Alteration And Mineralization Of Tiegelongnan Giant Cu(Au,Ag) Deposit In Tibet

Tiegelongnan,an important Porphyry-High Sulfidation epithermal Cu(Au,Ag)deposit located in the west of Bangong Co-Nujiang metallogenic belt,is the first giant one with over 10 million tons Cu metal in Tibet.It's very significant for improving the regional metallogenic theory and exploration to reveal characteristics of alteration,mineralization,diagenesis and ore-forming process of the Tiegelongnan deposit.Based on the detailed field geological survey,drilling logging and microscopy,two kinds of mineralization were identified,and the early porphyry mineralization was overlapped by the later epithermal mineralization.Veinlet,vein and disseminated pyrite,chalcopyrite,bornite and minor molybdenite,etc.were the typical mineralization in the deep and outer.Potassic,prophylitic,and phyllic were the main alterations and A,B,D types of vein occurred during the porphyry mineralization.Epithermal mineralization is mainly produced in the shallow,superimposed the porphyry mineralization with disseminated-vein pyrite,enargite,bomite,covellite,digenite,spionkopite,yarrowite,djurleite and other Cu-S series minerals.Advanced argillic is typical alteration with widely developed N type veins(kaolinite or aluminum sulfide veins).The diagenesis and mineralization of Tiegelongnan deposit can be subdivided into:diagenetic stage(I),magmatic-hydrothermal stage(?)and supergene stage(?),according to characteristics of alteration and mineralization,the crosscutting relationship of veins.The diorite porphyry intruded early(123 Ma),which represents the upper limit of magmatic activity.The granodiorite porphyry(122Ma-120 Ma)was the main ore-bearing intrusion,with the closest relationship with mineralization.These volcanic rocks,andesite or dacite,erupted late(111 Ma)and covered the surface,as the products of magmatism post mineralization.At the same time,these ore-bearing porphyries had the characteristics of arc magma,and their source is related to partial melting of mantle wedge metasomatized by the north subduction slab-derived fluids.The rollback of north subduction Bannu ocean slab is the main dynamical regime for the deep magma formation and emplacement.Geochronology showed that the 40Ar-39Ar ages of potassic biotite and phyllic sericite were 121.1±0.5 Ma and 120.8±0.9 Ma respectively,coincided with Re-Os ages(121.2±1.2 Ma)of molybdenite during porphyry mineralization.40Ar-39Ar age of advanced argillic alunite is 117.9±1.6 Ma,coincided with the Rb-Sr age(117.5±1.8 Ma)of pyrite from epithermal mineralization.The porphyry mineralization is dominated by magmatic hydrothermal fluids,and the homogenization temperature is about 340?.The ore-forming fluids of epithermal mineralization are mixed fluids between magmatic hydrothermal fluids and groundwater,with homogenization temperature 300?.So,temperature drop,phase separation and fluid mixing may be the key factors for metal precipitation during ore-forming process.In addition,sulfur of mineralization is mainly derived from deep mantle-derived magmatic sulfur.Lead mainly originates from mixed source between crust and mantle in the subduction orogen.Both the helium-argon isotope composition of pyrite and the high Re concentration of molybdenite are convinced the addition of mantle-derived components for the ore-forming materials.In situ analysis of pyrite by LA-ICP-MS showed that pyrites from porphyry mineralization with simple texture,are relatively enriched cobalt,nickel and selenium,and pyrites from HS-epithermal mineralization with zoned texture commonly,are relatively enriched in copper,arsenic,gold,silver,lead,antimony,bismuth and other elements.The differences in texture and elements composition of them can be used as an important indicator for the exploration of porphyry-epithermal system.In summary,the ore-forming process of the Tiegelongnan giant deposit can be divided into three stages:A)Porphyry mineralization(123-119Ma):diorite and granodiorite porphyries intruded successively,and formed the porphyry alterations and mineralization.B)Epithermal mineralization(118?117 Ma):magmatic vapour and hydrothermal fluid mixed with groundwater,superimposed and reformed the early porphyry alterations and mineralization,and formed the advanced argillic alteration and high-sulfidation Cu polymetallic mineralization widely.C)Volcanic rocks cover and protection(111?110Ma):the magmatism was actived again after long-time erosion of porphyry-epithermal ore-body(about 7 Ma).Large-sized andesite and dacite erupted from the deep magma and covered the surface to protect the ore-body from the following strong denudation.