Tungsten mineralization associated with the Argemela microgranite (Central Portugal)

Abstract

The Argemela microgranite is a late- to post-tectonic granite (305–300 Ma), and is an example of a late Variscan small intrusion of W-enriched granite with magmatic cassiterite, columbite and Li-micas. The occurrence of this type of magmatism is very rare in the European Variscan Belt. A quarry where this microgranite intrusion has been exploited for ceramic applications revealed the border of an aplite cross-cut by two types of quartz veinlets: type I with quartz, K-feldspar ± amblygonite; and type II with quartz and wolframite mineralization, thus allowing the study of a W-mineralization which is intimately associated with a highly-differentiated magmatic system. In this work, detailed mineralogical, geochemical and fluid inclusion studies are presented, in an attempt to reconstruct the P–T-x conditions responsible for different stages of fluid evolution. We have found magmatic and hydrothermal aqueous-carbonic fluids (H2O–CO2–CH4–N2–NaCl) associated with the late crystallization of the Argemela microgranite, and also responsible for the later formation of the two types of veinlets. The Argemela fluid system is characterized by an increase of CO2 in the volatile phase and a decrease in salinity throughout the transition from magmatic to hydrothermal stage. Most importantly, the late stages correspond to the period of tungsten deposition at pressures below 100 MPa.