Trace elements and textures of hydrothermal sphalerite and pyrite in Upper Permian (Zechstein) carbonates of the North German Basin
Introduction
For >150 years, the North German Basin (NGB) has been an area of economic interest due to its oil and gas resources. Research on hydrocarbon migration, drill cores, and reflection seismic data provide information relevant to the geologic community beyond the oil and gas industry. Numerous exploration and production drill cores reach depths of up to eight kilometers below the surface and penetrate the Lower (Rotliegend) and Upper (Zechstein) Permian units and provide drill core, e.g., for metallogenic studies. Thus, the lithotypes, subsurface structures and basin architecture of the NGB are well constrained (Brink et al., 1992; Littke et al., 2008; Maystrenko et al., 2008; Mazur et al., 2005; Otto, 2003; Scheck and Bayer, 1999; Schmidt Mumm and Wolfgramm, 2004).
Within the Ca2 carbonate, there is evidence of widespread hydrothermal sulfide mineralization of unknown age and origin in the subsurface of the Lower Saxony Basin (LSB). Mempel (1962) recognized the existence of mineralized zones and recent research efforts have elaborated on the characteristics, extent, and economic potential of these sediment-hosted Zn-Pb-Fe zones (Nadoll et al., 2018; Nadoll et al., 2019; Sośnicka and Lüders, 2018). However, many geochemical and mineralogical aspects of the sulfide mineralization have not been studied in detail. Industrial mining of mineral deposits (i.e., base metal sulfides) began as early as the 12th century in the LSB with the mining of silver-bearing galena in Zechstein sedimentary successions of the Ca1 formation in the Osning Mountains (Rose and Gödecke, 1984). The Ca1 marine sequence is further significant in the NGB and the Polish Trough in the east because the Kupferschiefer polymetallic mineralization occurs at the transition of Rotliegend terrestrial redbeds, volcanic rocks, and the Ca1 (Borg et al., 2012; Vaughan et al., 1989).
Here, we present a detailed petrographic and geochemical investigation of the carbonate-hosted (Ca2) Zn-Fe-Pb-sulfide mineralization in the LSB, with a focus on the chief sulfide minerals sphalerite and pyrite. In-situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses reveal compositional trends that help to elucidate the processes that led to the formation of these mineralized zones and associated physicochemical conditions (e.g., Cook et al., 2009; George et al., 2016; Keith et al., 2014; Ye et al., 2011), which can aid future exploration efforts. Compositional and sulfur isotopic signatures of sphalerite and pyrite are used to characterize and classify the mineralization and discuss genetic factors such as sulfur source, formation temperature (GGIMFis geothermometer), and timing.
Section snippets
The North German Basin
Northern Central Europe comprises several large-scale superimposed intracratonic basins that are commonly referred to as the Central European Basin System (CEBS) (Maystrenko et al., 2008). The CEBS spans an area from the Variscides in the south up to the Norwegian Danish Basin in the north, the North Sea in the west, and Poland in the east (Fig. 1A). The CEBS is subdivided into the Late Carboniferous to Early Permian WNW–ESE-trending Northern and Southern Permian Basin (SPB). Comprehensive
Sampling
Samples for this study were collected from oil and gas exploration drill cores, dating from the 1970s to 90s. The sampling was undertaken at the core storage facilities of ExxonMobil in Nienhagen, Germany and Wintershall Holding GmbH in Barnstorf, Germany. The majority of the drill cores are located in the center of the LSB with three drill cores at its outer perimeter (Fig. 1B).
The sample depth varies from 2.881 to 4.062 m below the surface. A total of 115 drill core specimens, ranging between
Mineralogy
The principal sulfide minerals hosted in the Ca2 carbonate, in decreasing order of abundance, are sphalerite, pyrite, and galena Figs. 3; 4A–H. These sulfides were encountered throughout all sampled wells demonstrating their widespread occurrence (Fig. 1B). Sphalerite is commonly associated with a younger generation of fluorite and barite (Fig. 4E). Some samples are monomineralic with respect to sulfides and solely contain sphalerite or pyrite (Fig. 4D, G), others show an assemblage of all
Sulfidation processes
Based on petrographic and geochemical evidence we can infer that sulfide mineralization formed during two major stages in the LSB—(i) early diagenetic and (ii) late hydrothermal, whereby the latter shows indications of successive mineralization with variable physicochemical conditions and varying fluid compositions. Early stage euhedral to subhedral pyrite formed in an early diagenetic setting. There are no indications that a hydrothermal fluid deposited this type of pyrite as it occurs evenly
Conclusion
There is evidence for a widespread epigenetic sediment-hosted hydrothermal Zn-Fe-Pb mineralization in the Permian Ca2 dolostone, where sphalerite, pyrite, and galena occur as massive hydrothermal ore on a millimeter to meter-scale. Four different pyrite types were classified, i.e., disseminated, clustered, stylolite-bound, and massive pyrite. The first two formed during diagenesis and the latter two during hydrothermal events. Pyrite has particularly low trace element contents, which do not
Acknowledgments
This research was funded by the German Ministry for Education and Research (BMBF), grant number [033R165]. We thank ExxonMobil and Wintershall Holding AG for providing the drill core samples. We would like to thank Helene Brätz for guidance with LA-ICP-MS analyses, Uli Schüßler for assistance during EPMA analyses, Max Frenzel for his support in the temperature calculations as well as Harald Strauss for sulfur isotope analyses. The editor Robert Ayuso, Gregor Borg, and two anonymous reviewers
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