MERN - Québec Mines - A new worldwide trend












November 2002
 


A new worldwide trend

Mineral potential assessment via integration of spatially georeferenced data is an approach that is now used on a global scale, and it keeps generating more and more interest. The growing number of articles dealing with this technique confirms this. This new trend was strongly influenced by the development of new software making the process possible on a personal computer, and by the availability of new digital databases.

Formerly, zones favourable to the discovery of new deposits were generally identified by overlapping several layers of polyester, each illustrating a geological feature potentially indicating the presence of a given type of ore deposit (fault, geophysical anomaly, geochemical anomaly, etc.). Target areas were selected among those showing the largest number of overlapping features. The digital integration of georeferenced data revolutionized this approach, by making it possible to target, with pinpoint accuracy, the type of data (or theme) to process, to adapt the processing based on data-driven or knowledge-driven methods, and finally, to combine the results according to the relative importance of each theme.

The various methods used to process the data are grouped into two categories: first, data-driven methods that rely on statistical analysis to determine relevant spatial associations of various geological, geophysical or geochemical elements with known ore deposits; second, knowledge-driven methods which, based on the expert assessment of the processing geologist, rely on the definition and spatial processing of relevant metallogenic parameters for a selected model.

Data-driven methods use a training set generally derived from a digital map where a sufficient number of known deposits corresponding to the selected ore deposit model are located in the study area. The processing consists in calculating the statistical correlation between this training set and a certain number of digital maps each representing a parameter (or theme) relevant to the model (map of felsic intrusions, iron alteration, synvolcanic dykes, etc.). Data-driven methods essentially use three techniques: regression analysis, neural networks and weight of evidence.

Knowledge-driven methods (used to produce MPMPS maps) are based on an ore deposit model that is relevant for the study area. This model is based on geological parameters (heat source, favourable lithologies, faults allowing fluid circulation, etc.) which are considered as essential or favourable elements for the formation of deposits. To these parameters are also added discrete indicators (mineral occurrences, alteration indicator minerals, geochemical anomalies, etc.) signalling the presence of mineralization. In each case, these elements constitute a modelling theme. Data processing is guided by the expert’s evaluation of the role and importance of each theme in the emplacement of potential deposits. The weight attributed to the various themes and their combination may be derived either from the arbitrary assessment of an ore deposit expert, or from combination methods based on concepts such as fuzzy logic, or the Dempster-Shafer belief theory.

Data-driven methods that require a significant number of occurrences related to the selected model can only be used in areas that have already been extensively explored. The advantage of these methods lies in the fact that they are based on a factual approach that may be demonstrated and verified. However, they are based on occasionally complex statistical correlations which, from a geological standpoint, are not always meaningful. Knowledge-driven methods are more suitable in poorly explored areas where little or no favourable mineralization has been recognized to date. Since the evolution of the product is guided by the expert judgment of a metallogenist, this method yields a product that closely resembles ore deposit models already known and studied by exploration companies. On the other hand, the arbitrary weight attributed to parameters in this approach is determined by the expert preferences of the metallogenist, which may vary from one expert to another.
















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