Mineralogy,Geochemical Characteristics And Genesis Of The Vesuvianite Jade In Hanzhong,Shaanxi

In this paper,the mineral petrology characteristics,major and trace elements,and rare earth elements composition of the vesuvianite jade in Hanzhong,Shaanxi were studied by means of polarizing microscope,electron probe,X-ray fluorescence spectrometer,inductively coupled plasma mass spectrometer and other instruments.Based on the geological characteristics of the study area,the formation mechanism of the vesuvianite jade in Hanzhong,Shaanxi was explored.The vesuvianite jade in Hanzhong,Shaanxi is produced in the skarn belt in the contact zone between Indosinian granodiorite and Middle Devonian carbonate rocks.The vesuvianite jade is compact and massive,mainly with micro crystalloblastic structure,and the unequal granular crystalloblastic structure,porphyroblastic texture and metasomatic residual structure can be seen locally.The main mineral of the jade is vesuvianite,which is mainly granular,followed by columnar and fibrous,indicating that it was formed at different stages.Secondary minerals include almandine,diopside,chlorite,epidote and calcite.The main chemical components of the docrase jade are Si O₂,Al₂O₃,and Ca O,with Si O₂ content ranging from 37.22%to 43.84%,Al₂O₃ content ranging from13.04%to 21.98%,and Ca O content ranging from 26.03%to 36.38%.The trace element content of the docrase jade in Hanzhong,Shaanxi is higher than that of the surrounding rock,but the trend of trace element changes between the jade and the surrounding rock is relatively consistent.The content of rare earth elements in the jade is relatively low,and overall,it shows a slightly higher degree of fractionation between light rare earth elements compared to heavy rare earth elements,while also exhibiting a positive anomaly of Eu.Granodiorite is one of the most important sources of the main chemical composition of the docrase jade.It provides the formation of jade with Si O₂,Al₂O₃and other major components,as well as Na₂O,K₂O and other oxide components regulating environmental properties.Ca O is mainly from calcareous carbonate rocks in the wall rocks,and Mg O is mainly from granodiorite followed by the wall rocks.The positive anomaly of Eu and the small fluctuation of Y/Ho values near 28indicate that the fluid during the formation stage of the Fushan jade mainly came from high-temperature magma.The vesuvianite jade in Hanzhong,Shaanxi belongs to a typical calcareous skarn type of the vesuvianite jade.The microcrystalline structure of the jade indicates that the formation of the vesuvianite jade went through two stages:diagenesis and jade formation（1）Diagenesis:The magmatic hydrothermal fluid differentiated during the upwelling process of intermediate acidic magma in the region,which contacts and metasomatizes with the Middle Devonian carbonate rocks along structural fractures to form coarse-grained vesuvianite crystals.（2）Jade formation:after the comprehensive collision with orogeny,the second intermediate acid magma intrusion occurred in the study area,and a part of vesuvianite crystals melted to form fine-grained vesuvianite under the effect of later hydrothermal solution;The other part is subjected to ductile deformation and stress,resulting in the formation of fine-grained vesuvianite,ultimately forming the vesuvianite jade.The ore-forming environment of jade has also undergone corresponding changes from early to late stages.In the early stage,it was in a weak acidic environment with high temperature,relative reduction,and high oxygen fugacity,while in the late stage,it was in a acidic environment with medium temperature and weak oxidation.