Petrogenesis Of The Yanshanian Granite And Metallogenic Characteristics Of The W-sn Polymetallic Deposits In The Laojunshan Region,southeastern Yunnan Province

Gejiu-Bozhushan-Laojunshan W-Sn metallogenic belt located in the southeast of Yunnan province,and its geotectonic position geotectonic position lies in the conjunction and transformation of South China plate,Yangtze plate and Sanjiang Tethyan tectonic belt which belongs to the Yanshanian orogenic of southeast Yunnan in the western of South China.The area contains rich W,Sn,Ag,Pb,Zn and other valuable materials.Laojunshan W-Sn ore concentration area located at the southern tip of the Gejiu-Bozhushan-Laojunshan W-Sn metallogenic belt,ore mineralization in the area is complex,and with development of Laojunshan composite granite.The dating of minerals such as wolframite and cassiterite has not been reported,and the magmatism and lithogenesy of Laojunshan composite granite are still controversial.In this thesis,LA-ICPMS zircon U-Pb ages of the wolframite and cassiterite,and Re-Os ages of the molybdenite are present to constrain the ages of major ore-forming stage,and LA-ICPMS zircon U-Pb ages of the zircon and monazite,in-situ zircon Hf-O isotopes,coupled with whole-rock major and trace elements and Sr-Nd-Hf isotopic data,are present to establish the three stages magma activity time limit of the Laojunshan composite granite and the intrusive units with different times,find out the sources and petrogenesis of in order to better understand the origin of granitic rocks and to further explore possibly associated crustal reworking process and regional tectonic evolution.1.According to the contact relation between the Laojunshan composite granite and each unit in field,and on the basis of petrographic and geochemical characteristics,5 units of the Laojunshan composite granite have been identified,including coarse grained two-mica adamellite,medium grain two-mica adamellite,medium-fine grained two-mica adamellite,fine-grained two-mica adamellite and granite porphyry,and mainly for intrusive contact between each unit.Detailed geochemical study indicates that the Laojunshan composite granite samples have high contents of Si O2 and alkalis,which belong to the peraluminous high-K calc-alkaline.The zircon saturation temperature and Ti saturation temperature of Laojunshan composite granite are concentrated in 740-770�C which is the formation temperature 4 of Laojunshan granite.2.The zircon and monazite LA-ICPMS U-Pb dating shows: 3 coarse grained two-mica adamellite have zircon U-Pb ages of 91.63�0.84 Ma,91.61�0.80 Ma and 91.44�0.85 Ma,the ages of monazite are 88.84�0.87 Ma,88.84�0.85 Ma,88.58�0.91 Ma respectively;3 medium grain two-mica adamellite have zircon U-Pb ages of 91.48�0.89 Ma,91.09�0.80 Ma,91.29�0.89 Ma,the ages of monazite are 88.57�0.98 Ma,88.25�0.90 Ma and 88.55�0.98 Ma respectively;2 medium-fine grained two-mica adamellite have zircon U-Pb ages of 87.37�0.58 Ma and 87.47�0.78 Ma,the ages of monazite are 85.89�0.82 Ma and 85.93�0.83 Ma respectively;2 fine-grained two-mica adamellite have zircon U-Pb ages of 87.10�0.51 Ma and 87.10�1.0 Ma,the ages of monazite are 85.68�0.97 Ma and 85.53�0.83 Ma;2 granite porphyry have zircon U-Pb ages of 83.17�0.75 Ma and 83.19�0.76 Ma,the ages of monazite are 82.09�0.81 Ma and 82.48�0.81 Ma.These ages highlight that the magmation of Laojunshan composite granite is divided into three periods,and the limits for each period are 91-88 Ma,87-85 Ma and 83-82 Ma respectively,while its age interval of 4-3 Ma.In inherited zircon measured a zircon U-Pb age is 99.3�1.1Ma,shows may be a small magmatic activity before the first phase of the Laojunshan granite magma activity whose emplacement is deeper,not surface.Then this study established a new and accurately chronology framework for Laojunshan composite granite.3.Whole-rock Sr-Nd-Pb isotope,zircon Hf-O isotope and monazite Nd isotopic data show that Laojunshan W-Sn ore-concentrated area attributes for the Yanshanian orogenic of southeast Yunnan in South China.Homologous magmatic zircon values and Hf model age of the inheritance zircon range from 1.95 to 1.55 Ga.The TDM2(Nd)ages of zircon and whole-rock range from 0.96-1.36 Ga,which are generally indicative the Laojunshan granite source belongs to the paleo Proterozoic crystallinebasement,with the age limit on 1.95-1.85 Ga.Furthermore,the O isotopic values of Laojunshan granite suggest a source from the old continental crust.4.In this thesis,wolframite from Dongfei W-Sn yield a weighted mean 206Pb/238 U age of 86.2 � 2.2 Ma,and cassiterite from Kouha Sn(Cu)polymetallic mine area yield a U-Pb age of 87.7�4.3 Ma,these U-Pb ages are also geologically consistent with the medium-grained granite age of 91-89 Ma.Moreover,wolframite and cassiterite ore bodies hosted in the medium-grained granite units of Laojunshan composite granite,likely indicated the mineralization related to the first stage magma activity of the Yanshanian Laojunshan granite.Molybdenite in the Nanwenhe scheelite deposit yield a Re-Os isochron age of 219.4 � 2.9 Ma,which associated with scheelite in the major ore-forming stage,then suggests the scheelite mineralization related to Indosinian tectonic movement.Hence,there an EW trend granite porphyry vein resulted in the ore superimposition and reformation,where hidden under the Nanwenhe scheelite deposit.