Petrology, geochemistry and the mechanisms determining the distribution of platinum-group element and base metal sulphide mineralisation in the Platreef at Overysel, northern Bushveld Complex, South Africa     

David A. Holwell  Iain McDonald 《Mineralium Deposita》2006,41(6):575-598

Platinum-group element (PGE) mineralisation within the Platreef at Overysel is controlled by the presence of base metal sulphides (BMS). The floor rocks at Overysel are Archean basement gneisses, and unlike other localities along the strike of the Platreef where the floor is comprised of Transvaal Supergroup sediments, the intimate PGE–BMS relationship holds strong into the footwall rocks. Decoupling of PGE from BMS is rare and the BMS and platinum-group mineral assemblages in the Platreef and the footwall are almost identical. There is minimal overprinting by hydrothermal fluids; therefore, the mineralisation style present at Overysel may represent the most ‘primary’ style of Platreef mineralisation preserved anywhere along the strike. Chondrite-normalised PGE profiles reveal a progressive fractionation of the PGE with depth into the footwall, with Ir, Ru and Rh dramatically depleted with depth compared to Pt, Pd and Au. This feature is not observed at Sandsloot and Zwartfontein, to the south of Overysel, where the footwall rocks are carbonates. There is evidence from rare earth element abundances and the amount of interstitial quartz towards the base of the Platreef pyroxenites that contamination by a felsic melt derived from partial melting of the gneissic footwall has taken place. Textural evidence in the gneisses suggests that a sulphide liquid percolated down into the footwall through a permeable, inter-granular network that was produced by partial melting around grain boundaries in the gneisses that was induced by the intrusion of the Platreef magma. PGE were originally concentrated within a sulphide liquid in the Platreef magma, and the crystallisation of monosulphide solid solution from the sulphide liquid removed the majority of the IPGE and Rh from it whilst still within the mafic Platreef. Transport of PGE into the gneisses, via downward migration of the residual sulphide liquid, fractionated out the remaining IPGE and Rh in the upper parts of the gneisses leaving a ‘slick’ of disseminated sulphides in the gneiss, with the residual liquid becoming progressively more depleted in these elements relative to Pt, Pd and Au. Highly sulphide-rich zones with massive sulphides formed where ponding of the sulphide liquid occurred due to permeability contrasts in the footwall. This study highlights the fact that there is a fundamental floor rock control on the mechanism of distribution of PGE from the Platreef into the footwall rocks. Where the floor rocks are sediments, fluid activity related to metamorphism, assimilation and later serpentinisation has decoupled PGE from BMS in places, and transport of PGE into the footwall is via hydrothermal fluids. In contrast, where the floor is comprised of anhydrous gneiss, such as at Overysel, there is limited fluid activity and PGE behaviour is controlled by the behaviour of sulphide liquids, producing an intimate PGE–BMS association. Xenoliths and irregular bands of chromitite within the Platreef are described in detail for the first time. These are rich in the IPGE and Rh, and evidence from laurite inclusions indicates they must have crystallised from a PGE-saturated magma. The disturbed and xenolithic nature of the chromitites would suggest they are rip-up clasts, either disturbed by later pulses of Platreef magma in a multi-phase emplacement or transported into the Platreef from a pre-existing source in a deeper staging chamber or conduit.  相似文献   

Zr-in-rutile thermometry in blueschists from Sifnos, Greece   总被引：1，自引：0，他引：1  

Frank S. Spear  David A. Wark  John T. Cheney  John C. Schumacher  E. Bruce Watson 《Contributions to Mineralogy and Petrology》2006,152(3):375-385

Zr-in-rutile thermometry on samples of blueschist from Sifnos, Greece, yields temperatures that reflect progressive crystallization of rutile from ca. 445 to 505°C with an analytical precision of + 18/−27 and ± 10°C using the electron microprobe and ± 1.5–3.5°C using the ion microprobe. Individual grains are generally homogeneous within analytical uncertainty. Different grains within a single sample record temperature differences as large 55°, although in most samples the range of temperatures is on the order of 25°. In several samples, Zr-in-rutile temperatures from grains within garnet are lower than temperatures from matrix grains, reflecting growth of rutile with increasing temperature of metamorphism. Although the specific rutile-producing reactions have not been identified, it is inferred that rutile grows from either continuous reaction involving the breakdown of lower grade phases (possibly ilmenite), or from pseudomorph reactions involving the breakdown of relic igneous precursors at blueschist-facies conditions. No systematic variation in rutile temperatures was observed across the blueschist belt of northern Sifnos, consistent with the belt having behaved as a coherent block during subduction.  相似文献   

Avalanche Risk During Backcountry Skiing – An Analysis of Risk Factors     

Harpa Grímsdóttir  David Mcclung 《Natural Hazards》2006,39(1):127-153

Skier-triggered avalanches are the main cause of avalanche accidents in backcountry skiing. The risk of accidents during backcountry skiing was analysed statistically and related to factors such as elevation level, aspect, stability rating and the time of the year. The analysis is based on a database about terrain usage and avalanche accidents from a large heli-skiing operator in Canada, which makes it possible to study the conditional probability of accidents given the recorded pattern of terrain usage. This study shows that the historical risk of accidentally triggering an avalanche greater than size 1 depends highly on the stability rating, with the highest risk occurring during “poor” stability. The risk is greater at high elevations, and it is lower during the late season than earlier on. Skier risk does not depend as much on aspect as may be indicated from avalanche data alone. However, it is relatively high in the N–NE–E sector. These factors are not independent of each other and therefore analyses of combined factors were also performed. Questionnaires and interviews were used to gain knowledge about the terrain selection of professional mountain guides. These results indicate that when selecting terrain, guides first look at the overall shape and size of the terrain, but avalanche history of terrain and inclination are also important factors. Finally, remarks in avalanche reports were analysed, and common human factors identified.  相似文献   

Multicollinearity and correlation among local regression coefficients in geographically weighted regression   总被引：5，自引：3，他引：5  

David Wheeler  Michael Tiefelsdorf 《Journal of Geographical Systems》2005,7(2):161-187

Present methodological research on geographically weighted regression (GWR) focuses primarily on extensions of the basic GWR model, while ignoring well-established diagnostics tests commonly used in standard global regression analysis. This paper investigates multicollinearity issues surrounding the local GWR coefficients at a single location and the overall correlation between GWR coefficients associated with two different exogenous variables. Results indicate that the local regression coefficients are potentially collinear even if the underlying exogenous variables in the data generating process are uncorrelated. Based on these findings, applied GWR research should practice caution in substantively interpreting the spatial patterns of local GWR coefficients. An empirical disease-mapping example is used to motivate the GWR multicollinearity problem. Controlled experiments are performed to systematically explore coefficient dependency issues in GWR. These experiments specify global models that use eigenvectors from a spatial link matrix as exogenous variables.This study was supported by grant number 1 R1 CA95982-01, Geographic-Based Research in Cancer Control and Epidermiology, from the National Cancer Institute. The author thank the anonymous reviewers and the editor for their helpful comments.  相似文献   

Geocomputation, Geodemographics and Resource Allocation for Local Policing   总被引：1，自引：0，他引：1  

David I Ashby  Paul A Longley 《Transactions in GIS》2005,9(1):53-72

The field of geodemographics is one of the most fertile applications areas of geocomputation research. Geodemographic profiles of the characteristics of individuals and small areas are pivotal to tactical and strategic resource management in many areas of business, and are becoming similarly central to efficient and effective deployment of resources by public services. In this context, this paper describes research that has been developed in partnership with the Devon and Cornwall Constabulary and Institute of Criminal Justice Studies (ICJS) at the University of Portsmouth to assess the potential use of geodemographics as a policing tool. Crime, the fear of crime and the efficacy of public services are issues that have moved to the top of the UK political agenda and have long been matters of serious public concern. This paper considers the ways in which police forces might use geodemographics to better deploy resources at a variety of spatial scales in England and Wales.  相似文献   

Combined Digital Photogrammetry and Time-of-Flight Laser Scanning for Monitoring Cliff Evolution   总被引：4，自引：0，他引：4  

Michael  Lim  David N.  Petley  Nicholas J.  Rosser  Robert J.  Allison  Antony J.  Long  David  Pybus 《The Photogrammetric Record》2005,20(110):109-129

Although cliffs form approximately 75% of the world's coastline, the understanding of the processes through which they evolve remains limited because of a lack of quantitative data on the morphological changes they undergo. In this paper the combination of terrestrial time-of-flight laser scanning with high-resolution digital photogrammetry is examined to generate high-quality data-sets pertaining to the geomorphic processes governing cliff development. The study was undertaken on a section of hard rock cliffs in North Yorkshire, UK, which has been monitored over a 12-month period. High-density, laser-scanned point clouds have been used to produce an accurate representation of these complex surfaces, free from the optical variations that degrade photographic data. These data-sets have been combined with high-resolution photographic monitoring, resampled with the fixed accuracies of the terrestrial laser survey, to generate a new approach to recording the volumetric changes in complex coastal cliffs. This has led to significant improvements in the understanding of the activity patterns of coastal cliffs.  相似文献   

High Resolution Earth Imaging for Geospatial Information     

David A.  Holland 《The Photogrammetric Record》2005,20(112):384-386

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1957-1996台风对中国降水的影响   总被引：15，自引：0，他引：15  

Ren Fumin  Byron Gleason  David Easterling 《大气科学进展》2002,19(5):943-952

台风活动中国气候的重要特点之一，它能带来大量降水并造成严重的财产损失。在一些地区，台风降水甚至可以在总降水量中占很大比例。本文目的是研究那些对中国产生影响的台风并重点关注台风对中国降水的影响。文中涉及四个方面的工作。首先，研究了影响中国台风的频率，结果表明台风影响的主要季节为5－11月，尤其以7－9月频繁；在过去40年中影响台风的频率没有明显的变化趋势。第二，对台站台风降水的气候特征分析结果显示，海南和东南沿海地区受台风的影响最大，而且长江以南大部地区每年都受到台风的影响；另外，影响区域大部分地区的台风降水在过去40年中表现出下降的趋势，但是只有东北地区南部这种趋势是显著的。第三，对台风个例的分析表明，个例降水总量和影响面积之间存在着显著的线性关系。最后，对台风造成的中国范围降水总量进行了分析，初步结果显示台风降水总量在1957－1996年间显著减少。  相似文献   

Bury, Burn or Both: A Two-for-One Deal on Biomass Carbon and Energy     

David W. Keith  James S. Rhodes 《Climatic change》2002,54(3):375-377

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A Stabilizing Dynamical Influence in the Deep Mantle due to the Radiative Thermal Conductivity and a high temperature at the Core-Mantle Boundary     

Erik O. Sevre  Fabien Dubuffet  David A. Yuen  Emma S. G. Rainey 《Visual Geosciences》2002,7(1):1-4

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