Metallogenic Geological Characteristics Of Granite-related Uranium Deposit In The Sanjiu Uranium Ore Field,middle Section Of Zhuguangshan Complex Massif

Zhuguangshan complex massif(ZGS)is located in Taoshan-Zhuguang uranium metallogenic belt of South China uranium metallogenic province,which is one of the important granite-related uranium ore concentration areas in China.Sanjiu area is located in the middle section of the ZGS,between Lujing and Chengkou uranium ore fields.In recent ten years,there have been many breakthroughs in prospecting work in this area,and its uranium resources have been upgraded to the level of ore field(SJOF).Predecessors have carried out a lot of work and achieved fruitful results in the SJOF.However,compared with the southern part of ZGS,the old uranium-rich strata,uranium-producing granites,uranium minerals and main associated minerals,mineralization indicators,ore formation age,ore-forming fluid properties and other important ore-forming contents in the SJOF are lack of research,which restricts the further understanding of the metallogenic geological characteristics of granite-related uranium deposits in the SJOF.Based on the previous work and research,this paper summarizes the uranium mineralization characteristics of some uranium deposits(occurrence)in the SJOF in the middle section of the ZGS.By means of SEM,EMPA,LA-ICP-MS,helix SFT and other high-precision observation and/or analysis techniques,the major and trace elements,isotope geochemistry,mineralogy,in-situ micro dating and fluid inclusion microscopic observation of various rocks and minerals were studied.This paper studies and discusses the uranium source strata and granites,main uranium minerals and associated minerals,ore-forming age,ore-forming fluid,involving the geochemical composition of major and trace elements of rocks and minerals,ore-forming period,ore-forming fluid properties and evolution,trying to perfect the metallogenic model of granite-related uranium deposits,and discuss the development direction of uranium resources exploration in the study area.The main new understandings in this paper are as follows:(1)There are favorable metallogenic geological conditions,such as superior structure and magmatic-hydrothermal activity,and good metallogenic and prospecting potential for uranium polymetallic deposits in the SJOF.The hydrothermal uranium deposits are mainly located in NE,near SN,NW-trending secondary faults,and the ore control elevation is roughly located at-330?1 160 m.The mineralization depth and denudation depth in the SJOF are relatively shallow,and there is still a large potential for ore exploration in the depth.Uranium ore bodies are usually present as veins,network veins and lenticular.The ores are mainly of siliceous vein type,altered cataclasite type and structural breccia type.The ore minerals are mainly pitchblende,and a variety of secondary uranium minerals are widely developed on the surface.Various geological body interfaces,such as the contact zone between pluton and stratum,the contact surface between pluton and pluton,the ore-controlling faults inside the pluton,the superimposed area of hydrothermal alteration,geophysical and geochemical anomalies are the important ore-forming and prospecting positions.(2)The source of granite-related uranium deposits in the SJOF is mainly uranium-rich Sinian-Cambrian strata and early Yanshan granites.The ore-forming fluids were formed by multi-stage mixing of crust-mantle-derived fluids,which had experienced a long-term deep thermal cycle and remixing of crust source fluid,with the characteristics of low salinity and low mantle source components.The ore-forming fluids were mainly formed in the temperature range of 180?220?,the density range of 0.86?0.94g/cm³and the pressure range of 16?20 MPa.At the later mineralization stage,the mantle component of ore-forming fluids gradually decreased and transformed into mixed fluids dominated by crust-derived fluid.The reducibility of ore-forming fluids was generally stable,which was conducive to the long-term migration,unloading,enrichment of uranium and stable preservation of ore bodies.(3)The main uranium-producing granites,such as Sanjiangkou pluton,belong to highly differentiated S-type granite.The Sanjiangkou pluton and South China uranium-producing plutons have the same or similar geophysical and chemical characteristics,such as mineralogical characteristics,magma crystallization temperature,oxygen fugacity,etc.The uranium production of granites in South China may not be related to their emplacement depth,denudation depth and mineralization temperature,but mainly depends on the characteristics of rock formation.(4)The common minerals that are closely related to uranium mineralization or can effectively indicate uranium mineralization in the SJOF include dark-red or variegated microcrystalline quartz with high REE content,ripidolite with high Fe content,colloidal pyrite with high REE content and less S,hematite with high Fe³⁺/Fe²⁺ratio and rich LREE,colloidal allomorphic and/or fine-grained euhedral pyrite and anhedral hematite,purple-black fluorite with low LREE content.(5)The main ore mineral in the SJOF is pitchblende occurs as oolitic granular and irregular veinlet.The in-situ dating of EMPA and LA-ICP-MS of pitchblende shows that the deposits may begin to form at?140 Ma,and there are four main mineralization stages,which were 15?25 Ma,35?45 Ma,55?65 Ma and 95?105 Ma.The mineralization stages of 55?65 Ma and 95?105 Ma may correspond to the metallogenic peak period under the Mesozoic Cenozoic extensional tectonic background in South China,and the mineralization stages of 35?45 Ma and 15?25 Ma may correspond to the later reformed mineralization stage.(6)The geochemical study of rocks and minerals shows that the metallogenic process of granite-related uranium deposits in the SJOF was complex.Taking the Sanjiu uranium ore field as an example,it is believed that the granite-related uranium mineralization in South China is characterized by multi-stage metallogenic transformation.