Wall rock metasomatism and formation conditions of the silver-polymetallicBol'shoi Kanimansur deposit (Tadjikistan)

Products of the hydrothermal alteration of rocks and vertical zonality at t he Kanimansur silver-polymetallic deposit (Tadjikistan) have been studied. On the basis of these studies, physicocochemical conditions of metasomatism and ore formation were reviewed. Condensed carbonaceous matter in wall roc k metasomatites and ores were found and carbon isotopic composition of this carbonaceous matter and composition of gases in open pores in rocks of the deposit were analyzed. As a result of geological observations, analysis of mineral parageneses, compositions of minerals, and chemistry of wall rock alteration, the following four stages C, tl of the hydrothermal process wer e distinguished: autometasomatic, propylitic, preore chlorite-sericite, and synore. Mineral geothermometers were used to estimate the temperatures of metasomatism, which varied within the range 200-250 degreesC for all stages except for the autometasomatic stage (450-500 degreesC). The formation of unhydrated light micas, quartz, and highly ferruginuous chlorites was relat ed to acid preore metasomatism. The stability of these chlorites testifies to a slight increase in the acidity of solutions during the preore stage (p H not less than 3.5). The wall rock metasomatites and sulfide ores are char acterized by a relatively rare occurrence of pyrite due to a low activity o f the sulfide sulfur during hydrothermal alterations( log a(S2) = -7...-10) . The low activity of the sulfide sulfur is confirmed by the fact that iron is tied in a highly ferruginuous chlorite. At upper ore levels, even zinc is tied in chlorite. This testifies to a much greater a(S2)/a(O2) value. Ju dging by the participation of carbonaceous matter in the process, ore compo nents were deposited due to a local increase in the alkalinity at the ore-f orming stage and oxidation of reduced fluids. The established features of m etasomatic transformations, are related to particulars of the regional magm atism.