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The Neoproterozoic Urucum/Mutún Fe and Mn deposits in W-Brazil/SE-Bolivia: assessment of ore deposit models

[Die neoproterozoischen Fe- und Mn-Lagerstätten Urucum/Mutún in W-Brasilien/SE-Bolivien: Bewertung der Modelle zur Lagerstättenbildung]

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The Neoproterozoic Fe and Mn deposits of the Urucum district in W-Brazil and Mutún in SE-Bolivia are associated with BIFs (banded iron formations) and unique as they represent (together with the Rapitan sequence of the Yukon, NWT of Canada, and the Otjosondu deposits of Namibia) the youngest BIF sequences in the world and also consist almost entirely of hematite and chert (jaspilite) with iron present only as FeO. The genetic models for these deposits are equivocal and comprise end member models such as epicontinental-marine basin, climate controlled sedimentary, and hydrothermal volcanism-seawater. The reinterpretation of the regional geological setting as a large graben structure and associated fault zones is congruent with evidence for fluid activity, during or post formation of the initial Fe-Mn enrichment (proto-ore), which include: (1) the occurrence of braunite that is indicative of an elevated temperature (metamorphic?) environment, (2) the existence of quartz-tourmaline veins crosscutting siliciclastic rocks of the Jacadigo Group, and (3) hydrothermal magnetite that occurs in graben-related fault zones. We believe that a comprehensive ore genesis model for the Urucum/Mutún district needs to be compatible with the geotectonic setting and must incorporate various signatures of hydrothermal activity during or after the formation of the proto-ore BIF.

Die neoproterozoischen gebänderten Fe- und Mn-Erze des Urucum-Distriktes in W-Brasilien und des Mutún-Distriktes in SE-Bolivien gehören mit den Abfolgen Rapitan (Kanada) und Otjosondu (Namibia) zu den jüngsten BIF-(,,banded iron formation")Vorkommen der Erde. Diese Vorkommen verfügen außerdem über eine gemeinsame mineralogische Besonderheit: sie sind aus einer Hämatit- und Jaspilitabfolge aufgebaut, in der Eisen nur in Form von Fe2O3 vorkommt. Über die Genese dieser spätproterozoischen Lagerstätte gibt es bisher noch sehr unterschiedliche Auffassungen, z. B.: Ablagerungen in epikontinentalen marinen Becken, klimagesteuerte Sedimentationsabläufe, Ausfällung der Metalle aus dem Meerwasser, angereichert durch distale hydrothermale Aktivitäten. Neue Erkenntnisse über die geologische Entwicklung des Gebietes von Corumbà als Grabenstruktur (Corumbá-Graben und Chiquitos-Tucavaca-Aulakogen) mit den entsprechenden Störungszonen stützen die Annahme, dass die ersten Fe-Mn-Anreicherungen über hydrothermale Fluide im Zuge der Grabenbildung erfolgt sind. Als besondere Hinweise gelten u. a.: 1. Vorkommen von Braunit als möglicher Hinweis auf höhere Temperaturen im Metamorphosebereich; 2. Quarz-Turmalingänge in der BIF-Abfolge; 3. Hydrothermal gebildeter Magnetit in den Störungszonen der Grabenstruktur. Diese Fakten sind nach unserer Auffassung unbedingt in die Diskussion über die Genese der Erzvorkommen von Urucum/Mutún einzubeziehen.
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Document Type: Research Article

Publication date: 2007-01-01

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  • Zeitschrift der Deutschen Gesellschaft für Geowissenschaften (ZDGG) is an international peer reviewed journal that accepts papers on research and applied topics in the Earth Sciences. It is published online and in print. One volume, consisting of four issues is published annually. The journal is continously published by German Society of Geosciences (DGG) since its foundation in 1848. It was relaunched in 2005.

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