Guilin Hu, Kim Dam-Johansen, Stig Wedel, Jens Peter Hansen. Decomposition and oxidation of pyrite, Progress in Energy and Combustion Science, Volume 32, Issue 3, 2006, Pages 295-314, ISSN 0360-1285, http://dx.doi.org/10.1016/j.pecs.2005.11.004.
"In inert atmospheres, the transformation of pyrite proceeds through a multi-step process in the sequence of pyrite→pyrrhotite→troilite→iron. The whole process is determined by the temperature and the total sulfur gas pressure in the system."
Since there isn't any oxygen in an inert atmosphere, the sulfur in the pyrite must be converted to elemental sulfur to maintain charge neutrality and mass balance as you proposed.