Compositional variation in Hg-Ag-rich tetrahedrite from the polymetallic Eskay Creek deposit, British Columbia, Canada

摘要

The Eskay Creek, British Columbia, Canada, is a polymetallic, gold- and silver-rich, volcanic-hosted, massive sulfide deposit. The ore in the deposit is divided into subzones distinguished by mineralogy, texture, grade and metallurgical characteristics. This study presents the results of a mineralogical examination of three composite field samples, with emphasis on the chemistry of tetrahedrite. Tetrahedrite is associated with variable amounts of Hg-bearing sphalerite and cinnabar, and an array of sul-fides, sulphosalts and non-opaque minerals. Electron probe micro analyses of tetrahedrite in the three composite samples reveal wide variations in compositions. The Hg content in tetrahedrite in the three samples varies between 1.17 wt. percent and 20.74 wt.percent, whereas the Ag content ranges from 6.17 wt.% to 20.97 wt.%. The average chemical formula of the fahlores approximates the formula (Cu_(7.8)Ag_(1.51))(Fe_(0.19)Zn_(0.59)Hg_(1.25))(Sb_(3.74)As_(0.19))S_(13). Tetrahedrite exhibits substitution of Hg and Ag, negative covariance between Ag and Cu, and between Hg and Zn + Fe. Two main types are distinguished based on the Hg contents; one with < 1 (atoms per 13 S) and one with > 1 (atoms per 13 S). Tetrahedrite with the highest Hg contents tends to have the lowest Zn and Fe concentrations. The highest Hg contents in tetrahedrite are recorded in the samples with the highest bulk Hg concentrations and with the highest cinnabar contents. The compositional variations of tetrahedrite are important for two reasons. First, because they can be used in mapping ore forming fluids and indicate possible temporal variations. Second, determination of the compositional variation of the tetrahedrite in the mine has metallurgical implications because the mineral is an important Ag source. The mineralogical data indicate that non-physical processes (e. g., pyrometallurgy) must be used to separate Hg from Ag, and also Cu and Zn concentrates.