Study On The Metallogenic Mechanism Of Tashisayi Nephrite From Xinjiang

The Tashisayi nephrite deposit,located in the middle and upper reaches of the Tashsayi River in Qiemo County,Xinjiang,has been mechanized mining recent years and yield more than half of the primary Xinjiang nephrite.Nephrites mined from this deposit are white,gray,green and brown in color,and is well known for its high-quality brown nephrite named sugar-nephrite by Chinese.However,seldom research has been done due to its hostile environment.The Tashsayi nephrite occurs in bedding faults and marble fissures near the contact zone between granite and magnesia marble.Petrography,geochemistry and zircon CL images show that the granite is a cataclastic A-type alkali-feldspar granite(Ca O 1.76-3.68 wt%)altered by Ca²⁺-rich hydrothermal fluids.It will be altered into a zoisitization granite.In the alteration zone,according to the sequence of alkali-feldspar granite?zoisitization granite?zoisite-tremolite rock?nephrite,the content of Si in epidote/zoisite increases,while the content of Fe decreases in turn.The Tashisayi nephrite is mainly composed of tremolite.Its trace elements and hydrogen and oxygen isotopes can be used to distinguish D-type nephrite from some other producing areas(such as Luodian,Dahua,Sanchakou,Golmud,etc.),but cannot be distinguished from those from other Xinjiang mines.Its individual impurity minerals include zoisite,prehnite,parisite,and magnesium hexasilicate.The symbiotic association zoisite,tremolite and chlorite indicates that the formation temperature of minerals in this area is?350?,which is in agreement with the calculation result of zircon-Ti thermometer(352?).Combining with field geological survey and petrographic analysis,it is believed that Si-bearing hydrothermal metasomatizes magnesia marble forming nephrite,and the by-product Ca²⁺is migrated to form epidote/zoisite in the nephrite,contact zone and granite.The U-Pb age of zircon in wall rocks and Tashisayi is analyzed.The intrusive age of A-type alkali-feldspar granite is 926±7 Ma,the formation age of zoisitization granite is 433±10 Ma,and the formation age of nephrite is 425.4±3.4 Ma.Among them,the latter two zircons are hydrothermal zircons,and the formation age is within the error range,which further indicates that both of them formed in the same hydrothermal process,and once again shows that A-type granite does not provide hydrothermal source for nephrite formation.Considering that the hydrothermal solution for nephrite formation does not come from contact zone intrusive rocks,combined with the characteristics of hydrogen and oxygen isotopes of the nephrite(?D=-89.3?-82.5,?¹⁸O=2.3?5.8),it is considered that the metallogenic hydrothermal solution migrated a long distance in the fault zone and exchanged components with the surrounding rocks during the migration process,showing characteristics of weak metasomatism.In addition,atmospheric water may also be involved in.Based on the regional geological characteristics and relevant age data,it is preliminarily speculated that the pluton in the?5 km northwest may provide hydrothermal fluids and heat source for the formation of nephrite.Geologically,the Tashisayi is part of the southern Altyn subduction plate.The thermal events of?926 Ma and?433 Ma are the two most important thermal events in South Altun,North Qaidam,West Kunlun,Qimantag,Qilian and East Kunlun.The former is thought to be related to the Glenville convergence which formed the Rodinia supercontinent,and the latter to the closure of the prototethys.Nephrite in West Kunlun East Kunlun Altyn and Northern Qaidam Basin was formed in the closing stage of the prototethys.