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951.
Archaean lode gold deposits in the Coolgardie Goldfield, Western Australia, occur in mafic and ultramafic rocks which have
been metamorphosed to the amphibolite facies. Mineralisation was broadly synchronous with peak metamorphism, the main phase
of granitoid emplacement, and regional deformation. Several different structural styles are represented by the deposits of
the Coolgardie Goldfield. Mineralisation occurs along sheared felsic porphyry-ultramafic rock contacts, in gabbro-hosted quartz-vein
sets, in fault-bounded quartzvein sets, and in laminated quartz reefs sited in brittleductile shear zones. The structures
hosting mineralisation formed in response to a progressive deformation event, related to granitoid emplacement in an east-west
compressional far-field stress régime, but with local heterogeneous stress orientations. This occurred after an earlier period
of thrust-stacking, with probable north-south directed tectonic transport. Two contrasting styles of goldrelated wallrock
alteration are associated with the auriferous lodes of the Coolgardie Goldfield. A high-temperature assemblage (formation
temperature >500°C), characterised by the proximal alteration assemblage garnet+ hornblende + plagioclase + pyrrhotite, contrasts
with a medium-temperature assemblage (formation temperature <500°C), consisting of calcic amphibole + biotite + plagioclase
+ calcite + arsenopyrite + pyrrhotite. The distribution of the two styles of gold-related alteration is controlled by distance
from voluminous syntectonic granitoids located to the west of the Coolgardie Goldfield, with the high-temperature style of
alteration more proximal to the granitoid-greenstone contact than the medium-temperature style. The occurrence of gold deposits
that formed under amphibolite facies conditions throughout the Coolgardie Goldfield supports a crustal continuum model for
Archaean lode-gold deposits, which proposes that gold is deposited in metamorphic environments that range from the sub-greenschist
to granulite facies. In addition, the data from Coolgardie suggest that syntectonic, synmineralisation granitic plutons may
play a significant role in controlling the style of gold associated wallrock-alteration at deep crustal levels. 相似文献
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953.
Mariya I. Yurkina 《Studia Geophysica et Geodaetica》1994,38(4):325-332
Summary A relation is established between coefficients of an expansion of the gravitational potential into a series of Legendre's
function of the second kind and coefficients of an expansion of gravity anomalies on the surface of the reference ellipsoid
into a series of the same functions. This connection can be useful in geodetic computations which take into account the Earth's
flattening. 相似文献
954.
With the atmosphere general circulation model ECHAM the passive transport of NOx emitted from global subsonic air traffic and the NOx concentration change due to these emissions are investigated. The source of NOx is prescribed according to an aircraft emission data base. The sink of NOx is parameterized as an exponential decay process with globally constant lifetime. Simulations in perpetual January and July modes are performed. Both the resulting mean and the standard deviation of the NOx mass mixing ratio are analysed. In January horizontal dispersion is more pronounced and vertical mixing is smaller than in July. In both cases the resulting quasi-stationary fields of the mass mixing ratio display a pronounced zonal asymmetry. The variability accounts up to 30% of the mean field. 相似文献
955.
I. Lerche 《Pure and Applied Geophysics》1994,143(4):537-562
Equations have been presented previously which predict that reflector curvature can affect significantly seismic reflection amplitudes at both zero and nonzero source-receiver offsets. Here the fact that faults are generally concave-upward is used to examine the curvature effect for compaction-driven faults which the sediments have both exponential and logarithmic porosity decreases with increasing depth.The curvature effect is generally larger over upwardly concave reflectors where amplitudes can either increase with offset (exposed focus) or decrease with offset (buried focus). The magnitude of the effect depends on the ratio between the depth to the structure and the radius of curvature of the structure. A phase change of 90° also occurs at a critical offset in the case of an exposed focus concave-upward reflector, with decreasing amplitude at offsets larger than the critical value. Dip move-out (DMO) dominantly removes the amplitude variation with offset due to curvature. These results suggest that when looking for amplitude variations with offset in a fault prospect, DMO should be applied as a preprocessing step. Compaction-driven faults have an exposed focus and, for an exponential variation of porosity with depth, there is a maximum curvature effect at a depth roughly the same as the scaling depth for the porosity. Logarithmic porosity with depth variations suggest a continued increase in the curvature effect of faults with increasing depth, to a maximum amplitude increase of about 35% relative to a plane interface. 相似文献
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