Genesis of Li-Donbassite and Tosudite in Echassières Granitic Cupola, Massif Central, France

抄録

Lithium-bearing donbassite and tosudite occur in veins in the Echassières granitic cupola (Massif Central, France). The two minerals are characterized by high Li contents and very low Fe and Mg contents; their structural formulae are :(Si_3.81Al_0.19)_4.00(Al_3.81Li_0.52Fe²⁺_0.01Ca_0.02Mg_0.01Mn_0.01)_4.38O₁₀(OH)₈(Na_0.07K_0.04)_0.11 for donbassite and(Si_3.50Al_0.50)_4.00(Al_2.95Li_0.22Fe³⁺_0.01Ti_0.01)_3.19O₁₀(OH)₅(Ca_0.01Na_0.15K_0.18)_0.34 for tosudite. Petrographic investigations of drill cores from the Echassières area indicate that these two minerals were formed in intermediate stages of a hydrothermal alteration series, muscovite-pyrophyllite-donbassite-tosudite-kaolinite, which occurred successively in the post-magmatic stage of granite. Fluid inclusion and stable isotope studies indicate that these aluminosilicates were formed at 400-150°C by interactions between low salinity (1-8 wt% NaCl) fluids of meteoric origin and the alkali granite.Thermodynamic calculations assured that Li-donbassite possessed the stability field from 300 to 200°C under acid conditions (3 < pH < 5) in the Echassieres hydrothermal system.