MAGMAS FROM EL TENIENTE CU-MO DEPOSIT (34ºS, CENTRAL CHILE): THE FINAL STAGES OF AN EVOLVING SOURCE THROUGH CENOZOIC CONSTRUCTION OF THE MODERN ANDEAN OROGEN

El Teniente porphyry Cu-Mo deposit (~34ºS, Andes of central Chile) is one of the largest deposits of this type presently known. It is the southernmost economic porphyry of the Miocene-early Pliocene Cu-Mo belt of the Andes (~32-34ºS), which includes other giant deposits and constitutes one of the most richly endowed copper provinces in the world (>220 Mt of contained Cu). Formation of these deposits occurred in a short time span during the Cenozoic constructive period of the modern orogen, and during the final stages of arc-related igneous activity in the current western Andean slope. This developed in the region at least since early Oligocene and until middle Miocene, to then progressively diminish and migrate to the east (~35 km) until reaching its current position at the active volcanic arc.

The main El Teniente intrusive units (~6.4-4.8 Ma) show a relatively radiogenic and restricted Hf isotopic composition (ƐHf_I: +6 to +10), which is also indistinguishable from that defined by preceding Cenozoic barren igneous rocks in central Chile. All these rocks derive from nearly 25 Myr of subduction-related magmatic activity developed under contrasting tectonic regimes (extensive to compressive) and margin configurations. This suggests a primary control of the isotopic signature by a stable long-lived MASH-type reservoir in the deep lithosphere. The long-lasting magmatism that preceded the El Teniente formation was probably responsible for the progressive enrichment of this MASH source.

Tectonic and magmatic evolution of central Chile during construction of the modern Andes supports the hypothesis of dehydration melting reactions in the enriched MASH reservoir, and this is considered fundamental in the origin of El Teniente fertile intrusions. Such reactions probably occurred as a consequence of increasing crustal thickness, and were prompted by a high-temperature thermal regime resulting from a long history of preceding magmatism. Dehydration melting reactions are also expected to occur at a regional scale, and thus can explain the simultaneous generation of porphyry deposits in the Miocene-Pliocene Cu-Mo belt of central Chile. The latter can be considered as an example of how, given the appropriate conditions, dehydration melting processes can enhance magma fertility in a particular metallogenic epoch.