Fracture-filling Calcite Research Articles

Rare earth elements (REE) in deep groundwater from granite and fracture-filling calcite in the Tono area, central Japan: Prediction of REE fractionation in paleo- to present-day groundwater

Rare earth elements (REEs) combined with yttrium (YREE) in deep groundwater from granite and fracture-filling calcite are being studied at the Mizunami Underground Research Laboratory (MIU, Tono area, central Japan). The groundwater was sampled from cored boreholes drilled from the MIU galleries at 200, 300, 400 and 500m depths. The fracture-filling calcite was collected from the same boreholes and dissolved in 1M NH4 acetate buffer solution (pH4.8). Major chemical composition of groundwater changed with depth; and the concentrations of some major elements (Na+, K+, Ca2+ Mg2+, Cl−, Br− and B) increased with depth, while alkalinity, SO42−, and F− decreased. Isotopic compositions of δ18O and δD indicated that the groundwater originated from meteoric water. YREE patterns of groundwater also changed with depth; with heavy REE (HREE) enrichment in “shallow” groundwater (−200 to −400m) and light REE (LREE) enrichment in “deep” groundwater (−500m). The predominant YREE species are YREECO3+ and YREE(CO3)2− (LaCO3+: 65–70%, La(CO3)2−: 8–27%, LuCO3+: 20–38% and Lu(CO3)2−: 53–77%). The LREE enrichment in the “shallow” groundwater resulted in preferential scavenging of positively charged YREECO3+ by negatively charged bedrock surfaces in slightly alkaline groundwater pH (7.4–8.6). Because of the construction and operation of the underground facility, the “shallow” groundwater has been influenced by infiltration of surface water undersaturated in calcite. In contrast, the “deep” groundwater was barely affected by the mixing of surface water, indicating that the YREE originated from YREE fractionation by bedrock granite. Irrespective of depth, a negative Ce anomaly preserved in the groundwater, is probably indicative of paleo-oxic conditions during paleo-recharge. The negative Ce anomaly in the groundwater was possibly fractionated from sedimentary calcite with positive Ce anomalies during the paleo-recharge. The YREE patterns of the fracture-filling calcite were enriched in LREE. The isotopic variations of δ18O and δ13C in the fracture-filling calcite indicated that it originated from a mixture of sea water and fresh water at a temperature <100°C. The YREE patterns of paleo-groundwater from which the fracture calcite precipitated were calculated using YREE abundances in the fracture calcite and distribution coefficient reported in previous laboratory experiments. The paleo-groundwater was found to be enriched in LREE and HREE depending on the aqueous YREE species. Our results suggest that changes in the paleo- to present-day YREE fractionation range roughly from that of present-day shallow to deep groundwater.

Rare earth element and yttrium (REY) geochemistry in carbonate reservoirs during deep burial diagenesis: Implications for REY mobility during thermochemical sulfate reduction

The impact of burial diagenesis (especially deep burial-related processes such as thermochemical sulfate reduction, TSR) on the distribution of rare earth elements and yttrium (REY) in diagenetic minerals in carbonate reservoir has gained little attention even though they may be unique indicators of the diagenetic system being closed or open to external influx of material. Trace element and REY concentrations, 87Sr/86Sr, δ18O and δ13C have been determined for limestone, host dolomite, pore-filling calcite (calcite-2), and late stage fracture-filling calcite (calcite-3), barite (barite-2) and anhydrite (anhydrite-3) from Lower Triassic Feixianguan Formation reservoirs in northeast Sichuan Basin, China. Calcite-2, calcite-3, barite-2, and anhydrite-3 precipitated during deep burial (from 110°C to 220°C), demonstrated by petrology and fluid inclusion thermometry. Sr isotope analysis revealed that diagenetic carbonate minerals and anhydrite have largely identical 87Sr/86Sr ratios to Triassic seawater, indicating negligible input of Sr from terrigenous sources and probably a relatively closed diagenetic environment for precipitation of these minerals. Carbon isotope analysis showed that calcite-2 has relatively low δ13C values (down to −18.9‰ V-PDB), suggesting that they are TSR calcites with the carbonate derived from oxidized, isotopically-light hydrocarbons. Unlike the seawater-like REY patterns of the limestone and dolomites, calcite-3 and anhydrite-3 are enriched in rare earth elements (ΣREE) and show light rare earth element enrichment and heavy rare earth element depletion, exhibiting a chevron-like pattern of shale normalized REY trends. In contrast, TSR calcite (calcite-2) has similar ΣREE to the host dolomite but a relatively flat REYSN pattern, suggesting strong variations in elemental and REE compositions of the burial fluids. Significantly, TSR calcite shows a prominent positive Eu anomaly and an unusually high-chondritic Y/Ho ratio. Both yttrium versus holmium fractionation and Eu2+ oxidation to Eu3+ must have occurred during thermochemical sulfate reduction. Hence, a positive Eu anomaly and an elevated Y/Ho ratio may be used as effective proxies to differentiate calcite resulting from TSR from ordinary calcite cement. This is especially useful when carbon isotope analysis cannot be used to give an unambiguous interpretation of the origin of the calcite.

Compilation and Review of 87Sr/86Sr and Stable Isotopes from Groundwater, Calcite Fracture Fillings, Mineral, and Whole‐Rock Sampling at Äspö, Sweden

Integrated isotopic and hydrochemical studies of groundwater at Äspö, Sweden, support mixing models involving deep saline water, low-solute infiltration, and Baltic Sea water. Carbon, oxygen, and strontium isotope analyses of calcite fracture fillings indicate that paleohydrologic conditions were different than those of today in terms of the isotopic composition of water flowing through fractures. Sr isotopes of whole-rock and mineral (plagioclase, microcline, biotite, and epidote) samples are used to assess the effects of water-rock interaction. Biotite is a major reactant in the early stages of water-rock interaction. Strontium isotope systematics of groundwater from deep in the Hard Rock Laboratory, and underground research facility, and from low conductivity zones revealed a first order mixing line defined by a high-chloride saline component with 66 mg/L Sr and an ancient sea water component with approximately 4.5 mg/L. Dilution with low salinity recharge has produced groundwater with variable Sr contents and 87Sr/86Sr values between 0.7186 and 0.7160. Differences between Äspö and Laxemar groundwater are shown by trends in Sr concentrations plotted against 87Sr/86Sr. The Äspö trend shows increasing 87Sr/86Sr values with increasing concentrations of Sr, whereas the Laxemar groundwater trend shows little variability in 87Sr/86Sr with increasing Sr concentrations. These trends are controlled by the differences in 87Sr/86Sr composition of the saline end members in the two areas.

U–Pb, Rb–Sr, and U-series isotope geochemistry of rocks and fracture minerals from the Chalk River Laboratories site, Grenville Province, Ontario, Canada

As part of the Geologic Waste Management Facility feasibility study, Atomic Energy of Canada Ltd. (AECL) is evaluating the suitability of the Chalk River Laboratories (CRL) site in Ontario, situated in crystalline rock of the southwestern Grenville Province, for the possible development of an underground repository for low- and intermediate-level nuclear waste. This paper presents petrographic and trace element analyses, U–Pb zircon dating results, and Rb–Sr, U–Pb and U-series isotopic analyses of gneissic drill core samples from the deep CRG-series characterization boreholes at the CRL site. The main rock types intersected in the boreholes include hornblende–biotite (±pyroxene) gneisses of granitic to granodioritic composition, leucocratic granitic gneisses with sparse mafic minerals, and garnet-bearing gneisses with variable amounts of biotite and/or hornblende. The trace element data for whole-rock samples plot in the fields of within-plate, syn-collision, and volcanic arc-type granites in discrimination diagrams used for the tectonic interpretation of granitic rocks.Zircons separated from biotite gneiss and metagranite samples yielded SHRIMP-RG U–Pb ages of 1472±14 (2σ) and 1045±6Ma, respectively, in very good agreement with widespread Early Mesoproterozoic plutonic ages and Ottawan orogeny ages in the Central Gneiss Belt. The Rb–Sr, U–Pb, and Pb–Pb whole-rock errorchron apparent ages of most of the CRL gneiss samples are consistent with zircon U–Pb age and do not indicate substantial large-scale preferential element mobility during superimposed metamorphic and water/rock interaction processes. This may confirm the integrity of the rock mass, which is a positive attribute for a potential nuclear waste repository. Most 234U/238U activity ratios (AR) in whole rock samples are within errors of the secular equilibrium value of one, indicating that the rocks have not experienced any appreciable U loss or gain within the past 1Ma. However, 234U/238U AR in fracture mineral samples collected down to borehole lengths of about 740m deviate from the secular equilibrium value and 234U/238U model ages calculated for fracture mineral samples showing excess 234U range from 593 to 1415ka, thus providing evidence of fracture flow in the associated bedrock during the past 1.5Ma. Rare earth element patterns are variable in fracture-filling calcites and Fe oxides/hydroxides but are similar to those observed in associated whole-rock samples. The observed Ce anomalies are very small (CeN/CeN∗≈1), do not vary with depth, and, therefore, do not contain evidence that the studied fracture minerals precipitated from oxidizing waters at the conceptual depth of a repository.

Diagenetic evolution and associated mineralization in Middle Devonian carbonates, southwestern Ontario, Canada

Abstract The Middle Devonian Lucas Formation of southwestern Ontario, Canada, forms the main hydrocarbon reservoir rocks in the region. Integrated field, petrographic, fluid inclusion and isotope geochemistry reveal a variety of diagenetic processes that include the formation of dolomite, calcite, celestine and fluorite under diverse geochemical conditions. The host carbonate strata display two types of replacive dolomite fabrics: 1) fine-crystalline matrix dolomite (5–20 μm); and 2) medium-crystalline dolomite (50–150 μm) with an average size of 100 μm. Stable oxygen and carbon isotope results for both dolomite fabrics show a wide range of values (δ18O = −9.4‰ to −3.9‰ VPDB, δ13C = 0.4‰ to 3.4‰ VPBD). For late fracture-filling calcite, δ 18O and δ 13C values range from −9.4 to −5.9‰ VPDB and −7.7‰ to −0.04‰ VPDB, respectively. The 87Sr/86Sr ratios of selected dolomite and calcite samples range from 0.70798–0.70838, preserving Middle Devonian seawater values while some samples have slightly more radiogenic ratios than contemporaneous seawater. Field and petrographic evidence and Sr isotope ratios of dolomite suggest early dolomitization from evaporated seawater in a sabkha setting (shallow seepage reflux or evaporitive pumping). However, the depletion in δ18O (varying from −3.9 to −9.4‰ VPDB), and radiogenic Sr isotope values of the Lucas Formation dolomite are considered to be the result of recrystallization, which is interpreted to occur at elevated temperatures in a burial environment. The 18O-depleted values and slightly radiogenic 87Sr/86Sr ratios of calcite cements suggest precipitation from hot, saline basinal/hydrothermal brines. In addition, the 13C-depleted calcites indicate that carbon was possibly formed by oxidation of organic matter. Fluid inclusions in some of the late calcite cements fluoresce under UV light, indicating the presence of hydrocarbons. Homogenization temperatures of these inclusions vary from 87.1°C to 171.9°C with an average value of 126°C. Average salinity of fluid inclusions is 19.7 wt. % NaCl eq. Celestine samples have lower salinities relative to calcite, averaging 14.5 wt. % NaCl eq. with an average homogenization temperature of 219.5°C. Based on the geochemical and fluid inclusion results, hot, saline basinal/hydrocarbon bearing brines originated from central part of the basin and migrated to Devonian carbonates via fractures and faults. Celestine is a product of thermochemical sulfate reduction and formed due to mixing of saline, Sr and sulfate bearing brines and meteoric waters.

Diagenetic Evolution and Porosity Destruction of Turbiditic Hybrid Arenites and Siliciclastic Sandstones of Foreland Basins: Evidence from the Eocene Hecho Group, Pyrenees, Spain

Abstract This study aims to unravel the impact of diagenetic alterations on porosity loss of foreland-basin turbiditic hybrid arenites and associated siliciclastic sandstones of the Eocene Hecho Group (south-central Pyrenees, Spain). In this succession, hybrid arenites and calclithites are extensively cemented by mesogenetic calcite cement (δ18OVPDB = −10.0‰ to −5.8‰; Th, mode = 80° C; salinity mode = 18.8 wt% eq. NaCl), Fe-dolomite (δ18OVPDB = −8.5‰ to −6.3‰) and trace amounts of siderite. The extent of carbonate cementation is interpreted to be related to the amounts of extrabasinal and intrabasinal carbonate grains, which provided nuclei and sources for the precipitation and growth of carbonate cements. Other diagenetic alterations, such as pyrite and albitization, had no impact on reservoir quality. Scarce early diagenetic cements, coupled with abundant ductile carbonate and siliciclastic framework grains, have led to rapid porosity loss owing to compaction. Conversely, abundant quartz in the sandstones prevented rapid loss of porosity by mechanical compaction. Reservoir quality was affected by mesogenetic cementation by quartz overgrowths, calcite and dolomite intergranular pressure dissolution of quartz grains, and formation of fracture-filling calcite cement (δ18OV-PDB values from −10.4‰ to −7.8‰; Th temperatures of 150° C), which are attributed to deep circulation of hot meteoric waters during extensional stages of tectonism. The results of this study illustrate that diagenetic evolution pathways of the arenites and sandstones are closely linked to the variation in detrital composition, particularly the proportion and types of extrabasinal noncarbonates, extrabasinal carbonates, and intrabasinal carbonate grains. These insights suggest that marine turbiditic hybrid arenites and calclithites of foreland basins are subjected to more rapid and extensive porosity loss owing to compaction and cementation than associated siliciclastic sandstones. Degradation of reservoir quality makes these hybrid arenites, calclithites, and sandstones suitable as tight gas reservoirs, but only if fracture porosity and permeability develop during tectonic deformation.

Fluid flow record from fracture-fill calcite and its relationship to oil shows (Upper Jurassic limestones, Maestrat Basin, Iberian Range)

Fluid flow record from fracture-fill calcite and its relationship to oil shows (Upper Jurassic limestones, Maestrat Basin, Iberian Range)

Origin, age, and paleoenvironmental significance of carbonate precipitates from a granitic environment, Akshayuk Pass, southern Baffin Island, Canada

This study documents the discovery of calcite crusts on the upper surface of clasts within morainic complexes in Akshayuk Pass, southern Cumberland Peninsula (Baffin Island), a region underlain by granitic and gneissic rocks of the Precambrian Canadian Shield. Strontium isotope ratios (87Sr/86Sr) indicate that the major source of calcium is derived from the local dissolution of fracture-filling calcite and a minor source is derived from silicate weathering. The calcite crusts have δ13C values between 1.6‰ and 12.0‰ and δ18O values in the –13.0‰ and –7.9‰ range. These values are highly enriched over the predicted isotopic values based on the δ13CDIC and δ18O of the local water and temperature at which calcite precipitation occurred. The isotopic enrichments are attributed to a combination of both equilibrium and kinetic evaporation and were verified experimentally. The series of evaporative experiments indicate that kinetic evaporation produces a far-from-equilibrium isotope effect on both the δ13C and δ18O composition of the precipitating calcite (13CKIE CaCO3–CO2 between 20.2‰ and 40.5‰ VPDB; 18OKIE averaging 34.9% ± 3.7% VSMOW). Based on these results, the formation of the calcite crusts is ascribed to the evaporation of stagnating ephemeral lakes and streams following the retreat of valley glaciers. Given the &gt;1500 m difference in heights between the highland and the valley, the katabatic winds originating from the Penny Ice Cap can act as a catalyst to increase the rate of evaporation. In addition, radiocarbon dating of ten of these crusts yielded Holocene ages (7640 cal (calibrated) BP to modern). These results suggest that the maximum expansion of Holocene valley glacier in Akshayuk Pass, previously thought to have occurred during the Little Ice Age, is probably much older.

A reconnaissance study of fluid inclusions in fracture-filling quartz and calcite from the Lopra-1/1A well, Faroe Islands

Fracture-filling calcite and quartz from the Lopra-1/1A well (at 2380 m and 3543 m depth) contains both aqueous low-salinity fluid inclusions and hydrocarbon-dominated fluid inclusions. Microthermometry indicates that the aqueous fluids contain 0.2 to 1.4 equivalent weight% NaCl and occasionally contain traces of hydrocarbons. Homogenisation to liquid occurred between 90°C and 150°C. Modelling based on these fluid inclusion observations indicates that during burial the basaltic section was subjected to temperatures of 160°C and 170°C, occasional pressures of 600–700 bars and the simultaneous percolation of aqueous and hydrocarbon fluids. These fluid conditions may also be relevant to the formation of zeolite observed in the Lopra-1/1A well.

Fluid flow record from fracture-fill calcite in the Eocene limestones from the South-Pyrenean Basin (NE Spain) and its relationship to oil shows

The presence of oil shows associated with fractures provides a significant opportunity to a) unravel the type, origin and evolution of fluids involved in fracture-fills, and b) examine how they relate to oil migration. Two stages of calcite cement (C1 and C2) were distinguished in the fractures of the Eocene Armàncies platform carbonates; C1 is characterised by fence-like crystals, exhibits dull red luminescence and contains abundant twin planes, inclusions and δ 18O values that range from − 6.2‰ to − 4.8‰ VPDB. C2 consists of blocky clean crystals, is characterized by dark brown-red luminescence that alternates with yellow bands, and contains hydrocarbon fluid inclusions with homogenisation temperatures of approximately 120 °C. δ 18O values range from − 9.6‰ to − 8.9‰ VPDB. The remaining porosity after C2 precipitation is filled with liquid oil that reached 115 °C. This would seem to indicate that free oil and fluid inclusions oil probably come from the same migration pulse. Oil migration timing was coeval with C2 and continued after calcite cementation was completed.

Fatty acids in sparry calcite fracture fills and microsparite cement of septarian diagenetic concretions

Sparry calcite fracture fills and concretion body cements in concretions from the Flodigarry Shale Member of the Staffin Shale Formation, Isle of Skye, Scotland, entrap and preserve mineral and organic materials of sedimentary and diagenetic origin. Fatty acids are a major component of the lipids recovered by decarbonation and comprise mainly n-alkanoic and α-ω dicarboxylic acids. Two generations of fracture-fill calcite (early brown and later yellow) and the concretion body microspar yield significantly different fatty acid profiles. Early brown calcites yield mainly medium-chain n-alkanoic acids with strong even predominance; later yellow calcites are dominated by α-ω dicarboxylic acids with no even predominance. Both fracture fills lack the long-chain n-alkanoic and α-ω dicarboxylic acids additionally recovered from the concretion bodies. The absence of longer chain acids in the calcite spar fracture fills is inferred to result from the transport of fatty acids by septarian mineralising fluids whereby low-aqueous solubility of longer chain acids or their salts accounts for their relative immobility. Comparative experiments have been carried out using conventional solvent extraction on the concretion body and associated shales, both decarbonated and untreated. Extracted lipid yields are higher, but the fatty acids probably derive from mixed locations in the rock including both kerogen- and carbonate-associated lipid pools. Only experiments involving decarbonation yielded α-ω dicarboxylic acids in molecular distributions probably controlled mainly by fluid transport. Alkane biomarker ratios indicate very low thermal maturity has been experienced by the concretions and their host sediments. Septarian cracks lined by brown calcite formed during early burial. Microbial CO 2 from sulphate-reducing bacteria was probably the main source of mineralising carbonate. Emplacement of the later septarian fills probably involved at least one episode of fluid invasion.

The geochemical implication of a variable Eu anomaly in a fractured gneiss core: application for understanding Am behavior in the geological environment

The rare earth elements (REEs) have been used as analogs for understanding the behavior of actinide elements in geological conditions. Here the authors discuss the usefulness of the change of Eu abundance in a fractured aquifer for deducing the behavior of Am under geological conditions. The REE abundance of fractured core samples from Precambrian crystalline rocks in the Namyangju area, Korea provides a good example of a water–rock interaction in the low temperature aqueous environment. The rock types studied are mainly biotite banded-gneisses with mineral assemblages dominated by biotite, K-feldspar, quartz and plagioclase. Calcite, chlorite, muscovite and sericite occur as secondary minerals, with calcite being the main filling material in fractures. In general, the core samples from 5 boreholes are enriched in light REE (LREE) and depleted in heavy REE (HREE), with (La/Yb) N=7.3 to 67.6 and have negative Eu anomalies. However, positive Eu anomalies also occur in the fractured core samples that contain secondary calcite. Results of leaching experiments show that a variation of chondrite-normalized REE patterns is associated with the fracture-filling calcite, where Eu has been reduced and selectively concentrated in the solutions from which calcite has precipitated. The cohesive energy of Am in actinide series is similar to that of Eu. Hence, the behavior of Eu in the fractured aquifer may be an important key to understanding and predicting the behavior of Am in the geological environment after radioactive waste disposal in the ground. In addition, a slight Yb anomaly from chondrite-normalized REE pattern can be observed by a calcite leaching test, which also suggests that extremely reducing conditions possibly existed in the past groundwater environment.

Sedimentologic, diagenetic and tectonic evolution of the Saint-Flavien gas reservoir at the structural front of the Quebec Appalachians

ABSTRACT The Beekmantown Group (Lower Ordovician) of the Saint-Flavien reservoir has produced 162x106 m3 (5.7 bcf) of natural gas between 1980 and 1994. The conversion of the field into gas storage was initiated in 1992 and the pool became operational in 1998. Integration of structural and sedimentologic features, carbonate and organic matter petrography and geochemistry for 13 drill holes is used to define a tectonic-sedimentologic-diagenetic model for porosity evolution in these reservoir dolostones. The Beekmantown Group consists of numerous fifth-order shallowing-upward cycles 1.0 to 7.0 m thick (average of 2.4 m). Each cycle consists of a basal shale deposited during the initial flooding of the platform which was subsequently covered by a shoaling succession of subtidal and intertidal limestones to intertidal dolostones. Early dolomitization has produced intercrystalline porosity and preserved some moldic pores in the intertidal facies. Near surface, post-dolomite karstification has created vugs that were subsequently filled by early marine calcite fibrous cement based on the 18O and 13C ratios of calcite. Early burial elements consist of horizontal stylolites, pyrite and sphalerite. Late migrated bitumen was thermally altered or vaporized as native coke under deep burial conditions exceeding 240°C, partly due to overthrusting of Appalachian nappes. Under these conditions, breccias and fractures were generated and subsequently filled with K-feldspar, quartz, illite, and xenomorphic and poikilotopic calcite. The 18O of the poikilotopic calcite and homogenization temperature of coeval fluid inclusions indicate formation under high temperatures (Th about 260°C). Horizontal shear zones and marcasite-rich vertical stylolites were produced during folding and thrusting. Dissolution has preferentially affected late fracture-filling calcite and generated most of the actual porosity during or soon after the Taconian Orogeny. The relationship between the occurrence of smectite and this type of porosity indicates the low temperature condition of this dissolution (T <100°C). Porosity in the Saint-Flavien reservoir has been mostly produced by fracture-controlled, late to post-Taconian dissolution of early to late calcite in the intertidal dolomitic slightly porous facies at the top of rhythmic cycles that compose the Beekmantown Group. End_Page 126------------------------

Palaeohydrogeological Evidences from Fracture Filling Minerals - Results from the Äspö/Laxemar Area

ABSTRACTTwo different EU projects: the EQUIP project (Evidences from Quaternary Infills from Palaeohydrogeology) running from 1997 to 2000 and the ongoing PADAMOT-project (Palaeohydrogeological Data Analysis and Model Testing) have addressed the possibility of using fracture minerals as recorders of past hydrochemical and hydrogeological environments. The Swedish contribution in these two projects has concentrated on studies of fracture filling calcites from the Äspö/Laxemar area, southeast Sweden.The analyses applied in the two projects comprise stable isotope ratios of carbon and oxygen and strontium together with trace element contents and textural studies. The results obtained so far (mainly from the EQUIP project) show that several generations of calcite can be identified, chemical zoning is common, and the influence on calcite precipitation of fresh and marine water decreases with depth. It can be concluded that fine scale zoning, possible dissolution/redistribution of calcite, and the disturbances caused by drilling (where loose material which may contain young calcite precipitates were lost), introduce uncertainties in the recognition of different calcite generations. However, despite these problems it is indicated that calcite fracture fillings in Precambrian bedrock environments can add information e.g. concerning changes in groundwater chemistry and redox conditions in the past. The possibilities and problems with palaeohydrogeological interpretations are discussed based on the available data.

Rare earth elements as indicators of groundwater environment changes in a fractured rock system: evidence from fracture-filling calcite

Rare earth element (REE) abundances in core samples from Precambrian crystalline rocks at the Samkwang Mine site provide evidence of the solution chemistry involved in precipitation of calcite on fractures. The rock types collected in core samples are mainly banded-gneisses, with mineral assemblages dominated by biotite, K-feldspar, quartz and plagioclase. Calcite, chlorite, muscovite and sericite occur as secondary minerals, with calcite being the main filling material in fractures. In general, the core samples from 4 boreholes are enriched in light REE (LREE) and depleted in heavy REE (HREE), with negative Eu anomalies. However, positive Eu anomalies also occur at specific depths within 3 boreholes. Variation of chondrite-normalized REE patterns results from the fracture-filling calcite in core samples. Calcite fracture fillings provide a record of paleo-hydrology, where Eu has been reduced and selectively concentrated in the solutions from which calcite has precipitated.

Diagenetic and oil migration history of the Kimmeridgian Ascla Formation, Maestrat Basin, Spain

The marine limestones of the Kimmeridgian Ascla Formation in the Maestrat Basin reached more than 3500 m in burial depth during the Cretaceous era. Despite containing organic-rich intervals, mature in parts of the basin, its potential as oil source-rock has been either overlooked or questioned. A petrographic, geochemical and fluid-inclusion (FI) study of the cements of the Ascla was performed in order to unravel its diagenetic and thermal evolution. We particularly sought evidence of oil migration and its timing. Three sequences of cement were distinguished. Sequence 1 fills the primary porosity and began with Fe-poor calcites with geochemistry and FIs consistent with precipitation from marine-derived waters during shallow burial. These calcites were followed by burial cements, including ferroan calcite, dolomite, and minor celestite and barite. Sequence 2 consists of Mg-rich, fracture-filling calcite cement zones. The earlier ones are ferroan and contain primary aqueous and oil FIs with homogenization temperatures suggesting precipitation at temperatures as high as 117°C. Sequence 3 is dominated by fracture-filling calcites with geochemistry and FIs indicating precipitation at low temperatures (less than ∼50°C) from meteoric waters. Cross-cutting relationships with compressional microstructures indicate that Sequence 3 formed after the Eocene–Oligocene tectonic inversion of the basin. Oil FIs in Sequence 2 provide evidence that light oils migrated through the Ascla Formation via fractures and microfractures. These oils were likely generated in the organic-rich marls of the basal part of the Ascla. The paragenetic sequence and burial history are consistent with oil generation when the Ascla was at or close to maximum burial depth, but before the Eocene Alpine tectonism, which likely formed the structural traps in the basin. Oil generation and migration occurred long before this event. Therefore, it is probable that early traps were breached by the Alpine structures and that potential in this basin sector is low.

Transient calcite fracture fillings in a welded tuff, Snowshoe Mountain, Colorado

The core from two boreholes (13.1 and 19.2 m depth) drilled 500 m apart in the fractured, welded tuff near the summit of the Snowshoe Mountain, Colorado (47°30′N, 106°55′W) had unique petrographic and hydrodynamic properties. Borehole SM-4 had highly variable annual water levels, in contrast to SM-1a, whose water level remained near the land surface. Core samples from both boreholes ( n=10 and 11) were examined petrographically in thin sections impregnated with epoxy containing rhodamine to mark the pore system features, and were analyzed for matrix porosity and permeability. Core from the borehole sampling the vadose zone was characterized by open fractures with enhanced porosity around phenocrysts due to chemical weathering. Fractures within the borehole sampling the phreatic zone were mineralized with calcite and had porosity characteristics similar to unweathered and unfractured rock. At the top of the phreatic zone petrography indicates that calcite is dissolving, thereby changing the hydrogeochemical character of the rock (i.e. permeability, porosity, reactive surface area, and mineralogy). Radiocarbon ages and C and O stable isotopes indicate that calcite mineralization occurred about 30 to 40 ka ago and that there was more than one mineralization event. Results of this study also provide some relationships between primary porosity development from 3 types of fracture in a welded tuff.

Geochemical study of calcite veins in the Silurian and Devonian of the Barrandian Basin (Czech Republic): evidence for widespread post-Variscan fluid flow in the central part of the Bohemian Massif

Carbonate fracture cements in limestones have been investigated by fluid inclusion and stable isotope analysis to provide insight into fluid evolution and deformation conditions of the Barrandian Basin (Silurian–Devonian) of the Czech Republic. The fractures strike generally north–south and appear to postdate major Variscan deformation. The most common fracture cement is calcite that is locally accompanied by quartz, natural bitumen, dolomite, Mn-oxides and fluorite. Three successive generations of fracture-filling calcite cements are distinguished based on their petrographical and geochemical characteristics. The oldest calcite cements (Stage 1) are moderate to dull brown cathodoluminescent, Fe-rich and exhibit intense cleavage, subgrain development and other features characteristic of tectonic deformation. Less tectonically deformed, variable luminescent Fe-poor calcite corresponds to a paragenetically younger Stage 2 cement. First melting temperatures, Te, of two-phase aqueous inclusions in Stages 1 and 2 calcites are often around −20°C, suggesting that precipitation of the cements occurred from H2O–NaCl fluids. The melting temperature, Tm, has values between 0 and −5.8°C, corresponding to a low salinity between 0 and 8.9 eq. wt% NaCl. Homogenization temperatures, Th, from calcite cements are interpreted to indicate precipitation at about 70°C or less. No distinction could be made between the calcite of Stages 1 and 2 based on their fluid inclusion characteristics. In some Stage 2 cements, inclusions of highly saline (up to 23 eq. wt% NaCl) brines appear to coexist with low-salinity inclusions. The low salinity fluid possibly contains Na-, K-, Mg- and Ca-chlorides. The high salinity fluid has a H2O–NaCl–CaCl2 composition. Blue-to-yellow-green fluorescing hydrocarbon inclusions composed of medium to higher API gravity oils are also identified in some Stages 1 and 2 calcite cements. Stage 1 and 2 calcites have δ18O values between −13.2‰ and −7.2‰ PDB. The lower range of the calculated δ18O values of the ambient fluids (−3.5‰ to +2.7‰ SMOW) indicate precipitation of these cements from deeply circulating meteoric waters. The presence of petroleum hydrocarbon inclusions in some samples is interpreted to reflect partial mixing with deeper basinal fluids. The paragenetically youngest Stage 3 calcite cement has only been encountered in a few veins. These calcites are characterised by an intensely zoned luminescence pattern, with bright yellow and non-luminescent zones. Inclusions of Mn-oxides and siliceous sinters are commonly associated with Stage 3 calcite, which is interpreted to have precipitated from shallower meteoric waters. Regional structural analysis revealed that the calcite veins of the Barrandian basin belong to a large-scale system of north–south-trending lineaments that run through the territory of the Czech Republic. The veins probably reflect episodes of fluid migration that occurred along these lineaments during late stages of the Variscan orogeny.

Calcite fracture fillings as indicators of paleohydrology at Laxemar at the Äspö Hard Rock Laboratory, southern Sweden

Isotopic compositions of C (δ 13C), O (δ 18O) and Sr (δ 87Sr) were determined for calcite fracture fillings in the crystalline rock penetrated by a 1.6 km drill hole at Laxemar, near the Äspö Hard Rock Laboratory (ÄHRL) in southern Sweden. These calcites precipitated from groundwater some time in the past, and their δ 13C, δ 18O and δ 87Sr values reflect those of the source waters. The present-day groundwater system is hydrochemically stratified with highly saline water underlying more shallow brackish and fresh water. The origin of this stratified system is probably related to past glaciations although the ultimate origin of the deep, highly saline water is still problematical. None of the calcite fracture fillings sampled below 900 m could have precipitated from any of the present-day ground waters which in view of the glacial history of the region is not surprising. However, several shallow calcite fracture fillings are formed by precipitation from the present-day groundwater. Coupled variations in δ 13C, δ 18O and δ 87Sr isotopes at depths in excess of 900 m suggest that these isotope systems in calcite are recording a time-dependent evolution of groundwater composition.

Radioactive waste containment in indurated shales: comparison between the chemical containment properties of matrix and fractures

Abstract The chemical containment properties of poorly consolidated, plastic clays have received considerable attention, particularly during investigations related to radioactive waste disposal. However, the chemical containment properties of indurated shales have received rather less consideration. This paper summarizes those properties of consolidated shales which make them suitable for the chemical containment of radionuclides, using the example of investigations at the Tournemire research site in the south of France. The problems associated with investigating such low-permeability media are highlighted, and solutions to these problems are presented. At this research site, tectonic processes have induced centimetre-scale fractures and decimetre-scale faults in the shales, whereas excavation of a tunnel has generated metre-scale fracturing. Some low water flows through the fracture networks have been detected. However, the Tournemire shales are characterized by very low matrix pore-water contents, and any limited fluid mobility through the matrix is caused mainly by diffusion. Slowly flowing waters in fractures are chemically analogous to these matrix pore-waters. The hydrochemistry is of Na-Cl-( HCO 3 − ) type, and the waters are slightly alkaline, strongly reducing and of low ionic strength. The resemblance between the chemistries of pore- and fracture-waters arises from the buffering of the pore-water chemistry by matrix mineral/water interactions, and possible subsequent diffusion of the pore-water into the fractures. The chemical buffering mechanisms controlling the water chemistries (especially pH, alkalinity and Eh ), are discussed and modelled. The chemical conditions will minimize the mobility of many radionuclides in both shale matrix and in fractures. Chemical evidence from the fracture-filling calcite is consistent with this hypothesis and demonstrates the past immobility of U and Cs. Batch experiments carried out in gloveboxes under a N 2 atmosphere and with synthetic fracture-water show weaker retention of Cs by the calcite fillings than by the shale matrix. Due to its very low porosity, the shale matrix should behave like a membrane filter for colloids, further reducing the mobility of radionuclides. In fractures, interface structures between matrix walls and calcite veins should also hinder colloid migration (except perhaps in occasional centimetre-scale apertures).