E. Cardoso Fonseca

The selective extraction of Pb and Zn in selected mineral and soil samples, application in geochemical exploration (Portugal)

The study described herein concerns the application of selective chemical extractions on metal-bearing minerals and soils to geochemical exploration. Specifically, the study addresses itself to the detection of anomalous soils in the vicinity of a mineralized zone. A kinetic study of selectivity of partial chemical extractions-applied to minerals of Pb and Zn was undertaken in order to establish a systematic procedure (reagents, time of extraction, etc.) which could be recommended for the soils of the study area in Portugal. It is shown that: (1) NH4 acetate dissolves smithsonite, willemite, cerussite, anglesite, pyromorphite completely and coronadite, galena, sphalerite only partially; (2) hydroxylamine hydrochloride dissolves coronadite only partially; (3) H2O2 dissolves sphalerite and galena only partially; (4) hot mixed acids dissolve residual galena, sphalerite and coronadite. About 850 soil and gossan samples were collected from two mineralized districts in Portugal, one in a carbonate environment (Portel) and the other in an aluminosilicate zone (Sanguinheiro). For the kinetic study in soil samples from Portel and Sanguinheiro zones, the minus 80 mesh fraction (−180 μm) was submitted to an extraction procedure using the following reagents, in sequence: NH4 acetate, hydroxylamine hydrochloride, NH4 oxalate in darkness, NH4 oxalate under U.V. irradiation and finally strong acids. The mineralogy of the soils from the two areas is different. In the aluminosilicate zone, the products of weathering of sulphide mineralization are mainly anglesite, plumbogummite, amorphous iron oxides, goethite and hematite rich in Pb and Zn. In the carbonate area, the Pb- and Zn-bearing phases generally found in the soil and gossan are Mn oxides, lepidocrocite and hematite. The gossan, in this case, also contains smithsonite and cerussite. In soils from the aluminosilicate zone the use of NH4 oxalate (dark) given a better geochemical response for Pb than NH4 acetate, NH4 oxalate (U.V.) and conventional acid digestion. On the other hand, NH4 oxalate (U.V.) gives better results for Pb and Zn than conventional acid digestion in samples from carbonate area. The analysis of NH4 oxalate (U.V.) extractions, instead of conventional acid digestion, can nevertheless be recommended for both environments.

Application of selective extraction techniques in metal-bearing phases identification: a South European case study

The study described herein concerns the application of selective extraction techniques on metal-bearing phases identified in gossan samples. Specifically, the study is directed towards the detection of Zn- and Pb-bearing phases in Balsa-Portel area (southern Portugal). In order to identify the metal-bearing phases, samples were submitted to sequential extraction procedures using the following reagents: NH4 acetate; hydroxylamine hydrochloride; oxalate in darkness; H2O2; oxalate under UV irradiation; and finally, mixed acids (HCl–HNO3–HF). In the Balsa-Portel area, the Zn- and Pb-bearing phases generally occurring in the gossan samples are lepidocrocite, Mn-oxides, cerussite and smithsonite, dissolved by oxalate under UV irradiation (lepidocrocite), hydroxylamine hydrochloride (Mn-oxides) and ammonium acetate (cerussite and smithsonite).

Application of geostatistical methods in gold geochemical anomalies identification (Montemor-O-Novo, Portugal)

Abstract The study described herein concerns the application of geostatistical methods to data soil from Montemor-O-Novo area (Southern Portugal). In the area, the gold mineralised zones (Banhos, Caeiras, Fales, Gamela, Malaca and Monfurado) are characterised by different geological settings and mineralogical assemblages. A total of 1211 soil samples were collected in Montemor-O-Novo area and analysed for Cu, Pb, Zn, As, Ba and Au by atomic absorption spectrometry. To account for spatial structure, simple and cross variograms were computed for the main directions of the grid sampling. From the experimental variograms a linear model of coregionalization composed of three structures, a nugget effect and two anisotropic spherical structures, was fitted to each of the six variables. The coregionalization matrices deduced from the theoretical model show the relationships between the variables at different scales. These matrices were compared with those obtained by principal component analysis (PCA). This methodology was the basis for estimating the corresponding spatial components (Y0, Y1 and Y2) using factorial kriging analysis (FKA). Maps of raw data, Y0, Y1 and Y2 were made for each variable. The use of multivariate analysis permit the study of the spatial structure intrinsic to geochemical data and the identification and refinement of significant anomalies related to Au-bearing mineral deposits.

Soil geochemical prospecting for gold at Marrancos (Northern Portugual)

Abstract This paper discusses some geochemical mapping techniques and their effectiveness in detecting of gold and its pathfinders geochemical anomalies. The study area (Marrancos area) is situated in Northern Portugal. Gold occurs in a quartz breccia with pyrite and arsenopyrite hosted by metamorphic rocks (hornfels) and it is present in these two sulphide minerals as inclusions or disseminated in the crystal lattice. In this area, the geochemical anomalies obtained by isoconcentration mapping of single elements were difficult to interpret. The use of alternative geochemical mapping such as multielement halos (additive halos) proved to be effective in the definition of Au, As, Bi, Sb, Te and Se secondary geochemical signature.

Environmental Assessment of the Caveira Abandoned Mine (Southern Portugal): Part 1: Characterization of Metal Contaminated Soil

Mine activity in Portugal had a huge impact on the growth of the regions where it took place, like in the Iberian Pyrite Belt. Nowadays, most of these mines are abandoned, as is the case for the Caveira mine. Soil geochemistry indicates that high contents of Cu, Pb, Zn, As, Cd, and Hg occur in the soils collected near the tailings. Multiple Correspondence Analysis identifies two areas with high concentrations of Pb, As, Hg, Sb, Mo, and Tl. However, the results suggest a different geochemistry for each of the areas. The non-site-specific methods (Hazard Index and GLC guidelines) classify all the areas as contaminated. Metal fractionation in the soil phases is different for the several elements studied. The articulation of the metal fractionation results with the GLC guidelines reduces the area of soil contaminated by Pb, but not by As.

Soil data analysis from central Portugal by Principal Component Analysis and geostatistical techniques

The area under study is located near Estarreja, in central Portugal where chemical industries are the main source of pollution. Until 1975, liquid effluents produced by these industrial units were discharged directly into several nearby streams. These effluents contained many different types of contaminants, such as aniline, benzene, monochlorobenzene, mononitrobenzene, As, Hg, Zn and Pb, amongst others. The industrial units also dumped a large amount of solid waste, which was stockpiled, directly on the surrounding land. This solid waste contains pyrite-rich ashes, dusts and sludge containing Hg and calcium hydroxide. A geochemical soil survey was conducted in a 49-km2 area where a total of 985 soil samples were collected during 1998 and 1999, on a regular grid of 200 × 200 m. The soil samples were analysed for Cu, Pb, Zn, Fe, Mn, Co, Ni, Cr and As, following an HCl–HNO3–HF digestion. To identify the main soil contaminants and to map the most important contaminated areas, Principal Component Analysis (PCA) and geostatistical techniques (Variography and Kriging map) were used. The results of PCA data analysis, compared to the available information in pollution indices (Contamination Index – Cidx), outline one important area contaminated by Cu, Pb, Zn and As as a result of effluent dumping by the chemical industry.

Selective chemical extraction of Cu from selected mineral and soil samples: enhancement of Cu geochemical anomalies in Southern Portugal

Abstract The study described herein concerns the application of selective chemical extractions on metal-bearing minerals and soils to geochemical exploration. Specifically, the study aims at the detection of anomalous soils in the vicinity of two mineralized zones in Southern Portugal. A kinetic study of the selectivity of partial chemical extractions applied to Cu minerals has been carried out in order to establish a systematic procedure (reagents, extraction plateau, etc.) which could be recommended for soils of the two study zones. It is shown that: (1) NH 4 acetate dissolves malachite, azurite and cuprite completely and chrysocolla, conicalcite and atacamite only partially; (2) hydroxylamine hydrochloride dissolves chrysocolla and conicalcite only partially; (3) H 2 O 2 dissolves chalcopyrite only partially; (4) NH 4 oxalate (U.V.) dissolves conicalcite and atacamite only partially; (5) mixed-acid attack dissolves residual chalcopyrite, chrysocolla, atacamite and conicalcite. A total of 740 soil samples were collected from the Salgadinho and Tinoca areas. The Cu mineralization in the Salgadinho area is situated in an Upper-Devonian Volcanic-Siliceous Complex. The weathering products of the mineralization are mainly amorphous iron oxides, goethite and hematite. The Cu mineralization in Tinoca area is located in the Arronches-Campo Maior belt, where stratiform Cu mineralization is found. The weathering products of the mineralization are mainly malachite, amorphous iron oxides, goethite and hematite. In order to identify the Cu-bearing phases and the extraction plateau, the −80 mesh fraction of two soil samples was submitted to an extraction procedure using the following reagents in sequence: NH 4 acetate, hydroxylamine hydrochloride, NH 4 oxalate (dark), NH 4 oxalate (U.V.) and finally strong acids. In soils from the Salgadinho and Tinoca areas, the use of NH 4 oxalate (U.V.) in single dissolution which would incorporate those phases dissolved by NH 4 acetate and NH 4 oxalate (dark), gives the broadest anomalous surface and higher contrast for Cu than acid digestion. The analysis of NH 4 oxalate (U.V.) extractions, instead of acid digestion, can thus be recommended for both areas.

Assessing soil quality using p-field simulation: application to the Marrancos mining area, Portugal

Abstract The purpose of this study was to assess the negative impact on soil caused by several mining episodes that occurred at Marrancos mine, an old gold/silver mine. The main aims were to: (i) assess the quality of the soil and the delineation of areas potentially contaminated by arsenic; (ii) assess the uncertainty attached to the assignment; and (iii) present a method of stochastic simulation (probability field simulation) as an alternative technique to predict pollutant concentrations of heavy metals, or metalloids, at unsampled locations. Indicator kriging was used to model the uncertainty attached to the concentration values at the unsampled sites and p-field simulation produced probability maps that allowed assessment of the probability that the arsenic concentrations did not exceed a regulatory threshold. The probability maps were then used to identify hazardous areas. About 90 realisations were generated for the variable, meaning 90 equiprobable scenarios for the spatial distribution of arsenic. The use of a potential risk-based standard as contamination threshold shows a wide area of soils classified as hazardous for ecosystems that need further investigation to determine the urgency for remediation.

Identification and mapping of pollution indices using a multivariate statistical methodology, Estarreja, central Portugal

Abstract In the Estarreja area of central Portugal, near an important focus of chemical industrial activity which is responsible for the eventual contamination of local aquifers by Cu, Pb, Zn, Co, Ni, Cd and Cr, a geochemical soil and groundwater survey was conducted, in order to identify and map meaning pollution indices (obtained by a specific multivariate statistical model) that could be used for pollution control and monitoring purposes. The main results of the research concern the statistical stability of two independent indices in three time periods and the linkage between soil and water data which assist in the interpretation of one of the indices.

Factorial Kriging Applied to the Study of Industrial Contaminated Soils

In this paper we study industrial contaminated soils in an area near Estarreja, close to Aveiro in central Portugal. The industries are the main source of pollution in this area, since one of the most important chemical industrial perimeters is located within the study area Those industries are responsible for the contamination of soils and local aquifers mainly by Cu, Pb, Zn and As. Until 1975 the liquid effluents, produced by these industrial units, were discharged directly to several streams. These discharges contained different types of contaminants. The industrial units have also rejected huge quantities of solid waste. A geological soil survey was conducted to identify the origin of the contaminationand to map the most important contaminated areas. Geochemical data were interpreted by multivariate geostatistical techniques and by Factorial Kriging, to understand and identify spatial components acting at several scales and allowing the identificationof trace metal common sources. A comparisonbetween the mapping of the different spatial components and of the initial variables logaritmized values as well as the results of the Factorial Analysis of Corregionalization allowed the perception of the variables different behaviour at different scales (punctual, local and regional).The methodology used is an important tool for the separation between the geological and anthropic inputs and for the identification of common pollution sources.