Geochemistry, geochronology and mineralisation potential of the granites in the Central Iberian Zone: The Jalama batholith

Abstract

The Jalama batholith (Spain and Portugal) is one of the numerous granites of the Central Iberian Zone with Sn- and W-associated mineralisation. On the basis of petrographical and geochemical characterisation three types of granite have been distinguished: inhomogeneous granitoids, porphyrytic granites and leucogranites, all of these being peraluminous and subalkaline. All the granites correspond to S-type granites. The field data, the petrography and lack of geochemical affinity relationships of the leucogranites with the remaining granites indicate that their geneses correspond to an independent magma batch and superimposed fractional crystallisation process. The granitic units show subparallel REE patterns. There is a decrease in total REE and an increase in the negative Eu anomaly from the inhomogeneous granitoids to leucogranites. Some leucogranites show relatively low contents of Sn and W almost certainly due to segregation in the magma of a melt rich in water carrying Sn–W. These elements are concentrated in the water phase, which eventually gives rise to Sn–W-associated mineralisation. The ages obtained by means of a whole-rock Rb–Sr isochron for the granites mainly indicate an early intrusion of the inhomogeneous granitoids (319±10 Ma), followed in time by porphyrytic granites (279±9 Ma), which can be associated to the late-post-kinematic granites within the third Variscan deformation phase (D 3). Apart from the average Sn content, the variations of trace elements, principally Sr, Ba, Rb, Th and P, establish that the porphyritic granites and the inhomogeneous granitoids will be barren granites while the leucogranites and the subfacies at the margin of the porphyritic granites correspond to granites with mineralisation potential. It is precisely in these granites of the Jálama batholith that the Sn–W mineralisation is located, for which the criteria utilised has been demonstrated to be effective.