GEOTHERMAL FLUID SOURCE DETERMINATION IN THE UZON CALDERA, KAMCHATKA, RUSSIA

Uzon Caldera has a complex hydrothermal plumbing system that supplies numerous active thermal fields.  Geochemical and stable isotope data reveal a diversity of fluid compositions ranging from pure meteoric to dominantly magmatic.  Outflow volumes from all thermal fields are low, indicating that the system is dominantly advective in terms of heat flow.  Mass transfer occurs predominantly via vapor plume ascent/condensation processes.

Forty-two water samples from hot springs, lakes, streams, and precipitation within the caldera were collected in September 2003.  Temperature ranged from 3°C (melt water streams) to 99°C (boiling pools).  Eh ranged from –350 to +250 mV, and pH ranged from 1 to 7.  Stable isotope data show significant mixing trends within individual thermal fields as well as between fields.  The North Thermal Field displays a meteoric signal (D/H=-119 to -113‰, d¹⁸O=-16 to -15‰), while East (ETF) and Central Thermal Fields (CTF) are slightly more magmatic (ETF: D/H=-108 to -82‰, d¹⁸O=-16 to -6‰; CTF: D/H=-112 to -88‰, d¹⁸O=-16 to -6‰).   The Orange (OF) and West Thermal Fields (WTF) have significant magmatic components (OF: D/H=-94 to -74‰, d¹⁸O=-7 to -2‰; WTF: D/H=-98 to -79‰, d¹⁸O=-9 to -2‰).  Chemically, the pools are also diverse and on an even more local scale.  Pools 2m apart show an order of magnitude difference in concentrations of numerous elements, including Al, As, B, Ba, Fe, K, Mg, Mn, Na, Sr, Li, Y, La, Ce, W, Au, Th, and SO₄. 

Magmatic vapor condensate is likely collecting in the shallow subsurface in various places along the WNW and NNE trending caldera faults and mixing with meteoric water.  Principle component analysis (PCA) of the geochemical data show that ca. 75% of the data are explained by PCA axes 1 and 2 alone, supporting the hypothesis that mixing of end member meteoric and magmatic fluids controls the geochemistry of the hot spring fluids.  Individual hot springs that are adjacent to one another and have markedly different chemistries reflect the channelized nature of the fluid flow and the heterogeneous permeability within individual thermal fields.