Rosario Jiménez-Espinosa

Numerical analysis of hydrogeochemical data: a case study (Alto Guadalentı́n, southeast Spain)

Abstract This study examines the spatial variability of the factors obtained from the application of correspondence analysis to a hydrogeochemical data set. The goal was to synthesize the hydrogeochemical information using this multivariate statistical technique, by setting a series of factors which clarified the main properties of one aquifer. Then, a geostatistical framework to obtain a probabilistic assessment of groundwater quality was established. Experimental and theoretical semivariograms of the selected factors, considered as regionalized variables, were computed. These variographic information and factor values in the experimental sites were used in the ordinary kriging, which provides unbiased and linear estimates of the regionalized variables. These estimates were used to compile maps of the chosen factors, which explain their spatial distribution. The selected case study was the alluvial aquifer of Alto Guadalentin which is situated in southeast Spain, in the Internal Zones of Betic Cordilleras. These waters are chiefly SO4 and Cl types, but HCO3 facies are common in the central sector of the basin. High temperature, acid pH, problems of overexploitation and pollution by CO2-gas characterise these waters. Available groundwater quality monitoring data were used to calibrate the numerical model. The present study focused on setting the main physical and chemical attributes and establishing the spatial pattern of groundwater quality and the temporal changes in this pattern.

Application of geostatistics to identify gold-rich areas in the Finisterre-Fervenza region, NW Spain

Three univariate geostatistical methods of estimation are applied to a geochemical data set. The studied methods are: ordinary kriging (cross-validation), factorial kriging, and indicator kriging. These techniques use the probabilistic and spatial behaviour of geochemical variables, giving a tool for identifying potential anomalous areas to locate mineralization. Ordinary kriging is easy to apply and to interpret the results. It has the advantage of using the same experimental grid points for its estimates, and no additional grid points are needed. Factorial kriging decomposes the raw variable into as many components as there are identified structures in the variogram. This, however, is a complex method and its application is more diAcult than that of ordinary or indicator kriging. The main advantages of indicator kriging are that data are used by their rank order, being more robust about outlier values, and that the presentation of results is simple. Nevertheless, indicator kriging is incapable of separating anomalous values and the high values from the background, which have a behaviour diAerent to the anomaly. In this work, the results of the application of these 3 kriging methods to a set of mineral exploration data obtained from a geochemical survey carried out in NW Spain are presented. This area is characterised by the presence of Au mineral occurrences. The kriging methods were applied to As, considered as a pathfinder of Au in this area. Numerical treatment of Au is not applicable, because it presents most values equal to the detection limit, and a series of extreme values. The results of the application of ordinary kriging, factorial kriging and indicator kriging to As make possible the location of a series of rich values, sited along a N‐S shear zone, considered a structure related to the presence of Au. # 1998 Elsevier Science Ltd. All rights reserved.

Influence of phyllosilicates and fluid–rock interaction on the deformation style and mechanical behaviour of quartz-rich rocks in the Carboneras and Palomares fault areas (SE Spain)

Abstract Deformed quartzitic rocks from the Carboneras and Palomares fault areas (SE Spain) are enriched in phyllosilicates compared to their respective protoliths. Deformation is mainly localized in highly foliated chlorite-rich bands. Quartz-rich bands show brittle deformation developing dolomite-rich cross-cutting veins re-cementing microcataclasite areas. Undamaged lenses within the cataclastic rocks contain patches of phyllosilicates with randomly oriented chlorite and mica. Mg, Fe, water, As and Zn enrichment of the damaged rocks suggests a process of hydrothermal chloritization associated with the Cabo de Gata volcanism. Petrographic characteristics indicate that hydrothermal alteration that produced chlorite and mica-enrichment occurred before faulting. Phyllosilicates provided lubricating properties to the quartzitic rocks, favouring the predominance of creep over seismic stick-slip and reducing the possibility of large seismogenic events. Dolomite cementation as a consequence of fluid-rock interaction processes would have a limited effect, due to the presence of weak phyllosilicate surfaces.

Use of Multivariate Indices to Characterise Hydrogeochemical Data. Application to the Alluvial Aquifer of Alto Guadalentín, SE Spain

Waters from the alluvial aquifer of Alto Guadalentin, which is situated in the Internal Zones of Betic Cordillera (SE Spain), have a series of human-induced and natural problems. These waters are chiefly sulphate and chloride types, but bicarbonate facies are common in the central sector of the basin. Also, high temperature, acid pH, problems of overexploitation and pollution by CO2-gas characterise these groundwaters. We have data taken between 1989 and 1992, totalling 337 samples and 12 variables (Ceron, 1997).