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Title: Graphite–(Mo,W)S2 intergrowth as a palaeoenvironmental proxy in metasedimentary rocks
Title of periodic: Lithos
metadata.dc.title.subtitlejournal: An International Journal of Petrology, Mineralogy and Geochemistry
Authors: Cabral, Alexandre Raphael
Zeh, Armin
Viana, Nívea Cristina da Silva
Schirmer, Thomas
Lehmann, Bernd
Affiliation: Centro de Desenvolvimento da Tecnologia Nuclear,CDTN, Belo Horizonte, MG, Brazil
Institut für Angewandte Geowissenschaften, Karlsruhe, Germany
Vale Manganês S.A., Conselheiro Lafaiete, MG, Brazil
Institute of Disposal Research, Technische Universität Clausthal, Clausthal-Zellerfeld, Germany
Institute of Disposal Research, Technische Universität Clausthal, Clausthal-Zellerfeld, Germany
Issue Date: 2017
Keywords: Tungsten-bearing molybdenite Gerais;Graphite;Molybdenum proxy;Queluzite;Barbacena greenstone belt;Minas Gerais
Abstract: Molybdenum enrichment in pristine organic-C-rich sedimentary rocks forms the basis for inferring the presence of dissolved oxygen in seawater. Organic matter removes dissolved hexavalent Mo from seawater where anoxic and euxinic conditions are attained. However, it is unknown whether this Mo-based proxy is retained under metamorphic conditions where organic C is no longer preserved. Here, we describe aggregates of graphite and molybdenite (MoS2) containing up to 21 mass per cent of W as tungstenite (WS2) in solid solution. These aggregates are disseminated in a sulfide-rich Mn-silicate-carbonate rock (queluzite), metamorphosed under amphibolite-facies conditions within the Archaean Barbacena greenstone belt in Minas Gerais, Brazil. Our finding indicates that: (i) W is, like Mo, a palaeoenvironmental proxy; (ii) the W proxy is sensitive to high fS2/fO2 environments; (iii) both Mo and W proxies survive amphibolite-facies overprint as (Mo,W)S2 intergrown with graphite. Archaean greenstones are potential candidates for storing palaeoenvironmental information as (Mo,W)S2–graphite intergrowths.
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