São Pedro das Águias skarn: A W-(Sn) deposit from the Douro Scheelite Belt, Northern Portugal

False colour mineral maps of the typical skarn lithologies in the S. Pedro das Águias deposit. A) L type; B) M type; C) F Type; D) L/M type. The scale bar corresponds to 4 mm. • S. Pedro das Águias (SPA) is a W(Sn) skarn deposit associated with a S-type peraluminous Variscan muscovite granite. • Main W mineral is scheelite occurring in different types of skarns. • Tin content in skarns is justified by the presence of malayaite, cassiterite and stokesite. • These skarns can be also distinguished by their rare earth elements (REE) contents. • Scheelite generations reveal different REE patterns recording the history of the SPA hydrothermal system. The São Pedro das Águias skarn deposit is one of the most significant tungsten deposits in the Douro Scheelite Belt, located in Northern Portugal. It is situated at the contact between the Bateiras Formation of the Schist-Greywacke Complex and contains scheelite as the primary tungsten mineral, along with some minor tin minerals. A recent evaluation suggests indicated resources of 1,495 kt with 0.55 wt% WO 3 and inferred mineral resources of 1,230 kt with 0.59 wt% WO 3 . The deposit hosts scheelite (CaWO 4 ) as the major tungsten mineral, along with minor tin minerals. Two distinct skarn layers are identified at the deposit, differentiated by their stratigraphic positions: an Upper Skarn and a Lower Skarn, separated by approximately a 50-meter-thick sequence of schists. Based on its structural, mineralogical, and geochemical characteristics, three main types of skarns can be distinguished: L Type, characterized by pink and green bands, fine to medium-grained with minor scheelite (0.5–0.6 vol%); M Type, banded, medium to coarse-grained, with more abundant scheelite (0.8–3.6 vol%); and F Type, fine-grained, scarcely banded to massive, with scheelite (1.5 vol%) and abundant tin minerals (3.1 vol%), primarily malayaite. A typical characteristic of this W-(Sn) skarn is its high content in W (from 2,260 to over 10,000 ppm), Sn (from 405 to 7,770 ppm), and F (from 6,820 to over 20,000 ppm). These skarns can be distinguished by their rare earth element (REE) contents, with higher values (average ΣREE = 130 ppm) in the L Type skarn and lower values (average ΣREE = 66 ppm) in the M Type skarn. The composition of scheelite grains can vary within the same sample, exhibiting different chemical compositions and REE patterns, suggesting a complex formation process. In the richest M Type skarn, scheelite is characterized by REE depletion. Its Mo content is always low: less than 123 ppm in the L Type and less than 26 ppm in the M Type. These scheelite compositions can be regarded as characteristic of reduced skarns. This study shows that scheelite records the history of the hydrothermal system related to the formation of São Pedro das Águias reduced skarns. Scheelite deposition is triggered by the crystallisation of F-rich minerals during skarn formation, which caused F and REE depletion in the fluid and consequent saturation in W.