| As important strategic mineral resources,molybdenum and uranium metals have an extremely important position in national defense construction,scientific and technological progress and social and economic development.As an important part of South China molybdenum metallogenic belt,there are porphyry type and hydrothermal vein type molybdenum deposits in Hainan Island.Although previous researches on geological characteristics,ore-controlling factors,geochemical characteristics and genesis of ore-bearing granites,deposit genetic types and diagenetic and metallogenic chronology of molybdenum deposits in Hainan Island,and has achieved fruitful results.However,research on key scientific issues such as the source of ore-forming materials,the property and evolution of ore-forming fluids and metallogenic mechanism for each molybdenum deposit on the island is relatively weak,especially the property and evolution of ore-forming fluids and metallogenic mechanism are almost blank.At the same time,previous studies have mainly focused on a single deposit and lacked research on the similarities,differences and regularities of molybdenum mineralization of different types of deposits in the island from a regional and systematic perspective.The existence of the above-mentioned problems restricts the understanding and deepening of the molybdenum mineralization theory and molybdenum precipitation enrichment mineralization mechanism in Hainan Island.In addition,the Hainan Island molybdenum deposits and the North Guangdong granite-type uranium deposits are located in the same South China plate,and all of them show a close spatial relationship with the Yanshan-age granite,and the mineralization age is similar.However,the molybdenum and uranium mineralization show relative independence in space.Therefore,the comparative study of the similarities and differences between the two mineralization can not only explore the potential of uranium mineralization in Hainan Island,but also has important theoretical and practical significance to deepen the theory of plate tectonic mineralization system.In this paper,we focus on two typical molybdenum deposits in the Shimenshan and Gaotongling in Hainan Island.Using high-precision modern analytical testing techniques and based on comprehensive studies of petrology,Lithogeochemistry,mineralogy,fluid inclusions and isotopic geochemical analysis,we conduct an in-depth study on the genesis of the ore-bearing granites,the source of ore-forming fluids,the property and evolution of ore-forming fluids in Shimenshan and Gaotongling molybdenum deposits,and analyze and discuss their metallogenic regularity and mechanism.At the same time,from the perspectives of metallogenic geological environment,geochemical characteristics and genesis of ore-bearing granites,sources of ore-forming fluids and metallogenic mechanism,the mineralization of molybdenum deposits in Hainan Island and granite-type uranium deposits in northern Guangdong province are compared,and discussed the uranium mineralization potential of Yanshanian granites in Hainan Island.The main results and understanding achieved are as follows.（1）The geochemical characteristics of host granites of Shimenshan deposit and Gaotongling deposit have been studied and compared,and the genetic types of host granites have been determined.The results show that the contents of major elements in the two ore-bearing granites are basically similar,with high Si O2（67.61%～72.84%）,Al2O3（13.70%～14.99%）and K2O（K2O/Na2O ratio greater than 1）and low Mg O（0.70%～1.40%）,A/CNK ratio is about 1,and consistent with the peraluminous or weakly peraluminous,high k calc-alkaline rock series.The normalized distribution curves of rare earth elements overlap with each other,showing a right-leaning pattern of LREE enrichment,HREE relative deficit and Weak Eu deficit（δEu=0.67～0.84）.The spider diagram of trace elements shows the characteristics of enrichment of large ion lithophile elements（K,Rb）,high field strength elements（Th,U,La,Zr,Hf,etc.）and obvious Nb-Ta deficit valley,high Sr(343.7×10-6～611.4×10-6),low Yb(0.72×10-6～1.07×10-6)and high Sr/Y ratio（37.03～57.09）,and similar ISrvalue,εNd（t）value and Sm/Nd ratio.The genetic types of the two rocks are consistent,and both belong to C-type Adakite.（2）Discussed the tectonic environment and formation mechanism of ore-bearing granite（C-type Adakite）.It is considered that the host granites of Shimenshan and Gaotongling deposit have similar magmatism source area,Which are series products of the same source area and partial melting evolution process.It is revealed that there was 40～67 km thickened crust in Qiongzhong area of Hainan Island during the early Yanshan period and crustal thinning occurred after the late Yanshan period.The ore-bearing granite（C-type Adakite）is a series of products of partial melting process of thickened lower crust material（enveloped plate group）caused by basaltic magma undertransgression under mantle plume mechanism,and the magmatic material has the characteristics of crust-mantle mixing.（3）It has been established that the properties and evolution of ore-forming fluids in Shimenshan and Gaotongling molybdenum deposits.The results show that the fluid inclusions of Shimenshan molybdenum deposit are mainly of rich liquid two-phase A-L type and AC-L type.The ore-forming fluid is H2O-Na Cl system with medium high temperature and low salinity containing CO2.The homogenization temperature showed a single peak（mainly 315℃～375℃）,while the salinity showed a double peak（range 1.5%～5.5%Na Cleqv.,6.5%～8.5%Na Cleqv.）,and the CO2content decreased significantly during the evolution from high temperature to low temperature,which was believed to be caused by boiling in the process of mineralization fluid action.The fluid inclusions of the Gaotongling molybdenum are mainly rich liquid two-phase（A-L type）.The ore-forming fluid is H2O-Na Cl system with low temperature and low salinity containing a small amount of CO2,CH4,N2and other gases.The homogenization temperature showed obvious a double peak（range 250℃～280℃and 180℃～220℃respectively）,and the salinity was between 4%～8%Na Cleqv,showing weak a double peak.The evolution process of CO2content decreased significantly from high temperature to low temperature,which was believed to be caused by the mixture of low-temperature and low-salinity fluid and fluid boiling.（4）Discussed the sources of ore-forming fluids in Shimenshan and Gaotongling molybdenum deposit.The results show that the Pb isotopes of the Shimenshan and Gaotongling molybdenum deposit are consistent,indicating that solutes in ore-forming fluids are mainly derived from mantle,and there may be a small amount of crust-derived materials mixed.The isotopic composition of Mo indicates that the ore-forming material Mo is a mixture of mantle material and crust-derived material.H-O isotope indicates that the initial ore-forming fluid of the two deposits is mainly magmatic water,and it is inferred that meteoric water mixing occurred in and around the ore-forming site.S isotope has mantle-derived properties.It is concluded that the source of ore-forming fluid of the Shimenshan porphyry molybdenum deposit and Gaotongling quartz vein molybdenum deposit is consistent,and the source of ore-forming fluid is mantle-derived.It is inferred that the ore-forming fluid is closely related to the magma source area of granite crust-mantle interaction in host rock.（5）The similarity and correlation of molybdenum mineralization in Shimenshan and Gaotongling molybdenum deposits are proposed by comparing their metallogenic characteristics.The comparison shows that the Shimenshan and Gaotongling deposits have the same genetic type and similar diagenetic and metallogenic age.The molybdenum ore-forming fluid has the characteristics of"sameness",which is contemporaneous,similar in source area and similar in origin.The central part of the vein is quartz vein type ore.The difference between the two deposits on the two sides of the quartz vein type ore corresponds to dolomite type ore and alteration type ore respectively,which is the result of the alteration type series change caused by the evolution of ore-forming fluid from deep to shallow and from high temperature to medium low temperature.The molybdenum mineralization of the two deposits is not isolated,the genesis of the deposits is related,which is a series of products of the action and evolution of ore-forming fluids of the same origin,the same properties and the same age.It is more appropriate to classify the Shimenshan and Gaotongling deposits as veined molybdenum metallogenic series related to adakite granites in late Yanshanian than porphyry type and quartz vein type.（6）Discussed the metallogenic mechanism of molybdenum deposits in the south-central hainan Island.It is considered that molybdenum ore-forming fluid and ore-bearing granite have the same molten magma source（crust-mantle source）.Under the dynamic background of the late Yanshanian paleo-Pacific plate retreating,mantle plume development and crustal extension and thinning,crust-mantle interaction led to partial melting of materials near the crust-mantle interface,and induced multi-stage granite magmatic intrusion,forming C-type Adakite.Shortly after the late granitic emplacement,the Mo-rich ore-forming fluid from the crust-mantle melting source migrated upward along the magmatic emplacement path and fault structure（it may have extracted part of Mo from the crust source rocks）.When the ore fluid migrated to the semi-consolidated or consolidated rock mass in the shallow part of the crust,it filled and accumulated in the fault structure,and gradually occurred infiltration metasomatism and crystal precipitation（filling）mineralization.The boiling and cryptic explosion of ore-forming fluid and the mixing of shallow fissure water lead to the exhaust,decompression and cooling of ore-forming fluid,which are the main factors that promote the ore-forming fluid components to reach supersaturation and crystallize precipitation mineralization.It is inferred from deep metallogenic dynamics that this mechanism can be extended to the mineralization of molybdenum deposits in the middle and south of Hainan Island.（7）Molybdenum mineralization in Hainan Island is compared with uranium mineralization in northern Guangdong Province.The comparison shows that there are significant differences in basement composition and genetic types of ore-bearing granite between Hainan molybdenum deposit and north Guangdong uranium deposit.There is a significant ore-rock time difference between uranium mineralization age and ore-bearing granite in northern Guangdong,while the two are basically the same in Hainan molybdenum deposit.Although the ore-forming fluids are related to the deep magmatic source or have mantle-derived properties,their compositions are significantly different.Filling and infiltration metasomatism are the main metallogenic modes.The precipitation mineralization of ore-forming materials is mainly related to the change of physical and chemical conditions of ore-forming fluid such as temperature,pressure,solubility（saturation）or p H value.It is concluded that the material basis of the molybdenum deposit in Hainan Island and the uranium deposit in northern Guangdong Province may be the key factor restricting the formation of molybdenum ore in granite in the south-central Hainan Island and uranium ore in granite in northern Guangdong Province.It is speculated that the granites in the middle and south of Hainan Island have no uranium mineralization potential. |
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