Diogo R. N. 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.

Mineralogy and geochemistry of tin- and germanium-bearing copper ore, Barrigão re-mobilized vein deposit, Iberian Pyrite Belt, Portugal

The Barrigão re-mobilized copper vein deposit, Iberian Pyrite Belt, southern Portugal, is located about 60 km south of Beja and 10 km southeast of the Neves Corvo ore deposit, in Alentejo Province. The deposit is structurally associated with a NE–SW striking fault zone inferred to have developed during late Variscan deformation. The copper ore itself is a breccia-type ore, characterized by up to four ore-forming stages, with the late stages showing evidence of fluid-driven element re-mobilization. The ore is dominated by chalcopyrite + tennantite-tetrahedrite, with minor arsenopyrite, pyrite, and löllingite. The supergene paragenesis is composed mainly of bornite, covellite, and digenite. Whole-rock analyses show anomalous tin and germanium contents, with averages of 320 and 61 ppm, respectively. Electron microprobe analysis of Barrigão ores revealed the germanium and tin to be restricted to chalcopyrite, which underwent late-stage hydrothermal fluid overprint along distinct vein-like zones. The measured zonal enrichment of tin and germanium is related to limited element re-mobilization associated with mineral replacement, which resulted in distinctive mineral disequilibrium. Fluid-driven element zoning affected chalcopyrite and tennantite coevally. The average contents of germanium and tin in chalcopyrite are of 0.19 and 0.55 wt.%, respectively, as confirmed through additional micro-proton-induced X-ray emission (micro-PIXE) analysis. The distribution of tin and germanium in chalcopyrite correlates strongly with iron. Tin and germanium covary. Minute sub-microscopic inclusions of an unknown Cu–Sn–Ge sulphide phase have been detected in chalcopyrite and in small vugs therein. These inclusions hint at a stanniferous sulphide as the most possible host for tin and germanium in chalcopyrite, although the idea of limited incorporation of these two elements through element substitution cannot be completely excluded.

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.