Partially decoupled magmatic and hydrothermal events in porphyry copper systems?

Porphyry Copper Systems (PCS) represent a significant source of metals, and will continue to play a key role in future with the development of green technology. Despite being one the most studied mineral systems, the primary controls on the ore tonnage of deposits (that varies up to 5 orders of magnitude in nature) remain poorly constrained. The Eocene Chuquicamata Intrusive Complex (CIC) in northern Chile hosts one of the world’s largest porphyry copper deposits and represent a perfect natural laboratory to explore the influence of timescales in controlling the formation and size of PCS.

Here we investigate the tempo of multiple magmatic-hydrothermal events in the CIC applying molybdenite geochronology (Re-Os ID-NTIMS) and high precision zircon petrochronology (U-Pb CA-ID-TIMS geochronology in tandem with LA-ICPMS trace element composition). Preliminary geochronological results may suggest a partial decoupling of the magmatic and hydrothermal events. Zircon U-Pb geochronology results point to a multi-million-year protracted magmatic history with at least two discrete pulses separated by 500 kyrs. The hydrothermal event appears slightly younger than the youngest magmatic pulse and lasted for ca. 1 Myrs.

The extensive duration of the mineralization scales with the behemothian size of the Chuquicamata deposit (more than 110 Mt of contained copper) and corresponds to predictions from numerical modelling of magma degassing. Interestingly, the apparent temporal decoupling between magmatism and hydrothermal activity at Chuquicamata suggests that syn-mineralization ore-forming magmas might not always intrude as dyke or stock at mineralization depth and can remain hidden at upper to mid-crustal depth. In the absence of high-precision geochronological data, this may bear consequences when assuming a direct genetic link between spatially associated porphyritic rocks and the mineralization.