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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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| Authors: | Alexandre Raphael Cabral Michael Wiedenbeck Francisco Javier Rios Ant?nio Augusto Seabra Gomes Jr Orlando Garcia Rocha Filho Richard David Jones |
| Institution: | 1. Exploration Geology, Rhodes University, PO Box 94, Grahamstown, 6140, South Africa 2. Technische Universit?t Clausthal, Lagerst?tten und Rohstoffe, Adolph-Roemer-Str. 2A, 38678, Clausthal-Zellerfeld, Germany 3. Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum, Telegrafenberg C128, 14473, Potsdam, Germany 4. Centro de Desenvolvimento da Tecnologia Nuclear (CDTN/CNEN), 30123-970, Belo Horizonte-MG, Brazil 5. Gerência de Explora??o Mineral de Ferrosos (GAEMF), Centro Tecnológico de Ferrosos–Miguel?o, VALE, Fazenda Rio do Peixe, Galp?o de Testemunhos, 34000-000, Nova Lima-MG, Brazil 6. Rua Ceará, 1338/701, 30150-311, Belo Horizonte-MG, Brazil 7. 1636 East Skyline Drive, Tucson, AZ, 85178, USA |
| 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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