Super-deep, TSR-controlled Phanerozoic MVT type Zn-Pb deposits hosted by Zechstein-2 gas reservoir carbonate (Ca2), Lower Saxony Basin, Germany

Abstract

The Earth's crust is endowed with outstanding mineral wealth, however, in the future near-surface ore deposits will become exhausted and mineral potential of deeper crustal levels will become a target for exploration. Knowledge of the fluid dynamics of deep-seated ore-forming systems in e.g. sedimentary basins is therefore crucial for developing genetic models, which would facilitate effective exploration of hidden orebodies. Here we utilized fluid inclusion and stable isotope analyses in order to decipher the ore-forming processes, which were responsible for deposition of considerable deep-seated (2.7–3.6 km) Zn-Pb mineralization in the Lower Saxony Basin (LSB). Massive sphalerite-rich stratiform/stratabound and vein-type mineralization in the LSB is hosted by Ca2 carbonate. Our data show that ore deposition was controlled by mixing of reservoir H2S with hot (T = 125–208 °C), highly-saline (21–32 wt% NaCl equiv.) metalliferous fluids ascending from greater depth along fault zones. Sulfur isotope ratios of sulfides (δ34S = −12.5 to +8.5‰) and carbon isotope ratios of fluid inclusion gases (δ13CCH4 = −6.2 to −22.7‰; δ13CCO2 = −0.8 to −6.2‰) reveal compelling evidence for TSR (thermochemical sulfate reduction)-derived origin of H2S. The ore-forming fluids were expelled from an over-pressurized system during Late Cretaceous basin inversion. Depth estimates show that the Zn-Pb mineralization in the LSB formed considerably deeper (3.3–4.4 km) than any other Phanerozoic MVT Zn-Pb deposit and therefore they can be classified as a super-deep, TSR-controlled MVT end-member.