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The Yaou and Couriège gold mineralisations (Guiana Shield) : from primary enrichment to secondary processes

Abstract : Gold deposits located in French Guiana (France, South America) within Rhyacian terranes (2.3-2.05 Ga), are the witnesses of the Trans-Amazonian orogeny. These deposits, in the Guiana Shield, are covered by a dense rainforest where well-developed lateritic profiles leave little outcrop of unaltered bedrock. Considering this specific context, this thesis is interested in defining (i) what are the primary gold mineralisation processes and (ii) what is their preservation and enrichment (or not) during secondary supergene processes and is based on two case studies: the Yaou deposit and the Couriège prospect. At Yaou, intrusive bodies (quartz monzodiorite), mainly hosting gold mineralisation, are aligned along a sinistral shear zone affecting a volcano-sedimentary sequence. A polyphase tectono-magmatic evolution, of 5 deformation phases, is proposed at the scale of the mineralized camp. Associated with this deformation framework are two gold events which control the overall gold budget of the deposit. The intrusion hosting the mineralisation is dated at 2130.6 ± 5.8 Ma (U-Pb on zircon). The modalities of supergene processes are studied at the Couriège prospect, which includes several gold occurrences associated either with pyrite-rich quartz veins within in situ lateritic profiles, or with colluvial and alluvial horizons. Two profiles were studied to assess the role of supergene processes in the modification of primary (hypogene) mineralisation. The Achman Giraud site is characterised by an authigenic alteration profile with gold-bearing quartz veins. The weak vertical and lateral dispersion of gold, the predominantly primary character of the gold grains, and the limited evidence of supergene gold reprecipitation, argue for minor supergene remobilisation without clear evidence for gold enrichment during lateritisation. The Lupe site exposes a gold-bearing detrital horizon truncating a barren lateritic profile. Gold is present as electrum inclusions in partially goethitised detrital pyrite crystals and as free gold grains exhibiting a sub-rounded shape, mechanical transport marks and Ag-depleted rim, suggesting transport and exposure to weathering over significant distance and time.
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Vincent Combes. The Yaou and Couriège gold mineralisations (Guiana Shield) : from primary enrichment to secondary processes. Earth Sciences. Université de Lorraine, 2020. English. ⟨NNT : 2020LORR0265⟩. ⟨tel-03272400⟩



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