An apparatus to mechanically stem (plug) a borehole or blast hole to contain or confine the gases generated by explosives is provided, thus preventing the energy generated by explosives to be released to the atmosphere. An assembly of the mechanical stemming apparatus for mining blasting operations according to the present invention comprises 2 pieces, placed one over the other: a base piece (1) and a central piece (6), and additionally includes two lateral wedge pieces (8) and four supporting pivots (11). The mechanical stemming apparatus according to the present invention and the entire apparatus set form a cylindrical body with a diameter lower than the borehole diameter and a length directly proportional to the diameter and depth of the boreholes or blast holes.