Abstract:

The Baranevskoye gold-silver deposit occurs within the caldera of the Balkhach volcano in the south-east part of the Central Kamchatka mining district. The host rocks are andesite, basalt of Miocene-Pliocene age. The hydrothermal alteration varies as a function of depth and is subdivided into deepest quartize zone, intermediate zones of quartz-sericite(alunite)-pyrite-Fe-Ti-oxides and quartz-sericite-illite-pyrite associations, and shallow quartz-adularia-hydromica-clay-carbonate zone. Mineralogical study reveals three major stages of mineralization. The early stage is present by a highly disseminated copper-rich mineralization with main ore minerals of chalcopyrite, bornite, tennantite-tetrahedrite, and rare gold occurring in the Rzhavaya vein. Tennantite-tetrahedrite series is represented by two end-members of tennantite and tetrahedrite with tetrahedrite preponderating. Later stages are characterized by advanced Au-Ag mineralization containing gold, pyrite, chalcopyrite, sphalerite, galena, tellurides, and sulfosalts. The early Cu-rich mineralization (stage I) is designated as an intermediated state, followed by later low-sulfidation Au-Ag mineralization (stages II and III). Gold was constantly deposited through from Stage I to Stage III. Crystalline gold was also found in void-space in the altered host rocks. Early gold is relatively Ag-rich with Au content of about 59-65% compared to later gold of Stage II and III (Au 64-72%). Fluid inclusion microthermometry shows homogenization temperatures between 190-280 ℃ for the Central vein, 190-240 ℃ for the Rzhavaya vein and 230-310 ℃ for the quartz of the altered host rocks associated with crystalline gold. Low salinity of the fluid inclusions (0.9-2.4 wt.% NaCl eq.) suggests an influx of the meteoric waters. Based on mineralogical and fluid inclusions data the physicochemical conditions of the ore formation are discussed.

Key words: gold, ore deposit, volcanic arc, bornite, fluid inclusion, Kamchatka