Hydrothermal Alteration of Host Rocks at the Schlema Uranium Deposit, Erzgebirge, Germany. (I. Principal Geochemical Characteristics)

摘要

For a long time, the world's largest Schlema uranium vein deposit in the Erzgebirge was referred to as a deposit in basic rocks. Indeed, at the uppermost levels of the Schlema, uranium ore veins are actually localized in mafic rocks of diabase composition. However, at depth, a considerable number of ore veins is hosted by carbonaceous schist, dark quartz-sericite, quartz-feldspar-sericite schists, and other country rocks relatively rich in silica. In general, about 75-80% of the uranium ore veins are hosted by "nonbasic" rocks. This calls for the consideration of interaction between the U-bearing solutions and rocks of contrasted composition, i.e., basic and silicic. The results of such a study, which was carried out for the first time, are reported in this paper. As was established, the silicic rocks, which host the major body of the uranium ore and possess a high reducing capacity, were in a relative equilibrium with the ore-bearing solution in terms of major components and underwent only a slight alteration near the fissure walls. The basic rocks reveal a severe disequilibrium with the ore-forming solution and reacted with it with the for- mation of astable silica-rich mineral assemblage quartz + K-(Na)-aluminosilicates + hematite. These relationships should be regarded as an important boundary condition for the reconstruction and physico- chemical modeling of ore formation at the deposit, including the modeling of hydrothermal alteration of host rocks.