The Metallogenic Significance Of Multistage Tectonic-Magmatic Themorchronology Of Yemaquan Iron-Zinc Deposit,Qinghai

Yemaquan iron-zinc deposit locates in Qimantage district,west of Qinghai.There are multistage tectonic-magmatic activities in this district.Early Devonian rock mass is constitute of granodiorite and monzonitic granite,which emplaces in the east part of Yemaquan deposit as the form of batholith.Late Triassic rock mass is constitute of monzodiorite,quartz diorite,monzonitic granite ect.,which emplaces in the west part of Yemaquan deposit as the form of stocks or apophyses.Yemaquan deposit was adopted as the subject in this study.Taking LA-ICP-MS zircon U-Pb dating,40Ar-39Ar isotope test,major and trace element test,rare earth element test methods to acquire the rock-forming age,metallogenic age and geochemical datum of major rock mass in this deposit.The temperature-time(T-t)evolutionary trajectory of rock mass in Late Triassic and the relationship of mineralization were discussed based on the theory of tectonic-magma-thermochronology.The study achieved following results and cognization.(1)Yemaquan Middle-Late Triassic rock masses belonged to high cal-alkaline series.Monzodiorite had I-A characteristics of complex rock mass,which occurred at the stage of post-collision or volcanic arc region in structural mapping.The two rock masses were formed in post-collision stage according to the regional background.The magma was formed from the melting course of part of lower crust,with the probability of mixing with mantle materials,and had the obvious characteristics of separation and crystallization in the process of evolution.The magma activities had relationship with large-scale underplating occurred in East Kunlun region in Late Triassic.Early Devonian rock masses were dispersed in structural mapping,and they had obvious characteristic of I type granite.Most of them were in volcanic arc region.They formed in volcanic arc environment according to characteristics of regional geological evolution.(2)The zircon U-Pb isochronal age of monzonitic granite from Yemaquan deposit was(229.4±2.5)Ma(MSWD=1.3).The zircon U-Pb isochronal age of monzodiorite was(226.5±3.6)Ma(MSWD=2.6).The 40Ar-39Ar plateau age of biotite was(219.5±5)Ma(MSWD=0.97).The 40Ar-39Ar plateau age of plagioclase was(216.5±1.3)Ma(MSWD=1.13).All of the above stock masses were formed in Late Triassic.The age from old to young were as followed:zircon U-Pb isochronal age of monzodiorite,40Ar-39Ar plateau age of biotite and 40Ar-39Ar plateau age of plagioclase.Therefore,the ages occurred according with the sequence of typical mineral forming temperature.The zircon U-Pb age of granodiorite was(402.8±5.4)Ma(MSWD=2.0).The 40Ar-39Ar dating for the plagioclase from granodioritewas failed to yield a meaningful plateau age.The age of sample YMQ507-1 phlogopite was(222.0±1.3)Ma(MSWD=0.41),which indicated the western ore belt of Yemaquan iron-zinc deposit was occurred in Late Triassic.(3)The closure temperatures of sample YMQ027's plagioclase,biotite and sample's YMZK507-1 phlogopite were calculated in this study.According to the calculation of closure temperature,the closure temperature of the plagioclase and biotite from sample YMQ027 were 300??350? and 350??400? under the condition of 20?/Ma cooling rate,and the closure temperature of sample YMZK507-1's phlogopite was 450?-500?.The feasibility of multi diffusion domain(MDD)model of sample YMZK507-1 was also discussed.The result shew that under the assumption of three diffusion domains,the simulated plateau age were consistent with the gas release model of 39Ar,but different plateau ages yielded different T-t path with large variance.(4)The invasion heat effect of rock masse was carried out in this study.The result shew a rock stock with radium of 2km could heat the surrounding area about 2km out up to about 200?,but the time of duration was short,which could sustaine about 0.1 Ma.After 1Ma period,the rock stock could only retain itself with the temperature of about 130?.A simulation for Ar headted diffusion had been carried out.It shew a period of 20Ma beginning from 240 Ma with a surrounding temperature higher than 230??250? could reproduce the meaning plateau age like sample YMQ020 plagioclase.The potential distribution space of T-t trace of sample YMQ027 were studied by combing the zircon U-Pb age,the 40Ar-39Ar plateau age of biotite and plagioclase.The result of simulation yielded a about 16Ma duration to achieve the environment temperature by the highest cooling rate,which was inconsistent with the result of another stimulation of heating affect of rock apophysis invasion,but it was in accordance with the temperature condition which yieldedthe meaningless platuae age of plagioclase from sample YMQ020.It indicated another heat source was expected in Yemaquan iron-zinc deposit.(5)The large scale polymetallic mineralization in Qimantage area was closely associated with late Paleozoic-middle Mesozoic tectonic-magma activitise.The rock-forming age concentrated in the period of 237-230Ma and 228?220Ma.The ore-forming age could be divided into two groups:the ore-forming rock of the first group always occurred as batholith with large amount of mafic microgranular enclaves rich in mafic iron at the stage of 230?220Ma;the second group always occurred at the stage of 220?204Ma,which had characteristics of I or A type and formed as stocks or apophyses.Most of the small stocks in the second group located at the north Kunlun belt,which was the product of the last large-scale underplating occurred at the late Triassic.Multistage magmatic activities could maintain stable heat source,which could effectively improve metallogenic material extraction.Moreover,a patch of small rocks could form a complex heat field which was constituted of multiple high-termprature centers.The high gradient and small scale heat field was easy to form in such complex heat field,which could give a strong affection to the evolution of geochemistry field and fluid.Therefore,the second group has better metallogenic potential.