The Study On Petrology And Geochemical Characteristics Of Nephrite From Dazaohuo,Qinghai

In this paper,the petrology,mineralogy,geochemical characteristics,and the genesis of nephrite from Dazaohuo,Qinghai province were systematically studied through gemological testing,infrared spectroscopy analysis,Raman spectroscopy analysis,polarized light microscopy,backscattered electron analysis,scanning electron microscope observation,electron probe microanalysis,and laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS).The color of the Qinghai Dazaohuo nephrites used in this article is mainly divided into three groups:green,yellow and brown.The overall color is uniform,the texture is fine,slightly transparent to opaque,and presends a oil-waxy luster.Through conventional gemological tests,the refractive index range of Qinghai Dazhaohuo nephrite is 1.61-1.63,and the relative density range is 2.92-3.0.It is fluorescently inert under LWUV and weak red fluorescence under SWUV.The main mineral of Qinghai Dazhaohuo nephrite is tremolite,other minerals include diopside,apatite,zircon,quartz,etc,the surrounding rock composition includes diopside,calcite,dolomite,and orthoclase.The tremolite particles in the green group and the sugar group are mainly felt-shaped crystalloblastic texture,and in the yellow group are mainly leaf-shaped crystalloblastic texture,while which close to surroudding rock are relatively large,and mainly sheaf crystalloblastic textureAccording to the data of electron probe and LA-ICP-MS,the main chemical components of Qinghai Dazhaohuo nephrite are Si O₂,Mg O,and Ca O.It is rich in Si⁴⁺,and the content of Si⁴⁺in nephrite in the surrounding rock group is higher than that in nephrite group;Ca²⁺/Mg²⁺is close to the theoretical value 0.40,but the nephrite in the surrounding rock group is affected by calcite,which is greater than the theoretical value,and in the jade group is lower than the theoretical value.Among the trace elements,Sr,Nb,Ti,and P showed negative anomaly,while U and Pb were positive anomaly.The sample?REE content is low(3.163-6.625 ppm),the value of LREE/HREE ranges from 0.99-1.79,La _N/Yb _N,(La/Sm)_Nand(Gd/Yb)_Nshow slight enrichment of LREE,HREE fractionation is more obvious than LREE.?Ce and?Eu are both positive anomaly to negative anomaly,but?Ce tends to show positive anomaly and weak negative anomaly,while?Eu is more negative anomaly in nephrite of surrounding rock formations.Fe²⁺/(Mg+Fe²⁺)value and Cr-Ni-Co content indicate that Qinghai Dazhaohuo nephrite is marble-type nephrite.The relationship between the mass fraction of Al₂O₃and Na₂O+K₂O,the correlation between Si and Al,and the content of Ti all indicate that the mineralization temperature of Qinghai Dazaohuo nephrite is relatively low.?Eu and?Ce,Th/U,the valence state of Fe,and Zr/Hf,Nb/Ta indicate Qinghai Dazaohuo nephrite is produced in a medium acid reducing environment.The main mineral of"waterline"is tremolite,which appears in some samples of green group and yellow group,and the altered edges of orther minerals indicate that the minerals undergone multiple metasomatisms in the process of mineralization,and the texture of nephrite became more delicate through metasomatism.The genesis of nephrite in Dazaohuo of Qinghai Province is due to the contact thermal metamorphism between dolomitic marble and intermediate-acid magmatic rock to form tremolite,and nephrite was formed under the repeated metasomatism of magma and hydrothermal fluid,it is a very typical intermediate-acid magmatic hydrothermal Type deposit.The colors of the green group and the yellow group are respectively caused by Fe and Ti,which belong to the primary colors,while the colors of the sugar group belong to the secondary colors.