Academic literature on the topic 'Modal mineralogy'

Nabbaren nepheline syenite, a silica-deficient intrusive rock with low Fe content, was the industrial mineral deposit study case in this study. The quality of industrial mineral products are generally based on their bulk chemistry, which are directly related to their modal mineralogy and mineral chemistry; however, these are costly and time-consuming to determine. A geometallurgical-based methodology, known as element-to-mineral conversion (EMC), was applied to estimate its modal mineralogy based on its given bulk and mineral chemistry. EMC is a convenient and cost-effective technique, which can be used to quickly estimate modal mineralogy. Two EMC methodologies were applied: one least square based, LS-XRD, and one regression based, R-XRD. Additionally, average and specific mineral chemistries were used during estimations. The R-XRD method, a method not yet used for EMC purposes, gave better modal mineralogy estimations than LS-XRD. Considering the restrictions in the method, R-XRD shows potential for improvement and implementation at operational scale, making it a valuable geometallurgical tool for increasing resource performance, easing decision-taking processes, and reducing risks. The use of different mineral chemistries did not influence the modal mineralogy estimation, unlike the method used for it.

SUMMARY Thermal conductivity is a physical parameter crucial to accurately estimating temperature and modelling thermally related processes within the lithosphere. Direct measurements are often impractical due to the high cost of comprehensive sampling or inaccessibility and thereby require indirect estimates. In this study, we report 340 new thermal conductivity measurements on igneous rocks spanning a wide range of compositions using an optical thermal conductivity scanning device. These are supplemented by a further 122 measurements from the literature. Using major element geochemistry and modal mineralogy, we produce broadly applicable empirical relationships between composition and thermal conductivity. Predictive models for thermal conductivity are developed using (in order of decreasing accuracy) major oxide composition, CIPW normative mineralogy and estimated modal mineralogy. Four common mixing relationships (arithmetic, geometric, square-root and harmonic) are tested and, while results are similar, the geometric model consistently produces the best fit. For our preferred model, $k_{\text{eff}} = \exp ( 1.72 \, C_{\text{SiO}_2} + 1.018 \, C_{\text{MgO}} - 3.652 \, C_{\text{Na}_2\text{O}} - 1.791 \, C_{\text{K}_2\text{O}})$, we find that SiO2 is the primary control on thermal conductivity with an RMS of 0.28 W m−1 K−1or ∼10 per cent. Estimates from normative mineralogy work to a similar degree but require a greater number of parameters, while forward and inverse modelling using estimated modal mineralogy produces less than satisfactory results owing to a number of complications. Using our model, we relate thermal conductivity to both P-wave velocity and density, revealing systematic trends across the compositional range. We determine that thermal conductivity can be calculated from P-wave velocity in the range 6–8 km s−1 to within 0.31 W m−1 K−1 using $k({V_p}) = 0.5822 \, V_p^2 - 8.263 \, V_p + 31.62$. This empirical model can be used to estimate thermal conductivity within the crust where direct sampling is impractical or simply not possible (e.g. at great depths). Our model represents an improved method for estimating lithospheric conductivity than present formulas which exist only for a limited range of compositions or are limited by infrequently measured parameters.

Abstract. The mineralogy of desert dust is important due to its effect on radiation, clouds and biogeochemical cycling of trace nutrients. This study presents the simulation of dust radiative forcing as a function of both mineral composition and size at the global scale using mineral soil maps for estimating emissions. Externally mixed mineral aerosols in the bulk aerosol module in the Community Atmosphere Model version 4 (CAM4) and internally mixed mineral aerosols in the modal aerosol module in the Community Atmosphere Model version 5.1 (CAM5) embedded in the Community Earth System Model version 1.0.5 (CESM) are speciated into common mineral components in place of total dust. The simulations with mineralogy are compared to available observations of mineral atmospheric distribution and deposition along with observations of clear-sky radiative forcing efficiency. Based on these simulations, we estimate the all-sky direct radiative forcing at the top of the atmosphere as +0.05 W m−2 for both CAM4 and CAM5 simulations with mineralogy and compare this both with simulations of dust in release versions of CAM4 and CAM5 (+0.08 and +0.17 W m−2) and of dust with optimized optical properties, wet scavenging and particle size distribution in CAM4 and CAM5, −0.05 and −0.17 W m−2, respectively. The ability to correctly include the mineralogy of dust in climate models is hindered by its spatial and temporal variability as well as insufficient global in-situ observations, incomplete and uncertain source mineralogies and the uncertainties associated with data retrieved from remote sensing methods.

Abstract. The mineralogy of desert dust is important due to its effect on radiation, clouds and biogeochemical cycling of trace nutrients. This study presents the simulation of dust radiative forcing as a function of both mineral composition and size at the global scale, using mineral soil maps for estimating emissions. Externally mixed mineral aerosols in the bulk aerosol module in the Community Atmosphere Model version 4 (CAM4) and internally mixed mineral aerosols in the modal aerosol module in the Community Atmosphere Model version 5.1 (CAM5) embedded in the Community Earth System Model version 1.0.5 (CESM) are speciated into common mineral components in place of total dust. The simulations with mineralogy are compared to available observations of mineral atmospheric distribution and deposition along with observations of clear-sky radiative forcing efficiency. Based on these simulations, we estimate the all-sky direct radiative forcing at the top of the atmosphere as + 0.05 Wm−2 for both CAM4 and CAM5 simulations with mineralogy. We compare this to the radiative forcing from simulations of dust in release versions of CAM4 and CAM5 (+0.08 and +0.17 Wm−2) and of dust with optimized optical properties, wet scavenging and particle size distribution in CAM4 and CAM5, −0.05 and −0.17 Wm−2, respectively. The ability to correctly include the mineralogy of dust in climate models is hindered by its spatial and temporal variability as well as insufficient global in situ observations, incomplete and uncertain source mineralogies and the uncertainties associated with data retrieved from remote sensing methods.

The main objectives of this study are to determine how various physical and chemical properties of geologic samples can be investigated by Fourier Transform InfraRed (FTIR) spectral analyses, and determine how each of these individual properties uniquely alter the mid-infrared spectrum. Of particular interest is how extraterrestrial samples differ (spectrally) from terrestrial samples, and how such findings can be applied to current and future missions to airless planetary bodies (such as Diviner Lunar Radiometer, aboard the Lunar Reconnaissance Orbiter, and the Mercury Thermal Radiometer on BepiColombo). As such, a range of geological samples have been analysed including terrestrial rocks (anorthosite, granite, grabbro etc.), mineral standards (common rock-forming minerals), lunar meteorites (from Miller Range, Antarctica), and Apollo 14, 15, and 16 soils. A new technique to analyse such samples has been developed and implemented as part of this study: FTIR spectral imaging of unconsolidated samples (powders and soils) to obtain modal mineralogy estimates. Such estimates are comparable to QEMSCAN analyses and spot point counting of the same samples. This is particularly relevant for the non-destructive analysis of Apollo soil samples (bulk and sieved fractions). Individual spectra of polished terrestrial and extraterrestrial samples have been obtained in preparation for the creation of a spectral database. Such samples also have coupled chemical composition information via Electron Probe MicroAnalysis (EPMA). To have a spectrum and an associated chemical composition for each mineral in a database is unique compared to other spectral databases. Analyses of lunar meteorites resulted in an understanding of how shock (caused by hypervelocity impacts) alters the physical and spectral properties of lunar minerals. FTIR microscopy of individual minerals and phases in the meteorites were coupled with optical and cathodoluminescence (CL) imaging to identify the level of shock obtained by each mineral and phase. The FTIR reflectance bands of plagioclase merge with increasing shock pressure until a single, low-reflectance broad peak is displayed by the most highly shocked plagioclase (>45 GPa), and a dark-red colour is present in CL images. FTIR and QEMSCAN analyses of Apollo regolith samples have provided an understanding of the spectral effects of bulk mineralogy, maturity (a measure of the time spent at the lunar surface), grain size, and mineral chemistry. Using such information, the modal mineralogy of each sample has been estimated, one of which had not previously been analysed for its modal mineralogy. Samples from the same Apollo missions present similar spectral features, meaning FTIR spectroscopy can be used to identify the origin of lunar soils. A weak correlation in maturity with a spectral feature termed the Christiansen Feature has been found for lunar samples. Related to maturity, FTIR spectra of individual agglutinates (a product of space weathering) have been obtained and the spectral properties of agglutinates (decreased %Reflectance values of the region sensitive to geological materials) resemble those of highly mature lunar soils.

The demands for efficient utilization of ore bodies and proper risk management in the mining industry have resulted in a new cross-disciplinary subject called geometallurgy. Geometallurgy connects geological, mineral processing and subsequent downstream processing information together to provide a comprehensive model to be used in production planning and management. A geometallurgical program is an industrial application of geometallurgy. Various approaches that are employed in geometallurgical programs include the traditional way, which uses chemical elements, the proxy method, which applies small-scale tests, and the mineralogical approach using mineralogy or the combination of those. The mineralogical approach provides the most comprehensive and versatile way to treat geometallurgical data. Therefore it was selected as a basis for this study. For the mineralogical approach, quantitative mineralogical information is needed both for the deposit and the process. The geological model must describe the minerals present, give their chemical composition, report their mass proportions (modal composition) in the ore body and describe the ore texture. The process model must be capable of using mineralogical information provided by the geological model to forecast the metallurgical performance of different geological volumes and periods. A literature survey showed that areas, where more development is needed for using the mineralogical approach, are: 1) quick and inexpensive techniques for reliable modal analysis of the ore samples; 2) ore textural characterization of the ore to forecast the liberation distribution of the ore when crushed and ground; 3) unit operation models based on particle properties (at mineral liberation level) and 4) a system capable of handling all this information and transferring it to production model. This study focuses on developing tools in these areas. A number of methods for obtaining mineral grades were evaluated with a focus on geometallurgical applicability, precision, and trueness. A new technique developed called combined method uses both quantitative X-ray powder diffraction with Rietveld refinement and the Element-to-Mineral Conversion method. The method not only delivers the required turnover for geometallurgy but also overcomes the shortcomings if X-ray powder diffraction or Element-to-Mineral Conversion were used alone. Characterization of ore texture before and after breakage provides valuable insights about the fracture pattern in comminution, the population of particles for specific ore texture and their relation to parent ore texture. In the context of the mineralogical approach to geometallurgy, predicting the particle population from ore texture is a critical step to establish an interface between geology and mineral processing. A new method called Association Indicator Matrix developed to assess breakage pattern of ore texture and analyze mineral association. The results of ore texture and particle analysis were used to generate particle population from ore texture by applying particle size distribution and breakage frequencies. The outcome matches well with experimental data specifically for magnetite ore texture. In geometallurgy, process models can be classified based on in which level the ore, i.e. the feed stream to the processing plant and each unit operation, is defined and what information subsequent streams carry. The most comprehensive level of mineral processing models is the particle-based one which includes practically all necessary information on streams for modeling unit operations. Within this study, a particle-based unit operation model was built for wet low-intensity magnetic separation, and existing size classification and grinding models were evaluated to be used in particle level. A property-based model of magnetic beneficiation plant was created based on one of the LKAB operating plants in mineral and particle level and the results were compared. Two different feeds to the plant were used. The results revealed that in the particle level, the process model is more sensitive to changes in feed property than any other levels. Particle level is more capable for process optimization for different geometallurgical domains.

Memoria para optar al título de Geólogo
El yacimiento Los Sulfatos corresponde a un depósito de tipo pórfido cuprífero emplazado en rocas volcánicas del Mioceno tardío-Plioceno temprano, aproximadamente a 69 km al NE de Santiago, Chile. Corresponde a un sistema hidrotermal de brechas con mineralización de calcopirita y bornita en matriz, diseminada y en vetillas. El proceso de concentración corresponde a la etapa de mayor consumo eléctrico en la minería del cobre, por lo que una cuantificación de los subprocesos que lo componen es de vital importancia para la disminución de costos (Comisión Chilena del Cobre, 2018). Dentro de la etapa de concentración pueden identificarse diferentes subprocesos tales como: conminución y flotación. El SPI (SAG Power Index) corresponde a un índice que cuantifica el rendimiento de los molinos semiautógenos SAG en etapa de conminución. Este mide el tiempo en minutos, que demora el 80% de una roca en pasar de una granulometría de 12,7 mm a una granulometría de 1,7 mm. En el presente estudio se cuantificó la mineralogía modal de las rocas por medio de un análisis de roca total y XRD, aplicando una transformación de elementos a minerales por medio de la estequiometría de los minerales y la concentración de elementos químicos en la roca. Luego de identificados los minerales predominantes se aplicaron técnicas estadísticas para correlacionar la mineralogía con el SPI. El principal problema que se presentó en la etapa de cuantificación corresponde a que, minerales como las micas blancas y los feldespatos potásicos, poseen en su estructura los mismos elementos químicos con proporciones estequiométricas diferentes, por lo que la forma de abordar la problemática se basó en identificar las muestras que tuvieran mica blanca o feldespato potásico, generando una regresión lineal con 26 muestras que se ajusta de buena forma el comportamiento de las muestras respecto a la composición mineral de esta. Luego de realizada la regresión lineal se interpoló la razón mica blanca/feldespato potásico para un total de 44 muestras. Una vez obtenida la razón Mb/Fk para un total de 70 muestras se realizó un modelo numérico en el software de modelamiento geológico Leapfrog, con el que se validaron el total de 30 muestras restantes. Los resultados obtenidos muestran que, los minerales que tendrían mayor incidencia en la dureza geometalúrgica SPI de las rocas, corresponden a los filosilicatos y a los tectosilicatos. Los primeros, influirían respecto a su comportamiento elástico y plástico en la etapa de molienda, por lo que a una mayor proporción de filosilicatos aumentaría la dureza geometalúrgica de las rocas. Caso contrario a lo que pasa con los tectosilicatos, los cuales, al poseer un comportamiento frágil, disminuyen el SPI a mayores proporciones. Se concluye que el parámetro geometalúrgico SPI estaría relacionado con la mineralogía de las rocas de yacimiento Los Sulfatos, en particular, con la relación entre filosilicatos y tectosilicatos.
Anglo American

O Distrito Mineiro de Salto do Jacuí (DMSJ) compreende a maior jazida de ágata em geodos do RS. O DMSJ está inserido na Supersequência Gondwana III (da Bacia do Paraná) onde são posicionadas a rochas vulcânicas da Formação Serra Geral (Cretáceo) e os arenitos da Formação Botucatu (Jurássico). Observa-se ainda a interação entre as lavas e os sedimentos (intertraps arenosos) formando feições como diques de arenito, fraturas e geodos preenchidos por sedimentos além de diversos tipos de brechas, sempre associados com a mineralização de ágata. Ideias que abrangem desde uma origem magmática a altas temperaturas como também uma possível formação a temperaturas mais baixas tem sido apresentadas, mas até o momento não há um consenso sobre a gênese deste mineral. Neste trabalho são aplicadas técnicas estratigráficas, químicas e isotópicas na análise da mineralização de ágata com objetivo de caracterizar e propor um modelo para a sua formação. As rochas vulcânicas, as sedimentares e amostras de ágata foram analisadas por técnicas petrográficas, por difratometria de raios X (DRX), por microscopia eletrônica de varredura (MEV) e análises químicas (em amostras de rocha total) e isotópicas (relações isotópicas do Sr, Nd, Nd e O). Em relação às rochas vulcânicas ficou caracterizada a presença de derrames de basalto e de dacito. Os arenitos dos intertraps são semelhantes ao arenito Botucatu. No entanto, localmente, foi descrita uma fácies mais fina, argilosa e micácea interpretada como possível ambiente do tipo interduna. A ágata tem diferentes características petrográficas, químicas e isotópicas o que mostra a complexidade dos processos envolvidos na sua gênese. O modelo de formação da ágata destaca a formação de um fluido silicoso a partir da lixiviação da sílica da matriz vítrea dos dacitos por águas meteóricas. A solução saturada em sílica gerada migra através do sistema permo-poroso onde predominam as fraturas e microfraturas. Esta solução ao encontrar um espaço maior, os geodos, precipita observando a sequência: opala – calcedônia – ágata – quartzo macrocristalino. Como a ágata normalmente é encontrada nas proximidades dos geodos com sedimentos (interduna??) e também por evidências isotópicas, pode-se supor que no modelo proposto, secundariamente houve influência de água/nível freático associada com a facies fina do intertrap. Assim, com base em diferentes técnicas analíticas foi possível propor um modelo genético de baixa temperatura para a mineralização de ágata do DMSJ.
Salto do Jacuí Mining District (DMSJ) comprises the largest deposit of agate geodes in the Rio Grande do Sul State, Brazil. The DMSJ is included in the Gondwana III Supersequence (Paraná Basin) where the volcanic rocks of the Serra Geral Formation (Cretaceous) are positioned, as well as the sandstones of the Botucatu Formation (Jurassic). Also, interaction between lavas and sediments (sandstone intertraps) is present, creating features such as sandstone dikes, fractures and geodes filled by sediment. Several types of breccias are observed too and they are always associated with the agate mineralization. Until now, there is no consensus about the genesis of this mineral and propositions include the influence of a magmatic source to high temperatures, as well as formation at lower temperatures. In the present work, we applied stratigraphic, chemical and isotopic techniques in the analysis of agate mineralization in order to characterize and propose a model for its formation. The agate samples, the volcanic and sedimentary rocks were analyzed with the help of petrographic techniques, X-ray diffraction (XRD), scanning electron microscopy (SEM), chemical analyses (in whole rock samples) and isotopic analyses (isotopic ratios of Sr Nd, Nd and O). The volcanic rocks were characterized by the presence of basaltic flows and dacite. The intertrap sandstones are similar to those observed in the Botucatu sandstone. However, locally, it has been described a finer, clay-rich and micaceous facies, interpreted as possible interdune environment. Agate displayed different petrographic, chemical and isotopic characteristics, showing the complexity of the processes involved in its genesis. The model of agate formation indicates that a siliceous fluid is generated from the leaching of silica of the dacites glass matrix by rainwater. The saturated silica solution migrates through the permo-porous system where fractures and microfractures predominate. When this solution flows into a larger space, like geodes, it precipitates according to the following sequence: opalchalcedony- agate-quartz. Usually, agate is found in the surroundings of geodes containing sediments (interdune?), but also by isotopic evidence. Taking these facts into account, we can assume that the proposed model was secondarily influenced by water/groundwater level associated with the thin facies of intertrap. Thus, based on different techniques, it was possible to propose a genetic model for the low temperature agate formation in DMSJ.

Lithium Cesium Tantalum (LCT) pegmatites are important resources for rare metals like Cesium, Lithium or Tantalum, whose demand increased markedly during the past decade. At present, Cs is known to occur in economic quantities only from the two LCT pegmatite deposits at Bikita located in Zimbabwe and Tanco in Canada. Host for this Cs mineralisation is the extreme rare zeolite group mineral pollucite. However, at Bikita and Tanco, pollucite forms huge massive, lensoid shaped and almost monomineralic pollucite mineralisations that occur within the upper portions of the pegmatite. In addition, both pegmatite deposits have a comparable regional geological background as they are hosted within greenstone belts and yield a Neoarchean age of about 2,600 Ma. Furthermore, at present the genesis of these massive pollucite mineralisations was not yet investigated in detail. Major portions of Western Australia consist of Meso- to Neoarchean crustal units (e.g., Yilgarn Craton, Pilbara Craton) that are known to host a large number of LCT pegmatite systems. Among them are the LCT pegmatite deposits Greenbushes (Li, Ta) and Wodgina (Ta, Sn). In addition, small amounts of pollucite were recovered from one single diamond drill core at the Londonderry pegmatite field. Despite that, no systematic investigations and/or exploration studies were conducted for the mode of occurrence of Cs and especially that of pollucite in Western Australia. In the course of the present study nineteen individual pegmatites and pegmatite fields located on the Yilgarn Craton, Pilbara Craton and Kimberley province have been visited and inspected for the occurrence of the Cs mineral pollucite. However, no pollucite could be detected in any of the investigated pegmatites. Four of the inspected LCT-pegmatite systems, namely the Londonderry pegmatite field, the Mount Deans pegmatite field, the Cattlin Creek LCT pegmatite deposit (Yilgarn Craton) and the Wodgina LCT pegmatite deposit (Pilbara Craton) was sampled and investigated in detail. In addition, samples from the Bikita pegmatite field (Zimbabwe Craton) were included into the present study in order to compare the Western Australian pegmatites with a massive pollucite mineralisation bearing LCT pegmatite system. This thesis presents new petrographical, mineralogical, mineralchemical, geochemical, geochronological, fluid inclusion and stable and radiogenic isotope data. The careful interpretation of this data enhances the understanding of the LCT pegmatite systems in Western Australia and Zimbabwe. All of the four investigated LCT pegmatite systems in Western Australia, crop out in similar geological settings, exhibit comparable internal structures, geochemistry and mineralogy to that of the Bikita pegmatite field in Zimbabwe. Furthermore, in all LCT pegmatite systems evidences for late stage hydrothermal processes (e.g., replacement of feldspars) and associated Cs enrichment (e.g., Cs enriched rims on mica, beryl and tourmaline) is documented. With the exception of the Wodgina LCT pegmatite deposit, that yield a Mesoarchean crystallisation age (approx. 2,850 Ma), all other LCT pegmatite systems gave comparable Neoarchean ages of 2,630 Ma to 2,600 Ma. The almost identical ages of the LCT pegmatite systems of the Yilgarn and Zimbabwe cratons suggests, that the process of LCT pegmatite formation at the end of the Neoarchean was active worldwide. Nevertheless, essential distinguishing feature of the Bikita pegmatite field is the presence of massive pollucite mineralisations that resulted from a process that is not part of the general development of LCT pegmatites and is associated with the extreme enrichment of Cs. The new findings of the present study obtained from the Bikita pegmatite field and the Western Australian LCT pegmatite systems significantly improve the knowledge of Cs behaviour in LCT pegmatite systems. Therefore, it is now possible to suggest a genetical model for the formation of massive pollucite mineralisations within LCT pegmatite systems. LCT pegmatites are generally granitic in composition and are interpreted to represent highly fractionated and geochemically specialised derivates from granitic melts. Massive pollucite mineralisation bearing LCT pegmatites evolve from large and voluminous pegmatite melts that intrude as single body along structures within an extensional tectonic setting. After emplacement, initial crystallisation will develop the border and wall zone of the pegmatites, while due to fractionated crystallisation immobile elements (i.e., Cs, Rb) become enriched within the remaining melt and associated hydrothermal fluids. Following this initial crystallisation, a relatively small portion (0.5–1 vol.%) of immiscible melt or fluid will separate during cooling. This immiscible partial melt/fluid is enriched in Al2O3 and Na2O, as well as depleted in SiO2 and will crystallise as analcime. In addition, this melt might allready contains up to 1–2 wt.% Cs2O. However, due to the effects of fluxing components (e.g., H2O, F, B) this analcime melt becomes undercooled which prevents crystallisation of the analcime as intergranular grains. Since this analcime melt exhibits a lower relative gravity when compared to the remaining pegmatite melt the less dense analcime melt will start to ascent gravitationally and accumulate within the upper portion of the pegmatite sheet. At the same time, the remaining melt will start to crystallise separately and form the inner portions of the pegmatite. This crystallisation is characterised by still ongoing fractionation and enrichment of incompatible elements (i.e., Cs, Rb) within the last crystallising minerals (e.g., lepidolite) or concentration of these incompatible elements within exsolving hydrothermal fluids. As analcime and pollucite form a continuous solid solution series, the analcime melt is able to incorporate any available Cs from the melt and/or associated hydrothermal fluids and crystallise as Cs-analcime in the upper portion of the pegmatite sheet. Continuing hydrothermal activity and ongoing substitution of Cs will then start to shift the composition from Cs-analcime composition towards Na-pollucite composition. In addition, if analcime is cooled below 400 °C it is subjected to a negative thermal expansion of about 1 vol.%. This contraction results in the formation of a prominent network of cracks that is filled by late stage minerals (e.g., lepidolite, quartz, feldspar and petalite). Certainly, prior to filling, this network of cracks enhances the available conduits for late stage hydrothermal fluids and the Cs substitution mechanism within the massive pollucite mineralisation. Furthermore, during cooling of the pegmatite, prominent late stage mineral replacement reactions (e.g., replacement of K-feldspar by lepidolite, cleavelandite, and quartz) as well as subsolidus self organisation processes in feldspars take place. These processes are suggested to release additional incompatible elements (e.g., Cs, Rb) into late stage hydrothermal fluids. As feldspar forms large portions of pegmatite a considerable amount of Cs is released and transported via the hydrothermal fluids towards the massive pollucite mineralisation in the upper portion of the pegmatite. Consequently, the initial analcime can accumulate enough Cs in order to shift its composition from the Cs-analcime member (>2 wt.% Cs2O) towards the Na-pollucite member (23–43 wt.% Cs2O) of the solid solution series. The timing of this late stage Cs enrichment is interpreted to be quasi contemporaneous or immediately after the complete crystallisation of the pegmatite melt. However, much younger hydrothermal events that overprint the pegmatite are also interpreted to cause similar results. Hence, it has been demonstrated that the combination of this magmatic and hydrothermal processes is capable to generate an extreme enrichment in Cs in order to explain the formation of massive pollucite mineralisations within LCT pegmatite systems. This genetic model can now be applied to evaluate the potential for occurrences of massive pollucite mineralisations within LCT pegmatite systems in Western Australia and worldwide
Lithium-Caesium-Tantal-(LCT) Pegmatite repräsentieren eine bedeutende Quelle für seltene Metalle, deren Bedarf im letzten Jahrzehnt beträchtlich angestiegen ist. Im Falle von Caesium sind zurzeit weltweit nur zwei LCT-Pegmatitlagerstätten bekannt, die abbauwürdige Vorräte an Cs enthalten. Dies sind die LCT-Pegmatitlagerstätten Bikita in Simbabwe und Tanco in Kanada. Das Wirtsmineral für diese Cs-Mineralisation ist das extrem selten auftretende Zeolith-Gruppen-Mineral Pollucit. In den Lagerstätten Bikita und Tanco bildet Pollucit dagegen massive, linsenförmige und fast monomineralische Pollucitmineralisationen, die in den oberen Bereichen der Pegmatitkörper anstehen. Zusätzlich befinden sich beide Lagerstätten in geologisch vergleichbaren Einheiten. Die Nebengesteine sind Grünsteingürtel die ein neoarchaisches Alter von ca. 2,600 Ma aufweisen. Die Bildung derartiger massiver Pollucitmineralisationen ist bis jetzt noch nicht detailliert untersucht worden. Große Bereiche von Westaustralien werden von meso- bis neoarchaischen Krusteneinheiten (z.B. Yilgarn Kraton, Pilbara Kraton) aufgebaut, von denen auch eine große Anzahl an LCT-Pegmatitsystemen bekannt sind. Darunter befinden sich unter anderem die LCT-Pegmatitlagerstätten Greenbushes (Li, Ta) und Wodgina (Ta, Sn). Zusätzlich wurden kleine Mengen an Pollucit in einer einzigen Kernbohrung im Londonderry Pegmatitfeld angetroffen. Ungeachtet dessen, wurden in Westaustralien bis jetzt keine systematischen Untersuchungen und/oder Explorationskampagnen auf Vorkommen von Cs und speziell der von Pollucit durchgeführt. Im Verlauf dieser Studie wurden insgesamt neunzehn verschiedene Pegmatitvorkommen und Pegmatitfelder des Yilgarn Kratons, Pilbara Kratons und der Kimberley Provinz auf das Vorkommen des Minerals Pollucit untersucht. Allerdings konnte in keinem der untersuchten LCT-Pegmatitsystemen Pollucit nachgewiesen werden. Von vier der untersuchten LCT-Pegmatitsystemen, dem Londonderry Pegmatitfeld, dem Mount Deans Pegmatitfeld, der Cattlin Creek LCT-Pegmatitlagerstätte (Yilgarn Kraton) und der Wodgina LCT-Pegmatitlagerstätte (Pilbara Kraton) wurden detailliert Proben entnommen und weitergehend untersucht. Zusätzlich wurden die massiven Pollucitmineralisationen im Bikita Pegmatitfeld beprobt und in die detailierten Untersuchungen einbezogen. Der Probensatz aus dem Bikita Pegmatitfeld dient als Referenzmaterial mit dem die Pegmatitproben aus Westaustralien verglichen werden. Die vorliegende Arbeit fasst die wesentlichen Ergebnisse der petrographischen, mineralogischen, mineralchemischen, geochemischen und geochronologischen Untersuchungen sowie der Flüssigkeitseinschlussuntersuchungen und stabilen und radiogenen Isotopenzusammensetzungen zusammen. Alle vier der in Westaustralien untersuchten LCT-Pegmatitsysteme kommen in geologisch ähnlichen Rahmengesteinen vor, weisen einen vergleichbaren internen Aufbau, geochemische Zusammensetzung und Mineralogie zu dem des Bikita Pegmatitfeldes in Simbabwe auf. Weiterhin konnten in allen LCT-Pegmatitsystemen Hinweise für späte hydrothermale Prozesse (z.B. Verdrängung von Feldspat) nachgewiesen werden, die einhergehend mit einer Anreicherung von Cs verbunden sind (z.B. Cs-angereicherte Säume um Glimmer, Beryll und Turmalin). Mit der Ausnahme der Wodgina LCT-Pegmatitlagerstätte, in der ein mesoarchaisches Kristallisationsalter (ca. 2,850 Ma) nachgewiesen wurde, lieferten die Altersdatierungen in den anderen LCT-Pegmatitsystemen übereinstimmende neoarchaische Alter von 2,630 Ma bis 2,600 Ma. Diese fast identischen Alter der LCT-Pegmatitsysteme des Yilgarn und Zimbabwe Kratons suggerieren, dass die Prozesse, die zur LCT-Pegmatitbildung am Ende des Neoarchaikums führten, weltweit aktiv waren. Ungeachtet dessen stellt das Vorhandensein von massiver Pollucitmineralisation das Alleinstellungsmerkmal des Bikita Pegmatitfeldes dar, welche sich infolge eines Prozesses gebildet haben der nicht Bestandteil der üblichen LCT-Pegmatitentwicklung ist und sich durch eine extreme Anreicherung an Cs unterscheidet. Die neuen Ergebnisse die in dieser Studie von den Bikita Pegmatitfeld und den Westaustralischen LCT-Pegmatitsystemen gewonnen wurden, verbessern das Verständnis des Verhaltens von Cs in LCT-Pegmatitsystemen deutlich. Somit ist es nun möglich, ein genetisches Modell für die Bildung von massiven Pollucitmineralisationen in LCT-Pegmatitsystemen vorzustellen. LCT-Pegmatite weisen im Allgemeinen eine granitische Zusammensetzung auf und werden als Kristallisat von hoch fraktionierten und geochemisch spezialisierten granitischen Restschmelzen interpretiert. Die Bildung von massiven Pollucitmineralisationen ist nur aus großen und voluminösen Pegmatitschmelzen, die als einzelner Körper entlang von Störungen in extensionalen Stressregimen intrudieren möglich. Nach Platznahme der Schmelze bildet die beginnende Kristallisation zunächst die Kontakt- und Randzone des Pegmatits, wobei infolge von fraktionierter Kristallisation die immobilen Elemente (v.a. Cs, Rb) in der verbleibenden Restschmelze angereichert werden. Im Anschluss an diese erste Kristallisation entmischt sich nach Abkühlung eine sehr kleine Menge (0.5–1 vol.%) Schmelze und/oder Fluid von der Restschmelze. Diese nicht mischbare Teilschmelze/-fluid ist angereichert an Al2O3 und Na2O sowie verarmt an SiO2 und kristallisiert als Analcim. Zusätzlich kann diese Schmelze bereits mit 1–2 wt.% Cs2O angereichert sein. Aufgrund der Auswirkung von Flussmitteln (z.B. H2O, F, B) wird allerdings der Schmelzpunkt dieser Analcimschmelze herabgesetzt und so die Kristallisation des Analcims als intergranulare Körner verhindert. Da diese Analcimschmelze im Vergleich zu der restlichen Schmelze eine geringere relative Dichte besitzt, beginnt sie gravitativ aufzusteigen und sich in den oberen Bereichen des Pegmatitkörpers zu akkumulieren. Währenddessen beginnt die restliche Schmelze separat zu kristallisieren und die inneren Bereiche des Pegmatits zu bilden. Diese Kristallisation ist einhergehend mit fortschreitender Fraktionierung und der Anreicherung von inkompatiblen Elementen (v.a. Cs, Rb) in den sich als letztes bildenden Mineralphasen (z.B. Lepidolit) oder der Konzentration der inkompatiblen Element in die sich entmischenden hydrothermalen Fluiden. Da Analcim und Pollucit eine lückenlose Mischungsreihe bilden, ist die Analcimschmelze in der Lage, alles verfügbare Cs von der Restschmelze und/oder assoziierten hydrothermalen Fluiden an sich zu binden und als Cs-Analcim im oberen Bereich des Pegmatitkörpers zu kristallisieren. Fortschreitende hydrothermale Aktivität und Substitution von Cs verschiebt dann die Zusammensetzung des Analcims von der Cs-Analcim- zu Na-Pollucitzusammensetzung. Zusätzlich erfährt der Analcim bei Abkühlung unter 400 °C eine negative thermische Expansion von ca. 1 vol.%. Diese Kontraktion führt zu der Bildung des markanten Rissnetzwerkes das durch späte Mineralphasen (z.B. Lepidolit, Quarz, Feldspat und Petalit) gefüllt wird. Vor der Mineralisation allerdings, erhöht dieses Netzwerk an Rissen die verfügbaren Wegsamkeiten für die späten hydrothermalen Fluide und begünstigt somit den Cs-Substitutionsmechanismus in der massiven Pollucitmineralisation. Weiterhin kommt es bei der Abkühlung des Pegmatits zu späten Mineralverdrängungsreaktionen (z.B. Verdrängung von K-Feldspat durch Lepidolit, Cleavelandit und Quarz), sowie zu Subsolidus-Selbstordnungsprozessen in Feldspäten. Diese Prozesse werden weiterhin interpretiert inkompatible Elemente (z.B. Cs, Rb) in die späten hydrothermalen Fluide freizusetzen. Da Feldspäte große Teile der Pegmatite bilden, kann somit eine beträchtliche Menge an Cs freigeben werden und durch die späten hydrothermalen Fluide in die massive Pollucitmineralisation in den oberen Bereichen des Pegmatitkörpers transportiert werden. Infolgedessen ist es möglich, dass genügend Cs frei gesetzt werden kann, um die Zusammensetzung innerhalb der Mischkristallreihe von Cs-Analcim (>2 wt.% Cs2O) zu Na-Pollucit (23–43 wt.% Cs2O) zu verschieben. Die zeitliche Einordnung dieser späten Cs-Anreicherung wird als quasi zeitgleich oder im direkten Anschluss an die vollständige Kristallisation der Pegmatitschmelze interpretiert. Es kann allerdings nicht vernachlässigt werden, dass auch jüngere hydrothermale Ereignisse, die den Pegmatitkörper nachträglich überprägen, ähnliche hydrothermale Prozesse hervorrufen können. Somit konnte gezeigt werden, dass es durch Kombination dieser magmatischen und hydrothermalen Prozessen möglich ist, genügend Cs anzureichern, um die Bildung von massiven Pollucitmineralisationen in LCT-Pegmatitsystemen zu ermöglichen. Dieses genetische Modell kann nun dazu genutzt werden, um das Potential von Vorkommen von massiven Pollucitmineralisationen in LCT-Pegmatitsystemen in Westaustralien und weltweit besser einzuschätzen

Des modèles géométriques ont été proposés pour reconstruire la géométrie de plis associés aux rampes (par exemple pli sur flexure de faille), en identifiant en particulier la profondeur de niveau de décollement et le déplacement total sur la rampe. Ces méthodes de reconstruction géométrique sont appliquées pour des plis partiellement érodés. Au cours de l'érosion, le cut-off de la rampe peut être érodé et, par conséquent, le déplacement sur la rampe est difficile à quantifier. Dans cette thèse, nous développons onze modèles thermo-géométriques. Les modèles combinent les données géométriques et les données d’enfouissement pour proposer une évolution cinématique d’un pli avec cut-off érodé. Nous supposons que la mise en place d'une unité tectonique produit une anomalie thermique dans le mur de la faille, et que cette anomalie thermique pourrait indiquer une épaisseur de bloc chevauchant. Les modèles fournissent une estimation de la profondeur de décollement et le déplacement total sur une rampe érodée, qui ne dépend pas de taux d’érosion. Dans le cas de chevauchements actifs, les modèles proposent un taux de déplacement et un âge de l'initiation de la faille en fonction de taux d'érosion. Ces données sont utilisées pour proposer un développement cinématique de coupes érodées. Nous appliquons les modèles sur les plis érodés et actif à Taiwan dans les zones de Choshui et Miaoli. On propose des coupes régionales équilibrées en utilisant la technique de modélisation directe. Dans la section Choshui, nous proposons un niveau de détachement de ~5 km à ~14 km, marquée par deux sauts successifs de rampes de ~5 km and ~4 km. En supposant un taux d'érosion à 4 mm/an, l'âge de l’initiation de chevauchement active est entre 3,3 Ma dans la partie intérieure de prisme (Chevauchement de Tili) à 0,9 Ma dans la partie extérieur (Chevauchement de Chelungpu). Le raccourcissement totale sur la coupe de Choshui est ~100 km et le taux de déplacement calculé est ~1 cm/an. Pour tester nos modèles thermo-géométriques dans une chaîne plissée inactive, on applique nos modèles sur les plis érodés associés aux failles de Pine Mountain et Jones Valley dans la chaîne plissée des Appalaches. L'application des modèles thermo-géométriques nous permet d’estimer une quantité de déplacement sur les deux failles et expliquer de manière satisfaisante l'anomalie thermique dans le mur des failles de Pine Mountain et Jones Valley. Afin d'améliorer la description de l’anomalie thermique qui se développe dans le soubassement des failles, on a étudié l'évolution des minéraux magnétiques des roches argileuses le long de quatre sections dans la chaîne plissée à Taiwan. On a remarqué que la greigite (Fe3S4) domine l'assemblage magnétique dans les roches enfouies à moins à moins de de 70°C. La magnétite (Fe3O4) se développe pour des températures d’enfouissement de ~50°C et domine l’assemblage magnétique jusqu'à ~350° C. A partir ~300°C, la pyrrhotite monoclinique (Fe7S8) se développe aux dépens de la magnétite, et à ~350°C, la magnétite n'est plus détecté. Ces résultats peuvent être utilisés en complément d'autres géothermomètres pour identifier les anomalies thermiques dans une gamme de de 50-70°C et de 300-350°C où les caractéristiques des minéraux magnétiques sont identifiées
Geometric models have been proposed to account satisfactorily for ramp-related folds (e.g. fault-bend fold), identifying in particular detachment depth and total shortening. These methods of geometric reconstruction are applied on partially eroded folds. During erosion, the fault cut-off may be removed and as a result, the displacement is difficult to quantify. In this thesis, we develop 11 thermo-geometric models combining geometric description of folds and burial data to propose kinematic evolution of folds with eroded cut-offs. We assume that the emplacement of a tectonic unit will result in a thermal anomaly in the footwall, and that this thermal anomaly might indicate a thickness of the overriding unit. The models provide an estimation of the detachment depth and the total shortening on an eroded ramp, independent of the erosion rate. In the case of active thrusts, the models provide an estimation of the slip rate and the age of the initiation of the thrust as a function of the erosion rate. These data are used to unravel the kinematic development of eroded cross-sections. We apply the models on eroded folds from Taiwan underlined by active thrusts in the Choshui and Miaoli sections. We propose regional balanced cross-sections using forward modeling technique. In the Choshui section, we propose a detachment profile with a depth between ~ 5 km and ~ 14 km, marked by two steps of ~ 5 km. Assuming erosion rate at 4 mm/a, the age of initiation of the active thrusts is ranging from 3.3 Ma inward (Tili thrust) to 0.9 Ma outward (Chelungpu thrust). The total shortening from the whole section is ~100 km and the calculated slip rate is about 1 cm/a. To test our models in a non-active fold-and-thrust belt, we study eroded folds associated to the Pine Mountain thrust and Jones Valley thrust from the Appalachian belt. The application of the thermo-geometric models provides a value of the total shortening and explains satisfactorily the thermal anomaly in the footwall of the Jones Valley thrust. In order to improve the description of the thermal anomaly, we have studied the evolution of magnetic minerals of argillaceous rocks in four sections from the Taiwan thrust belt. We found that the iron sulfide greigite (Fe3S4) is dominating the magnetic assemblage in the less buried rocks (<70°C). The magnetite (Fe3O4) develops at burial temperature of ~50°C and is dominating the magnetic assemblage up to ~350°C. By ~300°C, the monoclinic pyrrhotite (Fe7S8) develops at the expense of magnetite, and at ~350°C, the magnetite is no longer detected. These results can be used complementary to other geothermometers to identify thermal anomalies in the range 50-70°C and 300-350°C where characteristic magnetic minerals are identified

La caractérisation géochimique et isotopique des eaux souterraines de la " zone minéralisée de l'Entre-Deux-Mers " indique une origine commune de la minéralisation, directement liée à la minéralogie des formations captées par les forages.La géochimie montre que les interactions eau-roche sont majoritairement influencées par la présence d'évaporites, mais que d'autres interactions mettant en jeu des carbonates, des silicates et des argiles existent. Un modèle géochimique d'acquisition de la minéralisation reconstitue parfaitement la chimie des eaux souterraines à l'échelle de la zone d'étude. Ce modèle, construit en se basant sur la géochimie des eaux et sur la minéralogie des formations tertiaires du nord du Bassin aquitain, met à l'équilibre des eaux avec des formations carbonatées et évaporitiques. Afin de mieux comprendre la distribution latérale et verticale des formations tertiaires et leur minéralogie, une approche paléogéographique et sédimentologique a permis de localiser les différents horizons riches en sulfates et/ou en fluorures, mais aussi de comprendre leur origine de dépôt. En se basant sur l'hydrogéologie, la paléogéographie, la minéralogie et la géochimie, des hypothèses de répartition de la minéralisation à l'échelle du forage ont pu être testées. Les résultats de la modélisation couplée hydrodynamique-transport reconstituent la chimie des eaux prélevées par les forages de la " zone minéralisée de l'Entre-Deux-Mers ". Au vu de ces résultats, un modèle avec obturation des horizons riches en sulfates et en fluorures a été testé et les résultats obtenus ouvrent des perspectives pour des futures recherches. Ce travail a donc permis de comprendre l'origine de la minéralisation des eaux de " la zone minéralisée de l'Entre-Deux-Mers ", mais aussi de proposer des améliorations et des perspectives pour une meilleure gestion d'une des principales ressources en eau potable de la Gironde.

Les structures de combustion constituent un témoin de la fréquentation humaine et leur étude permet d'appréhender un aspect du mode d'occupation d'un lieu donné. Ainsi, pour compléter les approches classiques qui s'intéressent à la typologie des foyers, à la fréquence des feux, à la nature des combustibles, etc., une caractérisation thermique de ces structures a été proposée. Elle s'appuie sur les impacts thermiques enregistrés par les sédiments soumis aux feux et plus précisément sur les modifications des propriétés de thermoluminescence (TL) et de magnétisme avec la chauffe.Le site-laboratoire est celui de la grotte de Fraux (Dordogne), occupée à l'Âge du bronze, dont le statut et le mode d'occupation pose question puisqu'elle présente tant des vestiges domestiques (sols de circulation, foyers, mobiliers) que des vestiges symboliques (manifestations pariétales, dépôts de mobilier). La place importante des foyers parmi ces vestiges a induit une étude spécifique de ces structures. En effet, ce site recèle plus d'une soixantaine de structures de combustion et, aspect important pour notre approche archéométrique, présente un état de conservation exceptionnel puisque la grotte est restée fermée depuis l'occupation de l'Âge du bronze.L'étude de certains foyers de la grotte des Fraux a permis de tester le potentiel de paléothermomètres fondés sur ces deux propriétés indépendantes à savoir la TL des grains de quartz et le magnétisme des oxydes de fer contenus dans les sédiments. Le paléothermomètre TL a été élaboré en comparant les signaux TL d'échantillons provenant de foyers archéologiques à ceux de références thermiques chauffées en laboratoire. Pour le magnétisme deux pistes ont été exploitées : les températures de déblocage de l'aimantation rémanente et l'évolution de la signature magnétique -minéralogie et taille de grain) avec la chauffe. La détermination des paléotempératures atteintes par les sédiments substrats des structures de combustion apporte une première indication sur leur intensité de chauffe. Afin d'étalonner ces informations paléothermométriques en termes d'énergie mise en jeu, des feux expérimentaux ont été réalisés. Ils ont permis de comparer les impacts thermiques entre feux archéologiques et feux expérimentaux, de construire un échantillonnage d'histoire thermique connue, mais aussi d'estimer les températures atteintes, les épaisseurs de sédiments affectés, les quantités de combustibles consommés pendant un temps donné, la quantité d'énergie dégagée par la combustion... Ces expérimentations ont aussi servi de base à une modélisation de la propagation de la chaleur dans les sédiments. Les simulations effectuées dans ce modèle numérique permettent alors d'estimer un temps minimal de fonctionnement des structures de combustion.Nous disposons ainsi d'un nouvel outil pour la caractérisation thermique de foyers archéologiques.

The main regularities of the saturation of kimberlite rocks with the accessory mineral Mg-ilmenite (Ilm), the peculiarities of the distribution of Ilm compositions in individual pipes, in different clusters of pipes, in diamondiferous kimberlite fields, are considered as the example of studies carried out within the Yakutian kimberlite province (Siberian Craton). Interpretation of different crystallization trends in MgO-Cr2O3 coordinates (conventionally named “Haggerty’s parabola”, “Steplike”, “Hockey stick”, as well as the peculiarities of heterogeneity of individual zonal and polygranular Ilm macrocrysts made it possible to propose a three-stage model of crystallization Ilm: (1) Mg-Cr poor ilmenite crystallizing from a primitive asthenospheric melt; (2) Continuing crystallization in the lithospheric contaminated melt by MgO and Cr2O3; (3) Ilmenite subsequently underwent sub-solidus recrystallization in the presence of an evolved kimberlite melt under increasing oxygen fugacity (ƒO2) conditions.

Formation lithologies that are composed of several minerals require multiple porosity logs to be run in combination in order to evaluate volumetric porosity. In the most simple solution model, the proportions of multiple components together with porosity can be estimated from a set of simultaneous equations for the measured log responses. These equations can be written in matrix algebra form as: . . . CV = L . . . where C is a matrix of the component petrophysical properties, V is a vector of the component unknown proportions, and L is a vector of the log responses of the evaluated zone. The equation set describes a linear model that links the log measurements with the component mineral properties. Although porosity represents the proportion of voids within the rock, the pore space is filled with a fluid whose physical properties make it a “mineral” component. If the minerals, their petrophysical properties, and their proportions are either known or hypothesized, then log responses can be computed. In this case, the procedure is one of forward-modeling and is useful in situations of highly complex formations, where geological models are used to generate alternative log-response scenarios that can be matched with actual logging measurements in a search for the best reconciliation between composition and logs. However, more commonly, the set of equations is solved as an “inverse problem,” in which the rock composition is deduced from the logging measurements. Probably the earliest application of the compositional analysis of a formation by the inverse procedure applied to logs was by petrophysicists working in Permian carbonates of West Texas, who were frustrated by complex mineralogy in their attempts to obtain reliable porosity estimates from logs, as described by Savre (1963). Up to that time, porosities had been commonly evaluated from neutron logs, but the values were excessively high in zones that contained gypsum, caused by the hydrogen within the water of crystallization. The substitution of the density log for the porosity estimation was compromised by the occurrence of anhydrite as well as gypsum.

The early descriptions of chemical bonding in minerals and geological materials utilized purely ionic models. Crystals were regarded as being made up of charged atoms or ions that could be represented by spheres of a particular radius. Based on interatomic distances obtained from the early work on crystal structures, ionic radii were calculated for the alkali halides (Wasastjerna, 1923) and then for many elements of geochemical interest by Goldschmidt (1926). Modifications to these radius values by Pauling (1927), and others took account of such factors as different coordination numbers and their effects on radii. The widespread adoption of ionic models by geochemists resulted both from the simplicity and ease of application of these models and from the success of rules based upon them. Pauling’s rules (1929) enabled the complex crystal structures of mineral groups such as the silicates to be understood and to a limited extent be predicted; Goldschmidt’s rules (1937) to some degree enabled the distribution of elements between mineral phases or mineral and melt to be understood and predicted. Such rules are further discussed in later chapters. Ionic approaches have also been used more recently in attempts to simulate the structures of complex solids, a topic discussed in detail in Chapter 3. Chemical bonding theory has, of course, been an important component of geochemistry and mineralogy since their inception. Any field with a base of experimental data as broad as that of mineralogy is critically dependent upon theory to give order to the data and to suggest priorities for the accumulation of new data. Just as the bond with predominantly ionic character was the first to be quantitatively understood within solidstate science, the ionic bonding model was the first used to interpret mineral properties. Indeed, modern studies described herein indicate that structural and energetic properties of some minerals may be adequately understood using this model. However, there are numerous indications that an ionic model is inadequate to explain many mineral properties. It also appears that some properties that may be rationalized within an ionic model may also be rationalized assuming other limiting bond types.

Abstract The Cortez district is in one of the four major Carlin-type gold deposit trends in the Great Basin province of Nevada and contains three giant (&gt;10 Moz) gold orebodies: Pipeline, Cortez Hills, and Goldrush, including the recently discovered Fourmile extension of the Goldrush deposit. The district has produced &gt;21 Moz (653 t) of gold and contains an additional 26 Moz (809 t) in reserves and resources. The Carlin-type deposits occur in two large structural windows (Gold Acres and Cortez) of Ordovician through Devonian shelf- and slope-facies carbonate rocks exposed through deformed, time-equivalent lower Paleozoic siliciclastic rocks of the overlying Roberts Mountains thrust plate. Juxtaposition of these contrasting Paleozoic strata occurred during the late Paleozoic Antler orogeny along the Roberts Mountains thrust. Both upper and lower plate sequences were further deformed by Mesozoic compressional events. Regional extension, commencing in the Eocene, opened high- and low-angle structural conduits for mineralizing solutions and resulted in gold deposition in reactive carbonate units in structural traps, including antiforms and fault-propagated folds. The Pipeline and Cortez Hills deposits are located adjacent to the Cretaceous Gold Acres and Jurassic Mill Canyon granodioritic stocks, respectively; although these stocks are genetically unrelated to the later Carlin-type mineralization event, their thermal metamorphic aureoles may have influenced ground preparation for later gold deposition. Widespread decarbonatization, argillization, and silicification of the carbonate host rocks accompanied gold mineralization, with gold precipitated within As-rich rims on fine-grained pyrite. Pipeline and Cortez Hills also display deep supergene oxidation of the hypogene sulfide mineralization. Carlin-type mineralization in the district is believed to have been initiated in the late Eocene (&gt;35 Ma) based on the age of late- to postmineral rhyolite dikes at Cortez Hills. The Carlin-type gold deposits in the district share common structural, stratigraphic, alteration, and ore mineralogic characteristics that reflect common modes of orebody formation. Ore-forming fluids were channeled along both low-angle structures (Pipeline, Goldrush/Fourmile) and high-angle features (Cortez Hills), and gold mineralization was deposited in Late Ordovician through Devonian limestone, limy mudstone, and calcareous siltstone. The Carlin-type gold fluids are interpreted to be low-salinity (2–3 wt % NaCl equiv), low-temperature (220°–270°C), and weakly acidic, analogous to those in other Carlin-type gold deposits in the Great Basin. The observed characteristics of the Cortez Carlin-type gold deposits are consistent with the recently proposed deep magmatic genetic model. Although the deposits occur over a wide geographic area in the district, it is possible that they initially formed in greater proximity to each other and were then spatially separated during Miocene and post-Miocene regional extension.

A consistent approach to evaluate the mineralogy and petrophysical attributes throughout the extent and life of the field is essential. This ensures proper assessment of the reservoir potential and provides deeper insights about the sedimentological and depositional environment. However, different logging technologies with varying capabilities and applications are often used during field development phases, introducing differences and discrepancy in the assessment. In a study from North Sea, only few wells had high-end spectroscopy measurements to decipher complex mineralogy. Due to the lack of similar measurements in other wells, we explore the application of Intelligent Reservoir Characterization using Class-based Machine Learning (CbML) to provide a reliable and consistent evaluation. During training phase of CbML, key wells with complex mineralogy evaluated using advanced wireline spectroscopy logs are used. First training data is automatically reduced into interpretable facies using a novel Petrophysical Data-Driven Classification (PDDC) methodology. Then facie-wise learning models, including 1)outlier detection, 2)prediction of target complex mineralogy from input spectroscopy, 3) feedback QC loop of reconstructing spectroscopy from learnt mineralogy, and 4)corresponding uncertainties, are developed. During prediction phase, learnt models are applied to basic spectroscopy data in non-key wells to automatically identify outliers, assign facies, predict target complex mineralogy with uncertainties and reconstruct spectroscopy for a comprehensive QC. The procedure takes advantage of traditional petrophysical workflows as well as machine learning algorithms to quickly assess and deploy an intelligent reservoir characterization application. The methodology was used over a blind well where the predicted mineralogy showed a good match with that of an expert driven assessment using high-end spectroscopy data. In the non-key well, the mineralogy predicted from the CbML application was used to determine different petrophysical attributes like porosity, intrinsic permeability, and elemental concentrations, which matched well with the conventional core analysis and XRF data. Overall, the uncertainties estimated from the workflow were very low, establishing further the robustness and reliability of the results. While it is desired to have all the necessary measurements to address the formation complexity, the well logging program is mostly constrained by the logging environment or planned borehole trajectory. The case study above shows how the CbML workflow can be used to develop a fit-for-purpose intelligent solution as an alternative to traditional interpretation. Once a learnt model is developed, it can be shared among experts and applied to any new well to provide near-instant, consistent quantities of interest, such as mineralogy, grain density and elemental concentrations as in our present case.

Abstract Propped hydraulic fracturing is a stimulation technique used in tight formations to create conductive fractures. To predict the fractured well productivity, the conductivity of those propped fractures should be estimated. It is common to measure the conductivity of propped fractures in the laboratory under controlled conditions. Nonetheless, it is costly and time-consuming which encouraged developing many empirical and analytical propped fracture conductivity models. Previous empirical models, however, were based on limited datasets producing questionable correlations. We propose herein new empirical models based on an extensive data set utilizing machine learning (ML) methods. In this study, an artificial neural network (ANN) was utilized. A dataset comprised of 351 data points of propped hydraulic fracture experiments on different shale types with different mineralogy under various confining stresses was collected and studied. Several statistical and data science approaches such as box and whisker plots, correlation crossplots, and Z-score techniques were used to remove the outliers and extreme data points. The performance of the developed model was evaluated using powerful metrics such as correlation coefficient and root mean squared error. After several executions and function evaluations, an ANN was found to be the best technique to predict propped fracture conductivity for different mineralogy. The proposed ANN models resulted in less than 7% error between actual and predicted values. In this study, in addition to the development of an optimized ANN model, explicit empirical correlations are also extracted from the weights and biases of the fine-tuned model. The proposed model of propped fracture conductivity was then compared with the commonly available correlations. The results revealed that the proposed mineralogy based propped fracture conductivity models made the predictions with a high correlation coefficient of 94%. This work clearly shows the potential of computer-based ML techniques in the determination of mineralogy based propped fracture conductivity. The proposed empirical correlation can be implemented without requiring any ML-based software.

Abstract The influences of mineralogy on rock mechanical properties have profound application in oil and gas exploration and production processes, including hydraulic fracturing operations. In conventional resources, the rock mechanical properties are predominantly controlled by porosity; however, in unconventional tight formations, the importance of mineralogy as a function of rock mechanical properties has not been fully investigated. In unconventional tight formations, mechanical properties are often derived from mineralogy weight fraction together with the best estimate of porosity, assumption of fluid types, the extent of pore fillings, and fluid properties. These properties are then adjusted for their volumetric fractions and subsequently calibrated with acoustics or geomechanical lab measurements. A new method is presented that utilizes mineralogy weight fractions (determined from well logs or laboratory measurements). This process uses public domain information of minerals using Voigt and Reuss averaging algorithms as upper and lower bounds, respectively. An average of these bounds (also known as Hill average) provides a representative value for these parameters. Further, based on isotropic conditions, all the elastic properties are calculated. A typical output consisting of bulk-, shear-, and Young's - modulus, together with Poisson's ratio obtained from traditional methods of volume fractions and this new method using weight fractions is discussed and analyzed along with the sensitivity and the trends for individual rock properties. Furthermore, corresponding strengths, hardness, and fracture toughness could also be estimated using well known public domain algorithms. Data from carbonate reservoirs has been discussed in this work. This method shows how to estimate grain compressibility that can be challenging to be measured in the lab for unconventional tight rock samples. In low-porosity samples, the relative influence of porosity is negligible compared to the mineralogy composition. This approach reduces several assumptions and uncertainties associated with accurate porosity determination in tight rocks as it does not require the amount of pore fluids and fluid properties in calculations. The grain-compressibility and bulk-compressibility (measured by hydrostatic tests in the laboratory on core plugs or calculated from density and cross-dipole log) are used to calculate poroelastic Biot's coefficient, as this coefficient will be used to calculate in-situ principal effective stresses (overburden, minimum horizontal, and maximum horizontal stresses), which are, together with rock properties and pore pressure, constitutes the geomechanical model. The geomechanical model is used for drilling, completions, and hydraulic fracture modeling, including wellbore stability, and reservoir integrity analyses.

Abstract Drilling high pressure gas wells requires special consideration and precautions. The main goal is to analyze wellbore stability to plan drilling fluids and best drilling practices to minimize drilling risks. Ultimate objective is to provide a stable and smooth borehole environment for wireline logging runs and completion of a deep high pressure high temperature gas wells. Continuous collaboration and integration among all disciplines including Drilling, Geology, Geo-steering, Production Engineering and Reservoir Management is deemed key to a successful well.

Predicting drilling risks in advance is a major challenge in areas that lack drilling experience, and even when information from offset wells is available. Large overpressure was found at TD of an offshore exploratory well drilled mainly through shale. None of the other two previously drilled offset wells in the area had shown any sign of such a high overpressure. This study presents two complementary approaches to gain insight on the overpressure generation mechanisms. The effect of chemical compaction is first evaluated in terms of well cuttings analysis, including sample washing, high-resolution photo catalog, automated mineralogy and X-ray diffraction clay mineralogy analysis. The obtained mineralogical results confirm the presence of the dehydration diagenetic process involving the transformation of smectite to illite. Consequently, a numerical model is presented which combines the effect of mechanical and chemical compaction to predict pore pressure values very close to the overpressure observed during drilling. The model reproduces the depositional history of the lithological column by coupling mechanical and chemical compaction with fluid flow over geological time, and it allows predicting stress, porosity and pore pressure evolution at different geological ages. Calibration and verification of the results of the pore pressure model is done by comparison to drilling experience and logs (post-drill pore pressure profile, geology tops and density/porosity logs).

We present herein a summary of integrated data analyses aimed at investigating the effects of hydrothermal alteration on seismic reflectivity in the footwall of the Lalor volcanogenic massive-sulfide (VMS) deposit, Manitoba. Multivariate analyses of seismic rock properties, lithofacies, and hydrothermal alteration indices show an increase in P-wave velocity for altered volcanic and volcaniclastic lithofacies with respect to their least-altered equivalents. Scanning electron microscopy-energy dispersive X-ray spectrometry analyses of drill-core samples suggest that this P-wave velocity increase is due to the high abundance of high P-wave velocity aluminous minerals, including cordierite, Fe-Mg amphibole, and garnet, which in volcanic rocks are characteristic of VMS-associated hydrothermal alteration metamorphosed in the amphibolite facies. A seismic synthetic profile computed from a simple amphibolite-facies mineral assemblage model, consisting of mafic-felsic host rock contacts, a sulfide ore lens, and a discordant hydrothermal conduit, show enhanced seismic reflections at conduit-host rock contacts in comparison to the equivalent greenschist facies mineral assemblage model. Collectively our results suggest that VMS footwall hydrothermal alteration zones metamorphosed under middle- to upper-amphibolite facies conditions have enhanced potential for seismic detection.

A complete suite of bulk major- and trace-elements measurements combined with macroscopic/microscopic observations and mineralogy guided by scanning electron microscope-energy dispersive spectrometry (SEM-EDS) analyses were applied on Nekuashu (2.55 Ga) and Pelland (2.32 Ga) intrusions in northern Canada, near the Strange Lake rare earth elements (REE) deposit, to evaluate their magmatic evolution and possible relations to the Mesoproterozoic Strange Lake Peralkaline Complex (SLPC). These Neoarchean to earliest-Paleoproterozoic intrusions, part of the Core Zone in southeastern Churchill Province, comprise mainly hypersolvus suites, including hornblendite, gabbro, monzogabbro/monzodiorite, monzonite, syenite/augite-syenite, granodiorite, and mafic diabase/dyke. However, the linkage of the suites and their petrogenesis are poorly understood. Geochemical evidence suggests a combination of 'intra-crustal multi-stage differentiation', mainly controlled by fractional crystallization (to generate mafic to felsic suites), and 'accumulation' (to form hornblendite suite) was involved in the evolution history of this system. Our model proposes that hornblendite and mafic to felsic intrusive rocks of both intrusions share a similar basaltic parent magma, generated from melting of a hydrous metasomatized mantle source that triggered an initial REE and incompatible element enrichment that prepared the ground for the subsequent enrichment in the SLPC. Geochemical signature of the hornblendite suite is consistent with a cumulate origin and its formation during the early stages of the magma evolution, however, the remaining suites were mainly controlled by 'continued fractional crystallization' processes, producing more evolved suites: gabbronorite/hornblende-gabbro ? monzogabbro/monzodiorite ? monzonite ? syenite/augite-syenite. In this proposed model, the hydrous mantle-derived basaltic magma was partly solidified to form the mafic suites (gabbronorite/hornblende-gabbro) by early-stage plagioclase-pyroxene-amphibole fractionation in the deep crust while settling of the early crystallized hornblende (+pyroxene) led to the formation of the hornblendite cumulates. The subsequent fractionation of plagioclase, pyroxene, and amphibole from the residual melt produced the more intermediate suites of monzogabbro/monzodiorite. The evolved magma ascended upward into the shallow crust to form monzonite by K-feldspar fractionation. The residual melt then intruded at shallower depth to form syenite/augite-syenite with abundant microcline crystals. The granodiorite suite was probably generated from lower crustal melts associated with the mafic end members. Later mafic diabase/dykes were likely generated by further partial melting of the same source at depth that were injected into the other suites.