Studies On Granitic And Metallogenic Chronology And Geodynamics Of The Xitian Tin-tungsten And Dengfuxian Tungsten Deposits,Hunan Province,China

The Nanling metallogenic belt,located in the middle of the South China block,spans the southern Hunan,southern Jiangxi,eastern Guangxi and northern Guangdong provinces.It's a potential metallogenic belt with a lot of kinds of mineralizations,including tin,tungsten and radioactive mineralizations.A number of large-scale or super large-scale deposits have been found or prospected,including Shizhuyuan tungsten polymetallic deposit,Yaogangxian tungsten deposit,Qitianling tin deposit,Xianghualing tin deposit,Hehuaping tin deposit,Dayishan tin deposit and Da'ao tin deposit.These deposits are closely related to the Mesozoic granites which are widely distributed in the Nanling Metallogenic belt.The Xitian tin-tungsten polymetallic deposit and the Dengfuxian tungsten deposit,sited in the east of Hunan province,is located at the middle part of the Nanling metallogenic belt.In this thesis,we focus on these two deposits and study detailedly on petrology,geochronology,geochemistry and petrogenesis with zircon U-Pb dating method,muscovite Ar-Ar dating method and molybdenite Re-Os dating method.On the basis of detailed field geological survey,petrology,geochronology,whole-rock Sr-Nd and zircon Hf isotopic characteristics,we have built a model to interpret the process of magmatic activity and related mineralizations.Some conclusions have been drawn as below:1.The magmatism and mineralization ages of the Xitian and Dengfengxian deposits were determined.Zircon LA-ICP-MS U-Pb geochronology studies show that both of the Xitian and Dengfengxian granites consist of two stages magmatism,including Indosinian?224-226Ma?and Yanshanian?151-153 Ma?granties.The molybdenite Re-Os isochron age and Muscovite Ar-Ar isotope ages have shown that the mineralization ages of the two deposits are 147-151 Ma,which are consistent with the ages of Yanshanian granites,indicating that mineralizations of the Xitian and Dengfuxian deposits are closely related to the Yanshanian granites,and also consistent with the age peak of tungsten-tin mineralization?160-150 Ma?in South China.2.Ore metal materials of two deposits are derived from different sources.The Re contents of molybdenite in the Xitian and Dengfuxian deposits are separately 8.67-43.99 ppm and 0.001-0.008 ppm.The difference of Re contents in these two deposits indicates that the sources of ore-forming materials are significantly different.The ore-forming materials of the Xitian deposit are derived from mantle-crust mixture,and those of the Dengfuxian deposit are mainly from the crust.3.The Indosinian Xitian and Dengfuxian granites are the S-type granites.The Yanshanian Xitian granites and Dengfuxian granites are separately classified as the A-type and S-type granites.The Indosinian Xitian and Dengfuxian granites have similar geochemical characteristics,which are characterized by high silicon,aluminum,alkali and iron,and low titanium,magnesium,and calcium,indicating that they are differentiated S-type granites.Both of them are enriched with large-ion lithophile elements such as Cs,Rb and high field-strength elements such as Th,Ce,Nd,and depleting in Zr,Ba,Nb,Sr,Ti,and P with obvious negative anomalies of Ba,Sr,and Ti.Both of them exhibit a right-inclined chondrite normalized REE patterns.However,the Yanshanian Xitian and Dengfuxian granites associated with mineralization have distinct geochemical characteristics.The Yanshanian Xitian granites are classified as A-type granites,which are characterized by high silicon,aluminum,iron and alkali,low magnesium,calcium,and phosphorus.They have low ratios of CaO/?Na₂O+K₂O?and LREE/HREE,and extremely high ratio of Rb/Sr,and are enriched in Cs,Rb,Th,Ta,Ce,Nd,Zr,HREE,depleted in elements of Ba,Nb,Sr,Ti,P and Eu.The value of?Eu are very low with an obvious Eu negative anomaly.The major and trace elements features with seagull chondrite normalized REE patterns and high ratio of 10⁴Ga/Al suggest that they are grouped in high differentiational A-type granites.The Yanshanian Dengfuxian granites are S-type granites,which are characterized by high silicon,aluminum,potassium,alkali and iron,but low titanium,magnesium and calcium.They have high ratios of CaO/?Na₂O+K₂O?and LREE/HREE,and low ratio of Rb/Sr,and are enriched in Cs,Rb,Th,Ce,Nd,Zr,Pb,depleted in elements of Ba,Nb,Sr and Ti.The values of DI and CIPW?>1%?are much higher.The major and trace elements features with a right-inclined chondrite normalized REE patterns indicate that the Yanshanian Dengfuxian granites are classified as high differentiational S-type granites.4.Both Indosinian Xitian and Dengfuxian granites are products of partial crust remelting.The Yanshanian Xitian granites are products of crust-mantle mixture,and the Dengfuxian Yanshanian granites are products of crust remelting.The Indosinian granites from the Xitian and Dengfuxian deposits have similar Sr,Nd and Hf isotopic features indicating that they are derived from the same magma source.The Indosinian Xitian granites have the?Nd?t?values of-11.93^-10.07 and the?Hf?t?values of-14.9^-7.5.The Indosinian Dengfuxian granites have the?Nd?t?values of-12.33^-10.36 and the Hf?t?values of-19.4^-6.6.The two-stage Nd model ages of the Indosinian Xitian granites ranging from 1.85 to 2.00 Ga with an average age of 1.90Ga,are in agreement with the two-stage Hf model ages ranging from 1.73 to 2.20 Ga with an average age of 1.99 Ga.The two-stage Nd model ages of the Indosinian Dengfuxian granites ranging from 1.87 to 2.04 Ga with an average age of 1.93 Ga,are in agreement with the two-stage Hf model ages ranging from 1.74 to 2.48 Ga with an average age of 1.88 Ga.The low?Nd?t?and?Hf?t?values of the Indosinian Xitian and Dengfuxian granites with similar model ages demonstrate that they may have been mainly derived from the partial melting of the crust.The isotopic values of Sr,Nd and Hf from the Yanshanian Xitian granites are different from those of the Yanshanian Dengfuxian granites.The Yanshanian Xitian granites with the?Nd?t?values of-10.48^-7.29 and the?Hf?t?values of-14.7^-7.5,have the two-stage Nd model ages of 1.56¹.82 Ga with an average age of 1.65 Ga and the two-stage Hf model ages of 1.67-2.13 Ga with an average age of 1.81 Ga.These indicate that the granites may have been mainly derived from the melting of crustal and mantle materials.The Yanshanian Dengfuxian granites have lower?Nd?t?values of-13.79^-12.12 and?Hf?t?values of-16.3^-9.1.The two-stage Nd model ages of the Yanshanian Dengfuxian granites ranging from 1.96 to 2.10 Ga with an average age of2.03 Ga,are in agreement with the two-stage Hf model ages ranging from 1.78 to 2.23Ga with an average age of 1.94 Ga.This indicates that the granites are the products of partial melting of palaeo-Proterozoic crust.5.A model for explaining the process of magmatism and mineralization of the Xitian and Dengfuxian deposits is built.Based on studies geochemistry,geochronology,whole-rock Sr-Nd and zircon Hf isotopes of these two deposits,combined with data of other deposits in Nanling region,we've proposed that the Indosinian granites of the Xitian and Dengfuxian formed in the post-collisional setting and the Yanshanian granites occurred in a large-scale extensional environment induced by the lithosphere thinning of South China block owing to the paleao-pacific movement.Thus we have built a model to explain the formation of tin-tungsten mineralizations and related granites.The collision between the Indochina,North China and the South China plates resulted in the crust thickening in the Early Triassic.After the collision,the lithosphere partially extended and thinned under the action of the heat-stress effect,leading to the partial melting of the crust and forming granitic magma.The magma intruded along the deep faults and formed Indosinian S-type granites in the Xitian and Dengfuxian area.In the Middle Jurassic,the Paleo-Pacific plate began to subduct at a low angle to the South China Plate.Affected by the gravitational force,subducting slab began to break and resulted in upwelling of asthenosphere and large-scale extension of the lithosphere,which lead to formation of basaltic magma.The basaltic magma underplated and caused partial remelting of the crust,which formed the acidic magmas.The acidic magmas along the deep fault intruded in the Indosinian granites.Acidic magma mixed with basaltic magma intruded in the Xitian area and formed the Yanshanian A-type granites and tin-tungsten deposit.The acidic magmas emplaced in the Dengfuxian area and formed the Yanshanian S-type granites and tungsten deposit.