Mémoire (M.Sc.T)-- Université du Québec à Chicoutimi 1986. / "Mémoire présenté en vue de l'obtention du diplôme de M.Sc.A. (sciences de la terre)" CaQCU Document électronique également accessible en format PDF. CaQCU

Ph.D. / Petrographic, chemical and multiple sulfur isotope analyses were conducted on pyrite from argillaceous, arenaceous and rudaceous sedimentary rocks from the Mesoarchean Witwatersrand Supergroup. Following detailed petrographic analyses, four paragenetic associations of pyrite were identified. These include: 1) Detrital pyrite (derived from an existing rock via weathering and/or erosion). 2) Syngenetic pyrite (formed at the same time as the surrounding sediment). 3) Diagenetic pyrite (formed in the sediment before lithification and metamorphism). 4) Epigenetic pyrite (formed during metamorphism and hydrothermal alteration). It was found that the distribution of the pyrite varies with respect to the stratigraphic profile of the Witwatersrand Supergroup and depositional facies within the Witwatersrand depository. In this regard, the four paragenetic associations of pyrite are either scarce or absent in marine-dominated depositional environments, which occur in the lower parts of the succession and in geographically distal parts of the depository. Conversely, the four paragenetic associations are well represented in fluvial-dominated depositional environments, which occur in the middle and upper parts of the succession and in geographically proximal parts of the depository. However, it is worth noting that diagenetic pyrite in the West Rand Group occurs as in situ segregations in carbonaceous shale, whereas syngenetic and diagenetic pyrite in the Central Rand Group occurs as reworked and rounded fragments in fluvial quartz-pebble conglomerates. The strong association between fluvial depositional environments and sedimentary pyrite (syngenetic and diagenetic pyrite) infers a continental source of the sulfur (sulfide weathering or volcanic activity), whereas the lack of pyrite in marine depositional environments is consistent with the model of a sulfate-poor Archean ocean. The connection between epigenetic pyrite and the fluvial-dominated depofacies is probably related to the elevated concentrations of precursor sulfides (i.e., remobilization of syngenetic and early diagenetic pyrite) and the presence of organic carbon (conversion of metal-rich early diagenetic pyrite into pyrrhotite and base metal sulfides). In support of the petrographic observations above, it was found that the trace element chemistry of each paragenetic association of pyrite yields a distinctive set of chemical compositions and interelement variations (Co, Ni and As contents). Regarding detrital pyrite, two chemical populations can be distinguished according to grain size: 1) small grains (tens of μm’s) with high levels of metal substitution (up to wt. %) and interelement covariation and iv 2) large grains (>100 μm) with low levels of metal substitution (≤200 ppm). These two populations are thought to represent pyrite derived from sedimentary and metamorphosed source areas, respectively (see below). The trace element chemistry of diagenetic pyrite varies relative to the Fe-content of the host rock. Diagenetic pyrite from Fe-rich host rocks, such as magnetic mudstone and banded iron formation (BIF), generally contain low Ni contents (<500 ppm), moderate As contents (<1500 ppm) and relatively high Co contents (up to a few wt. %). Elevated concentrations of As probably reflect desorption of As from clays and Fe-oxyhydroxides during diagenetic phase transformations, whereas anomalous concentrations of Co are tentatively linked to the reductive dissolution of Mn-oxyhydroxides.

<p> The Lake Shore Mine has accounted for over a quarter of the gold produced in the Kirkland Lake camp. Petrography suggests that gold is a later phase in the paragenesis of ore minerals. Sulphides precipitated first followed by tellurides and gold. Wallrock alteration adjacent to the quartz veins was studied by thin section and geochemical analysis. Petrography enabled an alteration facies scheme based on predominant alteration minerals to be established. The alteration assemblages suggest the fluid had a high K and CO2 content, low Na, and reduced S content. Geochemical analysis of these samples confirm petrographic observations and suggests major additions of K2O, SiO2, Rb, Sb and Au to the wallrock, and losses of Na2O, Ba and Zr from the wallrock.</p> / Thesis / Bachelor of Science (BSc)

Pyrite porphyroblasts up to 300 mm in size are common in the polymetamorphosed, iron-rich, stratabound, massive sulfide ore at the Cherokee Mine, Ducktown, Tennessee. These porphyroblasts contain abundant inclusions of sphalerite, calcite, and micas that are used to determine the paragenesis of the porphyroblasts and the metamorphic history of the ore deposit. The ore mineralogy at the mine is: hexagonal pyrrhotite (60%), pyrite (30%), chalcopyrite (4%), sphalerite (3 %), and magnetite (3 %) with minor galena, molybdenum, tetrahedrite, bismuth, ilmenite, and rutile. The ore body is interpreted to have been syngenetic and to have contained both primary pyrrhotite and primary pyrite: additional pyrite may have formed as crusty accretions resulting from oxidation of primary pyrrhotite shortly after deposition. The early pyrites were later deformed and acted as seeds for the formation of the larger porphyroblasts by Ostwald ripening, and by the annealing of small pyrite grains into larger porphyroblasts, during isochemical metamorphism. Sphalerite geobarometry indicates initial growth of the pyrite porphyroblasts began at 6.8± 0.8 kilobars and that many sphalerite grains underwent some degree of re-equilibration at a later stage. Fluid inclusions formed during retrograde metamorphism have salinities near 12 % NaCl with a vapor phase rich in CO₂. Pyrrhotite-pyrite compositional profiles indicate at least partial re-equilibration of hexagonal pyrrhotite down to about 270°C. At lower temperatures further compositional re-equilibration was probably prevented, because the coexisting pyrite was too refractory to release the sulfur needed for the hexagonal pyrrhotite to react to monoclinic pyrrhotite. / Master of Science

This dissertation develops computational techniques to aid in efficiently studying petrologic systems that would otherwise be challenging. It then focuses on a case study in which the transition from diagenesis to syn-magmatic heating led to a recrystallization and sulfur mobilization. A Markov-chain Montecarlo-based methodology is developed to allow for the assessment of uncertainty in calculated phase assemblage diagrams. Such phase equilibria are ubiquitous in modern petrology, but uncertainties are rarely considered. Methods are discussed for visualizing and quantifying emergent patterns as phase diagrams are re-calculated with input data modified within permitted uncertainty bounds, and these are implemented in a new code. Results show that uncertainty varies significantly across pressure-temperature space and that in some conditions, estimates of stable mineral assemblage are known with very little confidence. A Machine-Learning (ML) based methodology is developed for automatically identifying unknown phases using Electron-dispersion spectra (EDS) in concert with a Random Forest Classification algorithm. This methodology allows for phase identification that it is insensitive to overfitting and noisy spectra. However, this tool is limited by the amount of reference spectra available in the dataset on which the ML algorithm is trained. The approximately 250 EDS spectra in the current training database must be supplemented to make the tool more widely useful, though it currently has an excellent success rate for correctly identifying various sulfide and oxide minerals. An analysis of paragenesis associated with Central Atlantic Magmatic Province (CAMP) intrusions helps to better constrain the dynamics of magma emplacement, while also providing a method for estimating the amount of sedimentary sulfide-sequestered sulfur mobilized as a result of magnetite formation associated with igneous activity. This method demonstrates that dike emplacement can trigger liberation of sedimentary sulfur with no direct cooling impact on climate. / Doctor of Philosophy / Determining how rocks and minerals form is fundamental to the geosciences. Here I present two computer-based techniques that can help address this essential problem. One method involves carefully determining uncertainty in thermodynamic modeling. Knowing the amount of uncertainty ultimately allows us to know the degree of confidence we can have in our model-based conclusions. The second computer-based technique involves using Machine Learning to automate the identification of minerals using an Electron-dispersion spectra (EDS) measured using a Scanning Electron Microscope (SEM). In theory, computers are much better than humans at quickly and repeatedly processing large sets of data such as EDS. This technique works well when the computer is successfully 'trained' on a large set of data but is somewhat limited in this case because there isn't diverse enough data available to train the computer. We therefore need better training data so that we can more fully benefit from this mineral identification tool. A third project I worked on involved assessing the impact of magma intruding into sedimentary rocks of the Culpeper Basin in northern Virginia. This occurred roughly 200 million years ago during the rifting of Pangea. The sedimentary rock around the magma heated up so much that water in the rock boiled and caused the rock to become fractured. After this a hydrothermal system was established that helped convert pyrite to magnetite, removing sulfur from the rocks in the process.

No description available.

No description available.

The geology, petrology and geochemistry of Catface porphyry Cu (Mo-Au) deposit, located on the west coast of Vancouver Island are examined in detail. Detailed core logging and sampling was carried out to characterize the geometry and identity of different intrusive phases and alteration styles prevalent during the emplacement and formation of the deposit, as well as their geochemical affinity. Early- and late-stage potassic alteration is identified, as well as main-stage sodic-calcic and calcic-sodic alteration. Four distinct Paleogene intrusive phases vary from quartz diorite to granodiorite in composition. The rocks are broadly calc-alkaline, weakly peraluminous to moderately metaluminous, and have typical arc geochemical affinity. The timing of emplacement and mineralization is constrained by U-Pb and Re-Os geochronology at 40.4-41.4 Ma and 40.9 ±0.2 Ma, respectively. All four Paleogene Catface intrusive phases were emplaced close in time with a direct temporal correlation to mineralization. The chalcopyrite- and pyrite-bearing miarolitic cavities in the Halo Porphyry intrusive, combined with U-Pb and Re-Os dates suggest this intrusive phase is the most likely source of mineralizing fluids. The intrusions were emplaced at depths of less four kilometers in the crust, as evidenced by the presence of miarolitic cavities and confirmed through amphibole-plagioclase thermobarometry, which record conditions of 615–700 °C and <200 MPa. The lack of primary anhydrite and hematite, and the presence of pyrrhotite in the ore system indicate a reduced magmatic-hydrothermal event. The SO3 contents in apatites are <450 ppm, indicative of a degassed and/or sulphate-free (reduced) magma. The assemblage K-feldspar-quartz-biotite-ilmenite yields oxygen fugacities (fO2) which are 0.5 to 3.0 log units below the quartz-fayalite-magnetite (QFM) buffer at an assumed pressure of 300 MPa; orders of magnitude more reduced than typical porphyry deposits. Parental magmas to the Catface deposit were either derived from intrinsically-reduced mantle, or more typical oxidized arc magma that was subsequently reduced during ascent and emplacement. Further isotopic work is required to determine which process contributed to the reduction of these magmas in an arc setting. Nevertheless, recognition of reduced porphyry-related magmatism on west-central Vancouver Island is of similar age to that of North Fork (~36.8-38.9 Ma) deposit in Washington suggesting a consanguinity of reduced magmatism with the Paleogene Cascade arc. / Graduate

No description available.

O Quadrilátero Ferrífero (MG) é uma importante unidade geotectônica do Cráton do São Francisco que alberga jazidas de ferro e ouro hospedadas na seqüência greenstone belt Rio das Velhas e cuja evolução geodinâmica esteve regida por vários eventos tectonometamórficos. Depósitos de ouro gerados por fluidos epitermais e remobilizações orogênicas proterozóicas se associam intimamente com camadas bandas de ferro e raramente disseminados em metapelitos no topo do greenstone. Nas mineralizações é comum uma paragênese que cristalizou mediante diversos processos e em distintos estágios com predomínio de sulfetos de ferro. Em geral os minérios possuem forte controle estrutural por dobras, falhas e cisalhamentos transcorrentes, desenvolvidos durante vários eventos deformacionais de diversa índole e estreita relação com processos de alteração hidrotermal como sericitização, carbonatação, cloritização, entre outros de menor incidência. O depósito de ouro Córrego do Sítio hospeda-se na seqüência silisiclástica de origem turbidítica metamorfisada em fácies xisto verde denominada Associação de Litofácies Ressedimentada que faz parte do Grupo Nova Lima, se apresenta disseminado em metapelitos e em veios de quartzo com carbonato. A mineralização que acompanha o ouro cristalizou em vários estágios principais nos que predominaram (i) pirita + pirrotita, (ii) arsenopirita + pirrotita + pirita com texturas finas e orientadas, (iii) arsenopirita com pirrotita e sulfosais em veios de quartzo, (iv) pirita grossa pervasiva em todas as rochas hospedeiras. O estudo da composição isotópica Pb, Sr e Nd, permitiu identificar que as fontes dos fluidos mineralizantes foram as rochas hospedeiras, que na evolução policíclica da mineralização participaram eventos metamórficos e hidrotermais a 2.2, 2.0 Ga. e sobreimpostos a ~600 Ma. os quais induziram os processos de circulação hidrotermal na área de estudo. Mediante comparativa com outros depósitos da região, sugere-se que os metais formadores do minério depositaram-se em Córrego do Sítio à idade de 2.2 Ga. originados a partir da BIF tipo São Bento e remobilizados por fluidos através de falhas inversas. Análises geocronológicas U-Pb (SHRIMP) e K-Ar embora não decisivos em quanto à determinação da idade da mineralização, revelaram a idade de ~2.7 Ga. como o limite temporal para a deposição das rochas hospedeiras. No contexto geológico aplicado se estabelece a potencialidade para a prospecção aurífera que apresentam as rochas metapelíticas do topo do Greenstone Belt Rio das Velas associadas a camadas bandadas de ferro com ouro, e que os alvos mais relevantes seriam zonas com falhas inversas e cisalhamentos transcorrentes. / The Quadrilátero Ferrífero is an important geotectonic unit of the São Francisco Craton that host iron and gold deposits within the vulcanosedimentary rocks of the greenstone belt Rio das Velhas. The geodynamic evolution of this greenstone includes several tectonometamorphic events. Gold deposits were formed by epithermal fluids, remobilized during the Proterozoic orogenic processes, are associated whit the banded iron formations and are sporadically disseminate in metapelitic rocks within the top of the greenstone sequence. In the ores is common the crystallization of iron sulfide minerals during diverse stages. Gold deposits have an important structural control by folds, reverse faults and strike-slip shearing zones, and are commons several hydrothermal alteration processes like sericitization, chloritization and carbonate usually related to the deformation. The Córrego do Sítio gold deposit is hosted in turbiditic siliciclastic rocks that are metamorphosed in the greenschist facies. The gold is associated with the iron sulfide arsenopyrite and pyrite, which are disseminated in metapelitic rocks and quartz-carbonate veins. The mineralization includes vary stages of crystallization, (1) pyrite and pyrrotite (2) arsenopyrite, pyrrotite and fine pyrite, (3) arsenopyrite whit pyrrotite and sulphosalts in quartz vein, and (4) pervasive pyrite. An integrated Pb, Sr e Nd, isotopic study of the Córrego do Sítio deposit allowed to identify that sources of the mineralizing fluids was the host rocks, that in the policiclic evolution the gold mineralizaton had participation events of metamorphism and hidrothermalism to age 2.2, 2.0 and 0.6 Ga. respectively the wich induced the hydrothermal circulation processes in the study area. The isotopic character of this deposit and its comparison with similar ones, suggest that the gold and its metals associated it is deposited in Córrego do Sítio to age 2.2 Ga. and that metals were derived from the banded iron formations, and remobilized by hidrothermal fluids across reverse faults. Although U-Pb (SHRIMP) and K-Ar geochronologic data were not conclusive to constrain the age of the mineralization, they provide to age ~2.7 Ga. that a good limit for the sedimentation of the host rock. It is suggest that there is a potential for gold prospection within the metapellitic rocks associated with banded iron formation of the Greenstone Rio das Velhas, and that the more relevant targets include areas with inverse and strike-slip fault.