Influence of fluid-rock interaction on gold mineralization in the Dongwan deposit, East Qinling, China: Constraints from systematic sulfur isotope and trace element geochemistry

The Dongwan gold deposit is located in the Xiong'ershan area, East Qinling, China, with gold ore bodies hosted in NE-trending hydrothermal alteration zone and quartz + sulfides veins in structures. Here we present systematic field investigation, petrographic observations, in-situ trace elements and sulfur isotope of pyrite, and sulfur isotope of galena and barite to elucidate the genesis and effect of fluid-rock interaction during hydrothermal evolution of the Dongwan gold deposit. Four types of pyrite are recognized based on their occurrences in different hydrothermal veins, structures and paragenetic sequences, as follows: i) Py1 in the stage I quartz-pyrite vein is composed of coarse-grained cubic pyrite (Py1-1) and medium-grained cubic pyrite (Py1-2); ii) Py2 is characterized by fine-grained disseminated pyrite in stage II vein; iii) Py3 is represented by pyrite that occurs together with galena, sphalerite and chalcopyrite in stage III vein. The early stage Py1-1 and Py1-2 are characterized by high Co/Ni ratios and low contents of As, Au and Ag. The main ore-forming stages represented by Py2 and Py3 show lower Co/Ni ratios and higher concentrations of As, Sb, Au, Ag, Pb and Cu. The Ni contents of all pyrite types are < 10 ppm, indicating that the ore-forming fluid was derived from felsic rocks. The delta S-34 values of the different pyrite generations range between -16.0% and -7.4%, which are attributed to the fractionation between barite and sulfide minerals. The sulfur isotopic composition of the hydrothermal fluid calculated from co-genetic barite and pyrite is 1.2%, suggesting that the ore-forming materials were possible derived from mantle source, and were correlated with the Late Mesozoic granitic magmatism. The main mechanisms of gold precipitation are fluid-rock interaction and decrease of temperature, which promote the Fe2+ rich Xiong'er Group wall rocks to reduce the hydrothermal fluid and disequilibrate the gold-sulfur complex causing the precipitation of gold. The overall trace elements and sulfur isotopic signatures suggest that the pyrite is of magmatichydrothermal origin, and that the ore-forming materials of the Dongwan gold deposit are probably related to post-magmatic fluids derived from the Early Cretaceous granitoid magmatism in the region.