Genetic Relationship between Subduction of Slab Topographic Anomalies and Porphyry Deposit Formation: Insight from the Source and Evolution of Rio Blanco Magmas

Abstract

AbstractThe Rio Blanco deposit, which is one of the largest porphyry Cu–Mo deposits in northern Peru, formed coevally with the subduction of the Inca Oceanic Plateau at 12–10 Ma. However, the genetic relationship between the subduction of oceanic plateaus and the porphyry deposit formation remains unclear. Igneous rocks emplaced at 23–12 Ma in northern Peru, including the Portachuela batholith (which hosts the Rio Blanco porphyry complex), are normal calc-alkaline to weakly adakitic. In comparison, the 12–8 Ma igneous rocks, including the ore-related Rio Blanco porphyry complex, have typical adakitic signatures, such as high Sr/Y ratios (up to 180) and LaN/YbN ratios (up to 32). The Rio Blanco igneous rocks (Portachuela batholith and Rio Blanco porphyry complex) have uniform zircon εHf(t) values (+0.3 ± 1.2) and δ18O values (6.5 ± 0.14‰). These geochemical characteristics indicate that the Rio Blanco igneous rocks evolved from mantle-derived parental melts in a long-lived, stable, homogeneous isotopic reservoir at the crust–mantle boundary. However, whereas both the Portachuela batholith and the Rio Blanco porphyry complex formed from hydrous parental magmas (>5 wt %; based on plagioclase hygrometry), the ones of the Rio Blanco porphyry complex seem to be more oxidized, hydrous, and sulfur-rich compared with the older batholitic rocks. Reverse zoning in plagioclase phenocrysts, with a systematic core–mantle–rim variation in An (anorthite) and Fe (total iron) contents, are common in the intermineralization rocks. The An content of the mantles of the plagioclase phenocrysts correlates positively with the Fe content, but in the rims, the An contents significantly decrease while Fe remains constant. The apatite inclusions in the mantles are richer in S (0.24 ± 0.06 wt %) and Cl (1.42 ± 0.32 wt %) than those in the phenocryst cores (S: 0.09 ± 0.07 wt %; Cl: 1.03 ± 0.56 wt %) and rims (S: 0.14 ± 0.09 wt %; Cl: 0.83 ± 0. 35 wt %). These systemic geochemical variations in the plagioclase phenocrysts suggest recharge by S- and Cl-rich melts followed by fluid exsolution. This magma recharge and subsequent fluid exsolution may have triggered porphyry Cu mineralization at Rio Blanco. The coincidence of timing between the geochemical transition and collision (initial subduction) of the Inca Oceanic Plateau with the South American plate may indicate a change in the tectonic regime to a compressional state of stress and a thickening of the crust during the collision. The tectonic transition would have facilitated the fractionation of mantle-derived magma in a deep crustal hot zone, resulting in oxidized, volatile-rich residual melts. Replenishment of the upper-crustal magma chamber by such volatile-rich magmas and the subsequent discharge of fluids are interpreted to be fundamental for porphyry Cu mineralization at Rio Blanco and plausibly for the formation of Late Miocene porphyry ore deposits in northern Peru in general.