The area-concentration multifractal modelling of a soil geochemical survey performed in the Alfarrobeira–Chança region (covering the Pulo do Lobo Terrane, PLT, and the Iberian Pyrite Belt, IPB) shows that the upper limit of Cu, Zn and Pb background concentrations in soils over PLT metasediments are c. 20, 55 and 20–30 ppm, respectively. In soils derived from the Volcano-Sedimentary Complex of IPB, the Cu, Zn and Pb regional threshold values are within the 25–45, 40–60 and 20–90 ppm intervals, respectively.
The lower limit of Cu, Zn and Pb soil anomalies overlying metasediments are within the 30–50, 90–115 and 45–60 ppm intervals, respectively; over soils with abundant volcanic-derived components those lower limits are scattered in the 30–70, 70–90 and 33–100 ppm intervals. Proximity to mineralized rocks is indicated by Cu, Pb and Zn soil contents above c. 100, 120–150 and 250 ppm, respectively. Most of the anomalies display strong anisotropy.
Separation of Cu, Zn and Pb soil anomalies reflects mostly the existence of distinct metal sources related to original compositional differences or to chemical changes developed during Variscan times. Metavolcanics represent the prevailing sources of Cu and, to a lesser extent, Pb; the prevalent sources of Zn are metasediments of the Phyllite-Quartzite (IPB) and Chança (PLT) Groups.
- Received January 2, 2013.
- Accepted August 16, 2013.
- © 2014 AAG/Geological Society of London