The first intra-oceanic island-arc porphyry Cu (Au) deposit in the western Bangong-Nujiang suture zone: Evidence from the Saidengnan Cu (Au) deposit

Abstract

The Saidengnan deposit, located in the Bangong-Nujiang suture zone, is proposed to be the first intra-oceanic island-arc porphyry Cu (Au) deposit in the Bangong-Nujiang metallogenic belt. The Cu (Au) mineralization is hosted in the quartz diorite porphyry as fine grained disseminated form. Zircon U-Pb ages of the ore-forming quartz diorite porphyry and the post-mineralization granodiorite are 118.1–118.8 Ma and 116.2 Ma, respectively, suggesting that mineralization occurred in the Early Cretaceous. The S isotopes (δ34S = −3.8 to −1.5) of ore sulfides and H-O isotopes (δD = −99‰ to −89.9‰, δ18OH2O = 4.9–9.4‰) of ore-bearing quartz veins indicate that the ore-forming fluids was sourced from the ore-forming intrusion. All intrusive rocks are enriched in light REE and LILE (Rb, K) and depleted in HFSE (Ni, Ta, Ti), similar with the island-arc magmas. Whole rock (87Sr/86Sr)i values (0.7040–0.7056), εNd(t) values (−0.35 to 1.94), and significantly positive zircon εHf(t) values (7.37–11.38) and young TDM2 of zircon Hf (455–705 Ma) indicate that the Saidengnan intrusive rocks are dominantly derived from mantle wedge metasomatized by subducted fluids and sediments. This is different from the Duolong continental arc magmas formed by crust-mantle mixing. Combined with stratigraphic data published, we propose that the Saidengnan porphyry Cu (Au) deposit should be formed in the intra-oceanic island-arc setting caused by the oceanic-oceanic subduction of the Bangong-Nujiang Ocean.