REVISTA DA FACULDADE DE CIÊNCIAS
NÚMERO 1
EDIÇÃO ESPECIAL
2004
THE USE OF GEOGRAPHIC INFORMATION SYSTEMS (GIS) IN THE
TREATMENT OF MULTIELEMENTAL GEOCHEMICAL DATA. AN EXAMPLE
FROM THE SANTIAGO DE CACÉM BASIN, SOUTHWEST PORTUGAL
Nogueira, P1., Inácio, D.2, Noronha, F.3 & Oliveira, V.4
1
Departamento de Geociências da Universidade de Évora.
2
Aluno Finalista do Curso de Engenharia de Recursos Geológicos.
3
Departamento de Geologia da Faculdade de Ciências da Universidade do Porto.
4
Instituto Geológico e Mineiro.
Resumo
Numa campanha de prospecção geoquímica o investigador tem de lidar com um conjunto complexo de dados
que necessitam de ser interpretados. Para tal pode ser efectuada uma abordagem baseada em métodos de
estatística multivariada. A análise factorial e de componentes principais são dois exemplos deste tipo de
abordagem. A informação geológica, tal como as litologias, as idades das formações, entre outros, devem ser
igualmente considerados e tomados em conta. O que estas técnicas não consideram é a distribuição espacial da
amostragem. Para este tipo de situações é necessária uma abordagem baseada no recurso a sistemas de
informação geográfica (SIG). Neste trabalho é apresentada uma abordagem deste tipo para estudar um conjunto
de dados geoquímicos para diversos elementos e aplicada ao caso de uma bacia Mesocenozóica do Sudoeste de
Portugal.
Abstract
After a geochemical campaign, the researcher is confronted with a multiset data that has to be interpreted. For
this, an approach based on the multivariate statistics methods can be applied. Factorial analysis and principal
components are two examples of such techniques. The geological information, such as lithologies, ages of
formations and others can be considered and taken into account. What these techniques cannot consider is the
spatial distribution of the samples. A GIS based approach is necessary for these situations. In this work, this
approach is applied to study a multielemental set of data from a Southwest Portuguese Mesocenozoic basin’s
case study.
Introduction
During a campaign of geochemical exploration for base metals in the
Mesocenozoic basin in Southwest Portugal a set of different techniques were applied to
evaluate its potential. Two types of samples were collected and treated: stream sediment
333
REVISTA DA FACULDADE DE CIÊNCIAS
NÚMERO 1
EDIÇÃO ESPECIAL
2004
samples and soil samples. These samples were treated for geochemical analysis and
mineralogical studies.
The geological context is of a marginal basin related to a pre-rift context
associated with the Atlantic aperture. This is a favourable context for the deposition of base
metals such as Cu, Pb, Zn and particularly of Gold.
A set of 29 elemental analyses of 109 samples for stream sediment samples and 28
elemental analyses of 27 soil samples were run in an international geochemical laboratory.
This data combined with the mineralogical studies (10 different mineral species identified) of
the samples would provide a total of 65 different thematic maps.
It is reasonable to accept that it is impossible to a researcher to combine by him
self all this information and to reliably interpret it.
Herein a tentative approach for the treatment and interpretation of such type of sets
is presented, using a multivariate statistic approach combined with the construction of a GIS
based model.
Geochemical univariate and multivariate analysis
In another presentation (Inácio et alia, this conference) the principles used in the
univariate and multivariate analysis are discussed. A principal factors analysis was selected
to describe the data. The cluster analysis demonstrated that gold does not correlate with any
element. This might be due to nugget effect on gold. Another effect that becomes obvious is
the presence of outlier and zero values that contribute to a poor correlation of the data.
On what concerns the geochemical exploration, this kind of studies are a previous
approach to understand elemental behaviour and is used to select which are the relevant
elements to the understanding of the geochemical pattern.
The GIS based approach to geochemical exploration
For the interpretation of the data a GIS project was developed. The topographic,
geological and structural information was collected from available cartography and digitised.
The geochemical information was digitised as a set of coordinated points and linked to the
table with the geochemical data (figure 1),
334
REVISTA DA FACULDADE DE CIÊNCIAS
NÚMERO 1
EDIÇÃO ESPECIAL
2004
Atributes
Faults
1
Geology
1
Atributes
Geology
1
1
Figure 1. The data structure for the GIS project.
Geodesic info
Atributes
Geochemical data
Topography
1
1
Samples
Atributes
1
1
Provenience
1
Concession
1
Atributes
1
Concentration factor
1
1
Atributes
1
Figure 1 (Continued). The data structure for the GIS project
Data analysis
The different thematic maps were analysed considering a mineralogical, geological
and geochemical basis. The analysis of the thematic maps allows us to consider two types of
elemental behaviour. Elements such as As and Fe that can be consider to reflect the geological
background, that is the separation between the different geological units (Palaeozoic,
Mesozoic and Cenozoic). The other type of behaviour is the one that does not relate directly
with the geologic background. Elements such as Au and Cu present this type of behaviour.
335
REVISTA DA FACULDADE DE CIÊNCIAS
NÚMERO 1
EDIÇÃO ESPECIAL
2004
This analysis points to the conclusion that thematic maps for Au and Cu represent
true concentration maps whereas thematic maps of elements such as Fe and As only reflect
the geologic background.
Suggestions for exploration
The geochemical maps of Cu and Au point to two areas of overlapping interest.
These areas are the potential areas for further mineral exploration campaigns. These areas are
the ones were the Mesozoic basin is more deep (the basin as a synclinal form and the areas are
near its axis). These should correspond to the maximum deep of the sandstone sedimentation
and therefore the places were the dynamic conditions of sedimentation promoted the
deposition of the gold.
Final considerations
The main objective of a mineral exploration campaign is to reduce the areas of
study, improving the knowledge on those areas and defining potential targets for further
exploration. Within a geochemical campaign a great amount of data is obtained and only the
combination of statistical and geographical oriented methods of analysis permits the full
assessment of the data significance.
Nowadays the ease of access and use of GIS systems provides the geochemical
exploration a tool for further developing the analysis of the data obtained.
References
Inácio, D., Nogueira, P., Noronha, F. & Oliveira, V. (2003). Sugestão de procedimento base para o
tratamento estatístico de dados referentes à prospecção geoquímica. VII Congresso de geoquímica dos países de
língua portuguesa. Maputo.
336