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Textural and fluid inclusion constraints on the origin of the banded-iron-formation-hosted gold deposits at Maevatanana, central Madagascar

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Abstract

The Maevatanana deposits consist of gold-bearing quartz–sulphide veins crosscutting banded iron formation (BIF) within a metamorphosed 2.5 Ga greenstone belt. The host rocks are dominated by a sequence of migmatites, gneisses, amphibolites, magnetite-rich quartzites and soapstones, intruded by large granitoid batholiths (e.g. the 0.8 Ga Beanana granodiorite). In the mineralised rocks, pyrite is the dominant sulphide, in addition to accessory chalcopyrite and galena. Outside the immediate ore zone, the BIF is dominated by quartz + magnetite ± hematite, accompanied by cummingtonite, albite and biotite. Gold occurs as globular grains (usually <500 μm) within quartz crystals close to the sulphides and as invisible inclusions within pyrite and chalcopyrite (up to 2,500 ppm Au content). Fluid inclusion textural and microthermometric studies indicate heterogeneous trapping of a low-salinity (∼3.6 wt.% eq. NaCl) aqueous fluid coexisting with a carbonic fluid. Evidence for fluid-phase immiscibility during ore formation includes variable L/V ratios in the inclusions and the fact that inclusions containing different phase proportions occur in the same area, growth zone, or plane. Laser Raman spectroscopy confirms that the vapour phase in these inclusions is dominated by CO2 but shows that it may contain small amounts of CH4 (<1 mol%), H2S (<0.05 mol%) and traces of N2. Fluid inclusion trapping conditions ranged from 220 to 380°C and averaged 250°C. Pressure was on the order of 1–2 kbar. The abundant CO2 and low salinity of the inclusions suggest a metamorphic origin for the fluid. Likewise, the presence of H2S in the fluid and pyritisation of the wall-rock indicate that gold was likely transported by sulphide complexing. Fluid immiscibility was probably triggered by the pressure released by fracturing of the quartzites during fault movements due to competence differences with the softer greenstones. Fracturing greatly enhanced fluid circulation through the BIF, allowing reaction of the sulphide-bearing fluids with the iron oxides. This caused pyrite deposition and concomitant Au precipitation, enhanced by fluid phase separation as H2S partitioned preferentially into the carbonic phase.

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Acknowledgment

This study was supported by the Corus project no. 98 518 221 “Géologie du socle cristallin de Madagascar et de ses minéralisations”. The authors thank the Malagasy government for the financial support to PA during his MSc in Toulouse. We are grateful for the technical assistance on the Raman and microprobe analyses, which was provided by Thérèse Lhomme and Philippe de Parseval, respectively. This work benefited from valuable discussions with A.E. Williams-Jones and critical review by Bernd Lehmann.

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Authors and Affiliations

  1. Laboratoire des Mécanismes et Transferts en Géologie-UMR 5563, CNRS/IRD/Université Toulouse 3, 14 avenue Edouard Belin, 31400, Toulouse, France

    Prosper Andrianjakavah, Stefano Salvi, Didier Béziat, Damien Guillaume & Bernard Moine

  2. Département des Sciences de la Terre, Faculté des Sciences, Université d’Antananarivo, Antananarivo, 101, Madagascar

    Prosper Andrianjakavah & Michel Rakotondrazafy

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  1. Prosper Andrianjakavah
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Correspondence to Stefano Salvi.

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Editorial handling: B. Lehmann

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Andrianjakavah, P., Salvi, S., Béziat, D. et al. Textural and fluid inclusion constraints on the origin of the banded-iron-formation-hosted gold deposits at Maevatanana, central Madagascar. Miner Deposita 42, 385–398 (2007). https://doi.org/10.1007/s00126-006-0119-x

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