The Aksug Porphyry Cu–Mo Deposit (Northeastern Tuva): Chronology of Magmatism and Ore Formation Processes (U–Pb and Re–Os Isotope Data) and Metallogenic Implications

Abstract —The Aksug porphyry Cu–Mo deposit is located in a region of long-lasting magmatic activity. Gabbroids of the Khoito-Oka complex are the earliest intrusive rocks, in which the Aksug granitoid pluton hosting ore-bearing small porphyry intrusions is localized. The intrusive activity was terminated with emplacement of late leucogranite dikes. There are different viewpoints on the age of magmatism and mineralization of the Aksug deposit, with the concept of their Devonian age prevailing. To solve the debatable issue, we performed isotope geochronological studies and analyzed new results of U–Pb (SHRIMP-II) zircon dating and previously published Re–Os molybdenite dates (518 ± 2, 516 ± 2, and 511 ± 2 Ma). The concordant U–Pb zircon ages for igneous rocks are younger than the Re–Os age for mineralization. New U–Pb dating of Khoito-Oka gabbro-diorites has yielded an age of 503 ± 2 Ma. The U–Pb SHRIMP zircon age of tonalites from the Aksug pluton has been estimated at 504 ± 5 Ma. The U–Pb zircon ages for ore-related tonalite porphyry I and tonalite porphyry II are 500 ± 6 and 499 ± 6 Ma, respectively. The obtained SHRIMP age for leucogranite dike is 509 ± 4 Ma. Two groups of U–Pb dates have been obtained for each of the analyzed zircon samples: close to the Re–Os dates (518–511 ± 2 Ma) and younger (507–486 Ma). The weighted average zircon ages calculated for early and late populations from post-ore leucogranites are 515 ± 4 and 500 ± 4 Ma, respectively. We suggest that zircons with an age close to the Re–Os dates found in post-ore leucogranites were assimilated from the underlying substrate and zircons with an age of 500 ± 4 Ma crystallized from melt. The oldest U–Pb dates (509–515 Ma) of individual zircon grains from ore-bearing tonalite porphyry are consistent with the Re–Os molybdenite ages. Zircons from tonalite, tonalite porphyry, and Khoito-Oka gabbroids sometimes show internal textures indicating secondary alteration. The younger U–Pb concordia zircon ages relative to the Re–Os dates might be due to the influence of late thermal processes on the U–Pb isotopic system. The younger dates (486–507 Ma) of individual zircon grains probably reflect the time of the impact of a thermal fluid process. The weighted average of these younger dates (502 ± 2 Ma) falls within the weighted average age of post-ore leucogranites (500 ± 4 Ma). According to the Re–Os dates, the Aksug deposit formed at the end of the early Cambrian. Ore occurrences similar in magmatism and mineralization to the Aksug ore deposit are widespread in Tuva and in the Lake Zone in Mongolia. Therefore, it is necessary to reassess the role of the Cambrian and Devonian magmatism in the development of porphyry Cu–Mo mineralization both in Tuva and in the Altai–Sayan orogenic area.