Abstract

Au-Ag-Te-Se-bearing epithermal mineralization at Prasolovskoye, northwest 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 into 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 formed from relatively high temperature (up to 340'C) fluids with salinities of 1.2 to 5.5 wt percent NaCl equiv. Following intrusion of rhyolitic dikes after stages I and II, gold-silver mineralization (stage III) formed during the late Pliocene (based on a K-Ar age of 1.94 + or - 0.10 Ma for a gold-bearing, adularia- rich vein sample). Vein-related, wall-rock alteration zones (0.5-2.5 m thick) occur along stage III veins, overprinting pervasive propylitie assemblages, and consist mainly of silicification and sericitization. Stage III ore mineralization is composed of four substages which show a progressive change in mineralogy with increasing paragenetic time: substage IIIA, bismuthinite-cassiterite-quartz; IIIB, polysulfide quartz; IIIC, telluride quartz; IIID, calcite-adularia-quartz. Each substage contains economic concentrations of gold and silver and shows distinct mineral assemblages with specific Au/Ag ratios and gold fineness values. Telluride quartz veins (usually 1-2 m thick) of substage IIIC are most typical of economic veins at Prasolovskoye and show characteristics of adularia-sericite-type deposits. Ore minerals in this substage consist of sulfides, tellurides, selenides, sulfosalts, native metals, sulfates oxides hydroxides, and tellurates. Fluid inclusion data indicate that the main Au-Ag-Te-Se mineralization during substage IIIC occurred at temperatures between 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 H isotope compositions of stage III fluids (delta 18 O = -4.7 to -9.49%; delta D = -80 to -83%) indicate meteoric water dominance during the Au-Ag-Te-Se mineralizing event.

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