Study On Genetic Relationship Of The Ore-forming Bodys In Skarn Iron-copper Deposit Of Nixiong Ore Field,Cuoqin County,Tibet

The Nixiong iron-copper ore field is divided into5mines from north west to south east: Gunjiu iron deposit-Nixiong iron deposit-Shasongnan iron deposit, Maojialai iron deposit and Ri’a copper deposit. Whole-rock major, trace and rare earth elements and Sr-Nd-Pb isotopic analyses, combined with zircon LA-ICPMS U-Pb dating, of granodiorite related to the Nixiong iron deposit and adamellite porphyry related to the Ri’a copper deposit, are carried out in this paper. Main achievements are as follows:(l)The petrogeochemical analysis results indicate that the granodiorite related to Nixiong iron deposit belongs to subaluminous%medium-high K calc-alkaline I-type granite. Geochemically the granodiorite is enriched in large ion lithophile elements(LILE) Rb, Ba, Sr, Th, U, K, Pb and depleted in high field strength elements(HFSE) Nb, Ta, Ti and shows essential characteristics of arc magmatic rocks. The rare earth element chondrite-normalized distribution patterns appear a notable enrichment of light rare earth elements relative to heavy rare earth elements without marked Eu anomalies and show signatures of magmatic rocks from an active continental margin.The petrogeochemical analysis results indicate that the adamellite porphyry related to Ri’a copper deposit belongs to subaluminous> high K calc-alkaline I-type granite. Geochemically the adamellite porphyry is enriched in large ion lithophile elements (LILE) Rb, Ba, Sr, Th, U, K, Pb and depleted in high field strength elements (HFSE) Nb, Ta, Ti.The rare earth element chondrite-normalized distribution patterns appear a notable enrichment of light rare earth elements relative to heavy rare earth elements with Eu negative abnormality and it also shows essential characteristics of arc magmatic rocks.(2)Zircon U-Pb LA-ICPMS dating of Nixiong granodiorite yields a weighted206Pb/238U mean age of112.09+0.54Ma with MSWD=0.45, indicating that the granodiorite was formed in late stage of Early Cretaceous. Zircon U-Pb LA-ICPMS dating of Ri’a adamellite porphyry yields a weighted206Pb/238U mean age of90.1±1.5Ma with MSWD=1.2, indicating that the adamellite porphyry was formed in early stage of Late Cretaceous. Thus, iron and copper mineralization of Nixiong ore field does not occured at the same time, but of two independent stages of mineralization:the iron mineralization of Early Cretaceous (112.09Ma) and the copper mineralization of Late Cretaceous (90.1Ma).(3)The Nixiong granodiorite and the Ri’a ore-forming adamellite porphyry both with characteristic:high radiogenic Sr isotopic ratios、low Nd isotopic ratios and high primitive Pb isotopic compositions. On the one hand, it displays the enriched mantle structures of EMII;on the other hand.it reflects an important influence of crustal components on the magmatic generation. The authors of the paper make a comprehensive analysis on the evolution histories of the Bangonghu-Nujiang Meso-Tethyan ocean and the Yaluzangbo Neo-Tethyan ocean. In late stage of Early Cretaceous and early stage of Late Cretaceous, the crust of Yaluzangbo Neo-Tethyan ocean is diving beneath the Lhasa block on the northern side, which conforms to the metallogenic tectonic background of Nixiong iron-copper ore field. Therefore, the authors of the paper reach a conclusion that the Nixiong iron deposit is genetically related to the northward subduction of the Yarlu-Zangbo oceanic crust.(4)In Nixiong iron deposit, the enrichment component of fluid released from sediments adhered to the subducted oceanic crust triggered the mantle wedge partially melt to generate the enriched magma. However, the enriched magma of Ri’a copper deposit is mainly composed of metasomatism of plate melt with mantle wedge. The mantle magma intruded and caused partical melting of lower crust and mixed with each other to form the metallogenic magma. The metallogenic magma of Nixiong iron deposit experienced fractional crystallization of plagioclase+feldspar and accessory minerals (allanite+monazite) and the metallogenic magma of Ri’a copper deposit experienced fractional crystallization of plagioclase+feldspar(±biotite) and accessory minerals(apatite).