Geochemistry of the La'erma and Qiongmo Au-Se deposits in the western Qinling Mountains, China

The microdisseminated gold deposits at La'erma and Qiongmo are typically ho sted in Cambrian chert formation in western Qinling Mountains, China. Both in the host rocks and the gold ores selenium is present in high abundance, as selenide minerals or isomorphous admixtures in sulfides, with independen t selenium orebodies can be delineated in some localities. Intergrowth of s elenides and native gold is commonly observed. Selenium is also noticed in considerably high content in fluid inclusions in quartz and barite, which a re closely related to gold mineralization. Studies show that gold and selen ium may have been transported together in ore solutions as Au-S-Se complex. In view of the wide difference in electrode potential between selenium and sulfur, the two elements would be separated under oxidizing conditions. Se lenium would mostly occur as isomorphic mixtures in sulfides during the ear ly stage of mineralization when the environment is more reducing but would precipitate as selenides during the late, oxidizing stage. Simultaneous enr ichment of gold and selenium would be favored by boiling of the ore solutio n or mixing of on solution with oxygen-rich water at shallow depths. (C) 20 00 Elsevier Science B.V. All rights reserved.