Study On Antimony Metallogenesis And Prospecting Prediction Of Yangla Copper-Polymetallic Deposit In Northwestern Yunnan,China

The Jinshajiang metallogenic belt in the“Sanjiang”region of Southwest China is famous for its enriched the Cu,Pb,and Zn minerals resources,but no antimony ore deposit has been reported so far.Yangla Cu-polymetallic deposit is located in the central part of Jinshajiang metallogenic belt.It is the largest and most typical Cu-polymetallic deposit in Jinshajiang metallogenic belt.Recently,new independently produced high-grade antimony orebodies have been discovered in the depth of Yangla deposit,which is the first discovery in Jinshajiang metallogenic belt.The antimony orebodies mineralization age,mineralization,and genetic relationship with the Cu orebodies have become urgent scientific problems to be solved.On the basis of National Science Foundation of China and cooperation project between the university and enterprise,this paper takes the newly discovered antimony orebodies as the object in the Yangla deposit.In this paper,the research techniques and methods,such as ore field structure analysis,in situ LA-ICP-MS trace elements,fluid inclusions,Sm-Nd isotopes of calcite,U-Pb isotopes of scheelite and minerals H-O,C-O,Sr,S,Pb isotopes and tectono-geochemistry are adopted.The metallogenic process,mineralization age and genetic mechanism of antimony orebodies are studied,and the genetic relationship between antimony orebodies and copper orebodies has been discussed.The copper-antimony metallogenic model of Yangla deposit has been established,which can guide the antimony ore prospecting and prediction of Yangla deposit.This study can not only enrich and improve the metallogenic theory of Yangla deposit,but also has important theoretical significance for the study of polymetallic mineralization and its evolution in the Jinshajiang metallogenic belt.The following achievements and understandings have been achieved:（ⅰ）The occurrence of newly discovered antimony orebodies are shown in NE30～50°∠38～55°NW.It is mainly distributed within NE-trending fracture zone and presented irregular veined,lenticular,saccular and beadlike.The ore-bearing lithology is marble of the second member of Linong Formation,followed by sandy slate.The main ore minerals are stibnite,followed by valentinite,pyrite,limonite,and romeite.The gangue minerals are mainly composed of calcite and quartz.（ⅱ）Antimony mineralization periods（Ⅰ）and（Ⅱ）stibnite have similar trace elements compositions,both of which are relatively enriched in Cu,As,Se,Ag,Sn,Pb and deficient in Zn,Ge,and W.Trace elements（i.e.,As As,Cu,and Pb）occur within stibnite mainly dominated by the form of isomorphism.The Fe,Ag and Mn mainly occur in the form of isomorphism and（or）microscopic minerals inclusions within stibnite.The Tl,Se,Zn,W and Sn mainly occur in the form of microscopic minerals inclusions within stibnite.PyriteⅠ,Ⅱ,andⅢin antimony ores all belong to hydrothermal genesis and are obviously enriched in antimony.The Sb mainly occur in the form of isomorphism and microscopic minerals inclusions within pyrite.（ⅲ）The stibnite,quartz,and calcite of Yangla antimony ores all have fluid inclusions.The homogenization temperature of fluid inclusions in stibnite is 148～191℃and the salinity is 1.74～4.96 wt.%Na Cl_eq.The homogenization temperature of fluid inclusions in quartz is 127～245℃,and the salinity is 1.74～14.04 wt.%Na Cl_eq.The homogenization temperature of fluid inclusions in calcite is 108～215℃,and the salinity is 2.57～11.46 wt.%Na Cl_eq.The antimony ore-forming fluid is present the low temperature-salinity-density,which is a simple weakly acidic Na Cl-H₂O±CO₂ system.The cooling and non-isothermal mixing actions of the ore-forming fluid may be the main metallogenic mechanism of antimony.（ⅳ）Theδ¹³C_PDB andδ¹⁸O_SMOW of antimony ore-related calcite are-4.53～-2.33‰and 14.98～16.30‰,respectively,with the initial(⁸⁷Sr/⁸⁶Sr)_t ratios ranging from0.719720 to 0.722080.TheδD_SMOW andδ¹⁸O_H2O values of quartz in the antimony mineralization period are-136.90～-123.10‰and 5.39～6.91‰,respectively.The isotopic compositions of C-O,Sr,and H-O in gangue minerals indicate that the antimony ore-forming fluid mainly originates from the dissolution of marine carbonate rocks（formation water）in the Yangla mining district,and there may be partial mixed magmatic components（magmatic water）.The S and Pb isotopic compositions of stibnite and pyrite indicate that the antimony metallogenic materials may be mainly derived from Devonian strata and ascribed to upper crustal materials in the Yangla mining district.（ⅴ）The Sm-Nd isochron age of the antimony ore-related calcite is 154.9±7.6 Ma,and in-situ U-Pb isotope ages of scheelite are 30.6±1.7 Ma and 30.3±2.4 Ma.These results indicate that there are two mineralization periods events of antimony orebodies（i.e.,Yanshanian and Himalayan）,which are obviously later than the granodiorite and Cu orebodies formation ages of Yangla deposit.Antimony orebodies of Yangla deposit belongs to low temperature hydrothermal genesis,and has no direct genetic relationship with Cu orebodies.The antimony and copper orebodies are the products of mineralization at different periods.The Yangla deposit mainly experienced three periods of mineralization events,such as skarn-porphyry Cu in Indochina,hydrothermal Sb in Yanshanian,and hydrothermal W-Sb in Himalaya.（ⅵ）The NE-trending faults(i.e.,F_j17 and F_j13)in Yangla deposit have undergone the transition from early extensional to late compressive shear,which is the ore-controlling structure of antimony orebodies and controls the spatial location and occurrence of antimony orebodies.The study of ore-field structure shows that Yangla deposit mainly experienced multiple tectonic movements such as Hercynian,Indosinian,Yanshanian and early-late Himalayan,and there are three ore-field structural system:NS,NE and NW structure belt.The antimony orebodies were formed later the NE-trending fracture zones,which indicates that the antimony orebodies may have been formed in Yanshanian-Himalayan period.（ⅶ）There are metallogenic elements anomalies of low temperature（i.e.,Sb,As,Hg,Mn）,medium temperature（i.e.,Cd,Zn,Pb,Sn,In,and Ag）and high temperature（i.e.,Bi,Te,Cu,and W）in the underground tunnel of 3,150 m section,which are obviously controlled by the NE-trending fracture zones.Three key prospecting targets areas can be delineated and optimized,indicating that there are great prospecting potentials of antimony in the depth of Yangla deposit.This study considers that antimony is a low-temperature hydrothermal deposit,controlled by fracture zone,and its genesis is related to Devonian marble and Himalaya concealed granitoid pluton,which based on the above-mentioned discussion on the antimony mineralization and prospecting prediction of Yangla Cu-polymetallic deposit.It is clear that the enrichment and mineralization of antimony requires the participation of ore-bearing wall rocks and its internal extension fracture zone and magmatic thermal driven activities.