Tungsten transport and deposition in magmatic-hydrothermal environments: the example of Panasqueira (Portugal)

Panasqueira W-Cu-Sn deposit (Portugal) consists of sub-horizontal mineralized veins of Qtz-Wfm-Cst-Ms-Toz-sulfurs. Qtz dissolution-recrystallization is the rule, complicating the setting up of the paragenetic sequence and the study of earliest stages over later superimposition. A considerable part of this thesis is devoted to the mineralogical study of the first wolframite (Wfm) depositional stage, which is the most valuable in terms of ore (W)-quantity. It is preceded by a tourmalinization coeval with W-rich rutile (305.25.7Ma). Studies on rutile have shown that oscillatory zoning is the result of the successive mixing between a W-rich, Nb + Ta poor fluid, with a Nb+Ta-rich and W-poor fluid. Also, Tur crystal composition (good recorder of the ore-fluid chemistry) has confirmed a multi-sources origin for W-transport and Wfm deposition. However, the hydrothermal transport and deposition of W are not well constraid. Here we use Raman spectroscopy coupled to the fused silica glass capillary technic to define the stability of the tungsten-polymers at different temperatures (up to 400°C) and pH. As a result, tungsten has a strong tendency to form complexes characterized by Kegging structures in acidic conditions which precipitate at high T, while its solubility increase with the pH increment. Fluid inclusion studies have been taken in consideration in this work, to evaluate the evolution of Panasqueira hydrothermal system. FI on quartz belonging to latest depositional stages have been examined and results have shown that trapping conditions are characterized by lower pressure and elevated temperatures. This allows to conclude that high geothermal gradient affected the studied zones at several stages and are related to heat advection produced by concealed granites from 305 to 295 Ma.