GEOLOGIC SETTING OF THE UZON CALDERA, KAMCHATKA, FAR EAST RUSSIA

The Uzon caldera is located 180 km northeast of Petropavlovsk, within the eastern volcanic belt of the Kamchatka Peninsula.  The Pleistocene/Holocene eruptive and depositional history includes 1) basaltic shield volcanism from 750-370 ka, 2) stratovolcano development from 370-225 ka, 3) ignimbrite production culminating in caldera collapse from 325-175 ka, 4) lacustrine infilling beginning at 100 ka, 5) faulting and extension at 40 ka, 6) resurgent dome formation at 12-10 ka, and 7) maar formation at 11 ka.  Geologically and geochemically diverse hydrothermal fluids now emanate from numerous hot springs within the caldera.

The distribution of hot springs is structurally controlled, with major thermal fields located either in the interior of the caldera along a major WNW-trending fault, or along the ring fracture system at the edge of the caldera. Individual hot springs vary from hard, constructional sinters that surround pools to gravel-hosted depressions to intensely acid-leached mounds. The majority of the springs are moderate temperature (40-70°C) although some are significantly hotter, including several boiling springs, as well as the vapor-only West Thermal Field.  pH ranges from 1 to circum-neutral and Eh ranges from -350 mv to +250 mv.  Sb-As-Hg  concentrations are elevated, and tend to co-vary with Cl-Li-Rb-Cs.  Sediments in and near active springs contain varying amounts of orpiment, realgar, stibnite, cinnabar, and pyrite.  Scorodite is present in several active pools.  Opal, kaolinite, smectite, barite, calcite, and zeolites are common as alteration products.  Alteration intensity varies from moderate to intense throughout much of the caldera.  Physicochemical gradients are steep, such that adjacent pools are in some instances mineralogically entirely different.  Such differences may be a function of very localized channelized fluid flow, resulting in precipitation of differing and discrete mineral assemblages.  D/H and oxygen isotope ratios of hot spring fluids reveals a broad spectrum of fluids, ranging from 100% meteoric (D/H=-112 per mil, d¹⁸O=-15 per mil) to dominantly magmatic (D/H=-74 per mil, d¹⁸O=-2 per mil), reflecting mixing of end-member fluids or significant amounts of exchange between meteoric fluids and igneous rocks in the subsurface.