Base metals in Abitibi: a
new approach to assessing potential
Daniel Lamothe1, Jean-Yves
Labbé1 et Jeff Harris2
1Direction de Géologie Québec
2Geological Survey of Canada
In April 2005, Géologie Québec will publish
an overview assessment of base metal potential in the Abitibi sub-province
and the Frotet-Evans belt. This large-scale survey covers all the Archean
volcanic rock located between the 47th and 51st degrees of latitude. This
is the first time in Québec that an area of this size has been
studied, with such a large range of integrated data and such high-resolution
treatment. The survey was a joint undertaking by the authors and resident
geologists in Abitibi, and is intended to define and document a number
of target areas in the territory covered that offer the most potential
for new discoveries.
Figure 1 – Area
covered by the assessment of base metal potential in the Abitibi
With this study, Géologie Québec hopes
to contribute to the discovery of new copper deposits that will compensate,
in the medium term, for the exhaustion of mineral reserves in several
mining camps. The final assessment will include a 1/500 000 map of the
sector, together with roughly 130 1/50 000 maps produced using ArcGIS
software and available on DVD in PDF format. Intermediate products in
digital format will also be available, such as a lithological map of Abitibi
using closed polygons, based on the 1/20 000 and 1/50 000 maps available
for the sector as a whole.
An innovative approach
The methodology uses a conceptual approach based on a
metallogenic model for volcanogenic massive sulphide (VMS) deposits.
In this approach, all the geological, geo-chemical and geophysical parameters
connected to the presence of VMS-type mineral deposits are processed using
a flowchart known as an “inference model” (Figure 2). The
modeller controls the final outcome of the process by determining, at
each stage, the evolution and parameters of the model based on the modeller’s
understanding of the role played by each.
Figure 2 – Inference
model showing all the parameters used to assess VSM-type mineral
This type of approach, however, has an inherent weakness
since the weighting (“importance”) of each parameter is fixed
in an entirely subjective way by the modeller. To compensate for this
arbitrary aspect of the process, the weighting of each parameter has been
determined by a statistical calculation for spatial association with 399 known VSM deposits in Abitibi, using the “weight of evidence”
technique (Wright and Bonham-Carter, 1996; Harris et al., 2001; Bonham-Carter,
1994). The final favourability of the region studied is obtained by combining
the weighted parameters using a fuzzy logic method, giving the modeller
a degree of control over the outcome (Porwal et al., 2003; D’Ercole
et al., 2000). Figure 3 is an example of a map showing favourability on
the basis of proximity to a site where rock analysis shows anomalic Cu,
Pb or Zn content. The map is one of the 17 parameters considered in assessing
Figure 3 – Map showing showing
favourability based on proxmity to a site where rock analysis
shows anomalic Cu, Pb or Zn content in the Rouyn-Noranda region.
The favourability calculation for this parameter is made by the
WofE module in ArcSDM. The result is converted into fuzzy values
(between 0 and 1) to allow the map to be combined with the 16 other parameters in the VMS model.
Exploration 2004: Concepts
A workshop, presented jointly by the author and Jeff
Harris, is one of the events programmed at Québec
Exploration 2004. Various concepts relating to the assessment of potential
based on data integration will be discussed, using specific examples from
the current assessment in the Abitibi region. The workshop will provide
an ideal opportunity to meet the authors of the study and obtain more
information on the methodology used and other possible applications for
mineral exploration in Abitibi and elsewhere.
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Systems for geoscientists-modeling with GIS: Pergamon, New York,
D’Ercole, C. - Groves, D.I. and Knox-Robinson, C.M., 2000, Using fuzzy
logic in a Geographic Information System environment to enhance conceptually
based prospectivity analysis of Mississippi Valley-type mineralisation,
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