Mineralogical Tracing Of Magmatic-Hydrothermal Evolution And Mineralization Of Jiajika Pegmatite Type Lithium Deposit,Western Sichuan,China

The Jiajika pegmatite type rare metal deposit in Western Sichuan is located in the Songpan-Ganzi lithium metallogenic belt in China.It is well known for its huge lithium resources.The study of its metallogenic mechanism has important theoretical and practical significance.In the Jiajika deposit,pegmatites are distributed in groups around the only exposed two mica granite in the area.From the granite outwards,microcline pegmatites(?type)? microcline-albite pegmatites(? type)? albite pegmatites(? type)?spodumene pegmatites(? type)? lepidolite(or muscovite)pegmatites(? type)formed successively regional zonations of pegmatites.In this paper,the representative pegmatite dikes in each regional zonation are selected to analyze the magmatic hydrothermal evolution process of Jiajika pegmatites.Among them,No.308 pegmatite(? type)is the largest exposed and the most well-zoned pegmatite in the area,and is also a large lithium deposit;the No.134 pegmatite(? type)is a super-large scale lithium deposit itself and has the highest ore grade.Thus we focus on these two pegmatite dikes,combined with other representative pegmatites in the area(No.34 vein-? type,No.33 vein-? type,No.104vein-? type,No.668 vein-? type No.528 vein-? type),using mainly the optical microscope,scanning electron microscope,electron microprobe analyzer and other mineralogical observation and analysis techniques,to study the crystallization and evolution history and mineralogical behavior of muscovite,spodumene,beryllium and phosphate minerals in these pegmatites.The physicochemical conditions of the melt and exsolved fluids during diagenesis and mineralization of the pegmatites and the degree of differentiation of the rare metal pegmatites in the Jiajika deposit are determined,and the magmatic-hydrothermal evolution process is traced,which provides an important theoretical basis for the lithium mineralization mechanism of Jiajika deposit and even the whole Songpan-Ganzi orogenic belt.The main achievements are as follows:(1)Through detailed mineralogical study,we found petalite and cesium mica for the first time in the Jiajika area,and proposed the genetic relationship between petalite and spodumene,which enriched the types of Li-rich pegmatite in China and expanded the mineralogical research content of pegmatite type lithium deposits;(2)Using the eucryptite-spodumene-petalite stability grid,combined with pegmatite phase equilibrium relationship(temperature and pressure conditions)and temperature measurement data of pegmatite spodumene fluid inclusions in Jiajika from previous studies,we limit the P-T conditions of the mineralization of the pegmatite in Jiajika deposit,which is unique compared with other pegmatite type lithium deposits in the world;(3)Proposed that the internal zoning of the pegmatites is mainly controlled by fractional crystallization;while for the high fractionated pegmatites(? type),the internal structural zonation formed as a result of the supercooling crystallization.(4)The initial melt properties and fluid evolution characteristics of the Jiajika pegmatite are determined: pegmatites from low to high fractionated type in the Jiajika ore field is characterized by increasing lithium content and steadily low F content in the initial melt,as well as increasing degree of magmatic-hydrothermal evolution.Among them,lithium mineralization depends mostly on the degree of internal fractional differentiation during evolution process of the intermediate differentiated pegmatites(? type);for those high fractionated pegmatites(? type),the initial melt had high lithium content,and the internal zonation and chemical differentiation within them are not obvious.Fluid evolution characteristics: according to the alteration sequence of spodumene and lithiophilite,and the mineral chemical characteristics of apatite,it is proposed that the late fluid evolved from Kand Na-rich fluids in an alkali metasomatic stage to H-and P-rich fluids in an acid metasomatic stage,and the chemical property(P rich)and scale of the late fluid are,at a certain extent,favorable for the preservation of spodumene.(5)The key controlling factors for the mineralization of methylcarbamate are discussed:(1)the initial melt is rich in lithium or even Li-supersaturated prior to crystallization;(2)The scale of exsoluted fluids is limited and the hydrothermal stage is not common;(3)The late exsoluted fluid is rich in P.