Surrounding Host Rock Research Articles

Mixed‐Mode Strain Localization Generated by Hydration Reaction at Crustal Conditions

AbstractHydration reactions influence rock density and rheology. For example, volume increases produced in hydration reactions may generate sufficient tensile and shear stress to fracture both the rock undergoing the reaction and the surrounding host rock. We performed in situ dynamic X‐ray synchrotron microtomography experiments to investigate reaction‐induced fracturing. Two experiments on hydration of periclase were performed at 180 or 190 °C, under a confinement of 10 or 80 MPa, a pore fluid pressure of 5 or 75 MPa, and with or without differential stress. The sample assembly consists of a periclase cylinder inserted into a central hole within a serpentinite cylinder. The reaction from periclase to brucite results in a large volume increase (110%), pushing the periclase/brucite against the serpentinite and ultimately breaking it. Using time‐resolved three‐dimensional imaging, we quantify the spatial and temporal distribution of the reaction‐induced fractures. We perform digital volume correlation analysis to obtain the incremental strain tensors throughout the hydration and fracturing process. We use numerical models to assess the distribution of stress within the serpentinite. The digital volume correlation results show mixed‐mode strain localization. The von Mises strain, indicative of shear, increases by a larger percentage than the contractive or dilatative strain components as the reaction‐induced fractures grow. The distribution of von Mises strain follows a power law relationship in the cumulative frequency‐magnitude domain, indicative of long‐range elastic stress interactions during fracturing. This experimental finding sheds insights on the mechanisms of microseismicity measured in areas undergoing active serpentinization.

Emplacement levels and pre-existing structures control mechanisms and host rock interactions of three granitic plutons, western Arabian Shield

Field relations, structural measurements, and microfabric analyses are used to explain the emplacement mechanisms of granitic plutons in the Asir terrane. These circular-to-elliptical granitic intrusions are prominent plutons scattered across the Arabian–Nubian Shield. Various structural and tectonic fabrics affecting three plutons in the western Arabian Shield encouraged studying the effect of pre-existing structures and emplacement levels on the emplacement mechanisms and related solid-state deformation. Two of these plutons, called Al Sayl and Um Ar Raka, are circular and enveloped by ductile or brittle solid-state deformation. Pre- and post-emplacement tectonic fabrics controlled the distribution and superposition of the solid-state deformation. Lateral expansion of magma during the emplacement of these plutons induced significant radial shortening that was associated with either high or low heat transfer into the host rocks and developed brittle or ductile solid-state deformation, respectively. Heat transfer was controlled by the emplacement level and magma pressure of each pluton. The third, oval-shaped Al Hawiyyah pluton was structurally controlled, where magma was emplaced first into twofold hinges on both sides of a thrust fault under low-to-intermediate differential stress. Syn-emplacement shortening was accommodated mainly by upward and lateral magma flows. Continuous magma flow caused uplift of the pluton and its surrounding host rocks and developed stretching structures in these rocks.

Long-Term Stability of a 13.7 × 30.5-m (45 × 100-ft) Undercut Span Beneath Cemented Rockfill at the Turquoise Ridge Mine, Nevada.

In 2001, researchers from the National Institute for Occupational Safety and Health (NIOSH) installed instruments at the Turquoise Ridge Mine in cooperation with Placer Dome, Inc. to monitor the geomechanical behavior and stability of a cemented rockfill (CRF) sill and the surrounding host rock during test mining of a large undercut span beneath backfill. Six parallel, adjacent drifts were mined and backfilled to construct a CRF sill, approximately 22.9 m (75 ft) wide by 30.5 m (100 ft) long. The sill was then partially undercut, successfully creating a 13.7-m (45-ft) wide by 30.5-m (100-ft) long span beneath the CRF. Only small vertical displacements were measured in the overlying host rock during mining, with most of the movement occurring at shallow depths in the mine roof. Because the back above the CRF sill remained stable, the majority of the mining-induced stress was transferred to the host rock abutments rather than to the backfilled drifts. During retreat mining of the undercut span, the CRF sill and the mine roof remained stable. Most of the measured vertical displacement was caused by separation of the backfill from the overlying host rock, or deflection of the CRF sill, which was comparable to the deflection of a monolithic, elastic plate having similar dimensions, material properties, and undercut spans. The CRF sill moved in mass as a single unit rather than as individual drift segments, and the vertical cold joints between adjacent backfill drifts did not adversely affect their stability. Additional measurements collected from the instruments have shown that the backfill span is still intact and in stable condition more than 16 years after the completion of undercut mining. Displacements in the mine roof and abutments have stabilized, and vertical stress and deformation within the CRF have generally leveled off or decreased. Although only slight mining-induced loads were transferred to the backfilled drifts, the CRF has confined the abutment ribs and mine roof, thereby improving their long-term stability. Results of compressive and tensile strength tests conducted with CRF samples from the test site indicate that the long-term compressive strength gain for CRF is similar to that of concrete, and that the tensile-to-compressive strength ratio for CRF is about 1/6 rather than 1/10. Assuming the in-place CRF gained strength at the same rate as the lab samples, an analytical analysis of the flexural stability of the CRF undercut span shows that the Factor of Safety for the span should have logically increased over time. By providing a better understanding of the long-term strength properties and geomechanical behavior of CRF, these research findings help improve the methods that are used for designing stable, long-term undercut entries beneath cemented backfill.

Thermal Evolution near Heat-Generating Nuclear Waste Canisters Disposed in Horizontal Drillholes

We consider the disposal of spent nuclear fuel and high-level radioactive waste in horizontal holes drilled into deep, low-permeable geologic formations using directional drilling technology. Residual decay heat emanating from these waste forms leads to temperature increases within the drillhole and the surrounding host rock. The spacing of waste canisters and the configuration of the various barrier components within the horizontal drillhole can be designed such that the maximum temperatures remain below limits that are set for each element of the engineered and natural repository system. We present design calculations that examine the thermal evolution around heat-generating waste for a wide range of material properties and disposal configurations. Moreover, we evaluate alternative layouts of a monitoring system to be part of an in situ heater test that helps determine the thermal properties of the as-built repository system. A data-worth analysis is performed to ensure that sufficient information will be collected during the heater test so that subsequent model predictions of the thermal evolution around horizontal deposition holes will reliably estimate the maximum temperatures in the drillhole. The simulations demonstrate that the proposed drillhole disposal strategy can be flexibly designed to ensure dissipation of the heat generated by decaying nuclear waste. Moreover, an in situ heater test can provide the relevant data needed to develop a reliable prediction model of repository performance under as-built conditions.

Formation and dynamics of magma reservoirs.

The emerging concept of a magma reservoir is one in which regions containing melt extend from the source of magma generation to the surface. The reservoir may contain regions of very low fraction intergranular melt, partially molten rock (mush) and melt lenses (or magma chambers) containing high melt fraction eruptible magma, as well as pockets of exsolved magmatic fluids. The various parts of the system may be separated by a sub-solidus rock or be connected and continuous. Magma reservoirs and their wall rocks span a vast array of rheological properties, covering as much as 25 orders of magnitude from high viscosity, sub-solidus crustal rocks to magmatic fluids. Time scales of processes within magma reservoirs range from very slow melt and fluid segregation within mush and magma chambers and deformation of surrounding host rocks to very rapid development of magma and fluid instability, transport and eruption. Developing a comprehensive model of these systems is a grand challenge that will require close collaboration between modellers, geophysicists, geochemists, geologists, volcanologists and petrologists. This article is part of the Theo Murphy meeting issue 'Magma reservoir architecture and dynamics'.

Conforming, non-conforming and non-matching discretization couplings in discrete fracture network simulations

Simulations of fluid flow in naturally fractured rocks have implications for several subsurface applications, including energy storage and extraction, and waste disposal. We are interested in flow in discrete fracture networks, which explicitly represent flow in fracture surfaces, but ignore the impact of the surrounding host rock. Fracture networks, generated from observations or stochastic simulations, will contain intersections of arbitrary length, and intersection lines can further cross, forming a highly complex geometry. As the flow exchange between fractures, thus in the network, takes place in these intersections, an adequate representation of the geometry is critical for simulation accuracy. In practice, the intersection dynamics must be handled by a combination of the simulation grid, which may or may not resolve the intersection lines, and the numerical methods applied on the grid. In this work, we review different classes of numerical approaches proposed in recent years, covering both methods that conform to the grid, and non-matching cases. Specific methods considered herein include finite elements, mixed and virtual elements and control volume methods. We expose our methods to an extensive set of test cases, ranging from artificial geometries designed to test difficult configurations, to a network extruded from a real fracture outcrop. The main outcome is guidances for choice of simulation models and numerical discretization with a trade off on the computational cost and solution accuracy.

Marine dipole–dipole controlled source electromagnetic and coincident-loop transient electromagnetic experiments to detect seafloor massive sulphides: effects of three-dimensional bathymetry

Seafloor massive sulphides (SMSs) are regarded as a potential future resource to satisfy the growing global demand of metals including copper, zinc and gold. Aside from mining and retrieving profitable amounts of massive sulphides from the seafloor, the present challenge is to detect and delineate significant SMS accumulations, which are generally located near mid-ocean ridges and along submarine volcanic arc and backarc spreading centres. Currently, several geophysical technologies are being developed to detect and quantify SMS occurrences that often exhibit measurable contrasts in their physical parameters compared to the surrounding host rock. Here, we use a short, fixed-offset controlled source electromagnetic (CSEM) system and a coincident-loop transient electromagnetic (TEM) system, which in theory allow the detection of SMS in the shallow seafloor due to a significant electrical conductivity contrast to their surroundings. In 2016, CSEM and TEM experiments were carried out at several locations near the Trans- Atlantic Geotraverse hydrothermal field to investigate shallow occurrences of massive sulphides below the seafloor. Measurements were conducted in an area that contains distinct SMS sites located several kilometres off-axis from the Mid-Atlantic ridge, some of which are still connected to hydrothermal activity and others where hydrothermal activity has ceased. Based on the quality of the acquired data, both experiments were operationally successful. However, the data analysis indicates bias caused by three-dimensional (3D) effects of the rough bathymetry in the study area and, thus, data interpretation remains challenging. Therefore, we study the influence of 3D bathymetry for marine CSEM and TEM experiments, focusing on shallow 3D conductors located beneath mound-like structures.We analyse synthetic inversion models for attributes associated with 3D distortions of CSEM and TEM data that are not sufficiently accounted for in conventional 1D (TEM) and 2D (CSEM) interpretation schemes. Before an adequate quantification of SMS in the region is feasible, these 3D effects need to be studied to avoid over/underestimation of SMS using the acquired EM data. The sensitivity of CSEM and TEM to bathymetry is investigated by means of 3D forward modelling, followed by 1D (TEM) and 2D (CSEM) inversion of the synthetic data using realistic error conditions. Subsequently, inversion models of the synthetic 3D data are analysed and compared to models derived from the measured data to illustrate that 3D distortions are evident in the recorded data sets.

Clastic Pipes and Soft-Sediment Deformation of the Jurassic Carmel Formation, Southern Utah, U.S.A.: Implications For Pipe Formation Mechanisms and Host-Rock Controls

Abstract Vertically oriented columns of sandstone, termed clastic pipes, are common in Jurassic deposits of the Colorado Plateau. Exposures in the sabkha to fluvial strata of the Carmel Formation in southern Utah provide an excellent opportunity to understand pipe-formation mechanisms and controls on their expressions. These clastic pipes demonstrate significant variability and fall into two major categories: (1) primary textural pipes and (2) non-textural pipes. Primary textural pipes show significant textural differences relative to the host rock, including an internal outward grain coarsening indicative of traction structures. The pipes contain entrained, brecciated host material, and the surrounding host rock has associated soft-sediment deformation. Primary textural pipes formed through multiple liquefaction and fluidization events as evident from crosscutting pipes, multiple eruption horizons, traction structures, and other evidence of fluid-suspended-sediment transport. Non-textural pipes do not have significant textural differences from the surrounding host rock but have clear diagenetic differences (e.g., mineral coloration and cement differences) relative to the host rock. These non-textural pipes formed through the upward expulsion of fluids similar to primary textural pipes, but the fluid forces were likely insufficient to fully fluidize the sand. Overall, the porous, sandstone host rock plays a key role in explaining pipe characteristics, including their cylindrical geometry (versus tabular geometries in mudstones) and diagenetic expression. Pipe variability typically corresponds to lithologic and stratigraphic changes in the surrounding host rock. These pipe examples are key paleoenvironmental indicators, which give valuable information about subsurface fluids and the presence of a near-surface groundwater system.

Self‐Organizing Fluid Convection Patterns in an en Echelon Fault Array

AbstractWe present three‐dimensional numerical simulations of natural convection in buried, vertical en echelon faults in impermeable host rock. Despite the fractures being hydraulically disconnected, convection within each fracture alters the temperature field in the surrounding host rock, altering convection in neighboring fractures. This leads to self‐organization of coherent patterns of upward/downward flow and heating/cooling of the host rock spanning the entire fault array. This “synchronization” effect occurs when fracture spacing is less than the width of convection cells within the fractures, which is controlled by fracture transmissivity (permeability times thickness) and heterogeneity. Narrow fracture spacing and synchronization enhance convective fluid flow within fractures and cause convection to initiate earlier, even lowering the critical transmissivity necessary for convection initiation. Heat flow through the en echelon region, however, is enhanced only in low‐transmissivity fractures, while heat flow in high‐permeability fractures is reduced due to thermal interference between fractures.

Chemical variability in mineralized veins observed by ChemCam on the lower slopes of Mount Sharp in Gale crater, Mars

ChemCam has observed a wide range of diagenetic features along the Curiosity rover traverse including pervasive Ca-sulfate veins. Observations by multiple instruments on Curiosity indicate that these veins are hydrated, formed during diagenetic fluid event(s). In this study, we delve into the chemical variability in these Ca-sulfate bearing veins and have identified two subsets in the Murray formation with enrichments in Fe and Fe + Mg. These chemical trends do not reflect a sampling mixture with the surrounding host rock but likely indicates the presence of authigenic phases formed during the emplacement of these veins. Based on passive reflectance spectral analysis and correlation with other elements, Fe3+ oxides and/or sulfates are proposed to account for the Fe-rich observations in the vicinity of the Naukluft Plateau whereas the Fe + Mg trend is also observed in adjacent dark-toned features with elevated Mn and P near the Old Soaker outcrop. The specific localization of these observations in the Gale stratigraphy implies changing pH and redox conditions in the groundwater at the time of formation of these veins, from oxidizing and likely more acidic near the Naukluft Plateau to more reducing conditions in the upper part of the Murray formation.

Multi-reservoir fluid mixing processes in rift-related hydrothermal veins, Schwarzwald, SW-Germany

Fluid mixing is an important process in the formation of many hydrothermal vein-type deposits. Here, we present evidence from hydrothermal fluorite-barite-quartz veins with Pb-Zn-Cu-(Ag)-sulfides and associated mineralization, indicating that mineral precipitation was initiated by mixing of fluids derived from multiple sources, including mixing between more than two end-member fluid compositions. Based on our observations, we relate the diversity of the hydrothermal veins of the Schwarzwald mining district in terms of mineral assemblage and fluid inclusion chemistry to the disturbed and transient geological environment during ongoing rifting. Literature data on the regional geology, current groundwater reservoirs, formation processes and hydraulic features are augmented by new fluid inclusion analyses from post-Cretaceous, hydrothermal vein minerals including microthermometry, crush leach, Microraman and LA-ICP-MS analyses of individual fluid inclusions.Petrography and microthermometry of fluid inclusions show complex sequences of alternating fluid signatures within different growth zones of one crystal. High (20–26wt% NaCl+CaCl2), moderate (5–20wt% NaCl+CaCl2) and low salinity (<5wt% NaCl+CaCl2), sulfate- and/or CO2-bearing primary fluids were trapped during crystal growth. Such variations are commonly observed in minerals from different localities. Bulk crush leach analyses show significant variations in major element composition of the trapped fluids, within the overall Na-Ca-Cl-SO4-HCO3-system. These variations are caused by mixing of fluids from different aquifers and in various proportions. Ancient fluids show chemical similarities to modern groundwater aquifers that are available for direct sampling, such as granitic basement, Lower Triassic sandstones or Middle Triassic limestones and evaporites. Analyses of individual fluid inclusions by LA-ICP-MS support this interpretation and document the multi-component fluid mixing processes at individual localities recorded on the scale of single crystal growth zones. The latter data are used in a diffusion model to obtain the duration of mineral growth (before the fluid is homogenized), which implies very short-lived fluid events on the order of seconds to hours.By defining end member fluids and their proportions, we show that nearly all fluid mixtures are saturated with respect to barite. By contrast, fluorite-saturated fluids can only be modelled by mixing of a basement brine with fluids from Triassic sandstones. All fluid mixtures are strongly undersaturated with respect to galena, chalcopyrite and sphalerite, the most commonly observed ore minerals in the hydrothermal veins. As the calculated fluid mixtures are typically relatively oxidized and contain high sulfate/sulfide ratios, precipitation of sulfides was probably related to short-lived reduction events caused by an influx of hydrocarbons, by reactions with graphitic wall rocks in fractures by sulfidation related to fluid-rock reaction with the surrounding host rocks or an external influx of hydrocarbon-bearing fluids. The multi-aquifer fluid mixing processes involving aquifers of different chemical and physical constitution were triggered by brittle deformation related to rifting of the Rhine graben. This appears to be essential for the formation of a large number of mineralogically diverse hydrothermal ore deposits.

Resonance oscillations of the Soufrière Hills Volcano (Montserrat, W.I.) magmatic system induced by forced magma flow from the reservoir into the upper plumbing dike

Short-period deformation cycles are a common phenomenon at active volcanoes and are often attributed to the instability of magma flow in the upper plumbing system caused by fluctuations in magma viscosity related to cooling, degassing, and crystallization. Here we present 20-min periodic oscillations in ground deformation based on high-precision continuous borehole strain data that were associated with the 2003 massive dome-collapse at the Soufrière Hills Volcano, Montserrat (West Indies). These high-frequency oscillations lasted ~80min and were preceded by a 4-hour episode of rapid expansion of the shallow magma reservoir. Strain amplitude ratios indicate that the deformational changes were generated by pressure variations in the shallow magma reservoir and – with reversed polarity – the adjacent plumbing dike. The unusually short period of the oscillations cannot be explained with thermally induced variations in magma properties. We investigate the underlying mechanism of the oscillations via a numerical model of forced magma flow through a reservoir-dike system accounting for time-dependent dilation/contraction of the dike due to a viscous response in the surrounding host rock. Our results suggest that the cyclic pressure variations are modulated by the dynamical interplay between rapid expansion of the magma chamber and the incapacity of the narrow dike to take up fast enough the magma volumes supplied by the reservoir. Our results allow us to place first order constraints on the viscosity of crustal host rocks and consequently its fractional melt content. Hence, we present for the first time crustal-scale in situ measurements of rheological properties of mush zones surrounding magmatic systems.

Downhole physical property logging for iron-oxide exploration, rock quality, and mining: An example from central Sweden

Several physical properties obtained from geophysical logging and laboratory measurements were analyzed in order to characterize iron-oxide mineralization and host rocks in the Blötberget mining area of central Sweden. Seven boreholes intersecting the mineralization between 300 and 600 m depth in a volcano-sedimentary setting were downhole logged for this purpose. The downhole logging included full-waveform triple sonic, natural gamma, magnetic susceptibility, formation resistivity, fluid temperature and conductivity while laboratory measurements consisted of density, rock quality designation and magnetite content measurements. Full-waveform sonic data were used for rock quality assessments of the mineralized zones and their host rocks and proved their potential to be used for mine planning purposes. The ore-bearing rocks are primarily distinguished by increased density and dynamic elastic moduli estimated from the full-waveform sonic logging. Although seismic velocities do not follow a linear increase with the density for the mineralized rocks, as expected for igneous rocks, it is observed that a strong seismic signal from the mineralization can be expected primarily due to their high density. In addition, the full-waveform sonic data could be used as a proxy for fracture delineations and in-situ rock quality assessment. For example, zones of washed-up amplitudes in these data correlate well with zones of poor quality rocks, identified by core logging. Based on the full-waveform sonic data, seismic attenuation and its reciprocal, the seismic quality factor, were calculated. A decrease in the seismic quality factor was observed at zones with low rock quality designation, but also correlated with the mineralized zones suggesting several fracture zones in the mineralization and that the ore-bearing rocks might be less competent than the surrounding host rocks. Eventual rock support and reinforcement might be required for future mining operations. Full-waveform sonic data have the potential to improve rock quality assessments for mine planning and exploration purposes.

Magmatic-hydrothermal evolution of the Kymi topaz granite stock, SE Finland: Mineral chemistry evidence for episodic fluid exsolution

The Kymi stock is a highly evolved topaz-bearing granite stock that has intruded into rapakivi granites in the central part of the Wiborg batholith (southwest Finland). The stock shows a well-defined zonation and intrusion sequence consisting of early porphyritic granites, subsequent equigranular granite and a late marginal stockscheider pegmatite. Miarolitic cavities occur within all rock types of the stock, but they are more abundant near the contact between the porphyritic and equigranular granites. Related hydrothermal greisen alteration, barren and Pb-Zn mineralized fluorite-bearing quartz veins crosscut the host rapakivi granites close to the Kymi stock. The major and trace element chemistry of biotite, K-feldspar, plagioclase and fluorite tracks the magmatic to hydrothermal evolution of the system. The biotite composition ranges from Al-rich annite to Li-rich siderophyllite, and the biotite chemistry shows an increase in Si, Li, AlVI, F, Mn, Rb, Cs, Zn, Be, Ga, Tl, Ta, and F, and a decrease in Fe, AlIV, Mg, Ti, Ba, Sr, Nb, and Cl along the evolution of the system. The Nb/Ta ratios in biotites suggest that magmatic fractionation is responsible for the progressive Ta enrichment in early porphyritic granites. The subsequent decrease in Nb at rather constant Ta in the more evolved equigranular granite and stockscheider pegmatite cannot be explained by magmatic fractionation, but requires selective partitioning of Nb into an aqueous fluid phase. The halogen composition of the biotites changes drastically from the early porphyritic granites to the later more evolved equigranular granite and stockscheider pegmatite. The Cl/F data of biotite from the miarolitic cavities plots at the transition between the porphyritic and equigranular granites, and is interpreted in terms of major fluid exsolution occurring at this stage. The minor and trace element data of K-feldspar and plagioclase show similar compositional trends as the biotites. The REEY patterns of fluorites from the equigranular granite and the stockscheider pegmatite, compared with those from the earlier porphyritic granites, show pronounced tetrad effects and correlated Y anomalies. This suggests that the F enrichment in the equigranular granite and the stockscheider pegmatite, in conjunction with major fluid exsolution and REEY complexing as fluoride species at this stage, have been key to development of the tetrad effect and Y anomalies. The fluorite from hydrothermal greisen alteration and quartz veins shows different chondrite normalized REEY patterns, indicating contributions from other sources such as meteoric fluids and/or interaction with the surrounding host rocks. The major and trace element data from the biotites of the Kymi stock are distinct from those of the host rapakivi granites, suggesting that the parental melts have not been entirely derived from fractionated rapakivi granite melts.

Structure and petrography of the Valle Mosso pluton, Sesia Magmatic System, Southern Alps

ABSTRACTThe Valle Mosso pluton (VMP) is a Permian granitic body intruded at intermediate to upper crustal levels in the rocks of the pre-Alpine basement of the Ivrea-Verbano Zone and Serie dei Laghi shortly after the end of Variscan orogeny. As a consequence of Triassic to Jurassic rifting and Alpine orogeny, the VMP and surrounding host rocks have been tilted more than 60° from their original Permian polarity. Thus at present day, the VMP offers the rare opportunity to study a roof-to-floor exposure of a granitic pluton, providing insights into pristine geometry of the magma chamber and its relations to the country rocks. This work presents a new drift and solid map of the VMP and its surrounding host rocks at 1:15.000 scale.

Reference level of the occupational radiation exposure in a deep geological disposal facility for high-level nuclear waste: A Monte Carlo study

In deep geological repositories for high-level nuclear waste, radiation field around the disposed nuclear waste package is characterized by highly scattered radiations due to the surrounding host rock layers or cement liner. Calculation of the reference level of the occupational radiation exposure in such a facility is hence of interest, since geometrical conditions of the occupational exposure in the facility cannot be readily represented by the standard irradiation geometries considered by ICRP. In this study, a horizontal emplacement drift inside rock salt was modeled to represent a deep geological disposal facility. A nuclear waste package, simulated with a shielding cask loaded with spent nuclear fuel, was placed on the ground of the rock salt drift. A “reference worker” inside the drift was represented by the ICRP/ICRU reference adult voxel phantom. The reference level of the occupational radiation exposure was then calculated with a Monte Carlo code in terms of the effective dose based on the ICRP 2007 recommendation. In order to investigate the occupational exposure of a worker during different working scenarios in the drift, the effective dose was calculated with the voxel phantom placed at various distances and different body orientations with respect to the shielding cask. Furthermore, the effective dose obtained with voxel phantom was compared with that obtained with the fluence-to-effective-dose conversion coefficients for the standard irradiation geometries provided by ICRP. It was found out that (1) usage of the dose conversion coefficients for the isotropic (ISO) geometry, which is recommended by ICRP for highly scattered radiation fields, generally underestimates the effective dose in the rock salt emplacement drift; (2) depending on the orientation of the worker in the drift, the dose conversion coefficients for the anterior-to-posterior (AP) or the rotational (ROT) geometry should be used, in order to obtain an adequate estimation of the effective dose in the rock salt drift.

Tracing the fluid evolution of the Kiruna iron oxide apatite deposits using zircon, monazite, and whole rock trace elements and isotopic studies

The ore genesis of the Paleoproterozoic iron oxide apatite deposits in the vicinity of Kiruna in northern Sweden is poorly understood, despite a century-long mining history and 2500Mt of iron ore with grades of 30 to 70wt% Fe produced in the region to date. Zircon grains from the ore, recently dated at ca. 1874Ma, show very different appearances compared to zircon from surrounding host rocks (ca. 1880Ma) and related intrusions (ca. 1880 and ca. 1874Ma), particularly an inclusion-rich rim. In contrast, zircon from the host rocks, and a proximal granite intrusion, exhibit typical igneous growth zoning. Electron microprobe results show near stoichiometric composition for Zr, Si, and Hf in the host rock zircon grains. The ore zircon crystals have low analytical totals with significant concentrations of Ca, Fe, Y, and P and infrared spectroscopy showed several weight percent of water. These ore zircon grains further show Fe-rich inclusions, zones and/or veins in elemental X-ray maps, and light rare earth elements (LREE) enrichment. Transmission electron microscopy (TEM) shows that the LREE are not due to micro- or nano-inclusions in the zircon, but are likely hosted as LREE oxides in amorphous regions of the grains. Based on these characteristics, the rims on ore zircon grains are interpreted to be of hydrothermal origin. Uranium-Pb in monazite from the ore, measured by SIMS, suggests a secondary event influencing the area at ca. 1624Ma, a period of known geologic activity in Fennoscandia. Electron microprobe X-ray mapping of these monazite grains shows no zoning and relatively low U and Th concentrations.Stark contrasts are visible between the ore (depleted mantle influence) and host rocks (crustal influence) in the whole rock Lu-Hf and Sm-Nd data. The depleted mantle signature of the ore could be related to the Kiruna greenstone group as a potential source region for the iron. The Sm-Nd isotopic composition of monazite from the ore shows a crustal influence, and indicates that the younger event has not disturbed the whole rock Sm-Nd signature of the ore. The hydrothermal nature of the ore zircon grains and the isotopic signatures point to a hydrothermal influence on the ore formation, with a high temperature magmatic fluid related to the intrusions as most likely heat and fluid source.

Application of Numerical Modelling and Genetic Programming in Hydrocarbon Seepage Prediction and Control for Crude Oil Storage Unlined Rock Caverns

Seepage control is a prerequisite for hydrocarbon storage in unlined rock caverns (URCs) where the seepage of stored products to the surrounding host rock and groundwater can cause serious environmental and financial problems. Practically seepage control is performed by permeability and hydrodynamic control methods. This paper employs numerical modelling and genetic programming (GP) for the purpose of seepage prediction and control method determination for the crude oil storage URCs based on the effective parameters including hydrogeologic characteristic of the rock and physicochemical properties of the hydrocarbons. Several levels for each parameter were considered and all the possible scenarios were modelled numerically for the two-phase mixture model formulation. The corresponding seepage values were evaluated to be used as genetic programming data base to generate representative equations for the hydrocarbon seepage value. The coefficients of determination (R2) and relative percent errors of the proposed equations show their ability in the seepage prediction and permeability or hydrodynamic control method determination and design. The results can be used for crude oil storage URCs worldwide.

The integration of physical rock properties, mineralogy and geochemistry for the exploration of large zinc silicate deposits: A case study of the Vazante zinc deposits, Minas Gerais, Brazil

The Vazante deposit, which is the world's largest zinc silicate deposit, occurs in brecciated dolomite and comprises mainly willemite with various proportions of hematite, Fe-carbonate, minor franklinite and magnetite. Exploration for this type of deposit is more challenging than zinc sulfide deposits, as they do not exhibit similar geophysical anomalies. To improve the application of geophysical surveys to the exploration of hypogene silicate zinc deposits, data from 475 samples were investigated from drill holes representative of the various types of ore and host rocks as well as barren zones of known geophysical anomalies in the Vazante District. Lithogeochemical and mineralogical (optical, SEM and MLA) data were integrated with physical rock properties (density, magnetic susceptibility and KUTh gamma-ray spectrometry) to assist in exploring for this type of deposit. The most distinct physical property of the ore is density, compared with the host rocks due to high proportion of denser minerals (hematite and willemite). However, barren hematite breccias also have high densities. The zinc ore and hematite breccias yielded higher magnetic susceptibilities than the surrounding host rocks, with the highest values associated with greater proportions of franklinite and magnetite. The density and magnetic susceptibility contrasts are a result of hydrothermal fluids interacting with and altering the carbonate host rocks. Zinc ore also yielded elevated U concentrations relative to the various host rocks, yielding higher gamma-ray spectrometric values. The results of this investigation indicate that an integration of magnetic, gravimetric and radiometric surveys would be required to identify zinc silicate ore zones.

Chemistry of diagenetic features analyzed by ChemCam at Pahrump Hills, Gale crater, Mars

The Curiosity rover's campaign at Pahrump Hills provides the first analyses of lower Mount Sharp strata. Here we report ChemCam elemental composition of a diverse assemblage of post-depositional features embedded in, or cross-cutting, the host rock. ChemCam results demonstrate their compositional diversity, especially compared to the surrounding host rock: (i) Dendritic aggregates and relief enhanced features, characterized by a magnesium enhancement and sulfur detection, and interpreted as Mg-sulfates; (ii) A localized observation that displays iron enrichment associated with sulfur, interpreted as Fe-sulfate; (iii) Dark raised ridges with varying Mg- and Ca-enriched compositions compared to host rock; (iv) Several dark-toned veins with calcium enhancement associated with fluorine detection, interpreted as fluorite veins. (v) Light-toned veins with enhanced calcium associated with sulfur detection, and interpreted as Ca-sulfates. The diversity of the Pahrump Hills diagenetic assemblage suggests a complex post-depositional history for fine-grained sediments for which the origin has been interpreted as fluvial and lacustrine. Assessment of the spatial and relative temporal distribution of these features shows that the Mg-sulfate features are predominant in the lower part of the section, suggesting local modification of the sediments by early diagenetic fluids. In contrast, light-toned Ca-sulfate veins occur in the whole section and cross-cut all other features. A relatively late stage shift in geochemical conditions could explain this observation. The Pahrump Hills diagenetic features have no equivalent compared to targets analyzed in other locations at Gale crater. Only the light-toned Ca-sulfate veins are present elsewhere, along Curiosity's path, suggesting they formed through a common late-stage process that occurred at over a broad area.