The Ni-Co Metallogenic Magmatism Of Qixin Mafic-Ultramafic Complex In Beishan,China

The Qixin mafic-ultramafic complex,which is the second largest mafic-ultramafic complex found after the Pobei in China,is composed of a large gabbro-norite body and more than 30 ultramafic intrusions.In recent years,mineral exploration has found disseminated-massive sulfide ores in several intrusions,and the metallogenic potentiality of Ni,Cu and Co is great.The magmatic source,magmatism and mantle plume contribution are lack of carbon and noble gas isotopes restrict deeply.In this paper,the whole-rock major and trace elements,carbon and noble gas isotope are analysed to explore the source of volatiles,magmatism,and the contribution of mantle plume for Qixin complex.The following are the mainly conclusions:（1）The Qixin mafic-ultramafic intrusions belong to subalkaline series,with Mg#is 60.40～86.27,Na₂O+K₂O is 0.02～2.81%.The REE content is 2.44～22.34ppm and it has a flat type standardized by chondrites.There are LILEs enriched like Rb,Ba,Sr and HFSEs depleted like Nb Ta,Zr and Hf.The Cu is relatively enriched,while Cr,Ni and Co are depleted in mafic rocks.However,the ultramafic rocks have similar content of Cr,Co and Ni with mantle,while there is a depleted content in mafic rocks.（2）The volatiles of Qixin intrusion of magmatic mineral fluids has the characteristics of deep mantle,crust and atmosphere endmember.Theδ¹³C values of CO₂ and CH₄ in the magmatic mineral fluid are-30.9‰to-3.6‰and-44.8‰to-14.9‰,respectively,indicating it has the characteristics of crust,methane oxidation and mantle components.（3）The noble gases isotopes in the magmatic minerals of olivine,pyroxene and plagioclase shows composition characteristics of deep mantle,crust and atmosphere:³He/⁴He ratio value is 0.63-6.78 Ra,²¹Ne/²²Ne ratio value is 0.0107-0.0810,²⁰Ne/²²Ne ratio value is 8.50-13.2,and ⁴⁰Ar/³⁶Ar ratio value is 217～542.（4）The magmatic source of the Qixin intrusion shows deep mantle characteristics:Whole rock（Sm/Yb）_N is 1.06～2.71,（La/Sm）_N is 0.37～3.33,and（Tb/Yb）_N is 0.67～1.76,which suggest the magmatic source region is lithospheric mantle;LILEs enriched,HFSEs depleted,high ³He/⁴He and the low ⁴⁰Ar/³⁶Ar indicate the contribution of deep mantle which also has been metasomatized by subduction fluid.（5）Qixin magma was contaminated by crustal obviousiy during evolution:The³He/⁴He released volatile is negatively correlated with（Th/Yb）_N and La/Yb,and positively correlated with Zr/Nb and whole rock（Th/Yb）_PM,（Ta/Th）_PM,（Th/Yb）_PM,（Nb/Th）_PM,Nb/La,Nb/Th,Nb/La and La/Ba show a distribution trend from mantle to crust,which all indicate that the Qixin magma suffered from obvious contamination of crust material.（6）The mafic parent magma with high Mg and high fluid components of Qixin intrusions is caused by the upwelling of asthenosphere caused by mantle plume,which resulted in partial melting of metasomatism lithospheric mantle.In the process of magmatic evolution,the ore-forming elements were enriched due to the contamination of crustal materials,and invaded the surface after the separation crystallization and sulfide segregation in the shallow magmatic chamber.