Geochemical Characteristics Of Hydrothermal Alteration Minerals And Uranium Mineralization In Mianhuakeng Uranium Deposit,northern Guangdong Province

There are five major uranium ore fields(Xiangshan,Taoshan,Zhuguang,Xiazhuang and Miaoershan)in South China.The Changjiang uranium mining area in North Guangdong Province is an important part of Zhuguang uranium ore field,which is located in the southeast of Zhuguangshan granitic pluton in the middle-east part of the Nanling Metallogenic Belt.It is one of the most important producing area of granite-type uranium deposits in South China.Presently,there are 6 proved reserves deposits(Mianhuakeng,Shulouqiu,Youdong,Changkeng,Changpai and Shuishi)and multiple uranium occurrences in the Changjiang uranium mining area,of which Mianhuakeng uranium deposit is the most typical granite-type uranium deposit with the deepest mining depth and the largest scale in South China.In this paper,the Mianhuakeng uranium deposit which in the Changjiang uranium mining area was taken as the research object.The representative rock and ore samples of the deposit were analyzed in petrography,fine mineralogy and major-,trace-and rare earth-elements geochemical characteristics by using modern analytical and testing techniques such as Polarized Light Microscopy(PLM),Scanning Electron Microscopy(SEM),Energy-dispersive X-ray Sectroscopy(EDS),Electron Probe Microanalyzer(EPMA),X-Ray Fluorescence Spectrometer(XRF)and Inductively Coupled Plasma Mass Spectrometry(ICP-MS).Combined with the mass balance calculation methods and the acid-alkali hydrothermal alteration theories,etc,the migration characteristics and laws of the components in the typical mineralization-alteration cross section of the deposit were discussed.The genetic mineralogy of chlorites,characteristics of"reddening"alteration,mineralogy and element geochemistry of fluorites,the relationship between hydrothermal alteration and uranium mineralization,etc were systematically studied.The main results are as follows:(1)The wall-rock alteration of Mianhuakeng uranium deposit was widespread and intense,and the mineralization-alteration cross section has obvious horizontal zoning characteristics.The typical horizontal mineralization-alteration cross section at-150m level of No.9 silicification zone in Mianhuakeng uranium deposit was studied,and the horizontal mineralization-alteration cross section can be divided into the fresh granite zone(V),the alkali metasomatic alteration zone(IV),the chloritization alteration zone(III),the hydromicatization alteration zone(II)and the hematitization alteration zone(I).The main alteration types and generation sequences were alkaline feldspathization at the premetallogenic stage?chloritization-illitization/hydromicazation-hematitization at the metallogenic stage?carbonation at postmineralization stage.Among them,chloritization,illitization/hydromicazation and hematitization can promote the accumulation of ore-forming elements,and the superposition of illitization/hydromicazation and hematitization were more favorable for uranium mineralization.(2)The horizontal mineralization-alteration cross section of Mianhuakeng uranium deposit,from the lateral alteration zone(the alkali metasomatic zone)to the the central mineralization zone(the hematitization alteration zone),the immigration rate of Si O₂(0.27%?0.21%?0.50%?0.70%)was positive to the immigration rate of U element(4.73%?8.07%?39.26%?98.29%).K⁺and Na⁺were mutually exclusive and incompatible,Mg O and Mn O exhibit a negative relationship,which maybe show a balanced convection migration.Th,Pb,Cs,Mo and As elements have the highest immigration rate in the central mineralization zone,while Ba,Sr,Co and V elements have the lowest migration rate in the central mineralization zone,which could be as a good indication for uranium mineralization.(3)The genetic mineralogy of chlorites in Mianhuakeng uranium deposit were studied.Chlorites in Mianhuakeng uranium deposit were classified according to their morphology and genesis in this paper.They are Chl₁-Chlorite closely associated with feldspar,Chl₂-Chlorite from biotites,Chl₃-Chlorite occurred in vein,Chl₄-Chlorite transformed from clay minerals and Chl₅-Uranium-associated chlorite,respectively.The Chl₁ and Chl₂ formed in the premetallogenic stage,Chl₃ and Chl₄ formed in the early stage of uranium mineralization,and Chl₅ formed in the main stage of uranium mineralization.According to the chemical composition of chlorite(analyzed by EPMA),the chlorites were classified and named by a relatively new and reasonable classification scheme in terms of their composition and structure.It shows that all of the chlorites in Mianhuakeng uranium deposit were trioctahedral-chamosite(chamosite).The formation environment of chlorites were discussed,which suggested to be from the premetallogenic stage?the early stage of uranium mineralization?the main stage of uranium mineralization,the formation temperatures(average value from 251.6??236.7??213.5?),oxygen fugacity([lg(fO₂/10^-5Pa)]average value from-42.0?-42.5?-43.8),sulfur fugacity([lg(fS₂/10^-5Pa)]average value0.6?-0.9?-4.2)were gradually decreased,which suggesting that the hydrothermal fluid evolving towards to the low temperature,low oxygen fugacity and low sulfur fugacity reduction environment.It also indicates the uranium metallogenic environment was a medium to low temperature,relatively acidic reduction environment.The contributions of chloritization for uranium mineralization were summarized from the geochemical behavior and the Source-Migration-Preservation processes of ore-forming elements,that is,chloritization provides favorable conditions for uranium mineralization by changing the occurrence states of uranium(Source),the physico-chemical properties of wallrocks and the physico-chemical equilibrium of uranium carriers(Migration and Preservation).Through the study of genetic mineralogy of chlorites,it is further proved that chlorite is an important,effective and reliable typomorphic mineral for indicating the uranium metallogenic environment.(4)The characteristics and essence of"reddening"alteration were studied by the means of fine mineralogy.It is considered in this paper that in the absence of phase analysis,it is not precise and rigorous to simply equate or attribute"reddening"alteration to hematitization,which belongs to"reddening"in a narrow sense.Iron hydroxides or iron oxides or Fe³⁺is the most important colorating mineral or colorating cation,which is an important factor for rock"reddening".A large number of microporosities(primary pores and secondary pores)in minerals providing places for the accumulation of colorating minerals in"reddening"alteration,which were the necessary condition for"reddening"alteration.Sericites and clay minerals(such as illite and hydromica)were likely to be the colorating minerals of the"reddening"alteration rocks.At the same time,they have a certain degree of adsorption,which plays a role as carriers and adsorption for other colorating minerals(iron hydroxides or iron oxides,chromium-and manganese-oxides)to a certain extent,and provides favorable conditions for the transport and accumulation of other colorating minerals.The isomorphic replacement of the transition-metal oxides(especially the transition-metal elements such as Cr,Mn,Ti,Co,etc in the?periodic elements of the Element Periodic Table)and their cations(Cr³⁺,etc)with Fe³⁺and Al³⁺,which result in the complexity and diversity of"reddening"alteration.Therefore,the phenomenon of"reddening"alteration in hydrothermal uranium deposits is not caused by a single factor,but by the coupling effect of many factors such as micropores in minerals,iron hydroxides or iron oxides,sericites and clay minerals,transition metal oxides and their cations.(5)The mineralogy and element geochemistry of fluorites in Mianhuakeng uranium deposit were studied.There are three types of fluorite in Mianhuakeng uranium deposit,they are the violet black fluorite formed in the main stage of uranium mineralization,the violet fluorite formed in the late stage of uranium mineralization and the light green fluorite formed in the post-stage of uranium mineralization,respectively.These three types of fluorite belong to hydrothermal genesis.The similar REE distribution pattern of violet black fluorite and violet fluorite suggesting that they have the same material source,while the material source of light green fluorite may be different from that of violet black fluorite and violet fluorite.From the main stage of uranium mineralization to the post-stage of uranium mineralization,the weak negative Ce anomaly,the obvious negative Eu anomaly of fluorite,the U content in fluorite and the variation characteristics of these parameters all indicating that the evolution of metallogenic environment and metallogenic fluid properties were from reduction to oxidation.Based on the study of trace elements and rare earth elements of fluorite trace in Mianhuakeng uranium deposit,combined with the metallogenic geological background of the Changjiang uranium mining area and the research results of the ore-forming fluids in the similar nearby uranium mining area,it is considered that the ore-forming fluids of Mianhuakeng uranium deposit were less likely derived from the Precambrian basement rocks which were rich in U,Ba.etc elements or to have an adequate water-rock interaction with them.There was the possibility of multi-source(mantle fluid and meteoric water)of ore-forming fluid,and relative to Precambrian basement rocks,it was a foreign fluid which experienced deep circulation.(6)According to the content,ratio and migration characteristics of the elements in each alteration zones of the Mianhuakeng uranium deposit,and combine with the basic geological characteristics,the basic properties of uranium and the previously published data(C-H-O-S-Sr-Sm-Nd isotopes),it is considered that the ore-forming materials of the deposit mainly contributed by the host rock,Changjiang granite.The ore-forming fluids were rich in volatiles and mineralization agents(CO₂,F,H₂O,etc.),alkali metals(K,Cs,Rb)and HREE with relatively high oxygen fugacity,and its sources were a mixture of mantle fluids and meteoric waters during the deep circulation.The introduction of volatiles and mineralization agents were the important mechanism for mineral migration of the deposit,and the environment from oxidation to reduction with CO₂ escape was an important mechanism for mineral precipitation of the deposit.