Trace element geochemistry by laser ablation ICP-MS of micas associated with Ta mineralization in the Tanco pegmatite, Manitoba, Canada
Résumé
In the Tanco pegmatite, one of the world's major Ta deposits, tantalum mineralization shows a complexity that reflects the complex petrogenesis of its host pegmatite. Micas are common in most of the pegmatite units and are intimately associated with the successive stages of Ta mineralization, from the wall zone to the central zones where micaceous replacement is pervasive. Different generations of micas, both primary and secondary, associated with Ta oxides, were selected for electron microprobe and laser ablation ICP-MS investigation. Their chemical trends are used to constrain the magmatic versus hydrothermal processes that played a role in their crystallization and their associated Ta mineralization. Micas range from dioctahedral muscovite to trioctahedral lepidolite through Al↔Li substitution. Unexpectedly, the most evolved compositions (low K/Rb ratios and high Li contents) occur in the wall zone; they are interpreted to reflect nonequilibrium crystallization from an undercooled melt, with or without boundary layer effects. In the central zones, the fine-grained mica–quartz assemblage hosts some coarser-grained Li-muscovite, which has the highest Ta contents (up to 400 ppm). These Li–F–a-rich micas are interpreted to originate from a magmatic metasomatic event, which was also at the origin of the MQM-style Ta mineralization at Tanco. However, the Li–Ta-poor, muscovite end-member compositions of fine-grained alteration micas suggest crystallization from an aqueous fluid, during a metasomatic (hydrothermal) event involving late pegmatitic fluids. The low Ta concentrations (around 50 ppm) of this fine-grained muscovite suggest that this fluid transported at least small amounts of Ta.