Geological Features and Genesis of the Jinduicheng Porphyry Molybdenum Deposit, Shaanxi Province, China

摘要

Situated in the Henan-Shaanxi fault-uplift area on the southern margin of the Sino-Korean Paraplatform,the jinduicheng porphyry molybdenum deposit is the most important molybdenum producer in China.During Yenshanian the Jinduicheng granite porphyry was emplaced in metaspilite of the Proterozoic XiongerGroup,controlled by a NW trending fault Mineral compositions are mainly quartz （25-40％）,microclineand microcline perthite（27-40％）and plagioclase（An8-14-,14-32％）,associated with minor biotite andmuscovite,and phenocrysts are made up of K-feldspar,quartz and phagioclase.Accessory minerals includemagmatite,apatite and zircon,The porphyry contains SiO2 73.83％ and K2O+Na2O 8.06％（withK2O/Na2O ratio being 1.82）,beloning to the calc-alkaline series Mineralization occurs in the porphyry body and biotitized and hornfelsized spilite Within the exocontactzone.The maximum depth of mineralization reaches 1000 meters below the surface.According to mineralassemblage,ore veins are classified into five types:（l） Pyrite-quartz;（2） Pyrite-K-feldspar-quartz; （3）pyrite-molybdenite-quartz:（4） pyrite-molybdenite-K-feldspar-quartz;and（5） Muscovitefluorite-pyrite-molybdenite-quartz. As the most important economic molybdenum mineral,molybdenite occurs in various forms.Wall-rockalterations show a gross zonation of K-feldspathization-greisenization→silicification→propylitization fromthe porphyry outwards,of which silicification is most intense and has close genetic relationship with Momineralization. Fluid inclusion studies yield homogeneous temperatures ranging from 240to240℃,with the main stagebetween 400 and 300℃.Gas inclusions frequently coexist with multiphase inclusions containing suchdaughter minerals as halite,sylvite,molybdenite and K-feldspar.Under moderate-high temperatures,lowerfo2,high fs2 and weak。acidity conditions,boiling of ore-forming fluids is prerequisite for the Precipitation ofmolybdenite. Sulfur,oxygen and carbon isotopic compositions suggest that at the high temperature stage（450℃）magmatic water is dominant and at the main ore-forming stage a mixture of mixture of magmatic water and meteoricwater is expected.At the late stage,the mixture is predominated by meteoric water.Sulfur and molybdenumare mostly of magmatic origin.