LA-ICP-MS analysis of single fluid inclusions in a quartz crystal (Madan ore district, Bulgaria)

Journal of Geochemical Exploration 108 (2011) 163–175

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Journal of Geochemical Exploration j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j g e o ex p

LA-ICP-MS analysis of single fluid inclusions in a quartz crystal (Madan ore district, Bulgaria) Boriana G. Kotzeva a,⁎, Marcel Guillong b, Elitsa Stefanova b, Nikolay B. Piperov a a b

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Institut für Isotopengeologie und Mineralische Rohstoffe, ETH Zürich, CH-8092 Zürich, Switzerland

a r t i c l e

i n f o

Article history: Received 1 December 2009 Accepted 7 January 2011 Available online 25 January 2011 Keywords: Fluid inclusions LA-ICP-MS Quartz crystal Kroushev Dol Pb-Zn ore deposit Trace elements Hydrothermal

a b s t r a c t A “long-living” crystal of barren quartz from Kroushev Dol Pb-Zn deposit (Madan district, Rhodope Mountains, Bulgaria) was studied. The semitransparent base part (the “root”) of the crystal contains abundant inclusions, predominantly along healed cracks, while the upper half or third of the crystal is clear and poor in inclusions. In order to analyze fluid inclusions in the quartz crystal, it was cut into 4 pieces across and along the c-axis and doubly-polished sections were prepared. Fluid inclusions trapped in this quartz supply information about the temporal evolution of paleofluids depositing ore minerals. More than a hundred inclusions from different assemblages were observed microscopically; about half of them were selected for further investigation. Most of the inclusions show rather similar homogenization temperature Th = 333-348 °C, however, those in the peripheral (late) zones of the crystal reveal lower Th: 312-336. The total salinity is between 6.0 and 9.1 wt. % in most of the inclusions, reducing to 4.0-5.7 wt. % NaCl equivalent closer to the crystal walls. Initially, LA-ICP-MS signals for 30 elements were evaluated. Eleven elements, which concentrations were above the limit of detection, were used for more detailed interpretation. The molar ratios X/Na (X= K, Ca, Mg) show no significant changes during the crystal growth. A small increase in pH seems to be responsible for significant decrease of PbS and ZnS solubility. Thus, the fluids become oversaturated in these substances and galena and sphalerite precipitate. The molar ratios Cu/Na, Zn/Na and Pb/Na in the fluid inclusions from the early to the late fluid inclusions assemblages suggest a decrease in ore-metal concentrations in the fluids during the last stages of mineral deposition. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The Pb-Zn deposits in Madan ore district (Bulgaria) are of great economic importance and have been a subject of intensive exploration and mineralogical, petrological, tectonic and geochemical research over the last 5 decades. Since 1941, three Mt of lead-metal and 2.5 Mt of zinc-metal have been yielded and more than 1 Mt of metal is still in reserve. The systematic study of Madan ore district began 50 years ago (Bogdanov, 1960) and was summarized recently by Bonev (2002). Several studies have characterized the physical and chemical evolution of the hydrothermal fluids in different Pb-Zn deposits in the Madan ore district. Bogdanov (1961) published the first data on homogenization temperatures of fluid inclusions in quartz from Madan and Nedelino ore districts. Later, Piperov (2002) compiled 9

⁎ Corresponding author at: Blvd, bl.11, Sofia 1113, Bulgaria. Tel.: + 359 2 979 39 01; fax: + 359 2 870 50 24. E-mail address: [email protected] (B.G. Kotzeva). 0375-6742/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2011.01.002

papers published before 2000, containing about 2000 Th values of fluid inclusions. Petrov (1988) gave an account of total salinity based on ice melting temperatures Tm of fluid inclusions in quartz. Kostova et al. (2004) determined the salinity of the hydrothermal ore-forming fluids from Yuzhna Petrovitsa Pb-Zn deposit. The mineral assemblages, as well as the structure of the deposits in Madan ore district, suggest the existence and operation of a single, large, multi-vent hydrothermal system in the past (Bonev, 2002). Fluid inclusion data also confirm this theory. Most of the primary and pseudosecondary fluid inclusions have Th between 300-350 °C and total salinities between 3-10 wt % NaCl equiv. with a mode about 5 wt. %. The latter is of great importance, since the conclusions made for one Pb-Zn deposit in the district could be also referred reasonably to the other deposits of the same type. However, data on the chemical nature of the fluids are very scarce. Dimitrov and Slavov (1972) published first results for the composition of fluid inclusions in quartz, galena and manganocalcite from Madan deposits. Unfortunately, the opening-inclusions technique they used (milling to powder) seems very inappropriate for this purpose and the results obtained are questionable, i.e. Ca2+ NN Na+, HCO-3 NN Cl-.

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Piperov et al. (1977) analyzed solution, trapped as single macroinclusions in galena (Bonev, 1969, 1977) from 5 important deposits in Madan ore district. The concentrations of the ore elements (Pb, Zn), however, were indicated only by limits of detection. Kotzeva et al. (2004) analyzed the fluid inclusion composition in barren quartz from Kroushev Dol deposit (see below) by leaching after crushing or decrepitation. The authors determined the principle cations Na, K, Mg, Ca and Mn. In a new study, Kotzeva (2010) published data for Cl--concentration obtained by ion chromatography after decrepitation-leach in fluid inclusions from the same material. Kostova et al. (2004) studied the stage of ore deposition over the vertical extent of the Yuzhna Petrovitsa mine. They used the LA-ICPMS method to determine the possible spatial variations of major, minor and trace elements in single inclusions in ore-stage quartz from different hypsometric levels. The aim of our study is to track the temporal P-T-X evolution of the hydrothermal fluids based on analyses of fluid inclusions from different generations in a single, long-living quartz crystal. The LA-ICPMS in situ technique is the best suited tool for the purpose (Audétat et al., 1998). Mass-spectrometry using inductively coupled plasma as an ion source and introducing the sample after laser ablation (LA-ICP-MS) is a powerful combination for analysis of single fluid inclusions in minerals (Allan et al., 2005; Günther et al., 1998; Heinrich et al., 2003). It is an efficient method for quantitative multi-elemental analysis of microscopic objects at high spatial resolution down to few micrometers (Heinrich et al., 2003). It is well known that even centimeter sized crystals may have trapped different fluid inclusion types, based on their phase proportion at room temperature, which may have different composition (Heinrich et al., 1999; Roedder, 1984). In this study we analyzed fluid inclusions in high-temperature hydrothermal quartz from the Kroushev Dol Pb-Zn deposit of Madan

ore district, Central Rhodope, Bulgaria (Dokov and Popov, 1961), using LA-ICP-MS, microthermometry and decriptometry. 2. Geological background A very important change happened with the geological evolution of the Rila-Rhodope massif in the end of Eocene, when the phase of collisional compression, accompanied by a long-term multistage metamorphism (Kostov et al., 1986), was followed by a new, extensional phase (Ivanov et al., 2000). This resulted in formation of faults, facilitating the penetration of rhyolitic lavas (32-30 Ma ago, Ovtcharova et al., 2001) and, finally, caused an intensive and prolonged hydrothermal activity (30.6 - 30.0 Ma ago, Kaiser-Rohrmeier et al., 2004). Thus, a system of six large, up to 10-15 km long, NNW trending subvertical faults was developed in the slope of the Madan Allochthone (Dome). This antiform is built up mainly of the PreCambrian high-grade metamorphic rocks of the Rhodope massif: migmatized gneisses, granite gneisses, biotite gneisses, amphibolites, micashists and layers of marbles. The latter are of great importance for the understanding of ore deposition, since they host the skarnreplacement ores. The Madan Pb-Zn deposits vary between two morphological types end-members: vein (open-space and composed stockwork zones) and metasomatic (replacement) ore bodies; various combinations of these two are observed. An idealized scheme of such a combination (open-space vein and replacement deposit) is shown in Fig. 1. The ore veins are steep to subvertical, 1 to 3 or more meters thick and up to 1-2 km long, filled with quartz-sulphide minerals. Open spaces (caverns) coated by quartz-sulfide, pure quartz or carbonate druses often occur in the central parts of the veins. It is worth mentioning that ore zones in depth (below 650 m above sea level (a.s.l.)) are followed by barren quartz only. Bonev and Piperov (1977) suggested boiling of the hydrothermal fluid as the

Fig. 1. Cross section of a Pb-Zn deposit from Madan ore district – a schematic drawing. The point and linear elements are presented out of the scale (enlarged). The well mineralized zones (massive ores) are in black. Italic denotes the presumed paleo-elements. Some important details are also enlarged: a replacement deposit (in the rectangle) and a quartz vein (in the circle). The scheme is compiled after Bonev and Piperov (1977) and Vassileva et al. (2005). The asterisk near to the boiling curve indicate the Th (289 – 293 and 310 – 318 °C) of fluid inclusions in a quartz specimen (Q2) from the same main Qrz-Gal-Sph stage in the Yuzhna Petrovitsa deposit from hypsometric levels (H) 1070 and 668 m (a.s.l.), respectively (Kostova et al., 2004, Table 1).

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reason for changing over the quartz crystallization to intensive sulfide minerals deposition. They based this suggestion on the evaluation of PT-conditions of ore precipitation. When Th is considered as temperature of mineral deposition, the estimation of pressure (P) remains problematic. As the vein has been opened to the surface, the hydrostatic pressure would be determined by the position of this surface, i.e. by paleorelief level. The remains of an ancient denudation plane are still preserved in the highest peaks and ridges over 1700 m a. s.l. (in the south of the district) up to 2000 m a.s.l. in the north (Dimitrov et al., 1967; Vaptsarov, 1976). This plane is built up of breccia-conglomerates and sandstones and it is well dated (by fossils) as Late Eocene (Kolkovski et al., 1996). The lower level of ore mineralization (650 m a.s.l.) corresponds to a depth of 1200-1300 m below the paleorelief and thus defines a hydrostatic (paleo) pressure of 90-110 bar at temperature of 310 °C for a fluid with a 5 wt% NaCl salinity (Haas, 1971). This state, however, fits exactly the liquid (5 w% NaCl equivalent)–vapour equilibrium (loc. cit.) and reveals the boiling level (cf. Fig. 1). The role of boiling in ore deposition is outlined in the extended study of Drummond and Ohmoto (1985). The Kroushev Dol deposit belongs exactly to this morphological type. Replacement type deposits are associated exclusively with the marble layers (2-3) of the Rhodope metamorphic complex. According to Bonev (2002), the early (pre-ore) skarns consist of highly Mn-rich clinopyroxenes. The relatively alkaline and reducing (Mn2+, Fe2+) environment of these materials favors sulfide ores deposition. In this case, the skarns are replaced by a retrograde aposkarn silicatecarbonate assemblage and almost simultaneously by the main sulfides and quartz (Bonev, 1995). The replacement of skarns with sulfide ores is a chemical interaction and does not depend on the boiling, i.e. on the hypsometric level. Finally, Bonev (2007) recognized three main evolution stages in the activity of the whole Madan hydrothermal system: 1) Penetration of pre-ore hydrothermal fluids that formed exoskarns of manganese clinopyroxenes developed only in marbles; 2) Main ore stage. Hydrothermal fluids of rather high T (300-350 °C) caused acid alteration of the gneissic rocks and skarns with formation of retrograde post-skarn Mn silicates and carbonates. Deposition of quartz (Qrz), pyrite, massive sulfide ores (sphalerite + galena) and minor chalcopyrite and arsenopyrite; 3) Late, post-ore stage. Deposition of quartz and chalcedony, carbonates (calcite ± barite), scarce sulphides (galena) and sulfosalts. T is lower: 260-200 °C.

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quartz crystals. Small pyrite grains (b0.5 cm) are disseminated in the roots of the quartz. On the higher levels of the veins, where the participation of sulfides increases, the first generation quartz occurres only as a thin (1-2 cm) crust on the walls, but inside the veins an intimate growth of quartz and ore minerals is observed, revealing synchronous deposition of minerals from the main ore stage (see Geological background). A detailed study of such ore quartz from the Yuzhna Petrovitsa deposit was done by Kostova et al. (2004). The quartz specimen was collected from a cavern in the barren quartz zone of the main ore vein of Kroushev Dol, on level 600. The gneissic walls of the cavity are coated by a crust of long prismatic columnar quartz (Fig. 1). The quartz crystals had been growing after geometric selection and formed dense sub-parallel aggregates. The single crystals are up to 8-10 cm long and 0.5-1.5 cm wide. The tip part (1/3 to 1/2) of the quartz crystals is semitransparent (Fig. 2), while the base part (“the root”) is nearly opaque; the transition zone is unruffled (unclear). This difference in the quartz clarity is due to the inclusion abundance. As it was found by decrepitometry (Fig. 3a and b) the opaque part contains at least 10 times more inclusions per gram material than the transparent upper part. The rhombohedral faces are overgrown by small needle-like quartz crystals (b0.5 mm) and small single pyrite crystals (b0.3 mm). These later minerals (stage 3, see above) are covered by a thin crust (up to 5 mm thick) of later calcite, whose deposition is accompanied by partial limonitization of the previously crystallized pyrite. Hence, all these observations reveal that the quartz is a “long-living” mineral and the inclusions trapped in it may have recorded the evolution of the ore-forming fluid at least during the most interesting main ore stage. The topographical study of fluid inclusions in quartz crystal, used by Audétat et al. (1998), was proven suitable for our examinations. In order to perform a detailed fluid inclusion study, the quartz crystal was cut as shown in Fig. 4a–c. The crystal was cut perpendicular to the c-axis in the opaque zone and was divided into two unequal pieces (~1:2) (Fig. 4a). Both pieces – base of the crystal and its body – were cut additionally, but now c-coaxial (see Fig. 4b). A total of 4 doubly-polished sections were prepared (Fig. 4c).

Clays should be also widespread, but their light (d = 2-3 g/ cm3) and small (b10 μm) crystals were taken out by the water flow, which hinders their genesis as well as their correlation with other minerals. Stefanov et al. (1988) studied hydrothermal clay minerals from 10 deposits of the Madan ore district. They divided them into four groups according to form, location, chemical composition and genesis: (1) Tectonic clays (mainly sericite), probably a pre-ore product; (2) High-temperature clay minerals formed during the hydrothermalmetasomatic process, synchronous with the ore deposition, represented by chlorite, sericite and kaolinite (dickite) (and quartz – in all cases). (3) Thin aggregate layers of scale-like crystals (up to 10-30 μm, sometimes ≤ 100 μm) deposited over the early-formed minerals. These clays (smectite, micas, kaolinite) are assumed to be products of the 3rd, post-ore stage; (4) Late dense clay masses of kaolinite (and smectite ≤ 10%). 3. Description of the studied quartz The barren quartz is a first generation mineral as it has grown directly on the vein walls. The products of hydrothermal quartzsericitization of the hosted gneisses supplied seeds (nuclei) for the

Fig. 2. The quartz specimen before cutting. Bar represents 1 cm. The thin crust of late carbonates, covering the rhombohedral tip, is removed by dissolution in 1 M HNO3.

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a

b

Fig. 3. a and b Decrepigrams of samples from the opaque (a) and transparent (b) parts of the crystal. “D” denotes to the record of the decrepitation activity (50 mg sample), the kinetic curve describes the release of H2O (0.500 g sample). "bis-" is the decrepitation activity and water release on reheating of the same samples. It is considered as a blank.

3.1. Topographic observation The fluid inclusions occur predominantly in the bases of the quartz crystals close to the contact with the wall rock, due to the fact that this is the area with largest density of cracks. These healed cracks host a large number of pseudosecondary fluid inclusions. The inclusions are so abundant and densely distributed that the quartz becomes semitransparent. The topography of the cracks (Fig. 5) reveals an episodic crystal growth followed by a healing, in a relatively short time (Fig. 4; Fig. 5, as. 2-7; as. = assemblage). Section C (Fig. 5) cuts across six cracks. The central cracks should be considered older, and the inclusions trapped in them should represent early stages of hydrothermal fluid evolution. However, we have to note that the time distance in the cracks formation is not evident.

More than 100 single fluid inclusions were examined microscopically. The studied fluid inclusions are mainly pseudosecondary, typically 10-100 μm in size (Fig. 6a). A group of inclusions, most likely primary in origin, randomly situated in the central area of the slice (the core), is indicated as “as.1”. Except the vacuoles from as.7, most of the inclusions are three-phase: vapour + liquid + solid, i.e. contain a very small solid phase (1-2 μm-sized). Solid phases in inclusions, and especially the accidentally trapped ones, may interfere with the analytical results. The laser ablation is a “total” method for introducing sample matter into plasma, without a possibility for separation of the inclusion phases and the surrounding host. Accidentally trapped solids in the inclusions may interfere with the analytical results for some elements; therefore their identification could contribute to the better interpretation of the results.

Fig. 4. Scheme of cutting the quartz crystal to obtain 2 (3) longitudinal and one cross section. Bar represents 1 cm. Section (C) denotes the base (B) (root zone) rather than the tip part. The body of the crystal (signed “P”) and especially the trigonal tip (“T”) represent a calmer stage of growth without visible fractures and relatively small number of fluid inclusions, randomly distributed in the crystal volume.

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these phases to be clay minerals (see above, Section 2. Geological background). 4. Analytical techniques About 90 of all observed fluid inclusions were measured by microthermometry. More than 50 of them were selected to be further analyzed by LA-ICP-MS. These inclusions were mainly pseudosecondary, typically 10-100 μm in size and they were representative for different assemblages of fluid inclusions. Several single inclusions from each assemblage were ablated; finally, a total of 10 different assemblages were studied. Only few of the analyzed inclusions were considered primary (Fig. 6b). 4.1. Microthermometry Fig. 5. Location of the fluid inclusion assemblages in the cross section C. Bar corresponds to 1 cm.

Unfortunately, the solid phases in the fluid inclusions are difficult for optical observation. In two or three inclusions only (not analyzed) aggregates of a few (sub-) micrometer-sized scale-like individuals were recognized. In transmitted light they are pale, beige to yellowgreen; in transparent light they look dark (green?). We assume

Prior to the LA-ICP-MS analyses, microthermometric measurements were performed on the selected fluid inclusions in order to determine their salinity and homogenization temperature. Salinities of the fluid inclusions were determined by final ice-melting temperatures. Three different microscopes were used for characterization of the samples. Most of the data were obtained by Olympus BX 60 and Nikon eclipse E 600 POL microscopes supplied with a “Linkam” THSMG-600 stage at the Institut für Isotopengeologie und Mineralische Rohstoffe, ETH Zürich. The calibration of Linkam heating-freezing stage was performed on synthetic fluid inclusion standards manufactured by SYN FLINC®. Temperature readings are considered to be accurate to ±0.2 °C for the melting point of H2O (0.0 °C) and to ±3 °C for the critical point of pure H2O (374.1 °C). Some of the microthermometric determinations were performed by “Amplival” (Carl Zeiss Jena) transmission microscope, equipped with laboratory-designed heating and freezing stages for measuring the homogenization and ice melting temperatures of the inclusions (Laboratory of Analytical Chemistry, Institute for General and Inorganic Chemistry, Sofia, Bulgaria). Some ideas and technical solutions to the construction of these stages were taken from Kormushin (1982) and Voznyak and Galaburda (1977), respectively. Sealed thin-wall glass capillaries (“spikes” - ca. 10 mm long, 0.05-0.10 mm in diameter) containing distilled water (0 wt % NaCl) or standard solutions of 1.00, 3.00, 6.00, 12.00 and 20.00 wt. % NaCl were used as standard “inclusions” in microcryometry. The calibration curve may be constructed either as EMF (mV) of the thermocouples vs. melting temperature of the last ice crystal, or directly vs. total salinity of the water phase, using Bodnar's (1993) equation. The estimated uncertainty is ± 0.3 wt % NaCl for salinity b 2%, and ±0.2 wt. % for higher salt content. The calibration curve for Th determination is based on the melting points of the “Signotherm” (Merck) standards 50, 120 and 180 and the following pure substances: phenolphthalein (261 °C), NaNO3 (307 °C) and K2Cr2O7 (398 °C). The uncertainty is within ±2 °C at T b 200 °C and ± 3 °C at higher temperatures. 4.2. LA-ICP-MS- system and measurements

Fig. 6. a) Typical pseudosecondary inclusions from C cross section (as. 4). The bar corresponds to 200 μm; b) Inclusions from as. 1, C cross section. This central zone of section C is assumed to be a domain of calmly growing, latter crystal body (P). The shape, the location of these inclusions (center of the section), as well as their random distribution, suggest a primary origin.

The utilized LA-ICP-MS system (at the Institut für Isotopengeologie und Mineralische Rohstoffe, ETH Zürich) is equipped with a homogenized 193-nm ArF laser prototype system similar to GeoLas (Günther et al., 1997). The gas flow (He), which carries the ablated material out of the sample chamber, infuses into the main Ar stream before entering the ICP-quadrupole mass spectrometer (Elan 6100, Perkin Elmer - SCIEX). Details about the conditions of LA-ICP-MS measurements are listed in Table 1. Data processing was made with LAMTRACE 1.52 and Excel 97 were used. A silicate glass of the National Institute of Standards and Technology (NIST 610) was utilized as a Certified Reference Material (CRM). It contains 65 elements of known concentration and was subjected to laser ablation before and after analysis for a maximum of 16 fluid

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inclusions in order to determine the relative sensitivity of the analytical tract for every element analyzed. The polished quartz sections, containing selected inclusions, were cleaned in an ultrasonic bath for 20-30 min before loading into the ablation chamber.

Table 1 LA-ICP-MS machine and data acquisition parameters. Quartz Excimer 193 nm ArF laser Compex 110I Output energy Homogeneous energy density on sample Pulse duration Repetition rate Pit sizes Ablation cell

40-50 mJ 14-18 J/cm2

Cell He gas flow

15 ns 10 Hz 30-40 μm In-house built plexiglass chamber with anti-reflection coated silica glass window 1.15 l/min He

ELAN 6100 quadrupol ICP-MS Nebulizer gas flow Auxiliary gas flow Cool gas flow rf-power Detector mode Quadrupole setting time

0.93-0.95 l/min Ar 0.80 l/min Ar 15,0 l/min Ar 1450 kV Dual checked on Al, Si 3 ms

Data acquisition parameters Sweeps per reading Readings per replicate Replicates Dwell time per isotope Points per peak Oxide production rate Isotopes analyzed Program "Fluid inclusions"

1 320-550 1 10 ms for every single element 1 per measurement tuned to b 0.4% ThO Li 7, B 11, Na 23, Mg 25, Al 27, Si 29, K 39, Ca 40, Sc 45, Mn 55, Fe 57, Co 59, Ni 62, Cu 65, Zn 66, Ge 73, As 75, Se 77, Rb 85, Sr 88, Ag 107, Cd 111, Sn 118, Sb 121, Te 125, Cs 133, Ba 136, Au 197, Pb 208, Bi 209

4.3. Chemical composition of the fluid inclusions In the first set of LA-ICPMS analyses we analyzed 30 major and trace elements in the fluid inclusions from three polished sections (Base B, Body P and Tip T) (see Table 2). Silicon (as an element of the host mineral) was also monitored. Generally, the limit of detection (LOD) of the specific elements, as determined by LA-ICP-MS, depends on the inclusion size. The larger the inclusion, the higher the achievable sensitivity and thus the lower calculated LOD. Some elements that were consistently around and below the limits of detection (Fe, Co, Ni, Sc, Ge, Ag, Cd, Au, Se and Te), were not measured again during subsequent runs. The concentrations of B, Al, As, Rb, Sr, Sn, Sb, Ba and Bi, which were above the LOD, were not traced regularly. The number of detected elements was limited in order to improve the mass-spectral detection sensitivity of some alkaline, alkaline-earth, and metal elements. That is why a reduced menu of only 11 elements (Na, Li, K, Mg, Ca, Mn, Cu, Zn, Pb, Cs, and Si) was used during the analyses of fluid inclusions from cross-section C (7 assemblages) (Table 3). Sodium is the most abundant cation in the fluid inclusions and its concentration can be estimated from fluid inclusion salinities determined by microthermometry prior to laser ablation. Its concentration was used as an internal standard. For multi-element measurements the wt% NaCl equivalent values were corrected for contributions of other cations (in this case K and Mn), using the well-established empirical salt correction (Heinrich et al., 2003). Many authors successfully used artificial (Günther et al., 1998; Shepherd and Chenery, 1995) or synthetic (halite-hosted: Moissette

Table 2 Element concentrations (ppm) in individual fluid inclusions from sections B (opaque root zone), P (body) and T (transparent tip part) of the crystal. Section

B

Inclusion size, μm Th, °C Total salinity, wt.% Li 7 B 11 Na 23 Mg 25 Al 27 K 39 Ca 42 Sc 45 Mn 55 Fe 57 Co 59 Ni 62 Cu 65 Zn 66 Ge 73 As 75 Se 77 Rb 85 Sr 88 Ag 107 Cd 111 Sn 118 Sb 121 Te 126 Cs 133 Ba 137 Au 197 Pb 208 Bi 209

N 100 330 4.7 b11 230 18400 b57 b44 3200 5400 12 140 b 330 b6 b 120 b24 61 b27 b18 76 51 17 b4 47 b25 22 b 1.4 79 b6 b4 5 12

P 60 329 6.5 b5 170 25400 b24 b29 6900 b2000 46 430 b 150 b1 b88 b6 b25 b16 51 9 135 b1 b 1.5 23 b14 b3 b 0.3 110 b3 b2 9 5

80 331 5.7 b5 240 22200 33 b 25 5400 4700 b3 160 b 140 b1 b 49 11 17 b 11 72 b0.7 91 16 b2 16 b 14 28 6 86 b2 b2 8 4

70 346 4.0 b1 210 22500 9 b7 7400 2300 2 300 b 50 b0.4 23 9 20 b4 39 0.5 150 36 b0.5 b3 b4 b0.6 b0.1 120 7 b0.5 12 1

T 90 341 4.5 b0.1 150 17700 60 720 2300 1500 b0.1 90 43 b0.1 b2 5 18 b0.5 24 0.1 53 15 0.2 b0.5 2 8 b0.02 74 0.2 b0.1 12 0.3

N 100 340 5.1 b2 180 20000 8 1300 2100 1700 b1.5 130 b 56 b0.5 b 19 5 b7.5 b 11 17 4 57 26 b0.5 b2 b4 4 b0.1 82 b1 b0.6 10 2

70 338 4.7 b6 160 18500 b 42 b 37 4600 b 3400 b7 180 b390 b2 b 75 23 b 36 b 16 b 18 b0.9 66 18 b4 b9 62 37 b0.6 67 4 b3 15 3

70 339 4.7 b 16 130 18500 b 45 b 72 2800 b 5200 b 11 130 b440 b4 b190 18 b 48 b 30 58 b1.9 33 18 b4 b 17 b 40 b3 b1 39 b7 b5 91 5

70 335 4.7 76 140 18600 b 16 b 22 4800 2200 40 210 b200 b1 b 70 b 11 32 b 21 23 b0.8 96 18 b1.5 b9 b 14 b1 b0.5 74 3 b2 9 3

60 348 5.7 19 140 22500 b 42 b 78 6200 b 5300 b 10 330 b460 b4 b130 22 b 41 b 39 42 7 94 30 b4 19 b 23 46 b1 110 b6 b4 13 6

60 312 4.7 1300 170 18500 b 140 b 100 4400 b 7950 180 b83 b 700 b12 b 225 b57 b 100 b48 b30 28 67 18 b6 b49 b67 b7 86 30 b15 b10 b6 2

60 313 4.9 b 29 b 82 19300 b110 b175 b12300 b 25 b104 b 1020 b9 b645 b 94 b 84 b 98 b 62 b2 32 b7 b9 b 59 280 b 10 b3 100 b 22 b 10 10 8

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Table 3 Element concentrations (ppm) in individual fluid inclusions from 7 assemblages (as.) in C cross section (Fig. 5). as.

1 1 1 1 1 1 2 2 2 2 3 3 3 3 3 3 3 4 4 4 4 4 4 5 5 5 5 5 6 6 6 6 6 7 7 7 7 7 7 B* B* B*

Inclusion's Size, μm

Salinity, wt.%

Th, °C

100 90 50 90 90 N 100 60 60 25 40 70 70 70 20 80 30 30 40 40 30 40 80 40 50 50 60 50 50 80 80 50 80 50 18 15 18 15 11 15 N 100 60 80

6.0 6.0 6.0 6.0 6.0 6.0 8.8 8.8 8.8 8.8 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.1 8.1 8.1 8.1 8.1 8.1 9.1 9.1 9.1 9.1 9.1 8.7 8.7 8.7 8.8 8.8 4.5 4.5 4.5 4.5 4.5 4.5 4.7 6.5 5.7

335 333 337 337 333 335 336 336 336 336 337 337 337 337 338 338 338 334 334 334 338 338 338 338 338 338 338 338 333 333 333 338 338 335 336 336 317 320 320 330 329 331

Li 7

Na 23

Mg 25

K 39

Ca 42

Mn 56

Cu 65

Zn 66

Cs 133

Pb 208

570 230 170 190 4 620 35 41 b9 b2 b 0.9 b 0.7 740 b17 b 0.4 b2 b9 b2 b4 b12 51 b 0.5 b2 b 1.4 b 1.1 b 1.2 b 0.6 b 0.6 b4 190 b3 b8 b23 b48 b 100 b55 b37 b24 b78 b11 b5 b5

22600 22700 18300 23400 22700 23300 34600 34600 35200 32500 46900 56100 32900 34600 33200 36600 35600 32100 31900 31300 32100 31500 32000 35900 35700 35800 33900 34200 33000 32600 32400 34400 32500 17700 17700 17000 17300 17600 17600 18400 25400 22200

31 b2 b 13 0.4 3 25 b4 4 b 25 7 b8 22 22 b 84 6 b 13 b 34 b 14 b 25 b 46 b 19 b2 b6 b9 b6 b5 b4 b5 b 21 23 b9 b 18 b 64 b210 b440 b150 b210 b 44 b400 b 57 b 24 33

1100 670 590 1000 1500 1600 1400 6000 13000 12000 9900 11300 8000 1800 1400 3800 4900 3000 5100 18200 6000 8500 4000 8000 6900 1700 5400 5400 2500 3000 2400 2000 2400 4200 4200 6800 2600 4300 3900 3200 6900 5400

1900 1400 b1600 1100 2100 1800 3200 4600 8900 4000 6700 4900 1600 b7700 4100 5400 5300 3300 3300 8400 2600 2300 1200 5900 960 3900 3200 4000 2600 2700 b1200 4300 b8000 b 11700 b 24700 b8500 b 11800 b5000 b 32200 5400 b2000 4700

360 190 120 180 390 240 380 560 940 1200 1300 1300 740 400 870 450 330 320 400 1600 1000 1100 320 1000 4900 730 670 660 77 200 380 140 240 b120 b240 180 b100 88 b330 140 430 160

7 0.7 b4 0.2 b 0.4 0.3 12 b 0.7 b7 7 22 b2 3 40 40 b5 b21 b4 6 b15 b3 b 0.8 b2 12 14 b 0.8 7 b 1.3 b6 15 26 b18 b19 b76 b 160 b33 b69 b19 b85 b24 b6 11

110 19 32 21 26 44 65 53 82 110 130 160 110 100 75 94 87 74 99 130 48 73 68 88 93 84 88 86 65 63 126 77 88 b 90 b190 b140 b110 b 45 b320 61 b 25 17

88 87 29 91 91 84 130 160 260 160 240 280 150 100 160 150 110 130 87 140 110 100 110 200 160 150 130 150 120 110 43 92 70 66 68 88 57 74 87 79 110 86

18 17 7 4 18 18 36 34 56 70 78 120 70 54 60 18 41 52 b1.4 54 55 47 40 64 58 52 54 57 b1.3 19 33 26 24 b9 b 19 13 b7 b2 b 28 5 9 8

* See also Table 1.

et al., 1996; or quartz-hosted: Allan et al., 2005; Günther et al., 1998; Shepherd and Chenery, 1995; Wilkinson et al., 1996) fluid inclusions to test the methods for fluid inclusion studies. The abilities of the analytical system utilized were also checked by analysis of artificial quartz-hosted inclusions. 4.3.1. Host correction Host correction was applied using the method of Heinrich et al. (2003). Every analysis cluster was comprised by up to 16 inclusions and one or two host mineral analyses. However, due to the inhomogeneity of the host quartz, host correction using quartz signals from separate ablations often yielded apparent high concentrations of some elements (Li, Al) in the fluid inclusions. Therefore, we used the signal from the same ablations, integrating the host signal before or after the signal of the inclusion itself. Finally, based on microthermometric data (total salinity) and X/Na ratios, obtained by LA-ICP-MS, the absolute element concentrations (ppm) were calculated (see Tables 2 and 3). 4.4. Decrepitometry The decrepitation activity of the both parts of the studied crystal is documented in Fig. 3a and b. This method is used for estimation of the inclusion abundance (cf. Hladky and Wilkins, 1987) of the trans-

parent tip and the opaque base. The rest of crystal was cut additionally separating the clear tip part very carefully. Grain fraction of 1.251.60 mm was prepared from both separates (transparent and opaque). Decrepitobarometry was performed in two ways: 1) Direct recording of decrepitation activity in vacuum, heating up to 600 °C (50 mg sample, p ≤ 10-5 Torr (0.0013 Pa), 10 deg/min heating rate). The micro-bursts of the decrepitating inclusions were detected by a fast response Penning-type gauge and registered through an electric “high-pass filter” on the recorder input. 2) Kinetics study. Samples of 0.5 g were heated in vacuum (p ≤ 10-3 Torr (0.13 Pa)) at 10 deg/min rate in a closed volume (~2 l). Decrepitation of inclusions usually releases volatile substances, predominantly water vapor (N99%), which causes a respective increase of the pressure. The readings of the vacuum meter (Pirani-type) were recorded continuously. After decrepitation abating (T = 600 °C), the water vapor was frozen in a cool trap (liquid nitrogen) and later it was determined compressometrically by a silicone-oil manometer. In general, the water amount released at decrepitation was used as an arbitrary measure for inclusion abundance (Roedder, 1984). The repeated heating of the same samples (“bis-”) was considered as a blank.

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5. Results 5.1. Microthermometric measurements The data obtained by microthermometry (Th and total salinity determination) from 51 fluid inclusions are presented in Tables 2 and 3. The range of the total salinity is between 4.0 and 9.1 wt. % NaCl equivalent. These values are in good agreement with the results obtained from fluid inclusions in quartz specimens from another deposit of the Madan ore field – “Yuzhna Petrovitsa” (Kostova et al., 2004). The temperature of first melting (Tfm ≈ eutectic T), when recognized, is always between -21 and -20 °C, suggesting low Ca concentrations. The homogenization temperatures range from 312 to 348 °C (Tables 2 and 3); the majority of inclusions have a Th between 330 and 340 °C (Fig. 7). Some Th values between 280 and 290 °C are recorded as well, most likely due to the presence of secondary fluid inclusions. A relatively low (≤100 bar) hydrostatic pressure is assumed (Fig. 1) which does not suggest significant corrections in the Th data. 6. Discussion 6.1. Homogenization temperature and salinity The evolution of temperature and salinity of the fluid inclusion assemblages is plotted on Fig. 8a and b respectively, sorted by their temporal entrapment in the quartz crystal. According to the measured Th, the quartz crystal grew generally at relatively high, but nearly constant temperature 329 – 348 °C, as indicated by inclusions in B and C (except as. 7). Some inclusions in the transparent tip (T) part and the peripheral assemblage (as. 7) display lower Th: 320–312 °C, suggesting a temperature decrease during the late stages of crystal growth (Fig. 8a). The spatial distribution of fluid inclusions in relation to the total salinity is similar in its arrangement to the Th of inclusions (Fig. 8b): early inclusions have total salinity of 5.7-9.1 wt.% NaCl - equiv., while the transparent late zones of the crystal contain fluid of lower salinity: 4.0–5.1 wt.% NaCl - equiv. Similarly to Th, the salinity of early inclusions (assemblages in sections “B” and “P”, and as. 1-6, crosssection “C”) is rather homogeneous, without unequivocal salinity change during entrapment. In contrast, the latter inclusions (tip “T” and as. 7) have lower salinity. The difference between the opaque and the clear parts of the crystal concerning Th and total salinity of the fluid inclusions is statistically

Fig. 7. A histogram of homogenization temperatures of the fluid inclusions.

Fig. 8. Time – resolved distribution of the homogenization temperature Th (a) and total salinity (b) of fluid inclusions. Concerning the figures and letters see Tables 2 and 3. Note that assemblage C1 groups primary inclusions randomly distributed in the central zone of the cross section (C). This zone belongs probably to the transparent part of the crystal, younger than the early opaque root zone. Thom. data for C7 (empty hexagons) and for tip part T (empty triangles) are presented individually. They would not be averaged, as the dispersion of the results overcomes the experimental error many times.

significant. The absolute error does not reach more than ±5 deg (±2 deg usually for Th and ±0.5 wt. % NaCl-equiv., respectively). This is proven by repeated measurements of Th and Tm in the same inclusion, and/or in the same artificial inclusion with known Th and Tm. The calm transition is confirmed by the absence of rough imperfections of the crystal body, suggesting a possible interruption in crystal growth, as well as a presence of a diffuse zone between the inclusion-abundant semitransparent base part and the inclusion-poor tip (Fig. 3).

6.2. Statistical pretreatment of the data All statistical pretreatment of the data were made by evaluating of two RSD (95% probability). Since the inclusions are destroyed during the experiment, a single inclusion can be measured only once. This is a drawback in the estimation of the random error (the precision of the analysis) by standard statistics and, hence, does not make the results from a single inclusion representative enough. This problem can be avoided if we consider the distinct assemblages as samples of a sufficiently homogeneous fluid (Heinrich et al., 2003). Then the results from analyses of several syngenetic inclusions should compensate for repetitive analyses of a single inclusion. This strategy allows presenting the analytical results from a group of inclusions as a mean value and the range (e.g. Gagnon et al., 2003) or estimating the precision (Wilkinson et al., 1994). Ulrich et al. (2001) say that “typical uncertainties … are estimated from analyses of several microthermometrically identical inclusions in one assemblage”. On that account Table 4 was built, where molar ratios of 11 elements were presented, normalized towards the main cation in the fluid inclusions: X/Na mol/mol. As it was mentioned above, the analyzed inclusions represent 10 clusters (assemblages) of 2 to 7 inclusions. Assuming identity of the fluid trapped in the inclusions of a distinct assemblage, the corresponding mean (or median) values for these ratios were calculated.

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171

Table 4 Mol ratios (medians) obtained from processed LA-ICP-MS- results. Section

Assemblages

Number of inclusions

K/ Na × 10-1

Ca/ Na × 10-2

Mg/ Na × 10-3

Mn/ Na × 10-3

Cu/ Na × 10-4

Zn/ Na × 10-4

Pb/ Na × 10-5

tip body base

T P B C1 C2 C3 C4 C5 C6 C7

2 7 3 6 4 7 6 5 5 6

0.8* 1.4 ± 0.4 1.4 ± 0.8 0.3 ± 0.1 2.2 ± 1.6 0.8 ± 0.4 0.9 ± 0.5 0.9 ± 0.5 0.44 ± 0.1 1.4 ± 0.6

≤4.3 5.4 ± 1.6 7.6 ± 2 4.2 ± 1.2 7±6 8±2 4.6 ± 2 7±2 5±3 b16

b6 0.4* 1.4* 0.4 ± 0.2 0.2* 0.4 ± 0.2 ≤0.4 ≤0.1 0.7* b 2.5

≤4 3.9 ± 1.3 4±2 3.9 ± 1.7 9±7 9±2 12 ± 7 9±2 3±2 3*

18* 2.8 ± 1.5 2 ± 1.5 b 0.1 1* 2÷5 b 0.3 0.8 ÷ 1.5 2.3* b3

b6 3±2 3±2 4±2 7±5 10 ± 1 8±2 9 ± 0.5 8±4 b8

6* 6 ± 1.4 4 ± 1.3 8±3 15 ± 9 19 ± 6 17 ± 2 18 ± 2 8±3 b1

Cross 1

Li/Na

Rb/ Na × 10-3

Cs/ Na × 10-4

Sr/ Na × 10-4

0.7* 1.0 ± 0.4 1.0 ± 1

8* 7±2 7 ± 1.2 6.7 ± 1.6 8±4 8 ± 1.2 6 ± 1.3 7.6 ± 1.4 5±2 7 ± 1.3

2.6* 2.6 ± 0.7 2.1*

0.0036*

* - single results – from 1 or 2 fluid inclusions. The mol ratios are represented as mol/mol data.

In order to check the statistical difference between the individual assemblages (“zero hypothesis”), the standard method of dispersion analysis was applied. The results obtained are plotted in Fig. 9. It is obvious that X/Na ratios (X = K, Ca and Mn) for all assemblages do not differ significantly. Thus, in the object under consideration, the inclusion content cannot be recognized as different in the limits of the confidence interval (20–30% relative error), independent on location in the crystal or on generation. Some problems with the precision of LA-ICP-MS in fluid inclusion analysis are discussed by Heijlen and Muchez (2000). The estimation of the analysis precision suggests integration of the results from many single inclusions in a body of evidence, with a general mean. A comparison with bulk methods is possible. Data from decrepitation-leach (D) and crushing-leach (C) analyses of the same material, as well as our unpublished results, obtained by D-ICP-AES are shown in Table 5. It is important to compare the data obtained (general mean) with a computed model of solution compositions from water/rock interactions (Ryzhenko and Krainov, 2000). The major element concentrations calculated from LA-ICP-MS are compared to the data evaluated for TPX-environment close to that of Madan quartz deposition (Table 6). The comparison of the results obtained by different methods display a good accordance within the confidence interval, concerning the ratio K/Na. The results for the three other elements need some additional explanation. Ca/Na data obtained by the three methods to be compared show difference of about one order of magnitude. An overview shows

(Table 5) that Ca-molality, calculated on the base of LA-ICP-MS measurements (6 × 10-2 mol/kg H2O), is significantly higher than the calculated values. Another indirect estimation of the Ca-values (only as a trend) may be done using the empirical method for determination of the temperature of a hydrothermal solution of Fournier and Truesdell (1973), where Na-, K- and Ca- molalities are used. In this case the temperature estimation, based on LA-ICP-MS data (270 ± 15 °C), also leads to values farthest from the Th-results (Th N 312 °C). At the same time the lower Ca-concentrations obtained by D-ICP(ES) provide higher T = 300 ± 15 °C, much closer to Th- data. Trapped Cacontaining solid phases, evaporated by laser ablation, could be the source of Ca in this case. Inclusions, in which solid phases are not observed (C7, Table 3), do not yield analytical signal for Ca: it is always below the limits of detection. In the case of D-ICP some solid phases could also enter the analytical tract (Chryssoulis, 1983), but they probably interfere less. The Mg-concentrations supplied from LA-ICP-MS and D-ICP analyses are considered identical within the error limits. These results conform very well to the theoretical values (Table 6). Manganese registered by LA-ICP-MS may be regarded to the solid phases mentioned as it is a typomorphic element for Madan ore district (Vassileva and Bonev, 2003). Possible changes in the concentration of ore elements (Mn, Cu, Zn, Pb) during the hydrothermal process are of special interest. Table 3 and Fig. 10 reveal that mean values of Mn/Na and Pb/Na, determined for core assemblages (C2 – C6) are higher than these ratios in the inclusions from the peripheral (C7) or transparent tip (T) zones. However, the broad dispersion of the results as well as the fact that analytical signal remains often below LOD, hinder the discussion on the mentioned observation. 6.3. The major cations of the hydrothermal solute: alkalis and alkaline-earths Processing the raw data shows that Na+ is the major cation in the fluid. The results for element concentrations (Tables 2 and 3) and for X/Na mol ratios (Table 4) point to Na+ as a major cation comprising

Table 5 Comparison between LA-ICP-MS and two methods of bulk analyses. Method

Fig. 9. Time-resolved element-to-sodium (X/Na) ratios, presented as heavy vertical bars. The bar height corresponds to the range of the values obtained. The empty circles denote single results. The unknown values below LOD are depicted as arrows from LOD to 0; dashed arrows correspond to out-of-scale LODs.

LA-ICP-MS Leach D C D - ICP

X/Na (mol/mol) × 10-3 (X = K, Ca, Mg and Mn) K

Ca

Mg

Mn

Reference

110 ± 20 70 ± 10 90 ± 20 90 ± 20

60 ± 10 19 ± 6 19* 6±2

0.5 ± 0.2 4 ± 1.6 6±2 0.6 ± 0.4

7.0 ± 1.6 1.4*

this paper Kotzeva et al. (2004)

1.2 ± 0.6

to be published

* - single results.

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Table 6 Comparison between analytical data (molality) for the fluid inclusions composition and model solutions, calculated by Ryzhenko and Krainov (2000). Ion

K Na Ca Mg

Water phase of fluid inclusions in Qrz MDN (KD)

Model solution (w/r = 0.01; a single event of straining) p (CO2) = 1 × 10-3.4 bar*

p (CO2) = 1 × 10-2 bar**

8 × 10-2 1 6 × 10-2 5 × 10-4

1.0 × 10-3 1.07 1.1 × 10-2 3.3 × 10-4

3.3 × 10-3 3.4 4.7 × 10-3 1.4 × 10-4

* - equal to p(CO2) = 0.0004 bar = 40 Pa = 0.3 Torr. ** - equal to p(CO2) = 0.01 bar = 1000 Pa = 7.5 Torr.

about 80-90% of the positive charges. Potassium is the second most abundant cation with 7-8% of the positive charges. Ca2+ is significantly less abundant (about 3%); the concentration of Mg2+ is negligible. Some extreme values of Ca/Na ≥ 0.10 and Mg/Na ≥ 0.001 may also be ascribed to the trapped solids. The Li signal in mass spectra could be referred to the host mineral: Al3+ may substitute Si4+ in the quartz lattice, which is chargebalanced by Li+. For some inclusions, however, the trapped solid phase seems to be an additional source of Li. Rb/Sr ratio varies from 1.8 to 8.0 in the fluid inclusions. This is similar to that observed for K-feldspars in felsic gneisses of the Rhodope metamorphic complex (Arnaudova et al., 1983; Cherneva et al., 2008). The hydrothermal alteration of those types of rocks resulting in K/Ca exchange may explain also the enrichment of K in the fluids (Kostov et al., 1986). 6.4. Ore elements Roedder (1972) summarized a lot of data obtained from analyses of fluid inclusions. Concentrations of the heavy metals vary from less than 1 ppm up to 10 000 ppm, typically 10 – 100 ppm. Seward and Barnes (1997) also compiled data based on fluid inclusion or geothermal fluid concentrations, or thermodynamic calculations of saturation conditions: 1.2 - 62 ppm for massive sulfide ores, 3.5 – 2200 ppm for Cu-Pb-Zn veins, and 740 – 7700 ppm for skarn type deposits. The results reported here fall well into the same range: Zn (b7.5 ÷ 160 ppm), Pb (b1.3 ÷ 120 ppm) and Cu (b0.2 ÷ 40 ppm) (Tables 2 and 3). An evaluation of equilibrium concentration of ore elements and comparison with analytical data is necessary for the understanding of ore deposition. Taking into account the very low solubility of PbS and ZnS in pure water, even at T = 300 °C, the solubility products are -22.7 -22.1 L300 and L300 , respectively (Rafalsky, 1973). PbS = 10 ZnS = 10

Under hydrothermal conditions, however, in the presence of chloride ions, the situation is quite different, due to the formation of stable chloride-complexes. Elevated temperature and pressure, as well as the ion strength of the solution, also effect on the solutiondeposition equilibrium. Barrett and Anderson (1988) determined experimentally sphalerite and galena solubilities in NaCl solutions to 300 °C and proposed an Eq. (1) for calculating molal solubilities: log mPb;Zn =



log mPb;Zn

 N

  − 3 + log mSðrÞ + 2ð4−pHÞ

ð1Þ

It is clear that two factors strongly effect on the sulfide equilibrium concentrations: the sulfur content and pH. Unfortunately, these factors cannot be measured directly; only an approximate evaluation is possible. It is evident now that sulfide ores in Madan district have been deposited from chloride-dominated solutions. In such a system the presence of HCl is possible, as HCl at 300 °C is a relatively stable complex: log K300 HCl =-2.5 (Helgeson, 1964, Fig. 17), where K is dissociation constant). The analytical results supply only scarce information about the HCl-content. The balance between cations and anions (chloride) points to log (mNaCl/mHCl) N 2 and hence, to a pH N 3.5 for the system PbS-NaCl-HCl-H2O at mNaCl(t) = 1 and T = 300 °C (Helgeson, 1964, Figs. 26 and 29). Therefore indirect methods for pH estimation must be used. Kostova et al. (2004) applied to the alteration assemblages in the Madan deposits. These authors used successfully stability boundary between sericite (muscovite) and K-feldspar for pH determination: at total K ≤ 0.1 mol/kg H2O (i.e. 0.25-0.3%) in the solution and K-activity about 0.01 mol/kg H2O they found that pH would be in the range 4-6. Taking also into account the lower limit of sulfur content (mS(t)) at the pyrite-pyrrhotite equilibrium, Kostova et al. (loc. cit.) evaluated more precisely pH = 5.2 for the ore depositing solution in another location (Yuzhna Petrovitsa), close to Kroushev Dol deposit. Details for these speculations as well as the sources referred may be found in the study cited above. Carbon dioxide is a common component of hydrothermal fluids and, hence, the carbonate equilibrium may be used for pH determination. Dorofeeva and Naumov (1974). pH-values are tabulated as a function of M/m(ΣCO2) ratio, where M ≈ m(HCO-3) + m(CO23 ) and when free CO2 predominates (i.e. m(H2CO3) NN m(HCO-3) + m(CO23 )), it approximates m(HCO-3). We have some unpublished data for CO2 released from quartz (MD–KD) by decrepitation: CO2/ H2O = 0.013 ( ± 0.002) mol/mol or ΣCO2 = 0.72 mol/kg H2O. Unfortunately, Cl- prevail over all other anions in Madan oredepositing solutions: Cl– ≥ mol % (Kotzeva, 2010), which hinders the HCO-3 - determination. If the last 3% are assumed to be entirely HCO-3, for 1 m NaCl M ≈ m(HCO -3 ) ≈ 0.03. Then M/m(ΣCO 2 ) ≤ 0.03/ 0.72 = 0.04, and at 300 °C (and ion strength μ ≈ 1), pH = 5.9. The inclusions in galena from Madan district contain F- ≤ 3% of the anions (Piperov et al., 1977), so it is likely that HCO-3 - ions are below 1% of the anions and pH ≤ 5.6. Finally, the HCO-3 - concentration could be controlled by dissociation of the dissolved CO2 only:  1 = 2 þ − þ 300 CO2 →H2 CO3 ↔H + HCO3 and H = KðIÞ × mðΣCO2 Þ × γ : ð2Þ Then, at T = 300 °C in 1 m NaCl (μ ≈ 1) and pK (I)300 = 8.78 (according to Rafalsky (1973) and m (ΣCO2) = 0.72 (see above) and the activity coefficient γ = 0.054 (according to Ryzhenko, 1963). þ

log H = 0:5½−8:78 + ð−0:14Þ + ð−1:27Þ = −5:1; i:e:pH = 5:1 ð3Þ Fig. 10. Analytical results for ore elements (Mn, Cu, Zn and Pb) presented as ratios X/Na in a graphical mode. Bars depict the precision evaluated for every quotation of data (95% probability). The corresponding inclusions assemblages are indicated on the abscissa. Empty circles denote single results.

We recognize this value as the lower limit of hydronium activity in the solution. The equilibrium concentrations of Pb and Zn in a 1 m NaCl solution at 300 °C and pH = 5.1, 5.2, 5.6, and 5.9, calculated according to

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Barrett and Anderson (1988) are presented in Table 7. Two model concentrations of the sulfide sulfur mS = 10-3 and 10-4 mol/kg H2O are also used. The comparison of tabulated values with analytical results does not answer directly the question how close to solution – deposition equilibrium of PbS and ZnS is the hydrothermal solute. It may be concluded only, that this point is in a slightly acid (pH = 5.1 – 5.2) medium (the neutral pH = 0.5 K300 w = 0.5 × 11.70 = 5.85 – Rafalsky (1973)) and low S2- - concentration (ms ≤ 0.001 mol/kg H2O). The equation of Barrett and Anderson (1988, Footnote of Table 5) reveals the extreme sensitivity of galena and sphalerite deposition to the hydronium activity. Even an increase in pH of half unit causes a decrease of one log unit in metal sulfides solubility. This finding is of great importance to the understanding of ore deposition. Such a small increase in pH may be caused by boiling (enrichment of the newformed vapor phase with CO2) or neutralization: chemical interaction with carbonates (marbles, skarns) (cf. Reed and Spycher, 1984). The strong dependence of the sulfide solubility on pH explains very well the situation in the Madan ore district where zones of boiling fluids and skarns contain massive sulfide ores. The permanently high content of Mn (77 ÷ 1600 ppm, one case 4900 ppm), being typomorphic element for the region (Bonev, 2003), is remarkable. The early pre-ore pyroxene skarns, entirely embedded in marble horizons are highly manganoan, as well as the products of their intensive retrograde alteration (Vassileva and Bonev, 2003). Thus, the increased Mn content in the fluid reflects the specific environment of Madan deposits, belonging to the largest known ore deposits of this type. Concentrations vs. time–resolved evolution (early → late) are represented in Fig. 11, suggesting a decrease in the ore-element concentrations at the end of the main ore stage (2) (see Section 2). The concentration of the major elements in the solution, following the changes in salinity, drops to a half. The ore elements concentrations, however, decrease to more than a half for Zn and about one order of magnitude for Pb and Mn. The concentration of Cu is very low and scattered in most assemblages and no statistically significant variation can be identified. 6.5. Other chalcophile elements The deposition of sulfide ores in Madan ore district suggests the presence of chalcophile elements. Trace amounts of Te were only detected in a few of the analyzed inclusions (Table 2). Arsenic (17 – 72 ppm), antimony (b1 – 46 ppm) and bismuth (0.3 – 12 ppm) were Table 7 Calculated sphalerite and galena solubilities as a function of pH and reduced sulphur molalities (according to Barrett and Anderson, 1988). log m Pb;Zn =

log m Pb;Zn


N

  − 3 + log m SðrÞ + 2ð4−pHÞ

For 1 m NaCl at 300-Cð log m Pb ÞN = −2:04 and ð log m Zn ÞN = −0:91 Subscript “N” denotes to “normalized” at 300 °C, pH = 4 and total reduced sulphur = 0.001 m (mol/kg H2O), i.e. log mS = -3. Concentration

log m c; ppm

pH

5.1

logmS

Zn

Pb

Zn

Pb

Zn

Pb

Zn

Pb

- 3.0

-3.11 50.8 -2.11 508

-4.24 11.9 -3.24 119

-3.31 32.0 -2.31 320

-4.44 7.5 -3.44 75

-4.11 5.1 -3.11 50.8

-5.24 1.2 -4.24 11.9

-5.62 0.16 -4.62 1.57

-5.84 0.3 -4.84 3.0

- 4.0

5.2

5.6

5.9

Analytical data are (mean values of results from more representative assemblages 2-6): Pb 44 (± 25) and Zn 86 (± 40) ppm.

173

Fig. 11. Time-resolved trend of the ore element (Cu, Pb, Zn and Mn) concentrations (ppm) in the fluid inclusions. The figures denote the identification number of fluid inclusion assemblages (as.) in cross section C from Fig. 8b and Table 3.

also found in the inclusions. These elements occur in galena from Madan ore district (Sb 60 ÷ 860 ppm, Te b 1 ÷ 79, Bi b 1 ÷ 2500), and are important modificators of the crystal habits (Bonev, 2007); and their source is undoubtedly the hydrothermal solute. 6.6. Boron The B content (see Table 2) of the fluid inclusions analyzed is rather uniform: 130 – 240 ppm (one case b82). These values, when compared with the B concentrations in some contemporary hydrothermal systems (Ellis, 1979: 0.5 – 140 ppm), seem somewhat high. They are very similar, however, to the results from fluid inclusions analysis, summarized by Roedder (1972): b3 up to 520 ppm. 6.7. Application to the mineral deposition As it was mentioned above, 10 assemblages of inclusions were identified. Although they are topographically distinct, the inclusions of 5 (6) of them do not differ significantly neither in the Th and total salinity, nor in the chemical composition (X/Na ratios). The location of these assemblages suggests the timing of the cracks healing (early – late), but the time distance between the appearance of the separate inclusion assemblages is unknown. It is most likely that these inclusion assemblages (section C, 2 ÷ 6) appeared in the beginning of the main ore stage, as a result of intensive quartz deposition, accompanied by thermal and mechanical stress. This caused cracks formation but their healing took place in a relatively short time. Most of the inclusions (N90%, cf. decrepitometry) have been trapped at the same time. Later a decrease in temperature (Th = 329 ÷ 312 °C) and in total salinity is recorded in the fluid inclusions, populated the other assemblages: C7, P, T. No changes in the chemical composition (X/Na ratios) were detected. This observation may be explained simply by involving a weak flux of cold and fresh waters, infiltrated into the ascending hydrothermal fluid. The transition from the starting conditions to the late ones is unclear, as it is evident by features of the crystal growth: the inclusion abundance decreases significantly making the tip parts of the crystals transparent; the faces are smooth, especially the terminal rhombohedra. The dilution reduces the degree of oversaturation, which causes a calmer and slower crystal growth. This suggests a longer mineral deposition with lower T and lower salinity. There are no data for interruption in the hydrothermal flow as well as in the crystallization process at least during the main ore stage. This confirms Bonev's conclusion about the development of the oreforming process during the geological time without interruptions (Bonev, 2007).

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Ore element concentrations in the ascending hydrothermal fluid seem near saturation under the given PTX – conditions. Any increase in pH – value, due to boiling or interaction with carbonates, causes a drop in the solubility of Pb and Zn sulfides. This way the solution becomes supersaturated concerning these phases, which initiates precipitation of galena and sphalerite. 7. Conclusions Single fluid inclusions from different assemblages in a quartz crystal were analyzed, in order to determine the temporal evolution of the ore-forming fluids. Neither the Th (330 ÷ 350 °C), nor the total salinity (5 ÷ 9 wt. %) of inclusions trapped in earlier fluid inclusion assemblages (more 90% of the total), vary significantly. The chemical composition of the same inclusions, determined by LA-ICP-MS, does not show statistically significant variation. The concentrations of the ore elements (Pb, Zn) in the ascending hydrothermal fluids seem to be close to equilibrium with PbS and ZnS under the existing PTX – conditions; in the zone of barren quartz, however, the fluids are still undersaturated. Even a small change (increase) in pH is assumed to be responsible for significant drop in the PbS and ZnS solubility, the solution becomes oversaturated in these substances and galena and sphalerite were precipitated. Late inclusions display lower salinity (4.5 ÷ 6.5 wt. %) and slightly lower Th (336 ÷ 312 °C) suggesting a small admix of low-saline (infiltrated?) water. The X/Na ratios of the main components do not change significantly; only Pb/Na and Mn/Na ratios seem to decrease. The dilution of the mineral-forming fluids caused a calmer growth of the barren quartz. The decrease of the activity of ore elements suggests also abating of the ore deposition and probably indicates the end of the main ore stage. Acknowledgements The LA-ICP-MS analyses as well as many of the microthermometry measurements were performed at ETH – Zürich. The authors are grateful to Prof. Dr Ch. Heinrich for providing access to the analytical facility. One of the authors (B. Kotzeva) is indebted to National Science Fund of Bulgaria (National Centre for New Materials UNION, Contract No DO-02-82/2008) for maintenance towards attendance at ETH Zürich. The authors are thankful to Dr. Milen Kadyiski for his assistance in preparation of the paper. In memoriam The authors dedicate this study to the memory of our late colleague and friend Prof. DSc Ivan Bonev (1936–2008) from Geological Institute, Bulgarian Academy of Sciences. More than 40 years he worked in the field of mineralogy; sulfide minerals have been his favorite subject. As a young geologist he held the position of a mine engineer in Madan ore district (Kroushev Dol mine) and has kindly supplied us with quartz specimen from his private collection. His valuable advices helped us very much with our studies. References Allan, M.M., Yardley, B.W.D., Forbes, L.J., Shmulovich, K.I., Banks, D.A., Shepherd, T.J., 2005. Validation of LA-ICP-MS fluid inclusion analysis with synthetic fluid inclusions. American Mineralogist 90, 1767–1775. Arnaudova, A., Bedrinov, I., Orlov, R., 1983. Structural state and geochemical characteristics of potassic feldspars from the Central Rhodope metamorphic complex. Geologica Balcanica 20 (1), 67–84. Audétat, A., Günther, D., Heinrich, Ch.A., 1998. Formation of a magmatic – hydrothermal ore deposit: insights with LA-ICP-MS analysis of fluid inclusions. Science 279, 2091–2094. Barrett, T.J., Anderson, G.M., 1988. The solubility of sphalerite and galena in 1-5 m NaCl solutions to 300 °C. Geochimica et Cosmochimica Acta 52, 813–820. Bodnar, R.J., 1993. Revised equation and table for determining the freezing point depression of H2O-NaCl solutions. Geochimica et Cosmochimica Acta 57, 683–684. Bogdanov, B., 1960. Geological structure and arrangement of Madan ore district, 6. Annales of Institute for Geology and Mining, Sofia, pp. 1–2.

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