Uranium minerals from the San Marcos District, Chihuahua, Mexico

The mineralogy of the two uranium deposits (Victorino and San Marcos I) of Sierra San Marcos, located 30 km northwest of Chihuahua City, Mexico, was studied by optical microscopy, powder X-ray diffraction with Rietveld analysis, scanning electron microscopy with energy dispersive X-ray analysis, inductively coupled plasma spectrometry, and gamma spectrometry. At the San Marcos I deposit, uranophane Ca(UO₂)₂Si₂O₇·6(H₂O) (the dominant mineral at both deposits) and metatyuyamunite Ca(UO₂)(V₂O₈)·3(H₂O) were observed. Uranophane, uraninite (UO_2+x), masuyite Pb(UO₂)₃O₃(OH)·3(H₂O), and becquerelite Ca(UO₂)₆O₄(OH)₆ ·(8H₂O) are present at the Victorino deposit. Field observations, coupled with analytical data, suggest the following sequence of mineralization: (1) deposition of uraninite, (2) alteration of uraninite to masuyite, (3) deposition of uranophane, (4) micro-fracturing, (5) calcite deposition in the micro-fractures, and (6) formation of becquerelite. The investigated deposits were formed by high-to low-temperature hydrothermal activity during post-orogenic evolution of Sierra San Marcos. The secondary mineralization occurred through a combination of hydrothermal and supergene alteration events. Becquerelite was formed in situ by reaction of uraninite with geothermal carbonated solutions, which led to almost complete dissolution of the precursor uraninite. The Victorino deposit represents the second known occurrence of becquerelite in Mexico, the other being the uranium deposits at Peña Blanca in Chihuahua State.     

Modeling non-steady state radioisotope transport in the vadose zone - A case study using uranium isotopes at Peña Blanca, Mexico

Current models using U- and Th-series disequilibria to study radioisotope transport in groundwater systems mostly consider a steady-state situation. These models have limited applicability to the vadose zone (UZ) where the concentration and migratory behavior of radioisotopes in fluid are often transitory. We present here, as a first attempt of its kind, a model simulating the non-steady state, intermittent fluid transport in vadose layers. It provides quantitative constraints on in-situ migration of dissolved and colloidal radioisotopes in terms of retardation factor and rock-water interaction (or water transit) time. For uranium, the simulation predicts that intermittent flushing in the UZ leads to a linear relationship between reciprocal U concentration and ²³⁴U/²³⁸U ratio in percolating waters, with the intercept and slope bearing information on the rates of dissolution and α-recoil of U isotopes, respectively. The general validity of the model appears to be borne out by the measurement of uranium isotopes in UZ waters collected at various times over a period during 1995-2006 from a site in the Peña Blanca mining district, Mexico, where the Nopal I uranium deposit is located. Enhanced ²³⁴U/²³⁸U ratios in vadose-zone waters resulting from lengthened non-flushing time as prescribed by the model provide an interpretative basis for using ²³⁴U/²³⁸U in cave calcites to reconstruct the regional changes in hydrology and climate. We also provide a theoretical account of the model’s potential applications using radium isotopes.     

Extension and magmatism in the Cerocahui basin,northern Sierra Madre Occidental,western Chihuahua,Mexico

The Sierra Madre Occidental of northwestern Mexico is the biggest silicic large igneous province of the Cenozoic, yet very little is known about its geology due to difficulties of access to much of this region. This study presents geologic maps and two new U-Pb zircon laser ablation inductively coupled plasma mass spectrometry ages from the Cerocahui basin, a previously unmapped and undated ~25 km-long by ~12 km-wide half-graben along the western edge of the relatively unextended core of the northern Sierra Madre Occidental silicic large igneous province. Five stratigraphic units are defined in the study area: (1) undated welded to non-welded silicic ignimbrites that underlie the rocks of the Cerocahui basin, likely correlative to Oligocene-age ignimbrites to the east and west; (2) the ca. 27.5–26 Ma Bahuichivo volcanics, comprising mafic-intermediate lavas and subvolcanic intrusions in the Cerocahui basin; (3) alluvial fan deposits and interbedded distal non-welded silicic ignimbrites of the Cerocahui clastic unit; (4) basalt lavas erupted into the Cerocahui basin following alluvial deposition; and (5) silicic hypabyssal intrusions emplaced along the eastern margin of the basin and to a lesser degree within the basin deposits.

The main geologic structures in the Cerocahui basin and surrounding region are NNW-trending normal faults, with the basin bounded on the east by the syndepositional W-dipping Bahuichivo–Bachamichi and Pañales faults. Evidence of syndepositional extension in the half-graben (e.g. fanning dips, unconformities, coarsening of clastic deposits toward basin-bounding faults) indicates that normal faulting was active during deposition in the Cerocahui basin (Bahuichivo volcanics, Cerocahui clastic unit, and basalt lavas), and may have been active earlier based on regional correlations.

The rocks in the Cerocahui basin and adjacent areas record: (1) the eruption of silicic outflow ignimbrite sheets, likely erupted from caldera sources to the east during the early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up, mostly prior to synextensional deposition in the Cerocahui basin (pre-27.5 Ma); (2) synextensional late Oligocene mafic-intermediate composition magmatism and alluvial fan sedimentation (ca. 27.5–24.5 Ma), which occurred during the lull between the Early Oligocene and early Miocene pulses of the ignimbrite flare-up; and (3) post-extensional emplacement of silicic hypabyssal intrusions along pre-existing normal faults, likely during the early Miocene pulse of the ignimbrite flare-up (younger than ca. 24.5 Ma). The timing of extensional faulting and magmatism in the Cerocahui basin and surrounding area generally coincides with previous models of regional-scale middle Eocene to early Miocene southwestward migration of active volcanism and crustal extension in the northern Sierra Madre Occidental controlled by post-late Eocene (ca. 40 Ma) rollback/fallback of the subducted Farallon slab.     

Hydrogeological conditions and quality of ground waters in northern Banat,Pannonian basin

Geological relationships, hydrogeology and chemical composition of ground water in northern Banat were studied through the period 2000–2004 using the available background data from published and unpublished sources. Northern Banat is the extreme northeastern part of the Republic of Serbia and a geotectonic part of the vast Pannonian depression. The source of domestic and industrial water supply is only groundwater from artesian and subartesian aquifers of Lower Pleistocene (Q₁¹) and Upper Pleistocene (Pl₃²) sand deposits. The ground water, “peculiar” in chemical composition, is the only source of drinking water in the arid area. A notable variation in the chemical composition of artesian waters within the same geotectonic unit (Pannonian basin), abstracted for municipal water supplies of Kikinda, Novi Knezevac and Djala, has attracted attention of these authors. Our paper attempts to interpret the variation in the chemical composition of ground water and the cause of the variation by the interaction of ground water and rocks forming the aquifers on the case example of the water supply sources for the three mentioned towns. With respect to the depth and lithology of the aquifers, we interpret the varied chemical compositions of waters in the mentioned sources as a consequence of natural factors (geological environment), geological relationships and hydrogeological conditions.     

Distribution of mineralization in time and space in Chihuahua,Mexico

Physiographically, Chihuahua includes Sierra Madre Occidental (western) and Mesa Central (eastern) provinces. Stratigraphically, coal occurs in Triassic-Jurassic strata and upper Cretaceous rocks, porphyry Cu-Mo deposits in stocks, fissure-vein Ag-Au lodes in andesite-rhyolite sequences and in Cretaceous sediments, Mn fissure-vein deposits in rhyolitic extrusives, U in fissures and contacts in rhyolites, gypsum in Late Jurassic and Early Cretaceous strata, manto Pb-Zn-Ag and stratiform Cu-Ba deposits separately in Lower Cretaceous strata, magmatic segregation and contact Fe deposits in trachyte and rhyolite and adjacent intrusives respectively, and fluorite as mantos and veins in Cretaceous sediments. Assemblages occur in zones that strike NW paralleling other mineral belts in adjacent northwestern Mexico. Temporal, spatial and genetic relationships link magmatic-hydrothermal origin deposits to magmatic suites that vary from calc-alkaline (west) to alkaline (east). Favorable localizing structures include doming by batholiths, normal faulting and contact metamorphism in the Sierra Madre Occidental Orogen and by folding, thrusting and stock emplacement in the eastern Chihuahua Tectonic Belt, all during Laramide time. Mineralization persisted through the Oligocene interval, although post-mineral volcanism continued until Late Cenozoic time. Contemporaneous events in the eastern Pacific basin suggest that tectonism, magmatism and metallogenesis are consistant with a convergent plate model along a continental margin.     

Naturally acidic surface and ground waters draining porphyry-related mineralized areas of the Southern Rocky Mountains,Colorado and New Mexico

Acidic, metal-rich waters produced by the oxidative weathering and resulting leaching of major and trace elements from pyritic rocks can adversely affect water quality in receiving streams and riparian ecosystems. Five study areas in the southern Rocky Mountains with naturally acidic waters associated with porphyry mineralization were studied to document variations in water chemistry and processes that control the chemical variations. Study areas include the Upper Animas River watershed, East Alpine Gulch, Mount Emmons, and Handcart Gulch in Colorado and the Red River in New Mexico. Although host-rock lithologies in all these areas range from Precambrian gneisses to Cretaceous sedimentary units to Tertiary volcanic complexes, the mineralization is Tertiary in age and associated with intermediate to felsic composition, porphyritic plutons. Pyrite is ubiquitous, ranging from ∼1 to >5 vol.%. Springs and headwater streams have pH values as low as 2.6, SO₄ up to 3700 mg/L and high dissolved metal concentrations (for example: Fe up to 400 mg/L; Cu up to 3.5 mg/L; and Zn up to 14.4 mg/L). Intensity of hydrothermal alteration and presence of sulfides are the primary controls of water chemistry of these naturally acidic waters. Subbasins underlain by intensely hydrothermally altered lithologies are poorly vegetated and quite susceptible to storm-induced surface runoff. Within the Red River study area, results from a storm runoff study documented downstream changes in river chemistry: pH decreased from 7.80 to 4.83, alkalinity decreased from 49.4 to <1 mg/L, SO₄ increased from 162 to 314 mg/L, dissolved Fe increased from to 0.011 to 0.596 mg/L, and dissolved Zn increased from 0.056 to 0.607 mg/L. Compared to mine drainage in the same study areas, the chemistry of naturally acidic waters tends to overlap but not reach the extreme concentrations of metals and acidity as some mine waters. The chemistry of waters draining these mineralized but unmined areas can be used to estimate premining conditions at sites with similar geologic and hydrologic conditions. For example, the US Geological Survey was asked to estimate premining ground-water chemistry at the Questa Mo mine, and the proximal analog approach was used because a mineralized but unmined area was located adjacent to the mine property. By comparing and contrasting water chemistry from different porphyry mineralized areas, this study not only documents the range in concentrations of constituents of interest but also provides insight into the primary controls of water chemistry.     

Transient electromagnetic study of basin fill sediments in the Upper San Pedro Basin,Mexico

The Upper San Pedro River Basin in Mexico and the United States is an important riparian corridor that is coming under increasing pressure from growing populations and the associated increase in groundwater withdrawal. Several studies have produced three-dimensional maps of the basin fill sediments in the US portion of the basin but little work has been done in the Mexican portion of the basin. Here, the results of a ground-based transient electromagnetic (TEM) survey in the Upper San Pedro Basin, Mexico are presented. These basin fill sediments are characterized by a 10–40 m deep unsaturated surficial zone which is composed primarily of sands and gravels. In the central portion of the basin this unsaturated zone is usually underlain by a shallow clay layer 20–50 m thick. Beneath this may be more clay, as is usually the case near the San Pedro River, or interbedded sand, silt, and clay to a depth of 200–250 m. As you move away from the river, the upper clay layer disappears and the amount of sand in the sediments increases. At 1–2 km away from the river, sands can occupy up to 50% of the upper 200–250 m of the sediment fill. Below this, clays are always present except where bedrock highs are observed. This lower clay layer begins at a depth of about 200 m in the central portion of the basin (250 m or more at distances greater than 1–2 km from the river) and extends to the bottom of most profiles to depths of 400 m. While the depth of the top of this lower clay layer is probably accurate, its thickness observed in the models may be overestimated due to the relatively low magnetic moment of the TEM system used in this study. The inversion routine used for interpretation is based on a one-dimensional geologic model. This is a layer based model that is isotropic in both the x and y directions. Several survey soundings did not meet this requirement which invalidates the inversion process and the resulting interpretation at these locations. The results from these locations were rejected.     

Uranium-series chronology for sediments of Lake Hovsgol, Mongolia, and the 1-Ma records of uranium and thorium isotopes from the HDP-04 drill core

Uranium and thorium isotopes in an 81-m long sediment core (HDP-04) of Lake Hovsgol, Mongolia, were measured to investigate their downcore distributions and to explore potential linkage to paleoenvironmental changes. Three-dimensional isochron techniques using isotope-ratio diagrams in ²³⁸U–²³⁴U–²³⁰Th–²³²Th system presented by Ludwig and Titterington were applied to age date the lake sediments at the depths of 11.42, 14.71 and 14.83 m in the HDP-04 section, the estimated ages of these horizons are 66 ± 8, 122 ± 11 and 128 ± 22 ka, respectively. The ²³⁸U concentration throughout the entire section fluctuated by a factor of 12, ranging from 19.9 to 232.1 mBq/g with anomalously high ²³⁸U peak at 23.8 m in depth, while the ²³²Th concentration varied only by a factor of about two between 24.3 and 54.0 mBq/g. The discrimination of the bulk ²³⁸U into authigenic and terrigenous ²³⁸U fractions was attempted, based on the measured ²³²Th as a correction index for terrigenous materials. In the upper 24 m corresponding to the last 250 ka, the authigenic ²³⁸U was higher in interglacials and lower in glacials. This depth profile of authigenic ²³⁸U contents was almost identical pattern with that found in a sediment core (VER98-1-6) from the Academician Ridge, Lake Baikal. Further, this profile can be correlated well with that of photosynthetic pigment contents, one of proxies of paleoproductivity, suggesting that the variation of authigenic ²³⁸U contents were associated with the environmental change around Lake Hovsgol.     

Evaluation and analysis of surface deformation in west Chapala basin,central Mexico

Vertical deformation was measured at 14 benchmarks within the urban area of Jocotepec Mexico using first-order leveling methods and then spatially analyzed in relation to land subsidence and soil discontinuity patterns. The study area is located within the western portion of the Chapala basin, middle-west Mexico. Observations of vertical surficial deformation were made at each benchmark (September and November 2012) relative to a fixed station and compared to an initial survey of each benchmark (April 2012). Results indicate that a maximum subsidence of 7.16 cm over the 8-month measurement interval occurs near downtown coincident with the largest levels of drawdown and translates to a maximum subsidence rate of 0.89 cm/month for the sampling period. Two benchmarks located northwest and southeast of the urban area exhibited uplift of 2.8 and 0.76 cm, respectively, suggesting a complex mechanical response between the sedimentary soil units and the factors causing deformation. A potential spatial relationship exists between subsidence patterns and soil discontinuities. Four separate cones of groundwater depression were observed with two being coincident with subsidence bowls downtown and south of the urban area; however, there is no clear relationship between drawdown and subsidence in the remaining areas. Hydrogeologic reconstructions reveal alternating sequences of alluvial aquifers and highly deformable lacustrine aquitards. An analysis of the soil discontinuities reveals that they are directly aligned with the patterns of vertical deformation.     

Isotopic characteristics of precipitation, surface and ground waters in the Yinchuan plain, Northwest China

The isotopic characteristics of hydrogen and oxygen of various water bodies in the Yinchuan plain were investigated. A total of 131 water samples were collected and another 99 water samples collected by other scholars and the International Atomic Energy Agency were referred to in the study. The stable isotopic compositions of precipitation and lake waters are influenced by dry climate and strong evaporation of the area, and the slope of local meteoric water line and lake water line are smaller than that of global meteoric water line. The isotopic compositions of the Yellow River water are significantly lower than the weighted averages of local atmospheric precipitation and are controlled by the runoff of upstream areas. The isotopes of phreatic water suggest that the single phreatic water is predominantly recharged by the bedrock fissure water of Helan Mountain, while the phreatic water in multilayer structure area is recharged by multiple sources. The confined waters in the area may be recharged under cooler climate conditions than the present, which makes the heavy isotopes depleted. The ³H contents of various water bodies are in the following order: the Yellow River water > multilayer phreatic water > single phreatic water > lake water > drainage ditch water > upper confined water > lower confined water. This is in accordance with the recharge, discharge and circulation conditions of the specific water bodies. The isotope compositions of waters in the Yinchuan plain are mainly affected by external water recharge rather than local precipitation. This study is meaningful and helpful in understanding the groundwater flow systems and water cycle in the area.     

Hydrogeological and mixing process of waters in aquifers in arid regions: a case study in San Luis Potosi Valley,Mexico

The climatic conditions of arid regions are characterized by high temperatures, low precipitation and high evapotranspiration rates that can explain the reduced recharge of aquifers. Thus, in these regions, there are some problems related to the groundwater quality and recharge that makes worse the problem of groundwater supply. A model, taking into account ternary mixtures, is presented and applied to a case study: the aquifer of San Luis Potosi valley located in the highlands of the central part of Mexico. In this valley, four hydrochemical facies were identified that correspond to the Ca–Na + K–HCO₃, Na + K–Ca–HCO₃, Ca–HCO₃ and Ca–SO₄ types. From this characterization, it was found out that the recharge area (known as Bledos Graben) is located at the SE of the valley; the deep water flow comes from there (Villa de Reyes and Alvarez Range) to the center of the valley. Mixture fractions were obtained by using chlorides and fluorides as conservative elements, from which it was possible to quantify the contribution of each member to the groundwater quality. According to these results, the contributions to the water extracted from this aquifer are as follows: shallow flows 50%, deep flows from Villa de Reyes 27%, and flows coming from the Alvarez Ranges about 15%.     

伊犁盆地乌库尔其矿床中下侏罗统水西沟群第Ⅶ沉积旋回铀矿富集因素

本文从铀矿化特征入手 ,重点对矿床的构造、古气候、岩相古地理、岩性、水文地质和地球化学及有效还原剂含量等与第Ⅶ沉积旋回铀富集有关的因素进行了探讨 ,认为乌库尔其铀矿富集的关键因素为含铀建造 (砂体 )的存在、含氧含铀水的长期补给以及有效还原剂的存在。     

Evaluation of the recharge processes and impacts of irrigation on groundwater using CFCs and radiogenic isotopes in the Silao-Romita basin, Mexico

The recharge processes in the overexploited aquifer of the Silao Romita basin, central Mexico, were investigated by means of gaseous tracers (chlorofluorocarbons, CFCs) and radioactive isotopes (C-14, tritium). CFC concentrations varied between 0.06 and 12 pmol/l (CFC-11), 0.03 and 1.7 pmol/l (CFC-12), and <0.01 and 0.23 pmol/l (CFC-113). CFC concentrations are controlled by irrigation return flow which became apparent by the comparison with tritium. Tritium activities ranged from 0 to 3.5 TU. The calculated mean residence times of 70 to more than 300 years are considerably lower than the ages estimated based on the CFCs data. These data showed that CFCs were not appropriate for groundwater dating in this particular area but the CFCs were suitable as a qualitative measure of the magnitude of irrigation return flow which proved to be a significant source of recharge in the irrigated areas. Radiocarbon activities were in the range of 6–109 pmC. Carbon-13 values varied between –11.9 and –7.2‰ VPDB. Modelling of carbon isotopes with NETPATH along a plausible flow path reveals considerable influences of exchange with soil CO₂ and carbonate dissolution. Radiocarbon data indicate, at least in one case, the existence of groundwaters with residence times of more than 10,000 years.     

二连盆地巴彦乌拉地区砂岩型铀矿含矿地层时代与古地理环境探讨

二连盆地砂岩型铀矿含矿层是一套以河流、沼泽相沉积为主的粗碎屑岩系,含丰富的孢粉化石.目前,该地区含矿地层的时代存在争论.本文通过对巴彦乌拉地区含矿地层中孢粉的综合研究,主要侧重孢子和花粉在地层中的分布特征及国内外孢粉组合的比较,确定其时代为早白垩世晚期的阿普第-阿尔必期,沉积时期为温暖潮湿的亚热带气候,且有向半潮湿、半干热的方向发展的趋势.古地理环境以低平、丘陵地势为主,盆地周围有少量中、高山分布.     

Seasonal variations of helium,radon, and uranium in lake waters near the Key Lake uranium deposit,Saskatchewan

To test the usefulness of the He method for the detection of buried U ore, a lake-water survey of the Key Lake area was carried out during March 1977 and repeated in June 1977. A set of 87 lake-water samples from 37 sites in eight different lakes were collected from the same sites in each survey. These samples were analyzed for He, Rn, O₂, Eh, pH, conductivity, alkalinity, and U.In the lakes the highest He concentrations, of nearly four times atmospheric equilibrium concentrations, were obtained during the winter survey. This was under the ice in two bottom lake samples where the sandy overburden extends for some 50 m directly to the ore zone. Radon and U values at these sites were also anomalous, but the highest values of Rn and U were obtained in a shallow lake about 2 km south of the ore zone where concentrations of radioactive boulders occur.The average net He and Rn contents dropped from 21 standard nanolitres/litre (nl/l) and 25 picocuries/litre (pCi/l) in the winter under the ice to 3 nl/l and 8 pCi/l in the summer, respectively. Average U and conductivity dropped from 2.3 ppb and 15 μmho/cm to 1.5 ppb and 11 μmho/cm, respectively, in the same suites of samples. The drop in the ionic species probably reflects the effect of snow and ice meltwater dilution and the much larger drop in the dissolved He and Rn reflects the combined effects of meltwater dilution, wind turbulence over ice-free lakes, and change in thermal gradients.The He results must be viewed with guarded optimism until a detailed investigation is completed to ascertain what fraction of the detected He is from the ore, what fraction from basement He that may have found its way into the lakes via the fracture zone in which the ore is located, and what effect drilling has had on the He flow into the lakes from the ore zone.     

Characterization of surface water and groundwater in the Damascus Ghotta basin: hydrochemical and environmental isotopes approaches

The hydrochemistry of major ions and environmental isotope compositions (¹⁸O, ²H and tritium) of water samples have been used to investigate the characteristics of rainfalls, surface water and groundwater in the Damascus Ghotta basin. The groundwater salinity in the Damascus Ghotta basin gradually increases, as the groundwater moves from western to south-eastern and north-eastern parts of the basin. A strong relationship exists between the Barada river and the surrounded groundwaters, mainly in terms of recharge by infiltration of surface waters. The groundwater quality in the Adra region has clearly become less saline as a result of establishment of the sewage-water-treatment station in this area since 1997. The uncommon depleted stable isotope concentrations in the vicinity of Al-Ateibeh Lake and Adra valley could be interpreted as a result of sub-flow recharge from the Cenomanian–Turonian aquifer, mostly prolonged along the Damascus Fault, which forms direct contact between this complex and the Quaternary alluvium aquifers. The extensive exploitation of water from the Cenomanian–Turonian aquifer for drinking water supply would shortly be reflected by a gradual decline of the groundwater table in the Damascus Ghotta basin. Amelioration of water quality in the Damascus basin still requires further management strategies and efforts to be taken within the forthcoming years.     

Saline waters and residual brines in the Shiraz-Sarvistan basin,Iran

Origin of saline waters in the Shiraz-Sarvistan area, Iran, is determined by a combined isotopic (¹⁸O and D) and chemical characterization. Four types are recognized: (a) fresh water of the anticlinal carbonatic aquifer; (b) fresh and brackish runoff in the synclinal basins; (c) salt springs originating through dissolution of rock salt by type (a) fresh water; and (d) residual brines formed in synclinal closed drainage basins, through evaporation of former water types and loss of the relatively less-soluble salts.     

Estimation of uranium endowment by subjective geological analysis—a comparison of methods and estimates for the San Juan Basin,New Mexico

This study investigates how estimates of uranium endowment made by a geologist using an appraisal system that is based upon a formalization of geoscience and decision rules compare with estimates made by informal and unconstrained intuitive processes. The motivation for this study derives from the premise that formalization of decisions would mitigate the heuristic biases and hedging that may result from the use of unconstrained intuitive processes. Estimates of the uranium endowment of the San Juan Basin of New Mexico by four methodologies are compared in this study. These methods, ranked from top to bottom by degree of decomposition (mitigating of heuristic bias)and control on hedging, are as follows Implicit 2 1.5 × 10⁶ s.t. of U₃O₈ Implicit 1 1.6 × 10⁶ s.t. of U₃O₈ NURE (1980) 2.4 × 10⁶ s.t. of U₃O₈ Appraisal system 3.9 × 10⁶ s.t. of U₃O₈ The magnitude of expected uranium endowment estimated by these methods, ranked from smallest to largest, is in this same order. With the exception of the NURE estimates, the magnitude of the variance (uncertainty)of uranium endowment, ranked from smallest to largest, also is in this same order. These results prompt the suggestion that the more decomposed and formalized the estimation procedure, the greater the expected value and the variance of uranium endowment. Equivalently, predicating U ₃ O ₈ endowment estimation strictly upon that part of the geologist's geoscience that is useful in making U ₃ O ₈ endowment estimates and upon his understanding of the region's history produced larger estimates than have previously been reported. However, this method of estimation also shows that uncertainty about the actual state of U ₃ O ₈ endowment is much greater than previously described.     

Processes controlling Sr in surface and ground waters of Tertiary tholeiitic flood basalts in Northern Iceland

Strontium concentrations of 253 natural water samples from Skagafjördur, a Tertiary tholeiitic flood basalt region in northern Iceland range between 0.10 and 28 ppb. Surface environments (rivers, lakes, and peat soil waters) include the whole range of observed Sr concentrations whereas the Sr concentrations of ground waters are, in most cases, <3.5 ppb. Concentrations of Sr derived from basalt dissolution (i.e., rock-derived Sr) in waters of rivers and lakes exhibit a near linear correlation with the concentration of rock-derived Ca with a median molar Ca/Sr ratio of 1350. This systematic correlation suggests that Ca and Sr concentrations are controlled by weathering processes, i.e., the extent of dissolution of the basalt. The relative mobility of Sr during weathering in Skagafjördur is approximately half that of Ca, which is consistent with observed relative mobilities of these elements elsewhere in Iceland and in other basaltic regions. Peat soil waters commonly have lower concentrations of Sr and higher Ca concentrations than rivers and lakes, and molar ratios of rock-derived Ca to Sr in peat soil waters exhibit no systematic pattern. In several cases calculated concentrations of rock-derived Sr in peat soil waters yield negative values, suggesting a mineralogic sink for Sr in these waters.The low Sr concentrations in cold and thermal ground waters (<3.5 ppb) suggest mineralogic control over Sr in the ground water systems. Precipitation of secondary Sr minerals such as strontianite and celestite is ruled out as the ground waters are understaturated with respect to these minerals. Ground waters are characterized by high Ca/Sr molar ratios (∼5000 compared to bedrock Ca/Sr ratio of 730) suggesting that Sr is being preferentially incorporated (relative to Ca) into secondary minerals. The secondary minerals present in the bedrock in Skagafjördur that can preferentially incorporate Sr include zeolites, such as heulandite, chabazite, and thomsonite, and smectite. Ion-exchange calculations demonstrate that activities of Sr²⁺ and Ca²⁺ in ground water solutions in Skagafjördur are consistent with ion-exchange equilibria between these waters and heulandite from other Tertiary basalts in Iceland suggesting that this mineral may play an important role in controlling the concentration of Sr in the Skagafjördur ground waters. Incorporation of Sr into calcite cannot explain the observed high Ca/Sr ratios of the Skagafjördur ground waters because calcite, when precipitating, only admits limited amounts of Sr. Aragonite is not considered a likely candidate either because it has only very slight preference for Sr over Ca and ground waters above 40 °C are undersaturated with respect to this phase. However, predicted Sr content of calcite in equilibrium with the Skagafjördur ground waters (0.5-83 ppm Sr) is in good agreement with measured Sr content of this mineral in Tertiary basalts elsewhere in Iceland (<0.1-63 ppm), suggesting that the Skagafjördur ground waters can be used as analogues for Tertiary crustal solutions involved in the zeolite facies metamorphism of the Icelandic crust.     

Arsenic and uranium transport in sediments near abandoned uranium mines in Harding County,South Dakota

Sediment samples were analyzed as part of ongoing environmental investigations of historical U mining impacts within Custer National Forest in Harding County, South Dakota. Correlations between As and U content, grain size and soil mineralogy were determined to identify contaminant fate and transport mechanisms. Soil samples collected near the mining source zone and up to 61 km downgradient of the minesites were analyzed. Samples were homogenized and wet sieved through polymer screens, and metal(loid) concentrations were determined using inductively coupled plasma mass spectrometry (ICP-MS). Powder X-ray diffraction (XRD) analysis identified quartz as the primary mineral for all size fractions, with varying amounts of analcime, indicative of volcanic origin. Selected samples were examined for trace mineral composition using scanning electron microscopy (SEM). The presence of Fe sulfides and Fe (hydr)oxides indicate heterogeneity in redox potentials on a microscopic scale. Elevated metal(loid) concentrations were associated with trace concentrations of Fe sulfide, indicating an influence on metal transport during weathering. Sequential chemical extractions (SCE) performed on source sediment fractions demonstrated that most As and U was adsorbed to Fe- and Mn-oxides and carbonates with lesser amounts bound by ion exchange, organics and Fe sulfides. Large changes in U/Th and As/Th ratios were observed to coincide with geochemical changes in the watershed, suggesting that metal(loid)–Th ratios may be used in environmental investigations to identify geochemically-significant watershed conditions.