Geological And Geochemical Characteristics Of Ore-hosting Strata In The Kalatag Ore Concentration Area,Xinjiang

The Kalatag region is located in the southern margin of the Turpan-Hami basin,which belong to the Dananhu-Tousuquan Paleozoic proliferativeisland arc,in Eastern Tianshan.Thedeposits in this region can be divided into fourcategories: VMS,porphyry-skarn,subvolcanic hydrothermal types and Cu-Ni sulphide.In the Paleozoic strata,the ore-bearing volcanic rocks can be divided into three lithologic sections in general,where VMS and subvolcanic hydrothermal deposits are mainly found.In this paper,we systematicallycollectedthe volcanic rock samplesthrough the measured geological section.Then,the main and trace elements,Sr-Nd isotopes and SIMS zircon U-Pb isotope dating were carried out to obtain the following innovative understanding:The ore-bearing strata ofcoreformation in Kalatagbelong to the Daliugou Formationin the late stage of early Paleozoic.The lower lithologic section of the marine basic volcanic rocks(unit 1):the rocks ofthis section is formedbefore the Late Ordovician(> 450Ma),which is enrichedin LREE and large ion lithophile elements as Rb,Ba,U,K,Pb.and also enrichedin Th,Zr,Hf,depleted Nb,Ta,P,Ti and Sr;The middle lithologic section of the marine intermediate volcanic rock-volcaniclastic rocks(unit 2): the rocks of this section is formed before the early Silurian(> 440Ma),whichis enrichedin LREE and large ion lithophile elements as Rb,Ba,U,K,and also enriched in Pb,Th,Zr,Hf,depleted Nb,Ta,P,Ti.Under the hydrothermal effection of the seabed,there is strong alteration zonationwith strong pyritized phyllite,chloritization and silicification occurred;The upperlithologic section of neritic-continentalintermediate-felsic volcanic rocks-volcaniclastic rocks(unit 3):the rocks of this section is formed before the Late Silurian-Early Devonian(> 420Ma).From the intermediate volcanic rocks to the acid volcanic rocks,total rare earths gradually increase with the evolution order of magma differentiation,which Eu elementrange from normality to obvious negative anomalies.And the trace elements show that large ion lithophile elements as Rb,Ba,Kenriched,and Nb,Ta,P,Ti,Sr aredepleted.The geochemical characteristics of this stratum show that it comes from island arc magma.The upper lithology is obviously enriched in sodium,and the Na2 O content of volcanic rocks with strong alteration is reduced in the lower-middle lithologic section.The Sr-Nd isotope show that volcanic rocks(87Sr/86Sr)i ranged from 0.704701 to 0.707351,?Nd(t)ranged from 2.12 to 7.44,and the(87Sr/86Sr)iof intrusion rocks ranged from 0.703770 to 0.705203,?Nd(t)at 5.66~6.84,which shows that it is derived from depleted mantle.Basically,volcanic rocks are affected by sea water with a little contaminationof the crustal materials.With the southward descending of the Kalamaili ocean,a series of island arc volcanic magmatic activitiesare brought into the Kalatagwhich are under the seawater environment at this time.At the late stage of the middlevolcanicmagma eruption,the Honghai VMS copper-zinc deposit is formed in the low-lying area.And this depositis quickly buried and preservedbythe uppervolcanicmagma eruption.