Carlos Inverno

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.

U-Pb geochronology of felsic volcanic rocks hosted in the Gafo Formation, South Portuguese Zone: the relationship with Iberian Pyrite Belt magmatism

Abstract Felsic volcanic rocks exposed in the Frasnian Gafo Formation, in the Azinhalinho area of Portugal, display very similar geochemical signatures to volcanic rocks from the Iberian Pyrite Belt (IPB), located immediately to the south. The similarities include anomalously low high field-strength elements (HFSE) concentrations, possibly caused by low-temperature crustal melting, which translate into classification problems. A geochronological study, using laser ablation-inductively coupled plasma-mass spectrometry (LA- ICP-MS) analyses of zircon grains from these rocks, has provided concordia ages of 356±1.5 Ma and 355±2.5 Ma for two samples of rhyodacite porphyry, and 356±1.4 Ma for a granular rhyodacite. These results show that volcanism at Azinhalinho was broadly contemporaneous with IPB volcanism, widely interpreted as being of Famennian to Visean age. Considering that the host rocks of the Azinhalinho volcanic rocks are Frasnian, and therefore deposited synchronously with the Upper Devonian Phyllite- Quartzite Group sedimentation in the IPB basin, the radiometric ages imply that the Azinhalinho felsic rocks are intrusive and likely represent conduits or feeders to the volcanism of the IPB.

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.

Metallogenesis of the São Martinho and Mosteiros Gold Deposits, Tomar Cordoba Shear Zone, Portugal

The São Martinho and Mosteiros orogenic gold deposits are located in east-central Portugal within the WNW-ESE-trending Tomar Cordoba Shear Zone (TCSZ). This important structure separates two major tectonostratigraphic zones within the Iberian massif, the Central Iberian Zone to the north and the Ossa Morena Zone to the south. The TCSZ comprises a geologically complex and diverse zone of intense Variscan deformation and metamorphism contemporaneous with sinistral displacements of up to 300 km. Within the shear zone, rocks are arranged in a flower structure with the oldest rocks, the Blastomylonitic Belt (BB), at its core. The TCSZ is intruded by both syntectonic (syn-Variscan) and late- to post-tectonic granitoids, and its NE sector is marked by Westphalian-age wrench faults. Gold deposits are hosted in the Neoproterozoic Série Negra rocks, which is metamorphosed to greenschist facies north of the Blastomylonitic Belt at Mosteiros, and to amphibolite facies to the south at São Martinho. São Martinho is characterized by two distinct episodes, RS I and RS II, of gold mineralization. The first, RS I, comprises both disseminated and veinlet styles of mineralization and is closely associated with foliation-parallel quartz I veinlets. The ore minerals in RS I are: deformed arsenopyrite I + pyrite I + pyrite II + chalcopyrite I + gold I. The second episode of mineralization (RS II), of probable magmatic origin, is associated with quartz II veins that cross-cut the foliation. The paragenesis is characterized by arsenopyrite II + pyrrhotite II + pyrite III + chalcopyrite II + loellingite + gold II. At São Martinho alteration is difficult to recognize but silicification, chloritization, muscovitization/sericitization ± tourmalinization, albitization, and carbonatization are recognized. Mineralization occurred after peak metamorphism in the Early Carboniferous, contemporaneous with the Variscan orogeny. An early contribution from metamorphic devolatilization to the ore-forming fluids initiated metal precipitation and alteration within the Série Negra metasediments, followed by a later episode of fluid circulation and mineralization linked to the emplacement of syntectonic granitoids. At São Martinho, the formation of Late Carboniferous wrench faults provided conduits for hypersaline magmatic fluids derived from large, late- to post-tectonic granitic magmas. These fluids remobilized early-formed orogenic lode gold forming a second generation of mineralization associated with high-temperature minerals such as loellingite. Paragenetic studies suggest that both the Mosteiros and the São Martinho deposits share the characteristics of the early RS I mineralizing event, but only the latter has been subjected to the later remobilization event associated with granitoid magmatism. The recognition of the different styles of alteration and mineralization provide valuable tools for precious mineral exploration within the TCSZ.