Deep epithermal Au mineralization at Tuanjiegou, northeast China

To better understand the physicochemical features of deeply mineralized intervals of low-sulfidation (LS) Au deposits, the Early Cretaceous Tuanjiegou Au deposit in northeastern China was characterized as it comprises alteration and mineralization styles that reflect deeper portions of the mineralizing system. From shallow to deeper portions of the Tuanjiegou deposit, the mineralization is zoned from sulfide-poor quartz bands through hydrothermal breccia to deep pyrite-marcasite-rich veinlets that occur along hydrofractures within a granodiorite porphyry stock. The zoned alteration halo is centred on the ore zones, and comprises an inner phyllic alteration zone (illite + quartz + muscovite) and an outer intermediate argillic alteration zone (illite +/- smectite), with propylitic alteration being limited. Given that apatite fission track ages are extremely sensitive to the ambient temperatures of 60-120 degrees C, the overlap in the closure dates of magmatic apatite (108.1 +/- 3.8 Ma) and zircon (108.2 +/- 1.2 Ma) from the granodiorite porphyry is indicative of shallow emplacement of the granodiorite porphyry, likely at depths slightly >0.7 km. A similar paleodepth (900 +/- 188 m) was estimated from homogenization temperatures (291 +/- 14 degrees C) of boiling fluids. Preservation of the deep portions of the Tuanjiegou ore system may imply that the productive ore intervals of poorly explored LS epithermal deposits globally are underestimated. Given the evidence for a magmatic origin of the ore-forming fluids at Tuanjiegou, we highlighted that ore-forming fluids are released initially from a water-saturated, oxidizing felsic melt under low-pressure (similar to 4 km) conditions.