The Study On Mineralogical Characteristics And Genesis Of Nephrite In Dahua, Guangxi,China

In this paper,we study the mineralogical characteristics and deposit genesis of tremolite jade in Dahua,Guangxi Province.By studying its mineralogical composition,structural characteristics,spectral characteristics,geological genesis and geochemistry,Compared with tremolite jade from different deposits,the differences betwen different deposits were analyzed.The overall color of Guangxi Dahua tremolite jade is uniform,and the main colors are grayish green,dark green and bluish white.Tremolite jade samples are slightly transparent-opaque,weak grease-waxy luster,uneven quality.Through the conventional gemological test to tremolite jade from Dahua,Guangxi,the refractivity is range from 1.60 to 1.61,relative density is range from 2.75 to 2.99,and Mohs hardness is range from 5.72 to 6.17.By observing the sample sheet under polarizing microscope,combining IR,scanning electron microscopy,micro-Raman spectrometer,XRD and EPMA test analysis shows that the main mineral component of Guangxi Dahua tremolite jade is tremolite,The Mg/(Mg+Fe²⁺)ratio ranges from 0.941 to 1.00;and the subordinate minerals include diopside,actinite,chlorite,allanite,albite,calcite and quartz.the textures are felted fibrous crystalloblastic,closely intertwined tremolite fibers,oriented microfiber crystalloblastic texture.The degree of crystallinity of tremolite jade is about 89.40%on average,which is bigger than Xinjiang nephrite and smaller than Qinghai nephrite.By EPMA and LA-ICP-MS test analysis showed that the main chemical components of Guangxi Dahua tremolite jade were SiO₂,MgO,CaO,minior contains FeO,Al₂O₃,MnO,Na₂O,K₂O,etc.The contents of trace elements Cr,Ni,and Co are much lower than in serpentinite-type nephrite.The rare earth element content is generally high?62.011-223.116 ppm?,with a positive Eu anomaly?1.090-2.343?,positive and negative Ce anomalies?0.913-1.077?,enrichment in light rare earth elements,and flatness in heavy rare earth elements.The distribution curves of the rare earth elements differ among the nephrite and surrounding rock samples,indicating multiple sources and the multi-period superposition of ore-forming fluids.The Fe²⁺/(Mg²⁺+Fe²⁺)ratio in Guangxi Dahua tremolite jade is lower than 0.06,indicating that the rocks surrounding the nephrite deposit are carbonates.By taking c(Ca²⁺),c(Mg²⁺),and c(Fe²⁺+Fe³⁺)as end-members for projection,amphibole can be concluded to have originated from contact metasomatism.Considering the findings of the geological survey,the Guangxi Dahua tremolite jade deposit is related to basic intrusive rocks such as diabase and hydrothermal fluid carried by magma.Contact metasomatism occurred with dolomitic limestone during the Variscan orogeny,This study on the genesis of the Guangxi Dahua tremolite jade deposit provides a theoretical basis for the formation of the nephrite deposit during the Variscan-Indosinian period,which is favorable for the exploration of new nephrite deposits in the region of limestone or dolomite marble intruded by silicon-rich fluids in the felsic rocks during the Variscan-Indosinian period..