Cherts with moganite in continental Mg-Clay deposits: an example of "False" Magadi-type cherts, Madrid basin, Spain.

During the Miocene the Madrid basin was a closed basin in which a succession of alluvial and lake deposits were replaced by different types of crusts (mainly calcretes, dolocretes, and silcretes). In a geographically restricted area to the south, Mg-clay deposits were precipitated in ponds on the sand flats. A discontinuous nodular and stratiform Mg-rich chert appears at the top of the Mg-clay deposits and displays visual, mineralogical, and petrological characteristics that are very different from the other silcretes of the Madrid basin. As revealed by X-ray diffraction, the cherts consist mainly of quartz and moganite, which are commonly found in Magadi-type cherts. The petrological characteristics reveal that the chert consists of a groundmass crossed by several networks of quartz veins. The groundmass is formed by mosaic quartz with different sizes of crystals (usually less than 20 mm) and length-slow chalcedony. The presence of gridwork extinction patterns, the differentiation of silica flakes, and the preservation of the micromorphological characteristics of the Mg-clay deposits indicate an early replacement by quartz and moganite. The quartz networks show veins of different shapes (rectilinear, irregular, branched, and V-shaped), sizes (from a few microns to several millimeters), and textures (mainly megaquartz, microquartz, and length-slow chalcedony). The V-shaped veins, and some irregular and branched veins, are considered to be a consequence of the silicification of cracks in the host rocks. The other veins represent silica filling cracks that formed by shrinkage during the various stages of chert formation. Many macroscopic attributes (e.g., some kinds of cracks), mineralogical features (the presence of moganite), and petrological characteristics (e.g., rectilinear or gridwork orientation of quartz crystals) are similar to those of Magadi-type cherts. In spite of this, the origin and the environment of formation of this Mg-rich chert, which crystallized by silicification of Mg-clays during deposition of silcrete, are very different from those of the typical Pleistocene Magadi cherts of African lakes. A pronounced shrinkage during silicification in a zone just below the water table and the presence of the moganite are the two factors that caused these Mg-rich cherts to appear similar to Magadi-type cherts

Diagenetic and hydrothermal transformations in marine biogenic-rich (siliceous and calcareous) deposits (Níjar, Almeria)

En este trabajo se aborda un estudio petrológico detallado de depósitos diatomíticos con ópalos, intercalados con biocalcilutitas/biocalcarenitas en dos secciones de la Cuenca de Níjar. Estos depósitos se encuentran asociados a rocas volcánicas, o están atravesados por venas opalinas hidrotermales. El objetivo es conocer la composición de los depósitos y establecer las diferencias entre las transformaciones diagenéticas y las hidrotermales. Las secciones estudiadas, por su composición litológica (biomicritas ricas en microforaminíferos, diatomitas y ópalos), ausencia de sapropeles y existencia de bioturbación, podrían atribuirse a la unidad inferior del Miembro Abad (Messiniense inferior). La diagénesis de las biomicritas y diatomitas provocó la formación de nódulos y capas opalinas (transformación ópalo A- ópalo CT), así como la aparición de calizas diagenéticas como consecuencia de procesos de calcitización de diatomeas facilitados por la presencia de CO2 y Ca provenientes de la silicificación de los microfósiles carbonáticos. Las venas opalinas, que incluyen sepiolita, se interpretan como relleno de fracturas y fallas por fluidos hidrotermales de baja temperatura. La sepiolita precipitaría inicialmente siendo posteriormente silicificada por ópalo. Este reemplazamiento de la sepiolita por ópalo generó la fuente de Mg para la dolomitización de las capas opalinas diagenéticas que son atravesadas por venas hidrotermalesA detailed petrological study of diatomites interlayered with biocalcilutites / biocalcarenites and opals (two sections) was performed in the Níjar Basin (SE Spain). The studied sections are associated with volcanic rocks, or they are crossed by faults and fractures with hydrothermal opaline veins. The objective of this work is to know the composition of the deposits, and to establish the differences between the diagenetic and hydrothermal transformations. The lithological composition of the sedimentary deposits (foraminifera-rich biomicrites, diatomites and opals), the absence of sapropels and the presence of bioturbation, could indicate that they can be attributed to the lower unit of the Abad Member (early Messinian). The diagenesis of biomicrites and diatomites caused the formation of opaline nodules and beds, by aging from opal A to opal CT. The silicification of carbonate microfossils produced the calcitization of the diatoms and the formation of diagenetic limestones because CO2 and Ca were released. The opal-sepiolite veins are interpreted as the result of regional low temperature hydrothermalism. Sepiolite was initially precipitated in fractures and faults, and was subsequently silicified by opal. The sepiolite-opal replacement generated the source of Mg for the dolomitization of the opaline diagenetic layers affected by hydrothermal vein

Bacterial dolomite in ephemeral lacustrine carbonate-gypsum facies (Deza Formation, upper Eocene,Almazán Basin)

The lower sequence of the Deza Formation represents an ephemeral carbonate lacustrine environment revealed by the presence of limnic fossil and evidence of dry periods marked by the presence of numerous carbonate pseudomorphs, after lenticular interstitial gypsum crystals. In this work, the results of a petrological study of some dolomite facies in the lower sequence are shown to highlight the characteristics of dolomite crystals and to demonstrate their origin by biomineralization. The dolostones have massive or grumelar texture, with microfossils (gastropods and charophytes less than 2%) and carbonate pseudomorphs (calcite or dolomite) after interstitial lenticular gypsum crystals (20-60%); they have always palyigorskite as cement and may have undergone dedolomitization and silicification. Under SEM dolomite crystals are cylindrical, with sizes up to 40 microns in high. Due to their concentric inner vacuolar structure, high fluorescence, and the occasional occurrence of "dumbbell" forms, the dolomite is interpreted as formed as a result of sulfate reducing bacteria and EPS activity. This dolomite occurred in a micrite/biomicrite mud with interstitial gypsum crystals formed during drought periods. The biodolomite was formed at the expenses of the original calcareous matrix by replacement of gypsum crystalsLa secuencia inferior de la Formación Deza representa un ambiente carbonático lacustre efímero puesto de manifiesto por la presencia de fósiles límnicos y evidencias de desecación periódica marcadas por la existencia de numerosos pseudomorfos carbonáticos de cristales lenticulares de yeso intersticial. En este trabajo se muestran los resultados del estudio petrológico de determinadas facies dolomíticas de la secuencia inferior para resaltar el carácter de los cristales de dolomita y demostrar su origen por biomineralización. Las dolomías presentan textura masiva o grumelar con menos del 2% de microfósiles (gasterópodos, y caráceas) y entre un 20 y un 60% de pseudomorfos carbonáticos (calcita o dolomita) de cristales de yeso; siempre tienen paligorskita como cemento y pueden presentar evidencias de procesos de dedolomitización y silicificación. Los cristales de dolomita en MEB son cilíndricos con tamaños de hasta 40 µm de altura. Debido a su estructura interior vacuolar concéntrica, alta fluorescencia, y en ocasiones, su forma “en mancuerna” se interpreta que se originaron en relación con bacterias sulfato-reductoras y sus EPS. Esta dolomita precipitó en una micrita/biomicrita con cristales de yeso intersticial formados en épocas de emersión. La biodolomita se formó por remplazo de la matriz calcárea original y del yes

Cristobalite formation in altered andesites (Los Escullos, Almeria)

El presente trabajo aborda el estudio de la composición mineralógica y petrológica de un afloramiento de andesitas alteradas en la región volcánica de Cabo de Gata (Los Escullos), mediante Difracción de rayos X, Microscopía Óptica (MO) y Electrónica (MEB) con Energía Dispersiva. El objetivo es determinar los procesos de alteración que fueron capaces de formar gran proporción de cristobalita, independientemente de los procesos de silicificación que generaron venas y diques silíceos en este afloramiento. La microscopía óptica revela que la alteración afectó solo a la matriz de la andesita, quedando los fenocristales (anfíbol, biotita, plagioclasa y opacos) preservados. La matriz está constituida mayoritariamente por cristobalita y pequeñas cantidades de zeolitas y esmectitas, que se formaron posteriormente a la cristobalita. Las maclas de cristales de cristobalita con hábitos cuadrangulares y tetragonales, identificados en MEB, sugieren que pudo haber una precipitación en fase de vapor. Se considera que la cristobalita se constituye por un proceso de desvitrificación, planteando la hipótesis de que también pudo existir cristalización de cristobalita en fase de vapor, durante la percolación de gases en el comienzo del enfriamiento del vidrio volcánicoThis paper studies the mineralogical and petrological composition of an outcrop of altered andesite in the volcanic region of Cabo de Gata (Los Escullos), using X-ray diffraction, optical microscopy and scanning electron microscopy (SEM) with energy dispersive analyses. The aim is to determine the alteration processes that were able to develop a large amount of cristobalite in the andesites. This cristobalite grew independent of the silicification that generated siliceous veins and dikes in this outcrop. Optical microscopy reveals that the alteration affected only the andesite groundmass, leaving the phenocrysts (amphibole, biotite, plagioclase and opaques) preserved. The paste is mainly composed of cristobalite and small amounts of zeolites and smectites, which were formed in a later stage. The twinnings of cubic and tetragonal cristobalite crystals, identified in SEM, suggest that they might be formed as a vapour-phase. It is considered the cristobalite was formed by devitrification and perhaps by vapour-phase mineralization and that may be necessary a cristobalite precursor formation, in order to provide the nuclei. This process would occur during the percolation of gas at the start of cooling of the glass in the andesitic flo

Differential features and genesis of silica levels mined in the Casa Montero archaeological site. (Vicálvaro, Madrid)

The research during the construction of the M-50 highway has allowed the study of the silica levels that constitute the most important Neolithic flint mine in the Iberian Peninsula. A group of more than 3.800 extraction pits were realized to obtain cherts and opals that appear among Mg smectites beds (Aragonian deposits). The description of the nodular and stratiform silica levels and the study of their types and distribution have been done at geological sections obtained during the archaeological excavations. Three petrological types of siliceous rocks (chert, opal and opaline chert) are established and defined according to their mineralogy and microfacies. Cherts are constituted only by quartz and locally minor amounts of moganite. Opals contain opal CT, Mg smectites and minor quartz. Opaline chert is defined when the proportion of quartz is higher than 50%. All the types of siliceous rocks reproduce bioturbation structures as well as other structures found in the host rocks (grains, intraclasts, brecciation). The silica levels at the base of the columns are constituted only by chert and were formed by the silicification of nodular and lenticular dolomite levels included in the Mg-smectites beds. The rest of the siliceous levels are composed of opals and opaline cherts and were formed by silicification of Mg-smectites. The outer part of the opaline chert has a higher content in opal and clay relicts whereas the inner part is mainly microcrystalline quartz. This change is consequence of the recrystallization of the opal (aging) producing a more compact silica rock. The main silicification occurred under the groundwater table because the structure of the host rocks is conserved in the silica rocks. A general question is whether the silica that precipites is introduced by groundwater or it is released from the dissolution of clays during their replacemen

Silicification and quartz types in volcanic rocks (Cabo de Gata volcanic region)

The original mineralogical and geochemical compositions of andesitic volcanic rocks from SE of Spain have been modified by different processes, under hydrothermal environment. In particular, silicification process increases the initial silica concentration of the volcanic rock from 69 % to 97%. One of the characteristics of this alteration is the breakdown of mafics minerals and feldspars with supply of Al, Fe, Mg…etc to the interstitial fluids which can be later introduce as traces in the new quartz formed. The silica rocks created in the process are mainly constituted by quartz with different crystal sizes and textures. The spectral analysis of the cathodoluminiscence (CL) emission in combination with the Scanning Electron microscopy observations show that the igneous quartz crystals of the volcanic rock are characterized by a dominant band at 2,95 eV (420 nm) while the quartz crystals formed by silicification have a dominant band at 2,54 eV (~ 500 nm). Although it is difficult to identify the causes of these spectral differences, it is considered that the 420 nm emission band is due to native defects of the igneous quartz and the ~ 500 nm emission band is due to the substitutional incorporations of impurity ions in the hydrothermal quartz crystals. The SEM-CL imagines show a non uniform luminescent emission. This is interpreted in terms of either microsilicifications affecting the quartz crystals and the groundmass of the volcanic rocks or as consequence of the pseudomorphic replacements by quartz of the primary minerals. The hydrothermal silicification is an early process that increase the amount of silica of the initial andesitic rock making other petrological and geochemical types of volcanic rock

Silicification and dolomitization of anhydrite nodules in argillaceous terrestrial deposits: an example of meteoric-dominated diagenesis from the Triassic of central Spain

Cauliflower-shaped nodules are widespread in a single red mudstone bed in the Buntsandstein (Triassic) facies of the Iberian Range. They consist mostly of quartz, dolomite and calcite, but other minerals, such as barite, kaolinite and iron oxyhydroxides, are also present. The nodules are spherical, ovoid or elongate in shape and range from 1 to 8 cm across. The surface of the nodules is irregular, and some show a pedogenic coating of microspar. The sedimentological and petrographic data suggest that the initial anhydrite nodules formed through a progressive increase in the porewater concentration of Ca2+ and SO4 2– in a vadose environment, occasionally under the influence of pedogenic processes. Partial replacement of the anhydrite by megaquartz occurred under more dilute conditions in the same type of setting, as indicated by the presence of organic filaments on the quartz crystal surfaces. In type A nodules, the dissolution of the innermost anhydrite was complete, and different types of quartz cement filled the porosity. Fracturing and meteoric cementation by calcite and minor amounts of kaolinite were the latest processes affecting these nodules. In type B nodules, the dissolution of the anhydrite was incomplete, inhibiting quartz cementation and enabling later dolomitization of the anhydrite. Dolomitization appears to have been driven by sulphate reduction, as indicated by the presence of bacterial bodies within the dolomite crystals. Dedolomitization and precipitation of barite, kaolinite and calcite spar cements occurred later under the influence of meteoric solutions. The nodules may mark the former locations of the water table and provide evidence for an episode of highly evaporitic conditions throughout wide areas of the basin. Their occurrence reveals not only a complex diagenetic history but is also indicative of palaeogeographic and palaeoclimatic conditions

Sedimentología y diagénesis de las secuencias lacustres someras en el área del yacimiento neolítico de Casa Montero (Vicálvaro, Madrid)

The sedimentary succession at Casa Montero flint archaeological site belongs to the green-clays unit of the Intermediate Unit of the Madrid Basin. A succession of silt-clay (mainly Mg-smectites), dolomite and silica rocks has been studied to know the sedimentary environment and diagene- tic processes around the site. The succession consists of silt-clay and carbonate sequences that represent a lacustrine environment with subaerial exposure events. The exposition events led to the development of a enlarged pseudomicrokarst in the topmost carbonate level. Vertisols were reworked at the top of the sequence. The diagenetic processes include early dolomitization of palustrine carbonates, partial transformation of the Mg-smectites and silicification of carbonates and clays in a groundwater environment. The presence of the archaeological site is due to the presence of opaline cherts with "aging" to quartz, that are uncommon in the silt-clay sequences of the Madrid Basin. The "aging" give these flints optimum properties for the Neolithic industry