Polymetallic CuBi-( Pb-Zn-CoAg ) mineralization of the Perin Potok locality near Bor , Serbia

Complex polymetallic Cu-Bi-(Pb-Zn-Co-Ag) mineralization of the Perin Potok 
 locality occurs as disseminations and fine nests in 
 quartz-ankerite-(sericite) veins. The veins are located in metamorphic rocks 
 of the outer contact zone of the Variscan Gornjane Granitoid. The 
 mineralization consists of (in decreasing abundance): chalcopyrite, aikinite, 
 bismuth, galena, Ag,Bi-bearing tetrahedrite, sphalerite, cobaltite and an 
 unnamed Bi2Te mineral. All these phases form distinctive exsolutions and 
 intergrowths and they simultaneously precipitated from a very complex 
 hydrothermal fluid. Silver shows elevated contents in tetrahedrite (3.3-4.4 
 wt. % Ag), galena (0.9-1.1 wt. % Ag) and in the unnamed Bi2Te mineral (0.9 
 wt. % Ag). Such high Ag concentrations can imply that Ag minerals could be 
 also present. Minor amounts of rutile showing fine intergrowths with sericite 
 also occur in this paragenesis. This is W-bearing rutile that shows zoning 
 caused by up to 2.2 wt. % W. The studied mineralization is probably 
 genetically related to the Variscan Gornjane Granitoid, although the 
 possibility of derivation from the metamorphic basement should be also taken 
 into account. [Projekat Ministarstva nauke Republike Srbije, br. OI176016]


Introduction
The locality of Perin Potok, situated about 10 km north-eastward of the town of Bor, has long been known as one of the occurrences of W-Mo mineralization in north-east Serbia.The mineralization is mostly represented by scheelite and molybdenite and, apart of these minerals, the presence of sulphides -chalco-itoids and their metamorphic basement are found to host numerous ore-bearing quartz veins with W, Au and sporadically with Cu, Fe and Pb-Zn mineralizations (SIMIĆ 1953;KALENIĆ et al. 1973KALENIĆ et al. , 1976)).In this context, the true origin of these mineralizations (either magmatic or metamorphism-related?) remained poorly constrained.
In this study we report new data on the polymetallic mineralization occurring in the Perin Potok locality, in particular on the assemblage dominated by Cu-Bi-(Pb-Zn-Co-Ag) metallogeny.To our best knowledge, this locality represents the very first occurrence of such type of mineralization in north-east Serbia.By presenting textural and mineral chemistry characteristics of the observed ore paragenesis we want to provide better understanding of the formation of W-Modominated mineralization in north-east Serbia.We also have reasons to believe that this association of metals could be present in economically significant proportions in this area, and can shed new light on the whole metallogeny of the entire region.

Geological Setting
The studied polymetallic mineralization occurs in a very complex area of the East Serbian Carpatho-Balkanides that represents an assemblage of the Lower Paleozoic terranes intruded by the late Variscan granitoids (e.g.KARAMATA & KRSTIĆ, 1996;KARAMATA, 2006).The Perin Potok mineralization is found along the southern contact zone between the Variscan Gornjane Granitoid and the surrounding metamorphic rocks of the Stara Planina-Poreč terrane.Further to the west there are occurrences of the Upper Cretaceous andesitic volcanics and volcaniclastics of the Timok Magmatic Complex that is famous of large porphyrycopper and epithermal-gold systems (e.g.BANJEŠEVIĆ, 2006;KOLB et al. 2013).A simplified geological sketch of the area is shown in Fig. 1.
Gornjane is a NNW-SSE elongated lens-shaped granitoid pluton that consists of quartz monzonite, granodiorite, quartz diorite, diorite and syenite displaying gradual transitions in composition (DIVLJAN & MIĆIĆ 1960;KALENIĆ et al. 1976;VASKOVIĆ et al. 2012).Central parts of the magmatic body are composed of quartz monzonite surrounded by granodiorite, while in the peripheral parts and contact zones dioritic rocks generally occur.This granitoid pluton contains numerous quartz veins, veins of pegmatite and aplite, and younger shallow granite intrusions.VASKOVIĆ et al. (2012) reported the U-Pb zircon ages of 323.3 ±2.6 Ma to 305.8 ±3.6 Ma.These ages confirm that the emplacement of this granitoid massive occurred during the late Variscan events.
The metamorphic basement consists of the Rifeo--Cambrian to the Lower Palaeozoic units (KALENIĆ et al. 1976) composed of various metabasic and metasedimentary rocks.The locality of Perin Potok is located along the contact zone between the Gornjane Granitoid and the Lower Palaeozoic unit.The latter unit starts with conglomerates and continues with sandstones, siltstones, metamorphosed clays and phyllites.
The mineralization is found in up to 30 cm thick quartz-ankerite and subordinate sericite veins.The veins can be found only in stream beds as stream flanks are covered by humus.They are mainly enclosed in adjacent metamorphic rocks.Sulphide minerals form disseminated mineralization and nests up to 5 mm in size in these veins.

Results
This complex mineralization is characterized by fine intergrowths of many sulphides (Fig. 2), among which chalcopyrite and aikinite predominate, whereas bismuth, galena, sphalerite, tetrahedrite and locally cobaltite are less abundant.In addition, fine exsolutions of an unnamed Bi 2 Te mineral also occur in this assemblage, but only as rare grains up to 10 µm in size.Chalcopyrite and aikinite occur as irregular grains mainly up to 1-2 mm in size.Exceptionally,  chalcopyrite can be up to 4-5 mm in size.Bismuth forms characteristic exsolutions in the central parts of aikinite grains (Fig. 2a).Tetrahedrite also forms exsolutions in aikinite (Fig. 2a), but it mainly appears as intergrowths with chalcopyrite, sphalerite and galena (Fig. 2c).Cobaltite forms individual subhedral to euhedral grains up to 0.1 mm in size or shows intergrowths with chalcopyrite (Fig. 2d).
Chemical compositions of all the above-mentioned ore minerals are given in Table 1.Aikinite, chalcopyrite and bismuth do not contain EMPA-detectable trace elements.Tetrahedrite displays chemical zoning caused by Sb-As solid solution.Apart of the essential elements, this mineral contains considerable amounts of silver (3.3-4.4 wt.% Ag) and bismuth (3.3-6.1 wt.% Bi).The presence of these metals characterizes this mineral as a variety of Ag,Bi-bearing tetrahedrite.The unnamed Bi 2 Te mineral contains 0.9 wt.% Ag and shows Bi:Te ratio close to 2:1.There is no known mineral of such composition.However, as this mineral occurs in very small grains, its detailed determination was not possible.Galena contains 0.9-1.1 wt.% Ag.Sphalerite contains common amount of iron (1.6 wt.% Fe) and relatively higher amounts of cadmium (7.5 wt.% Cd).Cobaltite shows slight zoning caused by its common impurities of iron (3.8-6.8 wt.% Fe) and nickel (1.0-1.3 wt.% Ni).
Negligible amounts of pyrite and pyrrhotite are also present.Pyrite occurs in anhedral to subhedral individual grains and in aggregates up to 0.3 mm in size, while pyrrhotite forms rare subhedral to euhedral grains up to 50 µm in size.
Minor amounts of rutile occur as irregular grains and aggregates up to 0.2 mm in size.Rutile shows fine intergrowths with sericite and displays crystal zoning caused by uncommonly high tungsten contents of up to 2.2 wt.% W (Fig. 3 and Table 2).Sericite, i.e. fine-grained muscovite that is coeval with rutile (Fig. 3), appears within cracks in sulphidebearing mineral aggregates (Fig. 2b).Thus, it could be concluded that this mineral assemblage formed in the stage of mineralization where sulphide minerals are   simultaneously deposited with quartz and ankerite, whereas sericite and rutile precipitated in a subsequent paragenetic sequence of the same stage.

Discussion and conclusions
The above reported data suggest that minerals carrying Cu and Bi -chalcopyrite and aikinite, are the most abundant ore phases in the studied sample suite, whereas bismuth, galena, sphalerite, cobaltite and tetrahedrite are subordinate.All these ore minerals of Cu, Bi, Pb, Zn and Co, which form distinctive exsolutions and intergrowths, simultaneously precipitated from a complex hydrothermal fluid.This fluid was enriched also in additional elements of interest, especially in Ag, and in lesser degree in Ni and Te.The presence of Ag minerals is not revealed in this assemblage, but this precious metal occurs is considerable amounts in other ore minerals, principally in tetrahedrite and galena.Nickel occurs in cobaltite, while tellurium forms fine exsolutions of an unnamed Bi 2 Te mineral.Sphalerite contains elevated contents of cadmium.Additionally, minor amounts of W-bearing rutile are also found.Thus, it could be suggested that this paragenesis was formed under high-to middletemperature hydrothermal conditions from a complex Cu-Bi-Fe-Pb-Zn-Ag-Co-Ni-Ti-W-As-Sb-S-Te-bearing fluid.The presence of such very complex ore paragenesis indicates that, in addition to already known occurrences of W-Mo mineralization (SAVIĆ 1956), this area probably contains other types of mineralization.These types can be variable at relatively short distances and they may host so far undiscovered ore phases, especially Ag-bearing minerals.This is inferred from elevated contents of this metal, which are found in some minerals investigated by this study.
The metal budget of the studied mineralization probably derived from the Gornjane granitoid.However, as this mineralization was revealed in adjacent metamorphic rocks, it lacks clear spatial and genetic relationships of the mineralization with the granitoid.Thus, the origin of the mineralization from the metamorphic basement could be also considered.It is much less plausible that this mineralization is related to the Upper Cretaceous Timok Magmatic Complex because, this very well-studied Cu-dominated mineralization is not characterized by the presence of Bi minerals or by the ore mineral assemblage present here.
As already stated above, Cu, W, Mo and in lesser degree Pb mineralization is widespread in quartz-and pegmatite veins of the Gornjane Granitoid.Moreover, SIMIĆ et al. (1953) found the presence of cassiterite in some placers of the streams surrounding the Gornjane granitoid.They reported an association of W, Mo and Sn, and presumed that the presence of still undiscovered Bi minerals could complete the already known metal assemblage typical of granitoid rocks.The oc-currence of Bi minerals reported by this study supports their speculation.
Partly similar mineralization regarding predominant metals was discovered in east Serbia only in the region of Stara Planina Mt.It is the Cu-Bi ore deposit of Aljin Do where vein ore bodies are found to occur in the large gabbro massive of Zaglavak, and this deposit is interpreted as having derived from the adjacent Variscan granitoids (JANKOVIĆ 1990).
On the other side, the Perin Potok mineralization occurs in quartz-ankerite-(sericite) veins located in metamorphic rocks that are generally rich in quartz and sericite.These metamorphic rocks are widespread in NE Serbia and represent also the basement of other granitoids in this region (SIMIĆ et al. 1953).Numerous ore-bearing quartz veins with W-, Au-and various polymetallic mineralizations are found in the metamorphic basement in other parts in NE Serbia (SIMIĆ 1953;KALENIĆ et al. 1973KALENIĆ et al. , 1976)).For instance, there is a similarity of the ore vein type found in Perin Potok with those occurring near Neresnica because both contain ankerite (SIMIĆ 1953).All these occurrences were often explained as genetically related to Variscan granitoids, even though a clear spatial relationship between the mineralized veins and the granitoids lacks in many cases.

Table 1 .
Results of electron microprobe analysis (wt.%) of the ore minerals from the Perin Potok locality.

Table 2 .
Results of electron microprobe analysis (in wt.%) of W-bearing rutile from the Perin Potok locality.