Mesozoic Volcanic Basins And Uranium Mineralization In The Eastern Nanling Range

A series of Mesozoic bimodal volcanic rocks occur in the eastern Nanling Range,which is one of the most important prospecting target areas for volcanic rock-type uranium deposits in South China.Many uranium deposits were discovered in the Baimianshi and Rencha volcanic basins.But no uranium orebodies are found in the nearby Xunwu and Changpu volcanic basins until now.There are few research works on the forming ages,petrogenesis,tectonic settings for volcanic rocks in these volcanic basins.The genetic relationship between volcanic rocks and uranium mineralization is also unclear.Thus,in this paper we carried out systematic studies on zircon U-Pb chronology,elemental geochemistry,and Sr-Nd-Hf isotopes of volcanic rocks from the four basins to constrain their petrogenesis and tectonic settings.In addition,we carried out pitchblende U-Pb dating and in-situ pyrite S-Pb isotopic and trace elemental analysis for U deposits in the Baimianshi and the Rencha basins to constrain ore genesis and.Basesd on the new data,we established the genetic models for the volcanic rocks-hosted uranium deposits in the eastern Nanling Range and provided suggestions for guiding the further exploration in this area.Zircon U-Pb chronology analysis show that the Xunwu volcanic rocks formed at 97⁹9 Ma and the Rencha volcanic rocks formed at two stages,188¹92 Ma and 95¹05 Ma,respectively.The Changpu basalt formed at 187 Ma.The granite in the Baimianshi basin was emplaced at 243 Ma and the basalt formed at 186 Ma.Thus,the volcanic rocks in the eastern Nanling Range mainly formed at two stages: early Jurassic?186¹92 Ma?and late Cretaceous?95¹05 Ma?.U-Pb dating results of pitchblende from the 278 deposit in the Rencha basin shows that the uranium mineralization ages focused on 53⁷1 Ma,which are younger than the forming ages of the hosting volcanic rocks.Uranium mineralization at the Baimianshi basin mainly occurred in three periods: 148-189 Ma,128-139 Ma,and 53-103 Ma.Thus,volcanic rock-type uranium deposits were formed later than the volcanic magmatism,and there exists a huge age gap between volcanic rocks and uranium mineralization.Early Jurassic acidic volcanic rocks in the Rencha basin and the Changpu basin have A-type geochemical characteristics: enriched in Na₂O+K₂O and HFSE contents,10000*Ga/Al > 2.6,Zr+Nb+Ce+Y>500×10^-6,Zr>250×10^-6,Nb>20×10^-6.The initial ⁸⁷Sr/⁸⁶Sr ratios of acidic magmatic rocks vary from 0.709508 to 0.711352,?_Nd?t?from-10.8 to-6.8,and Nd model ages vary from 1.5 to 1.98 Ga.The zircon ?Hf?t?value vary from-14.6 to-9.3 with two-stage Hf model ages of 1.8².1 Ga.Geochemical characteristics implied that these early Jurassic acidic volcanic rocks formed from partial melting of Meso-Proterozoic metamorphic rocks in the lower crust.The early Jurassic basalts in the Baimianshi basin and the Rencha basin show OIB-like geochemical characteristics.The early Jurassic basalts might form from partial melting of upwelling asthernospheric mantle and experience different degrees of crustal contamination.Based on the above description and discussion,the early Jurassic acidic and basic rocks in the eastern Nanling Range may form in the continental rift environment.Upwelling of the asthenospheric mantle triggered the partial melting of metamorphic rocks in the lower crust,eventually contributed to the eruption of early Jurassic bimodal volcanic rocks.Late Cretaceous acidic volcanic rocks in the eastern Nanling Range have A-type and I-type geochemical characteristics.Their Sr-Nd-Hf isotopes show that they were mainly derived from the partial melting of Middle Proterozoic metamorphic basement with variable mixing of mantle materials.Late Cretaceous mafic rocks in the eastern Nanling Range don't show negative Nb,Ta anomalies and have positive ?_Nd?t?values.It is significantly different from the contemporary island arc basalts in Southeast Coast in South China.Thus,these geochemical features indicate that Late Cretaceous volcanic basins in the Nanling Range were formed at back-arc extension environment.In-situ sulfur isotope analysis of pyrites in the Baimianshi uranium orefield exhibited that the pyrite from early-stage pyrite-quartz veins have relatively uniform ?³⁴S values,varying from-3‰ to +1‰,while ?³⁴S values of pyrite from late-stage pyrite-fluorite veins vary from-12‰ to-7‰.Sulfur in ore-forming fluids may come from magmatic rocks,while late-stage ore-forming fluids may have higher oxygen fugacity.Besides,the comprehensive lead isotope analysis of the basement granite,basalt,sulfide and fluorite from ores demonstrate that the uranium for mineralization mainly came from the basement Baimianshi granite.A new metallogenic model for the Baimianshi uranium deposits was proposed.The early-stage medium-to-high temperature,low-salinity,and high oxygen fugacity fluids upwelled along the structural fractures,which leached U from the Baimianshi granite.When the fluids went through adjacent carbonaceous sandstone strata and basic volcanic rocks,U precipitated to form the orebody from the U-rich fluid due to the decrease of oxygen fugacity.In-situ sulfur isotopic and trace elemental analysis of pyrite in the 278 deposit show that the ?³⁴S values of pyrite from the altered diabase vary from-3.1‰ to +3.6‰ with low U concentration of 0.02⁰.27×10^-6.But those from the altered rhyolite porphyry vary from-16.8‰ to-5‰ with high and variable U concentration of 0.62³06×10^-6.Sulfur maybe come from magmatic rocks.The lead isotope analysis of the metamorphic rocks,rhyolite,rhyolite porphyry and pyrite show that the ore-forming materials of the 278 uranium deposit maybe come from rhyolite porphyry.At Late Mesozoic?⁷0 Ma?,a series of red clastic rock basins formed in Rencha under regional extensional setting,which indicated a droughty and oxidic environment.Therefore,the fluids originated from the red basins have high oxygen fugacity,which flew along the tensional fractures in the volcanic basin and leached sulfur and lead from the rhyolite porphyry.When meeting with the geochemical reduction barrier,such as the diabase dikes,U was reduced and precipitated to form the ore body.The main controlling factors to forming the volcanic rock-type uranium deposits in the eastern Nanling Range include: Indosinian U-rich S-type granites or Yanshanian Urich acid volcanic rocks;Cretaceous-Tertiary crustal extension and red basins;regional extensional faultsand reduced barriers.