Study On Petro-mineral Features And Genetic Mechanism Of Ruoqiang Nephrite, Xinjiang Province

Ruoqiang area located in Altyn Tagh is one of the most important nephrite-producing area in China,which shows good metallogenic and prospecting prospects.However,due to the harsh natural environment in this region,the research on nephrite deposit,especially on the genesis and geological characteristics of the deposit,is almost blank.In this study,the latest theories and methods of mineralogy,petrology,geochemistry and chronology are used to systematically study the nephrite deposit in Ruoqiang area,in order to supplement the research blank of nephrite deposits from Ruoqiang area.The Yinggelike deposit produces nephrite with yellow tone.The nephrite is mainly composed of tremolite,with less minor minerals.The main microstructure of nephrite is fibroblastic structure.There are two main kinds of rocks outcrop in the nephrite mine:Precambrian dolomitic marble and diorite.Geochemical features show that this deposit is dolomite-related.The calculated hydrogen and oxygen isotopes of the ore-forming hydrothermal fluid,combining with the geological characteristics,indicate that the ore-forming fluid of nephrite is mainly magmatic fluid.Therefore,it can be inferred that the formation of nephrite has a strong genetic relationship with the metasomatism of dolomite by fluids derived from local igneous intrusions.Petrographic observation and chemical composition analysis show that the formation of nephrite mainly went through three stages:the rock forming stage,the nephrite forming stage,and the nephrite post-formation stage.The bulk-rock chemical compositions analyses of the high quality Yinggelike nephrite show that Fe³⁺,Fe²⁺,Ti,and Mn are the most abundant chromophore ion in all the samples,while other coloring elements are too low to affect the nephrite coloration.From the perspective of coefficient of variation and spectral analyses,Fe³⁺and Fe²⁺are probably the most important factors in the color of nephrite.The Fuguoling nephrite deposit mainly produces green and brown nephrite.This nephrite is mainly composed of tremolite,and the main accessory minerals are epidote and diopside.The lepidoblastic texture is the main structure of nephrite,which affects the uniformity of nephrite.The granite in the mining area is I type.Chondrite-normalized REE patterns of the nephrite is in the form of"M",which indicates that the rock has undergone hydrothermal alternation.The granite went through two stages of evolution.First,the crust-mantle material mixed in the source area to form the original magma.Then,the magma underwent a high degree of differentiation evolution and consolidated into rock.During the late magmatic evolution,the rock mass experienced a strong melt-fluid interaction.The Fuguoling granite belongs to orogenic granite and is the product of syn-collision.The formation age of granite should be early Paleozoic,and it may be the response of collision orogeny between Ordovician dunhuang block and Altyn Tagh.The appearance of pink vein rock in mining area is probably caused by granite alteration.Zircons in the Fuguoling granite are hydrothermal zircons.The age of hydrothermal zircons can be used to constrain the age of nephrite,and the reasons are as follows:1)the granite is closely related to nephrite in space,2)the magmatic hydrothermal of metasomatic granite is suitable for the formation of nephrite,3)the temperature of hydrothermal zircon is highly compatible with that of nephrite,4)the age of the hydrothermal zircon is no earlier than that of the granite,and there is no paradox between the measured data and the theoretical data.Therefore,the upper age limit of nephrite from Fuguoling is 471±16Ma.