Multi-fractal modelling and spatial Cu-soil anomaly analysis along the southern border of the Iberian Terrane in Portugal

The present study examines a wide Cu-soil geochemistry survey covering an area of ca. 570 km2 along the southern border of the Iberian Terrane in Portugal, from Ferreira do Alentejo to Serpa. The surveyed area covers the southern border of the Ossa Morena Zone (OMZ), largely dominated by the Beja Igneous Complex (BIC) and adjoining remnants of the Beja Acebuches Ophiolite Complex (BAOC), where different ore-forming systems are known to occur. The methodology employed combines geostatistical analysis and multi-fractal modelling to compute threshold values and assess the spatial variability of the Cu-soil data set.Values of regional threshold are variable and reflect the prevalence of different rock types occurring at the southern Iberian Terrane: (i) > 40 ppm when gabbroic (± diorite) rocks of BIC dominate over the country metavolcanic/metasedimentary sequences; (ii) 30–40 ppm when a mixing of gabbroic rocks of BIC and BAOC exists; (iii) 20–30 ppm when mesocratic to felsic BIC intrusive rocks predominate over gabbroic rocks; and (iv) < 20 ppm where the Cainozoic sediments cover a significant part of the surveyed area. Different types of Cu-soil anomalies are related to spatially co-existent but distinct (syngenetic or epigenetic/structurally controlled) Cu-sources, which can be distinguished on the basis of local anomalous values (Va) and anisotropy axial ratios (AAR) as follows: (i) Va ≤ 500 ppm and 1.8 ≤ AAR ≤ 2.5 trace Cu anomalies developed within gabbroic rocks without significant structural control; (ii) Va ≤ 500 ppm and 2.2 ≤ AAR ≤ 2.9 indicate Cu anomalies associated with major shear zones and/or subsidiary structures; (iii) Va ≤ 600 ppm and 1.5 ≤ AAR ≤ 3.3 point to very local Cu anomalies that usually define alignments related to strike–slip fault zones; and (iv) Va ≤ 600 ppm and 1.8 ≤ AAR ≤ 2.2 correspond to several anomalies following major lithological contacts, namely, between intrusive rocks of BIC and the metamorphic country rock sequences.

► A 570 km2 Cu-soil geochemistry survey in the southern Iberian Terrane is examined.
► Thresholds modelled with multi-fractals reflect the prevalence of different rocks.
► Data spatial variability and anisotropy are modelled with geostatistical analysis.
► The robustness of the methodology is systematically tested with field data.
► Validated anomalies relate to syngenetic or epigenetic/structurally controlled sources.