The growing demand for scandium (Sc), essential for several modern industrial applications, thrives the mining industry to develop alternative Sc sources. In such context, significant Sc concentrations (~100 ppm) were recently reported in several Ni-Co lateritic oxide ores developed after mafic-ultramafic rocks. This contribution examines the distribution of Sc in Ni-Co laterites from New Caledonia, the sixth largest Ni producer worldwide. Representative lateritic profiles were selected based on the protolith type and include dunite, harzburgite and lherzolite protoliths, wherein the Sc content, determined by the relative proportion of olivine and pyroxene, ranges from <5 ppm in dunite to >10 ppm in lherzolite. In Ni-Co laterites, dissolution and leaching of primary Mg-rich silicates leads to the residual enrichment of iron as ferric oxides/oxyhydroxides in the upper horizons. Downward remobilisation and trapping of Ni and Co lead to their local enrichment to economic concentrations, with maximum grades reached in the rocky saprolite and in the transition laterite horizons, respectively. In contrast, maximum Sc enrichment occurs in the yellow laterite horizon, where Sc-bearing goethite reaches about ten times the Sc content of the parent rock. Consequently, harzburgite-and lherzolite-derived yellow laterites yield maximum Sc concentrations up to 100 ppm, together with moderate Ni and Co concentrations. There, Sc is a potentially valuable by-product that could be successfully co-extracted along with Ni and Co through hydrometallurgical processing. In addition to peridotite-hosted laterites, hornblende-rich amphibolites yield elevated Sc up to 130 ppm. The saprolitisation of amphibolites leads to the formation of a goethite-gibbsitekaolinite mixture with Sc concentrations >200 ppm. There, goethite is the main Sc carrier with up to 800 ppm Sc. Therefore, despite their relatively limited volumes, amphibolite-derived saprolites may also represent attractive targets for Sc in New Caledonia.

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The Yaou deposit, located in French Guiana within the Guiana Shield, is one of the most promising gold deposits of the regional Palaeoproterozoic greenstone belt. It displays numerous quartz monzodiorite bodies aligned along a sinistral shear zone where a five-deformation phases model is established at the camp scale. The ductile D1/2YA phase is responsible for the main penetrative foliation while the D3YA phase is related to shearing. An intrusive event is identified as being pre to syn-D3YA. The following phase D4YA represents a brittle quartz-carbonate veining set hosted preferentially within intrusive bodies and along the shear zone. A local D5YA brecciation event crosscuts the D4YA veins. Among this deformation history, two auriferous events (D3YA and D4YA) control the overall grade of the Yaou gold deposit. More specifically, most of the Au grade is associated with the main economic D4YA veining event, where the gold is visible and linked to Py4 within an ankerite/hematite rich alteration halo. At the microscopic scale, results of in situ analyses using LA-ICP-MS on pyrite show that metasediment-hosted Py0 is a primary source of submicroscopic gold having a low contribution to the total endowment. Py3 shows some gold content due to possible remobilisation of AuD0YA. Gold in Py4 is found as submicroscopic gold, as micro-inclusions and as infilling fractures in association with elements such as Te, Ag and Bi. Most contribution to the Au grade is from micro-inclusions and, to a lesser extent, from free and submicroscopic gold. The ore shoot locations are lithologically controlled for AuD0YA (metasedimentary unit-hosted), structurally controlled (shear zone-hosted) for AuD3YA and rheologically controlled for the AuD4YA (intrusion-hosted). The deposit is clearly polyphase both at the macroscopic and the microscopic scales, invisible gold is associated with As whereas visible gold is observed as inclusions in pyrite with high contents of Ag, Te and Bi. We define an early low-grade enrichment of AuD0YA to AuD3YA followed by a later high-grade event, AuD4YA supporting polyphase mineralisation processes. This study confirms that orogenic gold deposits can be formed by remobilisation and/or new gold inputs during multiple deformation, veining and hydrothermal events.

Abstract. During the Archaean, the Sun's luminosity was 18 to 25 % lower than the present day. One-dimensional radiative convective models (RCM) generally infer that high concentrations of greenhouse gases (CO 2 , CH 4 ) are required to prevent the early Earth's surface temperature from dropping below the freezing point of liquid water and satisfying the faint young Sun paradox (FYSP, an Earth temperature at least as warm as today). Using a one-dimensional (1-D) model, it was proposed in 2010 that the association of a reduced albedo and less reflective clouds may have been responsible for the maintenance of a warm climate during the Archaean without requiring high concentrations of atmospheric CO 2 (pCO 2 ). More recently, 3-D climate simulations have been performed using atmospheric general circulation models (AGCM) and Earth system models of intermediate complexity (EMIC). These studies were able to solve the FYSP through a large range of carbon dioxide concentrations, from 0.6 bar with an EMIC to several millibars with AGCMs. To better understand this wide range in pCO 2 , we investigated the early Earth climate using an atmospheric GCM coupled to a slab ocean. Our simulations include the ice-albedo feedback and specific Archaean climatic factors such as a faster Earth rotation rate, high atmospheric concentrations of CO 2 and/or CH 4 , a reduced continental surface, a saltier ocean, and different cloudiness. We estimated full glaciation thresholds for the early Archaean and quantified positive radiative forcing required to solve the FYSP. We also demonstrated why RCM and EMIC tend to overestimate greenhouse gas concentrations required to avoid full glaciations or solve the FYSP. Carbon cycle-climate interplays and conditions for sustaining pCO 2 will be discussed in a companion paper.

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