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 © 2021 Gold Open Access: This paper is published under the terms of the CC-BY license. 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 Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/116/7/1649/5386619/4833_swinkels_et_al.pdf by guest on 28 September 2021 1650 SWINKELS ET AL. 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. Downloaded from http://pubs.geoscienceworld.org/segweb/economicgeology/article-pdf/116/7/1649/5386619/4833_swinkels_et_al.pdf by guest on 28 September 2021 EVOLUTION OF THE FREIBERG DISTRICT, GERMANY 1651 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

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