Trace elements and textures of hydrothermal sphalerite and pyrite in Upper Permian (Zechstein) carbonates of the North German Basin

Deeply buried Upper Permian (Zechstein) carbonates in the southwestern North German Basin host widespread Zn-Fe-Pb sulfide mineralization. The spatial extent of the sediment-hosted mineralization, the geochemical formation conditions, and the number of mineralizing events that are manifested in the Permian sedimentary rocks are an ongoing matter of debate and interest for mineral exploration efforts in the North German Basin. We present detailed petrographic and geochemical data of ore minerals from drill core samples from the Lower Saxony Basin that forms the southwestern part of the North German Basin. Based on these data, we propose and discuss a genetic model for the ore-forming processes. Of particular interest are the sources of metals and sulfur and the timing and temperature at which the sulfides formed. The main sulfide minerals are sphalerite, pyrite, and galena, which commonly occupy mm- to m-scale open space fillings and fractures. Petrographic observations further suggest hydrothermal replacement of the Zechstein Ca2 carbonates, by sulfides, secondary calcite, and anhydrite. Early diagenetic pyrite occurs disseminated in the Zechstein Ca2 carbonates. Scanning electron microscopy, electron microprobe, laser ablation ICP-MS, and sulfur isotope analyses were used to collect petrographic and compositional data on the sphalerite and pyrite. Hydrothermal sphalerite displays cyclical growth patterns and sectoral zoning characterized by colors varying from colorless to black and corresponding indicative variations in their minor and trace element concentrations, e.g., Fe concentrations ranging between below the limit of detection (7.7 ppm) to a maximum of 2.8 wt%. A detailed LA-ICP-MS multielement map illustrates the observed growth patterns and reveals that Mn, Co, Fe, Cu, Ge, Ag, Cd, Hg, Tl, and Pb are the elements that control the distinct color variations. Sectoral zoning is reflected in comparatively high Cu, Ag, and Pb contents, whereas the cyclical growth banding can be retraced by variations in Fe, Co, Cd, Hg, and Tl contents. Calculated temperatures from the trace element signatures using the GGIMFis geothermometer indicate a formation temperature of 148 ± 55 °C for sphalerite. Although the investigated mineralization bears many trademarks of a Mississippi Valley-type (MVT) deposit, the elevated temperatures, a burial depth of around 3.2 km, low concentrations of common strategic elements such as Ga, Ge, and In (< median of 1.3 ppm in sphalerite), as well as the abundance of pyrite represent the unique nature of the Ca2-hosted mineralization. The observed epigenetic ore zones are therefore best described as carbonate replacement mineralization and present an intriguing case study of geochemical exploration in a deeply covered terrane