ALBERT

Beschreibung: 〈span〉〈div〉Abstract〈/div〉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 〈δ〈sup〉34〈/sup〉S〈sub〉H〈sub〉2〈/sub〉S〈/sub〉 (–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 (〈sup〉206〈/sup〉Pb/〈sup〉204〈/sup〉Pb = 18.2366–18.3146, 〈sup〉207〈/sup〉Pb/〈sup〉204〈/sup〉Pb = 15.5404–15.5624, 〈sup〉208〈/sup〉Pb/〈sup〉204〈/sup〉Pb = 38.0901–38.2293). In contrast, the δ〈sup〉18〈/sup〉O〈sub〉H〈sub〉2〈/sub〉O〈/sub〉 (–13.6 to –7.6‰) and δD〈sub〉H〈sub〉2〈/sub〉O〈/sub〉 (–127 to –96‰) values indicate that the hydrothermal fluids were dominantly meteoric. Physicochemical modeling shows that sulfidation of the host rocks (decrease of 〈span〉α〈/span〉〈sub〉HS〈/sub〉〈sup〉–〈/sup〉〈sub〉(aq)〈/sub〉) and condensation of H〈sub〉2〈/sub〉Te-bearing vapors (increase of 〈span〉α〈/span〉〈sub〉HTe〈/sub〉〈sup〉–〈/sup〉〈sub〉(aq)〈/sub〉) 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 H〈sub〉2〈/sub〉Te-bearing magmatic fluids with meteoric waters in producing large Au and Ag telluride deposits.〈/span〉

Beschreibung: 〈span〉〈div〉Abstract〈/div〉We present the first systematic characterization of the Fe isotope composition of magmatic pyrrhotite for the application of Fe isotopes to the origin and evolution of magmatic sulfide deposits. Iron isotopes provide constraints on redox, temperature, fluid exsolution, fractional crystallization, and intermineral diffusion at magmatic or subsolidus conditions. Paired with S isotopes, Fe isotopes have proven to be useful tracers of crustal contamination in bulk magmatic sulfide. Relative and specific Fe isotope values exist for major Fe-bearing igneous minerals; however, there is a paucity of well-constrained Fe isotope data for sulfides. Recent studies indicate that pyrrhotite will strongly influence the Fe isotope systematics of magmatic systems, yet there are few published 〈span〉δ〈/span〉〈sup〉56〈/sup〉Fe values (〈sup〉56〈/sup〉Fe/〈sup〉54〈/sup〉Fe in the sample relative to IRMM-14) of natural pyrrhotite. To provide this fundamental information, we report the first dataset for magmatic pyrrhotite from nine deposits of various origins and ages. The 〈span〉δ〈/span〉〈sup〉56〈/sup〉Fe value of pyrrhotite samples (〈span〉n〈/span〉 = 17) ranges from –0.55 ± 0.04‰ to +0.05 ± 0.03‰, and reflects the composition of the sulfide, variable degrees of assimilation, and crystallization history of each deposit. Only the impact-related Sudbury deposit shows especially light 〈span〉δ〈/span〉〈sup〉56〈/sup〉Fe values for pyrrhotite (–0.89 ± 0.04‰; (–0.62 ± 0.04‰; 〈span〉n〈/span〉 = 2). A modeling approach using these new data confirms the potentially strong influence of pyrrhotite on the Fe isotope systematics of a magmatic system. A global dataset of pure individual sulfide minerals will aid in more accurately tracing the processes at play in the formation and evolution of deposits containing magmatic sulfide.〈/span〉