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  • Springer Science + Business Media  (3)
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  • 1
    Publication Date: 1986-07-01
    Description: The vein-type gold deposits of middle to late Archean age constitute one of the major sources of gold production in the world. These deposits are highly diverse in character, but are invariably found in proximity to mafic and ultramafic volcanic rocks, and closely associated with tectonically disturbed zones, or ‘breaks’, in the ancient crust. Using the Larder Lake ‘break’ of northern Ontario and Quebec as a model, a general theory of genesis for such deposits is proposed. The Larder Lake ‘break’ is of linear configuration, up to four miles wide, and extending over a distance of approximately 150 miles. Many rich and extensive vein-type gold deposits are associated with it. This ‘break’ is recognized as a highly folded and faulted, dominantly sedimentary stratigraphic unit, formed as part of the development of the Blake River geosyncline of the Abitibi greenstone belt. Faulting is pervasive within the ‘break’ but it is non-continuous. Under an oxygen-poor, somewhat acidic weathering regime believed extant in Archean time, gold and associated constituents, derived as weathering products from mafic and ultramafic volcanics, appear to have been concentrated as solutions and sols in muddy surficial accumulations along a paleo-continental margin. On transfer to a shallow marine environment, the gold-bearing solutions became concentrated mainly as cherty and pyritic evaporitic residues. These residues, infolded and variably metamorphosed and remobilized during ensuing geosynclinal development, formed the numerous and apparently heterogeneous gold deposits of the Larder Lake ‘break’. Mode of redistribution of gold-bearing residues and metamorphism of their host rocks varies widely. In mild form, little vein development is present and original sedimentary features of the host rock are often preserved. In more extreme cases, true hydrothermal veining is evident and complete recrystallization of the host rock has taken place, sometimes to the extent of formation of a melt with intrusive capabilities. In any case, the gold and related vein constituents remain closely associated with the primary host material and alteration effects are largely derived from the recrystallization of original host rock constituents. The suggested mode of genesis clarifies the relationship between various types of ‘break’-related, vein-type gold deposits and offers an explanation of the complex stratigraphy and structure of the ‘break’ environment. It also suggests a penecontemporaneous relationship between the Larder Lake ‘break’ and other auriferous ‘breaks’, of varying configuration, within the Superior Province of the Canadian Shield. The concept can be used advantageously in exploration for new ‘breaks’ and new deposits. ©1986 Springer-Verlag
    Print ISSN: 0026-4598
    Electronic ISSN: 1432-1866
    Topics: Geosciences
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  • 2
    Publication Date: 2001-03-26
    Description:  The gravitational attractions of terrestrial masses and condensed terrestrial masses were modeled in local regions of gravity stations in different ways. These differences in the models included the type of coordinate frame (Cartesian versus spherical), grid spacing (30 vs 3 arcseconds), and the shape of the terrain (“flat-topped” vs “sloped-topped” prisms). The effect of each of these variables is quantified for its overall impact on Helmert gravity anomalies. The combined effect of removing the masses and restoring the condensed masses is also compared to classical terrain corrections for suitability in computing Helmert anomalies. Some detailed conclusions are drawn from these test computations. The effect of the Earth's curvature has both a near-field effect (due to the differences in volume and shape between rectangular and spherical prisms) and a far-field effect (due to physical location of masses below the horizon). The near-field effect can achieve 0.4 mGal in the Rocky mountains, and affect the geoid by up to 7.5 cm. Additionally, the approximation of the terrain by flat-topped prisms (even at fine spacings such as 3 arcseconds) is inappropriate for terrain near the station, where errors of 20 mGal have been computed using 30-arcsecond data. It is concluded that when 30-arcsecond terrain is allowed to have a more curved (bilinear) prism top, its gravitational attraction is a significantly closer approximation of 3-arcsecond terrain, even for the prism surrounding the station, as compared to the case of 30-arcsecond flat-topped prisms. It is suggested that classical terrain corrections, for many reasons, should not be used to compute Helmert anomalies. Considering only the accuracy, and not the speed, of the computations, the following conclusions are drawn: terrain effects computed inside a local “cap” should be done exclusively in spherical coordinates with a 3-arcsecond Digital Elevation Model (DEM) out to 0.2∘ radius, and then a 30-arcsecond DEM from 0.2 out to 3.5∘. In all cases, bilinearly shaped prism tops should be used. ©2001 Springer-Verlag Berlin Heidelberg
    Print ISSN: 0949-7714
    Electronic ISSN: 1432-1394
    Topics: Architecture, Civil Engineering, Surveying , Geosciences
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  • 3
    Publication Date: 2003-03-01
    Description:  Operating an FG-5 absolute gravimeter in a helium atmosphere should reduce the noise that is caused by vibrations of the air–vacuum interface where the interferometer laser beam enters and leaves the vacuum dropping chamber. The helium atmosphere will decrease the change in refractivity across the interface by up to 88% over the value in air, depending on the purity of the helium. A marked reduction in the post-fit residuals is observed at frequencies of about 50 Hz and little or no effect at higher frequencies. A qualitative explanation of the frequency cutoff in terms of motion of air under compression is developed. Further experiments to measure and characterize the effects of the vibrations are planned. ©2003 Springer-Verlag Berlin Heidelberg
    Print ISSN: 0949-7714
    Electronic ISSN: 1432-1394
    Topics: Architecture, Civil Engineering, Surveying , Geosciences
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