The role of long-lived arc volcanism in the formation of the VMS deposits: A case study of the volcanic-sedimentary sequence of Kangbutiebao formation associated with VMS deposits, Altai Mountains

Abstract

Questions about the role of volcanism in the formation of VMS deposits and the origin of metals in pre-ore volcanic rocks in accretionary orogen generally puzzle scholars. The volcanic-sedimentary sequence of Lower Devonian Kangbutiebao Formation (KBTB Fm) associated with VMS deposits in the Altai Mountains is studied to attempt to solve these issues. The KBTB Fm is predominantly composed of felsic volcanic rocks with subordinate sedimentary rocks. Zircon U-Pb dating of rhyolite and rhyodacite suggests that felsic volcanism occurred in the Early Devonian (400.1 ± 0.8 Ma to 395.3 ± 0.9 Ma). Partial melting of juvenile crust under the environment of lower temperature (662 to 728 °C) and pressure (<∼15 km) resulted in volcanic rocks with arc-like trace elements and depleted Hf isotopes. Both compositional heterogeneity of source rocks and contrasting melting temperatures are responsible for the variety of petrochemistry. Sedimentary rocks exhibit low CIA values (51–66) and varied ICV values (0.76–1.78), implying that the provenance was affected by weak weathering. As suggested by detrital zircon geochronology, detrital materials from sedimentary rocks are dominated by pyroclastic materials that erupted from the volcano, together with minor eroded detrital materials from adjacent intrusions. It is proposed that the KBTB Fm was formed under continental rifted arc or back-arc condition, and local extensional setting was induced by slab-rollback of the Paleo-Asian Ocean, which produced bimodal volcanic rocks and VMS deposits. Large volumes of sequences with low metallic abundance were produced and protracted hydrothermal circulation was sustained under the condition of long-lived arc volcanism, leading to most VMS deposits with low to moderate grade, and rarely, high grade. All these parameters are grouped together (i.e., abundant but low contents of metals, long-lived heat source, and syn-volcanic faults), leading to most VMS deposits hosted by the KBTB Fm in the southern Chinese Altai.