Late Permian orogenic gold mineralization at Haoyaoerhudong, northern China: Constraints from hydrothermal titanite and apatite chemistry

The giant Haoyaoerhudong gold deposit is located on the north margin of the North China Craton. This deposit takes the form of lenticular orebodies hosted by the Mesoproterozoic carbonaceous schist, phyllite, and slate. Gold-related hydrothermal alteration types recognized at Haoyaoerhudong are silicification, sulfidation, bio-titization, and graphitization. In addition, titanite and apatite were recorded in hydrothermal system. There are four types of hydrothermal apatite, namely, rounded Ap-S in altered schist, fractured Ap-V1 in quartz--biotite-sulfide veins, anhedral Ap-V2 in quartz-sulfide stringers, and Ap-V3 with abundant fluid inclusions in quartz-sulfide veinlets. As for different titanite types, Ttn-S is found to replace ilmenite in altered schist. Clustered Ttn-V1 is associated with graphite and gold in quartz-biotite-sulfide veins. Anhedral Ttn-V2 generally coexists with pyrrhotite and chalcopyrite in quartz-sulfide stringers. Euhedral Ttn-V3 is intergrown with Ap-V3 in quartz-sulfide veinlets. A U-Pb age of 255.6 +/- 2.5 Ma (MSWD = 4.2, n = 27) was obtained for Ttn-V3 via in situ U-Pb analyses, which is younger than the nearby biotite granite (272.9 +/- 1.5 Ma MSWD = 2.1, N = 25). The Haoyaoerhudong deposit can therefore be considered the product of a postmagmatic mineralization event at around 256 Ma. Apatite and titanite trace elements are robust tools for tracking fluid pathways in orogenic gold deposits. The enrichment of Mn, Fe, Y, and rare earth elements (REE) is observed in Ap-S but is progressively depleted from Ap-V1 through Ap-V2 to Ap-V3. There are also higher concentrations of Mn, Fe, Mo, Bi, Nb, Ta, Zr, Hf, and REE for Ttn-S than other titanites. Nevertheless, the K, Ba, Sr, and V concentrations are enriched in Ttn-V3, similar to the enrichment of Sr in Ap-V3. The apatite and titanite compositions measured here indicate the low salinity but Sr and alkali metals-enriched ore-forming fluids, typical for orogenic gold systems. Systematic geochemical variations in Eu, Mn, and V concentrations may imply that the ore-forming fluids were slightly oxidized compared to altered schist. The interaction between the carbonaceous schist and hydrothermal fluids would have led to CO2 consumption and apatite, titanite, and graphite formation. We further concluded that Sn vs. Mo, V vs. Th/U, and Zr + Hf vs. Th/U diagrams help to distinguish hydrothermal titanite in orogenic gold deposits from those in porphyry, skarn, and iron oxide-copper-gold deposits.