Geostatistical approach to the study of the magnetic susceptibility variation: Lamas de Olo Pluton case study

Abstract

The Lamas de Olo Pluton, in northern Portugal, is a post-tectonic pluton located in the northern part of the Central Iberian Zone of the Iberian Variscan belt. It is a composite massif comprising different granites: Lamas de Olo, Alto dos Cabecos and Barragem. Magnetic susceptibility was measured in 349 oriented cores from 48 sampling sites across the pluton. The measured magnetic susceptibility (Km) values range between 21 µSI (or 21 × 10− 6 SI) and 2769 µSI for all studied granites. The application of the geostatistical methodologies leads to a more detailed mapping of the magnetic susceptibility data and provided a quantification of its spatial dispersion. Kriging interpolation allows the visualization of two different areas in the Lamas de Olo Pluton: (1) areas with higher magnetic susceptibility values, typical of magnetite-bearing granites (Km ≥ 1200 µSI) and (2) areas with lower values, which correspond to ilmenite-type behavior granites (21 µSI < Km < 1200 µSI), showing the heterogeneity of their granites, with a predominance of ilmenite-type granites. Alto dos Cabecos and Barragem are classified as ilmenite-type granites and Lamas de Olo granite presents both behaviors. The mineralogical and geochemical study of the Lamas de Olo granite with different behaviors does not show significant differences between the two types, allowing us to conclude that the Lamas de Olo granite was originated by the same magma and its mineralogical composition varied during the rise and emplacement of the magma implying the alteration of magnetite into hematite.