Magmatism and metallogenic mechanism of the Ga'ergiong and Galale Cu-Au deposits in the west central Lhasa subterrane, Tibet: Constraints from geochronology, geochemistry, and Sr-Nd-Pb-Hf isotopes

The Ga'ergiong-Galale ore district in the Bangong-Nujiang metallogenic belt hosts two large skarn Cu-Au deposits, associated with voluminous Late Cretaceous volcanism and plutonism. In this study, we present new zircon U-Pb age and Hf isotopic data, whole-rock geochemical and Sr-Nd-Pb isotopic data to constrain the geodynamic mechanisms of Late Cretaceous magmatism and metallogenesis within the Ga'ergiong-Galale ore district. The intrusive rocks in the district include diorite, quartz diorite, granodiorite, granite, and dioritic porphyrite. Zircon U-Pb analyses suggest those rocks mainly emplaced at early Late Cretaceous (84-95 Ma), which coincide with the ore-forming age of Ga'ergiong and Galale deposits (87 and 88 Ma, respectively). Geochemically, the granodiorite and dioritic porphyrite are metaluminous with depletion of Nb, Ta, and Ti and characterized by high MgO (2.68-4.16 wt%), Mg-# (55-87), Sr (414-616 ppm), Sr/Y (30-56), Cr (22.5-105.0 ppm), and Ni (26.6-59.7 ppm), low Y (10.6-14.0 ppm), and heavy rare earth elements (HREE), showing geochemical signatures of high Mg-# adakitic rocks. Four dioritic porphyrite and three granodiorite samples exhibit homologous Sr-Nd-Pb isotope compositions and have positive whole rock epsilon(Nd)(t) values of +1.1 to +3.0, (Sr-87/Sr-86)(i) values of 0.7045-0.7048, (Pb-206/Pb-204)(t) ratios of 18.42-18.73, (Pb-207/Pb-204)(t) ratios of 15.58-15.63, and (Pb-208/Pb-204)(t) ratios of 38.54-38.88. Besides, 116 analyses from the Ga'ergiong-Galale adakitic rocks give positive zircon epsilon(Hf)(t) values (+ 4.3 to +12.1). The geochemical and isotopic data indicate that the early Late Cretaceous high Mg-# adakitic rocks in the Ga'ergiong-Galale ore district originated from the delaminated juvenile lower mafic crust that has been modified by the previously subduction. Partial melting of previously subduction modified, delaminated juvenile lower crust not only formed the hydrous adakitic melts in highly oxidation state, but also provided the ore metals (e.g., Cu, Au, Mo) for the subsequent mineralization. Such melts reacted with surrounding mantle peridotite to elevate the contents of metallic elements again. Then the melts experienced varying degrees of fractional crystallization process in the shallow magma chamber to form the dioritic and granodioritic magmas. Those magmas emplaced into the limestone of Duo'ai and Jiega Formation to form the Ga'ergiong and Galale skarn Cu-Au deposits, respectively. Field observation, together with geochronological and geochemical data, indicate that the early Late Cretaceous mineralization events in the northern and central Lhasa subterranes took place in a transition tectonic setting from collisional orogeny to extension and the input of subduction modified juvenile crust play an important role in the Late Cretaceous ore-forming events.