Authigenic Assemblages Research Articles

Origin of Upper Cretaceous marine ironstones of Ayat Formation (Turgay depression, Northern Kazakhstan)

This work studies the mineralogical and geochemical features of ooidal ironstone formation of the Upper Cretaceous Ayat Formation located in the Turgay depression (Kazakhstan). The origin of ooidal ironstone deposits has been the subject of many discussions for a long time. These results present diagenetic conditions of the marine ironstone precipitation in the Ayat Formation. Various authigenic minerals, such as siderite, glauconite, goethite, pyrite, hydroxylapatite, wurtzite, and barite indicate fluctuations in the different geochemical conditions during the diagenesis of marine sediments of the Ayat Formation. The precipitation of in situ pyrite and wurtzite was controlled by an oxygen-deficient environment at the interface between water and sediment, which was accompanied by bacterial sulphate reduction under seabed conditions and, in consequence, by the concentration of sulfides. The authigenic assemblage of siderite, wurtzite, and barite indicates an integrated process of their input to site the sedimentation. The stable isotopic composition of siderite supports the microbial carbonate origin by decomposing the organic matter. However, geochemical features (the pattern Co/Zn vs (Co + Ni + Cu)) evidence the hydrogenous precipitation of iron. It involves the interaction of two central in situ processes to form Ayat ooidal ironstones. One of them is the iron precipitation from the bottom water, and the second is the microbial generation of hydrocarbons. Marine Ayat and channel Lisakovsk ironstones have similar features of bulk rock geochemistry. It reflects the exact environments of the ooidal iron ore formation, while the wall rocks have distinctive facies conditions.

Depositional and diagenetic controls on porosity evolution in sandstone reservoirs of the Stuttgart Formation (North German Basin)

The Mesozoic succession of the North German Basin comprises permeable sandstones operated by ‘conventional’ open doublet systems for geothermal heat production. The high permeability, exceeding 6 Darcy at individual localities, is the result of undercompacted grain fabrics, low abundance of authigenic minerals and large secondary porosity volumes. The discrepancy of undercompacted grain fabrics and porosity volumes of >20% at depth of up to 2500 m gave rise to a basin-scale study, in which classical petrographic methods were employed to reconstruct the diagenesis of sandstones on the example of the Stuttgart Formation (Schilfsandstein, Late Triassic). Associated to the deposition of sandstones in distinct fluvio-deltaic environments, four diagenetic pathways are indicated by systematic variations of detrital and authigenic assemblages. These four pathways are herein introduced as Delta Channel, Delta Plain, Fluvial Channel, and Floodplain Diagenesis Types. In response to the history of the North German Basin, high permeabilities of sandstones of the Delta Channel and Fluvial Channel Diagenesis Types are related to the subsequent control of depositional and diagenetic regimes on porosity evolution. Pervasive eogenetic calcite cementations contributed to preservation of pre-burial intergranular volumes and effectively prevented the mechanical compaction of grain fabrics during late Triassic–late Jurassic burial diagenesis. The uplift from deep to shallow burial depth, associated to latest Jurassic–Cenozoic structural differentiation and inversion of the North German Basin, triggered substantial dissolution processes that mainly affected carbonate cementations. This opened pores and contributed to the evolution of secondary porosity, in particular in sandstones of the Delta Channel and Fluvial Channel Diagenesis Types. Moderate post-inversion reburial rates resulted only in negligible mechanical compaction and reduction of secondary porosity. The improved knowledge of diagenetic pathways, herein exemplified by sandstones of the Stuttgart Formation, will significantly contribute to improved predictions of Mesozoic hydrothermal reservoir in the North German Basin.

Mineralogy, Geochemistry and Genesis of Zeolites in Cenozoic Pyroclastic Flows from the Asuni Area (Central Sardinia, Italy)

Natural zeolite occurrences have been recognized in several Cenozoic pyroclastic deposits in central Sardinia. This study concerns the mineralogical and geochemical characterization of the zeolitized tuffites in the Asuni area (Oristano province) and aims to complement information regarding the zeolitization processes developed in the nearby Allai deposits. Optical and scanning electron microscopy, X-ray powder diffraction, qualitative vs. quantitative microanalyses and bulk-rock geochemistry were performed. Analytical results allow defining the mineral distribution, textural relationships and geochemical features of the zeolite-bearing rocks. The most abundant secondary minerals are Ca-Na mordenites. Contrarily to the most common worldwide clinoptilolite + mordenite paragenesis, mordenite is dominant and occurs in different morphologies, rarely coexisting with clinoptilolite in the studied volcanic tuffites. Glauconite and dioctahedral smectite complete the authigenic assemblages. The primary volcanic components mostly include plagioclase, quartz and glass shards, roughly retaining their original appearance. The tuffites range in composition from dacite to rhyolite. The collected dataset shows that zeolitization is most abundant in coarser-grained deposits and points to a genetic process that mainly involves an open hydrothermal environment governed by aqueous fluids with significant marine component, in post eruption conditions.

Mineral and chemostratigraphy of a Toarcian black shale hosting Mn-carbonate microbialites (Úrkút, Hungary)

Toarcian black shale that hosts Mn-carbonate microbialites at Úrkút, Hungary was investigated by mineralogical, inorganic, and organic geochemical methods for characterization and comparison with other European black shales representative of the Toarcian Oceanic Anoxic Event. Based on the authigenic mineral composition, calculations were made to estimate environmental conditions during sediment accumulation and early diagenesis. Geochemical and petrographic results of organic, carbonate, and REE multiple-proxy analyses revealed a strong congruence between the host black shale and the Mn-carbonate ore beds. The Úrkút black shale is really a gray shale with moderate to low TOC contents that accumulated in a starved basin. The organic matter and anoxic characteristics resulted from rapid accumulation of organic matter from microbial booms, accompanied by a geothermally generated hydrothermal circulation system, and a high rate of authigenic mineral formation (clay minerals and proto-ore minerals). The inferred enzymatic Mn and Fe oxidation blocked carbonate formation by decreasing the pH. The system remained suboxic via syngenetic mineral accumulation (Fe-rich biomats), and became anoxic during diagenesis in conjunction with pyrite generation. The separation of black shale beds and Mn-ore beds is not distinct through the section. Instead, a distal hydrothermally induced clay-rich authigenic assemblage (marlstone) best describes the black shale, in which Mn-oxide proto-ore beds (Mn-rich laminae) formed from the beginning of black shale deposition, when the oxygen supply in the sedimentary basin was insufficient for enzymatic Mn(II) oxidation. Mn-oxide proto-ore was transformed to Mn-carbonate ore during microbially mediated processes during early diagenesis. The drivers for Mn-bearing organic matter-rich marlstones were most probably a combination of regional and local processes, with generation of a tectonic rift system that promoted geothermally generated hydrothermal fluids, which initiated microbial blooms. Black shale mineralogy, geochemistry, and organic matter at Úrkút differ from those of the epicontinental shelf black shales of the Tethyan Ocean.

Element Distribution Between Coexisting Authigenic Mineral Phases in Argillic and Zeolitic Altered Tephra, Olduvai Gorge, Tanzania

AbstractThe current study demonstrates how co-existing zeolite and clay minerals formed by the alteration of tephra in a closed-basin lacustrine and lake-margin environment can retain the overall composition of the original bulk tephra for many elements, even when diagenetic conditions and resulting authigenic mineral assemblages change. Zeolite and clay minerals co-exist in the closed-basin, saline-alkaline lacustrine altered tephra of Pleistocene Olduvai Gorge, Tanzania, and their diagenetic histories can be reconstructed using variations in their textures and compositions. The authigenic minerals in the altered tephra of the Olduvai paleolake form a classic ‘bull’s-eye’ pattern, with clay-dominated tephra in the distal lake margin, chabazite and phillipsite in the proximal margin, and phillipsite ± K-feldspar in the intermittently dry lake and lake center. Fifteen representative samples of altered volcanic ash lapilli (designated Tuff IF) were analyzed by X-ray diffraction (XRD), X-ray fluorescence (XRF), electron probe microanalysis (EPMA), and scanning electron microscopy (SEM) to determine their authigenic mineral assemblages and bulk compositions, and to texturally and compositionally compare their clay mineral and zeolite components.Textural observations indicate that clay minerals formed first, followed by zeolites and finally feldspars. Clay minerals, however, persist even in the most altered samples. The overall composition of Tuff IF shows only limited change in Fe, Si, Al, and Na between fresh, clay-altered, and zeolite-dominated diagenetic environments, despite significant differences in authigenic assemblage. Where zeolites dominate the assemblage, the remaining clay minerals are rich in Mg, Fe, and Ti, elements that are not readily incorporated in zeolite structures. Where clay minerals dominate, they are more Al-rich. A ‘mixing model’ combining clay-mineral and zeolite compositions yields a close approximation of the original volcanic glass for most elements (exceptions including Mg, Ca, and K). This initial composition was preserved in part by the redistribution of elements between co-existing clay minerals and zeolites.

Chloritization of Late Ordovician K-bentonites from the northern Baltic Palaeobasin–influence from source material or diagenetic environment?

The Baltic Ordovician–Silurian sedimentary succession embodies numerous altered volcanic ash beds that are illite–smectite dominated. Only a few beds rich in chlorite–smectite are known from the Upper Ordovician Pirgu Stage of Estonia. Mixed-layer chlorite–smectite occurs commonly in low-grade metamorphic and hydrothermal environments. However, chloritic K-bentonites of the Pirgu Stage have never been buried deeply and lack signs of metamorphic overprinting. In order to understand the origin of chloritization, three distinct beds were sampled in 14 drillcores from Estonia and Latvia and analysed by means of XRD and SEM. The principal authigenic assemblage of the bulk samples consists of chlorite–smectite (corrensite) together with illite–smectite and K-feldspar. The actual mineral composition of K-bentonites though varies from sample to sample. The clay mineral assemblages range from virtually pure chlorite–smectite to illite–smectite dominated assemblages with minor or no chlorite–smectite. The proportion of chlorite–smectite in K-bentonites shows systematic lateral variations: the share of chloritic phases is highest in the shallower-water part of the palaeobasin and decreases towards the deeper part of the basin. Such regular lateral variations suggest a possible link between chloritization and the configuration of ancient palaeobasin. The present study suggests that the chloritization of primary felsic ashes occurred during early diagenesis and that it was caused by an influx of Mg-rich water probably from a marine sabkha-type environment.

Origin and migration timing of hydrothermal fluids in sedimentary rocks of the Paraná Basin, South America

Clay authigenesis induced by migrating hydrothermal fluids related to the Early Cretaceous Serra Geral magmatism in the Paraná Basin was investigated by mineralogical, geochemical and isotopic means. Stratigraphically equivalent sedimentary samples from the Permian Rio Bonito (in southeastern Brazil) and San Miguel (in eastern Paraguay) formations were collected close to and far from major intrusive bodies. The rocks are generally cemented, in addition to the clay material, by a variety of minerals, such as albite, K-feldspar, calcite, dolomite, zeolite and quartz. The authigenic clay minerals mainly consist of different types of illite-rich mixed layers illite–smectite and chlorite. Corrensite and smectite were also observed. Measured and estimated K–Ar ages indicate that the varied authigenic illite assemblages were most probably generated 140 to 130 Ma ago, during the Serra Geral magmatic event. Hydrothermal fluids depleted in LREE and sometimes enriched in Eu promoted the formation of the well-crystallized illite minerals in rocks characterized by high water / rock ratios. Additional authigenic assemblages with PAAS-type REE patterns probably formed in less porous rocks with low water / rock ratios. Also, some of the authigenic clays yield HREE-depleted patterns, which point to the occurrence of organic matter during crystallization.

Mineral evolution and element mobility during episyenitization (dequartzification) and albitization in the postkinematic Bohus granite, southwest Sweden

The petrography and geochemistry of two subsurface occurrences of pervasively albitized and episyenitized (i.e. dequartzified) granite in the central part of the postkinematic Sveconorwegian Bohus granite, in southwestern Sweden, were studied. The altered granite is whitened and consists of an albite-microcline framework with a very fine-grained interstitial assemblage of illite + hematite + ankerite ± anatase. Textural relations indicate that the initial dissolution of magmatic quartz, accompanied by albitization of plagioclase, left a vuggy reservoir rock, which facilitated fluid penetration and thereby further alteration. The subsequent stage is characterized by infilling of vugs by the very fine-grained authigenic assemblage and by concomitant alteration of igneous minerals such as magnetite, ilmenite and biotite. Deuteric alteration products of magmatic minerals are affected by these two alteration stages, implying that episyenitization occurred widely after emplacement and cooling of the host granite. Mass balance calculations, with reference to adjacent unaffected granite, evince mainly Si loss and Na gain, reflecting the observed mineralogical changes. The volume decrease is estimated to be ~ 9%, owing to compaction accompanying the dequartzification and a slight porosity increase. The calculations revealed a slight enrichment in HREEs ± Y, essentially hosted by xenotime and a thorite-xenotime solid solution, both exsolved during recrystallization of monazite. Oxidation of ferrous iron and partial loss of chalcophile elements reflect oxidizing conditions. The alteration episode is Permo-Carboniferous and is possibly associated with circulation of oxidizing fluids through the Bohus granite.

Pedogenic and groundwater processes in a closed Miocene basin (northern Spain)

The Sepúlveda-Ayllón Basin (SAB) was an episodically closed Miocene basin adjacent to the Tertiary Duero Basin (northern Spain). The sediments studied here represent a depositional system developed from the margin to the centre of the basin by alluvial fan, mud flat and saline mud flat/ ephemeral lacustrine environments. An assemblage of authigenic lithotypes was formed in the basin, their distribution being influenced by that of the depositional environments within the basin. Authigenesis took place mainly before sedimentation of the next set of beds. In the distal alluuial fan, authigenesis of smectite (outer part), palygorskite (inner part) and calcrete predominates, and the original muddy deposits are still recognizable. In the mud flat, a more intense transformation of the sediments took place, with predominant formation of palygorskite (outer part), interstitial gypsum, calcrete (outer part), dolocrete (inner part) and brown silcrete. In the saline mud flat /ephemeral lake, original sediment was extensively disturbed and replaced by magnesian clays (sepiolite and magnesian smectite), interstitial gypsum, dolocrete and cream and white silcrete. Authigenic alteration within each cycle varied in time and place, and was both pedogenic and groundwater-related. The authigenic assemblages point to a closed basin under evaporation in a seasonally dry climate. At the end of this period the SAB became connected with the Duero Basin to the northeast, thus losing the endoreic regime. Consequently, the authigenic evaporitic assemblages were then unstable and were partially replaced by a late sparry calcrete.

Illite/Smectite Geothermometry of the Proterozoic Oronto Group, Midcontinent Rift System

AbstractCharacterization of the Nonesuch Formation, middle unit of the Proterozoic Oronto Group, as a potential hydrocarbon source for the Lake Superior basin portion of the Midcontinent Rift system requires an understanding of the thermal maturity of the region and its relationship to the thermal history. Illite/smectite (I/S) expandability data were collected from the Nonesuch Formation and the overlying Freda Sandstone and compared with organic thermal maturity data; both data sets coupled with a thermal and burial history for the White Pine area of Michigan allow regional interpretation of maximum formation temperatures of the Nonesuch Formation and the Freda Sandstone with respect to time. Samples collected from drill holes in northeastern Wisconsin display nearly pure smectite within the lower Freda Sandstone trending abruptly to ordered I/S within the Nonesuch Formation. Regular trends of decreasing expandability with depth occur in four other drill holes to the northeast. Comparison of I/S expandability between similar stratigraphic intervals reveals a significant trend of increasing thermal maturity to the northeast, with the lowest thermal maturities observed in the Iron River Syncline area just west of White Pine, Michigan.I/S geothermometry suggests maximum temperatures in the Nonesuch Formation of 140°C in Wisconsin, 115°C in the Iron River Syncline area, 160°C at White Pine, and 190°C near the southern portions of the Keweenaw Copper District. The geographic pattern of temperatures determined from I/S geothermometry is identical to that determined from organic thermal maturity indicators in the Nonesuch Formation (Imbus et al., 1988, 1990; Hieshima and Pratt, 1991; Pratt et al, 1991; Mauk and Hieshima, 1992).Regular variations in I/S expandability with depth occur in the Freda Sandstone and the Nonesuch Formation near the southern limits of the Keweenaw Copper District. These variations suggest a fossil geothermal gradient of 55°C/km and limit the thickness of sediment above the Nonesuch Formation to approximately 3 km. In comparison, 3.6 km of Freda Sandstone are presently exposed near the Wisconsin border, and numerical modeling suggests a range of 4–6 km of sediment overlying the Nonesuch Formation. None of the data indicate the presence of the Bayfield Group sediments above the Nonesuch Formation at the time of clay diagenesis. Samples from White Pine suggest a two-stage burial history: 1) clay reaction, possible hydrocarbon maturation, and copper-sulfide mineralization at maximum temperatures above 100°C during the main rifting and burial event, followed by 2) fracturing, reverse faulting, and fluid circulation during a rift-terminating compressional event that may have allowed petroleum migration and native copper mineralization at temperatures below 100°C. Abrupt changes in I/S expandability with depth and the presence of poorly crystalline I/S (greater than 80% expandable) and kaolinite in the Freda Sandstone in Wisconsin appear to represent later overprinting of the diagenetic assemblage by fluids that were probably cooler and of differing composition than earlier diagenetic fluids. However, the authigenic assemblage from the vicinity of White Pine, Michigan, which includes up to 25% expandable I/S, appears to represent a diagenetic profile formed during the main rifting and burial event. Therefore, these expandable I/S-type clays are essentially 1.0 billion years old.

Diagenesis of Cretaceous sandstones of the Kootenay Formation at Elk Valley (southeastern British Columbia) and Mt Allan (southwestern Alberta)

Vitrinite reflectance data for two sections of the Kootenay Formation are different, but increase with depth. Illite crystallinities (XRD) are irregular with depth but detailed S.E.M. examination shows that crystal size and morphology increase with depth. At Elk Valley (low reflectance) S.E.M. examination shows kaolinite + dolomite as the authigenic assemblage. At Mt Allan (45 km north, higher reflectance) kaolinite disappears at 265 m and chlorite + calcite appears implying the equilibrium 5 CaMg( CO 3) 2 + Al 2 Si 2 O 5( OH) 4 + SiO 2 + H 2 O ag Mg 5 Al 2 Si 3 O 10( OH) 8 + 5 CaCO 3 + 5 CO 2. (1) Equilibrium (1) involves CO 2 and H 2O which form an immiscible fluid at diagenetic conditions. Thermochemical data, activity coefficients for the fluid mixture (Margules solution model), and fugacity coefficients allow the position of intersection of the miscibility surface and equilibria similar to (1) to be calculated. Maximum temperatures (180–250°C) in water-rich fluids are in accord with geothermal gradients of 33°C/km and temperatures from vitrinite reflectance (220°C). It is suggested that produced CO 2 may be responsible for the development of secondary porosity during deep burial.