Calcite Zones Research Articles

Reactive Transport Modeling of Interactions between Acid Gas (CO2 + H2S) and Pozzolan-Amended Wellbore Cement under Geologic Carbon Sequestration Conditions

The implementation of acid gas cosequestration requires investigation of the potential for acid gas leakage along existing wellbores at sequestration sites. In this study, the interaction between pozzolan-amended wellbore cement (35 vol % pozzolan/65 vol % cement, hereafter referred to as 35:65 sample) and acid gas (e.g., a mixture of CO2 and H2S) was simulated using the reactive transport code CrunchFlow. The model was applied to describe, interpret, and extrapolate scanning electron microscopy-backscattered electron (SEM-BSE) and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) results on pozzolan-amended cement samples exposed to a 1 wt % NaCl solution saturated with an acid gas mixture of 21 mol % H2S and 79 mol % CO2 under the temperature of 50 °C and pressure of 150 bar. Simulation outputs included calcite volume percentage, total Ca and S weight percentages in the solid phase, porosity, and effective permeability from the surface to the interior of pozzolan-amended wellbore cement. The model reproduced the observed calcite zone formed in the brine-cement interface region of the sample after 2.5 days of exposure. The model also predicted that the calcite layer became dense (calcite vol % in the layer reached 55%) after 90 days of exposure, consistent with the experimental observation. C–S–H was the primary Ca2+ source to form the calcite layer, followed by C3S and Ca(OH)2. The main observed products of reaction between the 35:65 sample and H2S were pyrite and ettringite. Pyrite was primarily formed within 0.5 mm from the brine-cement interface; ettringite mainly formed within 1 mm from the interface. The model simulated these reactions that only the interface region (up to 2 mm distance from the surface) of the 35:65 sample became porous after 30 years of exposure. However, this narrow porous region could still serve as a migration pathway for acid gas, which was indicated by the increase in effective parallel permeability values determined from the simulation results. Those results show consistency with results of neat cement samples exposed under similar conditions. An increase in H2S content (in the range of 0 mol % to 40 mol %) results in more dissolution of Ca-bearing minerals in cement and more precipitation of calcite. Overall, this study indicates that an increase of porosity and permeability of pozzolan-amended wellbore cement at the cement interface with brine saturated with CO2 and H2S can cause significant changes in effective permeability of the cement.

Energy resources and feeding guild structure of macroinvertebrate assemblages in the hyporheic zone of calcite depositing lake outlets

Abstract We investigated the distribution of particulate organic matter (POM) and its relation to flow velocity and tufa deposition rates (TDR) in the 4–10 cm deep zone at tufa barriers. We expected surface conditions affect the hyporheic zone, even in habitats with non-moving substrate in calcite depositing lake outlet streams. Additionally, we analyzed feeding guild structure of macroinvertebrate assemblages against environmental conditions, food resources and predatory abundance. Overall, more POM was deposited in the hyporheic zone of slow flow and low TDR habitats. Mean energy stored in the hyporheic POM was 163.4 kJ dm−3. While least abundant in mass, coarse POM represented the majority of organic matter energy stock. Coarse particles accumulated more at fast flow habitats and finer particles accumulated more at slow flow habitats. We propose that fast flow partly flushes small particles and partly macroinvertebrate fauna ingests and converts them to larger particles (fecal pellets) and transports them to the hyporheic zone. Collector gatherers dominated the assemblages (76%) and as passive filterers (4%) they thrived at fast flow sites. Grazers were the second dominant feeding guild (14%) and were more abundant at slow flow sites. Surface flow and predation pressure were the most important controls of hyporheic assemblage structuring. POM content was not as important. We propose that in the stable, non-moving tufa hyporheic zone food is plenty for the scarce fauna so macroinvertebrates are more reactive to predatory pressure. Moog's functional feeding guild allocation system, while more complex, proved more suitable for our analyses than classification system derived from Cummins’.

Accurate Mg/Ca, Sr/Ca, and Ba/Ca ratio measurements in carbonates by SIMS and NanoSIMS and an assessment of heterogeneity in common calcium carbonate standards

As archives of past climate variability, the micron and sub-micron scales of element:calcium (Me/Ca) variability in both biogenic and inorganic carbonates contain important geochemical information. Ideally working at smaller and smaller scales leads to higher temporal resolution of past changes, but more often it has revealed the strong overprint of other processes on the climate signal. Therefore, the role of SIMS and NanoSIMS techniques in studying paleoenvironmental proxies continues to increase. We evaluate the accuracy and precision of the CAMECA ims 7F-GEO and NanoSIMS-50L ion probes for measurements of Sr/Ca, Mg/Ca, and Ba/Ca ratios in carbonate minerals. Nine carbonate reference materials were examined for their 88Sr/42Ca, 24Mg/42Ca, and 138Ba/42Ca ratios using a primary O− beam with spot sizes of 20–40μm (SIMS) and 0.8–2μm (NanoSIMS). To assess accuracy, seven of these standards were analyzed for Sr/Ca and Mg/Ca with ID-ICP-MS. Variability in the elemental ratios arising from drift and changes in the tuning of the ims 7F-GEO over a nine month period is smaller than the chemical heterogeneity of the most frequently analyzed standards (OKA and Blue-CC). Across a whole crystal, Blue-CC is more homogeneous (1σ of 2.39% and 1.60% for Sr/Ca and Mg/Ca respectively) than OKA, but in the bulk matrix of OKA there is even less variability (1σ of 0.85% and 0.83% respectively). We find that carbonate samples can be accurately normalized to carbonate standards with significantly different absolute Me/Ca ratios. NanoSIMS intensity ratios follow counting statistics better than ±1% (2σ) at any one spot, but conversion to Me/Ca ratios increases the uncertainty. Two factors give rise to this limitation. First, the spatial heterogeneity of nominally homogeneous standards can lead to accuracy offsets that are as large as the ranges quoted above. Second, the NanoSIMS generates higher instrumental mass fractionation relative to SIMS. The combination of three different analytical techniques demonstrates that Blue-CC and homogeneous calcite zones in OKA are promising reference materials for precise analyses. Accuracy is crucially dependent on making independent measurements on exactly the same crystal of standard used in the SIMS and NanoSIMS machines.

The Spar Lake Strata-Bound Cu-Ag Deposit Formed Across a Mixing Zone Between Trapped Natural Gas and Metals-Bearing Brine

Ore formation at the Spar Lake red bed-associated strata-bound Cu deposit took place across a mixing and reaction zone between a hot oxidized metals-transporting brine and a reservoir of “sour” (H2S-bearing) natural gas trapped in the host sandstones. Fluid inclusion volatile analyses have very high CH4 concentrations (≥1 mol % in most samples), and a sample from the fringe of the deposit has between 18 and 36 mol % CH4. The ratio of CH4/CO2 in fluid inclusions appears to vary regularly across the deposit, with the lowest CH4/CO2 ratios from high-grade chalcocite-bearing ore, and the highest from the chalcopyrite-bearing fringe. The helium R/Ra isotope ratios (0.23–0.98) and concentrations define a mixture between crustal and atmospheric helium. The volatiles in fluid inclusions (CH4, CO2, H2S, SO2, H2, H2O, and other organic gases) and values of f O2 and temperature calculated from the volatiles data all show gradations across the deposit that are completely consistent with such a mixing and reaction zone. Other volatiles from the fluid inclusions (HCl, HF, 3He, 4He, N2, Ar) characterize the brine and give evidence for only shallow crustal fluids with no magmatic influences. The brine entered the gas reservoir from below and along the axis of the deposit and migrated out along bedding to the southwest, northeast, and northwest. Metals-transporting brines may have been fed into the host sandstones from the East Fault, but that remains uncertain. Abundant ore-stage Fe and Mn calcite cements from the reduced fringe have δ 13C values as low as −18.4‰, and many values less than −10‰, which indicate that significant carbonate was generated by oxidation of organic carbon from the natural gas. The zone of calcite cements with very low δ 13C values approximately envelopes chalcocite-bearing ore. Sulfur isotope data of Cu, Pb, and Fe sulfides and barite indicate derivation of roughly half of the orebody sulfide directly from sour gas H2S. That sour gas H2S had developed in steps known from other sedimentary basins, starting with (1) bacterial sulfate reduction (BSR) of seawater sulfate having δ 34S of about 20‰ and sequestering of the sulfide in organic matter in source rocks stratigraphically below the deposit host rocks, followed by (2) maturation of the sulfide-bearing organic matter into liquid petroleum with relatively homogeneous sulfide having δ 34S of 5 ± 5‰, then by (3) thermal cracking of the oil to CH4 and H2S with relatively homogeneous sulfide having δ 34S closely distributed, about 6‰. The CH4 and H2S migrated and were trapped in sandstones of the upper member of the Revett Formation, where they were later met by the 200°C metals-transporting brine. There was additional contribution of sulfide to ore from later thermochemical sulfate reduction (TSR) operating on sulfate δ 34S of 20 to 29‰ in both formation waters and metals-transporting solutions. A large range of δ 34S in sulfides resulted as the 6‰ sour gas sulfide was supplemented with varying proportions of 20 to 29‰ sulfide from TSR.

Theoretical Equilibrium Shape of Calcite. 2. [4̅41] Zone and Its Role in Biomineralization

The Hartman–Perdok analysis on the ⟨441⟩ zone of calcite was carried out, and the calculation of the surface and attachment athermal energies was performed on both unrelaxed and relaxed surfaces by using the interatomic Rohl potential and the GULP simulation code. The flat (F) character of the {1014} and {1120} forms and the stepped (S) character of the {0118} and {2134} forms contradict the observed occurrence frequency of natural crystals: {2134} > {0118} > {1014} > {1120}. A minor reduction of the relaxed surface energy values of both {0118} and {2134} forms could be sufficient to make the theoretical equilibrium shape of the crystal composed by the {1014}, {0112}, {1010}, {0001}, {0118}, and {2134} forms, which can explain the richness of the growth morphology of calcite, even if water adsorption is not considered. The theoretical analysis is focused on the {2134} scalenohedron to understand the adsorption of the enantiomers of some amino acids. Epitaxy models are described: (i) betwe...

Relationship between CO2-dominated fluids, hydrothermal alterations and gold mineralization in the Red Lake greenstone belt, Canada

Abstract The Red Lake greenstone belt is one of the foremost Au mining camps in Canada and hosts the world-class Campbell-Red Lake Au deposit. Belt-scale hydrothermal alteration is characterized by proximal ferroan dolomite zones associated with Au mineralization surrounded by distal calcite zones, both being accompanied by potassic alterations (sericite, muscovite, and biotite). At the Campbell-Red Lake and Cochenour deposits Au mineralization (in particular high-grade ore) is associated with silica and sulfides (especially arsenopyrite) that replace carbonate ± quartz veins and the host rocks. The prevalence of carbonic fluid inclusions and rare occurrence of aqueous-bearing inclusions in carbonate–quartz–Au veins in the Campbell-Red Lake deposit, and the consistency of homogenization temperatures of carbonic inclusions within individual fluid inclusion assemblages (FIA), have been interpreted to indicate that H2O-poor, CO2-dominated fluids were responsible for the carbonate veining and Au mineralization. Further studies of fluid inclusions in carbonate–quartz veins within and outside the deformation zone hosting the Campbell-Red Lake deposit (the Red Lake Mine trend) including the Cochenour Au deposit, the Redcon Au prospect, and outcrops in the distal calcite zone also reveal the dominance of carbonic fluid inclusions in vein minerals. These studies indicate that CO2-dominated fluids were flowing through fractures during carbonate vein formation and Au mineralization both within and outside major structures. The carbonic fluid may have been initially undersaturated with water, or it may have resulted from phase separation of an H2O–CO2–NaCl fluid. In the latter case, phase separation modeling indicates that the initial fluid likely had X CO 2 values larger than 0.8. Calculations based on hydrothermal mineral assemblages indicate X CO 2 values in the host rocks from 0.025 to 0.85, reflecting a change from CO2-dominated fluids in the fractures (veins) to H2O-dominated fluids in the host rocks away from the fractures. The CO2-dominated fluids were likely advected from granulite facies in the deeper crust, whereas the H2O-dominated fluids were derived from the ambient host rocks of amphibole to greenschist facies. Calculations based on CO2 requirements and source constraints indicate that the mineralizing fluids were likely two orders of magnitude more enriched in Au than the commonly assumed values of a few μg/L, which may explain why the Campbell-Red Lake deposit has a very high-grade of Au (average 21 g/t for the whole deposit and 81 g/t for the Goldcorp High-Grade zone). Fluid inclusion data suggest that the carbonate veining and Au mineralization likely took place at depths from 7 to 14 km. The development of crustiform–colloform structures in the carbonate ± quartz veins, which was previously interpreted to indicate relatively shallow environments, may alternatively have been related to extremely high fluid pressures and the CO2-dominated nature of the fluids, which could have enhanced the brittle properties of the rocks due to their high wetting angles.

Sequence stratigraphic controls on synsedimentary cementation and preservation of dinosaur tracks: Example from the lower Cretaceous, (Upper Albian) Dakota Formation, Southeastern Nebraska, U.S.A.

A thin cemented sandstone bed in the Upper Albian Dakota Formation of southeastern Nebraska contains the first dinosaur tracks to be described from the state. Of equal importance to the tracks are stable-isotope (C, O) analyses of cements in the track bed, especially in the context of data derived from generally correlative strata (sandstones and sphaerosiderite-bearing paleosols) in the region. These data provide the framework for interpretations of paleoenvironmental conditions, as well as a novel approach to understanding mechanisms of terrestrial vertebrate track preservation. High minus-cement-porosity (> 47%) and low grain-to-grain contacts (∼2.5) in the track bed indicate early (pre-compaction) lithification. Although phreatic cements dominate, the history of cementation within this stratigraphic interval is complex. Cathodoluminescence petrography reveals two distinct calcite zones in the track-bearing horizon and four cement zones in stratigraphically equivalent strata from a nearby section. The earliest calcite cements from both localities are likely coeval because they exhibit identical positive covariant trends ( δ 18O values of − 9.89 to − 6.32‰ and δ 13C values of − 28.01 to − 19.33‰ VPDB) and record mixing of brackish and meteoric groundwaters. All other calcite cements define meteoric calcite lines with δ 18O values clustering around − 9.42‰ and − 8.21‰ VPDB from the track-bearing horizon, and − 7.74‰, − 5.81‰, and − 3.95‰ VPDB from the neighboring section. Distinct meteoric sphaerosiderite lines from roughly correlative paleosols serve as a proxy for locally recharged groundwaters. Back-calculated paleogroundwater δ 18O estimates from paleosol sphaerosiderites range from − 7.4 to − 4.2‰ SMOW; whereas, meteoric calcite lines from the track horizon are generally more depleted. Differences in cement δ 18O values record changes in paleogroundwater recharge areas over time. Early calcite cements indicate mixing of fresh and brackish groundwaters during the syndepositional lithification of the track horizon. Later calcite cements, however, indicate recharge from a larger catchment basin that extended far inland. Therefore, the cements likely record a rise and subsequent fall in relative sea level. We conclude that scrutiny of the cement isotope geochemistry of genetically significant surfaces, especially track beds, can provide new data for interpreting sea level change.

Organic- and carbonate-rich soil formation ∼2.6 billion years ago at Schagen, East Transvaal district, South Africa

A ∼17-m paleosol sequence at Schagen, South Africa, which developed on a serpentinized dunite intrusion in a granite-gneiss terrain ∼2.6 Ga ago, is characterized by an alternating succession of thick (∼1–3 m) carbonate-rich (dolomite and calcite) zones and silicate-rich (serpentines, talc, and quartz) zones; the upper ∼8 m section is especially rich in organic C (up to ∼1.4 wt.%). Petrologic and geochemical data suggest the upper ∼8 m section is composed of at least three soil profiles that developed on: (i) silicate-rich rock fragments (and minerals) that were transported from local sources (serpentinite and granite) by fluvial and/or eolian processes; and (ii) dolomite and calcite zones that formed by locally discharged groundwater. The Mg and Fe in the paleosol sequence were largely supplied from local sources (mostly serpentinite), but the Ca, Sr, and HCO 3 − were supplied by groundwater that originated from a surrounding granite-gneiss terrain. In the uppermost soil profile, the (Fe is retained, the Fe 3+/Fe 2+ ratio increases, and ferri-stilpnomelane is abundant. These data suggest the atmospheric pO 2 was much greater than ∼10 −3.7 atm (>0.1% present atmospheric level [PAL]). The carbonaceous matter in the soils is intimately associated with clays (talc, chlorite, and ferri-stilpnomelane) and occurs mostly as seams (20 μm to 1 mm thick) that parallel the soil horizons. These occurrences, crystallographic structures, H/C ratios, and δ 13C org values (−17.4 to −14.4‰ PDB) suggest that the carbonaceous matter is a remnant of in situ microbial mats, originally ∼1 to ∼20 mm thick. The microbial mats developed: (a) mostly on soil surfaces during the formation of silicate-rich soils, and (b) at the bottom of an evaporating, anoxic, alkaline pond during the precipitation of the Fe-rich dolomite. These δ 13C org values are difficult to be explained by a current popular idea of a methane- and organic haze-rich Archean atmosphere (Pavlov et al., 2001); these values, however, can be easily explained if the microbial mats were composed of cyanobacteria and heterotrophs that utilized the remnants of cyanobacteria in a strongly evaporating environment.

Petrology, U/Pb dating and (C-O) stable isotope constraints on the source and evolution of the adakite-related Mezcala Fe-Au skarn district, Guerrero, Mexico

The Mezcala gold district, Guerrero, Mexico, is a Cretaceous to early Paleocene oxidized Fe-Au skarn deposit, genetically associated with an adakitic magmatic event of 63±2 Ma, dated by ion-probe U/Pb on zircons. The inner and outer alteration patterns and the mineralogical sequence (high garnet/pyroxene ratio, high ferric/ferrous ratio >1, predominance of Fe-poor garnet and pyroxene, low total sulfides) found in Mezcala are compatible with the description of an oxidized gold skarn type. Carbon and oxygen isotope analyses of the inner calcite zone (–9.98 to –11.64 and +13.21 to +14.59, respectively) unequivocally support a magmatic signature of the hydrothermal fluid. Carbon and oxygen isotopes from the outer calcite zone (–8.81 to +3.45 and +12.95 to +22.77, respectively) suggest a complex mechanism of degassing and subsequent cooling/dilution of the resulting magmatic brine with the meteoric water. Gold appears to be closely associated with the adakite stock, whereas its transport is related with the outflow of highly oxidized magmatic brines from the intrusion. The gold precipitation is triggered by cooling/dilution of the degassed magmatic brine by the meteoric fluids

Relation Between Sedimentation and Diapirism at the Davis Hill Salt Dome, Liberty Co., Texas

ABSTRACT The Davis Hill dome is a relatively large salt stock located in the northeastern part of the Houston salt dome province. The minimum depth is about 800 ft to the cap rock and about 1,200 ft to salt. The source layer (Jurassic Louann Salt) lies at a depth of at least 30,000 ft. Seismic and well log interpretation suggests that diapir growth took place from Late Cretaceous to Plio-Pleistocene. Passive growth (downbuilding) of the dome ceased during the Early Oligocene (Vicksburg) and was followed by active growth (upbuilding) of the dome. Present-day topographic relief at Davis Hill is 175 ft above the surrounding coastal plain. A radial pattern of oxbow lakes above the dome suggests that the dome still is rising, but the topographic maximum does not correspond to the apex of the dome. Studies of cores indicate that the cap rock is about 440 ft thick and comprises, from bottom to top, anhydrite, gypsum and calcite zones. The anhydrite zone represents the less soluble components of the salt that accumulated as the halite was dissolved by pore fluids. The calcite zone formed by bacterial alteration of sulfate accompanying hydrocarbon destruction. The calcite zone in these cores contains significant amounts of sulfide minerals and oil. The sulfiderich calcite zone is approximately 40 ft thick and is dominated by locally massive iron sulfides. These sulfide concentrations resulted from the interaction of deep heated metal-bearing formational brines with bacterially derived reduced sulfur in the ambient cool ground waters.

The Petrogenetic Association of Carbonatite and Alkaline Magmatism: Constraints from the Spitskop Complex, South Africa

The 1341 Ma old Spitskop Complex in South Africa is one of a series of intrusions of alkaline affinity, which were emplaced into the central Kaapvaal Craton over the time period 1.4–1.2 Ga. Spitskop contains calcite and dolomite carbonatite closely associated with pyroxenite, ijolite and nepheline syenite, and provides an ideal opportunity to study the petrogenetic relationships between alkaline silicate and carbonatite magmatism. The pyroxenites are not alkalic and are preserved as xenoliths within a plug-like intrusion of ijolite. Nepheline syenites are highly peralkaline, though not agpaiitic, and intrude the ijolites as a series of sheets. These units are cut by a plug of carbonatite composed of an incomplete marginal zone of calcite and dolomite–calcite carbonatite, and a larger central zone of ferroan dolomite carbonatite. Clinopyroxene compositions change systematically from diopside-rich compositions in the pyroxenites to aegirine-augite–hedenbergite in the ijolites to acmite-dominated compositions in the nepheline syenites. Whole-rock chemical data indicate, however, that the nepheline syenites and ijolites are unlikely to be related through fractional crystallization of any reasonable combination of their component minerals (clinopyroxene, nepheline, perthite) from a common parental magma. Low total rare earth element (REE) concentrations and flat to convex-up normalized patterns in the syenites contrast strongly with the steep, light REE (LREE)-enriched patterns in the ijolites. The silicate and carbonatite components differ markedly in their εSr–εNd compositions, the carbonatites having more depleted values (εSr –10 to +10; εNd –1 to –8) than the silicates (εSr 0 to +33; εNd –8 to –13). In addition, the calcite-rich carbonatites have more negative εNd (–6 to –8) than the dolomite carbonatites (–1 to –4). Contrasting isotopic compositions along with the geochemical variations within and between the silicates and carbonatites argue against them being derived from conjugate immiscible liquids. Instead, it is proposed that the carbonatites evolved from primitive carbonate liquids produced directly by low-degree melting of carbonated mantle peridotite. A preliminary model is presented to explain how mantle carbonatite melts can ascend through the mantle and into the crust. It is proposed that the silicate magmatic rocks associated with the carbonatite are produced by melting of enriched mantle lithosphere induced by the influx of deeper-sourced carbonatite melts.

SIMS analyses of minor and trace element distributions in fracture calcite from Yucca Mountain, Nevada, USA

Fracture-lining calcite samples from Yucca Mountain, Nevada, obtained as part of the extensive vertical sampling in studies of this site as a potential high-level waste repository, have been characterized according to microbeam-scale (25–30 μm) trace and minor element chemistry, and cathodoluminescent zonation patterns. As bulk chemical analyses are limited in spatial resolution and are subject to contamination by intergrown phases, a technique for analysis by secondary ion mass spectrometry (SIMS) of minor (Mn, Fe, Sr) and trace (REE) elements in calcite was developed and applied to eighteen calcite samples from four boreholes and one trench. SIMS analyses of REE in calcite and dolomite have been shown to be quantitative to abundances < 1 × chondrite. Although the low secondary ion yields associated with carbonates forced higher counting times than is necessary in most silicates, Mn, Fe, Sr, and REE analyses were obtained with sub-ppm detection limits and 2–15% analytical precision. Bulk chemical signatures noted by Vaniman (1994) allowed correlation of minor and trace element signatures in Yucca Mountain calcite with location of calcite precipitation (saturated vs. unsaturated zone). For example, upper unsaturated zone calcite exhibits pronounced negative Ce and Eu anomalies not observed in calcite collected below in the deep unsaturated zone. These chemical distinctions served as fingerprints which were applied to growth zones in order to examine temporal changes in calcite crystallization histories; analyses of such fine-scale zonal variations are unattainable using bulk analytical techniques. In addition, LREE (particularly Ce) scavenging of calcite-precipitating solutions by manganese oxide phases is discussed as the mechanism for Ce-depletion in unsaturated zone calcite.

Geochemistry of post-uplift calcite in the Permian Basin of Texas and New Mexico

Integration whole-oil gas chromatography of produced oil and oil inclusions, formation-water chemistry, and stable isotopes has identified environment-diagnostic differences in calcite cements between oil field and outcrop environments in the Permian Basin of Texas and New Mexico. Calcite-δ13C and fluid-inclusion composition are the most diagnostic of pore-fluid evolution and can help interpret rock-fluid reactions. Late-stage calcite cement in the northwestern part of the basin formed in a meteoric aquifer that was emplaced by Neogene-age uplift and tilting of the Guadalupe Mountains. Where the confined aquifer intersects the Henderson oil field, the water, which is less saline than sea water, has 900-1,400 ppm bicarbonate alkalinity because of oil oxidation and contains 750 ppm H2S as a result of anhydrite calcitization and sulfate reduction. The oil field has been severely damaged by biodegradation. Modeling of δ13C in pore-filling calcite from the field (mean δ13C = -17‰ PDB) suggests oxidation of oil provided nearly 100% of the carbon in the cement. Comparison of gas chromatograms of produced oil and oil liberated from fluid inclusions in calcite shows that inclusion oil is older and more severely biodegraded (paraffin-free) than produced oil. This implies that oil in the reservoir was remobilized soon after Neogene-age meteoric invasion and carbonate cementation. The Algerita Escarpment in the Guadalupe Mountains is the site of active meteoric water recharge and growth of phreatic calcite cement. The phreatic cement contains single-phase, aqueous fluid inclusions. The cement is depleted in 13C to an extent that is diagnostic of a 1:1 mixture of soil-CO2 from decay of C4-type plants (desert grasses) and carbon derived from dolomite matrix by ground-water dissolution. This phreatic zone calcite displays a trend of 13C enrichment with depth due to increased rock-water interaction along the flow path.

Geology and geochemistry of wall-rock alteration at the Carlin gold deposit, Nevada

The Carlin disseminated gold deposit occurs in an autochthonous sequence of Paleozoic sedimentary rocks exposed in a structural window in the Roberts Mountains thrust in north-central Nevada. The upper 175 m of the Silurian Roberts Mountains Formation hosts the majority of ore at Carlin and is characterized by laminated, fine-grained, calcareous and/or dolomitic argillaceous siltstone with local coarser grained siltstones and 50-cm-thick lenticular interbeds of sand- and granule-sized calcareous bioclastic debris or fossil hash. Detailed studies of drill core and exposures in the East pit of the Carlin mine show that alteration and mineralization are zoned away from crosscutting fault conduits and these more permeable bioclastic beds, indicating that these two features were major inflow zones for hydrothermal fluid.In unoxidized rocks, unaltered calcareous siltstone (1) containing quartz, dolomite, calcite, illite, K feldspar, and pyrite is progressively converted to assemblages of (2) quartz + dolomite + calcite + illite + pyrite, (3) quartz + dolomite + illite-K mica + pyrite, (4) quartz + illite-K mica + pyrite, and (5) quartz + kaolinite-dickite + pyrite adjacent to inflow zones where jasperoids are developed. Gold most consistently enriches the zone of calcite and dolomite removal (3 and 4 above), though it occurs in all zones, locally in high concentrations. This zoned alteration was accomplished by a CO 2 -rich acidic fluid. This acidic alteration enhanced the passage of fluids by extensive carbonate removal to form zones of higher permeability.Oxidation is wholly a supergene effect related to deep weathering, because the oxidation is superimposed on both mineralized and altered rocks with only minor effect on the major element chemistry; it has produced low-temperature goethitic Fe oxides rather than higher temperature hematite and is not spatially related to Au distribution at the mine or on a district scale.Because of extensive carbonate removal leading to local volume reduction through collapse and/or compaction, geochemical effects are examined using ratios to relatively immobile elements such as Al and Ti. Extensive depletion of Ca, Mg, aud CO 2 and introduction of Si, Au, and S have occurred. Potassium is depleted in the conversion of illite to dickite-kaolinite in proximal silicified inflow zones, and Fe enriches some pyritized rock. Carbonate removal and silicification are two separate processes, both of which are spatially associated with mineralization. Mineralized decarbonated rocks and barren footwall rocks commonly are not silicified, and intensely silicified proximal alteration zones are generally low grade.

Effect of meteoric water flow on calcite cementation in the Middle Jurassic Oseberg Formation, well 30/3-2, Veslefrikk Field, Norwegian North Sea

The Middle Jurassic delta front sandstones of the Oseberg Formation in well 30/3-2 in the Veslefrikk oilfield contain abundant calcite cemented intervals with thicknesses of up to 2.15 m. Most of the calcite cement is characterized by δ 18O(PDB) values in the range −5 to −10‰ and by low strontium concentrations, mostly in the range 100–500 ppm. The low strontium concentration in the studied calcite cement can probably best be explained by the precipitation of calcite during meteoric water flushing. δ 18O(PDB) values of −5 to −10‰ have also been considered characteristic of calcite cement precipitated from meteoric water, and ubiquitous authigenic kaolinite and feldspar dissolution in the studied sandstones suggests that extensive meteoric water flushing of the Oseberg Formation has taken place during and soon after deposition. A few of the studied calcite cemented intervals contain a calcite cement with a much higher strontium content (750–1430 ppm) and more negative δ 18O(PDB) values (−11.1 to −14.3‰). These calcite cemented intervals are thought to have formed at greater depth after meteoric water flushing had terminated. The calcite cement with the highest strontium concentration and the most negative δ 18O values occurs within calcite cemented intervals with sharp boundaries. The calcite cement with a low strontium concentration and intermediate δ 18O values is found within calcite cemented intervals with gradational boundaries characterized by a transition zone of calcite cemented specks in uncemented sandstone. This difference in cementation pattern can be explained by the effect of pore water flow-rates on the nucleation and growth of calcite cement.

Evolution of metal ratios and δ34S composition of sulfide mineralization during anhydrite cap rock formation, Hockley Dome, Texas, U.S.A.

Metal zonation and sulfur isotope patterns of cap rock-hosted sulfides reveal a complex evolution of the sulfide-depositing system at the Hockley salt dome. The general metal trend observed is an overall increase of Zn relative to Pb downward in the anhydrite cap rock. In the upper cap rock section, the Pb ( Pb+Zn) ratios commonly range from 0.25 to 0.40. Ratios of <0.20 and commonly <0.10 are typical of the lower cap rock section. The concentration of Ag also appears to increase in the lower part of the anhydrite cap rock. Textural evidence indicates that some of the metal concentrations represent stratiform sulfides that were precipitated at the salt-cap rock contact as the anhydrite zone accumulated by sequential underplating. The metal ratio data suggest that the sulfide-precipitating fluid became more Zn-, and possibly Ag-rich, during anhydrite cap rock accumulation. δ 34S-values of sulfide minerals within the Hockley Dome cap rock range from −30 to +4‰ (CDT) (59 analyses). A profile of δ 34S-values from a single drill hole indicates that sulfide minerals become progressively heavier with depth to the approximate middle of the cap rock where the isotopic trend reverses. The inflection point corresponds with a local highly mineralized biogenic calcite zone. A mixing model involving variable contributions of isotopically heavy-and light-sulfur end-members appears to best account for the observed isotopic trends. Fault-leaked H 2S from mature oil and gas reservoirs is a possible extrinsic source of heavy sulfur, but local thermochemical sulfate reduction of cap rock sulfate cannot be eliminated by existing data. Bacterial reduction of cap rock sulfate is believed to be the source of light H 2S. The relative abundance of deep-sourced metalliferous brines appears to have increased with time during the accretion of the upper part of the anhydrite cap rock. Biogenic H 2S subsequently began to dominate the mixed sulfur reservoir resulting in a −15‰ shift and relatively light δ 34S-values of pyrite to the present cap rock-salt contact.

Compositional, Order/Disorder, and Stable Isotope Characteristics of Al‐Fe Epidote, State 2–14 Drill Hole, Salton Sea Geothermal System

Epidote (Ca2Fe3Si3O12(OH)‐Ca2Al3Si3O12(OH)) is a common hydrothermal mineral in metasediments and veins at depths &gt;900 m in the State 2–14 drill hole of the Salton Sea geothermal system. The mole fraction of Ca2Fe3Si3O12(OH) in epidotes (Xps) from this drill hole ranges from 0.11 to 0.42, and complex compositional zoning of octahedral Fe3+ and Al is typical within single grains. With increasing depth there is an overall, but irregular, decrease in the Fe3+ content of epidotes in metasandstones. In most samples, vein epidotes are more Fe3+ ‐rich and exhibit a wider compositional range than metasandstone epidotes from the same depth. Octahedral Fe3+ in the M(1) sites in four epidotes, evaluated by 57Fe Mössbauer spectroscopy, ranges from 7.5 ± 1.7% to 11.4 ± 1.5% of the total iron. The most ordered epidote formed in veins at 2618 m in the biotite zone. This epidote has an ordering parameter (σ = 1–2XFe3+,M(1)) of 0.85 ± 0.03 which corresponds to a calculated state of equilibrium order/disorder at ∼390° ± 60°C, in close agreement with the probable downhole temperature of ∼340°C, Two epidote samples from the chlorite + calcite zone (1420 m, ∼265°C; 1867 m, ∼300°C) are more disordered, corresponding to calculated states of equilibrium order/disorder of &gt;450°C, These data suggest that the analyzed epidotes from the chlorite + calcite zone are in a metastable state of substitutional order/disorder. The measured value of δDepidote from 1420 m is −96‰, which is 5–6‰ lighter than epidotes from 1867 m (δDepidote = −90‰), 2227 m (δDepidote = −91‰), and 2618 m (δDepidote = −90 ‰). When compared to the reservoir fluid from the December 1985 flow test, the derived fractionation of hydrogen isotopes between epidote and the geothermal brine is −19‰ at 1867 m and ∼340°C. This value is in accord with published experimental hydrogen isotope fractionations between iron‐rich epidote and aqueous electrolyte solutions. Therefore, considering the complexity of the Salton Sea brine, the isotopic temperature is similar to the inferred downhole temperature.

Electron microscope analysis of zoned dolomite rhombs in the Jet Rock Formation (Lower Toarcian) of the Whitby area, U.K.

AbstractZoned dolomite rhombs which occur in the organic-rich Jet Rock Formation (Toarcian) of northeast England have been studied using backscattered scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray microanalysis. The rhombs, which are 5–20 μm in size, are variable in chemical composition, but many consist of a dolomite core surrounded by a zone of ferroan dolomite, ankerite or ferroan calcite. Zoned rhombs occur in early-diagenetic calcite-cemented concretions and layers as well as dispersed throughout the shales. Distributional and textural evidence suggests that they formed mainly by primary precipitation from pore fluids within the zone of bacterial sulphate reduction. The ferroan outer zones probably formed after burial below the sulphate reduction zone when insufficient H2S was available to react with all the Fe2+in solution.

Dolomitization in a Mixing Zone of Near-Seawater Composition, Late Pleistocene, Northeastern Yucatan Peninsula

ABSTRACT Patches of dolomite occur in cores of reefal limestone from the shallow subsurface on the northeastern coast of the Yucatan Peninsula. This limestone accumulated during an interglacial high stand of sea level about 200,000 years ago. Dolomitization was preceded by freshwater diagenesis, including precipitation of sparry calcite cement, stabilization of Mg-calcitic skeletal fragments, and partial dissolution of aragonitic components. This suggests a predolomitization lowering of sea level with the consequent freshening of pore water. The subsequent precipitation of dolomite indicates a return to high sea level with the consequent increase in Mg/Ca ratio of pore water. Dolomitization took place during a brief high stand of sea level, either shortly after deposition about 200,000 yr BP, r, more likely, about 125,000 yr BP. Dolomite occurs both as microcrystalline replacement dolomite and as cement. The cement is part of the following diagenetic sequence: 1) limpid euhedral-subhedral calcian dolomite crystals, 2) zoned dolomite crystals with zones formed by variations of the calcium/magnesium ratio in dolomite, 3) layers of alternating calcian dolomite and magnesian calcite or calcite, and 4) calcite. This sequence represents the progressive freshening of ground water during the initial stage of a fall in sea level. Average cation composition of the limpid dolomite cement is Ca57Mg43 (electron microprobe analysis). Zoned cement crystals are composed of Ca57-59Mg43-41 layers and Ca62Mg38 layers. Most of the higher-calcium dolomite layers are dissolved, forming hollow-zone crystals. In cement with alternating dolomite and calcite zones, the calcite is Ca99-97Mg1-3 (low-Mg calcite) and Ca96-93Mg4-7 (Mg calcite). The dolomite and Mg calcite zones are partially to totally leached. 18O compositions of Yucatecan dolomite and of modern ground water suggest dolomite precipitation from mixed water ranging from about 75% seawater, 25% freshwater to nearly all seawater. (Isotope analyses are for the most stable calcian dolomites; more soluble, calcium-rich dolomite presumably is analyzed with calcite and thought to be isotopically lighter than the less soluble dolomite.) In the cement sequence, the most stable dolomite is followed by more soluble dolomite as ground water becomes less saline. Isotope analyses, together with position of dolomite in the cement sequence, suggest the most stable calcian dolomite (including limpid dolomite) precipitated from mixed water with large proportions of seawater, and the less stable, more calcian dolomite precipitated from fresher mixed water.

Preliminary Paleomagnetic Investigations of Winnfield Salt Dome Cap Rock, Louisiana: ABSTRACT

Quarrying operations at Winnfield salt dome in the North Louisiana basin provide access to calcite, gypsum, and anhydrite cap-rock zones. Sulfide laminae in the anhydrite zone are comprised dominantly of pyrhotite with lesser amounts of sphalerite, galena, pyrite, and marcasite. Sulfides cement euhedral anhydrite grains and represent the products of the episodic introduction of metalliferous waters along the salt-anhydrite interface during halite dissolution and residual anhydrite accumulation. Thus, sulfide laminae provide a chronological record of anhydrite cap-rock accretion. Two hundred oriented samples were collected in stratigraphic sequence, covering much of the exposed anhydrite section. Alternating field demagnetization readily revealed the magnetic polarity of most samples. Using only reversely magnetized samples with a well-defined stable magnetization (N = 50, ^agr95 = 6.2°) yields a pole position at 71.4°N, 125.7°E, which implies that the sampled cap-rock sequence formed in the Late Jurassic. This age is consistent with geologic evidence indicating that cap-rock formation began in the late Jurassic and was most intense during the Early Cretaceous. A densely sampled 45-ft stratigraphic interval contains a sequence of normal and reverse polarity zones. Assuming a constant formation rate, these zones can be compared with the M-anomaly sequence. A growth rate of about 67 ft/m.y. (20 m/m.y.) is indicated. This value is about 30 times less than estimates of salt dome growth rates. Because a 10 to 50-fold decrease in volume is associated with halite dissolution and anhydrite accretion, the paleomagnetically determined value for cap-rock formation rate is reasonable. This investigation is the first such study undertaken. The results are encouraging and offer a unique approach for investigating the timing of various geologic processes related to salt dome formation. End_of_Article - Last_Page 1422------------