DECODING THE RHYTHMS OF PORPHYRY COPPER SYSTEMS (Invited Presentation)

Porphyry copper systems are temporally and genetically linked with the dynamic evolution of underlying magmatic reservoirs ¹. The chamber supplies magma and metalliferous fluid that are essential for the formation of these systems. As one of the most studied and best understood deposit types, the temporal frame of ore formation, a fundamental question, remains controversial. Traditional wisdom holds that the formation of these deposits are characterized by a linear hydrothermal process, e.g., the mineralization process is continuous. In contrast, there are emerging chronological data and numerical modelling evidences argue for a pulsed release of magmatic fluids from underlying magmatic reservoirs at the timescales of thousand-year-level ^2-4.

Through integrating deposit geology, fluid inclusion microthermometry, high-precision Re-Os molybdenite dating and oxygen isotope analysis of quartz-molybdenite veins from the Qulong system, we show that fluid evolution under an absolute timeframe was cyclic. We further demonstrate that such a process can be decoded from high spatial resolution oxygen isotopic analysis in a relative timeframe fashion. Specifically, our SIMS oxygen isotope data of single-grain hydrothermal quartz from the Qulong porphyry copper system faithfully record meteoric water incursion during episodic fluxing of magmatic fluids.

In line with multiple generations of intrusions and recent advances in high resolution chronological/isotopic analysis and numerical modelling, we suggest that cyclicity is an ubiquitous feature for ore formation that ultimately linked to dynamic evolution of underlying magmatic reservoirs.

Key References

1 Sillitoe, R. H. Porphyry Copper Systems. Economic Geology 105, 3-41 (2010).

2 Li, Y., Li, X. H., Selby, D. & Li, J. W. Pulsed magmatic fluid release for the formation of porphyry deposits: Tracing fluid evolution in absolute time from the Tibetan Qulong Cu-Mo deposit. Geology 46, 7-10 (2018).

3 Li, Y., Selby, D., Condon, D. & Tapster, S. Cyclic Magmatic-Hydrothermal Evolution in Porphyry Systems: High-Precision U-Pb and Re-Os Geochronology Constraints on the Tibetan Qulong Porphyry Cu-Mo Deposit. Economic Geology 112, 1419-1440 (2017).

4 Chelle-Michou, C., Rottier, B., Caricchi, L. & Simpson, G. Tempo of magma degassing and the genesis of porphyry copper deposits. Scientific Reports 7, 40566 (2017).