One of the most essential resources for life on our planet is water. A concern for water resource sustainability has shifted towards the sustainable development of clean water body resource (SWDF, 2009). Data for the Vaal River water chemistry is in abundance. However, research on the historic natural conditions influencing the inorganic water quality, is not as extensive. Inorganic data was obtained from the Department of Water Affairs, for the period 1972 to 2011, for identified monitoring stations along the Vaal River. Water quality was evaluated using various geochemical techniques to analyse the data. The results of the study indicate that the water chemistry of the Vaal River is controlled by: 1. Chemical weathering of siliceous sediment, intrusive igneous rocks and metamorphic rocks (Na+, K+, Mg2+, Ca2+ and (HCO3)-). 2. Anthropogenic influences increasing the sulphate (SO4) concentration There is no major increase in ion concentrations for the stations. However the concentrations of bicarbonate (HCO3)- and SO4 change as it progresses downstream from the first upstream station to the last downstream station. Based on the chemical characterisation, three groups have been identified. (1) Group 1 stations appear to suggest a higher influence in chemical weathering than the group 2 stations. (2) Group 2 stations appear to suggest a greater influence from SO4. (3) Group 3 stations appear to suggest an influence from both the bicarbonate and the SO4 influences. Geographically the chemical weathering is an indication of the three different groups with strong anthropogenic influences in the middle group. The water chemistry for the Vaal River is controlled by two processes, namely chemical weathering and anthropogenic influences. The prominent indication of the difference in these two influences can be seen between group 1 and group 2. A secondary conclusion indicates that a total dissolved solid (TDS) alone is not an accurate representation of anthropogenic influence (or poor water quality) on inorganic water quality of the Vaal River. The natural weathering or geological influences appears to play a more dominant role in certain sections or catchments with lower contributions from anthropogenic influences. / MSc (Environmental Sciences), North-West University, Potchefstroom Campus, 2015

One of the most essential resources for life on our planet is water. A concern for water resource sustainability has shifted towards the sustainable development of clean water body resource (SWDF, 2009). Data for the Vaal River water chemistry is in abundance. However, research on the historic natural conditions influencing the inorganic water quality, is not as extensive. Inorganic data was obtained from the Department of Water Affairs, for the period 1972 to 2011, for identified monitoring stations along the Vaal River. Water quality was evaluated using various geochemical techniques to analyse the data. The results of the study indicate that the water chemistry of the Vaal River is controlled by: 1. Chemical weathering of siliceous sediment, intrusive igneous rocks and metamorphic rocks (Na+, K+, Mg2+, Ca2+ and (HCO3)-). 2. Anthropogenic influences increasing the sulphate (SO4) concentration There is no major increase in ion concentrations for the stations. However the concentrations of bicarbonate (HCO3)- and SO4 change as it progresses downstream from the first upstream station to the last downstream station. Based on the chemical characterisation, three groups have been identified. (1) Group 1 stations appear to suggest a higher influence in chemical weathering than the group 2 stations. (2) Group 2 stations appear to suggest a greater influence from SO4. (3) Group 3 stations appear to suggest an influence from both the bicarbonate and the SO4 influences. Geographically the chemical weathering is an indication of the three different groups with strong anthropogenic influences in the middle group. The water chemistry for the Vaal River is controlled by two processes, namely chemical weathering and anthropogenic influences. The prominent indication of the difference in these two influences can be seen between group 1 and group 2. A secondary conclusion indicates that a total dissolved solid (TDS) alone is not an accurate representation of anthropogenic influence (or poor water quality) on inorganic water quality of the Vaal River. The natural weathering or geological influences appears to play a more dominant role in certain sections or catchments with lower contributions from anthropogenic influences. / MSc (Environmental Sciences), North-West University, Potchefstroom Campus, 2015

Abstract Orogenic gold deposits from the Reefton goldfield in New Zealand hosted in Ordovician metasediments of the Greenland group have produce 67 tons of gold before 1951. The Blackwater mine in Waiuta account for about 1/3 of the gold production at Reefton prior 1951. The ore system at Blackwater consist of NE trending steeply dipping gold-bearing quartz veins (Birthday reef) occurring in faulted, sheared and folded alternating sequence of sandstone-mudstone metamorphosed to low greenschist facies and hydrothermally altered proximal to distal from the quartz vein. Host rock and ore forming fluids interaction resulted in a systematic change in the mineralogy and geochemistry of the wallrock developing a distinctive footprint of the ore system. The mineral assemblage subsequent to hydrothermal alteration is formed by quartz veining, chlorite, carbonates (minor calcite-dolomite-ankerite-siderite), albite, K-Mica and composite quartz-carbonate veining, carbonates spotting, pyrite, arsenopyrite, chalcopyrite, cobaltite, galena and in a lesser extent ullmannite, gersdorffite, pentlandite, millerite and sphalerite. Mass balance calculation based on geochemical data obtained by ultra-low detection analysis with a detection limit in part per trillion of Au, As and Sb provide new insight in the geochemical footprint of the ore system at Blackwater. A clear zone of roughly 40 meters (20 meters above and below the birthday reef) is enriched in Au, As and Sb up to respectively 6806%, 605% and 891% compared to the un-mineralised protolith. Furthermore, mass changes in K2O and Na2O indicate a consistent decrease in Na and increase in K in the vicinity of the Birthday reef reflecting the replacement of albite by K-mica. This is corroborated by pattern of alkali alteration index Na/Al for albite and 3K/Al for K-Mica showing similar trend. Carbonation and de/-hydration index also exhibits peaks in samples adjacent to the Birthday reef, though carbonation index is also influenced by carbonates content in the protolith or late carbonation that may not related to gold mineralisation. Using the indicators above-mentioned it is possible to define the mineralogical and geochemical “footprint” for the ore system in the host rock allowing to use this footprint as a tool for mineral exploration for orogenic gold deposits similar to Blackwater. Given that the geochemical footprint of orogenic gold deposit such as Blackwater is significantly wider than the economically viable part of the deposit defining the footprint of the ore system offers the potential for vectoring from sub-economic mineralisation towards higher-grade ore that is economically viable.

THEREDA (Thermodynamic Reference Database) is a collaborative project, which has been addressed this challenge. The partners are Helmholtz-Zentrum Dresden-Rossendorf, Karlsruhe Institute of Technology (KIT-INE), Gesellschaft für Anlagen- und Reaktorsicherheit Braunschweig mbH (GRS), TU Bergakademie Freiberg (TUBAF) and AF-Consult Switzerland AG (Baden, Switzerland). The aim of the project is the establishment of a consistent and quality assured database for all safety relevant elements, temperature and pressure ranges, with its focus on saline systems. This implied the use of the Pitzer approach to compute activity coefficients suitable for such conditions. Data access is possible via commonly available internet browsers under the address http://www.thereda.de. One part of the project - the data collection and evaluation for uranium – was a task of the Helmholtz-Zentrum Dresden-Rossendorf. The aquatic chemistry and thermodynamics of U(VI) and U(IV) is of great importance for geochemical modelling in repository-relevant systems. The OECD/NEA Thermochemical Database (NEA TDB) compilation is the major source for thermodynamic data of the aqueous and solid uranium species, even though this data selection does not utilize the Pitzer model for the ionic strength effect correction. As a result of the very stringent quality demands, the NEA TDB is rather restrictive and therefore incomplete for extensive modelling calculations of real systems. Therefore, the THEREDA compilation includes additional thermodynamic data of solid secondary phases formed in the waste material, the backfill and the host rock, though falling into quality assessment (QA) categories of lower accuracy. The data review process prefers log K values from solubility experiments (if available) to those calculated from thermochemical data.

Thesis (MSc)--University of Stellenbosch, 2007. / ENGLISH ABSTRACT: The aim of the study is to ascertain whether geochemical exploration techniques can be used in the search for tanzanite deposits in the Merelani area, NE Tanzania. Previous studies have successfully demonstrated a partial extraction method (in situ soil leaching) in identifying prospective ultramafic bodies at the Rockland ruby mine in the Mangare area, Kenya, thereby demonstrating the usefulness of geochemical methods in gemstone exploration. In this study, a partial extraction as well as a whole-rock geochemical method was used to determine the applicability of these methods in prospecting for tanzanite mineralisation using different sampling media, such as soil, stream sediment and calcrete. It is possible that this geochemical approach may not be as effective as physical methods such as the separation and examination of heavy mineral suites. However, its viability needs to be evaluated due to the potential efficiency and relative logistic ease of the method. In essence the scientific method employed is to compare overburden (soils, stream sediments and calcrete) chemistry with known underlying geology, the latter having been established via diamond core drilling. A positive correlation would allow the prediction of overburden covered tanzanite mineralisation. Soil samples were collected from a trench dug perpendicular to regional lithological strike over both barren and tanzanite-bearing horizons. XRF trace element data for the soils was compared to the chemistry of the underlying lithologies. ICP-AE data derived from 1 molar HCL soil leachate (12 hour leach) and soil XRF data, from the same samples, was compared, using a mass balance index, to discern any hydromorphic dispersion of selected trace elements and to evaluate the leachate as a viable alternative to XRF analysis. In general, a good correlation exists between the soil and rock trace element data profiles over the length of the section. However, Ti- and Zr-normalised mass balance calculations show some down-hill drift, but this does not disrupt the overall pattern. The ICP-AE acid leach data show that hydromorphic dispersion is low, that the trace elements of interest (V, Cr, Ni and Cu) are hosted within non-soluble phases. Consequently, the leach technique is not a viable alternative to XRF analysis of the soils. XRF analysis of the soils was shown to be potentially useful in identifying new areas of mineralisation as the soils overlying a graphitic calc-silicate schist, that always occurs adjacent to the tanzanite mineralisation in the Merelani area, was found to be easily identifiable based on anomalous concentrations of V. An exploration concession was chosen for stream sediment sampling on the basis of the presence of large streams, of a few tsavorite mines indicating high prospectivity for tanzanite, and because of a variation in geology on the property. Tanzanite and tsavorite are cogenetic in the known tanzanite deposits. In this case the aim was to investigate the possible occurrence of tanzanite-like geochemical anomolies (i.e. the anomalous V observed in the soil chemistry investigation) could be detected in the vicinity of the tsavorite mines. Tsavorite, the gem variety of grossular garnet, also contains high concentrations of V. The samples were analysed by XRF whole-rock methods for trace element content. The data shows a number of clear positive V anomalies in the study area. The data also shows that each of the existing or abandoned mines in the area is marked by a positive V anomaly. This section of the study also demonstrated a relatively low degree of stream sediment dispersion of the trace elements of interest – most likely a function of the semi-arid climate. The fine fraction (<90μm), however was shown to be mobilised to a relatively larger degree than the coarse (180μm – 300μm) and medium (90μm - 180μm) fractions. As is predictable from the leachate analysis, factor analysis of the data shows that the trace elements are dominated by heavy mineral geochemistry and that a study in heavy mineral exploration might provide a cheaper and more viable option to those explored in this study. Calcrete samples were taken from an abandoned, 10m deep mine shaft, which was sunk through the calcrete to reach the tanzanite deposit. The shaft was sampled from the bottom, closest to the tanzanite mineralisation, to the surface to investigate the association between trace element geochemistry and proximity to the deposit. There was no vertical association between the trace element geochemistry of the calcrete and proximity to the tanzanite deposit. There was also no clear indication in the geochemistry of the calcrete of the existence of the tanzanite deposit beneath it. This further indicates the immobility of the elements of interest in this environment. This study has demonstrated that properly constrained soil and stream sediment geochemical studies may be of use in tanzanite exploration. However, it must be stressed that this is only the case if the geochemical signature of the lithological package associated with the mineralisation is unique and well known. / AFRIKAANSE OPSOMMING: Die doel van hierdie studie is om te bepaal of geochemiese eksplorasie tegnieke vir die soek na tanzaniet afsettings in die Merelani area, noord-oos Tanzanië, gebruik kan word. Voorige studies het gewys dat ‘n gedeeltelike ekstraksie metode (in situ grond looging) gebruik kon word om prospektiewe ultramafiese liggame by the Rockland rubyn myn in die Mangare area, Kenia te identifiseer. Hierby is gedemonstreer dat geochemiese eksplorasie metodes suksesvol in edelsteen eksplorasie toegepas kan word. In hierdie studie is ‘n gedeeltelike ekstrasksie en heel-rots geochemiese metodes gebruik om die toepaslikheid van hierdie metodes op tanzaniet eksplorasie te toets. Verskillende geologiese materiale is gemonster, naamlik grond, stroom sedimente en kalkreet. Dit is moontlik dat hierdie geochemiese benadering nie so effektief soos fisiese metodes soos swaar mineraal skeidings mag wees nie. Dit is nogtans belangrik om die toepaslikheid van hierdie metodes op tanzanite eksplorasie te toests, as gevolg van die potensiële effektiwiteit en relatiewe logistiese gemak van die metodes. Die essensie van die wetenskaplike metodiek wat in hierdie studie gebruik is, is om die geochemie van die grond, stroom sedimente en kalkreet te vergelyk met die geochemie van die onderliggende geologie wat deur middel van diamant boorwerk vasgestel is. ‘n Positiewe korrelasie sou dan dui op ‘n bedekte tanzaniet afsetting. Grond monsters is van ‘n sloot geneem wat loodreg op die strekking van die tanzaniet gemineraliseerde en ongemineraliseerde horisonne gegrawe is. XRF spoor element data van die gronde is vergelyk met die chemie van die onderliggende gesteentes. IGP-AE data wat bekom is deur die monsters met 1 molaar HCl te loog (12 uur loging) is vergelyk met XRF data van dieselfde monsters deur middel van ‘n massa balans indeks om te bepaal of daar enige hidromorfiese dispersie van sekere spoor elemente is en om die toepaslikheid van loging as ‘n alternatief tot die heel-rots metode te bepaal. In die algemeen is daar ‘n goeie korrelasie tussen die grond en rots spoor element data profiele oor die lengte van die seksie. Alhoewel, Ti- en Zr-genormaliseerde massa balans data profiele wys dat daar ‘n mate van afwaartse beweging van grond na die voet van die heuwel is, maar dat hierdie ‘n breuk in die algemene patroon vorm nie. Die IGP-AE data dui daarop dat die hidromorfiese verspreiding van spoor elemente laag is en dat die spoor elemente wat van belang is (V, Cr, Ni en Cu) in nie-oplosbare fases gesetel is. Gevolglik is die logings metode nie ‘n toepaslike alternatief tot die heel-rots XRF metode op gronde nie. XRF analises op die gronde het gewys dat die XRF metode moontlik nuttig kan wees om nuwe areas van tanzanite mineralisasie aan te dui, omdat die gronde wat ‘n grafietiese kalk-silikaat skis oorlê, wat altyd langs die tanzaniet draende horisonne voorkom, is op grond van anomale konsentrasies van V geïdentifiseer. ‘n Eksplorasie konsessie is op die basis van die teenwoordigheid van groot strome, ‘n paar tsavoriet myne wat aanduidend is van hoë prospektiwiteit vir tanzaniet is en as gevolg van ‘n variasie in geologie in die area vir stroom sediment monstering gekies. Tanzaniet en tsavoriet is kogeneties in bekende tanzaniet afsettings. In hierdie geval was die doel om te ondersoek of tanzanietagtige anomalieë (nl. die anomale konsentrasies van V wat in die ondersoek van die grond chemie opgemerk is) in die omgewing van die tsavoriet myne geïdentifiseer kan word. Tsavoriet, die edelsteen variëteit van grossulaar granaat, bevat hoë konsentrasies V. Die monsters is deur middel van die XRF heel-rots metode vir spoor elemente geanaliseer. Die data dui op ‘n paar monsters met hoë V konsentrasies in die ondersoek area. Hierdie studie het ook gedui op ‘n lae stroom sediment verspreiding van die spoor elemente van belang, heel waarskynlik is dit ‘n funksie van die semi-ariede klimaat. Die fyn fraksie (< 90μm) blyk tot ‘n groter mate as die growwer (90μm tot 180μm en 180μm - 300μm) fraksies gemobiliseer te word. Soos voorspel kan word deur die loogings analise het faktor analise gewys dat die spoor elemente deur swaar mineraal geochemie gedomineer word en dat ‘n studie op swaar minerale moontlik ‘n goedkoper en meer toepaslike eksploraise metode is as die wat in hierdie studie ondersoek is. Kalkreet monsters is van ‘n ongebruikte, 10m diep myn skag wat deur die kalkreet gesink is om by die tanzaniet gemineraliseerde horison uit te kom geneem. Monsters is van die bodem van die skag, naaste aan die tanzaniet mineralisasie, tot die oppervlak geneem om die assosiasie tussen die spoor element geochemie en afstand van die tanzaniet mineralisasie te ondersoek. Geen vertikale assosiasie tussen spoor element geochemie en die nabyheid tot die tanzaniet afsetting kon vasgestel word nie. Daar was geen duidelike aanduiding in die geochemie van die kalkreet op die onderliggende tanzanite afsetting nie. Hierdie is ‘n verdere annduiding op die nie-mobiele toestand van spoor elemente in hierdie omgewing. Hierdie studie het suksesvol gedemonstreer dat goed gedefinieerde grond en stroom sediment geochemiese studies moontlik in geochemiese eksplorasie vir tanzaniet bruikbaar kan wees. Dit is belangrik om in gedagte te hou dat dit slegs die geval is as die geochemie van die litologiese paket wat met die mineralisasie geassosieer is uniek en goed bekend is.

The Prairie Creek Mine, NWT, is a non-producing Zn-Pb-Ag mine located within the Mackenzie Mountains. The 320 km2 area is surrounded by the Nahanni National Park Reserve, and is found approximately 40 km upstream of a UNESCO World Heritage Site. Geochemical characterization of weathered material in the area is useful from both an exploration geochemistry and environmental geochemistry perspective. This investigation attempted to characterize geochemical and mineralogical controls on metal mobility in natural and manufactured environments. Surface waters and sediments were sampled, analyzed, and interpreted. Detailed mineralogical analyses were also performed, including SEM, XRD, and synchrotron-based µXRF and µXRD. Prairie Creek contains sulphide and supergene mineralization hosted in carbonate rocks, which supply the area with a high pH, as well as a wealth of alkalinity and buffering capacity. This study aims to provide some insight as to the geochemical and mineralogical controls on the weathering processes which may enhance or inhibit mobility of metals downstream of known mineralization. Results show that the metals are found in much higher concentration in the stream sediments than in the stream waters. Pb and Zn show the highest concentrations, and Zn appears to be more mobile than Pb. Based on water versus sediment chemistry, it may be possible to differentiate between mineralization types. Dissolution textures and alteration of detrital grains indicates that chemical weathering does occur within the streams and releases metals to the aqueous environment, though they are likely quickly attenuated through adsorption or co-precipitation. The mine site hosts a historic ore stockpile and waste rock pile, established in the early 1980’s. Geochemical characterization of the reactions occurring within these piles and of the mineralogical controls on metal mobility can contribute to the mitigation of risk from leachate. Metal concentrations are variable and do not support a trend, which indicates that metal mobility within these piles may be attributed to micro-environments. There appears to be a trend in increasing alkalinity and decreasing sulphate towards the bottom of the piles. Therefore, metals appear to have limited mobility in the studied environments, although small scale chemical reactions are occurring which may release and attenuate metals. / Thesis (Master, Geological Sciences & Geological Engineering) -- Queen's University, 2013-07-04 20:00:21.774

Actuellement, la Loire, voit sa géodynamique de plus en plus détériorée. Le déficit du transport solide provoque un enfoncement du lit mineur sur plusieurs secteurs du bassin Amont. Ceci a pour conséquence une déstabilisation progressive des ouvrages (digues, ponts) et l'érosion ou le colmatage des berges ainsi qu'une baisse du niveau piézométrique des nappes alluviales. Le traitement de ces problèmes passe par une meilleure connaissance des processus d'érosion et d'incision fluviale, le premier laissant place au second une fois que la couche « alluviale » a été totalement emportée, laissant apparaître un affleurement rocheux dans le lit du cours d'eau. Ce phénomène complexe est actuellement constaté en divers endroits de la Loire, dans la plaine du Forez. Nous proposons trois approches pour l'analyser.La première approche est fondée sur la modélisation numérique utilisant les équations de Barré-Saint-Venant, pour l'écoulement, couplées aux équations d'Exner et de Meyer-Peter Müller pour le transport solide. Ce modèle monodimensionnel permet d'obtenir la côte du fond du lit de la rivière et le flux solide au droit de chaque point de calcul.La seconde approche, mécaniste, consiste à déterminer de façon semi-empirique le taux d'incision du substratum marneux à partir de deux variables majeures: la puissance hydraulique totale et le coefficient d'abrasion en un point donné. La puissance hydraulique est calculée à l'aide d'un modèle hydraulique tandis que le nombre d'abrasion est une propriété mécanique de la marne qui est déterminée à partir d'essais en laboratoire. La troisième approche relève de la géochimie. Elle consiste à déterminer les provenances des matériaux contribuant à la recharge latérale des sédiments, à partir de leurs signatures géochimiques. Ceci a fait l'objet d'analyses en laboratoire sur des échantillons prélevés sur l'ensemble du linéaire entre Grangent et Balbigny. Les résultats obtenus montrent que ces approches indépendantes sont complémentaires et permettent une description à la fois qualitative et quantitative de l'enfoncement du lit de la Loire dans le secteur d'étude. / Currently, the Loire river, sees its geodynamic increasingly deteriorated. The deficit of sediment transport causes erosion of the bed on several areas of the basin. This results in a gradual destabilization of structures (dams, bridges), erosion or clogging of banks or a decline in piezometric level alluvial.Treating these problems requires a better understanding of the processes of erosion and river incision, the second succeeding the first, once the alluvial material of the bottom was completely removed, revealing a bed outcrop.This complex phenomenon is currently found in various parts of the Loire river, in the plain of Forez.We propose three approaches for this analysis.The first approach is based on numerical modeling using the equations of Barre-de-Saint-Venant, for flow, coupled with the equations of Exner and Meyer-Peter Müller for sediment transport. This monodimensional model allows the simulation of riverbed changement and sediment discharge, right each calculation point of grid mesh.The second approach is mechanistic and consists of determining the rate of marly bedrock incision by a semi-empirical method by the use of two major variables: the total hydraulic power and an abrasion coefficient. The hydraulic power is calculated using a hydraulic model, while the abrasion coefficient is a mechanical property of the marl which is determined from laboratory tests.The third approach is the geochemistry. It consists in determining the provenance of the materials from tributaries and in the main chanel by analyzing their geochemical signatures. This has been the subject of laboratory tests on samples taken across the linear from Grangent to Balbigny.The results obtained show that these independent approaches are complementary and provide both a qualitative and quantitative description of the incision of the Loire river in the study area.

In this thesis, the response of uranium minerals and poorly crystalline phases to changes in geochemical conditions through geological time has been assessed in order to understand the mobility of natural uranium in the fracture network of a proposed site for a spent nuclear fuel repository in Forsmark, Sweden. Identification and characterization of solid phase uranium have been performed through electron microprobe analysis and optical petrography (Article I). The identified uraninite, haiweeite, and uranophane crystals have been dated using U-Pb and Pb-Pb isotope ratios obtained from secondary ion mass spectrometry and laser ablation-inductively coupled plasma-mass spectrometry (Article II). The mobility of uranium in current Forsmark groundwaters and fracture system has been modelled using the PHREEQC geochemical program and Ra and Rn isotope systematics (Article IV). The rate of submarine groundwater discharge (SGD) from the Forsmark coast to Öregrundsgrepen has also been modelled using Ra isotopes (Article V). Results from these studies support a geologically early (~1200 Ma) oxidation of U(IV) to U(VI). It is further suggested that the old U(VI) minerals present in the bedrock are soluble at the pe values and alkalinities observed in the Forsmark groundwaters. At pe &lt; −4.6 and alkalinity &lt; 60 mg/L, U(VI) can be reduced to U(IV) and deposited in the fracture network. Although a non-negligible rate of SGD has been observed, this cannot be attributed to the discharge of deep (&gt;200 m.b.s.l.) Forsmark groundwaters on the basis of current data. / <p>At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: In press. Paper 4: Manuscript. Paper 5: Manuscript.</p>

In this thesis, I have developed a new methodology for the accurate determination of mass-dependent variations in nickel (Ni) isotope compositions. Nickel is initially separated in a three-column ion-exchange procedure, and the purified solutions are analysed by multi-collector inductively coupled plasma mass spectrometry (MCICPMS) using a double-spike technique. Using this methodology, I have measured the first Ni isotope ratios for a wide variety of natural geological samples. Significant Ni isotope variations were observed, with an overall spread in delta 60Ni-values of -0.9 to 2.5 permil. In igneous rocks Ni isotopes appear to be largely homogeneous, with only small variations (0.2 permil) between different rock types. Weathering of silicate rocks does on the other hand appear to cause significant fractionation of Ni isotopes, probably producing an isotopically heavy riverine input to the ocean. A heavy isotope signature is also visible in hydrogenetic ferromanganese crusts, with surface scrapings from globally distributed crusts show an average delta 60Ni-value of 1.65 permil. However, the variation in these samples is over 1.5 permil, likely reflecting local sources or biological processes, or alternatively indicating a heterogeneous Ni isotopic composition of the ocean. Organic-rich sediments also show heavy isotopic compositions, which are possibly transferred to the crude oils originating in these types of sediments. The only significant reservoir of light Ni isotopes found during this project are sulphides from magmatic systems. Overall, this thesis demonstrates the potential of this system as a powerful new tracer for a variety of geochemical processes.

L'altération chimique de la croûte terrestre fournit à l'ensemble des cycles bio-géochimiques de la surface les éléments essentiels à leur fonctionnement. L'érosion de grands orogènes, comme la chaîne Himalayenne s'accompagne de flux d'érosion et d'altération significatifs, susceptibles d'avoir un impact à l'échelle globale. L'objectif de cette thèse est de comprendre comment les processus physiques et chimiques façonnent le signal sédimentaire afin de quantifier l'érosion et l'altération actuelle ainsi que leur variations passées. L'étude détaillée de la dynamique du transport sédimentaire et des caractéristiques physiques et géochimiques des sédiments dans le bassin du Gange montre qu'actuellement environ 10 % du flux sédimentaire érodé en Himalaya est séquestré dans la plaine alluviale du Gange. L'utilisation des isotopes cosmogéniques (10Be) dans les sédiments de rivières montrent des taux d'érosions stables entre 1.3 et 1.4 mm par an pour l'ensemble de la chaîne drainée par le Gange. De plus, le transfert de sédiments dans la plaine s'accompagne d'un appauvrissement en éléments mobiles marquant l'altération chimique de ceux-ci. Cette altération a été quantifié et suggère que la plaine du Gange joue un role dominant dans l'altération des sédiments Himalayens. Les échanges cationiques lors du passage des sédiments au domaine marin restent limités dans le cas du système Himalayen et ne permettent d'augmenter le bilan de stockage de carbone à long terme que de 20 % environ. Enfin, l'enregistrement de la Baie du Bengale, qui couvre les produits issus de l'érosion Himalayenne sur les derniers 20 000 ans, montre que les sédiments exportés au Dernier Maximum Glaciaire (DMG) étaient significativement moins altérés qu'à l'actuel. Le système Himalayen n'est donc pas tamponné vis-à-vis des forages climatiques à haute fréquence du Quaternaire et les taux d'altération actuels ne peuvent très extrapolés dans le passé. / Chemical weathering of the earth crust supplies the essential elements for numerous biogeochemical cycles. Physical erosion of large orogens, such as the Himalayan range, is accompanied by significant weathering fluxes possibly affecting the global environment. The objective of this PhD is to understand how surface processes affect river sediment properties in order to asses current erosion and weathering rates but also to decipher their past variations. To answer this question we studied the transport dynamics, the physical and the geochemical characteristics of the sediments in the Ganga basin. This study suggests that about 10 % of the flux eroded in the Himalayas is currently stored in the Ganga floodplain. Cosmogenic isotopes (10Be) measured in river sediments show stable erosion rates between 1.3 and 1.4 mm/yr for the entire Himalayan range drained by the Ganga. Furthermore, we show that River sediments are progressively depleted in the most mobile elements, as weathering proceeds during transfer in the floodplain. By comparing this flux to the weathering flux of the Himalayan range, we show that floodplain weathering is predominant in weathering Himalayan sediments. Cation exchange occurring when Ganga and Brahmaputra (G&B) sediments enter the marine environment are limited and enhances the long term carbon storage, linked to silicate weathering by only ca. 20 %. Finally, the Bay of Bengal sedimentary record, which documents the last 20 000 years of Himalayan erosion shows that the sediments exported during the last glacial maximum (LGM) were significantly less weathered compared to the sediments currently exported. The Himalayan system is thus not buffered towards the high frequency climate forcing changes of the Quaternary and modern weathering rates cannot easily be extrapolated over the past.