Economic Geology Volume 116, Number 7

Economic Geology, v. 116, no. 7, pp. 1649–1667

Spatial and Temporal Evolution of the Freiberg Epithermal Ag-Pb-Zn District, Germany

Laura J. Swinkels,1,† Jan Schulz-Isenbeck,1 Max Frenzel,2 Jens Gutzmer,1,2 and Mathias Burisch1,2 1TU Bergakademie Freiberg, Institute of Mineralogy, 09599 Freiberg, Germany 2Helmholtz-Zentrum Dresden-Rossendorf, Helmholtz Institute Freiberg for Resource Technology, 09599 Freiberg, Germany

Abstract The Freiberg district hosts one of the largest series of epithermal polymetallic vein deposits in Europe. The availability of a systematic collection of historical samples provides an excellent opportunity to study the anatomy of these epithermal systems. Detailed petrographic investigations, geochemical analyses, and fluid inclusion studies were conducted on several vertical profiles within the Freiberg district to decipher mineralogical and geochemical zoning patterns. Six distinctive mineral associations have been recognized within the Frei- berg epithermal veins; sphalerite-pyrite-quartz and galena-quartz±carbonate associations are most abundant in the central sector, as well as in the deepest sections of veins on the periphery of the district. A high-grade sphalerite-Ag-sulfides-carbonate association occurs laterally between the central and peripheral sectors and at intermediate depth in veins on the periphery. Shallow and peripheral zones are dominated by an exceptionally Ag-rich Ag-sulfides-quartz association, whereas the shallowest veins locally comprise Ag-poor stibnite-quartz and quartz-carbonate associations. Fluid inclusion assemblages returned low salinities (<6.0 wt % NaCl equiv), and homogenization temperatures successively decrease from ~320°C associated with the proximal and deep sphalerite-pyrite-quartz association, to ~170°C related to the distal and shallow Ag-sulfides-quartz association. The architecture of the Freiberg district is related to the temporal and spatial evolution of magmatic- hydrothermal fluid systems, including boiling and concomitant cooling, as well as CO2 loss. Constraints on the paleodepth indicate that the veins formed between 200 and 1,800 m below the paleowater table. High-grade Ag ore occurs over a vertical interval of at least 500 m and is bracketed by shallower stibnite-quartz and barren quartz, and deeper base metal-sulfide-quartz zones.

Introduction torical mining operations in the geoscientific collection of the Intermediate sulfidation Ag-Pb-Zn epithermal systems are TU Bergakademie Freiberg, the Freiberg district serves as an a major source of Ag and also contain economic amounts of excellent example to study the anatomy of such polymetallic Au, Zn, Pb, and Cu (Simmons et al., 2005). Many of the well- epithermal vein systems. known examples of this particular ore deposit type are located About 5,600 t (180 Moz) of Ag were produced in the Frei- in the Sierra Madre Occidental of Mexico, e.g., Fresnillo, Tay- berg district during at least 800 years of historical mining, oltita, and Pachuca-Real del Monte (Simmons, 1991; Albin- starting in 1168 and continuing to 1969 (Baumann et al., son et al., 2001; Camprubí and Albinson, 2007), with similar 2000), making it to one of the most significant silver resources epithermal deposits occurring in Peru (Petersen et al., 1977; in Europe. Silver and base metal mineralization in the dis- Candiotti de los Rios et al., 1990; Baumgartner et al., 2008; trict is mainly related to N-S– to NE-SW–striking polyphase Rottier et al., 2018) and Bolivia (Phillipson and Romberger, magmatic-hydrothermal veins, which are hosted by gneiss and 2004; Arce Burgoa, 2009), as well as in Spain (Concha et al., mica schist (Fig. 1; Müller 1901; Bauer et al., 2019a; Burisch 1992), Australia (Oliver et al., 2019), and elsewhere (Sillitoe et al., 2019a). and Hedenquist, 2003). Many of these deposits have a dis- Historical mining operations focused on the central part of tinct vertical and lateral zoning, which includes high-grade the district, in the immediate vicinity of the towns of Frei- Ag zones at shallow to intermediate depth (100–1,000 m) that berg and Brand-Erbisdorf. The peripheral parts of the dis- systematically grade into more base metal-rich sulfide veins trict, such as Bräunsdorf and Kleinvoigtsberg, saw less de- with increasing depth (Albinson et al., 2001; Simmons et al., velopment. Although the silver grades on the periphery were 2005; Camprubí and Albinson, 2007; Oliver et al., 2019). exceptionally high (1–4 kg/t; Müller, 1901), mining generally While these overall vertical trends appear to be characteristic ceased earlier (1860–1880), and most of the historical opera- for such Ag-Pb-Zn vein systems, detailed spatial and temporal tions were smaller and shallower compared to those in the district- and vein-scale zoning is typically not well constrained. center (Baumann, 1965). The Freiberg Ag-Pb-Zn district, Erzgebirge, Germany, has Most of the scientific concepts of the Freiberg district date only recently been identified as an example of an Ag-Pb-Zn back to the early work of Müller (1850, 1901) and von Cotta epithermal system (Burisch et al., 2019a). Because of the (1855, 1870), whereas the later studies of the mid- and late- th availability of numerous well-documented samples from his- 20 century focused on generic classification schemes, without producing significant advances in the genetic understanding of the mineral systems. As a consequence, the genesis remained †Corresponding author: e-mail, [email protected] poorly constrained until a suite of recent studies demonstrated

ISSN 0361-0128; doi:10.5382/econgeo.4833; 19 p. Digital appendices are available in the online Supplements section. 1649 Submitted: August 21, 2020 / Accepted: January 7, 2021

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Fig. 1. Overview map of the study area. A) Location of the Erzgebirge at the border between Germany (DE) and the Czech Republic (CZ). B) Simplified geologic map of the Erzgebirge after LfULG (1994). C) Simplified geologic map of the Freiberg district based on Hoth et al. (1980), with ATVC = Altenberg-Teplice Volcanic Complex, DB = Döhlen basin, EG = Eiben- stock granite, NG = Niederbobritscher granite, TWVC = Tharandter Wald Volcanic Complex. Known hydrothermal veins are indicated as red lines.

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that the bulk of the Ag-Pb-Zn ore of the Freiberg district is re- terozoic composite gneiss unit, comprising biotite-plagioclase lated to magmatic-hydrothermal activity (Bauer et al., 2019a; orthogneiss (locally referred to as lower gray gneiss) and bi- Burisch et al., 2019a) of Permian age (Ostendorf et al., 2019). otite-muscovite-plagioclase paragneiss (locally referred to as However, these recent studies either included only a small upper gray gneiss; Tichomirowa et al., 2012). The gneiss units number of samples or were restricted to individual deposits form an ellipsoid-shaped (dome-like) body, which is over- within the Freiberg district and thus did not take the district- thrusted by mica schists in the northwest and phyllites in the scale architecture of the mineralizing systems into consider- north and northeast and bordered by other gneiss units in the ation. This is the focus of the present investigation. south and west (Fig. 1C). In the north, gneiss, mica-schist, Three vertical profiles along well-mineralized veins in dif- and phyllites are locally alternated with metagabbro, serpen- ferent parts of the Freiberg district were investigated in or- tinite, and amphibolite schist (Baumann, 1965; Baumann et der to constrain vertical zonation along the profiles, down to al., 2000). a depth of 560 m below the present-day land surface. Ana- East of the town of Freiberg, the gneiss units were intruded lytical techniques used include detailed petrographic and by the late Variscan (ca. 325–320 Ma) Niederbobritzscher fluid inclusion studies and multielement geochemical assays. biotite granite (Tichomirowa, 1997). The granite is accompa- This data set, complemented by data from previous studies, nied in the east by a rhyolitic unit of the Tharandter Wald allows the vertical and lateral zonation as well as the para- Volcanic Complex (ca. 320 Ma; Breitkreuz et al., 2009). Nu- genetic evolution of the district to be constrained, providing merous rhyolite/microgranite and lamprophyre dikes crosscut important information for exploration targeting within the the metamorphic units in the area (Müller, 1901; Baumann, district, as well as insights into Ag-Pb-Zn epithermal systems 1965; von Seckendorff et al., 2004; Abdelfadil et al., 2014). in general. Hydrothermal mineralization in the Freiberg district Background Three fundamentally different types of hydrothermal veins have been recognized in the Freiberg district: (1) epither- Regional geology mal polymetallic sulfide-quartz-carbonate veins, (2) fluorite- The Erzgebirge metallogenic province (Fig. 1B) forms the barite-quartz-Pb-Zn veins, and (3) less abundant five element northern tip of the Bohemian Massif, part of the Variscan (Bi-Co-Ni-Ag-As) veins (Müller, 1901; Baumann et al., 2000; orogen in central Europe. The Variscan orogen resulted from Bauer et al., 2019a; Burisch et al., 2019a; Ostendorf et al., the collision of Gondwana and Laurussia between 400 and 2019). This study focuses only on the economically dominant 340 Ma (Kroner et al., 2010). The Erzgebirge consists of a polymetallic sulfide-quartz-carbonate veins, which are prob- diverse suite of metamorphosed nappe stacks, with Cadomian ably related to Permian magmatic-hydrothermal activity (276 and Paleozoic protoliths forming a large SW-dipping anticline ± 16 Ma; Ostendorf et al., 2019). The fluorite-barite and na- (Romer et al., 2010; Rötzler and Plessen, 2010). The meta- tive metal-arsenide veins are significantly younger and have morphic units were subsequently intruded by syn- to late- been tentatively associated with the opening of the northern collisional granitoids between 336 and 315 Ma, followed by Atlantic (Ostendorf et al., 2019). postcollisional bimodal magmatism (305–270 Ma; Kroner et Polymetallic epithermal mineralization in the Freiberg al., 2010; Hoffmann et al., 2013; Kroner and Romer, 2013; district occurs in steeply dipping N-S– and NE-SW–trend- Zhang et al., 2017). The late Carboniferous and early to mid- ing veins that are hosted by gneiss, mica-schist, metagabbro, dle Permian were characterized by intense rifting, volcanism, and less commonly phyllites. Mineralization can typically be and basin formation (e.g., Döhlen basin; Gaitzsch et al., 2010; traced over large vertical extents (>1 km; Kraft and Tischen- Schneider and Romer, 2010). The youngest record of Perm- dorf, 1960). Three mineral associations have historically been ian silica-rich volcanism is dated at ~270 Ma (Schneider and distinguished within the epithermal Ag-Zn-Pb veins of the Romer, 2010; Hoffmann et al., 2013). Continued subsidence Freiberg district: (1) a base metal-sulfides-quartz association, during the Mesozoic led to burial of the basement units un- referred to as “Kiesige Bleierzformation” (kb), comprising derneath thick sedimentary sequences (Ziegler, 1990). Even- mainly sphalerite, galena, arsenopyrite, pyrite, pyrrhotite, and tually, in the Cenozoic, the formation of the Eger Graben chalcopyrite, (2) a sphalerite-Ag-sulfides-carbonate associa- rift resulted in the exhumation of the Variscan basement and tion (“Edle Braunspatformation”; eb) with sphalerite, galena, associated hydrothermal deposits (Ziegler, 1990; Ziegler and fahlore, and silver sulfosalts, and (3) an Ag-sulfides-quartz as- Dèzes, 2007). sociation (“Edle Quarzformation”; eq) with abundant silver The Erzgebirge is host to numerous types of ore deposits sulfosalts, acanthite, arsenopyrite, pyrite, galena, sphalerite, (e.g., Baumann et al., 2000; Haschke et al., 2021; Reinhardt and Sb sulfides (Müller, 1901). et al., 2021; Guilcher, in press). Most prominent among these Individual veins commonly comprise multiple generations are magmatic-hydrothermal deposits such as skarns (Sch- of vein infill that may represent several distinct mineral as- uppan and Hiller, 2012; Bauer et al., 2019b; Burisch et al., sociations. The predominant association, however, varies 2019b; Korges et al., 2020; Reinhardt et al., 2021), greisens systematically on the district and vein scale (Müller, 1901; (Štemprok, 1967; Zhang et al., 2017; Korges et al., 2020), and Burisch et al., 2019a). The base metal-sulfides-quartz asso- epithermal veins (Bauer et al., 2019a; Burisch et al., 2019a). ciation is the dominant vein fill in the central part of the district. The sphalerite-Ag-sulfides-carbonate association is most District geology prominent at the historical mining camps of Brand-Erbisdorf The Freiberg district is located in the northeastern part of the and Kleinvoigtsberg, 6 km south and 10 km north of the town Erzgebirge (Fig. 1). The predominant lithology is a Neopro- of Freiberg, respectively (Fig. 1; Müller, 1901; Burisch et

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al., 2019a). The Ag-sulfides-quartz association prevails in the found in the contact zone between gneiss and schist and tend peripheral sectors and shallow vein sections (Müller, 1901; to be less continuous along strike and dip than in the cen- Burisch et al., 2019a). Veins that comprise Ag-sulfides-quartz tral sector; they frequently split or pinch out and locally form infill commonly also contain discrete Sb sulfides, such as stib- stockwork-like swarms of veinlets. The shallow levels of the nite and berthierite, which typically occupy a shallower posi- veins in this sector are dominated by an Ag-sulfides-quartz as- tion in the veins than the major Ag mineralization (Burisch sociation, whereas at deeper levels the sphalerite-Ag-sulfides- et al., 2019a). The intensity of host-rock alteration is variable carbonate and the base metal-sulfides-quartz associations pre- and mostly characterized by silicification and sericitization vail (Müller, 1901; Baumann, 1965; Baumann et al., 2000). (muscovite) with some disseminated pyrite, arsenopyrite, ga- The Alte Hoffnung Gottes mine near Kleinvoigtsberg was lena, and rarely chlorite (Rösler and Kühne, 1970). However, the economically most significant mine in the northern sec- a comprehensive study on host-rock alteration in the Freiberg tor and was mined to depths of 560 m below surface (Müller, district has never been conducted. 1901; Baumann, 1965). The area of Reinsberg comprises sev- Geochronologic, petrographic, geochemical, and micro- eral small mining camps with the Emanuel mine, which was thermometric observations indicate a magmatic-hydrother- mined to depth of 310 m below surface, as the most significant mal origin of much of the Freiberg epithermal district, af- operation (Müller, 1901; Baumann, 1965). filiated with early Permian magmatism (Bauer et al., 2019a; Western sector: The Neue Hoffnung Gottes mine close Burisch et al., 2019a; Ostendorf et al., 2019). However, a to the town of Bräunsdorf was the most important opera- causative intrusion has not yet been identified. Attempts to in- tion in the western sector (mined down to 290 m below the tersect a potential intrusion by drilling during Sn exploration surface). Similar to the northern sector, hydrothermal veins campaigns in the 1950s and 1970s were unsuccessful as all strike mainly northeast-southwest. In contrast to other sec- five drill holes with final lengths of 1,110, 1,317, 1,745, 1,826, tors, veins are often hosted by what has been described as a and 1,061 m, respectively, did not intersect an intrusive body. graphite-rich schist unit (Müller, 1901; Baumann, 1965; Bau- Notably, abundant veins with base metal-rich mineralization mann et al., 2000; Burisch et al., 2019a). Within the ~300-m were still present at the final depths of these drill cores (Kraft vertical profile of the Neue Hoffnung Gottes mine, a distinct and Tischendorf, 1960; Krutak, 1980). vertical zoning has been recognized, which includes a shallow Sb quartz cap grading into Ag-sulfides-quartz association with Sectors of the Freiberg district increasing depth (Burisch et al., 2019a). The Freiberg district has been subdivided into five sectors Southern and eastern sectors: Historical mining opera- based on geography (Fig. 1C) and distinct mineralogical and tions were much smaller in the southern and eastern sectors geochemical variations (Baumann et al., 2000). In the follow- of the Freiberg district. Veins are characterized by abundant ing, background information on the most important mines Ag-sulfides-quartz mineralization. An increase of base metals and characteristics of the ore deposits of each sector are brief- at depth is reported and is locally associated with economic ly introduced. Cu and Sn mineralization (Müller, 1901; Baumann, 1965). Central sector: The central sector, including the towns of The major operations in the southern sector were the Fried- Freiberg and Brand-Erbisdorf, comprises the deepest and rich August and Friedrich Christoph mines, southeast of the largest underground mines of the district (down to 600 m town of Frauenstein. These operations targeted an ~200- × below surface). Abundant N-S– to NE-SW– and E-W–strik- 2,000-m swarm of N-S–striking veins. The operations were ing hydrothermal veins form a dense fracture network with limited to depths of 170 m below surface (Müller, 1901). individual veins traceable over ~5 km along strike. Vein thickness may reach up to 4 m but usually ranges between Methods 0.1 and 0.8 m (Müller, 1901). The base metal-sulfides-quartz association is the most prominent vein infill in the central Sampling sector (e.g., Himmelfahrt mine, Freiberg) and is associated For this study, 152 samples were selected from the geosci- with elevated concentrations of Cu, Sn, and In (up to 71,000, entific collection of the TU Bergakademie Freiberg (App. 13,000, and 1,560 g/t, respectively; Müller, 1901; Seifert and Table A1), since most of the historical mines are no longer Sandmann, 2006). In the southern part of the central sec- accessible. Because comprehensive sample descriptions and tor (Brand-Erbisdorf) the sphalerite-Ag-sulfides-carbonate historical mine plans are available for a large number of the association prevails, commonly crosscutting or coating the samples, localities can be reconstructed in detail (±10 m ac- paragenetically older base metal-quartz association (Müller, curacy). Samples for this study were mostly collected from 1901). Because of the dominance of the sphalerite-Ag-sul- three vertical profiles (Fig. 2), located at the Himmelfahrt fides-carbonate association, the Himmelsfürst mine (Brand- mine, Freiberg (profile 1), in the central sector, the Alte Hoff- Erbisdorf) was one of the economically most significant nung Gottes mine, Kleinvoigtsberg (profile 2), and the Eman- mines of the entire Freiberg district (Müller 1901; Seifert uel mine, Reinsberg (profile 3), both in the northern sector. and Sandmann, 2006). Descriptions and data from a vertical profile near Bräunsdorf Northern sector: The northern sector includes the histori- (western sector), recently published by Burisch et al. (2019a), cal mining camps of Kleinvoigtsberg, Großvoigtsberg, Ober- are also considered in this study. The limited availability of gruna, Siebenlehn, Reinsberg, and Mohorn (Fig. 1C). Hy- well-documented samples in the eastern and southern sectors drothermal veins mainly strike northeast-southwest, can be of the district did not allow for the compilation of systematic traced along strike for up to 2 km, and have thicknesses be- profiles; thus, fewer samples from these areas are included in tween 0.1 and 4 m. The veins in the northern sector are often this study.

P1 P2 P3 Petrography

400 m Seventy-six doubly polished and 46 single-polished sections (200–250 μm thick) were produced at the sample preparation laboratories of the Helmholtz Institute Freiberg for Resource Technology. Mineral identification and petrography, including 300 m ER characterization of textures and paragenesis, were done using FG FG FG a Carl Zeiss Axio Imager M1m light microscope (transmitted WI FG and reflected light), equipped with an AxioCamMRc5 camera NG WA CH 200m RG for documentation. Mineral identification was complemented GL CH CH RG by scanning electron microscopy using a FEI Quanta 650F GL NG GL GL RG instrument. Both instruments used are located at the Depart- ment of Mineralogy, TU Bergakademie Freiberg (TUBAF). FG 100 m PS RG CR GL PS Geochemical analysis RG CH PS Seventy-seven samples were analyzed for their geochemi- 0m ER PS cal whole-rock composition at Activation Laboratories Ltd. (Actlabs; Ancaster, Canada). The samples analyzed included all mineralogical varieties of vein infill from across the different sectors of the Freiberg district. Between 50 and -100 m KA 500 g were ground to analytical fineness (95%, <105 µm) and analyzed for major, minor, and trace elements using instru- mental neutron activation analysis (INAA; Au, Ag, As, Ba, KA KA PS -200 m GL CA HS Br, Ca, Co, Cr, Cs, Fe, Hf, Hg, Ir, Mo, Na, Ni, Rb, Sb, Sc, Se, Sn, Sr , Ta, Th, U, W, Zn, La, Ce, Nd, Sm, Eu, Tb, Yb, Lu), CH HS Au fire assay, inductively coupled plasma-optical emission spectroscopy (ICP-OES), and inductively coupled plasma- Fig. 2. Overview plan of the samples used for vertical profile 1 (P1), profile 2 (P2), and profile 3 (P3). Profile 1 is located directly below the city of Frei- mass spectrometry (ICP-MS; digestion by sodium peroxide berg (P1) and covers the following veins: Gottlob Morgengang (GL), Erzen- fusion; Al, As, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, gel Stehender (ER), Wagfort Spat (WA), Wilhelm Stehender (WI), Christian Dy, Er, Eu, Fe, Ga, Gd, Ge, Ho, Hf, In, K, La, Li, Mg, Mn, Stehender (CR), Karl Stehender (KA), and Caspar Nord (CA). Profile 2 is Mo, Nb, Nd, Ni, Pb, Pr, Rb, S, Sb, Se, Si, Sm, Sn, Sr, Ta, located near the town of Kleinvoigtsberg (P2) and comprises the following Tb, Te, Th, Ti, Tl, Tm, U, V, W, Y, Yb, Zn). Information on veins: Peter Stehender (PS), Frisch Glück Stehender (FG), Christliche Hilfe Stehender (CH), Heinrich Stehender (HS), and Neuglück Stehender (NG). measurement parameters, standard materials used, and de- Profile 3 is located near Reinsberg and includes samples exclusively from the tection limits are provided as supplementary material (App. Reinsberger Glück Morgengang (RG). Table A2). The geochemical data set obtained for this study is complemented by whole-rock analyses (n = 82) of collec- Profile 1 consists of 15 samples from the Erzengel Ste- tion samples from the Freiberg district reported in Seifert hender, Wagfort Spat, Wilhelm Stehender, Gottlob Morgen- and Sandmann (2006). gang, Christian Stehender, Karl Stehender, and Caspar Nord Fluid inclusion analysis veins of the Himmelfahrt mine (Freiberg) between shaft Da- vid and shaft Rudolf (~1 km apart), covering a vertical interval Microthermometric analyses were performed on 19 doubly of 485 m (–185 to 300 m above sea level [a.s.l.]; Fig. 2). Pro- polished thin sections using an Olympus BX 53 microscope file 2 comprises 27 samples from the Peter Stehender, Frisch and a Linkam THMS 600 heating-freezing stage at the De- Glück Stehender, Christliche Hilfe Stehender, Heinrich Ste- partment of Mineralogy (TU Bergakademie Freiberg). Prior hender, and Neuglück Stehender veins of the Alte Hoffnung to microthermometric measurements, fluid inclusion assem- Gottes mine (Kleinvoigtsberg). The profile covers a vertical blages (FIAs) were petrographically classified as primary (P), transect of 520 m from –240 to 280 m a.s.l. Profile 3 includes secondary (S), pseudosecondary (PS), and clusters (C), ac- 13 samples of the Reinsberger Glück Morgengang vein of the cording to Goldstein (2001). Eutectic temperatures (Te), fi- Emanuel mine (Reinsberg), with the elevation ranging from nal melting temperatures of ice (Tm(ice)), and homogenization 68 to 210 m a.s.l., covering 142 vertical meters. temperatures (Th) were measured three times for each fluid Additional samples were selected from Freiberg (n = 4), inclusion upon heating. Results are reported as average values Brand-Erbisdorf (n = 24), Halsbrücke (n = 2), Kleinwalters- of the three individual heating runs for each inclusion. Inclu- dorf (n = 1), and Hohentanne (n = 1) in the central sector; sions affected by postentrapment processes and FIAs with Mohorn (n = 3), Großvoigtsberg (n = 10), Kleinvoigtsberg (n inconsistent Th (>20°C) but constant liquid-vapor ratios were = 23), Obergruna (n = 4), Burkersdorf (n = 1), Bieberstein (n omitted from the data set. The calibration of the stage was = 1), Siebenlehn (n = 2), Reinsberg (n = 1), Zella (n = 1), and checked daily by measuring synthetic Tm(CO2), Tm(ice), and Th in Dittmansdorf (n = 1) in the northern sector; Klingenberg (n H2O and H2O-CO2 fluid inclusion standards. The fluid salinity = 1) in the eastern sector; Frauenstein (n = 9) in the southern was calculated from the Tm(ice) according to Steele-MacInnis sector; and Bräunsdorf (n = 7) and Frankenstein (n = 1) in the et al. (2012). Liquid and vapor volume fractions were esti- western sector. mated for each fluid inclusion at room temperature.

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Seventeen fluid inclusions classified as primary, pseudosec- (Fig. 5A, G). The mineral paragenesis usually commences ondary, or cluster from different depths were analyzed with with fine-grained quartz I (~0.1–1 mm), followed by euhe- Raman spectroscopy at the Department of Geology at the TU dral arsenopyrite I, pyrite I, and dark sphalerite I (Fe-rich; Bergakademie Freiberg using a TriVista Raman spectrom- ~12 wt % Fe; Bauer et al., 2019a). Sphalerite I is typically an- eter with a 532-nm laser, 100-mW laser power, 100-µm slit hedral and usually includes fine-grained disseminated chal- width, and a 50× objective. The spectrometer was calibrat- copyrite and, less commonly, inclusions of galena and fahlore ed using the 520.7-cm–1 line of silicon. For each inclusion, (tetrahedrite). Quartz I, arsenopyrite, and pyrite also occur the measurement includes a z profile and individual spectra as impregnations in the host rock. Sphalerite I is overgrown for the vapor phase, the liquid phase, and the host mineral. by quartz II, which commonly forms large euhedral comb The acquisitions were performed using gratings of 500 and crystals (up to 30 mm long) exhibiting up to 0.5-mm-thick 1,500 lines/mm. The profile measurement ran at an integra- growth zones. Cracks and cavities in sphalerite I and quartz tion time of 200 s. Individual measurements were performed II are filled with galena, smaller amounts of fahlore, and rare at an integration time of 600 s accumulating one to six acquisi- carbonate minerals. tions. Identification of aqueous and gaseous compounds was Galena-quartz ± carbonate: Characteristic of the galena- done by comparing obtained Raman spectra with the data- quartz ± carbonate association are large abundances of galena base of Frezzotti et al. (2012) and references therein. as well as the presence of minor amounts of carbonate minerals and Ag minerals (Fig. 5B). Pyrite and sphalerite are also Results present, but in lesser amounts than in the sphalerite-pyrite- quartz association. Quartz II is the main gangue mineral; lo- Mineral associations cally quartz II is intergrown with minor acicular cassiterite, Six distinct mineral associations were recognized in the inves- predating the carbonate minerals (Fig. 5H). Galena I over- tigated sample suite (Table 1). The present classification dif- growths on quartz II commonly contain inclusions of silver- fers slightly from previous classification schemes. An overview rich fahlore (tetrahedrite-freibergite) and pyrargyrite, which of relationship between the different classification schemes is form elongate inclusions parallel to the cleavage planes of given in Table 1. Individual samples may contain several min- galena (Fig. 5I) or more massive aggregates (Fig. 5J). Carbon- eral associations superimposed (1) as gradual transition from ate minerals (carbonate I, including calcite, siderite, and an- one to another, (2) in sharp crosscutting veinlets, or (3) as kerite) typically postdate galena I and may form intergrowths breccias locally resulting in a complex vein architecture (Fig. with acanthite and polybasite. 3). The following petrographic descriptions provide a summa- Sphalerite-Ag-sulfides-carbonate: The abundance of car- ry of the characteristics of the six associations as observed in bonate minerals (calcite, siderite, ankerite, dolomite, Mn cal- the present study. A schematic paragenetic sequence combin- cite, and rhodochrosite) accompanied by Ag minerals is dis- ing all associations described below and their interrelation- tinctive for the sphalerite-Ag-sulfides-carbonate association ship is presented in Figure 4. and provides a sharp mineralogical contrast to the other asso- Sphalerite-pyrite-quartz: The oldest mineral associa- ciations (Fig. 3A). This association typically commences with tion, paragenetically, is observed in profiles 1 and 2. It is massive sphalerite II at the vein selvage or encapsulating brec- characterized by abundant pyrite, arsenopyrite, and sphal- ciated host-rock fragments (Fig. 5C). Macroscopically, sphalerite, accompanied by quartz as the major gangue mineral erite II has a black to dark-orange color and appears slightly

Table 1. Characteristic Minerals of the Associations Defining the Freiberg Epithermal System

Terminology Association Major ore minerals Minor ore minerals Major gangue Minor gangue Spatial distribution Müller (1901) Sphalerite- Sphalerite I (Fe-rich), Chalcopyrite, fahlore, Quartz I, II Center, kb pyrite-quartz arsenopyrite I, pyrite I cassiterite, stannite, periphery—deep hematite, jamesonite Galena-quartz Galena Pyrite I, fahlore, pyr- Quartz II Carbonate I Center, kb quartz ± carbonate argyrite, sphalerite, (primarily periphery—medium Base metal- acanthite calcite) and deep Sphalerite- Sphalerite II (Fe-rich), Pyrite II, chalcopyrite, Carbonate I (cal- Quartz II Center—shallow and eb Ag-sulfides- fahlore, pyrargyrite, galena, arsenopyrite II, cite, siderite, medium, carbonate acanthite miargyrite, stephanite, rhodochrosite, periphery—medium polybasite, native Ag ankerite) Ag-sulfides- Arsenopyrite III, pyrite Galena, chalcopyrite, Quartz III Carbonate II Center—shallow, eq quartz III, sphalerite III, boulangerite, freiesle- (ankerite, Mn Periphery—shallow pyrargyrite, fahlore, benite, cassiterite, calcite, rhodo- and medium polybasite acanthite, miargyrite, chrosite) native Ag, electrum Stibnite-quartz Stibnite, berthierite, jame- Sphalerite, arsenopyrite, Quartz III Carbonate II Periphery—shallow eq sonite, boulangerite pyrite, galena (Fe-rich) Quartz- Pyrite, arsenopyrite, Quartz IV, car- Periphery—shallow carbonate galena, sphalerite, bonate II electrum, pyrargyrite, hematite

A sphalerite-Ag-sulﬁdes-carbonate B CbI

sphalerite-pyrite-quartz

QzIII

SpII

Pyr

CbI

20 cm 1.5 cm

C Quart z I I D Q u a r t z SpI I I I bladedcb

Qz-Cb

Py CbII Gn

QzIII

galena-quartz-carbonate Ag-sulﬁdes-quartz sphalerite-Ag-sulﬁdes-carbonate 4 cm Apy E

CbI

5 cm 3 cm

Fig. 3. Representative hand specimens illustrating various mineral associations and their relationship to each other. A) Sample 63986 (Kleinvoigtsberg): sphalerite-pyrite-quartz with massive sphalerite (Sp), pyrite (Py), and galena (Gn) cut by sphalerite- Ag-sulfides-carbonate. B) Sample 50076 (Freiberg): brecciated sample with clasts of sphalerite-Ag-carbonate with carbonate I (Cb) and sphalerite II (Sp) cemented by quartz III (Qz; Ag-sulfides-quartz association). Quartz III (Qz) is overgrown by pyrargyrite (Pyr) crystals that preferentially occur as cavity infill. C) Sample 52713 (Kleinvoigtsberg): arsenopyrite (Apy), pyrite, and sphalerite followed by galena and quartz II, all related to the galena-quartz ± carbonate association. This association is brecciated and overgrown by carbonate and minor sulfides of the sphalerite-Ag-sulfides-carbonate association, which is again overgrown by quartz and fine-grained Ag minerals of the Ag-sulfides-quartz association. D) Sample 52936 (Reinsberg): clasts of sphalerite-Ag-sulfides-carbonate association overgrown by quartz (Qz III) and arsenopyrite (Apy) of the Ag-sulfides-quartz association. Quartz-carbonate association with minor sulfides occurs as late-stage vein infill. E) Sample 52634 (Großvoigts- berg): brecciated clasts of the sphalerite-Ag-sulfides-carbonate association cemented by barren quartz.

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Sphalerite-pyrite- Galena-quartz± Sphalerite-Ag- Ag-sulﬁdes- Stibnite-quartz Quartz-carbonate quartz carbonate sulﬁdes-carbonate quartz Quartz I II IIIa IIIb IV

Carbonates I II

Arsenopyrite I II III

Pyrite

Sphalerite I II

Galena

Chalcopyrite

Cassiterite

Fahlore

Pyrargyrite

Stephanite

Polybasite

Acanthite

Sb-sulfides

Fig. 4. Simplified paragenetic sequence of the different types of epithermal vein fill in the Freiberg district. Line thickness indicates the relative mineral abundance. Dotted lines indicate that the mineral was only observed rarely. Carbonates include calcite, siderite, dolomite, ankerite, Mn calcite, and rhodochrosite. The Fe concentrations in Sp I and Sp II are ~12 and 8%, respectively (Bauer et al., 2019a).

lighter than sphalerite I, consistent with a somewhat lower usually subordinate. The sequence typically starts with fine- Fe content compared to sphalerite I (~8 wt % Fe; Bauer et grained euhedral arsenopyrite III surrounded by early quartz al., 2019a). Carbonate I is coeval to or postdates sphalerite II III (Figs. 3D, 5D). Small amounts of pyrite, sphalerite, and and overprints older sulfide minerals as well as quartz II. Lo- galena are common in voids within aggregates of early quartz cally carbonate I replaces older quartz, in places resulting in III. Galena is followed by pyrargyrite, polybasite, freibergite, pseudomorphous replacement textures (Fig. 5K). Carbonate boulangerite, and acanthite. Silver minerals are accompanied I is coeval with minor amounts of arsenopyrite II, pyrite II, by coeval quartz III, which forms zoned euhedral crystals with and galena II but also with fahlore (freibergite), pyrargyrite, comb, plumose, and feathery textures (Fig. 5O). Bladed tex- miargyrite, acanthite, and polybasite (Fig. 5L). tures are locally present. Zoned quartz III crystals may contain Ag-sulfides-quartz: This association is characterized by bands of chalcedony. Voids between quartz III crystals and late abundant Ag minerals, whereas galena and sphalerite are veinlets are commonly filled with carbonate II (Fig. 5D).

Fig. 5. Representative ore textures. A) Sample 50006 (Freiberg): massive sphalerite (Sp), pyrite (Py), galena (Gn), arsenopyrite (Apy), and chalcopyrite from the sphalerite-pyrite-quartz association. B) Sample 52757 (Kleinvoigtsberg) with abundant galena overgrown by quartz (Qz II) and minor amounts of carbonate (Cb I) characteristic of the galena-quartz ± carbonate assemblage. C) Sample 63963 (Großvoigtsberg) containing host-rock clasts encapsulated by sphalerite (Sp II) and subsequent carbonate (Cb I) from the sphalerite-Ag-sulfides-carbonate association. D) Sample 52924 (Reinsberg) showing carbonate I related to the sphalerite-Ag-sulfides-carbonate association overgrown by arsenopyrite, polybasite (Pba), pyrargyrite (Pyr), and quartz (Qz III; visible growth zones) related to the Ag-sulfides-quartz association. Late-stage carbonate (Cb II) occurs in cavities. E) Sample 36680 (Kleinvoigtsberg) displaying a complete vein section, which comprises quartz II and jamesonite (Jms) typical for the stibnite-quartz association. F) Sample 53033 (Siebenlehn) of quartz and carbonate with bladed textures and minor sulfides forming the quartz-carbonate assemblage. G) Reflected-light photomicrograph in air of sample 53720 (Klein- voigtsberg) with quartz I (Qz I), arsenopyrite I (Apy I), and Fe-rich sphalerite I (Sp I). H) Reflected-light photomicrograph in air of sample 50017 (Freiberg) comprising quartz II with minor cassiterite (Cst), carbonate I, and galena from the galena- quartz ± carbonate association. I) Reflected-light photomicrograph in air of galena I of the galena-quartz ± carbonate association (sample 52713, Kleinvoigtsberg) with elongate inclusions of Ag-rich fahlore (Fhl) and pyrargyrite. J) Reflected-light photomicrograph in air of sample 52789 (Kleinvoigtsberg) with sphalerite and quartz II overgrown by galena and pyrargyrite. K) Reflected-light photomicrograph in air of sample 52755 (Kleinvoigtsberg) showing quartz II overgrown by sphalerite II and arsenopyrite. Carbonate I partly replaces quartz II (dotted line). L) Reflected-light photomicrograph in air of sphalerite and arsenopyrite overgrown by carbonate and subsequently by pyrargyrite, illustrating the sphalerite-Ag-sulfides-carbonate association (sample 52768, Kleinvoigtsberg). M) Reflected-light photomicrograph in air of sample 52377 (Kleinvoigtsberg) with pyrargyrite and carbonate II in cavities between euhderal quartz III (Qz III). N) Photomicrograph in air of sample 52612 (Großvoigtsberg) containing stibnite (Stb) together with sphalerite and marcasite (Mrc) with quartz III forming the stibnite- quartz association. O) Sample 52929 (Reinsberg) with arsenopyrite, sphalerite, galena, and later pyrargyrite in quartz III of the Ag-sulfides-quartz association overgrown by carbonate II and the quartz-carbonate association.

A B C

Sp I Cb I Host Host Gn ApyI Qz II Py I

Cb I Gn Sp II 2cm 1cm 5cm Cb I

D E Host F Qz II bladedcb Cb II

Apy Jms Qz IV

Qz III+Pba+Pyr Cb I

2cm 6cm 0.5cm

G H I ApyI Qz II

Sp I Qz I

Gn Fhl Cst

Cb I

Pyr Sp I 200µm 200µm 500µm

J K L Py Sp I

Cb I Sp

Pyr Cb I Gn Apy ApyII Qz II Qz II Pyr Sp II

500µm 500µm 500µm

M N O quartz-carbonate

Pyr Cb II Cb II Qz III Qz III Pyr

500µm 2000µm Apy 1cm Sp +Gn

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Stibnite-quartz: The stibnite-quartz association is marked limit of the analytical method used (5,000 and 10,000 g/t, by the presence of discrete Sb sulfides with abundant stibnite, respectively). As a consequence, the mean values for Pb in berthierite, boulangerite, and jamesonite (Fig. 5E). The Sb this data set would be underestimated. By using a different minerals typically overgrow euhedral quartz II and occur as analytical method, Seifert and Sandmann (2006) were able to needles or as dense masses, which appear to be cogenetic with determine high Pb concentrations. The means of Pb concen- quartz III (Fig. 5N). Minor amounts of marcasite, sphaler- trations are therefore calculated using only data from Seifert ite, and pyrite may be associated with the Sb minerals. Silver- and Sandman (2006) and analyses below the upper detection bearing phases are extremely rare. Fragments of the sphaler- limit in our own data set. ite-pyrite-quartz and galena-quartz ± carbonate associations Samples dominated by the base metal-quartz associa- are locally cemented by Sb minerals and quartz. tion (n = 5 samples from this study + 60 from Seifert and Quartz-carbonate: This association comprises quartz and Sandmann, 2006) contain the highest mean concentrations chalcedony that can be accompanied by carbonate minerals of Zn (13.9 wt %), As (3.5 wt %), Pb (3.1 wt %), and Cu (Fig. 5F). Only small amounts of usually very fine grained (0.9 wt %) and also significant concentrations of Sn (0.2 wt sulfide minerals are associated with this association. Acces- %) and In (219 g/t). Base metal-quartz–dominated samples sory minerals include pyrite, arsenopyrite, hematite, and elec- also contain some Ag (769 g/t) as well as some Au (0.3 g/t). trum. The quartz-carbonate association occurs as small vein- In contrast, the sphalerite-Ag-sulfides-carbonate associa- lets crosscutting paragenetically older mineral associations, as tion (n = 17 + 8), has a higher mean Ag content (4,923 g/t) late-stage vein infill or as individual veins (Figs. 3D, 5O). Mas- with only moderate base metal concentrations (4 wt % Cu, sive veins commonly consist of breccias, with angular clasts Pb, Zn combined). The mean Au content (0.3 g/t; Table 2) is of (altered) host rock, and quartz and carbonate as cement. similar to that of the base metal-quartz association. The Ag- Both quartz and carbonate commonly show bladed textures sulfides-quartz association (n = 42 + 14) is marked by the (Fig. 5F). highest reported mean concentration of Au (1.98 g/t) and as much Ag (4,910 g/t) as the sphalerite-Ag-sulfides-car- Whole-rock geochemistry bonate association, plus high Sb (3,377 g/t) concentrations. The 77 bulk-rock analyses of the present study are comple- Maximum silver grades in both the sphalerite-Ag-sulfides- mented with 82 previously published geochemical analyses carbonate and Ag-sulfides-quartz association may exceed (Seifert and Sandmann, 2006). The complete data set is pro- 3 wt % (Fig. 6; Table 2). The precious metal contents in vided in Appendix Table A2. The geochemical data are pre- the stibnite-quartz association (n = 4) are 410 g/t Ag and sented according to the dominant mineral association in the below detection limit for Au, but for Sb, grades are invari- analyzed samples (Fig. 6; Table 2), even though some samples ably above the upper detection limit (>1 wt %). Samples comprise more than one mineral association. Because the dominated by the quartz-carbonate association (n = 9), in sphalerite-pyrite-quartz and galena-quartz ± carbonate asso- contrast, have Au contents of 0.5 g/t but low Ag, Sb, and ciations are intimately associated in many cases, they are con- base metal concentrations. sidered together and labeled as base metal-quartz association for bulk chemical analysis. Several analyses of the sphalerite- Fluid inclusion analysis Ag-sulfides-carbonate and Ag-sulfides-quartz associations Microthermometric analyses were conducted on 108 FIAs yield Pb and As concentrations above the upper detection (655 individual fluid inclusions; Table 3). The data set in-

Table 2. Summary of Bulk-Rock Analyses from This Study and Data from Seifert and Sandmann (2006)

Au Ag Sb Sn As Zn Cu Pb (g/t) (g/t) (g/t) (g/t) (wt %) (wt %) (wt %) (wt %) Ag/Au Base metal-quartz Mean 0.28 769 485 1,729 3.5 13.9 0.9 3.1 2,746 n = 65 Min <0.005 <5 0.8 20 0.10 <0.01 0.04 <0.01 Max 2.37 4,670 2,530 13,000 34.1 46.2 7.1 7.2

Sphalerite-Ag-sulfides- Mean 0.28 4,923 1,709 339 1.0 2.7 0.2 1.11 17,582 carbonate Min <0.005 <5 9 3.7 0.002 0.007 0.002 0.016 n = 25 Max 1.70 32,500 >10,000 1,800 10.0 22.8 2.1 4.4

Ag-sulfides-quartz Mean 1.98 4,910 3,377 517 1.4 4.2 0.2 1.51 2,468 n = 56 Min <0.005 <5 63 1 0.054 0.015 0.003 0.007 Max 16.40 39,100 21,400 3,810 11.7 21.6 1.4 5.3

Stibnite-quartz Mean <0.005 410 >10,000 22 0.5 0.4 0.01 0.1 - n = 4 Min <0.005 <5 >10,000 15 0.047 0.042 0.004 0.087 Max <0.005 1,160 >10,000 38 1.1 0.7 0.03 0.1

Quartz-carbonate Mean 0.52 72 117 13 0.23 0.04 0.02 0.09 138 n = 9 Min 0.01 <5 6 1 0.002 0.003 0.001 0.002 Max 2.05 469 484 64 1.0 0.2 0.1 0.3

Dash indicates value not available 1Averages are calculated without values >upper limit of detection

cludes fluid inclusions hosted by quartz II in samples from are all ~21°C, indicating that the fluid inclusions can be best the central sector (profile 1; App. Table A3), as well as fluid described in the H2O-NaCl-system. inclusions hosted by quartz II and III from the peripheral sec- Homogenization temperatures of primary, clustered, and tors (profiles 2 and 3: App. Tables A4, A5). Quartz I, IV, and pseudosecondary (Fig. 7) FIAs hosted by quartz II from the carbonate minerals were not found to contain fluid inclu- profile 1 (Freiberg) range from 274° to 331°C with salinities sions suitable for analysis. An accurate measurement of eutec- between 2.2 and 6.1 wt % NaCl equiv. Primary, clustered, tic temperatures was not always possible because of the small and pseudosecondary FIAs in quartz II from profile 2 (Kle- size of the fluid inclusions. Measured eutectic temperatures invoigtsberg) have slightly lower homogenization tempera-

10000 10 10000 10000

1000 1 1000 ) 1000 (g/t (g/t) (g/t) (g/t) 100 100 Au Sn 0.1 Ag Sb 100 10 10 0.01 10 1 1

100 100 100 100

10 10 10 10 (kg/t) (kg/t) (kg/t) (kg/t) Zn Pb 1 1 1 Cu 1 As

0.1 0.1 0.1 0.1

1000

10 0.01 10 100 0.001 (g/t ) (g/t ) (g/t ) 10 In/Z n In

Ga 1 Ge 1 0.00001 1

0.1 0.1 0.1

base-metal-quartz Ag-sulﬁdes quartz quartz-carbonate

sphalerite-Ag-sulﬁdes-carbonate stibnite-quartz

Fig. 6. Box and whisker plots of element variation for the different associations. The plots contain data from this study and Seifert and Sandmann (2006).

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Table 3. Summary of Microthermometric Data of Fluid Inclusion Assemblages (FIAs)

Profile Generation Elevation (m) FIAs Th range Salinity range Mean Th Mean salinity (n) (°C) (wt % NaCl equiv) (°C) (wt % NaCl equiv) 1 Qz II 200–400 6 277–331 2.2–5.7 306 3.2 1 Qz II 0–200 9 274–326 2.0–6.1 305 4.2 2 Qz II 200–400 5 234–286 0.5–2.9 261 1.8 2 Qz II 0–200 10 235–300 0.4–2.4 266 1.4 2 Qz II <0 19 250–302 0.4–2.0 277 1.3 2 Qz III 200–400 18 202–286 0.3–1.5 233 0.9 3 Qz III 200–400 11 173–225 0.3–3.7 193 1.8 3 Qz III 0–200 9 156–226 0.0–3.7 202 1.2

tures, in the range from 234° to 302°C, with salinities of 0.3 to ing a breccia with clasts of the sphalerite-Ag-sulfides-carbon- 2.7 wt % NaCl equiv. Lower temperatures of 202° to 253°C ate association. The stibnite-quartz and quartz-carbonate as- and salinities of 0.5 to 1.5 wt % NaCl equiv were measured in sociations are absent in profile 1. FIAs in quartz II related FIAs related to quartz III from profile 2. In profile 3 (Reins- to shallow (200–400 m a.s.l.) samples have homogenization berg), FIAs in quartz III have the lowest range of homogeni- temperatures between 277° and 331°C, and homogenization zation temperatures of 164° to 238°C, with salinities between temperatures of inclusions in quartz II from intermediate 0.1 and 3.5 wt % NaCl equiv. Homogenization temperatures depths (0–200 m a.s.l.) are 274° to 326°C (Table 3). Because in secondary FIAs (in both quartz II and III, in all profiles) of a lack of suitable FIAs, no microthermometric data from range between 238° and 314°C. samples below 0 m a.s.l. could be obtained. Heterogeneously trapped FIAs (Fig. 7E) are recognized in quartz II and III predating carbonate I and II, respectively. Profile 2—Kleinvoigtsberg, Peter Stehender Heterogeneous FIAs (clusters) related to quartz II have liq- All observed mineral associations occur in samples uid/vapor ratios between 10 and 91 and have homogenization from profile 2 (Fig. 8). At depths between –240 and temperatures of 280° to 344°C. These heterogeneous as- –160 m a.s.l. the paragenetically oldest sphalerite-pyrite- semblages occur at ~95 m a.s.l. in profile 1 and at ~–160 and quartz association is predominant, typically accompanied by –240 m a.s.l. in profile 2. Heterogeneous FIAs (pseudosec- minor amounts of the galena-quartz ± carbonate association. ondary and clusters) related to quartz III have liquid/vapor Between 40 and 240 m a.s.l. the galena-quartz ± carbonate ratios between 7 and 83 and homogenization temperatures and sphalerite-Ag-sulfide-carbonate associations dominate, between 138° and 328°C. They occur at 260 m a.s.l. in profile with minor amounts of the sphalerite-pyrite-quartz associa- 2 and at ~180 and 200 m a.s.l. in profile 3. tion locally present on vein selvages. Samples from the upper Raman spectroscopy was performed on 17 primary and 80 m of the profile (240–320 m a.s.l.) are dominated by the pseudosecondary fluid inclusions in quartz II and III at Klein- Ag-sulfides-quartz association, with minor amounts of sphal- voigtsberg (profile 2; App. Table A6). In the liquid of the fluid erite-pyrite-quartz, galena-quartz ± carbonate, and/or sphal- inclusions, distinct peaks were detected between wavelengths erite-Ag-sulfide-carbonate present. Only a few samples from 2,571 and 2,591 cm–1 in nearly all sections. These peaks can be shallow depth contain the stibnite-quartz association. Al- – related to reduced sulfur species (HS or H2S; Frezzotti et al., though the stibnite-quartz association is mainly restricted to 2012). Most analyses also have peaks at wavelengths of 2,870 shallow depths (cf. Burisch et al., 2019a), minor amounts of Sb and 2,910 cm–1, which are typical for methane (Frezzotti et sulfides were reported down to the 13th level (–200 m a.s.l.). al., 2012). In two inclusions, one related to quartz II and one Intense brecciation of the host rock and vein infill as well as related to quartz III, peaks occur at wavelengths 1,281 and the occurrence of chalcedony, colloform, bladed, and plu- –1 1,385 cm , a characteristic of CO2 in liquid or vapor (Frez- mose quartz mainly occur above 180 m a.s.l. FIAs hosted zotti et al., 2012). by quartz II from shallow (200–400 m a.s.l.), intermediate (0–200 m a.s.l.), and deep (<0 m a.s.l.) levels have homog- Profile 1—Freiberg, Gottlob Morgengang enization temperatures of 234° to 286°, 235° to 300°, and The sphalerite-pyrite-quartz and galena-quartz ± carbonate 250° to 302°C, respectively (Table 3). Fluid inclusions re- associations are the dominant vein fill in profile 1 (Fig. 8). The lated to quartz III, limited to shallow vein sections (200–400 sphalerite-pyrite-quartz association prevails between –185 m a.s.l.), have homogenization temperatures between 202° and 170 m, whereas the galena-quartz ± carbonate association and 261°C. is dominant at 95 to 300 m. The transition from sphalerite-pyrite-quartz– to galena-quartz ± carbonate–dominated vein fill Profile 3—Reinsberg, Reinsberger Glück Morgengang is gradual between 95 and 170 m. The sphalerite-Ag-sulfides- All samples from the Reinsberg profile are dominated by the carbonate association may occur in variable abundance in the Ag-sulfides-quartz association, accompanied by sphalerite- entire vertical profile but generally becomes more abundant Ag-sulfides-carbonate and quartz-carbonate associations (Fig. as depth decreases. It typically crosscuts the paragenetically 8). Here, the sphalerite-Ag-sulfides-carbonate association older sphalerite-pyrite-quartz and galena-quartz ± carbonate comprises only subordinate fine-grained sulfides and typically associations. The Ag-sulfides-quartz association is rare within occurs as fragments cemented by minerals of the younger Ag- profile 1 and was only recognized at 215 m (Fig. 3B), cement- sulfides-quartz association (Fig. 3D). Quartz III is abundant

350 A C

300 C]

[° 250 h T Qz II P Qz III PS P1 C 50 µm 200 P2 Het. FIA P3 Burischetal. 2019a Baueretal. 2019a D 150 0123456 Salinity wt % NaCl equiv 400 B

300

200 50 µm

] 100 [m E aon

Elev 0

-100

P1 Qz II -200 P2 Qz III P3 V+L Averageand Th range V L foreachsample 50 µm -300 200 250 300 350 ° Th [ C] Fig. 7. A) Salinity (in wt % NaCl equiv) versus homogenization temperature (°C) of all fluid inclusion assemblages (FIAs). Quartz generations are indicated by the color of the rim, and sample location with the symbol color (P1 = profile 1, P2 = profile 2, P3 = profile 3). Symbol shape indicates type of FIA: primary (P), pseudosecondary (PS), and clusters (C). Heteroge- neously trapped FIAs are marked with a double rim. B) Elevation (m) versus homogenization temperature (°C) of analyzed FIAs averaged per quartz generation per sample. The range of homogenization temperatures is indicated by the line. C) Photomicrograph of primary FIA in sample 52719 parallel to the growth zones of quartz III. D) Pseudosecondary fluid inclusion trail in sample 52939. E) FIA in sample 52939 with different liquid-vapor ratios indicating boiling. L = liquid, V = vapor.

in profile 3 samples and often shows bladed, comb, and plu- curs as the latest vein fill. The stibnite-quartz association was mose textures, and chalcedony is also common. Carbonate II not observed in profile 3, although Müller (1901) and Bau- (commonly Mn-rich calcite and rhodochrosite) may exhibit mann (1965) described the occurrence of minor amounts lattice-bladed textures and in several cases occurs together Sb sulfides at this locality. FIAs in quartz III of shallow with kaolinite. The quartz-carbonate association typically oc- (200–400 m a.s.l.) samples have homogenization tempera-

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KleinvoigtsberKleinvoigtsbergPg P22 ReinsberReinsberggPP3 C III 200-260200-260°C°C C IIIIII 170-240°170-240°C 22550°0°C 200 ] 150-200°150-200°C 200 m [m ] IIII 230-290230-290°C°C [ n io t a

v 100 100 Elevation El e ] IIIIII 160-220°160-220°C m [m ] [ n o i IIII 230-300230-300°C°C 0 evat Elevation El

-100-100

IIII 250-300250-300°C°C

-200 C 280-290280-290°C°C

FreibergFreiberg PP11

300 IIII 280-300°280-300°C

200 IIII 290-330°290-330°C ] m [m ] BräunsdorfBräunsdorf [

n 100 IIII io 270-330°270-330°C t a v ] Elevation El e

m 300 IIIIII 0 [m ] [ 180-240°180-240°C n io

evat 200 --100100 Elevation El IIII 240-300°240-300°C N 02.5 57.5 10 km -200

Geology Mineralassociations Textures/ﬂuidinclusions Sedimentary units Sphalerite-pyrite-quartz C Chalcedony

Schist and phyllite Galena-quartz±carbonate Colloform quartz Volcanicunits Brecciation Sphalerite-Ag-sulﬁdes-carbonate Igneousunits Bladedquartz/carbonate Ag-sulﬁdes-quartz Gneiss Heterogeneous fluid inclusion Stibnite-quartz assemblage Hydrothermal vein III Homogenization Tin quartz III

Fig. 8. Summary of the mineral zoning, petrographic observations, and microthermometric measurements within the investigated profiles and their location in the district (map modified after Hoth et al., 1980). Observations from Bräunsdorf were added after Burisch et al. (2019a).

tures between 178° and 225°C, whereas homogenization and Ag-sulfides-quartz associations are 2H O-NaCl–dominat- temperatures of inclusions in quartz III from intermediate lev- ed fluids with salinities <6 wt % NaCl equiv. The presence els (0–200 m a.s.l.) range between 156° and 226°C (Table 3). of methane and reduced sulfur species indicates that the ore fluid was relatively reduced, which is also evident from the Discussion abundant occurrence of arsenopyrite. Homogenization tem- Microthermometric data indicate that the fluids associated peratures decrease from the central (275°–330°C; profile 1, with the sphalerite-pyrite-quartz, galena-quartz ± carbonate, quartz II) to the peripheral sectors (235°–300°C; profile 2,

quartz II) as well as from deep to shallow vein intersections shallow vein sections, i.e., within the sphalerite-pyrite-quartz, (283°–260°C in quartz II, profile 2; Fig. 7). Consistent with stibnite-quartz, and early Ag-sulfides-quartz associations. The microthermometric data, the abundance of temperature-sen- scarcity of carbonate minerals supports successive cooling of sitive elements such as In (cf. Frenzel et al., 2016) system- the fluid as an ore-forming mechanism for those mineral as- atically decreases from deep and proximal sphalerite-pyrite- sociations, since cooling without concomitant degassing does quartz to shallow and distal Ag-sulfides-quartz associations. not result in precipitation of carbonate minerals (Dong et Variable In/Zn ratios of the different associations indicate al., 1995; Simmons et al., 2005; Burisch et al., 2017). Boiling that this trend is not related to the decreasing abundance of indicators such as heterogeneously trapped FIAs (Fig. 7E), sphalerite from deep and proximal to shallow and distal, but bladed textures (Fig. 5F), chalcedony (Sander and Black, instead is related to a systematic decrease of the In content of 1988; Dong et al., 1995; Moncada et al., 2017), and carbonate sphalerite, probably as a result of decreasing temperature (cf. minerals (Fig. 5D) are also recognized at shallow depth within Frenzel et al., 2016). the profiles (profiles 2 and 3). However, these are invariably The first occurrence of discrete Ag minerals in the veins related to late-stage quartz III. is intimately related to the onset of hydrothermal carbonate precipitation. The major occurrence of carbonate minerals (in Temporal and vertical evolution of the Freiberg the sphalerite-Ag-sulfides-carbonate association) is restricted epithermal system to distinct zones occurring at intermediate positions in the Based on crosscutting and/or overgrowth relationships, three profiles (~0–200 m a.s.l.). The presence of carbonate minerals distinct mineralization stages (1, 2, 3) are recognized across is most likely related to drastic changes in pH due to boil- the district (Fig. 9) and document the continuous temporal ing and concomitant CO2 loss (Hedenquist and Henley, 1985; evolution of the Freiberg hydrothermal system. For each of Simmons and Christenson, 1994). In addition, the pseudo- these stages systematic spatial mineralogical variations occur morphous replacement of quartz by carbonate minerals (Fig. (labeled with A, B, C, D, E), which are largely similar in each 5K) strongly supports a pH increase of the fluid. Most carbon- stage and follow the paragenetic sequence. ate minerals likely precipitate in the early stages of boiling Stage 1: Stage 1 (Fig. 9) is characterized by the sphalerite- (Drummond and Ohmoto, 1985), which may account for the pyrite-quartz association. In the central sector of the Frei- low abundance of carbonate minerals in both the deepest and berg district stage 1 mineralization dominates but gradually

stage 1 stage 2 stage 3

Present-day ??? erosion surface

E F

D Boilingonset G m 300°C 250°C 200°C

00 C D G ~5 OOLIN C

B Boiling onset + ? COOLIN CO2 loss

A 350°C

SOURCE SOURCE SOURCE

sphalerite-pyrite-quartz sphalerite-Ag-sulﬁdes-carbonate stibnite-quartz

galena-quartz±carbonate Ag-sulﬁdes quartz quartz-carbonate

Fig. 9. Schematic model of the temporal (stages) and depth (A-F) evolution of the Freiberg epithermal systems.

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decreases in abundance toward distal veins and shallow parts stage 1 and 2 (Fig. 3B, D, and E). Stage 3 is characterized by of the system to a minor portion of the vein fill. Homogeni- abundant Ag-sulfides-quartz mineralization (stage 3D) that zation temperatures related to stage 1 range between ~330° changes to quartz-carbonate (stage 3F) in the shallow sections and 260°C; these systematically decrease from proximal and/ of the vein (Fig. 9). Homogenization temperatures related to or deep to distal and/or shallow parts of the district. Hydro- stage 3 are lower than in the previous stages (170°–250°C). In thermal carbonate minerals are absent. deeper vein samples, stage 3 is difficult to recognize, since at Stage 2: Commonly, stage 1 is truncated or overgrown by deeper intersections it may be mineralogically indistinguish- a sphalerite-Ag-sulfide association (Fig. 3A). In the deepest able from stage 1 or 2. Carbonate minerals late in the para- and/or proximal zones of the district, the onset of stage 2 is genesis, textures indicative for boiling, and heterogeneously analogous to stage 1, characterized by the sphalerite-pyrite- trapped fluid inclusions shallow in the profiles indicate a shal- quartz association (stage 2A; 270°–300°C), which gradually lower boiling onset than in stage 2. changes to the galena-quartz ± carbonate association (stage 2A; 260°–290°C). This transition marks the onset of the main Paleodepth constraints boiling that eventually results in the precipitation of the sphal- Heterogeneous FIAs can be used to constrain the paleodepth erite-Ag-sulfides-carbonate association (stage 2C; Fig. 9) with during ore formation, since vapor pressure during boiling corresponding homogenization temperatures between 240° equals the hydrostatic pressure (Fig. 10; Haas, 1971; Heden- and 270°C. As a result of further cooling of the fluid, the min- quist and Henley, 1985). Some uncertainty is, however, intro- eralogy changes from sphalerite-Ag-sulfides-carbonate to Ag- duced by the fact that the effect of CO2 in the fluid cannot be sulfides-quartz (stage 2D) in more distal and shallower vein estimated here, since quantitative CO2 analyses in the fluid sections. Fluid inclusions related to stage 2D are in the range inclusions have not been conducted. Carbon dioxide peaks in between 220° and 250°C. Even further cooling of the fluid to the Raman spectra do indicate minor concentrations of CO2 190° to 260°C may locally result in the transition from Ag-sul- in the fluid. Based on the absence of clathrates in all analyzed fides-quartz (stage 2D) to stibnite-quartz (stage 2E) in more fluid inclusions (Fig. 7), CO2 concentrations have to be below distal and shallow vein sections (cf. Burisch et al., 2019a). 1.5 mol % (Hedenquist and Henley, 1985; Diamond, 2001). Stage 3: Stage 3 is most abundant in distal and/or shallow Therefore, the reconstructed paleodepths (Fig. 10) have to be vein sections and is commonly associated with brecciation of regarded as minimum values. 0 A B P1 P2 P3

200 25 0 50 m ASL

400 30 0 ] 20 0 [m

600 10 0 0

able m ASL

800 -200

20 0m m ASL

paleo-wate rt 1000

Stage1 0

below Stage2

1200 300 Stage3 -200 m ASL Depth Th average and range of 1400 quartzgenerationper sample 100

1.0wt% CO2 1600 0 Th,min of heterogeneousQzII 5wt% NaCl m ASL

Th,min of heterogeneousQz III 1800 0wt% NaCl 1002150 200 50 300 Temperature[°C]Position of thestageswithin each proﬁle

Fig. 10. A) Temperature versus depth diagram with boiling curves for salinities of 0 and 5 wt % NaCl equiv and 1.0 wt % CO2 (Haas, 1971; Hedenquist and Henley, 1985). Depth estimates are based on the minimum homogenization temperature of heterogeneous fluid inclusion assemblages (FIAs) from profile 2 and Bauer et al. (2019a). B) Minimum paleodepth constraints for each profile (P1, P2, and P3) and stage (1, 2, 3), based on the minimum depth of the FIAs. Black bars represent the vertical extent of mineralization stages observed in the profile samples with an axis of their present-day elevation m a.s.l. Stage 1 of profile 1 could not be constrained.

Results are most comprehensively illustrated in profile 2 2000) and which may furthermore be significantly influenced (Fig. 10). Here, heterogeneous FIAs in quartz II (stage 2) by the admixture of meteoric fluids (Hedenquist and Low- from samples taken from present-day elevations of –160 and enstern, 1994). However, low-salinity fluids are also reported –240 m a.s.l. were formed at depths of at least 800 and 850 m from other magmatic-hydrothermal systems were mixing with below the paleowater table. The heterogeneous assemblage meteoric waters has been excluded (Catchpole et al., 2015; in quartz III (stage 3) from 260 m a.s.l., in contrast, yields a Rottier et al., 2018). The position of the source intrusions in minimum paleodepth of 440 m. The greatest minimum depths the Freiberg district as a key element in the architecture of of mineral formation of 1,250 m below the paleowater table the mineralizing system is yet unknown. are obtained for heterogeneously trapped FIAs from the central sector of the Freiberg district at a present-day elevation of Conclusions 230 m a.s.l. (quartz II from stage 2; Bauer et al., 2019a; Fig. 10). The Freiberg district comprises old (Permian) and well-pre- Shallow (~240 m a.s.l.) homogeneous FIAs in quartz II of served intermediate-sulfidation epithermal systems that share stage 1 (profile 2, sample 52677) having homogenization tem- many characteristics with well-studied Ag-Zn-Pb mineral peratures between 234° and 286°C must have formed at a systems elsewhere. Three ore-forming stages with distinct paleodepth below 850 m (below the liquid-vapor curve; Fig. mineralogical characteristics are recognized, reflecting the 10B). However, heterogeneous assemblages (–240 m a.s.l.) temporal and spatial evolution of the magmatic-hydrothermal in quartz II associated with stage 2 (profile 2, sample 52695) systems. Stage 1 mineralization is dominated by base metals must also have formed at roughly the same paleodepth. The (sphalerite-pyrite-quartz association). In stage 2, deep base fact that these assemblages now occur almost 500 vertical me- metal mineralization (sphalerite-pyrite-quartz and galena- ters apart, despite having originally formed at the same pa- quartz ± carbonate) is followed by a carbonate-rich zone with leodepth, indicates that some of the overburden must have high Ag concentrations (sphalerite-Ag-sulfides-carbonate), been eroded between stage 1 and 2. This denudation led initiated by boiling. Silver concentrations increase successive- to the emplacement of shallow-formed mineralization with ly toward shallower depths and in distal parts of the district. lower homogenization temperatures (stage 2) next to deep- Stage 3 mineralization is abundant at shallow depths and in er-formed mineralization with higher temperatures (stage distal areas, characterized by high-grade Ag mineralization, 1)—i.e., telescoping. Since the CO2 content of the fluid is not shallow onset of boiling, and late barren quartz. known, the extent of this exhumation cannot be constrained Petrographic and geochemical data indicate that high-grade precisely. It may lie between ~400 and 600 m. Ag mineralization (avg ~4,900 g/t)—including the sphalerite- The paleodepth of the boiling horizons varies laterally on Ag-sulfides-carbonate and Ag-sulfides-quartz associations— the district scale. According to the paleodepth calculations extended over a vertical interval of at least 500 m (top is above, in profile 1 (central sector) a present-day elevation of ~300 m below paleowater table). The presence stibnite-quartz 230 m corresponds to a minimum paleodepth of ~1,250 m, mineralization that formed at shallow depth in the north to whereas in profile 2 (northern sector, ~10 km north of profile northwest peripheral sector of the district indicates that here 1) the same elevation corresponds to a minimum paleodepth the entire vertical interval of Ag mineralization may be pre- of ~470 m (Fig. 10), resulting in a difference of 780 m. This served at depth. If so, these features may be used to target regional difference may be caused by asymmetric uplift and exploration efforts in the Freiberg district. net erosion related to Eger graben rifting or due to preexist- ing paleotopography. Acknowledgments The integration of minimum paleodepth estimates, micro- The authors are very grateful to Lluís Fontboté and Jeffrey thermometric data, and petrography reveals that base met- Hedenquist; their very detailed and constructive reviews im- al-rich mineralization (stage 1) formed preferentially at pa- proved a former version of this manuscript significantly. Many leodepths below ~800 m at temperatures between 240° and thanks also to Lawrence Meinert for editorial handling. We 330°C (Fig. 10). Conversely, major Ag (and Au) mineraliza- are indebted to Christin Kehrer for access to the geoscien- tion (stages 2 and 3) formed at depths of less than 800 m in the tific collections of the TU Bergakademie Freiberg (TUBAF) peripheral sectors, with the highest Ag grades above ~600-m and for support during sample selection, Roland Würkert paleodepth. and Michael Stoll (Helmholz Institute Freiberg for Resource Fluid source Technology; HIF) for sample preparation, and Björn Fritzke (TUBAF) for technical support with the sample photographs. Rubidium-strontium ages of sphalerite from the polymetal- Birk Härtel (TUBAF) is gratefully acknowledged for his sup- lic epithermal associations of the Freiberg district yielded port with Raman spectroscopy. Ben Pullinger, Richard Sillitoe, ages of 276 ± 16 Ma (Ostendorf et al., 2019). This age relates and Matthias Jurgeit provided insightful discussions. Parts of polymetallic epithermal mineralization to postorogenic rift- this project were funded by the European Social Fund and ing and extensive bimodal volcanism in the region (Ostendorf the Federal State of Saxony (ESF; project 100339454 re- et al., 2019). High concentrations of Sn (up to 1,729 g/t) as- ceived by M. Burisch), Globex Mining Enterprises Inc., and sociated with base metal-quartz association suggest that the Excellon Resources Inc. ore-forming fluids are possibly related to ilmenite series intrusions (i.e., reduced; Lehmann, 1990, 2020; Arce Burgoa, REFERENCES 2009). The low salinity of the ore fluid is typical for distal Abdelfadil, K.M., Romer, R.L., and Glodny, J., 2014, Mantle wedge metaso- magmatic-hydrothermal systems, which may occur as far as matism revealed by Li isotopes in orogenic lamprophyres: Lithos, v. 196– 6 km away from their source intrusion (Hedenquist et al., 197, p. 14–26.

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Laura Swinkels is a Ph.D. candidate at the Tech- Mathias Burisch is an assistant professor at the nische Universität Bergakademie Freiberg. After TU Bergakademie Freiberg, Germany. He received obtaining a B.Sc. degree in Utrecht, Netherlands, his Ph.D. degree from the University of Tübingen and an M.Sc. degree in Tromsø, Norway, she moved in 2016. Since moving to Freiberg (October 2016) to Freiberg to study the genesis of the epithermal he has built a research project portfolio centering vein systems of the Freiberg district. In her Ph.D. on different types of hydrothermal ore deposits. project, she integrates a wide range of methods, Together with his students and coworkers, Burisch including fluid inclusion microthermometry, whole-rock geochemistry, and constructs comprehensive mineral system models using a multitude of ana- trace element geochemistry, to better understand the vein architecture and lytical methods. underlying ore-forming processes of the Freiberg vein district. In addition to the scientific aspects, her work includes how these findings can be applied to exploration targeting.

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