Recognizing metasedimentary sequences potentially hosting concealed massive sulfide accumulations in the Iberian Pyrite Belt using geochemical fingerprints

Abstract

Promising advances in the development of new greenfield and brownfield exploration methods and methodologies for massive sulfide ore-systems in the Iberian Pyrite Belt (IPB) have been made over the last two decades. However, the effects of lateral and vertical facies variations in metasedimentary piles forming the two main lithostratigraphic units that potentially host massive sulfide deposits [the Phyllite-Quartzite Group (PQG) and the Volcanic-Sedimentary Complex (VSC)] on mineral exploration remain unknown. Herein we report on results of a multi-element geochemical study of 133 samples representing PQG and VSC metasedimentary rocks from the Albernoa area (≈713 km2); this dataset is complemented by whole-rock Sr, Nd and Pb isotopic data for a subset of 12 VSC metasedimentary rock samples.The prevalent siliciclastic component in the analyzed rocks derives from an evolved “felsic source” (as suggested by the Zr/Sc, Th/Sc, Eu/Eu* relationships) that, according to CIA values, should have been affected by significant weathering before they were deposited in the basin from Middle Devonian to Visean time. The siliciclastic component comprises clay and (fine-grained) sand whose relative abundance can be distinguished on the basis of Al2O3/TiO2, Sc/TiO2, Ga/TiO2, Zr/TiO2 and SiO2/Al2O3 ratios. These terrigenous components are, in places, variably mixed with volcanic-derived fractions, leading to the formation of tuffaceous metapelites typical of the VSC sequences but also present in the upper stratigraphic sections of the PQG. In general, VSC samples display the following parameters 2.5 ≤ SiO2/Al2O3 ≤ 6.5, 0.02 ≤ TiO2/Al2O3 ≤ 0.06 and 0.2 ≤ (CaO + Na2O + K2O)/Al2O3 ≤ 0.25, whereas PQG samples display 2.5 ≤ SiO2/Al2O3 ≤ 12, 0.08 ≤ TiO2/Al2O3 ≤ 0.58 and (CaO + Na2O + K2O)/Al2O3 ≤ 0.3. Fe2O3/TiO2 ratios ≥ 10 and Al2O3/(Al2O3 + Fe2O3 + MnO) ratios ≤ ≈0.6 indicate the strong influence of an early-developed (prior to Variscan metamorphism and deformation) hydrothermal component on the prevalent siliciclastic component sometimes mixed with a variable volcanic-derived fraction. Moreover, values of (As + Sb)/Sc, (Cu + Zn + Pb)/Sc and 5 × [(Fe2O3 + MgO + MnO)/Al2O3] ratios between 1 and 10 indicate the influence of seafloor metasomatism processes, and ratios ≥ 10 indicate close proximity to hydrothermal discharge potentially associated with massive sulfide mineralization.The Sr, Nd, Pb isotopic data show that VSC metasedimentary rocks are mostly derived from old, reworked, upper continental crust. Isotope ratios characterizing the main siliciclastic component are somewhat disturbed by the significant presence of volcanic-derived components and/or mineral changes due to early hydrothermal alteration/mineralization processes, which should have involved isotopically distinct fluids.