Au-Ag-Te Ores from Epithermal Quartz Veins of the Kushikino Mine, Kagoshima Prefecture

Abstract

The Kushikino mine is located at the northeastern part of Kushikino City in Kagoshima Prefecture, Japan. The ore deposit is of representative epithermal Au-Ag quartz vein type developed in Miocene andesite. Two Te-concentrated veins were investigated in the present study: the Arakawa No.5 and Shinpi No.1 veins. The veins are largely composed of quartz, calcite, chlorite/smectite interstratified mineral, apatite, and ore minerals. The following ore minerals were confirmed to occur, as determined with ore microscope and electron microprobe: sphalerite, chalcopyrite, galena-clausthalite solid solution, pyrite, hessite, altaite, calaverite, petzite, electrum, sylvanite, etc. in the order of decreasing abundance.
A tentative low-temperature phase diagram for the Au-Ag-Te system was constructed in the present study on the basis mainly of the observed mineral assemblages and the chemical compositions of the coexisting minerals. The equilibrium temperature for the assemblages, inferred from the experimental studies carried out by previous in-vestigators, is approximately 100°C (at least lower than 120°C). On the other hand, the homogenization tempera-tures, measured on fluid inclusions in quartz and calcite associated with ore minerals, range from 200° to 250°C. If it is the case that the ore minerals precipitated from fluids in this temperature range, there is a significant temperature difference, as large as more than 100°C, between the precipitation temperature and the equilibrium temperature. This may be attributed to the rapid reaction rate during retrograde process and the relatively slow cooling rate in natural environments.