Study On The Geological Characteristics And Enrichment Regularities Of Mineralization Of Nancuo Gold Polymetallic Deposit In Amdo Area,Tibet

Nancuo Gold polymetallic deposit is located in Amdo area of Tibet.The tectonic position is located in eastern part of Bangongco-Nujiang copper metallogenic belt which of the north and south sides are Lhasa plots and Qiangtang plots.This region has experienced a series of tectonic events from Mid Triassic to Late Cretaceous,such as formation,subduction,collision and final closure of the Bangongco–Nujiang ocean.The characters of Nancuo Gold polymetallic deposit such as Metallogenic site,geological conditions,alteration characteristics and metallogenic times are similar to YuLong mineralization area which is in the western of Bangongco-Nujiang copper metallogenic belt.This metallogenic geological conditions lead to large metallogenic and ore-prospecting potential in this area.The exposed strata in the area are the Mugagangri zoning of Qiang Nan-BaoShan area in Yunnan and Tibet district.Most of the exposed strata are the Upper Jurassic Sha Muluo Group calcareous fine sandstone,sandstone and mudstone,followed by the Quaternary of Holocene alluvial materials.The structure of this area is dominated by faults and folds.The faults mainly include the east-west structure in the north of the mining area,the north-east structure in the south of the mining area,the east-west thrust fault and the circular ore-bearing fault zone around the ore body.The north-South Fault is in the middle of the mining area,which dislocate the ore body with many paragraphs in the north,control of the modern valley in the south.It is the main ore-breaking structure.The fold structure is mainly distributed in the Middle East of the mining area,which is identical with ore body and ore-bearing tectonic alteration belts,the fold axis is mainly about 60 degrees.Magmatic rocks are mainly granodiorite,fine grained granodiorite porphyry,gray quartzdiorite.Zircon dating results demonstrate that granodiorite and granodiorite porphyry were formed in Middle-Late Jurassic(159.2±0.69 Ma and 161±1.5 Ma)are 161.5 ma ± 1.5Ma and 159.2 ma ± 0.69 Ma.The rocks have features similar to O-type adakitic rock with high Sr,low Y and Yb,Na enrichment,minor positive Eu anomalies,indicated the typical characteristics of arc magmatite.that suggest a mixing source from partial melting of oceanic crust and depleted mantle.Combined with the Mesozoic geodynamic evolution of the Bangongco—Nujiang suture zone,the rocks were formed Bangongco–Nujiang oceanic crust subducted northward active continental margin setting.There are four ore blocks in the mining area named-north I,north II,south I,south II block.It is a mesothermal deposit in the three of north I,north II,south II blocks.Mineral veins are "thin and rich",mainly controlled by fault structures and rock.The ore body in south II block is controlled by the north-west ore-carrying structure and east-west ore-depositing structures.Ore minerals are mainly arsenopyrite,pyrite,limonite,subordinately chalcopyrite and malachite.The types of alteration are sericitization,carbonatation,chloritization and silicification.Ore structure contains vein structure and disseminated structure.The study of the fluid inclusions shows that from early to late,the temperature,salinity,density decreased.The ore-forming fluid in the main mineralization stage is characterized by middle-high temperature and middle-high salinities.The ore-forming pressure can be calculated to almost 3 to 36 MPa,the ore-forming depth ranges from 0.3 to 4.6km.?18OH2O of quartz test value is 8.8‰~11.0‰,?DH2O of hydrothermal test value is-107.4‰~-117.3‰,the results show that ore-forming fluid is mainly from magma water,minor meteoric water.It is a skarn deposit in the south I blocks.The ore body appears veined and lenticular shapes,mainly controlled by interlayer fissure and contact zone.The types of alteration are skarnization,sericitization,carbonatation,chloritization and silicification.Ore minerals are mainly chalcopyrite,bornite,digenite,electrum,sphalerite,limonite and malachite.The ore can be classified into three types,that is,massive ore,breccia ore and disseminated ore.The study of the fluid inclusions shows that from early to late,the temperature,salinity,density decreased.The ore-forming fluid in garnet is characterized by high temperature and high salinities.The temperature is 258.3 ?~432.3?.The ore-forming fluid in calcite vein is characterized by middle-low temperature and middle-low salinities.The temperature is 186.2?~352.6?.The ore-forming pressure can be calculated to almost 6 to 33 MPa,the ore-forming depth ranges from 3.37 to 4.99 km.The deposits in north and south region have similar tectonic backgrounds,geological characteristics and fluid evolution process.The mineralization characteristics and genetic types are different,but these blocks should belong to the same metallogenic convergence,the same geochemical field,the same fluid field.Because different ore-controlling factors in different position result in different mineralization characteristics and genetic type,specific circumstance:1: It is a skarn deposit in the south I blocks because of carbonate strata and agenesis fault;2: It is a mesothermal deposit in the three of north I,north II,south II blocks because of developmental fault.Mineralization enrichment studies found that the ore-body is controlled by structure and magmatic rock which provide heat and ore-forming fluid.The distribution of ore-forming fluid is controlled by north-west ore-carrying structure and east-west ore-depositing structures.It is proposed that east-west ore-depositing structures are the main research direction in the future.