Delimiting geochemical anomalies in the exploration of covered deposits with multifractal methods and using stream sediment data from the Iberian Pyrite Belt, Southwest Iberia

Abstract

The Iberian Pyrite Belt (IPB) is a world-class province of Upper Paleozoic age for volcanic- and shale-hosted massive sulfides that includes 5 of the 10 supergiant (>150 Mt) deposits of their kind in the World. The volcano-sedimentary sequences hosting the ores are locally exposed, commonly covered by flysch-type successions of variable thickness (the Baixo Alentejo Flysch Group, BAFG) as a result of primary (sedimentary) features and superimposed effects due to Variscan folding and tectonic stacking. In addition and when affected by late-developed geomorphic depressions, the exhumed Paleozoic basement is also capped by Meso-Cenozoic sediments. The total thickness of cover sequences on top of the litho-stratigraphic sequence of interest can reach 100′s of meters, creating problems for mineral exploration.A large stream sediment (multi-element) geochemical database for the IPB was done in the 90′s during a wide-ranging regional survey. This database was reassessed by means of multifractal methods in view of their faculty to enhance the detection of faint geochemical anomalies and provide geochemical indicators to deep buried ore systems. Several anomalies were detected, namely for Cu, Zn, and Co along NW-SE to WNW-ESE trends. The main anomalous geochemical trends identified include zones that: (i) extend along the Mira Formation of BAFG, opening for the first time two non-traditional areas for further exploration studies; (ii) run along the boundary between the Mira and Mértola Formations of BAFG, probably related to different geochemical signatures and late-developed, fault-controlled, epigenetic ore systems; and (iii) overlap windows of volcanic-sedimentary sequences within the Cenozoic detrital cover and the site below which the deep buried deposit of Lagoa Salgada is located.