Geological And Geochemistrical Characteristics Of Zhaxikang Antimony Polymetallic Deposit, Southern Tibet

Zhaxikang antimony polymetallic ore deposit is the only super large deposit in Northern Himalayan Metallogenic Belt (NHMB), the total amount of resources Pb+ Zn=126.8million tons,,Sb=13.8 million tons, Ag=1800 tons,Au=3.9 tons,Ga=361 tons,and the Mn and Fe ores will be more than 1000 million tons. Many researcheres done a lot of work on the genesis of this deposit, but there are still a lot of controversy now. This paper will discussing the genetic types of Zhaxikang deposits from geological features, distribution of elements and geochemistry aspects.Zhaxikang deposit occurrences in low metamorphic Marine facies clastic rock, which interbebed with tuff recurrently in drilling hole. In the mining area, basic-felsic magma rocks are developed. The ore body is controlled by S-N high-angle normal faults and westward. The main ore minerals are sphalerite, galena, ferromanganese carbonates, antimony-lead sulfosalts, stibnite, and the main gangue mineral are quartz, calcite, mica. Based on the intersecting relationship between different veins and the mineral sequence, the metallogenic process can be divided into two Metallogenic, and the early exhalative-sedimentation period can be subdivided into five stages(ferromanganese carbonate-arsenopyrite-pyrite stage, brown-black fine grained sphalerite-ferromanganese carbonate-pyrite±fine grained galena stage, brown medium-coarse grained sphalerite-ferromanganese carbonate-pyrite-arsenopyrite stages, black coarse grained sphalerite-ferromanganese carbonate-pyrite-galena stages, galena-antimony and lead sulfosalts-pyriteiquartz stages); the late thermal spring superposed wih transform process period can subdivide into three stages(sphalerite-quartz-calcite stages, stibnite-quartz stages, quartz±alcite veins stages).Early ores and Ferromanganese carbonates have SEDEX characteristicals with low REE, LREE enrichment, positive Eu anomalie. The Cd(250-2300×10-6)contents, Ga/In> 1, Zn/Cd(mean=346) and Cd-In diagram of sphalerite in early ores is similar to SEDEX deposits, and markedly different the sphalerite(may be recrystallization) in the late ores.The ?13CPDB content of carbonates shows ferromanganese carbonates (?13CPDB=-7.2?-6.3‰) may be formed by exhalative-sedimentation, and the calcite (?13CPDB=-5.7?-0.5‰)may be formed by hot Springs.By the evidence of S isotope, the sulphur related with early Pb-Zn mineralization(834S=7.7%o-12%o) may be origin from sea water, while, the sulphur related with late stibnite mineralization are likely from magma. Pb isotope suggest the metallogenic material come from upper crust, H-O isotope show that ore-forming fluid related with the late ores(8Dv-SMOW=-142--162%o,?18OH2O‰=-1.9?-12.9‰) similar to geothermal water in Tibet, but ore-forming fluid related with the early ores may be from hot brine.The He-Ar isotope of sphalerite, ferromanganese carbonates and pyrite reveal theore-forming fluid to early Pb-Zn mineralization is from crust, however, the late period of mineralizaton ore-forming fluid has more mantle fluid and geothermal water. The 830Si content of geyserites indicate the late fluid have the properties of hot spring, is similar to the quartz veins formed by hot spring in Yunnan Tengchong.After the comprehensive study on Zhaxikang deposit,we draw conclusion it at least exist twice mineralization process.Late Jurassic-Early Cretaceous(147.2-150Ma), The Northern Himalayan Metallogenic Belt in the passive margin, and rift zone asthenosphere upwelling (plume) and mixed with the lithospheric mantle make a certain amount of volcanic magma intermittent eruptions on the seafloor because of tension decompression melting occurred in the context.This volcanic magma produce thermal anomaly, and drive the rift basins hot brine along with the students break up, finally exhalative-sedimentation form Zn-Pb mineralization. Miocene, in post-collisional extensional settings, numerous of leucogranites and acidic dykes along the dome center and EW-trending fault zone intrusion, which close with the Zhaxikang deposit in time and space, and possibly communicate the deep ore source, become mineralized carrier. Totally, Zhaxikang depospit should be classified as the SEDEX type and hot spring transform type.