Fluid evolution and ore deposition in the Harz Mountains (Germany)

Abstract

Quartz-stibnite, Pb-Zn, Ba-, and F-vein mineralization in the Harz Mts. over a long span of time from the Late Carboniferous to the Late Cretaceous. The economically most important quartz-sulphide and subsequent carbonate-sulphide ore deposits were deposited during the Jurassic. The homogenization temperatures of primary two-phase fluid inclusions in vein quartz lie between 120 and 180 o C on average. Secondary two-phase inclusions in calcite and fluorite show approximately the same range, whereas only few primary fluid inclusions have homogenization temperatures between 210 and 300 o C. Since boiling and effervescence can be excluded, precipitation of calcite may have been achieved from fluids when cooling from 300-200 o C as indicated by the positive δ 13 C vs. δ 18 O relationships in calcite. The evolution of chlorinity in the fluid inclusions reveals that, at different times, fluids from different sources were drained and mixed. Sulphides in the quartz-sulphide and carbonate-sulphide mineralizations were deposited by mixing of an ascending metal- and carbonate-bearing H 2 S-free metamorphic fluid with a H 2 S-bearing formation water. By contrast, the gangue minerals precipitated by simple cooling of the ascending fluid. Subsequently, barite was deposited by the mixing of a probably similar metamorphic fluid with descending SO 4 2− -bearing fluids from residual evaporites of the Mesozoic cover. Finally, fluorite locally precipitated from ascending brines originating from deeper sources than the carbonate-rich metamorphic fluid