Using the Selector PC application, we studied the process of formation of magnesian skarns at the contact of dolerites with carbonate-salt deposits. The physicochemical parameters of metasomatic processes were estimated by studying the localization of skarn ore shoots and mineral assemblages in the deposits of the Angara–Ilim type. The action of magmatic fluids on the system dolerite–magnesian salt deposits and dolerite–carbonate-salt deposits resulted in zonal columns of infiltration magnesian skarns. The computation was carried out using a dynamic multireservoir model of a flow reactor with a constant temperature gradient and a uniform pressure. We have established that changes in the C/H ratio and Cl content in the fluid source affect the composition of the produced mineral assemblages. Depending on the temperature during the formation of skarns, different mineral assemblages are produced: diopside, enstatite, anorthite, quartz, ilmenite, hercynite, and pyrrhotite at 1040–1010 °C; monticellite, forsterite, magnetite, geikielite, periclase, spinel, calcite, and graphite at 980–740 °C; and calcite, dolomite, phlogopite, halite, and graphite at 710–380 °C. Wollastonite is observed in the rear zone of magnesian skarns. We examined the temperature-dependent sequence of formation of different types of silicates, spinels, and Ti-containing minerals in the metasomatic column. The computation results show that during crystallization, the tholeiitic magma releases a fluid phase with C/H = 0.1–1.0, amounting to 1.5–2.0 wt.%.