ABSTRACT

A chloride complex model for Kuroko mineralization at the Uchinotai-nishi deposit of the Kosaka mine is constructed. Possible chemical composition of the ore solution and possible physicochemical properties of the ore-forming environments calculated on the basis of the thermochemical data of HELGESON (1969) can be limited to narrow ranges, if more than 1 ppm Cu is assumed to be dissolved in the ascending Kuroko-forming solution. The model ore-forming environments derived here are consistent with the data of temperature and sulfur fugacity estimated from studies of fluid inclusions, mineral equilibria and sulfur isotopes of the Kuroko deposits. The characteristic mineral zoning of the deposits are explicable by differential precipitation of ore-constituent minerals from the model solutions moving along the gradients of the model ore-forming environments. In order to explain the distribution of sulfate minerals in the deposits, it is necessary to assume mixing of the ascending solution and the coeval seawater. In comparison with the chemistry of the natural metal-rich brines, the model solutions are low in Na/K ratio, while high in Na/Ca and reduced sulfur to metal ratios. It is suggested that these features might be characteristic of volcanogenic ore-forming solutions, which are likely to retain equilibrium conditions at higher temperatures as a result of their rapid ascent from the source reservoir.