Este trabalho tem como objetivo investigar os processos controladores da mobilidade e concentração de níquel (Ni) em rochas ultramáficas serpentinizadas no Escudo Sul-Riograndense (ESrg), Rio Grande do Sul (RS), Brasil. A composição primária das rochas ultramáficas hospedeiras de Ni constitui um parâmetro relevante, mas os processos secundários são os controladores mais importantes dessas mineralizações, principalmente a serpentinização, pressupondo-se que o Ni possa migrar da olivina ou metamórfica para os minerais do grupo das serpentinas. Para a caracterização mineralógica e textural das rochas ultramáficas serpentinizadas da porção oeste do ESrg, foram aplicadas técnicas de petrografia e Microscopia Eletrônica de Varredura. Para a análise química dos elementos em rocha total foi realizada fluorescência de Raios-X, ICP e ICP-MS. Estas técnicas permitiram classificar quimicamente as rochas e relacionar a variação da intensidade dos eventos metassomáticos, metamórficos e hidrotermais que modificaram a mineralogia dos protólitos. No entanto, foram necessárias estudos de detalhe dos minerais individuais, notadamente olivinas e serpentinas com o uso da microssonda eletrônica para determinar as concentrações de Ni e suas variações nos diferentes corpos ultramáficos investigados. Dessa forma, foi possível quantificar e identificar os minerais concentradores de Ni e a relação dos eventos com a mobilidade e concentração dos elementos. Os resultados de microssonda indicam que as olivinas dos peridotitos do Maciço Pedras Pretas possuem baixos teores de níquel que variam de 0,13% a 0,21% e a média é 0,17%, enquanto que as olivinas dos harzburgitos da Sequência Cerro Mantiqueiras possuem teores mais elevados na média de 0,31%. As olivinas do Pedras Pretas possuem composição homogênea enquanto que aquelas do Cerro Mantiqueiras mostram variações composicionais importantes com conteúdo de Fo 92-98 e teores de níquel entre 0,3% e 0,4%. Tais variações no Cerro Mantiqueiras podem estar relacionadas a diferenças na composição do protólito ou a outro fator desconhecido que necessita futuras investigações. As olivinas e serpentinas no Cambaizinho e Serrinha mostram valores de níquel entre 0,19% a 0,3%, comparáveis ao Cerro Mantiqueiras, porém bem mais elevados que o Pedras Pretas. Este estudo mostra que os corpos ultramáficos do Cambaizinho- Serrinha e Cerro Mantiqueiras possuem potencial para desenvolver depósitos de Ni não sulfetado devido as concentrações relativamente elevadas de Ni nas olivinas. Entretanto, tais depósitos não se desenvolveram devido a dois fatores principais: o primeiro está ligado à superposição de eventos de metamorfismo e deformação recorrentes no tempo e no espaço que propiciaram a mobilização do Ni; o segundo fator é atribuído a ausência de agentes supergênicos favoráveis para a formação de depósitos lateríticos como os observados na região norte do Brasil. / This paper investigates the processes controlling the mobility and concentration of nickel (Ni) in serpentinized ultramafic rocks in the Sul- Riograndense Shield (ESrg), Rio Grande do Sul (RS), Brazil. The primary composition of the ultramafic Ni host is a relevant parameter, but the secondary processes are the almost important controllers of these mineralizations, mainly serpentinization, assuming that Ni may migrate from igneous or metamorphic olivine minerals to the group of serpentine this phase. For the mineralogical and textural characterization of the serpentinized ultramafic rocks of the western portion of ESrg, were applied techniques of petrographic and scanning electron microscopy. For chemical analysis of elements in rock whole was performed X-ray fluorescence, ICP and ICP-MS. Although these techniques allow chemically classification of rocks and relate the variation of intensity in which metasomatics, metamorphic and hydrothermal events, changed the that mineralogy of the rock, however analysis of individual mineral detail, notably olivine and serpentine minerals using the electron microprobe detail, were necessary to determine the concentrations of Ni and variations in different ultramafic bodies investigated. Thus, it was possible to quantify and identify the Ni concentrators minerals and the relationship of events with the mobility and concentration of the elements. The microprobe results indicate that the olivine of peridotite Pedras Pretas have low contents of NiO ranging from 0.13% to 0.21% and averaged 0.17%, while the olivine harzburgites Cerro Mantiqueiras have higher levels of NiO averaging 0.31%. The olivine of Pedras Pretas have a homogeneous composition as those of Cerro Mantiqueiras show important compositional variations with a content of forsterite the Fo 92-98 and NiO contents of between 0.20% and 0.40%. Such variations in Cerro Mantiqueiras may be related to differences in the composition of the protolith or another unknown factor that needs further investigation. The olivine and serpentine in Cambaizinho and Serrinha, show NiO values between 0.19 % to 0.3 %, values comparable to the Cerro Mantiqueiras, but higher than the Pedras Pretas. This study shows that the Cambaizinho, Serrinha and Cerro Mantiqueiras have the potential to develop non- sulphide Ni deposits, due high Ni concentrations in olivine. However, these deposits are not developed due two main factors: the first is linked to the superposition of events the metamorphism and deformation applicants in time and space, that enabled the Ni mobilization; the second factor is attributed to lack preservation of profiles suitable for the Ni concentration, due to uplift and erosion lateritic subsequent.

A ambiência geotectônica das rochas máfico-ultramáficas do sudoeste do Cráton Amazônico é, de uma maneira geral, pouco conhecida. A maioria dos trabalhos desta porção cratônica está enfocada nos estudos geocronológicos em granitóides, de modo que pouco se sabe sobre a origem e significado tectônico destas rochas. Neste contexto, esta pesquisa buscou contribuir para o conhecimento da evolução geotectônica do sudoeste do Cráton Amazônico, através da caracterização de um complexo ofiolítico Mesoproterozóico, correspondente ao Complexo Trincheira, de idade Calimiana. Desta forma, uma nova proposta de modelo tectônico é aqui apresentada, a qual explica muitas das características anteriormente enigmáticas da história Pré-cambriana desta área-chave e possibilita outras alternativas para a reconstrução do supercontinente Columbia. O ofiolito Trincheira é composto de rochas extrusivas (anfibolitos derivados de basaltos maciços e almofadados), intrusivas máfico-ultramáficas, chert, formação ferrífera bandada, pelitos, psamitos e pequena proporção de rochas cálcio-silicáticas. A composição geoquímica das rochas extrusivas e intrusivas máfico-ultramáficas mostra semelhanças com os basaltos toleiíticos modernos, as quais possuem moderado a forte fracionamento de elementos terras-raras leves, padrão quase horizontal dos elementos terras-raras pesados e moderada a forte anomalia negativa dos elementos de alto campo de força (especialmente Nb), uma assinatura geoquímica típica de zona de subducção. As unidades basais do ofiolito Trincheira são quimicamente similares aos modernos basaltos de cadeia meso-oceânica (MORB). Esse comportamento químico muda para as unidades de topo as quais apresentam uma assinatura similar aos toleiítos de arco-de- ilha (IAT). Portanto, o ofiolito Trincheira deve ter sido originado em um ambiente intra-oceânico de supra-subducção composto de um sistema de arco/retro-arco. Os dados isotópicos de Sm, Nd e Sr para essas rochas indicam valores iniciais de Nd de moderados a altamente positivos (+2.6 a +8.8) e muito baixa razão inicial de 87Sr/86Sr (0,7013 – 0,7033), sugerindo que esses magmas foram originados a partir de uma fonte mantélica empobrecida e nada ou fracamente contaminados por componentes de subducção. O complexo ofiolítico foi deformado, metassomatizado e metamorfisado durante o desenvolvimento da Faixa Móvel Guaporé, um orógeno acrescionário-colisional Mesoproterozóico (1,47-1,35 Ba), constituído pela zona de sutura Guaporé, a qual une o Cráton Amazônico com o Bloco Paraguá. A fase colisional que marca o encaixe final dessas duas massas continentais ocorreu por volta de 1,35 Ba, onde o metamorfismo atingiu temperaturas entre 780 a 853°C nos granulitos máficos e 680 a 720°C nos anfibolitos, com pressão média de 6,8 kbar. A sutura Guaporé foi reativada no final do Mesoproterozóico e evoluiu para a abertura de um rift intracontinental, com a sedimentação das rochas dos Grupos Nova Brasilândia e Aguapeí, o qual marca a fragmentação final do supercontinente Columbia, por volta de 1,3-1,2 Ba. Granulitos máficos, anfibolitos e trondhjemitos da porção meridional do Cinturão Nova Brasilândia, representativos da última fase compressional que afetou o sudoeste do Cráton Amazônico, forneceram idades U-Pb de 1110 Ma, as quais datam o metamorfismo de alto grau e o fechamento do rift, processo resultante da acresção do microcontinente Arequipa-Antofalla ao Cráton Amazônico. Portanto, a fragmentação do supercontinente Columbia foi seguida rapidamente pela aglomeração de outras massas continentais, formando o supercontinente Rodínia, por volta de 1100 Ma. / The tectonic framework of the ultramafic-mafic rocks of the southwestern Amazon Craton is generally little known. Most of work this cratonic portion is focused on the geochronological studies of granitoids, so that little is known about the origin and tectonic significance of these rocks. In this context, this study contributes to the knowledge of the tectonic evolution of the southwestern Amazon Craton, through the characterization of a Mesoproterozoic ophiolitic complex, corresponding to the Trincheira Complex of Calymmian age, and propose a tectonic model that explains many previously enigmatic features of the Precambrian history of this key craton, and discuss its role in the reconstruction of the Columbia supercontinent. The complex comprises extrusive rocks (fine-grained amphibolites derived from massive and pillowed basalts), mafic-ultramafic intrusive rocks, chert, banded iron formation, pelites, psammitic and a smaller proportion of calc-silicate rocks. The geochemical composition of the extrusive and intrusive rocks indicates that all noncumulus mafic-ultramafic rocks are tholeiitic basalts. These rocks display moderately to strongly fractionation of light rare earth elements (LREE), near-flat heavy rare earth elements (HREE) patterns and moderate to strong negative high field strength elements (HFSE) anomalies (especially Nb), a geochemical signature typical of subduction zones. The lowest units of the Trincheira ophiolite are similar to the modern mid-ocean ridge basalt (MORB). This behavior changes to an island arc tholeiites (IAT) signature in the upper units of the Trincheira ophiolite. Therefore, the Trincheira ophiolite appears to have originated in an intraoceanic supra-subduction setting composed of an arc-back-arc system. Mafic-ultramafic rocks of the Trincheira ophiolites display moderate to highly positive initial Nd values of +2.6 to +8.8 and very low values for the initial 87Sr/86Sr ratio (0.7013 - 0.7033). It is suggested that these magmas originated from a depleted mantle source, which experienced low degree of contamination by variable subduction components. The ophiolitic sequence was deformed, metasomatized and metamorphosed during the development of the Alto Guaporé Belt, a Mesoproterozoic accretionary-collisional orogen that represents the Guaporé suture zone. Metamorphism was pervasive and reached temperatures of 780-853°C in mafic granulites and 680-720°C in amphibolites under an overall pressure of 6.8 kbar. The Guaporé suture zone is defined by the ESE–WNW trending mafic-ultramafic belt formed during a Mesoproterozoic (ca. 1.47-1.43 Ga) accretionary phase, and overprinted by upper amphibolite-granulite facies metamorphism during collisional phase in the Ectasian (~1.35 Ga), which mark the docking final of the Amazon craton and Paraguá Block. This suture was reactivated and evolved from the development of an intracontinental rift environment, represented by Nova Brasilândia and Aguapeí Groups, which mark the final breakup of the supercontinent Columbia in the late Mesoproterozoic (ca. 1.3-1.2 Ga). Mafic granulites, amphibolites and trondhjemites in the northernmost portion of the Nova Brasilândia belt yield U-Pb zircon ages ca. 1110 Ma, which dates the high-grade metamorphism and the closure of the rift, due to the accretion of the Arequipa-Antofalla basement to the Amazon craton. Therefore, the breakup of supercontinent Columbia was followed in short sequence by the assembly of supercontinent Rodinia at ca. 1100 Ma.

Este trabalho tem como objetivo investigar os processos controladores da mobilidade e concentração de níquel (Ni) em rochas ultramáficas serpentinizadas no Escudo Sul-Riograndense (ESrg), Rio Grande do Sul (RS), Brasil. A composição primária das rochas ultramáficas hospedeiras de Ni constitui um parâmetro relevante, mas os processos secundários são os controladores mais importantes dessas mineralizações, principalmente a serpentinização, pressupondo-se que o Ni possa migrar da olivina ou metamórfica para os minerais do grupo das serpentinas. Para a caracterização mineralógica e textural das rochas ultramáficas serpentinizadas da porção oeste do ESrg, foram aplicadas técnicas de petrografia e Microscopia Eletrônica de Varredura. Para a análise química dos elementos em rocha total foi realizada fluorescência de Raios-X, ICP e ICP-MS. Estas técnicas permitiram classificar quimicamente as rochas e relacionar a variação da intensidade dos eventos metassomáticos, metamórficos e hidrotermais que modificaram a mineralogia dos protólitos. No entanto, foram necessárias estudos de detalhe dos minerais individuais, notadamente olivinas e serpentinas com o uso da microssonda eletrônica para determinar as concentrações de Ni e suas variações nos diferentes corpos ultramáficos investigados. Dessa forma, foi possível quantificar e identificar os minerais concentradores de Ni e a relação dos eventos com a mobilidade e concentração dos elementos. Os resultados de microssonda indicam que as olivinas dos peridotitos do Maciço Pedras Pretas possuem baixos teores de níquel que variam de 0,13% a 0,21% e a média é 0,17%, enquanto que as olivinas dos harzburgitos da Sequência Cerro Mantiqueiras possuem teores mais elevados na média de 0,31%. As olivinas do Pedras Pretas possuem composição homogênea enquanto que aquelas do Cerro Mantiqueiras mostram variações composicionais importantes com conteúdo de Fo 92-98 e teores de níquel entre 0,3% e 0,4%. Tais variações no Cerro Mantiqueiras podem estar relacionadas a diferenças na composição do protólito ou a outro fator desconhecido que necessita futuras investigações. As olivinas e serpentinas no Cambaizinho e Serrinha mostram valores de níquel entre 0,19% a 0,3%, comparáveis ao Cerro Mantiqueiras, porém bem mais elevados que o Pedras Pretas. Este estudo mostra que os corpos ultramáficos do Cambaizinho- Serrinha e Cerro Mantiqueiras possuem potencial para desenvolver depósitos de Ni não sulfetado devido as concentrações relativamente elevadas de Ni nas olivinas. Entretanto, tais depósitos não se desenvolveram devido a dois fatores principais: o primeiro está ligado à superposição de eventos de metamorfismo e deformação recorrentes no tempo e no espaço que propiciaram a mobilização do Ni; o segundo fator é atribuído a ausência de agentes supergênicos favoráveis para a formação de depósitos lateríticos como os observados na região norte do Brasil. / This paper investigates the processes controlling the mobility and concentration of nickel (Ni) in serpentinized ultramafic rocks in the Sul- Riograndense Shield (ESrg), Rio Grande do Sul (RS), Brazil. The primary composition of the ultramafic Ni host is a relevant parameter, but the secondary processes are the almost important controllers of these mineralizations, mainly serpentinization, assuming that Ni may migrate from igneous or metamorphic olivine minerals to the group of serpentine this phase. For the mineralogical and textural characterization of the serpentinized ultramafic rocks of the western portion of ESrg, were applied techniques of petrographic and scanning electron microscopy. For chemical analysis of elements in rock whole was performed X-ray fluorescence, ICP and ICP-MS. Although these techniques allow chemically classification of rocks and relate the variation of intensity in which metasomatics, metamorphic and hydrothermal events, changed the that mineralogy of the rock, however analysis of individual mineral detail, notably olivine and serpentine minerals using the electron microprobe detail, were necessary to determine the concentrations of Ni and variations in different ultramafic bodies investigated. Thus, it was possible to quantify and identify the Ni concentrators minerals and the relationship of events with the mobility and concentration of the elements. The microprobe results indicate that the olivine of peridotite Pedras Pretas have low contents of NiO ranging from 0.13% to 0.21% and averaged 0.17%, while the olivine harzburgites Cerro Mantiqueiras have higher levels of NiO averaging 0.31%. The olivine of Pedras Pretas have a homogeneous composition as those of Cerro Mantiqueiras show important compositional variations with a content of forsterite the Fo 92-98 and NiO contents of between 0.20% and 0.40%. Such variations in Cerro Mantiqueiras may be related to differences in the composition of the protolith or another unknown factor that needs further investigation. The olivine and serpentine in Cambaizinho and Serrinha, show NiO values between 0.19 % to 0.3 %, values comparable to the Cerro Mantiqueiras, but higher than the Pedras Pretas. This study shows that the Cambaizinho, Serrinha and Cerro Mantiqueiras have the potential to develop non- sulphide Ni deposits, due high Ni concentrations in olivine. However, these deposits are not developed due two main factors: the first is linked to the superposition of events the metamorphism and deformation applicants in time and space, that enabled the Ni mobilization; the second factor is attributed to lack preservation of profiles suitable for the Ni concentration, due to uplift and erosion lateritic subsequent.

Ces travaux de thèse apportent de nouvelles contraintes sur la concentration et le comportement de l’H dans le manteau lithosphérique et reposent sur l’étude pétro-géochimique de trois séries de xénolites de péridotites à spinelle, associant les concentrations en élément majeurs et en traces y compris l’H dans les minéraux dits anhydres (les NAMs, ici, olivine, pyroxènes) et hydratés (amphibole). Les effets de la remontée des xénolites dans du magma hôte sur les concentrations en H des NAMs ont pu être étudiés sur une série de xénolites de péridotite du champ volcanique d’Eifel (Allemagne). Une variation intra cristalline de concentration en H peut être identifiée dans l’olivine et non dans les pyroxènes coexistant. Ces profils de concentration en H peuvent être utilisés pour estimer des vitesses de remontées des magmas. Dans le cas des volcans étudiés, ces vitesses sont estimées entre 3.5 et 12 m.s-1. Ces résultats suggèrent que les pyroxènes sont de meilleurs proxy que l’olivine pour quantifier la concentration mantellique de l’H. Les xénolites de Ray Pic (Massif Central, France) ont permis de discuter dans un contexte de point chaud, l’effet de la fusion partielle et du métasomatisme à grande échelle sur les concentrations en H des NAMs. Les concentrations en H des minéraux ne suggèrent pas de lien avec le métasomatisme subit, qu’il soit modal, cryptique, à rapport liquide/roche élevé ou faible. Cependant, en comparant les concentrations en H avec un marqueur de la fusion partielle (Yb du cpx), l’H semble avoir un comportement similaire à une MREE (e.g., Sm ; D(cpx/liquide)~0.29). Enfin, des xénolites composites associant une péridotite accolée à un agent métasomatique ont permis de cibler l’influence du métasomatisme de petite échelle (pluri-centimétrique). L’interaction magma roche identifiée sur une harzburgite en contact avec du basalte montre dans les cas des olivines, des variations chimiques couplé entre éléments majeurs en fonction de leur proximité au filon et les concentrations en H. Plus l’olivine se rééquilibre avec le liquide moins elle contient d’H. Parallèlement dans cette étude, trois échantillons présentent une lherzolite en contact avec une pyroxénite avec 14% d’amphibole, une clinopyroxénite avec 40 % d’amphibole et une amphibolite (98% d’amphibole). Cette contiguïté avec un filon métasomatique permet l’étude du comportement de l’H en contexte de percolation en bordure de filon. Chaque échantillon présente des concentrations en H homogènes pour chacune des phases minérales. Cependant plus les filons contiennent d’amphibole moins il y a d’H dans les NAMs. D’autre part, une nouvelle fois, la corrélation positive entre les concentrations en H des NAMs et le Sm(cpx) en tant que marqueur de métasomatisme suggère que l’H se comporte comme une MREE.En conclusion, les minéraux des spl-harzburgites contiennent en moyenne un peu plus d’H que ceux des spl-lherzolites. Les concentrations en H des olivines sont sensibles à la dévolatilisation lors de la remontée dans le système magmatique et le rééquilibrage avec le magma. Au contraire, les concentrations en H des pyroxènes, spécialement l’opx, sont très homogènes suggérant des concentrations mantelliques. Le comportement de l’H lors de la fusion partielle et du métasomatisme reste complexe ; nos données suggèrent que l’H suit les MREE tel que le Sm. / This thesis provides new constrains on H concentrations and H behaviour in the lithospheric mantle and is based on a petro-geochemical study on 3 spinel-peridotite xenoliths series with major and traces elements analyses, including H in nominally anhydrous minerals (NAMs, olivine and pyroxenes) as well as hydrous minerals (amphibole).Ascent effects through a magmatic system on H concentration of NAMs are studied for a xenoliths series from the Eifel volcanic field (Germany). Intracrystalline variation in H concentration are observe in olivine but not in the coexisting pyroxenes. Such H concentration profiles are used to calculate the rate of magma ascent. For the studied volcanoes, the calculated rate of magma ascent is between 3.5 and 12 m.s-1. Such H concentration variations imply a devolatilisation affecting only olivine, whereas the pyroxenes are homogeneous and then can be used as a better proxy for mantle H concentrations.Ray Pic xenoliths (French Massif Central) belongs to a mantle plume setting, implying the possibility to assess the effect of partial melting and large scale metasomatism on H concentration of NAMs. The H concentrations determined do not suggest a strong link with the suffered metasomatism whether modal, cryptic, at low or high melt rock ratio. However, using H concentrations and a marker of the partial melting (Yb in cpx), H seems to behave as a MREE (e.g., Sm, D(cpx/melt) ~ 0.29).Finally, the composite xenoliths with a peridotite adjacent to a metasomatic agent allow to target the influence of small scale metasomatism (pluri-centimetric). A magma-rock interaction between a harzburgite and a basaltic patch shows that, for olivines, chemical variations in major element as a function of olivine proximity to the vein, is coupled to H concentrations of NAMs. More the olivines are close to equilibrium with the basalt, more the H concentrations are low. Alongside in this study, three samples consist of a lherzolite adjacent a pyroxenite (14% amphibole), to a clinopyroxenite (40% amphibole) and to an amphibolite (98% amphibole) respectively. This special relationship with a metasomatic vein allows to study the behaviour of H during wall rock percolation. Each sample display homogeneous H concentration within each NAMs. However, H concentration is inversely correlated to modal content in amphiboles in the peridotite. Furthermore, a positive correlation between H in NAMs and Sm(cpx) here as a marker of metasomatism suggests, again, that H behaves as a MREE.To conclude, minerals from harzburgite contain in average a bit more H than the one in lherzolite. The H concentration in olivine are sensitive to degassing during magma ascent toward the surface and reequilibrium with magma. On the contrary, H concentration in pyroxenes, especially opx, are very homogeneous suggesting mantle concentration. The behaviour of H during partial melting and metasomatism is complex. However, our data suggest that H broadly follows MREE.

The study area comprises the Archaean Manica Greenstone Belt and the Vumba Granite Gneiss, the Proterozoic Messica Granite Gneiss of the Zimbabwe Craton, the possibly allochthonous metasedimentary sequence of the Frontier Formation, the granitoids of the Mozambique Metamorphic Province, which are subdivided into Vanduzi Migmatite Gneiss, the Chimoio Granodiorite Gneiss, the Nhansipfe Granitic Orthogneiss and the Pan-African Tchinhadzandze Granodiorite Gneiss. The rock sequences in the two provinces are cut by mafic intrusions. The greenstone belt comprises mafic to ultramafic and pelitic schists and serpentinites of the Macequece Formation and metasediments ofthe M'BezaNengo Formation. The mafic to ultramafic schists and the serpentinites have chemical signatures of komatiites. The Vumba Granite Gneiss comprises the northern and southern Vumba granitoids dated at 3885±255 Ma, and 2527±632 Ma respectively. They vary from metaluminous to peraluminous, have normative QAP compositions of granodiorites and monzogranites and chemical signatures of mantle fractionates and volcanic-arc granitoids. The Early Proterozoic Messica Granite Gneiss is 2348±267 Ma old, is metaluminous and has QAP compositions of monzogranites and chemical signatures suggesting a crustal source and a volcanic-arc environment. The Frontier Formation comprises quartzite and pelitic schists. The Vanduzi Migmatite Gneiss comprises stromatic and stictolithic types. Two mineral assemblages are distinguished as they contain either garnet or hornblende. The Mid-Proterozoic Chimoio Granodioritic Gneiss is 1236±201 Ma old. It is granodioritic and metaluminous with a chemical signature of volcanic-arc granitoids. The Late Proterozoic Nhansipfe Granitic Orthogneiss is dated at 981 ±83 Ma and varies from metaluminous to peraluminous. The Rb, Nb and Y contents are typical of within-plate granitoids, whereas Ga, Zr, AI, Ce andY are typical of A-type granitoids. The age of the mafic intrusions falls between ~500 and ~11 00 Ma. The rocks typically contain plagioclase, hornblende and clinopyroxene with or without garnet and orthopyroxene. The chemistry of the rocks is typical of sub-alkaline tholeiites. The Tchinhadzandze Granodiorite Gneiss may be part of a Pan African event which lasted till ~450-~500 Ma. The normative feldspar compositions and Rb, Ba and Sr contents are typical of granodiorites. It is metaluminous and has Rb, Y and Nb contents typical of volcanic arc granitoids. The planar fabrics in the Archaean granite-greenstone belt are characterized by E-W to SW-NE strikes and steep dips to N and S and to NW and SE. The mineral lineations and fold axes plunge 60° and 30° respectively towards the E. Within the Mozambique belt, around the central part and in the extreme east of the study area, the planar fabrics have essentially N-S strikes and steep dips to E and Win contrast with complex deformation observed in the migmatites and megacrystic granitoids. The study area can be subdivided into three metamorphic blocks, namely, one of low-grade greenschist facies, one of medium-grade amphibolite facies and a third block of high-grade metamorphism. / Dissertation (MSc)--University of Pretoria, 2012. / Geology / unrestricted

The Bay of Islands Ophiolitic Complex (BOIC) is one of most well preserved and well-exposed ophiolites in the world. The BOIC consist of four massifs; these are the Table Mountain (TBL), North Arm Mountain (NAM), Blow-Me-Down Mountain (BMD) and Lewis Hills massifs. Proposed geological environments of the BOIC in Newfoundland are diverse; ranging from oceanic spreading ridge to supra-subduction setting. The BOIC has a complete ophiolite sequence as defined at the Penrose Conference (Anonymous, 1972) including ultramafic mantle rocks, ultramafic to gabbroic cumulate rocks, sheeted dikes, pillowed basaltic rocks and capping sedimentary rocks in structurally ascending order. We studied harzburgite and overlying massive dunite in the BOIC. Harzburgite is generally medium-grained, and contains olivine, orthopyroxene, Cr-spinel, clinopyroxene and rare sulfide minerals. Harzburgite is massive to strongly deformed, with local development of mylonitic shear zones. A foliation and lineation are defined by elongated and fragmented grains of orthopyroxene and Cr-spinel. Dikes, sills, veins, and irregularly-shaped bodies of dunite and pyroxenite are present throughout the harzburgite unit. Dunite is the predominant lithology of the Blow-Me-Down Mountain. It is typically fine- to medium-grained, massive, and contains minor Cr-spinel and rare sulfide minerals. Dunite contains olivine, Cr-spinel and minor pyroxenes in some samples. Olivine crystals are commonly partly replaced by serpentine along fractures and in outer rims. Bulk rock and mineral composition data suggest that harzburgites are mild to highly refractory mantle residues after partial melting. In contrast all dunite samples show a cumulate geochemical signature from a mafic melt that originated from highly refractory mantle peridotites. Our study suggest that the harzburgite in the BOIC originally formed as oceanic lithosphere at a slow spreading ridge, possibly in the vicinity of active arc systems, whereas the parental melt for dunites formed in subduction setting. The second part of this study measured trace element compositions for olivine, Cr-spinel and bulk rock of dunite. The measured bulk rock compositions are compared to those of calculated based on mineral chemistry and their abundance. This comparison suggests that the trapped melt fraction was negligible during the crystallization of the dunites. The calculated melt compositions for the dunites confirm that the melt formed in subduction setting.

Les écosystèmes ultramafiques (serpentiniques) de Nouvelle-Calédonie sont considérés comme des « hotspots » de la biodiversité, notamment en raison des pressions adaptatives exercées par des conditions édaphiques drastiques. En effet, ces sols, résultant de l'altération naturelle du manteau océanique, sont composés de plus de 85% d'oxydes de fer, sont déficients en N, P, K, déséquilibrés en Ca/Mg et riches en métaux lourds (Ni, Cr, Mn, Co). Dans les associations entre végétaux et microorganismes du sol, les deux partenaires jouent un rôle essentiel dans l'adaptation aux conditions édaphiques, essentiellement au niveau de la tolérance aux métaux lourds. Dans notre étude, nous avons choisi des espèces endémiques du genre Tristaniopsis (Myrtaceae) comme plantes modèles pour étudier le rôle des champignons ectomycorhiziens et des bactéries associées à l'adaptation des plantes au nickel. Pour étudier l'effet des sols ultramafiques sur la diversité des ectomycorhizes et des bactéries mycorhizosphériques, ainsi que sur les déterminants génétiques de résistance/adaptation des bactéries associées, environ 150 ectomycorhizes ont été échantillonnées à partir de quatre sites ultramafiques (trois au massif du Koniambo et un dans la forêt de Desmazures) et deux non-ultramafiques reposant sur des sols volcano-sédimentaires (site d'Arama). La caractérisation génotypique et phylogénétique des ectomycorhizes et des bactéries mycorhizosphériques obtenues a révélé la présence d'une grande diversité de champignons (principalement Cortinarius, Pisolithus, Russula, Boletellus) et de bactéries (Pseudomonas, Burkholderia, Bacillus) dans les deux types de sols, avec une richesse spécifique particulièrement élevée dans les sols ultramafiques. De plus, les bactéries mycorhizosphériques provenant des sols ultramafiques avaient des proportions significativement plus élevées d'isolats portant les gènes nreB et cnrT que celles issues des sols volcano-sédimentaires. Une forte corrélation positive a également été observée entre l'occurence de ces gènes, connus pour conférer la tolérance aux métaux lourds chez les bactéries, et la tolérance des isolats au nickel en culture pure. La récente mise en évidence de souches bactériennes mycorhizosphériques Ni-tolérantes et promotrices de la croissance de Pisolithus albus en co-culture doivent permettre d'identifier des bactéries auxiliaires de la mycorhization qui pourront être ensuite exploitées dans le cadre de programmes de revégétalisation de sites ultramafiques miniers en Nouvelle Calédonie. / New Caledonian ultramafic (serpentine) ecosystems are considered as hotspots of biodiversity, partly because of the adaptative pressure exerted by drastic edaphic conditions. Indeed these soils resulting from natural weathering of oceanic mantle could be composed of up to 85 % of iron oxides and are deficient in N.P.K., unbalance for the Ca/Mg ratio and rich in heavy metals Ni, Cr, Mn, Co. Both plant and soil microbes play a vital role in the adaptation to soil conditions mainly heavy metal uptake and tolerance. In our study, we choose endemic species of the genus Tristaniopsis (Myrtaceae) as model plant to study the role of ectomycorrhizal fungi and associated bacteria in plant adaptation to nickel. To investigate the effect of ultramafic soils on ectomycorrhiza and mycorrhizosphere bacteria diversities as well as on the genetic determinants of resistance/adaptation of associated mycorrhizosphere bacteria, 200 ectomycorrhizas were sampled from four different ultramafic sites (3 in Koniambo and 1 in Desmazures forest) vs two non-ultramafic ones from volcano-sedimentary soils (Arama). Molecular characterization of ectomycorrhiza (rRNA ITS) and associated mycorrhizosphere bacteria (16S rRNA) from these samples showed the presence of different fungi (Pisolithus albus, Russula spp., Boletellus spp.) and bacteria (Burkholderia spp., Bacillus spp., Pseudomonas spp.) that can be found in both soils. However, bacteria isolated from ultramafic soils could grow in the presence of Ni up to 20 mmol L-1 and contained cnrA and nreB genes, known to confer heavy metal tolerance, contrary to bacteria isolated from non-ultramafic soils. Moreover, we found a strong positive correlation between heavy metal tolerance and P-solubilizing ability. Further knowledge on functional diversity of ectomycorrhiza-mycorrhizosphere bacteria associations and its role in the adaptation of plants to ultramafic soils would help in the understanding of plant functioning on New Caledonian mine sites.

This thesis examines the bioleaching of six different Canadian nickel sulphide ores at pH levels above what is generally considered optimum (~ 2). The majority of work discussed in this thesis was conducted with a low-grade metamorphosed ultramafic nickel sulphide ore from Manitoba, Canada (Ore 3), which is not currently exploitable with conventional technologies. The ore contains 21% magnesium and 0.3% nickel. Nickel is the only significant metal value, and is present primarily as pentlandite. A substantial fraction of the magnesium is present as the serpentine mineral lizardite, making processing difficult with conventional pyro- and biohydrometallurgical techniques. The work with this ore has two equally important objectives: to minimize magnesium mobilization and to obtain an acceptable level of nickel extraction. Batch stirred-tank bioleaching experiments were conducted with finely ground ore ( 147 µm) with temperature and pH control. The first phase of experimentation examined the effect of pH (2 to 6) at 30 °C, and the second phase examined all combinations of three pH levels (3, 4 and 5) and five temperatures (5, 15, 22.5, 30, and 45 °C).

Lichens are among the most prominent and successful life forms of metal-rich habitats, including ultramafic rocks and soils; however, research on lichens of ultramafic habitats is limited, especially on the North American continent. A review of the published literature on lichens of ultramafic substrates in North America yielded a total of 437 lichen species reported from ultramafic rocks and soils. Lichen assemblages of ultramafic substrates vary in composition and are dominated by acidophytic (low pH preferring) taxa with a minor, but consistent, basiphytic (high pH preferring) component. Species lists from ultramafic habitats in different geographic regions varied widely, suggesting that factors unrelated to substrate, such as climate, have a large effect on lichen assemblage composition. However, several studies showed clear differentiation between lichen composition on nearby or adjacent ultramafic and nonultramafic habitats, suggesting that ultramafic substrates harbor regionally unique lichen assemblages. In a regional community ecology study, we sampled lichen biotas of eight ultramafic and eight sandstone outcrops along a 70 km maritime influence gradient in order to test three hypotheses: 1) a substrate effect hypothesis that saxicolous lichen communities of ultramafic and sandstone outcrops are compositionally distinct; 2) a maritime gradient hypothesis that coastal and inland communities are compositionally distinct; and 3) a maritime moderation hypothesis that coastal ultramafic and sandstone communities are more similar than those of inland ultramafic and sandstone. Relationships between lichen communities and abiotic variables were analyzed using perMANOVA. Ultramafic communities showed significant differentiation from sandstone communities in the study area. A total of 134 taxa were recorded - 81 taxa from ultramafic outcrops and 100 taxa from sandstone, with 47 taxa found on both rock types. Ultramafic outcrops were characterized by greater similarity between samples, lower lichen cover, larger differences in cover between north and south aspects, and higher abundance and diversity of cyanolichen taxa relative to sandstone. Coastal, intermediate, and inland communities were compositionally distinct from one another, and sandstone and ultramafic assemblages were significantly differentiated in all coastal distance groups. This study is one of few to quantitatively examine lichen communities of two rock types, and is unique in that it does so at a regional scale. These results add to our understanding of the interactive roles of substrate and maritime influence in lichen community assembly.

<p> Sulfide-poor platinum-group element (PGE) mineralization occurs within the Archean Boston Creek Flow ferropicrite, Abitibi greenstone belt, Canada. The PGE mineralization (∑PGE+Au = up to 1000 ppb) is manifest as metre-scale platinum-group mineral-bearing pods of disseminated chalcopyrite and pyrite (< 1 modal%) within titaniferous magnetite-rich gabbroic rock at the base of its central gabbroic layer. This mineralization is distinct in character from well known PGE mineralization associated with massive Fe-Ni-Cu sulfides at the base of komatiite flows at Kambalda, Western Australia and elsewhere. Exploration strategies presently used to search for PGE in mafic and ultramafic volcanic rock terrains are based on the geological and geophysical characteristics of sulfide-rich PGE mineralization. Consequently, refinements in exploration strategies are required if economic concentrations of sulfide-poor PGE mineralization are to be discovered in volcanic terrains.</p> <p> To begin development of such exploration criteria, ground-based magnetic and VLF surveys were conducted over the PGE mineralization along a single cross-section through the BCF. Drill core samples were collected along this transect to characterize the volume magnetic susceptibility and natural remanent magnetization (NRM) of the mineralization. Magnetic highs ranging in intensity from 64000 to 65000 nT were recorded for the base of the gabbroic layer, including mineralized outcrops. Susceptibilities of up to 9700 cgs and high remanence values with variable directions were determined. VLF, as expected, was not useful in identifying the mineralized horizons. The peridotite at the base of the flow appears to be the only conductive rock in the BCF.</p> <p> The magnetic highs associated with the base of the gabbroic layer define a positive anomaly that appears to be podiform in outline and up to ten metres in maximum dimension. This result suggests that the titaniferous magnetite-rich rock is itself podiform, like the enclosed PGE mineralization. This magnetic anomaly is not extensive enough to be evident on a regional scale aeromagnetic map as a separate anomaly within the BCF, and is not evident through VLF techniques. The rocks hosting the PGE mineralization is defined by the paleomagnetic results but this is not a practical field method.</p> <p> The geophysical characterization of the PGE mineralization host rocks, and of the BCF in general, demonstrates the potential of detailed magnetic and susceptibility mapping, together with petrographic and petrologic studies, in the search for economic PGE concentrations of sulfide-poor PGE mineralization in other volcanic and possibly plutonic rocks as well. Specifically, the results suggest that podiform magnetic anomalies within titaniferous magnetite-rich pyroxenites and gabbroic rocks may have potential use in the exploration for economic sulfide-poor PGE mineralization. Although the paleomagnetic methods used in this study are probably not of direct use in exploration, they were able to distinguish the different lithologies in the BCF. This substantiates the results of the susceptibility measurements in characterizing PGE mineralized, titaniferous magnetite-bearing rocks. The results of the paleomagnetic study also show that the NRM of the Ghost Range intrusive complex is not primary and therefore the Archean apparent polar wander path as it is currently defined is incorrect.</p> / Thesis / Bachelor of Science (BSc)