Metal budget and origin of aqueous brines depositing deep-seated Zn-Pb mineralization linked to hydrocarbon reservoirs, North German Basin

Abstract

The origin, evolution, and interplay of brine and hydrocarbon fluid systems play a crucial role in the formation of deep sediment-hosted base metal ore deposits. Here we investigate ratios of halogens, noble gases, stable C and S isotopes, and metal budgets of aqueous brines, which deposited deep-seated and near-surface hydrothermal Zn-Pb mineralization hosted by Zechstein carbonates in the Lower Saxony Basin (North German Basin), by studies of fluid inclusions in sphalerite and quartz. Major and trace element geochemistry and noble gas isotopic signatures of brine inclusions revealed that the ore-forming fluids were highly reactive and experienced prolonged interactions with host rocks in the constricted, over-pressured metal source regions and consequently evolved from near-neutral, oxidized brines towards more reduced, acidic high-salinity brines. Quartz-hosted halite-saturated fluid inclusions with Th <200°C contain Zn and Pb concentrations up to ca. 9400 μg g-1 and 5200 μg g-1, respectively, and indicate the efficiency of metal scavenging processes. The interactions with Westphalian coals and Corg-rich shales influenced the redox state as well as the trace and critical element budget of the sphalerite-hosted fluid inclusions, with enrichment in Ge, Pd, Sb, Tl, Bi, and Ag. The salinities of metalliferous fluids originated primarily from seawater evaporation, however in addition a significant halite-dissolution component is present in the southern part of the Lower Saxony Basin. High concentrations of radiogenic noble gases and potassium in the sphalerite-hosted fluid inclusions are ascribed to strong interactions with the Paleozoic siliciclastic sedimentary pile and crystalline basement rocks. Reflux of the strongly modified, sulfur-poor, Zn-Pb-bearing acidic brines, proceeded via re-activated structurally controlled pathways into sour gas or gas-saturated brine pools in the Zechstein Ca2 carbonate unit. Here, mixing of the ascending metal-rich brines with H2S derived from thermochemical sulfate reduction (TSR), resulted in the deposition of deep-seated Zn-Pb ores in the Lower Saxony Basin. The overall timing of the Zn-Pb ore formation can be constrained to the Upper Cretaceous basin inversion.