Mineralization Physical And Chemical Conditions Of Uranium Deposit In Haita Area,Miyi County,Sichuan Province

The paper is extended by the research of the migmatitic uranium mineralization mechanism in Panxi Region?Grant No.41472073?of National Natural Science Foundation,and also supported by the key research project,the uranium mineralization mechanism along the Kang-dian Axis and prospecting direction,of China Nuclear Industrial Geology Bureau.Based on the study of the characteristics of fluid inclusions and the composition of Hydrogen and Oxygen isotopes and metallogenic age,a certain understanding of the physical and chemical conditions of ore-forming fluids,its sources and the process of mineralization has been obtained as follows:?1?Four major types classified by the lithologic characteristic of minerals are migmatitic schist ore,migmatite ore,quartz-feldspar vein ore and quartz uraninite-rich vein ore.Through radiograph,uranium in quartz-feldspar vein ore found is mainly distributed as small particles of granular uranium minerals;uraninite-rich quartz vein ore has uneven distribution of particulate uranium minerals,finely divided uranium minerals,there may be uneven distribution of vein-like asphalt.Electron probe analysis shows that the uranium minerals are mainly uraninite and its secondary uranium minerals formed by in situ oxidation,as well as the pitchblende derived from metasomatized sphene in the late period.The main mineral assemblage of the ores in the Haita area is feldspar-quartz-uraninite-pitchblende-sphene-brannerite-molybdenite.?2?Three main types of fluid inclusions are liquid-phase-rich inclusions,H₂O-CO₂ inclusions and daughter-mineral-bearing three-phase inclusions in uraninite-rich mineral-bearing quartz-feldspar veins and quartz veins.Three types of inclusions are accompanied together,and liquid-rich inclusions accounted for the vast majority.?3?The results of microthermometry on fluid inclusions show that the ore-forming temperature of the extremely high grade uranium deposit in the Haita area can be divided into two major stages:the main range of the early ore-forming fluid temperature is 380?⁵00?in the stage of migmatization,which is high-temperature fluid;the main range of the late-stage ore-forming fluid temperature is 160²20?in hydrothermal superposition ore-forming process,and the fluid is low temperature fluid.?4?Through the empirical formula,the salinity of the ore-forming fluids of the extremely high grade uranium deposits varied greatly in the Haita area.The salinity of early ore-forming fluids in migmatization are predominately in the range 20 wt%²3.18 wt%NaCl and 33.48 wt%⁴5.33 wt%NaCl,which is low-medium salinity fluid.In the hydrothermal superposition ore-forming stage,the salinity predominated range of late-stage ore-forming fluid is 20 wt%²3.18 wt%NaCl,which is low salinity fluid.?5?Using the empirical formula of density,the density range of ore-forming fluids is 0.64⁰.84 g/cm³ in migmatization,which is low density fluid.The density range of late-stage ore-forming fluids is 0.92¹.09 g/cm³ in the hydrothermal superposition ore-forming stage,which is high density fluid.?6?According to the empirical formula,the ore-forming pressure range is108.37¹25.50MPa,averaged 118.42MPa in the early migmatized hydrothermal ore-forming stage,corresponding to mineralization depth range 3.61⁴.18Km,showing the hyperthermal,high-pressure and hypogenic ore-forming environment.The ore-forming pressure range is 41.29⁷9.87MPa in the late-stage hydrothermal superposition ore-forming phase,corresponding to the mineralization depth range1.38².70Km,inferring the low pressure,low-middle-pressure,hypergenic environment.?7?The results of laser Raman analysis show that the fluid consists of CH₄,H₂,CO₂,H₂O,N₂,indicating the reducibility of the fluid.The Eh?V?range of the fluid inclusions is-0.346⁰.181V,which indicates that the fluid is in the vicinity of the redox interface.The pH of the ore-forming fluid is 5.50⁶.02 in the Haita area,with an average of 5.87,which belongs to weak acid fluid.It is consistent with the main alteration types,silicification,pyritization,molybdenite mineralization etc.in the Haita area.?8?The mineralization in Haita area can be divided into two main stages.The first stage is the early migmatic hydrothermal mineralization stage.The ore-forming fluid is mainly migmatic hydrothermal fluid.The fluid has the characteristics of high temperature,low salinity and low density,and the mineralization takes place in the high-pressure and hypogenic environment.The second stage is the late-phase hydrothermal superposition mineralization stage.The fluid has the characteristics of low temperature,low salinity and high density,and the mineralization takes place in the low-pressure and hypergenic environment.In the early stage,fluid mixing occurred,and in the late stage hydrothermal superposition mineralization occurred.Mineralization is closely related to fluid mixing and fluid boiling.?9?Hydrogen and oxygen isotopic studies shows that ore-forming fluids may be the mixture of magmatic water and metamorphic water during the early stage,with the addition of atmospheric precipitation in the late stage.?10?The isotope dating shows that the molybdenite age,which is closely related to the crystalline uranium ores,is 761±19 Ma,indicating that there is uranium and molybdenum mineralization in the Chengjiang period.The age measured of uraninite is 210.5 Ma,which is consistent with the previously measured 228.6 Ma,inferring the existence of uranium mineralization in the late indosinian.Zircon age is219±14Ma in the fine-grained granite closed to the Haita A-10 uranium ore occurrence,showing that Indosinian Mineralization may have a relationship with granitic magma activity in late Indosinian.Therefore,there are two or more uranium mineralization,including Chengjiang and Indosinian mineralization,in Haita Area.