Rb-sr dating of epithermal vein mineralization stages in the eastern Harz Mountains (Germany) by paleomixing lines

A method is evaluated that enables the dating of binary isotopic paleomixing in hydrothermal alteration assemblages by iterative reconstruction and regression of corresponding two-component mixing lines from conventional Rb-Sr data. The model has been applied to illite-bearing granulometric fractions of hydrothermally altered sedimentary rocks associated with epithermal vein mineralization in the eastern Harz Mountains, Germany. These veins are characterized by a principal quartz-sulfide stage containing quartz, base metal sulfides, and siderite and a subsequent dominant calcite-fluorite-quartz stage. Rb-Sr analyses were performed on untreated granulometric fractions, HCl-treated residues, corresponding HCl leachates, and cogenetic vein minerals obtained from two silicified rock samples related to the quartz-sulfide and the calcite-fluorite-quartz mineralization, respectively.Neither the untreated nor the HCl-leached fractions of these rock samples yield statistically robust and geochronologically meaningful isochrons. When applying the binary mixing model, however, two well-defined paleomixing lines can be reconstructed for the HCl-treated fractions of the rock samples and associated vein minerals at model ages of 226 ± 1 Ma (quartz-sulfide alteration) and 209 ± 2 Ma (calcite-fluorite-quartz alteration). We examine the significance and reliability of these model ages and the corresponding paleomixing lines by means of statistical and geological criteria and discuss the effects of leaching experiments performed on the granulometric fractions using 1 N HCl as well as the nature and origin of the components dissolved in the acid leachates.Our results suggest that the mixing model provides a successful dating concept for isotopic disequilibrium processes during low-temperature hydrothermal events that produce binary mixtures among newly formed alteration phases.