hide
Free keywords:
-
Abstract:
The Panasqueira vein-type W-Sn-Cu deposit is hosted by Cambrian pelitic-psammitic metasediments and related to a greisenized S-type granite emplaced in the late stages of the Variscan orogeny. The origin of the mineralizing fluids, as well as the hydrothermal processes and structural control on ore transport (e.g., role of greisen cupola vs. fault zones) and deposition, is still debated.
We address the question of fluid source and evolution from the results of in situ microprobe (major and minor elements), LA-ICP-MS (minor and trace elements) and SIMS (B isotopes) measurements of coexisting tourmaline and white mica from different settings within the Panasqueira mine. White mica is one of the most abundant hydrothermal minerals in the deposit, including the ore veins, greisen and wall-rock alteration zones from the preore to the main ore stage. Tourmaline and quartz are the main phases forming the wall-rock alteration zones, but tourmaline is scarce in the veins, forming only locally at the vein margins and in late-stage fractures.
White mica has muscovitic to phengitic and celadonitic compositions and relatively high concentrations of lithophile trace elements (Li, Rb, Cs, Tl, Nb, W, Sn, and Ta) that may logically derive from granite-related fluids. Contrarily, tourmaline has zoned dravitic-schorlitic compositions and is richest in V, Sr, Ti, and Zr that were likely supplied by the country rocks.
Boron isotope compositions of tourmaline and white mica support a magmatic source for the early hydrothermal fluids. Furthermore, the fractionation of B isotopes between them provides estimates for the temperature of vein formation (ca. 460°C) and for a later fluid pulse recorded in crosscutting, mineralized fault zones (ca. 260°C). These results agree well with estimates from Ti-in-quartz thermometry from wall-rock alteration zones (500°C) and from the low end of homogenization temperatures for fluid inclusions in vein quartz (360°–230°C).
