Perseverance is a world-class, komatiite-hosted nickel sulphide deposit situated in the well-endowed Leinster nickel camp
of the Agnew–Wiluna greenstone belt, Western Australia. The mine stratigraphy at Perseverance trends north-northwest (NNW),
dips steeply to the west, and is overturned. Stratigraphic footwall units lie along the western margin of the Perseverance
Ultramafic Complex (PUC). The PUC comprises a basal nickel sulphide-bearing orthocumulate- to mesocumulate-textured komatiite
that is overlain by a thicker, nickel sulphide-poor, dunite lens. Hanging wall rocks include rhyodacite that is texturally
and compositionally similar to footwall volcanic rocks. These rocks separate the PUC from a second sequence of nickeliferous,
E-facing, spinifex-textured komatiite units (i.e. the East Perseverance komatiite). Past workers argue for a conformable stratigraphic
contact between the PUC and the East Perseverance komatiite and conclude that the PUC is extrusive. This study, however, clearly
demonstrates that these komatiite sequences are discordant, implying that the PUC may have intruded rhyodacite country rock
as a sill with subsequent structural juxtaposition against the East Perseverance komatiite. Early N–S shortening associated
with the regional DI deformation event (corresponding to the local DP1 to DP3 events at Perseverance) resulted in the heterogeneous partitioning of strain along the margins of the competent dunite. A
mylonite developed in the more ductile footwall rocks along the footwall margin of the PUC, while isoclinal F3 folds, such as the Hanging wall limb and Felsic Nose folds, formed in low-mean stress domains along the fringes of the elongated
dunite lens. Strata-bound massive and disseminated nickel sulphides were passively fold thickened in hinge areas of isoclinal
folds, whereas basal massive sulphides lubricated fold limbs and promoted thrust movement along shallowly dipping lithological
contacts. Massive sulphides were physically remobilised up to 20 m from their primary footwall position into deposit-scale
fold hinges to form the 1A and Felsic Nose orebodies. First-order controls on the geometry of the Perseverance deposit include
the thermomechanical erosion of footwall rocks and the channelling of the mineralised komatiitic magma. Second- or third-order
controls are several postvolcanic deformation events, which resulted in the progressive folding and shearing of the footwall
contact, as well as the passive fold thickening of massive and disseminated sulphide orebodies. Massive sulphides were physically
remobilised into multiple generations of fold hinges and shear zones. Important implications for near-mine exploration in
the Leinster camp include identifying nickeliferous komatiite units, defining their three-dimensional geometry, and targeting
fold hinge areas. Fold plunge directions and stretching lineations are indicators of potential plunge directions of massive
sulphide orebodies. 相似文献
Tephra layers with Icelandic provenance have been identified across the North Atlantic region in terrestrial, lacustrine, marine and glacial environments. These tephra layers are used as marker horizons in tephrochronology including climate studies, archaeology and environmental change. The major element chemistries of 19 proximally deposited Holocene Icelandic silicic tephra layers confirm that individual volcanic systems have unique geochemical signatures and that eruptions from the same system can often be distinguished. In addition, glass trace element chemistry highlights subtle geochemical variations between tephra layers which appear to have identical major element chemistry and thus allows for the identification of some, if not all, tephra layers previously considered identical in composition. This paper catalogues the compositional variation between the widespread Holocene Icelandic silicic tephra deposits. 相似文献
The structure and electronic properties of trigonal and orthorhombic boric oxide (B2O3) are studied using periodic ab initio Hartree-Fock method. The optimised structural parameters for two B2O3 polymorphs are in good agreement with experimental data. The analyses of their electronic structures provide insights into the chemical nature of the B–O bond and the way in which it changes with the coordination number around boron and oxygen. Our quantum-chemical study suggests that the orthorhombic form is more ionic than the trigonal form and that the coordination number of boron around oxygen plays a more dominant role than that of oxygen around boron in B2O3 crystals. 相似文献
The theoretical work presented here was stimulated by the interpretation of auroral field-aligned currents in terms of an Alfvén wave generated in the neutral sheet. Allowing for convection such a wave can be stationary relative to the Earth, and with an Alfvén Mach number of about 10?2, hydromagnetics predict that the wave normal should be nearly perpendicular to the magnetic field. All the theory presented here is limited to the cold plasma approximation, which is the next step after hydromagnetics, but should have validity here as the wave is propagating into the cold polar wind plasma.The approach is similar to that of Kellogg (1964) except here we consider only the Alfvén mode, and only for Alfvén Mach numbers of about 10?2. Initially a linear approach was adopted but further computation showed that non-linear effects were responsible for making the current density approximately uniform.The final section presents a plasma sheet boundary crossing selected to illustrate the theory, and is taken from ISEE 1 and 2. The data is such that it permits a first-order estimation of scale sizes to be made in the tail, which in this case was found to be about 1000 km. Subsequent mapping to ionospheric altitudes produced a scale of about a few tens of kilometers. 相似文献
We discuss preliminary results of an 11.7 m imaging survey of ultracompact H II regions from the Wood and Churchwell radio survey. We find that that the morphologies of ionized gas and warm dust are often significantly different, indicating that an H II region classification scheme should be based on more than radio data. 相似文献
Calculations of the Rigid Unit Modes (RUMs) allowed in the nepheline structure are used to explain the diffuse scattering
previously seen in electron diffraction experiments. The RUM calculations also show that the modulation wavelength for incommensurate
nephelines is essentially determined by the framework topology. X-ray diffraction is used to measure the intensity of the
diffuse scattering as a function of temperature. The diffuse intensity increases sharply at 308 K. This effect is interpreted
as being due to the softening of a phonon mode, indicating a phase transition. Measurements of this phase transition below
the transition temperature are made using hard mode infrared spectroscopy.
Received: 17 February 1999 / Revised, accepted. 15 October 1999 相似文献
We describe the instrumental corrections which have to be incorporated for reliable correction and deconvolution of images obtained in the 16–22 keV and 22–30 keV energy bands of the Hard X-Ray Imaging Spectrometer (HXIS) aboard the Solar Maximum Mission (SMM). These corrections include amplifier gain and collimator hole size variations across the field of view, amplifier/filter efficiency, variation in effective collimator hole size and angular response with photon energy, dead-time, and hard X-ray plate transmission. We also emphasise the substantial Poisson noise in these energy bands, and describe the maximum entropy deconvolution/correction routine we have developed to establish the spatial structure which can be reliably inferred from HXIS data.
Next we discuss the results of application of our routine to the three impulsive flare phases reported by Duijveman et al. (1982) as exhibiting hard X-ray ‘footpoints’, namely 1980, April 10, May 21, and November 5. Our main conclusions are:
(1)
Maximum entropy smoothing and Poisson noise data perturbations do not remove the main footpoint features in 16–30 keV nor change their basic morphology. However the results emphasise the asymmetry in footpoint size in the May 21 flare and confirm its possible presence in April 10. They also reveal the 3rd weak distant footpoint in the May 21 flare at an earlier time than found by Duijveman et al.
When the 16–22 and 22–30 keV bands are analysed separately, however, it is found that the footpoints are much less visible above noise in the harder band - i.e. the footpoint spectra are steep. In the April 10 and November 5 flares they are steeper than either the spectrum of intervening pixels or the spectrum at higher energies measured for the whole flare by the SMM Hard X-Ray Burst Spectrometer (HXRBS).
(2)
The footpoint contrast with surroundings is less than found by Duijveman et al., despite image deconvolution, because of the maximum entropy smoothing of noise.
(3)
The 16–30keV HXIS footpoint fluxes in the three flares are respectively 28%, 17%, and 15% of the simultaneous HXRBS flare power-law spectrum extrapolated into this energy range.
(4)
Where Poisson noise is taken into account we find, by cross-correlating pixel count rates, that footpoint synchronism was either not provable at all, or substantially less close than reported by Duijveman et al.
Next we considered the implications of these results for models of the footpoint emission. Contrary to Duijveman et al. we do not consider the HXIS ‘footpoint’ data as supporting a conventional thick target beam interpretation since:
(A)
The footpoint photon (and electron) fluxes are much less than expected from HXRBS extrapolation. This result casts doubt on recent models of chromospheric heating by electron beams which usually assume all of the HXRBS emission to come from HXIS footpoints.
(B)
The footpoint spectra for the April 10 and November 5 flares are much softer than the HXRBS spectrum and than the spectrum of intervening pixels, contrary to thick target predictions.
(C)
Contrary to Duijveman et al. footpoint synchronism does not demand an unreasonable Alfvén speed and so does not require non-thermal particles.
In spite of these objections we also re-considered the constraints placed on the acceleration site conditions in a beam interpretation by return current stability and footpoint contrast in the summed 16–30 keV range. Using the smoothed maximum entropy contrast and taking explicit account of coronal thermal emission, we find maximum densities somewhat larger than Duijveman et al. estimated, and much higher maximum values of Te/Ti.
Regarding thermal interpretations we found:
(a)
Models involving continuous production of short-lived hot kernels in the arch top with Maxwellian tail electrons escaping to the footpoints could explain the 16–30 keV contrast with a rather higher energetic efficiency than a pure beam model. However, whatever the temperature distribution of hot kernel production, the model predicts footpoints harder than the arch summit, contrary to HXIS data.
(b)
A model with hot kernels produced in one limb of an arch can explain the asymmetry in footpoint size observed in May 21, and probably April 10, and is energetically even more efficient than (a) but is also inconsistent with the spectral data.
(c)
Finally we point out that HXIS footpoint data may be consistent with a purely geometric interpretation in an almost uniform arch filled with hot plasma.