Magmatic-hydrothermal transition in the Tanco rare-element pegmatite: Evidence from fluid inclusions and phase-equilibrium experiments

Fluid inclusions in petalite, spodumene, eucryptite, and coexisting quartz from the Tanco mine, Manitoba, have been evaluated with respect to experimentally calibrated reaction relationships in the system LiAlSiO₄-SiO₂ to define the cooling path and fluid evolution within this pegmatite. At Tanco, the conversion of petalite to pseudomorphic intergrowths of spodumene + quartz occurred at approximately 500°C and 3000 bars in the presence of a dense hydrous alkali borosilicate fluid with a minor carbonate component. Between 470 and 420°C and between 2900 and 2600 bars, the borate component of this fluid was removed by the crystallization of tourmaline, resulting in the deposition of albite, micas, quartz, and ore minerals (e.g., microlite, beryl, and pollucite), and consequent evolution of a comparatively low-density, solute-poor, CO₂-bearing aqueous fluid (approximately 91 mol% H₂O, 5 mol% CO₂, 4 mol% NaCl equivalent). Reactions over this P-Tinterval mark the transition from magmatic to subsolidus hydrothermal conditions. Ore-bearing albitic (aplite or cleavelandite) units probably were deposited directly from the borosilicate fluid and are not the result of subsolidus metasomatism.