Computer-based reconstruction of the physicochemical conditions of formation of the Krasnov (16° N) and Ashadze (13° N) submarine systems in the Mid-Atlantic Ridge (MAR) has been performed using the equilibrium-thermodynamic approach to study samples from these sites. In the first case, a sphalerite-pyrite-barite association was considered, and in the second case, a sphalerite-pyrite association. In the modeling conducted, the composition of the sphalerite solid solution corresponding to the nonideal mixing of ZnS and FeS was used as a correlation parameter with the total composition of the Fe–Zn–Ba–S–H2O–NaCl–HCl hydrothermal system depending on temperature (200–300 °C) and a given pressure of 100 bar. The calculation results predict that at an iron content of 0.17–0.36 wt.% in sphalerite, the minimum formation temperatures of the equilibrium sphalerite–pyrite–barite association should correspond to the interval of 280–300 °C (Krasnov site). As the iron content in sphalerite increases to 4.15–13.28 wt.%, the occurrence of barite in the systems studied becomes impossible and the formation temperatures of the sphalerite–pyrite association become equal to or higher than 300 °C (Ashadze site).