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T-4518 59 as 5 mm) galena followed sphalerite deposition. It replaces pyrite (Figs. 326 and c) and occtus as open space fillings (Fig. 34b) and small (1-5 µm) blebs in the sphalerite. The galena as blebs could have been deposited with the host sphalerite or could be a replacement feature. Six microprobe analyses conducted on five galena grains indic,rte that galena contains less than 50 ppm silver. A second, more extensive, period of chalcopyrite deposition also began after sphalerite deposition (Fig. 346). The textural relationship between galena and chalcopyrite suggests that chalcopyrite was deposited towards the end of galena deposition. Chalcopyrite is anliedral and tends to be finer-grained than either galen~i or sphalerite; grain sizes typically range from 1 to 300 µm in diameter, but grains as large as 1 mm are not unusual. It replaces pyrite and occurs as open space Fillings. The chalcopyrite deposited at this stage has a variable texture due to incipient replacement by stromeyerite (Figs. 36a and 36b). This second stage of chalcopyrite can only be distinguished from the eazly, wg-lining chalcopyrite when the former is partially replaced by stmmeyerite. Electrum grains range from less than 1µm to over 200 µm in diameter (Appendix C). A significant majority of the grains are less than 20 µm, but larger grains are not uncommon. Most electrum grains occur at base-metal sulfide grain boundaries rather than as refractory grains fully enclosed in other minerals. About sixty per cent of electrum grains shaze a grain boundary with chalcopyrite, 40% with sphalerite, 20% with galena, and 10% with stromeyerite (Appendix C). Approximately 30% of electrum grains share grain boundaries with only chalcopyrite and, frequently, eazlier quartz and/or pyrite. About 15% of elecavm grains shaze grain boundaries with sphalerite and 15% with galena in the absence of chalcopyrite. All