Evolution and fertility of magmas associated with iron oxide-apatite (IOA) deposits, Coastal Cordillera, Northern Chile: A zircon petrochronology perspective

Iron oxide-apatite (IOA) deposits in the Coastal Cordillera of northern Chile are spatially associated with Late Jurassic to Early Cretaceous intrusive rocks of intermediate composition. To constrain the magmatic conditions and identify possible ore fertility indicators, we used simultaneous LA-ICP-MS U–Pb dating and trace element geochemistry of zircon crystals from intrusions related to IOA deposits in the El Algarrobo district which is contrasted against the data from zircon grains of granitoids spatially related to Cretaceous porphyry Cu–Mo ± Au east of the El Algarrobo district. Our data show that intrusive rocks related to IOA deposits in the El Algarrobo district formed during a ∼5 Myr period (128–123 Ma) and were associated with water-poor (Eu/Eu* < 0.3) and relatively reduced (ΔFMQ from −1.5 to +1.1) intrusions, which could explain the sulfur-poor nature of IOA deposits. In contrast, intrusive rocks associated with porphyry-type mineralization east of El Algarrobo formed between 97 and 95 Ma and exhibit higher water content (Eu/Eu* up to 0.7) and oxidizing conditions (ΔFMQ from 0.0 to +2.2). We interpret these contrasting signatures as reflecting a change in the geodynamic setting at 123–122 Ma—from extensional to compressional conditions—that led to higher residence times of magmas at deep crustal levels, resulting in an increase in their H2O content through episodic magma (and volatile) replenishment. Based on our zircon data, we propose potential IOA fertility indicators characterized by (Eu/Eu*)/(Dy/Yb)N ratios <3 and relatively reduced conditions, i.e., ΔFMQ < +1. The anomalously reduced conditions (ΔFMQ < 0) can signal fertility. The relation of reduced magmas with IOA deposits supports the hypothesis that it was the result of the crystallization of large amounts of igneous magnetite related to ore genesis. Reduced magmas in arc settings are uncommon, and hence, the proposed IOA fertility indicators are a promising exploration tool that needs further testing in IOA–IOCG districts.