Petrology And Metallogenesis Of The Porphyry Cu Deposits In The Northern Great Xing'an Range

The newly discovered Xiaokele and Fukeshan porphyry Cu deposits were located in the northern Great Xing'an Range.In this thesis,deposit geological characteristics,zircon U-Pb and molybdenite Re-Os isotopic ages,whole-rock geochemical and Sr-Nd isotopic data,zircon Hf,pyrite and chalcopyrite S,quartz H-O,and calcite C-O isotopic data,biotite,hornblende and pyrite in-situ geochemical data are reported,with the aims to constrain the petrogenesis and ore deposit genesis of the studied deposit,and discuss the metallogenic process and regional tectonic evolution.The research led to the following conclusions:(1)In the Xiaokele deposit,inner zone potassic alteration and outer zone phyllic alteration were identified and granodiorite porphyry was considered to be the ore-causative rocks.Cu-Mo ore bodies are mainly hosted in the potassic zone with some Mo ore bodies hosted in the phyllic alteration zone.In the Fukeshan deposit,at least seven stages igneous rocks were identified,and quartz diorite porphyry shows great affinity with Cu-Mo ore bodies,and coarse-grained monzogranite and quartz monzonite contacted with the ore-causative rocks are the main ore-bearing rocks.The alteration zones in the Fukeshan deposit are not as clear as those in the Xiaokele deposit due to the overprint of alterations by late stage igneous rocks.(2)The U–Pb ages of zircons from rhyolite,mineralized granodiorite porphyry,diorite porphyry and granite porphyry associated with the Xiaokele deposit are 152.5 ± 1.7,150.0 ± 1.6,147.9 ± 1.3,and 123.2 ± 1.7 Ma,respectively.The U-Pb ages of zircons from medium-grained monzogranite,coarse-grained monzogranite,ore-bearing quartz diorite porphyry,quartz monzonite,ganodiorite porphyry,diorite porphyry and andesitic porphyry at the Fukeshan deposit are 192.7 ± 1.9,192.2 ± 2.7,148.7 ± 0.8,148.8 ± 0.9,144.1 ± 1.1,144.9 ± 0.9,and 144.8 ± 1.3 Ma,respectively.The molybdenite Re-Os isotopic ages from the Xiaokele and Fukeshan deposit are 147.3 ± 0.8 Ma and 148.0 ± 2.8 Ma,respectively,constraining the formation of the study deposits during the Late Jurassic.(3)Whole-rock geochemical,Sr–Nd isotopic and zircon Hf isotopic data indicated that the rhyolite and granite porphyry in the Xiaokele deposit were derived from a basaltic lower crust.The ore-bearing granodiorite porphyry was formed by the partial melting of an oceanic slab.The diorite porphyry possibly originated from the melting of an enriched mantle.In the Fukeshan deposit,the coarse-grained monzogranite and granodiorite porphyry were derived from a thickened juvenile lower crust.The ore-related quartz diorite porphyry and quartz monzonite possibly originated from the melting of an oceanic slab.The medium-grained monzogranite was probably formed by partial melting of basaltic lower crust at a shallower depth.The diorite porphyry and andesitic porphyry were probably derived from the melting of a metasomatized depleted mantle.The ore-causative rocks in both deposits show adakite affinity,with high Sr,low Y and Yb contents and therefore high Sr/Y ratios.(4)Biotite,hornblende and pyrite in-situ geochemistry were used to estimate the T,P,H2 Omelt and Logf(O2)of the magma.The results show that the parent magmata of the ore-causative rocks in the xiaokele and Fukeshan deposits have high Log(f O2)(NNO+1-NNO+2)and H2Omelt(~3.5%-5%).The generation depths for the Xiaokele and Fukeshan deposits are 1.9 and 3.8 km,respectively.(5)Pyrite and chalcopyrite S isotopic,pyrite geochemical and molybdenite Re-Os isotopic data show that mantle materials have played an important role in the formation of the Xiaokele and Fukeshan deposits.(6)The late Mesozoic tectonic evolution in the northeastern Great Xing'an Range was dominated by the Mongol-Okhotsk Ocean.The Mongol-Okhotsk oceanic slab was subducted southward during the Early Jurassic(195–170 Ma)and underwent slab retreat during the early Late Jurassic(165–155 Ma).This was followed by the subduction of a ridge on the oceanic slab during the Late Jurassic(150–147 Ma).The Mongol–Okhotsk Ocean finally closed during the Early Cretaceous(145–143 Ma).(7)The petrogenesis of adakites and Cu-Mo mineralization in the eastern part of the Erguna Block in NE China were controlled mainly by the late-stage south-wards subduction of a ridge on the Mongol-Okhotsk Ocean.The following large-scale sinking of the residual oceanic slab led to the upwelling of the asthenosphere and is favorable for the generation of volcanic and sub-volcanic rocks and medium-low T hydrothermal Au,Ag,Pb-Zn-Cu deposits.