Geochemical Characteristics And Genesis Of Cassiterite And Sphalerite In The Tongkeng Tin Polymetallic Deposit,Northwestern Guangxi

Tongkeng deposit is a super large tin polymetallic deposit located in Dachang,Northwest Guangxi,which is composed of upper tin polymetallic ore body and deep zinc-copper ore body.Tongkeng deposit has been studied extensively.There is no dispute between the upper vein tin polymetallic ore body and the deep zinc-copper ore body.However,the source of mineralization of the stratoid tin polymetallic ore body is still divided.Most scholars believe that the metallogenesis is related to the Longxianggai granite buried deep in Dachang ore field,and some believe that the mineralization of stratoid ore bodies originates from Devonian strata and is superimposed by hydrothermal action at later stage.Geochemical characteristics of cassiterite of different occurrences（vein and stratoid）in tin-polymetallic ore bodies and sphalerite in different types of ore bodies（tin polymetallic ore body and zinc-copper ore body）of Tongkeng deposit have been systematically developed in this thesis.The ore-forming materials and sources of ore-forming fluids of Tongkeng tin polymetallic deposit are discussed further,which deepens the understanding of the genesis of the deposit and provides new data for the ore-forming mechanism of the constrained deposit.The following achievements and realizations are mainly obtained through this work:（1）The trace elements of cassiterite in tin polymetallic ore bodies of different occurrences（vein,stratoid）of the Tongkeng tin polymetallic ore body have similar characteristics,being Fe-rich and poor in W,Nb,Ta,Hf,Ga and In.The characteristic elements ratios and W-Fe diagrams indicate that the ore-forming materials of the tin-polymetallic ore body is mainly derived from magma.Cassiterite has low total rare earth elements and rich light rare earth elements.The rare earth element patterns of cassiterite in the stratoid ore body is different from that of vein ore body.However,the Y/Ho ratios of cassiterite are close to that of Longxianggai granite and Devonian ore bearing strata,indicating that in addition to the source of magma,some stratigraphoc materials also be added.（2）The trace elements of sphalerite in different types of ore bodies（tin polymetallic ore body and zinc-copper ore body）of Tongkeng deposit mainly show the features of rich in Fe,Sn,In,and poor in Ge.The element association and characteristic ratio indicate that the ore-forming materials of different types of ore bodies are similar,which is mainly derived from magma.Sphalerite has low total rare earth elements and rich light rare earth elements.The rare earth element patterns of sphalerite in different types of ore bodies are roughly similar.However,the Y/Ho ratios of sphalerite are significantly high,indicating that the ore-forming materials have similar sources,and the ore-forming fluid may be mainly magmatic hydrothermal fluid.（3）The study of hydrogen and oxygen isotopic shows that theδD=（-85‰～-73.4‰）and theδ¹⁸O_H2O=（-1.0‰～6.8‰）of sphalerite in the upper part of the Tongkeng tin polymetallic ore body,and theδD=（-106.7‰～-86.3‰）andδ¹⁸O_H2O=（-2.8‰～3.4‰）of sphalerite in deep zinc-copper ore body.The projection points in theδD-δ¹⁸O_H2Odiagram are mainly located in the transition area between magmatic water and atmospheric precipitation line,indicating that the ore-forming fluids of different types of ore bodies are mainly magmatic fluids,but there is mixture of atmospheric precipitation,and the proportion of magmatic fluid in the upper tin-polymetallic ore body is higher than that in the deep zinc-copper ore body.（4）The main metal minerals（cassiterite and sphalerite）in Tongkeng deposit may be formed in high-temperature environment,and the metallogenic environment may change from reducing environment to oxidating environment.（5）The Tongkeng tin polymetallic deposit is a magmatic hydrothermal polymetallic deposit related to the Late Cretaceous magmatic activity.The ore-forming material and ore-forming fluid mainly come from magma.The genesis of the deposit belongs to magmatic hydrothermal filling metasomatism.