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

Abstract

The 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.

Au-Bi-Te(-Cu) Mineralization in the Wawa Gold Corridor (Ontario, Canada): Implications for the Role of Bi-Rich Polymetallic Melts in Orogenic Au Systems

“…The following isotopes were measured: 34 S, 57 Fe, 59 Co, 60 Ni, 63 Cu, 66 Zn, 75 As, 78 Se, 82 Se, 95 Mo, 98 Mo, 107 Ag, 109 Ag, 121 Sb, 130 Te, 184 W, 197 Au, 208 Pb, and 209 Bi. Several isotopes were also measured to monitor contamination from adjacent gangue minerals, these being 27 Al, 29 Si, 44 Ca, and 48 Ti. Transects across individual mineral grains were performed with a PhotonMachines 193 nm nanosecond Ar-F Analyte Excite excimer laser coupled to an Agilent 7900 quadrupole mass spectrometer using a laser energy of 4.1 µJ and a pulse rate of 20 Hz.…”

“…Given the apparent similarity in the isotopic and chemical nature of the fluids derived from these two reservoirs [3,8,[13][14][15], the usefulness of the geochemical association of Au with various elements as evidence for a specific fluid source has been a subject of interest (see [16] and references therein), and mineral assemblages and compositions that reflect a Au-Bi-Te association are often taken, in part, to be diagnostic of a magmatic-hydrothermal origin for ore metals [17][18][19][20][21][22][23]. Indeed, both the transport of Au in Bi-rich polymetallic melts and its association with Bi and Te have been repeatedly documented in a variety of magmatic-hydrothermal environments, including skarns [24][25][26][27][28], iron oxide copper-gold (IOCG) and iron oxide cobalt-gold-bismuth deposits [29,30], intrusion-related gold systems [31], greisens [32], and porphyry-epithermal systems [28,33,34]. However, despite the obvious relevance of Birich polymetallic melts in orogenic deposits to discerning both the transport mechanism(s) for Au and potentially the source of Au-bearing fluids, only a few studies have investigated such processes in natural orogenic Au systems [7,17,21].…”