CR-DOMINANT EMERALD MINERALIZATION ASSOCIATED WITH PEGMATITIC GRANITE AT GHOST LAKE, ONTARIO

The Taylor (now Ghost Lake) beryl occurrence near Dryden in northwestern Ontario occurs in a 3.5 by 1.5 km area of pegmatitic granites of the Mavis Lake Pegmatite Group proximal to the 2685 Ma Ghost Lake Batholith. This region is part of the Sioux Lookout Terrane, which lies between the Winnipeg River Subprovince to the north, and Wabigoon Subprovince to the south. The beryl is concentrated in two pegmatite dikes situated 340 metres south of the Ghost Lake batholith; the dikes are ~60 m apart and are hosted by a talc-rich metaultramafic unit intercalated with low to medium-grade mafic metavolcanic rocks. Only one of these dikes contains emerald. The beryl and emerald mineralization occurs with muscovite, K-feldspar, quartz, tourmaline, and albite along the contact with the metaultramafic host, which has been metasomatized to a black schist composed of 95% phlogopite with minor plagioclase and black tourmaline. The beryl occurs as euhedral crystals with a maximum length of 2.3 cm and a maximum diameter of 1.8 cm. Most are opaque white to pale green, but about 10% of the crystals are emerald green in colour. A total of 26 electron microprobe analyses were obtained from four emerald samples and the analyses were recalculated on the basis of 18 O and three Be atoms per formula unit (apfu). It is important to note that this gives the maximum possible Be content and ignores possible substitution at the Be site. The average Cr₂O₃ concentration is 0.26 wt.% (maximum 0.47 wt.%), and the mean and maximum V₂O₃ concentrations are 0.02 and 0.10 wt.%, respectively. Both Cr and V are chromophoric in emerald; however, the concentration of Cr indicates the predominant importance of Cr. The average FeO and MgO concentrations are 0.36 and 0.56 wt.%, with maximum values of 0.50 and 0.70 wt.%, respectively. The average Na₂O concentration is 0.86 wt.% (maximum 1.31 wt.%). Future research will study the environment of emerald precipitation in the Ghost Lake pegmatites. Stable isotope studies (O and H) will help constrain the temperature and possible source of the mineralizing fluid, and its composition will be studied using fluid inclusion and microprobe analyses of the associated mineral phases.