A NEW METHOD OF FRACTAL AND MULTIFRACTAL ANALYSIS APPLIED TO PROSPECTIVITY IN THE OSSA-MORENA ZONE, VARISCAN MASSIF (SPAIN)

Based on the geological and metallogenic knowledge of 400 prospects and mineral deposits in the Ossa-Morena Zone (Variscan Massif), a spatial statistical analysis of several types of mineralization and their geological controls (host lithologies and related fracture systems) has been carried out in a GIS. For example, 78.4% of mineral are located within 500 m of a specific group of faults (>50% within 100 m) and different mineral types are associated with particular geological units. An analysis of these geological controls on the distribution of the mineralization was carried out using the Weights of Evidence method (WofE). Four posterior probability maps were produced for each selected mineralization (Fe, Cu, Pb-Zn and F-Ba), which provide a relative measure for prioritizing areas of potential exploration and can be considered as prospectivity maps for the area.

Most types of mineralization are clustered within regions of suitable geological control. Traditional fractal analysis using box-counting measures a combination of the distribution of the deposit and its control. In the Ossa-Morena region the capacity dimensions of each mineralization ranges between 0.84 (Fe) and 0. 94 (Pb-Zn-Ag, F-Ba) confirming the degree of clustering of the distributions, and their lithological controls range between 1.40 (F-Ba) and 1.66 (Fe). The capacity dimensions also discriminate fault orientations (structural controls) that exert influence over the mineralization (NW-SE for Fe and NE-SW for Pb-Zn-Ag).

A new method of calculating fractal dimensions has been developed that is based on box-counting sites of selected mineralization in comparison with their geological controls. Thus, the fractal dimensions are regionalized and measure the degree of scale invariant clustering of mineralization rather than the control. The regionalized fractal dimensions are between 1.59 and 1.97, varying from moderately clustered (Pb-Cu) to more randomly distributes (Fe and F-Ba) within the control regions.

Mineralizing processes are considered heterogeneous fractals or multifractals. We have generated realistic multifractal simulation models of the mineral distribution, based on multiplicative cascades, involving the geological controls and using the probability values obtained from WofE.