Fluid Evolution and Hydrothermal-Breccia Revealed High Energy Processes in Erdenet Cu-Mo deposit, Mongolia

Abstract:

High energy processes in the magmatic-hydrothermal environment are due to the existence of pressurized fluid underneath the self silica sealing. In this research, we investigate porphyry as an analog to the magmatic-hydrothermal system to provide natural evidence of such phenomena. We retrieved core samples from Erdenet mine, Mongolia, and analyzing the stockwork/vein system to elucidate the fluid evolution by using microthermometry, quantitative, and qualitative petrological approach. We apply particle size distribution (PSD) and several petrological analyses to investigate how the formation mechanism of the breccia and how high energy processes involved in this system.

Granodiorite hosted vein/stockwork was emplaced at a temperature of ~700-750℃ based on Ti-in-Biotite and Ti-in-Quartz. And the truncated vein provides relative fluid events that can be distinguished into three fluid stages. Qtz-molybdenite (qtz-mol) vein represents Initial fluid characterized high P-T, followed by quartz-py (qtz-py) vein with lower temperature formation, and barren-quartz vein (qtz) mark the last fluid stage within the system. The breccia occurs at variable depth, consist of monomict and polymict texture with a variety of clast and matrices. Cumulative PSD of clast shows that all breccia types have low Ds (<2) with a similar trend of the curve indicate fluid-assited breccia govern the formation mechanism of the breccia. We suggest that breccia mark the trasnsition of fluid activity in Erdenet, where the energy gain from impermeable sealing due to silica precipitation on surrounding intrusion and overpressured fluid underneath the silica layer initiate the rupture to form breccia.

Porphyry system provides a significant lesson learned about how fluid behavior and high energy processes on the magmatic-hydrothermal environment and how this lesson can be used for future energy extraction from the superhot geothermal system or enhance our understanding of spontaneous mineral deposition.