Magmatism and gold–quartz–tourmaline mineralization at the Klyuchevskoe deposit, Eastern Transbaikalia, Russia: A potential new epithermal type?

The large Klyuchevskoe gold deposit is a rare example of a mineral system in which native gold co-precipitated with pyrite in quartz-tourmaline metasomatites and veins along the fractured zones within altered granitoids. The first U-Pb zircon dating of igneous suites at the deposit reveals multi-pulse magmatic activity between 164 Ma and 146 Ma. Igneous rocks belong to the Middle-Late Jurassic Amanan (164-157 Ma) and the Late Jurassic Amudzhikan (151-146 Ma) complexes. The Amanan suites comprise quartz monzodiorite, granodiorite, and granite emplaced at 164-161 Ma and 159-157 Ma. The Amudzhikan suites include diorite, quartz monzodiorite, and granodiorite porphyries intruded at 151-150 Ma and at 149-146 Ma. Geological observations coupled with U-Pb zircon ages indicate that economic gold mineralization originated from hydrothermal fluids exsolved from the same magma chamber that generated the Amudzhikan suites within the 151-150 Ma interval. Whole-rock geochemistry and zircon mineral chemistry indicate that both complexes originated via partial melting of the lower crust, albeit with varying mantle contributions. The Amanan granitoids were derived from highly oxidized (ΔFMQ = +1.3 to +3.1), wet to super-wet magmas. In contrast, the Amudzhikan granitoids formed from low to moderately oxidized (ΔFMQ = -0.6 to +2.3) and moderately hydrous magmas. These magmas had initial oxidation states close to the FMQ buffer (ΔFMQ ≈ 0) but underwent progressive oxidation and hydration during their evolution. The key features of the Klyuchevskoe deposit include (1) a gold-bearing ore association dominated by pyrite (± arsenopyrite ± chalcopyrite); (2) pervasive quartz-tourmaline (± albite) alteration; (3) association with weakly oxidized, high-K calc-alkaline magmatism; and (4) generation of magmatic-hydrothermal fluids within a dynamic open magmatic system, with fluid flux derived from a magma chamber. The Klyuchevskoe deposit may represent a previously unrecognized subtype of epithermal mineralization.