The role of volatiles in forming high-grade porphyry Cu deposits: learnings from apatite inclusions in zircon

Volatiles, such as H₂O, S, Cl and CO₂, play a crucial role in regulating volcanic emissions and contribute significantly to the genesis of ore deposits. However, a comprehensive understanding of their role presents a significant challenge due to the intricate task of quantifying the volatile budget and assessing the timeframe linked to magma ascent and volatile exsolution in ore systems.

Apatite, Ca₅(PO₄)₃(F,Cl,OH), stands out as a well-established repository of various volatile species (H₂O, CO₂, S, halogens) in addition to numerous trace elements, making it an excellent mineral to study the volatile budget of magmas. When encapsulated in zircon, apatite inclusions can preserve a unique record of the abundance of volatile ore-forming constituents, eliminating potential influences from diffusion, degassing, and alteration effects associated with late-stage melts and/or hydrothermal fluids.

Our investigation focuses on thoroughly examining such inclusions, alongside groundmass apatite from the Escondida porphyry Cu-Mo±Au district in Northern Chile, which represents one of the most significant high-grade porphyry copper districts in the world. The ore-forming magmatism of the Escondida district took place in the framework of the protracted late Eocene-Oligocene arc magmatic activity, favouring the emplacement of multiphase porphyry stocks.

To resolve the volatile budget of the melt/fluid responsible for the Cu-Mo±Au mineralization in Escondida, we selected a suit of minimally altered samples from mineralized felsic-intermediate igneous intrusions as well as barren counterparts that are spatially associated with the copper mineralization but do not contain metals in sufficient quantity to be extracted and sold at a profit.

The employed multi-microanalytical approach on apatite (SEM-BSE-CL imaging, EPMA, δ³⁴S by SIMS) and the host zircon (Ti-in-zircon geothermometry, U-Pb geochronology), allowed us to determine:
a) the concentrations of halogens in the melt, which contribute to our understanding of the initial, optimal volatile composition for high-grade copper mineralization; b) the pathway and relative timing of volatile exsolution during the magmatic-hydrothermal stage; c) the hydrothermal volatile record of apatite associated with the mineralization stage.

Our work provides new insights into the role of volatiles in arc magmatic settings, demonstrating their potential in shaping economically viable deposits, a crucial cornerstone for a sustainable and environmentally conscious future.