Mineralogy And Genesis Of Green Nephrite In The Western Section Of Manas Region,Xinjiang

The Manas green nephrite deposit is located in the ophiolite mélange belt at the north Tianshan,Xinjiang,and is one of the most important deposits for producing green nephrite jade in China.The earliest record for mining this deposit was at the Qing Dynasty,and the jade articles were manufactured and dedicated to the emperors as tributes.However,seldom research about the petrology and ore deposit geology has been done due to the hostile environment(e.g.,high altitude area,complex terrain,harsh climate).In this paper,field observations,petrography,mineralogy,geochemistry,stable isotope,and zircon SIMS chronology were utilized to provides a comprehensive description of the green nephrite and surrounding rocks of Huangtaizi,Jilangde and Qinglangba deposits in the western part of Manas,and,therefore,the genesis of the green nephrite was deduced.The nephrite orebody,occurring as veins,wedge,and lenticular shapes,was found in wedge-shaped regions of the contact zone between the ultramafic rock and the mafic rock xenolith.It usually shows obvious zoning of the rocks,and that is,the sequence from the serpentinite to surrounding rock is serpentinite–chloritized rocks–tremolitic rocks–(diopsidite)–nephrite ore body–diopsidite–mafic rock.Fieldwork reveals that the formation of nephrite is probably related to slab subduction or the emplacement of ultramafic rocks.Fieldwork,petrography,and geochemistry show that the serpentinite,whose protolith is harzburgite,is the mantle wedge serpentinite in a fore-arc environment.The typical paragenetic mineral assemblages(antigorite,lizardite,and diopside)in the serpentine schist indicates they were formed at the temperature below 500?.The replacement and recrystallization occurred extensively after its formation.Brittle deformation is widely developed in the mafic wall rock which contains prehnite,diopside and titanite related to metasomatic processes.The presence of metamorphic diopside,actinolite and hydrothermal titanite indicates the upper greenschist facies(320-525?),while prehnite may be formed in the temperature range of 250 to 350?.Geochemistry of the whole rock and mineral chemistry of chromite indicate that the mafic rock derived by partial melting triggered by the infiltration of the fluids with crustal signature in subduction zone environments.The green nephrite is mainly composed of tremolite,with minor diopside,chromite,clinochlore,talc,serpentine,Ni-sulfide,and Ni-arsenide.The nephrite and related rocks show metasomatic and fibroblastic(e.g.,radial blastic and bunched)textures and deformation microstructures(brittle and ductile).The U-Pb chronology of zircon in mafic wall rock shows multiple ages of inherited igneous zircon core of the Precambrian,sometimes with the metamorphic zircon rims.Combined with the regional geology and the previous chronological studies of the ophiolite belt in the north Tianshan,the metallogenic epoch of Manas green nephrite is later than that of the ophiolite—Carboniferous.In conclusion,the metallogenic process of the nephrite is as follows.First,the diopside formed by the metasomatism of mafic rocks by serpentinizing fluids at the temperature of approximately 500? and low-moderate pressures.As the ore-forming fluids gradually evolved to increasing the Mg components and the temperature gradually decrease,the coarse-grained tremolite was formed at the expense of diopside under redox conditions and at high water/rock ratios through a dissolution–crystallization process.In the temperature range 250-400?,coarse-grained tremolite dynamic recrystallized to nephrite with fine matted-fibrous texture and a complex combination of brittle and plastic deformations in a complex dynamic environment under conditions of unilateral stress,repeatedly changing in magnitude and direction.Besides,the nephrite could be precipitated directly from the Mg-and Ca-bearing fluids in a complex dynamic environment.