Paleomagnetic Research Of The Sediment-hosted Ore Deposits

There is fair amount of world-class Pb-Zn deposits in Cenozoic sedimentary basins in China,such as the giant Jinding Pb-Zn deposit in Lanping county,Yunnan province.However,the specific metallogenic epoch of the Jinding lead-zinc deposit is not clear which directly limits the understanding of the metallogenic mechanism.It is quite rare for domestic sedimentary-hosted lead-zinc ore deposits in particular to use paleomagnetic method as an approach to study the long-exited dating problem.This thesis is aimed at introducing and practicing paleomagnetic dating techniques targeting at the Jinding Pb-Zn deposit,by getting the Jinding lead-zinc deposit metallogenic model of metallogenic age,provides a theoretical basis for further prospecting and becoming China's first paleomagnetic dating of sedimentary-hosted type lead-zinc deposit.Our dating of the sedimentary-hosted lead-zinc deposits of innovative knowledge will accelerate the combination of paleomagnetism and ore deposits in China and provide a new reference for future paleomagnetic dating.Most paleomagnetic dating application are on lead-zinc ore deposits,this thesis works on this direction to understand how the Jinding lead-zinc deposit formed in the Earth's crust and in what parts of the formation of the Earth's crust,and its metallogenic age,and establishes a metallogenic model.Sedimentary-hosted Pb-Zn deposits are ideal for paleomagnetic dating methods,in relative terms,these type of ore deposits are most likely suffered less alteration and in relatively stable tectonic environment.Paleomagnetic dating method relies on low concentrations of magnetite,hematite or pyrite in the growth process of recording the magnetic minerals in the Earth's magnetic field direction,these iron minerals don not have to be dominant in the rock.Using paleomagnetic dating method can bypass the effects of radioisotopes,dating the ore body and host rock directly,and other obvious advantages.By statistical analyzing the studies on sedimentary-hosted lead-zinc mineralization within the world,70% metallogenic age of those deposits are obtained using paleomagnetic dating method.The Jinding zinc-lead deposit is located in Lanping-Simao basin in Yunnan Province,the main ore-bearing strata are Cretaceous and Tertiary terrestrial strata,mainly about 200 Mt of Zn and Pb,Zn 6.08%,Pb grade 1.29%,which is the largest lead-zinc deposit in China,also the youngest sedimentary-hosted super large Pb-Zn deposit.The main metallogenic belt is located in the Jinding dome structure,the ore bodies are flat shaped hosted in the Paleocene Yunlong Formation(autochthonous system)of pebbly sandstone and the overlying Lower Cretaceous Jinxing Formation sandstones.Laminated character and eruptive sedimentary rocks can't be found in the orebody.There is significant alteration occurred in orebody accompanied by metasomatism and cavity filling,indicating that the mineralization of epigenetic origin.The forming of the Jinding lead-zinc deposit has a close relationship with the collision of Eurasia and India mass movements of the continental crust.Paleomagnetic analyses of 645 specimens from 42 sites using mostly thermal and then alternating field step demagnetization isolated a stable characteristic remanent magnetization(ChRM)for 29 ore sites.Step demagnetization and rock magnetic analysis of Zn and Pb concentrates and tailings show that the main remanence carriers are pyrrhotite and titanomagnetite inclusions in the sphalerite and galena.The randomly oriented ChRM directions of the 18 clast specimens collected from site 40 for a conglomerate test show that the clast ChRM directions have not been remagnetized by a metamorphic event.A paleomagnetic fold test on the mineralized specimens is negative,showing that the mineralization's epigenetic because its ChRM substantially(87±10%)postdates by 87±10% the major thrusting folding event that formed the dome structure of the Jinding deposit.Using the extensive regional geological and paleomagnetic data bases from other studies with the Jinding paleomagnetic data yields a mean age of 23±3 Ma for the mineralization,this thrusting activity is the main cause of the Jinding Zn-Pb deposit formed dome structure,indicate that mineralization is epigenetic.