Mineralogical Characteristics Of Typical Yanshanian Granites In Southern Zhuguang Composite:Implications For Petrogenesis And Uranium Metallogenic Potential

Granite-type uranium deposit is one of the important types of uranium deposits in China.The southern Zhuguang Composite that located in the northern Guangdong Province is an important part of Taoshan-Zhuguangshan granite-type uranium metallogenic belt.Uranium deposits mainly occur in the central and eastern parts of the southern Zhuguang Composite,while no one was found in the western part.The heterogeneous distribution of the uranium deposits implies that the rock bodies in the southern Zhuguang Composite have different uranium minerlization capacity.Three Yanshanian granites were selected for comparative studies in this paper,including two uranium-bearing granites and one uranium-barren granite.Through comparing their geochemical and mineralogical characteristics,this paper explored the deciding factors of uranium mineralization capacity of these granites and implications for petrogenesis and uranium metallogenic potential,and furthermore,systematically studied the mineralogical characteristics and contributions for uranium mineralization of uranium-source minerals of the uranium-bearing Changjiang granite,these were concluded as follow:There are differences between the uranium-bearing and the uranium-barren granite,in genetic types of granite,major and minor elements geochemistry,and mineral chemistry.The U content of the granite and whether it could provide ore-forming material,depend not only on the U content of its source region,more importantly on geochemical behavior of uranium in magma evolution.It is not easy to distinguish I-type or S-type granite for the highly fractionated granites with high-Si.However,mineralogical characteristics of apatite and biotite are important parameters for distinguishing them.It reveals concordantly here:the uranium-bearing Changjiang granite and Sanjiangkou granite belong to S-type granite which are derived from sedimentary rocks consisting of ancient crustal material,while the uranium-barren Jiufeng granite belongs to I-type granite which show its crust-mantle mixed origin and is derived from partial melting of either metamorphic greywacke or continental material through crust-mantle interaction.Uranium mineralization capacity of granites is closely related to the composition of the material source of granitoid magma,the granite which is derived from the U-riched sedimentary rocks has a higher capacity of uranium mineralization.Meanwhile,the geochemical behaviour of uranium in magma evolution determines its uranium metallogenic potential,if as the granitoid magma developed,the crystal fractionation of the early Th-riched minerals lead to the crystalization of late U-riched minerals,i.e.uraninite in which U can be easily released in later hydrothermal stage,and under this circumstance,the granite can provide metallogical material for uranium mineralization.In the uranium-bearing Changjiang granite,there are lots of U-riched accessory minerals included in biotites.The chloritization of biotite alter the original U-bearing accessory minerals of the granite and form new U-bearing minerals such as coffinite near the chlorite.Uranium in the newly formed U-bearing minerals can be easily released into fluid under circumstance of later hydrothermal fluid being in slightly oxidizing state in the mineralization stage.Accessory minerals in the Changjiang granite include zircon,apatite,coffinite-thorite,uraninite and monazite.Among these accessory minerals,uraninite,uranothorite and coffinite are thought of important uranium-source minerals of the Changjiang granite because of their high uranium content that is easy to be released during alterations.Monazite have a moderate quantity of uranium that is potentially able to be liberated into fluid when it is altered to synchsite.Such a process only occurs during chloritization of major minerals such as biotite that enclosed the monazite.Therefore monazite is a potential uranium-source mineral.Zircon is not a uranium-source mineral because uranium in zircon remains stable and not released even under hydrothermal alteration.Other minerals like allanite and apatite have little uranium which is even below the detection limit of electron-microprobe,are thus not uranium-source minerals of the Changjiang granite either.