Abstract

The assemblage electrum-sphalerite-pyrite-argentite is common in epithermal Au-Ag vein-type deposits in Japan. On the basis of the iron content of sphalerite and the silver content of electrum in this assemblage, the precipitation temperatures were estimated. The electrum-sphalerite temperatures were compared with the filling temperatures of fluid inclusions in quartz, which was considered to have grown nearly simultaneously with this mineral assemblage. Almost all electrum-sphalerite temperatures are within + or - 30 degrees C of the filling temperatures, showing a reasonably good agreement. This agreement indicates that (1) the f (sub S 2 ) -temperature relations in the Au-Ag-S and Fe-Zn-S systems can be extrapolated to temperatures below 200 degrees C from < or = 300 degrees C experimental data, (2) the electrum-sphalerite-pyrite-argentite assemblage probably formed at close to an equilibrium condition, and (3) the pressure corrections to filling temperatures of fluid inclusions necessary to obtain formation temperatures are relatively small (<30 degrees C). In contrast to the precipitation temperatures for the electrum-sphalerite-pyrite-argentite assemblage, the temperatures for the electrum-sphalerite-pyrite-Ag- Se Te mineral (naumannite, aguilarite, hessitc) assemblage are not in good agreement with the filling temperatures of fluid inclusions. A less than unity activity of Ag 2 S in Ag-Se-Te minerals may explain this disagreement.

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