Chemical and mineralogical changes of two-mica Hercynian granites of the Viseu region (central Portugal) induced by contact metamorphism of younger granites: implications for the genesis of Sn-W ore deposits

Late Hercynian porphyritic biotite granites of the Viseu region (Central Portugal) develop a 2 km wide contact aureole on older Hercynian muscovite-biotite granites. Several textural, mineralogical and geochemical transformations were recognized in the two-mica granites of the aureole. The most relevant mineralogical transformation is the progressive breakdown of muscovite, decreasing the modal abundance on average from ca. 11% to less than 2% near the contact; muscovite is replaced by K-feldspar, which increases modally from ca. 17 to 33%. Other dehydration reactions involving the breakdown of muscovite occur within 500 m of the contact producing andalusite, sillimanite and corundum, suggesting that temperatures higher than 600 ^oC were probably reached. K-feldspar is less ordered and dominantly monoclinic close to the contact. Geochemical data from the muscovite-biotite Salgueiral granite obtained along a traverse and corroborated with other regional data indicate an important decrease in the abundances of the following elements towards the contact: P₂O₅, H₂O, Li, F, Rb, Cs, Sn, Nb and probably W. The loss of most of these elements can be explained by the breakdown of muscovite, which is an important reservoir of water and many trace elements in the two-mica granites. Mineral δ¹⁸O and δD results show that no significant interaction with meteoric water has occurred; some degree of homogenization recognized through the aureole suggests, however, that interaction with dehydration fluids of composition close to magmatic was developed. From the data, a model emerges in which the breakdown of muscovite provided the release of ore forming elements and part of the fluid necessary for the development of a closed hydrothermal system, which resulted in the generation of a large number of Sn-W ore deposits, in the form of small mineralized quartz veins spatially related to the contact aureole.