Geochemical structure of base-metal filling veins and parameters of hydrothermal ore formation

The aim of this study was to determine at which parameters hydrothermal systems generate rich veins with bulk sphalerite contents of 30% and local concentrations in vein cross sections up to 60–70% and more. Such contents were found in the vein bodies of the Dzhimi deposit in the Sadon ore district, North Osetiya. For this purpose, we examined the thermodynamic models of the formation of base-metal filling veins. Ore-bearing fluids are formed in the root part of the hydrothermal system by the interaction of barren solutions with the host rocks (granites), which contain background contents of ore elements. The thermodynamic simulations were conducted for the system H-O-K-Na-Ca-Mg-Al-Si-Fe-C-Cl-S-Zn-Pb-Cu, which is described by 54 minerals of constant and variable composition and 78 aqueous species. The calculations for the mobilization zone were carried out for the temperature range of 360–440°C (through 10°C) and pressures of 600–1200 bar (with a 100 bar step). At each of the indicated temperature and pressure values, 100 waves (portions) of primary barren solution were subsequently passed through the granites. More than 20 complete models of the formation of filling veins (each model involving from 1000 to 1300 calculations) were constructed for individual T-P points in the mobilization zone, which was modeled by a sequence of multiwave step flowing reactors with a step of 10°C from 350–420 to 100°C at a constant pressure within the range of 600–1100 bar. We studied the effect of different background contents of Zn and Pb in granites on the efficiency of mobilization and ore formation and compared the relations in the naturally occurring distribution of ore elements along the continuous cross sections through Pb-Zb veins with the results of thermodynamic simulation. It was established that ore bodies with indicated bulk and local cross sectional contents of sphalerite could be formed in a narrow range of conditions in the mobilization zone (410–440°C and 900–1200 bar) and elevated background contents of Zn (more than 0.007 wt %) in the host granite. The maximum sphalerite contents (bulk and local in vein cross sections) are achieved updip the model veins within the temperature range of 150–200°C. Original Russian Text ? M.V. Borisov, D.A. Bychkov, Yu.V. Shvarov, 2006, published in Geokhimiya, 2006, No. 11, pp. 1218–1239.