Base metals in Abitibi: a new approach to assessing potential

Daniel Lamothe¹, Jean-Yves Labbé¹ et Jeff Harris^2
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 47^th and 51^st 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 region.

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 potential.

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 potential.

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 and methodology

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.

References

Bonham-Carter, G. E, 1994, Geographic Information Systems for geoscientists-modeling with GIS: Pergamon, New York, 398 pages.

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, Australian Journal of Earth Sciences, N^o 47, pages 913-927.

Harris, J.R. Wilkinson, L. Heather, K. Fumerton, S. Bernier, M.A. Ayer, J. and Dahn, R., 2001, Application of GIS processing Techniques for Producing Mineral Prospectivity Maps – A Case Study: Mesothermal Au in the Swayze Greenstone Belt, Ontario, Canada, Natural Resources Research, Vol. 10, N^o 2, pages 91-124.

Porwal A. Carranza, E.J.M. and Hale, M., 2003, Knowledge-driven and Data-driven Fuzzy models for redictive Mineral Potential Mapping, Natural Resources Research, Vol. 12, N^o 1, pages 1-25.

Wright, D. E, and Bonham-Carter, G. E, 1996, VHMS favourability mapping with GIS-based integration models, Chisel Lake-Anderson Lake Area, in Bonham-Carter, G. E, Galley, A. G., and Hall, G. E. M., eds, EXTECH J: A Multidisciplinary Approach to Massive Sulphide Research in the Rusty Lake-Snow Lake Greenstone Belts, Manitoba, Geol. Survey Canada Bull. 426, pages 339-376, 387-401.