In situ LA-ICP-MS U-Pb age of cassiterite from the Auxiliadora Sn-W vein type deposit, northwestern Spain Iván Losada 1 , Iñigo Borrajo 2 , Joaquín E. Ruiz-Mora 3 , Luis M. Rodriguez-Terente 4 , John M. Hanchar 5 , Fernando Tornos 2 1 University of Salamanca 2 Instituto de Geociencias (IGEO, CSIC-UCM) 3 Consultant. A Coruña 4 Museum of Geology, University of Oviedo 5 Department of Earth Sciences, Memorial University of Newfoundland Abstract. The Auxiliadora deposit is a small mine located in the town of Noia, Galicia (northwestern Spain). Here, mineralization occurs in several quartz veins which crop out along the coast. This work is focused in the cassiterite mineralization which occurs intergrown with a micaceous selvage along the veins. The obtained U-Pb ages in cassiterite of 285.2 ± 2.4 Ma, corresponds to the last stage of the Hercynian orogenesis and is coeval with that obtained in other deposits in the area. 1 Introduction Some of the most important Sn and W metallogenic districts in the world are found in the European Variscan belt. One of these districts is the Iberian massif (Spain and Portugal). Here, numerous Sn-W deposits occur in the northwesternmost region of Galicia, associated with the youngest Variscan magmatism at ca. 300 – 285 Ma (Borrajo et al. 2022; Losada et al. 2022). Several high grade mineralization like San Finx (Losada et al 2022), Santa Comba (Cuenin 1982), Fontao (Nesen 1979) and Penouta (Llorens et al. 2017) stand out, however there are dozens of small deposits that were mostly mined during the Second World War. The Auxiliadora Sn-W vein type deposit is one of these small deposits, and it is part of a group of Sn- W mineralization located in the municipalities of Noia and Lousame (Ruiz Mora 1982). Among them, the San Finx, located 7 km to the southwest is the most important (figure 1). In this study we present LA-ICP-MS U-Pb ages on cassiterite from the Auxiliadora deposit. This mineral is chemically and physically resistant, and its U-Pb isotopic composition is increasingly being used as a reliable geochronometer of the Sn mineralization event (e.g., Yuan et al. 2011; Carr et al. 2020). 2 History The mining history in the area of Noia can be start in various concessions requested in the area of Santa Cristina de Barro (Noia), around the second half of the 19 th century, with some prospecting work or mining in any case only testimonial. At the end of 1937, the "Auxiliadora" concession was requested, covering practically the entire peninsula immediately to Noia with 30 hectares. During the 1940s, due to the extremely high prices of tungsten and the good tin market, work was carried out clandestinely throughout the area, without mechanization or hardly technical supervision. In 1945, a hundred of workers produced between 40 and 50 kg per day of cassiterite concentrates. The ore was sent for magnetic separation to the facilities of a mine that dredged the sediments of the ria and that was located a few hundred meters to the north. The mining of the numerous veins outcropping on the coast was completely superficial, being more important than the underground mine, equipped with two shafts of 20 meters each. Another part of the mined deposit was the alluvium or coastal maritime sediments, which supported the bulk of the activity due to its ease of exploitation, and from which cassiterite, wolframite, scheelite and rutile concentrates were obtained. The mine worked during the Second World War and another 5 years in the following decade. The production obtained is difficult to assess, but perhaps it was around 60 t of cassiterite in total and about 20 t of wolframite during the years 1943-1944. Around 1956 the mine was closed. 3 Geological setting Auxiliadora is located on the Barro peninsula (Figure 1), in the town of Noia. The best outcrops are found at Abruñeiras-Punta da Cruz beach. The host rock is mainly the Borneiro orthogneiss, from the Malpica Tui high-pressure, low- temperature unit (Llana Fúnez 2020). It is a coarse- grained, heterogeneously deformed rock with a granodioritic composition. In an area of ca 15 km surrounding the mine, syn-late to post-variscan granites crop out. The deposit is composed of dozens of quartz veins with irregular thickness between 1 mm and 5 cm. The veins trend 090-120/80 NE. They develop a muscovite-rich selvage with cassiterite at the contact with the host rock (Figure 2). The quartz infilling is absent in the narrow zones of the veins where they are composed mainly of muscovite. Other minerals present are wolframite, scheelite, chalcopyrite and pyrite.