Geological And Geochemical Characteristics And Ore Genesis Of The Sadaowanzi Gold-telluride Deposit In Heilongjiang Province, NE China

The newly discovered Sandaowanzi gold-telluride deposit, with a total reserve of more than 25 t of Au and an average grade of 15 g/t, is located in the Great Hinggan Range Metallogenic Belt in NE China. This deposit is unique because it is the first reported case of a dominantly Au(±Ag)-telluride deposit in this area. Ore bodies principally occur in quartz veins and stockworks and minor in disseminations hosted by trachyandesites and andesitic breccias. Four paragenetic stages of mineralization are identified, demonstrating an early deposition of sulfides and subsequent precipitation of tellurides, which are mainly composed by petzite, sylvanite and to a lesser extent, hessite, calaverite, altaite, unnamed telluride (Au1.8Ag0.2Te), krennerite, empressite, stutzite and coloradoite. The telluride associations are mostly derived from decomposition of previous phases (i.e.γ-and χ-phases) during cooling..Fluid inclusions in quartz veins are mainly dominated by four distinct tyes, i.e., L-V, V-L, pure liquid or gas and L-V-S inclusions. Microthermometric data reveal that the Sandaowanzi ores precipitated under hydrostatic pressures of ～450 bars, and from moderate-temperature (240° to 280℃) and low to moderate salinity (0.2 to 14.2 wt% NaCl equivalent) hydrothermal fluids. As the proceding of mineralization, the hydrothermal system was characterized by subsequent cooling and existence of fluid boiling.The δ34SH2S (-2.2 to -0.2‰) values indicate a magmatic source for sulfur, whereas the δ18OH2O (-13.6 to -7.6‰) and δDH2O (-127 to -96‰) values are consistent with the dominance of meteoric waters for multiple quartz formation. Lead isotopes suggest the origin of ore materials and magmatism responsible for andesites and granites was partly attributed to mantle. Noble gas (He and Ar) isotopes demonstrate clear mixing trends between crustal and mantle-derived components, confirming a significant contribution of fluid produced from mantle-derived magmas (maximum value as 17%) into the epithermal system.Twelve pyrite samples from the alteration zone yield a well defined Rb-Sr isochron age of 119.1±3.9 Ma, which is indicative of the formation of alteration minerals. This age is in agreement with a robust Rb-Sr isochron age of 121.3±2.6 Ma derived from 10 auriferous quartz samples. The obtained isochron age of ～120 Ma represents the formation of the Sandaowanzi gold-telluride epithermal system, which is approximately synchronous with the host trachyandesite with an age of 124±4 Ma and much younger than the spatially associated monzogranite with an age of 182±1 Ma. Dating results indicate a close relationship between the Au-Ag-Te mineralization and the Early Cretaceous volcanism.Physicochemical conditions of the Sandaowanzi mineralizing fluids are pH=3.01 to 5.95,fS2=10-14.4 to 10-9.25, fO2=10-38.8 to 10-31,fTe2=10-15.2 to 10-7.9 and logaAu+(aq)αAg+(aq)=10-6.87 to 10-6.56, and gold is mostly scavenged from a HTe--dominant ore-forming fluid. The pH and αHTe-(aq)/αHS-(aq) increase coupled with fO2 decrease due to occasional boiling and subsequent condensation are the plausible mechanisms of precipitation of abundant Au- and Ag-telluride ores.