Late-mesozoic Tectonic-magama Evolution And Its Relationship With Mineralization In Luanchuan County

Luanchuan County is an important part of East Qinling Metallogenic Belt which is famous formolybdenum reverse in the world. This paper made a detailed research of structures, plutons andmolybdenum deposits in Luanchuan County, and then analyzed the spatial and temporal distribution,petrogenesis, source materials and tectonic environment of several typical plutons by field work,petrographic, rock geochemistry (major, trace and rare earth elements), zircon U-Pb geochronologyand Lu-Hf isotopic measurement. Besides, three typical porphyry-skarn molybdenumpoly-metallogenic deposits were chosen to discuss their differences. All above were discussed for thetectonic-magma evolution and metallogenic process in Luanchuan County. This paper has got thefollowing five achievements:(1) Late-Mesozoic magma activity can be divided into two stages in Luanchuan County: one isfrom late-Jurassic to early Cretaceous (160-130Ma) and another is from early to middle Cretaceous(130-100Ma). The former related to mineralization distributed in the north and middle part ofLuanchuan County, such as Nannihu, Shangfang, Shibaogou and Yuku plutons. Their lithologicalassociations mainly are intermediate-acid igneous rocks like granitoid porphyry, monzogranite andso on; while the latter represented by Laojunshan batholith distributed in the south part of LuanchuanCounty and mainly be composed of biotite monzogranite.(2) Late-Mesozoic plutons show the geochemical characteristics of the highly fractionatedI-type granites in Luanchuan County. They also show the characteristics of peraluminoushigh-potasium calc-alkaline and shoshonite series. There are obvious differentiation between lightand heavy rare earth elements of the plutons. Trace elements represent the enrichment of LILE (Rb,U, Th and Ta) and the depletion of HFSE (Ba, K, P and Ti).(3) Zircon Hf isotopic characteristics of two stages’ plutons above have obvious differences.The values of εHf(t) and tDM2are from-30to-8and from1.7to3.1Ga respectively in the early stage,which declared that its source materials should come from the remelting of Paleo-MesoproterozoicYangtze subduction crust; on the other hand, the values of εHf(t) and tDM2are from-3to3and from0.8to1.2Ga respectively in the late stage, which suggested that its source materials should comefrom partial melting of Meso-Neoproterozoic North Qinling older meta-basalts crust and a few juvenile crust materials.(4) By analyzing the coupling of magmatism and mineralization and deciphering the tectonicenvironment of granitoid plutons in Luanchuan County, it can be concluded that a series oftectonic-magma evolution caused the formation of late-Mesozoic Mo-W-Lead-Zinc polymetallicdeposits under the mechanism transiting from compression to extension in late Jurassic toearly-Cretaceous.(5) In the process of late-Mesozoic magmatism and mineralization, the source materials andwall rocks of the plutons are the key factors to determine mineralization or not. Meanwhile, the typesof wall rock are also the key factors to determine the associated minerals (W or Fe) in themolybdenum deposits.