Combined Hydrodynamic and Thermodynamic Model of a Convective Hydrothermal System. 2. Self-Mixing of Solutions.

(C) 1997 A. V. Tutubalin, D. V. Grichuk

Moscow State University
Department of Geology
Vorob'evy gory, Moscow, 119899 Russia

A combined model of a convective hydrothermal system in a mid-ocean ridge was developed with allowance made for self-mixing of hydrothermal solutions. The method of modeling is developed on the basis of the approach reported in [1] and includes two stages of calculations. At the first stage, the hydrodynamic model of the system is calculated, and temperatures, pressures, and filtration rates are found for the hydrothermal solution in each element of the system. At the second stage, compositions of equilibrium mineral associations and hydrothermal solitions are calculated for all these elements. The method makes it possible to find the spatial distribution of metasomatic and newly formed ore minerals in the system and the volume effects of metasomatic reactions. A method for estimating local rock/water ratios in a convective system with self-mixing of solutions is suggested.

Calculations concerning the spatial regions of mineral formation show that massive epidote-rich rocks appear in the region near the top of the intrusive body and of greenschist metasomatites --- at a distance from the contact with the intrusive, which agrees with the distribution observed in ophiolites. At early stages of the system evolution, ore minerals display stockwork ore formation in the zone of ascending flow of hydrothermal solutions. As the process develops, ore elements are effectively removed from the system. Maximum negative volume effects of metasomatic reactions (increasing porosity) are observed for the formation of epidote-rich rocks in direct vicinity of the intrusive body. Maximum positive volume effects are due to deposition of gangue minerals: anhydrite in zones characterized by intense descending flow of the solution and quartz in the region characterized by intense cooling of the solution.