Carlos J. P. Rosa

Geochemistry of Volcanic Rocks, Albernoa Area, Iberian Pyrite Belt, Portugal

Volcanic rocks from the Albernoa area essentially consist of calcalkaline quartz-feldspar-phyric coherent and hyaloclastic rhyodacites, and alkaline and tholeiitic basaltic rocks. Binary plots show that high-field-strength elements behaved as immobile elements, and allow for the identification of two felsic rock suites. Silica and alkali mobility, however, is reflected by compositional scatter on major-element diagrams: felsic rocks display rhyolitic to apparent andesitic compositions, and the mafic rocks display basaltic to apparent dacitic compositions. Silica and alkali mobility was focused along fracture networks and within the matrices of hyaloclastic breccias. Problematic classification of geotectonic setting for the felsic rocks is a reflection of anomalous high-field-strength element systematics; this probably results from a low temperature of crustal fusion, causing decreased solubility of the refractory phases in which these elements reside. The mafic rocks, however, evidently were generated in an extensional setting without involvement of subduction; the existence of apparent arc signatures was caused by crustal assimilation. This is compatible with volcanism in an attenuated continental lithosphere setting, due to strike-slip tectonics during oblique continental collision.

Felsic pyroclastic and effusive volcanic facies hosting the Neves Corvo massive sulfide deposit, Iberian Pyrite Belt, Portugal

The combination of biostra-tigraphic data with the facies architecture of the Neves Corvo massive sulfide deposit allows dating of the two major volcanic events identified in the mine stratigraphy. Both volcanic episodes are represented by submarine and proximal to source vent deposits, with intrabasinal origin and rhyolitic composition. Thick pyroclastic pumice-rich facies of the late Famennian and youngerlavas of the late Strunian (latest Famennian) constitute the host sequence of the massive sulfides. The hydrothermal mineralizing event occurred after the emplacement of the rhyolitic lavas.

Introduction and Geological Setting of the Iberian Pyrite Belt

The 250 × 20–70 km Iberian Pyrite Belt (IPB) is a Variscan metallogenic province in SW Portugal and Spain hosting the largest concentration of massive sulphide deposits worldwide. The lowermost stratigraphic unit is the early Givetian to late Famennian-Strunian (base unknown) Phyllite-Quartzite Group (PQG), with shales, quartz-sandstones, quartzwacke siltstones, minor conglomerate and limestones at the top. The PQG is overlain by the Volcanic Sedimentary Complex (VSC), of late Famennian to mid-late Visean age, with a lower part of mafic volcanic rocks, rhyolites, dacites and dark shales, hosting VHMS deposits on top (many times capped by a jasper/chert layer), and an upper part, with dark, purple and other shales and volcanogenic/volcaniclastic rocks, carrying Mn oxide deposits. The VSC is covered by the thousands of meters thick Baixo Alentejo Flysch Group of late Visean to Moscovian age. The VSC comprises a bimodal submarine volcanic succession, with VHMS deposits spatially associated to dacites and rhyolites corresponding to effusive/explosive lava-cryptodome-pumice cone volcanoes. The lava/domes consist of coherent lithofacies surrounded by clast-rotated hyaloclastite breccia and minor autobreccia, with massive VHMS ore at the top of the felsic effusive units and stockworks in the autoclastic and pyroclastic breccias. The eastern IPB rocks are intruded by the voluminous Sierra Norte Batholith (tonalite-trondhjemite-granodiorite, TTG series). Felsic volcanic rocks (dacite to high-silica rhyolite) predominating over basalts and dolerites, belong to the calc-alkaline series and plot mostly in the within-plate field in tectonic discriminative diagrams. Several periods of volcanism, from 384 to 359 Ma are recognized. Dacites and rhyolites exhibit Nd and Sr enrichment, typical of a crustal signature, and their overall geochemistry suggests generation by fractionation/partial melting of amphibolites at low pressure. Trace elemental modelling of the basic rocks, involving tholeiitic lavas and alkaline basaltic lavas and dolerites, points to mixing between E- and N-MORB and assimilation of crustal material. Variscan NW-SE/W-E-trending and SW- or S-verging folds (with NE- or N-dipping planar cleavage) and thrusts, occur in west-central and eastern IPB, respectively. In late to post-Variscan time strike-slip oblique faults formed, either N-S to NNW-SSE or NE-SW to ENE-WSW, dextral or sinistral (both extensional), respectively. The first set hosts late Variscan Cu-Pb-Ba veins and Mesozoic(?) dolerite dykes. IPB contains over 90 VHMS deposits, estimated before erosion at >1700 Million tonnes (Mt), with 14.6 Mt Cu, 34.9 Mt Zn, 13.0 Mt Pb, 46,100 t Ag, 880 t Au and many other metals, particularly Sn. Eight of these are giant (≥100 Mt) VHMS deposits, namely Rio Tinto, Tharsis, Aznalcollar-Los Frailes, Masa Valverde, Sotiel-Migollas and La Zarza (Spain) and Neves Corvo and Aljustrel (Portugal). The VHMS deposits are of the felsic-siliclastic type and mostly of the Zn–Pb–Cu and Zn–Cu–Pb metal content types. The deposits range in thickness from 1 m to tens of meters (plus increase from tectonic stacking) and up to a few kilometers in extension, and many are underlain by large stockwork zones. Their age is either Strunian (palynological age) in the southern IPB or mostly Tournaisian in the northern IPB. The major massive ore minerals are pyrite, sphalerite, chalcopyrite, galena (and cassiterite at Neves Corvo), also present with dominant quartz-chlorite-sericite-carbonate in the stockwork ore. Sericite and chlorite were also formed from additional alteration in the hanging wall rocks. Metal zonation in most VHMS deposits consists of a Cu-rich stockwork and base of the massive ore, with Zn–Pb massive ore above and extending laterally. S-, O-, H- and C-isotope data indicate that ore-forming fluids contain predominant or exclusive modified seawater. A magmatic fluid contribution to the dominant seawater has been proposed for some deposits. The deposits are exhalative or formed by shallow subsurface replacement of either muds/shales or coherent felsic volcanic rocks.