Mineralogy and mineral chemistry related to the Au mineralization in the Dunde Fe-Zn deposit, western Tianshan

The Dunde deposit, which is located in the Awulale Fe metallogenic belt, Xinjiang, China, is a uniquely large Fe-Zn deposit and also contains considerable Au resources. We present the results of a comprehensive study of this deposit based on fieldwork, mineralogy, and mineral geochemistry to constrain the Au mineralization. Our study indicates that the mineralization of Fe, Zn, and at least some Au precipitated in the same hydrothermal system. The Au mineralization, which occurs in the sulfide stage, is characterized by invisible Au at high temperatures and by electrum at low temperatures. Unusual sulfide is recognized in the deposit, such as cobaltite and bismuth minerals. Pyrrhotite compositions indicate that the early precipitation of sulfide generated in high temperatures (410–570 ℃) and high sulfur fugacities (−2.47 to −6.68). Arsenopyrite compositions indicate that the late precipitation of sulfide formed in low temperatures (326–393 ℃) and low sulfur fugacities (−7.41 to −12.00). Bismuth minerals exhibit a close relationship with the Au mineralization. In high temperatures and high oxygen fugacities, bismuth dissolved in the hydrothermal fluid as Bi3+ and precipitated as galenobismuthite and cosalite accompanying precipitation of the invisible Au. Accompanying the decrease of oxygen fugacities and temperatures, the Bi melt directly separated from the hydrothermal fluid and scavenged Au. The reduced condition further enriched Au in Bi melts through the decomposition of the invisible Au-bearing Bi-Pb sulfide. The decrease of temperatures caused the decomposition of the Bi-Au melt and the formation of native bismuth, bismuthinite, and electrum. In addition to the mineralization of Fe, Zn and Au, considerable Cu-, Co-, and Mn-bearing sulfide precipitated in the Zn ores. Integrated utilization of the Zn ores may get greater economic values.