The Genetic Study Of Narusongduo Lead Zinc Ore Deposit In Middle Gangdese Belt, Tibet

Study area was located to the center Lasha terrene, lied to the south coast of the Longge’er-Nyainqengtanglha back-arc faulted uplift area in the middle Gangdise metallogenic belt. There exited multi-tectonic episode activity, coexistence of tectonic magmatic processs and the formation of a complex multi-stage arc-forming accretionary by the transformation of ocean crust subduction, arc-arc collision or arc-continent collision in the Indo-Asian collisional orogenic processes in the Gangdise metallogenic belt, which formed a complex tectonic magmatic belt and owned extremely advantageous geological conditions. From geologic and resource survey on in our country, the Gangdise metallogenic belt has achieved a great major breakthrough in mineral resource exploration and evaluation. There are two important polymetallic ore belts, Cu-Mo-Au ore belt and Pb-Zn-Ag ore belt, occurred in the southern and northern parts of the Gangdise Cu-Fe-Pb-Zn polymetallic metallogenic belt, south Tibet, respectively. Then, Gangdise metallogenic belt become one of China’s national metallogenic belt and an important strategic resource reserve base to succeed. Extensive researches on the Cu-Mo-Au metallogeny and related porphyry have been carried out in the southern Gandese, however, little research has been done on the Pb-Zn-Ag polymetallic ore belt in the northern Gandese.The Narusongduo deposit is the representative one of the Pb-Zn-Ag polymetallic ore belt. Author select Narusongduo porphyry Pb-Zn ore deposit as the main subject. On the basis of detail geological field research and carefully well study, the author systematic compared ore-forming geological characteristics, mineralogy, petrology, chronology, isotope geochemistry, ore forming materials and fluids of east district to west district in the Narusongduo ore deposit by using many modern analysis and testing technologies, Combined with previous existing researches and laboratory analysis results and to consolidate. Author, then, revealed the metallogenic geodynamics backgrounds and mineralization characteristics, discussed the genesis of deposit and preliminary eatablished the genetic model for metallogenesis of the Narusongduo ore deposit. Thesis achieved the main results and understanding as follows:1. Mading a detail comparision of the different types of mineralization in the east district and west district.The west mine paragraph the case of the typical formed skam ore by hydrothermal carbonate. The carbonate construction of the lower Permian was the importance of ore formation. The ore bodies are strictly controlled by the structure of the contact zone, lied lenticular and layered, in which replacement fabric was so common. To be different from the west mine, the east district formed as typical porphyry Pb-Zn ore deposit by magmatism. Cryptoexplosive breccias tube in the east district controlled the main ore bodies, in which wall-rocks consist of Linzizong volcanic rocks and quartz-porphyry. It was the cryptoexplosive processs that leaded to enrichment and precipitation of ore-form ing metal and cryptoexplosive breccia tube controlled ore body occurrence which brecciated or massive structure ore were two typical ore structure.2. The analogous tectonic background, similar quartz-porphyry in each district, but different periods of the different types of mineralization.Narusongduo deposit was formed during the Indo-Eurasia collisional orogeny, when the Neo-Tethys subduction was in the completion. The average concordia age of U-Pb geochronology of porphyry genetically related to mineralization is61.56～63.38Ma in the west mine as well as mineralization age, while the chronology of porphyry in the east district ranges58.54to60.36Ma and mineralization age occurs58.54～57.15Ma by sericite Ar-Ar age. Author is convinced that the Narusongduo ore deposit related to the main collisional tectonic-magmatic event since periods of mineralization strongly consisted with the start time of Indo-Eurasia collisional orogeny.3. The systematic work of geology, mineralogy, petrology, isotope geochemistry about porphyry volcanic rocks.The porphyry in both district accompanies by phenocrysts (quartz, feldspar) and microscopic crystalline felsic matrix while their petrochemistry shows many different characteristics. The porphyry in the east district appears SiO2richer, has higher contents of alkalis, shows higher the K2O/Na2O ratio and CaO/(Mg+FeO·) ratio than that in the west mine, while Rb/Ba ratio and Rb/Sr ratio are lower than that in the west mine. The porphyry in the east district shows the shoshonitic calcium alkaline stongly praluminous characteristics while the porphyry in the west mine appears middle-high potassium calcium alkaline stongly praluminous. The porphyry is characterized by inconspicuous fractionation of LREE/HREE and the REE patterns are rightist gentle type in both district. The contents of REE, LREE/HREE ratio,(La/Yb)N ratio and the6Eu of porphyry in the east district are obviously higher than that in the west mine while both of each has no Ce anomaly. All the porphyry in the west mine demonstrate the characteristic negative anomalies in Ba, Nb, Sr, P, and Ti in the spidergram while the porphyry in the east mine shows the characteristic negative anomalies in Ba, Nb, Ta, Sr, and Ti in the spidergram. Both of them enrich large ion lithophile elements (LILE) and loss high field strength elements (HSFE). Nb/Ta ratio and Zr/Hf ratio of porphyry close to the the bulk continental crust, while Rb/Sr ratio seems further higher than that in the the bulk continental crust. These features imply that magmatic origin should be the best choice of the continental crust.However, Sr-Nd-Pb isotope geochemistry of both district show the most analogous characteristic to the the North India continent crust, which represented by the basement and granitoids from Tethyan and Higher Himalayas with the most enriched Pb compositions. Partialy, both porphyry and volcanic rocks in the Narusongduo ore deposit may more or less mix the the Neo-Tethyan mantle reservoir represented by Yarlung ophiolite. The feature of Sr-Nd-Pb isotope geochemistry revealed that Nyainqentanglha Group metasediments (Gangdise basement) may be the best choice of porphyric magma origin.It was Nyainqentanglha Group metasediments that partially melt by muscovite dehydration melting with anhydrous conditions at about20-35km depth by the underplating of high temperature basaltic magma which form in the start time of Indo-Asian collisional orogenic (-65Ma). Diorite which form in64.17±0.3Ma(Zircon U-Pb age) in study area and basalt, basaltic andesite, andesite in Dianzhong Formation are the product of AFC process caused by underplating basaltic magma mixing with middle-upper crustal felsic magma while porphyry in both district and rhyolite (61.7Ma) in Dianzhong Formation mainly represent the product of felsic magma. Felsic magma emplaced along the east-west thrust faults.4. Metal and sulfur Enrichment by H2S degassing in magma system in both districts.The834S of Porphyry in west district appears higher than the834S of ore as well as in in east district. What’s more, the834S of Porphyry in west district show further higher than that in east district and Author attributed this great difference to H2S degassing during magma intrusion. Obviously, cryptoexplosive mechanisms carry out more metals from in the magmatic system to fluid in east district. Both districts have the same total834S and Pb isotope in the ore of both district show the isotopic composition of consistent to the porphyry in both district, which show that minerals resource and sulfur comes mainly from porphyry.5. The crucial importance of magmatic fluids interaction with atmospheric fluids in both districts.REE patterns of galena, sphalerite and ore show the most analogous characteristic to the limestone in the lower Permian in west district and appear obviously negative Ce and Eu anomalies, which differed from REE patterns of porphyry. Fluids in in west district have deposition transformation fluids characteristics and the atmospheric fluids play an important role in ore-forming in west district.But REE patterns of galena, sphalerite and ore show the most similar characteristic to the porphyry in east district which only appear obviously negative Eu anomalies. Fluids still retain the nature of magmatic fluids, however the atmospheric fluids also play an important role in ore-forming in east district.