Abstract:

In recent years, studies on the porphyry metallogenic timescale have become more refined and diversified.By using high-precision isotope dating, element diffusion chronology and thermodynamic numerical simulation, the porphyry metallogenic timescale are precisely defined to be between tens to hundreds of thousands of years.Quartz is a chemically stable mineral in various mineralization stages, which can be utilized as an ideal object to explore episodic hydrothermal processes.Taking the Machangqing porphyry Cu-Mo deposit in the Sanjiang Tethyan Domain as the case study, and basing on the fine arrangement of the vein relationship and mineral assemblage at each stage, we used the diffusion chronology method of Ti and Al in quartz to constrain the timescale, and discussed the practical application of diffusion chronology.The Machangqing deposit contains multi-stage hydrothermal veins, which can be divided into multiple hydrothermal stages based on their crosscutting relationship and mineral composition.For the quartz in the early-ore A2 and main-ore B3 veins, our finding shows that the early-ore quartz samples have higher correlation between the CL intensity and Ti element, and its timescale defined by the Ti diffusion is 10.5-57.5 ka; for the main ore stage, the quartz CL intensity has a higher correlation with the Al element, and its timescale defined by the Al diffusion is 522.3 ka.Overall, we consider that using element diffusion chronology to constrain the porphyry metallogenic timescale may be affected by the temperature-pressure conditions, simulation location, analytical precision and accuracy.Therefore, it is necessary to construct a reasonable diffusion model based on the metallogenic conditions and mineral growth characteristics.

Key words: timescale, diffusion chronology, quartz, porphyry deposit, Machangqing, Sanjiang Tethan domain