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
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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
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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
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