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Study On Metallogenesis Of Porphyry Deposits In Eastern Kunlun Orogenic Belt, Qinghai Province

Posted on:2015-01-24Degree:DoctorType:Dissertation
Country:ChinaCandidate:Q L XuFull Text:PDF
GTID:1260330428483931Subject:Mineralogy, petrology, ore deposits
Abstract/Summary:PDF Full Text Request
The Eastern Kunlun Orogenic Belt (EKOB), one of the important metallogenic belt in China, hascomplicated geological conditions and rich mineral resources, which attracted much attention of thedomestic and foreign geologists. The EKOB is an important part of the China’s Central Orogenic Beltwith intense magmatism resulting from the frequent tectonic activities. The tectonic processes that theEKOB has undergone include Pre-Caledonian formation of Archean Paleo-continent, breakup ofProterozoic Paleo-continent and formation of oceanic basalt plateau; Caledonian closure of thePre-Prototethys Ocean, subduction and collision, and accretion; Hercynian-Early IndochineseAndes-type orogeny; Late Indochinese-Yanshanian lithosphere delamination and mantle-derivedmagma underplating; and Meso-Cenozoic EW tectonic differentiation.In this study, we present petrography, zircon U–Pb dating, and major and trace elements forvolcanic and intrusive rocks in the eastern Kunlun area, to constrain the tectonic evolution of theEKOB during Caledonian-Late Indochinese. Research shows that Early Caledonian (~475Ma) gneissicgranites were probably originated by partial melting of a mafic lower crust metasomatized bysubduction-related fluids, and formed in an active continental margin setting related to the northwardssubduction of the Proto-Tethys oceanic plate beneath the Qaidam Massif along the middle Kunlun faultbelt. Late Hercynian monzogranites (~268Ma), and Early Indochinese fine-grained granites (~245Ma)and felsic volcanic rocks were probably originated by partial melting of a mafic lower crust. The EarlyIndochinese mafic volcanic rocks were probably derived from partial melting of an enriched mantlesource metasomatized by subduction-related fluids. They all formed under an Andes-type activecontinental margin setting related to the later period of subduction of the Paleo-Tethys oceanic plate.Late Indochinese quartz diorite-porphyrites (~223Ma) were probably originated by partial melting of an ancient continental crust heated by the underplating of the mantle-derived mafic magma with mixingprocess involving mafic and felsic magmas. They formed in a post-collisional extensional environmentfollowing the closure of Paleo-Tethys Ocean in the Late Indosinian.This paper carries on systematic research on the field observations and laboratory investigationsfor seven deposits in the study area, including Ulan Uzhur Cu deposit, Aikengdelesite Mo-Cu deposit,Halongxiuma W-Mo deposit, Jiadanggen Cu-Mo deposit, Mohexiala Ag polymetallic deposit,Kaerqueka Yelasai Cu deposit, and Harizha Cu deposit. Studies suggest that Aikengdelesite Mo-Cudeposit, Halongxiuma W-Mo deposit, Jiadanggen Cu-Mo deposit and Ulan Uzhur Cu deposit areporphyry deposit; Mohexiala Ag polymetallic deposit is a storeyed deposit where hydrothermalvein-type and porphyry-type mineralization coexisted; Kaerqueka Yelasai Cu deposit belongs tohigh-temperature hydrothermal vein deposit, while Harizha Cu deposit is a hydrothermal vein depositinstead of previously considered porphyry deposit. According to the characteristics of each deposit andcombined with the classification of porphyry Mo deposit by Westra et al.(1981), we divided porphyryMo deposits in the EKOB into stock type derived from calc-alkaline rocks (Aikengdelesite Cu-Modeposit and Jiadanggen Cu-Mo deposit) and intrusion type (Halongxiuma W-Mo deposit). Thisclassification is more consistent with actual geological characteristics of deposits, which provides anew geological basis and broadens ideas for prospecting.Studies on fluid inclusions in each deposit revealed that the types of fluid inclusions in porphyrydeposits in EKOB contain mainly gas-liquid two-phase inclusions, daughter crystal-bearing three-phaseinclusions, and a small amount of CO2-bearing three-phase inclusions. Analysis of Laser Ramanspectrum for partial deposits shows that the compositions of gas phase contain a small amount of SO2and CH4, besides H2O and CO2, and the ore-forming fluid is mainly NaCl-H2O system, indicating thecharacteristics of the moderate to high temperature, high salinity, and moderate to high density.Characteristics of hydrogen and oxygen isotopes show that the ore-forming fluid is dominated by themagmatic water, and meteoric water involved in metallogenic process with different degrees. Metalsulfide has relatively single sulfur source, which is the deep origin magmatic sulfur with thecharacteristics of mantle source. Research of Lead isotope shows that the metallogenic material ofporphyry deposit in Late Indosinian has the crust-mantle mixing characteristics.Studies on chronology and geochemistry of metallogenic rocks show that SHRIMP zircon U-Pbage of metallogenic porphyry in Ulan Uzhur Cu deposit is416.7±3.3Ma, i.e., Late Caledonian, ratherIV than Indosinian previously considered. LA-ICP-MS zircon U-Pb age of metallogenic granite porphyryin Aikengdelesite Cu-Mo deposit is248.3±1.5Ma, i.e., Early Indosinian, and the magma source isgiven priority to crustal materials with imprint of subduction fluid. Metallogenic granodiorite porphyryin Jiadanggen Cu-Mo deposit, metallogenic granite porphyry in Mohexiala Ag polymetallic deposit,and metallogenic granodiorite porphyry in Halongxiuma W-Mo deposit, yield LA-ICP-MS zircon U-Pbages of227±1,222±1, and230±1Ma, respectively, i.e., Late Indosinian. Granodiorite porphyrys inJiadanggen and Halongxiuma deposits were derived from partial melting of the mafic lower crust, andgranite porphyrys in Mohexiala deposit were derived from partial melting of the metamorphicgraywacke in upper crust, all showing the crust-mantle mixing characteristics. The Indosinianmetallogenic rocks were I-type granites with high differentiation and oxidation, belonging to themagnetite type granites, mostly forming copper molybdenum deposit.Based on the above researches, this paper establishes the metallogenic model for porphyrydeposits in EKOB, summarizes the temporal and spatial distribution of porphyry deposits in EKOB,and analyzes their denudation and preservation conditions. It mainly divides into three metallogenicperiods, namely, the Late Caledonian, Early Indosinian and Late Indosinian, and the Late Indosinian isthe most important period. Mineralization distributes in each belt of East Kunlun, and lessens fromnorth to south. The north belt of East Kunlun (i.e., the north back-arc rift belt) has the most deposits,the middle belt of East Kunlun (i.e., the middle basement uplifting and granite belt) takes the secondplace, and the south belt of East Kunlun (the south composite collage belt) has the least deposits. Fromwest to east mineralization increased, the most western part, Qimantage area, has less deposits. To theeast side, the middle East Kunlun has none deposit, while the eastern East Kunlun has a large amountof deposits. The south belt with minimum denudation depth has good preservation condition for oredeposits, the preservation condition of north belt takes the second place, While, epizonal deposits aremost difficult to save in the middle belt with the biggest eroding depth.
Keywords/Search Tags:Fluid inclusion, Petrochemistry, Zircon U-Pb chronology, Mineralization, porphyry deposit, Eastern Kunlun Orogenic Belt
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