Multi-elemental and isotope geochemistry of metapelites from the Volcano-Sedimentary Complex (Iberian Pyrite Belt, Portugal) : unravelling fingerprints of different sources, sedimentary environments and exhalative-hydrothermal activity

Abstract

Over the last two decades, promising steps have been made in the development of new greenfield and brownfield exploration methods for massive sulphide ore-forming systems in the Iberian Pyrite Belt (IPB). 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. Massive sulphide deposits of the IPB document the progression of ore-forming processes in the SW Iberian Variscides, throughout uppermost Devonian to Early Carboniferous. Sulphide ores are usually hosted in volcanic-dominated or shale-dominated successions belonging to the VSC (Late Fammenian to Late Visean). Yet, several important orebodies at Neves Corvo are sitting within upper sections (Fammenian) of the early deposited Phyllite-Quartzite Group (PQG). Four key sectors of the Portuguese segment of IPB were revisited and 262 samples collected to characterize the geochemical features of metapelites from PQG and VSC sequences in barren and fertile settings. The selected sectors include active (Neves Corvo and Aljustrel) and old (Lousal) mines, besides promising brownfield (Gavião) and greenfield (Sesmarias) prospects, and other lithostratigraphic sections without known mineralization. The composition of PQG and VSC metapelites is similar and mainly controlled by a mixture of clayey-derived and quartz sandy-derived components largely resulting from different sources of granitic to granodiorite/quartz-diorite composition. Mineral transformations during diagenesis and/or hydrothermal alteration/mineralization partially overprinted the primary composition. During sedimentation/diagenesis, prevalent redox conditions were mostly confined to oxic environments, at places transiting to suboxic. Subsequent multi-stage interaction with reducing hydrothermal fluids (to which the sulphide mineralization is related) generated heterogeneous anoxic signs. Significant hydrothermal/mineralization imprints exist when Fe2O3/TiO2 ≥ 10 and Al2O3/(Al2O3+Fe2O3+MnO) ≤ 0.6. The 5x[(Fe2O3+MgO+MnO)/Al2O3], (Cu+Zn+Pb)/Sc and (As+Sb)/Sc geochemical ratios were recently proposed to separate barren from altered/mineralized metasedimentary successions in the IPB. When all these three ratios are above 10, a realistic vectoring towards massive sulphide accumulations occurs. From this dataset, 98 samples were chosen to conduct a Pb-Nd-Sr isotope study from different sections of the PQG and VSC successions (from Givetian to Upper Visean), including footwall and hanging wall domains of mineralized horizons in Neves Corvo, Aljustrel and Lousal. An extensive review of the isotopic data for pelite rocks in the IPB allow the evaluation of their sources and the potential of radiogenic isotopes as exploration tools, when combined with trace element geochemistry. The whole-rock Nd and Sr isotopic compositions along with Th/Sc ratios shows that PQG and VSC sediments are mostly composed of a silicilastic mix supplied by the dismantling of an old basement: -11 ≤ ƐNdi ≤ -8; 87Sr/86Sri up to 0.727; and TDM ages ranging from 1.08 to 1.91 Ga (̅ = 1.67 ± 0.16 Ga). The rising of ƐNdi values recorded by many samples of upper VSC (up to ± 0.2 in the Aljustrel sector) is interpreted as a result of local incorporation of volcanic-derived contributions completed during sedimentation processes during and after Upper Tournaisian. The PQG succession, at times complemented by VSC thick piles, represent the main crustal reservoirs of metals that supplied the hydrothermal inflows in IPB, which were dominated by modified seawater. Other sources, more radiogenic, should be involved in the IPB metal budget when the role of Sn- and Cu-rich fluids became central in the ore system, as documented for the Neves Corvo deposit. The increasing of 207Pb/204Pbi ratios in sediments is sensitive to the circulation of mineralizing fluids. On the contrary, significant rise in 206Pb/204Pbi (and 207Pb/204Pb) values in sediments bearing disseminated sulphides other than pyrite only occur when conditions for a protracted interaction with radiogenic hydrothermal inflows of moderate to high temperature existed. The proximity to massive sulphide ore systems could be recognized in sedimentary levels of PQG and VSC whenever 207Pb/204Pbi > 15.60 together with other multi-elemental geochemical ratios such as Fe2O3/TiO2 and (Cu+Zn+Pb)/Sc > 10