Hypogene Alteration of Base–Metal Mineralization at the Václav Vein (Březové Hory Deposit, Příbram, Czech Republic): The Result of Recurrent Infiltration of Oxidized Fluids

Abstract

The Václav vein (Březové Hory deposit, Příbram ore area, Czech Republic) is a base–metal vein containing minor Cu-Zn-Pb-Ag-Sb sulfidic mineralization in a usually hematitized gangue. A detailed mineralogical study using an electron microprobe revealed a complicated multistage evolution of the vein. Early siderite and Fe-rich dolomite were strongly replaced by assemblages of hematite+rhodochrosite and hematite+kutnohorite/Mn-rich dolomite, respectively. In addition, siderite also experienced strong silicification. These changes were associated with the dissolution of associated sulfides (sphalerite, galena). The following portion of the vein contains low-Mn dolomite and calcite gangue with Zn-rich chlorite, wittichenite, tetrahedrite-group minerals, chalcopyrite, bornite, and djurleite, again showing common replacement textures in case of sulfides. The latest stage was characterized by the input of Ag and Hg, giving rise to Ag-Cu sulfides, native silver (partly Hg-rich), balkanite, and (meta)cinnabar. We explain the formation of hematite-bearing oxidized assemblages at the expense of pre-existing “normal” Příbram mineralization due to repeated episodic infiltration of oxygenated surface waters during the vein evolution. Episodic mixing of ore fluids with surface waters was suggested from previous stable isotope and fluid inclusion studies in the Příbram ore area. Our mineralogical study thus strengthens this genetic scenario, illustrates the dynamics of fluid movement during the evolution of a distinct ore vein structure, and shows that the low content of ore minerals cannot be necessarily a primary feature of a vein.

1. Introduction

In the Bohemian Massif, for centuries, the famous Příbram ore area was the most important source of metallic ores bound to hydrothermal veins. Uranium, Ag, Pb, Zn, and Sb ores were mined here from ca. 1400 ore veins. The ore zone covers an area of ca. 40 km × 15 km and ore minerals were extracted from depths down to 1800 m. More than 500,000 t of Pb, 3500 t of Ag, and 48,000 t of U were mined out in the district [1]. The amount of mined U metal ranks the Příbram ore area among the largest vein-type U deposits in the world.

The ore veins in the Příbram area are also interesting from a scientific point of view. Although more than 1000 studies devoted to Příbram deposits and their mineralization exist, many principal questions remain unresolved. The end of modern mining activities in 1991 coincided with the change in the political system in the Czech Republic in 1989. The change in social structure was associated with the better availability of modern mineralogical research methods, which allow a much more detailed assessment of the paragenetic situation and compositional variations in individual minerals. However, the closure of mines was connected with a significant decrease in research activities in the district. In addition, the majority of recent papers was devoted to the description of some (presumably rare) newly found mineral phases. The number of systematic modern detailed studies evaluating the whole paragenesis together with assessing of genetic aspects of individual ore veins still remains very low in the Příbram area.

Many unresolved genetic problems are often associated with a complex paragenetic evolution of the Příbram base–metal ores. Widespread simple banded texture, in which bands composed of carbonates, quartz, and sulfides alternate, is often complicated by less or more intense metasomatic replacement of some vein components by later mineral phases. These processes sometimes led to a complete change in the mineral composition of some ore portions. A well-known example illustrating such features is the so-called “hard ores”, which occur in the part of the Příbram base–metal veins and are sometimes in direct contact with normal banded ores. The “hard ore” is a fine-grained black matter composed especially of quartz and sulfides. The detailed microscopic studies of polished sections in reflected light [2,3,4] confirmed that “hard ores” were initially siderite veins, in which siderite was almost completely replaced by grains of quartz and ore minerals. On the basis of careful evaluation of fossil grain boundaries, [2] was able to identify up to three generations of such replacement at a single place. The recent works dealing with the evaluation of ore textures by means of backscattered-electron (BSE) imaging fully confirmed the previously reported complicated paragenetic and textural evolution of ore veins in the Příbram area [5,6,7,8].

In this contribution, we provide results of new detailed mineralogical investigations of archive samples from the Václav vein. Although the Václav vein is a distinct base–metal bearing structure, its mineralization is characterized by only locally high ore grades, Cu enrichment, and Pb,Zn depletion in comparison with neighboring base–metal ore veins. In addition, uncommon black acicular to columnar ore aggregates were recognized in a late drusy cavity in one sample (Figure 1), whose mineralogical evaluation is also presented in this paper. The results significantly increase the existing fragmentary knowledge on the mineral composition of the Václav vein. Moreover, the textural investigations of the studied vein illustrate the complexity of genetic processes involving the ore veins in the Příbram area.

2. Geological Setting and Ore Mineralization of the Příbram Area

The Příbram ore area is situated in the central part of the Czech Republic close to the tectonic boundary dividing two major basement blocks of the Bohemian Massif: the deeply uplifted Moldanubian area in the east and the shallowly eroded Teplá-Barrandian unit in the west. The contact of both areas is hidden by a large granite intrusion of the Central Bohemian Plutonic Complex (CBPC). The ore area is hosted by volcano-sedimentary sequences of the Teplá-Barrandian unit near the contact with the igneous rocks of the CBPC [9] (Figure 2). The Teplá-Barrandian unit is dominated by folded siliciclastic sediments (greywackes, siltstones, and shales) with interpositions of volcanic rocks of the Upper Proterozoic to Lower Paleozoic age, which was contact-metamorphosed by the intrusion of the CBPC. The volcano-sedimentary sequence is intruded by numerous palaeobasalt (dolerite) dykes of presumed Upper Cambrian-to-Lower Silurian age [9]. The CBPC is a polyphase Variscan granite-dominated intrusion, which was emplaced between 354 and 335 Ma [10,11]. Gravitational collapse of the Variscan orogen led to rapid uplift, erosion of the basement, and formation of freshwater Piedmont basins, which were filled up by the molasse sediments of the Uppermost Carboniferous-to-Lower Permian age. Siliciclastic sediments prevail here, in basal parts with coal seams [12,13,14,15]. In the present erosional section, most of these sediments are already eroded.

The Příbram ore area consists of two principal districts comprising uranium and base–metal vein mineralizations: the westward lying Březové Hory ore district, where base–metal mineralization prevails over uranium ores, and the eastward Příbram uranium and base–metal district, where uranium ores predominate (Figure 2). The Březové Hory ore district (hosting the Václav vein under this study) was the largest source of hydrothermal Ag, Pb, Zn, and Sb ores in the Czech Republic and consists of three deposits: Březové Hory, Černojamské, and Bohutín [16]. The Březové Hory deposit with the major veins Vojtěšská hlavní, Ševčínská, Matkobožská, and Eusebská was the most important part of the district. The ore veins are hosted by Cambrian siliciclastic sediments and usually follow palaeobasalt (dolerite) dykes [9]. The Příbram uranium and base–metal district contains several deposits including the Bytíz, Háje, Jerusalem, Brod, and Lešetice deposits (Figure 2).

Four major mineralization stages were distinguished in the Příbram area: siderite-sulfidic, carbonate, uraninite, and carbonate-sulfidic, listed according to their age [17]. The ore veins are composed of carbonates (siderite, dolomite-ankerite series, calcite), quartz, and baryte, which host the predominating Pb-Zn sulfide mineralization with prevailing galena, sphalerite, pyrite, and locally with Pb-Sb, Ag-Cu-Sb, and/or Ag-Pb-(Sb) sulfosalts. The uranium mineralization is represented by uraninite, U-bearing anthraxolite, and minor coffinite hosted by calcite gangue. The radiometric dating of the Příbram ores gave ages between 330 and 224 Ma covering all major mineralization stages from siderite-sulfidic to the calcite-sulfidic one [18,19].

The early portion of the siderite-sulfidic stage had formed from high-temperature (200–300 °C), high-salinity (15–24 wt. % NaCl eq.), and high-δ¹⁸O (7–10‰ V-SMOW) Na>>Ca-Cl brines [1,7]. The source of these fluids is not known as their composition differs significantly from typical metamorphic and magmatic fluids in the wider area (cf. [6,20,21,22]). The flow of these early fluids was substituted by surface-derived fluids during the crystallization of later portions of the vein mineralization. Two types of surface-derived fluids were recognized: 1) low-temperature (Th < 150 °C), low-salinity (<7 wt. % NaCl eq.), and near-zero δ¹⁸O (−3 to 3‰ V-SMOW) solutions, whose source is interpreted in contemporaneous (Lower Permian) freshwater partly evaporated Piedmont basins developed above the originating ore district [1,7]; and 2) low-temperature (Th < 150 °C) and high salinity (up to 30 wt. % NaCl eq.) near-zero δ¹⁸O (−4 to 4‰ V-SMOW) brines with highly variable Ca/Na ratios, whose source is interpreted either in external marine sources or in in situ interactions of meteoric water with basement rocks producing “shield brines” [7,8].

The material under this study came from the Václav vein, which is one of the important vein structures of the Březové Hory deposit and is hosted by a dolerite dyke cutting the Lower Cambrian greywackes, sandstones, and conglomerates (Figure 3). The Václav vein was exploited by the Anna shaft between the surface and the 36th level. Despite the large depth extent (around 1300 m), the economic importance of the Václav vein was inferior due to the overall low proportion of ore mineralization [9]. The richest parts of the vein were concentrated on the crosscuts with the František, Jánská, and Karolínská veins in the upper and middle depths of the deposit. The Václav vein is considered to be one of the youngest ore veins, together with the Jánská, Karolínská, Prokop, and Zikmund veins [23]. The general direction of the Václav vein is 15° and its dip is 80° to the ESE [9]. The presence of quartzose graywacke in deeper horizons east of the Václav vein had a negative influence on the extent of ore mineralization [23]. Massive chalcocite and bornite ores were mined from the Václav vein in the 1850s from depths of about 350–400 m (15th and 16th level of the Anna shaft).

3. Material and Methods

Our study was conducted on a specimen deposited in the collection of the National Museum, Prague, under catalog number P1N 9430, which was found in the year 1852 at the 16th level of the Václav vein in a stope southern from the crosscut with the Jánská vein in the Anna shaft field. The specimen has dimensions 8 cm × 5.5 cm × 4.5 cm and represents the profile through the whole vein fill from the surrounding rock up to a drusy cavity in the youngest calcite fill (Figure 1). In addition, another historical sample originating from a private collection of J. Hyršl and showing an incomplete vein fill was studied only briefly in one polished section (Dy-816).

The surface morphological features of minerals were documented by the use of a Nikon SMZ 25 microscope equipped with a digital camera Nikon DS-Ri2 and the function of picture assembling in the software NIS Elements AR version 4.20.

The polished sections were prepared from minute (up to 1 cm in size) ore chips broken off from museum specimens. Polished sections were prepared by use of standard diamond polishing techniques. Optical properties of minerals in reflected light were observed with a Nikon Eclipse ME600 microscope with digital camera Nikon DXM1200F.

The quantitative chemical analyses were performed by use of a Cameca SX100 electron microprobe at the Department of Mineralogy and Petrology, National Museum, Prague, Czech Republic, operating in the wavelength-dispersive mode (WDS). Details on analytical conditions are given together with complete analytical data sets in a supplementary file (Table S0). They can also be found in a few of our recently published papers [5,6,7,8,24,25].

The Raman spectra of some mineral phases of interest were collected by means of a DXR dispersive Raman spectrometer (Thermo Scientific, Waltham, MA, USA) mounted on a confocal Olympus microscope. An unpolarised green 532 nm solid-state diode-pumped laser was used to excite the Raman signal, which was detected by a CCD detector in a spectral range of 50–3500 cm⁻¹. The measurement was calibrated by use of multiple lines of neon, Raman bands of polystyrene, and a standardized source of white light in order to calibrate the wavelength position, laser frequency, and intensity of the signal, respectively. The experimental settings used were 100× objective, 5 s exposure time, 100 exposures, 50 μm pinhole spectrograph aperture, and 5–10 mW laser power level. The possible thermal damage of the measured place was controlled by visual observation of the excited mineral surface after the measurement, by checking of possible decay of spectral features at the start of excitation and evaluation of the thermal downshift of Raman lines. Spectral operations were carried out by use of the Omnic 9 software (Thermo Scientific). The obtained spectra were compared with reference spectra of minerals from the RRUFF database [26].

The following mineral abbreviations are used throughout this paper: Ac—acanthite, Ag—silver, Ann—annivite, Bkn—balkanite, Bo—bornite, Ca—calcite, Cb—(meta)cinnabar, Chl—chlorite, Cpy—chalcopyrite, Cst—cassiterite, Cv—covellite, Dju—djurleite, Do—dolomite, Ga—galena, He—hematite, Jal—jalpaite, Ktn—kutnohorite, Mck—mckinstryite, Q—quartz, Rdc—rhodochrosite, Sid—siderite, Smy—stromeyerite, Sp—sphalerite, TGM—tetrahedrite-group mineral, Ttd—tetrahedrite, Wt—wittichenite. In addition, R refers to wall rock.

4. Results

A distinct zoned texture of the vein is recognized macroscopically on sample P1N 9430 (Figure 4). The host rock is greenish hydrothermally altered siltstone, which is strongly impregnated by carbonates (siderite, rhodochrosite, dolomite group) and ore minerals (hematite, chalcopyrite, bornite). The host rock is included in polished sections VA-1 and VA-2.

The host rock is overgrown by a 2 cm thick early fine-grained dark brown vein fill (Zone A in Figure 4). The early two-thirds of Zone A, represented by sections VA-2 and VA-3, are composed especially of predominating quartz, subordinate carbonates (dolomite series, siderite-rhodochrosite series, and calcite), common hematite, and minor sulfide minerals including sphalerite, chalcopyrite, and tetrahedrite-group minerals. The complicated textural relationships among minerals appear in Zone A, including crosscutting, intense replacement, and dissolution phenomena, which are observed especially in carbonates and sulfides. The uppermost one-third of Zone A (contained in sections VA-4 and lowermost part of VA-5) is formed by zoned aggregates of carbonates of the dolomite series rich in kutnohorite component with numerous aggregates of hematite. In addition, the upper part of Zone A contains irregularly shaped dark grey domains up to 1 cm in size, composed of fine-grained and often zoned aggregates of carbonates of dolomite group, calcite, hematite, sulfides (sphalerite, TGM, bornite, chalcopyrite), and chlorite. This domain is included in the polished section VA-6. Zones A and B are divided by an interposition of well-developed quartz crystals reaching up to 1 mm in size, documented in sample VA-5.

The following Zone B is formed by a ca. 1 cm thick distinct drusy coating of a dolomite-group carbonate, consisting of coarse individual rhombohedral crystals that reach up to 1 cm in size (Figure 4). Macroscopically, the carbonate is yellowish-white and finely zoned with the latest ca. 0.5 mm thick grey growth zone containing numerous disseminated minute grains of hematite. Intense replacement of earlier Fe-rich dolomite by an assemblage of hematite and Mn-rich members of the dolomite series is observed. This zone is included in samples VA-4, VA-5, and VA-7.

Zone C, represented by polished sections VA-8 and VA-9, is composed of beige to pinkish fine-grained (grain size ca. 0.5–1 mm) carbonates of dolomite series (Figure 4), showing distinct zoning and replacement textures in the bottom part of the Zone C.

Zone D is formed by the youngest portion of the vein, which is essentially composed of calcite containing small nests of sulfide minerals. Its crystallization began by deposition of a ca. 1 mm thick red growth zone (Figure 4), whose coloration is due to finely dispersed hematite, and which also contained minor chlorite. The majority of Zone D is formed by milky white coarsely grained (grain size ca. 1–3 mm) calcite with a maximum thickness of 1 cm, containing grains and up to 2 cm × 1 cm large clusters of sulfide minerals (chalcopyrite, bornite, TGM, minor Ag-Cu-S phases). This zone is included in sections VA-8 and VA-9.

Calcite-sulfide aggregates of Zone D contain a cm-sized drusy cavity (Zone E) lined by up to 5 mm colorless rhombohedral calcite crystals, from which dark grey acicular to columnar aggregates of ore minerals up to 4 mm × 0.8 mm in size stick out (Figure 1 and Figure 4). Sections Dy-818 and Dy-973 were prepared from material from the drusy cavity. A mineralogical study of this youngest ore assemblage showed prevailing chalcopyrite and bornite together with a suite of minor Cu, Ag-Cu, and Ag-Cu-Hg sulfides and native silver.

Below, there are characterized individual minerals recorded in the vein.

Siderite is present in altered wall rock and in the early part of Zone A (sections VA-1 and VA-2). It forms individual isolated grains, which are always strongly corroded and/or overgrown by rhodochrosite, carbonates of the dolomite series, hematite, quartz, and/or chalcopyrite (Figure 5a–d). Because of strong corrosion, the original grain size cannot be evaluated and only relics reaching up to 0.5 mm are preserved. No zonation was usually observed in BSE images in the grain scale; small darker zones enriched in Mg and Ca are exceptional (Figure 5a). The spot WDS analyses (Table S1) confirmed a limited chemical variability (Sid_66–77Rdc_12–26Mag_4–13Cal_0–5). In the classification scheme by [27], the projection points fall just around the boundary between siderite and Mn-rich siderite (Figure 6a). The low content of Na (up to 0.21 wt. % Na₂O), detected in a quarter of collected analyses, is difficult to explain in terms of siderite crystal chemistry. However, they may potentially be attributed to the salt content of fluid inclusions involved in the excitation volume. An average empirical formula of siderite, on the basis of 33 WDS analyses and 1 metallic cation, is (Fe_0.73Mn_0.20Mg_0.06Ca_0.01)_∑ = _1.00CO₃.

Rhodochrosite accompanies siderite in altered wall rock and in the early part of Zone A (in sections VA-1 and VA-2). It occurs in two morphological varieties. The most frequent are narrow (with thickness up to 20 µm) outer growth zones rimming corroded relics of siderite grains (Figure 5b,c). These rhodochrosite zones often enclose hematite aggregates (Figure 5c). Less common are individual isolated isometric grains or even euhedral rhombohedral crystals, up to 120 µm in size, which are enclosed by carbonates of the dolomite series, hematite, and/or quartz (Figure 5b,d). Rhodochrosite is strongly corroded, especially by carbonates of the dolomite group. Though no distinct zonation is observed in BSE images, the spot WDS analyses (Table S2) showed a wide chemical variability (Rdc_47–78Sid_2–44Mag_2–18Cal_0–11Smi_0–3), with the main differences associated with various mineral assemblages. In the classification scheme by [27], two distinct data clusters are observed (Figure 6a). Data classified as Fe-rich rhodochrosite were collected from hematite-hosted rhodochrosite crystals, grains, and veinlets, whereas Fe-depleted analyses occupying the rhodochrosite field are from rims around siderite and from isolated grains/relics hosted by carbonates of the dolomite series. In contrast, the elevated contents of smithsonite (up to 3.1 mol. %), sphaerocobaltite, and gaspéite (both up to 0.2 mol. %) components are almost exclusively hosted by narrow Fe-poor rhodochrosite rims around siderite. These observations likely suggest multiple compositionally distinct populations/generations of rhodochrosite, but clear textural evidence allowing us to distinguish multiple generations is missing. The mean empirical formula of rhodochrosite, on the basis of 34 WDS analyses and 1 metallic cation, is (Mn_0.68Fe_0.15Mg_0.12Ca_0.03Zn_0.02)_{∑ = 1.00}CO₃.

Minerals of the dolomite series are the dominating carbonates in the studied vein, which were recorded in all studied polished sections (VA-1 to VA-8) except for those from the late drusy cavity (Zone E). Similarly, it is also present in a big amount in the altered wall rock. The crystallization of the dolomite series undoubtedly postdates siderite and rhodochrosite as indicated overgrowths and crosscutting veinlets and predates the crystallization of calcite. Early siderite and rhodochrosite are strongly corroded and replaced by dolomite-group carbonates. The cross-cutting evidence clearly indicates that multiple generations of carbonates of the dolomite series occur in the Václav vein. However, a significant volume of dolomite in Zones A, B, and C displays replacements of an older dolomite series carbonate by a younger one with different compositions, which creates serious problems with the identification of individual generations on the basis of textural patterns. In terms of chemical composition, the whole collected dataset shows huge variations (Dol_28–86Ktn_6–70Ank_0–43Min_0–7) covering classification fields of dolomite, Fe-rich dolomite, Mn-rich dolomite, and Mg-rich kutnohorite in the classification scheme by [27] (Figure 7a,b). Nevertheless, most compositional data (Table S3) concentrate in three clusters (Figure 7c) denoted Do-I (characterized by Fe > Mn and classified as Fe-rich dolomite), Do-II (with wide Mg-Mn substitution, whereas Fe is essentially missing, classified as dolomite, Mn-rich dolomite, and Mg-rich kutnohorite), and Do-III (approximately Fe = Mn, classified as dolomite, Mn-rich dolomite, and Fe-rich dolomite). The occurrence of Do-I carbonate is restricted to Zone B, where it is instructively replaced by an assemblage of hematite and Do-II (Figure 5e,f). In addition, the slightly zoned massive Do-II carbonate forms the late portion of Zone A, where it is accompanied by a large amount of hematite (Figure 5e). Finally, some Do-II replacements and growth zones were also observed in two positions in basal portions of Zone C, with local manifestations of replacement of Do-II by Do-III (Figure 8a). The zoned Do-III carbonate predominates especially in the latest portions of the dolomite vein adjacent to calcite (Figure 8b–d) and minor occurrences are observed in Zones A and B as well as in altered wall rock. The Zn contents are usually elevated in Fe-poor Mn-bearing dolomite belonging to the Do-II compositional type. Low amounts of Na (up to 0.34 wt.% Na₂O) and Al (up to 0.18 wt.% Al₂O₃), recorded in a small part of WDS analyses, are probably related to anizomineral impurities. The mean empirical formula of carbonate from the dolomite series, on the basis of 290 WDS analyses and 2 metallic cations, is Ca_1.03(Mg_0.52Mn_0.33Fe_0.11Zn_0.01)_{∑ = 0.97}CO₃.

Calcite is the youngest gangue component and major mineral in Zones D (samples VA-8 and VA-9) and E (Figure 1, Figure 4, and Figure 8c). In addition, calcite is also the youngest component in dark hematite-rich domains represented by sample VA-6, which are hosted by Zone A (Figure 8b,d). In Zones D and E, white coarse-grained (grain size up to 5 mm) calcite partly fills the remaining open spaces, leaving drusy cavities lined by colorless calcite crystals (Figure 1). The narrow earliest portion of calcite is pink to red colored due to finely dispersed hematite pigment (Figure 4). In BSE images, calcite from both occurrences does not display chemical zoning. The chemical composition of calcite (Table S4) is characterized by low contents of magnesite and siderite components and elevated content of rhodochrosite (Cal_89–97Rdc_3–9Sid_0–2Mag_0–1). No distinct correlations appear among the contents of Fe, Mg, and Mn (Figure 6b). There are no differences in the composition of calcite occurring in Zone A in the youngest parts of the studied vein. The mean empirical formula of calcite, on the basis of 20 WDS analyses and 1 metallic cation, is (Ca_0.93Mn_0.06Fe_0.01)_{∑ = 1.00}CO₃.

Quartz occurs in two positions in the studied vein. Early quartz I is the major component of the early portion of Zone A (samples VA-1 to VA-4), where it strongly replaces siderite (Figure 5a,b). The quartz gangue often contains small drusy cavities lined by drusy coatings of quartz crystals, which were later completely filled up by hematite, rhodochrosite, dolomite-series carbonates, and/or Cu-Fe sulfides (bornite and chalcopyrite; Figure 8e,f). Inclusions of galena were rarely recorded to be enclosed in quartz I. Quartz II divides Zones A and B and was encountered in sample VA-5. It forms an interposition in carbonates of the dolomite series, which is composed of well-developed euhedral crystals grown into ancient open space and reaching up to 0.8 mm in size (Figure 5e).

Chlorite is an accessory component, which was found in two distinct positions in the vein, both showing, however, similar assemblages. A greater amount of chlorite was recorded in a dark nest hosted by dolomite-series carbonate in the upper part of Zone A (sample VA-6). Here, chlorite aggregates grow over aggregates of euhedral hematite crystals cemented by zoned dolomite (Do-II and Do-III) and calcite. Chlorite forms isolated spherical aggregates up to 80 µm in size (Figure 8d), composed of small sheets. In BSE images, chlorite aggregates are usually irregularly zoned. Bright cores contain higher amounts of Zn. A single 15 µm long sheet of chlorite was in addition found to be enclosed together with hematite and chalcopyrite in the earliest red-colored calcite of Zone D in sample VA-8 (Figure 4). The stoichiometry of collected WDS analyses (Table S5) proved the trioctahedral chlorites. Besides Si (2.97–3.14 apfu; anhydrous base of recalculation of 14 atoms of oxygen), Al (1.95–2.11 apfu), Mg (0.78–1.58 apfu), and Fe (1.28–3.26 apfu), there are also observed high contents of Zn (0.27–2.06 apfu) and less elevated contents of Mn (0.03–0.13 apfu), Ca (0.01–0.19 apfu), and K (up to 0.01 apfu). The Fe/(Fe + Mg) ratios range from 0.46 to 0.81. The predominating octahedral cations are either Fe or Zn, giving the studied chlorites to be classified as chamosite and baileychlore (Figure 9a). The markedly higher Zn contents were recorded in chlorite from Zone A (1.14–2.06 apfu) than in those from Zone D (0.27–0.59 apfu). The Ca contents are elevated especially in a tiny chlorite sheet from Zone D (sample VA-8), where they are probably influenced by surrounding calcite. The lack of correlation between the contents of Ca and Si (Figure 9b) suggests that Ca is bound in chlorite [30] and not in another phyllosilicate (e.g., Ca-rich smectite). The average empirical formula of chlorite (mean of 39 analyses) formulated on an anhydrous basis of 14 atoms of oxygen is (Fe_1.79Zn_1.55Mg_1.40Al_1.09Mn_0.05Ca_0.05)_Σ5.93(Si_3.05Al_0.95)_Σ4.00O₁₀(OH)₈.

Hematite occurs in Zones A to D as well as in altered wall rock (samples VA1 to VA-8). In general, its amount decreases from the early portion of the vein to the latest one. Hematite likely occurs in multiple generations, which can be distinguished based according to their paragenetic position but not according to their chemistry. In the early Zone A hematite postdates siderite and quartz I, whereas rhodochrosite seems to be coeval with hematite, and assemblage bornite + chalcopyrite is clearly younger than hematite (Figure 5a–d and Figure 8e,f). In Zone B, the assemblage dolomite Do-II + hematite postdates dolomitic carbonate Do-I (Figure 5e,f). The rest of the paragenesis contains only a small amount of hematite, including a narrow early pink-to-red zone in Zone D (Figure 4). The hemispherical aggregates reaching up to 0.2 mm with distinct zoning and radial fabric (Figure 8e,f and Figure 10a) as well as rosettes composed of euhedral lenticular crystals (Figure 8b,d) illustrate the growth of hematite into free space in Zone A. In BSE images, growth zoning is often observed in hematite crystals and aggregates (Figure 8f and Figure 10a,b). In part, it is caused by chemical impurities, but in part, this feature is attributed to increased porosity as is suggested by poorly polished surface (Figure 8e) and lowered analytical totals of WDS analyses (down to 84 wt. %). Although such totals resemble goethite, the identity of hematite was confirmed by Raman spectra even in the BSE-darkest parts. In contrast, the well-developed lenticular and non-porous crystals from section VA-6 (Figure 8b,d) showed analytical totals close to 100 wt. %. The dolomite-carbonate-hosted hematite from Zones A to C forms spherical or elongated, often branchy aggregates with patchy zonation, which are enclosed in Do-II dolomitic carbonate (Figure 5e,f and Figure 8a). This assemblage consumes early dolomite Do-I in Zone B (Figure 5e,f). Similar but hematite-richer textures occur also in the uppermost part of Zone A, but no relics of potential precursor were found here (Figure 5e), although rarely observed rhombohedral arrangement of hematite aggregates suggests a carbonate. Sporadic tiny hematite grains characterize pink to red calcite at the base of Zone D (Figure 4). The chemical composition of hematite (Table S6) is surprisingly complex, but no systematic differences were observed in the composition of hematite from various positions in the vein. Almost all WDS analyses contain variably elevated contents of Sb (up to 0.094 apfu; the base of recalculation is 3 atoms of oxygen). In addition, most analyses displayed minor Si (up to 0.073 apfu), Al (up to 0.047 apfu), Zn (up to 0.018 apfu), Ca (up to 0.015 apfu), and Mn (up to 0.012 apfu) and approximately half of them also Pb (up to 0.022 apfu). Neglecting two analyses with high contents of Cu correlating with high S (probably representing contamination of the analyzed volume by Cu(-Fe) sulfides), slightly enhanced amounts of S (up to 0.006 apfu) and Cu (up to 0.016 apfu) were also recorded in some analyses. Contents of Al and Si are strongly positively correlating (R² = 0.83) with a 1:2 atomic ratio (Figure 11a), which implies the smectite (montmorillonite) or pyrophyllite stoichiometry. Other cations do not correlate with either Si or Al. The contents of Sb do not correlate with Si (Figure 11b), the sum of divalent cations (Figure 11c), or other cations except for Pb, which tightly (R² = 0.74) correlates positively (Figure 11d). The average empirical formula of hematite (mean of 98 analyses) on the basis of 3 atoms of oxygen is (Fe_1.93Si_0.02Sb_0.02Al_0.01)_Σ1.98O₃.

Cassiterite occurred in the youngest part of Zone A in sample VA-4. A cluster of strongly corroded cassiterite grains reaching up to ca. 15 µm in size was situated in between dolomite (Do-II) gangue and a hematite rim around a corroded sphalerite grain (Figure 10c). The chemical composition (Table S7) corresponds well to cassiterite (analytical totals between 99.5 and 100.4 wt. %). The major substituent is Fe (0.014–0.027 apfu) and to a lesser extent Al (0.002–0.013 apfu). In addition, traces of Si (up to 0.010 apfu), W (up to 0.003 apfu), Mg, and/or S (both up to 0.002 apfu) were recorded in part of the analyses. The average empirical formula of cassiterite (mean of six analyses) on the basis of 2 atoms of oxygen is (Sn_0.97Fe_0.02Al_0.01)_Σ1.00O₂.

Chalcopyrite is a very common mineral in the studied ore mineralization. It is most frequent in the late calcite portion of the vein, where this mineral occurs in several morphological varieties. In sample Dy-817, rare chalcopyrite forms xenomorphic isometric grains up to 80 µm in size hosted by massive bornite (Figure 10d,e). These chalcopyrite grains are often concentrated in clusters reaching up to 0.5 mm in size. Around these clusters, chalcopyrite grains are also present in “veinlets” of non-porous bornite. Chalcopyrite largely shows fine porosity and its nature is identical to that of the host bornite (see below), except for grains hosted by non-porous bornite veinlets, which are also non-porous.

In contrast, only homogeneous non-porous chalcopyrite occurs in sample Dy-973, consisting of a few acicular to finger-like ore aggregates from the drusy cavity (Zone E). Three textural varieties of chalcopyrite appeared in this sample. The first type (denoted Cpy-I) forms up to several mm long finger-like aggregates. Some of them are built up by massive homogeneous chalcopyrite only, but most of them represent variably thick chalcopyrite crusts lining mineral aggregates composed of bornite, covellite, and Ag-Cu sulfides with some preserved residual porosity. These internal largely polymineral aggregates have acicular or isometric shapes (depending on the orientation of the section), and the thickness of their continuous and both sides symmetrically developed chalcopyrite rims range 20–300 µm at different objects (Figure 10f). Some of these smaller objects are either partly or completely enclosed in bornite and/or overgrown by tetrahedrite (Ttd-III) and/or Ag-Cu(-Hg) sulfides (Figure 12a,b). The second type of chalcopyrite (denoted Cpy-II) forms either non-continuous thin coatings on bornite or thin elongated (dimensions up to 10 µm × 50 µm) ribbon-like individuals enclosed mostly in outer parts of bornite aggregates (Figure 12a,b). These ribbons exhibit various orientations with respect to the outer surface of the bornite aggregate and in the grain scale, they seem to follow up to two crystallographic directions in the host mineral grain. The Cpy-II is, again, earlier than Ag-Cu-(Hg) sulfides and tetrahedrite Ttd-III. The third morphological type of chalcopyrite is represented by symplectitic intergrowths of chalcopyrite and Fe-enriched mckinstryite (Figure 12b,c). Symplectite forms a single isometric ca. 70 µm aggregate enclosed in bornite.

In almost all other samples (except for VA-7, where chalcopyrite is missing), common chalcopyrite usually overgrows and often also replaces sphalerite and bornite (Figure 12d,e). Chalcopyrite encloses grains of sphalerite, Bi-rich tetrahedrite-(Zn), wittichenite, and unnamed AgFe₂Cu₆S₈ (Figure 8e). In contrast to material from the drusy cavity, multiple generations of chalcopyrite cannot be distinguished on the basis of textural features in most polished sections from the earlier portion of the vein. Only in Zone D can there be distinguishable early porous Cpy-I and late homogeneous Cpy-II (Figure 12e). In one case (sample VA-8), a thin (up to 20 μm) growth zone of Ag-enriched chalcopyrite (up to 0.008 apfu Ag) hosted by chalcopyrite with Ag contents only up to 0.003 apfu was observed (Figure 12f).

The WDS analyses (Table S8) showed a relatively unified composition of chalcopyrite within a single polished section even in cases where various textural types (i.e., Cpy-I and Cpy-II) occur. Minor differences can be detected among individual polished sections. Except for Zn (up to 0.031 apfu), Sb and Ag contents (up to 0.015 apfu), and rare Pb (up to 0.002 apfu), no other minor elements were found. Silver and Sb contents partly correlate positively (Figure 13a). The average empirical formula of chalcopyrite on the basis of 4 apfu (83 analyses) is Cu_1.01Fe_0.98S_2.01.

Bornite is quite a common ore phase in the studied material. Similar to chalcopyrite, bornite also displays some textural differences among individual polished sections. Sample Dy-817 contains bornite as a major ore mineral, the majority of which displays fine porosity (Figure 10d,e). Pores usually reach around 5–10 µm in size and do not contain any late mineral fill. Since individual pores seem to have a tubular shape, it is evident that the massive bornite aggregate is composed of domains with apparently different spatial orientations because pores have a unified orientation within a single domain (Figure 10d,e). These domains (“grains”) have isometric shapes and sizes ranging from 50 to 300 µm. The “grain” boundaries are often highlighted by interpositions of homogeneous non-porous bornite, which are, at least in some cases, rather younger veinlets containing isolated grains of chalcopyrite. In reflected light, porous bornite often seems to contain small isometric domains characterized by an apparent bluish tint in comparison with neighboring “normal” bornite (Figure 10d). The reason for such anomalous optical behavior remains unresolved because WDS analyses showed the same compositions for both anomalous and normal domains in this sample.

Bornite is a common mineral in sample Dy-973 (Zone E), where it partly infills both the internal spaces within Cpy-I crusts and frequently also overgrows them (Figure 10f and Figure 12a,b). Fine porosity characterizing sample Dy-817 is completely missing here, but irregularly shaped larger elongated cavities reaching up to 100 µm in length are common in some samples, being oriented in parallel with the elongation of the whole aggregate. These cavities are mostly empty, but some of them are lined or fulfilled by aggregates of covellite and Ag-Cu-S minerals (Figure 10f and Figure 12a,b). From the outer margins, bornite aggregates are overgrown and partly replaced by chalcopyrite II, Bi-bearing tetrahedrite, and Ag-Cu(-Hg) sulfides (Figure 12a,b). In other samples (VA-1, VA-2, VA-5, VA-6, VA-9) bornite forms rims of sphalerite grains, which are partly replaced by younger chalcopyrite or relics in chalcopyrite aggregates (Figure 12d,e). In sample Dy-816, bornite commonly replaces subhedral grains of djurleite and is replaced by chalcopyrite rims and ribbons from the outer margin (Figure 14a). Bornite is homogeneous in the BSE images except for one case (sample VA-9), in which the bornite rim contains a brighter zone due to increased Ag content (Figure 14b).

The WDS analyses (Table S9) revealed rather unified compositions of bornite in all samples except for Ag-rich domains in sample VA-9. Bornite from the Václav vein contains Ag up to 0.12 apfu and, in the case of Ag-rich domains, 0.27–0.32 apfu; the Ag and Cu contents correlate negatively (Figure 13b). Similar increased Ag contents (up to 0.48 apfu) are described, e.g., in bornite from the Um Samiuki deposit, Egypt [38]. The average empirical formula of Ag-poor bornite on the basis of 10 apfu (77 analyses) is (Cu_4.98Ag_0.03)_Σ5.01Fe_0.99S_4.00 and those of Ag-rich domains (3 analyses) is (Cu_4.62Ag_0.29)_Σ4.91(Fe_1.02Zn_0.03)_Σ1.05S_4.03.

Sphalerite belongs to the abundant ore minerals in the studied material; it was identified in Zones A to D in samples VA-1 to VA-7 and VA-9. It forms anhedral corroded grains up to several mm in size, usually rimmed and commonly partly replaced by bornite and younger chalcopyrite (Figure 8e, Figure 10c, Figure 12d,e and Figure 14b–d). An unusual Sn,Cu-containing sphalerite occurs as domains up to 50 μm in size (Figure 14c) in Sn,Cu-free sphalerite (sample VA-4) in association with cassiterite, galena, chalcopyrite, and Bi-bearing tetrahedrite-(Zn). The increased Cd contents were found in sphalerite domains up to 50–100 μm in size in association with wittichenite, Bi-rich tetrahedrite-(Zn), covellite, chalcopyrite and hematite (sample VA-4; Figure 14d), or thin growth zones in one sphalerite grain from sample VA-7 (inset in Figure 14d). Finally, Mn-bearing sphalerite occurred as one anhedral grain up to 30 μm in size in a drusy cavity in kutnohorite-to-dolomite Do-II gangue in association with hematite (Figure 14e).

In the chemical composition of sphalerite, no correlations appear for pairs Fe-Sn, Cu-Sb, Cd-Sn, Sn-Sb, and Fe-Cd, whereas a strong 1:2 positive correlation is observed for Sn-Cu for most data (Figure 13c). Such significant Sn and Cu contents in sphalerites from the Příbram ore district have so far been observed only in samples from the H32A vein of the Háje uranium deposit [7] but with only the half extent of this substitution (up to 0.01 apfu Sn). Besides minor Sn (up to 0.023 apfu), Cu (up to 0.054 apfu), and Fe (up to 0.022 apfu) Sn, Cu-rich sphalerite from the Václav vein also contains traces of Cd (up to 0.006 apfu) and In and Sb (both up to 0.003 apfu). Cadmium-rich (0.02–0.04 apfu) sphalerite shows only low contents of Fe (up to 0.01 apfu; Figure 13d) and Cu (up to 0.04 apfu). In Mn-containing sphalerite, the contents of Mn, Fe, Cu, and Cd in amounts not exceeding 0.01 apfu were determined. The prevailing “usual” sphalerite contains minor Fe (up to 0.02 apfu), Cu, and Cd (both up to 0.01 apfu). The chemical analyses of all sphalerite types (101 point analyses) and corresponding empirical formulae are given in Table S10.

Djurleite was recorded only in sample Dy-816, where this Cu-sulfide substitutes early chalcopyrite Cpy-I. It forms aggregates up to 0.5 mm in size enclosed in bornite (Figure 14a). Djurleite is bluish-grey in reflected light and tarnished very fast to blue after polishing. Bornite apparently replaces djurleite from the margins. The chemical composition of the studied djurleite with Me/S = 1.92 (1.88–1.96) and 65.7 (65.2–66.2) at.% Me corresponds to ranges given for djurleite–Me/S = 1.94 (range 1.87–1.97) and 66.0 (65.2–66.3) at.% Me [39]. Besides Cu and S, the studied mineral (Table S11) contains minor Ag (up to 0.16 apfu), Fe (up to 0.24 apfu), and traces of Pb (up to 0.02 apfu) and In (up to 0.01 apfu). Its average empirical formula on the base of 47 apfu (mean of 58 analyses) is (Cu_30.66Ag_0.13Fe_0.10)_Σ30.89S_16.11.

Three generations (differing by paragenetic position, morphology, chemical zonality, and composition) of tetraehedrite group minerals were found in the studied samples. However, all are characterized by very low Ag contents in the triangular position (0.04–0.21 apfu), which correspond to the most Ag-poor TGM known in the Příbram ore area [7].

The earliest type of TGM (Ttd-I) is represented by anhedral grains up to 3 mm in size in association with chalcopyrite, bornite, sphalerite, mckinstryite, and Bi-containing TGM in Zones A and D (samples VA-3 and VA-9; Figure 12e). The Ttd-I is cut in places by veinlets composed of bornite and late chalcopyrite (Figure 14f). In the BSE images, its grains are irregularly blurred zones (Figure 15a) due to Sb-As and Zn-Fe-Cd substitutions. The tetrahedral position is occupied by Zn (up to 2.05 apfu), Fe (up to 1.29 apfu), Cd (up to 0.74 apfu), Cu²⁺ (up to 0.52 apfu), and local traces of Hg (up to 0.07 apfu) and Pb (up to 0.02 apfu). According to the recently published nomenclature scheme of minerals of the tetrahedrite group [40], tetrahedrite-(Zn) prevails over tetrahedrite-(Fe) and one analytical point corresponds to tetrahedrite-(Cd) (Figure 16a). In the trigonal pyramidal position, the dominant Sb (3.19–4.06 apfu) is accompanied by As up to 0.84 apfu; no correlation between As and Zn/Fe/Cd contents was observed.

A younger Bi-containing TGM (Ttd-II) occurs in Zones A, B, and D (samples VA-2, VA-3, VA-4, VA-5, VA-6, and VA-9) as strongly zoned (BiSb_-1 substitution) grains and rims up to 50 μm in size overgrowing and cutting Ttd-I, bornite, sphalerite, wittichenite, and chalcopyrite (Figure 15b–d), which are always in association with hematite crystals. The Ttd-II chemical compositions correspond to tetrahedrite-(Zn), with the dominant Zn (1.20–2.18 apfu) in the tetrahedral position accompanied by minor Fe (up to 0.54 apfu) and Cu²⁺ (up to 0.23 apfu) and local traces of Cd (up to 0.08 apfu), Pb, and In (up to 0.02 apfu). The trigonal pyramidal position is occupied by Bi (0.02–2.94 apfu) and Sb (1.15–4.06 apfu) with traces of As (up to 0.15 apfu). Such significant Bi contents in TGM are unusual and have so far only been observed in samples from localities Vindfall, Tary-Ekan, Rędziny, Hřebečná, and Jáchymov [41,42,43,44]. The dominant substitution in sample from the Václav vein is BiSb_-1 (Figure 16b) in contrast to the samples from Jáchymov and Hřebečná, where BiAs_-1 is highly predominant (Figure 16b). Bismuth dominant members, recently defined new mineral annivite-(Zn) [41] and the not-yet approved “annivite-(Cu)”, were determined as anhedral domains with size about 20 μm in association with other TGM (Figure 15c,d).

The youngest type of TGM (Ttd-III) is observed in sample Dy-973 (Zone E) as rare idiomorphic crystals up to 120 µm in size (Figure 12a), overgrowing bornite and chalcopyrite II. The Ttd-III crystals are rarely overgrown by the Ag-Cu-Hg sulfides. No zoning is observed in BSE images. Chemically, tetrahedrite-(Fe) prevails over tetrahedrite-(Zn). Within a single crystal, evolution from a Zn-dominated tetrahedrite toward an Fe-dominated one was recorded. In the tetrahedral position, Fe content is greater (max. 1.14 apfu) than that of Zn (max. 1.86 apfu) and the concentration of both elements strongly varies. Also, Cu²⁺ (up to 0.87 apfu), Hg (up to 0.05 apfu), and Pb (up to 0.02 apfu) occurred in this position. In the trigonal pyramidal position, Sb content varies between 4.06 and 4.25 apfu, and traces of arsenic were found only locally (up to 0.05 apfu). The chemical analyses of all TGM (165 point analyses) and corresponding empirical formulae are given in Table S12.

Mckinstryite is the most common Ag-Cu sulfide in the studied assemblage. The mineral is usually tightly associated with other Ag-Cu sulfides (stromeyerite and jalpaite). Mckinstryite is usually a major constituent of a few µm thick outermost rims on chalcopyrite Cpy-II and bornite as well as partial or complete fillings of residual porosity and rare cracks inside aggregates of Fe-Cu sulfides in sample Dy-973 in Zone E (Figure 12a–c and Figure 15e). In addition, Fe-rich mckinstryite forms symplectitic intergrowths with chalcopyrite, which was exceptionally recorded in the same sample (Figure 12b,c). Jalpaite and covellite are enclosed in mckinstryite (Figure 15e), whereas stromeyerite could be intimately intergrown with mckinstryite (Figure 12c). A variability in the Ag/Cu ratio is typical for mckinstryite; the Cu and Ag contents vary 0.60–0.87 apfu and 1.14–1.28 apfu, respectively (Figure 13e). Minor contents of Fe were observed (up to 0.08 apfu; in the case of the Fe-rich sample from symplectite 0.21 apfu) as well as traces of Cd, Te, and Cl (Table S13). The mean composition of the studied mckinstryite (calculated from 23 analyses on the base of 3 apfu) is Ag_1.21(Cu_0.79Fe_0.03)_Σ0.82S_0.97 and is close to the empirical formula of mckinstryite from Milín–Ag_1.19Cu_0.81S_0.99 [31] and type material from the Foster mine, Cobalt, Canada–Ag_1.18Cu_0.82S [45].

Stromeyerite was found in sample Dy-818 as a thin rim with a thickness of up to 20 µm around native silver and inclusions up to 60 µm × 30 µm in size in silver (Figure 15f). Stromeyerite also represents a subordinate constituent of Ag-Cu sulfide rims around finger-like or acicular aggregates of Cu-Fe sulfides in Zone E (sample Dy-973; Figure 12b,c). The chemical composition of stromeyerite from the Václav vein is close to the ideal composition AgCuS (Table S14). Only minor contents of Cd (max. 0.002 apfu), Te (max. 0.003 apfu), Fe (max. 0.004 apfu), and Cl (0.005 apfu) were found. The mean composition of the studied stromeyerite (calculated from 11 analyses on the base of 3 apfu) is Ag_0.99Cu_1.03S_0.98.

Jalpaite was recorded in aggregates of Ag-Cu sulfides in polymineral needle-like assemblage hosted by chalcopyrite in sample Dy-973 (Zone E) and composed of bornite, covellite, and Ag-Cu sulfides. Jalpaite forms xenomorphic isometric grains up to 8 µm in size, enclosed in mckinstryite (Figure 15e). In sample VA-3 (Zone A), jalpaite forms rare anhedral grains up to 30 μm in length in sphalerite in association with wittichenite, Bi-containing tetrahedrite-(Fe), and hematite. Chemical analyses of jalpaite (Table S15) agree with the ideal formula Ag₃CuS₂; only traces of Fe, Cd, Te, and Cl (up to 0.004–0.108 apfu) were determined. The increased Zn content (0.031 apfu) in a single point analysis can probably be influenced by the surrounding sphalerite. The average empirical formula of jalpaite on the basis of 6 apfu (4 analyses) is Ag_2.91(Cu_1.09Fe_0.07Zn_0.01Cd_0.01)_Σ1.18S_1.90.

Native silver was found as a 0.8 mm porous aggregate, rimmed by stromeyerite in Zone E (sample Dy-818; Figure 15f). It is homogenous in the BSE image. Anhedral porous grains of silver up to 20 μm in quartz gangue were also observed in sample VA-1 (Zone A). These types of silver (Table S16) contain only minor contents of Hg (up to 0.011 apfu), Cd, Fe (up to 0.001 apfu), As, Te (up to 0.002 apfu), and Cl (up to 0.003 apfu). In addition, a tiny (ca. 8 µm) inclusion of Hg-rich silver (with up to 0.12 apfu Hg) associated with Hg-poor silver (Table S16) was exceptionally encountered in the residual porosity of bornite in sample Dy-973.

Acanthite was only found as very rare anhedral grains up to 20 μm in size as part of veinlets in association with unidentified Ag-Cu-Hg-S phases in quartz gangue (sample VA-1, Zone A). In addition, an overgrowth on Cd-enriched sphalerite occurred in sample VA-7 (inset in Figure 14d). Besides dominant Ag and S, the studied acanthite (Table S17) contains minor contents of Hg (up to 0.022 apfu) and traces of Te and Cl. Its chemical composition on the basis of three apfu (two analyses) corresponds to the empirical formula (Ag_1.91Hg_0.02)_Σ1.93S_1.06Cl_0.01.

Wittichenite occurs as anhedral grains up to 50 μm in size (Figure 14d and Figure 15d) in association with sphalerite and Bi-containing Ttd-II, bornite, chalcopyrite, covellite, and hematite in Zone A (samples VA-3, VA-4, and VA-6). Its chemical composition is close to ideal stoichiometry Cu₃BiS₃ (Table S18), with minor contents of Ag in the range 0.04–0.37 apfu; similar Ag contents in wittichenite were published, e.g., for samples from Janjevo (0.4 apfu Ag [46]) or Rędziny (0.2 apfu Ag [47]). The locally observed Zn contents in wittichenite from the Václav vein (up to 0.31 apfu) can probably be influenced by the surrounding sphalerite. The average empirical formula of wittichenite on the basis of 7 apfu (14 analyses) is (Cu_2.75Ag_0.16Zn_0.11Fe_0.05)_Σ3.07Bi_0.92S_3.00.

Covellite forms strongly anisotropic grey to blue subhedral to anhedral isometric to tabular grains up to several µm in size (Figure 10f and Figure 12a,d). It occurs together with Ag-Cu sulfides either in residual porosity of bornite-chalcopyrite aggregates or overgrow needle-like bornite-chalcopyrite aggregates from drusy cavity together with Ag-Cu or unspecified Ag-Cu-Hg sulfides. Covellite enclosed by aggregates of Ag-Cu sulfides is often along grain boundaries slightly replaced by mckinstryite (sample Dy-973). Beside dominant Cu and S, the studied covellite (Table S19) contains minor contents of Ag (0.04–0.06 apfu) and Fe (up to 0.02 apfu). A similar minor Ag content is reported for covellite from, e.g., Um Samiuki deposit [38], Chah Nali, north Bazman [48], or Tismice near Český Brod [49]. The average composition of the studied covellite (calculated from five analyses on the basis of two apfu) is (Cu_0.99Ag_0.05Fe_0.01)_Σ1.05S_0.95.

Very rare galena forms a teeth-like overgrowth on chalcopyrite-II with dimensions 10 µm × 5 µm, which was not possible to analyze without contamination by the associated Ag-Cu-Hg-S phase (sample VA-6). It was also observed as anhedral grains up to 30 μm in size in Sn,Cu-sphalerite in association with cassiterite and chalcopyrite (sample VA-4; Figure 14c) and minute inclusions enclosed in early siderite and quartz-I (Zone A). In the chemical composition of galena (Table S20), besides dominant Pb and S, a minor content of Fe (up to 0.017 apfu) and traces of Bi, Ag, Cd, In, and Cl (0.001–0.006 apfu) were determined. Its chemical composition on the basis of two apfu (two analyses) corresponds to the empirical formula (Pb_0.96Fe_0.01Bi_0.01)_Σ0.98S_1.01.

A balkanite-like phase was recorded in a trace amount in assemblage with the youngest Ag-Cu(-Hg) sulfides in Zone E. In sample Dy-973, it forms a fine (up to 10 µm thick) rim on intergrowth of stromeyerite and mckinstryite, developed in a cavity within Fe-Cu sulfides (Figure 12b,c). In other samples, compositions corresponding to balkanite were collected from some grains of young Ag-Cu-Hg phases, overgrowing aggregates of djurleite, bornite, and chalcopyrite (Figure 17a–c). The chemical composition of the balkanite-like mineral from the Václav vein (Table S21) is characterized by minor Hg and Cu deficits and Ag excesses compared to the ideal formula, but its composition is consistent with published analyses of balkanite (Figure 13f and Figure 18a) with a wider range of CuAg_-1 substitution. The average composition of the studied balkanite-like mineral (calculated from 11 analyses on the base of 23 apfu) is Cu_9.74Ag_4.70Hg_0.67Fe_0.20S_7.66Cl_0.33.

Imprecisely identified phases include young Ag-Cu-Hg-S phases, overgrowing aggregates of bornite and chalcopyrite, and exceptionally, tetrahedrite crystals in Zones A, D, and E in samples Dy-973, VA-1, and VA-9 as well (Figure 17a–c). They likely include (meta)cinnabar, forming grains up to 12 μm in size on chalcopyrite II, which are rimmed and intergrown by balkanite or unidentified Ag-Cu-Hg-S phase (Figure 17a). Many grains from the outer surface of bornite-chalcopyrite aggregates belong to unidentified Ag-Cu-Hg-S phase(s) with highly variable compositions (Table S22). In BSE images, both homogeneous domains as well as distinctly zoned grains were recorded. The Hg-poorest compositions correspond to balkanite, but the more Hg-enriched analyses show wide compositional scattering (Figure 18b–d); we cannot exclude the possibility that they can represent submicroscopic intergrowths of several mineral phases. These phases overgrow covellite and (meta)cinnabar.

Unnamed phase AgCu₆Fe₂S₈ was found in Zone A (sample VA-1) as anhedral corroded grains reaching up to 17 μm × 27 μm in size, which are replaced by chalcopyrite in association with early sphalerite and younger tetrahedrite-(Zn) (Figure 8e). In reflected light, it is reddish, resembling bornite in color. Its chemical composition (Table S23) can be expressed on the base of 17 apfu by empirical formula Ag_0.96Cu_6.01(Fe_2.00Zn_0.07Hg_0.04)_Σ2.11S_7.92 (mean of 4 analyses). Minor Zn contents (0.05–0.09 apfu) may be caused by surrounding sphalerite. The determined chemical composition does not correspond to any known mineral or unnamed mineral phase and its cation/anion ratio (1.15) is different from that of bornite (1.5).

Unnamed phase (Cu,Ag)₄FeS₄ forms irregular veinlets and aggregates up to 30 μm in size in covellite (Figure 17d) in association with chalcopyrite, sphalerite, tetrahedrite-(Zn), and hematite (Zone A, sample VA-3). The chemical analyses (Table S24) indicate an AgCu_-1 substitution with Ag and Cu contents in the range 1.00–1.32 apfu and 2.63–3.20 apfu, respectively. Its average empirical formula on the base of nine apfu (mean of two analyses) (Cu_2.92Ag_1.16)_Σ4.08(Fe_0.96Zn_0.05)_Σ0.98S_3.93 does not correspond to any known mineral. Its stoichiometry is close to the so-called “anomalous bornite” from Bílá Voda near Javorník (Czech Republic) with the ideal formula Cu₄FeS₄ [50] but Ag contents in this phase do not exceed 0.01 apfu.

5. Discussion

5.1. Heavy Metal Enrichment of Hematite

Although clay minerals are well-known for their sorption capacity for heavy metals [51,52], the lack of correlation between Si and heavy metals in analyses of hematite from the Václav vein (Figure 11b,c) precludes the possibility that these metals could be bound to the admixture of these phyllosilicates hosted in certain growth zones of hematite. It is well known that iron oxides and hydroxides also have a high sorption capacity for Sb, including hematite, goethite, and amorphous iron (III) hydroxide [53,54,55,56,57,58,59,60]. Antimony can be sorbed in both the (III) and (V) oxidation states, especially in slightly acidic to near-neutral environments [53,59,61]. There is no doubt about the primary nature of euhedral homogeneous lenticular hematite crystals (Figure 8b,d), which implies enrichment in Sb associated with the direct growth of hematite. Moreover, on the basis of textural evidence, we cannot exclude the idea that a part of the hematite originated from an amorphous precursor: a spherical morphology with a distinct concentric growth zonation, radial arrangement, and increased residual porosity (Figure 8e,f and Figure 10a) resembles a recrystallized gel. In the latter case, the Sb enrichment might also be a feature of primary hydrated gel; or, Sb was introduced during the recrystallization process. Thus, multiple events associated with the sorption of Sb on iron oxides/hydroxides are probable in the studied vein.

The strong correlation between Sb and Pb (Figure 11d) suggests that a Pb-Sb sulfide might be the source of both metals. Although the occurrences of Pb-Sb sulfosalts are very local in the Příbram ore area, they are concentrated in the Březové Hory district [9,62]. Nevertheless, the 1:9 atomic ratio of both elements recorded in hematite from the Václav vein (Figure 11d) does not correspond to the composition of any Pb-Sb sulfosalt known from the Příbram ore area [7,9,62]. Therefore, either decoupling of both elements during the alteration process and/or independent sources of these elements can be suggested. In this context, it is also noteworthy that the transformation of hydrated iron oxides into hematite is associated with substantial (ca. 90%) removal of originally adsorbed Pb [63,64], pointing to the potential lowering of the original Pb/Sb ratio at least in part of the studied hematite, showing the morphology and internal texture indicative for its collomorphic precursor.

Hematite can also act as a sink for Zn. [65] reported Zn contents exceeding 0.10 apfu.

5.2. Textural Evolution of the Vein

The available textural evidence indicates that the studied vein mineralization initially precipitated in an open fracture, manifesting a common banded texture. However, this primary texture experienced relatively complicated superimposed evolution involving pronounced dissolution, re-filling of free spaces produced by dissolution, and replacement of earlier mineral phases by younger ones. Although the younger processes operated across the whole vein, their intensity was very different locally, often concentrated to certain horizons in the vein fill. Both gangue and sulfide minerals were subject to superimposed alterations.

5.2.1. Gangue

The most pronounced manifestations of multiple alteration processes can be observed in the Zone A. Here, siderite, preserved in rare relics in the early portion of the Zone A, is the oldest mineral phase, which was almost completely replaced first by quartz and then by rhodochrosite, carbonates of the dolomite series, and hematite (Figure 5a–d).

An interesting situation is developed in the uppermost part of Zone A, where the vein matrix is formed by Do-II dolomite with abundant aggregates of hematite (Figure 5e). Because identical assemblage (however, with a much lower content of hematite) was observed in an additional three horizons within the Zones B and C (Figure 5e,f and Figure 8a), we suggest that early three Do-II+He assemblages also represent replacements of earlier carbonate gangue by the given assemblage, which took place in chemically suitable (i.e., unstable at the given P-T-X conditions) horizons of the vein fill. The participation of the replacement process is undoubtedly proven in Zone B, where instructive replacement of coarsely crystalline Fe-enriched dolomite carbonate (Do-I) by Do-II+He assemblage was documented on BSE images (Figure 5e,f). In the late portion of Zone A, no relics of precursor minerals were found; however, a high amount of hematite suggests that it could be either a Fe-rich member of the dolomite series or siderite.

The black nests occurring in the youngest part of Zone A are also texturally interesting and mostly contain a late portion of carbonates of the dolomite series (Do-II, Do-III), calcite, chlorite, and euhedral crystals of hematite (Figure 8b,d). All these minerals are exceptionally well developed, suggesting that they grew into an open space but are associated with evidently strongly corroded grains of sphalerite rimmed by late Cu sulfides (chalcopyrite, bornite, TGM). We interpret these objects as dissolution cavities, formed at the expense of an unstable mineral (potentially nests of sphalerite or other sulfides?), which were subsequently cemented by later mineral phases.

5.2.2. Sulfides

The alteration and replacement processes also influenced sulfide minerals. This is clearly indicated in cases, where relics of an early mineral phase are still preserved in a newly formed mineral. This applies for (i) sphalerite, which is across the whole vein partly replaced by chalcopyrite or bornite rims (Figure 8e, Figure 12d,e and Figure 14b,d), (ii) covellite, which was partly replaced by Ag-Cu(-Hg) sulfides, (iii) djurleite, which is from its margins consumed by bornite (Figure 14a and Figure 17c), and (iv) bornite, which is from the margins partly replaced by overgrowths and ribbons of chalcopyrite II (Figure 12a,b, Figure 14a and Figure 17c). In addition, it is probable that the specific domain texture of sample Dy-817, formed by bornite and chalcopyrite with a variable orientation of the tubular micropores (Figure 10d,e), represents the feature of a pre-existing mineral, which was completely replaced by the mentioned Fe-Cu sulfides. As no relics of precursor minerals were found, its exact identification is not possible in this case. However, it was a mineral that forms aggregates composed of isometric xenomorphic grains, which may be compatible with quartz, carbonates, pyrite, galena, or sphalerite, mentioning at least the most common minerals of the Příbram base metal ores and/or in the Václav vein itself.

A nice example of alteration processes involving sulfide minerals can be documented in sample Dy-973, covering the mineral assemblage of the youngest acicular to finger-like ore aggregates from the drusy cavity (Zone E; Figure 1, Figure 10f and Figure 12a,b). We suggest that these specifically shaped ore aggregates involve pseudomorphs after an unknown acicular mineral, which served as a base for the crystallization of later minerals. The precursor acicular mineral remained not preserved and the only reason for its occurrence lies in the regular morphology of ancient cavities generated by its dissolution. The interpreted evolution of texture and mineral assemblage in sample Dy-973 is illustrated in Figure 19. The first step was characterized by the crystallization of an acicular mineral within the open space of the drusy cavity (Figure 19a). Then, its crystals were coated by early chalcopyrite (Cpy-I), which was deposited in the form of a continuous layer (Figure 19b), at least in cases observed today, because those formed by non-continuous coatings must be necessarily destroyed during the subsequent step, which included the complete dissolution of the acicular mineral (Figure 19c). The next step included the crystallization of bornite, which was deposited in both the internal space of chalcopyrite crusts as well as in the free space among these crusts (Figure 19d). One can presuppose a rapid precipitation of this mineral, which may be suggested by abundant residual open cavities within this mineral phase. Then, a second generation of chalcopyrite (Cpy-II) precipitated, forming largely non-continuous coatings on earlier minerals as well as frequent ribbons within the outermost parts of bornite (Figure 19e), which were best available for chalcopyrite-precipitating fluids. Afterward, minor tetrahedrite (Ttd-III) crystallized in rare well-developed crystals (Figure 19e). During the next step, residual porosity in bornite as well as newly formed cracks were partly or completely filled up by covellite and Ag-Cu-S sulfides, which were followed, especially at the outer surface of ore aggregates, by small amounts of galena, (meta)cinnabar, and compositionally heterogeneous Ag-Cu-Hg sulfides including balkanite (Figure 19f).

An unresolved issue represents the interpretation of the origin of the symplectitic intergrowth of mckinstryite and chalcopyrite (Figure 12b,c). Symplectite textures usually represent a product of the breakdown of an unstable mineral, which occurred under changed P-T-X conditions [66,67,68]. We did not record the description of symplectite with the same composition in the available literature. It cannot therefore be excluded that the mineral composition of the symplectite was modified during the late evolution of ore assemblage, with total replacement of one mineral phase by the other.

5.3. Paragenetic Sequence of the Vein

The complicated evolution of the studied vein poses a serious problem with the construction of a paragenetic scheme. In addition, further complications arise with respect to (i) partly non-equivalent textural features observed in various polished sections, (ii) missing contacts among many minerals due to the rare occurrence of some mineral phases, and (iii) simple chemistry of most minerals often occurring in various portion of the vein. Therefore, the resulting paragenetic scheme must be necessarily schematic in order to reduce possible mistakes. Another problem is the recognition of individual generations of a single mineral in a situation where a pronounced alteration of an early vein by late minerogenetic events occurred; also, the chemical composition of this mineral is very simple across the whole vein. This is valid especially for chalcopyrite, bornite, and hematite in the studied samples.

The interpreted simplified paragenetic scheme of the studied mineralization is illustrated in Figure 20. Multiple mineralizing events, separated by fracturing, characterize the history of the Václav vein. The overall evolution of the studied vein is characterized by early siderite with nests of sphalerite, which were both strongly corroded first by quartz-I+minor galena and later by the rhodochrosite+hematite assemblage. The next mineralizing stages are characterized by precipitation of quartz-II and then by the dolomite gangue: first, the Fe-rich (Do-I) step, then an episode producing Mn,Zn-enriched Fe-poor Do-II carbonate accompanied by common hematite, and finally, a Fe-Mn-bearing dolomite (Do-III). The Do-II+He assemblage also strongly replaced some parts of earlier dolomite and/or siderite gangue. The next stage deposited Zn-bearing chlorite and traces of hematite, both being coeval with the beginning of the calcite crystallization. The crystallization of Cu sulfides (chalcopyrite, bornite, djurleite, TGM) was likely coeval with the later portion of the calcite vein, displaying some repetitions in the precipitation of these sulfides. The latest portion of the studied assemblage involved Ag-Cu sulfides, covellite, native silver, and balkanite with an accompanying Ag-Cu-Hg phase(s).

5.4. Comparison with Other Příbram Veins

The possibilities of a detailed comparative study are strongly limited because there are only a few modern studies dealing with a detailed evaluation of the whole paragenesis and composition of individual mineral phases in individual veins in the Příbram ore area. Therefore, the below overview can be significantly updated in the future.

The mineralization of the Václav vein shows some similarities and differences when compared with typical base metal veins in the Příbram ore area. The carbonate succession (Figure 20) follows those usually reported from ore veins of the Příbram ore area (siderite → carbonates of the dolomite series → calcite [1,5,6,7,8]. The chemical composition of siderite and dolomite Do-1 from the Václav vein follows those from base–metal veins in the district well (Figure 6a and Figure 7a). In contrast, the major differences involve (i) a high amount of quartz replacing early siderite, (ii) the presence of rhodochrosite (it is sporadically known only from the Háje deposit, Příbram uranium district [7]; Figure 6a), (iii) a low amount of sphalerite and galena, (iv) a high amount of Cu(-Fe) sulfides and hematite in the vein, (v) a high amount of Mn and elevated Zn combined with a lack of Fe in part of dolomite-group carbonates, (vi) Zn-rich composition of chlorite, and (vii) a total lack of Ag and the presence of Cd, Hg, and Bi in the TGM.

Considering the ore mineralization (gangue minerals were not studied in detail yet), the best paragenetic similarities can be mentioned if the mineralization of the Václav vein is compared with mineralizations bound to a N-S trending zone in the southern part of the Příbram ore area. This zone hosts the deposits Vrančice and Milín and, in the northern continuation, the Lešetice deposit. The third paragenetic stage of the Pošepný vein in the Vrančice deposit contains abundant Cu sulfides (chalcocite, bornite, covellite, djurleite) associated with stromeyerite, jalpaite, mckinstryite, and native silver [69,70,71]; also, baileychlore was described from this vein [72]. Ore minerals are hosted by hematite-bearing quartz, dolomite, calcite, and minor baryte gangue. In the northern continuation of the Vrančice ore deposit, the Milín deposit is situated, with rare occurrence of mckinstryite, stromeyerite, tetrahedrite group minerals, bornite, chalcopyrite, and covellite [31] hosted by the predominant quartz and minor calcite gangue. In the Lešetice deposit, there is the most important vein L1 containing the Ag-Cu mineralization with the occurrence of stromeyerite, betekhtinite, native silver, and Cu-sulfides hosted by carbonates [71].

The ore mineralization very similar to those observed in the Václav vein and comprising sphalerite, djurleite, anilite, covellite, stromeyerite, balkanite, Hg-rich silver, and rare Cu-arsenides (koutekite and kutinaite) was recently described from a unique willemite-bearing sample, which was found in the dump material of the Jerusalem deposit in the Příbram uranium district [5]. The gangue, except for early siderite and late calcite, also contained several compositional types of dolomite-group carbonates including an Mn,Zn-enriched and Fe-depleted one (Figure 7a). Baileychlore (Figure 9a), Zn-enriched hematite (with up to 0.052 apfu Zn), hisingerite, minor quartz, and tiny inclusions of an unspecified oxidic Sn mineral were also detected [5]. According to the present state of knowledge, the willemite-bearing mineralization from the Jerusalem deposit shows the greatest similarities with the Václav vein in terms of both mineral assemblage and chemical composition of individual minerals.

Nevertheless, few specific compositional features can be mentioned in the Václav vein. First, Bi enrichment of base metal mineralization is extremely rare in the Příbram ore area [6,28] and is interpreted in terms of remobilization of Bi from old Au-bearing quartz veins occurring in the area, which contain some Bi tellurides [6,73,74]. The Hg mineralization is more frequent in the Příbram uranium district, where Hg enrichment is observed in native silver, dyscrasite, and allargentum, which originated during the latest calcite-sulfidic stage at several sites [7,75,76,77]. Rare late balkanite was described here too [5]. Low concentrations of Hg rarely reported from sphalerite (usually around 0.003 wt. % and up to 0.2 wt. % [62]) are considered to be a source of Hg for rare late mineralization containing Hg-enriched native silver and imiterite in the Březové Hory district [78]. Rare cinnabar is also reported from the Vrančice deposit [79,80]. The elevated content of Sn bound to accessory cassiterite is reported from the fine-grained “hard ores” of the Březové Hory ore district. In addition, low contents of Sn were detected in many ore minerals in the Březové Hory and Bohutín deposits [9].

5.5. T-X Formation Conditions

The collected mineralogical data offer a few lines of evidence for the specification of the temperature of ore formation, as follows:

(1)

Chlorite geothermometry, based on the amount of ^ivAl and Fe/(Fe + Mg) ratio in chlorite and according to [81], suggests formation temperatures of chlorite between 147 and 172 °C (Table S5).;

(2)

The presence of hematite and absence of goethite in the vein defines the formation temperatures above ca. 85–130 °C ([82] and references therein);

(3)

The occurrence of mckinstryite indicates formation temperatures below 94.4 °C [32,83] and stromeyerite is stable below 93.3 °C [83].

All of the above-mentioned estimates thus suggest a low-temperature (ca. 180–≤100 °C) nature of late-stage mineralization bound to Zones D and E. The direct evidence for the temperature of formation of earlier dolomite and siderite gangue is missing at the Václav vein, but we suggest that it would be comparable with other parts of the Příbram ore area. The previous fluid inclusion studies revealed a high-temperature nature of early siderite and accompanying quartz and sphalerite, which precipitated at ca. 200–300 °C across the whole Příbram ore area [1,7,8]. Similarly, these studies illustrated that dolomite-group carbonates formed at somewhat lower temperatures between ca. 110 and 260 °C and calcites at distinctly low temperatures between ca. 60 and 140 °C, with data being mutually well comparable for both base metal and uranium districts [1,7,8]. The Ag-Cu mineralization of the Vrančice deposit originated under the temperatures 180–100 °C, with its decrease under 100 °C in its latest phases [1].

The often-observed instability of early mineral assemblage during the subsequent evolution of the vein suggests that temperature was not the only variable, which changed substantially during the vein formation. In the context of the previously published results from the Příbram ore area, we cannot exclude the role of the changed composition of the fluids. The three major genetic types of ore-forming fluids identified in the Příbram ore area are briefly characterized in Chapter 2. The formation of dolomite can be related either to the mixing of early high-salinity high-δ¹⁸O “deep” fluids with low-salinity surface (presumably meteoric) waters and/or basinal waters derived from the contemporaneous freshwater Piedmont basins; or, these Lower Permian Piedmont basins were the only source of parent fluids, like in case of late calcite [1,7,8]. Besides changes in major genetic types of fluids, episodic fluctuations in pH, Eh, salt composition, and salinity can be especially expected in the case of waters derived from the Lower Permian Piedmont basins because these (semi)closed reservoirs developed in a hot (semi)arid climate leading to seasonal partial evaporation of the basins [13]. The solute chemistry of recent examples of these playa-type basins shows significant variations, with examples rich in chlorides, sulfates, (bi)carbonates, and/or borates, in dependence on climatic conditions and lithology of underlying rocks [84,85]. In addition, the production of a suite of volatiles (CO₂, N₂, CH₄, H₂, O₂, and other hydrocarbons) is likely in horizons rich in organic matter in these basins [8,24,25].

Second, the observed replacement of a rather large volume of early Fe²⁺-bearing carbonates by the assemblages rhodochrosite+hematite and dolomite-to-kutnohorite+hematite likely points to substantial changes in the redox state of the fluids. An increase in Eh is illustrated by the oxidation of almost all Fe²⁺ (originally bound in carbonates) to Fe³⁺ (precipitated in the form of hematite). In contrast, Mn²⁺ did not oxidize during the same process and re-precipitated in newly formed carbonates. In such a situation, a mildly high Eh of the alteration fluids can be expected [86,87]. The effect of redox-induced separation of Fe from Mn could be further highlighted by the presence of agents allowing for the complexation of Mn, such as bicarbonate and sulfate anions, in the alteration fluids [86]. The complex hematite chemistry showing high amounts of heavy metals and Sb (Figure 11) suggests coeval dissolution of sulfide minerals, which was confirmed by direct observation in the case of sphalerite. High contents of heavy metals in the alteration fluids, necessarily combined with low amounts of reduced sulfur under elevated Eh, were probably the reason for the formation of compositionally unusual Zn-rich chlorite and Zn-bearing Mn-rich carbonates. In any case, our detailed paragenetic study indicates that invasion of oxygenated fluids occurred at least three times during the evolution of the Václav vein: (i) during formation of the rhodochrosite+hematite assemblage, (ii) during formation of the dolomite-to-kutnohorite+hematite assemblage, and (iii) during formation of the calcite+hematite+chamosite-to-baileychlore assemblage. Redox changes influencing the mobility/availability of Fe can also be responsible for the substitution of chalcopyrite (observed in sample P1N 9430) by djurleite (observed in a sample donated by J. Hyršl).

The near-neutral to slightly alkaline pH of the alteration fluids can be expected with respect to the crystallization of Mn carbonates [86]. High alkalinity is improbable as it favors rapid precipitation of Mn oxides at elevated Eh [86] or crystallization of willemite [5,65,88], which were both never observed in the studied vein mineral assemblage. Similarly, the lack of evidence for the dissolution of quartz does not favor the highly alkaline fluids [5].

5.6. Implications for Ore Genesis in the Příbram Area

The available data suggest that the early evolution of the Václav vein followed the model proposed for the typical siderite-sulfidic stage of the Příbram ore area. Siderite, associated Zn-Pb sulfides, and early Fe-rich dolomite Do-1 formed from ascending hydrothermal ore fluids that migrated along an open fracture developed along a pre-existing palaeobasalt dyke in the case of the Václav vein (Figure 21a).

Repeated episodes characterized by the activity of oxygenated fluids might operate according to two scenarios. Both were related to the existence of a hydrothermal cell developed in the area of the originating ore deposit, which was supplied by surface waters from contemporaneous (Lower Permian) Piedmont basins [1,7,8]. First, the switching off of the flow of “deep” low-Eh ore fluids may lead to a predominance of surface-derived waters in the ascending branch of the hydrothermal cell, where the studied ore vein had formed (Figure 21b). A relatively long migration path hosted by lithologies rich in reductants including sulfides+Fe²⁺-rich carbonates (in the vein fill), Fe²⁺-rich silicates (in the host palaeobasalt dyke), and pyrite+organic matter (in the host siliciclastic sediments) requires a focused and rapid fluid flow in order to preserve its oxidative character. Second, tectonic movements along the existing vein can cause a temporary change in the direction of fluid flow in the cell, leading to the percolation of the early vein by descending oxygenated surface waters (Figure 21c). Much shorter migration paths of oxygenated fluids are an advantage of this model situation. Which of both these possibilities took place in the case of the Václav vein could be tested in the future by evaluation of the mineral assemblages with respect to depth: the second scenario requires more oxidized assemblage to be observed toward the surface.

The earliest fill of the Václav vein is formed by siderite, whose chemical composition is identical to those of other ore veins in the Příbram ore area (Figure 6a). Therefore, it is unlikely that the Václav vein should be younger than other base metal veins, as was suggested by [23]. Only the intensity of younger intra-vein overprint can be higher than in the case of other veins. Conversely, the spatial orientation of the Václav vein coincides with the earliest generation of normal faults, which formed after the initiation of the gravitational collapse of the Bohemian Massif after the ending of the Variscan Orogeny in the Uppermost Carboniferous. The movements along steep NNE-SSW trending normal faults delimited the architecture of all Uppermost Carboniferous-to-Permian Piedmont basins (i.e., Blanice, Jihlava, and Boskovice Grabens in the present erosional section) [12]. The importance of the fracture, within which the Václav vein had formed, is illustrated by a regular course and a large depth extent of the Václav vein, contrary to many other ore veins in the Příbram ore area [89].

Another ~N-S trending zone hosts the Vrančice, Milín, and Lešetice deposits, which show similar mineralization to those found in the Václav vein. Thus, this area likely has the potential to further test the above-mentioned genetic hypotheses dealing with the origin of hypogene “oxygenated facies” of the Příbram ores. Ref. [31] suggested the lithological control of the Vrančice, Milín, and Lešetice Ag-Cu sulfide occurrences, which are characterized by the proximity of the granitoids of the Central Bohemian Plutonic Complex. However, the newly described occurrence at the Václav vein is located in the Cambrian sediments with no relation to any granitoid occurrence. This favors the fluid processes and not the host lithology as a key factor controlling the occurrence of the Ag-Cu mineralization.

Although there are also known sulfide-free siderite veins in the wider area [89], the low content of Pb-Zn sulfides, which are usually a significant constituent of the siderite-sulfidic stage, is surprising in the Václav vein. Sphalerite hosted in early Zone A shows signs of pronounced corrosion, suggesting that it was significantly dissolved by late fluids. Galena, which is usually a predominating ore mineral in the Příbram ore veins [9,62], is almost missing in the studied samples. The observed occurrence of dissolution cavities containing corroded relics of sphalerite and filled by late-stage Cu-sulfides, hematite, and carbonates as well as enrichment of late minerals in Zn, Pb, and Sb point to a possibility that sulfide minerals were present but were extensively dissolved by oxygenated fluids. Therefore, the low amount of sulfides cannot be necessarily the primary feature of a vein but can be attributed to significant leaching by late hydrothermal fluids. Careful textural and geochemical investigations are necessary to identify these processes.

An intriguing question is the source of Cu and Ag, which are anomalously enriched in the studied mineralization in comparison with typical base–metal veins in the Příbram area. One can assume that these metals could be potentially leached from early base–metal mineralization during its interactions with late fluids. This assumption may be valid, especially in the case of Ag, which is present in trace amounts in galena (which is the dominating ore mineral) as well as in a number of accessory minerals hosted by base–metal sulfides containing Ag as a major constituent (i.e., Ag-bearing TGM, diaphorite, freieslebenite, owyheeite, Ag-poor Ag-Pb-Sb sulfosalts, etc. [4,6,7,9]). In contrast, more uncertain is the remobilization of a sufficient amount of Cu, because Cu minerals are much rare in the typical Pb-Zn veins in the Příbram area [4,6,7,9] and Cu represents a major ore element in late mineralization. Therefore, we cannot exclude the possibility that at least a part of the metals was brought by fluids from the Permian sedimentary basins, which contain local accumulations of stratiform Cu ores. At the northern margin of the Bohemian Massif, the World’s largest Permian stratiform Cu deposit occurs in the Zechstein basin, which is genetically related to the activity of brines associated with marine evaporites [90]. However, minor deposits and small occurrences of stratiform Cu mineralization also occur in the Permian sediments preserved in the inner parts of the Bohemian Massif [49,91,92,93], implying that such mineralization formed even in the freshwater environment. Besides predominating Cu, these stratiform ore mineralizations usually also contain minor Ag, and, locally, Hg [49,94]. Obviously, if the parent metal-bearing basinal fluids infiltrated along extensional faults into the subsurface, they could give rise to a basement-hosted vein-type Cu mineralization [95,96].

The manifestations of a poly-phase evolution and associated locally very intense intra-vein metasomatism and replacement are common in the Příbram ores, being described from both base–metal [2] and uranium [5,6,7,8,97] districts of the Příbram ore area. Such features are compatible with the geological position of the ore area close to a supra-crustal long-lived lineament dividing the Teplá-Barrandian and Moldanubian units representing two of the principal basement blocks of the Bohemian Massif [98].

6. Conclusions

• The Václav vein, representing one of the base–metal ore veins of the Březové Hory deposit, provided an interesting and mineralogically rich mineral assemblage. A total of 31 mineral species was found there, including three unknown sulfide phases.
• The vein is characterized by surprisingly complicated evolution. Early siderite and dolomite-ankerite gangue containing sphalerite and galena were subject to repeated superimposed hypogene alterations, manifested by the strong (i) dissolution of sulfides, (ii) silicification, and (iii) replacement of early Fe-bearing carbonates by assemblages of hematite and Mn-rich Fe-poor carbonates (rhodochrosite or members of the dolomite-kutnohorite series). In addition, the replacement of sphalerite by chalcopyrite and bornite is observed across the vein. Widespread multiple remobilizations of heavy metals are also manifested by unusual compositions of chlorite (Zn-rich chamosite to baileychlore), hematite (Sb-, Zn-, and Pb-enriched), and Mn-enriched carbonates (Zn-enriched). An uncommon Ag-Cu-Hg sulfide mineralization appeared in the late stage of vein evolution.
• The observed disequilibrium textures suggest repeated and significant changes in the chemical composition and/or physico-chemical conditions of the parent fluids. A moderate increase in the redox potential of fluids is inferred from the replacement of early Fe-rich carbonates by assemblages of Mn-rich carbonates and hematite. These findings are compatible with repeated episodic invasion of oxygenated surface waters, likely represented by local meteoric waters and/or basinal waters from contemporaneous (Lower Permian) partly evaporated freshwater Piedmont basins.
• Similar mineralizations are known from the Jerusalem, Vrančice, Milín, and Lešetice deposits in the Příbram ore area. Because the modern detailed studies were mostly not conducted at these localities, they represent suitable sites, at which the origin of the specific “oxygenated ore facies” can be further studied in the future.
• The low-grade ore mineralization is probably not the primary feature of the Václav vein. The amount of ore minerals was diminished by superimposed fluid events, which caused partial dissolution of at least a part of early base–metal sulfide minerals.