Iron formations are valuable archives of sedimentary conditions and post-depositional events.However,geochemical proxies commonly used to determine genetic characteristics can be variably modified dur-ing metamorphism and deformation,hampering their use as records of regional geological events.This work focuses on strongly reworked magnetite-quartz-rich rocks from the S?o José do Campestre Massif,one of the oldest fragments of preserved crust in South America.The genetic classification of these magnetite-quartz-rich rocks is not straightforward because primary assemblages and textures were vari-ably modified by granulite facies metamorphism during a regional Paleoproterozoic migmatization event.To address genetic ambiguities,we analyzed their magnetite and pyroxene chemistry,whole-rock geochemistry,and Sm-Nd isotopes.Magnetite chemistry indicates that pyroxene-poor iron forma-tions(Type B)are low in trace elements such as Ti,Al,V,and Mn,suggesting a chemical similarity to iron formations elsewhere.In contrast,magnetites from pyroxene-enriched Type A iron formations are rich in trace elements and more akin to magnetite crystallized from higher temperature systems,such as skarn and IOCG.The 147Sm/144Nd of these rocks show substantial variation even at the outcrop scale,indicating a locally-controlled,highly heterogeneous mixture of Archean,Paleoproterozoic,and Neoproterozoic sources.Therefore,our geochemical tools point out to heterogenous signatures of these magnetite-quartz rocks and proxies compatible with both low and high-temperature conditions and age of deposi-tion spanning sources from the Archean to the Neoproterozoic.We interpret that the studied S?o José do Campestre magnetite-quartz rocks represent Archean iron formations with original magnetite chemistry and isotopic signatures variably modified by metamorphism and by at least one deformation-related hydrothermal event.These results contrast with similar examples from China and Greenland where iron formations either preserved the magnetite chemistry or the primary isotopic signatures.Our study indi-cates that metamorphism can selectively affect chemical proxies used to study iron formations and undermine the genetic classification of iron ores.Thus,these proxies should be carefully applied in the interpretation of syn-depositional environments of polydeformed belts.

Joice Dias de Moraes;Pedro Cordeiro;Eduardo Abrah?o Filho;Juliana Rezende Oliveira;Carlos Victor Rios da Silva Filho

Instituto de Geociências,Universidade de Brasília(UnB),70910-900 Brasília,DF,Brazil;Department of Mining Engineering,Pontifical Catholic University of Chile,7820436 Santiago,Chile

地学前缘（英文版）

[1]Joice Dias de Moraes;Pedro Cordeiro;Eduardo Abrah?o Filho;Juliana Rezende Oliveira;Carlos Victor Rios da Silva Filho-.Metamorphic disturbances of magnetite chemistry and the Sm-Nd isotopic system of reworked Archean iron formations from NE Brazil)[J].地学前缘（英文版）,2022(05):227-241

reworked,Campestre,migmatization,Magnetite,magnetites,polydeformed

Metamorphic,disturbances,isotopic,Archean,formations,from,NE,Brazil,Iron,valuable,archives,sedimentary,conditions,post,depositional,events,However,geochemical,proxies,commonly,used,determine,genetic,characteristics,can,variably,modified,metamorphism,deformation,hampering,their,records,regional,geological,This,focuses,strongly,quartz,rocks,Jos,do,Massif,one,oldest,fragments,preserved,crust,South,America,classification,these,not,straightforward,because,primary,assemblages,textures,were,by,granulite,facies,during,Paleoproterozoic,To,address,ambiguities,analyzed,pyroxene,whole,geochemistry,isotopes,indicates,that,poor,Type,low,trace,elements,such,Ti,Mn,suggesting,similarity,elsewhere,In,contrast,enriched,more,akin,crystallized,higher,temperature,systems,skarn,IOCG,147Sm,144Nd,show,substantial,variation,outcrop,scale,indicating,locally,controlled,highly,heterogeneous,mixture,Neoproterozoic,sources,Therefore,tools,point,heterogenous,signatures,compatible,both,spanning,We,studied,represent,original,least,related,hydrothermal,These,results,examples,China,Greenland,either,Our,study,selectively,affect,undermine,ores,Thus,should,carefully,applied,interpretation,syn,environments,belts

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