Fluid Evolution and Controls on Gold Precipitation at the Dongga Au Deposit, Tibet, China: Insights from Pyrite Trace Elements

The Dongga Au deposit is located in the giant Xiongcun porphyry Cu-Au ore district within the Southern Lhasa terrane; however, the evolution of ore-forming fluids and the mechanisms of gold precipitation during the main mineralization stage remain poorly constrained. This study integrates geological observations and in situ LA-ICP-MS trace element analyses of pyrite to address the above issues. Three generations of pyrite are identified: Py1 occurring in quartz–sulfide veins, Py2 in chlorite–sulfide veins, and Py3 in pyrite veins. Trace element data show that Au and As contents are relatively low in all three pyrite generations and mainly occur as lattice-bound elements, whereas Pb, Ag, Bi, Cu, and Zn are predominantly hosted in micro- to nano-scale mineral inclusions. Ore-forming temperatures estimated from Se concentrations in pyrite indicate progressive cooling from ~400 °C to ~270 °C (Py1 to Py3). Combined with thermodynamic modeling and mineral assemblage constraints, this suggests that the ore-forming fluid experienced significant meteoric water input, accompanied by decreasing temperature, sulfur fugacity, and oxygen fugacity, as well as increasing pH. The principal gold mineralization stage occurred at approximately 340 °C, where temperature and pH conditions jointly stabilized Au transport primarily as Au(HS)2−. We propose the mixing between meteoric water and mineralized magmatic fluid caused a decrease in sulfur fugacity, oxygen fugacity and temperature, thereby limiting the availability of HS− required for stabilizing Au(HS)2− complexes and thus resulting in the decoupling of Au(HS)2−, which triggered gold precipitation.