| The Tiegelongnan ore deposite is the first discovered porphyry-epithermal copper(gold,silver)deposit with Cu resource over 10 million tons.The forming and pretection history of the Tiegelongnan ore deposit is of great significance to enrich the metallogenic theory and guide mineral exploration in blank areas.On the basis of detailed field geological survey,the CL,LA-ICP-MS,SHRIMP-RG,fission track and(U-Th)/He analytical techniques were used to study the Elemental Geochemistry,Geochronology and Low Temperature Thermochronology of zircon and apatie in the Tiegelongnan ore deposit.The evolution characteristics and metallogenic specialization of magma,magmatichydrothermal-thermal history,the forming and protection model of the Tiegelongnan ore deposit as well as the uplift-erosion mechamism were revealed.The main results are as follows:1.The systematic zircon geochemistry was studied on the basis of geological characteristic of ore deposit Two magma events were revealed including granodiorite porphyry,diorite porphyry,quartz-diorite porphyry,andesite porphyry emplaced at 124?119Ma as well as part of granodiorite porphyry and rhyolite porphyry emplaced at 127?124Ma and small amount of granodiorite porphyry and andesite emplaced at 114?110Ma.Zircon geochemistry data show that these two magma have similar geochemistry characteristics and they came from the same magma chamber.Ore-bearing granodiorite and quartz-diorite that evolved from mafic magma crystallization differentiation have high oxidization and high water content.SO2 degassing and re-intrusion/mixing of some S-rich magma under medium-low Fe/S values are the main reasons for high oxidation of magma.2.Through zircon(U-Th)/He,apatite fission track,(U-Th)/He studies,it is revealed that the mining area was affected by low-temperature thermal events from Late Cretaceous to Oligocene and late Miocene,The erosion rate between 98Ma and 47Ma is about 0.086km/my.,and it decreased to 0.039km/m.y.at 47Ma.The denudation thickness is as thick as 6.4km since the Late Cretaceous.The uplift-erosion of the Qiangtang terrane is inhomogeneous,the underplating of basaltic magma is the mechanism of uplift-cooling events in the central part of the Qiangtang terrane,and the detachment of the Ban-Nu subduction plate is the main mechanism of uplift-cooling at the Late Cretaceous.3.Seven cooling stages were divided combining with the age and anealing temperature data of zircon,apatite,biotite and alunite:(1)120Ma,rapid cooling stage;(2)120?117Ma,fast cooling stage;(3)117?100Ma,fast cooling stage;(4)100?80Ma,slow cooling stage;(5)80-45Ma fast cooling stage;(6)45?30Ma,slow cooling stage;(7)since 30Ma,including early slow cooling stage(30-7Ma)and late fast cooling stage(<7Ma).The stages(2)to(3)are magmatic-hydtothermal evolution stages,and stages(4)to(7)are uplift-erosion stages after metallogenesis.4.The forming and protecting model was first built:(1)120Ma,the granodiorite and quartzdiorite that are high crystalized and have high water content and low Fe/S ratio were strong oxidized by the SO2 degassing,the released S formed the porphyry mineralization during the cooling of magmatic-hydrothermal fluids.The potassic alteration was also formed at this stage.(2)The continuous cooling of magma-fluid and/or circulation of magma-fluid and atmospheric precipitation promote epithermal mineralization,sericitization and advanced argillic alteration.(3)The Meriqiecuo Group igneous rocks of 114?110Ma formed and overlapped the ore deposit,the multiple magmatism and orogeny made the deposit undergo a cooling process of 17m.y..The magmatic-hydrothermal evolution ended and uplift-erosion after ore-forming started.(4)100?80Ma,thrust nappe structure caused by collision orogeny of Qiangtang and Lhasa terranes thickened the upper strata of the deposit(5)45?30Ma,30?7Ma,<7Ma,the ore deposit underwent uplift-erosion caused by the collison between India and Eurasia continents.5.The ore indicating of the forming-pretection mechanism was confirmed.Overlying Meriqiecuo Group igneous rocks and the thrust nappe indicate the ore deposit had been pretected;zircons that have high EuN/EuN*(>0.54),(Ce/Nd)/Y(>0.06),Ce/Nd(>25)and low Dy/Yb(<0.14)indicate the ore-forming magma. |
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