Assimilation And Contamination Of Huangshandong Cu-Ni Sulfide Deposit In Northern Tianshan

In recent years,crust sulfur contamination is considered as the key to the mineraliza-tion of magmatic copper nickel deposit.However,the systematic study on the contami-nation process is still weak,especially whether the mafic magma assimilated the direct country rock or crustal rocks at depth is not clear,and the way of sulfur addition is still to be further studied.Therefore,petrography,petrochemistry,sulfur isotope and mineral compositions of the Huangshandong large Cu-Ni deposit in Northern Tianshan at the southern margin of Central Asian orogenic belt were carried out to deepen the under-standing of contamination process and further clarify the metallogenic process in this paper.The contents of Si O₂ and Al₂O₃ in mafic-ultramafic rocks with pegmatitic plagioclase xenocrysts in the Huangshandong intrusion are higher than those in adjacent rocks without pegmatitic plagioclase xenocrysts.The mafic-ultramafic rocks also have a high La/Sm ratios and positive correlation with La/Yb,Ce/Pb,Nb/Ce and Nb/U,indicating that the intrusive rock is contaminated with crustal materials.The Se(×10⁶)/S of sul-fide-bearing mafic-ultramafic rocks ranges from 41.38 to 198.25,with an average value of 95.76,which is between the ratios of crust rocks and mantle rocks,indicating that crustal sulfur has been added into the mafic magma.Magma mixing process occurred in the Huangshandong intrusive rock,but there are not difference of sulfides contents be-tween the mixed rocks and unmixed rocks,so magma mixing have not triggered sulfide segregation in the deposit.The ?³⁴S values of the sulfide-bearing mafic-ultramafic intrusive rocks in the Huang-shandong deposit lie in the mantle range(-0.86‰?1.33‰),while the?³⁴S values of the country rock vary widely(-22.23‰?18.75‰),indicating that the sulfur in the direct contacted country rock have not been added into the Huangshandong intrusion.The cummingtonite and cacite-quartz-pyrrhotite veins were found in the contact zone be-tween the intrusive rock and the country rock,suggesting that country rocks have been metasomatized by magmatic hydrothermal fluids.The cloddy pyrrhotites in the country rock(<50m)near the intrusive rock have high Ni,Co,Cr,Ti,Se and Ag contents,which are similar to those in the intrusive rock.However,the high Zn,Mn characteris-tics of the pyrrhotites are obviously different from those in the intrusive rock,and simi-lar to those in the country rock(>50m)far away from the intrusion,suggesting that pyr-rhotite in the country rock near the intrusion may be formed by magmatic hydrothermal metasomatic transformation of pyrites in the country rock.This is consistent with the high Se(×10⁶)/S(48.8?246.6)and mantle-like?³⁴S values(-11.83‰?0.24‰)in the country rock close to the intrusion.The trace element compositions of the acidic pegmatitic plagioclase xenoliths in the Huangshandong intrusive rock are distinctly different from the basic plagioclase in the intrusive rocks,but similar to the plagioclase in the volcanic clastic rocks of the Wutongwozi Formation,suggesting that the pegmatitic plagioclase xenoliths may be from the Wutongwozi Formation or deeper crustal rock,so magmatic contamination should occur in depth.Additionally,the large variation range of Sr and Eu contents in the pegmatitic plagioclase xenoliths indicates that their chemical composition were in-fluenced by mantle derived magma in different degrees.It is inferred that the metallo-genic process of the Huangshandong deposit is as follows:firstly Ni-rich mantle-derived magma intruded into the crust and formed a stage magma chamber.A large amount of crust sulfide was assimilated and mafic minerals have crystallizated in the stage magma chamber,which triggered sulfide segregation.The sulfides accumulated at the bottom of the magma chamber and experienced sulfur isotope homogenization,which made the sulfur isotope concentrated in the mantle range.When following mag-ma flowed though the stage magma chamber,the sulfides would be carried by the mag-ma into the shallow magma chamber.If the sulfide in the shallow magma chamber re-acted with the fresh magma,the copper,nickel and other ore-forming elements were continuously enriched into the sulfides and formed ore bodies in the Huangshandong deposit.