Genesis Of Tiegelongnan Super-large Copper (Gold And Silver) Deposit In Tibet,China

The Tiegelongnan ore deposit is located at the central part of the Duolong ore concentration area,about 110 kilometers northwest of the Gaize County,Tibet,China.It was hosted in the Zhapu-Duobuza magmatic arc of the Bangong Co.-Nujiang suture zone and with the Qiangtang terrane to the south,as the first super-large Cu(Au,Ag)deposit with typical high-sulfidation epithermal metallogenic characteristics founded in the Bangong Co-Nujiang metallogenic belt in decades.The amount of the controlled resource of Cu is over 11 million tons(mainly in 332+333 and a few in 334)@Cu>0.53%,associated with more than 120 tons of Au @0.08×10-6 and 2500 tons of Ag@1.80×10-6.Its discovery has broken through the previous understanding of the "porphyry Cu-Au deposit" in the Duolong ore concentration area.Based on the porphyry-epithermal metallogenic system,a creative exploration model has been constructed,which has an extremely important guiding significance in regional prospecting.Concentrating on the Tiegelongnan ore deposit,this paper has summarized geology of the deposit such as the mineralogical characteristics,alteration and mineralization,which enriched the mineralogy of the porphyry-epithermal system.Furthermore,the discussion among the alteration,mineralization and genetic model of the metallogenic system is reliable for prospecting prediction in depth.1)According to detailed geological catalog and the research of lithology and mineralogy,the characteristics of the ore are systematically investigated and the typical mineral assemblages and their structural features of the porphyry mineralization superimposed by epithermal mineralization are identified and confirmed.The composition of minerals and characteristics tpyes of structure and texture mentioned above clearly reflect that the Tiegelongnan mining area is characterized by dual mineralization of both porphyry mineralization and epithermal mineralization.2)Based on the assemblage of non-metallic minerals in the Irongregna mining area,six types of alteration are identified: potassium silicate,hornfelsic,beresitization,argillization,advanced argillization and propylitization,respectively.Because of multi-stage superposition of varies kinds of alteration,and in combination with their space location,the ore deposit can be divided into six alteration belts,from deep to shallow,which are listed as potassium silicate zone?beresitization zone?advanced argillization zone?oxidation leaching zone?barely altered Meiziqiecuo formation volcanic rock(cap rock),with propylitization zone located at the outboard of the ore deposit.3)The order of formation of metal minerals has been clarified,combined with their combination of features,then the mineralization of the times are divided: The forming sequence of metallic minerals indicates that the order of mineralization probably is pyrite ? molybdenite ? hematite ? chalcopyrite,bornite ? tennatite ? enargite?digenite?covellite.The mineralization process of mining area is devided into four periods combined with mineral assemblage characteristics,and they are listed as below,magma period(?)?porphyry mineralization period(?)? epithermal mineralization period(?)?hypergene period(?),among which the porphyry mineralization period(?)can be divided into molybdenite-chalcopyrite-bornite stage(?-1)and bornite-chalcopyrite-hematite stage(?-2),and the epithermal mineralization period(?)can be divided into tennatite-enargite stage(?-1),covellite-digenite minerals stage(?-2)and sphalerite-galena stage(?-3).The oxidation secondary enrichment of ore body barely developed,owing to the ore body was covered by Meiriqiecuo formation volcanic rock of 110 Ma shortly after mineralization and it didn't undergo the denudation or leaching.4)The relationship between alteration and mineralization has been clarified: The study shows that the alteration and mineralization in this mining area can correspond well.That is the mineral assemblage of enargite-digenite-covellite,standing for highextremely high sulfidation state,mainly formed in the advanced argillic alteration zone,partially in the upper part of the propylitic zone.The the mineral assemblage of enargite-tennantite-hematite-bornite,presenting middle-high sulfidation state,mostly formed in the bottom of the advanced argillic alteration zone and the middle-upper of the propylitic zone.And the mineral assemblage of bornite-chalcopyrite-hematite,representing the middle-low sulfidation state,produced in the the propylitic zone and potassic silicatization zone.5)Colusite was identified by the first time in the Tiegelongnan deposit.According to its combination with other ore minerals and alteration minerals,indicating it formed in acidic or slightly neutral,high-low temperature high sulphur state environment.From the shallow to deep of the deposit,with the enargite and tennatite Cu-As-S system minerals content diminishes,until to disappear,colusite has the same evolutionary trend,which implies a transition from high sulfidation epithermal to porphyry mineralization.6)The results of H-O isotope analysis suggest that the fluids related to the porphyry mineralization in the Tiegelongnan ore deposit are mainly derived from magmatic hydrothermal fluids mixed with a small amount of atmospheric precipitation,however the fluids of the epithermal metallogenic stage diverge clearly from the residual magmatic hydrothermal fluids and its output site is closer to the surface which indicated that it is likely that more atmospheric precipitation has been mixed.Both S and Pb isotope composition show that the magma from the depth provided the main ore-forming materials.This kind of magma may originate from the mixture of the molten subducted Bangong-Nujiang ocean slab and the mantle materials,and contaminated by some crustal materials.7)Based on the characteristics of metallogenetic assemblages,altered mineral assemblages and alteration belts in the Tigonglong deposit,the genesis patterns of the deposits were discussed for the first time: From120 Ma to 110 Ma,the southern margin of the Qiangtang rapidly uplifted and the palesurface gradually declined,the copper-containing hot fluid from the deep mixes with the heated atmospheric precipitation would form a dilute solution.These dilute solutions continued to infiltrate downward,flowing through the early formation of porphyry-type ore body,it would dissolve a large number of porphyry copper-bearing minerals,thereby generating a lower temperature,acidic,copper and sulfur-bearing ore-forming fluids.When it comed into contacting with the surrounding rock and undergoes boiling and cooling processes,new wall rock alteration(advanced argillization consisted of quartz,pyrophyllite,alunite,dickite,kaolinite and other acid-strongly acidic altered minerals)and ore minerals(high sulfidation to ultra-high sulfidation mineral assemblages composed of enargite-digenite-covellite etc.)superimposed on the early formed porphyry ore bodies(chalcopyrite-bornite and other low-sulfur sulfide minerals)above,making the copper grade increased,resulting in high-grade thick superposition of ore body,and to form a complex deposits of alteration,mineral assemblage and ore composition,but also inherited and retained the characteristics of porphyry deposits.In addition,volcanic eruptions around 110 Ma overlie the ore body,effectively protecting the ore body from further leaching and denudation,thus preserving the current status of the Tiegulongnan deposit.Above all,the Tiegelongnan super-large Cu(Au,Ag)deposit is defined as a kind of porphyry-high-sulfidation epithermal deposit and a genetic model of the deposit has been established based on the Tiegelongnan ore deposit.Combined with the existing exploration information and the inversion of the audio frequency magnetotellurics(AMT),the porphyry orebody in the depth of the Tiegelongnan deposit is more likely to be bigger than now.