Study On Diagenesis And Mineralization Of Devonian Mafic-ultramafic Intrusions In Beishan District,Gansu Province,and The Significance Of Tectonic Setting

The Beishan orogenic belt is one of the developed areas of the magma Cu-Ni sulfide deposit,which lie in the southern part of the Central Asian orogenic belt.The Heishan intrusion is the only large Cu-Ni sulfide deposit in the devonian which found in the Beishan orogenic belt,Gansu province.In this paper,we chioce two maficultramafic intrusions(Dashantou,Guaishishan)for the research.Using the petrology,mineralogy,petrogeochemistry and chronology,through the study of the magma source,the Parent magma properties,assimilation-contamination,the fractional crystallization and sulfide segregation,discussed the diagenetic mineralization process and the tectonic background of the Dashantou and Guaishishan i ntrusions.Mainly found the following points:1)The Dashantou intrusion is made up of Gabbro facies,Olivine gabbro and ultramafic rock,Olivine gabbronorite,Olivine Websterite and hornblende peridotite,The Guaishishan intrusion consists of pegmatoidal bojite,medium grain gabbro,contain olive pyroxenite,olivine pyroxenolite.The sulfide is occurred in ultramafic rock,it consists of pentlandite,chalcopyrite,pyrrhotite and pyrite.2)The olivine from Dashantou intrusion is Chrysolite(Fo=76.00~87.03);The pyroxite is mainly composed of clinopyroxene and orthopyroxene,clinopyroxene is mainly made up of enstatite and diopside,orthopyroxene is mainly composed of bronzite;Plagioclase has a higher grade(An=76.80-90.67)which is mainly composed of bytownite and anorthite;the hornblende appear in all kind of rocks.the sequence of mineral crystal is Olivine-Plagioclase-(Orthopyroxene+Clinopyroxene)-Hornblende.3)The study of petrogeochemistry shows that the rare earth elements partition in Dashantoubei instrusion leans to the right,LREE are more obviously fractionation than HREE,?Eu has,large ion lithophile element(Cs?Rb?Ba),deficiency high field strength elements(Nb?Ta?Zr?Hf).The rare earth elements partition in Heishan and Guaishishan instrusions lean to the right,non apparent abnormality in ?Eu,Enrichment Large Ion Lithophile Elements(Cs?Rb?Ba),deficiency high field strength elements(Nb?Ta),non apparent abnormality in Zr and Hf.showing that Dashantou instrusion is different from Guaishishan and Heishan instrusions in the metallogenic mineralization.4)The zircon of olivine gabbronorite and light gray fine-grained gabbro in DAshamtoubei instrusion were carried out dating by LA-ICP-MS U-Pb,it shows that 404.9±3.1Ma and 403.8±3.3Ma,which belong to the product of the early devonian magma activityies.It's different from Heishan(358~377Ma),Dashantounan(359~374Ma)and Guaishishan(358.6Ma)instrusion in dating,the latter was formed in the middle-late devonian.5)Through the study of petrogeochemistry,geochronology,metallogenic mineralization and tectonic setting shows that the early devonian Dshantoubei instrusion's tectonic backgrounds is island arc subduction environment,Heishan and Guaishishan instrusion in the later devonian were made in the active continental margin or the island arc subduction environment.