ABSTRACT

The relation in the chemical composition between coexisting sphalerite and carbonate, which occur together with pyrite, from some hydrothermal vein deposits, was investigated on the basis of the data of both theoretical considerations and electron microprobe analyses. The chemical environment of hydrothermal ore deposition was then discussed. Concentrations of total dissolved sulfur and total dissolved carbon, NaCl-equivalent concentration and pH play an important role as the factors limiting the hydrothermal condition, reflecting the chemical nature of ore-forming solution. Therefore, the relation between the activity of FeS in sphalerite and that of FeCO₃ in carbonate should be considered as a function not only of fugacities of gaseous phases concerned, but also of chemical composition of the ore-forming solution. For refined theoretical considerations, activity-concentration relations for components in the solid phases must be known. The relation between activity and concentration of FeS in sphalerite can be obtained from the recent experimental works in the Zn-Fe-S system. That of carbonate can be approximately obtained on the basis of the experimental work for the CaCO₃-FeCO₃ system by ROSENBERG (1963) and the theoretical study by THOMPSON (1967). Analytical data are plotted on logm'^sp_FeS (m' and sp mean gram formula weight and sphalerite, respectively)-logm'^cb_FeCO3 (cb means carbonate) diagram. In general, the results of the measurements are consistent with the theoretical consideration. Namely, a positive correlation of the logm'^sp_FeS and the logm'^cb_FeCO3 was obtained. Comparing the theoretical consideration with the analytical data obtained, it can be concluded that the fugacity of oxygen gas during ore deposition varies in a very wide range. About eight orders in the range for the fugacity of oxygen gas in logarithmic scale is deduced as a result. This result tells us that the fugacity of oxygen gas could be the most important parameter limiting the physicochemical environment among many parameters. The fugacity of oxygen gas at the time of deposition is increased from the Toyoha (Pb-Zn) thorough the Oe (Mn-Pb-Zn), Inakuraishi (Mn) and Jyokoku (Mn) to the Omori (Au-Ag) vein deposits. The relation may indicate decreasing order of the depth of formation.