Magnetic mineralogy of Variscan granites from northern Portugal: an approach to their petrogenesis and metallogenic potential

Abstract

Northern Portugal is characterized by the occurrence of numerous W hydrothermal deposits spatially associated with granites. The primary goal of this work is to establish a relationship between the magnetic behavior of the granites and the redox conditions during magma genesis, as this can influence the occurrence of mineralizations, namely of W (Mo). To this end, the magnetic mineralogy of the granites of the Lamas de Olo Pluton, a posttectonic pluton in northern Portugal, with associated W (Mo) occurrences was characterized and compared with the magnetic mineralogy of other post-tectonic Variscan plutons. This pluton is composed of different biotite granites: Lamas de Olo, Alto dos Cabeços and Barragem. To better characterize its magnetic behavior, different analytical techniques that complement previous magnetic susceptibility studies were performed. The magnetic mineralogy of Lamas de Olo Pluton was then compared with other post-tectonic Variscan plutons such as the Vila Pouca de Aguiar, Peneda-Gerês and Lavadores-Madalena plutons. The presence of magnetite in some of these granites is important because it points to melt-oxidized conditions not commonly found in Iberian Variscan granites. Our study shows that granite areas where magnetite and/or magnetite/ilmenite coexist are important targets for W (Mo) mineralizations. The results indicate that a few plutons have granites with a complex redox history which leads to the formation of magnetite and ilmenite. Magnetic mineralogy. Variscan granites. Post-D 3 biotite granites. Central Iberian Zone. W-Mo mineralizations. KEYWORDS

A Multi-Method Approach to Geophysical Imaging of a Composite Pluton in North Portugal

“…LOP granites are mostly composed of quartz, plagioclase (oligoclase, oligoclase-andesine, and albite), K-feldspars (perthitic orthoclase, and microcline), and biotite. Muscovite I (Ms I), muscovite II (Ms II; post-magmatic muscovite), zircon, sphene, allanite, fluorite, hematite, magnetite, ilmenite, chlorite, rutile, apatite, goethite, epidote, and tourmaline are present as accessory minerals (e.g., [14]). The LO granite is the most abundant in outcrop and is characterized by a medium-to-coarse-grained porphyritic granite (Biotite (Bt) > Muscovite (Ms)).…”

“…The AC corresponds to a fine-to-medium-grained porphyritic granite (Bt > Ms). The younger granite, which cut the other two granites, is BA which outcrops in the center of the pluton and near a dam and is classified as leucocratic fine-to-medium-grained and slightly porphyritic granite (Bt = Ms I + Ms II) [14,34,35].…”

“…In the present work, the geophysical methods were applied to post-orogenic Variscan granites from the Lamas de Olo pluton. The magnetic susceptibility and gravimetric studies were previously performed by Cruz et al [5,14,15], and the radiometric approach is now overlapped and analyzed together with the data previously obtained.…”

Geodynamics of the Central Tethyan Belt Revisited: Inferences From Crustal Magnetization in the Anatolia‐Caucasus‐Black Sea Region

“…In particular, ocean floor gabbros from a 1.2 Ma oceanic crust have Curie temperatures of 550–600°C; but volcanics and dikes from the same area have Curie temperatures of 100°–300°C due to differences in magnetic mineralogy (Varga et al., 2004). Differences in magnetic mineralogy also lead to significant variations in Curie temperatures of Variscan granites (Cruz et al., 2020) and rhyolites, andesites, and dacites from young volcanoes (Jackson & Bowles, 2014) (Figure 2c).…”

“…Black line labeled C = 4.0 marks the upper limit of reported empirical values. (c) Curie temperature of titanomagnetites as function of the x‐parameter (left) (Hunt et al., 1995); other magnetic minerals (middle); and young volcanoes in Cascadia, Variscan granites in Portugal, and rock examples from young oceanic crust (right) (Cruz et al., 2020; Jackson & Bowles, 2014; Varga et al., 2004). Vertical axis refers to all panels.…”