Abstract

Tourmaline is ubiquitous at the Tsa da Glisza emerald prospect, Yukon Territory, occurring as porphyroblasts in greenschistfacies metavolcanic and ultramafic rocks and as crystals in granites, aplites, and quartz-tourmaline veins. The principal occurrence of emerald is along contacts between the quartz-tourmaline veins and mafic country rocks, but it also occurs in the aplites. We assess the potential of using tourmaline compositions as a guide to emerald mineralization at Tsa da Glisza based on major element, trace element (Li, Be, Sc, Cr, V, Mn, Ni, Co, Cu, Zn, As, Rb, Sr, Y, Zr, Nb, Cd, Mo, Sn, Sb, Cs, Ba, Ta, W, Bi, Pb, Th, and U), REE (La, Ce, Pr, Nd, Sm, Eu, Yb, Lu), and boron isotope data. The tourmaline compositions encompass Na-Fe schorl, Na-Mg dravite, and Ca-Mg uvite and show a strong dependence on host-rock chemistry. Tourmalines closely associated with emeralds (intergrown in same hand specimen) are slightly more iron rich dravites compared with those from schists and alteration zones lacking emerald. Binary and ternary plots of major and trace element concentrations show that emerald-associated tourmalines occupy restricted ranges of composition, albeit considerably overlapping those not associated with emerald. Trace element concentrations in tourmaline associated with emerald mineralization include 20 to 40 ppm Co, ~50 ppm Ni, 0.5 ppm La, 0.5 ppm Y, 100 to 200 ppm Li, and 300 to 600 ppm Zn. Boron isotope (δ11B) values range from − 15.6 to − 4.2 per mil for all tourmalines (n = 80). Low values are most common and are consistent with a dominantly granitic source of boron. Higher isotope ratios (>− 8‰) are confined to tourmalines from country rocks and quartz veins, suggesting a contribution of isotopically heavy B from the maficultramafic series. The three emerald-related samples have a restricted, intermediate range of − 10.3 to − 7.9 per mil.

Multivariate cluster analysis defines two populations of tourmalines corresponding to host-rock type. One cluster includes tourmaline from country rocks and quartz-tourmaline veins and the other is dominated by samples from granites and aplites. The latter cluster also includes all tourmalines associated with emeralds. With trace element concentrations and boron isotope values as input variables, one stepwise discriminant function analysis model produces a 91 percent correct classification matrix and an 88 percent correct jackknifed classification matrix. This result suggests that tourmaline has potential as an indicator mineral at the Tsa da Glisza prospect and possibly also in the exploration for emeralds elsewhere.

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