External authors:

• A. Inostroza ( Universidad de O'Higgins )
• A. Olivares ( Universidad de O'Higgins )
• D. Hernandez-Uribe ( University of Illinois Chicago Hospital )
• M. Reich ( Universidad de Chile )
• Fernando Barra ( Universidad de Chile )
• R. Romero ( Universidad de Chile )
• Jay. J. Ague ( Yale University )
• Daniele Tardani ( Universidad de O'Higgins )
• C. de los Santos Valderrama ( Corp Nacl Cobre )

Abstract:

To investigate the redox and volatile evolution of arc magmas linked to supergiant porphyry copper deposits (PCDs), we study zircon and their apatite inclusions from the Miocene-Pliocene intrusive rocks in the Central Chilean Andes, including the El Teniente supergiant PCD. Our dataset includes 149 zircon-apatite inclusion pairs covering similar to 200 km along the arc bracketing El Teniente and capturing similar to 20 Myr of arc evolution leading to its formation. We show that magmatic volatile contents increased progressively during arc maturation, culminating in the generation of volatile-rich magmatic precursors to the El Teniente PCD (similar to 7.7 +/- 1.0 wt.% H2O, similar to 4000 +/- 700 ppm Cl, up to similar to 1750 ppm S). Despite their elevated volatile contents, these precursor magmas were only moderately oxidized relative to earlier barren arc magmas. The strongly oxidized conditions associated with ore formation (up to similar to Delta FMQ +3) developed during progressive open-system degassing, accompanied by declining melt Cl and S contents. At El Teniente, ore formation occurred within a narrow window during this redox-volatile evolution in which oxidation was sufficient to destabilize magmatic sulfides while volatile contents remained high enough to sustain extensive fluid exsolution and efficient Cu transfer. Magmas that were either less oxidized and volatile-rich, or strongly oxidized but volatile-depleted, did not produce significant mineralization.