Geochemical study of different-aged mining dump materials in the Freiberg mining district, Germany

Historical mining dumps are useful archives for the investigation of weathering processes. The objective of this study was to investigate the weathering behavior of waste-rock material derived from the 800-year-old silver ore mining in Freiberg, Germany. For identifying time-dependent weathering indices, dumped material of four dumps of different ages and corresponding rock was examined regarding the geochemical composition. The dumped material is characterized by high contents of heavy metal containing sulfidic ores, such as pyrite, arsenopyrite, sphalerite and galena. Acid mine drainage is produced by the oxidative weathering of the sulfide minerals and causes the increased dissolving of soluble metals with increasing age of dumps. As a result of these weathering processes, a clear depletion of chalcophile elements in the older dump material (800 years) compared to the youngest dump (100 years) was observed. In the soil horizons downstream the dumps, high quantities of heavy metals (e.g., up to 12,000 ppm As, 3,300 ppm Pb, 640 ppm Zn), mainly adsorbed on organic matter, were determined and indicate a time-dependent element transfer from the dumps into their surrounding soils.