HYDROTHERMAL ALTERATION FACIES IN THE UZON CALDERA, KAMCHATKA, FAR EAST RUSSIA

The Uzon caldera, located in Kamchatka, Far East Russia, contains a remarkable diversity of geothermal fluids both between and within individual geothermal fields.  This diversity has produced a broad array of alteration minerals in the pools, hot springs, mud pots, and gas vents within the caldera.

Six geothermal fields were studied during the summer of 2003, and the hydrothermal alteration assemblages were divided into two types as follows: 1) Type I fields where alteration has only developed immediately surrounding a hot spring, pool, mud pot, or gas vent (meter scale) and, 2) Type II fields that have been exposed to long term, steam heated acid volatile leaching, which has pervasively altered the entire field.

Type I bulk mineralogy reflects the relatively unaltered state of the original andesite/dacite “gravellite” substrate, with plagioclase (± pyroxene) being the dominant mineral.  Significant hydrothermal alteration minerals only occur in sediments within discrete pools.  The alteration assemblage includes kaolin group clay + smectitic clays + opal-A ± pyrite ± sulfur.  In areas saturated with low temperature meteoric water, iron oxides are common.

Type II bulk mineralogy is characterized by a wide variety of minerals derived from the alteration of acidic pyroclastics, as well as andesite/dacite.  Common minerals include opal-A, opal-C, quartz, and a kaolin group clay.  Alunite group minerals (jarosite and minimite) are major components in some fields, and only trace in others.  Trace amounts of pyrite, brushite, and mordenite are found only as singular occurrences.  Elemental sulfur is ubiquitous around gas vents.

Fluid chemical composition varies both between and within fields indicating a very complex plumbing system for the caldera.  Within any individual field, pH-Eh-T° can vary from pool to pool such that adjacent pools and hot springs, separated by only meters, can have completely different physiochemical conditions, in turn producing the wide variety of alteration minerals seen.