ et al. 2013B).
Their mutual structural and textural characteristics are complex, and characterized by small grain size (<5-100 µm), which beside intergrowths (effects of reaction and/or corrosive processes, high-temperature exsolution products, etc.) additionally makes it difficult to single it out for crystallographic (XRD) and chemical (spectrochemical and mass spectrometric) investigations.Chemical composition of the minerals was calculated according to ANTHONY et al. 1990.Besides so far determined sulfosalts, new minerals from the Pb-Ag-Bi-Sb-S system could be discovered (RADOSAVLJEVIĆ 1988).

Sulfosalts of the Pb-Sb(As)-S system, with ±Fe and ±Cu
This sulfosalt group is most abundant occurring in almost all deposits and mineralizations of the Boranja orefield.Sulfosalts of Pb-Sb(As) composition are the most common in the Rujevac polymetallic Sb-Zn-Pb-As deposit.In this deposit Pb was characteristically deposited after Sb (BORODAEV 1978), which genetically deviates from other Sb deposits and occurrences within the SMMP.There are various of sulfosalts belonging to the sphalerite-Pb-Sb(As) sulfosalt-As mineral assemblage, which are very dominant.So far, the following sulfosalts have been determined: zinkenite -Pb 8.93 (Sb 18.94The most abundant sulfosalt of the sphalerite-Pb-Sb(As) sulfosalt-As mineral assemblage is zinkenite.It was first discovered by JANKOVIĆ et al. (1977) and MOËLO et al. (1983), and later supplemented by new data by RADOSAVLJEVIĆ (1988), ZARIĆ et al. (1992), RADOSAVLJEVIĆ (2012), RADOSAVLJEVIĆ et al. (2012), and RADOSAVLJEVIĆ et al. (2014a).It occurs in the following mineral association: Pb-Sb(As) sulfosalts, sphalerite, arsenopyrite, realgar, duranusite, native As, stibarsen, dolomite, and quartz.It occurs as tabular, needle-and wool-like fibrous individuals (plumosite), forming larger individual aggregates mostly in the interstices of the quartz matrix.Its central zones are locally replaced by plagionite.Moreover, zinkenite intensively intersects, penetrates and overgrowths cataclased sphalerite aggregates (Fig. 2f).It also contains inclusions of quartz and duranusite.When replacing crystal aggregates of older stibnite, it is often penetrated and overgrown along its edges by dolomite metacrysts.Twinnite is characterized by polysynthetic twinning and commonly occurs as the youngest sulfosalt along the edges of stibnite, lesser plagionite.Although it is mentioned in the literature (JANKOVIĆ et al. 1977), robinsonite was not confirmed in our study.In addition, according to PRUSETH et al. (1997), robinsonite is unstable at temperatures below 300 °C, confirming its absence in this deposit.
The most abundant sulfosalt of the Veliki Majdan ore zone is jamesonite.It occurs as short-prismatic crystals, deposited in the interspaces of chalcopyrite and calcite aggregates.Bournonite most frequently occurs at the grain boundaries between galena and chalcopyrite replacing galena (Fig. 2b).It is Ag-free, and contains As from 0.5 to 7.7 wt%.As-bearing bournonite also occurs in the epithermal Au-Te vein system of the Sacarimb deposit in Romania (CIOBANU et al. 2005).EPMA data yielded stoichiometric composition, without presence of any other element except As.
Besides these Pb-Sb sulfosalts, new analyses confirmed presence of a Sb-member with the highest Sb content (46.4-46.7 wt%) in the Centralni revir locality within the Veliki Majdan ore zone.EPMA yielded following average crystallochemical formula of fülöppite Pb 3.02 Sb 7.97 S 15.01 (3 analyses).It is deposited in interspaces between pyrite grains in short prismatic forms (Fig. 2c).Unlike fülöppite from the Rujevac polymetallic deposit, this one is As-free.

Sulfosalts of the Cu(Ag)-Fe(Zn)-Sb(As)-S system
Tetrahedrite is a Cu-Sb sulfosalt mineral with following average crystallochemical formula: (Cu,Ag) 10 (Fe,Zn) 12 Sb 4 S 13 .It is the Sb end-member of the continuous solid solution series with As-bearing tennantite.Other elements also substitute in the structure, most notably Fe and Zn, along with less common Ag, Hg and Pb.Bismuth also substitutes Sb, and Bibearing tetrahedrite or annivite is a recognized variety.
Tetrahedrite-group minerals occur in coarse crystalline aggregates only within the Veliki Majdan ore zone.These minerals are closely related to chalcopyrite and bournonite, forming part of the galena-sphalerite mineral assemblage.Furthermore, they usually cement older cataclastic pyrite aggregates.Tetrahedrite-group minerals rim galena aggregates and are also replaced by boulangerite.According to both, optical observations and measurements in polished sections (microhardness and reflectance spectra) and chemical analyses, two types of tetrahedrite-group minerals can be recognized (RADOSAVLJEVIĆ et al. 1986).
The mineralogical investigation confirmed that the Bobija deposit consists of a simple mineral association composed of sulfides, sulfosalts (tetrahedrite-tennantite group), native Ag, barite and gangue minerals.Tetrahedrites are significantly abundant in the ore.According to the optical features they generally correspond to tennantite, and partly to tetrahedrite.These minerals are often in association with pyrite, sphalerite, and galena but in a lesser extent.Colloform grains of tetrahedrite-tennantite composition are not rare, and are separated from pyrite and sphalerite in the assemblage (Fig. 2e).Occurrence of disperse pyrite in tetrahedrite surfaces when it completely changes its optical features is not rare.Central parts of these aggregates are sometimes seized by zoned pyrite.
According to RADOSAVLJEVIĆ et al. (2013a), Ag content incorporated in the structure of galena amounts to approximately 15 wt%, while the rest is in a form of micron ("visible") and/or submicron ("invisible") particles of Ag minerals.The following Ag minerals diaphorite, fizélyite, freieslebenite, schirmerite, and Ag-bearing tetrahedrite were determined qualitatively into the insoluble residue using XRD method.

Sulfosalts of the Pb-Bi-S(Te) system
This group belongs to lillianite homotypic series.The definition and crystal chemistry of this homologous series were presented by MAKOVICKY (1977) and MAKOVICKY & KARUP-MØLLER (1977a, 1977b). .These cannot be mutually macroscopically distinguished owing to their very small grain-size.Aggregates were embedded in garnet-calcite matrix.Well-developed crystals have not been observed, only spherical and spindle-like forms up to 10 µm in length.In addition, fewer occurrences of these sulfosalts were also determined in the Kolarica locality (Veliki Majdan ore zone), associated with pyrrhotite-sphalerite-galena mineral assemblage (RA- DOSAVLJEVIĆ-MIHAJLOVIĆ et al. 1998, 2007;RADOSA-VLJEVIĆ et al. 2013a).

Sulfosalts occurring in the
Bursaite is characterized by complex intergrowths that appear along cracks and fissures of chalcopyrite and silicates in a form of lath-like grains (Fig. 2i).In comparison to the other accompanying Pb-Bi sulfosalts, it is harder.It often contains inclusions of native Bi as exsolution products.According to MOELO et al. (2008), bursaite has been discredited as a mineral species.From the Sn-W deposit (Shumilovskoe locality) MOZGOVA et al. (1988) described an almost identical mineral to the one from the Kram-Mlakva ore zone.In their detailed mineralogical work on bursaite and cannizzarite the authors proposed that bursaite should be retained as an intergrowth of two lillianiterelated phases, each with distinct unit-cell parameters.The EPMA composition, which represents a composite of two phases, indicates a Pb deficiency (n ≈ 3.83).Minerals of bursaite composition from four known localities (Uludag-Turkey, Shumilovskoe-Russia, Cofer-Virginia, and Kram-Mlakva-Serbia) still have a problem of unsolved crystal structure.However, our new evidences, led to confirm that bursaite is undoubtfully a distinct mineral.Unfortunately, numerous attempts to determine the crystal structure using XRD on both single-crystal and powdered samples from the Kram-Mlakva ore zone were not successful, due to a very low crystallinity degree (RADOSAVLJE-VIĆ-MIHAJLOVIĆet al. 2007).
Cannizzarite reflectance is moderately high, but lower than bursaite (Fig. 2i).Reflection pleochroism is distinct, light gray to creamy.The anisotropy is strong, similar to bursaite, and hardness is considerably lowered (similar to galena).According to optical (reflectance, bireflectance, anisotropy) and physical (hardness) characteristics the investigated sulfosalt corresponds to cannizzarite.It is determined on a basis of optical, crystallographic and chemical measurements (RADOSAVLJEVIĆ-MIHAJLOVIĆ et al. 2007).Cosalite is less abundant than bursaite and canniz-zarite.It occurs along sulfosalt aggregate rims as "jaggy" intergrowth forms.It is white, very similar to galena with trace of cream.Reflection pleochroism is weak and distinct only in oil, light gray to light green.The anisotropy is noticeable, but very distinct in oil with strong illumination.Reflectance and hardness are lower than in bursaite and cannizzarite (RADOSA-VLJEVIĆ 1988).
Aikinite is the least abundant.It occurs in a form of elongated crystals, in a contact with bursaite and chalcopyrite.Hardness is the highest of all sulfosalts of this group.It is white with a light tint of cream.Reflection pleochroism is distinct in air, in oil very striking, light yellow to gray.Anisotropy is also distinct in air, in oil rather high.It is determined on a basis of optical and chemical measurements (RADOSAVLJEVIĆ 1988).
Ustarasite occurs only in the Kolarica locality (Veliki Majdan ore zone) mostly as mutually parallel thin needle-like crystals (up to 100 µm in length), and rarely as rhombohedral crystals embedded in older galena and carbonate matrix (Fig. 2g).It is an exsolution product of galena and Bi-Sb-Ag complex compounds.Bireflectance is noticeable, //N strong luster like galena, ⊥ N darker with gray tint (quite to that of falkmanite), microhardness similar to cosalite, and anisotropy is strong without internal reflections (RADOSAVLJEVIĆ et al. 2013a).
Tetradymite also occurs in sulfosalt aggregates in form of fine needles.It is white, with faint yellowish tinge (Fig. 2i).Bireflection is weak, hardly visible at grain boundaries, light yellow to creamy.The anisotropy is distinct, and reflectance is high (R ~ 60 %).It is determined on a basis of optical and chemical measurements (RADOSAVLJEVIĆ-MIHAJLOVIĆ et al. 2007).

General genetic and paragenetic characteristics
Temperature of deposition of Pb-Zn mineral associations in the BOF range from 480-160 ºC (RADOSA-VLJEVIĆ et al. 2012).Galena and sphalerite of this orefield were formed in high-, middle-and low-temperature hydrothermal stage, while Pb-Zn mineral associations of the Pb(Au)-Zn-FeS 2 Veliki Cip and CaF 2 -Pb-Zn Ravnaja deposits unquestionably correspond to the high-(480 ºC) and low-temperature hydrothermal stage (230 ºC), respectively.In addition, the Pb(Ag)-Zn-FeS 2 Veliki Majdan ore zone corresponds to the temperature range from 450 to 370 ºC that is between high-and medium-temperature hydrothermal stages.Temperature decreases moving further from the Tertiary granodiorite of Boranja according to the following sequence: Kolarica-Centralni revir-Kojići.Moreover, in the Ravnaja deposit, NIKOLIĆ & GATTER (1986) determined two temperature intervals of formation of fluorite (275-245 ºC and 205-160 ºC), and density of fluids (0.98-0.80 g/cm 3 ).In addition, temperature range of deposition from 280 to 160 ºC was obtained on quartz and sphalerite from the Sb-Pb-Zn-As Rujevac deposit using cryometric method (MUDRINIĆ 1984).
Judging by the look of the exsolution structures of various compositions established in all levels of ore deposits, the temperature of crystallization of all skarn, high-, and middle-temperature hydrothermal associations was identical to formation of isometric coarsecrystalline grain-like structures.However, low-temperature hydrothermal associations characterize finegrained colloform and gel-like textures with regular appearance of recrystallization (RADOSAVLJEVIĆ 1988).
Based on paragenetic relations into the polymetallic deposits of the BOF, the beginning of crystallization is connected to the low partial pressure fS 2 , and deposition of low sulfidization minerals (pyrrhotite, Fe-rich sphalerite, tetrahedrite group of minerals, Pb-Sb and Pb-Bi sulfosalts, etc.).Minerals of high sulfidization (transformation of pyrrhotite into pyrite, pyrite, Fe-poor sphalerite, antimonite, realgar, etc.) began to crystalize with temperature decrease, partial pressure fS 2 increase, and spatial distancing from the Tertiary granodiorite of Boranja.Deposition areas were carbonates (mostly Triassic limestones) and silicates (dacite, andesite, slates), but in a lesser extent (RADOSAVLJEVIĆ et al. 2013a).
Silver is important and genetically significant metal which content varies from 10 to 820 g/t in the BOF.Its transport was achieved by polysulfide solutions enriched with Pb, Bi, Sb, and As.However, a possibility of carbonate-bicarbonate and halogen complex solutions should not be excluded.The best correlation is between Ag and Pb (r=0.828significant at the 95 % confidence level), while among other elements it does not exist.This is expected since galena is the main Ag-bearing mineral, while occurrence of Ag minerals (Ag-tetrahedrite, pyrargyrite, electrum, and native Ag) is limited (RADOSAVLJEVIĆ 1988).Besides Ag, typomorphic elements as Bi and Sb are significantly abundant in galena.Complex investigations of galena from various deposits yielded that it frequently occurs in a form of isostructural solid solutions with diaphorite, fizélyite, freieslebenite, and schirmerite (e.g.WERNICK 1960;ONTOYEV & KORSAKOVA 1967;HODA & CHANG 1975;WANG 1999;CHUTAS et al. 2008).
Crystallization of minerals in the BOF occurred in several successive stages, which together correspond to the unique mineralization cycle.According to the deposited minerals it can be concluded that hydrothermal solutions descend from common magmatic chamber connected to the Tertiary granodiorite of Boranja (RADOSAVLJEVIĆ et al. 2013a).

Conclusions
The areal extent of the SMMP covers around 30,000 km 2 in the territory of Serbia and extends over the three major geotectonic units: the Vardar ophiolite zone, the Serbo-Macedonian massif, and the inner Dinarides.It covers a small part of eastern Bosnia and Herzegovina (B&H), larger parts of Serbia and the Former Yugoslav Republic of Macedonia (FYRM), and also extends towards Bulgaria and Greece.The SMMP contains numerous volcanic-intrusive complexes of calcalkaline and shoshonitic affinity.These igneous complexes are directly associated with the development of numerous deposits and metal occurrences; primarily as Pb, Zn, Sb, then Cu and Mn, and to a lesser extent Fe, Bi, Ag, Hg, U, Sn, and W.
Polymetallic deposits of the BOF are genetically connected to the Tertiary granodiorite complex.It consists of a large number of Pb-Zn and Sb sulfide deposits, and in a lesser extent Cu, As, Bi and Ag.Magnetite deposits of lesser importance, connected to Ca-skarn stage, were formed along the contact of Triassic limestones and quartz diorite.Minerals of the BOF are characterized by very diverse types and are consisted of sulfides, sulfosalts, native metals, wolframates, molybdates, oxides, silicates and hydroxides.