T6 Mineralogical Society of America at 100: The Many Faces of Tourmaline—From Crystallographic Complexity to Recorder of Crustal Evolution

Sunday, 22 September 2019
8:00 AM - 12:00 PM
Room 131ABC, North Building

FLUOR-ELBAITE FROM THE LEDUC PEGMATITE IN QUÉBEC, CANADA

Paper number: T6-7
Program: Technical Sessions
Time: 9:40 AM – 9:55 AM

Authors

Abstract

Located near the town of St.-Pierre-de-Wakefield, Québec, Canada (~50 km north of Ottawa, Ontario), the Leduc pegmatite is the only known Li-bearing pegmatite in the Grenville Tectonic Province. Past mining ventures, first in 1884 for large mica sheets, and then in 1908 for gem tourmaline, were largely unsuccessful, the mica being too lithian, and the tourmaline, although abundant, being too fractured. Leduc is a small, well-zoned granitic pegmatite body, having a quartz core, an intermediate zone (comprising fluor-elbaite, microcline - var. amazonite, cleavelandite, lepidolite-zinnwaldite, and quartz), and a heterogeneous wall zone (comprising schorl, pink perthite and quartz).

Tourmaline crystals occur throughout the pegmatite and can range in size from <1 cm to ~75 cm in length. At least two categories of morphologically distinct tourmalines are observed: (1) narrow sprays of sub-parallel crystals emanating from a common base; and (2) massive, isolated euhedral crystals with terminal ends. In single specimens, of both types, extensive colour-zoning is observed, with the typical base-to-termination pattern being dark blue-to-black grading to regions of pinkish-red and greenish-blue. A mantle of black material is commonly observed, encasing the entire crystal. Chemical analyses show fluor-elbaite cores and schorl exteriors. The fluor-elbaite has average compositions of [X(Na0.6Ca0.2Vac0.2) Y(Li1.0Al1.5Mn0.1Fe0.4) ZAl6 (BO3)3 (Si6O18) V(OH) W((OH)0.3F0.7)] (bluish-green material), and [X(Na0.5Ca0.2Vac0.3) Y(Li1.0Al1.7Mn0.2Fe0.1) ZAl6 (BO3)3 (Si6O18) V(OH) W((OH)0.2F0.8)] (pinkish-red material).

The schorl/elbaite contact is texturally and compositionally discontinuous, showing the occurrence of resorption with the influx of a late-stage, Fe/Mn-rich fluid. The composition of Leduc tourmalines varies between fluor-elbaite and schorl near-end-member compositions along YAl3+ + YLi+ + WF- ↔ Y2Fe2+ + W(OH)-. A complex textural relationship between the schorl, F-elbaite cores, and co-precipitating feldspars (at the sub mm scale) is also observed. Investigation of these textures allows aspects of the geochemical conditions present during (and after) pegmatite emplacement to be better constrained, and Leduc to be placed in the broader context of regional geology.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-338759

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