| Xiangshan uranium orefield is the largest volcanic-type uranium orefield in China.Proven uranium deposits are unevenly distributed in the northern porphyry metallogenic belt and western volcanic lava metallogenic belt.Despite that lots of studies have been done and achivements have been obtained,there is still a controversy on the understanding of magma evolution sequence.In addition,three major ore-bearing rocks in Xiangshan are granite porphyry,rhyolite dacite and “porphyritic lava”.It is indisputable that the granite porphyry and rhyolite dacite porphyry belong to porphyry.However,whether "porphyry lava" is also porphyry remains to be solved.Porphyry is closely related to uranium deposit,so it is necessary to carry out work on the systematic determination of porphyry,ore-controlling function of porphyry and further prospecting direction in Xiangshan.Based on chronology and lithology of main rocks,magmatic evolution sequence is reclassified and divided into three volcanic subcycles.Geochemisty,different petrogenesis,relationship between typical uranium deposits and main mineralized rock within each volcanic subcycle are analyzed and then porphyry ore-control model is constructed.On the basis of constructed model,further prospecting directions are proposed.The completion of this paper can not only deepen the understanding of Xiangshan porphyry type uranium deposits with theoretical significance,but also promote the prospecting of Xiangshan uranium-popymetals with practical significance.Main achievements are summarized as follows:(1)Based on the evidence of chronology and lithofacies,the magmatic evolution sequence of the Xiangshan volcanic intrusive complex can be divided into three volcanic subcycles.The first volcanic subcycle(140~137Ma)consists of eruptive(sedimentary)facies welded tuff,overflow facies rhyolite dacite and subvolcanic facies rhyolite dacite porphyry.The second volcanic subcycle(137~133Ma)consists of eruptive(sedimentary)facies welded tuff and porphyroclastic lava containing metamorphic breccia,overflow facies porphyroclastic lava and subvolcanic facies porphyroclastic granite.The third volcanic subcycle(133~122Ma)consists of subvolcanic facies granite porphyry and a series of dykes,lacking eruptive(sedimentary)facies and overflow facies.(2)The three volcanic subcycles of Xiangshan have similar characteristics of siliconrich,alkali-rich,right-leaning REE(Rare earth elements)distribution,enrichment of LRLE(large ion lithophile elements)and depletion of HFSE(high-field strength elements),and all the three subcycles belong to the high-differentiated A-type granite series.But there are still difference among these subcycles.The first volcanic subcycle of eruptive(sedimentary)facies-overflow facies-subvolcanic facies has anti-sequence characteristics,strong fractional crystallization,high oxygen fugacity and obvious crust-mantle mixing characteristics,originating from the melt of Mesoproterozoic upper crust with the mix of mantle under intraplate extensional tectonic setting of post-collision.Distribution of the first volcanic subcycle is controlled by volcanic vent near Shutang in the western area of Xiangshan,without inheritance of the volcanic channels from the second volcanic subcycle.The second volcanic subcycle also has the characteristics of antisquence from eruptive(sedimentary)facies-overflow facies-subvolcanic facies,with the strongest fractional cycstallization,low oxygen fugidity without crust-mantle mixing,originating from the melt of Mesoproterozoic upper crust under intraplate extensional tectonic setting of post-collision.Distribution of the second volcanic subcycle is controlled by the high magmatic reservoir in the middle of Xiangshan.The third volcanic subcycle consists of granite porphyry,with strong fractional crystallization,low oxygen fugidity without crust-mantle mixing.Granite porphy originates from the melt of Mesoproterozoic upper crust under intraplate extensional tectonic setting of post-collision.Distribution of the third volcanic subcycle is controlled by deep stratigraphic and lithologic interfaces.Uranium deposits in western area of Xiangshan are controlled by rhylolite dacite porphyry(subvolcanic facies)from the first volcanic subcycle and porphyroclastic porphyry(subvolcanic facies)from the second volcanic subcycle.Uranium deposit in eastern area of Xiangshan is controlled by porphyroclastic granite(subvolcanic facies)from the second volcanic subcycle.Uranium deposits in northern area of Xiangshan are controlled by granite porphyry(subvolcanic facies)from the third volcanic subcycle.(3)Based on the systematic analysis and research,relationship between uranium deposits and porphyry on time,space and source are determined,and the ore-control model of Xiangshan porphyry is established.In terms of time,the alkaline mineralization age of the uranium deposits in the north of Xiangshan is consistent with the diagenetic age of the granite porphyry,while the rhyolite dacite porphyry and fragmentary granite porphyry in the west of Xiangshan are consistent with the alkaline mineralization age.In space,ore bodies are distributed in porphyry deformation,front zone and inner and outer contact zones.The consistency of trace elements,rare earth elements and Sr-Nd isotope between uranium orebodies and porphyry indicates that the porphyry provides the main ore-forming materials.Uranium mineralization began after the formation of subvolcanic facies in the late stage of each volcanic subcycle.After the magmatic period,the uranium-rich hydrothermal fluid evolves into ore-forming fluid through the water-rock interaction.The mixing of ore-forming fluid,mantle-derived material and meteoric water enable ore-forming fluid migrate,reduce and mineralize in favorable areas with geochemical barriers such as the occurrence variation zones of rhyolite dacite porphyry and "porphyroclastic lava",the occurrence mutation zones,the front zone and inner and outer contact zone of granite porphyry.Porphyry provides the main uranium source,heat source,ore-forming fluid and space for mineralization during uranium mineralization.(4)Based on the porphyry ore-control model of Xiangshan,further prospecting directions are proposed.The prospecting direction of porphyry type uranium deposit in the north area of Xiangshan is where the concealed granite porphyry or occurrence variation,the front zones and the inner and outer contact zones.The deep part of existing deposits,the intersection of porphyry and NE trending structure,and the contact zones of deep concealed granite porphyry are the favorable prospecting directions for uranium ores in the west of Xiangshan.In addition,the deep area of the western part of Xiangshan has lead-zinc potential.The deep morphologic changes of granite porphyry,front zones and internal and external contact zones in southern Xiangshan area have great prospecting potential.The conjected zones between bed rock and wall rock of eastern granite porphyry is a favorable direction for uranium prospecting. |
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