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 Freiberg 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.

Intermediate sulfidation Ag-Pb-Zn epithermal systems are a major source of Ag and also contain economic amounts of Au, Zn, Pb, and Cu (Simmons et al., 2005). Many of the well-known examples of this particular ore deposit type are located in the Sierra Madre Occidental of Mexico, e.g., Fresnillo, Tayoltita, and Pachuca-Real del Monte (Simmons, 1991; Albinson et al., 2001; Camprubí and Albinson, 2007), with similar epithermal deposits occurring in Peru (Petersen et al., 1977; Candiotti de los Rios et al., 1990; Baumgartner et al., 2008; Rottier et al., 2018) and Bolivia (Phillipson and Romberger, 2004; Arce Burgoa, 2009), as well as in Spain (Concha et al., 1992), Australia (Oliver et al., 2019), and elsewhere (Sillitoe and Hedenquist, 2003). Many of these deposits have a distinct vertical and lateral zoning, which includes high-grade Ag zones at shallow to intermediate depth (100–1,000 m) that systematically grade into more base metal-rich sulfide veins with increasing depth (Albinson et al., 2001; Simmons et al., 2005; Camprubí and Albinson, 2007; Oliver et al., 2019). While these overall vertical trends appear to be characteristic for such Ag-Pb-Zn vein systems, detailed spatial and temporal district- and vein-scale zoning is typically not well constrained. The Freiberg Ag-Pb-Zn district, Erzgebirge, Germany, has only recently been identified as an example of an Ag-Pb-Zn epithermal system (Burisch et al., 2019a). Because of the availability of numerous well-documented samples from historical mining operations in the geoscientific collection of the TU Bergakademie Freiberg, the Freiberg district serves as an excellent example to study the anatomy of such polymetallic epithermal vein systems.

About 5,600 t (180 Moz) of Ag were produced in the Freiberg district during at least 800 years of historical mining, starting in 1168 and continuing to 1969 (Baumann et al., 2000), making it to one of the most significant silver resources in Europe. Silver and base metal mineralization in the district is mainly related to N-S– to NE-SW–striking polyphase magmatic-hydrothermal veins, which are hosted by gneiss and mica schist (Fig. 1; Müller 1901; Bauer et al., 2019a; Burisch et al., 2019a).

Historical mining operations focused on the central part of the district, in the immediate vicinity of the towns of Freiberg and Brand-Erbisdorf. The peripheral parts of the district, such as Bräunsdorf and Kleinvoigtsberg, saw less development. Although the silver grades on the periphery were exceptionally high (1–4 kg/t; Müller, 1901), mining generally ceased earlier (1860–1880), and most of the historical operations were smaller and shallower compared to those in the center (Baumann, 1965).

Most of the scientific concepts of the Freiberg district date back to the early work of Müller (1850, 1901) and von Cotta (1855, 1870), whereas the later studies of the mid- and late-20th century focused on generic classification schemes, without producing significant advances in the genetic understanding of the mineral systems. As a consequence, the genesis remained poorly constrained until a suite of recent studies demonstrated that the bulk of the Ag-Pb-Zn ore of the Freiberg district is related to magmatic-hydrothermal activity (Bauer et al., 2019a; Burisch et al., 2019a) of Permian age (Ostendorf et al., 2019). However, these recent studies either included only a small number of samples or were restricted to individual deposits within the Freiberg district and thus did not take the district-scale architecture of the mineralizing systems into consideration. This is the focus of the present investigation.

Three vertical profiles along well-mineralized veins in different parts of the Freiberg district were investigated in order to constrain vertical zonation along the profiles, down to a depth of 560 m below the present-day land surface. Analytical techniques used include detailed petrographic and fluid inclusion studies and multielement geochemical assays. This data set, complemented by data from previous studies, allows the vertical and lateral zonation as well as the paragenetic evolution of the district to be constrained, providing important information for exploration targeting within the district, as well as insights into Ag-Pb-Zn epithermal systems in general.

Regional geology

The Erzgebirge metallogenic province (Fig. 1B) forms the northern tip of the Bohemian Massif, part of the Variscan orogen in central Europe. The Variscan orogen resulted from the collision of Gondwana and Laurussia between 400 and 340 Ma (Kroner et al., 2010). The Erzgebirge consists of a diverse suite of metamorphosed nappe stacks, with Cadomian and Paleozoic protoliths forming a large SW-dipping anticline (Romer et al., 2010; Rötzler and Plessen, 2010). The metamorphic units were subsequently intruded by syn- to late-collisional granitoids between 336 and 315 Ma, followed by postcollisional bimodal magmatism (305–270 Ma; Kroner et al., 2010; Hoffmann et al., 2013; Kroner and Romer, 2013; Zhang et al., 2017). The late Carboniferous and early to middle Permian were characterized by intense rifting, volcanism, and basin formation (e.g., Döhlen basin; Gaitzsch et al., 2010; Schneider and Romer, 2010). The youngest record of Permian silica-rich volcanism is dated at ~270 Ma (Schneider and Romer, 2010; Hoffmann et al., 2013). Continued subsidence during the Mesozoic led to burial of the basement units underneath thick sedimentary sequences (Ziegler, 1990). Eventually, in the Cenozoic, the formation of the Eger Graben rift resulted in the exhumation of the Variscan basement and associated hydrothermal deposits (Ziegler, 1990; Ziegler and Dèzes, 2007).

The Erzgebirge is host to numerous types of ore deposits (e.g., Baumann et al., 2000; Haschke et al., 2021; Reinhardt et al., 2021; Guilcher, in press). Most prominent among these are magmatic-hydrothermal deposits such as skarns (Schuppan and Hiller, 2012; Bauer et al., 2019b; Burisch et al., 2019b; Korges et al., 2020; Reinhardt et al., 2021), greisens (Štemprok, 1967; Zhang et al., 2017; Korges et al., 2020), and epithermal veins (Bauer et al., 2019a; Burisch et al., 2019a).

District geology

The Freiberg district is located in the northeastern part of the Erzgebirge (Fig. 1). The predominant lithology is a Neoproterozoic composite gneiss unit, comprising biotite-plagioclase orthogneiss (locally referred to as lower gray gneiss) and biotite-muscovite-plagioclase paragneiss (locally referred to as upper gray gneiss; Tichomirowa et al., 2012). The gneiss units form an ellipsoid-shaped (dome-like) body, which is overthrusted by mica schists in the northwest and phyllites in the north and northeast and bordered by other gneiss units in the south and west (Fig. 1C). In the north, gneiss, mica-schist, and phyllites are locally alternated with metagabbro, serpentinite, and amphibolite schist (Baumann, 1965; Baumann et al., 2000).

East of the town of Freiberg, the gneiss units were intruded by the late Variscan (ca. 325–320 Ma) Niederbobritzscher biotite granite (Tichomirowa, 1997). The granite is accompanied in the east by a rhyolitic unit of the Tharandter Wald Volcanic Complex (ca. 320 Ma; Breitkreuz et al., 2009). Numerous rhyolite/microgranite and lamprophyre dikes crosscut the metamorphic units in the area (Müller, 1901; Baumann, 1965; von Seckendorff et al., 2004; Abdelfadil et al., 2014).

Hydrothermal mineralization in the Freiberg district

Three fundamentally different types of hydrothermal veins have been recognized in the Freiberg district: (1) epithermal polymetallic sulfide-quartz-carbonate veins, (2) fluorite-barite-quartz-Pb-Zn veins, and (3) less abundant five element (Bi-Co-Ni-Ag-As) veins (Müller, 1901; Baumann et al., 2000; Bauer et al., 2019a; Burisch et al., 2019a; Ostendorf et al., 2019). This study focuses only on the economically dominant polymetallic sulfide-quartz-carbonate veins, which are probably related to Permian magmatic-hydrothermal activity (276 ± 16 Ma; Ostendorf et al., 2019). The fluorite-barite and native metal-arsenide veins are significantly younger and have been tentatively associated with the opening of the northern Atlantic (Ostendorf et al., 2019).

Polymetallic epithermal mineralization in the Freiberg district occurs in steeply dipping N-S– and NE-SW–trending veins that are hosted by gneiss, mica-schist, metagabbro, and less commonly phyllites. Mineralization can typically be traced over large vertical extents (>1 km; Kraft and Tischendorf, 1960). Three mineral associations have historically been distinguished within the epithermal Ag-Zn-Pb veins of the Freiberg district: (1) a base metal-sulfides-quartz association, referred to as “Kiesige Bleierzformation” (kb), comprising mainly sphalerite, galena, arsenopyrite, pyrite, pyrrhotite, and chalcopyrite, (2) a sphalerite-Ag-sulfides-carbonate association (“Edle Braunspatformation”; eb) with sphalerite, galena, fahlore, and silver sulfosalts, and (3) an Ag-sulfides-quartz association (“Edle Quarzformation”; eq) with abundant silver sulfosalts, acanthite, arsenopyrite, pyrite, galena, sphalerite, and Sb sulfides (Müller, 1901).

Individual veins commonly comprise multiple generations of vein infill that may represent several distinct mineral associations. The predominant association, however, varies systematically on the district and vein scale (Müller, 1901; Burisch et al., 2019a). The base metal-sulfides-quartz association is the dominant vein fill in the central part of the district. The sphalerite-Ag-sulfides-carbonate association is most prominent at the historical mining camps of Brand-Erbisdorf and Kleinvoigtsberg, 6 km south and 10 km north of the town of Freiberg, respectively (Fig. 1; Müller, 1901; Burisch et al., 2019a). The Ag-sulfides-quartz association prevails in the peripheral sectors and shallow vein sections (Müller, 1901; Burisch et al., 2019a). Veins that comprise Ag-sulfides-quartz infill commonly also contain discrete Sb sulfides, such as stibnite and berthierite, which typically occupy a shallower position in the veins than the major Ag mineralization (Burisch et al., 2019a). The intensity of host-rock alteration is variable and mostly characterized by silicification and sericitization (muscovite) with some disseminated pyrite, arsenopyrite, galena, and rarely chlorite (Rösler and Kühne, 1970). However, a comprehensive study on host-rock alteration in the Freiberg district has never been conducted.

Geochronologic, petrographic, geochemical, and microthermometric observations indicate a magmatic-hydrothermal origin of much of the Freiberg epithermal district, affiliated with early Permian magmatism (Bauer et al., 2019a; Burisch et al., 2019a; Ostendorf et al., 2019). However, a causative intrusion has not yet been identified. Attempts to intersect a potential intrusion by drilling during Sn exploration campaigns in the 1950s and 1970s were unsuccessful as all five drill holes with final lengths of 1,110, 1,317, 1,745, 1,826, and 1,061 m, respectively, did not intersect an intrusive body. Notably, abundant veins with base metal-rich mineralization were still present at the final depths of these drill cores (Kraft and Tischendorf, 1960; Krutak, 1980).

Sectors of the Freiberg district

The Freiberg district has been subdivided into five sectors based on geography (Fig. 1C) and distinct mineralogical and geochemical variations (Baumann et al., 2000). In the following, background information on the most important mines and characteristics of the ore deposits of each sector are briefly introduced.

Central sector: The central sector, including the towns of Freiberg and Brand-Erbisdorf, comprises the deepest and largest underground mines of the district (down to 600 m below surface). Abundant N-S– to NE-SW– and E-W–striking hydrothermal veins form a dense fracture network with individual veins traceable over ~5 km along strike. Vein thickness may reach up to 4 m but usually ranges between 0.1 and 0.8 m (Müller, 1901). The base metal-sulfides-quartz association is the most prominent vein infill in the central sector (e.g., Himmelfahrt mine, Freiberg) and is associated with elevated concentrations of Cu, Sn, and In (up to 71,000, 13,000, and 1,560 g/t, respectively; Müller, 1901; Seifert and Sandmann, 2006). In the southern part of the central sector (Brand-Erbisdorf) the sphalerite-Ag-sulfides-carbonate association prevails, commonly crosscutting or coating the paragenetically older base metal-quartz association (Müller, 1901). Because of the dominance of the sphalerite-Ag-sulfides-carbonate association, the Himmelsfürst mine (Brand-Erbisdorf) was one of the economically most significant mines of the entire Freiberg district (Müller 1901; Seifert and Sandmann, 2006).

Northern sector: The northern sector includes the historical mining camps of Kleinvoigtsberg, Großvoigtsberg, Obergruna, Siebenlehn, Reinsberg, and Mohorn (Fig. 1C). Hydrothermal veins mainly strike northeast-southwest, can be traced along strike for up to 2 km, and have thicknesses between 0.1 and 4 m. The veins in the northern sector are often found in the contact zone between gneiss and schist and tend to be less continuous along strike and dip than in the central sector; they frequently split or pinch out and locally form stockwork-like swarms of veinlets. The shallow levels of the veins in this sector are dominated by an Ag-sulfides-quartz association, whereas at deeper levels the sphalerite-Ag-sulfides-carbonate and the base metal-sulfides-quartz associations prevail (Müller, 1901; Baumann, 1965; Baumann et al., 2000). The Alte Hoffnung Gottes mine near Kleinvoigtsberg was the economically most significant mine in the northern sector and was mined to depths of 560 m below surface (Müller, 1901; Baumann, 1965). The area of Reinsberg comprises several small mining camps with the Emanuel mine, which was mined to depth of 310 m below surface, as the most significant operation (Müller, 1901; Baumann, 1965).

Western sector: The Neue Hoffnung Gottes mine close to the town of Bräunsdorf was the most important operation in the western sector (mined down to 290 m below the surface). Similar to the northern sector, hydrothermal veins strike mainly northeast-southwest. In contrast to other sectors, veins are often hosted by what has been described as a graphite-rich schist unit (Müller, 1901; Baumann, 1965; Baumann et al., 2000; Burisch et al., 2019a). Within the ~300-m vertical profile of the Neue Hoffnung Gottes mine, a distinct vertical zoning has been recognized, which includes a shallow Sb quartz cap grading into Ag-sulfides-quartz association with increasing depth (Burisch et al., 2019a).

Southern and eastern sectors: Historical mining operations were much smaller in the southern and eastern sectors of the Freiberg district. Veins are characterized by abundant Ag-sulfides-quartz mineralization. An increase of base metals at depth is reported and is locally associated with economic Cu and Sn mineralization (Müller, 1901; Baumann, 1965). The major operations in the southern sector were the Friedrich August and Friedrich Christoph mines, southeast of the town of Frauenstein. These operations targeted an ~200- × 2,000-m swarm of N-S–striking veins. The operations were limited to depths of 170 m below surface (Müller, 1901).

Sampling

For this study, 152 samples were selected from the geoscientific collection of the TU Bergakademie Freiberg (App. Table A1), since most of the historical mines are no longer accessible. Because comprehensive sample descriptions and historical mine plans are available for a large number of the samples, localities can be reconstructed in detail (±10 m accuracy). Samples for this study were mostly collected from three vertical profiles (Fig. 2), located at the Himmelfahrt mine, Freiberg (profile 1), in the central sector, the Alte Hoffnung Gottes mine, Kleinvoigtsberg (profile 2), and the Emanuel mine, Reinsberg (profile 3), both in the northern sector. Descriptions and data from a vertical profile near Bräunsdorf (western sector), recently published by Burisch et al. (2019a), are also considered in this study. The limited availability of well-documented samples in the eastern and southern sectors of the district did not allow for the compilation of systematic profiles; thus, fewer samples from these areas are included in this study.

Profile 1 consists of 15 samples from the Erzengel Stehender, Wagfort Spat, Wilhelm Stehender, Gottlob Morgengang, Christian Stehender, Karl Stehender, and Caspar Nord veins of the Himmelfahrt mine (Freiberg) between shaft David and shaft Rudolf (~1 km apart), covering a vertical interval of 485 m (–185 to 300 m above sea level [a.s.l.]; Fig. 2). Profile 2 comprises 27 samples from the Peter Stehender, Frisch Glück Stehender, Christliche Hilfe Stehender, Heinrich Stehender, and Neuglück Stehender veins of the Alte Hoffnung Gottes mine (Kleinvoigtsberg). The profile covers a vertical transect of 520 m from –240 to 280 m a.s.l. Profile 3 includes 13 samples of the Reinsberger Glück Morgengang vein of the Emanuel mine (Reinsberg), with the elevation ranging from 68 to 210 m a.s.l., covering 142 vertical meters.

Additional samples were selected from Freiberg (n = 4), Brand-Erbisdorf (n = 24), Halsbrücke (n = 2), Kleinwaltersdorf (n = 1), and Hohentanne (n = 1) in the central sector; Mohorn (n = 3), Großvoigtsberg (n = 10), Kleinvoigtsberg (n = 23), Obergruna (n = 4), Burkersdorf (n = 1), Bieberstein (n = 1), Siebenlehn (n = 2), Reinsberg (n = 1), Zella (n = 1), and Dittmansdorf (n = 1) in the northern sector; Klingenberg (n = 1) in the eastern sector; Frauenstein (n = 9) in the southern sector; and Bräunsdorf (n = 7) and Frankenstein (n = 1) in the western sector.

Petrography

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 characterization of textures and paragenesis, were done using a Carl Zeiss Axio Imager M1m light microscope (transmitted and reflected light), equipped with an AxioCamMRc5 camera for documentation. Mineral identification was complemented by scanning electron microscopy using a FEI Quanta 650F instrument. Both instruments used are located at the Department of Mineralogy, TU Bergakademie Freiberg (TUBAF).

Geochemical analysis

Seventy-seven samples were analyzed for their geochemical 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 500 g were ground to analytical fineness (95%, <105 µm) and analyzed for major, minor, and trace elements using instrumental neutron activation analysis (INAA; Au, Ag, As, Ba, 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), Au fire assay, inductively coupled plasma-optical emission spectroscopy (ICP-OES), and inductively coupled plasmamass spectrometry (ICP-MS; digestion by sodium peroxide fusion; Al, As, B, Ba, Be, Bi, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Ho, Hf, In, K, La, Li, Mg, Mn, Mo, Nb, Nd, Ni, Pb, Pr, Rb, S, Sb, Se, Si, Sm, Sn, Sr, Ta, Tb, Te, Th, Ti, Tl, Tm, U, V, W, Y, Yb, Zn). Information on measurement parameters, standard materials used, and detection limits are provided as supplementary material (App. Table A2). The geochemical data set obtained for this study is complemented by whole-rock analyses (n = 82) of collection samples from the Freiberg district reported in Seifert and Sandmann (2006).

Fluid inclusion analysis

Microthermometric analyses were performed on 19 doubly polished thin sections using an Olympus BX 53 microscope and a Linkam THMS 600 heating-freezing stage at the Department of Mineralogy (TU Bergakademie Freiberg). Prior to microthermometric measurements, fluid inclusion assemblages (FIAs) were petrographically classified as primary (P), secondary (S), pseudosecondary (PS), and clusters (C), according to Goldstein (2001). Eutectic temperatures (Te), final melting temperatures of ice (Tm(ice)), and homogenization temperatures (Th) were measured three times for each fluid inclusion upon heating. Results are reported as average values of the three individual heating runs for each inclusion. Inclusions affected by postentrapment processes and FIAs with inconsistent Th (>20°C) but constant liquid-vapor ratios were omitted from the data set. The calibration of the stage was checked daily by measuring synthetic Tm(CO2), Tm(ice), and Th in H2O and H2O-CO2 fluid inclusion standards. The fluid salinity was calculated from the Tm(ice) according to Steele-MacInnis et al. (2012). Liquid and vapor volume fractions were estimated for each fluid inclusion at room temperature.

Seventeen fluid inclusions classified as primary, pseudosecondary, or cluster from different depths were analyzed with Raman spectroscopy at the Department of Geology at the TU Bergakademie Freiberg using a TriVista Raman spectrometer with a 532-nm laser, 100-mW laser power, 100-µm slit width, and a 50× objective. The spectrometer was calibrated using the 520.7-cm–1 line of silicon. For each inclusion, the measurement includes a z profile and individual spectra for the vapor phase, the liquid phase, and the host mineral. The acquisitions were performed using gratings of 500 and 1,500 lines/mm. The profile measurement ran at an integration time of 200 s. Individual measurements were performed at an integration time of 600 s accumulating one to six acquisitions. Identification of aqueous and gaseous compounds was done by comparing obtained Raman spectra with the database of Frezzotti et al. (2012) and references therein.

Mineral associations

Six distinct mineral associations were recognized in the investigated sample suite (Table 1). The present classification differs slightly from previous classification schemes. An overview of relationship between the different classification schemes is given in Table 1. Individual samples may contain several mineral associations superimposed (1) as gradual transition from one to another, (2) in sharp crosscutting veinlets, or (3) as breccias locally resulting in a complex vein architecture (Fig. 3). The following petrographic descriptions provide a summary of the characteristics of the six associations as observed in the present study. A schematic paragenetic sequence combining all associations described below and their interrelationship is presented in Figure 4.

Sphalerite-pyrite-quartz: The oldest mineral association, paragenetically, is observed in profiles 1 and 2. It is characterized by abundant pyrite, arsenopyrite, and sphalerite, accompanied by quartz as the major gangue mineral (Fig. 5A, G). The mineral paragenesis usually commences with fine-grained quartz I (~0.1–1 mm), followed by euhedral arsenopyrite I, pyrite I, and dark sphalerite I (Fe-rich; ~12 wt % Fe; Bauer et al., 2019a). Sphalerite I is typically anhedral and usually includes fine-grained disseminated chalcopyrite and, less commonly, inclusions of galena and fahlore (tetrahedrite). Quartz I, arsenopyrite, and pyrite also occur as impregnations in the host rock. Sphalerite I is overgrown by quartz II, which commonly forms large euhedral comb crystals (up to 30 mm long) exhibiting up to 0.5-mm-thick growth zones. Cracks and cavities in sphalerite I and quartz II are filled with galena, smaller amounts of fahlore, and rare carbonate minerals.

Galena-quartz ± carbonate: Characteristic of the galena-quartz ± carbonate association are large abundances of galena as well as the presence of minor amounts of carbonate minerals and Ag minerals (Fig. 5B). Pyrite and sphalerite are also present, but in lesser amounts than in the sphalerite-pyrite-quartz association. Quartz II is the main gangue mineral; locally quartz II is intergrown with minor acicular cassiterite, predating the carbonate minerals (Fig. 5H). Galena I overgrowths on quartz II commonly contain inclusions of silver-rich fahlore (tetrahedrite-freibergite) and pyrargyrite, which form elongate inclusions parallel to the cleavage planes of galena (Fig. 5I) or more massive aggregates (Fig. 5J). Carbonate minerals (carbonate I, including calcite, siderite, and ankerite) typically postdate galena I and may form intergrowths with acanthite and polybasite.

Sphalerite-Ag-sulfides-carbonate: The abundance of carbonate minerals (calcite, siderite, ankerite, dolomite, Mn calcite, and rhodochrosite) accompanied by Ag minerals is distinctive for the sphalerite-Ag-sulfides-carbonate association and provides a sharp mineralogical contrast to the other associations (Fig. 3A). This association typically commences with massive sphalerite II at the vein selvage or encapsulating brecciated host-rock fragments (Fig. 5C). Macroscopically, sphalerite II has a black to dark-orange color and appears slightly lighter than sphalerite I, consistent with a somewhat lower Fe content compared to sphalerite I (~8 wt % Fe; Bauer et al., 2019a). Carbonate I is coeval to or postdates sphalerite II and overprints older sulfide minerals as well as quartz II. Locally carbonate I replaces older quartz, in places resulting in pseudomorphous replacement textures (Fig. 5K). Carbonate I is coeval with minor amounts of arsenopyrite II, pyrite II, and galena II but also with fahlore (freibergite), pyrargyrite, miargyrite, acanthite, and polybasite (Fig. 5L).

Ag-sulfides-quartz: This association is characterized by abundant Ag minerals, whereas galena and sphalerite are usually subordinate. The sequence typically starts with fine-grained euhedral arsenopyrite III surrounded by early quartz III (Figs. 3D, 5D). Small amounts of pyrite, sphalerite, and galena are common in voids within aggregates of early quartz III. Galena is followed by pyrargyrite, polybasite, freibergite, boulangerite, and acanthite. Silver minerals are accompanied by coeval quartz III, which forms zoned euhedral crystals with comb, plumose, and feathery textures (Fig. 5O). Bladed textures are locally present. Zoned quartz III crystals may contain bands of chalcedony. Voids between quartz III crystals and late veinlets are commonly filled with carbonate II (Fig. 5D).

Stibnite-quartz: The stibnite-quartz association is marked by the presence of discrete Sb sulfides with abundant stibnite, berthierite, boulangerite, and jamesonite (Fig. 5E). The Sb minerals typically overgrow euhedral quartz II and occur as needles or as dense masses, which appear to be cogenetic with quartz III (Fig. 5N). Minor amounts of marcasite, sphalerite, and pyrite may be associated with the Sb minerals. Silver-bearing phases are extremely rare. Fragments of the sphalerite-pyrite-quartz and galena-quartz ± carbonate associations are locally cemented by Sb minerals and quartz.

Quartz-carbonate: This association comprises quartz and chalcedony that can be accompanied by carbonate minerals (Fig. 5F). Only small amounts of usually very fine grained sulfide minerals are associated with this association. Accessory minerals include pyrite, arsenopyrite, hematite, and electrum. The quartz-carbonate association occurs as small veinlets crosscutting paragenetically older mineral associations, as late-stage vein infill or as individual veins (Figs. 3D, 5O). Massive veins commonly consist of breccias, with angular clasts of (altered) host rock, and quartz and carbonate as cement. Both quartz and carbonate commonly show bladed textures (Fig. 5F).

Whole-rock geochemistry

The 77 bulk-rock analyses of the present study are complemented with 82 previously published geochemical analyses (Seifert and Sandmann, 2006). The complete data set is provided in Appendix Table A2. The geochemical data are presented according to the dominant mineral association in the analyzed samples (Fig. 6; Table 2), even though some samples comprise more than one mineral association. Because the sphalerite-pyrite-quartz and galena-quartz ± carbonate associations are intimately associated in many cases, they are considered together and labeled as base metal-quartz association for bulk chemical analysis. Several analyses of the sphalerite-Ag-sulfides-carbonate and Ag-sulfides-quartz associations yield Pb and As concentrations above the upper detection limit of the analytical method used (5,000 and 10,000 g/t, respectively). As a consequence, the mean values for Pb in this data set would be underestimated. By using a different analytical method, Seifert and Sandmann (2006) were able to determine high Pb concentrations. The means of Pb concentrations are therefore calculated using only data from Seifert and Sandman (2006) and analyses below the upper detection limit in our own data set.

Samples dominated by the base metal-quartz association (n = 5 samples from this study + 60 from Seifert and Sandmann, 2006) contain the highest mean concentrations of Zn (13.9 wt %), As (3.5 wt %), Pb (3.1 wt %), and Cu (0.9 wt %) and also significant concentrations of Sn (0.2 wt %) and In (219 g/t). Base metal-quartz–dominated samples also contain some Ag (769 g/t) as well as some Au (0.3 g/t). In contrast, the sphalerite-Ag-sulfides-carbonate association (n = 17 + 8), has a higher mean Ag content (4,923 g/t) with only moderate base metal concentrations (4 wt % Cu, Pb, Zn combined). The mean Au content (0.3 g/t; Table 2) is similar to that of the base metal-quartz association. The Ag-sulfides-quartz association (n = 42 + 14) is marked by the highest reported mean concentration of Au (1.98 g/t) and as much Ag (4,910 g/t) as the sphalerite-Ag-sulfides-carbonate association, plus high Sb (3,377 g/t) concentrations. Maximum silver grades in both the sphalerite-Ag-sulfides-carbonate and Ag-sulfides-quartz association may exceed 3 wt % (Fig. 6; Table 2). The precious metal contents in the stibnite-quartz association (n = 4) are 410 g/t Ag and below detection limit for Au, but for Sb, grades are invariably above the upper detection limit (>1 wt %). Samples dominated by the quartz-carbonate association (n = 9), in contrast, have Au contents of 0.5 g/t but low Ag, Sb, and base metal concentrations.

Fluid inclusion analysis

Microthermometric analyses were conducted on 108 FIAs (655 individual fluid inclusions; Table 3). The data set includes fluid inclusions hosted by quartz II in samples from the central sector (profile 1; App. Table A3), as well as fluid inclusions hosted by quartz II and III from the peripheral sectors (profiles 2 and 3: App. Tables A4, A5). Quartz I, IV, and the carbonate minerals were not found to contain fluid inclusions suitable for analysis. An accurate measurement of eutectic temperatures was not always possible because of the small size of the fluid inclusions. Measured eutectic temperatures are all ~21°C, indicating that the fluid inclusions can be best described in the H2O-NaCl-system.

Homogenization temperatures of primary, clustered, and pseudosecondary (Fig. 7) FIAs hosted by quartz II from profile 1 (Freiberg) range from 274° to 331°C with salinities between 2.2 and 6.1 wt % NaCl equiv. Primary, clustered, and pseudosecondary FIAs in quartz II from profile 2 (Kleinvoigtsberg) have slightly lower homogenization temperatures, in the range from 234° to 302°C, with salinities of 0.3 to 2.7 wt % NaCl equiv. Lower temperatures of 202° to 253°C and salinities of 0.5 to 1.5 wt % NaCl equiv were measured in FIAs related to quartz III from profile 2. In profile 3 (Reinsberg), FIAs in quartz III have the lowest range of homogenization temperatures of 164° to 238°C, with salinities between 0.1 and 3.5 wt % NaCl equiv. Homogenization temperatures in secondary FIAs (in both quartz II and III, in all profiles) range between 238° and 314°C.

Heterogeneously trapped FIAs (Fig. 7E) are recognized in quartz II and III predating carbonate I and II, respectively. Heterogeneous FIAs (clusters) related to quartz II have liquid/vapor ratios between 10 and 91 and have homogenization temperatures of 280° to 344°C. These heterogeneous assemblages occur at ~95 m a.s.l. in profile 1 and at ~–160 and –240 m a.s.l. in profile 2. Heterogeneous FIAs (pseudosecondary and clusters) related to quartz III have liquid/vapor ratios between 7 and 83 and homogenization temperatures between 138° and 328°C. They occur at 260 m a.s.l. in profile 2 and at ~180 and 200 m a.s.l. in profile 3.

Raman spectroscopy was performed on 17 primary and pseudosecondary fluid inclusions in quartz II and III at Kleinvoigtsberg (profile 2; App. Table A6). In the liquid of the fluid inclusions, distinct peaks were detected between wavelengths 2,571 and 2,591 cm–1 in nearly all sections. These peaks can be related to reduced sulfur species (HS or H2S; Frezzotti et al., 2012). Most analyses also have peaks at wavelengths of 2,870 and 2,910 cm–1, which are typical for methane (Frezzotti et al., 2012). In two inclusions, one related to quartz II and one related to quartz III, peaks occur at wavelengths 1,281 and 1,385 cm–1, a characteristic of CO2 in liquid or vapor (Frezzotti et al., 2012).

Profile 1—Freiberg, Gottlob Morgengang

The sphalerite-pyrite-quartz and galena-quartz ± carbonate associations are the dominant vein fill in profile 1 (Fig. 8). The sphalerite-pyrite-quartz association prevails between –185 and 170 m, whereas the galena-quartz ± carbonate association is dominant at 95 to 300 m. The transition from sphalerite-pyrite-quartz– to galena-quartz ± carbonate–dominated vein fill is gradual between 95 and 170 m. The sphalerite-Ag-sulfides-carbonate association may occur in variable abundance in the entire vertical profile but generally becomes more abundant as depth decreases. It typically crosscuts the paragenetically older sphalerite-pyrite-quartz and galena-quartz ± carbonate associations. The Ag-sulfides-quartz association is rare within profile 1 and was only recognized at 215 m (Fig. 3B), cementing a breccia with clasts of the sphalerite-Ag-sulfides-carbonate association. The stibnite-quartz and quartz-carbonate associations are absent in profile 1. FIAs in quartz II related to shallow (200–400 m a.s.l.) samples have homogenization temperatures between 277° and 331°C, and homogenization temperatures of inclusions in quartz II from intermediate depths (0–200 m a.s.l.) are 274° to 326°C (Table 3). Because of a lack of suitable FIAs, no microthermometric data from samples below 0 m a.s.l. could be obtained.

Profile 2—Kleinvoigtsberg, Peter Stehender

All observed mineral associations occur in samples from profile 2 (Fig. 8). At depths between –240 and –160 m a.s.l. the paragenetically oldest sphalerite-pyrite-quartz association is predominant, typically accompanied by minor amounts of the galena-quartz ± carbonate association. Between 40 and 240 m a.s.l. the galena-quartz ± carbonate and sphalerite-Ag-sulfide-carbonate associations dominate, with minor amounts of the sphalerite-pyrite-quartz association locally present on vein selvages. Samples from the upper 80 m of the profile (240–320 m a.s.l.) are dominated by the Ag-sulfides-quartz association, with minor amounts of sphalerite-pyrite-quartz, galena-quartz ± carbonate, and/or sphalerite-Ag-sulfide-carbonate present. Only a few samples from shallow depth contain the stibnite-quartz association. Although the stibnite-quartz association is mainly restricted to shallow depths (cf. Burisch et al., 2019a), minor amounts of Sb sulfides were reported down to the 13th level (–200 m a.s.l.). Intense brecciation of the host rock and vein infill as well as the occurrence of chalcedony, colloform, bladed, and plumose quartz mainly occur above 180 m a.s.l. FIAs hosted 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 homogenization temperatures of 234° to 286°, 235° to 300°, and 250° to 302°C, respectively (Table 3). Fluid inclusions related to quartz III, limited to shallow vein sections (200–400 m a.s.l.), have homogenization temperatures between 202° and 261°C.

Profile 3—Reinsberg, Reinsberger Glück Morgengang

All samples from the Reinsberg profile are dominated by the Ag-sulfides-quartz association, accompanied by sphalerite-Ag-sulfides-carbonate and quartz-carbonate associations (Fig. 8). Here, the sphalerite-Ag-sulfides-carbonate association comprises only subordinate fine-grained sulfides and typically occurs as fragments cemented by minerals of the younger Ag-sulfides-quartz association (Fig. 3D). Quartz III is abundant in profile 3 samples and often shows bladed, comb, and plumose textures, and chalcedony is also common. Carbonate II (commonly Mn-rich calcite and rhodochrosite) may exhibit lattice-bladed textures and in several cases occurs together with kaolinite. The quartz-carbonate association typically occurs as the latest vein fill. The stibnite-quartz association was not observed in profile 3, although Müller (1901) and Baumann (1965) described the occurrence of minor amounts Sb sulfides at this locality. FIAs in quartz III of shallow (200–400 m a.s.l.) samples have homogenization temperatures between 178° and 225°C, whereas homogenization temperatures of inclusions in quartz III from intermediate levels (0–200 m a.s.l.) range between 156° and 226°C (Table 3).

Microthermometric data indicate that the fluids associated with the sphalerite-pyrite-quartz, galena-quartz ± carbonate, and Ag-sulfides-quartz associations are H2O-NaCl–dominated fluids with salinities <6 wt % NaCl equiv. The presence of methane and reduced sulfur species indicates that the ore fluid was relatively reduced, which is also evident from the abundant occurrence of arsenopyrite. Homogenization temperatures decrease from the central (275°–330°C; profile 1, quartz II) to the peripheral sectors (235°–300°C; profile 2, quartz II) as well as from deep to shallow vein intersections (283°–260°C in quartz II, profile 2; Fig. 7). Consistent with microthermometric data, the abundance of temperature-sensitive elements such as In (cf. Frenzel et al., 2016) systematically decreases from deep and proximal sphalerite-pyrite-quartz to shallow and distal Ag-sulfides-quartz associations. Variable In/Zn ratios of the different associations indicate that this trend is not related to the decreasing abundance of sphalerite from deep and proximal to shallow and distal, but instead is related to a systematic decrease of the In content of sphalerite, probably as a result of decreasing temperature (cf. Frenzel et al., 2016).

The first occurrence of discrete Ag minerals in the veins is intimately related to the onset of hydrothermal carbonate precipitation. The major occurrence of carbonate minerals (in the sphalerite-Ag-sulfides-carbonate association) is restricted to distinct zones occurring at intermediate positions in the profiles (~0–200 m a.s.l.). The presence of carbonate minerals is most likely related to drastic changes in pH due to boiling and concomitant CO2 loss (Hedenquist and Henley, 1985; Simmons and Christenson, 1994). In addition, the pseudomorphous replacement of quartz by carbonate minerals (Fig. 5K) strongly supports a pH increase of the fluid. Most carbonate minerals likely precipitate in the early stages of boiling (Drummond and Ohmoto, 1985), which may account for the low abundance of carbonate minerals in both the deepest and shallow vein sections, i.e., within the sphalerite-pyrite-quartz, stibnite-quartz, and early Ag-sulfides-quartz associations. The scarcity of carbonate minerals supports successive cooling of the fluid as an ore-forming mechanism for those mineral associations, since cooling without concomitant degassing does not result in precipitation of carbonate minerals (Dong et al., 1995; Simmons et al., 2005; Burisch et al., 2017). Boiling indicators such as heterogeneously trapped FIAs (Fig. 7E), bladed textures (Fig. 5F), chalcedony (Sander and Black, 1988; Dong et al., 1995; Moncada et al., 2017), and carbonate minerals (Fig. 5D) are also recognized at shallow depth within the profiles (profiles 2 and 3). However, these are invariably related to late-stage quartz III.

Temporal and vertical evolution of the Freiberg epithermal system

Based on crosscutting and/or overgrowth relationships, three distinct mineralization stages (1, 2, 3) are recognized across the district (Fig. 9) and document the continuous temporal evolution of the Freiberg hydrothermal system. For each of these stages systematic spatial mineralogical variations occur (labeled with A, B, C, D, E), which are largely similar in each stage and follow the paragenetic sequence.

Stage 1: Stage 1 (Fig. 9) is characterized by the sphalerite-pyrite-quartz association. In the central sector of the Freiberg district stage 1 mineralization dominates but gradually decreases in abundance toward distal veins and shallow parts of the system to a minor portion of the vein fill. Homogenization temperatures related to stage 1 range between ~330° and 260°C; these systematically decrease from proximal and/or deep to distal and/or shallow parts of the district. Hydrothermal carbonate minerals are absent.

Stage 2: Commonly, stage 1 is truncated or overgrown by a sphalerite-Ag-sulfide association (Fig. 3A). In the deepest and/or proximal zones of the district, the onset of stage 2 is analogous to stage 1, characterized by the sphalerite-pyrite-quartz association (stage 2A; 270°–300°C), which gradually changes to the galena-quartz ± carbonate association (stage 2A; 260°–290°C). This transition marks the onset of the main boiling that eventually results in the precipitation of the sphalerite-Ag-sulfides-carbonate association (stage 2C; Fig. 9) with corresponding homogenization temperatures between 240° and 270°C. As a result of further cooling of the fluid, the mineralogy changes from sphalerite-Ag-sulfides-carbonate to Ag-sulfides-quartz (stage 2D) in more distal and shallower vein sections. Fluid inclusions related to stage 2D are in the range between 220° and 250°C. Even further cooling of the fluid to 190° to 260°C may locally result in the transition from Ag-sulfides-quartz (stage 2D) to stibnite-quartz (stage 2E) in more distal and shallow vein sections (cf. Burisch et al., 2019a).

Stage 3: Stage 3 is most abundant in distal and/or shallow vein sections and is commonly associated with brecciation of stage 1 and 2 (Fig. 3B, D, and E). Stage 3 is characterized by abundant Ag-sulfides-quartz mineralization (stage 3D) that changes to quartz-carbonate (stage 3F) in the shallow sections of the vein (Fig. 9). Homogenization temperatures related to stage 3 are lower than in the previous stages (170°–250°C). In deeper vein samples, stage 3 is difficult to recognize, since at deeper intersections it may be mineralogically indistinguishable from stage 1 or 2. Carbonate minerals late in the paragenesis, textures indicative for boiling, and heterogeneously trapped fluid inclusions shallow in the profiles indicate a shallower boiling onset than in stage 2.

Paleodepth constraints

Heterogeneous FIAs can be used to constrain the paleodepth during ore formation, since vapor pressure during boiling equals the hydrostatic pressure (Fig. 10; Haas, 1971; Hedenquist and Henley, 1985). Some uncertainty is, however, introduced by the fact that the effect of CO2 in the fluid cannot be estimated here, since quantitative CO2 analyses in the fluid inclusions have not been conducted. Carbon dioxide peaks in the Raman spectra do indicate minor concentrations of CO2 in the fluid. Based on the absence of clathrates in all analyzed fluid inclusions (Fig. 7), CO2 concentrations have to be below 1.5 mol % (Hedenquist and Henley, 1985; Diamond, 2001). Therefore, the reconstructed paleodepths (Fig. 10) have to be regarded as minimum values.

Results are most comprehensively illustrated in profile 2 (Fig. 10). Here, heterogeneous FIAs in quartz II (stage 2) from samples taken from present-day elevations of –160 and –240 m a.s.l. were formed at depths of at least 800 and 850 m below the paleowater table. The heterogeneous assemblage in quartz III (stage 3) from 260 m a.s.l., in contrast, yields a minimum paleodepth of 440 m. The greatest minimum depths of mineral formation of 1,250 m below the paleowater table are obtained for heterogeneously trapped FIAs from the central sector of the Freiberg district at a present-day elevation of 230 m a.s.l. (quartz II from stage 2; Bauer et al., 2019a; Fig. 10).

Shallow (~240 m a.s.l.) homogeneous FIAs in quartz II of stage 1 (profile 2, sample 52677) having homogenization temperatures between 234° and 286°C must have formed at a paleodepth below 850 m (below the liquid-vapor curve; Fig. 10B). However, heterogeneous assemblages (–240 m a.s.l.) in quartz II associated with stage 2 (profile 2, sample 52695) must also have formed at roughly the same paleodepth. The fact that these assemblages now occur almost 500 vertical meters apart, despite having originally formed at the same paleodepth, indicates that some of the overburden must have been eroded between stage 1 and 2. This denudation led to the emplacement of shallow-formed mineralization with lower homogenization temperatures (stage 2) next to deeper-formed mineralization with higher temperatures (stage 1)—i.e., telescoping. Since the CO2 content of the fluid is not known, the extent of this exhumation cannot be constrained precisely. It may lie between ~400 and 600 m.

The paleodepth of the boiling horizons varies laterally on the district scale. According to the paleodepth calculations above, in profile 1 (central sector) a present-day elevation of 230 m corresponds to a minimum paleodepth of ~1,250 m, whereas in profile 2 (northern sector, ~10 km north of profile 1) the same elevation corresponds to a minimum paleodepth of ~470 m (Fig. 10), resulting in a difference of 780 m. This regional difference may be caused by asymmetric uplift and net erosion related to Eger graben rifting or due to preexisting paleotopography.

The integration of minimum paleodepth estimates, microthermometric data, and petrography reveals that base metal-rich mineralization (stage 1) formed preferentially at paleodepths below ~800 m at temperatures between 240° and 330°C (Fig. 10). Conversely, major Ag (and Au) mineralization (stages 2 and 3) formed at depths of less than 800 m in the peripheral sectors, with the highest Ag grades above ~600-m paleodepth.

Fluid source

Rubidium-strontium ages of sphalerite from the polymetallic epithermal associations of the Freiberg district yielded ages of 276 ± 16 Ma (Ostendorf et al., 2019). This age relates polymetallic epithermal mineralization to postorogenic rifting and extensive bimodal volcanism in the region (Ostendorf et al., 2019). High concentrations of Sn (up to 1,729 g/t) associated with base metal-quartz association suggest that the ore-forming fluids are possibly related to ilmenite series intrusions (i.e., reduced; Lehmann, 1990, 2020; Arce Burgoa, 2009). The low salinity of the ore fluid is typical for distal magmatic-hydrothermal systems, which may occur as far as 6 km away from their source intrusion (Hedenquist et al., 2000) and which may furthermore be significantly influenced by the admixture of meteoric fluids (Hedenquist and Lowenstern, 1994). However, low-salinity fluids are also reported from other magmatic-hydrothermal systems were mixing with meteoric waters has been excluded (Catchpole et al., 2015; Rottier et al., 2018). The position of the source intrusions in the Freiberg district as a key element in the architecture of the mineralizing system is yet unknown.

The Freiberg district comprises old (Permian) and well-preserved intermediate-sulfidation epithermal systems that share many characteristics with well-studied Ag-Zn-Pb mineral systems elsewhere. Three ore-forming stages with distinct mineralogical characteristics are recognized, reflecting the temporal and spatial evolution of the magmatic-hydrothermal systems. Stage 1 mineralization is dominated by base metals (sphalerite-pyrite-quartz association). In stage 2, deep base metal mineralization (sphalerite-pyrite-quartz and galena-quartz ± carbonate) is followed by a carbonate-rich zone with high Ag concentrations (sphalerite-Ag-sulfides-carbonate), initiated by boiling. Silver concentrations increase successively toward shallower depths and in distal parts of the district. Stage 3 mineralization is abundant at shallow depths and in distal areas, characterized by high-grade Ag mineralization, shallow onset of boiling, and late barren quartz.

Petrographic and geochemical data indicate that high-grade Ag mineralization (avg ~4,900 g/t)—including the sphalerite-Ag-sulfides-carbonate and Ag-sulfides-quartz associations—extended over a vertical interval of at least 500 m (top is ~300 m below paleowater table). The presence stibnite-quartz mineralization that formed at shallow depth in the north to northwest peripheral sector of the district indicates that here the entire vertical interval of Ag mineralization may be preserved at depth. If so, these features may be used to target exploration efforts in the Freiberg district.

The authors are very grateful to Lluís Fontboté and Jeffrey Hedenquist; their very detailed and constructive reviews improved a former version of this manuscript significantly. Many thanks also to Lawrence Meinert for editorial handling. We are indebted to Christin Kehrer for access to the geoscientific collections of the TU Bergakademie Freiberg (TUBAF) and for support during sample selection, Roland Würkert and Michael Stoll (Helmholz Institute Freiberg for Resource Technology; HIF) for sample preparation, and Björn Fritzke (TUBAF) for technical support with the sample photographs. Birk Härtel (TUBAF) is gratefully acknowledged for his support with Raman spectroscopy. Ben Pullinger, Richard Sillitoe, and Matthias Jurgeit provided insightful discussions. Parts of this project were funded by the European Social Fund and the Federal State of Saxony (ESF; project 100339454 received by M. Burisch), Globex Mining Enterprises Inc., and Excellon Resources Inc.

Laura Swinkels is a Ph.D. candidate at the Technische Universität Bergakademie Freiberg. After obtaining a B.Sc. degree in Utrecht, Netherlands, and an M.Sc. degree in Tromsø, Norway, she moved to Freiberg to study the genesis of the epithermal vein systems of the Freiberg district. In her Ph.D. project, she integrates a wide range of methods, including fluid inclusion microthermometry, whole-rock geochemistry, and trace element geochemistry, to better understand the vein architecture and 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.

Mathias Burisch is an assistant professor at the TU Bergakademie Freiberg, Germany. He received his Ph.D. degree from the University of Tübingen in 2016. Since moving to Freiberg (October 2016) he has built a research project portfolio centering on different types of hydrothermal ore deposits. Together with his students and coworkers, Burisch constructs comprehensive mineral system models using a multitude of analytical methods.

Gold Open Access: This paper is published under the terms of the CC-BY 3.0 license.

Supplementary data