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"' T-4518 g 1 <br />compression. Northeasterly primary compression resulted in the propagation of sigtnoidal <br />north-northeast-trending tensional cross-structures between left-lateral sheaz zone <br />boundaries. East-trending structures were then developed at left-lateral shear zone <br />boundaries. With continuing left-lateral differentia] movement, the blocks of rock between <br />them rotated slightly as dominoes with right-lateral offsets between them. The rotational <br />movement ceased after deposition of fine-grained quartz-pyrite mineralizadon. Late:, <br />unmineralized faults occur in a conjugate set at N23W/88W and N35E/57W. <br />Hypogene mineralization in veins occuaed in six distinct stages: 1)fine-grained <br />quartz-pyrite+lrtolybdenite; 2) medium- to coarse-grained quartz-pyrite~eticite+~tematite <br />±sphalerite±galena; 3) carbonate; 4) main base- and precious-metal stage sphalerite- <br />galena-chalcopyrite-electrum-acanthite; 5) sulfosalt stage teaahedrite-tennantite-proustite- <br />pyrazgyrite-famarinite-s~omeyerite-chalcopyrite; and 6) dolomite. Electrum occurs as 1 <br />to 200 µm grains with chalcopyrite, sphalerite, and galena, typically at grain boundaries. <br />Electrum was deposited with the second period of chalcopyrite in the base- and precious- <br />metal stage of mineralization. It appeazs to replace sphalerite, galena, and pyrite. Analysis <br />of electrum by microprobe revealed that compositions are variable: the majority of grains <br />have a Au:Ag ratio of about 70:30 atom percent but gold content varies from 37% to <br />85%. An appazent association exists between the 85% gold electrum and chalcopyrite. <br />The late supergene mineral assemblage is covellite-chalcocite-digenite-malachite-azttrite- <br />gold-silver-gcethite-lepidocrocite. <br />Several previously unreported ore minerals were identified during petrographic and <br />