Marine Evaporative Genesis And Formation Age Of Paleoproterozoic Borate Deposits,Eastern Liaoning Province:Constrains From Isotopic Geochemistry

A large amount of Paleoproterozoic borate deposits are hosted as layers or lens within the lower units of a volcanic-sedimentary sequence in eastern Liaoning province,Northeastern China,and form a significant source of boron to China.The mining areas include Houxianyu,Wengquangou and Zhuanmiao-Yangmugan,while the ore types can be divided into suanite-szaibelyite and ludwigite-szaibelyite.The metallogenic models for sedimentary-metamorphic borate deposits of eastern Liaoning province can be divided into nonmarine evaporite,hydrothermal sediment and pyrometasomatic replacement type.Ore-bearing wall rocks are serpentinized ultrabasic volcanic rocks and carbonates,and comprise layered leptynites,leptites and streaky migmatites with increasing distance from the ore.Based on SHRIMP and LA-MC-ICP-MS zircon U-Pb technology,this research analysed the zircons in streaky amphibolite migmatite,leptynite,leptite,leucogranite,amohibiote and ultra-basic volcanic rocks,and get the following the data and results:(1)SHRIMP zircon 207Pb/206Pb ages of leptynite and amphibolte in the hanging wall of Zhuanmiao-Yangmugan boron district are 2174.0± 9.9Ma and 1869 ± 28Ma,respectively;(2)LA-MC-ICP-MS zircon 207Pb/206Pb ages of leptite and serpentinized olivine basalt located in the hanging wall of the borate deposits are 2139±13 Ma and 2130±19 Ma,respectively.(3)The LA-MC-ICPMS zircon U-Pb analysis of the magmatic zircon cores of layered granites and leucogranites have weighted mean 207Pb/206Pb age of 2240±4Ma and 2157±6Ma respectively.The magmatic zircon rims of leucogranites have high Th/U ratios of 0.4159?1.0829 and weighted mean 207Pb/206Pb age of 1932±26Ma.Therefore,the magmatic zircon core(-2.17Ga)represents the crystallization age of volcanic rocks during the early stages of Liaoji rift.While the recrystallized zircons of amphibolite and magmatic zircon rims of leucogranite indicating that the late Palaeoproterozoic tectonothermal event of North China Craton may induce in situ melting of the lower volcanic-sedimentary stratums of Liaohe Group.Based on in situ boron isotope measurements by laser ablation-multicollector-inductively coupled plasma-mass spectrometry(LA-MC-ICP-MS),?11B values of borate ores and tourmalines range from 6.4‰ to 13.9‰(mean 10.2‰),indicates that boron in Paleoproterozic borate deposits was sourced from marine evaporites.?34Sv-CDT values of borate ores,serpentinized marbles,and anhydrites range from 16.1 ‰ to 24.79‰.The ?13Cv-PDB values of marbles range from-5.0‰ to 4.6‰,while the ? 13CV-PDB values of relative fresh marbles range from 3.2‰ to 4.6.These isotopic values are interpreted to represent the Paleoproterozic Lomagundi event.Petrography and geochemistry characteristics of the serpentinized ultrabasic rocks indicate it may originally been a type of erupted marine volcanic rock that covered and protected the evaporative boron-bearing sediments during the amphibole face metamorphism,and finally reacted with the evaporates and formed mineral assemblages of ludwigite-szaibelyite and suanite-szaibelyite.World-class magnesite deposit hosted at the stratums of North Liaohe Group,located at the north slope of Liaoji rift.The stable isotope geochemistry and zircon U-Pb chronology show:(1)The B content of marbles and ores in magnesite deposit range from 9.76×10-6 to 287×10-6,and ?11B values range from 7.8‰ to 13.3‰,average 11.1 ‰,exhibit a metamorphic marine sedimental characteristic;(2)the Values of ?34SV-CDT of marbles and ores in magnesite deposit range from 16.0‰ to 26.5‰,average 18.9‰,and the values of ?13CV-PDB of magnesite and its wall rock are range from 4.59‰ to 4.4‰,average 0.9‰,indicates a marine sediment genesis;(3)The B content and PAAS-normalized REE pattern of magnesite and borate deposits are similar;and(4)The CL images and U-Pb results of zircons in leptynite and leptite that locate outside the magnesite and borate deposit are similar,and yields a peak magmatic peak age of 2.2Ga.Therefore,the magnesite deposites and the borate deposits,which locate at the northern slope and central depression,respectively,are synchrono-heteropic facies of evaporative mineralization,formed in a rhythmic environment of evaporation and volcanic-sedimentation at about 2.2 Ga.