Potential of porphyry copper mineralization in the Triassic Yunxian-Jinggu arc, in the Sanjiang orogen, SW China

Abstract

Porphyry copper deposits are commonly associated with porphyries generated in arc environments. Assessing the contemporaneous arc volcanic rocks provides crucial insights into the potential for porphyry copper mineralization. The Triassic Yidun arc in the northern part of the Sanjiang orogen in southwest China, which formed in association with the subduction of the Paleo-Tethys Oceanic slab, hosts several porphyry copper deposits. However, the potential for porphyry copper mineralization in the Yunxian-Jinggu volcanic arc in the southern part of the Sanjiang orogen, which was also linked to the subduction of the Paleo-Tethys Oceanic slab, remains unclear. This study reports zircon U–Pb ages, trace element compositions and Hf isotopes, and whole-rock element compositions and Sr–Nd–Pb isotopes of the basaltic-andesitic-dacitic rocks in the Yunxian-Jinggu volcanic arc. Zircon SHRIMP U–Pb dating of the Wenyu basalts and andesites yielded ages of 244.3 ± 1.9 Ma and 242.5 ± 2.5 Ma, respectively. The basalts have arc geochemical affinities with εHf (t) values of 2.3 to 9.3, εNd(t) values of –0.6 to 0.2, and (87Sr/86Sr)i values of 0.7065 to 0.7068. They have high Ba concentrations (∼ 550 ppm), highly variable Ba/La ratios (0.01 to 0.10), and a narrow range of Th/Yb ratios (1.2 to 1.9). These characteristics suggest that the basalts were derived from mantle metasomatized by fluids likely released from the subducted Paleo-Tethys slab. The Wenyu andesites, with εHf (t) values range from 1.6 to 7.0, εNd (t) value of –0.4, and (87Sr/86Sr)i value of 0.7031, along with distinct Pb isotopes, were probably produced through basaltic magmas undergoing crustal fractional crystallization. The Minle dacites, sharing similar εHf (t) value (–1.6 to 10.7) but lower εNd (t) values (–3.1 to –4.9) compared to Wenyu volcanics, are interpreted as derived from partial melting of basaltic magmas with contamination from ancient crustal components. The zircon Eu/Eu* ratios (mostly < 0.4), zircon ΔFMQ value (–0.6 ± 1.1), and bulk-rock Sr/Y ratios (< 10.3) for the Yunxian-Jinggu volcanic rocks indicate that co-genetic intrusions, if present, would likely be less hydrous and less oxidized, making them less favorable for porphyry copper mineralization.