Se minerals in the continental and submarine oxidation zones of the South Urals volcanogenic-hosted massive sulfide deposits: A review

The occurrence of Se minerals in continental and submarine oxidation zones of the South Urals volcanogenic-hosted massive sulfide deposits is reviewed. Clausthalite, naumannite and Se-bearing galena occur both in continental and submarine oxidation zones. Native selenium, tiemannite and Se-bearing secondary copper sulfides, sulfosalts and chalcopyrite are found only in the continental oxidation zone, whereas Te-bearing naumannite, bohdanowiczite, and Se-bearing roquesite are identified in the submarine oxidation zone. The Se minerals more often occur in the lower parts of the continental and submarine oxidation zones: secondary copper enrichment subzone and the bottom part of the leaching subzone. In the continental supergene zone, the Se-rich sulfur sand subzone mostly contains Se in forms of native phase and various selenides. Rare Se minerals are found in continental gossans (iron cap) and submarine gossanites (ferruginous products of complete submarine oxidation of massive sulfide ores mixed with hyaloclasts and carbonates). As shown by thermodynamic calculations, presence of selenides in the lower part of the oxidation zone is due to their stability under more oxidizing conditions than the corresponding sulfides at low temperature. The formation of selenides and Se-bearing sulfides in assemblage with Fe(III)-oxyhydroxides can be a result of low activity of reduced S, when Se successfully competes with S for the formation of the chalcogenides. The formation of selenides in the upper part of the oxidation zone is probably related to local redox barrier, which is formed due to vital functions of living organisms. The primary massive sulfide ores with Se-, Pb-, Ag-, and Hg-bearing high-temperature chalcopyrite and another high-temperature primary sulfides, and low-temperature colloform pyrite are source of the selenium and metals for supergene selenides and Se-bearing chalcogenides.