Physicochemical Conditions of Ore Formation at the Kalguty Mo–W Deposit: Thermodynamic Modeling

Abstract

Thermodynamic modeling was carried out for the formation of Mo–W ores at the Kalguty deposit (Gornyi Altai). The modeling was based on the physicochemical conditions of Mo–W ore formation estimated from fluid inclusion data. Quartz–wolframite veins of the deposit were formed under the influence of homogeneous reduced carbonate–chloride fluids, which showed elevated W and Sb concentrations. Pyrite–chalcopyrite–molybdenite mineralization was formed under the influence of heterogeneous oxidized sulfate–carbonate–chloride fluids enriched in Cu, Mo, W, Bi, and S. The economic combined greisen–vein Mo–W (Be) mineralization of the Kalguty deposit was formed by the superposition of molybdenite–chalcopyrite mineralization on the mineral assemblages of earlier quartz–wolframite veins. Ore forming processes were modeled for the scenarios of isobaric cooling and rock–solution interaction in the presence of oxidized and reduced model solutions corresponding to the natural ore-forming fluids of the Kalguty deposit. The results of thermodynamic modeling allowed us to conclude that rock interaction with oxidizing acid solution enriched in Cu, Mo, Bi, W, and S is the most plausible model for the formation of the greisen Mo–W ores. The interaction was accompanied by the inversion of the Eh–pH parameters of oxidizing ore-forming fluid and changes in its metal content, salt composition, and gas components. The chalcopyrite–molybdenite mineralization was formed during cooling of hot (>400°C) metalliferous oxidizing acid fluids before the inversion of their Eh–pH parameters. Wolframite ores could be deposited from the same portion of ore-forming fluid after the Eh–pH inversion and cooling. The combined Mo–W mineralization of the Kalguty deposit could result from at least two hydrothermal rhythms characterized by similar physicochemical parameters of ascending ore-forming fluids. The level of ore formation of each successive hydrothermal rhythm moved upward. This resulted in the telescoping of the high-temperature chalcopyrite–molybdenite mineralization by the earlier wolframite mineralization. The obtained data indicate the significance of probable vertical movement of the ore formation zone during the multistage mineralization process for the development of certain sequences of formation of mineral assemblages observed in ore deposits.