Geochronology and genesis of the newly discovered Mengdehe orogenic-type Au deposit in the Xing’an-Mongolia orogenic Belt, NE China

Abstract

The Xing’an-Mongolia Orogenic Belt (XMOB) is a tectonic window to understand the subduction-orogenesis-magmatism and corresponding Au mineralization during the Mesozoic. Although epithermal gold mineralization associated with Paleo-oceanic plate subduction has been widely reported, information on orogenic Au metallogenesis during this period is insufficient. Geochronology and genesis study of the newly discovered orognenic Au deposit named as Mengdehe located at the Heihe-Nenjiang tectonic mélange in the eastern XMOB, can provide new information about regional metallogenetic mechanisms in this region. The Mengdehe Au deposit is the first discovered mylonitic rocks-hosted and brittle-ductile shear zone-dominated Au deposit in this area. Magmatic zircon U-Pb ages from the granitic mylonite and undeformed diorite in the Mengdehe deposit constrain that the strike-slip event probably has happened between 245.8 and 208.0 Ma. Rb-Sr dating of hydrothermal pyrites from the ore-bearing altered rocks indicates that Au mineralization occurred at 209.6 ± 3.1 Ma. We consider that the Mengdehe Au deposit was formed in the Late Triassic and associated with a strike-slip event. The trace elements, and He-Ar, S and Pb isotopic compositions of the ore-forming stage pyrites suggest that the ore-forming materials were derived from crustal sources with orogenic affinity. Combined with the early Mesozoic regional tectonic evolution in NE China and deformation characteristics of wall rocks, hydrothermal minerals and diorites, we conclude that the Au mineralization of the Mengdehe deposit was in response to the tectonic transformation from compression to extension associated with the extensional collapse of a previous thickened continental lithosphere related to the final closure of the Paleo-Asian Ocean.