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Talc mineralisation associated with soft hematite ore, Gongo Soco deposit, Minas Gerais, Brazil: petrography, mineral chemistry and boron-isotope composition of tourmaline

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Abstract

Talc mineralisation occurs as hematite–talc schist between soft hematite ore and dolomitic itabirite at Gongo Soco, Quadrilátero Ferrífero of Minas Gerais, Brazil. The hematite–talc schist and soft hematite have a prominent tectonic foliation of tabular hematite. Tabular hematite without preferential orientation is superimposed on the tectonic foliation. The talcose schist is enriched in F and has a constant Fe/S ratio. Electron-microprobe analyses indicate trace amounts of S in different generations of hematite. The whole-rock Fe/S ratio possibly represents sulfate S from hematite-hosted fluid inclusions. Fluid inclusions in foliation-overprinting hematite and chlorite geothermometry from talcose rocks suggest, respectively, temperatures from <200°C to ~300°C. Tourmaline, a rarely observed mineral in the hematite–talc schist, belongs to the alkali group and falls in the dravite compositional field. Boron-isotope determinations of tourmaline crystals, using secondary ion mass spectrometry, vary from −20‰ to −12‰ δ11B. This compositional isotopic range and the tourmaline chemical composition suggest a meta-evaporitic origin. A non-marine evaporitic setting is the most likely source of acidic, highly oxidising fluids, which resulted in the abundant F-bearing talc and the presence of otherwise immobile Ti in hematite. Oxidising brines were channelled along shear zones and converted dolomitic itabirite into the Gongo Soco soft hematite and the talc mineralisation. The latter is envisaged as the hydrothermal wall-rock alteration of dolomitic itabirite, which gave rise to the soft hematite ore.

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Acknowledgements

We gratefully acknowledge numerous visits to the Gongo Soco mine, initially under Mineração Socoimex, now operated by Companhia Vale do Rio Doce (VALE). Vagner Costa and Douglas José de Oliveira (VALE) are particularly thanked for logistical support and access to internal files. ARC expresses his gratitude to Prof. G. Beaudoin and Prof. B. Lehmann for access to the electron-microprobe facility at Université Laval and TU Clausthal, respectively. Reviews by two anonymous referees, as well as comments by Nikola Koglin, have greatly improved the manuscript.

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  1. Alexandre Raphael Cabral

    Present address: Technische Universität Clausthal, Lagerstätten und Rohstoffe, Adolph-Roemer-Str. 2A, 38678, Clausthal-Zellerfeld, Germany

Authors and Affiliations

  1. Exploration Geology, Rhodes University, PO Box 94, Grahamstown, 6140, South Africa

    Alexandre Raphael Cabral

  2. Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum, Telegrafenberg C128, 14473, Potsdam, Germany

    Michael Wiedenbeck

  3. Centro de Desenvolvimento da Tecnologia Nuclear (CDTN/CNEN), 30123-970, Belo Horizonte-MG, Brazil

    Francisco Javier Rios

  4. Gerência de Exploração Mineral de Ferrosos (GAEMF), Centro Tecnológico de Ferrosos–Miguelão, VALE, Fazenda Rio do Peixe, Galpão de Testemunhos, 34000-000, Nova Lima-MG, Brazil

    Antônio Augusto Seabra Gomes Jr

  5. Rua Ceará, 1338/701, 30150-311, Belo Horizonte-MG, Brazil

    Orlando Garcia Rocha Filho

  6. 1636 East Skyline Drive, Tucson, AZ, 85178, USA

    Richard David Jones

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Editorial handling: R. Perez Xavier

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Table 1

Electron-microprobe analyses of carbonate from fresh wall-rock itabirite (DOC 41 kb)

Table 2

Electron-microprobe analyses of chlorite from fresh wall-rock itabirite and hematite–talc schist (DOC 58 kb)

Table 3

Electron-microprobe analyses of talc from hematite–talc schist (DOC 46 kb)

Table 4

Electron-microprobe analyses for trace elements in hematite (DOC 46 kb)

Table 5

Electron-microprobe analyses of tourmaline from hematite–talc schist (DOC 70 kb)

Table 6

Whole-rock chemical analyses of hematite–talc schist and dolomite (DOC 84 kb)

Table 7

Boron-isotope ratios of tourmaline from hematite–talc schist (DOC 34 kb)

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Cabral, A.R., Wiedenbeck, M., Rios, F.J. et al. Talc mineralisation associated with soft hematite ore, Gongo Soco deposit, Minas Gerais, Brazil: petrography, mineral chemistry and boron-isotope composition of tourmaline. Miner Deposita 47, 411–424 (2012). https://doi.org/10.1007/s00126-011-0374-3

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