Source and evolution of ore-forming hydrothermal fluids in the northern Iberian Pyrite Belt massive sulphide deposits (SW Spain): evidence from fluid inclusions and stable isotopes

A fluid inclusion and stable isotopic study has been undertaken on some massive sulphide deposits (Aguas Teñidas Este, Concepción, San Miguel, San Telmo and Cueva de la Mora) located in the northern Iberian Pyrite Belt. The isotopic analyses were mainly performed on quartz, chlorite, carbonate and whole rock samples from the stockworks and altered footwall zones of the deposits, and also on some fluid inclusion waters. Homogenization temperatures of fluid inclusions in quartz mostly range from 120 to 280 °C. Salinity of most fluid inclusions ranges from 2 to 14 wt% NaCl equiv. A few cases with T _h=80–110 °C and salinity of 16–24 wt% NaCl equiv., have been also recognized. In addition, fluid inclusions from the Soloviejo Mn–Fe-jaspers (160–190 °C and ˜6 wt% NaCl equiv.) and some Late to Post-Hercynian quartz veins (130–270 °C and ˜4 wt% NaCl equiv.) were also studied. Isotopic results indicate that fluids in equilibrium with measured quartz (d ¹⁸O _fluid ˜–2 to 4‰), chlorites (d ¹⁸O _fluid ˜8–14‰, dD _fluid ˜–45 to –27‰), whole rocks (d ¹⁸O _fluid ˜4–7‰, dD _fluid ˜–15 to –10‰), and carbonates (d ¹⁸O _ankerite ˜14.5–16‰, d ¹³C _fluid =–11 to –5‰) evolved isotopically during the lifetime of the hydrothermal systems, following a waxing/waning cycle at different temperatures and water/rock ratios. The results (fluid inclusions, d ¹⁸O, dD and d ¹³C values) point to a highly evolved seawater, along with a variable (but significant) contribution of other fluid reservoirs such as magmatic and/or deep metamorphic waters, as the most probable sources for the ore-forming fluids. These fluids interacted with the underlying volcanic and sedimentary rocks during convective circulation through the upper crust.     

An analysis of water column distributions in Florida Bay

Florida Bay is a shallow, semi-enclosed lagoon that has recently experienced significant changes to its ecosystem. These include increased turbidity and the occurrence of cyanobacteria blooms in the central region of the bay. To accurately understand these changes we need to understand the spatial and temporal patterns in observed water quality parameters. To this end, we have used empirical orthogonal functions (EOFs) to analyze both the spatial and temporal variability in an 8-yr record of water quality variables. We have used the EOFs in two ways, one highlighting local changes occurring in the bay, the other emphasizing changes occurring on a bay-wide scale. The local analysis shows that the central region of the bay has the greatest variability in water quality parameters, especially with respect to chlorophyll and nutrient concentrations. The bay-wide analysis shows a different picture. The chlorophyll blooms in the central bay are not apparent bay-wide indicating that they are a local manifestation of processes occurring on a bay-wide scale. The spatial and temporal patterns for nitrate are dissimilar from the other nutrients raising the possibility that the mechanisms controlling nitrate differ from those controlling other nutrients. On a bay-wide scale, spatial patterns are similar to distributions of sediment type and show the significance of interactions between the water column and benthos. Time-series analysis of the EOFs shows that the dominant variation of many water quality parameters is seasonal, even though a system-wide shift occurred between 1994–1995 corresponding to an increase in rainfall and runoff from the Everglades.     

Limiting apparent-resistivity values for dipping-bed earth models

Consideration is given to some special features of normalized apparent-resistivity (NAR) curves, resulting from the deployment of Wenner electrode arrays on the surface above dipping earth structures. Limiting values of the potential are derived when a direct-current source is located at points on the surface above a simple two-region dipping-bed earth model and their influence on corresponding NAR curve characteristics is investigated. Particular attention is given to the exploitation of such features to provide a new and direct approach to model parameter estimation, either as an alternative to traditional curve-matching techniques or as a source of supporting information when other earth model characterization methods have been employed. Throughout, the emphasis is on the single-dipping-bed model, but application to more complex structures is discussed, including examples of two dipping beds, dipping dikes and more general tilted unconformities.     

Mapping the geochemistry of the northern Rub' Al Khali using multispectral remote sensing techniques

Spatial variations in sand sea geochemistry relate to mixing of different sediment sources and to variations in weathering. Due to problems of accessibility, adequate spatial coverage cannot be achieved using field surveys alone. However, maps of geochemical composition produced from remotely sensed data can be calibrated against limited field data and the results extrapolated over large, inaccessible areas. This technique is applied to part of the Rub' Al Khali in the northern United Arab Emirates. Trend surface analysis of the results suggests that the sand sea at this location can be modelled as an east–west mixing zone of two spectral components: terrestrial reddened quartz sands and marine carbonate sands. Optical dating of these sediments suggests that dune emplacement occurred rapidly around 10 ka BP , when sea level was rising rapidly. The spatial distribution of mineralogical components suggests that this phase of dune emplacement resulted from coastal dune sands being driven inland during marine transgression, thereby becoming mixed with rubified terrestrial sands. Copyright © 2001 John Wiley & Sons, Ltd.     

Surface joint systems, tectonic stresses and geomorphology: a reconciliation of conflicting observations

This note seeks to reconcile the widespread small-scale fractographic observations indicating that surface joints are extension fractures with the frequently observed occurrence of such joints as what appear to be nearly orthogonal conjugate sets, their strikes fitting the shear lines of the neotectonic stress field. It is suggested that the local appearance of joints as extension fractures may have nothing to do with their orientation in the large-scale neotectonic stress field, inasmuch as extrapolations from a local scale to plate-tectonic dimensions are quite speculative. Thus, the local extension-characteristics of the joint surfaces may be acquired at the latest stage of their genesis as a result of the corresponding rock faces becoming exposed, but the geometrically orientational attributes may be conditioned by the shear in the surrounding large-scale neotectonic stress field; a new possible mechanism for reconciling the conflicting local and large-scale observations is suggested.     

西澳大利亚州土地利用的实践与技术——新疆土地利用之借鉴

西澳大利亚州(西澳)大部分为年平均降水量200～500mm的干旱半干旱地区.这里的农田草场面临土地退化,包括植被衰败、地下水上升和土壤盐渍化等,主要由土地开垦及过牧所致.西澳在解决此问题及草场管理、矿区植被重建方面取得许多成功经验和相关技术.(1)盐渍化控制开垦土地、砍伐林木、种植小麦引起的地下水上升和次生盐渍化.大范围植树可控制地下水上升.适合植树的条件是a)较低的地下水含盐量(＜5 ds/m);b)种植汲取地下水能力适当(200mm/a)的树种,否则应对根带10年洗盐1-2次,以防根带积盐;c)地下水位较高的地区,应植树于＞5°的缓坡上.当地科学家应用GIS、空间分析和野外监测充分研究树木生长和地下水位、含盐量、土壤质地及地面坡度的关系,最终确定适宜树木生长的土地景观.(2)草场管理草场因过牧、野生动物危害、杂草蔓延、大风、洪水、火灾的破坏而退化,当地通过以下措施促使牧场实现有效恢复和更新a)以草定畜;b)修建多种截水、集水、引水工程,确保墒情;c)在地表营造适于牧草种子着床、萌芽生长的小生境;d)完善的种植技术和良好的种源;e)坚持围栏和定点监测,确定植被覆盖率和复牧的时间强度.(3)矿区植被重建和尘土控制西澳的两大金矿开采一个世纪后,占地400hm2的矿渣堆山丘似地高耸在城镇的东南部,成为该地尘土主要来源.沿矿渣堆等高线筑起阶梯状集水埂,并施加肥料,通过分阶段直播或移苗等种植实验,成功地在矿渣堆上重建起植被,既减少了风、水侵蚀,又遏制了扬尘势头.另外,西澳土地利用过程中的新技术和新方法还有a)牧场监测利用WARMS监测系统监测并解释土壤、植被的变化及发展趋势,为评价土地系统的载畜潜力、水土保持、牧业管理提供基础信息;b)水的有效利用集、储水工程、脱盐技术和使用管理;c)水土保持修建各种具有集、排、分流地表径流、控制盐渍化、防涝功能的土石工程.工程的建筑形式和技术指标应符合当地的气候特征.     

Analyse quantitative de l'hétérogénéité de la fracturation : application aux carrières souterrainesQuantitative analysis of fracturing heterogeneity: application to underground quarries

The quantitative analysis of fracturation heterogeneity from Geographic Information System (GIS) has become very important for the modelling of underground quarries stability, the reservoir properties deduced from fracturing and their consequences on the groundwater run-off. This paper aims to give a method of quantitative analysis of fracturing in three underground limestone quarries of Gironde. More than 1000 fractures were measured, mapped and numbered. The heterogeneity analysis and the fracturing connectivity are based on the spatial distribution of geometrical parameters of each fracture, such as: openness, length, direction, orientation, spatial distribution of different fracture classes and connexity. To cite this article: A. Cerepi, C. R. Geoscience 334 (2002) 163–170.     

Submarine groundwater discharge: an outlook of recent advances and current knowledge

Driven by the hydraulic gradient, terrestrial groundwater can be discharged directly into the coastal ocean where heads are above sea level, a phenomenon known as submarine groundwater discharge (SGD). Although long overlooked, in recent years SGD has received increasing attention due largely to its potential importance in the transport of chemical constituents to the sea. Indeed, SGD occurrence and control mechanisms are still poorly understood. Submarine discharge often represents only a minor component of regional total water budgets, but because estimated fluxes are highly variable and quantification remains challenging, the process deserves to be carefully evaluated when investigating marine ecosystems and/or the safety of geological repositories in coastal areas, notably for the underground storage of CO₂. In view of the focus the topic is gaining, reflected in the large amount of information continuously being published in this field, here we present a selected review of our current knowledge of key mechanisms determining submarine groundwater discharge, and an updated compilation of most recent advances in research both in Japan and other regions of the world. In addition, we identify some potential pitfalls that need to be borne in mind in future work on SGD. Particularly in view of globally increasing urbanization and climate change, this contribution should prove useful also to local governmental authorities and scientists involved in groundwater coastal ecosystem management.