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This paper describes and interprets the mineral and facies assemblages that occur in carbonate–evaporite shallow lacustrine deposits, considering the importance of the processes pathway (i.e. dolomitization, gypsum calcitization and silicification). The Palaeogene deposits of the Deza Formation (Almazán Basin, central‐northern Spain) are selected as a case study to determine the variety of physicochemical processes taking place in carbonate–evaporite shallow lakes and their resulting diagenetic features. Dolostones are the predominant lithology and are composed mainly of dolomite with variable amounts of secondary calcite (5 to 50%), which mainly mimic lenticular gypsum (pseudomorphs). Five morphological types of dolomite crystal were identified as follows: dolomite tubes, dolomite cylinders, rhombohedral dolomite, spheroidal and quasi‐rhombohedral dolomite, and cocoon‐shaped dolomite. The dolomite cylinders and tubes are interpreted as the dolomitized cells of a widespread microbial community. The sequence of diagenetic processes started with growth of microlenticular interstitial gypsum in a calcareous mud deposited on the playa margin mudflats, and that sometimes included microbial sediments. Immediately following growth of gypsum, dolomite replaced the original calcite (or possibly aragonite) muds, the microbial community and the gypsum. Partial or total replacement of gypsum by dolomite was related mainly to the biomineralization of endolithic microbial communities on gypsum crystals. Later calcitization took place under vadose, subaerial exposure conditions. The development of calcrete in distal alluvial settings favoured the release of silica and subsequent silicification on the playa margin mudflats. Stable isotope compositions of calcite range from −9·02 to −5·83‰ δ13CPDB and −7·10 to 1·22‰ δ18OPDB; for the dolomite, these values vary from −8·93 to −3·96‰ δ13CPDB and −5·53 to 2·4‰ δ18OPDB. Quartz from the cherts has δ18OSMOW values ranging from 27·1 to 31·1‰. Wide variation and relatively high δ18OSMOW values for dolomite indicate evaporitic and closed hydrological conditions; increased influx of meteoric waters reigned during the formation of secondary calcite spar., This research was funded by the Spanish Ministry of Economy and Competitiveness, projects CGL2011‐27826‐CO2‐02 and CGL2014‐54818‐P.

The volcanic rocks of the Cabo de Gata region have been transformed by hydrothermal alteration, forming in some areas, highly silicified and feldspatized rocks. Various samples from outcrops of unaltered and hydrothermally altered volcanic rocks were studied by conventional transmitted-light microscopy to determine their genesis (magmatic, inherited or neoformed) and scanning electron microscope cathodoluminescence to define their spectral features. The magmatic quartz shows emission bands at 400, 440, and 480nm., and these bands disappear or decrease in the secondary hydrothermal quartz, characterized also by an intense emission band at around 570–580 nm, and another around 460 nm. The inherited magmatic plagioclase and neoformed anorthoclase have some similar emission bands and their genesis is not marked by cathodoluminescence features. The magmatic K-feldspars (sanidine), have emission bands at around 425, 440, and 490 nm, constituting a strong blue emission (420–500 nm). This emission is wea...

This study reports the mineralogical and geochemical variations of surface saline deposits and their substrates along an arid valley with an ephemeral river system (Spanish: rambla) affected by pollution from the now closed Cabezo de San Cristobal mine. This mine, which is close to Mazarron, Spain, once produced Pb, Zn, Ag and Fe ores. Saline efflorescences and crusts were found on piles of tailings and on the mudflats surrounding acidic ponds associated with the mine. These were composed of jarosite (10–60%), haematite (up to 20%) and gypsum (5–20%), and the kieserite (20–40%) and halotrichite (5–40%) mineral groups. In the rambla deposits downstream of the mine, the mineralogy of the saline efflorescences was similar to that recorded upstream: gypsum up to 20%, thenardite 5–20%, halite 20–40% and bloedite 20–40%. However, the substrate was very different, being mainly composed of jarosite (40–60%), haematite (5–20%) and goethite (5–20%); these minerals were never found upstream of the mining area, where carbonates predominated. Downstream of the mine, the rambla deposits contained large quantities of Fe (158,251 ppm), Pb (2,720 ppm), Zn (3,064 ppm), Sb (240 ppm) and As (1,092 ppm), along with Co (29 ppm), Ce (323 ppm), V (112 ppm), Th (23 ppm) and Nb (19 ppm) (all figures are maxima). Upstream, these contents were insignificant. Neither were large quantities found in the actual stream water (only Cu, V and Zn were detected, and then at concentrations of

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 µm) 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.

Unusual mineralized and silicified carbonate-rich geodes were found hosted in Lower Triassic red mudstones in Central Spain. From their borders to their centres, the geodes display (a) a millimetric carbonate crust, (b) a quartz rim, (c) massive dolomite, (d) gold-bearing iron-rich infills (hematite laths and goethite with up to 7000 ppm of gold) and (e) calcite cements that sometimes seal the central hollow. Textural evidence indicates that the geodes were originally anhydrite, which was subsequently replaced by quartz and dolomite. The resultant porosity from this replacement, or by later dissolution, has been filled by epithermal gold-bearing iron-oxide hydroxides, romanechite and calcite.δ13C values indicate the participation of meteoric waters in an environment which was characterized by both a sub-desert climate and a temperate–tropical climate. Oxygen signatures reflect very variable temperatures for all minerals, with the exception of calcite, which appears to have precipitated at

Varieties of chalcedony (calcedonite, quartzine, and lutecite) and other optical micro- and cryptocrystalline textures of quartz and opal from Miocene silcretes of the Madrid Basin were studied using a Thermo Fisher DXR Raman microscope. The microscope has a point-and-shoot Raman capability of 1 µm spatial resolution and was coupled with a standard optical microscope. Our results show that all the varieties of chalcedony can have a composite Raman spectrum of both quartz and moganite. The spectra are independent of the chalcedony origin by ageing, direct replacement, and cementation processes. Moganite was absent only in some calcedonite cements. The presence of moganite is independent of the surface sedimentary setting of the host rocks in which the silcretes are formed

The xenoliths of dunites and other ultramafic rocks (peridotites ,wherlites..etc) are generated by contact metamorphism from sedimentary rocks, proyecto FEDER "Aspectos educativos del volcanismo canario Ministerio Educación Y Ciencia proyecto BTE2002-04017-Co2-1

7 páginas, 3 figuras, 3 tablas., Green opals have been identified in veins and vugs in hydrothermalized basalts from the area of Taganana (Tenerife). The aim of this paper is to document the petrology and geochemistry of these opals and to describe the formation process from amorphous components. Green opals were studied using optical microscopy, powder X-ray diffraction, electron microprobe, atomic absorption spectroscopy and induced coupled plasma mass spectrometer. Our textural and X-ray powder diffraction analyses indicate that green opals are composed of a major groundmass fraction of crystalline (opal CT and celadonite) and amorphous components (opal A and Fe-(hydro)oxides), which host minor siderite and dolomite crystals. The results indicate green opals were formed during the hydrothermal process from a Fe-rich silica amorphous phase (pseudoglass) which aging and alteration transformed into a groundmass of celadonite and opaline mixture rich in Fe-(hydro)oxides. The opaline–celadonite groundmass, or its amorphous precursor, served as a substrate for the growth of siderite and dolomite crystals., The research was supported by the project PB-98-0668-CO2-01 and the project about the research of terrestrial models for the exploration of Mars of the Centro de Astrobiologia (CSICINTA, Associated to the NASA Astrobiology Institute).

The radiolarite sequence of the middle Subbetic (Betic cordillera, southern Spain) is composed of three main lithologies: radiolarite, siliceous mudstone and marl, and pelagic limestone with radio larians. Virtually all transitional lithologies among the three listed exist. Clay minerals, calcite, and hematite make up the radiolarite impurities. Smectite, illite, and locally kaolinite are the clay minerals present, smectite always being most abundant. No radiolarite bed with more than 85% silica has been found; the total SiO2 content ranges from 61 to 85%. Frequently the compact appearance of the radiolarite results from a high carbonate content. The ratios Si/Si + Al + Fe + Ca, Si/Si + Al + Fe, and Al/Al + Fe + Mn are calculated and used to compare the beds of couplets in rhythmic sequences and to obtain the relative distance of sections from their terrigenous source. These three chemical indices do not give consistent results. Fe enrichment of radiolarite relative to siliceous mudstone is probably related to the low mean sedimentation rate of these sequences from the Subbetic (3.5 mm/1,000 y). Pelagic settling in a basin with irregular bottom topography and with areas topographically protected from sediment input is proposed for deposition of these deposits. Productivity cycles and terrigenous dilution gave rise locally to rhythmically bedded sequences, usually composed of radiolarite mudstone alternations. Locally, contour and distal dilute turbidity currents reworked and deposited the sediment, and gave rise to thin sequences of layered sedimentary rocks and to laminated beds. Deposition above a relatively shallow CCD is deduced, and a water depth shallower than previously thought is suggested for these deposits, which are considered as the deepest water deposits of the Betic External Zones.