Development of free thermal convection of fluids in permeable rock domain with two parallel vertical faults is considered. Permeability of the faults is much higher than in the enclosing rocks. Upper and lower boundaries of the rock domain are horizontal. The lower boundary of the domain is impermeable. The upper boundary is open. Temperatures at these boundaries are fixed, and the temperature at the lower boundary is higher than the upper one. Three-dimensional model is considered. The conditions for thermal convection onset were obtained by linear stability analysis. The boundary condition for the convection onset is defined as existence of a steady-state solution of the governing equations for the fluid flow and heat transfer with nonzero convection velocities. This takes place when Rayleigh number exceeds certain critical value. These critical Rayleigh numbers are determined for different values of permeability contrast between the faults and the enclosing rocks and different distances between the faults. It is shown that thermal convection in the considered system can develop at Rayleigh numbers which are lower than the critical value for homogeneous rocks. Influence of Rayleigh number on convection cell pattern is analyzed. The role of the thermal convection in the considered systems in formation of ore deposits is examined.