REP18451 - Laserfiche WebLink

Home Browse Search

T-4518 61 electrum-stromeyerite occurrences also contain chalcopyrite. Petrographic observations and the high percentage of electrum grains sharing boundaries with chalcopyrite indicate that electrum is most closely related to the second stage of chalcopyrite (Figs. 37a and b). The common intergrowth of electrum with galena and sphalerite may be attributed to replacement of these minerals during chalcopyrite-electrum deposition. Gold could not be correlated with copper, zinc, or lead geochemically but a weak correlation between gold and siiver was found (Appendix D). Electrum grains were analyzed by microprobe to determine the Au:Ag ratio and to determine any relationship between the ratio and associations with other minerals such as chalcopyrite, sphalerite, and galena. Thirty analyses on thirteen electrum grains (Appendix E) revealed a cluster of compositions at approximately 70% gold and 30% silver (Fig. 38). However, compositions range from 37% to 85% gold. Variable compositions were recorded within several grains (Fig. 39). It is unclear if the low and high gold electn~m reflect different stages of mineralization from the more common 70% gold or if the electrum composition is simply variable. Electrum with a gold:silver ratio of 85:15 shared grain boundaries with only chalcopyrite and gangue minerals. However, there was also electrum with lower At.:Ag ratios sharing grain boundaries with only chalcopyrite. A third period of chalcopyrite deposition resulted in the blebs in sphalerite (Fig. 35). The timing of this event is unclear. It is distinct from the second stage of chalcopyrite, however, in that it is not replaced by stromeyerite. One grain of acanthite was found occurring with galena and tetrahedrite during microprobe analysis. It was approximately 2µm in diameter, irregular in shape, and could not be distinguished from the tetrahedrite under the microscope (Fig. 40).