Magma mixing and Cu-Au mineralization in the Gangdese magmatic belt in response to India-Asia collision

Abstract

The Quxu pluton in the northern Yalu tsangpu suture of southern Gandese, Tibet, consists petrologically of gabbro, diorite, granodiorite and granite. Mafic macro-granular enclaves (MME) are widespread in the granitic intrusions. There are many Cu-Au deposits and mineralized occurrences in the pluton. Detailed geological investigations, systemic research, and petrological and geochemical studies, have been conducted. Results suggest the three types of rocks are likely to have formed during the same magmatic event; that is, magma mixing in the Eocene. The host granitoid is an acidic end member, gabbros are akin to basic end members and mafic microgranular enclaves represent incompletely mixed basic magma clots trapped in acidic magma involved in magma mixing. Zircon SHRIMP ? U-Pb dating gave a mixing age of 50 Ma. The isotopic dating also suggests that the magma mixing took place soon after the initiation of India-Eurasia continental collision related to underplating of collision-induced basic magma under continental crust. Underplating and magma mixing likely played an important role in Cu-Mo mineralization, forming skarn type Cu-Au (Fe) deposits and porphyry Cu-Au mineralization in southern Gangdese north to the collision zone. This paper describes the magma mixing event and the related mineralization, aims at the understanding of the petrogenesis of MME-bearing granitoids, the tectonic settings and related mineralization. Evidence from petrology and elemental geochemistry show that the Cu-Au mineralization is closely related to the magma mixing for chalcopyrite and malachite occurrences around the MME. Therefore, it is argued that the mineralization formed during Eocene (ca. 50 Ma). Magma mixing was likely the main process of mass-energy interaction between mantle and crust and of related Cu-Au mineralization due to crustal partial melting by mafic magma underplating during the continental collision.