Petrophysical attributes of the Seixo Amarelo-Gonçalo Li-bearing aplite-pegmatites (Guarda-Belmonte area, central Portugal)

Lithium-bearing pegmatites of the Seixo Amarelo-Gonçalo area are hosted within twomica granites of the Guarda-Belmonte area in central Portugal. The pegmatites, often concealed beneath cover material, occur at sizes difficult to detect via airborne geophysical methods but may be detectable at the prospect scale using high-resolution methods such as geoelectrical surveys. However, to maximize contributions from these methods, an understanding of the rock physical properties, e.g., electrical resistivity of pegmatites and their host rocks, is required. In this study, we present results from petrophysical data acquired from laboratory measurements of drill core samples, including volume magnetic susceptibility, electrical resistivity, and chargeability for a suite of pegmatites and their host granite from the Gonçalo Li mine. Though there was large overlap in all petrophysical characteristics between the pegmatites and granites, overall, granites indicated higher magnetic susceptibility than pegmatites, the degree of which increases with alteration associated with pegmatite halo formation and crystallization of Fe-rich minerals (e.g., zinnwaldite, tourmaline). Lepidolite-bearing pegmatites have lower magnetic susceptibility compared to their barren counterparts or those containing zinnwaldite. Unexpectedly, based on laboratory measurements, the electrical resistivity values of granites are higher than those of pegmatites, and highest in granites containing tourmaline, followed by the general two-mica granite of the area and lastly by zinnwaldite-bearing granites in alteration halos. In pegmatites, resistivity values were highest in pegmatites containing zinnwaldite, followed by barren pegmatites, with lepidolite-bearing pegmatites having the lowest resistivity values. The lone dolerite sample, representing late veins that crosscut the deposit, has the lowest resistivity value. Results from chargeability measurements presented as phase angle proved inconsistent owing to overall weak polarization potential and also samples dominated by coarse-grained minerals for studied lithologies, which yielded scattered data. The petrophysical work presented here serves as a supplement for future geophysical investigations in pegmatite exploration in the region and elsewhere with similar geological settings.