Magmatism And Metallogenesis At Duolong Ore District, Tibet

Duolong ore district is located in Bangong-Nujiang magmatic-tectonic zone, this is a newly found world-class porphyry-high sulfidation epithermal ore district with reserves over 1200 Mt Cu and 100 t Au. We’ve systematically conducted zircon U-Pb and geochemical analyses of the igneous rocks to reconstruct the magmatic processes.At the same time, we’ve conducted systematical analyses regarding the thermal history of ore-deposition and hydrothermal duration, precipitation of sulfides, as well as the source of the metals. We’ve also established the preliminary model for the ore-forming processes of Duolong district. Results of this study produced the following products:1. The combined EMPA, zircon U-Pb, whole rock geochemistry and Sr-Nd-Hf isotopes analyses reveal that the Duolong district was formed in the tectonic background of subducting, which released the water and sediments to the overlying mantle wedge, hence, caused the partial melting of the mantle wedge and formed primary magma, some of which were erupted during 143 Ma in to surface as the high Nb basalt. The primary magma was keeping uderplating and ponding at the base of the lower crust, then interacting with the mafic lower crust, which lead to the develop of intermediate magma with mantle-crust isotopic characteristics. Some of these magma erupted to the surface as the basaltic andesite(118Ma) and andesite(114Ma-105Ma), on the other hand, some of them ascending to the middle-upper crust, and might have experienced the AFC process before emplaced near the surface as dioritc and/or granodioritc intrusions.2. The thermobarometric formulations of amphibole suggest that the cause for the productive porphyries is the high oxygen fugacity compared with the barren ones.This study also shows that the oxygen fugacity is correlated with the crystallization temperature of amphiboles; therefore, the V+Ti contents in magnetite could be used as valuable indicator for mineral exploration.3. The integrated zircon U-Pb, molybdenite Re-Os, sericite 40Ar-39 Ar chronology suggests the ore-forming processes occurred during 121Ma-117 Ma, and the magmatic-hydrothermal events from emplacement of intrusions to the later sericite alteration last 1Ma.4. Fluid inclusions suggest that the ore-forming fluids in Bolong, Naruo,Diaonamugang are magmatic fluids. The various alterations are caused by magmatic fluids, though; the meteoric water might get involved in the chlorite alteration and sericitc alteration to some extent. The precipitation of metals was caused by the cooling of the magmatic fluids, as well as the oxygen fugacity. The Hf isotopes, Re contents of molybdenite and Sulfur isotopes imply the contribution of mantle materials for the porphyry deposits. Last but not the least, the formation of porphyry-type deposits was related to high temperature and high salinity magmatic fluids, however, the high sulfidation deposits might be related with medium salinity magmatic fluids.