Records of high temperature ore fluids through magmatic vapor contraction from the Dongji gold deposit, SE China

The Dongji Au deposit represents the largest gold deposit in the Zhenghe region of South China with a proven reserve of 12.5 t gold and 136 t Silver. The deposit occurs mostly in Upper Jurassic rhyolite porphyry and partly in volcaniclastic rocks and is controlled by NE trending breccia zone. Our investigations suggest that two distinct stages of gold mineralization occur at Dongji: the stage 1 is dominated by quartz, pyrite, and arsenopyrite, with visible Au − Ag minerals, and occurs as a wide variety of structural styles that include individual veins, fracture stockworks and breccias; and the late stage 2 is characterized by gold-bearing pyrite, illite, chalcedony, and apatite, mainly distributed in cement of breccia style ores. The host rocks near orebodies develop pronounced illitization, silicification, weak chloritization. Microthermometric results from fluid inclusions within multi-generation growth zones of quartz in stage 1 and secondary fractures associated with later stage yielded homogenization temperatures (320 ∼ 411℃ and 218 ∼ 332 ℃, respectively) and salinities (0.4 ∼ 4.2 wt% NaCl equivalents and 0.7 ∼ 3.9 wt% NaCl equivalents, respectively). Combined with quartz titanium thermometer results, the high temperature and low salinity ranges of liquid-rich two-phase fluid inclusion in quartz of stage 1 indicate that the ore-forming fluid originated from magmatic vapor, which contracted into an aqueous liquid by cooling at elevated lithostatic pressure above the critical curve of the salt-water fluid system. The δD_V-SMOW values of stage1 quartz range from − 69.9 ‰ to − 60.9 ‰, with δ¹⁸O_H2O‰ values between − 0.9 ‰ and 1.4 ‰, implying the auriferous fluids derive from a magmatic with minor meteoric mixed source. The δ³⁴S_V–CDT values of pyrite of stage 1 and 2 range from –0.3 to 8.0 ‰, revealing that sulfur mainly originates from the magma. The microthermometric data and quartz composition evolution suggest a fluid mixing contribution to metal precipitation. Combined with the geology, gangue and ore mineral assemblage, alteration, fluid inclusion, and H–O–S isotopic characteristics, we propose that the auriferous ore-forming fluids of the Dongji deposit may be the product of magma vapor contraction and migration in a porphyry–epithermal transitional gold system. These findings also indicate a great prospecting potential for the porphyry type mineralization at depth.