255 research outputs found
A large scale investigation into changes in coal quality caused by dolerite dykes in Secunda, South Africa-implications for the use of proximate analysis on a working mine
The coalfields of South Africa contain numerous dolerite intrusions, which affected the quality of
the surrounding coal through thermal processes, commonly believed to be controlled by the size of
the magmatic body. Data gathered from a working coalfield in Secunda, South Africa, suggest that
the relationship between intrusive sills and coal is complex and factors other than intrusion width
must be considered in relation to the contact metamorphic effect. The study area contains multiple
dolerite intrusions of Karoo age, of which three intrusions occur as sills intruded close to the main
coal seam of the. A large database (>8000 boreholes) of coal quality data was used to investigate
the presence or absence of a change in coal quality relative to dolerite proximity. Reduction in coal
quality was defined using three proximate analysis values, namely the ash, volatile content and dry
ash free volatile (DAFV) as defined in the coal industry. The resultant investigation showed no
correlation between the position and thickness of the dolerites, and changes in coal quality as measured by proximate analysis. In the absence of a linear relationship between coal quality and
dolerite proximity, two processes are proposed to explain the absence of the contact metamorphic
effects expected from previous studies. Firstly dolerite emplacement dynamics may influence the
size of the metamorphic aureole produced by an intrusion, invalidating intrusion size as a measure
of thermal output. Secondly, hydrothermal fluids mobilised by the dolerite intrusions, either from
the country rock or the intrusion itself may percolate through the coal and act as the metamorphic
agent responsible for changing coal quality, by dissolving the volatile and semi-volatile
components of the coal and transporting them to other locations. These two processes are sufficient
to explain the lack of a clear “metamorphic effect” related to the dolerite intrusions. However, the
perceived lack of a clear correlation between the coal quality parameters and the metamorphic
effects associated with dolerite intrusion may also reflect the inadequacies of proximate analysis
techniques in quantifying geological processes within the coal.Coaltech research organisation.http://www.elsevier.com/locate/jafrearsci2017-05-31hb2016Geolog
Detection of magnetite in the Roossenekal area of the Eastern Bushveld Complex, South Africa, using multispectral remote sensing data
Multispectral sensors, along with common and advanced algorithms, have become efficient tools for routine lithological
discrimination and mineral potential mapping. It is with this paradigm in mind that this paper sought to evaluate and
discuss the detection and mapping of magnetite on the Eastern Limb of the Bushveld Complex, using high spectral
resolution multispectral remote sensing imagery and GIS techniques. Despite the wide distribution of magnetite, its
economic importance, and its potential as an indicator of many important geological processes, not many studies
had looked at the detection and exploration of magnetite using remote sensing in this region. The Maximum
Likelihood and Support Vector Machine classification algorithms were assessed for their respective ability to detect
and map magnetite using the PlanetScope Analytic data. A K-fold cross-validation analysis was used to measure the
performance of the training as well as the test data. For each classification algorithm, a thematic landcover map was
created and an error matrix, depicting the user’s and producer’s accuracies as well as kappa statistics, was derived.
A pairwise comparison test of the image classification algorithms was conducted to determine whether the two
classification algorithms were significantly different from each other. The Maximum Likelihood Classifier significantly
outperformed the Support Vector Machine algorithm, achieving an overall classification accuracy of 84.58% and an
overall kappa value of 0.79. Magnetite was accurately discriminated from the other thematic landcover classes with a
user’s accuracy of 76.41% and a producer’s accuracy of 88.66%. The overall results of this study illustrated that remote
sensing techniques are effective instruments for geological mapping and mineral investigation, especially iron oxide
mineralization in the Eastern Limb of the Bushveld Complex.http://sajg.geoscienceworld.orgam2021Geography, Geoinformatics and MeteorologyGeolog
Distinguishing between legally and illegally produced gold in South Africa
The identification of gold-bearing material is essential for combating the theft of gold
in South Africa. Material seized in police operations is generally a mixture of gold
from different mines, and as such cannot be traced back to a single location. ICP-OES
analysis of material dissolved by acid dissolution provided a database of gold
compositions comprising gold from South African mines, illegal gold stolen from the
mines, and commercial gold alloys and jewelery. Discrimination between legal and
illegal gold was possible due to the presence of Pb, As, Sb, Sn, Se, and Te in the
stolen material, elements which are not present in legally produced gold. The presence
of these elements is a quick and simple way to distinguish between gold alloys based
on refined gold, such as in commercially manufactured jewelery, and gold alloys
containing a proportion of unrefined and therefore illegally obtained gold.Goldfields Security Serviceshttp://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1556-40292017-01-31hb2016Geolog
Immiscible iron- and silica-rich liquids in the Upper Zone of the Bushveld Complex
The Bushveld Complex (South Africa) is the largest layered intrusion on Earth and plays a
considerable role in our understanding of magmatic differentiation and ore-forming processes.
In this study, we present new geochemical data for apatite-hosted multiphase inclusions in
gabbroic cumulates from the Bushveld Upper Zone. Inclusions re-homogenized at high temperature
(1060-1100°C) display a range of compositions in each rock sample, from iron rich
(35 wt.% FeOtot; 28 wt.% SiO2) to silica-rich (5 wt.% FeOtot; 65 wt.% SiO2). This trend is
best explained by an immiscible process and trapping of contrasted melts in apatite crystals
during progressive cooling along the binodal of a two-liquid field. The coexistence of both Si rich
and Fe-rich immiscible melts in single apatite grains is used to discuss the ability of
immiscible melts to segregate from each other, and the implications for mineral and bulk
cumulate compositions. We argue that complete separation of immiscible liquids did not
occur, resulting in crystallization of similar phases from both melts but in different
proportions. However, partial segregation in a crystal mush and the production of contrasting
phase proportions from the Fe-rich melt and the Si-rich melt can be responsible for the cyclic
evolution from melanocratic (Fe-Ti-P-rich) to leucocratic (plagioclase-rich) gabbros which is
commonly observed in the Upper Zone of the Bushveld Complex where it occurs at a vertical
scale of 50 to 200 m.This project was supported by a DAAD-Doktorandenstipendium for L.A. Fischer. B. Charlier
and O. Namur acknowledge support from the Alexander von Humboldt Foundation. I.V.
Veksler has been supported by DFG grant VE 619/2-1.http://www.elsevier.com/locate/epsl2017-06-30hb2016Geolog
Fast linear algebra is stable
In an earlier paper, we showed that a large class of fast recursive matrix
multiplication algorithms is stable in a normwise sense, and that in fact if
multiplication of -by- matrices can be done by any algorithm in
operations for any , then it can be done
stably in operations for any . Here we extend
this result to show that essentially all standard linear algebra operations,
including LU decomposition, QR decomposition, linear equation solving, matrix
inversion, solving least squares problems, (generalized) eigenvalue problems
and the singular value decomposition can also be done stably (in a normwise
sense) in operations.Comment: 26 pages; final version; to appear in Numerische Mathemati
Clinopyroxene megacrysts from Marion Island, Antarctic Ocean: evidence for a late stage shallow origin
Clinopyroxene megacrysts (up to 5 cm) from a scoria cone on Marion Island, Antarctic Ocean are zoned, with compositionally distinct low (Al + Ti) and high (Al + Ti) patches arranged haphazardly throughout crystals. Inclusions of olivine, pyrrhotite, oxides, sulphides, and rounded inclusions with euhedral micro-crystals interpreted as former melt inclusions are observed. Olivine inclusions have variable compositions, ranging from primary Ti-poor crystals to Ti-rich crystals hosting secondary haematite crystals formed by hydrogenation. The crystals contain voids that are concentrated in the middle of each crystal indicating that the initial crystal growth was skeletal. Subsequent crystallisation filled in the skeletal framework creating the patchy zoning in the crystals. The Marion Island megacrysts are not homogenous, but the combination of crustal clinopyroxene compositions, primary and hydrogenated olivine, and the mode of eruption in scoria eruptions indicates that these crystals most likely formed in a shallow magma chamber. Primary olivines crystallised from a mafic magma and secondary altered olivines were incorporated into a rapidly growing megacryst in a super-saturated, fluid-rich environment, prior to being ejected onto surface in a scoria eruption.http://link.springer.com/journal/710hj2020Geolog
Complex relationships among personality traits, job characteristics, and work behaviors
The aim of the study was to investigate the additive, mediating, and moderating effects of personality traits and job characteristics on work behaviors. Job applicants (N = 161) completed personality questionnaires measuring extraversion, neuroticism, achievement motivation, and experience seeking. One and a half years later, supervisors rated the applicants' job performance, and the job incumbents completed questionnaires about skill variety, autonomy, and feedback, work stress, job satisfaction, work self-efficacy, and propensity to leave. LISREL was used to test 15 hypotheses. Perceived feedback mediated the relationship between achievement motivation and job performance. Extraversion predicted work self-efficacy and job satisfaction. Work stress mediated the relationship between neuroticism and job satisfaction. Job satisfaction and experience seeking were related to propensity to leave. Autonomy, skill variety, and feedback were related to job satisfaction
Reproducibility in the absence of selective reporting : An illustration from large-scale brain asymmetry research
Altres ajuts: Max Planck Society (Germany).The problem of poor reproducibility of scientific findings has received much attention over recent years, in a variety of fields including psychology and neuroscience. The problem has been partly attributed to publication bias and unwanted practices such as p-hacking. Low statistical power in individual studies is also understood to be an important factor. In a recent multisite collaborative study, we mapped brain anatomical left-right asymmetries for regional measures of surface area and cortical thickness, in 99 MRI datasets from around the world, for a total of over 17,000 participants. In the present study, we revisited these hemispheric effects from the perspective of reproducibility. Within each dataset, we considered that an effect had been reproduced when it matched the meta-analytic effect from the 98 other datasets, in terms of effect direction and significance threshold. In this sense, the results within each dataset were viewed as coming from separate studies in an "ideal publishing environment," that is, free from selective reporting and p hacking. We found an average reproducibility rate of 63.2% (SD = 22.9%, min = 22.2%, max = 97.0%). As expected, reproducibility was higher for larger effects and in larger datasets. Reproducibility was not obviously related to the age of participants, scanner field strength, FreeSurfer software version, cortical regional measurement reliability, or regional size. These findings constitute an empirical illustration of reproducibility in the absence of publication bias or p hacking, when assessing realistic biological effects in heterogeneous neuroscience data, and given typically-used sample sizes
The Earth: Plasma Sources, Losses, and Transport Processes
This paper reviews the state of knowledge concerning the source of magnetospheric plasma at Earth. Source of plasma, its acceleration and transport throughout the system, its consequences on system dynamics, and its loss are all discussed. Both observational and modeling advances since the last time this subject was covered in detail (Hultqvist et al., Magnetospheric Plasma Sources and Losses, 1999) are addressed
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