Mesozoic Magmatic And Hydrothermal Mineralization Of The Huayangchuan Uranium-polymetallic Deposit In Shaanxi Province

Mineralogical investigation in carbonatite is one of the important research contents of carbonatite,it is not only critical for revealing magmatic-hydrothermal evolution,but also has a significant implications for mineralization.In this study,uranium-niobium minerals,as well as accessory minerals in the Huayangchuan uranium-polymetallic deposit of Shaanxi province,were selected to conduct electron probe microanalyzer,scanning electron microscopy,and Laser ablation inductively coupled plasma mass spectrometry.Coupled with with petrographycal,geochemical features of minerals and host rock,mineral isotopic geochemical characteristics,this study explores mineral type,chemical composition,mineralization chronology,mineral evolution,and element migration characteristics.It provides a basis for further revealing the ore-forming evolution processes and formation mechanism of this deposit,as well as enriching uranium polymetallic ore-forming theory.According to mineral assemblage,mineralization type and vein body interpenetration,mineralization in the carbonatite of the Huayangchuan deposit can be divided into two stages.The first stage is the magmatic U-Nb mineralization stage,hosted in the aegirine-bearing carbonatite;and the second stage is high-temperature hydrothermal U-Nb mobilization and re-mineralization stage,and hosted in the biotite-bearing barite carbonatite.The magmatic U-Nb mineralization stage developed abundant pyrochlore and uraninite,with the mineral assemblage of calcite+quartz+pyrochlore(pcl3)+uraninite(Ur1)+allanite(Aln1)+aegirine+monazite+titanite(Ttn1)+apatite.Pcl3 displayed in the shapes of hemimorphic to allomorphic granular,or vein filling along the fracture between quartz and calcite.B-site of the Pcl3 is dominated by Nb,Na and U occur at the A-site.The edge and interior fracture of Pcl3 underwent low temperature alteration to form kencalciopyrochlore(Pcl4),and the alteration reaction can be expressed as:3Ca²⁺+^YO^2-?Ba²⁺+Sr²⁺+^A(?)+^Y(?).Uraninite(Ur1)in the Pcl3 is featured with high Pb O and low(Ca O+Fe O+Si O₂)contents with U/Th ratio of 28.7-92.3,?REE content of 4.8-6.7 wt.%.The edge and micro fracture of the Ur1 grains underwent late hydrothermal alteration,forming uraninite(Ur3)with low Pb O and high(Ca O+Fe O+Si O₂)content.In-situ LA-ICP-MS U-Pb isotopic age of Ur1 is(213±3.7)Ma,which are consistent with the carbonatite age(218.7±1.2)Ma,obtained from in-situ LA-ICP-MS U-Th-Pb of monazite in carbonatite.This indicates that the early Mesozoic U-Nb mineralization is the product of carbonatite magmatism.In the early Mesozoic,the Huayangchuan carbonatite magma was generated by melting of fertilized carbonated mantle during subduction processthe while the Qinling orogenic belt at the post collisional stage.The U and Nb are abundant in the carbonatite magma,and preferentially existed in the late stage of carbonatite magma,resulting in the formation of the pyrochlore(Pcl3)and uraninite(Ur1)and the main U-Nb mineralization stage of the Huayangchuan deposit.In the late stage,mineral assemblage of the biotite-bearing barite carbonatite is calcite+quartz+pyrochlore(Pcl5,Pcl7)+uraninite(Ur2)+allanite(Aln2)+titanite(Ttn2)+barite-celestite+fluorite+galena+sphalerite+pyrite.Oxycalciopyrochlore(Pcl5)is octahedral or semiidiomorphic granular aggregate.The internal fracture of the Pcl5 shows the low temperature hydrothermal alteration and generate kencalciopyrochlore(Pcl6),the alteration reaction can be listed as:3Ca²⁺+^YO^2-?Ba²⁺+Sr²⁺+^A(?)+^Y(?).The edge of the Pcl5 displays high temperature hydrothermal alteration,and generate hydropyrochlore(Pcl7),the alteration reaction can be expressed as:3Nb⁵⁺+4Ca²⁺?2Si⁴⁺+Ti⁴⁺+U⁴⁺+REE³⁺+Ba²⁺+Sr²⁺+^A(?).Uraninite(Ur2)shows allomorphic granular or vein in shapes,and together with allomorphic titanite(Ttn2)distribut along the edge and fracture of pyrochlore(Pcl5).In situ LA-ICP-MS U-Pb isotopic age of uraninite(Ur2)is(133±1.8)Ma,belonging to the Late Mesozoic mineralization.The late mineralized stage may be the products of hydrothermal fluid activity during the Late Mesozoic period.Magmatic activity in the late Mesozoic(130?146 Ma)in the East Qinling area may be an important source of heat and F-rich ore-forming fluid for the late mineralized stage.The dense fracture and fracture structures in the region provide a channel for the migration of F-rich hydrothermal fluids which extract uranium from pyrochlore(Pcl5)and early-formed uraninite(Ur1)to form F-and U-rich fluids.Until F in the hydrothermal fluid combines with Ca in the surrounding rock to form fluorite,the solubility of uranium in the hydrothermal fluid decreases,causing Ur2 precipitation along the edge and fracture of early pyrochlore(Pcl5).Ca,Ti,Nb and other elements migrated from pyrochlore(Pcl5)to form titanite(Ttn2).At the same time,hydrothermal alteration developed along the edge to the core of the Aln1 to generate the Aln2,and the reaction can be expressed as:(Mg,Mn,Sr,Fe)²⁺+REE³⁺?Ca²⁺+Al³⁺,forming albinite(Aln2).Therefore,the Huayangchuan U-polymetallic deposit developed magmatic mineralization and hydrothermal mobilization and re-mineralization event in Mesozoic.The early Mesozoic U-Nb mineralization took place at about 220 Ma,which is the product of carbonatite magmatism,while the late mobilization and re-mineralization took place at about 130 Ma,which is the result of hydrothermal fluid reformation of the early magmatic U-Nb mineralization.