Christopher Galbraith

Tourmaline, a boron-bearing ring silicate, is ubiquitous at the Tsa da Glisza (formerly Regal Ridge) emerald prospect in the Yukon. It occurs as porphyroblasts in greenschist facies meta-volcanic and ultramafic rocks, as phenocrysts in granite, in aplites and quartz-tourmaline veins that cut the schists, and as granular black masses in highly altered gossan or fault zones. Contact metamorphism and metasomatism of the Devonian-aged greenschist rocks by the Cretaceous-aged granite/aplite/quartz-tourmaline veins appear to be responsible for local emerald mineralization. The quartz-tourmaline veins are particularly important because emeralds form along their selvages, although not all such veins have associated visible emerald mineralization. Tourmaline compositions include solid solutions among Na-Fe schorl, Na-Mg dravite, and Ca-Mg uvite. The major-element compositional variations of the tourmalines correspond well with the bulk-rock chemistry of their respective host rocks. Dravitic tourmalines, with up to 2.12 apfu Y-site Mg, are most common on the property, occurring in country rocks, aplites, and quartz-tourmaline veins. Less commonly, uvites also occur in these same rocks, with up to 0.88 apfu X-site Ca. Schorls, with up to 2.06 apfu Y-site Fe, are limited to granites and aplites and, rarely, with emeralds in country rock. Tourmalines from veins and alteration zones associated with emerald mineralization have subtle differences in major-element compositions, compared with tourmalines from schists and veins with no emerald mineralization. Tourmalines associated with emeralds are slightly more iron-rich dravites compared with tourmalines of schists and alteration zones without emeralds. Analyzed trace elements in tourmalines include Li, Be, Sc, Cr, V, Mn, Ni, Co, Cu, Zn, As, Rb, Sr, Y, Zr, Nb, Cd, Mo, Sn, Sb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Yb, Lu, Ta, W, Bi, Pb, Th, and U. Binary plots of major- and trace-element concentrations show that tourmalines associated with emerald occupy restricted ranges of composition, albeit with considerable overlap with tourmalines not associated with emerald. Tourmaline trace-element concentrations that correspond to emerald mineralization include 20-40 ppm Co, ~50 ppm, Ni, 0.5 ppm La, 0.5 ppm Y, 100-200 ppm Li, and 300-600 ppm Zn. Trace elements Co, Ni, Zn, Pb, and Sb correlate with increased Fe, and probably enter the Y-site at the expense of Mg. Multivariate statistical analysis, particularly discriminant function analysis using the trace-element concentrations, indicates that tourmaline can act successfully (92-100% correct classification matrix) as a stand-alone indicator of emerald mineralization at Tsa da Glisza.

Keywords: tourmaline, emerald, indicator, beryl, metasomatic, chromium, schorl, dravite, felsic, Yukon, major element, trace element.
Pages: 136
Supervisor: Barrie Clarke