TE-BEARING AND SE-BEARING EPITHERMAL AU-AG MINERALIZATION, PRASOLOVSKOYE, KUNASHIR ISLAND, KURIL ISLAND-ARC

Au-Ag-Te-Se-bearing epithermal mineralization at Prasolovskoye, northw est Kunashir Island, Kurils, occurs along fault zones developed within a circular intrusive zone in Miocene to Quaternary age volcanic rocks . The ores show complex textures and mineralogy and can be divided int o three stages based on tectonic fracturing and mineral associations: pyritic (I), polymetallic (II), and gold-silver (III). Stages I and II occur as hydrothermal replacement zones, veins, and breccias and are base metal sulfide rich without economic gold and silver grades. Fluid inclusion and sulfur isotope data indicate that stages I and II forme d from relatively high temperature (up to 340 degrees C) fluids with s alinities of 1.2 to 5.5 wt percent NaCl equiv. Following intrusion of rhyolitic dikes after stages I and II, gold-silver mineralization (sta ge III) formed during the late Pliocene (based on a K-Ar age of 1.94 /- 0.10 Ma for a gold-bearing, adularia-rich vein sample). Vein-relate d, wall-rock alteration zones (0.5-2.5 m thick) occur along stage III veins, overprinting pervasive propylitic assemblages, and consist main ly of silicification and sericitization. Stage III ore mineralization is composed of four substages which show a progressive change in miner alogy with increasing paragenetic time: substage IIIA, bismuthinite-ca ssiterite-quartz; IIIB, polysulfide quartz; IIIC, telluride quartz; II ID, calcite-adularia-quartz. Each substage contains economic concentra tions of gold and silver and shows distinct mineral assemblages with s pecific Au/Ag ratios and gold fineness values. Telluride quartz veins (usually 1-2 m thick) of substage IIIC are most typical of economic ve ins at Prasolovskoye and show characteristics of adularia-sericite-typ e deposits. Ore minerals in this substage consist of sulfides, telluri des, selenides, sulfosalts, native metals, sulfates, oxides, hydroxide s, and tellurates. Fluid inclusion data indicate that the main Au-Ag-T e-Se mineralization during substage IIIC occurred at temperatures betw een 150 degrees and 245 degrees C from low-salinity (0.4-1.6 wt % NaCl equiv) fluids. Gold deposition was most likely the result of boiling of hydrothermal fluids. Measured and calculated O and II isotope compo sitions of stage III fluids (delta(18)O = -4.7 to -9.4 parts per thous and; delta D = -80 to -83 parts per thousand) indicate meteoric water dominance during the Au-Ag-Te-Se mineralizing event.