Hercynian acid magmatism and related mineralizations in Nortern Portugal

Abstract

Cabeceiras de Basto (CB), Vieira do Minho (VM) and Vila Pouca de Aguiar (VPA) granite plutons are located in the so-called Central Iberian Zone, northern Portugal. U-Pb zircon and monazite geochronology yielded the minimum emplacement age of 311 Ma for CB and the crystallization ages of 311 Ma for VM and 299 Ma for VPA, constraining the time interval of the third Hercynian deformation phase, relative to which these massifs can be grouped as follows: syn- to late-tectonic (CB and VM) and post-tectonic (VPA) granites.CB is a composite peraluminous two-mica granite pluton, occupying the core of a N 130° E antiform. Geochemical and isotopic data suggest a heterogeneous crustal source of mid-Proterozoic age (1.2 Ga). The granites are specialized in Sn, Li and, to a lesser extent, W mineralizations. Fractional crystallization is the main concentrator mechanism for these elements. Late-magmatic hydrothermal alteration processes (feldspar albitization and plagioclase and biotite muscovitization) enhanced the concentration promoted by the magmatic differentiation.Both the VM and VPA plutons are controlled by deep regional fault zones and consist of composite biotite granites, generated in the lower crust and installed in higher structural crustal levels, resulting in thermal metamorphism (P:2 kb; T: 500-600°C) capable of generating crustal convecting fluids. They are spatially related to important Sn, W and Au mineralizations. Although they are not highly specialized, they have contributed significantly as heat sources for the ore concentrations