Metallogenic System And Model Of Fe Deposits In Luzong Volcanic Basin,Anhui,Eastern China

The Middle and Lower reaches of the Yangtze metallogenic belt,located in the northern margin of the Yangtze plate,is one of the most important intracontinental copper,gold and iron polymetallic metallogenic belts in China.Luzong Basin is an iron ore concentration area characterized by volcanic-related and skarn iron deposits in the metallogenic belt.The geological exploration and research in this area has a long history,and have many achievements.Since 2013,a great breakthroughs have been made in deep exploration in the Luzong Basin,and new large iron ore bodies have been discovered 600 meters below the main ore body of the Luohe deposit;the Longqiao deposit,Dabaozhuang deposit,Makou deposit,Yangshan deposit and Daling deposit have also revealed new geological phenomena.These new discoveries and breakthroughs in these deposits are inconformity to existing metallogenic models.It shows that the metallogenic model of Luzong Basin is quite different from that porphyrite model in the adjacent Ningwu Basin.Therefore,the metallogenic system and metallogenic model of iron ore deposits in Luzong Basin need to be further study.On the basis of previous studies and combined with the latest exploration results,through field geological survey,core cataloguing and laboratory lithofacies work,combined with whole rock geochemistry,isotope geochemistry(whole rock,single mineral),isotope chronology,high-precision in-situ trace elements of minerals and isotope analysis and other methods,this thesis has studied longqiao,Luohe,Dabaozhuang,Makou,Yangshan and Daling to clarify the metallogenic process of different types of iron deposits in the basin.Clear and definite the relationship between different types of iron deposits in the Luzong basin.By comparing with other iron deposits in the middle and lower reaches of the Yangtze River metallogenic belt,better understand the metallogenic specificity of ore-forming intrusive rocks,the relationship between evaporate and iron mineralization,and the source of phosphorus in the deposit,and discussed the metallogenic dynamic background of the iron deposit.The main knowledge and progress are as follows:The predecessors classified the Longqiao as a deposit of sedimentary-hydrothermal reformation type,believing that the syenite in the mining area provided ore-forming fluid.In this work,a new diorite intrusion was discovered in Longqiao iron ore deposit,and its lithology was determined to be gabbro diorite.Its diagenetic age was 133.5±0.8Ma,earlier than the known syenite in the deposit.Thestudy of geological characteristics of the deposit shows that gabbro diorite is closely related to iron mineralization,while syenite is a post-metallogenic ore-breaking rock mass.The results of microelement analysis of magnetite in Longqiao iron deposit show that the content of V and Ti in the same magnetite ore sample is obviously different when the content of microelement is similar to that of Mg+Al+Si.It indicates that there are two stages of superposition of magnetite in the same sample.The samples near gabbro diorite have higher formation temperature(Ti,V)and lower water-rock reaction intensity(Mg+ Al+Si),with the distance from gabbro diorite,the formation temperature of magnetite decreases with the water-rock reaction increases.Therefore,Longqiao iron deposit belongs to stratabound skarn type iron deposit,in which some iron may come from the hydro-rock reaction between magmatic fluid and sedimentary siderite in host rock,but the main source of iron is still dioritic magmatic hydrothermal fluid.The total resources of Luohe iron ore deposit is expected to exceed 1 billion tons.It is the largest iron ore deposit in the metallogenic belt.Its volcanic rock has distinct metallogenic characteristics,and its related research has important prospecting and exploration value.Based on the systematic ore-deposit study of the Luohe iron ore deposit,it is determined that the host rocks of the newly discovered ore bodies in the deep part of the deposit and the shallow ore bodies are thick volcanic rocks(trachyandesite-pyroxene trachyandesite).It is clear that the genesis of the Luohe iron ore deposit is related to the deep concealed dioritic magmatic activity.The mineralization of Luohe iron deposit is divided into six stages: alkaline feldspar stage(I),diopside-anhydrite-magnetite stage(II),chlorite-epidote-carbonate stage(III),anhydrite-pyrite stage(IV),quartz-sulfide stage(V)and carbonate-sulfate stage(VI).The formation ages of the titanite in the lower and middle layers of the Luohe iron ore deposit are 130.0±0.8Ma and 129.7±0.8Ma,respectively,which are similar.The trace element characteristics of the titanite indicate that the ore-forming temperature is about 700-800?,and the oxygen fugacity of the ore-forming fluid increases from deep to shallow.The Nd isotope characteristics of the two types of titanite are similar to those of host rocks,indicating that the upper and lower ore bodies are the products of the same mineralization system.The in-situ S isotope characteristics of SHRIMP,a typical mineral in Luohe iron deposit at different stages,show that the ?34S value of pyrite in stage II is 8.2-9.3‰,the ?34S value of pyrite in stage III is 7.2-11.1‰,of which vein pyrite(7.2-7.4‰)islower than disseminated pyrite(8.7-11.1‰),the ?34S value of pyrite in stage IV is6.2-10.6‰,and the ?34S value of pyrite in stage V is-2.5—-4.6‰.Stage II anhydrite has a value of 16.1-17.7‰,and Stage IV anhydrite has a value of 18.3-19.2‰.Phase II,III and IV pyrites have relatively stable sulfur isotope values,and their associated anhydrite values have little change,while the sulfur isotopes of pyrites in phase V show a sudden trend of decreasing.The above sulfur isotope characteristics indicate that sulfate supply is continuously obtained from deep evaporate in the metallogenic system.Early sulfur isotope fractionation is only controlled by disproportionation reaction,and later sulfate reduction leads to an increase in pyrite ?34S value.SHRIMP in-situ C-O isotope characteristics of typical minerals in each stage of Luohe iron ore deposit show that stage II ore-forming fluid ?18O is obviously higher than magmatic water,and the value of ?18Ofluid decreases rapidly twice in the process of fluid evolution,indicating that there are two times of magmatic-hydrothermal pulsation or meteoric water addition in the ore-forming process,corresponding to massive precipitation of stage IIb and stage IV anhydrite respectively;Most of the C-O isotope binary maps of carbonate samples tested have a?13C value of-5?0‰,and there is no correlation between the value of ?13C and the value of ?18Ofluid,which indicates that the carbon source in ore deposit fluids mainly comes from Triassic sedimentary strata,and the decrease of oxygen isotope indicates the addition of meteoric water.Luohe iron ore deposit has undergone at least two stages of magmatic-hydrothermal pulsation.The second hydrothermal pulsation temperature decreases obviously and lasts for a short time.The large amount of meteoric water addition in the later stage is the key factor leading to the transformation of magnetite into pyrite-anhydrite assemblage.The trace elements and O isotope of magnetite in Luohe have dual characteristics of skarn and IOA type deposit.In conclusion,the Luohe iron deposit is different from the typical skarn type iron deposit and the typical IOA deposit.The shallow part of the deposit is also comparable with the porphyry type hydrothermal system.It belongs to the special Fe-P-SO42-system.Here we temporarily call it the "untypical" IOA deposit.The Dabaozhuang deposit is composed of hematite,pyrite and anhydrite ore bodies.It occurs in tuffaceous volcanic rocks of the Zhuanqiao Formation.It is very similar to VMS type deposit in geological characteristics,but it located in a volcanic basin and has distinct metallogenic characteristics.Through systematic ore depositgeology feature,SHRIMP in situ S isotope and LA-ICP-MS analysis of pyrite,it is determined that there are four types of pyrite in the deposit,and different types of pyrite have a large variation range of ?34S(-31.4-10.5‰).The pyrite vein(type I)in tuff ?34S is + 9.9‰ and + 10.5‰;fine-grained annular or elliptic pyrite(type II)?34S is-9.2—-2.0‰;pyrite replace tuff and hematite ?34S is + 3.1-+5.3;Granular pyrite(type IV)?34S is-29.7‰—-30.4‰.The ?34S range of anhydrite was narrow,which was 21.0-21.7‰.Type I pyrite has the characteristics of high Mn,Co,Ni,Zn,low As,Ti,Tl and Sb;type II pyrite has higher Al,Ti,V,Cu,As,Sb,Te,Tl,but lower Mn,Zn and Se;type III pyrite has higher Mg,Al,V,Ti,and a larger range of variation,with higher Se,and lower Cu,Te;Mn,Zn,As,Sb,Bi,Tl and other trace elements.It is between type I and type II;most of the trace elements in type IV are lower than those in other three types of pyrites.The above characteristics indicate that the initial high temperature fluid in the deep contains a large amount of formation sulfur,and the type I pyrite shows similar sulfur isotope characteristics to that of the Luohe iron deposit.Subsequently,the ejected hydrothermal fluid mixes with lake water to form sedimentary pyrite(type II)at temperatures not exceeding 300?.The non-ejected fluid metasomatizes the surrounding rock to form disseminated or vein pyrite(type III).At the end of hydrothermal activity,fluid activity weakens and temperature drops rapidly,forming a small amount of type IV pyrite.Unlike typical VMS deposits,the sulphur in Dabaozhuang deposit is derived from deep evaporate rather than mixing of seawater,and belongs to volcanic lake exhalation sedimentary deposit.Previous studies have suggested that the typical "mineral assemblages" of apatite-diopside-magnetite occur in the syenite of the Makou iron deposit,which is a special porphyry type iron deposit.Through systematic ore deposit and electron probe analysis,the ore-forming mother rock diorite and quartz syenite of Makou iron ore deposit are determined to be late-breaking minerals.The diagenetic age of diorite is131.2±3.3 Ma).The clear relationship between Makou iron ore and gabbro diorite magmatic activity further unifies the ore-forming specificity of iron ore deposit magmatic rocks in Luzong area.The trace element characteristics of magnetite in Makou deposit indicate that the hydrothermal properties of albite stage are close to magmatic water.During the mineralization of magnetite,the assimilation of magmatic hydrothermal fluid to Triassic strata is enhanced,and then the temperature of ore-forming fluid from diopside magnetite stage to quartz sulfide stage decreases rapidly with the addition of atmospheric water.The pyrite sulfur isotopecharacteristics indicate the total sulfur content in the deposit.From magmatic sulfur.Makou is similar to Meishan type in Ningwu porphyritic iron deposit in terms of ore-forming material source,ore body characteristics,mineral assemblage and precipitation conditions of magnetite.Through systematic comparative study of trace elements in magnetite of different iron deposits in Luzong basin,it is proposed that the crystallization process of hydromagnetite in iron deposits in the basin is influenced by diopside crystallization,and the Mg content has a wide range of variation.The formation temperature of hydromagnetite produced in volcanic rocks is similar,and the formation temperature of magnetite in Makou iron deposit is slightly higher,and the variation of trace elements is similar to that of typical IOA magma-hydrothermal model.The variation trend of trace elements in magnetite of Luohe and Yangshan iron deposits is between Makou and Longqiao.Most of the spots fall into porphyry area,some in skarn area,and the general trend is IOCG.Makou hydrothermal magnetite has the lowest oxygen isotope value,which is close to orthomagmatic magnetite.Longqiao skarn deposit has the highest oxygen isotope,while Luohe and Yangshan magnetite are between them.They have dual(transitional)characteristics of skarn and IOA.The results of this study show that the mineralization of iron ore deposits related to magmatic hydrothermal solution is a continuous changing process.The strong water-rock reaction of skarn deposits results in a more transverse trend of magnetite composition change,which is completely different from that of the Kiruna type iron ore deposit proposed by Knipping et al(2015).The geochemical composition of magnetite can not be strictly restricted.Compared with the boundary,it is more reliable to use the evolution trend of trace elements of magnetite to judge the type of deposit.Through regional comparison,we clearly points out that the intrusive rocks of iron ore deposits in the middle and lower reaches of the Yangtze River metallogenic belt have obvious metallogenic specificity.The dioritic intrusive rocks is the necessary condition for the formation of skarn and porphyrite iron deposits,while the formation of syenite intrusive rocks is later,it cut iron ore bodies in some mining areas,so it is not directly related to iron mineralization.The diagenetic age and geochemical characteristics of the iron-forming intrusive rocks in Luzong Basin,Ningwu Basin and southeastern Hubei are basically the same.The magmatic source area is a metasomatic mantle whose composition is close to EM I-type enriched mantle.During the process of magma ascension,it is less contaminated by lowercrustal materials and retains more of the characteristics of the mantle in the source area.SHRIMP in-situ O isotope and trace element characteristics of apatite in different types of iron deposits in Luzong basin indicate that the magmatic apatite in gabbro diorite from Makou and Longqiao iron deposits is mainly F-rich and Cl-rich apatite.Makou hydrothermal apatite inherits the geochemical characteristics of magmatic apatite,while the hydrothermal apatite in Luohe and Nihe deposits has higher SO3,indicating the ore-forming system assimilates the gypsum salt layer selectively,magma can assimilate a large number of salts,but the assimilation of gypsum is limited.The addition of gypsum mainly depends on the dissolution of hydrothermal solution.This assimilation mechanism results in obvious differences in mineral assemblages and apatite geochemical characteristics of deposits with different emplacement depths of rock masses in the Luzong Basin.Most ore-forming fluids of iron deposits in Luzong volcanic basin react with Triassic sedimentary strata in depth,and then rise along faults or branches into volcanic rocks to form a large number of Na-Ca alterations.As a result of skarn mineral development and high CO2 fugacity,apatite development is less than that in South America.Alteration characteristics,trace element characteristics of magnetite and oxygen isotope indicate that iron deposits in the basin should belong to the transitional part of skarn-IOA,similar to early Na-Ca alteration zone in magmatic-hydrothermal IOCG deposit.On this basis,a multi-layer metallogenic model of iron deposits in Luzong Basin has been established,which can be divided into Skarn-type iron deposits(Longqiao)occurring in Triassic sedimentary strata,IOA-type iron deposits(Makou)occurring in rock mass and volcanic rock contact zone,skarn-IOA type iron deposits(Luohe,Nihe,Yangshan)occurring in thick volcanic rocks,and hematite deposits occurring under medium-low temperature oxidation conditions.Bed(Daling)and exhalation sedimentary pyrite deposits(Dabaozhuang)occurring in tuff of late Zhuanqiao cycle.Although the occurrence locations of each deposit are different,they are all related to dioritic magma.The iron mineralization process in the basin is continuous and genetically interrelated.It is a product of magmatic hydrothermal activity in the Early Cretaceous.The Middle and Lower reaches of the Yangtze River belt is a foreland basin system between the Yangtze plate and the Dabie orogenic belt.As an important part of the metallogenic belt,Luzong Basin should belong to the lower front zone in thewhole system,which may be the collision suture site of the paleoplate.Its diagenesis and mineralization are restricted by the Mesozoic Yanshanian geodynamic background in eastern China.The magma in the source area began to activate at 152 Ma.After 135 Ma,a series of volcanic basins and iron-dominated deposits were formed between 135 Ma and 123 Ma due to the change of subduction stress direction of the Paleo-Pacific plate,the strengthening of regional extension and the activation of local suture zone.