1990 Volume 40 Issue 223 Pages 337-352
Epithermal gold deposits in Japan may be divided into Se-and Te-types. The Se-type occurs in sedimentary and volcanic rocks, while all deposits of the Te-type formed in volcanic rocks. The Se-type is accompanied by Ag, Pb, Zn and Mn mineralization, while the Te-type is, associated with Cu and Bi, and rarely Hg and Tl. The Se-type is characterized by the presence of large amounts of adularia, sericite, and carbonates but lacks barite. The Te-type is characterized by the presence of barite, sericite, and absence of carbonates. Cu minerals such as enargite are common and Bi minerals such as bismuthinite occur in the Te-type but Cu minerals are rare and there are no Bi minerals in the Se-type. Sericite alteration is common in Te-type gold mining districts, while propylitic and potassic alterations dominate in Se-type gold mining districts. Fluid inclusion data indicate that the Te-type and Se-type formed over temperature ranges of 200-300°C and 150-270°C, respectively. δ34S of sulfide sulfur for the Se-and Te-types are -8 to +5‰ (av. 0‰) and -3 to +7‰ (av. +2 to 3‰), respectively, indicating an igneous source of sulfide sulfur for the Te-type and igneous and sedimentary sources for the Se-type. δ34S of sulfate sulfur for the Te-type is in the range of +18 to +29‰, suggesting a contribution of SO2 gas of volcanic origin. Sulfur and oxygen activities of ore fluids for the Te-type may have been higher than those for the Se-type, while the pH of ore fluids for the Te-tvpe is estimated to have been lower than that for ore fluids of the Se-type.
These geologic, mineralogic and geochemical characteristic features and a comparison of these features with those of active geothermal systems accompanied by epithermal-style mineralization indicate that (1) the Se- and Te-types formed in two different parts of same geothermal system, (2) the Te-type formed closer to the heat source than the Se-type, (3) the Te-type formed at a mineralization site further from the volcanic center than acid-sulfate-type and (4) contribution of SO2 gas of volcanic origin and the type of host rocks are important factors in causing of the differences in the chemical nature (oxygen activity, sulfur activity, pH, sulfur isotopic compositions) of ore fluids responsible for the formations of the Te- and Se-bearing epithermal gold deposits in Japan.
