FIBROUS TOURMALINE: A SENSITIVE RECORDER OF FLUID COMPOSITION AND EVOLUTION

Tourmaline (tur) supergroup minerals are one of about 390 minerals to occur with fibrous morphology. Similar to other morphologies of tur, fibers respond to the host-rock assemblage but they are particularly responsive to chemical changes in the local hydrothermal fluids. New observations and chemical analyses of fibrous tourmalines from localities world-wide coupled with published data reveal commonalities. When fibers nucleate on a preexisting substrate of tur, fibers generally have a dramatically different composition than the substrate tur, and fibrous growth is restricted to the +c pole of the tur. Tourmaline fibers also nucleate on other minerals. Most fibrous tourmalines form late in the paragenetic sequence, typically where there is open fluid-filled space, e.g. at the end of the tur crystallization sequence in a pegmatite or in a fluid-rich fracture system.

Chemically, some fibers are homogeneous, but many fibers are zoned with the zoning tracking the chemical evolution in the host environment. Fibrous tur formed in hydrothermal/geothermal settings are typically foitite, magnesio-foitite, dravite or oxy-dravite and reflect the influence of both fluids and local mineral assemblage. Fibrous tur that develops in the late stages of pegmatite pockets/miarolitic cavities exhibits a more restricted range of compositions that generally progress from foititic to schorlitic to elbaitic compositions consistent with fractionation of trapped fluids. Using newly derived expressions relating X-site cationic occupancy in tur to aqueous fluid compositions, Na and Ca contents in aqueous fluids in local equilibrium with the fibrous tur can be calculated. These calculations suggest that in all petrologic settings fibrous tourmalines equilibrated with aqueous fluids having Na concentrations ranging from 0.07-0.48 mol/l Na, with the lower ranges associated with foititic fibrous tur, and that fluids contain low Ca concentrations (<0.16 mol/l Ca). Fibrous tur that has high oxy-species components are likely to form in fluids with relatively high salinities. Compositions of fibrous tourmaline provide an additional method to decipher the evolution of the hydrothermal environments, particularly those associated with a dynamic fluid phase that is no longer present.