ИСТИНА

Formation of porphyry-type ore deposits is associated with degassing of crystal magma chambers. Saline rich-in-metals magmatic fluid penetrates into a shallow region saturated with cold meteoric water where the metals concentrate in brine lenses. In order to investigate lenses formation, we developed an extension of our multiphase simulator MUFITS (www.mufits.imec.msu.ru) for NaCl–H2O binary mixture flows in a wide range of pressures, temperatures and salinities. A fast method for calculation of the properties of the mixture under sub- and supercritical thermodynamic conditions including three-phase gas-liquid-solid states is developed. We consider a degassing of magma chamber into axisymmetric host rock with a high permeable vein zone in the central part of the domain surrounded by low permeability material. The degassing is simulated as a point source of hot supercritical fluid that ascends rapidly along the vein undergoing the phase transitions. At relatively low pressures the evaporation of H2O results in enrichment of residual fluid in halite and metals. At a depth of 1–2 km halite precipitation as solid phase blocks the pore space and facilitates formation of concentrated brine lenses. Results of parametric studies indicate that the size of a brine lens increases with increase in injection temperature of magmatic fluid. At temperatures below ~ 650 oC evaporation of the brine does not lead to precipitation of salt and blockage of the pore space, thus, formation of the concentrated brine lens is suppressed. Low permeability of the host rocks decreases the intensity of meteoric water convection resulting in smaller dilution of magmatic fluid. It favors formation of a larger brine lens. This work was supported by the Russian Science Foundation under grant RSF-16-17- 10199.