The Aricheng South uranium occurrence is associated with Na metasomatism that affected the granitoids of the Kurupung Batholith
in western Guyana. The mineral paragenesis indicates that late-magmatic albitization was followed by chlorite alteration of
biotite. A minor amount of uraninite occurs in fractures in the newly formed albite crystals, often in company of calcite.
The main mineralization stage occurred later than albitization and chloritization and is represented by brannerite disseminated
in a groundmass of fine-grained hydrothermal zircon. Whole rock geochemistry supports the temporal dissociation of albitization
from the main ore stage. Brannerite, zircon, and uraninite are often partially altered to secondary brannerite, zircon, and
coffinite, respectively. Stable oxygen (chlorite, calcite) and hydrogen (chlorite) isotope compositions suggest that a highly
evolved meteoric fluid, or at least one corresponding to a very high rock/fluid ratio (δ18O of approx. 3.4% to 4‰ and δD of approx. −80‰) may have caused the pre-ore alteration assemblage. The fluids in equilibrium
with main ore stage zircon have δ18O of approx. 6.8‰ and appear to be of magmatic origin. The Aricheng occurrence geochemically, mineralogically, thermally,
and paragenetically resembles the Valhalla U deposit in northern Australia despite differences between the deposits’ host
lithologies, whereas the Lagoa Real and Espinharas U deposits in Brazil have host rock lithology that resembles that of Aricheng. 相似文献
General circulation models (GCMs) are unanimous in projecting warmer temperatures in an enhanced CO2 atmosphere, with amplification of this warming in higher latitudes. The Hudson Bay region, which is located in the Arctic and subarctic regions of Canada, should therefore be strongly influenced by global warming. In this study, we compare the response of Hudson Bay to a transient warming scenario provided by six-coupled atmosphere-ocean models. Our analysis focuses on surface temperature, precipitation, sea-ice coverage, and permafrost distribution. The results show that warming is expected to peak in winter over the ocean, because of a northward retreat of the sea-ice cover. Also, a secondary warming peak is observed in summer over land in the Canadian and Australian-coupled GCMs, which is associated with both a reduction in soil moisture conditions and changes in permafrost distribution. In addition, a relationship is identified between the retreat of the sea-ice cover and an enhancement of precipitation over both land and oceanic surfaces. The response of the sea-ice cover and permafrost layer to global warming varies considerably among models and thus large differences are observed in the projected regional increase in temperature and precipitation. In view of the important feedbacks that a retreat of the sea-ice cover and the distribution of permafrost are likely to play in the doubled and tripled CO2 climates of Hudson Bay, a good representation of these two parameters is necessary to provide realistic climate change scenarios. The use of higher resolution regional climate model is recommended to develop scenarios of climate change for the Hudson Bay region. 相似文献
Redox and nonredox reactions causing pseudomorphic replacement of hematite by magnetite and magnetite by hematite are compared.Pseudomorphic replacements resulting from redox reactions are known as martitization [replacement of magnetite by hematite due to oxidation; reaction (1)] and mushketovitization [replacement of hematite by magnetite due to reduction; reaction (2)]. These two replacements cause characteristic ore textures and volume changes (reaction (1): increase of 1.66%; reaction (2): decrease of 1.64%). These small volume changes are the reason that martitization and mushketovitization are widespread in many rocks under condition, however, that oxidizing or reducing fluids (solutions) are present.The same initial and end products may also be involved in nonredox reactions. Reaction (3) is the replacement of hematite by magnetite due to simple addition of Fe2+ atoms under basic conditions. This reaction causes an increase of the volume of 47.6%. Reaction (4), causing a volume decrease of 32.2%, is the replacement of magnetite by hematite due to leaching of Fe2+ atoms under acidic conditions. From these volume changes it is concluded that reaction (4) may occur in many rock types, whereas reaction (3) is restricted to unlithified sediments only. However, ore textures caused by nonredox reactions are unknown and therefore their occurrence in rocks is hypothetical. 相似文献
Landslides are one of the most damaging natural hazards and have killed tens of thousands of people around the world over the past decade. Slow-moving landslides, with surface velocities on the order of 10−2–102 m a−1, can damage buildings and infrastructure and be precursors to catastrophic collapses. However, due to their slow rates of deformation and at times subtle geomorphic signatures, they are often overlooked in local and large-scale hazard inventories. Here, we present a remote-sensing workflow to automatically map slow-moving landslides using feature tracking of freely and globally available optical satellite imagery. We evaluate this proof-of-concept workflow through three case studies from different environments: the extensively instrumented Slumgullion landslide in the United States, an unstable lateral moraine in Chilean Patagonia and a high-relief landscape in central Nepal. This workflow is able to delineate known landslides and identify previously unknown areas of hillslope deformation, which we consider as candidate slow-moving landslides. Improved mapping of the spatial distribution, character and surface displacement rates of slow-moving landslides will improve our understanding of their role in the multi-hazard chain and their sensitivity to climatic changes and can direct future detailed localised investigations into their dynamics. 相似文献
This paper presents an analysis of two large rock toppling/sliding events which occurred in January 2014 and February 2019 at the Cliets unstable slope (Savoie, French Alps). To understand the mechanism involved and its control by external forcings, a multi-technique analysis approach is used combining geological observations, meteorological data analysis, topographic measurements and simple physical modeling. The pre-failure stage of the events is more particularly analyzed. No direct relationships are found between triggering factors and surface motion though a kinematics analysis highlights the transition toppling-sliding. It showed that, at first order, this transition occurred 4 years before the first failure of 2014, while it happened 2 months before the second failure of 2019. From this date, the environment is considered like a block sliding on an inclined plane. By applying a frictional model (Helmstetter et al. in Journal of Geophysical Research: Solid Earth 109(B2), 2004), we illustrated that the two events belong to an unstable velocity-weakening sliding regime. The time to failure (Voight in Science 243(4888):200–203, 1989) is forecasted with the model, and the results are consistent with the observations. They confirm that the gravitational factor is predominant over the triggering factors for the two events.
The Cerrado biome is the second largest in Brazil, but the evolution of the Cerrado during the late Quaternary is not yet fully known. This study identifies paleoenvironmental changes during the last 23 000 years, based on a tropical mountain peatland record, in the Serra do Espinhaço Meridional in central-eastern Brazil. A multi-proxy approach was used that involved palynological analysis, stable isotopes (δ13C, δ15N), geochemistry, radiocarbon dating and multivariate statistics derived from a peatland core from Rio Preto (Minas Gerais state). The study reveals a very humid and cold climate during the late Pleistocene, with an increase in temperature and decrease in humidity at the Pleistocene–Holocene transition. During this period there was strong instability in the landscape (episodes of erosion). At the beginning of the Holocene there was a reduction in humidity with greater landscape stability. The current sub-humid climatic conditions seem to have been established in the mid-/late Holocene, with periods of landscape instability. Our findings agree with other Cerrado records that contradict previously established hypotheses, such as the Amazonian Refuge and the Pleistocene Arc. 相似文献