Geochemical Characteristics And Prospecting Significance Of Laochang Sn-Cu Polymetallic Deposit In Gejiu District,Yunnan Province

The Gejiu tin-copper polymetallic deposit in Yunnan is one of the super-large tin deposits in China and even in the world,with a long history of mining.Although the Gejiu district is rich in tin and copper resources,it is still faced with the problems of shortage of resources and insufficient subsequent reserves.So,it is necessary to increase the investment in prospecting,comprehensively improve the level of prospecting and prediction in the deep and peripheral parts of the Gejiu district,and deeply study the ore-forming geological background,metallogenic mechanism,mineralization characteristics and the enrichment regularity of the Sn and Cu.It is of great economic and practical significance to increase the guarantee of reserve resources.Laochang deposit is one of the five major deposits in Gejiu district,and there are many mineralization types in the area.According to the distance between orebody and granite,it can be divided into altered rock type Sn-Cu deposit,greisen type Sn deposit,skarn type Sn-Cu（Cu）deposit,cassiterite-sulfide deposit,basalt type Cu deposit,interlayer Pb-Zn deposit,vein type deposit and interlayer oxidation deposit.There is a lack of clear research on the ore-forming materials and fluids sources,genesis and their relationships between different mineralization types.So,in this paper,on the basis of detailed field geological survey,chronology,trace elements and isotope geochemistry analysis are used to study the micro-area geochemistry of ore minerals and gangue minerals in orebodies of different mineralization types.Combined with the research results of predecessors,the ore-forming physical and chemical conditions,ore-forming fluid and material sources and ore-forming age of the Laochang deposit are analyzed in order to determine the genetic type of the deposit and provide a direction for further prospecting.The main understandings of this thesis are as follows:（1）The cassiterite U-Pb ages of two altered-rock type tin mineralization samples and one tourmaline vein type mineralization sample collected from Xi’ao in Laochang deposit are 83.3±2.1 Ma,84.9±1.7 Ma and 84.0±5.6 Ma,respectively,which are consistent with the diagenetic age of the Laoka granite（85 Ma）.Theδ¹⁸O values of cassiterite samples range from 3.9‰to 4.7‰,and theδ¹⁸O_H2Ovalues of the corresponding ore-forming fluids vary little,ranging from 7.16‰to 8.25‰,which is consistent with theδ¹⁸O_H2Oof magmatic water of Laoka granite and is obviously lower than that of carbonate rocks of Gejiu formation,indicating that the ore-forming fluids come from magma.Cassiterite is rich in high field strength elements such as Nb,Ta and Ti,indicating that cassiterite was formed in an environment greatly influenced by magmatic hydrothermal fluid.Therefore,it is considered that the altered-rock type tin mineralization and tourmaline vein type tin mineralization in Xi’ao are magmatic hydrothermal genesis.（2）Tourmaline in Laochang deposit can be divided into three types:tourmaline veins in granite,mineralized tourmaline veins and tourmaline veins in strata.In the ternary classification diagram,they mainly fall in the region of alkali group.The Fe/（Fe+Mg）and Na/（Na+Ca）values of tourmalines varies from 0.122 to 1.000 and 0 to0.653,respectively.From early tourmaline in granite to late tourmaline in strata,there is a transformation from schorl to dravite,which reflects the change of fluid composition,which indicating that the farther away from the granite,tourmaline is the less affected by Fe-rich magmatic hydrothermal fluid,but more affected by Ca-rich and Mg-rich carbonate rocks.Theδ¹¹B values of tourmalines with different types range from-15.6‰to-14.8‰,indicating that the B element has a single source.The calculated range of B isotope composition in the fluid during tourmaline crystallization is-13.1‰～-9.5‰,indicating that the ore-forming fluid comes from granitic magmatic hydrothermal fluid.The value ofδ¹¹B in tourmaline fluid during tourmaline crystallization is larger than that of the other two,which shows that the value ofδ¹¹B in the fluid increases due to the water-rock reaction between the hydrothermal fluid and the strata in the process of migration.To sum up,the author thinks that the tourmaline in the Fe-rich granite and the tourmaline in the ore-bearing tourmaline vein are formed in the magmatic hydrothermal fluid which differentiated from the Laoka granite,while the tourmaline in the strata is formed in the fluid formed by the interaction between magmatic hydrothermal fluid and carbonate rocks.（3）Large-scale skarn Cu mineralization has been found in the contact zone between Laoka granite and carbonate rocks in the Laochang deposit.The ore is only Cu mineralized,and the tin grade is low.Trace element data show that chalcopyrite in skarn copper deposit is enrich in Zn,Ag,In and Sn,and is depleted in Co,Ni,Ga,Ge,As,Cd,Sb,Bi and Pb.The sphalerite is enriched in Fe,Co,Cu,Ag,Cd,In and Bi,and depleted in Ni,Ga,Ge,As,Sn,Sb,Tl and Pb.The sulfur isotopes of sulfides in skarn ores range from-2.9‰to+1.4‰,which is narrow and concentrated around 0‰,indicating a magmatic source.The Fe/Zn ratio of sphalerite is 0.09～0.16,and the crystallization temperature of sphalerite is from 298°C to 350°C.The high Sn and medium Cd/Zn ratios in chalcopyrite and the high In and Zn/Cd ratios in sphalerite indicate that the skarn copper mineralization in Laochang deposit was formed in high temperature magmatic hydrothermal fluid.Therefore,skarn mineralization is formed by the interaction between high temperature granitic magmatic hydrothermal fluid and carbonate rocks of Gejiu formation,and skarnization occurs in the contact zone between rock bodies and carbonate rocks.（4）In the Laochang deposit,not only the amount of tin metal has reached a large scale,but also that of copper and lead metals has reached a medium scale.The trace element concentrations and sulfur and lead isotope compositions of sulfides from the newly discovered Laochang Pb-Zn ore were analyzed.The results show that sphalerite is enrich in Fe,Cu,Ga,Ag,In and Sn,depleted in Co,Ni,Ge and As,and moderate in Mn and Cd.The LA-ICP-MS ablation profiles of sulfides show that Fe,Cd,Mn,Ga and In elements in sphalerite exist in the form of solid solution and/or lattice replacement,while Ag,Cu,Pb,Sb and Sn elements exist in the form of mineral inclusions and solid solutions.Pyrite is enrich in Ag,Sb,Pb and As and poor in Co,Ni,Se,Ti and Mn.The LA-ICP-MS ablation profiles show that Co,Ni and As elements enter the pyrite lattice in the form of lattice replacement,while Sn,Ag,Pb and Sb exist in the form of solid solutions and mineral inclusions.The crystallization temperature calculated by Fe/Zn in sphalerite is 299°C～344°C and the GGIMFis temperature of sphalerite is 268°C～312°C,indicating that the Laochang Pb-Zn deposit was formed in a medium-high temperature environment.Sphalerite is rich in Ga and In and poor in Ge,indicating that it is formed in magmatic hydrothermal fluid.Compared with the Laochang skarn copper deposit,the trace elements of sphalerite（Ga,In,Co）and pyrite（Co,Ni,Co/Ni）indicate that the Pb-Zn deposit is of magmatic hydrothermal origin.The sulfur isotopes of sulfides in the Laochang Pb-Zn deposit range from-1.2‰to+3.7‰,indicating that S is mainly derived from Laoka granite.The ²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb values of galena are 18.400～18.447,15.656～15.697 and38.478～38.968,respectively,indicating that the ore-forming metals are mainly from the upper crust.Therefore,the author thinks that the Pb-Zn mineralization is the magmatic hydrothermal genesis and the distal mineralization in the Laochang granite metallogenic system.（5）Through the study of different types of tin mineralization,skarn Cu mineralization,Pb-Zn mineralization and tourmaline related to Sn mineralization in Laochang deposit,the author thinks that the polymetallic mineralization in Gejiu Laochang is magmatic genesis,and the ore-forming material and fluid are derived from Laoka granite.During the upward migration of ore-forming fluid,there is a water-rock reaction with carbonate rocks,resulting in changes in the composition and physical and chemical properties of ore-forming fluid,which facilitates the formation of ore bodies.Therefore,it is considered that the ore-forming process of the Laochang deposit is as follows:during the Yanshanian magmatic activity,the hydrothermal fluid separated from granitic magma carried a large number of ore-forming elements（Sn,Cu,W,etc.）.Different mineralization types of ore deposits were formed when ore-forming fluids reacted with the surrounding rocks.And in the later stage of magmatic hydrothermal transport,atmospheric precipitation was involved in the formation of heat flow cycle,extracted materials from strata,and further selected precipitation according to the different physical and chemical properties of each element.Therefore,different mineralization types have relevance and inheritance in time and space.The mineralization takes granite as the core and is controlled by rock mass shape,fault,fracture zone,surrounding rock and other factors.（6）The altered rock type Sn-Cu deposit should be searched around the Laoka granite.And the favorable prospecting positions are the steep and gentle alternation of the granite,the basin and trough depression at the top of the granite,the intersection of branches,dikes,tongues and the main rock mass,and the intersection of structure and rock mass before and during mineralization.On the other hand,the favorable ore-bearing sites of skarn deposits are the slopes and grooves at the top of the rock mass,and the intersecting parts of the upper flanks of the rock mass and the strata,interbedding zones or different lithologic abrupt interfaces developed by the interlayer structure.The Pb-Zn deposit is controlled by the stratigraphic fracture zone and should be prospected in the periphery of tin and copper mineralization.