The application of lithogeochemistry in the search for unconformity-type uranium deposits, Northern Saskatchewan, Canada

Lithogeochemical-mineralogical haloes around unconformity-type uranium deposits in northern Saskatchewan can expand the size of drill targets up to fifteen times. The deposits are located at or near the unconformity between Aphebian metamorphosed basement rocks in close proximity to Archean granitoids and overlying unmetamorphosed sandstones of the Helikian Athabasca Group. Deposits studied include Key Lake (Deilmann), Midwest Lake and Eagle Point.Unconformity-type deposits are associated with broad alteration haloes in the overlying sandstones and more restricted haloes in the basement rocks. The haloes in the sandstone are localized around steeply dipping fault structures and are characterized by zones of intense leaching of specular hematite and detrital heavy mineral layers and by changes in chemistry related to clay mineral alteration and tourmalinization.Clay mineral alteration haloes are common in the sandstone and basement host rocks. Interstitial clays consist mainly of kaolinite and illite with lesser amounts of chlorite. The ratio K₂O/AI₂O₃ proved useful in delineating illite-kaolinite patterns in the sandstone and in the uppermost portions of the paleoweathering profile in basement rocks. The Midwest deposit is characterized by a broad bell-shaped zone (500 m across strike) of high K₂O/AI₂O₃ ratios (>0.18) in which illite is the dominant clay mineral. The sandstone above the Deilmann deposit, on the other hand, is characterized by silicification and kaolinitization with low K₂O/AI₂O₃ ratios (<0.04). This kaolinite cap overprints a preexisting illite zone.Anomalously high boron values are characteristic of the three deposits considered in this study. Boron anomalies are similar in extent to the anomalous clay mineral alteration haloes. Altered sandstones commonly contain aggregates of radiating magnesium-rich dravite needles within the clay matrix. The interpretation of boron patterns is problema tical however, mainly because of the detrital tourmaline component in sandstones and metamorphic tourmalines present in the Aphebian metasediments.Trace elements such as U, Ni, As and Co are generally of limited use in expanding targets in sandstone because their haloes are restricted to a few tens of metres horizon-anomalies along the steeply dipping zones of mineralization related to fault structures. Uranium (>3 ppm) does form an anomaly more than 200 metres laterally across the Midwest deposit. At the Deilmano deposit anomalous uranium dispersion is restricted to within a few metres of high grade ore.In the basement rocks, the various layers of the paleoweathering profile are geochemically overprinted up to 250 m from mineralization. Bleaching related to illitization and chloritization is associated with enrichment in K₂O, MgO, B, S, U, As, Ni and P₂O₃. The ratio Fe₂O₃/MgO is useful in delineating chloritization in the upper portion of the paleo weathering profile.Deposits with large root extensions in the Aphebian metasediments such as Eagle Point, have intense dravite-chlorite-illite alteration zones which are restricted to within a few metres of mineralization across strike. ‘Quartzite” units are alteration related. The complexity of the basement lithology inhibits the use of individual elements as alteration guides other than in the intensely altered zone. The application of multivariant techniques, element ratios and clay mineralogical work prove useful in identifying the mineralogical changes at Eagle Point.     

Determining hydraulic resistance in gravel‐bed rivers from the dynamics of their water surfaces

Traditionally, approaches to account for the effect of the boundary roughness of a gravel‐bed river have used a grain‐size index of the bed surface as a surrogate for hydraulic resistance. The use of a single grain‐size does not take into account the spatial heterogeneity in the bed surface and how this heterogeneity imparts resistance on the flow, nor the way in which this relationship changes with variables such as flow stage. A new technique to remotely quantify hydraulic resistance is proposed. It is based on measuring the dynamics of a river's water surface and relating this to the actual hydraulic resistance created by a rough sediment boundary. The water surface dynamics are measured using a new acoustic technique, grazing angle sound propagation (GRASP). This proposed method to measure hydraulic resistance is based on a greater degree of physical reasoning, and this is discussed in the letter. By measuring acoustically the temporal dynamics of turbulent water surfaces over a water‐worked gravel bed in a laboratory flume, a dependency is demonstrated between the temporal variation in the reflected acoustic pressure and measured hydraulic resistance. It is shown that the standard deviation in acoustic pressure decreases with increasing hydraulic resistance. This is shown to apply for a range of relative submergences and bed slopes that are typical of gravel‐bed rivers. This remote sensing technique is both rapid and inexpensive, and has the potential to be applied to natural river channels and to other environmental turbulent flows, such as overland flows. A whole new class of low‐cost, remote and non‐intrusive instruments could be developed as a result and used in a wide range of hydraulic and hydrological applications. Copyright © 2006 John Wiley & Sons, Ltd.     

Li isotope fractionation in peridotites and mafic melts

We have measured the Li isotope ratios of a range of co-existing phases from peridotites and mafic magmas to investigate high-temperature fractionations of ⁷Li/⁶Li. The Li isotopic compositions of seven mantle peridotites, reconstructed from analyses of mineral separates, show little variation (δ⁷Li 3.2-4.9‰) despite a wide range in fertility and radiogenic isotopic compositions. The most fertile samples yield a best estimate of δ⁷Li ∼ 3.5‰ for the upper mantle. Bulk analyses of olivine separates from the xenoliths are typically ∼1.5‰ isotopically lighter than co-existing orthopyroxenes, suggestive of a small, high-temperature equilibrium isotope fractionation. On the other hand, bulk analyses of olivine phenocrysts and their host melts are isotopically indistinguishable. Given these observations, equilibrium mantle melting should generate melts with δ⁷Li little different from their sources (<0.5‰ lighter). In contrast to olivine and orthopyroxene, that dominate peridotite Li budgets, bulk clinopyroxene analyses are highly variable (δ⁷Li = 6.6‰ to −8.1‰). Phlogopite separated from a modally metasomatised xenolith yielded an extreme δ⁷Li of −18.9‰. Such large Li isotope variability is indicative of isotopic disequilibrium. This inference is strongly reinforced by in situ, secondary ion mass-spectrometry analyses which show Li isotope zonation in peridotite minerals. The simplest zoning patterns show isotopically light rims. This style of zoning is also observed in the phenocrysts of holocrystalline Hawaiian lavas. More dramatically, a single orthopyroxene crystal from a San Carlos xenolith shows a W-shaped Li isotope profile with a 40‰ range in δ⁷Li, close to the isotope variability seen in all terrestrial whole rock analyses. We attribute Li isotope zonation in mineral phases to diffusive fractionation of Li isotopes, within mineral phases and along melt pathways that pervade xenoliths. Given the high diffusivity of Li, the Li isotope profiles we observe can persist, at most, only a few years at magmatic temperatures. Our results thus highlight the potential of Li isotopes as a high-resolution geospeedometer of the final phases of magmatic activity and cooling.     

A critical review and re-investigation of the Pleistocene deposits between Cranfield Point and Kilkeel,Northern Ireland: Implications for regional sea-level models and glacial reconstructions of the northern Irish Sea basin

The coastline of County Down includes sites that are pivotal to understanding the history of the last glaciation of the northern Irish Sea Basin in relation to relative sea level and regional glacial readvances. The cliff sections display evidence that has been used to underpin controversial models of glaciomarine sedimentation in isostatically-depressed basins followed by emergent marine and littoral environments. They also provide crucial evidence claimed to constrain millennial-scale ice sheet oscillations associated with uniquely large and rapid sea-level fluctuations. This paper reviews previous work and reports new findings that generally supports the ‘terrestrial’ model of glaciation, involving subglacial accretion and deformation of sediment beneath grounded ice. Deep troughs were incised into the till sheet during a post Late Glacial Maximum draw-down of ice into the Irish Sea Basin. Ice retreat was accompanied by glaciomarine accretion of mud in the troughs during a period of high relative sea level. The trough-fills were over-ridden, compacted, deformed and truncated during a glacial re-advance that is correlated with the Clogher Head Readvance. Grounding-line retreat accompanied by rapid subaqueous ice-proximal sedimentation preserved a widespread subglacial stone pavement. Raised beach gravels cap the sequence. The evidence supports an uninterrupted fall in relative sea level from c. 30?m that is consistent with sea level curves predicted by current glacio-isostatic adjustment modelling. Critical evidence previously cited in support of subaerial dissection of the troughs, and hence rapid fall and rise in relative sea level prior to the deposition of the glaciomarine muds, is not justified.     

Edgar Hynes Thompson: pragmatism, ingenuity and mechanisms

In this contribution the author looks for an explanation of an uncharacteristically scathing review, by E. H. Thompson towards the end of his life, of a book, Theory of Errors and Generalized Matrix Inverses, by Arne Bjerhammar, a leading European academic.     

Terracing potential field data

Terracing is an operator that is applied to potential field data to produce regions of constant field amplitude that are separated by sharp boundaries, as an aid to interpretation. When applied to map data, the boundaries are defined by the zero contour of the 2D Laplacian derivative operator. An improved method is described here that defines the boundaries by the zero contour of the profile curvature. This approach gives superior results because the 2D Laplacian operator is composed only of derivatives in the EW and NS directions, while the profile curvature uses the curvature in the 'uphill' direction at each point, whatever that direction may be. The method is demonstrated on gravity data from South Africa. Source code in Matlab format is available from the authors on request.     

Spatial contributions of diffuse inputs and within-channel processes to the form of stream water phosphorus over storm events

Concentrations of suspended particulate matter (SPM), NO₃-N and P fractions: PO₄-P, dissolved organic P (DOP), particulate P (PP) and bioavailable exchangeable P were examined over 5 storm events in two nested agricultural catchments in NE Scotland: a (51 km²) catchment and its headwater (4 km²). NO₃-N showed anticlockwise hysteresis for all storms in both catchments. In contrast, the headwater showed strong clockwise hysteresis of SPM, dissolved and particulate P concentrations, but which weakened through summer to spring. Less pronounced hysteresis of P forms in the larger catchment was attributed to a combination of factors: a less energetic system, nutrient leaching from the floodplain, a point source of a small sewage treatment works and the occurrence of coarser soil and sediment parent materials with less P adsorption and transport capacity. The headwater exhibited a strong ‘first flush’ effect of sediment and dissolved P, particularly following dry conditions, received a significant transfer of readily-solubilized organic P from the surrounding soils in late summer and after manure applications in winter, and was the likely cause of large sediment associated P signals observed in the 51 km² catchment. Our results suggest that steeper gradient headwaters should be targeted for riparian improvements to mitigate soil erosion from headwater fields. The efficiency of riparian erosion controls is also dependant on the size of the store of fine sediment material within the stream channel and this may be large.     

Turbulence in the presence of internal waves in the bottom boundary layer of the California inner shelf

Turbulence measurements were collected in the bottom boundary layer of the California inner shelf near Point Sal, CA, for 2 months during summer 2015. The water column at Point Sal is stratified by temperature, and internal bores propagate through the region regularly. We collected velocity, temperature, and turbulence data on the inner shelf at a 30-m deep site. We estimated the turbulent shear production (P), turbulent dissipation rate (ε), and vertical diffusive transport (T), to investigate the near-bed local turbulent kinetic energy (TKE) budget. We observed that the local TKE budget showed an approximate balance (P?≈?ε) during the observational period, and that buoyancy generally did not affect the TKE balance. On a finer resolution timescale, we explored the balance between dissipation and models for production and observed that internal waves did not affect the balance in TKE at this depth.