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Mineralium Deposita

, Volume 54, Issue 2, pp 155–174 | Cite as

Polyphase enrichment and redistribution processes in silver-rich mineral associations of the hydrothermal fluorite-barite-(Ag-Cu) Clara deposit, SW Germany

  • Maximilian F. KeimEmail author
  • Benjamin F. Walter
  • Udo Neumann
  • Stefan Kreissl
  • Richard Bayerl
  • Gregor Markl
Article

Abstract

The silver-copper sulfide mineralization associated with the fluorite-barite vein system at the Clara deposit in SW Germany shows large scale vertical zoning. Low to moderate silver contents prevail in the upper 350 m, whereas high silver contents occur in the subsequent 450 m of the currently known vein system. This change in Ag tenor is related to conspicuous mineralogical changes with depth. A detailed petrographic and fluid inclusion study identifies evidence for five subsequent hydrothermal and one alteration stage—all contributing to mineralogical diversity. The vertical Ag zoning, however, is attributed only to the first of these stages. During this first stage, increasing oxidation of ascending hydrothermal fluids (90–160 °C, 24.2–26.7 wt% NaCl+CaCl2) led to the formation of high-Ag tetrahedrite-tennantite in the lower parts and basically Ag-free enargite in the upper parts of the vein system. The subsequent hydrothermal stage led to significant mineralogical changes, but inherited the pre-existing Ag zonation. In this second hydrothermal stage, which was related to fluids similar in composition to those of the first stage (70–125 °C, 23.1–26.5 wt% NaCl+CaCl2), dissolution of high Ag-tetrahedrite-tennantite resulted in the formation of complex Ag-sulfosalts together with moderately Ag-bearing tetrahedrite-tennantite and chalcopyrite. The first two stages were formed by fluid mixing of a sedimentary and a hot basement fluid. The influx of fluids with high Ag, Bi and Pb activity during stage 3 and 5 resulted in the local replacement of earlier Ag-sulfosalts by galena and Ag-(Bi)-sulfosalts. The fourth stage is marked by partial dissolution of sulfides and sulfosalts by a late, hot, undiluted basement fluid (250 °C, 18.7–20.9 wt% NaCl+CaCl2) precipitating fluorite, barite and quartz. Finally, supergene alteration lead to the dissolution of silver-bearing phases and the precipitation of acanthite and native silver. The study illustrates, how metal tenor and mineralogy are decoupled in vertically extensive, polyphase hydrothermal vein systems. This may be pertinent to similarly zoned polymetallic vein systems.

Keywords

Fahlore Tennantite-tetrahedrite Polybasite-pearceite Hydrothermal Silver Schwarzwald ore district 

Introduction

The Schwarzwald ore district in SW Germany hosts numerous polymetallic hydrothermal veins consisting primarily of quartz, fluorite, barite, and minor calcite together with minor Ag, Pb, Zn, Sb, As, Cu, Co, and U mineralization (Bliedtner and Martin 1986; Staude et al. 2009). Various types of mineralization formed intermittently since the Permian with a maximum activity during the Jurassic-Cretaceous and Tertiary (Pfaff et al. 2009; Markl 2015). The vein deposits have been mined discontinuously during the last 5000 years (Goldenberg and Steuer 2004), but the Clara Mine currently represents the last active mine in the area (Staude et al. 2010a, b, and references therein; Markl 2015). The Clara vein system formed mainly during the Jurassic-Cretaceous by fluid mixing triggered by extensional processes during the opening of the Central Atlantic (Pfaff et al. 2009; Staude et al. 2009; Walter et al. 2016, 2017a). It is among the top five worldwide localities with respect to mineral diversity and type locality of 13 minerals (Markl 2015, and references therein). To a large extent, this is due to an extensive supergene overprint of an already unusually variable hydrothermal mineral assemblage (Bernard and Hyrsl 2006; Markl 2015). Several authors dealt with the Clara vein system focusing on the geology and structural relations (List 1969; Zirngast 1978; Maus et al. 1979; Huck 1984), mineralogy of hydrothermal and supergene minerals (e.g., Schmeltzer 1983; Kaiser 1984; Walenta 1984; Rehren 1985; Kolitsch et al. 1995; Baumgärtl and Burow 2002; Van der Heyde 2002; Staude et al. 2010a, b; Markl 2015), and the petrological-geochemical conditions of ore formation (Huck 1984, Pfaff et al. 2012; Walter et al. 2017a). The hydrothermal vein system, on which the Clara Mine operates today, has discontinuously been mined for copper, iron and silver since the seventeenth century (Bliedtner and Martin 1986; Werner and Dennert 2004). Since 1850, the vein system has been exploited for barite, from 1898 until today without any interruption. Since 1978, the newly discovered parallel striking fluorite-dominated vein has been mined for fluorite and since 1997, a Ag-Cu-concentrate is produced from sulfide-rich parts (called “silverspar” = Silberspat in German) of the barite vein. This silverspar becomes more prominent with depth (Baumgärtl and Burow 2002; Markl 2015). In 2015, about 150,000 t of raw ore were processed to 50,000 t of fluorite and 45,000 t of barite concentrate. Additionally, 170 t of an Ag-Cu-sulfide concentrate were produced in the same year, containing 4 t of silver and 50 t of copper (Elsner and Schmitz 2017).

The Clara vein system is remarkable for numerous reasons: it is one of few hydrothermal barite-fluorite vein systems with an exposed vertical extent of more than 800 m and it is marked by incredible mineralogical diversity (> 400 species known so far; Markl, 2015). These remarkable facts render detailed studies of this vein system scientifically relevant, as they contribute to the understanding of the origin and subsequent overprint of a complex, polyphase hydrothermal system. Here, we present a detailed description of the ore textures and paragenetic sequence of the silver-rich association hosted by the Clara barite vein, discuss the observed mineralogical variation with depth as well as redistribution processes using microscopy, electron microprobe, and fluid inclusion analyses.

Geology, mineralogy, and vein formation of the Clara deposit

Geological framework of the Schwarzwald ore district

The Schwarzwald is a mountain range of about 50 × 150 km size located in SW Germany, geologically consisting of exhumed Variscan basement rocks (mostly metasedimentary gneisses and granites) and Lower Permian to Upper Jurassic sedimentary cover rocks (Kalt et al. 2000). During Carboniferous collision processes, the paragneiss units (locally containing orthogneisses and amphibolites) were deformed and metamorphosed to upper amphibolite-facies grade (Geyer and Gwinner 2011). Between 335 Ma and 315 Ma, these gneiss units were intruded by post-collisional S-type granites (Altherr et al. 2000; Hann et al. 2003). Regional basins were filled by red bed arkoses and conglomerates during the Lower Permian. Due to subsidence from early Triassic to Jurassic, quarzitic sandstones, shales, and limestones were deposited unconformably on the basement rocks. During the Paleogene, rifting of the Upper Rhine Graben was initiated and the graben shoulders were exhumed due to uplift. The erosion of the sedimentary cover over the last 20–30 Ma resulted in the partial exposure of the basement (Geyer and Gwinner 2011, and references therein). Erosion was strongest in the southern parts; in the Central and Northern Schwarzwald, the basement-cover unconformity is still preserved. In the southern Schwarzwald, the basement gneisses have been eroded to a depth of about 1.5–2 km below the former unconformity (Nitsch and Rupf 2008). The Clara vein system is part of the Central Schwarzwald and extends from the basement gneisses into the Permian and Triassic red bed cover rocks.

Hydrothermal vein formation in the Schwarzwald

Various types of hydrothermal veins formed over the last 300 Ma in the Schwarzwald and are hosted by the Variscan basement rocks and their Permo-Triassic sedimentary cover (Pfaff et al. 2009; Staude et al. 2009; Walter et al. 2016). The over 1000 veins consist dominantly of barite, fluorite, quartz, and minor carbonates besides native metals, metal oxides, sulfides, and arsenides (Metz et al. 1957; Bliedtner and Martin 1986). Most of the hydrothermal activity occurred around 200–100 Ma during late pull-apart tectonics related to the break-up of the North Atlantic and during opening of the Rhine Graben rift system between 40 and 20 Ma (Staude et al. 2009; Burisch et al. 2018). Based on structural, mineralogical, and microthermometric studies, the hydrothermal veins have been subdivided into five formation stages comprising (i) Late Carboniferous quartz-tourmaline veins with W-Sb ores, (ii) Permian quartz veins with Sb-(Au)-(Ag) ores, (iii) Triassic-Jurassic quartz veins with Fe-ores, (iv) Jurassic-Cretaceous fluorite-barite-quartz-carbonate veins with Pb-Zn-Cu, Fe-Mn or Ag-Bi-Co-Ni-U ores and (v) Post-Cretaceous quartz-barite-fluorite or carbonate veins with Pb-Zn-Cu-Ag and Cu-Ni-Bi-Ag ores (Walter et al. 2016). Based on fluid inclusion data, Bons et al. (2014), Fusswinkel et al. (2013) and Walter et al. (2016, 2017a, and references therein) provide strong evidence for binary fluid mixing as the dominant mechanism of hydrothermal mineral formation. Based on fluid chemistry and isotope systematics, mixing occurred between a highly saline, metal-rich basement brine and an almost equally saline metal-poor brine presumably sourced from Middle Triassic limestone (Muschelkalk) (Staude et al. 2010a, b, 2011; Fusswinkel et al. 2013; Bons et al. 2014; Walter et al. 2015, 2016, 2017a). This binary fluid mixing led to precipitation of large amounts of fluorite, barite, and quartz during the Jurassic-Cretaceous (Burisch et al. 2016b). Late-stage quartz is thought to have formed by either fluid mixing or cooling (Burisch et al. 2017b). Further information on classification, genesis and conditions of formation of the Schwarzwald hydrothermal veins can be found in Behr and Gerler (1987), Behr et al. (1987), Staude et al. (2009, 2010a, 2011, 2012a, b), Pfaff et al. (2009, 2010), Markl (2015), Walter et al. (2015, 2016, 2017a, 2017b, 2018), Burisch et al. (2016a, 2016b, 2018).

Local geology of the Clara Mine

The complex vein system of the Clara Mine is located north of the city of Wolfach in the Central Schwarzwald (Fig. 1). The veins are hosted by paragneiss and metatexite units predominantly composed of quartz, plagioclase, biotite, orthoclase, and minor hornblende (age of metamorphism about 330 Ma (Kalt et al. 1994)). The minerals of the host rock gneisses show strong sericitization and/or chloritization accompanied by a strong silicification and precipitation of hematite (Okujeni 1980; Huck 1984; Kloos 1990). The sedimentary cover consists of Permian arkoses and Triassic sandstones that unconformably rest on the basement gneisses. The Clara vein system has been subdivided into five mineralization stages including numerous sub-stages (List 1969; Zirngast 1978; Maus et al. 1979; Huck 1984; For details see ESM and Table 1).
Fig. 1

a Geological map of the Clara vein system including main hydrothermal veins and major tectonics. b Cross-section of the Clara Mine with the projected mine development modified after Huck (1984) and Bucher et al. (2009)

Table 1

Compilation of the mineralization stages of the Clara veins after Huck (1984) including main stages, sub stages, important sulfides, and age constraints

Main stages

Sub stages

Sulfide/oxide minerals

Age constraints

1. Silicification

1.1–1.3 Quartz

Hematite, pyrite, marcasite

 

2. Fluorite main stage (Fluorite vein Fig. 1)

2.1–2.3 Fluorite + quartz + sellaite

Pyrite, chalcopyrite, fahlore, (ferberite)

130 ± 20 (fluorite/barite)1; 143 ± 2 (host rock sericite)1; 173 ± 2 (ferberite)2;

3. Barite main stage (Barite vein Fig. 1)

3.1 Barite + fluorite + (quartz)

Pyrite, marcasite, chalcopyrite

144 ± 5 (host-rock illite)1

3.2 Barite + (fluorite)

Pyrite, marcasite

 

3.3 Barite + fluorite + quartz

Minerals of the “silverspar”

 

3.4 Barite + (fluorite)

 

4. Barite interstage

4.1; 4.2 Barite + fluorite + siderite

Chalcopyrite, marcasite

 

5. Quartz main stage (Quartz vein; “Diagonaltrum” Fig. 1)

5.1–5.4 Quartz + barite + fluorite

Pyrite, galena, chalcopyrite

 

1Mertz (1987); 2Pfaff et al. (2009)

The first stage (1) is represented by intensive silicification and brecciation of the host gneiss and is followed by the second stage (2) (fluorite main stage) consisting of fluorite-quartz-sellaite (up to 12 m thickness; Werner and Dennert (2004)) together with minor pyrite, chalcopyrite, ferberite, and minerals of the tennantite-tetrahedrite solid-solution series (hereafter referred to as fahlore; chemical formulae and abbreviations for the most important minerals are listed in Table 2).
Table 2

Typical minerals of the “silverspar” with stoichiometric formulae and abbreviations (Abbr) used in Figs. 3, 4, and 8

Mineral

Formula

Abbr

Mineral

Formula

Abbr

Barite

BaSO4

Brt

Famatinite

Cu3SbS4

Fam

Calcite

CaCO3

 

Galena

PbS

Gn

Fluorite

CaF2

Fl

Geocronite

Pb14(Sb,As)6S23

 

Quartz

SiO2

Qtz

Hematite

Fe2O3

 

Sellaite

MgF2

 

Jordanite

Pb14(As,Sb)6S23

 

Siderite

FeCO3

 

Luzonite

Cu3AsS4

Luz

Acanthite

AgS

Act

Marcasite

FeS2

Mrc

Arsenopyrite

FeAsS

Apy

Matildite

AgBiS2

 

Berryite

Cu3Ag2Pb3Bi7S16

 

Mckinstryite

Ag5Cu3S4

 

Bornite

Cu5FeS4

Brn

Native Silver

Ag

Ag

Chalcopyrite

CuFeS2

Cp

Polybasite

(Ag,Cu)16(As)2S11

Plb

Cobaltite

CoAsS

 

Pearceite

(Ag,Cu)16(Sb)2S11

Prc

Covellite

CuS

Cv

Proustite

Ag3AsS3

Prs

Diaphorite

Pb2Ag3Sb3S8

 

Pyrargyrite

Ag3SbS3

Pya

Emplektite

CuBiS2

 

Pyrite

FeS2

Py

Enargite

Cu3AsS4

Eng

Stephanite

Ag5SbS4

 

Fahlore

(Cu,Ag)10(Fe,Zn)2(As,Sb)4S13

Fh

Stibnite

Sb2S3

 

The third stage (3) (barite main stage) crosscuts and follows the older fluorite vein particularly in the SE parts but mostly runs parallel to it and discordantly crosscuts the sedimentary cover (Huck 1984). The barite main stage is subdivided into 4 different substages (3.1–3.4; List 1969; Zirngast 1978; Huck 1984). The first and second substages (3.1, 3.2) consist of barite and fluorite together with small amounts of botryoidal pyrite-marcasite aggregates and minor chalcopyrite (Huck 1984). The third substage (3.3) is characterized by alternating layers of medium-grained white barite, gray fluorite, and quartz together with interbedded sulfides. The thickness of the sulfide layers varies from a few mm to some dm. Towards greater depth, the proportion of sulfides increases. The sulfide minerals predominantly occur in the southern part of the vein and consist of chalcopyrite and fahlore mostly intergrown with fluorite (List 1969). This substage is commonly referred to as “silverspar,” which is processed at Clara Mine to produce Ag and Cu concentrates. It is also this substage that the present contribution is mainly concerned with. The “silverspar” can be subdivided into two mineralogically different groups: the upper level group (UG) is characterized by a relatively simple mineralogy (fahlore + chalcopyrite and rare enargite) and is present between the 4th and the 10th level (690–450 m above mean sea level (amsl); Huck 1984; List 1969; Zirngast 1978). The lower level group (LG) was found only below the 10th level down to the present mining level (2017: 20th level, 10 m below mean sea level) and shows a more complex mineralogy including Ag-sulfosalts besides fahlore and chalcopyrite (Schmeltzer 1983; Huck 1984; Walenta 1984; Rehren 1985; Kolitsch et al. 1995; Van der Heyde 2002). Besides the normal Ag-Cu sulfide mineralization, the “silverspar” shows spatially confined mineralogical variations by the additional occurrence of large amounts of arsenopyrite and/or galena. Although it is likely that the “silverspar” also occurred above the 4th level, it is not preserve due to intensive weathering.

During the fourth substage (3.4) coarse-grained barite crystals grew together with fluorite and subordinate fahlore and chalcopyrite. Whether these sulfide grains represent remobilization products from substage 3 or can be attributed to a distinct mineralization stage remains unclear (List 1969; Huck 1984). The fourth stage (4) is called “barite interstage” and is characterized by different generations of barite, fluorite, siderite, and chalcopyrite plus marcasite.

The fifth stage (5) is dominated by quartz and is called “quartz main stage”; it crosscuts the older fluorite and barite veins. This stage developed in close relation to the ENE-WSW striking, major regional shear zone, the Northern Kinzigtal fault (NKF) (Huck 1984; Fig. 1). The formation of stage five is subdivided into 4 different substages (Huck 1984). The most important is the fourth substage (5.4), which contains galena and chalcopyrite often crosscutting the earlier fluorite and barite veins leading to a prominent silicification of the gangue minerals, especially of barite. This substage reaches up to 5 m thickness in the northern part forming a separate vein, referred to as “Diagonaltrum.”

Age dating of the “fluorite main stage” reveals ages of 130 ± 20 Ma (fluorite, barite; Rb-Sr method), 143 ± 2 Ma (host rock sericite; Rb-Sr method), and 173 ± 2 Ma (ferberite; U-Pb method), for the “barite main stage” of 144 ± 5 Ma (host rock illite; Rb-Sr method) (Pfaff et al. 2012, and references therein). This shows that the main mineralization of the Clara vein system belongs to the Jurassic-Cretaceous vein group, which according to Behr and Gerler (1987), Staude et al. (2009) and Walter et al. (2016) formed by fluid mixing.

Methods

For this study, a sample suite of 52 thin sections was investigated by light and electron microscopy, besides macroscopic observations on dozens of hand specimens. Fluid inclusion sections (4 samples) were prepared from the counterpart of the thin sections. Samples come from the mineral collection of the University of Tübingen and were sampled over the years on different mining levels. Also, samples collected from the mine dumps over the last 30 years were included. It was only roughly possible to assign the latter ones to a certain mining level.

Electron microprobe analysis (EMPA)

For qualitative and quantitative determination of the major and minor element compositions of sulfides and native elements, a JEOL 8900 electron microprobe was used at the Department of Geosciences, University of Tübingen, Germany, and a Cameca SX100 electron microprobe at the University of Stuttgart, Germany. Both microprobes running in wavelength-dispersive (WD) mode used a focused beam with 20 nA current and a 20 kV acceleration voltage. Due to the rapid decomposition of polybasite-pearceite, pyrargyrite-proustite, and billingsleyite under the electron beam, a beam diameter of 10 μm and a beam current of 5 nA were used. Matrix corrections were performed according to the JEOL ZAF method (Armstrong 1991). The elements for the quantitative program were selected after qualitative analyses in energy dispersive mode (EDS). Details of the WD-configuration used, including standards, counting times of the peak/background, analyzed fluorescence lines, and the average detection limits and the whole dataset are presented in the ESM.

Fluid inclusion microthermometry

Microthermometric analyses were performed in fluorite and quartz (n = 132) from one UG (MK-38; 6th level; 600 m amsl) and three LG samples (MK-7, MK-16, MK-34; below the 10th level; 450 m amsl) to determine the chronological sequence of fluid inclusion (FI) assemblages using a Linkam stage (model THMS 600) at the University of Tübingen. Observations were made in double polished thick sections (100 to 200 μm). The FI were classified as primary, pseudo-secondary, secondary, isolated inclusions, and clusters of inclusions (see ESM) with no geometrical relation to former crystal surfaces or fractures according to the FI-assemblages approach of Goldstein and Reynolds (1994). Each FI was analyzed triply to determine the final melting temperature of ice (Tm-ice) and hydrohalite (Tm-hh) and the homogenization temperature (Th). The presented data include only FI for which all three analyses differ less than 0.1 °C for Tm-ice and Tm-hh and less than 1 °C for Th. Synthetic H2O, H2O-NaCl and H2O-CO2 standards were used for calibration. The salinity in the ternary NaCl-CaCl2-H2O system was determined according to Steele-MacInnes et al. (2011). The volume proportion for each FI was estimated based on filling degree tables and reported in the volume proportion notation (Shepherd et al. 1985; Bakker and Diamond 2006). FI showing evidence of post-entrapment modifications were excluded from analysis and data interpretation. A pressure correction considering the method of Bodnar and Vityk (1994) was applied, assuming hydrostatic conditions with a depth of the water column inferred from the paleo-depth of 1.0–1.5 km as estimated by Geyer and Gwinner (2011). Uncertainties of this approach are discussed in Walter et al. (2015, 2016, 2017a, 2017b). Since hydrostatic conditions can be assumed and overburden is negligible for the Clara vein system, the pressure correction has only minor effects on the homogenization temperature (5–10 °C). Therefore, homogenization temperatures were presented as uncorrected values. Microthermometry of FI in barite is difficult, since they are easily destroyed during freezing or heating (decrepitation, leakage, necking-down). Consequently, microthermometric data obtained from FI in barite were only used for estimating salinities. All FI data can be found in the ESM.

Cathodoluminescence microscopy

Cathodoluminescence (CL) microscopy studies were performed to obtain additional qualitative information on the fluid petrography of FI-assemblages and on the paragenetic sequence of the gangue minerals. A hot cathode CL microscope (type HC1-LM) at the University of Tübingen was used with an acceleration voltage of typically ~ 14 kV and a beam current density of ~ 9 μA/mm2 on the sample surface.

Geochemical modeling

For calculating predominance diagrams, The Geochemist’s Workbench version 10.0 (Bethke and Yeakel 2015) was used. Calculations are based on the Thermoddem database (Blanc et al. 2012). Data for enargite were implemented to the Thermoddem database from the SOLTHERM database (Palandri and Reed 2017), data for tetrahedrite, freibergite, Ag-polybasite, Ag-Cu-polybasite were from Sack (2000) and Sack (2017). Combining the thermodynamic data from different sources is a potential source of error. However, Keim et al. (2017) showed that such errors are relatively small and have no influence on the general interpretation of the predominance diagrams in this work.

Results

Paragenetic/petrographic description of the “silverspar” in the barite vein

The mineralogical and petrographic description of the mineral textures in this study focuses on the “silverspar” (sub stage 3.3) of the “barite main stage” (stage 3). Samples of this study come from the 4th to 18th level of the Clara Mine. In the following, the paragenetic sequences of the UG and LG of the “silverspar” are presented separately. In the UG and LG, the “silverspar” can be subdivided into six different paragenetic stages (S1-S6; Fig. 2), although S3 and S5 are missing in the UG.
Fig. 2

Paragenetic sequence of the “silverspar” mineralization. Minerals occurring in both groups are shaded in blue, minerals occurring exclusively in the LG are not shaded, and minerals exclusively present in the UG are shaded in gray. Black bars represent minerals that occur frequently, gray bars represent minerals that are rare during a paragenetic stage. The circled lettered numeration refers to thin section photographs (Figs. 4 and 5)

The upper level group (UG)

Macroscopically, the UG “silverspar” shows a smaller amount of sulfides compared to the LG and is characterized by meter-thick sequences of alternating, cm- to dm-thick bands of barite, fluorite, and quartz, which are accentuated by horizontally aligned, small, unconnected sulfide pods predominantly within fluorite II zones (Fig. 3a). Sulfides are also present as more massive aggregates of several cm to dm thickness, again predominantly in the fluorite-dominated parts of the vein (Fig. 3b), albeit much less common than in the LG.
Fig. 3

Photographs of hand specimens showing typical textures of the “silverspar.” a Top: Barite dominated sample with bands of fluorite with sulfides; bottom: Banded barite-fluorite ore exposed in the Clara Mine (photograph from Werner and Dennert 2004). b Sulfide bands in fluorite together with barite. c Nodular aggregate of enargite plus fluorite in barite. d Sulfide bands in a barite-dominated sample. e Quartz and sulfides randomly distributed in barite. f Massive sulfides together with barite, quartz, and fluorite

The first paragenetic stage S1 is characterized by the occurrence of columnar enargite together with nodular fluorite I enclosed in barite I plus quartz I (Figs. 3c and 4a). Enargite is overgrown by radial marcasite (Fig. 4b) crystals and associated with small amounts of bornite (Fig. 4c).
Fig. 4

Photomicrographs of the “silverspar” of the Clara Mine illustrating important ore textures. a Enargite replaced by fahlore plus chalcopyrite (MK-35). b Radial marcasite crystals overgrown by fahlore and chalcopyrite (MK-35). c Bornite and covellite together with minor fahlore enclosing fine-grained chalcopyrite (MK-35). d Galena and polybasite-pearceite together with enargite and fahlore (MK-35). e Fahlore enclosed in interstitial fluorite (MK-39). f Euhedral chalcopyrite and alterated fahlore (MK-37). g Large fahlore aggregate together with different generations of quartz and fluorite (MK-3). h Fahlore and chalcopyrite together with arsenopyrite (MK-3). For mineral abbreviation, see Table 2; Refl = reflected

During the second paragenetic stage S2 enargite crystals are partly or entirely replaced by fahlore II and chalcopyrite I (Fig. 4a, d). Fahlore II and chalcopyrite I are accompanied by rare polybasite-pearceite I, galena II and covellite (Fig. 4d). This occurrence of polybasite-pearceite is the only example of a distinct primary Ag-phase in the UG samples. Their occurrence appears to be confined to the transition between UG and LG. Fahlore II and chalcopyrite I show no clear age relation and grow either on barite II crystals (Fig. 4e) or are enclosed by interstitial fluorite II (Fig. 4f). As accessory mineral, arsenopyrite II occurs enclosed in fahlore II (Fig. 4h). The following ore stages S3 and S5 are missing in the UG, but quartz II, fluorite II, and fahlore II show partial dissolution by later fluorite III and quartz III during S4 (Fig. 4g). In paragenetic stage S6, fahlore II and chalcopyrite I are replaced by covellite (Fig. 4f) and/or famatinite-luzonite which are rarely intergrown with small μm-sized acanthite and native silver grains.

The lower level group (LG)

Macroscopically, the LG samples show alternating bands of barite, fluorite, and quartz. The sulfides form connected bands, mostly in the fluorite- and quartz-dominated zones (Fig. 3d). In some samples, however, a clear banding is not present and nest-like sulfide aggregates occur, usually in quartz-rich parts of the barite vein (Fig. 3e). Massively developed sulfide “pods” up to several dm in size or thickness are typical of the LG and contribute the largest portion to the overall sulfide content (Fig. 3f).

In S1, an early pyrite/marcasite phase is accompanied by arsenopyrite I (Fig. 5a, b), fahlore I, galena I (Fig. 5c), and exolved matildite-galena aggregates. These sulfides are typically replaced by later chalcopyrite I of S2.
Fig. 5

Photomicrographs of the “silverspar” of the Clara Mine illustrating important textures. a Pyrite/marcasite enclosed by arsenopyrite (MK-43). b Pyrite and arsenopyrite enclosed by euhedral chalcopyrite (MK-43). c Fahlore, arsenopyrite, pyrite and galena enclosed by chalcopyrite (MK-17.1). d Chalcopyrite together with cogenetic fahlore (MK-12). e Polybasite-pearceite overgrown/replaced by fahlore (MK-12). f Polybasite-pearceite replaced by fahlore together with chalcopyrite (MK-36). g Polybasite pearceite together with galena (MK-48). h CL-image of polybasite-pearceite plus galena replacing fluorite (MK-1). i Polybasite-pearceite and fahlore replaced by pyrargyrite-(proustite) (MK-4). j CL-image of fluorite replacing galena (MK-1). k Fluorite replacing galena, polybasite-pearceite, and pyrargyrite-(proustite) (MK-2R). l Late quartz together with euhedral arsenopyrite (MK-15). For mineral abbreviation, see Table 2; Refl = reflected

Chalcopyrite I of S2 forms either euhedral crystals (Fig. 5b), or is intergrown with fahlore II. Fahlore II frequently overgrows polybasite-pearceite I (Figs. 5e, f and 3g). Galena II is relatively rare and cogenetic with chalcopyrite I. Euhedral grains of arsenopyrite II are variably abundant and crystallized during the entire stage (Fig. 5c).

In some samples, S1 and S2 are overprinted by the younger mineralization of stage three (S3). Here, aggregates of polybasite-pearceite II ± galena III (Fig. 5g) replace fluorite I/II (Fig. 5h), fahlore II and polybasite-pearceite II (Fig. 4i). Additionally, polybasite-pearceite II enclose minerals of S1 + S2 (Fig. 5i). In this association, also billingsleyite, diaphorite, stephanite, jordanite, and geocronite occur in trace amounts (Kolitsch, pers comm 2017). Polybasite-pearceite II and chalcopyrite I are commonly replaced by later pyrargyrite-proustite solid solution (Fig. 5i). This paragenetic stage represents a galena-rich variety of the “silverspar” that occurs locally (Markl 2015).

During S4, fluorite III, barite III, and quartz III partially replace the earlier S1-S3 sulfides. This replacement process can be observed mostly in fluorite III, which encloses small roundish and amoeboid-like grains of the earlier sulfides (Fig. 5j, k). Fluorite III is accompanied by needle-like matildite I and galena IV.

The subsequent paragenetic sequence S5 is characterized by fine-grained quartz IV together with variable amounts of predominantly arsenopyrite III (Fig. 5l). Arsenopyrite III occurs either enclosed in quartz IV or as prismatic rhombs in vugs and on the surface of late-stage chalcopyrite II. Matildite II, cobaltite, berryite, stibnite, and scheelite also occur, either growing in vugs or enclosed by chalcopyrite II. This paragenetic stage is responsible for the (arsenopyrite + quartz)-rich variety of the “silverspar.” During S6, the sulfides undergo alteration to famatinite-luzonite, covellite and—especially in the vicinity of the silver sulfosalts—larger amounts native silver and acanthite occur.

Compositional variation of sulfides

The following minerals were analyzed by electron microprobe: Fahlore (n = 434), polybasite-pearceite (n = 130), pyrargyrite-proustite (n = 37), billingsleyite (n = 7), pyrite (n = 30), marcasite (n = 5), chalcopyrite (n = 81), galena (n = 79), matildite (n = 17), arsenopyrite (n = 74), famatinite-luzonite (n = 83), enargite (n = 11), covellite (n = 14), native Ag (n = 15), acanthite (n = 5). Representative analyses with calculated formulae for each mineral can be found in the ESM.

Tennantite-tetrahedrite solid-solution series ((Cu,Ag)10(Zn,Fe)2(As,Sb,Bi)4S13)

Fahlore I and II can be distinguished by their molar XAg (defined as: Ag/(Cu + Ag)) (Fig. 6a). For fahlore I, XAg ranges between 0.19 and 0.37 (average of 0.28) and for fahlore II between < 0.01 and 0.18 (average of 0.05). About 20 (of 80) individual analyses of fahlore I show freibergitic compositions (> 20 wt% Ag; Riley 1974).
Fig. 6

Compositional variation of the sulfide minerals of the “silverspar.” a Molar As/(As+Sb) versus molar Ag/(Ag + Cu) showing the chemical variation of fahlore I and fahlore II. Gray stars show distinct samples for both groups. b Molar As/(As+Sb) versus molar Ag/(Ag + Cu) showing chemical variation of fahlore in the UG and the LG. c Molar As/(As+Sb) versus Zn/(Zn + Fe) showing the chemical variation of fahlore from the UG and the LG. d Molar As/(As+Sb) versus molar Ag/(Ag + Cu) showing the chemical variation of polybasite–pearceite I and II. e Molar As versus Sb showing the chemical variation of famatinite and luzonite/enargite. Stars mark end-member compositions, gray points stand for enargite, blue points for pyrargyrite-proustite analyses

The molar XAs (defined as: As/(As+Sb + Bi)) of both fahlore generations is highly variable and ranges for fahlore I between 0.13 and 0.69 and for fahlore II between 0.28 and 0.99. The Bi contents are generally low with maximum contents of 4.3 wt%. Fahlore I has a higher average Bi content (0.13 wt%) than fahlore II (0.02 wt%). Within individual samples, increasing XAg is correlated with decreasing XAs (see stars in Fig. 6a). This has been observed by several authors (e.g., Hackbarth and Petersen 1984; Kemkin and Kemkina 2013) and was explained by the little tolerance of both atoms in the fahlore structure (Johnson et al. 1986, and references therein). A separation of the fahlore generations by molar XZn (defined as: Zn/(Zn + Fe)) ratios is not possible, since they widely overlap. XZn for fahlore I ranges between 0.04 and 0.68 (average of 0.41) and for fahlore II between < 0.01 and 0.77 (average of 0.40). Mercury contents reach up to 0.3 wt% and Se contents up to 0.2 wt%. All other elements are below 0.1 wt%.

XAg of fahlore from the UG ranges between 0.01 and 0.37 (average of 0.13) and for LG between < 0.01 and 0.06 (average of 0.01) (Fig. 6b). The molar XAs for fahlore in the UG and LG overlaps over a large range, but LG fahlore reaches lower XAs than the UG samples. The Bi content in both groups averages at 0.02 wt%. A separation of the UG and LG by XZn is not possible (Fig. 6c).

Polybasite-pearceite solid solution series ((Ag,Cu)16(As,Sb)2S11)

For the minerals of the polybasite-pearceite solid solution series, the term pearceite is used for molar XAs > 0.5 and polybasite for XAs < 0.5 (Bindi et al. 2007a). If Cu contents are > 4.0 atoms per formula unit (apfu), a structural change can be observed and the minerals are then named cupropolybasite and cupropearceite (Bindi et al. 2007b).

Polybasite-pearceite I and II can be clearly distinguished based on their molar XAg ratio (Fig. 6d). XAg for polybasite-pearceite I ranges between 0.63 and 0.87 (average of 0.72), for polybasite-pearceite II between 0.73 and 1.00 (average of 0.85). XAs ratios vary for polybasite-pearceite I between 0.46 and 0.89 (average of 0.70), for polybasite-pearceite II between 0.25 and 0.98 (average of 0.58). Most of our analyses (< 80%) belong to pearceite and cupropearceite with XAs between 0.5 and 0.98 and XAg between 0.63 and 0.98 corresponding to maximum copper contents of 19.0 wt% (Fig. 6d). Cupropolybasites were not observed. In examples of the polybasite-pearceite solid solution series from both LG and UG minor and trace element contents are consistently low with maximum Se contents of 0.2 wt%, Zn contents of 1.4 wt% and Bi contents of 0.5 wt%.

Pyrargyrite-proustite solid-solution series (Ag3(Sb,As)S3) and billingsleyite (Ag7(As,Sb)S6)

Minerals of the pyrargyrite-proustite series show variable XAs ranging from 0.01 to 0.37 with an average of 0.17 and are, therefore, Sb-dominated. Maximum Cu and Bi contents are 0.1 wt% and 1.5 wt%, respectively. All other trace elements are < 0.1 wt%. The chemical formula for billingsleyite (normalized to 14 apfu) is (Ag7.14Cu0.11)∑7.25(As0.74Sb0.10Bi0.01)∑0.84S5.90) (average of 7 analyses). Maximum Cu contents are 1.0 wt%, maximum Sb and Bi contents each 1.2 wt%. All other minor elements are < 0.1 wt% and/or below the element-specific detection limit.

Pyrite (FeS2)/marcasite (FeS2), chalcopyrite (CuFeS2), galena (PbS), matildite (AgBiS2), and arsenopyrite (FeAsS)

Galena contains variable amounts of Ag, Cu, Fe, Hg, Sb, and Bi. Silver contents reach up to 2.1 wt%, Cu contents up to 1.8 wt%, Bi contents up to 4.3 wt%, Fe contents up to 2.7 wt%, Sb contents up to 0.9 wt%, and Hg contents up to 0.2 wt%. All other minor element contents are < 0.1 wt%. The different galena generations are chemically indistinguishable. Arsenopyrite shows variable molar As/S ratios ranging from 0.62 to 0.92. Co contents are generally low with maximum contents of 0.4 wt%. Se occurs in all analyzed grains between 0.1 and 0.2 wt%. The concentrations of other minor elements is generally below 0.1 wt% and/or below the element specific detection limit. Chalcopyrite contains Ag as major impurity with average contents of 0.2 wt% and a maximum of 2.7 wt%. As and Sb contents reach 0.9 and 1.0 wt%, respectively. Pyrite commonly shows As contents up to 7.4 wt%, Ni and Co contents up to 2.0 wt%, and Cu contents up to 4.0 wt%. Matildite contains Pb, Cu, and Fe as major impurities with average contents of 3.2, 1.3, and 0.8 wt%, respectively.

Famatinite-luzonite (Cu3(As,Sb)S4) solid-solution series, enargite (Cu3AsS4), covellite (CuS) and native silver (Ag)

As can be seen in Fig. 6e, famatinite-luzonite shows an extensive exchange of As and Sb, with molar XAs (As/As+Sb) varying between 0.02 and 0.98. Most important minor elements are Ag with average contents of 3.1 wt% and Fe with average contents of 0.51 wt%. Enargite is clearly As-dominated with maximum Sb contents of 1.74 wt% (Fig. 6e). Silver contents are up to 0.9 wt%, all other elements are below 0.1 wt%. Native silver contains on average 0.3 wt% and a maximum of 0.7 wt% Cu. All other element concentrations remain < 0.1 wt%. The most important trace element in covellite is silver with average contents of 2.1 wt%.

Fluid inclusion characteristics of the “silverspar”-related gangue minerals

Fluid inclusions (FI) in fluorite (n = 71) and quartz (n = 61) cogenetic to sulphide mineral assemblages in the “silverspar” were studied. FI-assemblages contain primary (p), pseudosecondary (ps) and isolated (iso) inclusions (For exemplary FI petrography see Fig. 1 in ESM). Primary FI are situated on growth zones of the host minerals and typically show angular shapes and are of small size (< 20 μm in diameter). Pseudosecondary inclusions were recognized on sealed cracks strictly bordered by the grain boundaries of a host mineral generation; they typically have rounded shapes and are larger in size (up to 70 μm in diameter) than the primary FI. Numerous secondary mono-phase aqueous inclusions were recognized in all gangue minerals. These were not investigated as part of this study.

Both primary and pseudosecondary FI show high-salinity aqueous fluids (Fig. 7), which freeze between − 70 and − 100 °C. First melting can be detected above − 50 °C implying a ternary NaCl-CaCl2-H2O system with a eutectic temperature of − 52.0 °C. Ice and hydrohalite are last-dissolving phases (for data plotted in the ternary NaCl-CaCl2-H2O diagram see Fig. 2 in ESM). The final melting temperature of ice is in the range of − 15.9 °C to − 33.0 °C, of hydrohalite between − 14.7 and − 41.0 °C, which records a salinity of 18.7 to 26.7 wt% (NaCl+CaCl2). Uncorrected homogenization temperatures vary from 70 to 250 °C. Calculated Ca/(Ca + Na) molar ratios of the fluids vary between 0.21 and 0.82. Within single FIAs (independent of their fluid petrographic position), salinity and Th are almost constant, but they vary significantly between different trails within one sample. The degree of fill is constant at 0.95. As Fig. 7 shows, fluorite III and quartz III have lower salinities (18.7–20.9 wt% NaCl+CaCl2) compared to FI in fluorite I/II and quartz I/II of the earlier stages (23.1–26.7 wt% NaCl+CaCl2). Furthermore, the uncorrected homogenization temperatures of fluorite III are much higher (190–250 °C) than those of the earlier fluorite I/II and quartz I/II (70–160 °C) and also of quartz III (90–100 °C).
Fig. 7

Uncorrected homogenization temperatures versus salinity (NaCl+CaCl2) for FI of different quartz (triangles) and fluorite (circles) generations (S1, S2, S3) of the “silverspar”

Discussion

Depth zonation of the “silverspar” and polyphase redistribution of silver within the vein

Fahlore I is the first silver-bearing sulfide in the LG of the Clara vein system. In the UG, it is very rare or missing and enargite occurs instead (Fig. 8a). The presence of As5+ in enargite compared to As3+ in fahlore indicates that an oxidation process during fluid ascent could be responsible for this mineralogical zoning. The calculated predominance fields (Fig. 9) support this assumption as they show that oxidation (e.g., at neutral pH) stabilizes enargite instead of tetrahedrite (see also Eq. 1 in Table 3).
Fig. 8

Schematic illustration of the vein formation of the “silverspar” showing the different paragenetic stages S1 (a); S2 (b); S3 (c); S4 (d); S6 (e) over a depth profile of 700 m (note that S4 is not pictured). Labeled, blue arrows mark fluids involved in the formation of the different stages. Black bars beside the legend show the schematic sulfide portion, Ag-contents, and Pb-contents of the “silverspar” over depth. For further explanation of the paragenetic stages, see text

Fig. 9

a log fO2(g)-pH dependent predominance diagram of enargite (Cu3AsS4) and tetrahedrite (Cu12Fe2Sb4S13) and aqueous species. Blue arrow shows qualitative fluid evolution during paragenetic stage S1. As input parameters, following values were used: T = 120 °C; P = 300 bar (Note: pressure only used for calculation of water stability); log aCu2+ = 10−5; log fS2 = 10−17; log aFe2+ = 10−4; log aSb(OH)3 = 10−4; log aH2AsO4- = 10−6; log aCl- = 10−1. b log f(O2)-pH dependent predominance diagram of Ag-polybasite (Ag16Sb2S11), Ag-tetrahedrite (Ag12Fe2Sb4S13), Ag-Cu-polybasite (Ag8Cu8Sb2S11), tetrahedrite (Cu12Fe2Sb4S13), and aqueous species. Blue arrow shows qualitative fluid evolution during paragenetic stage S2 and S3. As input parameters, following values were used: pH = 6; log fO2(g) = −55; T = 120 °C; P = 300 bar (Note: pressure only used for calculation of water stability); log fS2 = 10−17; log aFe2+ = 10−4; log aSb(OH)3 = 10−4; log aH2AsO4- = 10−6; log aCl- = 10−1

Table 3

Equations 1–4 used in the text

Equation 1

Cu7Ag3(Fe,Zn)As4S13 + 3H2S + 5Cu+ + 2H+ + 2O2 = 4Cu3AsS4 + Fe2+ + Zn2+ + 3Ag+ + 4H2O

fahlore I + fluid phase 1 = enargite + fluid phase 2

Equation 2

23Cu7Ag3(Fe, Zn)(As1Sb3)S13 + 148Cu+ + 43As(OH)3 + 37Fe2+ + 7Zn2+ + 184H2S + 7.5O2 = 30Cu9Ag1(Fe, Zn)(As2Sb2)S13 + 3Ag13Cu3As2S11 + 30CuFeS2 + 9Sb(OH)3 + 117H2O + 236H+

fahlore I + fluid phase 1 = fahlore II + polybasite-pearceite I + chalcopyrite I + fluid phase 2

Equation 3

4Cu3AsS4 + H2S + 3H2O + 3Fe2+ + Zn2+ = 2CuFeS2 + Cu10(Fe1Zn1)As4S13 + 1.5O2 + 8H+

enargite + fluid phase 1 = chalcopyrite I + fahlore II + fluid phase 2

Equation 4

Ag16As2S11 + 6Cu+ + O2 + 10H+ = 2Cu3AsS4 + 16Ag+ + 2H2O + 3H2S

pearceite + fluid phase 1 = luzonite + fluid phase 2

Since enargite does not incorporate silver in relevant amounts, the oxidation is believed to be the reason for the large scale silver zoning during the initial stage. A gradual silver depletion in an ascending fluid system seems to be unlikely, since there is no systematic decrease in silver contents of fahlore I.

The oxidation is probably due to the increasing portion of a more oxidized (meteoric) fluid from a sedimentary fluid aquifer during fluid mixing in the vicinity of the basement/cover unconformity, which is situated at about 800 m amsl. The shift in mineral stability seems to be more or less abrupt, since enargite and fahlore I do not occur in the same samples. However, higher sulfur fugacities also favor the presence of enargite instead of fahlore (Einaudi et al. 2003). Sulfur may be derived from the sediment-hosted fluid aquifer (Walter et al. 2017a, 2018, and references therein), again supporting the occurrence of enargite at shallower depth of the Clara vein system.

Sulfides of S2 are ubiquitous in both the UG and LG samples. In terms of quantity, they represent the main sulfide mineralization of the “silverspar.” Interestingly, the proportion of massive to low-volume banded sulfides clearly increases with depth, which would fit with the idea of an ascending fluid system precipitating sulfides and therefore depleting the system in metal(oids) on ascent. Alternatively, changes in the mixing process responsible for mineral precipitation (e.g., due to involvement of different aquifers or different proportions of the respective end members) may be invoked to explain this zonation in the amount of sulfides. In addition to the different amounts present in the “silverspar”, fahlore II shows a chemical division between the UG and the LG with regards to its silver content (Fig. 6b). This is most likely caused by the pre-enrichment of silver in the LG by fahlore I (Fig. 8b), which is replaced by chalcopyrite I, polybasite-pearceite I, and fahlore II with moderate Ag-contents (see Eq. 2 in Table 3; comprising the stoichiometry of the analyzed sulfides). In the UG, where fahlore I is missing, a silver-poor association of chalcopyrite I and silver-poor fahlore II occurs.

The equation shows that the fluid leading to the replacement of fahlore I in the LG had to introduce Cu, Zn, Fe, As, and S, under the assumption that silver behaves conservatively. High Cu activity during the replacement process is also reflected by the high Cu contents in polybasite-pearceite I (Fig. 6d). We suggest a similar fluid with high Zn, Fe, and S-activity for the replacement of enargite by chalcopyrite I and silver-poor fahlore II in the UG (Eq. 3 in Table 3; Fig. 8b)

In the LG, Cu-rich polybasite-pearceite I is typically overgrown by fahlore II with low to moderate Ag-contents. This texture is interpreted to reflect the evolution of a fluid with high Ag activity (by the dissolution of fahlore I) to a fluid with lower Ag activity due to the precipitation of polybasite-pearceite I. This interpretation is supported by the calculated predominance fields (Fig. 9b) as they show that during increasing Cu activity (see arrow) Ag-rich fahlore is not stable anymore and Cu-rich polybasite will form instead. Due to polybasite precipitation, the system will develop towards lower Ag-activity ending up with Ag-poor fahlore. Note that in some LG samples, polybasite-pearceite I is missing and fahlore II shows relatively high Ag contents. This observation probably records too high Cu activity to stabilize polybasite-pearceite.

During S3, the local replacement of Cu-rich polybasite I and fahlore II by Cu-poor polybasite II and Ag-rich galena III (rarely accompanied by jordanite, stephanite, diaphorite, and geocronite) records the presence of a fluid with high Ag and Pb activity (Fig. 8c). The subordinate appearance of galena during S1 and S2 makes a redistribution of earlier (Pb)-sulfides unlikely and renders an influx of a new metal-bearing fluid more probable. The subsequent replacement of polybasite-pearceite II and I by pyrargyrite-proustite (which have the lowest Cu contents of all observed phases) again supports a significant increase of the fluid Ag activity during this stage. The calculated predominance diagram of Fig. 9b shows that due to increasing Ag-activity (see arrow) Cu-rich polybasite and fahlore are not stable anymore and Ag-rich polybasite will form instead. The replacement of primary fahlore by polybasite-pearceite and pyrargyrite-proustite was also observed e.g., by Petrova and Ilieev (2007) in a Ag-Pb deposit in Bulgaria.

During S4, the earlier formed sulfides are partially dissolved and replaced by gangue minerals and no new sulfides form; silver is locally depleted during this process (Fig. 8d; see discussion below). Note that during paragenetic stage S5, the minerals of S1-S4 show no modification in terms of Ag- redistribution. Lastly, during S6, the Ag-sulfosalts underwent alteration to famatinite-luzonite (Fig. 8e). Equation 4 (Table 3) shows that this process requires the influx of relatively oxidized, Cu-rich fluids, which are probably related to the increasing influence of oxidized near-surface, possibly meteoric fluids.

Such fluids percolating in the uppermost parts of the vein system may be enriched in copper due to reaction with chalcopyrite or fahlore. Since famatinite-luzonite incorporates only minor amounts of silver, the released Ag re-precipitates as distinct silver minerals (native silver, acanthite) in the vicinity (see also Keim et al. 2016).

The source of silver for the Schwarzwald deposits are probably biotite and plagioclases since they carry up to 0.3 ppm and 0.5 ppm Ag respectively (Kortenbruck 2014). If these minerals become altered by fluids in the root zone of the hydrothermal vein systems, this Ag is probably mobilized.

Dissolution of sulfides

In paragenetic stage S4, fluids precipitate fluorite III and quartz III and dissolve existing sulfides of stages S2 and S3 (see Figs. 5k and 8d). Primary and pseudosecondary FI in these fluorite III and quartz III crystals show salinities of 18.7–20.9 wt% NaCl+CaCl2 and uncorrected homogenization temperatures of 180–250 °C, which are significantly different from the fluids responsible for the formation of the sulfides of S1 and S2 (~ 23.1–26.7 wt% NaCl+CaCl2 and 70–160 °C see above).

Walter et al. (2016, 2017a, and references therein) suggested the formation of the Jurassic-Cretaceous vein system in the Schwarzwald by mixing of relatively hot basement fluids (300–350 °C; Schwinn et al. 2006) with salinities around 21 wt% NaCl+CaCl2 with a cooler sedimentary cover-related fluid aquifer with salinities up to 28 wt% NaCl+CaCl2. This mixing process results in a typical temperature range of 60–180 °C (depending on the mixing ratio) and salinities between 23 and 26 wt% NaCl+CaCl2. This corresponds well to FI-analyses from the literature for the Clara fluorite vein on quartz and fluorite (Staude et al. 2010a; Pfaff et al. 2012) and in the gangue minerals related to the main sulfide precipitation in paragenetic stage S1 and S2 reveal homogenization temperatures between 70 °C and 160 °C and salinities between 23.1 and 26.7 wt% NaCl+CaCl2.

In contrast, the FI in fluorite III and quartz III (Fig. 7) show the lowest salinities (18.7–20.9 wt% NaCl+CaCl2 wt%) and the highest temperatures (up to 250 °C) reported for any Jurassic-Cretaceous vein in the Schwarzwald so far (50–180 °C, 20–28 wt% NaCl+CaCl2 (Walter et al. 2016)). A tentative explanation would involve a higher than usual proportion or even a pure basement brine with moderate salinity, which would not be in equilibrium with the sulfides (as shown by their dissolution).

The temperature decrease from reservoir temperatures (300–350 °C, Schwinn et al. 2006) to fluorite III (around 210 °C) to quartz III (around 90 °C) would reflect ascent and cooling of such a basement brine, which would explain the amount of observed quartz (e.g., Burisch et al. 2017b). Only small amounts of galena and matildite precipitated during this stage, indicating that most metals remained in the hot fluid. These metals may have been re-precipitated in parts of the vein system that were not sampled during this study.

Influence of the quartz main stage on the “silverspar”

During the paragenetic stages S3 and S5, a Pb-Ag-Bi-As-rich mineral assemblage overprints the “silverspar” locally; this assemblage is quartz-rich and crosscuts the mineralization stages of S1 and S2. The abundance of quartz suggests that this overprint could be related to the quartz main stage (“Diagonaltrum”; Fig. 1), the formation of which is associated to the NKF (Huck 1984). Other NKF-related deposits show similar mineral assemblages of Pb-Ag-Bi-(As) sulfosalts (Staude et al. 2010a) comprised of benjaminite, berryite, galena, gustavite, and matildite together with other Ag-Bi-Pb-Cu minerals. Several of these minerals (galena, matildite, emplektite, and berryite) have also been observed in S3 and S5 of the “silverspar”. Huck (1984) and Van der Heyde (2002) described chalcopyrite, fahlore, pyrite, and polybasite-pearceite as typical sulfides associated with the quartz main stage. Based on these mineralogical similarities of the quartz main stage with S3 and S5, the “silverspar” appears to have been locally overprinted by fluids from the quartz main stage. The local influence of these fluids lead to the spatially limited enrichment of Ag-Pb (S3) and As-(Bi) (S5), compared to the primary and the main stage (S1, S2). The enrichment especially of Bi in NKF-related mineral assemblages remains poorly understood (see also Staude et al. 2010a).

Formation of the banded barite-fluorite-sulfide texture

A striking feature of the “silverspar” is the alternating sequence of early euhedral-subhedral barite with little or no sulfides and interstitial fluorite with much larger amounts of sulfides (Fig. 3a). Barite forms during the mixing of a hot, Ba-bearing, sulfur-poor basement fluid with the sulfate-bearing sediment-related fluid aquifer (dissolution of evaporites like gypsum; Walter et al. 2018). This mixing process also leads to the precipitation of fluorite. In contrast to calcium, which is present in both fluids (carbonate dissolution, plagioclase alteration), fluorine is present in the basement fluid aquifer only (Bucher and Stober 2010; Burisch et al. 2016a; Walter et al. 2018). For the precipitation of the sulfides, an additional reducing agent like methane and/or H2S is needed (Markl et al. 2016; Burisch et al. 2017a; Walter et al. 2018).

The calculated predominance field diagram (Fig. 10) reveals that a relatively oxidized fluid system first precipitates barite and fluorite without sulfides. If chalcopyrite is chosen as a representative sulfide, the diagrams indicate that chalcopyrite stability is reached at an exemplary pH of 7 around a log fO2 of −45. At these conditions, barite + fluorite + chalcopyrite can precipitate. If reduction continues, barite is not stable anymore (at pH = 7 at log fO2 of −52) and fluorite plus chalcopyrite precipitate instead. Thus, the rare association of barite with sulfides and the common association of fluorite with sulfides can be explained by a fast reduction process during the influx of a reducing fluid (see e.g., Markl et al. 2016). For the multiple changes from barite to fluorite plus sulfides (in some parts of the “silverspar” more than 100 times, c. f. Fig. 3a), we suggest the following scenario: An ascending basement fluid first forms barite and fluorite without sulfides by fluid mixing with a sediment-related fluid. Due to the influx of a reducing fluid, barite becomes unstable and the association of fluorite + sulfides precipitates instead. A likely source of such a reduced fluid is the interaction of a fluid in with the (in parts) pyrite- and graphite-bearing host rocks. Calculations from Pfaff et al. (2012) support this assumption, since calculated oxygen fugacities for a fluid in equilibrium with the Clara Mine host rock gneisses are low (log fO2 ≤ −75.9). Methane as redox agent seems unlikely since no carbonates precipitated during the formation of the “silverspar”, which would be the case, if methane would be oxidized to CO2/HCO3 in a Ca-rich fluid system (see e.g., Burisch et al. 2017a). The recurrent banding of the “silverspar” may, therefore, be caused by the pulsed influx of a reducing fluid in a relatively oxidized system precipitating fluorite and barite.
Fig. 10

a pH-fO2(g) dependent predominance diagram of fluorite, barite, and chalcopyrite. Solid dark-gray line marks the limits of chalcopyrite stability, green dashed line of fluorite stability, and brown dashed line of barite stability. Black arrow symbolizes repeated reduction of an oxidized system by the influx of a reducing agent. Gray area marks oxidized, blue area a reduced fluid system. Input parameters are based on Burisch et al. (2017a) (T = 120 °C; P = 300 bar (Note: pressure only used for calculation of water stability); log aH4SiO4 = − 3; log aCu2+ = − 5; log aCl- = − 1; log aSO42- = − 3; log aCa2+ = − 2; log aBa2+ = − 2; log aFe2+ = − 5). All elements were speciated over pH and fO2. b Illustration of the alternating sequence including the pulsed reduction. Ox. = oxidation; Red. = reduction. c Photograph of the banded “silverspar”

Conclusions

The Cu-Ag mineralization hosted by the Clara vein system in the Central Schwarzwald, South Germany, shows a pronounced mineralogical and chemical zoning. During the first paragenetic stage (S1), an ascending fluid system with relatively high Ag/Cu ratios lead to a Ag-rich mineralization in the lower parts (below 450 m amsl) comprising fahlore I with up to freibergitic composition. This fluid became increasingly oxidized towards shallower depth of the vein and enargite became stable instead of fahlore. The typical, meter-thick alternating sequences of sulfide-free barite and sulfide-bearing fluorite reflect a recurrent influx of a reducing agent in a fluid system precipitating barite and fluorite.

The vertical zonation imprinted by the first hydrothermal stage was overprinted by a second hydrothermal stage (S2) that involved fluids with high Cu and Fe activity, leading to precipitation of chalcopyrite I, fahlore II with intermediate Ag contents and Ag sulfosalts in the lower parts of the vein, and fahlore with low Ag contents and no discrete Ag sulfosalts in the upper parts. Fluid inclusion temperatures reveal that the primary silver mineralization (S1) and the following main sulfide stage (S2) formed at temperatures between 70 and 160 °C and from fluids with high salinities, as they are typical of the Jurassic-Cretaceous veins in the Schwarzwald. The local overprint by Ag-Pb and/or As-Bi fluids (S3, S5) is likely to be connected to Ag-Bi-Pb mineralization related to the large-scale North Kinzigtal fault system (NKF). A further hydrothermal stage (S4) is related to the influx of a hot (190–250 °C) undiluted basement fluid; this is expressed by paragenetically late fluorite + quartz mineralization and partial dissolution and replacement of pre-existing sulfides. Lastly, the Ag-sulfosalts underwent alteration by a Cu-rich, more oxidized fluid (S6), which released Ag and re-precipitated it as native metal or acanthite in the vicinity. This may be related to the exhumation of the vein system during the Paleogene (see also Burisch et al. 2018).

Despite profound mineralogical changes, large-scale silver zonation of the first stage (S1) appears to have been preserved through successive hydrothermal overprints of the vein system at the Clara Mine (S2-S6). Based on these findings, a similar enrichment of silver towards deeper parts of other hydrothermal veins of the Central and Northern Schwarzwald may be expected, where similar vein systems of similar age and architecture have been less deeply eroded and mined. More generally speaking, the study illustrates, how metal tenor and mineralogy may remain decoupled in vertically extensive, polyphase hydrothermal vein systems. This may be pertinent to similarly zoned hydrothermal vein systems elsewhere.

Notes

Acknowledgments

We are grateful to T. Wenzel and T. Theye for their friendly assistance during EMP analysis, S. Staude, U. Kolitsch, and K. Huck for their helpful input during discussions and interpretation of the results, and S. Schafflick for the professional sample preparation. This work is a contribution of the r4 project “ResErVar—Ressourcenpotential hydrothermaler Lagerstätten der Varisziden” funded by the German Ministry of Education and Research (BMBF; Project reference number 033R129E).

Supplementary material

126_2018_799_Fig11_ESM.gif (184 kb)
Fig. 1

ESM Photographs, showing the exemplary FI petrography including (a) Primary FI in quartz (b) Isolated and primary FI in quartz and (c) Isolated FI in fluorite. (GIF 183 kb)

126_2018_799_MOESM1_ESM.tif (10.5 mb)
High resolution image (TIFF 10768 kb)
126_2018_799_Fig12_ESM.gif (41 kb)
Fig. 2

ESM Ternary H2O-NaCl-CaCl2 phase diagram including the measured FI as circles. (GIF 40 kb)

126_2018_799_MOESM2_ESM.tif (1.4 mb)
High resolution image (TIFF 1403 kb)
126_2018_799_MOESM3_ESM.xlsx (162 kb)
ESM 1 (XLSX 162 kb)
126_2018_799_MOESM4_ESM.docx (17 kb)
ESM 2 (DOCX 17 kb)
126_2018_799_MOESM5_ESM.xlsx (40 kb)
ESM 3 (XLSX 39 kb)
126_2018_799_MOESM6_ESM.docx (29 kb)
ESM 4 (DOCX 28 kb)

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  • Maximilian F. Keim
    • 1
    Email author
  • Benjamin F. Walter
    • 1
  • Udo Neumann
    • 1
  • Stefan Kreissl
    • 1
  • Richard Bayerl
    • 2
  • Gregor Markl
    • 1
  1. 1.Mathematisch-Naturwissenschaftliche Fakultät, Fachbereich GeowissenschaftenUniversität TübingenTübingenGermany
  2. 2.StuttgartGermany

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