| Abstract |
The Rodnikovy epithermal Au-Ag vein deposit is one of the largest gold fields of South Kamchatka. Its foundation began over 1 100 000 years ago. During the 200 000 years there have been formed three main vein zones with estimated reserve accounting for 40 t of Au and 470 t. of Ag. Hydrothermal activity that continues up to date is resulting from the evolved Vilyuchinsky hydrothermal ore-forming system. At recent surface it appears as a series of thermal springs, small-sized heated-up locations of “hot” ground and zones of anomalous thermal flow – «hot zones» which were discovered by adits. The largest springs are Vilyuchinsky hydrotherms. Along the flood-plane terrace at more than 1.5 km- distance they form several travertine domes and a series of travertine fields, and boggy areas overgrown by thermophilic plants, and thermal water discharges at maximum temperature of 54.5 °C. Hydrogeological wells drilled to 500-1,000 m depths reached down a hydrothermal reservoir with the temperature of 55.5 – 78.5 °C. The study of the epithermal Au-Ag vein deposit made by the adit revealed zones of elevated jointing and brecciation at a depth of 500 m away from the wells. These zones produced strong hydrothermal activity. Hot water, coming through fractures at 91.5 °C and at a rate of 2.4 liters per second, heated dry rocks bedding the walls of the adit. During 2 years at some specific sites called “hot zones” the temperature of heating-up has reached 110 – 125 °C. The roof and walls of the adit have been covered with various deposits and new mineral formations in form of sublimates. Depending on weather conditions mineralogical and geochemical features of the “hot zones” have changed. In winter the inflow of meteoric water has been dramatically decreasing. This resulted in increasing the rate of heating and temperature and caused a decrease of humidity. In conditions of “drying” the walls of the adit were covered with crystals of a broad range of minerals: sulphates, phosphates, carbonates, quartz, adularia, and sulfides. In summer, the quantity of atmospheric water infiltrating from the surface to “hot zones" increases. In consequences of this some minerals formed in earlier arid time such as complex sulphates with accumulated gypsum, calcite and chlorite, were dissolved. Mineral associations of primary productive veins affected by current hydrothermal solutions are involved in process of contact between water and rock and form earlier unknown new phases. The aggregates of argentite stibiopearseite-arsenpolybasite with inclusions of native gold have been overgrown with thin 2-3 mm crust with crystals of proustite, xanthoconite, pyrite, quartz and adularia. The aggregates themselves change by segregation of reasonably large grains of electrum (Au=60-50 %) into small amount of gold in thin associations of pyrite and altaite. Here with there have been formed new submicroscopic hard detected mineral phases containing Ti, Bi, Sb, As, Se, O, and S. In drainage pipes the new mineral formations appear as concentric zonal aggregates of calcite, aragonite, quartz, adular with framboids and crystals of pyrite containing arsenic and separate micrograins of gold. Observations made from 1991 to 2005 revealed significant elevation of mercury content in newly formed minerals and altered rocks. |