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

Abstract

The present study examines a wide Cu-soil geochemistry survey covering an area of ca. 570km2 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) >40ppm when gabbroic (±diorite) rocks of BIC dominate over the country metavolcanic/metasedimentary sequences; (ii) 30–40ppm when a mixing of gabbroic rocks of BIC and BAOC exists; (iii) 20–30ppm when mesocratic to felsic BIC intrusive rocks predominate over gabbroic rocks; and (iv) <20ppm 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≤500ppm and 1.8≤AAR≤2.5 trace Cu anomalies developed within gabbroic rocks without significant structural control; (ii) Va≤500ppm and 2.2≤AAR≤2.9 indicate Cu anomalies associated with major shear zones and/or subsidiary structures; (iii) Va≤600ppm 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≤600ppm 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.