Les systèmes ultramafiques sont souvent synonymes de fortes teneurs en chrome (Cr) dans des roches, qui est naturellement lixivié vers les eaux de surface et souterraines. En raison de cet enrichissement naturel, les zones ultramafiques font souvent l’objet d’extractions massives. Le processus minier inclut l'exploitation et les activités d'enrichissement, dans lesquelles la grande quantité de résidus produits riches en métaux, tels que les gangues et les terrils, sont stockés à l’air libre. Ces processus peuvent considérablement augmenter les teneurs en Cr trivalent (Cr (III)) et hexavalent (Cr (VI)) disséminé dans l'environnement. Ce dernier (Cr (VI)), est connu pour être fortement soluble dans l'eau, biodisponible et toxique. Ce travail de thèse a porté sur i) la zone d’exploitation minière du nickel de Barro Alto (BA, Etat du Goiás, Brésil); ii) la zone d'exploitation minière historique de chromite de Cromínia (CA, Etat du Goiás, Brésil) et iii) une exploitation minière actuelle dans la vallée de Sukinda (Odisha, l'Inde). Le principal objectif de ces travaux est l'identification de l'impact d'activités minières (extraction du nickel et du chrome) sur la mobilité de Cr et sa disponibilité dans des zones ultramafiques, à l'aide des techniques isotopiques. Les compartiments chimiquement et isotopiquement échangeables de Cr(VI) (ECr (VI)) les plus hauts ont été mesurés dans des échantillons de minerai à BA, où l'enrichissement en isotopes légers de Cr (-0.76 à -0.16 ‰) a été attribué à la perte de Cr isotopiquement lourd et échangeable Cr(VI) pendant l’altération. Des valeurs étonnamment lourdes de δ53Cr ont été aussi trouvées dans des minerais saprolitiques à BA et dans des sols affectés par l’extraction à CA, jusqu’à +3.9 ‰ alors qu’ils étaient fortement enrichis en Cr(III). Les causes principales ont été attribuées à l'existence de chromite hydrothermale à CA et/ou à l’altération naturelle suivie par la réduction de Cr(VI), induisant la reprécipitation de Cr mobile et isotopiquement lourd. Le chrome était présent dans les lixiviats des échantillons de minerais saprolitiques et lateritiques de BA, sous la forme de Cr(VI) isotopiquement lourd (jusqu’à +4.84 ‰), en cohérence avec le Cr échangeable (ECr(VI)) (jusqu’à +4.37 ‰). Ces valeurs étaient dans la même gamme de compositions isotopiques que celles mesurées dans les eaux douces de surface (ruisseaux et réservoirs) situés dans la zone ultramafique. Ces résultats impliquent que Cr est principalement disséminé sous sa forme toxique Cr(VI), dont la disponibilité augmente depuis i) le profil de sol ii) les minerais et iii) les résidus miniers. Lors de tests de bioaltération réalisés sur les terrils avec Acidithiobacillus thiooxidans (pH~2) ou Pseudomonas putida (pH~9), Cr a été initialement extrait sous sa forme Cr(VI) puis réduit en Cr (III). Dans les expériences avec A. thiooxidans, la réduction du chrome est dû à l’important pouvoir réducteur d'une série de composés soufrés, tandis que P. putida utilise probablement pour la réduction des chromates une variété d'accepteurs d’électrons. Ces mécanismes, de même que l’augmentation de la concentration en matière organique et la carbonatation minérale, pourraient expliquer la plus faible concentration en Cr(VI) échangeable dans les terrils âgés par rapport aux terrils récents. Par la technique d’échange isotopique, la contribution des SPM au transport du chrome échangeable, ainsi que l’impact des colloïdes contenant des phases porteuses de Cr sur la mesure de ce compartiment échangeable associé aux SPM (EWCr) a été mise en évidence. Les plus grosses particules (> 0.2 µm) jouent un rôle prépondérant dans la zone impactée par l’activité minière, tandis que le chrome est principalement associé aux colloïdes (1 kDa-500 kDa) dans la zone vierge. La présence de colloïdes organiques et inorganiques contenant du chrome non-échangeable induit une surestimation de la valeur de EWCr, qui peut être corrigée par le biais d’un scénario du pire / Ultramafic systems are often synonym of high chromium (Cr) content in rocks, which is naturally leached to surface and groundwater. Due to this natural enrichment, ultramafic areas are massively mined. The mining process includes exploitation and beneficiation activities, in which large amount of metal-rich residues, such as overburden, waste rock and tailings, stored in open air, are produced. These processes may considerably increase the amount of both trivalent (Cr(III)) and hexavalent chromium (Cr(VI)) released to the environment. The later (Cr(VI)), known to be highly soluble in water, bioavailable and toxic. Within these mining residues, many chemical and biological processes may take place, which will control Cr speciation, mobility and availability. The present work was conducted on i) the nickel exploitation and metallurgic area of Barro Alto (BA, Goiás state, Brazil); ii) the antique chromite exploitation area of Cromínia (CA, Goiás State, Brazil) and iii) the current chromite mine in the Sukinda valley (Odisha, India). The main focus of this research is the identification of the impact of mining activities (nickel and chromium mining) on Cr mobility and availability in ultramafic environments, through the use of isotopic techniques. The chemically and isotopically exchangeable pools of Cr(VI) (ECr(VI)) were higher in BA ore samples, where the enrichment in light chromium isotopes (-0.76 to -0.16‰) was attributed to the loose of isotopically heavy and exchangeable Cr(VI) during weathering. Astonishingly, heavy δ53Cr values were also found in saprolitic ores in BA and in mining-affected soils in CA up to +3.9‰, strongly enriched in Cr(III). The main causes have been attributed to the existence of hydrothermal chromite in CA and/or to natural weathering followed by Cr(VI) reduction that induces reprecipitation of mobile and isotopically heavy Cr. In such a mining context, the accelerated weathering would play an important role in this process. Chromium in the leachate, of BA lateritic and saprolitic ores samples, was present as Cr(VI) isotopically enriched in heavy isotopes (up to +4.84‰), consistently with the exchangeable Cr (ECr(VI)) (up to +4.37‰). These values were in the same range as isotopic compositions measured in the fresh waters (streams and ponds) in the ultramafic area. These results imply that Cr is mainly released as the toxic Cr(VI) specie, which availability increases from i) the soil profile to ii) the ores and iii) the mining residues. This also suggests that δ53Cr could be used as a tracer of Cr leaching in environmental studies in the dissolved phase.In bioleaching tests on tailings with Acidithiobacillus thiooxidans (pH ~ 2) or Pseudomonas putida (pH ~ 9), Cr was initially extracted as Cr(VI) and later reduced to Cr(III). In the experiments with A. thiooxidans the Cr reduction is due to the production of a series of sulfur compounds with high reducing power, while for P. putida probably uses a variety of electron acceptor for chromate reduction, enhanced by the presence of extracellular polymeric substances. Those mechanisms together with the increase of natural organic matter (NOM) and mineral carbonation, could explain the lower exchangeable pool of Cr(VI) in stockpiled chromite tailings compared to fresh tailings. Through the use of isotopic exchange, the contribution of SPM to the transport of exchangeable chromium and the impact of colloids containing Cr-bearing phases on the determination of the exchangeable pool of Cr associated to SPM (EWCr) was highlighted. Larger particles (>0.2 μm) were dominant in the impacted area, while chromium was mainly associated with colloids (1 kDa–500 kDa) in the pristine area. The presence of organic and inorganic colloids containing non-exchangeable chromium induces an overestimation of the EWCr values, which can be over-come thanks to a worst-case scenario correction

MESMEG / Department of Mining and Environmental Geology / The increase in the demand and market price of gold has led to reprocessing of gold tailings in many parts of the world. Mines are recently closing down due to depletion of resources and increasing mining costs leading to the reprocessing of old tailings dams. The cost of rehabilitation is high, and therefore a more convenient way of rehabilitation is required. The most convenient strategy identified here was to reprocess tailings for gold and use waste rocks as construction materials. The tailings residues (waste remaining after reprocessing) will be relocated to a more convenient place to avoid pollution. Gold reprocessing from tailings dams has gained momentum in South Africa especially in the Witwatersrand Basin where there are large volumes of tailings. Gold is being reprocessed from tailings in this area using hydraulic monitors. This study focused on the evaluation of gold and heavy metals within the tailings at Consolidated Murchison Mine tailings in Gravelotte, Limpopo province. Augering was conducted over the tailings up to a depth of 8 m along four sampling Profiles. The first profile had two sampling points, the second profile with three sampling points, the third and fourth profiles consisted of four and five sampling points respectively. Samples were collected at 1 m interval, therefore a total of 112 samples were collected and analysed for heavy metals using X-Ray Fluorescence spectrometry and 84 samples were analysed for gold using fire assaying. Tailings sampling was accompanied with tailings logging, taking note of colour, texture and moisture content. Based on this, the oxidation status of the tailings dam was determined. Oxidation zone of this tailings dam was mainly from top down to a depth of 3 m. The transitional zone was not identified, hence after the oxidation zone, the rest was unoxidized zone. This study established that gold was erratically distributed within the tailings dam with the lowest and highest values of 200 mg/kg and 1880 mg/kg respectively and the average was 670 mg/kg. The tonnage of tailings within the dam was found to be 13 280 310 tons with a total gold amount of 8 897. 81 kg. At the current world market, this interprets to US$ 306 932 396.00 (R 4 281 706 924.20). It was concluded that this tailings dam is economically viable for reprocessing, although previous studies have indicated that it is not possible to extract gold from tailings dams completely. The heavy metal content of Pb, Ni and Cr were found to be high with average values of (ppm); 5631.5, 2062.6 and 1345 v respectively. The metals with the lowest values were Cd, Co and Cu, averaging (ppm); 0.01 ppm, 19.8 ppm and 42.1 ppm respectively. Heavy metal content in soil around the tailings dam was gradually decreasing with distance from the tailings dam. Waste rocks have been used in some parts of the world as sub-base material for engineering construction, hence in this study, a total of 6 waste rock samples were collected using grab sampling method for geostatistical investigation. Such samples were subjected to various geotechnical tests which included particle size distribution analysis (sieve analysis), Atterberg limit tests and laboratory compaction test to determine their suitability for construction. The waste rock material was found to be suitable for road construction as it was classified under Group A-1-a using the AASHTO classification system. The material consisted mainly of rock fragments, gravel and sand material with minor silt/clay. In general, Consolidated Murchison mine waste was found to be suitable for road construction. / NRF