Timing of S-saturation in the formation of the Oubulage porphyry Cu-Au deposit, Inner Mongolia, Northern China

The Oubulage porphyry Cu-Au deposit, Inner Mongolia, North China, is located in the Langshan Tectonic Belt on the southern margin of the Central Asian Orogenic Belt (CAOB). The ore-forming quartz porphyry intrusion was formed during the Middle Permian (268.4 ± 1.6 Ma). Randomly and directionally distributed sulfide blebs are observed in porphyries, which are spherical or ellipsoidal in shape. There are two groups of such blebs: polymetallic sulfide blebs and silicate-carbonate-sulfide blebs. Sulfide minerals in these blebs include pyrite, chalcopyrite, and pyrrhotite. Silicate-carbonate-sulfide blebs also contain calcite, epidote, plagioclase, chlorite, fluorite, and bastnaesite. Pyrite in sulfide blebs has δ⁵⁶Fe ranging from −0.38 ‰ to 1.19 ‰. Calcite in sulfide blebs has C-O-Sr isotopic compositions (δ¹³C_VPDB = −7.35 to −5.35 ‰, δ¹⁸O_VSMOW = 8.35to9.92 ‰, ⁸⁷Sr/⁸⁶Sr = 0.7032to0.7064) indicative of a magmatic origin. We propose a new possible genetic model in which sulfide saturation occurred in a magma chamber, leading to the formation of sulfide droplets. Assimilation in sulfur (S) in the magma chamber caused S-saturation, and the sulfide melts are transported to the shallow crust by volatile-rich fluids.