Brand A. A., Groat L. A., Linnen R. L., Garland M. I., Breaks F. W., Giuliani Gaston. (2009). Emerald mineralization associated with the mavis lake pegmatite group, near Dryden, Ontario. Canadian Mineralogist, 47 (Part 2), p. 315-336. ISSN 0008-4476.
Titre du document
Emerald mineralization associated with the mavis lake pegmatite group, near Dryden, Ontario
Brand A. A., Groat L. A., Linnen R. L., Garland M. I., Breaks F. W., Giuliani Gaston
Source
Canadian Mineralogist, 2009,
47 (Part 2), p. 315-336 ISSN 0008-4476
The Taylor 2 (also known as Ghost Lake) emerald showing near Dryden, in northwestern Ontario, is associated with a granitic pegmatite of the Mavis Lake Pegmatite Group proximal to the 2685 Ma Ghost Lake Batholith. The Taylor 2 pegmatite consists of three separate limbs that intruded a wide zone of chlorite schist near the eastern end of an altered ultramafic sill. Most of the beryl and emerald occurs in a "zone of mixing" between the southern and central limbs of the pegmatite. The rock in the "zone of mixing" consists of relict orange K-feldspar crystals (to 30 cm) in a matrix of anhedral bluish plagioclase, quartz, fine-grained black phlogopite, blue apatite crystals (to 1 cm), and black tourmaline crystals (to 2 cm). The beryl occurs as euhedral crystals up to 2.3 x 1.8 cm; most are opaque to translucent and white to pale green in color, but about 10% are emerald. The emerald compositions show average Cr concentrations of 0.25 wt.% Cr2O3 (maximum 0.42 wt.% Cr2O3 or 0.03 Cr apfu) and maximum V concentrations of 0.05 wt.% V2O3. Primary fluid inclusions in the emerald crystals show variable compositions and high temperatures of homogenization (to 490 degrees C). Oxygen isotope compositions of the emerald crystals, which average 9.6(7)%, suggest a high degree of homogenization and fluid buffering during fluid-rock interaction. Whole-rock compositions show high concentrations of Be (89 ppm) in the the Taylor 2 pegmatite and elevated Cr in the chlorite schist (2610 ppm) and the altered ultramafic sill (3050 ppm). Geochemical similarities support the hypothesis that the chlorite schist is the faulted analogue of the altered ultramafic sill. The absence of beryl in the latter unit may be due to lower amounts of fluid or F concentrations (similar to 150 ppm versus similar to 1300 ppm for the chlorite schist). The emerald in the Taylor 2 showing and the associated phlogopite most likely formed through metasomatism of the chlorite schist, which was driven by granitic magmatism.
