Mineralogical, fluid inclusion, and multiple isotope (H-O-S-Pb) constraints on the genesis of the Sandaowanzi epithermal Au-Ag-Te deposit, NE China

Abstract

The Sandaowanzi gold deposit in the Great Hinggan Range metallogenic belt, northeast China, is unusual because the mineralization takes the form of Au- and Ag-bearing tellurides. This mineralization, which constitutes a resource of 28 t Au, is hosted in 20 NW-trending, syntaxial, layered quartz veins and tension gashes within almost coeval trachyandesites and andesitic breccias that overlie the Sandaowanzi monzogranite. Alteration halos are developed around the quartz veins and tension gashes. They consist of an inner silicic-pyritic zone, two intermediate zones containing quartz-illite-sericite and quartz-adularia, and an outer zone containing a quartz-calcite-kaolinite-chlorite assemblage. Fluid inclusion microthermometric data reveal that the Sandaowanzi ores precipitated from moderate-temperature (200°–280°C), low-pressure (70–130 bar), and low- to moderate-salinity (mostly <6.0 wt % NaCl equiv) hydrothermal fluids. Temperature and pressure are interpreted to have decreased with the evolution of the system. The <δ34SH2S (–2.2 to –0.2‰) values indicate that the sulfur is of magmatic origin and suggest that it was leached by the ore fluid from the host volcanic rocks. A magmatic origin is also interpreted for the metals, based on Pb isotope data (206Pb/204Pb = 18.2366–18.3146, 207Pb/204Pb = 15.5404–15.5624, 208Pb/204Pb = 38.0901–38.2293). In contrast, the δ18OH2O (–13.6 to –7.6‰) and δDH2O (–127 to –96‰) values indicate that the hydrothermal fluids were dominantly meteoric. Physicochemical modeling shows that sulfidation of the host rocks (decrease of αHS–(aq)) and condensation of H2Te-bearing vapors (increase of αHTe–(aq)) were the dominant controls on precipitation of the Au and Ag telluride ores. This study emphasizes the importance of fluid-rock interaction (sulfidation) and the mixing of low-density H2Te-bearing magmatic fluids with meteoric waters in producing large Au and Ag telluride deposits. GeoRef Subject Asia D/H Da Hinggan Ling copper ores hydrogen lead China fluid inclusions inclusions isotopes Far East gold ores igneous rocks isotope ratios molybdenum ores Pb-206/Pb-204 metal ores metals metamorphic rocks O-18/O-16 Pb-208/Pb-204 radioactive isotopes silver ores mineral deposits, genesis iron ores skarn stable isotopes metasomatic rocks metasomatism Pb-207/Pb-204 oxygen paragenesis tellurides S-34/S-32 volcanic rocks sulfur trachyandesites tin ores