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ESTUDOS PETROGRAFICOS E GEOQUIMICOS DOS GIGANTES DEPOSITOS DE MINERIO DE FERRO DA SERRA NORTE, PROVINCIA MINERAL DE CARAJAS, PARA, BRASIL. Os depositos de minerio de ferro de Carajas, localizados no sudeste do estado do Para no Brasil, estao hospedados na sequencia metavulcanossedimentar do Grupo Grao Para, Supergrupo Itacaiunas. Os protolitos da mineralizacao de ferro sao jaspilitos, soto- e sobrepostos por basaltos, ambos metamorfisados em facies xisto verde. Os maiores depositos de minerio de ferro da Serra Norte sao N1, N4E, N4W, N5E e N5S, estao distribuidos ao longo e estruturalmente controlados pelo flanco norte da dobra Carajas. Minerio de alto teor (> 64 % Fe) consiste em minerios compactos e friaveis. O contato basal dos minerios de alto teor e definido por rocha basaltica alterada hidrotermalmente, composta por clorita e hematita microlamelar. Diferentes estagios de alteracao hidrotermal afetaram jaspilitos para formar minerios de ferro, da zona de alteracao distal, representada pelo estagio cedo-hidrotermal, as zonas de alteracao intermediaria e proximal, concomitantes com o evento principal de formacao de minerio de ferro. A zona proximal representa o estagio de alteracao avancado (i.e., minerio de ferro de alto). Jaspilitos dos depositos N4W, N5E e N5S e minerios compactos dos depositos N1 e N4E tem baixo conteudo ETR, sao enriquecidos em REE leves e exibem anomalias positivas de europios (Eu/Eu*> 1), padrao tipico de formacoes ferriferas bandadas arqueanas. O padrao de ETR definido por minerios de N5E e quase horizontal e apresenta aumento em ETR e ausencia de anomalia positiva de Eu. O aumento acentuado em ETRL ocorreu durante a formacao de magnetita e hematita microlamelar. Ja aumento em ETRP e padrao quase horizontal de ETR coincidem com o avanco da martitizacao formando hematita anedrica, o qual pode ter favorecido o aumento relativo de ETR pesados no fluido residual, resultando em precipitados do estagio avancado de mineralizacao (formacao de hematitas euedrica e tabular). As mudancas mineralogicas, geoquimicas e isotopicas de jaspilitos a minerios de ferro de alto teor sugerem uma origem hidrotermal para minerio compacto via interacao de fluido magmatico reduzido em estagio cedo-hidrotermal, lixiviando silica e formando magnetita. Este fluido evoluiu para condicoes mais oxidantes, com avanco da martitizacao, aumento na concentracao de ETR e formacao de hematita microlamelar em veios e bordas de martita, a partir de mistura com aguas meteoricas modificadas. Palavras-chave: jaspilito, minerio de ferro, Carajas, elementos terras raras. ABSTRACT: The Carajas iron ore deposits, located in the southern part of the state of Para in Brazil, are hosted by the metavolcano-sedimentary sequence of the Grao Para Group, Itacaiunas Supergroup. The protoliths to iron mineralization are jaspilites, under- and overlaid by basalts, both greenschist facies metamorphosed. The major Serra Norte N1, N4E, N4W, N5E and N5S iron ore deposits of the Carajas Mineral Province are distributed along, and structurally controlled by, the northern flank of the Carajas fold. High-grade iron mineralization (> 64 % Fe) is made up of hard and soft ores. The basal contact of the high-grade iron ore is defined by a hydrothermally altered basaltic rock mainly composed by chlorite and microplaty hematite. Varying degrees of hydrothermal alteration have affected jaspilites to form iron ores, from distal alteration zone, representing an early alteration stage, to intermediate and proximal alteration zones, synchronous with the main iron-ore forming event. The latter represents an advanced alteration stage (i.e., the high-grade iron ore). Jaspilites from the N4W, N5E and N5S deposits, and hard ores from N1 and N4E have a low REE content, are enriched in light REE and exhibit positive europium anomalies (Eu/Eu*> 1), which is typical of Archean banded iron formations. The REE pattern defined by N5E ores is nearly flat and displays an increase in REE and absence of the positive Eu anomaly. The increase in LREE was accentuated during the formation of magnetite and microplaty hematite, and the advance of martitization to form anhedral hematite, which may have favoured the relative increase of HREE in the residual fluid, resulting in an increase in HREE in advanced-stage precipitates and almost flat REE patterns associated with the advanced stage of mineralization (euhedral and tabular hematite formation). The mineralogical, geochemical and isotopic changes from jaspilites to high-grade iron ores suggests a hydrothermal origin for hard ore via interaction with an early-stage, relatively reduced magmatic fluid, which leached silica and formed magnetite, which evolved to more oxidizing conditions, with the advance of martitization, increase in the REE concentration and microplaty hematite precipitation in veins and martite borders , from interaction with modified meteoric waters. Keywords: jaspilite, iron ore, Carajas, rare earth elements

PETROGRAPHIC AND GEOCHEMICAL STUDIES AT GIANT SERRA NORTE IRON ORE DEPOSITS IN THE CARAJÁS MINERAL PROVINCE, PARÁ STATE, BRAZIL