1. Introduction
Although skarn deposits represent products of interaction of a silicate melt (proximal skarns) or magmatic fluids (distal skarns) with a carbonate rich lithology, hydrothermal fluids play a significant role in evolution of all types of skarn deposits. Late hydrothermal overprints particularly affect the skarn deposit geometry, type of alteration products and ore distribution [
1,
2,
3,
4].
The Sasa Pb-Zn-Ag deposit is a typical distal skarn deposit and has been selected as a site to study processes that involve transport of base metals by magmatic and hydrothermal fluids as well as physicochemical factors that control the deposition of base metal-bearing mineral phases. The Sasa deposit (42.0° N, 22.5° E) is located on the Balkan Peninsula, approximately 150 km east from Skopje, Republic of Macedonia (
Figure 1). It consists of three ore-bearing localities: Svinja Reka, Golema Reka and Kozja Reka (
Figure 2a). The deposit hosts approximately 23.4 million metric tons of ore at 7.5% of Pb and Zn and up to 22 g/t Ag. Mining activities in the area date back to ancient times. The first geological investigations began in the 19th century and industrial production started in 1966. Since November 2017, the deposit is operated by Central Asia Metals.
The Sasa Pb-Zn-Ag skarn deposit is hosted by the Serbo-Macedonian massif, a large elongate basement complex situated along the eastern part of the Balkan Peninsula. It extends southward from Serbia through Kosovo, Macedonia and Bulgaria to the Chalkidiki Peninsula in northern Greece (
Figure 1) and holds numerous economically important ore deposits of Cu, Au, Pb and Zn (e.g., Bor and Majdanpek, Serbia; Toranica, Sasa and Bucim, Macedonia; Osogovo, Bulgaria; Skouries, Greece [
5,
6,
7,
8,
9]).
The Sasa deposit is spatially and temporally associated with the Tertiary calc-alkaline magmatism [
10]. It comprises prograde and retrograde mineral assemblages hosted by a sequence of Paleozoic marbles intercalated with quartz–graphite schists [
11]. The prograde mineralization is represented by anhydrous Ca-Fe-Mn-silicate minerals (pyroxenes and pyroxenoids). A subsequent retrograde stage contains amphiboles, epidote, chlorites and ilvaite [
12]. The principal ore minerals, galena and sphalerite, are accompanied by variable amounts of hydrothermal quartz and carbonates.
This study presents the mineral chemistry, fluid inclusion and stable isotope data obtained on the skarn and hydrothermal mineral assemblages to give an insight into the evolution of the mineralizing fluids, to constrain the physiochemical conditions during the skarn formation and ore deposition and to refine the metallogenic model of the deposit.
3. Materials and Methods
A total of 30 samples were collected from the Svinja Reka locality, an active underground mine at the Sasa deposit. The representative samples of host rocks, associated magmatic rocks and ore mineralization were selected for further mineralogical and geochemical studies.
Paragenetic relationships were studied in thin polished sections by transmitted and reflected polarized light microscopy. The X-ray powder diffraction (XRD) was conducted at the University of Zagreb on a Philips PW 3040/60 X’Pert PRO powder diffractometer (45 kV, 40 μA), with CuKα-monochromatized radiation (λ = 1.54056 Å) and θ–θ geometry. The area between 4° and 63° 2θ, with 0.02° steps, was measured with a 0.5° primary beam divergence. Compound identifications were based on a computer program X’ Pert high score of 1.0 B and literature data.
The textural features and semi-quantitative analyses of mineralized samples were examined by a Zeiss Merlin Compact VP field emission Scanning Electron Microscope (SEM) equipped with an Energy-Dispersive X-Ray (EDX) spectrometer and an Electron Backscattered Diffraction (EBSD) detector at UiT The Arctic University of Norway. EDX analyses were conducted with an X-Max80 EDX detector by Oxford instruments at a working distance of 8.5 mm, using an accelerating voltage of 20 kV and an aperture of 60 µm. The samples were mechanically polished and carbon-coated. The retrieved data were further processed by applying the AZtec software also provided by Oxford instruments. EBSD analyses for phase identification and distribution were conducted on a Nordlys EBSD detector in combination with the Aztec data processing software, both provided by Oxford instruments. The analyzed samples were mechanically and chemically polished with a colloidal silica solution and coated with a carbon layer. The samples were tilted to 70°. An acceleration voltage of 20 kV was applied in combination with a 240 µm aperture. Step sizes for EBSD mapping were from 4.5 µm to 6 µm; six bands were detected with refined accuracy as indexing mode. Indexing rates were from 74.0% to 88.4%. A camera exposure time of 21 ms was applied in both cases.
Petrographic and microthermometric measurements of fluid inclusions within transparent minerals (quartz, calcite, sphalerite and pyroxene) were performed at the University of Zagreb and at UiT The Arctic University of Norway. Double polished, 0.1 mm to 0.3 mm thick, transparent mineral wafers were studied. Measurements were carried out on Linkam THMS 600 stages mounted on an Olympus BX 51 (University of Zagreb) and an Olympus BX 2 (UiT) using 10× and 50× Olympus long-working distance objectives. Two synthetic fluid inclusion standards (SYN FLINC; pure H
2O and mixed H
2O–CO
2) were used to calibrate the equipment. The precision of the system was ± 2.0 °C for homogenization temperatures, and ± 0.2 °C in the temperature range between −60° and +10 °C. Microthermometric measurements were made on carefully defined fluid inclusion assemblages, representing groups of inclusions that were trapped simultaneously. The fluid inclusion assemblages were identified based on petrography prior to heating and freezing. If all of the fluid inclusions within the assemblage showed similar homogenization temperature, the inclusions were assumed to have trapped the same fluid and to have not been modified by leakage or necking; these fluid inclusions thus record the original trapping conditions [
42,
43,
44].
Carbon and oxygen isotope analyses of calcite separated from the host marble, as well as from skarn and hydrothermal mineral associations, were performed at the University of Lausanne and at UiT The Arctic University of Norway. In both laboratories, calcite powder was extracted from hand-picked samples using a dentist’s drill. A mass of 250 μg of powder was loaded in sealed reaction vessels, then flushed with helium gas and reacted at 72 °C with phosphoric acid. The evolved carbon dioxide was sampled using a ThermoFinnigan Gas-Bench and isotope ratios were measured in continuous flow mode using a ThermoFinnigan Delta + XP mass spectrometer. Data was extracted to an EXCEL file by using the ISODAT NT EXCEL export utility and further calculation steps were carried out using a predefined EXCEL Worksheet. Linearity corrections were applied based on the relationships between the intensity of the first sample peak (m/z 44) and δ 18O value of the standards. Due to calibration based directly on standard, which were part of each run (Carrara marble), correction for calcite runs was unnecessary. The stable carbon and oxygen isotope ratios are reported in the delta (notation as per mil (‰) deviation relative to the Vienna Standard Mean Ocean Water (V-SMOW) for oxygen and Vienna Pee Dee Belemnite (V-PDB) for carbon. The analytical reproducibility was better than ±0.05‰ for δ 13C and ± 0.1‰ for δ 18O.
5. Discussion
Geological, mineralogical and geochemical features of the Sasa Pb-Zn-Ag deposit classify this deposit to the group of calcic Pb-Zn skarn deposits [
45]. Although the mineralization is closely associated with magmatic rocks, direct contacts between the mineralization and the magmatic rocks are obscure (
Figure 2), suggesting a distal character of the deposit and the interaction of mineralizing fluids with the host carbonate rocks (cipollino marble) as the major mineralizing mechanism.
Geochemical features (trachytic to trachydacitic composition; calc-alkaline character; Na
2O/K
2O < 1; high large-ion lithophile element to high field strength element ratios (LILE/HFSE); strong enrichment in K, Pb and U) as well as their K/Ar age (31–24 Ma) suggest that magmatic rocks associated with the Sasa deposit are a product of the calc-alkaline to shoshonitic post-collisional magmatism that affected the Balkan Peninsula during the Oligocene–Miocene period [
21,
23,
38,
39,
40,
41,
53], resulting in the formation of numerous magmatic–hydrothermal ore deposits along the Vardar Zone and the Serbo-Macedonian Massif (
Figure 1; [
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
54]).
The paragenetic sequence (
Figure 4) indicates that, during its formation, the Sasa Pb-Zn-Ag deposit underwent three main stages similar to other known skarn deposits worldwide: (1) a stage of isochemical metamorphism; (2) an anhydrous prograde stage and (3) a retrograde/hydrothermal stage [
1,
55,
56]. As the deposit is hosted by a highly metamorphosed terrain, it is difficult to distinguish the regional metamorphic signature from the metamorphism associated with the emplacement of Tertiary magmatic bodies. However, the isotopic composition of preserved lenses of the host marble overlap with values typical for marine carbonates, suggesting that the metamorphism has not disturbed its primary isotopic composition.
The prograde mineral assemblage is marked by the presence of anhydrous Ca-Fe-Mg-Mn silicates, predominantly pyroxenes from the hedenbergite–johannsenite series. The predominance of anhydrous minerals reflects a low water activity, whereas the prevalence of pyroxene (hedenbergite) over garnets (andradite) suggests a high ferrous/ferric ratio and a relatively reductive environment [
57]. Absence of Fe-sulfides indicates a low sulfur fugacity during the prograde stage. Although our SEM/EDS analyses (
Table 1) revealed that pyroxenes are characterized by a relatively high and uniform FeO/MnO ratio, previously published data [
12,
58] suggest greater variations in the pyroxene composition among different ore bodies and local predomination of Mn-rich pyroxenes. The variations may be controlled by periodical variations in the chemistry of infiltrating magmatic fluids:
and/or in local temperature oscillations:
* Calculated for standard conditions using thermodynamic data published by [
59,
60].
In contrast to the carbonate component of the host cipollino marble that was replaced by pyroxenes, grey mica has not been significantly affected by metasomatic processes during the prograde stage of the mineralization, probably due to the insufficient water activity and a relatively high K
+/H
+ molar ratio:
According to the fluid inclusion data, the prograde stage occurred at temperatures above 405 °C and at pressures above 30 MPa under the influence of moderate salinity and low density Ca-Na-chloride bearing aqueous fluids. The absence of liquid CO
2 indicates that XCO
2 was below 0.1 [
61,
62,
63]. Due to absence of any reliable independent geothermometer and/or geobarometer, only the minimum P-T conditions can be set (
Figure 14).
Textural relations (
Figure 5,
Figure 8 and
Figure 9) indicate that pyroxenes were replaced by mixtures of hydrous silicates, carbonates, quartz and magnetite, reflecting an increase in water activity and oxygen and/or CO
2 fugacities:
Although minerals formed during the prograde phase do not incorporate Al, the retrograde mineral paragenesis contains aluminosilicates (mostly chlorites and epidote group minerals). The presence of carbonates points to near-neutral pH conditions and a limited capability for hydrothermal transport of aluminum [
64]. However, layers of grey mica within the host cipollino marble might have served as a local source of Al:
The stable isotope composition of the carbonates revealed a significant contribution of magmatic CO
2 during the retrograde stage of the Sasa deposit (
Figure 13). The fluid inclusion studies suggest that infiltrating fluids were Mg-Na-chloride or Fe
2+-chloride solutions. Their salinities are slightly greater compared to the prograde fluids and the homogenization temperatures point to the gradual cooling of the system. The coexistence of rhodonite, rhodochrosite and quartz implies that temperature and pressure did not exceed 375 °C and 200 MPa (
Figure 14). Cooling of the system below 400 °C resulted with the ductile-to-brittle transition [
65,
66,
67], promoted reactivation of old (pre-Tertiary) faults and shifted the system from the lithostatic to hydrostatic regime. Such conditions allowed progressive infiltration of ground water and therefore increased the water activity and oxygen fugacity. At the same time, due to the lithostatic to hydrostatic transition, the pressure dropped by approximately 2.7 times, triggering a more efficient degassing of the emplaced magmatic body and increasing fugacity of numerous volatiles including H
2O, CO
2, H
2S and/or SO
2. The progressive contribution of magmatic CO
2 has been recognized from the retrograde mineral paragenesis (Equations (5)–(9);
Figure 4), as well as from the isotopic composition of associated carbonates (
Table 4;
Figure 13). As Cl preferentially partitions into the fluid phase [
68,
69], fluxes of magmatic fluids will increase the total salinity of the circulating fluids. This increase in the salinity during the retrograde stage has been recorded by fluid inclusions entrapped by retrograde quartz, i.e., quartz that crystallized as the retrograde alteration product after prograde pyroxenes (
Figure 15). The greater salinity promoted the metal–chloride complexing which, together with the higher water activity in the system, enhanced the hydrothermal transport of base metals, including Pb and Zn [
64], and moved the Sasa deposit to the hydrothermal ore-forming stage (
Figure 4).
The retrograde stage is marked by an increase in the sulfur fugacity that resulted in a replacement of Fe-silicates, mostly hedenbergite, by pyrrhotite during the early retrograde stage and by pyrite during the later retrograde stage and the hydrothermal stage (
Figure 4). The previously published sulfur isotope data show that sulfur is predominantly magmatic in origin [
70]. The textural features also point to a slight time lag between intensive degassing of magmatic CO
2 and S-bearing volatiles, probably controlled by the difference in solubility of CO
2 and S in silicate melts [
71].
The isotopic composition of hydrothermal carbonates reflects diminishing influence of magmatic CO
2 and more significant contribution of the host cipollino marble (
Figure 13). The fluid inclusions studies revealed that the syn-ore mineralization was deposited from Mg-Na-chloride or Fe
2+-chloride hydrothermal fluids. The wide range of recorded homogenization temperatures, together with the variable salinities (
Figure 15), suggest cooling under the influence of cold ground waters as a plausible mechanism for the ore deposition:
However, neutralization of the mineralizing fluids in reactions with host carbonates contributed to the ore deposition (
Figure 16):
The post-ore stage is marked by the abundant deposition of calcite, revealing near-neutral pH conditions. The isotopic composition of post-ore carbonates overlaps with the values obtained from syn-ore carbonates (
Figure 13). The fluid inclusion data suggest deposition temperatures below 300 °C from relatively diluted Ca-Na-Cl-bearing fluids.
7. Conclusions
The geological setting of the Sasa Pb-Zn-Ag skarn deposit and previously published geochemical studies on the associated magmatic rocks (trachytic to trachydacitic composition; calc-alkaline character; Na2O/K2O < 1; high LILE/HFSE ratios; strong enrichment in K, Pb and U; the K/Ar age of 31–24 Ma) suggest that the Sasa deposit is a product of the calc-alkaline to shoshonitic post-collisional magmatism that affected the Balkan Peninsula during the Oligocene–Miocene period and resulted in formation of numerous magmatic-hydrothermal ore deposits along the Vardar Zone and the Serbo-Macedonian Massif.
The relatively simple, pyroxene-dominated, prograde mineralization at the Sasa Pb-Zn-Ag skarn deposit resulted from an interaction of magmatic fluids with the host cipollino marble. The absence of direct contacts between the mineralization and the magmatic rocks as well as the textural features of the skarn paragenesis reflect the infiltration-driven metasomatism. Obtained mineralogical and geochemical data suggest that the prograde stage occurred under conditions of low water activity, low oxygen, sulfur and CO2 fugacities and a high K+/H+ molar ratio. Fluid inclusion data set the minimum P–T conditions at 30 MPa and approximately 405 °C. Mineralizing fluids were moderately saline and low density Ca-Na-chloride bearing aqueous solutions.
The transition from the prograde to the retrograde stage was initiated by cooling of the system below 400 °C and the associated ductile-to-brittle transition that shifted the system from the lithostatic to hydrostatic regime. The retrograde mineral assemblages reflect conditions of a high water activity, high oxygen and CO2 fugacities, a gradual increase in the sulfur fugacity and a low K+/H+ molar ratio. The isotopic composition of retrograde carbonates revealed a significant contribution of magmatic CO2. Infiltration fluids carried MgCl2 and had a slightly higher salinity compared to the prograde fluids. The maximum formation conditions are set to 375 °C and 200 MPa.
The deposition of ore minerals (predominantly Bi, In and Ag- enriched galena and Fe and Mn-bearing sphalerite) occurred during the hydrothermal phase under a diminishing influence of magmatic CO2. The mixing of ore-bearing (Mg-Na-chloride or Fe2+-chloride) aqueous solutions with cold and diluted ground waters is the most plausible reason for the destabilization of metal–chloride complexes. However, neutralization of relatively acidic ore-bearing fluids in the interaction with the host lithology could have significantly contributed to the deposition.
The post-ore, predominantly carbonate, mineralization was deposited from diluted Ca-Na-Cl-bearing fluids of a near-neutral pH composition. The depositional temperature is estimated to be below 300 °C.