WALL ROCK METASOMATISM AND FORMATION CONDITIONS OF THE SILVER-POLYMETALLIC BOL'SHOI KANIMANSUR DEPOSIT (TADJIKISTAN)

Abstract

Products of the hydrothermal alteration of rocks and vertical zonality at the 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 rock 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 of the hydrothermal process were 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°C for all stages except for the autometasomatic stage (450-500°C). The formation of unhydrated light micas, quartz, and highly ferruginuous chlorites was related to acid preore metasomatism. The stability of these chlorites testifies to a slight increase in the acidity of solutions during the preore stage (pH not less than 3.5). The wall rock metasomatites and sulfide ores are characterized by a relatively rare occurrence of pyrite due to a low activity of the sulfide sulfur during hydrothermal alterations (logaS2 = -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 aS2 /aO2 value. Judging by the participation of carbonaceous matter in the process, ore components were deposited due to a local increase in the alkalinity at the ore-forming stage and oxidation of reduced fluids. The established features of metasomatic transformations, are related to particulars of the regional magmatism.