Porphyry Cu fertility of eastern Paleo-Tethyan arc magmas: Evidence from zircon and apatite compositions

Abstract

The Paleo-Tethyan arc belt is host to a small number of porphyry Cu deposits the key controls on which remains poorly understood. It has been hypothesized that the Paleo-Tethyan arc magmas were reduced and therefore less fertile for porphyry Cu formation but few studies have been conducted to directly assess the magmatic oxygen fugacity. We investigate the magma fertility of eastern Paleo-Tethyan arc igneous rocks in the Sanjiang region, SW China, represented by the Maoding (268.3 ± 2.3 Ma), Suwalong (275.6 ± 1.4 Ma to 264.7 ± 1.4 Ma) and Baimaxueshan (255.7 ± 1.5 Ma to 249.3 ± 1.1 Ma) granitoids. They have geochemical features consistent with typical continental arc rocks, formed from partial melting of the mantle wedge, with increasing contamination of crust-derived melts over time. The presence of amphibole, high Sr/Y ratios (35 ± 20, n = 17), and lack of negative Eu anomalies (EuN/EuN⁎ = 1.04 ± 0.3, n = 17) are consistent with derivation from hydrous magmas. Zircons from the intrusive rocks have relatively low EuN/EuN⁎ (mostly <0.4) ratios and calculated ΔFMQ values (−2.1 ± 1.2, n = 86). Combined with apatite microphenocrysts with extremely low sulfur contents (< 0.12 wt% SO3), it suggests that the intrusions originated from water-rich but reduced arc magmas, that were less fertile for porphyry Cu formation. Our study highlights that magmatic fO2 is probably a critical control on the prospectivity of arc systems and confirms that zircon and apatite compositions can be used to evaluate porphyry Cu fertility.