Gold-telluride-sulfide association in the Sandaowanzi epithermal Au-Ag-Te deposit,NE China: implications for phase equilibrium and physicochemical conditions

The Sandaowanzi gold-telluride deposit, with a total reserve of ?≥?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 the first reported case of a dominantly Au (±Ag)-telluride deposit in this area and it reveals highly economic bonanza Au- and Ag-telluride ores. 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 (Au_1.8Ag_0.2Te), krennerite, empressite, stützite and coloradoite. Abundant telluride assemblages identified from Sandaowanzi ores are mostly attributed to breakdown of early tellurium-bearing phases (i.e., γ- and χ-phases) during cooling. The deposition of substantial Au-Ag-Te minerals are constructed under physicochemical conditions of T?=?240 to 280 °C, pH?=?4.39 to 5.64, logfO₂?=–44.8 to –41.8, logfTe₂?=–9.75 to –9.43, logαAu⁺ _(aq)/αAg⁺ _(aq)?=??6.87 to –6.56, and gold is mostly scavenged from a HTe^?-dominant ore-forming fluid. The unusually high Te concentrations in the Sandaowanzi epithermal system are likely attributed to alkaline to calc-alkaline magmatic degassing.