Newly Discovered Native Gold and Bismuth in the Cihai Iron‐Cobalt Deposit, Eastern Tianshan, Northwest China

Abstract

AbstractThe Cihai iron‐cobalt deposit is located in the southern part of the eastern Tianshan iron‐polymetallic metallogenic belt. Anomalous native gold and bismuth have been newly identified in Cinan mining section of the Cihai deposit. Ore formation in the deposit can be divided into three stages based on geological and petrographical observations: (I) skarn, with the main mineral assemblage being garnet‐pyroxene‐magnetite; (II) retrograde alteration, forming the main iron ores and including massive magnetite, native gold, native bismuth, and cobalt‐bearing minerals, with the main mineral assemblage being ilvaite‐magnetite‐native gold‐native bismuth; and (III) quartz‐calcite‐sulfide assemblage that contains quartz, calcite, pyrrhotite, cobaltite, and safflorite. Native gold mainly coexists with native bismuth, and they are paragenetically related. The temperature of initial skarn formation was higher than 340°C, and then subsequently decreased to ∼312°C and ∼266°C. The temperature of the hydrothermal fluid during the iron ore depositional event was higher than the melting point of native bismuth (271°C), and native bismuth melt scavenged gold in the hydrothermal fluid, forming a Bi‐Au melt. As the temperature decreased, the Bi‐Au melt was decomposed into native gold and native bismuth. The native gold and native bismuth identified during this study can provide a scientific basis for prospecting and exploration for both gold‐ and bismuth‐bearing deposits in the Cihai mining area. The gold mineralization in Cihai is a part of the Early Permian Cu‐Ni‐Au‐Fe polymetallic ore‐forming event, and its discovery has implications for the resource potential of other iron skarn deposits in the eastern Tianshan.