Mostafa Fayek

O and Pb isotopic analyses of uranium minerals by ion microprobe and U-Pb ages from the Cigar Lake deposit

We apply a rapid and accurate in situ technique to make U–Pb isotopic measurements of complexly intergrown uranium minerals and oxygen isotopic analyes of uraninite from the unconformity-type Cigar Lake uranium deposit. Secondary uranium minerals intergrown with uraninite, such as coffinite, USiO4� nH2O and calciouranoite, CaU2O7� 5H2O, were identified by highresolution transmission electron microscopy (HRTEM). In situ U–Pb results from three stages of uraninite and coffinite define well-correlated arrays on concordia with upper intercepts of 1461F47, 1176F9, and 876F14 Ma (F1r). These ages are interpreted as the minimum ages of mineralization correlate with the timing of clay mineral alteration (f1477 Ma) associated with these unconformity-type uranium deposits, the ages of magnetization events at 1600–1450 and 900 Ma from the Athabasca Basin, and the Grenvillian Orogeny at f1100 Ma. In situ U–Pb isotopic analyses of uraninite and coffinite can document the Pb*/U heterogeneities that occur on the scale of 15–30 Am, thus providing relatively accurate information regarding the timing of fluid interactions associated with the evolution of these deposits. The high spatial resolution and precision of the ion microprobe allow us to measure d 18 O values of 20–100 Am unaltered portions of uraninites from Cigar Lake. The range of d 18 O values ( � 33.9 to � 20.4x) are among the lowest reported for unconformity-type deposits, confirming that conventional fluorination analyses of material sampled at the mm-scale are insufficient to avoid contamination from isotopically heavier coffinite and calciouranoite. D 2002 Elsevier Science B.V. All rights reserved.

Coupled cation and oxygen-isotope exchange between alkali feldspar and aqueous chloride solution

Abstract Nanoscale isotope and chemical images of grains of Amelia albite that were reacted with 2 m 18O-enriched solution of KCl show a correspondence between O-isotope exchange and K-Na exchange. Experiments were conducted for 4.6 d at 600 °C and 200 MPa. After 6 d, the 150 µm diameter albite grains had 5.20 µm rims in which Na was nearly completely replaced by K and in which the O was strongly enriched in 18O. The boundary between the core albite and the K-feldspar replacement is sharp and decorated with numerous pores. The distribution of Na and K, determined by electron probe microanalysis, is uniform within the core and rim and has an abrupt discontinuity at the interface. No evidence exists for K-Na interdiffusion at the resolution of electron probe. The NanoSIMS shows that the interface is also sharp in the distribution of 18O and 16O. The NanoSIMS image data and the electron probe data were coregistered; principal components analysis of the merged data set shows that 86% of the total variance in the data result from a single principal component loaded by the replacement of Na by K and 18O. The combined electron probe and NanoSIMS analyses indicate that both cation and isotope exchange occurred during solution and reprecipitation of the feldspar.

Mineral chemistry and oxygen isotopic analyses of uraninite, pitchblende and uranium alteration minerals from the Cigar Lake deposit, Saskatchewan, Canada

Abstract The Cigar Lake unconformity-type U deposit is one of the largest and highest grade U deposits in the Proterozoic Athabasca Basin, northern Saskatchewan, Canada. Cigar Lake has recently been the focus of an international, 3-a, collaborative program in which this U deposit was studied as a natural analogue for a spent nuclear fuel repository. The deposit is located near the eastern margin of the Athabasca Basin, 430 m below the surface, at the intersection between Hudsonian-age faults and the unconformity between Athabasca group sandstones and Aphebian metasediments. Three stages of U mineralization have been identified based on cross-cutting relationships and textures observed in thin section and back-scattered electron (BSE) images, O isotope data and chemical compositions. All stages of U mineralization have been variably altered to Ca-rich, U-hydrate minerals or uranyl oxide hydrate minerals and coffinite. U Pb chemical ages of the 3 stages of U mineralization from Cigar Lake coincide with the 3 major fluid events that precipitated clay and silicate minerals at 1500 Ma, 950 Ma, and 300 Ma, throughout the entire Athabasca Basin. Stage 1 and 2 uraninite and pitchblende have the lowest δ18O values that range from −30.1 ‰ to −15.2‰; whereas, stage 3 uraninite has δ18O values ranging from −10.0‰ to −3.4‰. Uranyl oxide hydrate minerals have δ18O values that range from −11.3‰ to −8.2‰; whereas, uranyl minerals have much higher δ18O values. Based on U Pb chemical ages,δ18O values, and petrographic relationships of U alteration minerals associated with primary U mineralization, the Cigar Lake U ore is similar to U ore from other unconformity-type U deposits in the Athabasca Basin. Therefore, the Cigar Lake ore deposit, although surrounded by clay and sandstone barriers, has been effected by the same fluid events that have altered other unconformity-type U deposits in the Athabasca Basin. The 3 stages of ore formation and associated alteration minerals permit the detailed study of fluids responsible for U deposition and alteration. This information provides the necessary context for the evaluation of the Cigar Lake deposit as a “natural analogue” for the disposal of spent nuclear fuel in underground vaults in rocks of the Canadian Shield.

IN SITU STABLE ISOTOPIC EVIDENCE FOR PROTRACTED AND COMPLEX CARBONATE CEMENTATION IN A PETROLEUM RESERVOIR, NORTH COLES LEVEE, SAN JOAQUIN BASIN, CALIFORNIA, U.S.A

ABSTRACT Knowledge of the evolution of carbonate cementation in hydrocarbon reservoirs is key to understanding the history of fluid flow during petroleum accumulation. The Stevens sands is a sequence of marine shales and deep-sea fan sands that was deposited within the Miocene Monterey Formation in the south-central part of the San Joaquin basin, California, during the upper Miocene (10-6 Ma). Rapid, high-precision in situ oxygen and carbon isotopic analyses of carbonate phases using the ion microprobe operated in multi-collection mode, in conjunction with electron microprobe analyses, indicate that carbonate cement zones within the Stevens sands at North Coles Levee (NCL) have had a complex and protracted fluid history. Three main generations of carbonate cement were identified. The relative timing of carbonate cement precipitation within the Stevens sands at NCL was estimated using the thermal and burial history of the San Joaquin basin, in situ oxygen isotope data, and cementation temperatures derived from equilibrium oxygen isotope fractionation factors for calcite-water and dolomite-water. Precipitation of these cement zones began soon after sediment deposition ( 7 Ma) and is ongoing. Early dolomite was precipitated at a temperature of 10°C, near the sediment-water interface, and soon after sediment deposition. Calcite cements, which are the most abundant variety, precipitated semicontinuously between 4 Ma and 5 Ma, at temperatures between 50°C and 65°C, and depths of 800 m to 1300 m. Fe-dolomite, which is paragenetically late, appears to have precipitated at temperatures near 100°C in response to pore-pressure reduction, which accompanied exploitation of the gas cap within the last 35 years. Carbon in these cements was likely derived from several sources including marine, maturing hydrocarbons, and a zone of methanogenesis.

Sulfur isotope microanalysis of sphalerite by SIMS: constraints on the genesis of Mississippi valley-type mineralization, from the Mascot-Jefferson City district, East Tennessee

Abstract The Mascot-Jefferson City (MJC) district is the most productive zinc district in East Tennessee. The deposits are of Mississippi Valley-type (MVT), hosted by carbonate rocks and dominated by sphalerite mineralization in strata-bound breccia bodies. We have utilized the high spatial resolution (20–30 μm) of the ion microprobe to obtain in situ sulfur isotopic analyses from discrete growth zones of sphalerite and analyses of associated pyrite. Two types of pyrite were noted: pre-sphalerite, diagenetic pyrite ( δ 34 S of −16.1‰ and −20.0‰) and syn-sphalerite pyrite that is intergrown with sphalerite ( δ 34 S of 31.3‰ to 33.7‰). Two textural varieties of sphalerite mineralization (banded and non-banded) were characterized. Banded sphalerite exhibits fine (μm to cm) banding that has grown around a carbonate substrate. Banded sphalerite has δ 34 S values from 27.8‰ to 51.0‰, high Cd contents (up to 0.96 wt.%) and dark areas that are likely due to minute inclusions of organic carbon. The non-banded sphalerite has δ 34 S values from 20.2‰ to 39.5‰, high Fe content and no organic inclusions. Regardless of the textural variety of sphalerite mineralization, our results show that the sulfur isotopic composition within a single polished thin section is heterogeneous and can vary by as much as 15‰. The δ 34 S values recorded in this study are among the heaviest ever reported for MVT deposits. The microscale δ 34 S variations and presence of high δ 34 S values have been previously undocumented for East Tennessee. The data presented here suggest multiple sulfur sources and sulfide precipitation by fluid mixing. The most probable scenario involves significant sulfur input from a sulfate- and metal-bearing fluid of variable δ 34 S composition mixing with a gas cap containing H 2 S of relatively homogeneous δ 34 S composition. The gas cap provided lesser amounts of sulfur to the system. Mixing of two isotopically different sulfur sources of variable proportions can account for the observed microscale variation in δ 34 S.

Diffusion of C and O in calcite from 0.1 to 200 MPa

Abstract We measured the diffusivity of C and O in calcite over the pressure range 0.1-200 MPa at 600-800°C in a pure CO2 atmosphere. The experiments were conducted on single, preannealed crystals of Chihuahuan calcite in an isotopically labeled atmosphere, and the diffusion profiles were measured by secondary ionization mass spectrometry (SIMS). At 800 °C, DC and DO are identical at 0.1 MPa at a value of ~10-13.5 cm2/s. The value of DC decreases to ~10-16 cm2/s with an increase in pressure to ~50 MPa and remains at that value to 200 MPa, but DO remains nearly constant at a value of ~10-14 cm2/s to 200 MPa. The identical values at low pressure indicate that C and O are migrating together as a carbonate anion. A simple model relates the diffusivity of carbonate anions to the formation of vacancies at the crystal surface, which predicts that DC ∞ 1/fCO₂. The prediction matches the observed decrease in DC with increasing pressure to 50 MPa. The shapes of the diffusion profiles for the low-pressure experiments indicate compositional dependence of D, which also suggests the influence of CO2 sorption on the diffusivity. The value of DC at 0.1 MPa can be fitted to the relation DC = 0.62 exp[(-291 kJ/mol)/RT]. The activation energy is nearly twice the value determined for DO at 100 MPa, ~166 kJ/mol. The change in slope for log DC vs. P and the change in Ea between 0.1 and 100 MPa suggest that the migrating C species changes from carbonate anions at low pressure to carbon atoms at P ≥ 50 MPa. The values of DO at 0.1 MPa can be fitted to DO = 0.017 exp[(-261 kJ/mol)/RT], approximately the same as for C at 0.1 MPa and similar to the relation for DO at 100 MPa: DO = 0.008 exp[(-242 kJ/mol)/RT]

Micro-structures associated with uraninite alteration

Abstract Primary uraninite from the Canadian Proterozoic unconformity-type uranium deposits have exceptionally low, but variable, δ 18 O values (−32 to −15‰). Although these uranium deposits have been studied extensively, the oxygen isotope systematics in uraninite from these deposits are poorly understood. X-ray powder-diffraction patterns of uraninite with both low and high δ 18 O values show that these uraninite samples are consistent with a composition of UO 2 and are texturally similar. Micro-diffraction and lattice images obtained using high-resolution transmission electron microscopy (HRTEM) of uraninite with the lowest δ 18 O values show that this uraninite is well crystallized and essentially defect-free. Energy dispersive X-ray spectroscopy of well-crystallized uraninite indicates relatively high Pb contents but low Si and Ca contents. In contrast, micro-diffraction and lattice images of uraninite with δ 18 O values near −18‰ show that this uraninite is polycrystalline, with micro-diffraction patterns that often show both streaking and concentric patterns. High-resolution images reveal sub-grain formation and rotation, formation of edge dislocations, low-angle grain-boundaries, and bent lattice-fringes. The uraninite is also characterized by relatively high Si and Ca contents and variable Pb contents. This indicates that incipient alteration occurs on a micro-scale as revealed by HRTEM. The sub-grains in this uraninite were preserved either because the annealing process in uraninite was retarded by the Si and Ca impurities, or they were formed during a late alteration event and the uraninite has had little time to anneal.

Obsidian hydration: A new paleothermometer

The natural hydration of obsidian was first proposed as a dating technique for young geological and archaeological specimens by Friedman and Smith (1960), who noted that the thickness of the hydrated layer on obsidian artifacts increases with time. This approach is, however, sensitive to temperature and humidity under earth-surface conditions. This has made obsidian hydration dating more difficult, but potentially provides a unique tool for paleoclimatic reconstructions. In this paper we present the first successful application of this approach, based on combining laboratory-based experimental calibrations with archaeological samples from the Chalco site in the Basin of Mexico, dated using stratigraphically correlated 14 C results and measuring hydration depths by secondary ion mass spectrometry. The resultant data suggest, first, that this approach is viable, even given the existing uncertainties, and that a cooling trend occurred in the Basin of Mexico over the past 1450 yr, a result corroborated by other paleoclimatic data.

A Rapid In Situ Method for Determining the Ages of Uranium Oxide Minerals: Evolution of the Cigar Lake Deposit, Athabasca Basin

We present a rapid and accurate technique for making in situ U-Pb isotopic measurements of uranium oxide minerals that utilizes both electron and ion microprobes. U and Pb concentrations are determined using an electron microprobe, whereas the isotopic composition of Pb for the same area is measured using a high-resolution ion microprobe. The advantages of this approach are: mineral separation and chemical digestion are unnecessary; homogenous uranium oxide standards, which are difficult to obtain, are not required; and precise and accurate U-Pb ages on ∼10 μm spots can be obtained in a matter of hours. We have applied our method to study the distribution of U-Pb ages in complexly intergrown uranium oxides from the unconformity-type Cigar Lake uranium deposit, Saskatchewan, Canada. In situ U-Pb results from early formed uraninite define a well-correlated array on concordia with upper and lower intercepts of 1467 ± 63 Ma and 443 ± 96 Ma (±lσ), respectively. The 1467 Ma age is interpreted as the minimum age of mineralization and is consistent with the age of clay-mineral alteration (∼1477 Ma) and magnetization of diagenetic hematite (1650 to 1450 Ma) that is associated with these unconformity-type uranium deposits and early diagenesis of the Athabasca Basin sediments. In situ U-Pb isotopic analyses of uraninite and coffinite can document the Pb∗/U heterogeneities that can occur on a scale of 15 to 30 μm, thus providing relatively accurate information regarding the timing of fluid interactions associated with the evolution of these deposits.

Oxygen isotopic composition of nano-scale uraninite at the Oklo-Okélobondo natural fission reactors, Gabon

Abstract High spatial resolution (10-30 μm), in situ oxygen isotopic analyses by secondary ion mass spectrometry (SIMS), coupled with high-resolution transmission electron microscopy (HRTEM), were used to show that uraninite from the Oklo-Okélobondo natural fission reactors that occur in near surface environments, have low δ18O values and nanotextures that are consistent with interaction with ground water. These low δ18O values (-14.4 to -8.5‰) suggest that the minerals exchanged with meteoric groundwater. In contrast, reactor zones that occur at depth have largely retained their original O isotopic composition (-10.2 to -5.6‰) and uraninites are well-crystallized and essentially defect-free. These observations clearly demonstrate that by combining both HRTEM and in situ O isotopic analyses by SIMS, it is possible to characterize the nano-scale porosity and postdepositional alteration of U-bearing phases.