Data analysis and geostatistics applied to soil geochemical data of
Transcrição
Data analysis and geostatistics applied to soil geochemical data of
WORKSHOP - SOLOS EM PROSPEÇÃO MINEIRA CASOS DE ESTUDO SOBRE PROSPEÇÃO GEOQUÍMICA DE SOLOS Centro Ciência Viva do Lousal | 11 de Dezembro de 2015 INICIATIVA COMEMORATIVA DO ANO INTERNACIONAL DOS SOLOS Data analysis and geostatistics applied to soil geochemical data of the Marrancos mineralization Amélia P. Marinho Reis1*, Eduardo Ferreira da Silva1, António Jorge Sousa2 1 GEOBIOTEC, Departamento de Geociências, Campus Universitário de Santiago, Universidade de Aveiro, 3810-193 Aveiro, Portugal 2 CERENA, Instituto Superior Técnico, Av. Rovisco Pais, 1049-001, Lisboa, Portugal *[email protected] Keywords: gold, surface dispersion patterns, pathfinders, soild-phase fractionation, potentially toxic elements Study area The Marrancos–Portela das Cabras mineralization is located in the Vila Verde municipality, Braga district (NW Portugal), within a gold (Au) metallogenic province that extends from Galicia, Asturias and Léon in Spain to Minho, Trás-os-Montes and Beira in north–central Portugal. The main lithologies in the region are Silurian metasediments, a medium-to fine grained porphyritic biotite–muscovite Hercynian granite and hornfels developed at the metasediments–granitoid contact. The study area is situated in the hornfels sector. Within the study area, the hornfels are mainly banded coarse-grained quartz–feldspar but, closer to the contact with the granitoid (northern sector of the study area), pelitic fine-grained biotite or muscovite hornfels are more abundant. Small quartz–sulphide veins frequently crosscut the pelitic hornfels. There are some evidence that this mineralization, formerly known as Cova dos Mouros mine, was exploited for gold by the romans (Braz et al. 2011) and tentatively exploited for tungsten (W) during the II World War. The Marrancos mineralization is hosted by a N40°E quartz-breccia, with a 70°-75° NW dip, which is spatially related to a major shear zone (Viegas et al. 1991). The primary geochemical signature is chemically defined by an assemblage of Fe–As–Bi–Au–Ag–(W-Sn–Mo–Cu–Pb–Zn) corresponding to a mineralogical assemblage of arsenopyrite–pyrite–bismuthinite–bismuth–gold–electrum-(tungstates– cassiterite–estanite–molybdenite–chalcopyrite–sphalerite–galena–sulphosalts) (Noronha and Ramos 1993; Reis et al. 2001). Excluding quartz, arsenopyrite and pyrite are the most important minerals of the deposit. Chalcopyrite, sphalerite, pyrrhotite and galena are minor 27 sulphides. Gold occurs mostly as electrum, mainly in arsenopyrite and pyrite, either as small inclusions (average grain size 1-3 μm) or disseminated in their crystal lattice. Although the maximum Au grade found by Reis (1997) barely reaches 7 g t-1, Viegas et al (1991) indicate an approximate value of 35 g t-1. Evidence of past mining, exploration and other human activities can be found at the S sector the study area, and soil disturbances are not uncommon (remnants of open pits, trenches and drill holes, the football field, edification). Some have partially exposed the mineralised structure to weathering, promoting the dispersion of the metals at the surface. However, the soils of the northern part of the area are far less disturbed. Early studies The first studies report on a dataset resulting from 286 soil samples collected in a rectilinear grid, at equal distances (40 m) along evenly spaced lines (50 m). The lines have an ≈N45°W orientation, which is perpendicular to the NE–SW trending quartz-breccia. The samples were collected at slightly variable depths, normally between 10 and 30 cm in depth (horizon A), depending on the soil characteristics at the site. The <77 μm soil size fraction was used for chemical analysis. Soil concentrations of Fe, Cu, Zn, Pb, Co, Ni, Mn and Bi were determined by atomic absorption spectrometry (AAS), while As, Sb, Se and Te were analysed by AAS-hydride generation following an HClO4-HF attack. Gold was analysed by Inductively Coupled Plasma–Atomic Emission Spectroscopy (ICP–AES) with a detection limit of 7 ppb (Reis et al., 2001). Values for precision (expressed as RSD%) are <10 % for Fe, Cu, Zn, Pb, Co, Ni, Mn and Bi, but >10% for Au, As, Sb, Se and Te. Exploration studies carried out using this dataset aimed at detecting and interpreting ‘‘multi-element’’ anomalies related to the Au mineralization. Statistical techniques such as multiple correspondence analysis (MCA), variography and factorial kriging analysis were used to identify spatial patterns of dispersion and surface anomalies in the soil. However, the elevated As concentrations found in these soils triggered an environmental study to assess the soil vulnerability to arsenic contamination. Since risk mapping can provide important information for science-based decision-making in the evaluation of contaminated sites, indicator kriging was used to perform a risk probability mapping. Late studies These studies used 144 soils from 286 archived samples in the University of Aveiro. The original sampling grid used to collect the soils was enlarged to 80 m × 50 m, a decision based on previous studies carried out in the area (Reis 1997). Sample splits of 15 g were digested in a hot (95°C) modified ‘aqua regia’ leach and analysed for 55 elements, including 28 Au, by ICP-mass spectrometry (ICP-MS) at ACME Analytical Laboratory (ISO 9002 Accredited Co.). Excepting Au (13%), Bi (11%) and Te (12%), values for precision are <10 % for all elements. Given the larger number of chemical elements determined in the soil samples and the better reproducibility values obtained, new exploration studies were carried out to identify surface dispersion patterns of Au and other relevant metal(oid)s. Principal components analysis (PCA) and ordinary kriging were combined to: (1) identify associations between chemical elements; (2) estimate spatial patterns of variation for such associations in the soil. Sequential extractions are widely used for exploration purposes as well as for environmental assessments. Then, a sub-set of 10 topsoil samples were selected to carry out a solid-phase fractionation study using a chemical extraction method. The objective was to identify and quantify the fractions of As, Cd and Pb occurring in different soil phases (clay minerals, carbonates, amorphous Fe, Mn oxides, organic matter and more resistant mineral phases). The studies carried out allowed identifying and interpreting soil geochemical anomalies of Au and other relevant metal(oid)s occurring at the surface. References: Braz C.M., Fontes Silva V.M., Gomes Braga A.C., 2011. A mineração em época romana no concelho de Vila Verde (Braga, Portugal). Férvedes, 7: 235 – 242 Noronha F, Ramos JMF, 1993. Mineralizações auríferas no Norte de Portugal. Algumas reflexões. Cuaderno, Laboratório Xeolóxico de Laxe, Coruña, 18, 133–146. Reis A.P., 1997. Soil geochemistry in the Marrancos gold mineralization. Study of dispersion mechanism and optimisation of the geochemical gold exploration parameters in the Vila Verde-Ponte da Barca ore belt. Ph.D. Thesis, Departamento de Geociências, Universidade de Aveiro, (In Portuguese). Reis A.P., Sousa A.J., Cardoso Fonseca E., 2001. Soil geochemical prospecting for gold at Marrancos (Northern Portugal). J. Geochem. Explor. 73, 1-10. Reis A.P., Silva E.F., Sousa A.J., Patinha C., Martins E., Guimarães C., Azevedo M.R., Nogueira P., 2009. Geochemical associations and their spatial patterns of variation in soil data from the Marrancos gold-tungsten deposit: a pilot analysis. Geochemistry: Environment, Exploration, Analysis, 9: 319-340 Viegas L.F.S., Andrade R.S:, Rodrigues L.V., Martins L.P., 1991. Projecto de prospecção de metais nobres (ouro e prata) faixa Vila Verde / Ponte da Barca. Relatório final. Serviço de Fomento Mineiro. S. Mamede de Infesta. Relatório interno (arquivo do LNEG, S. Mamede de Infesta). 70 pp 29