Genesis Of The Makeng Giant Iron Deposit In The Yong'an-Meizhou Depression Belt,Southwestern Fujian

Yong'an-Meizhou depression belt in Southwestern Fujian,geologically located in eastern part of Cathaysia Block,is an important Fe-Mo polymetallic belt in southeastern China.The belt hosts the large scale Longfengchang and Luoyang iron deposit,and the largest scale Makeng iron deposit.Although their geological and geochemical aspects have been investigated by many workers,other aspects such as ore-forming fluid evolution,magnetite mineralization mechanism,and ore deposit genesis have been rarely studied intensively,especially for the Makeng iron deposit.Therefore,on the basis of detailed field investigation,combined with the paragenetic relationship between skarn mineral and ore mineral and fluid inclusions petrographic observation,this paper selected the Makeng iron deposit to carry out on fluid inclusion geochemistry,stable isotope geochemistry,and magnetite mineral chemistry,in order to discuss ore fluid genesis and evolution,magnetite mineralization mechanism,and ore deposit genesis.The Makeng deposit is hosted in marble of Jingshe Formation-Qixia Formation.The host rock develops voluminous skarn mineral alteration.The mineralization process in the Makeng area have undergone anhydrous skarn stage(stage 1)(garnetdiopside)?hydrous skarn-magnetite stage(stage 2)(amphibole-chloritehedenbergite-magnetite±johannsenite±epidote)?sulfide stage(stage 3)(quartzcalcite-fluorite-galena-sphalerite±chlorite)?carbonate stage(stage 4)(quartzcalcite).Magnetite in the hydrous skarn-magnetite stage shows lower contents of Ti+V and higher contents of Ca+Al+Mn.Hedenbergite in the hydrous skarn-magnetite stage consist of halite-bearing three phase inclusions and vapor-rich two phase inclusions,with similar homogenization temperatures(458 to 587? and 451 to 512?,respectively)and contrasted salinities(26.5 to 48.4 wt%NaCl equiv.and 2.4 to 6.9 wt%NaCl equiv.,respectively),indicating fluid boiling took place in this stage,whereas quartz,calcite,and fluorite in the sulfide stage contain liquid-rich aqueous inclusions with a lower temperature and lower salinity of 182 to 343? and 1.9 to 20.1 wt%NaCl equiv.,suggesting fluid mixing occurred in this stage.H-O isotopic compositions of garnet(?18Ofluid=6.2 to 8.0 ‰ and ?D=-77 to-101 ‰)in the anhydrous skarn stage,magnetite(?18Ofluid=6.7 to 9.6 ‰ and ?D=-96 to-128 ‰)in the hydrous skarnmagnetite stage,and quartz(?18Ofluid=-0.85 to-1.04 ‰ and ?D=-110 to-124 ‰)in the sulfide stage indicate that ore-forming fluid responsible for the magnetite formation was derived from magmatic water and for the sulfide formation was generated by mixing between magmatic water and meteoric water.Fluid inclusions and H-O isotopic results indicate fluid boiling induced formation of magnetite mineralization,whereas fluid mixing resulted in formation of sulfide mineralization.Over all,ore fluid in the Makeng area experienced fluid boiling in the early magnetite mineralization stage and then fluid mixing between magmatic water and meteoric water in the later sulfide mineralization stage.The former resulted in formation of magnetite and the latter resulted in formation of sulfide.The making iron deposit shares a number of common features with typical skarn deposit,such as wall rock alteration,mineralization process,ore-forming fluid,and magnetite mineral chemistry,marked different from volcanogenic massive deposit,indicating a skarn type iron deposit.