Whole Language and Phonics

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Composición mineral y relaciones de fase de los arseniuros de Co-Fe-Ni del yacimiento de Aït-Ahmane (Bou-Azzer, Marruecos). Diferencias con otros depósitos.

El yacimiento de Aït-Ahmane, localizado en el extremo este del distrito de Bou-Azzer, presenta notables variaciones mineralógicas en función de la etapa de mineralización en el que nos encontramos. El estado I está constituido por skutterudita I y calcita. El estado II, por rammelsbergita junto con miembros de la solución sólida rammelsbergita-safflorita, miembros de la solución sólida rammelsbergita-safflorita-löllingita, mienbros de la solución sólida rammelsbergita-löllingita y safflorita, niquelina, miembros de la solución sólida cobaltita-gersdorffita y dolomita. En el estado III precipitó skutterudita II y dolomita. El último estado IV, está constituido por miembros de la solución sólida safflorita-löllingita, arsenopirita, alloclasita, skutterudita III y löllingita. La skutterudita del estado I indica temperaturas de formación de 650ºC, obtenidas a partir de campos experimentales de estabilidad de solciones sólidas. En el estado II, a partir de la precipitación de rammelsbergita, los fluidos van enriqueciéndose en Co y Fe, y tras la precipitación de los miembros de la serie cobaltita-gerdorffita, aumenta bruscamente la fugacidad del As junto con el aporte de Co, dando lugar a la precipitación de skutterudita del estado III. El estado IV indica un aumento de la fugacidad del S, de manera que trás la precipitación de diarseniuros se produce una etapa de sulfoarseniuros que finaliza con un nuevo aporte de Co y As que supone una nueva precipitación de skutterudita y löllingita. En esta última etapa las temperaturas estarían en torno a los 500ºC según los digramas de fases experimentales. Todas estas etapas de mineralización, con fluidos de composición y fugacidades de As variable produjeron la desestabilización de las fases previas que se reequilibraron bajo las nuevas condiciones

Formaciones bandeadas de hierro del SE de Angola: Caracterización mineralógica y textural del yacimiento de Tchamutete

Las formaciones de hierro están compuestas por rocas sedimentarias de gran interés económico. La mayoría de éstas se formaron en el fondo marino durante el Arcaico y Paleoproterozoico inferior, con una composición química heterogénea de minerales ricos en hierro en los que encontramos óxidos (hematites o magnetita), silicatos (greenalita, minnesotaita o estilpnomelana), carbonatos (siderita y ankerita ) y sulfuros (pirita). El yacimiento de Tchamutete, situado en el Grupo Sur del distrito minero de Cassinga (al sur de la provincia angoleña de Huila) es un yacimiento tip BIF (Formaciones Bandeadas de Hierro), de edad arcaica (alrededor de 2700 M Grupo Sur de Cassinga se puede dividir en 2 subgrupos diferentes: Grupo Jamba, en donde se encuentra Tchamutete, y Grupo Cuandja, ambos compuestos por unidades volcano-sedimentarias. Con el estudio de las muestras mediante diferentes técnicas instrumentales como Rayos-X, microscopía óptica y microscopía electrónica (SEM), se ha obtenido la mineralogía principal: hematites, martita, magnetita residual y cuarzo. Las muestras se ha agrupado en función de su proporción en Fe y de su posición estratigráfica. Así, desde las más profundas a las más superficiales hemos definido 3 facies: Facies A: Con una concentración baja en Fe y finas bandas de hematites. Facies B: Con mayores proporciones en hematites que las muestras anteriores y menores cantidades de cuarzo, el cual presenta numerosas inclusiones de hematites. Facies C: Hematites masiva la cual ha sufrido una intensa meteorización que le da una apariencia de hematites pulverulenta. Los resultados generales sugieren que el yacimiento de Tchamutete es un depósito BIF de tipo Superior, enriquecido mediante procesos supergénicos. Respecto a su clasificación mineralógica se consideraría como un yacimiento de hematites de alto grado (más del 60% en Fe), concretamente del tipo microplaty-hematites

Investigaciones mineralógicas de muestras procedentes de campañas de exploración minera en Bolivia

El objetivo principal de este trabajo es realizar una caracterización mineralógica de muestras de dos proyectos de Bolivia a través de microscopía óptica de ojos, microscopía óptica y microscopía electrónica de barrido (FESEM). A partir de los datos adquiridos, se obtendrá la posible secuencia paragenética junto con un modelo genético plausible y los mejores criterios de exploración para el tipo de mineralización sugerido. Se han recibido un total de ocho muestras de dos objetivos de exploración diferentes que son "SJ" y "PO". Presentan un carácter multiepisódico con tres etapas de mineralización. La primera etapa se caracteriza por la alteración propilítica, sericítica y de silicificación de la roca huésped. La segunda etapa es la más importante desde el punto de vista económico con la precipitación de los principales minerales económicos: la esfalerita, la galena, la estanita y los sulfos de Ag y Pb. El geotermómetro de freibergita nos permite acercarnos a la temperatura de cristalización de la etapa 2 alrededor de 1700C. La asociación mineral y los tipos de alteración de las muestras estudiadas sugieren zonación vertical dentro de los depósitos y se extraerían de las zonas de superficie. La mineralización estudiada puede ser clasificada como un depósito polimetálico de metal base de Ag Sn, perteneciente al cinturón de estaño de la zona central de la Cordillera Oriental, donde cientos de depósitos de Sn-W-Ag, clase, definir la providencia. Para una mayor exploración de estos proyectos se recomienda realizar una cartografía detallada prestando especial atención a la mineralización conocida y alteración hidrotérmica, recolección de muestras y análisis de los elementos que pueden estar asociados con el depósito mineral de interés. La subsiguiente exploración subterránea (perforación y métodos geofísicos), permitirá a los geólogos realizar la evaluación de los depósitos minerales

Caracterización mineralógica del yacimiento Candelaria (Gallinero de Cameros, La Rioja)

At the end of the 19th century several small underground metal mines were exploited in the Cameros Massif. Now, one of them, the Candelaria ore deposit, located in Gallinero de Cameros (La Rioja), has been studied. The main objective of this study is to characterize the geology and mineralogy of this mine. A mapping of the inside of the mine has also been done.During the Titonian-Albian, there was a rifting period that caused the formation of the Cameros Basin, with the deposition of more than 6500 meters of sediments in continental and transitional environments. The mineralisation is hosted in the Tera Group, which is the first depositional sequence in which the basin is divided. After the Alpine orogeny, the normal faults that had formed during the extensional stage were reactivated as thrusts and the Cameros Massif was formed.The samples taken in the mine were analysed with optical and electronic microscope to determine the mineralogy, composition and mineral paragenesis. The host rock is mainly an organic-rich shale layer, and the microconglomerates and sandstones around it. The mineralisation consists of disseminations, fracture-filling and replacements of copper sulphides and sulfosalts, mainly tennantite, chalcopyrite and bornite, but also arsenopyrite, with supergene covellite, azurite and malachite. The arsenic-rich mineral tennantite was the first copper mineral to form, while arsenopyrite was the last one to precipitate, originating large euhedral crystals.The event has been linked to the Albian-Coniacian hydrothermal metamorphism event, although its origin remains unclear. The connate fluids and meteoric water that leaked into the sediments would have been mobilized by temperature and pressure gradients originated by the infilling of the basin. During their circulation, they would have leached the oxidized metals of the continental sediments and would have been transformed into brines and enriched in sulphur due to the interaction with the evaporites of the Keuper facies. These oxidized brines would have been driven preferentially through the faults that were originated during the rifting stage, which would work as high-permeability conduits. The precipitation of the sulphides was induced by the presence of reduced organic-rich layers, pyrite, shale clasts and carbonates, which acted as metal traps. These characteristics enable us to classify the Candelaria mine as a redbed-hosted stratiform deposit.<br /

Mecanismos de reequilibrio mineral en arseniuros de Co-Fe-Ni en Tamdrost (Bou-Azzer, Marruecos)

El yacimiento de Tamdrost, en el distrito de Bou-Azzer (Anti-Atlas Marroquí) está formado por triarseniuros, diarseniuros y sulfoarseniuros de Co-Ni-Fe, y fases minoritarias de sulfuros de Cu. Debido a que el yacimiento ha sufrido diversas etapas de fracturación, los minerales han estado sometidos a procesos de disolución y reequilibrio. Se han diferenciado 3 asociaciones: la primera asociación está compuesta por skutterudita, rammelsbergita, niquelita, miembros de la solución sólida rammelsbergita-safflorita, rammelsbergita-lollingita, rammelsbergita-lollingita-safflorita, safflorita-lollingita y lollingita. La segunda asociación la componen los minerales: skutterudita, safflorita, miembros de la solución sólida rammelsbergita-safflorita, miembros de la solución sólida cobaltita-gersdorffita, cobaltita y alloclasita. La tercera asociación está compuesta por arsenopirita, bornita y calcopirita y aparece asociada a las últimas etapas de fracturación englobando fases previas. Mientras que la primera skutterudita presenta una tendencia Ni+Fe con posibles temperaturas de formación a 650ºC, la segunda muestra una tendencia de Ni sin una clara relación con las temperaturas. Los diarseniuros de la asociación I definen por completo el campo de solución sólida a 625ºC, demostrando la no existencia del solvus previamente propuesto por Gervilla y Ronsbo (1992). Los diarseniuros de la asociación II muestran una solución sólida completa a lo largo del binario NiAs2-CoAs2, que se extiende más allá de los campos experimentales y con unas temperaturas de formación similares a las de la asociación I. La tercera asociación se formó a una temperatura de entre 440ºC y 460ºC

Geología y mineralogía del yacimiento de Serrana Segunda (Gallinero de Cameros, La Rioja)

The main goal in this work is the study of Serrana Segunda ore deposit. In order to achieve this purpose, a petrographic study (with optical and electronic microscope) of rock samples collected in the mine, chemical analysis using dispersive X-ray energy and a bibliographic search of the geological context have been doneSerrana Segunda is located in the Cameros Massif, an ancient Mesozoic basin created during an extensive period of rupture. The host rocks, Titonian in age, were deposited during an active early rifting stage and are mainly formed by fluvial and lacustrine sediments. This rifting stage generated a stress forces with normal faults systems and high subsidence rates with a thicker sedimentary infill (more than 6,500 meters). Later, with the Alpine orogeny, faults were reactivated as thrusts and folded structures. The very low- to low grade metamorphism that suffered the region was caused by thermal subsidence and the creation of two different hydrothermal systems, which resulted in the circulation of high-temperature fluids from lower levels through fault planes, fractures and permeable rocks. The host rocks are a mix of subarkose and feldspathic wacke with significant amounts of organic matter and high porosity that allows the circulation of fluids. Ore minerals occur as cements and replacements. Sedimentary-diagenetic fine-grained bornite, anilite and chalcocite were overprinted by a late hydrothermal mineralization made up of chalcopyrite and tennantite as the predominant mineralogy, along with minor arsenopyrite, galena and cobaltite-gerdorsffite. As products of a supergenic alteration azurite, malachite, thyrolite, chenevixite and stromeyerite were precipitated. Copper sulfides have crystallized replacing plant fragments or in areas with a high accumulation of organic matter.The mineralization occurs as stratabound to stratiform disseminations and is related with significant accumulations of organic matter along bedding, which served as an in situ reductant that would cause the precipitation of copper from an oxidized sedimentary brine. The key attributes of Serrana Segunda ore deposit allow us classified it as a Redbed-hosted copper model.<br /

La mina de Anglas (Pirineneos Centrales, Francia): Caracterización mineralógica y textural

Se ha estudiado el yacimiento de Anglas, un filón de Zn-Ba-(Pb) localizado en los Pirineos Centrales. Se han reconocido dos tipos de mineralizaciones: una stratabound compuesta por magnetita, pirrotita y siderita, que encaja en los hastiales devónicos, y otra filoniana que rellena una falla extensional E-W y de fuerte buzamiento. Su mineralogía está formada por esfalerita con cantidades menores de pirita, galena y calcopirita. La alteración supergénica dio lugar a un pequeño nivel de calaminas en la parte superior del filón. La comparación de este depósito con otros similares tanto en los Pirineos como en la Europa occidental indica un tiempo de formación entre el Triásico y Cretácico Inferior. La precipitación del depósito se produjo cuando salmueras calientes ascendieron del basamento Paleozoico y se mezclaron con aguas marinas y meteóricas próximas a los márgenes de una cuenca subsidiente

Modelos de alteración de cromititas ofiolíticas durante el metamorfismo

The composition of chromite is commonly used to interpret the petrogenesis and the geodynamic setting of its host ultramafic rocks. Its high resistance to alteration compared to the primary silicates has made this oxide particularly useful as a petrogenetic indicator in ultramafic rocks in which metamorphic alteration has obliterated other primary fingerprints. However, a great body of work, based on the study of chromitites from ophiolites, layered complexes, Alaskian-Ural complexes and komatiites, have shown that the chemistry and structure of chromite can be also significantly modified during both prograde and retrograde metamorphism. One of the main targets of the scientific community working on chromite in ultramafic host rocks is to know what processes may produce the alteration of chromite, and more importantly, what are the implications for the petrogenetic interpretations derived from them. Of particular interest is to develop a general alteration model to constrain the processes, mechanisms and conditions (temperature, pressure and chemical potential of the species) able to alter the chromite and to test if the alteration occurs during the prograde and/or retrograde metamorphism. In this work I will use a combination of petrological and geochemical tools to model the mechanisms that produce the alteration of chromite during metamorphism. A set of chromite-bearing ophiolite complex showing variable metamorphic pathway were selected to undertake this study. The chromitite samples were selected from chromite deposits of three ophiolite complexes: i) the Eastern Rhodope in Bulgaria (retrograde metamorphism from eclogite- to amphibolite-facies), ii) Tehuitzingo serpentinites in Mexico (retrograde metamorphism from eclogite- to greenschist-facies), and iii) Los Congos and Los Guanacos ultramafic massifs in Argentina (prograde metamorphism up to granulite-facies with amphibolite-facies overprint). Additionally, chromitite samples of unmetamorphosed ophiolites from Mercedita (Eastern Cuba) and Dyne (New Caledonia) were used as examples of unaltered chromites for comparison. Chromite in the metamorphosed chromitites from Eastern Rhodope and Tehuitzingo exhibits up to four textural types: i) partly altered chromite, with primary cores surrounded by porous chromite enriched in Cr2O3 and FeO and depleted in Al2O3 and MgO; ii) porous chromite, with a well-developed porosity and chlorite filling the pores; iii) zoned chromite, characterized by primary cores surrounded by non-porous chromite enriched in Fe2O3 (i.e. ferrian chromite), occasionally rimmed by magnetite; and iv) non-porous chromite, with polygonal mosaic-like texture. The different pattern of zoning are interpreted as a consequence of two-stage processes associated with the infiltration of reducing and SiO2-rich fluids which evolve to more oxidizing and SiO2-rich conditions. P-T-X diagrams performed for these chromitites predict that the first alteration stage took placed as a result of the reaction of primary chromite with highly reducing SiO2-rich fluids. This would result in the formation of porous chromite in equilibrium with chlorite. The theoretical model reproduces well the changes of Cr# and Mg# observed from primary to porous chromite in the natural samples. It suggest that porous chromite take place from ca. 700 to 450ºC in Eastern Rhodope while its occurs from ca. 700 to 250ºC in Tehuitzingo, during the retrograde metamorphism from eclogite- to amphibolite- and greenschist-facies, respectively. T-¿SiO2 pseudosection suggests that the addition of highly reducing SiO2-rich fluids was crucial for the alteration of chromite. I suggest that such kind of fluids could emanate from the surrounding peridotite during serpentinization when silica minerals (olivine and pyroxene) were hydrated and dissolved. The temperatures for the second stage were estimated using isotermal Al3+-Cr3+-Fe3+ sections and range between 450 to and 600ºC. These overlap the range of temperatures estimated for the first stage. This suggests that oxidizing fluids evolved from previous reducing fluids, stabilizing magnetite and form non-porous ferrian chromite by infiltration through the network of pores in the porous chromite. The analysis of a suite of minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V, Sc) in the chromite grains from Eastern Rhodope and Tehuitzingo using LA-ICP-MS in this work is revealed as a powerful tool to identify metamorphic fingerprints. Thus when the distribution of these elements in the altered chromite grains is compared with those from unmetamorphosed chromitites, I observed that partly altered chromites after primary high-Cr chromite are enriched in Zn, Co and Mn (ZCM-anomaly) but strongly depleted in Ga, Ni and Sc. This distribution of minor and trace elements is related to a decrease in Mg# [Mg/(Mg+Fe2+)] and Al2O3, produced by the crystallization of chlorite in the pores of porous chromite. Non-porous chromite is enriched in Ti, Ni, Zn, Co, Mn and Sc but depleted in Ga, suggesting that fluid-assisted processes have obliterated the primary magmatic signature. Zoned chromites have cores depleted in Ni, (Ga) and Sc but are progressively enriched in Zn and Co as Mg# and Al2O3 decrease toward the rims. They have overall lower concentrations in Ga, Ni and Sc and higher Zn and Co than the non-porous rims of ferrian chromite. Magnetite rims of zoned chromite from Tehuitzingo are strongly depleted in all minor and trace elements, denoting their subsequently formation. The complex variation of the minor and trace elements vs Fe3+/(Fe3++Fe2+) in the different types of chromite suggests a complex interplay of substitutions, linked with the ability of fluids to infiltrate the chromite and the extent of the re-equilibration between pre-existing cores and newly-formed rims. The results demonstrate that metamorphism can seriously disturb the original magmatic distribution of minor and trace elements in chromite. The abundances of these elements, and by inference the major elements, can be strongly modified even in the cores of grains that appear ¿unaltered¿ in terms of major elements. In the samples from Los Congos and Los Guanacos (Argentina) there are different textural types of chromite: i) Type I homogeneous Fe2O3-rich chromite in contact with relicts of clinopyroxene; ii) Type II, with spinel grains hosting a complex intergrowth of blebs and very fine lamellae of magnetite; iii) Type III, with symplectic texture composed of variable proportion of magnetite and spinel. Isochemical phase diagrams (pseudosection) computed for chromitite samples from Los Guanacos predict that the formation of non-porous ferrian chromite take place as a result of the infiltration of more oxidizing fluids through porous chromite probably, during seafloor metamorphism (ca. 300ºC and 4 kbar). According to the calculated phase relations, prograde metamorphism produces the reaction of non-porous chromite with chlorite to form a homogeneous chromite with higher Al2O3, Fe2O3 and MgO at ca. 780ºC and 10kbar, which according to regional studies can be associated with the granulite-facies peak conditions. The blocking temperature obtained in Type III chromite from Los Guanacos (ca. 600ºC) suggests that the exsolution of non-porous chromite in Type II and Type III chromite grains, which have higher Fe2O3 and lower Cr2O3 than those from Type I take place during the amphibolite-facies metamorphism. Partition coefficient of major, minor and trace elements (Ga, Ti, Ni, Zn, Co, Mn, V, Sc) in the equilibrium pair spinel-magnetite in Type III chromite grains from Los Guanacos indicate that Ga3+, Co2+ and Zn2+ are compatible, while Ti4+, Ni2+, V3+, Sc3+ and, lesser extent Mn2+ are incompatible in spinel. I suggest that such distribution is the consequence of the affinity of these metal ions for an octahedral or tetrahedral coordination site in the spinel structure and, therefore, their preference to form normal or inverse spinel structure. The ideal behavior of minor and trace elements in the equilibrium pair spinel-magnetite explains why the formation of non-porous chromite rims, with inverse spinel structure, modified the composition of cores from zoned chromite, with normal spinel structure

Caracterización mineralógica, textural y química de la mina "El Fraile", Cordillera Ibérica

The goals of this work are to perform a mineralogical, textural and chemistry characterization of the El Fraile mine (Iberian Range) and to make a comparison between several similar ore deposits. In order to achieve these objectives, different techniques and methodologies (field work, optical microscopy and field emission scanning electron microscopy) have been carried out. The El Fraile mine consists of a mineralization vein-type that fills a trend fault E-W with an approximate length of 150m. It is hosted by Cambric-Ordovician materials. The host rock is a quartzarenite with bioclasts. The mineralogy consist on primary minerals as galena (PbS), bournonite (PbCuSbS3) and minor amounts of sphalerite (ZnS) and plumosite (~Pb2Sb2S5). The gangue minerals are quartz and siderite, along with secondary minerals as anglesite (PbSO4) and covellite (CuS). From a textural and chemical point of view, two types of galena have been recognized: the earlier (Gn1), with an almost stoichiometric composition and related with bournonite, and the later (Gn2), Sb-rich and formed by replacement of plumosite. Taking into account the mineral assemblage, the form and structure of the deposit, as well as the host rock lithology, a comparison with the classification proposed by Gumiel and Arribas (1987) for the antimony deposits in the Iberian Peninsula has been made. Although there are several significant differences, the Fraile mine could be compared to the association quartz-Pb-Sb-Ag. Furthermore, García et al. (1988) studied two similar deposits close to El Fraile mine, Leonor and Aragón mines, stablishing four depositional stages, with a different mineral assemblage from that studied in this work, which could indicate either a different hydrothermal system or sampling problems