Association of Electrum and Calcite and Its Significance to the Genesis of the Hishikari Gold Deposits, Southern Kyushu, Japan

Abstract

Abstract. Mineral assemblage, precipitation sequence and textures of the gold‐bearing veins from the Hishikari epithermal vein‐type deposits, southern Kyushu, Japan, were examined. In addition, fluid inclusion microthermometry and carbon and oxygen isotopic compositions of calcite were determined. Calcite, and that replaced by quartz, were commonly observed throughout the precipitation sequence of the veins. Thus, calcite must be a more common gangue constituent initially than observed presently. Association of calcite and electrum is observed immediately subsequent to columnar adularia in some vein samples. In addition, close association of electrum with pseudo‐acicular quartz, and electrum with truscottite were observed. The initial coprecipitation of electrum and calcite might be a common phenomenon in the gold‐bearing veins at the Hishikari deposits.The Th (homogenization temperature) data from the Honko‐Sanjin deposits are generally higher than those from the Yamada deposit. Samples that show association of calcite and electrum yielded higher Th (206–217°C, average) than the Th data from calcite associated with low‐grade Au ore or barren (180–204°C, average). The measured Tm (temperature of last melting point of ice) range from ‐0.4 to 0.0°C. The result suggests that the salinity of the hydrothermal solution was low during the precipitation both of calcite associated with Au mineralization and of barren calcite. Fluid inclusion evidence suggestive of boiling of hydrothermal solution for the precipitation of calcite was not recognized in the present work. The δ13C and δ18O values of calcite range from ‐10.8 to —4.7 % and from +3.2 to +15.2 %, respectively. The δ13C value of H2CO3 and the δ18O value of H2O in the hydrothermal fluids calculated assuming isotopic equilibrium with calcite using the temperature obtained by fluid inclusion microthermometry, range from ‐14.4 to ‐9.1 %, and from ‐6.2 to +5.5 %, respectively. Thus, the calculated δ18O values of H2O for calcite further confirm the presence of the 18O‐enriched ore fluids during the mineralization at the Hishikari deposits. The hydrothermal solution isotopically equilibrated with the sedimentary basement rocks was responsible for the gold mineralization associated with calcite.