FIBROUS TOURMALINE: A SENSITIVE PROBE OF FLUID COMPOSITIONS AND PETROLOGIC ENVIRONMENTS

Tourmaline supergroup minerals with fibrous morphology record and respond to changing conditions in fluid-rich hydrothermal environments. Based on published data, hand -specimen and optical observations, and new chemical analyses of fibrous tourmalines from localities worldwide, several commonalities are apparent. Fibers typically nucleate on a preexisting substrate of tourmaline, but the fibers generally have a dramatically different composition than the substrate tourmaline. When fibers nucleate on a single tourmaline crystal, fibrous growth is restricted to the +c pole of the tourmaline. Tourmaline fibers also nucleate on or in other minerals without preexisting tourmaline. Most fibrous tourmalines form late in the paragenetic sequence of a geologic environment and generally where there is an open fluid-filled space, e.g., at the end of the tourmaline crystallization sequence in a pegmatite pocket or in a hydrous fluid-rich fracture system. Fibrous tourmaline compositions can be foititic, schorlitic, dravitic, or elbaitic, reflecting the dissolved components in the fluxing aqueous fluids. While some fibers are homogeneous, many fibers arc chemically zoned with irregular, patchy, or oscillatory zoning, and the zoning tracks the chemical evolution trends in the host environment. Using newly derived expressions relating X-site cationic occupancy to aqueous fluid compositions, Na and Ca contents in aqueous fluids in local equilibrium with fibrous tourmaline suggest that in all petrologic settings fibrous tourmalines equilibrated with aqueous fluids having variable Na concentrations (0.07-0.48 mol/l Na with the lower ranges associated with foititic fibrous tourmaline) and with generally low Ca concentrations (<0.16 mol/l Ca). The fibrous tourmalines that have high oxy-species components are suggestive of formation in fluids with relatively high salinities. The compositions of fibrous tourmaline provide an additional method for deciphering the evolution of hydrothermal environments, particularly those associated with a dynamic fluid phase that is no longer present.