Paleo-redox boundaries in fractured granite

At the Earth’s surface, Fe(II) often oxidises and forms insoluble Fe(III)-(oxyhydr)oxides, whose particle size and structure depend on solution composition and temperature during formation and afterwards. Bacterial processes and exposure to reducing environments reduces them again, releasing dissolved iron to the groundwater. During such cycling, the Fe isotopes fractionate to an extent that is expected to depend on temperature. In this study, we report on the use of Fe-oxides as paleo-redox indicators, using their structure, morphology and Fe-composition as a clue for formation conditions. In samples taken from ∼120 m drill cores in granite from SE Sweden, X-ray amorphous, superparamagnetic, nanometre-sized Fe-oxides are confined to fractures of the upper ∼50 m, whereas well-crystalline Fe-oxides, with particle sizes typical for soils, occur down to ∼110 m. We also identified hematite with a particle size of 100 nm, similar to hematite of hydrothermal origin. The Fe isotope composition of the fine-grained Fe-oxides (−1‰ < δ⁵⁶Fe < 1‰, IRMM-14 referenced) scatter significantly compared to the distribution previously observed for hydrothermal material (−0.26‰ < δ⁵⁶Fe < 0.12‰) and they are dominantly heavier than Fe-bearing silicates from fractures (−0.56‰ < δ⁵⁶Fe < −0.35‰). This is consistent with formation by low-temperature weathering, where Fe-silicates dissolve, Fe(II) oxidises and Fe(III)-oxides precipitate. The X-ray amorphous, nanometre-sized nature of near-surface Fe-oxides suggests recent formation. The deeper situated, well-crystalline Fe-oxides are more mature and we interpret that they record earlier oxidising events. They exist in fractures that are not significantly altered, indicating formation during periods of oxidation. Our results show that oxygenated water may reach depths of ∼110 m in fractured granite. The absence of natural, low-temperature Fe-oxides from deeper drill cores suggests that oxygenated waters do not readily penetrate beyond about 100 m and suggests that radioactive waste repositories located at a depth of ∼500 m should be well-protected from oxygenated waters.     

Water level variations in fractured,semi-confined aquifers of Anantapur district,Southern India

The study on water level conditions of fractured aquifer system in northeastern part of Anantapur district is of immense importance as the area is covered by varied geological formations and has different irrigation patterns. The monthly groundwater level data of 154 observation wells for five year period (2001–06) is analyzed to decipher the behavior of water levels in different seasons and geo-environments. The hydrographs of the average water level data of each Mandal (group of villages) indicate steady declining trend ranging from 0.50 to 2.91m/yr. Yellanuru Mandal has both the shallowest and the deepest water levels among eight Mandals, highly undulating terrain could be one of the reasons for this contrasting condition. The pre-monsoon water levels show decline of 8.22 m in one year from May 2002 to 2003. A negative seasonal fluctuation of ?1.49m has occurred in the year 2002 during which the area received 32% less than normal rainfall. The mean water levels are deeper by 42% in areas covered by sedimentary formations than those of granite terrain. Raise in water levels is significant where monthly rainfall is more than 200 mm. Due to erratic rainfall in space and time, deeper water levels are noticed even in post-monsoon period and shallow in February month at some locations. The water levels in command areas are deep and exhibit falling trend as the area forms the tail end part of the Tunga Bhadra High Level Canal. The deeper water level conditions and its declining feature is directly related to groundwater development in the form of increased agriculture activity, reduced area under rain-fed crops, high horticulture development. Arid climatic conditions, low precipitation and continuous exploitation of groundwater resources could be other factors contributing for steady decline in water levels in the area. The wide variations in groundwater levels could be due to uneven topography, heterogeneous and anisotropic conditions of granites and poor porosity — permeability of shales, lack of vegetation, and increased groundwater extraction.     

Geochemical patterns of arsenic-enriched ground water in fractured,crystalline bedrock,Northport, Maine,USA

High mean As concentrations of up to 26.6 μmol/L (1990 μg/L) occur in ground water collected from a fractured-bedrock system composed of sulfidic schist with granitic to dioritic intrusions. Sulfides in the bedrock are the primary source of the As in the ground water, but the presence of arsenopyrite in rock core retrieved from a borehole with As concentrations in the ground water barely above the detection limit of 2.0 μmol/L, shows that there are complicating factors. Chemical analyses of water from 35 bedrock wells throughout a small watershed reveal spatial clustering of wells with high As concentrations. Stiff diagrams and box plots distinguish three distinct types; calcium-bicarbonate-dominated water with low As concentrations (CaHCO₃ type), sodium-bicarbonate-dominated water with moderately high As concentrations (NaHCO₃ type), and calcium-bicarbonate-dominated water with very high As concentrations (High-As type). It is proposed that differences in recharge area and ground-water evolution, and possible bedrock composition difference are responsible for the chemical distinctions within the watershed. Lack of correlation of As concentrations with pH indicates that desorption of As is an insignificant control on As concentration. Correlations of As concentrations with Fe and redox parameters indicates that reductive dissolution of Fe(III) oxyhydroxides may play a role in the occurrence of high As concentrations in the NaHCO₃ and High-As type water. The oxidation of sulfide minerals occurs within the ground-water system and is ultimately responsible for the existence of As in the ground water, but there is no correlation between As and SO₄ concentrations, probably due to precipitation of Fe(III) oxyhydroxides and adsorption of As under oxidizing conditions.     

Helium, carbon dioxide and oxygen soil gases: Small-scale variations over fractured ground

To investigate the control on small-scale variation of He in soil gas exercised by minor fracturing, shallow surveys have been carried out over a cave system formed along an approximately orthogonal set of fractures in Devonian limestone in southwest England. The possibilities that He variation could be related to deep-seated, major fractures or hidden mineralisation, and that other soil gases may also be affected by minor fracturing were assessed by contemporary surveys for CO₂ and O₂. Comparisons of soil gas values with variations in electrical apparent resistivity were also carried out. Location of fractures with direct connection to the cave system was determined by spiking the cave atmosphere with He and then resurveying after equilibration.The results for CO₂ and O₂ show anomalies of low magnitude (with respect to atmospheric concentrations), and although they display an antithetic relationship, this is generally poor. There is also no strong correlation with the results of the He surveys either before or after spiking, or with the apparent resistivity values. Moreover, anomalies in CO₂ + O₂ do not support the pattern of variation shown by the individual gases. These negative results suggest the absence of deep-seated fractures or hidden mineralisation, and show that CO₂ and O₂ values are independent of minor fracturing. The origin of the variation is attributed to bacteriological productivity.He soil gas concentrations were obtained as disequilibrium values relative to Field Atmospheric Air (ΔHe/ppb-FAA). Positive ΔHe values were generally found to correlate with areas of thin, dry soil cover, enhancement occurring along fracture lines. Negative ΔHe values were also found to correlate with fracture lines, but in these areas the fractures are overlain by a thick soil cover with a high moisture content. It is considered that distinctions can be drawn between water-conducting and dry fractures, and that negative ΔHe values are likely only to be encountered with shallow soil gas samples. Results of spiking with He show a clear distribution of enhanced values along the set of orthogonal fractures, irrespective of the sign of the natural He anomaly. It is considered, therefore, that major deep-seated fractures may result in positive He anomalies superimposed upon negative ΔHe values. This implies that He anomalies must always be related to a local datum value. It is concluded that resolution of interacting variables in the interpretation of soil gas data is facilitated by integrated soil gas surveys.     

Multiple-methods investigation of recharge at a humid-region fractured rock site,Pennsylvania, USA

Lysimeter-percolate and well-hydrograph analyses were combined to evaluate recharge for the Masser Recharge Site (central Pennsylvania, USA). In humid regions, aquifer recharge through an unconfined low-porosity fractured-rock aquifer can cause large magnitude water-table fluctuations over short time scales. The unsaturated hydraulic characteristics of the subsurface porous media control the magnitude and timing of these fluctuations. Data from multiple sets of lysimeters at the site show a highly seasonal pattern of percolate and exhibit variability due to both installation factors and hydraulic property heterogeneity. Individual event analysis of well hydrograph data reveals the primary influences on water-table response, namely rainfall depth, rainfall intensity, and initial water-table depth. Spatial and seasonal variability in well response is also evident. A new approach for calculating recharge from continuous water-table elevation records using a master recession curve (MRC) is demonstrated. The recharge estimated by the MRC approach when assuming a constant specific yield is seasonal to a lesser degree than the recharge estimate resulting from the lysimeter analysis. Partial reconciliation of the two recharge estimates is achieved by considering a conceptual model of flow processes in the highly-heterogeneous underlying fractured porous medium.     

Data-driven modeling for groundwater exploration in fractured crystalline terrain, northeast Brazil

It is not possible, using numerical methods, to model groundwater flow and transport in the fractured crystalline rock of northeastern Brazil. As an alternative, the usefulness of self-organizing map (SOM), k-means clustering, and Davies-Bouldin techniques to conceptualize the hydrogeology was evaluated. Also estimated was the well yield and groundwater quality across the Juá region. This process relies on relations in the underlying multivariate density function associated with a sparse local set of hydrogeologic (electrical conductivity, geology, temperature, and well yield) and a complete regional set of airborne geophysical (electromagnetic, magnetic, and radiometric) and satellite spectrometric measurements. Resampling of the regional well yield and electrical conductivity estimates provides sufficient resolution to construct variograms for stochastic modeling of the hydrogeologic variables. The combination of these stochastic maps provides a way to identify potential drilling targets for future groundwater development. The data-driven estimation approach, when applied to available airborne electromagnetic and water-well hydrogeologic measurements, provides a low-cost alternative to numerical groundwater flow modeling. In addition to fractured rock environments, the alternative modeling framework can provide spatial parameter estimates and associated variograms for constraints to improve the traditional calibration of equivalent groundwater-porous-media models.     

Theoretical study of hydraulically fractured penny-shaped cracks in hot,dry rocks

A simple and analytical method of solving the problem of heat extraction from a penny-shaped crack having both inlet and outlet holes is developed by using a two-dimensional flow model when fluid is injected at a constant flow rate. Temperature distributions in both rock and fluid are obtained by taking into account the hydraulic and thermal growth of the crack. The outlet fluid temperature and energy extraction rate versus time are shown by some illustrative examples. Although the effect of thermal, contraction of rock upon the crack width is large, its influence upon flow rate and crack tip stress intensity factor is sufficiently small within the first several years so that the crack radius can be determined mainly by the mechanical deformation of, the rock.     

Modeling nonlinear response of fractured rocks and reservoirs

Since the early work of Athey (1930), there have been many attempts to describe the various nonlinear behaviors of rocks and soils in terms of functionals having only a few parameters, while nevertheless being able to fit the complicated available data with satisfactory accuracy. Such approaches have not been universally applied however, and the present analyses are intended to draw attention to the possibility of using such nonlinear fitting methods on old as well as new data sets. In particular, some special emphasis is placed here on re‐examining the well‐known laboratory data of Coyner (1984) on rocks in light of such modeling tools, and we find that the nonlinear approach again has several clear advantages – especially in terms of reducing the number of variables needed to describe the observed behavior of both bulk modulus and porosity of rocks undergoing large changes in pressure. Copyright © 2016 John Wiley & Sons, Ltd.     

The groundwater geochemistry in a semi-arid, fractured crystalline basement area of Dodoma, Tanzania

The chemical character of groundwater is generated through processes including the leaching of surficial and near-surface soil salts, cation exchange and mineral dissolution of, in this case, mainly amphiboles and sodic plagioclase. Molar ratios thermodynamic analysis using a NETPATH geochemical model and standard Piper trilinear and Gibbs diagrams have been used to establish these geochemical processes. Though the groundwater is dominantly of NaCl type, it is apparently suitable for general irrigation purposes.     

Groundwater in fractured crystalline rocks, the Clara mine, Black Forest (Germany)

The chemical composition of water sampled in a 700 m deep underground barite-fluorite mine in the crystalline basement of the Black Forest area (SW Germany) varies systematically with depth and the length of flow paths trough, the fracture porosity of the gneiss matrix. Calcium and sulfate increase as a result of a combined sulfide oxidation and plagioclase alteration reaction. The gneiss contains andesine–plagioclase (An₂₀–An₄₀) and is rich in primary sulfide. As an effect of Ca and SO₄ release by the prime water–rock reaction, dissolved oxygen decreases and the waters become more reduced. The waters have Cl/Br mass ratios of about 50, which is very close to that of experimentally leached gneiss powders indicating that the rock matrix is the source of the halogens. The waters are undersaturated with respect to calcite in the upper parts of the mine. With increasing reaction progress, calcite saturation is reached and carbonate forms as a reaction product of the prime reaction that also controls the partial pressure of CO₂ to progressively lower values. The chemical evolution of groundwater in fractured basement of the Clara mine suggests that the partial pressure of CO₂ is an internally buffered parameter rather than a controlling external variable.     

Delineation of capture zones for municipal wells in fractured dolomite, Sturgeon Bay, Wisconsin, USA

A wellhead protection study for the city of Sturgeon Bay, Wisconsin, USA, demonstrates the necessity of combining detailed hydrostratigraphic analysis with groundwater modeling to delineate zones of contribution for municipal wells in a fractured dolomite aquifer. A numerical model (MODFLOW) was combined with a particle tracking code (MODPATH) to simulate the regional groundwater system and to delineate capture zones for municipal wells. The hydrostratigraphic model included vertical and horizontal fractures and high-permeability zones. Correlating stratigraphic interpretations with field data such as geophysical logs, packer tests, and fracture mapping resulted in the construction of a numerical model with five high-permeability zones related to bedding planes or facies changes. These zones serve as major conduits for horizontal groundwater flow. Dipping fracture zones were simulated as thin high-permeability layers. The locations of exposed bedrock and surficial karst features were used to identify areas of enhanced recharge. Model results show the vulnerability of the municipal wells to pollution. Capture zones for the wells extend several kilometers north and south from the city. Travel times from recharge areas to all wells were generally less than one year. The high seasonal variability of recharge in the study area made the use of a transient model necessary. Electronic Publication     

Using saline tracers to evaluate preferential recharge in fractured rocks,Floyd County,Virginia, USA

Potassium chloride (KCl) and potassium bromide (KBr) tracers were used to explore the role of geologic structure on groundwater recharge and flow at the Fractured Rock Research Site in Floyd County, Virginia, USA. Tracer migration was monitored through soil, saprolite, and fractured crystalline bedrock for a period of 3 months with chemical, physical, and geophysical techniques. The tracers were applied at specific locations on the ground surface to directly test flow pathways in a shallow saprolite and deep fractured-rock aquifer. Tracer monitoring was accomplished with differential electrical resistivity, chemical sampling, and physical monitoring of water levels and spring discharge. KCl, applied at a concentration of 10,000 mg/L, traveled 160 m downgradient through the thrust fault aquifer to a spring outlet in 24 days. KBr, applied at a concentration of 5,000 mg/L, traveled 90 m downgradient through the saprolite aquifer in 19 days. Tracer breakthrough curves indicate diffuse flow through the saprolite aquifer and fracture flow through the crystalline thrust fault aquifer. Monitoring saline tracer migration through soil, saprolite, and fractured rock provided data on groundwater recharge that would not have been available using other traditional hydrologic methods. Travel times and flowpaths observed during this study support preferential groundwater recharge controlled by geologic structure.     

Elastic moduli for fractured rock mass

Summary The presence of joint discontinuities has long been recognized as an important factor influencing the mechanical behavior of rock masses. This paper proposes a stress-strain model for an assemblage of intact rock blocks separated by joint planes. The stress-strain relationship accounts for spacings and orientations of the joint sets. Closed-form expressions of elastic moduli for a rock mass with three intersecting sets of joints are derived explicitly in terms of properties of joints and intact rock. Applicability of the derived expressions are evaluated by comparing the predicted results with experimental results from physical model tests.     

The sources and budget for dissolved sulfate in a fractured carbonate aquifer,southern Sacramento Mountains,New Mexico,USA

Climate change in the SW USA is likely to involve drier conditions and higher surface temperatures. In order to better understand the evolution of water chemistry and the sources of aqueous SO₄ in these semi-arid settings, chemical and S isotope compositions were determined of springs, groundwater, and bedrock associated with a Permian fractured carbonate aquifer located in the southern Sacramento Mountains, New Mexico, USA. The results suggest that the evolution of water chemistry in the semi-arid carbonate aquifer is mainly controlled by dedolomitization of bedrock, which was magnified by increasing temperature and increasing dissolution of gypsum/anhydrite along the groundwater flow path. The δ³⁴S of dissolved SO₄ in spring and groundwater samples varied from +9.0‰ to +12.8‰, reflecting the mixing of SO₄ from the dissolution of Permian gypsum/anhydrite (+12.3‰ to +13.4‰) and oxidation of sulfide minerals (−24.5‰ to −4.2‰). According to S isotope mass balance constraints, the contribution of sulfide-derived SO₄ was considerable in the High Mountain recharge areas, accounting for up to ∼10% of the total SO₄ load. However, sulfide weathering decreased in importance in the lower reaches of the watershed. A smaller SO₄ input of ∼2–4% was contributed by atmospheric wet deposition. This study implies that the δ³⁴S variation of SO₄ in semi-arid environments can be complex, but that S isotopes can be used to distinguish among the different sources of weathering. Here it was found that H₂SO₄ dissolution due to sulfide oxidation contributes up to 5% of the total carbonate weathering budget, while most of the SO₄ is released from bedrock sources during dedolomitization.     

赣南八仙脑破碎带型钨锡多金属矿床成矿流体和年代学研究

八仙脑矿床为赣南近年确认的较为典型的破碎带型钨锡多金属矿床。本文通过流体包裹体岩相学、显微测温、氢、氧同位素和年代学研究,探讨了八仙脑钨矿成矿流体来源及演化、成岩成矿时代和形成构造环境。结果表明,流体包裹体以液体包裹体为主,含少量CO2包裹体,流体具低(150～170℃和200～220℃)、中(250～270℃)和高(290～310℃)等不同的显微测温峰,显示出多期次流体活动相互叠加的特征,成矿流体为低盐度、低密度的H2O-CO2-NaCl体系。矿石中石英的δD为-78‰～-65‰,δ18OH2O为4.33‰～6.44‰,表明成矿流体以岩浆水为主并有大气降水的混合。利用SHRIMP锆石U-Pb法和辉钼矿Re-Os等时线法,分别获得八仙脑矿区成矿黑云母花岗岩和破碎蚀变带中黑钨矿-石英脉的形成年龄为157.2±2.2Ma和157.9±1.5Ma,成岩成矿年龄一致,均为晚侏罗世。结合区域最新研究成果,认为赣南钨矿形成于华南中生代岩石圈伸展时期的侏罗纪板内拉张环境的地球动力学背景,为同一成岩成矿系统的产物。     

基于Ruger近似式预测裂缝型储层的可行性研究

叠前纵波方位各向异性检测裂缝是目前应用最为广泛的方法,该方法主要是利用AVAZ地震资料椭圆拟合的长轴与短轴信息来评价裂缝,该方法在具体应用时,认为裂缝型储层AVAZ响应特征的变化仅由各向异性参数决定,而没有考虑非各向异性参数的影响,从而引起裂缝评价的多解性。为提高该技术的裂缝评价精度,从模型分析入手,将模型中各向异性参数及岩性参数设置为概率密度分布函数,然后采用Monte Carlo随机方法进行叠前AVAZ正演模拟。首先,对各向异性参数对椭圆拟合的影响进行简单分析,得出各向异性参数γ对椭圆扁率B/A与各向异性因子B影响最大、δ次之、ε最小。然后,进一步重点分析不同标准差的速度与密度模型的B/A与B响应特征,得出地层的速度与密度的变化在较小范围内时,该技术评价裂缝才具有一定的可行性,其中,地层纵波和横波速度的变化（尤其是纵波速度）对叠前AVAZ响应影响较大,地层密度几乎没有影响,故当横向岩性变化较大时,该方法在评价裂缝型储层时的结果具有一定的不确定性。