Mineralogical and chemical evolution of the Ernest Henry Fe oxide–Cu–Au ore system, Cloncurry district, northwest Queensland, Australia     

Geordie Mark  Nicholas H. S. Oliver  Patrick J. Williams 《Mineralium Deposita》2006,40(8):769-801

The Ernest Henry Cu–Au deposit was formed within a zoned, post-peak metamorphic hydrothermal system that overprinted metamorphosed dacite, andesite and diorite (ca 1740–1660 Ma). The Ernest Henry hydrothermal system was formed by two cycles of sodic and potassic alteration where biotite–magnetite alteration produced in the first cycle formed ca 1514±24 Ma, whereas paragenetically later Na–Ca veining formed ca 1529 +11/−8 Ma. These new U–Pb_titanite age dates support textural evidence for incursion of hydrothermal fluids after the metamorphic peak, and overlap with earlier estimates for the timing of Cu–Au mineralization (ca 1540–1500 Ma). A distal to proximal potassic alteration zone correlates with a large (up to 1.5 km) K–Fe–Mn–Ba enriched alteration zone that overprints earlier sodic alteration. Mass balance analysis indicates that K–Fe–Mn–Ba alteration—largely produced during pre-ore biotite- and magnetite-rich alteration—is associated with K–Rb–Cl–Ba–Fe–Mn and As enrichment and Na, Ca and Sr depletion. The aforementioned chemical exchange almost precisely counterbalances the mass changes associated with regional Na–Ca alteration. This initial transition from sodic to potassic alteration may have been formed during the evolution of a single fluid that evolved via alkali exchange during progressive fluid-rock interaction. Cu–Au ore, dominated by co-precipitated magnetite, minor specular hematite, and chalcopyrite as breccia matrix, forms a pipe-like body at the core of a proximal alteration zone dominated by K-feldspar alteration. Both the core and K-feldspar alteration overprint Na–Ca alteration and biotite–magnetite (K–Fe) alteration. Ore was associated with the concentration of a diverse range of elements (e.g. Cu, Au, Fe, Mo, U, Sb, W, Sn, Bi, Ag, F, REE, K, S, As, Co, Ba and Ca). Mineralization also involved the deposition of significant barite, K(–Ba)–feldspar, calcite, fluorite and complexly zoned pyrite. The complexly zoned pyrite and variable K–(Ba)–feldspar versus barite associations are interpreted to indicate fluctuating sulphur and/or barium supply. Together with the alteration zonation geochemistry and overprinting criteria, these data are interpreted to indicate that Cu–Au mineralization occurred as a result of fluid mixing during dilation and brecciation, in the location of the most intense initial potassic alteration. A link between early alteration (Na–Ca and K–Fe) and the later K-feldspathization and the Cu–Au ore is possible. However, the ore-related enrichments in particular elements (especially Ba, Mn, As, Mo, Ag, U, Sb and Bi) are so extreme compared with earlier alteration that another fluid, possibly magmatic in origin, contributed the diverse element suite geochemically independently of the earlier stages. Structural focussing of successive stages produced the distinctive alteration zoning, providing a basis both for exploration for similar deposits, and for an understanding of ore genesis.  相似文献   

Combined Digital Photogrammetry and Time-of-Flight Laser Scanning for Monitoring Cliff Evolution   总被引：4，自引：0，他引：4  

Michael  Lim  David N.  Petley  Nicholas J.  Rosser  Robert J.  Allison  Antony J.  Long  David  Pybus 《The Photogrammetric Record》2005,20(110):109-129

Although cliffs form approximately 75% of the world's coastline, the understanding of the processes through which they evolve remains limited because of a lack of quantitative data on the morphological changes they undergo. In this paper the combination of terrestrial time-of-flight laser scanning with high-resolution digital photogrammetry is examined to generate high-quality data-sets pertaining to the geomorphic processes governing cliff development. The study was undertaken on a section of hard rock cliffs in North Yorkshire, UK, which has been monitored over a 12-month period. High-density, laser-scanned point clouds have been used to produce an accurate representation of these complex surfaces, free from the optical variations that degrade photographic data. These data-sets have been combined with high-resolution photographic monitoring, resampled with the fixed accuracies of the terrestrial laser survey, to generate a new approach to recording the volumetric changes in complex coastal cliffs. This has led to significant improvements in the understanding of the activity patterns of coastal cliffs.  相似文献   

Carbon budget for a subtropical seagrass dominated coastal lagoon: How important are seagrasses to total ecosystem net primary production?     

James E. Kaldy  Christopher P. Onuf  Peter M. Eldridge  Luis A. Cifuentes 《Estuaries and Coasts》2002,25(4):528-539

It has been assumed that because seagrasses dominate macrophyte biomass in many estuaries they also dominate primary production. We tested this assumption by developing three carbon budgets to examine the contribution of autotrophic components to the total ecosystem net primary production (TENPP) of Lower Laguna Madre, Texas. The first budget coupled average photosynthetic parameters with average daily irradiance to calculate daily production. The second budget used average photosynthetic parameters and hourly in situ irradiance to estimate productivity. The third budget integrated temperature-adjusted photosynthetic parameters (using Q₁₀=2) and hourly in situ irradiance to estimate productivity. For each budget TENPP was calculated by integrating production from each autotroph based on the producers’ areal distribution within the entire Lower Laguna Madre. All budgets indicated that macroalgae account for 33–42% of TENPP and seagrasses consistently accounted for about 33–38%. The contribution by phytoplankton was consistently about 15–20%, and the contribution from the benthic microalgae varied between 8% and 36% of TENPP, although this may have been underestimated due to our exclusion of the within bed microphytobenthos component. The water column over the seagrass beds was net heterotrophic and consequently was a carbon sink consuming between 5% and 22% of TENPP, TENPP ranged between 5.41×10¹⁰ and 2.53×10¹¹ g C yr⁻¹, depending on which budget was used. The simplest, most idealized budget predicted the highest TENPP, while the more realistic budgets predicted lower values. Annual production rates estimated using the third budget forHalodule urightii andThalassia testudinum compare well with field data. Macroalgae and microalgae contribute 50–60% of TENPP, and seagrass may be more important as three-dimensional habitat (i.e., structure) than as a source of organic carbon to the water column in Lower Laguna Madre.  相似文献   

Oxygen fugacity and geochemical variations in the martian basalts: implications for martian basalt petrogenesis and the oxidation state of the upper mantle of Mars     

Christopher D.K Herd  Lars E BorgJohn H Jones  James J Papike 《Geochimica et cosmochimica acta》2002,66(11):2025-2036

The oxygen fugacity of the Dar al Gani 476 martian basalt is determined to be quartz-fayalite-magnetite (QFM) −2.3 ± 0.4 through analysis of olivine, low-Ca pyroxene, and Cr-spinel and is in good agreement with revised results from Fe-Ti oxides that yield QFM −2.5 ± 0.7. This estimate falls within the range of oxygen fugacity for the other martian basalts, QFM −3 to QFM −1. Oxygen fugacity in martian basalts correlates with ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and La/Yb ratios, indicating that the mantle source of the basalts is reduced and that assimilation of crust-like material controls the oxygen fugacity. This allows constraints to be placed on the oxidation state of the martian mantle and on the nature of assimilated crustal material. The assimilated material may be the product of early and extensive hydrothermal alteration of the martian crust, or it may be amphibole- or phlogopite-bearing basaltic rock within the crust. In either case, water may play a significant role in the oxidation of basaltic magmas on Mars, although it may be secondary to assimilation of ferric iron-rich material.  相似文献   

Changes in the mode of Southern Ocean circulation over the last glacial cycle revealed by foraminiferal stable isotopic variability     

Ulysses S Ninnemann  Christopher D Charles 《Earth and Planetary Science Letters》2002,201(2):383-396

Benthic foraminiferal oxygen and carbon isotopic records from Southern Ocean sediment cores show that during the last glacial period, the South Atlantic sector of the deep Southern Ocean filled to roughly 2500 m with water uniformly low in δ¹³C, resulting in the appearance of a strong mid-depth nutricline similar to those observed in glacial northern oceans. Concomitantly, deep water isotopic gradients developed between the Pacific and Atlantic sectors of the Southern Ocean; the δ¹³C of benthic foraminifera in Pacific sediments remained significantly higher than those in the Atlantic during the glacial episode. These two observations help to define the extent of what has become known as the ‘Southern Ocean low δ¹³C problem’. One explanation for this glacial distribution of δ¹³C calls upon surface productivity overprints or changes in the microhabitat of benthic foraminifera to lower glacial age δ¹³C values. We show here, however, that glacial-interglacial δ¹³C shifts are similarly large everywhere in the deep South Atlantic, regardless of productivity regime or sedimentary environment. Furthermore, the degree of isotopic decoupling between the Atlantic and Pacific basins is proportional to the magnitude of δ¹³C change in the Atlantic on all time scales. Thus, we conclude that the profoundly altered distribution of δ¹³C in the glacial Southern Ocean is most likely the result of deep ocean circulation changes. While the characteristics of the Southern Ocean δ¹³C records clearly point to reduced North Atlantic Deep Water input during glacial periods, the basinal differences suggest that the mode of Southern Ocean deep water formation must have been altered as well.  相似文献   

Fluvial sediment transport and deposition following the 1991 eruption of Mount Pinatubo   总被引：1，自引：0，他引：1  

Shannon K. Hayes  David R. Montgomery  Christopher G. Newhall 《Geomorphology》2002,45(3-4)

The 1991 eruption of Mount Pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows. Bedload sampling in the Pasig–Potrero River, one of the most heavily impacted rivers, revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth, fine-grained bed. Dimensionless bedload transport rates in the Pasig–Potrero River differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress, but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments. The similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply; in arid rivers, this is attributed to a flashy hydrograph, whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery. This work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced.  相似文献   

The ins and outs of skateboarding and transgression in public space in Newcastle,Australia     

Nicholas Nolan 《The Australian geographer》2003,34(3):311-327

This study examines skateboarding as a transgressive activity in different inner Newcastle public spaces, highlighting the way certain places are constructed, and the values and meanings attached to them. Skateboarding has been sanctioned in some places, but is considered to be inappropriate in others, resulting in the implementation of skating restrictions in specific areas. Transgressive conduct is different from the norm and appears to be ‘out of place’. However, labels of ‘in’ and ‘out’ of place are too simplistic, because transgression is more nuanced and can simultaneously operate at multiple scales. Some skateboarding activities and locations are seen as more legitimate than others, and so a skater can be both ‘in place’ and ‘out of place’ at the same time. Problems and inconsistencies in the regulation of public space are revealed, because although skateboarding may be illegal in some places, the regulations are blunted by limited enforcement, justified by distinctions between ‘good’ and ‘bad’ skateboarding.  相似文献   

Flow and fracture maps for basaltic rock deformation at high temperatures     

Valentina Rocchi  Peter R. Sammonds  Christopher R.J. Kilburn   《Journal of Volcanology and Geothermal Research》2003,120(1-2):25-42

Understanding how the strength of basaltic rock varies with the extrinsic conditions of stress state, pressure and temperature, and the intrinsic rock physical properties is fundamental to understanding the dynamics of volcanic systems. In particular it is essential to understand how rock strength at high temperatures is limited by fracture. We have collated and analysed laboratory data for basaltic rocks from over 500 rock deformation experiments and plotted these on principal stress failure maps. We have fitted an empirical flow law (Norton’s law) and a theoretical fracture criterion to these data. The principal stress failure map is a graphical representation of ductile and brittle experimental data together with flow and fracture envelopes under varying strain rate, temperature and pressure. We have used these maps to re-interpret the ductile–brittle transition in basaltic rocks at high temperatures and show, conceptually, how these failure maps can be applied to volcanic systems, using lava flows as an example.  相似文献   

Investigation of spatially distributed braided river flows using a two‐dimensional hydraulic model     

A. P. Nicholas 《地球表面变化过程与地形》2003,28(6):655-674

This paper reports the application of a two‐dimensional hydraulic model to a braided reach of the Avoca River, New Zealand. Field measurements of water surface elevation, depth and velocity obtained at low flow were used to validate the model and to optimize the parameterization of bed friction. The main systematic trends in the measured flow variables are reproduced by the model. However, field data are characterized by greater spatial variability than model output reflecting differences in the scale of processes measured in the field and represented by the model. Additional model runs were conducted to simulate flow patterns within the study reach at five higher discharges. The purpose of these simulations was to evaluate the potential for using two‐dimensional hydraulic models to quantify the reach‐scale hydraulic characteristics of braided rivers and their dependence on discharge. Changes in flow depth and velocity with increasing discharge exhibit trends that are consistent with the results of previous field investigations, although the tendency for the wetted area of the braidplain within particular depth and velocity categories to remain fixed as discharge rises, as has been noted for several braided rivers in New Zealand, was not observed. Modelled shear stress frequency distributions fit gamma functions that incorporate a distribution shape parameter, the value of which follows clear systematic trends with rising discharge. These results illustrate both the problems of, and potential for, using two‐dimensional hydraulic models in braided river applications. This leads to something of a paradox in that while such models provide a means of generating hydraulic information that would be difficult to obtain in the field at an equivalent spatial resolution, they are, due to the problems inherent to data collection, difficult to validate conclusively. Despite this limitation, the application of spatially distributed models to investigate relationships between discharge and reach‐scale form and process variables appears to have considerable potential. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

Short‐term erosion rates from a 7Be inventory balance     

Christopher G. Wilson  Gerald Matisoff  Peter J. Whiting 《地球表面变化过程与地形》2003,28(9):967-977

Detailed soil erosion studies bene?t from the ability to quantify the magnitude of erosion over time scales appropriate to the process. An inventory balance for ⁷Be was used to calculate sediment erosion in a 30·73 m² plot during a series of runoff‐producing thunderstorms occurring over three days at the Deep Loess Research Station in Treynor, Iowa, USA. The inventory balance included determination of the pre‐ and post‐storm ⁷Be inventories in the soil, the atmospheric in?ux of ⁷Be during the event, and pro?les of the ⁷Be activity in the soil following the atmospheric deposition. The erosion calculated in the plot using the ⁷Be inventory balance was 0·058 g cm^?2, which is 23 per cent of the annual average erosion determined using ¹³⁷Cs inventories. The calculated erosion from the mass balance is similar to the 0·059 g cm^?2 of erosion estimated from the amount of sediment collected at the outlet of the 6 ha ?eld during the study period and the delivery ratio (0·64). The inventory balance of ⁷Be provides a new means for evaluating soil erosion over the time period most relevant to quantifying the prediction of erosion from runoff. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献