Where Is The Volcanic Caldera In Xiangshan Uranium Ore Field? Implictions From The Magnetic Fabrics

As the world's third largest volcanic type uranium ore field,the Xiangshan volcanic basin has attracted much attention for its large industrial value.The ore hosting rocks are mainly the early Cretaceous rhyodacite and porphyroclastic lava,as well as small amounts of high level intrusive acidic rocks and metamorphic rocks.The vertical alteration of polymetallic mineralization-related uranium reaches 1000 m.The uranium mineralization in the Xiangshan basin is closely related to the volcano structure,and the flow structure is not obvious for the high degree of crystallization of the lava.In addition,volcanic topography is not clear due to the later erosional effect.Therefore,there are different understandings about the volcanic caldera or magma channel.The rhyodacite and the main body of porphyroclastic lava are from different lava channels,or they are genetic inherited?Is porphyroclastic lava a product of a large caldera,or is it from multiple volcanic-magmatic channels? This work tried to recover the direction of the ancient magmatic flow through the study of anisotropy of magnetic susceptibility(AMS)of the rhyodacite and porphyroclastic lava,and further determined the location of the ancient volcanic calderas combined with remote sensing,geological,geophysical and drilling data.This project aims to discover the uranium-bearing ancient volcanic calderas using AMS,which is firstly conducted in China.Here are the primary results as follows:(1)Rhyodacite and porphyroclastic lava show high magnetic susceptibility(Km),the majority concentrated in(1000-3000)×10-6SI.K-T curve and hysteresis loop shows that the magnetic mineral in rhyodacite is mainly magnetite and maghemite(or pyrrhotite),as well as small amount of sulfides and ilmenite mineral.Magnetic minerals in porphyroclastic lava is mainly magnetite,however,it may contain a small amount of sulfide and ilmenite minerals.Most of the samples showed a flat magnetic susceptibility ellipsoid(T>0)and relatively lower HC and Mrs/Ms ratios(0.07-0.3).For particle size,most of rhyodacite and porphyroclastic lava form PSD(pseudo-single-domain)to MD(multi-domain),while somesamples containing SP(superparamagnetic particles).(2)Both rhyodacite and porphyroclastic lava's degree of anisotropy of magnetic susceptibility(Pj)shows no positive correlation with magnetic susceptibility(Km).Most of the Pj values are 1.01-1.04,reflecting tectonic movements doesn't change the magnetic fabric of lava in Xiangshan volcanic basin.(3)In the study area,the rhyodacite outcrops to the west and could be divided into two parts by Huxi town.Most rhyodacite samples in east of Huxi twon show low angle or nearly horizontal NW-SE direction of the magnetic lineation,whereas rhyodacite in the west show NNE-SSW direction of the magnetic lineation.It indicate that rhyodacite's crater probably located at Sutang,south of Zoujiashan uranium deposit.However,Xiangshan peak may still be a crater of rhyodacite.The magnetic lineation of porphyroclastic lava shows great variation,which associated with several craters.The magnetic lineation of porphyroclastic lava from Xiangshan peak to Huxi and Youfang-Xiangshi shows NWW-SW and NE-SW respectively.Combined with drilling cores and MT data,it may conclude that Xiangshan peak is a crater.Magnetic lineation of porphyroclastic lava is nearly circular in south of Ruyiting,Diaoyuan and Duishang,which suggest 3 crater.Comprehensively speaking,Xiangshan peak is the main crater of porphyroclastic lava and there are still 4 relatively small craters around it.