Apatite sulfur isotopes reveal anhydrite saturation in collisional porphyry systems, southern Tibet

Abstract Porphyry deposits are vital in providing copper and gold to the industry. While it is clear that a substantial amount of sulfur (S) is required for mineralization, how magmas transport the required amount of S from depth remains a subject of debate. Some have reported the presence of anhydrite as mineral inclusions. However, to date, no geochemical evidence exists to show if porphyry magma can reach anhydrite saturation during fractionation. To provide new insight, we report new apatite S isotope data from porphyry Cu systems, barren granitoids, deep-crustal xenoliths, and mantle-derived mafic intrusions collected in the Gangdese belt, southern Tibet. The consistently high apatite δ34S of Jiama (+4.29‰) agrees with the previously reported δ34S of subcontinental lithospheric mantle–derived alkaline magma, showing a distinctive contribution from mantle components. Moreover, anhydrite saturation occurred during the evolution of collisional mineralizing magmas. This is evidenced by the coherence between modeled anhydrite crystallization curves and the Jiama apatite data, as well as the presence of anhydrite inclusions within apatite grains. Since S content at anhydrite saturation (SCAS) is generally higher than S content at sulfide saturation under the same magma composition, sulfide saturation may normally precede sulfate saturation. To sum up, our study suggests that porphyry mineralizing magmas received an influx of S from mantle-derived magma and evolved to possess sufficient S content and oxygen fugacity to reach SCAS.