Geochemical Study On Typical Skarn Profile In Tongshan,Ningzhen

Ningzhen ore district is located in the easternmost section of the polymetallic metallogenic belt in the middle and lower reaches of the Yangtze River.The Tongshan Cu-Mo deposits is located in the northeast of the Ningzhen ore district,developed a good skarn profile,Previous studies have focused on skarn deposits,but there is a lack of simple skarn profile to study the physical and chemical process of skarnization.Therefore,this paper will introduce the migration behavior of elements in the process of skarnization through detailed mineral geochemistry of typical skarn profiles developed in Tongshan deposit,and finely describe the formation process of skarn combined with whole rock geochemistry and mineral geochemistry,through the distribution characteristics of the elements in the whole rock,garnet and pyroxene to determine the skarn boundary,guide the search and development of skarn deposits.The typical skarn profile developed in Tongshan district has obvious zoning characteristics,including granodiorite belt,endoskarn belt,extroskarn belt,sulfide belt and marble belt.A large number of garnets and pyroxenes are developed in endoskarn and extroskarn.Garnets are mainly divided into two categories:andradite and grossular.The andradite(And_23-56Gro_42-74)are developed in the endoskarn,and crystallizes in the weak reduction and open system environment through infiltration.It has obvious zonal characteristics,showing HREE-enriched and LREE depleted patterns.,the Eu anomaly changes greatly,δEu=0.42–3.95,with an average of 1.43.Grossular(And_55-99Gro_1-43)is developed in the external skarn and formed under the condition of weak oxidation-oxidation.In the early stage,andradite was formed in the closed system,showing a homogeneous state.With the opening of the system and the increase of water-rock ratio,the total amount of rare earths in garnet increases,In the later stage,the system is gradually closed,a lot of oxygen was consumed,resulted in the Al-riched rims of andradite.All andradite show LREE depleted and HREE riched patterns,strong positive Eu anomalies,δEu=0.36-46.83,and the average value is 7.82.The Al3⁺and REE³⁺of garnet in the extroskarn are positively correlated,indicating that the rare earth elements entering garnet follow the YAG substitution mechanism.The REE characteristics of garnet in endoskarn skarn are mainly controlled by fluid and petrochemistry.Pyroxenes are widely distributed throughout the whole stage of skarn development.Pyroxenes from endoskarn(Di_69-79Hd_19-29)are Fe-riched and belong to diopside,compared with those from extroskarn(Di_77-92Hd_7-21).From endoskarn to extroskarn,the contents of Co,Ni,Mn and Fe in pyroxene showed an overall upward trend,while the contents of Si、Mg、Ca show an opposite phenomenon.According to the whole rock date,the contents of Na、Al、K、Ti、Sr shows an obvious decresing trend,while V、Ni、Fe、Mg、Zn shows an increasing trend from endoskarn to extroskarn.It is found that Al and Sr elements in the whole rock data,Ti,Zr,Ni elements in garnet,and Co,Ni,Mn elements in pyroxene have obvious different distribution characteristics in endoskarn and extroskarn,which can be used to distinguish the type of skarn.So as to limit the boundary of rock mass and surrounding rock in the process of skarnization.