Présentation G. Launay

Deciphering fluid flow at the magmatic-hydrothermal transition: case of the world-class W-Sn-(Cu) ore deposit of Panasqueira (Portugal)

Deciphering the behavior of fluid flow at the magmatic-hydrothermal transition is crucial to understand physical processes leading to the formation of intrusion-related ore deposits and hence to predict the localization of mineralized bodies. However, the direction and the velocity of fluid flow related to this transitional stage remains poorly constrained. Here we present a coupled textural and chemical study performed on tourmaline growth bands to constrain fluid flow at the incipience of the hydrothermal system of the W-Sn-(Cu) Panasqueira deposit. This exceptional deposit consists of a dense network of flat wolframite and cassiterite-bearing quartz veins intensely developed above a well-known hidden greisen cupola. The W-Sn mineralization is preceded by a tourmalinization stage occurring as metasomatic halos around veins and as selvage along the vein-wall-rocks contacts. Results emphasize the key role of the greisen cupola on fluid focusing and the role of fluid overpressure conditions during the vein opening. Velocity values highlight an efficient transport of metals in veins largely dominated by advection process (10^-4-10^-3 m.s^-1), whereas fluid flow in the altered wall-rocks is pervasive and mainly driven by diffusion (10^-6-10^-5 m.s^-1). LA-ICP-MS analyses of tourmaline growth bands reveal that fluids coming from the cupola are enriched in Na, K, Li, Sr and Sn, thus emphasizing the contribution of magmatic fluids during the vein opening and the metasomatic alteration of the wall-rocks. More generally, this study demonstrates the critical role of granitic apexes as emanative centers of mineralized fluids, and therefore analysis of mineral growth bands may serve as a possible pathfinder for the research of hidden granitic cupola related to mineralized bodies.