An investigation of the rock magnetic properties using stepwise isothermal remanence (IRM) acquisition, thermomagnetic analysis
and temperature-dependent susceptibility history, identifies magnetite as the carrier of the main fraction of the remanence,
associated with maghemite and hematite in Malan loess (L1), Holocene soil (S0) and last-glacial paleosol (S1). The presence
of short-lived direction fluctuations indicates that no significant smoothing occurs in L1 when its remanence is locked, and
thus L1 is capable of recording the geomagnetic secular variation (PSV), while the PSV has been severely smoothed or wiped
out by pedogenic processes during S1 formation. It has been suggested that the Mono Lake and Laschamp excursions are two independent
geomagnetic events based on this study. 相似文献
There are six distinct classes of gold deposits, each represented by metallogenic provinces, having 100's to >1000 tonne gold production. The deposit classes are: (1) orogenic gold; (2) Carlin and Carlin-like gold deposits; (3) epithermal gold-silver deposits; (4) copper-gold porphyry deposits; (5) iron-oxide copper-gold deposits; and (6) gold-rich volcanic hosted massive sulfide (VMS) to sedimentary exhalative (SEDEX) deposits. This classification is based on ore and alteration mineral assemblages; ore and alteration metal budgets; ore fluid pressure(s) and compositions; crustal depth or depth ranges of formation; relationship to structures and/or magmatic intrusions at a variety of scales; and relationship to the P-T-t evolution of the host terrane. These classes reflect distinct geodynamic settings. Orogenic gold deposits are generated at mid-crustal (4–16 km) levels proximal to terrane boundaries, in transpressional subduction-accretion complexes of Cordilleran style orogenic belts; other orogenic gold provinces form inboard, by delamination of mantle lithosphere, or plume impingement. Carlin and Carlin-like gold deposits develop at shallow crustal levels (<4 km) in extensional convergent margin continental arcs or back arcs; some provinces may involve asthenosphere plume impingement on the base of the lithosphere. Epithermal gold and copper-gold porphyry deposits are sited at shallow crustal levels in continental margin or intraoceanic arcs. Iron oxide copper-gold deposits form at mid to shallow crustal levels; they are associated with extensional intracratonic anorogenic magmatism. Proterozoic examples are sited at the transition from thick refractory Archean mantle lithosphere to thinner Proterozoic mantle lithosphere. Gold-rich VMS deposits are hydrothermal accumulations on or near the seafloor in continental or intraoceanic back arcs.
The compressional tectonics of orogenic gold deposits is generated by terrane accretion; high heat flow stems from crustal thickening, delamination of overthickened mantle lithosphere inducing advection of hot asthenosphere, or asthenosphere plume impingement. Ore fluids advect at lithostatic pressures. The extensional settings of Carlin, epithermal, and copper-gold porphyry deposits result from slab rollback driven by negative buoyancy of the subducting plate, and associated induced convection in asthenosphere below the over-riding lithospheric plate. Extension thins the lithosphere, advecting asthenosphere heat, promotes advection of mantle lithosphere and crustal magmas to shallow crustal levels, and enhances hydraulic conductivity. Siting of some copper-gold porphyry deposits is controlled by arc parallel or orthogonal structures that in turn reflect deflections or windows in the slab. Ore fluids in Carlin and epithermal deposits were at near hydrostatic pressures, with unconstrained magmatic fluid input, whereas ore fluids generating porphyry copper-gold deposits were initially magmatic and lithostatic, evolving to hydrostatic pressures. Fertilization of previously depleted sub-arc mantle lithosphere by fluids or melts from the subducting plate, or incompatible element enriched asthenosphere plumes, is likely a factor in generation of these gold deposits. Iron oxide copper-gold deposits involve prior fertilization of Archean mantle lithosphere by incompatible element enriched asthenospheric plume liquids, and subsequent intracontinental anorogenic magmatism driven by decompressional extension from far-field plate forces. Halogen rich mantle lithosphere and crustal magmas likely are the causative intrusions for the deposits, with a deep crustal proximal to shallow crustal distal association. Gold-rich VMS deposits develop in extensional geodynamic settings, where thinned lithosphere extension drives high heat flow and enhanced hydraulic conductivity, as for epithermal deposits. Ore fluids induced hydrostatic convection of modified seawater, with unconstrained magmatic input. Some gold-rich VMS deposits with an epithermal metal budget may be submarine counterparts of terrestrial epithermal gold deposits. Real time analogs for all of these gold deposit classes are known in the geodynamic settings described, excepting iron oxide copper-gold deposits.
The global mean vertical energy flux of the (1,1,1) mode of atmospheric oscillation is evaluated at 80 km altitude by classical tidal theory for mean January, April, July and October conditions using revised profiles of water vapour and ozone heating. Fluxes calculated for January and July are lower than those for April and October due to seasonal changes in water vapour, solar declination and Sun-Earth distance. Flux values obtained are compared with a previously stated requirement for maintaining the residual thermosphere and are adequate unless damping, which is ignored in the present calculations, introduces a factor of more than an order of 10 in magnitude. The relative changes of flux between the above four months are noted to be similar in form to the semi-annual variation of thermospheric air densities. 相似文献
Accretionary orogens are the sites of long-lived convergent margin tectonics, both compressional and extensional. They are
also the hosts to the majority of the world’s important gold deposits. A very diverse range of deposit types occurs within
accretionary orogens, commonly in close proximity in space and time to each other. These include porphyry and associated high-sulphidation
Au–Cu–Ag deposits, classic low-sulphidation Au–Ag deposits, low-sulphidation Au deposits centred on alkalic intrusive complexes,
Carlin-type Au deposits, Au-rich volcanic-hosted massive sulphide deposits, orogenic Au deposits, intrusion-related Au deposits
and iron oxide Cu–Au deposits. Empirical patterns of spatial distribution of these deposits suggest there must be fundamental
generic controls on gold metallogeny. Various lines of evidence lead to the proposal that the underlying key generic factor
controlling accretionary orogen gold metallogeny is regional-scale, long-term, pre- and syn-subduction heterogeneous fertilisation
of the lithospheric mantle that becomes a source of mineralisation-associated arc magma or hydrothermal fluid components.
This process provides a gold-enriched reservoir that can be accessed later in a diverse range of tectonomagmatic settings.
Based on this concept, a unified model is proposed in which the formation of a major gold deposit of any type requires the
conjunction in time and space of three essential factors: a fertile upper-mantle source region, a favourable transient remobilisation
event, and favourable lithospheric-scale plumbing structure. This framework provides the basis for a practical regional-scale
targeting methodology that is applicable to data-poor regions. 相似文献
Potholes represent areas where the normally planar PGE-rich Merensky Reef of the upper Critical Zone of the Bushveld Complex
transgresses its footwall, such geometric relationships being unusual in layered intrusions. The recognition of vertical dykes
of Merensky pyroxenite in the footwall suggests downward collapse of crystal mush into pull-apart sites resulting from tensional deformation due to the loading effects of major new
magma additions. In contrast, crosscutting anorthosite veins display physical and isotopic evidence of upward emplacement. The Merensky Reef and its footwall have distinct initial Sr-isotope ratios (R0 > 0.7066 and <0.7066, respectively), which may be used to constrain these processes related to pothole formation. Merensky
Reef in potholes (R0 = 0.7069−0.7078) shows no isotopic evidence of assimilation of, or reaction with, footwall material. Discrete, discordant
replacement bodies of anorthosite extend from the footwall lithologies to cross-cut the Merensky Reef and its hanging wall.
The initial Sr-isotope ratio in these replaced rocks is totally reset to footwall values (R0 = 0.7066), and immediately adjacent stratiform lithologies are slightly modified towards footwall values. In contrast, Neptunian
pyroxenitic (Merensky) dykes cross-cutting the footwall lithologies, with a large surface area to volume ratio, and low Sr
content, do not display footwall-like Sr-isotope initial-ratios (R0 = 0.7077), and thus show no evidence for assimilation of or reaction with footwall material. Furthermore, pegmatoidal replacement
pyroxenite (“replacement pegmatoid”), at the base of the Merensky Reef within potholes, has a high initial-ratio (R0 > 0.7071), and so models of pervasive metasomatism by footwall material are not applicable. This isotopic evidence indicates
that there was no active interaction of footwall material with the overlying magma during, or after, the formation of Merensky
Reef potholes, a basic tenet of existing pothole formation hypotheses involving footwall mass-transfer. In contrast, the isotopic
data are entirely consistent with an extensional model for pothole formation, with the more radiogenic Merensky magma migrating
laterally to fill extensional zones in the footwall layers.
Received: 11 October 1997 / Accepted: 21 December 1998 相似文献
Marine capture fisheries face major and complex challenges: habitat degradation, poor economic returns, social hardships from depleted stocks, illegal fishing, and climate change, among others. The key factors that prevent the transition to sustainable fisheries are information failures, transition costs, use and non-use conflicts and capacity constraints. Using the experiences of fisheries successes and failures it is argued only through better governance and institutional change that encompasses the public good of the oceans (biodiversity, ecosystem integrity, sustainability) and societal values (existence, aesthetic and amenity) will fisheries be made sustainable. 相似文献
Summary The world-class Paleoproterozoic Vergenoeg fluorite deposit in South Africa is hosted in a breccia pipe comprising units with varying proportions of pegmatoidal fayalite, magnetite, fluorite and siderite. The adjacent A-type Bushveld granites also have associated fluorite deposits containing fluorite with similar REE patterns, fluid inclusion and Sr isotope compositions to those at Vergenoeg, leading to the proposal that there is a genetic relationship. This is despite the silica-undersaturated nature (SiO2<30%) of the Pipe and its extreme enrichment in Ca, F, Fe, Nb, P and REE compared to granites. Both liquid immiscibility from a granitic melt and granitic magmato-hydrothermal activity have been proposed as genetic mechanisms to explain this exotic composition. However, the Vergenoeg Pipe shows greater similarities to alkaline rocks, in particular the Phalaborwa carbonatite of similar age, including: i) size and shape, ii) associated maars, iii) mineralogical zoning, iv) geochemical, radiogenic and stable isotope composition, and v) presence of both high-T and low-T fluid inclusions. This suggests an alternative genetic relationship with alkaline magmas, in which some geochemical and radiogenic isotopic similarities to Bushveld granites are the consequence of broadly contemporaneous development in the same tectonic setting within the same lithosphere, whereas others may be due to hydrothermal overprinting. Similarities with Phalaborwa and also with Bayan Obo, Mongolia, indicate that the Vergenoeg pegmatoid pipe could be an extreme carbonatite-associated member of the Fe-oxide Cu–Au (±REE±P) group of deposits.Supplementary material to this paper is available in electronic form at http://dx.doi.org/10.1007/s00710-003-0012-6Tables 1-4 available as electronic supplementary material 相似文献