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121.
J. L. Mair V. J. Ojala B. P. Salier D. I. Groves S. M. Brown 《Australian Journal of Earth Sciences》2013,60(5):895-912
Stress mapping is a numerical modelling technique used to determine the distribution and relative magnitude of stress during deformation in a mineralised terrane. It is based on the general principle that fluid flow in the Earth's crust is primarily related to pressure gradients. It is best applied to epigenetic hydrothermal mineral deposits, where fluid flow and fluid flux are enhanced in dilational sections of structures and in sites of enhanced rock permeability due to high fracture density. These are defined by sites of low minimum principal stress (σ3). Most stress mapping is carried out in two dimensions in plan view using geological maps. This is suitable for terranes with steeply dipping lithostratigraphy and structures in which the distribution of mineral deposits is largely controlled by fault structures portrayed on the maps. However, for terranes with gently dipping sequences and structures, and for situations where deposits are sited in and near the hinges of complex fold structures, stress mapping in cross‐section is preferable. The effectiveness of stress mapping is maximised if mineralisation was late in the evolutionary history of the host terrane, and hence the structural geometry of the terrane and contained deposits were essentially that expressed today. The orientation of syn‐mineralisation far‐field stresses must also be inferred. Two examples of orogenic gold deposits, which meet the above criteria, are used to illustrate the potential of stress mapping in cross‐section. Sunrise Dam, located in the Archaean Yilgarn Craton, is a lode‐gold deposit sited in a thrust‐fold belt. Stress mapping illustrates the heterogeneity of stress distribution in the complex structural geometry of the deposit, and predicts the preferential siting of ore zones around the intersections of more steeply dipping, linking thrusts and banded iron‐formation units, and below the controlling more gently dipping basal thrust, the Sunrise Shear. The Howley Anticline in the Pine Creek block hosts several Palaeoproterozoic gold deposits, sited in complex anticlinal structures in greywacke sequences. Stress mapping indicates that gold ores should develop in the hinge zones of symmetrical anticlines, in the hinge zones and more steeply dipping to overturned limbs of asymmetric anticlines, and in and around thrusts in both anticlines and parasitic synclines. The strong correlation between the predictions of the stress mapping, based on the distribution of low σ3, and the location of gold ores emphasises the potential of stress mapping in cross‐section, not only as an exploration tool for the discovery of additional resources or deposits, but also as a test of geological models. Knowledge of the potential siting of gold ores and their probable orientations also provides a guide to drilling strategies in both mine‐ and regional‐scale exploration. 相似文献
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Marian Munteanu Allan H. Wilson Yong Yao Shao-Yong Jiang Gordon Chunnett Yaonan Luo Leonard Mafurutu Rofhiwa Phadagi 《Journal of Volcanology and Geothermal Research》2010,189(1-2):118-130
Lengshuiqing is part of the late Proterozoic igneous province from the western margin of the Yangtze craton. The Lengshuiqing area comprises five ultramafic–mafic intrusions, emplaced in the late Proterozoic Yanbian Group. The intrusions from Lengshuiqing contain cumulate ultramafic zones (peridotite + olivine pyroxenite), with cumulus olivine and Cr-spinel, and intercumulus pyroxenes, hornblende, phlogopite and plagioclase. Ni–Cu ore (pyrrhotite + pentlandite + chalcopyrite) is hosted in the ultramafic zones. Olivine-free diorite–quartz diorite ± gabbro and granite zones commonly occur above the ultramafic rocks. The genesis of the intrusions (conduit-related accumulation or differentiation from stagnant magma) was investigated. The amount of sulphides in the intrusions from Lengshuiqing is one order of magnitude bigger than the sulphides that can be dissolved by a volume of mafic magma similar with the volume of the intrusions. Most intrusions from Lengshuiqing have bulk composition (peridotite ± diorite ± granite) more magnesian (MgO = 21–22%; Mg# > 78) than the deduced composition of their parental magma (MgO = 9–11%; Mg# = 64–67). This indicates the accumulation of sulphide and mafic silicates from a volume of magma much bigger than the volume of the intrusions, which can be explained by the fractionation from magma ascending through the intrusions to shallower depths. A continuous supply and vent of magma is consistent with the lack of chilled margins, the melting of the wall rocks and the generation of high-temperature mineral assemblages (K-feldspar, diopside, and sillimanite) in the Yanbian Group. The intrusions from Lengshuiqing are seen as microchambers on conduits draining olivine-, Cr-spinel-, and sulphide-bearing mafic magma from a larger staging chamber. 相似文献
124.
Olufunso A. Somorin H. Carolyn Peach Brown Ingrid J. Visseren-Hamakers Denis J. Sonwa Bas Arts Johnson Nkem 《Global Environmental Change》2012,22(1):288-298
This paper discusses the discourses on climate change adaptation and mitigation that are currently at the forefront in the Congo Basin. On mitigation, the forests have enormous opportunities to contribute to the reducing emissions from deforestation and forest degradation (REDD+) mechanism. But the forest itself and its multiple dependent societies and sectors need to adapt to potential climate risks. Hence, actors are debating the design of climate change policy in the forest sector. Theoretically, we combine the agency-focus of frame analysis and discourse theory to analyze how different agents hold frames on climate change adaptation and mitigation policies in the region. This paper draws upon interviews with 103 different actors from government, international organizations, non-governmental organizations, research institutions and private sector in three countries: Cameroon, Central African Republic (CAR) and Democratic Republic of Congo (DRC). Three discourses were found on policy response to climate change in the forest sector: mitigation policy only, separated policy on adaptation and mitigation, and an integrated policy on adaptation and mitigation. The various frames articulated around each discourse by the coalitions include elements of: costs and benefits, scale of operation, effectiveness, financial resources and implementation mechanisms. Overall, the mitigation discourse, through its mix of actors, resources and interests seems to be stronger than the adaptation discourse. The paper finally outlines a number of implications of the discourses for policy design. 相似文献
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In this paper we present a theoretical investigation of the structures and relative stability of the olivine and spinel phases of Mg2SiO4. We use both a purely ionic model, based on the Modified Electron Gas (MEG) model of intermolecular forces, and a bond polarization model, developed for low pressure silica phases, to investigate the role of covalency in these compounds. The standard MEG ionic model gives adequate structural results for the two phases but incorrectly predicts the spinel phase to be more stable at zero pressure. This is mainly because the ionic modeling of Mg2SiO4 only accounts for 95 percent of the lattice energy. The remainder can be attributed to covalency and many-body effects. An extension of the MEG ionic model using “many-body” pair potentials corrects the phase stability error, but predicts structures which are in poorer agreement with experiment than the standard ionic approach. In addition, calculations using these many-body pair potentials can only account for 10 percent of the missing lattice energy. This model predicts an olivine-spinel phase transition of 8 GPa, below the experimental value of 20 GPa. Therefore, in order to understand more fully the stability of these structures we must consider polarization. A two-shell bond polarization model enhances the stability of both structures, with the olivine structure being stabilized more. This model predicts a phase transition at about 80 GPa, well above the observed value. Also, the olivine and spinel structures calculated with this approach are in poorer agreement with experiment than the ionic model. Therefore, based on our investigations, to properly model covalency in Mg2SiO4, a treatment more sophisticated than the two-shell model is needed. 相似文献
128.
Barry Smit Michael Brklacich Robert B. Stewart Ray McBride Murray Brown Deborah Bond 《Climatic change》1989,14(2):153-174
Increasing concentrations of atmospheric CO2 and other greenhouse gases are expected to contribute to a global warming. This paper examines the potential implications of a climatic change corresponding to a doubling of atmospheric concentrations of CO2 on crop production opportunities throughout Ontario, a major food producing region in Canada. The climate is projected to become warmer and drier, but the extent of these shifts are expected to vary from region to region within Ontario. The effect of this altered climate on crop yields and the area of land capable of supporting specific crops varies according to region, soil quality and crop type. Most notable are the enhanced opportunities for grains and oilseeds in the northern regions, and the diminished production prospects for most crops in the most southerly parts of Ontario. 相似文献
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130.
Jaidong Ko Nancy E. Brown Alexandra Navrotsky Charles T. Prewitt Tibor Gasparik 《Physics and Chemistry of Minerals》1989,16(8):727-733
The phase boundary between MnTiO3 I (ilmenite structure) and MnTiO3 II (lithium niobate structure) has been determined by analysis of quench products from reversal experiments in a cubic anvil
apparatus at 1073–1673 K and 43–75 kbar using mixtures of MnTiO3 I and II as starting materials. Tight brackets of the boundary give P(kbar)=121.2−0.045 T(K). Thermodynamic analysis of this boundary gives ΔHo=5300±1000 J·mol−1, ΔSo = 1.98 ±1J·K−1· mol−1. The enthalpy of transformation obtained directly by transposed-temperature-drop calorimetry is 8359 ±2575 J·mol−1. Possible topologies of the phase relations among the ilmenite, lithium niobate, and perovskite polymorphs are constrained
using the above data and the observed (reversible with hysteresis) transformation of II to III at 298 K and 20–30 kbar (Ross
et al. 1989). The observed II–III transition is likely to lie on a metastable extension of the II–III boundary into the ilmenite
field. However the reversed I–II boundary, with its negative dP/ dT does represent stable equilibrium between ilmenite and lithium niobate, as opposed to the lithium niobate being a quench
product of perovskite. We suggest a topology in which the perovskite occurs stably at low T and high P with a triple point (I, II, III) at or below 1073 K near 70 kbar. The I–II boundary would have a negative P-T slope while the II–III and I–III boundaries would be positive, implying that entropy decreases in the order lithium niobate,
ilmenite, perovskite. The inferred positive slope of the ilmenite-perovskite transition in MnTiO3 is different from the negative slopes in silicates and germanates. These thermochemical parameters are discussed in terms
of crystal structure and lattice vibrations. 相似文献