Supergene processes on ore deposits?a source of heavy metals

Abstract

The study of supergene processes (i.e., secondary processes running in ore deposits and driven by thermodynamic nonequilibrium between ore-and rock-forming minerals and natural waters, gasses, etc.) is important in order to understand the migration of heavy metals from ore into their adjacent surroundings. The contamination of the local environment can be characterized by the composition of pore waters. The Pb-Zn-Cu ore deposits of Zlate Hory (Czech Republic) have been chosen for a detailed study of pore solutions. A simple model has been created to describe the evolution of supergene processes in the ore deposits. This model is based on the determination of chemical composition of pore solutions. The dilution of pore solutions of such mineral deposits results in acid mine drainage. Pore solutions can have, during specific stages of their evolution, relatively high concentrations of Cu (0.09 mol/kg), Zn (0.1 mol/kg), SO4 (0.8 mol/kg) and an extremely low pH (1.38). The supergene alteration of pyrite is the most important process determining the character of pore water. This reaction causes significant acidification and is a leading source of acid mine drainage. The leached zone originates from the interaction of pyrite and limonite. Increased concentrations of heavy metals and sulfates occur in pore waters. The dynamic composition of pore waters within ore deposits undergoing the supergene process can be used to distinguish: (1) three main zoneslimonite, transition, and primary zone and (2) two areas—an area with the highest intensity of weathering processes and an area of weathering initiation. In these areas the rate of sulfide oxidation is higher as a result of low pH. From the study of these zones and areas we can further our knowledge of ore body, pore solution, acid mine drainage, and contamination of the local environment.