Early Cretaceous magmatic-hydrothermal processes for the lode Yingdongpo gold and Poshan silver deposits in the Tongbai Orogen, central China

Numerous lode gold and silver deposits occurred in orogenic belts in China, although some were related to intrusions within anorogenic settings. Genesis of lode gold and silver deposits in the Tongbai-Dabie orogen remains equivocal. In this study, Rb-Sr dating, electron microprobe and LA-ICP-MS element analyses of sulfides, fluid inclusion microthermometry and H-O isotopic analyses were carried out to decipher the formation and fluids evolutions producing the super-large Yingdongpo gold and giant Poshan silver deposits. Sphalerite Rb-Sr dating yields isochron ages of 133.3 +/- 3.0 Ma and 136.1 +/- 1.8 Ma for the Yingdongpo and Poshan deposits, respectively, suggesting that both deposits formed coevally with Early Cretaceous granitic magmatism. H-O isotopic data indicate that the ore-forming fluids of both deposits were primarily of magmatic origin with meteoric water inputting. Their initial ore-forming fluids were CO2-rich, with high temperatures and intermediate salinities. At the Yingdongpo gold deposit, native gold and electrum coexist with polysulfides; invisible Au mostly occurs as nanoparticle inclusions in sulfides and is associated with As. At the Poshan silver deposit, Ag gradually increases in early to late generations of sphalerite, and is positively correlated with Pb. Silver minerals likely formed via exsolution from galena due to progressively decreasing temperatures. There are discrete processes of ore-forming fluids, with different mechanisms responsible for gold and silver mineralization. Au mineralization is related to an abrupt change of the physicochemical properties in fluids immiscibility at 190 degrees to 265 degrees C; whereas Ag mineralization is favored by progressive deposition of sulfides in a slowly cooling hydrothermal system with temperatures from 317 degrees to 88 degrees C.