Geochemistry Of The Ore-bearing Porphyry Of Qulong Cu(Mo) Ore Deposit In Gendese Belt, Tibet, And Its Petrogenesis

Qulong porphyry copper (molybdenum) deposit is located in Mozugongka County, Tibet. It situated in the southern margin of Lhasa-Riduo localization basin of the continental margin magmatic arc in east Gangdese belt. The middle Jurassic Yeba formation volcanics, Paleocene biotite granite porphyry, Miocene quartz monzonite and Oligocene granite porphyry could be found in the ore field. Four mineralized porphyry bodies and five copper ore bodys, which locate in the depth over 500 m, have been distinguished in Qulong. It is one of the biggest breakthrough of ore prospecting in Tibet. Qulong also have the potentiality of being a super large porphyry copper (molybdenum) deposit in the eastern Gangdese porphyry copper belt.This research mainly aims at the ore-bearing porphyry bodies, by studying the mineral chemistry, major and the trace element geochemistry, and isotope geochemistry. By comparing to other typical deposits in this metallogenic belt, the characters of spatiotemporal distribution, mineralization, alteration and structural setting, and petrogenesis are also summarized.The ore-bearing biotite granite porphyry and quartz monzonite emplaced in the Jurassic Yeba formation volcanics. The biotite granite porphyry emplaced in the tuff and rhyolite porphyry, and the quartz monzonitic emplaced in the biotite granite porphyry and the rhyolite porphyry. The copper (molybdenum) ore bodies are mainly situated in or out of the contact zone between the mineralized porphyry bodies and the biotite granite porphyry.The Re-Os model ages yielded by molybdenite from the Qulong porphyry deposit range from 15.82±0.19 to 16.85 ± 0.19 Ma. The SHRIMP zircon U-Pb measurement of the biotite granite porphyry yielded the age of 15.9±0.7 Ma.This is very consistent with the published data (14-20Ma) of the quartz monzonite in the whole Gangdese belt.The porphyries characterized by high - SiO₂, K₂O and N₂O and low CaO and TiO₂. The aluminium saturation indexes (A/CNK) are less than 1, the Rittmann Indexes are mostly less than 3.3. So they belong to dominative aluminous high-K cale-alkaline series rocks. The rocks are enriched in LILE (e.g. K, Rb, Sr, U) and LREE, and depleted in HFSE (e.g., Nb, Ta, Ti, Y) and HREE. The Eu negative anomaly is inconspicuous. The porphyries show adakite magmatic affinity withparticularity, which suggest the complexity of the magma origin.The common alterations are potash feldsparize, biotitize, silicification, sericitization and partial argillation with the mineralized porphyries. The alteration zoning is analogous to the model of Lowell and Guilbert, with typical "face type" feature. The concentric alteration zones from the inner outward are successively the K-silicate zone, the quartz-sericite zone, and the propylitic zone, but the argillatio zone is on rare occasion. The orebodies mostly occur in the K-silicate alteration periods, and the later quartz-sericite periods take second place.Different from the most porphyry deposits which generate in extruding environment (mainly continental margin arc and island arc), the diagenesis and mineralization of the Gangdese porphyry copper belt took place during post-CoUis ional crustal extension period. The mineralized porphyry bodies were locally controlled by the near NS-striking extension tectonics.The characteristics of the geognosy, the rare elements compositions and the signatures of Sr, Nd and Pb isotope of the rocks inferred that the mineralized porphyry related to the tethys subducted oceanic crust, the basaltic lower crust and the oceanic deposit, coupled with the exchange of substance with the depletion mantle and the enrichment mantle II (EM II) as well as the lower crust partial melting and the up-crust contamination. All of that show the complexity of magmatic origin.