The use of microchemical analysis of alluvial gold grains in mineral exploration: experiences in Britain and Ireland

The examination of populations of gold grains by SEM and EPMA generates a microchemical signature based upon the assemblage of opaque mineral inclusions within the gold and the concentration of minor alloying metals. Duplicate samples of alluvial gold from the same locality have yielded the same microchemical signature independent of the field worker who collected the sample, or the date of collection. A study of five separate alluvial localities yielded a microchemical signature consistent with the mineralogy of the adjacent host mineralisation. This result has permitted informed speculation on the styles and complexity of gold mineralisation contributing to the alluvial population of gold grains where the style of source mineralisation is unknown.The technique can yield information about several aspects of the source mineralisation of an alluvial population at an early stage in the exploration process. Distinctive microchemical signatures can be used to evaluate the dispersion of gold either by fluvial or glacial action. The technique can identify gold derived from separate mineralising events either within a region or along a watercourse, thereby allowing speculation on the magnitude of the source mineralisation. It is also possible to establish whether specific types of gold mineralisation of potential economic interest have contributed to an alluvial population. In areas subjected to glaciation, analysis of the microchemical signature of populations of alluvial gold may indicate whether the gold grains are of proximal or distal origin.The volume of information made available through the study of alluvial gold grains has contributed greatly to the understanding of regional gold mineralisation. Interpretation of microchemical signatures of populations of alluvial gold from about 130 localities throughout Great Britain and Ireland has facilitated characterisation of gold in terms of the style of the source mineralisation. This information has augmented that available from about ten bedrock gold localities and has permitted a more complete classification of gold occurrences throughout the region.Quantitative analysis of some minerals within the opaque inclusion assemblage can provide information on the chemistry of the mineralising fluid, which may be related to the capacity for gold transportation.In Devon, South West England, a model for the emplacement of gold mineralisation was developed from interpretation of the microchemical signature of alluvial gold grains. The selenide-rich inclusion suite containing no sulphides suggested that oxidising conditions prevailed during precipitation. The model of transport of gold in solution through the oxidising environment of Permian red beds and its subsequent precipitation in underlying rocks due to reduction in Eh was consistent with the observed distribution of gold in alluvial sediment, residential overburden and drill core. Using this model other targets for gold mineralisation were successfully predicted.     

Adaptive optics near-infrared imaging of NGC 2992 – unveiling core structures related to radio figure-of-8 loops

We present near-infrared adaptive optics, Very Large Array (VLA) radio and Hubble Space Telescope ( HST ) optical imaging of the nearby Seyfert galaxy NGC 2992. Spiral structure and an extension to the west are traced down to the core region at the limiting resolution of our near-infrared images. A faint, diffuse loop of near-infrared and radio emission is also observed to the north, embedded within the prominent 2-arcsec radio loop previously observed to the north-west. Near-infrared colour maps and CO narrow-band imaging are then used to identify which regions may not be purely reddened stellar populations. Our new data provide evidence that the VLA radio-loop morphology in the shape of a figure of 8 represents two components superimposed: (1) outflow bubbles out of the plane of the disc, coincident with the extended emission-line region (EELR); (2) star formation along the spiral arm within the galaxy disc and through the dust lane. The near-infrared continuum emission associated with the outflowing radio bubbles suggests that the radio loops are driven by the active nucleus.     

The dynamics of natural pipe hydrological behaviour in blanket peat

Natural soil pipes are found in peatlands, but little is known about their hydrological role. This paper presents the most complete set of pipe discharge data to date from a deep blanket peatland in Northern England. In a 17.4‐ha catchment, we identified 24 perennially flowing and 60 ephemerally flowing pipe outlets. Eight pipe outlets along with the catchment outlet were continuously gauged over an 18‐month period. The pipes in the catchment were estimated to produce around 13.7% of annual streamflow, with individual pipes often producing large peak flows (maximum peak of 3.8 l s^?1). Almost all pipes, whether ephemerally or perennially flowing, shallow or deep (outlets > 1 m below the peat surface), showed increased discharge within a mean of 3 h after rainfall commencement and were dominated by stormflow, indicating good connectivity between the peatland surface and the pipes. However, almost all pipes had a longer period between the hydrograph peak and the return to base flow compared with the stream (mean of 23.9 h for pipes, 19.7 h for stream). As a result, the proportion of streamflow produced by the pipes at any given time increased at low flows and formed the most important component of stream discharge for the lowest 10% of flows. Thus, a small number of perennially flowing pipes became more important to the stream system under low‐flow conditions and probably received water via matrix flow during periods between storms. Given the importance of pipes to streamflow in blanket peatlands, further research is required into their wider role in influencing stream water chemistry, water temperature and fluvial carbon fluxes, as well as their role in altering local hydrochemical cycling within the peat mass itself. Enhanced piping within peatlands caused by environmental change may lead to changes in the streamflow regime with larger low flows and more prolonged drainage of the peat. Copyright © 2012 John Wiley & Sons, Ltd.     

Estimation of the elastic properties of seafloor sediments byinversion of precritical reflection data

A method has been presented for estimating the elastic properties of sea-floor sediment by inverting the amplitude-range information of precritical reflection data. The method was tested using synthetic data for reflection from a half-space sediment model and the estimates values for the P- and S-wave speeds and the density were in good agreement with the geoacoustic model parameters. The synthetic data were also used to determine the sensitivity of the method to interference from reflections from subbottom layers. In most practical cases the interference is not likely to be a problem for precritical reflection data. The method was then applied to measurements of seafloor reflection obtained in an experiment carried out off the west coast of Canada. The estimated values of the elastic properties of the topmost sediment layer were consistent with values available from the literature, and with estimates from other experiments carried out in the same region     

Geoacoustic model parameter estimation using a bottom-moored hydrophone array

This paper describes results from an experiment carried out to investigate geoacoustic inversion with a bottom-moored hydrophone array located in the shallow waters of the Timor Sea off the northern coast of Australia. The array consisted of two arms in a V shape, horizontally moored at a site that was essentially flat over a large area. Hydrophone positions were estimated using an array element localization (AEL) technique that established relative uncertainties of less than 1 m on the seafloor. The data used for geoacoustic inversion were from experiments with continuous wave (CW) tones in the 80- to 195-Hz band transmitted from a towed projector. A hybrid search algorithm determined the set of geoacoustic model parameters that maximized the Bartlett fit (averaged coherently spatially at each tone and incoherently over frequency) between the measured and modeled data at the array. Due to the long range experimental geometry, the inversion was sensitive to attenuation in the sediment. The inverted geoacoustic profile performed well in a simple test for localizing the sound source at other sites in the vicinity of the array. Range-depth localization performance for the horizontal array was comparable to that for an equivalent vertical array.     

Epsilon Aurigae

In April 1984, fourth contact ended the two year long eclipse of Epsilon Aurigae. An astrometric study of the study of the system was carried out by Van de Kamp (1978) leading to the conclusion that the orbit is seen very close to edge on. The eclipse was monitored by a number of groups from the ground and from spacecraft such as the IUE. Ultraviolet observations of the system from IUE have thrown new light on the nature of the system that lead us to conclude that the secondary object is probably a cold, dusty accretion disk surrounding a star that is completely hidden inside the diskCommunication presented at the International Conference on Astrometric Binaries, held on 13–15 June, 1984, at the Remeis-Sternwarte Bamberg, Germany, to commemorate the 200th anniversary of the birth of Friedrich Wihelm Bessel (1784–1846).     

Storylines: an alternative approach to representing uncertainty in physical aspects of climate change

Investigating the relationships between climate extremes and crop yield can help us understand how unfavourable climatic conditions affect crop production. In this study, two statistical models, multiple linear regression and random forest, were used to identify rainfall extremes indices affecting wheat yield in three different regions of the New South Wales wheat belt. The results show that the random forest model explained 41–67% of the year-to-year yield variation, whereas the multiple linear regression model explained 34–58%. In the two models, 3-month timescale standardized precipitation index of Jun.–Aug. (SPI_JJA), Sep.–Nov. (SPI_SON), and consecutive dry days (CDDs) were identified as the three most important indices which can explain yield variability for most of the wheat belt. Our results indicated that the inter-annual variability of rainfall in winter and spring was largely responsible for wheat yield variation, and pre-growing season rainfall played a secondary role. Frequent shortages of rainfall posed a greater threat to crop growth than excessive rainfall in eastern Australia. We concluded that the comparison between multiple linear regression and machine learning algorithm proposed in the present study would be useful to provide robust prediction of yields and new insights of the effects of various rainfall extremes, when suitable climate and yield datasets are available.