Trace-elements Variations Of Quartz In The Shizhuyuan W-Sn Polymetallic Deposit Of Southern Hunan Province,south China:Implications For The Ore-Forming Processes

The Shizhuyuan polymetallic deposit,located in the middle of the Nanling metallogenic belt,southern Hunan Province of South China,is a super-large skarn type tungsten-tin-mo-bismuth polymetallic deposit.It is known as the"World museum of non-ferrous Metals"because of its large scale,high grade,various minerals,abundant symbiosis association and complex metallogenic conditions.Although the Shizhuyuan deposit has been explored and exploited for many years and the genesis of the deposit has been studied,there are still significant differences on the ore-forming process of the deposit and the mechanism of tungsten,tin,molybdenum and bismuth polymetallic enrichment,which seriously restrict the breakthrough of further prospecting and exploration.In order to solve these key scientific problems,this paper takes Shizhuyuan polymetallic deposit as the research object.On the basis of systematic field geological survey and petrographic observation,the characteristics of mineral assemblages in different ore-forming stages of the Shizhuyuan deposit and the characteristics of trace elements in quartz in magmatic-hydrothermal stage were studied by means of EPMA and LA-ICP-MS.This paper has obtained the following aspects of understanding:（1）The hydrothermal evolution has experienced four stages,including greisen stage,skarn stage,degradation metamorphic stage and sulfide stage,from early to late.The greisen stage forms gangue minerals quartz,muscovite,K-feldspar,and topaz,as well as the important ore mineral wolframite.The skarn stage,with less quartz,mainly produced skarn minerals such as garnet,diopside,wollastonite,and a small amount of scheelite in the later stage.The degradation metamorphic stage mainly forms a large number of oxides such as scheelite and magnetite,and at the same time,strong W-Mo-Bi mineralization occurred.In addition,the sulfide stage mainly yielded pyrite,chalcopyrite,etc.,with the quartz obviously increased.（2）According to the change of the content of trace element Ti in quartz,we calculated the temperature changes in the magmatic stage and the hydrothermal stage,respectively.The temperature variation range of the granite stage is 425°C～650°C,the greisen stage is 299°C～456°C,the skarn stage is 348°C～542°C,and the temperature variation range of the sulfide stage is 266℃～402℃.As a result,the temperatures from magmatic stage to hydrothermal stage show an overall downward trend.（3）Trace elements can enter the crystal lattice of quartz（SiO₂）in three ways:single atom substitution,coupled substitution and interstitial charge compensation substitution.The contents of Al,Li,Na,K,Ge and Ti in quartz are more enriched than that of other trace elements.Al,Ge and Ti preferentially enter the crystal lattice of quartz,followed by Li,K,Na and P.（4）The ratio of quartz trace elements can clarify the degree of magmatic evolution.The most sensitive elements for magmatic differentiation are Li,Rb,Al,Ge and Ti,while Ge/Ti and Al/Ti can well reflect the degree of magmatic evolution.（5）The enrichment of tungsten and tin is related to the continuous evolution of magma,and the precipitation of tungsten and tin is caused by the decrease of fluid temperature and the repeated change of pH value in the fluid.To sum up,the evolution process of the Shizhuyuan skarn can be divided into greisen stage,skarn stage,degradation metamorphic stage and sulfide stage;The change of metallogenic environment is reflected by the change of trace element content.The temperature generally shows a downward trend and the p H decreases.The degradation metamorphic stage is the main metallogenic stage of scheelite,and the characteristics of fluid in the precipitation stage of metal sulfide tend to be stable.Ge/Ti and Al/Ti can well reflect the degree of magmatic evolution.In the Shizhuyuan deposit,the concentrations of Ge,Li,Al and Rb increase regularly with the evolution of melt.The precipitation of tungsten and tin is caused by the decrease of fluid temperature and the repeated change of p H value in the fluid.