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Student Number : 7729948 - MSc dissertation - School of Animal, Plant and Environmental Sciences - Faculty of Science / As early as 1583 an ultramafic plant was described (Proctor & Woodell, 1975). Since the early 1900s a number of works documenting ultramafic vegetation in various parts of the world have been published (Proctor & Woodell, 1975). The vegetation of the Great Dyke, Zimbabwe was only described in 1965 (Wild, 1965). Much has been written since then, however. It was only as recently as 1989 that any work on ultramafic vegetation in South Africa was published (Morrey et al., 1989), possibly because ultramafic vegetation in the Barberton Greenstone Belt is not noticeably different from that of the surroundings and outcrops are much smaller than the Great Dyke. Studies since then have documented the flora of the ultramafic soils of the Barberton Greenstone Belt (BGB) (Morrey et al., 1992; Williamson, 1994; Balkwill et al., 1997; Williamson et al., 1997; Changwe & Balkwill, 2003; Williamson & Balkwill in prep.). There are around 40 larger outcrops of ultramafic soil in the BGB, the largest of which are shown in Figure 1.1, and to date 29 endemic taxa have been discovered, 5 of which hyperaccumulate Ni (Williamson & Balkwill, in prep). Using IUCN criteria 21 of these taxa have recommended conservation status in the vulnerable categories and four are data deficient. With a number of threatened taxa and scientifically interesting and potentially useful Ni hyperaccumulators on the South African part of the BGB, it was likely that there were additional endemic taxa in Swaziland or additional populations of species collected in South Africa. The ultramafic sites in Swaziland (Figure 1.2) range in altitude from 4750 m above sea level (Figure 1.3) in the south to below 2250 m in the Komati River valley (Figure 1.4), higher than the South African sites which range from 354 – 1648 m above mean sea level (Balkwill et al., 1997). The Swaziland sites thus provide an opportunity to discover the effect of altitude on the vegetation of ultramafic soils in the BGB. The higher sites are cooler than the lowveld sites, with frost at night in winter and even snow on rare occasions. Rainfall averages 127 to 152 mm per year, the highest rainfall being recorded at the higher altitudes, where frequent fog also supplements the rainfall (Compton, 1966). The Swaziland sites also show a range of topography with the lower altitude sites often situated on the slopes of mountains, but higher altitude sites comparatively level. Very little of the ultramafic area in Swaziland has any form of protection and half the area has already been lost to agriculture and forestry. A previous study (Witkowski et al., 2001) identified Kniphofia umbrina Codd. as a critically endangered ultramafic endemic. There was thus an urgent need to study the remaining ultramafic areas and document the vegetation before more of this unique and important habitat is lost, and possibly some endemic plant species too. High population growth, expanding forestry and black wattle encroachment could all impact negatively on the remaining diversity of the ultramafic areas.

L'objectif de cette thèse est l'étude de l'association magnétite-skarn-magmatisme. Dans ce but, deux gisements de magnétite (Yamansu et Cihai) ont été sélectionnés dans l'Est du Tianshan (NW Chine). Dans les deux gisements, l'assemblage grenat-pyroxène-magnétite a été reconnu, mais les mécanismes de formation de la minéralisation y sont différents. Cet assemblage a ensuite été alteré par un stade hydrothermal. Les études minéralogiques et géochimiques ainsi que les zonations observées suggèrent, que dans le gisement de Yamansu (YMD), le skarn et la minéralisation sont liés à la circulation d'un fluide riche en fer; alors que pour le gisement de Cihai (CMD), la mise en place du minerai est probablement associée à un magmatisme mafique/ultramafique. A YMD, selon l'anisotropie de la susceptibilité magnétique (AMS) et une étude paléomagnétique, le basalte et le skarn massif à grenat, tout deux encaissant les corps à magnétite massive, étaient en position horizontale lors de leur formation. L'attitude actuelle de ce gisement serait donc due à la phase tectonique majeure. La magnétite dans CMD présente les caractéristiques d'une magnetite magmatique, plutôt qu'hydrothermale. Dans ce gisement, la contamination du magma par des formations sédimentaires a pu contribuer à séparer efficacement un liquide riche en oxyde de fer d'un bain silicaté. / The aim of this thesis was to investigate the magnetite-skarn-magmatism association, of which two study objects (Yamansu and Cihai magnetite deposits) were selected from eastern Tianshan, China. Both of the Yamansu and Cihai magnetite deposit illustrate garnet-pyroxene-magnetite assemblage in prograde stage and hydrothermal assemblage in retrograde stage which essentially defined as skarn-related deposit, but with different forming mechanisms. Yamansu magnetite deposit (YMD) is a typical metasomatic skarn deposit confirmed by the mineral assemblage and chemical composition, zonation etc., whereas to Cihai magnetite deposit (CMD) probably associated to sedimentary assimilation into mafic/ultramafic magma and fractionation. According to anisotropy of magnetic susceptibility (AMS) and paleomagnetic study, the basalt and the massive garnet skarn which clamp massive magnetite ore bodies were coincidently horizontal in YMD. In addition, it was the Fe-rich fluid caused extensive skarnization in YMD. The magnetite in CMD is magmatic origin, rather than hydrothermal according to mineralogy and geochemical study. External material contamination of the mafic/ultramafic magma probably efficiently separated the magnetite and silicate at the peripheral part of the large mafic/ultramafic pluton in high temperature.

<p> intrusive Ultramafic sill of the Cape Smith-Wakeham Bay Fold Belt, located in the Ungava Peninsula P.Q., was studied. Measurements of olivine crystals were made on enlarged images of thin sections to determine the average olivine grain size and volume across the intrusion. </p> <p> The Bravo ultramafic sills exhibit a peculiar petrographic and chemical zoning, the rocks becoming increasingly rich in olivine as one moves toward the centre of the intrusion. Rock Compositions range from gabbro at the margin to olivine rich peridotite near the centre. The olivine grain size distribution exhibits n similar zoning, the grain size increasing toward the centre. However, the maximum value is skewed somewhat to the south along the edge of the central olivine rich zone, coinciding with the maximum value of Ns (Nickel in sulfides). </p> <p> The zoning is a consequence of flowage differentiation. Along the margins of the intrusion grain dispersive pressure (the pressure due to mechanical interaction between phenocrysts is dominant and grains are forced toward the centre. Within the centre of the intrusion, where the increased crystal concentration results in plug flow, the force of gravity is dominant and the largest grains make their way to the base of the plug. </p> <p> The Bravo Ultramafic Sills are pre-tectonic in origin being intruded into a group of eugeosynclinal strata in a sub-horizontal attitude. </p> / Thesis / Bachelor of Science (BSc)

La région Centre Ouest du Brésil possède des massifs ultrabasiques (UB) avec des manteaux latéritiques représentants des réserves et ainsi des ressources économiques importantes de Ni. Ces massifs nickélifères permettent d’étudier le cycle du Ni sous contraintes anthropiques par une approche pluridisciplinaire, associant caractérisations chimiques, mesures physiques de sa spéciation solide, et traçage isotopique des sources et des processus affectant le nickel. Cette thèse focalisée sur l’utilisation des isotopes du Ni comme traceur a eu pour but d’identifier la signature isotopique au sein de plusieurs compartiments en interactions les uns avec les autres (roche mère, saprolite, latérite, sol et plantes) et d’associer les fractionnements observés aux processus biogéochimiques. De plus, du fait de leurs ressources économiques, les massifs étudiés, Barro Alto et Niquelândia, subissent une pression anthropique importante, minière et métallurgique. Ainsi, ils offrent une opportunité unique d’étudier le fractionnement isotopique du nickel associé à son cycle anthropique, et voir s’il est possible d’identifier la contribution anthropique dans le cycle naturel du Ni par sa signature isotopique. Les résultats ont montré que l’altération de roches UB conduit à un fractionnement isotopique du Ni, se traduisant par une perte en isotopes lourds de la phase solide, soit un Δ⁶⁰Ni de - 0,47 ‰ entre la roche mère et le top-sol. Ce fractionnement semble associé au moins en partie à l’incorporation et à la sorption des isotopes légers du Ni sur les oxydes de Fe lors de la remobilisation du Ni au cours de l’altération. Cet enrichissement en isotopes légers du Ni dans la partie solide conduit à une composition isotopique en Ni plus lourde dans la phase dissoute (eaux du massifs : 0.50 ‰ < δ⁶⁰Ni < 0,70 ‰). La zone saprolitique présente une variation importante de signature isotopique allant pour δ⁶⁰Ni de -0,04 ‰ à 1,41 ‰. Pour les échantillons présentant une signature isotopique lourde, le Ni se trouve principalement au sein de la serpentine fortement substitué, quand les signatures plus légères sont en lien avec une proportion de Ni plus importante au sein de la goethite. Dans un système comme celui-ci avec un nombre important de phases porteuses de Ni, il est cependant très difficile d’établir un lien entre la spéciation solide et l’isotopie du Ni. Le rôle des plantes dans le cycle du Ni a été abordé en étudiant le fractionnement isotopique du Ni dans trois espèces de plantes hyperaccumulatrices de Ni et deux espèces de plantes tolérantes. Il existe un fractionnement isotopique lors du transfert de Ni de la tige vers la feuille, qui est systématiquement enrichie en isotopes lourds du Ni. Les feuilles sont les compartiments des plantes où les teneurs en Ni sont les plus importantes. L’enrichissement en isotopes lourds du Ni des feuilles par rapport aux sols (- 1,05 ± 0,03 ‰ < Δ⁶⁰Ni_sol-feuilles < - 0,06 ± 0,12 ‰) semble indiquer que la restitution et la décomposition de cette matière organique au niveau du sol s’accompagnera d’un apport en isotopes lourds du Ni dans le sol. L’activité pyrométallurgique entraîne un fractionnement isotopique lors du processus de fusion en conditions réductrices qui conduit à la formation des scories de réduction. Ces résidus présentent un enrichissement en isotopes lourds (δ⁶⁰Ni = 0,18 ± 0,05 ‰) par rapport au matériel entrant (δ⁶⁰Ni = 0,08 ± 0,08 ‰) et au produit final, le FeNi (δ⁶⁰Ni = 0,06 ± 0,02 ‰). Enfin, cette étude montre que l’utilisation des isotopes du Ni pour tracer le Ni « anthropique » et le distinguer du Ni naturel possède certaines limites en raison du faible fractionnement induit par les processus pyrométallurgiques aux regards de la grande variabilité des échantillons naturels au sein de la littérature (-1.03 ‰ < δ⁶⁰Ni < 2.50 ‰). / The Centre region the West of Brazil possesses ultramafic massifs (UM) with coats lateritic representatives of the reserves and so the important economic resources of Ni. These nickelifere massifs allow to study the cycle of Ni under anthropological constraints by a multidisciplinary approach, associating chemical characterizations, physical measures, solid speciation, and the isotopic drawing of sources and the processes affecting the nickel. This work focused on the use of the isotopes of Ni as tracer aimed at identifying the isotopic signature within several compartments in interactions some with the others (source rock, saprolite, laterite, soil and plants) and to associate the fractionation observed in the biogeochemical processes. Furthermore, because of their economic resources, the studied massifs, Barro Alto and Niquelândia, undergo an important, mining and metallurgical anthropological pressure. So, they offer an opportunity to study the isotopic fractionation of the nickel associated with its anthropological cycle, and to see if it is possible to identify the anthropological contribution in the natural cycle of Ni by its isotopic signature. The results showed that the weathering of rocks UB leads to an isotopic fractionation of Ni, being translated by a loss in heavy isotopes of the solid phase with a Δ⁶⁰Ni of - 0,47 ‰ between the bedrock and the top-soil. This division seems associated at least partially with the incorporation and with the sorption of the light isotopes in iron oxides during the remobilization of Ni. This enrichment in light isotopes in the solid part leads to a heavier isotopic composition in the dissolved phase (waters of massifs: 0.50 ‰ < δ⁶⁰Ni < 0,70 ‰). The saprolitic zone presents an important variation of isotopic signature (δ⁶⁰Ni) from -0,04 ‰ to 1,41 ‰. For samples presenting a heavy isotopic signature, Ni is mainly within the serpentine strongly substituted, when the lighter signatures are in connection with a proportion of Ni more important within the goethite. In a system as this one with a significant number of Ni-bearing phases, it is however very difficult to establish a link between the solid speciation and the isotopic composition. The role of plants in the cycle of Ni was approached by studying the isotopic fractionation of Ni in three species of hyperaccumulating plants of Ni and two species of tolerant plants. There is an isotopic fractionation during the transfer of Ni between the stem and the leaf, which is systematically enriched in heavy isotopes of Ni. The leaves are the compartments of the plants where the Ni contents are the most important. The enrichment in heavy isotopes of Ni in leaves with regard to soils (- 1,05 ± 0,03 ‰ < Δ⁶⁰Ni_sol-feuilles < - 0,06 ± 0,12 ‰) seems to indicate that the return and the decomposition of this organic matter at soil level will come along with a contribution in heavy isotopes of Ni in the soil. The pyrometallurgical activity lead to an isotopic fractionation during the smelting process in reducing conditions which leads to the formaton of the reduction slag. These slags present an enrichment in heavy isotopes (δ⁶⁰Ni = 0,18 ± 0,05 ‰) compared with the feeding material (δ⁶⁰Ni = 0,08 ± 0,08 ‰) and the end product, the FeNi (δ⁶⁰Ni = 0,06 ± 0,02 ‰). Finally, this study shows that the use of the isotopes of Ni to decipher the Ni anthropogenic and natural is limited because of the low fractionation induced by the pyrometallurgical processes in the looks of the big variability of the natural samples within the literature (-1.03 ‰ < δ⁶⁰Ni < 2.50 ‰).

Rare earth elements (REE) are an increasingly important group of metals due to their role in the development of modern technologies. Despite being abundant within the Earth’s crust recoverable ores are uncommon, and their mineralization isn’t as well understood as other abundant ore types. In this work, the mineralization of REE occurring in outcrop samples at Hicks Dome, a cryptoexplosive feature that resides in the Wauboukigou Alnöite Province in southeastern Illinois, was studied to determine the mode of mineralization and the origin of the REE. Xenotime-(Y) was identified with a scanning electron microscope in an intrusive breccia and emplaced during or shortly after the uplift which created the oval doming of the Paleozoic section. Whole rock REE concentrations from ultramafic dikes at Hicks Dome closely match global averages of lamprophyres and carbonatites, with a steep La to Lu slope, and enrichment of light rare earth elements. Hicks Dome has unique characteristics relative to the other intrusions in the Province, such as elevated REE, Th, and doming. These traits indicate that the dome was more closely related to an intrusive body at depth that sourced heat, volatile gases, and a suite of rare elements. Based on these data, the REE mineralization and thorium associated with the siliceous breccia is directly related to the alkaline ultramafic intrusion at Hicks Dome. The volatile rich, hot fluids emanating from the ultramafic magma supplied REE and thorium were mixed with the regional fluids responsible for the IKFD.

Although soils developed from ultramafic parent materials have significance to agriculture, ecology and health, their bio-geochemistry is poorly understood. The mineralogical and bio-geochemistry of soils formed from the ultramafic parent materials of the Great Dyke, Zimbabwe and Gillespie County, Texas was investigated. The objectives were to determine the mineralogical and bio-geochemical properties of the soils in order to assess the potential impact and challenges to agriculture, and environmental quality. Soil samples were taken from the crest, shoulder, footslope and the toeslope. Chemical analyses were performed by nuclear and spectroscopic techniques. Mineral characterization was conducted by x-ray diffraction (XRD) and spectroscopic techniques. Microbial whole-community structure was determined by the fatty acid methyl esters (FAME) technique. The results indicate wide chemical and mineralogical compositions among the studied sites. The soils contain relatively high concentrations of heavy metals (some sites contain Cr(VI)), but low levels of K and Ca. The highest concentrations of trace metal were associated with chromite, Fe oxides and serpentinite. The concentrations of Mg were higher than those of Ca and varied between Zimbabwe and Texas soils largely due to the parent materials. Unique to these soils is the occurrence of talc, serpentine, chlorite, Fe-rich smectite, amphiboles, pyroxenes, Fe and Cr oxides in relatively large amounts. These soils also lack micas and have neglible amounts of kaolinite and feldspars. Palygorskite and serpentine occurred in specific soil horizons and at specific landscape positions. FAME profiles indicate that the soil microbial community structure is predominantly bacteria and fungi (including arbuscular mycorrhiza fungi) at each landscape position across the transect. Biomarkers for actinomycetes were undetectable. The proportions of Gram-positive bacteria were higher than those of the Gram-negative bacteria. Very low levels of nutrients (Ca and K), higher Mg/Ca molar ratios, and the relatively high concentrations of heavy metals in these soils impact agricultural productivity. High concentrations of heavy metals, the presence of the Cr(VI) as well as its great potential to form in these soils might impact microbial activity and environmental quality. The occurrence of fibrous minerals (e.g serpentine and amphiboles) in these soils will likely impact human health.

Ultramafic soils enriched in nickel, such as found in Australia and New Caledonia, are associated with unique, diverse and poorly known vegetation communities. Re-establishment of these highly specific ecosystems is still a challenge for Ni mining companies. Ultramafic vegetation communities are the outcome of a long evolution process resulting in their adaptation to the extreme soil conditions found on ultramafic outcrops. Mycorrhizal fungi, a very common plant symbiont, are generally thought to be beneficial to plants in other ecosystems, providing plants with phosphorus and even promoting metal tolerance in plants in some cases. We examined the hypothesis that mycorrhizal fungi may contribute to the survival of plants in ultramafic soil conditions. Bandalup Hill, an ultramafic outcrop enriched in Ni (South West of Western Australia) was selected to assess the contribution of mycorrhizal fungi to ultramafic plants. Soil constraints, in particular the degree of Ni toxicity, were assessed at two sites with ultramafic soils within the outcrop. Total metal, nutrient, DTPA extractable Ni and available P were measured in soil while Ni, Ca and Mg were tested in the soil solution. In addition, nutrients and metals were analyzed in shoots of some plant species occurring at each site: Eucalyptus flocktoniae, Melaleuca pomphostoma, Melaleuca coronicarpa and Hakea verucosa. Topsoils in Bandalup Hill and plant shoots had high levels of Ni, and very low levels of P, K and N. Variation in DTPA extractable Ni between sites reflected the variation in shoot Ni level of E. flocktoniae and M. pomphostoma. Variations in soil solution Ni levels reflected variations in shoot Ni levels of M. coronicarpa and H. verucosa between sites. The germination requirements of the plant species used to assess the soil constraints was assessed. Species selected included Eucalyptus flocktoniae, Melaleuca coronicarpa, and Hakea verucosa. Seeds of E. flocktoniae and M. coronicarpa had a higher germination rate if pre-treated with smoke water, while no pre-treatment was required to germinate H. verucosa seeds. The unusual germination requirement of E. flocktoniae and M. coronicarpa involve complex chemical signals that may be present in the soil when the conditions are more favorable for plant establishment. Such unusual germination requirement may represent an adaptation to the hostile conditions of the ultramafic soils of Bandalup Hill. The mycorrhizal association and root characteristics of the selected plant species was also assessed after 8 weeks of growth in undisturbed ultramafic topsoil cores from Bandalup Hill. Roots of these species (including H. verucosa from a previously designated non-mycorrhizal family, Proteaceae) were associated with mycorrhizal fungi. Roots of E. flocktoniae and M. coronicarpa were colonized by both arbuscular mycorrhizal fungi (AMF) and ectomycorrhizal fungi (ECM), while roots of H. verucosa only contained some AM fungal structures. All species had high shoot to root ratios and their root characteristics reflected their association with mycorrhizal fungi. Based on the previous observations, uninoculated and inoculated E. flocktoniae seedlings were grown for 10 to 16 weeks in sand amended with Ni at 0, 0.2, 1 and 2.3 mg/kg. Mycorrhizal inoculum consisted of spores of Pisolithus sp. (ECM) or a mix of AMF spores and colonized root fragments, both originating from Bandalup Hill. Another inoculum consisted in Pisolithus sp. spores from a site with ultramafic soils in New Caledonia. Inoculation with AM and ECM fungi from Bandalup Hill was beneficial to E. flocktoniae. Benefits consisted mainly of a reduction of Ni shoot translocation at the highest Ni soil level. At 1 mg/kg soil Ni, E. flocktoniae exhibited a certain degree of tolerance to Ni. A substantial increase in growth and nutrient uptake with Pisolithus sp. from Western Australia was also observed. The contribution of AM fungi from Bandalup Hill to E. flocktoniae, M. coronicarpa, H. verucosa, and Trifolium subterraneum (clover) was then examined in ultramafic soil from Bandalup Hill.Steaming of ultramafic soil increased the availability and plant uptake of P. Consequently, uninoculated seedlings grew better, and inoculation with AM fungi decreased the growth of native plant species but did not affect their shoot Ni concentration. The presence of AM fungi increased the concentration of P in shoots of native plants species. Inoculation had no effect on the growth and nutrient content of subterranean clover. As mining activities have the potential to reduce the infectivity of AM fungi in topsoils, the effect of disturbance and storage practices on the AM infectivity of ultramafic topsoils collected in summer or winter from Bandalup Hill was investigated. Disturbance consisted in passing topsoil through a 2mm seive and cutting roots into 1cm fragments. Disturbed topsoil was then stored at room temperature in pots that were either sealed from the atmosphere or left open, and pots were maintained at field capacity. E. flocktoniae seedlings were planted in undisturbed and disturbed topsoil just after topsoil collect and then after 3, 6 and 9 months of topsoil storage. AM fungi present in the topsoil collected in summer was less susceptible to initial disturbance than AM fungi present in topsoil collected during winter. Also, storage of topsoil in sealed pots watered to field capacity was more detrimental to its infectivity than storage of topsoil in dry conditions. Mycorrhizal fungi can contribute to the survival of some native plant species in the ultramafic soils of Bandalup Hill and they may represent another strategy to improve the success of Ni mine revegetation. However, such contribution may not be the unique avenue for native plants to survive in ultramafic soils of Bandalup Hill.

Submitted by LENON MELO ILHA null (lenonilha@unipampa.edu.br) on 2017-08-22T14:11:00Z No. of bitstreams: 1 Levantamento Magnetométrico em Sequência Metaultramáfica Mineralizada em Cromita, São Sepé (RS).pdf: 4941337 bytes, checksum: 4bae6be70b020d210ee384046ccfeadd (MD5) / Approved for entry into archive by Monique Sasaki (sayumi_sasaki@hotmail.com) on 2017-08-23T18:49:11Z (GMT) No. of bitstreams: 1 ilha_lm_me_rcla.pdf: 4941337 bytes, checksum: 4bae6be70b020d210ee384046ccfeadd (MD5) / Made available in DSpace on 2017-08-23T18:49:11Z (GMT). No. of bitstreams: 1 ilha_lm_me_rcla.pdf: 4941337 bytes, checksum: 4bae6be70b020d210ee384046ccfeadd (MD5) Previous issue date: 2017-06-30 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Estudos de prospecção e pesquisa mineral por meio da geofísica são fundamentais para o reconhecimento e incorporação de novas reservas, devido ao baixo custo em relação aos métodos diretos de investigação e a possibilidade de cobertura de amplas áreas. Este trabalho consiste na aplicação do método da Magnetometria Terrestre, na caracterização de um corpo ultramáfico metamorfizado e mineralizado em cromo, a área de estudo se localiza na zona rural do município de São Sepé (RS) e situada nos limites das rochas do Escudo Sul Riograndense. Os litotipos presentes na área são arenitos, rochas metavulcânicas e rochas máficas/ultramáficas. Foram realizadas 1677 leituras em linhas perpendiculares a direção principal do corpo ultramáfico, com espaçamento de 25 metros entre pontos de medida. Os mapas magnetométricos gerados revelam anomalias de alta intensidade, acima de 420nT no mapa de intensidade de campo total, relacionadas com as rochas ultramáficas. Os resultados demonstraram a aplicabilidade do método da magnetometria na individualização de ocorrências minerais magnéticas de dimensões métricas, e a possibilidade de uso na seleção de alvos para perfuração, amostragem e quantificação de teores, em rotinas de pesquisas por depósitos de cromo. / Studies of mineral research and exploration through geophysics are fundamental for the recognition and incorporation of new reserves, due to the low cost in relation to the direct methods of investigation and the possibility of covering large areas. This work consists of the application of the Terrestrial Magnetometry method, in the characterization of a metamorphic and mineralized chrome mineralized body, the study area is located in the rural area of the city of São Sepé (RS) and located in the limits of the rocks of the Escudo Sul Riograndense. The lithotypes present in the area are sandstones, metavulcanic rocks and mafic / ultramafic rocks. There were 1677 readings in lines perpendicular to the main direction of the ultramafic body, with spacing of 25 meters between measurement points. The generated magnetometric maps show high intensity anomalies, above 420nT in the map of total field strength, related to ultramafic rocks. The results demonstrated the applicability of the magnetometry method in the individualization of magnetic mineral occurrences of metric dimensions, and the possibility of use in the selection of targets for drilling, sampling and quantification of contents, in search routines for chromium deposits. / CNPq: 470821/2013

L'approvisionnement en métaux rares est devenu un sujet de préoccupation majeur du fait de nombreuses incertitudes géopolitiques et de leur utilisation croissante dans des produits de haute technologie. Cependant, pour assurer l'approvisionnement, des progrès doivent être accomplis dans la compréhension de la ressource. Parmi les métaux rares, la demande potentielle pour le scandium (Sc) est croissante, du fait de son utilisation dans des alliages de haute technologie ainsi que dans les piles à combustible. L'approvisionnement n'est actuellement pas assuré mais les gisements latéritiques sont des cibles prometteuses. Le Sc pourrait devenir un sous-produit de nombreuses latérites nickélifères, et même le principal produit des minerais latéritiques à fortes teneurs découverts dans l'est australien. Pour comprendre l'origine de ces enrichissements latéritiques en Sc, nous avons étudié les processus géochimiques et cristallochimiques qui forment l'un de ces gisements (Syerston–Flemington, Nouvelle-Galle du Sud). Une méta-analyse des études géochimiques portant sur les roches et minéraux mantelliques, qui forment les roches mères de toutes les latérites enrichies en Sc connues à ce jour, révèle que les lithotypes les plus favorables sont les cumulats à clinopyroxène ou amphibole. Cette étude montre aussi un changement drastique dans la compatibilité du Sc avec la profondeur dans le manteau, allant d'incompatible dans le faciès à spinelle à compatible dans le faciès à grenat. Cette dualité est particulièrement intéressante pour tracer la profondeur de fusion partielle des roches extrusives.Une étude intégrée combinant diffraction des rayons X, analyses chimiques en roche totale et à l'échelle microscopique, ainsi que l'absorption des rayons X, a été menée dans le but de comprendre les processus de concentration du Sc durant l'altération latéritique. Les profils latéritiques riches en Sc résultent de la dissolution des minéraux primaires présentant des enrichissements en Sc légers, mais significatifs. Ensuite, le Sc est piégé successivement par les smectites, puis les oxydes de fer après la dissolution des phases argileuses. Les teneurs en Sc reflètent la haute capacité d'adsorption de cet élément par la goethite. Avec les mécanismes de dissolution–précipitation formant des générations successives de goethite, cette capacité préserve l'immobilité du Sc alors que la plupart des autres éléments sont lessivés, amenant à de hautes concentrations résiduelles. / The supply of rare metals has become a great concern due to the combination of geopolitical uncertainties and increasing use in new high-technology materials. However, to secure supply, progress on the understanding of the resource is needed. Among rare metals, there is a growing potential demand for scandium (Sc), resulting from its use in high-performance alloys and solid oxide fuel cells. The current supply is not guaranteed but lateritic deposits are a promising target. Scandium could become a by-product of many nickel laterites, and even the main product of high-grade lateritic Sc ores in eastern Australia. To understand the origin of lateritic Sc enrichment, we investigated the geochemical and crystal-chemical processes forming one of these deposits (Syerston–Flemington, New South Wales). A meta-analysis of geochemical studies on mantle-derived rocks and minerals, which form the parent rocks of all known Sc-enriched laterites reveals that the most favourable lithotypes are cumulates of clinopyroxene or amphibole. It also shows a drastic change in the compatibility of Sc with increasing depth in the mantle, ranging from incompatible in the spinel facies to compatible in the garnet facies. This dichotomy is of great interest for tracing the depth of partial melting of extrusive rocks.Integrated X-ray diffraction, whole-rock and microscopic chemical analyses and X-ray absorption spectroscopy have been used to understand the processes that concentrate Sc during lateritisation. Scandium-rich lateritic profiles result from the dissolution of primary minerals containing slight but significant Sc enrichment, followed by successive trapping by smectite and iron oxides after dissolution of the clay phases. Scandium grades reflect the high capacity of goethite to adsorb this element. Along with mechanisms of dissolution–precipitation forming successive generations of goethite, it preserves Sc immobility while most other elements are leached, leading to high residual concentrations of Sc.