Genesis Of Fluorite Deposits In Xingguo-Ningdu Metallogenic Belt, Southern Jiangxi

Southern Jiangxi is rich in fluorite mineral resources.The Xingguo-Ningdu metallogenic belt is the main area of fluorite deposits in Southern Jiangxi,and the genesis of the deposits is representative.The fluorite deposits in southern Jiangxi can be divided into "Nankeng-type" hydrothermal fluorite deposits and "Xiefang-type" hydrothermal fluorite deposits according to the characteristics of surrounding rocks.The fluorite orebodies of "Nankeng-type" hydrothermal fluorite deposits are mostly located in silicified fractured zones between the Late Jurassic biotite granite and Sinian calcium-bearing shallow metamorphic rocks.Most of the fluorite orebodies in "Xiefang-type" hydrothermal fluorite deposit occur in the silicified fractured zones and fissures of the Late Cretaceous strata,the Late Caledonian biotite granite and the Early Yanshan Late Jurassic biotite granite.Strong geothermal activity accompanies the structural belt of "Xiefang-type" fluorite deposit.Although the predecessors have done a lot of summary and research work on the genesis of fluorite deposits in southern Jiangxi,no isotope datas have been reported on the precise metallogenic epoch of fluorite.Most fluorite deposits are determined by field characteristics and comparison of fluorite deposits in adjacent areas of Zhejiang Province.The relationship between fluorite mineralization and surrounding rocks,especially magmatic rocks,is not clear.The genesis of fluorite deposits is weak and controversial.In view of the above problems,this paper chooses Xingguo Longping fluorite deposit and Tongda fluorite deposit in Xingguo-Ningdu fluorite metallogenic belt as the research objects.They are representative fluorite deposits of "Nankeng-type" fluorite deposit and "Xiefang-type" fluorite deposit in southern Jiangxi.In this paper,the genesis and tectonic setting of ore-bearing magmatic rocks,precise age of fluorite mineralization,source of F and temperature measurement of fluid inclusions of the two fluorite deposits are studied in depth,and the genesis of fluorite deposits in southern Jiangxi Province is further discussed,and the fluorite mineralization model is established,which provides scientific basis for ore-forming prediction and guides the next fluorite prospecting.The LA-ICP-MS zircon U-Pb dating results show that Yongfeng granite,the ore-hosting magmatic rock of Longping fluorite deposit,was formed at 155 Ma and Longshi granite was formed at 154~157 Ma,which are the products of magmatic activity in Late Jurassic.Zircon U-Pb ages of coarse-grained porphyry-like biotite monzonite and fine-grained biotite syenite in the main body of Tongda fluorite deposit are 410 Ma and 400 Ma respectively,indicating that the main stage granite was formed in the Late Silurian-Early Devonian,while the supplementary stage fine-grained biotite syenite was formed in the Early Devonian.Zircon U-Pb ages of Qishan granite with early Caledonian ductile deformation range from445 Ma to 440 Ma,suggesting that it was formed in the Late Ordovician to Early Silurian and belongs to the product of early Caledonian magmatism.Therefore,the dating results in this paper support the understanding that Caledonian granites have early ductile deformation granites and late massive granites.The age of 458 Ma deep-melting zircon is obtained at the edge of Qishan granite and 460 Ma magmatic zircon age is obtained at the center of the granite,suggesting that a period of deep-melting-magmatic activity may occur in the southern Jiangxi area around 460 Ma in the early Caledonian period,which provides a constraint for the initiation age of the Caledonian deep-melting movement in this area.The Yongfeng complex has geochemical characteristics of aluminous A-type granite,such as rich silicon,high Zr+Nb+Ce+Y content,high FeO~T/MgO value,relatively high Ga/Al value,enrichment of high field strength elements such as Th,U,Hf and light rare earth elements,strong depletion of Ba,Sr,P,Ti and other ion lithophile elements,relative depletion of Nb and Ta elements.Zircon saturation temperature,biotite composition and zircon Hf isotope characteristics indicate that the Yongfeng complex was formed in an environment of relatively high temperature,low pressure and high oxygen fugacity in intraplate tension,and was formed by partial melting of ancient crustal materials,which may be related to subduction and retreat of the Paleo-Pacific plate.The Huitong granite has geochemical characteristics of S-type granite,such as enrichment of high field strength elements such as Rb,K and Zr,Hf,and depletion of high field strength elements such as Ba,Sr,P and Nb,Ti.Zircon saturation temperature,biotite composition and zircon Hf isotope characteristics indicate that the Huitong granite may be formed under the environment of relatively high temperature,low pressure and high oxygen fugacity in intraplate tension,and formed by deep melting of ancient crustal materials,but there may be a small amount of local mixing of mantle-derived magma.The results of electron probe microanalysis of biotite show that,similar to other fluorite-producing granites in southern Jiangxi,the compositions of biotite in the Yongfeng complex and Huitong granite are rich in Ti,Mg and F,but poor in Al and Fe.The content of biotite F in Longshi granite is more than 1%except for individual samples,while the content of biotite F in Yongfeng complex is less than 1%except for individual samples,which indicates that the hydrothermal fluid in late stage of crystallization differentiation enriches more volatile components such as F after magmatic stage,suggesting that the hydrothermal fluid in late stage of magmatic stage provides the source of F for fluorite mineralization in Longping fluorite deposit.The F content of biotite in Huitong granite(0.71~1.02%)is close to that of other fluorite-producing granites in southern Jiangxi,slightly lower than that of Longshi granite,but significantly higher than that of Yongfeng granite,which provides the source of F for fluorite mineralization in Tongda fluorite deposit.The classification of biotite in The Yongfeng complex belongs to iron-rich biotite.The classification of biotite in Huitong granite belongs to iron biotite area and the boundary area between iron biotite and iron biotite.The biotite crystallizes in a higher oxygen fugacity environment.The biotite crystallizes in Longshi biotite monzonitic granite in a higher oxygen fugacity environment.These characteristics are similar to those in the northern part of Nanling and southern Jiangxi.Fluorite-producing granites are similar.The Sm-Nd isochron dating results show that the formation age of Longping fluorite deposit is 158±3Ma,and that of Tongda fluorite deposit is 94±3Ma and 43±4Ma,respectively.The age of these three groups coincides with the geological field characteristics,representing the fluorite metallogenic age of the two fluorite deposits,indicating that the Longping fluorite deposit has late Jurassic fluorite mineralization and the Tongda fluorite deposit has late Cretaceous and Paleocene fluorite mineralization.Therefore,the metallogenic epoch of Longping fluorite deposit and Tongda fluorite deposit may represent the metallogenic epoch of "Nankeng-type" hydrothermal fluorite deposit and "Xiefang-type" hydrothermal fluorite deposit in southern Jiangxi.It is suggested that Nankeng-type hydrothermal fluorite deposits in southern Jiangxi were mainly formed in the late Jurassic,and there may be superimposition of late fluorite mineralization.There are at least two stages of fluorite mineralization in "Xiefang-type" hydrothermal fluorite deposit,namely,Late Cretaceous and Paleocene.The temperature measurement results of fluid inclusions show that the ore-forming fluids in the early stage of Longping fluorite deposit belong to medium-high temperature,medium-low salinity and low density fluids.The temperature of inclusions occurs in a high temperature range,especially in the temperature ranges of 310~320?and 340~350?,with relatively high salinity.The discovery of high temperature,relatively high salinity fluid end-elements and low and medium temperature fluid end-elements in the early stage of Longping fluorite deposit,and the weak negative Eu anomaly of fluorite in Longping fluorite deposit indicate that the ore-forming fluid of Longping fluorite deposit has experienced relatively high temperature(>250?).Fluorite in the fluorite deposit has a weak negative Eu anomaly.The fluorite metallogenic age of Longping fluorite deposit(158±3Ma)is consistent with the diagenetic age of the ore-forming magmatic rock complex(155Ma and 154~157Ma),suggesting that the fluorite mineralization is closely related to the magmatic activity of the Late Jurassic.The high F content of biotite in the late stage Longshi rock mass of the Yongfeng complex indicates that the hydrothermal fluid after the late stage of crystallization differentiation is more enriched with F and other volatile components,suggesting that the hydrothermal fluid after the late stage of magma provides the source of F for the fluorite mineralization of Longping fluorite deposit.Many evidences show that the hydrothermal fluid of Yongfeng complex participates in the fluorite mineralization of Longping fluorite deposit after the late magmatic stage,which provides F source and heat source for the fluorite mineralization of Longping fluorite deposit.The salinity-homogeneous temperature covariance diagram of fluid inclusions in Longping fluorite deposit shows that there is an obvious linear relationship between fluid salinity and homogeneous temperature in the early stage of mineralization of the deposit.The main manifestation is that with the temperature decreasing gradually,the salinity tends to decrease gradually,and the trend of fluid evolution is consistent with the trend of miscibility.It further indicates that there is a mixing of two fluids in the early stage of mineralization.Medium-low temperature and low salinity fluids have generally low homogenization temperature and salinity values.Only when a large amount of atmospheric precipitation is added can they be formed,or the ore-forming fluids themselves are heated atmospheric precipitation.Therefore,the ore-forming fluid of Longping fluorite deposit is a mixture of high-temperature and relatively high-salinity magmatic later source fluid and low-temperature and low-salinity meteoric water source fluid.With the development of mineralization,the precipitation mechanism of Longping fluorite deposit is mainly fluid cooling from the main stage of mineralization to the late stage of mineralization.Therefore,Longping fluorite deposit belongs to medium-low temperature hydrothermal filling type fluorite deposit,which mainly comes from post-magmatic hydrothermal and atmospheric precipitation.The precipitation mechanism of fluorite in the early stage of mineralization is mainly the mixing of hydrothermal fluid and atmospheric precipitation after magmatic stage,and the water-rock reaction takes place.The precipitation mechanism of fluorite in the main and late stage of mineralization is mainly the cooling of fluid and the water-rock reaction takes place.The REE characteristics of fluorite in Tongda fluorite deposit are similar to those of Huitong granite,suggesting that the ore-forming materials,especially F,are derived from Huitong granite.The results of EPMA analysis show that the content of biotite F in Huitong medium-coarse porphyritic biotite monzonitic granite is higher(0.71~1.02%,average 0.87%),which further proves that Huitong biotite monzonitic granite provides F source for fluorite mineralization in Tongda fluorite deposit.The fluorite of Tongda fluorite deposit has a strong negative Eu anomaly,suggesting that the ore-forming fluid of the fluorite deposit is low temperature(200~250?).The results of inclusion microthermometry further show that the ore-forming fluids of Tongda fluorite deposit belong to medium-low temperature,low salinity and low isodensity ore-bearing hot water solution.This low salinity medium-low temperature solution can be formed only when a large amount of meteoric water is added,or the ore-forming fluid itself is heated meteoric water(paleogeothermal water).The homogenization temperature-salinity covariance diagram of inclusions in Tongda fluorite deposit shows that there is a clear linear relationship between homogenization temperature of fluid inclusions and salinity.From the early stage of mineralization through the main stage of mineralization to the late stage of mineralization,the fluid temperature and salinity decrease slightly.The trend of fluid evolution is consistent with the trend line of miscibility,indicating that the precipitation mechanism of Tongda fluorite deposit is mainly paleogene.Geothermal water mixes with late atmospheric precipitation and undergoes water-rock reaction.Based on the evidences of fluorite deposit geological field characteristics,magmatic rock characteristics,metallogenic epoch,rare earth element characteristics of fluorite and fluid inclusions,a comprehensive metallogenic model map of fluorite deposits in southern Jiangxi is established.The "Nankeng-type" fluorite deposits,including Longping fluorite deposit,belong to magmatic-hydrothermal metallogenic system,while the "Xiefang-type" fluorite deposits such as Tongda fluorite deposit belong to "Xiefang-type".In the paleogeothermal metallogenic system,they have different fluid sources,metallogenic epochs and ore genesis.Comprehensive analysis shows that the radiogenic heat of the high-potassium Huitong intrusion and the late Cretaceous intermediate-basic volcanic activity in southern Jiangxi provide heat source for the late Cretaceous fluorite mineralization of the Tongda fluorite deposit,while the Paleocene fluorite mineralization heat source comes from the radiogenic heat of the high-potassium Huitong granite.It can well explain the geothermal phenomena near Tongda fluorite deposit,suggesting that the fluorite deposit and the modern geothermal may be the continuation of the hydrothermal activity system formed by radioactive heat of granite.