PROGRAMA DE PESQUISA E PÓS-GRADUAÇÃO EM GEOLOGIA ÁREA DE CONCENTRAÇÃO:
PETROLOGIA, METALOGÊNESE E EXPLORAÇÃO MINERAL
DISSERTAÇÃO DE MESTRADO
CONTROLE ESTRUTURAL E CARACTERIZAÇÃO DOS
DEPÓSITOS DE ALTO TEOR DE HEMATITA
HIDROTERMAL EM DOMÍNIOS TRANSPRESSIONAIS DO
CINTURÃO DE DOBRAMENTOS E CAVALGAMENTOS DA
CHAPADA DIAMANTINA ORIENTAL
VANDERLÚCIA DOS ANJOS CRUZ
SALVADOR 2016
CONTROLE ESTRUTURAL E CARACTERIZAÇÃO DOS
DEPÓSITOS DE ALTO TEOR DE HEMATITA
HIDROTERMAL EM DOMÍNIOS TRANSPRESSIONAIS DO
CINTURÃO DE DOBRAMENTOS E CAVALGAMENTOS DA
CHAPADA DIAMANTINA ORIENTAL
Vanderlúcia dos Anjos Cruz
Orientadora: Profa. Dra. Simone Cerqueira Pereira Cruz Co-orientadora: Profa. Dra. Lydia Maria Lobato
Tese de Mestrado apresentada ao Programa de Pós-Graduação em Geologia do Instituto de Geociências da Universidade Federal da Bahia como requisito parcial à obtenção do Título de Mestre em Geologia, Área de Concentração: Petrologia, Metalogênese e Exploração Mineral.
SALVADOR
“Eu tentei 99 vezes e falhei, mas na centésima tentativa eu consegui. Nunca desista dos seus objetivos mesmo que esses pareçam impossíveis. A próxima tentativa pode ser a vitoriosa” (Albert Einstein)
(Valdeque) por me acompanhar nessa luta, as minhas avós, tios, tias, primos e afilhados, as minhas amigas de longa data, que apesar de não entederem nada de rocha sempre me ajudaram com uma palavra de carinho e incentivo, e as minhas tias do coração Aurora, Janu e Neiram. Gostaria de prestar um agradecimento especial a Rilza, a pessoa mais incrível que já conheci em minha vida e que me fez entender que eu sou capaz de alcançar todos os meus abjetivos.
A empresa Cabral Resources Ltda. pelo apoio no campo, na litogeoquímica e na petrografia, em especial a Paulo Alexandre Ribeiro, Daniel e Luis Carlos por estarem sempre dispostos a ajudar.
Aos mestres por me prepararem e ensinarem tudo que sei sobre a geologia, principalmente aos professores Ernade, Telésforo, Ângela, Jailma, Hailton e Olívia, em especial, a minha orientadora e a amiga, Simone Cruz, que me ajudou muito para o meu desenvolvimento na universidade e por está comigo em todos os momentos. Aos meus amigos da geologia que estiveram sempre ao meu lado e que me deram muita força nessa caminhada.
RESUMO
Os depósitos de ferro estudados estão localizados no município de Ibicoara, porção sul da Serra do Sincorá, centro oeste do Estado da Bahia. Do ponto de vista tectônico, estão inseridos no setor epidérmico do Cinturão de Dobramentos e Cavalgamentos da Chapada Diamantina, especificamente no domínio transpressional de Ibicoara, com interação de dois campos de tensão principal máximos, orientados segundo WSW-ENE e NW-SE, respectivamente, de idade Edicariana. O objetivo principal deste trabalho é contribuir com o entendimento dos processos e controles na formação de domínios com alto teor em ferro encaixados em sequências metavulcanossedimentares estéreis. A principal motivação deste trabalho está relacionada com o fato de que depósitos representam importantes fontes de ferro em todo o mundo e que as novas descobertas de depósitos desse tipo na Bahia abrem uma promissora fronteira para a exploração mineral no Estado. Visando alcançar os objetivos propostos, foram realizados estudos bibliográficos, trabalhos de campo, petrografia, litogeoquímica e análise química em hematitas por LA-ICPMS. Arenito quartzoso e arcoseano da Formação Tombador são as rochas encaixante de domínios hematíticos na área. Representa nova descoberta de depósitos hidrotermais de alto teor (high grade) de ferro na Bahia, o que abre promissora fronteira para a exploração mineral no Estado. Três fases deformacionais são identificadas. A primeira (D1) desenvolveu dobras do tipo kink, com chevron subordinada, zonas de cisalhamento relacionadas a deslizamento flexural com estruturas S/C, lineação de estiramento mineral (Lx1), slickenline e
slickensides. A segunda fase (D2) nucleou zonas de cisalhamentos destrais e sinistrais, reversas de alto
ângulo, com orientação geral segundo NNW-SSE e NW-SE, respectivamente, fraturas de tração de
alto ângulo com orientação ENE-WSW. Na terceira fase deformacional (D3) tem-se o
desenvolvimento de zonas de cisalhamento destrais e sinistrais com orientação segundo ENE-WSW e NE-SW. Os domínios hematitizados são caracterizados como laminar, maciço em bolsões, stringer, maciço venular, stockwork e brechóide. Os três primeiros são fortemente controlados pela estratificação da rocha encaixante e pela foliação S0//S2. Os domínios maciços são os que possuem os
mais altos teores de Fe2O3, atingindo 98%, e os mais baixos teores de Al2O3 e P2O5. Nesses domínios
há correlação negativa entre Fe2O3 e SiO2, Zr, Th, Sr, Nb, Hf, e Ba e positiva desse óxido com Zn, V,
U, Cu, Co, e com o somatório de Elementos Terras Raras (ETR). Quando normalizados pelo Post-Archean Australian Shale (PAAS), a distribuição dos ETR mostra, em geral, anomalias negativas de Eu, além de um fraco enriquecimento de Elementos Terras Raras leves (ETRL) em relação aos Elementos Terras Raras pesados (ETRP). Os elevados teores em Fe2O3 e os padrões de ETR
assemelham-se com os minérios de alto teor dos depósitos N5E da Província Carajás e do depósito Pau Branco do Quadrilátero Ferrífero e diferem significativamente de Formações Ferríferas Bandadas (BIF) de diversas regiões do mundo. O estudo da sucessão paragenética demonstra a existência de três grupos de hematita. Os grupos 1 e 2 relacionam-se com a formação, principalmente, dos domínios laminares e maciços em bolsões controlados pelas estratificação da rocha e pela foliação S0//S2. A
composição das hematitas aponta para a herança/influência da rocha encaixante. O terceiro grupo relaciona-se com os domínios maciços venulares. Para os grupos 1, 2 e 3 são identificadas, respectivamente, 3, 4 e 2 gerações de hematita, sendo que as hematitas da primeira geração são as que possuem a menor granulação. Determinações por LA-ICPMS mostram que os elementos traço nas hematitas têm picos de U, Pb e Ni. Para os ETRs tem-se padrões horizontais para a primeira geração de hematitas dos grupos 1 e 2 e enriquecimento em ETRP em relação aos ETRL para a geração mais tardia dos grupos 1 e 2 e as gerações do grupo 3. A alteração hematítica, hidrotermal, possui controle estrutural associado com zonas de cisalhamento da fase D2, nas quais se desenvolvem setores
dilatacionais com formação de veios. Além disso, tem-se o controle litológico relacionado com a porosidade e a permeabilidade da rocha, bem como pela presença de estruturas sedimentares que favorecem à difusão do fluido hidrotermal a partir das zonas de cisalhamento. A formação dos domínios de hematita envolveu processos de substituição das encaixantes e precipitação direta do fluido hidrotermal, preliminarmente interpretado como bacinal. O mesmo provavelmente lixiviou componentes químicos das unidades metassedimentares do Supergrupo Espinhaço e de diques que ocorreu regionalmente, se movimentando durante as fases deformacionais que inverteram o Aulacógeno do Paramirim.
host rocks of hematite domains in the area. They represent a new discovery of high-grade hydrothermal iron ore in Bahia, opening new opportunities for mineral exploration in the State. Three deformational phases are identified. The first one (D1) developed kink folds with subordinate chevron folds, shear zones related to flexural slip with S/C structures, stretch lines (LX1) slickenline and slickensides. The second phase (D2) is represented by dextral and sinistral reverse high-angle shear zones with NNW-SSE and NW-SE orientation trends, respectively, and high-angle tension fractures trending ENE-WSW. During the the third deformation phase (D3), dextral and sinistral shear zones with ENE-WSW and NE-SW orientation were developed. Laminar, massive, stringer, venular, stockwork and breccia hematite domains are characterized. The first three are strongly controlled by the rock layering and the foliation S0//S2. Massive domains are those with the higher Fe2O3 content, reaching 98%, with low Al2O3 and P2O5 contents. In these domains Fe2O3 is negatively correlatated with SiO2, Zr, Th, Sr, Nb, Hf and Ba. On the other hand, there Fe2O3 is positively correlated with Zn, V, U, Cu, Co and with the sum of Rare Earth Elements (REE). When normalized to the post-Archean Australian Shale (PAAS), the distribution of REE show generally negative Eu anomalies, and a weak enrichment of light rare earth elements (LREE) compared to heavy rare earth elements (HREE). The high Fe2O3 content and REE patterns are similar to the high-grade iron ores at the N5E deposit, Carajás Province, and are significantly different from banded iron formation (BIF) of other regions around the world. The paragenetic study shows the existence of three hematite groups. Groups 1 and 2 are related to the formation mainly of massive domains controlled by the rock stratification and foliation S0//S1. Their hematite composition points to the inheritance/influence of the host rock. The third group is related to the venular domains. For groups 1, 2 and 3 respectively 3, 4 and 2 generations of hematite are identified. The first generation of hematite is the finest of them all. LA-ICPMS trace element analyses show U, Pb and Ni peaks in hematite. The REE have a horizontal pattern for the first generation of hematite in groups 1 and 2, and HREE enrichment relative to LREE for the later generation of groups 1 and 2 and the generations of Group 3. Hematite hydrothermal alteration has structural features associated with shear zones of phase D2, in which dilational sectors developed and vein were formed. Furthermore, there is the lithologic control associated with rock porosity and permeability, as well as the presence of sedimentary structures that assisted hydrothermal fluid diffusion nuclated from shear zones. The hematite domains involved host rock replacement and hydrothermal fluid precipitation, preliminarly interpreted as basinal. The fluid probably leached chemicals components of the Espinhaço Supergroup metasedimentary units and dikes, moving during the deformational stages that inverted the Paramirim Aulacogen.
SUMÁRIO
CAPÍTULO 1 INTRODUÇÃO GERAL 9
CAPÍTULO 2 - ARTIGO 1 18
1. Introdução 20
2. Contexto Geológico Regional 21
3. Materiais e Métodos 24
4. Sistema Transpressional de Ibicoara 25
5. Geologia dos Depósitos de Ferro de Ibicoara 28
5.1 Rocha Hospedeira 28
5.2 Localização e Geometria dos Domínios Enriquecidos em Hematita 31
6. Sucessão Paragenética 34
7. Geoquímica dos Depósitos 38
8. Análise por LA-ICPMS de Hematita 46
8.1 Diagramas Multielementares 46
8.2 Elementos Terras Raras 46
9. Discussão 51
9.1. Evolução deformacional 51
9.2. Sucessão Paragenética 52
9.3. Variação Química dos Domínios Hematitizados e a Influência da Rocha Hospedeira
54
9.4. Alteração Hidrotermal, Controle e Modelo Evolutivo para o Depósito 58
10. Conclusão 64
CAPÍTULO 3 – CONCLUSÃO GERAL 74
APÊNDICE A – JUSTIFICATIVA DA PARTICIPAÇÃO DOS AUTORES 77
APÊNDICE B – TABELA DE DADOS GEOQUÍMICOS 78
APÊNDICE C – TABELA DE DADOS GEOQUÍMICOS 79
APÊNDICE D- TABELA DE DADOS DA ANÁLISE LA-ICPMS HEMATITAS HEM1-G1
80
ANEXO A- REGRAS DE FORMATAÇÃO DA REVISTA 109
Cavalgamentos da Chapada Diamantina Ocidental (CDO). Esse cinturão representa o domínio das deformações epidérmicas relacionadas com a inversão do Aulacógeno do Paramirim (Sensu Pedrosa Soares et al., 2001).
Figura 1: Mapa de situação (a) e localização das áreas de estudo (b). c) Mapa geológico regional simplificado
com as principais unidades geológicas. Fontes: (a) Modificado de GOOGLE Maps 2014; Santos (2011); c) modificado de Danderfer Filho (1990).
O Aulacógeno do Paramirim possui como unidades de preenchimento as rochas metassedimentares de idades paleo, meso e neoproterozoicas dos supergrupos Espinhaço e São Francisco. Os depósitos da região de Ibicoara são de alto teor de ferro (high-grade iron) e estão hospedados em metarenitos estéreis, por vezes arcoseano, da Formação Tombador, do Supergrupo Espinhaço. As unidades desse Supergrupo, com idade entre 1,78-1,1 Ga (Danderfer Filho et al., 2009, 2015; Chemale Jr et al., 2012; Guadagnin et al., 2013), compreendem predominantemente rochas siliciclásticas com metavulcânicas félsicas subordinadas (Guimarães et al., 2005, 2012; Danderfer Filho et al., 2015). Algumas propostas de empilhamento estratigráfico já foram aventadas para esse Supergrupo na região da Chapada Diamantina (Fig. 2). A Formação Tombador, que hospeda os depósitos de ferro de Ibicoara, situa-se no Grupo Chapada Diamantina e nela predominam metarenito continental costeiro, depositado sob clima semiárido, através de sistemas de leques aluviais, fluvial entrelaçado multicanalizado e eólico (Guimarães et. al., 2012) (Fig. 3).
No Supergrupo Espinhaço algumas unidades são ferruginosas mas, em geral, predominam arenitos quartzosos e arcoseanos, estéreis. No entanto, trabalhos exploratórios por empresas privadas realizados na porção setentrional da Chapada Diamantina, especificamente na região de Ibicoara, mostram a ocorrências de domínios hematizados, que atingem teores de até 98,26% de Fe2O3t. Esses depósitos são muito particulares por se
tratarem de zonas enriquecidas em hematita hospedadas em metarenitos estéries, arcoseanos, diferentes de outros exemplos de enriquecimento hospedados em formações ferríferas bandadas (Rosière e Chemale Jr, 2000, Klein, 2005; Rosière e Rios, 2006, Beukes e Gutzmer, 2008, Figueiredo e Silva et al., 2011).
Os principais minérios de ferro de classe mundial estão associados com processos químico-exalativos em sequências metavulcanossedimentares arqueanas e paleoproterozoicas (Rosière e Chemale Jr, 2000, Klein, 2005; Rosière e Rios, 2006, Beukes e Gutzmer, 2008;), mas minérios de ferro de alto teor (high-grade iron ore) e associados com fluidos hidrotermais hipogênicos vêm ganhando cada vez mais destaque no cenário das mineralizações de ferro no mundo (Beukes et al., 2003, Rosière e Rios, 2004, Rosière et al., 2008; Dalastra e Rosière, 2008; Kock et al., 2008, Gutzmer et al., 2008, Figueiredo e Silva et al., 2011). A existência de depósitos de ferro de alto teor encaixados na Formação Tombador (Grupo Chapada Diamantina) é de amplo interesse científico, tendo em vista que trata-se de uma formação mesoproterozoica depositada em bacia intracontinental e em ambiente fluvial,
aplicado para a exploração de ferro na Chapada Diamantina.
O objetivo principal deste trabalho é contribuir cientificamente com o entendimento dos processos e controles na formação de depósitos de ferro de alto teor hospedados em sequências metassedimentares estéreis, tendo como principal laboratório os corpos de hematita da região de Ibicoara. Os objetivos específicos são: (i) detreminar o contexto geológico das ocorrências de hematita nos dois depósitos selecionados para estudo; (ii) levantar as características petrográficas/paragenéticas e geoquímicas dos domínios hematitizados; (iii) determinar os controles desses domínios; e (iii) elaborar um modelo evolutivo para a mineralização.
A principal justificativa do trabalho é que na Bahia, em especial na Chapada Diamantina, alguns depósitos de ferro vêm sendo descobertos ao logo da última década, mas pouco ou nada se sabe sobre a geometria desses depósitos, bem como sobre seus processos formadores. Dessa forma, o estudo pode fornecer elementos científicos para elucidar os processos formadores dos depósitos de ferro da região de Ibicoara, além de colaborar com um tema de amplo interesse mundial e relacionado com as condições geológicas para a formação de minérios de alto teor, além de fornecer subsídios para pesquisa exploratória de outros depósitos da região.
Para alcançar tais objetivos, utilizamos as seguintes etapas:
(i) Estudos bibliográficos com levantamento das principais publicações sobre alteração hidrotermal, depósitos de ferro, em especial os de alto teor e da geologia da área de estudo;
(ii) Trabalhos de campo, totalizando quinze dias, com visitas a 35 afloramentos na região de Ibicoara, nos quais foram identificados e descritos os tipos de domínios ricos em hematita. Além disso, nessa etapa foi realizada a coleta de amostras para estudos petrográficos, geoquímicos de rocha total e da hematita;
(iii) Análise estrutural com descrição de seções geológicas regionais e locais para a determinação das fases deformacionais e controle estrutural dos domínios hematitizados.
Utilizou-se o método clássico de aquisição de dados estruturais tridimensionais com bússola tipo Clar (Modelo Gekom-Breithaupt). Os dados foram obtidos pelo sistema Dip-Direction. O tratamento estatístico foi feito através do geosoftware STEREONET 9.5 (Allmendinger et al., 2012);
(iv) Estudos petrográficos, que colimaram com a descrição de quarenta e cinco lâminas delgadas polidas na qual determinou-se a mineralogia e as características microestruturais das zonas enriquecidas em hematita. Além disso, nesses estudos procedeu-se a determinação da sucessão paragenética;
(v) Estudos geoquímicos em rocha total em vinte e nove amostras de domínios enriquecidos em hematita, bem como em uma amostra da rocha hospedeira da mineralização. As análises foram realizadas no laboratório Acme Analytical Laboratories LTDA. Os resultados obtidos foram organizados e tratados em diagramas específicos utilizando o software EXCEL (2013). O normalizador utilizado foi o Post Archean Australian Shale (PAAS) (Mclennan, 1989);
(vi) Análises químicas de hematitas por LA-ICP-MS em sete amostras no laboratório da Universidade Federal de Ouro Preto (UFOP);
(vii) Análises da hematita em Microssonda Eletrônica;
(viii) Microscopia eletrônica de varredura (MEV) em cinco amostras para o estudo de minerais traço. Detalhes das análises acima mencionadas estão apresentados na seção de materiais e método do artigo no capítulo 2. O estudo foi desenvolvido no laboratório da UFOP em um MEV da marca JEOL, JSM-6510. O equipamento permitiu a produção de imagens dos minerais traços de alta resolução e sua composição química qualitativa.
Depois de interpretada a sucessão paragenética, algumas amostras foram selecionadas para o estudo de inclusões fluidas em hematita. Entretanto, não obteve-se sucesso nessas análises, pois as amostras não apresentam inclusões e/ou as inclusões são muito pequenas (nanoscópicas) para a realização de análises. Também tentou-se a realização de estudos geocronológicos em monazitas, identificadas no MEV, através de Microssonda Eletrônica, mas também não houve sucesso nessa etapa, uma vez que os grãos de monazita apresentam granulação inferior a 5 micras.
Essa dissertação está organizada em três capítulos. No capítulo 1 apresenta-se a introdução Geral, com a apresentação do tema, problemas, objetivos, justificativas e materiais e métodos. No capítulo 2 apresenta-se o artigo científico submetido na revista Ore Geology Reviews e no capítulo 3 têm-se as Conclusões da Dissertação, com algumas recomendações.
Figura 3: Mapa (a) e colunas estratigráficas (b, c, d) mostrando o empilhamento das unidades do Grupo Chapada Diamantina. Modificado de Guimarães et al. (2012).
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CAPÍTULO 2
STRUCTURAL CONTROL AND CHARACTERIZATION OF
HIGH-HEMATITE CONTENT HYDROTHERMAL DEPOSITS
IN TRANSPRESSIONAL DOMAINS OF THE EASTERN
CHAPADA DIAMANTINA THRUST AND FOLD BELT
V. Anjos Cruza, S. C. P. Cruza, L. Lobatob, J. S. Santosc, G. M. P. Limaa, F. J. Riosd,
a Universidade Federal da Bahia, Instituto de Geociências, Salvador, BA, Brasil; b Universidade Federal de
Minas Gerais, Centro de Pesquisas Prof. Manoel Teixeira da Costa, Instituto de Geociências, Belo Horizonte, MG, Brazil, c Geólogo pela Universidade Federal da Bahia, d Centro de Desenvolvimento de Tecnologia Nuclear (CDTN), Vista do Sol, Belo Horizonte, MG, Brasil,
RESUMO
Os depósitos hematíticos de alto teor de Ibicoara estão inseridos no setor epidérmico do Cinturão de Dobramentos e Cavalgamentos da Chapada Diamantina, no Cráton do São Francisco. As rochas hospedeiras da mineralização são arenitos quartzosos ou arcoseanos, da Formação Tombador, que apresentam estratificação plano paralela. Regionalmente, três fases deformacionais foram identificadas. A primeira (D1) desenvolveu dobras do tipo kink, com chevron subordinada, zonas de
cisalhamento relacionadas a deslizamento flexural com estruturas S/C, linhas de estiramento (Lx1), slickenline e slickensides. A segunda fase (D2) nucleou zonas de cisalhamentos destrais e sinistrais
reversas de alto ângulo com orientação geral segundo NNW-SSE e NW-SE, respectivamente, fraturas de tração de alto ângulo com orientação ENE-WSW. Na terceira fase deformacional (D3) ve-se o
desenvolvimento de zonas de cisalhamento destrais e sinistrais com orientação segundo ENE-WSW e NE-SW. Os domínios hematitizados mais ricos são maciços e neles o teor de Fe2O3 atinge 98%. Esses
domínios distribuem-se ao longo da foliação primaria sedimentar (estratificação plano paralela e cruzada tangencial), nas foliações das zonas de cisalhamento da fase D2 ou em fraturas de tração
geradas nessa fase deformacional. Os domínios maciços em bolsões gradam lateralmente para domínios laminar e stringer. Além disso, tem-se o maciço venular, stock work e brechóide que truncam os domínios maciços em bolsões ou a rocha hospedeira. Análises químicas dos domínios hematitizados mostram que há uma correlação negativa entre Fe2O3 e SiO2, Zr, Th, Sr, Nb, Hf, e Ba e
positiva desse óxido com Zn, V, U, Cu, Co e com o somatório de Elementos Terras Raras (ETR). Quando normalizados pelo Post-Archean Australian Shale (PAAS), as assinaturas dos Elementos Terras Raras nos domínios hematitizados mostram, em geral, anomalias negativas de Eu, além de um fraco enriquecimento de Elementos Terras Raras leves (ETRL) em relação aos Elementos Terras Raras pesados (ETRP). O estudo da sucessão paragenética demonstrou a existência de três grupos de hematita. Os grupos 1 e 2 relacionam-se com a formação, principalmente, dos domínios laminares maciços em bolsões controlados pelas estratificação da rocha e pelas foliações S0//S2 em zonas de
cisalhamento. A distinção entre eles se dá pela menor granulação da primeira geração de hematita e pela presença de domínios brechóides no segundo grupo. O terceiro grupo relaciona-se com os domínios venulares. Para cada grupo nota-se um aumento sucessivo da granulação da hematita e um aumento relativo dos Elementos Terras Raras pesados em relação aos leves. Além disso, semelhanças entre as assinaturas químicas das hematitas de primeira geração dos grupos 1 e 2 com as assinaturas da rocha hospedeira revelam uma influência da composição química da rocha original na composição das primeiras hematitas geradas no depósito. Essa influência não é observada nem para as hematitas tardias dos domínios maciços em bolsões, nem para as hematitas dos domínios maciços venulares. A
Chapada Diamantin epidermal fold and thrust belt. Iron mineralization host rocks are quartz and arcosean sandstones from the Tombador Formation, which present planar stratification. Regionally, three deformational phases are identified. The first D1 phase developed kink with
subordinate chevron folds, shear zones related to flexural slip with S/C structures, stretch lines (LX1), slickenlines and slickensides. The second phase D2 is represented by dextral and
sinistral, reverse high-angle shear zones with NNW-SSE and NW-SE orientation trends, respectively, high-angle tension fractures with an ENE-WSW orientation. During the third deformation phase D3, dextral and sinistral shear zones with ENE-WSW and NE-SW
orientations were developed. The richest hematite domains are massive, having Fe2O3
contents to 98%. These domains are distributed along the primary sedimentary foliation (plane-parallel stratification and tangential cross-bedding), the foliations of D2 shear zones, or
traction fractures generated during D2. The massive hematite domains grade laterally into
platy and stringer domains. In addition, there are vein, stockwork and breccia mineralization styles that crosscut massive domains or host rock. Chemical analysis of hematite domains show that Fe2O3 correlates negatively with SiO2, Zr, Th, Sr, Nb, Hf and Ba. On the other
hand, it correlates positively with Zn, V, U, Cu, Co, and with the sum of rare earth elements (REE). When normalized to the post-Archean Australian Shale (PAAS), the distribution of REE generally show negative Eu anomalies, and a weak enrichment of light rare earth elements (LREE) compared to the heavy rare earth elements (HREE). The paragenetic succession shows the existence of three groups of hematite. Groups 1 and 2 are mainly related to the massive domains controlled by rock stratification and the S0//S2 foliation in shear zones.
The distinction between them is given by the finer grain size of the first generation of hematite and the presence of breccia domain in the second group. The third group is related to vein style domains. For each group, the hematite grain size increases successively, and there is a relative increase in the HREE in relation to the LREE. Furthermore, similarities between the chemical signatures of the first groups 1 and 2 hematite generation with the signatures of the host rock reveal an important influence of the chemical composition of the original rock. This is not the case for the late hematite of massive domains, as well as vein-style hematite, suggesting high fluid-rock conditions. The hematite hydrothermal alteration at Ibicoara has structural control, with dilational structures associated with D2 shear zones. The host lithotype
was also a contributing factor due to its porosity and permeability, and the presence of sedimentary structures that favored flow of the hydrothermal fluid from shear zones. Replacement and direct precipitation took place and were responsible for the transfer of solutions and formation of hematite-rich areas. The hydrothermal fluid probably leached components from the Espinhaço Supergroup metasedimentary units and dikes that are regionally present, and carried these during the deformational phases that inverted the Paramirim aulacogen.
1. Introdução
The high iron content deposits of Ibicoara, Brazil, are located within the Western Chapada Diamantina Thrust and Fold Belt, in the São Francisco Craton (Almeida, 1977) (Fig.1). This epidermal belt was generated by the inversion of the Paramirim Aulacogen (Pedrosa Soares et al., 2001) (Fig. 2) during the Neoproterozoic (Cruz and Alkmim, 2006). The Ibicoara transpressional domain is observed within this feature. The main filling unit of this aulacogen is the Espinhaço Supergroup, aged between 1.78 Ga and 1.1 Ga (Danderfer et al., 2009, 2015; Chemale Jr et al., 2012; Guadagnin et al., 2013), with up to 8,000 m in thickness (Cruz and Alkmim 2016). The Tombador Formation is among the most relevant components of the aulacogen. This formation encompasses mainly metasandstones (either pure or arkosic) and metaconglomerates deposited in fluvial and aeolian environments (Guimarães et al. 2012).
Iron deposits mainly formed by hypogenic processes are hosted in banded iron formations (BIF). Examples of deposits of this nature occur in the Carajás Province and Quadrilátero Ferrífero district (Brazil), in the Hamersley Province and Yilgarn and Pilbara cratons (Western Australia), in the Kaapvaal Province (Africa), in the Western-Africa Craton, and in India, Ukraine, and China (Dalstra and Guedes, 2004, Rosière and Rios, 2004; Cope et al., 2008, Dalstra and Rosière, 2008; Mukhopadhyay et al., 2008, Rosière et al., 2008; Figueiredo and Silva et al., 2011, 2013a, 2013b; Angerer et al., 2012, Zhang et al., 2012, Teitler et al., 2014, Hagemann et al., 2016). The formation of these deposits is related to sequential mineral substitution of BIF by hydrothermal processes, resulting in changes in mineralogy and texture, and also in the development of hydrothermal alteration zones with various intensities of mineral substitution (Beukes et al., 2003; Dalstra and Guedes, 2004, Dalstra and Rosière, 2008, Angerer and Hagemann, 2010, Duuring and Hagemann, 2013a,b, Figueiredo and Silva et al., 2013a,b, Maskell et al., 2013). Deformational structures, such as fold and shear zones, represent important ducts for the formation of this type of deposit (Holdsworth and Pinheiro, 2000, Lobato et al., 2005, Dalstra and Rosière, 2008, Figueiredo and Silva et al., 2008, 2013a, b, Santos et al., 2010, Thorne et al., 2014, Angerer et al., 2016). In turn, the high iron content hematite deposits of the Ibicoara region that are hosted in the metasandstones of the Tombador Formation reach Fe2O3 contents of up to 98.26%, and are quite particular considering that they represent hematite-enriched zones hosted in sterile metasandstones. These deposits are regionally discontinuous, a unique example in the literature, and have been the focus of several exploratory studies conducted by companies of
Figure 1: Schematic geological map of the eastern portion of Brazil showing the São Francisco Craton limits and the location of the studied deposits.
The objective of the present study was to present the geological context of the high iron content deposits in the area of Ibicoara, as well as the petrographic and chemical characteristics of the mineralized hematite domains. In addition, the present study proposes an evolution model in order to collaborate with the comprehension of processes related to the formation of high iron content ores hosted in sterile metavolcano-sedimentary sequences.
2. Regional Geological Context
The Paramirim Aulacogen was developed by the succession of six rifts that evolved between 1.75 and 0.65 Ga (Pedrosa Soares and Alkmim, 2011). The inversion of this
aulacogen occurred during the Ediacaran through the development of thrust and fold belts (Danderfer Filho, 1990, 2000; Cruz, 2004), among which is the Western Chapada Diamantina Thrust and Fold Belt (Danderfer Filho, 2000) (Fig.2).
The Espinhaço and São Francisco supergroups are the filling units of the Paramirim Aulacogen. The Espinhaço Supergroup is characterized by a sequence of siliciclastic rocks, predominantly metasandstones, and felsic metavolcanic rocks, which were deposited between the Statherian and Stenian periods (Shobbenhaus et al., 1994; Babinsky et al., 1999; Danderfer Filho et al., 2009, 2015; Chemale Jr et al., 2012; Guadagnin et al., 2013). The Espinhaço Supergroup is comprised from its base to its top in the Chapada Diamantina area by the Serra da Gameleira Formation and by the Rio dos Remédios, Paraguaçu, and Chapada Diamantina groups (Guimarães et al., 2012). The Tombador Formation is among the units that comprise the Chapada Diamantina Group, hosting the high iron content deposits investigated in the present study. According to Guimarães et al. (2012) and Guadagnin et al. (2013, 2015), this formation was deposited in a coastal environment by intertwined alluvial and fluvial fan systems, with aeolian reworking at a low base sea level. The same authors subdivided the formation into: (i) Lower Unit, which encompasses metaconglomerates, poorly sorted impure and feldspar metasandstones; meta-quartz arenites with trough and tangential cross-bedding, and crystal-rich metavolcanoclastic rocks and metaconglomerates, which host metavolcanic rock clasts. Guadagnin et al. (2015) obtained crystallization ages for the crystal-rich metavolcanoclastic rocks (U-Pb, zircon, LA-ICPMS) of 1,436±26 Ma, 1,437±50 Ma, and 1,416±28 Ma; and (ii) Upper Unit, which consists of well-sorted bimodal meta-quartz arenites with cross and tangential bedding, and levels of feldspar metasandstones and metaconglomerates.
The São Francisco Supergroup in the Chapada Diamantina encompasses the Bebedouro and Salitre formations. The first of these formations consists of four lithofacies of siliciclastic rocks and one lithofacies of carbonate rock (Guimarães et al., 2012). The results of a study on the origin of detrital zircons of diamictites from the Bebedouro Formation, conducted by Figueiredo et al. (2009), suggested maximum deposition age of 874±9 Ma. The Salitre Formation is subdivided into four units of carbonate and siliciclastic lithofacies with predominance of calcarenites and dolomites (Guimarães et al., 2012). Felsic metavolcanic rocks were dated as 669±14 Ma by Santana (2016; zircon, U-Pb, LA-ICPMS).
Figure 2: Simplified geological map of the Espinhaço and São Francisco supergroups in the Paramirim Aulacogen (Guimarães et al., 2012). OA – Araçuaí Orogen, SFC – São Francisco Craton, Gr – Group, Fm – Formation.
The epidermal Western Chapada Diamantina Thrust and Fold Belt (Fig. 2) (Danderfer-Filho 1990, 2000; Lagoeiro 1990; Cruz et al. 2012) is located within the intracontinental sector of the Araçuaí-Congo Orogen. The Ibicoara transpressional domain, defined in the present study, is situated in easternmost area of the orogen. The structural framework of this feature was also studied by Pedreira and Margalho (1990), Santos (2011), Moitinho (2011), Santana (2011), and Maia (2011). The results obtained by these authors converge towards the existence of two regional tension fields: an older one, following a WSW-ENE trend; and a younger one, following a NW-SE trend. These tension fields were responsible for the nucleation of reverse shear zones, push-up and transcurrent zones, and duplexes and folds generally oriented NNW-SSE and E-W.
3. Material and Methods
During field campaigns, outcrops of the studied deposits were visited in order to identify the types of hematite-rich domains and describe the sampling wells owned by Cabral Resources LTDA, which measured between 1 and 7 meters in depth. Moreover, structural data and samples were collected for petrographic and lithogeochemical analyses.
The structural analysis was conducted by describing regional and local geological cross-sections in order to determine deformational phases and the structural control of hematite-enriched zones and their relationship with deformational phases. The classic method for tridimensional structural data acquisition was performed using a Brunton compass and a Clar Com-Pro Transit compass. Data were obtained using the Dip-Direction system. Statistical analysis was performed using the geosoftware STEREONET 9.5 (Allmendinger et al., 2012).
Petrographic analyses involved the description of 45 polished thin sections in order to determine the mineralogy and microstructural characteristics of hematite-enriched zones. In addition, paragenetic succession was also determined in the present study.
A total of 29 samples from hematite-rich domains and a sample from the country rock were forwarded to Acme Analytical Laboratories LTDA. The results obtained were organized and processed using specific diagrams of the software EXCEL (2013). Major element oxides (SiO2, Al2O3, Fe2O3, MgO, CaO, TiO2, P2O5, MnO, Na2O, Cr2O5 and K2O) were analyzed using the ICP-AES method. Minor elements (Ba, Rb, Sr, Y, Zr, Nb, Th, Pb, Ga, Zn, Cu, Ni, V, Cr, Hf, Cs, Sc, Ta, Co, Be, U, W, Sn, Mo and Au) and rare earth elements (REE) (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) were analyzed using mass spectrometry (ICP-MS). Iron oxide (FeO) was determined through titration followed by an
system coupled to a mass spectrometer (ICP-MS Agilent Technologies 7700x) at the laboratory of the Universidade Federal de Ouro Preto (UFOP). Data was acquired in 55 s, with 30-µm spots, pulse intensity of 8-9 J/cm2, and laser light frequency of 10 Hz. Samples underwent ablation in a ultra-high purity He atmosphere, which was later mixed with Ar for the transportation of the material to the ICP-MS. Analyses were performed in inclusion-free hematite crystals and crystals that were larger than the spot.
An electronic microprobe was used in order to determine the internal pattern to be used in the LA-ICP-MS analyses. The oxides SiO2, Fe2O3, MnO, P2O5, Cr2O3, V2O5, NiO and TiO2 were analyzed in order to determine this internal pattern. V2O5 was chosen as the internal pattern for the LA-ICP-MS analyses aiming to determine which of the aforementioned oxides presented the lowest variation among the samples analyzed. The JEOL 8230 SuperProbe from the UFOP laboratory was used for this analysis. The microprobe was equipped with five dispersive wavelength spectrometers, under a 5 µm-wide beam for larger hematites, and a focused beam for smaller hematites, using a voltage of 15 kV and current of 20 nA.
A Scanning Electron Microscope (SEM) equipped with an Energy-Dispersive X-ray Spectroscope (EDS) was used to assess trace elements in five samples. This evaluation took place at the UFOP laboratory using a JEOL JSM-6510 SEM. The equipment produced high-resolution images of trace minerals and their qualitative chemical composition. This information was used to identify the best areas for LA-ICP-MS analysis, which are those free from inclusions and zonings.
4. Ibicoara Transpressional System
The analysis of structural information yielded the identification of three families of deformational structures. The oldest one (phase D1) was represented by regional folds, such as the Campo Redondo and Mundo Novo anticlinal folds (Pedreira and Margalho, 1990). Kink folds predominated (Fig. 3a), with subordinated chevron fold. Fault-propagation folds
were also observed (Santos, 2011). Primary foliation (S0) presented maximum plane oriented 056°/03° (Fig. 4a). The hinge line (Lb1) was generally positioned following a NNW-SSE orientation, with northward plunging. These folds vary from open to smooth (Sensu Fleuty, 1964). Moreover, they are harmonic, cylindrical, and are associated with reverse shear zones anchored in sub-horizontal displacements, forming a flat-ramp type of geometry (Suppe and Mendwedeff, 1990; Rolim and Alkmim, 2004). Stretching lineation lines (Lx1) and slickenlines were found along the movement planes, both of which presented high rake and slickensides (Fig. 3b). The polar isodensity diagram (Fig.4b) showed that the preferential orientation of Lx1 was 252°/32°. Shear surfaces related with flexural flows with well-developed S/C structures can also be found, as well as a spaced S0//S1 foliation with maximum planes positioned following 050°/06° (Santos 2011) (Fig.4c). Sub-horizontal quartz veins integrate the structural framework related to the oldest deformation phase identified in this sector of the Chapada Diamantina. These structures corresponded to the Eastern Chapada Diamantina Thrust and Fold Belt.
The second family of structures (phase D2) encompassed high-angle shear zones, with dextral and sinistral kinematics and a reverse component (Fig. 3c), which presented maximum planes positioned at 085°/79° and 217°/88° (Fig.4d-e), respectively. Stretching lineation was preferentially positioned according to a WSW-ENE trend (Fig. 4f). The main movement indicators were S/C structures, tension gashes, traction fractures, and echelon veins. These structures were filled with quartz, hematite, and subordinately, pseudotachylite (Fig. 3d-e). The main orientation of these veins was 334°/84° (Fig. 4g). Some of them were folded with sub-vertical hinge lines. Slickensides complete the structural framework of this family.
The third family of structures (phase D3) either truncated or reactivated the structures of family D2. This family encompassed high-angle shear zones, with dextral and sinistral kinematics (Figs. 5a-b), and maximum plane positioned at 354°/86° and 290°/79°, respectively (Fig. 4h-i). Quartz veins, in either parallel or en-èchelon systems, with maximum planes oriented 247°/79° (Fig. 4j), and tension gashes also occurred associated with this group of structures (Fig. 5c). Along with the previously described structures of the second family, this set of structures from phase D3 integrated the structural framework of the Ibicoara transpressional domain, where the most expressive regional representatives are the Sincorá and Rio Una shear zones (Fig. 6).
Figure 3: a) Kink fold of D1 phase developed in arcosean metarenite of the Tombador Formation; b) Motion plane associated with reverse shear zone of D1 phase with mineral stretching lineation (Lx1); c) Overview of the Sincorá shear zone developed in D2 phase; d) Hematite vein of D2 phase; e) Shear zone of D2 phase developed in arcosean metarenite of the Tombador Formation with specularite vein concordant with the foliation (S0//S2).
Figure 4: Structure diagrams of phases D1, D2 and D3 shown by polar isodensity diagrams: a) for foliations S0 (data obtained in Santos, 2011); b) for mineral stretching lineation (Lx1); c) for foliations S0//S1; d) of dextral shear zone of phase D2; e) of sinistral shear zone of phase D2; f) for mineral stretching lineation (Lx2); g) of hematite and pseudotachylyte veins of phase D2; h) of dextral shear zone of phase D3; i) of sinistral shear zone of phase D3, and; j) of quartz veins of phase D3.
5. Geology of the Ibicoara Iron Deposits
5.1 Host Rock
The host rocks of the hematite-rich domains were white arkosic metasandstones, with grain size ranging from medium to coarse sand from the Tombador Formation (Chapada Diamantina Group, Espinhaço Supergroup), which were intercalated with micro-conglomerates. Metamorphism was considered incipient. Moreover, the primary sedimentary structure (S0) was recognized by the presence of small plane-parallel and trough stratifications (Figs. 6 and 7).
Figure 5: a) Staggered veins indicating sinistral movement for the sinistral shear zone of phase D3; b) Motion plane of dextral shear zone of phase D3 with slikenlines and slickensides completing the structural framework; c) Quartz veins in parallel system and tension gashes indicating sinistral movement of the shear zone of phase D3.
Figure 6: Geological map of the study area with the location of deposits 1 and 2. Based on Pedreira and Margalho (1990) and Santos (2011).
Figure 7: Arcosean metasandstone of the Tombador formation: a) Small-scale tangential cross stratification; b) Plane-parallel stratification; the yellow line represents the primary rock stratification (S0).
5.2 Location and Geometry of Hematite-Rich Domains
Two deposits were defined: (i) Deposit 1, westwards from the Rio Una Shear Zone; and (ii) Deposit 2, eastwards from the Sincorá Shear Zone (Fig. 6).
Hematite-rich domains are classified as:
(a) Laminar, in which disseminated discontinuous levels are distributed according to the deposition surface (S0) of the country rock (Figs. 8a-d);
(b) Massive, which truncate rock stratification and occur in two ways: (i) forming irregular features, presenting various sizes and geometries (irregular massive), which are laterally connected with laminar domains. In this case, they are subdivided as fine-grained domains (0.05-1.00 mm) and coarse-grained domains (0.25-2.20 mm) (Figs. 8e-f); and (ii) forming veins (vein massive), presenting structures with lengths ranging from 3 to 7 cm, and thickness ranging from 0.2 mm to 2 cm (Figs. 10a-b). These domains follow the orientation of the cross-bed stratifications present in host rocks, which are discordant in relation to S0;
(c) Stringer, which is mainly associated with the laminar domain, presenting lengths ranging from 11 mm to 23 cm, and thickness from 0.5 to 3 cm (Figs. 9a-c). This domain is discordant in relation to rock stratification cossets (S0) and is positioned following the sets;
(d) Stockwork, with fragments from either the massive and laminar hematite domains or from the country rock, sealed by smaller hematite crystals (Figs. 10c-d); and
(e) Breccia, where either hematite breccia with metasandstone grains (Fig. 10e) or grains from hematite-rich massive domains predominate (Fig. 10f). Clasts range from sub-angular to sub-angular, with sizes ranging from 2.5 to 5.0 cm. Modal values and the main petrographic characteristics of each type of hematite-rich domain are presented in Table 1.
Some trace minerals (mode < 1%) with grain size lower that 5 µm were found in all domains using SEM, such as: uraninite, chromite, baddeleyite, cassiterite, monazite, and jamesonite.
Figure 8: a) Macroscopic characteristics of lamellar hematite domain; b) Lamellar domain grades to massive domain with hematite veins; c) Lamellar domain shows lateral relationship with massive domain and venular massive; d) Microscopic aspect of the lamellar domain; e) Macroscopic aspect of the massive domain; f) Microscopic characteristics of the massive domain. S0 - Primary structure, LD – lamellar domain, VD – venular, massive domain, MD – irregular massive domain, Hem – hematite, Qz – quartz (Abbreviations: Whitney and Evans, 2010).
Figure 9: Macroscopic aspect of the stringer domain; b) Hand sample of the stringer domain; c) Microscopic aspects of the stringer domain. S0 - primary structure, SD – stringer domain, Hem – hematite, Qz – quartz (abbreviations: Whitney and Evans, 2010).
Table 1: Petrographic characteristics of hematite-rich domains of the Ibicoara region. Hem – hematite, Qz – quartz (Abbreviations: Whitney e Evans, 2010).
6. Sucessão Paragenética
Three groups were defined considering the vein and laminar/irregular massive domains, the truncation relationships between the hematite crystals found within these same domains, and their grain sizes:
(i) Group I (G1) – associated with coarse-grained irregular massive domains;
(ii) Group II (G2) – related to fine-grained irregular massive domains, which evolve to breccia domains; and
(iii) Group III (G3) – associated with vein massive domains.
The characteristics of the hematites of each group are presented in Table 2 and are associated with the structure family of the second deformational phase (D2). All hematites were gray under reflected light and, in general, did not present preferential orientation, with predomination of radial arrangements. Hematites found in veins were exceptions. However, when they occurred, they presented orthogonal mineral growth lineation positioned in relation to the fracture wall. Nonetheless, structures of this kind are not very common. Hematite-hematite contact was straight but presented interlobate quartz grains. Corrosion edges were a common finding in this mineral (Figs. 11a-b). Polysynthetic twinning were observed in hematites 1 and 2 of groups 1 and 2, respectively.
Figure 10: a) Microscopic aspect of the massive venular domain; b) Photomicrograph of the venular domain; c) Stockwork domain with hematite veins; d) Photomicrograph of the stockwork domain; e) Macroscopic characteristics of the breccia domain; f) Photomicrograph of the breccia domain. VD – venular massive domain, MD – massive domain, SWD – stockwork domain, BD – breccia domain, Hem – hematite, Qz – quartz (Abbreviations: Whitney e Evans, 2010). Photomicrographs b, d and f taken under reflected, plane, polarized light.
Group 1- Crystals from the first generation of hematites (Hem1-G1) were formed without a preferential orientation, presenting an acicular habit, but followed the primary foliation (S0)
of the country rock. In these domains, hematites were disseminated among grains of quartz, while the country rock predominated (Fig. 12a). As the percentage of hematite increased, crystals became coarser (Table 2), euhedral and lamellar, with a predominant radial arrangement. Hematites 2 (Fig. 12b) (Hem2-G1) and 3 (Hem3-G1) (Fig. 12c) refer to the second and third generations of hematite. With the enrichment of this domain with Hem2-G1 and Hem3-G1, quartz-hematite contacts become gradually more interlobate and edge corrosion features became more common. Skeletal quartz grains were observed in the domains that were richer in Hem3-G1 (Fig. 11a-b).
Group 2- The first generation of this group (Hem1-G2) did not present preferential orientation either and was the finest of the group, mostly presenting an acicular habit. Comparatively to Group 1, the hematites of Group 2 were finer. In addition, regarding Hem1-G1, the domain that presented the first generation of hematite grains (Hem1-G2) (Fig. 12d) also presented the lowest volume of country rock. The second hematite generation (Hem2-G2) caused pseudomorphism in rock fragments, originating angular to round hematite pseudomorphs with sizes ranging between 0.75 and 1.75 mm (Fig. 12e-f). There was no significant increase in hematite Hem2-G2 grain size in relation to Hem1-G2, and the percentage of country rock practically did not vary between these two generations. The third hematite generation (Hem3-G2) (Fig. 13a) was coarser than the other generations of this group and, in this case, lamellar hematites predominated. The fourth hematite generation (Hem4-G2) (Fig. 13b) was related to the breccia domain, and composed the matrix of the breccia. This generation was finer than the previous ones (0.04 to 0.5 mm) and presented dominant acicular habit.
Group 3- This hematite group occurred in veins. The first generation (Hem1-G3) was the finest and presented acicular habit (Fig.13c). The second generation (Hem2 -G3) formed polycrystalline agglomerates, coarser than Hem1-G3, with predominating radial habit (Fig. 13d). Similarities were observed between the grain sizes of hematites from Group 1 and Group 3.
Figure 12: Photomicrographs of groups 1 and 2: a) Hematite 1 from Group 1 (Hem1-G1); b) Hematite agglomerates generating hematite 2 (Hem2-G1); c) Hematite 3 from group 1 (Hem3-G1); d) Hematite 1 from Group 2 (Hem1-G2); e) Hematite 2 from Group 2 (Hem2-G2) forming rounded pseudomorphs with domains of Hem1-G2. Photomicrographs taken under reflected, plane polarized light.
Figure 13: Photomicrographs of hematite from Groups 2 and 3. a) Hematite 3 from Group 2 (Hem3-G2); b) Hematite 4 from Group 2 (Hem4-G2) with fragments of Hem3-G2; c) Hematite 1 from Group 3 (Hem1-G3); d) Hematite 2 from Group 3 (Hem2-G3) with domains of Hem1-G1. Photomicrographs taken under reflected, plane polarized light.
7. Geochemistry of the deposits
Major, trace, and REE were analyzed in 29 samples of the studied deposits. Twenty-eight of these samples were divided into hematite-rich laminar domains, irregular massive domains, vein massive domains, and host rock of the hematitized domains.
Variations of SiO2 and Fe2O3total in deposit 1 ranged between 0.52% and 47.27%, and
between 44.41% and 97.59%, respectively. In turn, in deposit 2, this variation ranged between 0.38% and 3.71% for SiO2, and between 84.32% and 98.26% for Fe2O3total. Deposit 2
presented the highest iron contents and the lowest silica contents. These values were higher in Fe2O3total and lower in SiO2 (Fig. 14) when compared to the mean variations of contents in
several BIF, such as those studied by Klein (2005), Lindemayer et al. (2001) in the Serra de Carajás (Brazil), Pickard, (2003) in the Kuruman deposit (South Africa), Dauphas (2007) in
Figure 14: Comparison of the Fe2O3 and SiO2 contents of the classic BIF and the Carajás ore with the hematite domains from the Ibicoara region. MSJC- São José do Campestre Massive, BIFs.
The binary diagrams for major elements presented in Figure 15 showed a negative correlation between Fe2O3 and SiO2, and between this component and Al2O3. In the host rock
sample analyzed, contents of Fe2O3 and SiO2 were 0.32 and 97.24%, respectively. Irregular
massive and vein massive hematite domains presented the highest contents of Fe2O3, while
the laminar domain presented the lowest contents (Figs. 15a and 15b). Al2O3 enrichment was
observed in the hematite-rich laminar domain, followed by impoverishment towards the irregular and vein massive domains. There was no correlation pattern between P2O5 and
Fe2O3 data, but the low content of this component was striking (Fig. 15c).
Figure 15: Binary diagrams for the high-Fe2O3 domains in the deposits of the Ibicoara region. a) Fe2O3 versus SiO2; B) Fe2O3 versus Al2O3; and c) Fe2O3 versus P2O5.
Negative correlations were observed in trace elements between Fe2O3 and Zr, Th, Sr,
Nb, Hf, and Ba (Fig. 16a-f). The hematite vein massive domain presented the lowest contents of these elements, while laminar and irregular massive domains presented the highest contents. In turn, positive correlations were observed between Fe2O3 and Zn, V, U, Cu, Co,
and the sum of REE. The highest contents were found in irregular massive domains (Figs. 17a-e). Host rock samples presented ppm values lower than 1, except for Zr, Sr, and Ba, which were 27.7, 73.9, and 4 ppm, respectively.
Figure 16: Binary diagrams for trace elements of the hematite domains of the Ibicoara region. a) Zr versus Fe2O3; b) Th versus Fe2O3; c) Sr versus Fe2O3; d) Nb versus Fe2O3; e) Hf versus Fe2O3; e f) Ba versus Fe2O3.
Trace and REE diagrams in the present study were all normalized using the Post-Archean Australian Shale (PAAS) (Mclennan, 1989). In the spider diagram (Fig. 18a), hematite-rich domains from deposits 1 and 2 presented high contents of Cu, Co, V, and Ba, and low contents of Sr in relation to the host rock. There was not a great variation of Rb and Cs contents in these deposits. The values of the host rock samples were overcome by
hematite-rich domains for the remaining elements. Samples of the irregular massive domain presented the highest concentrations of Cu, V, Y, and Th.
Figure 17: Binary diagrams for trace elementsof hematite domainsfromIbicoara region. a) Zn versus Fe2O3; b) V versus Fe2O3; c) U versus Fe2O3; d) Cu versus Fe2O3; e) Coversus Fe2O3 f) ETR versus Fe2O3 sum.
A similar behavior was observed between the pattern of minor and trace elements of the Ibicoara deposits and the high-content ore from deposit N5E in the Carajás Province (Figueiredo and Silva, 2011) and in Pau Branco from the Quadrilátero Ferrífero area (Hensler, 2013) (Fig. 18b). Higher contents of Cu, V, Sr, Ba, Y, and U were observed, while there were lower contents of Co, Rb, and Hf. In addition, hematite domains from Ibicoara were found to
irregular massive domains, and positive anomalies (Eu/Eu*>1) for the vein massive domain and host rock. Positive anomalies were also observed for Sm and Gd. Moreover, Ce presented negative anomalies for all types of domains, but presented a positive anomaly for the host rock.
Another comparison between the REE patterns of the Ibicoara hematite-rich domains and the high-content iron ores from N5E, in the Carajás Province (Figueiredo and Silva et al., 2011), and the Pau Branco deposit from the Quadrilátero Ferrífero area (Hensler, 2013), is illustrated in Figure 18d. The results obtained showed that all deposits presented weak enrichment of LREE when compared to HREE. However, while the N5E and Pau Branco deposits presented a positive anomaly of Eu, the deposits of the area of Ibicoara presented predominantly negative anomalies of this element.
Additionally, the mean values of the REE analyses were compared to the mean values of the analyses of: (i) Archean itabirites from Nova Lima, Minas Gerais, Brazil (Raposo and Ladeira, 1993); (ii) Kuruman BIF (Bau and Dulski, 1996); (iii) high-temperature hydrothermal fluids (> 350°C) from the Mid-Atlantic Ridge (Bau and Dulski, 1999); (iv) shallow sea water from the Pacific Ocean (< 500 m) (Alibo and Nozaki, 1999); (v) Lake Superior-type BIF oxide-silicate facies, from the Itabira Group, Quadrilátero Ferrífero (Veríssimo et al., 2002); (vi) Algoma de Isua-type BIF, western Greenland (Bolhar et al., 2004); and (vii) itabirites modified by the Mina Lagoa D’Anta hydrothermalism in the Licínio de Almeida Metavolcano-sedimentary Sequence, Caetité, Bahia (Borges, 2012) (Fig. 19a). From these comparisons, the following can be understood: (i) while in Isua, Itabira, Nova Lima, Lagoa D’anta, and Kuruman high-temperature hydrothermal fluids and shallow sea water show positive anomalies of Eu, in the Ibicoara deposits, anomalies of Eu were predominantly negative for the hematitized laminar and massive irregular domains, and slightly positive for the vein massive domain (Fig. 19b); (ii) regarding Ce, the itabirite ore from Lagoa D’Anta and shallow sea water (Fig. 19a) showed a strongly negative anomaly,
while in the remaining cases this pattern ranged from horizontal to weakly positive. For the studied hematite-rich domains, the anomaly in Ce was negative; and (iii) HREE enrichment was observed for the samples of Figure 19a, unlike what was observed in the samples of the studied hematitized domains and high-temperature hydrothermal fluids, which presented patterns of LREE enrichment.
Figure 18: a) Trace elements distribution of hematite domains from the Ibicoara region; b) Trace elements distribution of hematite high-grade ore of N5E deposits, Carajás Province (Figueiredo e Silva, 2009) and Pau Branco in the Quadrilátero Ferrífero (Hensler, 2013). Standardized to PAAS (McLennan, 1989); c) REE diagrams for hematite domains from the Ibicoara region; d) REE diagram for high-grade hematite ores of N5E deposits, Carajás Province (Figueiredo e Silva, 2009) and Pau Branco at the Quadrilátero Ferrífero (Hensler, 2013). Standardized to PAAS (McLennan, 1989).
Table 3: Sum variation of ETR, and of the (La/Yb)N, (La/Sm)N, (Sm/Yb)N and Eu/Eu* ratios for the samples of hematite-rich domains, Ibicoara region.
Figure 19: REE pattern of: a) Kuruman BIF (Bau and Dulski, 1996), high temperature hydrothermal fluids (>350ºC) (Bau and Dulski, 1999), shallow Pacific seawater (<500 m) (Alibo and Nozaki 1999), Archean BIFs from the Nova Lima Group, Rio das Velhas greenstone belt (Raposo and Ladeira, 1993), Isua BIF, west of Greenland (Bolhar et al., 2004), hydrothermally altered itabirites from the Lagoa D’Anta, Licínio de Almeida metavolcano-sedimentary sequence, Caetité, Bahia (Borges, 2012), Superior Lake itabirites, Itabira Group, Quadrilátero Ferrífero (Veríssimo et al., 2002); b) Samples from hematite- rich domains from the Ibicoara region. Standardized to PAAS (McLennan, 1989). IF – Iron Formation.
Contents of La in the marine environment can mask anomalies of Ce both in these locations and also in resulting chemical precipitates. In order to assess the oxidant nature in seawater and chemical precipitates by determining real anomalies in Ce, Bau and Dulski (1996a) proposed a diagram combining values of Ce/Ce* and Pr/Pr*. Therefore, negative anomalies in Ce occur in samples that present (Pr/Pr*) >1, while positive anomalies occur when (Pr/Pr*) <1 (Fig. 20).
Figure 20: (Ce/Ce*) vs (Pr/Pr *) diagram standardized to PAAS for samples of the hematite-rich domains from the Ibicoara deposits, indicating fields of true positive and negative Ce anomalies. Klein and Beukes (1989) and Beukes and Klein (1990) use similar data on the 3.8 Ga Isua iron formation, Greenland, to argue towards the presence of an oxidizing environment in the Archean seas.
8. Hematite LA-ICPMS Analyses
Hematites from the three groups of the paragenetic succession were analyzed and their results are presented in Appendices 1-18.
8.1. Multi-element Diagrams
The diagrams of Figure 21 show that the highest variations in element content occurred in the first generations of groups 1 and 2. Generations 2 and 3 of the same groups presented lower variations in the value of these elements.
Group 1 – For subgroups Hem1-G1 and Hem2-G1 (Figs. 21a-b), Nb presented peaks ranging from positive to negative, but negative peaks predominated in Hem3-G1 (Fig. 21c). The element Ba ranged from positive to negative peaks in Hem1-G1, from weakly positive to negative in Hem2-G1, and was negative in Hem3-G1. Enrichment of U and Pb was observed in all subgroups in relation to PAAS. In general, Y was poorer.
Group 2 – In general, positive peaks were observed for Zr, U, and Pb, while negative peaks were observed for Nb, Y, Co, and Ni (Fig. 21d-g). The element Cu varied widely, with positive and negative peaks, though it was predominantly positive in subgroup Hem4-G2. The highest contents of Sr were found in Hem1-G2 hematites, reaching values 100 times higher than PAAS.
Group 3 – Both subgroups presented positive peaks of U and Pb. In Hem1-G3 hematites, Ba and Ni ranged from positive to negative. In turn, in Hem2-G3, Ba and Ni were respectively negative and positive. The element Y, in general, was negative. The element Cu ranged from positive to negative in Hem1-G3, and was negative in Hem2-G3 (Fig. 21h-i). The behavior of hematites from Group 3 was similar to the behavior of hematites in subgroups Hem2-G1 and Hem3-G1.
The values of Co, Ni, Cu, Zn and Pb from the Ibicoara hematite deposits showed higher variation than what was observed for the micro-lamellar hematites of the Carajás Province (Figueiredo and Silva, 2009). The values of Ni, Zn and Pb obtained in the hematites were higher than the total chemistry of the rock (Fig. 22).
8.2. Rare Earth Elements
Group 1 – Two different patterns were observed in subgroup Hem1-G1: a less fractioned one, with a horizontal pattern; and a more fractioned one, which was weakly enriched with LREE in relation to HREE. Values of La/YbN ranged between 0.02 and 1.97,
