Research On The Diagenetic And Metallogenic Epoch Of The Dabaoshan Polymetallic Deposit,Guangdong Province

The Dabaoshan polymetallic deposit,which located in south China,is an important ore deposit.It is also an important producing area for copper,lead,zinc,tungsten and molybdenum of Qinhang Metallogenic Belt.Although the academic circles have studied deeply about this deposit,there are still a lot of untouched problems.On the one hand,there are still much dispute about the formation age of pluton intrusions of this deposit.On the other hand,the formation age and the mineralization mechanism of porphyry-skarn type W-Mo and SEDEX type Cu-Pb-Zn orebodies need further research.The formation age of the pluton intrusion and orebody play an important role in the study of deposit genesis,so it is necessary to make it clear.In this study,after the extensive filed work,the results of zircon U-Pb geochronology and Re-Os geochronology of molybdenite and black carbonaceous mudstone,we attempt to investigate the formation ages of the pluton intrusions and orebodies.Furthermore,combining with the previous date,we propose the metallogenic model and prospecting direction.The main results and conclusions are as the follows:The results of zircon U-Pb dating,combining with the extensive filed work,show that there exists two kinds of intrusions: secondary dacite-porphyry and granodiorite.Further study shows that these two intrusions have obvious intrusive-contact relationship: the granodiorite intruded into the secondary dacite-porphyry.Zircon U-Pb dating of the two intrusions show that the secondary dacite-porphyry formed at 440 Ma,which is the result of magmatic induced by Wuchuan-Sihui fault of Caledonian,while the granodiorite formed at 165 Ma,which is the result of magmatic induced by crustal thinning of Yanshanian.The results of molybdenite Re-Os dating,combining with other researchers' study shows that the porphyry-skarn type W-Mo orebody formed at 165 Ma,which owns the same formation age with other deposits in south China.This age indicates this deposit was derived from intermediated-acidic intrusions of Yanshanian,induced by crustal thinning and lithospheric extension.Filed work about the stratiform-substratiform orebody indicates this orebody has no direct relation with magma intrusion.This orebody presents stratiform,substratiform or brecciaous in sharp,and it occurs mainly in Qiziqiao Formation.The ore lie parallel with the stratum and keep step with the stratum in sharp,with synsedimentary characteristics.These characteristics indicates that this deposit is an SEDEX type deposit.The results of Re-Os geochronology of black carbonaceous mudstone from Qiziqiao Formation show it was formed at 385 Ma,represents the formation age of stratiform-substratiform orebody.These conclusions are different from other researchers' opinions that this orebody formed in Caledonian or Yanshanian,and it will be of great significance in further exploration.With the formation ages of the intrusions and orebodies,combining with the tectonic settings of south China in Caledonian and Yanshanian,we can make a clear conclusion that there exists two metallogenetic events.The former event about submarine exhalative sedimentary happened in Hecynian while the latter metallogenetic event about intermediate-acid magmatic happened in Yanshanian.South China stayed away from the subduction zone in Hecynian,moreover,this area is in infraneritic environment for a long time.So it is beneficial for this area to form some big deposits.On the other hand,because of the large-scale magmatic activities happened in south China in Yanshanian,many polymetallic deposits related to granite porphyry are formed.The intrusion of magma brought much hydrothermal solution to the stratum and formed many skarn type deposits.By all aspects presented above,we think next work to exploration in the Dabaoshan polymetallic deposit needs focusing two areas: the periphery of the intermediate-acid intrusions in Dabaoshan and Chuandu,and the Qiziqiao Formation.The north-east area of the Dabaoshan poyphyry-type Mo deposit in the north contact zone is a good exploration prospect.