Hydrothermal alteration,geochemistry and 40Ar-39Ar geochronology of the dikes in south Zhuguang uranium district:Implications for the regional uranium mineralization event
The southeast part of South China,is one of the most important uranium producing area in China,which occurs numerous granite-hosted vein-type hydrothermal uranium deposits represented by the south Zhuguang and Guidong uranium district.The regional mafic and felsic dikes have experienced passive hydrothermal alteration,with the types of silicification,sericitization,chloritization,carbonation,and hematitization that similar to the types around U orebodies.The mafic dikes have high ignition loss,indicating rich volatile composition,and their major element contents retained the original geochemical characteristics of provenance to some extent,i.e.,relatively high contents of K2O,low contents of TiO2 and ∑FeO,and Na2O＜K2O.Compared with the altered mafic dikes,the mineralized one has obviously higher value of Fe2O3/FeO,contents of Zn,Mo,Th and REE,and obviously lower contents of V,Cr,and Ni.The chondrite-normalized rare earth element patterns of these mafic dikes have negative slopes from LREE to HREE and are characterized by negative δEu and barely δCe anomalies,indicating that the reformation of hydrothermal alteration is appropriate.The ore shows enrichment of heavy and light REE relative to the un-mineralized altered samples,except Eu which is probably related to the deposition of REE-rich pitchblende.The 40Ar/39Ar dating of mafic dikes are probably older than 131 Ma and might be～140 Ma,white mica pegmatite veins yielded a reasonable plateau age of 161.81±0.85 Ma.The 40Ar/39Ar age spectra diagram of all samples suggest the regional hydrothermal alteration event of south Zhuguang uranium district is no earlier than 66.0±1.1 Ma.In view of previous U-Pb dating,the uranium mineralization of southeast part of South China are mainly including two stages,which are 100-110 Ma for Guidong uranium district,and 52-65 Ma for south Zhuguang uranium district,respectively.The mineralization ages of these uranium districts are commonly tens million of years later than the emplacement of the regional dikes,therefore the regional uranium mineralization are not genetically related to the formation of mafic dikes.
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