Subduction-related Late Triassic Luerma porphyry copper deposit, western Gangdese, Tibet, China: Evidence from geology, geochemistry, and geochronology

Abstract

Due to the difference of lithospheric structure between the eastern and western segment of Gangdese metallogenic belt, it is generally believed that porphyry copper deposits are mainly developed in the east. However, the newly discovered of Luerma porphyry copper deposit in the western segment of Gangdese provides an excellent research object for us to supplement this understanding. The molybdenite re-Os isotope dating of two samples from quartz veinlet ores (ca. 213.1 ± 5.2 Ma and ca. 212.0 ± 3.7 Ma), combined with the LA-ICP-MS zircon U-Pb dating of three ore-bearing quartz monzonite porphyry samples (ca. 212.7 ± 0.9 Ma, 213.1 ± 1.1 Ma, and ca. 211.8 ± 1.8 Ma), indicates a new generation of Late Triassic Luerma copper porphyry-type mineralization events in the western Gangdese magmatic belt. The Luerma porphyry is a strongly peraluminous shoshonite series rock, is rich in silicon total alkalis, potassium and aluminum, has low contents of calcium, magnesium, and iron. It is enriched in light rare earth elements and large ion lithophile elements, relatively depleted in heavy rare earth elements and high field strength elements, and has negative Eu anomalies and no Ce anomalies. The porphyry exhibits relatively low (87Sr/86Sr)i values (0.705617 on average), high εNd(t) values (0.63 on average), high εHf(t) values (11.98 on average), and young two-stage Hf model ages. The Luerma ore-related porphyries are most likely generated dominantly from partial melting of juvenile crust that formed by metasomatized mantle during the subduction of the NeoTethys Ocean to the Lhasa terrane and underwent crystal fractionation during the process. The deposit is currently the oldest and westernmost porphyry deposit in the Gangdese porphyry copper belt, providing evidence that porphyry copper mineralization in this belt began in at least the Late Triassic.